ZOOLOGY D->_?I-

BIOLOGY
LIBRARY

General and Professional Biology

GENERAL AND PROFESSIONAL

BIOLOGY

with Special Reference to Man

(A One or Two- Year Course, Including Introductory Embryology and Comparative Anatomy)

BY

EDWARD J. MENGE, Ph. D.

If
Director of the Department of Zoology, Marquette University,

Late Professor of Biology, University of Dallas.

THE BRUCE PUBLISHING COMPANY
MILWAUKEE, WISCONSIN

IJBRARf
G

Copyright 1922
EDWARD J. MENGE

Printed in the United States
of America

To My Students

whose loyalty and appreciation have been my
inspiration throughout the years this
book has been in preparation.

-

PREFACE.

Teachers of the Biological Sciences have often observed that:

(1) The majority of American College Students are the children
of parents who have not had a college training and, therefore, have no
proper conception of what a college course means nor an understanding
of the reasons that lead, educators to choose certain particular studies for
the curriculum instead of others.

(2) The work done by the average student up to his entry into Col-
lege has neither taught him HOW to. study, nor how to coordinate the
work of the various courses he has had.

(3) The technical words he has met with have not been analyzed,
so that he has no conception of their derivation, and, consequently, of
their true meaning. He has largely memorized whatever was learned
with little understanding of meanings.

(4) The professional world (especially of Medicine and Dentistry)
is in general accord with the idea that "General Biology" or "General
Zoology" should be followed by "Introductory Embryology" and "Com-
parative Anatomy."

(5) Most -texts on the Biological Sciences either try to make the
subject matter entirely too easy, and thus forget to mention the many
points of prime interest to professional students, or they try to cover the
entire range of animal biology, thus burdening both book and student
with matter that is going to be forgotten as soon as examinations are
passed.

(6) The student is, therefore, confronted with several alternatives:
either he takes the "easy" course and feels that because he Avas told so
little, there is but little to be told, or, if the more detailed course has
been taken, he finds that it has helped but little, if any, to assist him
in his chosen field, and he is rightfully disappointed.

(7) The terminology in Botany, Zoology, and Medicine is by no
means identical, and much must be re-learned by professional students.

(8) A textbook usually confines itself to "Type Forms" or to
"General Principles." In either case the student suffers for want of the
half that is left untold.

(9) Results of scientific work are often given, such- as the life-
cycle of the Malarial Parasite, without showing in detail the type of
work necessary to bring about .those results, thus preventing the student
from gaining one of the most valuable lessons of his scientific course.

(10) Medical and Dental educators, as well as students themselves,
are constantly complaining of the insufficient stress placed on Histology

and Neurology in the preliminary courses, as it is in these fields that so
many students later find their greatest difficulties.

(11) Medical educators insist that in a few years all medical schools
must add a course in Medical Zoology. The students who are now be-
ing prepared for these courses must obtain an adequate number of ex-
amples of animal parasites in their pre-medical studies or they will not
be able to profit fully when such later course is taken.

(12) The student now purchases three, and often four, texts for his
biological work, none of which is a true continuation of its predecessor.

(13). When studying a given biological problem, constant reference
must be made to facts and findings of various kinds for the purpose of
checking up and coordinating the work one is doing. If a student must
seek through many volumes for such references he is all too apt not to
look for any at all, whereas if he has but to turn a few pages, .he will
almost invariably search out many.

Being confronted with points such as these, and wishing to obtain
the professional student's point of view as well as an understanding of
his difficulties, the author took the regular laboratory courses offered in
American Schools of Medicine and has built this book on what experi-
ence taught him to hold most valuable.

Therefore, he begins (1) by showing the student WHY to study,
and (2) HOW to study and HOW to COORDINATE the various
courses of the curriculum. (3) The glossary is made quite complete by
giving both derivations and pronunciations of all technical words used
in the text and the student is then obliged to write them out in the
parentheses left blank for that purpose. (4) "General Biology" is fol-
lowed by "Introductory Embryology" of the Chick and Frog, together
with a general statement regarding Mammalian Forms, thus presenting
to the student the beginnings of a Comparative Study. This, then, is
followed by "Comparative Anatomy" where constant comparisons are
not only made, but where back references are always being brought into
play, so as to force a repetition, so essential to a full understanding of
all scientific work.

(5) One subject (The Frog) is treated exhaustively so that the
student will not be burdened with too good an opinion of his own knowl-
edge of even so humble a thing as the frog, while principles are always
presented AFTER the facts have been shown upon which those prin-
ciples rest.

(6) The entire work is concentrated and by no means "easy." The
goal of the student is kept in mind.

(7) The terminology that the professional student is going to use
later is always borne in mind and stressed.

(8) "Type Forms" are studied, but only in so far as these are nec-
essary to a full and complete understanding of both the anatomy and

physiology of the animal and to furnish the facts on which to build in-
terpretations and principles.

(9) In such work as that on the Malarial Parasite the result of
scientific work is first shown so as to cause the student to wonder how
such a mass of intricate detail could ever have been discovered. Then
a detailed account of the painstaking and intelligent effort necessary to
make such findings valuable is given.

(10) Histology and Genetics are stressed because in all biological
work a thorough knowledge of the cell and tissues is a prerequisite for
further work, and Neurology, because of its tremendous importance in
all Biological, Psychological, and Medical fields.

(11) Examples, wherever possible, have been chosen in so far as
they add to, or detract from human welfare, for, after all, Students of
Education, Law, Philosophy, Psychology, Sociology, Theology, Econom-
ics, Engineering, Medicine, and Dentistry are, and must be, most inter-
ested in Man.

(12) All that is needed for two complete years of biological work
is contained within this one volume, each part logically following the
part preceding, thus not only saving the student considerable time and
expense, but also serving him as a sort of continual reference work in
his future professional years of study. Both the Bee and the Grass-
hopper have been included so that teachers may use their preferred form.

(13) Then, too, the student, having his entire course of work be-
fore him in a single volume, often, of his own volition, reads much more
than he normally would were the subject matter scattered through sev-
eral texts, for it is an easy matter to refe.r to another closely related
subject if the reference can be found by a mere turning of a few pages,
rather than consulting a number of separate books.

The book is so written that it can be used as a text for General
Biology, General Zoology (by merely omitting Chapters XV and XVI)
for Introductory Embryology, and for Comparative Anatomy.

Where only one year is given to biological work, such as in many
Dental Schools, it is suggested that the first semester be given to "Gen-
eral Biology" or "General Zoology" made up of selected chapters from
the first half of the Text, while the second semester be confined to the
higher forms such as Dogfish, Turtle, and Cat or Rabbit, as found in
''Comparative Anatomy."

The Laboratory Manual to accompany "General Biology" and "In-
troductory Embryology," written by Professor John Giesen, will be
ready for general distribution by June, 1923.

Dr. L. H. Hyman's "A Laboratory Manual for Comparative Verte-
brate Anatomy" (University of Chicago Press), is being used for the
Comparative Work in Anatomy.

Long Bibliographies have not been given in this book, as these are
seldom consulted by a student during the first two years of his college

career. However, as all of the books mentioned below should be in
every College Library, those who wish such bibliographies can find the
best of them in Kellicott's "Chordate Development," Patten's "The
Early Embryology of the Chick," and Kingsley's "Comparative Anatomy
of Vertebrates."

It is much more important for the student to know HOW to Com-
pile a Bibliography than to look over one already made. Therefore at
Marquette University a different subject is assigned each student to
look up for the purpose of compiling a bibliography of everything writ-
ten on that subject for the past forty years. Such subject may be taken
from any Index of the Journal of the American Medical Association.

Forty years are chosen because it is about that many years ago that
some of the larger indices were compiled, and it is essential that the stu-
dent be forced to go through all the indices year by year. If the indices
are not found in the smaller towns and cities, the bibliography can be
made during one of the vacations when the student passes through some
of the larger cities where there is a Medical or Scientific Library.

The more important Indices published in the English Language are :

The Zoological Record (published yearly by the Zoological Society of London.
Each volume gives a complete list of the works and publications relating to
zoology in all its branches that have appeared during the preceding year. The
first volume was for the year 1864).

The Index Medicus (Found in any Medical Library).

Index Catalogue of the Surgeon General's Office.

International Catalogue of Scientific Literature, "Zoology," "Botany," ''General
Biology." (Pub. by Harrison &.Sons, 45 St. Martin's Lane, London.)

For Popular Articles:

The Reader's Guide to Periodical Literature.

The International Index to Periodicals. (Before Jan., 1921, The Reader's Guide
to Periodical Literature Supplement.)

The books which have been of greatest service to the author are:

On General Biology

Parker and Haswell, "Text-book of Zoology."

L. A. Borradaile, "A Manual of Elementary Zoology."

Shipley and MacBride, "Zoology."

R. W. Hegner, "College Zoology."

J. G. Needham, "General Biology."

Linville and Kelly, "A Text-book in General Zoology."

O. H. Latter, "The Natural History of Some Common Animals."

Schull, Larue, and Ruthven, "Principles of Animal Biology."

A. M. Marshall, "The Frog."

S. J. Holmes, "The Biology of the Frog."

H. S. Pratt, "A Manual of the Common Invertebrate Animals."

Ward and Whipple, "Fresh-Water Biology."

Sanderson and Jackson, "Elementary Entomology."

Leland O.. Howard, "The Insect Book."

J. H. and Anna B. Comstock, "A Manual of the Study of Insects."

Frank E. Lutz, "Fieldbook of Insects."

J. W. Folsom, "Entomology with special reference to its Biological and

Economic Aspects."

Riley and Johannsen, "Handbook of Medical Entomology."
W. T. Caiman, "The Life of Crustacea."
R. W. Hegner, "The Germ-Cell Cycle in Animals."

W. E. Agar, "Cytology, with Special Reference to the Metazoan Nucleus."
L. Doncaster, "An Introduction to the Study of Cytology."
L. W. Sharp, "Introduction to Cytology."

C. Hill, "A Manual of Normal Histology and Organography."
Krause-Schmahl, "A Course in Normal Histology."
W. E. Castle, "Genetics and Eugenics."

E. G. Conkin, "Heredity and Environment in the Development of Man."
C. B. Davenport, "Heredity in Relation to Eugenics."

East and Jones, "Inbreeding and Outbreeding."

H. E. Walter, "Genetics."

T. H. Morgan, "A Critique of the Theory of Evolution."

S7~JT~Holme!;7 "The Evolution of Animal Intelligence."

M. F. Washburn, "The Animal Mind."

H. S. Jennings, "Behavior of the Lower Organisms."

Eric Wasmann, "Instinct and Intelligence in the Animal Kingdom."

James Johnstone, "The Philosophy of Biology/'

A. D. Darbishire, "An Introduction to a Biology and Other Papers."
Vernon L. Kellogg, "Darwinism Today."

Wm. A. Locy, "Biology and Its Makers."

H. F. Osborn, "From the Greeks to Darwin."

C. E. and E. A. Bessey, "Essentials of College Botany."

Bergen and Davis, "Principles of Botany."

C. S. Gager, ''Fundamentals of Botany."
Wm. C. Stevens, "Plant Anatomy."
Strasburghers "Textbook of Botany."

D. H. Campbell, "A University Textbook of Botany."
Coulter, Barnes, and Cowles, "Textbook of Botany."

I. F. and W. D. Henderson, "A Dictionary of Scientific Terms."

On Kmbryology

F. R. Lillie> "The Development of the Chick."
W. E. Kellicott, "Chordate Development."

B. M. Patten, "The Early Embryology of the Chick."
Prentiss and Arey, "Textbook of Embryology."

On Comparative Anatomy

G. C. Bourne, "An Introduction to the Study of Comparative Anatomy."
J. S. Kingsley, "Comparative Anatomy of Vertebrates."

L. Vialleton, "Elements de Morphologic des Vertebres."

Schimkewitch, "Lehrbuch d. vergl. Anatomic d. Wirbelthiere."

H. H. Newman, "Vertebrate Zoology."

H. W. Wilder, "History of the Human Body."

Parker and Haswell, "A Textbook of Zoology."

L. H. Hyman, "A Laboratory Manual for Comparative Vertebrate Anatomy."

H. S. Pratt, "A Course in Vertebrate Zoology."

Reighard and Jennings, "Anatomy of the Cat."

Davison and Stromsten, "Mammalian Anatomy, with special reference to

the Cat."

O. C. Bradley, "A Guide to the Dissection of the Dog,"
Hans Gadow, "Amphibia and Reptiles."

B. F. Kaupp, "The Anatomy of the Domestic Fowl."

C. J. Herrick, "An Introduction to Neurology."
Emil Villiger, "Brain and Spinal Cord."

S. W. Ransom, "The Anatomy of the Nervous System."

The author wishes at this point to thank all those who have as-
sisted him in any way.

Thanks are due to Professors Wm. A. Locy, F. R. Lillie. H. S.
Pratt, C. W. Ballard, Dr. L. H. Hyman, and Mr. W. C. Clute, and their
publishers, as well as to Professor J. H. McGregor, for permission to use
various cuts from their published works. Credit is given in the legend
of each cut.

Thanks are due Professors J. A. Bick of Loyola University, Edward
Menager of the University of Santa Clara, Wm. Atwood of the Mil-
waukee Normal School, and Dr. Peter P. Finney of the University- of
Dallas for reading much of the manuscript and offering valuable sug-
gestions.

Thanks are due for detailed reading and technical criticism of the
manuscript to the following:

Professor Richard A. Muttkowski, of the University of Idaho, for
going over the greater portion of the entire manuscript; Dr. L. H.
Hyman, of the University of Chicago, for going over the portion de-
voted to "Comparative Anatomy" ; Professor Eben J. Carey, of Mar-
quette University, for going over the entire portion devoted to "Embry-
ology"; Professors W. N. Steil of the University of Wisconsin, J. G.
Brown of the University of Arizona and the Carnegie Desert Botanical
Laboratory, and Sister Mary Ellen, of Santa Clara College, for going
over the portions on Botany ; Professors Joseph Jastrow, of the Uni-
versity of Wisconsin, and George A. Deglman, of Marquette University,
for reading the portions devoted to Psychology; Professor John B.
Kremer, of Marquette University, for reading the portions devoted to
Geology and Paleontology; Professor Edward Miloslavich, late of the
University of Vienna, for reading those portions on Immunity and Path-
ology; Professor Alfred V. Boursy, for reading the glossary, and Pro-
fessor Robert Bauer, for reading the chapter on Coordination.

Thanks are due Mr. Leo Massopust, Mr. Lane Newberry, and Mr.
Frank Leibly for the many and painstaking drawings they have made,
and to Mr. Arthur Vollert, Mr. Frank Krause, Mr. Gervase Flaherty,
Mr. Norman O'Neill, Mr. Frank Freiburger, Mr. Robert Schodron, Miss
Phyllis Schnader, Miss Irma Gall, and Miss Nathalie Hart for the many
hours of assistance rendered in seeing the book through the laborious
processes of printing.

Thanks are due the publishers, not only for their excellent work,
but for their constant willingness to render every assistance possible to
lighten the work of the author.

The author's appreciation must be extended to his fellow charter
members of the Baconian Society, Professors Walter Abel, Alfred V.

Boursy, John Giesen, and Edward Miloslavich, for assistance and criti-
cism rendered in discussing innumerable points at their meetings.

And lastly, the author would be remiss in his duty did he not ex-
press his special thanks and appreciation to his co-worker of many years,
Professor John Giesen, whose loyalty and willingness to assist in every
way have made many additional hours of work possible on this book.

EDWARD J. MENGE.

Marquette University,

Milwaukee, Wisconsin,

June 25, 1922.

CONTENTS

Chapter

Page

Part I— General Biology 19-432

I. Why To Study 19

II. How To Study 27

III. The Co-ordination of Subjects Studied 34

IV. The Frog 43

V. The Cell 88

VI. The Chemistry of Living Matter and Cell Division 94

VII. Histology of the Frog 108

VIII. Summary of the Frog 117

IX. The Protozoa 121

X. Interpretation of the Facts thus far presented. 157

XI. Genetics 165

XII. Animal Psychology 172

XIII. Intermediate Organisms 185

XIV. Immunity 194

XV. The Plant World 202

XVI. The Plant World Continued 215

XVII. The Coelenterata 247

XVIII. Introduction to the Coelomata 258

XIX. The Earthworm 262

XX. Flatworms (Platyhelminthes) and Threadworms

(Nemathelminthes) 285

XXI. The Arthropoda 312

XXII. Insects at Large 328

XXIII. The Grasshopper 332

XXIV. The Honey Bee 353

XXV. The History of Biology 375

XXVI. Paleontology 393

XXVII. Evolution 402

XXVIII. Classification 414

Part II — Introductory Embryology 433-663

(Chick, Frog, Mammal)

EMBRYOLOGY OF THE CHICK 435-542

XXIX. The Development of the Embryo before the Egg is laid. 435

XXX. The Primitive Streak and Origin of the Mesoderm .... 455

XXXI. The Four to Six Somite Stage (About 24 Hours) 465

XXXII. The First Half of the Second Day (24-36 Hours) 470

XXXIII. The Second Half of the Second Day (36-48 Hours) .... 476

XXXIV. Extra-Embryonic Membranes '. . . . 485

XXXV. Development of the Third Day 489

Chapter Page

XXXVI. The Differentiation of the Somites 502

XXXVII. The Development of the Fourth Day 508

XXXVIII. The Coelom and the Mesenteries 536

XXXIX. Development of the Fifth Day 538

EMBRYOLOGY OF THE FROG 543-618

XL. The General Embryology of the Tadpole as compared

with that of the Chick 543

XLI. The Digestive Tract ... 585

XLII. The Mesodermal Somites 591

XLIII. The Circulatory System 594

XLIV. The Urogenital System. 605

XLV. The Skeletal System 612

MAMMALIAN EMBRYOLOGY 619-633

XLVI. Mammalian Embryology 619

Part III — Comparative Anatomy. 634-886

XLVII. Introduction to Comparative Anatomy 637

XLVIII. Classification of Fishes, Amphibians, and Mammals. . . . 639

XLIX. The Integument 664

L. The Endoskeleton 681

LI. The Digestive System 722

LII. The Respiratory System 757

LIII. The Circulatory System. 772

LIV. The Urogenital System 806

LV. The Muscular System 826

LVI. The Nervous System 833

PRONOUNCING INDEX-GLOSSARY . . 887

Part I
General Biology

CHAPTER I.

WHY TO STUDY

Two hundred and sixteen (216) separate and distinct combinations
can be formed by three dice of different design, as shown by the
drawing (Fig. 1).

On the principal of chance, if these three dice are thrown an infinite
number of times, each one of the 216 combinations will appear just as
often as every other one.

This is true only if the dice are not weighted. Combinations being
formed by three dice have been chosen because there are usually at least
three alternatives in any case Avhere a man's judgment or opinion is
required or asked for. Further, an analogy can be found in the com-
plete human individual where the

Physical

Mental and

Moral

must ever We considered; while on the strictly scientific basis, everything
that a man is, or* can be, depends upon the three factors:

Inheritance

Environment and

Training.

Or, again, no opinion worth anything can be formed without the fol-
lowing three factors being taken into consideration :

Obtaining the facts

Reasoning thereon

Forming a judgment or conclusion.

Each dice possessing six sides may be compared to the many facts,
conditions, or possibilities that go to make up any one of the three great
factors appearing in the tables above.

It is self-evident from this that in any given case where there are
three factors with six possibilities contingent upon each, unless life's
dice are weighted by knowledge, a man's opinion stands only one chance
in 216 of being correct.

The almost ideal laboratory evidence that substantiates these
statements is found in the fact that out of three thousand cases
at one of our leading hospitals, it has been found the diagnosticians
were correct only 53.5% of the time.* If, at our most important insti-
tutions, the ablest and best trained men, working with the finest equip-
ment obtainable, are correct only approximately one-half the time, it
means that on the principle of chance, when anyone passes an opinion

*"Diagnostic Pitfalls Identified During a Study of Three Thousand Autopsies," by Richard C.
Cabot, M.D. Journal of the American Medical Association, pp. 2295-2298, Dec. 28, 1912.

V ::'* -GENERAL BIOLOGY

BHHHHH C

[OI!010II01I01O

* •II* • ! (• • ) I* •! U •] U •! I* •! (• «| !• •! [TT1 [•"•] (TT1 (TTl (TVI fTT] ITTl (VTJ r

ilUllIJy [LiJIIIJ 1XJ|XJ[X)[XJ|XJ|XJ liiliUlillilllilJJI

HBS0H QQQQQH 1Z00SIZI

[3 Olds 13 Id 0Ix][xllxHxi|x] [ElOimillOlIO

Fig. 1.

Probability of Error Chart (showing there are 216 different combinations pos-
sible with three dice of different design).

WHY To STUDY

21

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prri p-ii [7T] p-ri rrsi pn ra [vl [vl [vl Ivl [vl [T:irr:]f:T|fr:|fr:l|:Ti

!• •[ [• •! |« •] ]• «| [• •[ |» •[ [• V| [• •[ [• *| !• »| |» •! !• »| l» •! I* •! !• »l !• »l |» »l !• •!

HHHEHH Qf3QQI3'Q 00S21E1IZ]

• I (••ll**H**H**ll**ll*

• • I* •! I* •) l< ill* SI

HHE1QEIE] HHOHaQQ 000000

Fig. i.

GENERAL BIOLOGY

or conies to a conclusion without all obtainable knowledge, he cannot
approach correctness even this often.

The evidence forces the conclusion that, under present conditions,
if we should know all that is possible for a human being to know, we
could be right only about one-half the time. As knowledge is the only
way in which we can be right even as frequently as this, it follows that
in instances wrhere an opinion is called forth without any knowledge, a
man forms approximately 215 erroneous conclusions to every one that
is correct. The scriptural command becomes intelligible : "Get ye there-
fore knowledge."

It has been said that the evidence from diagnostic sources is almost
ideal to illustrate the point here made. Everything we do that requires
an opinion is pure diagnosis. In other words, every time one passes
a judgment upon the facts presented, it is diagnosis of some kind, and
any error in our diagnosis means that no intelligent suggestion can
come forth as to a remedy, except on the basis of one correct one to
215 erroneous ones. The diagnosis must be correct or the remedy is
absurd with the only possible exception of a guess accidentally correct.

No intelligent person wishes to have his government run, his estate
adjusted, his house built, or his farm managed upon pure guess work
in which the chances are two hundred and fifteen times more wrong-
things being done than right ones. And this is not only the case in
medicine, dentistry, and the professions at large, but in the every-day
business world as well. Dun and Bradstreet, who keep a record of every
individual entering, as well as every one failing in business, tell us that
95 out of every 100 men wrho enter a commercial line for themselves
fail at some time in their lives. This is due, not only to an ignorance
of the particular line of work they may enter, but also to ignorance of
business principles and methods at large.

To many persons it seems that the purely practically-trained in-
dividual is better equipped than he whose training has been theoretical,
and individuals usually mentioned as examples to illustrate this point
<of view are always some of the ablest practically-trained men to be
found, who are then compared with some of the poorest theoretically-
trained. Because a boy is sent to college does not mean anything
except, that, if he has a capacity for the work that he there takes up;
he will be able to get the practical side of his study, while in addition
he will learn why he does what he does, when he does it. Any man
with great ability along a given line will naturally know more, and be
able to work better along that line than any man without such capacity
who has merely taken some theoretical course. But, if we take two
men of equal intelligence and capacity, who take up, let us say, the
plumber's trade, he who has mastered both the practical and the theo-
retical side of his work will always be superior to him who has become
interested in only one or the other. It must be remembered that

WHY To STUDY 23

(1) Capacity

(2) Opportunity, and

(3) Application

are essential to make a master of anyone in anything.

It takes considerable time to show the fruits of any study, and men
are impatient tor results.

Someone has truly said that the value of education consists in
knowing a man when you meet one, which means, of course, that any-
one knowing- his subject-matter in a given case will be able to know
whether anyone claiming to be an expert in that field speaks truth-
fully or not. It means that we must KNOW about a matter ourselves
or we cannot intelligently choose worthy leaders. It means, we must
be able to distinguish the gold from the dross, real ability from adver-
tising, real scientific men from those who are simply well-known. It
means we must be able to distinguish between the real expert and him
who calls himself one, remembering that experts do not disagree very
much, but those wrho call themselves experts do.

Whether we like it or not, we must acknowledge that every man
who has ever lived and exercised any kind of leadership, good or bad,
has left his impress upon our generation, for all those who have gone
on before us and now sleep the eternal sleep, together with those living
now, as well as those who are to come after us, really form a great
intellectual democracy, from which all but the present generation are
removed only in person. The past is with us in an overwhelming mass.
The unborn are those for whom we now labor. All of our customs, our
traditions, our ideas of conventional correctness and wrongness, and our
laws were given us by men long since passed away. In other words,
we are actually ruled by dead men.

The men who have long since passed away have given us their
ideas and their thoughts but, to us those ideas and thoughts, those laws
and traditions must be interpreted and our interpreters of these things
are our courts. Mr. Taft has said, "I care not who makes the laws, if
I can but interpret them." It is always meanings, interpretations, that
are of most value. Now, we know that our judges (our legal interpre-
ters), are practically all college men, which means that, in the final
analysis, everyone of us is controlled by what our institutions of higher
learning teach.

It therefore behooves each and everyone of us to obtain the requisite
knowledge before forming an opinion as to whom we shall follow as
leaders in every walk of life, whether this be in politics or in war, in
civil or religious life, in law or in medicine, in farming or commercial
pursuits, or we shall be wrong in nearly every thing we do.

Not possessed of knowledge, a man confuses sincerity with truth,
forgetting that the most insane of men are intensely sincere, and that
anyone following sincere but insane theories of life must quite naturally

24 GENERAL BIOLOGY

reap destruction ; forgetting that to obtain the truth in anything, all the
facts must be known and a valid interpretation placed upon the facts,
and this can only be done by considering man in his entirety — in his
physical, mental, and ethical aspects.

One must therefore weight life's dice with knowledge if correct-
ness is to be formed in any walk of life.

Nearly every parent desirous of his child's welfare, wishes he could
leave the child the benefit of his own experience, so that the child could
profit by his parent's mistakes and not make the same ones. The value
of this may be appreciated when it is remembered that if this cannot
be done, there is absolutely no progress. For, each individual, instead
of starting where his parents left off and continuing onward, would
necessarily have to begin where they began, and, consequently, when
life came to a close they would be exactly where their parents had been
at the same time.

Men of the past have therefore written their experience in books,
and we of to-day can profit not only by the experience of our immediate
parents but by the experience of all our forefathers.

The laboratory has gone even a little further than this.

As no one man could wrork out every detail in the study of a single
plant or animal without having to take the work of all those into con-
sideration who had gone before and who had contributed something to
the knowledge of the particular plant or animal under discussion, so,
men have gathered into a single grouping the important physical experi-
ences which have been found convincing to their minds and have called
such grouping a textbook.

The study of the subject-matter of such textbook, plus the actual
working out of these same convincing experiences (now called experi-
ments) in the laboratory, cause the student actually to see the way in
which proof is obtained for the various conclusions men hold.

In fact, laboratory work, plus a study of the text, is the fulfillment of
the parent's wish that his child inherit the parent's experience.

From the experiment which thus gives us conclusive evidence of the
way some physical process works, we draw our principles.

Principles are mental tools without which no mental progress would
be possible.

In fact, a principle is a law of nature, proved by physical experi-
ment, to which no exception has been found. Physics presents an
excellent illustration of the value of principles.*

Everyone knows that if a substance, such as iron, which is heavier
than water, is placed in water it will sink. Yet iron ships do not sink.

*An old Greek named Archimedes, while taking a bath, discovered that when he immersed his
body in a tub filled with water, his body lost considerable weight. Later he was able to prove
experimentally that the weight of the water that ran over the top of the tub was exactly the same
as the weight his body lost while immersed. It was this discovery which made iron ships possible.

WHY To STUDY 25

Why? Because what we mean by "heavier than water" is that the same
quantity of a given substance is heavier than the same quantity of water.

After Archimedes discovered his principle, we knew immediately
that if we could bend iron so that it would occupy more cubic feet of
space than that same number of cubic feet of water would weigh, it
would be "lighter than water" and would float.

Heavy iron ships could not, however, be of practical use until some
one again, discovered the principles of steam or electricity, and so they
were not used until such principle was found. This shows well the
inter-relationship of things that men do, no matter how many years
apart the doing may be.

In Biology, which means the Science of Life, (Gr. Bios=life+
logos=discourse) we are interested in finding principles, so that no
matter in what position we may be placed in later life, we can always
think back, find our principle^ and apply it in a thousand and one
different ways.

The finding of principles, which is real science, must never be con-
fused with the application of these principles.

The former is what is meant by science while the latter is merely
ordinary labor.

Inventors apply scientific principles, they are therefore not scientists.

Another point to be remembered is that animals (as well as children
before they reach the so-called age of reason) learn by doing a thing
over and over, until success or failure comes. If success conies, they
make such successful endeavor a part of their later life. This is called
the trial and error method of learning.

Educated men and women do not try out each and everything, but
come to their conclusions by weighing the evidence for and against a
principle, and if the principle is found to be worthy of consideration,
adjust their actions accordingly.

This is well illustrated if one finds a man attempting the invention
of a perpetual motion machine.

It is a well established principle of physics that no more work can
be obtained from a machine than is put into it, and even then a little
loss must be allowed for on account of friction.

All educated men know this law of nature, and consequently do
not waste their time on such a fruitless undertaking.

But, should any one refuse to accept this principle, there is nothing
left but to continue trying and trying, and coming to the conclusion of
its uselessness by personal failure. The obtaining of principles is then
the great work of science. Science itself has had many definitions.
Some of the best are:

"Systematized knowledge."

"Classified common sense."

"Checking up and getting rid of one's prepossessions."

26 GENERAL BIOLOGY

"Knowledge gained and verified by exact observation and correct
thinking, especially when methodically formulated and arranged in a
rational system."

In other words, science means a gathering of facts, plus the logical
meanings or interpretations of these facts.

The object or purpose of getting the principles which science thus
finds, is to control nature and to prophesy what will occur when given
acts are performed.

As Biology is the science of life, everything that has anything what-
ever to do with life is really a branch of this science. For example,
Biology is not a branch of medicine or dentistry, but medicine and
dentistry are branches of biology where men enter into particular details
of some division of it.

Biology, therefore, to the medical student, the dental student, the
law student, the student of sociology or the student of education, presents
a sort of bird's-eye view of every one of the fundamental processes upon
which his whole detailed study must be built, so that he can the better
and more understandingly read the valuable and meaningful literature
of the day and discuss intelligently the very basis on which his every
conclusion must rest its truth. In other words, it makes it possible for
the student the better to be able to prevent men who call themselves
experts, but who are not, from deceptively making people believe they
are.

CHAPTER II.

HOW TO STUDY

Someone has said that to be a cultured man or woman one must
know something about everything ; everything about something ; and
never wilfully or maliciously cause suffering to others.

No better ideal of what a student of biology should try to attain has
ever been written. If all biological principles now known are grasped
by the student he can most certainly be said to come as near knowing-
something about everything as it is possible for him to come.

If he will learn the Frog thoroughly, he can come under the second
division of importance, and if he will bear the final of the three injunc-
tions in mind, Biology will be a humanizing and cultural as well as a
scientific and laboratory study.

Never read a book, article, or paper, without fountain pen and note-
book or note-paper beside you.

(Probably 95% of all you are ever going to read, you will want to
forget, but the remaining 5% you will need, and need badly when that
need comes.)

I. Notes are of no vafue unless they are usable. There are differ-
ent types of records for different purposes, but those found most con-
venient by the author are as follows :

For regular lectures and for general reading: A series of ordinary
bond paper cut to the size of 4x6 inches.

Many of these can be carried in the pocket constantly if a little
heavier paper, or even a piece of cardboard 4x13 inches is bent in the
mid-line so as to form a covering for the loose cards. A rubber band
is placed about the packet.

A paste-board file for this size of card can be obtained at any
stationery store, and the cards then held together by fasteners can be
placed under subject headings, ready for access at all times.

No book or article of value should be read without its name, sub-
ject, author, and edition of the book stated. (This latter is very im-
portant, because in a year or two another edition may have all its pages
differently numbered, and even additions and deletions made, so that
should you quote such a volume, and the one to whom you are quoting,
looks it up in another edition, you will be considered not only inaccurate
but absolutely untruthful.)

For clippings — If you own the journal or paper in which an excellent
article appears, cut it out. Be sure, however, to write upon it immed-
iately the name of the periodical from which you clipped it, as well as the
year, month, and day it appeared.

Until you have a large accumulation take ordinary long (envelopes

28 GENERAL BIOLOGY

inches. Fold each clipping and place it in such envelope,
writing its title on the upper left-hand corner under the subject like this —

Biology
Frogs

"Different kinds of Frogs."

If you do not own the periodical, make out a subject-card just as
you would for a lecture, and file it under the proper subject.

For your regular laboratory work you will always have your draw-
ing book. All notes that pertain to the laboratory will be made in that.

II. Students will find that it is easier not to take notes on lectures.*
Many think they will remember more without notes. This is true if
the notes are not looked at again ; but, if they are gone over from time
to time, although some of the lecture may be lost by the writing, still,
much that is important will be brought back to mind that would other-
wise be lost.

III. Suppose you see an animal, let us say a frog. You must
observe :

(1) Its external characteristics.

(2) Its similarities to the human being.

(3) Its dissimilarities to the human being.

(4) Its normal home.

(5) Its method of life.

(6) Its relations to its surroundings.

(7) The conditions under which you came to see it.

(8) Its actions, normal and when disturbed.

(9) Its own food and whether it is food in turn for other animals.

(10) How it comes to be where it is?

(11) Whether or not it remains in the same surroundings through-
out the year?

This is all studied on the living frog. Kvery action of a living thing
comes under "physiology," which is the science of functions. Physiology
must, therefore, be studied on the living plant or animal.

V. In the laboratory you take up the internal structure, as well as
the development of the organism, which means in the case of the frog,
how the animal's eggs grow into a tadpole, and this into a frog in turn.

VI. You are trying to get a complete picture in your mind of what
is known about life and you are trying to get a gauge by which to know
when a thing is true and when it is not ; consequently, you must think of
these things you study just as you would if you were trying to use your1
camera instead of your mind for the taking of the picture.

(1.) Clear away all that is unimportant.

"Of course, each day, after the lecture, the student is to attempt to repeat all that was said in
the lecture he has heard, and when he conies to a forgotten point he must consult his notes.

How To STUDY 29

(2.) Choose a subject. In this case biology.
(3.) Have proper perspective (that is, have the relationship of
everything about your subject, in proper form, and do not unduly stress
any one point). In other words, have you taken everything into consid-
eration? For example, you must see to it that some other branch of
science does not have some points and conclusions which may destroy
yours, for if a single exception can be found in science, it has disproved
the law.

(4.) There must be sufficient light to make the subject stand
forth and be seen clearly. This implies a proper background — a back-
ground built up in the study of science by ascertaining what has gone
before, and what causes have produced the particular historical soil upon
which the seed of men's ideas have been able to grow. In other words,
you must have all the facts that can be found, if you would have this
light throw your subject into the full glare of day. You must exclude
shadows as much as possible.

(5.) You must see that your subject is in focus, which means
that in any given case, it must stand forth in bold relief. It must not fade
away in the distance and become blurred by your prejudices or desires.
No vagaries of thought must be permitted. Your reasoning must be
clear and definite. Your whole system must be built up philosophically
and logically.

(6.) You must decide upon how large an opening you are to
allow your lens. That is, within what narrow limits you are to discuss
the subject under consideration.

(7.) You must decide upon the length of time for your ex-
posure. In a scientific treatise this means that you must know that suf-
ficient time has elapsed to make your experiments valid and positive.

(8.) If you read of the results of others you must take into
consideration the temperamental makeup of the individual writing as well
as of yourself and other readers who later pass judgment thereon.

And remember that just as a football player or a
musician must keep in constant practice, or lose his
proficiency, so your brain must have its DAILY
EXERCISE or it will likewise lose its proficiency.
Further, as the distinguishing difference between man
and the lower animals consists in man having an in-
tellect, form your own conclusions.

Remember also, that the man who can, but does not
read, has not as high an intellectual ranking as he who
can not read ; and he who has the capacity to think but
does not, ranks lower than he who is born without such
capacity.

VII. See that your note book contains complete drawings and de-
scriptions of each of the following subjects for every form studied:

30 GENERAL BIOLOGY

I. ANATOMY.

1. External Makeup.

2. Internal Makeup.

(a) Digestive.

(b) Circulatory.

(c) Respiratory.

(d) Excretory.

(e) Nervous.

(f) Skeletal.

(g) Muscular.
(h) Reproductive.

II. PHYSIOLOGY

1. Processes Pertaining to Food.

(a) Ingestion (Taking in food).

Egestion (Throwing out of undigested food).

(b) Digestion (Fermentation of ingested substance brought
about by various secretions).

(c) Absorption (Passing of digested food by osmosis through
body membranes and making use of inspired oxygen).

(d) Circulation (Carrying absorbed substance to all parts of
the body).

(e) Assimilation (The conversion of absorbed non-living mat-
ter into protoplasm).

(f) Growth (The increase in size due to additional protoplasm
being made).

(g) Reproduction (The production of new organisms similar
to the parent).

2. Processes Pertaining to Oxygen.

(a) Inspiration (Taking in oxygen which aerates blood as it
passes through lungs).

(b) Oxidation (Burning of ingested food).

(1) Secretion (Pouring out substances to be used again).

(2) Expiration (Throwing out CO2).

(3) Excretion (Throwing out used substances in the form
of urea, uric acid, etc.)

(Response

(4) Energy (a) Applied < Behavior.

[Locomotion.

[Heat.
(b) Unapplied^ Light.

, [Electricity.

III. STIMULI

Any of the processes mentioned in this outline may be made more

How To STUDY 31

rapid or may be slowed even to the point of complete stoppage by me-
chanical, chemical, and sometimes by mental stimuli.

IV. CLASSIFICATION

(a) Phylum.

(b) Class.

(c) Order.

(d) Family.

(e) Genus.

(f) Species.

V. Finally, remember that your study may be interpreted from any or
all of the following points of view :

Both plants and animals must always be thought of when discussing
living matter, and both are studied in practically the same manner. But
as no one man can study all about plants or animals, or even about every-
thing that pertains to only one plant or animal, scientific men have
divided their work into group-studies as follows :

I. Morphology: (Gr. morphe=form) Study of Form.

1. Promorphology (Gr. pro=First+morphe=^Form) which
treats of General Form.

2. Anatomy (Gr. anatemno— to cut up) The study of organ-
isms by dissection. Usually studied on the dead individual; that is, a
study of Structure.

Subdivision

(a) Gross, or Macroscopic (Gr. macro— large) that which can
be seen with the naked eye.

(b) Microscopic (Gr. micro=small) embracing the study of
the more minute structures with a microscope.

Splanchnology (Gr. splanchnos=organs)
Histology (Gr. histos=web, or tissue)
Cytology (Gr. cytos=cell)
Neurology (Gr. neuron=nerve)

3. History of Development, that is, a study of the different
stages through which an organism passes from the moment the fertilized
cell begins dividing.

Individual Evolution, Embryology (Gr. en=in-f bruo— bud,
Ontogeny (Gr. on=existing-f genna=to begin), the study of the indi-
vidual before birth.

Racial Evolution (Phylogeny). (Gr. phylon=tribe.) The
study of the race.

4. Teratology (Gr. teras=wonder) the study of malformations
and monstrosities in organisms.

II. Physiology. The Study of Functions.

1. Physiology Proper, (Gr. physis=nature) the functional rela-
tion of part to part and to the whole.

32 GENERAL BIOLOGY

2. Ecology, (Gr. oikos=house) Relations of the individual to
its whole surroundings, or the study of its environment.

3. Pathology, (Gr. pathos=suffering). The study of disease.
This study also belongs under microscopic anatomy in that disease makes
many changes in the actual structure of the cells and tissues.

III. Distribution.

In Space, (Geographical Distribution, often called Zoogeography
in so far as it affects animals).

In Time, Paleontology, (Gr. palaios=ancient-|-onta=beings),
also called Paleozoology, in so far as it affects the study of fossil remains
of animals.

IV. Economic Zoology or Botany, the study of everything living in
so far as it touches human welfare.

V. Classification, or Taxonomy (Gr. taxis=arrangement-)-nomos=
law) The grouping of plants and animals according to likeness or rela-
tionship.

VI. Psychology, (Gr. psyche—mind or soul) The study of the
mind.

VII. Sociology, (L. socius=companion), the study of animal
societies and their relation of each member of the society to the other.
This relationship is sometimes said to be due to the so-called Herd In-
stinct.

VIII. Genetics (Gr. genesis=birth) The science "which seeks to
account for the resemblances and differences which are exhibited among
organisms related by descent."

Heredity,

Individual : •{ Environment, , _

T^, , .. \ Instruction.

Education, \ ...

( Emubung or training.

Racial: Archeology, (Gr. archaios=ancient) The study of
ancient findings to ascertain the cultural state of man at different epochs.

IX. Biometrics (Gr. bios=life+metron=measure) The statistical
study of life's events or happenings so as to be able to gauge how often
these same events are likely to take place in the future.

Summarizing the points that must be kept in mind we may say :

1. You must be able to distinguish between conspicuousness and
importance.

2. You must be able to distinguish between fact and interpreta-
tion,

3. You must be able to distinguish between principles and their
application.

How To STUDY 33

4. You must be able to discuss all sides of a subject so as to over-
come scientific bigotry and narrowness.

5. You must have a knowledge of type-forms as well as the gen-
eralized biological principles which can be drawn from such knowledge.

6. You must be able to apply all the principles you have learned
to the human body.

7. You must know (not only memorize) the meaning of the scien-
tific words which you are called upon to use. This will be accomplished
by writing in the derivations of the words in the parenthesis left open
for that purpose. All these can be found in the glossary at the end ol
the volume.

CHAPTER III.

THE CO-ORDINATION OF SUBJECTS STUDIED

Comparatively few students either grasp or understand the value of
the various studies laid out for them by college authorities, and it is this
lack of grasp and understanding which causes them to slur over much of
the subject-matter which it is necessary to know a little later.

As a starting point for this understanding it is essential that one
grasp fully the underlying object of scientific study.

There used to be a marvelous clock exhibited in our smaller cities
which had some ten or fifteen dials upon it, each dial recording some im-
portant time-element. One, for example, showed the hour of the day,
another the time of the rising and setting of the sun, another did the
same for the moon, while still another gave the time of the ebb and flow
of the tides. There was a dial showing the day of the month, so nicely
arranged that even the 29th day of February in leap year would be
noted. This clock was so adjusted that there was not only the intonation
of a chime every quarter-hour, but several interesting events were re-
corded at this same time ; for instance, a cuckoo announcing the quarter-
hour was followed by a rooster strutting forth and crowing, while on
the hour, a tiny door at one side of the clock opened, and the twelve
apostles solemnly inarched across and disappeared on the opposite side,
while strangest of all, these apostles did not drag their feet, but actually
lifted them as they took their hourly walk.

And all of this elaborate adjustment was the result of a single clock-
work running by a single winding.

A living organism is something like that clock, except that it is a
thousand-fold more complicated, for man can do many times the number
of things that the clock did. Now, the science of biology is directed just
toward the one end of attempting to find what original mechanism
makes all these complicated actions possible. In other words, in the
study of biology we are attempting to find not only life's clock-work
and the unit structures with which such a mechanism is built, but we
also try to approach that particular place or time in the history of the
universe when the first winding took place.

Chemistry presents an excellent starting-point for further explana-
tions. In chemistry a compound is analyzed by finding the type of
molecules that are contained within it, a molecule being the smallest
obtainable particle of a chemical compound. The molecule, in turn, is
then reduced to atoms of the various chemical elements, an atom being
the smallest obtainable particle of a chemical element. The mind can
readily conceive, however, that there are smaller particles than atoms.

CO-ORDINATION OF SUBJECTS 35

As soon as we realize that the black soot which comes from a
smoking-chimney and the purest of white diamonds are both composed
of exactly the same chemical atoms — that is, are both pure carbon — we
find we are not satisfied until some explanation of this remarkable dif-
ference can be given.

At this point we must pass to the study of physics, for it is physics
that deals with the laws of movement and of energy. In reality, it is
from the physicist, rather than from the chemist, that we obtain an ex-
planation as to why the different chemical formulae are what they are.
The physicist has found that atoms can be broken up into very tiny par-
ticles, each fragment having the power of attracting or repelling other
fragments. Such tiny particles are called electrons. •

Knowing this we can evolve a great underlying principle that
can be applied in as many different ways as was the principle of Archi-
medes, referred to in a former chapter.

In fact, the theory of electrons tells us not only why two elements
chemically alike have a totally different appearance, but it also gives us
an explanation as to why there are different chemical elements to begin
with.

It has been found that if pure carbon is subjected to tremendous
heat in electric furnaces, followed by the application of thousands of
pounds of pressure, this carbon will become a diamond.

The scientific man, learning that this is true, immediately attempts
to bring about a wider application of his knowledge for the purpose of
evolving still other principles. Such an one comes to the conclusion,
then, that it is quite likely that all matter is composed of electrons, and
that the different forms that matter assumes are due only to varying de-
grees of heat and pressure.

This means that everything physical — everything that occupies
space — be it wood, iron, coal, radium, hydrogen, oxygen, or \vhat not,
is quite probably composed of the same ultimate material or substance,
but in each case such ultimate substance has been exposed to a different
quantity of heat and pressure.

Now, the object of science is to control nature and to prophesy
what will occur when certain acts are performed.

It is well to note at this point that science can never explain the
fundamental why of anything. It can not tell why metal becomes soft
when heated while an egg becomes hard. What science can do, and
aims to do, is to find how things can be changed from \vhat they are by
performing a combination of certain very definite acts.

Just as the explanation of how chemical elements come to be what
they are, was found by physicists and not by chemists, thus showing the
inter-relationship of the two sciences, so man, being a complete entity
made up of both the physical and the mental, must be considered in this
double capacity if we are to study him scientifically.

36

GENERAL BIOLOGY

On the physical side physics and chemistry form man's most impor-
tant studies. Everyone knows that food is composed of chemical sub-
stances which, after being taken into the body, passes through many
chemical changes before it is converted into new blood to keep life going.
The student, however, probably does not know that Louis Pasteur, the
Father of Bacteriology, was a chemist, and that the whole modern con-
ception of medicine is based on the bacteriological findings he obtained
while working on fermentation experiments in the chemical laboratory.
The knowledge he there gained was later applied by Lister, who made
aseptic surgery possible.

The discovery of oxygen, by a chemist, directly underlies practically
every experiment in physiology that can be performed, while the great
modern surgical advances are largely due to our ability to anesthetize
the patient. The anesthetics used are nitrous oxide, chloroform, and
ether — all products of the chemical laboratory.

Fig. 2. Diagrams to illustrate the different types of levers in their relations to
the mechanical' action of muscles.

A. Comparison between head, foot and elbow.

B. Comparison between different actions of foot.

Most muscles act on bones as levers. In physics there are three types of levers recognized. In
the first type (I) the fulcrum (F) lies between the place where power (P) is exerted on the lever
and the point of resistance or load (L). Levers of this kind are frequently met with in the body.
In A, (1) the weight of the skull tends to bend the head forward, while the force exerted by the
dorsal muscles of the neck serves to keep the head in position.

In levers of the second class (II) the point on which power is exerted moves through a greater
distance than the point of resistance. Speed is thus sacrificed to power. Such levers are rare in
the body. An example is the body raised on the toes.

In levers of the third class (III) the point on which force is exerted, moves a less distance
than the point of resistance, power thus being sacrificed to speed. This is the most common form
of leverage in the body. The example A (III) is that of the elbow. Here the biceps and brachialis
muscles are attached only a short distance from the elbow-joint or fulcrum, while the hand is the
region on which force is exerted. A movement at the point P through a short distance will cause
L to move a great distance.

The ability to analyze any product or portion of the body must lie
with analytical chemistry, while the study of how to build up new
products comes under synthetic chemistry. All digestion takes place by

CO-ORDINATION OF SUBJECTS 37

enzymes, and the enzymes of the stomach, for example, will not function
unless they are placed in an acid medium, such as the gastric juice.
Changes in food, or abnormalities of various kinds, may cause an excess
of this acid, or may prevent a sufficient quantity of the proper quality
being formed — all such changes are chemical.

The study of physics in its application to one's body is not so self-
evident and is often the bug-bear of students. Unfortunately most of
the textbooks on physics lay their greatest stress upon mechanical laws
only in their industrial applications, and fail to show how these same
laws apply to the human body.

The mechanics of the living body are, however, quite similar and
much more important than all the industrial applications which can be
found.

The three types of levers with the fulcrum in different positions is
the same in the body (Fig. 2) as it is in general mechanics, and a
knowledge of the exact points where stress and pull are applied, with a
consequent ability .to "figure out" where new growth structures will de-
velop, is of prime importance in broken, misplaced, and re-set structures,
if the patient is not to suffer untold agony and sorrow in future years.

In this connection, the laws governing pulleys, the combination of
rolling "and sliding movements of joints, as well as the principles of
gravity, must be thoroughly understood ; for, it is simply and solely on
these principles that the various movements of the body can take place,
and consequently, it is only a knowledge of such principles which can in
turn make possible the correction of abnormalities of joints.

The principles governing friction are applied- in the correction of
both internal and external injuries, while experimental physiology would
be impossible without a knowledge of centripetal and centrifugal forces
and the laws governing liquid and gaseous pressures.

The la\vs governing liquids apply throughout the entire body in
great detail, for there is scarcely a spot as large as the point of a needle
anywhere in the body that liquid nourishment (blood or lymph) does
not enter. Pressure in any region causes swellings, varicose veins,
dropsy, and a host of other ills ; while bed-sores are nothing more or
less than the effect of continued pressure of blood in the same vessels
of the side or back on which the person lies, gravity causing the blood
to sink to the lowest level and be held there.

An understanding of the difference in densities makes many physio-
logical experiments possible, which would otherwise result fatally to
the patient. A solution, if it is to be injected, must not only have the
proper density so as not to cause a too rapid change in the blood, but
the whole subject of osmosis, diffusion, and capillary attraction must be
understood before such an experiment can be intelligently applied.

The place where parasites are most likely to lodge, is largely deter-
mined by the rapidity, direction, and pressure of the blood-stream.

38 GENERAL BIOLOGY

Hydrometers and urinometers for testing liquid densities are built
on the principles just mentioned.

Air is a gas, and as such comes under the laws governing' gaseous
pressure and gaseous diffusion. When it is remembered that the whole
process of life is snuffed out whenever the breathing apparatus ceases to
work properly, it will be seen that the aeration of the blood to keep it
red and healthy, the working of the lungs under normal and abnormal
conditions (the latter in chest puncture), the being overcome by gas,
externally or internally, as well as the changes in breathing at different
heights and at different depths (as on mountain-tops and in subma-
rines) ; all these can only be understood and helped by a thorough study
of the laws and principles applying to gasses.

The principle of the force-pump makes the pumping of the heart
and the one-way valves in heart and veins understandable.

All food eaten can only be reduced to blood by a burning process,
called oxidation, so that unless the principles governing heat are under-
stood, the processes of digestion and the consequent abnormality — indi-
gestion— must go on unremedied.

The principles of ventilation in the home, office, work-shop, or sick
room, make for health or disease, just as one applies them or leaves them
unapplied. A window opened at the top warms the incoming air before
it strikes the patient. The knowledge that warm air ascends and cold
air descends lets us know that a heating plant must be placed in the
basement and a cooling plant in the attic, while the principle of evapo-
ration explains how outpourings of the sweat glands, by being drawn
off rapidly into the surrounding atmosphere, make it possible for warm-
blooded animals to retain an even temperature, regardless of varying
environments.

In "chills" the body really produces more heat than ordinarily, but
it is the heat-regulating apparatus which is out of order.

Thermometers and hygrometers are measuring instruments by
which we are able to note the degree of heat and moisture respectively
in the atmosphere. They are, of course, simply an application of some
of the laws learned in physics.

Then, too, boiling and sterilizing make food, normally unpalatable
and sometimes even injurious, palatable and non-injurious. It is sterili-
zation also which makes antiseptic surgery possible.

The laws governing liquids under varying* conditions of heat give
us our basis for understanding evaporation, condensation, distillation,
conductivity, convection, radiation, and even plumbing and heating. It
explains why germs can be killed at a much lower temperature when
there is moisture in the air (steam) than otherwise. In fact, a human
being can live in a boiling-point temperature if the air is dry, but he
cannot live in anywhere near so high a temperature if there is moisture.

The entire understanding of the working of the ear is a matter of

CO-ORDINATION OF SUBJECTS 39

physics, in that "sound" is a branch of that science. And, as the larynx
is the instrument through which our vocal sounds are produced, this,
too, must be studied in the light of physics.

All knowledge of the eye, such as our ability to fit glasses, opera-
tions for ocular defects, and all the instruments with which the modern
oculist examines and remedies eye-troubles, are the result of direct ap-
plications of the principles of "light," which, like "sound," is a branch
of physics. Any assistance in improving hearing or sight must, there-
fore, be looked for only in the laws of physics.

The microscope, without which practically none of our modern
scientific work would be possible, is the direct application of the laws
found in physics, and there cannot be a single improvement in that in-
strument until a new principle of physics is discovered.

Likewise, the microtome, the instrument by which we are able to
cut minute sections for the microscope, could not cut with precision the
thin slices that it does (1/25,000 of an inch in thickness), if it were not
made in accordance with the laws of physics.

Electricity, used so much now in the treatment of disease, the X-ray,
the fluoroscope, and radium — all these come under the science of
physics.

That same science explains why the blood-platelets gather along
the blood-vessel where the blood stream is slowest; it explains how
coagulation is thus assisted so that we do not bleed to death when
wounded ; it tells us why one can crawl over thin ice when walking
across the same ice-sheet would be impossible ; why we can safely crawl
on the floor in a room filled with smoke, when standing erect would be
fatal ; it makes an intelligent understanding possible of how to drain
wounds ; it tells us why water-pipes burst when the water in them
freezes ; it tells us why a quilt or comforter of cotton is warmer than
a woolen blanket ; it tells us why men's voices are different from those
of women's, and why the pupil of the eye can accommodate itself to
changing distances and intensity of light ; and, just as it tells us that an
electric bell will not ring until the proper connection is made, so it makes
possible the locating of lesions in the body by noting where nerve con-
nections are functioning properly and where they are not.

Probably the mathematical sciences may seem somewhat remote
from the study of life in general, yet calculus is needed in the study of
physical chemistry, and the laws of refraction in the fitting of glasses.
The relationships of structures in the body must be studied both as to
their quantity and quality. The various names given to .the different
forms and shapes of the parts of the body are largely taken from
geometry.

Surely so remote a subject as ancient Greek is far removed from
modern scientific study, and yet the student need but turn to the
glossary of this book, and go over the thousands of names there given,

40 GENERAL BIOLOGY

to see that ancient Greek is not only valuable, but extremely essential ;
for, practically every name that plants and animals possess comes from
the Greek, and unless the meaning of the word itself is known, the entire
subject-matter becomes pure memory work.

The reason each student must draw a picture of what he sees in his
laboratory experiment, is to force him to observe so well and so accu-
rately that he can make a drawing of a structure so accurate, that another
may in turn recognize the object from the drawing. Drawing a picture
of what he sees also forces the student to keep the subject in mind for
a greater period of time than would otherwise be the case, and gives him
a definite graphic mental picture of what he has seen.

A knowledge of English makes it possible for the student to present
a word-picture of the same matter that the drawing presents. A de-
scription is, therefore, demanded of the student in addition to the draw-
ings, thus again causing him to call to mind all that he has seen and
noted. This not only means that the repetition thus forced upon him
will cause him to remember the subject-matter the better, but it means
that he learns to do that particular thing upon which much of his future
reputation as a professional man depends, namely, to prove to others in
good clear and telling language what he knows.

The mere gathering of facts is of no more value than the mere
gathering of bricks. The important thing in science is to be able to
coordinate the facts that one finds, and to read into these facts their real
meanings. Meanings, however, require the use of the intellect, and the
laws which govern the intellect are embodied in that branch of study
called philosophy. The most important -philosophic studies for the
scientific student are logic, psychology, and ethics.

Every valid conclusion which anyone may form must be built up
logically. Logic is merely the grammar of reason. In fact, every diag-
nosis that a medical man makes, must be built entirely upon logic if it is
to be worth anything, or to stand the test of truth.

The study of the way in which the mind works is called psychology,
and no man can intelligently study, nor clearly understand, any of the
abnormal workings of the human mind, unless he first knows the nor-
mal. He can know little about mental or nervous diseases unless he
knows the way in which the mind works when it is not diseased. In
his study of neurology the medical student follows the various nerve-
tracts of brain and spinal cord, but he cannot understand the real mean-
ing of these nerve-tracts unless he knows his psychology. He will
become a follower of fads and fancies while he misses the underlying
truths and facts that the real scientific man should have.

From the philosophical realm we obtain the validity of our ethics.
Ethics is the science of conduct. We know that holding an air-breathing
animal under water will drown it. And just as death to the animal fol-
lows such an act, so, too, many of our acts bring a definite punishment

CO-ORDINATION OF SUBJECTS 41

of some kind with them. It is to know what acts bring punishments,
so as to know what acts to avoid, which is the distinct province of ethics.
In other words, it helps us to arrange a definite "philosophy of life" for
ourselves.

And lastly, one or two modern foreign languages should be known
in order that we can the better obtain another angle and another point
of view to the many possible explanations that the same facts may seem
to prove. One has but to read through any ordinary school textbook
of science to find what an overwhelmingly large number of foreign
names and papers are there quoted. This means that no one can deem
himself a master of his subject unless he knowrs at least many of the
thousands of observations that have been made by the great scientific
minds of other lands. Unless he knows this, he is bound to spend a
large part of his life in the attempt at proving or disproving many things
that have already been proved or disproved by others. He is wasting
the time which should be given to more valuable work.

From what has been said above it will be seen that all the sciences
must be studied to throw light on the different workings of the body.

At this point it is necessary for the student to grasp the fact that
every living thing must be considered as a complete unity, like the clock
mentioned, and that every organ and every part of an organ which a liv-
ing thing possesses, is definitely connected to, and with, every other part
of the body.

One may suffer from headaches, or with eye-trouble caused by dis-
placed bones in the feet, which in an indirect way press against nerves
connecting with the head ; or, one may have a backache or earache, or
even rheumatic difficulties, due to ulcers beneath the teeth.

It is for reasons such as these that it is as necessary for a dentist as
well as an oculist to study biology, and learn the unity of the living
being. For there is no more reason for a student of dentistry to confine
all his study to the teeth alone, or an oculist to the eye alone, than it
is for a nerve specialist to study the nerves alone. Any such one-sided
study leaves out of consideration the most important factors necessary
to a legitimate diagnosis. And, with a wrong diagnosis, the treatment is
bound to be wrong, or at most, mere guess-work.

It is well also for the student to bear in mind that, though he may
not immediately see the relationship of some things to the general
course he is taking, it does not follow that such relationship does not
exist.

One can learn to start and stop a locomotive in twenty minutes, but
this does not make one an engineer. It takes years to do this. It is
not when all things go well that the expert is called in, but when things
go wrong. This is true of the engineer, the physician, the dentist, the
lawyer, and other professional men. And it is only he, who knows the
relationship of all the parts, who can hope to be an expert.

42 GENERAL BIOLOGY

in conclusion, the student should remember that all college courses
are arranged on a minimum basis. That is, the work laid out for the
student is the least amount of work he can do and yet obtain a passing
grade. It is, therefore, only the student who actually does more than
is required, who deserves any credit worth mentioning.

CHAPTER IV.

THE FROG

The frog lends itself to laboratory work in biology probably better
than any other animal. It is sufficiently common so that it is at least
somewhat familiar to the student, and it can be procured practically
at any season of the year. It is a vertebrate (Latin — vertebratus-
jointed) which means it has a back bone, and an amphibian (Greek
amphi=both, bios=life), meaning that it lives a double life. This
latter statement refers to the animal's inability to live either on land
alone or entirely submerged in water. This inability to live entirely
in the air or in water is well shown by the fact that if the frog's skin
becomes dry, as it does when the animal is away from water and in a
dry atmosphere, the animal dies, because the skin is then no longer
capable of serving as an organ of respiration (Latin re=back-f-spiro=
breathe). Contrariwise if it be constantly immersed in water it will
also die, because it must breathe air.

The particular species (Rana pipiens) that we are describing
(though any other of the common forms would answer the same pur-
pose) is found in or about fresh-water lakes, ponds, or streams. The
species is fairly well distributed over the entire North American con-
tinent, except the Pacific Slope.

Everyone has noticed the longer and stronger hind legs of the frog
and the squatting position it assumes on land, as well as the rapidity
with which he leaps into the water when disturbed along the banks.
If one observes him while in water that is beyond his depth, it will be
noted that the hind legs hang out straight and the tip of the nose is
exposed to the air. Should he be disturbed while in this position, the
hind legs are flexed, (L. flecto-bend) which throws the body downward.
The fore legs are used in arranging the direction in which the animal
will go; the hind legs are then extended, (L. ex=out-|-tendo=stretch)
completing the movement which forces him forward.

Everyone also knows the sound of croaking frogs at night, especially
when the atmosphere becomes damp, though it is not so generally known
that the frog croaks far more frequently during the breeding season than
at other times. The croaking can be accomplished both in and out of
the water. The croaking under water is produced by the air from the
lungs being forced past the vocal cords into the cavity of the mouth,
and then back again into the lungs.

There is another reason why the frog may be considered as leading
two lives (Fig. 3) beside the fact that it needs both air and water, and
that is that it lives a different type of life when young than when grown.
This comes about as follows : The eggs of the female frog are almost
always laid in water and hatched there. Little tadpoles develop from
these eggs and breathe by gills in the larval ( ) condition.

44

GENERAL BIOLOGY

Some species of frogs retain these gills all through life, even though
lungs may be present in the adult forms. The tadpole gradually develops
into the mature frog, losing its tail and developing the long hind, and
the short fore legs so familiar in the adult animal.

EXTERNAL FEATURES

It is essential that one examine quite carefully the external struc-
ture of any plant or animal one may wish to study ; for, unless this
knowledge is borne in mind, internal structure cannot be interpreted
correctly. It is well also to keep in mind our own bodies, and to observe
similarities and differences wherever they may occur in animals and
plants.

C, egg con-

4, eggs before £?, eggs after they taining young D, young tadpoles attaching
they are laid are laid tadpole themselves to a plant

B. young tadpole with ex- F, young tadpole with

ternal gills internal gills G, young tadpole with hind legs

H, tadpole with webbed
feet

7, tadpole with legs and arma

J, young frog

Fig.

Eggs, Tadpole, and Adult Stages of Frog.
(After Brehm and other authors.)

It will be noted immediately, that the frog has no neck. The head
is broadly united to the trunk. The eyes protrude somewhat, but can
readily be withdrawn into the orbits. A pressure put upon the eyes if
the mouth of a frog is open will extend the inner membrane lining of
the roof of the mouth quite prominently, showing that the orbit, or eye
socket, is not separated from the mouth by any of the bones of the skull
as in ourselves. The dark oval opening of the eye, the pupil, is sur-
rounded by the iris, a more brightly colored ring. There are upper and
lower lids, the upper one moving but slightly, the lower one thin and
transparent, and capable of covering the entire eye. This lower eyelid
is different from that of most animals, and this type of lid will be met
with again in other animal forms to be studied. The nictitating mem-
brane ( ) is separated from the lower lid (Fig. 4), but
appears to form a continuation. In birds, for example, this membrane is
also very thin and can be thrown over the eye from the inner angle of the
orbit. Behind the eye is a more or less circular area called the tympanic

THE FROG

membrane, ( ) which covers the ear drum. There

is a slight prominence in the center of this membrane produced by one
of the small bones called the columella ( ). This

bone connects with the inner ear and when any sound-wave strikes the
tympanic membrane the vibrations are communicated through this bone
into the internal ear. This gives rise to the sensation of hearing. On
the inner side of the tympanic membrane we find a little cavity known
as the Eustachian tube ( ), which opens internally

into the mouth. There is no external ear present as in ourselves.

The two openings immediately
above and behind the tip of the nose
are called nostrils or external nares.
Sometimes, in front of the eyes
there is a little light area known as
the brow-spot, which was connected
with the brain in the embryo
( ). The brow-spot

is a feature of considerable interest
from the fact that in the embryonic
development of the frog, it connects
with a peculiar outgrowth of the
brain known as the epiphysis
( ) or pineal gland.

This is supposed to be a rudiment
( ) of a stalk

which formerly connected with the
medial eye ( )

which still persists in certain forms
of reptiles (Fig. 5), ( ).

The nostrils are guarded by valves
which open and close during respira-
tion.

The mouth extends from one
side of the head to the other, and the
anus ( ) is situated

) end of the body. The fore
limbs are divided into an upper arm, a fore arm, and a manus ( )

or hand, the latter possessing four digits and the so-called thumb, a rudi-
ment of the fifth. In the male, the inner digit is thicker than the corre-
sponding one of the female, especially during the breeding season. The
entire fore arm is also relatively thicker in the male than in the female.
The hind limbs are well adapted for jumping and swimming. These
are divided into three portions, the upper portion known, however, as
the thigh, the middle as the cms or shank, and the distal ( )

portion as the foot or pes. The foot is well developed, there being five

Fig. 4. Examples of Nictitating Membranes.
(From various authors.)

at the posterior (

BIOLOGY

toes and the rudiment of a sixth, called the prehallux ( ),

situated on the inner side of the foot. The toes themselves are con-
nected with a web, making the foot quite efficient as a swimming organ.
There are also small cushions called subarticular pads ( )

between the bones of the toes.

Fig. 5. Pineal eye of a Lizard ; diagrammatic. A brain and upper wall of the
skull, the latter cut through ; B, pineal eye alone, in section. V, Z, M, H, cerebrum,
thalamencephalon, optic lobes, cerebellum ; h skin, s roof of skull, o unpigmented
portion of skin below which the pineal eye lies, in a hole in the roof of the skull ;
p epiphysis, i hypophysis, 2 optic nerve. L lens, R retina, N nerve of pineal eye.
(After Boas.)

The skin is smooth and loose, containing large black pigment spots
( ) and some green and golden pigments as well.

As with other vertebrates, the skin has .two layers, an outer called the
epidermis ( ) and the inner, the dermis ( ).

Nothing similar to hair or scales can be found on the frog. There are
large mucous glands ( ) in the skin which keep the

surface slimy and there are also some poison glands secreting
( ) a whitish fluid, supposedly for defensive pur-

poses. Behind the eyes there are usually two light colored ridges
formed by a thickening of the skin and called the dorso-lateral dermal
plicae ( ) or folds. There may be some smaller,

irregular, longitudinal folds ( ) of skin between

these. It will also be observed that the color of the skin is much darker
on the upper or dorsal surface than below, where it is almost white.

INTERNAL STRUCTURE

As with us, the various organs ( ) and tissues

( ) of the frog's body are supported by an internal

skeleton of bones. This is called an endoskeleton ( )

to distinguish it from such types of animals as the crayfish which have
their entire skeletal structure on the outside of the body, forcing that
animal to grow an entirely new skeleton whenever the animal itself
grows larger than its skeleton- jacket will stretch. The higher forms of
animals all have endoskeletons. The different parts of the body are
moved by the action of muscles, which in turn are innervated ( )

THE FROG 4?

by nerves. To know the internal structure of an animal one must know
all that can be known in regard to the following systems :

1. Digestive

2. Circulatory

3. Respiratory

4. Excretory

5. Nervous

6. Skeletal

7. Muscular

8. Reproductive.

After an incision is made along the mid-line (Fig. 6) of the ventral
( ) surface of the animal from the lower angle of

the jaw to its most posterior end, the internal organs are seen. These
are called the viscera ( ). The cavity in which they

are found is known as the coelom ( ) or body cavity.

If the animal has just been chloroformed, the heart will still be beat-
ing. The heart is contained in a sac-like structure called the pericardium

( ).

Surrounding at least a portion of the pericardium, are three promi-
nent lobes of the reddish-brown liver, while the' lungs, looking like small
strawberries, lie, one on each side, near the anterior end of the abdomi-
nal cavity.

The stomach is easily recognizable, together with the coiled intes-
tine attached to it.

The kidneys are flattened reddish bodies attached to the dorsal body
wall.

If it is the breeding season, and the frog is a female, almost the
entire body-cavity may be filled with thousands of eggs. The eggs in
turn are contained in a film-like covering known as the ovary ( )

and oviducts ( ), the latter organs serving as tubes

through which the eggs leave the body. If the specimen should be a
male, the two testes ( ) will be suspended by little

membranes at the side of the digestive canal (t, Fig. 6). The entire
lining of the abdominal cavity in all the higher forms of animals is called
the peritoneum ( ). When one or two layers of

this peritoneum suspend, or hold up an organ, such as the digestive canal
and the reproductive organs, such suspending peritoneum is called a
mesentery ( ).

THE DIGESTIVE SYSTEM

It will be noticed that the tongue is what is called extensile
( ), that is, it can be thrown forward and outward.

There is a sticky substance secreted on the tongue which causes objects
with which it comes in contact to adhere. It is also interesting to ob-
serve that unless an object is moving, the frog pays no attention to it.

GENERAL BIOLOGY

The mouth or oral opening is relatively large. The opening on the in-
terior is called the buccal ( ) cavity. There are
teeth on the maxilla ( ), premaxilla ( ),
and vomer bones ( ). These assist in holding, but

Fig. 6. A Male Frog Dissected from the Ventral Side.

a.ab.v., Anterior abdominal vein, cut short, ligatured, and turned back ; a.musc.,
cut edge of abdominal muscles ; bl., urinary bladder ; c.d., common duct of gall-
bladder and pancreas ; d.ao., dorsal aorta ; du., duodenum ; f.b., fat body ; fem.v.,
femoral vein ; g.b., gall-bladder ; ht., heart ; hy.n., hypoglossal nerve ; im., ileum ;
i.v.c., inferior vena cava ; k, kidney ; k.d. kidney duct with vesicula seminalis ;
lr., liver ; o, point at which c.d. enters the duodenum ; pcs., pancreas ; pl.v., pelvic
vein ; r.L, right lung ; rm., rectum ; r.p.v., renal portal vein ; sar., sartorius muscle ;
sm., mylohyoid muscle ; sp., spleen ; st., stomach ; t., testis ; v.v., vesical vein. (After
Borradaile. )

not in masticating the food. Immediately back of the tongue on the
floor of the mouth is a narrow slit called the glottis ( )

leading to a tube passing to the lungs, and directly behind the glottis,

THE FROG

a larger opening is found, leading to the oesophagus, which empties into
the stomach. The stomach itself is crescent-shaped, lying mostly on the
left side of the body. The larger anterior portion is called the cardiac
end ( ), while the constricted or posterior portion,

meeting with the intestine, is known as the pyloric ( )

end.

msnt.

Fig.

A. A Diagram of a Transverse
Section Through the Ileum

.of a Frog.

c.mM Circular muscle layer ; c.t.,
submucosa ; ep., epithelium which
lines the gut ; l.m., longitudinal
muscle layer ; msnt., mesentery ;
per., peritoneum ; rid , longitudinal
ridges of ileum composing mucosa.

B. A Portion of the Section Shown

in A, More Highly Magnified.
b.v., Blood vessel ; c.t., connective
tissue of mucous membrane or sub-
mucosa ; c.m., circular layer of mus-
cle fibres ; ep., epithelium ; g.c.,
goblet cell ; l.m., longitudinal layer
of muscle fibres ; let., "lacteal" or
lymph vessel of the intestine ; leu.r
leucocyte or lymph corpuscle ; p «.,
peritoneal epithelium. (After
Bourne. )

The first portion of the intestine, a sort of U-shaped band, is
known as the duodenum ( ). The several coils fol-

lowing it are the intestine proper. This intestine finds its way into a
large but short chamber known as the rectum, which in turn communi-
cates with the exterior through what is called the cloacal opening-
( ). The walls of the stomach are composed of five

layers (Fig. 7), the outer portion quite thin, called peritoneum, then two-
muscular layers, the outer being called the longitudinal, and the inner
the circular muscle layer, followed by a spongy division called the sub-
mucosa and an inner folded mucous layer, the mucosa itself. This latter
is made up of glands lying in connective tissue. These glands are longer
at the cardiac than at the pyloric end. The inner layer of the intestine,,
the mucosa, is considerably folded and consists of absorptive and goblet
cells. The urinary bladder, reproductive ducts and rectum, open into
the cloaca.

The digestive glands themselves are the pancreas and liver, the for-
mer lying immediately between the duodenum and the stomach. It is a
much branched tubular gland, secreting an alkaline digestive fluid and

50

GENERAL BIOLOGY

empties into the common bile duct. The three-lobed liver, already men-
tioned, also secretes an alkaline digestive fluid, known as bile. This is
carried by the little bile capillaries into the gall bladder where it is stored
until food enters the intestine, when it passes into the duodenum through
the common bile duct. Digestion begins in the stomach.

According to Latter, "the alkaline fluid secreted by the mucosa
layer of the oesophagus and the acid gastric juice secreted by the glan-
dular walls of the stomach digest out the proteid portion of the food by
means of a ferment, ( ) called pepsin, which changes

proteids into soluble peptones. The food then passes through the pyloric
constriction into the intestine. Here it is attacked by the pancreatic
juice and the bile. The pancreatic juice contains three ferments: (1)
trypsin, which converts proteids into peptones ; (2) amylopsi^ which
converts starch into sugar; and (3) steapsin, which splits up fats into
fatty acid and glycerin. The bile emulsifies fats and converts starch into
sugar. The intestinal wall produces a secretion which probably aids in
converting starch into sugar.

"Absorption begins in the stomach, but takes place principally in
the intestine. The food substances which have been dissolved by the
digestive juices are taken up by the mucosa layer, passed into the blood
and lymph, ( ) and are transported to various parts

of the body (C, Fig. 8). The undigested particles of food pass out

Fig. 8. Diagrams of Important Relationships.

A. The relation of the hepatic portal system to the stomach, intestine,
pancreas and liver.

B. Diagrammatic transverse section through the abdominal region of a frog.

C. Diagram of the two main channels by which food enters the general circulation
in mammals, e, intestine with villi ; v, v, in its walls ; r a, right auricle of the
heart ; m, postcava ; n, precava ; o, thoracic lymph duct ; p, pancreas ; q, pancreatin
duct ; r, hepatic vein ; s, portal vein ; t, bile duct from I, liver ; arrows indicate the
course of secretions entering the intestine, and of the absorbed food departing
therefrom.

(A, after Howes; B, after Parker; C, from Needham's General Biology," by
permission of The Comstock Publishing Co.)

of the intestine into the cloaca and are then discharged through the anus
as faeces."

The absorbed food is used by the frog to build up new protoplasm
to take the place of that consumed in the various life activities, and to
increase the size of the body. Food is stored up in the liver-cells as

THE FROG 51

glycogen, a carbohydrate similar to starch and often called "animal
starch." The absorbed food is conveyed to the liver by the portal vein
and there converted into glycogen, pending the demands of the general
tissues of the body. As occasion arises it is converted into more soluble
material, a sugar, and sent into the main bloodstream via the hepatic
veins and inferior vena cava. Fat globules are also contained by the
liver cells. The storage function of the liver is one of considerable im-
portance, especially during hibernation and at the breeding season ; the
weight of the organ exhibits a well-marked seasonal variation in accord-
ance with the amount of reserve food contained. The details of this
phenomenon have been worked out by Alice Gaule in Rana esculenta.
The breeding season of this form is in May, June, and July. The table
shows the average weight of the liver in the two sexes month by month.
"It will be observed that the liver is most depleted in both sexes
in June, the middle of the breeding season, and that it reaches its maxi-
mum weight in September when the system has recovered from the ex-
hauStion of spawning. Throughout the winter the reserves are being
steadily used up, with no recovery by the female, the average weight
of whose liver is greater than that of the male, but with a slight recovery
in March and in May by the male. It is probable that this general dif-
ference depends upon the fact that the ovaries of the female make a
great and continuous demand upon her system throughout the whole
period of maturation, so that in spite of renewed feeding in the Spring
there is no recuperation in the liver. In the male, however, there is no
such continuous drain but rather a sudden call upon the reserves at the
actual time of pairing — a call due not only to the discharge of the sper-
matozoa but also to the muscular exertions of the male at that season.
This call is marked in vigor by the sudden reduction of the liver to
rather less than half its weight 'in June.

Weight of Weight of

Month male liver female liver

January 10 grms: 13 grms.

"February 10 12.5

March ' 13 11

April 10 10

May 12 9

Tune 5.5 7.5

July 7.5 11

August 6 12

September 22.5 27

October 18 25

November 22 25

December 18 22

"Reserves of food are also laid up in the fat-bodies. These have no

52 GENERAL BIOLOGY

direct connection with the digestive system, but may conveniently be
dealt with here. They are bright yellow, finger-like bodies grouped in
front of the testes or ovaries as the sex may be. They develop from
the anterior portion of the genital ridges whose posterior portions alone
give rise to the sexual organs. In the autumn they are of great size and
loaded with fat-cells, a certain amount of lymphatic tissue being also
present. In the spring they are much reduced. It is probable that they
also perform other functions at all seasons of the year, but on this point
we have no precise knowledge."

GLANDS

Glands may be conveniently classed into two groups :
Exocrine glands, that is, glands whose products are used externally or
on substances entering the body, and which generally leave by way of
ducts.

Endocrine glands, that is, glands whose products act on the body
itself, not on substances brought into it. This type of gland generally
has no duct ; or if so, as in the case of ovaries and pancreas, the
"endocrine" portion of the secretion is absorbed by the blood vessels
and does not leave the gland by way of a duct. The term "ductless
glands" has been used to designate these glands, but has been found
inappropriate. The products of the endocrine glands are known fre-
quently as "internal secretions," and are composed of active agents simi-
lar to enzymes. The name of "hormone" (excitant) has been given to
these agents, which differ from enzymes primarily in that their activity
is not destroyed by boiling.

It has been found, however, that the action of the endocrines may
be both stimulatory and inhibitory, as, for instance, in certain experi-
ments on tadpoles which were fed with thymus and thyroid gland.
Gudernatsch (1912-14) found that the thyroid food stimulated develop-
ment and inhibited growth, while thymus stimulated growth and in-
hibited development. Thyroid-fed tadpoles matured in four weeks, as
contrasted with the normal period of twelve weeks, but were dwarfs and
pigmies in size ; while thymus-fed tadpoles were gigantic in size, but
after sixteen weeks showed no indication of transformation, in fact, had
not yet developed their hind legs. From certain other experiments on
the sexual glands it has been similarly concluded that the internal secre-
tion from the sex glands (specifically that of the interstitial cells) acts
both as a stimulant to the body so that it will develop the characteristics
pertaining to its proper sex, and as an inhibitor in suppressing those of
the other sex. The excitant has been named hormone, the inhibitor
chalone.

The products of the various endocrines are regulatory in nature, and
control or affect such processes of the body as growth, puberty, sec-
ondary sexual characters, blood pressure, metabolism, distribution and

THE FROG 53

concentration of substances, muscle tone, blood sugar, etc. Instincts,
emotions, mental and psychic states are stimulated, inhibited and com-
plicated by endocrine action.

The two thyroid glands are situated on either side of the hyoid, and
secrete quite a quantity of iodin. In man, its atrophy is associated with
the disease called cretinism ( ) where certain parts

of the body, such as the head, may become very large. Cretins are
almost always idiots.

The two thymus glands ( ) lie one behind each

tympanum. They are small and oval in shape, usually reddish in color,
and are placed directly beneath the depressor mandibular muscle. The
thymus, like the thyroid, diminishes in size with age.

The adrenal bodies ( ) are little bands of a

yellowish color extending along the mid ventral surface of the kidneys.
They secrete adrenalin, a substance necessary for the life of the animal.
This substance is used to a considerable extent in medicine at the pres-
ent time as it will cause a contraction of the blood vessels and raise the
blood pressure after injection. However, a little later a reaction sets in,
and a lowering of blood pressure follows.

The spleen ( ) is a reddish organ lying imme-

diately dorsal to the anterior end of the cloaca. It is supposed to act as
a sort of filter for the blood. The old corpuscles are destroyed and new
colorless ones are formed. It must be remembered that all that is known
in regard to the ductless glands demonstrates that they are of vital im-
portance, but that no absolute conclusions can be drawn as to definite
functions of any of .them ; for, while one or two of their functions are
known, there are probably many more functions that are not yet even
dreamed of.

THE CIRCULATORY SYSTEM

It is essential that the student grasp the fact that there are several
types of circulation. The systemic proper is that closed system of blood
vessels by which the blood leaves the heart and passes through the large
arteries into the capillaries to carry nourishment to every point in the
body (Fig. 9). These arterial capillaries then meet with the venous
capillaries, and waste-matter is collected in the blood and carried bv the
veins into two anterior and one posterior venae cavae by which the blood
is returned to the heart.

The heart itself, however, must have blood vessels carrying nour-
ishment to the heart-walls just as an engine run by steam and supplying
water to different parts of a building, must have water supplied to its
own boiler in order that the steam which gives the engine its power may
continue to be generated.

The heart muscle, which is the engine of the body, must similarly
have its supply of blood to its own walls in order that the heart may

54

GENERAL BIOLOGY

be able to pump the blood to all parts of the body after it has entered
the heart from the lungs where it was aerated. The blood sent to be
aerated forms the second type or pulmonary circulation. The digested
food which the individual has absorbed must now be taken into the
blood and be made a part of that blood, so that there is a replacement of
lost substances. This explains why the blood which goes to the digestive
tract by the coeliac axis passes through two series of capillaries before
returning to the heart :

IT-.O.

Fig. 9. Diagram Representing the General Course of Blood in the Frog and the

Principal Sets of Capillaries (cp.) Through Which the Blood Flows.
The vessels through which impure blood goes are dark, while those carrying
pure blood are left unshaded. The arrows indicate the direction of blood flow.
ant.ab, anterior abdominal vein; ao' , aorta; au' , right auricle; au", left auricle;
c.c, common carotid artery ; err, coeliaco-mesenteric artery ; cp.a, anterior systemic
center ; cp.al, alimentary center ; cp.cu, cutaneous center ; cp.hp, hepatic center ;
cp.p, posterior systemic center ; cp.pl, pulmonary center ; cp.re, renal center ; cu,
great cutaneous artery; d.ao, dorsal-aorta; l.h' , anterior lymph heart; I h", posterior
lymph heart; m.cu, musculo-cutaneous vein; p, hepatic portal -vein; pcu, pulmo-
cutaneous vein; pr.c, precaval vein; pt.c, postcaval vein; pul, pulmonary vein;
re, renal artery ; rp, renal portal vein ; s.v., sinus venosus ; tr.a, truncus arteriosus ;
v, ventricle. (After Howes.)

First, into the capillaries of the intestine wrhere it. receives the nutri-
ment absorbed from the food ; and, after being collected into the large
portal vein, enters the liver.

Second, after entering the liver the portal vein breaks up into an-
other system of capillaries writhin that organ. vj *

After the blood has passed through the liver this second set of capil-
laries unites to form the hepatic vein which empties into the large pos-
terior vena cava leading to the sinus venosus. This whole system, where
vein's break up into capillaries but are again united to form a second vein,
is called a portal system.

Part of the blood that goes to the legs also has a double system—
first entering the capillaries in the leg muscles and on its way back pass-
ing through the kidneys where it is; broken up into capillaries: The
bl6od that takes this route returns from the leg through the renal-portal
vein, but the rest of the blood from the legs is diverted to the abdominal
vein which passes through the liver (but not the kidneys) on its way to
the heart. :

THE FROG

55

Now, just as has been explained in regard to the necessity of the
heart having an arterial supply of blood to its own walls in addition to
that which it pumps from its cavities, so the liver and kidneys must also

r.au.-- —

tr.a.— }• -

si.

--i.v.c.

Fig. 10. The Frog's Heart.

A, seen from the ventral side ; B, from the dorsal side ; C, heart opened and viewed from ventral side.
(The ventral wall of the truncus, ventricle, and auricles has been removed).

A. c.a, carotid arch ; car, carotid artery ; c.gl, carotid gland ; La, lingual artery ;
pc.a, pulmocutaneous arch ; pm, pericardium ; r.au, l.au, right and left auricles ;
s.v.c, superior vena cava ; »y.a, systematic arch ; tr.a, truncus arteriosus ; v, ventricle.
B. i.v.c., inferior vena cava ; p.v., pulmonary veins ; r.au., l.au., right and left
auricles ; s.au., opening from sinus to right auricle ; s.v.c., superior vena cava ; s.v.,
sinus venosus ; tr' ., branches of truncus cut across; v., ventricle.

, C. au.v., Auriculo-ventricular valves ; c.a., carotid arch ; c.ao., cavum aorticum ;
ch.i., chordae tendine'se ; l.au., left auricle ; o.p.v., opening of pulmonary vein ; 'o.pc.,
opening of dorsal division of synangium, by which blood passes from the cavum
pulmocutaneum to the pulmocutaneous arch ; pc.a., pulmocutaneous arch ; r.au.,
right auricle ; s.au., sinu-auricular opening with valves; si., first row of semilunar
valves; si'., semilunar Valves of second row; si' .1, the semilunar valve from which
the spiral valve starts ; the line points to a small portion of the valve which has
been cut open; si' .2, small semilunar valve at end of cavum pulmocutaneum; si',3,
a small part of a large semilunar valve, of which the rest extends across that por-
tion of the front wall of the truncus which has been removed ; sp.v., spiral valve ;
sy.a., systemic arch; tr.a., wall of truncus arteriosus; tr' ., one of the two bundles
of arteries into which the truncus divides ; v., ventricle. (Redrawn from Borradaile. )

56 GENERAL BIOLOGY

have their own supply of blood to feed the liver and kidney substance,
in addition to that received from their respective portal veins.

All the blood coming from the heart, and passing directly back to
the heart, whether it flows through the portal, renal-portal, abdominal or
other veins, is classified as the systemic circulation. This is to be dis-
tinguished from the pulmonary circulation, which deals with the blood
which, having been returned by the veins to the heart, is now sent to
the lungs to be purified and aerated. This blood leaves the heart ven-
tricle through the pulmonary arteries and is returned to the heart auricle
through the pulmonary vein.

It is interesting to note, that in the frog, a part of the already-used-
blood (venous blood) which in the human being all goes to the lung
through the pulmonary artery, passes through the cutaneous artery, a
branch of the pulmonary, to the skin under the arm, where it is also
purified by the oxygen in the water. It will be remembered that the
frog needs both air and water for breathing purposes, and breathes
through both lungs and skin.

The frog's heart is composed of three compartments (Fig. 10), in-
stead of four, as in the higher forms of animals. The blood that has
been purified in the lungs flows into the left auricle through the pul-
monary vein and is thus kept separate from the impure blood in the right
auricle ;x but, as there is only one ventricle, and as blood is always re-
ceived by the auricles, and always expelled from a ventricle, the impure
blood from the right auricle, as well as the pure blood from the left
auricle, is all emptied into one ventricle so that it is bound to intermingle.
However, the blood from the right side is a little more impure than on
the left, because the left side is directly connected with the left auricle
and it is the left auricle which has the purest blood. The pure and im-
pure blood are also kept partly separated by various irregular muscular
partitions called trabeculae extending through the ventricle.

The action of the heart is as follows : The two auricles filled with
blood contract at the same time, thus forcing arterial and venous blood
into the ventricle. Here the two kinds of blood are kept from mixing
by the muscular trabeculae just mentioned. At the systole of the ven-
tricle, the venous blood, which lies nearest the bulbus arteriosus, is first
forced forward. This takes the most direct course through the wide and
short pulmonary arteries which are practically empty at the time. The
mixed arterial and venous blood follows the next easiest course through
the aortic arches, while the last blood to leave the. ventricle (the pure
arterial blood), can only go to the carotids, where the resistance is
greater on account of the small size of the vessels and the obstacles pre-
sented by the carotid glands.

Blood usually looks red. This is due to the large red corpuscles
(Gr. erythrocytes=rerythros— red+cytos=cell). The redness itself Js
due to what is called haemoglobin, a chemical substance contained within

THE FROG 5?

the corpuscles. There are also white corpuscles called leucocytes (Gr.
leukos=white+cytos=cell) in the blood. These are often able to force
themselves through the walls of the capillaries and then wander about
through the tissues. There is still a third type of tiny bodies in the
liquid part of the blood which are called platelets. Of these little is
known.

Beside the arteries and veins there are also the lymph vessels in the
skin, intestine, and other parts of the body which belong to the circula-
tory system. The liquid part of the blood in which the corpuscles float
is known as plasma. When the blood passes into all parts of the body
to nourish it, some of this plasma finds its way through the little arterial
capillaries, bathing the intercapillary spaces. This plasma which has
left the blood vessel proper to bathe the body tissues is called lymph.
This lymph must be* gathered again and made a part of the blood, so
various little lymph capillaries drain the body and pour the lymph back
into the veins. These lymphatic vessels are very delicate, and must be
prepared in a special way to be seen. The little open spaces where
lymph gathers and from which the lymphatics carry it to the veins are
known as a lacunae. These lacunae also connect with larger cavities in
the body.

The lymph vessels in the intestine have a special name, being called
lacteals ( ). There are also lymphatic glands found

,.- lymph

lymph

Fig. 11. Section Through a Lymph Heart. (After Weliky.)
Id — Tube-like valve at entrance of vein into the heart.
v— Vein.
lymph — Lymph-heart.

in connection with the lymph vessels, and in the frog there are two pairs
of lymph hearts (Figs. 11, 347) whose contraction propels the lymph in
its circulation.

With this in mind, we take up the principal divisions of the circula-
tory system.

There is a true heart consisting, however, of only three cavities
(Fig. 10), two thin-walled auricles, ( ) one on the

58 GENERAL BIOLOGY

right and one on the left side, and a muscular ventricle. There is also
a thick-walled tube called the truncus arteriosus ( )..

which arises from the base of the ventricle, and a thin-walled triangular
sac, the sinus venosus ( ) from the dorsal side. The

heart is the central pumping station of the entire circulatory system,
which furthermore consists of all the arteries, veins, and lymphatic
structures in the body. Arteries always carry blood away from the
heart, veins to the heart. The fibres of the heart muscles run in every
direction, so that in systole, ( ) that is, when the

heart contracts, its size is diminished and the blood in the various cavi-.
ties is forced out; then in diastole, ( ) when the

heart again expands, the blood flows into it. The openings of both the
auricles and ventricles are guarded by valves, little flaps of membrane
which permit the blood to flow through the opening quite readily, but
close up when the blood begins to flow backward, as it would be bound
to do when the ventricle contracts, if the valves did not block th£ pas-
sage. The large truncus arteriosifs (the proxim,al portion of which is
called bulbus arteriosus), has two large branches called aorta'e ( ).

___- inter-no I £. /as? it
jne (fienle)

Fig. 12. Femoral Nerve, Artery and Vein of Puppy.

The truncus receives the blood as it is forced out of the heart when it

^-ct>ntracts, and from whence it is distributed to all parts of the body.

\Tjie sinus venosus on the dorsal surface of the heart is the'cavity into

which the veins bring back the bloocT from all parts of the body. The

sinus itself opens'into the right auricle and thus receives all the blood

which flows back to the heart from all parts of the body, except 'the

lungs.

THE FROG

The blood from the lungs empties into the left auricle by two small
veins, one from each lung> —

Blood vessels pass to every part of the body. We know they are
everywhere because one cannot insert the point of the finest needle in
any part of the body without piercing them, showing they are so close
together that one cannot get in between them. Arteries are always rela-
tively thicker-walled and more elastic than veins (Fig. 12).

car.dr

subcl

pulm —

, vert

Fig. 13. The Arterial System of the Frog.

/., Carotid artery; //., systemic artery: ///., Pulmocutaneous artery. (The"
three together being the aortic arches.) ao.c., dorsal aorta; car, carotid artery;
car.dr., carotid gland ; coel mes., coeliaco-mesenteric artery ; cut., cutaneous artery ;
d, intestine ; gen., spermatic artery ; h., testis ; if., iliac artery ; Leb., Liver ; ing.,
lung; ling., lingual artery; n., kidney ; occ., occipital artery; occ.vert., occipito-
vertebral artery ; pulm., pulmonary artery ; ren., renal artery ; subcl., subclavian
artery; tr., truncus arteriosus ; v., ventricle of heart; vert., vertebral artery. (After
Meissner. )

The principal divisions of the arterial system (Fig. 13) may be sum-
marized as follows :

I. The common carotid ( ) divides into the

lingual or external carojid, supplying the tongue and neighboring parts,
and the internal carotid which gives off the palatine ( )

artery to the roof of the mouth, the cerebral carotid to the brain and the

GO GENERAL BIOLOGY

ophthalmic artery to the eye. There is a little swelling known as the
carotid gland at the point where the common carotid branches.

II. The pulmo-cutaneous ( ) artery forms the
pulmonary artery, passing to the lungs, and the cutaneous artery. The
cutaneous in turn gives off the auricularis ( ) dis-
tributed to the lower body and neighboring parts, the dorsalis which
supplies the skin of the back, and the lateralis which supplies the skin on
the sides. Most of these branches also carry blood to the various respi-
ratory organs, lungs, skin, and mouth.

III. The systemic arches pass outward, around the digestive canal,
and then unite to form the dorsal aorta. Each systemic arch gives off
an occipito-vertebral artery which divides ; one branch, the occipital,
( ) supplying the jaws and nose; the other, again
dividing forms the vertebral, supplying the spinal cord and muscles of
the body wall, and the subclavian which is distributed to the shoulder,
body-wall and arm. The dorsal aorta gives off the coeliaco-mesenteric
artery. This divides, forming the coeliac which supplies the stomach,
pancreas, and liver, and the anterior mesenteric, which is distributed un-
der the intestine, the spleen, and the cloaca. Back of the origin of the
coeliaco-mesenteric, the dorsal aorta gives off four to six urinogenital
arteries which supply the kidneys, reproductive organs, and fat bodies.
A small posterior mesenteric artery arises near the posterior end of the
dorsal aorta passing into the rectum. In the female this artery also sup-
plies the uterus. The dorsal finally divides into two common iliac
( ) arteries which are distributed into the ventral
body-wall, the rectum, bladder, the anterior part of the thigh (here called
femoral artery), and other parts of the hind limbs (sciatic artery).

All the arteries finally break up into a vast number of microscopic
thin-walled vessels called capillaries (Lat. capillus=hair) by which
every part of the body is reached.

THE VEINS

The veins (Fig. 14) return the blood to the heart by draining all
parts through venous capillaries ; the veins reversing the arterial system
by constantly becoming larger and larger. It will be, noted here that the
blood vessels thus form a closed system and the blood that leaves the
heart returns without leaving the vessels. "The blood from the lungs is
collected in the pulmonary veins and poured into the left auricle. The
rest of the venous blood is carried to the sinus venosus by three large
trunks, the two anterior venae cavae ( ) and the

posterior vena cava. The anterior venae cavae receive blood from the
external jugulars ( ) which collect blood from the

tongue, thyroid, and neighboring parts, the innominates which collect
blood from the head by means of the internal jugulars and from the
shoulder by means of the subscapulars, and the subclavians which col-

THE FROG

61

lect blood from the fore limbs by means of the brachial, and from the
side of the body and head by means of the musculocutaneous veins. The
posterior vena cava receives blood from the liver by means of two hepatic

jug.inl

pultn

port .Hep

Fig. 14. The Venous System of the Frog.

abd., Abdominal vein ; br., brachial vein ; card., cardiac vein ; cav.i., post caval
vein ; cav.s., precaval vein ; cut.m., musculo-cutaneous vein ; d., intestine ; d., lumb..
dorso-lumbar veins ; f.sin., opening between sinus venosus and auricle ; h, liver ;
hep., hepatic vein ; il., iliac or femoral vein ; tt.tr., transverse iliac vein ; leb., liver ;
Ing., lung; nr., kidney; pelv., pelvic vein; port.hep., hepatic portal vein; port.ren.,
renal portal vein ; pulm., pulmonary vein ; ren., renal vein ; sc., sciatic vein ; test.,
testis ; ves., vesicle veins. (After Meissner.)

veins, from the kidneys by means of four to six pairs of renal veins, and
from the reproductive organs by means of spermatic or ovarian veins.

"The veins which carry blood to the kidneys constitute the renal-
portal ( ) system. The renal-portal vein receives

62 GENERAL BIOLOGY

the blood from the hind legs by means of the sciatic and femoral veins,
and blood from the body wall by means of the dorso-lumbar vein.

"The liver receives blood from the hepatic-portal system. The
femoral veins from the hind limbs divide, and their branches unite to
form the abdominal vein. The abdominal vein also collects blood from
the bladder, ventral body wall, and heart. The portal vein carries blood
into the liver from the stomach, intestine, spleen, and pancreas."

\^The sinus venosus contracts first, forcing the impure venous blood
into the right auricle; then both auricles contract and the oxygenated
( ) blood brought to the left auricle by the pulmonary

veins is forced into the left part of the ventricle, while the impure blood
from the right auricle is forced into the right side of the ventricle. The
ventricle then contracts and the impure blood is forced out, first passing
principally into the pulmocutaneous arteries and thence to the lungs and
skin, and the oxygenated blood is pushed but later through the carotid
and systemic arteries to the other parts of the body." The blood then
passes through the various blood vessels which become smaller and
smaller. These minute vessels are called capillaries. It is here that the
food and the oxygen of the blood bathe the tissues, and waste products
are taken up.

The renal-portal system carries the blood from the legs and pos-
terior portions of the body to the kidneys where urea and similar impuri-
ties are taken out. The hepatic-portal system carries all the blood from
the digestive tract into the liver where bile and glycogen are formed.
All blood brought to the lungs and skin is oxygenated and carried back
to the heart.

The liquid in which the blood corpuscles float is called blood plasma
as long as it is contained within the walls of the blood vessel. When
it leaves the blood vessel and bathes various parts of the intervening
spaces, it is called lymph; while, if it should be taken out of the body
entirely, it would be called serum.

The lymph spaces in the frog's body are very large and communi-
cate with one another as well as with the veins. There are four so-called
lymph-hearts (Figs. 11, 347) ; two near the third vertebra, and two near
the end of the vertebral column. These lymph-hearts force the lymph
into the internal jugular and transverse iliac veins by their pulsation.
The lymph itself is colorless, and whatever corpuscles it may contain are
likewise colorless.

RESPIRATION

As has already been mentioned, breathing takes place through the
skin, both in water and air, although the lungs are naturally the prin-
cipal organs of respiration. The air is taken jn through the external
nares into the olfactory ( ) chamber, then through

the internal or posterior nares into the mouth cavity. The valves, which

THE FROG -63

have already been mentioned, then close ; the floor of the mouth is raised,
the air being" forced through the larynx ( ) to the

lungs themselves. The contraction of the body-wall forces the air back
from the lungs into the mouth. It is interesting to note that the glottis
closes, while the floor of the mouth alternately raises and lowers — thus
drawing in and expelling air through the nares into the mouth cavity
by what are called throat movements.

The lungs themselves (Fig. 15) are formed of
minute chambers called alveoli ( )

tne walls of which are filled with little blood capil-
laries. The larynx is strengthened by five carti-
lages, ( ) across which the vocal
cords are stretched. The expulsion of air from the
lungs across the free ends of the vocal cords pro-
Aiveoii of Lungs. duces the sound known as croakmg. The laryngeal

muscles regulate the tension of the cords, causing the particular pitch
of the sound made.

Male frogs often have a pair of vocal sacs opening into the mouth
cavity, serving as resonators ( ) and increasing

the volume of the sound.

THE EXCRETORY SYSTEM

The food taken into the body is said to be ingested. The part of
the food which is actually taken into the blood as nutriment is said to
be digested, and that part of the food which passes directly through .the
body without becoming a part of it is said to be egested. Every living
cell ingests and must assimilate food in order to live ; consequently, it
must also get rid of that material which has already served a nutrient
purpose, and this getting rid of a substance that has been digested and
that has served a purpose is called excretion. This word must not be
confused with secretion, which means that a substance is given off from
the cell or gland which is to be used again by some part of the body.
The waste matter that is eliminated from the body in the form of carbon
dioxide is thrown off through the organs of respiration, but the solid
products have specialized organs for their removal. The skin serves as
such an organ to a small extent. The frog does not use the skin in this
way anywhere nearly to the extent that human beings do, because the
amphibia do not possess sweat glands. The liver and the walls of the
intestine are also excretory in character.

The most important organs for excretory purposes, however, are
the kidneys, two oval, flattened dark red bodies lying behind the peri-
toneum in the dorsal portion of the body-cavity. It is well to know that
the kidneys are about the only abdominal organs, even in the higher
forms, that lie between the dorsal peritoneum and body- wall. The kid-
neys are abundantly supplied with blood vessels, though they, them-
selves, are composed of connective tissue. The fact that so manv blood

64 GENERAL BIOLOGY

vessels run to them shows that they are decidedly important organs.
Each kidney contains a great number of coiled tubes called uriniferous
tubules, each one of which begins in a Malpighian body near the ventral
surface (Fig. 16). This body consists of a knot of blood vessels called
the glomerulus and a surrounding membrane known as Bowman's cap-
sule. This capsule is really the thinned out and expanded end of a
uriniferous tubule which has become pushed in by the glomerulus. All
excretions are carried by the uriniferous tubules to a collecting tubule,
and thence to the ureter. The ureter of each kidney passes caudad
( ) toward the cloaca, emptying therein, thence into

the bladder, a large two-lobed sac. This latter organ may be collapsed

Fig. 16. A, Diagram Showing Formation of Renal Tubules and Bowman's Capsule.

(After Borradaile.)

cap., Capillary plexus , col.t., collecting tubule ; me. and M cp., Bowman's cap-
sule (Malphighian capsule) ; r.a., renal artery; r.v., renal vein; r.p.v., renal portal
vein ; ur.t., uriniferous tubules ; w.d., Wolffian duct.

B. Diagram Showing Relation of Glomerulus and Renal Tubules to the Blood Vessels.

(After Guyer.)

if empty, or, if filled with the urine secreted by the kidney, may be con-
siderably distended. The ventral surface of the kidney has a great many
ciliated ( ) funnels called nephrostomes (Fig. 168)

whose expanded ends open into the coelom. In the young frog these are
connected with the renal tubules, while in the adult they open into
branches of the renal vein. The renal arteries and the renal-portal vein
carry the blood to the kidney, leaving again by the renal veins. The
glomeruli are supplied only with arterial blood, but the renal tubules re-
ceive blood from the renal portal veins and to a slight extent from the
renal arteries.

The functions of the kidney, as already stated, is the elimination of
waste matter from the blood. The excretion itself, known as urine, is
composed of a large number of compounds in solution. Most of the
nitrogen leaves the body in the form of urea (NH2)2CO, a white, crys-
talline compound, very soluble in water.

It is interesting to remember that this was the first organic chemi-
cal compound actually manufactured in the laboratory.

Urea represents the final product of the breaking down of the nitro-

THE FROG

65

Vll.p&l,

V.x.

genous substances of the body, and it has been shown that the forma-
tion of this substance takes place to a large extent in the liver from

which it is given to the
blood by a process of inter-
nal secretion. Beside urea,
urine contains various salts
in solution such as chlorides,
sulphates, phosphates of so-
dium, potassium, calcium,
and magnesium, as well as
other substances.

As far as we know at
this moment, practically all
of the excreted substances
of the kidney pass through
the glomeruli. The exact
function of the glomeruli
are not known, though there
are many theories regarding
it.

The bladder arises as an
outpushing of the ventral
wall of the cloaca. It is re-
garded as homologous
( ) with

the allantois (Fig. 363)
of the embryo of higher ver-
tebrates. It is very distensi-
ble. There are circular mus-
cles at the mouth of the
bladder which are able to
contract and expand, the
contraction closing the clo-

-a pal nnpnincr tn mab-f it nr»Q

sible for urine to collect in
the bladder.

f.t.

Fig. 17. The Central Nervous System and Principal
Nerves of a Frog, Seen From Below.

/., Olfactory lobes ; //., Optic chiasma ; I.-X., cranial
nerves ; 1-10, spinal nerves ; V.md., V.mx., V.op.,
mandibular, maxillary, and opthalmic branches of
fifth cranial nerve; VI', sixth cranial nerve after
leaving the Gasserian ganglion ; VH.hd., VII-pol.,
hyoidean and palatine branches of seventh cranial
nerve; IX'., branch from ninth cranial nerve to seventh;
IX"., main branch of ninth cranial nerve ; X.v., tenth
cranial nerve passing to viscera ; V.x., a small twig from
the undivided main branch of the fifth cranial nerve ; X.x,
a branch from the vagus to certain muscles ; an.V.,
annulus of Vieussens through which the subclavian artery
passes ; f.t., filum terminale ; G.g., Gasserian ganglion ;
hy.n., hypoglossal (first spinal) nerve; inf., infundi-
bulum ; pit., pituitary body ; r.c., ramus communicans ;
sci.n., sciatic nerve ; sy.c., longitudinal commissure of
sympathetic chain ; sy.g., sympathetic ganglion ; v.g.,
vagus ganglion. (Redrawn from Borradaile.)

Compare with Figures 472C, 478, 480, 483.

THE NERVOUS SYSTEM

One of the necessary
conditions of life is what is
commonly called irritability,

• AxrVi^n r» r r> r> #» r 1 v

[ c*n> Wnen properly

cti'rnnla t^rl r^rfr^rm ^Artatn

stimulated, periorm certain

66 GENERAL BIOLOGY

movements. In the higher forms of animals a definite nervous system
does this work and permits a co-ordination of activities in different
parts of the body. For example : In order to leap when danger threat-
ens, the frog must be able to send the necessary nervous impulses to
both hind legs at one time, for if only one leg should get an impulse,
the frog would fall over on one side instead of propelling his body for
some distance ahead.

There is also another function the nervous system has to perform,
and that is the accumulation of the effects of experiences which the ani-
mal in question has had, so that such animal may profit by the memory
of these experiences in new situations. When this ability is highly de-
veloped, we speak of it as reasoning or intelligence, whereas when the
animal only remembers, let us say, a physical punishment for having
performed a given act, and by sheer association of the punishment and
the act ceases to perform the act which brought about the punishment,
such an association is not known as intelligence, but as association
memory.

Practically all parts of the body have nerves running to them.
There are three closely associated divisions in the nervous system (Fig.
17) known as :

1. The central, consisting of brain and spinal cord.

2. The peripheral, consisting of cerebral and spinal nerves, and

3. The sympathetic, supplying non-striated muscles.

THE CENTRAL NERVOUS SYSTEM

As in all of the vertebrates, the brain and spinal cord are on the
dorsal side of the animal, being contained within a bony case known as
the skull and neural canal. It will be noted that beginning at the an-
terior end, the brain consists of quite distinct parts, namely, the olfac-
tory lobes, the cerebral hemispheres, the two large optic lobes, a well
developed mid brain, a small cerebellum, and a broadening of the spinal
cord itself called the medulla oblongata. From the ventral surface, we
may see in addition the crossing from one side to the other 6f the optic
nerves, known as the optic chiasma.

A small process directly behind the optic chiasma called the infundi-
bulum ( ) ends in another small body, the pituitary

body ( ) or hypophysis ( ).

On the dorsal side of the mid brain is found the pineal gland
( ) or epiphysis ( ), already

mentioned as a rudimentary organ which, in some forms of the reptiles,
forms a dorsal median eye. The cerebrum and optic lobes (thalamen-
cephalon) ( ) together constitute the fore brain,

the optic lobes form the mid brain, the cerebellum and medulla form the
hind brain.

It is not clear what functions each part of the frog's brain can per-

THE FROG 67

form. From various experiments, however, it is known that the frog
loses the power of spontaneous movement if the mid brain and cerebral
hemispheres are removed, while the spinal cord becomes very irritable
if the optic lobes are cut away. No function has yet been definitely
ascribed to the cerebellum and even when all of the brain, with the ex-
ception of the medulla, is removed, the animal "breathes normally, snaps
at and swallows food, leaps and swims regularly, and is able to right
itself when thrown on its back." If the posterior portion of the medulla
is removed, the frog dies.

THE SPINAL CORD

The spinal cord passes down through the bony vertebral or spinal
column. It is short and somewhat flattened. There is an enlargement
in the brachial region where the nerves pass off to the fore limbs, and
one further back where the large nerves originate which supply the hind
legs. The cord tapers to a narrow thread called the filum terminate
which extends into the urostyle. There is a median fissure on both dor-
sal and ventral sides of the cord, while from the sides of it, the roots
of the spinal nerves are given off. The cord itself is surrounded by two
membranes, an outer, the dura mater, and an inner known as the pia
mater. There is an H-shaped central mass of gray matter consisting- of
nerve cells, and an outer mass of white matter composed of nerve fibers.

There is a little opening through the center of the cord called the
central canal. The various cavities in the brain are a continuation and
expansion of this central canal.

THE PERIPHERAL NERVOUS SYSTEM

There are ten pairs of spinal nerves in the frog, each arising by a
dorsal and ventral root and springing from the horns of the gray matter
of the cord (Fig. 470). The two roots unite to form a trunk, passing
out between the arches of the vertebrae.

The brachial, or arm branches, are made up of the second, as well as
branches from the first and third pairs of spinal nerves, and pass to the
fore limbs and shoulder, while the sciatics arise from plexuses composed
of the seventh, eighth, and ninth spinal nerves, and run to the legs.

There are also ten pairs of cranial nerves which supply the organs
of special sense, certain muscles, various organs of the head, the heart,
lungs, and stomach. They are named as follows :*

*There are two additional cranial nerves in the higher animals, the spinal accessory and hypo-
glossal, and medical students remember them by the following verse, the first letter of each word
being the initial letter of the correspondingly numbered nerve:

I. On VII. Finn

II. Old VIII. And

III. Olympus IX. German

IV. Towering X. Picked
V. Tops XT. Some

VI. A XII. Hops

68 GENERAL BIOLOGY

1. The olfactory ( ), nerves running from the
olfactory lobes to the nasal cavities.

2. The optic nerves, running from the optic lobes, crossing each
other to form the optic chiasma and passing to the eye on the opposite
side of the head.

3. Oculomotor, supplying the muscles of the eye.

4. Trochlearis ( ), sometimes called the patheti-
cus, supplying the muscles of the eye.

5. The Trigeminus ( ), or trifacial, a sensory
nerve, supplying the sides of the head.

6. The Abducens ( ), supplying the muscles
of the eye.

7. Facial, chiefly motor in its action and supplying the sides of the
head.

8. Auditory, supplying the inner ear.

9. Glossopharyngeal ( ), a sensory nerve, sup-
plying the pharynx and tongue.

10. Pneumogastric ( ), or vagus, supplying the

larynx, heart, and stomach.

THE SYMPATHETIC SYSTEM

The main trunks of this system consist of a nervous strand on each
side of the spinal column (Fig. 337). Throughout the abdominal cavity
one may see the chain of minute nerve ganglia, ten in number, which are
also connected with the spinal nerves. From these chains of ganglia tiny
nerves are given off, supplying the intestine, the kidney, and other ab-
dominal organs.

Although the sympathetic system is connected with the spinal
nerves, it has entirely distinct and separate functions. Microscopically,
one finds quantities of neurones, each with its little cell-body, dendrites
( ) and axon. These are massed in the brain and

cord, as well as in the ganglia outside of the cord. Some of them carry
impulses to the center and some away from it. There are several
branches where a vast intermingling of the sympathetic strands is seen,
the principal ones being called the coeliac ( ) or

solar plexus, supplying the stomach, intestine, liver, pancreas, spleen,
and sending fibers to the gonads and kidneys, and the urogenital plexus,
supplying kidneys and gonads primarily.

THE FROG

69

THE SENSE ORGANS

If one marks a series of spaces on the volar ( )

surface of the fore arm of a human being about a millimeter square, and
such person is then blindfolded, it will be found that when a cold needle
touches certain squares he will feel a sensation of cold, whereas if it
touches certain other squares, he will feel a sensation of heat. From
this experiment it is learned that a great many, if not all, nerves have a
very special and definite work to perform.

Where a great mass of such specialized nerve endings is grouped in
one place, it produces an organ of special sense such as the eye, the ear,
the nose, the tongue. All of these organs are groups of nerves whose
endings are on the surface of some part of the body, and carry sensations
inward to the central nervous system. These are called sensory nerves.

The nerves which begin in the central nervous system and go out-
ward to some of the muscles, producing various movements of those
muscles, are called motor nerves.

Both sensory and motor cells may unite in a ganglion and have
both types of fibers run in the same sheath from there on; these are
called mixed nerves.

m.n. p. sup. d.n.l. gl.n.t.
I'lr

B

Fig. 18. The Eye.

A. Eye in position. d.n.l., lachrimal duct leading from eye to interior of
nose ; gl.n.L, lachrimal gland ; m.n., nictitating membrane ; no,., nares ; p.i., lower
eye-lid; p.sup., upper eye-lid. (After Schimkewitsch.)

B. Diagrammatic section through the optical axis of the eye of the frog.

C. Diagrammatic horizontal section of the eye of man. (After Guyer.)

THE EYE

Probably the most important special sense organ is the eye (Fig.
18). Practically only one type of sensation is carried by the nerves of
this special sense organ, and that is light perception. The eye of the
frog is a large spherical organ similar to the eye of all of the higher
animals. The walls of the organ are opaque, with the exception of a
transparent portion directly in the foreground occupying about one-
third of the eye ball and called the cornea ( ).

70

GENERAL BIOLOGY

The darker portion of the eye acts as does the dark chamber of a
camera. This chamber takes up about two-thirds of the posterior part
of the eyeball and consists of three layers. Toward the exterior is found
the sclerotic ( ) coat made up of fibrous tissue and

cartilage. Then follows a thin pigment-containing coat, known as the
choroid ( ) and in the inside of this a very thin

layer, known a.s the retina ( ). It is the retina which

is sensitive to light. Almost in the center, but a little to one side of the
back chamber, the optic nerve enters, spreading out on the retina, so
that it has a considerable area that light may affect. The chamber of
the eye itself is divided in two parts by a transparent spherical, crystal-
line lens which is held in
position by several bands of
fibers. The lens is partly
covered anteriorly by an
opaque membrane, in reality
a continuation of the
choroid, growing out of the
wall of the chamber on all
sides. This membrane is
known as the iris ( ),

and it covers the entire
outer portion of the lens
with the exception of the
center. This central uncov-
ered portion is called the
pupil, and it is through this
the light enters. There are
pigment cells in the iris
which give the color to the
eye.

Both of the eye-cham-
bers are filled with a trans-
parent liquid. That between
the cornea and the lens is
called the aqueous humor
( ) and that

back of the lens, which is
quite thick, is called the vit-
reous humor ( ).
The retina itself is quite
complicated, being com-
posed of thousands of end
organs of sensory nerves,
highly sensitive to the light

ZV-

Fig. 19.

A. Diagrammatic transverse section of the head
of the toad showing arrangement of the parts of the
ear. (After Guyer from Jammes. )

B. The labyrinth of the right ear of the frog,
seen from the outer side.

C. A diagram of the ear of the frog. col.,
Columella ; f.o., fenestra ovalis ; Eu., Eustachian tube ;
lab., part of the membranous labyrinth, containing
endolymph ; m., mouth ; md., mandible ; peril., peri-
lymph ; sk., skull; tym., tympanic membrane. (B
and C, from Borradaile.)

THE FROG 71

which is focused upon it by the lens. There are six muscles attached
to the eyeball by means of which it can be moved in practically any
direction (Fig". 466).

THE EAR

As has already been noted, there are really no external ears on
the frog though there is a rounded, flat membrane covering the real ear
(Fig. 19). Directly beneath this outer membrane there is another
tougher one which is known as the tympanic membrane ( ).

It extends over a shallow, cone-shaped cavity called the tympanum, or
ear-drum, and connects with the mouth through the Eustachian tube

( )-

The columella ( ), a slender bar of bone and

cartilage, extends across this, being attached to the membrane at one
end and connected with the inner ear at the other. It is by this little
bar that vibrations of the outer membrane are carried to the inner ear.
This inner ear is the real organ of hearing and is made up of the sensory
end of the auditory nerve. The auditory nerve lies embedded within the
skull itself.

There are several semi-circular canals present which function as a
balancing organ so that the animal can keep an upright position. These
form what are often called an "organ of the sense of equilibrium."

THE OLFACTORY ORGAN

There is little known regarding what effect the sense of smell has
in the life of a frog, but it is known that there are little olfactory sacs
just within the bones into which the openings from the nostrils lead.
The air enters these and then passes through the bones into the mouth
by the internal nares. The ending of the olfactory nerve is in this little
sac, where it is spread out to a considerable extent and where vapors
of various kinds in the air may affect it.

THE TONGUE

The sense of taste probably resides in the tongue, though there are
various small structures on the roof and floor of the mouth which may
have similar functions.

Conclusions of this kind are based on observations of what the fre£
does when different tasting liquids are brought in contact with the struc-
tures mentioned.

The fact that the animal does react differently to different tastes
is again accounted for by the finding of nerve endings in these supposed
taste organs.

TOUCH AND PRESSURE

These senses are located in the skin in various parts of the body.
This is due to the fact that there are many sensory nerves whose end-
organs terminate in the skin. Just as the experiment of the cold needla

GENERAL BIOLOGY

H

Fig. 20. The Axial Skeleton of the Frog.

A. The skull and vertebral column of frog viewed from dorsal surface.

B. The same -from the ventral surface.

C. Lateral view of the urostyle ; a bristle is passed through the foramen for
the tenth spinal nerve.

D. The branchial skeleton of the frog : O., orbital fossa ; pmx., premaxilla ;
mx., maxilla ; .q-j., quadrato-jugal ; na., nasal ; pf., parieto-frontal ; ex., exoccipital ;
fm., foramen magnum ; pro., pro-otic ; sq., squamosal ; sp.et., sphenethmoid ;
par., parasphenoid ; pal., palatine ; vo., vomer ; ptg., pterygoid ; av., atlas ; c.,
centrum ; ar., neural arch ; zyg., zygapophysis ; trv., transverse process ; ur., uro-
style; H., body of hyoid ; Ha., anterior cornu ; H .p., posterior cornu of hyoid.

E. The skull of a frog, seen from the right side : a.c., Anterior cornu of hyoid ;
a.sp., angulo-splenial ; 6., body of hyoid ; col., columella ; d., dentary ; e.n., external
nasal opening ; f.p., fronto-parietal ; m., maxilla ; mm., mentomeckelian ; n., nasal ;
o.c., occipital condyle ; p.c., posterior cornu of hyoid ; p.m., premaxilla ; pro.,
prootic ; pt., pterygoid ; q., quadrate ; q.j., quadratojugal ; sp., sphenethmoid ; sq.,
squamosal.

F. The skull of a frog seen from behind : col., Columella ; ex., exoccipital ;
/.m., foramen magnum ; o.c., occipital condyle ; pro., prootic ; pt., pterygoid ; q.,
quadrate ; q.j., quadratojugal ; sq., squamosal ; IX.X., foramen for ninth and tenth
cranial nerves.

G. The cartilaginous skull of a frog, seen from above after the removal of
most of the bones : a./., Anterior fontanelle ; au., auditory capsule ; cr., cranium ;
ex., exoccipital ; l.p.f., left posterior fontanelle ; nas., nasal capsule ; o.c., occipital
condyle ; pro., prootic ; pt., pterygoid ; q., quadrate ; q.j., quadratojugal ; sp.,
sphenethmoid ; u.j., upper jaw bar.

THE FROG 73

has demonstrated particular sensations for particular nerve endings in
the arm of man, so it may be supposed that these different end-organs
in the skin may have similar definite functions.

THE SKELETON

The frog is possessed of an endoskeleton as is man. The bones and
cartilages constituting this endoskeleton furnish a support, holding in
position all the muscles and organs of the body.

For convenience's sake the skeleton is divided into two parts, the
axial portion (Fig. 20), comprising skull and vertebral column, and the
appendicular portion (Figs. 21, 22), consisting of the pectoral or shoul-
der, and pelvic or hip girdles, together with the bones of the limbs which
these girdles support.

The frog's skeleton possesses about ninety articulated bones (united
at the joints). The skull has the various bones comprising it so firmly
fused that they appear as a single bone. Even the seemingly single
bone of the fore arm will be found to consist of two bones which have
also fused together.

THE AXIAL SKELETON

This is divided into the skull [cranium ( ) and

visceral skeleton ( )], and vertebral column. The

two divisions of the skull mentioned above are made up of the brain case
together with the auditory ( ) and olfactory cap-

sules ( ) which constitute the cranium. The jaws

and hyoid arch ( ) together, form the visceral skel-

eton.

The inside of the cranium where the brain is placed is known as
the cranial cavity. The skull itself is composed of thirty-two bones and
cartilages fused together so as to appear almost a solid structure. The
cranial bones form the roof, walls, and floor of the cranial cavity.

The floor is composed of the basioccipital ( )

and the parasphenoid ( ).

The walls consist of the parietals ( ), the otic

bones ( ), and the exoccipital ( ).

The roof is made up of the supraoccipital ( )

and the frontals.

The facial bones, forming the face, consist of nasals ( ),

the premaxillas ( ), and the maxillas ( )

above, and vomers ( ) below. The premaxillas and

the maxillas, however, are a part of the visceral skeleton comprising, to-
gether with a pair of quadrangulars, the upper jaws.

H and I. Vertebrae of a frog. H, fourth vertebra, seen from in front ; I, sixth
and seventh vertebrae from the right, az., Prezygapophysis ; cen., centrum ; n.a.,
neural arch ; n.c., vertebral foramen ; n.s., neural spine ; r>z., postzygapophysis ; r.c.,
cartilage at end of transverse process ; tr., transverse process.

(A, B, C and D from Bourne, after Ecker. E, F, G, H and I, after Borradaile.)

74 GENERAL BIOLOGY

The maxilla and the premaxilla bear teeth. The lower jaw or
mandibular arch ( ), is made up of a pair of carti-

laginous rods (Meckel's cartilages), enforced by a pair of dentary bones
( ) and a pair of angulo-splenials ( ).

The jaws themselves are attached to the cranium by an apparatus con-
sisting of squamosals ( ), pterygoids ( )
and palatines ( ), the whole often known as a sus-
pensory apparatus, or a suspensorium ( ). These
bones, though attached to the cranium in the adult frog, are at first free
from it, being in reality the upper parts of what are called visceral
arches, which lie below the cranium. The second arch is called the
hyoid, and is quite rudimentary, only a small part of it being left in the
adult frog. In the higher forms such as man, this is a well-developed
V-shaped arch to which the tongue is attached, but in the frog it remains
only as a flat plate, partly bone and partly cartilage, so loosely attached
to the skull that it is quite easily, and one might add, usually, lost. It
lies directly beneath the larynx ( ) in the frog, giv-
ing this support and rigidity, being connected with the skull by liga-
ments ( ) only.

In the young frog all parts of the skull are soft, but true bone forms
as development goes on. A part of the skull forms originally as carti-
lage, a material that is harder than membrane but softer than bone.
Mineral matter is deposited a little later in the cartilage, causing ossifi-
cation ( ) or true manufacture of bone.

Bones such as the occipitals, parietals, pterygoids, and the mandi-
bles, formed from cartilage, are known as cartilaginous bones, the other
ones being manufactured first as membranes. Here, too, mineral matter
is laid down and the structures become hardened. Such bones as
frontals, parietals, parasphenoids, squamosals, nasals, vomers, pre-
maxillas, and maxilla are of the latter kind and are called membrane
bones. The projections at the posterior end of the skull where it con-
nects with the vertebral column are called occipital condyles ( ) ;
and the large opening directly between these through which the spinal
cord continues down through the bony canal of the spinal column is
called the foramen magnum ( ).

THE VERTEBRAL COLUMN

This consists of nine separate segments of bone (H and I, Fig. 20),
each known as a vertebra ( ), and a long platelike

posterior extension, the urostyle ( ). Each verte-

bra consists of a centrum and a neural arch ( ), the

latter enclosing the neural foramen; on each side of all but the first
vertebra is found a transverse process, while all vertebrae possess a dor-
sal spine and a pair of smooth surfaces where each successive vertebra
rests upon the next following. These articulating processes are called

THE FROG

75

zygapophyses ( ). The little bones themselves are

held together by ligaments and move on one another by means of the
centrum and zygapophyses. This permits a firm axial support, while
also allowing for the bending of the body. • By having all the vertebrae,
one immediately above the other, the neural opening is continuous, so
that the spinal cord not only lies free, but the vertebrae themselves are
thus prevented from bending sufficiently to damage the cord.

The surfaces of the centra unite by a ball and socket joint. Each of

the first seven vertebrae possesses a
ball on the posterior and a socket
on the anterior surface. The eighth,
however, is concave on both sur-
faces and the ninth is convex on
both. It is important to know the
difference in action that this entails.
Although all nine vertebrae are
much alike, they can easily be distin-
guished from each other. The first
possesses no transverse process,
while the centrum of the ninth has
two convex posterior surfaces and
very large transverse processes. It
is from this last vertebra that the
urostyle, the long slender bone, ex-
tends backward to the end of the
body.

The urostyle is supposed to rep-
resent the tail found in allied ani-
mals, such as the salamanders. The
spinal cord actually extends into
the urostyle, but passes out almost
immediately through two small
openings on either side, as two
rather tiny filaments.

There are no ribs in the frog,
and the transverse processes end
rather abruptly a very short distance from the centrum.

THE APPENDICULAR SKELETON

The shoulder or pectoral girdle ( ) (Fig. 21)

serves as an attachment for the muscles that move the fore limbs, and
also as a protection for the organs in the anterior portion of the trunk.

The girdle itself surrounds the body just back of the head, consist-
ing of a paired scapula ( ), the dorsal part of which
is made of cartilage, a coracoid ( ), a precoracoid

Fig. 21.

Pectoral Girdle, Arm, and Hand, of Frog.

A. The shoulder girdle of the frog ; the
scapula and suprascapula are turned outwards.
ep., episternum ; os., omosternum ; ep.c.,
epicoracoids ; mes., mesosternum ; xi., xiphister-
num ; s.sc., suprascapula ; sc., scapula ; gl.,
glenoid cavity ; cor., coracoid ; el., clavicle.

B. Forearm and hand of right side, as seen
from above : ru., radio-ulna ; / — V., the
five digits ; r., radiale ; im., intermedium ; u.,
ulnare ; a., first distal carpal bone ; b., second
distal; c., third distal.

C. Radio-ulna of right side : o., olecranon ;
r., radius ; u., ulna.

D. Humerus : h., head ; sh., shaft ; or., dis-
tal articular knob ; £., trochlea. ( From Bourne,
after Ecker.)

76

GENERAL BIOLOGY

or epicoracoid, and a clavicle ( ) fused together.

At the meeting of coracoid and scapula there is a little smooth cavity
where the fore arm joins the- girdle called the glenoid fossa ( ).

Where coracoid and clavicle meet at the mid line on the ventral side
of the body, there are four bones. These four actually are a part of the
axial skeleton, but are usually classified as a part of the appendicular
as well. The most anterior one of the bones is called the episternum,
the one between this and the clavicle is the omosternum, while the pos-
terior one closest to the omosternum is the mesosternum, and the one
projecting farthest backward is the xiphisternum.

The fore limbs are made up of a long bone, the humerus ( ) ,

joining the pectoral girdle in the glenoid fossa at its proximal end
( ) and with the radio^ulna at the distal end

( ). This latter bone constitutes the skeleton of

the fore arm and in reality consists of two bones, the radius and the ulna,
fused together.

Fig. 22. The Pelvic Girdle and Leg.

A. Pelvic girdle complete.

B. One side of pelvic girdle : II., ilium ; Isch., ischium ; Pu., cartilaginous
pubis ; Ac., acetabulum.

C. Femur of the frog: p., proximal; d., distal articulating surfaces; s., shaft. ^

D. Tibio-fibula, seen from below : p., proximal ; d., distal articulating sur-
faces ; t., tibial half of the bone separated by a groove from /., the fibular half.

E. The right ankle and foot of the frog, seen from below: This figure is
drawn to a larger scale than A and B. a., astragalus ; c., calcaneum ; / — V., the
five principal digits; X., the minute accessory digit. (From Bourne after Ecker.)

The wrist possesses six bones, the ulnare ( ),

radiale, ( ), intermedium, and three carpals

The hand has five proximal metacarpal ( )

bones, followed in digits ( ) II and III by two

phalanges ( ), and in digits IV and V by three
phalanges.

The pollex ( ) or thumb is rudimentary.

The pelvic ( ) or hip girdle (Fig. 22) supports
the hind limbs, and consists of two sets of three parts each, the ischium

THE FROG 77

( ), ilium ( ), and the pubis

( ), the latter being cartilaginous, strongly united.

The edge of the hip girdle is called the crest. The meeting of the two
pubic bones forms a symphysis ( ). The anterior

end of each bone is attached to one of the transverse processes of the
arched vertebra. The little cup-shaped opening where the three bones
just mentioned meet, is called the acetabulurn ( ).

It is in this concavity that the head of the femur ( ),

the long bone in the thigh, lies.

The hind limb consists of a thigh ( ) with the

femur as its solitary bone. The leg proper, running from knee to ankle,
is made up of the tibia ( ) and fibula ( )

fused together, called the tibio-fibula, or leg bone.

Note the ridges on these long bones for the attachment of muscles.

There are four tarsal bones ( ), the astragalus

( ), the calcaneum ( ), and two

smaller ones.

The foot has five complete digits as well as an extra or super-
numerary toe. Each digit has one proximal metatarsal bone, while be-
yond these there are a variable number of phalanges. The hallux
( ) corresponding to the great toe of man is the

smallest of the series. It has one metatarsal and two phalanges. On
the inner side of the hallux is the calcar ( ), the

extra toe. It may have one or two joints and a short metatarsal.

THE MUSCULAR SYSTEM

All movements in the body are produced through the contraction
of some one or more muscles. The muscles in turn are innervated
( ) by one or more nerves. The muscle is usually

attached by one or both ends to a bone, so that a good leverage is ob-
tained. In some cases, the attachment is direct, in others by means of
a tendon, a band of tough somewhat inelastic connective tissue which
is in reality the continuation of the muscle fascia after the muscle itself
ends.

Contraction may be brought about by many causes, such as heat,
pressure, electrical, or chemical stimuli ( ).

There are three distinct types of muscles (Fig. 23) ; each type has
a more or less individual, cellular arrangement, and these three types
are known as heart muscle, voluntary or striated muscle, and involun-
tary or nonstriated muscle.

Striated muscle can be moved when the individual possessing it so
desires. Such are the muscles of the arm and hand. Examples of non-
striated muscle may be found in the blood vessels, where the desire of
the individual has little or nothing to do with the contraction and ex-

78

GENERAL BIOLOGY

pansion of circular and longitudinal muscles contained within the walls
of the blood vessels themselves.

The outer surface of all muscles is covered by a connective tissue
membrane called fascia ( ), which is not very elastic,

and usually becomes thicker toward the end of the muscle, graduating
in a dense, fibrous band called a tendon, or if this tendon is broad and
flat an aponeurosis ( ).

That part of the muscle most thoroughly attached — usually to a
relatively immovable part — and most frequently toward the center of
the body, is called its origin. The more movable and distal attachment
is known as its insertion,

The action of a muscle in con-
tracting is to draw origin and inser-
tion closer together.

Whenever a muscle moves any
part of the body in its normal direc-
tion or as one may say, with the
joint, such movement is called
flexion ( ) ; against

the joint extension ( ).

A muscle which pulls any limb or
portion of a limb away from the
central axis of the body is an ab-
ductor ( ), and
one which draws the limbs or their
appendages toward the center of the
body is an adductor ( ).
Rotators ( ) aro
those which cause the limb to rotate
about its axis such as those turning
the femur at the hip ; levators raise
a part such as the lower jaw, and
depressors produce the opposite
movement.

To know a muscle there are five
points which must be remembered:

(1) Its Origin.

(2) Its Insertion.

Its Relation to other struc-

Fig. 23. Different Types of Muscle-Fibres.

A., embryonic striped muscle-fibre from
the tail of a tadpole, showing the nuclei nn.,
and the protoplasm p., of the ccenocyte from
which the fibres are developed. The fibres ex-
hibit alternate dark and light bands, and in
the centre of each dark band is a light line,
the line of Hensen.

B., cardiac muscle-fibre showing the short
branched nucleated cells.

C., a. single cell from cardiac muscle-fibre
more highly magnified, showing the cross-
striation and the nucleus n.

D., group of unstriped muscle-fibres from
the bladder : a., the nuclei ; p., the granular
remains of the cell protoplasm ; /., the longi-
tudinally striated contractile portion. (A and
D, from Bourne. B and C from Schafer.)

(3)
tures.

(4)

(5)

Its Innervation.
Its Action.

The following list will give the student a clear and accurate idea
of what is essential in the study of the muscular system (Fig. 24). The
relation of each muscle to surrounding structures can be obtained only

THE FROG 79

by a dissection of them in the animal, and a thorough study of the draw-
ings.

A. MUSCLES OF THE TRUNK

1. Muscles of the lower or ventral side.

(a) Muscles of the abdomen.

e. g. Rectus abdominis, a wide band running along the abdo-
men divided lengthwise down the middle by the connective tissue linea
alba and transversely by tendinous intersections.

Obliquus externus, a broad sheet at each side of the body,
arising from an aponeurosis known as the dorsal fascia which covers
the muscles of the back, and inserted into the linea alba above the rectus
abdominis.

Obliquus internus and transversus, muscular sheets below the
external oblique.

By their contraction all these muscles lessen the size of the body
cavity and compress the organs within it.

Innervation : All of these muscles are innervated by twigs from
IV, V, VI and VII spinal nerves.

(b) Muscles of the Breast Region.

e. g. Pectoralis, large and fan-shaped, inserted into the deltoid
ridge of the humerus and consisting of a sternal portion which arises
from the pectoral girdle, and an abdominal portion which arises from
the aponeurosis at the side of the rectus abdominis.

It draws down the arm.

Innervation : Twig from II spinal nerve.

Coraco-radialis, arising from the coracoid and inserted into the
upper end of the radius. It bends the arm.

Innervation: Twig from II spinal nerve.

2. Muscles of the Back.

(a) Muscle inserted into the lower jaw.

Depressor mandibulae, triangular, arising from the supra-
scapula and inserted into the angle of the lower jaw, which it draws
downwards and backwards, thus opening the mouth.

(b) Muscles inserted on the fore-limb.

e. g. Latissimus dorsi ( ) triangular, aris-

ing from the dorsal fascia and inserted into the deltoid ridge. It draws
back the arm.

Infraspinatus, in front of and similar to the latissimus dorsi. It
raises the arm.

Innervation : Twig from II spinal nerve.

(c) Muscles inserted into the shoulder girdle.

e. g. Levatur anguli scapulae, arising from the skull and in-
serted into the under side of the suprascapula, which it draws forward.
Innervation : Twig from I spinal nerve.

80

GENERAL BIOLOGY

Serratus, arising from the little knobs on the transverse
processes of the vertebrae which represent the ribs, and inserted into the
under side of the suprascapula, which it draws backwards, outwards, or
inwards, according to the division which is contracted.

(d) Muscles inserted into the hind-limb.

e. g. Gluteus (iliacus externus, or gluteus medius), arising

cor. ; at/.

Pet. st.

f. A.

Fig. 24. A — A Ventral View of the Muscular System of a Frog.

ad.long,, Adductor longus ; ad.mag., adductor magnus ; one., anconaeus ; cor.rad.,
coraco-radialis ; dtd., deltoid : c.ob., external oblique ; e.cr., extensor cruris ; gast.,
gastrocnemius ; grac., gracilis ; La., linea alba ; pct.ab., abdominal part of the
pectoral muscle ; pct.st., sternal part of the same ; r.ab., rectus abdominis ; sar.,
sartorius ; sm., mylohyoid ; t.i., tendinous intersections ; t.A., tendo Achillis ; t.f.,
tibiofibula ; tib.ant., tibialis anterior ; tib.post., tibialis posterior ; v.int., vastus
internus ; x.c., xiphoid cartilage. (After Borradaile.)

B. Dissection of special muscles of the left hind leg of the toad (redrawn from
Jammes). Muscles shaded in black are extensors; in gray, flexors.

THE FROG 81

from the ilium and inserted into the head of the femur, which it rotates
inwards.

(e) Muscles inserted into the hip girdle.

e. g. Coccygeo-iliacus, arising from the urostyle and inserted
into the ilium, which it holds firm as a fulcrum for the movements of the
hind-limb.

(f) Muscles of the Backbone.

e. g. Longissimus dorsi, a band running the whole length of
the back, divided by tendinous intersections, which are attached to the
transverse processes, and inserted in front into the skull. It straightens
the back.

Innervation : Twig from I spinal nerve.

B. MUSCLES OF THE HEAD

1. Muscles underneath the Head.

e. g. Sternohyoid, from hyoid to pectoral girdle.

Geniohyoid, from hyoid to chin.

Hyoglossus, from hyoid to tongue.

Petrohyoid, from hyoid to auditory capsule.

Mylohyoid, submandibular, or submaxillaris ( ),

a sheet of muscle running from side to side of the lower jaw. These
muscles alter the position of the floor of the mouth.

Innervation : Twigs from I spinal nerve.

2. Muscles of the Lower Jaw.

e. g. Temporalis and masseter ( ), arising

from the skull and inserted into the lower jaw, which they raise.
Innervation : Mandibular branch of the V cranial nerve.

3. Muscles of the Eyeball (Fig. 466).

Rectus superior, r. inferior, r. externus, r. internus, arising from
the skull in the hinder part of the orbit and inserted into the eyeball.

Innervation: All but the rectus externus from the III cranial
nerve. The r. externus by the VI cranial nerve.

Obliquus superior and o. inferior, arising from the skull in the
front part of the orbit and inserted into the eyeball.

Innervation : Obliquus superior by the IV cranial nerve and o.
inferior by the III cranial nerve. ,-,,.

C. MUSCLES OF THE FORE-LIMB

1. Muscles for the Upper Arm.

e. g. Deltoideus, arising from the scapula and inserted into the
humerus. It raises the arm.

2. Muscles for the Fore-Arm.

Triceps brachii or anconaeus ( ), arising

from the scapula and humerus, and inserted into the upper end of the
ulna. It straightens the arm.

There is no Biceps muscle in the arm of the frog.

B2 GENERAL BIOLOGY

3. The muscles of the Wrist and Fingers are numerous and com-
plicated.

Innervation: Branches and twigs of II spinal or Brachial
nerves innervate all arm and finger muscles.

D. MUSCLES OiF THE HIND-LIMB

(1) Superficial muscles of the Thigh on the Preaxial (apparent
ventral)* Surface.

1. Sartorius ( ), a long, narrow band arising
from the lower end of the ilium, lying obliquely upon the adductor mag-
nus, and inserted into the tibia on its inner side near the end. It bends
the knee.

2. Adductor magnus, a large muscle arising from the pubis and
ischium, lying along the inner border of the sartorius and inserted into
the femur near its lower end. It draws the thigh towards the body.

3. Adductor longus is a long narrow muscle lying along the outer
side of the adductor magnus, and often completely hidden by the sar-
torius ; it arises from the iliac symphysis beneath the sartorius, and unites
a little way beyond the middle of the thigh with the adductor magnus.
It adducts the thigh and draws it ventrally.

4. Gracilis major ( ) or rectus internus major,
a large muscle arising from the ischium, lying along the inner side of the
adductor magnus, and inserted into the inner-side of the head of the
tibia. It bends the knee.

5. Gracilis minor or rectus internus minor is a narrow flat band of
muscle running along the inner, or flexor margin of the thigh ; it rises
from a tendinous expansion connected with the ischial symphysis, and is
inserted into the inner side of the tibia, just below its head. Action is
the same as for gracilis major.

Innervation: Branches and twigs from the sciatic nerve and
plexus.

(2) Superficial muscles of the Extensor Surface of the Thigh.

1. Triceps extensor femoris, or cruris, a very large muscle in-
serted into the front of the tibia just below the head of the latter, but
arising from the pelvic girdle as three separate muscles, the rectus an-
terior femoris ( ), vastus externus ( ),
and vastus internus, or crureus ( ). All these lie
on the front of the thigh, and their action is to straighten the knee.

Innervation: Branches and twigs from the sciatic nerve and
plexus.

(3) Superficial muscles of the postaxial (apparent dorsal) Surface
of the Thigh.

*The femur of the frog rotates away from the midline more than does the femur of man.
Consequently the true outer border of the frog's thigh is equivalent to the inner border of man's.
In other words the preaxial surface of the frog's thigh is equivalent to the inner surface of man s.

THE FROG 83

1. The gluteus (iliacus externus), already mentioned, lies in
the thigh between the rectus anticus and the vastus externus.

2. The biceps (ileo-fibularis) is a long slender muscle which
arises from the crest of the ilium just above the acetabulum ; it lies in
the thigh along the inner border of the vastus externus, and is inserted
by a flattened tendinous expansion into the distal end of the femur aiSc
the head of the tibia-fibula.

3. The semimembranosus is a stout muscle lying along tlu
inner side of the biceps, between it and the rectus internus minor. It
arises from the dorsal angle of the ischial symphysis just beneath the
cloacal opening, and is inserted into the back of the head of the tibia.
It is divided about its middle by an oblique tendinous intersection.

It adducts the thigh and flexes or extends leg according to
whether the leg is in a flexed or extended position.

4. The pyriformis is a slender muscle which arises from the
tip of the urostyle, passes backwards and outwards between the biceps
and the semimembranosus, and is inserted into the femur at the junc-
tion of its proximal and middle thirds. It pulls the urostyle to one side
and draws the femur dorsally.

Innervation : Branches and twigs from sciatic nerve and
plexus.

(4) Deep muscles of the Thigh.

1. The semitendinosus is a long thin muscle which arise* by
two heads ; an anterior one from the ischium close to the ventral angle
of the ischial symphysis and the acetabulum ; and a posterior one from
the ischial symphysis. The anterior head passes through a slit in the
adductor magnus and unites with the posterior head in the distal third
of the thigh. The tendon of insertion is long and thin, and joins that
of the rectus internus minor to be inserted into the tibia just below its
head. It adducts the thigh and flexes the leg.

2. The adductor brevis is a short wide muscle, lying beneath
the upper end of the adductor magnus. It arises from the pubic and
ischial symphyses, and is inserted into the preaxial surface of the proxi-
mal half of the femur.

3. The pectineus ( ) is a rather smaller
muscle, lying along the outer (extensor) side of the adductor brevis.
It arises from the anterior half of the pubic symphysis in front of the
adductor brevis, and is inserted like it into the proximal half of the
femur.

4. The ilio-psoas (iliacus internus) arises by a wide origin
from the inner surface of the acetabular portion of the ilium ; it turns
round the anterior border of the ilium, and crosses in front of the hip-
joint, where, for a short part of its course, it is superficial between the
heads of the vastus internus and of the rectus anticus femoris ; it then

84 GENERAL BIOLOGY

passes down the thigh beneath these muscles, and is inserted into the
back of the proximal half of the femur. It draws the thigh forward.

5. The quadratus femoris is a small muscle on the back of the
upper part of the thigh ; it arises from the ilium above the acetabulum,
and from the base of the iliac crest; it lies beneath the pyriformis and
behind the biceps, and is inserted into the inner surface of the proximal
third of the femur between the pyriformis and the ilio-psoas.

6. The obturator is a deeply situated muscle which arises from
the whole length of the ischial symphysis and the adjacent parts of the
iliac and pubic symphyses, and is inserted into the head of the femur
close to the gluteus.

Innervation : Branches and twigs from sciatic nerve and
plexus.

5. Muscles of the Leg or Shank.

e. g. (1) Peroneus, a long muscle which arises from the end
of the femur, lies along the side of the tibio-fibula, and is inserted into
the end of the tibia and the calcaneum ( ). It ex-

tends leg and foot and flexes foot.

Innervation : Peroneus nerve.

(2) Gastrocnemius ( ), a large, spindle-
shaped muscle which forms the "calf." It arises from the hinder side
of the end of femur and tapers into the long tendo Achillis, which passes
under the ankle joint and ends in the sole of the foot. It straightens the
foot on the shank.

Innervation : Tibialis nerve.

(3) Tibialis anticus, arising from the front of the femur by a
long tendon, lying in front of the shank, and dividing into two bellies,
which are respectively inserted into the astragalus and calcaneus. It
bends the foot on the shank.

Innervation : Peroneus nerve.

(4) Tibialis posticus arises from the whole length of the flexor
surface of the tibia; it ends in a tendon which passes round the inner
malleolus ( ), lying in a groove in the lower end
of the tibia, and is inserted into the dorsal surface of the astragalus. It
extends the foot when flexed, and flexes foot when extended.

Innervation : Tibialis nerve.

(5) Extensor cruris lies along the preaxial side of the tibialis
anticus, partly covered by this and partly by the strong fascia of the
leg. It arises by a long tendon from the preaxial condyle of the femur,
runs in a groove in the upper end of the tibia, and is inserted into the
extensor surface of the tibia along nearly its whole length. It extends
the foot.

Innervation : Tibialis nerve.

6. Muscles of the Foot.

These, just as the muscles of the wrist and hand are many and

THE FROG 85

complicated, but the student should know at least the general location of
the following:

Aponeurosis plantaris.

The flattened and broadened continuation of the tendon of the
gastrocnemius muscle passing over the heel and spreading out on the
sole of the foot in a sort of triangle with the base toward the toes.

Where this aponeurosis crosses the heel it is known as the
tendon of Achilles.

Flexor digitorum I, II, III, IV, V.

Each digit usually has a flexor, extensor, abductor, and ad-
ductor bearing the number of the toe to which it is attached, the great
toe being I.

There are also small Interosseus muscles between the various
tarsal bones.

For a detailed account of every muscle of the frog see : Ecker's
"The Anatomy of the Frog." — (Oxford University Press.)

REPRODUCTIVE ORGANS

The sexes are separate in the frog. The male has a rather thick pad
on the underside of its thumb, larger in the spring at the breeding sea-
son, than at any other time of the year. The two rounded or oval sperma-
ries (A, Fig. 25) of a light yellow color are found at the upper end
of the kidneys, while branching masses of a yellow shade are usually at-
tached to them. The sperm, the male gamete ( ),
is produced in the spermaries, being carried through slender ducts, the
vasa efferentia, through the kidneys, emptying into the ureters. It will
be observed, therefore, that in the male frog the ureters serve both as
an exit for the excretion of the kidneys and the secretion of the sperma-
ries. In some species of frogs, the ureters are slightly enlarged, forming
a small sac just where they enter the cloaca, and these sacs are known
as seminal vesicles. The sperm are held there until ready to be dis-
charged.

In the breeding season, if the body of a female (B, Fig. 25), be
opened, the ovaries are filled with eggs and seem to fill almost the en-
tire body-cavity. The ovaries, the female gonads ( ),
are placed in a position corresponding to the spermaries in the male. If
it is not the breeding season, the ovaries are rather small, slightly folded
and leaf-life, not very much larger than the spermaries, but of a dif-
ferent shape. The eggs break out of the ovary into the body-cavity and
make their way into the coiled oviduct through a small opening, passing
down into the thin-walled distensible uterus ( ).
The oviducts themselves are not directly connected with the ovaries,
but lie coiled next to the kidneys, the anterior end being a funnel-shaped
opening. The tube itself passes caudad beside the kidneys, opening
into the cloaca. The uterus is the rather large thin-walled chamber at

86

GENERAL BIOLOGY

its termination, in which the eggs are stored after passing through the
oviducts until the final egg laying. The oviducts themselves, like the
ovaries, vary in size at different seasons of the year.

The gelatinous substance covering the eggs is secreted by little
glands in the oviducts called nidamental glands (Lat. nidus=a nest).
It is to be observed that the sexual organs and kidneys lie close together
and have a common opening, and in the male the same duct, namely,
the ureter, serves for an exit of both sperm and urine. A similar close

ost.abd.

I* ••

neb.n.

Fig. 25. The Urogenital Organs of the Frog. A, Male ; B, Female.

ao.b., systemic arteries ; ao.c., main aortic trunk ; cav.i., vena cava inferior ;
cl., cloaca (dissected from the ventral side) ; coel.mes., coeliaco mesenteric artery;
d., large intestine; f.k., fat bodies; h.s.L, urogenital duct; h.s.l.' , entrance of uro-
genital duct into cloaca ; il., iliac artery ; n., kidney ; neb.n., adrenal bodies ; ost.abd.,
funnel-shaped opening of oviduct; ov., ovary; ovid., oviduct; ovid. ', entrance of
oviduct into cloaca; test., testes ; ut., uterus; ves., urinary bladder; ves.' , opening
of bladder into cloaca; ves.sem., seminal vesicle; w., Wolfian duct; w.' , opening of
Wolfian ducts. (After W. Meissner.)

relation is found in nearly all other vertebrates, and when the study of
embryology is taken up it will be found that the ducts and kidneys were
originally derived from the same region of the embryo. It is therefore
common to speak of the excretory and reproductive system together as
the urogenital system.

THE FAT BODIES

Directly in front of the gonads, we find a yellow organ with many
finger-like processes known as a fat body. It has a broader and closer

THE FROG 87

attachment to the anterior end of the male gonad than it has to the
female ovary. It is supposed to serve as a storehouse of nutriment, for
it varies in size and shape at different seasons of the year. Nearly all
the fat disappears from the cells in spring, while as soon as the feeding
period begins the fat increases.
References :

Ecker, "The Anatomy of the Frog."

Holmes, "The Frog."

Parker & Haswell, "Textbook of Zoology."

Bourne, "Comparative Anatomy of Animals."

Borradaile, "Manual of Zoology."

Schaefer, "The Endocrine Organs."

Bandler, "The Endocrines."

CHAPTER V.

THE CELL

It will be observed later in the study of the histology of the frog
that the different types of cells vary in size and shape, some being round,
others more or less cuboidal, still others cylindrical, etc. As there are
animals possessed of but a single cell which can nevertheless perform
all acts necessary to a complete organism and, consequently, can lead an
independent existence, the cell is called the biological unit, and things
in the biological world are not considered explained until they have been
reduced to terms of cell units.

There is not a living thing, plant or animal, which comes into ex-
istence that does not start life as a single cell. It is therefore an axiom
( ) of science that there can be no living cell unless

it sprang from a previous cell. Therefore, an egg, regardless of whether
it be the small egg of a frog or so large a one as that of the ostrich, is
only a single cell. In fact, in the hen's egg usually used in the laboratory
for experimentation, the yolk represents the food for the offspring, the
egg proper being that little portion, about the size of a dime, which
always floats on the top of the yolk, regardless of the position of the

egg.

The following drawing (Fig. 26) is that of an ideal cell. This means
that everything which the student will ever find in any cell, plant or
animal, is contained in this drawing. One must remember, however,
that search may be made from now until the end of time and no one cell
may ever be found with all of the parts shown in this ideal cell.

C»ll Htrnfc

Fig. 26. An Ideal Cell.

THE CELL 89

The entire substance surrounded by the cell wall is called proto-
plasm. This is a jelly-like or viscous material something like the white
of an egg. Probably most cells have a definite wall, though many animal
cells do not. On the inside of this cell wall there is a network, or reticu-
lum, in which are found little foreign bodies, plastids, and open spaces
called vacuoles. The network itself is called spongioplasm, because it
somewhat resembles a sponge. The liquid protoplasm on the inside of
this network is called hyaloplasm ( ). On the inside

of the cell there is a seemingly smaller cell, called the nucleus. This
nucleus is considered the most important part of a cell. A cell may have
one nucleus, or it may have many. There is a nuclear wall just as there
is a cell wall, and on the inside of the nucleus there is also a network
or reticulum.

When a cell has been chemically stained with various substances,
it is found that a portion of the network in the nucleus takes the stain,
while a portion does not, showing that this nuclear network is composed
of at least two different substances. The part which takes the stain is
called the chromatin ( ) network, and the part

which does not take the stain is called linin ( )

network. This nuclear network taking the stain usually stands out quite
distinctly from the rest of the cell, making it appear at first glance as
though the entire nucleus had taken a great quantity of stain to itself.

The substance lying within the network of the nucleus is called
nucleoplasm. It may happen that some cells do not have a definitely
outlined nucleus with a nuclear wall, but nevertheless these cells have
nuclear material scattered throughout the cell itself in the form of
granules; such granules are known as distributed nuclei. In the red
blood corpuscles of the human being there are no nuclei in the adult
form, although these cells are nucleated when they originally begin
growing.

On the inside of the nucleus there is a smaller nucleus in turn which
is called the nucleolus ( ).

At certain places in the nucleus where the various fibers of network
cross each other, there may be little knots, called net-knots, and these
must not be confused with the nucleoli. The chromatin itself appears in
a granular form, and the granules are called chromomeres ( ).

There may even be two nucleoli in one nucleus. These stain quite
readily also, but appear somewhat different from the chromatin after
such staining. Exactly what the nucleolus does, biologists do not know.
It disappears during the time the cell divides and consequently has been
thought to be for the purpose of holding something in reserve for this
division process.

All of the material within the cell walls but outside of the nucleus
is known as cytoplasm, to distinguish it from the nuclear material .within
the nuclear wall or membrane.

90 GENERAL BIOLOGY

Just outside of the nucleus and within the cytoplasm, there is usu-
ally found a tiny circle with a dot in the center. The dot itself is called
the centrosome ( ) and the circle about it the at-

traction sphere, or centrosphere.

There are little perforations through the nuclear wall so that there
is a direct connection between nucleoplasm and cytoplasm.

CELL INCLUSIONS AND CELL PRODUCTS

Bodies of a solid nature, not protoplasmic, are common to many
cells. These are pigments, oil, fat, crystals, glycogen, starch, chlorophyl,
etc., and are commonly spoken of as cell inclusions, though as a matter
of fact only foreign substances such as bacteria, etc., should be called
inclusions. Starch and chlorophyl are found almost exclusively in plant
cells. By these inclusions the shape of the cell is often changed, and
particularly the position of the nucleus. Fat gathers at one end of the
cell, crowding the nucleus to the opposite extremity and displacing the
cytoplasm to the periphery, mostly to that end of the cell occupied by
the nucleus. Pigment may be in solution, more frequently in granules,
and always is found in the cytoplasm, not in the nucleus. Vacuoles are
very common to most cells. These vary in number and size and are
usually spherical cavities filled with fluid secreted by the protoplasm.
The vacuoles contract, often with considerable regularity, and, as a rule,
empty to the surface of the cell. Waste products are in this way elimi-
nated from the body of the cell.

The constituents of a typical cell may then be summarized as fol-
lows:

1. Cytoplasm, the protoplasm that surrounds the nucleus, consist-
ing of:

(a) Spongioplasm, a reticulum or fibrillar network ;

(b) Hyaloplasm, a fluid portion, also called cytolymph ;

(c) Cell membrane, often absent in animal cells.1

2. Nucleoplasm or karyoplasm, the protoplasm of the nucleus :

(a) Nuclear membrane, frequently absent ;

(b) Chromatin, network that stains easily ;

(c) Limn, closely allied to the chromatin but does not stain ; dis-
solves in distilled water ;

(d) Nuclear sap, a fluid perhaps analogous to the hyaloplasm ;

(e) Nucleolus, spherical body that stains heavily ;

(f) Nuclear net knots, or karyosomes, false nuclei that are nodal
points formed by interlacing chromatin network;

(g) Centrosome, a small spherical body often found in the cyto-

JRegarding the cell membrane, it is well to know that, this is a purely relative 'term, just as a
drop of chloroform in water, or a drop of water in chloroform, or a bubble of air in water, can
be said to have a cell membrane. These are really surface tension phenomena, where the inter-
phases of water-chloroform, etc., have equal resistance to each other. In the "cell membrane" we
really have naked protoplasm, tending to round up just as the drop of water does in chloroform.

THE CELL

91

plasm of animal cells near the nucleus. It is looked upon as the dynamic
center in cell division.

If the student is to study Medicine he will probably find an advan-
tage in dividing the various definitely discernible substances in the
cytoplasm, into Mitochondria, Plasmosomes, and Paraplasmic sub-
stances.

Mitochondria* (Fig. 27). These are little granules, rods, and
threads in the protoplasm, quite constant in the various cell bodies, at
least, of the animal world. In fact, one investigator insists that it is.
these mitochondria rather than the chromosomes which are the bearers
of heredity; while another insists that they accumulate at both poles of
the cell, and are converted into secretory granules.

Plasmosome's.f These are tiny granules distinguished from the
mitochrondia because they are concerned with the housekeeping of the
cell, that is, with the assimilation of food materials, with forming vari-
ous secretions, and with the excretion of waste matter. Plasmosomes
have not been seen but are supposed to be present because there are
certain substances produced in the cells which must be due to something

Fig. 27.

a b c

Mitochondria as They Appear in the Sex Cells of Dividing Sperm of Blaps.

a. Scattered granular mitochondria.

b. Rod-shaped.

c. Rods drawn out around spindle.

(After Duesberg.)

physical or chemical. This is shown by the fact that the products of
the cell form little swellings of various kinds. These swellings take a
stain and it is the particles which cause these swellings or cell-products
which are known as plasmosomes. The cell-products consist largely of
fat and carbohydrates, and may be stored in the cells. Cell products are
called cytofacts or metaplasm. (This latter term because such substance
is due to metabolism.)

Golgi apparatus (Fig. 28). Very recently by a special staining
method known as Golgi's silver impregnation method, it has been found
that there is an "internal reticular apparatus" consisting of a system of

*While some medical men usually speak of mitochondria, and some of the older writers use
the term bioplasts, plastidules, archoplasmic granules, plastosomes, plastochondria, chondrioconts,
plastoconts and chondriomites, depending on the shape of the mitochondria, the name cytologists
use is that of Chondriosom.es, so that the student must think of mitochondria and chondriosomes as
interchangeable terms.

tMedical men are inclined to use the term plasmosomes as here given, but cytologists use the
term only to mean true nucleoli. These latter workers never use it in the sense we have given it
in this book.

92 GENERAL BIOLOGY

rods or network close to the nucleus, but associated especially with the
dense protoplasm which surrounds the centrosome. In epithelial cells
the network lies close to the free ends of the cells. The Golgi apparatus
is probably found in all animal cells, though little is as yet known about
it, except that there is a continuity from parent-cell to daughter-cells
by a sort of mitotic division of it quite similar to the regular chromosome
division. Prolonged treatment with osmic acid will make the Golgi
apparatus visible.

Plastids are differentiated portions of protoplasm representing cer-
tain regions in which physiological processes are localized. They are
quite common in plants and protozoa. In the former they are usually
colored, such as the chloroplasts which are the chlorophyl-carrying or-
gans. Each kind of plastid is supposed to serve a separate type of func-
tion.

Attraction sphere and Centrosome. These may be quite conspicu-

Fig. 28. Golgi Apparatus in Epidermal Cells.

a. Golgi network beside the nucleus in cell of a horse.

b. Same in skin of cat, but broken into small rods around the mitotic figure
in the large central cell. (After Deinecka.)

ous although it is not known whether they are important or not in cell-
division, shortly to be described.

Paraplasmic substances. These are the foreign substances which
can be seen in the cytoplasm, but which have not become part of the
living cell itself. Such are granules of pigment or calcium, fat globules,
various vacuoles filled with fluid, etc.

"It is clear that the construction of the cell is highly specialized in
most cases for the function which it is to carry out, and that it is sup-
plied with the most perfect mechanisms for these purposes. Some of
these are evident in the form of contracted bands in the protoplasm, or
in long, nerve processes ( ) like electric wires care-

fully insulated by sheaths of fatty material, or in mobile cilia which me-
chanically perform duties in the transportation of foreign particles. In
others, the tools of their trade are recognizable in the form of the
granules which seem to prepare ferments by which the chemical pro-
cesses which the cells effect are carried out. While these are visible in

THE CELL 93

many cases, there are others, even when we know that the most mul-
tifarious chemical reactions are being carried on, in which nothing of the
mechanism is recognizable to our eyes."

In plant cells where the cell wall is quite thick, and in some of the
animal cells, this cell wall is made up of cellulose, a substance quite
clearly related to the starches, although there are other substances, such
as lignin or silica, often associated with it, while in the cell walls of
animals there is a nitrogen containing substance, such as chitin, keratin,
and gelatin.

Where there is no distinct cell wall, there may be a cuticle, or
pellicle, covering the entire cell. This may be considered a lifeless secre-
tion, just the same as is the cell wall produced by some of the vital
activities of the cell itself. The vacuoles are little open spaces or vesi-
cles of liquid enclosed within the protoplasm. They may be persistent
or merely temporary. In protozoa, vacuoles are quite common. If they
enclose food particles, they are called food vacuoles. They may, by con-
tracting suddenly, eject their contents and serve thus as. excretory or-
gans. As these vacuoles which eject their contents usually are formed
again in the same place, they are called pulsating or contractile vacuoles.

CHAPTER VI.

THE CHEMISTRY OF LIVING MATTER
AND CELL DIVISION

Organic Chemistry, although named after the organs of living
things, has come to be the study of carbon compounds. But as the three
great chemical groupings of a living organism consist of proteins, carbo-
hydrates and fats and all of these contain carbon, a large part of the
study of organic chemistry is still devoted to living matter.

One of the great problems of biology is to solve the riddle of how
and where life originated. If the stars and planets surrounding our
globe were at one time masses of intensely heated matter no life could
have been sent from one planet to another. Still it is interesting to
know that the first elements appearing on a cooling star are the very
ones which go to make up proteins ; namely, carbon, oxygen, hydrogen,
nitrogen, and sulphur.

It will be remembered that oxygen is the source of most of the en-
ergy of an organism, and that the cell is the unit of biology, this cell
being made up of various substances called protoplasm.

If a substance is of the consistency of glue and non-crystalloid,
it is called a colloid.

Colloids are contrasted with crystalloids, such as sugar, salt, urea,
etc., in fact, any of those substances which, when in solution, will pass
through a membrane.

An emulsion is one fluid phase suspended in another. The fluids are
said to be in suspension.

Most organic matter is colloidal and some biologists believe that a
colloid substance will ultimately be accepted as the biological unit in
place of the cell.

Protoplasm, the substance of the entire cell, has somewhat the form
of foam, although it differs from foam in having the alveoli filled with
a thick liquid substance about the consistency of the white of an egg.
The alveoli which make up the foam-like protoplasm, although having
very thin walls, have walls thick enough so that diffusion is very slow
and the substance itself is different in the alveoli themselves and the
spaces between the alveoli.

All protoplasm does not show such alveolar composition. With the
ultra-microscope much of the protoplasm appears as tiny particles. It
is, therefore, supposed that this homogeneous mass is colloid in charac-
ter, that is, consists of tiny granules which are suspended in a liquid
medium. As there isn't very much difference between a colloid and an
emulsion in this case, and as there are cases in which no alveoli can be

ORGANIC CHEMISTRY AND CELL DIVISION 95

seen, it is possible that alveolar substance and interalveolar substance
may differ about as much or as slightly as a colloid and an emulsion.

The early workers on the cell saw very thin fibers in the proto-
plasm, and established the "filar" or "reticular" theories of protoplasmic
structure. We now know that if the alveoli are arranged in rows the
liquid between the alveoli will appear like threads, although we have
not been able to find that these so-called fibers have any important func-
tion. These theories, therefore, are not among the important biological
problems now.

When cells are prepared and stained for study in the laboratory
they have many granules distributed within them. These may be coag-
ulation products of the interalveolar protoplasm, or the cut ends of
fibers or cell inclusions of various kinds.

The great mass of protoplasm is really an emulsion. The tiny bub-
ble-like particles or alveoli and the liquid in which these float are called
by the physical chemist "phases" of a "system." It can, therefore, be
understood that the various surface phenomena which interest the
physical chemist are to be found in the living cell, and any chemical
knowledge of this nature which the student of the cell can obtain will
stand him in good stead. Much of the activity of protoplasm can be
explained by a study of surface tension.

It is to be borne in mind that protoplasm is never solid, although
solid particles may and most often are included within its liquid or semi-
liquid mass.

Protoplasm is made up of both organic and inorganic substances.
Organic.

A. Always present.
Enzymes.
Non-enzymes

Carbohydrates,
Lipoids,
Extractors,

Intermediate products of metabolism.
B. Not always present.
Pigments,
Hormones,

Aromatic compounds,
Toxic compounds.

The enzymes are continually attempting to produce an equilibrium
in the cells. They are chiefly protein in nature and speed up the chem-
ical reaction. They may be killed by light or heat. Their activities
are specific, each type of enzyme doing only one particular type of work.
Every step in the breaking down of proteids is done by a specific
enzyme.

96 GENERAL BIOLOGY

Inorganic.

A. Always present, and called essential elements.

H2O

Plus C, H, N, K, Ca, Na, Fe, NH4.
Minus CO2, SO4, Cl, PO4.

B. Sometimes present.

I, Br, NO2, NO3, Zn, Ba, Cu, Mn, As, Fl, Si, Mg.
Muttkowski has summarized the chemical elements concerned in
living matter as follows :

I. Elements concerned with Food.

1. Those which compose food.

A. Proteins— C, O, H, N, (S, P)— build protoplasm.

B. Fats — C, H, O — energy and reserve. Certain P-fats
enter into building up of all protoplasm (lecithin).

C. Carbohydrates — C, O, H — energy, and reserve.

2. Elements concerned in food synthesis.
Mg, CO2 (in plants only).

3. Concerned with food storage — K.

4. Katalysts— Fe, Ca, Mn, I.

II. Elements concerned with Physiological Processes.

1. Regulation (turgor, toxicity) — K, Cl, Na, Ca, I, Br.

2. Sensory — P.

III. Elements concerned with structural relations.

1. Form relations — elasticity — N, Cl.

2. Supporting tissues — C, Ca, Si, Mg, P, Fl, (S) in form of
phosphates, carbonates, oxalates.

CELL DIVISION

Every living thing, plant or animal, begins its life as a single cell.
Therefore it follows that if one wishes to understand how a many-celled
animal (metazoan) ( ) comes to its adult form of

life, one must find an original single cell and follow it throughout all its
changes until it has come to adultship.

Every living cell grows if it obtains food, and when it reaches its
maximum size splits in two. It may do this equally or unequally;
that is, it may split into a very large and a very small part, or it may split
equally into halves of like size and shape. There are then two cells where
there was only one before. These two cells then grow until the time
they attain their maximum size when the same process is gone through
again, so that in a short time there are four cells, then eight, sixteen,
thirty-two, sixty-four, one hundred twenty-eight, and so on.

When the children's story is remembered of the blacksmith who
was willing to shoe the king's horse on Sunday provided the king xvould
pay one cent for the first nail, and double that for each nail he drove,
so that by the time the blacksmith had put in twenty-eight nails, he had

ORGANIC CHEMISTRY AND CELL DIVISION

97

won more than a million dollars for the twenty-eighth nail alone, it is
easy to understand what a division of cells may bring about in a short
time ; especially when it is remembered that the tiny bacteria, which
are single-celled plants, may multiply and divide in such way every few
minutes. In the course of one or two hours, where division is suf-
ficiently rapid, there are millions upon millions of cells where there was
only one before.

Textbooks say there are two ways in which cell division conies
about, but recent investigations tend to show that this is in error and
that all cell division is mitotic. One method was said to be the shorter

Fig. 29. Diagrams Representing the Essential Phenomena of Mitosis.
A, a cell with resting nucleus containing a chromatic reticulum and a single
nucleolus. The centrosome is double and surrounded by the centrosphere. B, the
centrosomes are separating and each is surrounded by astral rays ; the chromatin
forms a convoluted thread or spireme. C, the spireme is broken up into a number
of V-shaped chromosomes, the polar spindle is formed between the now widely
separated centrosomes. D, the chromosomes attached to the spindle-fibres are
arranged at the equator of the spindle. E, division of the chromosomes, which are
viewed end on. F, divergence of the chromosomes. G, chromosomes collecting at the
poles of the spindle, the space between them occupied by interzonal fibres ; commence-
ment of division of cell-body. H, I, complete division of the cell, and reconstitution of
the nuclei. In / the centrosomes are dividing preparatory to a new mitosis. Note
A-Z>=prophase ; £=:metaphase ; F, G=anaphase; H, 7=telophase. (After Bourne.)

and simpler way, in which the cell, without any previous changes that
could be observed, split in two parts. But the longer method, known
as mitosis (Fig. 29), is the more common, and is the one whicti must
be studied in detail if any understanding whatever is to be obtained as
to how plants and animals evolve from the single original cell to the
marvelous complex organisms into which they develop in adult life.

The cell, as has just been described in the last chapter, has a net-
work in the nucleus that stains quite easily and readily. In the normal
condition such a cell is said to be in the resting stage. In the higher
forms cell division takes place only after fertilization, that is, after the
male sperm has united with the female egg. The chromatin, or stained
nuclear network, begins a process by which the stained part separates
from all of the other network, taking upon itself the shape of a single
thread or skein. A little later, this skein of chromatin breaks up into small

98 GENERAL BIOLOGY

particles which may be shaped like a horseshoe or like the capital letter
L, or merely appear as little straight or bent rods. These little broken
up particles of chromatin are called chromosomes. As these chromo-
somes are in all probability the most important physical particles in the
study of biology, one must get this subject of mitosis and chromosomes
clearly in mind or all that follows will necessarily be lost.

Just before the cell goes from the resting stage into the skein or
spireme stage, the little centrosomes lying within the centrosphere break
into two parts, one part migrating around the nuclear wall until it lies
opposite the first half.

Formerly it was thought that it was due to these polar bodies that
the chromatin breaks up into chromosomes, but as no centrosomes are
found in higher plants, although the chromatin acts just as it does in
animal cells, this explanation must be given up. Between these two
polar bodies in the animal cell there develops a series of very fine lines
which may be only a reflection of some kind, but which one very fre-
quently sees when the cell is undergoing mitosis. These fine lines are
called a spindle, readily recognized in the drawing. There are four
periods usually mentioned in cell division :

The Prophases. This is the skein stage already mentioned.

The Metaphase. Immediately after the chromosomes have appeared
as small broken particles of chromatin, they gather at the mid-line or
equatorial region of the spindles. Then the chromosomes split in two
.lengthwise and the cell is said to be in the metaphase stage.

The Anaphases. Immediately after the chromosomes have divided
lengthwise, one-half of them move toward one polar body and the other
half toward the other. During the time the chromosomes have split
and the time they have united about the polar bodies, the cell wall has
indented until it meets the opposite indentation, thus forming two sepa-
rate daughter cells,* This stage is called the anaphase.

The Telophases. This phase lasts from the anaphases until the
time the cells again resume the resting stage.

It. will be noted that the metaphase is used in the singular, whereas
the other three have been used in the plural. This will be readily under-
stood when it is remembered that these terms are only convenient names
enabling us to discuss intelligently with others the whole subject of
mitosis, and, so that when a given thing or event is observed during
any particular time of the division of the cells, it can be written and
spoken about in an understandable way.

The metaphase is only that particular moment when the chromo-
somes have gathered at the equatorial plane and are dividing. All the
other phases cover a much longer period, and, passing through various
, stages, are therefore used in the plural.

*In plants a new cross cell-wall often originates by a thickening of the central spindle fibres.

ORGANIC CHEMISTRY AND CELL DIVISION 99

In different types of cells, all of these stages vary a little as to length
of time and as to the method in which and by which particular cen-
trosomes, skeins (also called spiremes), spindles, and chromosomes, ar-
range themselves. It is well to note also that in the higher forms of
plants the centrosomes have not been seen, and that there is a difference
between plants and animals in the way the cytoplasm divides. In the
animal cells, as shown in the drawing, the cell walls indent until the two
indented portions meet, and the separation takes place in that way;
whereas, in the plant cell this does not indent, but the cell-wall becomes
thicker and thicker until a definite cell wall has been grown for the two
new cells.

There are also exceptions as to just when and how the spindle
forms. In some species of salamander, the spindle begins outside of the
nucleus, and then as the nuclear membrane disappears the fibers pass
through the nucleus itself.

THE REAL MEANING OF MITOSIS

The real significance of mitosis is found in the fact that the chro-
mosomes, a more detailed study of which will be taken up as soon as the
protozoa have been studied, split in two lengthwise and that the chro-
mosomes are practically the only visible things that pass from a parent
cell to become a new individual. Whatever an offspring is to obtain
from its parents must therefore be already present in the chromosomes
of the various germ cells of the parents, or it cannot be inherited by the
offspring.

A little later it will be explained also how this lengthwise dividing
of the chromosome means that each new individual obtains one-half of
whatever it is from its mother and one-half from its father, although
one's inheritance on the mother's or father's side is usually not evenly
distributed as to quantity, and possibly, quality. For example, we may,
as far as external appearance go, resemble our fathers, yet have our
mother's mental characteristics. One must therefore not confuse the
characteristics which can be seen and are very conspicuous, with those
which may not be seen, but which may nevertheless be much more im-
portant.

By remembering this statement one may understand the biologist's
division of all cells in the body into two great groups. These two
groups are known as somatoplasm ( ) and germplasm

( ), the latter consisting of those particular cells

which are going to reproduce offspring like the parent, while the somato-
plasm consists of all the other cells of the body. It can be imagined
from this that it is quite possible for the somatoplasm or outer portion
of the body (which is the only portion visible) to cover up many im-
portant or, at least, latent and dormant characteristics that an individual
may have inherited, but which characteristics may come forth at .any

100

GENERAL BIOLOGY

Lygatu

moment. In fact one can understand that such characteristics may lie
dormant throughout the entire life of a parent and come forth only in
the offspring.

MATURATION AND ELEMENTARY EMBRYOLOGY

Very low in the scale of life there is a differentiation into sexes ; the
smaller more active particle is known as the male gamete, while the
larger passive portion is the female gamete.

In all higher forms fertilization is our starting point in any discus-
sion of embryology or development.

There are apparent exceptions to this rule, such as those insects
which give rise to young by virgin birth, a process called parthenogene-

Actu.i sis ( ), and in the

number of V ' '

"rf^eMu spmr, EBf. zMott.*SS? case °^ tnose animals in which sev-

eral (as high as three) immature
generations may be present at the
time of birth. This latter condition
is known as paedogenesis ( ).

Before fertilization various
changes take place in the germ cells
which are to produce the mature egg
and sperm. This process is called
maturation ( ), (Fig. 30).

The early cells are called pri-
mordial germ cells. They are in a
state of rest in all the higher animals
for several years, or until the indi-
vidual grows to sexual maturity.
When this time has been reached,
there are three stages through which
the primordial cell passes before
producing the mature ovum or
sperm.

1. The primordial germ begins
to divide mitotically (Fig. 31). The
resultant cells are called oogonia
and spermatogonia.

2. After a varying number of
divisions the many new cells thus
produced go through a process of

A. Diagram illustrating the behavior of the g^OWth. They are then called Pri-
"accessory," sex-accompanying chromosome in mary OOCVtCS and SpermatOCytCS.
fertilization. For the sake of clearness, but rv\\, « << • » «
four other chromosomes are shown, and these 3. 1 heSC then ripen OT ma-
four diagram ma tically ; accessory (x) , solid .. P 1-1 r^-i- .•

black. (After Wilson.) ture," after which fertilization can

B. A diagram of the gametogenesis and , i i
fertilization. take plaCC.

Premeiot-ic
divisions

ORGANIC CHEMISTRY AND CELJ, DIVISION: : 19 1

From what we shall soon learn regarding Paramoecia we know that
the chromosomes are the important carriers of all physical traits inher-
ited by a child from the parent. But, unless there is some method by
which the chromosomes throw off one-half their number, each child,
being the result of an egg and a sperm mating, would possess every-
thing its mother possessed, plus everything its father had. A super-race
would thus be produced which in a very few generations would be to-
tally unlike any of its parents. One can imagine what it would mean
to have every child twice as strong, and twice as tall, as its parents.
It would not be long before men would be thousands of feet tall, and
there would be little room for more than one of two people in the world.
But Nature apparently loves an average, and so somewhere, the chro-
mosomes are halved.

The ripening process is known as the maturation division (Fig. 30).
The egg varies from the sperm in the number of complete function-
ing cells it produces, although the chromatin acts alike in both cases.

From the primordial egg cell only one mature egg develops, while
three undeveloped eggs, called polar bodies, are formed. These latter

degenerate and have no known func-
tion. Each sperm cell, however, de-
velops into four complete functional
spermatozoa, any one of which may
fertilize an egg.

Notwithstanding this difference,
both sperm and egg cell have the same
number of chromosomes characteristic
of the species. This full quota of
chromosomes is called the diploid num-
ber.

The primordial cells (those which
are to become eggs) begin their growth
very early in the embryo. Usually,
there is a quantity of yolk deposited to
serve as food for the embryo which is
in turn to develop from the egg.

The chromatin in the nucleus

sperm\

\ /

I I I I

Fig. 31.

A. Diagram of the derivation of the sex gathers in a thick mass towards one

side of the nucleus. This is known as
the synapsis stage. From this thick
mass of chromatin there will emerge

crease in number and diminution in size just one-half the number of

cells. 2., the fertilized egg (zygote)
som., the body plasm (soma) ; t., the de-
velopment period during which the germ
plasm and the body plasm are indistinguish-

(the number of divisions is much greater

than shown) : r., the period of increase in SOmCS USUally found in Cells of the

dze with differentiation of cytoplasm ; w.,

the two maturation divisions; pb., polar ticular SpCClCS WC are Studying. Such

cells are said to have the haploid num-

bodies ; e., egg. (After Boveri.)

B. Spermatozoa of Rana esculenta.

C. Spermatozoa of Rana fusca.
Leydig.) mp., middle piece.

(After

chromosomes.

102 ; GENERAL BIOLOGY

Each of these chromosomes is double, the two parts either lying
side by side, or end to end. This stage of half the number of chromo-
somes (but where each is a double chromosome), is called pseudo-reduc-
tion. Real reduction then follows. The two parallel portions of each
chromosome divide longitudinally, while the entire chromosome con-
tracts into small four-portioned chromosomes, each of which is called a
tetrad (Figs. 31 and 33). A mitotic figure now forms and moves toward
the outer rim of the egg, the nucleus divides equally, so that one-half
of each tetrad passes to a daughter nucleus.

Although the nucleus divides equally, the cytoplasm does not. This
produces one large egg cell and one small particle, this latter with one-

-chr

C.
Fig. 32. Fertilization of the Amphibian Ovum.

A, outline drawing of a section parallel to the axis of the egg ; the superficial
pigment of the animal hemispheres of the egg is indicated, but the yolk granules
are omitted, co., entrance cone ; spz., spermatozoon lying at the bottom of the
entrance funnel ; s.sp., spermsphere.

B., a meridional section through the egg at a later stage ; cT , sperm nucleus,
also called the male pro-nucleus ; ? , egg-nucleus, also called the female pro-nucleus ;
as., sperm-aster ; pb., polar body. The size sperm-and egg-nuclei has been exag-
gerated.

C, portion of a section through an egg showing an early stage in the forma-
tion of the fertilization spindle, highly magnified ; tf sperm-nucleus ; ? , egg-
nucleus ; cs., centre ernes.

D, portion of a section of an egg showing the early stage of the metaphase of
the fertilization spindle ; chr., the chromosomes derived from the sperm- and egg-
nuclei lying unevenly, but still in two distinct groups, in the equatorial plane.
(After Jenkinson.)

half the chromatin, but with little or no cytoplasm. The smaller portion
is the first polar body. This is pinched off from the egg cell proper.

Both egg cell and polar body now begin to divide again. It is in
this second division that each remaining half-tetrad (now called a dyad),
separates into its two component parts, one going to each daughter
nucleus. Thus the second polar body is formed which is also pinched
off from the egg cell proper. Often the first polar body again divides to
form two tiny cells, but none of the polar bodies perform any actual
known function for the organism. From this account we note that there
are four cells which have formed from the primordial egg cell — the egg

ORGANIC CHEMISTRY AND CELL DIVISION

103:

proper and three polar bodies. Two of the polar bodies are the result of
the first polar body dividing in turn.

It is of great importance to note that the order of development may-
change in different species. For example, some polar bodies never di-
vide, while in some species maturation takes place before, and in others
after fertilization.

We shall see in our study of
plants that this reduction-division is
not confined to the animal world.

The male cell — the sperm —
passes through similar changes to
that described for the egg cell, ex-
cept that there are no polar bodies
formed.

In biology we always think of
the reproductive cells as the germ
plasm \vhich alone carries on from
parent to offspring all things that
can be inherited. It must therefore
follow that there is something in the
germ plasm which determines what
the offspring is to be. These de-
termining factors must be in the
chromosomes, as it is only the chro-
mosomes which pass from parent to
child. But there can be a consid-
erable "change-about" of the chro-
mosomes. For example, if we have
four chromosomes numbered like
this: 1, 2, 3, 4, either 1 and 2 may
be thrown out in the reduction divi-
sion, thus leaving 3 and 4 ; or 1 and 3 may be thrown out, leaving 2 and
4; or 2 and 3 may be thrown out, leaving 1 and 4; and so on.

If it be remembered that quite a number of combinations can be
made in this way in both the egg and the sperm, it is readily understood
that there can be several times this number of combinations brought
about by a mingling of sperm and egg after fertilization, when the re-
duced sperm cell unites with the reduced egg cell. When we come to-
the study of Genetics we shall enter into this phase more thoroughly^

It will, of course, depend upon what characters are thus carried by
the two mating chromosomes as to what characters the new organism
will possess. It is this assorting and rearrangement of chromosomes
which is in all probability the cause of variations within a given species.
This is.by.no means the same as saying that it is the cause of new
species. This distinction must be kept clear.

Fig. 33. Maturation of the Egg of Cyclops
(the full number of chromosomes is

not shown ) .

A, chromosomes already split longitudinally ;
B, chromatin masses with indication of trans-
verse fission to form the tetrads ; C, the young
tetrads arranging themselves on the first polar
body spindle ; D, tetrads in first body spindle ;
E, separation of the dyads in the same ; F,
position of the dyads in the second polar body
spindle, the first polar body being really above
the margin of the egg. (After Riickert.)

104 GENERAL BIOLOGY

The diploid number of chromosomes is reduced to the haploid num-
ber by a union of the chromosomes, two by two. And this union in twos
is by no means haphazard. An understanding of this can best be seen
in animals where the chromosomes are different both as to shape and
size. The squash-bug (Anasa tritis) is a good example.

In these bugs the chromosomes occur in two sets, larger ones and
smaller ones (Fig. 30). During pseudo:reduction, the larger unites with
a larger one, and the smaller with a smaller one, and so on. All the
resulting tetrads are symmetrical.

The- sum total of all the character-factors which are received from
the parents of an animal at the time the egg is fertilized are contained
in these two sets of chromosomes. In some insects virgin birth is not
uncommon. In these cases a complete individual develops from the
mature egg alone — that is, from the one having only one-half the definite
number of chromosomes normally present in each cell of that species.
This shows that each set of chromosomes contains all that is necessary
for a complete individual. We, therefore, think that the linking of a
similar chromosome from the male and a similar one from the female
must be for the purpose of bringing similar important factors together
so as to strengthen such factors. A fuller discussion of inheritance will
be left for the chapter on Genetics.

FERTILIZATION

The union or fusion of the sperm nucleus and the egg nucleus is
known as fertilization ( ). The spermatozoan is

composed of three parts, head, tail, and mid-piece (Fig. 31). The head
is largely nuclear material and is the only portion which actually en-
ters the egg and fuses with it. Sperm may enter an egg either before
or after maturation of the egg is completed.

After the sperm cells have passed through the maturation process
a great mass of them are secreted at one time from the spermaries. If
an animal lives in water the sperm float about in that fluid, otherwise
enough liquid is excreted to make it possible for the sperm to float about
until coming in contact with an egg.

Among all higher animals there are special copulatory organs which
vary considerably in different animals but which, in all cases, serve to
bring egg and sperm together.

There is a great attraction between these germ cells of the different
sexes which cause their union and fusion, though what this attraction
is has not yet been discovered.

If the sperm enters the egg after the latter has matured (which
is by far the more common method) certain changes begin taking place
at once.

The sperm nucleus is called tfie male pronucleus (Fig. 32) after it
enters the egg while the nucleus of the egg is known as the female pro-

ORGANIC CHEMISTRY AND CELL DIVISION

105

nucleus. There is often a special aperture in the wall of the egg called
a micropyle ( ) through which the sperm enters.

Usually only one sperm cell enters an egg. Various changes are set up
at the very moment the sperm enters the egg causing the egg membrane
to become impervious to other sperm, though sometimes, if the egg be
old or diseased, this process may not begin soon enough, so that several
sperm enter the same egg. This is called multiple fertilization. There

O

A, one-celled stage B, two-celled stage C. four-celled stage D. eight-celled stag*

//, many-celled
stage

Fig. 34. Cleavage of Frog's Egg.

are some species in which this multiple fertilization occurs normally.
Monstrosities are often formed in this way.

When the two pronuclei unite they form a fusion nucleus (Fig. 32),
also called the first segmentation nucleus. The egg is then said to be
fertilized, or impregnated.

The full quantity of chromosomes is now again present and there
seems to be an impulse brought with them which starts the egg dividing.
This division of the fertilized egg is known as segmentation or cleavage
(Fig. 34). This is brought about by ordinary mitosis, and these first
cells which come into being by the splitting of the fertilized cell are
called blastomeres ( ). The chromosomes do not

divide longitudinally in these blastomeres but each new cell receives
one-half of the material brought by each of the parent cells. In this
way every cell in the body gets an equal amount of chromosome mate-
rial from each of its parents. And in this way also, every cell in the
body of an individual has exactly the same number of chromosomes
within it that every other cell has.

Each succeeding division of cells produces cells a trifle smaller than
the parent cell.

The cells divide differently with different quantities of yolk. Usu-
ally the first thre« cleavage planes are perpendicular to each other. If
the yolk is evenly distributed the newly formed cells will be more or
less of equal size.

Often the yolk collects at the lower portion of the egg. This is
undoubtedly due to the force of gravity. In such cases the protoplasm
gathers at the upper end. The upper end is then called the active,
formative, or animal pole and the lower the passive, nutritive, or veg-
etable pole. The polar bodies are usually freed at the formative pole.

106

GENERAL BIOLOGY

This causes the blastomeres at the nutritive pole to become larger, and
divide less rapidly than those in the region where the protoplasm is in
excess. In fact the yolk may be so excessive as not to permit any divi-
sion at all within it.

Two forms of segmentation are usually given :

A. Total segmentation.

I. Equal: In which there is little yolk material and that
well distributed. (Illustrated in most of the lower invertebrates and
mammals.)

II. Unequal: In which there is a moderate amount of yolk
which accumulates at the passive pole. The cells at the active pole are
more numerous and smaller than at the passive. (Illustrated in many
mollusks and in Amphibia.)

B. Partial segmentation.

I. Discoidal: In which there is an excessive amount of yolk

with the nucleus and a small mass of
protoplasm occupying a disc at the ac-
tive pole. This disc alone segments
and the embryo lies upon the yolk.
(Illustrated in the eggs of fishes, birds,
and reptiles.)

II. Peripheral: In which an
excess of yolk collects at the center of
the ovum, with the protoplasm at the
periphery. The dividing nuclei as-
sume a superficial position and sur-
round the unsegmented yolk. (Illus-
trated in the eggs of insects and other
arthropods.)

As segmentation continues the
b^astomeres remain attached to each
other and from a spherical mass (Fig.
35). If the individual cells project out from the mass and the sphere is
more or less solid, it resembles a mulberry and is called a morula (

), but if it becomes a single layer of cells and is hollow
it is known as a blastula ( ). In the latter case the hollow

portion in the center is filled with a fluid. The hollow space itself is
called the segmentation cavity.

If this blastula indents (just as though one were to take a hollow
rubber ball and push in one side with a finger), there are two layers
in the indented region. The outer layer is called the ectoderm or
epiblast, and the inner the entoderm, endoderm, or hypoblast, while the
entire two layered mass is known as a gastrula ( ).

The indentation is also called invagination and gastrulation (Fig. 36).
Having indented, the indented portion draws together to form a

Fig. 35.

A, vertical section through a segmenting
ovum in the blastula stage. B, C and D,
similar sections through later stages. Bl.,
segmentation cavity or blastocoele ; bp.r
blastopore. (After Morgan.)

ORGANIC CHEMISTRY AND CELL DIVISION

107

single mouth-like opening. This open-
ing is the blastopore ( ),
and the newly made cavity surrounded
by entoderm is the primitive intestinal
tract or archenteron ( ).
In our study of the hydra it will be
found that that animal grows thus far
and then remains throughout its entire
career in the gastrula stage.

In higher forms a third layer is
formed between the ectoderm and en-
toderm known as the mesoderm. Ani-
mals having these three germ layers
(Fig. 37) are called triploblastic
( ). All tissues

and organs are derived from some one
or more of these germ layers. To study
this development is the special province
of Embryology.

Often certain blastomeres grow
more rapidly than others in the same
embryo. Such is the case with frog's eggs (Fig. 38). This results in
the more rapidly growing cells surrounding those which divide more
slowly. A growing of one set of cells over another is called epibole
( ). The separation of the germ layers or mem-

branes by splitting apart is known as delamination.

Fig. 36. Formation of the Gastrula in
Amphibia, Diagrammatic Longi-
tudinal Section.

1, Blastula ; 2, the invagination has be-
gun at i (the corresponding place in 1 is
indicated by an arrow) ; the invagination
is in the form of a furrow, but does not
yet surround the egg ; 3, the invagination
is proceeding : 4, perfect gastrula ; the
archenteron is almost filled with a project-
ing part of the hypoblast, which is later
dissolved and absorbed by the embryo, ek.,
ectoderm (light) ; en., entoderm (shaded) ;
g., mouth of gastrula ; h., segmentation
cavity ; t., invagination furrow ; n., archen-
teron. (After Boas.)

Fig. 38. Frog's Egg,

Fig. 37. Diagrammatic Figures in Explanation of the Formation of the Showing Proportion-

Third Germ Layer — the Mesoderm. ate Increase of

1, youngest, and 4, the oldest stage. Smaller Cells at

ek., ectoderm; en., endoderm ; m., mesoderm. (After Boas). Top of Egg.

References :

E. W. MacBride, "Textbook of Embryology."
E. B. Wilson, "The Cell in Development and Inheritance/'
Kellicott, "Chordate Development."
Gurwitsch, "Morphologic and Biologic der Zelle."
Heidenhaim, "Plasma und Zelle."
Buchner, "Prakticum der Zellenleh're."
L. W. Sharp, "An Introduction to Cytology."

W. E. Agar, "Cytology, with Special Reference to the Metazoan
Nucleus."

L. Doncaster, "An Introduction to the Study of Cytology."

CHAPTER VII.

HISTOLOGY OF THE FROG

Every living individual, plant or animal, being- able to live an inde-
pendent existence and possessing the ; four characteristics of irritability
( ), ability to take and digest food, to grow by in-

tussusception ( ), and to reproduce its own kind,

is called an organism.

The higher organisms are made up of separate specialized organs,
each organ consisting of a series of tissues, and each tissue, in turn, is
made up of a sheet- of similar functioning cells., ^.^

The cell is the biological unit, and the modern world attempts to
explain all living things in terms of cellular construction.

It can be appreciated quite readily that the cell is intensely impor-
tant in the study of all living organisms when it is realized that every
living thing, plant or animal, originally grows from a solitary cell, and
any tiny structure capable of producing so wondrous an animal as the
frog or still more wondrous an animal as the human being, is certainly
of importance.

In fact, if one could find all the possibilities of any given cell, and
then find why it has these possibilities, and just how and why it devel-
ops into the particular structure that it does and no other, the riddle
of life would be solved.

It must be remembered that every living thing starts life as a single
cell and then if it is to become a multicellular animal it passes through
a cell-dividing stage. Some plants and animals remain in the one-celled
stage, while others, as soon as they begin to divide, adhere together and
form tissues, which in turn develop into organs. This means that a
study of the origin, development, and content of the unit cell gives us
a sort of bird's eye view of how living things work and grow. A study
such as this presents a more complete view than could be procured in
any other way.

First, therefore, it is necessary to know the different kinds of tissues
that may be encountered ; these are grouped under four distinct heads :

1. Epithelial.

2. Connective.

3. Muscular.

4. Nervous.

1. Epithelial tissues (Fig. 39) are always surface tissues. They
lie in layers with a small amount of intercellular substance. The sur-
faces of organs, the linings of cavities of organs, and the lining of glands,
blood vessels, and ducts of all kinds, possess this tissue. In fact, it is

HISTOLOGY OF THE FROG

109

surface tissue whether lying on the internal or external surface of an
organ.

There are, however, various types of epithelial tissue and these are

named from their shape.
For example: Flattened or
squamous epithelium easily
obtained from the outermost
skin of the frog during the
time it molts, or from the
peritoneum, is composed of
cells which are broad and
flat with a rounded nucleus
near the center.

In the mucous layer of
the intestine, we find what
is known as columnar epi-
thelium, because the cells
are shaped like columns,
while in many places such
as in the outer skin, there
are transitional stages be-
tween these two types of
tissues which have some of
the characteristic shape of both flat and columnar epithelium.

If these cells are several layers deep they are called stratified epi-
thelium.

Should they have tiny hairlike substances called cilia at their outer
ends they are known as ciliated epithelium. These may be almost any
shape — columnar, cuboid or flattened. Ciliated epithelium is found in
the mouth, throat, parts of the peritoneal lining of the body-cavity, inner
lining of the oviducts, in the mouths of the ciliated funnels of the kid-
ney, in the ventricles of the brain, and, in very early life, even on the
outer surface of the body.

2. Connective tissue (Fig. 40) serves to support and hold together
various parts of the body. In this type of tissue, the intercellular sub-
stance is quite abundant as contradistinguished from nearly all other
types, and it is interesting to note that nearly all of the connective tissue
is derived from the middle germ-layer or mesoderm ( ).

The intercellular substance changes in many ways, remaining soft, or
becoming fibrous and even changing into bone. The principal types of
this tissue are as follows:

White fibrous connective tissue, most widely distributed, and easily
obtained from the membranes connecting skin and body-wall. Under
the microscope it appears as a clear gelatinous substance in which many
fibrils are embedded. The fibrils are unbranched but have a character-

Fig. 39.

A, stratified epithelium from the oesophagus of the
rabbit, seen in section. In the lower part of the figure
the connective tissue and muscular layers are shown. B,
squamous epithelium from the mesentery of the Frog,
silver nitrate preparation ; El, E2, goblet cells from the
frog's mouth ; Dl, D2, isolated ciliated epithelium cells
from the frog's mouth ; D3, an isolated ciliated cell from
the gill of the mussel. C, columnar epithelium from the
intestine of the frog. (From Bourne, after a drawing
by Dr. E. H. Schuster.)

110

GENERAL BIOLOGY

istic wavy appearance; often they are united in bundles and run in all
directions. A few yellow elastic fibers may be scattered among the
white. These are always straight, however, and not wavy. If the tissue
should be treated with acetic acid, the white fibers swell up and disap-

XfpsSK^^ •^'••:-s4?^:;'J?^»

m-sXM

illlii

!>•'>£.' ._' V - ••»•<— x. ... v • -* ?-».V<O

O«/«r circumjeren-
Ital lamellae.

Haversian or con-
centric lamellae.

~ Haver sian canal.

Fig. 40.

A. Elastic Cartilage.

B. Haversian system with one lacuna sketched.

C. Segment of transversely ground section from shaft of a long bone, showing
all lamellar systems. (From Bohm and Davidhoff).

pear. The yellow are not affected. The yellow fibers may also branch,
and when cut they do not curl as do the white fibers. In the various
spaces of the matrix ( ) connective tissue corpuscles

or cells may also be found, varying in form and appearance, often united
with neighboring cells to form an irregular network, the meshes of
which are filled with intercellular substance. White fibrous tissue varies
in consistency and texture in different parts of the same animal. The
loose tissue binding muscles together is known as areolar ( ),

being composed of sheets and strands intersecting each other in all
planes. It is this that forms the fascia for each muscle, being modified
into a tendon at the end.

HISTOLOGY OF THE FROG 111

The looser tissue of the lymphatic glands is called adenoid
( ) and is composed of an irregular network of sheets

and strands which forms a fine meshwork of supporting cells.

The various ligaments uniting the bones are formed of a dense and
non-elastic variety of white fibrous tissue. It is also found in the cutis
of the skin, the submucosa of the alimentary canal, in the walls of the
blood vessels, in the substance of glands, and in the capsules covering
various organs.

Adipose tissue is regarded as a form of connective tissue in which
the cells have enlarged by being gorged with fat. The nucleus here lies
toward one side of the cell, while the cell-wall and a thin pellicle of pro-
toplasm surround the fat globule.

Cartilage is a dense and massive variety of connective tissue. The
predominant type in the frog is known as hyaline ( ),

the matrix of which appears transparent and homogeneous ( ),

although it really consists of numerous fibers of different types which
can only be observed after various chemical experiments have been per-
formed upon it. The cells in this type of tissue are contained in little
rounded spaces or lacunae scattered quite irregularly through the mat-
rix. There may be two or more cells in one lacuna, causing us to believe
that the cells may have quite recently been formed by a division of the
parent cell. An intercellular substance is deposited around each cell,
there being a sort of partition grown between each of the cells which
gradually increases in thickness and presses them farther and farther
apart. The outer surface of the cartilage is covered by a thin layer
called the perichondrium ( ).

Hyaline cartilage is found at the ends of the bones of the limbs,
between the spinal vertebrae and the ends of their transverse processes
at the tip of urostyle, in the pubis of the pelvic girdle, in the hyoid, and
the cartilage of the larynx and of both ends of the sternum. It also
forms the basis of the cranium and the central axis of the lower jaw.

Calcined cartilage is that which contains a deposit of lime salts in
the matrix. It is found in the pelvis of old frogs, in the suprascapula
and at the ends of the larger bones in the limbs such as the head of the
humerus and femur.

Bone structure is quite similar to that of cartilage and also contains
cells embedded in a solid matrix. In bone, however, the matrix is made
more firm by a deposit of carbonate and sulphate of lime. If the bone
is immersed in acid the lime solids are removed, the histological struc-
ture of bone is quite like that of cartilage. It does not follow from this,
however, that bone is merely calcified cartilage, for bone and cartilage
differ from each other both histologically and chemically. Cartilage
often is followed by bone, but when it is, the cartilage has been broken
down and the bony tissue has taken its place. We speak of two types
of bone, namely, compact, and spongy or cancellous. The former is firm

112 GENERAL BIOLOGY

and dense; the latter being composed of a comparatively loose arrange-
ment of plates and parts, lacks the strength of compact bone. The
spongy or cancellous type is found in the center of the vertebrae and to
a small extent within some of the long bones. Bones such as the femur
and, in fact, all of the long supporting bones in the body, must be rather
compact. A cross section of any of these long bones will show the outer
hard portion of a compact bone with an inner soft marrow and a thin
surface layer over the outside called the periosteum ( ),

quite similar in structure to the perichondrium surrounding cartilage.
The arrangement of the layers in compact bone is concentric, and the
layers themselves are known as lamellae ( ). These

lamellae contain numerous lacunae in which the bone-cells proper are
found. Fine branching tubes or canaliculi containing processes from
the bone-cells are given off from the lacunae and extend in all directions,
often anastomosing ( ) with the neighboring

canaliculi.

Bones grow like trees in that there are succeesive layers added to
the outside. The cells forming the inner layer of the periosteum, known
as osteoblasts ( ), are continually giving rise to new

bone cells, which cause new layers of bony substances to be deposited
between the periosteum and the old bone. New layers may, however,
be added on the inner surface between the walls and the marrow cavity.

Muscle tissue (Fig. 41) is composed of elongated cells or fibers
united by connective tissue, as already mentioned. There are three
types, the voluntary or striated, the involuntary or unstriated, and the
automatic or branched, a sort of combination of these two known as
heart-muscle.

The nonstriated fibers are rather simple in structure, commonly
spindle-shaped with a single nucleus near the center, often elongated.
The ends of the fibers may be branched, but are not usually so. The
length of the fibers varies to a considerable extent. They may be very
narrow, or short and comparatively thick. In the involuntary muscle
fibers there is usually no cross striation, but one may find delicate longi-
tudinal strands called fibrillae, usually considered to be the contractile
elements of the cell. The cell wall itself is thin and transparent. Non-
striated muscles respond to stimuli quite slowly, being also somewhat
slow to relax after the function has been performed. It is found particu-
larly in those branches of the body where sudden movement is not re-
quired, such as in the muscular coats of the alimentary canal, in the
walls of blood vessels, in various ducts, in the lungs, urinary and gall
bladders, and around glands in the skin, and also in the iris and ciliary
muscle of the eye.

Striated muscle fibers are more complicated in structure. They
possess several spindle-shaped nuclei scattered throughout the cell, each
nucleus surrounded by a small amount of undivided cytoplasm ( ).

HISTOLOGY OF THE FROG

113

There is a thin but well defined cell wall called the sarcolemma
( ), best seen where the contents of the fiber are

crushed or broken apart.

Each fiber of voluntary muscle is regarded as a single cell with
numerous nuclei scattered throughout its cytoplasm. In the early stages
of development there is but one nucleus in each cell in the voluntary
muscle, but as the fiber grows and the nucleus rapidly divides, while the

B.

C.
Fig. 41.

A. Smooth muscle fibers from the bladder of a Frog.

B. Heart-muscle syncitium.

C. Striated muscle fibers from the muscle of a cat. Q, cross discs separate*
from each other by interposed discs 11, 12. zz shows the stripe in which granules

"are visible. 'H, is shown as a center-disc, situated within the cross-disc. (From
Krause-Schmahl "Histology," by permission of The Rebman Co.)

cytoplasm does not, there are naturally a number of nuclei within a sin-
gle cell wall.

There is here both a longitudinal and a cross striation consisting of
alternate light and dark bands. Sarcostyles ( ) or

fibrillae which extend the entire length of the cell are the cause of the
longitudinal striations. These fibrillae, as in the unstriated muscle
fibers, are the contractile elements and they are kept apart by a semi-
fluid substance called the sarcoplasm. The fibrillae themselves are ar-
ranged in bundles or muscle columns separated from each other by a
thicker layer of sarcoplasm than is found between the fibrillae. The
cross striation is due to the fact that the fibrillae really consist of seg-

114 GENERAL BIOLOGY

ments or sarcomeres ( ). The segments are sepa-

rated from each other by a very fine dark line called Krause's membrane.
This membrane extends not only across the individual fibrillae but
across the entire sarcoplasm between the fibrillae of the fiber. Krause's
membrane is bordered on each side by a more or less clear and lightly
stained band formed by the ends of the two adjoining segments. The
middle portion of each segment forms a so-called dark band, and across
the center of this band there extends a second very delicate membrane
known as the line of Hensen. Should the muscle fiber be cut trans-
versely, the cut ends of the muscle columns present a number of polygo-
neal areas known as Cohnheim's fields. The spaces between the fields
are filled with sarcoplasm and the dotted appearance is due to the cut
ends of the tiny individual fibrillae.

The muscle fibers of the heart are different from either the striated
or unstriated fibers. Heart muscle presents cross striations, although,
contrary to the ordinary striated muscle, each fiber possesses more than
one nucleus. Further, every heart muscle cell has branches which con-
nect with other branches, thus forming a continuous network called a
syncitium ( ). (A syncitium represents a group of

cells whose separating walls or membranes have been lost, resorbed, or
failed to form.)

4. Nerve tissue (Fig. 42) is made up of nerve fibers and ganglion
cells ( ). A nerve cell, together with all of its pro-

cesses, is called a neuron. Each nerve is made up of a bundle of fibers
held together by connective nerve tissue and surrounded by a common
sheath. The central strand of a nerve fiber is called the axis cylinder.
About this is found the medullary sheath ( ). (also

called the white substance of Schwann) ; then a delicate external mem-
brane called the neurilemma or sheath of Schwann.

There are various constrictions to be seen in any long nerve known
as the nodes of Ranvier. This is where the white substance is inter-
rupted although the axis cylinder and neurilemma continue.

The nuclei surrounded by a small amount of protoplasm are found
immediately beneath the neurilemma. There are also various oblique
markings across the medullary sheath between the nodes of Ranvier
known as incisures of Schmidt. The axis cylinder of a nerve is merely a
continuation of a ganglion cell, being made up of very fine fibrillae, with
an intervening fluid substance. The white or medullary substance con-
tains a large amount of fatty material called myelin ( ).
This sheath is supposed to act somewhat as an insulator.

Nerve fibers and muscle fibers develop differently, the former being
a composite structure formed of cellular elements that originate in vari-
ous ways. For example, the nerve sheaths, though coming in contact
with, and surrounding the axis cylinder, have a totally different origin
from. the cylinder. It is interesting to know that in its development the

HISTOLOGY OF THE FROG

115

axis cylinder is the first to make its appearance and comes from the ex-
terior or ectodermic layer of the organism in which it develops, while
the cells forming the sheath come from the mid or mesodermic layer.

The negative manner of testing any of our scientific laws and prin-
ciples may be illustrated here by calling attention to the fact that much
of our knowledge of the position of nerves in various parts of the body
does not come from our ability actually to trace them throughout their
entire course, but by tracing the dying portion of an injured nerve.
Having found that the cell is the important part of a nerve, arid that
whenever a fiber is cut between its cell and the termination of its

Vend rites
-Cell Body

.Yode of Bani-ier
,'evrilemma

Fig. 42. — Neurons of Various Types from Higher Animals. .

A, a complex of neurons from the cerebrum ; B and C, neurons from the
cerebellum ; D, a single neuron from the cerebrum. E, diagram of a neuron or
nerve unit.

processes it is that part still attached to the cell which will grow again,
experimenters have cut nerve fibers and then watched that portion no
longer connected with the cell proper, degenerate. By watching this and
then observing those parts of the body that degenerate along with the
dying nerve fiber, it is easy to see where the fibers actually pass and
terminate.

Nerve centers is the name given to those parts where several nerve
cells are grouped together such as the brain, spinal cord, ganglia, and
the various ganglionic masses of the sympathetic system. The centers
themselves consist of ganglion cells and their fibers, together with the
connective tissue which holds them together, and the little vessels which
supply them with nutriment and carry away waste products.

Ganglion cells are usually quite irregular in outline with a single
nucleus near the center. The cytoplasm is rather granular and with
certain stains shows a network of tiny fibers connected directly with
the fibrillae of the nerve fiber as well as with other processes of the

116 GENERAL BIOLOGY

cell. There are several kinds of processes. The axis cylinder process
already mentioned, which requires a sheath and becomes part of a nerve
fiber, and the protoplasmic processes, sometimes several in number,
which are shorter than the axis cylinder and usually branched.

The cells themselves are designated as unipolar, bipolar, or multi-
polar, in accordance with the number of processes they may give forth.
Unipolar ganglion cells are found in the sympathetic ganglia.

CHAPTER VIII.

SUMMARY OF THE FROG

It has now been seen that the frog is a cold-blooded animal, an am-
phibian, and a vertebrate.

Its external features have been observed.

Its internal structure consisting of a series of organs known as sys-
tems have been studied. These were:

(a) Digestive. 1

(b) Circulatory. I Concerned with Metabolism.

(c) Respiratory. |

(d) Excretory.

(e) Nervous. 1 Concerned with regulation and

(f) Endocrine secretions. J control.

(g) Muscular. "} „ , . , t

uv ci i * i I Concerned with locomotion,

(h) Skeletal.

,.*. T support and protection,

(i) Integumentary. J

(j) Reproductive. Concerned with the propagation of the
race.

It has been learned that organs are composed of tissues, and tissues
in turn, of sheets of similar functioning cells.

There were four general types of tissues :

(a) Epithelial.

(b) Connective.

(c) Muscular.

(d) Nervous.

Tissues may also be classified according to their functional and
structural character. For example, according to function, the epithelium
is grouped as follows :

(a) Glandular, which consists of secreting cells.

(b) Sensory, which consists of sensory nerve cells and their

fibers.

(c) Germinal, which consists of those cells having especial

growth or reproductive ability.

(d) Protective, which goes to make up an outer covering of an

organ or of the body itself.
According to structure:

(a) Cuboidal,

(b) Cylindrical,

(c) Columnar,

(d) Squamous,

(e) Stratified,

118

GENERAL BIOLOGY

Connective Tissue is known as

(a)

(b)

(c)
(d)
(e)

Cellular, when it is composed almost entirely of cells with

little substance in between them.
Homogeneous, if the entire substance looks very much

alike.
Fibrous,
Cartilage,
Bone.
The Muscular Tissue is divided into :

(a) Striated or Voluntary.

(b) Non-striated or Involuntary.

(c) Heart Muscle.

The Nervous Tissue consists of cells known as :

(a) Unipolar,

(b) Bipolar,

(c) Multipolar.

Some writers call blood and lymph cells (Fig. 43) a fifth type of
tissue.

Red Corpuscles (ery-
throcytes) ( ) (caus-

ing the characteristic red color
of the blood) occur almost
only in vertebrates. (In inver-
tebrates, such as the earth-
worm, the blood-plasma is
red.) The red corpuscle has
no nucleus in the mammal
while in other vertebrates it
has.

White Corpuscles (leu-
cocytes) ( ) are wan-
dering cells in the blood and
lymph which are phagocytic
( ) in their action,
that is, they assist in keeping
the body in health by devour-
ing foreign substances.

The various organs of the body are responsible for the particular
size, shape, and function of the animal possessing them.

There are two ways of looking at an organ :

(a) Morphologically, or according to its structure or anatomy.

(b) Physiologically, according to the function such organ
may perform.

If organs of different animals are physiologically equivalent, that
is, if they function similarly, they are known as analogous organs.

Fig. 43.

A, Red blood corpuscles (haematids) of the
frog, stained with safranin and much magni-
fied, to show the nucleus and nuclear net-
work, bl, an amoeboid coarsely granular
leucocyte from the frog's blood, showing trifid
nucleus ; b2, b3, b4, other forms of leucocyte
from the frog's blood. c, discoid non-
nucleated haematids from human blood, much
magnified; cl, c2, c3, different forms of leu-
cocytes from human blood. (After Bourne).

SUMMARY OF THE FROG 119

If the organs of different animals are morphologically equivalent,
that is, if they have developed in a similar manner in relation to the
other structures immediately surrounding them, they are called homo-
logous organs.

There are three possibilities in comparing animals :

(a) The organs may at the same time be homologous and
analogous.

(b) They may be homologous but not analogous, as for exam-
ple the swim-bladder of fishes and lungs of mammals.

(c) They may be analogous but not homologous, as for ex-
ample the gills of fishes and the lungs of mammals.

The functions of organs are classified as follows :

(a) Vegetative, found in plants. These have to do principally
with growth.

(b) Animal, being those functions which are absent in plants
or but very slightly developed, while in the animal kingdom they are
considerably increased or are totally separate and distinct from any-
thing the vegetable world may possess.'

The vegetative functions are equally complete in both man and the
lower animals although they may develop quite differently in the two
groups.

Animal functions are those of motion and sensation. The work of
the various specialized sense organs, such as the eye and ear, come under
this grouping, while the work of those organs which pertain to nutri-
tion and reproduction, which both plants and animals possess equally
well, are vegetative.

Living matter has been shown to have four distinguishing charac-
teristics:

(a) Irritability,

(b) Growth by intussusception,

(c) Reproduction,

(d) Nutrition.

When nutrition is discussed biologically it must be thought of in
its widest sense as including not only the taking in of food and drink,
and the digestive process consisting largely of fermentation and the
absorption of such digested food, but also as including the taking in
of oxygen through the respiratory tract to cause heat and energy, and
the distribution through the circulatory system of the blood (the real
nourishment of the body) and finally, there must be included the ex-
cretory system which eliminates all that for which the 'body has no
further use.

An organism was defined as any living thing capable of leading an
independent existence.

If it is living matter it must have the four characteristics men-
tioned.

120 GENERAL BIOLOGY

The frog is one of the higher organisms made up of organs, which
in turn are made up of tissues consisting of sheets of similar functioning
cells,

In the frog each group of tissues has a definite work to perform,
i. e., the eye only sees and the ear only hears, the bones only support,
and the heart only pumps blood.

This specialization in the work of an organ is known as a division
of labor.

There are hundreds of thousands of animals so small that they can-
not be seen with the naked eye, many of which are composed of only a
single cell.

As they have only one cell they can have no tissues and conse-
quently no organs. But if they are living things they must possess the
four characteristics which distinguish living matter.

They do have these four distinguishing characteristics. Conse-
quently the single-celled animal is as truly a living organism as is the
frog.

But, as there are no organs and no tissues, the protoplasm in this
single cell must be able to do all the different kinds of work that the
.different organs in the frog do.

Therefore, in one sense of the word, the single-celled animal which
is able to do all that a many-celled animal can do without any of that
many-celled animal's organs, is much more complex and remarkable
.than is the so-called higher form.

And lastly, even those organisms highest in the scale of life begin
that life with a single cell which in turn grows by that cell dividing
and becoming two, these twro four, these four eight, and so on until the
complete adult is reached.

With this summary in mind we may take up the study of the single-
celled organisms.

CHAPTER IX.

CV

THE PROTOZOA
AMOEBA

Just as the frog is easily obtainable and most frequently studied in
the laboratory, so the Amoeba (Fig. 44), because it may be found any-
where, is one of the most clas-
sic forms of uni-cellular life
that is made use of in the lab-
oratory. This single-celled
animal has all of the four char-
acteristics necessary for a liv-
ing being. It is found almost
anywhere but. it does not fol-
low that it is found every-
where. In fact, unless particu-
lar arrangements are made to
have the Amoebae ready at the
time they are wished for study,
the probabilities are that they
will not be found where one is
looking for them.

Just as with the frog, so
with a single-celled animal, we
attempt first to study its ana-
tomy or morphology. We want to ascertain what seeable parts go to
make up this tiny animal. We find in it all the constituents of a cell and
all the needed characteristics to make an organism.

There is an outer colorless layer of clear cytoplasm, called the ecto-
sarc ( ), then a large central mass of granular cyto-

plasm, known as the endosarc ( ). A contractile

or pulsating vacuole will usually be found lying in that part of the ani-
mal opposite the part that is moved rnost frequently. There may be
Several food vacuoles, various foreign substances such as grains of sand,
and undigested particles (this latter depending, of course, upon whether
the animal is studied immediately after it has been feeding extensively).
Then there is also some material which has been digested and is ready
for excretion, and a nucleus. The nucleus is not easily distinguishable
in living Amoebae. For this purpose animals are killed and stained,
mounted upon slides, and studied very carefully with the compound
microscope.

The nucleus changes with the various movements of the animal, so
that it will not be found in the same location in all Amoebae studied. It

Fig. 44. Amoeba Proteus.

A, the animal in its natural condition ; B, an ani-
mal that has swallowed a long filamentous plant ; C,
the animal in the state of division.
cv, contractile vacuole ;
ec, ectosarc ;
in, endosarc ;

ex, remains of undigested food ;
p, protoplasm. (After Conn.)

122 GENERAL BIOLOGY

has a rather firm nuclear wall or membrane and there are quite a num-
ber of spherical particles of chromatin scattered about in the nuclear
sap. The contractile vacuole usually lies near the nucleus, but as the
vacuole grows it becomes further and further separated from the latter,
and by the time it is ready to contract and expel its contents, lies close
to the end farthest from the pseudopodia, or, what is commonly called
the posterior end. It then re-appears close to the point of its disappear-
ance, being carried along by the streaming protoplasm back to a posi-
tion near the nucleus, again passing through the same stages just de-
scribed. <-

The fluid content of the contractile vacuole contains urea. As this
is the common excretory substance of all animals we know that the
contractile vacuole is excretory. It is also respiratory because COX in
all probability passes to the exterior of the body by way of this organ,
the oxygen itself being taken in through the outer surface of the body.
It is well to compare Amoeba's physiological functions with the
respiration of a h'igher animal, such as the frog. The food vacuoles come
into existence whenever food is taken into the organism, each vacuole
seemingly acting as a temporary stomach.

MOVEMENT

The ectosarc, also called ectoplasm, sends out finger-like projections
into which the cytoplasm of the cell then flows. These out-pushings are
known as pseudo-pods ( ' ), or rhizopods ( ).

Often there are several of these pseudopods thrust out at one time, al-
though it is usually the one which comes in contact with some object
which gains the mastery, all of the animal then rolling forward so that
the cytoplasm extends into the out-pushing mentioned.

There have been various theories advanced to account for their
movements (Fig. 45), they being as follows:

1. The adherence theory. This merely means that if a drop of
water or any inorganic liquid be placed upon a flat surface, a part of it
coming in contact with some other substance, the entire drop will gravi-
tate toward the attached end. There are many pseudopods which are
extended out into the surrounding liquid, however, and do not come in
contact with any other solid substance; so, while this theory might
explain those pseudopods which do become attached, it does not explain
those which are known as free and which do not come in contact with
solid objects.

2. The surface tension theory. This theory is also taken from the
study of physics and chemistry and supposes that ther.e are various cur-
rents which move forward or outward in the central axis and backward
along the surface. Unfortunately for this theory, the currents in the
amoeba do not run that way.

3. The contractile theory. This theory has had a varying history,

THE PROTOZOA

133

although apparently it holds the stage as well as any, and better than
most theories at this particular moment. The current seems to start at
the foremost part of the animal and extends backward. Jennings has
shown that Amoeba verrucosa resembles an elastic sac filled with fluid.
By placing this animal in a substance such as soot, which he caused to

surround one of them, it was
shown that the streaming fol-
lowed the ectosarc toward the
forepart of the animal, and just
as it got beneath the Amoeba,
remained there until the ani-
mal had moved over it, when
it again moved upward at the
posterior end.

Bellinger has shown that
whether on floor or ceiling,
wherever Amoebae are found
to move, there is a sort of
creeping walk by which one or
more outer parts of the animal
are extended at random. When
this projecting part comes in
contact with a solid substance
the most posterior attachment
relinquishes its pseudopod. It
is therefore assumed that there
is a contractile substance with-
in the animal.

All the various experiments that have been performed along this
line have depended upon surface tension for their explanation. How-
ever, even if the animal moves in a similar manner to a drop of liquid
that is not living, it does not follow that it is the same force in each case
that causes the movement.

It is .essential that all;of the subject headings under which the frog
was studied should also be borne in mind when the single-celled animals
come in for investigation. For example, in regard to metabolism the
following subjects must be studied just as in a more complex organism:
ingestion, digestion, eges-tion, absorption, circulation, assimilation, dis-
similation, secretion, excretion, and respiration.

It can readily be understood that there must be some instinctive
process by which Amoebae know what food to ingest and what not to,
or they might continue to take in sand particles and indigestible sub-
stances which would cause the body to become so extended and heavy
that the animal would die from this effect alone. There are of course
no organs such as a mouth and intestinal tract as there are in the frog.

Fig. 45. — Locomotion of Amoeba proteus.

Photographs in side view. A and B show a speci-
men attached at two points, a and 6, and a pseudopod
which projects from one end and bends down to the
substratum as in B at d ; C shows the extension of a
long pseudopod. (From Hegner after Bellinger.)

124

GENERAL BIOLOGY

The food is taken in at any part of the body. This food consists of tiny
aquatic plants, other single-celled animals, bacteria, and various animal
and vegetable matter.

It is of special interest to note that when food is taken by Amoeba
the animal really places its body around the food (Fig. 46). Experi-
ments with inorganic substances, such as a drop of chloroform in a
watch glass of water have shown that the chloroform will take in sub-
stances like shellac and parraffine and reject wood, glass, etc. It must
not be forgotten, however, that these substances which are thus accepted
by the inorganic drop of liquid are those which normally adhere to
chloroform. But with Amoebae the majority of food substances
do not adhere to the surface of the animal, and so again there is con-
siderable dissimilarity between the experiment and the actual facts in
the case.

In digestion the food vacuoles have been embedded in the endo-

plasm. The cell wall pours out
a secretion of some mineral
acid supposedly HC1. This di-
gestive fluid seems to dissolve
only proteid substances and has
no effects upon fats and carbo-
hydrates. Hofer performed an
interesting experiment by cut-
ting an Amoeba in two parts
after it had just been well fed
and the part that did not have
the nucleus was unable to di-

A, Amoeba encysted.

B, Amoeba ingesting a plant, p, retracted pseudo- gCSt lOOd.

podium; dt, plant (diatom) taken in as food, cv, A u • -i j-

contractile vacuole ; f.v., food vacuole ; n, nucleus. A SOmewnat Similar COndl-

tion will be found a little later

in the study of the earthworm, in which case, if the animal be cut in
two behind certain segments the forepart of the animal, which contains
the important organs, will regenerate a new tail. Whereas, the tail-part
which has been cut off will regenerate another tail. This latter animal
having no organs, must necessarily starve to death as it has no way of
ingesting food.

After digestion has taken place in Amoeba, any indigestible particle
may be thrown out at almost any point on the surface of the animal.
These indigestible substances are probably heavier than the protoplasm
itself, so this heavy portion sinks through the lower part of the animal
body. Then as the animal moves away it leaves the indigestible solid
particle behind.

There is no circulatory system proper in a one-celled animal, so that
after the food has been digested it must be absorbed and passed into
the body substance proper of the animal. Here we come to a new term,

Fig. 46.

THE PROTOZOA

125

that of assimilation, which means that now that new food matter has
been digested and is within the body, there must be a rearrangement of
some kind to form new particles and to add them to those already exist-
ing. It is this ability to manufacture protoplasm from unorganized mat-
ter that is one of the very fundamental properties of living matter.

Any movement or energy expended by an animal is due to the
breaking down of complex molecules by what is known as oxidation.*
The process of tearing down is called katabolism or dissimilation. This
is a slow combustion process giving out heat and producing energy by
which the animal can perform its various functions of life. The sub-
stance thus broken down and "used up" must also be accounted for in
any scientific study. This residual matter usually consists of solids and
fluids ; namely, water and some mineral substance, urea and carbon
dioxide (CO,). Under this heading we include all secretions, excre-
tions, and products of respiration.

Whenever glands produce a substance which is to be used again by
the animal, such product is called a secretion, while substances which
are thrown out of the body entirely are called excretions.

The contractile vacuole, by virtue of the fact of its containing urea,
is considered an excretory organ, and because CO2 also makes its way
to the exterior of the organ, it is supposed to be respiratory likewise.
Amoeba, like any animal, grows more rapidly than otherwise if

food is plentiful. The food being taken in-
ternally the growth therefore comes from
the center outward, in other words by intus-
susception.

Whenever a cell reaches its full growth,
its outer shell, membrane, or whatever its
external covering may be called, not having
infinite possibilities in the way of extension
and stretching, usually breaks if more food
is taken. Cell division is the process by
which the plant or animal starts anew, thus
saving the parent cell from breaking. A
simple division into two (Fig. 47), is called
binary or amitotic division. It will be re-
membered that this mere splitting in two
parts is the shortest method by which cells
divide, but, which, as we have already said,
probably does not occur at all, and it is only
due to our observational methods not being
sufficiently delicate to note the exact processes, that we speak of ami-
totic division at all. In Amoeba proteus, however, which we are study-

Fig. 47.

So-called amitotic division of
Amoeba, showing the changes which
take place during division. The
dark body in each figure is the nu-
cleus ; the transparent circle, the
contractile vacuole ; the outer, clear
portion of the body, the ectoplasm ;
the granular portion, the endo-
plasm ; the granular masses, food
vacuoles. Much magnified.

*Oxidation may be likened to a series of infinitesimal explosions, which could be detected if
we had instruments delicate enough for such a purpose.

126 GENERAL BIOLOGY

ing, there are two methods of division. The so-called simple binary or
amitotic, just mentioned, and a process known as speculation. There
have been a few instances reported where the animal formed definite
mitotic figures. Very few investigators have observed sporulation.
"This latter process lasted from two and one-half to three months ; the
pseudopodia were first drawn in, and the animal became spherical. A
three-layered cyst was then secreted. The Amoeba rotated within these
for several days after which all movement ceased. The nucleus divided
until there were twenty or thirty nuclei present, arranged near the sur-
face. Continued division resulted in an increase of nuclei from five
hundred to six hundred. Walls now appeared at the periphery, cutting
off the nuclei, each with a small amount of cytoplasm. The wall of the
cyst became soft, broken, and allowed the small amoeba to escape. Hun-
dreds of these amaebulae or pseudo-podiospores, as they are sometimes
called, broke out at one time. They became recognizable as Amoeba
proteus in from two and one-half to three weeks. No reason could be
discovered for sporulation, although experiments were conducted in
which specimens were starved, were given an excess of food, were al-
lowed to dessicate, and were transferred to water from different locali-
ties ; none of these resulted in encystment and sporulation."

However, whichever way the animal may divide it is simply a mat-
ter of growth before it is ready to divide again. We have here the
interesting fact confronting us that these little single-celled animals are
practically immortal. That is, they do not die. One may kill them by
boiling and in other ways ; but, left to themselves, they will continue
until they have reached their limit of adultship when they divide, each
individual becoming two new and separate animals.

It is important that the fact be grasped that in these little unicellu-
lar animals a parent does not give birth to its offspring. The parent
itself becomes the offspring. That is, there are no ancestors. Each and
every animal carries its complete and total ancestry with it.

BEHAVIOR

The way in which an animal reacts to a stimulus is called its be-
havior; and when that behavior has not been learned, but comes forth
without consciousness on the part of the animal, yet is protective to the
animal, such behavior or reaction is called instinct. In these lower one-
celled animals two words are used in discussing behavior and instincts.
These are tropisms or taxis, which merely mean a movement of some
kind. To these words one adds the generic name of the stimulating
cause, using the word positive and negative to explain one's meaning.
For example, there are usually eight tropisms or taxis mentioned :

(1) Thigmotropism, meaning a reaction to contact of some kind;

(2) Chemotropism, meaning a reaction to a chemical;

THE PROTOZOA

1-27

(3) Thermotropism, meaning a reaction to heat;

(4) Phototropism, meaning a reaction to light;

(5) Electrotropism, meaning a reaction to an electric current;

(6) Geotropism, a reaction to gravity;

(7) Chromotropism, a reaction to color;

(8) Rheotropism, a reaction to current.

Amoebae move away from strong light, so that they are said to be
negatively phototropic. They are also negatively thigmotropic.

If an action is self-imposed it is said to be autogenous ( ) ;

if an external object causes a reaction, whether such object be located
within or without the body, the action is known as etiogenous ( ).

EUGLENA

This little organism (Fig. 48) moves about like a full-fledged ani-
mal although it has chlorophyl in its body and manufactures its food

as does the plant; and it does this notwith-
standing the fact that it has a mouth and
gullet.

Euglena belongs to the Class Mas-
tigophora ( ), which

means that there is a whip-like flagellum
protruding from its anterior end. Several
animals must be grouped together in order
that the naked eye may see any organisms
present. When there are many about there
is a characteristic green color given the sur-
rounding water.

EXTERNAL AND INTERNAL
FEATURES

: Euglena viridis is a single celled
elongated animal pointed at the posterior,
and blunt at the anterior end. As in
the stigma; r, reservoir; c.v., con- Amoeba, there are two layers in the cyto-

tractile vacuoles ; chr, chromato- « , , ,

phors; am, pyrenoids with sheaths plasm, the ectosarc, a dense outer layer, and

the endosarc, a more fluid central mass.
There is a thin cuticle running in parallel
E a free thickenings around the bodv of the animal

undergoing1

longitudinal division. F and G, divi- in an oblique direction so that it appears

sion of an encysted form. (A-D,

after Bourne; E-G, after Stein.) Striated.

The mouth is a funnel-shaped depression lying a little to one side
of the center of the anterior blunt end. The gullet is a continuation of
this depression. It looks like a duct, and connects with a large spher-
ical vesicle, the reservoir, into which several minute contractile vacuoles
discharge their contents.

Fig. 48.

A. Euglena viridis; m, the so-
called mouth ; n, the nucleus ; e,

128 GENERAL BIOLOGY

There is also a red dot, called an eye-spot or stigma, close to the
reservoir, near the inner end of the gullet. It is made up of small
granules of haematochrome ( ). Because the an-

terior end of the animal seems to be more sensitive to light than other
parts, it is supposed that this red stigma functions somewhat as an eye.
It has also been suggested that this red haematochrome is not the sensi-
tive part at all but that the protoplasm immediately beneath it is. As
haematochrome has many of the characteristics of pigment granules
of the eyes of higher animals, it is likely that we meet here with a sort
of beginning eye.

There is a single nucleus a little posterior to the center of the body.
It has a distinct membrane. On the inside of the nucleus there is a so-
called nucleolus. However, as this latter functions as a division center
it is probably not a nucleolus.

There are a number of oval discs called chromatophores ( )

suspended in the protoplasm. These contain chlorophyl. In Euglena
we meet with our first example of photosynthesis ( ).

A little later, when plant-life is studied, it will be noted that this is the
accepted method among plants of manufacturing their food. Photosyn-
thesis means that "the chlorophyl is able, in the presence of light, to
break down the carbonic acid (CO2), thus setting free the oxygen, and
to unite the carbon with water, forming a substance allied to starch
called paramylum ( ). If specimens are kept in

good light continually a large amount of paramylum will be stored up
for future use, being laid down around some granules of proteid sub-
stance near the center of the body. These granules are called pyrenoids
( ). Both the pyrenoids and chromatophores are

permanent cell structures and increase in number by division and not by
the origin of new ones from the other parts of the body."

LOCOMOTION

The animal moves by means of its flagellum, which appears as a
single whiplike structure, although really it is composed of four separate
fibrils wound together. This flagellum begins in the body proper and
extends through the wall of the mouth depression. It is often as long
as the animal itself. In addition to the assistance rendered the animal
in locomotion by this appendage, the entire animal is elastic, contracting
and expanding, so that the body looks much like a worm in movement.

NUTRITION

As already stated, Euglena is like a plant in that it possesses
chlorophyl and manufactures its own food. When an animal manu-
factures its food in this way it is said to be holophytic ( ).
But as Euglena can live in the dark, and chlorophyl does not permit
the manufacture of food without light, the animal must be able to feed

THE PROTOZOA 129

in some other way also When organic substance in solution is taken
in through the body wall, as probably happens in the case of Euglena
when in the dark, such method of obtaining food is said to be sapro-
phytic ( ), while those animals which ingest solid

particles of food like the frog are said to be holozoic ( ).

ENCYSTMENT

Euglena, like Amoeba, when food becomes scarce, as well as for
unknown reasons, may encyst. It does this by becoming spherical,
secreting a rather thick gelatinous covering, and throwing off the
flagellum.

REPRODUCTION

This takes place by binary longitudinal division. "The nucleus
divides by a primitive sort of mitosis. The body begins to divide at
the anterior end. The old flagellum is retained by one-half, while a
new flagellum is developed by the other. Often division takes place
while the animals are in an encysted condition. One cyst usually pro-
duces two Euglenae, although these may divide while still within the
old cyst wall, making four in all, while recent observers have recorded
as many as thirty-one young flagellated Euglenae which escaped from a
single cyst."

BEHAVIOR ' : i

Euglena swims in a spiral manner as does the paramoecium. Like
the paramoecium, too, it has only two reactions to stimuli. But in the
case of Euglena, the forepart of the animal swings about in a circle
while the posterior part remains more or less stationary, thus forming
a sort of pivot around wrhich the forepart moves.

It is positively phototropic though direct sunlight will kill it. As
all plants and animals need certain quantities of air, moisture and heat,
in which they thrive best, being injuriously affected if too much of these
substances is applied, the phototropic. action, as well as the killing by an
overabundance of light, is understood. The environmental conditions
in which an organism thrives best is called the optimum ( )

for such organism.

VOLVOX

All the unicellular animals having wrhip-like flagella come under a
sub-grouping known as Mastigophora. This group is particularly inter-
esting in that it furnishes us with our first example of unicellular ani-
mals forming colonies. The best known and studied of this group of
colonial flagellata is Volvox (Fig. 49) found in fresh water ponds.
Doflei found as many as twenty-two thousand cells in a single colony.
There is a division of labor in the colonies, for the various cells are not
all a1ike. though each is a separate and distinct animal. Some of these

130

GENERAL BIOLOC.Y

cells are somatic and nutritive, while others are germ-cells or reproduc-
tive. Here also we come in contact with our first and lowest form of sex
life. Any organ which produces sex cells is known as a gonad. There are
certain germ, cells in a colony of volvox called parthenogonadia
( - ). These divide into many cells which drop into

the center of the mother colony and finally escape through a break in

the wall. There are other germ cells,
however, which are also produced; the
smaller are called spermatazoa or
microgametes. These are the male
germ cells, while the larger ones called
macrogametes or eggs are the female
germ cells. The eggs are fertilized by
the spermatazoa, and, after passing
through a resting stage, develop into
new colonies.

Colonies may be of one sex only.
In such cases the male colonies can be
recognized by the sperm pockets ar-
ranged in a wide belt around the mid-
dle of the colony with the poles free
from cells.

There is a distinct difference in a
colony of single-celled animals of this
kind and a tissue which is a sheet of
similar functioning cells ; such sheet
being combined with others to form an organ while the organs form a
complete single individual. In Volvox there is no such grouping of
sheets of similar functioning cells. Each cell is complete and distinct
in itself and is as much an individual as Amoeba or Euglena just studied,
except that it is attached to its fello\vs.

PLASMODIUM MALARIAE

*'" The malarial organism, "Plasmodium malariae (Fig. 50), a member
of the class Sporozoa, nearly all of which are parasitic, lives in the hu-
man body. Human blood contains minute circular disks known as red
blood corpuscles, within which the malarial organisms may be found
in persons who are suffering from malaria, or chills and fever. The or-
ganism first appears as an extremely minute body, in shape somewhat
like the Amoeba, though much smaller. It increases in size. After
reaching a size which nearly fills up the red blood corpuscles, it breaks
up into twelve to sixteen small spores. The blood corpuscles now break
into pieces and the spores are liberated into the liquid blood. Each
may then make its way into a corpuscle and repeat again the history as
already described.

Fig. 49. Volvox.

The individual cells are united by radi-
ating strands of protoplasm. A, a mature
cdlony ; a. spermaries ; g, ovaries. B, zy-
gote resulting from the fusion of the
gametes. C, two sperm. D, egg. (From
West, after Klein.)

THE PROTOZOA

131

All of the following details of the life-cycle of Plasmodium malariae
must be thoroughly understood and memorized by the student, because,
(1) Plasmodium malariae is the classic example of an animal-parasite

Fig. 50. Life Cycle of Plasmodium Malariae.

of tremendous importance to man ; (2) it is our best known example
of a parasite which requires an intermediate host before being able to
carry infection; (3) it presents an excellent illustration of the methods

133 GENERAL BIOLOGY

used in obtaining experimental proof for scientific theories, and (4) it
brings home an understanding of the vast quantity of painstaking effort
necessary to obtain that proof.

The malarial organism, Plasmodium malariae, is a member of the
class Sporozoa, nearly all of which are parasitic. It lives in the red
corpuscles (therefore called Haematozoa) of human blood where it
grows for a time, and then breaks up into from twelve to sixteen spores
which rupture the corpuscle. The corpuscle itself then breaks up into
tiny particles and the spores are thrown into the blood-stream.

The malarial parasite has two life-cycles (Fig. 50), so to speak, one
the sexual cycle, which develops in mosquitos, and the other the asexual
cycle, which develops in man.

1. The sexual development of the malarial parasite within the body
of the mosquito takes from eight to ten days. These sexual forms are
known as gametes.

The male cell (gamete) is also called a gametocyte. This gameto-
cyte develops from four to eight microgametes which force their way
into the large female cells (macrogametes).

A sort of fertilization is thus set up. This fertilized cell is now
called a migrating cell or ookinete. The ookinete penetrates the stom-
ach-wall of a mosquito and builds a cyst (oocyst). Grassi says there
may be as many as five hundred oocysts at a time in the stomach-wall
of a single mosquito.

In the oocyst many tiny spherical bodies develop, (sporulation.)
These spherical bodies are the sporoblasts (primitive spore-cells) which
develop into thousands of sporozoites. These latter are merely tiny
filaments which get into the lymph system of the entire body of the
mosquito. As they reach the mouth-parts they are ready to be injected
into any human being which the mosquito bites. Once inside man, they
enter the red-blood corpuscles and are "known as schizonts (asexual
forms).

.From here on we must trace the asexual cycle of development in
man. The injected organism which has been placed in the blood-stream
of man is called sporozoite. It finds its way into the red blood cor-
puscles and becomes rounded and more or less ring-shaped while it is
amoeboid in movement. It is now a full fledged schizont ( ).

The schizont lives at the expense of the red corpuscle and deposits scat-
tered black or reddish, so-called melanin granules. These granules
should properly be called haematozoin granules ( ).

The schizont now matures and becomes rosette-shaped when it is
known as the morula. Its nucleus breaks into daughter nuclei, or
rounded spores, known as merozoites ( ), the num-

ber of which may vary from six (in the Quartan fever parasite), to
twenty (in Tertian fever type).

The red corpuscle is finally broken up. This liberates the mero-

THE PROTOZOA 133

zoites into the blood-stream where (within an hour) they may again
attack and enter other red blood cells.

The breaking up of the red blood cells takes place at the moment
the merozoite is mature, and the chills and fever likewise appear at this
time.

It will be readily understood that when thousands of red corpuscles
are thus removed from the circulation, the patient will be pale
(anaemic). The chills may be due to the haematozoin granules, which
possibly contain poisonous substances.

This process of the merozoites being thrown out of the red cor-
puscles into the blood-stream may continue for some time, but after
a while, rounded forms, which throw off tiny filaments, appear. These
are the male sexual forms which require a mosquito as host before
being able to develop further. The female forms may, however, either
go on developing in man or remain dormant, and come forth again at
some later date when conditions are favorable. The sexual cycle, al-
ready described in the mosquito's body, now begins if the infected
individual is bitten by a female Anopheles mosquito.

The chills always appear at regular intervals, because the incuba-
tion period of each of the three kinds of malarial parasites (although
differing for each species) is always the same for the same species.
Thus the tertian fever species (Plasmodium vivax) "hatches" every
third day — hence its name ; the quartan (Plasmodium malariae) every
fourth day, while the aestivo-autumnal type (Plasmodium immaculatum
Laverania), at irregular intervals. In fact, the physician uses this
definite incubation period as his clue in diagnosing the case, to find
what particular type of malarial parasite has infected his patient.

After the spores or merozoites are thrown into the blood-stream,
many are devoured by the white corpuscles (leukocytes), but those not
devoured, again enter new red corpuscles and so continue reinfecting
the same patient, although they are unable to infect another.

The method of communication from one person to another can only
come about in the following manner :

A female mosquito of the genus Anopheles must suck the blood
of an infected person if the disease is to be communicated. As soon
as the infected blood reaches the changed environment of the mos-
quito's stomach, the series of changes begin in the merozoites, which
have been described above. It will thus be seen that Plasmodium ma-
lariae must not only pass through two stages of a life-cycle, sexual and
asexual, but these two stages are unable to develop in a single host, the
asexual stage developing in man, and the sexual in mosquitos.

At this point the question will occur, "How do we know all this?"
It is the answer to this question which will give the student (1) the
finest illustration of what modern -laboratory methods mean; (2) it will
acquaint him with the exhaustive investigations which students of sci-

134 GENERAL BIOLOGY

ence are always performing, and (3) it will show him what great quan-
tities of material must be sifted before one can prove an accepted scien-
tific theory, or advance a new one.

It was on November 6, 1880, that Dr. Laveran, a French army
surgeon serving in Algeria, plainly saw the living parasites under the
microscope in the blood of a malarial patient. But it was not until five
years later that medical men accepted his findings. Then several
Italian pathologists, prominent among them being Golgi, Marchiafava,
and Celli, worked out the behavior of the parasite in human blood.
These men found that the fever and chills always came at definite
periods of development in the parasite.

But they could not find how the parasite got into the blood of the
patient. The name "Malaria" is Italian and means "bad-air'.' (malaria)
and the disease had always been associated with swamps and stagnant
water, so it is not strange that mosquitos had been thought of as hav-
ing some relationship to the disease. Medical men were, however,
inclined to consider such a thought as savoring too much of superstition
to consider it.

Notwithstanding this general attitude, Dr. A. F. A. King, an Amer-
ican physician, in 1883 summed up the evidence which to him seemed
quite conclusive for such an association.

Riley and Johannsen have put Dr. King's argument in the follow-
ing words :

1. Malaria, like mosquitoes, affects by preference low and moist
localities, such as swamps, fens, jungles, marshes, etc.

2. Malaria is hardly ever developed at a lower temperature than
sixty degrees Fahr., and such a temperature is necessary for the devel-
opment of the mosquito.

3. Mosquitoes, like malaria, may both accumulate in and be ob-
structed by forests lying in the course of winds blowing from malarious
localities.

4. By atmospheric currents malaria and mosquitoes are alike capa-
ble of being transported for considerable distances.

5. Malaria may be developed in previously healthy places by turn-
ing up the soil, as in making excavations for the foundation of houses,
tracks for railroads, and beds for canals, because these operations afford
breeding places for mosquitoes.

6. In proportion as countries, previously malarious, are cleaned up
and thickly settled, periodical fevers disappear, because swamps and
pools are drained so that the mosquito cannot readily find a place suit-
able to deposit her eggs.

7. Malaria is most dangerous when the sun is down and the dan-
ger of exposure after sunset is greatly increased by the person exposed
sleeping in the night air. Both facts are readily explicable by the mos-
quito malaria theory.

THE PROTOZOA 135

8. In malarial districts the use of fire, both indoors and to those
who sleep out, affords a comparative security against malaria, because
of the destruction of mosquitoes.

9. It is claimed that the air of cities in some way renders the poison-
innocuous, for, though a malarial disease may be raging outside, it does
not penetrate far into the interior. We may easily conceive that mos-
quitoes, while invading cities during their nocturnal pilgrimages, will
be so far arrested by walls and houses, as well as attracted by lights in
the suburbs, that many of them will in this way be prevented from pen-
etrating "far into the interior."

10. Malarial diseases and likewise mosquitoes are most prevalent
toward the latter part of the summer and in the autumn.

11. Various writers have maintained that malaria is arrested by
canvas curtains, gauze veils and mosquito nets, and have recommended
the use of mosquito curtains, "through which malaria can seldom or
never pass." It can hardly be conceived that these intercept marsii-air
but they certainly do protect from mosquitoes.

12. Malaria spares no age, but it affects infants much less fre-
quently than adults, because young infants are usually carefully housed
and protected from mosquito inoculation.

King's work does not seem to have come under the notice of the
European and Asiatic workers, so it was not until 1894 that Sir Patrick
Manson, who had done pioneer work in filariasis (See Chapter XX),
came to the conclusion that there must be an intermediate host for a
parasite so similar in its general functioning as malaria is to filaria.

It was already known that long thread-like processes formed as
soon as the parasite escaped from the blood, and became free-swimming
in the surrounding media.

At first it was thought that water containing the parasite was the
carrier of infection but no one who drank the water developed malaria ;
in fact, they did not even develop the disease when this water was
actually injected into the veins.

Manson then suggested that these motile forms must have some-
thing to do with the manner of communicating the disease, and it was
he who also thought a blood-sucking insect the most likely intermediate
host. After so much progress had been made it was a simple matter
to think of the old association of mosquitoes and malaria.

It is interesting to note also, that Laveran working independently,
come to similar conclusions in the same year that Manson did.

Major Ronald Ross, in India, without any knowledge of the form
or appearance of the parasite during the time it is developing within
its intermediate host, and without a knowledge of the species of the
insect he was looking for, spent two and a half years of intensely ardu-
ous work following out experiments largely suggested by Manson.

136 GENERAL BIOLOGY

Finally, in August, 1897, seventeen years after the parasite was
first discovered in man, he obtained his first clue.

While he was dissecting a "dappled-winged" mosquito and had
searched every cell and found nothing, he came to the insect's stomach.
In writing of this, Major Ross says: "Here, however, just as I was
about to abandon the examination, I saw a very delicate circular cell,
apparently lying amongst the ordinary cells of the organ and scarcely
distinguishable from them. On looking further, another and another
similar object presented itself. I now focused the lens carefully on one
of these, and found that it contained a few minute granules of some
black substance, exactly like the pigment of the parasite of malaria. I
counted altogether twelve of these cells in the insect."

As he searched further he found the mature pigment cells contained
multitudes of thread-like bodies which, when the parent cell was rup-
tured, poured into the body of the insect. These were the spores formed
in the sexual generation.

Major Ross did his experimental work on birds which are infected
with malaria, but his results were soon found to apply to man as well.
vSo complicated a scheme of things can never appeal to men at
large, and yet it is just men at large who must assist in any preventive
measures which are to wipe out diseases of this nature. For this reason
a series of popular experiments were tried out.

Drs. Sambon and Low, of the London School of Tropical Medicine,
went to the most malarial portion of Rome in the most dangerous sea-
son. Here they lived with three or four others from July until the 19th
of October in a specially constructed mosquito-proof hut near Ostia.
They were thus protected from sunset to sunrise from the bites of mos-
quitoes. Not one of them became infected, while mosquitoes sent from
here to London were allowed to bite several people (Dr. Manson's own
son being one of the subjects who volunteered for the experiment), all
of whom came down with the disease.

Again, in Italy, railroad employees who were housed in mosquito-
proof huts did not develop the disease while those not so housed did.

Our own experience in cleaning up the Panama Canal Zone of Ma-
laria and Yellow Fever are notable examples of preventive measures
being used most effectively, from the knowledge gained in the study of
the life-cycle of the malarial parasite.

In Cuba, Yellow fever (also a disease caused by an infecting para-
site carried by the mosquito), was shown, likewise, to be carried only
through an intermediate host. Major Walter Reed had workmen sleep
in beds and use the clothing of those who died of yellow fever, but kept
such men housed in mosquito-proof huts, and not one developed the
disease, while those who were bitten by the infecting mosquito and hav-
ing perfectly clean bedding and linen took the disease. Dr. Charles J.
Finlay of Havana, Cuba, and Major Walter Reed are the Manson and
Ross of the Yellow-fever parasite.

THE PROTOZOA

137

There are a hundred and twenty-five species of mosquitoes in North
America, but it is only the female of the genus Anopheles which can
transmit malaria to man, though some members of the genus Culex do
transmit it to birds. (At least this is true in India.)

The distinguishing characteristics of the two groups is as follows
(Fig. 51) : In Culex the wings are clear, while in Anopheles they have

Fig. 51.

A. — Life history of house mosquito (Culex).

B. — Life history of malaria mosquito (Ano-
pheles). (From Howard, U. S. Dept. of Agri-
culture).

C. — Culex larva, showing details of external
structure. (After Riley and Johannsen.)

brown spots. In Culex the axis of the body forms a curved line as
though the insect were hump-backed, while Anopheles presents a
straight line when resting. For those familiar with insect anatomy we
may add that Culex has short maxillary palpi while Anopheles has them
almost as long as its proboscis ; and lastly, for those with a musical ear,
we may add that the female Anopheles, which is the only one which
carries the malarial parasite, sings several tones lower than the Culex.

The eggs of Culex are always laid in a mass, while those of
Anopheles are laid singly. As the eggs hatch, the larvae of Culex hang
from the surface of the water at about an angle of 45 degrees, while the
larvae of Anopheles lie almost parallel to the surface of the water.

Prevention is always the scientific method of overcoming disease.
Because mosquitoes lay their eggs in quiet pools, men conceived the
idea of preventing these eggs from hatching. This has not been possi-
ble, but oil poured on the water will kill the little wrigglers after they
hatch.

The breathing tubes of wrigglers are provided with hydrofuge
plates at their openings which will not permit water to enter. Since
these hydrofuge surfaces are due to the presence of oil, it is obvious that
oil poured on the surface of the water will mix with this and cause the

138

GENERAL BIOLOGY

entry of oil into the breathing tubes, thus asphyxiating the wrigglers.
It has been thought that certain kinds of fish destroy eggs and wrig-
glers. Muttkowski personally examined over 6,000 of these fish-stom-
achs and found only one mosquito wriggler. Another observer exam-
ined about 2,000 specimens of Gambusa, the so-called "mosquito-de-
stroying top minnow," and found mosquito wrrigglers in only about 2%
of the fish.

PARAMOECIUM

The. animal now to be studied is the Paramoecium (Fig. 52), a
member of the class Infusoria.* Paramoecia are often called slipper

animalculae because they are shaped like a
slipper, or more correctly like a cigar. The
distinctive characteristic of this animal is that
its entire body is covered with little hair-like
projections called cilia. The rapid movement
back and forth of these cilia (especially those
of the oral groove which beat faster than those
on other parts of the body), causes the animal
to be propelled through the water in which it
swims. There is an oral groove extending
obliquely backward from the forward end
which empties just a little behind the middle
portion of the body. The mouth is situated at
the end of the oral groove, so that as the ani-
mal is swimming along and constantly revolv-
ing, various substances are forced down the
oral groove and as they reach the end of this
groove are thrust into the mouth proper. There
is also an endosarc and ectosarc just as there
is in Amoeba, but there is also an additional
membrane or pellicle, sometimes called the
cuticle. This is demonstrated by placing a
drop or two of 35% alcohol in a drop of water
where some of the Paramoecia are found. The
pellicle will then be raised like a blister, show-
ing that this part is separate and distinct from the rest of the animal.
Immediately beneath the cuticle there is found a layer of spindle-shaped
cavities in the ectoplasm filled with a semi-fluid substance. These are
known as trichocysts ( ), supposed to be weapons

of offense and defense (Fig. 53). If a little acetic acid or even ordinary
blue or green fountain pen ink is added to the water these trichocysts are
exploded, and the long threads which they contain are discharged.

Fig. 52.

Paramoecium caudatum.

1. Mouth at bottom of
groove. 2. Oesophagus. 3.
Food vacuole just being
formed. 4. Contractile vacu-
oles. 5. Trichocysts which
have exploded ; the unex-
ploded ones line the cuticle.
6. Cilia. 7. Meganucleus. 8.
Micronucleus. 9. Contractile
fibrils. (After Butschli.)

"The early workers in biology took vegetable matter, such as dried grass, and steeped it in
boiling water, then letting this infusion stand in the air. The animals found therein were called
Infusoria.

THE PROTOZOA 139

There are two contractile vacuoles, one close to each end of the
body, while six to ten radiating canals communicate with these vacuoles
and other portions of the body. These canals collect the fluid from the
surrounding protoplasm and pour it into the vacuoles, after which the
vacuoles contract alternatively at intervals of twenty seconds and dis-
charge their contents to the outside of the body.

This, it will be remembered, is as it is in Amoeba, where the con-
tractile vacuoles act as organs of both excretion and respiration. Para-
mcecia feed on bacteria and minute unicellular ani-
mals. The animal moves back and forth rapidly, caus-
ing a current of water to be sent down the gullet so
that various food particles are swept in. Along the
gullet there is a row of cilia which have fused together
forming what is called an undulating membrane. As
the food enters the end of the gullet a food vacuole is
moedum de produced, which as soon as fully formed, separates
fending itself from an from the gullet and is swept awav bv the rotary

attack by a Proto- . " . J

zoon, Didinium. The streaming movement of the endoplasm. i his process

triochysts are dis- « . . , N ,-,,,. ,.

charged and mechan- IS known as CyClOSlS ( ). ihe digestion

myllyawaye ^From occurs within the food vacuole, while the undigested
Hegner, after Mast.) particles are cast out at a definite anal spot which
can only be seen when these particles are discharged.

BEHAVIOR

Conjugation and division of Paramoecia will be discussed in the
following chapter. Here the ordinary reactions of this animal will be
taken up.

While Paramoecia normally swim by means of cilia, they can, when
forced to, exhibit great elasticity and pass through very small openings.
The body goes forward, turning round and round on its long axis, al-
ways toward the left as it is propelled forward. This is the result of
the cilia in the oral groove growing more rapidly and effectively than
elsewhere, so that one obtains approximately the same effect as rowing
in a boat in which the oars on one side are applied much more strongly
than on the other. The animal would naturally swim in a circle if this
were the only force applied, but as it rotates on its long axis continu-
ally, it goes forward. This produces a spiral course. "The swerving,
when the oral side is to the left, is to the right ; when the oral side is
above, the body swerves downward; when the oral side is to the right,
the body swerves to the left, etc. Hence the swerving in any given
direction is compensated by an equal swerving in the opposite direc-
tion ; the resultant is a spiral path having a straight axis."

Paramoecium responds to stimuli negatively and positively just as
do other forms of unicellular animals. This animal has been particu-
larly well studied in the laboratory as to its reactions to various stimuli,

140 GENERAL BIOLOGY

and it is intensely interesting to note that whenever any injurious sub-
stance or stimulus is applied at its anterior end; the cilia reverse them-
selves and the animal swims backward for a short distance away from
the reason of stimulation. The forepart of the animal then swings
about, using the posterior part as a pivot, when the animal again moves
forward. If it again comes into an undesirable medium, the same pro-
cess is repeated. As the animal backs up from an unpleasant stimulus,
using its posterior end as the pivot upon which to turn, various samples
of the surrounding medium are brought into the oral groove, so that,
as soon as these samples of liquid no longer contain the unpleasant
stimulus, the animal moves forward.

The important point to remember here is that Paramoecium has
only two reactions, the going forward and the going backward. Much
erroneous interpretation may be avoided if this be remembered when
the study of the animal mind, or animal psychology, is taken up later ;
for, no matter how many hundreds of times an animal of this kind may
try an experiment, it always continues this trial and error method of
going forward, bumping into something that is antagonistic to itself,
backing up, and again coming forward until it accidentally gets into a
medium that is satisfactory. In fact, there are some substances, such
as acetic acid, to which Paramoecia react in a peculiar manner. If a
drop of this acid be placed before the animal, it will enter the liquid ;
but once within the acid-drop it will react to the surrounding water
in a negative manner; that is, it will come to the edge of the acid-drop
and then back away again and again. Then, the trial and error method
may be observed when heat is applied to the surrounding media. The
animal tries almost every direction until it finds some method of escap-
ing from the unfavorable stimulation, the optimum temperature is nor-
mally between 24 and 28 degrees C.

There are positive reactions of Paramoecium also, such for example
as its habit of lying against solid objects. Paramoecia are negatively
geotropic, in that they usually come toward the upper portion of the
water in which they are placed. The animals usually swim upstream
and it is supposed that the reason for this is that the current might
interfere with the beating of the cilia if another direction were taken.

It is generally supposed that it is the physiological condition of
Paramoecium which determines the character of any response to a given
stimulus. This means merely that the actions are more or less spon-
taneous and due to the internal condition of the animal (autogenous).
This internal condition changes, however, with the different amounts
and qualities of food and digestion. "Thus one physiological state re-
solves itself into another. This 'becomes easier and more rapid after
it has taken place a number of times/ giving us grounds for the beliei
that stimuli and reaction have a distinct effect upon succeeding re-
sponses."

THE PROTOZOA

141

"We may sum up the external factors that produce or determine
reactions as follows :

1. The organisms may react to a change even though neither ben-
eficial nor injurious.

2. Anything that tends to interfere with the normal current of life
activities produces reactions of a certain sort (negative).

3. Any change that tends to restore or save the normal life pro-
cesses may produce reactions of a different sort (positive).

4. Changes that in themselves neither interfere with nor assist the
normal stream of life processes may produce negative or positive reac-
tions, according as they are usually followed by changes that are injuri-
ous or beneficial.

5. Whether a given change shall produce a reaction or not often
depends upon the completeness or incompleteness of the performance
of the metabolic processes of the organism under the existing condi-
tions. This makes the behavior fundamentally regulatory."

When one organism causes disease in another it is said to be patho-
genic to the organism affected. For example, Amoebae bucallis are
found in pyorrhea, a disease of the teeth. The drug emetine kills
Amoebae bucallis and when these are killed the diseased condition im-
proves. From these facts it has been concluded that this particular
protozoan is the cause of pyorrhea, although this is not strictly true.

While, as we shall shortly see. most of the pathogenic organisms
belong to the plant kingdom, still the following animal organisms which
cause disease in man, are rather important factors in the study of
biology :

Fig. 54.

Entamoeba histolytica from a case of amoebic dysentery in man. Ectp., ecto-
plasm ; Endp., endoplasm ; V, vacuoles ; A7, nucleus, cy, encysted amoebae. (After
Rivas).

GENERAL BIOLOGY

PATHOGENIC PROTOZOA

Class I. Rhizopoda

(a) Entamoeba hystolytica (also called entamoeba dysenteriae
(Fig. 54).

Entamoeba histolytica causes a chronic ulcerative process in
the large intestine, so-called amoebic dysentery. The organisms are
frequently carried to the liver by the portal circulation and give rise to
abscesses which may attain a large size and may extend to a pleural
cavity or to a lung.

It is a common infection in the tropics, but occurs 'also more
or less frequently in temperate zones.

The organism measures fifteen to twenty-five micra in diarn-

Entamoeba gingivali
1914).

(buc calls) .

Fig. 55.
(After A. J. Smith in Dental Cosmos, Sept.,

eter. It contains a small round vesicular nucleus which stains but
poorly with the ordinary basic dyes and with alum hematoxylin. The
nucleus contains a minute nucleolus. The cytoplasm around the nucleus
is finely granular and is surrounded by an outer zone or ectosarc which
is transparent and refractive, and which sharply defines the outer limits
of the organism.

The entamoeba hystolytica is examined on a warm stage, in
order to detect the characteristic movements ; but it is readily identified
in properly fixed tissues owing to its characteristic morphology.

"The organism is quite phagocytic and frequently contains red
blood corpuscles, bacteria, or cellular debris. It is able to penetrate
fibrous and other tissues and is frequently found in the walls of blood
vessels and inside of them."

It secretes a mild toxin (which may be a waste product) which
'slowly kills the cells in its neighborhood and then gradually dissolves
them.

Amoebae are, however, often found in normal tissues. Some-
times the nuclei seem to be fading out.

It is found principally in the intestines and sometimes in the
liver.

Cultures of these amoebae have been shown to withstand dry-
ing from eleven to fifteen months.

(b) Entamoeba buccallis (also called E. gingivalis and E. den-

THE PROTOZOA

Ins-

has been said to cause pyorrhea alveolaris (Fig. 55), but Rivas
holds that these Amoebae are the effect of infection and thus represent
a secondary infection which aggravates the primary infection.
Class II. Sporozoa

Subclass 1. Telosporidia ( )

Order 3. Haemosporidia ( )

Plasmodium which causes malaria. (See Fig. 50.)
Subclass 2. Neosporidia ( )

Order 2. Sarcosporidia ( )

Sarcocystis miescheriana. (Fig. 56.)

Medical men often call these organisms "Rainey's tubes."
They are found in the muscles of pigs.

These tubes are ovoid bodies filled with small sickle-shaped
unicellular organisms — the sarcocystis miescheri. It sometimes is found
in man, causing serious disease called psorospermiasis, usually fatal.
Class III. Mastigophora
Order 1. Flagellata
Trypanosoma gambiense (Fig. 57) causes the disease known

Fig. 56.

Longitudinal section
through muscle of a Pig,
c o n t a ining Sarcocystis
Miescheriana ( K u h n ) .

(After Braun.)

Fig. 57.

Trypanosoma gambiense, from
a case of sleeping sickness.
Different forms. (After Man-
son.)

as trypanosomiasis, commonly known as sleeping sickness.

These parasites are found in many invertebrates and verte-
brates.

Life history in two stages. One "a flagellate monadine
( ) phase, in which they live in the blood-stream

of vertebrates, and in some of which they cause serious disease; the
other is a gregarine ( ) non-flagellate phase which

may also be parasitic and which is met with in forms of Kala-Azar."

Causes sleeping sickness, which is common in West Africa.
Those living on "wooded shores of lakes and rivers, such as fishermen
and canoe men are subject to it. The parasite is carried "by the bite

144

GENERAL BIOLOGY

of the tsetse fly (glossina palpalis), and where this insect exists the dis-
ease is liable to prevail. The fly lives on the bushes on the lake shores
or river banks, and feeds on the blood of crocodiles, antelopes, etc. The
trypanosomes undergo changes in the body of the fly and the infectivity
does not appear until the thirty-second day, but continues for at least
seventy-five days."

The parasite is found mostly in the cerebro-spinal fluid, though
less commonly in the blood. Hope of exterminating the disease seems
to lie in exterminating the game (crocodiles especially) on which the
tsetse fly feeds.

FLAGELLATES OF UNCERTAIN POSITION. (Fig. 58.;

12 34

*

6 7

Fig, 58.

Kala-Azar organism. 1, from a patient in India; 2 and 3, individual flagellate,
(Leishmania Jonovani) ; 4, 5, 6 and 7, Lcishmania infantum. (From Kolle-
Wassermann).

Leishmania donovani.
Leishmania infantum.
Leishmania tropica.

Causes Indian Kala Azar (dum-dum fever), Infantile Kala
Azar, and tropical boil, respectively. Common in Asia. Causes lesions
on exposed surfaces of body and enlarged spleen and anaemia.

The bed-bug or a blood-sucking bug is
probably the common carrier because ingested para-
sites undergoing development into flagellate forms
have been found in the bed-bug.

The infantile disease affects children only :
probably through dog fleas, as dogs are spontane-
ously infected in the epidermic regions.
Class IV. Infusoria

Balantidium coli (or Entamoeba coli;.
(Fig. 59.)

A ciliated, oval-shaped infusorian. There
is a "bean-shaped macro-nucleus and a spherical
micro-nucleus. In tissues the organisms frequently

Fig. 59.

Balantidium coli, from
an ulcer of man's intes-
tine. After Braun and
Luhe.)

THE PROTOZOA

145

exhibit changes in form due to ameboid motion, as in penetrating the
epithelial lining of the intestinal glands.

"The organism is a common inhabitant of the intestine of the
hog where it causes no lesion. On rare occasions it is apparently trans-
ferred to man and gives rise to more or less extensive ulcerations in
the large intestine (rarely in the lower end of the small intestine) ac-
companied by persistent diarrhea which may terminate fatally."

They are found in the lumen and walls of the intestine but
usually they penetrate the epithelial wall and lie next to a gland. Some
collect in the lymph-nodules while often they are found in lymph-ves-
sels and veins, but they do not seem to be distributed by streams of
these vessels. They have been found in the liver.

They do not seem to produce a toxin but do a mechanical i^i-
jury only, although this injury opens paths through which bacteria often
cause infections.

Fig.

A. Partially schematic drawing of Trichomonas iwtestinalis.

B. Trichomonas muris dividing (5 stages).

C. Lamblia intestinalis. a, flagellated form ; b, cyst ; c, flagellated form viewed
from the side.

D. Cercomonas hominis. a and b, show different forms of the organism ;
c, cyst.

(From Kolle-Wassermann ; B, after von Kuczynski ; C, after Betison and
Grassi and Schewiakoff ; D, after Wenyon.)

146

GENERAL BIOLOGY

The ulcers caused by this organism resemble those caused by
entamoeba histolytica.

Trichomonas hominis (Fig 60).
Cercomonas hominis

In intestines, causing acute or chronic diarrhea.
Lamblia intestinalis

Larger than the trichomonas. Flagellated forms have been
found in the sputum of cases of gangrene of lung, and in those having
pleurisy.

Spirochaeta recurrentis (Fig. 61).

Causes Relapsing Fever
(also called Famine Fever, Seven
Day Fever, and Tick Fever), prob-
ably transmitted by mosquitoes or
bugs. From five to seven relapses
take place after all symptoms have
disappeared.

The spirillum or spirochete
is 15 to 40 micra long, shaped like
a corkscrew. Quite motile and pres-
ent in blood during the febrile
paroxysms, disappearing at intervals.

"The disease has been reproduced by injecting into a healthy
monkey blood sucked by a bug from the infected animal."
Treponema Pallidum (Fig. 62).

1. 2.

Fig. 61.

1. Spirochaete recurrentis, found in Russia.

2. Same as 1, but from a patient in Africa.
(From Kolle-Wassermann ) .

Fig. 62.

Schematic drawing of undulating membrane of Spirochaetes. a and b Spiro-

chaeta pallida ; c, S. refrinyens ; d, a small Spirochaete of the same species ;

e, Spirochaete found in an ulcerated carcinoma ; /, Spirochaete dentium ; g, Spi-

rochaeta plicatilis merely showing the extremity of a rather long individual.

(After Schaudinn).

Cause of syphillis.

Acquired syphillis is due to a mucous membrane coming in
contact with the spirochete.

Congenital syphillis is transmitted through the mother to the
child.

THE PROTOZOA 147

The treponema is a spiral, curved organism from 5 to 15
microns in length, showing active movements in fresh specimens.

Syphillis is one of the most serious and far-reaching of all dis-
eases, in fact so far-reaching that one of the world's greatest diagnos-
ticians has said that if one could know every ramification of this dis-
ease he would know nearly all there was to medicine. Doubtful if
curable.

Though all symptoms are gone, the disease may appear again.

In fact, in prisons, where there was little likelihood of a second
infection, symptoms have appeared ten years after a supposed cure.

THE PROTOZOA, SUMMARY OF IMPORTANT FACTS

(Table Modified from R. Hertwig and R. W. Hegner.)

1. The Protozoa are unicellular organisms without true organs or
true tissues.

2. All vital processes are accomplished by the protoplasm (sar-
code), digestion directly by its substance, locomotion and the taking of
food by means of protoplasmic processes (pseudopodia), or by appen-
dages (cilia and flagella).

3. Excretion takes place by special accumulation of fluid, the con-
tractile vacuoles.

4. Reproduction is by budding or by fission. Conjugation has
been witnessed in many, and possibly occurs in all. True conjugation
is a process of fertilization (caryogamy), in contrast to fusion of plasma
(plasmogamy).

5. Protozoa are aquatic, a few living in moist earth ; they can only
exist in dry air in the encysted condition, surrounded by a capsule which
prevents desiccation.

6. Since encysted Protozoa are easily carried by the wind, the oc-
currence of these animals in water which originally contained none is
easily explained.

7. The mode of locomotion serves largely as a basis for division
of the Protozoa into the classes Rhizopoda, Mastigophora, Infusoria
and Sporozoa.

8. The Rhizopoda are subdivided into the following orders :
Lobosa, Heliozoa, Radiolaria, and Foraminifera.

9. The Rhizopoda have changeable protoplasmic processes, the
pseudopodia.

10. Order 1. Lobosa ( ). Rhizopoda with
fmgerlike (lobose) pseudopodia. Most of the Lobosa occur in fresh
water, a few in moist earth, and some are parasites.

Examples: Amoeba, Arcella and Difflugia. (Fig. 63 .)

Arcella ( ) is common in the ooze on the bot-

toms of fresh-water ponds and ditches. It has a dome-shaped brownish
shell of chitin which it secretes. The lobose pseudopodia protrude from

148

GENERAL BIOLOGY

a circular opening in the center of the flattened surface.

Difflugia ( ) is another common member of the

order Lobosa, and is also found in the ooze of ponds. Its shell consists

Fig. 63.

A. Amoeba proteus. (After Gruber).

B. and C. Arcella discoides. (After Leidy).
D. Difflugia urceolata. (After Leidy).

of minute particles of sand and other foreign objects held together by
chitin.

11. Order 2. Heliozoa ( ) Rhizopoda with

thin, radially arranged pseudopodia, which are usually supported by
axial threads.

Examples : Actinophrys. (Fig. 64.)

Actinophrys ( ), the sun animalcule, lives

among the aquatic plants in fresh water ponds
and ditches. The body appears vesicular, be-
ing crowded with vacuoles. The small organ-
isms which serve as food strike the pseudo-
podia, pass down to the body, and are en-
gulfed ; larger organisms are drawn in by sev-
eral neighboring pseudopodia acting to-
gether.

12. Order 3. Radiolaria ( )

Marine Rhizopoda with raylike pseudopodia, a
central perforated capsule of chitin, and usually
a larger enclosing skeleton of silica.
Examples: Actinomma, Thalassicola. (Fig. 65.)
The shells of the radiolarians, upon sinking to the sea bottom, form
radiolarian ooze ; this becomes hardened, producing rock strata as much
as 1,000 feet thick. These rocks may take the form of quartzites, flint,
or chert concretions.

13. Order 4. Foraminifera ( ) Rhizopoda,

mostly marine, with fine, branching pseudopodia which fuse forming a
protoplasmic network.

Examples: Allogromia, Globigerina. (Fig. 66.)

Allogromia ( ) lives in fresh water and has a

Fig. 64.

Actinophrys sol X about 800.
(From Bronn.)

THE PROTO/OA

149

Fig. 65.

A. to H. Isolated Nucleus of Thalassicola nucleata Hux. (After Verworn.)

A. to D. Regenerative changes.

E. to H. Degenerative changes.

I. Actinomonas Pusilla (Kent) n, nucleus; /, flagellum ; p, pseudopoJia.

chitinous shell. The shells of many Foraminifera consist of numerous
chambers connected by openings (foramina), and are composed of cal-
cium carbonate. When these shells sink to the sea-bottom, they become
Globigerina ooze, which solidifies, forming gray chalk.

14. The Mastigophora ( ) may easily be dis-

tinguished from other Protozoa by the presence of one or more flagella.
Four orders are usually recognized: (1) Flagellata,
(2) Choanaflagellata, (3) Dinoflagellata, (4) Cysto-
flagellata.

15. Order 1. Flagellata ( )

Mastigophora with one or more flagella at the an-
terior end of the body.

Examples : Euglena, Mastigamoeba, Chilomo-
nos, Uroglena, Volvox. (Fig. 67.)
Fig. 66. Mastigameba ( ) is of

Protoplasm of Giabi- special interest, since it appears to combine the dis-

<jerina, after the shell has f . . . r .

been dissolved, n, nu- tiiipfiiishing' characteristics ot both -the Rhizopoda

cleus. (After Hertwig.) V - , . , . .

and Mastigophora, that is, it possesses pseudopodia

Fig. 67.

A. Uroglena americana Calkins, a sphaeroid colony.

B. Mastigamoeba aspera. (After Schultze).

150

GENERAL BIOLOGY

and also a distinct flagellum. It is therefore able to creep about on a
solid object or swim directly through the water.

Chilomonas ( ) is a very common Flagellate in

laboratory cultures. Uroglena forms spheroidal colonies consisting of a
great number of individuals held together by a gelatinous matrix. This
form is often responsible for the "oily odor" of drinking water caused
by the escape of small droplets of an oil-like substance from the cells.

Volvox ( ), (Fig. 49), is a colonial Flagellate

found in fresh-water ponds. It may consist of as many as twelve thou-
sand cells. Protoplasmic strands connect each cell with those that sur-
round it ; physiological continuity is thus established. All of the cells
are not alike, since some of them, the germ cells, are able to produce new

A. B.

Fig. 68. Fig. 69.

Frotcrosponffia haeckeli S. K. A. Peredinium divergens, chr.

(S. Kent). B. Ceraiium tripos (Calkins). (From Pratt's "Manual" by

permission of A. C. McClurg & Co.)

colonies, but others, called somatic or body cells, have no reproductive
power.

Some of the germ cells, the parthenogonidia ( ),

grow large, divide into many cells, drop into the center of the mother
colony, and finally escape through a break in the wall. Other germ
cells produce by division a great number of minute microgametes or

Fig. 70.

Noctiluca millaris. A, entire animal ; /, flagellum ; n, nucleus ; o, cytostome
and beside it the tooth and lip ; t, tentacle ; B, C, upper end with two stages in
the formation of zoospores; D, zoospores. (After Hertwig.)

THE PROTOZOA

151

T.

spermatozoa, and still others grow large, becoming macrogametes or
eggs. The eggs are fertilized by the spermatozoa, and, after a resting
stage, develop into new colonies.

16. Order 2. Choano-
flagellata ( )
Mastigophora with a con-
tractile protoplasmic collar
from the bottom of which
extends a single flagellum.

Examples : Monosiga,
Proterospongia. (Fig. 68.)

17. Order 3. Dino-
flagellata ( )
Mastigophora w i t h two
flagella, one at the anterior
end, the other passing
around the body, often in a
groove.

Examples : Peridinium,
Ceratium. (Fig. 69.)

18. Order 4. Cystoflag-
ellata ( )
Mastigophora with two
flagella, one resembling a
tentacle, the other lying in
the gullet.

Examples : Noctiluca,
Leptodiscus. (Fig. 70.)

Enormous numbers of
Noctiluca ( )

are often found floating near
the surface of the sea, giving
it the appearance, as
Haeckel says, of "tomato
At night they are

longicollis more enlarged; IV copulation of gametes; V phosphorescent, emitting a

longicolhs more enlarged; IV, copulation of gametes; V, v . r \

A, Clepsidrine, blattarum. 1-4, Monocystis magna. 1, blllish OT PTCCnish li°"ht.

two individuals copulating while in the spermatheca of

an earthworm, surrounded by spermatozoa; 2, encysted; 19. The SpOTOZOa

3 and 4, parts of cysts, formation and conjugation of the

more enlarged gametes, cu, cuticula ; dm, deutomerite ; ( ) are

ek, ectosarc ; en, entosarc ; g, gametes; gl, zoospores ; -,-j .

g2, oospore; pm, protomerite ; n, nucleus; r, residual -TTOtOZOa WltflOUt motile Or-

body ; s, sperm of earthworm; z, zygote. (From Hertwig <-!-! • -

after various authors.) gans. 1 hey are parasitic in

Metazo*?. Reproduction is

mainly by spore formation. The following classification is simplified
from Minchin's account in Lankester's Treatise on Zoology, Part 1 :

Different Gregarina. I-VII, development of Stylor-
hynchus; 1, S. longicollis; II, encysted S. oblongatus (two
animals) beginning gamete formation; ///, same later
when the sexually differentiated gametes are copulating ; gOUD
IV -VI I, formation and development of zygote of S.

152

Fig. 72.

Development of Coccidium schubergi 1, entrance of sporozoites in cell; 2, its
growth ; 3, nuclear multiplication ; 4, division into merozoites ; 5, macro-and
microgametes ; 6, zygote divided into four sporozoites. 8-11, Emeria stiedae. 8,
auto-infection ; 9, formation of sporoblasts ; 10, change of spores into sporqzoites ;
11, spore with two sporozoites, much enlarged; c, z, sporozoite ; e, epithelial cell;
k, n, nucleus ; mi, microgamete ; o, macrogamete ; r, residual body ; sp, spore ;
sp', sporoblasts. (1-7 after Schaudinn ; 8-11 after Wasielewsky and Metzner).

20. Subclass 1. Telosporidia ( ) Sporozoa in
which the life of the individual ends in spore formation.

21. Order 1. Gregarinida ( ) Telosporidia
possessing a firm pellicle and complex ectosarc ; intracellular during the
early stages of the life cycle, later free in the body cavities of inverte-
brates.

f Examples: Monocystis, Porospora, Gregarina. (Fig. 71.)

Fig. 73. Plasmodium malariae.

A. Parasites of tertian malaria.

B. Parasites of estivo-autumnal malaria.

C. Parasites of quartan malaria. (After Thayer and Hewetson ) .
(From supplement No. 18 to the Public Health Service, Jan. 20, 1915.)

THE PROTOZOA

153

Monocysts ( ) may be found in the seminal

vesicles of almost every earth-worm ; Gregarina is a common parasite of
the cockroach ; and Porospora gigantea, which reaches a length of two-
thirds of an inch, inhabits the alimentary canal of the lobster.

22. Order 2. Coccidiidea ( ) Telosporidia simple

in structure; trophozoite is a minute intracellular parasite.

Example: Coccidium. (Fig. 72.)

Members of this order are sometimes found in the liver and intestine
of man and other vertebrates, and in Arthropoda and Mollusca.

23. Order 3. Haemosporidia (
parasitic in the blood of vertebrates.

Example: Plasmodium. (Fig. 73.)

24. Subclass 2. Neosporidia (

give rise to spores at intervals during active life.

25. Order

) Telosporidia

) Sporozoa which

Myxosporidia

( ) Neosporidia

with ameboid intercellular tropho-
zoite.

Example: Nosema. (Fig. 74.)
The Myxosporidia are parasitic
especially in Arthropoda and fish,
frequently causing serious epidemics
in aquaria. Nosema bombycis pro-
duces the silkworm disease, pebrine.
26. Order 2. Sarcosporidia
( ) Neosporidia

usually parasitic in the muscles of
vertebrates.

Example: Sarcocystis. (Fig. 75.)
The most common Sarcosporidia
are Sarcocystis miescheriana in
the muscle of the pig, S. muris in that of the mouse, S. lindemanni, rarely
occurring in the muscles of human beings.

27. The Infusoria ( ) are Protozoa with cilia
which serve as locomotor organs and for procuring food. Paramecium
is a typical member of the class. There are two subclasses, (1) Ciliata,
and (2) Suctoria.

28. Subclass 1. Ciliata ( ) Infusoria with cilia
in the adult stage, a mouth, and usually undulating membranes or cirri.
Many ciliates are confined to fresh water, others occur either in fresh or
salt water, and still others are parasitic in Metozoa.

29. There are four orders: (1) Holotricha, (2) Heterotricha, (3)
Hypotricha, (4) Peritricha.

30. Order 1. Holotricha ( ) Ciliata with cilia
all over the body and of approximately equal length and thickness.

Fig. 74,

Nosema. Longitudinal section of stomach of
honeybee showing infection with Nosema apis :
<>;>. Epithelial portion, containing spores of the
parasite stained black. (The younger para-
sites, not differentiated so easily by staining.
are not shown ; they are found toward the base
of the cells reaching the basement membrane
(6m) , but do not extend beyond it. Younger
spores sometimes show an unstained area at
one end and occasionally at both ends, m,
muscular portion of stomach wall showing an
outer and an inner longitudinal muscular layer
and a middle circular one. (After G. F. White,
U. S. Dept. of Agriculture Bulletin No. 780.)

154

GENERAL BIOLOGY

Examples : Paramoecium, Coleps, Loxophyllum, Colpoda, Opalina.
(Fig. 76.)

The Holotricha are probably the most primitive Infusoria. Para-
moecium caudatum is the best known species. Members of the follow-
ing genera are frequently found in fresh-water cultures : Coleps, Lox-
ophyllum, and Colpoda. Opalina ranarum is a large multi-nucleate spe-
cies living in the intestine of the frog. It has no mouth, but absorbs

Fig. 75. Sarcocystis
miescheriana.

(Doflein.)

A, a cyst; B, Pork
containing cysts.
(From Pratt's "Man-
ual" by permission of
A. C. McClurg & Co.)

A.

B.

Fig. 76,

A. Coleps hirtus Ehr. (After Maupas).

B. Division phase of A.

C. Opalina ranarum, (After Bronn).

D. Colpidium colpoda. (Calkins).

E. Loxophyllum rostratum (Conn.)

digested foods through the surface.

31. Order 2. Heterotricha ( ) Ciliata whose
cilia cover the entire body, but are larger and stronger about the mouth
opening than elsewhere. This adoral ciliated spiral consists of rows of
cilia fused into membranelles and leads into the mouth.

Examples: Spirostomum, Bursaria, and Stentor. (Fig. 77.)
Stentor ( ) may be either fixed or free swim-

ming. It is trumpet-shaped when attached and pear-shaped when swim-
ming. The cuticle is striated and just beneath it are muscle fibers
(myonemes). The nucleus is ellipsoidal, or like a row of beads.

32. Order 3. Hypotricha ( ) Ciliata with a
flattened body and dorsal and ventral surfaces. The dorsal surface is
free from cilia, but spines may be present. The ventral surface is pro-
vided with longitudinal rows of cilia and also spines and hooked cirri,
which are used as locomotor organs in creeping about. The cilia around
the oral groove aid in swimming as well as in food taking. There is a
macronucleus, often divided, and two or four micronuclei.

Examples: Oxytricha, Stylonychia. (Fig. 78.)

33. Order 4. Peritricha ( ) Ciliata with an

THE PROTOZOA

155

adoral ciliated spiral, the rest of the body is without cilia, except in a
few species where a circlet of cilia occurs near the aboral end.

Examples : Vorticella, Carchesium, Zoothamnium. (Fig. 79.)
The common members of this order are bell-shaped and attached by
a contractile stalk. Certain species are solitary (Vorticella), others
form tree-like colonies (Carchesium), and still others are colonial but

A.

Fig. 11.

A. Spirostomum teres (Conn).

B. Bursaria truncateUa (Conn).

C. Condylostoma patens. (Cal-
kins.)

(From Pratt's "Manual" by per-
mission of A. C. McClurg & Co.)

B.

Fig. 78.

\. Oxytricha bifaria (Conn).

J5. Stylonychia mytilus (Dof-
lein).

(From Pratt'a "Manual" by
permission of A. C. McClurg &
Co.) '

with an enveloping mass of jelly (Zoothamnium). The stalk contains
a winding fiber composed of myoneme fibrils ; this fiber, on contracting,
draws the stalk into a shape like a coil spring.

34. Subclass 2. Suctoria ( ) Infusoria without

cilia in the adult stage. No locomotor organs are present and the ani-
mals are attached either directly or by a stalk. No oral groove nor
mouth occurs, but a number of tube-like tentacles extend out through
the cuticle.

Examples: Podophyra, Sphaerophyra. (Fig. 80.)

Ciliates are captured by their tentacles and the substance of the

Fig. 79.

A. Vorticella nebulifera (Bronn).
B. Vorticella patellitM (Calkins).

C. Carchesium polypinum (Doflein).

D. Diagram of Vorticella. The cilia at the side of the mouth have be n
omitted. (From Pratt's "Manual" by permission of A. C. McClurg & Co.)

captured prey is sucked into the body. Both fresh-water and marine
species are known. Podophyra is a well-known fresh-water form.
Sphaerophyra is parasitic in other Infusoria.

Fig. 80.

Podopkyra gracUis. (Calkins). (From Pratt's "Manual" by permission of A. C.
MeClurg & Co.)

CHAPTER X.

INTERPRETATIONS OF THE FACTS THUS FAR PRESENTED.

The far-reaching importance of biology may be shown by obtaining
an understanding of this fact : that, when anyone wishes to discuss in-
heritance, environment, training, or any of the many philosophies, or
theories of life, some physical (biological) background must be found
or the discussion is not likely to impress many. A conception of such
background may be gained by reviewing the following facts just studied:

The little cigar-shaped animal known as Paramoecium is found in
fresh water. It moves about rapidly by means of tiny hair-like projec-
tions which cover its entire body. There are in reality only two move-
ments it can perform. It goes forward and background constantly, turn-
ing its body over and over so that its path is spiral-shaped. A groove
extends half way down the length of the body into which particles of
food are swept as the animal moves forward. The mouth being located
at the lower end of this groove, the food is thus conveniently forced into
it and swallowed.

The entire animal is composed of a thick substance looking some-
thing like the white of an egg, but that this thickened material is not
all alike is attested by the fact that a drop of alcohol placed upon it
causes the outer portion of the animal's body to swell up like a blister
while the same alcohol apparently has no effect upon the internal struc-
ture. Then, too, if Paramoecia are placed in a staining fluid, two spots
take the color much better and much deeper than do other parts of the
body, showing that the two spots which thus take the stain are of dif-
ferent chemical composition from the other parts. Were all the sub-
stance alike it would all stain alike. These stainable spots we call nuclei.

Everyone has observed that all living things who fulfill their nor-
mal span of life are subject to the same natural laws, such as being born,
growing to maturity, and dying. The nearest thing to an exception to
this general rule is found in the little single-celled animal of which we
are speaking. This little fellow is not born. When it is time for its
parent to pass from this earthly region as an individual, it merely divides
into two separate and distinct animals (Fig. 81).

There are now two Paramoecia where there was only one before.
This is significant. The two new animals (each consisting of one-half
of its parent) again divide into two separate animals, and so continue
dividing indefinitely. The greatest number of divisions observed so far
is six thousand. This means that Paramoecia do not die, though they
can be killed, for example, by boiling, by acids of various descriptions,
and in other ways. It means further, that every Paramoecium now in

158

GENERAL BIOLOGY

existence is actually a part of all its ancestors, or to be more accurate,
it is its ancestors, for these ancestors have never ceased to be. This
must necessarily be true, because each ancestor merely divided into two

offspring, the offspring thus being in reality
the parent itself. This is vastly different
from a parent giving birth to an offspring
and then dying.

It is an established and incontrovertible
biological fact that no living cell can come
from anything but a previous living cell.
No organism or living thing can possibly
come into existence except from some pre-
viously existing living parent form.

Now, if sufficient food is given Para-

Fig. 81.

Stage

Paramotcium caudatum. A, Stage
A, the micronucleus in each gamete
preparing for division

moecia they will keep on dividing several
B stage B hundred times, but then, if they are with

the daughter nuclei in each gamete others of their kind an interesting- event

dividing. C, Four micronuclei in °

each gamete. D, three of the four takes place. TwO of the animals Will SW11TI

pas-)

micronuclei are disintegrating ; the r , .

surviving nucleus in each gamete around and around, Finally attaching tnem-

has divided to form d". 'the male, « 1,1 i .1 i -i ,»

and j. the female pronucleus. E, Selves to each Other length W1SC while the

wa" of each animal that comes in contact

with its mate seems to disappear, the two
animals becoming almost, but not quite, one
individual.

The smaller colorable spot in each animal now begins to divide into
two parts as shown in the drawings. These parts again divide, making
four pieces to each nucleus. Three of these pieces disappear (probably
they are dissolved in the body substance), but the one remaining piece
then again divides into two pieces, one of which remains more or less
stationary while the other (often partially connected with the first)
moves toward the midline of the two connected animals to meet with
a similar movable piece of stainable matter from the attached individual.
The two pieces of movable-stainable-matter become one for a short pe-
riod, seemingly exchanging some of their substance, then they again
separate and go back to form a nucleus like the one from which they
sprang.

The animals themselves now separate, and each begins its division
into two new animals, which again divide, such division continuing as
already mentioned, several hundred times, until this same conjugation
or joining process is brought about again. The larger stainable-spot is
dissolved at the time of conjugation and is thought to have some nutri-
ent function.

It is the nuclear material which seems to be the important physical
matter in the formation of any living thing — plant or animal — and in
turn it is only the colorable matter inside the nucleus known as chro-

INTERPRETATION OF FACTS 159

matin, which breaks up and divides, and is carried on from parent to
offspring. These little broken pieces of chromatin are called chromo-
somes. The chromosomes have come to be considered the most impor-
tant factors throwing light on the many problems of inheritance — that
is, on all problems that pertain to what we actually obtain from our par-
ents, whether these be physical, emotional, or intellectual.

It is therefore of decided importance that we obtain a clear concep-
tion of chromosomes, because in the final analysis every detail of wtiat
we are and can be, that has any relation whatever to our physical, emo-
tional, and mental makeup must come from our parents through the
chromosomes in the egg-cell of the mother and the sperm-cell of the
father. In other words, the chromosomes that were ours at the moment
of mixture of sperm and egg, possessed the sum total of all the .actual
physical, mental, and emotional endowment with which we were pos-
sessed when ushered into the world (except food and environment
needed for growth, as well as a place to grow).

In the case of Paramoecia the animal does not inherit anything from
its parent — it is half of its parent. Each Paramoecium is thus equivalent
to an egg-cell or a sperm-cell, though there is no sex present in Para-
moecia. The offspring is not a chip from the old block — it is half the
block.

An interesting application follows.

In every living thing where observation of chromosome material
lias been possible, life begins from an egg-cell of some kind, and in the
higher forms this egg-cell receives one-half the chromosomes from the
sperm after the egg-cell itself has cast out one-half of its own chromo-
somes. There is thus a constant trend toward forming an average in-
dividual of 'the species to which each such individual belongs, for, each
new living thing that comes into being is made up of one-half the chro-
mosomes \vhich the maternal egg-cell possessed, and one-half of those
which the paternal sperm-cell contained.

If this were not so, then in those cases in the animal world where
we have virgin-birth, there would be an ever lessening quantity of the
chromosome material in each next generation, so that each offspring
would become more and more unlike its parent, until in time, when no
fertilization takes place to restore the proper quantity of chromosome
material by a paternal sperm-cell being added to the maternal egg-cell,
the offspring would not be recognized as a member of the species to
\vhich its ancestors belonged.

Every female at the time of her birth has every egg in her body that
she will ever have. This is as true of a bird as of a human being. In
the human there are about 35,000 eggs in each of the two ovaries, though
only about 100 to 200 of these -actually ripen and pass out of the body
during the sexual life of the individual. This means that the mother ha»

160 GENERAL BIOLOGY

little or no influence on the formation of the egg, it being already com-
plete by the time she herself is born.

The eggs lie dormant and do not begin to ripen until sexual life
begins (averaging from twelve to fifteen years in the human being).
But, when an egg does ripen, an interesting process takes place, it is
expelled from the ovary immediately and, just as Paramoecia split into
two parts, so does the egg. But the egg does not divide equally. A
little piece called the polar-body separates from the main part of the egg.
This polar-body may divide again, but even so, it deteriorates and can
be seen no more in a short time. The stainable nuclear material breaks
up into a number of chromosomes just as does the chromatin of the
Paramoecium, and one-half of these chromosomes remain in the larger
portion, the other half passing into the polar-body to deteriorate with
that part.

The head of the male cell (spermatozoan) is practically all nuclear
material and goes through approximately the same process as the egg
does except that the sperm divide equally as to size, thus forming two
definite, living sperm-cells where there was only one before — again, this
is just like Paramoecia. And here, too, the chromosomes divide equally,
so that each sperm has only one-half the full number of chromosomes it
had before it divided.

As every plant and animal that lives comes into existence in prac-
tically the same way, that is, through a single cell of the father and a
single cell of the mother uniting, we see that this is nature's way of
bringing together the normal number of chromosomes needed to make a
complete individual. This again, means that each individual thus comes
into possession of one-half the traits or capacities of each parent-cell
(not necessarily one-half of the traits or capacities of the parent) from
which he sprang. Were this not true, each and every one of us would
be quite unlike our parents, because each would be less than either par-
ent, instead of each taking one-half from each parent and thus becoming
a complete human being like both. As our parents can give us only the
single egg-cell and the single sperm-cell, everything else being merely
food and environment, it follows that everything we can possibly inherit
as to our physical and mental makeup must be in the chromosomes that
these eggs and sperm contain ; for, it is only the chromosome part that
intermingles, divides, and causes new cells to form.

For anyone wishing to study life, therefore, the study of chromo-
somes looms up as the most important factor.

The laboratory study of the fertilized cell of which we are speaking
has shown that each such fertilized cell divides into two cells, these
two into four, each of these four into two, making eight, these eight into
sixteen, and so on indefinitely until the entire body has finished its
growth.

The first group or sheet of cells becomes a hollow sphere called a

INTERPRETATION OF FACTS 161

blastula. Some animals stop growing at this stage. Others continue
growing, which means that this single-layered sphere indents and this
indentation extends into the sphere until two layers of cells are formed.
This is called the gastrula stage. Animals having two layers stop
growth when this stage is reached, while all higher forms produce a
third layer of cells between these two.

Every living thing passes through one or more of these develop-
mental processes. It was this fact which led so many of the early biol-
ogists to suppose that each developmental stage meant that each one
of the higher forms of animals must have sprung from those which
stopped in the one and two-layer stage just beneath the higher form.
What it does mean, however, is that all living forms pass through a
similar state of growth.*

Very early in this development of an egg, after it begins to grow
(fertilization apparently furnishes this growth impulse), certain cells
divide much more rapidly than do others. The rapid-growing cells con-
sequently, soon surround the less-rapidly growing ones, thus forming a
sort of protecting case or capsule for them. Now, some of the very first
cells that are thus protected and grow into the very innermost portions
of the growing embryo, are the egg-cells and the sperm-mother cells.
This occurs long before one can even distinguish what kind of an animal
.the embryo is to become.

It was Professor August Weismann of the University of Freiburg1
in Baden, who in 1892 gave the world his book, "The Germ-plasm, a
Theory of Heredity," which has made us interpret the various facts so
far mentioned in a different way from what had been done before. Up
to that time men said that the reason a boy so closely resembled his
father was because he was "a chip from the old block," Professor Weis-
mann has shown us that this is incorrect, and that both father and son
are pieces from the same block. That is, the sex-cells in both mother
and father being a part of the earliest differentiation in the growing
embryo as already shown, are really placed in position in the child be-
fore he is born, so that a parent simply considered as a parent has abso-
lutely nothing whatever to do with the matter, such parent's body acting
only as a case or capsule which carries the germ-cell to the next genera-
tion.

This is made clearer when it is remembered that every egg in every
female is already present at the time of such individual's birth. All that
happens during her life is a ripening-, or maturing, of such egg, and fer-
tilization by the male sperm. The sperm-mother-cells that are to divide
and form sperm, are already present in the male child when he is bornr
though they begin to divide only after puberty.

*It does not follow that because a man builds a school, a barn, and a church, that the church
must therefore have first been a school and a barn, even though such builder used exactly the
same tools and similar material in the building of each structure ; in fact, it would not follow,
even though he build the foundation and the first story of each structure exactly alike in each case.

162 GENERAL BIOLOGY

The sex-cells are therefore present at birth in each person, and no
one can either change or add anything to them, unless, again it be
merely the food and drink he takes that may or may not nourish such
sex-cell properly.

This means, then, that just as with Paramoecia, each and every
one of us cannot obtain from our parents one more particle of physical,
emotional, or mental ability than our parents may have had, because we
get only what was present in the egg-cell of our mothers and the sperm-
cell of our fathers.

It means further, that when we go back even twenty-five genera-
tions, considering our two parents, four grand-parents, eight great-
grand-parents, etc., we are related in actual blood-relationship to more
people than there are in the world at the present: time. It means that
just as Paramoecia are really their grand-parents and all their ancestors
in one, so we are also actually and truly our own ancestors in so far as
our sex-cells are concerned.

An actual living particle of every one of our forefathers is really
present in each one of us. It means that the entire animal world, in-
cluding the human family, by constant intermingling of chromosomes,
is always tending toward an average, so that no matter how many cen-
turies elapse there is no real individual physical or biological progress
possible. Always will the next succeeding generation, or at least the
next after that, have some sex-cells in their bodies that will again pro-
duce an average being.

This sex-bridge which connects every human being with every
other human being in this way, is sometimes referred to as the Weis-
mannian bridge. It is this bridge which is both the hope of an oppressed
people and the despair to those who would change human nature from
what it is. We can build only upon instincts ; upon human desires and
upon wishes which afre ours at birth, though we may develop such in-
stincts and desires, bur* no actual change; in human nature can possibly
ever come into existence. Human nature is the same now as it has ever
been and always must be, until some method be obtained by which
we can tell in advance by looking at a chromosome, what good and
bad characteristics such chromosome contains, and then be able to de-
stroy the bad therein. This means that we are aeons and aeons removed
from any solution to our eugenic problem on a truly scientific basis.
Even then, were we able to accomplish this practically impossible task,
we should still have to evolve some plan by which we could see the
egg and the sperm before they unite, a task again practically impossible
until new human beings can be grown in the laboratory.

Professor Weismann also demonstrated to the scientific world that
the germ-plasm early separates from that part which is to become the
outer portion of the body and which is called the somatoplasm.

The Abbott Mendel has proved that no matter how much inter-

INTERPRETATION OF FACTS 163

breeding there may be among plants or animals, there are only two types
of offspring produced, i. e., pure stock and half breeds. The eggs and
sperm in the germ-plasm always remain pure. That is, if a white and
black animal mate, a portion of the eggs in the ovary of any female off-
spring from such union will be carriers of pure black and a portion will
be carriers of pure white characteristics. The sperm of the male, like-
wise, are carriers of one or the other colors, but are not themselves half-
breed. It will be noticed, therefore, that from this Mendelian theory
additional evidence is brought forth to substantiate the Weismannian
theory of germ-plasm, wrhich holds that the germ-plasm is separate and
distinct from the rest of the body.

The color of the skin of any offspring of black and white animals
may be of any shade, from pure black, to almost, or entirely white. But
the sperm and the egg have not intermingled in so far as color is con-
cerned. The color shows up on the outer part of the body, or in what
we call the somatoplasm. The germ-plasm always remains pure, so that
in the next succeeding generation, if any of these half-breeds in turn
mate with each other we have the four possibilities of a white sperm
meeting with a black egg and again producing a half breed, or a black
sperm mixing with a black egg which produces a pure black, while a
black sperm with a white egg produces a half breed, and a white sperm
with a white egg produces a pure white.

From observation, however, it is found that the half breeds will
look like one or the other of their parents in so far as color of skin, eyes,
and hair is concerned. Whatever color the offspring shows is known
as the dominant color.* We cannot tell, however, until we observe the
first brood of half breeds which is the dominant color.

We do not know why one characteristic is thus dominant, but the
important thing to remember is that this entire possibility of any of
the four possible matings mentioned above, taking place in any mixed
offspring is all a matter of chance. Having observed thousands of in-
stances of this kind among both plants and animals, scientific men now
accept it as a fact that we do obtain two pure bloods, and two half
breeds from matings of mixed ancestry. It will be noticed that this is
pure chance, there being approximately half as many carriers of either
color in each sperm of the male as there is in each egg of the female.
Therefore, there is just as much likelihood at any given time of a black

*Recessive is the word set in opposition to dominant. A recessive characteristic is always pres-
ent in the germ-plasm of an animal or plant of mixed ancestry, but it does not show in the
somato-plasm — in any part of the body proper outside the germ-plasm. The dominant character-
istics cover up the recessive characteristics. For example, in half-breed offspring — a cross between
white and black parents — if all these half-breeds are black, we call black the dominant charac-
teristic as to color, though such half-breed has just as much white in him as he has black. The
white which is present but which is not seen is called the recessive charasteristic.

It is very important, however, to remember that in so far as the germ-plasm — the sex cells
themselves, that is, the eggs and sperm — is concerned, each egg and each sperm has roughly speaking,
one-half black and one-half white characteristics ; but the dominant characteristic is the only one
which shows, and that only in that part of one's make-up which is not germ-plasm. .

To clarify the matter ; if half-breeds, which are the offspring of black and white parents are all
black, we call black the dominant color.

164 GENERAL BIOLOGY

sperm meeting a white egg as of a white one meeting a black, and vice
versa. But it must not be forgotten that not only the half breeds, but
also the pure bloods of the dominant type will all probably look alike
as to color. This appearance of the same color in the half-breeds that
appeared in the dominant pure-blood, is the thing which confused men
for many years, and it was only after Abbott Mendel gave us his ex-
planation that we have been able to understand why this is so.f

Mendelism has also added some interesting biological speculations
to the earlier idea of naturalists.

If we define species, as meaning all those particular plants and ani-
mals which can interbreed and in turn give birth to fertile offspring, it
can be seen immediately that we cannot have any new species at all,
because, if the offspring of such plants or animals can give birth in turn
to other offspring, they belong to the same species as do their parents,
and if they differ in appearance from their parents they can only be
called variations of the parent species. If they do not interbreed, or, if
after interbreeding, they give birth to non-fertile offspring (such as the
mule, which is the non-fertile offspring of a mare and a jack), then of
course there can be no further offspring, and we can have no further
species.

Mendelism has added a very important and interesting fact to such
theorizing. For example, in the dominant type of offspring, there is
always a pure recessive sperm and egg, so that it follows, that at any
time in the future, if by chance such pure egg and sperm meet, a totally
different type of plant or animal than its parent may be produced. But
this may be merely the coming forth of a plant or animal similar to
some ancestral form, which was the result of two recessive germ-cells
meeting. Therefore, although these recessive germ-cells were always
present in all ancestors, they were covered up in so far as external
characteristics are concerned by the dominant characteristics. A new
species, such as this which comes forth suddenly, is called a "sport"
in nature, and the theory that all new forms come forth in this way is
called the mutation theory. But, as these so-called new forms may be
explained as being recessives, again coming forth after lying dormant for
ages, there may be here no new species at all.

fit is well to remember however, as Professor Darbishire has said, that, while on the Mendelian
theory we know there are such things as dominants and recessives and that unit characteristics of
some types are transmitted from parent to offspring, still, all the evidence we have so far is based
upon color of eyes, straightness or curliness of hair, color blindness, and one or two other obser-
vations'of this kind. It will be seen, therefore, that these are not vital, and may not be so important
as we have thought them.

CHAPTER XL

GENETICS

With a clear understanding of what has been said regarding the
division of chromosomes, in maturation described in a former chapter,
and the discussion of interpretations in Chapter X, we are in a position
to understand the terminology of heredity, genetics, and Mendelism,
which is met with quite commonly in modern biological literature.

While genetics really means the "origin of things" it has come to
be used as the name of that science which studies the ways and means
by which minor inheritable characters can be judged. It must never be
forgotten that to inherit anything from one's parents in the biological
sense, means that the "something" which is inherited must already be
present in the egg of the mother, or the sperm of the father, or in both
these germ cells at the time the egg is fertilized. Every factor that
may influence an organism, which is not already present in the gametes,
is due to environmental conditions and cannot be said to be inherited.

At this point we must also remember the distinction made in a
former chapter that germplasm and somatoplasm are entirely separate
and distinct.

Mendelism, or rather Mendel's "law," merely means that each char-
acter that we may inherit must be considered as a single unit; that is,
we must not think of 'a child as inheriting its father's hair because it
has dark curly hair like its father, but we must think of darkness in
color as one character of inheritance and curliness as another; for, a
child may inherit the darkness in color from his mother and the curliness
from his father.

Thinking in terms of unit characters will throw much light upon
many of the interesting problems of life. We may thus account for one
artist, for example, having a very decided sense of form and another of
color.

It is now generally conceded by biologists that acquired character-
istics are not transmitted to the offspring. We know, however, that
brothers and sisters of the same family differ from each other in many
respects. We know that no two leaves of grass are exactly alike; in
other words, that all living things springing from the same parents vary
somewhat from each other. It is the purpose of genetics to find the
mechanism by which such variation takes place and then to be able to
apply the knowledge thus gained toward bringing about the types of
variations one wishes. Every variation represents a single unit charac-
ter or a combination of these unit characters. One may use as an exam-
ple the various species of cattle. Cows of a certain breed may produce

166 GENERAL BIOLO<;Y

a very rich milk but not a great quantity. Cows from another breed
may produce great quantities of less rich milk, while those of still an-
other breed living in the tropics may be more or less immune to heat
and tropical disease. If one wishes to bring cattle into a rather hot
clime, it will, therefore, be to one's advantage to obtain that breed which
will produce the greatest quantity of good rich milk and likewise be
able to withstand the great environmental change necessitated by re-
moval from a temperate or cold clime to one of great heat.

We have already seen that the inheritable characters are contained
within the chromosomes. The definite factors, whatever they may be,
which carry the unit characters within the chromosomes are called
genes.

From our knowledge, at the present moment, of the way the chro-
mosomes divide in cell division, and the way they throw off one-half
of their number during maturation just before 'fertilization so that fer-
tilization can again restore the regular number, we are led to believe
that no unit character can be inherited unless a gene from the father
and a gene from the mother unite in the chromosomes.

We may say, for example, that all the unit characters which any
individual can possibly inherit are contained within the chromosomes of
the germplasm of its parents ; that each chromosome may contain thou-
sands of genes which may occur in any combination, the individual him-
self actually inheriting only those unit characters which happen to be
the result of the particular gene of paternal and maternal chromosomes
which met at the time of fertilization.

To make this clear let us assume that a white and black guinea
pig are mated. The whiteness and blackness that we see, lie, of course,
in the somatoplasm ; but, in order that either color be inherited, there
must bf» £enes in the chromosomes of the germplasm which determine
the sor^atic character of whiteness and blackness. We know that if a
black guinea pig is mated with another black guinea pier, both of which
are in turn the offspring of an entire race of black animals, that only
black guinea pigs will be produced. However, if a black and white
animal mate, the offspring are really half-breeds, in regard to their
germ-cells, though their somatoplasm may show some variation in color.
We, therefore, assume, from the experimental evidence obtained through
breeding experiments of many kinds, that in order to produce a black
animal, both the paternal and maternal genes, which carry the determi-
nation of color, must have carried blackness. In the production of a
half-breed, one of the genes determining color must be white and the
other black. (Fig. 82.)

In other words, two genes always meet to produce any character
sufficiently powerful to be carried on, in turn, through succeeding gen-
erations, and the character which is thus carried on and which shows
itself in the somatoplasm is called dominant. Blackness would thus be

GENETICS

167

dominant when a white and black animal mate and produce half-breeds
which are all black in color. However, as has already been stated, the
color is only in the somatoplasm. It is important to remember that the
gerrmlasm remains pure; that is, some of the eggs in the mother and

some of the sperm in the father will
be of the black variety and others
will be of the white variety. To put
this in other words, there will be no
half-breed eggs or sperm.

Whatever unit character shows
up in the somatoplasm in animals
of different breeds is said to be
dominant, while that unit character
which is present in the germplasm
but is not seen in the somatoplasm
is said to be recessive.

In our example of the mating
of a white and black guinea pig the
offspring, though black, have white-
ness in their germplasm even if it
does not show externally in the
somatoplasm. Blackness in this
case is therefore dominant; white-
ness, recessive.

In their accounts of breeding ex-
periments, geneticists use a formula
to represent dominant and recessive
genes. The capital letter represents the dominant gene and the small
letter the recessive. In the example we have been discussing, the capital
letter — B — would represent the gene which carries blackness, the domi-
nant color ; and the small letter — w — will represent the gene carrying
the recessive white. The formula in our example of a halfbreed black
and white, therefore, is — Bw — .

In those cases where pure blooded blacks would meet with pure
blooded blacks the formula would be BB, while in the breeding of two
pure whites the formula would be WW. It will therefore be noted that
we may have the various formulas BB, Bw, WW, Wb, provided, of
course, that a recessive black could be found.

Wherever two genes are alike, so that either has BB, or WW, the
resultant zygote is called a homozygote, while the organism resulting
from a homozygote is said to be homozygous. If the two genes of the
mating pair are different, such as Bw or Wb, the zygote is called hetero-
zygote, the resultant animal being called heterozygous. It is, of course,
quite common for the same animal to be homozygous for some charac-
ters and heterozygous for others.

Fig. 82.

Diagram of two chromosomes, each square

representing a gene.

An insect, for example, with these two
chromosomes would possess a 'normal
wing, a minature wing, a rudimentary
wing, and forked bristles. All these char-
acters could be transmitted to the offspring.
The insects' body and eyes, however,
would be heterozygous.

168 GENERAL BIOLOGY

The parents are often represented by the capital letter P. The first
generation (which means the offspring from these parents) are repre-
sented by the formula, F±. The offspring of F1 in turn are known as
F2, and so on, the F representing a final generation.

In many cases the various characteristics that the genes determine
may be independent of each other, but, just as certain chemical elements
have an affinity for each other, so there are various types of characters
that often link themselves in the same way. This is known as linkage.
Color of hair and the direction in which the hair grows, such as curli-
ness, straightness, or whorls, are often linked. Then there are also cer-
tain types of sex linkage by which we mean that there are certain char-
acters such as plumage in fowls and eye-color in flies which are almost
always concomitant with the sex of the individual.

Much has been written on sex-determination in the past, though
it is only recently that any progress has actually been made in this field.
It has been found that sperm cells possess an extra or accessory chro-
mosome (called an X-chromosome by American writers and a hetero-
tropic chromosome by Europeans). (Fig. 30, A.) When such a sperm
cell fertilizes an egg, a male is produced, while, when an egg containing
a regular even number of chromosomes is fertilized by a sperm with an
•even number of chromosomes, a female is produced.

Interpreting these findings, of the cytologists, biologists now be-
lieve that there is such an extra chromosome in both egg and sperm
but that in the egg, this X-chromosome divides as do the others, al-
though this division is delayed until some time after the other chromo-
somes have divided in the maturation divisions. This means that the
X-chromosome of the sperm is really a double chromosome which fails
to separate during spermatogenesis and consequently goes over to one
of the two sperm-cells entire.

Then, in some organisms this X-chromosome has actually been seen
to be made up of a larger and a smaller portion, while in the female of
the same species both parts of the chromosome are of equal size. When
the accessory chromosome is thus divided into two parts of different
sizes, the smaller is called the Y-chromosome.

It follows from this, that if unit characters are carried by the genes
of the X-chromosome, all organisms in which the sperm carry an
X-chromosome, must necessarily transmit the characters of the X-chro-
mosome to the female offspring only, while females can transmit them
equally to all offspring. Similarly in those organisms in which eggs
may lack one chromosome, the female can transmit characters only to
their sons, while males can transmit to their offspring of both sexes.
This is the explanation of sex-linked transmission as shown in men who
are color-blind. Such men transmit this defect to their daughters, and
the daughters can in turn transmit it to all of their sons and daughters.

GENETICS 169

There are exceptions to this. A usual sex-linked character, such
as color-blindness, is sometimes transmitted from father to son directly.
This is explained by Bridges as being due to what he terms a "non-dis-
junction" of the sex-chromosomes in the polar divisions of the egg dur-
ing the maturation division. In other words, such a non-disjunction may
come about by the two X-chromosomes in the egg pairing, but then
failing to separate, so that either both remain in the mature egg or both
are extruded with the polar bodies.

In a study of parthenogenesis (virgin-birth) further evidence is
brought forth in regard to the function of the X-chromosome.

For example, there may be one maturation division without a reduc-
tion of chromosomes. In this case the single polar-body and the egg
nucleus will both contain the diploid number of chromosomes. This is
quite common in the Crustacea and a few other forms.

Or, there may be two polar divisions, after which one of the polar
bodies reunites with the egg nucleus. Here, again, the full number
{diploid number) of chromosomes are found.

Or, in some forms (Hymenoptera and the male-producing eggs of
Rotifers) two polar divisions really take place, which reduce the chro-
mosomes to the haploid number. If these eggs are unfertilized, they
give rise to males. Such eggs already have only half the full number
of chromosomes. Consequently in their germ cells, in turn, there is no
further reduction. The first spermatocyte division in these is really
suppressed. If the eggs are fertilized they produce females.

In those Hymenoptera where there are two divisions, but the chro-
mosomes divide at the equator and not longitudinally, the diploid num-
ber is retained and females are usually produced.

Various other evidences of great value and interest will be found
in the books on Cytology mentioned at 'the end of this chapter.

The diagram of the chromosome-cycle of Phyllaphis coweni (Fig.
83) will throw light on this subject.

The top group shows a fertilized egg with four ordinary chromo-
somes and two X-chromosomes. Three lines of descent pass downward
from the egg. On the left, this line of descent leads to a female which
will produce a sexual egg. The central line of descent leads to a female
which will reproduce parthenogentically, and on the right the line of
descent leads to a male.

The second and third groupings from the top represent the meta-
phase groups as well as the diagrammatic anaphases of three eggs, of
which the left and middle will produce females, the right a male. In
the female-producing eggs, the X-chromosomes divide at the equator
and not longitudinally ; while in the male-producing eggs they pair and
separate so that the male has only one X-chromosome.

The fifth grouping from the top at the left, is the metaphase group
of the first polar division of the sexual egg. All the chromosomes are

170

GENERAL BIOLOGY

paired. Below this, the anaphase of the first, and the telophase of the
second polar division, leaves three chromosomes in the egg.

The fifth grouping on the right is the metaphase group of the first
spermatocyte division with paired regular chromosomes and a single
X-chromosome. Below this, the diagrams of the first (unequal) and the

,;,::-:;•,; :,-;..., :.,;'"

a

I Ml

Fig. 83.

Diagram of chromosome-cycle of Phyllaphis coweni. See text for explanation.
( After Doncaster. )

second (equal) spermatocyte divisions, leading to the ultimate sperm-
cell with three chromosomes and a small degenerate cell with two.
When the egg and sperm again unite in fertilization, the original six
chromosomes are restored and the egg is again as we see it at the top.

From what has been said, it can be plainly seen that in all organ-
isms where there is an X-chromosome, this extra particle (as it does not

GENETICS 171

.

divide as do the other chromosomes) must result in some sperm-cells
having an even number of chromosomes and others an odd number.

For example, let us say there are 21 chromosomes in the original
germ-cell from which the sperm is to develop. One of the newly form-
ing sperm would possess ten and the other eleven chromosomes. The
regular somatic number of chromosomes in such an organism would be
twenty-two. The egg will, therefore, regularly divide and throw off
eleven to obtain the haploid number. Those eggs which are then fer-
tilized by a sperm containing ten chromosomes become males (as the
diploid number in such a case would again be twenty-one) and those
eggs fertilized by a sperm containing eleven would possess the full
somatic number of twenty-two chromosomes and become a female.

This means that in those cases where there are X-chromosomes,
the odd chromosome never pairs in the maturation division with another
chromosome, nor does it produce a tetrad. It simply passes undivided
to the daughter sperm.

References :

L. Doncaster, "An Introduction to the Study of Cytology."

W. E. Agar, "Cytology."

W. E. Castle, "Genetics and Eugenics."

East and Jones, "Inbreeding and Outbreeding."

CHAPTER XII.

ANIMAL PSYCHOLOGY

In no branch of study is the student confronted with more difficul-
ties in the way of separating fact from interpretation, and explanation
from description, than in the field of Animal Psychology, and this, not-
withstanding the fact that Animal Psychology owes its entire value to
its ability to explain and not -to describe.

The tendency of the human mind to read into -an animal's actions
the same motives and reasons that cause man to react in a similar man-
ner is difficult to overcome. In fact a definite word, anthropomorphism,
is in common use among psychologists to describe just this tendency to
humanize animals.

Still, the only way we have of interpreting the behavior of an ani-
mal must be in terms of human understanding, for we have neither lan-
guage nor imagery which can bring to us the sensations, emotions, and
driving force of an organism so totally unlike ourselves as an insect, for
example.

As one writer has said, anger with us is always associated with an
increase in heart beat and a more rapid breathing, and our nerves are
all "set on edge," but an insect has a totally different set of blood-ves-
sels, an entirely different breathing apparatus and a different nervous
system. What are its accompanying sensations when it feels angry?
In fact, a wasp often bites off its own abdomen when angry. How can
we, when our respective organisms are so unlike, know much about how<
such animals feel?

Further, all of us have observed that probably most plays and nov-
els hinge their plot entirely on some misunderstanding. If human be-
ings, who have a common language to make themselves understood,
are so frequently misunderstood, how much more will we not misun-
derstand and misread the actions of animals entirely unable to tell us
anything in terms which are understandable to both?

It is for reasons of this kind that many throw up their hands in
despair and insist that we never can know anything at all about the
animal mind, but that if we wish to establish an animal psychology
anyway, there is only one way to go about it, and that is, merely to
study the behavior in the laboratory under set conditions so that we
can learn just how each animal reacts to a given stimulus. Such a
method assumes that all animals of the same sex, of the same age, in
the same state of health, will always react in exactly the same way when
the same stimulus is applied under the same conditions.

We shall go on from this point a little later, after the student un-
derstands several important terms.

ANIMAL PSYCHOLOGY 173

Objective and Subjective are two of the most important. The for-
mer is the term applied to all things which come under the senses. That
is, a thing is objective when it can be observed and measured in the
laboratory. It is anything, in other words, which occupies space. Sub-
jective refers to those things which make no observable difference in
space and which cannot therefore be measured in terms of the rule and
scale of the laboratory. For example, changes in the mental world,
such as thought and feeling, are subjective. In the classic sense, sub-
jective means the act of mind itself or what is in the mind, while ob-
jective refers to the matter with which the mind works.

An illustration of these two terms as they are commonly used comes
to mind. Suppose a neurologist were to examine the optic nerve and
the optic centers of the brain of a student while the latter is reading
a letter. The neurologist could probably tell that the optic nerve and
center were functioning, but he could never tell what the latter con-
tained, nor could he see what emotions were called forth in the mind
of the student. The movement in the nerve and nerve center would be
objective, while the emotional impression made on the student would be
subjective.

Not only would the neurologist be unable to observe the emotional
impression made upon the student, but he would be unable to tell why
certain vibrations which, as far as observation goes, are all alike, should
produce sensations of red or green in one case, and another color in
another case.

All our emotions, longings, ambitions, thoughts and ideas, as long
as they remain mental states, are subjective, while when they express
themselves as acts they become objective.

Psychology is the study of the subjective world. The word
Pschyology (Greek psyche=soul+Logcs=discourse) actually means
the -study of the soul, but. since laboratory methods have come into ex-
istence in psychology, and laboratory men think only in terms of meas-
urable substances, it is commonly said to be the study of mental phe-
nomena.

Since the laboratory methods of studying everything objectively
under set conditions has made its way into psychology, the workers in
this field have become divided into various camps or schools. First,
come the Behaviorists, who insist that the results of mental activity are
actions and reactions to given stimuli, and it is only these results which
can be measured, and which, therefore, may validly be used as data on
which to form any theories of the mental life of animals. Second, come
the Introspectionists, who follow the classic method of antiquity. They
insist that the only real way of studying mental life is to introspect —
to look into our own mental life and try to understand how and why
we do what we do under varying conditions. They insist that we must
analyze our own thoughts, motives, and emotions, and then if an animal

174 GENERAL BIOLOGY

has an organization quite like our own, we may validly assume that it,
too, functions somewhat like our own.

Since extremists on any side of a discussion are likely to go astray,
it is always best not to confine oneself entirely to following any single
group to the exclusion of another. To be fair, one must use anything
and everything that will throw light on the problem one is trying to
solve.

The word Mind is another confusing term. By the older writers
it was used to designate the personality of an individual. That is, if one
say with Descartes "I think" therefore "I exist," the "I" which does the
thinking and which does exist is the true personality, the true mind.
Or, one may note that it is quite common to dream that one hns died
and attends one's own funeral. That which can look at its own physical
body as the physical co-partner of the true ego — of the individual's per-
sonality— is the mind, or as the older writers called it — the soul.

Not only do we here see a distinction of the ego or personality
proper, as mind, but we also note that the mind is separate from the
thoughts which the mind brings forth. We can, therefore, understand
these writers when they tell us that the brain is in turn the organ of
the mind, but not the mind itself.

The average laboratory man will have little* of this, however. He
insists that mind does not exist as distinct from thought and emotion.
He means by mind the whole "stream of consciousness" of the individ-
ual— all thoughts such as one has ever had, plus all one's emotions, such
as pleasures and pains — accumulated experiences of the individual, in
other words.

The laboratory men do, however, admit two divisions of this mental
life, namely, consciousness (awareness) and feelings (emotions or affec-
tions, such as pleasure and pain).

The student can understand these two divisions easily if he -will
think of breaking a bone in his body. It is one thing to know (be con-
scious of) that the bone is broken, and another thing entirely to feel the
pain it may cause.

The idea of a difference between the mind and the physical body
containing it, leads us to note the distinction between mind and matter.
Those who accept this distinction are called dualists.

Great conflicts have been waged by the learned of all times as to
which is the more important of the two — mind or matter — and which
was first upon .the scene of existence. Some have contended that mind
(spirit) came first, and this, then, was the cause of the physical universe
(matter). Such contenders are known in philosophy as spiritualists.
Others contended that matter was first on the scene, and that mind was
late in its arrival, because it is only an emanation of some kind from the
physical. That is, mind is something like the secretion from ductless
glands which we know little about, but which we know to exist. Such

ANIMAL PSYCHOLOGY 175

men are called materialists. Yet another group insisted that as mind
and matter are always together, neither may be given the preference.
Both are different sides of the same coin. Each thought-wave is always
associated with a nerve-wave of some kind, and neither can -exist with-
out the other. Such men are called monists. It will be seen that the
term "monists" is applied to this group because they do not accept a
dualism in life.

These different groups of contenders attack psychological problems
with different prepossessions. The spiritualist is likely to call himself
an interactionist in psychology, the materialist a behaviorist, and the
monist a parallelist.

As it makes a profound difference to a patient as to which one of
these theories his physician holds, the student must know what each
term means or he will be totally unable to pass judgment on the many
and conflicting discussions which are ever coming before him.

The interactionist holds that the state of mind of an individual can
and does influence his physical being and vice versa. An example of
this is a man worrying over financial losses, whose body becomes run
down until disease clutches him.

The behaviorist insists that only a definite physical reaction, meas-
urable in the laboratory, is valid data on which to base a scientific con-
clusion, and that until the individual mentioned above shows a definite
measurable reaction, there is no change which we as scientists can use
or accept.

The parallelist, insisting as he does that both the mentality and the
physical organ which is associated with it are different sides of the same
thing, must necessarily consistently claim that the mind is totally un-
able to influence the body and the body totally unable to influence the
mind. In fact, one of their prominent writers says that one may as
well expect a piece of beefsteak put into a sausage machine to come out
a moonlight sonata as to expect either body or mind to influence each
other.

It is therefore only the interactionist who can consistently speak of
nervous and mental diseases, and who can consistently use both physical
and psychic remedies.

At this point we may consider what is commonly designated as*
structural and functional psychology.

Structural psychology concerns itself with (1) the general organi-
zation of an organism, (2) the general organization of its nervous sys-
tem, and (3) the organization of the specialized nerve parts such as the
eye, ear, nose, etc.

Functional psychology is interested in (1) the general way an or-
ganism reacts (discrimination), (2) whether the organism can modify
its action (docility), and (3) in how many ways and in what way its
behavior will vary (initiative).

176 GENERAL BIOLOGY

Again the student must be cautioned not to let. one side of a prob-
lem cause him to discard much that is of value in opposing schools of
thought.

Just as those who are primarily interested in nothing but anatomy
are likely to leave out important functional causes in a disease, so those
primarily interested in physiology are likely to forget the structural ele-
ments which may contribute points of tremendous importance.

All schools of science have been drilling into the student the sup-
posed "fact" that "Structure determines Function," but since the very
recent work of Carey, who converted unstriated bladder-muscle of a liv-
ing dog into living heart-muscle by simulating heart conditions in the
bladder, we must insist that function has just as important a part in
changing and determining structure as structure has in determining
function.

However, we must not forget that even in the case just mentioned,,
the substance to be changed was already present and must have pos-
sessed the potentiality of change before it could be worked upon.

With this introduction we can the better understand the two ways
in which the study of comparative psychology is approached by the
modern laboratory worker. First, we may take a highly developed indi-
vidual, such as man, and after analyzing his mental-world, apply the
knowledge thus gained to the lower forms, or

Second, we may follow up, step by step, the increasing learning-
ability on the part of all phyla of animals, beginning with the unicellular
and passing upward through an ever-increasing scale of ability.

It is this second method which seems to have found most favor with
animal psychologists.

But, in reading works on animal psychology, one is always con-
fronted with a great confusion of terms. In fact, one finds here the
same difficulties that confront the student in any of the biological sci-
ences. The first workers in all these fields were philosophers, and were
interested primarily, and sometimes only, in the human family. The
terms, therefore, which these men used, although worked out with great
precision, applied only to man.

The newer writers took many of the older labels and placed them
on new bottles, so to speak. This has caused a world of confusion, not
only to new students, but to many well versed in language and litera-
ture.

Such words as Mind, Intelligence, Reason, Memory, Consciousness,,
Sensory or Associative Memory, Instincts, and Reflex Actions, are some
of the terms which the student must use, and which have many con-
flicting meanings in modern literature. It is imperative that the student
obtain a clear and concise definition of these terms and use them only
in this restricted sense. Then only can he understand the meaning
which different writers assign these words, and then only can he know

ANIMAL PSYCHOLOGY 177'

whether they are calling other things by the same name or giving dif-
ferent names to the same thing.

We shall have to speak of Instincts immediately, so it is well to-
begin with this term. Instincts are defined as inherited tendencies in
an organism which cause protective reactions when harmful stimuli are
applied. For example, a frog, even after both cerebral hemispheres are
removed, will still scratch the part of its body to which a drop of acid
is applied, and it will even snap at and swallow a fly if placed on the
tip of its nose. Again, a fly will walk, fly, and clean its legs and wings,
after its head is entirely removed, and the writer has kept a decapitated
cat alive for many hours by artificial respiration and caused it to per-
form many instinctive actions such as scratching itself, waving its tail,
etc.

In order to understand Reflex Action,
it is first necessary to know the meaning of
a Nerve Arc (Fig. 84). This latter is
merely the entire nerve-path over which an
impulse passes to a nerve-center and out
again to a muscle cell. It must, therefore,
consist of the nerve-ending of a sensory

Fig. 84. Diagram of the path of a . . 11-11 • "

simple nervous reflex action. nerve (receptor) through Which the inV

pulse is received, the sensory nerve-fiber

which carries the impulse to the nerve-center to join at this point with
a motor-nerve fiber which in turn carries the motor impulse to the
motor-nerve-ending (affector). This motor-nerve-ending is always
located in some muscle fiber. The textbooks often speak of a nerve-arc,
as "a perception with a motor impulse."

A Simple Reflex Action is one that passes over such a simple nerve-
arc without first passing to the higher nerve-centers, or, we may say,
one which does not come into the consciousness of the individual in
whom the action takes place. Such a Reflex Action is, therefore, purely
physical. There is no need of assuming any mental state or sensation
as an accompaniment.

When an individual is born, his nerve arcs are set in some form
or another, so that with one individual the same stimulus will cause
quite a different reaction, than it will in another. But, just because
these nerve-arcs are set in the way they are, the same nerve-arc will
always react in the same way to the same stimulus, if all other condi-
tions are equal. For example, a child may have grown accustomed to
saying "I is" for "I am" and have said it so often that it finds it very
difficult to correct itself. Now, if we constantly force the child to use
the form "I am," the particular nerve-arc which carried the "I am" re-
action will become relatively stronger than the one which carried the
reaction "I is," and then, and not until then, does the latter phrase be-
come a sort of second nature of the child.

178 GENERAL BIOLOGY

So, too, a puppy that has the vicious habit of snapping at pass-
ers-by, can be made to react differently by giving him a whipping sev-
eral times, immediately after he does the undesirable act.

In both these cases memory enters, but only a simple sensory mem-
ory (association memory) which has little to do -with any thought The
impulse (inner stimulus) in the puppy to snap, is great, and so the
"snapping nerve-arc" carries the impulse and the snapping is done; but
the punishment which has been meted out has set up an impulse of an
opposing nature, and as soon as this latter becomes the stronger im-
pulse, the puppy has been trained.

We may say in this case, that the puppy has the desire to snap, and
the nerve-arc which carries this snapping-impulse begins to function ;
but the whipping has caused a new nerve-arc to function at the same
moment, so that a third nerve-arc, that of inhibition, comes into play
and the animal does nothing. This is quite similar to the reaction of
persons in hypnosis. Here an individual is told he cannot bend his arm.
The impulse not to bend the arm is just as strong as the one to bend it,
and so no movement takes place.

An impulse is denned as an inner stimulus.

It is well to bear in mind the foregoing paragraphs as these show
the possibility of two opposing impulses and even two opposing reac-
tions taking place at one and the same time over different nerve-arcs.
Often we read of lower organisms possessing discriminating powers
of various kinds which can be interpreted in quite different ways from
what the writer of such an account would have us believe. We need
only remember that there can easily be one set of nerve-arcs function-
ing for the acceptance of food and another set for the rejection of it, so
that it depends on which set carries the stronger impulse as to whether
the animal accepts or rejects the food. It is by no means necessary to
assume any discriminating ability.

There are also Complex Reflex Actions making use of several nerve-
arcs, sometimes often forming regular chains of reactions. In these
cases, the result of one stimulus sets up another, and so on. In fact
we call such continued setting up of stimuli reflex chains. As an exam-
ple we may refer back to the frog whose cerebral hemispheres have been
removed. If we place a fly on the tip of its nose, that stimulus sets the
''snapping" nerve-arc functioning. Then as the fly is taken into the
'mouth, a new nerve-arc causes a swallowing impulse, which sets up re-
actions of still other nerve-arcs which in turn cause the digestion glands
to pour out digestive substances.

It will be noted that such reflexes are quite useful to the animal,
and it will be remembered that our definition of instinct called attention
to the protective value of inherited nerve-arc actions.

Instincts may be deferred. That is, they may not be observable at

ANIMAL PSYCHOLOGY 179

birth, but come forth only later in life when various glands begin to
pour out secretions which affect many parts of the body.

It is often stated that instincts are the "inherited habits" of the in-
dividual's ancestors. This was Lamarck's idea. But this cannot be,
because many animals lay" their eggs before they themselves have de-
veloped the later characters, which the offspring possess. Consequently
the young would have to inherit a habit that the parent was going to
form later. This is somewhat like saying that because both a mother
and her daughter are divorced, the daughter inherited her divorce from
her mother.

While instincts are made up of reflexes, the reflex proper is said
to affect only one part of the organism while instinct affects the entire
body. That is, we should say the winking of the eye, when danger
threatens, is reflex, while running way from the danger is instinctive.

Instincts really consist of inner driving forces which make the ani-
mal possessing them restless until the instinctive act is performed.
However, we must remember here also that just as with the impulses
already mentioned, there may be conflicting instincts. In such cases the
stronger will come forth. Or both may be equally strong, so that no
action at all will take place.

Recent psychology often speaks of tropisms. As we have seen from
our study of former chapters, a tropism is a movement of some kind on
the part of a living organism. Those who wish to interpret all action
of living organisms in terms of physics and chemistry are fond of using
this term. Such men prefer to cast the term "instinct" to the four winds
of heaven and explain everything in physico-chemical terms. They in-
sist that a caterpillar climbs to the end of a twig on account of the chem-
ical change in its body that is caused by hunger, let us say. New chem-
ical molecules and adjustments are forming, and this makes one part of
the body lighter than another, so that the laws of physics enter and
the heavier part will be followed by the lighter in going downward,
or a chemical affinity of some nature will draw the chemical substance
of the animal toward it. After having eaten the tiny bud on the twig,
a new chemical change takes place and so the animal must, whether
it will or not, obey the next chemical and physical change and descend
from the twig.

Dr. Vernon Kellogg, recently in this connection, called attention
to a scientific friend who explained to Dr. Kellogg that the reason he
took a corner seat in a restaurant was due to a primeval impulse which
made him want to have his body in close contact with the wall. But,
as Dr. Kellogg says, the reason he chose that particular seat was be-
cause he had made an appointment to meet a friend there.

As the principal test of an animal's mental ability is the rapidity with
which it learns, we must know what Learning means. Learning means
the ability on the part of a living organism to vary its actions according

180 GENERAL BIOLOGY

to some definite plan which will show that it has profited by past ex-
perience.

In this connection one is also confronted with difficulties as in
other fields. Suppose one is attempting to see wheth'er an animal can
distinguish between colors and then learn to go to one hue rather than
to another. Suppose now, that the animal does not show any more
inclination of going toward one color than toward another. This by no
means proves that the animal cannot distinguish, or is unable to learn,
that there are two colors. It may mean nothing more than that colors
make so little difference to the animal that there is no reason (motive)
for his choosing one rather than the other. In such an instance the ani-
mal's reaction to both colors would be identical, and one could prove
little or nothing from its behavior.

Another animal may be thought unable to learn because it tries a
problem a few times and then ceases to react at all to the stimulus. This
may be due entirely to fatigue on the part of the organs used, and not
to inability to learn. That is, the nerve receptors may become dulled
or tired by new stimuli which are foreign to the animal in its native
career.

Or again, some sensation may be pleasant to an animal only if sec-
ondary factors are present, such as the taking of food only when fmngry
or when the body is in good health. But surely the rejection of food
does not mean that the animal either can or cannot discriminate between
foods. We often will not eat one kind of food, while another is relished,
or we often will just as readily eat ice-cream, candy, or fruit, and show
just as much desire for the one as for the other, but this certainly does
not mean that we do not know the difference between these three types
of edibles.

Then, too, the state of health makes a tremendous difference in
what an animal will choose. Dogs and cats eat certain plants at certain
times but at other times they will not touch them. But do they not
know the difference between these plants and other food?

Then, too, an animal may be trained to do certain things, but sup-
pose it does these things without having been trained. Can one not
argue as well that the animal merely stumbled upon doing the act, and
then doing it often, the nerve-arcs became fixed and the animal can no
longer help itself? It is now a habit.

Habits are but acts performed by fixed nerve-arcs.

The question may arise as to what difference there would be be-
tween psychology and physiology if all we are to study consists of
nerves and reactions. Really, there would be no difference in content
of the two sciences, the difference would consist in emphasis. The
psychologist lays stress on emotions, feelings, etc., and the physiologist
on the simple observable reaction which follow a given stimulus. The
psychologist, in other words, wants to know how the animal feels and

ANIMAL PSYCHOLOGY 181

what it has in its consciousness when a stimulus is applied and its ac-
tions are changed.

We have seen in our study of past chapters that the unicellular
Paramoecium has only two simple reactions, namely, a backward and
forward movement, while the vertebrate frog can move in many and
varying ways in order to get out of haim's way. Probably all animals
can learn. That is, they can be taught to make some change in their
behavior, but the rate of speed with which they can learn probably be-
comes greater as we ascend from lower to higher phyla. The same may
be said of the complexity of the problems to be learned.

All animals and children when left to themselves learn whatever
they do learn by what is called the trial and error method. This simply
means that they try something and if this is unpleasant or painful, they
try something else. Contrariwise, if a reaction produces pleasure, it is
done again and again until it becomes a habit:

All learning, no matter what it may be, must, however, be based
on instinct in its widest sense. That is, the problem presented must
be something which can be solved by making use of some instinctive
behavior of the animal upon which we wish to experiment. For exam-
ple, a cat can be placed in a closed box in which there is a lever, which,
when pressed, will open a door. Now, cats are excitable and when
excited will begin to leap about. This is an instinctive action. If, while
leaping about, the animal strikes the lever and the door opens, it can
be trained, by enclosing it often enough in the same or a similar box, to
press the lever without going through the leaping first.

Learning, then, really means profiting by past experience. But no
profiting by past experience is possible unless such past experience ia
remembered. Now, such memory by no means must be a definite think-
ing out of a past event and then sitting back and saying "I will" or "I
will not do this again." Most physical experiences, even in man, are
merely non-conscious functioning of nerve-arcs. Neither men nor the
lower animals do any thinking in regard to these simple or complex-
chain-reflex-actions. It is a mere association of one stimulus starting
another and is called, as already stated, sensory, or associative memory.

There may, or may not, be an awareness of doing an act at the time
it is being done. That is, there may be consciousness not only of the
fact that an act is being performed, but there may also be an awareness
of pleasure and pain, accompanying it, though there is little proof that a
definite thought — that is, reasoning — is performed, and that it is then
due to such reasoning that changes of action are made. These learning
acts are in all probability due only to sensory memory.

An example comes to mind. We have all heard some one tell of
a horse that knows when Sunday comes, that being the only day when
the animal does not come out of the stable to be harnessed as soon as
its master appears.

182 GENERAL BIOLOGY

But does this show that the horse can count up to seven and has
a sort of mental calendar on which he checks off the days? By no
means ! All it may mean is that if a horse works six days in the week,
there is a certain feeling of tiredness which has become associated with
it, just as a blind man can tell by his "feeling" how many blocks he
walked and where it is time to turn without counting the blocks.

We can then conclude that all animals may be conscious to some
extent; that is, they may be aware of their actions, although this has
nothing to do with reasoning — with thinking.

The veteran experimental psychologist, the late Professor Wm.
Wundt, said, "Animals never think and humans but seldom," and most
animal psychologists hold to this dictum, if, by "reason" is meant true
thinking, that is, a weighing of two or more sides of a problem and then
by a definite mental act decide or conclude what is to be done. In other
words, thinking means to use abstract ideas and to form conclusions.

There are many writers who mean by the term "thinking" only an
ability to profit by past experience, so we must always find what an
author means by his terms before we attempt to pass judgment on what
he says. Others likewise speak of "Intelligence" which should mean
only the ability to think, as any associative memory. This is really
placing old labels on new bottles and is very confusing to the student
who wishes to know both the past and the present of his science.

The desires of the different men in animal psychology must also be
taken into consideration when reading their respective works. There
are those who wish to show that there is no real difference between man
and the lower animals. These insist that man has nothing distinct from
the lower animals except language, but that man's seeming difference in
the .mental world is only a little greater development of animal charac-
teristics. Language by them is often said to be the cause of man's
greater mental ability in that he can by this means write down his find-
ings so that others may profit by them.

Those who hold that- man is something separate and distinct from
the animal, call attention to the fact that language but expresses
thought, and one must have thought before he can develop a language,
rather than language being the cause of thought. These men also insist
that there is no proof that any animal has ever "reasoned" out a prob-
lem in the way mentioned in an earlier paragraph, and therefore no ani-
mal lower than man can be said to have any "Intelligence" in the classic
sense.

These latter men would say that hundreds of thousands of cats,
dogs, and even apes (which are considered the more intelligent animals)
are very fond of warm places. Such animals have lain before hundreds
of thousands of open fires and enjoyed the warmth. They have seen
their masters keep the fire aglow by placing fuel upon it, and yet not
in a single instance has any animal drawn the very simple conclusion

ANIMAL PSYCHOLOGY 183

that it is the fuel which keeps the fire going, and has, therefore, placed
(without being taught), a single stick of wood on the dying embers.

Not only this, the child when it grows up teaches others, and our
schools and colleges are all arranged for the sole purpose of making a
young man and woman at an early age know what it would take a very
old person centuries to learn by personal experience. No animal is
known to teach another a trick which it itself has learned from a third
individual, unless, of course, the act is instinctive and would have been
learned anyway.

Whether one thinks of man as but a more highly developed lower
animal, or whether one looks at man as a being apart, all agree that,
man can reason, whether he often does or not. All agree that man has
larger brain-hemispheres of finer texture than organisms on a lower
scale ; that he has an upright posture and a more delicate hand ; that
he can use tools, and has the foresight to be able to raise his own food
and to live in cold climes by understanding the use of fire ; and above
all that he is set apart from other creatures not only in having a lan-
guage, but in also having a knowledge of what he should and should not
do — in other words, that he has a moral sense.

So, too, all are agreed that the trial and error method of learning
shows infantile or animal intelligence and not human intelligence. All
education, all colleges and universities have been brought into existence
to present principles, that is, to present a mental and cultural gauge,
so that each individual need not try out every detail of experience for
himself; but, by learning the principles and laws which govern nature,,
he can sit back and "figure out" or " reason out" whether a given con-
clusion can or cannot be true.

This is just as workable in the political and religious world as it
is in the scientific. Here is shown the difference between the educated
and the uneducated man. One must not feel hurt or surprised if an edu-
cated man, knowing his principles and his laws, laughs at one who pro-
poses a problem or a solution of a problem which can be seen to be
erroneous immediately. The uneducated man cannot understand or see
this until it has been tried and found unworkable.

From what has been said in this chapter, if we wish to be sure that
we are right, we must be sure of what a writer means by his terms;
we must be sure that we are not reading too much of our own thoughts
into an animal's acts ; we must be sure that we are consistent in our
interpretations and that if we explain an animal's behavior in terms of
tropisms that we must also interpret man's in much the same way; we
must insist that the observer who is attempting to convince us that his
theories are correct, has a scientific training and knows how to distin-
guish all these things of which we have been speaking. That is, he
must be able to separate fact from inference. We must insist that he
be intimately acquainted with the habits of the animal he is discussing,

184 GENERAL BIOLOGY

so that he will not assume, for example, that an animal, like many in-
sects which have an instinctive impulse to cover up obstructions, is
showing great intelligence when it covers up a minute stream and thus
forms a bridge and crosses it; we must insist that he know the past
experience of the particular animal he is discussing so that he will not
confuse an associative memory with true intelligence ; we must insist
that he has no personal affection for the animal and thus wants to make
it "show up" well ; and lastly, we must insist that he do not let his de-
sire to tell a good story gloss over important details and leave out
others.
References :

John Watson, "Behavior an Introduction to Comparative Psychol-
ogy-"

M. F. Washburr, "The Animal Mind."

S. J. Holmes, "1 he Evolution of Animal Intelligence."
Eric Wasmann, "Instinct and Intelligence."

CHAPTER XIII.

INTERMEDIATE ORGANISMS

One of the interesting findings of biology is that it is sometimes
impossible to distinguish between certain plants and animals in some
of the lower forms. The classic example of this is the plant-animal
Haematococcus consisting of a single cell and moving about by flagella.
It will be remembered that Euglena viridis has chlorophyl in the
body and is classified as an animal. One of the great and outstanding
characteristics of plants" is that most of them possess chlorophyl if they
grow in the light, and that they are capable of manufacturing their own
food by virtue of this fact. (See Chapter on the Chemistry of Living
Matter.)

Pleurococcus (Fig. 85) commonly
studied in the laboratory is a close rela-
tive to Haematococcus. It is a one-celled
organism found commonly on the north
. side of trees, moist rocks, and wooden
fences, dull green in color, and powdery
when dry. When moist it becomes
brighter in color and slimy to the touch.
It is found in practically every part of the
world on the shady and moist sides of the
objects mentioned above.

Under the microscope it is found
that this substance consists of thousands
of tiny single-celled organisms to which

the name of Pleurococcus has been given. There is a definite cell wall
and a nucleus. The chloroplast, however, obstructs a view of the nucleus
in the unprepared cell. The organism reproduces by simple fission and
has a tendency to form clusters or colonies usually of from two to ten
or twelve cells. When this occurs the cells assume a more or less irreg-
ular shape due to the pressure of the adjoining cells. The nucleus lies
near the center of the cell and contains one or more nuleoli. The net-
work of the nucleus can also be distinguished.

In the cytoplasm will be found the chlorophyl-bearing organ or re-
gion called the chloroplast. Due to the chlorophyl this will appear
bright green, but if the cell be placed in alcohol the chlorophyl will be
dissolved out leaving the chloroplast grayish. It is important to note the
distinction between the chloroplast, which is a living organ of the proto-
plasm, and the chlorophyl, which is simply the green pigment contained
in the chloroplast.

Fig. 85.

A. Pleurococcus.
B and C. Haematococcus Cells.
(Greatly magnified.)

186 GENERAL BIOLOGY

Photosynthesis.

In the final analysis every particle of food an animal eats must come
from and through the plant world. For example, when man eats a piece
of steak, the animal from which it was taken lived either directly on
plants or on other animals which fed on plant-life.

Here it is well to appreciate the interesting way Nature has of
keeping a sort of balanced quantity of all needed organisms, for the
meat-eating animals or carnivores do not allow an overproduction of,
plant-eating animals or herbivores, and are prevented from multiplying
too rapidly by parasites in their own ranks, while much of the vegetable
world is saved because animals eat each other.

The thought of these facts has led to the statement that the impor-
tant thing in life was to get enough to eat and to-prevent one's self from
being eaten.

The plants manufacture their own food from the substances they
can extract from the surrounding soil and the air. Plants are there-
fore not dependent upon other animals or plants for their food as ani-
mals are. Those organisms dependent upon other living organisms for
their food are said to be heterotrophic ( ) in nutrition

while those which can manufacture their own food are said to be auto-
trophic ( ) in nutrition.

But only those plants which possess' chlorophyl are autotrophic.
Therefore fungi, molds, and most bacteria, which are plants, but which
have no chlorophyl, are heterotrophic; and, being obliged to live upon
other organisms, they are parasitic ( ) or sapro-

phytic ( ).

Chlorophyl is either contained in a chloroplast, as already stated, or,
in the simplest form of green plants, it is scattered throughout the pro-
toplasm. Chemically "chlorophyl is a complex compound of carbon,
hydrogen, oxygen, nitrogen, and magnesium ; its probable empirical
formula is given by one investigator as Cr,4H72OBN4Mg. While not
a constituent of chlorophyl, iron is always present in the chloroplast
and seems to be essential to chlorophyl formation. Either in solution
or in the living plant chlorophyl absorbs part of the light which falls
upon it." The energy of the light thus absorbed by the chloroplast is
what enables the plant to perform its work. As light is required this
process goes on only during the day.

"The materials from which carbohydrate food is manufactured by
green plants are two in number, carbon dioxide and water. Carbon
dioxide is present in the atmosphere in the small but constant concen-
tration of about 3 parts per 10,000 parts of air, and is therefore readily
available to such plants as the Pleurococcus. Water is absorbed directly
from the substratum through the cell wall into the protoplast. The car-
bon dioxide taken in is dissolved in the water in which it is readily
soluble. While the exact steps in the process of formation of carbohy-

INTERMEDIATE ORGANISMS

1ST

drate foods from these substances are not yet clear, the essential facts
are well established. The carbon dioxide and water are partially or com-
pletely reduced to their elements, which immediately recombine to form
a monosaccharide sugar (probably dextrose) with the freeing of oxygen.
These two processes are represented by the reaction 6CO5-|-6H2O—
C6H12O6-|-6O2. The oxygen is given off into the atmosphere through
the cell wall. The sugar is the primary food of the plant, being the
principal material used in the synthesis of other foods and in the pro-
cesses of metabolism. When
it is produced in excess of the
immediate requirements a fur-
ther reaction takes place by
which some of the water is
eliminated and the sugar is
"condensed" into starch ; this
reaction is n(C6H12O0) =
(C6H1005)n+n(H20).

"This starch is deposited in
the chloroplast as granules or
"starch grains" and forms a re-
serve food supply for the cell ;
in green plants kept in dark-
ness the starch grains soon
disappear ~ and reappear only
after the plant has again been
in the light for a considerable
period of time. In some plants,
e. g., Vaucheria (Fig. 86), the
excess food is stored in the
form of a fat or oil, but it is
probable that here also the first
food formed is a sugar.

"This process by which car-
bohydrates are manufactured
in green plants is called photo-
synthesis; its essential fea-
tures are summarized as fol-
lows: The materials used are
carbon dioxide and water; the
energy is obtained from sun-
light absorbed by chlorophyl ;
the chloroplast by the use of
this energy brings about a
chemical synthesis of the materials, resulting in the freeing of oxygen

Fig. 86.

I. Asexual Reproduction of the Green Felt

(Vaucheria).

A, formation and discharge of the large, many-
ciliate zoospore from the terminal sporangium ; B,
the zoospore showing the ciliated surface ; C, sec-
tion through the surface of the zoospore showing the
pairs of cilia above the nuclei and the layer of plastids
beneath ; D, germination of zoospore ; E, young plant
of Vaucheria, the two filaments having arisen at op-
posite ends of the zoospore, one having developed an
organ of attachment or holdfast h ; F, a group of
plastids, the lower in process of division. (A, B,
after Gotz ; C, after Strasburger ; D, E, after Sachs. )

II. Sexual Reproduction of the Green Felt

(Vaucheria).

A, Vaucheria sessilia ; o, oogonium ; a, antheridi-
um ; 08, the thick-walled oospore, and beside it an
empty antheridium ; B, Vaucheria geminata, a short
lateral branch developing a cluster of oogonia and a
later stage with mature oogonia o and empty an-
theridium a ; C, sperms ; D, germinating oospore.
(From Bergen & Davis "Principles of Botany" by
permission of Ginn & Co., Publishers. C, after
Woronin ; D, after Sachs. )

188

GENERAL BIOLOGY

and the production of a sugar, some of which is usually transformed into
starch and stored in that form."

There are mineral substances also necessary for the plant to carry
on its life-processes such as magnesium and iron. Nitrogen, potassium,
phosphorus, calcium, and sulphur are also required by most plants, al-
though it is to be understood that there are very minute quantities of
these in so simple a plant as Pleurococcus.

The process by which proteins and fats are built up is not known
in detail, but it is supposed to be due to the action of enzymes. The
fats occur in Pleurococcus at those times when the plants become dry
and are inactive or in a resting condition. At such times little or no
starch is formed while fats are present in quantity.

YEASTS

The Pleurococcus just studied, though a simple single-celled plant,
is quite complex when compared with a yeast cell. The yeast cell is
merely a small mass of granular cytoplasm with various vacuoles scat-
tered about. These vacuoles must not be mistaken for nuclei. Often
there are little buds (Fig. 87) on the side where a new cell is forming,

:•'-.'•?••;•
ViVvf

Fig. 87. Yeast Cells.
n, Nucleus ; v, vacuole ; a, ascms.

Fig. 88. Various Forms of Bacteria.

a, Spirillum \ b, Bacillus typhosus ; c.
Staphylococcus ; d, e, j, h, Micrococcus ; f, k, I,
Bacillus; g, Pauedomonas pycocyanea; i, strep-
tococcus. (From G. Stuart Gager's "Fundamen-
tals of Botany," by permission of P. Blakis-
ton's Sons & Co., Publishers.)

and sometimes three or four cells will form surrounded by a single wall,
in which case the outer wall forms an ascus ( ),

and the cells contained therein are ascospores.

The nucleus may be shown by special staining processes.

Yeasts have been called organized ferments because fermentation is
actually associated with the life of the yeast-cell. That is, there are
enzymes within the cell (intracellular) which act through the living
protoplasm which produced them. They are not poured out as in the
saliva or the pepsin (extracellular).

This power of producing fermentation possessed by yeasts is still
retained even though the plant itself be killed with alcohol, ether, or
acetone. So, too, the bacteria which cause lactic acid in milk may be
apparently killed, thus losing their power to perform any of the normal
vital actions such as growing and dividing, and yet be able to produce
lactic acid.

INTERMEDIATE ORGANISMS 180

Yeast reproduces by budding [(also called gemmation) ( )].

A valuable study by the great French bacteriologist, Louis Pasteur,
has shown that various inorganic substances could be made into a fluid
and if the yeast cells were placed therein they could utilize it for growth
and reproductive purposes. This ability to use and manufacture new
substances from wholly inorganic matter sets the yeasts apart as being
a sort of intermediate grouping between even the lowest plants and the
inorganic world.

Yeasts must have oxygen, however, to carry on their work, the
anaerobic ( ) bacteria are the only exception among

living organisms in not needing oxygen.

It must be remembered that the yeast cell is an organism and is
already existent, only making use of these inorganic substances by con-
verting them into proteins and carbohydrates, by virtue of the chemical
enzymes within the yeast cell itself.

Yeasts work at temperatures from 9 to 60 degrees C. When fer-
mentation takes place, as in bread, the temperature is raised during the
fermentation process by the release of energy.

Yeast secretes an enzyme which is a sugar ferment. This enzyme
may, for example, convert starch into sugar, although yeast "utilizes
only about 1% of the sugar, and decomposes the remainder into carbon
dioxide and alcohol. The reaction of the fermentative decomposition
may be expressed as follows :

Sugar Alcohol Carbon dioxide

C6H1209 2C2H80 + 2C02

It is the production of these two by-products that makes yeast com-
mercially important. Yeast produces the same reaction in the sugars
of cider and wines, and in the metamorphosed starches of the cereal
grains, that are chiefly used in industry in the production of alcohol.
The carbon dioxide is also utilized in the making of bread. Yeast is
mixed with the dough, and, fermenting in it, evolves the carbon dioxide
gas, which "raises" it, making it porous, and improving its digestibility
and flavor.

"If a little fresh yeast be sown in a bottle of Pasteur's solution (or
even in a 15% sugar solution made with tap water, which will be likely
to contain enough of the mineral salts for considerable growth), and
kept in a moderately warm place, within twenty-four hours abundant
growth will be evidenced by the increasing turbidity of the liquid, and
by the taste of the alcohol in it and by the odor of the escaping carbon
dioxide* arising from it. It may be demonstrated by examination of a
drop of the fluid with the microscope."

*A simple chemical test of the presence of CO2 in the escaping gas may be made by thrusting a
glass rod with a drop of lime water suspended on it into the mouth of the culture bottle. The calcium
oxide (CaO), of which lime watf-r is a solution, readily unites with free carbon dioxide to form a
white precipitate of calcium carbonate CaCO3 (CaO -|-COa=CaCO3) which may be seen to form
in the drop.

190 GENERAL BIOLOGY

BACTERIA

It is common to hear discussions regarding- germs of various kinds.
Such discussions usually pertain to all those plants and animals which
are likely to cause disease. Bacteria, however, refer to very minute plant
organisms classified under the chlorophyl-less fungi (mycetes), under
the general grouping of schizomycetes ( ).

While most diseases are probably due to, or associated with bac-
teria, very few bacteria, relatively speaking, cause disease, the great ma-
jority of them being of undoubted value to other living organisms.

There are three general shapes after which bacteria are named. The
bacillus is rod-shaped (b, f, k, 1, Fig. 88), the coccus (c, d, e, f, h, Fig.
88), (sometimes micrococcus), is berry-shaped or spherical, and the
spirillum is spiral-shaped or merely curved something like a comma
(A, Fig. 88). Bacteria may be so small that only many thousands
together form a spot sufficiently large to be seen under a high power
microscope, or they may be of relatively large size. That is, they vary
from less than 1 micron (the measurement used in microscopy, meaning
1-1000 of a millimeter, or 1-25000 part of an inch) to 30 or 40 microns.
It has been estimated (Migula) that there are 1272 distinct species of
bacteria.

Not only do bacteria vary according to shape, but as to their method
of growth under varying conditions of temperature and surrounding
substance.

Bacteria may possess cilia or flagella and move quite rapidly. They
reproduce by simple binary fission. The spirillum and bacillus divide at
right angles, usually lengthening slightly before division. Cocci may
divide in different planes and various names have been assigned to them
on this account. If they divide into two parts but remain attached they
are called diplococci ( ). If they continue dividing

in one plane in this way but remain attached so as to form chains they
are called streptococci ( ), if they divide in two

C-.

\

e\

f *M' «8>?. ^*t
' •? *aM? «**

Fig. 89. Various Groupings of Spherical Forms of Bacteria.
a, Tendency to lancet-shape; b, coffee-bean shape; c, in packets (sar-
cina), d, in tetrads; e, in chains (streptococcus) ; /, in irregular masses
(staphylococcus). Magnified 1000 diameters. (After Fliigge. )

INTERMEDIATE ORGANISMS 191

planes they are called staphylococci ( ), and if in

three, sarcina ( ). ( Fig 89.)

Sometimes the protoplasm of bacteria breaks up into a number of
bodies within the cell. These bodies are called endospores ( ).

The value of this breaking up is supposed to be similar to that of
encysted amoeba ; namely, to permit the organisms to await some more
favorable feeding period and environment. During this spore state bac-
teria are very resistant.

The sterilization of various substances in the laboratory takes spor-
ulation into consideration, so that when a substance is to be sterilized,
it is placed in a temperature of 50 or 60 degrees C. for several days in
succession, rather than at a higher temperature at one time. This per-
mits the spores to germinate. As spores are hard to kill while in the
spore-state, but readily succumb when placed in a 50 to 60 degrees C.
temperature after germination, it wrill be seen that this intermittent
sterilization is the best method so far known.

"Fischer divided bacteria into three groups, according to the na-
ture of their metabolism. (1) Bacteria which are like the green plants
in requiring neither organic carbon nor organic nitrogen. These are the
so-called prototrophic bacteria, which possess the remarkable property
of being able to build up both carbohydrates and protein out of carbon
dioxide and inorganic salts. (2) Bacteria which need organic carbon
and nitrogenous compounds. These are called the metatrophic bacteria.
(3) The paratrophic bacteria which live as true parasites and can exist
only within the living tissue. This group cannot manufacture its own
food and is like other animals in this respect. The metabolism of bac-
teria may then show all of the phases already described for green plant
cells and for animal cells as well as certain additional phases. The food
is absorbed directly through the cell wall and is as varied as is their
habitat. There seems to be no form of organic substance living or dead
that may not serve as a source of food supply for bacteria, so that the
enumeration of their foods becomes practically impossible. A special
phase of the metabolism of bacteria is illustrated in their relation to
nitrogen compounds. Nitrogen in an uncombined state cannot be used
as food energy by most plants. It is obvious that the amount of am-
monia, nitrites, and nitrates would soon become exhausted unless there
were some way of supplying more of the nitrogen compounds. Many
of the soil bacteria are prototrophic in habit and carry on the important
work of combining the free nitrogen into a form that can be used by
other organisms. The several nitrogen combinations are effected
through the agency of several kinds of bacteria. There are also bacteria
which live in the roots of certain plants, like clover, beans, and peas,
which are able to utilize the nitrogen of the air. All of the higher forms
of plants and all of the animals are dependent upon microscopic bacteria
for their nitrogen. It would be very strange if the character of meta-

192 GENERAL BIOLOGY

holism which is so fundamental in living things should be essentially
different in bacteria; it probably is not, and so the usual steps in assimi-
lation and dissimilation may be assumed to take place in bacteria. Dur-
ing this process enzymes are utilized and toxins produced."

Bacteria increase with marvelous rapidity by becoming larger in
size, followed by a division of each organism into two. If each of these
divide every half hour, in twenty-four hours a single bacterium will have
become something like 17,000,000 individuals. It can be seen quite
readily that such a tremendous increase in so short a time means that
vast quantities of food must be at the bacteria's disposal, or the organ-
isms themselves must die. If they are then in the body of an animal,
the effects of the poisons produced by their dead bodies may be an im-
portant factor in injuring the host.

However, comparatively few types of
bacteria are pathogenic. Most of them
have some useful function. They are the
chief agency . in decomposition and decay
by which they help to restore organic ma-
terials into the general circulation of na-
ture's economy.

Bacteria spoil food and rot substances
which then become soil fertilizers ; they
sour milk and ripen cheese; they break

Fi*' 90RoX^dM(cS£.°n the down tissues in disease, and aid in diges-
i, section of ascending branches; tion. They, therefore, do much that makes

6, enlarged base of stem ; *, root- ...

tubercles containing bacteria. life in the higher organisms possible, while

at the same time doing many things which cut that life short.

While it was only after microscopes were invented that bacteriology
could become a science, still it has always been known that acid solu-
tions and salt solutions keep food from spoiling and that heavy sugar
solutions do the same. Thus it was possible to pickle and preserve foods
and to make jellies.

Bacteria require heat and moisture for their growth, so that fruit
and meats can be dried, and by preventing one of the important factors
for bacterial life from being available, such meat can be preserved for
great periods.

Drugs and chemicals which prevent the growth of bacteria are
known as antiseptics. Thus, wine was used as an antiseptic by the an-
cients which they poured on wounds. We use alcohol to-day instead
of wine.

In agriculture there are certain soil-bacteria which produce tiny

INTERMEDIATE ORGANISMS 193

galls ( ), commonly known as tubercles (Fig. 90),

on the roots of clover and other leguminous ( )

plants. These serve a very important purpose in that they derive nitro-
gen directly from the air and supply it to the clover. This makes it
possible for clover to grow in soil very poor in nitrogen, while the over-
production of nitrogen leaves the soil richer than it was before.

The galls themselves are filled with rather large x- and y-shaped
bacteria, easily seen under the microscope. These bacteria die, and
the nitrogen which they contain is added to the surrounding soil, some-
times directly, and sometimes through the intermediate plant to which
it is attached.

CHAPTER XIV.

IMMUNITY

Whether the study of biology be taken up by those who intend
practicing medicine, or for general cultural purposes, the fact remains
that all of us, at some time in our lives, require the se vices of a medical
man. Likewise, all of us who make any pretense whatever at being
college men and women feel, and rightfully so, that unless we can in-
telligently follow at least the ordinary scientific articles appearing in
various magazines and journals written for educated men and women,
there has been some radical defect in our instruction.

In view of the fact that practically all modern medicine is based
upon the theory of immunity, neither the medical man, the medical stu-
dent, nor the educated man at large, can intelligently discuss or intelli-
gently understand anything that muy be told him regarding himself or
the method of treatment suggested when disease comes to him, unless
the theory of immunity is understood.

The subject of immunity is rather difficult, in fact, probably one
of the most difficult that confronts the first and second year student of
biology ; but his ability to grasp and understand the theory is, in a way,
a test of his ability at understanding and applying the knowledge he has
gained in biology.

As all coelomates have their bodies arranged as one tube lying
within another, if one could draw out any coelomate body lengthwise,
the outer part would appear as a tube with very thick walls, while the
gastro-intestinal-tract would form an opening through the entire body.
In fact the whole body would appear quite similar to an ordinary thick-
ened gas pipe (Fig. 164).

One can readily understand that the opening in the gas pipe is really
subject to the same atmosphere and environmental conditions that the
'outside of the pipe may be. So, too, the intestinal tract with all its
<iiverticula is really outside of the body in so far as the atmospheric sur-
roundings are concerned. In fact, the interior portion of the gastro-
intestinal-tract is just as much outside the body (although not quite as
much exposed) as is the skin on the outer surface.

Now, the surface on the inner side of the gastro-intestinal-tract, just
as the skin, forms a layer that can, under certain conditions, be pene-
trated by either physical, chemical, or living substances. We know that
we can scratch or cut ourselves. This results in a physical injury. We
know that poisons are chemicals which can injure tissues whenever
such poisons get into the system, and we also know that living organ
isms such as bacteria (unicellular organisms from the plant world), and

IMMUNITY 195

protozoa (unicellular organisms from the animal world) can take up a
sort of parasitic life within other living forms. Whether a given sub-
stance injures chemically, or whether a living organism is to find its
way into another and injure it, depends upon whether or not the foreign
substance or organism can penetrate through the outer skin surface of
the body or through the surface of the gastro-intestinal-tract. It is only
when such foreign objects are able to get within the body proper (that
is, within the space between body surface and intestinal surface) that
injury results.

Probably most of such injurious substances are taken in through
the mouth and later find their way through the more or less delicate
lining of the gastro-intestinal-tract. It is of value to note that the
seventy of a burn, in the area affected, depends upon the length of time
the particular area remains in contact with either fire or acid. This
means that a deep or severe burn in a localized area may cause death,
while a less severe burn spread over a greater area might not.

Now, if a liquid, which can neutralize or wash away the given sub-
stance, could be thrown upon the acid at the time it is spilled, such acid
would be washed away immediately and little if any harm would be
done. So, too, in the gastro-intestinal-tract, an injurious substance that
may find its way therein, may be neutralized or washed away if a suf-
ficient quantity of neutralizing fluid is secreted or passed through the
intestinal tract.

Therefore, two things must be kept in mind when discussing a sub-
ject of this kind: the strength or power of the injuring agent, and the
length of time the injuring agent is in contact with a susceptible surface.
In fact one may add a third factor, for there is a possibility of a foreign
substance being taken into the system which may so affect the regen-
erative abilities of the host, as not only to prevent healing of a wound,
but which will actually continue to irritate and injure more than the
original injuring agent.

Once the injuring agent has entered the body the question arises
as to the method by which it injures -the host. It must be remembered
that living organisms, whether they be bacteria or protozoa, are in turn
subject to the same laws that govern the life of the host itself. Some
of the larger parasites, such as tapeworms, really remain within the in-
testinal tract and use the food of the host before the host himself derives
the benefit of what he has eaten. There are also parasitic protozoans,
such as the malarial parasite, which, once it has entered the blood
stream, actually eats out the center of the blood cells. Then there are
those which use some part of either the blood or other tissues of the
body for food and in this way injure the host; or again, there are those
which use but a very small quantity of the host's food and are conse-
quently not particularly injurious to the host on that account, but the
various excreta ejected by these parasites may prove injurious either as

196 GENERAL BIOLOGY

a mechanical obstruction of some kind or as a chemical poisoning. And
still again, there may be various poisonous substances formed by the
parasites themselves which will injure the immediately surrounding
tissues of the host only in the location of the parasites ; or, the poisons
may be soluble in the blood stream and in this way pass throughout the
entire body, injuring many regions. And there is still another way by
which injury is brought about by parasitical invaders. There are cer-
tain bacteria and protozoa which require considerable oxygen for their
life processes. The red blood corpuscles have become red by coming in
contact with the air in the lungs and absorbing oxygen which they then
distribute throughout the body. If the parasite, however, takes this
oxygen from the red blood cells, only carbon dioxide and carbon monox-
ide will remain. Carbon monoxide is "coal gas," a gas which often
asphyxiates men working in the coal mines. It therefore follows that
one may actually be "gassed" and die of this "gassing," should there be
parasites in the body which remove the much needed oxygen from the
red blood corpuscles. In such instances where oxygen is withdrawn,
death results almost immediately.

Then there may be all manner of mixed infections, as they are
called. Just as it requires fire in order to cause powder to explode, so
there are certain chemical substances as well as living organisms which
by themselves do little harm or injury; but, when a second or third sub-
stance mixes with them, may prove quite injurious. Conversely, a single
substance may be quite injurious, such as either an acid or an alkali,
but when the two are mixed they neutralize each other and no active
injury is brought about.

Everyone knows that no two people are exactly alike, in their ability
to resist disease and that one person may tolerate a much greater injury
than another without succumbing to it. Most of us have probably read
of the ancient king who, being afraid that an enemy might poison him,
took small doses of various poisons daily so that in due time he could
take great quantities without its having any injurious effect upon him.
That is, his toleration for this specific poison grew, and his body was
able to resist the usual injury caused by such poison. That is, his sys-
tem became insensible to specific poisons which were thus unable to
affect him injuriously because an immunity to these poisons had been
set up.

Resistance, tolerance, and immunity may be classified in various
ways, such as racial, familial, and individual. As an example of race
immunity we have those groups of individuals living in the tropics who
do not succumb to the various tropical fevers that affect a stranger al-
most immediately. The classic example, however, is that of the Jews,
who having fought tuberculosis for thousands of years, are now more
immune than any other known race of mankind. The negroes and In-

IMMUNITY 197

dians, on the contrary, never having had tuberculosis until the white
man brought it to them, succumb quickly.

If certain families seem to be more or less immune we call this a
familial immunity, and if only an individual is intensely resistant to a
given disease or injury, we speak of it as individual immunity. Im-
munity is also divided (1) natural immunity (under which racial im-
munity can be classified), and (2) artificial immunity. The second of
these divisions is again subdivided into active and passive immunity.

Bacteria either contain poisonous substances (endotoxmes), as
does the Typhoid fever bacillus, or they produce poisonous substances
(merely called toxins or ectotoxines) as does the Diphtheria bacillus,
Tetanus bacillus, etc.

Now, if these toxic substances are distributed within the body, the
body tries to protect itself by manufacturing antitoxins, which are op-
posing substances for the purpose of neutralizing the poisons and thus
preventing them from injuring the system. If the toxic poisons are not
too severe, the antitoxin prevents a disease from forming.

As an example we may cite Typhoid fever. Here the antitoxin is
not only manufactured, but actually remains in the body of the patient
for some years after the disease has passed away. The great quantity
of antitoxin present can, during these years, thus prevent another attack
of the disease if the Typhoid bacillus again gets into the body. Such
an individual is, therefore, during the time the antitoxin is present in his
body, immune to new attacks of Typhoid fever. He is immune because
his body produced immunizing bodies which protect him. His body is
active in producing these immunizing bodies, and such immunity is
therefore called an active immunity.

An animal which, in a specific disease, also builds up such anti-
bodies or immunizing bodies, may have these antibodies removed from
its blood. The liquid part of the blood which contains the antibodies
is called an antibacteriological or antitoxic serum. Such sera are then
injected into human beings, and as the person into whom they are in-
jected, then has protective substances which he does not otherwise have,
he (without having his own body manufacture the antibodies), becomes
immune for a certain length of time to the specific disease for which the
antibodies were manufactured in the animal's body. Such an immunity
is therefore called a passive immunity.

It is upon principles evolved from these facts that the various
vaccines have been brought forth against Cholera, Typhoid fever, Small-
pox, etc., as preventive measures, as well as the therapeutic sera injected
after the disease is present, as in Meningitis, Diphtheria, Tetanus, etc.

We know that various chemical substances have definite affinities
for each other, and to explain the mechanism of immunity it is assumed
that every cell in the body has some particular chemical attachment or
affinity. Let us say a certain molecule connected with each cell has an

198 GENERAL BIOLOGY

affinity for various substances that pass through the body. This mole-
cule is called a receptor. We know that normally, as blood passes the
different cells of the body, the cells have a selective action, that is, they
practically reach out and drink in what they need. One of the experi-
ments performed on Pararnoecia demonstrates what is mea,nt by this
chemical selective action. It is there shown that certain chemical sub-
stances such as a sugar solution may cause the animal to go in an oppo-
site direction, but if it has once gone into the solution it will not again
leave. This selective action which all cells probably possess to some
degree, may work on a similar basis ; that is, normally, the molecule
(the receptor) draws to itself the particular food that it needs as the
blood passes. But, just as Pararnoecia may actually enter the sugar solu-
tion or even various injurious solutions, so the molecule or receptor may
also sometimes take or select from the passing blood poisonous or toxic
substances and unite them with itself. This, of course, injures the cell
to which the receptor is attached.

We know from ordinary observation that whenever we injure our-
selves sufficiently, a scar forms. Then, too, it will be noticed that the
scar is almost always slightly elevated. This means that more scar
tissue has actually formed than there was skin before. From micro-
scopic studies we find that whenever those particular cells known as
fibroblasts (which form a goodly portion of the connective tissue ele-
ment of the body) are injured, they grow much more rapidly and pro-
fusely than they did before such injury took place; in other words, if the
fibroblasts are injured, more connective tissue will grow in the region
of injury than grew originally. Once an injury takes place and regen-

Oeration or regrowth begins, there
i "l^ c: "***^\ *s usually an excess of such re-

l j-J ft ._ jJr generation or of such growth.

With this in mind it is easy
d. a. c. to understand that when a mole-

Fig. 91. Diagram Illustrating the Factors cule Or TCCCptOr has anchored t6

Concerned in Immunity. . 1f . /-,-,. r\i \ i • i

cl.. the cell to be dissolved; c., the com- *tself a POlSOn (Fig. 91) which

plement or solvent by which it is dissolved; injures the Cell tO which the FC-
a., the amboceptor or intermediate body by

which the two can be brought together. CCptOr IS attached, the Cell

grow several receptors where it had only one before. Such excessive
production of receptors causes a portion of the receptors to be thrown
off from the cell. These separated receptors then find their way into
the blood-stream. The receptors in the blood-stream are able to anchor
poisons to themselves just as when attached to cells. This means that
there are great quantities of these receptors taking up the poison that
would otherwise injure the various cells with which the poisons might
come in contact. The receptors thus prevent injury to cells which nor-
mally would be open to attack.

Certain conditions, however, must be fulfilled before the receptors

IMMUNITY

can unite to themselves the poisonous substance, and the condition nec~
essary in this instance is that a certain ferment-like substance called a
complement (or alexin) be present. These are protectors against infec-
tion.

Complements can be demonstrated to exist in the laboratory. When
blood serum is placed in a test tube, the receptors do not 'unite with the
toxin if a very small amount of heat is applied to the serum proper.
Heat kills or paralyzes whatever it is which makes the union possible.
If, however, we add but a very small amount of unheated serum, the
union takes place almost immediately. Whatever it is that has been
destroyed by the heating and which permits or causes the receptor to
unite with the poisonous substance is called the complement.

It is quite possible that certain cells of the body, or even all the
cells of some animals, may have no receptors at all for certain poisons,
and therefore, such cells and animals would have a natural immunity
toward those poisons. It is because the thrown-off receptor needs the
complement before it can anchor the poison that they have been called
amboceptors.

The foreign body or substance is called the antigen, while the am-
boceptor produced by the action of injurious antigens is known as
the antibody.

It is of great importance to know that the molecule which is the
amboceptor is decidedly specific. That is, an amboceptor will react only
to one specific foreign substance, so that antibodies formed in diphtheria,
for example, will not be the same as those formed in tetanus, nor will
they be able to assist in anchoring poisons produced in tetanus.

Quite naturally, the rate and ability of metabolism in a cell will
determine how rapidly receptors are formed, and consequently will de-
termine how rapidly immunity can be brought about. This means, in
turn, that if the poisons can act more rapidly than the cells, the cells
as well as the possessor of those cells will succumb.

Phagocytes (white blood cells which devour foreign substances) are
also subject to this same rate and ability of metabolism. Some phago-
cytes may devour a foreign body before the latter has time to bring-
about an injury.

Some phagocytes may have no chemical substance within them
which can dissolve the invader, and so the invader may continue to live
even though engulfed by a phagocyte, or, the invader may even kill the
phagocyte.

Then it must be remembered that in all parasitic organisms the
same conditions largely apply which apply in the host, so that just as
the host may strengthen his resistance so the parasite may strengthen
its virulence so as to overcome the increasing resistance of the host.
For example, capsules form about the bodies of the anthrax bacillus and
the pneumococcus, which makes them more resistant to any injurious

200 GENERAL BIOLOGY

substances of the host. And these capsules only form in the body of a
host where some kind of immunity is possible. In cultures in the labora-
tory, where no immunity is brought into play, capsules do not form on
the groups mentioned.

The encapsulated forms are not subject to phagocytic action, and
some even continue to produce more and more powerful poisons to in-
jure the unlucky phagocyte which may devour it.

It is assumed that inflammations and fevers probably cause an in-
creased production of phagocytes and chemical neutralizations to pro-
tect the body in injury and disease.

The amboceptors anchor soluble poisons only when the complement
is present.

Similarly, phagocytes will not engulf bacteria unless the bacteria
have first been prepared for such engulfing by a substance in the normal
blood serum similar to the complement called opsonins. If an
animal has already been immunized by repeated introduction of bac-
teria, still more resistant bodies called bacteriotropins appear. These
bacteriotropins (which are only a sort of outstanding opsonin in immune
sera) act as opsonins and prepare the bacteria for the phagocytes.
Opsonins, bacteriotropins, and in fact all substances which prepare for-
eign substances for the phagocytes are called cytotropins.

When foreign bodies of any kind dissolve body substances they arc
said to be cytolytic (cytes-cell-j-lysis-dissolving) if they dissolve the
cytoplasm ; haemolytic, if they dissolve the red substance (haema—
blood) in the blood cell ; hepatolytic (hepar=Hver) if they dissolve liver
cells, etc., etc. Such lysins are usually antibodies.

If a reaction can be produced which will cause bacteria or cells to
clump together, such clumping is called agglutination, while the sub-
stances in immune sera which cause agglutination are called agglutin-
ins. This is commonly called the Widal reaction. Agglutination is so
specific that the serum of an individual suffering from typhoid fever, or
even the serum of one who has had the disease, will cause the clumping
of typhoid bacilli when a few drops of it are placed in a culture of the
bacilli.

If any foreign protein substance is injected into any of the higher
animals, new substances similar to antibodies are formed, which are also
specific in acting on the same protein substances by causing a cloudy
precipitate, and sometimes by changing the protein by breaking it up into
simpler substances, some of which are poisonous. This fact makes it
possible to tell whether blood stains are those of a human being or not.
For example, the clear serum of a rabbit can be treated with human
blood serum and if even a portion of the dissolved human blood stain is
then added, a cloudy precipitate forms, although this precipitate will
not form when blood from a lower animal is added. Similarly if the

IMMUNITY 201

rabbit's blood be first treated with the blood of a lower animal, human
blood will not cause the precipitate.

A strange phenomenon has also come forth in recent years known
as an anaphylactic shock. This is probably connected in some way with
the precipitation reaction. It means that an animal already immunized
to a protein may die when additional protein of the same kind is in-
jected. This condition in an animal is known as anaphylaxis ( ).
In other words, we may say anaphylaxis is an oversensitiveness of the
organism toward bacterial toxins and foreign sera. The reason for ana-
phylaxis is not yet satisfactorily explained.

The principle of anaphylaxis is used to diagnose certain diseases.
The tuberculin reaction is nothing more nor less than the injection of
the proteins of the tubercle bacillus into the skin, which (because the
dose is very small) does not overwhelm the entire nervous system, but
produces only a fall in temperature and a slight fever, if the disease is
present.

An antitoxin is, as already stated, the soluble substance produced
which neutralizes quantitatively fresh injections of the same poison.
The commercial diphtheric antitoxin is merely the serum of a horse
which has had repeated doses of diphtheria toxin injected until it has
been brought into a state of active antitoxic immunity. Like all im-
munizing agents antitoxins are all specific for some single toxin.

In this connection it is interesting to note that while sheep are very
susceptible to the toxin formed by the tubercle bacilli, they are not sus-
ceptible to the injection of the dead bacilli themselves. Guinea pigs are
quite susceptible to the bacilli but not to the toxin. The proteins of
one's own body injected into ourselves are poisonous.

The various ways in which immunity is produced by injection of
foreign substances may be summarized as follows :

(1) The virulent parasites are administered in small doses so as
to give the individual the disease in a mild form (active immunity).

(2) Weakened parasites may be injected in larger doses and pro-
duce the same result.

(3.) Dead bacteria may be given in place of living, so as to produce
a feebler but similar result.

(4) The poisons may be isolated from the parasites, and gradually
increasing doses are injected, thus increasing the normal neutralizing
ability of an individual.

(5) Serum from an animal, immunized by one of the above pro-
cesses, may be placed directly into another individual and thus permit
him to become immune without going through any form of the disease
himself (passive immunity).

CHAPTER XV.

THE PLANT-WORLD
SIMPLE PLANTS

Everyone is already familiar with some of the higher groups of
plants known as "flowering plants," but everyone is not familiar with
the fact that flowering plants are few in number, indeed, when compared
with the thousands of different kinds of minute plants that cannot even
be seen with the naked eye, and which do not bear flowers.

Prominent among these latter are such single celled plants as Pleu-
rococcus, the yeasts, and bacteria already studied. But there are others
also, which, though commonly seen, must remain unknown unless ob-
served under the microscope.

To be able to discuss the plant-world intelligently one must know
certain terms commonly used, just as it was necessary to know the vari-
ous names of the many parts of the frog before the animal-world could
be intelligently discussed.

The following outline and drawing (Fig. 92) will give such a knowl-
edge of terms :

Root (with or without branches)

Stem (with or without branches)

Plant Body "

Shoot

Blade

Leaves

Apex

Margin

Base of the Blade

Veins

Petiole (the leaf stalk)
Leaf-base

Stipules. (Small leaf-like structures
at the base of the petiole).

There are as many and varying classifications of plants as there are
of animals, but, four great groupings hold their own because these group-
ings are simple and easily understood.

SIMPLE PLANTS

203

They are as follows :

Thallophytes (
plant body.

Algae (Chlorophyl-bearing thallophytes).

Fungi (Thallophytes without chlorophyl).

Bryophytes or Moss Plants (

Pteridophytes and their allies (
and their allies.

Spermatophytes or Seed Plants (

). Plants possessing a simple

The Ferns

Fig. 92. Leaf, Root,
Shoot and Flower.

A. Leaf of a wil-
low (Salix 8p.). b.,
blade ; p., petiole ; s..
stipules.

B. Diagram to show
the essential parts of
a "flowering" plant.
t.r., tap-root ; b.r.,
branch root ; cot., seed-
leaf (cotyledon) ; i.,
internode ; a.l., leaf-
axil ; «., node ; a.b.,
axillary bud ; r., re-
ceptacle of floral or-
gans ; co., calyx ; per.,
perianth ; co., corolla ;
st., stamens (androe-
c i u m ) ; pi., pistil

(gynoecium).

b r

tr

(From C. Stuart Gager's "Fundamentals of Botany," by pei
mission of P. Blakiston's Sons & Co.. Publishers.)

204

GENERAL BIOLOGY

THALLOPHYTES

Thallophytes — These plants have a mere plant body, there being no
true stem, roots, or leaves, though there may be parts that resemble
stems, roots and leaves. They may be very fragile as are some of the
thread-like green Spirogyra (Fig. 93), (also called pond-scum and frog-
spit), commonly found in fresh-water creeks and ponds, or tough sea-
weeds like the brown kelp many feet in length. The cells usually grow
end to end.

As has been seen by this time, when
living organisms are discussed, there are no
hard and fast rules by which one may
classify anything. There are some Thallo-
phytes which really have stem-like and
leaf-like structures, but the classification
originally based on structures, must now
be thought of more from a functional or
life-cycle point of view. All thallophytes
are alike in having a more or less simple
life-cycle, so this must serve us as a basis.
The various algae (Fig. 94), are named
after some distinctive characteristic ; thus
those which are green are called Chloro-
phyceae ( ), those which

), those which are slimy
), etc.
) is in turn a representative of the

— chlorophyll Unit

pyrenoids

Fig. 93.

The band-like chloroplasts extend
in a. spiral from one end of the cell
to the other. In them are imbedded
nodule-like bodies (pyrenoids), and
near the center of the cell the
nucleus is swung by radiating
strand* of cytoplasm. (After Stras-
burger. )

are red, Rhodophyceae (
Myxophyceae (
Spirogyra (

chlorophyceae. The cells are elongated and attached end to end. There
are spirally arranged bands (chromatophores or chloroplasts) which con-
tain chlorophyl. The number of these bands and the method of coiling
depend upon the species to which each belongs. The cytoplasm is rather
thin and lies next to the cell-wall, while fine threads of it extend to the
nucleus. The special centers in the chloroplasts where starch is stored
are called pyrenoids ( ). Nearly 98 per cent of the

cell is water, yet the 2 per cent remaining can perform every one of the
four vital processes. The bubbles often seen are filled with oxygen
which is a waste product of photosynthesis.

Reproduction takes place in two ways, either by the individual cell
dividing at right angles to. the length of the cell, or after two individuals
have conjugated. The latter is seen when two plants lying close to-
gether send out projections (Fig. 95) which unite, forming bridges
through which the cytoplasm of one plant mixes with another.
In fact these two cells may unite so thoroughly that they become one,

SIMPLE PLANTS

205

Fig. 94. Chlorophycae, Rhodophycae, and Myxophycae.

A. Cladophora, a branching green alga, a very
small part of the plant being shown. The branches
arise at the upper ends of cells, and the cells are
ccenocytic.

B. A red alga (Gigartina) , showing branching
habit, and "fruit bodies."

C. Three common slime moulds (Myxomycetes)
on decaying wood : To the left above, groups of the
sessile sporangia of Trichia; to the right above, a
group of 'the stalked sporangia of Stemonitis, with
remnant of old plasmodium at base ; below, groups of
sporangia of Hemiarcyria, with a plasmodium mass
at upper left hand. (A, after Caldwell ; B, after
Schenck ; C, after Goldberger.)

Fig. 95.
The Union of the Gametes in Spirogyra.

A, two filaments of Spirogyra qiiinina,
side by side, showing stages in the union of
the cells (gametes) to form the zygospores ;
B, another species (S. longata) , in which the
cell unions occur between adjacent gametes in
the same filament. (After Schenck.)

becoming smaller as a thick-
ened wall is secreted about it.
When this latter event takes
place the organism is said to
be in a spore state, and because
the spore has been formed by
the fusion of two cells it is
often called a zygospore
( ). Conjugation

is thus a preparatory process
to permit a mixing of the par-
ent chromatin before actual re-
production takes place.

It has already been ex-
plained that a sexual germ-cell
is known as a gamete. The
zygospore is therefore now one
cell, the product of the fusion
of two gametes. There is here
then the beginning of sex-life
in the plant-world; this is why
the two conjugating and fusing
parent cells are known as
gametes.*

The spore cannot escape
from the parent cell, however,
until such parent-cell decays.

Artificial Fertilization. —
"More than a hundred years

Fig. 96. A Common Foliose Lichen (Parmelia)

Growing Upon a Board, and Snowing

Apothecia. (After Goldberger.)

*This is the first sign of two sexes we shall see in the laboratory, although the very first sexual
differentiation in plants probably lies in the Volvacales (Fig. 97).

206

GENERAL BIQL

,oc,v

Fig. 97.
Volvox Globator, a Colonial Form of the Volvocaceae.

(See Fig. 49, where this same form is considered an
animal.)

A, mature colony, with four daughter colonies devel-
oping in its interior ; B, section of the edge of the colony,
snowing three vegetative cells and a developing egg ;
C, a packet of sperms within the parent cell and a single
sperm very much magnified at the side ; D, an egg sur-
rounded by a swarm of sperms ; E, an oospore with
heavy protective wall. (From Bergen & Davis' "Princi-
ples of Botany," by permission of Ginn & Co., Pub-
lishers. )

ago Spallanzani succeeded
in artificially fertilizing eggs
of various animals ; but more
recently several workers
have succeeded in causing
the egg to begin growing by
chemical means. These men
saw that the formation of
the fertilization-membrane is
purely physical, so biologists
began to reason that it ought
to be possible to induce it
artificially. The experiment
was first successfully made
by a zoologist, Loeb, with
the eggs of sea-urchins and
other marine animals. In
1913 it was successfully accomplished by Overtoil with the eggs of
Fucus. The eggs were dipped for about a minute, or a minute and a half
to two minutes, in a mixture of 50 cc. of sea-water plus 3 cc. of a very

weak solution of acetic, butyric, or
other fatty acid, and then trans-
ferred to normal sea-water. This
treatment caused the formation of
the fertilization-membrane, quite as
in natural fertilization by the sperm.
If, after the formation of the mem-
brane, the eggs are placed for 30
minutes in hypertonic sea-water
(50 cc. of normal sea-water plus 8
to 10 cc. of a weak solution of so-
dium chloride (common salt), or
potassium chloride), and then back
into normal sea-water, the eggs be-
gin to divide and continue to de-

Fig. 99. Phycomycetes.

These are the alga-like fungi without septa
in the mycelium, except in the sporing
branches, where they occur to cut off the spore-
bearing cells. The septa also occur in old
filaments. The mycelium is therefore continu-
ous.

Common water mold (Saprolegnia.) : A,
a. fly from which mycelial filaments of the par-
asite are growing ; B, tip of branch organized
as a sporangium ; C, sporangium discharging
biciliate zocispores ; F, oogonium with an-
theridium in contact, the tube having pene-
trated to the egg ; D and E, oogonia with
several eggs. (A-C after Thuret ; D-f after
De Barry.)

Fig. 98. Growth Habit of the Bread Mold

( Rhizopus Nigricans ) .

Sketch showing two groups of erect hyphae
bearing sporangia, with root-like clusters of
filaments at their bases.

SIMPLE PLANTS

207

velop into young plants. The question as to the chromosome number
in the cells of plants formed by artificial fertilization is of very great in-
terest, but has not yet been investigated."

It must be remembered that the egg was already there. Artificial
fertilization merely hastened a normal action. This does not throw any
light on the origin of life, as is popularly supposed.

VAUCHERIA

This is the common "green felt" (Fig. 86), usually found on soil,
though it is often found in water. The thread-like filaments are coarser
and longer than spirogyra and they also branch. Vaucheria are tube
algae.

There is an interesting difference here from the spirogyra in that
there are no transverse cell-walls throughout the entire filament, but
there are many nuclei scattered about. Such a form is called a coenocyte
( ) or syncytium (I. E., Fig. 86).

Fig. 100. Ascomycetes (Sac-like Fungi).

The figure shows the characteristic group-
ing of asci. The layer in which the asci ap-
pear is called a hymenium. In these th«
mycelium has dividing septae and the spores
are contained in asci. (After Chamberlain.)

Fig. 101. Basidiomycetes.

Typical basidium with sterigmata (distal
short stalks), showing spores in different
stages of development. (After De Bary.)

In basidiomycetes the spores develop on
little club-shaped hyphae. Smuts, rusts and
mushrooms belong in this group.

Reproduction takes place both sexually and asexually (Fig. 86). In
the latter case the old end of the filament dies, setting free the branches
which become separate plants, or a cross wall forms in one of the
branches. A thickening occurs beyond this cross wall and this thick-
ening is known as a zoospore. The zoospore breaks away from the par-
ent plant, swimming about for a time and then becomes a new plant.

208 GENERAL BIOLOGY

It is made up of many cells but forms only one plant.

Sexual reproduction occurs when one or more large oval protrusions
form on branches which have grown out apparently for this purpose. At
the very end of this branch is the terminal cell in which many small cells
are formed. These small cells escape into the water. Each one possesses
long cilia by means of which it swims about. A single one of these
ciliated forms enters the oval mass. The little ciliated form is known as
the male gamete, and the large oval protrusion as the female gamete.
The organ which produces a gamete is called a gonad* ( ),

The oval body is consequently known as an oogonium ( )

or egg-gonad. Two gametes, uniting as have the two just mentioned,
form a single cell known as an oospore. This oospore, after a short
period of rest, forms a new plant. It will be noted that in Vaucheria the
gametes are of unequal size. In Spirogyra they were of equal size. In
fact, whenever gametes are formed, it is the smaller and more active one,
regardless of whether there are any other distinguishing features or char-
acteristics, which is called the male gamete or sperm, while the larger
and more passive one is known as the female gamete or egg.

The union of sperm and egg is called the process of fertilization.

The male gonad is called the antheridium ( )

and the female gonad is known as an oogonium.

Some algae live with various fungi. These symbiotic ( )

plants are the lichens ( ). (Fig. 96.)

THE FUNGI

The Algae-like, or tube fungi, make up the Phycomycetes, while the
higher fungi such as mushrooms, toad-stools, puff-balls, rusts, and smuts,
are known as Carpomycetes.

The fungi, no matter how differently they may appear or in what
out-of-the-ordinary place they may grow, are alike in two great charac-
teristics. 1. They possess no chlorophyl, and 2. They reproduce by
spores.

They live either upon decaying matter, in which case they are called
saprophytes, or at the expense of another organism when they are called
parasites.

Bread Mold is easily obtainable, but that from fruits or from dead
flies serves just as well for study. (Fig. 98.) There is a tangled mass
of thread-like structure which is the working body of the plant. This
tangled mass is known as mycelium ( ), while the

individual threads are known as hyphae ( ). If the

hyphae send out threads in turn these are called rhizoids ( ),

and it is these little root-hairs which penetrate the substance on which
the mold forms and through which it absorbs what is needed. It is sup-

*Botanists do not look with favor on the term "gonad" in plants, but it has seemed advisable
to use this term here, for, in zoology the student must use it constantly.

SIMPLE PLANTS 209

posed that enzymes are produced in the hyphae which can make the
bread or fruit utilizable to the plant.

Reproduction takes place by a number of upright stalks called spor-
angiophores ( ), growing from the mycelium. There

is formed a spore-case or sporangium at the very tip of the stalk. In
this the spores are formed and when the spore-case bursts the dust-like
particles which are really spores are scattered about by air currents.

There may be sexual reproduction in the molds quite similar to that
in Spirogyra. Two hphae unite by their free ends and a wall forms, thus
producing two end cells which eventually become a single spore with a
very dark heavy wall. Here again, the gametes being similar, the re-
sulting body is a zygospore. The sexual process does not occur very
often.

It is to be remembered that molds are plants. .But growing as they
do in the dark, they have no chlorophyl and do not make their own food,
but feed on food already prepared ; not on ordinary plant or animal food
as does man, for example, but on decaying matter or on food that has
already been digested by the host, either before or after assimilation.

The so-called water-mold is both parasitic and saprophytic as it can
thrive either on dead or living fish. This means that molds are a degen-
erate form of green plants which have acquired a habit of making some
other organism do their work for them, rather than build their own food
by photosynthesis as plants usually do.

PATHOGENIC FUNGI

Some of the difficulties of classification may be observed here by
noting that botanists classify fungi or mycetes ( ) as

follows :

1. Phycomycetes : algae-like fungi. (Fig. 99.)

2. Ascomycetes : sac-fungi (asci) ; Asexual spores formed in sacs.
(Fig. 100.)

3. Basidiomycetes : spores, born on little club-shaped hyphae, or
basidia. (Fig. 101.) (Includes smuts, rusts, and mushrooms.) The
pathogenic fungi bear many names and cannot be accurately placed, be-
cause pathologists use other than the regular botanical terms and mean-
ings, as shown by the table below.

Most infectious diseases due to vegetable parasites are caused by
bacteria, but a few owe their origin to micro-organisms of a higher type,
namely, to the yeasts and molds. Two of the infectious processes caused
by yeasts, although comparatively rare, deserve brief consideration.
Both the organisms and the lesions they produce, microscopically and in
gross, resemble each other more or less closely. For this reason they
were for a long time confused with each other, but the differential char-
acteristics are now fairly generally recognized.

210 GENERAL BIOLOGY

The relation of the yeasts or blastomycetes to the bacteria and the
molds is shown in the following diagram :

Pathogenic Protophytes.

(These are all Chlorophyl-less plants.)

Schizomycetes ( )

(bacteria)

Hyphomycetes (
(molds)

Blastomycetes (
Saccharomycetes (

or

Ascomycetes (
(yeasts.)

Oidia (

(transition form)
Oidium.

Blastomyces.

A living organism may be injured either mechanically or chemically.

Mechanical or physical injury may be the result of violence, pres-
sure, heat (burning), cold (freezing), light rays, and electricity.

Chemical injury results from poisons (toxins), whether by auto-
intoxication, or toxins produced by animal or vegetable parasites.

It is readily understood that a parasite may cause either mechanical
or chemical injury to its host: the former by propagating so rapidly
that it causes a physical obstruction, as is the case with the Tubercular
bacillus ; and the latter by a definite poison that the foreign organism
produces. The poison, or the mere mechanical effect, may in turn cause
some out-of-the-ordinary conditions, as for example when it furnishes a
stimulus to overgrowth on some part of the host, as galls on trees (a
growth caused by the stimulus of aphids), or tumors in human beings.
These latter are merely overgrown healthy cells.

These overgrown healthy cells may press against neighboring
blood vessels, obstructing the blood-flow and thus causing the death of
the tissue which fails to receive its required amount of blood.

It may be stated in this connection that there are some hosts which
are not affected at all, or but little, by the poisons parasites produce, al-
though the parasite itself does many such hosts a great deal of damage.

The subject of toxins is among those of which no great knowledge
has yet been obtained. They are of great interest and importance as can
readily be judged from what has been said.

Probably, if toxins and enzymes are thought of here in relation to

SIMPLE PLANTS

each other, it will lead to a clearer understanding of each. Both can be
studied only by the effects they produce, the one injurious, the other
beneficial.

One may also think of several other possibilities on the part of the
invading organism. For example, it may live entirely at the expense
of its host, in which case it is a parasite ; it may live on decayed matter
and do little if any injury, and thus be a saprocyte, or it may actively
engage in killing and devouring parasitic invaders and thus be of great
value to its host, when it is known as a phagocyte.

True yeasts grow by budding (Fig. 87) ; they rarely form mycelia;
under unfavorable conditions of growth they may form endospores.

Oidia grow by budding and as mycelia with spore formation. (Fig.
102.)

Hyphomycetes (Fig. 98) grow as mycelia with spore formation of
asexual or sexual origin.

All authorities seem agreed that there is no sharp line of demarca-
tion between the blastomycetes and the hyphomycetes, and most of them
place the oidia as a transition form.

Fig. 102.

Oidium, showing spores
being cut off from the
tip of the branch. Such
spores are called conidio-
ipores.

Fig. 103. Aspergillus Fumigatus. (After Brumpt. )

Blastomycosis (also called Saccharomycosis), is the term applied to
the lesions produced by a blastomyces. A variety of organisms have
been cultivated from the lesions, and different names have been assigned
to them. "It is not known if these are distinct entities. Most study of
this type of organism has been about Chicago.

"There is an infection in the skin, usually remaining localized there.

212 GENERAL BIOLOGY

but it may invade the circulation and cause lesions in other parts of the
body.

"The blastomyces occur in human tissues only in the blastomycetoid
form, that is, as small round bodies with granular protoplasm, and with
thick hyaline capsules. They multiply by budding only. In cultures
they may develop mycelia or grow by budding, or in both ways. They
may be numerous in the lesions which they produce, or few and hard
to find."

They produce a fairly strong toxin.

Aspergillus fumigatus (Fig. 103) is an example of the pathogenic
ascomycetes. It is a fungus widely distributed, usually as a harmless
parasite, having been found in the auditary canal, nose, and throat.

In birds, in cattle, more rarely in dogs, Aspergillus may cause lesions
of the lungs, resembling tuberculosis, and there have been of late years
a good many cases reported in man, particularly pigeon keepers and hair
sorters. "In the majority of cases the infection is secondary to some
long-standing affection of the lungs," though it also causes a primary
lesion resembling broncho-pneumonia, usually quite serious. The patient
coughs up a grayish brown mass the size of a bean made up entirely of
mycelium and spores.

Oidiomycosis (granuloma coccidioides) is the term applied to the
lesions produced by an Oidium variously named in the past immities,
coccydioides, etc., but not yet definitely classified by the botanists. In-
fection with this organism is rare and is confined almost exclusively to
California. The disease is practically fatal.

The oidium occurs in human lesions in the form of spherical bodies
which may reach a size of thirty microns. They consist of an irregularly
staining mass of protoplasm enclosed within a double contoured capsule
which is occasionaMy covered with prick1 es, or even long spines. The
organisms multiply in tissues only by endosporulation, never budding.
The spores may number as high as a hundred or more. They are lib-
erated by the bursting of the capsule. The number of parasites in the
lesions varies. They may be many or few and hard to find. In cultures
the oidium grows as long septate branching hyphae. In time, spores
develop in the ends of the hyphae and are infectious if inoculated in ani-
mals; the hyphae themselves are not.

The lesions produced by Oidium often bear a close resemblance to
those caused by the tubercle bacillus, and have probably been mistaken
for them more than once on histologic examination. If the organisms
are few in number, a. cheesy region may be formed, and if numerous,
even abscesses and ulcers.

Blood and lymph streams seem to carry the organism so that it is
widely distributed. It is as likely to be found primarily within, as on
the skin of the body.

SIMPLE PLANTS

213

Like the tubercle bacillus, Oidium involves the same organs; lungs,
lymph-nodes, adrenal glands, meninges, seminal vesicles, etc.

The skin lesions are chronic and consist of nodules, abscesses, and
ulcerations.

DISEASES CAUSED BY FUNGI OF MORE OR LESS UNCER-
TAIN POSITION
Actinomycosis or lumpy jaw.

SPOROTRICHOSES

The Sporotrichoses.

(a) Subcutaneous. Small solid nodules, becoming abscesses, ulcer-
ating the skin.

(b) Cutaneous. Principally in arms, hands and legs, though it
may occur on other parts of body. Ulcers form in groups of two or
three.

(c) Localized, hard and eroded on surface.

Fig. 104. Sporotrichum Beurmanni. (After Brumpt.)
-1, Single lateral conidiospore ; 2, terminal conidiospores ; 3,
collection of laterial conidiospores.

The parasite (Fig. 104) is "introduced by accidental inoculation, and
possibly through grains and fruit." Acts like bacteria, producing toxins,
toward which toxins there are active reactions of the body-fluids. It is
a short rod 3 to 5 microns long and 2 to 3 microns in breadth. In cul-
tures it grows in filaments of about 2 microns in diameter and forms
characteristic ovoid spores.

"The points of differentiation between the forms are due largely to
variation in the modes of sporulation."

GENERAL BIOLOGY

Fig. 105. Actinomyces Bovis.

(After Rivas.)

Parasite producing actinomycosis (lumpy
jaw). Actinomyces bovis, also called
nocardia actinomyces, nocardia bovis, strep-
tothrix actinomyces, streptothrix israeli,
ocspora bovis, cladothrix act inomy coses,
and bacterium actinocladothrix.

Nocardiosis. "On the one hand
the parasites (Fig. 105) resemble bac-
teria, on the other hand the hyphomy-
cetes or molds, in forming branching,
thread-like filaments, and the produc-
tion of fine conidia. They represent a
transition between the bacteria and the
lower fungi." Only a dozen cases have
been reported. Very like tuberculosis
or multiple abscesses, three cases have
appeared as abscesses of the brain.

Mycetoma or Madura disease.
Largely in India, though it does occur
in other parts of the world. Ther.e is
great swelling of the foot, generally
on the sole, nodular growths and mul-
tiple abscesses. Black, brownish, or
yellow granules are formed, one micron
in diameter.

A v a r i e t y of
Streptothrix ( F i g.
106) has been found
in the pale colored
granules, closely re-
sembling Actinomy-
ces. "It is held by
most observers that
this S t r ep t ot hrix
madurae and Acti-
nomyces are distinct
species." "From the
black variety of
granules a hypomy-
cete has been grown,
an organism closely
allied to Aspergil-
lus."

In medicine, all diseases caused by any non-bacterial fungi are some-
times called Mycosis.

It is worthy of note that medical men, being most interested in dis-
ease, take similar appearing lesions as their basis for classifying organ-
isms, while biologists classify organisms according to structure and de-
velopment, often with little reference to what disease such organisms
may cause.

Fig. 106. Madurella Mycetoma. (After Brumpt.)

CHAPTER XVI.

THE PLANT WORLD CONTINUED

THE THREE HIGHER GROUPINGS
BRYOPHYTES

Bryophytes are usually said to possess archegonia ( ;

or primitive egg gonads composed of many cells as contradistinguished
from the thallophytes which, when they possess gonads at all, are prac-
tically always composed of single cells.

Bryophytes are moss-plants and liver worts, and their life-cycle con-
sists of two stages, the sexual and the sexless. When these stages follow
each other it is called an "alternation of generations." The sexual plant
or gametophyte forms eggs and sperm which unite, while the asexual
plant or sporophyte is the plant which grows from the fertilized egg of
the sexual plant. This non-sexual plant forms asexual spores which in
turn grow into gametophytes.

Bryophytes may be quite simple resembling the thallophytes, or
form a leafy stem as in the mosses.

There are some 12,000 different species of mosses or Musci, as they
are technically known. These are divided into three distinct orders :

1. Sphagnales ( ). The peat-mosses. (Fig-.

107.)

Fig. 107.

The Peat Moss,

Sphagnum.

Fig. 108. Andreaea Petrophila.
A, plant with mature sporophyte.
B, longitudinal section of sporophyte.
Ps, pseudopodium ; col, columella.
(From D. H. Campbell's "A University
Text-Book of Botany," by permission
of The Macmillan Co., Publishers.)

216

2. Andreaeales (

108.)

3. Bryales (

GENERAL BIOLOGY

). The black-mosses. (Fig.

). The true mosses. (Fig. 109.)

Fig. 109. A Common Moss

( Catharinea Undulata ) .
Showing the branching leafy moss
plants ( gametophytes ) attached to
the root-like mass of protonemal
filaments and bearing sporophytes.
(After Sachs.)

Fig. 110. Sphagnum Acutifolium, Ehrb.
A., prothallus (pr.) with a young leafy
branch just developing from it ; B., portion of
a leafy plant ; a., male cones ; ch., female
branches ; C., male branch or cone, enlarged
with a portion of the vegetative branch adher-
ing to its base ; D., the same, with a portion
of the leaves removed so as to disclose the
antheridia ; E., antheridium discharging spores ;
F., a single sperm ; G., longitudinal section of
a female branch, showing the archegonia
(or.) ; H., longitudinal section through a
sporongonium, with dome of sporogeneous tis-
sue ; or., old neck of the archegonium ; «/.,
Sphagnum squarrosum Pers. ; d., operculum ;
c., remains of calyptra ; qs., mature pseudo-
podium ; ch., perichsetium. (After Schimper. )

The Sphagnales are the most primitive, and the Bryales the most
highly developed.

Sphagnum (Fig. 107) is a peat-moss, growing, as its name implies,
in swamps and along the margin of lakes. The peat-bogs of northern
regions are made up of thick clumps of this plant. Peat-mosses are
usually light green in color bordering on white, and sometimes have a
slight tinge of red and yellow.

The plant has certain branches which bear reproductive cells and
other branches which are sterile. (Fig. 110.)

The gamete plant (the one bearing the gametes or reproductive
cells) has an upright stem with a mass of pith in the center. The outer-
most portion is called the cortex. The cell walls in the cortex are thicker
than those in the center and often contain pigment. The thickness of
the cortex varies from two to four cells in thickness. The leaves are
only one cell in thickness and never have a mid-rib or other veins. In
other words, there are no fibre-vascular bundles in the stem at all. This
is one of the great characteristics which distinguish this whole group
of plants from the next higher grouping — the Ferns.

PLANT WORLD CONTINUED

211

As the leaves mature, a goodly portion of the cells increase in size,
the entire protoplasm being added to the walls of the cell so that these
become very thick. This leaves the cell filled with nothing but air and
water. In fact, this hygroscopic ( ) ability of the

cells is the reason florists use the sponge-like Sphagnum in packing
flowers for shipment.

In those branches which are set aside for reproductive purposes,
each sex uses individual branches for the antheridia (male branch) and
the archegonia (female branch), (Fig. 111). In some species, entire

Fig. Ill

A Common Moss
(Polytrichum
commune ) .
A., male plant,
showing cup-like
tip containing the
antheridia. B., fe-
male plant with
the sporophyte ;
col., cap, or calyp-
tra, over the de-
veloping spore
case ; C., a. mature
spore case with
the calyptra re-
moved.

Antheridia and Archegonia.
Section Through Section Through

the Tip of the the Tip

Male Plant of a
Moss ( Funaria ) .

a., antheridium ;
f., sterile filament,
or paraphysis ; I.,
leaf.

the Tip of
Female Plant of
a Moss (Funa-
ria).

A., group of
archegonia a. ; I.,
leaf. B., an arche-
gonium in detail,
showing enlarged
basal portion e.
with the egg, and
the neck n. above
with its row of
canal cells ; m.,
mouth. (After

Sachs.)

(From Bergen & Davis' "Principles of Botany," by permis-
\ sion of Ginn & Co., Publishers).

plants are of one sex or the other. In these, therefore, antheridia and
archegonia are never found on the same plant. Such plants are said to
be dioecious (from two households), while those plants on which both
male and female reproductive branches appear are said to be monoecious
(from one household).

The branches bearing antheridia are called antheridophores. An
antheridium is found in the axil ( ) of each leaf of the

head and consists of a stalk composed of not more than four rows of cells.
When the antherium is mature it contains many sperm.* (Fig. 112.)
The sperm are coiled, and bear two long thread-like cilia at their anterior

*Botanists use
for, kept the term

'sperms" for the plural of "sperm," while zoologists do not. We have, there-
'sperm" throughout as meaning both singular and plural.

-218

GENERAL BIOLOGY

are

end. There is a small appendage called a vesicle which contains starch
granules. As the antheridia ripen, the sperm-sac is forced open and the
sperm discharged. It is important that this sperm-sac be not confused
with the spore-capsule to be mentioned later.

The branches bearing archegonia are called archegoniophores and
usually found toward the upper portion of the plant, while the
a archegonia themselves are at the tip

of the archegoniophores. (Fig. 113.)
Each archegonia has a neck, neck-
canal, a venter which contains the egg,
and a basal or pedicel. The archegonia
of ferns will be found to be quite like
this, except that the pedicel is missing.
Usually, several archegonia are found
on a single branch. A number of en-
larged leaves surround the archegonia.
They constitute a perichaetium

Fig. 112.

The Antheridium of a
Common Moss

( Funaria ) .
a., Antheridium ; b.,
escaping sperms ; c.,

a single sperm in its
parent cell.
Sachs.)

(After

Both archegonial and antheridial
branches begin growing close together Fig ng

but the main branch from \vhich both Archegonium of

Sphagnum,

develop continues growing between showing a young em-

tVifMn nnH cp>r»n refiner tVi^i-n f nrtViP>r- onrl bryo sporophyte (em.)

tner alia developing in the ven-
ter. ( After Schimper.)

H c^r»n r^ finer tln^ivi furtViPn- o

a separating tnem lurtner
further

Fertilization probably occurs in winter as young embryos are found
in abundance in the spring. A film of wrater is needed for this purpose

Fig. 114. The Sporophyte of the Peat Moss

(Sphagnum).

A., group of the sporophytes on stalks, which are
really growths from the gametophyte. B., longi-
tudinal section through a sporophyte, showing the
large foot imbedded in the top of the stalk ; a., the
remains of the parent archegonium. with the neck
still present; s., a spore chamber; c., cover. (From
Bergen & Davis' "Principles of Botany," by permis-
sion of Ginn & Co., Publishers.)

PLANT WORLD CONTINUED

because sperm must swim to the neck-canal and pass through this into
the venter. Here it enters the egg and the nuclei of sperm and egg unite.
The fertilized egg now divides -by mitosis very rapidly, the upper
cells form a large globular spore-case with a thick central column within
known as the columella. This is surrounded by a dome of spores, around
which the wall of the sporangium is formed. The spore-case later
pushes against the wall of the archegonium by enlarging. The wall is
then ruptured, the top portion remaining as the calyptra ( ),

(Fig. 119), while the spore case later opens by means of a lid. The lower
cells produced by the dividing oosperm becomes a swollen foot, which
is imbedded in the tissues below. It remains connected with the spore-
case by a short stalk.

The structure which thus develops from the fertilized egg-cell is
called the sporophyte (Fig. 114) stage of Sphagnum. In fact, all such
simple plants which develop spores are called sporophytes.

Simultaneously with the maturing of the sporophyte. the apex of
the female branch elongates into a leafless stalk about half an inch or
more in length, known as the pseudopodium. It is supposed that the
reason that the pseudopodium and sporophyte grow thus simultane-

Fig. 115.

Antheridium of Pteris (B.), showing wall Fig. 116. Sphagnum sp.

cells (a.), opening for escape of sperm mother

cells (e.), escaped mother cells (c.), sperms A- B- .young protonemata ; C., older pro-
free from mother cells (6.), showing spiral t™*™* wlt* Ieaj^ bud,' k- • r- »»**M»I
and multiciliate character. (After Caldwell.) rhizoids. (After Campbell.)

ously is probably due to the cells in the foot secreting a substance which
stimulates the cells in which it is imbedded to divide and enlarge, result-
ing finally in the formation of the pseudopodium. The advantage the
plant gains is that the spore-case is raised to a higher plane and it can
thus throw its spores much farther than would otherwise be the case.

As Sphagnum possesses no chlorophyl, it does not manufacture its
own food and must therefore live on the absorption of food-matter from
the gamete plant through the foot.

The spores themselves develop in the following manner. (Fig.
115.) In the spore-case the inner cells differentiate into two kinds, one
making up the larger portion of the tissues, and the other larger and
richer in protoplasm, forming a dome of sporogenous or spore-forming
tissue near the upper wall. It is from this latter type of cell that the
spore-mother-cells are developed.

220

GENERAL BIOLOGY

These spore-mother cells are divided twice, thus producing four
spores each, and it is these spores which eventually germinate and pro-
duce the gametophytes. In ferns, we shall see that a quite similar
process of spore formation takes place.

'During the time the spores are maturing, a circular groove, called
an annulus, forms near the apex of the spore-case. The cells in this
region have thinner walls than the surrounding cells. These cells later

become dry, and the thinnest part becomes torn
to form a lid or operculum ( )

at the summit of the spore-case. As the opercu-
lum falls away, the sperm are dispersed.

If they find suitable soil a short green proto-
nema (Fig. 116) germinates The tip of the
protonema broadens to form the prothallus
wrhich is one cell in thickness. Tiny rhizoids
( ) form on the under

side and from the margin, while other threads
c o n t a i n i ng chlorophyl
then develop. Often a
thallus forms at the tip of
each of these threads.
From this thallus a leafy
branch grows upward and
the sphagnum plant de-
scribed is again a full-
fledged adult organism.
The plant, from the time
it germinates from the

Fig. 117.

Sphagnum Cuspidatum,
showing innovation, or
short, branches. ( After
Schimper.)

Fig. 118. I.
The Sporophyte of a Common Moss (Funaria).

A., young sporophyte s. attached to the leafy moss
plant and covered by the calyptra cat. B., sporophyte
with mature spore case sc. and calyptra cal. at the
tip. C., spore case with calyptra removed ; o., the
cover (operculum). D., a stoma from the surface of
the spore case. E., section of young spore case, show-
ing the cylindrical central region of spore-producing
tissue sp. F., the spore-producing tissue in detail.
(From Bergen & Davis "Principles of Botany," by
permission of Ginn & Co., Publishers.)

Fig. 118. II Developing Sporophytes
of a Common Moss (Funaria).
A, very young stage, showing the
early cell divisions of the egg ; /?.,
older sporophyte just before the
archegonium o. is torn away from
the gametophyte and carried up-
ward as the calyptra. The base of
the sporophyte has now grown down
into the tip of the leafy moss plant
(gametophyte) and is firmly an-
chored to it. (After Sachs.)

PLANT WORLD CONTINUED 221

spore until the thallus develops, is the gametophyte. This is to be distin-
guished from the adult plant which, as we have seen, is called the sporo-
phyte.

There is an asexual multiplication of Sphagnum also. This is
brought about by a sterile branch developing more powerfully than the
surrounding ones. Then, each year, as the old stern dies off below,
the young branch becomes a new plant. Sometimes little plantlets,
known as innovation branches (Fig. 117), strike root and be-
come independent plants. These innovation branches spring from close
to the tip of the sterile branches.

The life-cycle of Sphagnum may be summarized as follows :

OUTLINE OF LIFE HISTORY OF SPHAGNUM

Sphagnum-plant (gametophyte)

Antheridial branch Archegonial branch

I I

Antheridia Archegonia

1 1

Sperm (male gamete) Egg (female gamete)
Fertilization

I I

Oosperm (zygote)

I I

Embryo

I I

Mature Sporophyte

I I

Sporangium

I I

Spore-mother-cell

c c 0 ~ f Reduction

Spore Spore Spore Spore )

I
I

Protonema

I

Thallus

I

Sphagnum-plant (gametophyte)

222

GENERAL BIOLOC.V

Fig. 119.

A Moss ( Tetraphia
sp.) , showing gemmse ;
G., a gemma enlarged.
(From C. Stuart Gager's
"Fundamentals of Bot-
any," by permission of
P/Blakiston's Son & Co.,
Publishers.)

The so-called true mosses (Fig. 109) have life-histories quite like
that of Sphagnum, although there are differences. In true mosses the
protonema produces leafy branches (the true moss-plants), but it does
not produce a thallus. The leafy branches arise directly from the fila-
mentous protonema. True mosses are both monoe-
cious and dioecious. There is no pseudopodium
(Fig. 118), but the stalk of the sporophyte which
is very short in Sphagnum, here elongates to form
a seta, often more than an inch in length.

The true mosses have little breathing pores
called stomata at the base of the capsule. Sphagnum
has the stomata, but they do not function. Chloro-
phyl-bearing cells surround these stomata, so that
in the true-mosses there is some food actually man-
ufactured by photosynthesis.

The sporophyte of the true mosses seems to
occupy an intermediate position between Sphagnum
and the next higher group of plants, the Ferns.

. .

There is an increase in sterile tissue as we approach

r ., . . ,

the terns, and a decrease in fertile tissue in the
sporophytes.

From experiments so far performed it seems
that every cell of the moss-plant can, like the tissue-animal Hydra, which
we shall soon study, develop a protonema — that is, each cell is a poten-
tial spore. Each protonema produces buds which become mature plants.

There are certain species of mosses in which the leafy-shoot, and
in others, the protonemata, give rise to a special type of small bodies
called gemmae (Fig. 119), ( ), which "become sepa-

rated from the parent plant and give rise to new plants.

A comparison of Sphagnum and a fern (to be studied next) is of
value here.

The commonly known "fern-plant" is a sporophyte while the Sphag-
num-plant is a gametophyte.

The fern sporophyte is dependent on the gametophyte for nutrition,
at first, then the sporophyte becomes entirely independent, while the
simple gametophyte perishes.

The Sphagnum sporophyte is the simpler plant and it is this sporo-
phyte which must depend upon the gametophyte for nutrition through-
out its entire life.

Reproduction is quite alike in Fern and Sphagnum. Each produces
haploid gametes of two sexes, which then unite in fertilization, the zygote
being diploid. It is the zygote which produces the spore-bearing phase.
The spores, which are in turn haploid due to a reduction having taken
place, then give rise to the haploid gametophytes, so that we may sum
tip the life-cycle in both Fern and Sphagnum by saying: Gametophyte

PI.A.VT \YOKLD CONTINUED 22:}

alternates with sporophyte, fertilization with reduction, gametes with
spores, haploid cells with diploid.

It will be seen from what has been said that this whole group of
plants shows a differentiation of cells into tissues, while in the higher
forms leaf-like structures appear. Then the rhizoids (specialized ab-
sorbing organs) are developed, and the plant tissues themselves contain
chlorophyl. It is supposed that bryophytes have evolved from aquatic
forms to land forms and consequently, as parts of the plant have dried,
various structural adaptations ( ) have been brought

about.

. ^ PTERIDOPHYTES

These are the Ferns (Fig. 120) and their allies in which the dis-
tinguishing feature is that these plants possess nearly everything that
thallophytes and bryophytes possess, plus a conducting or vascular
system.

These plants are supposed to have arisen "from a bryophyte ances-
try where the sporophyte (sexless) generation, in some plants capable
of doing chlorophyl work, developed a root system and vascular tissue,
and taking the land habit, became independent of the gametophyte.
This was one of the most important forward steps in the evolution of
the higher plants, for it gave the sporophyte complete freedom to live
and grow to its maximum size. This change marked a turning point
in plant evolution, for, after the sporophyte became the most complex
and conspicuous phase of the life-history, the gametophyte grew less
prominent, until in the seed-plants the sexual generation became actually
dependent or parasitic upon the asexual generation. This is a relation
which is exactly the reverse of that which exists between the gameto-
phyte and sporophyte in the liverwort and mosses."

"After the sporophyte became independent of the gametophyte, the
next important advance was the development of the lateral spore-bear-
ing and vegetative organs called fronds ( ). Then
came the differentiation of the frond into vegetative leaves, given up
entirely to chlorophyl work, and spore-leaves (sporophyls) devoted
chiefly or wholly to spore production. With this also, came the massing
of the sporophyls into cones, which was really the beginning of the
structures called flowers in seed plants."

The pteridophytes have underground stems (root-stocks or
rhizomes) so that only the leaves appear above ground. There is a
terminal bud at the tip of the fern-stem. The rhizome bears true roots
and its tissues are differentiated into epidermal, fundamental, mechanical,
and conducting systems. In the tropics there are tree ferns, many of
which have been found among the fossil plants.

The spore-cases grow in groups called son ( )

on the underside of the leaves (Fig. 121). As the annular ring about

224

GENERAL BIOLOGY

each individual spore-case dries up, that side which is thinnest and has
become dried most, splits open, throwing out the spores. There are
usually 64 in each sporangium. These spores drop about the moist
earth and grow into a minute plant, by first absorbing moisture, and
then as the osmotic pressure becomes too great on the inner portion of
the spore it breaks, sending out a tiny tube (Fig. 122). This process is
called germination ( ). Then a smaller tube ap-

pears close to the spore body, but from the tiny tube, mentioned above,

B

D

Fig. 120. The Ferns and Their Allies.

A. Fern plant (Aspidium), showing roots, rhizome, and frond: A., section
of fruit dot (sorus), showing spore cases, some of which are ejecting their spores;
B., portion of a leaflet, showing unripe fruit dots ; C., portion of a leaflet, showing
ripe fruit dots. (After Strasburger. )

B. Order I. Salviniales (Floating Allies of ferns). Salvinia natans.

C. Order II. Equisetales. Branched Equisetum. Equisetum Funstoni, com-
monly called "Scouring Rushes," as distinguished from the "Horsetails" (also
called Equisetales). The stems of Horsetails die each year and the fruiting cones
have no terminal point.

D. Field Horsetail, showing buds and tubers.

E. Order III. Lycopodiales (Club-mosses). Common Club-moss, Lycopodium
clavatum.

F. Order IV. Isoetales (Quillworts). Braun's Quill wort, Isoetes echinospora
Braunii.

(A, after Stra^burger : B to F, from W. C. Clute's "The Fern Allies," by
permission of The Frederick A. Stokes Co.)

PLANT WORLD CONTINUED

Fig. 121.

A., a leaflet of the frond viewed from be-
low to show the position of the sori. B., de-
tails of the sori and veining on a portion of a
leaflet. C., section of a sorus ; t., indusium ;
a., sporangia. D., a spore case or sporangium,
snowing the opening from which the spores
(sp.) have been discharged; r., ring. (From
Bergen & Davis' "Principles of Botany," by
permission of Ginn and Co., Publishers.)

and this is the beginning of the root-like bodies, the rhizoids, which are
to hold the plant in place and absorb moisture and food material from
the ground.

This minute plant developing
from the spore is called the prothal-
lium ( ). It is

often heart-shaped with a portion
just posterior to the notch called
the cushion, several cells thick, and
the outer part called the wings
which are only one cell in thickness.
Near the notch of the heart,
close to the cushion, several flask-
shaped bodies, the archegonia are
formed. Each archegonium contains
an egg cell. Among the rhizoids
are the sperm gonads called anthe-
ridia ( ). Many tiny motile

cells are found in the antheridia at
maturity, but as these are dis-
charged and find a small amount of
moisture they reach the egg and
fertilize it.

It will thus be seen that here, too, as in the mosses, there is an alter-
nation of generations, the ordinary Fern being the asexual plant and
the prothallus the sexual.

SPERMATOPHYTES

This group includes the plants which bear flowers like the rose and
lily, as well as such flowerless groups as the pines which have their re-
productive organs in cones or clusters, and are by no means so conspicu-
ous as are those contained in a real flower.

Two older groupings of these higher plants are :
Phanerogams ( ). (The flowering plants.)

Cryptogams ( ). (The non-flowering plants.)

This grouping is one that came into existence before the sexual
processes of plants had been studied to any extent, and so is not accu-
rate, because the so-called hidden processes of the Cryptogams is in
reality more evident than those of the complicated Phanerogams. As
the seed is the all-important part of a plant from the reproductive point
of view, the name spermatophyte has become popular. Seed plants, like
Ferns, are sporophytes, though there is a gametophyte generation in
their life-history, but it is so reduced in structure that it is quite difficult
to see. The seed must, therefore, be studied.

It can readily be understood that the seed having a hard covering,
which is wonderfully adapted for a protective purpose, lends itself well

226

GENERAL BIOLOGY

S VON

. ; , Fig. 122.
I. The Fern Prothallium and Archegonium.

A., stages in the germination of the spore. B., young prothallium, showing
first appearance of wedged-shaped, apical cell x. C., tip of prothallium beginning
to take on the heart-shaped form ; x., apical cell. D., mature prothallium, showing
group of archegonia on the cushion just back of the notch, and antheridia further
back: rh., rhizoids. E., an open archegonium with egg ready for fertilization,
and two sperms near the entrance of the neck. (A., B., C., E., after Campbell;
D., after Schenck.)

II. The Antheridium and Sperms of a Fern (Onoclea) .

A., small prothallium with many antheridia an. : s., old spore wall. B.,
antheridium, showing cover cell c., ring cell r., and basal cell b., inclosing the sperm
mother cells. C., antheridium opening. D., sperms. (After Campbell.)

III. Diagram of a cytological life-cycle, based on a hypothetical fern with four
chromosomes in the sporophyte. The nuclear phenomena are based on those of the
thread- worm (Ascaris) . Each chromosome is designated by a characteristic mark
so that it may be traced throughout the diagram. (After R. F. Griggs).

£o long vitality, and makes it possible for the embryo to develop so far
within its protective covering that it can take root and establish itself
r.eadily when the time is ripe. Then, too, the seed is a storage organ
oi condensed food for the embryo.

•j(uorjrhe pollen grain of seed plants produces a male gametophyte which
bears either sperm or sperm nuclei.

-ojjj-Jn the ovule of seed plants there is a megaspore which produces an
sac in which the egg is formed. The pollen grain produces an

PLANT WORLD CONTINUED

227

Fig. 123. — Morphology of typical monoco-
tyledonous plant. A, leaf, parallel-ve ned ; B,
portion of stem, showing irregular distribu-
tion of vascular bundles ; C, ground plan of
flower (the parts in 3's) ; D, top view of
flower ; E, seed, showing monocotyledonous
embryo. (From C. Stuart Gager's "Funda-
mentals of Botany" by permission of- P.
Blakiston's Son & Co., Publishers.)

Fig. 124. — Morphology of a typical dico-
tyledonous plant. A, leaf, pinnately-netted
veined ; B, portion of stem, showing concen-
tric layers of wood ; C, ground-plan of flower
(the parts in 5's) ; D, perspective of flower;
E, longitudinal section of seed, showing dico-
tyledonous embryo. (From C. Stuart Gager's
"Fundamentals of Botany" by permission of
P. Blakiston's Son & So., Publishers.)

outgrowth or pollen tube which penetrates the tissues surrounding the
egg and thus the sperm is carried to the egg, fertilizing it.

Seed plants are commonly divided into

Monocotyledons ( ) example, lilies, corn and

grasses. (Fig. 123.)

Dicotyledons ( ) example, beans and cotton.

(Fig. 124.)

The drawings of various stem cross sections will illustrate the dif-
erence in the structure of the two types of seed plants. (Fig. 125.)

Angiosperms. — In this type of plant the ovules are produced in a
closed ovary composed of one or more carpels ( ).

The ovules become seeds, and the carpels and surrounding parts are
what constitute a fruit. This fruit may consist of the ripened ovary
only, or it may include the calyx ( ) and receptacle

also.

As no seed can be formed unless the reproductive organs, stamen

228

GENERAL BIOLOGY

Fig. 125. A. — Diagrammatic Cross-section of Stem of Indian Corn (endogenous or
Monocotyledonous Plant), cv, fibro-vascular bundles; gc, pithy material between
bundles. B. Diagrams of stem sections (exogens or dicotyledonous plant). a,
cross-section of chickweed stem, the inner circle representing the cambium ring,
the two radial lines indicating the portion enlarged in b ; e, epidermis ; h, hair ;
c, cambium-separating between p, phloem and w, woody portions of bundles ;
v, spiral vessels in the woody portion ; x, pith and y, common parenchyma of
bark ; c, segment of a sunflower stem ; p, parenchyma ; 6, bast fibres ; s, sieve
tube ; c, cambium ; g, vessels, pitted and spiral ; h, wood fibres ; d, one year, and
e, four year old woody stems, illustrating the increase of vascular bundles. (From
Needham's "General Biology" after Wettstein, by permission of The Comstock
Publishing Co.)

( ) and pistils ( ) are present,

these are called essential organs, and plants having both in a single
flower are called perfect flowers, while those having only one or the
other essential organs are called imperfect.

If a flower possess in addition to the essential organs a calyx
( ) and a corolla ( ), it is called

a complete flower.

All of these parts are better understood from a study of Figure 146
than from any description that could be given.

PLANT HISTOLOGY

A correct understanding of plant tissues can, however, come only
from a knowledge of how such tissues develop.

Just as we shall soon see, hydra (because
it is composed of tissues only) can regenerate
almost any portion of the body, so, too, the
early embryonic substance of plants is all
quite alike, and can develop into many and
varying types of cells. This early undiffer-
entiated embryonic plant tissue is known as
meristem. It is from this meristem that

t Three Growth Zones, showing

the SO-Called primary tiSSUeS develop. HOW- arrangement of the Fundam n-
, , , i -1 11 1 tal Tissue Layers in roots and

ever, in the early embryo, even while all the stems, i, Dermatogen zone. 2,

11 ., 1-1 -, • .11 Periblem zone. 3, Plerom zone.

cells are quite alike, it is possible to suggest (After c w. Baiiard's
a division into three zones (Fig. 126), in each

°*

PLANT WORLD CONTINUED

229

of which certain particular structures will ulti-
mately grow.

The diagram shows an outer, or dermato-
gen region, a more interior or periblem region,
and an innermost, or plerom region. It is in
the dermatogen zone that the first covering-
tissues develop, while the periblem zone gives
rise to the covering-tissues of the mature
plant. All other structures arise in the plerom
zone.

The original cell-masses which constitute
the three zones mentioned above, are known
as fundamental tissues up to the time the pri-
mary tissues can be seen.

Fig. 127.

A, longitudinal section through
the root tip of spiderwort, showing
the plerome (pi), surrounded by
the periblem (p) , outside of peri-
blem the epidermis (e) which dis-
appears in the older parts of the
root, and the prominent root-cap
(c). (After Land.)

B, diagram of a root hair : CM,
cell membrane ; CS, cell sap ; CW,
cell wall ; P, protoplasm ; N,
nucleus ; S, soil particles.

Fig. 128. Arrangement of the Pri-
mary Tissues in the Root.
1. Epidermis. 2. Hypodermis. 3.
Primary Cortex. 4. Endodermis.
5. Xylem bundle. 6. Pith. 7.
Phloem bundle. (After C. W. Bal-
lard's "Vegetable Histology." Cour-
tesy of John Wiley and Sons ) .

In the dermatogen of the root, three distinct primary tissues de-
velop. The outermost layer at the root-tip (Fig. 127) is the root-cap.
This becomes thickened and protects the more delicate structures as the
process of growth forces the root-cap through the soil.

The epidermal cells above the root-cap give rise to root-hairs, which
are important absorption organs.

Above that portion of the root, which is covered with root-hairs,
there are thick-walled epidermal cells. These form the primary epi-
dermis.

In the periblem zone there are also three primary tissues. (Fig.
128.) The layer bordering on the primary epidermis is known as the
hypodermis ( ). This layer is made up of thick-

230

GENERAL BIOLOGY

walled cells which are usually angled. The layer joining the plerome
zone is the endodermis ( ). The cells in this layer

are also thick-walled and resemble those of
the hypodermal layer. Between hypoder-
mal and endodermal layers there are several
layers of cells which constitute the primary
cortex or cortical parenchyma ( ).

The cortical parenchyma is made up
largely of undifferentiated original periblem
tissue.

It is in the plerom zone (Fig. 129)
where the most striking changes in the cell
walls take place. Groups of cells have
their walls thickened by the deposition of
lignin ( ), which forms the

growth in a. dicotyledonous stem fibrous elements that give strength to the

plant. Such fibrous elements are known as
prosenchyma ( ). The con-

Fig. 129.

Diagram to illustrate secondary

which takes place in the plerome
zone.

R, the first-formed bark; p,
mass of sieve cells ; ifp, mass of
sieve cells between the original

wedges of wood; fc, cambium of ducting elements are developed in the midst

wedges of wood ; ic, cambium be-

cells ; fh, wood of the original

wedges ; ifh, wood formed between

wedges ; x, earliest wood formed ;
(c). (After Land.)

tween wedges; b, groups of bast of theSC Hgnified Cells.

Each group of Hgnified cells, together
with its associated ducts .constitutes the
xylem ( ). This is usually arranged in a very

definite order in the plerom region. There are other cells forming tubes,
also in the plerom zone. The end walls of these cells are perforated.
These form the sieve tubes. Each group of sieve tubes with its asso-
ciated companion cells, parenchyma cells, and Hgnified tissue, constitutes
the phloem ( ). These bundles are also often ar-

ranged in a very definite order.

The Hgnified cells of xylem are called wood fibers (Fig. 130), and
the Hgnified cells of phloem are called bast fibers.

Xylem and phloem are made up of both fibrous and vascular (con-
ducting) elements to form fibre-vascular bundles.

The xylem and phloem are located in a circle near the outer
boundary of the plerom region, and as they begin to develop, usually
alternate with one another.

As there are narrow strips of unchanged plerom parenchyma ex-
tending between the fibro-vascular bundles, (Fig. 131) these strips
present the appearance of rays, and consequently are known as

PLANT WORLD CONTINUED

231

Fig. 130. Types of Wood and Bast Fibers.
A, cross section of bast fibers from stem of Aristolochia
Sip/io showing stratification. B, Portion of bast fiber,
showing oblique striation. C, Portion of bast fiber show-
ing transverse striation. D, Bast fiber from the bark of
Cinchona Calisaya, showing longitudinal striae and small
tubes connecting the lumen of the cell with the exterior.
(From Bastin's "College Botany." Courtesy of G. P.
Engelhard & Co.)

medullary rays, while the
unchanged parenchyma in
the center of the plerom is
the pith.

In many orders of
plants it is these primary
tissues which remain with
but little change, through-
out life, but in the higher
orders these primary tissues
change to secondary or
permanent tissues. (Fig.
132.)

The epidermis is re-
placed by a bark structure
which originates in the
periblem region.

Some of the primary cortical
cells become meristematic, thus
constituting the cork cambium or
phellogen ( ) ; these

cells subdivide rapidly to form a
new tissue on their outer surface,
the cork, and on the inner surface,
phelloderm.

Bark is everything outside of the
true cambium (not the cork cam-
bium), excluding the cambium and
epidermis.

The phellogen retains its meris-
tematic power throughout the entire
life of the plant so that new pro-
tective tissues can keep pace with
the internal growth.

The primary nbro-vascular bundles consist of xylem and phloem,
but in the change to secondary structures, a meristematic tissue called
cambium ( ) develops in connection with these.

The cambium develops on the outer face of the xylem (Fig. 133),
and on the inner face of the phloem, so that the cambium arc on each
xylem bundle produces xylem on its inner face and phloem on its outer
side. Similarly, the cambium arc on the phloem bundle develops xylem
on the inner side and phloem upon the outer.

Fig. 131. Medullary Rays and Pith.

A, Pinus Virginiana, cross section of
two-year-old branch. P, pith ; x, wood, show-
ing two annual rings ; cam, cambium ; ph,
phloem ; r, resin-ducts in the cortex. B, Pinus
insignis, cross-section of the inner part of the
wood. P, pith ; t1, primary tracheae ; ta,
secondary tracheids ; r, resin-ducts ; m, medul-
lary ray. (From D. H. Campbell's "A Uni-
versity Text-book of Botany," by permission
of The MacMillan Co., Publishers).

232

GENERAL BIOLOGY

Fig. 132. Arrangement of Secondary Tissues

in Roots and Stems.

1. Peridem (bark). 2. Phellogen. 3.
Phelloderm (bark). 4. Phloem elements. 6.
Cambium. 6. Xylem elements. 7. Medullary
rays. Compare with Fig. 129. (After C. W.
Ballard's "Vegetable Histology." Courtesy of
John Wiley and Sons).

This causes each fibro-vascular
bundle now to consist of xylem and
phloem elements, separated from
each other by a thin strip of cam-
bium. Such bundles which have
been completed by the cambium are
called complete fibro-vascular bun-
dles, while those not so completed
are known as incomplete fibro-vas-
cular bundles. (Fig. 134.)

As the cambium continues
growing constantly, the plerom
parenchyma becomes almost en-
tirely replaced by xylem. The new

fibro-vascular bundles develop in the broad primary medullary rays.
The stem and root development differ somewhat. There are no

root hairs or root-caps on the stem. The primary stem epidermis often

possesses stomata (breathing pores)

while the root does not. The

parenchymal cells of the stem often

contain chloroplasts which the

parenchymal cells of the root never Fig. 133. Diagram showing the Method by

do. Then, too, the root has no hypo- wh^sth0en Srt^^f B^^L^

dermis (mechanical tissue immedi- bc the cells J^ES??'*. cambium ceih:

ately underneath the epidermis), vac, the wood ceils.

There is usually no endodermis in the stem though there is in the root.

The plerom zone of primary stems differs considerably from that of

primary roots both in the arrange-
ments and development of tissues.
All fibro-vascular bundles in the
plerom region of the primary stem
are complete, showing phloem,
xylem, and cambium elements
throughout their entire period of
growth. This means that the primary
fibro-vascular bundles of the stem
are really equivalent to the sec-
The primary stem structures

Fig. 134. Completion of Fibrovascular Bundles.
F, Completed fibrovascular bundle. 1.
Xylem elements. 2. Cambium. 3. Phloem ele-
ments. (From C. W. Ballard's "Vegetable
Histology," Courtesy of John Wiley & Sons).

bundles of the

ondary bundles of the root,
described above serve throughout the life of the plant only, if such
plant is an annual. In perennials ( ), a better and

more durable covering tissue must be developed. In these the primary
epidermis is replaced by periderm tissues which have been produced by
a phellogen which in turn developed in the primary cortex. The peri-
derm of stems is often ruptured and cast off as the inner tissue expands.
This does not occur in roots. When such casting off takes place, the

PLANT WORLD CONTINUED

233

primary periderm is replaced by secondary periderm which develops
directly from the original phellogen or secondary phellogen layers. The
hypodermal and endodermal layers disappear as soon as the phellogen
is formed in the primary cortex. The primary fibre-vascular bundles

become larger by new
xylem and phloem ele-
ments being added by the
cambium and the cam-
bium arcs extend until
they become a complete
ring or circle.

New fibre - vascular
bundles form in the broad
medullary rays extending
between the original bun-
dles while new woody ele-
ments are being added to
the xylem. These woody
elements, however, never
entirely replace the
original plerom tissue in
the center of the stem.
This unchanged central
plerom tissue is the pith.
As the plerom paren-
chyma is entirely replaced
by woody tissues in roots,
the presence of pith is val-
uable in distinguishing
stem from root.

The secondary or per-
manent stem tissues are
often divided into parenchyma ( ) and prosenchyma.

Parenchymal cells may be found in all three zones of the embryo. They
have thin walls and protoplasmic contents. Prosenchymal cells are
formed in the plerom region of the embryo. They have thick walls, and
the protoplasmic contents are very inconspicuous or even entirely lack-
ing. While these distinctions are by no means absolute, they are of
great importance. Further, prosenchymal cells are usually spindle-
shaped while parenchymal cells are more inclined to be spherical or
cubical with rounded corners. (Fig. 135.)

The final tissues are usually grouped according to their functions.
They are:

Fig. 135.

A. Early undifferentiated cells known as Embryonic or
Meristem tissue.

B. The secondary (permanent) tissues are divided into
parenchyma and prosenchyma. The former have thin walls
and protplasmic contents. They are found in the undiffer-
entiated cellular structures of all three zones in the embryo.
They are usually spherical in shape, or at least "as broad
as they are long." Prosenchyma cells are formed in the
Plerom region of the embryo. They have thick walls and
little or no cell content. The cells are usually long fiber
cells with sharp-pointed ends.

a. Transverse Section, Triticum Rhizome. 1. Epidermis.
2. Hypodermis. 3. Cortical parenchyma. 4. Endodermis.
5. Fibers, surrounding sieve and ducts. 6. Sieve. 7. Ducts.
8. Concentric fibrovascular bundle. 9. Pith parenchyma,

b. Powdered Triticum Rhizome. 1. Epidermis. 2. Hypo-
dermis. 3. Parenchyma, longitudinal view. 4. Endodernvis.
5. Fibers. 6. Vessels. (From C. W. Ballard's "Vegetable
Histology," Courtesy of John Wiley & Sons).

*Xylem and phloem both carry water, but the former carries food material as such, while the
latter carries food in the water.

234

GENERAL BIOLOGY

Covering or Protective Tissues. (Fig. 136.) Epidermis and peri-
derm.

Supporting or Mechanical Tissues. (Fig. 137.) All fibrous tissues,
such as wood and bast fibers, stone cells (schlerenchyma), polygonal cells
with very thick cellulose walls, especially thick at the angles (collen-
chyma) which take the place of woody tissue in annual herbaceous or
green stems, fruits, seeds, and leaves. Collenchyma is usually associated
with the fibrous tissues in the midrib of leaves.

Fig. 136. Epidermal Tissues.

A, Sectional views of Leaf Epidermis. 1. Upper epidermis, Ficus leaf. 2. Lower
epidermis, Ficus leaf. 3. Upper epidermis, Eucalyptus leaf. 4. Epidermis of Pine
leaf. 5. Upper epidermis, Orange leaf. 6. Upper epidermis, Geranium (Pelargo-
nium), leaf. E, epidermis. H, hair.

B, Surface views of Leaf Epidermis. 1. Hepatica leaf (wavy walls). 2. Chima-
philla leaf (beaded walls). 3. Henbane leaf ( v/avy and striated walls). 4. Senna
leaf, angled cells). 5. Convallaria leaf (beaded walls). (From C. W. Ballard's
"Vegetable Histology,'' courtesy of John Wiley & Sons.)

Absorption Tissues. (Fig. 138.) Root-hairs for liquids, and
stomata (openings usually on the underside of leaves surrounded by two
sausage-shaped guard-cells) and lenticels (openings in the periderm or
corky coverings of mature woody plants).

Conducting Tissues. (Fig. 139.) Ducts (tracheae are continuous
tubes formed by the absorption of the connecting cell's end-walls, and
the disappearance of the cell contents.) These tubes may be pitted
(when there are numerous pores through the cell wall), reticulate (when
the lignin laid down on the inner side of the cell wall is in the shape of
a network), scalariform (when the non-lignified portions of the cell walls
form long narrow slits which are quite uniform). Such cells are often
angled (no others are). Annular (thin-walled tubes with rings of \\g-
nified tissue within the lumen of the tube), and spiral (where the lig-

PLANT WORLD CONTINUED

235

nified tissue is arranged in the form of a continuous spiral-band, or col-
lection of bands).

Tracheids are merely single cells which have lost their protoplasmic
contents, but not their entire end-walls. Communication is carried on
by pores in the vessel walls.

Sieve Tubes, unlike all the other ducts mentioned above, usually
carry materials away from the leaves. They are merely individual cells
whose end-walls have not completely
broken down, as in the tracheids, but 1

have formed sieve plates with many
pores or perforations connecting one
such individual cell with the next be-
low, and so continuing for great
lengths in the plant. The walls of

Fig. 137. Mechanical and Supporting Tissues.

These tissues consist of wood and bast fi-bers (See Fig. 130), schlercnchyma
(stone-cells), and collenchyma.

A, Portion of epidermis and collenchyma from the stem of Rumex crispus.
Cross t< ction, ep, epidermis ; c, collenchyma.

B, Sclerotic cells from the root of Apocynum androsaemifolium. All highly
magnified. (From Bastin's "College Botany" courtesy of G. P. Engelhard & Co.)

C, 1. Peppermint stem. Arrangement of collenchymatic (C), tissues at angles
of the stem. 2. Peppermint stem. 3. Sabal seed. 4. Colchicum seed. (Porous
type). 5. Nux Vomica seed. (Striated type). 6. Arrangement of collenchymatic
tissues around the midvein of a leaf. C, collenchyma. (From C. W. Ballard's
"Vegetable Histology," Courtesy of John Wiley & Sons).

sieve tubes are composed of cellulose, there being no trace of lignifica-
tion.

Medullary Rays furnish the method by which material is transported
from the inner-tube region of the plant, laterally, to the tissues which
lie closer to the outside of the stem, and from these to the pith where
food may be stored.

Latex Tubes. These are non-porous tubes in certain plants and
contain a milk-like fluid.

. Porous Parenchyma. In the pith region the parenchyma, which is
very porous, may possibly assist in permitting the nutrients which are
in solution to pass back and forth.

236

GENERAL BIOLOGY

As already stated, each group of vessels with its connecting mechan-
ical or supporting tissue forms a fibre-vascular bundle. These may be

either complete or incomplete; com-
plete, if they possess xylem, phloem,
and cambium elements, and incom-
plete if they possess either xylem or
phloem without the cambium ele-
ment. The xylem is always sup-
ported by wood fibers and the
phloem by bast fibers. There are
five different arrangements of fibro-
vascular bundles :

(1) Radial (common in all
young roots, and sometimes in
mature monocotyledonous roots).
These are always incomplete, con-
sisting of either xylem or phloem.
The xylem or phloem elements are
arranged in a circle within the endo-
dermis, a xylem bundle alternating
with a phloem bundle.

(2) Concentric fibre-vascular
bundles (common in monocotyledo-
nous roots and stems), are bundles
consisting of both xylem and
phloem, so arranged that either
the xylem surrounds the phloem or

the phloem surrounds the xylem. The former arrangement is the more
common. The bundles are irregularly scattered in the pith region.

(3, 4, and 5) Collateral fibre-vascular bundles are complete, having
both xylem and phloem elements, as well as a cambium-arc. These are
in turn divided into three types, known as open, closed, and bi-collateral.

(3) Closed Collateral bundles (usually found only in the pith of
monocotyledonous stems and rhizomes and the leaves of all seed plants),
are made up of a xylem portion and a phloem portion, never separated
from each other by a strip of cambium.

(4) Open Collateral bundles (most frequently found in dicotyledo-
nous roots and stems) are made up of xylem elements within a cambium
zone and phloem elements on the outer side of the cambium.

(5) Bi-Collateral bundles (found in some dicotyledonous roots and
stems) are made up of a xylem element and the associated cambium, but
with two phloem elements, one on each surface of the xylem.

Assimilating and Synthesis Tissues (Fig. 140). The Chloroplasts

Fig. 138. Absorption Tissues.
1. Root hairs (H) on rootlet of germinat-
ing Fenugrek seed. C, root cap. 2. Stomata,
surface view. A, breathing pore. G, guard
cells. B, bordering, neighboring or surround-
ing cells. 3. Stomata, sectional view. A,
breathing pore. G, guard cells. B, bordering
cells. 4. Lenticel (A). (From C. W. Bal-
lard's "Vegetable Histology," Courtesy of
John Wiley & Sons).

PLANT WORLD CONTINUED

F

237

Fig. 139. Conducting Tissues.

A. Collateral type, Bamboo stem. 1. Fibrous tissue. 2. Ducts. 3. Sieve.

B. Collateral Bundle, arrangement of fibrovascular elements. 1. Xylem. 2.
Endodermis. 3. Phloem.

C. Bicollateral Bundle, arrangement of fibrovascular elements. 1. Phloem.
2. Cambium. 3. Xylem. 4. Cambium. 5. Phloem.

Z>. Open collateral type, Aconite tuber. 1. Bast fibers. 2. Sieve cells. 3. Cam-
bium. 4. Wood fibers. 5. Ducts. 6. Medullary ray.

E. Open Collateral Bundle, arrangement of fibrovascular elements. 1. Phloem.
2. Cambium. 3. Xylem. 4. Medullary ray.

F. Radial type, Sarsaparilla root. 1. Endodermis. 2. Sieve surrounded by
bast fibers. 3. Wood fibers surrounding sieve and ducts. 4. Ducts.

G. Radial Bundle, arrangement of fibrovascular elements. 1. Endodermis.
2. Xylem. 3. Phloem. 4. Pith.

H. Concentric type, Fern rhizome. 1. Endodermal sheath. 2. Sieve sur-
rounded by small parenchyma. 3. Fibrous tissues. 4. Ducts.

/. Concentric Bundle, arrangement of firo vascular elements. 1. Endodermal
sheath. 2. Phloem. 3. Xylem. (From C. W. Ballard's "Vegetable Histology,"
Courtesy of John Wiley & Sons).

(the tiny divisions in the cell of plants which contain chlorophyl), are
important structures in synthesis by converting (when in the sunlight)

B

Fig. 140. Assimilating and Synthesis Tissues.

A. Plastids (chloroplasts) in a cell.

B. Diagram to illustrate the processes of
oreathing, food-making, and transpiration
which may take place in the cells of a green
leaf in the sunlight. (After Stevens).

carbon dioxide and other sub-
stances into starches and sugars ;
and the Leukoplasts (similar struc-
tures which do not contain
chlorophyl), by assisting in form-
ing storage- or reserve-starch from
the sugar manufactured by the
chloroplasts.

Secreting cells and hairs which
are really structures quite like the
glands of animals.
In plants which continue their life

Storage Tissues (Fig. 141).
throughout many seasons, there must be a method of storing the food
which is made primarily in the summer. The organs are the Paren-

238

GENERAL BIOLOGY

chyma cells of the cortical and pith regions. Here the reserve starch
made by the leukoplasts is stored, as also are many other plant nutrients.

Secretion cavities of various kinds carry oils and other products of
gland cells.

Colenchyma cell walls, especially in seeds and fruits, contain much
cellulose. This means that collenchymal cells are supporting and storage
tissue as well as synthesis tissue.

Fig. 141. Storage Tissues.
These are the parenchyma cells of the
cortical and pith regions of the plant ; the
cellulose in the collen chyma cells (which
makes collenchyma a synthesis, supporting,
and storage tissue), and cavities of stone
cells and fibers.

A, grain of corn, cut lengthwise ; C, coty-
ledon ; E, endosperm ; H, hypocotyl ; P,
plumule.

B, starch grains in the cells of a potato
tuber.

Fig. 142. Reproductive Tissues.
Diagrammatic sections of sporogonia of
liverworts : A, Riccia, the whole capsule being
archesporium except the sterile wall layer ;
B, Marchantia, one half the capsule being
sterile, the archesporium restricted to the other
half ; D, Anthoceros, archesporium still more
restricted, being dome-shaped and capping a
central sterile tissue, the columella (col)
(After Goebel).

Cavities of Stone cells and Fibers may contain nutrient material in
a few cases, but in such instances it is not readily available for the plant.

Reproductive Tissue (Fig. 142). From inner tissues of anther and
ovary in flowering plants.

When pollen is transferred from anther ( ) to

stigma ( ) the process is called pollination. Wind,

insects, and water are means by which pollen is carried from one plant
to another. Bees are common carriers, and the remarkable way some
plants are adapted to forcing any intruder to carry pollen with it, is one
of the most astounding of all adaptations in nature (Fig. 239).

POLLINATION

Pollination can probably best be understood by considering the mod-
ern pines. In the common Scotch Pine (Fig. 143), (Pinus silvestris)
the microsporophyls (called stamens in the flowering plants) are
massed into cones about 1 centimeter in length, and these cones are in
turn massed in clusters.

There are two sporangia on the lower surface of each microsporo-
phyll. These microspores or pollen escape from the sporangia and are
carried by the wind (often for many miles) to the megaspore (carpellate)
cones.

The megaspore cones grow singly or in clusters near the ends of the
upper twigs of the season's growth, and are also about one centimeter in
length. There is a general axis on which flat megasporophyls are borne.
Each of these megasporophyls bear two inverted megasporangia or
ovules (Fig. 144).

PLANT WORLD CONTINUED

239

The pollen falls between the megasporophyls (called carpels in the
flowering plants), and each microspore then pushes out a pollen tube
which penetrates the ovule tissue. This process stimulates the growth
of the cone tissues and the cone, therefore, increases in size. The ovules
also enlarge and the upper end of the ovule develops a thickened mass
of green tissue which grows beyond the end of the sporophyl, to form
the seed scale. These seed scales are merely the distal ends of the ovules,
and function as organs of photosynthesis for a year or so.

Fig. 143. Scotch Pine (Pinus sylvestris ) .

A-D, stages in the development of the carpellate cone, and its car-
potropic movements. E, very young carpellate cone much enlarged ; F,
ventral, G, dorsal views of a scale from E ; 1, ovuliferous scale; 2, ovule
(in longitudinal section) ; 3, pollen chamber and micropyle leading, to
the apex of the nucellus (megasporangium) ; 4, integument of the ovule;-
G, 1, tip of ovuliferous scale ; 5, bract ; 4, integument ; H, longitudinal
section at right angles to the surface of the ovuliferous scale (diagram-
matic) ; 6, megaspore ; 7, pollen chamber, /, longitudinal section of a
mature cone ; 6, ovule ; J, scale from a mature cone ; 6, seed ; w, wing of
seed ; K, dissection of mature seed ; h, hard seed coat ; c, dry mem-
branous remains of the nucellus, here folded back to show the endosperm
and embryo ; e, embryo ; p, remains of nucellus ; L, embryo ; c, coty-
ledons ; e, hypocotyl ; r, root-end. (From C. Stuart Gager's "Funda-
mentals of Botany," by permission of P. Blakiston's Son & Co., Publishers).

The following summer or autumn a spore-mother-cell, also known
as an archespore, arises in the interior tissues of the ovule. This arche-
spore then divides into four cells which are really four young mega-
spores, although only the one lying in the lowest position actually de-
velops into a full-fledged megaspore.

This megaspore then divides and subdivides until a rather solid
cellular mass is formed. This cellular mass is the gametophyte. (Fig.
145.)

It is from this gametophyte that several (usually four) sunken
archegones arise. The completing process of fertilization may now take
place.

240

GENERAL BIOLOGY

After fertilization the gametophyte becomes stored with food and
functions as the endosperm. HI

The pollen-tube
has also resumed its
growth by this time
and has brought the
two non - ciliated
sperm to the mouth
of an archegone. One
of the . sperm fuses
with the egg which
completes fertiliza-
tion. This fertiliza-

tion takes place in
the pines more than
a year after pollina-
tion.

The fertilized
egg, now called a
zygote, gives rise to
the embryo consist-
ing of a cylindrical
stem with narrow
whorled leaves and a

Fig. 144. I. Carpellate cone, carpels, and seed of the Sco^cn pine (Firms sylveatris) .

A, young growth with carpellate cones, about three weeks after the opening of
the terminal bud : n, young pine needles. B, inner and side view of a cone scale
at the time of pollination as shown in A : b, bract ; o, ovules. C, inner and side
view of scales from a mature cone as shown in D : b, bract ; o, fertilized ovules
now rapidly maturing into winged seeds ; w, the developing wings. D, a. mature
cone. E, a mature winged seed. F, section of mature seed ; t, hard seed coat,
or testa, developed from the integument of the ovule, n, a membranous seed coat
which is the remains of the nucellus ; en, endosperm or tissue of the female
gametophyte ; em, embryo with group of cotyledons c and the suspensor s ; m,
micropylar end of seed.
II. The staminate cone, stamen, and pollen of the Scotch pine (Pinus sylvestria ) .

A, young growth, with staminate cones about two weeks after the opening of
the terminal bud. B, details of cone. C, end view of stamen. D, side view of
stamen. E, pollen mother cell developing four pollen grains in a tetrad. F, pollen
grain showing the two wings ; p, prothallial cell ; g, generative cell ; t, tube
nucleus. — E, (After Miss Ferguson).

III. White pine.

(Pinus Strobus) . Longitudinal section through an archegonium at the time of
fertilization. Above the fusing nuclei are various other elements emptied into the egg
from the pollen-tube. Collected June 21, 1898. X about 62. s.g., starch grains ; p.r.,
prothallium ; c.p.t, cytoplasm from pollen-tube ; st.c., stalk-cell ; t.n, tube-nucleus ;
s.n, sperm-nucleus; e.n, egg-nucleus; n.s, nutritive spheres. (After Margaret C.
Ferguson). I, II, (From Bergen & Davis "Principles of Botany," by permission
of Ginn & Co. Publishers). Ill, (From C. Stuart Gager's "Fundamentals of
Botany," by permission of P. Blakiston's Son & Co. Publishers).

PLANT WORLD CONTINUED

241

root. It is still imbedded in the gametophyte tissue from which it draws
its nourishment.

The ovule, seed-scale, and cone, have increased in size in the mean-
time, the seed-scales losing their chlorophyl and becoming woody. As

Fig. 145. I. The Gametophytes of the Pine.

A, diagram of a section of a year-old ovule ; embryo sac with mature arche-
gonia ar imbedded in the tissue of the endosperm (female gametophyte) ; pollen
tubes (male gametophytes ) growing down through the tissue of the nucellus n ;
p c, pollen chamber ; m, micropyle ; i, integument. B, germinating pollen grains,
showing young male gametophyte ; t, tube nucleus ; g, generative nucleus ; p, pro-
thallial cell. C, tip of pollen tube applied to the egg ; t, tube nucleus ; s, the two
sperm nuclei. D, a mature archegonium sunken in the tissue of the endosperm,
showing the large egg surrounded by a jacket of cells rich in protoplasm : two neck
cells of the archegonium shown just above the egg. — B, C, (After Miss Ferguson).
II. The Sperm and Ovule of a Cycad (Zamia) .

A, lower surface of a stamen, with numerous pollen sacs in two groups. B,
the two large top-shaped motile sperms at the end of the pollen tube ready to be
discharged above the archegonia. C, a sperm viewed from the end, showing the
spiral band which bears the cilia. D, diagram of a section of an ovule after polli-
nation : m, micropyle ; i, integument ; p, pollen chamber ; n, nucellus containing
developing pollen tubes ; a, archegonia, with large eggs imbedded in the endosperm
female gametophyte). — B, C, (After Webber).

III. — Diagram of the life-cycle of a pine. (After Schaffner).

I., II., (From Bergen & Davis "Principles of Botany," by permission of
Ginn & Co. Publishers).

242

GENERAL BIOLOGY

the supply of water becomes less and less the cone becomes dry and
consequently the young sporophyte stops growing. The cone and seeds
are now said to be ripe, so that as the dry seed scales are spread out and
blown away the part of the seed which contains the embryo is carried
with them, and as soon as water is again supplied the embryo again
begins to grow, breaking the brittle integument or indusium covering it,
and the root is ready to penetrate the soil and carry water to stem and
leaves of the new plant.

FLOWERS

The flowers of flowering plants (Fig. 146) consist of cone-like clus-
ters of closed megasporophyls (carpels) above, and microsporophyls

(stamens) below, subtended
by a perianth. The plant on
which the flowers grow is the
sporophyte.

The microspores or pol-
len-cells (Fig. 147) each pro-
duce a mature gametophyte
which consists of a pollen tube
with three nuclei (Fig. 148 B) ;
one, the nucleus of the pollen
tube itself, and the other two
sperm nuclei.

The megaspore is retained
within the ovule (Fig. 148 A),
(megasporangium). A gameto-
phyte with a single egg devel-
ops within the ovule. After
fertilization, the zygote devel-
ops into an embryo and an en-
dosperm, to be described
shortly, while the entire ovule
becomes covered with one or
two coats to form the seed.
With proper moisture and soil,
the sporophyte escapes from
the seed as with the pine. (Fig.

ciples of Botany," by permission of Ginn & Co. Pub- 1/1Q \
lishers). 4^0

The purpose of a flower is the production of seed. The ripened
carpel with its contained seed is known as a fruit. (Fig. 150.)

The Buttercup (Fig. 151) will serve as an excellent example of the
flowering plants. Here we have many carpels (simple pistils) each
made up of an ovary (the simple closed cavity below) which gradually
tapers to a soft terminal stigma. The carpels are flat and open when

pe \~

Fig. 146. Floral Organs.

A, Orange blossom. (After Bailey).

B, Hydrophyttum, col, lobe of calyx ; cor, lobe of
corolla; st, stamens; p, pistil. (After Lindley).

C, Diagrams of flower, showing face-view and
dissection, r, receptacle ; se, sepal ; pe, petal ; st,
stamen ; pi, pistil ; o, ovule.

The parts of a complete bisexual flower of the
higher seed plants (angiosperms) are sepals, petals,
stamens, and pistils. The sepals, taken together,
constitute the calyx ; the petals, taken together, con-
stiute the corolla. The calyx and corolla are col-
lectively known as the floral envelopes, or perianth.

Many angiospermous flowers consist of five cir-
cles, or whorls, two of which belong to the perianth,
two to the stamens, and one to the pistils. The parts
of each circle alternate in position with those of the
preceding or following one, and all the members of
each circle are alike. (From Bergen & Davis "Prin-

PLANT WORLD CONTINUED

243

the plant is young, but they gradually have their margins curve upwards
and close. During the time the carpel is closing, an ovule grows out
from the base and becomes enclosed by the carpel walls.

There are several rows of stamens encircling the pistils. Each sta-
men or microsporophyl bears four elongated, parallel, sporangia con-
taining pollen or microspores. The stalk of the stamen is called the fila-
ament, while the four pollen-sacs (sporangia) are known collectively as

Fig. 147.

A. Different kinds of pollen grains, highly magnified, two
of them forming pollen tubes. ( After Duggar ) .

B, C. Parts of a stamen.

A, front ; B, back ; a, anther ; c, connective ; /, filament.
(After Strasburger ) .

D, E, F, Modes of discharging pollen.

A, by longitudinal slits in the anther cells (amaryllis) ; B,
by uplifting valves (barberry) ; C, by a pore at the top of each
anther lobe (nightshade). (After Baillon).

the anther. When mature, the sporangia split longitudinally and permit
the escape of the pollen.

There are two series of leaf-like structures below these we have just
been discussing. These two series together form the perianth. The
upper series is made up of yellow petals. The petals collectively form
the corolla. The lower series consists of five pointed, green sepals, and
collectively forms the calyx.

A spore-mother-cell or archespore arises in the ovule (Fig. 148A).
This then divides into four young megaspores, only the deeper one de-
veloping. The other three perish. There is thus only a single megaspore
in the ovule. The nucleus of the megaspore later divides into two, each

244

GENERAL BIOLOGY

portion moving toward opposite poles of the megaspore cavity. Each
of these portions divides twice, thus forming four nuclei at each pole.

One nucleus from each pole (often called the polar nuclei) then
moves toward the center and these two meeting, unite.

One of the nuclei about the pole functions as an egg nucleus. The
two companion cells are called synergids. The cells at the opposite pole
are called the antipodal cells.

It is at this time that the pollen, which has fallen on the carpel
stigma, germinates to produce a reduced gametophyte and a pollen tube.
This pollen tube penetrates the soft stigma tissues and carries two sperm
toward the ovary cavity. As the pollen tube reaches the ovule it enters
a tiny pore called the micropyle between the two integuments, and then
passes through the nucellus. The ovule is thus penetrated, and one of
the sperm unites with the egg and fertilizes it.

The zygote now divides continually and soon there is developed a

A.

Fig. 148.

A. At the left, diagram of the anatomy of an angiospermous flower shortly
after pollination ; anth., anther ; fil., filament ; st., stamen ; stig., stigma ; p. g.,
pollen grains germinating; sty., style; pt., pollen tube; o. w., ovary wall; o., ovule,
containing embryo-sac; pet., petal; sep., sepal. 1-8, Stages in the development of
the female gametophyte (embryo-sac) ; meg.sp., megaspore-mother-cell ; i.L, inner
integument ; o.i., outer integument ; fun., funiculus ; chal., chalaza ; nu., nucellus
(megasporangium) ; emb., embryo-sac. All diagrammatic. (From C. Stuart
Gager's "Fundamentals of Botany," by permission of P. Blakiston's Son & Co.
Publishers).

B. Diagrammatic Representation of Fertilization of an Ovule.

t, inner coating of megasporangium (ovule) ; o, outer coating of ovule; p,
pollen tube proceeding from one of the pollen grains on the stigma ; c, the place
where the two coats of the ovule blend. (The kind of ovule here shown is inverted,
its opening m being at the bottom, and the stalk / adhering along one side of the
ovule), a to e, embryo sac, full of protoplasm; a, so-called antipodal cells of
embryo sac ; n, central nucleus of the embryo sac ; e, nucleated cells, one of which,
the egg cell, receives the male nucleus of the pollen tube ; /, funiculus or stalk of
ovule; m, micropyle or opening into the ovule. — (After Luerssen).

PLANT WORLD CONTINUED

Fig. 149. Diagram of Life-cycle of an Angiosperm (Alisma

Plantagoaquatica) .

9, female gametophyte (embryo-sac) ; 8a and 9a, male gameto-
phyte (pollen-grain). (After J. H. Schaffner).

Fig. 150.

Development of the pea fruit from the pea flower. (After
Yung's Chart).

tiny stem with a little root at one end and two rudimentary leaves at
the other.

The gametophyte has, in the meantime, resumed its development
on account of the union of the second sperm nucleus with the two polar
nuclei to form the endosperm nucleus. This endosperm nucleus divides
rapidly, although the cell walls are much delayed in this development.
In a short time the endosperm has surrounded the embryo sporophyte
and has rilled in the growing ovule. This surrounding and nourishing
cell mass is now called the endosperm, which is neither gametophyte
nor sporophyte.

As the ovule grows in size, its outer coat becomes thickened and
hardened, and the endosperm within has enlarged and solidified. A
layer of cells at the base of the ovule now becomes corky and checks

246 GENERAL BIOLOGY

the supply of water, so that the whole ovule becomes hardened to form
the seed.

It \vill be noted, therefore, that the spermatophytes also show an
alternation of generations, the ordinary plant being- the sexless type. It

is this ordinary flowering plant
which produces the microspores, or
pollen grains, and megaspores. In
the nuclear divisions which produce
these cells, the chromosome number
is reduced to half the original num-
Fig. i5i. ber.

The pollen grains produce one

ofThe" &Z wfthTn" tM °f the S6XUal PhaS£S °f the life his-

large petals of the corolla of which three are tory, the male gametophyte, which

shown ; within this and seated higher on the J \ J

axis are the numerous club-shaped stamens, forms the Sperm nuclei ', the mega-

each of which bears four pollen-sacs. Cen- , , ,

trally in the flower are the numerous carpels, Spore prOdUCCS the Other SCX-

one of which is dissected so as to show its -, _i__ 4.1,^ r^.^,^^1^. .^t,

single ovule, or future seed. (From Bower Ual phase, the female gametophyte

which bears an egg. Fertilization

occurs by the fusion of a sperm cell with the egg; thus the nucleus of
the fertilized egg contains twice the number of the reduced amount of
chromosomes, one-half of which has been contributed by the sperm and
one-half by the egg. The fertilized egg develops into the embryo of the
seed which, upon germination, becomes the mature sporophyte or sexless
phase of the life history with its characteristic number of chromosomes.

References :

Strasburger, Noll, Schenck and Karsten, "A Textbook of Botany."

Coulter and Chamberlain, "Textbook of Botany." Vols. I and II.

Coulter, "Plant Structures."

Wm. C. Stevens, "Plant Anatomy."

C. W. Ballard, "The Elements of Vegetable Histology."

C. S. Gager, "Fundamentals of Botany."

Berger and Davis, "Principles of Botany."

C. E. and E. A. Bessey, "Essentials of College Botany."

Edson S. Bastin, "College Botany."

Geo. Massee, "A Textbook of Fungi."

F. L. Stevens, "The Fungi which Cause Plant Disease."

Elizabeth M. Dunham, "How to Know the Mosses."

Wm. N. Clute, "The Fern Allies."

CHAPTER XVII.

THE COELENTERATA

The Coelenterata (Gr. koilos==hollow+enteron— intestine) are all
aquatic (mostly marine) animals, possessing a single system of internal
chambers called a gastro-vascular-cavity, having a single opening which
serves both as a mouth and a vent for egestion and excretion. In other
words, digestion and circulation all occur in this single tubular cavity.
In all the higher forms of animal life there is a coelom ( ),

that is, a cavity between the intestinal tract and the body wall. This
was observed in the frog where all the viscera ( ) are

inside the body but outside the intestinal tract.

In the Coelenterata there is a radial symmetry as contradis-
guished from the bilateral symmetry of the frog.

The animals belonging to this phylum are diploblastic, that is, they
have gone through the gastrula stage in developing and remained sta-
tionary at the end of that stage, with this exception, that they just begin
forming a third layer which, however, never becomes a regular tissue.
The entoderm and ectoderm are separated from each other by a thick
mucilaginous mesoglea ( ) or mesenchyme

( ). The point of value here is that in the higher

forms this midlayer becomes an actual tissue by forming a very definite
sheet of cells called the mesoderm, while in the Coelenterata the layer
does not become cellular. The midlayer here acts as though it were
about to form into a tripoblastic animal but has not succeeded.

There may be a few migratory cells found in the mesoglea, but as
a whole it is non-cellular.

The phylum is further distinguished by the fact that in practically
all its members there are stinging cells [(sometimes called nettle-cells
or nematocysts ( )].

Nerve cells (sensory) and muscle cells both occur.

Reproduction by non-sexual methods is the more common, though
sexual methods may alternate, forming individuals of quite unlike ap-
pearance.

HYDRA FUSCA

The classic coelenterate for laboratory study is this little animal
(Fig. 152), found in ponds and streams attached by its basal end to vari-
ous types of aquatic vegetation. It is from 2 to 20 mm. long; conse-
quently it can be seen by the naked eye.

The entoderm contains the brown bodies from which the animal
receives its name. The animal itself has a mouth opposite the basal disk.
About the mouth, there is a varying number of tentacles, usually four

248

GENERAL BIOLOGY

to seven. These are closed at their free ends, but their interior channels
are a direct continuation of the gastro-vascular cavity.

At the distal third of the body, the male gonads, the testes, are seen
as cone-shaped elevations during the breeding season (September and
October), while the female gonads, the ovaries, are knoblike projections

close to the basal disk. In addition
to these sexual organs one may find
buds on various parts of the body.

As the Hydra is a diploblastic
animal, that is, one which has re-
mained in the gastrula stage, it
means that the simple indentation
of the original blastula has given the
animal only epithelial tissue, for
epithelium is surface tissue, and
both inner and outer portions of this
animal are surface tissues.

The ectoderm is primarily pro-
tective and sensory, and is made up
of two principal kinds of cells: (1)
epitheliomuscular, and (2) intersti-
tial. The former are shaped like in-
verted cones, and possess long (up
to .38 mm.), unstriped contractile
fibrils at their inner ends ; these
enable the animal to expand and
contract.

The interstitial cells lie among
the bases of the epitheliomuscular
cells ; they give rise to three kinds
of nematocysts or stinging cells
(Fig. 153). Nematocysts are present on all parts of the body except the
basal disc, being most numerous on the tentacles. The interstitial cell
in which the nematocyst develops is called a cnidoblast ( ) ;

it contains a nucleus and develops a trigger-like process, the cnidocil
( ), at its outer end, but is almost completely filled

by the pear-shaped nematocyst. Within this structure is an inverted
coiled thread-like tube with barbs at the base. When the nematocyst
explodes, this tube turns rapidly inside out and is able to penetrate the
tissues of other animals. The explosion is probably due to internal pres-
sure produced by osmosis, and may be brought about by various methods
such as the application of a little acetic acid or methyl green. Many
animals when "shot" by nematocysts are immediately paralyzed and
sometimes killed by a poison called hypnotoxin which is injected into it
by the tube.

Fig. 152. Hydra.

A, an animal in its expanded form ; B,
the same animal contracted ; C, a diagram of
the longitudinal section of the animal, show-
ing the internal structure ; D, an epithelio-
muscle cell ; E, & bit of the body wall highly
magnified showing the two layers of the body ;
F, a digestive cell ; G, one of the nemato-
cysts with its thread extruded ; H , & second
type of nematocyst with the coiled thread
within the sac ; I, nematocyst of the third
type with its thread extruded ; J, a bit of the
tentacle, very highly magnified, showing the
batteries of the nematocysts ; K, two of the
secreting cells of the basal disk, en, cnidocil ;
ec, ectoderm ; en, endoderm ; m, mouth ; mes,
mesogloea ; o, ovary ; s, spermary ; t, new ten-
tacle forming. (After Conn.)

THE COEI.ENTERATA

249

Two kinds of nematocysts smaller than that just described are also
found in the ectoderm of Hydra. One of these is cylindrical and con-
tains a barbless thread which, when discharged, aids in the capture of
prey by coiling around the spines or other structures that may be
present.

Certain ectoderm cells of the basal disk of Hydra are glandular and
secrete a sticky substance for the attachment of the animal.

The entoderm, the inner layer of cells, is primarily digestive, ab-
sorptive, and secretory. The digestive cells are large, with muscle fibrils
at their base, and flagella or pseudo-
podia at the end which projects into
the gastrovascular cavity. The flagella
create currents in the gastrovascular
fluid, and the pseudopodia capture solid
food particles. The glandular cells
are small and without muscle fibrils.
Interstitial cells are foun& lying at the
base -of the other entoderm cells.

Fig. 153. Transverse Section of Hydra fusca.
1. Ectoderm cells (myo-epithelial) . 2.
Interstitial cells. 3. Nematocysts. 4. Coelen-
teron. 5. Endoderm cells. 6. Vacuoles. 7.
Food granules. 8. Nuclei. (After Shipley
and MacBride).

Fig. 154.

Hydra moving like a measuring worm and
using tentacles as feet. (From Jennings, after
Wagner).

The mesoglea is an extremely thin layer of jelly-like substance situ-
ated between the other two layers.

From recent investigations it seems well established that Hydra
possesses a nervous system, though complicated staining methods are
necessary to make it visible. In the ectoderm there is a sort of plexus
of nerve-cells connected by nerve-fibers with centers in the region of
the mouth and foot. Sensory cells in the surface layer of cells serve as
external organs of stimulation, and are in direct continuity with fibers
from the nerve cells. Some of the nerve-cells send processes to the mus-
cle fibers of the epitheliomuscular cells, and are therefore motor in func-
tion. No processes from the nerve-cells to the nematocysts have yet
been discovered, though they probably occur. The entoderm of the body
also contains nerve-cells, but not so many as are present in the ecto-
derm.

250 GENERAL BIOLOGY

Hydra obtains its food by throwing out nematocysts and paralyzing
its prey. The surface of the tentacle itself is somewhat sticky which
assists in keeping food from getting away, once the tentacle bends about
it and carries it to the animal's mouth. After the food enters the mouth
the forepart of the animal contracts to send it downward.

There are gland-cells in the entoderm which secrete a digestive
fluid, and it is probable that some digestion takes place in the entoderm
cells themselves. These latter .have little flagella by which food is
whipped about. When digestion takes place within these entoderm
cells, digestion is said to be intracellular.

It is interesting to note that Hydra will not respond to food stimuli
or capture prey after being fed.

The normal position of Hydra is an attachment to some solid object
by its basal disk. When the animal moves from one attached place to
another, it uses its tentacles as feet, slowly moving them along as though
walking upon them, and when a suitable location has been found, releas-
ing its body at the basal end and attaching it to the newly-found spot.
(Fig. 154.)

The reproduction of Hydra is especially interesting in that it fur-
nishes us with excellent proof for Weismann's insistence on the separa-
tion of somato-plasm and germ-plasm.

This animal usually reproduces by budding, as does yeast, except
that the bud in this instance pushes out and becomes stalk-shaped. The
tentacles of the bud grow from the distal end of the new stalk bud, and
the entire new organism is pinched off from the mother stalk or body.
(Fig. 152, C.)

In fact, it is not uncommon for one of these buds to form new buds
on its body before it is ready for an independent existence itself. At all
times, the cavity of the newly forming animal is in direct continuation
with the mother cavity, until the pinching-off process occurs.

There is a division of the body sometimes, though very infrequently,
by simple fission ( ), that is, by a splitting of the

entire animal lengthwise, commencing from the distal end and extending
to the basal disk. Sometimes, also, even the buds reproduce in this man-
ner, while transverse fission is not unknown.

In the sexual method, the spermatozoa from the testes escape into
the surrounding water. The eggs arise in the ovary from ectodermal
interstitial cells. There is usually only one egg in the ovary that grows
to maturity, though several may begin growth, only to have one of them
— the stronger by virtue of position, or ability to obtain more food — •
absorbing the others. Two polar bodies are given off from this egg
when it is ready for fertilization and then it is said to be mature.

The cleavage of the egg is total, and almost equal. After this origi-
nal egg has divided .several times the blastula is formed with a cavity
called the blastocoel ( ). Cells from the inner por-

THE COELENTERATA 251

; • !

tion bud off and make a sort of solid gastrula-like structure; this later
becomes the entoderm. The "Ectoderm now secretes a thick chitinous
( ) shell covered with sharp projections. The embryo

then separates from its parent and falls to the bottom, where it remains
unchanged for several weeks. Then interstitial cells make their appear-
ance. A subsequent resting period is followed by the breaking away of
the outer chitinous envelope, and the elongation of the escaped embryo.
Mesoglea is now secreted by the ectoderm and entoderm cells ; a circlet
of tentacles arises at one end, and a mouth appears in their midst. The
young Hydra thus formed soon grows into the adult condition." Almost
any part of the Hydra may be cut off and each part will grow into a com-
plete new animal. This is supposed to be due to the fact that Hydra is
an animal composed of tissues which have not yet become organs as in
the higher animals. Therefore the original germ-cells have not divided
as often as in higher animals, and each cell contains a little portion of
germ-plasm, which causes each cell to have the power or potentiality of
producing a complete organism. This theory receives additional weight
from the fact that the Hydra can and does reproduce in practically every
known way, sexual, asexual, by budding, by longitudinal and transverse
fission, in addition to having the ability of restoring any lost part, and
of forming a complete new animal from the smallest part.

When, however, an animal is classified in one of the higher phyla
and its somatoplasm is therefore further removed from the germplasm,
the regenerative ability decreases. This is shown in man, where a piece

of skin will grow again
when removed, though an
entire finger will not be re-
placed.

Regeneration means that
a part of an organism can
reproduce the whole or at
least a portion of the lost
part. This is distinguished

Fig. 155. Medusa, showing gastrula-form. from reproduction, though

Diagrams showing the similarities of a polyp (A) :n ITtrHt-a +Vio +,,,~ ~~,

and a medusa (B). circ, circular canal; ect, ectoderm; in ^V^a the tWO are mtl-

end, entoderm; ent. cav, gastrovascular cavity; hyp, matelv rflatprl
hypostome; mnb, manubrium ; msgl, mesoglea; mth, J

mouth ; nv, nerve rings ; rad, radial canal ; v, velum. As has been Stated there

(From Parker and Has well).

is an alternation of genera-
tions in this animal. The form we have been discussing so far is called
the Hydroid form or the polyp ( ), while the asexual

form, so different in appearance from the hydroid, is umbrella shaped
and Called a medusa ( ), (Fig. 155.) The convex

portion is usually the upper surface, and tentacles hang down from the
edges. At first glance the two forms appear totally dissimilar, but with
a clear conception of what a gastula really is, we can readily imagine

252

GENERAL BIOLOGY

grasping the hydroid form by the mouth and pushing this portion of the
animal in upon itself, when we have the gastrula still, and also the
medusoid form.

It must be remembered that some species may always retain the
medusoid form and others the hydroid, while still a third may alternate
regularly or irregularly between the two.

Obelia ( )

This is a colonial form of Hydra found attached to rocks, wharves,
and to various algae, in which budding began, but the newly-formed ani-
mals remained attached to the parent stalks. (Fig. 156.)

Such a colony consists of a basal stem, the hydrorhiza ( ),

which is attached to the substratum. At intervals, upright branches
known as hydrocauli ( ) are given off. At every

bend in the hydrocaulus a branch arises. The stem of this side-branch

Fig. 156. Hydrozoa.

A, part of a colonial Hydrozoan, Obelia. B, Longitudinal sec-
tion through a single hydranth. C, Cross section through medusoid
individual. 1, ectoderm ; 2, entoderm ; 3, mouth ; 4, coelenteron ; 5,
coenosarc ; 6, perisarc ; 7, hydrotheca ; 8, blastostyle ; 9, medusa-
bud ; 10, gonotheca ; or.c, mouth region ; end. and endt., entoderm ;
ect., ectoderm ; st.l., mesoglea lying between ectoderm and ento-
derm; hyth, hydrotheca. (From Borradaile after various authors).

is ringed and expanded at the end into a hydra-like structure, the
hydranth ( ). Each individual polyp consists of a

hydranth and the part of the stalk between the hydranth and the point
of origin of the preceding branch. Full-grown colonies usually bear re-
productive members (gonangia) in the angles where the hydranths arise
from the hydrocaulus.

All of the soft parts of the Obelia colony are protected by a chitinous
covering called the perisarc ( ) which is ringed at

THE COELENTERATA

253

various places, and is expanded into gonothecae and cup-shaped hydro-
thecae ( ) to accommodate the hydranths.

A shell extends across the base of the hydrotheca which serves to
support the hydranth. The soft parts of the hydrocaulus and of the
stalks of the. hydranths constitute- the coenosarc ( )

and are attached to the perisarc by minute projections. The coenosarcal
cavities of the hydrocaulus open into those of the branches and thence
into the hydranths, producing in this way a common gastro-vascular
cavity.

The coenosarc consists of two layers of cells — an outer layer, the
ectoderm, and an inner layer, the entoderm. These layers are continued
into the hydranth. The mouth is situated in the center of the large
knob-like hypostome ( ) and the tentacles ( ),

about thirty in number, are arranged around the base of the hypostome
in a single circle. Each tentacle is solid and consists of an outer layer
of ectodermal cells and a single axial row of entodermal cells with a
large number of nematocysts at the extremity. The hydranth captures,
ingests, and digests food just as does Hydra.

The reproductive organs develop quite like the hydranths, as buds
from the hydrocaulus. They thus represent modified hydranths. The
central axis of each is called a blastostyle ( ), and

together with the gonothecal covering, is known as the gonangium
( ). The blastostyle gives rise to medusa-buds

which soon become detached and pass out of the gonotheca through the
opening in the distal end.

Some medusae produce eggs, and others, sperm. The fertilized eggs
again develop into colonies like those which gave
rise to the medusae. The medusae provide for the
dispersal of the species, since they swim about in
the water and establish colonies in new habitats.
The medusae of Obelia are shaped like an umbrella
with a fringe of tentacles and a number of organs
of equilibrium on the edge. Hanging down from
the center is the manubrium
( ), with the

mouth at the end. The gastrovas-
cular cavity extends out from the
cavity of the manubrium into four
radial canals on which are situated
the reproductive organs.

"The germ-cells of the medusae
of Obelia arise in the ectoderm of

A.

B.

Fig. 157.
A. Liriope Exigua (family Geryoniidae) .

(Mayer).

B. Hydraiike stage in the development of the manubrium, and then migrate

along the radial canals to the repro-
ductive organs. When mature, they

Gonionemus. One of the tentacles is carrying
a worm (w) to the mouth. Tentacles in con-
tracted state. (From the Cambridge Natural
History, after Perkins).

254 GENERAL BIOLOGY

break out into the water. The eggs are fertilized by spermatozoa which
have escaped from other medusae. Cleavage is similar to that of Hydra,
and a hollow blastula and solid gastrula-like structure are formed. The
gastrula-like structure soon becomes ciliated and elongates into a free-
swimming larva called a planula ( ). This soon acquires a
central cavity, becomes fixed to some object, and proceeds to found a
new colony."

When there is an alternation of generations, the one being sexual,
reproducing by eggs and spermatozoa, and the other asexual, reproduc-
ing by division or budding, such as alternation of generations, is called
metagenesis ( ).

In Obelia the -asexual generation (the colony of polyps) forms buds
of two kinds, the hydranths and the gonangia ( ). The

sexual generation (the medusoid) reproduces only by eggs and spermat-
ozoa.

Hydra do not have a regular medusoid stage and Geryonia (Fig.
157A), ( ), no polyp or hydroid stage.

Gonionemus (Fig. 157B), ( )

The structure of a medusa or hydrozoan jellyfish is well illustrated
by Gonionemus, which is quite common along the eastern coast of the
United States. It is about half an inch in diameter. In general form
it is similar to the medusa of Obelia. The convex or aboral surface is
called the exumbrella ( ), and the concave, or oral

surface, the subumbrella ( ). The subumbrella is

partly closed by a perforated membrane called the velum ( ).

The animal takes in water into the subumbrella-cavity, and then forces
it out through the central opening in the velum by the contracting of
its body, thus propelling the animal in the opposite direction. This
method of locomotion is called hydraulic. It is common to all medusae.
The tentacles (from sixteen to about eighty) are capable of great
contraction. Adhesive or suctorial pads are found near their tips. Hang-
ing down into the subumbrellar cavity is the manubrium with the mouth
at its end surrounded by four frilled oral lobes. The mouth opens into
a gastrovascular cavity which consists of a central "stomach" and four
radial canals. The radial canals enter a circumferential canal which lies
near the margin of the umbrella,

The cellular structure of Gonionemus is similar to that of Hydra,
but the mesoglea is thicker which gives the animal a jelly-like consist-
ency. Then there are many nerve cells scattered about beneath the
ectoderm, and a nerve ring is placed about the velum and there are
sensory cells with a tactile function on the tentacles. At the margin of
the umbrella there are two kinds of sense organs: (1) Those at the base
of the tentacles are round bodies which contain pigmented entoderm
cells and communicate with the circumferential canal ; (2) those between
the bases of the tentacles, Avhich are small outgrowths, probably organs

THE COELENTERATA

255

B

of equilibrium, that is, statocysts ( • ). Muscle fibers,

in both exumbrella and subumbrella, are present.

Beneath the radial canals the sinuously folded reproductive organs
or gonads, are suspended. Gonionemus is dioecious ( ).

each individual producing either eggs or spermatozoa. These reproduc-
tive cells break out directly into the water, where fertilization takes
place. A ciliated planula develops from the egg as in Obelia. This soon
becomes fixed to some object, and a mouth appears at the unattached
end. Then four tentacles grow out around the mouth and the Hydra-
like larva is able to feed. Other similar Hydra-like larvae bud from its
walls. How the medusae arise from these larvae is not known, but
probably there is a direct change from the hydroid form to the medusa.

POLYMORPHISM

Whenever there is a division of labor among the different members
of the same colony so that each does a particular work, such colony is
said to be polymorphic ( ) if there are more than

two kinds of specialized individuals ; dimorphic if only two different
specializations have taken place.

The "Portuguese man-of-war"
(Fig. 158) is an excellent example
of the former, in that it is a bladder-
like structure to which many tenta-
cles are attached. It floats upon the
water. Some of these tentacles are
nutritive, others are tactile
( ) ; some contain

batteries of nematocysts, others are
male reproductive zooids, and still
others give rise to egg-producing
medusae.

The Coelenterata (together with
the Echinoderma) were formerly
called Radiata on account of their
radial form. It is now known that in
the higher groups of coelenterates
this radial form may be transformed

A, Physalia or Portuguese man-of-war, a 1 . .. . 1 ., ,
•colonial Hydrozoon. (From Hegner, after into a Diradial Or bilateral Sym-
Agassiz).

B, Diagram showing possible modifica-
tions of medusoids and hydroids of a hydro- Older writers often SDoke of the
zoan colony of the order Siphonophora. c. gas-

frozooid with branched, grappling tentacle, /; Coelenterates as ZoOphyta (ammal-

g, dactylozooid with attached tentacle, h ; i, . .. - , .

generative medusoid ; k, nectophores (swim- plants) On aCCOUnt OI their resem-

ming bells) ; I, hydrophyllium (covering 11 i i ,1 •

niece); m, stem or corm ; «, pneumatophore. DlanCC tO plants DOttl in appearance

The thick black line repre^nts rntoderm, the otn/J • fU^.V mofV,^.^ nf off o^Vir>i <*«f

thinner line ectoderm. (From Hegner after and m tneir method Ot attachment.

Allman)- Then, too, these animals simulate

Fig. 158.

256

GENERAL BIOLOGY

Fig. 159. Scyphozoa.
A, Tessera princeps, order Stauromedusae.

B, Periphylla hyacinthia, order Peromedusae.

C, Charybdea marsupialis, order Cubomedusae.
G, gonads ; Gf, gastral filaments ; ov, gonads ;
Rf, annular groove ; Rk, marginal bodies ; Rm,
circular muscle; T. tentacles. (From Sedgwick,
after Haeckel.)

Fig. 160. Examples of Alcyonoria.
Coral. A, Tubipora musica, organ-pipe
coral, a young colony. Hp, connecting hori-
zontal platforms ; p, skeletal tubes of the
zooids ; St, the basal stolon. B, Alcyonium
digitatum, with some zooids expanded. C,
Corallium, a branch of precious coral. P,
polyp. D, Pennatula sulcata, a sea-feather.
(A and B, from Cambridge Natural History;
C, from Sedgwick, after Lacaze Duthiers ; D,
from Sedgwick, after Kolliker).

plant-conditions by their method of reproduction, namely, by fission and
budding, as well as often forming colonies.

CLASSIFICATION

There are three great classes of coelenterates - - Hydrozoa
( ), Scyphozoa ( ), and An-

thozoa ( ).

The Hydrozoa possess neither stomodaeum nor mesenteries
( ), and their sex-cells are discharged directly to

the exterior. Hydra and Obelia belong to this class.

The Scyphozoa may, or may not, possess a stomodaeum and mesen-
teries. The stomodaeum is more or less equivalent to the gullet in
coelenterates, serving as the passageway between mouth and the gas-
tro-vascular cavity or "stomach." The membranes which hold this
stomodaeum in place are called mesenteries.

The position of tentacles and tentaculocysts is made use of in sepa-
rating the coelenterates into the various classes.

Examples of Scyphozoa (Fig. 159) are: Tessera, order Stauro-
medusae; Periphylla, order Peromedusae; and Charybdea, order Cubo-
medusae.

The Anthozoa are divided into two sub-classes as follows : Sub-
class I. Alcyonaria (Fig. 160), all of which have eight hollow, pinnate,
tentacles and eight complete mesenteries. They also possess one
siphonoglyphe, which is ventral in position, while all the retractor mus-
cles of the mesenteries lie on the side toward the siphonoglyphe

( ).

Examples of an Alcyonaria are the organ-pipe coral known as Tubi-
pora of the order Stolonifera, and the pretty sea-fans and the red coral

THE COELEXTERATA

257

A.

B.

Fig. 161. Examples of Zoantharia.

A, Oculina speciosa, & branch of madreporarian coral. (After Sedgwick).

B, Meandrina, a rose-coral of the order Madreporaria. (After Weysse).

C, A group of sea anemones. (After Andres).

used in jewelry. The latter is known as Corallium of the order Gor-
gonacea.

Sub-class II. Zoantharia (Fig. 161). These usually possess many
simple, hollow tentacles, generally arranged in multiples of five or six.
There are two siphonoglyphes as a rule, and the mesenteries vary in
number. The retractor muscles are never arranged as in the Alcyonaria.
A skeleton may or may not be present. The animals may be simple or
colonial.

Examples of Zoantharia are the sea-anemones such as Actiniaria,
and the stony corals such as Oculina of the order Madreporaria, and the
rose-coral Meandrina, order Madreporaria.

CHAPTER XVIII.

INTRODUCTION TO THE COELOMATA

From what has already been learned it is known that animals may
be divided, according to whether or not they have a backbone, into two
great groups — the vertebrates and the invertebrates. Also, according to
whether they are composed of one or more cells into protozoa and meta-
zoa, and the latter according to the number of germ layers each form
develops, into diploblastic and triploblastic organisms.

Now we come to another common method of classifying them into
two groups — the coelomata and the acoelomata.

With the exception of the frog, all of the animals studied so far — •
the protozoa and coelenterata — belong to the acoelomata, because they
have no additional cavity between the digestive tract and the body wall.
Coelomata have such a body-cavity. All animals higher in the scale of
life than hydra are coelomates.

It will be remembered that in hydra there was a thick mucilaginous
substance — the mesoglea — formed between the ectoderm and entoderm.
In some of the lower forms of acoelomata there are processes stretching
across from inner to outer germ-layer, which often secrete fibers which
become "connective tissue or may be developed into muscular fibers.
Where the cells and fibers are sparse, this space is said to be a primary
body-cavity. Where they are abundant, it is called parenchyma
( ) or connective tissue."

This body cavity, also known as the coelomic cavity (Fig. 162) or
coelom (Gr. koiloma=a thing hollowed out), consists of "one or more
pairs of sacs with perfectly defined walls lying at the sides of the ento-
dermic tube. In the adult these sacs join each other above and below
the entoderm, and the adjacent walls entirely or partly break down, and
thus one continuous cavity results. The wall of the coelom and the tis-
sues derived from it are known as mesoderm."

The distinctive difference between the primary body cavity of the
coelenterates and this secondary body-cavity of the coelomates, is a dif-
ference in the walls of the cavities and not in the space between the
walls. The outer wall of the primary body cavity is merely ectoderm.

It will be remembered that this primary body cavity serves both as
a digestive and circulatory system in the coelenterates. In the higher
animals, therefore, it may be said that the blood-vessels are really part
of the primary body cavity.

In triploblastic animals the mesoderm does not form a completely
solid mass extending the entire length of the body. A slight cavity is
left in its center extending along the long axis of the organism.

INTRODUCTION TO THE COELOMATA

259

This mesoderm forms in two ways (1) either by little pouches grow-
ing from the entoderm which are then nipped off, or (2) by two large
cells which grow as buds from the entoderm, which once formed, grow
rapidly, forming the so-called mesodermic bands, which bands later be-
come hollowed out. The two cells forming the original bud are termed
pole-cells. This hollowed out portion is the coelom. A close study of
Figure 163 will make a better understanding of the above possible.

It must be understood that these two forms are not likely to be found
in any one animal.

The open space thus formed which we have called the coelom has
thus a layer toward the outside of the body and a layer of cells or wall
toward the entoderm from which it sprang. The outer wall of the coelom
is called the somatic layer or the somatopleure ( ),

while the inner is known as the splanchnopleure ( ).

hep

JZtrvph

Jiephrost

Fig. 162.

Transverse section through the middle
region of the body of the earthworm, Lum-
bricus, circ.mus, circular muscle fibers ; coel,
coelom ; dors.v, dorsal vessel ; epid, epidermis ;
ext.neph, nephridopore ; hep, chlorogogen cells ;
long.mus, longitudinal muscles ; neph, nephri-
dium ; nephrost, nephrostome ; n.co , nerve-
cord ; set, setae ; sub.n.vess, subneural vessel ;
typh, typhlosole ; vent.v, ventral vessel. (From
Parker and Haswell, after Marshall and
Hurst):

Fig. 163.

Two stages in the early development of a
common fresh-water mollusc, Planorbia, to
show the origin of the mesoderm cells.

The ectoderm cells are deeply shaded, the
endoderm cells are unshaded. A. Young stage
in which the endoderm has not begun to be
invaginated ; it is a lateral optical section.
B. Older stage, optical section seen in front
view ; the endoderm cells are invaginating,
and the two mesoderm cells are seen on each
side. 1. Mesoderm or pole-cells ; in B, each
has budded off another mesoderm cell. (After
Rabl).

When pole-cells form, the cavity of the digestive canal is small in
proportion to the thickness of its wall, so that the pole-cell may be con-
sidered as "a solid pouch."

In most Coelomata the mesoderm or coelomic wall forms by far the
greatest portion of the body. There are sometimes cells which form in
the primary body-cavity, to which some writers have also applied the
term mesoderm. This term should, however, be reserved for the walls
of the coelom as just described, while mesenchyme ( )

should be used for the cells forming within the primary body-cavity.

Mesenchyme arises from different germ-layers in different phyla of

260 GENERAL BIOLOGY

animals. It may arise from the entoderm or ectoderm or both, or even
from the walls of the coelom. In this latter case it may spring from
ectoderm, entoderm, and mesoderm. In the higher coelomata it arises,
however, "partly from the ectoderm but principally from the outer wall
of the coelom. Everywhere it gives rise to connective tissue and to the
tissues developed from this (tendon, cartilage; bone, etc.), whereas the
coelomic wall or true mesoderm gives rise to the generative cells and
their ducts, and the main parts of the muscular system, including the
muscular coats of the principal blood-vessels.

The entoderm, after the mesoderm has separated from it, forms the
lining of the digestive tube and of its appendages, which in the higher
Vertebra.ta are the organs known as lungs, liver, pancreas, and urinary
bladder. The basis of the skeleton of Vertebrata, the gelatinous rod
called the notochord, also arises from the entoderm.

After gastrulation has taken place in the growing embryo, there are
only two germ layers, ectoderm and entoderm. The inner layer under-
goes various changes, as it is to be used for
a totally different purpose from its outer
protective layer. It must be remembered,
however, that just after indentation, both
Fig 164 layers are alike in that they have both con-

Diagrammatic cross section of the stituted the simple blastula. The blastula,

body of a coelenterate (such as the .

hydra) and of a coeiomate. The it will be remembered, is but a single layer

latter forms a tube within a tube. r ,, .. , 1-1

of cells forming a more or less spherical

body. The opening formed by gastrulation and known as the mouth or
stomodeum ( ) does not undergo the same change that does the

part on the more interior portion of what is now called the entoderm.
In fact, the mouth region remains ectodermal. As soon as an organism
has formed three germ-layers and has both an opening in its body for
ingestion as well as egestion of food, there com.es another infolding of
ectoderm in the gastrula at the opposite end from the stomodeum. This
forms an anal opening which is called a proctodeum ( )•

This infolding, just as the stomodeal infolding, is also ectoderm.

It is of interest and value here to know that the entire brain and
nervous system arise from ectoderm. It will be readily understood why
this is so, when it is realized that no organism from the simplest flower
up to man, could possibly live unless there were some method by which
such organism could protect itself when danger threatened. Any me-
chanical injury, such as pressure or laceration, cannot affect the body un-
less it strikes the outer portions first. Therefore the sensory nerve end-
ings must be placed close to the outer portion of the body so that they
will receive the message of threatened danger first. These danger mes-
sages are then carried to the central nervous system where a co-ordina-
tion must be brought about between the sensory fibers and the motor

INTRODUCTION TO THE COELOMATA 261

nerves, thus making it possible for any or all parts of the body to be
withdrawn from the zone of danger.

For toxic injuries, as well as parasitical invasions, which come
through the intestinal tract, the student must think of the body, when
drawn out completely, as forming a tube within a tube. (Fig. 164.)

The inner one called the intestinal, or digestive tract, has an open-
ing straight through the body. This means that the inside of the diges-
tive tract is really outside the body in so far as exterior environmental
conditions may affect it, such as temperature, air, etc. In other words,
it is as though one took an ordinary small gas or water pipe and placed
it in water. There would be the same kind and quality of water on the
inside as there would be on the outside of the pipe.

The larger outer tube is the outer body wall.

"The internal anatomy of the lower animals was first studied by
physicians and others primarily interested in human anatomy. An un-
fortunate consequence is that a large number of names are used in the
description of simpler animals which are based on fanciful resemblances
between their organs and those of man. As a consequence many of these
names are quite misleading. To give some instances : The word stomach
in the Lobster denotes part of the stomodaeum, in the vertebrata it sig-
nifies part of the entodermic tube. The pharynx ( )
of an earthworm is the stomodeum, in a fish it includes both stomodeum
and the first part of the entodermic tube. The term liver has also been
much abused.

"The names taken from the higher animals, which are customarily
used in the description of the alimentary canal, are as follows: Mouth
or buccal-cavity, pharynx, oesophagus, stomach or crop, gizzard, intes-
tine, and rectum. They are applied generally to parts of it succeeding
one another in the order above given. The significance of these will be
explained in each case : it would perhaps be more logical to sweep away
altogether these and a host of similar terms employed to designate other
parts of the body, but so deeply are they engrained in zoological litera-
ture that such a course would render unintelligible most anatomical de-
scriptions of species that we possess."

CHAPTER XIX.

THE EARTHWORM

Earthworms are found in practically all parts of the country, living
in burrows not lower than 12 or 18 inches beneath the earth's surface.
It is in about these depths that they find the richest portions of decay-
ing vegetable and animal substances upon which they feed. Professor
Latter has given us a most interesting account of these animals. During
"periods of prolonged drought or frost they descend to greater depths
and undergo aestivation ( ) or hibernation

( ), as the case may be, coiled up into a com-

pact spiral and lying in a small excavated chamber. This is lined with
small stones which prevent close contact with the surrounding earth
and so permit free respiration. The sides of the burrow are kept moist
by slime discharged from the glandular cells of the skin, and perhaps
by liquid discharged from the body-cavity through the dorsal pores
which occur in the grooves that separate segment from segment. The
slime is said to possess antiseptic properties, and thus preserve the skin
of the worm from harmful bacteria.

"The mouth of the burrow is guarded by small stones or more fre-
quently by one or more leaves pulled in to a greater or less distance.
Fir-needles, stalks of horse-chestnut leaves and other similar things are
often to be seen standing nearly erect upon the ground, their lower ends
having been forcibly dragged into the mouth of a burrow by a worm.
On still, warm nights in early autumn the rustling noise of fallen leaves
being dragged along by worms is often plainly audible in favorable
localities. Darwin has pointed out that worms exhibit considerable in-
telligence in drawing the narrow end of leaves of various shapes fore-
most into the burrow : the leaves with broad bases and narrow apices
are generally pulled in tip first, whereas when the base is narrower than
the apex the reverse position is usually found. There is no doubt that
worms can judge which end of any leaf is the better to seize. The
reason for thus pulling objects into the entrance of the burrow is prob-
ably to prevent the entry of foes, centipedes, parasitic flies, etc., to keep
the burrow moist by preventing evaporation, to keep out the cold lower
strata of air at night, to bring food supplies within safe reach, and also
to enable the worms to lie near the mouth of the burrow unobserved.
Here, however, they are not secure from all attack, for the quick ears
of the thrush and other birds enable them to detect the slightest move-
ment and, with a quick plunge of the beak, to seize, and after a brief
tug-of-war, to extract the worm from its refuge. Frequently the well-
known worm-castings are thrown up on the surface, and when this is so,

THE EARTHWORM 263

leaves are not, as a rule, drawn into the burrows, the heap of castings
serving the purpose.

"The burrow is made partly by the awl-like, tapering anterior end
pushing aside the earth on all sides, and partly by the actual swallowing
of the earth as the worm advances, so that the animal literally eats its
way into the soil. The organic material in the swallowed soil serves
as food, and the residue in a state of very fine division passes out at the
anus, and is used either to form the above mentioned castings or as a
lining to the burrow, especially where this passes through hard, coarse
earth.

"Perfectly healthy worms seldom leave their burrows completely
except perhaps after a very heavy rain. The majority of those so fre-
quently found traveling over the surface of roads and paths after rain
are infected by the larvae of parasitic flies and doomed to die. On warm,
moist evenings, however, worms may be seen in hundreds lying stretched
on the surface of the ground with only the broad flattened posterior end
remaining in the burrow. Here we see one of the uses of this modifica-
tion in the shape of the hinder segments of the body : their greater width
enables them to obtain a firm purchase on both sides of the burrow, and
thus the worm is provided with a sure anchor on which it can pull, and
at the slightest alarm, shoot back like stretched elastic into the security
of its burrow. At other times the flat tail is employed trowelwise in
smoothing the excrement against the walls of the burrow or in disposing
the castings on this side and on that of the mouth of the burrow."

"The effects produced on the surface soil by the action of earth-
worms have been fully pointed out by Charles Darwin in his well-known
book, 'Vegetable Mould and Earthworms.' It will be sufficient here to
call attention to a few facts only. Worms, play a most important part
in maintaining the soil in a state suitable to vegetation. The burrows
form ventilating tubes whereby the soil is aerated and respiration by the
roots of plants rendered possible ; at the same time they open up drain-
age channels, preventing the surface from becoming waterlogged.
Doubtless also roots find an easy passage through the soil along the
lines of burrows even after the walls have more or less fallen in. More-
over, the excrementitious earth with which the burrows are lined is
peculiarly suited to root fibers, being moist, loose and fertile. Micro-
scopic examination of the earth deposited by worms shows it to resem-
ble two-year-old leaf mould such as gardeners use for seed-pans and
pricking-out young seedlings ; most of the plant-cells are destroyed,
shreds and fragments alone remaining, discolored and friable, mingled
with sand grains and brown organic particles. In chemical composition,
too, worm-castings are very similar to fertile humus.

"The castings which are thrown up on the surface materially im-
prove the quality of the upper soil, and render it more fit for the germi-
nation of seeds, many of which directly or indirectly get covered by the

264 GENERAL BIOLOGY

upturned earth. It has been reckoned that there are upwards of 50,000
worms in an acre of soil of average quality : hence the total effect of
the work of this vast host must be very considerable. Each worm ejects
annually about 20 ozs. of earth. The weights of earth thrown up in a
single year on two separate square yards observed by Darwin were
respectively 6.75 and 8.387 Ibs., amounts which represent respectively
14.58 tons and 18.12 tons per acre per annum.

"In addition to this tilling action worms improve the quality of the
soil by the leaves and other organic debris which they drag into their
burrows, and thus bring within reach of bacteria. These, as it is well
known, especially abound in the upper soil, and effect the speedy decom-
position of dead animals and vegetable tissues.

"Archaeologists are indebted to worms for the preservation of many
ancient objects, such as coins, implements, ornaments, and even the
floors and remains of ancient buildings that have become buried by the
soil thrown up as worm-castings. The process of disappearance is of
course hastened by the excavations effected by the worms below the
surface, for the collapse of the burrows slowly but surely allows objects
on the surface to sink downwards.

"In the disinteg'ration of rocks, and the denudation of the land,
worms play an important part. The penetration of the burrows, and
the lining with castings, carries down the humus-acids to a considerable
depth and exposes the underlying rocks to their solvent action. Within
the body of the worm itself small stones and grains of sand are reduced
to yet finer dimensions and rendered the more easy to transport by wind
and water. On sloping surfaces the upturned castings, at first semi-
fluid, flow down, and when dry roll down the incline, or are washed by
the rain into the valleys and ultimately carried out to sea, while on
level ground the dried castings are blown away to lower spots by the
wind. The more or less parallel ridges that are frequently found on
' the sloping sides of grass-clad hills are in part, at any rate, formed by
the material derived from worm-castings, which has temporarily lodged
against tufts of grass, etc., and in turn furnishes a richer and deeper soil
for stronger growth which arrests yet more and so increases the ledge.
All land surfaces, whether level or sloping, provided they are occupied
by worms, are reduced in altitude by their action. In no small degree,
then, may earthworms be held responsible for our valleys and hills and
all the softer features of our scenery."

EXTERNAL APPEARANCE

There are rings or segments (Fig. 165) extending along the entire
length of the animal's body formed by constrictions or annuli. The seg-
ments themselves are known as somites or metameres. It is from these
ring-like (L. Annulus-ring) constrictions and segments that the animals
belonging to this group are named Annelids or Annulata. Worms are

THE EARTHWORM

265

divided into annelids, or segmented worms, plathyhelminthes or flat
worms — (Gr. platy=flat-j-helminthes= worms) ; and nemathelminthes
or thread-worms (Gr. nema— thread-]- helminthes=:worms).

The important external characteristic in the annelids is, then, a re-
gional differentiation. That is, the forming of separate segments or
regions externally, and a separation and segmentation of many internal

structures. Metamerism is com-
mon in all higher forms of organ-
isms except the soft-bodied animals
such as the Molluscs and the spiny-
skinned Echinoderms. In Man this
metamerism is distinctly shown in
the separate segments of the spinal
column.

There are many differentiations
in various regions of the earth-
worm's body. For example, the
anterior end is sensitive to touch
and light to a much greater degree
than the middle and posterior por-
tions. On the eighth, ninth, four-
teenth and fifteenth segments there
are openings of the reproductive
system, while from the twenty-
eighth to the thirty-seventh seg-
ments a broad band surrounds the
dorsal and lateral portions of the
worm called a clitellurn, the func-
tion of which will be explained un-
der Reproduction.

There are from 140 to 180 seg-
ments in the earthworm. All of the
differentiation just mentioned oc-
curs toward the anterior end of the worm. We therefore say the earth-
worm has an anterior-posterior differentiation.

As the earthworm will always place itself in a definite position
when crawling along — that is, will "right" itself if it be turned about,
we speak of that portion toward the surface on which it moves as the
ventral surface and the surface away from this as the dorsal. If an
animal thus rights itself there must be a difference between the ventral
and dorsal surfaces. This difference is spoken of as a dorso-ventral dif-
ferentiation or dorsiventrality.

The ventral surface will be found to be more flattened than the dor-
sal, while many little whitish glands are present toward the anterior end.
On the ventral surface are also found the mouth, anus, reproductive,

Fig. 165.

Latero-ventral view of Lumbricus ter-
restris, slightly smaller than life-size. (From
Hatschek and Cori).

1. Prostomium. 2. Mouth. 3. Anus. 4.
Opening of oviduct. 5. Opening of vas de-
ferens. 6. Genital chaetae. 7. Lateral and
ventral pairs of chaetae.

XV, XXXII, and XXXVII are the 15th,
32nd, and 37th segments. The 32nd to the 37th
form the clitellum. (After Latter).

266 GENERAL BIOLOGY

and excretory openings, as well as peculiar bristle-like setae. These lat-
ter will be discussed under locomotion.

The earthworm, like the frog, is bilaterally symmetrical. A median
dorso-ventral line drawn through the worm divides it into two equal
parts. This will be understood the better when it is remembered that
all unpaired parts of the animal, such as mouth, anus, central blood
vessel, etc., would be cut into two equal parts by a medial section, while
all paired portions such as setae and reproductive openings would have
one-half of such paired portion on each side of the animal.

The dorsal excretory pores, one to each somite posterior to th-e
tenth, lie in the constrictions and are difficult to find, but on the ventral
surface various openings can readily be seen. These are principally, two
pairs of minute pores between the ninth and tenth and the tenth and
eleventh somites coming from the seminal receptacles. The male genital
openings are on the fifteenth and the pair of female genital openings on
the fourteenth ; the excretory organs, called nephridia, have two open-
ings on each somite behind the first three or four and anterior to the
last. Practically all of the ventral openings posterior to the male genital
pore, with the exception of the anus, are too small to be seen with the
unaided eye.

The animal moves along primarily by alternate rhythmic constric-
tions of the longitudinal and circular muscles of the body-wall which
contract and elongate successive regions of the body. There are eight
chitinous setae to each somite, easily felt if the animal be drawn between
the fingers. An ordinary hand-lens will show them quite clearly. There
is then a double way in which the worm moves, the muscular action
furnishing the contraction and expansion and the setae furnishing cog-
like projections by which the worm can make forward progress. This
is well exemplified by the fact that if an earthworm be placed on a highly
polished surface there is little if any progress made by it.

Muscles are attached to the inner parts of the setae, making it pos-
sible to shift their positions. The flattened tail of Lumbricus terrestris,
serves as an anchor while the anterior portion of the animal's body lies
on the surface of the earth.

INTERNAL STRUCTURE

Probably the earthworm illustrates a coelom (Fig. 162) as well as
any form which could be given the student, for upon making either
dorsal or ventral longitudinal incision the animal will giv« the appear-
ance of a tube within a tube, the central one being the digestive tract
held in its central position by little thin membranes or walls running
from each outer constriction. These walls are called septa ( )

or dissepiments ( ). There are here, then, many

coelomic cavities which can be clearly seen, it being remembered that a

THE EARTHWORM

26?

coelom is defined as the cavity lying between the digestive tract and the
outer body wall.

There are muscles, nerves, glands, connective tissue, blood-vessels,
epithelium, and endothelium, just as in the frog, though not developed
as elaborately. There is also a delicate lifeless coat called the cuticle.

THE DIGESTIVE SYSTEM

The alimentary canal (Fig. 166) begins at the anterior end with a
mouth cavity or buccal pouch, extending from the first to the third
somite inclusively, the thick muscular pharynx ( )

lies in somites four and five ; the oesophagus, a narrow straight tube,

B

Fig. 166.

A, Longitudinal vertical section through the anterior portion of an earthworm.
br., brain ; cr., crop ; /tt., seminal funnel ; giz., gizzard ; int., intestine ; n.c., nerve
cord ; neph., nephridia ; oes., oesophagus ; oes. gl., oesophageal gland ; ph., pharynx.

(From Parker and Haswell after Marshall and Hurst).

B, Section of the Alimentary Canal, c, chlorogogen cells : cm, circular muscles ;
ep, epithelium, lining the canal; Im, longitudinal muscles; v, blood vessels.' (From
Conn, modified from Sedgwick and Wilson).

extends through the sixth to the fourteenth somite ; a thick muscular-
walled gizzard in somites seventeen and eighteen ; and a thin-walled
intestine from somite nineteen to the anal opening.

The dorsal wall of the intestine is folded in, forming a longitudinal
ridge, called the typhlosole ( ). This gives the in-

testine considerable expansion and affords additional surface for diges-
tion.

The wall of the intestine, as in the frog, is composed of five layers,
(Fig. 166, B):

(1) An inner lining of ciliated epithelium,

(2) A vascular layer containing many small blood vessels,

(3) A thin layer of circular muscle fibers,

(4) A layer consisting of a very few longitudinal muscle fibers,

(5) An outer thick coat of chlorogogen cells ( >
modified from ,the coelomic epithelium.

It is supposed that, because these chlorogogen cells lie in the typhlo-
sole close to the dorsal blood vessel that they may aid in some digestive
process. Then, because chlorogogen granules are present in the coelomic
fluid of adult worms and make their way to the outer part of the body

268 GENERAL BIOLOGY

through the dorsal pores, it has likewise been suggested that they may
have some excretory function.

Three pairs of calciferous glands (L\>f\<? C AX**^ & )» one Pa^r
in each of the somites from ten to twelve are found at the sides of the
oesophagus. The first pair are pouches pushed out from the alimentary
canal which open directly into the oesophagus. The other two pairs are
swellings of the oesophageal wall. They have a number of small cavities
which open directly through the epithelium into the oesophagus in
somite fifteen.

One writer thinks these glands manufacture carbonate of lime which
is then secreted into the alimentary tract to neutralize the acid foods,
while another suggests that the primary function of the glands is merely
to excrete calcareous matter derived from leaves on which the animal
feeds. This opinion he bases on the fact that such matter accumulates
in leaf-tissue and remains in the leaf when it falls. The worms taking
in large quantities of calcareous matter but having no' shell or bone,
there is no use for it, and so "some special excretory apparatus seems
necessary." This latter opinion does not oppose the one given imme-
diately preceding it. But the gizzard and intestinal content of worms
is, as a rule, acid, so this would seem to oppose both of the above ideas.
However, this acidification "may be the result of fermentations occur-
ring in the later stages of digestion."

It will thus be seen, that many things must be considered before
one can speak on subjects such as these with any degree of authority
and positiveness.

As stated, the earthworm feeds on decaying leaves and animal mat-
ter. This food is sucked into the buccal cavity.

Here it receives a secretion from the pharyngeal glands, after which
it passes through the oesophagus to the crop to be stored temporarily.
Secretions from the calciferous glands in the oespohageal walls neutral-
ize the acids. The gizzard is a grinding organ in which the food is
broken up into minute fragments by being squeezed and rolled about.
Then, too, solid particles, such as rough pebbles, which are frequently
swallowed, may aid in the grinding process. The food then passes to
the intestine, where most of the digestion and absorption takes place.

Digestion in the earthworm is very similar to that of higher ani-
mals. The digestive fluids act upon proteins, carbohydrates, and fats;
in them are special compounds called ferments or enzymes, which break
up complex molecules without themselves becoming changed chemically.
The three most important enzymes are (1) trypsin ( ),

which dissolves protein; (2) diastase ( ), which

breaks up molecules of carbohydrates1', and (3) steapsin ( ),

which acts upon fats. These three enzymes are probably in the digestive
fluids of the earthworm. The proteins are changed into peptones, the

THE EARTHWORM 269

carbohydrates into a sugar compound, and the fats are divided into
glycerin and fatty acids."

After this process has taken place the food is ready for absorption.
This takes place through the wall of the intestine by osmosis, assisted
by an amoeboid activity of some of the epithelial cells.

It will be remembered from our study of the frog that all parts of
a living organism must be nourished. The food absorbed is now taken
into the circulation and made an actual part of the blood. As there are
no blood vessels in some parts of the earthworm, some of the absorbed
food is also taken into the coelomic fluid so as to bathe the bloodless
areas.

THE CIRCULATORY SYSTEM

The blood of the earthworm, unlike that of man, is actually red,
while the corpuscles are colorless. In man the blood-liquid is colorless
and the corpuscles floating about in the blood-plasma are red. This
means that the pigment haemoglobin ( ) is within

the corpuscle in man and the higher animals, while it is in solution in
earthworms.

The following table mentions most of the important longitudinal
blood-vessels (Fig. 167) :

(1) The dorsal or sur>ri-intestinal, running along- the dorsal surface
of the alimentary canal, from the posterior end of the body to the
pharynx. It then divides into many small branches.

(2) The ventral or sub-intestinal trunk lying just beneath the ali-
mentary canal. It also extends from the posterior end of the body to
the pharynx where it divides into many small branches.

(3) The sub-neural trunk, as its name implies, passes along under
the ventral nerve cord the entire length of the body.

(4) A pair of lateral-neural trunks (smaller than those above)
lying, one on each side of the ventral nerve cord.

As in the frog and all other vertebrates, paired arteries, veins, and
nerves, p#ss out of and into the spinal cord between the various verte-
brae, so in each segment of the earthworm tiny branches of the dorsal
and ventral trunks called parietal ( ) branches pass

along the various septa dividing the somites, and connect with the body
wall, where they split unto fine branching capillaries supplying and
draining the dermal musculature and epithelium.

Capillaries from the dorsal branch also supply the digestive tract,
while in the anterior region two lateral vessels supply the reproductive
organs.

It will be remembered that in the study of the frog, the circulatory
system began with a three-chambered heart. In the earthworm there
is no separate and distinct organ such as the heart. In its place there

270

GENERAL BIOLOGY

are five pairs of enlarged vessels
called aortic arches, aortic loops, or
"hearts," running from dorsal trunk
to ventral through the seventh,
eighth, ninth, tenth and eleventh
somites.

These "hearts," as well as the
dorsal trunk, furnish the muscular
contraction and elongation of circu-
lar and longitudinal muscles which
force the blood through the vessels.
Such rhythmic contraction and ex-
pansion in either blood vessels or
intestines is known as peristalsis

( ):

In the frog there is a systemic

and pulmonary circulation. The
earthworm possessing no lungs can
have no pulmonary circulation.

The blood of the earthworm is
continuous in closed blood-vessels,
so it is called a closed systemic cir-
culation.

But just as there is the closed
circulation consisting of heart,*
arteries, veins and capillaries in the
frog, as well as a lymphatic, open
circulation, by which the lymph passing out of the blood-vessels is able
to bathe every part of the body, so we speak of a coelomic circulation
in the earthworm, which is equivalent to the lymph-like substance out-
side of the blood-vessels, but within the coelomic cavity of the frog.

The blood is collected from the intestine by two pairs of vessels
which enter a longitudinal typhlosolar tube. This tube is in turn con-
nected with the dorsal trunk by three or four short tubes in each somite.
As there are no circular muscles in the walls of the ventral trunk
this cannot contract, so the propelling of blood is caused by the dorsal
trunk and "hearts" as already stated. This ability of the dorsal trunk
and "hearts," together with the fact that there are valves in both of
these vessels which permit blood to flow forward but not backward,
determines the direction of flow. These valves are just behind the open-
ings of the parietal vessels and in front of the openings of the hearts.
There are other valves also, in some of the other vessels, but these just
mentioned are most important to show how and why the blood flows as
it does.

The blood must, therefore, flow forward toward the anterior end of

S/i

Fig. 167.

A series of diagrams to illustrate the ar-
rangement of the blood-vessels and the course
of the circulation in Lumbricus herculeus.
A. Longitudinal view of the vessels in somites
8, 9 and 10. B. The blood-vessels as seen in
transverse section in the same region. C.
Longitudinal view of the vessels in the intesti-
nal region. D. Transverse section through the
intestinal region. sp, supra-intestinal ; sb,
sub-intestinal, and sn, sub-neural longitudinal
trunks ; nl, lateral neural vessels ; ht, ht, con-
tractile vessels or "hearts ;" it, intestino-tegu-
mentary vessels ; cv, commissural vessels ; af.i,
afferent intestinal vessels ; ef.i, efferent intesti-
nal vessels ; ty, typhlosolar vessel ; i, intestine ;
oe, oesophagus; s.s. septa. (After Bourne
from a drawing by Dr. W. B. Benham).

THE EARTHWORM 271

the animal in the dorsal trunk. It is thus forced through the "hearts"
and, as it reaches the ventral trunk, is sent both in an anterior and a
posterior direction. From the ventral trunk the blood passes to the body
wall and nephridia. The lateral neural trunks then receive the blood
which has gone to the body-wall, while that having gone to the nephridia
has been expelled. The blood in the sub-neural trunk flows posteriorly,
then upward through the parietal vessels into the dorsal trunk. The
anterior portion of the body receives its nourishment from both dorsal
and ventral trunks.

The Coelomic circulation consists of the fluid in the coelomic cavi-
ties. These cavities are continuous throughout all the somites through
dorsal apertures or slits occurring between the various septa and the
digestive tract. The fluid itself is made up of colorless plasma with
white blood cells or leucocytes ( . ). This fluid is

washed back and forth by the movements of the worm and thus bathes
the endothelial lining of the coelom.

The amoeboid corpuscles in the coelomic fluid have a remarkable
power of attacking bacteria and other microscopic organisms such as
Gregarines and Infusorians or even small Nematode worms. If such
parasites enter the coelom the amoeboid cells surround and destroy them.
Their operations are, however, not confined to the inside of the earth-
worm. The slime of the body surface is in part composed of mucus
secreted by the skin, and in part of coelomic fluid and its corpuscles
which find exit through the dorsal pores. The corpuscles are thus able
to attack and destroy bacteria before they effect an entry into the body.
There is no doubt that a worm is constantly exposed to these minute
organisms for the upper layers of the soil teem with them. The slime
itself is a protection, for it both arrests the bacteria and holds them
stranded in the trail which the worm leaves behind it in its progress.
The application of a grain of some irritant, such as corrosive sublimate,
enables one to see how a worm protects itself. As soon as the irritant
touches the skin the segments in front and behind the seat of injury
are forcibly constricted, while the affected segment itself swells up in
consequence of the increased pressure brought to bear upon it from both
sides. At the same time there is a conspicuous gush of coelomic fluid
from the dorsal pores in that region and an abundant secretion of mucus
from the skin itself. Thus the threatened region is, as it were, isolated
by ligatures from the rest of the body and all the defensive resources
at once brought to bear upon the enemy. The coelomic fluid is alkaline
and contains crystals of calcium carbonate, and also contains micro-
organisms which when isolated and reared in artificial cultures emit the
characteristic smell of earthworms. It is, therefore, not improbable that
this odor is due to the micro-organisms and not really a feature of the
worm itself.

272 GENERAL BIOLOGY

From what has been said above it will be seen that there is in reality
no true circulation in the earthworm.

RESPIRATION

The earthworm needs oxygen just as do all animals ; but, as it has
no lungs it obtains its oxygen through its moist outer membrane. There
are many capillaries lying immediately beneath the cuticle, thus pre-
senting a great expanse of blood area which is somewhat similar to the
many capillaries in the lungs of higher forms. The oxygen here com-
bines with haemoglobin. The blood gets to these capillaries through
the vessels supplying the body wall and is then returned to the dorsal
trunk by way of the sub-neural trunk and the intestinal connectives.

As the nervous system must co-ordinate every movement of the
body, it requires an excellent blood-supply, which is furnished the bet-
ter in the earthworm by the sub-neural trunk lying very close to the
ventral nerve cord. The nervous system is thus continually supplied
with fresh nourishment.

THE EXCRETORY SYSTEM

Most of the excretory matter is carried outside the body by a num-
ber of coiled tubes called nephridia, a pair of which lies in each somite
except the first three and the last. The dorsal pores also serve as ex-
cretory organs to a minor extent.

A clear understanding of the nephridia is important, because such
an understanding will serve the student in good stead in his future
studies of the excretory organs of vertebrates. This is the better un-
derstood when it is known that the excretory organs of all higher forms
develop from embryological beginnings quite similar to those of the
earthworm.

Each nephridium (Fig. 168) consists of:

(1) The funnel or nephrostome ( ),

(2) The ciliated neck,

(3) The coiled narrow tube,

(4) The wide glandular tube,

(5) The ejaculatory duct opening to the outside.

"The ciliated neck of the nephrostome passes through the anterior
wall of the somite, close to the mid-ventral line. The nephrostome,
therefore, lies in the somite anterior to the one containing its own
nephridium, so that waste matters of any one somite are expelled to the
outside by the nephridium of the next posterior somite. The nephro-
stomes, or mouths, of the nephridia are flattened fan-like structures, con-
sisting of two flattened lamellae or plates with a narrow slit-like opening
between them ; the great cells lining the opening are covered with pow-
erful cilia which maintain a constant current toward the tubular part
of the nephridium. These tubes are developed in coils which lie in the

THE EARTHWORM

273

posterior parts of the somites, three coils or turns in each, the third
ending in an enlarged portion opening to the outside on the ventral wall
of the somite. All of the turns are richly supplied with blood vessels."
An excellent way to demonstrate the action of these nephridic
organs is that of injecting carmine powder into the coelom. It will then
be observed that this foreign substance is taken up by the chlorogogen
cells, which then break down, freeing the carmine together with frag-

Fig. 168. Nephridium.

ments of the chlorogogen cells, and all are caught up by the current
made by the nephrostome, and carried through the nephridium to the
outside. From this experiment the conclusion has been drawn that
some, at least, of the waste matters of the tissues are brought to the
chlorogogen cells by the circulation and are acted upon by the fluids of
those cells. The products of this activity are liberated into the coelom*
by the fragmentation of the cells, and then excreted from the worm by
the nephridia. • ?:V " "~'^uy

THE NERVOUS SYSTEM

Notwithstanding the nerve cells scattered about in the Hydra, it is
in the Earthworm that we meet with our first organized nervous system
(Fig. 169). That is, of course, excluding our study of the frog. And it
will be remembered that the nerve cord was on the dorsal side of the
frog. In the earthworm, and all animals lower than vertebrates, it lies
on the ventral surface. This is quite important and will be of use in our
later study of evolutionary theories.

Nerves are sensory, motor, or mixed as noted in the frog. Both
sensory and motor nerves run to the muscles of the earthworm, causing
reflex action. A reflex action means that an impulse sent toward the
central nervous system through a sensory nerve, meets a motor nerve
(the meeting place being called a ganglion), and the motor impulse is
then returned to the place from whence the sensory impulse originated,
permitting an organ to move. If such ganglion lies in the lower nerve
centers, that is, if it lies caudad to the brain, so that an impulse from

274

GENERAL BIOLOGY

a sensory fiber need not first pass to the brain before meeting the motor
fiber, it is called a reflex.

The ventral nerve cord is in reality a series of ganglia, one pair lying
in each somite posterior to the fourth. Each pair is connected by a
nerve cord to the one preceding and following it. In somite four this
nerve cord divides into two parts, one passing on each side of the ali-
mentary tract to again unite
above the pharynx in the third
somite. This dorsal union is
the brain, while the two por-
tions forming it are known as
the circum-pharyngeal con-
nectives. The segmental
ganglia forming the nerve cord
are called the sub-pharyngeal
ganglia. The brain and ventral
cord form the central nervous
system. The nerves passing
from the central nervous sys-
tem to the various parts of the
body constitute the peripheral
nervous system.

The supra - pharyngeal
ganglia supply the prostomium

with two large nerves which give off many branches ; they also send
nerves into somites two and three. One nerve extends out from each
circum-pharyngeal connective. In each somite from four to the pos-
terior end of the body, three pairs of nerves arise, -two pairs from the
ganglionic mass and one pair from the sides of the nerve cord just be-
hind the septum which separates the somite from the one preceding.

Each enlargement of the ventral nerve cord really consists of two
ganglia, which are closely fused together. In transverse section these
fused ganglia are seen to be surrounded by an outer thin layer of epi-
thelium, the peritoneum, and an inner muscular sheath containing blood
vessels and connective tissue as well as muscle fibers. Near the dorsal
surface are three large areas, each surrounded by a thick double sheath
and containing a bundle of nerve fibers. These are called neurochords-
or "giant fibers." Large pear-shaped nerve cells are visible near the
periphery in the lateral and ventral parts of the ganglion.

The nerves of the peripheral nervous system are either efferent or
afferent. Efferent nerve fibers are extensions from cells in the ganglia
of the central nervous system. They pass out to the muscles or other
organs, and, since impulses sent along them give rise to movements,
the cells of which they are a part, are said to be motor nerve cells. The

Fig. 169. Diagram of the Anterior End of Lum-

bricus Herculeus to show the Arrangement

of the Nervous System.

I, II, III, IV. The first, second, third, and
fourth segments.

1. The prostomium. 2. The cerebral ganglia.
3. The circumoral commissure. 4. The first ven-
tral ganglion. 5. The mouth. 6. The pharynx.
7. The dorsal and ventral pair of chaetae. "8. The
tactile nerves to the prostomium. 9. The anterior,
middle and posterior dorsal nerves. 10. The an-
terior, middle and posterior ventral nerves.
(After Hesse).

THE EARTHWORM 275

'afferent fibers originate from nerve cells in the epidermis which are
-sensory in function, and extend into the ventral nerve cord.

SENSE ORGANS

The sensitiveness of Lumbricus to light and other stimuli is due to
the presence of a great number of epidermal sense organs. These are
groups of sense cells connected with the central nervous system by
means of nerve fibers, and communicating with the outside world
through sense hairs which penetrate the cuticle. More of these sense
organs occur at the anterior and posterior ends than in any other region
of the body. The epidermis of the earthworm is also supplied with
efferent nerve fibers which penetrate between the epidermal cells forming
a sub-epidermal network.

THE REPRODUCTIVE SYSTEM

The earthworm, like Hydra, is hermaphroditic (Fig. 170)
( ), that is, has both sexes in each animal.

The female reproductive organs, the ovaries, lie in somite thirteen,
the oviducts in somites thirteen and fourteen, while two pairs of seminal
receptacles or spermathecae lie in somites nine and ten.

The ovaries are small pear-shaped bodies lying on either side of
the mid-ventral line, being attached by their larger ends to the ventral
part of the anterior septum.

The oviducts are made up of various parts : the ciliated funnel just
posterior to each ovary which passes through the septum, dividing
somites thirteen and fourteen, where it has an enlargement known as
the egg sac. It then narrows into a thin duct which opens to the external
part of, the body on the ventral surface near the center of somite four-
teen.

The spermathecae or seminal receptacles are white spherical sacs
near the ventral body-wall, one pair each in somites nine and ten. These
open to the outside through the spermathecal pores lying between
somites nine and ten, and ten and eleven.

The male reproductive organs consist of two pairs of glove-shaped
testes, one pair each in somites ten and eleven. Their positions in the
somites are similar to the ovaries. The vas deferens ( ),

the male organ homologous to the female oviduct, is likewise a ciliated
funnel serving as the mouth of the duct through which the sperm pass.
This lies immediately behind each testis. The duct itself passes through
the septum just back of the funnel, where it forms several convolutions,
then extending backward near the ventral surface. The two sperm ducts
which arise on either side of the midventral line, unite in somite twelve
and then run back as a single tube, opening to the outside through the
spermiducal pore on somite fifteen. In a sexually mature earthworm,
the testes and funnel-shaped inner openings of the sperm ducts are in-

276

GENERAL BIOLOGY

closed by large white sacs, the seminal vesicles lying in somites nine to
twelve. There are three pairs of these sperm sacs, one in somite nine,
one in somite eleven, and the third in somite twelve. In somites ten
and eleven there are central reservoirs.

The testes are rather difficult
to find in a mature worm because
they are quite small and the dor-
sal wall of the vesicle must first
be removed.

The sperm are developed in
the testes and stored in the sem-
inal vesicles from which they are,
during the period of copulation,
injected into the seminal recep-
tacles of another worm. Fertili-
zation actually taking place out-
side the body, however.

When the earthworm is sex-
ually mature there is a clitellum

Fig. 170. Lumbncus Herculeua. Of Cmgulum ' formed, COVCring

firstlweViZ s°omHes, SfSn wnen^tt Inimal SOme six Or Seven Segments. This

L^%&l^^^t^S^^S&^ is a thickened portion often sup-
septa. The pins are placed in the 3rd, 9ih, and posed to be a SCar formed by the
18th somites. B. View ot the first sixteen somites . . *
of the same worm after removal of the alimen- worm after having been injured
tary tract, to show the nervous system and re- . _ .. . t
productive organs, be, buccal cavity, cut across; Or CUt in tWO. Mating may take
eg, cerebral ganglia ; g, gizzard ; int, intestine ; < f , i

nph, nephridia; od, oviduct; oe, oesophagus; ov, place at any season of the year,

ovary in somite 13; ph, pharynx with radiating i f r>mnrc tnr»rA ^r^nn^ntKr in

muscular strands; prv, proventriculus ; s, septa; DUt OCCUrS more IrcqUCntly in

sd, sperm duct; sf, seminal funnels; spth, sper- -warm rlamn i/upatVipr

mathecae in somites 9 and 10; sp.s, sperm sacs; Wdl -ner<

t, testis. (After Bourne). Again quoting Latter. i Two

worms from adjacent burrows, "each retaining a firm hold in its own
burrow by means of the flattened tail, apply their ventral surfaces to
one another so as to overlap for about a third of the length of the body.
The head of each worm points toward the tail of the other. The clitel-
lum of each secretes a band of mucus which binds the two worms firmly
together, so firmly, indeed, as to cause two well-marked constrictions,
while a slimy covering, the slime tube, surrounds the two worms from
the 8th to the 33rd segments. The seminal fluid, containing spermatozoa
( ) and spermatophores ( ),

flows within the slime-tube, and during sexual union, in the early stages
of the formation of the cocoons spermatophores cover the dorsal and
lateral surfaces of segments 9, 10, and 11 of each worm and are packed
between the two worms. The spermatozoa flow backwards from the
male aperture in a longitudinal groove on each side to the receptacula
(spermathecae) of the other worm, the grooves of the two animals to-
gether forming a temporary tube. Hence only one worm can emit sper-

THE EARTHWORM 277

matozoa at any given time, otherwise there would be opposing currents.
The worms are so placed that the 9th segment of each is opposite the
32nd (1st clitellar) of its mate, then the thickened clitellum forms a bar-
rier past which no flow of seminal fluid can take place."

'The long genital setae in the 'tubercula pubertatis' ( )

of the clitellum, and of segments 10 to 15, are probably used, the former
to liberate the coupon from its seat of origin, and the latter series to
hold the coc6on off the ventral surface in the region of the oviducal
openings and those of the spermathecae, and thus allow ova and sper-
matophores to pass into the cocoon as it passes forwards. These special-
ized setae replace those of ordinary form as the worm reaches maturity.
The eggs do not pass out of the oviduct till near the end of the act of
mating. Each of the two worms forms a cocoon, and slips out of it
backwards, passing it forward over its head. The cocoon being elastic
closes its two open ends as soon as the body of the worm is withdrawn,
and becomes more or less lemon-shaped, its bulging center being occu-
pied by about four eggs, spermatozoa and albuminous material produced
by the so-called capsulogenous glands, which may be seen on the ventral
side of some of the segments in front of the clitellum. The cocoons, at
first white but soon becoming yellow, are left in the earth, and as a rule
only one of the contained eggs produces a young worm. The size of the
cocoons differs in the various species, those of L. terrestris are from
6 to 8 mm. long by 4 to 6 mm. broad, of Eisenia foetida from 4 to 6
mm. long by 2 to 3 mm. broad. There is some doubt as to the precise
function of the spermathecae. It seems certain that the spermatozoa
contained in them are derived from some other worm. It is also the
case that these organs are full of spermatozoa prior to sexual union, and
are empty subsequent to that act, at any rate when cocoons are formed
and eggs deposited. Worms have been observed to separate without
producing cocoons, and though perhaps in some instances the separation
may have been due to disturbance caused by observation, yet there is
reason to think that two unions are necessary, one to fill the spermathe-
cae, and a second to form cocoons. In such a case it is probable that
each worm acts as a carrier of spermatozoa from its first to its second
mate, i. e., worm A gets its spermathecae filled by the spermatozoa of
B in the first union, and passes these spermatozoa to C in the second.
The actions are probably often reciprocal. According to Goehlich, while
spermatozoa are flowing from one worm to the spermathecae of the
other, there is given out from the spermathecae of the former a -small
quantity of mucus which hardens when it reaches the air : a second por-
tion of mucus containing a group of spermatozoa is then emitted, this
becomes attached to the first mass, and with it forms a spermatophore.
The whole spermatophore is attached to the body of the other worm
close to the clitellum. When the cocoon is made the spermatophores
are rubbed off into it as the animal withdraws itself.

GENERAL BIOLOGY

"Light could probably be thrown on this matter by some such ex-
periments as follow : keep a number of worms, each in a separate flower-
pot, from infancy to maturity; kill a few and examine the contents of
their spermathecae (it is conceivable that a worm may be able to pass
spermatozoa into its own spermathecae) ; allow remainder to mate once,,
note if cocoons are deposited ; kill some and examine contents of sper-
mathecae; allow rest to mate a second time, pairing some with their
former mates and others with different mates : kill all and examine sper-
mathecae."

In plants and animals where both sperm and eggs are found in the
same individual there is usually a different period for the maturing of
each or some apparatus like this of the earthworm is brought into play
so that it is very seldom that the same organism can fertilize itself.

The sperm-mother cells are derived from the testes and deposited in
the seminal vesicles. They are not fully developed, or as we say, "ma-
ture," however, when they leave the testes, and so must continue their
development in the seminal vesicles.

The sperm-mother cells or primordial germ-cells from which the
sperm are developed in the testes, have their nuclei divide into 2, 4, 8,
or 16 daughter nuclei which become arranged in a single layer near the
periphery of the protoplasm which has not divided. Cell walls then ap-
pear extending inward into the undivided protoplasmic mass. These
newly-formed cells now divide again, forming as high as from 32 to 128
cells when the whole mass breaks up into smaller colonies. These
nucleated cells which are to become sperm are called spermatogonia.
These spermatogonial colonies become spherical, each containing 32 pri-
mary spermatocytes, all of which are still fastened by cytoplasmic
threads to the central protoplasm. This whole 32 celled colony is now
called a blastophore.

Each colony of primary spermatocytes "gives rise to 64 secondary
spermatocytes, and these divide into .128 spermatids. The latter then
metamorphose ( ) into spermatozoa, The number

of chromosomes in the spermatozoa is sixteen ; this is one-half the num-
ber contained in the somatic cells, the reduction having taken place dur-
ing maturation by the union of the chromosomes two by two in the sec-
ondary spermatocytes, and a subsequent separation when the spermatids.
were formed."

The head of the spermatozoon is practically all nuclear material,
the mid-piece is what was formerly the centrosome, while the cytoplasm
formed the tail. But as it is only the head which actually enters and
fertilizes the egg, the tail being used only for locomotive purposes, it
will be seen why nuclear material is considered so very important.

THE EARTHWORM

279

OOGENESIS

The egg-mother cells are found in the ovary in various stages of
growth, beginning at the basal end of each ovary with the most primitive

germ-cells, the ova increasing
in size toward the extreme
end, where the germ-cells are
distinctly recognizable as eggs.
Each egg is surrounded by a
follicle ( ) of nutritive

cells. The eggs separate from
the end of the ovary dropping
into the body-cavity, then pass-
ing into the ciliated end of the
oviduct which goes to the egg-
sac where part of the matura-
tion takes place. From here
they either pass out into the
cavity of the slime-tube and are
conveyed from the external
openings of the oviduct in
somite 14 to the cocoon, or they
enter the cocoon itself when it
passes over this somite during*
deposition, the eggs actually
being fertilized by the spermat-
ozoa after the cocoon is shed,
and before the egg has com-
pleted its maturation process.

H.

K. J.

Fig. 171.

Segmentation and early stages of development of
Lumbricus. A, B, C, D, successive stages of
segmentation. E. Blastula stage. F. Com-
mencement of invagination ; the macromeres
form a flat plate on the ventral side. G. An embryo
somewhat younger than F viewed from above, show-
ing the mesomeres and mesoblast rows derived from
them. H. Gastrula stage viewed from below, -show-
ing the wide oval blastopore bounded by macromeres ;
at the sides the rficromeres are growing over the
macromeres. J. Later stage, showing the elongated
blastopore and the further overgrowth of the macro-
meres by the micromeres. K. Optical longitudinal
section through a later stage after the closure of the
blastopore. bp, blastopore ; ec, ectoderm ; en, endo-
derm ; ent, enteron ; mac, macromeres ; mes, meso-
blast; mic, micromeres; mm, mesomeres. (From
Bourne after Wilsonl)

EMBRYOLOGY

The egg of the earthworm is holoblastic (Fig. 171) although cleav-
age is unequal, the first division resulting in one large and one small cell.
The second cleavage divides the small cell into two equal parts but
cuts off only a small portion from the larger one. The small cells are
called micromeres and the large ones macromeres. Cleavage is very
irregular after this second division. The micromeres are the animal cells
and the macromeres the vegetative cells.

A cavity, the blastocoele, soon forms between micromeres and mac-
romeres resulting in a blastula.

Two of the larger cells of the blastula project down into the blas-
tocoele. These continue dividing and form two rows of small cells from
which the mesoderm is to form. They are therefore called mesomeres,
while the two rows formed from them are known as mesoblastic bands.
During the time these bands are forming the blastula becomes flattened,

280

GENERAL BIOLOGY

the larger cells forming a plate "of clear columnar cells, and the small
cells spread out into a thin dome-shaped epithelium."

The mesomeres lie toward the posterior end of the blastula and the
mesoblastic bands lie along the longitudinal axis of the worm, showing
the beginnings of bilateral symmetry.

A gastrula is now formed by the invagination of the plate of large
cells, this invagination continuing until only a slit remains. This tiny
opening or slit is called the blastopore, while the cavity is the enteron.

Fig. 172. Polygordius Appendiculatus.

A, dorsal view, an, anus ; ct., cephalic
tentacles; h, head. B, trochosphere larva, an, Fig. 173. Nereis Pelagica.'^. (After Oersted),

anus ; e, eye-spot ; m., mouth. C and D, stages
in development of trochosphere into the worm.
pnp, pronephridium. (From Bourne, after
Fraipont).

There are now three germ-layers. The mid-layer or mesoderm al-
ready began forming before gastrulation.

The large clear cells which invaginated have become the inner lining
of the enteron and form the entoderm; the outer portion is ectoderm,
while the mesoderm is made up of the two mesoblastic bands which lie
between ectoderm and entoderm.

As the earthworm is to be our example of the coelomates it is of
value here to observe how the coelom is formed.

The mesoderm separates into the two layers on each side of the
body. A cavity forms between these layers. This cavity is the coelom.
The outer portion of the divided mesoderm is called the somatopleure
( ), the inner layer the splanchnopleure ( ).

The muscles of the body-wall are formed from the somatopleure,
while the splanchnopleure forms the muscles of the alimentary canal.
After the germ-layers are formed, the embryo elongates, the anterior-
posterior axis passing through the blastopore. There are various in-
pushings from the ectoderm which become the elements of the nervous
system. Such beginning cells are called neuroblasts if they form nerves.

THE EARTHWORM 281

There are also separations from the mesoderm forming nephroblasts
if they form nephridia, somatoblasts which form muscles, etc.

The ectoderm turns in at both anterior and posterior ends, the for-
mer forming the mouth or stomodeum ( ), the latter
the anal opening or proctodeum ( ).

The chlorogogen cells are formed from mesoderm, as are also the
blood-vessels, muscles, reproductive organs and seta sacs. The young
worm is now ready for an independent life, and it leaves the cocoon
after from two to three weeks.

The following table will give a summary of the important tissues
derived from the various germ-layers :

ENTODERM ECTODERM MESODERM

Oesophagus, Outer Epithelium, Muscles,

Crop, Nervous System, Coelomic Endothelium,

Gizzard. Stomodeum, Chlorogogen Cells,

Proctodeum, Calciferous Glands,

Ends of Nephridia. Blood vessels,
Septa,

Nephridia, functional parts,
Seta Sacs,

Reproductive Organs.
BEHAVIOR

As already seen, worms are apparently fond of having their bodies
in contact with solid objects as shown by their home-life. Moisture
causes a positive reaction if such moisture comes in direct contact with
the worm's body. This is well illustrated by placing the earthworm,
Allobophora foetida (the small manure worm), on a piece of dry filter
paper when it will not react, but as soon as moisture is applied it begins
to burrow, provided this moisture or liquid is taken from manure.

Darwin supposed that the earthworm's ability to distinguish edible
from inedible food lay in the sense of contact. This would make contact
in the earthworm act as a sort of taste organ. Various chemicals which
cause a reaction may be due to this sort of secondary taste-ability.

While there are no eyes, light causes the animal to react as shown
by its moving away from lighted areas though the manure worm will
respond positively to a very faint light. The preferable colors, when
very faint, are red, green and blue in the order given, though it does not
follow from this that the earthworm can distinguish 'colors ; its ability
consisting, in all probability, of "feeling" different rays of light as well
as different intensities.

It has also been noted that if a previous stimulus is much stronger
than a succeeding one, the first will naturally continue to react and cause
either no reaction to a second or at least lessen such reaction. An exam-
ple of this is found when the animal is feeding or mating. Light which

282

GENERAL BIOLOGY

under normal conditions causes a negative reaction, may have no effect
whatever under such circumstances, the instinctive reaction of the pri-
mary instinct being stronger than the artificial secondary stimulus.

REGENERATION

Any part of an earthworm may be cut off at any point between the
end of the prostomium and the fifteenth to the eighteenth segment and
a new anterior end will grow out from the cut end of the body consisting
of a single segment if only one segment was removed ; two segments, if
two segments were removed ; and of three, four, or five segments, if

three, four, or five segments were
removed. But never more than
segments one to five are regen-
erated, regardless of the number re-
moved, and no new reproductive
organs appear if the original ones
were contained in the severed piece.
If the cut is made behind segment
eighteen, a tail will grow out from
the cut surface of the posterior
piece, thus producing a worm con-
sisting of two tails joined at the
center. Such a creature cannot take
in food, and must slowly starve to
death. When the regenerated part
is different from the part removed,
as in the case just cited, the term
heteromorphosis is given to the phe-
nomenon.

Regeneration of a tail differs
from that of a head, since more than
five segments can be replaced. The
anal segment develops first, and then
a number of new segments are intro-
duced between it and the old tissue.
The rate of regenerative growth
depends upon the amount of old
tissue removed. If only a few seg-
ments of the posterior end are cut off, a new tail regenerates very
slowly ; if more are removed, the new tissue is added more rapidly. In
fact, the rate of growth increases up to a certain point as the amount
removed increases. The factors regulating the rate of regeneration have
not yet been fully determined, although several possible explanations
have been suggested.

Fig. T74.

A. Hirudo medicinalis, about life size.

1. Mouth. 2. Posterior sucker. 3. Sen-
sory papillae on the anterior annulus of each
segment. The remaining four annuli which
make up each true segment are indicated by
the markings on the dorsal surface.

B. View of the internal organs of Hirudo
medicinalis. On the left side the alimentary
canal is shown, but the right half of this
organ has been removed to show the excretory
and reproductive organs.

1. Head with eye spots. 2. Muscular
pharynx. 3. 1st diverticulum of the crop. 4.
llth diverticulum of the crop. 5. Stomach.
6. Rectum. 7. Anus. 8. Cerebral ganglia. 9.
Ventral nerve cord. 10. Nephridium. 11.
Lateral blood-vessel. 12. Testis. 13. Vas de-
ferens. 14. Prostate gland. 15. Penis. 16.
Ovary. 17. Uterus — a dilatation formed by
the conjoined oviducts. (After Shipley and
MacBride).

THE EARTHWORM 28$

GRAFTING

Pieces of earthworms may be grafted upon other worms without
much difficulty. Three pieces may be so united as to produce a very
long worm ; the tail of one animal may be grafted upon the side of an-
other, producing a double-tailed worm ; or the anterior end of one indi-
vidual may be united with that of another. In all such experiments the
parts must be held together by threads until they become united.

The Annelida are divided into three classes, as follows :

(1) Class Archiannelida (Gr. arche, beginning — Lat. annellus,
ring). The Polygordius (Fig. 172) is the typical example. This class
is without setae or parapodia.

(2) Class Chaetopoda (Gr. chaite, bristle — pous, foot). Nereis^
the common sand-worm, and the earthworm are classic examples.
Nereis differs from the earthworm in having a pair of chitinous jaws,
a pair of tentacles, and two pairs of eyes on the prostomium, as well as-
in having a pair of palpi, and four pairs of tentacles on the peristome.
The parapodia are used for locomotion, while the lobes of the parapodia
are well supplied with blood-vessels and serve as gills. Then, too, there
are jointed locomotor-setae on each parapodium, while the muscles which
move the parapodium are attached to two buried bristles, called aciculae,
which serve as a sort of internal skeleton. The sense organs of Nereis
are also developed more highly than those of Lumbricus, the tentacles
serving as organs of touch, while the palpi are thought to act as organs
of taste, and the eyes, of sight.

Nereis (Fig. 173) is the example of the Sub-class known as Poly-
chaeta (on account of its many foot-like structures), while such worm-
like water-animals as Tubifex, Dero, and Nais, usually serve as the ex-
ample of the sub-class, Oligochaeta (having few setae).

(3) Class Hirudinea. (Lat. hirudo, leech.) These are worm-like
animals living in fresh water and on land. They are commonly called
leeches. They are flattened dorso-ventrally. The external segmentation
does not correspond to the internal segmentation. The leeches are dis-
tinguished from the earth-worm by having definitely thirty-three seg-
ments, two suckers (one at each end), and no setae (except in one
genus). They are hermaphrodites.

The most important example is the medicinal leech known as
Hirudo medicinalis (Fig. 174), normally about four inches long, though
capable of much contraction and expansion. Not only are these animals
used to draw blood from patients, but Lambart advises against drinking
water which is not filtered, especially in the tropics, as the small leeches
may be swallowed. They then attach themselves below the larynx and
instead of releasing themselves when filled with blood as they do on an
external surface, they seem to draw a small amount of blood and then
migrate to another spot close by and begin the same process, thus caus-
ing considerable anaemia (loss of blood).

284 GENERAL BIOLOGY

This is readily understandable when it is realized that the leech
has three chitinous jaws to form the mouth (which lies within the an-
terior sucker). These jaws bite into a region, and a secretion from the
mouth-glands is poured out which prevents the host's blood from coagu-
lating. It is thus difficult to stop the bleeding after the animal has
moved to a new location.

The digestive tract of the leech is especially adapted to the diges-
tion of blood of vertebrates, upon which the leech feeds. There is a
muscular pharynx and a short oesophagus leading to the crop. This
crop has eleven branches or diverticulae. Then there is a stomach, an
intestine and an anus. The leech can ingest about three times its own
weight of blood.

There is a peculiar kind of connective tissue known as botryoidal
( ) tissue which develops in what should be the

coelom. This body-cavity is therefore very small, although there are
spaces in the coelom which are not filled with this tissue, these spaces
being called sinuses.

There are seventeen pairs of nephridia, quite like those of the earth-
worm (except that they sometimes do not have an internal opening)
which carry waste products from the coelomic fluid and from the blood.
Respiration takes place at the surface of the body through the many
blood-capillaries found in the skin.

There are nine pairs of segmentally arranged testes which empty
their sperm into the vas deferens, then into a much-folded tubule called
the epididymis. Here they are fastened into bundles known as sper-
matophores. They are then ready to fertilize the eggs of another leech,
after passing out of the copulatory organ.

The eggs develop in a single pair of ovaries, from which they pass
through the oviducts into the uterus, and finally out through the genital
pore situated on the ventral side of the ninth segment. A cocoon is
formed after copulation quite like that in earthworms.

CHAPTER XX.

FLATWORMS (PLATYHELMINTHES) AND THREADWORMS
(NEMATHELMINTHES)

Systematically the flat worms and round worms should be placed
before the earthworm as they are not coelomates, but, as the average
man always thinks of a sort of segmented animal similar to an earth-
worm when worms are mentioned, and medical men likewise are not
very accurate when they discuss these animals, the student is more likely
to remember the three types of worms if he thinks of them all at once
and notes their similarities and differences.

The Annelids are of little importance from a medical standpoint
with the exception of the leech (Hirudo Medicinalis) commonly used to
draw blood, but the flat worms and round unsegmented worms have
come to have a very considerable bearing on the human being from a
pathological standpoint.

THE FLATWORMS

The flatworms (which constitute the phylum Platyhelminthes) are
subdivided into the following three classes :

Class I. Turbellaria (Lat. turbo, I disturb), with ciliated ectoderm;
free-living habit, example : Planaria.

Class II. Trematoda (Gr. trema, a pore; eidos, resemblance), with
non-ciliated ectoderm ; suckers ; parasitic habit, example : Fasciola
hepatica (liver fluke), and

Class III. Cestoda (Gr. kestos, a girdle; eidos, resemblance), with
body of segments ; without mouth or alimentary canal ; parasitic, exam-
ple Taenia (tapeworm).

TURBELLARIA

Turbellaria are the only flatworms which are not parasitic. They
live on the lower surface of submerged stones and debris close to the
margin of ponds, springs and lakes. Most of these are Planaria (Fig.
175), but often a longer worm is found (from ten to fifteen millimeters)
which is called Dendrocoelum lacteum.

Planaria crawls about among aquatic plants to seek its food. The
cilia covering the ectoderm assist in this movement, though the animal
also contracts and expands its body. As soon as a planarian finds a
small animal suitable for its food, the proboscis, lying near the center
of the body, is practically turned inside out through the mouth. This
proboscis grasps the food and draws it into the body. As the mouth
is near the center of the ventral surface, the proboscis can be extended
in any direction.

•286 GENERAL BIOLOGY

The digestive system consists of the mouth, proboscis or pharynx
(which lies in a muscular sheath), and three chief interior intestinal
branches, one running forward to the head end of the body and two
leading tailward. Many small side pouches or diverticula protrude. In
fact, every part of the body has such a pouch. This means that all
parts of the body can take nourishment immediately from the digestive
tract so that planaria needs no circulatory system. All non-digested
food must be egested through the mouth, as there is no anal opening.

In some forms a definite green substance appears which is due to
the zoQchlorellae or symbiotic one-celled plants which live in the middle
.germ-layer.

Fopd is digested both intercellularly and intracellularly, which
means that a part of the food is digested in the intestine proper by secre-
tions which are poured out
from cells in the intestinal
walls; and, that food may also
be digested by pseudopodia
extending from cells in the in-
testinal walls, which pseudo-
podia take in the undigested
food to the cell which then
B. digests it.

Fig. 175. A. Planaria polychroa X about 4.

1, Eye. 2. Ciliated slit at side of head. 3. Mouth External Annearanre

of proboscis. 4. Outline of the pharynx sheath into ^Xierr ince-

tTvheChporhee pharynx ™ * withdrawn- 5' Reproduc- Planaria is bilaterally

B. Dendrocoeium 9raffi. (Woodworth). symmetrical and dorso-ven-

trally flattened. The head-end is blunt and the tail-end tapers. It is
usually less than half an inch in length. The common American species
is known as Planaria maculata. It has a definite pair of eye-spots.

Turbellaria are metazoans and triploblastic. The mesoderm con-
sists mostly of muscles and loose parenchyma cells. The coelom is rep-
resented by the genital sacs.

Turbellaria are classified according to the type and number of
.branches found in the digestive tract.

In some of the turbellaria, though not in Planaria, there are special
'ectodermal cells which secrete mucus, or produce rod-like bodies called
rhabdites.
The Excretory System.

The excretory system (Fig. 176) consists of two irregular longi-
tudinal much-coiled tubes, one on each side of the body. Near the an-
terior end these two tubes are connected by a transverse vessel. The
longitudinal vessels open to the exterior by two small pores on the
dorsal surface of the animal.

Many fine tubules branch off from these main tubes and ramify
through all parts of the body, terminating in large flame-cells (Fig. 177).

FLATWORMS AND THREADWORMS

Each of these flame-cells (which are characteristic of the flatworms)
consists of a central cavity into which a bundle of cilia project. The
flickering of the cilia look something like a candle-flame, and it is from
this fact they are named. It is the flame-cell which is considered the

real excretory organ of the animal, though
some writers think it may also have some
respiratory functions.

The Nervous System.

There are two lobes (Fig. 176) of nerv-
ous tissue beneath the eye-spots. These are
usually called the brain. There are also two
longitudinal nerve-cords, one on each side

Fig. 176. Anatomy of a Flatworiu.
en, brain ; e, eye ; g, ovary ; iv i2, iy
branches of intestine ; In, lateral nerve ; m,
mouth ; od, oviduct ; ph, pharynx ; t, testis ;
u, uterus ; v, yolk glands ; vd, vas deferens ;
<j" , penis ; ? , vagina ; c? , ? , common genital
pore. (From Lankester's Treatise, after v.
Graff).

Fig. 177.
c, cilia ; e,

Flame-cell of Planaria.
opening into the excretory

tubule. (From Lankester's Treatise).

of the body, connected by transverse nerves. Nerve branches pass into
the head proper from the brain region, so that the anterior end becomes
the more sensitive.

The Muscular System.

Immediately beneath the ectoderm, a group of muscles form a
dermo-muscular sac around the internal organs. There are two layers,
an inner longitudinal and an outer circular.

The Reproductive System.

Planaria are hermaphroditic, having both male and female repro-
ductive organs (Fig. 176). These animals nevertheless often reproduce
by fission. Each animal has numerous spherical testes which are con-
nected by small tubules called vasa deferentia. The single vas deferens
from each side of the body empties into, or through, the cirrus into the
genital cloaca.

At the base of the cirrus there is a seminal vesicle and several uni-
cellular prostate glands.

288

GENERAL BIOLOGY

After the sperm are formed in the testes, they pass to the seminal
vesicle through the vasa deferentia, and remain there until needed for
fertilization.

The ovaries are two in number. From these the two long oviducts

(which possess many yolk-glands) connect with the vagina The vagina

opens into the genital cloaca. The uterus also connects with the cloaca.

After the eggs ripen, they pass from the ovary through the oviducts

(where they collect yolk from the yolk-glands) and finally reach the

uterus. Fertilization occurs in the
uterus. Cocoons are formed, each
containing from four to twenty eggs
and several hundred yolk-cells.

As already stated, planaria ma\
also reproduce by fission. This
means in this instance that when
the hindermost portion of the ani-
mal is grown, it breaks off from the
fore part to produce a new animal.

Fig. 178. Regeneration of Planaria maculata.
A, normal worm. B, B1, regeneration of
anterior half. C, C1, regeneration of posterior
half. D, cross-piece of worm. D1, D2, D3, D*,
regeneration of same. E, old head. E1, E2, E3,
regeneration of same. F , F1, regeneration of
new head on posterior end of old head. (From
Hegner after Morgan ) .

Regeneration.

From the laboratory point of
view, Planaria is probably the most
available animal one can find to
show regeneration experiments.
This is especially true because the parts to be regenerated grow very
rapidly, each day marking a definite growth region.

Almost any part of the animal will re-grow, but there are portions
quite specialized in what is re-grown. If, for example, the head is cut
off directly behind the eyes, the more anterior part will regenerate a
new head but no body, thus making a two-headed animal. Such speciali-
zation is called polarity. (Fig. 178.)

There are two types of eggs laid. In the summer the eggs are thin-
shelled and develop quickly, while in the autumn the "winter eggs" are
laid. These are thick-shelled and lie dormant until spring before hatch-
ing.

TREMATODA

All the trematodes are parasitic. Some are monogenetic; that is,
the adults lay eggs which hatch into forms like their parents, all living
on the outside of their host. These are said to have a simple life-history.
This type of animal is usually found on cold-blooded vertebrates, such as
frogs, fishes, etc.

The endoparasitic trematodes (those which live in the internal or-
gans of a host, whether that be in the liver, lungs, intestines, bladder
or other similar internal structure), are mostly digenetic. This means

FLAT WORMS AND THREADWORMS

that the parasite must pass through more hosts than one to complete its
life cycle.

The liver fluke, Fasciola hepatica (Fig. 179) is the form usually
studied in the laboratory.

The adult liver fluke lives in the bile-ducts of the sheep's liver and

4

Fig.

Stages in the Life-History of the Liver Fluke,
Distomum Hepaticum.

1, Egg filled with large vitelline cells in which the segmenting
ovum, em., is embedded ; o., operculum ; 2, Miracidium larva with
large ciliated cells, the eyespot e., and the interior papilla, pa.
I, Miracidium boring its way into the tissues of Limnaea; /-/., flame
cells. 4, a sporocyst containing one fully developed and several de-
veloping rediae (R.) ; e., the degenerate eyes. 5, a redia containing
several daughter rediae in various stages of development ; m.,
mouth ; ph., pharynx ; ent., enteron ; r., muscle collar ; p., posterior
processes. 6, a cercaria ; m., mouth; s' ., anterior, and a"., poster-
ior suckers; cs., cystogenous cells. (After Thomas.)

is continually laying eggs which are carried through the intestine of
the host to the outside in the faeces. If these eggs become moist they
hatch into tiny ciliated larvae called miracidia. These larvae swim about
until they find a pond snail. This being found, the larvae bore their way

290

GENERAL BIOLOGY

into the snail where a complete change in the parasite takes place. It
takes about two weeks for the fluke larvae to form a sac-like sporocyst.
Each germ-cell in this sporocyst passes through a blastula and gastrula
stage and then becomes a second kind of larva which is now called a
redia. These rediae then break through the sporocyst and enter the
host's liver. The rediae have germ-cells within them, and these germ-
cells give rise to little cercaria which look something like tiny tadpoles.
These tadpole-like cercaria leave the snail and swim to the shore to form
cysts on surrounding vegetation.

As the sheep pass along and eat the vegetation bearing these cysts,

-Eggs.
-Parasite.

-Changes
Due to
Inflammation.

Fig. 180. Schistosomum Haematobium.

(Distoma Haematobium.)

From the submucosa of the large intestine of man.
(From a photograph lent to the author by
Dr. E. L. Miloslavich. )

the life-cycle is again begun. It will be noted from the account just
given that the larval stages breed in cold-blooded animals, while the
adult stages must have warm-blooded animals for their hosts.

The liver fluke is by no means unknown to affect the human liver,
and where this is known to be the case, great care must be exercised in
eating uncooked vegetables.

From the complicated life-cycle the liver fluke displays, it can
readily be understood that many thousands of eggs must be produced by
a singrle animal if liver flukes are not to die out; for, it is not at all likely
that many of the miracidia will find a snail host; and then again, it is
not very likely that many of the cysts on the shore vegetation will be
eaten by sheep.

One liver fluke will produce as high as five hundred thousand eggs,
and a single sheep may contain over two hundred adult flukes. This
means that over a hundred millions of eggs may develop in a single
sheep.

FLATWORMS AND THREADWORMS

291

f

Trematode Infections.

Schistosomum haematobium (Fig. 180), (also called Bilharzia
haematobia), which causes the disease known as bilharziosis, is by no
means uncommon in tropical countries such as Asia and Africa, and is
sometimes found in Europe and America.

The mature worm lives in the branches of the portal veins so that
the eggs are easily distributed (with the blood) into the liver and other
organs of the body. The eggs, which are the true cause of the disease,
have a tendency to affect the urinary apparatus, causing a bloody urine

to be discharged and also causing de-
structive and over-growth processes in
the bladder, urethra, and surrounding
parts. All these infected parts are
loaded with eggs so that abscesses and
fistulas form. Similar conditions may
take place in the rectum. Ten per cent
of all patients in Cairo were found to
be infected, while seven and a half per
cent of all army recruits in Egypt
showed the eggs in their urine.

Schistosomum Japonicum is the
Japanese species.
This blood-fluke is peculiar in that it has separate sexes, the male
being carried about by the female in a gynaecophorous canal (Fig. 181).
The eggs are oval and a terminal spine is found at one end. The
eggs hatch in water, so they may be taken in with raw vegetables or even
with drinking wrater.

It is an interesting fact that animal parasites often cause no pain,
but are on that very account the more dangerous because the patient
infected pays no attention to his infection and the disease thus grows
constantly worse without any attention being given it.

Schistosoma Japonicum vel cattoi. This species is common in China,
Japan, and the Philippines. The disease produced by it is called Kata-
yama disease. The liver hardens and the spleen enlarges. There is dys-
entery and a loss of blood.

The eggs are smaller than S. haematobium and they do not have the
terminal spine.

In Formosa, Paragonimus Westermani (Fig. 182), (Asiatic lung-
fluke or bronchial fluke), is often found as a parasite infecting the lungs
of man. It is also found in the brain where it causes death from
pressure.

The worm is 8-16 mm. long and 4-8 mm. broad, and is pinkish or red
in color. The disease it causes is often confused with tuberculosis, al-
though the microscope shows many eggs in the sputum. The liver,
brain and eyelid are the most common points affected.

292

( iKNKRAL I >I()L()C,V

The common liver-fluke Fasciola hepatica, though rare in this coun-
try, is common in Syria where men eat raw goat-livers. The disease is
called Halzoun.

Opisthorchis (Distoma) felineus is common in cats. It has been
found in Prussia, Siberia, and Nebraska.

Opisthorchis noverca (Distomum conjunctum) is the Indian liver-
fluke.

D

in.

Fig. 182. Infective Trematodes.

I. Opisthorchis felineus. Os., oral sucker; Ph., pharynx; /.,
intestine ; Vs., ventral sucker ; Ut., uterus ; Vg., vitelline glands ;
Vd., vitelline duct ; O., ovary ; T., testes ; EC., excretory canal.

II. Opisthorchis noverca. A., greatly enlarged. B., almost na-
tural size, m., mouth (oral sucker) ; ph., pharynx; ac., acetabulum
(ventral sucker) ; ut., uterus; vt., vitelline glands; ov., ovary; vd.,
vas deferens ; t., testes ; i., intestine ; cxp., excretory pore.

III. Fasciolopsis buski.

IV. Heterophyes heterophyes. a., schematic and highly en-
larged ; b., about twice natural size ; e., eggs, greatly magnified ;
d., spine greatly magnified. (I, after Stiles and Hassal ; II, after
Manson ; III, after Rivas ; IV, after Loose. )

V. Paragonimus Westermani (Asiatic Lung Fluke) : 1, oral
sucker ; 4, intestine ; 7, acetabulum ; 8, ovary ; 9, excretory canal ;
11, yolkglands ; 12, testis ; 14, uterus. (After Pratt.)

FLAT WORMS AXD THREADWORMS 293

Opisthorchis (Distoma) sinensis. This is one of the most important
of liver-flukes. It occurs extensively in Japan, China and India. It is
10-20 mm. long and 2-5 mm. broad. The eggs are oval and dark-brown
with sharply defined operculum. O. sinensis are also found in Canada
and the United States. Children are usually affected, and whole villages
succumb to its ravages.

Fasciolopsis (Distoma) buski is common in India, and

Mesogonimus heterophyes in Egypt and Japan.

CESTODA

The common tapeworm, Taenia solium (Fig. 183) is the best labora-
tory example of Cestoda. It lives in the digestive tract of man and feeds
upon the already digested food of its host. The tapeworm therefore
needs no digestive system of its own, and it has none.

Taenia is a long flatworm consisting of a knob-like head called the
scolex, and a great number of segments which are all like each other but
different from the scolex. These segments are known as proglottids.

Hooks and suckers on the scolex permit the animal to fasten itself
to the walls of the digestive tract of its host. A small constriction be-
tween head and proglottids is called the neck. The proglottids usually
increase in size the further they are from the scolex. It is not uncom-
mon to have a tapeworm reach ten or more feet in length and have some
eight or nine hundred proglottids. The proglottids are budded off from
the neck, so that the segments furthest from the head are the older. The
process of forming new proglottids is called strobilization.

The body of the simplest type of tapeworm is not segmented, though
most forms are.

Each proglottid contains a set of both male and female reproductive
organs, but the nervous and excretory systems are usually quite con-
tinuous through head and proglottids. The question often arises as to
whether each segment is not a complete individual, but the best authori-
ties believe that the scolex is an asexual individual which buds off the
sexual individuals which we have called proglottids.

There are a good many species of tapeworms, but all of them live
as parasites in the intestinal tract of other animals, and nearly all of
them require two hosts before their life cycle is completed. And, just as
the liver flukes require a cold-blooded and a warm-blooded animal as
their hosts, so the tapeworms usually require some herbivorous animal
as a host for the larval stages, and an animal which eats the flesh of the
herbivorous animal for the adult stages. We therefore have tapeworms
using pig and man, cow and man, fish and man, mealworm and rat, fleas
and dog, rabbit and wolf, etc., as the two hosts.

An adult tapeworm in the intestine of man will continually develop
new proglottids which pass out of the body and shed the eggs upon the
ground. Each proglottid may produce thousands of eggs. If these eggs

294

GENERAL BIOLOGY

Fig. 183. Tapeworms.

A. The Life-History of Tcenia solium. 1, six-hooked embryo in egg-case; 2,
proscolex or bladder-worm stage, with invaginated head ; 3, bladder-worm with
evaginated head ; 4, enlarged head of adult, showing suckers and hooks ; 5, general
view of the tapeworm, from small head and thin neck to the ripe joints ; 6, a ripe
joint or proglottis with branched uterus ; all other organs are now lost.

B. A proglottis of Toenia solium with the reproductive organs at the stage
of complete development, cs., Cirrus sac ; excr., excretory canals ; g.o., genital
opening ; n.c., nerve cord ; ov., ovary ; sh.g., shell gland ; t., testes ; v.d., vas
deferens ; ut., uterus ; vat]., vagina ; y-g., yolk gland.

C. Diagrams of Bladder-Worms. I. The ordinary Cysticercus type, with one . ,.
head. II. The Coenurus type, with many heads. IIL The Echinococcus type, with
many heads, and with blood capsules producing many heads.

D. Portion of hog's liver infested with echinococcus bladder-worm. A, after
Leuckart; B and C, after Borradaile ; D, after Stiles.)

then come in contact with grass, weeds, hay, or any vegetation which
cattle eat, they hatch in the intestine of the animal eating such vegeta-
tion. Each egg will develop a little six-hooked embryo which leaves the
egg and bores its way into the cow's body. It comes to rest either in
the liver or muscle tissue.

In about three months a bladder-worm known as a cysticercus has
developed, and if flesh containing these bladder worms is eaten by man,
he is in turn infected.

The cysticercus is really a tiny bladder-like sac with a scolex pushed
in on one side. When this gets into man's intestine, the scolex is pushed

FLATWORMS AND THREADWORMS [ \ 295

outward so that it can fasten its hooks into its new host's intestine. It
is now ready to bud off proglottids again.

At least one per cent of all cattle slaughtered in this country have
tapeworms. Certain species are also found in pork. All meat should
therefore be well cooked before eating.

The structure of the tapeworm is quite similar to Planaria, the flat-
worm which served as our introduction to this phylum.

It is well, however, to obtain a good description of the way tape-
worms reproduce, as it is due to their reproduction that infection takes
place.

The mature proglottid is almost entirely filled with reproductive
organs. From the spherical testes (which are scattered throughout the
entire proglottid) the sperm cells are carried through the vas deferens,
after being gathered into fine tubules, and pass to the genital pore.

Eggs arise in the two-lobed ovary, and pass into the oviduct. Yolk
from the yolk-gland then enters the oviduct and surrounds the eggs.
After this a shell is provided for the egg by the secretions from the shell-
gland, and the eggs pass into the uterus. The eggs have by this time
been fertilized and pass into the vagina. As the proglottid grows older,
the uterus becomes extended with eggs and even sends off uterine
branches likewise filled with eggs, while the rest of the reproductive or-
gans are absorbed. The proglotti-d is then said to be ripe. When ripen-
ing occurs, the proglottid is very likely to break off and be thrown out
with the faeces.

Cestode Infections.

There are four principal types of cestode worms (Fig. 184) which
infect the human being. These are :

Taenia saginata or mediocanellata,

Taenia solium,

Bothriocephalus latus,

Taenia echinococcus.

Each of these requires an intermediate host for the development of
the larval forms. The eating of the flesh of the intermediate host re-
leases the larval forms and the mature worm forms in the human host.

Taenia saginata (the common beef-tapeworm) is common in the
small intestine of man. As the segments (which are loaded with eggs)
ripen, they are discharged. The eggs are taken up with the food of the
ox. Then the embryo pierces the intestinal wall with the six hooks on
the worm's head. As it bores its way through into the blood-stream,
this blood-stream carries it throughout the entire system. Finally, they
come to rest in various muscles and develop into a cystic larval form.
It is at this point that mari becomes infected if raw beef is eaten which
contains these larvae.

Taenia solium has less uterine pouches filled with eggs than

296

GENERAL BIOLOGY

Taenia saginata. These eggs are ingested by pigs. This type of tape-
worm is rare in the human intestine in America, although it does occur.
The process of development is quite like that of Taenia saginata. The
cystic larvae of Taenia solium are called Cysticercus cellulosae.

Bothriocephalus Latus is found in many types of fish, such as sal-
mon, trout, perch, etc., and if this is ingested by man it passes through

ir.

Fig. 184. Types of Cestoda.
I. Heads of 1, Taenia Solium; 2, T. Saginata; 3, Dibothriocephalus latus;
4, Dipylidium caninum, this latter showing rostrum both evaginated and invagi-
nated ; 5, immature and 6, mature cysticercoid. (From various authors.)

II. Diagram of the anatomy of Tapeworms. 1, Taenia saginata; 2, Dibothrioce-
phalus latus. T, testes ; Vd., vas deferens ; C., cirrus ; Gp., genital pore ; Va., vagina ;
Rs.f receptaculum seminis ; Vtg., vitelline glands ; Vtd., vitelline duct ; Sg., shell
gland ; Ov., ovaries ; Ovd., Oviduct ; Ut., uterus ; Ot., ootype ; Exd., excretory duct ;
Mt., metraterm. (After Rivas.)

FLATWORMS AND THREADWORMS

297

the various stages already mentioned and produces considerable anaemia.
The genital openings are on the face of each segment in Bothriocephalus
latus instead of at the edges as in Taenia.

Taenia echinococcus differs from the three forms just mentioned in
that man is the intermediate host and the dog the true host

It also differs in size from those mentioned. Tapeworms in the
human being may reach a length of thirty to forty feet, but Taenia
echinococcus is only three mm. to six mm. in length. In cold countries
where men and dogs live in the same room and where dogs lick their
master's faces, eggs are transmitted to. the human digestive tract, al-
though intermediate hosts other than man are possible.

The developing cyst in the instance of the small worm is very large,
and there is a closely allied form known as Taenia multilocularis which
cften is present with Taenia echinococcus, and when this is the case, a
great mass of ramifying spongy tissue, full of small cavities, forms. If
these cysts grow in the brain the sheer pressure of the cysts cause injury
and then, too, if the first cyst ruptures it pours out poisons in the sys-
tem, as well as again spreading new larvae which form secondary cysts.

The eggs, when in the human intestine, hatch and bore through the
intestinal wall and are swept along by the blood-stream to their lodging
place. A thin, pearl-colored covering then surrounds it and about this
the tissues of the host react so as to form a capsule. A liquid is formed
in the thin membrane while buds grow out of the membrane. These
buds are finally recognizable as the heads of new worms. The heads
turn inside out, causing the hooks to face inward. This makes it possi-
ble for the worm to be swallowed by dogs and pigs. Then the head
turns back again to make use of its hooks and suckers. If no interme-
diate host is found, the worms may die, but in such a case there is a
large cyst filled with a mortar-like white material remaining.

Following is a summary of all the important
Tapeworms and their hosts:

Name
Taenia solium .

Taenia saginata

Final Host

Man

Man

Taenia elliptica

Taenia cucumerina

(Both of these are also!
called
Dipylidium caninum . . . . |

Taenia flavo-punctata . . .

cat mostly,
but also man .

Common in rats. .

Intermediate Host
Hog (in liver, mus-
cles, brain and
eye).

Ox and Giraffe (in
muscles).

In body-cavity of
dog, fleas and lice.

Moths and beetles.

298

(Hymenolepsis diminuta)

Taenia nana

(Hymenolepsis nana)

Taenia confusa

Dibothriocephalus latus .

Drepanidotaenia setigera

GENERAL BIOLOGY

Twelve cases known

in man.
Common in Italy

and known in

America.

A few cases in Man.

Man and Dog

Common in Fin-
land and regions
where fish is a
common food.

Goose

In peritoneum and
muscles of pike,
perch, and trout.

Water-flea and Cy-
clops brevicauda-
tus.

THE THREAD-WORMS
NEMATODA.

The nematodes are the thread-worms or round worms which make
up the phylum Nemathelminthes.

ovy

Fig. 185. A Cross Section, Ascaris Lumbricoiftes.

A, Transverse section, cu., cuticle ; dl., dorsal line ; der. epthm., epidermis ; ex.v.,
excretory tube ; int., intestine ; lat. 1., lateral line ; m., muscular layer ; ovy., ovary ;
ut., uterus ; v.v., ventral line.

. B. A female cut open to show internal structures. 1, pharynx; 2, intestine;
3, ovary ; 4, uterus, 5, vagina ; 6, genital pore ; 7, excretory tube ; 8, excretory pore.
(A, after Vogt and Yung; B, after Shipley and MacBride.)

FLATWORMS AND THREADWORMS

299

This phylum is likely to prove confusing to students as there are
various systematists who classify thread-worms under different phyla
and under groups which they call uncertain.

Nematodes form the single class of Nemathelminthes, and the two
best knowrn forms used in the laboratory are Ascaris lumbricoides (Figs.

Fig. 186. Tuberculous Cavity in Oesophageal
Wall of Man Containing an Ascaris
Lumbricoides. (From a photo-
graph lent the author by
Dr. E. L. Miloslavich.)

Fig. 187. Trichinella Spiralia.
A. Encysted Trichina Embryo.
B. Adult female from Intestinal wall. 1,
parasite ; 2, membrane of cyst ; 3, muscle-fiber
of pig. (After Leuckart.)

185 and 186), a parasitic worm found in the digestive tract of pigs, horses,
and man, belonging to the family Ascaridae; and Trichinella spiralis
(Fig. 187), of the family Trichinellidae, which causes a very dangerous
disease called trichinosis in rats, pigs, and man.

The female Ascaris is the larger of the sexes ; in fact, it may grow
to a length of from five to eleven inches and a fourth of an inch in diam-
eter. The body is of a light brown color with a narrow white stripe
along the dorsal and ventral surface, and a broader white line lying on
each side of the dorsal and ventral stripe.

The mouth-opening (which is surrounded by one dorsal and two
ventral lips) lies at the anterior end of the animal. The anal opening
lies at the posterior end. The tail-end of the female is straight, while
in the male it is slightly bent. In the male also there are penial setae,
which extend through the anal opening and which are used for copula-
tion.

300 GENERAL BIOLOGY

The Digestive System.

The digestive system is very simple, consisting of a mere straight
tube into which the already digested food of the host enters. A definite
coelom may also be seen. The more anterior portion of the digestive
tube is known as the pharynx. This is muscular, so that by contraction
and expansion it can draw the host's food into itself. At the posterior
end of the digestive tube the intestine becomes smaller. This is the
rectum, which empties through the anal opening.

The Excretory System.

This system consists of two longitudinal canals, one being located
in each lateral line. These open through a single pore near the anterior
end of the ventral body-wall.

The Nervous System.

A definite ring of nervous tissue surrounds the pharynx. From this
ring a dorsal and a ventral nerve cord are given off, as well as a number
of fine nerve strands and connections.

The Reproductive System.

In the male there is but a single testis, which is coiled and thread-
like. The sperm cells pass from this through a vas deferens to a seminal
vesicle and from here through the ejaculatory duct to the rectum.

In the female the reproductive system is Y shaped, the two arms
of the Y being the coiled ovaries which are continuous with the uterus.
It is the two uteri which unite in the stem of the Y to form a muscular
tube, the vagina, which opens to the outside of the body by a genital
aperture.

The egg is fertilized in the uterus, after which a chitinous shell
surrounds it, and the egg is then thrown out through the genital pore. It
is this chitinous shell which prevents the egg being digested in the in-
testine of the host where it must necessarily fall when being laid.

As nematodes are triploblastic animals with three definite germ
layers, these animals also have a coelom. Consequently, the body of
these worms must be thought of as a tube within a tube, with the re-
productive system lying between digestive tract and the body wall-
that is, within the coelom.

However, the coelom is quite different in worms from what it is
in higher animals.

In the higher forms, the coelom is a cavity between the two layers
of mesoderm. The excretory organs open into it and from its walls the
reproductive cells originate. In Ascaris the coelom has only the meso-

FLAT WORMS AND THREADWORMS

301

Fig. 188.

Oxyuris Vermicularis

The male is on the left, the

female on the right.

(After Glaus.)

17

Fig. 189.

Eggs of the More Important Worms Which Are
Parasitic to Man.

As all are of the same magnification, a comparison of the rela-
tive sizes is possible.

1, Fasciolopsis buskii; 2, Schistosoma mansoni; 3, Schistosoma
hacmatobium ; 4, Schistosoma japonicum; 5, Paraffonimus wester-
manii; 6, Clonorchis sinensis; 1, Metagonimus yokogawai; 8, Taenia
saginata; 9, Taenia solium; 10. Hymenolepsis nana; 11, Hymeno-
lepsis diminuta; 12, Diphyllobothrium latum (Dibothriocephalvs
latus) ; 13, Ascaris lumbricoides (egg without outer coating) ; 14,
Ascaris lumbricoides (abnormal egg) ; 15, Ascaris lumbricoides; 1C.
Trichuris trichiura; 17 and 18, Hookworm eggs; 19, Enterobius
vermicularis oxyuris vermicularis ; 20, Oxyuris incognita; 21, Tricho-
strongylus orientalis. (After Hegner and Cort's "Diagnosis of Pro-
tozoa and Worms Parasitic to Man." Bull. Johns Hopkins Uni-
versity school of Hygiene and Public Health.)

derm of the body wall as a lining. There is no mesodermal lining sur-
rounding the intestines. Then, too, the excretory organs open directly
to the outside through the excretory pore, and the reproductive cells do
not originate from the epithelium of the coelom. Notwithstanding this

302 GENERAL BIOLOGY

difference, the space between intestinal tract and body wall is called a
coelom in worms.

Nematode Infections (Figs. 185, 186, 187, 188).

Ascaris lumbricoides is found chiefly in children. The female is
from seven to twelve inches in length and the male from four to eight
inches. The worm is pointed at both ends and of a yellowish-brown or
slightly reddish color. There is no intermediate host. The animal oc-
cupies the upper portion of the small intestine. Usually one or two are
found in a single location, although sometimes vast numbers of them
may be found. The worm may pass to the stomach and be vomited
forth, or -it may crawl up the oesophagus and then pass into the larynx
and asphyxiate the patient. In fact, it may enter any ducts or tubes
in the body.

Oxyuris vermicularis (commonly called pin-worms
or thread- worms), (Fig. 188), are parasites of the rec-
tum and colon. The male is about 4 mm. long and
the female about 10 mm. The parasites migrate and
come close to the surface during the night, thus caus-
ing accentuated irritation and itching about the rec-
tum and genital organs. Many eggs are found in the
faeces of infected children. It is essential that the dis-
tinguishing and diagnostic difference between oxyuris
The Hookworm. eggs and trichocephalus eggs be known. Both tvpes

a., male; &., fe- . 1M 1-1 , i

male; o., mouth; v., are quite alike except that trichocephalus eggs have a
oPfenengggfsOT disfAafrtS button-like lighter area (Fig. 189, 16). Re-infection
must be guarded against. These worms often find their
way into the appendix of children where they drill into the mucous mem-
brane and cause appendicitis. Trichina (Fig. 187), (also called Trichi-
nella spiralis), lives in the small intestine when adult. The disease
trichiniasis is caused by the embryos after they pass from the intestines
to the voluntary muscles where they encapsulate themselves as larvae.

The female is 3 to 4 mm. long and the male 1.5 mm. There are two
tiny projections from the posterior end of the worm. The larvae, when
encased in the muscle, is about 1 mm. long. Trichina have a pointed
head and a somewhat rounded tail. The parasites are ingested by man
when eating inadequately cooked pork. Each worm may produce as
high as 10,000 young, which are either placed directly into the lymphatics
by the female or burrow through the intestinal wall. They then encyst
in the muscle tissue. Pigs acquire the disease by eating offal or infected
rats.

Twenty-six different kinds of animals have been found in which
trichinae grow, and as many as 15,000 of these parasites have been found
in one gram of muscle.

FLATWORMS AND THREADWORMS

303

It may take about six
weeks for complete encap-
sulation, but once encap-
sulated they may remain
alive for twenty or twenty-
five years in the muscle.

Pigs may be literally
"filled" with these parasites,
causing what is known as
"measly pork," although
they may show no external
sign of infection. Many
countries now insist on pork
inspection to prevent a
spread of infection.

The patient usually suf-
fers with a fever, anaemia,
muscle pains (myositis),
which are often mistaken
for rheumatism, and intesti-
nal disturbances (gastro-
enteritis).

Ankylostoma duodenale in
the old world, and Necator
americanus in this country
are the Hook-worms (Fig.
190). The disease caused by
hook-worm is variously
known as ankylostomiasis,
uncinariasis, hook-worm dis-
ease, tropical or Egyptian
chlorosis, and anaemia of
bricklayers and tunnel-
workers.

The old-world animal is
small and cylindrical, the
male being about 10 mm. in
length and the female from
10 to 18 mm. There are chitinous plates about the mouth and there
are two pairs of sharp, hook-shaped teeth with which the mucosa of the
intestine is pierced. On the male there is a prominent caudal, umbrella-
like expansion. The American species is slightly more slender, with
a globular mouth and a different arrangement of teeth. The eggs of
the American form are slightly larger than those of the European forms.
The larvae of the hook-worm develop in moist earth and dig their

Fig. 191. Forms of Worms Parasitic to Man.

1. Larval stage of Filaria ozzardi (F. demarquayi).

2. Larval stage of Loa loa (Microfilaria diurna) .

3. Larval stage of Filaria bancrofti (Microfilaria
nocturna ) .

4. Larval stage of Acanthocheilonema perstans (Mi-
crofilaria perstans).

5. Adult parasite female of Strongyloides stercoralis.
6 and 7. Adults, male and female, of the free-living

generation of Strongyloides stercolaris.

8. Rhabditiform larva of Strongyloides stercoralis.
just hatched from egg.

9. Filariform infective larva of Strongyloides ster-
coralis.

10. Rhabditiform larva of Ancylostoma duodenale,
just hatched from the egg.

11. Filariform infective larva of Ancylostoma duo-
denale. (From Hegner and Cort's "Diagnosis of Proto-
zoa and Worms Parasitic to Man" ; 1-4, after Fulleborn ;
5-11, after Looss.)

304

GENERAL BIOLOGY

way through the soles of the feet of persons who go barefoot. Once
in the blood-stream they are carried along by it to the heart, thence to
the lungs, and many lodge in the windpipe from whence they are swal-
lowed, thus reaching the stomach and intestines. The larval forms here
attach themselves to the intestinal walls and feed on the blood of their
host. But as they puncture the intestinal wall, they exude a small
amount of poison which prevents the host's blood from coagulating.
There is thus a constant loss of tiny droplets of blood and the patient
naturally becomes anaemic. Not only do persons infected with hook-
worm suffer from such loss of blood, but the parasites injure the lungs
in passing through them, and thus make tuberculosis infections easy.

The writer was recently told by a worker

jBMfew JB m the medical corps of the army that more

iF- ftj| than 75 per cent of the examined southern

J^H^rV negroes showed hook-worm infection.

It is of great importance to dispose of all
!*5t\ human faeces in rural districts, in mines, brick-

yards, etc., so that the soil will not become
polluted. This will kill the eggs and thus pre-

& vent hatching of the parasites. Strong sun-

light seems to be quite effective in doing this
•pH
®BP^

The family Filariidae is also important
from a pathological point of view.

Filaria bancrofti (Fig. 191) is a parasite

Fig> (TromE1"Newn%1enLman- in human blood. It is interesting to know that

these parasites live in the lungs and larger

arteries throughout the day and in the blood-vessels in the skin at night.
Mosquitoes, which are active at night, suck the blood of infected per-
sons and thus carry the infection. In fact, it was the knowledge of this
which led to the discovery of the malarial parasite's life-cycle.

As the organism is placed in another person by the mosquito, after
the larvae have developed in that mosquito's body, they enter the
lymphatics and cause serious difficulties, probably by blocking the lymph
passage. If there is such a blocking, elephantiasis results. This is a
practically incurable disease in which the limbs or other portions of the
body swell to an enormous size, although producing little or no pain.
(Fig. 192.) In certain portions of the South Sea Islands almost a third
of the population is affected.

Medical men speak of Filaria diurna and Filaria perstans. The first
of these differs from F. bancrofti in not having granules in the axis of the
body, and the second by having its embryos smaller (namely, about
200 microns) than the preceding. Only the embryos have been seen.
The embryos of F. Bancrofti are about 270 to 340 microns in length.
The adult is about 83 mm. long and the female some 155 mm. The tail

FLAT WORMS AND THREADWORMS

305

in.

Fig. 193. Other Nematode Parasites.

I. a., Dracunculus (filaria) medinensis (female) , showing mouth and embryo.
b., Transverse section through adult female of I, a, showing many embryos

in the uterus.

II. Cyclops. This animal is the intermediate host of Dracunculus.

III. Trichocephalus dispar (also called Trichuris trichiura) of the Family
Trichinellidae. a., egg ; 6., female ; c.f male attached to the intestine, showing the
long, slender, cephalic end buried in the submucosa ; sp., spicule.

IV. Gigantorhynchus gigas, of the Class Acanthocephala, and Family Echinor-
hynchidae. A., two males and one female adult attached to the mucosa of the
intestine; B., eggs as seen in preparation; C., eggs as found in feces. (I, after
Bastian and Leuckart ; II, after Riley and Johannsen ; III, after Leuckart ; IV,
after Brumpt and Perrier.)

in the male has two spiral turns. The female produces vast numbers
of young which enter the blood-stream through the lymphatics. Each
embryo is enclosed in a tiny shell about one-ninetieth of an inch in
length. They can thus pass through the capillaries quite readily. They
can be seen in a blood-drop under the microscope. As many as 2,100
embryos have been seen in 1 cc. of blood.

Dracunculus medinensis is a peculiar worm, the female being about
a yard long. It is probably taken in with food. It makes its way down-
ward, and, when arriving at the ankle, usually pushes its head through
the skin, causing an abscess. As the eggs are then deposited, it leaves

306 GENERAL BIOLOGY

the infected person of its own accord. Few of these have been found
in America.

Trichocephalus dispar (Fig. 193), or whip-worm, is found in the
caecum and large intestine of man. It is 4 to 5 cm. in length, the male
being a trifle shorter than the female. The parasite is remarkable in
that there is a great differentiation between the two ends of the body.
The anterior end, which forms about three-fifths of the body, is very
thin and hair-like, while the posterior portion is thick, and, in the female,
conical and pointed. In the male it is blunter and rolled like a spring.
The eggs are lemon-shaped, 0.05 mm. in length. Each has a button-like
projection. There may be as many as a thousand parasites in one per-
son. The parasite produces no known symptoms in the patient, although,
patients who have been infected have become anaemic and suffered with
diarrhoea.

Dicotophyme renale, the male of which is over a foot long and the
female over three feet. These are seldom met with, but when present
may destroy the entire kidney.

Anguillula aceti'or (vinegar eel) has been found in the urine of man,
although it is supposed to have been in the bottle in which the urine was
collected.

Strongyloides intestinalis is found in the small intestines of man in
the tropics. Three per cent of the medical patients of the Isthmus of
Panama were found to be infected, and 20 to 30 per cent of the insane
division.

Acanthocephalus (thorn-headed worms) are also called Giganto-
rhynchus or Echinorhynchus. These are quite common in the intestine
of the hog, where they attach themselves by means of a protrusible pro-
boscis covered with hooks. In the old world the larva develops in cock-
roach grubs, while in America the larva devlops in the June bug.

The Acanthocephalia are distinguished from the Nematodes and
the Nematomorpha by the presence of a proboscis and the absence, of an
alimentary canal.

! ; INTERMEDIATE AND UNCERTAIN FORMS

In addition to the rather definite groups of worms mentioned in this
book, there are also various forms of uncertain position.

The term Mesozoa (Fig. 194) — (Gr. mesos, middle — zoon, animal)—
is often used as a general grouping for the three following families of
parasites: (1) Dicyemidae, (2) Orthonectidae, (3) Heterocyemidae.

They are called Mesozoa because they are regarded as intermediate
forms between the protozoa and the metazoa. They are closely allied
to the flat worms.

The Nemertinae (Gr. nemertes, true), are usually placed with the
flat worms. They may reach a length of ninety feet and are mostly
marine, though a few live in fresh water and in moist earth.

FLAT WORMS AND THREADWORMS

307

Fig. 194.

A. A Mesozoon,
Dicyema Paradoxum.

( From Parker and

Haswell, after

Kolliker.)

B. A Mesozoon,
Rhopalura giardii,

male.

(From Sedgwick,
After v. Beneden.)

C D

A. Malacobdetta grossa (Ver-
rill), entire worm. 1, proboscis;
2, mouth; 3, intestine; 4,
sucker.

B. Section through forward
end. 1, mouth ; 2, proboscis ; 3,
proboscis sheath.

C. Micrura leidy, (Verrill.)

D. Cerebratulus lacteus (Ver-
rill). (From Pratt's "Manual"
by permission of A. C. McClurg
& Co.)

. 195.

E. Cerebratulus fuscus, a Ne-
mertine. 1, cephalic slits ; 2,
opening leading into retracted
proboscis ; 3. dorsal commis-
sure of nervous . system ; 4,
ventral commissure ; 5, brain ; 6,
posterior lobe of brain ; 7,
mouth ; 8, proboscis ; 9, lateral
vessel ; 10, proboscis ; 11,
pouches of alimentary canal ; 12,
stomach. (From Shipley and
MacBride, after Burger.)

Cerebratulus ( ), and Micrura ( ),

(Fig. 195), are the usual examples of marine Nemertinae. Other forms
are not common. Malacobdella ( ) is parasitic in

some mollusks.

The Nemertinae are considered the lowest form of animal life in
which the blood-vascular system appears. There is a definite mesoderm
and a nervous and an excretory system quite like those in flatworms.
but they all have a long proboscis just above the digestive tract which
lies within a sheath. This can be everted. The body is covered with
cilia. These animals feed on other animals both dead and alive. They
usually live in burrows of mud and sand, though Cerebratulus is free
swimming.

A peculiar larval stage known as the Pilidium (Fig. 198 D) re-
sembling a helmet with cilia and a long tuft at the apex, is a distinguish-
ing feature of the development of Nemertinae. Ectodermal invagina-
tions surround the alimentary tract of the Pilidium. This invaginated
portion escapes from the larval form and becomes an adult.

308

GENERAL BIOLOGY

Fig. 19(5.

III.

I. Gordius aquaticus; hinder end of male.

II. Gordius lineatus; hinder end of male.

III. Paragordius variua ; A, hinder end of female ; B, of male

IV. Nectonema agile; (From Pratt's "Manual" by permission
McClurg & Co.)

IV.

of A. C.

A. B. Two species of Rotifera. A, Philodina. B, Hydatina.
(From Parker and Haswell, after Hudson and Gosse.)

C. Diagram showing the anatomy of a Rotifer, a, anus ;
br, brain ; c1, preoral, and c2, postoral circlet of cilia ; c.gl, ce-
ment gland ; cl, cloaca ; d.ep, dermic epithelium ; d.f, dorsal
feeler ; e, eye ; fl.c, flame-cells ; int, intestine ; m, muscles ; mth,
mouth ; nph, nephridial tube ; ov, ovum ; ovd, oviduct ; ovy, ger-
marium ; ph, pharynx; st, stomach; vt, vitellarium. (From
Parker and Haswell.)

D. Pilidium larva of a Nemertine. D, alimentary canal ;
E, E', the two pairs of ectodermal invaginations. (From
Sedgwick, after Metschnikoff.)

Fig. 197.

The arrowworm, Sagitta hexap-
tera (of the group Chaetognatha ) ,
ventral view, a, mouth ; b, intes-
tine ; c, anus ; d, ventral ganglion ;
e, movable bristles on the head ; f ,
spines on the head ; g, ovary ; h,
oviduct; i, vas def erens ; j, testis ;
k, seminal vesicle. (After Hert-
wig).

The Nematomorpha (Gr. nema, thread — morphe, form), is made up
of the single family Gordiidae (Fig. 196). These are the common horse-

FLAT WORMS AND THREADWORMS

309

A.

Av,

hair snakes. Various authors classify them under the order of Nema-
toda, while others classify them under the Phylum Nemathelminthes.
There are two genera : Gordius, which lives in fresh water, and Necto-
nema, a marine form. The internal anatomy is somewhat different from
the Nematodes, as there is a distinct epithelium lining the body cavity
and no lateral lines. There is also a pharyngeal nerve-ring, and a single

ventral nerve-cord, while the ovaries
discharge the eggs into the body-
cavity. Then, too, the larvae of
Gordius usually enter immature
stages of aquatic insects. These in-
sect larval-forms are then devoured
by other animals, and it is in the
intestines of the host where they de-
velop until they finally escape into
the water.

The Acanthocephala ( G r.
akantha, spine — kephale, head) are
the parasitic worms already men-
tioned above (Fig. 193), which may
infect man. They fasten themselves
to the intestinal wall of their host
by means of a protrusible proboscis
covered with hooks. In fact, it is
the presence of the proboscis and a
reproductive system as well as the

absence of an alimentary system which distinguishes the Acantho-
cephala from the Nematoda and the Nematomorpha. There is an alter-
nation of hosts during the developmental stages.

The Chaetognatha (Gr. chaite, horse-hair-gnathos, jaw) are marine
forms swimming about near the surface of the water. The arrow-worm
(Fig. 197) is the classic example. This is a member of the genus Sagitta.
The Chaetognatha are quite often included under the Phylum Nemathel-
minthes.

The Rotifera or Rotatoria (Fig. 198), (Lat. rota, wheel-fero, I
carry), are usually called the wheel-animalcules. They are very small
and were formerly thought to belong to the Infusoria. Most of them
live in fresh water. A few are parasitic. The sexes are separate. There
are summer and winter eggs produced by the female. The former are
thin-shelled and develop without fertilization (parthenogenetically).
The larger eggs produce only females and the small males. The winter
eggs are fertilized, have thick shells, and all develop into females. The
eggs of most mollusks pass through a larval stage known as a trocho-
phore ( ). which looks quite like the helmet-shaped

larva .described above. Now, Rotifers often resemble these trochophores.

Fig. 199.

Bugula avicularia, a. Bryozoon.
avicularia ; D, alimentary canal ; F, funiculus ;
Oes, oesophagus ; Ovz, ovicells ; R, retractor
muscle; Te, tentacular crown. (From Sedg-
vdck, after V. Nordmann.)

B. Phorords architecta. Young individ-
ual with about 30 tentacles. 1, epistome ; 2,
lophophore ; 3, digestive tract. (From Pratt's
•'Manual" by permission of A. C. McClurg
& Co.)

310 GENERAL BIOLOGY

Consequently, it is thought by some zoologists that they must be closely
related to the mollusks. Rotifers have a peculiar ability to secrete a
gelatinous envelope about themselves in times of drought, which pro-
tects them for great lengths of time and thus prevents them from per-
ishing.

The Bryozoa (Gr. Bryon, moss — zoon, animal) are moss-animals
(Fig. 199), which practically all live a colonial life. They look some-
thing like the hydroid form of Obelia, but their general structure is quite
unlike Obelia. Most of them are marine animals, though there are a few
types which inhabit fresh water. The polypide is the name given to the
soft parts which lie within a coelomic cavity and which is surrounded
by the zooecium (body-wall).

The lophophore ( ) is the crown of ciliated

tentacles surrounding the mouth. The alimentary tract, retractor mus-
cle, and the funiculus (a strand of mesodermal-tissue attached to the
stomach), are shown in Figure 199. There are no circulatory or excre-
tory organs. The eggs develop in the ooecium, which is a modified por-
tion of the body-wall.

Bugula is the usual laboratory example. Certain members of a col-
ony develop jaws for protective purposes. Such jaw-possessing mem-
bers are called aviculariae.

Bryozoa are divided into Ectoprocta in which the anus opens out-
side the lophophore, and a coelom is present as in Bugula; and Ento-
procta, in which the anal opening lies within the lophophore, while the
portion which should be a coelom is filled with mesodermal cells. Exam-
ples of this type are Pedicellina and Urnatella.

The Phoronidea, named after an ancient king, Phoronis, is made up
of the single genus Phoronis. The animals belonging to this group are
worm-like and are enclosed in membranous tubes. They live in sand
and are supposed to be related to the Ectoprocta.

The Brachiopoda (Gr. brachion, arm — pous, foot) are shelled marine
animals (Fig. 200), but with the shell on the dorsal and ventral portions
of the animal, instead of on the sides as with bi-valves. They are usually
attached to some object by a peduncle. An excellent example is Lingula,
a very old type, having been found in some of the oldest geological
strata, and which differ but little today from their oldest fossil-remains.
1 The Brachiopoda are not worm-like in any way, but they have an
uncertain position in classification, and so are included here.

The Gephyrea (Fig. 201), (Gr. gephyra, mound), often classified
under the annelids, are now believed by zoologists to be unrelated, but
there is even doubt that the various sub-grouping of Gephyrea them-
selves bear any very close relationship.

Three groups are usually noted:

(1) The Echiuroidea, in which the adult shows traces of segmenta-
tion, a proboscis, and a pair of ventral-hooked-setae and terminal anus.

FLATWORMS AND THREADWORMS

311

There is a larval trochophore stage. They ordinarily live in crevices
of rocks.

(2) The Sipunculoidea, which are unsegmented. They possess one
pair of nephridia, a large coelom and anal opening on the dorsal surface,
near the head-end. They usually possess tentacles at the anterior end.
They live in sand or bore their way into coral rock.

(3) The Priapuloidea are also unsegmented, having an anterior-
mouth surrounded by chitinous teeth and the anal opening in the pos-

Fig. 200.

Magellania flavescens (of the group
Brachiopoda). A, dorsal aspect of shell. B,
shell as seen from the left side, b, beak ; d.v.,
dorsal valve ; /, foramen ; v.v., ventral valve.
(From Weysse, after Davidson.)

Anatomy of a Brachiopod, Waldheimia
austrdlis. 1, mouth ; 2, lophophore ; 3, stom-
ach ; 4, liver tubes ; 5, median ridge on shell ;
6, heart ; 7, intestine ; 8, muscle from dorsal
valve of shell to stalk ; 9, opening of nephrid-
ium ; 10, stalk ; 11, body-wall ; 12, tentacles ;
13, coil of lip ; 14, terminal tentacles. (From
Shipley and MacBride.)

Fig. 201.

A. Echiurus pattasii (of the group
Gephyrea). a, mouth at the end of the grooved
proboscis; b, ventral hooks; c, anus. (From
the Cambridge Natural History.)

B. Sipunculus nudus (of the group
Gephyrea) laid open from the side. A, anus;
BD, brown tubes (nephridia); D, intestine;
G, brain ; Te, tentacles ; VG, ventral nerve-
cord. (From Sedgwick, after Keferstein.)

C. Priapulus candatus (of the group
Gephyrea ) . a, mouth surrounded by spines.
(From the Cambridge Natural History.)

terior region. They live in mud and sand. The head-end usually pro-
jects above the surface of the mud in which they lie.

References :

Ward and Whipple, "Fresh Water Biology."
Hegner's "College Zoology."

Pfatt's "Manual of the Common Invertebrate Animals."
Braun & Liihe, "A Handbook of Practical Parasitology."
W. H. MacCallunn, "A Text-book of Pathology."
Damaso Rivas, "Human Parasitology."

Kolle & Wassermann, "Hand-buch der Pathologenen Mikroorganis-
men."

Hegner & Coit, "Diagnosis of Protozoa and Worms Parasitic in
Man."

CHAPTER XXI.

THE ARTHROPODA
THE CRAYFISH

As an example of a gill-breathing arthropod, the crayfish has become
the classic laboratory type, and this because, like the frog, it is already
known to the student to some extent.

The phylum to which man and the frog belong — the Vertebrate —
is in point of numbers much smaller than the phylum Arthropoda, to
which the crayfish belongs — a group embracing more than three- fourths
of all living animals.

The Arthropoda are usually divided into branchiata* ( )

— commonly called Crustacea ( ) — those animals

possessing a hard chitinous ( ) exoskeleton and

breathing with gills, practically all of which live in water ; and tracheata
( ), consisting of those animals breathing through

little tubules called tracheae. The tracheata include grasshoppers, bees,
wasps, ants, spiders, and insects of all kinds. While 400,000 of the
600,000 known species of animals belong to the Arthropoda, the greatest
sub-group of these is, in turn, the insects.

The crayfish is large enough to be studied profitably in the labora-
tory. All who have lived or spent any of their youth near ponds and
rivers, know at least one or two species of crayfish. These they have
found lying quietly under the stones in running streams, and when such
stones were lifted, the animal's pincers were threateningly brought for-
ward to clasp the fingers of the supposed attacker. Then followed a
darting backward until the animal again pushed itself under some shel-
tering object or was able to find some close corner in which its body
could be pressed.

The exterior skeleton so prominent in the Arthropoda, is in thor-
ough contrast to that of the frog, whose supporting tissues are placed
on the innermost portion of its body ; yet it is not from this character-
istic that the phylum is named, but from the fact that the animals be-
longing to this group have jointed legs. The word arthropoda means
jointed feet.

The crayfish will be used in this book more as a type to introduce
nomenclature and general arrangement of the phylum Arthropoda than
as a study of detail.

The entry into a more minute investigation of the phylum will come
with a study of the grasshopper. The larger and more convenient size

*This classification into Branchiata and Tracheata lacks scientific foundation, but is convenient
for the beginner and for the student of medicine. As an example of why this classification is not
scientific, we may mention the true spiders which have no tracheae and yet are called Tracheates.

THE ARTHROPOD A 313

of the crayfish serves to show in gross much that is otherwise difficult
to observe in the insects and lends itself well to an illustration of serial
Homology and the so-called Savigny's law. The crayfish has not been
well studied, then, unless, after completing this chapter, these things are
definitely known.

EXTERNAL APPEARANCE

The crayfish is found nearly everywhere in this country and Europe ;
in the eastern part of the United States Cambarus affinis ( )

is prevalent, while Cambarus virilis ( ) is more plentiful

in the Middle States, and the European specimen found most frequently
is the Astacus fluviatilis ( ). There is little differ-

ence, however, in their external or internal makeup. It will be remem-

brc

Fig. 202. The common Crayfish, Astacus fluviatilis, seen from
the side.

abd. Abdomen. ami*. 1. First walking leg. amb. 4.
Fourth walking leg. an'. First antenna or antennule. an".
Second antenna, be. Branchiostegite. br.c. Branchiocardiac
groove, e. Carapace, ch. Chela, cv.g. Cervical groove. e.s.
Eye-stalks. g.g. Opening of green gland, mxp. 3. Third
maxillipede. rs. Rostrum, sw. Swimmerets. t. Telson. 15.
First segment of abdomen. 20. Last segment of abdomen, xx.
The last appendage. (After Shipley & MacBride.)

bered that the segmentation of the frog is found in the spinal column.
With the crayfish, however, segmentation can be observed externally
Tunning from anterior to posterior end, though there is a peculiar con-
dition of fusing of a number of the anterior segments (Fig. 202) which
thus form what is known as the cephalothorax ( ).

As one may observe in the embryological study of the crayfish that
each embryonic segment possesses a pair of appendages, it is but neces-
sary to count the appendages in an adult arthropod in order to find how
many segments have fused in any given region. This is known as
Savigny's Law.

Beginning at the anterior end of the animal we find the first seg-
ment having two pairs of long feelers, the longer ones being the antennae
?.nd the shorter the antennules.

Directly behind these there is a series of modified appendages (Figs.

202, 203) directly in front of the large mandibles. These cover the mouth

itself. Two pair of tiny appendages — the maxillae — lie anterior to the

mandible, whik three pairs of appendages — the maxillipeds — lie pos-

314

GENERAL BIOLOGY

terior to it, so modified as to form jaws. The two pair of maxillae and
the three pair of maxillipeds, together with the mandible, thus make six
pairs of jaws altogether.

Back of these six jaws, a pair of pincers is attached to the thorax
proper. These are known as chelipeds ( ), and be-

hind the chelipeds are four pair of walking legs. By observing these
legs it will be noticed that they are very much akin to the cheale proper
in that each has a broad attachment, the protopodite ( ),

Fig. 203.

A. Mandible. B. First maxilla. C. Second maxilla.
bs. Basipodite. ex. Coxopodite. en. Endopodite. ep. Epipodite.
ex. Exopodite. sc. Scaphognathite.

D. and E. First and second Maxillipedes. br. Branchial
filaments, cp. Carpopodite. dp. Dactylopodite. is. Ischiopodite.
me. Meropodite. prp. Propodite portions of endopodite.

F. Third Maxillipede. cs. Coxopodite setae.

G. Gill (=;epipodite.) (After Latter.)

where it meets the body, composed of two portions, a coxopodite
( ) and a basipodite ( ) which

then join the pincer proper. These pincers consist of a solid immovable
portion, the exopodite ( ) and a smaller movable and

inner portion, the endopodite ( ).

It will be observed that the pincers are only an enlarged walking
leg.

The portion of the crayfish directly behind the cephalothorax, with
the definite segmentation, is known as the abdomen and consists of six
segments, beside the tail. This latter consists of a central portion of the
tail called the telson ( ), and two pairs of leaf-like

structures on each side called uropods, which assist in forming a broad
wing-like tail and which, when the crayfish is frightened, can be bent

THE ARTHROPOD A

315

rapidly forward, thus sending the animal's body backward from the posi-
tion it occupied.

A typical segment of the abdomen (Fig. 204) consists of the upper
portion called the tergum ( •), a ventral portion,

the .sternum, two pleura (the extended portions continuing ventrally be-
hind the sternum), and two epimera, these latter forming the roof which
extends from the pleura to the appendage.

A. Diagram of skeleton of an abdominal segment of Astacus.
bs. Basipodite. ex. Coxopodite of swimmeret. ep. Epimeron. jt.
Point of articulation with skeleton of adjacent segment. pi.
Pleuron. st. Sternum, tg. Tergum. (After Latter.)

B. Section through cephalothorax of a crab. (After Pearson.)
H., Heart ; Te., extension of the tergum ; ST., sternum ; PL.,
pleuron ; T., tendons ; 1st W. L., insertion of first walking leg ; Br.,
gill in gill-chamber ; g., gut ; d.a., descending artery ; A., afferent
branchial ; E., efferent branchial.

There are thirteen segments in the cephalothorax. The eyes are
not counted as appendages. A cervical groove forms the separating line
between head and thorax. The entire dorsal shield of the cephalothorax
is called the carapace (. ), the jointed end extending

between the eyes being known as the rostrum, while the portion on the
sides covering the gills are the branchiostegites ( ).

The entire crayfish possesses twenty segments, counting telson and uro-
pods as one.

Each pair of the appendages is slightly different in appearance from
any other pair, though there is much similarity between them. The
three distinguishing types of crayfish appendages are known as (1)
foliaceous ( ), (second maxilla); (2) biramous

( ), (swimmerettes) ; (3) uniramous ( ),

(walking legs).

The female has an. opening at the base of the third walking leg
through which eggs are exuded. She also possesses a single opening
in the midline through which sperm may be inserted. Immediately be-

316 GENERAL BIOLOGY

hind the left walking leg, on the first abdominal segment, a peculiar
atrophied ( ) pair of appendages are found.

In the male, however, these appendages on the first and second ab-
dominal segment are wide, and the left walking leg possesses a small
opening through which the sperm are ejected. In the male the first pair
of swimmerettes are also transformed into "copulating organs." The
anal opening is found on the ventral surface in the midline of the telson.

SERIAL HOMOLOGIES AND ADAPTATIONS

When two parts of an organism develop alike as to structure, for
example the femur in the thigh and the humerus in the upper arm, we
call such bones or parts homologues ( ).

And when two parts function similarly, regardless of whether they
are alike structurally, we call such organs or parts analogues ( ).

While if any organ or part of an organ changes, due to a change of en-
vironment so as to better or benefit an organism, we call such change an
adaptation. In the crayfish there is what is called a serial homology.

This type of "homology" is characteristic of the group of the higher
Crustacea known as the sub-class Malacostraca ( ),

and this group well illustrates how a single plan of structure may run
through a series of forms of the utmost diversity in appearance, and how
parts essentially alike may be adapted to the most diverse ends.

"The Malacostracan body, be it an amphipod ( ),

an isopod ( ), a decapod ( ), or

-what not — is composed of a series of twenty* segments, each of which
is essentially of the skeletal plan shown in the diagram, except that the
appendages of the foremost segment are typically unbranched and the
hindmost segment (the telson) is rudimentary and bears no appendages
at all. Some of these segments may become fused together and con-
solidated on the dorsal side, only the appendages and ventral margins
remaining free. This may occur at either end of the body, but it occurs
constantly in the five front segments, these by fusion forming the head.
The appendages of these five segments always consist of two pairs of
antennae at the front, one pair of mandibles beside the mouth, and two
pairs of maxillae following the mandibles." These parts and their func-
tions will readily be understood a little later because of their likeness
to the parts bearing the same names in the insects shortly to be studied.
•"Immediately following the maxillae are one or more pairs of maxilli-
peds, likewise directed forward beneath the mouth to assist in the manip-
ulation of the food. Then follow legs and swimmerettes in more or less
variety, the terminal joints of some of the legs being modified in many
cases into highly specialized grasping organs called pincers, or chelipeds,
and the swimmerettes being frequently modified to serve reproductive or

*This is not counting a vestigial segment in the head region, that is discoverable only during
.embryonic life.

THE ARTHROPODA

317

A to D. Diagram of model gastric mill which can easily be made. After W.
E. Roth, A, Cardboard as first cut out ; B, Model complete at rest ; C, Model
complete ; muscles contracted ; D, Median vertical section of model to show folds.

Instructions :

Cut out a piece of card shaped as in Fig. A. Along ab, cd, ef, hi, and tnn
cut just the surface of the card with a penknife ; do the same, but on the opposite
face of the card, along gk and lo. Then bend slightly downwards the triangular
pieces 2, 2 ; turn 9, 9 under the piece 6, 5, 6 until the lower surfaces of 9, 9 are flat
against that of 6, 5, 6: stitch the shaded part of 9, 9 firmly by thread or fine
wire to 6, 5, 6 ; then bend the unshaded part of 9, 9 till at right angles to the
shaded part, using lo as hingeline. These projecting pieces of 9, 9 then represent
the lateral teeth.

Next bend the piece 1, 8, 4 upon hinge-line gk, until the shaded portion is
flat upon the surface of 4, where it must be securely stitched ; this done bend back
1, 3 on hinge-line cf until 3 is at right angles to 4. The projecting end of 4 made
prominent by these folds represents the central tooth. The piece 1 must now
be bent gently downwards upon 3, using cd as hinge-line, and 4 must be bent
sharply on 5, using mn as hinge-line. Lastly, perforate the corner of 6, 6 and
of 2, 2, and by a single wire (to allow a certain amount of rotation) unite right
hand 2 to right hand 6, and left hand 2 to left hand 6, in each case 2 being
outside 6. To do this 6, 5, 6 must be bent like a bow, its right and left arm*
being thrust downwards and inwards. The model will then be as in Fig. B.

If now the pieces 8, 8 and 7, 7, which represent the anterior and posterior
gastric muscles, are pulled so as to represent the effect of a muscular contraction
the three teeth come sharply together, but are separated again and the whole
model brought back to its original condition by the elasticity of the cardboard.
Of course in the actual stomach of the crayfish the gaps between the ossicles are
filled in with thin, flexible chitin. By carefully adjusting the size and direction
of the 3 teeth in the model and further by hardening them with sealing-wax or
similar material, they may be made to grind bread, etc., into small fragments.
A sectional view is shown in Fig. D.

E. Stomach or "gastric mill" of the crayfish cut through the middle, e.
cardiac regions of stomach ; d.L, duct from the liver ; g, gastrolith, or calcareous
disk secreted by the walls of the stomach ; i, intestine ; l.t., lateral teeth of grind-
ing apparatus ; m.t., median tooth ; oe, oesophagus ; py, pyloric region ; v, valve be-
tween cardiac and pyloric regions of stomach. (After Hatschek and Cori.)

318 GENERAL BIOLOGY

respiratory functions. The eight segments following the head consti-
tute the thorax and the seven last segments (counting the rudimentary
twentieth segment), the abdomen.

"Crustaceans being primitively free-swimming aquatic animals, it is
their swimming appendages that are least altered by adaptations. The
legs are the stoutest of the appendages, and these offer but one branch
arising from the basal piece, and that composed of a reduced number of
highly differentiated segments. A comparison of a leg with the last
maxilliped in the crayfish will show which appendage has been lost and
which preserved and specialized. The best clues to interpretation of
homologies in any appendage are likely to be found in other adjacent
appendages, which, because of proximity, have been subject to somewhat
similar influences."

THE DIGESTIVE SYSTEM

Crayfish live chiefly on living snails, tadpoles, young insects, and
the like, but sometimes eat one another, and may also devour decaying
organic matter. They feed at night, being most active at dusk and day-
break. The maxillipeds and maxillae hold the food while it is being
crushed into small pieces by the mandibles. The food particles pass
down the oesophagus into the anterior, cardiac chamber of the stomach,
where they are ground up by a number of chitinous ossicles forming the
gastric mill (Fig. 205). When fine enough, the food passes through a
sieve-like strainer of hair-like setae into the pyloric chamber of the stom-
ach ; here it is mixed with a secretion from the digestive glands brought
in by the hepatic ducts. The dissolved food is absorbed by the walls of
the intestine. Undigested particles pass on into the posterior end of the
intestine, where they are gathered together into faeces, and egested
through the anus.

THE CIRCULATORY SYSTEM

As in the frog, the liquid nourishing fluid, the blood, is pumped by
the heart (Figs. 206, 207) through the arterial system to the different
parts of the body. The blood of crayfish is generally colorless, or pinkish
in hue, but on standing, especially if exposed to air, it assumes a bluish
color. This is due to Haemocyanin, a respiratory protein, which has cop-
per in its nucleus.

Before moulting, the blood of the crayfish is pink in color, due to a
dissolved pigment, Tetronerythrin, a lipochrome, which is probably de-
posited in the new chitinous covering, since it is present in less quantity
in the blood after the complete formation of the new exoskeleton.

The blood of the crayfish transports food, gases, and wastes, similar
to the frog.

The crayfish does not possess a true venous system and the heart
has only a single large cavity. The open spaces in the animal's body
through which the blood is returned to the heart are called sinuses.

.
THE ARTHROPODA 31,.

The heart itself, lying close to the dorsal surface of the midline,
constricts when filled with blood. This constriction sends blood pos-
teriorly through the dorsal abdominal artery, which lies on the dorsal
surface of the intestinal tract, and through a short branch known as
the sternal artery, which passes downward crossing the intestinal tract.
The blood is also thus sent to the ventral thoracic artery anteriorly, and
posteriorly to the ventral part of the body through the abdominal artery.

The arteries passing out of the
anterior portion of the heart are the
ophthalmic, supplying the stomach.
oesophagus, and head, and the two
antennary, carrying blood to the
stomach, antennae, excretory or-
gans, and the various other tissues
of the head. The two hepatic
arteries lead to the digestive glands.
When the blood is forced
Fig- 20,i through the arterial system, the

Astacus fluviatilis. The heart A, From '

above; B, from below; C, from the left side: heart naturally COllapSCS, and the

a.a., Antennary artery ; a.c., alae cordis, or , , t , . , , . ..

fibrous bands connecting the heart with the blood which has been Sent OUt forCCS

the blood which is then present in
the arteries to be sent forward

abdominal artery ; st.a., sternal artery in B through the glands. ThcSC glands
cut off close to its origin. (From Dougherty ° °

after Huxley.) act similarly to the lungs in the

higher forms of animals, aerating the blood and sending it to the large
open place around the heart known as the pericardial sinus. The heart
itself has two openings on both dorsal and ventral surfaces, and one on
each side. The heart muscles, after constriction, again assume their nor-
mal state when the blood in the pericardial sinus seeps through the six
heart openings, filling the cavity. Each of the openings possesses a valve
which prevents the blood from passing out, except through the arterial
channels.

It is interesting to note that this method is just the reverse of that
occurring in the fishes where the blood passes through the heart first
and thence to the gills, while in the crayfish it is the returned blood that
passes .through the gills before reaching the heart.

Unless colored matter of some kind is injected into the circulatory
system the student will probably have some difficulty in finding either
heart or arteries.

Valves are present in all the arteries at the point of connection with
the heart, and blood passes into numerous capillaries and thence into the
open spaces between the tissues, until it reaches the external sinuses,
from which it enters the gill channels, to pass into the gill filaments
where oxygen from the water in the branchial chambers is- exchanged for
the carbonic acid that is held in solution in the blood. From here it

32-0

GENERAL BIOLOGY

passes by way of other gill channels into the 'branchio-cardiac sinuses;
thence to the pericardial sinus into the heart.

THE RESPIRATORY SYSTEM

The crayfish, living in, and breathing through water, has branchial
chambers which contain gills (Fig. 204, B) instead of lungs to form its
respiratory system. These gills are pyramidal in shape and are thrown
out into many flaps or lamellae closely packed together. Each gill has a
ventral and a dorsal vessel through which the blood from the body cavity
passes into the gills, spreading out through tiny capillaries into the
lamellae ( ), being continuous with similar capillaries

emptying into the dorsal vessel.

Fig. 207.

Semi-diagrammatic view of internal organs, and some limbs of right side of
a male Crayfish. Astacus ftuviatilis. 1. Antennule. 2. Antenna. 3. Mandible. 4.
Mouth. 5. Scale or squama of antenna, exopodite. 6. Arius. 7. Telson. 8. Opening
of vas deferens. 9. Chela. 10. 1st walking leg. 11. 2nd walking leg. 12. 3rd
walking leg. 13. 4th walking leg. 14. 1st abdominal leg, modified. 15. 2nd
abdominal leg, slightly modified. 16. 3rd abdominal leg. 17. 4th abdominal leg. 18.
5th abdominal leg. 19. 6th abdominal leg, forming with telson the swimming
paddle. 20. (Esophagus. 21. Stomach. 22. Mesenteron, mid-gut. 23. Cervical
groove. 24. Intestine. 25. Cerebral ganglion. 26. Para-oasophageal cords. 27,
Ventral nerve-cord. 28. Eye. 29. Heart. 30. Sternal artery. 31. Dorsal abdom-
inal artery. 32. Ventral abdominal artery. 33. Ventral thoracic artery. 34.
Ophthalmic artery. 35. Antennary artery. 36. Hepatic artery. 37. Testis. 38.
Vas deferens. 39. Internal skeleton. 40. Green gland. 41. Bladder. 42. External
opening of green gland. (From Latter after Howes.)

The venous blood in all parts of the body other than the gills, passes
through what is called an open sinus system, whereas in the gills them-
selves the anastomosing arch of the arterial and venous capillaries forms
a closed system.

The thin-walled flaps of the gills are in contact with the water, which
is sent through the branchial chamber by the muscles of the scaphogna-
thite ( ), a sort of scoop consisting of the fused

bract and exopodite of the second maxillae. This scoop bales the water
out of the forward end of the gill chambers. The swimmerettes, being

THE ARTHROPODA 321

in constant motion, send water forward to the gill chambers. The blood
thus comes in contact with fresh water, is aerated, and gives off its car-
bon dioxide. Some of the gills are on the appendages themselves, these
being the podobranches ( ), while those on the basal

part of the appendix are called arthrobranches ( ),

on account of being on the joint itself, while those which originate on
the body-wall are the pleurobranch.es ( ).

THE EXCRETORY SYSTEM

Contrasting interestingly with many of the other animals studied
in the laboratory, the excretory organs of the crayfish are in the head
region. They consist of two rather large green glands (Fig. 207), just
in front of the oesophagus, with a thin-walled dilated portion called the
bladder, and a duct opening to the exterior through a pore at the top of
a little elevation on the basal segment of the antenna.

THE NERVOUS SYSTEM

The nervous system (Fig. 208, B) is very much like that of the
earthworm. The central nervous system is made up of a ventral chain
of nerve ganglia, though it lies dorsal to the central blood vessel. The
ventral chain possesses a ganglion for practically every segment, from
its posterior end forward. The seventh is called the sub-oesophageal
ganglion.

The brain sends nerves to the eyes, antennules, and antennae. The
sub-oesophageal ganglion, lying in segment seven, is made up of the
ganglia from segments three to seven fused together. These send nerves
to the mandibles, maxillae, and first and second maxillipeds. Visceral
nerves are also supplied from the brain, extending posteriorly to the
viscera.

THE SPECIAL SENSE ORGANS

Each eye .(Fig. 208, A) is made up of some 2,500 little square facets.
The long rod extending immediately behind each facet is called an om-
matidium. It is supposed that the crayfish can thus see moving objects
much better than it could did it have an eye similar to higher forms.
But there being so many facets, it is assumed that the animal obtains
what is called a mosaic image, an image made up of a great many sepa-
rate and distinct views. However, as Latter says, "We must not con-
fuse this image that we think the animal obtains with the impression
that is given it, for the human eye sees an inverted image but the im-
pression is just the opposite."

Although each ommatidium has a small range of vision and forms
a stiple or mosaic image, it has been calculated that the range of adjoin-
ing ommatidia overlaps so that a continuous picture or image is formed.

322

GENERAL BIOLOGY

Fig. 208. Ommatidium and Central Nervous
System.

A. An ommatidium or eye-element from
the eye of the Lobster (after G. H. Parker).
c, cornea (cuticle) ; c.h., corneal hypodermis,
which secretes the cuticle ; co., cone cells ; cr.,
crystalline cone ; n, nuclei ; «./., nerve fibres ;
r.d., distal or outer retinula cells ; r.p., prox-
imal or inner retinula cells ; rh., rhabdome.

B

B. A semi-diagrammatic view of central
nervous system of a crayfish, ob.l, ab.6, The
first and sixth abdominal ganglia ; cer., cere-
bral ganglion ; c.ces., circumoesophageal com-
missure ; I.e., longitudinal commissures of
ventral cord ; n.ab.L, nerves to abdominal
limbs; n.at.l, nerve to antennule ; n.at.2,
nerve to antenna ; n.ch., nerve to cheliped ;
n.m., nerves to limbs adjoining the mouth ;
o.n., optic nerve; s.ces., suboesophageal
ganglion; st.a., sternal artery; th.l, th.6, first
and sixth thoracic ganglia ; v.n., nerve to
proventriculus ; v.n'., nerve to hind-gut.
(After Borradaile.)

Thus, three adjoining facets might view the word "Biology" in this way:

Bio olo ogT-

That is, facet one, would see the first three letters, facet two the
middle three, and facet three the last three. But since the range of each
facet overlaps that of the adjoining, the image formed is actually this :

Bio ogy

olo

In other words, instead of an apposition image or mosaic, a super-
position image or continuous picture is formed.*

*Microphotographic
correct one.

tudies have definitely demonstrated that the account here given is the

THE ARTHROPODA

It is doubtful whether the crayfish can hear. Some of the older texts
speak of an otocyst ( ), but the newer ones have

discarded this name entirely, for that organ, \vhich was supposed to be
used for hearing, has come to be considered a balancing organ by which
the animal knows whether or not it is right side up and which, thereby,
makes it possible for the crayfish to adjust its position and direction.

These little chitinous lined sacs on the basal segment of each anten-
nule are now called statocysts ( ).. There are a

number of sensory hairs in this sac and a few grains of sand called stato-
liths. These latter are placed there by the crayfish itself. These little
sand grains coming in contact with the sensory hairs make it possible
for the animal to determine its direction and position while swimming.
The statocysts are therefore called organs of equilibrium. The statocyst
is shed whenever the animal molts.

We do not know whether the crayfish has a definite sense of smell
or not. When meat juices or tiny particles of meat are so placed in
the water that a slight current carrying some of the meat comes close
to the animal's feelers, it begins working its jaws. This may be either
a. sense of touch, or taste or smell.

MUSCULAR SYSTEM

As the crayfish possesses an exoskeleton all of the muscles are at-
tached to the interior of its casing, the strongest ones being in the abdo-
men by which that part of the body can be bent quickly and easily,
producing a powerful stroke in the water and shooting the body back-
ward rapidly. All of the appendages likewise are supplied with muscles.
The muscles are very beautifully arranged, quite complicated and rather
difficult to work out by the student.

REPRODUCTION

Crayfish are dioecious, that is, the two sexes are separate (Fig. 209).
The male (cambarus) possesses tri-lobed testis (an anterior pair and a
single posterior lobe) in which the spermatozoa arise which pass through
the vasa deferentia ( ) out of the paired genital

openings, in the base of the first abdominal appendage.

In the female there is a bi-lobed ovary in which the eggs are found.
These, upon ripening, pass through the parent oviducts out of the genital
openings, one of which is located in each base of the third walking leg.
The sperm are transferred from the male to the seminal receptacle of
the female during copulation, which takes place most frequently in the
autumn. The seminal receptacle itself is a cavity in the fold of the
cuticle between the fourth and the fifth pairs of walking legs.

The eggs are usually laid in April and probably fertilized at that

324 GENERAL BIOLOGY

time. The female exudes a sticky substance upon the swimmerettes
after lying upon her side for several days and cleaning and polishing
them very thoroughly. When the eggs are laid they adhere to the swim-
merettes which are moved back and forward through the water, thus
aerating them. It takes from five to eight weeks for the eggs to hatch,

Fig. 209.

A. Male reproductive organs of crayfish. — After Huxley, t., Testes ; vd., vas
deferens on last walking leg.

B. Female reproductive organs of crayfish. — After Suckow. ov., Ovaries ;
ov' ., fused posterior part; od., oviduct; vu., female aperture on the second walking
leg.

C. Spermatozoa of a crayfish. C. Whole spermatozoon from above ; D, part,
enlarged, from the side, cps., Capsule; pr., stiff processes. (After Borradaile. )

the larvae clinging to the egg shell. In about two days the first molting
or ecdysis takes place ; for any animal possessing an exoskeleton finds it
impossible to grow without splitting its exterior covering and getting
a new one to take its place.

The young stay with the mother about a month, then shift for them-
selves. Crayfish attain an age of approximately three or four years.
They molt at least seven times during the first summer.

REGENERATION

We have seen how the earthworm, if it is divided in a region pos-
terior to the vital organs, will grow a new tail for the forepart, as well
as a new tail-like portion on the tail itself. In the latter case, the animal
starves to death, because there is no way of eating.

With the flat\vorm planaria, all manner of fantastic forms may be
grown by cutting off, or splitting, or grafting. The crayfish, too, pos-
sesses the power of regeneration to some extent, though nowhere nearly
as much as the worms. If a leg, eye, or pincer is destroyed (Fig. 210),
the animal grows a new appendage, though in place of an eye, it may,
and often does, grow an appendage quite similar to one of the walking
feet, or even a pincer, depending on how much of the original appendage
was destroyed.

AUTOTOMY

An interesting condition of the crayfish, as well as of some of the
other crustaceans, is the breaking off, by the animal itself, of one or more

THE ARTHROPOD A 325

of its legs when caught in a position where it seems incapable of extri-
cating itself.

At certain parts of the legs, there is a thick diaphragm with a tiny
hole through which blood passes, and it is here that the animal breaks
off its own leg, tire tiny drop of blood there exposed coagulating almost

immediately and thus preventing its
bleeding to death.

With an open blood system,
such as the crayfish has, bleeding to
death would be an easy matter were
this special arrangement not made

Diagram showing antenna-like organ re- in the animal. A new leg, as large
generated in place of an eye of Palcemon. ., < -11 j i c

(From Morgan, after Herbst.) aS the One lost, Will develop from

the stump thus remaining.

Reed says, "Autotomy is not due to a weakness at the breaking
point, but to a reflex action, and that it may be brought about by a stim-
ulation of the thoracic ganglion as well as by a stimulation of the nerve
of the leg itself."

It wrill be seen quite readily that this power of autotomy is of con-
siderable advantage to an animal.

PARASITIC CRUSTACEA

Sacculina ( ). (Fig. 212.)

The young are active free-swimming larvae "much like a young
prawn ( )" or young crab. But the adult bears abso-

lutely no resemblance to such a typical crustacean as a crayfish or crab.
The Sacculina, after a short period of independent existence, penetrates
to the abdomen of a crab, and completes its development while living
as a parasite on the crab. In its adult condition it is simply a great
tumor-like sac, bearing many delicate root-like suckers which penetrate
the body of the crab host and absorb nutriment. The Sacculina has no
eyes, no mouth parts, no legs, or other appendages, and hardly any of
the usual organs except reproductive organs. Degeneration here is car-
ried very far.

"Other parasitic Crustacea, as the numerous kinds of fish lice which
live attached to the gills or other parts of fish, and derive all their nutri-
ment from the body of the fish, show various degrees of degeneration.
With some of these fish lice the female, which looks like a puffed-out
worm, is attached to the fish or other aquatic animal, while the male,
which is perhaps only a tenth of the size of the female, is permanently
attached to the female, living parasitically on her."

PLANKTON

One may, with a fine-meshed net, sweep in a considerable collection
of organisms from the surface of ponds, lakes, rivers, or ocean. There

326

GENERAL BIOLOGY

will be thousands of minute creatures of varying shape and size. Some
of them are too small to be seen with the naked eye, while others are
easily noticed. Collections of this kind may be made from any waters
at any time of the year, from thousands of miles out at sea, and over
depths of thousands of feet, to the shore line itself. The reason organ-
isms can be found everywhere in water is due to the fact that their whole
life is spent afloat, beginning with the egg and reaching through the
adult stage. Living organisms of this type have been called plankton,
and comprise Protozoa, Algae, Diatoms, Rotifiera, and small Crustacea,
the latter being especially noticeable.

To permit a life afloat, organisms are provided with various types
of adaptations, such as minute droplets of oil, long spines to add
buoyancy, and gelatinous envelopes. Among the small Crustacea, spines
and oil drops are. especially abundant. Upon analysis it has been shown
that the oil of fish is derived from these small Crustacea. The reason
for this is easily understood when it is known that the sole food of sev-
eral species of whale and of many fish is plankton.

TERRESTRIAL CRUSTACEANS

The Class Malacostraca ( ) are Arthropoda,

usually of large size, with five segments in the head, eight in the thorax,
and six in the abdomen and a gastric mill in the stomach. These, like all
other classes, are divided into Orders. Prominent among these orders

Fig. 211.

A. Ascellus aquaticus a1, a2 antennae ; br, brood-pouch ; k, pleopoda modified
to gills; md, mandibles; p^-p7, thoracic feet; pa^pa*, abdominal feet (pleopoda) ;
I- VI, head; VII-XIII, thoracic segments; XIV-XX, abdominal segments partly
fused. (After Hertwig).

B. Oniscus asellus, a terrestrial species. (After Paulmier).

are the Decapoda ( ). The crayfish comes under this

grouping. All members of this order have the first three pairs of thoracic
limbs specialized as maxillipeds, and possess five pairs of thoracic walk-
ing-legs, while all the thoracic segments are generally covered by the
carapace. They also have stalked, compound eyes.

The Isopoda ( ) have a body that is long and

flat (Fig. 211, A), seven free thoracic segments, leaf-like legs and no
carapace. There are no gills in the thorax.

The five anterior pairs of pleopods are modified for breathing pur-

THE ARTHROPODA

327

poses, the endopodites are thin-walled plates, and the exopodites and

the whole first pair of pleopods serve as a gill-cover.

In the terrestrial Isopoda (Fig.
211, B) — the wood-lice — the gills are
adapted for breathing damp air. In
these, the first and second gill-covers
have air-tubes within them. These
function like the tracheae of insects
and are therefore physiologically,
but not morphologically, compara-
ble to tracheae.

The many different species of
Isopoda (except the wood-lice) are

aquatic. There are many which are parasitic, feeding .on both dead and

living fish, and fish in turn feed on them.

A very remarkable finding in the parasite Cymothoidae ( ),

by Buller, is that the same individual can be developed first as a male

and then as a female.

Cryptoniscus ( ) is a more or less shapeless

sac which attaches itself to the stalk of Sacculina (Fig. 212), and after

the host (which is itself a parasite) is killed, the new parasite uses the

"roots" of Sacculina to draw forth its own nourishment.

The Entoniscidae ( ), parasitic, are usually

hermaphroditic, although they have small males, called "complemental

males," attached to themselves.

Development of the parasitic crustacean,
Sacculina carcinus : A, Nauplius stage ; B,
cypris stage ; C, adult attached to its host, the
crab. Carcinus maenas. (After Hertwig).

CHAPTER XXII.

INSECTS AT LARGE

It is well first to note that insects (often wrongly called Hexapoda,
on account of their having three pairs of legs), are winged six-legged
arthropods (Pterygogenea), ( ), (Fig. 213). The

body is divided into three distinct regions — the head, the thorax, and
the abdomen. The head has the following appendages : a single pair of
antennae ( ) ; usually two compound eyes ; three

simple eyes called ocelli ( ) ; and four different

kinds of mouth-parts. These mouth-parts consist of a labrum (single,
and not one of the series of metameric appendages), mandibles, maxillae,
and labium; these last three being paired.

The thorax is composed of three segments — prothorax, mesothorax,
and metathorax. Each segment is protected by four exoskeleton plates
— a dorsal tergum, a ventral sternum, and two lateral pleura. There is a
pair of walking legs on each thoracic metamere, while the last two usu-
ally also have a pair of wings attached.

The abdomen usually consists of eleven segments, on which there
are no appendages except accessory reproductive organs and sometimes
a sting at the posterior end.

In general there are two types of mouth-parts. These may vary
considerably. Grasshoppers and beetles have biting mouths, while the
true bugs have mouths arranged for sucking, and some insects such as
the bee have specialized mouth-parts which may be used for either biting
or sucking.

The walking legs have five parts: a proximal coxa ( ),

often fixed immovably to the sternum to which it is attached ; a short
trochanter ( ) ; a long femur ; a slender tibia ; and a

jointed tarsus, which is usually provided with little hooks or pads, at its
free ends. As insects have varying modes of life, such as swimming,
flying, digging, and leaping, the legs of each type of insect are adapted
to the particular functions of each.

It is from the last two thoracic segments that the wings arise. The
wings are of two types. Broad ones, such as the butterfly possesses, are
used for sailing, while smaller ones like those on flies can be moved
quickly, thus causing more rapid movement of the animal. There may
be scales or hairs on the wings. Likewise, wings may be thick or thin,
light or heavy, and vary in many other ways. The so-called "veins" in
insect wings are not veins at all, but thickenings supporting the wings.

As insects are complex organisms, all the interior structures nor-

INSECTS AT LARGE

32!)

mally found in any animal, are also found in
them, though these may vary considerably
as to shape and size. For example, those
insects which feed on vegetation have longer
digestive tracts than do those feeding on
animal matter.

The parts of the digestive system (Fig.
214, E) are: The mouth or buccal cavity; a
slender oesophagus, dilated to form a thin-
walled crop; a muscular gizzard or proven-
triculus, a glandular stomach or ventriculus
from which little pouches or caeca branch
out, and a long slender intestine. At the
junction of the stomach and intestine the
slender Malpighian tubules discharge their
excretions into the alimentary canal.

Contrary to the higher forms of life no
air-breathing insects have lungs. They re-
ceive their oxygen through a network of
tubes, called tracheae which open through
little spiracles ( ) along

the sides of abdomen and thorax (Fig. 215).

If, therefore, one wished to chloroform
or drown an insect it could not be done by
covering the head or placing the head un-
der water. The abdomen and thorax would
have to be covered with the anaesthetic or
the water.

Fig. 213.

I. External anatomy of Calopte'nus spre'tus, the head and thorax disjointed;
up, Uropatagium ; /, f urcula ; c, cercus. (Drawn by J. S. Kingsley).

II. An adult mosquito, much enlarged, with all the parts that are used in classi-
fication named. (Smith, N. J. Experiment Station, Bulletin 171, 1904).

III. Side view of Locust with the Thorax separate from the head and abdomen
divided into three segments. (I, III, from Packard's "Zoology," by permission of
Henry Holt & Co.)

If the insect flies a great deal these tracheae are expanded into air
sacs, which adds to the lightness of its body.

330

GENERAL BIOLOGY

However, those insects living in water have tracheal or blood gills,
or both, or at least some specialized adaptation by which oxygen may be
used.

A peculiar feature of all animals possessing an exoskeleton is, that
as soon as the inside of such skeleton grows but slightly, it becomes too
large for its skeletal jacket, so that it must split and a new one form. This

A.-D. Successive stages in the concentration of the central nervous system of
Diptera. A, Chironomus; B, Empis; C, Tabanus; D, Sarcophaga. (After Brandt).

E. Internal anatomy of Calopte'nus fc'mur-rubrum: at, Antenna and nerve
leading to it from the "brain" or supra-esophageal ganglion (sp) ; oc, ocelli,
anterior and vertical ones, with ocellar nerves leading to them from the "brain ;"
oe, oesophagus ; m, mouth ; Ib, labium or under lip ; if, infra-esophageal ganglion,
sending three pairs of nerves to the mandibles, maxillae, and labium respectively
(not clearly shown in the engraving) ; sm, sympathetic or vagus nerve, starting
from a ganglion resting above the oesophagus, and connecting with another ganglion
(eg) near the hinder end of the crop; sal, salivary glands (the termination of
the salivary duct not clearly shown by the engraver) ; nv, nervous cord and gan-
glia; ov, ovary; ur, urinary tubes (cut off, leaving the stumps); ovt, oviduct;
sb, sebaceous gland ; be, bursa copulatrix ; ovt', site of opening of the oviduct (the
left oviduct cut away) ; 1-10, abdominal segments. All other organs labeled in
full. (Drawn from his original dissections by Mr. Edward Burgess). (From
Packard's "Zoology," Henry Holt & Co., Publishers).

is called ecdysis ( ), or moult (Fig. 227), and the

periods between moults are called "instars."

It will be remembered that we spoke of a double-life in the frog,
not only as applied to its living in water and on land, but as to its be-
ginning life looking very much different from what it does as an adult.
Practically all insects go through a metamorphosis ( )

of some sort, and this is much more complicated than the change under-
gone by the frog.

When insects hatch from eggs (Fig. 241, I, II), and are unlike their

INSECTS AT LARGE

331

parent-forms, they are said to be heterometabolous ( ) ;

such insects hatch as nymphs ( ), a wingless form

gradually growing larger and larger wings after each ecdysis until the
adult form is reached; holometabolous ( ), if there

is a complete metamorphosis, such as being born a worm-like larva
( ), which takes food for a short time and then goes

TniSc

HtTraSc

Fig. 215.

A. Respiratory system of worker honey-bee as seen from above, one anterior
pair of abdominal sacs removed and transverse ventral commissures of abdomen
not shown. / sp, III sp, VII sp, spiracles ; HtTraSc, Tra Sc, 1, 2, 4, 7, 8, 10,
tracheal sacs ; Tra, tracheae. (From Snodgrass, Tech. Series 18, Bur. Ent., U. S.
Dep't of Agric.)

B. A portion of the tracheal tissue of a cockroach, highly magnified. Only
parts of the tubes are in focus.

cu., Cuticular lining with spiral thickening ; nu., nuclei of the protoplasmic
layer; ppm., protoplasmic layer continuous with the epidermis ( "hypodermis" ) of
the surface of the body. (After Borradaile).

into a resting or pupal stage during which no food is taken, and during
which time it loses all its larval structures, finally developing into a com-
plete adult insect, known then as an imago ( ).

In those cases where there is no metamorphosis, the animals are
said to be ametabolous ( ).

CHAPTER XXIII.

THE GRASSHOPPER

We have seen from our study of the crayfish that it was an arthro-
pod— that is, had hollow jointed feet, and that the Phylum Arthropoda
is often divided for convenience into branchiata — (gill-breathing) and
tracheata (breathing by air tubes).

The two tracheata most commonly studied in the laboratory are the
bee and the grasshopper in this country, and the cockroach in England.
Each of these organisms well represents the group to which it belongs.
The bee is the more highly specialized and many books have been writ-
ten about this interesting animal ; 'in fact, so much so that the subject
matter covering it is almost inexhaustible. The grasshopper, however,
because it is considerably larger than the bee, is preferred by many
teachers.

The study of this animal is representative of the greater part of the
animal kingdom, for this is an insect, and there are more different kinds
of insects than there are of all other animals put together.

Some of our most important garden pests are insects, and it has
been estimated by competent authorities, that one-tenth of all farrr
products are destroyed by such pests. Now, there are very few of us
who would not object to being obliged to pay one-tenth of al] we earned
to anyone for the privilege of working; yet, how low our average intel-
ligence still is may be noted from the fact, that while a loss of one-tenth
of all our food is constant, year in and year out, the average farmer
would object very strenuously to paying out even one-tenth of the tenth
he loses to pay the salary of a group of trained men to prevent this loss
from occurring, although he would thus be increasing his income to a
considerable extent.

Let us put this into actual figures. The average farmer, let us say,
has an income from all his crops (and this income, of course, includes
his living expenses, as he raises the greater portion of his food) at the
lowest estimate, $2,000 each year. He should have, if the insect pest
were controlled, $2,200. Yet, if he were asked to contribute $20 each
year to such control he would rebel. But as each and every one of us
must live on what the farmer produces, we must pay $2,200 for $2,000
worth of food. That is, we must pay $100 a year extra for every thou-
sand dollars we spend. Let us consider clothes alone. These may be of
cotton, wool, or silk. Cotton and wool are direct farm products, and
the silk grower also must have this extra $100 to pay his own expenses
in purchasing food for himself and family. In the silk industry Pebrine

THE GRASSHOPPER

333

— a very serious silk-worm disease — causes thousands of yards less of
silk to be produced than would otherwise, thereby raising silk prices.

To make this clear to the student, suppose you are employed for a
certain number of days each week and a certain number of weeks in the
year, and are paid $5 a day for such work ; it follows that your employer
must receive enough money, when selling the product you produce, to
pay you $5 each day, plus a proportionate amount of the rent he pays
for the use of the building, taxes, bookkeeping, salesmen's salaries and
traveling expenses, as well as allowing interest on the investment. That
is, what you get $5 for, will cost the ultimate user at least $10, for, it is
just as difficult to sell and to deliver goods as it is to make them. But
now suppose a storm comes up and destroys the plant, and you still

Fig. 216. Head and Foot of Fly.
The Foot shows hooks, hairs, and pads. (Head after Herms).

work, receiving your $5 each day, the traveling salesmen still work, the
bookkeeper, stenographer, foreman, engineer, fireman, night watchman,
all still are kept on the job, and receive their stated pay, but the work
is all put into clearing away the debris and in rebuilding. It follows
that all of this expense of keeping these men employed must be added
to the cost of the article. This loss may be spread over a great many
years, it is true, only a penny or two being added to the selling price
of the article, but it must nevertheless be paid.

Now, suppose for a moment, that such a fire takes place regularly
every year, and that therefore one must work one-tenth of the entire
year without producing anything. This is equivalent to taking your
salary away for this tenth of the year though still obliging you to do the
work. Here is a parallel to the financial loss caused by insect pests
alone, to you. For this is your loss. You must work an extra five weeks
each year to pay for the fact that men at large rank so low in the intel-
lectual scale that they refuse to pay out $10 a year for each $1,000 they
receive to prevent tremendous food and clothing losses.

But this mere working of about five weeks each year for nothing

334 GENERAL BIOLOGY

is of little importance compared to the millions of lives lost each year
by the working out of the self-same principle that makes men think only
of the dollar they receive to-day, rather than of the ten-times-that-
amount they may have to-morrow, if they will but lay the foundations
to-day.

Every worker who dies of a disease which could have been pre-
vented, causes each and every one of us to do a portion of his work.
This means that we must actually pay the expenses of keeping up such
a one's family without anything being contributed on their part.

There is thus an underlying unity among all human beings, in that,
whether we will or not, we are our brother's keeper.

This .is again well illustrated by taking into consideration the fact
that your own home and property may be as clean as it is possible to
keep it, but your neighbor's is not. The flies which breed in his manure
pile, or in his garbage heap, will come into your home and deposit the
neighbor's filth on your food. That this deposit is no mere trifle is
shown by an enlarged sketch of the fly's proboscis (Fig. 216).

EXTERNAL APPEARANCE

The hard exoskeleton has already been mentioned as well as the seg-
mentation of the grasshopper's body. The segments in this animal are
unlike those of the earthworm in not being all alike.

There is a head, thorax, and abdomen, to which various jointed ap-
pendages are attached, a pair to each segment, where any appendages
are found at all.

The three pairs of legs formerly gave them the name of Hexapoda.
Two pairs of wings are usually found upon the dorsal side of the second
and third segments of the thorax, while the tiny outer openings of the
tracheae — known as breathing pores, spiracles or stigmata — are arranged
in pairs on each side of two thoracic segments and on all the abdominal
segments except the last two or three.

Grasshoppers as well as crickets and cockroaches are members of
the order Orthoptera ( ). All of this group have

mouth-parts (Fig. 217), or jaws formed for biting and gnawing as well
as two pairs of straight wings, the first pair thickened, the second pair
thin, and, when at rest, folded like a fan under the first pair.

A pair of jointed antennae or feelers extend forward from the head,
while a pair of large compound eyes located on the dorsal epicranium
and three ocelli or simple eyes are readily observed. The mouth-parts
consist of the labrum or upper lips, being hinged to the clypeus
( ), a pair of heavy strong mandibles and a first

pair of maxillae with feelers or palps ( ) at the sides,

while the second pair of maxillae are fused together to form the lower
lip, called the labium, and are attached to crescent-shaped genae

THE GRASSHOPPER

335

( ). The cheeks are called genae ( ),
while narrow postgenae are back of these.

The maxillae are the accessory jaws, being composed of three re-
gions, the lacinia or maxillae proper, the gulea ( ),

Three ocelli or simple eyes

Mandible*

Maxillary palpi

Maxilla

Labial palp

Compound eyes
Antennae
Clypeus (c).

I.»bruin

Palpifer or palpus bear

Paraglosstc or lateral lobei
of the tongue

mgulc. or tongue attached it
the base of the labium

Fig. 217.
A. and B. Skull of grasshopper ; C. Melanoplus differentialis. a, Antennae,

clypeus ; e, compound eye ; /, front ; g, gena ; I, labrum ; Ip, labial palpus ; m,
indible ; mp, maxillary palpus ; o, ocelli ; oc, occiput ; pg, post-gena ; v, vertex

c,

mandil

(After Folsom).

C. Head and Mouth-parts of an insect. (After Tenney).

the middle spoon-shaped part and the maxillary palpus, a special sense
organ. This palpus is in turn composed of various segments, the broad
basal piece being called the stipes ( ) which joins

in turn with a smaller cardo ( ).

The lower lip or labium is composed of two broad terminal flaps
called the ligula ( ). The mentum ( )

is the basal portion, while the small immovable submentum lies between
the mentum and the gula.

The right wing of a male mosquito, Anopheles maculipennis. A, anal area ;
1st A, anal nervure ; C, costa ; Cu, cubitus ; H, humeral cross-nervure ; 7, cross-
nervure between R2 and K4+5; J, cross-nervure between radial and medial sys-
tems ; K, cross-nervure between medial and cubital systems ; M, media ; O, cross-
nervure between Rl and R.,; R, radius; Sc, sub-costa. (From Sedgwick's Zoology,
after Nuttall and Shipley) ".

The thorax is divided into a prothorax, mesothorax and metathorax,
easily distinguished by the three pairs of legs, one pair of which is at-
tached to each of the three thoracic divisions. The prothorax constitutes
a collar which is drawn out into a shield above. The wings, as already
stated, are attached to the dorsal side of the mesothorax and metathorax.

The wings are divided by veins or nervures (Fig. 218) into so-called

336

GENERAL BIOLOGY

cells. Although these veins or nervures vary considerably in different
species they are quite constant in members of the same species and so
are often used as a basis of classification.

The principal longitudinal veins are the costa ( ),

subcosta, radius, media, cubitus ( ), and anal.

There are also cross veins. Any variations are the result of either
additional and lessened numbers of those just mentioned. In beetles the
fore-wings are sheath-like and called elytra ( ).

The fore-wings of grasshoppers and all members of orthoptera are
leathery and called tegmina ( ).

The abdomen consists of eleven segments, the posterior one less
clearly defined than the others.

The entire exoskeleton is divided by sutures ( )

into distinct pieces, the sclerites ( ), though several

of these sclerites may fuse.

Fig. 219. Ear of Locust (Caloptenus italicus) as seen from the inner side.

T, tympanum ; TR, its border ; o, u, two bone-like processes : bi, pear-shaped
vesicle ; n, auditory nerve ; ga, terminal ganglion ; si, stigma, or spiracle ; m, open-
ing muscle, and m1, closing muscle of same ; M, tensor muscle of tympanic mem-
brane. (After Graber).

The sclerites (Fig. 204, A) on the dorsal surface are called tergites
( ). These are often fused together in various

insects. The sclerites on the ventral surface are known as sternites
( ), while the side walls connecting dorsal and

ventral sclerites are called pleurites ( ).

Should one wish to speak of the entire dorsal portion it is spoken
of as the tergum or notum ( ), while the entire ven-

tral wall is called the sternum and the lateral wall the pleuron.

The last tergum is sometimes called the suranal ( )

THE GRASSHOPPER 337

plate, while the last sternite forms the subgenital plate. Below the level
of the eleventh tergite, on each side, there is a triangular podical plate
( ), and just above each podical plate and project-

ing backward from the hind margin of the tenth tergite there is a small
copulatory organ, the cercus. In the female this is extremely small.

The auditory ( ) organs (Fig. 219) lie on the

first abdominal segment, which is larger than the others, though not
forming a complete ring on account of the hind legs being inserted there.
This auditory organ is merely an oval spot of thin skin stretched across
a small cavity and connected with a nerve. This is the ear or auditory
apparatus.

The posterior portion of a female's abdomen is more tapering than
that of the male and is furnished with four blunt spines (six including
the inner guide), to form the egg-laying organ, the ovipositor. The
tip of the abdomen in the male is turned upward.

The first two pairs of legs on the grasshopper are walking legs,
while the third pair is used for jumping.

Taking one of the first walking legs, we find five separate divisions
(compare Fig. 203 and 213) into which it can easily be separated, namely,
the coxa ( ), the shortest joint in close proximity to

the body; the trochanter ( ), the next succeeding

small joint almost entirely fused with the coxa in the grasshopper; the
femur ( ), a long stout section, the tibia ( )

following this, also long and quite narrow, and finally the most distal
portion, the foot, called the tarsus ( ) composed of

four joints. i

There are spines on the leg and claws [also called ungues
( )] on the foot, while a suction disc, the pulvillus

( ), lies between the claws. The longer jumping

leg has the same five divisions but the trochanter has fused with the
femur, forming a small knob on the inside of the leg.

INTERNAL ANATOMY
THE DIGESTIVE SYSTEM

This consists as in all the other animals studied, of the alimentary
canal and the collateral or accessory organs, the salivary glands, and
gastric caeca.

The alimentary canal itself is a long tube extending throughout the
entire body. The mouth is the first division and is guarded on each side
by laterally moving mandibles. Between these mandibles, and aris-
ing from the inner side of the labium, is the short tongue-like organ
known as the hypopharynx, at the base of which a tube opens from the
several salivary glands. The epipharynx is the organ of taste, and is
located on the slightly convex surface of the inner side of the labrum.

338 GENERAL BIOLOGY

The continuation of the mouth leads into the short curved oesopha-
gus which in turn leads to the large ingluvies ( ) or
crop. Here are seen various rows of spine-like teeth. The proventric-
ulus or gizzard ( ) follows. This is a very small
organ also furnished with spines ; it empties into the large, thin-walled
ventriculus or stomach. Six tubular gastric caeca or blind sacs are at-
tached to the anterior end of the stomach. Posterior to the stomach the
alimentary canal forms the intestine, which is divided into three por-
tions: the ileum ( ), rather slender, with longitudi-
nal ridges on the inside (the infolding ridges increase the absorbing sur-
face) ; the colon, smaller than the ileum and possessing a smooth lining,
and the rectum, which has six longitudinal rectal glands of unknown
function.

The food of the red-legged locust, which feeds quite freely by day
(unlike the crickets and katydids which are more active at night), con-
sists of grass and little drops of dew. The pads at the tips of the legs,
and the claws, enable the animal to climb stalks of all kinds very readily.
This eating of dew rather than drinking at pools of water, has given us
the idea that there is something about standing-water that is fatal to
the grasshopper. That this idea is correct is evidenced by the fact that
grasshoppers kept in captivity must be sprinkled- with drops of water or
they usually perish.

The food once taken in the mouth finds the salivary glands pouring
their secretions forth which thus assist in preparing the food for the
crop to which it passes through the oesophagus. Here it is mixed with a
molasses-colored digestive fluid. It then passes on, being again ground
by the spinous processes in the muscular gizzard. The various gastric
caeca, each of which has an anterior and a posterior pocket, increase the
stomach space.

Once the food has passed through this stage it must become part
of the blood of the grasshopper. This it does by being absorbed through
the walls of the alimentary tract.

f THE CIRCULATORY SYSTEM

The grasshopper has a long tubular heart (Fig. 214, E) lying along
the dorsal surface just beneath the body wall. From this there are
arteries and sinuses connecting the various parts of the body. From its
position the heart is often called the dorsal vessel.

Anteriorly the heart is prolonged into a tube leading to the head and
is partially divided by valves into eight chambers. The position of the
heart-valves allows blood to flow headward only.

The propulsion of the muscular heart sends the blood forward
through various sinuses so that every part of the body may be nourished
by it. It then returns by a closed tube, the ventral sinus, to the peri-
cardial sinus or chamber, and enters the heart through several pairs of

THE GRASSHOPPER 339

lateral ostia ( ). If more food has been absorbed

than can be used, it is stored up as fat in the fat bodies on either side
of the heart.

THE RESPIRATORY SYSTEM

The blood of all insects (Fig. 220) contains a respiratory protein,
hemocyanin, similar to that of the crayfish. In some few species
(bloodworms=midge larvae, Chironomidae) hemoglobin is also found.
Since the hemocyanin is capable of absorbing oxygen and carbon dioxide,
it is probable that in the insects this respiratory protein aids the tracheae
in distributing oxygen and collecting CO2. The tracheae are kept open
and extended by a spiral thickening of chitinous lining and extend to all
parts of the body, even including the legs and wings (Fig. 215, B).

This is, no doubt, one of the reasons why the circulatory system is

i so poorly developed, for, unlike the
>O\ higher forms of animal life where
>^ the circulatory and respiratory sys-
* terns are dependent upon each other,
the systems in the insects are sepa-

Fig. 220. Blood Corpuscles of the Grasshop- J

per, stenobothrus. rate and distinct, so that everv part

JceHne; of the body can be supplied with
showing nucleus. (After Graber). oxygen at any time, regardless of

what may happen to another part. The disadvantage of such a method
consists in the necessity of having both a respiratory system and a cir-
culatory system in every part of the body, instead of having all respira-
tory work done in one place. The air sacs with which the tracheae are
connected are of value in making the animal light for flying and jumping
purposes. The grasshopper can beat any professional human jumper by
the distance it covers in a single leap when comparative size is con-
sidered.

If one notices a grasshopper when it breathes rapidly, it will be seen
that the abdomen lengthens and shortens, thus forcing air in and out of
the spiracles on the thorax and abdomen.

THE EXCRETORY SYSTEM

Like all animals, the grasshopper needs oxygen to carry on its meta-
bolic processes, and like all animals gives off carbon-dioxide as a waste
product, as well as water and a nitrogen-containing-substance called urea
(if in solution) or uric acid (if crystalline). It is interesting to note that
those grasshoppers which live in dry places excrete the crystalline
product while those which live in damp places excrete the soluble form.*

These excretory products leave the body through the urinary or
Malpighian tubules which empty into the intestine just posterior to the
stomach, thus causing both the excreted and egested material to leave

*Doubt has been thrown on former investigations by recent work, so it is well not to assume
that our opinions in regard to the work of the Malpighian tubules or of the formation of urea
are final.

340

GENERAL BIOLOGY

the body in the same way. These tubules ramify throughout the body
in the animal and are very conspicuous when the body is opened.

THE NERVOUS SYSTEM

The nervous system closely resembles that studied in the crayfish,
there being a series of ganglia along the ventral nerve cord which split

Fig. 222.

Reproductive system of the Queen honey
bee. a, accessory sac of vagina ; b, bulb of
stinging apparatus ; c, colleterial, or cement
gland ; o, ovary ; od, oviduct ; p, poison glands ;
pr, poison reservoir ; r, receptaculum seminis ;
re, rectum; v, vagina. (After Leuckart).

Fig. 221.

A, diagram to illustrate the action of
wing-muscles of an insect.

B, diagram of wing-muscles, a, alimen-
tary canal ; en, muscle for contracting thorax,
to depress wings ; d, depressor of wing ; e,
elevator of wing ; ex., expander of thorax to
elevate wing ; id, indirect depressor ; ie, in-
direct elevator ; I, leg muscle ; p, pivot or ful-
crum ; s, sternum ; t, tergum ; wg, wing.
(After Grabers).

at the oesophagus, one-half of the cord passing dorsalward on each side
of that organ, uniting again on the dorsal surface and forming the supra-
oesophageal ganglion or brain, while the ganglion below the oesophagus,
which branched to permit the passing around to form the brain, is known
as the suboesophageal ganglion. It is from the brain that nerves go for-
ward to supply the special sense organs, such as the eyes, antennae, and
labrum, while the mandibles and maxillae are supplied from the sub-
oesophageal ganglion.

Nerves are given off from the thoracic and abdominal ganglia to all
parts of the respective segments. The interesting thing about insects
is that these nerve centers seem to be as independent as are the separate
respiratory tracheae in that the head may be removed while the other
parts of the body continue their work almost as well as before. In addi-
tion to the Central Nervous System and the regular Peripheral Nervous
System, consisting of these segmental nerve filaments, there is also a
Sympathetic System, divided into two parts, one lying dorsal to the ali-
mentary tract and controlling processes while the other lies ventral to
the alimentary tract and controls the spiracle muscles.

THE GRASSHOPPER

341

THE SENSES OF INSECTS

We have already seen that there are simple and compound eyes in
an insect. An ocellus, or simple eye (Fig. 223), is made up of a lens,
vitreous body, retina, and nerve, quite like that of the frog, except that
the insect's eye is definitely fixed. It cannot accommodate itself to dis-
tance. Its power of vision is therefore more
limited. The lens being quite convex and
only able to focus at one distance, it is as-
sumed that insects must be very near-
sighted.

The surface of the compound eye is
made up of numerous facets each at the end
of a single eye-element called an ommatid-
ium (Fig. 208, A), which, as already de-
scribed for the crayfish, is, in a way, a sep-
arate and distinct eye.

Recent investigations of the structure
of ommatidia show that these are more or
less conical, the narrow end at the base be-
ing connected with the nerve fiber. From
this it can be assumed that the field of vision
of each ommatidium overlaps slightly that
of the adjoining ones. This assumption is
further supported by the fact that the lens of each ommatidium is con-
vex, so that not only rays in direct line but lateral rays are refracted
on the nerve fiber. In this way a superposition image is formed, not the
apposition image or mosaic described by older authors.

Recent work on the ocelli and compound eyes indicates both of these
structures work together to increase recognition of movement. This is
due to the fact that the rays of light reach the ocelli and compound eyes
at different angles. There is additional evidence that the ocelli are used
to distinguish light from darkness. Certain night-flying bees and dra-
gonflies have greatly enlarged ocelli. Because of the fixed focus of the
ocelli and the great convexity of the lens, the object to be seen must be
very near.

Whether insects perceive color as such is a question of much dis-
pute. Very little direct evidence is available, most of it circumstantial.
Many authors and experimenters hold that insects recognize colors only
as shades of gray, much as a color-blind person does. On the other hand,
not a single experiment to prove color vision has demonstrated such a
fact. It is not a necessary correlation that because flowers are colored,
insects see colors. Half of the good pollinators are night fliers.

Fig. 223.

Median ocellus of honey bea.
(Longitudinal section). h, hypo-
dermis ; I, lens ; n, nerve ; p, iris
pigment ; r, retinal cells ; v, vitreous
body. (After Redikorzew).

342

GENERAL BIOLOGY
TOUCH

The sense of touch is probably developed very highly in most in-
sects as there are sensory tactile hairs over the entire body, as well as
antennae, palpi and cerci which are especially developed tactile organs.

Fig. 224.

A. The common cricket, Gryllus Pennsylvanicus, female. Line indicates
natural size.

B. Oblong leaf-winged Katydid, Amblycorypha oblongifolia, female. (From
Kellogg's "American Insects," by permission of Henry Holt & Co.)

TASTE

The sense of taste is located in the sensory hairs or microscopic
elevation borne upon the tongue or hypopharynx, on the epipharynx
(which lies on the roof of the pharynx), something like the palate in
higher animals, and on the maxillary and labial palpi. From the experi-
ments so far performed it seems insects can detect tastes that man can-
not.

SMELL

Insects may depend upon the sense of smell to find their food more
than upon sight, but the usual experiments to demonstrate this are far
from satisfactory. The cutting away of antennae with the attendant
tearing of many tiny nerves, will certainly not cause any organism to
react normally.

Mclndoo has recently shown that the chief olfactory organs (at
least in the honey-bee) are located near, or on the base of the leg.

THE GRASSHOPPER 343

HEARING

As various insects produce noises of many kinds, we infer they must
hear, though definite evidence has not been forthcoming up to this time.
Flies and bees "buzz" by a rapid motion of the wings, while the singing of
the male cicada is produced by a rapid vibration of a pair of membranes
on the first abdominal segment, and a resounding drum-like membrane
within the thorax. Many beetles form a squeaking noise by rubbing
their wing-covers against some rasp-like portion of their body, while
grasshoppers rub their hind legs against the wing-covers as well as rub-
bing front and hind wings together.

Crickets and katydids (Fig. 224) have a definite scraper on the base
of one wing-cover and a file-like apparatus on the base of the other.
These are rubbed together which causes the neighboring membrane to
vibrate and produce the "chirp."

As such "chirps" or calls are answered by their mates, it must be
assumed that some hearing takes place.

The grasshoppers have a large auditory organ on each side of the
first abdominal segment consisting of a surface membrane or tympanum
stretched across a cavity, on the inside of which two tiny processes some-
thing like the ear-bones of the frog are found. There are also similar
membranes on the tibia of some insects which may also serve as auditory
organs.

A male mosquito will vibrate its antennae when a tone is produced
on a tuning fork of the same pitch as that made by the wings of the
female, so that it may be that in the mosquito the antennae have some
auditory function.

THE MUSCULAR SYSTEM

As in all animals possessing an exoskeleton the muscles must be
attached on the inner surface of the skeleton (Fig. 221). Each of these
muscles is innervated by nerves, however, just as in animals possessing
endoskeletons and move by a series of complicated pulley-like arrange-
ments as already seen in the crayfish.

THE REPRODUCTIVE SYSTEM

Among all insects there are two sexes, the male usually being the
smaller, more active and more brightly colored. It has been suggested
that the reason for this is that the handsomer males are thus able to
attract mates more often than those less handsome, and consequently
the young born of such more handsome fathers, were also handsome,
thus eliminating, by natural selection, the less handsome. It has been
suggested by some also that the female, who carries the eggs, by being
less gaudy in appearance is also less conspicuous, and therefore not so
likely to be caught by natural enemies.

344

GENERAL BIOLOGY

In all female insects there are a pair of ovaries (Fig. 222) usually
formed of many small tubes called ovarioles. From the ovaries the
oviducts pass out into a terminal region, the vagina, which is sometimes
also paired. This latter organ is usually formed by an invagination from
the outer part of the body until it meets the oviducts, while near this,
or branching off from it, there is a receptaculum for receiving and hold-
ing the male sperm received during copulation.

Then there are accessory glands which secrete a sticky substance or
cement as the eggs pass through the oviduct. These glands are known
as colleterial or sebific ( ) glands which open in turn

into the dorsal portion of a capacious pouch, the bursa copulatrix,

through a duct. This bursa rests
on, and opens directly into, the ovi-
duct of the female. Grasshoppers
have an external hard posterior re-
gion of the body known as an ovi-
positor (Fig. 225).

The males possess a pair of
testes usually formed of many small
tubes connecting with two ducts, the
vasa deferentia which carry the
sperm to the terminal portion called
an ejaculatory duct, which may have
one or two openings. This external
opening may be formed by the union
of both vasa deferentia or by an in-
vagination meeting these ducts.
The seminal vesicles, usually paired, open from either the vasa
deferentia or the ejaculatory duct. Here sperm are stored. Often there
are accessory glands whose secretion unites the sperm into packets
known as spermatophores. There may or may not be an external copu-
latory organ though in the grasshopper there are a pair of these, called
cerci. Often there are also external hard parts as in the female though,
of course, these are not ovipositors.

The sperm are placed in the seminal receptaculum of the female by
the male and may remain there for many years. The queen bee only
copulates once, and that on her first and only flight, and yet the sperm
have remained alive so that eggs which were laid thirteen years after-
ward were fertile.

There are a few insects which give birth to living young, such as
the "parthenogenetic summer aphids, a few flies, the little bee parasites
Strepsiptera, a few beetles and cockroaches," but by far the greater por-
tion lay eggs, the young then developing from these.

When eggs develop which have not been fertilized, birth is said to
be by parthenogenesis ( ). This occurs normally,

Fig. 225.

Rocky Mountain locust : a, a, a, Female in
different positions, ovipositing ; b, egg-pod ex-
tracted from ground, with the end broken
open ; c, a few eggs lying loose on the ground ;
d, e, show the earth partially removed to il-
lustrate an egg-mass already in place and one
being placed ; /, shows where such a mass has
been covered up. (After Riley).

THE GRASSHOPPER

345

at least for a number of generations, in two Lepidoptera and one beetle,
in some coccus insects and aphids, and in certain saw-flies and gall-
wasps. It occurs casually in the silk-moth, in some grouse, locusts, and
several other Lepidoptera, seasonally in aphids, in larval life in some
flies [Miastor ( ), Chironomus ( )]

and partially or "voluntarily" when the queen-bee lays eggs which be-
come drones.

PAEDOGENESIS

Among certain tiny flies hardly one millimeter in length and known
as midges (Fig. 226) there are pupae which produce eggs without fertili-

Fig. 226. Order Hymenoptera. D.

A, gall-fly, Rhodiles rosoe, female. B, galls produced by a bug. (A, from the
Cambridge Natural History; B, from Davenport, after Kerner).

C. Order Diptera. Hessian fly, Cecidomyia destructor (one of the midges).
a, larva. 6, pupa. (From Davenport, after Standard Natural History).

D. Young paedogenetic larvae of Miastorca genus of the family Cecidomyiidae
in the body of the mother larva. (After Pagenstecher).

zation. The larvae of the gall-gnat, the related members of this family,
and related Chironomidae likewise do this so that here we have a case
of a granddaughter commencing to grow and develop not only without
fertilization, but before the mother and grandmother themselves become
full-fledged imagoes or adult insects.

The larvae in such cases are hatched within the parent larva and
"in some cases escape by the rupture of the body."

Such development of one, two, or three generations within the im-
mature animal is called paedogenesis ( ).

POLYEMBRYONY

In 1904 P. Marchal described an interesting observation. He found

that in two small parasitic Hymenoptera ( ), a

Chalcid ( )— Encyrtus ( )—
which lay eggs in the developing eggs of the small moth Hyponomeuta

( ) and a Proctotrypid ( )—

Polygnotus — which infests a gall-midge — Cecidomyid ( )

346 GENERAL BIOLOGY

— larva, the nucleus of the egg of the insects divided and each such par-
ticle of nucleus became a complete new embryo. "Thus a mass or chain
of embryos is produced, lying in a common cyst, and developing as their
larval host develops. In this way over a hundred embryos may result
from a single egg. Marchal points out the analogy of this phenomenon
to the artificial polyembryomy that has been induced in Echinoderm
( ) and other eggs by separating the blastomeres,

and suggests that the abundant food-supply afforded by the host-larva
is favorable for this multiplication of embryos, which may be, in the first
instance, incited by the abnormal osmotic pressure on the egg."

When many embryos develop from a single egg in the way just de-
scribed it is called polyembryony.

H. H. Newman has shown that in the ant-eater, armadillo, in which
three to nine embryos commonly form different species, all develop from
a single egg. The fertilized egg does not split into separate parts but
evaginates in different portions to form separate embryos.

ALTERNATION OF GENERATIONS

A true alternation of generation has been found in Hymenopterous
gall flies (Fig. 226), in which a complete asexual generation (complete
from egg to adult) succeeds a complete sexual generation (egg to adult),
each generation being parasitic on a different host plant. The adults
in each case bear no resemblance to each other; in fact, they have not
only been described as different species, but actually as different genera.

EMBRYOLOGY

The flapping of wings or the "singing" of the male grasshopper
attracts the "unfertilized" females. The sperm are then injected into the
female receptacle, from whence they work their way into the various
eggs.

The zygote thus formed, begins to segment mitotically, forming the
embryo on top of the yolk close to the egg-shell. There are two pro-
tective membranes, the innermost being known as an amnion
( ) or chorion ( ), and the

outer as the sero&a ( ). As soon as the embryo has

used up the yolk as food it is ready to hatch.

However, all of this process does not take place in the body of the
grasshopper. Soon after fertilization the female drills a hole in the
ground with the hard portions of the ovipositor and deposits the eggs
which are then covered. These hatch in the spring. It is here, in its
warm underground cage, that most of the development described above
takes place.

"By opening and shutting the ovipositor a hole (Fig. 225), slightly
curved, is quickly drilled in the ground. This drilling process goes on

THE GRASSHOPPER

347

until nearly the entire abdomen is buried. Ovipositing females may
frequently be found in October. A frothy matter is first laid down from
the cement glands, then the eggs and cement are alternatively deposited
until some twenty to thirty-five eggs have been laid. Each individual
egg is elongated and slightly curved. The female ordinarily oviposits
more than once, averaging from 100 to 150 eggs in all. The eggs are
placed side by side in four rows, but standing obliquely to the wall in
such a way that all slant upward. Since they are all pushed tightly
against the wall of the cylindrical burrow the outside rows must project
beyond the two inner rows. In this way a channel filled with frothy
matter is left along the tops of the rows. Such a grooved arrangement
insures the escape of the young from the lower eggs in case those in the
upper ones die or are delayed in hatching.

"Each egg is covered by two membranes: (1) an outer thin, semi-

Fig. 227. Calopt'enus Spre'tus.

Process of acquiring wings : a, Pupa with skin just
split on the back ; 6, the imago extending ; c, the imago
nearly out ; d, the imago with wings expanded ; e, the imago
with all parts perfect, natural size. (After Riley).

opaque one which under a lens may be seen to be pitted or thrown into
ridges, and (2) an inner membrane (chorion) which is smooth and thick,
but so translucent that the young insect can be seen through it after
development has begun. While the outer covering is easily broken, the
inner is very resistant, requiring strong pressure between the fingers to
crush it.

"At hatching time in the spring the struggles of the young locust,
together with the swelling of parts within the chorion, burst the latter,
generally along the ventral side, and the young locust struggles out of
its burrow. Once out, it rests a few minutes, generally lying on one side.
The limbs are at first limp and directed backward. The animal is still
enveloped in a thin veil or pellicle which has aided it in forcing its way
out of the ground. This covering shortly splits along the middle of the
back and works off behind. Within an hour the locust takes its natural
gray color. The foregoing account applies particularly to the Rocky
Mountain locust."

The young grasshopper (like all exoskeletonous animals), though
able to feed immediately when its normal form has been completed, can-
not grow until it throws off its outer covering. This ecdysis occurs

348 GENERAL BIOLOGY

periodically. Of course, it takes time for the new skeleton to harden,
so that immediately after shedding its covering the animal is rather soft.
The wings appear after the first moult (Fig. 227). They increase in
size with each moult but become functional only after the final moult.
An insect which at birth resembles its parent, but is not entirely like it,
as the young grasshopper, is called a nymph ( ).

The last moult takes place in the late summer. The nymph then
"climbs up some grass stem or similar object, and, taking firm hold,
often with its head pointing downward, remains motionless for several
hours, till the skin swells over the head and thorax and finally splits
open along a median dorsal line. From this old skin the new head,
thorax, legs, wings and abdomen are slowly withdrawn while soft, ex-
panding and hardening within half to three-quarters of an hour."

It is then a full-fledged adult and is called imago. After the eggs
have been laid in the fall most of the locusts die.

BEHAVIOR

As there are more different species of insects than there are of all
other animals together, it is not strange that insects should be of con-
siderable interest and importance.

They illustrate better than any other type of animal the interrela-
tionships and interdependence of all living things.

Pollen is carried from one plant to another by insects (Fig. 239),
thus permitting vegetation to grow wherever there is sufficient heat
and moisture. This makes food more plentiful. Injurious animals and
pests are kept down even among their own kind. For example, the swift
little tachina fly (Fig. 240) pokes its egg between the segments of the
grasshopper's abdomen, which egg then develops into a maggot, and
this maggot bores its way into the interior of its host, feeding on the
living substance as it goes. It leaves the vital organs until last, so that
the grasshopper does not die until the maggot has abundantly supplied
itself with nourishment. Then, too, insects furnish the most abundant
food for birds, worms, toads, fish, and other animals. Even man has
not hesitated to use them as food. The Bible speaks of John in the
desert feeding on locusts and wild honey; one itself, the insect, the other,
the product of an insect.

In the markets of Manila large piles of grasshoppers with their
appendages removed are offered for sale, ready for cooking. The Moors
fry locusts in butter and they are said to make a very palatable dish.
In fact, many of the Indian tribes have been known to use not only
grasshoppers, but ants as well, as a part of their diet, while the natives
of Uganda keep crickets in a warm oven for their musical sounds. In
China it is said that fights are staged between crickets and that this is
a favorite method of gambling.

The larvae or grub of the warble fly is eaten by the Dog Rib In-

THE GRASSHOPPER 349

dians, who are fond of caribou which in turn is thoroughly infected with
these grubs. The grubs are eaten raw and the children consider them a
great delicacy.

To this list may be added moths and caterpillars, eaten by both Pai
Ute Indians and the Australian Bushmen, while bugs, beetles and the
eggs of these insects complete the list. The Manna of the Old Testa-
ment is considered by entomologists to be the secretion, somewhat, like
honey, from an insect. These manna insects, now called Gossyparia
mannifers ( ), "infested the smaller branches

of Tamarix gallica ( ) in large numbers, sucked up

sap in quantities, and exuded manna in the form of a sugary secretion
which, in the cool of the evening, fell to the ground in solid form, but,
after sunrise, melted and percolated the soil."

Conditions of the past have been changed since man has learned
to till the soil ; for, insects now receive other food, their conditions of
life have changed and comparisons of ancient times when men lived
under different conditions from the present are often likely to lead one
astray. This is particularly noticeable among agricultural peoples who
seldom use insects as food. In famines, anything could be relished and
it was no wonder that such peoples often turned to a diet not commonly
used, and then after an acquired taste had been brought about (just as
it is known that practically no one likes olives the first time he eats
them, but can acquire a very considerable taste for them later) the chil-
dren who had been fed upon such diet actually relished it. No better
proof of this could be found than the fact that pigeons and rabbits never
normally eat meat, but, if they are fed meat alone from birth, they will
die rather than eat a normal pigeon's or rabbit's food when they have
become fully grown.

In addition to being used as food, insects have formed a great source
from which various oils and other medicinal substances have been ab-
stracted from time immemorial. All historical literature is filled with
references to this use of insects.

Because all of us need a physician at some time or other, it is of
great interest and value to know as much about this subject as possible.

Over against this beneficial use of insects may be placed the great
devastations in our own country by the periodical locusts which sweep
every grain field bare before them, and other crop-injuring pests such
as boll weevils, which injure thousands of dollars worth of cotton an-
nually, while almost every grain has some sort of insect which uses such
grain as its food.

As carriers of developing eggs or various immature forms of para-
sites, insects are now known to do great injury to man as well as the
animal world at large. The classic example is that of the anopheles mos-
quito, carrying malaria, as also the tse-tse fly, already referred to as the
carrier of the germ of sleeping sickness.

350

GENERAL BIOLOGY

Lice (Fig. 228), and other so-called vermin (all of these belong to
the insect group), are not only injurious to higher forms of life by their
acts, but also as carriers of disease.

The common house fly carries dirt and filth from the garbage can

A. Female of flea, Putex irri
tans, infesting man.
Herms.)

Fig. 228.

Lice — both animal and plant.

B. Sarcoptes scabei, C. Order Acarina. Harvestmites

(After or "chiggers." Leptus irritans on
the right; L. americana on the left.
(From Osborn, after Riley).

(After female itch mite.
Leuckart).

D. Common cat and dog flea (Pu'lex ser-
rat'iceps) : a, Eggs: b, larva in cocoon; c,
pupa ; d, adult ; e, mouth parts of same from
side ; /, labium of same from below ; g, an-
tenna of same ; all much enlarged. ( Howard,
Bull. U. S. Dept. of Agriculture, 1896).

Rat Fleas. — It is believed that in tropical
countries the disease germs of the bubonic
plague may be transmitted from rats to men
by the bites or punctures of rat fleas.

E. Phylloxe'ra vasta'trix : a, Leaf with
galls ; b, section of gall showing mother
louse at center with young clustered about ;
c, egg ; d, larva ; e. adult female ; /, same
from side. (a. Natural size; b-f, much en-
larged). (Marlatt).

F. Phylloxe'ra vasta'trix: G. Pediculoides ventricosus, H. Head and Pronotum of (a) dog

a, Root-galls; b, enlarge- male. Grain louse which flea; (b) of cat flea; (c) hen flea,

ment of same, showing affects farmers and thresh- (After Rotschild.) (d) Nycteridiphilus

disposition of lice; c, a ers. (After Braun). '(Ishnopsyhus) hexactenus. (After Ou-

root-gall louse, much en- demans.)
larged. (Marlott.)

and manure pile to the food it lights upon, as well as to the baby's drink-
ing bottle. In this filth, it thus deposits, there are hundreds of tiny eggs
and seeds which as soon as they receive the necessary moisture and heat

THE GRASSHOPPER 351

of the interior of the human or animal body, begin to develop. This is
the common way in which typhoid fever is carried, for one can hardly
get this disease unless some excreted matter from a typhoid patient has
been eaten in this way.

An excellent way to demonstrate the fact of insects' eggs being
almost everywhere on our foodstuff is to take any fruit, such as a banana,
apple, cherry, or grape, and place this in a bottle plugged with cotton,
so that air may pass in but nothing else can. In a short time various
forms of animal life will be found therein. As these forms of life hatched
from eggs, the eggs must have been on the fruit before it was placed in
the bottle. It is of value to note that even after one has washed the
fruit well, such hatching will almost always occur, showing how thor-
oughly these insects fasten their eggs either on or into the surface struc-
tures of these fruits.

When different kinds of crops are planted different kinds of insects
will thrive, as those alone will survive which have a sufficient food sup-
ply. Those not feeding on the new plants either leave for satisfactory
fields, or die. If it is remembered that a duck, which feeds on fish, tastes
totally different from one not so fed, which shows that the food of an
animal makes a great chemical difference to the body tissues, it can
be understood how different diseases may come forth when parasites
change their food and environment. For, if it makes a chemical differ-
ence in the flesh of an animal as to what it eats, it means also that if a
new chemical substance in a parasite is poisonous to man, then the same
parasite when feeding on one food may not be poisonous and not cause
disease, whereas when feeding on another type of food, such chemical
poison may cause disease. Then there is the interesting fact that many
diseases of birds will not affect a frog normally when such disease germs
are injected, but, if the frog be placed in an incubator where its blood
becomes of the same temperature as that of the bird from which the
disease is taken, the disease will develop. This illustrates how different
temperatures change the susceptibility of different organisms to differ-
ent diseases.

The animals commonly called grasshoppers are really of varying
types (Fig. 229), the true grasshopper being long-horned; that is, it has
two antennae as long or longer than its entire body. The family to
which these belong is known as Locustidae, while the short-horned
grasshoppers belong to the family Acridiidae.

In America the Rocky Mountain Locust is the one which does the
great damage to crops. The exact time of laying and hatching of eggs
varies somewhat with the region of the country.

Often the young, until after the second or third moult, content
themselves with feeding on whatever food is close at hand, but as soon
as this food becomes scarce the animals congregate and "in solid bodies,
sometimes as much as a mile wide, march across the country, devouring

352 GENERAL BIOLOGY

every green crop and weed as they go. During cold or damp weather
and at night they collect under rubbish, in stools of grass, etc., and at
such times almost seem to have disappeared; but a few hours of sun-
shine brings them forth as voracious as ever. When, on account of the
immense numbers assembled together, it becomes impossible for all to
obtain green food, the unfortunate ones first clean out the underbrush
and then feed upon the dead leaves and bark of timber lands, and have
often been known to gnaw fences and frame buildings. Stories of their

Fig. 229. Long and Short Horned Grasshoppers.

A. Order Orthoptera. Katydid, Microcentrum
retinerve. (From Sedgwick's Zoology, after Riley. )

B. Red-legged grasshopper (Melanoplus femur-
rubrum) : Ab, abdomen ; Ant, antennae ; E, eye ; M,
mouth ; T, thorax ; S, spiracles.

incredible appetites are legion; a friend informs me that he still pos-
sesses a rawhide whip which they quite noticeably gnawed in a single
night.

"By mathematical computation it has been shown that such a swarm
could not reach a point over thirty miles from its birthplace, and as a
matter of fact they have never been known to proceed over ten miles."

There are other species and genera which do not migrate from their
native haunts at all. Many ingenious ways have been used to extermi-
nate them. Certain fungus growths on plants which the grasshopper
uses for food are fatal to him. So, too, is the little tachina fly already
mentioned. Such fungus growth and flies are sometimes developed to
assist in controlling injurious insects.

The effect of a difference of temperature on insects is well illustrated
by the fact that there is only one annual generation of grasshoppers in
New England, while there are two in Missouri.

Ditches are often dug in which the animals fall, or kerosene emulsion
is poured on water standing about, or placed in simple trough-like
wooden movable ditches. Even if the grasshopper crawls out of the oil
it dies shortly after.

For the control of grasshoppers, see any of the books mentioned on
Economic Entomology, at the end of Chapter XXIV.

CHAPTER XXIV.

THE HONEY BEE

The Honey Bee (Fig. 230) has been studied and written about for
centuries as one of the most interesting" of insects. It lives a decidedly
complex social life and has lent many examples to prophets and teachers
of all times.

The bee is intensely specialized in almost all parts of its body, and
as such is of great value to any comparative study of the arthropods.

Fig. 230.

Hive bees and comb (after Schmeil). A, Worker; K, queen;
D, drone ; 1, worker with cells filled with honey and covered ;
2, cells containing eggs, larvae, and pupa?; 3, cells containing
pollen ; 4, below 4 are regular cells ; 5 drone cells ; 6-10, queen
cells.

Foremost in rank in the hive is the Queen. She is the mother of
every member of the hive, for she alone, of all the inhabitants, lays
eggs.

With her, in the summer time, there are some sixty thousand work-
ers and several hundred drones. The latter are killed during the winter.

The abdomen of the queen is longer than that of a worker and there
is no pollen basket on the tibia of her hind leg's.

The drone is the male. He lives upon the food gathered by the
females. His body is heavy and broad and no pollen baskets are found
on the hind legs. His eyes are larger than those of either queen or
worker.

The worker is an undeveloped female, which can, however, by proper

354

GENERAL BIOLOGY

food, nourishment and care, become a queen in case the old queen dies.
The workers are smaller than either queen or drones. They are the ones
usually seen hovering about flowers.

Bees have mouth parts (Fig. 231), modified both for biting and suck-
ing, and two pairs of membranous wings.

B. Side view of mouth parts of the honey
bee, Apis Mellifera. a, base of antenna ; br,
brain ; c, clypeus ; h, hypopharynx ; I, labrum ;
lp, labial palpus ; m, mentum ; mo, mouth ;
mx, maxilla; sm, submentum. (After Chesh-
ire.)

Fig. 231.

A. Front view of the head and mouth
parts of a bee. — After Cheshire, a, An- •
tenna ; m, mandible ; g, labrum and epi-
pharynx ; mx.p., rudiment of maxillary
palp ; mx,, lamina of maxilla ; lp., labial
palp ; I., ligula ; 6., bouton at end. The
paraglossae lie concealed between the basal
portions of the labial palps and the ligula.

C. Tongue of honey
bee. p., protecting bris-
tles ; s., terminal spoon ;
t., taste setae. (After
Will.)

EXTERNAL APPEARANCE

The body is divided into head, thorax, and abdomen. (Fig. 213.)
The body is covered with a skin or cuticle which is composed of a thin
chitinous layer produced by the secretion from the cells lying beneath
it. This serves as a protection, but it is cast off at various intervals
during the early stages of growth.

There are a pair of large compound eyes and three ocelli or simple
eyes. The arrangement of the ocelli are somewhat different in queen,
worker, and drone. Two feelers (antennae) project from the front of
the head.

The mouth is made up of an upper lip or labrum, an epipharynx, a
pair of mandibles, two maxillae, and a labium. This latter is the under

HP.

The labrum is joined to the clypeus (the dome-shaped portion of
the skull), (Figs. 217 and 231), lying just above it. The epipharynx is

THE HONEY BEE

355

the fleshy projection extending beneath the labrum. It serves as an
organ of taste. The jaws or mandibles lie on each side of the labrum,
being notched in the queen and drone, and smooth in the worker.

The labium lies medially and extends downward from beneath the

labrum and is quite compli-
cated. The sub-mentum, which
is triangular in shape, joins the
labium to the back of the head.
The mentum lies next to the
sub-mentum. The mentum is
chitinous and contains muscle.
The tongue, or ligula, lies im-
mediately beyond the mentum.
The tongue has a spoon-shaped
end known as a bouton. A
labial palpus lies at each side
of the tongue, while tiny hairs,
used as organs of taste and
touch, as well as for gathering-
nectar, are arranged in regular
rows upon it.

The lower jaws or maxil-
lae extend over the mentum on
both sides. There are stiff hairs
on their edges, and maxillary
palpi on each side.

The thorax is divided into
prothorax, mesothorax, and
metathorax (Fig. 213), the last
two divisions each supporting
a pair of wings, while hairs,
which are used in gathering
pollen, cover the outside of the
entire thorax.

The legs of the bee are
highly specialized (Fig. 232).
The prothoracic legs have both
femur and tibia covered with branched hairs which are used in gather-
ing pollen. A pollen brush made up of curved bristles is seen at the
distal end on one side of the tibia. This brush is used to brush up the
pollen which has been loosened by some of the coarser spines.

On the other side of the tibia, a flat movable spine known as the
velum, fits over a curved indentation in the first tarsal joint. The whole
structure, brush and velum, is known as the antenna cleaner, while the.
row of teeth lining the indentation is called the antenna comb.

Fig. 232.

Legs of worker honey-bee. A., outer side of
metathoracic leg. p., metatarsus ; t., tarsus ; ti., tibia.
B., inner side of metathoracic leg. c., coxa ; p., meta-
tarsus ; t., tarsus ; ti., tibia ; tr., trochanter ; wp.,
wax pinchers. C, prothoracic leg. b., pollen brush ;
«6., eye brush ; p., metatarsus ; t, tarsus ti, tibia ; v.,
velum. D., mesothoracic leg ; lettering as in C. s.,
pollen spur. E., joint of prothoracic leg ; lettering
as in C. F., teeth of antenna comb. G., transverse
section of tibia through pollen basket, fa., pollen ;
h., holding hairs ; n., nerve. H., antenna in process
of cleaning, a., antenna ; s., antenna comb ; I., sec-
tion of leg; s., scraping edge of v., velum. (From
Root, after Cheshire.)

356

GENERAL BIOLOGY

The antennae are cleaned by being pulled through the indentation
between the teeth and the edge of the velum.

On this first tarsal joint also there is found a row of spines called
the eye brush. This structure is used to brush out pollen which has
lodged about the compound eyes.

On the last tarsal joint of each leg there is a pair of notched claws
by which the insect holds on to rough surfaces. Between these claws
there is a fleshy, glandular lobule known as the pulvillus, which is cov-
ered with a sticky secretion from the
glands. It is by this sticky substance
that the insect can attach itself to
smooth surfaces. Then, too, there are
tactile or touch hairs present.

The meso thoracic legs do not have
an antennae cleaner, but at the distal
end of the tibia there is a spur which
is used to pry the pollen out of the
pollen baskets on the third pair of legs,
as well as to clean the wings.

The metathoracic legs are prob-
ably the most interesting, in that they
possess a pollen basket, a wax pincher,
and the pollen combs. The pollen bas-
ket is a concavity in the outer surface
of the tibia. There are rows of curved
bristles along the edges. Pollen is
stored in this basket. The filling takes
place by the pollen combs scraping out
the pollen from the hairs on the thorax
into the basket on the opposite leg.

The wax pinchers consist of a row
of wide spines located at the distal end
of the tibia. These lie in opposition to
a smooth plate on the proximal end of the metatarsus. The pinchers re-
move the wax plates from the abdomen of the worker.

As already stated, a pair of membranous wings are attached to meso-
thorax and metathorax. There are hollow ribs called nerves or veins
passing through each wing. Often a row of little booklets on the an-
terior margin of the hind wing is inserted into a trough-like fold in
the posterior margin of the fore wing and thus join them together.

The abdomen is made up of six segments, each segment consisting of
a tergum or dorsal plate, and sternum or ventral plate. A pair of wax
glands is located on each of the four hindermost sternal plates. Both
queen and worker possess a sting (Fig. 233) at the end of the abdomen,
while the drone possesses a copulatory organ instead. There are also

Fig. 233.

Sting of worker honey-bee, b., barbs on
"darts ; i., k., 1., levers to move darts ; . n.t
nerves ; p., sting-feeler; pg., poison .gland ;
ps., -poison sac ; sh., sheath ; 5th g., fifth
abdominal ganglion. (From Packard,
after Cheshire.)

THE HONEY BEE 35?

slit-like openings of the reproductive system and an anal opening in
queen and worker.

The sting has a pair of sting feelers by which the bee seems to choose
a favorable location for the deposit of the sting. Two barbed darts are
then sent out. There is a sheath which guides the darts and aids in con-
ducting the poison. The poison is secreted in a pair of glands, one acid
and one alkaline, and it is then stored in a reservoir. It is commonly
believed that if a bee stings, it dies. This is not necessarily true ; but,
very often a part of the intestine and the poison glands are pulled out
of the body with the sting, and then, of course, the insect cannot live.

Oueens usually do not sting except in combat with other queens.

INTERNAL ANATOMY AND PHYSIOLOGY
THE DIGESTIVE SYSTEM

Beginning at the anterior end, the digestive system (Fig. 234), is
made up of mouth, oesophagus or gullet, honey-sac or honey-stomach,
true stomach, small intestine or ileum, and large intestine or colon.

The oesophagus passes through the thorax and is expanded into a
honey-sac at the anterior end of the abdomen. A stomach-mouth with
four triangular lips is found at the posterior portion of the honey-sac.
A number of bristles extends backward from the top of the lips. If the
alimentary canal be placed in a one-half of one per cent salt solution im-
mediately after the bee is killed, these lips will open and close for about
thirty minutes. Both circular and longitudinal muscles surround the
lips.

The glands in the walls of the stomach secrete digestive juices which
change the food into chyme. Part of this is absorbed and part forced
back into the ileum by muscular contractions. Here undigested food is
dissolved and also absorbed, while that which is not digested is thrown
into the colon, and from here, out of the body. No faeces are deposited
in the hive if bees are kept in proper condition.

Two pairs of salivary glands may be found : one pair within the
head lying against the cranium, and one pair in the ventral portion of
the anterior half of the thorax. The substances secreted from these
glands are weakly alkaline and are poured out upon the labium. Here
they act on the food as it is ingested.

THE CIRCULATORY SYSTEM

The blood of the honey bee is quite like that of the crayfish and
grasshopper, as it is colorless and contains amoeboid corpuscles. Little,
if any, oxygen is contained within it.

The crayfish is also like the bee in that it has a dorsal blood vessel
and many sinuses, but the'bee's circulatory system is even less. complete
than that of the crayfish.

358

GENERAL BIOLOGY

The heart, or dorsal vessel, is a tube in the median dorsal region
just below the surface, closed posteriorly and open in the head-region.
The walls being muscular, the heart contracts at intervals.

x The blood itself enters through five pairs of ostia, one into each of
the five compartments into which the heart is divided. Each compart-

ment is called a ventricle. Each
contraction sends the blood toward
the heart. There are valves which
prevent it from flowing backward.
It then passes through the various
spaces in the body to bathe the tis-
sues. As the blood passes ventrally,
it is gathered into the pericardia!
sinus, and when the muscles sur-
rounding this sinus contract, the
blood is forced through the ostia
back into the heart when it is again
ready to be sent out.

Fig 234.

A. Internal organs of the honeybee, bt.,
malpighian tubules ; c.s., true stomach ; dv.,
dorsal vessel ; e., eye ; g., ganglia of nerve
chain ; hs., honey sac ; it., rectum ; lp., labial
palpus ; mesa, t., mesothorax ; meta, t., meta-
thorax ; mx., maxilla; n., nerves. No. 1,
No. 2, No. 3, salivary glands ; oe.t oesophagus ;
p., stomach mouth ; pro.t., prothorax ; si.,
small intestine (ileum) ; v., ventricles of dorsal
vessel.

B. Ideal transverse section of an insect.
h., dorsal vessel ; i., intestine ; n., ventral
nerve-cord ; t.t., stigmata leading into the
branched tracheal tubes ; w.w., wings ; a.,
coxa of one leg ; b., trochanter ; c., femur ; d.,
tibia; e., tarsus. (After Packard, A, from
Cheshire. )

THE RESPIRATORY SYSTEM

Along each side of certain thoracic and abdominal segments there
appear openings called spiracles (Fig. 215). It is through these open-
ings that the bee breathes. One pair of these spiracles may be found in
the prothorax, one pair in the metathorax, and five pairs in the abdomen.

The spiracles open into little tubes known as tracheae which unite
in turn with other tubes running in a longitudinal manner. These longi-
tudinal tubes are called the trunks, and from the trunks many branches
are given off to all parts of the body. The tracheary tubes (though only
one cell f in thickness) have thickened rings arranged spirally, and it is
these rings which keep the tubes open.

THE HONEY BEE 359

Air-sacs are found in the abdominal region. These are expanded
portions of the tracheae and probably make the bee lighter as it flies,
for the bee can apparently increase and decrease the size of the air-sacs
at will. There are tiny valves in the spiracles and the bee takes in and
expels air by expansions and contractions of its abdomen. Hairs sur-
round the spiracles so as to prevent dust from entering* The rate of
respiration increases with the fatigue of the insect. Air is carried di-
rectly to the tissues through the tracheae so that no lung system is
needed in which blood and oxygen must mix.

THE EXCRETORY SYSTEM

There are Malpighian or urinary tubules (Fig. 234, A) which are
long, fine, hair-like structures, opening into the anterior end of the in-
testine. These are the excretory organs. Excretions are taken from
the blood in the form of urates, and pass through these urinary tubules
to the intestine from whence they are thrown out of the body with the
faeces.

THE NERVOUS SYSTEM

The nervous system (Fig. 214, 235) of the bee is made up of a chain
of paired ganglia with two groups of smaller ganglia. The first are
called the stomatogastric and the latter the sympathetic ganglia, re-
spectively. These ganglia are made up in turn of seven masses of nerve
tissue : two in the head, two in the thorax, and five in the abdomen.

Each mass is composed of two ganglia which lie side by side, and
these ganglia are connected with the mass in front and behind by two
nerve cords. Only the brain (the most anterior pair of ganglia) also
called the supraoesophageal ganglia, lies dorsal to the digestive tract.

The compound eyes, the ocelli, the antennae, and the labrum, are
connected with the brain by nerve twigs, while the mandibles, labium,
and other mouth-parts are connected with the suboesophageal ganglion
lying directly beneath the oesophagus.

The most anterior ganglia in the thorax innervate the muscles of
the first pair of legs, while the posterior thoracic ganglion is larger and
composed of several ganglia which have grown together. From the
fore part of this latter ganglion, nerves run to the fore wings and middle
pair of legs, while twigs from the posterior portion of this, same ganglion
pass to the hind wings and legs.

The organs and walls of the abdominal region are supplied by
twigs from the various abdominal ganglia; but, as with most animals,
the more posterior abdominal ganglia are the larger.

The stomatogastric portion of the nervous system is composed of
many small ganglia which are in direct connection with the organs of
digestion, circulation, and respiration, while the sympathetic nervous

GENERAL BIOLOGY

Fig. 235.

A. Nervous system of honey-bee, at a., and of
its larva, at 6., showing the simple type of the larva
and the specialization in the adult due to fusion of
the ganglia. (From Sanderson and Jackson, "Ele-
mentary Entomology/' by permission of Ginn & Co.)

C. Nervous system of the head of cock-
roach, a., antennal nerve ; ag., anterior later-
al ganglion of sympathetic system ; 6, brain
d.t salivary duct ; /., frontal ganglion ; h.
hypopharynx ; 1., labrum ; li., labium ; m.
mandibular nerve ; mx., maxillary nerve ; nl.
nerve to labrum ; nli., nerve to labium ; o.
optic nerve ; oc., oesophageal commissure ; oe.
oesophagus ; pg., posterior lateral ganglion of
sympathetic nervous system ; r., recurrent
nerve of sympathetic system ; s., suboeso-
phageal ganglion. (After Hofer.)

B. Sympathetic nervous system
of an insect, diagrammatically rep-
resented, a., antennal nerve ; 6.,
brain ; /., frontal ganglion ; l.L,
paired lateral ganglia ; m., nerves
to upper mouth-parts ; o., optic
nerve ; r., recurrent nerve »., nerve
to salivary glands ; st., stomachic
ganglion. (After Kolbe. )

system is made up of the many
fibers which pass to all parts of the
body from the triangular ganglia
lying in each segment.

ORGANS OF SPECIAL SENSE

These have already been dis-
cussed very thoroughly under the
general term, "The Senses of In-
sects," in Chapter XXIII.

THE MUSCULAR SYSTEM

As in the crayfish, the muscles
of the honey bee are attached to the
inner walls of the body. The num-

ber of muscles is very large, and the
largest muscles are those which move the wings and legs.

Muscles are both voluntary and involuntary. A good example of
the latter has already been noted in the experiment suggested of the
intestine being placed in a one-half of one per cent salt solution when
the lips of the stomach-mouth will open and close for some time.

Insects usually have much greater muscular strength proportion-
ately than larger animals. This is accounted for by the fact that the
weight of muscle increases as the cube of its diameter, while its strength
increases only as the square of its diameter.

THE HONEY BEE

361

THE REPRODUCTIVE SYSTEM

Only the queen (Fig. 236, A) can lay
eggs, although the workers have rudi-
mentary ovaries.

The two ovaries almost fill the "ab-
domen of the queen. Each of the ovaries
is made up of a great number of ovarian
tubules which contain eggs of different
sizes. The eggs pass into the oviduct
from the tubules, thence into the vagina
and out of the body through the genital
aperture.

There is an opening into the vagina
which connects with the spermatheca or
sac in which the sperm are stored, and
sperm from ~this sac may apparently be
released at will by the queen as the eggs
pass through. If the sperm is not re-
leased the egg is not fertilized and then
drones hatch. Only females hatch from
fertilized eggs.

In the drone (Fig. 236, B) two testes
are seen which are made up of several
hundred spermatic tubules in which the
F. 23g sperm are formed. A pair of fine tubes

A. Reproductive organs, sting, and called vasa dcfcrentta connect these sper-

poison gland of queen honey-bee. AGL, mofi,- tiiKpc -until tVif> ^pminal VP^lirlf*^
acid gland ; AGIO., duct of acid gland ; matlC tUDCS Wit VCSlClCS.

BGL, alkaline gland; Ov., ovary; ov., These latter in turn open into a pair of

ovarian tubules ; Ov.D., oviduct ; Pan.- x

Sc., poison sac; Spm., spermatheca; large mUCOUS glands which Unite. It IS

vagina!1" ' at this union that the ejaculatory duct

bee,' d^rsa^^ew, ^ffJrai °'P Jtton! begins. This duct ends in the copulatory

AcGl., accessory gland; B., bulb of nrcran

penis; EjD., ejaculatory duct; Pen, ^S*1*-

penis; TVs., testis ; vDef., vas deferens ; TVi^ cr^rm r\f tVi^ ma If ^re> rklsrprl in

Vea., seminal vesicle; it., uu., yy., zz., L ne Sperm Ot tne male are piaCCC

parts of penis (From Snodgrass, the spermatheca (seminal receptaculum)

lech, beries, 18, r>ur. h,nt., U. o.

Dep't. of Agric.) of the queen by a single drone, where

they remain alive for many years, in fact as long as the queen lives and
lays eggs. While the average life of a queen is probably somewhere
around three to four years, there is on record a queen which continued
laying fertile eggs for thirteen and a half years.

About five to eight days after emerging from the egg, a queen will
leave the hive. First, she crawls about and takes very short flights, and
then goes on a nuptial trip of about thirty minutes. One of the drones
copulates with her during the nuptial trip, after which the queen returns

8

362 GENERAL BIOLOGY

The eggs are bluish-white and oblong in shape. They are fertilized
just before leaving the queen's body. The eggs are deposited at the base
of the cells and then fastened into position in the cells by a secretion.
Fertilized eggs are laid in cells that have already been arranged to re-
ceive them, some being in queen cells, and some in worker cells, while
unfertilized eggs are placed in drone cells. But there seems to be evi-
dence that mistakes are made, and the right type of egg is not always
placed in the right cell.

EMBRYOLOGY

After the nuclei of the sperm and egg have united into a single
nucleus a. chitinous covering, the chorion, surrounds the entire egg. As
cleavage takes place, no definite cell walls appear. This means that a
great mass of protoplasm is present with many nuclei. These nuclei

migrate to the periphery to form a
single layer of cells, called the blas-
toderm, while the remaining portion
of the yolk remains as yolk-sub-
stance until it is converted either
into food for the developing em-
Fig. 237. bryo, or into further cellular sub-
Cross section of germ-band of Clytra at
gastrulation. g., germ-band; i., inner layer. Stance.

A germ-band or primitive streak

(Fig. 237) now forms on one side of the egg where the blastoderm be-
comes thickened. This is to become the ventral side pf the bee. The
brain develops separately. A median groove arises in the germ-band,
and so two germ layers are formed, an outer layer called the ectoderm,
and an inner known as the entomesoderm. It is the latter layer from
which both entoderm and mesoderm arise. Now the germ-band grows
around the entire egg.

It is of interest to know that while the antennae and four pair of
appendages can be seen near the anterior end of the embryo, one pair
of the anterior appendages disappear and the others become mouth
parts. Then, three pair of appendages develop on the thorax, all of
which disappear before hatching.

METAMORPHOSIS

The life-history of the bee is divided into four periods : egg, larva,
pupa, and adult or imago.

Queens, workers, and drones remain in the egg three days, but the
queens remain in the larval stage five and a half days, and in the pupal
stage seven days, while the workers remain in the larval stage five days,
and in the pupal stage thirteen. The drones remain in the larval stage
six days, and in the pupal stage fifteen days.

THE HONEY BEE 363

During the fourth day the larva hatches from the egg as a white,
footless, soft, grub-like form floating in "bee-milk," also called "royal
jelly." This "milk" is composed of digested honey and pollen with
probably some glandular secretions. The "milk" is formed in the true
stomachs of special "nurse" workers who place it in the cells.

All larvae are fed this royal jelly for about three days by the nurse
workers, but then a change takes place. Those which are to become
workers are fed honey and digested pollen, while those which are to
become queens alone continue to get the richer royal jelly until they
change to the pupal stage. The drone larvae, after the fourth day, re-
ceive undigested pollen and honey.

The young larvae grow rapidly and shed their exoskeleton several
times. In fact, during the last molt, even the lining of the alimentary
canal and all its contents is shed with the exoskeleton.
y>; Some five or six days after hatching, the nurse worker places a
quantity of food in the cell with the larva and places a cap on the cell.
The larva spins a cocoon of silk about itself some two or three days later.
It is now in a resting stage and is called the pupa.

The spinning-glands are in the mouth region, and later become the
salivary glands of the adult.

Almost the entire structure is made over during this pupal stage
and the full-fledged bee emerges in its adult form and shape.

BEHAVIOR

As the queen emerges from the pupal stage the eggs have not yet
distended her abdomen, so she is about the same size as a worker. As
soon as she becomes accustomed to her surroundings she starts on a
hunt for other queen cells. She breaks through these and stings the
pupa within or tears the cell down and lets the workers remove such
destroyed structures with the other debris. There is thus only one queen
left. It is after this time that the nuptial-flight, already mentioned, takes
place. By the ninth or tenth day she is busy laying eggs. The number
of eggs laid, or at least the rapidity with which eggs are laid, is deter-
mined by the amount of food the workers bring home. More eggs are
laid when more food is obtained.

The workers, when young, act as nurse maids for a week or two
before taking up the regular duties of gathering food. Some of these
also defend the hive against outside attacks, clean the hive, and even go
scouting to find suitable new quarters before swarming.

The workers really work themselves to death, and probably live
only some five or six weeks. New ones are being hatched continually
to keep the normal number of bees in the hive. Those which hatch in
the fall may live five or six months.

If a queen should die, any one of the workers may with proper

364 GENERAL BIOLOGY

feeding, be able to develop and lay eggs, but in such cases the new queen
would not have had the nuptial-flight, and therefore no eggs would be
•fertile. Consequently drones alone are hatched from the eggs. 'V*J*::

Drones hatch in the same way that queens and workers do, but take
no part in the work of the hive. One of them alone acts as queen-con-
sort. As soon as food is scarce they are starved to death and their dead
bodies removed with the remaining debris. At such a time even the
drone pupae, larvae, and eggs are destroyed.

As new bees are constantly being hatched, the hive may become
cvercrowded. When this occurs it is the old queen which collects several
thousand bees about her and goes through a complicated preparation to
start a new colony. Scouts are sent out to seek a fitting location,
and after first settling on a tree-branch or other object in a very dense
cluster, the whole colony takes up its new abode.

The cells are made of wax. Those \vhich are to have eggs placed
in them are hexagonal in shape, although a careful examination will
show they all vary slightly from each other. The cells which are to
contain honey are rounded.

The wax is produced by a secretion from the smooth, paired patches
called wax-glands on the ventral surface of the abdominal metameres.
The process gone through is as follows: The bees gorge themselves
with honey. Great clusters of such bees then hang from the top of the
hive for several hours when thin scales of wax form on the plates.
These scales of wax are then removed by the hind legs, while the fore-
legs transport it to the mouth. Here the wax scales are mixed with
saliva and kneaded by the mandibles. The wax is then ready either to
repair old cells or build new ones.

The cells may be built especially for honey or for breeding, but
often drone cells, even when the cocoon is still present, are used for
honey cells. However, cells made especially for honey have the open-
ings somewhat above their bases so that the honey will not run "but.

The cells which fasten the comb to the top and sides of the hive are
called attachment cells.

Bees gather nectar (not honey) from flowers. The maxillae and
the labial palpi form a tube through which the tongue can move back-
ward and forward. As the epipharynx is lowered, a definite passag'e
connects this tube with the oesophagus. The nectar itself becomes at-
tached to the hairs on the tongue, and is forced upward by pressing
maxillae and palpi together. It is then swallowed into the honey-sac,
where the necessary chemical changes which convert it into honey take
place. Here it is retained until the bee reaches the hive, when it is re-
gurgitated into the cells made to receive it. As there is much water
'contained in the newly-formed honey, the cells are left open until the
water is considerably evaporated. This is called the "ripening process."
'When the honey is "ripe" the cell is capped with wax.

THE HONEY BEE 365

The bees keep their wings moving while in the hive both to keep
air circulating and (in winter) to produce heat.

About thirty to fifty pounds of honey are produced a season by one
hive if conditions are favorable.

As honey lacks proteins, bees gather pollen by means of their mouth
parts and legs, and mix it with either saliva or even nectar to make it
sticky. It is then placed by the hind legs in the pollen baskets. As
the bee enters the hive, it backs up to a cell in which a larva is placed,
and scrapes the pollen into such cell by aid of the spur already men-
tioned. The deposited substance is known as "bee-bread." The young
workers then pack this bee-bread into the cells by using their heads as
tampers.

Still another substance known as propolis or "bee-glue" is gathered
by bees for the purpose of filling up cracks, for strengthening weak
parts, or even, probably, as a sort of varnish. Propolis is merely the
resinous material gathered from various plants which is then inserted
into the pollen basket. In the case of propolis, another worker removes
it from the gatherer, and it is this other worker \vhich also applies it
where needed.

In warm, dry weather, water is often sucked into the honey-sac from
dew, brooks, or ponds, and then carried to the larvae in the hive. In
cool weather enough water usually condenses in the hive. In fact, so
much moisture may condense as to injure the occupants.

All debris is removed immediately, so that cleanliness is insured.

ENEMIES OF THE HONEY BEE

There is a Bee-Moth, Galleria mellonella, which, when it can find
an entry, lays its eggs in the hive. The larvae then feed on pollen
cocoons and even cast-off larval skins. They burrow into the comb and
line their burrow with a silk which protects them from the bees, much
as a spider's web can either keep out or entrap insects.

There are also bee-lice which attach themselves to the queen and
weaken her by sucking the juices from her body. The bee lice, while
common along the Mediterranean Sea, are uncommon in America.
Spiders often catch bees in their webs.

Other insects such as dragon-flies, ants, and wasps may attack bees.
Toads and lizards also attack them, but these latter can be removed to
some distance from the hive and will then serve the important function
of devouring really noxious insects.

Mice prey upon pollen, honey, and even bees in the winter time.
One may note here, as we have already noted in the relation of insects
to man, that there may be various ways of insuring a "balance in nature.'*
As cats devour mice, and- mice bees, the number of cats may be the de-
ciding factor of the number of bees there are in a given neighborhood.
In fact, Huxley even suggested that this idea could be carried still fur-

366

GENERAL BIOLOGY

ther by considering- the number of old maids who were fond of cats,
these cat lovers then becoming the deciding factor as to the number of
bees a given region might have.

Various diseases afflict bees, probably largely of a bacterial nature
brought about by too long confinement in the hive. Once a disease has
taken hold of a hive it may infect any or all other hives in the region.

GYNANDROMORPHS

It has been found that among butterflies, ants and bees, it is not un-
common to have an abnormal individual which has male characteristics
in one part of its body and female
characteristics in another. The term
gynandromorphs (Fig. 238) has been
given such individuals. The more
common form such gynandromorphs
assume is that of the anterior part of
the body being one type and the pos-
terior another, or the entire right side
may be of one sex and the entire left
side another.

Bees are particularly valuable in
bringing about cross fertilization of
flowers. In fact, the Bumble Bee is
about the only insect visiting red clover
which has its mouth parts long enough
to reach down for the nectar of that
plant, so that if it were not for the bum-
ble bee, red clover would probably not
grow at all.

Fig. 239.

Salvia ap. (One of the Labiatse). a.,
flower bud ; b-f., various views of the open
flower ; an., anther ; st., stigma ; *., projec-
tions near the base of the filaments. The lead
pencil is made to imitate an insect visiting
the flower for pollen. By pressure at the base
of the filaments, the anthers are brought into
contact with the surface of the pencil, which
thus becomes covered with pollen. When the
next flower is visited the stigma, having then
bent down and spread apart, receives the
pollen from the other flower. Thus is ac-
complished cross-pollination. In 6., before the
visit of the insect, the stigmatic surfaces are
still in contact, so that pollination is not pos-
sible. (From C. Stuart Gager's "Funda-
mentals of Botany" by permission of P. Blak-
iston's Son & Co., Publishers.)

Fig. 238.

External appearance of gynandromorph.
Lateral hermaphroditism of gypsy moth. Left
side female; right side male. (After Tasch-
enberg. )

Orchards which have hives of bees usually show a better harvest
of fruit than those without hives.

THE HONEY BEE 367

It is probably color, odor, and the structure of both insect and plant
which determine which plants are visited most.

Many plants are so constructed that an insect entering the flower
for nectar conies in contact with the pollen of the plant which thus
brushes off on the insect's back (Fig. 239). Then as another flower
is visited this pollen is brushed off by the stigma thus bringing about
fertilization.

CLASSIFICATION

The Summary of the Arthropoda will show under what phylum,
class and order bees are classified. But here it is necessary to mention
the following five types of honey bees found in the United States, though
none are native.

German, with black-colored abdomen. These are the so-called wild
honey bees.

Italian, with yellow-striped abdomen.

Carniolan, with gray abdomen.

Cyprian, with yellow abdomen.

Caucasian, with yellow-gray abdomen.

All bees are included in the great family Apidae, but there are both
solitary and social species. Then, too, some are miners, carpenters, leaf-
cutters, etc.

As different species of bees have different length of tongues their
food must vary accordingly. This was seen in our discussion of the
Bumble Bee, which alone of all the bees, has a long enough tongue to
obtain the nectar from red clover. Short-tongued bees must seek a
flower with a less deeply placed nectar.

The list of books at the end of this chapter will furnish many chap-
ters of interest as to the more detailed life and habits of all species of
bees.

THE FLY

As flies may carry "tuberculosis, cholera, enteritis (including epi-
demic dysentery and cholera infantum — the fly-time 'summer complaint'
of infants), spinal meningitis, bubonic plague, smallpox, leprosy,
syphillis, gonorrhea, ophthalmia, and the eggs of tapeworms, hookworm,
and a number of other parasitic worms," they are certainly worthy of
our attention, and should be thought of here, although it must not be
thought that flies are the only carriers of these diseases. This is
especially interesting when it is noted that while only about two persons
die each year in the United States from the bites of poisonous snakes,
about one hundred from the bites of rabid dogs, nearly 100,000 die an-
nually from diseases carried by flies.

There are more than 43,000 different kinds of flies, gnats and mos-
quitoes which have been described in entomological literature, and there
is no telling how many more are still unknown. Tachina flies (Fig. 240),

368

GENERAL BIOLOGY

already described as killing grasshoppers, and Syrphus flies ( )

feed on insects and are therefore of value to man, but nearly all others
should be exterminated. Over ninety per cent of the flies found in and
about homes are the regular typhoid flies. When it is remembered that
the feet of these are furnished with claws for climbing over rough sur-
faces as well as with two pads, the pulvilli, covered with sticky, tubular
hairs by which the animal can attach itself to ceilings and glass surfaces,
one can understand the excellent summing up of what this means — that
"No more effective mechanisms for collecting dust could be designed

Fig. 240.

The Friend of Farmers. Red-tailed tachina-fly (Winthemia 4-pustulata.) a.,
natural size ; b., much enlarged ; c., army worm on which fly has laid eggs, natural
size; d., same, much enlarged. (After S. Singerland.)

than a fly's feet and proboscis (Fig. 216), a combination of six feather
dusters and thirteen damp sponges. While the constant 'cleaning' move-
ments of flies are clearly designed to rub off and scatter the adhering
germs everywhere they go."

There are "little house flies" (Fannia canicularis) which probably
most people believe grow into the regular house fly. Their breeding
habits and feeding places are quite similar to the house fly, but, as flies
hatch in the adult form they do not grow after once becoming flies.

Other flies such as bluebottles, greenbottles, and flesh flies or blow-
flies are also found about the home and frequently lay their eggs on meat.
These flies are scavengers.

In the South there is the screw-worm fly (Chrysomyia macellaria)
which deposits its eggs on wounds, for the maggots of this species feed
on living flesh. It is these flies also which are likely to lay their eggs
in the nostrils and ears of children or even of adults as they sleep out of

THE HONEY BEE

369'

doors. The maggots then cause intense pain as they feed on the sur-
rounding flesh.

The stable fly (Stomoxys calcitrans) looking something like a
housefly, except that it has a strong piercing beak, sucks blood from ani-
mals. It is supposed to be the animal which carries the germs of infan-
tile paralysis in addition to the injury it causes cattle.

The smaller horn fly (Haematobia serrata) swarms about the bases
of the horns of cattle, biting constantly.

ii.

in.

Fig. 241.

I. Typhoid fever or house-fly (Mus'ca domes' tica :) a, Adult male; b., pro-
boscis and palpus of same ; c., terminal joints of antennse ; d., head of female ; e.,
puparium ; /., anterior spiracle; all enlarged. (Howard and Marlatt, Bull. U. S..
Dept. of Agriculture, 1896.)

II. Metamorphosis of Saw-Fly.

III. Tsetse fly, which causes a disease of cattle in Africa, enlarged. (L..
O. Howard.)

IV. Larvae of bot flies attached to the walls of the stomach of a horse. (After-
Osborn. )

LIFE HISTORY */

Flies (Fig. 241) breed about filth, and any decaying matter, though
they can breed and do in any wet, fermenting vegetable or animal mat-
ter. The maggots are hard to kill; they will live in pure kerosene for
over an hour, and even more than thirty minutes in alcohol. They have

370 GENERAL BIOLOGY

even been bred from the open boxes of snuff on a druggist's counter,
though tobacco is supposed to be quite injurious to insects.

After the housefly's eggs are laid it takes about eight hours for them
to hatch into maggots. These finish their growth in six to seven days,
burrowing into the ground "under the manure pile" (hence the need of
concrete floors) and transform into brown puparia, from which they
emerge as adult flies in three days.

Hodge and Dawson have summed up the rapid increase in flies most
tellingly in the following words :

"After coming out as adults they fly about over an area not gen-
erally more than one thousand yards in diameter, and feed and drink
from two hundred to three hundred times a day for from ten to fourteen
days before maturing their first batch of eggs. This actually delivers
the enemy into our hands. It means that, with flytraps on every garbage
can and swill barrel, and with everything most attractive to flies very
carefully kept in these receptacles, not a single fly will succeed in feed-
ing for two weeks without getting caught. In this case no more eggs
will be laid, and the pests will vanish.

"Allowing ten days of feeding between emergence and oviposition,
figuring that a fly lays one hundred and fifty eggs at a batch and lives
to lay six batches, compute the increase of a pair of flies beginning to. lay
May 1. Half the progeny are supposed to be females. Test the follow-
ing figures :

May 10 152 flies.

May 20 302 flies.

May 30 11,702 flies.

June 10 34,302 flies.

June 20 911,952 flies.

June 30 6,484,700 flies.

July 10 72,280,800 flies.

July 20 325,633,300 flies.

July 30 5,746,670,500 flies.

"As this last amount makes 143,675 bushels of flies resulting from
a single pair of flies in three months, one can estimate what the result
will be if allowed to breed unrestrained during August and September
beside.

"The common sense question, then, is, why not let this pair of flies
catch themselves in May? This rapid increase also means that anything
short of extermination is hardly worth the effort. A fly is possessed of
no more cunning than shot rolling down a board, and the last pair will
run into a trap as easily as the first. Why not let them all catch them-
selves?"

During the winter, especially in cold climes, most of the flies are

THE HONEY BEE 371

killed, but probably some maggots pass the winter underground and in
stables where it is sufficiently warm, coming forth in the spring when
the weather warms up.

It has often been assumed that burying debris of various kinds would
kill the maggots. This is not true, as the maggots have crawled up
through six feet of earth, with which they were covered.

The best method of handling debris, such as manure, is to spread it
on the land daily. This is especially valuable, as manure loses almost
half its fertilizing power if stored. The sun will dry it and this will also
prevent the moisture which maggots need in order to thrive. However,
if this cannot be done, then a solution of iron sulphate (copperas), two
pounds to the gallon of water, may be thrown over such matter. Chloride
of lime is expensive and the fumes (chlorine) are likely to injure the farm
animals.

FLY KILLERS

The Kansas Board of Health Bulletin gives the following methods
of killing flies :

"A cheap and perfectly reliable fly poison, one which is not danger-
ous to human life, is bichromate of potash in solution. Dissolve one
dram, which can be bought at any drug store, in two ounces of water,
and add a little sugar. Put some of this solution in shallow dishes and
distribute them about the house."

"One of the best fly killers that can be used in the home is a tea-
spoonful of formalin in a quarter of a pint of water. When this is ex-
posed in a room it will be sufficient to kill all flies. They seem to be fond
of the water. Care should be taken to place it beyond the reach of chil-
dren."

"To quickly clean a room where there are many flies, burn pyreth-
rum powder. This stupefies the flies, when they may be swept up and
burned."

And the Agricultural Extension Department of the International
Harvester Company suggests the following ointments and sprays to keep
flies away from cattle :

(Any of the following must be applied frequently, as few will keep
flies away for more than a day or two following their application.)

One pound rancid lard, ^ pint kerosene.

Mix until a creamy mass forms. Best applied with cloth or with bare
hand. Rub thinly over the backs of the cows.

Three parts fish oil, one part kerosene. Apply with small spray
pump. . '""•

Two parts crude cottonseed oil or fish oil, one pint pine tar. Apply
with large paint brush.

372 GENERAL BIOLOGY

PARASITIC INSECTS

"Among the insects there are many kinds that live parasitically for
part of their life, and not a few that live as parasites for their whole life.
The true sucking lice and the bird lice live for their whole lives as exter-
nal parasites on the bodies of their host, but they are not fixed that is,

they retain their legs and power of locomotion, although they have lost

IT.

III.
Fig. 242.

I. Ichneumon-fly. Natural size.

II. Thalessa boring in an ash tree to deposit its eggs in the
burrow of a horntail larva, a wood borer. From photograph,
natural size. (After Dsrvison.)

III. Corn root aphis (Aphis maidiradicis) , winged and wing-
less female. The two black processes at the rear are Cornicles.
(From Needham's "General Biology" by permission The Comstock
Pub. Co.)

THE HONEY BEE 373

their wings through degeneration. The eggs of the lice are deposited on
the hair of the mammal or bird that serves as host ; the young hatch and
immediately begin life as parasites, either sucking the blood or feeding
on the hair and feathers of the host. In the order Hymenoptera there
are several families, all of whose members live during their larval stage
as parasites. We may call these hymenopterous parasites, ichneumon
( ) flies. (Fig. 242.) The ichneumon flies are par-

asites on other insects, especially of the larvae of beetles and moths and
butterflies. In fact, the ichneumon flies do more to keep in check the
increase of injurious and destructive caterpillars than do all our artificial
remedies for these pests. The adult ichneumon fly is four-winged and
lives an active, independent life. It lays its eggs either in or on or near
some caterpillar or beetle grub, and the young ichneumon, when hatched,
burrows into the body of its host, feeding on its tissues, but not attack-
ing such organs as the heart and nervous ganglia, w^hose injury might
mean immediate death to the host. The caterpillar lives with the ichneu-
mon grub within it, usually until nearly time for its pupation. In many
instances, indeed, it pupates with the parasite still feeding within its
body, but it never comes to maturity. The larval ichneumon fly pupates
either w'ithin the body of its host or in a tiny silken cocoon outside of
its body. From the cocoons the adult winged ichneumon flies emerge,
and after mating find another host on whose body to lay their eggs."

As an example of a parasite living upon another parasite, though
one of these uses a tree as its host, the remarkable ichneumon fly
Thalessa (Fig. 242) is an excellent example. This animal, which has a
very long, slender, flexible ovipositor, finds a spot in a tree \vhere the
insect Tremex columba ( ), commonly called the

pigeon horntail, has deposited its eggs about a half inch below the sur-
face of a growing tree. When these eggs are converted into larva, the
larva digs still deeper into the tree, filling up the open space behind it
with tiny chips. Through a very extraordinary instinct the Thalessa
finds the spot opposite where the Tremex larva lies and "elevating her
long ovipositor in a loop over her back, with its tip on the bark of the
tree, she makes a derrick out of her body and proceeds with great skill
and precision to drill a hole into the tree. When the Tremex burrow is
reached she deposits an egg in it. The larva that hatches from this egg
creeps along this burrow until it reaches its victim, and then fastens itself
to the horntail larva, which it destroys by sucking its blood. The larva
of Thalessa, when full grown, changes to a pupa within the burrow of
its host, and the adult gnaws a hole out through the bark if it does not
find the hole already made by the tremex."

Practically all the mites ( ) and ticks ( ),

animals closely allied to the spiders, live parasitically.

374 GENERAL BIOLOGY

Truly Dean Swift was right when he said :

"Great fleas have little fleas

Upon their backs to bite 'em,
And little fleas have lesser fleas,
And so ad infinitum."

References.

Sanderson and Jackson's "Elementary Entomology."
J. Arthur Thomson, "Outlines of Zoology."
Linville and Kelly, "A Text-book in General Zoology."
Leland O. Howard, "The Insect Book."
Vernon L. Kellogg, "American Insects."
Robert W. Hegner, "College Zoology."
J. H. Comstock, "Insect Life."

J. H. and A. B. Comstock, "A Manual for the Study of Insects."
A. S. Pearse, "General Zoology."
C. A. E'aland, "Insects and Man."

E. Dwight Sanderson, "Insect Pests of Farm, Garden and Orchard."
L. S. and M. C. Daugherty, "Principles of Economic Zoology."
Joseph Lane Hancock, "Nature Sketches in Temperate America."
Riley and Johannsen, "Handbook of Medical Entomology."
Jordan and Kellogg, "Animal Life."
Jordan and Kellogg, "Evolution and Animal Life."
Riley, "Destructive Locusts." U. S. Department of Agriculture,
Bulletin No. 25, 1891.

C. F. Hodge and S. Dawson, "Civic Biology."

James A. Nelson, "The Embryology of the Honey Bee."

CHAPTER XXV.

THE HISTORY OF BIOLOGY

It is generally conceded that those who have been with a business
organization throughout its growth period know most about that busi-
ness. Such men not only understand a thousand details of the work
that others do not, but they know why they do what they do. The
same truth holds good in science. But as none of us was present when
science began, the only way we can obtain such an understanding is to
read the story of those who were present ; as a consequence, the history
of any branch of science becomes an important study in the college cur-
riculum.

In reading history we are always inclined to pass some sort of a
judgment on the characters there found. This judgment is, however,
quite likely to prove erroneous, unless we first know something of the
times in which they lived, the obstacles they had to overcome, and the
reasons they had for beginning work in new fields.

We must weigh the evidence on all sides of a question very care-
fully, so as not to confuse conspicuousness with importance. For exam-
ple, an inventor is likely to be widely known because men at large can
see, use, and understand his invention ; but, as soon as another inventor
improves, or brings about another apparatus which takes the place of the
first invention, the first inventor ceases to interest men, and is then soon
forgotten.

Such a state does not apply to the real scientist — the discoverer of
a new principle — for, every invention and every application that his
principle brings about in future time, proves that principle to be just so
much the more important, and causes the scientist to be considered
greater and greater through onflowing years.

It is therefore the real scientists, the true originators and discov-
erers of principles who must be known and honored.

First, then, let us try to catch a glimpse of the times in which men
of past ages worked.

From the very earliest period of which we possess records, men have
been interested in agriculture and medicine — which means, botany and
zoology. Botany, in so far as a practical knowledge of food-plants was
essential to successful agriculture, and in so far as a practical knowledge
of medicinal plants was essential for the health of man and his animal
servants. Zoology, in so far as a practical knowledge of the breeding of
cattle and sheep was essential to a successful livelihood, and in so far
as a knowledge of the human body was essential to prevent wounded
men from bleeding to death.

376 GENERAL BIOLOGY

Aristotle (384-322 B. C.), who was the pupil of Plato, was one of
the first men to think of botany and zoology as a definite branch of
study. His great contribution to biology was that nature worked by
definite fixed laws — what we now call the law of continuity.

This discovery is intensely important because it made experimental
science possible. There would be but little use in spending months and
years in attempting to prove anything, if the laws of nature worked
differently at different times, under the same conditions ; for, the real
value of experimenting is found in one's ability to prophesy that the
same result will always take place if the same experiment is performed
under the same conditions.

The first mark of a true scientist is accurate observation and perfect
description, and the second is the power of visualization, by which he
can build up and mold his interpretations into a principle.

Aristotle had a mind of the highest type, and so his generalizations
still hold good after a lapse of thousands of years, provided, always, that
his facts were correct. He did not have the instruments for accurate
observation that we now have, so he often had to take many things .for
granted which have since been proved erroneous. But, his logic never
failed him when his facts were right.

Theophrastus (370-286 B. C.) laid the foundations of botany. The
astounding point that meets one in the reading of these old philosophers
is that they were able to work out so great an amount of detail with
the poor equipment they had, when we, with all our modern improved
apparatus, must search most diligently before we can accomplish the
same results.

As medical men were the first workers in biology proper, Hip-
pocrates (460-370 B. C.), the Father of Medicine, must be mentioned.
He made medicine into a separate science and set forth the ideals of the
medical man which are still an inspiration to all.

Dioscorides (about 64 A. D.), an army surgeon under Nero, and
Galen (131-201 A. D.), physician to Marcus Aurelius and his son Com-
modus, were both Greek physicians. The former originated the pharma-
copoeia, which was the standard textbook of botany for some fifteen
centuries. The latter wrote an anatomy and physiology which also was
a standard textbook for medical students for the same length of time.

Pliny the Elder (23-79 A. D.) wrote a book which, although sup-
posed to be accurate, had fact and fancy blended to so considerable an
extent that it is hard to separate them.

The men mentioned above are the only biological workers of whom
we have any record up to the time Christianity began to function.

The Roman Empire was mistress of the World at this time, and
pleasure was the Roman ideal. Christianity strenuously opposed such
an ideal, and soon won Emperor and people to its side. The moment
this occurred, all efforts on the part of both student and soldier were

THE HISTORY OF BIOLOGY 377

directed toward performing such acts as would bring glory to the God
they had accepted. And, as always, when the ideal of a nation is thrown
aside, the pendulum swings completely over to the other side. Conse-
quently, after Christianity was adopted, suffering, from having been
considered a burden and a nuisance to men who held pleasure as their
ideal, became something to be endured and practically enjoyed, inas-
much as he who suffered wras thus imitating in some small measure the
sufferings of the founder of Christianity. It follows that no great im-
petus was given to work that had for its object the relief of physical
discomforts. At this time, also, barbarian hordes were a constant
menace, and wars and rumors of wars not only kept men in the field,
but forced all energy to be directed toward the end of setting up some
kind of military and defensive stability. And, while many scientific ap-
plications are produced for destructive purposes in war, there can be no
true science at such time. Little serious studious work can be accom-
plished unless there is leisure and freedom from danger.

At this time there were only two fields of work in which a youth
of ambition might enter — the army and the Church. The first attracted
men who sought physical power, while the second attracted those who
sought knowledge.

The Church therefore established universities and libraries in the
monasteries — the only place where one could find men interested in
learning. It was here that the works of the great writers of antiquity
were preserved and used during the times when wars were not being
waged.

Even during these trying times some of the monks compiled animal
stories which were, however, concerned principally with pointing out
a moral. Such stories \vere collected in book form and became known
as the Physiologus. The Physiologus in turn developed into another
book of similar import called the Bestiaries, while on the botanical side
a book, which may be compared with the Bestiaries, was the Hortus
Sanitatis.

Later, another botanical work appeared, called the Herbals.

In the thirteenth century, Europe became somewhat settled. There
was then sufficient leisure and safety to permit men to take up a studious
life. The fame of the great scholars of that day spread rapidly. Every-
where studious men sought whatever books they could find, and read
them. Printing had not yet been invented, so it was only in the monas-
tery libraries that books (written by hand) could be found. These were
read with avidity, and much which had lain neglected during the years
of war and turmoil now was made known to the new generation. This
period from about 1250 to 1500 is therefore called the Renaissance or
Re-birth Period.

During the thirteenth century, the Dominican Monk Albertus Mag-
nus (1193-1280), began working on physical experiments while the

378 GENERAL BIOLOGY

Dominican Thomas Aquinas (1225-1274) began to collect and coordinate
all the scientific and philosophical knowledge of his day.

Following these came the Franciscan Monk, Roger Bacon (1214-
1294), the real father of modern science. Among his many writings
we find the first clear and unmistakable statements from which our
knowledge of modern lenses date. His work is like a modern mono-
graph in that it gives recognition to the opinions of others.

The old Romans had, it is true, used pieces of glass with water in
between for magnifying purposes, but it was Bacon who set men on
the right path regarding true observation, description, and the use of
modern laboratory instruments.

Gesner (1516-1565) wrote his Historia Animalium in several vol-
umes between 1551 and 1587, which was widely read, although he had
but little influence on successive generations.

The next truly great name in the history of biology is that of
Vesalius (1514-1564). He wrote the De Human! Corporis Fabrica in
1543. Up to this time the surgeon would not soil his hands by touching
and cutting the body. Such work was left for barbers, who performed
their dissections and operations under the direction of the surgeon.
Vesalius dissected with his own hands, and then described and pictured
what he found. Vesalius' old master, Jacobus Sylvius, was a strenuous
opponent of his pupil, as was also Vesalius' own pupil, Columbus. How-
ever, another pupil of Vesalius, who later became his successor at the
University of Padua, was Fallopius (1523-1562), who built upon the
work of his master.

Harvey (1578-1657) in 1628 published his Excercitatio Anatomica
de Motu Cordis et Sanguinis in Animalibus in which he showed con-
clusively that the blood flows in a circle from the heart throughout the
blood-vessels and back again to the heart.

In about 1600 compound microscopes were invented, and it is from
this time forward that the great microscopical discoveries were made
which have changed our modern conception of many ancient problems.

Robert Hooke (1635-1703) wrote his Micrographia in 1665 in which
he called attention to the "little boxes or cells" of which plants are com-
posed. It is he, therefore, who gave us our first notion of the cell.

The next important name is that of Van Leeuwenhoek (1632-1723)
who first saw Bacteria, Infusoria, Yeast, Rotifers, Hydra, and a host of
other organisms which were totally unknown up to his time. His work,
which attracted most attention in the scientific world, however, \vas his
description of spermatozoa. His imagination ran away with him, for he
was sure he saw definitely-formed tiny human beings in the spermatozoa.
A great conflict was waged by those who agreed with him and those who
opposed Van Leeuwenhoek and his school (the spermists), insisting that
it was the sperm which was the all important factor in producing life,

THE HISTORY OF BIOLOGY 379

while his opponents (the ovists) insisted that it was the egg which de-
veloped into new offspring.

Swammerdam (1637-1680), in his Biblia Naturae, compiled long and
painstaking researches on the anatomy of insects which, up to his time,
were considered unorganized physical masses.

Malpighi (1628-1694) of Bologna worked on plants and animals. He
made elaborate studies and illustrated them, on the development of the
plant-embryo, as well as on the embryology of the chick, the anatomy of
the silk-worm, and the structure of glands.

Chronologically, the systematists should be mentioned at this point,
but logically, it is better to introduce the student to the whole subject
of Classification and the men who did the classifying at the same time,
so this subject will be given in the next chapter.

However, as soon as there is any considerable classification and de-
scription of a subject, men begin to divide that subject into individual
parts or units, so that workers may narrow their field and confine their
work to such limited group or unit.

Comparative Anatomy, Physiology, Histology, Embryology, Genet-
ics, and Organic Evolution, are the main divisions into which Biology is
thus divided.

The work done by first-year students of biology, as set forth in this
book, consists of studying a type-form of the principal phyla of plants
and animals, and then attempting to develop biological principles from
the knowledge thus gained. This first-year work therefore includes the
fundamentals of Botany and Zoology. The Third Semester's work is
confined to the specialized study of Embryology, and the Fourth Semes-
ter's work is Comparative Anatomy and Physiology. In this last Semes-
ter's work the student studies in detail each organ or organ-system of
the great divisions of Zoology and then compares these, system by sys-
tem.

Probably the first man to attempt this latter method was Severinus
(1580-1656) of Naples. In 1645 he published a volume suggesting that
all vertebrates and man had much in common, structurally. However,
over a century before this time Belon had made drawings of the skeletons
of birds and man and placed them side by side so that differences and
similarities could be noted. Then came Tyson (1650-1708) of Cam-
bridge, who is the father of our modern meth'od of treating comparative
findings in monograph form. His work was a comparison of man and
monkeys.

Cuvier (1769-1832) of Paris is, however, the first of the great men
in this field of work. He was the first to embrace both living and extinct
forms in his comparisons, and he also obtained a wider grasp of the
problem confronting him than any of his predecessors. A good illustra-
tion of the synthesis sought for, and the breadth of knowledge desired

380 GENERAL BIOLOGY

in this department of research, can be found in his famous statement,
"Give me a tooth, and I will construct the whole animal."

This is the key-note to comparative study. It means that every
change in function modifies a structure, and that if we can know thor-
oughly all there can be known about function and its effect on structure,
and every change in one structure which may change a related structure,
we can, if we are given a structure, tell what the functions must have
been, and vice versa.

There are men who were lesser lights in the field of Comparative
Anatomy even before Cuvier's time, whose names it is well to know.
John Hunter (1728-1793), who founded the Hunterian Collection in
England; Camper (1722-1789) of Groningen, and Vicq d'Azyr (1748-
1794) in Paris. All of these did synthetic work, but their breadth of
knowledge, view, and vision fell far short of that of Cuvier.

Following Cuvier came Milne-Edwards and Lacaze-Duthiers in
France; Meckel, Rathke, Johannes Muller, and Gegenbaur in Germany;
Owen and Huxley in England ; Aggassiz, Cope, and Marsh in America.
When men once became interested in the great structural problems of
Zoology it was but natural that others should become interested in those
that were functional. Here was the birth of modern physiology. The
medical men were the first to do work in these fields. They established
systems of thought known as the iatro-mechanical and iatro-chemical
schools.

Haller (1708-1777) took the work of these men, surveyed it and
evaluated it, so that he may really be called the father of modern
physiology.

The first work in this new field was done on nutrition and respira-
tion. Reaumur (1683-1757) of Paris, and the Abbe Spallanzani (1729-
1799) of Pavia, did the most remarkable work in this field, although they
had forerunners on whose work they built in turn.

Such forerunners were van Helmont (1577-1644), Sylvius (1614-
1672). Bishop Stensen (1638-1686), de Graft7 (1641-1673), Peyer (1653-
1712), and Brunner (1653-1727).

The great names in chemistry whose work affected biological stu-
dents are primarily Boyle (1627-1691), Priestley (1732-1804), Lavoisier
(1743-1794).

In physiology proper the greatest names in Germany are: Liebig
(1803-1873), Wohler (1800-1882), the brothers Weber (E. H., 1795-1878,
and W. E., 1804-1891), Ludwig (1816-1895), Helmholtz (1821-1894,
Johannes Muller (1801-1858), and du Bois-Reymond (1818-1896). In
France, Dumas (1800-1884), Magendie (1783-1889), and in England, Hall
(1790-1857). The greatest of the physiologists is undoubtedly Johannes
Muller.

In Botanical physiology, Hale (1677-1761), is the greatest, while

THE HISTORY OF BIOLOGY 3S1

Cesalpino (1519-1603), Jung (1587-1657), and van Helmont (1577-1644),
occupy high places.

Ingen-Housz (1730-1799) was the first to show that carbon dioxide
from the air is broken down in the leaf when the plant receives sunlight,
and that the carbon is retained which thus assists materially in nutrition
and growth.

De Saussure (1740-1799) showed further that water and various salts
from the soil produced the remaining factors in this process, while Bous-
singault (1802-1887) gave us our knowledge of chlorophyl.

Haller and van Leeuwenhoek were what is called pre-formationists.
They supposed that each sperm or egg-cell already contained an embryo
somewhat fully formed, and that all that occurred during the growth
period was an enlarging of the parts which were already present. Such
an idea meant that every human germ-cell must have every other com-
plete human being that could ever descend from it, within itself, fully
formed, but very small. We know now that both those who held this
theory and those who opposed it were wrong. There must, of course,
be a potentiality present in each germ-cell which .can develop into what
it is to become, but this by no means signifies that the embryo possesses
a definitely formed embryo within it in turn. The new embryo is always
organized little by little until it becomes the completed individual adult
organism.

However, it is natural to see how and why observers thought they
saw the complete embryo in the egg. In our study of Embryology we
shall see that when the hen lays an egg, it is already from twenty-four
to thirty-six hours old, and consequently, even when wre have a freshly
laid egg (provided it is fertile) there is already an embryo which can be
seen. It was with material of this kind that these men had to work.

Wolff (1733-1794) had proved that the performationists were in
error, but Haller, who held the intellectual reins of workers in zoology at
the time, refused to accept it and so the lesser lights also refused.

It was but natural that after Hooke had observed that plants were
composed of cells that something should be done with such a finding.
Brown (1773-1858), working on the cell, discovered the cell nucleus in
1831, and the botanist Schleiden (1804-1881), and the zoologist Schwann
(1810-1882) published their works in 1838 and 1839, respectively, show-
ing that plants are developed from cells and that plants and animals are
alike in being composed of cells.

An important point was made in suggesting that each cell has two
functions : one to perform the work of itself and the other to perform a
task which makes it an integral part of a larger organism.

Schultze (1825-1874) in the early sixties established the idea of pro-
toplasm as the living substance of all cells. This protoplasm was called
by Huxley the "physical basis of Life."

In Embryology Fabricius (1537-1619) published a paper describing

382 GENERAL BIOLOGY

the sequences of development in the hen's egg up to the time of hatching.
Harvey was a pupil of Fabricius, and built upon the work of his master.
These men opposed the preformationists, and called their theory epigene-
sis — which simply means that the embryo arises by a gradual differen-
tiation of unformed material in the egg.

Malpighi in 1672 sent two important papers on Embryology to the
Royal Society, but apparently the time was not yet ripe for his work
and it was neglected for nearly a century. He stood with the epigenetic
group.

Bonnet (1720-1793) was one of the important men at this time who
threw the weight of his influence with Haller toward the preforma-
tionists.

At present embryologists hold, as was stated above, that there really
is an organization of some kind in both egg and sperm, but that no
embryonic shape has yet been established. The definite shape comes
forth only by the gradual differentiation of the unformed (but not un-
organized) matter. We may therefore say "the whole future organism
is potentially and materially implicit in the fertilized egg cell," which
means that both sides were partially right.

However, the greatest name in embryology is von Baer (1792-1876).
His work was done in the thirties of the last century. He is the father
of comparative embryology. It was he who first noted and described
cleavage, germ-layers, tissue and organ differentiation, and gave us the
well-known "recapitulation theory," now often called Haeckers "Law of
Biogenesis," on account of Haeckel's popularization of it. It will be
remembered that this theory holds that embryos pass through the adult
stages of the race to which they belong.

The origin of life has always been an interesting speculative subject
for thinking men, and many and mysterious are the ways in which life
was supposed to spring forth spontaneously. Aristotle thought that mice
developed from the river's mud, while later writers suggested that old
rags and cheese combined in a dark cellar would produce the same result.
The history of this subject makes more than fascinating reading.

Francesco Redi (1626-1698) was probably the first man to demon-
strate experimentally that life did not spring forth spontaneously as com-
monly supposed. He placed very thin cloth over a dish containing de-
caying meat and found that when flies were thus prevented from coming
in contact with the meat, no maggots formed, although maggots were
always supposed to arise spontaneously from decaying meat. But Redi
himself found parasites of various kinds within the bodies of other ani-
mals, and these he could not account for, so his experiment, while a
classic, did not settle the problem for others any more than it did for
himself. The settling of this vexed question was left for Louis Pasteur
(1822-1895), who first showed that decay was not the cause of micro-
organisms but the result of them. His experiments were made while

THE HISTORY OF BIOLOGY

383

Aristotle, 384-322 B. C.

Cuvier, 1768-1832.

Francesco Redi, 1626-1697.

Lazzaro Spallanzani, 1729-1799. Johannes Miiller, 1801-1858. Robt- Brown, 1773-1858.

Max Schultze, 1825-1874.

August Weismann, 1834-1914.

Louis Pasteur, 1822-1895.

Fig. 243.

(Aristotle and Max Schultze, from Needham's "General Biology" by permission
of The Comstock Publishing Co., Publishers. Pasteur, from G. Stuart Gager's
"Fundamentals of Botany" by permission of P. Blakiston's Sons & Co., Publishers.
Remaining photographs from Wm. A. Locy's "Biology and its Makers" by per-
mission of Henry Holt & Co., Publishers.)

384 GENERAL BIOLOGY

working on fermentation problems, and it is from his work that all mod-
ern medicine dates, for he was the founder of the science of Bacteriology.

In Genetics or Inheritance, from a purely biological angle, August
Weismann's (1834-1914) work, The Germ Plasm, stands out prominently.
It was Weismann who called our attention to the fact that the bodily
characteristics of any individual have but little, if any, effect on succeed-
ing generations. He held that germ-plasm alone carries inheritance.. In
other words, that acquired characteristics are not likely to be inherited,
and that if we are to make any change in future generations, we must
first learn how to make a change in the germ-cells.

Francis Galton (1822-1911) gathered a great quantity of statistics
on the stature of parents and children and published the result of his
research in the eighties.

The most important name in the study of inheritance is that of the
Augustinian Monk, Johann Gregor Mendel (1822-1884), who combined
experimental breeding of plants with a thoroughly scientific philosophy
and evolved from this combination the Mendelian laws which are now
used wherever breeding experiments are performed, whether on plants
or animals.

In the field of Organic Evolution, one may find among the ancients
many thoughts which show conclusively that they were not unaware of
a gradual change from smaller beginnings to greater and more developed
products. And St. Augustine (died 604) also calls attention to the fact
that a God is the greater, the more potentialities he can enclose within a
smaller area, which potentialities can then unfold and evolve.

Among the moderns, Buffon (1707-1778), was the first to obtain a
clear inkling of geographical isolation, struggle for existence, and arti-
ficial and natural selection, and he propounded a theory of how variations
came about through environment.

Erasmus Darwin (1731-1802) wrote on changes going on in the ani-
mal world and embodied his ideas in verse.

Lamarck (1744-1829) is the most philosophical, which means the
most profound, of all the writers of the evolutionary school, as he actually
tried to explain WHY changes took place in the organic world.

Cuvier (1769-1832), who was a contemporary of Lamarck, and who
at that time held the highest attainable place in the zoological world, was
a consistent opponent of Lamarck, but Geoffrey Saint-Hilaire (1772-
1844), though never attaining the rank of Lamarck, was a staunch up-
holder of the Lamarckian principles, and Goethe (1749-1832), the famous
poet, who was also a famous scientist of his day, became a disciple of
the new doctrine.

Lyell (1797-1875), the Englishman, in the early thirties of the last
century wrote his Principles of Geology which convinced men that the
same causes now in action always had been, and that we could therefore,

THE HISTORY OF BIOLOGY 385

by studying the time it took to make present changes in the earth's sur-
face, estimate the length of time and the age of the various strata of the
earth.

With the intellectual soil prepared in this way Charles Darwin (1809-
1882), published his epoch-making book, The Origin of Species by Nat-
ural Selection, in 1859. Darwin accepted, without explaining, the fact
that variations do occur. He assumed that the origin of existing species
could be explained by accepting the fact that variations did occur, and
that nature then selected which organisms should continue to exist by
killing off those which did not inherit as many variations of a survival
value. He assumed that acquired characteristics were inheritable, and
that the struggle for existence eliminated the unfit. Darwin had spent
twenty years in gathering the facts on which he based his theory, but
Alfred Russel Wallace (1822-1913) had reasoned out a similar theory
without having the facts that Darwin had, and it is an interesting coinci-
dence that both men were working on the same thought at the same time,
though independently. Darwin was willing to surrender all his work to
the younger man, but Wallace insisted that Darwin was to have the
credit as the latter had done such an immense amount of work on the
matter.

Evolution now serves the biological world as a sort of general plan
of the results of heredity, while genetics deals with the factors which
produce these results.

Thomas Huxley (1825-1895), though not a believer in the Darwinian
theory of Natural Selection, sprang to the defense of Darwin, primarily,
as Professor Poulton says, because Darwin was so constantly and per-
sistently treated unjustly. And it was Huxley who made Darwinism
popular. Hooker (1817-1911) in England, Haeckel (1834-1919) and
Weismann in Germany, and the Botanist Gray (1810-1888) in America,
were early converts. Haeckel, however, was too much of the showman,
and was always willing to sacrifice truth and accuracy to win his point.

Summing up what has been said, we may say that the basis of great-
ness in science is not the brilliancy of an individual discovery, but the
finding and enunciating of a principle which can find many applications
by those who follow.

The great findings, considered from this point of view of obtaining
principles and wide influence in biology, may be said to be the discovery
of protoplasm ; the establishment of the cell-theory ; the theory of organic
evolution ; the demonstration that germs are a tremendous factor in dis-
ease; and the experimental study of inheritance as suggested by the work
of Mendel and Weismann.

And the most important writings of the most important men may
be summarized here by following Professor Wm. Locy's account, which
we have modified slightly.

386 GENERAL BIOLOGY

THE MOST NOTABLE MEN AND WRITINGS IN BIOLOGY

The progress of biology has been owing to the efforts of men of very
human qualities, yet each with some special distinguishing feature of
eminence. Certain of their publications are the mile-stones of the way.
It may be worth while, therefore, in a brief recapitulation to name the
books of widest general influence in the progress of biology. Only those
publications will be mentioned that have formed the starting point of
some new movement, or have laid the foundation of some new theory.

Beginning with the revival of learning, the books of Vesalius, "De
Corpora Human! Fabrica" (1543), and Harvey, "De motu Cordis et
Sanguinis" (1628), laid the foundations of scientific method in biology.

The pioneer researches of Malpighi on the minute anatomy of plants
and animals, and on the development of the chick, best represent the
progress of investigation between Harvey and Linnaeus. The three con-
tributions referred to are those on the "Anatomy of Plants" (Anatome
Plantarum), (1675-1679) ; on the "Anatomy of the Silkworm" (De Bom-
byce, 1669) ; and on the "Development of the Chick" (De Formatione
Pulli in Ovo and De Ovo Incubato, both in 1672).

We then pass to the "Systema Naturae" (twelve editions, 1735-
1768) of Linnaeus, a work that had such wide influence in stimulating
activity in systematic botany and zoology.

Wolff's "Theoria Generationis," 1759, and his "De Formatione In-
testinorum," 1764, especially the latter, were pieces of observation mark-
ing the highest level of investigation of development prior to that of
Pander and von Baer.

Cuvier, in "Le Regne Animal," 1816, applied the principles of com-
parative anatomy to the entire animal kingdom.

The publication in 1800 of Bichat' s "Traite des Membranes" created
a new department of anatomy called histology.

Lamarck's book, "La Philosophic Zoologique," 1809, must have a
place among the great works of biology. Its influence was delayed for
more than fifty years after its publication.

The monumental work of von Baer "On Development" (Ueber
Entwicklungsgeschichte der Thiere), 1828, is an almost ideal combina-
tion of observation and conclusion in embryology.

The "Mikroscopische Untersuchungen," 1839, of Schwann marks
the foundation of the cell-theory.

The "Handbook" of Johannes Miiller (Handbuch der Physiologic
des Menschen), 1846, remains unsurpassed as to its plan and its execu-
tion.

Max Schultze in his treatise, "Ueber Muskelkoerperchen und das
was man eine Zelle zu nennen habe," 1861, established one of the most
important conceptions with which biology has been enriched, viz : the
protoplasm doctrine.

THE HISTORY OF BIOLOGY

387

Charles Darwin, 1809-1882 Alfred Russel Wallace, 1823-1913 Thomas Henry Huxley,

1825-1895

Lamarck, 1744-1829.

Johann Gregor Mendel, Hugo DeVries. 1848 —

Theodor Schwann, 1810-1882.

Karl Ernst von Baer, M. Schleiden, 1804-1881.

1792-1876.

Fig. 244.

(De Vries and Mendel, from G. Stuart Gager's "Fundamentals of Botany" by
permission of P. Blakiston's Son & Co., Publishers. Remaining photographs from
Wm. A. Locy's "Biology and its Makers" by permission of Henry Holt & Co
Publishers. )

388 GENERAL BIOLOGY

Darwin's "Origin of Species," 1859, is, from our present outlook,
the greatest classic in biology.

Pasteur's "Studies on Fermentation," 1876, is typical of the quality
of his work, though his later investigations on inoculations for the pre-
vention of hydrophobia and other maladies are of greater importance
to mankind.

Mendel's "Versuche iiber Pflanzen-Hybriden" appeared in 1865 in a
little Journal in Briinn, Austria, where Mendel was Abbot of the
Augustinian Monastery. It remained entirely unknown to the scientific
world until 1900 when three workers in the natural sciences rediscovered
it. These men were De Vries, Torrens, and Tschermak.

Mendel's work has become the foundation upon which all modern
research along genetic lines is based. Castle says, "Mendel had an ana-
lytical mind of the first order which enabled him to plan and carry
through successfully the most original and instructive series of studies
in heredity ever executed," and Bateson suggests that "had Mendel's
work come into the hands of Darwin, it is not too much to say that the
history of development of evolutionary philosophy would have been very
different from that which we have witnessed."

Weismann's "The Germ-Plasm, A Theory of Heredity," appeared
in 1893. It demonstrated the "continuity of the germ-plasm," a valuable
starting point for theorizing upon Mendel's Laws.

De Vries' "Die Mutationstheorie," published in 1901, caused much
of Darwin's theory, that evolution comes about gradually, to be set aside.
The sudden springing forth of new forms, rather than a slow change re-
quiring thousands of years, won many scientific men to it. In fact, all
modern evolutionary theories follow either the Darwinian or the De
Vriesian type, or build new ones on modifications of these.

It is somewhat puzzling to select a man to represent the study of
fossil life. One is tempted to- name E. D. Cope (1840-1897), whose re-
searches were conceived on the highest plane. Zittel (1839-1904), how-
ever, covered the entire field of fossil life, and his "Handbook of Paleon-
tology" (1876-1893) is designated as a mile-post in the development of
that science.

Before the Christian era, the works of Aristotle and Galen should
be included.

From the viewpoint suggested, the most notable figures in the de-
velopment of biology are : Aristotle, Galen, Vesalius, Harvey, Malpighi,
Linnaeus, Wolff, Cuvier, Bichat, Lamarck, von Baer, J. Miiller, Schwann,
Schultze, Darwin, Pasteur, Zittel, and Mendel.

Such a list is, as a matter of course, arbitrary, and can serve no use-
ful purpose except that of bringing together into a single group the
names of the most illustrious founders of biological science. The indi-
viduals mentioned are not all of the same relative rank, and the list should
be extended rather than contracted. Schwann, when the entire output

THE HISTORY OF BIOLOGY 389

of the two is considered, would rank lower as a scientific man than Koel-
liker, who is not mentioned, but the former must stand in the list on
account of his connection with the cell-theory. Virchow, the presumptive
founder of pathology, is omitted, as are also investigators like Koch,
whose line of activity has been chiefly medical.
References.

Henry F. Osborn, "From the Greeks to Darwin."

L. C. Miall, "History of Biology."

William C. Locy, "Biology and Its Makers." \

Garrison, "The History of Medicine."

Albert H. Buck, "The Growth of Medicine from the Earliest Times
to about 1800."

Albert H. Buck, "The Dawn of Modern Medicine."

Lorande L. Woodruff, "History of Biology," in The Scientific
Monthly, March, 1921.

A. G. Little, "Roger Bacon. Essays contributed by various writers
on the occasion of the commemoration of the seventh century of his
birth." (1914.)

CHRONOLOGICAL TABLE OF IMPORTANT BIOLOGICAL

EVENTS

B.C.
540 Xenophanes : first to recognize fossils as proving that the earth

was formed under the sea and rose out of it.
500 Heraclitus : often called the first evolutionist ; he first advanced the

principle that "all things flow."
450 Empedocles : first to suggest natural selection and survival of the

fittest.

400 Hippocrates : called "the Father of Medicine."
350 Aristotle: founder of zoology.
320 Theophrastus : first botanist.
320 Erasistratus : first to give mechanical explanation of disease

symptoms.

300 Herophilus : first anatomist.
A. D.

79 Pliny : wrote first popular natural history.
160 Galen : founded medical physiology.
1266 Bacon : wrote his Opus Majus.
1542 Vesalius: founder of modern anatomy.
1548 Falloppio : anatomist.
1551 Gesner: gathered first botanical garden (of fruits and flowers) and

first zoological museum.
1560 Eustachio : anatomist.
1583 Caesalpinus : classified plants by flowers.
1590 Janssen, J. and Z. : discovered compound microscope.

390 GENERAL BIOLOGY

1603 Fabricius : discovered valves in the veins.

1603 Harvey : discovered circulation of the blood.

1622 Ascello : discovered the lacteals.

1649 Rudbeck : discovered the lymphatics.

1650 Swammerdam : first great student of insects in relation to plants

and medicine.

1661 Malpighi : founder of pathology; discovered the capillaries in the
lungs ; founded modern embryology by a study of the incuba-
tion of the chick (1672).

1667 Leeuwenhoek : first to see bacteria.

1668 Redi: disproved spontaneous generation of insects by the discov-

ery of eggs and larvae ; wrote "Esperienze intorno alia Gcnera-
zione degl' Insetti."

1670 Mayow : studied animal respiration.

1671 Hooke : worked out microscopical structure of plants.

1680 Borelli: proved that all the movements of animals are caused by
muscles pulling on bone levers ; wrote "De Motu Animalium."

1682 Grew: studied structure of plants.

1693 Ray : classified plants.

1727 Hales: investigated respiration of plants.

1743 Haller: father of modern physiology.

1744 Reaumur: studied insects.

1749 Buff on: wrote a natural history.

1753 Linnaeus: founder of modern botany; classified plants.

1761 Koelreuter : studied hybridization of plants.

1764 Bonnet: evolutionist ; grouped animals in an ascending series.

1764 Wolff, Friedrich, Caspar: overcame the preformation doctrine.

1772 Rutherford: discovered nitrogen.

1774 Priestley: discovered oxygen and studied the breathing of plants.

1775 Spallanzani : disproved spontaneous generation of bacteria and

molds and demonstrated presence of living germs in the air.

1789 Galvani : discovered animal electricity.

1790 Goethe: worked out a scheme for the metamorphosis of the parts

of plants.
1794 Darwin, Erasmus: grandfather of Charles Darwin; wrote "Zoono-

mia," a long poem outlining evolution of life.
1796 Jenner: discovered vaccination.
1796 Sprengel : studied fertilization of plants.
1800 Cuvier: founder of modern comparative anatomy; wrote "Le,

Regne animal," 1817.

1800 Bichat: founder of modern histology.

1801 Lamarck: invented a scheme for the evolution of animals (by con-

scious effort and inheritance of acquired characters ; not
proved).

THE HISTORY OF BIOLOGY 391

1801 Treviranus : introduced the name "biology" as distinguished from

"botany," "zoology," "physiology," "anatomy," etc.
1804 Humboldt: studied distribution of plants.
1807 Rumford, Count: demonstrated absorption of carbonic acid by

plants.
1811 Bell, Charles: discovered motor and sensory nerve roots; founder

of modern neurology.

1818 G. St. Hilaire: pointed out unity of plan in animals.
1823 Von Baer: discovered the law of embryological development; (all

higher forms pass through somewhat similar forms to lower

ones in the embryological period).
1830 Brown: described cell nucleus.
1833 Muller, Johannes: founder of modern comparative physiology.

Wrote Handbuch der Physiologic des Menschen.
1835 Dujardin: studied protoplasm.
1838 Schleiden : discovered the cell as unit of structure in plants.

1838 Schwann: discovered the cell as unit of structure in animals.

1839 Agassiz : wrote on fresh-water fishes.

1841 Helmholtz: discovered rate ot nerve impulse.

1853 Mohl: studied protoplasm (living substance).

1857 Pasteur: founder of bacteriology; studied fermentation.

1858 Darwin: reported his work upon the origin of species by natural

selection and applied evolution to man.

1858 Wallace : reported his work upon the origin of species by natural
selection.

1858 Virchow: worked out cellular pathology; founder of modern cellu-
lar pathology.

1861 Schultze, Max: established protoplasm doctrine.

1863 Huxley: wrote "Evidence as to Man's Place in Nature."

1863 Lyell : wrote "The Antiquity of Man."

1865 Sachs : studied structural botany.

1865 Mendel: founder of modern genetics; discovered the law of
heredity.

1867 Lister : worked out aseptic surgery.

1875 Galton: studied inheritance.

1875 Hertwig, O. : studied fertilization.

1880 Koch : proved the relation of bacteria to disease.

1880 Laveran: discovered malarial parasite (in the mosquito).

1886 Leuckart : settled the modern classification of animals ; specialized
on parasites.

1893 Weismann: showed that germ-plasm and somatoplasm nre dis-
tinct.

1893 Zittel : wrote most important work on fossils.

392 GENERAL BIOLOGY

1888 Finlay [ discovered relation

1898 Reed 4 between yellow fever

1898 Lazear [and the mosquito.

1898 Howard : discovered relation between typhoid fever and the house
fly.

1900 De Vries, Correns, Tschermak : all working independently, redis-
covered Mendel's law of heredity.

1903 Stiles : discovered hookworm in the United States.

1914 Goddard : proved feeble-mindedness a unit character.

1915 Stockard: discovered influence of alcohol on offspring.

References.

W. A. Locy, "Biology and Its Makers."

L. C. Miall, "History of Biology."

Baas, "Outlines of the History of Medicine."

Garrison, "History of Medicine."

Hodge and Dawson, "Civic Biology."

Batesen, "Mendel's Principles of Heredity."

CHAPTER XXVI.

PALEONTOLOGY

Just as we attempt to read and interpret the history of man's prog-
ress in the handicrafts, and in the remnants of tools and pottery which
are found in various parts of the world, so we attempt to read and in-
terpret the changing conditions which have taken place in the earth's
crust by the study of geological and paleontological findings. Geology
concerns itself with the changes in the earth itself, while paleontology
seeks to build up a meaningful account of the changes which may have
taken place in living organisms throughout the past, as demonstrated
by their fossil remains (Fig. 245).

There are two general ways in which layers of rock and soil have
been laid down. The first has come about by great masses of molten
substance forming within the earth which were then thrown out by
volcanic action. Such masses harden to form minerals and other heat
products. If the minerals then become concentrated, they are called
ores. All such products formed by heat are known as igneous forma-
tions.

The second way in which changes have come about is this : Various
horizontal soil-layers have been shifted about by climatic changes such
as a subsiding of land surfaces and an elevation of the edges of the ocean.
This causes the lowered continent to be covered by shallow water, and
later, when this condition is again reversed, a layer of sediment is left
behind. It is in this sediment that millions of marine-forms of life are
stranded. If now, the sediment hardens, and these marine organisms
are safely protected from air and superficial decay, their bodies will be
preserved as fossil remains.

Fossil remains are, therefore, observed most frequently in the de-
posits on the floors of lakes, in peat-marshes, in the deltas of river-
mouths, and under the stalagmites in caverns in lime-stone districts.

The exceptional conditions necessary to preserve organic forms will
rarely be found everywhere, so that we must remember that no matter
how many fossil remains may be found, only a very infinitesimal por-
tion of the living forms of any given period will become known to us.
Then, too, in those which are preserved most, if not all, of the softer
parts of the organism are destroyed, only the hard portions remaining.

The necessity for coordinating the facts found in many and vary-
ing ways is of prime importance in the science of Paleontology, for
without such coordination there is neither sense nor value in its study.
This will be demonstrated quite clearly in what follows.

Geology and climatology attempt to explain each other, the former

394

GENERAL BIOLOGY

by its effect upon climate, and the latter by its effect on the changing
strata which go to make up the earth's surface. In fact, it is the
changing climatic conditions which give us the terms "ages" or "peri-
ods," such as the carboniferous age and the glacial period.

PSYCMO2OIC

U.oooto 150.000 YEARS

RECENT

CENOZOIC
or
(T<?rtiM-y)
5% of Geolocnc Time
5 Miles Pelp

Age fit Man , Irviwts and til
tKose Mammals now living
All Phyla oP Plants it\clu3-
hjjhjjtet flowery

Ou«rterriAryor Pleistocene

CrUciAl Period Alto
cMI«d Mct-tl »y> of
RsUeolithic as<«

Tapir; Pccaf/, Bison, LUma, Fjquus,
Megatherium, Mylodon.

(G-jjinrit sloths) (Gi^ntic sloths)

All trace of run lost
Mamma Ls abundant, many
of which are now extinct
Pithecanthropus
'Pi»hCKOS=apc+ anthropoj=
taj Found in Java _r
Probably at close or
Pliocene Period

RTIARY
of Hdmrnals)
)00,ooo year's

PLIOCENE
(pl«ioo-more
Ktinui>K«cent

Equus Beds fauus-.nont)
E<juus, Tapirus, ElepKas

Pliohippus Beds Hippus-HoKsc
Pliohippus, Nftstoden, Bos., PTC

PrimaU'i Hftd nude Jreat
Progress. Ancestral
Stock of gibbons
Dryopithecus (Druj.Twt

Large Anthropoids
Increase in higher
flowering Plants

i

Miohippus Beds
Miohippus, OicerAtherium&horwd),Th\t\ohyus

OrTOdon BedS.(Or*€.nou * in.odont-Too+K)

Edendates. Hyocnodon.Hyrocoden.
Brontothcrium Beds.(Bront«.ThunderVTh«rion Be^st)
Mesohippus.flenodus, Elotherium

Mammalian Forms
abundant Many now
extinct-
Increase in flowcnao-
Plants

^s^

^

Sfl

Diplacodon Beds Epihippus. Amynodon
Oinocerus Beds. Tinoceras, Uintatherium

Limnonyus, Orohippus.Helalefes. Colonoceras.
Coryphodon Beds. Eohippus, Monkeys,

(CorlJl>he -. Summit todont. Tooth

Gvrni vores. L/njulates, Til/odonts, Rodenls.

.1
0 — ^ %T
O ^ ^c>

CM

Bird like Ifcpfiiw
Flying Reptiles
Toofhed Birds'
FiVsf Srwlfes.
Bony FisKes abound;
SharKs nuraerou5;

Rapid increase
of lower Flowering
Plants

CRETACEOUS

CChMKy)
(A^'e of Reptiles)
5,000,000 year's

Lignite Series
HydMSAurus, Oryplosaurus

Pter^nodon Beds.(Pterodflc(u5-wmffd rcp»;ie r

Jinodon- foothlfss)

Birds with Teeth, Hesperorms.
Ichthyornis, nosASAiirs
Pfcrodacthyls. P/ejiojAurs.

DdKof-d (ffoup

First Birds,
Giant Reptiles,
Ammonites,
Cl»ms and Snails
Abundant

Ferns , Cycads,
and Conifers.

ill 1

13 £» °,

Atlantasaui'us Beds

Dinosaui's, ApAtosaui'us.
Narvosaufus, furtles, DiploSAUi'US.

First Mammal Found.
(ft Marsupial);
Sharks reduced to
new Forms,
Bony Fishes appear

Ferns, qcads,
and Coni-ftrs.

i/j S.S ^ o
<f|*|

Connecticuf River Beds.
Dinosaur Foot-prints. (Amphisaurus)
Crocodiles, (Belodon).

Fig. 245.

Composite Palaeontological Chart, compiled from many authors, showing geo-
logical strata and fossil-forms found in each. It will be noted that the number of
years assigned each stratum varies from any given amount to ten and even a
hundred times that number. The student must therefore realize that all such
estimates are only guesses. What he must know is the relative percentage of
time and the relative percentage of depth of each layer and speak only in terms
of "eras" and depths.

Professor Osborn has just described (Natural History, for November-Decem-
ber, 1921) a Tertiary man living long before the ice-age.

PALEONTOLOGY

395

If the deposition of the earth's layers have been laid down by water
and air, the various strata show such causes by forming a coarse sand-
layer, followed by a layer of finer sand or mud; or, two sandy layers
will be found separated by thin layers of muddy shale, the exact forma-
tion depending upon the velocity of both sand and water.

Or, there may be mechanical and chemical changes which produce
beds of rock-sand or gypsum between beds of marl. Likewise organic
activities may have their influence as shown by the fine beds of coal
succeeding layers of sand, or by a layer of large fossils imbedded in lime-
stone.

PAL/tOZOIC
(Primary)
19% of geologic time
18.5 Miles Peep

Earliest of true
Reptiles.
Amphibions;
Lung Fishes;
Fringe Fins;
First Cray Fishes,
Insects abundant;
Spiders,
Fresh Water Mussels.

Ferns;
Caiamites.
Lycopods,
Cycads,
Conifers.

CARBONIFEROUS
(Age of Amphibians)
5,000,000 Years

Permiesrv
Red Sandstone , etc
nag'nesian Limestone

Coal Measures.
First Keptiles(?)

Subcarboniferous
Carboniferous Limestone

First Amphibians.
(FrogliKe Animals)
SharKs;
Ostracophores.
First Long Shells
(Snail?)
nollw*s abundant
First Crabs
Ferns
Caiamites
Lycopods
Cycads
Conifers

"2^ ^.
< ul

11

ILJ <r

Corniferous
Schohe\ric Grit

First truly
terrestrial or air-
breathing animals;
First Insects;
Corals abundant ,
Hailed Fishes
Probably some
Land Vegetation

First Known Fishes:
Ostracophores;
(Fishes having no j»w».
Segmented B&cKbones or
limbarches i Forepart
protected by bony Pintes
Cartilaginous Skeleton ;
Brachiopods ,
Tribolites;no1lusKs.tti

:

c.

j

5"

Sandstones,
Shales,
Some Limestone

ID

_J

00

Ordovic^n
or
Lower Silurian

Slates, Sandstones,
Volcaruc KocKs, etc

Invertebrates only.
Probably some
Higher Aigae

J

-0

e
a
ft

Primordial

Slates Sandstones, etc

ARCHfcN
64% of geologic time
62 Miles Peep

Simple Marine
Invertebrates

Probably very
simple Algae

ARCH/tN

Huroniar\

Slates, Volcanic Rocks, etc
No Vertebrates Known

c.

Fig. 245.

396 GENERAL BIOLOGY

As there is a tremendous pressure of the superincumbent layers
upon the underlying strata, the lower layers as well as their fossil con-
tents are often crushed and injured. Extreme care must, therefore, be
taken to interpret one's findings. One can readily grasp what such
pressure would accomplish in the delicate layers of shale (called paper-
shales) which range from sheets as thin as paper to layers of such sheets
fifty feet or more in thickness.

A study of the fossil remains of plants and animals should show us
in what order these organisms lived and followed each other in times
long past, and it is usually conceded that they do ; but, it is not an un-
common thing to find an earlier fossil layer lying above a later one.
Geologists explain this by saying that changes have again taken place
which reversed these lower beds, or thrust earlier strata between other
layers. All this complicated arrangement lends itself to deceptive inter-
pretation. For example, those who oppose the usually accepted geo-
logical evidence of "periods of time" and "successive ages" say that the
arrangement of the various strata is so deceptive that it can only be
explained by a world-upheaval of some kind, and that, therefore, no evi-
dence of successive ages is worth anything.*

An interesting example of the order in which certain strata have
been formed, is found in instances where trees and their stumps are
found lying in a more or less semi-upright position. Often the stump-
part and roots still lie in their position of growth or at least they lie in
a deeper stratum than the upper and less heavy portion. Such trees
were either pushed over by streams of water or carried along by the
stream. The heavier end became caught or weighted, and sank, while
the upper end remained in a position of slant in the direction of the
current. It is, of course, also possible that the trees were entirely sub-
merged while still. growing, but in the latter case the rate of sand-deposit
must have been sufficiently rapid to lay down an accumulation of at
least forty feet (enough to cover the erect tree) before the wood de-
cayed.

Former regions have been identified by the occurrence of great
quantities of drift-wood found in the strata, as having been quite close
to land, while differences in climate are evidenced by the finding of
tropic plant and animal remains in cold regions, and arctic plants and
animals in tropic regions.

Migrations of plants and animals from one region to another are
demonstrated by their fossil remains being found in different types of
strata in different ages.

However, no one can tell the number of years required to lay down
the various strata any more than he can tell how many years elapsed to
form the intervals between such laying down; and these intervals

*G. McC. Price, "The Fundamentals of Geology."

PALEONTOLOGY 397

no doubt were often much longer than the time it took to form the
strata.

Intense cold or heat, resulting from a climatic change, undoubtedly
killed many organisms which were unable to adapt themselves to the
changing conditions of the past, while mountain ranges becoming ele-
vated cut off the moisture-supply of others who went the same way.

The glacial period is considered synonymous with the permian, and
represents the extreme of cold, while the tropical period, the extreme of
heat, is represented by coal beds (Fig. 245).

The mechanics of adaptation of living organisms to new climatic
and environmental changes has given rise to much speculation.

Lamarck thought that the organism was directly affected by any
change in environment and that this change then affected the germ-
plasm so that it could be inherited by the organism's offspring, and thus
result in a permanent racial change.

Others taught that both somatoplasm and germplasm are simulta-
neously affected. This theory is known as that of parallel induction.

Darwin, like Lamarck, believed that small environmental changes
became large ones as they were successively inherited. In fact, this was
held by nearly all the early workers since the time of Darwin ; but, as
no evidence was forthcoming which could explain how such environ-
mental changes could affect the germplasm and thus be inherited, biolo-
gists are inclined to hold with Professor H. H. Newman, that "external
factors accelerate or retard processes that were already under way in
the germplasm, so that the response appears to be something new in
kind when it is only the result of a sudden acceleration of a character
evolution already under way. Whatever be the underlying mechanism
involved in adaptive changes, there is no hope of explaining adaptations
on the Darwinian basis, through the selection of the best out of a vast
area of purely fortuitous variations ; for if the historical study of verte-
brate evolution reveals one thing more clearly than any other, it is that
evolutionary changes are ordinarily progressive, and determinate in
character, and that in many respects these ordinary processes of evolu-
tion are independent of each other and of environmental changes."

This means that we need not hold that animals always adapt them-
selves to their environment, but that they can migrate to environments
which are best suited to them. And there is ample evidence to show
that such migrations took place quite often. Some of these are shown
by the land-bridges (over which animals passed) now destroyed, which
connected islands and continents with each other. The animals were
then shut off from their original home by the destruction of the bridges.
Such animals are said to be geographically isolated.

Not only have animals migrated, but as already stated, the climate
itself migrated. This is shown by the fact that the marine and glacial

398

GENERAL BIOLOGY

coverings of the land's surface took place at much later periods in some
places than in others.

To return to the fossils themselves, it is necessary for the student
to understand the various forms in which fossil remains come down to
us. Bones may be buried in silt which then hardens. Later, water, con-
taining" minerals, may make its way through the silt and bit by bit dis-
solve the bone, and deposit a mineral in its place. This is petrification.
The shape and form of the bone remain intact, though the original bone-
substance is replaced by a mineral.

Fig. 246.

Mammoth found frozen in Siberia. The skin is mounted
in the museum of Petrograd in the posture in which, it was
found. (From Lull's 'Organic Evolution,' by permission of
the Macmillan Co., Publishers.)

Or, an organism may retain its form long enough to have the sur-
rounding substance completely encase it and harden. As time goes by
the organism is dissolved and disappears, while the hollow space it occu-
pied remains. Such hollow forms, in the shape of organisms no longer
present, are called molds. Investigators fill these molds with a material
such as plaster of paris (which hardens easily) and obtain a cast of the
original organism.

Then, too, as stated above, the tremendous pressure of the upper
layers may crush the fossil forms beneath, or the minerals which caused
petrification may be re-dissolved, so as to expose the fossil remains to
conditions which destroy them, and this may happen after they have
been so encased for thousands of years. Weathering and erosion may
also expose fossils to the harmful effects of the weather.

When very definite fossil remains are always found in certain strata,
they are often called "index fossils," as such fossils can be used to de-
termine the place and period of the strata from which they are taken.

One of the most interesting finds in 1901 was that of an ancient

PALEONTOLOGY

399

animal, whose species is now extinct, that of a mammoth found frozen
in the ice of Siberia (Fig. 246), whose flesh was in excellent condition.

In oil-bearing" sands many excellent fossil specimens have also been1
well preserved.

But the fossil remains which have excited most discussion and spec-
ulation are those which are supposed to have belonged to human beings
higher in the grade of life than the highest apes \ve now know, and yet

A.

C.
Fig. 247.

A. Remains of Pithecanthropus erectus; the single femur shown in different
aspects.

B. Remains of the Neanderthal man in the Provincial Museum at Bonn.

C. The Heidelberg Jaw.

(A. From "The Open Court", B. from "Weltall V. Mensehheit", C. from
Bryee after Schoet Ensack.)

distinctly lower than man. Authorities, however, disagree considerably
as to what type of being these bones represented, some insisting their
possessor was human, and some that he was not.

One of the most important of these "finds" (1891-1892) is that of
a part of a skull, two teeth, and a femur (Fig. 247, A). These parts lay
at some distance from each other, so that we cannot be certain that they
belonged to the same individual, but it is assumed they do. The shape

400

GENERAL BIOLOGY

of the femur indicates that its owner walked in an erect position and
was about as tall as men now are. From the parts thus found a so-called
"reconstruction" was made to show what the reconstructor thought the
individual must have looked like. The name Pithecanthropus erectus is
applied to the individual who once possessed these bones (Fig. 248).
The bones are presumably from the early Pleistocene period.

At another time a lower jaw with its teeth was found near Heidel-
berg in Germany (Fig. 247, C). As the teeth are not ape-like, but ap-
proach those of man, the individual who possessed them has been called
Homo-heidelbergensis. The fossil remains of various animals found in
the same region with the Heidelberg jaw give us the age of this find
as that of the second interglacial period, which means that this jaw is
only about one-half the age of Pithecanthropus erectus.

In 1856 there was found in Prussia the skeleton of what is called the
Neanderthal man (Fig. 247 B, and 248), or Homo-neanderthalensis
which comes from about the fourth glacial period, so that it is about one-
third the age of the Heidelberg man.

Fig. 248.

Restoration of prehistoric men. Left, Pithecanthropus
erectus; middle, Homo neanderthalensis, modeled on the
Chapelle-aux-Saints skull ; right, Cro-Magnon man, modeled
on type skull of the race. (From the original busts of, and
by courtesy of, Professor J. H. McGregor.)

Then in France and Wales a number of skeletons have been discov-
ered in which the skull is narrow and the face broad, something like that
of the Esquimaux. The cheek bones and chin are also prominent. Pro-
fessor J. H. McGregor has molded busts in accordance with his idea of
what such men must have looked like (Fig. 248).

There is no connection whatever between these various forms so we
cannot in any way prove they are a genetically continuous series. All
conclusions built upon these finds must, therefore, be purely conjectural.

From the evidence presented here, we note the fact that many pres-
ent-day forms of both plants and animals are unlike their ancestors. We
are, therefore, confronted with four possible explanations of why they

PALEONTOLOGY 401

are different: (1) that present-day forms are the lineal descendants of
ancestral forms unlike themselves and that all new forms with ever in-
creasing complexity spring from older ones ; (2) new forms different
from the older ones have been created at different periods ; (3) all forms
were brought into existence at about the same time, but due to a great
world upheaval the fossiliferous strata have been so confusedly arranged
that while all fossils are of one age, it is our mistaken interpretation
which makes us believe they are of different ages ; or (4) organisms came
into existence which at the time of their origin had the possibility of
change placed within their germ-plasm, but which had to await the
proper conditions of food and environment before they could come forth
to produce present-day forms.

If it be accepted that any present-day forms are different from their
ancestors, and that these new forms can produce offspring capable of
transmitting that change to their posterity in turn, we must speak of an
evolution as having taken place.

References.

Karl von Zittel's "History of Geology and Paleontology."

A. Dendy, "Outlines of Evolutionary Biology."

H. A. Nicholson, "Manual of Paleontology."

Charles Schuchert, "Historical Geology."

A. Morley Davies, "An Introduction to Paleontology."

H. S. Williams, "Geological Biology."

Articles on Geology, Paleontology, etc., in Encyclopedia Britannica.

CHAPTER XXVII.

EVOLUTION

The student must not forget when discussing evolution that this
term means only that some present-day forms have become unlike their
ancestors, and that such difference is then transmittable — in other words,
that it has affected the germ-plasm.

Evolution as it applies to the individual is therefore the name given,
a processj^y, and through, which an organi.gyn p^1w<f r^ rhan^es^ into
a different type of being from its parents.

In the chapter on Genetics, we have seen how all offspring vary to a
small extent, not only from their parents, but from each other. When
such differences are slight they are called variations and the organisms
possessing them are known as varieties. When such variations become
sufficient to set aside the new organism as a quite different type from
its parents, the new types are known as different species.

It will be noted that this is quite vague; for, what one man may
consider a difference sufficient to form a new species, another may not.
There is, therefore, no good definition of the term species. Biologists
disagree to a very marked extent as to what it means.

Members of the higher groups of animals are often considered as
belonging to the same species if they can inter-breed and give birth to
fertile offspring in turn. But, if we are to accept this definition, there
never can be any strictly new species; for, if animals can inter-breed,
their offspring will belong to the species to which their parents belong,
and if they cannot, there will be no offspring.

Then, there are those who take the position that only those indi-
viduals form true species which always breed true. If we accept this
definition, it may be said that whenever a so-called new-form or mutation
(as it is called) comes forth, such new-form is in reality only the return
of some ancestral type, which has been formed by the meeting of an egg
and a sperm, both of which carried recessive characteristics. From this
angle one may always explain new species as being old ones, again com-
ing forth.

However, species generally mean groups of individuals who possess
similar outstanding, characteristics, which characteristics can be trans-
mitted to their offspring.

First, then, in discussing evolution, one must be convinced that new
species really do come into existence, otherwise there can be no evolu-
tion. Practically all biologists now hold that new species do come into
being, which means that they accept evolution as a fact. There is, how-
ever, a vast difference of opinion as to the limitations within which evo-

EVOLUTION ~~~ * 403

lution operates, both in the individual and in the race, as well as to the
method by which evolutionary changes come to be what they are.

Secondly, if all present-day forms have sprung from ancestors \un1ike
themselves, the question arises as to whether all the different phyla
sprang from one original living being (whether evolution is monophyletic
or monogenetic), or whether there were numerous "first forms" from
which all successive forms spring (polyphyletic or polygenetic evolu-
tion).

Having settled for ourselves as to whether evolution is a fac^ weA
set about trying to find a theory which will account for that fact. \

There have been only two great theories advanced. One by Charles
Darwin, known as Darwinism or Natural Selection, and the other by

«* . ^, — - -— "-- — t — "* r

Korschinsky, and De Vries, known as the Saltation or Mutation theory,
or heterogenesis.

Darwinism holds that, as we have seen, the offspring of a single pair
of flies will be almost six billion in ninety days if all eggs were to hatch,
it follows that were such increase to continue in all animals and plants,
the food-supply would soon become exhausted. There must, therefore,
be a struggle for existence to determine which plants and animals are
fittest to survive.

Everyone has noted that millions of eggs, maggots, and insects never
reach the adult form of life on account of their being eaten by various
animals. The number of flies and other insects are, therefore, dependent
upon the number and activity of their natural enemies as well as their
own physical ability to avoid such enemies, together with their ability to
obtain a sufficient supply of food and water for themselves.

It follows that there will be a struggle even among the same group
of organisms for food and water, while the whole group must struggle
against their many natural enemies. Nature, through such struggle, se-
lects the strongest and most active (as these are the only ones which
will not succumb to the struggle) to carry on the race. The particular
characteristics which make it possible for plants and animals to survive
in this struggle for existence, are said to have a survival value. ^

Darwin accepted variations in all living organisms as a fact, and
built his theory on that fact. Re contended that useful variations by
possessing a survival value were transmitted to the offspring of such
organisms, so that each succeeding generation received the advantage of
its parents' acquired characteristics.

However, it is generally held now that acquired characteristics are
practically never inherited, and that natural selection only explains why
certain organisms did not die and others did. It cannot explain the origin
of new species.

Darwinism is based on the assumption that there are very minute
changes constantly taking place in the organism. These changes have
an effect on the germ-plasm of the individual, thus altering it, causing

404 • GENERAL BIOLOGY

the change to be transmitted. For example, a giraffe by constantly eat-
ing food from trees, finds it necessary to reach higher and higher. This
stretching of the neck will, then, in each generation cause the young
giraffes to be born with a slightly longer neck.

If such a change makes the individual better able to adapt itself to
its surroundings and thus gives it an advantage in the struggle for exist-
ence, it is said to be a selective factor.

The mutation theory, contrary to the Darwinian, insists on sudden
jumps or great changes taking place which are then transmitted. This
theory is based on the fact that there are so-called "freaks" or "sports"
in nature which suddenly spring forth.

The crooked-legged sheep is the classic example. A New England
ewe gave birth to a peculiar crooked-legged ram. The shrewd Yankee
farmer, who owned the sheep, saw in this crooked-legged ram an animal
that could not jump fences, and so kept it. The crooked-leggedness
proved to be a Mendelian dominant character which is transmitted from
parent to offspring. There are now great numbers of the descendants
of this single New England crooked-legged ram, which are in turn
crooked-legged.

Such mutations can be explained by assuming that the recessive
characters in egg and sperm have met after lying dormant for many
centuries.

In fact, there are a number of examples of characters lying dormant
and being carried on from parent to offspring, only coming forth at cer-
tain times when the mating organism likewise has a similar dormant
or recessive character. For instance, many mullein plants will, in certain
years, suddenly produce longer leaves than is normal for that plant, thus
showing that the cause of the longer leaves must be in the germ plasm
of the varying plants. For if such were not the case, there would not
be so many to develop longer leaves in the same generation.

In other words, this means that there is a peculiar arrangement of
genes in the chromosomes of the mating-plants, and these genes have
united to cause a similar abnormal development in all those plants which
spring from similar zygotes.

If is for reasons of this kind that biologists have come to the con-
clusion that environment does not change the organism to any appre-
ciable extent in its genetic value, but, that whenever changes come forth,
these are due to changes in the germ plasm.

To the two great theories mentioned above, there have been added
at various times what might be called sub-theories or part-theories to
account for certain developmental characteristics. The most prominent
of these is known as Orthogenesis. Eimer and Nageli are its sponsors.
Orthogenesis means that there is something in the organism which, once
a line of development has begun, this something will enable the organism
to continue in that certain line of development even though it kill the

EVOLUTION 405

individual. An example which comes to mind is the teeth of rodents.
These, when the animal becomes older and is unable to chew the hard
substances it did when young, continue to grow with the same undimin-
ished vigor that they did when they were constantly being worn down
by contact with hard substances, so that they may, as in the beaver, force
the mouth open and starve the animal to death.

But Orthogenesis must be explained, and various reasons have been
suggested to account for it, the reasons varying according to the "phi-
losophy of life" of the one who is doing the explaining.

Those who hold that all things are to be explained in terms of physics
and chemistry, attempt to explain orthogenesis in physico-chemical
terms, while those who hold that there is an inner driving force in all
living matter which cannot be explained by physics and chemistry, insist
that it is this inner driving force, or "vitalistic principle," which alone can
account for it.

Both sides, however, agree that the cause for this development is
not in the organism's environment, but must be sought for in the organ-
ism itself. As Professor Borradaile puts it, "The part of the environment
is to decide which of the experiments of the organism are failures." And
there is sufficient evidence to accept his statement. For instance, the
fertilized egg-cells of nearly all the higher organisms are quite alike, yet
they develop quite differently. And they retain this difference in devel-
opment, even if the egg is transplanted into the body of a different kind
of animal and is there allowed to develop to maturity. There must be
a difference in the environment of the organs within the bodies of differ-
ent animals, yet the egg grows on as it would have done under its nor-
mal environment.

When such definite direction takes place it is called purposiveness.
The objection raised against the physico-mechanists (those who be-
lieve that all things can be explained in terms of physics and chemistry)
by those who do not hold to their point of view (vitalists) is that the
body cannot be accepted as a machine in any true sense of the word. A
machine produces a very definite and single type of work, while the
living organism has all its work directed toward its own welfare, and
unlike any machine known, can, when it is injured, direct its entire work-
ing system toward repairing itself in addition to continuing its regular
work. It not only heals the wound inflicted, but actually grows new
material, as we have seen by the regeneration experiments in Planaria
and Arthropoda.

Then, too, there is a decided chasm dividing living and non-living
matter ; so much so, that it is a common dictum of biology that abso-
lutely no life can come from non-living matter, there being no single
case on record of any organism coming into existence except as the off-
spring from some other organism.

However, as we know living things did not exist always, there must

406 GENERAL BIOLOGY

have been a time when they did come into existence. The physico-
mechanists say, that while no living matter comes from non-living to-
day, yet, as there must have been a time when it did, we must assume
that different conditions once held sway from those we now know.

But, such being the case, we break the most important law known
to science — that of continuity. It is for this reason that it has been said,
that the breaking of this law of continuity is the only heresy known to
evolutionary science.

The theory that life always comes from life is known as biogenesis,
while the theory which holds that life can come from non-living matter
is called abiogenesis.

It will be noted that the evolutionary theories so far discussed have
only taken the physical side of the individual into consideration to the
entire neglect of the mental and intellectual.

It was Alfred Russel Wallace, co-founder of the Natural Selection
theory with Darwin, who saw this quite early, and insisted that the
psychical or mental side must also be considered if we are to form truly
valid conclusions. He contended that once mentality enters, as it does in
man, such an organism could use this mentality to set aside or change
the physical selection which Nature carried on. In other words, the
earlier evolutionists were interested in the structure of nerves and nerve
elements, while Wallace saw the necessity of taking the thought which
is carried by the higher nerve centers ifito consideration.

It is well for the student to know both the evidence adduced in sup-
port of evolution and evolutionary theories, and the objections which
have been hurled against it. We have, therefore, sutnnied up the argu-
ments of both sides, whether such support and objections are always con-
clusive or not.

EVIDENCES FOR EVOLUTION

1 . Paleontological.

(a) There are many new kinds of plants and animals found in each
successive strata as shown by their fossil remains.

(b) The later organisms are more complex than earlier ones.

(c) The more recent fossils prove that they are quite closely re-
lated to the modern forms now living.

2. Genetics.

Breeding experiments as well as observation prove that all organ-
isms are constantly varying, and that constant variations in the same
group of organisms are transmitted to succeeding generations.

3. Comparative Anatomy.

The similarity of structure in different individuals is precisely what
would be experienced if evolution did take place.

EVOLUTION 407

Homology (similarity of structural development) is to be regarded
as a sign of relationship, as it is assumed that in such cases little or no
structural change has taken place in times past. Contrariwise, organ-
isms which are quite dissimilar in structure are assumed to have diverged
many ages ago.

4. Comparative Embryology.

(a) Likenesses between embryos of different animals are assumed
to demonstrate a close fundamental relationship and a common ancestor.
An example often quoted is that of Sacculina (Fig. 212), a parasite on
the abdomen of the crayfish. This parasite is merely a rounded, pulpy
mass with no clearly defined structure except a little root-like projection,
which extends into the body of the host to absorb the fluids. The em-
bryo of Sacculina, however, is a very definitely shaped three-cornered
little organism with jointed legs and all other necessary features which
bring it under the crustacean classification. In fact, it is practically a
degenerated barnacle.

(b) All higher forms of vertebrates possess so-called gill-pouches
during the embryonic stage, although the higher forms do not retain
them in the adult stage. This would lead to the assumption that the
common ancestors of vertebrates must have been fish-like.

(c) According to von Baer and Haeckel, all animals during the em-
bryonic period pass through the adult forms of the race to which they
belong, thus presenting conclusive evidence of the history of their de-
scent.

5. Comparative Physiology.

Animals which are closely related genetically have a somewhat sim-
ilar blood-composition, as proved by the fact that the blood of one such
related animal can be successfully transfused to another without harm.

6. Geographical Distribution.

Animals such as marsupials (pouched animals) which have as much
in common structurally as the Australian kangaroo and the American
opossum, while yet quite unlike in general appearance, can only be ac-
counted for by taking the geological evidence for a land-bridge into con-
sideration which once connected Australia and America. The two ani-
mals having had the same ancestry, changed their appearance because of
a changed geographical environment, although their general structure has
remained quite as it was.

There are no native ungulates in Australia, although there is no rea-
son why there should not be if other than evolutionary methods have
been factors in producing new types of animals.

Or, again, one finds, for example, on the west coast of South America,
peculiar animals found nowhere else in the world, while on the neigh-

408 GENERAL BIOLOGY

boring islands there are animals resembling those on the coast of the
continent both in structure and habit, yet sufficiently different to be
called new species.
7. Natural Selection.

This is an attempted explanation of why present-day forms are what
they are, by showing that food is never equal to the possible rate of in-
crease in living forms. Such lack of food causes a struggle for existence
through which struggle the weakest (the ones being least adaptive), go
down, while the stronger (those best able to adapt themselves to their
environment), survive.

Natural selection describes the causes which have prevented surviv-
ing forms .from becoming extinct.

OPPOSING ARGUMENTS

The arguments which are usually brought forth to oppose these evi-
dences for evolution are as follows :

1 . Paleontology.

(a) The different kinds of plants and animals found in various
geological strata can only demonstrate that similar organisms were either
larger or smaller than others, or varied in ways which can be accounted
for by a difference in the temperature and food supply of different ages.
Examples of this are the horse and mammoth. Then, too, paleontologists
insist that their finds can only be explained by assuming that acquired
characteristics are inherited, although experimental evidence seems to
point against this being true.

(b) The so-called increasing complexity on the part of so-called
"younger" fossils as compared with so-called "older" ones, may always
be explained by assuming that Mendelian recessive characteristics have
again come forth, and that consequently the so-called "new forms" are
really a return of old ones.

(c) Recent fossils are like modern forms because the climatic
changes and the food supply have not varied much during the interval
between our own time and the time when the prototypes of these recent
fossils lived, and that examples of so-called older forms (those which lie
above the recent forms) can be considered evidence for this statement.

(d) Those who insist on experimental evidence which is always
under the control of the experimenter, say that fossil-remains furnish us
only with "descriptions" of what is found. It is a "dead** account. It
can never give us an explanation. Explanation and interpretation can
only come through our logic. Paleontological evidence is therefore all
logical and not experimental. A strictly scientific explanation from the
experimentalist's point of view must also present experimental evidence.
This has not been, and cannot be, done in paleontology.

EVOLUTION 409

2. Genetics.

(a) Inherited changes can always be referred back to ancient Men-
delian recessives meeting, and thus producing a "past" type. There can
be no strictly "new" types ever formed because the chromosomes never
die (as long as there are living offspring), and all that ever happens is
that some part of them is thrown out. But, from what is known of
biology, it is impossible to add anything to the offspring which is not
already present in the chromosome content of the germ cells.

(b) Mendelian characters themselves are only concerned with
minor details such as eye-color, hair-color, and similar matters. No
change of a definite survival value has yet been shown to come under its
laws.

3. Comparative Anatomy.

Similarity of structure by no means proves relationship, as shown
by examples of convergent evolution, where two quite dissimilar struc-
tures come to look alike in various aspects, due to similar functioning.
Witness such experiments as Carey's in which bladder-muscle was con-
verted into beating heart-muscle by causing the bladder to simulate
heart-conditions.

The argument from comparative anatomy holds good only if one
accept the dictum that "structure determines function," while the experi-
ment just mentioned shows that function determines structure, once one
has the material with which to work.

y

4. Comparative Embryology.

(a) Any organ not used is likely to degenerate. This accounts for
Sacculina degenerating when it assumed a parasitic habit where it no
longer uses the various organs it once used. This is not remarkable, and
if it proves anything, it proves only that an organism can lose something
it once possessed,, though it by no means proves that what we have been
considering a more complex organism, can arise from one that is less
complex.

(b) The so-called gill-pouches demonstrate only as in (c) that ver-
tebrate forms pass through similar stages of growth and not that one
springs from the other.

(c) If the Haeckelian law is to hold good, that embryos pass
through the adult stages of the race to which they belong, we are con-
fronted with some unacceptable conclusions. For instance, only the
human being walks in an entirely upright position. In man alone there
are three complete bends in the developing brain which remain through-
out adult life. It is assumed that only his upright position can account
for the third bend, which brings the cerebral hemispheres back over the
brain-stem. But in the chick, and in practically, if not all, vertebrates,

410 GENERAL BIOLOGY

these three bends take place in the embryo. It is later that at least one,
and sometimes two, of the bends disappear. We must therefore assume
that frogs, chicks, lizards, etc., once walked erect like man, an assump-
tion that not even the most ardent defenders of the Haeckelian law will
admit.

5. Comparative Physiology.

Similarity of blood-composition in quite similar forms is to be ex-
plained again on the principle that all similar forms go through a similar
development, and that with similar food and temperature, the blood must
necessarily have to be quite similar because it must draw its component
substance from the same food material.

6. Geographical Distribution.

This, like Natural Selection, can only show why some organisms
survived. It throws no light on origins. It can show how parts of an
organism may be lost, but not how additional complexity has come.

7. Natural Selection.

This explains nothing of importance. It fails utterly to explain the
degeneration of useless organs, and why variations of great magnitude
do not occur more often, as well as why and how a simultaneous varia-
tion in different parts of the body takes place to improve a definite
organ.

8. Psychology.

All the evidence evolutionists adduce to prove their arguments is
invalid because they take only the physical side of the organism into
consideration, forgetting the most important part — the mental.

9. Logic.

We have been reversing the order of things, by forgetting that if
a tiny cell or organism has the ability or potentiality of becoming a highly
complex animal, it must be much more complex than the later organism
into which it is to grow. For, surely the smaller an object may be, which
can contain all that it is later to become, the greater in complexity it
must be. And, if such a tiny object is so intensely complex, it could not
have suddenly sprung into existence without an intelligence of some kind
arranging it.

10. Physics.

The student of depth has been driven. into out and out skepticism of
anything being true in science, or has gone over entirely to mysticism,
because he cannot overcome the obstacle which the acceptance of the

EVOLUTION 411

laws of the different laboratory sciences place in the way of his biological
findings. For example, Physics tells him that no more work can be ob-
tained from a machine than is put into it, and that nothing can rise higher
than its source. Then the evolutionist tells him (in contradiction to
these laws) that more complex forms come from those less complex.
This belies both laws, for Intelligence is certainly something higher, and
more than non-living matter. And intelligence cannot be explained in
terms of either physics or chemistry.

If it be told the student that the energy of the sun furnishes the
energy which can do all these things, and that there is a law known as
the conservation of energy, he will read the statement of various emi-
nent physicists who tell him that the sun's heat will gradually become
less and less, finally becoming entirely dispersed. Consequently there
will not be as much as there once was, and the law is broken. This
brings him back to the place from which he started. *How did the
energy and the potentiality of a simple organism become complex, and
how did the developing intelligence become what it is, unless it got it
from something still greater?"

1 1 . Language and Intelligence.

If it be proved that plants and animals have arisen in an ever-ascend-
ing plane, how account for language and intelligence (true ability at
thinking which is then expressed in words) ? Can this psuchus or
psychon, or real intellectual part of man, have come from anything less
than a still greater intelligence?

12. Continuity.

As was shown in the chapter on the "History of Biology," the very
foundation of science, as now understood, is based on the law of con-
tinuity, namely, that the laws of nature never vary. Yet we find all
biologists agreeing that the law of continuity has been broken, by the
fact that living forms must have once sprung from non-living, a con-
dition now no longer true. This is the great "heresy" of evolutionary
science. As life and mentality do not now7 operate as they once did,
when, where and how did they begin?

13. No Satisfactory Theory of Evolution.

No theory of evolution yet propounded is satisfactory because none
has satisfied the requirements set forth above.

14. Impossibility of a Satisfactory Physical Explanation.

There exist certain rays known as infra-red, and ultra-violet, which
no human eye can see ; yet, these rays can be proved to exist by the
physicist. If the ultra-violet rays are thrown upon a group of brown
ants they will immediately scatter quite hurriedly, thus demonstrating
their ability to sense rays which man cannot.

412 GENERAL BIOLOGY

Now, it is probably from such evidence as this that one biologist, at
least, draws the conclusion that just as there are undoubtedly thousands
of colors which no human eye can see, and thousands of sounds no human
ear can hear, so there must be thousands of factors in every explanation
which the human mind cannot grasp. This being true, it follows that
if we can find any explanation which is plausible, and which fits in with
every nook and crevice of our mind, we know that such a theory is not
likely to be true, because there are thousands of points that we must
necessarily have neglected taking into consideration due to our sheer
intellectual inability. Thus even the most plausible arguments are
vitiated.

SUMMARY

We have presented practically all of the important arguments for
and against evolution itself and the various theories which attempt to
account for it, because it is just as essential for a well-educated man to
know the opposing arguments in any given case as it is for him to know
the supporting ones. The theories which the student is to accept are
those which he finds sufficient evidence for in his work throughout his
laboratory course.

Regardless of what one may believe the evidence has brought forth,
all biological workers must accept evolution as a scientific hypothesis,
though this does not mean' that they must accept ( any of the theories
propounded to account for it. The above statement is true, because there
is much more evidence to show that an evolution has taken place than
there is to show how and why it took place.

Then, too, the student must note the difference between the cause
of evolution (which the various evolutionary theories try to explain) and
the course of evolution. This latter is only a description of what has
been found, as, for instance, the charts which show the various fossil-
remains of what is considered the ancestors of the horse and mammoth.
Such charts, of course, explain nothing.

The Darwinians originally held to the doctrine that all variations
must possess some function of a survival value, but we now know that
characters which are a decided hindrance in the survival sense, are in-
herited and passed on from one generation to another just as readily as
those which are of value.

Two different types of organisms may often grow to be quite alike,
or at least certain organs may develop so as to appear alike if they func-
tion alike. This growing alike is known as convergent evolution. In-
dividuals originally structurally alike, which later become dissimilar, are
said to do so through divergent evolution.

From what has been said above, one is likely to agree with the writer
who said that every biologist seems to have his own pet theory to ac-
count for the evolutionary process.

EVOLUTION 413

Notwithstanding this fact, one must, however, have some kind of
a gauge by which to measure the plausibility of a proposed theory.
Otherwise there is not even an approach toward finding whether any
given evidence is of value.

It is to assist the student in forming such a gauge that the follow-
ing seven questions are here tabulated. These must be answered by any
theory which is to win complete and final acceptance.

CRITERIA FOR A SATISFACTORY EVOLUTIONARY THEORY

These questions refer to organic evolution in its widest significa-
tion, as referring to both the individual and the race.

(1) How did life originate?

(2) How can a more complex individual develop from ancestors
which were less complex?

(3) How can an organism adapt itself to its surroundings?

(4) What causes the so-called mechanically directed types of varia-
tions known as Orthogenesis? .

(.5) What causes the series of the many undoubtedly purposive
adaptations ?

(6) What causes the factors of heredity to behave as they do?

(7) What factors can account for mentality and intelligence (which
are non-physical things) arising from physical and non-mental matter?

References.

Charles Darwin, "The Origin of Species by Natural Selection."

Delage and Goldsmith, "The Theories of Evolution."

Vernon L. Kellogg, "Darwinism To-day."

Lull, Barrell, Schuchert, Woodruff, and Huntington, "The Evolution
of the Earth and Its Inhabitants."

Henry M. Bernard, "Some Neglected Factors in Evolution."

Lawrence J. Henderson, "The Order of Nature."

S. Herbert, "The First Principles of Evolution."

Thomas Hunt Morgan, "A Critique of the Theory of Evolution."

Erich Wasmann, "Modern Biology and the Theory of Evolution."

Erich Wasmann, "The Problem of Evolution."

N. C. Macnamara, "The Evolution and Function of Living Purposive
Matter."

A. D. Darbishire, "An Introduction to a Biology."

George McCready Price, "The Fundamentals of Geology."

George McCready Price, "The Q. E. D."

James Johnstone, "The Philosophy of Biology."

CHAPTER XXVIII.

CLASSIFICATION

It has already been shown that one may classify living things as to
structure or function, that is, as to anatomy or physiology. The early
naturalists felt that the most important thing in the study of living mat-
ter consisted in finding names and assigning definite places for every
distinct individual. A little later morphology, or anatomy, was consid-
ered most important. Still later physiology, or the way an animal per-
forms vital activities, was the all important thing. Then with the dis-
covery that urea, an organic compound, could be manufactured in the

laboratory, much stress was laid upon chem-
istry. It was formerly quite common for nat-
uralists to look for differences, in order to
classify the individual, while now we look
primarily for similarities in order to understand
the close relationships which bind individuals
into a common group.

Classification is now no longer the prime
factor in the study of biology and men who are
interested only in assigning names and group-
ings are not considered scientists. It must not
be forgotten, however, that there could be no
science possible, and biologists would be un-
able to discuss their work intelligently with
each other unless some method could be found

Fig. 249.

Carl von Linne, 1707-1778.
From G. Stuart Gager's

"Fundamentals of Botany" by by which each would know what the other was

permission of P. Blakiston's «« • «

Son & Co., Publishers. talking aDOUt.

It is therefore well to know several of the important naturalists
whose names are most intimately associated with this particular phase
of Biology.

John Ray (1627-1705), an Englishman, was the first real systematist.

Following him came Carolus Linnaeus (Carl von Linne, 1707-1778),
who is in reality the found.er of our present method of classifying. In
fact, one of the distinguished honors that may come to a Botanist is to
be elected a Fellow of the Linnean Society. Linne's important work
was his Systema Naturae, consisting of twelve volumes, which appeared
between the years 1735 and 1768. There was a thirteenth volume added
after his death. Linnaeus practically completed Ray's classification. He
used structure as the basis of classification. There were six classes, four
of which were vertebrate and two invertebrate. These classes were in

CLASSIFICATION 415

turn divided into orders, the orders into genera, and the genera into
species. However, the Linnaean Genus sometimes includes three or four
orders of our present arrangement of groups.

Following Linnaeus came Georges Cuvier (1769-1832), who in turn
was followed by De Blainville (1777-1850). The latter's method is con-
sidered superior to that of Cuvier.

Lamarck (1744-1829) classified animals according to their nervous
sensibilities, speaking of apathetic animals, that is, those without nervous
systems or apparent sensations among the invertebrates and the sensitive
animals, largely also among the invertebrates, while the intelligent ani-
mals corresponded to the vertebrates.

Then came Oken (1779-1851), who suggested two different methods
of classifying; neither one received much recognition. One of his sys-
tems was based upon the arrangement of organs, while the other was
based upon the senses. The latter were divided into such interesting
but valueless groups as Dermatozoa (literally, skin or touch animals),
by which he meant the invertebrates; the Glossozoa (literally, tongue
animals), the fishes; the Rhinozoa (nose animals), which included the
reptiles ; the Otozoa (ear animals) or the birds ; and another class, which
appears to have been called interchangeably the Ophthalmozoa (eye ani-
mals) or Trichozoa (hair animals), the mammals. It would be hard to
name a set of distinctions less applicable as classification marks than
most of these.

Pierre Latreille (1762-1833), Johannes Mtiller (1801-1858), and Louis
Agassiz (1807-1873), should also be mentioned among the systematists.

The Linnaean system has been adopted because it introduced a
sharply defined grouping and a definite terminology. In other words,
this system permits a grouping of forms which resemble each other, as
well as a grouping according to relationships other than physical resem-
blance.

As already stated, Linnaeus used four general groupings : class, or-
der, genus (plural, genera), and species. Modern systematists have
added phylum (plural, phyla), subphylum (assemblies greater than the
class), subclass, suborder, family, subfamily, genus, subgenus, species,
subspecies, and sometimes others.

The following table will illustrate the present method of naming and
classifying animals:

Phylum. Protozoa
Class. Rhizopoda
ORDER. Lobosa
Family. Amoebidae
Genus. Amoeba

Species. Proteus

The Botanists use a somewhat different classification, but the one
here given is the one of greatest value and importance to the student.

416 GENERAL, BIOLOGY

All Zoologists, although accepting this classification, do not necessarily
classify the same animals under the same heading. This often leads to
considerable confusion for the beginner.

The student of medicine will find that during the past twenty years
a definite nomenclature has been adopted in the study of human anatomy
known as the B. N. A., so called because it was brought about at an
International Anatomical Conference at Basle, Switzerland, and there-
fore called the Basle Nomenclatura Anatomica.

The Linnaean system designates the species by two Latin or Latin-
ized names; the generic name a noun, and the specific name usually an
adjective. To this is added a third, if a subspecies is recognized. A sub-
species is usually more or less synonymous with variety in classification,
although variety is sometimes used ; in fact, in one group, ants (family
Formicidae), there are usually four words in the name.

The rules applying to the nomenclature, although following Linnaeus
are set forth in various codes. These are the British Association Code,
the American Ornithological Union Code, the Code of the German
Zoological Society, and the Code of the International Zoological Con-
gress. "The code now almost universally in use is the International Code
of Zoological Nomenclature, adopted by the International Zoological
Congress and governed through a Commission on Nomenclature." The
principal rules are as follows :

"The first name proposed for a genus or species prevails on the con-
dition that it was published and accompanied by an adequate descrip-
tion, definition or indication, and that the author has applied the princi-
ples of binomial nomenclature. This is the so-called law of priority.
The tenth edition of the Systema Naturae of Linnaeus is the basis of the
nomenclature. The author of a genus or species is the person who first
publishes the name in connection with a definition, indication, or de-
scription, and his name in full or abbreviated is given with the name ;
thus, Bascanion anthonyi Stejneger. In citations the generic name of
an animal is written with a capital letter, the specific and subspecific
name without initial capital letter. The name of the author follows the
specific name (or subspecific name if there is one) without intervening
punctuation. If a species is transferred to a genus other than the one
under which it was first described, or if the name of a genus is changed,
the author's name is included in parentheses. For example, Bascanion
anthonyi Stejneger should now be written Coluber anthonyi (Stejneger),
the generic name of this snake having been changed. One species con-
stitutes the type of the genus ; that is, it is formally designated as typical
of the genus. One genus constitutes the type of the subfamily (when a
subfamily exists), and one genus forms the type of the family. The type
is indicated by the describer, or if not indicated by him is fixed by an-
other author. The name of a subfamily is formed by adding the ending
inae, and the name of a family by adding idae to the root of the name ot

CLASSIFICATION 417

the type genus. For example, Colubrinae and Colubridae are the sub-
family and family of snakes of which Coluber is the type genus."

Since evolution has become more or less of a keynote in the study of
Biology, it has been the desire of Biologists to group living structures
according to a common ancestry. This idea has been in the minds of
systematists since Darwin's time.

Similarity of species of a given genus is supposed to indicate kinship,
so that the individuals among any given genus show greater diversity
than do the members of the species going to make up that genus, al-
though all members of the genus have something in common. We may
take as an example the vertebrates, which constitute the so-called highest
phylum, and the protozoa — the single-celled animals — which constitute
the so-called lowest phylum. Frogs being vertebrates, that is, having a
backbone, are classified in the same phylum as man, who also has a back-
bone, but there is much greater difference between a frog and a man than
there is between the many different species of frogs.

As already stated, Systematists have usually used structure for their
important clue to affinities. "However, the evidential value of similarity
in one or several structures unaccompanied by the similarity of all parts
is to be distrusted, since animals widely separated and dissimilar in most
characters may have certain other features in common. Thus, the coots
( ), phalaropes ( ), and grebes

( ), among birds have lobate feet, but, as indicated

by other features, they are not closely related ; and there are certain liz-
ards (Amphisbaenidae), ( ), which closely resemble
certain snakes (Typhlopidae), ( ), in being blind, limb-
less, and having a short tail. The early systematists were very liable to
bring together in their classification analogous forms, that is, those which
are functionally similar; or animals which are only superficially similar.
In contrast with the early practice, the aim of taxonomists at the present
time is to group forms according to homology, which is considered an in-
dication of actual relationship. Since a genetic classification must take
into consideration the entire animal, the search for affinities becomes an
attempt to evaluate the results of all morphological knowledge, and it
is also becoming evident that other things besides structure may throw
light upon relationships. The fossil records, geographical distribution,
ecology, and experimental breeding may all assist in establishing affini-
ties."

It is, of course, necessary that before any final classification can be
made one must know the various forms that exist and have existed in the
past, and one of the greatest obstacles in this field is that most animals
having a soft body have decayed and left no record of themselves among
the fossil remains thus far found. Only those which possessed an in-
tensely hard substance, or lived and died in regions where, due to the

418 GENERAL BIOLOGY

peculiar character of the soil or water, they were preserved, can furnish
us with any accurate record of the past.

There are men who have taken up individual studies in order to
ascertain all the details of their given specialties, and such men are named
after the study-group they have adopted as such specialty; for. example,
one who specializes on the protozoa is called a Protozoologist ; one who
studies worms is known as a Helminthologist ; one who studies mollusks,
a Conchologist ; one who studies insects, an Entomologist, while he who
studies birds is an Ornithologist, and he who studies mammals, a Mam-
mologist.

It is, of course, understood that these men may not be interested in
classification alone, but that they may be Anatomists, Physiologists,
Ecologists, etc., also in regard to their favorite study.

The checking up of the different conclusions which different workers
in the same field, and different workers in different fields have arrived at,
is one of the most interesting and valuable studies possible. This is
particularly true, because so frequently all the evidence that a Paleon-
tologist accepts, points to a totally different conclusion from that which
the student of experimental genetics finds to be true. The history of
science is replete with cases of groups of men having held and defended
doctrines most valiantly, and with seeming correctness, entirely opposite
to those of men in other fields of study.

References.

Schull, "Principles of Animal Biology."

H. C. Oberholser, "The Nomenclature of Families and Subfamilies
in Zoology." Science, Aug. 13, 1920.

TABULAR VIEW OF THE CLASSIFICATION OF ANIMALS AS

FAR AS ORDERS

(After Hegner, Schull, Handlirsch, Brues, Melander, Muttkowskr,

and Wheeler.)

PHYLUM I. PROTOZOA

Class I. Rhizopoda ( )

Order 1. Lobosa ( )

Order 2. Heliozoa ( )

Order 3. Radiolaria ( )

Order 4. Foraminifera ( )

Class II. Mastigophora ( )

Order 1. Flagellata ( )

Order 2. Choanaflagellata ( )

CLASSIFICATION 419

Order 3. Dinoflagellata ( )

Order 4. Cystoflagellata ( )

Class III. Sporozoa ( )

Subclass I. Telosporidia ( )

Order 1. Gregarinida ( )

Order 2. Coccidiidea ( )

Order 3. Haemosporidia ( )

Subclass II. Neosporidia ( )

Order 1. Myxosporidia ( )

Order 2. Sarcosporidia ( )

Class IV. Infusoria ( )

Subclass I. Ciliata ( )

Order 1. Holotricha ( )

Order 2. Heterotricha ( ) .

Order 3. Hypotricha ( )

Order 4. Peritricha ( )

Subclass II. Suctoria ( )

PHYLUM II. PORIFERA

Class I. Calcarea ( )

Order 1. Homocoela ( )

Order 2. Heterocoela ( )

Class II. Hexactinellida ( )

Class III. Demospongiae ( )

Order 1. Tetraxonida ( )

Order 2. Monaxonida ( )

Order 3. Keratosa ( )

PHYLUM III. COELENTERATA

Class I. Hydrozoa ( )

Order 1. Anthomedusae ( )

Order 2. Leptomedusae ( )

Order 3. Trachymedusae )J

Order 4. Narcomedusae ( )

Order 5. Hydrocorallinae ( )

Order 6. Siphonophora ( )

Class II. Scyphozoa ( )

Order 1. Stauromedusae ( )
Order 2. Peromedusae ( V , )

Order 3. Cubomedusae ( )

Order 4. Discomedusae ( )

420 GENERAL BIOLOGY

Class III. Anthozoa ( )

Subclass I. Alcyonaria ( )

Order 1. Stolonifera ( )

Order 2. Alcyonacea ( )

Order 3. Gorgonacea ( )

Order 4. Pennatulacea ( )

Subclass II. Zoantharia ( )

Order 1. Edwardsiidea ( )

Order 2. Actiniaria ( )

Order 3. Madreporaria ( )

Order 4. Zoanthidea ( )

Order 5. Antipathidea ( )

Order 6. Cerianthidea ( )

PHYLUM IV. CTENOPHORA
PHYLUM V. PLATYHELMINTHES

Class I. Turbellaria ( )

Order 1. Rhabdocoelida ( )

Order 2. Tricladida ( )

Order 3. Polycladida ( )

Class II. Trematoda ( )

Order 1. Monogenea ( )

Order 2. Digenea )

Class III. Cestoda ( )

PHYLUM VI. NEMATHELMINTHES
PHYLUM VII. ECHINODERMATA

Class I. Asteroidea ( )

Class II. Ophiuroidea ( )

Class III. Echinoidea ( )

Class IV. Holothuroidea ( )

Class V. Crinoidea ( )

PHYLUM VIII. ANNELIDA

Class I. Archiannelida ( )

Class II. Chaetopoda ( )

Subclass I. Polychaeta ( )

Order 1. Phanerocephala ( )

Order 2. Cryptocephala ( )

CLASSIFICATION 421

Subclass II. Oligochaeta ( )

Order 1. Microdrili ( )

Order 2. Macrodrili ( )

Class III. Hirudinea ( )

Class IV. Onycophora ( )

PHYLUM IX. MOLLUSCA

Class I. Amphineura ( )

Order 1. Polyplacophora ( )

Order 2. Aplacophora ( )

Class II. Gastropoda ( )

Subclass I. Streptoneura ( )

Order 1. Aspidobranchia ( )

Order 2. Pectinibranchia ( )

Subclass II. Euthyneura ( )

Order 1. Opisthobranchia ( )

Order 2. Pulmonata ( )

Class III. Scaphopoda ( )

Class IV. Pelecypoda ( )

Order 1. Protobranchia ( )

Order 2. Filibranchia ( )

Order 3. Eulamellibranchia ( )

Order 4. Septibranchia ( )

Class V. Cephalopoda ( )

Order 1. Tetrabranchia ( )

Order 2. Dibranchia ( )

PHYLUM X. ARTHROPODA

Class I. Crustacea ( )

Subclass 1. Branchiopoda ( )

Subclass 2. Ostracoda ( )

Subclass 3. Copepoda ( )

Subclass 4. Cirripedia ( )

Subclass 5. Leptostraca ( )

Subclass 6. Malacostraca ( )

Class II. Merostomata ( )

Order 1. Gigantostraca )

Class III. Poecilopoda ( )

Order 1. Xiphosura ( )

422 GENERAL BIOLOGY

Class IV. Linguatulida ( )

Order 1. Pentastomoidea ( )

Class V. Pantopoda ( )

Order 1. Clossendromorpha ( )

Order 2. Nymphomorpha )

Order 3. Pycnogomorpha ( )

Class VI. Arachnoidea ( )

Subclass 1. Cteiphora ( )

Order 1. Scorpiones ( )

Subclass 2. Lipoctena ( )

Order 1. Pedipalpi ( )

Order 2. Araneae ( )

Order 3. Meridogastres ( )

Order 4. Opiliones ( )

Order 5. Acarina ( )

Order 6. Cheloneti ( )

Order 7. Solifugae ( )

Class VII. Myriapoda ( )

Subclass 1. Opisthogoneata ( )

Order 1. Chilopoda ( )

Subclass 2. Progoneata ( )

Order 1. Symphyla ( )

Order 2. Pauropoda ( )

Order 3. Diplopoda ( )

Class VIII. Mirientomata ( )

Order 1. Protura ( )

Class IX. Collembola ( )

Order 1. Arthropleona ( )

Order 2. Symphopleona ( )

Class X. Campodeoidea ( )

Order 1. Rhabdura ( ^>

Order 2. Dicellura ( )

Class XI. Thysanura ( )

Order 1. Lepismatoidea ( )

Order 2. Machiloidea ( )

CLASSIFICATION 423

Class XII. Pterygogenea (Insecta sensu stricto), ( )

Subclass 1. Orthopteroidea ( )

Order 1. Grylloblattoidea ( )

Order 2. Orthoptera ( )

Order 3. Phasmoidea ( )

Order 4. Diploglossata ( )

Order 5. Dermaptera ( )

Order 6. Thysanoptera ( )

Subclass 2. Blattaeformia ( )

Order 7. Mantoidea ( ' )

Order 8. Blattoidea ( )

Order 9. Zoraptera ( )

Order 10. Isoptera ( )

Order 11. Corrodentia ( . )

Order 12. Mallophaga ( )

Order 13. Siphunculata ( )

Subclass 3. Hymenoptera ( )

Order 14. Hymenoptera ( )

Subclass 4. Coleopteroidea ( . )

Order 15. Coleoptera ( )

Order 16. Strepsiptera ( )

Subclass 5. Embidaria ( )

Order 17. Embiidina ( )

Subclass 6. Libelluloidea ( )

Order 18. Odonata ( )

Subclass 7. Ephemeroidea ( )

Order 19. Plectoptera ( )

Subclass 8. Perloidea ( )

Order 20. Plecoptera ( )

Subclass 9. Neuropteroidea ( • )

Order 21. Megaloptera ( )

Order 22. Raphidoidea ( )

Order 23. Neuroptera ( )

Subclass 10. Panorpoidea ( )

Order 24. Panorpatae ( )

Order 25. Trichoptera ( )

Order 26. Lepidoptera ( )

424 GENERAL BIOLOGY

Order 27. Diptera ( )

Order 28. Suctoria ( )

Subclass 11. Rhynchota ( )

Order 29. Homoptera ( )

Order 30. Hemiptera ( )

GROUPS OF INVERTEBRATES OF MORE OR LESS UNCER-
TAIN SYSTEMATIC POSITION

Group 1. Mesozoa )

Group 2. Nemertinea ( )

Group 3. Nematomorpha ( )

Group 4. Acanthocephala ( )

Group 5. Chaetognatha ( )

Group 6. Rotifera ( )

Group 7. Bryozoa ( )

Group 8. Phoronidea ( )

Group 9. Brachiopoda ( )

Group 10. Gephyrea ( )

PHYLUM XI. CHORDATA

Subphylum I. Cephalochorda or Adelochorda ( )

Subphylum II. Urochordata or Tunicata ( )

Order 1. Larvacea ( )

Order 2. Ascidiacea ( )

Order 3. Thaliacea ( )

Subphylum III. Hemichordata ( )

Order 1. Enteropneusta ( )

Order 2. Pterobranchiata ( )

Order 3. Phoronidia ( )

Subphylum IV. Vertebrata or Craniata ( )

Class I. Cyclostomata ( )

Subclass 1. Myxinoidea ( )

Subclass 2. Petromyzontia ( )

Class II. Pisces or Gnathostomata ( )

Subclass 1. Elasmobranchii ( )

Order 1. Plagiostomi ( )

Suborder I. Selachii ( )

Suborder II. Batoidei ( )

Order 2. Holocephali ( )

CLASSIFICATION

425

Subclass II. Teleostomi (

Order 1. Crossopterygii (

Order 2. Chondrostei (

Order 3. Holostei (

Order 4. Teleostei (

Subclass III. Dipneusti (Dipnoi), (

Class III. Amphibia (

Subclass I. Stegocephali (
Subclass II. Lissamphibia (

Order 1. Apoda (Gymnophiona), (

Order 2. Urodela (

Order 3. Anura (

Class IV. Reptilia (

Order 1. Chelonia (

Order 2. Crocodilia (

Order 3. Sauria (Squamata), (

Division I. Lacertilia (

Division II. Ophidia (

Class V. Aves
Subclass
Subclass

Order 1.

Order 2.

Order 3.

Order 4.

Order 5.

Order 6.

Order 7.

Order 8.

Order 9.

Order 10.

Order 11.

Order 12.

Order 13.

Order 14.

Order 15.

Order 16.

Order 17.

Order 18.

Order 19.

Order 20.

Order 21.

I. Archaeornithes (

II. Neornithes (
Hesperornithiformes (
Ichthyornithiformes (
Struthioniformes (
Rheiformes (
Casuariiformes (
Crypturiformes (
Dinornithiformes (
Aepyornithiformes (
Apterygiformes (
Sphenisciformes (
Colymbiformes (
Procellariiformes (
Ciconiiformes (
Anseriformes (
Falconiformes (
Galliformes (
Gruiformes (
Charadriiformes (
Cuculiformes (
Coraciiformes (
Passeriformes (

426 GENERAL BIOLOGY

Class VI. Mammalia ( )

Subclass I. Prototheria ( )

Order 1. Monotremata ( }

Subclass II. Eutheria ( >

Division I. Didelphia ( )

Order 1. Marsupialia ( )

Division II. Monodelphia ( )

Section A. Unguiculata ( )

Order 1. Insectivora ( )

Order 2. Dermoptera ( )

Order 3. Chiroptera ( )

Order 4. Carnivora ( )

Order 5. Rodentia ( )

Order 6. Edentata ( )

Order 7. Pholidota ( )

Order 8. Tubulidentata ( )

Section B. Primates ( )

Order 9. Primates ( )

Section C. Ungulata ( )

Order 10. Artiodactyla ( )

Order 11. Perissodactyla ( )

Order 12. Proboscidea ( )

Order 13. Sirenia ( . )

Order 14. Hyracoidea ( )

Section D. Cetacea ( )

Order 15. Odontoceti ( )

Order 16. Mystacoceti ( )

BRIEF CHARACTERIZATIONS OF THE MAJOR GROUPS

OF ANIMALS

The principal groups of animals are given below with brief diag-
noses which may serve as definitions. It must be understood that the
characters given will often not be sufficient to distinguish all the forms
in a group, for there is much variation within the groups. They are in-
tended to give the student a general conception of the phyla, subphyla
and classes.

Phylum PROTOZOA ( ). Single celled ani-

mals without true organs or true tissues. If colonial, the cells are all
potentially alike.

Class RHIZOPODA ( ). Protozoa with

changeable protoplasmic processes (pseudopodia). Amoeba.

CLASSIFICATION 437

Class MASTIGOPHORA ( ). Protozoa with

one or more vibratile processes (flagella) which serve for locomotion and
for taking food. Euglena.

Class SPOROZOA ( ). Parasitic Protozoa,

usually without motile organs or mouth, reproducing by spores.

Class INFUSORIA ( ). Protozoa having

numerous slender vibratile processes (cilia), a cuticle, and fixed openings
for the ingestion of food and the extrusion of indigestible matter. Para-
moecium.

Phylum PORIFERA ( ). Diploblastic, radially

symmetrical animals with body wrall penetrated by numerous pores.
Body usually supported by a skeleton of spicules or spongin. Sponges.

Class CALCAREA ( ). Sponges with spicules

composed of calcium carbonate, monaxon or tetraxon in form.

Class HEXACTINELLIDA ( ). Sponges with

spicules composed of silicon, triaxon in form.

Class DEMOSPONGIAE ( ). Sponges with

spicules composed of silicon, not triaxon in form, or skeleton composed
of spongin, or with skeleton of both spicules and spongin.

Phylum COELENTERATA ( ). Diploblastic,

radially symmetrical animals with tentacles, stinging cells, single gastro-
vascular cavity, no anus. Two body forms are prevalent, the hydroid
and the medusa. Jellyfishes, polyps and corals.

Class HYDROZOA ( ). Coelenterates without

stomodaeum and mesenteries; sexual cells discharged to the exterior;
hydroid and medusa forms in the life history of same species, or only the
medusa, the latter having a velum. Polyps (including Hydra), a few
corals, small Jellyfishes.

Class SCYPHOZOA ( ). Coelenterates with

only the medusoid, not hydroid form ; velum lacking ; notches at margin
of umbrella. Larger Jellyfishes.

Class ANTHOZOA ( ). Coelenterates without

medusoid forms, with well developed stomodaeum and mesenteries. Sea
anemones, most corals.

Phylum CTENOPHORA ( ). Tripoblastic

animals ; symmetry partly radial, partly bilateral ; eight rows of vibratile
plates radially arranged. Sea walnuts or comb jellies.

Phylum PLATYHELMINTHES ( ). Triplo-

blastic, bilaterally symmetrical animals with body flattened, with a single
gastrovascular cavity (sometimes wanting) and no anus. Flatworms.

Class TURBELLARIA ( ). Free-living flat-

worms with ciliated epidermis. Planaria.

Class TREMATODA ( ). Parasitic flatworms

without cilia but with a hardened ectoderm, usually parasitic and with
attaching suckers. Flukes.

428 GENERAL BIOLOGY

Class CESTODA ( ). Parasitic flatworms with

the body differentiated into a head (scolex) and a chain of similar joints
(proglottides), the whole being usually regarded as a colony. Tape-
worms.

Phylum NEMATHELMINTHES ( ). Bilat-

erally symmetrical, triploblastic animals with an elongated cylindrical
body covered with a cuticle, with a true body cavity, and a digestive tract
with both mouth and anus. Roundworms.

Phylum ECHINODERMATA ( ). Radially

symmetrical (with minor exceptions), triploblastic animals with well de-
veloped coelom, and usually with five antimeres, spiny skeleton of cal-
careous plates, and organs of locomotion known as "tube feet" operated
by a water-vascular system. Starfishes, sea urchins, sea cucumbers.

Class ASTEROIDEA .( ). Free-living, typically

pentamerous echinoderms with wide arms not sharply marked off from
disc and with ambulacral grooves. Starfishes.

Class OPHIUROIDEA ( ). Free-living, typically

pentamerous echinoderms with slender arms sharply marked off from
disc and no ambulacral grooves. Brittle stars.

Class ECHINOIDEA ( ). Free-living, pent-

amerous echinoderms without arms ; the outer covering composed of cal-
careous plates bearing movable spines. Sea urchins, sand dollars.

Class HOLOTHURIOIDEA ( ). Free-living,

elongated, soft-bodied echinoderms with muscular body wall and tenta-
cles around mouth. Sea cucumbers.

Class CRINOIDEA ( ). Sessile echinoderms

with five arms generally branched with pinnules, aboral pole usually with
cirri, sometimes with jointed stalk for attachment to substratum. Feather
stars, sea lilies.

Phylum ANNELIDA ( ). Triploblastic, bilat-

erally symmetrical elongated animals with external and internal seg-
mentation; coelom usually present; setae common. True worms.

Class ARCHIANNELIDA ( ). Marine Annelida

with no setae nor parapodia.

Class CHAETOPODA ( ). Annelida with

setae and a perivisceral coelom; marine, fresh-water, or terrestrial in
habitat. Earthworms.

Class HIRUDINEA ( ). Annelida without

setae, and with anterior and posterior suckers. Leeches.

Class ONYCOPHORA ( ). Annelida breath-

ing by means of tracheal tufts, numbering from 10 to 40 per segment in
irregular arrangement, with non-jointed papillate legs, nerve cords ven-
tro-lateral, and without segmental ganglia, eyes of vesicular, annelid
type, skin with chitin. This group is often placed with the Arthropoda,
or as a separate phylum Proarthropoda, since its members have devel-

CLASSIFICATION 429

oped somewhat in the Arthropodan direction. Lankester thinks their
evolution is as follows :

Group Articulata

1. Rotifera to Tardigrada

2. Chaetopoda

a. Proarthropoda (Peripatus) developing independently.

b. Crustacea — separate origin from Chaetopoda.
From Crustacea by separate origin

a. Myriapoda

b. Insecta

c. Arachnida.

Paleontologists such as Walcott, the specialist on trilobites and
worms, derive all Arthropoda classes by separate lines from trilobites.

Phylum MOLLUSCA ( ). Triploblastic, bilat-

erally symmetrical (symmetry often obscured) unsegmented animals
with a coelom, a muscular foot and usually a shell. Mollusks.

Class AMPHINEURA ( ). Mollusks with ob-

vious bilateral symmetry, sometimes an eight-parted calcareous shell and
several pairs of gills.

Class GASTROPODA ( ). Mollusks with a

head and with bilateral symmetry usually obscured by a spiral shell of
one piece. Snails.

Class SCAPHOPODA ( ). Mollusks with

conical tubular shell and mantle.

Class PELECYPODA ( ). Mollusks without

a head, with bilateral symmetry, a shell of two lateral valves and a man-
tle of two lobes. Clams, mussels.

Class CEPHALOPODA ( ). Mollusks with

distinct bilateral symmetry and a foot bearing eyes and divided into
arms usually with suckers. Cuttlefishes, octopods.

Phylum ARTHROPODA ( ). Triploblastic,

bilaterally symmetrical, segmented animals with usually more or less
dissimilar somites, a coelom very much reduced, paired jointed ap-
pendages, and chitinous exoskeleton.

Class CRUSTACEA ( ). Arthropods breathing

by means of gills, two pairs of antennae, Crayfishes, crabs, shrimps. Cer-
tain terrestrial species with tracheae (Oniscidae — sowbugs).

Class MEROSTOMATA ( ). Fossil Arthro-

poda of gigantic size (2 meters in length), without antennae, short
cephalothorax, 12-segments in abdomen, and pointed telson. Euryp-
terus.

Class POECILOPODA ( ). Arthropoda with

large shield-shaped cephalothorax, abdomen with six pair lamellate leers,
with extremely long pointed telson. Limulus, king crabs.

430 GENERAL BIOLOGY

Class LINGUATULIDA ( ). Parasitic Arthro-

poda (Pentostomidae) of worm-like build, body with metameric circular
muscles, two pairs of hooks in region of mouth, mouth without mandi-
bles. Affinities uncertain.

Class PANTOPODA ( ). Marine Arthropoda,

body segmented, abdomen vestigial, with not more than seven pairs of
legs, mouth a beak.

Class ARACHNOIDEA ( ). Arthropods with

either tracheae, book lungs or book gills, or both, and no antennae. Har-
vest-men; spiders, mites,, ticks, scorpions.

Class MYRIAPODA ( ). Arthropods with

distinct head, one pair antennae, breathing through tracheae, whose stig-
mata are placed in linear metameric arrangement, many legs. Myriapods
and millipeds, centipeds.

Class MIRIENTOMATA ( ). Minute micro-

scopic Arthropoda (600-1600 micra), with six legs, a three-segmented
thorax (?), no antennae, post-embryonic increase of segments, first pair
of legs transformed into sense .organ. These minute forms were only
recently discovered, and their affinity is uncertain.

Class COLLEMBOLA ( ). Arthropods with

6-segmented abdomen, no post-embryonic increase in segments, one-
jointed tarsi, few tracheae, these opening in one pair of stigmata at the
throat, abdomen generally with spring. Spring-tails.

Class CAMPODEOIDEA ( ). Arthropods with

long body, abdomen 10 segments, with cerci. No eyes, mouth-parts with-
drawn, no post-embryonic change in abdominal segments. Spring-tails.

Class THYSANURA ( ). Arthropods with

free mouth-parts and palpi, three caudal appendages, abdomen 11 seg-
ments and covered with silvery scales, frequently with spring beneath.
Silver fish, fish moths.

Class PTERYGOGENEA ( ). Insecta, Hex-

opoda. Winged Arthropods, with three pairs of legs, embryos with 12
segments to abdomen, adults with all degrees of post-embryonic reduc-
tion from 12 to 6 segments. Breathe through tracheae, stigmata linear
and metameric in arrangements. True insects — i. e., winged Arthropods.

Phylum CHORD ATA ( ). Animals having at

some time during their life's history a notochord lying between the nerv-
ous system and the alimentary tract, a hollow central nervous system
lying entirely on one side of the digestive canal, and pharyngeal slits
extending from the pharynx to the exterior.

Subphylum CEPHALOCHORDA or ADELOCHORDA
( ). Fish-like chordates with a permanent noto-

chord composed of vacuolated cells, such as Amphioxus.

Subphylum UROCHORDATA or TUNICATA ( ).

Sac-like marine animals with a cuticular covering known as a tunic or

CLASSIFICATION 431

test. This group possesses a notochord only in the caudal region. Ex-
ample, Tunicates.

Subphylum HEMICHORDATA ( ). Worm-

like chordates of doubtful systematic position. There is a projection
from the mid-dorsal region of the alimentary canal similar to a noto-
chord. These animals possess a collar and a proboscis. Example,
Balanoglossus.

Subphylum CRANIATA or VERTEBRATA ( ).

Chordates in which the notochord either persists or becomes invested
with cartilage. Vertebrates have a segmented spinal column.

Class CYCLOSTOMATA ( ). Eel-like verte-

brates without functional jaws or lateral appendages. Examples, hag-
fishes and lampreys.

Class PISCES or GNATHOSTOMATA ( ).

Fishes with a lower jaw and paired pectoral and pelvic fins, scales and
paired nostrils. The heart has an auricle, a ventricle, a conus arteriosus
and a sinus venosus.

Class AMPHIBIA ( ). Cold-blood vertebrates

breathing by means of gills at some stage of their life-cycle. Skin not
usually covered with scales. Three chambers in heart beside the conus
arteriosus and sinus venosus. Frogs, toads, newts, and salamanders.

Class REPTILIA ( ). Cold-blooded verte-

brates breathing by means of lungs throughout their life-cycle. Usually
covered with scales. Lizards, snakes, crocodilians, and turtles.

Class AVES ( ). Warm-blooded vertebrates,

whose body is usually covered with feathers and the fore-limbs modified
far wings. Heart of four chambers. Birds.

Class MAMMALIA ( ). Warm-blooded ani-

mals with hair covering at some stage in their life-cycle. They suckle
their young and have a diaphragm between thorax and abdomen.

Subclass PROTOTHERIA ( ). Egg-laying

mammals. Example, monotremes, such as the Australian duck-bill.

Subclass EUTHERIA ( ). Mammals which

give birth to living young. These are the true mammals.

Division DIDELPHIA or METATHERIA ( ).

These are the marsupials, such as the opossum and kangaroo.

Division MONODELPHIA ( ). These are the

placental animals which are nourished in the body of the mother through
a true placenta.

INVERTEBRATE GROUPS OF UNCERTAIN POSITION

Certain groups of invertebrates have not been assigned a definite
relation to other groups. Opinion differs so widely as to their affinities
that they may well be kept out of the classification for the present.

Mesozoa. Parasites apparently intermediate between the Protozoa
and Metazoa. Not improbably degenerate relatives of the flatworms.

432 GENERAL BIOLOGY

Nemertinea. Terrestrial, fresh water and marine animals resem-
bling flatworms but with a proboscis, blood vascular system, and ali-
mentary canal with two openings.

Nematomorpha. Long thread-like animals with the body cavity
lined with epithelium, a pharyngeal nerve ring and a single ventral nerve
cord.

Acanthocephala. Parasitic worms with spiny proboscis, a complex
reproductive system and no alimentary canal.

Chaetognatha. Marine invertebrates with a distinct coelom, alimen-
tary canal, nervous system and two eyes.

Rotifera. Invertebrates with a head provided with cilia, usually a
cylindrical or conical body often with a shell-like covering, and a bifur-
cated tail or foot provided with a cement gland.

Bryozoa. Mostly colonial invertebrates resembling hydroids in
form, with distinct coelom, and with digestive tract bent in the form of
a letter U.

Phoronidea. A single genus of worm-like animals having tentacles
and living in membranous tubes in the sand.

Brachiopoda. Marine tentaculate animals with a calcareous shell,
composed of two unequal shell-parts (commonly called valves), a dorsal
and a ventral.

Gephyrea. Worm-like animals of doubtful affinities.

Part II
Introductory Embryology

(Chick, Frog, and Mammal)

CHAPTER XXIX.

EMBRYOLOGY OF THE CHICK

THE DEVELOPMENT OF THE EMBRYO BEFORE THE

EGG IS LAID

Before beginning the work proper in Introductory Embryology ii
is quite essential that the student turn back to earlier Chapters and re-
read what is said there on mitosis, fertilization, and the histology of the
frog. Such a review will lay a foundation for the detailed study of the
following pages.

When we come to take up Comparative Anatomy in the next semes-
ter's work, it will be found that the Haeckelian law of biogenesis (also*
called the "recapitulation theory"), although untrue in its usual applica-
tion, is a very convenient supposition in that it makes many points clear
if we accept it as a working hypothesis. This so-called law is denned
as follows : All animals, during their embryonic period, pass through
the same adult-stages that the various members of the race to which
they belong have passed. For practical purposes it is necessary to keep
this theory in mind in the study of embryology; for, it is the simplest
way of bringing home to the student the fact, that in any biological study
that is to be scientific, one must first study the more simple organisms
and then compare such simple forms with those that are more complex —
the so-called higher forms.

All living animals pass through a quite similar stage of development
in their embryonic period, so that the next succeeding higher form prac-
tically possesses everything that the immediately next succeeding lower
form possesses, plus something additional. And it is this "plus some-
thing" that we are trying to arrange in proper order when we study
embryology.

The value of this is not always clear to the student, but if he will
remember that a human being and a chick pass through quite similar
stages during their embryonic periods, the human being, however, devel-
oping further, he can understand how an obstruction may prevent any
individual part of an organism from receiving the proper nourishment
and environment, and thus cause such part to cease developing, thereby
producing what is called a rudimentary structure. (Fig. 250.)

Now, while all animals differ slightly from each other, there are
certain type-forms, in which the greatest differences can be clearly ob-
served. Such type-forms as commonly used in the laboratory are the
dog-fish, as a representative of the cartilaginous fishes; the frog, as an
example of amphibia; the chick or pigeon, as an example of birds; the
turtle, as an example of reptilia ; and the cat, rabbit, or pig, as an exam-
ple of the mammals.

436 EMBRYOLOGY OF THE CHICK

Fig. 250.

There is a membrane covering the pupil of the eye which, in man, normally dis-
appears when the embryo is seven months old. In the case here shown portions of
the membrane have persisted as an irregular network over the pupil. Such
persistent structures are called rudimentary. (From a drawing lent by Dr. G. N.
Brazeau.)

As we have been using the frog as a norm, or standard type, with
which to compare the other forms studied, it would probably seem best
to begin embryology with that animal. However, for the same reason
that the frog was used as an introductory subject for study (because
it can be procured easily and because it is a fairly complex form which
possesses structures with which the student is already familiar), so, the
hen's egg, which is much larger than that of the frog, can also be pro-
cured easily and is already somewhat familiar to the student. And in
addition to this, the chick embryo develops upon the surface of the yolk,
which makes the various germ layers very distinct, thus serving much
better than the frog as a beginning-type.

The first and foremost point in the study of embryology is accuracy
of observation; the second is the obtaining of a clear and understandable
concept of what has been observed ; and th'e third point is to show by
drawings that the first and second have been fully grasped.

There is considerable need for legitimate imagination in embryo-
logical work, because the entire study of embryology is for the purpose
of giving the student a more or less comprehensive idea of the process
through which, and by which, all the organ-systems in the body of living
things have come to be what they are. The study of embryology is
therefore different from our later work in pure anatomy, where each
structure is definite, and where such structure is studied only after it is
completely formed. In embryology we see the beginnings and develop-
ment of these later anatomical structures.

One should first take the complete embryo, and get a good grasp of
the general structure. Then, sections must be cut at various intervals
and studied microscopically. It must never be forgotten, however, that
our imagination must constantly remind us that there are three dimen-
sions to the living animal, and that what we are looking at in our section,

DEVELOPMENT OF THE EMBRYO 437

is but a series of still pictures, and that there is little value or meaning
in such observation unless one can, with imagination and logic, plus
preceding biological knowledge, build up a completed structure, so that
the mind's eye can see the entire animal as it actually is.

It must be remembered at this point that events which have already
taken place in the past, are the cause or causes of events that are now
taking place, and that will take place later. This is as true in embry-
ology as it is in such a study as history, for example. This means that
the various events of development are caused by preceding develop-
mental events and that these cause later steps in development in turn.

Another important point for the student to remember is that he
must not only be able to recognize histologically the type of cells he
may find in the section he is studying, but he must know the definite
location in the complete embryo from which his section is cut.

The complete bird-like form of the chick can be clearly seen before
the eighth day of incubation, all the principal changes having taken
place by that time. It will, therefore, be understood that these changes
are rather minute in their origins, for the eight-day embryo is only about
seven milHmeters in length. During, and after the eighth day,- the
changes which take place are primarily enlargements, or growth of por-
tions already present.

In the study of embryology we are not only interested in the devel-
opment of the chick from the egg, but we also wish to know how the
egg came, into existence.

The hen's egg is usually said to be a single cell. This is, however,
only true if the egg is unfertilized.

As birds' eggs are laid with shells upon them, it is necessary that
fertilization take place before the shell is formed. Fertilization in these
cases is internal. It takes about twenty-two hours for the egg to have
the various layers of yolk and white laid down, and for the shell then
to surround it. If the egg has been fertilized, the warmth of the mother's
body has already caused development throughout these days, so that
by the time the egg is laid, the little chick is already approximately two,
or two and a half, days of age. There is a variation in the age because
if the hen's egg is ready for laying during the main part of the day, it
is laid then, but if it is not ready for laying until, let us say, about four
or five o'clock in the afternoon, it is retained within the mother's body
until the following day, thus causing some embryos to be developed from
ten to fifteen hours more than others.

The so-called spoiling of eggs is usually due to the embryo chick
dying and then decaying.

In birds, where the eggs leave the mother's body, the yolk must
be quite large in order to furnish sufficient food for the embryo during
the two or three weeks intervening between the time the egg leaves the
mother's body and the time of hatching. In the viviparous animals, the

438

EMBRYOLOGY OF THE CHICK

egg remains extremely small because the nourishment of the embryo is
derived directly from the mother.

During the very first day of incubation the outlines of the embryo
are defined. During the second day a rather complicated series of folds
appear, separating the embryo from the yolk. The embryo, however,
remains in contact with the yolk-mass by a narrow stalk. The circula-
tory system now develops, through which nourishment is carried from
the yolk-mass to the embryo. Embryonic membranes and appendages
appear during the second and third days of incubation. These assist in
respiration and also in forming a larger area from which the food supply
may be brought from the yolk to the embryo.

Development usually begins at the head end and extends tailward,
so that the brain and other head structures are often quite well devel-
oped when there is little semblance of any other well-defined structure
toward the tail end. The chick usually makes a small opening in the
egg shell on about the twentieth day of incubation, and from then on
the lungs actually take in air and begin their regular external work, while
on the twenty-first day the chick breaks through the shell entirely.

With this introductory general outline, we shall take up the study
of the egg itself, working backward to its very simplest cell origin in
the mother's ovary.

THE EGG

The true ovum (Fig. 251, v.), or egg-cell proper, is the large yolk
or vitellum. This is surrounded by a tough vitelline membrane. The

ad

*/

Blastopore
(crescentic
groove)

yy

Fig. 251.

Semidiagrammatic illustration of the hen's egg at the time of laying. A.
Entire "egg." B. Diagram of a vertical section through the vitellus or ovum
proper, showing the 'concentric layers of white and yellow yolk, a, Air chamber ;
ac, chalaziferous layer of albumen ; ad, dense layer of albumen ; a}, fluid layer of
albumen ; 6, blastoderm ; c, chalaza ; I, latebra ; nl, neck of latebra ; P, nucleus of
Pander ; pv, perivitelline space ; smi, inner layer of shell membrane ; smo, outer
layer of shell membrane ; v, vitellus or "yolk" ; vm, vitelline membrane ; wy, layers
of white yolk ; yy, layers of yellow yolk. C. Surface view of Blastoderm of un-
incubated hen's egg. (A and B. after Marshall ; C, after Hertwig.)

DEVELOPMENT OF THE EMBRYO 439

end of the ovum, where the embryo is to develop, is called the animal
pole. It is nearly free from yolk, and appears at the time of laying as
a circular whitish area known as the blastoderm (Fig. 251, b), and
measures from three to four millimeters in diameter. As the animal
pole is not as dense as the surrounding material, it is always found on
top of the yolk, no matter which way the entire egg be turned, provided,
of course, that the yolk is free to rotate.

The more central portion of the animal pole is rather translucent
or pellucid and therefore is called the area pellucida (Fig. 251, C). This
central portion is surrounded by a whitish or opaque region called the
area opaca. The yolk itself is called deutoplasm, and is divided into two
types of material, white and yellow yolk. The white yolk occupies the
region just below the blastoderm, and is rather shaped like a flask, as
shown in the figure. It extends to the center of the yolk. It will be
noticed that the yolk is thus arranged in various concentric layers. A
layer of thick yellow yolk alternates with a thinner stratum of white
yolk. The two types of yolk differ in physical characteristics and in
chemical composition.

The vitellus or true egg-cell alone is formed in the ovary. Such
structures as develop within the ovary proper are called primary. Struc-
tures, such as chorionic membranes (found in most of the higher forms),
are known as secondary structures, while those particular regions which
are formed by accessory reproductive organs, such as the white of the
egg and the shell, are said to be tertiary structures. The white of the
egg is composed of albuminous matter which is chemically quite com-
plex. It will be remembered that the protoplasm in all living cells is
largely albuminous.

Toward each end of the newly-laid egg, one finds a dense, opaque
twisted cord extending through the white of the egg from opposite sides
of the yolk toward the apices of the shell. These twisted cords are called
chalazae (Fig. 251, C). They are continuous with a very thin, dense layer
of albumen surrounding the yolk. This thin layer is called the chalazif-
erous layer. It is generally assumed that the chalazae assist in holding
the yolk in position, though this has been disputed by several biologists,
primarly because the ends nearest the shell are not attached. Immedi-
ately outside the chalaziferous layer there is another thick, dense layer
of albumen, and superficial to this is a still thicker layer of a more fluid
albumen. The hard-boiled egg in which the albumen has coagulated
lends itself well for the observation of these various layers. Usually, in
observing such hard-boiled eggs the albumen is seen to be arranged in
spiral sheets.

The ovoid shell which surrounds the entire egg is quite resistant to
gradually applied pressure, but easily broken if the blow be sharp. The
shell in turn is covered superficially by a thin cuticle perforated by many
pores. The main substance of the shell is made up of loosely arranged

440

EMBRYOLOGY OF THE CHICK

particles of carbonates and phosphates of calcium and magnesium. The
inner surface of the shell is composed of a thin but dense layer of in-
organic salts. After the shell has dried, it is quite porous, thus making
the passing of gases and water-vapor quite easy.

There is a tough shell membrane lining the inner portion of the
shell. It is composed of a double sheet of fibrous connective tissue which
separates at the blunt end of the egg into an air space, becoming larger
as time goes by,

THE REPRODUCTIVE

One obtains a more thorough
the egg in the ovary if a review of

el

Fig. 262.

The reproductive system of the fowl. The
figure shows two eggs in the oviduct, whereas
normally only one egg is in the oviduct at a
time. 6, Blastoderm ; c, cicatrix ; cl, cloaca ;
da, dense layer of albumen ; /, empty egg
follicle from which the ovum has escaped; a,
glandular portion of oviduct ; i, isthmus ; m,
mesovarium ; o1-o4, ovarian ova in various
stages of growth ; O1, ovum in upper end of
oviduct ; Oa, ovum in middle portion of ovi-
duct (the oviduct has been cut open to show
the structure of this ovum) ;os, ostium or in-
fundibulum ; ov, ovary containing ova in
various stages of growth ; r, rectum ; u.
uterus ; v, vitellus ; w, ventral body wall,
opened and reflected. (From Duval. )

ORGANS OF THE FOWL

understanding of the development of
the entire reproductive organs is un-
dertaken. The reproductive organs
of the fowl do not develop equally
on each side, though they begin de-
veloping symmetrically. The right
ovary ultimately degenerates, and as
far as we know does not function.
The left ovary (Fig. 252) and ovi-
duct alone carry on the work of the
organs. The left organs therefore
become quite large.

A microscopical section of the
ovary shows this organ to be com-
posed of a great quantity of ova,
each ovum being contained in a
Graafian follicle (Fig. 253). The
ovary itself is suspended from the
dorsal abdominal wall by a double
fold of the peritoneum called a
mesovarium.

In the hen, the ova vary in size
from a very small cell up to the full
sized yolk. The oviduct is large,
thick-walled and muscular, being
convoluted, and having a different
structural form in the different
parts. As the oviduct is to carry
eggs from the ovary to the uterus,
there are two openings, one the ab-
dominal, which is rather wide and
flaring and funnel-shaped, coming
in close contact with the ovary.
This opening is called the ostium or
infundibulum, or the fimbriated
opening. This last name is due to

DEVELOPMENT OF THE EMBRYO

441

its fringe-like margin. This region is thin and muscular and lined with
cilia. The oviduct proper into which the ostium leads is known as the
convoluted glandular portion, which is followed by a short, third portion
called the isthmus. It is after passing through the isthmus that the egg
enters the so-called uterus, which is merely a dilated portion of the
glandular tube. The uterus in turn opens into a short terminal region,
a rather thin- walled vagina, and this again opens into the cloaca, just
dorsal to the opening of the rectum.

It is easier for the student to understand a developmental history
of the egg if it be thought of as passing through three periods. First,
from the beginning of the development of the ovum to the time of ovula-
tion. Second, from the time of ovulation through the period of fertiliza-
tion, and third, from the beginning of cleavage to the time the egg is
laid.

First Period. (From the beginning of the development of the egg
to the time of ovulation.) Most animals produce a large number of eggs
Avithin a very brief period, while in the hen there is a long period of egg
formation and laying, which extends over several months, after which
there is a period of almost complete cessation. Undoubtedly the reason
for this is that when an egg becomes as large as that of the hen, which

requires so much food in its
making, it is a considerable
drain upon the animal, and
secondarily, there isn't room
enough in the body of an
animal no larger than a hen
for many eggs of such size.
However, the fact that the
hen's ova develop in the way
they do, makes it possible
for us to observe almost a
complete succession of de-
velopmental changes from
the minute forms up to the
fully developed egg.

In our course of general
biology, we learned that very early in an organism, especially in triplo-
blastic forms, the germplasm and somatoplasm differentiate. A few cells
are set aside in the innermost portion of the body of the growing embryo
for reproductive purposes. The development in the growing embryo of
the germplasm is called oogenesis in the female, and spermatogenesis
in the male. It will also be remembered that all the eggs are already
present in the ovary at the time the female is born.

Ovum

follitular
epithelium.

Fig. 253.

Section from ovary of adult dog. The more or less
star-shaped figure on the right is a collapsed follicle
with its contents. Below and at the right are seen the
tubules of the Parovarium. (After Waldeyer.)

442

EMBRYOLOGY OF THE CHICK

OOGENESIS

The process by which the eggs already present in the ovary of the
new-born chick originally came to be what they are is known as oogene-
sis. The first event in oogenesis is known as the multiplication of the
oogonia. This occurs during the embryonic period of the animal. There
are two types of cells which develop from the original primary cells
set aside for reproductive purposes. How and why these differentiate
in the way they do we do not know, but we do know that there is a dif-
ferentiation.

As soon as these original cells commence to divide, some of them
develop into centrally located eggs or ova (Fig. 254), while others,
known as germinal epithelium, surround the more centrally located ova
and form a sort of case or capsule around them. The primitive egg
surrounded by this epithelial case is known as an oogonium. Some of
these leave the epithelium and pass into the stroma of the ovary. There
they degenerate. Those remaining, however, begin enlarging even while
they are dividing and multiplying. The epithelial cells also divide very
rapidly, forming long strands or cords which in turn extend into the

stroma. There comes a
time when these primitive
ova or oogonia stop multi-
plying; they are then called
primary oocytes. At this
time the strands or cords of
germinal epithelium break
up into little groups some-
times called nests. Each
nest consists of a single pri-
mary oocyte (Fig. 255) sur-
rounded by a number of the
original epithelial cells.
These latter cells form a
definite case surrounding the
oocyte. The case thus
formed is called the primi-
tive egg follicle. This final
arrangement takes place
within a few days after
hatching. It will thus be
seen that all the eggs which
enlarge, ripen, and pass out of the ovary are merely enlarged and devel-
oped primary oocytes.

Both the nucleus and the cytoplasm of the egg cell now begin to
enlarge, and yolk granules are laid down all about the centrally located

86.

Section of the Germinal Epithelium and Adjacent Stroma

in a Chick-Embryo.

g.ep., germinal epithelium forming a thickened
ridge-like projection ; pr.ov., primitive ova of various
sizes, some in the germinal epithelium and others some-
what beyond the limit of this epithelium ; st., strands of
cells which have grown from the germinal epithelium,
and one of which appears connected with an enlarged
primitive ovum. (From Semon.)

DEVELOPMENT OF THE EMBRYO

443

nucleus as well as throughout the cytoplasm, except in the peripheral
region. This- region remains comparatively free from yolk. At the point
where the ovum or follicle is attached (Fig. 256), there is a thicker por-
tion in the periphery known as the germinal disc or spot. As soon as
the ovum reaches a diameter of about five-tenths of a millimeter, the
nucleus migrates into the germinal disc, where it remains as long as the
egg continues in the ovary. An important point to remember is that
the animal pole of the ovum is toward the attached surface, that is, at
the point where the nucleus is located.

From this time onward, the yolk accumulates very rapidly. The
surface of the ovum is in the form of zona radiata (Fig. 256, B), in wrhich
there are many pores through which nutritive substances may easily
diffuse from the follicle cells. These follicle cells may therefore be called
nurse cells.

The manner in which we estimate the rate of growth in all final
stages of the hen's egg is by the arrangement of the concentric layers
of white and yellow yolk. This is possible, because layers mark daily
additions in the formation of yolk or food-matter.

When the follicle has completed its growth, it becomes somewhat
membranous. Directly opposite its point of attachment there are very

Fig. 255.

Young Oocyte surrounded by a single
layer of Follicular Cells. (Van der
Stricht.)

Showing attraction-sphere, centro-
some, and mitochondria.

Corona radiata.
Zona pellucida.
Germinal spot.

Germinal vesicle or
nucleus.

Fig. 256.
A, ripe Graafian follicle. B, ovum.

few blood vessels, and it is at this point that a modification takes place
in the form of a band appearing, known as the cicatrix. It is at the cicatrix
that the follicle ruptures to permit the escape of the egg into the oviduct.
The nucleus lies flat against the vitelline membrane, and becomes
very large just before the egg leaves the ovary. It is then called a
germinal vesicle, because the chromatin condenses, which leaves the
nucleus appearing as a large clear spot. The nuclear wall now breaks

444 EMBRYOLOGY OF THE CHICK

down and forms the first polar spindle. This rotates into position and
the primary oocyte is ready for its first maturation division, and later,
for ovulation.

Second Period. (Ovulation, Maturation, and Fertilization.)
The. coordination of different functions in the body is well shown
by the fact that at about the time a completed egg is ready to pass into
the oviduct, the region of the ostium of the oviduct becomes very active
and actually seems to grasp the ovarian follicle which contains the pri-
mary oocyte. This may be due to muscular or ciliary action or it may
be a combination of both. The follicle then ruptures, permitting the
egg to be thrown out. It seems that the pressure exerted by the con-
traction of the fringed end of the ostium may have had something to
do with such rupture. The throwing of the eggs out of the follicle is
called ovulation. The oocyte always enters the infundibulum of the
oviduct with its chief axis transverse to the long axis of the oviduct, and
throughout its entire passage down the tube, this relation is retained.

After the sperm has been injected into the female, they make their
way up the oviduct toward the ovary, seeming to gather at its end.
They may remain- alive and function for at least two weeks, sometimes
even longer. It will thus be noted that as soon as the egg has been dis-
charged from the follicle and has been taken into the oviduct, there are
millions of sperm floating about in the fluid surrounding it. A single
egg of the hen, unlike that in most animals, has from five to twenty-four
spermatozoa enter it. Such a process is known as polyspermy.
Polyspermy is abnormal in most animals, but it is the normal condition
in the hen. The egg is now fertilized. The sperm apparently affords the
stimulus which causes the egg to begin dividing and to form an embryo.

The egg, after the entrance of the various spermatozoa, is not yet
completely mature. A process of maturation now takes place. This
means that the egg divides into a larger and a smaller portion, both of
which portions may again divide into two parts. All of the smaller por-
tions degenerate, one large one alone developing into a complete, fer-
tilized, hen's egg. The reason for these small polar bodies (as the de-
generating portions are called) is that one-half of the chromatin must
be lost in order that the new-born young may be a normal individual
like its parent, as explained in our studies of mitosis, maturation, and
genetics.

After the second maturation division, the remaining nucleus unites
with a single sperm nucleus to form the first cleavage spindle, and the
egg is now ready to begin dividing and form a true embryo.

Third Period. (From the Beginning of Cleavage to the time the
egg is laid.)

It must be remembered here that the fertilized egg, which is to
become the embryo, is present in the hen's body quite a number of hours
before the egg is laid. In fact, from two to three days before the various

DEVELOPMENT OF THE EMBRYO

445

layers of white, yolk, and shell, have encircled it. The heat from the
mother's body has caused the embryo to begin to form, so that by the
time the egg is laid, the embryo is already several days old. It is there-
fore essential that the student understands in detail, exactly what has
already happened in the mother's body before the egg passes to the outer
world.

The first cleavage furrow can be seen about three hours after the
ovum has been discharged from the follicle. During this period the egg-
has passed along the entire glandular portion of the oviduct. The glands
themselves have secreted the most dense portions of albumen and also
the chalazae, it being remembered that the yolk was already laid down
before ovulation. The egg is carried along principally by peristaltic
action of the walls of the oviduct. Then, as the egg itself rotates, the
germ disc comes to describe a spiral path, which explains the spiral ar-
rangement of the albumen around the yolk. The egg then traverses the
isthmus for approximately an hour where the shell membrane is secreted
over the dense albumen. The fluid layer of albumen is secreted both in
the isthmus and the upper part of the uterus. . The fluid layer of the
albumen passes through the shell membrane which has already been laid
down, and it takes from five to seven hours after the egg enters the
uterus before this is completed. But, before this takes place, the shell
substance itself has already begun to be laid down on the shell mem-
brane. Usually twelve to sixteen hours are necessary to complete the
passage through the uterus and vagina. At the end of this time twenty-
one to twenty-seven hours have already elapsed since ovulation took
place. Gastrulation has begun, and the egg is laid.

We have already mentioned that if the egg reaches the vagina, ready
to be laid, during the main portion of the day, it will be laid that day,
but if it should be ready for laying after four or five o'clock in the after-
noon, it will be retained in the vagina until the following day, thus caus-
ing some embryos in freshly-laid eggs to be approximately twelve hours
older than others. It is for this reason that there is always considerable
variation, even when eggs have been incubated for the same number of
davs.

TABLE (After Kellicott) SHOWING THE CHIEF EVENTS
THE EARLY HISTORY OF THE HEN'S EGG

IN

Hours after

Ovulation

0

0 to 3

Location in

Oviduct
Infundibulum

Glandular
Portion

Action of Oviduct
Reception of Ovum

Secretion of chala-
zae, chalaziferous
and dense albumen
layers.

Action of Germ Disc

Maturation and Fer-
tilization.

First cleavage fur-
row.

EMBRYOLOGY OF THE CHICK

3 to 4

Isthmus

Secretion of shell
membrane and

Formation of eight
cells.

fluid albumen.

4 to 21 (27)

Uterus and
Vagina

Secretion of shell
and fluid albumen.
Retention prior to
laying.

Gastrulation begun,
or completed if
egg is long re-
tained.

With what has just been said in mind, the developmental processes
of an embryo become more understandable." The unicellular germ disc
is composed of a very definite area at the animal pole. The disc itself
is about three millimeters in diameter, and less than five-tenths milli-
meters in thickness. Directly beneath this disc, there is a merging of
the protoplasm with the white yolk. This well-marked region is called
the nucleus of Pander (Fig. 251, P), and this connects the central white
yolk by a narrow stalk called the latebra. It is necessary to study all
the figures carefully to understand these and successive terms, and to
grasp the relationship of each to the other.

There are two regions in the disc itself : the larger central portion

in

Fig. 257.

Cleavage. Upper Row, Amphioxus. (After Hatschek. ) 1, Unfertilized egg; 2,
stage of two blastomeres ; 3, stage of four blastomeres ; 4, stage of eight blast-
omeres ; 5, stage of seventy-two blastomeres ; 6, section of blastula ; p.b., polar body.
Middle Row, Frog. B, segmentation cavity, v, nucleus. Lower Row, Hen's egg.
(After Patterson.) Surface views of the blastoderm and the inner part of the
marginal periblast only. The anterior margin of the blastodisc is toward the top
of the page. A. Two-cell stage, About three hours after fertilization. B. Four
cells. About three and one-fourth hours after fertilization. C. Eight cells. About
four hours after fertilization. D. Thirty-four cells. About four and three-fourths
hours after fertilization. E. One hundred and fifty-four cells upon the surface ;
the blastoderm averages about three cells in thickness at this stage. About seven
hours after fertilization, ac. Accessory cleavage furrows ; m, radial furrow ; p,
inner part of marginal periblast ; sac, small cell formed by the accessory cleavage
furrows.

DEVELOPMENT OF THE EMBRYO

447

which is to form the blastoderm proper, and the narrow denser area
known as the periblast, which forms the outer margin. The periblast
forms a continuous layer over the yolk, peripherally.

In the center of the germ disc, the first cleavage furrow appears.
(Fig. 257). It is short and shallow, running about one-half the diameter
of the disc. We do not even know whether the first cleavage extends
directly through the central portion of the embryo. The main embry-
onic axis lies almost at right angles to the long axis of the whole egg,
the head end of the embryo being directed toward the left when the
sharp end of the egg is held pointing away from the observer. The first
cleavage plane does not seem to have any definite relation to either of
these axes.

The second cleavage is also vertical, and almost at right angles to
the first, so that we have four adequal cells, all, however, incomplete.
The third cleavage appears about an hour after the first. This is usually
parallel to the first. It divides the disc into two rows of four cells each.
This cleavage may be quite irregular in form, and from now on it is im-
possible to tell exactly how and when, in relation to time especially,
these egg cells divide. Consequently, after they have divided and formed
sixteen cells, all of these cells are very irregular, and there is a tendency
in the fourth cleavage plane to separate the eight cells into a central and
a distal one.

' Fig. 258.

Diagrams showing the blastulae : A, of Amphioxus ; B,
of frog, and C, of chick ; D, blastodermic vesicle of mammal.
(After Semon.)

A group of central cells now becomes circumscribed, but these must
not be confused with the marginal cells which remain incomplete both
below and distally, retaining their connection with the periblast. These
central cells have been separated by a horizontal cleavage plane, and
this cleavage plane separates the more superficial cellular elements from

448 EMBRYOLOGY OF THE CHICK

the underlying undivided protoplasm, leaving a little space, which is the
beginning of the segmentation cavity or blastocoele (Figs. 257, II B, and
258). The undivided protoplasm beneath is called the central periblast,
the original periblastic region being now known as the marginal periblast.
Both of the periblastic regions retain their connection with each other
peripherally in the deeper regions of the marginal cells.

The question that may arise here is, "What has become of the ac-
cessory or supernumerary spermatozoa?" Between the time of fertili-
zation and the first cleavage, these have formed nuclei which migrated
to the outlying portion of the blastodisc. There they probably divided
once or twice to form small groups of daughter nuclei. There even
seems to be an attempt of the cytoplasm to divide, and sometimes short
superficial growths are actually formed. These are called accessory
cleavages. They can be seen during the four and eight cell stage, usually
radial in direction, lying just across or outside the margin of the blasto-
disc. No true cells, however, are formed by such cleavages. The ac-
cessory sperm nuclei all degenerate rather rapidly, the accessory cleav-
ages fading away, so that by the time the embryo has reached the thirty-
two cell stage, no traces of these accessory structures can be found at all.
As cleavage continues, the number of central cells increases very
rapidly, by the marginal cells dividing and being added to the central
cells, although the central cells divide likewise. This latter multiplica-
tion is very rapid, the cells diminishing in size. For example, cleavages
appear in the central cells, causing the roof of the blastocoele to become
several cells in thickness. There are no cells added to the germ disc
from the floor of the segmentation cavity. The continual cutting off of
central cells from the marginal cells causes these latter to be consid-
erably shortened, until finally they are limited to the extreme margin of
the blastodisc only.

After division has taken place so that two or three hundred cells
have been formed, there are intercellular furrows extending out into the
marginal periblast. Up to this time, there have been no nuclei what-
ever in either central or marginal periblast, but two areas which are
continuous now become converted into a nucleated syncytium. Our
knowledge of this developing process comes from the study of the
pigeon. It has not been worked out in the chick. The process is some-
what like this: The marginal cells have become spherical in form, by
having the central cells cut off from them. Their nuclei now divide,
although the cytoplasmic divisions are either completely lacking or do
not completely divide. The free nuclei therefore become quite extensive
in the margin of the blastodisc, and as these nuclei continue multiplying,
they wander off into the marginal periblast so that nuclei are scattered
about quite thickly, though the structure itself is non-cellular. Some
of the nuclei also migrate inward below the blastodisc, so that the cen-
tral periblast is likewise converted into a nucleated structure with the

DEVELOPMENT OF THE EMBRYO 449

exception of the middle area above the nucleus of Pander. This area
continues to remain free from nuclei ; in fact, what is later to be known
as the germ wall is partly composed of the nucleated rim of the peri-
blast.

The blastoderm, which is rather circular, extends radially., both on
account of the growth of its own cells, and by the addition of cells from
the marginal periblast. The original region of the blastodisc becomes
thinner and transparent. It is then called the area pellucida. The cir-
cular margin which is derived from the periblast is called the area opaca.
The ring-like periblast keeps on growing, while additional nuclei are
formed peripherally. At the same time, the periblast is contributing
cells to the blastoderm also, so that it steadily increases in diameter.
The inner nucleated margin of the periblast, which later becomes cellu-
lar, contributes to the later extra-embryonic tissues and is called the
germ wall. The cells of the blastoderm later extend peripherally so that
they overlap the inner margin of the germ wall, to form a narrow region
transitional between pellucid and opaque areas.

It should be noted here that the lower surface of the periblast is
directly continuous with the yolk mass, and peripherally it is continuous
with a very thin superficial tissue of protoplasm. This latter is also
often referred to as a part of the germ wall.

As soon as the blastoderm has become thinned out as mentioned
above, the blastula stage is completed.

It is well at this point partially to summarize the development
through the morula and blastula stage before taking up gastrulation.

THE MORULA STAGE

While text-books usually speak of an "end" to the segmentation
process, it must not be supposed that the cells of the embryo stop divid-
ing. The whole process is continuous, and the word "end" here means
only that the general process of cell-division is now "general" no longer,
but that differentiation begins. The ending of the segmentation stage
means only that one can from this period on find a grouping or aggre-
gation of cells which are not all alike.

In eggs in which there is but little yolk, the segmentation results
in a rounded, closely packed mass of embryonic cells (blastomeres)
called a morula. This name has been given such a cell mass because it
resembles a mulberry. This morula stage, in eggs with little yolk, cor-
responds to the stage in the "end" of segmentation in the chick embryo.
At this time the embryo is a simple disc-shaped mass of cells, several
layers in thickness. This whole mass is the blastoderm. It lies closely
applied to the yolk.

The cells in the center of the blastoderm are smaller and quite
clearly defined, while the surrounding or peripheral cells are flattened,
larger, and in more intimate contact with the yolk beneath.

450

EMBRYOLOGY OF THE CHICK
BLASTULATION

The chick embryo remains in the morula stage for a very short
period, when there is a rearrangement of cells preliminary to the blastula
formation. First, a cavity forms beneath the blastoderm due to the
smaller central cells separating from the underlying yolk. The outlying
cells remain attached. This space is called the segmentation cavity or
blastocoel, while the marginal area of the blastoderm which remains
attached to the yolk is called the zone of junction. As soon as the seg-
mentation cavity is thus formed the embryo is said to be in the blastula
stage. .

From Figure 259, which shows only the blastoderm and a portion
of the yolk (the yolk being about three feet in diameter at this magnifi-
cation), a good understanding of the difference a larger amount of yolk
makes in the blastula-formation may be had.

In eggs with little yolk a definite morula or solid sphere of cells can
easily form, which may then develop into a hollow sphere or blastula.
But in eggs with a large quantity of yolk, as in the pigeon and the chick
(Fig. 258), the blastomeres are forced to grow on the surface of the

SECTION AT e-c 1ft C

Fig. 259.

Diagrams to show various stages in the gastrulation of a bird embryo. In
the surface-view plans, the blastoderm is supposed to be transparent so the under-
lying structures may be located. A, surface view of blastoderm, just before
invagination ; B, surface view of blastoderm, invagination well advanced ; C,
surface view of blastoderm at end of gastrulation ; D, vertical section through
blastoderm of stage represented in A; The plane of section is indicated by the line
a-a in A. E, vertical section through blastoderm of stage represented in B. The
plane of the section is indicated by the line b-b in B. F, vertical section through
blastoderm of stage represented in C. The plane of the section is indicated by the
lines c-c in C. (From Patten, after Patterson's figures for the pigeon.)

DEVELOPMENT OF THE EMBRYO

451

yolk, which is the mechanical reason for the disc-shaped blastoderm
being where it is and what it is in the bird's egg. That is, if the large
yolk of a bird's egg were removed and the blastoderm were allowed to
assume the spherical shape it would naturally take due to surface ten-
sion, there would be a decided similarity between the disc-shaped blasto-
derm and the ordinary morula stage of eggs with little yolk, such as in
amphioxus and in man.

Not only does the great quantity of yolk make this change in the
morula stage, but it is evident that a large amount of yolk does not per-
mit a simple hollow sphere formation by any method of cell arrange-
ment. Nevertheless, the central cells do separate somewhat from the
yolk and form the slight segmentation cavity mentioned above.

Imagining, now, that the yolk could be removed and the ends of the
blastoderm drawn together, we should have a true blastula form of the
simpler type.

GASTRULATION

It is essential that one remember that a large quantity of yolk will
make a considerable change in the process of gastrulation. The simpler

Y*//C

folk

Fig. 260.

Gastrulation in egg with different quantities of yolk. 1-5, Amphioxus (little
yolk) ; 6-8 Amphibian (moderate amount of yolk) ; 9-10, Birds (large amount of
yolk); blc, blastocoele ; future dorsal side; ect., ectoderm; end., entoderm ; ent.
and ach., archenteron ; blp., blastopore ; y.p., yolk plug. (After various authors.)

452

EMBRYOLOGY OF THE CHICK

.L,^ •/^/*J*.s.rf

-Y/r— ,

_j C2^-*-* 7-

Fig. 261.

A to £>. Diagrams illustrating the idea of
confluence (concrescence) as applied to the chick.
The central area bounded by the broken line rep-
resents the area pellucida ; external to this is the
area opaca, showing the germ wall (G. W.),
zone of junction (Z.J.), and margin of over-
growth (M.O.). m.n., Marginal notch.

E to G, Diagrammatic relations of the germ
layers at the time the primitive streak is formed
by concrescence of the blastoporal margins. E,
section of stage B ; F, section of stage D ; G,
section through blastoderm of a 16 hour chick
embryo. (A to D from Lillie's "The Development
of the Chick," by permission of Henry Holt &
Co.)

forms are brought about by an
inpushing of the outer layer of
the blastula as one indents a
rubber ball. This forms a two
walled (ectodermal and ento-
dermal) cup with a cavity in
the center called a gastrocoele.
The opening itself is known as
the blastopore (Fig. 260).

In birds with a large
amount of yolk, the blastula
cannot indent completely in-
to the blastocoele, due to the
disc-shaped blastoderm not be-
ing a true hollow sphere. The
very small blastocoele formed
between the blastoderm and
the yolk, allows but little in-
folding. The blastopore in the
case of an indented sphere is
relatively large. In the chick
there is but a tiny blastocoele,
while the blastopore is but a
small crescent-shaped slit at
the margin of the blastoderm
(Fig. 251, C). This slit is to
be thought of, however, as
similar to the regular round
opening in simpler forms,
which has been pushed to-
gether by the yolk not yielding.
The infolding entoderm is also
naturally compressed and flat-
tened by the tiny blastocoele
into which it can grow. In
fact, the lower layer of the in-
folding entoderm seems to be
prevented from growing nor-
mally by the unyielding yolk,
and so is broken and lies on the
yolk as scattered cells. These
scattered cells then shortly dis-
appear so that the yolk itself
forms the floor of the gastro-
coele.

DEVELOPMENT OF THE EMBRYO 453

Figure 260 presents a diagrammatic scheme which makes it possible
to see the general outlines of gastrulation in eggs with varying quantities
of yolk.

The zone of junction, where the peripheral region of the blastoderm
remains attached to the yolk, is called the area opaca, because when the
blastoderm is removed from the yolk-surface for laboratory study, the
yolk is so closely attached to this region that it adheres to the blastoderm
and renders the area more opaque. The more central portion which
thus has no yolk attached, is more translucent and is therefore called the
area pellucida.

The area opaca later differentiates into the following three more or
less distinct zones (Fig. 261) :

(1) The margin of overgrowth: Being a peripheral zone where
rapid proliferation pushes the cells out over the yolk without their ad-
hering to it.

(2) The zone of junction: Which has an intermediate zone in
which the deeper lying cells have no complete cell boundaries, so that
they form a syncytium which blends (without a definite boundary) with
the superficial layer of white yolk to which it adheres by many pene-
trating strands of cytoplasm.

(3) The germ wall: Being an inner zone made up of cells derived
from the inner border of the zone of junction, which have acquired defi-
nite boundaries and become more or less free from the yolk. Numerous
small yolk granules are usually found in the germ wall, due to the fact
that these were contained in the cytoplasm when they were still con-
nected with the yolk as cells of the zone of junction. It is the inner
margin of the germ wall which separates the area opaca from the area
pellucida.

When the chick embryo is ready for gastrulation, there is a thinning
of the blastoderm at the caudal margin with a consequent freeing of the
blastoderm at the caudal margin from the yolk (Fig. 259, D). In a sur-
face view, the crescent shaped gap in the posterior quadrant of the zone
of junction marks the separation of the blastoderm from the yolk (Fig.
259, A). The blastopore is that region where the blastoderm is free
from yolk and where it is likewise very thin.

It will be remembered that cell proliferation is continuous through-
out the entire blastoderm. The surface extent has now become much
greater by a general spreading out of the peripheral margins over the
yolk, but this extension, while taking place uniformly at the margins,
varies at the blastopore. This being at the posterior free end of the
blastoderm, the cells, as they proliferate, grow inward to form the ento-
derm. Once this differentiation has taken place, the part of the margin

454 EMBRYOLOGY OF THE CHICK

forming this entodermal portion takes no further part in the peripheral
expansion, though this entodermal part grows back toward the center
of the blastoderm, leaving the blastopore region behind. The marginal
region continues to grow and soon encloses it, so that by the time the
blastopore conies to close, it lies within the recompleted circle of the
germ wall (Fig. 259, C).

CHAPTER XXX.

THE PRIMITIVE STREAK AND ORIGIN OF THE MESODERM

All that has been described so far has actually taken place before
the egg" is laid. The real beginnings of a distinguishable embryonic area
may be said to start with the primitive streak. While there are various
theories as to just how this thickened streak is formed, the most logical
and intelligible is that it is a thickening formed by the two lips of the
blastopore meeting and growing downward.

To make this clear, the student will remember that throughout this
entire volume, the blastula has been considered a hollow sphere com-
posed of a single layer of cells, and the gastrula was this same hollow
sphere after it had indented so as to form two layers. The opening
where the indentation took place was called the blastopore.

In the chick-embryo we are to
think of this blastula, however, not
as a sphere, but as sausage-shaped,
with the indentation taking place
from about the center of the long
axis to one end. Thus we do not
have a round blastopore, but an
elongated one. And it is the closing
of the lips along this elongated slit
which forms the thickening called
tue primitive streak (Fig. 262). It
is clearly setn at sixteen hours of
incubation, not only as a thickening,
but as an indentation — the primitive
groove — with ridge-like thickenings
flanking each side, and extending
from the area opaca to almost the
center of the blastoderm. The part lying closest to the area opaca is
the caudal end, and the direction of the streak forms the long axis of
the embryo. At the cephalic end of the primitive groove there is a deep-
ening called the primitive pit, and directly anterior to this the two lips
of the primitive folds meet in the midline to form a small rounded eleva-
tion known as Hensen's node. This node serves as the region of demar-
cation separating the fast disappearing primitive streak from the noto-
chord, which forms cephalad to it in the long axis of the embryonic
area. The growth of the embryo is much greater headward than cau-
dally or laterally, so that the antero-posterior axis becomes considerably
elongated.

Fig 262.

Dorsal view of 16 to 20 hour chick embryo
showing primitive streak, primitive groove,
primitive node, beginning of neural groove,
blood-islands, and extent of mesoderm. (After
Duval.)

456

EMBRYOLOGY OF THE CHICK

The lips of the blastopore form a region of rapid cell proliferation,
though all the cells look quite alike. Nevertheless, it is from these rap-
idly proliferating cells that the various germ layers are derived.

Figure 263 shows an enlarged longitudinal, as well as a cross section
of an early embryo. As the lips of the blastopore grow closer and
closer together, they finally fuse, forming the primitive streak. Ecto-
derm and entoderm cannot be distinguished, but from the thickened ap-
proximation of the lips of the blastopore there is an inward growth of

y.f. l.b.

Fig. 263.

A. From medial longitudinal section through embryonic disk of Chick.
Bonnet.

B. From transverse section through Hensen's node — germ disk of chick of 2
to 6 hours' incubation. Duval. For lettering see Fig. C.

C. From transverse section through primitive groove — germ disk of chick of 2
to 6 hours' incubation. Duval. arc., Archenteron ; ec., ectoderm ; en., entoderm ;
l.b., lip of blastopore ; p.g., primitive groove ; y., yolk ; y.p., yolk plug.

a single layer of cells now called entoderm, and from between these two
layers some rather loosely arranged cells form a third layer, considered
the primitive origin of what is later to be called mesoderm.

At the same time this mesodermal outgrowth appears, the dipping
down of the outer layer occurs, forming the primitive groove.

The three layers which have thus been established are very impor-
tant, because in all forms of animal life so far studied, there is a decided
similarity in the origins and developments of the various organ systems.
Therefore, an understanding of the way the germ-layers and the organ
systems arise, alone permits an understanding of the ever increasing
perplexities coming forth as these in turn develop further.

In our study of comparative anatomy we shall see why it is that

ORIGIN OF THE MESODERM

457

all outer coverings of the body as well as the nervous system are de-
rived from the ectoderm ; why the lining of both digestive and respira-
tory organs comes from the entoderm ; and why the circulatory system
as well as the blood, lymph, muscle and connective tissue (except neu-
roglia) are derived from the mesoderm.

The primitive streak, relatively, seems to become pushed further
and further tailward, but this is due to the greater growth in the cephalic
region of the embryo. (Compare Figures 262 and 264.)

The entoderm spreads out as a very definite layer of cells, and
merges peripherally with the inner margin of the germ wall, even over-
lapping it slightly. The little cavity between the yolk and this ento-
dermal layer which has been called the gastrocoele will henceforth be
known as the archenteron or primitive gut (Fig. 265). The student is
not to look for a cavity in his sections, however, as the yolk in this

region, by the very fact that it is separated
from the entoderm and forms the floor of
the primitive gut cavity, will not adhere to
the embryo when it is removed for section-
ing purposes.

At eighteen hours of incubation the cell
boundaries of the germ wall cannot usually
be seen, though there are many nuclei and
yolk granules, the latter in various stages
of absorption. Because the nuclei of the
germ wall arise by division of the nuclei of
the cells lying at the margins of the expand-
ing blastoderm, it is assumed they are in-
strumental in breaking up the yolk in ad-
vance of the arrival of the spreading ento-
derm about the yolk sphere.

At about twenty-two to twenty-three

hours of incubation a pocket of entoderm can be seen in the anterior
region by examining the whole mount, and focusing through the ecto-
derm. This is the first formation of a gut floor in addition to the yolk
which has been answering that purpose up to this moment. This pocket
forms the fore-gut.

The mesoderm grows laterad and then extends cephalad, so that
an area between the two cephalad growing portions of mesoderm is
formed, which area is called the proamnion (Fig. 266, P). It is merely
an open space and must not be thought of as forming the later true
amnion. It is to be noted primarily, because it permits a better study
of just how the mesoderm grows in relation to it. It will be well to ob-
serve the difference in this space in eighteen and twenty-three hour
embryos.

As the mesoderm begins its growth where it does, there is none of

Fig. 264.

Surface view of a twenty-one
hour chick embryo, in which the
head-fold and first pair of primi-
tive mesodermal segments are pres-
ent. (After Duval.)

458

EMBRYOLOGY OF THE CHICK

it in the midline except posterior to the primitive streak; but, immedi-
ately on each side of the midline, the mesoderm is quite thickened, thin-
ning out as it extends toward each side. The dorsal mesodermic plates
are to develop from these thickened portions of the mesoderm, and as
they will then segment, they are called segmental zones of mesoderm.
The first somites will appear cephalad to Hensen's node, extending can-
daily along each side of the primitive streak and becoming less and less
distinct.

Fig. 265.

From medial vertical sections through embryonic disk of lizard,
showing five successive stages in gastrulation (Wenckeback,
Bonnet).

It is important -to note here that the sheet-like layers of mesoderm
so characteristic in the mid-body region do not extend to the head region
of the embryo. The mesoderm of the head region develops from quite
definitely organized layers immediately behind the future head. The
reason that the mesoderm of the head is separate in origin from that of
the remaining portion of the body may be accounted for by the fact that
the head is not segmented as is the mesoderm of the body-region.

THE NOTOCHORD

From the cephalic end of the primitive streak the rapidly prolif-
erating cells extend in an anterior manner. In non-bird-like vertebrates,

ORIGIN OF THE MESODERM

459

the notochord extends from the region of the anterior lip of the blasto-
pore, so it is assumed that this is also the case in birds.

If the student will think of assumptions and incidents of this kind,
and note the manner in which hundreds of such assumptions and inci-
dents must be gathered from all angles and from hundreds of experi-
ments by hundreds of different investigators, to make such a. study as
embryology possible, he will obtain at least some slight appreciation of
what scientific investigation means and what scientific method means.

In reading the literature of the subject, the student will note that
probably most writers insist that the notochord develops from the ento-
derm, though there are those who believe it comes from either of the
other two layers, and some even that it comes from all three.

Fig. 266.

B

A. — Surface view of Embryo at the Twenty-third Hour of Incubation.
A., anterior limit of head; AP., area pellucida ; AV., area vasculosa ; B.,
border of mesoderm ; C.A., yolk crescent ; H., Hensen's node ; P., pro-
amnion ; PP., primitive streak ; PV., mesoblastic somites ; St., sinus ter-
minalis bounding the vascular area ; U., unsegmented mesoderm. I, region
where the medullary folds have almost met to form the medullary canal.

B. — Anterior part of the preceding figure more highly magnified to show
details. A, ectoderm of anterior end of head ; B, mesenchyme ; C, sub-
cephalic pocket: 1, region where the medullary folds will begin fusing to form
medullary canal; 2, margin of the anterior intestinal portal; 3 and 4,
posterior regions of medullary folds ; 5, lateral limits of head region ; 6,
border of foregut. (From Duval.)

In all forms studied, however, the notochord is not seen to arise
from any definite layer, but it arises either at the same time the meso-
derm does (Fig. 267), or from the undifferentiated growth of cells about
the closed blastopore which gives rise to both entoderm and mesoderm.

The notochord itself is a rod-shaped structure, circular in cross sec-
tion, extending headward from Hensen's node.

THE NEURAL PLATE

A thickening of the ectoderm at about eighteen hours' incubation
causes a greater density along each side of the notochord. This denser
area is several cells in thickness, and forms what is called the neural or

460 EMBRYOLOGY OF THE CHICK

medullary plate. The cephalic ends of these plates seem to bend; but
from Hensen's node caudad, they diverge into thickenings on each side
of the primitive streak.

At twenty-one to twenty-two hours the outer portions of the neural
plate bend dorsally toward the midline and form the neural or medul-
lary groove, the ridges thus formed being called the neural or medullary
folds (Fig. 266, B). This is the first differentiation of the nervous
system.

After this period of incubation the denser portion which has formed
by all the cell differentiation mentioned above, is called the embryonal
area, and the outer peripheral region of the blastoderm is called the
extra-embryonic area, because from this extra-embryonic region arise
those structures which are not part and parcel of the embryo itself, but
serve as protective and nutritive layers.

At this period the anlage of the head appears as a rounded eleva-
tion with a definite crescent-shaped head-fold, the first definite boundary
of the growing embryo.

It is well at this point to

know what is to become of trie
mesoderm, so that we may have
several landmarks which will

chc'

wr. I*. \_ ^^ -^»*-- -^~—^_ ••"— r'Ti'ftTawimigy tun iuu'Jiift »u' >'JL_LL_ E « • 1

stand us in good stead.

In the earthworm, it will be
recalled, the entire animal is seg-

Sagittal section through region of primitive niented, that IS, COmpOSed of
node and caudal end of chordal canal of guinea mfz+imfrtto ™rVii1^ i'n tVi^ frr\rr coo-

pig (isy, days after fertilization) to show be- metameres, while in me trog, seg-

ginning of notochordal cells and ectodermal cells mentation SHOWS itself primarily

in one layer. Let., ectoaerm ; ent., entoderm ;

ch.c., chordal canal, dorsal and ventral wall clos- in the Spinal Column.

ing lumen; pr.pt., primitive pit. (After Huber

in The Anatomical Record, April 20, 1918.) In both earthworm and fl'Og

the segments are composed of an outer layer of ectoderm, an inner layer
of entoderm, and a middle layer of mesoderm.

When one speaks of metameres, one always means segments lying-
one behind the other, but now we must think of a sort of segmentation
also in each metamere, one below the other (Fig. 268). In fact, this we
must do if we are to understand that which follows.

Figure 268 shows a combination transverse and longitudinal ar-
rangement of metameres with the mesoderm divided into an outer
(somatopleure) and an inner (splanchnopleure) layer, and the segments
also divided horizontally.

The more dorsal portion of the horizontal divisions is called an
epimere, the mid-portion a mesomere (which is the beginning of the ex-
cretory system), and the more ventral portion is known as a hypomere.
The whole metamere is called a mesomeric somite.

In vertebrates, as we have seen, segmentation is observable primarily

ORIGIN OF THE MESODERM

461

?n the region of the spinal column. Therefore, in the study of verte-
brates, such as the chick, we shall find that, while segmentation begins
along the future spinal region, only the more dorsal portion of the meso-
derm is segmented, and that only partially. The epimeres alone, that
is, the paired parts lying at the side of the notochord, are truly seg-
mented, though the opening in them, the epicoele, shortly disappears.
The mesomeres with their mesocoeles develop into the excretory sys-
tem, and the hypomeres, which have not segmented, but whose opening,
the hypocoele, is continuous throughout the entire region where there
has been any segmentation, is now to be known as the coelom or body
cavity, into which the internal organs are to grow.

Fig. 268.

Stereogram showing the segmentation of the mesothelium. The dorsal and
ventral walls of the coelom later fuse to form the dorsal and ventral mesenteries.
AJ, alimentary canal ; EM, epimere ; Fb, forebrain ; Hb, hindbrain ; M, (under
SA-.c. ), myotome ; Mb, midbrain ; MM, mesomere ; sk.c., sclerotome. Sp, splanchnic
layer of the mesoderm (splanchnopleure). (Modified from JCingsley. )

It is to be remembered that epimere, mesomere, and hypomere are
composed of mesoderm only.

As the mesoderm begins to grow laterad and ventrad, and while it
is yet unsplit into an outer and inner layer, the thickened portion lying
on each side of the neural groove is called the vertebral plate, and the
more distal portion, the lateral plate.

The outer layer of the lateral plate, after it splits into two sheets,
is called somatic mesoderm (and after connecting with the ectoderm the
somatopleure) while the inner layer, the splanchnic mesoderm, connects
with the entoderm and is known as the splanchnopleure (Fig. 268).

In the head region, the cells of the vertebral plate scatter and com-
bine with cells which are continually being budded off from the walls
of the fore-gut to form the mesenchyme of the head region (Fig. 269).
It will thus be seen that mesenchyme is made up of a combination of

462

EMBRYOLOGY OF THE CHICK

Mestnchymal
ceil

Fibrillae in

fcTop/asptic

matrix

B

C* i

I

ettof
wjtM
Elasti

cells from both mesoderm and entoderm, and even of ectoderm, for,
scattered cells later join from the ectoderm of the head region.

The somites begin forming in the region of the more anterior end
of the primitive streak, the first one to develop remaining the more an-
terior. The first four pair of somites take part in the development of
the hinder part of the head region of the embryo.

A further important factor to remember at this point is that seg-
mentation is fundamental,
and that consequently any
structures in the body which
show segmentation, only fol-
low out some plan of the
original segmentation. This
is of value in tracing the
growth of various body-
parts, such as muscles, for
instance, in that the nerve
supply, which we shall
shortly see is also of seg-
mental origin, definitely tells
us where a muscle springs
from, because nerves always
follow muscles, and not vice
versa.

The somatopleure,
splanchnopleure, and coelom
become separated into em-
bryonic and extra-embryonic
regions later, although at
this early stage of which we
are writing they form con-
tinuous structures which ex-
tend laterally out from the
germ wall, and anteriorly
into the head region.
The following structures are developed from the embryonic portion:

body-wall,

gut-wall,

vascular organs,

pericardial cavity,

pleural cavity,

peritoneal cavity.

Cartilage cell

Eciodt
Mts

Fig. 269.

Figures showing the differentiation of the supporting
tissues (after Mall). A, white fibers forming in the
dermis of a 5 cm. pig embryo ; B, elastic fibers forming
in the syncytium of the umbilical cord from a 7 cm.
embryo ; C, developing cartilage from the occipital bone
of a 20 mm. pig embryo. Mesenchyma from the head of
a thirty-six hour chick embryo.

ORIGIN OF THE MESODERM

463

From the extra-embryonic portion the following are developed :
embryonic membranes and appendages,
extra-embryonic portions of the vascular system,
extra-embryonic coelom (exocoelom).

Probably the most understandable method of making much of what
has been said clear, is to use Professor Reese's method of illustration :

"An understanding of the way in which the embryo becomes folded
off from the rest of the egg, may perhaps be obtained in the following
way : Cut out four circles of cloth, say 75 cm. in diameter, of three dif-
ferent colors. Put the two circles that are of the same color together
and then put these two circles between the other two.

"Let these superimposed circles represent a greatly enlarged blas-
toderm that has been removed from the yolk to which it was originally

Fig. 270.

Schematic diagrams showing the extra-embryonic membranes of the chick.
The egg is cut longitudinally while the embryo (which lies at almost right angles
to the egg), is cut transversely. A, embryo at about 48 hours ; B, same at about 72
hours; C, same at about five days; D, same at about fourteen days. (After
Duval. )

attached. The upper layer of cloth will represent the ectoblast, the bot-
tom layer will represent the entoblast, and the two similarly colored
layers in the middle will represent the two layers of the mesoblast after
their separation.

"As the yolk takes no actual part in the formation of the embryo
other than as a supply of the food for the growth of the constantly en-
larging chick, it may be omitted from our model.

"Now spread the cloth-blastoderm upon a table and place under
its centers a small object, such as a bottle. If now, the fingers of one
hand be pushed under one end of the bottle, carrying, of course, the three

464 EMBRYOLOGY OF THE CHICK

germ layers with them, we shall have represented the formation of the
head fold. By pushing under the cloth at the other end of the bottle,
in the same way, we may represent the formation of the tail fold; and
in a like manner the lateral folds may be formed. If these folds, the
head, tail, and lateral be pushed under far enough, they will meet under
the center of the bottle, and we shall have the bottle, with its surround-
ing layers of cloth, connected with the rest of the model by only a sort
of stalk, which is hollow and composed of the three layers of cloth. The
bottle is used simply to give a solid object around which the folding
may be more easily done, but we are to consider the space occupied by
the bottle as an empty space.

"We have now represented what is sometimes called the embryo-
sac, or simply the embryo, in contradistinction to the yolk-sac, or simply
the yolk. The embryo remains connected with the yolk throughout the
period of incubation by the yolk- or somatic-stalk, and as the embryo
increases in size, the yolk-sac is, by absorption, constantly diminished.
The space occupied by the bottle, in our model, represents the digestive
tract of the chick, and is lined, as will be seen by examination of the
model, by the lower germ layer, or entoblast. The body cavity would
be difficult to represent in the cloth model, but it can be imagined to
exist as the narrow space between the two layers of similarly colored
cloth which we have just called the mesoblast.

"The formation of the amnion may be represented in our model by
lifting up with the fingers a small fold of the upper and second layers of
cloth, and pulling these two layers back over the head end of the embryo,
this fold will correspond to the head fold of the amnion. Similar folds
might be lifted up at the posterior end and at the sides of the embryo
model, to represent the tail and lateral folds of the amnion. The way in
which these folds fuse together will be explained later."

The allantois cannot be explained from the model, but can be un-
derstood by studying Figure 270. It arises as a thin-walled pouch from
the posterior end of the digestive tract, and as it increases in size, it ex-
tends around the upper side of the embryo, between the inner and outer
layers of the amnion.

Both amnion and allantois are thrown off at hatching, so take no
permanent part in the actual embryo.

CHAPTER XXXI.

THE FOUR TO SIX SOMITE STAGE
(About Twenty-four Hours)

As the embryo is already well on its way in development at the
time the egg is laid, and as it has been shown that the extent of devel-
opment varies considerably on account of the retention of the egg in
the hen for an extra twelve to sixteen hours if it is not ready for laying
sufficiently early in the day, the formation of the block-like portions
of mesoderm — the somites — becomes the more accurate measurement
of the age of an embryo. Chicks with the same number of somites do
not usually vary much among themselves in general, though individual
parts often do ; while chicks having been incubated for the same number
of hours vary considerably in all parts.

The twenty-four hour stage (four to six somites), (compare Figs.
264 and 266), is of great importance, for it is during this very early period
of the chick's life that the interesting and important differentiating
processes are noted. Up to the time the first four somites form the hind-
brain, the entire growth of the embryo from Hensen's node cephalad,
has been a formation of the head-region only.

There has been some question in the past as to whether or not ad-
ditional somites are formed anterior to the first ones thus laid down.
Patterson performed an interesting experiment which seems to warrant
our saying that such is not the case.

Professor Patterson incubated six eggs up to the one somite forma-
tion period, and then with the most asceptic precautions, opened the
eggs and marked the first somite by injuring it with an electric needle,
or inserted a minute glass pin therein. The shell was then again closed
by a small piece of egg-shell, and the eggs again incubated for varying
number of hours before being reopened. No new somites appeared an-
terior to the injured one.

In the study of whole mounts under the microscope, it must be re-
membered that reflected light coming up from below the object shows
different densities as darker or lighter areas. Any portions of the em-
bryo which have become thickened or folded over will therefore appear
extremely dark and be thus distinguished from the thinner and lighter
areas.

At the end of about twenty-four hours we then have :

1. Three definite germ layers. (Fig. 271.)

2. Four to five somites, forming in the vertebral plates, which verte-

bral plates have separated from the lateral plates.

3. The mesoderm divided into a somatic and a splanchnic layer.

466

EMBRYOLOGY OF THE CHICK

4. The neural groove almost but not quite closed.

5. A clearly outlined fore-gut and mid-gut.

6. Clearly defined head-folds marking the anterior limit of the embryo.

7. A definite notochord extending from the anterior end of the primi-

tive streak to what is to become the mid-brain.

8. The pellucid area is more or less pear-shaped and the vascular area

is seen as an inner zone of the area opaca.

9. The primitive streak is rapidly becoming relatively shorter and is

soon to disappear, the cells of which it is composed probably be-
coming rearranged to form other structures.

Transverse section through the primitive streak of a chick with six pairs of
mesodermal somites (about twenty- four hours), showing the formation of the blood-
vessels and blood. The section extends from the mid-line, nearly half across the
area vasculosa. b, Blood cells ; c, coelomic spaces ; e, empty endothelial tubes ; ec.
ectoderm ; en, endoderm ; gw, germ wall ; i, solid blood island ; m, axial mesoderm ;
i , . • a, primitive streak ; si, vascular sinuses of area vasculosa ; so, somatic mesoderm ;
sp, splanchnic mesoderm. (After Riickert.)

From the twenty-fourth hour on, the texture of the embryo becomes
firmer, and, whereas, it is difficult to remove an eighteen-hour embryo
without tearing, the twenty-four hour embryo can easily be removed.
All outlines become clearer now also.

The anterior part of the embryonal area has thickened, and is
slightly lifted above the remaining blastoderm as shown by the crescent-
shaped anterior boundary (Fig. 266, B). The embryo grows forward
oyer this crescent-shaped fold which thus comes to lie under the embryo
and forms a little pocket between the embryo and the fold called the
subcephalic pocket.

The neural folds now unite in the region of the future mid-brain,
closing rapidly posteriorly, and slowly anteriorly. The closed portion
is called the neural tube. The most anterior portion where the neural
tube will close is known as the neuropore, which is the region of what
is later to become the lamina terminalis. This lamina terminalis is
usually regarded as the morphologically anterior limit of the brain.
Topographically, however, this is not the case, for the fore-brain grows
forward and then bends back downward in front of the fore-gut, the
whole thing becoming bent like a shepherd's crook, so that the morpho-
logically anterior end comes to lie on its antero-ventral aspect.

The neural folds have a somewhat flattened crest, and these fold in-
ward, forming a vertical contact. The neural tube is thus formed by the

FOUR TO Six SOMITE STAGE 467

fusion of the lower or inner margins of 'these surfaces, the upper £>nes
again forming a continuous ectoderm, so that the neural tube becomes
entirely separated from it. The cells lying between this ectodermal and
lower margin, and which have been derived approximately from the
apices of the neural ridges, become the neural crests (Fig. 272). These
crests do not fuse in the midline, but remain as a pair- of longitudinal
bands along the dorsal-lateral surfaces of the neural tube, and are the
rudiments of the ganglia of the cranial and spinal nerves. They are not
uniformly developed, and appear much clearer in some sections than in
others.

Already very early in the area opaca considerable modification has

been going on. The area itself has
broadened and in both lateral and
posterior regions^it is mottled. This
mottled portion Ts due to jthe-Jprm-
ing of differentiated cell groups
forming what are called blood-
islands (Figs. 262, 264), which are
.the beginning of the vascular sys-
tem. The portion of the area opaca
in which these blood-islands occur
is known as the area vasculosa. This
area vasculosa begins immediately

Sapmbr»Bth««l pt»c

Fig 2?2 - behind the embryo, but then extends

Transverse section through the head of a laterally and anteriorly, while

7 day Ammoccetes in the region of the tri- . . • i .-: . : t , r .

geminai ganglion, von Kupffer. around its periphery a single definite

blood vessel forms, known as the sinus terminalis (Fig. 284, C). Beyond
this sinus terminalis, all the remaining area opaca consists of ectoderm
and entoderm, and extends around the yolk. It is then called the area
vitellina.

-The compact cell masses forming these blood-islands have been
formed throughout the deeper portion of the germ wall, becoming cov-
ered superficially with a deep layer of scattered germ wall cells. This
superficial layer comes to be known as coelomic "mesoderm."

This superficial layer and the blood islands become continuous very
early with the mesoderm of the pellucid area derived from the primitive
streak (Fig. 273). The blood islands become hollowed out, forming
lacunae. The cells which have formed the blood islands become both
the blood-vessel walls arid the blood cells. The lacunae then anastomose,
forming a complete network extending to meet the vascular structure of
the pellucid area .and later of the embryo.

!o -The cellular portion of the germ wall which remains after the
coelomic "mesoderm" and the blood islands have differentiated, forms
the rudiment of the yolk-sac entoderm, to be described later.

As soon as 'the extra-embryonic coelom forms, dividing the meso-

468

EMBRYOLOGY OF THE CHICK-

derm into somatic and splanchnic layers, the blood vessels remain asso-
ciated with the splanchnic layer.

Just as the somites are beginning to form, blood vessels will be
found at the margin of the pellucid area beginning to grow toward the
embryo. Only the tubular vessels develop in the area pellucida ; the
blood islands, as already noted, develop in the area opaca. This means
that the cellular or corpuscular elements of the blood arise in the pos-
terior region of the area opaca.

Fig. 273.

Beginning of the vascular system in the chick embryo. A, complete blood-
island ; B and C, beginning of vacuole formation ; D, vacuoles becoming confluent
to form the lumen of the blood vessel. b.hem., primitive red blood cells ; I, lumen ;
p, vessel- wall ; v, vacuole or lacunae. (After Uskow). E. A portion of the
vascular network destined to become the aorta in the chick embryo, gv, primitive
blood forming cells ; Ip, cytoplasmic lamina persisting in the lumen of the blood
vessel that has formed ; mar, blood vessels ; p, vessel wall ; p.ac., point of enlarge-
ment of network; vac, vacuoles. (From Vialleton.)

Professor Riickert has worked out the arrangement of the blood
vessels (Fig. 284). He says the vessels in the area pellucida are formed
by a rearrangement of small groups of cells in the splanchnic mesoderm
of the area. First, short sections of tubular vessels are formed, which
then connect with the more peripheral vessels of the opaque area, thus
forming a continuous vascular network extending toward the embryo
and finally reaching it at about the time, six pairs of somites are formed.

Soon after this, vessels appear in the embryo itself, the first being
the paired dorsal aortae in the body region. These are regarded as
merely straightened axial margins of the vascular network of the area
pellucida. They diverge widely posteriorly, passing as the vitelline
arteries into the general vascular network. Anteriorly they are pro-
longed forward to the heart region where they connect with a pair of
vessels differentiated in the mesenchyme of the head.

It will be remembered that the coelom is really paired, and extends
downward ventrally on both sides until at a later period it meets in the

FOUR TO Six SOMITE STAGE

469

ventral midline and fuses to form the one cavity which it later becomes
in all vertebrates. The crescent-shaped line where fore-gut and mid-gut
meet is called the anterior intestinal portal (Fig. 266, B).

It is in the region of the anterior intestinal portal that the coelomic
chambers on both sides show a marked enlargement. The enlargement
of each side extends mesiad toward the other, and finally both break
through into each other ventral to the fore-gut, to form the pericardial
cavity. These enlarged regions are called the amnio-cardiac vesicles
(Fig. 274) in their early stages; however, it is better to remember what

X -^

opVes

Fig. 274.

Ventral views of the head ends of chick embryos. A. Embryo with five pairs
of somites (about twenty-three hours). B. Embryo with seven pairs of somites
(about twenty-five hours), a.c.v., Amnio-cardiac vesicle; a. i. p., anterior intestinal
portal ; End'c.s., endocardinal septum ; FG., fore-gut ; Ht., heart ; My'C., myo-
cardium ; N'ch., notochord ; N'ch.T., anterior tip of notochord ; n.F., neural fold,
op. Ves., optic vesicle; p.C., parietal cavity (coelomic) ; Pr'a., proamnion ; a.2, s.4,
second and fourth mesodermal somites ; V.o.m., omphalo-mesenteric vein. (From
Lillie's "Development of the Chick" by permission of Henry Holt & Co.,
Publishers. )

they are to become, and think of them as the pericardial region of the
coelom.

The splanchnic mesoderm is thickened at the point where it lies
closely applied to the entoderm at the lateral margins of the portal. It is
from these thickened areas that the paired primordia of the heart will
arise later.

It will thus be noted that the heart develops from the ventral and
not the dorsal aspect.

The amnio-cardiac vesicles also become vascularized quite like the
rest of the pellucid area, and a pair of ventral aortae is formed beneath
the fore-gut. Immediately posterior to the anterior intestinal portal
these vessels diverge, passing into the vascular network as the rudiments
of the vitelline veins.

CHAPTER XXXII.

THE FIRST HALF OF THE SECOND DAY
(Twenty-four to Thirty-six Hours)

It is well at this point to continue with the vascular system, thus
giving a connected account of how the heart and the various blood ves-
sels are formed.

The paired primordia of the heart, already mentioned, grow mesiad
and fuse, to form a thin-walled tube which becomes the endothelial lining
of the heart (Fig. 275). The muscular walls of the heart are formed by
the addition of an external layer of mesoderm. This is understood the
better by noting that the splanchnic mesoderm on each side forms a fold
around the endothelial rudiment and fuses both dorsally and ventrally

Fig. 275.

Cross section of A, through head of 2 day chick embryo in the region
of the mid brain. B, through posterior region of head at the end of 3 days.
ao, aorta ; and, otic anlage ; c, heart anlage ; ch, notochord ; d, fore-gut ;
ec., endocardium ; ect, ectoderm ; ent, entoderm ; g.pl., neural crest ; h.h.,
hindbrain ; m.c., myocardium; m.n., midbrain. (After Marshall.)

in the midline. For a very short period this fusion remains on the dor-
sal aspect, being called the dorsal mesocardium. The ventral fusion
forms the ventral mesocardium. The ventral mesocardium breaks away
almost immediately, the dorsal mesocardium remaining for a longer time,
but then it also' disappears with the exception of the portion at the an-
terior and posterior extremities of the heart. The heart is now a short
median tube made up of endothelial and rudimentary muscular layers,
suspended in a cavity, later to be called the pericardial cavity. Ante-
riorly, the heart-tube is continuous with a short pair of vessels extending
into the head-fold — the ventral aortae already mentioned. Posteriorly

FIRST HALF OF SECOND DAY

471

the heart-tube is continuous with the vitelline or omphalo mesenteric
veins (Fig. 276).

The two vessels of the heart (Fig. 283) come in contact so as to form
the letter V, the point of the V being toward the head of the embryo.
The arms continue fusing until a Y-shaped stem has been developed,
with the stem toward the head.

Although the two tubes unite in the manner just mentigned, their
cavities remain distinct for a short time, the endothelial lining forming
two distinct cavities until a short time after the muscular walls have
fused. The muscular walls themselves are not complete on the dorsal
side for a short time, but as soon as the tubes have
thoroughly fused, the walls also complete their
function.

It is the stem of the Y which forms the true
heart, the two arms being continuous with the large
vitelline veins which carry blood to the heart from
the vascular area. The caudal end of the heart is
then said to be venous, while the cephalic end is
known as the arterial end.

At the thirty hour period the heart is a short
straight tube attached to the ventral wall of the
fore-gut or pharynx. The point where the vitelline
veins diverge is at the hindermost angle of the head-
fold. As the head-fold is pushed farther and farther
back, the straight portion of the Y is lengthened,
but as the tubular heart seems to grow more rap-
idly than does the place to which it is attached, it
is bent into a loop, with its convexity toward the
right side of the embryo. This looping is made pos-
sible by the fact that the heart has by this time lost
all connection with the wall of the fore-gut and re-
mains attached only at both ends.

It is even before this period that the heart begins to beat, the pulsa-
tions beginning at the venous end and passing to the arterial end. In
fact, the beating began before any muscular differentiation could be ob-
served in the heart region.

The cephalic end of the heart is known as the bulbus arteriosus.
The bulbus branches immediately into two narrow vessels, the aortic
arches, one passing upward on each side of the digestive tract to the
dorsal side of the embryo and then running tailward as the paired dorsal
aortae (Fig. 277). These vessels lie close to the notochord under the
somites, and extend as separate vessels almost to the tail, where a larger
branch than the vessel itself is given off from each. These two large
branches are the vitelline arteries, which carry the blood from the heart
back to the vascular area.

au.

Fig. 276.

Anterior region of
day chick embryo,
optic vesicle ; crbl, cere-
bellum ; h, heart ; m.h.,
vesicle of midbrain ;
med.obl., medulla oblon-
gata ; r.m., spinal chord ;
u.8., primitive segment ;
v.h'., primary vesicle of
the forebrain ; v.o.m.,
omphalo-mesenteric vein ;
x, anterior wall of prim-
itive forebrain which
later expands into the
cerebrum. (After von
Mihalkovics.)

472 EMBRYOLOGY OF THE CHICK

THE DIFFERENTIATION OF THE BRAIN REGION

At twenty-seven hours, the more cephalic end of the neural tube has
become considerably enlarged as compared with the more caudal por-
tion. The walls are thicker and the lumen larger. This portion is to
become the brain proper, the portion in which the lumen has not en-
larged becoming the spinal cord. A picture of the brain at this time
(Figs. 278 and 282) will show three primary vesicles or lumen-enlarge-

Fig. 277.

Diagrammatic ventral view of a 35-36 hour chick embryo. Compare with Figures
279 and 280. (Modified from Prentiss.)

ments together with what these three vesicles later become. The most
anterior of the three primary vesicles is known as the fore-brain, or
prosencephalon. The mid portion is called the mid-brain or mesencepha-
lon, while the most posterior vesicle forms the hind-brain or rhomben-
cephalori. The rhombencephalon is continuous with the spinal cord.

As all further developments of the brain arise from these three pri-
mary regions, it is of the utmost importance that these primary regions
be grasped fully.

FIRST HALF OF SECOND DAY

473

anterior neuropore

osenccphalon

metencephaJon

myctencephalon

E

Fig. 278.

Diagrams showing neuromeres in brain region of the neural tube. A, lateral
view of neuralplate of 24 hour chick embryo. B, dorsal view of brain from a 26-
27 hour (7 somite) chick embryo. C, dorsal view of brain from a 30 hour (10
somite) chick embryo. D, Dorsal view of brain from a 36 hour (14 somite) chick
embryo. E, Diagram of the segments (neuromeres, myotomes, etc.) of the head
in longitudinal section. A, anterior myotome ; a, abducens nerve ; b, branchial
clefts ; /, facial nerve ; g, glossopharyngeal nerve ; h, hypoglossal nerve ; I, lens
surrounded by layers of eye ; n, nasal pit with the terminal nerve nearby ; o,
oculumotor nerve ; op5, ophthalmicus profundus part of fifth nerve ; O85 ; ophthal-
micus superficialis part of the fifth nerve ; ot, otocyst ; a, spiracular cleft ; t,
trigeminal nerve ; to, truncus arteriosus ; tr, trochlearis nerve ; I- VIII, neuromeres ;
1-6 myotomes. (A-D, from Patten after Hill, E, from Kingsley after Neal.)

474

EMBRYOLOGY OF THE CHICK

From the lateral walls of the prosencephalon the primary optic vesi-
cles push out as a pair of rounded pockets, the lumen of each being di-
rectly continuous with that of the fore-brain.

The notochord extends as far as the infundibulum (Fig. 282, A),
(a depression in the floor of the fore-brain), so that all regions of the
brain lying anterior to it are called pre-chordal, while the rhomben-
cephalon, mesencephalon, and the part of the prosencephalon posterior
to the infundibulum, which lie dorsal to the notochord are called epi-
chordal.

Fig. 279.
33 hour chick embryo (12 somites).

As has been noted some time back, the most anterior region where
the neural tube closes is called the neuropore. The neuropore is still
open at this time and remains so, although gradually becoming smaller
until after the thirty-third hour period, but even then, there is a scar-like
fissure. As we -know of no structure arising from the neuropore, it is
important only as a sort of landmark in describing the location of brain
structures.

At this time the neural tube is closed back as far as the somites, and
it is of nearly uniform diameter, although, posterior to the last formed
somites, the neural tube is still open, and the neural folds can be seen
to diverge on either side of Hensen's node (Fig. 279).

It will be remembered that the first four somites formed are also a
part of the head region. Therefore it can be understood that as the
neural folds approach each other in the midline, in the region of Hen-

FIRST HALF OF SECOND DAY 475

sen's node, an opening remains, rhomboidal in shape. This is the rhom-
boid sinus.

In lower forms, there is an opening from the neural canal into the
digestive tract known as the neurenteric canal, or posterior neuropore,
at the point where the blastopore does not close until after it is sur-
rounded by the neural folds. In the chick the primitive pit represents
this region of the neurenteric canal.

Shortly after the twenty-seventh hour period, and as soon as the
caudal end of the chick can be definitely outlined, the primitive streak
disappears entirely.

LENGTHENING OF THE FORE^GUT

The crescent-shaped margin of the anterior intestinal portal grows
more and more caudad, first because the margin from each side grows
toward the midline to fuse with the other side, thus lengthening the fore-
gut by adding to its floor, and pushing the crescentic margin caudad ;
and second, all structures anterior to the anterior intestinal portal are
elongating so rapidly that the portal is bound to lie further and further
caudad from the cephalic end of the embryo.

These two processes together cause the space between the sub-
cephalic pocket and the margin of the anterior intestinal portal to be-
come elongated, and it is in this enlarging space that the pericardial por-
tions of the coelom extend, and in which the heart comes to lie.

CHAPTER XXXIII.

THE SECOND HALF OF THE SECOND DAY

(Thirty-six to Forty-eight Hours)

Fig. 280.
38 to 43 hour chick embryo. (15 somites.)

It is at this period where the caudal end of the embryo becomes
definitely outlined by the formation of a tail fold and lateral folds similar
to the head fold.

THE BRAIN

Going on from where we left off in our discussion of the formation
of the three primary brain vesicles, we find that at this period the neural
canal has completely closed, even the rhomboidal sinus has fused. The
primary vesicles have enlarged, and their lines of demarcation have be-
come more definite. The fore-brain has grown forward as an unpaired
vesicle from which the cerebral hemispheres are to develop. The walls
of the brain itself lie under the ectoblast, while between the walls and
the ectoblast there can be seen a small amount of mesoblast which is to
form the skull.

The optic vesicles have become elongated and definite constrictions

SECOND HALF OF SECOND DAV

477

Fig. 281.

Transverse sections through a 36 to 38 hour chick embryo. A, Through forebrain ; B, through
the pharyngeal membrane ; C, through hindbrain and auditory placodes ; D, through posterior end
of heart; E, through the intestinal portal; F, just posterior to E; G, through the fourteenth pair
of segments ; H, through the rhomboidal sinus ; I, through Hensen's node ; J, through the primi-
tive streak; K, medial longitudinal section of a 36 to 38 hour chick embryo. (This drawing must
be studied very carefully and thoroughly to understand the transverse sections which are cut
through the levels marked.)

478

EMBRYOLOGY OF THE CHICK

are formed at their bases so that they now form optic stalks, which bend
downward and backward.

The cranial nerves can be seen developing at this period also.

It is at this time that the- first bend or flexure takes place in the
brain, cephalad to the notochord (Fig. 282). This is the cephalic flexure.

If the neural plate be examined at the end of the first day, eleven
enlargements (Fig. 278) will be seen with definite constrictions between

metacoele
( ventricle V
thin roof of rnyelcncephalon

myelococlc

IV)

ventral cephalic fold

e«c«netencei>halic fold

mesocoele
(Sylvi.n.qu«ducl)

location of
posterior commissure
meso-diencephalic-fold

lateral telencephalic
• vesicle

B

Fig. 282.

Diagrams of brain of 4-day chick embryo. Dotted lines show arbitrary
boundaries between vesicles. A, longitudinal ; B, right side ; C, horizontal sec-
tion. (From Patten, after V. Kupffer.)

them. These enlargements are known as neuromeres and are really an
uncompleted segmentation.

The literature is filled with many varying and unsatisfactory theories
as to what becomes of each neuromere, but as yet nothing can be demon-
strated satisfactorily. It is conceivable, however, that as in the crayfish,
for example, where we assume that each separate appendage or pair of
appendages bespeak an embryological segment, so in vertebrates, where
optic vesicles grow from the fore-brain, we may assume a fusion of sev-
eral segments.

At about thirty-three hours, the floor of the prosencephalon has a

SECOND HALF OF SECOND DAY

479

depression formed in it which is to become the infundibulum. This is
an important seat of future development. It must, therefore, be studied
carefully so that it can be recognized in future work.

lv

le

Fig. 283.

I. Views to show the posterior displacements of the heart in the chick
embryo. A, The heart lies ventral to the first segment. This is the region where
the future hindbrain will form. B, The point of bending loop of the ventricle is
at the seventh cervical segment. C, The bending of the loop of the ventricle is
now at the ninth thoracic segment. (From Corning after Duval.)

II. The development of the heart of the chick. A-E, ventral views of the
heart; A, of a forty-hour embryo; B, of a 2.1 mm. embryo; C, of a 3.0 mm.
embryo ; D, of a 5.0 mm. embryo ; E, of a 6.5 mm. embryo. F. Frontal section
through the heart of a 9 mm. embryo, a, Auricle ; b, bulbus ; d, roots of dorsal
aorta ; e, median endothelial cushion ; i, interventricular groove ; la, left auricle ;
le, lateral endothelial cushion ; lv, left ventricle ; om, vitelline artery ; p, left pul-
monary artery ; ra, right auricle ; rv, right ventricle ; s, interventricular septum, sa,
interauricular septum; t, roots of aortic arches; v, ventricle. (A, F, after
Hochstetter; B to E after Greil.)

480 EMBRYOLOGY OF THE CHICK

At about thirty-eight hours, the three primary vesicles divide to
form five vesicles (Fig. 278, 282).

The prosencephalon divides into telencephalon (end-brain), and
diencephalon ('twixt-brain) ; the mesencephalon remains undivided, while
the rhombencephalon divides into metencephalon (cerebellum and
pons), and the myelencephalon (medulla oblongata).

The telencephalon has not yet completely separated from the dien-
cephalon, but there is a median enlargement showing where the division
will take place.

The two most anterior neuromeres of the original rhomencephalon
form the metencephalon, and the posterior four neuromeres form the
myelencephalon.

At thirty-five hours, the auditory pits begin growing as thickened
ectoderm known as auditory placodes on the dorso-lateral surface oppo-
site the most posterior inter-neuromeric construction of the myelen-
cephalon. At thirty-eight hours, the general level of the ectoderm has
become depressed to form a pair of cavities known as auditory pits. The
pits seem to recede until they become closed vesicles, and separate from
the superficial ectoderm, although it will not be until later that they
form a definite connection with the central nervous system.

TORSION

At about the same time the cephalic flexure begins, there is also the
beginning of a twisting of the entire embryo (Fig. 280), although at
this time the twisting is only observable in the head region. The bend-
ing of the cephalic region downward is, as already stated, called
"flexion." The twisting of the embryo from its ventral aspect to its side
is known as "torsion."

As the yolk lies directly beneath the embryo, it can easily be under-
stood that any bending ventrad would be stopped by the large mass of
inert yolk beneath, so that if there is to be any considerable bending at
all, the entire embryp must turn on its side, and this it does in all eggs
which possess considerable yolk, though it does not necessarily come to
lie on the same side in all amniotes. The chick turns so that its left side
lies next to the yolk.

Torsion begins in the head region and gradually and slowly extends
the full length of the body, so that a whole mount, after torsion is com-
pleted, shows the embryo lying on its left side with head and tail close
together, the entire embryo forming from one-half to about three-fourths
of a circle.

THE CIRCULATORY SYSTEM

By the end of the second day, the heart has become still more
twisted, and is now S-shaped with the venous end above and behind the
arterial end, so that both ends lie close together with the loop as an

SECOND HALF OF SECOND DAY

481

intermediate portion between. The venous portion forms a swelling
which later becomes the auricles, while the arterial end also enlarges to
form the bulbus arteriosus. The point of the loop becomes the ventri-
cles (Fig. 283).

It is toward the end of the second day that the pair of aortic arches

Fig. 284.

Diagrams of the circulation in the chick embryo and area
vasculosa. The vascular network of the area vasculosa is omitted
for the most part. A. Anterior and central parts of the em-
bryo and vascular area at about thirty-eight hours (sixteen pairs
of somites). Viewed from beneath. B. Median and anterior parts
of vascular area and embryo at about seventy-two hours (twenty-
seven pairs of somites). Viewed from beneath. C. The main
vascular trunks of the fourth day. a, Dorsal aorta ; aa, aortic
arches (first and second in A, second, third and fourth in C) ;
ac, anterior cardinal vein ; al, allantois ; au, auricle ; 6, bulbus
arteriosus ; dC, ductus Cuvieri ; dv, ductus venosus ; ec, external
cartoid artery ; h, heart ; ic, internal cartoid artery ; la, lateral
dorsal aorta ; Iv, left anterior vitelline vein ; p, anterior intestinal
portal;, pc, posterior cardinal vein; pv, posterior vitelline vein;
TV, right anterior vitelline vein ; s, sinus venosus ; t, sinus ter-
minalis ; tr, venous trunks of the area vasculosa ; v, ventricle ; «o»
vitelline artery; vv, vitelline or omphalomesenteric vein. (From
Kellicott after Popoff and Lillie.)

which have bent dorsad (and continue separately as the paired dorsal
aortae) unite behind the head to form a single vessel which comes to
lie directly beneath the notochord. However, after running but a short

4:82 EMBRYOLOGY OF THE CHICK

distance caudad, the single aorta again divides into two vessels from
which the large vitelline artery, already mentioned, is given off on each
side. The dorsal aortae, now greatly diminished in diameter, continue
into the tail.

The first pair of aortic arches formed are called the mandibular
aortic arches (Fig. 284).

A second pair now form behind the first, and before the close of
the day there may be still a third pair, all of which connect in a similar
manner to the first with the bulbus arteriosus and the dorsal aorta.

The sinus terminalis is now also much better developed than before
and a true circulation has been established, which can carry the yolk-
food-granules (after these have been converted into usable food) to the
embryo.

It is essential that a somewhat detailed knowledge of the circulation
be obtained.

The blood is brought to the heart by the vitelline veins (Figs. 277,
284). The heart then contracts and forces it through the aortic arches
into the dorsal aorta. Here it passes tailward, a small portion going into
the tail itself, but the greater part is carried to the vascular area. There
are two ways in which the blood now gets back into the vitelline veins.
First, it may pass directly to the veins from the arteries through the
connecting capillaries, or, second, it may pass into the sinus terminalis
at a middle point on each side, and then pass forward and backward
through this large vessel. The greater portion, however, in this second
method passes forward toward the head from where it is returned to
the heart through two large parallel vessels. The part which passes
backward is again distributed to the vascular area as there are no con-
necting vessels with the tail of the embryo. The vitelline veins and
arteries run parallel to each other, though the veins lie a little forward
from the arteries.

In the embryo itself, the cardinal veins are the main afferent ves-
sels. At thirty-eight hours the anterior cardinals can be seen. These
are a pair of vessels which return the blood from the head of the embryo
to the heart. The posterior cardinals are also paired, and return blood
from the caudal region.

Both anterior and posterior cardinal veins unite on each side of the
body to form a short common vessel before entering the heart, the
right and left ducts of Cuvier, or common cardinal veins. These Cuvier-
ian ducts then turn ventrad on each side of the fore-gut and enter the
sinus venosus at the same point the omphalomesenteric veins enter.

The omphalomesenteric veins [so called because they pass through
the umbilicus (navel) as umbilical vessels connecting the offspring with
the mother in the higher forms], are established in the chick from thirty-
three to thirty-six hours' incubation. They are postero-lateral exten-
sions of the self-same endocardial tubes which formed the heart. They

SECOND HALF OF SECOND DAY 483

extend laterad to meet the vessels which develop in the vitelline plexus
outside the embryo, and which extend inward toward the embryo. The
omphalomesenteric veins (those lying within the embryo) eventually
become one with the vitelline veins (those lying in the extra-embryonic
area) and thus establish the afferent vessels of the vitelline circulation.

The efferent vessels develop at about forty hours. They have a dual
origin. The embryonic vessels consist of the branches of the dorsal
aortae which extend outward where they meet with the extra-embryonic
arteries growing toward the embryo to meet with, and become confluent
with, the embryonic efferent vessels, now being known as the
omphalomesenteric arteries.

It is at about the thirty-second hour that the heart begins to con-
tract irregularly, although the maximum rate (150 to 180 per minute),
is not reached until after 100 hours of incubation.

A portion of a. cross section of a 54 hour chick embryo

through the solid anlage of the pronephric tubules in the

region of the beginning of the Wolffian duct. The nephros-

tomes are just beginning to form, neph.st., nephrostome ;

u.n. pronephric ducts; w, Wolffian duct. (After Kolliker.)

THE PATH OF A BLOOD-CORPUSCLE

It is well to follow a corpuscle through its entire circulation at this
time. With the contraction of the heart, the corpuscle will be sent
through the ventral aortae, along the dorsal aortae, out through the
omphalomesenteric arteries to the plexus of vessels on the yolk.

It will be remembered that there are various membranes surround-
ing the yolk. These contain many small vessels which absorb the yolk.
As there must be an oxygenation of the blood, this vitelline circulation
must also assist in this function until the allantois, shortly to be de-
scribed, is formed. This aeration can be accomplished on account of the
great area these membranes cover, which permits a wide field from

484 EMBRYOLOGY OF THE CHICK

which to draw the oxygen that permeates through the egg shell and the
albumen surrounding the yolk.

After the yolk has been absorbed as food-material, and the blood
has become oxygenated, the blood is collected into the sinus terminalis
and the vitelline veins. The latter converge toward the embryo from
all parts of the vascular layer, and empty into the omphalomesenteric
veins, which return the blood to the heart.

The blood which has been sent to the various parts of the embryo
has in the meantime been returned from the head region by the anterior
cardinals, and from the caudal end by the posterior cardinals, the an-
terior and posterior cardinal of each side having met to form a short
common cardinal (duct of Cuvier) through which the blood flows into
the sinus portion of the heart.

There is therefore a mixed circulation in the heart, consisting of
both embryonic and extra-embryonic blood. The extra-embryonic, of
course, is the richer in both food and oxygen supply.

THE EXCRETORY SYSTEM

After about ten somites' have been formed, the beginnings of the
excretory system are visible.

It will be remembered that the mid-region of the partially seg-
mented mesoderm, known as the mesomere, is to become the excretory
system. It can be noted first as a solid cord of cells extending for two
or three somites (Fig. 285). This will be called the Wolffian Duct as
soon as a lumen forms.

During the second half of the second day, this solid rod elongates
both headward and tailward, the more tailward portion becoming free
and lying between ectoblast and mesoblast. A lumen appears toward
its center and extends headward and tailward simultaneously. About
the beginning of the fourth day the duct definitely opens into the cloaca.

The Wolffian body also makes its appearance on the second day, but
it will be better understood if the description is reserved until later.

CHAPTER XXXIV.

EXTRA-EMBRYONIC MEMBRANES

It will be remembered that when the mesoderm splits into a somatic
and a splanchnic layer, it extends out over the yolk so that there is no
definite line of demarcation separating embryo from the surrounding
media. First, the head fold appears, delimiting the embryo at the
cephalic end, and later the tail-fold and lateral fold do the same for the

pi bl -

Fig. 286.

A, B, C, D, four stages of development of the embryonic membranes in birds.
al, allantois; am, amnion, (in Fig. B., this forms folds which give rise to both
amnion and serosa) ; am.h., amniotic cavity; d, digestive cavity; do, yolk-sac.

E. Cross section through entire egg (including shell), all, allantois which
begins developing at the blunt end of egg ; am.h., amniotic cavity ; coel.ex., extra-
embryonic coelom ; do.s., yolk-sac showing development of mid-gut — do.s.d. ; do.h.,
covering of yolk ; l.k., air-chamber ; mes.w., extensions of the mesoderm between
the communicating opening of yolk sac and amniotic cavity — pl.bl. The remaining
portion of the yolk covering (do.h.) closes the passage. These mesodermal exten-
sions as well as the lower tips, at the pointed end of the egg, close later and thus
form a closed amniotic cavity, pl.bl., amniotic cavity which develops from the
ectoderm with tiny projections on the inner side. It is in this cavity that the
remaining yolk is found ; pl.bl.-do.s., Communicating passage between amniotic
cavity and yolk-sac. (A, B, C, D, after Boas, E, after Duval.)

caudal and lateral regions, so that after these folds have bent downward
and under the embryo, and almost separated the embryo from the yolk,
we speak of the space between the somatic and splanchnic layers as the
intra-embryonic coelom and the extra-embryonic coelom, according to
which portion lies within, and which portion lies outside, the embryo.

The limiting folds which are continuous with the head fold and ex-
tend on each side of the embryo as the lateral folds, form the line of de-

48fi EMBRYOLOGY OF THE CHICK

marcation known as the lateral limiting sulci.

In this chapter we are concerned with the extra-embryonic mem-
branes which are developed from the various layers in the extra-embry-
onic region. The membranes themselves are four in number: the yolk-
sac, the amnion, the serosa, and the allantois (Fig. 270).

THE YOLK-SAC

This is the first of the extra-embryonic membranes to appear. It
must be remembered, that as the splanchnopleure grows outward from
the embryo, it surrounds the yolk, thus forming the yolk-sac. The yolk
itself forms the floor of the primitive gut.

As the underfolding in the head-region separates the head from the
remaining blastoderm, it grows caudally and forms an entodermal floor
to the primitive gut, and the part which thus obtains this entodermal
floor is called the fore-gut. So, too, in the tail region a little later (about
the third day), the tail folds under the posterior end of the embryo and
the part which thus obtains an entodermal floor in that region is called
the hind-gut. The portion between fore-gut and hind-gut is the mid-
gut, which is, of course, that region where the yolk is still the floor. As
the fore-gut and hind-gut become larger and extend toward each other,
the mid-gut occupies less and less area, until there is merely a little duct
something like the small end of a funnel, the larger end of the funnel
being the extended splanchnopleure surrounding the yolk. In other
words, the mid-gut consists only of the opening of the yolk-stalk, which
latter is made up in turn of the walls of the splanchnopleure drawn to-
gether at this point.

As the neck of the yolk-sac is thus constricted, the omphalomesen-
teric arteries and veins, which extend throughout the region where the
constriction takes place, have likewise been drawn into the constricted
area, and pass to and from the embryo through the yolk-stalk, side by
side.

The yolk is now covered with a vascular network spreading through-
out the splanchnopleure of the yolk-sac, so that the entire food supply of
the embryo comes to lie in a sac with this circulation of its own definitely
attached to the mid-body region, though as far as we know, no yolk
granules pass directly into the embryo, all of it being absorbed by the
vascular network. In older embryos, the yolk-sac even folds consid-
erably, so that a still greater expanse of vascular area is established.

The white albuminous portion of the egg rapidly loses the water it
contains, and is absorbed by the extra-embryonic membranes.

Ultimately (about the nineteenth day) the yolk-sac is completely
enclosed within the embryo, and then rapidly disappears until it is en-
tirely gone by the sixth day after hatching.

EXTRA-EMBRYONIC MEMBRANES 487

THE AMNION AND THE SEROSA

While the splanchnopleure forms the yolk-sac^ it is the somato-
pleure, lying outside the embryo, from which both amnion and serosa are
derived.

At about thirty hours, the first observable portions of the amnion
appear as a crescentic fold with the concavity toward the head of the
embryo. This fold must not be confused with the head fold of the chick
which folds under the embryo.

The head at this time sinks into the yolk to a slight degree, just as
the extra-embryonic somatopleure anterior to the head is thrown into
the head-fold of the amnion. As the embryo grows anteriorly and the
somatopleure caudally, the amniotic fold which is thus folded upon itself,
forms a double-walled cap over the head of the embryo, gradually ex-
tending more and more caudad. The caudally directed limbs of the
head-fold of the amnion continue growing posteriorly on each side of
the embryo, where they are known as lateral amniotic folds. These grow
dorsad and mesiad, finally meeting in the midline.

During the third day, the amniotic tail-fold develops and grows
cephalad to meet the structures just mentioned, thus forming a complete
envelope for the embryo. The place where the various amniotic folds
meet is called the amniotic raphe.

The amnion is now a completed saccular structure filled with a
fluid in which the embryo is free to move about and change positions.
In all probability, this ability of free movement also prevents adhesions
and consequent malformations.

It is to be noted that the manner in which the amniotic folds came
into existence has caused the innermost portions to be ectodermal. This
ectodermal layer is continuous with the ectoderm of the embryo.

Likewise, the manner of the somatopleure folding upon itself, as it
does, causes two walls to cover the embryo. The inner one is the ecto-
dermal layer just mentioned and the outer one is known as the serosa.
There is a sero-amniotic cavity between the two.

The somatopleure now extends peripherally until the entire yolk-
sac, and eventually the embryo as well, is covered with serosa.

The allantois extends between serosa and amnion.

THE ALLANTOIS

This structure differs from the amnion and yolk-sac in that it de-
velops within the embryo proper, though it does extend out into the
extra-embryonic region as it develops.

About the third day, the allantois develops by an outpushing of the
ventral wall of the hind-gut (entoderm), pushing the splanchnopleure
ahead of it, so that we may say it is composed of splanchnopleure with
an entodermal lining. The following day it pushes out of the embryo

488 EMBRYOLOGY OF THE CHICK

into the extra-embryonic coelom, the attached end lying caudal to, and
parallel to, the yolk-stalk. The proximal portion is called the allantoic
stalk, and the extended bladder-like distal portion the allantoic vesicle.

It grows very rapidly from the fourth to the tenth day, and extends
into the sero-amniotic cavity in a flattened manner. Ultimately it en-
compasses the entire embryo and yolk-sac, and in so doing the mesoder-
mic layer of the allantois fuses with the layer of mesoderm of the serosa
which comes to lie in direct contact with it. This means that there is
thus formed a double layer of mesoderm, the serosal portion being de-
rived from the somatic mesoderm, and the allantoic portion derived
from the splanchnic mesoderm. A very rich vascular network now de-
velops between these two layers connected with the vascular circulation
by the allantoic arteries and veins.

The allantois thus becomes an organ of respiration, as well as cir-
culation, to the developing embryo. As the allantois lies just beneath
the porous shell, there is a wide area presented for an exchange of the
carbon dioxide developed within the embryo and the oxygen from the
outer world.

In addition to this function, however, the allantois also serves as a
reservoir for the secretions from the excretory organs of the embryo,
and likewise takes part in absorbing the albumen.

THE CHORION

The serosa will become a part of the chorion in the higher forms,
and consequently, should be clearly understood at this point. The
allantoic vessels mentioned above and the mesoderm which lies between
the serosa and amnion, later fuse with the inner layer of the serosa to
form the chorion, which is the embryo's organ of attachment to the
uterine wall of the mother. How very important the allantoic circu-
lation becomes in mammals may be surmised by realizing that there
is little yolk in mammalian eggs. This forces the embryo to receive all
of its nourishment from the blood of the mother through the uterine
walls. The allantoic circulation thus performs the function of the vitel-
line circulation also.

CHAPTER XXXV.
DEVELOPMENT OF THE THIRD DAY

Fig. 287.
64-hour chick embryo. (41 somites.)

It is upon this day that more structures make their first appearance
than on any other single day of the chick's entire embryonic life. The
blastoderm itself has increased in size so that it covers almost one-half
the yolk surface. The white of the egg has decreased in amount so that
the vascular area has been brought closer to the surface under the shell,
making aeration of the blood easier.

The sinus terminalis reaches its greatest functional activity during
this day, and the vitelline veins have been brought in close contact with
the vitelline arteries by the growth of the embryo.

The blood, which the vitelline or omphalomesenteric arteries bring
to the sinus terminalis, still flows headward and tailward as before. The
portion flowing toward the head returns to the embryo through two
large vessels lying parallel to the long axis of the embryo, but some-
times there is only one of these — the one emptying into the left vitelline
vein. Even if the two vessels are present, the left is the larger.

It is on the third day also that the single posterior vessel, which

490 EMBRYOLOGY OF THE CHICK

also empties into the left vitelline vein and carries blood from the pos-
terior region of the sinus terminalis, makes its appearance.

It will be remembered that it is during this day that the torsion of
the embryo takes place from the head region posteriorly, so that cross
sections made from the anterior end will show the embryo turned upon
its left side, while in the posterior region it still lies upon its ventral
surface.

The flexion continues also, so that the mid-brain becomes the most
anterior region of the embryo. This flexion not only brings the fore-
brain in close relation to the heart, but brings optic and otic vesicles
opposite each other, it being remembered that the eye-pits form in the
fore-brain and the auditory pits in the medulla oblongata.

THE NERVOUS SYSTEM

All parts are growing, and have become larger than on the second
day (Fig. 288). The important new developments are as follows: the
epiphysis appears as a small evagination in the midline on the dorsal
surface of the diencephalon. It later becomes the pineal gland.

Rathke's Pocket (Fig. 301, 1) is an ectodermal invagination which
has folded in just beneath the infundibulum. This pocket soon loses its
connection with the outer epithelium, and then becomes permanently
associated with the infundibulum to form the hypophysis or pituitary
body.

THE OPTIC VESICLES (Fig. 289)

It will be remembered that these were originally broad stalks
directly continuous with the cavity of the fore-brain. The cavity or
lumen of the optic stalks is then called an optocoele and the cavity or
lumen of the prosencephalon is called the prosocoele. A constriction
formed earlier is very marked at about fifty-five hours. The distal ends
have invaginated, forming a double-layered cup. The newly indented
layer is termed the sensory layer, because it is from this that the sensory
layer of the retina is to be formed. The underlying layer is called the
pigment layer, because it is from this that the pigmented layer of the
retina is to arise. The invaginated cups are often called secondary optic
vesicles to distinguish them from the original vesicles before invagina-
tion, the original vesicles being then known as primitive vesicles.

The optic cup does not invaginate so as to form an equally rounded
edge. The invagination begins at the ventral surface and continues
dorsally and toward the midline, so that at the place where the invagi-
nation began, there is a region which has no definite lip. The cup, there-
fore, looks more like a cup that has had this portion broken out. This
lipless region is known as the choroid fissure.

The invagination continues for the length of the optic stalk, thus

DEVELOPMENT OF THIRD DAY

491

forming a fissure in the stalk
along which, and in which
the optic nerves and blood-
vessels come to lie. This is
on the ventral surface of the
stalk. In the meantime, the
optocoele has practically be-
come obliterated, a very
small portion alone remain-
ing between sensory and
pigment layers in the optic
cup. Even these fuse short-
ly, and then the optocoele
disappears entirely.

The eye lens arises in-
dependently of the optic
vesicles in the ectoderm,
close to the vesicle. At
forty hours the ectoderm in
this region has thickened.
The placodes thus formed
grow toward and into the
optic cups after themselves
forming vesicles. The super-
ficial ectoderm from which
they arise soon closes again
at the point where the lenses
have arisen, although a very
small opening may still be
seen for a short time.

It is well to call partic-
ular attention at this point
to the similarity of the way
in which the lens of the eye
and the auditory vesicle de-
velop by a thickening of
ectoderm, then insinking
and finally completely sep-
arating from the superficial ectoderm from which it sprang. The lesson
to be brought home, is that once these structures have separated from
the superficial ectoderm, regardless of their original similarity, each fol-
lows a totally different line of development and differentiation so as to
become structurally and functionally unlike in the adult condition. This
original similarity and adult divergence should be noted throughout em-
bryological and comparative studies.

Fig. 288.

Diagrams showing brain development in vertebrates.
Longitudinal sections.

I. Before the blastopore closes.

II. At the time three brain regions can be seen.

III. At the time five brain regions have formed.
(Compare with Fig. 281.)

A, prosencepnalon ; aa, dividing line between telen-
cephalon and diencephalon ; c. cerebellum ; cc, cerebellar
commissure; ch, (in I and II) dorsal nerve cord; (in
III) habenular commissure; en, neurenteric canal; cp,
posterior commissure ; cw, thickening on optic nerve
due to the crossing of fibers (this is the chiasma) ; D,
diencephalon ; dd, line separating diencephalon and
mesencephalon; e, epiphysis ; e', paraphysis ; ect, ecto-
derm ; ent, entoderm ; ff, line dividing mesencephalon and
metencephalon ; J, infundibulum ; It, lamina terminalis ;
M, mesencephalon ; Ml, myelencephalon ; Ms, spinal chord ;
Mt, metencephalon ; np, neuropore ; P, prosencephalon ;
pn, neuroporic process ; pv, ventral brain-fold ; R,
rhombencephalon ; r, thickening of ectoderm which is
sometimes said to be the anlage of an unpaired olfactory
groove ; ro, optic recess; si, the groove (sulcus intraen-
cephalicus) which forms the hindermost boundary of the
midbrain ; T, telencephalon ; tp, tuberculum posterius
(After von Kupffer.)

492

EMBRYOLOGY OF THE CHICK

In the myelencephalic portion of the brain, the neuromeres have lost
their dorsal constrictions, though they can still be seen on the lateral and
ventral surfaces, while the whole cord has thickened. This thickening-
constricts the lumen so that it is quite slit-like at this time. The neural
tube has closed completely at both anterior and posterior ends at this
period.

It will be remembered that the neural or medullary plates have
formed, and their lateral folds have begun to unite to form the neural
groove. This union has now been completed. The ectoderm, dorsal to

Diagrams of sections through the eye of the chick embryo at the end of the
second day. The dorsal margin is toward the top of the page in A and B. A. Eye
as viewed directly. B. Vertical section through the line x-cf, in A. C. Horizontal
section through the line y-y, in A. cf. Choroid fissure ; cv, cavity of primary
optic vesicle ; ec, superficial ectoderm of head ; i, inner or retinal layer of optic
cup ; I, lens ; o, outer or pigmented layer of optic cup ; ol, opening of lens sac from
surface of head ; pc, posterior chamber of eye ; s, optic stalk, continuous with the
floor and lateral wall of the diencephalon. (From Lillie "Development of the
Chick." by permission of Henry Holt & Co., Publishers.)

the groove, has again become continuous, leaving a slight area between
neural groove and superficial ectoderm.

It will also be remembered that there are small groups of cells on
each side of the midline lying within this area which we called neural
crests, to distinguish them from the neural folds with which they were
in close connection.

The two crests lying on each side of the midline fuse for a time, but
because they began as two separate groups, they again become separate
in a short time. They also form a sort of column on each side of the
midline, running along the long axis of the embryo, but soon they seg-
ment and become the dorsal root ganglia or sensory ganglia of the spinal
nerves (Fig. 290). As the segmented portions of these neural crests also
extend into the head region, they there give rise to the ganglia of the
sensory cranial nerves.

THE DIGESTIVE TRACT

At the period we are describing, the fore-gut has extended from
the anterior intestinal portal as its posterior limit to the infundibulum
as its anterior limit. It is divided into a pharyngeal portion, lying ven-

DEVELOPMENT OF THIRD DAY

493

tral to the myelencephalon and encircled by aortic arches, and an
oesophageal portion, lying posterior to the pharyngeal with a much
smaller lumen than the pharynx.

At this time there is an outpushing of the ventral portion of the
pharynx and an inpushing from the ectoderm close to this region, which
will soon meet and form the mouth-opening. The ectodermal inpushing

Spinal con!

Spina! ganglion —

Ventral root —

Mixed spinal nerve —
Myotome —

Sympathetic ganglion --

Fig. 290.

Developing nerve roots in a chick embryo of 4% days. (After
Neumayer. )

is known as the stomodaeum and the thin layer of tissue between the
inpushing and outpushing which is later to break through to complete
the mouth-opening is called the oral plate. (Fig. 301, I, Seessel's
pocket.) It is this oral plate region in the adult which separates the
oral cavity from the pharynx.

The fore-gut extends into the head region cephalad to the
stomodaeum, and this portion is called the pre-oral gut. This pre-oral
gut, however, disappears shortly after the oral plate breaks through,
leaving only a small diverticulum which is then called Seessel's pocket.

The digestive tract has been lying close to the notochord up to this
time, being separated from the notochord and the aortae by a broad thin
layer of mesoderm. Now it begins to draw ventrad from this position,
remaining attached, however, by the mesentery, a constantly narrowing
band of tissue.

This mesentery is composed of mesoblast continuous with that
which surrounds the entoderm of the digestive canal. The mesoblast
consists of an undifferentiated middle layer (Fig. 291, b), in which blood

494

EMBRYOLOGY OF THE CHICK

pig.
pig.

Fig. 291.

Development of Digestive Tract.

1 Transverse section of descending colon 10-mm. pig.

2 Transverse section of descending colon 14-mm. pig.
Transverse section of descending colon 20-mm.
Transverse section of descending colon 25-mm.
Transverse section of descending colon 31-mm. pig.
Transverse section of descending colon 46-mm. pig.

ABBREVIATIONS,
serosa.
undifferentiated middle layer.

dm., dorsal mesentery.

cm., inner circular smooth-muscle layer.

lm., outer longitudinal smooth-muscle layer.

mt., mesenteric taenia muscle band.

sp., Meissner's plexus (submucous.)

ap., Auerbach's plexus (intermuscular.)

sm., serosa.

subm., submucosa.

p.m., primordial mucosae cells.

N. B. — Note especially rapid increase in
tube and the absolute decrease in thickness of mesenchymal wall
due to tension stresses elicited by the growth of the former. (Ebep
J. Carey in The Anatomical Record, Vol, 19. No. 4.)

ridth of epithelial

DEVELOPMENT OF THIRD DAY

495

vessels are developed later, and a superficial layer (Fig. 291, a), of epi-
thelium, continuous with the epithelial lining of the peritoneal cavity.
The withdrawal of the anterior part of the fore-gut from the notochord
is slight, as little or no mesentery is developed in that region.

It is interesting to note here that the oesophagus has its lumen
closed for almost its entire length during the sixth day, only to reopen
from the posterior region anteriorly again in about two days by the epi-
thelial tube growing rapidly. This latter grows in a circular direction
on account of the outer pressure.

The portion of the intestinal tract immediately posterior to the
oesophagus becomes dilated on this day to form the stomach. This is
followed posteriorly with a short region recognized as the duodenum
because the beginnings of the liver and pancreas can be observed.
Mesenchymal cells gather about the oesophagus and stomach from which
their muscular and connective tissue coats will be derived.

There will be seen a small pitting in of the ectoderm to meet the
underlying entoderm where the anal opening is to appear. However,
this posterior opening does not open into the digestive tract until about
the fifteenth day of incubation. The indenture which is to become the
anal opening is called the proctodaeum.

The digestive tract is almost straight until the sixth day, when the
various loops form and the gizzard develops as a thick-walled outgrowth
from the end of the stomach.

THE LUNGS

Two small hollow outgrowths from the
ventral side of the oesophagus near its anterior
end are seen on this day, the oesophagus itself be-
coming constricted at the point of outgrowth.

These constrictions form two divisions, the
more dorsal being the oesophagus and the ventral
portion the trachea. At the point where oesopha-
gus and tracheae are continuous, the glottis will
be formed.

The trachea grows caudad and bifurcates to
form pairs of lung-buds. These lung-buds extend
outward into the surrounding mesenchyme lying
on either side of the midline. The splanchnic
mesoderm is pushed ahead of the growing lung-
buds until it covers them and forms their outer
investment layer, or pleural covering. The ento-
derm of the intestinal tract from which the
trachea evaginated, forms the entire lining of
trachea, bronchi,, and all air-chambers in the
adult lungs. The connective tissue stroma of the

Fig. 292.

Ventral view of lungs and
air-sacs of 12 day chick
embryo. at, anterior in-
termediate sac ; a, abdominal
sac ; c, cervical sac ; I, lat-
eral part of interclavicular
sac ; lu, lung ; m, mesial
part of interclavicular sac ;
oe, oesophagus ; p, posterior
sac; t, trachea. (From
Kingsley after Locy and
Larsell. )

496

EMBRYOLOGY OF THE CHICK

lungs, however, is derived from the mesenchyme surrounding the lung-
buds.

In the chick and all birds, there is a characteristic thin-walled, sac-
like outgrowth from the hinder edges of the lungs forming the air-sacs
(Fig. 292). These do not appear until about the eighth day.

THE LIVER

The liver arises as a ventral diverticulum from the duodenum. It
can be seen for a short time on the lip of the anterior intestinal portal

growing cephalad toward
the fork where the omphalo-
mesentric veins enter the
sinus venosus. The liver
grows out as a series of
cords pushing the splanchnic
mesoderm ahead of it as its
investing layer.

The liver evagination as
it forms, retains its opening
into the duodenum (Fig.
293), which later differen-
tiates somewhat to become
the common bile duct, the
hepatic and cystic ducts, as
well as the gall bladder.
Cellular cords bud off from
the diverticulum and be-
come the hepatic tubnles
which have secretory func-
tions.

As the intestinal portal
moves caudad when the
fore-gut lengthens, the
proximal portions of the
omphalomesenteric veins
come together and fuse in
midline. The fusion extends
caudad nearly to the level of
the yolk-stalk, beyond which
they still remain separate.
The liver now surrounds
the fused portion of the
omphalomesenteric veins.

It will be noticed, there-
fore, that the yolk materials

Fig. 293.

Two upper cuts are diagrams to show the develop-
ment of the liver, pancreas, and hepatic ligaments.
d, intestine; ect, ectoderm; leb, liver anlage ; lig.hep.ent.,
ligamentum hepato-entericum ; lig.susp.hep ; ventral mes-
entery or ligamentum suspensorum of the liver ; mesent.
dors., dorsal mesentery ; pancr. dors, and pancr. ventr.,
dorsal and ventral pancreas. (After Schmikewitsch. )

Lower cut is a diagram to show the development of
the liver. Lobule 1 shows the principal parts of the
gall capillaries ; Lobule 2, shows the anastomoses of these
gall capillaries ; in Lobule 3, only the efferent bile capil-
laries are shown, together with the arterial and venous
capillaries, a, arteries; b, veins. (After Stohr.)

DEVELOPMENT OF THIRD DAY

497

must already at this early period pass directly into and through the liver.
If this is remembered, it will make the adult portal circulation the better
understood.

THE PANCREAS

The pancreas arises as three diverticula from the duodenum at the
approximate level of the liver diverticulum. There are three pancreatic
buds, one medial, lying dorsal to the duodenum, and a pair of ventro-
lateral buds.

The median bud appears at about seventy-two hours, while the two
ventro-lateral buds can be seen at the end of the fourth day. The
dorsal bud arises directly opposite the liver, and grows into the dorsal
mesentery, while the ventro-lateral buds arise at the point where the
liver connects with the intestine, so that both the liver duct and the
ventral pancreatic duct open into the duodenum by a common duct
called the ductus choledochus. Cellular cords grow into masses from the
three buds, fusing into one glandular mass with two ducts remaining,
although sometimes all three remain.

THE THYROID GLAND (Fig. 294)

This arises as a median diverticulum from the floor of the pharynx
at the level of the second pair of gill pouches. By the close of the fourth
day, the solid, rod-like diverticulum lying in a longitudinal position
under the floor of the pharynx has become saccular, remaining connected
with the point of origin as the thyro-glossal duct opening at the root
of the tongue. In mammals, there are additional evaginations at the lat-
eral region of the fourth gill pouch. By the sixth day, the thyroid body
in the chick becomes bi-lobed, the lobes sending out cords of tissue
which become hollowed out to form the regular adult thyroid tissue.

The gland then shifts backward and
becomes surrounded with a sheath
of vascular connective tissue.

THE THYMUS GLAND
(Fig. 294)

This organ arises from the pos-
terior faces of the third and fourth
gill pouches after the fourth day of
incubation. While the organ is orig-
inally epithelial in character, there is
soon an ingrowth of mesenchyme
and the thymus then becomes chiefly
lymphoid in structure.

Fig. 294.

Diagrams to show the development of the
derivatives of the digestive tract in the
branchial region. A, Anura, B, lizard, cd,
carotid gland ; e1-ea, epithelial bodies ; Krd.
Krm, Krv, dorsal, mid, and ventral remains of
the axial portions of the gill pouches ; p.
postbranchial bodies ; Tml=3, Thymus anlage ;
Tr, Thyroid gland; I-V, Gill slits (After
Maurer. )

4:98

EMBRYOLOGY OF THE CHICK

THE VISCERAL CLEFTS AND VISCERAL ARCHES

Different parts of the embryo grow at different rates of speed, and
while the heart was formed directly under the anterior end of the diges-
tive tract on the second day, on the third day the heart has shifted its
position so far posteriorly that there is a distinct space between it and
the head proper. This space we may call the neck or pharynx.

It is in this region that the mesoderm has not divided into the two
layers — the somatic and splanchnic. We therefore still have a sort of
sheet consisting of the three primitive layers of ectoderm, mesoderm,
and entoderm, extending outward from the embryo.

The entodermal lining of this neck region becomes pushed out into
four narrow pockets (Fig. 295, A), called the visceral or gill pouches,
during the latter part of the second or the early part of the third day.
These meet with ectodermal depressions formed as furrows which grow
inward to meet the gill pouches. The thin wall between the outpushings
and the ectodermal inpushings breaks through in the lower forms, such
as in fish and amphibia, and there remains open throughout life, but in
the chick the opening is seen in the first three pairs during the first three
or four days. It remains open for about two days. These openings, or
places where openings usually occur, are known variously as visceral
clefts, gill clefts, or branchial clefts.

As the neck is considerably curved, these clefts do not lie parallel
to each other, but converge toward the ventral part of the neck. The
fourth cleft never opens in the chick.

Numbering and naming of these clefts begins with the most anterior
and continues caudad.

A

Fig. 295.

A, Horizontal, and B longitudinal section through the head region of Ammo-
coetes (larval stage of lamprey.) ao.b., anterior aortic arch; ao.d., dorsal aorta;
oo.v., ventral aorta ; di, invagination which separates the anlage of the thyroid
gland from the digestive tract; m, anlage of mouth; thyr, thyroid anlage; 1,
ciliated gill region which probably becomes the spiracle ; 2-8, gill pouches.
(A, after Vialleton ; B, after Dohrn.)

DEVELOPMENT OF THIRD DAY 499

The first one is called the hyomandibular cleft, while the remaining
ones are known respectively as the II, III, and IV gill clefts.

Between these clefts, as well as immediately anterior and posterior
to them, there is a pair of thickened regions, each pair of wrhich meets
ventrally in the midline and merges with its mate from the opposite side
of the body. These thicknesses are called visceral arches, gill arches,
or branchial arches, also numbered from the anterior end, caudad. The
first is called the mandibular, the second the hyoid, and from here caudad
the III, IV, and V.

The hyomandibular cleft lies between the mandibular and hyoid
folds or arches.

It is well at this point to anticipate a little as to what is to become
of these structures later (Fig. 296).

All the clefts close, with the exception of the hyomandibular. This,
too, begins closing at the end farthest from the pharyngeal opening, but
retains the opening into the pharynx. The unclosed end itself becomes
the tympanic cavity, while the remaining portion of the cleft becomes
the Eustachian tube.

The external auditory meatus is formed by a depression in the sur-

Fig. 296.

Head of a 5l/2 day chick embryo, a.n.f., lateral nasal process ;
au, eye ; bulb.ao.~ bulbus aortae ; i,n.f., inner nasal process ; fc.6.1
and fc.fo,2 mandibular and hyoid arches ; max, upper process of the
mandibular arch ; nas.gr., nasal groove ; st.f., frontal process ;
tr.nas., tear-duct running to nasal cavity ; ventr., ventricle ; v.h.,
forebrain. (After Duval.)

face ectoderm opposite the position of the tympanic cavity. The outer
end of the closed hyomandibular cleft thus lies between the tympanic
cavity and the external auditory meatus, the tissue formed by the closure
of the cleft forming the tympanic membrane.

The most posterior two gill arches or folds entirely disappear in
adult stages of the chick.

The pair of mandibular arches grow toward each other on the ven-
tral side and fuse to form the basis of the mandible, or lower jaw. From

500 EMBRYOLOGY OF THE CHICK

the dorsal end of each mandibular arch and at their anterior edge, a small
branch grows downward and forward during the fourth and fifth days.
Such branch or branches are called maxillary processes. There is a tri-
angular median process growing toward these maxillary processes from
the front of the head, known as the fronto-nasal process. The maxillary
processes form the upper jaw or the maxillary bones. The maxillary
processes do not fuse with each other, but to each side of the fronto-
nasal process. When this union does not become complete, the well-
known abnormality of hare-lip results.

The formation of clefts and arches may be understood the better by
the following illustration from Professor Reese :

With the hands in front of the body (the palmar aspect of each
hand directed mesiad), and pointed downward, "bring the tips of the
fingers together, the fingers of each hand being slightly separated. The
thumbs should, at first, be closely pressed against the forefingers, and
should be considered as fused with them. If the fingers and hands are
slightly bent, there will be a space between the two hands that may be
taken to represent the pharynx of the chick, while the four fingers will
represent the first four gill arches, and the spaces between the fingers
will represent the first three gill clefts. The closure of the visceral clefts
may be represented by bringing the fingers of each hand together. The
forefingers, which should, in reality, be the only ones which actually
meet in the midventral line, will represent the mandibular arch, forming
the lower half of the mouth. The formation of the maxillary arch, by
processes budded out from the upper ends of the mandibular arch, may
be represented by separating the thumbs from the forefingers, and point-
ing them toward each other without letting them come in contact; the
triangular space between the thumbs, thus held, being fulfilled in the
imagination by the fronto-nasal process. The angles between the thumbs
and forefingers will represent the angles of the mouth. Of course, to
make the comparison more striking, there should be one more finger to
represent the hindermost arch and cleft, but as the hinder arches and
clefts form no part of the adult chick, this omission is of little impor-
tance."

THE CIRCULATORY SYSTEM (Fig. 297)

As has been stated, there are already two or three pairs of aortic
arches present, by which blood is carried from the bulbus arteriosus
around the pharynx to the dorsal aorta. It will be noted that the first
aortic arch lies in the first (mandibular gill arch), the second in the hyoid
fold, and so on, each bearing a distinct relation to the correspondingly
numbered gill-fold.

The heart, 'which it will be remembered is attached only at the
cephalic and caudal ends, is growing rapidly and twisting upon itself.
The venous or atrial side is the more stationary. This side, originally,
lay caudal to the arterial or conus end of the heart, but in the twisting,

DEVELOPMENT OF THIRD DAY 501

the conus end comes to lie caudal to the sinus or venous end, a position
that the higher vertebrates all retain in the adult stage. In fishes, the
atrial region of the heart remains caudal to the ventricular portion even
in the adults.

The point where the two vitelline veins meet to empty into the
heart becomes pushed farther and farther caudad, so that the two veins
unite and form a common opening into the heart. All blood from the
vascular area to the heart passes through this single common tube,
though in a short time the right vein will dwindle away and disappear.
The tube is then an opening for the left vitelline vein only. This com-

wr

Fig. 297.

Diagrammatic lateral view of the chief embryonic blood-vessels of the chick,
during the sixth day. a, Auricle ; al, allantoic stalk ; ao, dorsal aorta ; c,
cceliac artery ; ca, caudal artery ; cl, cloaca ; cv, caudal vein ; da, ductus arteriosus ;
dv, ductus venosus ; ec, external carotid artery ; ej, external jugular vein ; t, in-
testine ; ic, internal carotid artery ; ij, internal jugular vein : I, liver ; m, meso-
nephros ; ma, mesenteric artery ; mv, mesenteric vein ; p, pulmonary artery ; pc,
posterior cardinal vein ; pv, pulmonary vein ; 8, sciatic artery ; sc, subclavian
artery ; scv, subclavian vein ; st, yolk-stalk ; sv, subcardinal vein ; ul, left umbilical
artery ; ur, right umbilical artery ; uv, left umbilical vein ; v, ventricle ; va, vitelline
artery; vca, anterior vena cava (anterior cardinal vein) ; vp, posterior vena cava;
vv, vitelline vein ; y, yolk-sac ; 3, 4, 6, third, fourth, and sixth aortic arches.
(From Lillie's "Development of the Chick," by permission of Henry Holt & Co.,
Publishers.)

mon tube-like entry into the heart is called the meatus venosus ; the por-
tion nearest the heart is the sinus venosus, and the portion lying more
distal, the ductus venosus.

The dorsal aorta gives off numerous branches supplying various
portions of the body of the embryo, the blood being returned by two
large veins on each side of the body. That from the anterior part of the
embryo being carried through the anterior cardinal veins and that from
the posterior part of the body being carried by the posterior cardinal
veins, the anterior and posterior cardinals then unite into a common vein
before emptying into the sinus venosus, and this common vein is called
the duct of Cuvier.

XXXVI.

THE DIFFERENTIATION OF THE SOMITES

The somites have already been described as almost solid triangular
blocks of cells derived from the dorsal mesoderm. There is a tiny open-
ing in the center running horizontally through the somite. Oftentimes
the opening cannot be seen at all. This opening is called the myocoele.
As the embryo continues to increase in size, the triangular block
becomes more or less circular and there are two layers of cells, an outer

epithelial layer and an inner
portion (Fig. 298). The inner
portion has its cells irregularly
arranged.

It is this ill-defined group
of cells which is known as the
sclerotome. The cells are
mesenchymal.

The sclerotomes of each
side now grow still farther
toward the notochord and sur-
round it. Later they develop
into the vertebrae.

The dorsal portion of the
outer cell mass whose more
medial portion became the
sclerotome, has retained its
definite outlines and epithelial
characteristics. This portion,
now called the dermatome, is to
become the deeper layer of the
integument. It is important to
remember at this point that the ectoderm gives rise to the epithelial layer
of the integument only.

The portion of the cell mass which lies medial and slightly ventral
to the dermatome is called the myotome. The myocoele now lies between
the dermatome and the myotome. It is from the myotome that the
entire skeletal musculature is developed by the ventral walls of the
myotome becoming converted into longitudinal muscle fibers. These
bands of fibres then remain divided into blocks which correspond to the
original somites. Here again we have a metameric arrangement of mus-
cles in the embryo of the chick which corresponds to the segmental ar-
rangement of muscles in the adult fish.

Fig. 298.

Diagram of Myotome and Nerve Development.
The more dorsal portion of the somatopleure is
known as the dermatome while the dorsal portion of
the splanchnopleure lying closest to the dermatome
forms the sclerotome.

DIFFERENTIATION OF SOMITES

503

The outer portion of the myotome gives rise to the muscles of the
neck and trunk, the muscles of the appendages arising independently of
the myotomes.

>ve.P.

77

Fig. 299.

The development of the mesonephros. A, B. Transverse sections through the
mesonephric tubules of the duck embryo with forty-five pairs of somites. After
Schreiner. C. Transverse section through the middle of the mesonephros of a
chick of ninety-six hours. From Lillie (Development of the Chick). Ao., Dorsal
aorta ; B., rudiment of Bowman's capsule ; c., collecting duct ; Cod., crelom ; Col. T.,
collecting tubule ; d., dorsal outgrowth of the Wolffian duct ; Glom., glomerulus ;
germ. Ep., germinal epithelium ; M's't., mesentery ; «.£., nephrogenous tissue ; r.,
rudiment of conducting portion of primary tubule ; T. 1, 2, 3, primary, secondary,
and tertiary mesonephric tubules ; V.c.p., posterior cardinal vein ; W.D., Wolffian
duct. D, Cross section through the head kidney in the region of the gonads of a 4
day chick embryo, a, germinal epithelium showing the primary germ-cells c and o;
a, portion of the peritoneal epithelium which forms the Mullerian duct ; E, the
tissue immediately surrounding the germ cells which forms the stroma of the
gonads later ; L, Somatopleure ; m, mesentery ; WK, Pronephros ; y, Wolffian duct ;
z, Mullerian duct. (After Waldeyer and O. Hertwig.)

THE EXCRETORY SYSTEM

It is on the third day that the intermediate cell mass — the mesomere
(Fig. 268, mm) — lying between the somite proper and the point where
the mesoderm splits into somatic and splanchnic layers, becomes very"
prominent, being covered with sharply defined epithelial cells (Fig.
299).

It is of great importance for one's future study of embryology as
well as for the study of comparative anatomy that the development of
the excretory system be thoroughly understood.

It is this intermediate cell mass or mesomere, now called the nephro-
tome, which is to develop into both urinary and reproductive systems.

504

EMBRYOLOGY OF THE CHICK

The Wolffian duct has already been mentioned. The embryonic kidney
in the chick is called the Wolffian body or mesonephros. This embryonic
kidney ceases to function very soon after hatching, and is then replaced
by the metanephros.

One of the lowest forms of a chordate (an animal which possesses a

notochord), is the small fish-
like amphioxus or lanceola-
tus. In this animal a primi-
tive form of excretory system

•fg, B,^ develops and persists

£$, ^© throughout the adult life of

the animal. It is called a
pronephros, or head kidney.
This structure develops in
the frog and other amphibia
during the embryonic pe-
riod, but it is followed by
the mesonephros or Wolffian
body, which becomes the
permanent kidney of the
amphibian, while in the
chick, as in all amniotes, the
mesonephros serves as the
embryonal kidney, which is
then followed by the devel-
opment of a metanephros or
permanent amniote kidney.

Schematic arrangement to show relationship of v^1^' «JvAj.)

metanephros and mesonephros. I, in Gymnophiona (trop- Nntwith«:tpnrlinrr + V, &

ical amphibians without tails or legs). II, in ad- INOtWlt

vanced chick embryo. Ill, one type of its appearance in
man. IV, in rabbit. The Wolffian duct and ureters are
black. The canaliculae of the mesonephros are hatched.
The canaliculae of the metanephros are dotted. (After
Felix.)

higher animal forms develop what the immediately lower animal form
possesses, plus the next succeeding type of pronephros, mesonephros, or
metanephros.

Amphioxus therefore has the pronephros as its permanent kidney,
amphibians have the pronephros as a sort of embryonic kidney with the
mesonephros in the adult form, while all higher types of animals have
a pronephros (which just appears and degenerates during the early em-
bryonic period) with a mesonephros acting as an embryonic organ of
excretion, and then, later, from the caudal region of the mesonephric
duct the adult permanent kidney or metanephros develops.

To obtain a clear and accurate view of the functional and structural
relations of the three kidney-forms, it is important to summarize the
nephridial theory here.

9

Fig. 300.

type of these three kidneys
which an animal may pos-
sess in adult life, all of the

DIFFERENTIATION OF SOMITES 505

Theoretically, it appears that the waste matter containing nitrogen
which is elaborated in the primitive liver and collected in the coelom,
together with the coelomic fluid itself, passes outward through the
nephrostomes and tubules in each segment. In higher forms all the
parts are more differentiated and some of the segmentation is lost.

Figure 168 gives a clear understanding of the earthworm's segmented
excretory system which represents the pronephridic type of kidney.

Such a primitive type of nephridia, if completely developed, may be
described as follows : At the proximal end of the tubule, a ciliated fun-
nel, the nephrostome, opens into the coelom. The cilia may continue
into the tubule to produce a current which will carry the coelomic fluid
into and through the tubule. The tubule expands into a Malpighian or
renal corpuscle. This corpuscle consists of a vesicle known as Bow-
man's capsule, one side of which projects into the other, so that the
cavity is nearly filled. The inturned portion is the glomerulus, consist-
ing of a network of capillary blood vessels, supplied by an artery and
drained by a vein. Beyond the Malpighian corpuscle the tubule becomes
convoluted, while its cells become glandular. The first convoluted tubule
is followed by a straightened portion forming a simple U shape. The
arms of the U form the ascending and descending limbs. The entire U
is called Henle's loop. Then follows a second convoluted tubule which
passes by means of a short connecting tubule into the non-glandular
collecting tubule. Other groups of similar-formed excretory units enter
this same collecting tubule, which then leads into a urinary duct through
which the waste matter is carried out of the body.

Various parts of the complete system just described may be miss-
ing in different groups of animals. For example, in Amniotes, the
nephrostomes are never formed, though they are in Ichthyopsida.

In the pronephros, the Malpighian corpuscle is quite rudimentary
and often entirely lacking, and there is also no differentiation into con-
voluted tubules and Henle's loop.

The renal corpuscles form a sort of filtering apparatus by which
water is passed from the blood-vessels of the glomerulus into the tubules
near their beginning, which liquid thus serves to carry out the urea, uric
acid, etc., which has been secreted by the glandular portions of the walls
of the tubules.

A varying number of nephrotomes form in different animal forms,
and so also a varying number of nephrostomes form. Figure 300 will
give the student a general idea of how mesonephros, and metanephros,
follow each other and just what their relations are.

The tiny tubules must not, however, be confused with the ducts.
The ducts represent the collecting tubule described above.

The pronephric tubules grow first and then join the, pronephric
ducts. Later the mesonephric tubules grow caudad to the pronephric
tubules and join the same ducts. The original pronephric tubules then

506 EMBRYOLOGY OF THE CHICK

degenerate, so that now the ducts which were originally called pronephric
become the mesonephric ducts.

In the real kidney, or metanephros, the tubules do not grow toward
the mesonephric ducts, but from these ducts. They grow headward and
laterad and ultimately connect with the tubules of the mesonephros,
after which the mesonephros itself degenerates with the exception of
the Wolffian or mesonephric ducts, which in the male become the tubules
through which the sperm pass.

With this in mind, the excretory system of the chick can be studied
with some understanding.

At about thirty-six hours, it will be remembered, the pronephric
tubules were seen to arise from the nephrotome, one pair lateral to each
somite from the fifth to the sixteenth. Each tubule arises as a solid bud
of cells with the free ends growing dorsad, close to the posterior cardinal
veins. The distal end of each tubule is bent caudad later, until it reaches
the tubule directly posterior to it. Thus is formed a continuous cord of
cells which is to become the pronephric duct. These ducts continue to
extend caudad beyond the region where the tubules were formed, and
soon develop a lumen. The ducts ultimately reach the cloaca, extending
ventrally and opening into it.

The best way to study a series of cross sections is to begin caudad
and observe them serially toward the head, because the posterior por-
tions are not so well developed as are the anterior.

The pronephros (Figs. 285 and 299, D) varies in its development,
although it usually can be noted in from the fifth to the fifteenth or six-
teenth somite. Typically it develops from the tenth to fifteenth, inclu-
sive. No duct is formed anterior to the tenth somite, the pronephric
buds in that region disappearing by the end of the second day.

. Mesonephric tubules (Fig. 299, A, B, C), develop in all segments
from the thirteenth or fourteenth to the thirtieth, so that the most an-
terior mesonephric tubules develop in the same segments where the
pronephric tubules also developed, although it is only posterior to the
twentieth segment that the mesonephros develops typically.

The mesonephric tubules, which are to connect with the ducts, are
developed from radially arranged cell masses lying ventral and medial
to the ducts. The most anterior of these tubules acquire a lumen by
the time the ducts have developed their lumen. These tubules grow
toward and connect with the duct. Later they remain as isolated vesi-
cles. The grouping of the mesonephric tubules constitutes the
mesonephros or Wolffian. body. Some of the more cephalad mesonephric
tubules seem to develop nephrostomes opening into the coelom.

The tubules themselves, having formed separately from the ducts
and then grown outward and connected with them, have had their out-
ward ends develop a cluster of closely packed cells which lies in close
relationship to the dorsal aorta. This cluster becomes the glomeruli.

DIFFERENTIATION OF SOMITES 507

In fact, by the fifth day, circulation has already been established in the
glomeruli, and from then until the eleventh day, the mesonephros is at
the height of its functional activity. Then the metanephros takes its
place.

The pronephric tubules which attain even a degree of completeness
lie in the tenth to fifteenth somites. It is interesting to observe that
when these tubules begin to degenerate, the glomeruli begin to form at
the points of the tubules, close to the coelom and actually project into
the coelom. These bud-like structures are extremely variable, both as to
number and degree of development. They even develop differently on
both sides of the chick. They appear to be best developed on the third
and fourth days. It is for reasons such as the ones mentioned above
that former writers insisted that in the chick the pronephros really de-
veloped later than the mesonephros.

XXXVII.

THE DEVELOPMENT OF THE FOURTH DAY

On opening an egg which has been incubated for four days, the
great increase in size of the embryo is the most noticeable feature. The
germinal membrane now covers almost one-half of the yolk, and the

L

Fig. 301.

1, Median sagittal section of 82 hour chick embryo. 2. Whole mount to show

regions from which A to O are cut. Sections A=A — A; B— B — B; etc. (Re-
drawn from Duval.)

DEVELOPMENT OF FOURTH DAY

510 EMBRYOLOGY OF THE CHICK

vascular area is very prominent, although the sinus terminalis has al-
ready begun to diminish in distinctness.

The amnion covers the entire chick, but as there is as yet little fluid
in the amniotic cavity, the amnion lies close to the embryo.

The splanchnic stalk forms a narrow tube connecting yolk-sac and
mid-gut, but the somatic stalk has not kept even pace with the splanch-
nic, so that there is a ring-shaped space between the two through which
space the allantois projects. The allantois is connected by a narrow
stalk with the hind-gut just cephalad to the tail.

mv The cranial flexure increases

to a considerable extent as does
also the body flexure, so that the
embryo now describes a half-
circle.

The muscle plates are nearly
rig. 302. vertical in position, extending al-

Appendage muscles being budded off from most to the point of Separation of
myotomes in the European Dogfish, Pristiurus. b, « j 1 i.

muscle buds; my, myotomes. (From Kingsley SOmatOplCUre and SplanChnO-

pleure, while just beyond this

point of separation the somatopleure is raised to form a longitudinal
ridge on each side, which is called the Wolffian ridge.

It is on this day also that the beginnings of the appendages, the
wings and legs, can be seen as local swellings of the Wolffian ridge.

These arise (the wing-buds just posterior to the heart region, and
the hind-limb-buds just anterior to the tail) as conical or triangular
groups of mesoderm covered by ectoderm (Fig. 302). By the end of the
day the wing-buds have become elongated and narrow, while the limb-
buds are short and broad.

The embryo now lies on its left side, torsion being complete to the
extent of ninety degrees.

It is on this day also that a fourth gill cleft appears. The gill arches
become so thick now that one can scarcely see the aortic arches in any
of them.

In the head region, the cephalic flexure presses the ventral surface
of the head so tightly against the pharynx that the head and pharyngeal
region must be removed and studied from their ventral aspects or little
can be observed.

Figure 296 will show that the mandibular arch forms the more
caudal boundary of the oral depression, while on each side, the arch
forms an elevation, the maxillary processes, which grow mesiad and
form the antero-lateral boundaries of the mouth opening.

The nasal pits form as hollow depressions in the ectoderm of the
anterior part of the head overhanging the mouth region with U-shaped
elevations surrounding them. The median limb is the naso-medial
process and the lateral limb is the naso-lateral process. The two naso-

DEVELOPMENT OF FOURTH DAY

511

medial processes grow toward the mouth and meet the maxillary
processes which grow inward from each side. It is the fusion of these
two naso-medial processes with each other in the midline and with the
maxillary processes laterally that forms the upper jaw, the maxilla.

The lower jaw is formed by the fusion in the midline of the right
and left portions of the mandibular arch.

Foramen of Monro
Corpus striatum

Eye

III ventricle
Ohonoid fissure

Me'sodermal tissue,
forming later the
chorioid plexus

Pharynx
Tongue

Transverse section through the forebrain of a 16 mm. human
embryo (six to seven weeks) to show the relationship of the
ventricles.

THE NERVOUS SYSTEM

Figures 282 and 288 show how the two lateral evaginations of the
fore-brain stand in relation to the cephalic end of the central nervous
system, and why it is that the ears come to lie on practically the same
dorso-ventral level with the eyes, although they begin forming so far
ipart.

The development which brings this about has already been dis-
;ussed. Here it is important for the student to observe that the two
evaginations forming the telencephalic vesicles have an open space
within them, known as the I and II ventricles, also called lateral ven-
tricles. The portion between them is the III ventricle, which is later to
become a mere connecting slit-like tube to connect the lateral and more
posterior ventricles. The entire opening in the fore-brain is called the
telocoele, that in the diencephalon the diocoele, that of the mesencepha-
lon the mesocoele (later called the aqueduct of Sylvius), that of the
metencephalon the metacoele, and that in the myelencephalon the
myelocoele.

Figure 282 also shows that what was once the most anterior part
of the fore-brain, i. e., the lamina terminalis, is no longer so, the lateral
vesicles having extended further forward. The telencephalic vesicles be-
come the cerebral hemispheres in the adult. These become so large that
they cover the entire diencephalon and mesencephalon.

512 EMBRYOLOGY OF THE CHICK

All discussion of the central nervous system in our future study of
comparative anatomy will depend upon the student's thorough under-
standing- of the development of the brain regions and vesicles as here
discussed. Consequently, the various arbitrary lines used as demarca-
tions must be carefully studied.

The division between telencephalon and diencephalon is the imag-
inary line drawn from the velum transversum to the recessus opticus.
The velum is that slight extension marking the point where the primary
fore-brain is to divide, while the recessus is that transverse furrow in
the floor of the brain which leads directly into the lumina of the optic-
stalks.

The Diencephalon: There is little change in this on the fourth day,
except that the infundibular depression in the diencephalon has deep-
ened, and lies close to Rathke's pocket (Fig. 301, I), with which it later
fuses to form the hypophysis. Later the lateral walls of the diencepha-
lon are to become thickened to form the thalami. As these thalami
grow inward toward each other, they will cause the diocoele or third
ventricle to become quite small. The anterior part of the roof of the
diencephalon remains thin, and the blood-vessels grow downward into
the diocoele as the choroid plexus.

The division between diencephalon and mesencephalon is an imag-
inary line drawn between the tuberculum posterius (a rounded elevation
in the floor of the brain, of importance only as a landmark of this kind),
and the internal ridge formed by the original dorsal constriction between
the primary fore-brain and mid-brain.

The Mesencephalon: There is little change in this portion of the
brain, though a little later, dorsal and lateral walls become thickened
to form either the optic lobes or the corpora quadrigemina. Optic lobes
and optic vesicles must not be confused, as these are two separate and
distinct structures.

The floor of the mesencephalon thickens to form the cerebral pedun-
cles of the adult, which serve as the main pathway for the fiber tracts
which connect the cerebral hemispheres with the posterior part of the
brain and spinal cord. The mesocoele becomes quite small by these
various thickenings and is now called the aqueduct of Sylvius.

The Metencephalon: The metencephalon is separated from the
mesencephalon by the original inter-neuromeric constrictions which
arose early and marked off this portion of the brain. The caudal boun-
dary is not well defined, though it is supposed to merge in the myelen-
cephalon where the roof changes from its thickened state to the thinner
condition observed more posteriorly. There is little change on the
fourth day in this region, though later an extensive ingrowth of fiber
tracts develops both on the ventral and lateral walls. These fiber tracts
form the pons and the cerebellar peduncles, while the roof of the meten-
cephalon enlarges and becomes the cerebellum.

DEVELOPMENT OF FOURTH DAY 513

The Myelencephalon : There is also little change in this region, but
later the roof becomes thinner and blood-vessels push their way into
the opening now called the fourth ventricle, as the posterior choroid
plexus, while the ventral and side-walls become floor and lateral walls
of the medulla.

THE GANGLIA OF THE CRANIAL NERVES (Fig. 282, B)

Along the neural crests already discussed, various ganglia are
formed. The largest on the fourth day is known as the Gasserian
ganglion of the fifth cranial nerve. (The fifth is also called the trigemi-
nal nerve.) It lies ventral and lateral, as well as opposite to the most
anterior neuromere of the myelencephalon. It forms the sensory nerve
fibers which grow from the brain mesially and distally into the mouth
and face region. This fifth cranial nerve is divided into three great
branches: the ophthalmic, the maxillary, and the mandibular.

The first branch, the ophthalmic, can be seen on the fourth day
extending toward the eye, while the other two are just beginning to
grow toward the mouth angle.

Just anterior to the auditory vesicle a mass of neural-crest cells is
developing into what is to become the facial or seventh cranial nerve
and the acoustic or eighth cranial nerve. This cell mass divides on the
fourth day to form the geniculate ganglion of the seventh and the
acoustic ganglion of the eighth nerve.

Caudad to the auditory vesicle, the ganglion of the glossopharyngeal
or ninth cranial nerve can be seen, and the ganglion of the vagus or tenth
nerve may just be observed. The ninth can be seen in whole mounts,
the tenth probably cannot.

THE SPINAL CORD

Throughout the spinal cord there is a compressed, slit-like lumen
known as the central canal. Just as the ganglia of the cranial nerves
make their appearance on the fourth day, so, too, do the spinal nerves.

It requires special methods of staining to study the growth of the
nerve fibers from the neuroblasts, but the development of the spinal
nerve roots can be studied in ordinarily stained slides.

It is important to understand that in the adult there will be two
roots to each spinal nerve (Fig. 290), one ventral, which is motor in
function, and one dorsal, which is sensory in function. Both of these
unite lateral to the spinal cord. Immediately distal to this union there
is a branch extending to the sympathetic nerve cord. This branch is
known as the ramus communicans, and extends ventrad.

Before the union of dorsal and ventral nerve roots takes place a
spinal ganglion or dorsal ganglion is seen lying in the dorsal roots. This
ganglion is formed from the neural crests, and grows toward the cord,
thus forming the dorsal root, but there are also fibers growing away

514 EMBRYOLOGY OF THE CHICK

from the cord from this same ganglionic region which are known as
peripheral nerves.

The ventral roots (Fig. 290) are formed by fibers growing out from
the lateral portions of the cord itself, and are thus efferent nerves carry-
ing motor impulses from the brain and spinal cord to the muscles.

The sympathetic ganglia (Figs. 268, 290, 298) arise from cells which
have migrated ventrally from the neural crests to form cell masses on
each side of the midline on a level with the dorsal aorta. They are con-
nected to form cords, and on the fourth day enlargements can be seen
on these cords opposite the dorsal ganglia. These enlargements are
the primary sympathetic ganglia, each one of which is connected by a
ramus communicans to the corresponding spinal nerve. Later, both
sensory and motor fibers will extend to the sympathetic ganglia from
the spinal nerve roots as rami communicantes, while fibers running out
from the sympathetic ganglia connect with the various organs of the
body.

THE ORGANS OF SPECIAL SENSE

THE EYE ( Fig. 289)

We have already discussed the projections from the fore-brain
which are to form the optic cups as well as how the ectoderm directly
opposite the optic cup thickens to form the lens, this lens then meeting
with the cup.

On the fourth day the beginning of almost all the adult structures
of the eye can be seen.

The thickened internal layer of the optic cup will give rise to the
sensory layer of the retina.

The fibers which arise from the nerve cells in the retina grow along
the groove in the ventral surface of the optic stalk toward the brain to
form the optic nerve.

The external layer of the optic cup will, become the pigment layer
of the retina.

About the inside of the optic cup a grouping of mesenchymal cells
can be seen which gives rise to the sclera and the choroid coat.

Some of the mesenchymal cells even make their way into the optic
cup through the choroid fissure, and give rise to the cellular elements
of the vitreous body.

From the margins of the optic cup closest to the lens, the ciliary
apparatus of the eye is derived.

From the superficial ectoderm which overlies the eye, the corneal
and conjunctival epithelium are derived.

The mesenchymal cells which migrate to the region between the
lens and the corneal epithelium give rise to the substantia propria of
the cornea. The lens forms as a thickening of the superficial ectoderm,
which then becomes depressed so that it forms an invagination into the

DEVELOPMENT OF FOURTH DAY

515

optic cup. The margins of the cup narrow, and converge toward the
lens, while the lens itself loses its connection with the superficial ecto-
derm and forms a completely closed vesicle. A microscopic study of
sections of the lens show an elongation of the cells on that side of the
lens which lies toward the center of the optic cup. These elongating
cells are to become the lens fibers.

THE EAR

The auditory placode has already been mentioned as forming on
the second day. This thickening of the ectoderm sinks below the sur-
rounding ectoderm and becomes the floor of the auditory pit. This sep-
arates from the superficial layer from which it formed. It will be re-
membered that this causes the auditory pit to lie close to the myelen-
cephalon. The tubular connection formed by the constriction of the
region between the sunken placode and the superficial layer where it
originally forms, remains open for a time as the endolymphatic duct.

It is by a series of complicated changes that this placode, which
forms a vesicle, gives rise to the entire epithelial portion of the internal
ear mechanism.

Nerve fibers from the acoustic ganglion grow inward to the brain
and outward to the internal ear, thus forming its nerve connections. The
external auditory meatus cannot yet be seen, nor has the dorsal and
inner part of the hyomandibular cleft as yet given rise to the Eustachian
tube, which is to form later.

THE NOSE (Fig. 304)

The olfactory pits are merely paired depressions in the ectoderm
of the head, ventral to the vesicles of the fore-brain and just anterior
to the mouth. These pits become deepened by the growth of the sur-
rounding parts. The epithelium of the pits ultimately comes to lie at
the extreme upper part of the nasal chambers, and there constitutes the
sensory epithelium. Nerve fibers grow inward from these cells to the
lobes of the fore-brain, constituting the olfactory or first cranial nerves.

A

Fig. 304.

Olfactory region of the hen, A in transverse and B in longitudinal section.
c, middle concha; ch, choana ; i, inferior (anterior) concha; o, connection of air
cavity with head; p, septum of nose; s, superior concha. (From Kingsley after
Gegenbaur. )

516

EMBRYOLOGY OF THE CHICK

THE SKELETAL STRUCTURE

On the fourth day the mesoderm surrounding the brain has increased
and begins to show slight traces of the skull formation toward the an-
terior portion of the head, and extending posteriorly. The fronto-nasal
process has already been discussed as well as the formation of the upper
and lower jaws.

The beginnings of the vertebral column are also in evidence, though
only to a slight extent. Nevertheless, it is well at this point to sum-
marize what will occur, so that future changes will be understandable.

During the fourth day the somites have increased from about thirty
to forty, each somite showing a more or less distinctive division into an
outward lying muscle plate, and an inner region which is to form the
vertebral column. It is from these inner portions that processes of
mesoderm are sent out by both dorsal and ventral regions to the neural
canal, as well as below the notochord, until these structures are com-

i.

Occip.-
SkleroK

Myocoel

Vert 3

Fig. 305.

I, Redividing of the spinal segments. On the left side of the cut the sclerotomes
and myotomes are seen in their original state. On the right they are seen in their
final state. The cephalic portions are dotted and the caudal portions hatched. The
arrows show the line of demarcation between head and neck. II, Ventral view of
spinal column to show redivided parts of each vertebra. (From Corning, after
Kollmann.)

pletely surrounded by mesoderm. By the end of the fourth day these
processes have become thickened, and are often called the membranous
vertebral column. The membranous vertebral column is still segmented,
each segment corresponding to the original somite from which it sprang.

On the fifth day these lines of segmentation disappear in the meso-
derm which becomes continuous in its surrounding of the neural canal
and notochord, though the muscle plates retain their segmentation.

On the fifth day also, the mesoderm lying in immediate contact
with the notochord becomes cartilaginous, to form a cartilaginous sheath

DEVELOPMENT OF FOURTH DAY 517

around the notochord throughout its entire length, while at each side of
the spinal cord paired bars of cartilage form, which will shortly fuse
with the cartilaginous sheath of the notochord to form the beginnings of
the neural arches.

Toward the end of the fifth day the points opposite the attachment
of the neural arches become thickened and more mature, but the por-
tions between the neural arches retain their embryonic character. This
causes what has been called a secondary segmentation of the cartilag-
inous tube. Later this segmentation becomes still greater until the en-
tire cartilaginous tube is made up of a series of vertebral rings or seg-
ments, each segment consisting of a vertebral ring with its attached
neural arch, and the anterior-posterior halves, respectively, of the suc-
ceeding and preceding intervertebral rings. Each of these segments
becomes one of the vertebrae which constitute the spinal column.

-Med. ossif. center
Fig. 306.

Thoracic vertebra and ribs of human embryo of 55 mm.

(Middle of 3rd month) to show ossification centers. Cartilage is

indicated by stippled areas, and ossification centers by irregular
black lines. (After Kollmann.)

It must be understood, however, that these so-called secondary seg-
ments do not correspond with the somites from which they were formed.
The secondary lines of segmentation lie at about the center of the mus-
cle plates (Fig. 305), so that each of these secondary segments obtains
approximately one-half of the muscle action from the immediately an-
terior muscle plate, and one-half from the immediately posterior muscle
plate, thus making it possible for each one of the vertebrae to have the
muscles from two regions act upon it.

The spinal column develops around the notochord.

Ossification of the vertebrae begins about the twelfth day in the
centrum of the second or third cervical vertebra, gradually extending
caudad. The neural arches ossify still later, there being two centers of
ossification for these. (Fig. 306.)

On about the seventh day, the centrum of the first cervical vertebra,
or atlas, separates from the rest of the bony ring and becomes attached
to the axis to form the odontoid process.

On the seventh day there are present about forty-five vertebrae.

518 EMBRYOLOGY OF THE CHICK

The most posterior five or six fuse a little later and form the pygostyle
(Fig. 418).

THE EXCRETORY SYSTEM

The anterior tubules of the Wolffian body disappear before the end
of the fourth day, while the posterior tubules have increased in size and
become convoluted. The intermediate cell mass from which they arise
is quite prominent. In cross-sections the convoluted tubules will nat-
urally be cut at all angles, but they can be distinguished from the duct
by observing their much thicker walls. The glomeruli can also be seen
filled with blood vessels.

The permanent kidney, or metanephros (Fig. 307), begins its de-

Fig. 307.

Diagram of Urogenital Organs. A, in Indifferent stage. B, development of
the male from the indifferent anlagen, and C, development in the female from the
indifferent anlagen.

The dotted lines represent the organs in their relative positions in the adult
stage with the exception of the Miillerian duct in the male and the mesonephric
duct in the female. These latter ducts disappear for the most part. (After
Hertwig. )

velopment toward the fourth day in the region lying between the
Wolffian body and the cloaca, that is, between the thirtieth and thirty-
fourth segment.

The metanephric duct or ureter forms first, as did the ducts of the
pronephros and the mesonephros. This duct grows forward on the outer
side of the mesoderm lying in the region just mentioned. It grows
from the dorsal side of the posterior end of the Wolffian duct anteriorly.
Naturally it has an opening into the Wolffian duct from which it is a
diverticulum, but on the sixth day it develops a separate opening into
the cloaca.

It is from these ureters that lateral outgrowths arise which join
with the rods of tissue now forming in the surrounding mesoderm.
These outgrowths then develop into the tubules and Malpighian bodies
of the metanephros in a similar manner to the way the Wolffian bodies
developed.

The permanent kidney is quite small when compared with tlie
metanephros but increases in size to a considerable extent just before
hatching.

DEVELOPMENT OF FOURTH DAY 519

THE REPRODUCTIVE SYSTEM (Fig. 307)

On the fourth day a thickened strip of peritoneum forms on the
lateral and superior face of the Wolffian body, which later extends all
the way to the cloaca. This may be called the tubal ridge. It appears
first at the anterior end of the Wolffian body and grows posteriorly, im-
mediately external to the Wolfnan duct. This tubal ridge invaginates,
forming a groove-like arrangement at the cephalic end of the Wolffian
body, and the lips of this groove then fuse and form a tube — the
Mullerian duct. Fusion takes place on the fifth day. The anterior end
of this Mullerian duct remains open and is to become the opening into
the coelom of the oviduct which the Mullerian duct later becomes. There
are several openings which will develop at the anterior end in addition
to the main one, but these latter close normally. Should they remain
open, the abnormal condition of having two openings in the duct results
in the adult stage. The posterior end remains closed.

The older embryologists considered these two or three openings in
the Mullerian duct as homologous with the nephrostomes of the
pronephros, and so insisted that the pronephros followed the
mesonephros in the chick. Modern embryologists consider that these
openings lie entirely too far posteriorly and laterally to permit of this
older interpretation.

In both sexes so far, development has been alike, but on the eighth
day the Mullerian ducts begin to degenerate. They disappear almost
entirely by the eleventh day in the male.

In the female chick, the left Mullerian duct forms the oviduct, while
the right Mullerian duct degenerates. The left one alone remains
functional.

The Wolffian body disappears almost entirely in the male, though
a small group of tubules covering the anterior head of the testes remains
as the epididymis. In the female it also disappears almost entirely, the
part remaining being the parovarium, a small body lying in the mes-
entery between the ovary and the kidney.

The Wolffian duct disappears entirely in the female, but it acts as
the vas deferens or sperm duct in the male.

The germ cells probably arise from the entoderm in vertebrates.
The entoderm is never metameric, though some of the older embryolo-
gists spoke of metameric gonotomes as primitive segmented regions
which were to form the gonads.

At about the time the somites form, the portion of the entoderm
which is to become the gonads, migrates through the developing meso-
derm in the epithelium of the genital ridges which have formed imme-
diately lateral to the mesentery. The primitive or primordial ova or
sperm can be recognized not only from their size but from their reactions
to microscopic stains (Fig. 254).

520 EMBRYOLOGY OF. THE CHICK

In the female, the epithelium increases in thickness to an enormous
extent. The primitive ova multiply, and the products of this multiplica-
tion, accompanied by some of the epithelial cells, sink into the deeper
stroma of the connective tissue, and thus form ovarial or medullary
cords, each such cord containing a number of ova. The cords then break
up, each egg becoming surrounded by a layer of epithelial cells, the
whole forming a Graafian follicle. The follicle cells supply the nourish-
ment to the egg lying within.

This whole growth takes place only on the left side of the chick, as
the right ovary is not functional.

In the male, the beginnings of the gonad formation are similar to
that of the female, but instead of the cords breaking up into separate
follicles, each cord, develops a lumen which becomes converted into the
seminiferous tubule. One can, however, see in the walls of these tubules
both types of cells that were seen in the Graafian follicle. Indeed, there
is found a third type of cell called Sertoli's cell, which is supposed to
act as a sort of nutritive or nurse cell to the developing sperm.

THE ADRENAL BODIES

While these bodies lie closely attached to the kidney, they have not
developed as a part of the urinary system.

It is important to know that the adrenal organs, which are among
the prominent ductless glands now studied in the schools, arise from two
separate and distinct origins :

First, by a proliferation of peritoneum, and second, by a prolifera-
tion of the sympathetic ganglion cells. It is the portion arising from the
p'eritoneum which connects with the mesonephros.

The peritoneal proliferations begin as cords or strands of cells along
the dorsal aorta. These then connect with the renal vesicles of the
mesonephros. Later, the sympathetic proliferations extend within the
peritoneal cords, so that the peritoneal cords now become the cortex and
the sympathetic portions become the medulla of the adult adrenal
glands.

THE. CIRCULATORY SYSTEM

At this point it is well for the student not only to realize, but to
appreciate the great number of experiments necessary to demonstrate
biological facts, as well as to understand the great number of possible
errors and objections which men may bring forth to oppose the inter-
pretation of these facts after the facts themselves have been demon-
strated.

Suppose the question be raised as to whether the first beating of
the heart of an embryo is muscular or nervous in type. What experi-
ments, for example, would be necessary to answer such a question sat-
isfactorilv?

DEVELOPMENT. OF FOURTH DAY 521

Off hand, one might say, that as nerves carry impulses to all mus-
cles, and as there are nerves in the heart muscle, the action must be
nervous.

Nerve fibers grow into the heart muscle from the nerve cells close
by, but the very finest nerve stains known, have been unable to demon-
strate that there are any nerves whatever in the heart muscle at the time
of its earliest beating. It may be objected that however fine our nerve
stains may be, they are not sufficiently so to demonstrate possible nerve
cells or parts of nerve cells. And, that if we improve in our technical
ability by obtaining new stains, we may expect to find nerve-cell-sub-
stance heretofore unseeable. This objection is not well taken, because
if any muscle be removed from the body and placed in normal salt solu-
tion, the muscle fibers do not lose their contracting ability, although
in a few days the nerves degenerate and can be dissected out. If, then,
our present stains do show the nerve fibers clearly in embryos, and
these can be seen to be in exactly the same position as in the- adult heart^
as demonstrated by the experiment just cited, it is quite reasonable to
assume that the stains do show all the nerve fibers that are actually
present. If this be true, we can demonstrate that all such nerve fibers,
which normally take a stain, have been destroyed. But, the new nerve-
less muscle still contracts and expands.

It could, of course, be argued that in so far as this is embryonic
material not yet far removed from the germ plasm, that, therefore, every
particle of the embryonic material still retains some of the undifferen-
tiated nerve cells, and consequently every part of the embryo does
actually retain some slight nervous substance which may, under extra-
ordinary circumstances, be brought forth.

This objection is overcome by an experiment performed some years
ago by taking a portion of the adult intestinal tract, chopping it up very
finely, and placing it in a test tube. Notwithstanding the fact that it
was thoroughly chopped up, this substance still was ab1e to digest food
placed in the tube with it. Those who insist upon all action being
nervous in type, then contended that the different particles of the intes-
tine still retained some of the essential parts of the nerve cell, so that,
notwithstanding the parts being cut up into very tiny particles, the
essential nervous elements were still doing the work.

A portion of intestine was then kept in a chemical medium similar
to that mentioned in the heart-experiment, and, as with the muscle-ex-
periment, the nerves degenerated and were dissected out, but the intestine
itself continued performing its normal functions.

If the tough adult nerve-structures are so easily degenerated
in a normal salt solution, it is surely safe to assume that the hundred-
fold more delicate embryonic nerve structures will also be destroyed in
such a medium.

It will be remembered that the heart grows as a simple straight

522 EMBRYOLOGY OF THE CHICK

tube, and that the blood is formed in the blood-islands by the cavities
flowing together. As these cavities fuse they became tubular, forming
the vitelline veins which carry the blood to the heart.

It is of the utmost importance to remember that this early heart
tube, even before the blood passes through it, has a slow, irregular
"beat." This, however, is not a true heart-beat, but merely the func-
tional movement of living muscle.

The true heart-beat is established at that particular moment when
the thin membrane which separates the anterior from the posterior por-
tion in the tubular heart breaks through by the greater pressure of the
blood from the posterior region pressing forward. The tiny membrane
can be seen to bulge out toward the head region until it finally breaks.

From that moment on, the blood forces its way through the heart
and begins a rhythmic muscular reaction on the part of the heart.

The architecture of all muscles is such that various muscle cells
are antagonistic to other muscle cells in the same group, so that each
muscle can, if it elongates, also contract and shorten, the two sets of
fibers being mutually antagonistic, so as to retain a normal balance.
The heart muscle shows this principle admirably in that it is composed
of two groups of spirally wound muscle fibers, the one unwinding as
the other winds up, thus causing a mutual interaction which keeps up
by the rhythm of the heart-beat.

From the study of physics we know that when two streams which
run in different directions meet, a vortetf is formed. If we now turn to
our earlier description of the development of the circulatory system
during the first two days, we shall find that there are two openings into
the heart from which streams of blood are brought into that organ. As
these two blood vessels send their streams together, a vortex is formed.
We thus find a physical explanation as to why the heart muscles follow
in their growth the optimum stretching caused by the spirally running
stream of blood.

From all that has been said above, it follows that when a heart is
removed from an animal body and kept "alive" for days or weeks, it
is but the physical continuation of the normal muscular antagonistic re-
action of the two spiral shaped groups which have been wound up quite
as a clock is wound.

As months and years have elapsed in the winding of these spiral
muscles, it is quite natural to understand that they are still sufficiently
wound when removed from the body so that they will continue in action
for some days if no external conditions exist to cause a stoppage sooner.
Such external conditions may be pressure, friction of various kinds, or a
drying up of the tissues when not retained in proper media.

If the immediately preceding paragraph be remembered, one can
always explain such objections as this: "If potassium is removed from
the medium in which a heart is placed, it ceases to function, thereby

DEVELOPMENT OF FOURTH DAY 523

proving that it is the potassium solution which causes the reaction."
It will be remembered that it was stated in the preceding paragraphs
that the action of the muscles will continue for some time until external
conditions cause a stoppage. The removal of potassium solution from
the surrounding medium has nothing to do with the reaction ability in
the muscle cell itself, but its removal removes a factor necessary to re-
action, by making the medium one in which it cannot react. An exam-
ple may make the matter clearer. A living human being has the power
to move his arms and walk about. This power is retained for many
years. Let us suppose that we remove certain substances from the air
which are needed for his lungs to function. An individual breathing
such an atmosphere would either slowly or rapidly (depending upon
what gases are removed) grow less and less able to move his arms or
to walk, and in a short time this ability would cease entirely. In other
words, such an individual needs a certain kind of atmosphere for breath-
ing purposes, without which he cannot perform his normal functions.
This, however, is vastly different from saying that the constituents of
air are the cause of his being able to move.

From what has been said above, all that we can say, in regard to
nerve-and-muscle-action, is that experiments tend to demonstrate that
muscle cells have the ability to act and react, and that the nerves are
only the connectors and impulse carriers, by which a coordination of
muscle cells, which are not in contact with each other, may be brought
about.

In the embryo the yolk is converted into blood, and the pressure
of that blood as it passes through the various vessels with its greater
posterior and its less anterior pressure, brings about the results men-
tioned above. In the adult, the food that is taken in and converted into
blood, works on quite similar principles by continuing to produce a
greater posterior than an anterior pressure.

The embryonic circulation can only be understood when it is realized
that it varies from the adult circulation in a manner that is accounted
for by the difference between embryonic and adult feeding. In the
embryo, due to the food coming entirely from the yolk, there is devel-
oped a yolk or vitelline-circulation. As the chick's lungs are non-func-
tional before birth, and the allantois functions as a respiratory organ,
there is developed an allantoic circulation, while a third type is the cir-
culation of the embryo itself. The vitelline and the allantoic together
constitute the extra-embryonic circulation.

All food material to the embryo comes from the yolk (although the
yolk particles do not turn directly into blood. It is the action of the
entodermal cells which line the yolk-sac and pour out a secretion of
enzymes, which breaks down the yolk granules). It is thus seen that it
is the vitelline vessels which carry food into the embryo, and it is the
allantois which serves both as a respiratory and excretory organ (at

524 EMBRYOLOGY OF THE CHICK

least until the nephroi are formed). It is the allantoic circulation which
permits the escape of carbon dioxide and other waste matters.

Therefore, the intra-embryonic circulation has nothing to do with
either manufacturing blood or throwing out waste matter (until the
nephroi are formed) ; it serves only as the carrier, distributor, and col-
lecting system of both food and waste materials.

As all three systems, intra-embryonic, vitelline, and allantoic send
their vessels to and from the heart, the contents of all three systems
mingle in that organ, although of course the vitelline circulation is the
richer in food material, and the allantoic the richer in waste matter.

It is at this point that the student must again remember that arteries
need not necessarily carry blood rich in food matter, but that an artery
is any blood-vessel carrying blood away from the heart under a high
pressure. This pressure probably accounts for the fact that arterial
walls are thicker and stronger than venous walls.

Veins are the carriers of blood to the heart.

THE VITELLINE CIRCULATION

This has been described in detail at an earlier period.

THE ALLANTOIC CIRCULATION

We have already spoken of paired vessels extending through each
segment of the embryo which arise from the aorta at about the level
of the allantoic stalk. One pair of these segmental vessels increases in
size as the allantois grows, and is distributed over the allantois in a rich
plexus. As the allantois lies close under the shell, there is thus afforded
a large area where gases can easily be exchanged and oxygenation be
brought about. After such oxygenation and the extrusion of the carbon
dioxide, the allantoic blood is gathered by the allantoic veins, and car-
ried back to the heart.

The excretory ducts later develop in the embryo and then empty
into the allantoic stalk close to its cloacal end. It is at this time that
the allantois begins to function as a receptacle for solid waste matters,
which, after the fluid parts have been evaporated, retains this waste-
matter until it is thrown off at the birth of the animal.

The right and left allantoic veins run cephalad in the lateral body-
walls of the chick, and enter the sinus venosus, one on each side of the
omphalomesenteric vein. These two allantoic veins will shortly fuse
and form a single umbilical vein (Fig. 308).

The yolk-sac is regarded as a diverticulum of the intestine, and the
allantois as a diverticulum of the urinary bladder, which itself is an out-
growth of the alimentary tract.

These outgrowths carry their blood vessels with them. Therefore,
the omphalomesenteric artery and the vitelline veins (these latter are

DEVELOPMENT OF FOURTH DAY

ft /r~~H3^)

9

525

Fig. 308.

A to F, Diagrams illustrating the formation of the omphalomesenteric and
umbilical veins, in the chick. A. At about fifty-eight hours. B. At about sixty-
five hours. Veins joined dorsal to the gut. C. At about seventy-five hours.
Veins again separate. D. At about eighty hours. Secondary union of veins around
the gut. E. At about one hundred hours. Definite arrangement of the vessels. F.
Relationship of liver vessels, c. Vena cava posterior (inferior) ; dC, ductus

EMBRYOLOGY OF THE CHICK

diverticula of the omphalomesenteric veins) extend out over the yolk,
constantly increasing as to both absolute numbers and as to branches,
as the yolk-sac spreads over the yolk.

The allantoic arteries are also called umbilical arteries. They are
what will later be known as hypogastric arteries. In birds and reptiles
five vessels, three arteries (one omphalomesenteric and two allantoic;,
and two veins (one vitelline, really omphalomesenteric, and one allan-
toic), connect the embryo freely through the umbilical stalk (Figs. 284,
297, 308).

In mammals, where there is little or no yolk, the yolk-sac is reduced
or absent entirely and the omphalomesenteric and vitelline vessels dis-
appear very early, so that the umbilical cord or stalk contains only the
two allantoic arteries and one allantoic vein.

In the dogfish and all elasmobranchs, where there is a large yolk-sac
but no allantois, the vitelline circulation alone is found, the allantoic
not being present.

THE INTRA-EMBRYONIC CIRCULATION

The large vessels communicating with the heart are the first ones to
appear in the chick embryo. At thirty-three hours the ventral aorta ex-
tends headward, bifurcating ventral to the pharynx to form a single pair
of aortic arches. This pair of arches passes dorsad around the pharynx,
then running tailward on the dorsal wall of the gut as the paired dorsal
aortae (Fig. 277).

On the second day, as the visceral arches and clefts appear, this
original pair of aortic arches comes to lie in the mandibular arch. In
each of the visceral arches posterior to the mandibular, new aortic arches
are formed, which connect the ventral aortae with the dorsal aortae.

At fifty-five hours we saw there were three pairs of these aortic

Cuvieri ; dv, ductus venosus ; g, gut ; hi, left hepatic vein ; hr, right hepatic vein ;

I, liver ; o, omphalo-mesenteric vein ; p, anterior intestinal portal ; pa, rudiment
of pancreas ; ul, left umbilical vein ; ur, right umbilical vein ; v, vitelline vein ; I,

II, primary and secondary venous rings around the gut. (After Hochstetter. )
G to J, Diagrams to show the origin of the postcaval vein and the changes
in the abdominal vein in amphibians and reptiles. G, elasmobranch stage. The
lateral abdominal veins i enter the common cardinal veins c and are not connected
with the renal portal veins p. H, the lateral abdominals i have joined the renal
portals at t posteriorly, and anteriorly pass into the liver /, where they unite with
the hepatic portal vein h; a new vein, the postcaval vein g, is seen growing caudad
from the liver /, where it arises from the hepatic veins o. I, condition in the
adults of urodele amphibians ; the postcaval vein g, has reached and fused with the
posterior cardinals e and and the subcardinals j at the point r; the two lateral
abdominal veins have united to form the ventral abdominal vein i which empties
into the hepatic portal h. J, condition in adult reptiles ; the anterior portions of
the posterior cardinal veins n are obliterated, leaving the postcaval vein g as
the sole drainage for the subcardinals j and the kidneys k; the two lateral ab-
dominal veins remain separate as in elasmobranchs. a, anterior cardinal vein ; b,
sinus venosus ; c, common cardinal vein ; d, subclavian vein ; e, posterior cardinal
vein; /, liver; g, postcaval vein; h, hepatic portal vein; i, lateral (or in I, ventral)
abdominal vein ; j, subcardinal vein ; k, kidney ; I, iliac or femoral vein ; m, caudal
vein ; n, obliterated part of the posterior cardinals ; o, hepatic veins ; p, renal
portal veins ; q, pelvic veins ; r, union of postcaval, posterior cardinals, the sub-
cardinals ; s, union of postcaval and subcardinals ; t, union of abdominal vein with
the renal portal system. (From Hyman's "A Laboratory Manual for Comparative
Vertebrate Anatomy," by permission of The Chicago University Press.)

DEVELOPMENT OF FOURTH DAY

527

arches with a fourth pair just beginning to form. It is at about this
period also that there is an extension headward from the dorsal aortic
roots. These extensions form the internal carotid arteries which supply
the brain.

The external carotid arteries arise later from the ventral aortic roots.
They also grow cephalad as do the internal carotid arteries, but, unlike
the internal carotids, the external carotids supply the face.

By the end of the fourth day two more pairs of aortic arches appear

o

00

i

2
J-
*

il

Fig. 309.

Schematic diagrams illustrating the changes which take place in the aortic
arches. A, embryonic prototype ; B, Fishes ; C, Urodeles ; D, Lizard ; E, Birds ; F.
Mammals.

The dotted lines show the portions which have become obliterated in the adult
forms of the animals mentioned, ao.asc., ascending aorta which branches into the
following aortic arches : 0,00, 1, 2, 3, 4, ; ao.desc., descending aorta ; bot, duct of
Botallus ; pulm, pulmonary artery ; subcl, subclavian artery ; 0,00, 1, 2, 3, and 4,
the six aortic arches. (After Boas.)

posterior to those already present. The fifth pair of aortic arches is
very small and disappears in a short time.

The first and second arches have become smaller and also finally
disappear. Probably most often the entire first arch has disappeared
by this time and sometimes the second has also gone.

Consequently, there are present only the third, fourth, and sixth
pairs. While these arches do not remain intact permanently, though

528 EMBRYOLOGY OF THE CHICK

parts of them do, it is from these three pairs that the main blood vessels
arise.

In reptiles, birds and mammals, all the main vessels of the adult
connecting the heart with the dorsal aorta are derived from the fourth
pair of embryonic aortic arches.

It is important to remember this, as our studies in comparative
anatomy will consist of the study of an amphibian, a dogfish, a turtle,
and a cat or rabbit, and the student will be required to show similarities
and differences of this nature in the different groups.

In reptiles the aortic arches remain in pairs (Fig. 309), but in birds
the left arch degenerates, while in mammals it is the right arch which
degenerates. The dorsal aortae, which began as paired vessels, now
fuse close to the sinus venosus. The portion extending cephalad is
fused for a very short distance, though never involving the region of
the aortic arches.

Quite early in development there are segmental vessels arising from
the aorta which extend into the dorsal body-wall. The pair at a level
with the anterior appendage-buds enlarge and extend into the wing-
buds as the subclavian arteries.

We have already mentioned the pair opposite the allantoic stalk
which has enlarged to become the allantoic arteries.

The external iliac arteries which supply the posterior appendage-
buds arise as branches from the allantoic arteries close to the origin of
the aorta.

At four days, the chick embryo still has the omphalomesenteric
arteries as its main visceral supply. It will be remembered that these
arteries are paired originally, but as the embryo (which must be con-
sidered as having its ventral portion open and thus lying extended over
the yolk of the egg), comes to have its ventral walls meet and grow
together, the omphalomesenteric arteries, like the heart and other paired
structures which later become fused to form a single vessel or organ,
are brought together and fused, thus forming a single vessel which
comes to lie in the mesentery and runs from the aorta to the yolk-stalk.

The proximal portion of the omphalomesenteric artery persists as
the superior mesenteric of the adult, after the atrophy of the yolk-sac.

The inferior mesenteric artery and the coeliac artery arise from the
aorta independently at a later stage.

The cardinal veins are the main afferent systems of the early em-
bryo. They form on the second day as paired vessels on each side of the
midline and extend both headward and tailward. The anterior and pos-
terior cardinal veins on the same side come together to form the duct
of Cuvier, which duct runs ventrally and enters the sinus venosus. On
the fourth day there is practically no change in the cardinal veins.

Later, the proximal portions of the anterior cardinal veins become
connected by a new transverse vessel which forms, and enters into the

DEVELOPMENT OF FOURTH DAY 529

venous atrium of the heart, while the distal portions remain as the jugu-
lar veins of the head region.

The posterior cardinal veins (Figs. 301, 308), lie in the angle be-
tween the somites and the lateral mesoderm. It is of importance to
locate these vessels and understand their position, as the excretory sys-
tem develops in close relationship to them later, and their relation to
the excretory system cannot be understood unless their developmental
process is closely followed at this stage.

The mesonephroi develop from the intermediate mesoderm so that
the posterior cardinal veins lie just dorsal to them throughout their
length (Fig-. 301).

In fact, the posterior cardinal veins are the principal afferent ves-
sels of young embryos. However, in the adult these posterior cardinal
veins are going to be replaced by the large vena cava.

With the foregoing in mind as a sort of general bird's-eye view of
what has taken place and what will take place in the main blood vessels,
we shall enter into a little more detail.

THE HEART

The heart began as a paired structure. When the ventral walls of
the embryo came together, the two portions of the heart also came to-
gether, and formed a single tube in the midline of the body, close to
the ventral portion.

After this fusion, the heart is nearly straight and double-walled.
The endothelial lining of the heart has the same structure and is con-
tinuous with the entering- and outgoing blood-vesse1s.

There is a thickened layer over the heart called the epimyocardium,
which later separates into a thickened muscular layer, the myocardium,
and a thin non-muscular covering called the epicardium.

As the paired tubes have come together to form the single heart,
the splanchnic mesoderm from each side of the body has also come
together to form the dorsal and ventral mesocardia (Fig. 275).

The ventral mesocardium disappears almost immediately after its
formation, but the dorsal mesocardium continues suspending the heart
for some little time, also disappearing ultimately, except at the more
caudal portion of the heart.

As already described, the heart, now lying in the pericardial cavity,
is attached at both ends and grows much more rapidly than the sur-
rounding body, so that it begins to fold upon itself. The bending of
the organ must be carefully studied or later work upon the heart will
have little meaning. (Figs. 274, 276, 279, 280, 283, 287.)

It will be noted that the cephalic end of the heart is attached just
where the aortae leave it, while the caudal end of the heart is attached
where the omphalomesenteric veins and the dorsal mesocardium meet.

530 EMBRYOLOGY OF THE CHICK

It will also be noticed that the caudal or ventricular end grows
toward the right.

The physical restriction placed upon the growing heart by the dor-
sal bending of the entire embryo, and the pushing in of the yolk dor-
sally, plus the fact that the entire embryo (by torsion) comes to lie
upon its left side, accounts for the particular shape and direction of the
heart's bending.

As the U-shaped bend continues to grow, the closed portion of the
U is forced caudad, and twisted upon itself to form a loop. This forces
the atrial (arterial region) portion slightly to the left (that is, toward
the yolk) and the conus arteriosus is thrown across the atrial region
by being bent to the right (or away from the yolk), and then caudad.
The closed portion of the loop is the ventricular region. By this twist-
ing process the original cephalo-caudal relations of the atrial and ven-
tricular regions have become reversed, the atrial region now lying
cephalad to the ventricle.

Not only has the position of the two regions become reversed, but
there is a constriction forming which divides atrium from ventricle (Fig.
283). The constriction itself is called the atrio- ventricular canal.

It is on the fourth day that the bulbus arteriosus becomes attached
to the ventral surface of the atrium. The bulbus pressing inward, the
atrium grows as a seeming expansion around each side of the bulbus,
and it is these lateral growings which indicate right and left divisions
of the atrium, which later separate entirely.

The ventricle has an indication of a right and left division also at
this same time, caused by a longitudinal groove appearing on its sur-
face.

The bulbus later divides to form the root of the aorta and the pul-
monary artery.

Though the heart began its formation at the level of the hind-brain,
it has come to lie now at a level with the anterior appendage-buds, and
as the ventricular portion which is the more unattached, it is this ven-
tricular region which extends the more caudad.

Histologically, the endocardium of a four-day chick is still a single
layer of cells, while the myocardium can be distinguished from the outer
epicardium. The myocardium is composed of elongated cells which
show some resemblance to the muscle cells which they are to form.
They are arranged in bundles extending toward the lumen. These bun-
dles will become the trabeculae carnae of the adult heart.

The cells of the epimyocardium are becoming flattened to form the
true epicardium, while loosely placed mesenchymal cells lie in the region
between endocardium and myocardium near the atrio-ventricular canal.
These mesenchymal cells will take part at a later period in forming the
various septa which are to divide the heart into chambers as well as the
connective tissue frame-work of the valves.

DEVELOPMENT OF FOURTH DAY 531

The ventricular septum is completed at about the sixth day, its
anterior edge fusing with the posterior edge of the septum which divides
the truncus arteriosus into right and left halves.

The anterior edge of the septum of the truncus arises between the
fourth and fifth aortic arches in a manner which causes the blood com-
ing from the left side of the truncus (that is, from the left ventricle)
to pass through the third and fourth aortic arches, while the blood from
the right ventricle passes into the fifth aortic arch.

About the seventh day the right and left parts of the truncus sepa-
rate completely from each other. The right branch remains connected
with the fifth aortic arch as the pulmonary trunk, and the left is con-
nected with the third and fourth arches as the systemic trunk.

The ventral ends of the third arches become the subclavian arteries,
carrying blood to the anterior appendages, while the dorsal communica-
tion between third and fourth arches disappears.

This means that the blood now passes from the left side of the heart
through the third arch to the anterior appendages, and through the
fourth arch to the dorsal aorta.

About the fifth day, the fourth pair of arches are the larger of any
arches remaining, the left one, however, becoming smaller and smaller
in size until it disappears almost entirely. The right fourth aortic arch
grows larger and larger to form the systemic arch of the adult chick.

It has already been stated that in mammals it is the right arch which
disappears, the left alone persisting as the systemic arch (Fig. 309).

Early on the third day the pulmonary arteries form in the walls of
the lungs, and extend toward the fifth arch with which they connect
at the ventral ends of these arches. The dorsal end of the fifth arch
between the point of union of the pulmonary artery and the dorsal aorta
is called the duct of Botallus (Fig. 309). This ductus Botalli offers the
blood from the right side of the heart a passage into the dorsal aorta
so that little passes through the capillaries. The duct, however, shrivels
up at the time of hatching, and becomes entirely closed so that all the
blood from the right side of the heart must pass into the pulmonary
circulation. It is at this time that the lower portion of the aortic arch
becomes the pulmonary artery.

THE VEINS

As has been stated several times, the anterior and posterior cardinal
veins unite with each other on a side to form the duct of Cuvier and
then enter into the meatus venosus. These anterior and posterior cardi-
nals bring back the blood to the heart from practically all parts of the
body except the digestive organs.

The anterior cardinals persist as the jugular veins to which the
pectoral veins from the anterior appendages soon become joined. From
the head and neck the vertebral veins also join the jugulars.

532 EMBRYOLOGY OF THE CHICK

The posterior cardinals remain large as long as the Wolffian body
is functional, but as the permanent kidneys develop, these veins become
smaller and smaller and ultimately disappear.

The ducts of Cuvier persist in the adult chick as the anterior venae
cavae.

The posterior or inferior vena cava develops from the meatus veno-
sus, which was formed by the union of the two omphalomesenteric
veins. To understand the evolving process by which the posterior vena
cava comes into existence, it is necessary to follow carefully the devel-
opment of the surrounding organs.

The liver forms as a diverticulum from the digestive tract. This
diverticulum then grows around the meatus venosus until it completely
surrounds the meatus. Blood-vessels form in the liver, extending toward
the meatus venosus, into which they open by the fifth day.

At the posterior edge of the liver, there are a number of afferent
hepatic vessels coming from the meatus venosus through which some
of the blood coming to the heart from the vascular area may enter the
capillaries formed in the liver substance.

At the anterior edge of the liver, where the meatus venosus might
be said to be leaving the liver, there is a collection of efferent hepatic
vessels whose distal ends are in direct connection with the capillaries
of the afferent hepatic vessels.

The blood passing through the liver has two courses it may take.
Most of it passes through the large meatus venosus into the heart, but
some of it passes through the afferent hepatic vessels into the liver sub-
stance where it is collected by the efferent hepatic vessels and carried
to the meatus venosus.

That part of the meatus venosus lying between the afferent and
efferent hepatic vessels is often called the ductus venosus.

The two allantoic veins already described unite on entering the
body to form a single vein emptying into the left (persistent) omphalo-
tnesenteric vein. It is well to remember that as the yolk-sac decreases
in size, the allantois increases, and so, too, the relative size of omphalo-
mesenteric veins and allantoic veins changes ; the omphalomesenteric
becomes smaller and the allantoic becomes larger, so that it almost
seems as though the omphalomesenteric were a branch of the allantoic.
Both of these veins disappear at the time of hatching.

The superior mesenteric artery was formed by the closure of the
ventral body-wall so as to bring the paired omphalomesenteric veins
together, to form a single vessel running from the aorta to the yolk-
stalk. As the yolk-sac atrophies, the proximal portion of the omphalo-
mesenteric artery becomes the superior mesenteric artery.

The mesenteric vein is formed by a union of the veins from the walls
of the hinder part of the digestive tract, which there form a single vein.
This vein is at first quite small, and empties into the omphalomesenteric

DEVELOPMENT OF FOURTH DAY 533

vein just before the latter enters the liver. The point of entry may be
said to be the beginning of where the omphalomesenteric vein becomes
the meatus venosus.

It will, therefore, be noted that the blood which goes to the liver
comes from three sources :

(1) Through the omphalomesenteric vein, from the yolk-sac. This
blood is rich in food material and has been oxidized in the vascular area.

(2) Through the allantoic vein from the allantois. This blood is
very rich in oxygen.

(3) Through the mesenteric vein from the digestive tract of the
embryo. This blood is venous in character.

The mesenteric vein increases in size with the growth of the em-
bryo, and after the omphalomesenteric and allantoic veins disappear at
the time of hatching, it persists as the hepatic portal vein of the adult
chick. This large vessel brings blood back from the hinder parts of the
digestive canal to the liver.

On the fourth day the posterior or inferior vena cava proper arises.
It forms between the posterior ends of the Wolffian bodies, and runs for-
ward in the midline, ventral to the aorta. It joins the meatus venosus
anteriorly between the heart and the anterior edge of the liver, and pos-
teriorly it connects with the permanent kidney as soon as these are
formed. It also connects posteriorly with the hind limbs and the caudal
region.

The posterior vena cava is at first quite small, but as more and more
blood is being sent from the developing metanephroi and the caudal
region, it becomes even larger than the meatus venosus of which it was
originally but a branch.

Just before the vena cava becomes larger than the meatus venosus,
the efferent hepatic vessels have shifted their position so that they now
enter directly into the vena cava instead of the meatus as formerly. In
fact, before the time of hatching the entire portion of the meatus venosus
lying between the heart and liver becomes obliterated, so that all blood
flowing into the posterior end of the liver through the portal vein, passes
into the posterior vena cava through the hepatic vein (Fig. 308, I, J).

The relative changes in the size of blood vessels must be clearly
understood and followed, or the circulatory system of the embryo, and
consequently, also the circulation of the adult will be hopelessly con-
fused.

It is well at this point to obtain an idea of the embryonic circulation
of a little later time than that of the fourth day which we have been
discussing.

By the beginning of the sixth day, the septa which have already been
mentioned have divided both auricles and ventricles into right and left
halves (Fig. 283). However, neither of these septa are complete. The
septum separating the two parts of the auricle develops perforations, and

534 EMBRYOLOGY OF THE CHICK

in the human heart these perforations form an oval-shaped opening called
the foramen ovale, which may, in abnormal cases, remain open and thus
cause a constant intermingling of venous and arterial blood. Usually,
such persons do not live long, although there are notable exceptions.
This inter-auricular foramen closes at the time of hatching, so that the
blood from the right auricle can be sent to the lungs for aeration as soon
as these organs become functional at birth.

The septa are sufficiently developed so that we may speak of four,
divisions or cavities in the heart. This makes a double circulation pos-
sible, namely, the systemic and the pulmonary (up to the time of hatch-
ing, the allantoic circulation takes the place of the pulmonary).

By this time, then, the heart is fully formed. The sinus venosus has
been absorbed into the right auricle, of which it forms a part. The open
foramina allow blood to pass back and forth between the auricles. The
ventricular septum is more complete. The truncus arteriosus is divided
into two separate vessels, the pulmonary trunk arising from the right ven-
tricle, and the systemic trunk from the left ventricle.

The aortic arches which are still present are the third, fourth, and
fifth, and small portions of the first and second.

The systemic trunk from the left ventricle leads to the third and
fourth pairs of aortic arches, from which the head and fore-limbs are
supplied.

The pulmonary trunk, arising from the right ventricle, leads to the
fifth pair of aortic arches, which are directly continuous with the dorsal
aorta. It is from these that the small pulmonary arteries arise.

It will be remembered that as the lungs are not yet functional, there
is little use for these vessels until later. An omphalomesenteric artery
carries blood to the yolk-sac and a large allantoic artery passes from the
aorta to the allantois.

The venous system consists of the right and left anterior venae cavae,
and the posterior vena cava. The former drain the head and fore-limbs,
and the latter the posterior portions of the body, the limbs, and the kid-
neys.

Before reaching the heart, the posterior vena cava is joined by the
ductus venosus, through which blood is returned from the yolk-sac, allan-
tois, and embryonic alimentary canal, by the omphalomesenteric, allan-
toic, and mesenteric veins respectively.

All three venae cavae open into the right auricle of the heart, but
due to the position and direction of the opening, and to a valve, tne t)lood
from the posterior vena cava is directed through the foramen ovale into
the left auricle, while the blood from the right and left venae cavae (an-
terior) remains in the right auricle.

As the auricles now contract, the blood which has come from the
posterior vena cava is forced into the left ventricle and passes out through
the systemic trunk through the third and fourth pairs of aortic arches

DEVELOPMENT OF FOURTH DAY 535

to the head and fore-limbs, while the blood from the anterior venae cavae
passes out through the right ventricle through the pulmonary trunk and
thus through the fifth aortic arches into the dorsal aorta, from where the
blood goes to the body and hind-limbs of the embryo. A small portion,
however, is carried out along the omphalomesenteric arteries to the yolk-
sac and through the allantoic arteries to the allantois to take up nutriment
and oxygen. In the early embryo, a much greater portion of this pul-
monary circulation goes to yolk-sac and allantois.

It is assumed that the vastly greater proportion of blood supply to
the anterior region, as contrasted with the smaller quantity to the pos-
terior portions, accounts for the greater and more rapid development of
the head region, which it will be remembered is the first part of the chick
to develop.

The disproportionate development of the head may be realized when
it is known that the human child at birth has a head about one-fourth the
length of its entire body.

At about the time of hatching the ductus Botalli (which it will be
remembered is that portion of the fifth aortic arch lying between the
dorsal aorta and the point of origin of the vessel that runs to the lung)
— (Fig. 309) — closes up entirely, so that the blood from the right ven-
tricle must pass through the pulmonary veins back to the left auricle.

The lungs now become functional, and the true pulmonary circulation
is established. The allantoic circulation, being no longer needed, ceases,
while the allantoic arteries and veins disappear, as do also the omphalo-
mesenteric arteries and veins when the yolk-sac has finished its work, and
the hatched chick can take in its own food.

It is at this time also that the entire supply of blood which goes to
the liver passes through the mesenteric vein, which is now called the
hepatic portal vein. The ductus venosus has closed, and so all blood
brought to the liver must pass through the hepatic capillaries before
reaching the heart.

The foramen ovale does not close immediately after hatching, but
does so in a few days ; but, as soon as it does, all blood returned to the
heart by the three venae cavae is emptied into the right auricle from
which it is then forced into the right ventricle, thence through the pul-
monary artery to the lungs, and back through the pulmonary veins to the
left auricle, from which it is forced into the left ventricle, and finally
through the systemic trunk. Such an entire separation of venous and
arterial blood is called a double circulation.

XXXVIII.

THE COELOM AND THE MESENTERIES

In our account of the earthworm, the student was introduced to all
higher forms of animals possessing a coelom or body-cavity. The chap-
ter on the earthworm should be reviewed at this point.

Then,, too, in the early part of our work on chick embryology, we
have seen how the mesoderm divided into splanchnopleure or somato-
pleure and how the organs growing out from their respective beginnings
pushed a layer of one of these coverings before them. And we have also
seen how the chick embryo is quite similar to an animal which has had
a ventral incision made along the midline and then had these two halves
stretched over a yolk-sphere, so that its organs or portions of organs
which developed from two primordia or beginnings, could later come to-
gether when the fusion of the ventral body walls produced a single organ
of the two separated halves.

In adult birds and mammals, the coelom or body-cavity consists of
three regions, known as pericardia!, pleural, and peritoneal. The pleural
region is paired, each half containing one lung. The other two chambers
are unpaired. The pericardial region contains the heart, and the peri-
toneal contains all the abdominal viscera.

As the coelom arises by a splitting of the mesoderm, and the two
halves of the chick are spread out over the yolk, the coelom is naturally
a paired cavity, only becoming a single cavity when the ventral body
walls of the embryo come together, and the ventral mesentery then dis-
appears.

There are no segmental pouches in the chick coelom as there are in
some of the lower vertebrates, though it cannot be said that this is unlike
the lower forms, for, by the time the coelom appears in the chick, the
pouches would already be broken through anyway, and have become
connected.

As the mesoderm splits and the splanchnopleure and somatopleure
extend out over nearly the entire yolk-sac, it is to be understood that
much of this split mesoderm is extra-embryonic. This has already been
described in an earlier chapter.

Here we are concerned with the embryonic coelom.

The portion of the embryonic coelom which gives rise to the three,
body-cavities mentioned above is marked off by a series of folds which
separate the body of the embryo from the yolk. With the closure of
the ventral body walls, the embryonic coelom becomes completely sepa-
rated from the extra-embryonic, though in the yolk-stalk region it re-
mains open much longer than in other portions (Fig. 281, C to G).

COELOM AND MESENTERIES 537

It is this same closure of the ventral body walls which also brings
the two portions of the gut together ventrally. This causes the newly-
closed gut to lie between the two layers of splanchnic mesoderm while
the body-spaces on each side form a right and left coelomic chamber.
In fact, there are double layers of mesoderm which enclose and support
the gut. These double layered supports are called mesenteries. The
dorsal mesentery remains as a continuous support — at least the greater
portion of it does — but the ventral mesentery soon disappears, causing
the right and left coelomic cavities to become confluent.

In the liver region, however, the ventral mesentery does not dis-
appear (Fig. 293). The liver arose by a ventral outgrowth of the gut
and extended into the ventral mesentery. As the liver grows ventrally
from the digestive tract, there is a portion of the ventral mesentery lying
dorsal to the liver, that is, between the liver and the gut. This persists
as the gastro-hepatic amentum while the portion ventral to the liver is
called the ventral ligament or the falciform ligament.

The dorsal mesentery persists, as stated, but has different names in
different parts, i. e., mesocolon, where it supports the colon, mesogaster,
where it supports the stomach, etc.

Septa grow out from the body wall to divide the body-cavity into
the pericardial, pleural, and peritoneal chambers mentioned above.

XXXIX.

DEVELOPMENT OF THE FIFTH DAY

On this day the head and tail of the embryo have nearly come to-
gether by the great curving of the chick. The yolk is completely cov-
ered by the blastoderm, and the vascular area covers nearly two-thirds
of the blastoderm.

THE LIMBS

It is during this day that the limb-buds increase considerably in size,
and are marked off into a proximal rounded portion and an expanded
distal region. It is in the expanded distal region that the digits can be
seen to form in cartilage. The rounded proximal portion is slightly bent
at the points where elbow and knee joints will be formed.

The elbow and knee-angles at first are directed almost straight out
from the body, but on about the eighth day both fore and hind-limbs
rotate until the elbow-joint points caudad, while the knee-joint points
cephalad.

By the end of the tenth day, both pairs of appendages have their
definite outlines, though feathers and nails are not yet formed.

Although the structures which are to become bones are first out-
lined in cartilage, they later become ossified. There are three well-
formed digits in the expanded distal portion of the fore-limb at this time
with a possible fourth in a rudimentary condition, while in the expanded
distal portion of the hind-limb there are also three well-defined digits
with two in a rudimentary condition.

The development of the bony vertebrae has already been discussed.
Here it is well to state that the ribs develop as cartilaginous bars in the
body wall of the chick. The ventral ends of these fuse ventrally, and
after fusion, a portion of each of the fused ends separates from the re-
maining ribs from which they formed. It is this portion which has sep-
arated that becomes the sternum.

THE DEVELOPMENT OF THE SKULL

The skull is divided into two regions: (1) The skull proper, and
(2) the visceral skull, this latter being that portion which has developed
from the visceral arches.

THE SKULL PROPER

The notochord forms a sort of central portion around which the
vertebrae form. The anterior end of the notochord serves a sort of
similar function in the head region.

DEVELOPMENT OF FIFTH DAY

539

On each side of the notochord a sheet of cartilage develops. These
wo sheets are known as parachordal plates (Fig. 310). They form a

tr

%>•-''

,o.ch.

p.ch

p.C.h.

tectcn

bas. cr

- ---sph.lat.

tect s.at

Fig. 310.

Diagrams of skull formation in Salmon. A, First anlage of cranium. B, C, D,
successive stages in cranial development. Left half of D is an advance in
development of right half, au, eye; bas.cr., base of cranium; e.b.c.a., e.b.e.p.,
anterior and posterior basicapsular commissure (ascending process of palato-
quadrate cartilage) ; ch, notochord; c.tr, trabecular cornu ; hyp, opening in which
hypophysis develops ; n.k., nasal capsule ; o.bl., and f.b.c., fenestra basicapsularis ;
o.k., ear capsule; occ, occipital region; p.ch., parachordal plates; sph.lat., lateral
sphenoid; tect.cr., roof of cranium; tect.s.ot., Cartilaginous arch between otic
capsules representing the cartilaginous roof of higher vertebrates (tectum
synoticum) ; tr., cranial trabeculae. (A, D, after Waskoboynikow ; B, C, from
Gaupp after Stohr's model.)

540

EMBRYOLOGY OF THE CHICK

floor for the mid and hind-brains. These plates then fuse both dorsally
and ventrally around the notochord, and consequently enclose it. The
fused plate is then known as the basilar plate, forming the floor of the
hinder portion of the skull.

The auditory capsules which enclose the auditory organs form and
fuse to the sides of the basilar plate. It is from growths of the basilar
plate and the auditory capsules that the floor and occipital portions of
the skull are formed.

The anterior portion of the skull is formed from two slender rods
lying cephalad to the notochord, but which are in connection with the
parachordal plates. These rods are known as trabeculae cranii.

The pituitary body lies between these trabeculae cranii, so that in
fusing as they now do to form the ethmoid plate, the pituitary body

comes to He in the position where it will
be found when we study Comparative An-
atomy.

The ethmoid plate (Fig. 311) extends
cephalad to the tip of the beak, and fuses
anteriorly with the olfactory capsules.

The interorbital septum develops as a

Profile viewFTf 2-day chick-skuii. large vertical plate from the dorsal surface
as. aiisphenoid; d, dentary ; e, of the ethmoid plate, along the whole median

ethmoid; /, frontal; I, prefrontal ; **

mx, maxillary; n, nasal; ol, lateral hne. It IS quite COmniOn tO Speak of Car-

occipital ; os, superior occipital ; pa, ... 1 i i r A 1

palatine; pm, premaxiiiary ; pt, tilagmous and membranous bones of the

skull. This means only that some of the
bones there formed (in fact, all these we
have just been describing) were first pre-
formed in cartilage, and then became bone,
while the membranous bones were first cartilage, and then, by being
placed where there was considerable stretching, they became quite thin
membranes before they finally ossified.

The membranous bones form the roof of the skull, such, for exam-
ple, as the parietals, frontals, etc.

THE VISCERAL SKULL

It will be remembered that the first visceral arch was also called the
mandibular arch, because it is from this arch that the mandible or lower
jaw is formed, and that the second visceral arch was known as the hyoid
arch, because it is from this that the hyoid bone, or cartilage which sup-
ports the tongue, has developed. The parts of the skull which are thus
developed from the visceral arches form the visceral skull.

THE HEART

It is during the fifth day that the interventricular septum is almost
completed, fusing with the posterior edge of the septum which now

portion of skull just becoming con-

verted into bone. (Cartilage is

stippled.) (After Boas.)

DEVELOPMENT OF FIFTH DAY . 541

develops in the truncus arteriosus. This latter septum is formed be-
tween the fourth and fifth pairs of aortic arches and follows a sort of
spiral course caudally to where it joins the interventricular septum. It
is the position and shape of these septa which cause the blood to course
into the different channels as described for the fourth day.

Two sets of semilunar valves have now formed between the two
divisions of the truncus arteriosus and the two ventricles into which
they open. The heart continues growing, but it is not until about the
twelfth day that the interauricular septum has almost completely closed,
leaving only the foramen ovale as a small opening between the two
auricles. The foramen ovale develops a little fold of membrane which
some days after hatching closes the opening entirely.

The ventricles now become thickened to a very considerable extent.
The auricles likewise thicken, but not to so great an extent as the ven-
tricles. The ventricular thickenings on the inside of the heart form as an
inward growth of ridges which are called trabeculae carnae. They are
really separate muscle-bundles which help to open and close the valves.

On the sixth and seventh day the distinctly bird-like characteristics
appear. Up to this time the beginner cannot tell the difference between
a chick embryo and that of practically any other one of the higher ver-
tebrates.

The nasal region now begins to lengthen and the fore-limbs will be
seen to develop into wings.

The allantois has become very large and contains a considerable
amount of fluid.

The omphalomesenteric arteries and veins now pass from the body
of the embryo as single vessels. The yolk, though seemingly as large
as before, is quite liquid in form.

The flexion of the body is less marked than before, while the head
is not so large in proportion to the remainder of the body as formerly.

The cerebral hemispheres can be seen quite plainly, as well as the
beginnings of the tongue-bud.

On the next three or four days the little sac-like regions in which
the feathers develop make their appearance as protrusions from the sur-
face, especially on the dorsal side of the chick, while a chalky patch at
the tip of the nose marks the beginning of the horny beak. The yolk
has become wrinkled and flabby.

After the eleventh day, the abdominal walls become firmer and the
intestines are enclosed in the peritoneal cavity. The body is now com-
plete except for the narrow stalks of the umbilicus and yolk-sac. The
amniotic fluid tends to disappear, making the amnion less prominent.

By the thirteenth day the feathers are well distributed over the
entire body, although they do not break through their sacs until about
the nineteenth day, by which time they are approximately an inch in
'ength.

542 DEVELOPMENT OF FIFTH DAY

On the thirteenth day the nails and scales appear on the toes, and
by the sixteenth day nails, scales and beak are firm and well developed.
On this day also the cartilaginous skeleton completes its growth, and
various centers of ossification make their appearance.

By the sixteenth day the white of the egg has disappeared and the
mesoderm has divided completely into the splanchnopleure and somato-
pleure all the way around the yolk.

On the nineteenth day the remains of the yolk are drawn into the
body cavity of the embryo.

The embryo begins its development originally by lying with its
axis transverse to the long axis of the egg, but by the fourteenth day it
turns so that its head is toward the air space at the larger end of the
egg, and at about the twentieth day the chick's beak is pushed through
the inner covering of the air space so that it can now begin using its
lungs. It is at this time that the pulmonary circulation begins. The
blood stops flowing into the umbilical vessels, and the allantois conse-
quently shrivels up and is left inside the shell as the chick pecks its way
out.

XL.

THE EMBRYOLOGY OF THE FROG

THE GENERAL EMBRYOLOGY OF THE TADPOLE AS COM-
PARED WITH THAT OF THE CHICK

As in our study of the embryology of the chick, it is essential that
the student again read the chapters on mitosis, fertilization, and the
summary on embryology, and then go over each system in the develop-
ing chick corresponding to the system he may be studying in the frog,
for only in this way can the comparison of the developmental processes
be understood.

After this has been done, the following groups of Craniata must be
kept clearly in mind to make clear the various embryological relation-
ships which must be referred to, not only in the study of embryology,
but also in Comparative Anatomy.

CLASSIFICATION OF CHORDATA
(After Newman)

Sub-Phylum I. Cephalochordata (Adelochorda), (Fig. 312).
i. n.

limit If {'

MIM 1

.<&'

'Zr\

Fig. 312.

I. Examples of Amphioxus ( Branchiostoma and Lancelet) , Tunicates (First
two upper figures), Lamprey (The large, lower, left-hand figure-adult; and the
embryo lamprey, usually called Ammocoetes — 2 upper right-hand figures), and the
Hag fish (2 lower right-hand figures).

II. Sketch of chief kinds of Urochordata showing distribution in sea. Dotted
lines on left indicate life-zones. The surface is called the pelagic zone. (From
Herdman.)

This includes but a single family of fish-like creatures, of which
there are about twelve species. The type form is Amphioxus
more correctly known as Branchiostoma).

544

THE EMBRYOLOGY OF THE FROG

Sub-Phylum II. Urochordata (Figs. 312, 313).

Order 1. Larvacea (Appendicularia), free-swimming forms with
permanent tail.

Order 2. Ascidiacea (Tunicates or Sea-Squirts), fixed forms with-
out tail in the adult.

Order 3. Thaliacea (Salpians), free-swimming forms without tail
in the adult.

ii.

III. IV.

Fig. 313.
UROCHORDATA.

I. Oikopleura in 'house'. The arrow shows course of current.

II. Diagram of Appendicularia from the right side, an, anus ; ht, heart ; int,
intestine, ne, nerve; ne' , caudal portion of nerve; ne.gn' , principal nerve
ganglion; ne.gn"., ne.gn"'., first two ganglia of tail nerve; noto., notochord ;
oes., oesophagus; or.ap., oral aperture; oto. otocyst (statocyst) ; peri.bd., peri-
pharyngeal band ; ph., pharynx ; tes., testis ; stig., one of the stigmata ; stom.
stomach. (After Herdman. )

III. and IV. Ascidia. Entire animal as seen from the right side and dissection
from the same side, an, anus ; atr.ap, atrial aperture ; end, endostyle ; gon, gonad ;
gonad, gonoduct ; hyp, neural gland ; hyp.d, duct of neural gland ; mant, mantle ;
ne.gn. nerve-ganglion ; oes ap, aperture of oesophagus ; or ap, oral aperture ;
ph, pharynx; stom, stomach; tent, tenacles ; test, testes. (After Herdman.)

Sub-Phylum III. Hemichordata (Fig. 314).

Order 1. Enteropneusta, including worm-like forms such as Balano-
glossus.

EMBRYOLOGY OF TADPOLE AND CHICK

545

Order 2. Pterobranchiata, sessile, tube-dwelling forms — Cephalo-

discus and Rhabdopleura.

Order 3. Phoronidia, tubicolous forms — Phoronis (Fig. 199).
Sub-Phylum IV. Vertebrata (Craniata).

v.

VI.

VII. VIII.

Fig. 313.

V. Botryllus violaceus.

VI. Composite Ascidian. 'Diagram of an individual member of a colony of
composite Ascidians. The zooids are in pairs and • seen in vertical section, an,
anus; at, atrium; at', atrium of adjoining zooid ; cl, cloaca common to both
zooids ; end, endostyle ; gld, digestive gland ; gn, nerve ganglion ; ht, heart ; hyp,
neural gland ; lang, languets ; mant, mantle ; or.ap, oral aperture ; ov, ovary ;
periph, peripharyngeal band ; ph, pharynx ; reet, rectum ; stom, stomach ; te, testis ;
tent, tentacles ; tst, test or common gelatinous mass in which individuals are im-
bedded ; v.d, vas deferens ( V, after Mile-Edwards ; VI, after Herdman. )

VII. Salpa democratica, asexual form, ventral view, and VIII lateral view in
section, at, atrial cavity ; atr.ap, atrial aperture ; br, branchia ; branch, dorsal
lamina ; c.c, ciliated crests on edge of branchia ; c.f, ciliated funnel ; d.l, dorsal
lip ; end, endostyle ; ey, eye ; gl, digestive gland ; gn, ganglion ; ht, heart ; int,
intestine ; Ing, languet ; mo, mouth ; mus.bds, muscular bands ; ne.gn, nerve-
ganglion ; 02, oesophagus ; oe.ap, and or.ap, apertures of oesophagus and mouth ;
ph, pharynx ; pp, peripharyngeal band ; proc, processes at posterior end ; sens.org,
sensory organ ; st, and stol, stolon ; st, on left, stomach ; v.l, ventral lip. ( VII,
after Vogt and Jung, VIII, after Delage and Herouard.)

Order 1. Cyclostomata (round mouth eels), such as hagfish and

Lampreys (Fig. 312).
Order 2. Pisces (true fish with jaws).

546

THE EMBRYOLOGY OF THE FROG

Order 3. Amphibia (vertebrates with aquatic larvae, but usually

air breathing in the adult condition), (Fig. 315).
Order 4. Reptilia (cold-blooded, air-breathing vertebrates).
Order 5. Aves (birds, feathered vertebrates).
Order 6. Mammalia (beasts or quadrupeds).

A. B. C.

II.

Fig. 314.
HEMICHORDATA.

I. Various types of Enteropncusta which are relatives of Balanoglossus. A,
Balanoglossus clavigerus; B, Glandiceps hacksi; C, Schizocardium brasiliense ; D,
Dolichoglossus kowalevskii; a, anus; ab, abdominal and caudal regions; b,
branchial region ; c, collar ; g, genital region ; gp, gill-pore or branchial cleft ;
gn, genital wing ; h, hepatic region ; m, position of mouth ; p, proboscis ; t,
trunk. (From Newman, A, B, C, after Spengel, D, Bateson.)

II. A and B Rhabdopleura, C Cephalodiscus dodecalophus. A and C, entire
animals. B, diagram of median longitudinal section of A. A, a, mouth ; b, anus ;
c, stalk of zooid ; d, proboscis f e, intestine ; /, anterior region of trunk ; g, one
of the tentacles. (After Ray Lank ester. ) B, a, arm; an, anal prominence; col,
collar ; col.ne, collar nerve, c.s, cardiac sac ; int, intestine ; m, mouth ; ntc, noto-
chord ; oe, oesophagus ; pr, proboscis ; pr.c, proboscis-coelom ; ret, rectum ; st,
stomach; te, tentacles; tr.c, trunk-coelom ; v.n, ventral nerve. (B, after Sche-
potieff; C, after Mclntosh.)

The frog is usually considered a transitional form separating the
lower from the higher craniata both embryologically and anatomically.
And although the craniates vary considerably among themselves, the
frog has enough in common with all such variations to make it a norm
or standard type for constant and valid reference, both as to anatomy

EMBRYOLOGY OF TADPOLE AND CHICK

517

and development. The adoption of a standard makes an understanding
possible of any special modifications one may find.

Then, so much work has been done in this field that a thorough
understanding of the embryology of the frog is essential to anyone who
wishes to do advanced work in the zoological sciences.

Just as there are various groups of birds with different hatching
periods, so different species of frogs also vary as to the length of time
the embryo requires before being able to emerge from the egg. But,

Fig. 315.
Examples of tailed and tailless Amphibia.

unlike the birds, the frog passes through a process of metamorphosis,
which simply means that even after the embryo's emergence from the
egg, it does not have the adult form, but must pass through still further
changes before becoming a full-fledged frog. In the frog the form that
is assumed at the time of hatching and which later changes on arrival
at adult life, is called the tadpole stage.

Temperature has much to do with both the rapidity with which a
frog's egg develops, and with which the tadpole develops into an adult
frog. This, however, is not unlike the hen's egg, for, it will be remem-
bered that after the hen's egg was fertilized and the embryo had already
begun to develop (before the egg has been laid), such an egg could be
placed in a moderately cool place for many days, which would result in
all development ceasing. If the egg is then placed under a hen, or in
an incubator, the embryo again begins to develop.

We shall arrange our study of the frog under two headings : First,
the true embryonic period. This extends from the time the egg is fer-
tilized through blastulation and gastrulation. During this time the germ
layers as well as the larval and tadpole organs form. This period ex-
tends up to the time the tadpole emerges from the egg.

Second, the larval period, from the time of hatching to the time the
legs are formed, the tail is thrown off, and the animal has become a full-

548

THE EMBRYOLOGY OF THE FROG

fledged frog with all its various organs and the form that it is to retain
throughout adult life.

To grasp fully that which follows, it is necessary to review the ac-
count of the reproductive organs in the chapter on the frog in the early
part of this book.

As the sperm from the male frog never enter the female body, the
egg must be fertilized after it has been laid. This is quite different from
fertilization in the hen. During the breeding season, as the eggs are
squeezed from the female, the male passes over the eggs and deposits
his sperm upon them. The fertilized eggs begin to divide almost imme-
diately, and within approximately thirty-six hours the blastula stage has
been reached. In about six days, when the embryo is five millimeters in
length, there is already a twitching within the egg, showing that life is
present, and within two weeks after fertilization the embryo wriggles
its way out of the surrounding jelly and becomes a free-living larva or
tadpole. This is the end of the true embryonic period.

If the temperature is
greater than normal, such as
it usually is in the labora-
tory, then the larvae may
hatch in five days. In either
case, however, suckers
(Figs. 316, 317, 318) are
formed in the head of a tad-
pole by which it attaches
itself to a jelly-like substance
surrounding the eggs al-
though it may attach itself
to other objects in the water
as well. Sometimes the tad-
poles will even fall to the
bottom of the water and lie there.

As the mouth opening does not form until two to five days after
hatching, the tadpole naturally cannot take in any food from the out-
side, and so is still dependent upon the yolk, still undigested, within its
body.

As soon as the
tadpole begins to take
in food from the out-
side, the suckers de-
teriorate and disap-
pear. From this time
on the tadpoles are
very active They feed
on almost any plant or

Fig 316.

A frog embryo at the stage of hatching. an.,
Proctodseum ; au.c., slight swelling over the rudiment of
the ear ; e.g., external gills on gill arches ; na., invagina-
tion to form nasal capsule ; o.c., slight swelling over the
rudiment of eye; s., sucker; atm., stomodseum (in-
vagination which will form the mouth . (After Bor-
radaile. )

SpT

Fig. 317.

Four stages of the development of the adhesive apparatus;
(suckers) of Bufo vulgaris; A, suckers; M, mouth; SpT,
spiracular tube. In 3 the gills are almost completely hidden by
the united right and left opercular folds.

The small outline figures indicate the shape and approxi-
mate size of the tadpoles. (After Thiele.)

EMBRYOLOGY OF TADPOLE AND CHICK 549

animal debris, and in the laboratory will thrive on a diet of cereals. As
the egg is dependent upon temperature for its rate of speed in develop-
ing, so the rate of speed at which the tadpole grows is dependent upon the
quantity of food it obtains.

External gills develop shortly after hatching, being used as respira-
tory organs. These disappear as soon as the mouth opens, and the true
internal gills are formed. When the true gills form, they are protected

by a cover called the operculum. The por-
tion underneath the operculum remains con-
nected with the outside by only a single pore
on the left side, known as a spiracle. The
limb-buds appear normally at about four or
five weeks, although in the laboratory at a
1 rig. sis. higher temperature, much sooner. The an-

i, Front view of the mouth of a tcrior pair develop within the opcrcular

tadpole of Rana temporaria show- ., , ,

ing the transverse rows of tiny cavity, and consequently cannot be seen
horny *£& \i^T SS&& from the outside. The posterior develop,

(After Gutzeit.) Qne Qn each sjde Q£ the cloaca a little later,

and become quite large and jointed by the end of the second month.

In the meantime the lungs have been growing and the young tad-
pole comes to the surface of the water to expel small bubbles of air and
to take in a fresh supply. In the common species of frogs, metamor-
phosis begins at about the end of the third month. It is at this time
that the tadpole ceases feeding, and the outer layer of skin, as well as
the horny jaws (Fig. 318), are thrown off. The lips shrink, the mouth
is no longer suctoreal, and becomes much wider, while the tongue in-
creases in size. The eyes also become prominent. The fore-limbs ap-
pear, the left one pushing through the opening of the gill chamber, while
the right pushes its way through the opercular fold on that side, leaving
a ragged hole. The stomach and liver enlarge, while the intestine be-
comes shorter and smaller in diameter than before, and the animal be-
comes carnivorous. The gill-clefts close and many changes occur in the
blood vessels due to the change in the animal's mode of breathing. The
bladder is formed, the kidneys undergo changes, and there is a definite
sexual differentiation. The tail shortens and is finally lost as the hind
legs continue to lengthen.

If, however, the water has been particularly cold, the metamor-
phosis may be put off until the following spring ; in fact, it seems normal
with some species to wait even -longer than this, namely, as long as two
years, and sometimes three (Necturus) before the adult form is assumed.

If one thinks of the hen's egg being laid without the normal shell,
such a shell-less egg would be somewhat akin to the frog's egg. Here
the yolk is a rather blackened mass with a jelly-like substance surround-
ing it, similar to the white of the hen's egg, but without a solid shell.
Great masses of the eggs are found in one place, appearing very much

550 THE EMBRYOLOGY OF THE FROG

as though dozens of hen's eggs were broken, but with the yolks entire.
The eggs vary in different species from one and five-tenths millimeters
in diameter to twice that size. A little over half of the egg is a dense
black due to the pigment granules contained therein, while the remain-
der is rather white, although again, in different species the quantity of
pigment may vary greatly. The darker portion is commonly known as
the animal pole, and the lighter as the vegetal pole.

There are three membranes covering the egg: Primary. That
known as the vitelline membrane. This can sometimes be distinguished
from the pigmented substance lying directly beneath it, although some
writers deny that it exists at all.

The secondary membrane (sometimes called the chorion) is a rather
thin but tough layer secreted from the follicle cells of the ovaries.

The tertiary membrane is a thick, jelly-like layer derived from the
walls of the oviduct, lying close to the chorion, first as a dense layer,
#ut later as it enlarges it becomes quite clear.

It will be remembered that the yolk granules were quite evenly dis-
tributed in the yolk of the hen's egg and that the embryo developed upon
the yolk. In the frog, however, the deutoplasm or food part of the yolk
all lies at one end — the vegetal pole. Frog's eggs are therefore said to
be telolecithal.

The nucleus lies in the animal pole and has already commenced to
divide by the time the egg is laid. In fact, it is already in the metaphase
of the second polar division at that time. The first polar body has been
thrown out, and can be seen as a very tiny light spot in the flattened
area of the upper pole.

As the reproductive organs of the adult have just been reviewed,
we shall not again discuss them here, but the development of these or-
gans will be taken up individually at a little later period. The general
development of egg and sperm are quite like that which occurs in the
germ cells of the chick.

FERTILIZATION

The sperm drills its way through the thin jelly of the chorion and
normally enters the egg substance in the pigmented region. The point
of entry is a meridian, passing through both poles of the egg. The
meridian which passes through the animal and vegetal poles of the egg,
as well as through the point where the sperm enters, is called the fertili-
zation meridian. Only one sperm norma-lly enters the egg. Polyspermy
is not, however, rare, but as far as we know, always results in some ab-
normal development when it takes place.

It will be remembered that after one-half of the chromosome mate-
rial of the nucleus of the egg has been thrown out by the two polar
divisions, the nucleus which then contains one-half the normal number
of chromosomes is called the female pro-nucleus. The head of the sperm

EMBRYOLOGY OF TADPOLE AND CHICK 551

(which also has only half of the normal number of chromosomes) now
enters the egg and leaves a trail of pigment behind it. This sperm, after
entry into the egg, becomes the male pro-nucleus. The head of the sperm
makes its way directly to the female pro-nucleus. The tail of the sperm
is thrown off although both tail and midpiece enter the penetration path
(Fig. 319). The head and midpiece after traveling for some distance in
the egg, rotate so that the midpiece is placed in advance of the head.
The midpiece then begins to dissolve and to form a typical nucleus with
an opening within which is called a vesicle. The sperm then changes its
course, and moves toward the point where male and female pro-nuclei
will unite, unless, of course, the penetration path has already led in that
direction. The path made by the changing of direction of the head of the
sperm is called the copulation path (Fig. 319). This path is also marked
by a trail of pigment as the head of the sperm passes through the cyto-
plasm to reach the female pro-nucleus.

Fig. 319.

Sections through the egg of R. fusca, showing penetration and copulation
paths, and the symmetry of the first cleavage plane. A. Sagittal section through
the egg before the appearance of the first cleavage ; B. Frontal section of the
same stage as A, showing the symmetrical distribution of the egg material. C.
Frontal section through egg in two-cell stage, showing the symmetry of the egg ;
the penetration path is not shown, a. Anterior ; cp, copulation path ; I, left ; p,
posterior ; pp, penetration path ; r, right ; «, remains of first cleavage spindle ;
sp, superficial pigment; 1, first cleavage furrow. (After O. Schultze.)

After the sperm has entered the egg, some of the fluid from the egg
proper is withdrawn into a space between the egg itself and the chorion.
This is known as a perivitelline space. The egg can thus rotate within
its membrane. From this time onward the pigmented pole is always
uppermost. In unfertilized eggs, the membranes are more or less ad-
herent. The jelly-like covering of the egg absorbs considerable fluid and
swells up in about a minute after the egg touches water. A close ob-
servation of the jelly shows that it is made up of various layers, whose
function is not only to protect the egg from chemical and mechanical
injuries, and from being eaten by other organisms, but also to elevate the
temperature of the egg. They accomplish this latter by being trans-
parent spheres which condense the heat rays from the sun and at the
same time check the radiation from the egg itself.

552

THE EMBRYOLOGY OF THE FROG

MATURATION

It has been stated that by the time the sperm enters the egg, the
second polar division has already taken place, or rather, the metaphase
of the second division. is in the process of taking place. This division
is completed rapidly and cuts off the second polar body in about thirty
minutes after the sperm enters the egg. The second polar body is either
the same size or smaller than the first. The egg-nucleus then assumes
its normal form. The polar bodies are often seen floating about in the
perivitelline space.

The male and female pro-nuclei now move toward the center of the
egg and meet in the usual manner. The female pro-nucleus does not
leave any pigment in its trail as does the male. The sperm centrosome
and centrpsphere divide to form the poles of a small but typical cleavage
figure, which is always located toward the animal pole, never in the
center of the egg.

Immediately after fertilization, there is a streaming of the formative
protoplasm upward, and the deutoplasm downward, so that the animal
pole obtains practically no yolk, and the vegetal pole is composed almost
entirely of it. At this time also, the pigment granules directly opposite
the point where the sperm enters the egg are carried away, which leaves
a somewhat crescent shaped lighter area. This crescentic area extends
from half to two-thirds the distance around the egg, and is known as
the gray crescent (Fig. 320). This moving of the heavier portion to one
side changes the specific gravity of the egg so that the portion possess-
ing least weight lies uppermost and close to the gray crescent just oppo-
site the point of entry of the sperm.

Fig. 320.

Frog's eggs showing formation of gray crescent from side and from vegetal
pole. The animal pole is heavily pigmented.

In about an hour and a half after the entrance of the sperm, in Rana
fusca, according to Bracheti, the egg has arranged itself in the manner
described, and is now ready for the first cleavage. A vertical plane is
drawn through the point where the sperm enters the egg and passes
over the top of the egg through the egg-crescent. This becomes the mid-
line, on both sides of which the bilateral embryo is to develop.

There are three distinct substances of varying specific gravity in
the frog's egg, namely, protoplasm, pigment, and deutoplasm, and these

EMBRYOLOGY OF TADPOLE AND CHICK

553

do not arrange themselves in the manner described until after fertiliza-
tion, so that we may say that bilateral symmetry in the frog's egg is
potential but not actual until after fertilization and the rearrangement
of these three different substances.

The original cleavage plane lies at right angles to the egg axis, but
not at right angles to what is to become the axis of the embryo itselL
There is no direct relation between the plane of the first furrow in
cleavage and the fertilization meridian. The midline of the developing
embryo and the penetration path of the sperm normally correspond to a
vertical plane known as a gravitational plane, drawn through the egg
after the particles of protoplasm and deutoplasm have rearranged them-
selves according to gravity, and all of these correspond to the first cleav-
age furrow, though many variations of this occur.

THE FORMATION OF THE BLASTULA

One of the chief reasons for studying the embryology of the chick
before that of the frog is that the three germ layers of the chick are
more readily seen. The frog's egg divides into two portions, then into
four, eight, etc., quite like the hen's egg, and by the time there are eight
cells present, the four cells in the region of the animal pole are found

Fig. 321.

Cleavage of the frog's egg. A Eight-cell stage ; B, beginning of
sixteen-cell stage ; C, thirty-two-cell stage ; D, forty-eight-cell stage
( more regular than usual ) ; E, F, G, later stages ; H, I, formation of
blastopore. The central light area in / is the yolk-plug while the
rinpr which encases the yolk-plug is the margin of the blastopore.
(After Morgan.)

554

THE EMBRYOLOGY OF THE FROG

to be smaller than the
four in the region of the
vegetal pole. The
smaller ones are called
micromeres and the
larger ones macro-
meres. The micro-
meres, after the fifth
cleavage begins, divide
more rapidly than do
the macromeres. By a
continually more rapid
growth of this kind, the
smaller animal cells
soon almost surround
the vegetal or yolk sub-
stance. As the yolk be-
comes surrounded more
and more, there is a
somewhat central re-
gion where the yolk
can still be seen from
the exterior. This por-
tion of the yolk is
called the yolk-plug,
while the . margin of
darker animal cells im-
mediately surrounding
the yolk-plug is called
the blastopore (Figs.
321, 323).

Fig. 322.

First two figures, photographs of- Frog Blastulas. A to F, median sections
through Blastulas and Gastrulas of A, Cynthia bipartia with 64 cells (this is a
member of the tunicates). B, Gastrula of dona intestinalis (also a tunicate).
C, Gastrula of Amphioxus; D, Blastula of Axolotl (Mexican salamander which
breeds in the larval stage) ; E, Gastrula of an early stage of Turtle (Chdonia
caouana), F, Gastrula of a later reptilian stage (in the Gecko). A to F after
Rabl and Van Beneden.)

EMBRYOLOGY OF TADPOLE AND CHICK

555

The fact that the entire yolk comes to lie within the blastoderm
causes much of the growth process to be hidden from view. In fact,
Professor Johnstone of Cambridge University suggests that the frog em-
bryo may not have the three regular germ layers at all. We think they
are present, although pressed together so closely that it is practically
impossible to distinguish them.

A slight separation of the darker cell layer in the yolk-plug region
leaves an opening called the segmentation cavity or blastocoele (Figs.
322, 323). The portion of the pigmented layer which has separated from
the yolk-plug will now be known as the dorsal lip of the blastopore (Fig.
323).

The segmentation cavity forms near the animal pole. The entire
blastula, however, is not much larger than the original egg because,
although there are now thirty-two to sixty-four cells definitely formed,
these have been formed by constant cell division without much growth
after dividing. The roof and walls of the segmentation cavity are, there-
fore, composed of the external pigmented animal cells. These cells are
of different shapes and sizes, rather irregular and loosely arranged, and
really divided into two sheets, one lining the blastocoele, and the other
forming the true outer layer of the blastula. It will thus be seen that

Fig. 323.

Median sagittal sections through a series of gastrulas of the frog (R. tem-
poraria). The figures illustrate the change in position of the whole gastrula, as
well as the phenomena of gastrulation proper. A. Commencement of gastrulation ;
earliest appearance of the dorsal lip of the blastopore. Internally the gastrular
cleavage is indicated. B. Invagination more pronounced; beginning of epiboly.
C. Invagination, epiboly and involution in progress. The gastrular cleavage is
now indicated on the side opposite the blastopore. Rotation of the gastrula. D.
Just before the ventral lip of the blastopore reaches the median line. The in-
dentation of the wall of the segmentation cavity is an artifact. E. Blastopore
circular and filled with yolk plug. Gastrula beginning to rotate back to its
original position. Peristomial mesoderm differentiating. F. Segmentation cavity
nearly obliterated. Neural plate established. G. Gastrulation completed. o,
Archenteron ; b, blastopore ; c, rudiment of notochord ; ec, ectoderm ; en, endoderm ;
gc, gastrular cleavage ; ge, gut endoderm ; m, peristomial mesoderm ; np, neural
plate ; nt, transverse neural ridge ; s, segmentation cavity or blastocoele. (After
Brachet.)

556 THE EMBRYOLOGY OF THE FROG

the frog blastula is not made up of a single layer of cells, but of a double
layer of animal cells. This double layer is the ectoderm. The floor of
the blastocoele is made up of the large vegetal cells.

As the pigmented animal micromeres divide more rapidly from now
on, they naturally must grow toward the equator. This causes a thin-
ning of the roof, but a thickening of the walls of the segmentation cavity.
The equator of the blastula seems to be the region in which the cells
multiply most rapidly, and this equatorial region is called the germ-ring
or growth zone (Fig. 324).

At the same time the germ-ring begins its rapid multiplication of
cells, the gray crescent extends downward rather rapidly. This region
is to become the posterior or caudal side of the embryo. The germ-ring
from now on continues extending beyond the equator into the vegetal
region until it lies approximately half way between the equator and veg-
etal pole. This growing of the germ-ring pushes the yolk more and
more within the overgrowing animal cells, as already mentioned. The
yolk thus being pushed within, naturally forces the floor of the segmenta-
tion cavity into a convex arch.

This is considered the end of blastulation in the frog's egg.

THE FORMATION OF THE GASTRULA

We have at this point, then: most of the yolk withdrawn within
the overgrowing animal cells, two layers of which form the outer cover-
ing of the blastula at the animal pole; a segmentation cavity with its
floor convexly arched, and a definite antero-posterior differentiation, the
posterior side being marked by the gray crescent.

The development of the gastrula begins just beneath the posterior
lip of the blastopore by a groove which forms directly between the ani-
mal cells and the yolk cells (Fig. 323, A, gc). The groove itself is lined
by both kinds of cells on its opposite faces. We know from the develop-
ment which takes place later that this groove is the real beginning of
invagination. The groove itself becomes the archenteron or primitive
intestinal tract (Fig. 323, C, E, F, G, a). The upper lip of this groove is
the rim of the blastopore (Fig. 323, b). The animal cells become the
ectoderm and the yolk cells become entoderm.

As the yolk is pushed within the blastula it causes a narrow groove
to form in the region of the blastocoele, which separates the rising floor
from the remaining yolk. This groove finally becomes a definite narrow
slit, which splits off the ectoderm and entoderm at the point of invagina-
tion. The original groove is known as the gastrular groove, and the
splitting off into ectoderm and entoderm is called gastrular cleavage
(Fig. 323, A, gc). This gastrular cleavage extends from the dorsal lip
of the blastopore entirely around the gastrula to the opposite side from
where invagination takes place.

In the invagination area, a definite tongue of ectodermal cells pushes

EMBRYOLOGY OF TADPOLE AND CHICK 557

inward (Fig. 323, C, en) and joins directly with the inner yolk cells to
form the entoderm. Due to their position, the inner yolk cells are also
entoderm, although they do not form by a true invagination.

Viewing the entire egg externally during the process of gastrula-
tion, we may consider the germ-ring as something like a rubber band
placed about an ordinary ball in an equatorial plane. By sliding the
rubber band off the ball toward one side, we may understand how the
germ-ring brings its lateral region together in the mid-rim in the pos-
terior or caudal region. This coming together not only pushes the un-
derlying cells within, but causes the entoderm to extend further inward
and thus increases the cortical extent of the archenteron.

We, therefore, have the first invagination of the pigmented cells
forming the dorsal lip of the blastopore; the invagination then extends
laterally in both directions to form the lateral lips of the blastopore; and
finally the process of invagination continues around to the side of the
gastrula, practically to a point almost opposite to where it began, and
where the ventral lip of the blastopore is formed. This completes the
entire blastopore. After the yolk plug has disappeared within the gas-
trula, the blastopore remains a narrow, elongated slit connected with
the archenteron.

The method by which the blastopore grows by concrescence and
forms the primitive streak has already been described in the chick, a re-
reading of which should be done at this point. Comparisons should con-
stantly be made between the development of frog and chick embryos.

It will be noted from what has been said that gastrulation is not so
much formed by invagination in the frog as it is by a delamination within
the gastrula itself.

In fact, among the higher chordates, invagination is sometimes en-
tirely lacking, so that gastrulation may be accompanied by either involu-
tion, such as takes place in the chick, or by epiboly, which occurs to some
extent in the frog, and by delamination, a process just described.

At this point Figure 325 must be studied to understand the varying
changes of positions of the blastopore brought about by the rotation of
the egg. It is also to be remembered that the blastopore marks the cau-
dal extremity of the embryo.

The two-layered stage in the frog is of very short duration.

In the inner region, where the germ-ring and the yolk-cells which
line the blastocoele are continuous, there are transitional cells which are
to become the mesoderm (Fig. 324, m). These cells are continuous with
ectoderm on one side and entoderm or yolk-cells on the other, and can
not be distinguished as definite mesodermal cells until the blastopore is
completely formed.

In other words, the mesoderm first appears as a ring of cells just
within the margin of the blastopore. This mesodermal region is broad-
ened considerably in the dorsal region.

558

THE EMBRYOLOGY OF THE FROG

As the blastopore closes, it carries mesodermal cells toward the mid-
line (Fig. 324), where they form a broad median band extending forward
from the dorsal lip of the blastopore. These cells then multiply, and as
the dorsal lip extends downward, an axial thickening is formed. At this
same time, the archenteron extends and carries yolk-cells outward toward
the animal pole, so that the extent of the mesoderm is almost as great as
that of the entoderm.

Fig. 324.

Frontal and transverse sections through gastrulas of the frog (R. temporaries)
of various ages. A. Frontal section through gastrula of same age as Fig 323.
C. B. Frontal section through gastrula of same age as Fig. 323, D. C. Frontal
section through gastrula slightly older than Fig. 323, F. D. Frontal section
through gastrula of same age as Fig. 32, G. E. Transverse section through gastrula
slightly older than Fig 323, D. F. Transverse section through gastrula slightly
older than Fig 323, G. a, Archenteron ; b, blastopore ; c, notochord ; ge, gut
endoderm ; m, peristomial mesoderm ; np, neural plate ; s, segmentation cavity or
blastocoele. (After Brachet.)

Then a rearrangement of cells takes place, so that an irregular de-
lamination commences in the dorsal lateral regions on each side of the
thickened axial mass, and extends from there anteriorly and laterally
around the sides of the archenteron. Thus, a thick layer of mesoderm is
formed between the entodermal lining of the archenteron and the outer
ectoderm (Fig. 324, m).

At the lower pole, that is, toward the place where the yolk-plug is
being drawn into the interior of the egg (Fig. 324, b), the lower surface
of the yolk is also delaminated so that a circular margin of the mesoderm
is formed there. It is from this layer of mesoderm that cells and groups
of cells bud off and pass toward the lower pole — (it is to be remembered
that these cells and groups of cells begin their growth in the lower pole

EMBRYOLOGY OF TADPOLE AND CHICK

559

region, but lie above the lower pole itself) — toward the ventral portion
of the blastopore, so that a more or less completely continuous layer of
mesoderm is formed between the ectoderm and entoderm.

In the dorsal region of the blastopore, and extending along the dor-
sal axial mass, delamination does not occur as rapidly as in the ventral
region. The course of delamination is also modified, here. This modi-
fication is no doubt due to the fact that in this dorsal axial region the
cells which are to become mesoderm are derived from the cells which
invaginated from the outer layer, wrhile in other regions this is not the
case. Then, too, it is in this region that the notochord forms, which
further complicates matters.

In fact, cross sections in the region of the blastopore do not show

B
Fig. 325.

A, B, C, Transverse sections of Amblystoma tadpole. A, through suckers ;
B, through optic and olfactory region ; C, through gill region.

560

THE EMBRYOLOGY OF THE FROG

lines of demarcation between the notochord, mesoderm, and the dor-
sal entoderm for some little time ; but sections through the blastopore
(while the yolk-plug is still protruding) show the rim of the blastopore
to be composed of thick, undifferentiated cells which are a part of the
contracted germ-ring. A little laterally, the ectoderm is separated by
a narrow space or line which has formed during gastrulation, and the

D

Fig. 325.

D, E, F, Photographs of transverse sections through 12 mm. frog tadpole.
The sections are cut diagonally, the left eye (right side of figure) lying more
cephalad ; is the most anterior. E immediately posterior to D, but lens was lost
sectioning ; F, immediately posterior to middle cut.

EMBRYOLOGY OF TADPOLE AND CHICK

561

thin entoderm is separated from the middle layer. This separation of
layers is the delamination process we have been discussing.

As the outer animal cells are pigmented, we assume that whenever
pigment is found in any of the inner cells, it is an indication that such
pigmented cells are derived from the ectoderm.

As the yolk-plug is withdrawn, the blastopore becomes a mere slit,
while the ectoderm and mesoderm separate considerably, and dip down-
ward toward the entoderm in the dorsal midline, although never so far

562

THE EMBRYOLOGY OF THE FROG

as to reach the entoderm. This leaves a narrow, vertical ridge of cells
at the midline.

A pair of slight depressions from the archenteron push upward and
appear only as virtual grooves formed by pigmented cells. Forward
from this, the lower margins of these grooves come to look like lips
which approach the midline. The mesoderm in the meantime separates
from the archenteron to a considerable extent (Fig. 324).

Still farther forward, the grooves disappear, while the spaces be-
tween mesoderm and entoderm, as well as between mesoderm and ecto-
derm, extend vertically and delimit the pair of mesoderm masses.

The cells thus left in the midline between the mesodermal sheets
form a wedge-shaped elevation which is continuous with the entoderm.
This is the beginning of the notochord. (Figs. 323, G, c, and 324, F, c,).

The notochord is now cut off from the entoderm by a narrow slit
which leaves the archenteron with a dorsal roof, one cell in thickness.
(Fig. 324.)

From this point posteriorly (toward the blastopore), the grooves
of the archenteron are better marked.

THE MEDULLARY PLATE

At the same time that the notochord is developing, the medullary
plate is also being formed (Figs. 324, np, and 326, everything above
mes.), partly from the medial band of cells which extends from the
region of the dorsal lip of the blastopore nearly to the animal pole, and
partially from the axial thickening caused by the confluence of the lateral
portions of the germ-ring.

The inner portion of the former region is called the nervous layer
of ectoderm. It is this nervous layer which begins to thicken, and by
the time the blastopore has begun to close, a thickened medullary plate
has formed over the entire dorsal surface of the gastrula, essentially like
that described in the chick.

At about the time the yolk-plug has disappeared, the lateral ridges
have become elevated and form the lateral neural ridges (Fig. 327, n, f).

Fig. 326.

Transverse section through frog tadpole in the an-
terior region of the neural groove, au, eye-pits. (Note the
pigment in the outer cells) ; ect, outer layer of ectoderm:
ent, entoderm ; mes, mesoderm ; nr. anlage of the neural
crest. (After Eyelesheymer. )

EMBRYOLOGY OF TADPOLE AND CHICK

563

These extend from the blastopore to almost directly opposite on the
dorso-lateral portion of the embryo. Here they turn sharply to pass
toward the midline where they meet to form the transverse neural fold.
It is this latter fold which marks the anterior limit of the medullary
plate. The median groove becomes more pronounced and is then called
the neural groove.

The important points to bear in mind here are :

Firstj that as in the chick and all chordate animals, so, too, in the
frog, the closing of the blastopore by confluence (extending in an antero-
posterior direction, on the dorsal aspect of the embryo) forms a definite

Fig. 327.

Frog embryos. A, from behind and above ; B, from in front ; C, slightly earlier
than A and B. an., proctodeum (the invagination from which anus will form) ;
blp., blastopore ; ga, gill arches ; gp., gill plate ; n/., neural fold ; a, sucker ; ap.,
sense plate. (After Borradaile.)

axis, which means that by this confluence, the germ-ring becomes the
axis. It is on each side of this axis that organs and important struc-
tures develop.

Second, that gastrulation involves only the forming of two layers
(ectoderm and entoderm) from the single layered blastula.

Third, that notogenesis includes all the processes involved in the
formation of the medullary plate, notochord, and mesoderm.

Gastrulation is accomplished in the frog chiefly by a delamination
and rearrangement of the yolk cells and only to a slight extent by in-
vagination. The process of invagination is chiefly concerned in forming
the beginning of the notochord and the mesoderm in the dorsal and
dorso-lateral regions. These latter structures are not formed entirely,
however, by invagination, but also by material from the germ-ring which

564 EMBRYOLOGY OF TADPOLE AND CHICK

has been carried to the axial region. Invagination, in the frog, is there-
fore considered of minor importance.

Amphioxus is the only chordate which remains in a two-layered
state for some time. All other chordates retain this condition for a short
period, and this is so because the mesoderm begins its development al-
most at the moment of gastrulation.

In fact, in the frog, mesodermal cells are found almost immediately
after the entoderm cells begin, to form. They are found, first, all around
the margin of the blastopore where they form an important part of the
germ-ring. Mesoderm which forms in this way is called blastoporal or
peristomial.

As confluence begins, the lateral portions of the germ-ring are
brought .closer to the mid-dorsal region where they become a part of
the axis of the elongating embryo. That is, they form the mesodermal
bands, already referred to, and it is these bands of the germ-ring which
become axial in position, then to be known as gastral mesoderm. This
gastral mesoderm is nothing more nor less than mesoderm derived in
turn from the blastoporal mesoderm. It is no different from any other
mesoderm derived from the same origin, although it lies, of course, in
a different position from the remaining mesoderm. In Amphioxus, the
above statement, however, is not true ; for, in that animal, gastral meso-
derm and blastoporal mesoderm have different origins. This difference
in origins may be traced to the fact that in the frog the mesoderm dif-
ferentiates before gastrulation and confluence, while in Amphioxus, the
mesoderm does not differentiate until after these two processes have
begun.

THE FORMATION OF THE EMBRYO

All that has been discussed so far has taken place within two days
after fertilization. Now the embryo can be seen either lying in a straight
line on the dorsal surface or slightly concave, while the ventral surface
appears' convex. The ectoderm still forms the entire covering epithelium,
although some of these ectodermal cells have developed cilia which
appear just before the fusion of the neural folds. These cilia beat in a
posterior direction, thus giving the embryo a slow, rotary motion within
the egg membranes.

-The transverse neural fold marks the anterior limit of the nervous
system, whiler,,the posterior limit is located just anterior to the dorsal
lip of the blastopore;.

As confluence continues, the nervous system comes to extend from
almost pole to pole on the posterior surface of the gastrula, even- before
the blastopore has entirely contracted. The -gastrula then rotates so
that this posterior portion becomes dorsal, thus bringing the transverse
neural fold to the anterior end of the nervous system, while the medul-
lary plate occupies nearly the entire dorsal surface of the embryo.
fj .•.:,- The ridges which form on the neural plate and later fuse in a similar

EMBRYOLOGY OF TADPOLE AND CHICK

565

manner to that described in the chick, begin fusion in the region of the
medulla (Fig. 327), and continue both anteriorly and posteriorly. The
transverse fold extends backward so as to form the roof of the expanded
brain end of the developing nervous system. It meets the lateral folds
in the region lying between the fore and mid-brain, and as it is the
last region of the neural tube to close/ it is called the neuropore. The
neuropore lies just posterior to where the epiphysis is to appear. It is
quite transitory.

Neural crests are formed in a similar manner to the way they were
formed in the chick embryo.

The blastopore has become a narrow slit, the lateral walls of which
fuse, leaving two openings, an upper and a lower. The upper one is
directly continuous with the archenteron, while the lower one is known
as the proctodaeum. The proctodaeum is but a slight depression lined
with ectoderm.

The posterior ends of the neural folds extend out from the middle
regions where they fuse, to form the neural tube which then covers the
iipper blastoporal opening, thus forming a connection between arch-

hi

Fig. 328.

Sections of an embryo frog. A, transverse; B, longitudinal, an.,
Anus., ccel., crclom ; cct., ectoderm or epiblast ; end., endoderm or hypo-
blast ; f.br., fore-brain ; h.br., hind-brain ; ht., rudiment of heart ; int.,
intestine ; l.p., lateral plate of mesoblast ; lr., rudiment of liver ; 7n.br.,
mid-brain; m.s., mesoblastic somite; mes., mesoblast; mes' ., mesoblast
continuous with epiblast of neural canal and hypoblast of notochord ;
ne.c., neurenteric canal ; neh., notochord ; ph., pharynx ; pit., rudiment
of pituitary body ; so-.m., somatic mesoblast ; sp.c., spinal cord ; sp m.,
splanchnic mesoblast; stm., stomodaeum. (After Borradaile.)

566

THE EMBRYOLOGY OF THE FROG

enteron and neural tube. This opening is called the neurenteric canal.
(Fig. 328.)

Just as with the chick, the confluence of the two lateral walls of
the blastopore (which have been formed from the remains of the germ-
ring) bring a median cell mass together in the axial region, in which
ectoderm, entoderm, and mesoderm, are fused in a quite undifferentiated
mass. This is the primitive streak. The groove which lies in the mid-
line of the primitive streak is called the primitive groove.

It is from this primitive streak that ectodermal cells are budded
forth into the neural folds and upon the surface of the body. It is from
the primitive streak, also, that mesodermal cells are budded off into the
lateral bands, and entodermal cells into the walls of the archenteron.

At this time the chief characteristic of the brain is the single flexure
around the tip of the notochord (Fig. 329). The hypophysis (pituitary
body) can be seen as a tongue of ectodermal cells just beneath the end
of the fore-brain ; it extends inward a short distance.

The rudiments of the eyes (Fig. 326, au) are indicated as small
patches of the deeply pigmented ectodermal epithelium in an antero-
lateral region of the medullary plate.

The rudiments of the ears (Fig. 282, C) are seen as thickened
patches of the inner or nervous layer of ectoderm opposite the region
of the hind-brain. They are difficult to see externally as yet.

The rudiments of the olfactory organs are formed as thickened
patches of ectoderm below and in front of the optic rudiments. The tiny
depressions on the surface which are to form the future olfactory pits
may sometimes be seen at this period.

The notochord is completely delaminated except in the region of
the primitive streak, by the time the neural tube has closed.

By the time the neural tube is completed, the archenteron is called
a mesenteron, the anterior enlarged end forming the fore-gut, the walls
of which are but one cell in thickness (Fig. 329). This is also called the
pharynx in the embryo. The stomach and oesophagus are later to be
developed from this region.

Fig. 329.

Sagittal section of Anterior end of a frog tadpole 3.6mm.
.ong. (Redrawn from Corning.)

EMBRYOLOGY OF TADPOLE AND CHICK

567

Just in front of the neurenteric canal there is an enlargement also,
which forms the hind-gut or rectal portion of the intestine.

The mid-gut is, as in the chick embryo, that small portion in direct
connection with the yolk.

It will be remembered that the true mouth forms in chordates as
a secondary inpocketing of ectoderm. In the frog, the outpocketing
from the fore-gut which is to meet the ectodermal inpocketing is seen
just below the fore-brain (Fig. 329, pharyngeal membrane). This is the
region where the mouth will form later.

The liver will be seen as a ventral outgrowth beneath the anterior
end of the yolk mass (Fig. 328).

In sections, the rudiments of the first two or three visceral pouches
can be seen as vertical outgrowths from the sides of the pharyngeal
walls (Figs. 295 and 330). The pouches extend to the ectoderm with

B.

Fig. 330.

A. Horizontal section through an embryo frog some time before hatch-
ing, showing the optic vesicles springing from the sides of the fore-brain the
three anterior pairs of gill-slits, and five pairs of mesoblastic somites. B A
similar section through a tadpole shortly after hatching. The head is cut in
a lower plane than in A, so only a small part of the anterior end of the brain
appears in the section, a1, the mandibular arch ; oa, the hyoid arch ; a3, the
first branchial arch ; bv, blood-vessel in first and second branchial arch ; eg
external gills ; ent, enteron ; fbr, fore-brain; mch, branching mesenchyme
cells ; na, nasal pits ; nch, notochord ; nps, peritoneal funnel ; op, optic vesicle ;
ph, pharynx ; pnp, pronephros ; som, mesoblastic somites which in B are
converted into muscle. I, mandibulo-hyoid slit; //, hyo-branchial slit; III-V
branchial slits. (After Bourne.)

568

THE EMHRYOLOGY OF THE FROG

which they fuse. It is this fusion which causes the depressions in the
ectoderm just back of the head on the external surface.

THE SOMITES

Just as with the chick, somites are formed by transverse division
along the dorsal portion of the embryo, except in the primitive streak

an:

Jt-

A,

y*-

ol

lr.-

Fig. 331.

A. Tadpole of the frog at the time of hatching, an, anus ; ex. a, external
gills na, nasal pit ; s, sucker ; som, somites ; st, stomodseum ; yk, yolk-sac.

B. Diagram of a frontal section of a frog larva at the time of hatching,
(modified.) c, Crelom ; d, pronephric duct; F, fore-brain; i, infundibulum ;
in, intestine; n, nephrostome; o, base of optic stalk; ol, olfactory pit (pla-
code) ; p, pharynx ; t, pronephric tubules ; II, hyoid arch ; III -VI, first to fourth
branchial arches ; 1, hyomandibular pouch ; 2-6, first to fifth branchial
pouches.

C. A diagram of a transverse section of the frog embryo at the hatching
stage, ccel., Crelom ; ect., ectoderm ; int., intestine ; lr., liver ; m.pl., muscle
plate ; nch., notochord ; nst., nephrostome ; s.d., segmental duct ; sp.c., spinal
cord. The glomeruli are seen opposite the nephrostomes. (A and C, after
Borradaile ; B, after Marshall.)

EMBRYOLOGY OF TADPOLE AND CHICK

5G9

ep

region and in the head region. A complete sheet of mesoderm now sep-
arates ectoderm and entoderm in the embryo from the head region to me
primitive streak region, and this sheet separates into two layers, an outer
or somatopleure, and an inner known as the splanchnopleure. (Fig.
268.)

The thickened region where the somites form on the dorsal surtace
of the embryo along the notochord is called the segmental plate or
myotomal region, while the portion extending laterally (which is much
thinner) forms the lateral plates.

The coelom develops between somatopleure and splanchnopleure.

As the frog's egg has the yolk-
mass packed within the embryo, this
mass pushes the germ layers close
together, so that they are by no
means as clearly set apart as in the
chick embryo.

Between the second, third, and
fourth somites and the lateral plates,
small masses of cells remain closely
related to the somatopleure of the
lateral plate. It is these cell masses
which are to develop into the pro-
nephric tubules. All of these struc-
tures must be compared with similar
developmental structures in the
chick embryo, at this point.

The coelom proper can be seen
as a definite space only below the
pharynx in front of the liver (Fig.
331). The heart will develop in the
region where the loosely scattered
cells are seen, ventral to the
pharynx.

At

Fig. 332.

Diagrams of median sagittal sections of
the anterior ends of frog larva?. A. Of a
larva just before the opening of the mouth.
B. Of a 12 mm. larva (at the appearance of
the hind-limb buds), a, Auricle; ao, dorsal
aorta ; b, gall bladder ; bh, basihyal cartilage ;
eh, cavity of cerebral hemisphere (lateral
ventricle) ; e, epithelial plug closing the
O3sophagus ; ep, epiphysis ; g, glottis ; h,
hypophysis ; H, hind-brain ; hr, cerebral
hemisphere; ht, horny "teeth" ; hv, hepatic
vein ; i, intestine ; if, infundibulum ; j, lower
jaw ; i, liver ; ly, laryngeal chamber ; m,
mouth ; M, mid-brain ; mb, oral membrane

(oral septum) ; n, notochord; o, median por-
tion of opercular cavity ; ce, oesophagus ; p,
pharynx ; pb, pineal body ; pc, pericardia!
cavity; pd, pronephric (mesonephric) duct;
pt, pituitary body ; pv, pulmonary vein ; pill,
choroid plexus of third ventricle ; pIV,
choroid plexus of fourth ventricle ; r, rostral
cartilage ; ro, optic recess ; s, stomodaeum ; sv,
sinus venosus ; t, thyroid body ; ta, truncus
arteriosus ; tp, tuberculum posterius ; v,
ventricle; vc, inferior (posterior) vena cava.

(After Marshall.)

THE LATER DEVELOPMENT
OF THE TADPOLE

There are details in which the
various species of frogs vary, but all
pass through the following general
method of development.

It is both interesting and prof-
itable to call attention at this point
to the fact that, while the frog was
one of the earliest forms of animal
life studied in the laboratory, and

570 THE EMBRYOLOGY OF THE FROG

while many hundreds of volumes and articles have been written on it
from many angles, there are nevertheless hundreds of interesting points
in that animal's development which are unknown. In fact, one writer
says that the gaps that confront one in the study of the frog assume
"remarkable proportions" when one thinks of how much work has really
been done on this much-studied animal.

Then, too, as there is no accurate method of obtaining the age of
a frog, owing to the remarkable influence temperature and food play in
its development, it is often difficult to make clear much that should be
made clear to the student.

Roughly speaking, at the time of hatching, namely, about one or
two weeks after fertilization, the larvae of most species are about six
or seven millimeters in length. The tadpole is usually about nine or ten
millimeters in length at the time of the opening of the mouth, and eleven
or twelve millimeters when the limb-buds appear.

THE NERVOUS SYSTEM

There are no neuromeres in the brain region of the frog, though
otherwise the same brain divisions take place that we have discussed in
the chick.

The tuberculum posterius (Fig. 332, tp) is a thickening opposite the
tip of the notochord in the floor of the brain, while a dorsal thickening
appears in the roof of the brain obliquely upward and forward from this.
A plane passing from the tuberculum posterius in front of the dorsal
thickening separates the fore-brain from the mid-brain, while a plane
passing from the same tuberculum behind the dorsal thickening sepa-
rates the mid-brain from the hind-brain.

The beginning of the brain divisions is quite similar to the three
primary regions mentioned in the chick embryo, and a review of the
matter there given will make the divisions and cavities in the frog un-
derstandable.

It will be remembered that the olfactory lobes and the cerebral hem-
ispheres form the telencephalon, and that the telencephalon and the
"tween-brain" (diencephalon) together form the fore-brain, while the
optic lobes and optic chiasma form the main portions of the mesencepha-
lon. The cerebellum takes up most of the metencephaloti, while the
medulla oblongata forms the myelencephalon.

THE FORE-BRAIN

Opposite the neuropore (Fig. 328) the cells of the ectodermal cone
scatter, due to the pushing out of the head tissues in advance of the
brain. Soon all trace of the neuropore disappears except for a slight in-
dentation known as the olfactory recess, and this also disappears a short
time later.

The lamina terminalis (Fig. 282, C) is a thickening just below the

EMBRYOLOGY OF TADPOLE AND CHICK

571

e.

hm.

level of the olfactory recess in the anterior wall of the fore-brain. It
extends ventrally to where the optic stalks protrude. From the drawing
(Fig. 333) it will be noticed that a thickening occurs in the region of the
attachment of the optic stalks to form the torus transversus (Fig. 333, tr)
and the beginnings of the optic chiasma (Fig. 333, cw) and thalami.
The torsus transversus becomes the seat of the anterior commissure
(Fig. 333, cpa) as well as other commissures of the brain.

The narrow depression between the thickenings just mentioned
forms the recessus opticus (Fig. 333, ro), which is the passage to the
cavities in the optic stalks.

The infundibulum (Fig. 333, J) is an outgrowth of the posterior

portion of the fore-
brain where it extends
backward under the tip
of the notochord.

The epiphysis or
pineal body (Fig.
333, e) is an evagina-
tion from the dorsal
wall of the fore-brain
at its posterior limit
where the wall has be-
come quite thin.

The choroid plexus
(Fig. 333, pch) is the
non-nervous portion of
the roof of the fore-
brain which has be-
come thinned out con-
siderably, and in which

the blood vessels lie.
This whole region is
pushed into the third
ventricle.

The habenular
ganglia and the haben-
ular commissure (Fig.
333, ch) develop be-
tween the choroid
plexus and the
epiphysis.

The paraphysis (a
dorsal growth) devel-
ops in front of the habenular ganglia and commmissure considerably
later.

Fig. 333.

Median sagittal sections through the brain of the frog. A.
Of a larva of R. fusca of 7 mm. in which the mouth was open*.
B. R. esculenta at the end of metamorphosis, c, Cerebellum ; co,
anterior commissure ; cd, notochord ; ch, habentflar commissure ;
cp, posterior commissure ; cpa, anterior pallial commissure ; cq,
posterior corpus quadrigeminum ; ct, tubercular commissure ; cw,
optic chiasma ; d, diencephalon ; dt, tract of IV cranial nerve ;
e, epiphysis ; hm, cerebral hemisphere ; Ay, hypophysis ; J, ijn-
f undibulum ; M, mesencephalon ; Ml, myelencephalon ; Aft,
metencephalon ; p, antero-dorsal extension of diencephalon ; pch,
choroid plexus of third ventricle ; R, rhombencephalon ; rm,
recessus mammillaris ; ro, optic recess ; se, roof of diencephalon ;
t, telencephalon ; tp, tuberculum posterius ; tr, torus trans^versus
(telencephali) ; vc, valvula cerebelli ; vi, ventriculus impar
(telencephali) (third ventricle"). (From Von Kupffer, Hert-
wig's Handbuch, etc.)

572 THE EMBRYOLOGY OF THE FROG

The cerebral hemispheres appear when the tadpole is about seven
millimeters in length, that is, when it is ready to hatch.

The ventricles are similar to those in the chick.

As all these thickenings and outgrowths appear, the brain itself
seems to straighten the original flexure, but this is only apparent, as the
flexure remains, and the infundibulum still extends below and in front
of the tip of the notochord.

The hypophysis (Fig. 333, hy) grows as an inward extension of the
surface ectoderm* to meet with the infundibulum.

THE MID-BRAIN

There is not much change in this region of the brain except that
the ventro-lateral walls thicken, and these thickened portions are known
as the crura cerebri. They connect with the wall of the fore-brain. The
dorso-lateral walls of the mid-brain form the large, rounded optic lobes.

The posterior commissure (Fig. 333, cp) forms the anterior limit of
the mid-brain.

The aqueduct of Sylvius (Fig. 303) is the cavity in the mid-brain
connecting the third ventricle with the cavity in the rhombenceplralon.

THE HIND-BRAIN

There is little, if any, line of demarcation in the frog which divides
the hind-brain into metencephalon and myelencephalon.

The cerebellum (Fig. 333, c) is in the region which is commonly
designated as the metencephalon. This organ is quite small in the frog,
and appears late in larval life on the dorsal side of the hind-brain.

The non-nervous thinned-out roof of the fourth ventricle (which
covers the dorsal part of the region of the medulla oblongata or myelen-
cephalon), (Fig. 333, Ml), forms the choroid plexus of the fourth ven-
tricle.

The floor of the ventro-lateral walls of the hind-brain becomes thick-
ened and forms the main nervous pathways to and from the nuclei of
origin of most of the cranial nerves.

The brain gradually tapers into the spinal cord proper at the medulla
oblongata.

The central canal is the cen'tral opening running throughout the
length of the spinal cord. It is continuous with the cavities of the brain.
This central canal is lined with non-nervous cells known as ependymal
cells. The true nerve-cells which go to make up the main portions of
the wall of the spinal cord are called germinal cells. These latter are
in turn divided into supporting cells or glia cells, and the true functional
nerve cells or neuroblasts.

There is no dorsal fissure in the frog's spinal cord as there is in
higher forms, though there is a ventral fissure.

The gray matter of the cord is formed by the neuroblasts.

EMBRYOLOGY OF TADPOLE AND CHICK
THE PERIPHERAL NERVOUS SYSTEM

573

The peripheral nervous system has been discussed in some detail in
the chick, and will be taken up in comparative anatomy later.

In the tadpole there are some forty pairs of spinal nerves but only
ten pairs in the adult. They arise by a dorsal and ventral root, which
unite to form the trunk of the spinal nerve, after which this trunk divides
into a dorsal and a ventral ramus, while a ramus communicans connects
the trunk with the sympathetic system.

There are also ten cranial nerves instead of twelve as in the higher
forms.

Fig. 334.

Transverse and frontal sections of frog embryo to show position and division of
neural crest in head region.

The V, VII, IX, and X are called branchiomeric nerves on account
of their close relationship to the branchial clefts.

The cranial nerves take their substance from three embryological
elements, namely: (1) the cell masses derived from the neural crests
as described in the study of the chick, (2) cells from ectodermal patches
on the surface of the head, and (3) from the cell processes which extend
outward from the neuroblasts in the ventro-lateral walls of the spinal
chord.

They differ, therefore, from the spinal nerves, for in these (2) is
lacking".

The V, VII, IX and X cranial nerves arise by a single root (though
this may be mixed, i. e., it may be both sensory and motor in function)
passing into a large ganglion, beyond which a large horizontal branch
is given off wrhich in turn branches into two rami which pass anteriorly
and posteriorly to the gill cleft with which the particular nerve is asso-
ciated.

As in the chick, so in the frog, the cranial nerves develop from the
neural crests left on each side of the central canal after the neural folds
fuse, and the indented ectoderm again returns to its normal condition.

574 THE EMBRYOLOGY OF THE FROG

The neural crests are thus left between the central canal and the outer
ectoderm.

The neural crests are quite large in the head region, becoming
smaller toward the posterior region of the embryo. Each crest becomes
divided into three masses as the neural plate begins to close. (Fig. 334.)

The more anterior of these divisions which lies in the region of the
mid-brain is the beginning of the V nerve, and is to form the trigeminal
ganglion. The middle section is the beginning of the VIII and VII

Fig. 335.

The Nerve Placodes in the head of an Ammocoetes 4 mm. long. V1, first •
ganglion of the V cranial nerve ; Va, second ganglion of the same nerve ; VII,
ganglion of the VII cranial nerve ; IX, ganglion of the IX nerve ; X, ganglion of
the X (Vagus) nerve. 1, 8, 13, first, eighth and thirteenth ganglia in the epi-
branchial series, for1, 6r8, first and eighth branchial pouches ; c, ciliary nerve ; eh,
notochord ; I, lateral ramus of the X cranial nerve ; N, milage of hypophysis ; n.a.,
VI cranial nerve (abducens) ; o., ophthalmic nerve; r.f., recurrent branch of VII
cranial nerve (facial) ; r.v., recurrent branch of X (vagus) nerve; t., IV cranial
(trochlear) nerve; ves.op., optic vesicle. (From Vialleton after von Kupffer.)

nerves, known also as the acustico- facialis ganglion, while the posterior
division forms the beginning of the IX and X nerves, or the glosso-
pharyngeal and vagus ganglia. The three divisions do not separate en-
tirely from the medullary plate, but remain connected by a very slender
chain of cells to the medullar region of the brain.

When the tadpole has developed three or four somites, the inner or
nervous layer of the ectoderm opposite the crest ganglia proliferates to
form a patch, sometimes three or four cells in thickness. Such patches
are known as placodes (Figs. 268, 335), and are thought to be vestigial
sense organs.

In the placode there is found a superficial sensory element which
may disappear, and a deep ganglionic element usually retained. It is
the ganglionic portion which fuses with the nearest crest-ganglion, thus
forming the principal sensory portions of the nerve.

THE TRIGEMINAL OR V NERVE

This is the principal nerve of the mouth and mandibular arch. The
trigeminal portion of the neural crest is large and extends from the eye
to the hyomandibular cleft (Fig. 335). The ectodermal and mesodermal
cell-groups fuse as the crest ganglion grows downward. In the ventral
region it meets the mesoderm of the mandibular arch.

EMBRYOLOGY OF TADPOLE AND CHICK 575

It is important that the student note how the mesenchyme of the
mandibular arch is formed by the process of growth just described. The
mesodermal and ectodermal cells have so intermingled at the point of
fusion that the separate cells of ectoderm and mesoderm are now indis-
tinguishable. (Fig. 334.)

The dorsal and superficial cells of the crest ganglion retain their
nervous character and come into close relation to the large placode close
to them, but the superficial portion of the placode, which is sensory, now
disappears. The deep or ganglionic portion not only enlarges, but
divides into two parts. The anterior portion becomes the ophthalmic
ganglion of the ophthalmic branch of the V nerve. The fibers of this
branch grow cephalad through the dorsal head region, and also grow
medially and connect with the medulla oblongata.

The posterior portion of the placode ganglion fuses with the crest
ganglion to form the Gasserian ganglion or trigeminal ganglion.

It is from the cells of the trigeminal ganglion that fibers arise which
run to the medulla on the dorsal side, and these fibers form the sensory
root of the V nerve.

Then, too, there are fibers which grow out from the ganglion to pass
to the surface of the head to form the cutaneous branch of the V nerve,
while the fibers which pass in front of and behind the mouth are called
the mandibular and maxillary branches respectively.

All of these branches as well as those from most of the branchi-
omeric nerves can be seen before the opening of the mouth.

THE FACIAL AND AUDITORY, OR THE VII AND VIII NERVES

Both of these nerves are derived from the acustico-facialis crest
ganglion and the placode associated with it. The VII nerve is connected
with the hyomandibular cleft, while the VIII nerve is a purely sensory
(auditory) nerve, and so not one of the branchiomeric series.

The greater portion of the crest ganglion, as with the V nerve, con-
tributes to the mesenchyme of the hyoid arch, although the nervous por-
tion of the crest ganglion is more extensive than that of the V nerve,
which is due to the fact that a greater portion of the original ganglion
retains its nervous function.

The superficial or nervous character of the placode does not dis-
appear in this case, but keeps on becoming larger, after which it sinks
below the surface of the head and invaginates to form the auditory sac.
(Fig. 334.)

The deep placode ganglion cells which are in connection with this
sensory epithelium remain in contact with the sac to form the root of
the VIII nerve.

The remaining portion of the placode ganglion joins with the nerv-
ous portion of the crest ganglion to form the ganglion of the VII nerve.
It is from this ganglion that fibers pass to the medulla and to the hyoid

576 THE EMBRYOLOGY OF THE FROG

arch and associated regions, to form the hyomandibular and palatine
nerves.

THE GLOSSOPHARYNGEAL AND VAGUS (PNEUMOGASTRIC)
OR IX AND X NERVES

The remaining visceral clefts, that is, the first to fourth clefts, or
third to sixth visceral arches, are associated with the IX and X nerves.

The IX nerve is limited to the first gill cleft alone, but the X nerve
is associated and distributed to the others. It is to be considered a com-
pound nerve, as it is made up of several branchiomeric nerves.

The large posterior part of the neural crest in the head region is the
portion associated with the IX and X nerves.

Its growth is much like that of the V nerve, though it does not
assist in forming so much mesenchyme.

The superficial sensory portion of the placode of the IX nerve dis-
appears, and its ganglionic portion is only slightly related to the crest
ganglion.

Posterior to this, the larger placode of the X nerve appears simulta-
neously, and passes through similar stages, but in this case there is a
more extensive fusion between it and the nervous portion of the crest
ganglion.

The fibers from the IX and X ganglion pass out together to the
medulla as a single root. The anterior cardinal vein partially separates
the IX from the X ganglion.

The fibers which pass out from the IX nerve portion of the ganglion
are practically all placodal in origin, and pass to the first branchial cleft,
while the fibers coming from the mixed ganglion of the X nerve are
connected with all the remaining clefts.

It is well to pay considerable attention to the X nerve, as it is one
of the most important nerves in the body, being connected: with many
important, structures.

.„,..,, It is from the X ganglion that other processes than those just men-
tioned, also grow. A considerable tongue of cells grows -out posteriorly
to form the sense organs of the lateral line, shortly to be discussed, while
the fibers which are to become the lateral line nerves accompan}^ this
tongue (Fig. 340). These latter fibers are present only during the tad-
pole stage.

Then there are branches which pass to the thoracic and abdominal
organs to form the visceral branch of the X nerve.

As sensory nerves pass to a general center, and motor nerves pass
from a center to some outlying region, it is well to appreciate how some
of the nerves mentioned above come to be mixed, that is, having both
sensory and motor fibers running along side by side.

The motor fibers of the branchiomeric series do not arise by sepa-
rate roots (Fig. 336) as do the sensory, but from neuroblasts in the walls

EMBRYOLOGY OF TADPOLE AND CHICK

577

of the medulla oblongata which send out processes called axons, which
leave the medulla in close association with the sensory roots already de-
scribed. These are then distributed with the branches passing posterior
to the gill clefts.

The III cranial nerve is the first of the remaining III, IV, and VI
to appear, although all three of these form later than the ones discussed
above, that is, they form when the tadpole is five to six millimeters in
length.

The III is called the oculo-motor, the IV the trochlear, and the VI
the abducens. All are motor nerves, which innervate the muscles of the
eye-ball.

The I cranial nerve is the purely sensory olfactory nerve and the II
is likewise a purely sensory nerve, namely, the optic.

THE SPINAL NERVES

These nerves, unlike the cranial nerves, are related to the somites,
and not to the visceral clefts, and there are no placodes connected with
them.

The two most anterior myotomes
do not have spinal nerves connected
with them, and the myotomes soon dis-
appear, but the segments formed in the
neural crests, posterior to the head re-
gion (with the exception of the two
just mentioned), have cell processes
grow out into the cord to form the dor-
sal root of the spinal nerves, while
others grow away from the cord to
form the peripheral strands which are
distributed to the skin and other
sensory surfaces.

The ventral root of the spinal
nerve is formed by outgrowths or
axons from the neuroblasts on the ven-
tral side of the cord, and appear when
the tadpole is about four millimeters
in length. These then meet the dorsal
root a little distance beyond the ganglion, and pass partly to the meso-
dermal myotomes and partly to the sympathetic system.

THE SYMPATHETIC SYSTEM (Fig. 337)

When the tadpole is about six millimeters in length, one may see a
slight collection of cells on the spinal nerves at about the level of the
dorsal aorta. From our study of the sympathetic system in higher ani-
mals, we assume that these cell-groups are composed of elements from
the spinal ganglia, and from some of the posterior cranial ganglia. • ,.t

Fig. 336.

Schematic arrangement to show the
composition of the central nerve roots in
shark fins. The motor fibers run to the
muscles, and each motor spinal root is
made up of the fibers of three spinal seg-
ments. NI, II, III. IV, V, Neuromeres;
N2, N3, N4, corresponding motor roots ;
M1-M5, Myomeres; 1-10 Divisions of
Mydmeres. (From Rabl.)

578

THE EMBRYOLOGY OF THE FROG

The cells themselves migrate ventro-medially to form a pair of longi-
tudinal sympathetic cords, along each side of the dorsal aorta. It is from
these cords, then, that processes grow back to the spinal ganglia to form
the rami communicantes, as well as outwardly to the various organs and
surfaces.

Other fibers from other spinal ganglia grow out and follow the paths
thus laid down for them, while cells from the sympathetic cord probably
also migrate to form the large sympathetic ganglia found in close con-
nection with the large blood vessels, and the thoracic and abdominal
viscera. The fact of the matter is that the sympathetic nervous system

B.

ao.c.

— iscA

Fig. 337.

A. Sympathetic nervous system of the Frog, ao, aortic arch ;
ao.c., common aorta ; isch, ischial nerve ; sp.sy., communicating
branches between the spinal and sympathetic nerves ; ay, the two
branches of the sympathetic system; 1-10, spinal nerves. (After
Meissner.)

B. One-half of a transverse section of Ammocoetes, in the
head-region. Schematic. ao, aorta ; br.a., branchial branch of
nerves ; ch, notochord ; ep.br.pl., ganglion anlage which develops
where the epibranchial placode forms ; ect, ectoderm ; ent, ento-
derm ; lat.pl., lateral placode ; med.pl., medial placode ; med.obl.,
medulla oblongata (hind-brain) ; mt, myotome ; s.c.a., subcutaneous
branch of the epibranchial nerve placode ; ap.a., spinal branch
running inward ; s.pl., lateral plate of mesoderm ; symp.g., sym-
pathetic ganglion. (After von Kupffer.)

of the frog has not been worked out with any degree of thoroughness,
and we can only suppose many things from our knowledge of other forms
where more is known of this system.

The ganglion of the III cranial nerve is sympathetic in character,
as other cranial nerves may be, but this must be left for future workers
to demonstrate.

EMBRYOLOGY OF TADPOLE AND CHICK

579

•: L- THE EYE (Fig. 338)

The general method of the eye formation is quite similar to that
already described in the chick.

As the outer free rim of the optic cup draws tog-ether, it leaves a
small opening" which is the rudiment of the pupil. At this time we can
distinguish the inner and outer layer to the cup, and a central cavity.
These are the beginnings of the true retinal layer, the pigment layer, and
the posterior chamber of the eye respectively.

There is a choroid fissure formed, just as in the chick.

The lens forms as a thickening of the ectoderm opposite the pupil,
but this thickening involves only the nervous layer of the ectoderm.
It develops quite similarly to the ectodermal placodes in the formation
of the cranial nerves. In fact, the lens placode lies immediately anterior
to the placode of the V cranial nerve. About the time of hatching, the
lens has formed a prominent rounded thickening entirely cut <Mf from
the ectoderm.

~ cor

jnes

Fig. 338.

Section through the eye of a tadpole at the time when the operculum is
forming, much magnified, ch, choroid ; cor, mesoblast cells which will give
rise to the cornea ; dm, Descemet's membrane ; ep, pigmented external
epithelium ; L, lens ; mes, branched mesoblast cells ; op.n., optic nerve ;
pch, pigmented epithelium of the choroid ; R, retina ; rd, rods and cones
pulled away from the pigmented epithelium of the choroid by contraction
of the preparation; vit, cells of the vitreous humour. (After Bourne.)

This spheroidal mass hollows out, although it again becomes solid
by the cells on its inner side elongating, while the outer side remains as
a thin epithelial layer covering the distal surface of the lens. Then, as
the pupil narrows, the lens comes to lie just within the openings of the
cup.

580

THE EMBRYOLOGY OF THE FROG

-.dr.med.

Fig. 339.

Reconstruction of smelling apparatus of Frog and three transverse sections (on
the right) of tadpole through the regions marked A .4, B B, and C C. dr.nted.,
glands on the medial side ; lat, lateral view of the olfactory cavity ; lat-u, com-

EMBRYOLOGY OF TADPOLE AND CHICK 581

In the chick, and in fact in all vertebrates except the Teleosts, the
lens forms as a hollow vesicle caused by the surface ectoderm invagi-
nating. The choroid fissure of the optic cup closes a day or two before
hatching. The closing begins opposite the pupil.

THE EAR

The auditory placode appears just as the neural folds close. These
placodes then become depressed below the surface of the head and invagi-
nate to form the auditory sac or otocyst.

This sac at the time of hatching has become completely closed and
separated from the ectoderm from which it arose. Therefore, it comes
to lie in close relation to the lateral surface of the medulla.

The superficial layer of ectoderm continues to remain as a covering
of the region where the placode invaginated. The wall of the auditory
sac is but one cell in thickness, except in the medio-ventral region. It
is in this region that the ganglionic part of the placode is located. There
is a small finger-like outgrowth from the sac, which extends dorsally
from the medio-dorsal region. This is to become the endolymphatic
duct.

There is little change in the ear region from this time to the opening
of the mouth, that is, until the tadpole is ten to twelve millimeters in
length. Then development seems to begin again.

The remaining complicated changes in the formation of the inner
divisions of the ear are beyond the scope of this book. The VIII cranial
nerve connects with the ear.

THE NOSE

The olfactory organs appear quite early, in fact before the brain
closes (Fig. 339, no). A pair of ectodermal thickenings appears on each
side of the head just above and anterior to the future mouth region.
Again, only the deeper nervous layer is involved, as with the formation
of the ear; only in this case, the superficial layer of ectoderm does not
remain as a covering but disappears entirely. The thickenings them-
selves lie immediately anterior to the lens placodes, and are called olfac-
tory placodes.

The placodes invaginate, forming the olfactory pits which are later
to become the true nasal cavities (Fig. 339). A few cells from the inner
surface of the olfactory placode become detached and come into com-
munication with the surface of the fore-brain to form a sort of crest-

munication of the lateral and lower region of the olfactory cavity ; md.h., wall of
the mouth-cavity ; no,, external nares ; o, upper region of the olfactory cavity ;
olf, olfactory nerve ; o-u, communication between the upper and lower regions of
the nasal cavities ; u, lower region of the nasal cavity. ( After Bancroft. )

D, Frontal section of human fetus of 29 mm. (After Tourneux.)

E, Sections through the nasal region. Bone black and cartilage dotted, ds, den-
shelf ; g, Jacobson's glands; j, Jacobson's organ; n. main cavity of nose; oj,
opening of Jacobson's organ; t, tooth-germ. (After Schimkewitsch.)

582 THE EMBRYOLOGY OF THE FROG ,

ganglion. It is from these cells that the sheath cells of the true olfactory
nerves (I cranial nerves) seem to be formed, the olfactory nerves them-
selves forming from the sensory cells of the olfactory placode.

The olfactory pits are often said to form the anterior nares, while
the openings from the anterior nares into the mouth cavity are called
the posterior nares, internal nares, or choanae, all three terms meaning
approximately the same thing.

The olfactory development is quite complicated and cannot be
worked out in all details by the student in an elementary course such as
this, but it is important that the main features be understood, so that
light may be thrown upon later studies.

The epithelial lining of the olfactory pits forms a cavity on the dor-
sal side soon after hatching. This cavity then closes to form a separate
dorso-lateral lobe, which disappears entirely as metamorphosis con-
tinues.

During metamorphosis, various thickenings and outpocketings ap-
pear in the olfactory organ, and there is a sharp bend in the main axis.
The most important of the outgrowths is an extension from the ventral
side of the olfactory chamber, where a solid mass of cells proliferates.
This extended portion then acquires a cavity, grows rapidly, and turns
transversely toward the medial side. This structure is to become Jacob-
son's organ (Fig. 339). A large glandular mass develops upon the medial
end of this organ.

Opposite Jacobson's organ another growth appears which is non-
nervous, that is, is not lined with nerve cells. This becomes a large sac,
and the cavity of the sac is then added to the olfactory chamber. Still
another growth appears anteriorly, close to the base of the olfactory
duct. It is into this latter structure that the duct from the lachrymal
glands enters. Still later (about the time of metamorphosis) a dorsal sac
grows out from the medial and posterior walls of the tube.

During the late metamorphosis, the axis of the olfactory organ is
sharply bent by the shifting of the internal nares, and other glands ap-
pear as outgrowths, both in the olfactory chamber and in the posterior
walls of the internal nares.

THE SENSE ORGANS OF THE LATERAL LINE (Fig. 340)

In all gill-bearing animals, and in all animals that have gills at any
time during their development, such as the frog, a series of sensory
drgans develop which are known as lateral line organs. These organs
vary to a very considerable extent, but three or four of them are rather
constant. These are: (1) the supraorbital line, which runs forward from
the ear region over the eye to the tip of the snout. Twigs from the
ophthalmic branch of the VII cranial nerve innervate it,

(2) The infraorbital line, which also runs from the ear region, but

EMBRYOLOGY OF TADPOLE AND CHICK

583

passes under the eye to the snout. It is innervated by the buccalis nerve
which is a branch of VII. •';«.'*'

(3) The hyomandibular line runs along the jaw and operculum.
This is innervated by twigs from the mandibularis externus, and finally,
(4) the lateral line proper, which may be double. This extends back to
the tail on both sides of the animal, being innervated by twigs from the
lateralis, a branch of the X cranial nerve.

Fig. 340.

The development of the lateral line organs in R. sylvatica. A. Part of a
frontal section through the level of the ndtochord of a 3.3 mm. embryo. B. Part
of a transverse section through the vagus region of a 4 mm. embryo. C. Part
of a frontal section through a 4 mm. embryo of R. virescens. D. Section through
the lateral line organ of a 15.5 mm. larva of R. sylvatica. a. Auditory vesicle (in
A, its rudiment) ; 6, basement membrane of epidermis; eh, notochord ; g, gut; gV,
trigeminal ganglion, of V cranial nerve; gVlll, acoustic ganglion of VIII
cranial nerve ; gX, vagus ganglion ; gXl, ganglion of the lateral nerve (branch of
the vagus) : t, intersegmental thickenings of the epidermis (ectoderm) ; I, rudi-
ment of lateral line nerve ; lp, lateral plate of mesoderm ; my, myotomes ; n, inner
or nervous - layer of epidermis (ectoderm) ; nc, nerve cord; p, pigment in epi-
dermis ; si, inner sheath cells of lateral line organ ; sn, sensory cells of lateral
line organ ; so, outer sheath cells of lateral line organ. . K, Lateral line canals
and their nerves in a pollack (a fish belonging to the cod group). Canals and
brain are dotted while the lateral nerves are black, b, buccalis ramus of the
VII nerve; dl, dorsal rarnu* of lateralis of X nerve; h, hyomandibularis nerve;
Km, hyomandibular line of organs; to, infraorbital line; 1, lateral-line canal; n,
nares ; o, olfactory lobe ; op, operculum ; os, ophthalmicus superncialis nerve ; aoc,
commissure which connects the lines of both sides; so, supraorbital line of organs;
st, supratemporal part of lateral line vl, ventral- ramus of lateralis of X nerve ;
*, visceralis portion of X nerve. '(A. to D, from Harrison; E, from Kingsley after
Cole.)

584 THE EMBRYOLOGY OF THE FROG

There may be also a supratemporal line, connecting the systems of
both sides and extending across the posterior portion of the skull from
one side to the other.

These lateral line organs sink beneath the skin, and usually degen-
erate in water-forms of animals as soon as they are ready to live on
land. In a few cases such as Tritons, Amblystoma, etc., they are said to
reappear when the animals return to water to deposit their eggs.

Various functions are assigned to these sensory line organs, but
none has been clearly demonstrated. Probably they assist in recognizing
differences in the vibrations of the 'water and may permit the animal to
determine currents. They have been called a "sixth sense."

In the frog tadpole the sense organs of the lateral line are derived
from the placode of the X cranial nerve. The ramus lateralis of the X
nerve innervates the organ.

When the embryo is about four millimeters in length, there is a
small dorso-lateral section of the vagus ganglion separated from (though
lying close to) the ectodermal placode. The placode then begins to
elongate posteriorly, while the deep cells proliferate rapidly to form a
long, narrow tongue, which then pushes through the epidermis just out-
side the basement membrane. This tongue reaches as far back as the
tip of the tail by the time of hatching.

Along this line, groups of cells form at intervals, each group rep-
resenting the beginnings of a definite sense organ of the lateral line. In
each group there are a few central sensory cells surrounded by a layer
of enveloping cells. These groups then push up through the epidermis
to the surface of the body, and the sensory cells develop hair-processes.

There are other tegumentary sense organs developing in definite
rows on the head as well as dorsally from the mid-line to form those men-
tioned above, which are innervated by twigs from the VII, IX, and X
cranial nerves.

All of the lateral line organs disappear, however, when the tadpole
becomes an adult frog.

CHAPTER XLI.

THE DIGESTIVE TRACT

After the splitting of the mesoderm and notochord from the ento-
derm, the wall of the digestive tract is but one cell in thickness, except
in the region of the mid-gut, where the yolk-mass is very large.

The stomodaeum, already discussed, is a shallow depression just
below the olfactory and fore-brain region. This, by the time of hatch-
ing, while still very shallow, has its floor come to lie in contact with the
wall of the fore-gut. The region in which these fuse is called the oral
plate (Figs. 301, I, and 329, Pharyngeal membrane). It is at this point
that the mouth forms a few days after hatching. The oral sucker (Fig.
337) forms just below the stomodaeal invagination.

The margins of the mouth form mandibular ridges which become
drawn out as an upper and lower lip, the lower being the larger and
freely movable. (Figs. 316, 317, 318.)

Strands of cells from the deep layer of the epidermis push toward
the surfaces and as each cell arrives at the surface it becomes cornified
into a so-called "tooth." The upper lip has three rows of these
"teeth," while the lower has four rows. All of these are lost when the
tadpole assumes its adult shape. In the adult form, true teeth and jaws
form.

From the fore-gut the following structures are derived. (Figs. 328,
329, 337) :

The Pharyngeal Cavity, the large expansion in the anterior portion
of the fore-gut.

The Oesophagus, that portion of the fore-gut narrowed immediately
dorsal to the yolk.

The Visceral Pouches are seen as vertical solid foldings extending
to the surface ectoderm. Six pairs of these (compare this number with
those found in the chick) develop, increasing in size and importance pos-
teriorly.

The Visceral Arches, not to be confused with the pouches. The
arches are the vertical rods of mesoderm lying between the pouches.

The hyomandibular pouch is the name given the first pouch.

The Mandibular arch is the first arch, lying in front of the hyoman-
dibular pouch, or in other words, between the hyomandibular pouch and
the mouth.

The I to V Branchial, or gill pouches, are the remaining ones run-
ning posteriorly from the hyomandibular. The hyomandibular is the
first in point of position, but bears a separate name. The numbers thus
begin with the true second pouch.

586

THE EMBRYOLOGY OF THE FROG

a.k.'

The Hyoid Arch is the arch lying between the hyomandibular and I
branchial pouches.

Branchial Arches, or Gill Arches, are the names given to the remain-
ing arches.

The Branchial Clefts, or Gill Clefts, are the names given the various
pouches after they have opened to the surface of the ectoderm, and in-
ternally into the pharynx. The second and third clefts are the earliest
to become perforated, while the hyomandibular pouch does not become
perforated at all, but disappears shortly after the first perforations occur.

The Eustachian tube
or Tubo-tympanic cavity
of the ear, forms from the
dorsal wall of the hyo-
mandibular pouch.

The External Gills
(Figs, 337, 341), which
appear just before hatch-
ing, are small outgrowths
from the outer surfaces
of the dorsal ends of the
first and second branchial
(second and third vis-
ceral) arches. A small ex-
ternal gill also appears
later on the third bran-
chial arch. The two an-
terior gills grow rapidly
and form large lobed
processes by the time the
mouth opens. They be-
come vascular and form the first breathing or respiratory organs of the
tadpole. The posterior pair remain in a much more undeveloped state.
The Operculum (Fig. 341) is the name given to the covering of the
external gills. It makes its appearance before the mouth opens, growing
out from the posterior borders of the hyomandibular arches. These out-
growths extend backward so rapidly that by the time the anterior gills
have reached their maximum size, the operculum covers them in what
is called the opercular cavity.

The right opercular fold becomes fused with the body, while the left
remains partly open to form the opercular tube or spiracle.

The Internal Gills form as tiny elevations on the postero-external
faces of the branchial arches, just as the gill clefts are perforated. A
thin layer of ectodermal cells covers them, just as it does the external
gills. These gill coverings become doubled on the first three branchial
arches and remain a single row on the fourth branchial arch.

Fig. 341.

Ideal diagrammatic transverse section through a 13 mm.
frog tadpole, showing the first gill arch.

The right side shows the external gills (a.fc.) extending
through the gill opening which is almost closed. As the
opening closes the gills draw inward. On the left side the
gill opening is wide open with the external gills showing
themselves free, a.b., the first gill artery ; a.k., external
gills ; ao, aortic roots ; aud, otic capsule ; c.e., external caro-
tid ; d, pharynx ; s, lumen of the intestine ; filtr, anlage of
the filter apparatus (gill rakers); h, heart ; i.k., internal
gills ; nr, anlage of the neural canal ; op, gill operculum ;
v.b.c., common branchial vein ; v.b.e., external branchial
vein, v.b.i., internal branchial vein. (After Maurer.)

THE DIGESTIVE TRACT

58T

The Gill Filaments are the branched processes of the gills which
become quite vascular, and as they project into the opercular cavity, they
permit the exchange of gases from the respiratory current of water en-
tering the mouth, which current then passes through the gill clefts and
the opercular tube. n

th _ The Velar Plates and

Gill Rakers (Fig. 341,
filtr) form as tiny folds in
the pharyngeal region
which can be best under-
stood by a study of the
cuts. (The velar plates
are the smooth inturnings
of the floor of the pharynx
while the jagged tooth-
like foldings are the gill
rakers.)

The structures men-
tioned above are prac-
tically lost when the frog
reaches adultship, al-
though portions of these
structures are used in
building and forming
other structures which
will be discussed shortly.
The thymus body
(Fig. 342) appears just
prior to hatching. It is a
solid internal proliferation
from the epithelium of the
upper side of the first
branchial pouch (second
visceral, or hyobranchial
pouch). There is also a
similar outgrowth from the hyomandibular pouch, but this disappears in
a short time. The outgrowth which is to become the thymus grows
slowly and separates entirely from the wall of the pouch when the tad-
pole is about twelve millimeters in length.

Both permanent and transient thymus bodies lie close to the outer
surface of the head, immediately behind the auditory capsule and the
articulation of the jaw.

Epithelioid bodies are similar to the thymus glands. These are
formed from the dorsal ends of the other branchial clefts and become
lymphoid in character.

D

Fig. 342.

I. Diagram of the branchial pouch derivatives in the
frog. (After Maurer, with Greil's modification.) eg. Carotid
gland ; ev e^, e3, epithelioid bodies ; th, thyroid body ;
tmlt tmy, thymus bodies ; ub, ultimobranchial body ; I-V1,
first to sixth visceral pouches. (7, hyomandibular ; II-V1,
first to fifth branchial pouches.)

II. Diagrams of the derivatives of the visceral pouches
and arches in the frog. A. Lateral view, frog larva. B.
Lateral view, after metamorphosis. C. Transverse section
through gill of frog larva. D. Transverse section through
gill region, just after metamorphosis ; the gills have not
quite disappeared, a, Afferent branchial arteries ; c, carotid
gland ; d, dorsal gill remainder ; e, epithelioid bodies ; g,
internal gills ; m, middle gill remainder ; o, operculum ; «,
suprapericardial or postbranchial body ; t, thyroid body ; th,
thymus bodies ; v, ventral gill remainder ; I-VI, visceral
arches ; /, mandibular arch ; II, hyoid arch ; III-VI, first to
fourth branchial arches ; 1-6, visceral pouches ; 1, hyo-
mandibular pouch ; 2, hyobranchial pouch ; 3-6, first to
fourth branchial pouches. (Greil's Modification of Maurer.)

588

THE EMBRYOLOGY OF THE FROG

The Carotid Glands form from the ventral ends of the anterior gill
pouches as epithelial proliferations at about the time the internal gills
appear.

The Pseudothyroid Body is a small outgrowth in the postero-ventral
branchial region, apparently having no relation with the disappearing
gill-clefts. This body disappears with the exception of traces of the mid-
dle and ventral portions, which persist for a short time after metamor-
phosis, but then they, too, disappear.

The Ultimobranchial Bodies
(also called post-branchial or supra-
pericardial bodies) lie posterior to
the fifth visceral pouch. They have
formed as solid proliferations from
the pharyngeal wall just behind the
visceral (fifth) pouch, (fourth
branchial pouch). These bodies are
supposed to represent vestigial por-
tions of a sixth pair of visceral
pouches, although they do not ex-
tend to the surface ectoderm. They
separate from the pharynx and
acquire a lumen, coming to lie on
the floor of the pharynx in a supra-
pericardial position. (Fig. 342.)

The Thyroid Body is formed as
a medial invagination from the floor of the pharynx just a short time
before hatching. It forms as a solid rod of cells, but a few days after
the opening of the mouth it forms a pair of bodies which grows very
rapidly and becomes very vascular. The thyroid body has no genetic
relationship to the branchial, structures nor do any of the following
structures possess such relationship.

The lungs. These develop just before hatching as a pair of solid
proliferations from the ventral wall of the posterior portion of the fore-
gut, just between the yolk-mass and the heart. The cavities begin to
form in the proximal region.

The Laryngeal Chamber (Fig. 332, B) is formed by the wall of the
fore-gut between and around the lung diverticula which become de-
pressed and form a groove, which then (at least partially) constricts off
from the alimentary tract.

The Glottis is the opening which remains in the laryngeal chamber
as it constricts.

The Tongue appears just before metamorphosis as an elevation in
the floor of the anterior portion of the pharynx, just behind the thyroid
region. There is a glandular depression directly in front of the elevation

Fig. 343.

From a model of the duodenum and the
primary evaginations of the liver and pan-
creas in a 5 mm. sheep embryo. D.pan., Dor-
sal pancreas ; Du., duodenum ; D.ch., ductus
choledochus ; G.bl., gall bladder ; H.du., hepatic
duct. (After Stoss.)

Tm«; !)I(;I-:STIVE TRACT 589

which is carried forward by the anterior growth of the tongue, so that
the glandular portion becomes the tip of the tongue.

The Liver is one of the earliest diverticula of the alimentary tract,
appearing even before the embryo itself has begun to elongate. It lies
beneath the yolk mass and develops from the ventral portion of the fore-
gut, just posterior to the heart. A group of scattered mesodermal cells
lies between' liver and heart, which will soon be added to the anterior
wall of the liver rudiment. The anterior portion of the liver becomes
folded after hatching.

The Gall-bladder is formed as a postero-ventral extension of the
liver diverticulum which becomes more or less separated from the an-
terior portion of the liver.

The Bile-duct (Fig. 332) is the original opening of the liver divertic-
ulum into the alimentary tract from which it grew.

The Pancreas (Figs. 343, 293) develops in close proximity to the
liver diverticulum from three separate rudiments, a dorsal and two ven-
tral. The dorsal rudiment is a solid outgrowth from the dorsal wall
of the fore-gut. A complete separation between outgrowth and origin
soon takes place.

The right and left ventral rudiments grow out of the fore-gut at the
posterior margin of the bile-duct. These retain their connection with
the gut, enlarge, and after passing around the bile-duct, fuse together
in front of it. The dorsal portion later also fuses with this fused right
and left portion and connects with the gut by the pancreatic duct. At
this period, the pancreatic duct forms the boundary between the fore-
gut and the mid-gut, although later the pancreatic duct comes to lie
within the margin of the bile-duct. Oesophagus and stomach develop
quite as they do in the chick. It will be remembered that the oesophagus
closes for a time in the chick. It does likewise in the frog just after
hatching, when the tadpole is about eight millimeters in length.

The oesophagus, however, again opens just before the mouth is
formed, that is, when the tadpole is about ten or eleven millimeters
long.

The stomach is at first longitudinal, but it soon bends, and comes
to lie transversely, a matter of importance when nerves are to be traced,
for the nerves and blood vessels which are paired and known as right
and left will now be dorsal and ventral, depending upon the side toward
which the caudal end of the stomach turns.

THE DERIVATIVES OF THE MID-GUT

As with the chick, the mid-gut is confined to that more or less cen-
tral space where the yolk continues being absorbed and converted into
other substances in the growing embryo and tadpole. After hatching,
the yolk is absorbed very rapidly, some of the yolk-cells becoming the
epithelial lining of the growing intestine. The intestine is bent into a

590 THE EMBRYOLOGY OF THE FROG

transverse loop called the duodenal loop, with its proximal end at the
posterior portion of the stomach. In fact, the intestine grows to almost
nine times the length of the tadpole's body, but is shortened later to
about one-third the body-length by the time metamorphosis takes place.
The Hypochordal rod is a structure which forms from the mid-gut
as a medial ridge along the surface of its entodermal wall just under-
neath the notochord, although it has no relation to the notochord. It ap-
pears when the tadpole is three or four millimeters in length, and ex-
tends both cephalad and caudad, coming to lie free of the gut wall when
the tadpole reaches a length of about 4.6 millimeters. Finally, it can
be seen as a caudal extension from the dorsal pancreas posterior through
the tail. The rod itself is narrow, being only two or three cells in diam-
eter. Just before the opening of the mouth, it breaks into pieces and
disappears entirely.

THE DERIVATIVES OF THE HIND-GUT

It is in this region that the neurenteric canal is found .(Fig. 328),
as well as the proctodaeum. The terminal end of the hind-gut, which
is to become the rectum, fuses with the proctodaeum, and the anal open-
ing perforates the thin sheet which has separated these two structures.
This fusion and perforation takes place at about the time the tail begins
to elongate, namely, when the embryo is about four millimeters in length.
It is the proctodaeal region of both tadpole and frog which forms the
cloaca into which rectal, excretory, and reproductive ducts enter. The
Bladder forms as a ventral outgrowth from the cloaca just before meta-
morphosis.

The Post-Anal gut is formed by the true hind-gut remaining within
the embryo body proper as the tail continues growing. The nerve cord
and notochord, however, are carried along in the growing tail so that
the neurenteric canal is drawn out caudad. This neurenteric canal is
then cut off from the nerve cord, but for a short period its antero-ven-
tral portion opens into the rectum and it is this which is known as the
post-anal gut. This gradually closes, although a strand of cells can still
be seen in the region at the time of hatching, extending nearly to the tip
of the tail. It finally disappears altogether.

CHAPTER XLII.

THE MESODERMAL SOMITES

All the remaining systems to be described are intimately related to
the mesoderm.

The mesodermal region, in the chick, posterior to the head, divides
into block-like segments by the formation of connective tissue septa
which form at right angles to the long axis of the embryo. So, too, in
the frog embryo and tadpole, the mesoderm divides into block-like seg-
ments. These are the somites. And, just as with the chick, so with
the frog embryo, the mesoderm divides into an outer somatopleure and
an inner splanchnopleure layer with an open space between them known
as the myocoele.

That portion of the somites which lies directly on each side of the
notochord is known as the segmental or vertebral plate, while the por-
tion extending laterad from this segmental plate is called the lateral
plate.

The lateral plates are therefore merely direct continuations of the
segmental plates. However, as the somites divide off into blocks, the
vertebral plates become thickened and not only is the myocoele closed
which lies within them, but a constriction along the long axis of the
embryo separates the vertebral plate from the lateral plate.

As there are no somites in the head region, the mesoderm lies in
the form of scattered groups of mesenchymal cells in the head and
pharyngeal regions.

The somatopleure and splanchnopleure are not of equal thickness.
The outer somatopleure is only one cell in thickness, while the inner
splanchnopleure is much thicker. ' Therefore, the coelom which lies be-
tween these two layers, lies much closer to the outer portion of the em-
bryo than to the inner portion.

The one cell-layer which forms the outer somatopleure in the region
where the somites have formed is naturally segmented, as that layer is
an actual portion of the somite proper. These one cell-layered segments
of somatopleure are now called dermatomes or cutis plates, because they
will soon join with the ectoderm lying immediately above them to form
the outer wall of the embryo.

The segments of the splanchnopleure which lie toward the center
of the embryo are called myotomes, or muscle plates, because it is from
these that the muscle cells will form. In fact, the formation of the mus-
cle fibrils can already be observed when the embryo is scarcely five milli-
meters in length.

It is the thickening of the myotomes which obliterates the myocoele
quite early.

592

THE EMBRYOLOGY OF THE FROG

From the lower ends of each myotome, that is, from those regions
lying ventral and close to the mid-line, cells proliferate and move down-
ward beneath the notochord as well as upward between the notochord
and the myotome. These proliferations form the sclerotomes which are
to become the cartilaginous vertebral column.

Very early, that is, when the tadpole is only about rive millimeters
long, the somite proper separates from the lateral plates, and the sclero-
tomes separate from the lateral plates.

Immediately after this separation, there are ventro-lateral out-
growths from both myotomes and dermatomes. Those from the myo-
tomes become the ventral musculature and extend into the limbs as
voluntary muscles, while that from the dermatomes break up into groups
of mesenchymal cells, some of which become connected with the inner
surface of ectoderm, and form the dorsal covering of the embryo, while
some pass between the myotomes to form the connective tissue septa,
also called myocommata. (Fig. 423.)

There are thirteen pairs of somites formed in the trunk of the frog,
the two most anterior pairs disappearing in the adult. The full-fledged
frog, therefore, has but eleven definite segments. The two anterior
somites become part of the occipital region of the head. In the tadpole,
there are many more somites than the amount stated above, but with
the loss of the tail and the conversion of tadpole into frog, these are lost.

The following table will not only summarize the history of the
somites and spinal nerves, but if carefully studied will show how the
adult vertebral musculature which connects the posterior half of one
vertebra with the anterior half of the next succeeding one, receives its
innervation from more than one spinal nerve. A clear understanding
of this will help the student very materially in his comparative anatomy.
(See Fig. 336 also.)

TABLE OF SOMITES, VERTEBRAE, AND RELATED NERVES

OF THE TADPOLE

(From Kellicott, after Elliot.)

Cartilaginous
Elements In
Sclerotome

SOMITES

NERVES

Embryo Adult

Adult

Occipital
Region of
Skull

1

Absent (Disap-
pears at forma-
tion of limbs)

Root of vagus nerve

2

Absent (Disap-
pears at forma-
tion of vertebrae)

No nerve. Ganglion
in embryo only

THE MESODERMAL SOMITES

593

3

1

Ganglion and Nerve
in embryo
Absent in adult

1 Vertebra

4

2

1 Spinal Nerve (Hypo-
glossal)

2 Vertebra

5

3

2
Brachial Plexus
3

3 Vertebra

6

4

4 Vertebra

7

5

4
5 to body wall
6

5 Vertebra

8

6

6 Vertebra

9

7

7 Vertebra

10

8

7
8 Sciatic plexus
9

8 Vertebra

11

9

9 Vertebra

12

10

Part of
urostyle

13

11

10 to pelvic region

Even before the lateral plates are separated from the somites, there
is a tendency to segment in the lateral plate itself, close to the somite.
It is in this region that the excretory system is to be formed, and so
these portions are called nephrotomes, or the intermediate cell mass.
(Fig. 268.)

This mere trace of segmentation in the lateral plate lasts a very
short time. In fact, the lateral plate itself never segments.

The cavity within the lateral plates is the true coelom ; and, just
as in the chick, when the lateral plates extend further and further ven-
trally, they finally meet in the midline and fuse, thus making a single
coelom where there had been one on each side before.

This ventral fusion forms the ventral mesentery, which soon disap-
pears except in the heart region. The paired coelomic cavities also come
together dorsally between the notochord and the digestive tract, and
fuse to become the dorsal mesentery, which remains as a suspensory
arrangement for the gut (Fig. 293). The gut itself later sinks more and
more ventrally and the mesentery is pulled ventrad with it. The blood
vessels supplying the digestive tract then grow downward through these
two layers of mesentery.

CHAPTER XLIII.

THE CIRCULATORY SYSTEM
THE HEART

Immediately cephalad to the liver, just ventral to the hinder part of
the pharynx "(Fig. 328), is the heart region. Here the somatic and
splanchnic layers of mesoderm have separated by a wide cavity which
is to become the pericardial cavity (Fig. 344), continuous for a time with
the general body cavity, but later closed off.

cnd.cartt. mes.mrti.v.

Fig. 344.

Cross sections of frog tadpole. A and B in the region of the anterior and
posterior portions of the heart respectively ; C, through a more or less mid-
section of a more advanced tadpole, for comparative purposes, ect, ectoderm ;
end.card., endocardium ; ent, entoderm ; mes.card.d. and mes.card.v, dorsal
and ventral mesocardium ; par, somatic layer of mesoderm (somatopleure) ;
per. card., pericardial cavity; vise, visceral layer of mesoderm (splanch-
nopleure ) . ( From Rudneff . )

Both pericardial wall and muscular wall of the heart are derived
from the lateral plate mesoderm, while the endothelium — the inner lining
of the heart — arises from scattered mesodermal cells lying between the
splanchnic mesoderm and the digestive tract. These cells can be seen in
the two somite stage.

From Figure 344 one can get a good understanding of how the lat-

THE CIRCULATORY SYSTEM 595

eral plates extend beneath the pharynx and fuse in the midline to form
the ventral mesocardium.

The splanchnic layer of the pericardium becomes folded dorsally so
as to enclose the endothelial cells which have formed a more or less
short tubular arrangement. Then the folds meet and fuse dorsally to
form another tube on the outside of the endothelial tube. These latter
tubes thus enclose the endothelial tubes, and are connected with the
dorsal wall of the pericardial cavity. The connection forms the dorsal
mesocardium. The outer tube forms the muscle of the heart walls.

Much difficulty will be avoided in our later studies if, after a review
of the chick's circulatory system, the following account of the heart be
mastered :

The paired endothelial tubes fuse together cephalad to form the
bulbus aortae. Then there are outgrowths which form the beginnings
of the truncus arteriosus or ventral aortae. The endothelial tubes do
not fuse entirely in the posterior region, and are not alike on each side.
The right one is bent and forms the beginnings of the ventricle and the
right vitelline vein.

The left one is slightly longer and larger in diameter in the more
naudal portion. This is to become the auricle, while its continuation in
a caudal direction will become the left vitelline vein. Both vitelline
veins are connected directly with the yolk-mass and the liver.

As the embryo develops, the two endothelial tubes fuse quite like
the letter S, after which the dorsal mesocardium disappears, so that, as
in the chick, the heart tube is attached only at its two ends.

The heart now comes to have its caudal end toward the left side
and resting against the liver. This caudal portion is the region of the
sinus venosus and the auricles.

However, as the heart continues growing much more rapidly than
the surrounding portions, and as it is attached only at its two ends, the
cephalic region of the heart and the ventricle region swing downward
and come to lie ventral in position. This naturally forces the auricle
dorsad, and the auricle thus comes to occupy the greater portion of the
dorsal side of the adult heart.

Constrictions separate the heart early into two limbs, but it is only
after the tadpole's mouth has opened that the auricle is divided into
right and left halves by the inter-auricular septum which grows ventrad
from the dorsal wall. The sinus venosus remains connected with the
right auricle, while the pulmonary veins later enter the left auricle. The
pulmonary veins can hardly be seen during the tadpole stage.

The ventricle walls thicken, and a few days after the mouth opens,
the bulbus aortae divide into an anterior and a posterior portion. The
anterior portion is called the truncus arteriosus. This truncus arteriosus
is also divided into right and left channels.

596

THE EMBRYOLOGY OF THE FROG
THE ARTERIAL SYSTEM (Fig. 345)

The Lateral Dorsal Aortae, which are the first to appear, are paired
and lie dorsal to the pharynx. They are formed from a series of sepa-
rate spaces in the mesenchyme of the head region which then connect
and form the vessels of the cranial region.

The Dorsal Aorta (Fig. 345, ao) is the name given the lateral dorsal
aortae as soon as they fuse medially on the dorsal side of the embryo tc
form a single vessel. This vessel extends to the caudal extremity of the
embryo.

The Aortic Arch. An aortic arch forms in each branchial arch, firsl
as a little vascular space when the embryo is about 4.5 millimeters long

Diagrams of the heart and chief arteries of a tadpole. A, The vessels
of a tadpole at the stage when three external gills are present ; B, the
arrangement when secondary gills are in use ; C, the adult arrangement.
o.c., Anterior commissural vessel ; a.cb., anterior cerebral artery ; a/.,
afferent branchial arteries ; ao., dorsal aorta ; car., carotid artery ; e.g.,
carotid gland ; cu., cutaneous artery ; d.b., ductus Botalli ; ef., efferent
branchial arteries ; ht., heart ; hy., efferent hyoidean artery ; {., connecting
vessel ; I., lingual artery ; md., efferent mandibular artery ; p.c., posterior
commissural vessel ; pl.c., pulmocutaneous arch ; pul., pulmonary artery ;

sys., systemic arch; tr., truncus arteriosus ; v., ventricle; I-IV., branchial

aortic arches. (After Bourne.)

This space connects ventrally with the truncus arteriosus and dorsall}
with the lateral dorsal aorta (Fig. 341, ab). It is now called an aortic
arch. Thus there are four pairs of these aortic arches, being in reality
the third to sixth pair, and the first and second are formed in the mandib
ular and hyoid arches.

The Afferent Branchial Arteries (Fig. 345, af). As the externa

Tin-: CIRCULATORY SYSTEM 597

gills develop, a vessel forms dorso-laterally to the aortic arch, along the
base of the gill to supply it. The vessel opens out of the ventral end
of the aortic arch only to join it again at its upper end. The lower end
of the aortic arch is then called the afferent branchial artery.

The Efferent Branchial Arteries (Fig. 345, ef). These are but the
dorsal ends of the aortic arches. Loops of tiny capillaries formed in the
external gill and connect afferent and efferent vessels. Later, after the ex-
ternal gills are lost and the internal ones developed, part of the aortic
arches disappear, so that afferent and efferent vessels are connected by
vessels of the internal gills. This causes the original aortic arch to be-
come almost entirely an efferent branchial artery, while the vessel which
connected ventral and dorsal ends of the aortic arch becomes the afferent
branchial artery (Fig. 345).

As the internal gills disappear when the tadpole becomes a frog, the
lower end of the efferent branchial artery, which is the original aortic
arch, again acquires a direct connection with the afferent branchial artery
so that the blood again passes from truncus arteriosus to dorsal aorta.

The gill capillaries diminish and the connection which has thus been
re-acquired becomes larger, and forms the adult persistent vessels of the
branchial arches. As the fourth branchial arch has no external gills de-
veloped upon it, the part described above regarding external gills does
not apply to it. In all other respects, its blood supply is similar to those
which do have external gills.

The above description applies to Rana esculenta, which, although
different in detail, is essentially alike in all species.

The following account of the mandibular and hyoid vascular ar-
rangement is for Rana temporaria:

Hyoidean Vein. This is a small outgrowth of the lateral dorsal
aorta which extends toward, but never reaches the vessel which repre-
sents the aortic arch of the hyoid arch. It disappears at the time the
mouth opens.

At the time of hatching there is a small outgrowth from the truncus
arteriosus extending into the lower end of the hyoid arch, but this, too,
disappears shortly, though it is at this time that the vestige of the
aortic arch has already become divided into dorsal and ventral portions.
The dorsal portion also disappears, and the ventral becomes the hyoidean
vein. This disappears with the oral sucker.

Pharyngeal Artery. Just before hatching, the vessels of the mandib-
ular arch appear. Here," too, there is a vestigeal aortic arch in the lower
portion of the mandibular arch, which soon unites with an outgrowth
from the lateral dorsal aorta. After union, this vessel joins the hyoidal
vein.

After the mouth opens, the outgrowth from the lateral dorsal aorta
separates from the other vessels, growing forward. It is then known

598 THE EMBRYOLOGY OF THE FROC.

as the pharyngeal artery, while, as mentioned above, the hyoidean vein
disappears with the oral sucker.

The Anterior, or Internal Carotid Arteries (Fig. 345, car). These
are the extensions of the lateral dorsal aortae into the head.

Commissural arteries (Fig. 345, ac). These are the two transverse
connections between the internal carotids, which pass anterior and pos-
terior to the infundibulum.

Lingual, or External Carotid Arteries (Fig. 345, 1.). These appear
before the time of the mouth opening as a pair of sinuses in the buccal
cavity. As the mouth opens, they extend backward to connect with the
ventral ends of the efferent branchial arteries of the first branchial arch
at the point where the carotid gland is to develop.

Changes in Aortic Arches. As soon as the gills disappear, there
must naturally be a great modification in the branchial aortic arches
(Fig. 345).

Carotid Arch. As already stated, a continuous aortic arch is re-
established in each of the four branchial arches when the afferent and
efferent arteries fuse. The first branchial aortic arch, which is the third
arch of the entire series, remains as the carotid arch.

Systemic Arch. The lateral dorsal aorta between the carotid arch
and the aortic arch immediately posterior to it, is reduced to a solid
strand of connective tissue and no longer functions, so that the second
pair of aortic arches (the fourth of the whole series) form the roots of
the dorsal aorta and are called the systemic arches.

Pulmocutaneous Arch. The third aortic arch (the fifth of the whole
series) also becomes a solid strand of tissue and then disappears entirely,
while the fourth (the sixth of the whole series) becomes the pulmo-
cutaneous arch.

Pulmonary Arteries. These appear just after hatching as small out-
growths from the upper ends of the efferent branchial arteries of the
fourth branchial arch, which then extend backward to the lung rudi-
ments.

Cutaneous Arteries. These leave the pulmonary arteries and extend
dorsally to spread out over the skin of both sides and back.

Ductus Botalli. This, it will be remembered, from our study of the
chick, is that part of the fourth aortic arch between the lateral dorsal
aorta and the origin of the pulmonary arteries (Fig. 345). This portion
slowly atrophies and also becomes a solid strand. Three channels are
now formed in the truncus arteriosus by various septa which divide it
longitudinally. The carotid arches lead from one of these channels,
which receives blood from the left side, while another carries venous
blood from the right side of the heart to the pulmocutaneous arches.
The remaining one connects the systemic arches.

THE CIRCULATORY SYSTEM

599

ORIGIN OF THE CIRCULATORY SYSTEM AND THE BLOOD

About the time of hatching, outgrowths of the dorsal aorta, just
back of the pharyngeal region, extend laterally into the region of the
pronephros or head kidney. These later become very large and form
the vascular glomi of the kidney, traces of which remain long after the
pronephros itself has disappeared.

The origin of the blood itself is by no means well established in the
frog. The student should review the work on the chick in this respect.
All we can say in regard to the frog is that when the embryo is almost
ready for hatching, that is, when it is four to 4.5 millimeters in length,
irregular spaces appear in the mesenchyme and splanchnic mesoderm,
which later form continuous vessels. The corpuscles may arise as out-
growths from the endothelial lining of the blood vessels themselves, or
from blood-islands which form on the ventral side of the yolk mass, or
they may possibly form from both these sources.

Fig. 346.

The development of the posterior part of the venous system in the
frog. A. Portion of a transverse section through the posterior meso-
nephric region of an 18 mm. tadpole. B. Diagram of the veins of a 25-30
mm. tadpole. C. Diagram of the veins of the adult frog, a, Dorsal aorta;
c, vena cava ; e, nuclei of the endothelial lining of the mesopheric sinus,
continuous with the vascular endothelium ; /, femoral vein ; i, iliac vein ; Ic,
lateral mesonephric channel of the posterior cardinal vein ; m, mesentery,
mn, mesonephros ; n, mesonephric tubules ; p, posterior cardinal veins (in
C showing their original location ) ; pv, pelvic vein ; rp, renal-portal vein ;
rr, revehent renal veins ; sc, sciatic vein ; st, nephrostome ; u, caudal vein ;
vcm, median mesonephric channel of the posterior cardinal vein ; W,
Wolffian duct ; x connection between caudal vein and the lateral meso-
nephric channels ; 1-1, part of the renal-portal vein formed from the 'lateral
channel of the posterior cardinal ; 2-2, part of the renal-portal vein formed
from the median channels of the posterior cardinal vein. (After Shore.)

THE VENOUS SYSTEM (Figs. 308, 346)

Omphalomesenteric, Vitello-intestinal, or Vitelline Veins. These
veins are really the first part of the vascular system to form in the re-

600 THE EMBRYOLOGY OF THE FR<><;

gion of the blood-islands. They are paired but not alike on both sides.
They pass along the lateral surface of the yolk and liver, and enter the
sinus venosus. In fact, the sinus venosus is really formed by a fusion
of these vessels from each side.

Ductus Cuvieri or Cuvierian Sinuses. These are a pair of large veins
which enter into the sinus venosus also and may even form part of that
organ. They come from the body-wall opposite the sinus venosus.

Hepatic Vein. As the liver develops, the omphalomesenteric veins
which pass through that organ break up into capillaries within the sub-
stance of the liver. Then the parts of both omphalomesenteric veins,
which lie between the liver and the heart, fuse into a single hepatic vein.

Hepatic Portal Vein. The right omphalomesenteric vein disappears
caudad to the liver, while the left partly remains as the root of the future
hepatic, portal vein. This vein will later receive branches from the
digestive tract as well as from those organs which have arisen from the
digestive tract.

Anterior Cardinal Veins. As the ducts of Cuvier pass dorsad to the
dorsal body wall, they divide. One branch passes headward as the an-
terior cardinal vein.

Superior Jugular Veins. This is the name given the anterior cardi-
nal veins as they pass forward into the head, where they drain the brain
and the dorsal portions of the head.

Inferior Jugular Veins. These drain the mouth, sucker, and ventral
surface of the head, and open into the roots of the duct of Cuvier just
before these in turn enter the sinus venosus.

Posterior Cardinal Veins. These are the posterior or caudal por-
tions of the divided ducts of Cuvier, and are primarily the drainage sys-
tem of the body-wall and excretory system. They pass caudad along the
medial side of the pronephric ducts and receive the veins from the body-
wall known as the segmental veins. The posterior cardinal veins form
large sinusoids in the region of the pronephros, but as the metanephros
develops, all this is modified, so that at fifteen millimeters the caudal
ends of the veins fuse to form the single median cardinal vein.

Caudal Vein, This begins at the tip of the tail and drains that
region. It is unpaired, but upon reaching the body cavity, divides above
the cloacal region, and then empties into the posterior cardinal veins.

Posterior or Inferior Vena Cava, or Postcaval Vein. This begins
as a branching of the left omphalomesenteric vein lying dorsal to the
liver. From here, the postcaval vein passes through the suspensory fold
of the liver to the right posterior cardinal vein and connects with it just
anterior to the point where the median cardinal vein begins. This vessel
enlarges rapidly and becomes the largest blood vessel in the body. It
passes through the liver to the sinus venosus. The hepatic vein then
opens into it instead of into the sinus venosus as formerly.

Anterior, Superior, or Precaval Veins. The pronephric portions of

THE CIRCULATORY SYSTEM 601

the postcaval veins degenerate as the pronephroi degenerate, and ulti-
mately disappear entirely, even before metamorphosis is complete These
leave the ductus Cuvieri as the proximal portions of the anterior cardi-
nal veins, and it is these remaining proximal portions which are called
the anterior, superior, or precaval veins. All blood from the posterior
parts of the body-wall and from the tail now passes directly to the heart
through the median cardinal and postcaval veins.

Iliac Veins. The pronephroi are followed by the mesonephroi as
in the chick, and an alteration in the relation of the median cardinal vein
follows. On each side of the body the developing mesonephroi push
into the median cardinal vein, so that this vein is divided into one me-
dian and two lateral parallel channels. The caudal vein empties into
the median channel and finally disappears, and the iliac veins which
:ome from the hind-legs open into the lateral channels. It is the iliac
ve'ms which become the chief vessels leading to the mesonephric region
after the caudal vein disappears.

Adult Venous System. After an understanding of the formation
and change which takes place in the venous system during the embryonic
period, the adult system can be understood.

Afferent or Advehent Mesonephric Veins, or Renal Portal Veins.
These are merely the iliac veins, together with the lateral channels of
the median cardinal vein, with which they are continuous.

Posterior Vertebral Veins. These are the small veins from the pos-
terior body wrall which open into the renal portal vein.

Renal Veins, or Revehent Mesonephric Veins. These are the short
connecting vessels which connect the vascular space in the mesonephroi
with the median channel of the median cardinal vein, so that only this
median channel remains as a posterior continuation of the postcaval
vein.

Lateral Veins. A pair of these develop late in the ventral abdominal
walls, and open into the sinus venosus. These connect with the iliac
veins posteriorly, then fuse medially.

Anterior Abdominal Vein. The anterior ends of the lateral veins
lose their connection with the sinus venosus, while the anterior portion
of the right lateral disappears entirely. The left lateral vein forms a
new connection with the hepatic portal vein, and is then called the an-
terior abdominal vein.

Pulmonary Veins. These can be seen when the tadpole is about
six millimeters in length as projections of the endothelium on the dorsal
side of the sinus venosus. These projections form a tube,* opening proxi-
mally into the left side of the auricle, which distally leaves the wall of
the sinus venosus, and passes dorsally to the lung rudiments. This tube
bifurcates at the base of the lungs, where each branch then passes along
the medio-ventral side of the lung rudiment. After the lungs begin to
function, the pulmonary veins empty into the left auricle.

602

THE EMBRYOLOGY OF THE FROG
THE LYMPHATIC SYSTEM

By the time the tadpole is 6.5 millimeters in length, one may see
a single pair of "lymph hearts" (Fig. 11). They are sac-like, and grow
out of the intersegmental veins, usually from the fourth pair. That is,
they are outgrowths from the veins which run between the fourth and
fifth myotomes. These "lymph hearts" empty into the posterior cardinal
veins at the more caudal end of the pronephros.

The "hearts" themselves lie between the peritoneum and the outer
covering, and below the level of the myotomes. The endothelial lining
of the "hearts" and the blood vessels is continuous.

The "beating" of the lymph hearts is due to a syncytial layer or net-
work of striated muscle fibers immediately outside of the endothelium.
The "beating" begins about the time the mouth opens.

A short time after hatching, that is, when the tadpole is about 7.5
to 8.0 millimeters in length, two lymphatic vessels develop from each
heart. They are known as anterior and posterior lymph vessels. They
follow the lateral nerve in direction, the anterior vessel extending into
the head, and the posterior along the sides of the trunk. Valves guard
the openings of the lymph vessels into the "hearts" as well as into the
veins where they empty.

Fig. 347.

Frog. A, showing anterior lymph-hearts, from the dor-
sal side. B, showing posterior pair of lymph-hearts seen
from the ventral side, gl, gluteus muscle ; ic, iliococcygeal
muscle ; L, lymph-heart ; Is, levator scapvlae muscle ; N,
spinal nerve; p, piriformis muscle: r, vastus muscles; ta,
transverse scapularis major muscle ; ve, vastus externus
muscle; 1-5, vertebrae. (After Wiedersheim.)

Immediately after these lymph vessels begin growing, they develop
a rich network of capillaries which spread out in all directions, being
greatest in number close to the skin. Later, as the tadpole becomes
quite large (about twenty-six millimeters), the lymphatic system be-
comes well developed.

The anterior lymph vessel, running downward and forward, connects
with a large lymph sinus, around the mouth, heart, and branchial
region. The posterior vessel passes caudad into the tail, and there
divides into dorsal and ventral branches. These dorsal and ventral

THE CIRCULATORY SYSTEM

603

branches of each side then unite to form two large vessels which extend
through the tail, one of them above and the other below the myotomes.

The walls disappear in the network that has grown out from the
lymphatic vessels to form the large subcutaneous lymph sacs already
noticed in the dissection of the adult frog.

The thoracic ducts extend posteriorly from the "lymph hearts" and
are probably outgrowths from them. They lie between the dorsal aorta
and the posterior cardinal veins.

The posterior lymph hearts (Fig. 347) — (one to three pairs in num-
ber)— develop from the intersegmental vein just as did the anterior
hearts, but their development is postponed until the hind-legs appear.

Fig. 348.

Diagrams to illustrate the divisions of the coelom in the various vertebrate
classes. The transverse septum and its derivatives are indicated by thick lines.
A, fishes, showing the division of the coelom into pericardial cavity a, and pleuro-
peritoneal cavity g, by means of the transverse septum d. B., urodeles ; similar
to fishes with the addition of the lung h which projects into the pleuroperitoneal
cavity g. C, turtle ; the pericardial cavity a has descended posteriorly until it
lies ventral to the anterior part of the pleuroperitoneal cavity g ; the anterior face
of the transverse septum, d, has now become part of the wall of the pericardial
sac ; the lung, h, is retroperitoneal. D, early stage of Mammals, showing the
beginning of the coelomic fold (pleuroperitoneal membrane), j, descending from
the dorsal body-wall, and the liver, /, enclosed within the transverse septum, d.
E, later stage of mammals, showing union of the coelomic fold, j, with the
transverse septum d, the two together forming the diaphragm which separates the
pleura! cavity k from the peritoneal cavity, m; the liver has constricted from the
main part of the transverse septum, the constriction becoming the coronary liga-
ment, i, a, pericardial cavity ; 6, heart ; c, parietal pericardium or pericardial sac ;
d, transverse septum ; e, serosa of the liver, this being a part of the transverse
septum originally ; /, liver ; g, pleuroperitoneal cavity ; h, lung ; i, coronary liga-
ment of the liver ; j, coelomic fold which forms part of the diaphragm ; k, pleural
cavity ; I, pleuropericardial membrane or anterior continuation of the transverse
septum; m, peritoneal cavity. (From Hyman's "A Laboratory Manual for Com-
parative Vertebrate Anatomy," by permission of the Chicago University Press.)

604 THE EMBRYOLOGY OF THE FROG

Their first openings are into the posterior cardinal vein, which means
that later they empty into the renal portal veins.

The Spleen. This ductless gland is first seen in the developing tad-
pole at about ten millimeters. It appears as a mass of mesenchymal
lymphoid cells in the mesentery, immediately dorsal and posterior to the
stomach and around the mesenteric artery.

These cells then multiply and project from the mesentery so as to
have a peritoneal covering, as do all the organs in the body cavity.
Later, as the spleen enlarges, there seem to be various wandering cells
from the intestinal epithelium added to it. The spleen is complete by the
time the tadpole is twenty-five to twenty-seven millimeters in length
and is extremely vascular.

THE SEPTUM TRANSVERSUM (Fig. 348)

The pericardial cavity, already discussed, has remained open pos-
teriorly into the abdominal cavity, with the exception of the region
covered by the liver. Now, as the ducts of Cuvier form and pass from
the body-wall to the sinus venosus, they pass through this open region
and carry with them incomplete peritoneal folds from the body-wall.
These folds are called the lateral mesocardia. They remain incomplete
dorsally for a long time, but gradually extend ventrally so as to form a
complete separation between pericardial and peritoneal cavities This
transverse partition is called the pericardio-peritoneal septum or the
septum transversum. To this septum transversum is added a medial
portion of peritoneum from the anterior face of the liver, while on the
right side the septum becomes continuous with the posterior suspensory
fold of the liver commonly called the mesohepaticum.

After metamorphosis, the septum unites dorsally with the dorsal
mesentery and completes the separation between pericardial and peri-
toneal cavities.

CHAPTER XLIV.

THE UROGENITAL SYSTEM

From our study of the urogenital system of the chick, we learned
that while in birds and mammals, a pronephros, a mesonephros, and a
metanephros form, in amphibians the first two forms of nephridic organs
alone make their appearance, the mesonephros then remaining as the
permanent adult functioning kidney.

We have already spoken of the nephrotomes, which are also called
the intermediate cell mass (Fig. 268).

Some time before hatching, when the embryo is only about three
to four millimeters in length, this intermediate cell mass can be seen as
longitudinal thickenings on each side of the notochord. These thicken-
ings then form a groove, the lips of which soon fuse to form a tube or
duct. This is the pronephric or segmental duct. It is at the anterior
end of this pronephric duct (in the region of the second to fourth
somites) that the pronephros or head-kidney forms as a ventro-lateral
outgrowth.

At this anterior end of the pronephric duct there are three tiny
openings left as the lips of the duct fuse. These three openings become
the three pronephric tubules (Figs. 349, 350), and the openings of these
tubules into the coelom are called the nephrostomes.

The nephrostomes become lined with large cilia which produce a
current out of the coelom, which current then passes by way of the
pronephric duct to the cloaca.

The pronephros itself becomes quite vascular. In the discussion
of the posterior cardinal veins, mention \vas made of the close relation
of these veins to the excretory system. It will be remembered that the^
lie along the pronephric ducts. The elongating of the pronephric tubules
pushes them upward into the posterior cardinal sinus until the sinus is
nearly rilled. This means, of course, that the tubules are really bathed
in venous blood.

At the same time that this occurs, arterial blood is brought from
the dorsal aorta to the excretory system by arteries in the form of
glomeruli.

The manner in wThich the glomeruli form is rather complicated.
Opposite the second nephrostome, a fold appears in the splanchnic meso-
derm, when the embryo is about 4.5 millimeters long. This fold lies
parallel to the pronephros itself, becoming elevated and projecting into
the coelom opposite the nephrostomes. Vascular spaces appear in this

606

THE EMBRYOLOGY OF THE FROG

fold, which develop the long convoluted vessels of the glomerulus
proper, and also a definite vessel which connects with a branch from tne
dorsal aorta.

This region of the body cavity is later cut off from the pronephric
chamber by the lungs, projecting laterally, and carrying a fold of peri-
toneum across to the peritoneum which covers the pronephros, fusing
with it for a short distance. The pronephric chamber remains open into
the coelom both anteriorly and posteriorly to the lung region.

The pronephric capsule is derived from two sources, namely: from

otphrostome

gonad

MESONEPIIROS

Fig. 349.

A and B. Diagrams of the development of the excretory system of the frog.
A, The system of a tadpole about 12 mm, Jong, showing the pronephros and
origin of the mesonephric tubules ; B, the system at the end of metamorphosis.
The broken line represents approximately the position of the strip of peritoneal
epithelium which gives rise to the oviduct, cl., Cloaca ; d.ao., dorsal aorta ; f.b.,
fat body ; gl., glomerulus ; gr., gential ridge ; mcs., mesonephro.3 ; ms.t.,
mesonephric tubules ; bd., oviduct ; ov/., position of oviducal opening ; pn.f.,
pronephric funnels; pnp., pronephros; sg., segmental duct. (From Bourne.)

C, Diagram to show the structure of the pronephros and the mesonephros.
Pronephros on the right and mesonephros on the left. The chief difference is in
the relation of the glomerulus ; in the pronephros it projects into the coelom ;
in the mesonephros it projects into the tubule, which forms the Bowman's capsule
about it. (From Wiedersheim. )

THE UROGEXITAL SYSTEM

607

the ventro-lateral walls of the myotomes, which normally give rise to
mesenchyme but which here evaginate in the pronephric region over
both dorsal and lateral surfaces of the head-kidney to meet with folds
coming up from the somatic layer of the lateral plate. This forms a
capsule of connective tissue which encloses not only the pronephros
proper, but also the pronephric sinus of the posterior cardinal vein.

The pronephros is largest when the tadpole is about twelve milli-
meters long. It remains stationary until the twenty millimeter stage,
when it begins to degenerate. Degeneration is not quite complete at
metamorphosis. Various blind outgrowths of the three original tubules

Fig. 350.

Diagrams to show the development of the three kidneys and their ducts and
their relation to the male gonad. A, early stage rhovring the pronephros a, de-
veloping from the anterior end of the mesomere c and the pronephric duct b, which
has not yet reached the cloaca e. B, next stage illustrating the degeneration of
the pronephros at /, the development of the mesonephros h, from the middle portion
of the mesomere, the junction of the pronephric duct, now the mesonephric duct
O with the cloaca and the beginning of the metanephric evagination t from the
mesonephric duct. C, later stage, showing connection between certain tubules of
the mesonephros and the testis j by means of tubules, the vasa efferentia, p,
which grow out from the mesonephros ; and the penetration of the metanephric
evagination into the posterior end of the mesomere where it is subdividing to
form the collecting apparatus I, which becomes associated with the secretory
metanephric tubules m, developed in the mesomere. />, final stage, in which tjie
mesonephros has disappeared except for the remnant y, which connects with the
testis j by means of the vasa efferentia p; the tnesonephric duct g persists as the
vas deferens ; the two parts of the metanephros shown in C have united to form
a single organ r. a, pronephros ; 6, pronephric duct ; c, mesomere or nephrotome ;
d, intestine ; e, cloaca ; /, degenerating pronephros ; g, mesonephric or Wolffian
duct ; h, mesonephros or Wolffian body ; t, metanephric evagination from the
Wolffian duct in B, ureter in C and D; j, testis; fc, coiled portion of the vas
deferens forming part of the epididymis ; I, collecting part of the metanephros
derived from the Wolffian duct.; ra, excretory tubules of the metanephros derived
from the mesomere ; «, nephrostome ; o, renal corpuscle or Malpighian body ; p,
vasa efferentia ; q, remnant of the mesonephros, forming part of the epididymis ; r,
metanephros. (From Hyman's "A Laboratory Manual for Comparative Vertebrate
Anatomy," by permission of The Chicago University Press.)

008 THE EMBRYOLOGY OF THE

can be seen before degeneration sets in. The pronephric duct close:
just posterior to the pronephros, and the tubules break up and disappear
The nephrostomes, however, approach one another, and finally mee
to open into a common cavity called the common nephrostome. This
then, also closes so that the nephrostomes are entirely cut off from al
communication with the body cavity. The glomeruli also disappear
although traces of these can still be seen for some months after meta
morphosis.

THE MESONEPHROS OR WOLFFIAN BODY

Just about the time the pronephros attains its full size, or a littl
before, the mesonephros or Wolffian body begins to form in the regioi
of the nephrotomes or intermediate cell-mass of the seventh to twelftl
somites. It is both somatic and splanchnic in origin. These nephro
tomes fuse in a continuous longitudinal strip of irregularly .arrange<
cells which lie between the pronephric duct and the dorsal aorta, alonj
the posterior cardinal vein. Little swellings appear in this mass, whicl
are the beginnings of the mesonephric vesicles. They are more numerou
than the mesodermal segments, and so are not strictly metameric.

Each of these swellings divides into a large ventral and a smalle
dorsal chamber ; the larger one being called the primary mesonephri
unit and the smaller the secondary mesonephric unit.

The secondary units divide later to form a tertiary mesonephri
unit.

Each of the three units develops much alike. Figures 349 and 35'
show this development. There are two outgrowths, an inner tubul
which grows dorso-laterally to the pronephric duct where it opens, an<
an outer tubule which grows ventro-medially to the peritoneum wit'
which it fuses, and then it empties into the body cavity.

The inner tubules connecting with the mesonephric duct conver
the portion where such connections are formed into the mesonephros o
Wolffian body — the true kidney of the adult frog. The duct itself, i
the region of the connections, is known as the Mesonephric or Wolffia:
Duct, while the duct posterior to this region is now the ureter.

The inner tubules become elongated and coiled to. form the tubula
portion of the mesonephros, wrhile the outer tubules form outgrowth
which later form the capsule around the glomeruli, known as Bowman'
capsules.

A small twig from the dorsal aorta connects with each glomerulu
in a similar manner to the way the glomeruli were formed in the prc
nephros. The proximal portion of the outer tubule, from which the oul
growths arise to form the capsule, now separates from the remainin
tubule, but retains its connection with the inner tubule, and the distc
portion of the outer tubule comes to lie in connection with the bod
cavity. This latter connection becomes ciliated and forms a typicr

THE UROGENITAL SYSTEM 609

nephrostome as in the pronephros. The nephrostomal region now forms
another connection at its inner end with the sinus of the posterior cardi-
nal vein, in which sinus the mesonephric tubules lie surrounded by
venous blood.

Many, as high as two hundred, outer tubules and nephrostomes may
be formed from the three units described, and possibly some may be
formed also by independent evaginations from the peritoneum, or even
by splitting of those previously formed.

The urinary bladder is a median ventral outgrowth from the wall
of the cloaca nearly opposite the openings of the ureters.

THE REPRODUCTIVE SYSTEM

The mesonephric duct becomes divided somewhat obliquely into
two portions in front of the mesonephros, the more anterior portion now
being the Miillerian duct, while the posterior portion forms the Wolffian
duct. The Miillerian duct connects with the peritoneal epithelium an-
teriorly and empties into the cloaca posteriorly. In the male frog, this
duct persists as a mere longitudinal streak on the outer side of the kid-
ney, and extends some distance in front of it. In the female frog, this
Miillerian duct becomes the oviduct (Fig. 350).

The Wolffian duct functions as the ureter in both sexes, but in the
male, the posterior end of it becomes dilated into a glandular enlarge-
ment called the seminal vesicle.

Already at the six millimeter stage, as the mesentery is being formed
by the coming together of the somites from both sides just under the
dorsal aorta, a small group of entodermal cells is pinched off from the
yolk, which, after completely separating from the yolk, divide longi-
tudinally, each half moving laterally. These longitudinal halves are the
genital ridges (Fig. 349, B). They lie close to the mesenteric attach-
ment, and just beneath the cardinal veins.

The genital ridges become quite conspicuous in a short time by
germ-cell proliferations as well as by the peritoneal cells which cover
them, and the mesenchymal cells from the body wall which migrate to
this region.

The mesenchymal cells form the stroma of the ridge ; the peritoneal
cells form a thin superficial covering at first, while later they also form
the suspending folds (mesorchia of the testes, and mesovaria of the
ovaries) of the "fonads. The germ-cells now begin to proliferate and
form the nests of cells (Fig. 254), which are to develop into gonads and
gametes as already described early in the embryology of the chick.

The anterior portion of the genital ridge becomes the fat body
shortly before metamorphosis, while the posterior portion connects with
the mesonephric duct.

In this posterior region several outgrowths from the Malpighian
bodies known as sexual cords can be seen. These become tubular, and

610 THE EMBRYOLOGY OF THE FROG

extend into the substance of the gonad. In the male, these sexual cords,
after metamorphosis, form the vasa efferentia or efferent ducts by which
spermatozoa are carried from the gonad proper to the real sperm duct,
the vas deferens. In the female, the portions between ovary and meso-
nephros degenerate, remaining only as a vestige, called Bidder's organ
(Fig. 457).

The tadpole must be of considerable size before the sexes can be dis-
tinguished, Bouin giving the length as thirty millimeters in Rana Tem-
poraria. In the male gonad, the cells all look alike, while in the female
gonad, the follicle arrangement can be made out, and the ovary acquires
a central lumen.

THE ADRENAL BODIES OR EPINEPHROI

Figure 351 will show how the adrenal bodies grow on -the meso-
nephros of the frog. The important point to remember is that there are

9*-

Fig. 351.

Parts of sections through young R. temporaries, show-
ing the origin of the adrenal bodies. A. Through 30 mm.
tadpole. B. Through 11 mm. frog after metamorphosis,
o, Dorsal aorta ; ac, cortical cells of adrenal body ; am,
medullary cells of adrenal body ; ct, connective tissue ; g,
gonad ; gs, sympathetic ganglion ; m, mesentery ; n, mesoneph-
phros ; rv, revehent renal vein ; v, vena cava ; x, point where
ganglion cells enter mesonephros and adrenal body. (After
Srdinko.)

two kinds of cellular substances in these organs. The adrenal bodies lie
*rm the ventral surface of the mesonephros in the frog.

Histologically, one may see a coarse network of cell strands with
occasional groups of darkly staining tissue called phaeochrome tissue.
Blood from the median posterior cardinal vein occupies the spaces in
the adrenal body. The coarse network forms what is called the cortical
tissue, and the more darkly staining portions are known as medullary
tissue, because in the higher forms of animal life, the darkly staining
portion lies toward the inner region of the organ, and the coarse network
lies toward the outer or cortical region.

When the tadpole is about twelve millimeters in length, the cortical
region appears as small groups of cells lying along both sides of the
wall of the median posterior vein, below the level of the mesonephros,

THE UROGENITAL SYSTEM 611

as well as beneath the peritoneal epithelium from which they seem to
arise.

Just after metamorphosis, these cell-groups separate from the peri-
toneum to form the network mentioned.

The medullary portion, however, has a totally different origin, and
one which may throw light on further work in our study of ductless
glands, whose secretions have become an important factor in modern
medicine. This portion is derived from the ganglia of the sympathetic
nervous system by groups of cells whose precise origin is not clear.
Some of these cell groups remain in the sympathetic ganglia, but others
migrate to the adrenal body and become scattered about.

CHAPTER XLV.

THE SKELETAL SYSTEM

The notochord extends from the blastopore to the pituitary body,
as a rod of vacuolated cells rilled with fluid, around which three layers
or sheaths form.

The primary, or elastic sheath, is an outgrowth of superficial cells
of the notochord itself and forms the superficial surface sheath of the
notochord.

The secondary or fibrous sheath is formed between the primary
sheath and the notochord also by cells from the notochord itself.

The third, or skeletogenous sheath, forms on the outside of the pri-
mary layer at a later period as a thin sheath which is formed by the
sclerotomes. The sclerotomes, it will be remembered, are outgrowths
from the somites. This skeletogenous layer extends dorsally, entirely
around the neural tube, and laterally, from the notochord, in between
the successive myotomes. It is in this skeletogenous layer that the ver-
tebrae are to be formed.

When the tadpole is about fifteen millimeters long, a series of
metameric cartilages can be observed along the medio-ventral surface of
the notochord. They lie in the skeletogenous sheath (Fig. 352, cs).
These segments fuse longitudinally to form a pair of dorsal and ventral
strips which extend along the entire notochord.

These strips now become metameric by constrictions of fibrous tissue
which form rings. The rings are the beginnings of the inter-vertebral
ligaments, which, just as in the chick, appear opposite the middle of each
mesodermal segment.

The mesodermal segments become the vertebrae, so that the liga-
ments which form as separate segments between the vertebrae, are able
to act on both the vertebra lying immediately anterior and immediately
posterior to each mesodermal segment, after the muscle has developed
in connection with these ligaments (Figs. 305, 352).

The notochord becomes segmented and surrounded by cartilage, the
notochordal segments form the soft centrum of the vertebrae, and prob-
ably also portions of the intervertebral discs.

The ventral cartilages now grow around the sides of the notochord
and meet to fuse with the dorsal series.

The transverse processes of the vertebrae grow out from the ventral
cartilages. It is toward the lateral ends of these that the transverse
processes of the ribs later develop.

The neural arch is formed from outgrowths of the dorsal series
which grow inward beneath the neural cord and also dorsally and ven-

THE SKELETAL SYSTEM 613

trally. Later, when ossification begins, short processes called interverte-
bral articulatory processes develop from the neural arches, by which
each vertebra joins with the next succeeding vertebra.

Ossification begins in the tadpole between the dorsal and ventral
series of cartilages just described. There are nine vertebrae formed in
the frog, plus the urostyle, the latter being unsegmented.

771

Fig. 352.

Cross section through a. developing vertebra, rib and
exoskeleton of a Turtle, c, corium in which the dermal
plates are developed ; cs, primitive vertebral body ; ep,
epidermis ; m, external oblique muscle ; p, perichondrium ; r,
rib; sp, spinous process. (From Kingsley after Gotte. )

THE SKULL

The skull is commonly formed from various embryological elements,
which may be listed as follows :

(1) Cranium.

(2) Sense Capsules.

(3) Visceral Arches.

(4) Notochord.

(5) Vertebrae.

(6) Membrane or Derm Bones.

It will be remembered that there are no true segments in the head
region of the frog. Consequently, the list just given is only assumed
from a comparison of other forms.

When the cranial region begins its cartilage formation, the tadpole
Is about seven millimeters in length. A pair of dense strands of tissue
form along the ventro-lateral surfaces of the fore-brain. These then
become cartilaginous and form the beginnings of the trabeculae, or
trabecular cartilages (Fig. 310). These trabeculae extend forward and
fuse across the midline between the olfactory organs. The fusion forms
the internasal plate. The trabeculae continue extending forward, and
these extensions are known as the trabecular cornua, at the ends of
which the olfactory capsules form.

A pair of labial or suprarostral cartilages which have formed in the
upper lip meet with the olfactory capsules.

The notochord which extends into the brain up to the pituitary body
has a pair of tissue thickenings beginning in the region of the hind-brain

614

THE EMBRYOLOGY OF THE FROG

and extending anteriorly as parachordae, or parachordal cartilages.
These parachordal cartilages now fuse with the posterior ends of the
trabeculae to enclose the tip of the notochord, and the entire continuous
plate beneath the fore-brain is then called the parachordal plate.

These parts can be made understandable only by a careful examina-
tion of Figures 310 and 353, which must be studied with great thorough-
ness or much of our later work in comparative anatomy will be valueless.

From the visceral arches, the palato-quadrates are formed as a pair
of flattened rods, lateral to the trabeculae. These are in intimate rela-

Fig. 353.

A. Chondrocranium of 29 mm. larva of R. fusva. To the left, the ventral
surface ; to the right, the dorsal surface, a, Auditory capsule ; bp, basal plate ; c,
notochord ; ct, trabecular cornu ; /, basicranial fontanelle ; fa, foramen for carotid
artery ; fin, foramen magnum ; fo, foramen for olfactory nerve ; ir infrarostral
cartilage ; j, jugular foramen for IX and X cranial nerves ; I, perilymphatic
foramina ; m, muscular process ; M, Meckel's cartilage ; o, otic process of palato-
quadrate ; pf, palatine foramen ; pg, palato-quadrate cartilage ; sr, suprarostral
cartilage ; t, trabecular cartilage ; v, secondary fenestra vestibuli. B, Anterior
portion of chondrocranium of JR. fusca during metamorphosis. Lateral view.
C. Skull of 2 cm. R. fusca, after metamorphosis. Dorsal view. Membrane bones
removed from left side, a, Auditory capsule ; am, anterior maxillary process ;
on, annulus tympanicus ; art, articular process of palato-quadrate cartilage ; eo,
exoccipital bone ; /, fronto-parietal bone ; fpo, prootic foramen ; mx, maxillary
bone ; n, nasal bone ; o, olfactory cartilages ; on, orbito-nasal foramen ; pa,
anterior ascending process of palato-quadrate ; pg, pterygoid bone ; pi, plectrum ;
pm, posterior maxillary process ; pp, posterior ascending process of palato-
quadrate ; pq, palato-quadrate cartilage ; pt, pterygoid process of palato-quadrate ;
px, premaxillary bone ; qj, quadrato-jugal bone ; II, foramen for optic nerve ;
///, foramen for III cranial nerve; IV, foramen for IV cranial nerve. (From
Ziegler.)

tion to the cranium proper. They connect with the trabeculae by an-
terior ascending processes back of the olfactory region, and by posterior
ascending processes opposite the end of the notochord.

The remaining portion of the skull which develops from the visceral
arches is connected with the jaw, and will be described shortly.

The infundibulum and pituitary body lie within the basi-cranial
fontanelle, which is the open space just anterior to the tip of the noto-
chord. From now on, development continues mostly in the posterior
portion of the cranium.

Figures 310 and 353 show how the auditory organ is formed by a
connective tissue capsule which soon becomes cartilage, while the
mesotic cartilage grows out posteriorly and laterally from the para-

THE SKELETAL SYSTEM 615

chordal plate to unite with the auditory capsule ventrally, both anteriorly
and posteriorly.

The occipital cartilage is a continuation of the mesotic cartilage
which fuses with the auditory capsule, and leaves a small opening
through which the IX and X cranial nerve pass. This opening is called
the jugular foramen.

The basal plate is the name given to the floor of the posterior por-
tion of the cranium, which consists of occipital and mesotic cartilages
together with the parachordal plate.

The occipital cartilage extends dorsally around the neural cord to
form the foramen magnum.

The auditory capsule remains open into the cranial cavity internally
by a large foramen, but closes externally.

The trabeculae now grow across the basicranial fontanelle so that
it becomes entirely closed. This closed portion is the floor of the cranial
cavity. The trabeculae then extend laterally and form the lateral walls
of the cranial cavity, thus separating the cavity from the orbits.

Cartilages from the trabeculae also extend dorsally across the mid-
line in the anterior region, thus forming a narrow dorsal bridge, leaving
a large supracranial fontanelle between this bridge and the supra-
occipital region.

The internasal septum extends dorsally and becomes the anterior
wall of the cranial cavity, while the trabecular cornua remain separate
from the olfactory capsules, but connect anteriorly with the suprarostral
or labial cartilages. During metamorphosis, however, both labial car-
tilages and anterior ends of the cornua disappear in front of the olfactory
capsules.

True bones form late in the frog, the following being the more im-
portant ones which have developed from cartilage :

Exoccipitals, or Lateral Occipitals. These form from the posterior
portions of the occipital cartilage and auditory capsule. The occipital
condyles themselves as well as the median dorsal and ventral portions
of the occipital region remain as cartilage.

Pro-otics. These form from the more anterior portion of the audi-
tory capsules as well as from the basal plate and orbital region.

Ethmoids. These form in the anterior portion of the wall of the
orbit. They then unite both above and below so as to form a band around
the cranium, often also called the sphenethmoid or orbito-sphenoid.

Quadrato-jugal. The palato-quadrate cartilage forms bone only in
the region of the lower jaw. Then a connection is formed with a mem-
brane bone and these two together form the quadrate- jugal. All these
bones form before metamorphosis, the ethmoids alone developing some
weeks after metamorphosis has taken place.

The Visceral Skeleton. In the mandibular and hvoid arches as well

616 THE EMBRYOLOGY OF THE FROG

as the three branchial arches, the various skeletal elements appear as
condensations in the mesenchyme, which soon become cartilaginous.

First, a short rod appears in the mandibular arch, transverse to the
axis of the embryo.

This divides the dorsal portion into the beginnings of the upper jaw
or palato-quadrate, and the ventral portion which is the beginning of
the lower jaw. The lower jaw element becomes subdivided into Meckel's
cartilage, which comes to form the true jaw, and the infra-rostral car-
tilage.

The palato-quadrate has grown rapidly, as already described, and
then fused with the trabeculae.

When the tadpole is about twenty-one millimeters long, the pos-
terior or quadrate portion of this same cartilage connects with the audi-
tory capsule.

With metamorphosis, the mouth enlarges, and this pushes back
many of these structures, while the part of the palato-quadrate which
lies in the orbital region, softens and disappears to a considerable extent.
The anterior connection of palato-quadrate and trabeculae becomes the
future pterygoid and palatine regions. All these changes draw the jaw
to the posterior portion of the cranium from its original anterior position.

The infra-rostral cartilages, which have fused together across the
midline, now fuse with the Meckelian cartilages to form the apex or
mental portion of the chin. The fused cartilages are now known as
mento-Meckelian cartilages. As these ossify, they fuse with the dentary,
which is really the chief membrane bone of the lower jaw. There is a
small median element between the infra-rostrals which also fuses with
them.

The annulus tympanicus is the outgrowth from the quadrate car-
tilage which surrounds the tympanic membrane of the frog. It does not
complete its growth until long after metamorphosis.

The hyoid arch, like the three branchial arches lying posterior
to it, makes its appearance as a pair of rods of dense tissue in the corre-
sponding visceral arches, though not at the same time as the others.

The hyoid cartilage, also called the ceratohyaal cartilage, extends
dorsad and connects with the palato-quadrate immediately behind where
the jaw articulates. Ventrally, it unites with the hyoid cartilage of the
opposite side. (Fig. 354.)

The first branchial cartilage also unites in the ventral midline, while
the remaining branchial arches do not unite in the midline ventrally,
but have their lower anterior ends unite with the one lying immediately
anterior to it, and, finally, they connect dorsally in a similar manner.

The copula, which is a medial element, then appears in the ventral

THE SKELETAL SYSTEM 617

region of the pharynx between the hyoid and the first branchial, and
connects the ventral ends of both these arches.

The hypo-branchial plate consists of the lower ends of the first
branchials which have become flattened and expanded. The ventral
ends of the other three branchials fuse with the hypo-branchial plate.

The cerato-branchials are the lateral and mid-

u

die sections of the branchial cartilages between the
visceral pouches which remain separate from one
another.

At metamorphosis, when the gill slits close,
many changes naturally must take place in the
Fig. 354. structures just described. For example, the hyoid

archXidofanl 29anmm! bar loses its connection with the palato-quadrate,
tral* vfiew'. iUbb" Basl- an(^ Decomes smaller in diameter, while the copula
5a^chw? (basihyai^ c°ch likewise becomes smaller and a pair of new car-

ceratohyal; ho hypo- tilagCS develop On each side of it, which then COn-
branchial plate ; 1-4, first

nect t^ie hypo-branchial plate with the hyoid por-
tions. These are the manubrial cartilages.

The hyo-branchial apparatus of an adult frog is made up of a broad
median plate of cartilage which has been formed by the fusion of
manubrium, copula, and hypo-branchial plate. The hyoid cartilages re-
main as slender processes called the hyoid cornua. The remaining por-
tions practically disappear.

The membrane bones. In those portions of the cranium where con-
siderable stretching has taken place, such as in the roof of the skull and
the lining of the mouth, the substance is thinner than in the cartilaginous
portions, and is then called membrane.

Membrane is nothing more than stretched-out-cartilage.

The Parasphenoid. This is a single median bone, and tne first 01
all bones of the skull to appear, whether cartilaginous or membranous.
It forms in the roof of the mouth when the tadpole is about twenty
millimeters long. The parasphenoid becomes dagger-shaped and covers
the entire basicranial fontanelle.

The frontals and parietals, which are paired, appear later and cover
the supracranial fontanelle. They later fuse to form the fronto-parietals.

The nasals form the roof of the olfactory capsules and the septo-
nasals or intra-nasals appear within the capsules.

The premaxillae and maxillae are the membranous parts which be-
come the margins of the upper jaw.

The dentary and angular cartilages surround Meckel's cartilage ;
the dentary connects with the infra-rostrals of Meckel's cartilage.

The vomers are paired, and appear beneath the olfactory capsules.

The palatines form across the anterior margins of the orbits.

618 THE EMBRYOLOGY OF THE FROG

The pterygoids form along the inner faces of the palato-quadrate
cartilages.

The squamo&als form along the outer face of the palato-quadrate
cartilages, and ultimately reach back to the auditory capsules. The
quadrat ojugal is that portion which has developed at the posterior angle
of the palato-quadrate cartilage and then fused with the quadrate bone.
This is the only one of the membranous bones which cannot be distin-
guished from the cartilaginous bones of the skull.

CHAPTER XL VI.

MAMMALIAN EMBRYOLOGY

In both the chick and the frog — the two forms we have thus far dis-
cussed— the eggs have passed out of the body of the mother. In the
frog, the entire embryo developed after the egg left the mother, while
in the chick where fertilization is internal, development began before
the shell was formed, so that an embryo, approximately twenty-four
hours old, was already present when the egg was laid.

Now we shall deal with viviparous animals, that is, with those which
give birth to living young.

It will be understood quite readily that in those cases where living
young are brought forth, the development must take place within the
body of the mother, but, even in viviparous animals there are sub-
divisions. One sub-division is made up of such animals as the duckbill,
the Australian ant-eater, the Australian kangaroo, and the American
opossum. In these animals, known as Marsupials, the female bears a
pouch in the abdominal region in which the young are placed at a very
premature age. In fact, in the opossum, the embryo may be only about
eight days of age when it is born for the first time so to speak. The
mother then places it within the pouch or marsupium, and here the young
continue their development until able to lead an independent existence.

In all the higher forms of mammals with which the student is
familiar, fertilization is internal, as in the chick, and the embryo passes
through a process similar to that of the chick, except that this embryonic
process takes place within the mother's body.

There is, however, in viviparous animals no real yolk-supply as in
both the chick and the frog egg. Consequently, there must be some
kind of an arrangement by which the young not only become attached
to the uterus of the mother, but there must also be an arrangement by
which a blood-supply can pass from mother to offspring, thus taking
the place of the nourishment the yolk furnishes in egg-laying animals.

The mammalian egg, not possessing a yolk, is very small.

The original development of egg and sperm, however, is not very
dissimilar to that already described for the chick.

Before entering into the study of mammalian embryology proper,
it is well, at this point, to understand the terminology usually applied to
the life-history of a mammal.

First, the period of gestation or true embryonic period. It is during
this time that the embryo depends upon its connection with the mother's
uterus for nourishment. Gestation extends from the time of the fertili-
zation of the egg to the time of birth.

620

MAMMALIAN EMBRYOLOGY

Second, parturition, or the actual time of birth. The condition of
the offspring at the time of parturition varies to a considerable extent.
Some animals are born with the ability to walk and take reasonable
care of themselves within a very short time after birth, while some are
quite dependent upon their mother for a long time.

Third, the period of adolescence, which is that period of life in the
young devoted entirely to growth and development. It extends from
birth to sexual maturity.

Fourth, adult life, or the period of sexual maturity. During this
period many physiological changes often take place in the individual
entirely aside from those of the reproductive system.

FERTILIZATION

As the egg is thrown out of the Graafian follicle (Fig. 355), it passes
into the oviduct (Fallopian tube) and is carried by the cilia in the oviduct
to the uterus. If fertilization takes place, the sperm, which finds its way

A

C.

Fig. 355.

A. Section of well-developed Graafian follicle from human embryo (von Herff) ; the
enclosed ovum contains two nuclei. B. Ovary with mature Graafian follicle about
ready to burst (Ribemont-Dessaignes). C. Section of human ovary, showing
mature Graafian follicle ready to rupture. Kollmann's Atlas.

MAMMALIAN EMBRYOLOGY

621

into the uterus, passes into the oviduct in an opposite direction from
that which the egg takes. This causes a meeting of egg and sperm. The
length of time it takes the egg to reach the uterus, after ovulation, varies
in different species of animals. It may vary from a few hours to sev-
eral weeks. It is therefore practically impossible to state the exact
time when fertilization actually takes place. This is especially true in
the human being; but, as soon as the sperm does meet the egg, and fer-
tilization does take place, the embryo begins developing. Consequently
by the time the fertilized egg reaches the uterus, it has already passed
through, or is just passing through, a stage that is even a little advanced
beyond the gastrula stage. There are, in fact, several germ-layers already
present at this time.

THE BLASTODERM

As soon as fertilization takes place, the egg divides equally into
two cells, these two into four, and so on in the usual way. However,
very early, some of the cells divide more rapidly than others, so that
there is an overgrowth of those which grow most rapidly. This gives
rise to several terms. The more rapidly growing, or outer layer, is called
the sub-zonal layer, while the central mass is called the inner cell mass.

The sub-zonal layer is only one cell in thickness, so it is easily dis-
tinguished from the inner cell mass. Then, too, a cavity forms between
the two layers.

The entire structure is now called a morula (Fig. 356). As soon as

Fig. 356.

Morula and early blastodermic vesicles of the rabbit. Th.2 zona radiata and
ftlbuminous layer are not shown. A. Section through morula stage, forty-seven
hours after coitus. B. Section through very young vesicle, eighty hours after
coitus. Taken from uterus ; ordinarily the ova have not reached the uterus at
this age. C. Section through more advanced vesicle, eighty-three hours after
coitus. Taken from uterus, c, Cavity of blastodermic vesicle ; i, inner cell mass ;
w, wall of blastodermic vesicle (subzonal layer, trophoblast ) . (From Assheton.)

D. Section through the fully formed blastodermic vesicle of the rabbit,
/cm, Granular cells of the inner cell mass ; troph, trophoblast cells ; zp, zona
pellucida. (From Quain's Anatomy, after Van jJeneden.)

the cavity has definitely formed between the inner cell mass and the sub-
zonal layer, the morula is known as a blastodermic vesicle. This cavity
contains a fluid which is supposed to represent the yolk-mass of the
blastula and gastrula in the lower forms.

622 MAMMALIAN EMBRYOLOGY

It will be noted from what has just been said that considerable de-
velopment has already taken place by the time the fertilized egg reaches
the uterus. Or, to repeat what was said above, it is as a blastodermic
vesicle that the mammalian egg reaches the uterus after fertilization.

At this stage, two important points must be considered :

First, the method of the formation of germ-layers, and

Second, the method by which the blastodermic vesicle attaches itself
to the uterus of the mother.

Formation of the Germ-layers.

The inner cell mass spreads out rapidly so as to form an inner lining
to the sub-zonal layer. It is this inner lining which is the entoderm.

The sub-zonal layer becomes the ectoderm.

As there is a tremendous variation in the way germ-layers are
formed in mammals, it may be well to think of the following example
as a help in understanding some of these variations.

Suppose a group of football players who had already played together
in previous years were to come together again. Each would immedi-
ately take his place without any preliminary instruction. So, we may
think of the embryonic cells in the higher mammals taking a definite
place and then developing from there on, rather than passing through
all the stages of gastrula formation, and this gastrula then actually in-
denting to make two layers. That is, we may think of those embryonic
cells which are to develop into ectoderm and mesoderm actually taking
the proper position to develop into these structures without first becom-
ing a single sheet and then indenting.

This would mean that the undifferentiated cells which are to become
ectoderm would arrange themselves on the outer portion, those which
are to become entoderm would arrange themselves more inwardly, and
those which are to become mesoderm would take their place between
these two layers and then all three could begin developing at about the
same time and grow simultaneously.

In the lower mammals such as the cat, dog, rabbit, etc., this inner
cell mass (entoderm) keeps pace with the sub-zonal layer, so that the
original cavity which has formed between the jnner cell mass and the
sub-zonal layer is now surrounded by an inner layer of entoderm, while
the outer layer still remains sub-zonal. In the higher forms such as the
primates, that is, in man and the higher apes, the inner cell mass does
not grow as rapidly as the sub-zonal layer. There is, therefore, a second
cavity formed within the inner cell mass of entodermal cells.

It is the remaining portion of the inner cell mass, after the entoderm
has thus separated from it, which is the ectoderm. The sub-zonal layer
is then called the trophoblast (Fig. 356, D). This trophoblast serves
as the attachment of the blastodermic vesicle to the walls of the uterus.

We see from what has been said that a true mammalian gastrula

MAMMALIAN EMBRYOLOGY

(although formed in a different manner from either that of the chick or
the frog) has been established with two definite cell or germ-layers.

ATTACHMENT OF THE BLASTODERMIC VESICLE TO THE

UTERINE WALL

There are two general ways in which the blastoderm may become
attached to the uterus.

The trophoblast or sub-zonal layer may remain as an outer layer
around the entire blastoderm, or the developing embryo within the inner
cell mass may push through this outer layer and come to lie in close
relationship to the uterine wall.

At about the same time that attachment of blastoderm and uterine
wall takes place, the amniotic cavity is formed (Fig. 357). The tropho-

Fig. 357.

Diagrams of the relations of the cavities and layers in the rat, showing
the "inversion" of the germ layers. Median sagittal sections. Embryo and
amnion, black ; ectodermal knob or "trager" in light tone ; endoderm and
mesoderm in darker tone. A. Early stage before the formation of the false
amnionic cavity. B. Late stage showing false and true amnionic cavities
and the interamnionic cavity. a, Amnion ; ac, true amnionic cavity ; c,
chorion ; E, embryo (anterior end; ea, endodermal rudiment of allantois) ; /,
false amnionic cavity ; i, interamnionic cavity ; m, mesoderm ; ma, mesoderm
of allantois; n, endoderm; o, trophoblast (ectoderm); p, anterior intestinal
portal ; ra, rudiment of true amnionic cavity ; rf, rudiment of false amnionic
cavity; s, marginal sinus; t, "trager" (ectoderm) ; y, yolk-sac; ye, yolk-sac
endoderm; x, amnionic folds. (After Salenka.)

blast remains as an outer covering in man, in many primates, and in
such animals as the mouse, rats and guinea pig. When the trophoblast
remains as the complete outer covering such a condition is known as
entypy, and it is in animals in which this condition occurs that a definite
space is formed between the germ layers and the trophoblast. This
cavity is known as the amniotic cavity.

Sometimes the trophoblast thickens in this particular region and

624 MAMMALIAN EMBRYOLOGY

a second or false amniotic cavity may develop. Figure 357 will make
this clear.

In those cases, however, where the embryo pushes through the
trophoblast and comes to lie as a disc upon its surface, the amnion is
formed quite similarly to that already described in our study of the chick.

The region in which the embryo develops is known as the embryonic
shield. The primitive head-node lies about in the middle of the em-
bryonic shield. The primitive streak and the primitive grooves form
quite as in the chick, and as there stated, all structures lying anterior to
the head-node lie in the head proper.

A definite notochord also forms, and the neurenteric canal can be
seen quite plainly at the posterior limits of the embryonic rudiment.

There are scarcely more than half a dozen human embryos which
have ever been seen prior to the time of the formation of the medullary
plate. Then, too, none of these were of the same size, so we do not even
have a basis for valid comparison, and consequently, we are unable to
judge as to whether any of these were normal as to size and form.

Mesoderm is formed in the mammal as it is in the chick, each meso-
dermal somite dividing into a somatic and a splanchnic layer. A layer
of entoderm joins with the splanchnic mesoderm to form the yolk-sac,
although no yolk is present. The trophoblast joins with the somatic
mesoderm to form the chorion.

Here we may note that the term "ovum" is used in mammalian de-
velopment to designate any early stage in the embryo, even to the in-
clusion of the entire blastodermic vesicle. The term "embryo" in man
is given the embryo only during the first two months of its existence,
thereafter (that is, when the face and body are quite well formed) it is
known as a "foetus."

The smallest human embryo yet seen was 1.54 mm. in length, while
the entire blastoderm was about 1 cm. in diameter.

IMPLANTATION

There are three methods by which the blastoderm attaches itself to
the walls of the uterus.

First, by what is called central implantation. This occurs in the
ungulates and carnivores as well as in the lower primates and in some
rodents, such as the rabbit. In these the blastoderm becomes super-
ficially attached to the uterine wall, and, consequently, projects freely
into the lumen of the uterus.

Second, eccentric implantation. This type is found in the mouse
and in some insectivora. In these forms the blastodermic vesicle lies
in a furrow or groove in the uterine wall. This groove is then closed up
so that the vesicle comes to lie in the walls of the uterus.

Third, Interstitial implantation. In this type the blastodermic ves-

MAMMALIAN EMBRYOLOGY 625

icle actually burrows its way into the mucous membrane lining of the
uterus.

It is this third type which occurs in man and in some of the rodents
such as the guinea pig and the gopher.

The trophoblast, in the region where it is to meet with the uterine
wall, has become highly specialized physiologically in the eccentric and
interstitial types of implantation. Its cells form a layer of considerable
thickness and it is then called a trophcderm (Fig. 358). These cells are
supposed to dissolve or digest the uterine mucosa so as to permit a defi-
nite implantation and also, probably, to digest some of the mucosa as
food for the growing embryo.

The blastoderm attaches itself to the uterine wall between the two
oviducts, and it is in the region of implantation that the maternal tissues
come into contact with the embryo. We must, therefore, look for the
beginnings of the placenta in this region.

Fig. 358.

A. Diagrammatic section of placenta. (After Strahl, Bonnet.) i

B. Section through an embryo of 1 mm. embedded in the uterine mucosa
(semidiagrammatic after Peters). Am:, amniotic cavity; b.c., blood-clot; b.s.,
body-stalk ; cct., embryonic ectoderm ; ent., entoderm ; mes., mesoderm ; m.v.,
maternal vessels ; tr., trophoderm ; u. e., uterine epithelium ; u.g., uterine glands ;
y.s., yolk-sac.

In fact, it is the trophoderm which later becomes vascularized from
the mesoderm of the chorion or allantois, to act as the chief absorptive
surface through which, and by which, material from the maternal tissues
and blood is taken to the embryo.

THE EMBRYONIC MEMBRANES

It will be remembered that in the chick embryo, the amnion has as
one of its functions the protection of the embryo from drying and from
becoming deformed by the outer shell pressing against it. The chick's
yolk-sac contains a large quantity of food-substance which the develop-
ing embryo uses, and the allantois serves as a respiratory and (partially)
as an excretory organ. In the chick the serosa or chorion was of little
importance.

MAM M AL I A N E M BRYOLOGY

In the mammal it is the amnion which is of secondary importance.
The yolk-sac has no yolk in it and in so far as we know has little func-
tional value. The allantois has but little respiratory and excretory sig-
nificance. Its work is practically to bring the embryo in relation to its
food supply. It is the chorion which becomes the principal organ by
which nutritive material and excreted substances between maternal and
embryonic circulations take place.

It is this connection between mother and embryo which brings about
the formation of what is called the placenta. All mammals which de-
velop a placenta — that is, all mammals except those which lay eggs —
are known as placentals.

THE PLACENTA

The -detailed development of mammals must be left to much larger
volumes than this one, especially as there are so many variations occur-
ring even in quite similar groups of animals.

D.

Fig. 359.

Diagrams illustrating the development of the blastocyst and formation of the
placenta in Mammalia. A, a blastocyst at the end of segmentation ; B, an older
blastocyst, in which a cavity has appeared to one side of the inner mass of cells ;
C, a. later stage, showing the formation of the trager and growth of the yolk
epithelium round the yolk cavity ; D, formation of lacunae in the trager and
commencement of the embryo ; E, further development of the trager, the mesoblast
has split and the amnion and extra-embryonic coelom are formed ; F, longitudinal
section of uterus, showing the position of the embryo in a pit in the uterine wall.
G. longitudinal section of a later stage, showing the obliteration of the old
lumen and formation of a new lumen in the uterus, all, allantois ; am, amniotic
cavity ; COB, embryonic ccelom ; ec, epiblast ; eec, extra embryonic coelom ; ek,
embryonic knob ; em, embryo ; gl, uterine glands ; hy, hypoblast ; im, inner mass of
cells ; lac, lacunae in trager ; lu1, original lumen of uterus ; ht3, secondary lumen
of uterus ; nch, notochord ; ng, neural groove ; sbm, thickened sub-mucous layer
of uterus ; tr, trager ; tro, trophoblast ; yk, yolk-sac : yk.e, yolk epithelium. In
all the figures the trophoblast is shaded with dots, and the embryonic mesoblast is
represented in black. (After Bourne.)

MAMMALIAN EMBRYOLOGY 627

But as the student must know the placental animals in order to make
the most of his study in Comparative Anatomy, it is essential that he at
least obtain a clear and accurate understanding of the two principal
types of placental formation.

In the first place, the placenta may be defined as consisting of all
structures affecting nutritive, respiratory, and excretory interchanges
between the embryo and its mother in viviparous animals. It is evident,
then, that the placenta must form in the region where the trophoblast
comes in contact with the uterine mucosa, and that the trophoderm
itself plus the vascularization in the yolk-sac, allantois, or chorion, will
be the elements from which the placenta is developed. (Figs. 358, 359.)

At this stage the student must review the chapter on the develop-
ment of the extra-embryonic membranes in the chick.

The rabbit is often used as an example of a form of mammalian em-
bryology which can be contrasted with the embryological development
of the chick. In the rabbit the extra-embryonic membranes develop quite
like those in the chick, except that the point of fusion of these mem-
branes consists of only a small knot, whereas in the chick this fusion
takes the form of an elongated seam. In both rabbit and chick the tail-
fold grows more rapidly than the head-fold.

In man, where entypy takes place (that is, where the trophoblast
remains continuous about the entire blastoderm), the extra-embryonic
membranes do not grow as in the rabbit and chick, but by a splitting of
the ectoderm to form the beginning of the amniotic cavity.

The forming of the extra-embryonic membranes in man quite natur-
ally causes the embryo to remain connected with the blastodermic wall
by a body-stalk (Fig. 358, B). The separating of the ectoderm imme-
diately above the embryo to form the amniotic cavity causes the embryo
to form the floor of this cavity, while the trophoblast forms the roof.
The sides, or walls, of the cavity meet the embryo at the edges of the
embryonic shield.

But, whether the amniotic cavity is formed as in the rabbit or as in
man, the walls of the cavity extend ventrally until they surround the
umbilicus.

The yolk-sac and the yolk-stalk, as well as the allantois, although
quite small in man, are pushed into this body-stalk or umbilical-stalk.
The amniotic cavity grows large in man and contains from one-half to
one liter of liquor amnii.

THE YOLK-SAC

The open space on the interior of the mammalian blastodermic vesi-
cle is supposed to represent the yolk-sac (Fig. 359, G) of such animals
as the chick and the frog; and, as this open space is relatively very large,
the yolk-sac occupies the main portion of the early mammalian blasto-
dermic vesicle. The cavity of the vesicle opens into the mid-gut region

628 MAMMALIAN EMBRYOLOGY

by the broad yolk-stalk just as with the chick. Its wall is separated from
the chorion by the extra-embryonic coelom — also called the exocoelom.
(Fig. 359, E.)

The amnion and chorion are formed from somatopleure, while the
yolk-sac is formed from splanchnopleure.

The blood vessels and the sinus terminalis arise in the yolk-sac of
'.he rabbit just as they did in the chick.

In the higher primates, including man, the yolk-sac never fills the
entire blastodermic vesicle and is very slow to grow. In fact, during
the first month it has a diameter about the length of the embryo, and
after increasing this diameter to a little over a centimeter, it decreases in
size. The yolk-stalk is formed, however, and elongates considerably to
enter the proximal end of the umbilical cord.

The amniotic membrane now' expands and pushes against the exo-
coelom until that is eliminated and the yolk-sac disappears in the pla-
cental region. The yolk-stalk itself becomes a solid cord during the
second month. However, the proximal end sometimes remains open.
In such a case it appears as a diverticulum from the intestine, and is
called Meckel's diverticulum.

THE ALLANTOIS

This structure also varies in size to a considerable extent, from
filling the entire exocoelom as in the lower primates such as the Lemurs,
to occupying but a small portion of the umbilical cord as in man and the
higher primates (Fig. 359, G).

The early development of the allantois in the mammals is quite sim-
ilar to that in the chick, but its later development is varied, the variation
being ascribed to the changed conditions brought about by the formation
of placental structures.

The later history of the allantois is limited to the placental struc-
tures only. In the rabbit the allantois extends into the exocoelom and
comes in direct contact with the chorion in the region where chorion
and uterus unite. It thus lies in the direct pathway of connection be-
tween mother and offspring. Blood vessels now develop in the allantoic
mesoderm to form the umbilical arteries and the umbilical veins, and it
is through these allantoic blood vessels that the embryonic circulation
*;.s related to the placental circulation.

In man the development is quite different ; for, here there is nothing
.vhich interrupts the connection of chorion with the maternal tissues.
The way in which the body-stalk develops in man has been described
already. This is often said to be equivalent to a modified allantoic stalk.
There is, therefore, in man, no true allantois as a free vesicle. Only a
small tubular outgrowth from the entodermal lining of the yolk-sac can
be seen, and this outgrowth, in turn, is not distinguishaable from the
hind-gut. It extends in to the body-stalk. As the embryo grows, and the

MAMMALIAN EMBRYOLOGY

body-stalk extends, the allantoic stalk extends further along in the body
stalk as well, and so remains during foetal life (Fig. 360).

As the ventral body-walls of the embryo are formed and approacl
each other, the proximal end of the allantoic stalk becomes the urinar
bladder and the beginning of the urogenital sinus. From the bladde
region to the body-wall it is reduced as a mere solid strand of connectiv
tissue known as the urachus.

Vascularization is quite alike in the various mammalian forms.

The development of the placenta depends upon the manner an<
type of implantation, which in turn causes different relationships betweei
the growing embryo and the maternal tissues.

THE DECIDUAL MEMBRANES

We have been describing the embryonic placenta. Now we shal
describe the maternal placenta. There is a change which takes place ii
the lining of the uterine walls when the trophoderm unites with thi
uterus. The uterine lining which bulges out into the uterine cavity t<
cover the blastoderm is called the decidua capsularis (formerly, decidu;

reflexa), while the uterine lining a
the point where blastoderm am
uterus unite is called the deciduj
basalis or decidua serotina, the re
maining portion of the lining being
known as the decidua vera (Fig
361).

The chorion is at first composec
of an inner mesodermal layer anc
an outer epithelial layer (this lattei
being called the trophectoderm)
From the trophectoderm there de
velops an outer syncytial layei
which is called the trophoderm. It
is this trophoderm which invades
the maternal tissues. Large lacunae of blood are formed in the mater-
nal tissues by the syncytial tissue directly, or by the rupture of the blood
vessels which are under great pressure in this region.

The trophoderm then thickens at intervals and forms little villi or
finger-like projections, and the chorionic mesoderm grows out into these
villi so that there is a branching of the primary villi into secondary villi
or true villi (Fig. 358)).

In the meantime the blood lacunae run together and surround and
bathe the villi, while the trophoderm, which began as a spongy network,
is now a continuous layer covering the entire outer surfaces of the villi
and chorion.

Branches of the umbilical vessels develop in the mesoderm of the

Fig. 360.

Medial section of early human embryo.
( After von Spec, Kollmann. )

630

MAMMALIAN EMBRYOLOGY

chorion and villi. This means that there are now two layers of epi-
thelium covering the mesodermal core of all the villi, and that it is in
these villi that the chorionic circulation of the embryo is established.

The blood vessels of the uterus 'open into the little blood-lacunae,
which is another way of saying that the syncytial trophoderm which
covers the villi is bathed in maternal blood. This is where the nourish-
ment of the embryo takes place. The maternal blood itself does not pass
into the developing embryo.

Fig. 361.
Diagram to show relationship of mammalian embryo and maternal membranes.

At first the villi cover the entire surface of the chorion, but in man,
after a few weeks, the villi located away from the point of attachment
begin to degenerate and finally leave that portion smooth. This smooth
region is called the chorion laeve, while the attached portion which re-
tains the villi is known as the chorion frondosum (Fig. 362). It is the
chorion frondosum, together with the decidua basalis, which constitutes
the placenta. And it is the chorion frondosum to which the embryo is
attached by the body-stalk which later comes to be called the umbilical-
stalk or umbilical cord.

MAM MALI AN EM BRYOLOGY

631

The decidua basalis forms what is called the maternal placenta and
the chorion frondosum the foetal placenta.

The decidual membranes and
their attachments form the after-
birth. This afterbirth consists of
amnion, chorion, decidua vera,
placenta, and a part of the decidua
basalis.

THE UMBILICAL CORD

As the body-stalk becomes
longer and longer, finally reaching a
length of some fifty centimeters,
there must be some circulatory con-
nection between the embryo and the
chorion frondosum. This connec-
tion is brought about by the development of four blood vessels, two
veins and two arteries, known as the umbilical vessels or allantoic vessels
(Fig. 363). The two veins push their way into the embryo to open into
the heart. The arteries likewise grow in the same direction as do the
veins, but connect with the dorsal aorta. Their distal ends extend

Fig. 362.

Human Embryo. Age seven weeks.
(From Kollmann. ) cf, chorion frondosum. cl,
chorion laeve.

Fig. 363.

1 to 6, Diagrams representing six stages in the development of the foetal
membranes in a mammal.

The ectoderm is indicated by solid black lines ; the entoderm by broken lines ;
the mesoderm by dotted lines and areas. (After Kolliker.)

7, Diagram of nurture of young through embryonic membranes, g, gill circula-
tion of embryo ; h, heart ; i, dorsal aorta ; j, postcava ; k, allantoic artery ;
i, allantoic vein ; m, indicating the course of the blood of the mother, parallel
to n; n, that of the embryo; w, wall of uterus. (After Needham.)

632

MAMMALIAN EMBRYOLOGY

through the body-stalk into the villi to connect with the vascularization
there established.

The two veins later fuse, so that a cross section of a mature umbili-
cal cord (Fig. 364), shows two arteries and a single large vein.

Aa. umbilicales

V. umbilicalis

V. umbilicalis
A. umbilicalis | A. umbilicalis

Whartonsche
Sulze

Amnion
Coelom
Ductus omphaloentericus (vitellinus)

Ductus omphalo- Whartonsche Amnion
entericus (vitellinus) Sulze" '

Fig. 364.

I, Umbilical cord of human embryo at three months.

II, Same at birth. (After Corning.)

In addition to the umbilical vessels just mentioned, the yolk-stalk
(in the early stages only), and the allantoic stalk can be seen in cross
sections of the cord while the cord itself is filled with a mesenchymal,
mucous-like substance called Wharton's jelly.

The cord is twisted and is attached to the umbilicus or navel of the
foetus and to the placenta. The outer covering of the umbilical cord is
a layer of ectoderm which is continuous with that of the amnion of the
embryo.

The following table shows the relative increase in size and weight
of the human embryo and foetus throughout the period of gestation :

Ovum (estimated)

23 days

66 days

84 days
112 days
140 days
168 days
196 days
224 days
252 days

Weight
0.000004 grm.
0.04
3.0
36.0
120.0
330.0
600.0
1000.0
1500.0
2200.0

270 days

280 days 3200.0

C. H.

2.5 mm.

30.0

98.0
180.0
250.0
315.0
371.0
425.0
470.0
500.0

Length

C. R.

2.5 mm.

25.0

68.0
121.0
167.0
210.0
245.0
284.0
316.0
336.0

MAMMALIAN EMBRYOLOGY

633

If the student has thoroughly mastered the subject matter of this
semester's work in embryology he will not only
be able to understand how a normal embryo de-
velops, but he will also know how and why many
and varying types of abnormal development oc-
cur by either mechanical or chemical injury of
some kind, which injury may cause any portion
of the embryo to stop growing, while other parts
continue in the usual manner. Monstrosities of
many kinds may thus be formed, and even in ap-
parently normal individuals it is by no means
rare for the surgeon to find individual internal
organs underdeveloped or overdeveloped. All
such deviations from the normal are of the ut-
most importance to the medical man, and it is
only through a study of embryology that they are
made understandable.

Fig. 365.

Figure to illustrate the
''vertex-breech" method of
measuring human embryos.
a-b, vertex-breech length of
the embryo.

Part III
Comparative Anatomy

CHAPTER XLVII

INTRODUCTION TO COMPARATIVE ANATOMY

In the study of Comparative Anatomy a method somewhat different
from our study up to this moment must be brought into play.

In the forepart of this book the frog was studied as a type-form of
vertebrates, and the earth-worm as a type-form of annelids, as well as
of coelomates, and then, after each such type-form had been studied, it
was compared with other forms likewise studied in the laboratory.

Now we are to take an entire system in each of the leading groups of
vertebrates and compare system by system, always reviewing the devel-
opment of the particular system studied, and comparing such develop-
ment with the development of the respective systems in both frog and
chick, as shown in part three (embryology) of this book.

Three distinct points of view must be kept in mind in comparative
anatomy :

Structure (both gross and microscopic).

Development (embryology).

Comparison of organ systems.

Just as in any account of man's history we attempt to study those
races which we now consider as living under primitive conditions, be-
lieving that they will throw some light upon the problems that our an-
cestors had to overcome in order to bring about our present state of
civilization, so, in comparative anatomy we attempt to study the so-called
lower-animal life in order that this may throw light upon the develop-
ment of our own bodies. This may be brought home the better by re-
membering that all higher forms of life practically possess all organs and
systems of organs that the lower forms possess, plus an additional some-
thing. This does not prove by any means that any of the higher systems
must have necessarily come from the lower. All it does mean is that
all forms of animals which walk on the earth must have much that is
similar. For instance, legs that are used for the same purpose in all
animals must have muscles that will function alike ; because, regardless
of what position we systematically assign these animals, they, by virtue
of the fact that they must walk, must necessarily have leg muscles, and
having these, there must be a supporting structure for such muscles,
so that the skeletal systems of walking animals will be closely akin.

Comparative Anatomy proper, then, will consist of a comparison of
the organ systems of four great groups of vertebrates. The classic ex-
amples used for such comparison are :

The dogfish as an example of a group of living organisms whose
skeletal tissues are largely cartilaginous.

The turtle as an example of the reptilia.

638 COMPARATIVE ANATOMY

'I he cat as an example of the mammalia.

The frog is the classic example of the amphibian. This animal has
already been studied in the early part of the course, but must be kept in
mind so as to be compared with the above three types.

It is usual to exclude the aves, because reptile and bird have so many
structural similarities that the study of one suffices for that of the other.
In fact the single word Sauropsida has come into common biological
usage as meaning both reptiles and birds.

It is necessary first for us to have some conception of what is
meant by the phylum Chordata and to appreciate that there are inter-
mediate types between invertebrates and vertebrates. Such intermedi-
ate types are known as pro-chordata. The pro-chordata and the verte-
brata together form what Zoologists call the pyhlum Chordata.

The vertebrates possess a spinal or vertebral column which con-
sists of a great number of similar portions called vertebrae arranged in
a longitudinal series. In the early embryo of all vertebrata there appears
a rod-like notochord. This probably serves as a sort of stiffening to the
animal and in this respect only is it similar to the spinal column proper.
It is neither cartilage nor bone, and probably develops from the entoderm
or mesoderm. As the spinal cord develops from the ectoderm, and the
bones of the spinal column from the mesoderm, it will be seen that
neither of these three just-mentioned portions are alike in either origin,
function, or position.

In all vertebrates the main nerve cord lies on the dorsal side while
in invertebrates it lies on the ventral side.

There are certain groups of animals which possess no spinal col-
umn, yet, during the embryonic period have a notochord, a dorsal nerve
cord, and a gill-slit apparatus (Figs. 313, 314, 315, 316). The classic
examples of these forms are Amphioxus, Balanoglossus, and the
tunicate or sea squirt, all of which are comparatively small in size and
live in the sea. These forms are grouped together under the name of
pro-chordata. Professor Patton of Dartmouth College has described a
scorpion in which he is sure he has found a notochord. If he is correct it
will be seen that no classification of this kind is absolute, in that inverte-
brates of very early geologic times may have possessed such an embryo-
logical structure.

CHAPTER XLVIII

CLASSIFICATION OF FISHES, AMPHIBIANS, REPTILES,

AND MAMMALS.

Unless this chapter is mastered, there can be no understanding of
the textual matter which follows, as the scientific terms there used are
all based on what this chapter contains.

CHORDATA

The Chordata possess a notochord at sometime during their life's
history (the notochord lying between the nervous system and the ali-
mentary tract), a hollow central nervous system lying entirely on one
side of the digestive canal, and pharyngeal slits extending from the
pharynx to the exterior.

The Chordata are divided into four sub-phyla, all of which develop a
notochord during their embryonic period, though all do not later develop
a bony vertebral column.

The sub-divisions of the Chordata (Figs. 313, 314, 315, 316) are as
follows :

Sub-Phylum I. Cephalochordata (Adelochorda, Fig. 312).

The notochord runs only up to the head proper in most chordates,
but in the Cephalochordata, of which Amphioxus is the classic example,
the notochord extends to the very anterior end of the body. Amphioxus
is fish-like in form and is used as an example of the most primitive form
of the chordates. It will be remembered that there was reference made
to the simplicity of the embryology of Amphioxus in Part III of this
book.

Amphioxus has no skull or vertebral column. The pharyngeal slits
are quite numerous. The true scientific name of Amphioxus is Branchi-
ostoma. In popular language it is often called lancelet, on account of its
sharp, lance-like appearance.

Sub-Phylum II. Urochordata (Tunicates, Figs. 312, 313).

This group possesses a notochord only in the caudal region. The
young are tadpole-like, and there is a metamorphosis converting the
tadpole into a sac-like structure.

Order I. Larvacea (Appendicularia), free-swimming forms with
permanent tail.

Order II. Ascidiacea (Tunicates or Sea-Squirts), fixed forms with-
out tail in the adult.

Order III. Thaliacea (Salpians), free-swimming forms without tail
in the adult.

The neurenteric canal is permanent.

640 COMPARATIVE ANATOMY

Sub-Phylum III. Hemichordata (Fig. 314).

A rather doubtful form. There is a projection from the mid-dorsal
region of the digestive canal which looks somewhat similar to a noto-
chord. These animals have a collar and a proboscis.

Order 1. Enteropneusta, which include worm-like forms such as

Balanoglossus.
Order II. Pterobranchiata, sessile, tube-dwelling forms such as

Cephalodiscus, and Rhabdopleura.

Order III. Phoronidia, tubicolous forms such as Phoronis (Fig.
199).

Sub-Phylum IV. Vertebrata (Craniata).

1. The vertebrates show their segmentation in the adult form only
on the interior of the body, as for example, the metameric arrangement
of myotomes, sclerotomes, etc.

2. A cuticular skeleton is absent, but there may be cornifications
of the epithelium, or ossifications in the dermal regions, such as the scales
of fishes, etc.

3. An axial skeleton is present consisting of skull and vertebral
column.

4. There are two kinds of appendages supported by the axial skele-
ton, namely, the unpaired fins (which occur only in fishes and Amphibia),
and the paired appendages (anterior and posterior), which are usually
present.

5. The central nervous system is dorsal in position. The brain it-
self consists of five parts : the cerebrum, "twixt-brain," mid-brain, cere-
bellum, and medulla oblongata.

6. Of the sensory organs, the eyes and ears are the most highly
developed.

7. The respiratory organs arise from the entoderm of the pharynx.
Pharyngeal slits are present in the embryo. In terrestrial animals these
pharyngeal slits are later functionally replaced by lungs which develop
from the hinder portion of the pharynx.

8. The heart lies ventrally in the pericardium. In gill-breathing
species it contains only venous blood, but in lung-breathing animals it is
divided into venous and arterial halves. The circulation is closed.

9. The sexes are usually separate, while in most species the excre-
tory (nephridial) system forms the ducts for the reproductive (genital)
system.

10. Reproduction is strictly sexual.
The classes of Vertebrata are as follows :

Class I. Cyclostomata (Fig. 366).

These are the round-mouthed eels without a lower jaw. Examples
are the lampreys and hagfishes. It is in this group that we find the only
vertebrate parasites.

CLASSIFICATION

641

There is a primitive skull, but no true vertebrae (only bony arches).
Paired fins, true scales, and teeth are lacking. The gill-pouches are sac-
cular and the nose is unpaired.

Sub-Class I. Myxinoidea (Fig. 366).

These are the "hag-fishes" or "borers" which give off a slimy,

Bdellostoma dombeyi (Pacific hagfish)

3fyxinc r/lulinom (Atlantic lag fish)

Pelromyzon marinns (sea lamprey)
Fig. 366.

Cyclostomes. The light openings along the sides are mucous canals, the dark
ones are branchial openings.

mucous jelly when captured . It is from this fact they receive their name
of Myxinoidea.

Sub-Class II. Petromyzontia (Fig. 366).

These are the lampreys, which live in both salt and fresh water.
The myxinoids attack principally dead and disabled fishes, but the
petromyzons attack decidedly active fish much larger than themselves,
attaching themselves to their host and making great inroads with their
rasping tongues.

Class 2. Pisces (Gnathostomata). All fish having true lower jaws.

Fishes are distinguished from the Cyclostomes not only by having
true lower jaws but also by having a vertebral column (amphicoele ver-
tebrae, Fig. 404), by having scales, paired pectoral and pelvic fins, and
paired nostrils. They breathe by gills and have a heart with .venous
blood therein only, although the heart has auricle, ventricle, sinus
venosus, and some have a conus arteriosus.

Sub-Class I. Elasmobranchii.

These are the sharks and their near relatives. They have a car-
tilaginous skeleton, usually a heterocercal tail, placoid scales (thornlike),,
but in Mustelus (the dog-shark, which is used in the laboratory), pointed,,
overlapping scales. There are five to seven slit-like gill-openings on
each side. The eggs are few and hatched within a sac inside the body.
The skates also belong to this group. They are merely flattened out
sharks.

642 COMPARATIVE ANATOMY

There are various extinct orders and sub-orders of elasmobranchs,
but we shall deal only with two orders and two sub-orders.

Order, Plagiostomi.

Sub-Order I. Selachii (12 living and 3 extinct families of sharks

and dog-fishes, Fig. 367).
Sub-Order II. Batoidei (Saw-fishes, skates, rays and torpedoes, 7

families, Fig. 367).

Cltimaera monstrosa

Kain crinacea (common skate)

Fig. 367. Elasmobranchii.
(A, after Goode ; C, after Claus.)

Order Holocephali (Chimaera, Fig. 367, 1 living and 3 extinct families).
The Holocephali are very grotesque looking animals and are of
great antiquity. There are peculiar grinding plates in the mouth in-
stead of teeth.

Sub-Class II. Teleostomi. (The true bony fishes).

Skeleton partly or entirely bony, a single gill-opening on each side
leading to gill-arches on which there are gill filaments. There is also
a swim-bladder, although this may disappear with age.

In the higher forms where the skeleton is entirely ossified, the
pelvic girdle approaches the pectoral one, so that the pelvic fins may
be directly beneath the pectoral fins. It is this approach of the girdles
and fins which is used in classifying fish, because this is supposed to
show different degrees of specialization.

The position of the fins in the higher fishes is supposed to furnish
evidence to show that amphibians and higher fishes are not closely re-
lated.

Order I. Crossopterygii.

Sub-Order I. Osteolepida. (4 extinct families).

Sub-Order II. Cladista. Polypterus and Calamichthys are the

usual examples. (Fig. 368).
Order II. Chondrostei (5 extinct and 2 living families).

These include the paddle-fishes and sturgeons (Fig. 368).

CLASSIFICATION

643

Pulyi>liTus larva

Lepidosteus osseus (gar pike)

f

petf

Anna calva (buu- fn\)

Fig. 368. Ganoids.

In B, e.g., Large external gill of the hyoid arch ; PC., pectoral fins ; Pv., pelvic
fins. The larva is drawn in a very characteristic attitude.

In C note the elongated snout, the barbules bounding the ventral mouth, the
operculum covering the gills, the rows of bony scutes, the markedly heterocercal
tail.

D, Ventral and side view.

F, Amia calva (Bow fin), c.f., caudal fin; d.f., dorsal fin; pct.f., pectoral fin;
pv.f., pelvic fin; v.f., ventral fin. (B, after Budgett ; D, after Goode ; E, after Ten-
ney ; F, after Giinther. )

Order III.* Holostei (6 extinct and 2 living families).

These include the bow-fins and gar-pikes. (Fig. 368).

Order IV. Teleostei.

Sub-Order I. Malacopterygii (21 families).

These include tarpons, herring, salmon, etc. (Fig. 369).

Sub-Order II. Ostariophysi (6 families).

These include carp, tench, cat-fishes, etc. (Fig. 369).

Sub-Order III. Symbranchii (2 families).

A small group of eel-like fishes having characteristics of both

Ostariophysi and Apodes.
Sub-Order IV. Apodes (5 families).
These are the eels. (Fig. 369).
Sub-Order V. Haplomi (14 families).
These are the pickerel, killifishes (mud-minnows), etc.
Sub-Order VI. Heteromi (5 families).
These are the Fierasfer, etc. (Fig. 370).
Sub-Order VII. Catosteomi (11 families).

These are the stickle-backs, pipe-fishes, sea-horses, etc. (Fig. 370).
Sub-Order VIII. Percesoces (Flying fishes) (12 families).
These include the Belone, sand-eels, rag-fishes, etc. (Fig. 370).

*The student will meet with the term "Ganoid" in his reading. This merely refers to a shiny
scale. In the United States the gar-pike (Lepidosleus) found in the Mississippi Valley, is commonly
mentioned, although older writers made a distinct grouping of Ganoids, consisting of Orders I, II
and III, using the African Polypterus as the classic example. In Lepidosteus, Ganoid scales have
a sort of peg and socket arrangement.

644

COM PARATIVE Ax ATOM V

Fig. 369. Teleostei.

A, Brook Trout, a sub-genus of the Salmon family, a.L, adipose lobe of pelvic
fin ; an., anus ; c.f., caudal fin ; d. f. 1, first dorsal fin ; d. f. 2, second dorsal or
adipose fin ; LI., lateral line ; op., operculum ; pct.f., pectoral fin ; pv.f., pelvic fin ;
v.f., ventral fin. (A, after Vardine ; B and C, after Goode ; D from Bull.
U. S. F. C. 1895.)

Sub-Order IX. Anacanthini (3 families).
These are the cod, etc. (Fig. 370).

UiupocttatfHin barboun (sea horse}

(cod)

(jUbrrli (/tying fish)

Fig. 370. Teleostei.

A, F'ierasfer acus penetrating anal openings of holothurians. D, an, anus ; c.f.,
caudal fin ; d.f.l — 5, dorsal fins ; mx., maxilla ; pct.f., pectoral fin ; pmx. pre-
maxilla ; pv.f., pelvic fin; v.f. 1 and 2, ventral fins. (A, after Emery; B, after
Bull. U. S. F. C. 1907 ; C, after Jordan and Evermann ; D, after Cuvier. )

CLASSIFICATION

645

Sub-Order X. Acanthopterygii (78 families).

These include a great majority of our more common fishes, such as
perch, bass, mackerel, flounders, gobies, shark-suckers, climb-
ing perch, etc. (Fig. 371).

Sub-Order XI. Opisthomi (1 family).
These are the eel-like fishes.

Fig. 371. Teleostei.

B, Dissection of head of Climbing Perch to show accessory respiratory organ ;
F, normal and G, inflated porcupine fish. (A and E, after Cuvier ; B, F. and G,
after Giinther ; C, D, after Baskett.)

Sub-Order XII. Pediculati (5 families).

These are the Anglers, Bathymal Sea-Devils, etc. (Fig. 371).
Sub-Order XIII. Plectognathi (7 families).

These include the file-fishes, trunk-fishes, puffers, porcupine fishes
and sun-fishes. (Fig. 371).

Sub-Class III. Dipneusti (Dipnoi) The Lung-Fishes (Fig. 372).
(2 extinct and 2 living families. These include the Neoceratodus,

r>46

COMPARATIVE ANATOMY

Protopterus and Lepidosiren.

The skeleton of these lung-fish is largely cartilaginous but there is
a tendency toward ossification. The swim-bladder serves as lungs. The
very young individuals have long, feather-like external gills.
Appendix to the True Fishes.

I. Palaeospondylidae (1 family between cyclostomes and fishes).

II. Ostracodermi (3 orders of 8 families, mostly armored fishes).

III. Antiarchi (1 family of mailed fishes).

IV. Arthrodira (1 family of mailed fishes).

e rntodiiit fimlr.ri (Australian lung-fsii)

I.L

Proloplerits anneclen* (African lung-fish)

Fig. 372. Dipneusti.

In C, snt., sensory tubes ; / /.. lateral line ; e.br., external gills ; pc.L, pectoral
fin ; op, operculum. In D and E, eg ., external gills ; PC., pectoral fin ; Pv., pelvic
fin. (A, after Giinther ; B, after Glaus ; C, after W. N. Parker ; D, after Budgett ;
E and F, after Graham Kerr.)

It is well to note that 172 families of the 226 families of true fishes
are members of the order Teleostei.

Of the Elasmobranchii there are 23 families now in existence and
9 extinct.

The ganoids and dipnoi number 22 families.

Amphibia.

Contrasted with fishes, the amphibia have pentadactyl appendages,
while contrasted with reptiles, they possess double occipital condyles.
There are external gills in the larvae, though these do not always per-
sist. The adults breathe by lungs. The heart consists of two auricles,
one ventricle, a conus arteriosus, and a sinus venosus.

Sub-Class I. Stegocephali.

These are the extinct amphibia, many of which attained consid-
erable size.

CLASSIFICATION

647

Sub-Class II. Lissamphibia. (About 1,000 species, nearly 900 of which
are frogs and toads. Figs. 315, 376).

Order I. Apoda (Gymnophiona) Limbless Amphibia. (Fig. 373).

These are also called caecilians and sometimes "blind-worms." They
are without limbs or limb-girdles. They burrow in the earth and are
found in warm climes. The cranium is like that of the reptile in out-
ward appearance but the bones which constitute it are the same as those
which go to form any amphibian cranium. The skin is smooth and
slimy with many ring-like folds. There are as many as 200 to 300
vertebrae in some species. The eyes are rudimentary and probably
functionless. There is a feeling-organ protruded from between eye and
nose which serves to guide the animal. Some are oviparous, while
others are viviparous.

Fig. 373. Apoda.

Ichthyophis glutinosa. 1, nearly ripe embryo, with gills tail-fin, and with
considerable amount of yolk ; 2, a female guarding her eggs, coiled up in an under-
ground hole ; 3, a group of newly laid eggs ; 4, a single egg, enlarged and
schematised to show the twisted albuminous strings or chalazae inside the outer
membrane, which surrounds the white of the egg. 5, Caecilia, emerging from
burrow. (After P. and F. Sarasin.)

Order II. Urodela (Tailed Amphibia). (Figs. 315, 374).

These are the mud-puppies (Necturus), salamanders, newts, and
efts. Many authors call all urodeles with adult external gills, Perenni-
branchiata, though the following grouping is the more common.

Family I. Amphiumidae (Fig. 374).

This family is without external gills in the adult stage. There are
only two genera, Cryptobranchus and Amphiuma (Fig. 374).

Cryptobranchus allegheniensis (Fig. 374), is the well-known "hell-
bender" of our Eastern States.

Cryptobranchus japonicus is the giant salamander of Japan.

Amphiuma (Fig. 374) has only one species which ranges from Caro-

648

COMPARATIVE ANATOMY

lina to Mississippi in our Southeastern States. This is known as Am-
phiuma means, and is eel-shaped with much reduced limbs and a small
pair of inconspicuous gill-clefts guarded by skin-flaps. Some of these
animals are three feet in length, living in swamps and muddy water.
The female protects the eggs by coiling about them.
Family II. Salamandridae. (Salamanders and Newts) (Fig. 374).

These animals have no gills in the adult stage. Practically three-
fourths of all tailed amphibia belong to this family.

The most common type is the Desmognathus fuscus (Fig. 374).
The female coils about the eggs when laid. The young, after hatching,
look quite like adult forms.

Amblystoma tigrinum or "tiger salamander" (Fig. 374) is very com-
mon in North America. It has large yellow spots which may merge
into broad stripes or bands. The ground color is black. It may be

Fig. 374. Salamandridae.

In C, I, Female; 2, Male at the breeding season with well-developed frills.
E, Desmognathus fuscus (American newt). Female with eggs in underground hole.
(A, after Holder ; B, from Cambridge Natural History ; C, after Gadow, E, after
Wilder.)

found in damp places under stones and logs, or even in cellars of houses.
It is one of the classic forms used in the laboratory for various reasons,
one of them being that it is an animal which becomes sexually mature
while still in the larval stage, a condition called paedogenesis or neoteny.
Another very interesting fact is brought out in the life of the larval
forms of Amblystoma. The larva itself is called Axolotl, and was for-
merly considered to be a fully adult form. It is quite common near
Mexico City. However, when some of the Axolotls were taken to
Paris, and kept in aquaria, they metamorphosed into regular, full-fledged

CLASSIFICATION

649

Amblystoma. Not only this, but some of them could be made to revert
back to the larval Axolotl form.

Salamander maculosa, commonly called the "spotted" or "fire sala-
mander" is the most common of the European salamanders.

Salamandra atra is much darker than S. maculosa and is found in
the Alps at altitudes from 2,000 to 9,000 feet. This animal is interesting
in that it produces only two young at a time, which, while still in the
uterus, feed upon the surrounding eggs and pass through their entire
metamorphosis before being born.

Kammerer claims that S. atra will change to S. maculosa if brought
to lowland waters and then after being kept there for several genera-
tions, and later returned to the higher altitudes, they well retain the
breeding habits acquired as the lowland type. This fact has led some
authors to insist that here is a case of acquired characteristics being in-
herited.

Diemictylus viridescens is the "vermilion spotted eft" or newt. It
takes several years to reach the adult form. For three years it lives in
water and has external gills. During this time it is green in color. Upon
leaving the water it becomes yellow with vermilion spots, and at the
breeding season returns to water and again becomes green.

Triton cristatus (Fig. 374) Js the "crested newt." The male has a
decided crest during the breeding season.

Family III. Proteidae. (The Mud-Puppies) (Fig. 375).

These have three pairs of fringed external gills throughout life and
some authors call them perennibranchii.

There are only three genera with a single species each. Two of
these genera occur in America and one in Europe.

lacerlina (mud-eel)

Fig. 375. Proteidae and Sirenidae.
(After Chapin and Rettger.)

Necturus maculatus is the comm.on American "mud-puppy." It is
assumed that this may be an animal which has remained in the larval
stage.

Proteus anguineus (the Germans call them "olms") are blind, cave,

G50

COMPARATIVE ANATOMY

mud-puppies nearly white in color. But, if brought into the light they
become at first grayish and then jet-black.

Typhlomolge rathbuni, is a form quite like Proteus, and is found
in subterranean caves and sometimes is brought up from deep artesian
wells. They are found in Texas.

Family IV. Sirenidae. (The Sirens) (Fig. 375).

These have three pair of permanent fringed external gills, and the
body is eel-like. There are no hind limbs. There are two genera each
with a single species.

Siren lacertina, commonly called the "mud-eel." It may reach a
length of thirty inches. It is black in color dorsally and lighter ventral-
ly. It is found in the southeastern part of the United States.

Pseudobranchus striatus, is much smaller than Siren, hardly ever
reaching a length of more than seven inches. It has one pair of gill-
clefts and only three fingers. There is a broad yellow band along each
side. It is assigned the lowest place among the urodeles.

Order III. Anura (Tailless Amphibia) (Figs. 315, 376).

These are the frogs and toads.

Sub-Order I. Aglossa. -

These animals have no tongue.

This group is not yet commonly known as it occurs only in South
America and in Africa.

Pipa americana (also known as the "Surinam toad," Fig. 376), has
rather remarkable methods of carrying its eggs after they have been
laid. There are holes in the back of the female into which they are
deposited.

Xenopus, and Hymcnochirus are the African genera.

Pi/m americana

Rhacophorus
(fti/ing Irce-load of Borneo)

Atytes obstctricaiis
(obstetrical toad)

Fig. 376. Anura.

A, Pipa americana with young in skin pockets of. back. C, Male obstetrical
toad with string of eggs. (A, after Ludwig ; B, after Wallace; C, after Claus.)

CLASSIFICATION 651

Sub-Order II. Phaneroglossa.

These are the frogs and toads with tongues. There are seven
families.

The best known of these families are the Bufonidae which are the
common toads and the Ranidae, the "true frogs."

There is a peculiar species of toads in France and Switzerland called
Alytes obstetricans (Fig. 376), in which the male takes the eggs when
laid and wraps them around his hind legs, after which he deposits them
in a hole in the ground. These eggs are then moistened by him with
dew and taken out once-in-a-while in the water. When the eggs are
ready for hatching, he takes them all to the water and remains with
them until hatching is complete.

Class Reptilia.

There are four orders of living reptiles. These are cold-blooded
vertebrates, breathing by means of lungs throughout their life cycle.
Lizards, snakes, crocodilians, and turtles come under the heading of
Reptilia.

The fossil records of the past show that the four living orders are
but a small portion of the variations within this class which have con-
tinued their existence.

In the Mesozoic era (Fig. 245), commonly called the "age of Rep-
tiles," there have been found many skeleton-remains of immensely large
lizard-like animals. In fact, the name given to the largest of these
animals of the past is Dinosaur which means "terrible Lizard."

Sphenodon punctatum
Fig. 377. Reptilia.

(Sphenodon is considered the most primitive
type of living reptiles.) (After Gadow.)

There were many flying reptiles at that time while Plesiosaurs
lived in the water and had long paddles for swimming instead of legs.

Reptiles with wings are called pterosaurs. Some of their fossil
remains show these animals to have been twenty feet from tip to tip of
wings wrhen spread. It is assumed that these animals so overspecialized
various parts of their body that when great climatic and earth-changes
came about, they could not cope with the new conditions. It has also
been suggested that the eggs of many of these great animals may have
been used for food by very small mammals which caused the largest of
all beasts to die out entirely.

652

COMPARATIVE ANATOMY

The reptile most closely resembling extinct forms is thought to be
Sphenodon (Fig. 377). It is confined to a few small islands off the coast
of New Zealand and is hunted and eaten by the Maoris. It is also
called "tuatara" and lives in burrows. Externally it looks like a lizard,
though skeleton and viscera are quite unlike those of other living
lizards.

Order Chelonia. (Turtles and Tortoises).

These have a bony covering and toothless jaws. The covering con-
sists of a dorsal or upper portion called a carapace, and a ventral plas-
tron. These plates are soft in very young animals. The surface is cov-
ered with horny shields which Gadow believes to be phylogenetically
older than the underlying bony plates. These latter do not correspond
with the former in either number or position.

There are two sub-orders.

Sub-Order I. Athecae.

These are without a true carapace. There is only one living repre-
sentative of this type, namely the Leather-back Turtle, known as Der-

mochelys (sphargis) coriacea. (Fig. 378.) In-
stead of the regular carapace there are five
dorsal, five ventral, and two lateral dermal
plates. The tail is rudimentary and the limbs
are larger flipper-like paddles. Only large and
very small specimens have ever been found.
No one knows where they live between these
stages.

Sub-Order II. Thecophora.

These are the true turtles (Fig. 379) which
are divided into two groupings known as
Cryptodira to which group most of the turtles
of the Northern Hemisphere belong. The
head is retractile and the pelvis is not fused to
the shell, while in Division II, are the Pleuro-
dira, representing a large group of the South-
ern Hemisphere. These do not retract the
head but bend it sideways under the shell. The
pelvis is fused to the shell.

Sphargis coriacea
(leather-bade turtle)

Fig. 378.

The only chelonian without
a true carapace. (From Gadow.)

The more commonly known American Turtles belong to Division
Cryptodira. The snapping turtles being members of the family Chely-
dridae. The skunk- or musk-turtle is a member of the family Cinoster-
nidae, and the common pond tortoises are members of the family Testu-
dinidae. The "tortoise-shell" turtle belongs to those commonly called
"sea-turtles" and is a member of the family Chelonidae.

CLASSIFICATION

653

Order Crocodilia.

Crocodilia are character-
ized by well-developed limbs,
long tail, fixed quadrate
bone, and teeth fixed sepa-
rately in alveoli. The vari-
ous extinct forms, however,
do not have all these charac-
teristics.

There are only two fam-
ilies of Crocodilia (Fig. 380).
Family I. Gavialidae.

There is only one living
species of this family. It is
called Gavialis gangeticus.
It is found in the River
Ganges and other large
rivers of India.

Family II. Crocodilidae.

This family inludes both
old and new world crocodiles
and alligators. The latter
animals do not grow^as large
as the crocodiles. The alli-
gator is distinguished from
the crocodile by having a
broad, rounded snout.

Order Sauria. (Squamata.)

These are the lizards
and snakes. Their main dif-
ferentiating characteristics
are: a movable quadrate

A, grows to weigh 800 pounds; B, 40 pounds; C, bone which permits of a

***** ™ the tortoise" wide mouth-opening, a trans-
verse cloacal aperture and
double copulatory organs.
Division I. Lacertilia. (Lizards).

While normally the ordinary forms of lizards are scaly and have
four well-developed legs, there are many species which do not have
these characteristics. This latter type appear quite like snakes but the
bones of the skull always serve to distinguish them. Then too, the
lizards have no elastic ligament between the two halves of the lower
jaw, as snakes have.

654

COMPARATIVE ANATOMY

There are three
orders (Fig. 381).

sub-

Crocodttus (intcricanus (crocodile)

Fig. 380.
Crocodjlia. (After Basket! and Ditmars.)

Sub-Order I. Geckones.

These are the primitive
types having amphicoelous
vertebrae (Fig. 404), and no
bony temporal arches. They
have dilated clavicles, sepa-
rate parietals, eyes with
movable lids, broad, fleshy,
protrusible tongue, which is
nicked at the end. Usually
harmless. The tail is loosely
articulated and comes off
when seized although a new
one grows quite readily

Sub-Order II. Lacertae.

Most modern lizards be-
long to this group. Their
vertebrae are precocious and
solid. The ventral portions
of the clavicle are not
dilated.

There are cursorial types, arboreal types, volant types, an aquatic
type, a fossorial type and an ant-eating type, so-called from their vary-
ing modes of life.

The Gila Monster (Heloderma horridum) of our Southwestern
States is the only poisonous lizard known, while the monitor (Varanus
salvator) grows to the greatest length, something like seven feet or more.

Sub-Order III. Chamaeleontes (Chameleons). (Fig. 382).

These animals are the ones so well known on account of their
ability to change color, and the enormously long tongue by which they
readily catch insects at a distance of some seven inches.

Chameleons are highly specialized, the body is laterally compressed,
the tail is prehensile, and the toes are parted in the middle so as to be
used for grasping. They are found mostly in Madagascar. One species
is found in Southern Europe.

Division II. Ophidia (Snakes).

Snakes are really Sauria or Squamata in which the right and left
halves of the lower jaw are connected with an elastic ligament, thus per-
mitting the mouth to stretch greatly. They are usually limbless or have

CLASSIFICATION

G55

Hem>dac1ylus turicus
find TjtrenloJfi nitnirilnnica (yeckoncs)

Heloderma (Gila monster)

Angnis fragilis (limbless lizard)

Iguana tuberculata (common iguana)
Fig. 381. Lacertilia.
(A and D, after Gadow ; B, after Ditmars ; C, after Shipley and MacBride).

Cliainaelfon vulyaru

Anolis principalus (American chamaeleon)

Fig. 382. Chamaeleontes.
(A, after Gadow; B, Ditmars.)

656

COMPARATIVE ANATOMY

rudimentary limbs under the skin as has the python. The eyes are with-
out eye-lids.

Class V. Aves. (Birds).

These are closely related to the reptiles. In fact, reptiles and birds
are often grouped together as Sauropsida. They have a single occipital
condyle as do the reptiles. The heart of birds is, however, divided into
right and left halves. Birds are warm-blooded. There is a fusion of
the bones of the manus and there is the formation of a tibio-tarsus and
tarso-metatarsus (intratarsal joint). Feathers cover the body.

Birds are commonly divided into Ratitae or "running birds" such as
the ostrich, rheas, cassowaries, etc., which lack a furcula (wish-bone)
and a keel (Fig. 418) to the sternum, and the Carinatae, or the "flying
birds." These latter have the sternum keeled and the clavicles are
united to form the furcula.

There are two extinct groups which had teeth.

Class VI. Mammalia.

These are warm-blooded animals having a covering of hair, two
occipital condyles, and milk-glands in the female.
Mammals are divided into two sub-classes.
Sub-Class I. Prototheria, or egg-laying mammals.

Order I. Monotremata, which consists of two families (Fig. 383).

Ecliidna aciJeala (spiny ant-eater)

Fig. 383. Monotremata.

C. Echidna hystrix. I, lower surface of brooding female ; //, dissection showing
a dorsal view of the marsupium and mammary glands ; t t» the two tufts of hair
projecting from the mammary pouches from which the secretion flows ; 6m,
brood-pouch or marsupium; cl, cloaca; g.m., groups of mammary glands. (A, after
Shipley and MacBride ; B, after Claus ; C, after Haake.)

CLASSIFICATION

657

Family I. Ornithorhynchidae. (The duck-bill of Australia.)

There is no corpus callosum (Fig. 471), and the brain is the most
primitive of all living mammals.

The eggs, two or three in number, and covered with a hard shell,
are reptilian in form and are laid in a nest of grasses. The heat of the
mother's body hatches them.

Family II. Echidnidae.

These are the Australian Ant-Eaters. There is a temporary mar-
supial pouch. Only one egg, about half an inch long, is laid at a time
and placed in the marsupial pouch by the mouth of the mother. Here
the young hatch in a very immature condition, the mother being obliged
to remove the egg-shell after the young has come forth. The young
Echidna obtains its food by licking the milk-like secretion exuding from
the hairs in the pouch.

Sub-Class II. Eutheria.

These are the viviparous mammals which are divided into two divi-
sions.

Division I. Didelphia (Metatheria) (Fig. 384).
These are the marsupials.

Order I.. Marsupialia. Mammals having a pouch to carry their
young which are born in a rather immature condition. There is usually

Didftphyt dorsiyeru (South American opossum)

Petrogale xanthopus (rock wallaby
with young in pouch)

Fig. 384. Didelphia.

(A, after Vogt Specht ; B, after Nicholson.)

658

COMPARATIVE ANATOMY

no placenta. Australia furnishes us with most Marsupialia, such as the
kangaroo, wombat, phalangerer, pouched mole, and many other forms,
while the opossum is the only example in America.

Division II. Monodelphia. (Placental Mammals).

The young are never carried in a pouch, but a true placenta nour-
ishes the unborn fetus.

Scalops acqwificus (common mole) sorer riltgar;s (common sl.rnr)

Fig. 385. Jnsectivora.
(After Coucs.)

The placental animals are divided into the following sections: Un-
guiculates, Primates, Ungulates, and Cetacea.

Section A, Unguiculates. (Clawed animals).

Order I. Insectivora, such as moles, shrews, and hedgehogs (Fig.

385).

Order II. Chiroptera, such as bats (Fig. 386).

Order III. Carnivora, possess sharp teeth and claws.

Under this heading come the
cat, (Felidae) and dog (Canidae)
families, for example, and many
others.

Order IV. Rodentia are the

gnawing animals. Rabbits, guinea
pigs (Cavia), rats, mice, squirrels,
etc., come under this heading.

Order V. Edentata. This name
means toothless, but the animals
with the exception of the ant-eaters,
belonging to this group do possess
teeth. Different authors classify
the Edentata in various ways. The
animals usually coming within this
group are sloths, ant-eaters, and
armadillos. (Fig. 387.)
Fig. 386. Chiroptera. Section B, Primates. (Mam-

(After Sclater.) ™als With nails.)

CLASSIFICATION

G59

Cliotii

Tamil ml ti'i li-trail'ii-lylii ('/

Fig. 387. Edentata.
(A and B, after Vogt and Specht ; C, from Proc. Zool. Soc. 1871.)

Order VI. Primates. Mostly tree-inhabiting animals with
nails on fingers and toes instead ofc claws or hoofs. The monkeys which
are to be included under this heading are divided into Platyrrhine
(broad-nostril) and Catarrhine (narrow-nostril) groups. The former
are peculiar to the New World and the latter to the Old World. The
higher apes belong to the Old World group. New World monkeys
have a prehensile tail (Fig. 3-8) while no Old World monkeys possess

C'cbus h ifi>oli'ncnn
( wli \le-t

Fig. 388.

Note the prehensile tail so characteristic
of New World Monkeys.

660

COMPARATIVE ANATOMY

this. In the anthropoid or manlike apes (Simiidae) (Fig. 389), there is
no tail at all.

Section C, Ungulates. (Hoofed animals).

Order VII. Artiodactyla (even-toed ungulates), are pigs, (Suidae),
deer (Cervidae), giraffes (Giraffidae), cattle, sheep, goats (Bovidae).

Artiodactyla are all terrestrial or mud-inhabiting animals, usually
of large size, having hoofs on two or four toes. Their stomachs usually
have several chambers and are peculiarly adapted for an herbivorous
diet.

Artiodactyla are often divided into two groups :

Hylobfites entelloides
(dun-colored gibbon)

Pan (Ant 1\ ropopif h ecus)
troglodytes (chimpanzee)

Sinua satyr us (orang-utan)
Fig. 389. Simiidae.

(A and B, after Flower and Lydekker ; C, after Vogt and Specht ; D, after
Shipley and MacBride.)

CLASSIFICATION

661

Fig. 390. Sirenia.
(A, from Brehm, B, from Ingersoll. )

Group I. Suina. (Swine-like).

All the swine family come under this heading including the hippo-
potamus which is really a,n aquatic hog.

Group II. Ruminantia? (Ruminants).

The animals belonging to this group swallow their food rapidly and
later regurgitate it into the mouth for further chewing. Such animals
are said to "chew a cud." Camels, llamas, antelopes, cows, giraffes,
goats and sheep belong here.

662

COM PARATIYE ANATOMY

Order VIII. Perissodactyla. (Odd toed ungulates).

In this order the animal walks on the middle digit of fore and hind
feet. The following three families make up the entire order: Equidae
(horses, asses and, zebras). Tapiridae (tapirs) ; and Rhinocerotidae
(rhinoceroses).

Order IX. Proboscidia (Elephants).

Order X. Sirenia (Sea-cows such as Dugongs and Manatees, Fig.
390).

Aquatic offshoots of ungulate stock.

Hyrax abyssinicus (coneys or hyraces)

Fig. 391. Hyracoidea.
(From Lull after Brehm.)

Order XI. Hyracoidea (Coneys, Fig. 391).

Short-eared, rodent-like, primitive ungulates, usually living among
rocks, although some are tree-inhabiting.

Section D. Cetacea (Whales and Dolphins, Fig. 392).

Order XII. Odontoceti (Toothed-whales).

Examples of these are : sperm-whales, narwhals, beaked whales,

!>,!,, In,,,,* ,lfli>liis (dolphin)

Jialaena myglecetus lu'lialrbonf tefiate)

Fig. 392. Cetacea.

In D, a, upper arm; b, blow-hole; fa, forearm; h, hand; p.thl, small remains
of pelvis, thigh, and leg ; r, roof of palate ; w.w., plates of whalebone ; /, whalebone
fringe. (A, after Flower and Lydekker ; B, after Cuvier ; C, after Sedgwick ; D.
after Holder.)

CLASSIFICATION 663

porpoises, and dolphins. They have teeth but no whale-bone. They
possess a single nostril or "blow-hole" and some of the ribs are two-
headed.

Order XIII. Mystacoceti (Whale-bone whales).

These are also called baleen whales.

For convenience sake the following terms are often used :
Ichthyopsida.

This is a name given to Cyclostomes, Gnathostomes, and Amphibia
combined. The distinctive characteristics of the Ichthyopsida are that
all animals belonging to this group breathe by means of gills at some
period of their life's history. They are therefore, aquatic vertebrates.

They are sometimes called Anamniota or Anamnia because they do
not develop an amnion, and Anallantoida because they do not develop an
allantois.
Sauropsida.

This is the name given to birds and reptiles combined. The dis-
tinctive characteristics of the Sauropsida are, that all animals belonging
to this group breath with lungs and never develop functional gills : They
are therefore terrestrial vertebrates. Sauropsida, together with the
Mammalia, are called Amniota on account of their developing an am-
nion, and Allantoida on account of their developing an allantois.
Tetrapoda.

This is the collective name assigned to all four limbed animals,
whether they are amphibians, reptiles, or mammals.

CHAPTER XLIX

THE INTEGUMENT.

As has been the custom of this book throughout, in examining any
organism, we first observe its external appearance. It is thus the outer
covering of the body which becomes the first object of our study. In
fishes we therefore study scales ; in the frog and the human being, the
skin, while on other mammals, fur, and on birds, feathers. Yet, what-
ever forms such external parts may assume they are a covering of the
body, and as such form what is called an integument ( ).

This term includes the skin or cutis and all the structures derived from
it. If an animal lives in water the effect of water upon such covering
must be considered; likewise, consideration must be given to whether
it lives in a cold climate or in a warm, and whether it lives in the air or
burrows beneath the earth. All these things are bound to have their
effects upon modifying an animal's outer covering.

Microscopically the integument of vertebrates consists of two layers
(Fig. 393), an outer, epidermis, which is the remainder of the ectoderm
after the nervous system has been separated from it, and a deeper layer,
the corium or derma composed of mesenchyme which has been derived
from the somatic portion of the somite. It is into this deeper structure
that the nerves and blood vessels extend.

Accessory organs are developed in both layers, and may begin
growth in one and extend through the other. In all cases, however,
each element of the accessory organs has a very definite place of origin.

The integumental glands thus arise from the epidermis, though
dipping down into the corium to receive a fibrous covering.

Pigment usually develops in the corium and often then migrates
to the epidermis, although it does sometimes develop in the latter layer
itself.

Blood vessels (except in the mucous membrane of the pharynx of
lungless salamanders) develop in the corium.

Sensory nerve endings are quite freely distributed throughout the
epidermis, but the more specialized forms remain in the corium, often
pushed up into the epidermic zone in the form of papillae. The epider-
mis is thus a bloodless, protective, covering with but slight sensitive-
ness, all the more delicate structures being found in the lower layer.

Both skin layers have the power to form hard parts known as
cxoskeleton.

True bone, for example, develops from the corium, while horn and
enamel originate in the epidermis.

Horny structures, such as hairs or feathers (Fig. 394), are formed
from the epidermis alone, but, dip down into the richly vascular corium

THE INTEGUMENT

665

to obtain nourishment. The dermal scutes of ganoids, and the dermal
bones of higher forms arise entirely within the corium. Teeth are com-
posite structures composed of dentine, a hard sort of bone, from the
corium, overlaid with enamel from the epidermis.

It is important that one does not confuse the term integument with
mere portions of the integument ; for example, the epidermis is merely
an outer histologic layer. The ectoderm is merely one of the germ
layers from which both integument and the nervous system arise. The
skin alone on such animals as have feathers, scales or fur, likewise
would not be the integument, but both skin and its immediate outer
covering would constitute such protective substance. The following
schematic arrangement is that commonly used in medicine:

INTEGUMENT.

Epidermis.

1. Stratum corneum.

2. Stratum lucidum.

3. Stratum granulosum.

4. Stratum mucosum.

5. Stratum germinativum (Malpi-

ghian layer).

Secondary Epidermal Structures.

Exoskeleton. H

Hair.

Nails, claws, hoofs, beaks.

Feathers.

Epidermal scales.

Sensory nerve-endings

Dermis.
(Commonly
called Corium)

1. Papillary layer, made up of

dense connective tissue.

2. Reticular layer, made up of

looser connective tissue.

Secondary Dermal Structures.
Glands.
Pigment.
Blood vessels.
Lymph spaces.
Nerves.
Dermal scales.

Note: The exoskeleton of vertebrates consists of bone, horn and
enamel.

Bone originates in the corium (mesodermal).

Horn and Enamel originate in the epidermis (ectodermal).

In comparative anatomy the epidermis in turn is divided into two

666

COMPARATIVE ANATOMY

layers, the lower one being known as the Malpighian layer or stratum
germinativum (Fig. 393). Usually this layer rests on the corium and
is nourished by the fluids from such corium. The cells therefore grow
outward as they divide, forming a second or outer layer, the stratum
corneum. These outer cells being the ones which come in contact with
the surrouading media, are worn away almost as fast as new ones are

•1 J£>c ft on)

Fig. 393.

Diagram of a section through the skin of a mammal to show various layers,
hair, and sebaceous and sweat glands.

added from below. If these outer cells come off in large sheets we find
such a condition as that of a snake shedding its skin.

In land animals the first layer of cells budding off from the Mal-
pighian stratum seems a continuous sheet which is likely to be shed as
a whole. This is called the periderm (Fig. 395). Older books call this
the epitrichiurru but as this word means "above the hair" it is not accu-
rate when it refers to reptiles and birds which have no hair.

Malpighian layer is that in and from which the glands of the

THE INTEGUMENT

06'

skin are formed, while the corresponding part of the ectoderm con-
tributes to such sensory structures as the nose and ear.

The hair, nails, claws, feathers and other outgrowths of the cutis
come from the epidermis (Figs. 393, 394). Land animals usually have a
thicker epidermis than those which live in water. The latter keep the
outer portion of the body constantly moist and so show less of the hard-
ened or horny consistency Avhich is found in animals living in the air.
The corium lies directly beneath the epidermis, there being a loose layer
of connective tissue separating it from deeper structures. The corium
itself is a mass of fibrous connective tissue in which there is an inter-
mingling of elastic tissue, blood vessels, nerves, smooth muscle fibers,
etc. It is much thicker in mammals than in the lower vertebrates. It is

the corium which is commonly
known as leather. "Pigment cells
may be found in both epidermis
and corium. These are mesen-
chyme cells loaded with pigment
which are frequently under the
control of the nervous (sympa-
thetic) system and can be altered
in shape (chromatophores), thus
producing color changes, which
as in the chameleons, may be very
marked."

If the epidermis becomes
cornified, scales are produced.
This takes place by certain cells
in both corium and epidermis be-
ginning to multiply in certain
definite regions. These thicken-
ings become future scales by the
stratum corneum turning into a horny material. In snakes and
lizards these scales, together with all of the stratum corneum (even the
covering of the eye), are periodically moulted, the separation taking place
at the surface of the stratum Malpighii. In turtles and alligators there
is a gradual wearing away of the surface.

Claws, hoofs and nails are closely allied in their manner of growth
to scales (Fig. 396). In fact, a claw is formed by two scales. The dorsal
one is called the unguis and the ventral the sub-unguis. The dorsal scale
grows continually from a root and in mammals is forced over its bed.
The unguis is curved both transversally and longitudinally, while the
sub-unguis forms its lower surface.

In the human nail the unguis is nearly flat in both directions, and
the sub-unguis is reduced to a narrow plate just beneath the tip of the
nail. In the hoof the nnguis is rolled around the tip of the toe, while the

Fig. 394.

A diagram of a developing feather, highly
magnified. der., Dermis ; epL, epidermis ; fol.,
follicle; fth., feather; Mp., Malpighian layer of
epidermis ; pap., papilla by the growth of whose
epidermis the feather is formed. (From Shipley
and MacBride.)

668

COMPARATIVE ANATOMY

sub-unguis forms the 'sole' inside it. The 'frog' is the reduced ball of
the toe which projects into the hoof from behind.

The comparisons in this semester's work will be between fishes,
amphibians, reptiles, birds, and mammals, as these represent the great
type-forms of vertebrates.

FISHES

The life in water makes horny cornification very rare. The epider-
mis of fishes is therefore soft. "Pearl organs," however, appear during
the breeding season in some teleosts. Glands are quite abundant, the
secretion furnishing the slime on the surface. Some groups of fishes also
possess poison glands, usually in close relation to the spines of the fins.
The elasmobranchs have large pterygopodial glands in the "claspers" of
the males, although what these are for we do not know.

Photophores are some of the most interesting and striking of all
epidermal organs. They are usually found in elasmobranchs and teleosts

from the deep seas, where
sunlight does not penetrate.
In reality, they are formed
very much like an eye by the
cells of the Malpighian layer
dipping into the corium.
Here they are cut off from
their origin, forming a
deeper glandular layer, and
the outer rounded body
called the lens. The corium
then forms a reflecting layer,
which in turn is enclosed by
a coat of pigment.
In the myxinoids, there are many thread-cells in little pockets
located in various portions of the skin. Each of these cells contains a
long thread which is discharged upon stimulation, the threads forming
a network in which the mucus secreted by the ordinary gland cells is
entangled. Artificial pearls are made from "essence of pearl," which is
formed in the fibrous tissue of the corium of some fishes. :;i,

AMPHIBIA

The interesting-point about these animals is that during the early
larval stage, the epidermis is often ciliated, and two cells in thickness.
There are numerous mucus and poison glands, sometimes enlargements
of the neck called "parotid glands." These occur on the anura, and there
is likewise a gland on the back near the base of the tail. It will be re-
membered that the large lymph spaces under the skin of the frog make
it possible to remove that animal's skin quite readily. As amphibians

Fig. 395.

Section through the scale of a Lizard. 1. Peridermal
layer. 2. Heavily cornified cells forming the scale. 3.
Pigment cell. 4. Ordinary cells of horny layer. 5.
Innermost Malpighian layer. 6. Dermis. (After Ship-
ley and MacBride.)

THE INTEGUMENT

669

and the lungless salamanders respire largely by the skin, the corium is
richly supplied with blood vessels, which, at the time of the metamorpho-
sis of anura, penetrate into the epidermis. It is at this time that the
lungs are not yet functioning, and the gills are being absorbed. The
stratum corneum is shed periodically, either as a whole, as in urodeles,
or in patches. The "warts" of toads are partially cornifications of the
epidermis. A similar hardening of the skin at the ends of the toes results
in claws.

REPTILES

All these have horny scales and sometimes bony plates though some
of the fossil groups have a naked skin.

Glands are rare, though some "turtles have scent glands beneath
the lower jaw along the line between carapace and plastron ; snakes and
crocodilians have them connected with the cloaca, while the latter have
others, of unknown function, between the first and second rows of plates
along the back, as well as protrusible musk glands on the lower jaw."

Fig. 396.

Comparison of human finger nail (A)
and hoof of horse (B).

d.pt

Fig. 398.

Feather tracts of the pigeon. A, ventral ; B,
dorsal, al.pt, alar pteryla or wing tract ; c.pt,
cephalic pteryla or head tract; cd.pt, caudal
pteryla or tail tract ; cr.pt, crural pteryla ; cr.apt,
cervical apterium or neck-space ; fm.pt, femoral
pteryla ; hu.pt, humeral pteryla ; lat.apt, lateral
apterium ; sp.pt, spinal pteryla ; v.apt, ventral
apterium; v.pt, ventral pteryla. (From Parker
and Haswell, after Nitzsch.)

These latter are not true glands, as they produce no secretion, but cast
out the living cells. Color changes are not remarkable except in a few
snakes and lizards. Claws are common on the toes.

The so-called "femoral pores" on the under surface of the legs of
lizards are not true glands. They are epidermal structures composed of
horny cells, possibly having a sexual function.

BIRDS

The distinguishing characteristic of birds is that they possess
feathers. Both layers of skin are quite thin, there being both scales and

670

COMPARATIVE ANATOMY

feathers developed from the epidermis, although there are extremely few
glands. Some birds, like the ostrich, possess none, though a great many
species have the so-called uropygial gland at the base of the tail which
pours out an oily secretion for dressing the feathers. In a few rasores
(scratching birds), there are modified sebaceous glands near the ear. The
scales on the legs, as well as the claws on the feet and sometimes on the
wings, are often said to be derived from reptilian ancestors.

Feathers are closely related to scales. There are several kinds of
feathers, conveniently grouped under three heads :

1

Fig. 397.

Feathers of a pigeon. A, Down feather ; B, filoplume ; C, quill feather. a.s.,
Aftershaft ; i.u., inferior umbilicus ; qu., quill or calamus ; rch, rhachis or shaft ;
s.u., superior umbilicus; vex., vexillum or' vane. (After Borradaile.)

/, II, HI. — Parts of a feather. /., Four barbs (B.) bearing anterior barbules
(ABB.) and posterior barbules (PBB) ; II., six barbs (B.) in section showing
interlocking of barbules; ///., anterior barbule with barbicels (H.) (After
Nitzsch.)

(1) Filoplumes (hairy feathers).

(2) Plumulae (down feathers).

(3) Plumae (contour feathers).

It is the plumae that have the typical form consisting of shaft and
vane. (Fig- 397.) The base of the shaft is the hollow quill in which
a small amount of loose pith is found. The shaft or rhachis is solid, and

THE INTEGUMENT ti71

a groove runs the length of its lower surface. This is the umbilical
groove. The vane consists of lateral branches or barbs on either side,
which have, in turn, still smaller side branches called barbules. These
latter usually have small hooks at their sides and tips. These hooks
interlock to give iirmness and continuity to the whole vane. In down-
feathers, where hooks are lacking, the barbs arise directly from the end
of the quill, the barbs do not interlock, and no vane is formed. Hair-
feathers consist of long slender shafts \vith a few terminal barbs.

Archaeopteryx, the oldest known fossil bird, had well developed
contour feathers. In most birds, feathers are not equally distributed,
but are gathered in tracts known as pterylae (Fig. 398), and separated
by apteria or featherless regions where there are but few down or hair
feathers. These feather-tracts vary, however, in different groups of
birds, but are used to a considerable extent in classification.

There is a great similarity in the method in which the integument
develops in the different type forms we are studying (Fig. 394). For
example : "A down-feather begins as a thickening of the corium, push-
ing the epidermis before it. By continued growth this, forms a long,
finger-like papilla projecting from the skin. The corium extends into
the outgrowth, carrying blood-vessels with it, while an annular pit, the
beginning of the feather follicle, forms around the base of the papilla.
Next, the corium or pulp of the distal part of the papilla forms several
longitudinal ridges which gradually increase in height, growing into the
epidermis and pressing the Malpighian layer above them against the
periderm. As a rule, the stratum corneum is divided distally into a num-
ber of slender rods arising from the base (quill), which at last are only
held together by the periderm. Then the pulp retracts, carrying with it
the Malpighian layer. With the blood-supply removed, the epidermal
parts dry rapidly, and the periderm ruptures, allowing the rods to sepa-
rate, forming the down."

Contour-feathers are quite like down-feathers in their development
up to a certain point.

It is to be remembered that the dorsal and ventral sides of the
feather were the outside and inside of the stratum corneum of the
papilla. Scales of lizard skin show extreme similarity in their develop-
ment to the feather just described (Fig. 395). Many smooth muscle
fibers act to elevate the feathers in the corium of birds, and there are
also tactile or sense organs. The colors of feathers depend partly upon
red, yellow, orange, brown, and black pigment deposited in them, but the
iridescent colors are due to interference spectra.

MAMMALS

Mammals have a relatively thicker skin than other vertebrates (Fig.
393). There are many glands and considerable hair, except in a few
orders such as the whales and sirenians. There are likewise horns and

672 COMPARATIVE ANATOMY

claws as well as scales, though the latter are not so conspicuous in the
higher forms.

The corium is quite thick and is composed of irregular fibers inter-
laced with muscles, blood vessels, etc. Its outer surface often forms
papillae or ridges; especially on the palms and soles. These ridges carry
the epidermis with them. Several strata may usually be recognized
under the epidermis, namely : a thick Malpighian layer at the base, then a
thin stratum lucidum in which distinct cells cannot be recognized, and
the stratum corneum on the outside. One or more other layers may be
present. A cell must pass through all of these layers "before it is worn
from the surface of the skin.

HAIR

It is important that the histological structure of a hair (Figs. 393,
394) be compared with that of a feather already described.

Scales are found in many orders, usually best developed on the tail
and feet. They are rounded, quadrangular, or hexagonal, the square
scales being arranged in rings around the part, the others in groups of
five known as quincunx. These latter are closely similar to the scales
of reptiles. It seems, from recent investigations, that there is a close
relation between scales and hair, since in mammals with scales, hairs are
usually arranged in groups of three or five behind each scale ; and in
those without scales, the hairs are also grouped in the same manner. In
the early embryo, the hairs are arranged in longitudinal rows so that
grouping seems to come later.

GLANDS

These are of various kinds and types. The structural shapes and
forms into which they may be grouped have already been studied in the
frog and should be recalled, but we must also think of five divisions or
groups, classified not according to structure, but according to function.
Thus we have the following grouping :

(1) Sweat (tubular in shape extending from the Malpighian layer
down through the corium where they are coiled).

Schematic arrangement of varying types of mammary glands. 1, Echidna,
primitive type; 2, Halmaturus (a genus of Kangaroo) forming pouch in lacta-
tion ; 3, Didelphys, forming of nipple before lactation ; 4, Same during lactation
(quite like man) ; 5, Mammilary pouch in cow embryo; 6, in adult cow. (After
Max Weber.)

THE INTEGUMENT

673

(2) Sebaceous (acinous in shape), connected with each hair (Fig.
393).

(3) Mammary (modified tubular glands producing milk (Fig. 399).

(4) Tarsal or Meibomian (modified sebaceous glands in eyelid,
producing oil to keep tears from overflowing (Fig. 400).

(5) Anal (acinous in form, commonly scent glands, secreting a sub^
stance either for sexual attraction or for protection (Fig. 400).

Glands are often also divided according to the method by which
they furnish their secretions. First, into necrobiotic glands if they burst
when liberating their fluids. The individual gland is then destroyed.
And, second, vitally secretory glands if the secretions are poured through
the walls of the gland while the gland itself remains functional for an
indefinite period. In fact, this physiological distinction is often used to
determine homologies when other methods cannot be used.

Each animal class seems to develop integumental glands in its own
peculiar way, there being no definite and continuous history of gland
development found throughout the various groups. Those animals living
in the water, such as fish and amphibians, have glands that secrete pro-
tective substances which are often poisonous. The Sauropsida seldom

Fig. 400.

A, Sweat gland; B, Acinous gland. Complete gland and cross section. The
cross section is cut at the level of the arrow. (Compare with Fig. 393.)

have any integumental glands at all, and snakes have characteristic
cloacal glands secreting a particular nauseating substance. Certain
turtles have so-called musk-glands, probably for sexual attraction. In
some lizards there is a row of so-called glands (really femoral pores)
along the inner portion of the femora that secrete a substance at breeding
time which hardens into short spines or teeth. In birds there are only
the uropygial glands in the caudal region which furnishes an oil for the
feathers.

In mammals there are many and varying glands in the skin, but
they may all be placed into two groups (Fig. 400), namely, the sweat-

674 COMPARATIVE ANATOMY

glands, which are vitally secretory and tubular, and the acinous glands,
many of which are lobed and necrobiotic, although both originally arise
in connection with the hair.

The secretion from the sweat glands is usually thin and watery,
although it may vary from this to a thick viscous pinkish fluid, the so-
called "blood-sweat" of the hippopotamus.

The sweat glands may be found almost anywhere on the entire body
or they may be localized. Localization takes place most frequently in
the paws or on the palms of the hand and soles of the foot. Here they
serve to assist in grasping a given object more solidly.

A modification of these glands also furnishes the oily secretion of
the ear.

It probably has been observed that in hot weather horses sweat
quite pirofusely while dogs do not. This is due to the fact that horses
have sweat glands in the skin while dogs have not, so that dogs can only
obtain the same relief that other mammals obtain, during such weather,
by opening their mouths and panting, as it is only in this way that the
constantly accumulating moisture finds its way to a surface where evap-
oration then brings about a cooling. Muzzles should, therefore, always
permit the oxpening of the mouth.

There are some races of men, such as the Fuegians, who likewise
have few sweat glands.

The acinous glands furnish an oily secretion, apparently for the orig-
inal purpose of lubricating the hair, regardless of how far removed from
this function such glands may ultimately come to be. These glands are
called sebaceous. The tarsal or meibomian glands of the eyelids are
practically hypertrophied sebaceous glands of the eyelashes. These
meibomian glands pour out an oily secretion which lubricates the edge
of the lids and prevents tears from overflowing. There are modified
sebaceous glands in the various orifices^ of the body such as in the lips
and about the anus.

Then there are groups of glands which are localized for quite specific
functions, such as the anal-sacs of the skunk which secrete a protective
substance, and the sexually attractive glands such as that of musk or
civet. Musk is often used in the manufacture of perfumes.

Glands usually open as an elevation at a single place known as a
glandular area. The milk-glands of mammals are typical examples, but
there are cases where there is a sinking of the area so that instead of
the young taking a nipple in their mouths, the lips of the sunken area
fit Closely about the nose of the young and thus prevent the secretion
from being lost. Such is the case in Echidna (Figs. 383, 399). In the
opossum the nipple is really a sac like that in Echidna but turned inside
out.

;, •-! It is a common observation that in many of the domesticated ani-
mals there is a row of nipples extending from axilla to the groin. In the

THE INTEGUMENT 675

embryo of many placental animals there is an entire ridge along which
the mammary glands are to appear. In a short time there are suppres-
sions at regular intervals which leave protruding nipples. These nipples
in turn become reduced and eventually become actual depressions.

The varying position of the nipples in different groups of animals
is due to the retention of some of the nipples in a particular region and
the suppression of the remaining ones along this lateral ridge, which,
as mentioned, extends from axilla to groin.

It is of interest that the aquatic Sirenia (Fig. 390) have pectoral
mammary glands. They bear but a single young at a time and. nurse
their offspring by standing erect in the water while clasping the young
in their flippers. It is supposed that many of the mermaid stories had
their origin from an observation of this animal nursing its young.

It is by no means uncommon to find animals (including man) having
a peculiar arrangement of nipples on their bodies. Supernumerary nip-
ples are termed hyperthelism and supernumerary mammae are termed
hypermastism. These supernumerary developments sometimes occur on
the thigh and other parts of the body. They are considered displace-
ments and not reversions if they occur in out-of-the-ordinary regions,
and reversions if they occur in regions where they normally develop em-
bryologically.

While rudimentary nipples occur in the male of placental mammals,
and may even prove to be functional in some instances, monotromes and
marsupial males do not develop them at all.

SCALES

As the dog-fish has what is called the indifferent type of an exoskel-
eton, it is this animal which forms the classic example for a preliminary
study. Here we find imbricated rows of pointed scales (which merely
means that one row of scales covers the intervals of the next row).
(Fig. 401.) The scales of other fishes, as well as of reptiles, and even
the feather papillae of birds, and the hair of mammals, are all arranged
in a similar manner.

The scales of the dog-fish are said to be placoid (Fig. 401), which
means that each has an approximately flat base from which a sharp-
pointed cusp arises. This cusp is inclined in the direction of the free
edge of the scale. When the scale is in place, the inclination is toward
the posterior portion.

The scale itself consists of a core of dentine which is overlaid with
enamel. In fact, the cusp is almost all enamel. The papilla from which
nourishment comes to the scale lies beneath. In the embryo the scale
forms between epidermis and corium, the dentine arising from the corium
and the enamel from the epidermis.

In the selachians there are several rows of pointed teeth arranged
quite like the scales on the surface. These develop from the same layers

676

COMPARATIVE ANATOMY

and in the same manner as the scales, and consist of a similar structure,
so they are assumed to be merely placoid scales modified by different
usage.

All higher vertebrates inherit teeth. In birds and turtles they are
supposed to have been secondarily lost.

viyc.

B

Fig. 401.

Placoid scales. A, A portion of the skin of the dog fish as seen under a hand
lens; B, a single scale removed from the skin; C, the same in section (diagram-
matic), b., Base of the scale; c., the same in section; d, dentine; e., enamel;
p., pulp cavity. D, Part of the tail of a dogfish seen from the left side, with a
piece of the skin removed. LI., Tube of the lateral line ; myc., myocommata or
septa of connective tissue; mym., myomeres. (After Borradaile.)

E, ctenoid; F, ganoid; and G, cycloid scales. (From the Cambridge Natural
History; E, F after Giinther ; G, after Parker and Haswell.)

Ganoids (Fig. 368) develop their scales (Fig. 401) from the corium
alone, the epidermis playing no part. Consequently the ganoid scale is
all dentine. Ganoid scales are shiny, which is the very meaning of the
term "ganoid." They are usually rhomboidal in shape and do not pos-
sess a cusp. In the sturgeons (Fig. 368) the scales consolidate into
large bony shields called scutes. The former mailed or armoured fishes
merely carried this consolidation to great extremes and the plates were
continuous. In the sturgeons the plates are not continuous but are placed
in rows along the back and sides so that there are large areas unpro-
tected.

It is important to note at this point that in all ganoids (Polypterus,
sturgeons, paddle-fishes, gar-pikes, and bow-fins, Fig. 368), the plates
or scutes cover the entire head. The coming together of the edges forms

THE INTEGUMENT 677

sutures, while the structures lying between the sutures are commonly
called the dermal bones of the skull.

Frontals, parietals, maxillaries and squamosals . are found in all
higher groups, though the opercular and rostral series disappear entirely.
Most of the orbital series also disappear, with the exception of the lacri-
mal. Then, too, there are the dermal bones of the mouth cavity such
as vorners, palatines, and parabasal, which are supposed to have retained
the original character inasmuch as teeth often form on and in these
bones.

In the higher forms the dermal bones, however, arise from various
centers of ossification in the cutaneous mesenchyme, and while this dif-
ference has been explained as a curtailing of the previous race history,
it is quite likely that there is little difference between dermal and car-
tilaginous bone formation in the highest mammalian forms, the dermal
being merely more "stretched out" portions, as will be learned when the
endoskeleton is studied.

Scales in the teleosts, although often rhomboid when quite young,
become circular later and are then called cycloid. Ctenoid scales are
quite similar to cycloid except that they are set in diagonal rows in pock-
ets of the dermis with their free edges overlapping. (Fig. 401.)

Amphibians do not have scales and hard exoskeletons, although there
are extinct forms in which the body was covered with them.

In reptiles the scales arise only from the epidermis and .are there-
fore composed of horn or keratin. There is no trace of bone in them.
The corium, however, furnishes the nourishment to these keratin scales,
although it does not furnish any of the hard parts. There is no definite
knowledge as to the relationship between the scales of reptiles and those
of fishes.

Reptiles also have other integumental structures beside the keratin
scales, namely : spines, combs, and claws ; all, however, also made of
keratin.

The birds are structurally and developmentally quite like the rep-
tiles in that they possess feathers which are homologous to the reptile
scales, and in having their beaks and claws composed of keratin. There
have been toothed-birds in the past, and it is said that tooth-germs have
even been found in the embryonic jaws of some of our modern birds.

In mammals, while there are tiny scales covering the body, these
are seen as definite hard structures mainly on the claws, tails, and some-
times on the backs of such animals as the armadillos (Fig. 387). They
are always only epidermal in origin. In the armadillos the corium sec-
ondarily supplies the hardening portions so that the covering of those
animals becomes very thick, hard, and osseous.

It is assumed, very often, that formerly all mammals were covered
by hard scales because the hair arrangement of mammals is quite like
that of the scales. For example, on the tail of a rat, the scattered hairs

678

COMPARATIVE ANATOMY

will appear among the scales in a very definite relationship, namely, a
group of three hairs (one medial and two lateral) will project beneath
the margin of each. The median hair is the longer and stouter. In
addition to this, there are similar arrangements of hairs in groups of
three even upon areas not definitely associated with scales. The hairs,
however, are arranged in an imbricated series like scales. Even where
the hair is very thick, and forms a heavy fur, this arrangement can often
be made out.

As scales in their simplest form are tiny elevations, the pads on
mammalian feet are often used to illustrate the arrangement and transi-
tion of scales in different mammalian groups.

These pads are usually eleven in number, five for the tips of the
digits, four for the distal margins of palm and sole beneath the inter-
digital intervals, and two for the wrist or ankle.

The scale, rudiments are arranged in rows upon these pads. They fuse
to form "friction ridges," so-called because they prevent the animal from
slipping (Fig. 402). These friction ridges are always arranged at right

angles to the direction in which
there is considerable tendency
to slip. In the arboreal types
of mammals, such as the lemurs
and monkeys, the scale rudi-
ments are arranged in concen-
tric circles as in such animals
there is a tendency to slip in
any and all directions. The
ridges form only on the actual
contact surfaces.

While structure always de-
termines function, yet in integ-
umental studies we have found
that function very decidedly
modifies the various structures,
and later, we shall see that such
modification is not confined to
integument alone.

Now, to be truly scientific,
means to retain an open mind
to all truth wherever and whenever found. But our prejudices and
wishes all too often influence us as readily toward a too conservative as
toward a too radical point of view. We must face the facts as they are,
pleasant or unpleasant, but we must not forget that many different in-
terpretations can be drawn from the self-same facts. An example of
this is brought home at this very point.

There is no question about the facts so far presented, which anyone

A. B.

Fig. 402.

Ventral view of the palm of the hand of an
insectivore and of a primate to show correspondence
between relief and arrangement of friction ridges.
A, Crocidura coerulea (shrew-mouse). Forepaw
showing walking-pads enclosed by triangular folds
of skin. B, Macacus sp? (Old World monkey), Hand,
covered by friction ridges, the arrangement of which
corresponds to the relief of A. The pads are rep-
resented by concentric circles, and the triangular
folds by triradii. These latter features are here
designated by heavy lines, although in the animal
they are no more conspicuous than the others. (From
Wilder after Miss Whipple.)

THE INTEGUMENT 679

can demonstrate for himself. The question that presents itself is simply
this : Does it follow that because a bird has all the characteristics of a
reptile, plus some additional features, that, therefore, it had reptile an-
cestors?

If one accept the so-called Haeckelian law of Biogenesis, that each
individual in the embryonic stage passes through the adult stages of the
race to which it belongs, then such a conclusion is valid; but, if we re-
member that all this so-called law means is that all forms pass through
similar stages, the higher forms then continuing, while the lower ones
remain stationary, another interpretation is still more valid. And our
difficulty is by no means lessened when we remember that biologists at
large are agreed that acquired characteristics are not transmitted. What,
then, becomes of even a reasonable explanation of how any modifications
can be carried on from parent to offspring?

Still further, we have seen from Professor de Vries' work that all
newly appearing structures may be but the return of some recessive por-
tions which have long lain dormant, while in the so-called rudimentary
structures there is always the alternative of considering such structure
an overgrowth or a hypertrophy of some smaller organ valuable at some
time in embryonic life, or, it may even be a true remnant of a structure
no longer needed by modern methods of life, modern foods and modern
environment. Or, still a third alternative suggested by Professor Bate-
son, that just as a complex
structure is the more com-
plex, the smaller and simpler
it can be made to appear, so
the original fertilized egg-
Fig; 40»- cell, from which an entire

Diagram to show growth of bone. A, animal recent-
ly fed madder which causes a layer of bone (black) to Vertebrate develops, is much
be colored by the dye ; B, no madder fed for a time,

when a deposit of colorless bone on outside of colored more C O m p le X than the

layer is formed ; C, later the outer layer becomes thick- /• «« 1 i i_ j • i_

ened and the inner layer is absorbed. Finally Completed DOdy, DC-

cause the single cell had all the possibilities of the complete body within
its tiny self, and consequently, we are always really losing something
as development proceeds.

What is meant by a normal development of a cell into what it is
later to become, is simply, that commonly, certain obstacles are removed
by which these possibilities can come forth. If then, either environment
(external or internal), food, atmosphere, position, injury, or chemical
stimulus removes certain factors which hold back growth, any such pos-
sible factor already present in the cell may come forth ; but its possibility
must have been already present in the primitive cell.

This is well shown by the fa'ct that normally the skin finishes growth
at a certain time, but if a portion of skin is torn, the injury stimulates
the connective-tissue cells which then divide and fill the wound with
scar-tissue, that is, the original injury removes an obstacle to such con-

080 COMPARATIVE ANATOMY

nective-tissue-ceirs growth. What particular factors, then, can be said
to explain modifications? We do not know.

It is the province of science to press a problem further and further
back and thus raise more problems. There is, and can be, nothing abso-
lute about any scientific interpretation.

The student, as does the average man, wants something definite,
something he can be sure of; but this is just what he cannot find in any
biological study; and, unless he can appreciate this and still love science
— science is not for him.

If he should nevertheless go into a scientific field such as medicine
or dentistry, he will be a practitioner who will ever seek and follow the
opinions of the least scientific and least trustworthy men, simply because
these speak with definiteness and absoluteness, albeit, likewise with ab-
surdity.

CHAPTER L.

THE ENDOSKELETON

By the term skeleton we mean all hard parts used for support and
protection outside of what has already been termed the integument. The
skeleton develops only from mesenchyme. It will be recalled that after
the mesoderm has divided into a somatic and a splanchnic layer, these
two layers together are called mesothelium to distinguish them from the
Tiesenchyme, which, while also lying in the segmentation cavity, devel-
ops as separate cells from both the mesothelium and the entoderm. Some
even believe that ectoderm has a part in its formation.

When bone forms from cartilage the lime salts may be laid down
on the inner portion of the perichondrium and from there invade the
cartilage. This is called ossification by ectochondrostosis.

In the other way in which bones form from cartilage, the osteoblasts
are formed from the more interior cells, and then with this group of
osteoblasts as an ossification center, ossification extends in all directions.
This latter method is known as entochondrostosis.

Often the long bones increase by smaller bones forming and then
becoming attached to the ends of the long bones. Such joining is called
an epiphysis.

If madder is fed to an animal the actual bone formation is colored.
This makes it possible to see just how the new bone is formed. The new
bone is laid down outside of that already grown, and with such growth
the "marrow cavity" becomes larger by a resorption of the bone which
has already formed. The osteoblasts are laid down in between the newly
forming layers of bone (Fig. 403).

THE VERTEBRAL COLUMN

We have already seen in our study of the embryology of the frog
and chick how the centra of the vertebrae are formed around the noto-
chord and that possibly some parts of the chorda remain as the inter-
vertebral discs. Here we are to study and compare the adult form in
the various groups.

The most complete vertebrae may be found in the tails of some of
the lower vertebrates. Figure 404 shows a comparison of several types.

It will be noticed that dorsally there is a neural arch, while ventrally
a similar outgrowth from the centrum is known as the haemal arch,
while the pointed end in each case is known as a spine.

The haemal arch is quite incomplete or even entirely absent in the
regions anterior to the tail.

In the higher vertebrates (Fig-. 401) there are articular processes.

682

COMPARATIVE ANATOMY

Fig. 404.

/, A and B. Diagram of a vertebra of a bony fish. A, caudal ; B, trunk ; C,
amphicoelous ; D, procoelous ; E, opisthocoelous ; F, amphiplatyan vertebrae. The
head is supposed to lie at the left, c, centrum or body of vertebra ; ch, notochord ;
h.a., haemal arch ; h.c., haemal canal ; h.s., haemal spine ; h.z., haemal zygapophysis
or articulating facet ; m.b., intermuscular bone ; n.a., neural arch ; n.c., neural canal ;
n.s., neural spine ; n.z., neural zygapophysis ; r, rib.

//. Composition of vertebrae of Reptiles, illustrated by the first and second
cervical vertebrae. (1) Atlas (first cervical) and axis (second) vertebrae of
Crocodile. (2) Atlas and axis of Metriorhynchus, a Jurassic Crocodile. (3)
Analysis of the first two vervical vertebrae of a Crocodile. 2, second basiventral
complex or "intercentrum" continued upwards into the meniscus or intervertebral
pad. (4) Diagram of the fundamental composition of a Reptilian vertebra or
other amniotic, gastrocentrous vertebra. (5) The first three cervical vertebrae
of Sphenodon. (6) Trunk- vertebrae of Eryops, a Permian Proreptile, typically
temnospondylous. cp, articular facet of the capitulum of a rib. ( 7 ) The complete

THE ENDOSKELETON 68$

also called zygapophyses, both on the anterior and posterior sides of each
vertebra, and usually transverse processes extending laterally in the
planes of the original divisions between the muscles.

Where true ribs occur, there are two additional transverse processes
to which these attach.

The centrum where it meets \vith the intervertebral disc, has four
distinct forms (Fig. 404).

If the face of the centra at each end where it is to meet with the
intervertebral disc of the centra lying immediately anterior and imme-
diately posterior to it (as in fishes), is hollow at both ends, it is called
amphicoelous (Fig. 404). If one end is like a ball, namely, convex, and
the other concave, so that the ball-like portion can fit into it, the condition
is known as precocious if the socket lies on its anterior surface, and
opisthocoelous if on the posterior surface, while if the ends of the centra
are flat, as they usually are in mammals, such a condition is known as
amphiplatyan.

The arches of the vertebrae form first (Fig. 352), and the centra
later, and the sclerotome divides into a caudal and cranial half which
thus makes possible an advantageous condition to the animal in permit-
ting interaction of skeleton and muscles (Fig. 305).

It must be remembered, however, in this connection, that in some
animals, normally, and in others abnormally, the two halves of the
sclerotome may unite (as in some fishes), and thus not have this inter-
play of muscles ; or two neural arches may form by the rudiment which
normally becomes one arch, dividing as does the sclerotome, and thus
produce a greater quantity of vertebrae than usual. And, not only may
this happen to the neural arches, but also to the centra. In fact, almost
any variation in the spinal column may be accounted for by an embryo-
logical condition remaining in the adult form.

As the ventral nerve root usually penetrates the caudal division of
the halved sclerotome, and the dorsal root passes between the two divi-
sions of each sclerotome but penetrates the cranial portion, one can tell
in the adult, from following these nerve roots, which of the adult struc-
tures come from cranial and which from caudal halves.

atlas of an adult Trionyx hurum. The second basi ventral ( intercentrum ) is
attached to the posterior end of the first centrum which, not being fused with the
second centrum, is not yet an odontoid process. (8) The complete atlas of an
adult Trionyx gangeticus, still typically temnospondylous. (9) The first and second
cervical vertebrae of an adult Platemys. (10) The complete atlas of a Chelys
fimbriata. Az, Anterior zygapophysis; B.D, basidorsal ; B.V, basi ventral ;
Cv Ca, C3, first, second and third centra, formed by the interventralia ; Cpl, Cp*.
articular facets of the capitular portions of the first and second ribs ; I, V, In-
terventral ; Nv N2, N3, first, second and third neural arch ; formed by the basi-
dorsalia (B.D) ; Od, odontoid process (which is the first centrum) ; Pz, posterior
zygapophysis ; Rlt R2, ribs ; Sp, detached spinous process of the first neural arch
tv t2, tubercular attachments of the first and second ribs ; 1, 2, 3, 4, "intercentra"
(which are the basiventrals) ; 7, 77, 777, position of the exit of the first, second
and third spinal nerves.

777, Trunk vertebrae of a tropical Skate, h, haemal process ; t, intercalary
plate; n, neural process; r, rib; a, spinous process. (77, After Gadow. 777, from
Kingsley after Dumeril.)

£84 COMPARATIVE ANATOMY

The different shaped ends of the centra which have already been
mentioned are brought about after the vertebrae are quite definitely
formed. The centra with their arches are in a quite definite position
and the centra cannot therefore grow any more except at the ends.
These may have more substance laid down in the intervertebral regions,
however, and thus ultimately come to be amphicoelous. procoelous,
opisthocoelous, or amphiplatyan.

REGIONS OF THE VERTEBRAL COLUMN

The regions of the spinal column are :

(1) Cervical. The neck region, either without ribs of any kind or
the ribs are smaller than in the other regions.

(2) Thoracic. These have distinct ribs attached.

(3) Lumbar. Following the thoracic, and without ribs.

(4) Sacral. This region includes one or more vertebrae with which
the pelvic girdle is connected.

(5) Caudal. The tail-portion immediately following the sacrum.
These divisions are quite distinct in the higher vertebrates, but in

;he lower, any and all combinations may form, so that the ribs may ex-
tend almost the entire length of the spinal column. In such cases all
vertebrae having ribs are called dorsal.

In fishes, snakes, and whales, the sacral region cannot be distin-
guished, and in the fishes the dorsal and cervical vertebrae are quite
indistinguishable. In this latter case there are, therefore, only trunk or
abdominal vertebrae, and caudal vertebra, the line being drawn where
the haemal arches begin to have ribs attached.

The first cervical vertebra to which the skull is articulated is called
the atlas in all higher vertebrata, while the second cervical vertebra, at
least in the amniotes, is called the axis or epistropheus (Fig. 404).

In mammals the atlas can always be distinguished by the two an-
terior articulating surfaces for the two condyles of the skull, and the
axis, by the tooth-like projection known as the odontoid process, on
which the atlas turns.

It is interesting to note that embryologically, this tooth-like process
develops from the atlas, but then separates and later becomes attached
to the next succeeding vertebra.

In a few reptiles there is a so-called proatlas, consisting of one or
two plates lying between the atlas and the skull. It is not known just
what relationship this bears to the other vertebrae.

In fin-bearing animals, if the spinal column runs to the end of the
body in a straight line (Fig. 405), the caudal fins are known as diphycer-
cal, a condition found in the young of all fishes and in adult cyclostomes,
dipnoans, and crossopterygians.

If, as in the elasmobranchs and ganoids, the tail axis bends abruptly
upward at the end, but retains the dorsal fin-part and a portion of the

THE ENDOSKELETON

685

ventral region, it forms what is called a heterocercal tail, while if there
is the same upward bend of the spinal column but the ventral and dor-
sal fin-portions of the tail become alike as to size and shape, the tail is

Fig. 405.

Diagrams of the principal forms of tails in fishes. A, proto-
cercal fin (as in polypterus) ; B, heterocercal (as in sharks) ; C, homo-
cereal (as in most teleosts) ; D, homocercal (as in Amia). (After
Folsom. )

said to be homocercal. Homocercal tails are brought about by the neural
arches becoming smaller and the haemal arches becoming larger.

THE SKULL

Bone either forms in cartilage or membrane and it is quite common
to hear biologists speak of cartilaginous and membranous bone. How-
ever, recent investigations lead us to believe that the so-called membrane
is nothing more or less than cartilage drawn out very thin in those parts
where the greatest pressure is produced. This can be understood the
better if Figure 406 be carefully studied. It will be noticed that all the
superior and inferior boundaries are membranous, for here there is noth-
ing to prevent a considerable extension of growth, while in the inner-
most portions, where pressure comes from practically all sides, it is
cartilage.

Babies have a very soft spot on top of and close to the center of
the head for about one and one-half years after birth. Places such as
these are called fontanelles. These fontanelles are found during the em-
bryonic period at all spots in the skull where several points of ossifica-
tion come together. Ossification begins at many points, each center of
ossification extending and growing toward each other. The fontanelle
is the unossified spot that constantly becomes smaller until ossification
is complete.

Professor Eben J. Carey has recently shown, that contrary to the
usually accepted idea that bone grows simply because there is an inner
something which makes it assume definite forms, that it is the stress and

4386

COMPARATIVE ANATOMY

pull and pressure of its location which determines its shape, size, rapidity
of growth, and even its joints.

The reason this has not been understood heretofore is because for-
mer experimenters took only sections from the growing bone itself for
their study, whereas Professor Carey has taken the complete embryo-
logical structure, including all muscles and related portions, which might
throw light upon the pull and pressure which affects such bone during
its growing period.

Observing the ossification centers in the skull will throw light on
this subject (Fig. 406). There are many such centers, and they are
always found at exactly those points where there is an especial stress or
pressure. At these points it may be that sharper bends in the blood
vessels cause a slowing of the blood stream, which slowing in turn causes
lime salts to be laid down at the angles to a much greater extent than
-where the blood stream can rush past more swiftly. Then, with each

Pin* H f M

*£%*

Fig. 406.

A diagram of the skull bones of a mammal, the mem-
brane bones shaded. BO., Basioccipital ; EO., exoccipital ; C,
condyle ; SO., supraoccipital ; Par., parietal ; Fr., frontal ; Na,
nasal ; Pmx., premaxilla ; ME., mesethmpid ; L, lachrymal ; Tu.,
turbinal ; PS., presphenoid ; OC., orbitosphenoid ; AS., alis-
phenoid ; BS., basisphenoid ; SQ., squamosal ; P., periotic ; T.,
tympanic ; PI., palatine ; Pt., pterygoid ; MX., maxilla ; Ju.,
jugal ; T.H., tympanohyal ; S.H., stylohyal ; E.H., epihyal ; C.H.,
ceratohyal; B.H., basihyal ; Th.H., thyrohyal ; vomer ; MN.,
mandible. (From Borradaile, modified from Flower and
Weber.)

succeeding deposit of such a hardening substance, a still greater number
of blood capillaries is affected so that more lime is laid down, and so
on, until all of the capillaries have been more or less obliterated and the
'entire cartilage or membrane has become ossified.

Beginning with the axial skeleton, the skull becomes our first object
of attention. The cranium is that part of it which ericloses the brain
as well as the bony p^rts forming the eye-socket, the ear and nose, while
the more caudal portion, which is directly connected with the cephalic
<end of the digestive tract, is called the visceral skeleton.

That portion of the skull which is cartilaginous is known as the

THE ENDOSKELETON 687

chondrocranium, while the membranous portion is called a membrano-
cranium.

On each side of the notochord (which in the embryo extends as far
forward as the infundibulum of the brain) a horizontal plate of cartilage
is formed. These plates are known as parachordal plates (Figs. 310,
353). These extend laterally to the ears, forward to the end of the noto-
chord, and backward to the exit of the tenth nerve. The otic capsule
(cartilaginous) then grows about each internal ear and joins the para-
chordals. This forms a sort of trough in which the most caudal portion
of the brain lies. The floor of this trough is known as the basilar plate,
being formed of the parachordals and notochord as a floor, while the
sense capsules constitute the sides.

From this basilar plate two cartilages pass forward on each side
forming a similar trough for the anterior part of the brain. According
to Professor Kingsley, the lower of these, called the trabeculae cranii,
"join the anterior margin of the basal plate, while the dorsal bars, the
alae temporales or alisphenoid cartilages are eventually connected with
the anterior wall of the otic capsules. In most vertebrates the trabecu-
lae and alisphenoids develop as a continuum, but in some elasmobranchs
they are at first distinct. The two trabeculae unite in front to form a
median ethmoid plate beneath the olfactory lobes of the brain, beyond
which they diverge as two horns, the cornua trabeculae, ventral to the
nasal organs. The floor of the trough in front of the ears is formed by
the ethmoid plate anteriorly, while behind, it is usually of membrane,
but in the elasmobranchs, cartilage gradually extends from one trabec-
ula to the other, closing last below the infundibulum and hypophysis,
these lying for a time in an opening (fenestra, later fossa hypophyseos),
and after the closure, in a pocket in the floor of the chondrocranium, one
of the cranial landmarks, the sella turcica."

"In the more primitive vertebrates the trough is converted into a
tube around the brain by the extension of cartilages between the
alisphenoid cartilages and the otic capsules of the two sides dorsal to the
brain. This roof or tegmen cranii is usually incomplete, having one or
more gaps or fontanelles, closed only by membrane. In the higher ver-
tebrates the cartilage roof is at most restricted to a mere arch, the synotic
tectum, between the otic capsules of the two sides.

"Later a pair of nasal capsules develop around the olfactory organs.
These are usually fenestrated and become united to the cornua, alisphe-
noids and ethmoid plate. In a similar way a sclera (sclerotic coat) forms
around each eye, but since the eye must move, this sense capsule
never unites with the rest of the cranium. Behind the otic capsules
a varying number of (four in some sharks and most teleosts ; in others
three; in amphibia two), occipital vertebrae are developed, which later
fuse with the rest of the chondrocranium. They alternate with myotomes
and nerves in this region as do the vertebrae of the vertebral column.

688

COMPARATIVE ANATOMY

"The cartilaginous visceral skeleton arises in the pharyngeal region
which is weakened by the presence of the gill clefts. It consists of a
series of pairs of bars, the visceral arches lying in the septa between the
clefts, the bars of a pair being connected below the pharynx. Each
bar, at first, is a continuous structure, but to allow for changes of size in
the pharynx, each becomes divided into separate parts, while the arches
become connected in the mid-ventral line by unpaired elements, the

orbit °Uc7sule

chondrocranium \_^ N k hj^andibular *uralardi intercalary arch

olfactory capsule

pharyngobranchial
epibranchial
-ceratobranchial
• hyobranchial

basibranchial
or copula

pterygoquadrate
cartilage

mandibular (first)
gill arch

Meckel's
cartilage

hyoid (second)
gill arch.

V.Vl.VIl, of. I'll.
\p. n.a, ».,/. c. r.r. ^ ^£r. an.c. t.c.J. ! Lo.c. \ of. V. V-£- f^ ^

... fw^m^m

»

///. hy.a.

£1.
Fig. 407.

I. Diagram of the chondrocranium vertebral column, and gill arches of an
elasmobranch to show particularly the parts and relations of the seven gill arches.
(Hyman's modification of Vialleton. )

II. Skull and part of the backbone of a dogfish, seen from the right side. The
skeleton of the visceral arches has been pulled a little downwards, au.c., Auditory
capsule ; 6.6., basibranchial cartilage ; b.h., basihyal cartilage ; c., centrum ; cer.h.,
ceratohyal cartilage ; cer.b., ceratobranchial cartilages ; d.r.,v.r., foramina for the
dorsal and ventral roots of a spinal nerve ; e.m., extrabranchial cartilages ; e.c.f.,
»xternal carotid foramen ; ep.b., epibranchial cartilages ; e.l., ethmopalatine liga-
nent ; gr., groove for vein which connects orbital and anterior cardinal sinuses ;
g.r., gill rays ; hy.a., foramen for hyoidean artery ; hymd., hyomandibular cartilage ;
i.o.c., interorbital canal ; i.p., intercalary plate ; M.c., Meckel's cartilage ; I.e.,
labial cartilages ; nas.c., nasal capsule ; n.a., neural arch ; n.sp., neural spine ; o.n.f.,
orbitonasal foramen; op.V., opVII., foramina for ophthalmic, branches of fifth
and seventh nerves; op'., foramen through which combined ophthalmic nerves
pass from the orbit to the snout; op.g., grooves for op.V., VII.; orb., orbit;
p.sp.l., postspiracular ligament ; pal.b., palatine bar ; ph.b., pharyngobranchial
cartilages; r., rib; rost., rostrum; tr., ^entrilateral (so-called transverse) process.
(After Borradaile.)

THE ENDOSKELETON

689

copulae (Fig. 407). The two anterior arches are specialized and have
received special names, the first being the mandibular, the second the
hyoid arch, the others, in the region of the functional gills, being called
collectively gill or branchial arches. The number of these last varies
with the number of gill clefts, there being seven in the primitive sharks,
a smaller number in the higher groups, in which, with the loss of
branchial respiration, their form and functions may be altered. At first
all are clearly in the head region, but by the unequal growth of cranium

Fig 408.

A, B, C, Bones of early human skull to show their compound- nature. A,
occipital bone at birth showing the five elements of which it is composed. B,
sphenoid bone in an embryo of four months. C, temporal bone at birth, showing its
three components. Cartilage represented in black. (Redrawn from Sappey and
Hyman.)

D, Diagram of skull of new born child. White areas represent bones of
intramembranous origin; dotted areas represent bones (not derived from branchial
arches ) of intracartilaginous origin ; black areas represent derivatives of branchial
arches. (Combined from McMurrich and Kollmann.)

690

COMPARATIVE ANATOMY

and pharynx the gill arches are carried back some distance behind the
head. All of the arches are cartilaginous at first."

The mandibular arch lies in the region of the fifth nerve, behind
the mouth^nd between it and the first visceral cleft — the cleft that be-
comes the spiracle or Eustachian tube. "The arch is divided into dorsal
and ventral halves, known respectively as the pterygoquadrate (palato-
quadrate) and Meckelian cartilages. In the elasmobranch and chondro-

Fig. 409.

Side view of a mackerel, ar, articulare ; as, alisphenoid ; bo,
basioccipital ; d, dentary ; enp, entopterygoid ; eo, exoccipital ; ep,
ectopterygoid ; es, extrascapular ; epo, epiotic ; /, frontal ; io, in-
teropercular ; eth, ethmoid ; I, lacrimal ; mx, maxillary ; mxp,
metapterygoid ; n, nasal ; op, opercular ; p, parietal ; pe, petrosal ;
pf, postfrontal ; pi, palatine ; pm, premaxillary ; po, preoperculum ;
poo, postorbital ; prf, prefrontal ; ps, parasphenoid ; q, quadrate ;
sbo, suborbital ; so, supraoccipital ; sop, suboperculum ; spo,
sphenotic ; sq, squamosal ; sac, suprascapular ; sy, symplectic.
(From Kingsley after AUis.)

steous ganoids the pterygoquadrate forms the upper jaw, lying parallel
to and joined to the cranium by ligaments or in chimaeroids by continu-
ous growth. With the appearance of bones a new upper jaw is formed,
as described below, and the pterygoquadrate becomes more or less re-
duced, and ossifies as two or more bones with greatly modified functions.
Meckel's cartilage is the lower jaw of the lower vertebrates, while in the
higher it forms the axis around which the membrane bones of the defini-
tive jaw are arranged.

"The hyoid arch lies between the spiracle and the first true gill cleft,
in the region of the seventh nerve. It divides into an upper element, the
hyomandibular cartilage, and a ventral portion, the hyoid proper, which
may subdivide into several parts. In the lower elasmobranchs the
hyomandibular and the rest of the hyoid arch are closely connected, but
in the higher fishes the hyomandibular becomes more separated from
the ventral portion and tends to intervene between the mandibular arch
and the cranium, becoming a suspensor of the jaws. Still higher it loses
its suspensorial functions, becomes greatly reduced, and apparently is

THE ENDOSKELETON

691

Fig. 410.

A, Dorsal and B, Ventral views of cranium of Branchiosaurus aalamandr aides,
C, Posterior view of cranium of Trematosaurus. Br, branchial arches ; C, condyle ;
Ep, epiotic ; F, frontal ; J, Jugal ; L.O, lateral occipital ; M, maxillary ; N, nasal ;
No, nostril ; Pa, parietal ; PI, palatine ; Pm, premaxillary ; P.o, postorbital ; Pr.f,
prefrontal ; Pa, parasphenoid ; Pt, pterygoid ; Ptf, postf rental ; Q, quadrate ; Qj,
quadrate- jugal ; S.o, supraoccipital ; Sq, squamosal ; St, Supratemporal ; V, vomer.
(After Gadow.)

D, Chondrocranium of a frog shortly after metamorphosis. fov, fenestra
vestibuli ; m, Meckel's cartilage ; mtg, metapterygoid ; nc, nasal capsule ; ptgq,
pterygoquadrate ; tnas, tectum nasalis ; tsyn, tectum synoticum ; timed, taenia
tecti medialis ; III- V, nerve exits. (From Kingsley after Gaupp.)

subsidiary to the sense of hearing or it may be lost, the question not be-
ing decided. The hyoid proper becomes more or less intimately con-
nected with the arches behind and also is largely concerned in affording
a support for the tongue.

"The branchial arches are all similar to each other in the lower ver-
tebrates, but with the loss of branchial respiration in the higher groups,
they tend to become reduced, the reduction beginning behind. Some may
entirely disappear, others give rise to the laryngeal cartilages and the
first may fuse with the hyoid. The first arch is in the region of the ninth
nerve ; the others in that supplied by the tenth."

*Here it will be necessary to review the frog skull very thoroughly',
bearing in mind that, as in all vertebrates, the occipitals, the two sets of

"The frog does not have all the bones mentioned. All head bones that any vertebrate pos-
sesses are given in the list, so that the student must not expect to find any animal with all these
structures, though he will find no vertebrate with head-bones not mentioned here.

COM PARATIVK ANATOMY

sphenoids, and the ethmoids, are the four groups constituting the car-
tilaginous cranium. (Compare Figs. 407, 409, 410, 411, 412, 413, 414.)

Fig. 411.

Upper two views, skull of snake, an, angulare ; av, articular ; bo, basioccipital :
bs, basisphenoid ; d, dentary ; eo, exoccipital ; epo, epiotic ; /, frontal ; mx,
maxillary ; n, nasal ; oo, opisthotic ; p, parietal ; pi, palatine ; pm, premaxillary ;
pro, prootic ; pa, parasphenoid ; pt, pterygoid ; q, quadrate ; sa, surangulare ; so,
supraoccipital, sq, squamosal ; tr, transversum. ( From Kingsley after W. K.
Parker.)

Lower two views, skull of turtle. A, lateral view. B, from below, ang.
angulare, art, articulare ; au, orbit ; b.sph., basisphenoid ; ch, choanae ; d, dentary ;
fr, frontal ; j, jugular ; max, maxillae ; occ.b, basi-occipital ; occ.L, lateral occipital ;
occ.s., superior occipital ; op.ot., opisthotic ; pal, palatine ; par, parietal ; p.fr.,
postfrontal ; pr.mx., premaxillae ; pr.fr., prefrontal ; pt, pterygoid ; q, quadrate ;
q.j., quadratojugal ; sq, squamosal; vom, vomer. (After Schimkewitsch.)

THE ENDOSKELETON

693

l.Oc.

Fig. 412.

Upper view, a diagram of a bird's skull, disarticulated.
(After Gadow. ) Membrane bones shaded. B.Oc., basioccipital ;
E.Oc., exoccipital ; S.Oc., supraoccipital'; Pa., parietal ; Fr., frontal ;
Na., nasal ; pm., premaxilla ; M., maxilla ; Jit., jugal ; Qj., qua-
drato-jugal ; Qu., quadrate ; PL, palatine ; Pt., pterygoid ; pe.,
periotic ; Sq., squamosal ; AS., alisphenoid ; B.S., basisphenoid ; O.S.,
orbitosphenoid ; Pr.Sph., presphenoid ; vo., vomer ; iOS., interorbital
septum ; E., ethmoid ; Se., nasal septum ; De., dentary ; Sp .,
splenial ; An., angular ; SA., supra-angular ; Ar., articular ; M.K.,
Meckel's cartilage.

Lower views, model of developing chick-skull. A, profile view,
and B, hyoid apparatus, as seen from below, ang., Angulare ; b.br.,
basibranchial ; b.hy., basihyal ; c.aud., auditory (otic) capsule;
c.ent., entoglossal cartilage ; c.meck., Meckel's cartilage ; coi.au.,
auditory columella ; cor.hy., cornua of the hyoid ; dent, dentary ;
e.br., epibranchial, 1 ; fr, frontal ; fc.br1., ceratobranchial, 1 ; max,
maxillary ; was, nasal ; operc, opercular ; pa, parietal ; pal, palatine ;
pl.a.o., anteorbital plane ; pl.s.sept., supraseptal plane ; pl.sph.l.,
sphenolateral plane ; pr.fr., prefrontal ; pr.r.art., retroarticular
process; pr.tect., Processus tectalis (roof -forming) ; p.sph., para-
sphenoid ; pter, pterygoid ; qu, quadrate ; qu.j., quadrato-jugal ;
s.ang., supraangular ; sept.i.o., interorbital septum ; sq, squamosal ;
vom, vomer; zyg, zygomatic process. (After Gaupp.)

694 COMPARATIVE ANATOMY

The occipital is divided into:

A supraoccipital, an exoccipital on each side, and a basioccipital.
These four bones form the borders of the foramen magnum.

dermosupraoccipital

e*occipital
foramen magnum

rental
frontal

orbit
jugaJ

postfrontal
postorbita!
intertemporal
squamosal
supratemporal
parietal

quadratojugal

tabulare
dermosupraoccipital

nans
aremaxilla

prefrontal

lacrimal

frontal
jugal

orbit
lateral temporal

arcade

postprbital bar
postfrontal
parietal
lateral temporal

fossa

squamosal
quadrate

supratemporal
arcade

interparietal
occipital

quadratojugal

Fig. 413.

Dorsal view of the skulls of four representative vertebrates to show changes
and reduction of the membrane bones of the roof. A, skull of an extinct amphibian,
Capitosaurus, belonging to the Stegocephala; note the large number of membrane
bones completely roofing the skull. B, skull of one of the most ancient reptiles,
Seymouria, belonging to the Cotylosauria ; the membrane bones are nearly as
numerous as in the amphibian, are similarly arranged, and completely roof the
skull. C, skull of a modern reptile, the alligator ; several of the membrane bones
which were present in the extinct forms have been lost, and the roof has several
openings. D, skull of a modern mammal, the dog, showing still greater loss of
membrane bones. Membrane bones blank; cartilage bones stippled. (From
Hyman's "A Laboratory Manual for Comparative Vertebrate Anatomy," A, after
Reynolds, B, after Williston, by permission of the Chicago University Press.)

THE ENDOSKELETON

695

The sphenoids are:

The basisphenoid, extending forward to the sella turcica.
The presphenoid, extending from the trabeculae to the ethmoid
plate.

The alisphenoid, closely related to the basisphenoid.

The orbitosphenoid, in close relation with the presphenoid.

Fig. 414.

A,' A ventral view of the skull of a rabbit, ol., External process of the
alisphenoid ; b.oc., basioccipital ; b.ap., basisphenoid ; e.a.m., external auditory
meatus ; ex.oc., exoccipital ; f.m., foramen magnum ; inc., incisors ; ju., jugal ; mr.,
molars ; m.x., maxilla ; oc., occipital condyle ; pi., palatine ; pm., premaxilla ;
pmr., premolars; pr.sp., presphenoid; pt., pterygoid ; s.oc., supraoccipital ; ty.b.,
tympanic bulla ; v, vomer ; zy.mx., zygomatic process of maxilla ; zy.s., zygomatic
process of squamosal. (From Borradaile.)

B, A side view of a rabbit's skull. Pmx., Premaxilla ; No., nasal ; Fr., frontal ;
Pa., parietal ; Sq., squamosal ; S.O., supraoccipital ; Per., periotic ; T., tympanic
(the reference line points to the bony external auditory meatus, beneath it lies
the inflated bulla) ; PO., paroccipital process. (From Thomson.)

The ethmoids are divided as follows :

Mesethmoid, lying medially.

Ectethmoid, one on each side of the mesethmoid.

The ectethmoid becomes the ectethmoid labyrinth which comes to
lie between ectethmoid and mesethmoid, and the turbinate bones are.
sometimes added here.

From the otic capsule are developed various otic or petrosal bones.

These are usually divided into:

Prootic, lying in front of the ear.

Opisthotic, lying behind, although usually meeting below with a

696 COMPARATIVE ANATOMY

Epiotic and a sphenotic, in the teleosts and a few other forms de-
veloped from the lateral wall of the otic capsule and lying- in front and

Pterotic, lying behind and directly above the horizontal semicircular
canal of the ear.

The otic bones usually fuse and form a petrosal bone in all the
higher forms. This lies directly between the lateral parts of the basi-
occipital and the sphenoid.

A ring of sclerotic bones is often formed from the sclera of the eye
of birds and reptiles, though these never unite with the regular bones.
From the nasal capsules a lateral ethmoid often forms on the upper wall,
while the turbinate bones form on the medial and lateral walls. In man
the single occipital bone is formed by the four occipitals mentioned
above. The single sphenoid is a fusion of the six sphenoids mentioned
above, the alisphenoids form the greater wings and the orbito sphenoids
the lesser ones. (Fig. 408.) The ethmoid is similarly made up of the
various ethmoids mentioned.

The mernbranocranium gives rise to the following bones :

Nasal, covering the olfactory region.

Frontals, between the orbits.

Parietals, on the same level with the otic capsules.

Inter-parietal, unpaired, between parietal and supraocciptal.

While these are practically all of the membranocranial bones in the
roof of the cranium of the higher forms, others may appear in the lower
groups. For example :

Supratemporal, lying lateral to each parietal.

Postfrontal, behind the orbit.

Postorbital, forming the posterior wall of the orbit.

Supraorfcital, taking the place of the frontal in forming the superior
or medial wall of the orbit.

Prefrontal, bounding the orbit in front.

Lacrimal, lying lateral to the prefrontal.

Intel-temporal, lying dorsal or medial to the alisphenoid.

Postparietal, between parietals and interparietals.

Epiotic, lying above each otic capsule and usually called the tabu-
lare.

If as in some of the fish, birds, and reptiles, the basilar plate and
trabeculae fail to ossify, then the roof of the mouth, which is also, of
course, the frontal of the cranium, is also a membrane bone called the
parasphenoid, while farther forward the vomers or plough-share bones
are also membranous and lie in the nasal region. Both parasphenoid
and vomers may bear teeth.

As soon as bone commences to form, the pterygoquadrate changes
considerably, becoming closely connected with the cranium in front.
The middle portion disappears and the palatines, a pair of membrane
bones, replace the disappearing part. The remaining portion of the

THE ENDOSKELETON 697

visceral arch ossifies, usually though not always, forming two bones on
each side, the paired anterior pterygoid and the paired posterior quadrate.
These now become the suspensor of the lower jaw. In some forms such
as the teleosts and the reptiles, there appears an entire series of ptery-
goid bones.

Outside of the pterygoquadrate a second arch of membrane bone
develops to form the functional upper-jaw in all bony vertebrates. This
when fully developed consists of:

Squamosal, underlying the quadrate.

Quadratojugal, which follows immediately.

Zygomatic, also called malar or jugal.

Maxillary.

Premaxillary, which forms the tip of the jaw.

Only the maxillary and premaxillary bear teeth.

When, as in the higher forms of human life, the roof of the skull is
not continuous, but openings of various kinds are seen, such openings
are known as fossae. The more common and constant are as follows :

Infratemporal, being the most lateral.

Supratemporal, which is separated from the foregoing by the
squamoso-postorbital bars.

Posttemporal, lying between the parietal, supratemporal, and occipi-
tal bones.

Temporal, when infra and supra temporal fossae unite.

Any of the bones mentioned above may fuse or disappear entirely
in certain groups, while in others there may be connections quite differ-
ent from the usual type.

The lower jaw is by no means modified as extensively as the upper.
Meckel's cartilage, by an ossification, gives rise to two bones in each
half of the lower jaw. There is an articular bone (articulare) where the
jaw meets with the quadrate, and at the tip where both sides unite — the
symphysis — there may be a mento-Meckelian bone, although this does
not occur often. The rest of Meckel's cartilage forms an axis about
which the membrane bones of the lower jaw are arranged. These are
as follows :

(1) Dentary, surrounding the Meckelian in front and bearing teeth.

(2) Splenial, on the inner side behind the dentary and often bear-
ing teeth.

(3) Angulare, on the lower side usually extending back to the
hinder end of the jaw.

(4) Surangulare, lying on the outer posterior part of the jaw.

(5) Coronoid, on the upper side attach the muscles which close the
jaws.

(6) Goniale, also called antarticular or dermarticulare, lies on the
medial and ventral sides of the articulare with which it usually fuses.

CUM r AKATIVE ANATOMY

The dentary, splenial, and angulare are usually found, but very few
vertebrates have them all.

In the hyoid and branchial arches the outside portions are known
by the same names as the corresponding cartilages, membrane bones
never being found here.

The method by which the jaws are suspended varies. If the ptery-
goquadrate is directly connected with the cranium as in a few elasmo-
branchs, the suspension is called amphistylic. If it is held in place by
ligaments and the hyomandibular is interposed between the otic capsule
and the hinder end of the jaw, it is called hyostylic, while, if the pterygo-
quadrate is more or less fused with the cranium, as in all the higher forms
beyond the fishes, it is known as autostylic.

THE APPENDICULAR SKELETON

There are two types of appendages, namely: median or azygos,
which are found in aquatic vertebrates, and the regular paired appen-
dages found in all other classes except the cyclostomes. Several theories
have been advanced to account for the two types. One of these is that
the two types of appendages have no relation to each other, and devel-
oped independently, the pelvic and pectoral girdles being supposed to
have originated from the gill arches, while the appendage bones have
been derived from that portion which normally supports the gills. The
other view assumes that two longitudinal folds ran the full length
of the body behind the head (Fig. 415), each of these folds being sup-

Fig. 415.

Diagrams showing A, the undifferentiated condition
of the paired and unpaired fins in the embryo, and B,
the manner in which the permanent fins are formed
from the continuous folds. AF, anal fin ; An, anus ; BF,
pelvic fin ; BrF, pectoral fin ; D, dorsal fin-fold ; FF,
• dorsal fin ; RF, dorsal fin ; SF, tail-fin ; S, S, lateral folds
which unite at S' to form ventral fold. (From Wieders-
heim.)

ported by a series of skeletal rods. The two dorsal and two ventral
folds then fused to form the dorsal and ventral fin. The anal opening
is, however, on the ventral side. Consequently, the caudal fins had to
be formed from the ventral fusion behind the anal opening, while the
portion anterior to the anal opening develops into the appendages proper.

THE ENDOSKELETON

699

Both theories are unsatisfactory, the latter because there is no double
origin of the dorsal fins, and the former, known as the gill-arch-theory,
is unsatisfactory due to the fact that the paired appendages develop out-
side the body musculature, while the visceral arches are always internal.

All median appendages have the form of fins, and are termed dorsal,
terminal (caudal), or ventral (anal). They may occur as a continuous
fin, or they may be broken up with intervals between. Fins occur in all
fishes, in the larval and tailed amphibians, and even in rather isolated
groups, such as the Ichthyosaurs and the whales, although in the am-
phibians and the higher groups, there is no skeleton in the median fins.

The skeleton of the fins usually consists of definite metameric car-
tilaginous or osseous bars, each of which is divided into a proximal
basale and a distal radiale. The basale often articulates or alternates
with the spinous processes of the vertebrae. The radiale supports the
fins proper.

In the higher forms of vertebrates, when the skeleton of the fins is
not composed of cartilage or bone, there is a horny substance known
as elastoidin which forms of a number of slender rods in greater number
than the somites. They arise from the corium just below the epidermis,
often being united in bundles, and thus form soft-finned rays, often re-
placing the radiale.

PAIRED APPENDAGES

An extinct shark had a pair of fins approximately in the region where
pectoral and pelvic girdles normally form, but no satisfactory theory has
yet developed as to how arms and legs were derived from this type of fin.

700

COMPARATIVE ANATOMY

Fig. 416.

Shoulder girdles. I. Ventral views of the shoulder girdles of various Anura.
(Slightly enlarged.) 1, Bombinator igneus (a species of Frog) and 2, Bufo vul-

THE ENDOSKELETOX 701

The internal support of both shoulder and pelvic girdle consists of
inverted arches across the ventral side of the body, the limbs of the
arch extending dorsally. The part to which the limbs are attached is
called the girdle. The arch itself always forms in cartilage, though mem-
brane bone or dermal bone may be added.

The typical girdle consists of three elementary parts, one dorsal and
two ventral, all of which meet at the point of attachment of the limbs,
and all contribute to form the socket, in the forelimb called the glenoid,
and in the hindlimb known as the acetabulum, and shows why we con-
sider pectoral and pelvic girdles and appendages homologues.
THE SHOULDER GIRDLE (Fig. 416)

In fishes, the shoulder girdle is more or less U-shaped, with the
glenoid fossa at the dorsal end, and immediately dorsal to the fossa lies
the scapular region. Quite often, the dorsal part of the scapular region
is again divided, so that a supra-scapula is formed. In the skates the
supra-scapula articulates with the adjacent vertebrae; usually, however,
the entire girdle lies free from the axial skeleton. A pair of clavicles form
from the skin. These overlay the coracoid region of the girdle, and meet
in the midline, while a cleithrum, a second bone above the glenoid fossa,
forms. In some of the ganoids, it is the cleithrum which extends toward
the midline so as to take the strain ; in fact, it is assumed that in higher
groups, where the two halves of the cartilaginous girdle have separated,
the separation is due to the stress laid upon these parts.

In the higher ganoids and in the teleosts, the cleithrum increases in

garis (Toad) as examples of the arciferous type. 3, Adult, and 4, metamorphosing
Frog, Rana temporaria showing change from the arciferous into the firmisternal
type. 5, Hemisus guttatum (narrow-mouthed toad). 6, Breviceps gibbosus (tailless
amphibia). 7, Cacopus systoma (narrow-mouthed toad). Cartilaginous parts are
dotted. Ossified parts are white. Cl, clavicle ; Co, coracoid ; E, epicoracoidal
cartilage ; H, hiimerus, M, metasternum ; O, omosternum ; P, precoracoid ; Sc,
scapula; S.S, suprascapula. (From Gadow and Boulenger.)

II. A, The skeleton of the pectoral fins and girdle of a dogfish, seen from the
ventral side. (After Borradaile.) cor., Coracoid region; gl., glenoid surface; h.r.,
horny rays ; mpt., metapterygium ; mspt., mesopterygium ; ppt., propterygium ;
rad., cartilaginous rays ; sc., scapula. B. Ventral view of the shoulder-girdle
and sternum of a Lizard, Loemanctus longipes. (After Parker.) 1. Inter-
clavicle. 2. Clavicle. 3. Scapula. 4. Coracoid. 5. Precoracoidal process. 6.
Glenoid cavity. 7. Sternum. 8. Sternal bands not united. 9. Sternal rib.
C. Sternum and associated membrane bones of a Crocodile, C. palustris. (After
Shipley and MacBride. ) The last pair of abdominal ribs which are united with
the epipubes by a plate of cartilage have been omitted. 1. Interclavicle. 2.
Sternum. 3. Sternal rib. 4. Abdominal splint rib. 5. Sternal band. D. Lateral
view of the pelvis and sacrum of a Duck, Anas boschas (After Shipley and
MacBride.) 1. Ilium. 2. Ischium. 3. Pubis. 4. Pectineal process, the rudi-
ment of the prepubis corresponding to the pubis of the Lizard. 5. Acetabulum.
6. Ilio-ischiatic foramen. 7. Fused vertebrae. 8. Facet on which the projection
on the femur, the trochanter, plays. E, The breastbone and shoulder girdle of
a rabbit, seen from below and somewhat from in front. (After Borradaile).
acr., Acromion ; cl., clavicle ; cor., coracoid process ; cp., capitulum ; g.c., glenoid
cavity ; mb., manubrium ; mer., metacromion ; sc., scapula ; st.r., sternal portion
of a rib ; st. 2, st.6, second and sixth sternebrae ; tb., tuberculum ; v.r., vertebral
portion of a rib ; x., xiphisternum ; x.c., xiphoid cartilage. F. Sternum and sternal
ribs of a Dog, Canis familiaris. (After Shipley and MacBride.) 1. Presternum.
2. First sternebra of mesosternum. 3. Last sternebra of mesosternum. 4.
Xiphisternum. The flattened cartilaginous plate terminating the xiphisternum is
not shown. 5. First sternal rib.

III. Pectoral girdles of two types of ganoids. A, Acipenser (Sturgeon) and
B, Polypterus (African ganoid), ct, cleithrum; cv, clavicula ; dr, dermal rays; g,
glenoid" surface ; r, cartilaginous radialia. (From Kingsley after Gegenbaur.)

702

Co M I' A R A T I V E ANATOMY

size and usurps the function of the clavicle, while the clavicles themselves
disappear.

Other bones from the skin may develop, such as the supracleithra
(post-temporals or supra-temporals). These connect the girdle with the
skull ; sometimes also post-clavicles and infra-clavicles develop.

A review of the
appendicular skeleton,
as formed embryolog-
ically in the chick and
frog, will give one a
good idea of the parts
as they appear in am-
phibia. In the reptiles,
there is a considerable
variation in the shoul-
der girdle. In the
turtle, its position in-
side the carapace and
internal to the ribs is
supposed to be due to
the fact that the girdle
begins its development
in front of the ribs, and
later sinks to the posi-
tion it is to occupy in
adult life. The scap-
ula, procoracoid, and
Fig- 417- coracoid, are well de-

Tortoise skeleton Cistudo lutaria. Ventral side with plas- velopcd. The median
tron removed and placed at one side. C, costal plate ; Co, cora-
coid; e, endoplastron ; ep, epiplastron (clavicle) ; F, fibula; Fe, ends OI the latter are
femur ; H, humerus ; Hyp, hyoplastron ; Hpp, hypoplastron ; II, , «

ilium ; Js, ischium ; M, marginal plates ; Nu, nuchal plates ; Py, Connected by a Cartlla-
pygal plates; R, radius; sc, scapula; T, tibia; U, ulna; Xp, crinmtc GrMV/~vf i/-™'^1 T,-.
xiphiplastron. (From Parker and Haswell after Zittel). gttlOUS eplCOraCOld. Ill

t>, erf.

THE ENDOSKELETON

703

other forms of reptiles, the procoracoid usually is reduced, and the
clavicle takes its place, though in the lizards, the procoracoid still re-
mains in its reduced condition. Clavicles may or may not be present in
turtles. If they are, they are represented by the epiplastron (Fig. 417),
which is an element of the carapace. In the chameleons and crocodiles

Fig. 418. A.— The skeleton of a rabbit.

acr., Acromion ; cd.t., condyles for tibia ; cm., calcaneus ; cn.c., cnemial crest ;
fe., shaft of femur ; fi., fibula ; g. t., great trochanter ; gr.t., premolar and molar
teeth ; h., head of humerus, fitting into glenoid cavity ; hu, shaft of humerus ;
il., ilium ; inc., upper incisor teeth of the left side ; ind., lower incisor tooth ; is.,
ischium; ju., jugal bone ; lac., lacrymal bone; mcr., metacromion ; mx., maxilla;
o.f., obturator foramen ; ol., olecranon process ; os., orbitosphenoid bone ; pa., knee-
cap ; pis., pisiform bone ; pu., pubis ; ra., radius ; sc., scapula ; sp.s., spine of
scapula ; st., sternum ; st.r., sternal ribs ; sup., suprascapula ; £.3, third trochanter ;
ti., tibia ; tro., trochlea ; ul. ulna ; v.cd., v.cer., v.l., v.sac., v.th., caudal, cervical,
lumbar, sacral, and thoracic regions of the backbone ; v.r., vertebral ribs ; x., xiphi-
sternum ; x.c., xiphoid cartilage ; II, foramen for optic nerve. The clavicle and
hyoid are not shown.

B. — The skeleton of a pigeon, seen from the left side.

c.r., Fixed cervical rib ; c.r1., free cervical ribs ; cl., clavicle ; cor., coracoid ; d.,
dentary ; Eu., Eustachian tube ; e.oc., exoccipital ; f.r., fenestral recess ; fe., femur ;
fi., fibula ; fr., frontal ; hu., humerus ; i.o.s., interorbital septum ; il., ilium ; is.,
ischium ; lac., lacrymal ; me. 1-3, metacarpals ; mt. 1-4, metatarsals ; n., nasal ; o.f.,
obturator foramen ; pa., patella ; par., parietal ; ph. 1-4, phalanges ; pi., palatine ;
pm., premaxilla ; p.c.p., postorbital process of frontal ; pt., pterygoid ; pu., pubis ;
pyg., pygostyle ; q., quadrate ; r.c., radial carpal ; ra., radius ; s.o.b., suborbital bar ;
s.oc., supraoccipital ; sa., supra-angular ; sc., scapula ; sq., squamosal ; st., sternum ;
st.r., sternal ribs ; ti., tibia ; u.c., ulnar carpal ; u.p., uncinate process ; ul., ulna ;
v.cd., caudal vertebrae ; v.r., vertebral rib ; x.p., xiphoid process ; zy., zygomatic pro-
cess of the squamosal ; J., II., foramina for first two cranial nerves ; 1-3, first three
cervical vertebrae. (From Borradaile).

704 COMPARATIVE ANATOMY

the clavicle is entirely lost. In limbless lizards, the girdles are greatly
reduced, and in fact in the Ophidians the girdle itself has completely
vanished.

In birds (Fig. 418), the scapula is formed as a sword-shaped bar
overlying the ribs, while the coracoid extends from the glenoid fossa to
the anterior end of the sternum. The procoracoid has entirely disap-
peared. The two clavicles unite ventrally to form the wishbone, called
the furcula (Fig. 418, B.cl.). This may either articulate with the ster-
num or lie free.

In the monotromes the shoulder girdle is quite like that of the lizard.
This is also true of the young marsupials, but in the adult it becomes
quite like that in all other adult mammals. The coracoid in this instance
is reduced to the small coracoid process, definitely ankylosed to the ven-
tral end of the scapula. The scapula is well developed, forming a crest
called the spina scapulae on its external surface, which in turn culminates
in an acromion process (Fig. 416, II, E, acr.). The clavicle varies with
the manner in which the limb is used.

In the higher forms of mammals, the clavicle serves as a strong
brace between shoulder and sternum. However, in the ungulates, in the
whales, and in a few carnivores it has entirely disappeared. In some
mammals it appears as a mere rudiment, without apparent functional
value.

Two small, cartilaginous elements often intervene between clavicle
and sternum, called episternalia, or suprasternalia. Their homologies
are unknown.

THE HIP GIRDLE

The hip or pelvic girdle (Fig. 419) is quite homologous to the shoul-
der girdle, the acetabulum representing the glenoid fossa. The ilium
represents the scapula, while pubis and ischium represent the procora-
coid and coracoid. The gap or open space between pubis and ischium
is known as the ischio-pubic fenestra. In the lower forms there is an-
other opening, called the obturator foramen, through which the obturator
nerve passes to the pelvis. In the higher forms, this usually unites with
the ischio-pubic fenestra, the entire opening then being called the obtura-
tor foramen.

In the lower forms, such as the fishes, the basalia are on the inside,
and fused to form a single basal, through which the obturator nerve may
pass. The radialia are on its distal surface. The basalia of the two sides
do not meet, though there is often a small (or a pair of small) cartilage
plates between them. These are supposed to be the homologues of the
epipubis of the higher forms. There is no acetabular joint.

In the ganoids and teleosts, ossification begins, but there are no epi-
pubic elements. The pelvic fins may migrate so as to lie in front of the
pectoral.

THE ENDOSKELETOX

705

The elasmobranchs have a true girdle, although there are no sepa-
rate elements in it, and it does not ossify, there being but a continuous
ischiopubic bar running from one acetabulum to the other, with an
elongated iliac process running dorsad.

The pelvic girdle lies free of the vertebral column in all fishes, but

isc.pu.

Fig. 419.

A — The skeleton of the pelvic fins and B. — The pelvic girdle of a rabbit,

girdle of a female dogfish. from beneath.

ac., Acetabular surface ; bp., basiptery-
gium ; h.r., horny rays ; tZ., iliac process ;
isc.pu., ischio-pubic region ; rad., cartilagi-
nous rays.

ac., Acetabulum ; tt., ilium ; is.,
ischium ; ob.f., obturator foramen ;
pu., pubis ; sym., symphisis pubis.
(From Borradaile).

Isdi

C.

C. — Anlage of pelvic girdle of 6-day
chick embryo to show develop-
ment.

II, ileum ; Isch., ischium ; pb.,

pubis ; pp., pectineal process. (After *+

Johnsohn).

in animals that have to support the body-weight upon their limbs, the
pelvic girdle becomes definitely attached to the sacrum by the develop-
ment of one or more sacral ribs.

In the mud puppy (Necturus, Fig. 375), the median cartilage extends
forward as an epipubic process, while from the antero-lateral portion of
each pubic bone or cartilage, a pectineal process extends, and, in the
salamanders, to the extent of two or three somites, there is a cartilage

706 COMPARATIVE ANATOMY

formed independently of the pubis, in the linea alba,. called the ypsiloid
cartilage.

In the frog and other anura, the three pelvic bones are present, all
of which participate in the forming of the acetabulum, although the ilium
is very long and the ischio-pubis strongly compressed, so that the ob-
turator foramen and ischio-pubic fenestra are absent.

In reptiles, the pelvic bones are more solid and distinct than in any
of the lower forms. The ilium is often expanded, the ischio-pubic fenes-
tra large, and the ischium and pubis united from side to side by an epi-
pubic cartilage or a modification of this, known as the ligamentum me-
dium pelvis.

In some turtles, the epipubic cartilage bounds the fenestra on the
median side, but in all turtles, the fenestra and the obturator foramen
are merged into one. In lizards, there may be a separate bone ossified
from the posterior part of the epipubis. This bone is called the os cloacae
or hypo-ischium.

In legless lizards, the pelvis is greatly reduced, while all trace of it
is lost in the snakes, except the boas, and some opoterodonts (worm-like
serpents). In the crocodiles, due to the oblique position of the pubes,
the obturator foramen is very large. The pubes themselves do not unite
with each other. There are cartilaginous tips on the medial end which
may be separate epipubes. The lower end of the ilium also separates
as a distinct bone.

It is interesting to note that the pelvis of Dinosaurs has the ilium
arranged quite similar to that in birds. The sacrum also is somewhat
similar, due, apparently, to the upright position in which these animals
walked. The ischia are elongated, extending backward, and often uniting
below, while the pubic bones extend forward and downward, and have
strong post-pubic processes running parallel to the ischium, while often
the ilium gives off an iliac spine near the acetabulum.

The Pteryodactyls also had elongated ilia, similar to the Dinosaurs.
The ischium was then fused with the ilium, so that the pubis took no
part in the forming of the acetabulum. In these the pelvic opening was
very small.

In birds of the present time, the pelvic bones are fused ; the ilium is
quite long, and usually fused with the synsacrum, while the ischium and
pubis extend backward.

The pubes lie in the position of the postpubes of Dinosaurs, and
never meet below, except in ostriches. However, in the embryo, the
pubes run forward, only gaining their final position later on. There is
a pectineal process which arises in the acetabular region and extends
forward, quite like the pubis in Dinosaurs. In the mammals, the obtura-

THE ENDOSKELETON 707

tor foramen and ischio-pubic fenestra are united ; all three pelvic bones
unite to form the acetabulum, although the ilium and ischium may ex-
tend in such a manner as to exclude the pubis from taking part in the
formation of the fossa. Often an acetabular or cotyloid bone is formed
between the ilium and pubic bones and this may fuse with any of the
bones with which it comes in contact.

The inter-pubic cartilage in marsupials and monotremes may or
may not persist throughout adult life. When it disappears and the bones
unite solidly, but do not definitely ankylose, such union is called a sym-
physis.

In these non-placental mammals (the marsupials and monotremes
just mentioned) there are also marsupial bones which first form in car-
tilage, and then extend forward from each pubis in the ventral abdominal
wall. Their homology is unknown.

THE FREE APPENDAGES

In those animals, such as fishes living entirely in the water, the ap-
pendages are called ichthyopterygia. These are always paired fins.
When definite legs or arms are formed, such as in all classes of the
tetrapoda, such limbs and their modifications are known as chiropterygia.
It is commonly supposed that the latter have developed from the former,
but no one has yet been able to explain a method by which it came about.
All explanations, however, assume that certain parts of primitive fins
were retained and others likely modified, or, that certain parts were lost,
which were originally present, the remaining parts then becoming mod-
ified. The lower ganoids have a primitive form of fin but with increas-
ing complexity, there is a reduction of the basalia, either by entire dis-
appearance or by fusion. The remaining ones are then modified, so that
in elasmobranchs of the present time, we find the basalia usually number
three in the pectoral and two in the pelvic fins, being named from before,
backwards, as the pro, meso-, and meta-pterygium (Fig. 416)). The
middle one is absent in the hind limbs. The radiales are jointed trans-
versely, so as to give more flexibility. If these are arranged entirely
on one side of the basalia, they are called uniserial, but if they occur also
on the post-axial side, they are called biserial. The male elasmobranch
has the pelvic fin divided into two lobes, the medial being called the
clasper, or mixipterygium.

The anterior portion of the pectoral fin may develop as a strong
defensive spine, sometimes connected with the poison gland. In eels the
pelvic fin is lacking.

THE LIMBS

The legs (chiropterygia) of all tetrapoda are essentially alike (Fig.
420). Each consists of several regions, comparable in detail with each
other. The proximal is the upper arm (brachium) or thigh (femur) con-

708

COMPARATIVE ANATOMY

Fig. 420. — Comparisons of fore-limbs and hind-limbs.

1. Wing of a dove ; c, carpals ; h, humerus ; me, carpo-
metacarpus ; p.f., primary feathers ; r., radius ; s ./., secondary
feathers ; u., ulna.

A, and B. — The fore-limb and hind-limb of a bird compared.

H., Humerus ; R., radius ; U., ulna ; r., radiale ; u., ulnare ; C., distal carpals
united to carpo-metacarpus ; CO., the whole carpal region ; MC.L, metacarpal of the
thumb ; /., phalanx of the thumb ; MC.II. ; second metacarpus ; //., second digit ;
MC.IIL, third metacarpus; ///., third digit. F., femur; T.T., tibio-tarsus ; Fi.,
fibula ; Pi., proximal tarsals united to lower end of tibia ; dt., distal tarsals united
to upper end of metatarsus, forming a tarso-metatarsus (T.MT.) ; T., entire tarsal
region ; MT.L, first metatarsal, free ; I. -IV., toes.

C, D, E, F, G, — Anterior limb of man, dog, hog, sheep, and horse ; Sc, Shoulder-
blade ; c, coracoid ; a, b, bones of forearm ; 5, bones of the wrist ; 6, bones of the
hand ; 7, bones of the fingers.

H, I, J, K, L, — Posterior limb of man, monkey, dog, sheep, and horse: 1, Hip-
joint ; 2, thigh bone ; 3, knee-joint ; 4, bones of leg ; 5, ankle-joint ; 6, bones of foot ;
7, bones of toes. (I, A, B, After Thomson, C to L, after Le Conte).

taining a single bone, the humerus or femur in the fore or hind limb
respectively. The next region, the forearm (antebrachium) or shank
(cms) contains two bones, a radius or tibia on the pre-axial, and an ulna
or fibula on the postaxial side. Next follows the podium or hand (manus)
in front and the foot (pes) behind, each consisting of three portions.
The basal podial region, the wrist (carpus), or ankle (tarsus), consists
of several small bones; the second division (metapodium) is the palm
(metacarpus), or instep (metatarsus), and lastly come the fingers or toes
(digits), each digit consisting of several bones, the phalanges. These
separate parts are included in the accompanying table, in which the
terms given to the separate elements of the wrist and ankle of man are
included.

THE ENDOSKELETON

FORE LIMB (Arm) HIND LIMB (Leg)

Humerus — Femur

Radius = Tibia
Ulna— Fibula
Radiale = Tibiale

Upper arm (Brachium)

1
Fore arm (Antebrachium)

Basi-
podium

Wrist
(Carpus)

709

Thigh

Shank
(Crus)

Naviculare

(Scaphoid)
Lunatum
Triquetrum

Pisiforme

Multangulum

majus
Trapezium
Multangulum

minus

(Trapezoides)
Capitatum

Hamatum

I nter medium = Intermedium
Ulnare = Tibiale

Carpale1 — Tarsale1

Carpale2 = Tarsale2
Carpale3 = Tarsale3

Astragalus
(Talus)

Calcaneus
Naviculare

pedis
(Scaphoid)

Cuneiform1

Cuneiform2

Cuneiform3

Cuboides

Basi-
- podium
Ankle
(Tarsus)

Palm

(Metapo-

dium)

^
I

j Carpale4 = Tarsale4

I. Carpale5 = Tarsale5

(Metapo-

Metacarpale1'5 = Metatarsale1-5 dium)

Instep

Fingers (Phalanges)

Digits1-5^ Digits1'5

(Phalanges) Toes

The basal podial region, which is nearly typical in some reptiles,
tirodeles (Fig. 421), and man, consists of three rows of bones: a proximal
of three bones, a radiale or tibiale on the anterior side, an ulnare or
fibulare on the other, and an intermedium (not shown in the figure) be-
tween them. The distal row now consists of five carpales or tarsales
numbered from the anterior side.

The third row is composed of one or two centrales between the other
rows. The metapodials (Metacarpals and Metatarsals) and the digits,

710

COMPARATIVE ANATOMY

also numbered from one to five, have in some cases special names. The
thumb (digit 1) is the pollex, the corresponding great toe being the
hallux, while the fifth digit is called minimus, the second finger in the
hand the index, and the fourth the annulus.

From this typical condition all forms of chiropterygia — legs, arms,
wings — are derived by modification, fusion, or disappearance of parts.

^c.l.calc.

c.

II

V IV

Fig. 421. — The skeleton of the left fore- and hind-feet of a rabbit.
A, fore-foot ; B, hind-foot.

a.. Astragalus ; c.l, first distal carpal or trapezium ; c.2, second
distal carpal or trapezoid ; c.3, third distal carpal or os magnum ; c.4,5,
fused fourth and fifth distal carpals or unciform ; ce., centrale ; ce' .,
centrale of hind-foot or navicular ; cm , fibulare or calcaneus ; im., in-
termedium or semilunar ; me., metacarpals ; met., metatarsals ; ph.,
phalanges ; ra., lower end of radius with its epiphysis ; r.c., radiale or
scaphoid ; t.2, second distal tarsal or mesocuneiform ; £.3, third distal
tarsal or ectocuneiform ; £.4, 5, fused fourth and fifth distal tarsals or
cuboid ; u.c., ulnare or cuneiform ; ul., lower end of ulnar with its
epiphysis; I.-V., digits. (From Borradaile).

C. Hindlimb of the frog tadpole shortly before metamorphosis.

centr, centralis ; cl.calc., cartilaginous calcaneus ligament ; c.l.tars.s.,
supplementary cartilaginous tarsal ligament ; c.ses., sesamoid cartilage ;
F, fibula ; fib, fibulare ; pr.h., prehallux ; ses, sesamoid bone ; T, tibia ;
t.ach., tendon of Achilles ; tars, II and ///, second and third tarsals ;
tib, tibiale; I-V, phalanges. (After Tschernoff).

The more distal a part is, the more variable it is ; reduction takes place
on the margins of the appendage, the axial portions being the last to
disappear. Occasionally, in various groups (amphibia, mammals) there
occur what are interpreted as rudimentary additional digits — prehallux,
prepollex, and postminimus — but their status is uncertain. There are
also certain membrane-bones developed in the appendages, such as the
patella (knee-cap) in some reptiles, birds, and many mammals, in the
tendon that passes over the knee joint, the fabellae in the angle of the
knee of a few mammals, and the pisiforme in the carpus of man and some
other mammals.

THE ENDOSKELETON 711

We have already seen that in the frog the radius and ulna as well
as tibia and fibula are fused together, while the tarsals are considerably
elongated. Such fusion is not uncommon in many of the animals. The
extent of fusion varies, however, considerably. In the reptile limb there
is an intratarsal joint, so that the motion of the foot upon the leg lies
between the two rows of tarsal bones, instead of between the tarsals and
the bones of the shank. This is quite similar to the condition in birds.

Although limbs are lacking in the snakes and in some of the lizards,
there is nevertheless a considerable increase in the number of phalanges
in those reptiles where limbs do occur, while the more proximal bones
shorten. In some of the ichthyosaurs there may be as many as a hun-
dred phalanges in a single digit.

The skeleton of pterydactyls shows the fifth digit greatly developed,
which forms a definite support for the wings, while the other digits re-
main more or less normal. In birds the wings are considerably modified
(Fig. 420), although the structure is practically normal, up to the region
of the carpus, the carpal bones being greatly reduced by fusion, while
the metacarpa's and digits, no matter what their modification, are only
three in number.

Embryological studies of the chick show us that, although the first
digit begins to develop, it is entirely lost, and the fifth metacarpal, which
is present in the embryo, fuses early with the fourth, so that the digital
formula is II, III, IV. Added to this, there is an extensive fusion of
the bones of the tarsus and foot ; the ankle joint is intratarsal, the basal
row of tarsal bones fuses with the tibia, while the fibula is considerably
reduced to form the tibio-tarsus. The tarsales unite in the same way
with the fused metatarsals to form the tarso-metatarsus.

There are hardly ever more than four toes, but the number of
phalanges increases from two in digit II, to five in digit V. Ostriches
only have two toes, and many other birds three. In the mammals,
especially in the higher forms, there is considerable motion of both hand
and foot, rotation in the hand being especially noticeable by the motion
of the lower end of the radius around the u}na. In the whales, the basal
part of the forelimb is greatly shortened, while there is considerable
multiplication of the phalanges. The hind limb is entirely lacking in
some whales, while in others, there are two vestigeal bones supposed to
be the femur and tibia, imbedded in the muscles of the trunk.

A supra- or entepicondylar foramen frequently perforates the inner
lower end of the mammalian humerus, while in many forms the ulna is
fused with the radius in varying degrees. However, the ulna, whether
fused or not, always has on its proximal end a strong olecranon process
which extends beyond the elbow joint for the attachment of the exten-
sor muscles of the forearm.

The earliest prominences for the attachments of the muscles on the
femur are known as trochanters. They vary from one to three. The

712

COMPARATIVE ANATOMY

Fig. 422.

Cyclostomes, as exemplified by the marine lamprey, (Petromyzon marinus) ,
from 60 cm. to 1 m. long, of European, West African, and North American waters,
which goes up stream of the river in spring to lay eggs in the calm waters, and

THE EXDOSKELETOX 713

patella, or knee-cap bone, is analogous to the olecranon process, though
it never joins the other bones.

The ankle joint in mammals is never intratarsal, but always between
tarsal and crural bones.

Where the bones of the foot rest on the ground as in man and in
the bear, such a foot is known as plantigrade. Where the toe of the
foot includes only the distal phalanges such as in the dog and the cat,

comes down again toward the sea in autumn ; and the Planarian Lamprey,
(Petromyzon planari) , from 20 to 30 cm. long which inhabits the calm waters
entirely, and is commonly found in rivers.

Fig. 1. A lamprey (Petromyzon planari) with its mouth fixed to a rock. Pg.,
Genital papillae.

Fig. 2. Anterior part of the body of Petromyzon marinus, showing the seven
branchial openings and the buccal cupping glass surrounded by little papillae.
The olfactory opening lies in front of the eye.

Fig. 3. Section through the anterior region of Petromyzon marinus. The section,
slightly oblique, is nearly sagittal toward the front ; it deviates dorsad and down-
ward in order to take in the last of the left branchial sacs ; vb., buccal cupping
glass ; ca., ringed cartilage carrying the principal teeth ; cf., cartilaginous pieces of
the face ; I, lingual sucker, (the posterior part of the sucker has not been taken into
the section) is shown surrounded by its sheath ; ml., muscles of the lingual sucker ;
ph., pharynx ; oe., oesophagus ; m.oe., sphincter closing the entrance of the oeso-
phagus ; a.br., branchial aqueduct, showing the seven openings to the branchial
sacs ; va., valvular apparatus closing the entrance to the aqueduct ; brs-br., branchial
pockets continuing into the coelomic peribranchial cavities, the one being separated
from the other by septa ; C, heart : the auricle has been raised partly to show the
openings by which it communicates with (1) the ventricle (2) with the sinus
venosus, sv., as one sees them both from behind ; vc., entrance of the cardinal veins
into the sinus ; j., jugular vein ; vh., hepatic vein ; tao., aortic trunk, with the
conus arteriosus and its valvular apparatus at the base of the trunk ; ao., aortic
roots, reuniting on a level with the fifth branchial opening to form the aorta ; n.,
nostril ; sh., bottom of the hypophysial sac with a valvular sac lying before it ;
cer., brain ; m., medulla ; cd., dorsal cord ; /, liver ; ov., ovary ; p., posterior cul-
de-sac of the branchial enclosure which protects the pericardium.

Fig. 4. Mouth of the Marine Lamprey, de., teeth of the head of the lingual
sucker ; di., lower median tooth ; ap., principal lateral teeth grouped in twos ; da.,
accessory teeth ; ph, sensory papillae of the buccal lip ; os., cutaneous sensory
organs.

Fig. 5. Anterior region of the skeleton : ca., ringed cartilage carrying th,e
principal teeth, d., cf., cartilaginous parts of the face ; cr., brain box ; I, lingual
cartilage ; ol, olfactory capsule ; cd., dorsal cord ; an., the two neural arches, anterior
and posterior, of the same metamere ; cbr., branchial enclosure ; a, cartilaginous
rings surrounding the opening of the external gills ; p, posterior cul-de-sac of the
branchial enclosure; holding the heart (after Parker) ; the left half only of the
skeleton is represented.

Fig. 6. Section of a horny tooth (odontoid) of lamprey: ep., buccal epithe-
lium ; pa., dermal papillae ; D, tooth in use ; d, replacing tooth, in process of
development; k, horn producing cellules (After Warren).

Fig. 7. Sagittal section of the Pineal Eye. : ep., epidermis ; de., dermis ; op.,
pineal eye ; np., pineal nerve ; pp., parapineal eye ; ha., commissure and habenular
ganglion ; ch, chorioid curtain and lamella concealing the mesencephalon ; ca.,
anterior commissure ; on the walls of the thalamencephalon ; cp., posterior com-
missure, cr., cranial cartilage (after Studnicka).

Fig. 8. Section of a branchial pocket, passing through internal and external
openings. (By reason of the situation of these two openings, the section of the
left side of the figure is practically on a plane which places the face to the observer
and forms an angle of 45° with the median plane of the animal ; the rest of the
section, which is only drawn in, is entirely transverse, and therefore seems short-
ened), cd, dorsal cord; an, neural arch; m, section of the medulla; g, fatty tissue
completing the padding of the neural canal ; oe, oesophagus and beneath, the aorta ;
abr, branchial aqueduct ; t.ao., aortic trunk ; I, lingual cartilage and its muscular
sheath; j, jugular vein; caet appendages of the general cavity; cbr., section of the
divers pieces of the branchial enclosure ; o, internal branchial orifice at the in-
terior of the pocket; o', external branchial opening, with its threefold valves and
cartilaginous ring, a.br., wall of the branchial pocket ; fbd, branchial leaves ; par.,
peribranchial cavity slightly taken in section ; mu, muscles.

(1) Vignettes of the title: Scheme of the respiratory apparatus of two cy-
clostomes seen from the ventral surface : the oesophagus and the respiratory sacs
of the left side (G) only are represented; the horizontal flesh is turned toward the
caudal end of the animal ; the oesophageo-cutaneous canal, which exists only on the
left side is figured in discontinued tracts. To the left the respiratory apparatus of
Myxine is seen (six branchial pockets with efferent canals running to a single
opening) to the right, the respiratory apparatus of Bdellostoma polytrema (10-14
branchial pockets). (After Dean in Goodrich). (From the chart of Remy Per-
rier & Cepede).

714 COMPARATIVE ANATOMY

it is called a digitigrade foot, while, if the animal, such as the horse or
cow, walks upon hoofs which are homologous to the nails on the hands
and feet of higher forms, such a foot is called an unguligrade foot.

There may be variations and fusions in all these animals. For ex-
ample, in the horse, it is only the third digit which persists in a func-
tional condition.

SUMMARY OF THE CRANIUM
CYCLOSTOMATA

The cranium lies entirely beneath the brain, and forms neither side
nor roof for the latter (Fig. 422). The cranial cartilages are sometimes
said to be homologous with those of the embryonic fish skull.

DOGFISH

The investing bones are closely applied to the roof and floor of the
chondrocranium, and modify its form considerably by projecting beyond
the cartilaginous part, and concealing apertures and cavities (Fig. 407).
The large frontals cover the greater part of the roof of the skull, con-
cealing the fontanelles and furnishing roofs to the orbits. Immediately
behind the frontals is a pair of very small parietals; in front of them
is an unpaired supra-ethmoid, to the sides of which are attached a pair
of small nasals. On the ventral surface is the large parasphenoid, which
forms a kind of clamp to the whole cartilaginous skull-floor ; and in front
of, and below, the parasphenoid is the toothed vomer. Encircling the
orbit is a ring of scale-like bones, the sub-orbitals.

PISCES

The fish skull (Fig. 409) is covered above and below by numerous
dermal investment bones which are much like those of the primitive
extinct amphibia Stegocephali. By boiling, all the investment bones
are loosened and when removed a chondrocranium like that of the dog-
fish is seen.

In fishes there are primary and secondary structures in the jaw as
in the cranium. The primary upper jaw (palatoquadrate) is considered
homologous with the upper jaw of the dogfish. It does not, however,
remain cartilaginous but is ossified by five replacing bones : the toothed
palatine in front which articulates with the olfactory capsule; the ptery-
goid on the ventral edge; the mesopterygoids on the dorsal edge of the
original cartilaginous bar, and the quadrate at the posterior end of the
latter. These bones do not enter into the gap, and consequently do not
constitute the actual upper jaw of the adult fish. External to them are
two large investing bones, the premaxilla and the maxilla, which to-
gether, form the actual or secondary upper jaw. They both bear teeth.

THE ENDOSKELETON 715

A small scale-like bone, the jugal, is attached to the posterior end of
the maxilla.

The lower jaw is quite similarly modified. The articulare articu-
lates with the quadrate and is continued forward by a narrow pointed
rod of cartilage which is really the unossified distal end of the primary
Meckel's cartilage (the primary lower jaw). The articulare is the ossi-
fied proximal end, therefore, a replacing bone. Then there is a large
toothed investing bone which ensheaths Meckel's cartilage and forms
the main part of the secondary lower jaw. This is the dentary. There
is also a small investing bone, the angular, which is attached to the
lower and hinder end of the articulare.

The upper jaw connects with the cranium partly by the articulation
of the palatine with the olfactory region and partly by means of a sus-
pensorium formed of two bones separated by a cartilaginous interval.
The larger, usually called the hyomandibular, articulates with the audi-
tory capsule by a facet and the small, pointed symplectic fits into a
groove in the quadrate. Both bones are attached by fibrous tissue to
the quadrate and metapterygoid, and in this way the suspensorium and
palatoquadrate together form an inverted arch which articulates freely
in front with the olfactory, and behind with the auditory capsule. This
gives rise to an extremely mobile upper jaw.

The chondrocranium is a solid one-piece capsule which completely
encloses the brain and the principal sense organs. The cranium proper
is fused with paired nasal capsules and paired auditory capsules.

Closely associated with the skull, but not fused with it, is the man-
dibular skeleton, consisting of an upper jaw (pterygoquadrate cartilages)
and a lower jaw (Meckel's Cartilage). Back of the jaw are the visceral
arches. These are composed of upper and lower parts like the jaws.
The first pair is specialized as the hyoid arch, the five others being the
more generalized branchial arches that afford support for the gills.

AMPHIBIA

The skull (Fig. 410) Articulates with the atlas by two condyles
which are formed by the exoccipitals. There is an auditory columellar
apparatus fitting into the fenestra ovalis.

AVES

The skull (Fig. 412) is rounded, has large orbits, and the facial
bones are extended out upon the beak. The quadrate is movable and
articulates with the squamosal. There is a single occipital condyle.
There are no teeth in modern forms. The cervical vertebrae have pad-
dle-shaped articular surfaces which give the neck great flexibility, and
thus make the beak a very versatile instrument.

716 COMPARATIVE ANATOMY

REPTILIA

The special features in the turtle skull (Figs. 411, 413) are these:
teeth are absent, the maxillary, premaxillary, and dentary bones are cov-
ered with hard, chitinous sheaths which form the upper and lower
members of the cutting beak; the vomer is a single unpaired median
bone, and there are no lachrymals or ectopterygoids. The pterygoids
send wings of bone inward. The wings and the palatines form a con-
tinuous roof of the mouth ; the supraoccipital is prolonged backward
into a large, narrow process upon which are inserted the heavy neck
muscles. All of these bones, even the quadrate, are firmly united into
a solid cranium. There is only one occipital condyle.

MAMMALIA

The skull of the mammal (Figs. 413, 414) is more compact than that
of lower forms; consequently, it contains fewer elements than the skull
of reptiles. The following reptilian bones are not found in the adult
mammalian cranium : pre- and post- orbitals, pre- and post- frontals,
basi-pterygoids, quadrato-jugals, and supra temporals. The lower jaw
is reduced to a single pair of bones in the mammal, the angulare, splenial,
and articulare being absent. The latter is often said to have been drawn
in to form the malleus of the ear bonelets and the quadrate has been
drawn in to form the incus bonelet, while a remnant of the hyomandibu-
lar cartilage forms the stapes. The whalebone whale (baleen whale,
Fig. 392), shows the highest type of the so-called adaptive specialization
among mammals. Here the teeth are rudimentary and functionless
though present in the young. In the adult they are replaced by baleen.
The nostrils are paired, the skull symmetrical, the sternum is single,
while the ribs are one-headed and articulate only with the transverse
processes of the vertebrae.

SUMMARY OF THE SKELETAL SYSTEM
THE DOGFISH

The vertebrae develop at the intersection of the myosepta with the
mesenchyme that surrounds the notochord and neural tube. Each indi-
vidual vertebra is formed by the union of the two caudal halves of the
two sclerotomes of one segment with the cephalic halves of the two
sclerotomes of the next succeeding segment (Fig. 305). The vertebrae
therefore alternate. with the myotomes.

As the vertebrae and ribs are first formed in cartilage produced by
the activity of mesenchyme, so also, bones which form later a,re true
cartilage bones. In the elasmobranchs, the entire skeleton is made up
of cartilage with only a slight impregnation of calcareous matter.

Each vertebra begins as four pair of cartilages (called arcualia)

THE ENDOSKELETOX

717

which surround the notochord. Dorsally these are an anterior pair of
basidorsals and a posterior pair of interdorsals. Ventrally there is an
anterior pair of basiventrals and a posterior pair of interventrals.

In some fish and in extinct Amphibia and reptiles these cartilages
remain more or less separate. In most vertebrates, however, parts are
lost, while the remaining portions fuse together to form a single vertebra
which is then composed of a centrum (which encloses the notochord) ;
a dorsally directed neural arch (which encloses the spinal cord) ; and a
haemal arch (enclosing the blood vessels). (Figs. 352, 404.)

The neural arch is made up of the fused basidorsals, and the haemal
arch of the fused basiventrals, while the centrum develops from varying
parts in different groups of animals.

In the elasmobranchs the centrum is formed within the notochordal
sheath, thus forming a chordal centrum as contradistinguished from that
of nearly all other vertebrates where the centra are produced by the
fusion of certain arcualia to form a perichordal or arch centrum.

There are two kinds of ribs, namely : those which arise at the inter-
section of the myosepta with the horizontal skeletogenous septum (true
or intermuscular ribs), (Fig. 423, q), and those which arise at the inter-

Fig. 423. — Diagram to show the skeleton-forming septa in the trunk region of a

vertebrate.

o, skin ; 6, neural tube ; c, notochord ; d, blood vessel ; e, dorsal skeletogenous
septum ; /, ventral skeletogenous septum ; g, horizontal skeletogenous septum ; h,
myoseptum ; i, epaxial part of the myotome ; j, hy'paxial part of the myotome ;
k, coelom ; {, intestine ; m-p, cartilages from which the vertebrae are formed ; m,
basidorsal ; n, interventral ; o, basiventral ; p, interdorsal ; q, intermuscular rib ;
r, subperitoneal rib. Note the positions of the vertebral cartilages and ribs with
respect to the skeletogenous septa. (From Hyman after Goodrich).

section of the myosepta with the ventral skeletogenous septum or its de-
rivatives (false or subperitoneal ribs), (Fig. 423, r). Teleosts develop
the latter type, while all other vertebrates develop true ribs.

Some fishes (such as trout, salmon, herring, and polypterus), how-

718 COMPARATIVE ANATOMY

ever, develop both types of ribs and even additional ones at various levels
of the myosepta.

The vertebrae are connected with each other by a strand of noto-
chordal tissue that perforates all the vertebrae like the string through
a chain of beads.

The fins have ray-like supports of cartilage and the pectoral and
pelvic limb-skeletons are supported upon simple horseshoe-shaped pec-
toral and pelvic girdles, each composed of a single piece of cartilage.

AMPHIBIA

A short cervical and sacral region appear in Amphibia, the cervical
becoming longer in the higher forms of vertebrates. The pelvic girdle
lies free, in fishes, but in all other vertebrae it is immovably attached to
the sacrum. Three fingers develop early in the amphibian foot, although
a fourth appears quite late in development (Fig. 421). This fourth finger
lies well down on the ulnar side of the hand. Then a rudiment of the
fifth (the little finger) appears as a mere bump. The thumb, index, and
second finger, therefore, seem to be phylogenetically the oldest digits.
This is important in connection with the loss of fingers in other verte-
brates, as the last to develop is usually the first to be lost. In the am-
phibian we find feet instead of fins. This brings a change in the type
of movements in the animal, for with fins, an animal can only paddle
backwards and forwards. The muscles are, therefore, decidedly differ-
ent and nearly all trace of the segmental arrangement in them is lost.
Animals which live on land are relatively heavier than those whicn live
in water, so there is need of a much more rigid axial skeleton as well as
stronger limb girdles, and limb skeleton. This condition is brought
about by a more complete ossification of the parts of the skeleton that
bear the most weight. Exoskeleton parts also tend to disappear so that
in modern amphibia, the exoskeleton is entirely absent with the excep-
tion of the Caecilians, where it is rudimentary. In the Stegocephalians
there is a head armor while the exoskeleton is lacking on the rest of the
body. The sternum first appears in amphibia.

REPTILIA

In reptiles, birds and mammals the cervical region is longer than
that of the Amphibia and the trunk region is divided into an anterior
thoracic region with long ribs and a more caudal lumbar region with
short ribs or with none.

In all vertebrates rudimentary ribs are usually found on the cervical
and sacral vertebrae when these regions are present.

Fossil remains show that there were many more plates and scutes
on the turtles of the past than on those of the present. Both longitudinal
and transverse rows of elements have disappeared, the whole system

THE EXDOSKELETOX

719

now being" greatly simplified. Most species of turtles to-day show a cer-
tain percentage of individuals with supernumerary scutes and plates.
(Fig. 424.)

Fig. 424. Various plastra and their horny shields.

1, Testudo ibera; 2, Macroclemmys temmincki ; 3, Cinosternum odoratum;
4, Sternothaerus nigricans; 5, Chelodina longicollis ; 6, Chelone mydas. a or
an, anal shield ; abd, abdominal shield ; / or fern, femoral ; g or gul, gular,
unpaired in 3 ; h or hum, humeral shield ; i or int.g, intergular ; im, infra-
marginals ; m, marginals; p or pect, pectoral; x (in 1) inguinal shield con-
stituting with the axillary xx, the last trace of inframarginals. (After
Gadow).

In the trunk region the vertebrae are rigidly united to the narrow,
paddle-like ribs (Fig. 425). There are eight cervical, ten thoracic, two
sacral, and a variable number of caudal vertebrae, which are procoelous
in form (Fig. 404). A peculiar thing in the turtle is that both pectoral
and pelvic girdles are inside instead of outside the ribs. They actually
arise from primordia internal to the ribs so it is not a case of migration.
No one has yet been able to give a satisfactory explanation of this.

The pectoral girdle (Figs. 416, 417) is made up of a triradiate group
of flattened bones : the scapula, the procoracoid, and the coracoid, the
last being the largest. These three bones unite to form a socket which
receives the head of the humerus. The pelvic arch is more compact. It
consists of pubis, ischium, and ilium, which unite to form the acetabulum
for the head of the femur. Membrane bones are never found in the pelvic
girdle of any animal.

AVES

The sternum is keeled (Figs. 416, 418), except in such birds as the
ostrich, and the ribs have uncinate processes (Fig. 418, B, u.p.) except in
Screamers (members of the family Palamedeidae). The trunk vertebrae

720

COMPARATIVE ANATOMY

Neural Ptate

Costal
Shield

Bpiderm

Cutis.

Fig. 425.

A. Diagrammatic transverse section through the shell of Testudo. The horny
shields have been removed from the right side. On the left side one can see the
neural, costal, marginal, and pectoral shields. The bony dermal plates are dotted.
Cap, capitular portion of rib ; Sp.C, position of spinal cord. B, Vertical section
through part of the shell, magnified and diagrammatic. B, Bony layer of cutis ;
L, leathery layer of the cutis ; M, cells of the Malpighian layer ; P, star-shaped
pigment-cells ; Sc, stratum corneum composing the horny shields. C. Diagram of
skeleton of Testudo elephantopus, after removal of the left half of the carapace.
The plastron is indicated by a section through the middle line. Fe, femur, fore-
shortened ; Fi, fibula ; H, humerus ; II, ilium ; Is, ischium ; P.P., pubis ; R, radius ;
Scap, scapula ; Tb, tibia ; u, ulna ; 3, third cervical vertebra ; 1, 3, 5, first, third and
fifth fingers; XIII, thirteenth (fifth thoracic) vertebra. (After Gadow).

are mostly fused. There are three or four pre-caudal vertebrae with
terminal pygostyle (Fig. 418), two cervical, and three to nine thoracic
ribs, the latter attached to the sternum. The pectoral girdle is made up
of paired, blade-like scapulae, paired coracoids which unite with the
sternum, and three clavicles fused in the middle to form the "wishbone"
or furcula. The pelvic girdle is a solid bone, composed of the fused
ischia, ilia, and pubes. The pelvis is firmly fused with the sacral verte-
brae. The leg skeleton consists of a large femur, a slender fibula, and a
long, stout tibiotarsus, made up of the fused tibia and proximal tarsal
bones; the ankle joint is between the tibio-tarsus and the tarso-meta-
tarsus.

The foot has four digits, the hallux usually being directed backward.

THE ENDOSKELETON 721

MAMMALIA

The coracoid portion of the pectoral girdle (Fig. 416) is reduced
to a small coracoid process in all placentals, while the scapula of all mam-
mals possesses a spinous process.

There are usually paired clavicles and a median unpaired interclav-
icle in all land mammals. These are membrane bones.

CHAPTER LI.

THE DIGESTIVE SYSTEM

It will be remembered that all multicellular animals pass through a
blastula stage consisting of a hollow sphere composed of a single layer
of cells, which then indents to form a gastrula.

This means that there are now two layers of cells where there was
only one before. The outer layer is called the ectoderm and the inner
the endoderm. The indented end closes up, leaving a hollow tube com-
posed entirely of endoderm in the center, which, due to its being used
for other purposes than the ectoderm, and lying within the body, under-
goes totally different experiences than does the outer part of the body,
and these different experiences modify its structure. This hollow tube
is the primitive digestive tract. It will thus be seen that the digestive
apparatus is the very first one of the various systems of an organism
to differentiate.

This distinctive cavity is called the gastrocoele. In the lower in-
vertebrates this gastrocoele remains as a blind cavity with but a single
opening. It is among the worms that it first becomes converted into a
complete canal by the formation of an anal opening. In animals up to
this stage the same opening serves both for ingestion and egestion.

What is considered a distinct advance in the development of multi-
cellular animals is the development of a coelom, or body cavity, lying
between the digestive tract just mentioned, and the body wall. Up to
the time this coelom has developed, the body of the animal consists of a
single tube and its wall. But after the coelom has developed there is
established a secondary open space between the hollow digestive tube
and the body wall.

The coelom is developed by protrusions or diverticula pushing off
from the original digestive tube (Fig. 426). This means that the diges-
tive canal of the higher animals only represents a portion of the digestive
system of lower animals.

Another departure from the lower organisms consists in the fact that
the mouth and anal opening are not developed in the same way in the
vertebrates as they are in the lower forms of animals.

In the lower forms, after gastrulation, the indented end remains
open, thus serving as both mouth and anal opening at the same time.
In the higher forms, however, this indented end closes so that there is
a completely closed hollow tube composed of endoderm on the inside of
the body. To form the mouth and anal opening a new indentation at
both the cephalic and caudal ends takes place.

This indentation coming from the outer layer of the body means

DIGESTIVE SYSTEM

723

that mouth and anus are composed of ectoderm and not endoderm as is
the central digestive tube. After the indentation has gone far enough,
the thin plate of cells separating the central digestive tube from the
mouth and anus breaks through, so that a continuous opening is formed,
from the mouth through the digestive canal to the anal opening.

All the additional structures that go to make up the digestive system
as well as the respiratory system are formed by inpushings or outpush-

iii.

Fig. 426.

I. Diagrams to show method of outpushings in digestive tract. A, 6mm. pig
embryo ; B, same at 8 mm. ; C, same at 10 mm. t, trachea ; e, oesophagus ; s,
stomach ; I, liver ; d.p., dorsal pancreas ; v.p., ventral pancreas ; s.i., small intestine ;
c, caecum; v.d., vitelline duct. (From Carey, Journal of General Physiology. Vol.
III. No. 1).

II. Three schematic views of variations in the ducts leading from the gall-blad-
der, c and s, cystic duct ; ch, ductus choledochus ; h, hepatic duct ; he, hepato-
cystic duct; he, hepato-enteric duct; vf, gall-bladder. (From Schimkewitsch after
Wiedersheim ) .

III. A diagrammatic section of the cloaca of a male bird. (After Gadow.)
cd., Upper region of cloaca into which rectum opens ; ud., median region into
which ureter (u.) and vas deferens (vd.) open from each side; pd., posterior
region into which the bursa Fabricii (B.F.) opens.

ings (Fig. 426) of this elementary digestive tract and it will be necessary
to remember in one's study of all the higher forms, that no matter how
many of these inpushings or outpushings there may develop, and no
matter how lengthy the digestive tube may growT and coil, if it be
straightened out it will to all intents and purposes be a continuous hol-
low tube whose interior is really outside the body in so far as it is sub-
ject to all the external conditions to which the body itself is subject.
In other words, one may the better understand this if a hollow gas pipe,
open at both ends, is thought of. The hollow, straight opening, through
which the eye can see, represents the digestive canal. The metal of
which the pipe is composed represents the walls of the digestive tube.

724 COMPARATIVE ANATOMY

It can, therefore, easily be seen that any conditions such as dust and
moisture that may be in the atmosphere surrounding the outside of this
pipe will quite likely be found on the inside.

During the development of the nervous system there is a connection
between the lumen of the neural tube and the gastrular mouth so that
there is a temporary connection between the neural tube and the gas-
trocoele. This connection is called the neurenteric canal (Fig. 328, B,
ne.c). This connection, however, soon disappears so that the gastrocoele
is a closed sac with no opening whatever to the outside of the body until
the mouth and anal openings are pushed in from the ectoderm as already
mentioned.

Definite names are given to the various structures of the growing
embryo. The mouth opening is called the stomatodeum ( ).

The mid portion connecting the mouth with the anal opening is called
the mesodeum ( ), and the caudal part, which like

the stomatodeum is ectodermic, is known as the proctodeum.

This does not mean that in every animal in which an ectodermal
mouth and anus has developed that the ectodermal structures take up
the same length of the digestive system. In the articulates (crustaceans,
insects, and spiders) the stomatodeum and proctodeum are much longer
and larger proportionately to the mesodeum than in the higher forms of
life, in fact, in the vertebrates, the digestive canal is mainly mesodeal and
therefore endodermic. The mouth and anal regions composed of ecto-
derm are but a small portion of the entire digestive system.

The jaws, teeth, and tongue, which will be taken up separately, do
not develop from the simple digestive tube which has just been described,
but the other parts of the digestive system, even the most complicated
ones, have come from this tube alone by a growing in length, by enlarge-
ments of various kinds, by foldings, by outpushings and inpushings. Not
only have such complex organs as the liver and spleen, thyroid and
thymus glands, as well as many others, come from this endodermal tube,
but the entire breathing apparatus of chordates has arisen from its
cephalic end.

As some chordates, such as fishes, live in water, they require a to-
tally different type of breathing mechanism than those which live on
land, yet their branchial or gill system and the land living pulmonary
or lung system have in each case developed from the same simple diges-
tive tube. It must be remembered that this is only true of chordates.
Animals not chordates do not show such close relationship between the
digestive and respiratory systems.

The intestinal tract, if cut in cross section and examined microscop-
ically will be found to be composed of four layers of different types of

muscularia J

DIGESTIVE SYSTEM 725

cells. Starting from the inner layer we find them in the following order
(Fig. 291):

mucosa
submucosa

circular
longitudinal
serosa

It is in the mucosa or the inner layer that the glands which produce
the digestive juices are found. Here, too, are a few scattered involuntary
muscle fibers and lymphatic vessels for carrying away the nutriment after
it has been changed into a condition so it can be assimilated.

The submucosa is a thin layer of connective tissue supporting the
mucosa.

The musculosa varies a good deal, but essentially it is composed of
two layers of involuntary muscle cells both circular and longitudinal, the
former lying toward the lumen.

The circular muscles by contracting, lengthen the intestines while
the contraction of the longitudinal muscles shorten and thicken it. These
two actions cause the peristaltic movement occurring during digestion,
pushing the food forward and also permitting the various folds and little
finger-like projections, called villi, to come in contact with all of the ma-
terial that has been ingested.

In the higher forms, especially in the human being, there' are some
thirty feet or more of small intestine as contrasted with three or four feet
of large. The reason for this can be understood quite readily when it is
appreciated that a two-inch water pipe holds eight times as much as a
pipe one inch in diameter. The great mass of material that is ingested
is of no value whatever to the animal ingesting it unless such food can
be reduced to a more or less liquid state and be absorbed by the mucous
lining of the intestinal tract. Digestion, though beginning in the stom-
ach, really takes place in the small intestine. The smaller this intestine
is in diameter, therefore, and the more folds the mucosa has, the more
readily will the food, after it is sent through the digestive canal, be likely
to come in contact with the mucosa and be absorbed.

It is important to understand this as it will throw much light upon
various physiological functions of digestion, for, it will be seen that the
little finger-like villi must actually do the work of absorbing. That is,
after ingested material is ready for absorption, it does not pass by any
rule of gravity or mechanics into any definite opening; but these little
projections must actually reach out and drink in the necessary material.
As these little villi must in turn be kept in good condition and capable of
performing their functions by their nerve and blood supplies, it follows,
that where the nerve and blood supply is either weakened or lost, the
animal may die of starvation regardless of how much food it may ingest.
Many glands are found in the mucosa. Some of the larger ones have

726

COMPARATIVE ANATOMY

pushed their way further and further back so that they have not only
passed through the submucosa and musculosa, but have gone far beyond.
The liver and pancreas are good examples of those which have left the

III.

Fig. 427.

I. Diagrams to show formation of greater omentum in mammals and the
fusion of the mesogaster and the mesocolon. A, early stage in which the mesogas-
ter is beginning to form a bag at g. B, the mesogaster is drawn posteriorly, into a

DIGESTIVE SYSTEM 727

main digestive tract almost entirely but are still connected with it by
small ducts.

The glands push their way through both submucosa and musculosa
but, as they push against the serosa this seems to stretch out ahead of
all these outpushings, forming a covering for the outgrowths. This is
why not only the liver and pancreas, but every organ in the abdominal
cavity, is completely covered by this serous layer, which when thought
of in its entirety is called the peritoneum.

The kidneys form a single exception to the statement that all organs
in the abdominal cavity are completely covered with peritoneum. These
do not spring from the digestive tract, however, and will be discussed
later with the uro-genital system.

The entire digestive canal is covered with this serous layer. Figure
427 shows just how this develops and why it is that, while there is a
single layer of serosa over the ventral side of the intestinal tract there
are two layers running dorsalward which are attached close to the ven-
tral portion of the spinal region forming the sustaining ligaments.

Probably this will be made clear if one place an ordinary sheet of
paper on the desk before him and lay a pencil at right angles to the long
axis of the sheet. By picking up the two ends of the paper so that the
pencil is held within the fold it will be seen that under the pencil there
is only one layer of paper but above it there are two. The various out-
pushings of the intestinal tract push the serosa before them just as the
pencil does the paper in this case.

The two layers running dorsalward from the organ and forming the
sustaining ligament are called the mesentery, and it is between these two
sheets of mesentery that the blood supply of the organ is carried.

If it be remembered that the digestive tract begins as a single tube,
approximately the same length as that of the body in which it grows,
and if the various elongations, outpushings, and inpushings are then fol-
lowed through the embryonic period, considerable light will be thrown
upon our understanding of the adult structure (Fig. 428).

One must, however, be wary in comparing different type-forms of
animals, as well as animals of the same species at different stages of their
development, or there will be little validity in the comparisons.

The first portions of the digestive tract to differentiate are the

long bag g which is the greater omentum ; the mesogaster and mesocolon are fusing
at i. C, completion of the fusion of mesogaster and mesocolon at i. a, liver ; b,
serosa of the liver ; c, lesser omentum or gastro-hepato-duodenal ligament ; d,
stomach ; e, lesser peritoneal sac or cavity of the greater omentum ; /, mesocolon ;
g, portion of the mesogaster which forms the greater omentum ; h, intestine ;
i, fusion of the mesogaster and mesocolon. ( From Hyman after Hertwig ) .

II. Scheme of digestive canal and mesenteries in human embryos, 30 and 50
mm. long, ac, ascending colon ; c, caecum ; co, colon ; d, duodenum ; dc, descending
colon ; k, kidney ; r, rectum ; rd, recto-duodenal ligament ; rl, recto-lienal ligament ;
rrd, recto-duodenal recess; s, stomach; sp, spleen; tc, transverse colon. (From
Kingsley after Klaatsh).

III. Transverse section of a salamander embryo in the region of the liver.
(Redrawn from Maurer).

IV. Schematic arrangement to show the development of the omental bursa.
(After Corning). P, Pancreas; Ao, Aorta; L.H.G., Hepatogastric ligament.

728

COMPARATIVE ANATOMY

vi-

pharynx and stomach. The former is a fun-
nel-shaped enlargement at the cephalic end
with several pairs of lateral diverticula called
the pharyngeal pouches. These pouches in
some animals break through to the outside of
the body to form slits (Fig. 295). The
stomach may be of many shapes and sizes in
the various animals. That portion of the
stomach that meets with the oesophagus (the
narrow tube connecting pharynx and stomach)
is known as the cardiac portion, while the cau-
dal opening of the stomach is called the py-
lorus (Fig. 438). It will be found that this
pyloric end is rather thick and tough. There
is a valve here which closes so that the stom-
ach can be converted into a closed sac. A
rather thick short portion of the intestine im-
mediately-caudal to the pylorus is known as the
duodenum. Then follows the small intestine,
varying in length in all the animals, which ends
in the large intestine, and this in turn connects
directly with the anal opening to the exterior
of the body or in a terminal enlargement
which quite often receives the openings of the
urinary and reproductive systems before con-
necting with the anal opening. In such cases
as the latter the thickened portion of the large
intestine is called the cloacal chamber, or sim-
ply the cloaca. (Fig. 426, III.)

In fishes, amphibians, and sauropsida, the
cloaca is an important structure. In none of
the mammals except the montremes does it ap-
pear as a distinct organ.
There are various important diverticula of various shapes thrown
out along the digestive tract. The lateral pharyngeal pouches have al-
ready been mentioned. In fishes one often finds quite numerous pyloric
caeca. In mammals at the beginning of the large intestine where the
small one enters it there are colic caeca. In man as well as in several
other forms of mammals one of these little blind sacs is called the appen-
dix vermif ormis. . In birds one finds cloacal caeca.

DETAIL STUDY

The pharynx is that open portion behind the nose and mouth in
mammals which extends down to the voice box. From there downward
(including the voice box) the open portion is called the larynx.

Fig. 428.

Reconstruction of the diges-
tive canal of man. al, allantoic
stalk ; cl, cloaca ; g, glottis ; h,
hyoici arch ; li, liver ; lu, lung ;
md, mx, mandibular and maxil-
lary arches ; n, nasal pit ; o,
omphalomesenteric vein ; «,
stomach ; v, visceral arches ; vi,
vitelline stalk ; w, Wolffian body.
(From Kingsley after His).

DIGESTIVE SYSTEM 729

There are two general types of mouth forms. The first is found in
the great group of Cyclostomata (cycle mouths). There are no true jaws
(Fig. 422). The mouth is round and cannot be closed. Examples of this
form are the lampreys and hagfishes. This type of mouth is called suc-
torial. The cyclostomata are the only vertebrate parasites known. They
attach themselves to a living fish and suck their way directly through
the muscles of the host.

The second type of mouth belongs to that group called the Gnathos-
tomata (jaw-mouths). This type of animal has movable bones or carti-
laginous jaws, usually possessing teeth formed of dentine and underlaid
with enamel. The jaws are developed from one pair of visceral arches.
The teeth are quite similar to the placoid scales of certain fishes which
have been modified in various ways. There are two theories held in re-
gard to the Gnathostomata or jaw-mouth fishes. First, that the mouth
is like that of the cylostomes, to which the gill arches with their asso-
ciated teeth have been added; and the second, that this jaw-mouth is a
new opening which originally consisted of a pair of gill-slits which later
became fused in the mid ventral line, the first mouth then being lost.
Probably the latter view has more supporters, because in the selachians,
where there is supposed to be a more primitive condition, the jaw-mouth
is not at the extreme cephalic end of the animal, but on the ventral side
with a long rostrum extending cephalad to it. Later, in some of the
ganoids this jaw-mouth has a secondary position at the very tip or termi-
nal end.

The pharyngeal pockets develop from a row of outpushings meeting
a similar set of inpushings from the outside (Fig. 295). If the point of
contact is broken through, as in fishes, such openings are called gill-slits.
These are four to eight in number which permanently form a communica-
tion between the pharynx and the exterior, thus allowing the escape of
water taken in by the mouth for use in breathing.

One or more of these slits appear in the early stages of amphibians
and in a few forms persist throughout life. In reptiles, birds, and mam-
mals, there are similar inpushings and outpushings during the embryonic
period, but only two or three contact-points ever form openings, and then
only for a short time. However, the most anterior of these which ap-
pears in the selachians as the spiraculum or blow hole, persists in all
higher vertebrates as the Eustachian tube and the greater part of the
middle ear. The other pouches disappear, although cartilages, muscles,
arteries, and glands arise in the embryo in connection with these pouches.
Sometimes there is an arrested development so that an open com-
munication persists between the pharynx and the exterior of the jaw
either upon one or both sides, as a cervical fistula. This is supposed
to be a permanent gill-slit that for some mechanical or chemical reason
has not continued growing as it normally does in the higher forms.

The nasal cavities in fishes lie above the stomato-pharyngeal cavity

730 COMPARATIVE ANATOMY

and are unconnected therewith, while in the amphibians (Fig. 339) there
develops a pair of openings called the posterior-nares or choanae connect-
ing these two portions by openings in the roof of the mouth. This com-
munication is supposed to be "one of the changes inaugurated during the
transition from water to land and allows the ingress and egress ot air
to the pharynx and then to the lungs without opening the mouth, since
this action, although harmless for an animal immersed in water, would
soon cause the drying up of a mucous membrane lining a mouth cavity
if resorted to in air with anywhere near the same frequency. In the case
of the nasal cavities this is prevented in part by the small size of the
external openings, but still more by the -formation of slime glands capable
of producing an abundant secretion. The waste lacrimal fluid diverted
from the eyes to the nose is undoubtedly also of assistance in this re-
spect."

There is a tendency in the pharynx to form diverticula in the median
line (Fig. 426), that is, there is here an expansion into large sacs or res-
ervoirs which may or may not remain in communication with the pharynx
itself. The air-bladders in fish are examples (Fig. 441). While this air-
bladder is usually a closed sac filled by gases extracted from the blood,
in a few animals one finds a rather small air-duct coming from these air-
bladders to the pharynx. In fishes, where this occurs, the animal comes to
the surface of the water and makes a snapping or swallowing movement.
In the higher forms of animals this develops into the pulmonary system,
the lateral sacs being the lungs and bronchia and the median duct, the
trachea. The opening of the trachea into the pharynx is called the glottis
(Fig. 428), which together with the various cartilages and muscles de-
rived from the visceral system, forms the larynx. While there are always
two lungs in lung-breathing animals there is only one air-bladder in
those forms of vertebrates which are not lung breathing. Then, too,
the air-sac or air-bladder is almost always dorsal to the pharynx, while
the lungs lie ventral to it.

The flat plate of bone forming the roof of the mouth and thus sepa-
rating the nasal cavities from it, is called the hard palate. The soft cover
of this bony plate which extends backward beyond the palate is known
as the soft palate or velum palati.

Just as one can easily see the sulcus ( ) in the

median line immediately under the nose, so, in looking into the mouth,
a ridge will be seen which is formed right through the center of its roof.
As the human being, as well as all of the higher forms of animals, is
bilaterally symmetrical, and as different portions of the jaw begin their
growth from distinctly separate centers, which then grow toward the
midline and unite, one can readily understand not only why the ridge is
formed on the roof of the mouth but also why there is a sulcus immedi-
ately below the nose on the outer upper lip. If, due to a mechanical or
chemical obstruction of some kind, the two lateral portions of palate or

DIGESTIVE SYSTEM

731

Epithelium.

Fig. 429.

I, II, III, IV, Diagrams of developing
tooth (After Hill).

V, Section through the skin of an Elas-

upper lip do not meet, a harelip and
cleft palate result.

In some birds the two halves
of the palate never unite. In some
mammals, such as the cat and dog,
the two portions forming the upper
lip have not united as well as they
have in the human being, and, con-
sequently, a deep median groove
called the philtrum, remains. This
line can be seen to run along the
entire septum of the nose extern-
ally.

Next in order of study come the
teeth, tongue, tonsils, glands of the
mouth cavity, and glands of the
pharyngeal pockets.

There are two types of teeth
which have no relationship to each
other in their origin. The true
teeth are akin to placoid scales
(Fig. 429). They arise by a calcar-
eous secretion at the junction where
ectoderm and mesenchyme meet
and are thus a product of both
layers

The other type comes purely
and simply from the cuticle and is
formed by what is known as cornifi-
cation or hardening of the epithe-
lium (Fig. 422, 6). The parts which
have invaginated to form the stoma-
todeum retain the capacity to form
hard structures, consequently, any
portion of the mouth-walls may se-
crete scale substances. It is neces-
sary to appreciate this in order to
understand that in the different
types of fish and amphibia, teeth of
almost any number, size, and shape

mobranch showing formation of a dermal
spine. Highly magnified.

1. Horny layer of ectoderm. 2. Malpig-
hian layer. 3. Columnar cells of ectoderm
secreting. 4. Enamel. 5. Dentine (black).
6. Dentinal pulp. 7. Connective tissue.
(From Shipley and MacBride).

732 COMPARATIVE ANATOMY

may be found wherever there is cartilage or bone to hold them. In the
higher forms of animals, in fact, in all the amniotes, with the exception
of some squamata, teeth are found only on the margin of the jaws.
Turtles and all present forms of birds are toothless, though many extinct
birds, of which fossil remains have been found, did have teeth. It is in-
teresting to note that even in turtles and birds that have no teeth, the
dental ridge in which the teeth, of toothed animals do develop, is never-
theless present in the embryonic stages, it being assumed that this is
proof of their descent from toothed ancestors.

It will be observed in Figure 429 that at first ectoderm thickens.
The layer of cells immediately beneath the ectoderm pushes downward
into the mesenchyme. These latter cells by multiplying rapidly, push
portions of this ingrowing plate of cells back up and form a sort of finger-
like projection covered by the plate it has pushed before it. The mesen-
chymal finger-like projection forms the pulp of the tooth, while the plate
of cells which covers it becomes what is known as the enamel organ.
The pulp forms several layers of cells, the outer ones becoming odonto-
blasts, so called because it is from these that the bone-layer-substance
dentine or ivory of the tooth is formed. This latter substance is a secre-
tion from the ends of the odontoblasts and it is this which causes it to
be somewhat prismatic in form.

At the base of the enamel organ a denser substance called enamel
is secreted. This fits like a cap over the top and sides of the dentine.
The dentine continues to grow and forces the tooth up through the epi-
thelium so that the tip or crown then comes into position for use. The
nerve supply of the tooth comes from branches of the trigeminal or fifth
cranial nerve. Both nerves and blood vessels enter through the base of
the tooth. Usually, as soon as the teeth are fully formed, the odonto-
blasts cease growing. However, there are exceptions to this rule. The
tusks of elephants and the incisors of rodents function through life and
therefore continue to grow.

In mammals an additional layer of modified bone, the cement, is
formed around the root of the tooth. It may even extend through the
crown. The teeth in the mouths of skates and some other elasmobranchs
are arranged very much like the scales on the surface of the jaw, that is,
in groups of five. In most of the vertebrates there is a succession of
teeth.

Some animals, such as the shark and turtle, continue to renew and
shed their teeth. Such teeth are called deciduous.

In mammals a second set of teeth usually arises directly behind or
above and below the first set, so that the ends of the second set, which
are to force their way through the jaw, push against the roots of the
teeth which are already in use.

The group of first teeth which is formed in man is called the milk
dentition. The second is known as the permanent dentition. In some

DIGESTIVE SYSTEM

733

mammals such as the monotremes, sirenians, and cetacea, there is only
one dentition, while in some groups there are an indefinite number of
successive dentitions. In such animals as Guinea pigs, and in some bats,
the milk dentition is lost even before birth.

Practically all fishes with few exceptions have teeth, and these ex-
tend not only to the lining bones of the mouth, but in some even to the
hyoid and branchial arches. These latter are known as pharyngeal teeth.

Fig. 430.

Biting mechanism of the rattlesnake. la, and Ib, position of the ap»
paratus when mouth is shut. Ha, and lib, position of the apparatus when
mouth is opened widely, showing the spheno-pterygoid muscle (P.e.) con-
tracted, the pterygoid (Pt) pulled forwards, the transverse bone or
ectopterygoid (Tr) pushing the maxillary (M) rotating it and thereby
causing the poison-fang (J) to assume an upright position. Di, Digastric
muscle, the contraction of which lowers, or opens the lower jaw ; G, the
groove or pit characteristic of the Crotaline snakes ; J, poison fang ; M,
maxillary ; P, palatine ; P.e, spheno-pterygoid ; Pm, premaxillary ; Pt,
pterygoid ; Q, quadrate ; Sq, squamosal ; T, a, insertion of the an-
terior temporal muscle, by contraction of which the mouth is shut ; Tr,
transversum or ectopterygoid ; X, origin and insertion of a muscle and a
strong ligament, contraction of which draws the maxillary and its tooth
back into the position of rest and assists in shutting the mouth. (After
Gadow.)

The teeth may be cone shaped or flat, sometimes they even form large
plates as though a number of primitive teeth had grown together. Teeth
may be anchylosed to the summit of the jaws, attached to their inner
side, or have their roots implanted in grooves or pockets as in the human
being. The grooves in the jaw, in which teeth grow, are called alveoli.*

*Mammals are said to be monophyodont if they develop only one set of teeth, and diphyodont
if they develop two. However, even in momophyodont mammals, a second set usually develops,
although this set later becomes absorbed or remains in a vestigial condition.

When all the teeth are uniform they are said to be homodont, while if they vary in shape they
are heterodont.

Teeth are said to be acrodont, if anchylosed to the summit of the jaws, pleurodont, if fastened
to the jaw's inner side, and thecodont, if the roots are implanted in alveoli.

Teeth have also received names according to their function or their peculiar physical appear-

734

COMPARATIVE ANATOMY

The poison fangs of certain serpents are really specialized teeth on
the maxillary bones. They may be permanently erect, or turn as on a
pivot, so that when the mouth is closed the teeth lie along the roof of
the mouth. Vipers and rattlesnakes are examples of this latter type.
(Fig. 430.)

Fig. 431.

The types shown have been chosen from the principal families of the Carnivora in such a
manner as to present a complete view of the changes in dentition in that order. They are :

P, Proviverra (Cynohyaenodon) Cayluxi (Creodonts) ; V, Viverra (Civets) (Viverridae) ; H,
Hyaena crocuta L, (Hyaenidae) ; F, Felis leo L. (lion) (F 'elides) ; M Michairodus cultridens
Cuvier (Ancient saber-toothed tiger of the Tertiary age) (F elides) ; C, Canis familiaris L. (com-
mon dogs) (Canides) ; U, Ursus arctos, L, (Bears) (Ur sides) .

Letters used in common for the figures of the groups of teeth : i, incisors ; c, canines ; p,
pre-molars ; m., molars ; k, carnivores ; cm, inferior maxillary condyle ; gl, glenoid fossa ; co,
occipital condyle.

I. CREODONTS. Extinct order of the Eocene and of the lower Oligocene ; supposed to be
the common stem of all the carnivora.

Type form: Proviverra (Cynohyaenodon) Cayluxi Filhol, of the phosphorite of Quercy (upper
Eocene and lower Oligocene).

P — Right half of the base of the skull seen from below. Below, fragment of the right half
of the lower jaw, internal aspect.

3143

Complete Dental Formula

3143

ance. For example, they are said to be secodont, if used for cutting purposes, such as those of cats ;
bunodont, if used for crushing as in man ; lophodont, if they possess well-marked transverse ridges
as in the elephant ; and selenodont when they possess longitudinal crescent-shaped crests as in the
horse.

DIGESTIVE SYSTEM 735

.Premolars becoming complicated from before backward, to pointed tuberosities, and compressed
laterally (secodont type) ; the fourth premolar (p4) and the three upper molars united to three
tuberosities by sharp ridges. On the lower jaw, these teeth present the entire anterior surface to
three pointed tubercles and a flattened posterior heel (Type: tuberculo-sectorial of Cope).

II. CARNIVORA. They are distinguished from the Oeodonts, from the point of view of
dentition, by the differentiation in the two jaws, of a carnassial, or tooth of slicing action, made
apparent by its greater development than that of the other molars. It is the 4th premolar of the
upper jaw ; in the lower jaw it is the 1st molar.

In a general manner, all the teeth placed before the carnassial, that is to say all the premolars,
are sharp-pointed ; all those which are behind it are tuberculated.

1. Viverrides (Civets) : The most primitive of the Carnivora properly speaking (true Oligo-
cene), from which all the other forms are usually considered to have been derived. Type shown:
Viverra indica Desm.

V, skull, seen from the right side. The maxillae have been dissected away sufficiently, as in
the other figures, so as to show the roots of the teeth.

Vs, left superior carnassial tooth, seen from the crown.
Vi, left inferior carnassial, seen from the internal aspect.
3 1 4 (3) 2

Dental formula

3 1 4 (3) 2

The number of molars is in general reduced to two, characteristic of a carnivorous specializa-
tion.

2. The Mustelids (marten, sable, polecat, weasel, stoat) are very close to the primitive type.
The carnivorous tendency is strongly developed, as shown by the great reduction of the molars
and the higher development of the carnassial tooth.

Starting from the Viverrides, the various forms of the carnivora show changes in two clearly
divergent directions : one, in which the meat-eating nature of the animal becomes more and more
evident, as in Hyaenidae, Felides, and Pinnipedes (seals, eared seals, walrusses) and the other which
returns somewhat to the omnivorous order, separated from the Hyaenidae and Felides, and giving
rise to the Canides and Ursides.

3. Hyaenides: These form a branch supposedly derived in a direct line from the Viverrides
(Hyaenictia) as they appear in the upper Miocene.

Type Figure: Hyaena Crocuta L.

H, the two jaws, seen from the left side.

Hs, left superior carnassial tooth, seen from the crown.

Hi, left inferior carnassial tooth, internal view.

3141
Dental Formula :

3131
dentition quite like that of felides, and not well developed in a carnivorous sense.

4. Felides (cats) : The most characteristic of the Carnivora. Their most typical representa-
tives appear in the Miocene type, but they are preceded by others, which connect them with the
viverrides.

Type Figure: The lion (Felis Leo L.) ; Machairodus cultridens Cuvier, (a fossil Feline of
the European Pliocene age).

Flt Skull of a lion, seen from the left side.

F^, Left half of the same skull, seen from below.

F3, Left superior carnassial, seen from the crown.

Fi, Left inferior carnassial, seen from the internal aspect.
3131

Dental Formula

3121

Of the tuberculated molars, a single one persists, very much reduced (m1). The premolars,
although secondont, have undergone a certain reduction in their number as well as in their size,
leaving all the functional importance to the carnassials, which have become enormous. The
canines are likewise very strong, and are much longer than their neighbors. On the other hand,
the incisors, whose cutting function is done much more efficiently by the carnassials, have di-
minished. The jaw is, all in all, greatly shortened. Notice also the great development and widen-
ing of the zygomatic arch, giving a large surface for the levator muscles of the lower jaw (the
temporalis, which passes under the arch, and the masseter, which takes its origin from the entire
length of the arch). It has thus acquired considerable size and strength. The lower jaw is
greatly hollowed out on its external aspect, to permit insertion to the fibers of the large masseter
muscle.

M, Skull of Machairodus cultridens (extinct saber-toothed tigers), seen from the left side.

Ms, left superior carnassial, seen from the crown.

An exaggeration of the Feline type.
3120

Dental formula

3111

Huge development of the superior canine teeth, which surpass so far those of the lower jaw that
they limit closely, on each side, the buccal gap, no longer permitting free use of the canines and
carnassials in tearing off meat.

5. Canides (dog-like carnivora). A mixed group, both meat-eating and omnivorous. The
canides appear early in the Oligocene, their first forms being closely related to the primitive
Viverra or to the Creodonts, some of whose characteristics are even more primitive than those of
the typical Viverrides. The dog family appears in the early Pliocene.

Type Figure : Canis familiaris, L.
C. Left half of the skull, seen from below.

C', Left inferior maxillnry condyle. seen as a horizontal cylinder (characteristic of all the
Carnivora) in relationship with the j?l~nr>M cavity which is hollowed out cylindrically.
C , The skull, seen from the right side.

736 COMPARATIVE ANATOMY

Cs, Left superior carnassial, seen from the crown.

Ci, Left inferior carnassial, seen from the internal aspect.
3 1 4 3 (2)

Dental formula :

3133

6. Ur sides (Bears). The least carnivorous of all the Carnivora. They originated, apparently,
in the upper Miocene, from the primitive Canides (Amphicyon).

Type Figure : Ursus arctos L.

17, , The two jaws seen from the right side.

Un, Left half of the base of the skull seen from the lower aspect.

This figure has been placed near the corresponding figure of the Lion in such a manner as to
render apparent the comparison between these two extreme types. The comparison must be
limited to the portion included in each figure between the incisive teeth and the occipital condyle
(co). Behind this there is a very large hollowed out area which projects from the posterior
aspect, to serve as the inservation of the posterior muscles of the skull and neck.

Us, Left superior carnassial, seen from the crown.

Ui, Left inferior carnassial, seen from the internal aspect.
3132

Dental Formula :

3123

This is probably a regressive adaption to the omnivorous regime. The enormous development of the
molars have become quadrituberculated and complicated by. the appearance of little tubercles on or
between the greater tubercles. Regression of the cutting function of the teeth has followed,
the flesh-eating character becoming hardly apparent except in the remarkable power of the upper
canines, "with their very oblique insertion and long root. The skull also lengthens.

Changes which take place in the Superior Carnassial Teeth: (Figs. Vs, Hs, Fs, Ms, Cs, Us.
In the figures the arrow indicates the upper teeth, l;he arrow's point being directed toward the
opening of the mouth).

a, paracone or antero-external cusp; a', anterior accessory cusp; 5, protocone, or antero-
internal cusp ; c, metacone, or postero-external cusp.

Primitive form ( Viverra, Vs) ; type trigodont (triconodont). The tooth contains two ex-
ternal cusps (a,c) compressed laterally and united in a single cutting edge, and a third tubercle
(6) placed anterior and forward. A fourth tubercle (a') is often found in front of the two
external cusps on the same line with them. The cutting edge formed by these last is lengthened in
such a manner as to place these tubercles together and thus present three points. The tooth has
three roots, two anterior and one posterior.

Hyaenidae and F 'elides (Hs and Fs) : The external cutting edge is developed highly in these.
The internal anterior tubercle remains conical and blunt, but disappears completely in Machairodus
(Ms).

Canides: (Cs) : Changes in the omnivorous group. The tubercle a' has disappeared, the
tubercle 6 remaining prominent.

Ur sides: (Us) : There is the same type of accentuation as in the Canides. Three conical
tubercles.

Changes in the Lower Carnassial: (Figs. Ui, Ci, Hi, Fi, Vi) . The arrows are placed above
the tooth to indicate the lower ones. The point indicates the anterior direction, a, paraconid or
anterio-internal cusp; B, protoconid or external-anterior cusp; B', metaconid or internal posterior
cusp ; Y, hypoconid, or posterior talon.

The primitive type (Viverrides, Vi) is here the tuberculo-sectorial type of Cope. It contains
(1), An anterior part with three tuberculated points: two internal, A, B' , and one external,
B, (2) a posterior talon Y, low and flattened, carrying one or more blunt tubercles. The tooth
presents, in other words, a secodont anterior portion (carnivorous) and a tuberculated posterior
portion (omnivorous). It has two roots corresponding to the two parts.

This tuberculo-sectorial type is common to all the lower molars of the creodonts and is limited
more or less to the true carnassial tooth in the true Carnivora.

Canides. Ci : The tuberculo-sectorial type is preserved but with accentuation of the carnassial
character. Then there is a reduction of. the talon predominance of B, reduction of B' , and the
anterior root is somewhat stronger than the posterior.

Hyaenidae, Hi. : Regression of the talon ; the tubercles A and B compressed and united in
a sharp cutting edge, and bicuspid; B' notably reduced. The anterior root is much stronger
than the posterior.

F elides, Fi.: The talon of B' has nearly disappeared and there is predominance of the anterior
root.

Ursides, Ui. : Omnivorous type ; the secodont part is smooth ; its tubercles are blunt and
conical ; the talon contains more than half of the crown ; it is covered by the secondary tubercles,
which are elevated almost to the level of the anterior cusps. The whole of the talon has this one
surface entirely similar to that of the tuberculated molars which are placed next in position.

Title Figures: P, head of Panther (FELIS) ; Ci, head of Civet (VIVERRA) . (From the
charts of Remy, Perrier & Cepede).

There are four kinds of teeth in mammals (Fig. 431). In the human
being they are alike in both upper and lower jaws, as well as alike in both
halves of upper and lower jaws. For classification of teeth, we use only
one-half of the teeth in either jaw. Thus in man we find the two teeth
nearest the midline — the incisors — are followed by a single canine. This
is distinctly cone shaped and has a single root. Back of this come the
two pre-molars commonly called bicuspids, having1 two roots and com-
plicated crowns. They appear both in the milk and permanent den-

DIGESTIVE SYSTEM 737

titions ; and lastly three molars quite like the pre-molars in form, with
several roots, but appearing only in the permanent dentition. The num-
ber and kind of teeth is expressed by what is known as a dental formula.
As already stated, the number and kinds of teeth in the two halves of the
jaw are the same, so only one side need be represented in the formula,
but, as in some animals the upper and lower jaws do not have the same
types and forms of teeth, the formula must take both upper and lower
jaws into consideration. The upper figures, therefore, represent one-half
the upper jaw and the lower figures one-half the lower jaw.

DENTAL FORMULA

i-f, c-K pm-f , m-f

This is the dental formula for man ; that for the opossum being :

i-f-, c— »

EPIDERMAL TEETH

Epidermal teeth occur in cyclostomes (Fig. 422, 6) and various lar-
val stages of amphibia and monotremes. In the cyclostome these are
little cone-like projections of cornified epithelium with an underlying
core of integument. These epidermal teeth are differently arranged in
the lampreys and myxinoids. "In the latter they are few, there being
a single tooth on the palate and two chevron-shaped rows on the top.
In the lampreys, nearly the whole inner surface of the oral hood is lined
with these teeth of varying shape and there are a varying number upon
the tongue. These teeth are used as a means of fastening the animals to
their prey and those of the myxinoid tongue are used for boring into the
fishes on which those animals feed. In the larval anura (Fig. 318), the
edges of the jaws are armed with cornified papillae serving as teeth, the
arrangement of which varies in different genera. They are frequently
aggregated in dental plates used in scraping the algae from submerged
objects. They are not related to the teeth of cyclostomes."

Baleen or whalebone should be mentioned here. This is formed in
large plates of horny material attached to the margins of the upper jaw
(Fig. 392). The fringed ends and edges of these plates serve as strainers
to extract the food products from the various materials taken in with the
water.

In the embryos of certain lizards and snakes there is a median tooth
which projects from the mouth and which is used to rupture the egg cell
when the young is ready to escape. Such a tooth is called an egg-tooth.
An egg-tooth is formed in the turtles, Sphenodon, crocodiles, birds, and
monotromes, but in these cases it is only a thick (sometimes calcified)
portion of the epidermis.

738 COMPARATIVE ANATOMY

THE TONGUE

The tongue varies to a very considerable extent in the different
groups of vertebrates (Fig. 432). In mammals the hyo-branchial sup-
port consists simply of a basi-hyal (body) and two pairs of horns
(cornua). The most cephalad pair are the longer and usually consist of
four bony structures, the cerato-hyal, the epi-hyal, the stylo-hyal, and
the tympano-hyal, the latter bone attached to the skull in the tympanic
region. The pair of horns lying caudal consists of only a single skeletal
piece of bone known as the thyro-hyal which connects the body with the
thyroid cartilage of the larynx. In the human being, the anterior or
cephalad horns are considerably modified from those in other mammals.

The tympano-hyal and the stylo-hyals have fused with the otic re-
gion of the skull to form the styloid process, while the hypo-hyal is a
mere rudiment connecting with the styloid process by a ligament; the
cerato-hyal is not present. The anterior horns, though typically
longer and more complex than the others, are called the ''lesser" in man,
because the earlier anatomists took all of their names from the human
being without any comparisons with other forms.

There is no functional tongue in fishes, although the material which
develops into a tongue in the higher forms is present. This is known as
the anterior part of the hyo-branchial apparatus. The part of this com-
plex apparatus lying most cephalad, is found in the floor of the mouth
cavity. It is naturally shaped according to the outlines of the jaw which
border it. It may even be pushed forward so as to form a slight eleva-
tion by the action of the visceral muscles.

In amphibians where the gill-bearing function has more or less
ceased, this region forms the basis of the tongue, while a fleshy organ of
some kind may develop.

In the higher forms of vertebrates two to four of the visceral arches
form the skeletal basis of the tongue. The hyoid arch is the structural
foundation to which the muscles of the tongue are attached. Here one
usually finds, although there are many varieties, a median basi-branchial
piece called the os entoglossum and two caudad projecting horns — the
cornua. In Sauropsida the tongue is a direct condition of this, and the
principle motion consists in protruding and withdrawing the entire organ
by means of the two caudal horns which lie in sheaths from which they
may be everted. The tongue is sometimes quite long and then the
sheaths and the enclosed horns are, of course, of corresponding length.
If they are very long there must be some disposition made of them when
retracted. This is interestingly observed in the salamander Spelerpes
fuscus, where the sheaths of the horns run down the sides of the body
until they are attached to the pelvic bones, the ilia. In the woodpecker
they pass around the occipital region over the top of the head and end
near the anterior nares of the base of the upper beak. In such cases the

DIGESTIVE SYSTEM

739

ends of the horns are fastened to the bottom of the sheaths so that the
sheath is turned inside out when they are withdrawn.

The tongue develops between the hyoid and mandibular arches (Fig.
432). The hyoid often extends into and supports the tongue. Conse-
quently, the organ itself cannot be moved unless its supporting skeleton
is likewise moved. The tongue is a sensory organ but can be used as an
organ of touch and taste. There are little elevations (Fig. 433) known
as papillae, in many if not most animal tongues. Some of these are

mth

Fig. 432.

Two stages in the development of the tongue and pharyngeal
floor of man. c, copula (basihyal element) ; cs, cervical sinus; ep,
epiglottis ; g, glottis ; h, hyoid arch ; md, mandibular arch ; mth,
median anlage of thyroid gland ; t, tuberculum impar ; tg, tongue.
(P'rom Kingsley after His.)

Fig. 433.

Papillary surfaces of the human
tongue showing fauces and tonsils.
1, 1, circumvallate papillae, in front
of 2, the foramen caecum ; 3, fungi-
form papillae ; 4, filiform and con-
ical papillae ; 5, transverse and
oblique rugae; 6, mucous glands at
the base of the tongue and in the
fauces ; 7, tonsils ; 8, part of the
epiglottis ; 9, median glosso-epi-
glottidean fold (frenum epiglot-
tis). (From Hill after Sappey.)

sensory while others have become hardened and serve as rasping organs.
In the cyclostomes the tongue is thick and fleshy and is supported
by a cartilaginous skeleton. The muscles which throw out the tongue
are called protractor muscles and those which draw the tongue back to
its normal position are known as retractors. These muscles are devel-
oped from the postotic myotomes and their nerve supply comes from the
hypo-glossal nerve. In the myxinoids the terminal end of the tongue
possesses epidermal teeth which thus form a boring organ by which these
animals obtain entrance into their prey. In the lampreys the surface
has a rasping organ and also forms part of the sucking apparatus. In
the amphibians, there are a few anura (aglossa), in which the tongue is
practically absent, but in most of them the tongue actually contains in-
trinsic muscles supplied by the. hypo-glossal nerve and in such case the
tongue can be moved quite readily. The tongue of amphibians is made
up of a small basal portion quite similar to that of the fish, but to this

740 COMPARATIVE ANATOMY

is added a large glandular part which develops between that portion
called the copula or medial region and the lower body. The amphibian
tongue secretes slime which is rather useful in capturing its prey. In
the anura the tongue is fastened to the margin of the jaw, while its free
end when not in use lies on the floor of the mouth. In urodeles a much
greater portion of the tongue is attached than in anura, where not only
the anterior margin of the tongue, but a part of the ventral surface as
well, is held quite definitely in place by attachments.

The supporting skeleton of the tongue, as mentioned above, usually
consists of two pairs of horns largely formed from the ventral ends of
the hyoid and first branchial arches. The median portion or body which
unites these horns is known as the copula. The reptilian tongue includes
the parts already mentioned which are found in the amphibia, and in ad-
dition a median growth which arises between the basi-hyal and the lower
jaw known as the tuberculum impar (Fig. 432, t). Added to this, there
is found a pair of lateral folds lying above the first visceral (mandibular)
arch. From now on, as these parts develop, the trigeminal nerve sends
twigs to the tongue in addition to the hypoglossal and glossopharyngeal
as in the lower groups. In turtles and crocodiles, the tongue lies on the
floor of the mouth and cannot be protruded. In reptiles possessing a
retractile tongue, the hyoid apparatus extends into that organ. The un-
paired cephalad portion which we have called the os entoglossum is
equivalent to the term copula or basi-hyal; the retractor muscles are
usually attached to the two horns. In the tongues of birds, the lateral
parts of the reptilian tongue are not to be found and consequently there
is no branch from the trigeminal nerve. It is to be remembered that
during the development of a part, the nerves follow the growing muscle.
The bird's tongue has no intrinsic muscles. It has many varieties of
form, but is usually slender and covered with horny papillae. Even its
skeleton is reduced, there being only an os entoglossum with a pair of
structural elements attached in front, known as the paraglossae, while,
on the sides, a pair of horns form the first branchial arch, and, in the
median line behind, there is a portion called the urohyal. This is well
marked in the woodpecker, as has been stated.

Now, as to the use of the tongue. With the exception of the whale
the tongue is very mobile in all forms of mammalian life. The mobility
reaches its extreme in the ant-eaters (Fig. 387). It is largely due to the
intrinsic muscles which have been derived to a considerable extent from
the hypo-branchial musculature. The tongue itself is developed from
the unpaired elevation — the tuberculum impar, and from two thickenings
on the mandibular arch, which, together with the fleshy ridges above the
hyoid arch, form the tongue. These fleshy ridges above the hyoid arch
form the back part of the tongue. The line formed between the anterior
and posterior parts cannot very readily be seen in the adult, but it is quite
close to the circumvallate papillae and the foramen caecum. This latter

DIGESTIVE SYSTEM 741

is a little open place or pit in close relationship to the development of
the thyroid gland.

It will thus be seen that the mammalian tongue is quite similar to
that of reptiles, and exceeds that of birds by having portions in it that
come from the mandibular arch.

Two views are usually stated as to the relations of the mammalian
and amphibian tongue. One holds that the amphibian tongue is entirely
unrepresented in the mammals, unless it be by the sublingua. This is a
fleshy fold beneath the tongue of marsupials and lemurs, traces of which
occur in other mammals, even in man, as folds (plicae fimbriatae) be-
neath the tongue. In some cases (Stenops) the sublingua is supported
by cartilage, which may be the entoglossum. The other view is that at
least the anterior part of the tongue in amniotes is quite like that of am-
phibia. This view holds that the lyssa (a vermiform mass of cartilage,
muscle, and connective tissue, lying ventral to the median septum of the
tongue), is the equivalent of the entoglossum and its associated struc-
tures.

The dorsal surface of the tongue is covered with a soft epithelium
with many mucous glands. There are also varying forms of papillae
(Fig. 433), some of which, the taste buds for example, are sensory, while
some become cornified to form epidermal teeth. A rasping type of tongue
in which many of the papillae have become cornified is that of the cat.

GLANDS

In animals that live under water it is quite natural that compara-
tively few glands should be found in the mouth cavity other than the
very simplest kind. These pour out a slight amount of mucus. If glands
were to exist there to any extent their secretionss would be washed away
with the incoming and outgoing water that passes through the mouth
cavity of such animals. Then, too, one can easily understand that, where
a secretion of an animal gland is soluble in water, if such animal lives
in water, the secretion could be of no value whatever. Contrasted to
this, it can also readily be understood that animals which breathe air
must have many glands moistening all surfaces constantly, or the ab-
sorption, which is always going on, would soon have all parts of our
bodies so dry that they could no longer function. For this reason ter-
restial animals have many more secreting glands than water animals.

Mammals therefore have salivary glands. The saliva, which these
secrete, contains not only mucus, but a digestive ferment known as
ptyalin which changes starch into sugar.

Glands are named largely after the position they occupy, such as
labial, lingual, sub-lingual, etc.

In the air breathing amphibia, snakes, and lizards, there are labial
glands opening at the basis of the teeth as well as an intermaxillary or
internasal gland, in the septum between the nasal cavities, as well as

742 COMPARATIVE ANATOMY

palatal glands near the choanae, although these latter glands are lacking
in the caecilians. There is a sub-lingual gland on either side in many
reptiles. Probably all secretions from salivary glands in snakes are poi-
sonous. There is only one known poisonous lizard (Heloderma). In
these the sub-lingual glands furnish the poison. Birds do not have
labial and internasal glands, but they do have numerous other ones which
open separately into the roof of the mouth. They also have anterior and
posterior sub-linguals and even sometimes "angle" glands at the angle
of the mouth, a condition sometimes supposed to be a remnant of the
labial glands in the Sauropsida. Mammals possess small labial, buccal,
lingual, and palatine glands imbedded in the mucous membrane of the
mouth, each of which opens through a separate duct. All of these glands
serve to keep the various surfaces moist.

Many glands, however, have become specialized; for example, the
intermaxillary glands of frogs and toads (opening into the roof of the
mouth) secrete a viscid and sticky fluid which the tongue uses as it is
thrown out to catch and hold insects and other moving objects. So, too,
the buccal glands of poisonous serpents furnish the venom which is sent
forth through the poison fangs. These poison fangs, it will be remem-
bered, are teeth, and they are provided either with a groove along the
external surface or else they have a very small lumen through the center
of the tooth and act similarly to a hypodermic needle. Those glands
which assist in throwing out a thin, watery lubricant are called serous
glands, while those assisting in softening and dissolving dry food so that
it can be more easily swallowed are called salivary glands without regard
to their position.

In mammals the salivary glands are the parotid, lying ventral to the
ear (swelling up in man when he has mumps), the submandibular (called
submaxillary in human anatomy), the sublingual, and the retrolingual.
This last one is closely associated with the submandibular. It is not
found in all mammals.

The serous glands secrete a clear fluid without any salivary at-
tribute. The molar gland of ungulates or the voluminous orbital gland
of dogs are examples of this type. The orbital glands open into the
mouth-cavity close to the last upper molar. The submandibular is found
in the lower jaw beneath the mylohyoid muscle. Its duct (Wharton's
duct) opens near the lower incisor teeth. The retrolingual gland is near
the submandibular, its duct opening close to Wharton's. The sublingual
gland lies between the tongue and the alveolar margin of the lower jaw.
It empties through several ducts. The parotid opens through Stenson's
duct near the molars of the upper jaw.

THE PHARYNX

We have already described the pharynx as the cephalic end of the
digestive canal which is found between the cavity of the mouth and the

DIGESTIVE SYSTEM

743

oesophagus from which the respiratory system develops. It will be de-
scribed in more detail in our discussion of the respiratory system.
But, as there are certain more or less significant organs developed in the
pharyngeal region, it may be \vell to discuss them at this point. These
are especially the thymus and thyroid glands. It is customary to trace
the development of these two glands from the cyclostomes upward, be-
cause the cyclostomes furnish the first (more generalized) stage of de-
velopment of such glands, and thus make it possible to follow up con-
secutively any so-called advance from a lower developmental type to a
higher one. (Figs. 294, 434.)

There are six pharyngeal pockets (except in the cyclostomes) de-
veloped on each side. Each of these pockets possesses a dorsal and a
ventral recess. It is around these recesses that a group of epithelial
cells develops an organ-anlage, all seemingly alike in the lower forms.
In the higher forms, however, the dorsal group soon forms the thymus
and the ventral forms what are called epithelial corpuscles.

These thymus-anlagen may separate from the layer where they
originated or they may fuse into a single elongated organ, or they may
become constricted in number, the anterior ones disappearing.

In the cyclostomes, there are seven anlagen. In the teleosts there
are six pharyngeal pockets, but only four anlagen, and these are all the
more posterior ones.

tttyr

Fig. 434.

Thyroid and thymus glands with closely related organs. A, lizard; B,
Hen; C, Calf. car, carotid artery; h, heart; p.br.k., postbranchial bodies;
thym, thymus; thym' , point of thymus attachment; thyr, thyroid gland; tr,
trachea; v.jug., jugular vein. (After DeMeuron.)

In the mammals it is the third pocket which produces the thymus-
anlage, although sometimes there is a tiny addition from the fourth.

The epithelial corpuscles tend to disappear, but in amphibians they
become glandular and associate with the carotid artery to form carotid
glands. The number of these carotid glands varies in different groups of
animals.

Immediately behind the last gill slit, in the floor of the pharynx,
there is a pair of evaginations. These have been termed supraperi-

744 COMPARATIVE ANATOMY

cardial bodies because they secondarily become associated with the peri-
cardium of the selachians. At present they are usually called post-
branchial bodies. In selachians a complete pair of these bodies develops,
but in urodeles and lizards only the left one ever completes its develop-
ment, the right ultimately disappearing. Whether these bodies occur in
birds and mammals is not known, although there are somewhat similar
growths which are called parathyroid bodies, which do develop in these
animals and then become lost in the lobes of the thyroid gland.

Explanations of these bodies are not yet satisfactory.

The thyroid has come to be considered a very important organ since
endocrinology looms up so large in the medical world. This gland is
an evagination of the pharynx. It is first seen in the selachians, but
makes its appearance regularly in the higher forms. It arises from the
floor of the pharynx at about the level of the interval between the first
and second pockets. It becomes compact, and, like the thymus, does
not develop a duct. In the larva of Petromyzon (one of the cyclostomes)
the thyroid appears as an open trough, lined with cilia, which is in open
communication with the pharynx, a position quite like that in Amphi-
oxus. This trough is called the hypo-branchial groove or endostyle, an
organ which assists the passage of food down the pharynx by exuding
a slimy secretion and by furnishing a definite track, with cilia, which
can thus facilitate its movement.

Professor Wilder thinks the thyroid gland is primarily a digestive
organ, though in the true vertebrates its structure, as well as its func-
tion, has nothing to do with digestion. It is now generally taught
that the internal secretions of the thyroid gland stimulate growth and
inhibit development, while the secretions of the thymus gland stimulate
development and inhibit growth.

THE OESOPHAGUS

This is the swallowing tube connecting the mouth with the stomach.
It lies directly against the interior of the dorsal wrall of the body-cavity
and thus lacks a serous coat. There are no digestive glands in its walls
as a rule. Its length quite naturally varies with the length of the neck
of the animal in which it occurs. Usually, its internal lining is a smooth
epithelium. In the chelonians one finds cornified papillae pointing back-
wards. The oesophagus, like other parts of the digestive tract, consists
of five layers ; however, as it will be remembered that the ectoderm has
indented to form the anterior and posterior openings into the digestive
tract, a histological examination in the region where these two divisions
merge into each other will show a change of structure.

The muscles contained in the walls are striated at the cephalic end
and extend back in some cases even into the stomach. The oesophagus
usually has the same diameter throughout, but in many, if not most
birds, there is a dilation called the crop or ingluvies. This may either

DIGESTIVE SYSTEM 745

be an expansion on one side, or, as in pigeons, it may consist of median
as well as a pair of lateral chambers. The crop may be a simple res-
ervoir for food, or it may be a real glandular organ where secretions
are poured forth and digestion started. In fact, during the breeding
season pigeons secrete a milky fluid here which is used in feeding the
young.

THE STOMACH

The various portions of the stomach have already been named and
described in the frog. To the terms there given, should be added the
small curvature at the top or anterior surface of the stomach, usually
called the "lesser" curvature, and the posterior curvature called the
"greater."

In some forms of animals such as amphibians, the lining of the
mouth, oesophagus, and stomach is covered with cilia. In birds the
stomach is divided into an anterior glandular region called the proven-
triculus, and a posterior muscular region called the gizzard. After the
food has passed through the proventricular region and has mixed with
the secretion from its glands it passes into the gizzard. This latter
organ is not only muscular but the muscles have developed into a pair
of disks with tendinous centers. There is a secretion in the gizzard
which hardens the lining sometimes, even raising little elevations which
are used in grinding the food. One might almost consider them teeth.
Remembering that birds have no true teeth, one can readily understand
the advantage such an animal has in a gizzard of this type. Grain-eating
birds swallow small pebbles which enter the gizzard and are thus also
made use of for grinding purposes.

In fact, in the fossil pterodactyl there have been pebbles "found in
such portions as to lead to the supposition that these reptiles also had
a gizzard." It is well to note in this regard that the grain-eating birds
have the best developed gizzards, while birds of prey have gizzards much
less fully developed. In one species of pigeons a part of the wall of the
gizzard is ossified. In mammals there are more varying forms of stom-
achs. These are divided in from one to four regions. The ruminants
have two well developed divisions of the stomach (Fig. 435), the rumen
or paunch, and the reticulum or honey comb, though these two divi-
sions are really enlarged portions of the oesophagus and serve as res-
ervoirs of food. The food is regurgitated into the mouth for mastication,
and after it is swallowed a second time passes into the true stomach, the
psalterium (also called omasus or manyplies), and then to the abomasum
or rennet. The latter is used for gastric digestion.

It is of interest here to trace the embryonic changes of the mesentery
in mammals. The mesentery supporting the stomach is called the meso-
gastrium. The first curvature of the stomach, which is toward the left,
broadens the corresponding part of the mesogastrium, an effect which is

746 COMPARATIVE ANATOMY

still further increased by the lateral torsion of the entire stomach. The
spleen develops within this widened part and by its weight produces a
fullness which in turn causes a sagging down behind (dorsal to) the
lesser curvature, although attached to the greater. This tendency con-
tinues and causes the free lower fold of the bag-like extension to hang
down behind the stomach (Fig. 427).

This fold is called the greater omentum (omentum majus), which,
as all mesenteries are essentially double, must consist of four layers of
serous membranes, applied two and two, each pair holding between them
the blood vessels and absorbent vessels naturally belonging to a mesen-

Fig. 435.

Stomach of a Sheep. 4 abom, abomasum ; d, intestine ; /J,/2,
two folds which divide the rumen (paunch) into three regions;
kl, plyoric valve; o (3-4) opening which leads from the third
to the fourth stomach region ; oes, oesophagus ; 3, psalt,
psalterium (omasus or manyplies) ; 2.ret, reticulum (honey-
comb) ; l,rum, rumen (paunch) ; schl.r., pharyngeal groove.

The piece of wire marked I shows the direction the un-
masticated food takes, while // shows the direction of the re-
masticated food. (After Carus and Otto.)

tery. The cavity of the bag is the lesser peritoneal cavity of human
anatomy, and the opening into it (behind the stomach) is the foramen
epiploicum (foramen of Winslow). While the bag is widely open in
most mammals, in man the foramen is considerably reduced in size, and
the layers forming the pendulous fold are fused together to form a four-
layered apron that hangs below the stomach and covers the intestinal
folds.

THE INTESTINE

The duodenum takes its name from the twelve inches of rather large
diameter intestine which immediately follows the stomach. The word
"duodenum" is a name which was taken from human anatomy though

DIGESTIVE SYSTEM

747

even in man it is closer to eleven inches in length than it is to twelve
inches in the adult. In the lower forms of animals it varies in length
and shape, as do all the other parts of the intestinal tract.

Growing from this portion of the intestinal tract, immediately be-
yond the pylorus, in some of the ganoids and teleosts there may be as
many as one to two hundred blind tubes extending. These are known
as the pyloric caeca. There are a few elasmobranchs which have only
one pair of these caeca. These may be expanded into a pouch called
a bursa Entiana. The region of the intestine running caudad from the
duodenum is also called the post-hepatic intestine, and it is in this region
caudad to the liver, in which most of the digestive processes, as well as
most of the absorption of the products of digestion, take place.

The food having been more or less mixed with various salivary
secretions, and having been reduced to a semi-liquid state, receives the
bile from the liver, and the pancreatic juice from the pancreas (Fig. 436).

Fig. 436.

A, The duodenum of a rabbit. P., Pyloric end of stomach ; yb., gall bladder
with bile duct and hepatic ducts ; p.d., pancreatic duct. (From Krause after
Claude Bernard.)

B, Appendix vermiformis of kangaroo ; C, Appendix vermiformis of human
embryo. (After Wiedersheim. )

It is then ready, after being moved back and forth by the peristaltic
movement of the intestine, to be taken up and absorbed by the little
finger-like processes (villi) which extend from the inner surface of the
small intestine. As this has already been discussed it need not be en-
tered into again here. It is well, however, to remember that the length
of the intestine varies with the type of food the individual eats. It is
longer in plant-eating animals than in meat-eating animals. A classic
illustration of this is to compare the length of the intestine in the adult
frog with that of the tadpole. The adult frog's intestine is no longer
than that of a tadpole half the size a frog would be if it had kept up its
relative increase in size.

Any blind pouch is called a caecum. At the beginning of the large
intestine where the small intestine enters into it, the joining itself is
called the ileo-caecal junction, and the little projecting end of the large

748 COMPARATIVE ANATOMY

intestine is the caecum. It is the tapering end of this caecum which
forms the appendix (Fig. 436) in man and in some of the other verte-
brates. There is also a valve at the junction of the ileum and caecum
known as the ileo-caecal valve.

The first part of the small intestine which follows the duodenum
is known as the jejunum, while the more distal portion is the ileum.

Professor Wilder gives the following interesting account of the ap-
pendix and succeeding structures :

"At the junction of the small intestine with the large, there is a
strong tendency to form one or more caeca, or blind sacs, which often
become digestive organs of great physiological efficiency. The charac-
teristic form in reptiles is that of a single rather short and wide caecum,
symmetrically placed. In birds there are usually two symmetrical ones,
which attain great length in scratching birds (e. g., the common fowl),
and in ducks and geese, but are quite rudimentary in certain others
(woodpeckers, parrots, etc.). Ostriches possess a single caecum of
great length (seven to eight meters) and furnished with an internal
spiral partition, which greatly increases its effective surface.

"In mammals a single caecum is developed, which varies greatly
in size and functional importance. Rudimentary in edentates, most in-
sectivores, and bats, it frequently attains an enormous size in herbivorous
or graminivorous forms. In certain rodents (e. g., muskrat, woodchuck),
its total capacity equals- or exceeds that of the remainder of the alimen-
tary canal, and in the marsupial Phascolarctus it is three times the
length of the body. In the rabbit it is provided with an internal spiral
valve ; in certain other rodents and in the higher apes and man, the free
end becomes rudimentary, restricts its lumen, and forms a worm-like
process, the processus (appendix) vermiformis, which like all rudimen-
tary organs, is subject to a large amount of individual variation.

"Thus in the human subject the appendix varies in length between
the limits of 2-23 cm., the average for an adult being 8-9 cm. It is longest
proportionally during fetal life, its length relative to that of the large
intestine being 1 :10, while in adult life it is 1 :20. It is longest abso-
lutely between the ages of ten and twenty, after which it shows a slight
reduction. Its status as a rudiment of slight functional value is shown
by the tendency toward the obliteration of its lumen, a tendency which
increases steadily with age. Furthermore, these two characters, reduc-
tion in length and obliteration of the lumen, go hand in hand, short ap-
pendices being usually solid, while large ones are apt to possess a lumen.

"The position and arrangement of the colon varies considerably
among various mammals. In man it begins low down on the right side,
from which there proceed in order an ascending, transverse, and de-
scending portion, connected with the rectum by a sigmoid flexure,
through which the tube attains the median line ; a similar disposal is
seen in many other anthropoids, in lemurs and rodents, the majority of

DIGESTIVE SYSTEM 749

carnivores, and a few others. A more complex condition than this is
produced by the formation of long, narrow loops along the course of
either the ascending or transverse colons, or both, and these loops may
remain simple or roll into spirals. Such colon labyrinths are seen in
ruminants, in certain rodents as the lemmings and jumping mice, and
in a few lemurs.

"From this brief review of the alimentary canal and its modifications
the impression is gained that in this array of enlargements, elongations,
diverticula, spiral valves, and other devices, we have to do, not with a
consecutive anatomical history, but with numerous special cases of
physiological adaptations, developed in response to need ; and that a
similarity in one of these particulars implies, not genetic relationship
necessarily, but a similar demand responded to in a similar way. The
main object to be achieved in all cases is to regulate the amount of diges-
tive surface to the demands offered by the various kinds of food, and
as there is but a limited number of mechanical or architectural devices
possible, the same ones are employed in unrelated groups of animals,
having arisen independently in response to a similar physiological need.
This phenomenon of parallel development (or 'analogical resemblance,'
as Darwin calls it), may appear in any system or part and has been a fre-
quent source of error in the estimation of the inter-relationship of ani-
mals."

It has already been shown that the intestinal tract begins as a
straight tube, and enlargements take place in various portions of this
tube, the most prominent enlargement being the stomach. This enlarge-
ment has various paired nerves passing down each side of the digestive
canal, prominent among which are the vagus nerves. When the stomach
has become sufficiently large and extends some distance ventral, it turns,
so that what was the ventral region now points toward the right side
of the individual. This means that any nerves or blood vessels which
lie along the right side of the embryonic digestive tract will then lie on
the dorsal surface of the stomach and, of course, the left nerves and
blood vessels then become ventral. It will save considerable confusion
of thought if this be remembered.

THE LIVER (HEPAR)

The liver, as well as the pancreas (Fig. 293) — the two largest diges-
tive glands — are derived from the mucosa of the intestine. The former
grow by a ventral, and the latter by a dorsal, evagination. These
organs are in one sense of the word really enlarged intestinal glands
which pushed their way through the mucosa, submucosa, and musculosa
of the intestine, and then, as they pushed against the serosa (that tissue
being held down very loosely), the serosa stretched and grew directly
ahead of the two glands. The liver and pancreas are therefore covered
by a serous membrane, and both are connected on the side from which

75U COMPARATIVE ANATOMY

they pushed forth by a double layer of serosa which form their respec-
tive mesenteries. This serous covering is continuous with the covering
of the entire intestinal tract and is known as the visceral peritoneum
which also forms the lining of the abdominal cavity.

The liver is the largest gland in the body, but no matter how many
lobes it may develop or how large it may grow, a layer of serosa covers
every part of the gland except that part lying toward the side from
which it grew. Here, as stated, the layers coming from each side nat-
urally unite and form the double layer of serosa — the mesentery. The
two large suspensory mesenteries of the liver are called the ligamentum
hepato-gastricum (sometimes also called the lesser omentum) and the
ligamentum suspensorium-hepatis.

Practically the entire length of the digestive canal which passes
through the body-cavity is attached by both a dorsal and ventral mesen-
tery. The ventral mesentery becomes lost below the region of the liver,
leaving a sharp ventral edge to the two hepatic ligaments.

The function of the liver is to secrete bile (gall), as well as to form
various internal products such as glycogen, urea, and uric acid, all of
which substances are of great importance to the living animal. The
bile is sent to the intestines through the bile duct (also called choledochal
duct), while the other products are carried away by the blood. Sub-
stances which are not sent through a duct but are carried throughout
the body by the blood stream are known as substances of internal secre-
tion.

The liver is a compound tubular gland. The many little tubules in
the liver which form the gall capillaries empty into the bile duct. This
tubular condition of the liver is easily seen in ichthyopsida, but is dim-
cult to observe in mammals because of the tubular anastomosis and be-
cause of the close interrelation of the bile vessels and blood vessels.

The liver begins its growth cephalad, at about the same time the
blood vessels have already developed into the large sinus venosus and
hepatic veins. These blood vessels also contribute to the septum trans-
versum (Fig. 348). The growth of these latter organs prevents the liver
from continuing the cephalad growth so that from now on it increases
in size in an opposite direction.

Concomitant with its increase in size there is an immigration of
mesenchyme between the lobules of the liver. The blood vessels enter
at this time. The bile duct (if there are several, this is only true of one
of them) has a lateral diverticulum or enlargement. This is the gall
bladder (Figs. 426, II, 436), which serves as a reservoir for the bile. It
may be found in the substance of the liver itself, but is usually more or
less separate and lies dorsal to the liver substance. It is lacking in some
mammals. In fact, it is not uncommon in man to have it removed sur-
gically.

Both the liver and the gall bladder have ducts leading from them.

DIGESTIVE SYSTEM 751

Those coming from the liver are called hepatic ducts; those from the
gall bladder are called cystic ducts (Fig-. 436). These unite, forming
the common duct which is also called the choledochal duct. It is this
common or choledochal duct which empties into the intestine. The liver
has many and varying shapes in the different animals, depending to a
large extent not only upon the shape of the body but on the shape and
size of the organs which press upon it. The color of the gall may vary
from a brown, yellow, purple or green, to a vermilion.

THE PANCREAS

This is the second largest of the digestive glands (Fig. 436) and
secretes digestive ferments of great strength, such as trypsin, steapsin,
and amylopsin, which digest both proteins and carbohydrates.

In some respects the pancreas resembles the salivary glands and so
compensates in part for the absence of such glands in the lower verte-
brates. This pancreas arises, as already mentioned, from the dorsal wall
of the intestine, close to the liver. There are usually three diverticula,
one dorsal and two ventral. These latter soon unite. In sharks there
is only a single diverticulum, while in the sturgeon there are not only
two dorsal but also an equal number of ventral. The proximal portion
forms the ducts, the distal, the glands. The number of ducts that per-
sists varies immensely. In some forms of animals all but one disap-
pears, while in the lampreys all may be lost. However, in the mammals
there are usually two ducts persisting, the ventral known as the pan-
creatic or Wirsung's duct, and the dorsal called the accessory or San-
torini's duct. Again, the ducts may all remain distinct, or they may
unite before they enter the intestines. One of them may even unite
with the bile duct. While not absolutely proved, it seems that all verte-
brates have some form of pancreas. This may be only a slender tube in
the mesentery, as in teleosts, or it may lie outside the muscles in the
intestinal walls as in dipnoi. In the cyclostomes it is partly concealed
at the insertion of the spiral valve, partly in the liver. In these forms,
however, the duct has entirely disappeared so that it forms one of the
ductless glands, or in other words a gland of internal secretion. The
pancreas varies in shape and size. It may be long and straight or pos-
sess many lobules. Almost always it is placed between the duodenum
and the stomach. There is a question as to whether or not the gland
is composed of two separate and distinct structures.

SUMMARY OF THE DIGESTIVE SYSTEM
AMPHIOXUS

The mouth lies at the bottom of a vestibule (Fig. 437) and is an
oral funnel bounded by ciliated buccal tentacles, with cartilaginous sup-

752

COMPARATIVE ANATOMY

Fig. 437.

Amphiox ' us lanceola ' tus :
a, Anus ; au, eye ; 6, ventral
muscles ; c, body cavity ; ch,
notochord ; d, intestine ; do
and du, dorsal and ventral
walls of intestine ; /, fin-
seam ; h, skin ; k, gills ; ka,
gill-artery ; Ib, liver ; Iv,
liver-vein ; m1, brain vesicle ;
m2, spinal marrow ; tng,
stomach ; o, mouth ; p, ven-
tral pore ; r, dorsal muscle ;
s, tail fin ; t, t, aorta ; v, in-
testinal vein ; x, boundary
between gill intestine and
stomach intestine; y, hypo-
branchial groove. (After
Hackel.)

ports that serve to funnel the water into the
pharynx. The mouth is surrounded by a mem-
brane, the velum, which acts as a sphincter
muscle. A set of velar tentacles that serve as a
grating to strain out the larger particles is de-
veloped on the free edges of the velum.

The pharynx has sometimes upward of fifty
or more pairs of gill-clefts (also called branchial
apertures) that are separated by partitions in
which lie cartilaginous skeletal rods, connected
across with one another, forming a sort of bran-
chial basket. These apertures serve as means of
communication between the pharynx and the
atrium (the space between the pharynx and the
body-wall). The endostyle (a longitudinal
groove on the ventral side of the pharynx), the
peripharyngeal and hyperpharyngeal grooves, all
secrete mucus in the form of a continuous rope
which carries the food along with it to the stom-
ach. The atrium is a sort of mantle, composed
of folds of the body-wall that enclose the whole
branchial apparatus in a voluminous water-filled
chamber, the atrial cavity. The atrium is lined
with ectoderm and has but one opening to the ex-
terior, a posteriorly directed atriopore, which
carries off the water that comes through the
pharyngeal clefts. The atrium is a protection
for the delicate pharynx while the animal is in its
sandy burrow and helps to maintain an uninter-
rupted current of water.

ASCIDIANS (TUNICATES)

In the Ascidians (Fig. 313, IV) the method
of food concentration and transportation is simi-
lar to that of Amphioxus, although the apparatus
which carries on this function seems to be of an
improved type more appropriate for a sedentary
life. An atrial cavity surrounds the pharynx
which in turn is enclosed by a mantle that sur-
rounds the whole body. A thick tunic (after
which the animal takes its name) covers this
mantle. The atriopore is not posterior in direc-
tion, but lies close to the mouth and is forwardly
directed. The stomach opens near the bottom of
the pharynx, and the intestine takes a complete
turn and opens forward into the atrium. There is

DIGESTIVE SYSTEM

753

no notochord and no neural tube. Practically none of the structures
characteristic of the dorsal side of Amphioxus are present.

FISHES

The mouth opens directly into the capacious pharynx, which is per-
forated by five gill-clefts and the paired spiracles. A short oesophagus
of large caliber leads into a U-shaped stomach (Fig. 438), which in turn

Fig. 438.

A female dogfish in which the abdominal and pericardial cavities have been
opened from the ventral side, and the viscera somewhat displaced. The pericardium
has been opened slightly to the left of the middle line, and the right lobe of the
liver has been cut away.

ab. p., Abdominal pores ; 6., bile duct ; c., cardiac limb of stomach ; c.ar., caudal

754 COMPARATIVE ANATOMY

communicates with the intestine through a valve-shaped opening con-
trolled by a sphincter muscle. The cardiac end of the stomach may end
as a blind pouch. The organ is often sufficiently distensile to permit one
animal to swallow another as large as itself. The intestine is short but
of large diameter and has a secreting surface greatly enlarged by a fold
in the shape of a spiral staircase (present, however, in very few teleostei)
called the spiral valve. All primitive fish have this spiral valve. A large
bi-lobed liver, which is provided with a gall bladder and a bile duct,
opens into the intestine. The pancreas also pours its secretion into the
intestine.

TURTLES

The digestive system of reptiles varies somewhat in carnivorous
and herbivorous forms, but in all turtles it is comparatively simple In the
turtle there are no teeth. The tongue is broad and soft and cannot be
protruded. The stomach is a simple U-shaped enlargement of the ali-
mentary tract. The intestine is without a caecum ; it is clearly divided
into large and small intestines. The cloaca is proportionately large:

AVES

The mouth is hard aad narrow and the tongue is hard and often of
great functional value. The oesophagus which has many large cornified
papillae, develops an enlargement called the crop. The stomach has
a proventriculus which secretes the gastric juice, and a muscular gizzard
or gastric mill. The intestine is U-shaped, and is composed of duo-
denum, ileiim, and rectum. Between the ileum and rectum there are
two caeca. The rectum opens into a cloaca. There are two bile ducts
but no gall bladder. The pancreas empties into the duodenum. The
intestine is found to be longer and more coiled in ascending vertebrates.
In cyclostomes, teleostomes, and all non-placental mammals, the intes-
tine terminates in a cloaca, as do also the urinary and genital ducts.
In placental mammals and in cyclostomes and teleostomes the urinary
and genital ducts have a distinct and separate opening from that of the
intestine.

MAMMALS

In all vertebrates (except birds and mammals) the coelom consists
of the following two compartments :

artery ; c v., caudal vein ; d, bursa Entiana ; f.L, falciform ligament appearing
on surface of left lobe of liver in which it is embedded ; i., intestine ; i.a., intestinal
branch of anterior mesenteric artery ; 1., lienogastric artery ; not., notochord ; ov.,
ovary ; p., portal vein lying beside hepatic artery ; ps., pancreas with duct opening
into intestine ; py., pyloric limb of stomach ; r., rectum, between hinder ends of
oviducts, with rectal gland (r.gl.) attached to its dorsal side; sh., right shell gland
on course of right oviduct ; sp., spleen ; sp.c., spinal cord ; ur.p., urinary papilla ;
v., branch of portal vein formed by junction of intestinal and splenic veins.

Besides the above, note — nostrils : oronasal grooves ; mouth ; pectoral and
pelvic fins ; pericardial and abdominal cavities ; heart, consisting of sinus venosus
(behind), ventricle, auricle (showing at sides of ventricle), and conus ; cloaca, and
transverse section of tail, showing at the sides the myomeres, above the anterior
dorsal fin, and in the middle the cartilage of the backbone enclosing spinal cord,
notochord, and blood vessels. (After Borradaile. )

DIGESTIVE SYSTEM

755

(1) The pericardial cavity which contains the heart only.

(2) The pleuroperitoneal cavity which contains the other viscera.

Fig. 439.

A dissection of the neck and thorax of a rabbit. The heart has
been displaced a little to the right, and the pericardium removed.

ao.a., Aortic arch ; c.c., common carotid arteries ; c.sy., cervical
sympathetic nerve ; d.ao., dorsal aorta ; dep., depressor nerve ; di.,
diaphragm ; du.ar., ductus arteriosus ; ex.j , external jugular vein ; f.c.,
point at which the common carotid divides ; hy., hypoglossal nerve ;
i.c.g., inferior or posterior cervical sympathetic ganglion ; inn., in-
nominate artery ; i.v.c., inferior vena cava, lying in mediastinum ; l.au.,
left auricle ; l.l., left lung ; l.phr., left phrenic nerve ; l.pl c., left pleural
cavity; l.v., left ventricle; lar., larynx; aes., oesophagus in neck; ces'.,
the same in mediastinum ; p.c., posterior cornu of the hyoid ; pul.a.,
pulmonary artery ; pul.v., pulmonary vein ; r.au., right auricle ; r.d.,
ramus descendeus ; r.L, right lung, one part bulging into mediastinum;
r.lar., recurrent laryngeal nerve ; r.pl.c., right pleural cavity ; T.V., right
ventricle ; s.c.g., superior cervical sympathetic ganglion ; s.lar., superior
iaryngeal branch of vagus ; s.v.c., superior vena cava ; scl., subclavian
artery and vein ; smx., submaxillary gland ; t.m., tendon of mandibular
muscle ; thy., thyroid gland ; tra., trachea ; v.£., vagus ganglion ; vag.,
vagus ; W.d., duct of submaxillary gland (Wharton's duct) ; X., XII.,
cranial nerves. (From Borradaile. )

756 COMPARATIVE ANATOMY

A partition, the transverse septum (Fig. 348), separates the two
cavities. In vertebrates lower than Anura the pericardial cavity lies
cephalad to the pleuroperitoneal cavity, but beginning with the Anura,
the pericardial cavity comes to lie ventral to even the cephalic end of
the pleuroperitoneal cavity, because the heart and the pericardial cavity
descend and carry the transverse septum with them. This descent
causes the wall of the pericardial cavity, together with the transverse
septum, to form a sac — the pericardial sac — around the heart.

The part of the pleurocardial cavity dorsal to the heart later be-
comes the pleural cavities.

In birds and mammals the pleuroperitoneal cavity divides into an-
terior and posterior regions by a partition which descends from the dor-
sal body-wall to unite with the transverse septum. This partition is
known as the oblique septum in birds and the diaphragm in mammals
(Fig. 439). In mammals this diaphragm contains a great amount of
striated muscle.

In birds and mammals the coelom has become divided into four
;ompartments : one pericardial, two pleural and one peritoneal cavity.

While a dorsal mesentery supports the digestive tract in all verte-
brates, the ventral mesentery is absent in the adult, except in the regions
of the liver and bladder.

In mammals, the mesentery of the stomach is prolonged posteriorly
to become the greater omentum. An ileo-colic valve and a single caecum
are usually found where small and large intestine, meet in mammals,
although there are a few instances where there are two caeca. In some
edentates, in bats, in some carnivorous animals, and in many whales,
neither valve nor caecum are found.

In some rodents and marsupials the caecum grows as long or longer
than the animal's body (it is of great value in digestion here), while in
man, it degenerates into the vermiform appendix, the lumen of which
tends to close with increasing age.

In mammals the intestine and colon are straight tubes at first but
grow into folds later.

In monotremes the rectum terminates in a cloaca as it does in the
Sauropsida. This condition also occurs in the young of all mammals,
but, in all these, the urogenital and digestive openings become separated
later, and a perineal fold develops between the openings.

CHAPTER LII.

THE RESPIRATORY SYSTEM

It will be remembered that in studying the frog, the trachea and
oesophagus have their beginning close together at the caudal end of the
pharynx which is also the beginning of the cephalic end of the larynx.
In the higher forms of animals the trachea divides into two bronchii.
These bronchii again continually subdivide until there are many tiny
tubules called bronchioles spreading out to all parts of the lungs. These
bronchioles form a sort of an air-capillary system through which the
inspired air is sent to all parts of the lungs, there to assist in aerating
the entire pulmonary blood which has been sent to the lungs from the
heart, through the pulmonary artery. In order that the oxygen in the
inspired air can come in direct contact with the blood itself, there must
be a rather thin, more or less porous, membrane separating the blood
and air.

The lungs, liver, spleen, and kidneys are known as parenchymatous
organs. It is well to bear this in mind constantly, for many diseases
find their way from one of these organs to another. A parenchymatous.
organ is more or less sponge-like and consists of loosely woven tissue
in which there are many porous openings. Such organs are invariably
supplied with great quantities of blood.

These organs, especially the lungs, have a decidedly thin membrane
surrounding the sac-like ends of the bronchioles. Here the oxygen
passes through the thin walls to come into direct contact with the venous
blood which has been sent there through the pulmonary artery.

What has been said so far regarding the respiratory system applies
to vertebrates at large, although those who live a part of their lives in
water, have no lungs during that period, and in this respect resemble fish
and other animals which spend all of their time in the water. In such
forms gills (also called branchiae) develop on the walls of some of the
visceral clefts (these are also called gill clefts (Fig.. 295) or branchial
clefts). The clefts come from the sides of the pharynx and begin as a
pair of pouches or grooves of the pharyngeal entoderm. Extending
toward the sides of the animal they push aside the mesoderm until they
reach the ectoderm. The ectoderm and entoderm then fuse to form a
plate. This plate becomes perforated, thus connecting the pharynx with
the exterior of the body by a number of openings. These openings or
clefts begin development at the cephalic end and successively continue
caudad.

The visceral pouches, although developing in all verteberates, do
not as a rule break through in the mammals. In fact, the pouches may

758

COMPARATIVE ANATOMY

disappear without leaving any trace whatever except a Eustachian tube
and the various ductless glands already mentioned. In the true verte-
brates fourteen pairs of these clefts is the largest number found although
there are more than this in Amphioxus and Balanoglossus. In the
cyclostomes there are usually seven (eight to seven in notidanid sharks,
five or six in teleostomes, and five in birds and mammals). In this num-
bering the oral cleft is not included though there is some evidence that
the mouth arose by the coalescence of a pair of gill clefts.

The gill clefts do not form a serial repetition in the same manner
as does segmentation in other parts of the body, and it may even be .that
the metamerism of the head is not of the same character as the meta-
merism of the gill clefts. In the amniotes where gills are never devel-
oped the branchial pouches or clefts, however, appear and bear prac-
tically the same relation to the aortic and branchial arches as in the lower
forms. From this, it is often assumed that all of these higher forms
having this relation have had ancestors with gills.

There is an interbranchial septum covered externally with ectoderm
and internally with entoderm between every two successive gill clefts.
The inner portion of this septum is composed of mesoderm which in its
earlier stages contains a diverticulum of the coelom. Later, blood-ves-
sels (aortic arches) and skeletal elements (visceral arches) are developed
in each septum, the visceral arches forming on the splanchnic side of
the coelom and hence are not comparable to girdles or ribs.

In Cyclostomes and fishes the gills are either filamentous or lamellar
outgrowths of epithelium, which have developed on both anterior and
posterior walls of the interbranchial septa. Each gill contains a loop
of blood-vessel. There are two very thin layers between the blood and
the surrounding water, which thus permit an exchange of gases.

The filaments (sometimes called
gill-plates), (Fig. 440), which bound
each gill anteriorly and posteriorly
on one side, form a demibranch, and
it is the two demibranchs of a sep-
tum which then constitute a gill.
This means that each cleft is
bounded by demibranchs belonging
to two gills.

Some forms have external gill-
filaments in the very young which
are later absorbed.

In sharks that have more than
B five gill clefts, as well as in the

Cyclostomes, the first cleft bears
gills, but in many elasmobranchs, as
•veil as in the ganoids (sturgeon and

A.

Diagram of a gill, a, gill-arteries ; br,
branchial ray ; d, demibranchs ; kb, cross sec-
tion of bone of branchial arch ; s, septum ; v,
veins. (B, after Cuvier.)

RESPIRATORY SYSTEM 759

Polypterus), this cleft becomes smaller and smaller until there is only
a dorsal opening on the head — the spiracle. In most vertebrates this
spiracle is closed in the adult but in the tailless amphibia and the higher
mammals the inner portion persists as the Eustachian tube and the
greater part of the middle ear.

There are two types of gills in fishes. Practically all the elasmo-
branchs with the exception of the chimaeroids, have the interbranchial
septum well developed so that it extends beyond the demibranchs and
thus differentiates an excurrent canal in the cleft. The prolonged sep-
tum bends caudally at the outer end to protect the gills from injury.

In teleostomes and chimaeroids the broad fold of the posterior end
of the hyoid arch grows backwards over the clefts to form a gill-cover
or opercular apparatus. The gill-cover encloses an extrabranchial or
atrial chamber into which the clefts empty. The chamber opens by a
single slit behind the operculum.

In those instances, just mentioned, when an operculum is developed
the interbranchial septum is always reduced in size until there is only a
slender bar from which the demibranchs extend into the atrial cham-
ber. The two opercular folds are usually continuous beneath the
pharynx.

In teleosts and ganoids the operculum (gill-cover proper) is usually
differentiated from a more ventral portion known as the branchiostegal
membrane which is quite flexible, and possesses a skeleton of slender
branchiostegal rays. The ventral wall of the pharynx in these cases is
nothing but a slender bar and is called the isthmus.

Just as the air in the lungs in the higher forms of animals is taken
in through the outer air passages and then passes through the trachea,
bronchia, and bronchioles to the delicate septa in the lungs, so in animals
possessing gills there is likewise a delicate septum which separates the
blood from the stream of water which is constantly being passed over
the gills. Water is as a rule drawn into the mouth and as the enlarged
oral cavity contracts it is forced out through the clefts, passing over the
gills on its way. In the Myxinoids the oesophageo-cutaneous duct prob-
ably acts as the incurrent passage when the animal has the front of the
head immersed in the flesh of a fish. In the lampreys the water is prob-
ably taken and forced out through the gill clefts when the animal is
attached to some object. The spiracle serves as an incurrent opening
in many elasmobranchs, and it is provided with a valve which develops
from the anterior wall and closes to prevent any backflow. Sturgeons
and Polypterus have spiracles throughout life.

Sharks have the gill clefts on the side in the so-called neck region
while skates have them on the lower surface of the body. This differ-
ence is brought about by the union of the anterior appendages with the
head in skates.

Many teleosts have breathing valves at the mouth opening which

760 COMPARATIVE ANATOMY

permit water to enter but not flow out again. In such cases there is
a more posterior pair formed by the branchiostegal membrane closing
the opercular opening through which the outflow of water may occur.

In some of the teleosts and in such forms as Polypterus there is an
opercular gill with respiratory functions developed on the inner surface
of the operculum, while in some of the elasmobranchs (even those in
which the spiracle is closed), pseudobranchs composed of vertical folds
are developed on the anterior wall of the cleft. These are homologous
with gills but they are not respiratory, as they receive only arterial blood
which passes from the pseudobranch to the choroid coat of the eye and
sometimes even to the brain.

AMPHIBIA

In the amphibia, although the gill pouches form just as they do in
fishes, the first and fifth never break through, while in nearly all adult
forms all the clefts are closed. Exceptions occur in perennibranchs and
the derotremes in which from one to three external openings persist.

In the tailed amphibia and in the caecilians the operculum is merely
a fold of integument in front of the gill-area (Fig. 341). The operculum
develops without a skeleton support in the larva of tailless amphibians.
This fold grows backward over the gills and fuses. Thus there are
atrial chambers formed which usually open by a single excurrent pore
to the exterior. In a few forms, however, both right and left excurrent
openings occur.

It is usually conceded that the gills of amphibia are of ectodermal
origin, and that there may be both external and internal gills present at
the same time. In the tailless amphibia, such as frogs, the operculum
grows over the gill clefts, and the external gills are folded into the atrial
chamber where they are gradually reduced, while the gills which de-
veloped from the walls of the clefts become functional. At the time of
metamorphosis the clefts are entirely closed and the gills absorbed.

It has usually been taught that the gills of fishes were entodermal in
origin, but if this is true, they cannot be homologues of the amphibian
gills. However, the structures are so much alike in appearance, in struc-
ture and in function, that it seems they must be homologous. Neverthe-
less, more evidence will have to be awaited before positive assertions
of value can be made.

!!f It may be interesting, and it may with further knowledge some time
prove of value, to note from the foregoing, that amniotes have visceral
pouches in the embryo, though gills are never developed in the adult ;
that reptiles have five of these pouches — birds and mammals four. In
man only the first breaks through to form a cleft, while in many of the
higher forms there are grooves on the outside of the neck which show
their original position. The manner of obliterating these external
grooves is as follows : The arches most cephalad, especially the hyoid,

RESPIRATORY SYSTEM 761

after enlarging, slides back over those lying more caudad so that at least
the external branchial grooves lie in a pocket called the cervical sinus.
This sinus is later closed by a process from the hyoid arch extending
over it quite similarly to the development as shown in Anura. Inter-
nally the entodermal branchial pouches, with the exception of the first,
disappear, but the first persists as the Eustachian tube and the greater
part of the middle ear.

THE SWIM BLADDER

The swim bladder arises as a diverticulum of the alimentary canal
remaining in contact with that canal by a pneumatic duct in the ganoids
and one group of teleosts (Physostomi), (Fig. 441). This duct, although

Fig. 441.

Swim-bladders of those fresh-water fish whose air-bladders have
a duct (physostomous) . A, Pickerel; B, Carp; C, Eel. b, swim-
bladder; d, duct; g, red gland; oe, oesophagus. (From Kingbley
after Tracy.)

usually emptying into the oesophagus, may connect with the stomach.
However, in most teleosts the duct disappears entirely at an early date.
The swim bladder lies dorsal to the digestive duct outside of the peri-
toneum, although below the vertebrae and excretory organs. It may be
of almost any dimensions, sometimes extending the entire length of the
body. In some forms of teleosts, which remain almost constantly at the
bottom, it is absent entirely. The swim bladder, although usually un-
paired, is paired in most ganoids and may even form three divisions of
connecting sacs. There may be diverticula of any and all kinds. The
internal part of it may be smooth and simple or it may be subdivided by
various septa, or it may even be alveolar resembling the lungs of higher
vertebrates. There may be striated muscle fibers in the walls, and in
some Siluroids and Cyprinoids they are even somewhat calcified on ac-
count of some of the vertebral processes being included.

The blood supply of the swim bladder is arterial, and comes from
either the aorta or the coeliac axis ; sometimes different portions receive
blood from both these vessels. The arteries break up in the walls to
become networks of minute vessels known as retia mirabilia. These often
form "red spots" on the inner surface. From the retia the blood passes
to the postcardinal, hepatic or vertebral body-veins, in the ganoids and
phystomous species, especially those with a wide pneumatic duct.

The swim bladder contains a greater quantity of O2 than is found
in solution in the water in which the fish lives. It is therefore probably
a storage organ for O2 for use when the fish dives to lake bottoms in

762 COMPARATIVE ANATOMY

the summer for food. This can be understood the better when it is re-
membered that there is no O, at all at the bottoms of lakes in summer.
The swim bladder is supposed to make it possible for its possessor
to regulate its equilibrium while in its watery medium. This supposition
has the following- facts upon which to rest its validity : ground-feeding
teleosts do not have it, but those who must adjust their position in such
a way as to obtain the requisite food do have it, while in many of these
there is a diverticulum from it to various portions of the ear.

LUNGS AND AIR DUCTS

In all the higher forms of animals and in some few fishes — dipnoids
—the lungs arise as an outpushing from the ventral side of the pharynx
immediately behind the last gill pouch. This outpushing divides almost
immediately into a right and left half, and just as the outgrowing from
the digestive tract carried the covering of that tract before it, so, too, a
peritoneal covering is carried before the respiratory organs.

As development goes on, the growing part protrudes into the coelom
so that the parts lying therein have an entodermal lining which was
derived from the epithelium of the pharynx, while the outer layer of
peritoneum is serous mesenchyme carrying blood and lymph vessels,
nerve and smooth-muscle fibers between the two. That portion of the
respiratory system from the pharynx to the lungs consists of trachea,
bronchi, and their accessories. These together constitute what are com-
monly called air ducts. The lungs are treated as distinct from these.

On the ventral side of the trachea, in air-breathing animals, there
is a separation which forms the larynx (Fig. 442), the beginning of which
can be studied in amphibia, in the lower forms of which, a simple pair
of cartilages are developed on the sides of the glottis (the glottis simply
being an elongated slit connecting the pharynx with the air ducts). These
cartilages develop in the position of a reduced visceral arch. In other
forms, such as the Urodeles, the more cephalic ends of the lateral car-
tilages separate from the rest and form an arytenoid which is the first
of the laryngeal cartilages, and is imbedded in the walls of the glottis.
The balance of the lateral cartilages may remain as it is, or divide into
any number of pieces. However, the more cephalic pair of these pieces
often fuse in the mid ventral line to form the cricoid, which is the sec-
ond element of the laryngeal framework. There are various antagonistic
muscles attached to these cartilages which make it possible to open and
close the opening.

The vocal cords are formed by a pair of folds of the laryngeal lining
which extend parallel to the margins of the glottis. Sound is produced
by the vibrations caused by the air passing over these cords as they are
relaxed or tightened in different degrees. The larynx is quite rudimen-
tary in reptiles and birds. In the latter the syrinx, shortly to be de-
scribed, takes the place of the larynx.

RESPIRATORY SYSTEM

763

In the mammals one or more thyroid cartilages are added on the
dorsal side to those already described. In the monotremes the hyoid
apparatus and the larynx are most intimately connected, but in the
higher forms of mammals such an association is not so intimate even in
the embryo.

The thyroid cartilage forms a half ring on the ventral side of the
anterior end of the larynx in the higher mammals. The anterior dorsal
angles form cornua which connect with the hyoid by a ligament. Dorsal

A.

Fig. 442.

A, Muscles of larynx (voice box) of Rana esculenta. Dorsal view, aryt.,
arytenoid cartilage ; dtt.lar., dilator muscle ; hy.lar., hyo-laryngeus muscle ;
lig.i.crie., intercricoideum ligament ; s., tendon of posterior sphincter muscle ;
sph.ant., anterior sphincter muscle ; sph.post., posterior sphincter muscle.

(After Gaupp.)

B, Laryngeal apparatus of a Turtle, ar, arytenoid ; 61-2, first and second
branchial arches ; cr, cricoid ; d, dilator laryngis muscle ; g, glottis ; h, hyoid ;
he, hyoid cornua ; sph, sphincter laryngis ; tr, trachea. Cartilage is dotted,
bone is black. (From Kingsley, after Gb'ppert.)

to the thyroid is the glottis with the arytenoids in its walls. Posterior
to the glottis is the ring-shaped cricoid, which is followed by the trachea.
Anterior to the glottis lies the epiglottis, which is a fold of mucous mem-
brane supported by an internal cartilage which articulates with the an-
terior margin of the thyroid. Trie epiglottis usually stands erect, thus
leaving the glottis open during respiration, while during deglutition it is
pulled back into the glottis, supposedly preventing the entrance of food
into the trachea, but there are numerous cases on record where the epi-
glottis has been removed and such individuals seem to have no difficulty
with their food getting into the "wrong throat."

The cavity of the larynx bears a vocal cord on either side internally.
These are folds of the mucous membrane which extend from the thyroid
to the arytenoids, and by moving these latter cartilages they can be

764

COMPARATIVE ANATOMY

tightened or relaxed to alter the pitch of the note caused by their vibra-
tion. A pocket lies anterior to the cords, the laryngeal ventricle (sinus
of Morgagni), one on each side, quite small in most mammals, but well
developed in the anthropoid apes to large vocal sacs. In the chimpanzee
there is a median vocal sac in addition. These act as resonators and add
strength to the voice.

The larynx is prolonged in whales and marsupials so that it projects
into the choana behind the soft palate. This is an adaptation to the
manner of taking food from the water and breathing at the same time
in the whales, while in the young marsupials the milk is forced into the
mouth by the muscles of the mammae of the mother, an arrangement
that prevents strangulation.

The trachea (Fig. 442) in the higher forms has a series of cartilagi-
nous rings forming its walls. It varies in length and size as well as the
quantity of cartilage strengthening its walls in the different genera. It
is as a rule shortest in lizards and often convoluted in turtles. The carti-
laginous rings may be entirely complete or the dorsal part of the ring
may be of membrane. It is usually longest in birds.

It is interesting to note that the larynx never forms the voice organs
of birds. In this form of animal life, the sound producing parts are
formed from membranes which also vibrate by the passage of air, but
this voice organ is located at the point where the trachea divides into
bronchi and is known as a syrinx (Fig. 443). The most common form
of this organ is that in which the last rings of the trachea unite to form

a resonating chamber, the tym-

9y f* **i$ panum, while folds of mem-

brane, called internal and ex-
ternal tympanic membranes
(not to be confused with the
similarly named structure in
the ear), extend into the cavity
from the median and lateral
wall of each bronchus.

In some instances there is
also an internal skeletal ele-
ment called a pessulus, bearing
a semilunar membrane on its
lower surface. This type of
syrinx may be a symmetrical
and even may form a bony
resonating vesicle. There are
various muscles attached to
trachea and bronchi which per-
mit an alteration of the tension
of the folds in all forms of

Fig. 443.

Columba livia. The lungs with the posterior end
of the trachea, ventral aspect, a.in., aperture of
anterior thoracic air-sac ; br., principal bronchus ;
br' ,br.' rbr"'., secondary bronchi; p. aperture of ab-
dominal air-sac ; p.a., pulmonary artery entering
lung ; p.in., aperture of posterior thoracic air-sac ;
p.v., pulmonary vein leaving lung-; sb.b., aperture
of interclavicular air-sac; sp. b., aperture of cervical
air-sac; sy., syrinx; tr., trachea. (From Parker's
Zootomy. )

RESPIRATORY SYSTEM 765

syrinx, thus making it possible to change the sounds uttered.

In the mammals the cartilaginous rings of the trachea are dorsally
incomplete, this position being closed by membrane. A structure of this
kind permits the tube to remain open and yet also permits it to "give"
a little when food passes down behind it through the oesophagus.

THE LUNGS

In the lung fishes there is usually a single sac, although several
types of these animals have paired lungs. The pulmonary arteries spring
from the last efferent branchial artery of both sides. The blood supply,
therefore, under normal conditions, is arterial, and the lungs cannot act
as respiratory organs. "In times of drought (Protopterus), or of foul
water (Ceratodus), the gills no longer function, and the pulmonary
arteries bring venous blood to the lungs."

In amphibia the two lungs are elongated. They are united at their
bases though true bronchi are absent. They may or may not have
alveoli. In the frog the two lungs are distinct, the walls being divided
into a series of sacs or infundibula lined with alveoli. The infundibula
open into a central chamber, which, since it is ciliated and has numerous
glands in its walls, may be compared to a bronchiole.

In those terrestrial urodeles which are lungless in all stages of de-
velopment, no traces of larynx or trachea occur at all, even after the gills
are absorbed. In such species there is a considerable development of
capillaries in the skin as well as in the walls of the mouth and pharynx,
so that the respiratory functions are transferred to these parts. In the
frogs, as already shown, the skin is respiratory and largely supplied by
the cutaneous arteries arising from the same arch as the pulmonary
arteries.

The air ducts enter the anterior end of the lungs in amphibia, while
in higher forms the lungs extend anteriorly to the entrance of the
bronchi on the medial side. This change is in part the result of the
transfer of the heart into the thorax, the position of the pulmonary
arteries, thus forcing the bronchi toward the center of the lungs. In am-
niotes, also, the ducts are characterized by the presence of cartilage in
their walls, so that they are true bronchi. The bronchi may extend in-
side of the lungs and divide into secondary and tertiary bronchi.

In reptiles the lungs are often non-symmetrical, sometimes one even
being absent. In the snakes the lungs consist of a single sac lined with
infundibula either in part or throughout. In the lizards there are one
or more verticle septa dividing the lung into chambers lined with alveoli
while a part of the bronchus may extend to the extremity of the lungs.
In the chameleons the septa do not reach the distal wall, consequently
the chambers communicate so that the bronchus enters a cavity known
as the atrium. This connects with the various chambers separated by
the septa and these in turn open into a terminal vesicle. This whole

766 COMPARATIVE ANATOMY

structure seeming to anticipate the parabronchi — the small uniform sized
air-tubes in the lungs of birds which connect the larger secondary
branches of the bronchial tubes. "This resemblance is heightened by
the development in these same lizards of long, thin-walled sacs from
the posterior part of the lungs wh-ich extend among the viscera, even
into the pelvic region." The air sacs are used to inflate the body. It is
well to remember what has just been said in the study of similarly named
structures in the bird. In turtles and crocodiles there is no atrium and
the whole lung has a spongy texture. The bronchus in turtles enters
on the ventral side of the lung and not as in lizards in the medial.

SUMMARY OF THE RESPIRATORY SYSTEM
BALANOGLOSSUS

The pharyngeal clefts take the form of gill sacs, each of which opens
into the pharynx in a U-shaped slit, resembling that of Amphioxus, and
opens to the exterior by a small pore. These gill-slit openings to the
pharynx are supported by thin, chitinous bars resembling the gill bar
system of Amphioxus.

FISHES

The characteristic respiratory organs of aquatic vertebrates are gills
or branchiae. Gills are finely divided comb-like outgrowths of the ecto-
dermal or endodermal epithelium lining the branchial clefts. The num-
ber of clefts or gill slits vary from five to seven in number. Each cleft
is separated from its neighbor by branchial septa. The more primitive
the fish, the larger number of branchial clefts it is likely to have. The
modern types have regularly five. Heptanchus, sometimes mentioned
as the most primitive living species of shark, has seven clefts, while
Hexanchus, another primitive shark, has six. and elasmobranchs in gen-
eral have five fully developed clefts and a vestigial anterior first cleft
called a spiracle.

The spiracle is the rudimentary first cleft, which is also found
among the most primitive teleostomi (Crossopterygii and Chondrostei).
It is present in the embryos of Teleostei and Holostei, although here it
is closed before hatching. In the Holocephali, an aberrant group of
elasmobranch fishes, the fifth cleft is closed in the adult, which reduces
the number of functional clefts to four. The cyclostomes have on the
whole a larger number of clefts than the true fishes. However, the hag-
fishes (Fig. 366) of the family Myxinidae have no more than six pairs,
while those of the family Bdellostomidae (Fig. 366) have as many as
fourteen pairs, and the lampreys all have seven pairs.

The direction of change in fishes appears to be one of reduction in
the number of clefts from fifty or more in Amphioxus (Fig. 437) and
Ascidians (Fig. 313), fourteen to six in the cyclostomes, seven to five in
the true fishes, and four in the Holocephali.

RESPIRATORY SYSTEM 767

The openings of the clefts to the exterior differ in different groups
of fishes. Among the elasmobranchs each cleft usually opens separately
and is not covered by any flap or operculum, although in Chlameidose-
lachus, the primitive frilled shark, each cleft has a backwardly directed
flap or gill cover. In the Holocephali the first three clefts are covered
"by an operculum, and only the fourth or the last functional cleft opens
freely to the outside. In the great majority of teleostomi and in the
Dipneusti the five clefts are covered with a flap-like operculum, capable
of opening and closing, thus effectively protecting the branchial filaments
irom injury. In some of the eels and in other specialized teleosts, the
gills are completely covered with a fold of skin, the only exit being
through one or two small water pores. There are two quite different
and distinct kinds of gills found among fishes, namely : external and in-
ternal gills.

External gills are purely larval or embryonic organs. They are not
functional in any adult fish; although their homologues are found in the
perennibranchiate amphibia, believed to be paedogenetic or permanent
larval types. External gills are finely branched processes of the ectoder-
mal epithelium of the branchial tract. They are found in the embryos
of many elasmobranchs and in some teleosts. A notable case of larval
trills is seen in the advanced larva of Polypterus (Fig. 368).

The true functional gills of adult fishes are internal. They are finely
divided diverticula of the endodermal epithelium of the branchial clefts.
The nasal cavities are blind sacs which do not communicate with the
mouth. Such communication begins with amphibia.

THE AIR-BLADDER AND ACCESSORY ORGANS OF
RESPIRATION

In all of the groups of fishes above the elasmobranchs, there is a
single or paired air-bladder (probably homologous with the lungs of
"higher forms), a sac-like diverticulum of the pharynx, derived from
either dorsal or ventral sides of the alimentary tract. It is in all cases
supplied with blood from the pulmonary artery (which, in turn, arises
from the last efferent artery of either side), and, primitively at least,
subserves two functions: (1) that of a hydrostatic or buoyancy organ,
and (2) that of an accessory respiratory organ or primitive lung. In
the most primitive teleostome fishes, the Crossopterygii, it is used as a
lung when the water is foul ; in Amia, it is constantly functional as an
air-breathing apparatus ; while in the Dipneusti (lung-fishes) it is an
elaborately pouched lung, used to tide the fish over a period of drought.

In certain other fishes that have acquired terrestrial habits, such
as the climbing perch, Anabas (Fig. 371), which will drown if immersed
in water, and the air-breathing eel, Clarias, there is an extensive post-
"branchial chamber, provided with labyrinthine or arborescent elabora-

768 COMPARATIVE ANATOMY

lions of the epithelium that are highly vascular and play a pulmonary
role.

DOGFISH

Here branchial respiration is carried on in the six pairs of branchial
clefts. These branchi are primitive respiratory organs, consisting of
mere diverticula of mucus membrane, richly vascular, and supported by
cartilaginous processes called gill-rays. The water enters the mouth
and is forced out through the gill slits. In doing so it aerates the gill
filaments, and provides oxygen for the blood that circulates rapidly
through them.

AMPHIBIA

External gills are found in the perennibranchiate urodeles (Fig.
374) throughout life and in practically all amphibians while in the larval
stage.

The epithelium covering these external gills is ectodermal so that
they are really cutaneous and not pharyngeal gills. They are, therefore,
of a totally different nature from the so-called external gills of the em-
bryos of Elasmobranchs and Holocephali, in which case the external
gills are only filaments of the internal gills prolonged through the
branchial openings.

Internal gills develop only in the larvae of Anura and are probably
homologous with the internal gills of fishes, although even here the
epithelium may be ectodermal. In many species of Salamanders lungs
are absent, but in most amphibians they develop as ventral outgrowths*
from the oesophagus. The left is usually the longer. The lungs are
united at their base although true bronchi are absent. In the lungless
Salamanders respiration is exclusively cutaneous and pharyngeal. The
lings are supposed to have secondarily disappeared in these animals.
The air ducts enter the anterior end of the lungs in amphibia, while in
amniotes the lungs extend cephalad to the entrance of the bronchi which
is on the medial side. This change is due to the transfer of the heart
into the thorax so that the pulmonary arteries then force the bronchi
toward the center of the lungs. In the amniotes the ducts have cartilage
in their walls, thus being true bronchi. These bronchi often extend
into the lungs where they divide into secondary and tertiary bronchi.

REPTILIA

Gills are absent, and gill-slits disappear, in all animals higher than
arodeles. The lungs are large and complicated and often non-symmet-
rical, sometimes one is even lacking. In the snakes the lungs consist
of a single sac lined with infundibula either in part or throughout. In
the lizards there are one or nftore verticle septa dividing the lung into
chambers lined with alveoli, while a part of the bronchus may extend

RESPIRATORY SYSTEM 769

to the extremity of the lungs. In the chameleons the septa do not reach
the distal wall, consequently the chambers communicate so that the
bronchus enters a cavity known as the atrium. This connects with the
various chambers separated by the septa and these in turn open into a
terminal vesicle. This whole structure seems to anticipate the
parabronchi — the small uniform sized air-tubes in the lungs of birds
which connect the larger secondary branches of the bronchial tubes.
There develops in these lizards, long, thin-walled air-sacs from the
caudal portion of the lung. These extend among the viscera, even into
the pelvic region. The air sacs are used to inflate the body. In turtles
and crocodiles there is no atrium and the whole lung has a spongy
texture. The bronchus in turtles enters on the ventral side of the lung
and not as in lizards in the medial.

Inhalation and exhalation are effected partly by drawing in the neck
and thrusting it out again, thus decreasing and increasing the volume
of the thoracic cavity. The air is also swallowed into the lungs by filling
and then emptying the throat.

BIRDS

Birds have large lungs, each with nine small air-sacs. The air en-
ters the bronchi and passes to the air-sacs. The air is thus warmed be-
fore being taken into the alveoli of the lungs. It makes its exit through
the excurrent bronchi. A complete change of air occurs at each inspira-
tion and expiration. The trachea and the larger bronchi are kept open
by means of rings of cartilage ; the trachea is enlarged, just before it
divides, into a syrinx, or voice box (Fig. 443), a structure limited to
birds, and in no way homologous with the larynx of mammals ; the me-
chanics of voice production in the birds depends upon forcing the air
through a flexible valve, which is set into vibration. The lungs also
connect with visceral air-sacs, and with air-spaces in the bones.

MAMMALS

There are two points of view regarding the relationship of mam-
malian lungs to the respiratory apparatus of the lower forms of animals.
One view holds that the lungs are merely a further development of the
air-bladder of fishes, while the other insists that they are more likely to
be modified gill-pouches which have grown caudally into the coelom;
rather than opening to the exterior by growing laterally.

The fact, however, that the pneumatic duct is dorsal in position and
the blood supply is arterial, makes the first view seem improbable. The
latter view is supported by the fact that the lungs are paired outgrowths
from the pharynx immediately caudal to the last gill clefts and in serial
order with them. The blood supply fromithe sixth arterial arch would
be in full accord with this view. Then, too, both in the earlier stages

770 COMPARATIVE ANATOMY

and in the primitive forms the skeletal support of larynx and trachea
has the relations and appearance of rudimentary gill-arches and the
muscles surrounding this region are modified from those of the visceral
arches.

Each lung is enclosed by a pleural membrane, and the pleural cavity
in which it lies, is cut off entirely from the rest of the coelom by
the muscular diaphragm. This muscle usually lies transverse to the
main axis of the body. It is attached close to the inner margin of the
lower ribs and extends headward as a sort of tent or dome. The lungs
may be divided into lobes and lobules. The right one usually has the
greater number. In whales, elephants, and odd-toed ungulates there
may be no lobules at all, while in the monotremes only the right lung
has lobes.

From the main bronchial tube there are dorsal and ventral sec-
ondary bronchi, the ventral redividing. When the more cephalic
bronchi lie in front of, or above, the pulmonary artery they are called
eparterial bronchi, while the others are known as hyparterial.

Respiration is made up of Inspiration and Expiration. This has
already been described in the study of the frog. Little is known regard-
ing this process in the turtles and other reptiles. In birds the lungs are
definitely attached to the ribs and vertebrae so that with every motion
there is both a change in shape and size.

In the mammals the ribs lie at an oblique angle to the vertebral
column. As the intercostal muscles are contracted and relaxed the ribs
turn slightly and can increase and diminish the size of the thoracic
cavity.

The diaphragm forms a complete partition between the thoracic and
abdominal cavities and aids materially in respiration as it flattens when
contracted. This increases the size. of the pleural cavity and draws in air
through the trachea. The abdominal muscles likewise play a part. Ex-
piration is caused in part by the action of the intercostal and abdominal
muscles, in part by the elastic tissue and smooth muscles in the lungs
themselves.

J ' ACCESSORY RESPIRATORY APPARATUS

It will be recalled that the entire respiratory tract grows from the
primitive digestive tract. It is, therefore, not difficult to understand
that there are certain fishes which use a more caudal portion of the diges-
tive tract for respiration. In Cobitis, water is drawn in and expelled
through the anal opening, the more caudal end of the digestive canal
l>eing very vascular and used in respiration.

Among the amniotes the lungs are not functional either before
hatching or before birth. Still, oxygen is necessary for the development
of the embryo and the carbon dioxide which has formed must have an

RESPIRATORY SYSTEM 771

outlet. The organ used for this combined respiratory and excretory
function is called the allantois (Fig. 363). This is the ventral diverticu-
lum from the more caudal part of the digestive canal, which has already
been studied in Embryology. It becomes larger with the growing em-
bryo. It is extremely vascular and is absorbed in some forms such as
the Sauropsida or is drawn off with the placenta in mammals. The basal
part, however, persists as the urinary bladder.

CHAPTER LIIL

THE CIRCULATORY SYSTEM

To understand the modern interpretation of the circulatory system
it is necessary to have clearly in mind what is called the probable an-
cestral condition of this system in the lower forms of animals. Thus
one may observe how in each of the succeeding higher forms, something
is added to the development of the animal of the next succeeding scale
below.

Some have thought that the original circulation consisted of a
lymphoidal liquid alone, and as time went by this type of circulation
specialized into what we now term a blood circulation. It is thus sup-
posed that the lymph vessels, as we find them in modern forms of living
animals, furnish a clue as to how the primitive systems of vessels ap-
peared. It is all quite speculative, however. Another explanation, which
has more plausibility in its favor, is that the main blood vessels are the
remnants of the segmentation cavity which has become obliterated by
the growth of mesoderm, the part not obliterated then becoming the
blood vessels.

In any explanation that is built upon the Haeckelian "law" of bio-
genesis there not only remains much to be explained, but various occur-
rences even must be explained away. In this theory it is supposed that
much of the race history has been lost in development, while a develop-
ment of additional vessels of various kinds have covered up some of the
older developmental processes.

Many blood vessels which should arise as fissures between other
tissues are found to be formed as solid cords of cells. These may later
form a lumen and be converted into tubes, or in other instances, vessels
which originate separately in the embryo, may fuse together during de-
velopment to form a single one.

There are various main points, however, which must be understood
in any discussion of the blood system (Fig. 444, A, B). There is a dor-
sal tube carrying the blood toward the tail, from which various vessels
extend toward the right and left at almost right angles through the dor-
sal tube. Those that pass toward the outer side of the animal are called
somatic, those that pass toward the inner region are called splanchnic.
These transverse vessels connect with two ventral, longitudinal tubes,
one of which is in the wall of the digestive tract which runs headward
and unites with the other one which has passed through the ventral body
wall, so that, after the union of the two, a single tube is found coursing
to the head end of the body. In one of the lowest forms of chordates,
namely, Amphioxus, various parts of this system develop muscle walls
and then act as pumping organs.

C[RCLTLATORY SYSTEM

A.

D.

Fig. 444.

774

COM PAKATI VK ANATOMY

B^ne.ur-a.1 ctnasfemosii

Post- cos fa.! antiitomosis
o f clorsa.1 ra.mus of den

'"ferjfymefitai Arfcry

Dorja.1 interseamtntal
artery

Ventral splanchnic

Ventral anasTcmcstf of

ventral div of dorjal iri/trjet/menfa.j aj-fery

G.
Fig. 444.

Comparisons of Circulatory systems. A, a diagram of the vascular system of
.Amphioxus, from the right side, a.b.o., Afferent branchial arteries; ar., carotid
continuations of the suprabranchial arteries ; cons.s., contractile swellings on the
afferent branchial arteries ; d.ao., dorsal aorta ; eff.br. a., efferent branchial arteries ;
hep.v., hepatic vein ; int., intestine ; lr., liver ; m.v., "moniliform" vessel from left
carotid ; nph.pl., nephridial plexus ; ph., pharynx ; s.i.v., subintestinal vein ; sbr.a.,
suprabranchial arteries ; tr.v., transverse vessel joining the carotids ; v.ao., ventral
aorta ; v.cr., vessels of cirri ; v.sy., vessels of synapticulse ; v.t.b., vessels of tongue-
bars.

B, A diagram of the arterial system of a dogfish, seen from the right side.
a.b.a., Afferent branchial arteries ; a.mes., anterior mesenteric artery ; c.c., common
carotid artery ; cd., caudal artery ; coel.a., coeliac artery ; d.ao., dorsal aorta ; e.b.a.,
efferent branchial arteries ; e.c., external carotid artery ; epibr., epibranchial
arteries; ht, heart; hep., hepatic artery; hy.a., hyoidean artery (this joins the
internal carotid of the opposite side, which is not shown) ; i.e., internal carotid
artery ; il.a., iliac artery ; In.g., lienogastric artery ; p.c., posterior carotid artery ;
p.mes., posterior mesenteric artery ; ren., renal arteries ; scl.a., subclavian artery ;
v.ao., ventral aorta. (From Borradaile.)

C and D, Comparison of the venous systems of the dogfish and a teleost. anas,
anastomosis between the two posterior cardinals ; ao.v., ventral aorta ; atr, atrium ;
brack, brachial vein ; card.ant. and card.post., anterior and posterior cardinals ;
caud, caudal vein ; cl, cloacal vein ; con.art., conus arteriorsus ; duct.cuv., duct of
Cuvier ; hep, hepatic vein ; hy, hyoid vein ; il, iliac vein ; jug.inf., inferior jugular
vein ; lat, lateral vein ; leb., liver ; mand., manibular vein ; n., kidney ; port., hepatic
portal vein ; port.ren., renal portal vein ; segm., segmental veins ; s.int., subin-
testinal vein ; sin.hy., hyoid sinus ; into which the veins from the mandibular and
hyoid arches open ; sin.orb., orbital sinus ; sin.ven., sinus venosus ; sperm, spermatic
vein ; subsc., subscapularis vein ; subcl., subclavian vein ; schw.bl., veins of the
swim-bladder; ventr., ventricle. (Both figures from Boulenger, A after Parker.)

D, A diagram of the venous system of the dogfish, a.c.s., Anterior cardinal
sinus ; c.v., caudal vein ; d.C., ductus Cuvieri ; h.p.v., hepatic portal vein ; h.8.,
hepatic sinus ; hy.s., hyoidean sinus ; i.j.8., inferior jugular sinus ; i.o.s., interorbital
sinus ; U.S., iliac sinus ; int., intestine ; k., kidney ; lat.s., lateral sinus ; lr., liver ;
n.8., nasal sinus ; or.s., orbital sinus ; p.c.s., posterior cardinal sinus ; r.p.v., renal
portal vein; s.v., sinus venosus; scl.8., subclavian sinus. (From Borradaile.)

E, A diagram of the principal arteries and veins of a pigeon, ao., Aortic
arch ; Br.a., brachial artery ; Br.v., brachial vein ; C., carotid artery ; c.m.,
coccygeo-mesenteric vein ; d.a., dorsal aorta ; F., femoral vein adjoining femoral
artery ; h.v., hepatic veins ; il., internal iliac artery and vein ; i.v.c., inferior vena
cava ; }., jugular vein ; l.a., left auricle ; P., right pulmonary artery ; Pc.a., pectoral
artery ; Pc.v., pectoral vein ; ra., right auricle ; rp., hypogastric vein ; rv., renal
vein ; sc., sciatic artery and vein. Near the apex of the ventricle the coeliac and
anterior mesenteric arteries and the epigastric vein are shown, but not lettered. At
the hinder end of the figure the caudal and posterior mesenteric vessels are shown,
but not lettered.

F, The circulatory system of the rabbit, (a) Letters to right — e.c., External
carotid ; i.c., internal carotid ; e.j., external jugular ; scl.d., subclavian artery ;
scl.v., subclavian vein; p.a., pulmonary artery (cut short) ; p.v., pulmonary vein;
L.A., left auricle; L.V., left ventricle; d.ao., dorsal aorta; h.v., hepatic veins;
c, coeliac artery ; a.m., anterior mesenteric ; s.r.b., suprarenal body ; l.r.a., left
renal artery ; l.r.v., left renal vein ; K, kidney ; p.m., posterior mesenteric artery

(incorrectly shown as if paired); spm., spermatic arteries and veins; c.il.a.,
common iliac artery. (6) Letters to left — p.f. and a./., posterior and anterior
facial ; e.j., external jugular vein ; i.j., internal jugular ; R.Scl., right subclavian
artery; S.V.C., superior vena cava; R.A., right auricle; R.V., right ventricle;
I.V.C., inferior vena cava ; r.r.a., right renal artery ; r.r.v., right renal vein ;
s.r.b., suprarenal body ; spm., spermatic arteries and veins ; il., ilio-lumbar vein ;
f.v., femoral or external iliac vein; i.il.v., internal iliac veins. (From Thomson.)

G, Diagram of intercostal (intersegmental) arteries.

CIRCULATORY SYSTEM 775

In all vertebrates the heart lies on the ventral side of the digestive
tract covered by a pericardial sac. This sac is really a part of the
coelomic lining. The various large blood vessels carrying blood from
the heart to the general system are known as aortae, and the large veins
returning the blood directly into the heart are usually called venae cavae.

The ventral aorta gives off various pairs of vessels called the aortic
arches, which are situated on each side of the pharynx in the grooves
called the gill septa. These arches run from the ventral aorta around
the digestive canal to the dorsal side where they unite to form a longi-
tudinal canal. That is, the arches along each side form a separate canal
at first, then the two canals unite to form the dorsal aorta, which runs
caudad the entire length of the body. There may be, and usually are,
various small arteries arising from any or all of these arterial arches.
It is necessary that the student know what becomes of the aortic arches
and in what groups of animals certain ones disappear and others remain
functional. The first pair of arches lying toward the head end give rise
to both the internal carotid artery which goes to the brain, and the ex-
ternal carotid supplying the more superficial portion of the head. The
arteries which arise from the dorsal aorta are either somatic or splanch-
nic, that is, either supply outer or internal portions of the body. Exam-
ples of somatic blood vessels are the intercostal (intersegmental) arteries
running between the ribs, while the mesenteric arteries, which are dis-
tributed primarily to the alimentary canal, are of the splanchnic type.

The subclavian artery, which supplies the arms of the animal, and
the iliac artery, which supplies the hind limbs, are some of the larger and
more common of the somatic arteries.

The splanchnic or visceral arteries do not show much trace of seg-
mentation. They are distributed to the walls of the digestive tract. Two
pairs, however, of these vessels are of special importance, namely, a pair
of omphalomesenteric arteries in front, and a pair of hypogastric
arteries (internal iliac) near the origin of the iliac arteries (Fig. 450).

There are really no end arteries or veins. All arteries carry blood to*
certain parts of the body through minute capillaries which then anasta-
mose with the venous capillaries which drain the various parts which
the arteries supply.

The head is drained by a pair of jugular veins which are found above
the mouth. In fishes there are also a pair of inferior jugulars in the
region of the lower jaw and the lower side of the gill arches. These run;
caudad to the level of the sinus venosus, where they are joined by a post
cardinal coming from the excretory organs. The jugular and post cardi-
nal on each side unite to form a trunk which rims transversely and
empties into the sinus venosus. This is called the Cuvierian duct. A
pair of omphalomesenteric veins enters the sinus venosus from the caudal
side. These are continuations of a subintestinal vein running* alongside
of the liver after having passed along the ventral side of the digestive

776 COMPARATIVE ANATOMY

tract. This subintestinal vein forms a loop around the anal opening and
extends to the end of the tail as the caudal vein. The subclavian vein
from the arm may empty either into the jugulars or the post cardinal
near the Cuvierian duct. The blood from the hind limb leaves by an
iliac vein on each side, runs forward on the lateral body wall and is called
the lateral abdominal vein. This also enters the Cuvierian duct.

Omphalomesenteric and subintestinal veins belong to the visceral
or splanchnic group. The others are somatic. The vessels mentioned
are important, and should be known thoroughly because they develop
very early in the embryo, and, practically all later developments as well
as modifications that take place in them, can only be discussed intelli-
gently when the basic structures just mentioned are known. There is
probably no more variable system in the body (even in the same species)
than the vascular.

DETAILED STUDIES
THE HEART

The heart itself is a muscle having a distinctive cellular structure,
this being a sort of "cross" between voluntary and involuntary muscle,
the muscle fibers are striated but run in a syncitial form (Fig. 23).
The muscular walls of the heart itself are known as myocardium. The
inner layer of the heart, corresponding to the endothelium of the blood
vessels and continuous with them, is called endocardium, while the cov-
ering of the heart is known as the pericardium. Lying between the
myocardium and the pericardium is a serous liquid called pericardial
fluid.

We have already discussed a part of the embryonic method of heart
development, but it is necessary here to enter into more detail. The
lateral plates of the walls of the coelom grow centrally beneath the diges-
tive canal. There are four regions discernible in these lateral plates,
namely, the splanchnic or visceral, the mesenterial and somatic walls,
as well as the coelomic cavity.

Between the coelomic walls and the endoderm one may observe vari-
ous cells called vascular cells. It is supposed that they find their origin
from the mesothelium. Those that lie most dorsalward assist in form-
ing the dorsal blood vessels, while those lying ventrad contribute to the
heart and the ventral trunks. The two lateral plates just mentioned con-
tinue until they meet in a ventrad region. This forms the ventral meso-
cardium (Fig. 344). A little later the dorsal region comes together form-
ing the dorsal mesocardium, so that now, that which was formerly a
groove has become a definite tube. The ventral fusion has disappeared,
leaving the dorsal part attached and causing the two coelomic cavities
to unite, forming the pericardial cavity.

In turtles and crocodiles there is a small portion of this ventral

CIRCULATORY SYSTEM 777

mesocardium remaining- which connects the apex of the heart to the
pericardial wall. The walls of this tube are now called the myoepicardial
mantle and the vascular cells which are enclosed within this mantle are
those which form a continuous sheet and become the endocardium or
lining of the heart.

There are still some vascular cells cephalad and caudad to this tubu-
lar heart. These furnish a lining for the blood vessels which arise from
the edges of the lateral plates and connect with the heart. The first ves-
sels toward the head end (the anterior pair) become the mandibular
arteries, while those vessels lying caudad to the heart (the posterior)
are called the omphalo-mesenteric veins.

It is at this time also that immediately cephalad to the omphalo-
mesenteric vein a transverse tube appears on each side connecting with
the heart tube, and it is these tubes which are the ducts of Cuvier. The
ridge where the Cuvierian ducts grow, becomes larger until it forms
a transverse partition known as the septum transversum (Fig. 348).
It is this septum or partition which separates the heart cavity or peri-
cardial region from the abdominal or peritoneal cavity. In the myxinoids
and elasmobranchs this septum never completely closes dorsally, but
leaves one or two openings known as the pericardia-peritoneal canal.

Where the early embryo is closely appressed to the very large yolk
sac, as in the bony fishes and in all amniotes, the development of the
heart is modified. The pharynx is not complete below at first, but com-
municates ventrally with the yolk. The two hypomeres are thus pre-
vented, for a time, from meeting ventrally. "Each, however, is accom-
panied by its vascular cells ; its edge becomes grooved and the grooves
are rolled into a pair of tubes, lined with endocardium, so that for a time
the anlage of the heart consists of two vessels (Fig. 283), each connected
in front and behind with its own mandibular artery and omphalo-mesen-
teric vein, and is surrounded with its pericardial sac. Later the two
tubes approach and fuse with the formation of mesocardia as before :
these latter soon disappear, leaving the whole much as in the small-
yolked forms."

While the pericardium is relatively large at first, in adult forms it
is usually quite close fitting to the heart when the heart is expanded.

It must not be forgotten that in systole the heart contracts and be-
comes considerably smaller than normal ; that in diastole it expands and
attains its full size, filling the pericardium accordingly.

It can readily be understood that as long as the mesocardia are
present the heart tube will be a straight canal connected with the peri-
cardial sac in front and behind. However, as the mesocardia entirely
disappear in due time and the heart tube continues to grow, it bends upon
itself something like a capital letter "S," the bending or flexure being
largely in a vertical plane (Fig. 283).

At the middle point of the bend the tube remains quite small and

778

COMPARATIVE ANATOMY

here is formed what is called tr e atrio-ventricular canal (Fig. 445). It
is in front and behind this canal that the walls become thickened and
the lumen enlarged. The caudal end, which is also the dorsal in this
case, forms the chambers known as the atrium or auricle. The ventral
end becomes the ventricle. Caudad in the atrium, there is a constriction
forming a second chamber called the sinus venosus and it is into this that
the Cuvierian duct and the omphalo-mesenteric vein enter. The ventral
parts of the heart-tubes also form a smaller trunk called the truncus
arteriosus, while the ventral aorta connects this portion of the heart with
the mandibular arteries already mentioned.

While the heart is really a muscle, or rather many interwound bun-
dles of muscles, there are certain parts such as the sinus venosus where
the muscle cells themselves
are somewhat scanty as
compared with other parts
of the heart.

The endocardium devel-
ops folds or valves (Fig.
445) in certain places so that
blood may flow forward but
not backward, and this
valvular part of the truncus
is known as the conus arte-
riosus. In the vertebrates
this conus is reduced to a
single row of valves with
the exception of the elasmo-
branchs, ganoids, and am-
phibia. The valves lie be-
tween the auricle and the
ventricle and are prevented
from being pushed up into
the auricle (when the heart contracts and immense pressure is brought
to bear upon them) by little ligaments called chordae tendineae, which
extend from the edges of the valve to the opposite wall of the ventricle.
They are kept taut during systole by capillary muscles called columnae
carneae. There is also a valve between the auricle and the sinus in some
vertebrates where the hepatic vein enters into the sinus.

If the conus arteriosus is followed by a strong muscular region this
is called the bulbus arteriosus. The bulbus is composed of regular heart
muscle, while the truncus is composed of muscle like the rest of the
blood vessels. It is for this reason that both conus and bulbus are re-
garded as a part of the heart, while anything cephalad to these is con-
sidered a part of the ventral aorta.

Fig. 445.

A and b, Reduction of the bulbo-ventricular fold of
the heart. Ao, aortic bulb; Au, atrium ; B, bulbus cordis;
RV, right ventricle; LV, left ventricle; P (in b) pul-
monary artery. (After Keith.)

A, B, C, D, Scheme showing division of bulbus cordis
and its thickenings into aorta and pulmonary artery
with their valves. The division begins in B, the lateral
thickenings dividing respectively into a, e, and c,f. Ro-
tation from right to left shown in D. (After Heisler.)

CIRCULATORY SYSTEM 779

THE VASCULAR SYSTEM

After food has been taken into the digestive tract and digested and
the little villi of the small intestines have absorbed the semi-liquid food,
this newly absorbed food is ready to become a part of the blood. An
elaborate system of blood vessels with a wonderfully intricate and elab-
orate pumping apparatus — the heart — carries this nourishment to every
part of the body.

Before taking up the development of this system, known variously
as circulatory or vascular, it is necessary that the student understand
quite thoroughly what the adult organs are like and what their function
is. Only then may one validly attempt to ascertain how and why the or-
gans are placed where they are and how and why the function is what it
is. The central part of the vascular system is the heart. In the mammal
this consists of four definite chambers — two auricles at the broad end of
the heart, and two ventricles toward the lower or apex region. The
structure of the heart itself is muscular. The compartments of the heart
and the work they do belong to the circulatory system proper and will
be described here.

Every blood vessel leaving the heart, no matter whether it carries
arterial or venous blood, is called an artery, and every blood vessel en-
tering the heart is called a vein. This distinction must be kept very
clear.

Then, too, it must never be forgotten that blood entering the heart
through a vein always enters a sinus or auricle. This auricle acts as
a reception-chamber for all blood entering the organ. After the blood
has entered this chamber it passes downward through an opening into
one of the ventricles, and it is from the ventricle that the blood leaves
the heart.

In the higher forms of mammals, such as man (Fig. 445), blood en-
ters the right auricle through the large venae cavae, then passing down-
ward through the auricular-ventricular opening into the right ventricle.
From here it passes through the pulmonary artery to the lungs to be
aerated (that is, to be thoroughly mixed writh oxygen and to lose the
carbon dioxide that it has gathered in draining the entire body). After
being aerated, the blood passes- back to the left side of the heart through
the pulmonary vein to enter the left auricle, and then passes down
through a left auricular-ventricular opening into the ventricle from
which it sends forth the blood stream through the aorta to all parts of
the body, supplying the parts with food and nourishment.

The system just described is known as the systemic because the
blood which leaves the heart through the aorta nourishes all parts of
the body.

The arteries break up into smaller arterioles and capillaries. The
liquid part of the blood is called blood-plasma as long as it is contained

780 COMPARATIVE ANATOMY

within the blood vessels, and lymph as soon as it has seeped through the
walls of the blood vessels and bathed the surrounding tissues. It is
gathered up from here by the various lymphatic vessels which unite to
form the large lymph duct. This duct empties into one of the veins of
the neck. The blood which has remained within the blood vessels and
passed through the capillaries is taken up by the venous capillaries and
passes toward the heart either directly or indirectly through a portal
system.

It is essential that one appreciate that the arteries supply all parts
of the body with nourishment and that the veins do the draining. It
follows, then, that the arteries begin as vessels of some size and become
smaller and smaller as the blood supply from the heart becomes dis-
tributed more and more, while veins begin as capillaries and continu-
ally increase in size. An artery and a vein often lie side by side, but
the blood current in the vessels is running in an opposite direction.

In addition to the systemic circulation there is also the pulmonary
circulation, which is the name given to the blood stream leaving the
right ventricle of the heart, passing through the pulmonary arteries to
the lungs, and after being aerated returning, through the pulmonary
veins, to the left auricle, from whence it flows downward into the left
ventricle and is then again ready for the systemic circulation. ^^

Whenever a vein splits up into capillaries so that the venous blood
must pass through an organ on its way back to the heart (either to have
waste substances removed as in the kidneys or to take up new sub-
stances as in the liver), and this blood is then again collected by venous
capillaries and sent on its way, a portal system is formed.

The renal-portal system and the hepato-portal system are the two
important ones in the body's economy.

When the circulatory system of the frog was discussed it was stated
that one must not forget that the material with which the heart works
is blood, but that the heart is similar to a pump or an engine, and, that,
consequently, just as a pump or an engine which is used for the purpose
of forcing water through a great hydraulic system requires water in two
places and in two ways to continue its work, so the heart requires blood
in two places and in two ways to do its work.

The engine requires water in its boiler so that steam can be pro-
duced. This steam then supplies the force for its work of pumping
water, let us say, through the water-pipes of the building in which it
is installed. So, too, the heart must have a blood supply to furnish it
with energy just as the engine requires water to manufacture its steam.
Therefore, there are blood vessels running into the heart-walls and into
the walls of blood vessels themselves so as to furnish these with material
to produce the required energy to continue their pumping power. The
blood vessels that supply the heart walls are known as coronary vessels.
The coronary arteries leave the aorta immediately after the aorta, in

CIRCULATORY SYSTEM 781

turn, has left the left ventricle. The blood vessels in the walls of blood
vessels are known as vasa-vasorum.

It is essential that these two systems be kept separate and distinct.
The mere pouring of blood into the cavities of the heart is equivalent
to the water in the tank of an hydraulic system, while the blood which
enters the heart muscle itself is equivalent to the water in the engine's
own boiler that furnishes the steam from which the energy, in turn,
comes that makes pumping possible.

This analogy may be carried a little further; for, just as the water
in the hydraulic system, if it be used for drinking purposes, must be
filtered, so, before the blood, which is pumped through the vascular
system, can be used again, it must likewise be filtered. This is the work
of the portal systems.

A final point is to be borne in mind, before taking up the circulatory
system in detail, is that the vertebrate circulatory system is known as a
closed circulation, as contradistinguished from the open system seen in
some of the lower forms of animals such as the crayfish.

What is meant by a closed system, is that the blood from the time
it leaves the heart, until it returns, is always in direct communication
by means of arteries, capillaries and veins There are no open 'spaces
through which the blood can pass out of these vessels. A seeming ex-
ception is the lymph. This does not pass through an opening, however,
but seeps directly through the Avails and bathes all parts of the inter-
capillary region.

DEVELOPMENT

Everyone knows that the mammalian heart has its point or apex
to the left, but the student must know how this has come about. He
must also know why it is that, just as with the digestive tract, certain
nerves which lie in the right and left side of the early embryo, come to
lie on the dorsal and ventral sides in the adult. The heart, like the
digestive tract, grows something on the order of a straight tube, although
made up of separate cephalic and caudal ends which have become fused
together. As the embryo continues developing, the heart turns to the
left, so that the nerves which lie upon the right side will now be ventral
and those which lie upon the left side will be dorsal, while the right
auricle and ventricle which have been brought ventral by this turning
to the left now occupy almost the entire ventral portion of the heart,
the left auricle and ventricle being dorsal. It is for this reason that a
very small portion of the left auricle and ventricle can be seen from
the ventral side of the body. It will be remembered that in the. embryo
of the chick we spoke of mesoblastic cells which were derived from
three separate sources. One source of these is from the primitive streak.
The second source is from that scattered group of cells that was left
between the ectoblast and entoblast when the entoblast became a dis-

782 COMPARATIVE ANATOMY

tinct layer of cells. Thirdly, in the middle and lateral parts of the area
pellucida, cells are budded off from the upper side of the entoblast to be-
come mesoblast, at about the very time the primitive streak is forming.

Now, all of these mesoblastic cells together unite to form a con-
tinuous layer. This layer continues expanding until it passes beyond
the boundaries of the area pellucida and forms a middle layer in the
inner zone of the area opaca. This zone is the vascular area (Fig. 264).
It is in this area that the blood vessels begin to form. This occurs in the
chick on the very first day. A network appears in the entire vascular
area which surrounds the embryo. Here irregular reddish blotches are
formed which are called blood islands, and it is from these blood islands
that the red corpuscles are formed. This network develops into a sys-
tem of cords, at first solid, but soon a lumen is acquired and, as the ves-
sels unite, there is a continuous but indefinite blood vessel formed. The
very first vessel which becomes definitely shaped so that it can be rec-
ognized as a part of the vascular system is formed around the entire
vascular area as a sort of boundary and is called the sinus terminalis.
(Fig. 284, C.)

The blood islands appear in cross section as little local thickenings
on the dorsal walls of the blood vessels. These bud off into the cavities
of the vessels and form the first blood corpuscles, and it is supposed
that from these all the colored corpuscles of the blood are descended.

The network of vessels continues to grow, some of the vessels later
becoming arteries, some veins, and still others remain small as capil-
laries. These unite and extend toward the embryo, while, within the
embryo proper, there has been a growth of the vascular system also,
which has extended outward toward this vascular area. All these ves-
sels unite to form the entire vascular system.

Larger vessels of the vascular area unite with the posterior end of
the heart which by this time has already commenced to beat. The
other vessels unite with the anterior or cephalad end of the heart and
these become the arterial system, so that by the end of the second day,
in the chick, a complete vascular system has already been formed with a
beating heart.

At first the heart consists of only two longitudinal vessels which
are connected at the cephalic end. These spread out caudad like an in-
verted "V" (Fig. 283). The arms of this "V" shaped portion soon fuse
together and look like an inverted "Y." The cavities of these two fusing
tubes remain apart for some time and then form one cavity. That is, the
endothelial lining remains separate as two distinct cavities after the mus-
cular walls have united.

On the dorsal surface the muscular walls are incomplete also for a
short time, but after complete fusion the walls also are completed. It
is the stem of this "Y" which forms the heart. The two diverging arms
of the "Y" unite, or rather have united some time before this, so that

CIRCULATORY SYSTEM

783

they are continuous with the large vitelline veins which bring the blood
back to the heart from the vascular area.

The heart is now a short straight tube attached to the ventral wall
of the pharynx and consists of the muscular, united part of this "Y,"
the two arms being the ends of the diverging vitelline veins which run
backward or caudad at the hindermost angle of the head fold. As this
fold is pushed farther and farther back, the straight part of the "Y" is
naturally pushed back also and lengthened. Not only this, but this
straight part of the heart grows more rapidly than does the place to
•which it is attached, so that it does not even find room enough to con-
tinue its growth with the heartfold, but must bend into a loop with its
convexity toward the right side of the embryo. The heart has now lost
its attachment to the pharynx (with exception of its two ends). The

Fig. 446.

I, Schematic longitudinal sections of the heart. A, dogfish, B, Ganoids, and C,
Teolosts. a, atrium; 6, bulbus arteriosus (an enlarged portion of the truncus
arteriosus) ; c, conus arteriosus; k, valves; s, sinus venosus ; t, truncus arteriosus;
v, ventricle. (After Boas.)

//, The circulatory system in the amphibians. A, Urodele, and B, Anura.
a.l. and a.r., left and right atria; ao.w., aortic root (radice) ; ca., carotid arteries,
which spring from the conus arteriosus together with the ao.w.; l.a., pulmonary
arteries which carry venous blood from the ventricle to the lungs ; l.v., pulmonary
veins which carry arterial blood to the left atrium ; v, the veins which carry venous
blood from the general body system to the right atrium ; ventr, ventricle. (From
Schimkewitsch after Wiedersheim.)

caudal end of the heart, in which the vitelline veins empty, is called the
venous; the cephalic end the arterial end of the heart. Beating of the
heart begins as soon as a connection has been made between this "Y"
shaped tubular vessel with the vessels which have been formed in the
vascular area. The palpitation starts at the venous or caudal end and
passes to the arterial or cephalic end. The palpitation of the heart already
begins before one can distinguish any definite muscular tissue which has
developed from the mesoblast.

The arterial end of the heart is known as the bulbus arteriosus.
From this, two narrow vessels, the aortic arches, pass around the diges-
tive tract to the dorsal side, turning caudad and becoming the dorsal
aortae. These two dorsal aortae run along each side of the notocord
under ;he mesoblastic somites and pass toward the tail unconnected

784

COMPARATIVE ANATOMY

with each other, but just before reaching the tail a large branch is given
off, in fact, the branch is much larger than the aorta itself from which it
arises. This large branch is called the vitelline artery. It is the vitelline
arteries that carry the blood back to the vascular area from which it was
brought by the vitelline veins.

The heart we have just been describing is that of the chick. In
cyclostomes and fishes (except the dipnoi) there is what is known as a
branchial or venous heart (Fig. 446). All of the blood which enters such
a heart is venous blood. This venous blood is pumped directly to the
gills where it loses its carbon dioxide and takes up oxygen before being
distributed to the various parts of the body. The important thing to
note is that in such cases the blood only passes through the heart once
in making its complete circuit. It is not, however, correct to consider
the embryo of higher forms as being the same as this type of "one-heart-
circulation," for only oxygenated blood passes through the heart in such
embryos when it is in this stage.

In the dipnoi and amphibia (Fig. 446), where lungs are formed
which take up part of the work of the gills, the heart divides in an
arterial or systemic and a venous or respiratory half. This division is
caused by a septum or partition in the auricle which divides the cham-
bers. It will be remembered that blood always enters through a vein
and always enters into a sinus or an auricle of the heart. The venae
cavae which return the systemic blood to the heart, therefore, empty

srpt.ao.pulm.

ao.d.

anasl
cod

H.

Fig. 447.

CIRCULATORY SYSTEM

785

jug.d. .x

subcl

cav.sup.d.

co el

c.

Fig. 447.

Comparisons of heart and connecting blood vessels in the crocodiles and birds.
A, Dorsal view of Crocodilian heart. A.M., mesenteric artery ; Ad and As, aortic
arches ; D.C.d. and D.C.8., right and left ducts of Cuvier ; through which the venae
cavae enter the heart; LV., pulmonary veins; L.V.h. and R.V.h., left and right
atria ; P.d. and P.8., right and left pulmonary arteries ; S.d. and S.s., right and
left subclavian arteries ; Sp.i., region of the interseptal valves ; Tr.cc., common
carotid artery; V.C.C., Coronary vein; V.c.i., inferior vena cava. (After Rose.)

B, Ventral View of Crocodilian Heart, anast., so-called dorsal anastomosis of
the two roots of the aorta ; anon.l. and anon.r,, left and right innominate trunks ;
ao.l., left aortic arch ; ao.d., dorsal aorta ; ao.r., right aortic arch ; atr.l. and
atr.r., left and right atria ; coel, coeliac artery ; lig.bot., Botalli's Ligament
ost.atr.v., atrio-ventricular opening ; pidm., pulmonary artery ; pan., foramen of
Panizzae ; sept.ao., aortic septum ; sept.ao.pulm., aortic-pulmonary septum ; sept,
ventr., ventricular septum ; ventr.l. and ventr.r., left and right ventricle. (After
Greil.)

C, Heart and communicating vessels of bird. (Swan). anon, innominate
artery ; ao, aorta ; ao.b., aortic arch ; brach.a. and brach.v., brachial artery and
vein ; car, carotid artery ; cav.sup.a. and cav.sup.s., right and left superior venae
cavae ; coel, coeliac artery ; cut.abd.pect., cutaneous abdominal-pectoral vein ; jug.d.
and jug.a., right and left jugular veins ; Ing, lung ; mam.i., internal mammary
artery ; mes, mesenteric artery ; oes, oesophagus ; oes.i., inferior oesophageal artery ;
pulm.s., left pulmonary artery ; st.cl., sternoclavicularis artery ; subcl, subclavian
artery ; thor.inf. and thor.sup., thoracalis ( ) inferior and superior
arteries ; thyr, thyroid gland ; tr, trachea ; tr.car.f carotid trunk ; vert, vertebral
artery. (After Gadow.)

786 COMPARATIVE ANATOMY

into che right auricle, while the pulmonary veins which carry blood from
the lungs to the heart enter the left auricle. As this blood which has
returned from the lungs is now oxygenated and ready for distribution
to the general system, it is the left side of the heart which becomes the
arterial side.

In the higher forms the ventricle is also divided by a septum. The
valves on the right side which separate the auricle from the ventricle
are called the tricuspid valves, while those on the left side separating
the left auricle and left ventricle are known as the mitral or bicuspid
valves.

In the crocodiles there is an opening between the two sides of the
aortic trunk known as the foramen Pannizae (Fig. 447, B. pan), so that
there is really «some mixture of arterial and venous blood in these ani-
mals.

The separation into four compartments is complete in birds and
mammals (Figs. 445, 447, C), so that the blood must pass through the
heart twice. Once through the venous, and once through the arterial
half, in order to make a complete circuit of the body.

The heart is formed directly behind the mandibular artery which is
the first aortic arch (Fig. 309), so that as other vessels grow, it is forced
back further and further until it lies ventrad and caudad to the pharynx,
while in the adult higher forms of mammals it is carried back as a result
of this unequal growth even into the thorax. (The extreme of migration
being seen in the giraffe and the long-necked birds.)

THE ARTERIES
AORTA AND AORTIC ARCHES

The ventral aorta is that large artery running headwarcl from the
heart. It extends to the mandibular artery, which is another name for
the first aortic arch (Fig. 309). The mandibular arteries, like other
arches, pass dorsad (one on each side of the pharynx) until they meet
and form a pair of dorsal longitudinal tubes called the radices aortae.
Between the first aortic arch (mandibular artery) and the heart there
arise some six or more pairs of arches similar to those forming the man-
dibular artery. The number of such arches depends upon the number of
gill-clefts the animal has, for these arches develop in the septum be-
tween the gill-clefts. (The number of arches is greater in the myxinoids
where the number of clefts varies ; seven or eight in the notidanid sharks ;
and, as recent investigations tend to show, probably six in the embryos
of all other vertebrates.)

As the embryo continues to grow, the number of these arches, which
remain or degenerate, seems to be influenced to a considerable extent
by the various changes of the respiratory system the particular animal
in question may develop. When gills develop, each aortic arch divides

CIRCULATORY SYSTEM

787

into two portions. An afferent branchial artery which carries the blood
from the ventral aorta to the gills and an efferent branchial artery which
carries it from the gills to the radix aortae. Both afferent and efferent
vessels run parallel to each other for a part of their course, and are con-
nected with each other by numerous capillary loops running through
the gill filaments. As the blood passes through the gills it loses its
carbon dioxide and takes up oxygen so that it is changed from venous
to arterial blood. In all animals that develop an amnion one cannot
distinguish between afferent and efferent branchial arteries, the aortic
arches running directly from the ventral aorta to the dorsal longitudinal
radices aortae.

With the possible exception of cyclostomes, no gills are ever de-
veloped in the region of the first arch, so no afferent and efferent vessels

ir.

Fig. 448.

I, Aortic arches of amniotes. Compare with Figure 309. A, African Lizard
(Varanus) ; B, Snake; C, Alligator; D, Bird; E, Mammal. 6, basilar artery; ec,
common carotid ; ei, ce, internal and external carotids ; da, dorsal aorta ; p, pul-
monary, «, subclavian. (From Kingsley after Hochstetter. )

II, Comparison of Heart and aortic arch of crocodile and bird, a, right auricle ;
a', left auricle; ao, descending aorta; c, small connecting vessel; m, intestinal
branches. (c and m, before they separate, form the left aortic arch) ; v, right
ventricle: v' , left ventricle; 1 and 1', carotid arteries; 2 and 2', right and left
aortic ardh ; 4 and 4', pulmonary arteries. The right aortic arch, together with the
tiny branch, c. form the descending aorta. (After Boas.)

788 COMPARATIVE ANATOMY

arise from the mandibular arteries. There is, however, an external and
internal carotid artery supplying the head and brain coming from each
half of this first arch (Fig. 448). As various changes take place, how-
ever, the relation of the carotids makes them appear as though they
arose from the first functional arch.

In the cyclostomes and fishes the various arches do not undergo
much, if any, modification. Whatever modification does occur depends
upon changes in the gills.

In all land vertebrates, and many of the fishes, the first arch on both
sides disappears at the point where the external carotid artery begins.

When the spiracular gill is reduced, the second pair of arches is
partially or completely lost in the adult.

The third pair persists. The blood for the internal carotids flows
through these. In fishes, gymnophiona, and a few urodeles, the blood
for the radices aortae also flows through the third pair. In all four-
footed animals the radix disappears between the third and fourth arches
so that it leaves the third arch purely carotid. In such a case that part
of the ventral aorta between the third and fourth arches carries the
carotid blood along and hence is known as a common carotid artery (Fig.
448, I, B), which usually then divides into a right and left branch.

The fourth pair are the systemic trunks, in all four-footed animals,
and carry blood from the ventral to the dorsal aortae.

The fifth has become smaller, disappearing in nearly all animals
except lizards and urodeles. In reptiles the left side of the fourth arch
becomes separated from the rest of the ventral aorta, having its own
trunk connected with the right side of the partially-divided ventricle,
thus carrying a mixture of arterial and venous blood. The blood from
the left fourth arch on- the dorsal side is largely distributed to the diges-
tive tract. The right side of the arch and the carotids are connected
with the left side of the heart and are consequently purely arterial, the
arch itself forming the main trunk connecting the heart with the dorsal
aorta. In birds the radix on the left side of the adult disappears caudad
to where the subclavian artery begins, so that this arch supplies blood
only to the left arm. The right arch is purely aortic in character. In
the mammals this is entirely reversed, the right arch being subclavian,
the left supplying the dorsal aorta and the subclavian of that side.

The bird in its embryonic growth (as exemplified by the chick em-
bryo) turns upon its left side in about 80 per cent of all cases, and the
right arch persists, while in mammals, the embryo usually turns upon its
right side and the left arch persists.

When lungs are developed, whether that be in the lung fishes or any
of the higher forms of animals, a pair of pulmonary arteries develop
from the sixth pair of arches on the ventral side of the pharynx. These
arteries grow caudad into the lungs. That part of the arch dorsal to
these newly formed pulmonary arteries becomes reduced to a small

CIRCULATORY SYSTEM

789

vessel known as the ductus arteriosus or the duct of Botallus (Fig. 345,
C, d, b) in some of the urodeles. In some of the higher vertebrates one
occasionally finds a vestige of this persisting also, otherwise it entirely
disappears.

The ductus Botalli is quite important in the embryonic circulation
of amniotes, because the greater part of the blood goes through it to
reach the dorsal aorta during the time the allantois is the organ of res-
piration, while only enough blood goes through the pulmonary artery
to nourish the lung. The duct closes with the first inspiration of air,
while all blood passing into the last arch goes to the lung.

In the lung fishes and amphibia where there is but a single ventricle
in the heart, the pulmonary arteries are connected with the ventral aorta
just as are the other aortic arches. In higher forms, however, such as
the amniotes, where there is either a partial or complete division of the

Fig. 449.

777.

Schematic diagrams of circulatory systems in I, Petromyzon, II, Teleosts, HI,
in the higher vertebrates. I to VIII, gill arches ; A, dorsal aorta ; A, atrium ; Ab,
gill veins ; Acd, caudal artery ; All, allantoic artery ; Am, omphalo-mesenteric
artery; RA, and rad.ao., Aortic radices (roots of the aorta) ; S. S1, two branches
card., anterior arid posterior cardinal veins ; car. ex., c, and car. int., c' external
and internal carotid arteries ; coel.mes., coeliaco-mesenteric artery ; duct.cuv., and
D, duct of Cuvier ; E, external iliac artery ; H C, posterior cardinal vein ; Ic,
common iliac artery ; hy.v., gill veins of the hyoid arch ; k.art., gill arteries ;
k.ven., gill veins ; KL., gill clefts ; ophth, ophthalmic artery ; orb.nas., orbito-nasal
artery; RA, and rad.ao., Aortic radices (roots of the aorta) ; S. S1, two branches
of the gill veins which pass into the aortic radices ; Sb, Sb, and subl., subclavian
artery ; Sb1, subclavian vein ; si, and sin.ven., sinus venosus ; spr.k., spiracle gill ;
v, and ventr., ventricle ; VC, anterior cardinal vein ; Vm, omphalo-mesenteric vein.
(From Schimkewitsch, I, after Vogt, Jung and Bridge; II, Parker, III, Wieders-
heim.)

790 COMPARATIVE ANATOMY

ventricle into two portions, the conus arteriosus and the ventral aorta
are divided so that those portions which are derived from the sixth arch
are connected with the right side of the heart, while the rest of the
ventral aorta, with the exception already noticed in the reptiles, receives
its blood from the left side of the heart.

It is necessary to study the figures very carefully in order to see
how, already in the vertebrates such as the elasmobranchs, there is a
differentiation of the fifth and sixth arches from the rest of the series.
It will be remembered that the fifth arch is almost completely obliterated
in vertebrates possessing lungs, and that the sixth is completely sepa-
rated from the rest.

The dorsal aorta comes into existence by the fusion of two primitive
vessels running caudad. These lie dorsal to the mesentery and run al-
most parallel to the notochord to the very end of the body. This fusion
varies; it may extend as far forward as the aortic arch. It will be re-
membered that the portions which would normally be called the dorsal
aortae, when these segment, or when there is a division between the
various arches, are called radices aortae. Sometimes the dorsal aorta
extends still farther forward than the last aortic arch and involves the
whole of the radices, so that the dorsal aorta in this case extends to the
first arch.

As many students studying comparative anatomy are preparing for
medicine, dentistry, and other professions, it is necessary to call atten-
tion here to the fact that the names in human anatomy are somewhat
different from those adopted in books on Comparative Anatomy. In
the study of human anatomy that part of the ventral aorta which per-
sists is called the ascending aorta; that part of the fourth arch which
continues in existence is known as the arch of the aorta; and the rest
of the dorsal aorta running downward toward the feet is called the
descending aorta. This, in turn, is divided into one portion (passing
into the thorax) known as the thoracic aorta, while from the diaphragm
downward (as it passes into the abdominal cavity) it is known as the
abdominal aorta. The last two names are thus only convenient terms
to show the location of the descending aorta.

ARTERIES OF THE DORSAL AORTA

These are known as visceral (splanchnic), and somatic. As these
terms are already familiar to the student it is merely necessary here to
state that the visceral arteries run through the mesenteries where the
double layers of serosa are found, and furnish the blood supply of the
digestive tract. Many of the blood vessels are in a primitive condition
though they are not metameric. Usually, especially in vertebrates, these
smaller vessels become united into larger trunks. The principal ones are
as follows :

CIRCULATORY SYSTEM 791

VISCERAL ARTERIES (Fig. 444)

Coeliac Artery:

Origin, Radix or adjacent dorsal aorta.
Branches, Gastric, splenic, hepatic.

Superior Mesenteric Artery (running to cephalic portion of intestines).

Develops with the omphalomesenteric.
Inferior Mesenteric Artery (to caudal portion of intestine).

(Not always present, while other mesenteric arteries appear.)

Coeliac Axis — is the name applied if the superior mesenteric fuses with
the coeliac artery.

Hypogastric Arteries:

Originally connect dorsal aorta with subintestinal vein near anus,
later supplying rectum.

In animals higher than vertebrates a urinary bladder grows from
the rectal region and this is supplied by hypogastric branches called
vesical arteries.

In amniotes, where the distal end of the bladder becomes the allan-
tois, parts of the vesical arteries become allantoic arteries or umbilical
arteries, because they pass through the umbilicus. With the disappear-
ance of the allantois these arteries degenerate, leaving only the rectal
and vesical branches of the hypogastric trunk.

Caudal Aorta:

That portion of the dorsal aorta caudad to the hypogastric arteries.

SOMATIC ARTERIES

Distributed to body wall and its derivatives. (Contrary to the
visceral arteries, the somatic arteries are arranged metamerically.)
Intercostal Arteries (Fig. 444) :

One pair develop between each pair of myotomes, beginning at the
radices and the -dorsal aorta. As the aortic arches disappear and change,
the intercostals become connected close to their origin by a pair of ver-
tebral arteries, running through the openings in the transverse processes
of the vertebrae. The intercostals have different names, depending on
their location, as thoracic, lumbar, sacal, etc.

Vertebral Arteries:

In both man and other vertebrates the vertebral arteries pass in a
cephalad direction toward the ventral side of the medulla oblongata
where the right and left arteries unite to form one trunk called the
basilar artery. This runs straight forward underneath the brain. Two
branches of the vertebrals extend caudad from points just before where
the two vertebrals unite.

792 COMPARATIVE ANATOMY

Circle of Willis:

Just before the basilar artery reaches the hypophysis (pituitary
body) it divides, so that one-half of the basilar passes on each side of
the hypophysis. The internal carotid artery meets this divided basilar
on each side and the trunks thus formed meet near the optic chiasma
forming a complete arterial ring called the circle of Willis. It will be
noticed that the brain has thus a supply of arterial blood from both ven-
tral and dorsal regions, making it less likely to suffer from anything that
might impede the circulation in any one part.

Subclavian Arteries:

As the limbs grow out, a segmental artery for each somite concerned
in the appendages, extends into the member. These arteries become
connected with each other distally as well as with the veins of the limb
by a network of small vessels. The parts of these main trunks and the
connecting network enlarge while other portions atrophy. There are
numerous variations in the blood supply of the limbs. This explains
the shiftings of the subclavian artery shown in Figure 448.

The subclavian has the following names applied to different por-
tions :

Axillary — that part lying in the axilla.

Brachial — that part lying in the upper arm.

Radial and Ulnar — the ones lying adjacent to the bones of these
names.

Epigastric Arteries:

The development of the arteries of the hind leg is somewhat com-
plex. There is the same formation of -a capillary network as with the
fore-limb. Two of the arteries become prominent. The epigastric
artery lies forward. It descends from the aorta to the ventral side of
the body and forward, supplying the lower portion of the myotomes. It
becomes connected with the epigastric veins at first, although later these
may anastomose with the hinder ends of the cutaneous arteries. When
the hind limb grows out, the external iliac or femoral artery (a branch
of the epigastric), is sent into its anterior side. As the leg increases in
size this sometimes surpasses the parent epigastric in size, so that the
latter appears as a mere side branch.

Sciatic Arteries:

The sciatic or ischiadic arteries descend into the posterior side of
the leg, the name changing at the angle of the knee to popliteal artery.
Farther down this artery divides into peroneal and anterior and posterior
tibial arteries. The peroneal supplies the calf of the leg and the others
continue into the foot.

The arrangement of vessels here outlined is characteristic of the

CIRCULATORY SYSTEM 793

lower tetrapoda where the femoral artery is small. It is likewise char-
acteristic of the embryos of mammals. In the latter, however, b.efore
birth, the femoral artery grows down to join the popliteal, and so be-
comes the chief supply of the limb. These trunks and the hypogastric
do not always remain distinct. They often fuse in different ways at the
base. Epigastric and hypogastric arteries are distinct in many reptiles
and in birds, but in other vertebrates they fuse to form the common iliac
artery, so called because the proximal portion of the femoral is often
called the external, and the hypogastric the internal iliac artery. The
sciatic, likewise, may remain distinct, or it may fuse with the other3 at
the base, and then its independent portion will appear as a branch of the
common iliac artery.

A cutaneous artery arises from either the subclavian or the pul-
monary artery of either side (both conditions occur in the amphibia) to
run backward in the skin of the trunk. It may extend back and unite
with the epigastric artery. If, as in the amphibia, these arise from the
pulmonary artery, they contain venous blood and the skin acts as a sub-
sidiary respiratory organ.

Renal Arteries:

Renal arteries are paired and show metamerism in the primitive
state, details of which are given in the description of the organs they
supply. It is well to note that metamerism is well shown in these arteries
going to the pronephros and the mesonephros, while in the true kidney —
the metanephros — only a single pair of renal arteries furnishes the blood
supply.

Genital Arteries:

These, like the renal arteries, are paired and metameric in the primi-
tive state and are called
Spermatic in the male.
Ovarian in the female.
These are more numerous in lower animal forms than in higher.

THE VEINS
Omphalomesenteric Veins :

It will be remembered that the heart is developed in the pericardial
cavity. Caudad to the heart region the liver begins developing and tHus
prevents the lateral plates from coming together on the ventral side
as they did in the case of the heart. The lateral plates, however, become
grooved, and each one forms a tube, so that there are two vessels, called
the omphalomesenteric veins extending caudad from the heart, passing
around the liver where they meet with the extensions of the omphalo-
mesenteric arteries already described (Fig. 277).

•94

COMPARATIVE ANATOMY

Subintestinal Veins:

Caudad to this connection a pair of subintestinal veins (Fig. 450)
run toward the tail end on the ventral side of the digestive canal. These
fuse together into a medial tube just behind the anal opening, which ex-

Fig. 450.

Diagrams to show the development of the postcayal vein in the cat. The
cardinal system of veins is cross-hatched, the subcardinal veins closely stippled,
the hepatic veins are indicated by cross, vertical, and oblique hatching combined,
the supracardinal veins by open stippling, and the renal cottar by vertical hatching.

A, early stage, showing the anterior and posterior cardinal veins, a, b, c,
the common cardinal vein d, the subcardinal veins /, and the outgrowth e from the
hepatic veins of the liver. B, next stage, showing the union of the hepatic
outgrowth e with the subcardinal veins /, to form the proximal part of the
postcaval vein ; the two subcardinals have united with each other at h.

C, the anterior part of the posterior cardinal vein 6 has separated from the
posterior part c, c now being the renal portal vein ; the postcaval vein is seen to
be formed of the hepatic vein e, the right subcardinal /, and to be united by meajis
of the two subcardinals below h with the renal portals c.

CIRCULATORY SYSTEM 795

tends to the end of the tail. This fused portion is known as the caudal
vein. In the cyclostomes this connection persists. It disappears in other
vertebrates.

The left Omphalomesenteric vein, which passes along the left side
of the liver, continues to carry blood from the caudal or posterior part
of the body to the heart, while the right disappears with the exception of
the small portion between the sinus venosus and the liver.
The Portal System:

It will be remembered that the liver develops from a simple sac into
a compound tubular glandular structure. The left omphalomesenteric
breaks up into a great mass of capillary-like tubules or sinusoids, which
pass among the tubules of the liver and end by reconnecting at .the
cephalic end of the liver. As the liver increases in complexity so do
these sinusoids. The left omphalomesenteric is consequently quite im-
portant during this period and it is known as the ductus venosus
(Arantii), (Fig. 451). A little later, however, this importance is lost
by a part of the omphalomesenteric becoming the portal vein, which
brings all the blood from the posterior regions of the body to the liver,
sending it through the tiny sinusoids. The ends between the heart and
the liver, formerly called the ductus venosus, now become the hepatic
veins. It is in and through the hepatic veins that the collected blood
from the liver sinusoids is sent to the heart.

When a vein breaks up into capillaries of this kind, as in the liver
and kidneys, and its contents are again gathered in a vein, it is called a
portal system. That of the liver is the Hepatic portal, while that of the
kidney the Renal portal system.

In elasmobranchs and sauropsida, which produce eggs with large

D, the supracardinal system of veins i, represented by open stippling, has
appeared and has united anteriorly with the anterior parts of the posterior
cardinals b, medially with the subcardinals by an anastomosis fc, named the renal
collar, and posteriorly with the renal portals c.

E, union of the two anterior cardinals by a cross-connection p, and develop-
ment of the renal veins from the renal collar fc; the supracardinal veins have
separated into anterior parts connected with the posterior cardinals b and posterior
parts connected with the subcardinals and renal portals c.

F, continuation of E.

G, adult stage ; the left anterior cardinal joins the right by means of the
cross-vein p which is the left innominate vein ; the common stem a, which is the
right anterior cardinal, enters the heart by way of n, which is the right common
cardinal vein ; the left common cardinal vein persists as the coronary sinus o;
the right anterior parts of the posterior cardinal vein and supracardinal form
the azygos vein, b and i, while on the left side these are obliterated at v; the
postcaval vein is now complete and is seen to be composed of the hepatic vein
e, the right subcardinal, the anastomosis between the two subcardinals at h,
the right renal collar k, the posterior part of the supracardinal vein i, and the
posterior parts of the renal portals (posterior cardinals) c: the left subcardinal
and posterior cardinal contribute to the vein of the left gonad, hence the
asymmetrical arrangement of the genital veins in mammals.

H, composite diagram of the veins of a cat. a, anterior cardinal ; b, anterior
part of the posterior cardinal ; c, posterior part of posterior cardinal or renal
portal ; d, common cardinal ; e, hepatic portion of the postcaval (this is partly re-
moved in Figs. D-G ;) /, subcardinal; g, gonad; h, union between the two sub-
cardinals; i, supracardinal; j, kidney (metanephros) ; fc, renal collar or union
between subcardinals and supracardinals ; I, adrenal gland ; m, vein to adrenal
gland ; n, base of the precaval vein or right common cardinal ; o, coronary sinus
or left common cardinal ; omph.mes., omphalomesenteric artery ; p, left innominate
or connection between the two anterior cardinals ; q. internal jugular ; r, sub-
clavian ; 8, external jugular; t, external iliac; u, internal iliac. (Partly from
Hyman after Huntington and McClure in Anatomical Record, Vol. XX.)

796

COMPARATIVE ANATOMY

- card.p.

omph m.d, -

Fig. 451.

— ymph.m.s.

omph.-

yolk, the presence of a large food supply exercises a modifying influence
on these ventral veins. A pair of large vitelline veins runs out into the
yolk sac, over the yolk, from the junction of the omphalomesenteric and
the subintestinal veins to play a large part in the transfer of material to

the growing embryo (Fig. 284). The distal
parts of these veins follow the margin of the
yolk sac, forming a tube (sinus terminalis),
into which smaller veins empty. Blood is
brought to the yolk by the omphalo-mesen-
teric arteries. These arteries are also dis-
tributed to the yolk sac and divide up dis-
tally into a network of capillaries which
connect distally with the vitelline veins. The
blood is carried by these vitelline veins to
the liver and through the portal circulation
to the lieart. In the mammals a similar
vitelline circulation is developed, but here
the yolk sac contains no yolk, and so is of
minor importance and soon lost.

In amniotes there is an outgrowth, the
allantois, which arises as a diverticulum
from the hinder end of the alimentary canal.
This increases in extent by growing down-
ward and carrying the ventral body wall be-
fore it. Allantoic arteries (branches of the hypogastric arteries) extend
into it and are connected by capillaries with umbilical veins which arise
from the subintestinal vein behind the vitelline veins. This forms an
allantoic circulation which is both respiratory and nutritive in character.
In the reptiles, both of the umbilical veins persist through foetal life,
while in birds and mammals, one aborts, leaving the other as the efferent
vessel of the allantois. With the end of foetal life (at hatching or at
birth) both the vitelline and the allantoic circulations disappear, leaving
only inconspicuous rudiments.

Anterior Cardinal Veins (Superior Jugular or Jugular) :

(The inferior jugulars are found only in fishes and salamanders,
where they drain lateral and ventral branchial regions.) The superior
jugular vein lies dorsal to the gill-clefts and returns blood from the dor-
sal regions of the head.

Post Cardinal Veins (Figs. 450, 456).

These are very clearly related in development with the excretory
system and lie dorsal to the coelom and dorsal to the nephridial arteries.
Nearly all of the thoracic portion of the post cardinal veins soon disap-
pears in the higher forms, while a supra cardinal system develops, as
shown in the figures. This supra cardinal system in turn disappears with

Diagram showing development of
the mammalian hepatic portal
system. The omphalo-mesenteric
and the umbilical veins are reduced.
card.a. and card.p., anterior and
posterior cardinals ; d, intestine ;
d.ar., ductus venosus (Arantii) : I,
liver ; omph.m.d. and omph.m.s.,
right and left omphalo-mesenteric
veins ; umb.d. and umb.8., right and
left umbilical veins. (After Hoch-
stetter. )

CIRCULATORY SYSTEM 797

the exception of a posterior portion which takes part in the forming
of the post cava, and the right anterior portion which connects with the
remnant of the post cardinal to become the azygous vein. If the an-
terior left side persists also, this is known as the hemiazygous.

In the lower vertebrates they retain their function of draining the
excretory system.

Cuvierian Ducts:

These are formed by the meeting of the anterior cardinal and the
post-cardinal vein on each side to form short tubes for the emptying
of the cardinal veins into the sinus venosus.

Subcardinal Veins:

These are closely associated with the post-cardinals.

As the mesonephroi in their development reach the hinder end of
the coelom, the caudal vein loses its primitive connection with the sub-
intestinal vein and becomes connected with a pair of vessels, the sub-
cardinal veins, which develop in a ventral-medial position to the two
mesonephroi. The blood from the tail now goes through the subcardi-
nals and from them into the excretory organs, passing through a system
of capillaries to be gathered again in the postcardinals and by them to
be returned to the heart. Here, then, there is another portal system, the
first renal-portal system, which may be modified later.

Subclavian Veins:

One of these drains each forelimb. It originally empties into the
post-cardinal but later may empty into the Cuvierian duct or jugular
vein.

Common Iliac Vein:

This drains the hind limb and empties into the epigastric (lateral
abdominal) vein, which in turn empties into the post-cardinal or duct of
Cuvier.

While this is the condition in some elasmobranchs, in the reptiles
and amphibia the common iliac sends part of its blood as above, and part
through the post-cardinal of its ow-n side, so that blood from the hind
limbs has two routes to the heart.

Anterior Abdominal Vein:

In amphibia and some reptiles the two epigastric veins fuse in the
midline to form an anterior abdominal vein, which passes through the
remains of the ventral mesentery (ligamentum teres), to the liver and
forward.

In one mammal, Echidna alone, has such an anterior abdominal vein
been found.

The vessels of the appendages are but slightly developed in fishes.
There is a subclavian vein which enters the Cuvierian duct, and some-

798

COMPARATIVE ANATOMY

times a branchial vein which may empty into the sinus venosus. In the
amphibia a cutaneous magnus vein comes from the skin of the trunk
which may enter the subclavian. In all tetrapoda, the subclavian, after
it leaves the limb, receives a superficial cephalic and an axillary vein.
The latter, however, changes its name in the appendage to the brachial
vein. The common iliac vein is formed in the limb by a union of the
femoral and sciatic (ischiadic) veins, as well as the hypogastric (inter-
nal iliac) vein.

In all classes above fishes, such as dipnoi, amphibia, and amniotes,
a new vein, the postcava (vena cava inferior) arises in part from scat-
tered spaces and in part as a diverticulum of the sinus venosus and the
hepatic veins. It grows backward, dorsal to the liver, until it meets and
fuses with the right subcardinal vein, a portion of which now forms a
new trunk to carry blood from the posterior part of the body to the
heart.

The following changes are introduced in the embryonic renal portal
circulation whenever a postcaval vein develops. The subcardinals no
longer connect with the caudal vein but are connected with each other
by transverse vessels (interrenal veins). Portions of the postcardinals
grow backward to connect with the caudal vein. These posterior parts
of the postcardinals then become the advehent veins (Fig. 452) of a sec-

B.

Diagram of Renal Portal System in A, Alligator, and B, Bird. (After
Gegenbaur. )

CIRCULATORY SYSTEM 799

end renal portal system. They bring blood from the tail and hind limbs
to the excretory organs (mesonephroi). The subcardinals of both sides
usually fuse in the middle line. The fusion is initiated by the appearance
of the interrenal veins, which now act as revehent vessels to carry blood
from the excretory organs to the postcava and to the anterior portion
of the postcardinals which have joined the anterior ends of the subcardi-
nals. In mammals there is also a change in the postcardinals and in the
renal portal system.

In the lung fish Ceratodus there are some differences from the above
account. Here the cephalic portion of the right postcardinal loses its
connection with the vessels behind, a*nd acts as a vertebral vein, taking
the blood from the intercostal veins of that side back of the heart. The
caudal and the subcardinals form a continuous trunk, while the revehent
vessels form side branches. The caval portions of the postcardinals
grow back into the tail as paired vessels, forming no connection with
the caudal vein. In Protopterus there is no vertebral vein and the sub-
cardinals are not fused behind, while the advehent veins are connected
with the caudal.

Pulmonary Veins:

There may be various pairs of these. They carry blood from the
lungs to the left auricle of the heart. They never empty into the sinus
venosus.

THE LYMPHATIC SYSTEM

In addition to the arterial and venous divisions of the circulatory
system, all craniates develop lymph-vessels or lymphatics.

These consist of a network of lymph capillaries which are inter-
woven with, but independent of, the blood-capillaries. The lymphatic
system is not closed like the blood-vascular system, for there are not
only definite lymph vessels, but there are large open spaces — the lymph-
sinuses. Then, too, there are connections by little apertures, called
stomata, between the lymphatics and the coelom.

Lymph sinuses are found beneath the skin, as in the frog, between
muscles, in the mesenteries, in the walls of the alimentary tract, around
the central nervous system, and in many other parts of the body.

The lymph (which is practically the liquid part of the blood which
seeps through the blood vessel walls) is gathered in these sinuses and
then passes into more or less definite lymph vessels which, in turn, open
into the veins (Fig. 453).

Leukocytes are added to the plasma from the various lymphatic
glands (Fig. 453), such as the tonsils, thymus, and spleen.

In the lower craniates, such as the frog, lymph-hearts occur (Fig.
347). These are muscular dilations found in the course of certain
vessels.

800

COMPARATIVE ANATOMY

The lymph glands (Fig. 453) are made up of a network of connec-
tive tissue in which the lymph leukocytes (lymphocytes) are formed.

The function of lymph glands, therefore, seems to be that of de-
stroying foreign bodies and to add white blood corpuscles to the general

Ant. cardinal vein- "•

Right

lymphatic duct
Subclavian vein

Ant. lymph heart

Post, cardinal vein -I—

'V /?

*;r -t

B.

Post, lymph heart

Fig. 453.

A, Diagram showing arrangement of lymphatic vessels in a 20 mm. pig
embryo. (After Sabin.)

B, Diagram illustrating a stage in the development of a lymph gland. (After
Stohr.)

circulation. The lymph itself bathes all the cells of the body. There are
no red blood corpuscles in lymph.

The lymphatics of the intestine are called lacteals and perform the
important function of absorbing fats from the ingested food. These
lacteals combine with the lymphatic vessels from the hind limbs and
body to form a receptacle known as the receptaculum chyli, from which
a tube (thoracic duct) passes cephalad to open into one of the large veins
of the precaval system by a valvular opening. The thoracic duct is often
double.

In mammals the lymphatic system ramifies throughout all portions
of the body. The lymphatic system is too delicate to be worked out by
the ordinary laboratory dissection.

SUMMARY OF THE CIRCULATORY SYSTEM
AMPHIOXUS (Fig. 444)

The blood vessels are all of one kind, but due to various homologies
with the more complex vessels of higher animal forms, some are called
arteries and others veins.

The circulatory system consists of a ventral pulsating vessel, with-
out a specialized heart enlargement. This pulsating vessel pumps the
colorless blood forward and through the branchial arches to be aerated.
The blood then collects in paired dorsal aortae which unite back of the
pharynx into a single dorsal aorta. Branches are sent from this dorsal
aorta to the walls of the intestine where they break up into capillaries.

CIRCULATORY SYSTEM 801

The blood is collected from these capillaries into a median longitudinal
sub-intestinal vein, through which the blood flows forward to pass into
the hepatic portal vein at the origin of the liver. This portal vein breaks
up into capillaries within the liver and is then collected in the hepatic
vein which extends along the dorsal portion of the digestive gland, where
it turns downward and forward to join the caudal end of the ventral
pulsating vessel.

The vascular system of Amphioxus, therefore, consists primarily of

(I) a dorsal vessel represented by the paired and unpaired dorsal aortae,

(II) a ventral vessel represented by the subintestinal vein and the ven-
tral aorta, and (III) commissural vessels represented by the afferent and
efferent branchial arteries and the intestinal capillaries. • This is quite
similar to the circulation in the earthworm except for two important
differences. The blood in the ventral vessel of Amphioxus travels for-
ward, that in the dorsal vessel backward — just the reverse of what
occurs in the earthworm, while the ventral vessel is broken up into two
parts, by the interposition in its course of the capillaries of the liver, so
that all the blood from the intestine has to pass through the liver before
reaching the ventral aorta. This passage of the intestinal blood through
the vessels of the liver constitutes what is called the hepatic portal sys-
tem, which is characteristic of all vertebrates.

FISHES

The circulation in fishes corresponds quite closely in the main to
that of the chick's embryonic circulation. It is built about the gill sys-
tem. The blood is pumped forward from the ventral heart through the
gills, and is then, as arterial blood, carried backward in the dorsal aorta.
This scheme of circulation wherever found is interpreted as primarily
aquatic.

The heart consists of four chambers : (a) sinus venosus, (b) auricle,
(c) ventricle, and (d) conus arteriosus, through which blood passes in
the order given. The sinus and auricle lie dorsal to the ventricle.

In the lampreys there is no portal system.

In the dogfish (Figs. 446, 449, 454), the circulation is laid out in
accord with the branchial system. The blood brought to the heart by
the venae cavae is pumped forward through a common ventral aorta
which divides into five pairs of afferent branchial arteries, each of which
carries blood to one set of branchiae. A corresponding efferent branchial
vessel picks up the aerated blood from the branchiae and carries it to a
dorsal aorta, through which it is distributed to all parts of the body, both
anteriorly and posteriorly. The general systemic, hepatic-portal, and
renal-portal systems return the blood to the heart along dorsal vessels,
called anterior and posterior cardinal veins.

The fish-type of circulation is built primarily along lines laid down
by the branchial respiration and the heart pumps blood forward and

802-

COMPARATIVE ANATOMY

Fig. 454.

A, The forepart of the body of a dogfish, dissected to show the heart and
ventral arterial system, a.b.a., Afferent branchial arteries ; em., auricle ; c.a., conus
arteriosus ; ch, ceratohyal cartilage ; d.C., ductus Cuvieri ; g., gills ; y.c., gill clefts ;
i.e., internal opening of the first gill cleft ; M.c., Meckel's cartilage ; mu. muscles
from coracoid region of shoulder girdle to various parts of visceral skeleton ;
pm., pericardium; s.v., sinus venosus ; sc.f scapula; thy., thyroid gland (dis-
placed) ; v., ventricle; v.ao., ventral aorta.

B, The forepart of a dogfish, dissected from the ventral side, to show the
dorsal arterial system, the olfactory organs, and certain structures in the orbits.
The middle part of the floor of the -mouth has been removed, a.b.a., Afferent
branchial arteries ; c.c., common carotid artery ; coe.a., creliac artery ; d.ao., dorsal
arota ; e.b.a., efferent branchial arteries ; e.c., external carotid ; en., nostril ;
epibr., epibranchial artery ; hm., hyomandibular cartilage ; hy.a., hyoidean artery ;
i.e., internal carotid arteries ; inf., infundibulum ; M.c., Meckel's cartilage in lower
jaw ; o.i., inferior oblique muscle ; o.s., superior oblique muscle ; olf.o., olfactory
organ; p.c., -posterior carotid artery; sc., scapula; scl., subclavian artery; sic.,
skull ; sp., spiracle ; V.md., V.mx., mandibular and maxillary branches of fifth
nerve; //., optic nerve. (After Borradaile.)

through the branchial arches. This involves as many pairs of branchial
arches as there are paired functional afferent vessels carrying blood to
the gills, and efferent vessels carrying the oxygenated blood from the
gills to the dorsal aorta.

AMPHIBIA (Figs. 446, 456)

The principal changes in the amphibian circulation are concerned
with the branchial arches. These are remodeled to become blood ves-
sels that can function in an air-breathing animal. The branchial vessels
of lobe-finned ganoids and of amphibia in the larval stage consist of
four pairs; known from the region in which they develop as the third,
fourth, fifth and sixth. The third pair becomes the carotid arteries that
supply the head; the fourth becomes the systemic arches that supply
most of the body ; the fifth disappears, and the sixth becomes mainly the

CIRCULATORY SYSTEM

803

pulmonary arches. It is of interest to note that in all lung-breathing
fishes, the lungs are supplied from the branch of the sixth branchial arch.
In most amphibia, a branch of the sixth arch becomes cutaneous, for the
skin respiration is almost as important as the pulmonary. The heart is
carried back into the trunk and consists of a sinus venosus, right and left
auricle, ventricle, and conus arteriosus. The auricle has divided into a
systemic half and a pulmonary half which lie in front of the ventricle.
The single ventricle receives both arterial and venous blood, but there is
very little mixture of the two.

The postcava is well developed and the lateral abdominal veins (also
called epigastric) unite to form an anterior abdominal vein. This latter
vein permits the return of blood from the hind legs to the heart either
through the anterior abdominal and the hepatic portal system or the
renal portal system and the postcava.

REPTILIA (Figs. 447, 448, 452, 455)

Fig. 455.

Embryonic circulation of a Snapping Turtle (Chelydra) to show the relations
of allantois. a, right auricle ; al, allantois ; av, allantoic vessels ; c, caudal vein ;
da, dorsal aorta ; h, hypogastric artery ; j, jugular, I, liver ; oa, ov, omphalo-
mesenteric artery and vein ; pc, post-cardinal ; sc, subcardinal vein ; uv, umbilical
vein; w, Wolffian body; y, yolk-sac; 3-6, aortic arches. (From Kingsley after
Agassiz and Clarke.)

The heart is very broad laterally and consists of a sinus venosus
(although only distinguishable in Sphenodon externally), two quite dis-
tinct auricles (the right receiving venous blood from the body, and the
left aerated blood from the lungs), and a ventricle always more or less
completely divided into right and left portions. (In the crocodile the
partition is complete.)

804

COMPARATIVE ANATOMY

A.

B.

c.

Fig. 456.

Diagrams to show arrangement of principal veins in A, Urodele, B, Anura and
Reptilia, C, Bird, D, Mammal. 1, Sinus venosus, gradually disappearing in the
higher forms; 2, Ductus Cuvieri (superior vena cava) ; 3, Internal jugular (anterior
cardinal sinus or vein) ; 4, External jugular (sub-branchial) ; 5, Subclavian; 6,
Posterior cardinal, front part (venae azygos and hemiazygos of higher forms) ;
7, Inferior vena cava; 8, Renal portal (hinder part of posterior cardinal) ; 9,
Caudal; 10, Sciatic (internal iliac) ; 11, Femoral in A, Pelvic in B; 12, Anterior
abdominal in A and B, coccygeomesenteric in C; 13, Femoral (external iliac) in
B, C, and D; 14, Anastomosis of jugulars in C. (From Shipley and MacBride.)

The sinus venosus receives the venous blood from two precaval
(really the Cuvierian duct) and one postcaval vein. The blood passes
through the right auricle into the right half of the ventricle, after which
it passes through the pulmonary arteries to the lungs. From the lungs
it returns through the pulmonary veins to the left auricle and thence to
the left ventricle. From here it is pumped out through the paired aortic
arches to all parts of the body. There is both a renal and a hepatic
portal system.

Often there is a foramen (of Panizza) connecting the right and left
fourth aortic arches, so that blood can pass from one side to the other.

BIRDS (Figs. 444, 446, 447, 448, 452)

The heart is large and has two definite auricles, two ventricles, and
no distinct sinus venosus. The right auricle receives the venous blood
from the general body, while the left receives the aerated blood as it is
returned from the lungs. The right aortic arch carries all of the arterial
blood to the system. The renal-portal system is vestigial.

MAMMALIA

The mammals retain the left aortic arch and lose the right, while
birds retain the right arch and lose the left. Modern reptiles show a
tendency to reduce the left arch.

The valves between auricle and ventricle are tricuspid on the right

CIRCULATORY SYSTEM 805

side and bicuspid (mitral) on the left. In the monotremes, however,
both valves have three cusps.

The pulmonary artery and aorta all have three-lobed semilunar
valves.

In the monotremes the renal portal system is better developed than
in other mammals although in all mammals it functions but for a short
time and disappears with the degeneration of the mesonephroi (Wolffian
bodies).

A part of the capillary system of the mesonephroi enlarge during
the degenerative process to form a main trunk which connects the post-
cava with the caudal portions of the postcardinal veins. It is the post-
cardinals that drain the tail, iliacs, and metanephroi.

The left postcardinal largely disappears later with the exception of
that portion which connects with the suprarenal and gonad of the left
side. All the blood from the posterior part of the body is therefore re-
turned through the right postcardinal and the postcava, whose origin
appears to be at the union of the iliac veins.

In the turtle the postcaval vein unites with that part of the renal
portal system which lies caudal to the kidneys and the renal portal sys-
tem then passes out of existence.

This can be understood the better if it be remembered that the renal
portal veins are the caudal portions of the posterior cardinal veins, and
that the subcardinal veins (particularly the right subcardinal) form the
postcaval vein which lies between the kidneys.

In mammals the postcaval vein is formed principally of the distal
ends of the posterior cardinal veins, and of the right subcardinal, of the
(vitelline) hepatic veins close to, and cephalad to, the liver, as well as
of the hepatic veins which lie between the liver and the hind limbs. As
the postcaval vein is made up of so many different sources, there are
bound to be many variations in the adult state due to more or less per-
sistent embryonic conditions.

The more anterior portion of the postcardinal veins loses its con-
nection with the portion connecting with the excretory organs, and with
the thoracic portion of the supracardinals, to become the azygous vein
on the right side and the hemiazygous on the left. Either of these may
disappear or, as in man, there may be a cross connection between these
two veins. In such a case the anterior part of the hemiazygous is known
as the superior intercostal vein.

The abdominal veins are quite important in foetal life as they bring
blood from the placenta to the embryo.

In the higher vertebrates, including man, an innominate vein ex-
tends across from the carotid-subclavian trunk from one side to the other.
All the blood is thus returned to the heart by means of the base of the
right trunk, which is now called the precava or vena cava anterior.

The Cuvierian duct remains only as the coronary sinus.

CHAPTER LIV.

THE UROGENITAL SYSTEM.

As has already been noted, not only in the frog but in several of the
type-forms studied, there is an intimate connection between the excretory
and the reproductive systems. In fact this is so intimate that it is im-
possible to take up either subject without touching upon the other. For
this reason it is customary to treat both under the head of the Urogenital
System. The excretory organs consisting of the paired kidneys, or
nephridic organs and their ducts, serve the purpose of casting out of the
body the waste matter containing nitrogen,, and occasionally other
substances.

The gonads (ovaries or testes) are the reproductive glands. To any
and all of these, accessory structures are frequently added. The nephri-
dic organs proper have already been quite fully described in the frog, a
review of which is essential to the understanding of that which follows.

It will be remembered that the kidneys are parenchymatous glands,
being composed of a soft, more or less spongy tissue in which there is a
profuse quantity of blood. The great quantity of blood is sent through
the tiny venules which anastamose with the arterial capillaries in the
Malpighian corpuscles (Fig. 16.)

Some of the typical parts which go to make up the kidneys of higher
forms are lacking in certain groups of animals. In the amniotes, neph-
rostomes are never formed, although they do occur in most ichthyopsida.
In the pronephros the Malpighian corpuscle is rudimentary or lacking at
all stages, while there is no differentiation of convoluted tubules and
Henle's loop.

Professor Kingsley's excellent account of the urogenital system
is followed here.

Theoretically the function of the various parts of the nephridial
tubules is in outline as follows : In the primitive condition the nitro-
genous waste, is elaborated in the liver, collected in the coelom and,
together with the coelomic fluid is passed outward through the neph-
rostomes and the tubules, which act merely as ducts. "Higher in the
scale the parts become more differentiated and specialized. The renal
corpuscles form a filtering apparatus by which water is passed from the
blood-vessels of the glomerulus into the tubules near their beginning,
and this serves to carry out the urea, uric acid, etc., secreted by the
glandular portions of the walls of the tubules (convoluted tubules,
ascending limb of Henle's loop)."

"All three nephridia arise from the mesomeric somites or from the
Wolffian ridge which appears on either side of the median line where the
mesomeres separate from the rest of the wall of the body cavity, the

UROGENITAL SYSTEM 807

mesomeric cells furnishing the nephrogenous tissue from which the
definitive organs develop."

"Three views are held as to their relations one to another. Accord-
ing to one they are parts of an originally continuous excretory organ
(holonephros) which extended the length of the body cavity. This has
become broken up into the separate parts which differ merely in time of
development and function, with minor modifications in details. A
second view is that they are three separate organs, while a third regards
them as superimposed structures which occasionally overlap (birds,
gymnophiona) and thus are not, strictly speaking, homologous but
rather homodynamous. The first view has the most in its support, but
for convenience the three structures are kept distinct."

It is of considerable value to trace the successive series of these
excretory structures in the different types of animals. It will be re-
membered that in some of the forms studied, such as the earthworm,
there was an excretory organ in practically every segment of the animal's
body. It will be further remembered that insistence has constantly been
laid upon the fact that the so-called higher animal forms have practi-
cally every structure that the lower forms possess, plus something addi-
tional. This is well exemplified in the study of the nephridic organs.

The nephridic organs of the amniotes pass through a three-fold
development. The first excretory organs which grow, form the
pronephros or head-kidney, the next succeeding being known as the
mesonephros or Wolffian body, while the last to form, which becomes
the permanent kidney of the higher forms, is called the metanephros.

While all three are closely related both in their development and their
structure, there is a difference in their origin and in some of the details.

THE PRONEPHROS.

As its name implies, that is the first of the excretory organs to
appear. A review of the embryology of the excretory system must be
had at this point.

As the myotome is being formed from the epimere, the dorsal end of
each mesomere closes. This forms a sac which opens into the coelom.
Each of these is called a nephrotome and lies a little behind the head. It
is from these nephrotomes that the pronephros is formed. The number
of pronephridic organs varies from one in the teleosts, to a dozen or more
in the caecilians. The usual number, however, in the higher forms is
two. From the somatic walls of these nephrotomes there is an out-
growth toward the ectoderm. This forms slender pronephric tubules as
in the amphibia, or solid cords which later have a lumen from within
them as in elasmobranchs and amniotes. They thus all become tubules,
the proximal ends of each communicating with the metacoele by way of
the cavity in the nephrotome. The opening to the metacoele is called a
nephrostome, and as already noted, there will be as many tubules and
nephrostomes as there are somites. The distal ends of the nephrotomes

808 COMPARATIVE ANATOMY

grow outward until they reach just below the ectoderm when they bend
toward the caudal end of the body. Here the more cephalic tubules fuse
with those behind and it is at this meeting place of the tube that the
pronephric duct, sometimes called the archinephric duct, grows back-
ward immediately beneath the ectoderm. This backward growth con-
tinues until the caudal end of the metacoele has been reached. It is here
that the pronephric duct fuses with the caudal end of the digestive tract
and empties into the cloaca, as in the frog, where it meets with the
ectoderm close to the anal opening. In either case an opening then
breaks through so that the contents of the duct can be expelled.

The question is often asked as to whether the ducts thus formed
are of mesothelial origin or whether the ectoderm contributes a share.
From present evidence it is assumed that the ectoderm has no share
in their formation.

"The pronephros is functional for a time in the embryos of some
lower vertebrates ; in other groups it is a rudimentary and transitory
structure save for its participation in the oviducts and the ostium tubae
abdominale. When functional, it takes the nitrogenous waste from
the body cavity, while its filtering apparatus consists either of separate
glomeruli (one for each tubule) or the glomeruli of the separate somites
may run together, forming a glomus. These glomeruli or the glomus
of the pronephros do not project into a Bowman's capsule, but lie imme-
diately above the dorsal wall of the coelom, between the mesentery and
the nephrostomes, pushing the epithelium before them. Later, as in the
caecilians, they and the nephrostomes may be enclosed in a cavity cut
off from the coelom, so that the whole resembles a renal corpuscle, but is
different in origin. In either case the exuding fluid passes into the meta-
coele, from which it is drawn by the cilia of the nephrostomes and passed
into the tubules.

"The blood is brought to the glomus or glomeruli by short seg-
mental arteries arising from the dorsal aorta and, after passing through
the capillaries, it is carried away by the postcardinal veins of the cor-
responding side to the heart, these veins keeping pace in their backward
development with the development of the nephridial tubules."

In all vertebrate adults, with the possible exception of Bdellostoma
(Fig. 366), the pronephros has been replaced by the mesonephros and
later still in the amniotes by the metanephros. In the cyclostomes and
a few teleosts the pronephros, however, persists.

THE MESONEPHROS.

The mesonephros, also called the Wolffian body, is formed by a
series of mesonephric tubules which are developed after the pronephros
and its ducts are completely formed. The mesonephric tubules grow
out from the nephrotomes behind those which form the pronephros. The
tubules extend toward each side of the animal until they meet and fuse
with the pronephric duct. This duct is then the excretory canal for the

UROGENITAL SYSTEM 809

mesonephros. The point of origin of the mesonephric tubules varies in
different animals. Some lie dorsal to the pronephric tubules while two
arise from the same nephrotome one above the other. In fish and
amphibians the nephrostome consists of the opening of the nephrotome
into the metacoele. As this opening, however, is closed in the amniotes,
even before the tubules are formed, there are no nephrostomes, and con-
sequently there is no connection between tubules and the peritoneal
cavity.

"Segmental arteries grow out from the aorta to the splanchnic wall
of each nephrotome, forming there a network of capillaries at a higher
level than the pronephric glomeruli. The glomerulus thus formed
presses the wall before it, while the rest of the nephrotome closes around
it as a Bowman's capsule, the whole forming a Malpighian body (in some
rodents the glomeruli are rudimentary or absent). In most ichthyopsida
the Malpighian body is connected on one side with the metacoele by the
nephrostome, and on the other with the mesonephric tubule.

"Thus at first the mesonephros is a metameric structure, extending
over a much larger number of somites than does the pronephros and
reaching nearly to the posterior limits of the metacoele. As the develop-
ment of the embryo proceeds, the number of tubules, in all vertebrates
except the myxinoids, increases by budding in a manner not readily
described. These tubules unite with those first formed, so that the distal
part of these become collecting tubules. Each of these secondary tubules
forms its own Malpighian body and all of the tubules elongate, become
convoluted and the mesonephros loses its primitive metameric character.

"At the same time changes are introduced into the mesonephric
circulation. The veins emerging from the renal corpuscles extend out
into the region of the tubules, each breaking up there into a second sys-
tem of capillaries which envelop the tubules before returning the blood
to the postcardinal vein. The subcardinal vein brings the blood from the
caudal region (and usually from the hind limbs) to the Wolffian body
and this is also returned via the postcardinals to the heart."

THE MESONEPHRIC DUCTS.

In those animals seemingly more primitive, such as the elasmo-
branchs and in some of the amphibia, the pronephric duct divides longi-
tudinally from its most caudal end forward, almost to the cephalic end of
the Wolffian body. This occurs at the time the mesonephros develops.
There are thus two ducts formed one of which, called Wolffian or Ley-
dig's duct, remains connected with the tubules of the mesonephros and
forms its excretory canal, while the other called the Mullerian duct is
also quite closely related to the pronephros, but forms the oviduct in the
female. In the amniotes the pronephric duct does not divide but be-
comes the Wolffian duct, while the oviduct arises in another manner.
This same thing holds true in many of the amphibians and in all of the
teleosts.

810 COMPARATIVE ANATOMY

THE METANEPHROS.

While the mesonephros functions in all vertebrate embryos and
throughout the entire life of fish and amphibians as well as a short time
after birth in the lizards and opposum, this organ becomes replaced in
the adult of all amniotes by the two metanephridic organs which form
the true kidneys. Each of this pair of kidneys takes its origin directly
behind the mesonephros of the same side, while from the caudal end of
the Wolffian duct close to its entrance into the cloaca a tube, the ureter,
grows forward parallel to the parent duct, into the tissue caudal and
dorsal to the mesonephros. It is supposed that this is more or less
metameric, although all trace of such metamerism has disappeared, the
kidneys not being segmented in any stage of their development. The
cephalic end of the ureter has a varying number of branches, whose tips
expand to form what is called a primary renal vesicle. Around each
primary vesicle a group of cells develops, the aggregate of which grows
into an "S" shaped tubule, one end of which connects with the primary
renal vesicle, the other developing into a glomerulus. There are no neph-
rostomes. Still later these tubules multiply extensively, while the blood
capillary system of the glomerulus increases also.

THE URINARY BLADDER.

This reservoir for urine, also called a urocyst, forms toward the
caudal end of the excretory ducts. There are three kinds of urocysts.

1. A bladder arising by the fusion of the caudal ends of the Wolfrian
ducts plus a portion of the digestive tract. This is the cloaca type. The
Wolfrian ducts in this instance empty into the cloaca, the cloaca then
opening to the exterior.

2. The usual urinary bladder formed by a diverticulum from the
dorsal wall of the cloaca, cephalad to the openings of the Wolfrian ducts.
It is supposed however that this form may be homologous with the
rectal gland of the elasmobranchs.

3. The allantoic bladder occurring in all higher forms as a ventral
diverticulum from the cloaca. The entire outgrowth forms the bladder
in the amphibia, while in the amniotes only the proximal portion becomes
the bladder. The distal portion is used in the embryo as a respiratory
organ, the allantois. The allantois is quite extensive, forming a part
of the placenta in mammals. It is either absorbed or lost at or before
the time of birth.

In the higher forms in which a bladder is present, the ureters open
directly into it, the urine being conveyed to the exterior through the
single tube, the urethra, while in amphibia the urine must first pass
through the cloaca before entering the bladder, as the Wolffian ducts
do not directly enter the urocyst. In many birds and reptiles there is no
urinary bladder at all although these have an allantois during their em-
bryonic development.

"There is great difficulty in comparing the excretory system of the
vertebrates with anything known in the invertebrates. In general the

UROGENITAL SYSTEM 811

nephridial tubules may be compared with those of the annelids. Both
have nephrostomes opening into the coelom, and convoluted tubules
enveloped in a network of capillary blood-vessels, but in the annelid each
tubule opens separately to the exterior in the somite behind that in which
the nephrostome lies, while in the vertebrate the series of tubules empty
into a common duct. When it was thought that the ectoderm con-
tributed to the pronephric duct, the homologies appeared easy. The
duct was originally a groove on the outer surface into which the separate
tubules opened. Then the groove was rolled into a tube which continued
backward to the vicinity of the anus. By the downgrowth of the myo-
tomes the duct became cut off from its primitive position and came to lie
just outside the peritoneal lining. When, however, it is considered that
in all probability the pronephric duct is formed solely from the meso-
derm the homology falls to the ground and an explanation is still a
desideratum."

THE REPRODUCTIVE ORGANS.

A detailed study of the embryological beginnings and development
of the reproductive organs has already been covered in that part of this
book devoted to embryology. After this has been reviewed it will be
understood how the germ plasm is early set aside in the growing embryo
from which the gonads develop . The gonads are not segmented notwith-
standing the fact that earlier writers have taken another view.

These sexual organs in their growth, push a layer of peritoneum be-
fore them just as do the other outgrowths in the body. Such peritoneum
covering the male gonad, which serves as a support for the testes is
called a mesorchium, while that supporting the ovaries is known as a
mesovarium. In all the higher forms gonads are paired. In many fishes
and birds they are unpaired, but this is due to a fusion of two or degen-
eration on one side.

We have seen in our embryological study how the eggs are formed
in the female and lie within a Graafian follicle which, after rupture, leaves
a scar in the form of a corpus luteurn, while in the male, instead of the
primordial ova and the epithelial cells becoming separate follicles they
develop into a cord which later on has a lumen open through it and be-
comes the seminiferous tubule. Both epithelial cells and primordial
spermatagonia may be found in the walls of this tubule. A third type,
known as Sertoli's cell, is also found here. These latter are called nutri-
tive or nurse cells for the developing spermatozoa. Just what function,
aside from this supposed nursing, these cells have is unknown. The
testes remain in the position where they first appear in most vertebrates,
while in nearly all the mammals they descend, assuming a position out-
side the body cavity, being enclosed in a special pouch called the
scrotum.

THE REPRODUCTIVE DUCTS.

As fertilization is necessary in at least all the higher forms of
animals, there must be some method by which the sperm or the eggs

812

COMPARATIVE ANATOMY

as well as the young in viviparous animals may be carried to the outside
of the body. The sperm-ducts of the mammal are known as vasa defer-
entia. (Fig. 457.) The egg ducts of the female are called oviducts
(Fallopian tubes). The vasa deferentia are usually the Wolffian ducts,
but in the female the oviducts may be either the Mullerian ducts, or
specially developed tubes or even merely abdominal pores. In prac-
tically all the forms we are studying the Wolffian ducts serve as the out-
let for the sperm.

At the same time that the tubules, which are to carry the sperm,
are developing, there is an outgrowth of cells from the Bowman's cap-
sules at the cephalic end of the mesonephros, forming what are called
medullary cords. These latter continue their growth into the genital
ridge until they connect with the seminiferous tubules. All of these
acquire a lumen and both together form a continuous transverse
tubule, known as the vas efferens. (Fig. 458.) This continuous
tube leads from the genital cells to the Malpighian corpuscles and thence
by the mesonephric tubules to the Wolffian duct. The vasa efferentia
become connected by a longitudinal canal before actually entering the
Wolffian body ; and there is also usually a second longitudinal canal
which connects them in the body of the testes. The connection of testis

Fig. 457.

I, Diagrams of urogenital systems of female fishes. A, Afri-
can lungfish Protopterus; B, African ganoid Polypterus; Ameri-
can ganoid Amia; D, the garpike Lepidosteus ; E, most teleosts ;
F, trout and salmon, ap, abdominal pore ; cb, cloacal bladder ;
cl, cloaca ; /, funnel of oviduct ; gp, genital pore or papilla ; m,
mesonephros ; o, ovary ; od, oviduct ; r, rectum ; s, urogenital
sinus; up, urinary pore (papilla) ; ugp, urogential pore (papil-
la) ; w, Wolffian ducts. (From Kingsley after Goodrich.)

UROGENITAL SYSTEM
n.

813

Ov.

Fig. 457.

II. Diagrammatic representation of the modifications of the
urogenital ducts. 1, 2, male and female Newt. 3, a tubule of
the kidney. 4, Male Frog. 5, Male Toad. 6, Male Bombinator
(European Frog). 7, Male Discoglossus (Fire-bellied toad).
8, Male Alytes (obstetrical toad), d, artery entering and produc-
ing a coil in the Malpighian body, M; B, Bidder's organ;
ef.s.c., efferent segmental canal ; F.B, fat-body ; gl, glomerulus ;
K, kidney ; l.c.c., longitudinal collecting tubule ; M, Malpighian
body ; Md, Miillerian duct ; N, nephrostome ; O, ovary ; ov, ovi-
duct ; s.d., segmental duct ; T, testis ; Ur, ureter ; V.d., vas
def erens ; V.8., seminal vesicle. (After Gadow.)

and Wolffian body, while usually taking place at the cephalic end of the
mesonephros, may, as in some of the lung fishes, take place at the caudal
end.

At about this time the glomeruli of the tubules degenerate. This
means that the part of the mesonephros in which these glomeruli de-
generate is no longer excretory, but has become a part of the repro-

814: COMPARATIVE ANATOMY

ductive system. It will be noted, then, that sperm can pass throughout
the lumen of a tube the entire distance from the origin to its exit from
the body.

The cephalic end of the Wolffian duct also becomes purely repro-
ductive in the male, it being considerably coiled to form the epididymis.
(Fig. 458.) In the amniotes, where the hinder portion of the mesone-
phros is supplanted by the true kidney (metanephros), the whole
Wolffian duct is a sperm duct (vas deferens) in the male, while in the
female it largely or completely degenerates. In the amphibia and elas-
mobranchs the hinder end of the duct is both reproductive and excretory
in the male. In the female it is purely excretory.

"In the ichthyopsida, other than elasmobranchs and amphibia, the
sperm is carried to the exterior in other ways, and there is no connection
of the testes with the excretory organs. In the cyclostomes the sperm
escapes from the testes into the coelom and then is passed to the exterior
by way of the abdominal pores which in the lampreys open into a cavity
(sinus urogenitalis, Fig. 457) which also receives the hinder ends of the
Wolffian ducts. In the myxinoids the pores are united, and they open to
the exterior behind the anus and between it and the urinary openings."

OVIDUCTS

As already stated, in many forms the Miillerian duct is the direct
result of the splitting in two of the pronephric duct which then serves as
the oviduct. At its separation from the Wolffian duct the Miillerian
duct opens into the coelom by means of the pronephric tubules and their
nephrostomes. These then flow together and form a larger opening
called the ostium tubae abdominale, on each side. (Fig. 458.) In the
elasmobranchs the ostia are usually united ventral to the liver. The eggs
which are thrown out of the ovaries into the coelom are picked up by the
somewhat trumpet shaped extensions around the ostia and carried into
the oviduct. In some forms the pronephric tubules and nephrostomes
take part in the formation of the ostium tubae and the beginning of the
Dviduct ; however, as in all the higher forms, the rest of the oviduct arises
by the formation of a groove of the peritoneal membrane close beside
the Wolffian duct. This becomes rolled into a tube, the Miillerian duct.
In the amniotes the anterior end of the groove does not close, but re-
mains open as the ostium tubae. (Fig. 458.)

It is difficult to trace the successive stages from the most primitive
to the most highly developed. By some the condition of the oviduct in
the elasmobranchs is regarded as the most primitive. Some contend that
we have here a homologous condition — a condition resulting from similar
primitive structures ; others that it is rather analogous and an example
of convergent evolution in that these organs, having been used for similar
functions, have come to appear somewhat alike structurally.

It can be seen how difficult valid comparisons are when we have
such varying conditions in the lower, but nevertheless supposedly related

UROGENITAL SYSTEM

815

forms, as this : In the cyclostomes the eggs are thrown from the ovaries
directly into the coelom, being passed out through abdominal pores, but
in the teleosts alone there are several conditions, the ovaries of some are

B

testis
vasa efferentia

tpididymis (vestige
«f mesonephros)

metanephros

Fig. 458.

Diagrams to illustrate the urogenital system of male and female anamniotes and
amniotes. A, male elasmobranch or amphibian ; the mesonepheros is differentiated
into anterior genital and posterio-excretory portions ; the genital part is connected
with the testis by means of the vasa efferentia, which are outgrowths from the
mesonepheros, the mesonepheric or WolfRan duct serves as both genital and ex-
cretory duct ; the oviduct or Miillerian duct is vestigial. B, female elasmobranch or
amphibian ; the ovary is not qonnected with the mesonephros ; the mesonephros
and mesonephric duct serve only excretory functions ; the oviduct is well developed
and opens into the coelom by the ostium near the ovary. C, male reptile, bird, or
mammal. The excretory part of the mesonephros has disappeared but the genital
part persists as the epididymis (in part) which is connected as in anamniotes with
the testis by means of the vasa efferentia ; the Wolffian duct is purely genital and
is renamed the vas deferens ; the excretory function is served by the metanephroi
and ureters. D, female reptile, bird or mammal ; the mesonephros and Wolffian
duct have entirely vanished ; the condition of the ovary and oviduct is the same
as in anamniotes ; the excretory function is served by the metanephroi and ureters
exactly as in the male. (From Hyman's modification of Wilder.)

816 COMPARATIVE ANATOMY

simple and composed of solid bands or are sac-like, having an internal
lumen. In the simple forms the eggs pass into the coelom and thence to
the exterior by abdominal pores or by oviducts of varying lengths. We
do not know whether these ducts are true Miillerian ducts or whether
they are new formations.

The sacular condition of the ovaries may come about by the free
edge of the ovary bending laterally and fusing with the wall of the
coelom. This forms a cavity called the parovarial canal, closed in front.
Or there may be a groove in the covering epithelium forming on the
surface of the ovary. . In this case, as it closes and sinks inward, it forms
what is called an entovarial canal. In either case the canal may extend
to the most caudal end of the body cavity and form an oviduct in this
manner, "or the oviduct may be formed from both kinds of canals, one
in front, the other behind."

"From this it would appear that the ovary originally extended back
to the hinder end of the coelom (as it does in Cyclopterus) or that the
par- or entovarial canal had united with a Miillerian duct which has
otherwise been entirely lost. The oviducts thus formed usually unite
before opening to the exterior, either directly or via a urogenital sinus."

It will be remembered that there are shell glands (likewise called
nidamental glands) in those animals which are oviparous, although these
may appear in viviparous forms also, though they are very slight in
these latter instances. It is interesting to note that in some species of
elasmobranchs the eggs are larger than those of an ostrich. In this same
type of animal the caudal or inner side of the pelvic fin is specialized
for a copulatory organ, for in the elasmobranch fertilization is internal.
In the amphibia there are many interesting accessory reproductive re-
lations as mentioned in the chapter on classification of vertebrates. The
caecilians and Amphiuma lay their eggs in long strings in the soil and
the female incubates them although the male often takes charge of the
eggs. In Pipa each egg undergoes development in a pit in the skin of the
back of the female, and in Nototrema and Opisthodelphys (South
American tree-toads), there is a large pocket in the skin of the back,
opening near the coccyx, where the eggs are carried until partially
(Nototrema) or entirely developed. Salamandra maculosa and S. atra
bring forth living young, the former possessing gills at birth, the latter in
the adult form.

In the higher forms of vertebrates there is a definite single copula-
tory organ. Among the sauropsida the Sphenodon. alone lacks all copu-
latory organs, while in most birds they are incomplete. The males of
crocodiles, turtles, ostriches, ducks, geese, and swans are among the very
few that have a definite structure homologous to that of mammals for
this purpose. In snakes and lizards several sacs are developed from the
caudal wall of the transverse anus. They resemble appendages in the
embryo and form real copulatory organs called hemipenes. They are
present in both sexes though very small in the female. As growth con-

UROGENITAL SYSTEM

817

tinues, retractor muscles are developed which draw the organ back into
pockets where they are retained at all times except when used for copu-
lation. The simplest form of the copulatory organ is produced by a
thickening of the ventral wall of the cloaca. There is a longitudinal
groove formed in the upper surface of this through which the sperm may
pass. It may be divided into right and left halves, the tip of which
forms the glans penis. The homologous structure is the clitoris which
forms in the female though all parts but the glans are lacking.

In the mammals, while there are two pronephric tubules outlined in
the embryo, they never are functional and the pronephros degenerates.
The mesonephros, however, is definitely used during foetal life, and in
the marsupials and monotremes it even functions sometimes after birth.
However, in all forms of mammals it disappears in time, with the ex-
ception of the efferent ductules of the testes and a few remnants in both
sexes. The metanephros, which becomes the permanent kidney, has
several lobes in the early stages. A definite lobe is formed for each
end branch of the ureter so that each lobule has its own duct. This con-
dition is retained in "adult elephants, some ungulates, carnivores and
primates, and especially in the aquatic species (whales, seals), the lobules
being most numerous (200-|-) in some whales. In all other species the
ducts fuse and the lobules unite later into a compact mass lying in the
lumbar region near the last rib." These lobules are the cause of the
cortex and medulla of the kidneys forming two series of interlocking
pyramids. (Fig. 459.)

In the early embryonic stages the
gonads lie cephalad to the kidneys.

The ovaries are usually equally de-
veloped in the mammals, except in the
monotremes where the left is the larger. "It
is of interest that eggs — one in the Echidna,
two in Ornithorhynchus — have been found
only in the left oviduct." The ovaries, con-
trary to the testes, always remain in the
body, and in the monotremes retain their
early position. "They are supported by the
mesovaria which are attached to the median
side of the double fold of the .peritoneum
which supports the mesonephros. When the
Wolffian body degenerates, the fold becomes
the broad ligament while another fold con-
tinues down the Mullerian duct as the
ligament of the ovary. In some mammals
the ovaries have, in addition, a special fold
of the peritoneum, which in the rats and
mice encloses the ovary and the ostium
tubae connected with its opening."

. "The testes are relatively small and are

Fig. 459.

Longitudinal section through
kidney. 1, cortex, 1', medullary
rays; 1", labyrinth; 2, medulla; 2',
papillary portion of medulla ; 2",
boundary of medulla ; 3, transverse
section of tubules in the boundary
layer ; 4, fat of renal sinus ; 5,
artery ; transverse medullary rays ;
A, branch of renal artery ; C,
renal calyx ; U, ureter. The pyra-
mids are located between the fat
portions and form the papillae.
(From Hill after Tyson and
Henle. )

818

COMPARATIVE ANATOMY

shaped much like the ovaries and at first they are at about the same
level. The outer surface is smooth, a fibrous envelope, the tunica
albuginea, having developed around them, which sends trabeculae in-
^ward, dividing the seminiferous tubules into lobules. Except in the
monotremes the testes descend farther into the pelvic cavity, remaining
permanently in the pelvis in many insectivores, some edentates,
elephants, whales and Hyrax. In other groups they pass outside the
pelvic cavity to be enclosed in a special sac, the scrotum. The testes are
supported by a cord, the gubernaculum, the homologue of both liga-
ments of the ovary.

"The change in position of ovary and testis is accomplished in part
by the unequal growth of body wall and the supporting ligaments. In
the case of the male this descent of the testes is complicated. (Fig. 460.)
In outline it is as follows : By the unequal growth of gubernaculum and
body wall the testes are drawn down into the scrotum which is a pro-
truding part of the body wall into which a part of the coelom extends.
This wall is formed in part from the genital folds which surround the
genital eminence. It lies in front of the penis in the marsupials, behind
it in all placental mammals. When the canal connecting the cavity of
the scrotum (bursa inguinalis) remains open as it does in marsupials,
bats, rodents, insectivores, etc., the descent is temporary, the testes being
withdrawn into the peritoneal cavity at the close of the breeding season
by the cremaster muscle, developed from the transverse abdominal
muscle. In other mammals the descent is permanent, though sometimes
it does not occur until the time -of sexual maturity."

In the monotremes the
Miillerian duct is divided into
a cephalic portion known as the
Fallopian tube and a caudal
portion, the uterus, although
the line separating these two is
not very definite. The broad
trumpet-shaped end of the
Fallopian tube connects with
the coelom while the tube itself
secretes the albuminous cover-
ing of the eggs. The uterus is more muscular than the Fallopian tube
and it is here that the horny shell is formed. The uterus then opens
directly into the urogenital sinus to connect the cloaca with the exterior.
In other forms of mammals the end of the Mullerian duct between
the uterus and the urogenital sinus forms a vagina. In marsupials there
are two vaginae and sometimes three.

When the two caudal ends of the Mullerian ducts fuse as in many

Fig. 460.

Descent of the testis ac, abdominal cavity ; g,
gubernaculum; pv ; processus vaginalis ; t, testis; «,
scrotum ; tv, tunica vaginalis ; x, rudiment of pro-
cessus vaginalis.

UROGENITAL SYSTEM

819

placental animals such as in rodents, there are two uteri formed each
with a separate opening into the vagina. (Fig. 461.) In the carnivores
and ruminants where the fusion is carried still farther back, forming in
reality two uteri with only one opening it is called a uterus bipartibus,
or where it is carried still farther forming two horns it is called uterus
bicornuus. For the uterus simplex the fusion is entirely complete, as in
all primates, the two Fallopian tubes alone remaining as evidence of its
bilateral formation.

In the female the Wolffian duct and the mesonephros are largely

Five varying uteri. A Monotreme; B, Marsupials; C, duplex uterus; D,
bicornuate uterus, and E, Simple uterus, ost.abd., abdominal opening (ostium)
into oviduct ; ovid., oviduct ; s.u.g., urogenital sinus ; ut, uterus ; vag., vagina ;
ves., urinary bladder. Such uteri as A and B open into the urogenital sinus,
while C, D, and E, open into the vagina. (After M. Weber.)

lost in the adult, the mesonephros forming a small collection of tubules
near the anterior end of the ovary which is known as the parovarium.
In the male the Miillerian duct is also largely lost, the lower portion
sometimes persisting as a small blind tubule imbedded in the prostate
gland and known as the uterus masculinus. (Fig. 462.)

Between the tubules in the testes there are small aggregates of cells
known as interstitial cells, which are glands of internal secretion. In
man their products, which pass into the blood, apparently cause the
assumption of the secondary male characters — growth of hair on the
face, change of voice, etc. — at the time of puberty. There would also
seem to be some analogous structure in the ovary governing the develop-
ment of female characteristics and controlling some of the features of
menstruation.

There are also a number of accessory glands (Fig. 462) connected
with the genital ducts, usually better developed in the male than in the
female. The more prominent ones are: the seminal vesicles (present in
some rodents, bats, insectivores and in ungulates and primates) are a
pair of tubular or saccular glands opening into the vasa deferentia just
before these enter into the urogenital canal ; the prostate glands, (occur-
ing in all placental mammals with the exception of edentates and
whales) are connected with the urogenital canal, while farther along

820

COMPARATIVE ANATOMY

the canal Cowper's glands are found. These occur in almost all mammals
as scattered bodies or aggregated into larger masses surrounded by
smooth muscle.

Considerable uncertainty exists as to the exact functions of these
glands. The removal of the prostate and the seminal vesicle in rats pre-
vents fertilization, while the secretion of the seminal vesicles increases
the activity of the spermatozoa. It seems probable that they are of great
importance in connection with fertilization. It has also recently
been shown that in some instances the* coagulation of the secretion of
these, glands closes the vagina after copulation and, thus prevents the
exit of the sperm.

/,.. -//

p. 9

Fig. 462.

A, B, The reproductive organs of the rabbit. A, male ; B, female. In each
case the dissection is made from the left side, the animal lying on its back, bl.,
Bladder ; c.cav., corpus cavernosum ; c.cav.cl., corpus cavernosum of the clitoris ;
Cp., Cowper's gland; epd., cauda epididymis ; epd' ., caput epididymis ; F.t.,
Fallopian tube ; f.o., fimbriated opening of the same ; ov., ovary ; p.g., perineal
gland ; pn., penis ; pr., prostate ; r.g., rectal gland ; rm., rectum ; sc.s., scrotal sac ;
sp.c., spermatic cord (cut short) ; sym., symphysis pubis ; t., testis ; ur, ureter;
ut., uterus ; ut.m., uterus masculinus ; uth., urethra ; v.d., vas deferens ; vag,,
vagina ; vest., vestibule.

C, male, and D, female reproductive organs of dogfish, cl., Cloaca ; f.L, "falci-
form" ligament ; i.o.d., rudiment of the internal opening of the oviducts ; l.t., left
testis ; msn., mesonephros ; mso., mesorchium ; mtn., metanephros ; od., oviduct ;
oes., oesophagus ; ov., ovary ; r.t., right testis ; rm., rectum ; sh, shell gland ; sp.s.,
sperm sac ; u.g.p. and v.p., urinogenital papilla ; u.g.s. and ur.s., urinogenital
sinus; ur., ureter; ur'., ducts of metanephros; v.eff., vasa efferentia ; ves.sem.,
vesicula seminalis ; W.d., Wolffian duct or vas deferens.

In the monotremes the cloaca serves as a general gathering place
for both the products of the urogenital sinus and the excreted matter
from the digestive canal and kidneys. This cloaca has only a single
opening to the exterior and it is from this fact that the name monotreme
has been taken. In all other mammals there is a definite and complete

UROGENITAL SYSTEM

821

separation of the faecal and urogenital matter. This separation is
brought about by a horizontal partition dividing the cloaca into a dorsal
rectum and a ventral urogenital portion. This space between rectum
and urogenital portion is called the perineum.

us.

Fig. 462.

E, The urogenital organs of a female pigeon. K., kidney (metanephros)
with three lobes ; u., ureter ; cl., cloaca ; ov., ovary ; od., oviduct ; f.t., funnel at
end of oviduct ; r.r.od., rudimentary right oviduct.

F, The urogenital organs of a male pigeon. T., testes ; V., base of inferior
vena cava ; S.R., suprarenal glands; K., kidneys with three lobes (1, 2, 3);
u., ureter; v.d., vas deferens ; v.s., seminal vesicle; cl., cloaca. (A, B, C, D, from
Borradaile, E, F, from Thomson.)

ORGANS OF COPULATION.

In both sexes of mammals the same anlagen of the external genitalia
are found as already studied in embryology. These consist of a
genital prominence which is formed from the ventral or anterior wall of
the cloaca. This then protrudes from the opening and, when the per-
ineum is formed, two thickenings appear on each side, a medial genital
fold and a larger and lateral genital ridge, which extends back nearly to
the level of the anus. The genital prominence never develops much
farther in the female, while the folds and ridges become the labia minora
and majora. In the male, however a groove is formed on the primitively
dorsal surface of the prominence which continues into the cloaca. Then
the folds grow together behind the prominence, closing the groove to a
tube, the urethra, while the prominence becomes the glans penis. A
similar growth of the genital ridges toward the median line results in
the formation of the outer wall of the scrotum.

$22

COMPARATIVE ANATOMY

ovid.r. .

f.k. .-

test.r. -

ovid.l.

ovid.r JJL/1 —

cvid.l.

Fig. 463.

Hermaphrodite Frog, f.k., fat-bodies ; n.Z.
and n.r., left and right mesonephroi ; ovid.l.
and ovid.r., left and right oviducts ; teat.l. and
test.r., left and right testes. (After Mitro-
phanoff.)

While internal fertilization
takes place in most of the higher
forms of animals, there are many
vertebrates such as the cyclostomes,
most fishes with the exception of the
elasmobranchs, and many amphibia
in which fertilization does not take
place until after the eggs have
passed from the body of the female.
The organs by which sperm is
passed to the female are formed in
many ways and are not considered
homologous in the different forms.
As we already know from our
study of the earthworm, there are
animals possessing both ovaries and
testes. Such animals are commonly
termed hermaphrodites. True her-
maphrodites must have both ovaries
and testes functional. (Fig 463.)
It is interesting to note that while there are occasional hermaphro-
dites among the lampreys, this is a rather common occurrence in the
myxinoids. In these the cephalic end of the gonad is male, and the caudal
end female. However, usually one or the other of these functions, so
that the animal is either predominantly male or female. Hermaphroditism
has been found among the frogs, while in toads there is often a "pidder's
organ" lying directly in front of the gonads of the male but containing
immature ova. (Fig. 457.) Cases of hermaphroditism, although possi-
ble, are seldom found in mammals, the so-called cases being merely
arrested growth in the male, preventing the two portions of the scrotum
from joining in the mid-line, or an hypertrophy of the clitoris in the
female.

ADRENAL ORGANS.

Closely associated with the nephridial structures lie two. small duct-
less glands, one connected with each renal organ in the higher forms. In
the lower vertebrates each one of these is in turn composed of two
structures. In the amphibia and amniotes one portion, called the
suprarenal, forms the medulla while the interrenal forms the cortex of
the mammalian adrenals. (Fig 351.) The suprarenal portion is always
connected with the sympathetic nerve ganglia, some of the cells always
retaining their nervous character. Other cells, because they stain brown
or yellow with chromic salts are called chromaphile or phaeochrome cells.
(Fig. 464.) These are usually quite closely related to blood vessels.

UROGENITAL SYSTEM

823

A

Fig. 464.

A, The phaeochrome system of a just-born
rabbit. B, The same in a forty-five day girl, o,
aorta ; k, kidney ; p, phaeochrome bodies ; r,
rectum ; «, suprarenal ; u, ureter. The connection
between the bodies and the central portion of the
suprarenal is shown in A, (From Kingsley after
Kohn.)

The interrenals arise from the
epithelium of the coelom, there
being as yet considerable doubt as
to whether they are connected
with pro- or meso-nephros, or
whether they are totally distinct
in origin. They arise as isolated
clusters or bands of cells near the
dorsal margin of the mesentery.
Sometimes they are bilaterally
symmetrical, and in the lower
vertebrates may extend through-
out the entire length of the
coelom in the early stages.

Both interrenals -and supra-
renals are separate in the fishes.
The interrenals are the more com-
pact of the two and lie between
the excretory organs of the two sides of the body.

The suprarenal tissue forms- the medulla of the adrenals from a
series of tubules through which the blood from the suprarenal artery
circulates before it is carried away by the vein. The adrenals are closely
associated with the Wolffian bodies in amphibia, (Fig. 351) either
being attached to the inner margins (urodeles), or forming yellow stripes
(anura) on the ventral surface. In the reptiles they are lobulated bodies
near the gonads.

It is from the medullary portion in mammals that adrenalin, some-
times also called epinephrin, is procured. This is an activator or hor-
mone, which acts directly on the muscles and causes an increase in blood
pressure.

SUMMARY OF THE UROGENITAL SYSTEM.

Fishes (Fig. 457) :

The excretory system consists of elongated bodies situated in the
median dorsal part of the coelom. These bodies are composed of nephric
tubules that have funnel-like nephrostomes opening into the coelom.
The functional kidney is a mesonephros. The ovaries and testes (with
the exception of the teleosts) are sac-like structures that have ducts,
oviducts and vasa efferentia developed in connection with the primitive
nephridial duct, as in other groups.

In teleosts there are no vasa efferentia or true oviducts, for the
posterior ends of both testes and ovaries are continued into a duct direct ;
the duct from the testes unites with its fellow on the opposite side to
empty into either a urogenital sinus or directly to the outside, and the
one from the ovary takes the eggs direct from the ovary before they enter
the coelom as in most of the higher forms.

824 COMPARATIVE ANATOMY

The eggs of different fishes range from large heavily-yolked eggs
with chitinous shells, as in the modern elasmobranchs, to the small
pelagic eggs of many modern teleosts. The eggs pass out through the
ducts of teleosts as mentioned in the preceding paragraph or through
abdominal pores as in ganoids and in some Physostomi.

For the most part, fish-eggs are fertilized in the open water, although
there are many orders which practice internal impregnation and are
viviparous.

Most of the teleosts are dioecious but some are hermaphroditic.
Serranus, a member of the Perch family is even self-impregnating, while
Chrysophrys is successively male and female, while cod and herring
often exhibit the hermaphrodite condition, though this is abnormal.

Dogfish (Fig 462) :

The pronephros is never functional as an excretory organ. The
nephrostomes fuse to form the ostium tubae in the female.

The pronephric duct splits into both a Wolffian and a Miillerian
duct. The nephrostomes close in the adult. The anterior end of each
mesonephros is narrowed and in the male this connects with the anterior
end of the Wolffian duct to form a connection with the testes. The
epididymis consists of the coiled anterior end of this connection.

The Miillerian ducts become known as oviducts. The oviducts of
both sides connect with the coelom. The common opening thus formed
is the ostium tubae abdominale.

The eggs leave the ovary, pass to the ostium and are then carried
backward to a shell-gland. The enlarged portion of the tube forms the
uterus.

In the male, the anterior end of the mesonephros forms the epididy-
mis while the vasa deferentia of both sides unite to form a urogenital
sinus. There is an oval sperm-sac connected on each side. Fertilization
is internal.

The suprarenals are metameric and may be imbedded in the mesone-
phroi.
Amphibia (Fig. 457) :

The pronephros functions until metamorphosis. The tubules then
degenerate. In the adult frog and other tailless amphibians the nephros-
tomes of the mesonephros separate from the nephridial tubules to join
with branches of the renal blood vessels so that the coelom is in direct
connection with the excretory system.

The Wolffian duct carries the nephridial waste and the same duct
also acts as the vas deferens in the male just as it does in the dogfish.
Where these ducts enter the cloaca there is an enlargement on each to
form the seminal vesicle.

The urinary bladder is ventral to the cloaca. The eggs pass into the
body cavity and thence into the ostium tubae.

Fertilization is external in the tailless amphibians but internal in
tailed amphibians. The male of the tailed amphibians secretes a sub-

UROGENITAL SYSTEM

825

stance which binds the spermatozoa into little packets called spermato-
phores. There are various accessory reproductive relations as mentioned
in the chapter on classification.

Reptilia and Aves (Figs. 462, 465) :

The kidneys are metanephric bodies which
pass their excretion through paired ureters
directly to the cloaca in the reptiles and
from here into a urinary bladder which, in turn,
empties into the cloaca. The pronephros never
functions, and the mesonephros (always lack-
ing nephrostomes) may function after hatch-
ing for a time in some reptiles. In the female
the mesonephros, after degenerating, is pre-
served as the "yellow-body." The male repro-
ductive organs consist of a pair of testes, a
pair of much coiled vasa deferentia through
which the sperm passes to the grooved penis;
the latter organ being attached to the front of
the cloaca. The female organs consist of
paired ovaries and large oviducts, provided
with albuminous and shell glands. The eggs
when laid are covered with a tough shell, and
those of reptiles are usually buried in the
ground. Many reptiles are, however, vivi-
parous. The Wolffian duct is the urinary tube
while the mesonephros functions, but later degenerates in the female. It
persists in the male as the vas deferens.

In birds the left ovary alone remains functional.

Mammalia :

Only two pronephric tubules form and these never function. The
mesonephroi function in foetal life and in marsupials and monotremes
for sometime after birth. Nephrostomes never form except in Echidna.
In some rodents no glomeruli occur. The kidneys are of the metanephros
type. They are usually asymmetrical in position, one lying anterior to
the other. The ureters lead directly to the urinary bladder, which is
formed out of the remains of the allantois.

The ovaries are never single as in birds. They are very small on
account of producing minute eggs with little or no yolk. This small
size of ovaries and eggs is well fitted to the habit of uterine gestation.
The paired oviducts enlarge to form paired uteri and in some groups
these unite into a single median uterus.

The testes at firs.t lie in the body cavity as in reptiles, and occupy
positions homologous writh those of the ovaries. In most mammals with
the exception of monotremes, whales, elephants, armadillos, and a few
others, the testes descend into the scrotum. The penis of the male
mammal is homologous with the clitoris of the female.

Fig. 465.

Cloaca and urogenital
organs of a turtle, Chelydra
gerpentina. c, c' , blind sacs of
cloaca ; cl, cloaca ; e, epididymis
and vas deferens ; p, penis, r,
kidneys ; re, rectum ; «, groove
on penis ; t, testis ; u, ureter ;
ug.t cloacal opening of bladder ;
v, bladder. (From Sedgwick's
Zoology, after Gegenbaur.)

CHAPTER LV.

THE MUSCULAR SYSTEM.

The general Muscular system has been discussed in considerable
detail in our study of the frog while the development of the muscles was
taken up in the study of embryology.

It will be remembered that histologically there are voluntary and in-
voluntary muscles, the former being striated, the latter smooth, while
the heart muscles are a sort of combination of these two.

The smooth muscles have their beginnings in mesenchyme, and,
being involuntary, are innervated by the sympathetic nervous system.
Their action is also much slower than that of the striated muscle. They
are found in the skin, in the walls of blood vessels, in the walls of the
digestive canal and in the urogenital system.

The striated muscles have their origin in the walls of the coelom
and are of mesothelial origin. They are supplied by the motor nerves of
the central nervous system. They are all voluntary except those at the
more cephalic end of the digestive tract. Striated fibres may be found
in the body walls, in all organs of locomotion, in the head, in the
diaphragm, and in the cephalic end of the alimentary canal.

The voluntary muscles arise from the somites (which divide into
myotomes and lateral plates, after the epimeres have given rise to the
sclerotomes and dermatomes).

The myotome grows downward between the hypomere and the skin
to meet its fellow on the opposite side in the median ventral line. This
produces a completed coat of voluntary muscles which lies beneath the
skin. The muscle coat is divided into a dorsal and ventral part by the
horizontal skeletogenous partition (Fig. 423) which intersects the skin
at the lateral line. The dorsal muscles are called epaxial, and those ven-
tral to the septum, hypaxial.

The muscles originating from the lateral plates, in the gill-arch
region, which move the gill arches are called visceral muscles.

The muscles originating from the myotomes are called parietal or
somatic muscles.

All muscles except the diaphragm and heart (the heart is always
included under the circulatory system) are divided into three groups
known as parietal, visceral, and dermal muscles.

From our embryological study it will be remembered that the myo-
tomes were cut off from the walls of the coelom, each one forming a
closed sac, the inner wall being called the splanchnic layer and the outer
the somatic layer. The more dorsal cells of the splanchnic layer develop
many nuclei which can be seen in the interior of the cell in the lower
vertebrates. They are quite close to the surface in the muscle fibres of
mammals. Each myotome has its splanchnic wall converted into a

MUSCULAR SYSTEM

82?

muscle so that there are as many primitive muscles as there were
myotomes.

The somatic wall of the myotomes does not become muscle but
changes into mesenchyme from which the corium of the skin develops.
Some of the mesenchyme protrudes between the various myotomes and
there forms fibrous connective tissues that later become the ligaments
which connect the various muscles of a side.

This primitive muscle segmentation can still be seen in the inter-
costal and rectus abdominis muscles.

The myotomes lie close to the level of the notochord and spinal cordr
but they grow both dorsally and ventrally, working their way between
the skin and the walls of the coelom to become an actual part of the
somatopleure.

Ventrally the muscles from both sides grow toward each other and
practically meet at the mid-ventral line, the direct mid-ventral line which
is filled with connective tissues being known as the linea alba.

In the fishes the trunk and tail muscles are arranged in myomeres
which take a zig-zag course. (Fig 401). The muscles are divided hori-
zontally into dorsal and ventral portions (Fig. 423), the epaxial and hy-
paxial muscles, a line of division which follows more or less closely the
lateral -line. The plates of muscle do not retain their flat ends in the
adult, but one end becomes conical and fits into a corresponding hollow
in the next plate. In the tail of the amphibia epaxial and hypaxial
muscles are clearly recognizable, but farther forward the hypaxials are
greatly reduced, and in the amniotes the reduction is carried so far that
the epaxial muscles, greatly modified, can only be recognized in the cer-
vical and pelvic regions, the "tender-loin" being epaxial.

The developmental conditions are more complicated in the head than
in the trunk. In the head region of fish and birds ten coelomic pouches

develop while in amniotes the
number is apparently twelve.
These are known by number, with
the exception of the most anterior,
which was not known when the
numbers were applied and is
called A. These coelomic or head
cavities differ from the myotomes
farther back by having no un-
divided portion of the coelom be-
low, corresponding to the hypo-
meral zone. This difference is
possibly due to the existence of
visceral clefts in this region.

Four of these cavities lie in
front of the ear of which A dis-
appears completely, its cells join-

rig. 466.

The head of a dogfish, seen from above with
the right orbit opened, e., eyeball ; o.f., o.s., in-
ferior and superior oblique muscles ; r.e., r.int.,
r.s., external, inferior, internal, and superior recti
muscles; e.p., spiracle; //., optic nerve; IV.,
fourth nerve. (From Borradaile).

828 COMPARATIVE ANATOMY

ing the mesenchyme. The other three give rise to the "eye muscles"
which move the eye ball. (Fig. 466.) In general, 1, which lies in front
of the mouth, gives rise to four muscles, the inferior oblique and three
of the rectus muscles; 2, which lies in the region of the jaws, forms the
superior oblique muscles; while 3, in the hyoid region, develops the
lateral (external) rectus (in some animals also a retractor bulbi). The
origin of these muscles explains the distribution of the eye-muscle-
nerves, as each nerve supplies only the derivatives of a single myotome.
Several of the other myotomes disappear in development, but the
posterior becomes the so-called hypoglossal musculature.

We have been describing only the origin of the contractile tissue of
the muscles. There is also a connective tissue to be considered.

Mesenchyme cells invade the muscle fibres to form envelopes
(perimysium) which bind the fibres into bundles (fasciculi), these in
turn, are united by other envelopes called fascia. These connective-
tissue envelopes continue beyond where the contractile tissue leaves
off to form the cords or tendons by which the muscle is attached. The
more fixed point of attachment is called the origin, the less fixed the
insertion. Tendons may be of any shape ; such as long and slender, so as
to allow the muscle to lie in or near the trunk, the part to be moved being
in the appendage; or they may form broad flat sheets (aponeuroses).
These latter may occur not only at the ends but in the middle of a
muscle. Sometimes parts of tendons ossify, as in the patella or in the
"drum-stick" of the turkey. Such small rounded ossifications of this
kind are called sesamoid bones. In a few cases, even, as for example,
around the eye and mouth of mammals, the parietal muscles are without
attachment. Here they form rings which are used to diminish the size
of an opening (sphincter muscles).

Muscles vary considerably as to shape, size, number of "heads" or
points of origin, and numbers of contractile portions.

Muscles are usually arranged in antagonistic groups so that any
given action may likewise be reversed. We thus have flexors to bend a
limb and extensors to straighten it; elevators to close the jaw, depressors
to open it, etc.

It is rather difficult to trace exact homologies. The test usually con-
sidered best is to trace the nerve supply, for every muscle derived from a
given myotome is innervated by branches of the nerve which also
originally connected with that segment. A further test is the origin and
insertion. The action of a muscle is of little value in a test for homo-
logies.

A difficulty in the drawing of conclusions from specimens before one,
comes from the fact that a muscle may split into various layers either
longitudinally or transversely, and some even, though entirely different
in origin, may fuse together, while others, either in part or in whole, may
degenerate and disappear entirely. Should one take nerve supply as a
guide, as is usually done, it will be seen that the facial muscles, especially

MUSCULAR SYSTEM 829

those of the higher mammals, have certainly wandered a long way from
their embryologic origin.

The names and location of muscles of the frog should be thoroughly
reviewed at this point.

In the higher vertebrates the anterior spinalis differs from the frog
by being divided into several rectus capitis muscles which connect the
first vertebra with the skull.

The longissimus dorsi group lie on each side of the vertebral spines
in the angle between spinous and transverse processes and extends from
the pelvis to the head. This group is made up of a longissimus dorsi
proper in the lumbar region, an ileo-costalis (inserted on the dorsal part
of the ribs), and a longissimus capitis along the side of the neck to the
temporal region of the skull.

The muscles of the appendages are divided into intrinsic and extrin-
sic groups. The former have their origin in or on the appendicular skele-
ton itself, the latter have their origin on the trunk or axial skeleton
and their insertion on the girdle or base of the limb. Intrinsic muscles
therefore move parts of the limb ; extrinsic move the limb as a whole.
Muscles are often divided according to their action as already seen. Pro-
tractors draw a member forward ; retractors pull it back against the
body ; levators lift it, and depressors pull it down ; flexors bend a limb
or its parts ; extensors straighten it and rotators turn it upon its axis.

Some of the more prominent muscles are as follows :
Levators :

trapezius (for fore limb).

levator scapulae (for fore limb).

Depressors :

pectoralis (for fore limb):
serratus anterior (for fore limb).

Protractors :

pectineus (for hind limb),
adductors (for hind limb). •
sternocleidomastoid (for fore limb),
levator scapulae anterior (for fore limb).

Retractors :

pyriformis (for hind limb),
pectoralis minor (for fore limb),
latissimus dorsi (for fore limb).

The pubofemoralis draws the hind limb toward the mid line while
the gluteus muscle acts as a retractor and elevator.

THE VISCERAL MUSCLES.

As already stated the gill-bearing vertebrates develop a special
svstem of muscles in connection with the visceral arches which are used

830

COMPARATIVE ANATOMY

A.

branch of facial nerve
parotid duct
masseter

digastric

mylohyoid
submaxillary gland
sternohyoid

sternomastoid
cleidomastoid
basioclavicularis

cephalohumeral
first deltoid

lateral head of the triceps
long head of the triceps

pectoralis major

serratus ventralis

B.

parotid gland

rhomboideus capitis

splenius

external jugular vein
levator scapulae ventralis

pectoralis minor
anterior trapezius

second deltoid

posterior trapezius .

latissimus dorsi

external oblique

Fig. 467.

A, Superficial muscles of the cephalic part of the tailed amphibian Salamandra
maculata. a, anconeus ; bi, humero-brachialis inferior (biceps) ; be, levator
scapulae ; cue, cucularis ; dtr, dorso-tachealis ; dg, diagastric ; ds, dorsalis scapulae ;

eo, external oblique; Id, latissimus dorsi; m, petro-tympano-maxillaris (masseter) ;
inh, mylohyoid ; mhp, mylohyoid posterior ; pc, pectoralis ; ph, procoraco-humeralis ;
ra, rectus abdominis ; spc, supracoracoid ; 8th, sternohyoid. (From Kingsley after
Fiirbringer. )

B, Lateral view of the anterior part of the rabbit to show the muscles. The
head is turned slightly so as to give a ventral view of the throat. All dermal
muscles have been removed. (From Hyman's "A Laboratory Manual for Com-
parative Vertebrate Anatomy," by permission of the University of Chicago Press.)

MUSCULAR SYSTEM

831

for the purpose of opening and closing the clefts and also the mouth. In
the higher forms many of these muscles have disappeared although some
do retain their connection with the hyoid.

Visceral muscles are often divided into two sets according to their
derivation, as some develop from muscles which originally ran in a
transverse (circular) and others from those which ran in a longitudinal
direction.

The epibranchial muscles, the .sub-spinals and interbasales (which
lie in the dorsal part of the branchial region), and the coraco-arcuales (in

genioglossus
styloglossus
geniohyoid
hyoglossus
constrictor pharyngis

thyrohyoid
cut edge of
sternomastoid
sternohyoid
sternothyroid
cleidomastoid
cut edge of
clavotrapezius
sealeries
biceps brachii
subscapularis
teres major M.

mandible
molar gland
masseter
digastnc
mylohyoid
parotid gland
ibmaxillary gland
lymph glands

clavotrapezius

external jugular vein

clavobrachialis

pectoantibrachialis

long head of
the triceps

tiansversus costarum
serratus ventralis

long head of
the triceps

external oblique

Fig. 468.

Ventral view of the anterior part of a cat to show the muscles. All dermal
muscles have been removed. Superficial muscles on the right side, deeper layer of
muscles on the left side, after removal of the pectoral muscles, sternomastoid,
mylohyoid, and digastric. The nerves and blood vessels which cross the axilla have
been omitted. The epitrochlearis is also called extensor antibrachii. From Hyman's
"A Laboratory Manual for Comparative Vertebrate Anatomy," by permission of
The Chicago University Press.)

832

C() M PARATIVE ANATOM Y

the ventral or hypobranchial half) are derived from the circular group.
The most anterior of this circular group (Figs. 467, 468) are those which
open (digastric or depressor mandibulae) or close (adductors) the
mouth, and the mylohyoid which extends between the two rami of the
lower jaw. There are usually several adductors, known as masseter,
temporalis, pterygoideus, accordingly as they have their origin from
different parts of the skull.

The longitudinal muscles are largely confined to small slips which
pass from one arch to the next. These muscles undergo considerable
variation in the amphibians. In the amniotes there is also much varia-
tion but some of them are reduced on account of the loss of branchial
respiration with a consequent degeneration of the parts connected
with it. The most noticeable visceral muscles, therefore, in the higher
groups are those connected with the opening and closing of the mouth.

Up to this point all muscles mentioned have had a direct connection
with the skeletal system. With an increasing degree of development
there develops a dermal musculature. Here the muscles are inserted
directly in the skin although being derived from skeletal muscles.
Primitive conditions of this kind are found in reptiles and birds and serve
to move scales, scutes and feathers. This musculature attains its highest
development in many of the four-footed animals, who use it to twitch
the skin when insects attack them. In the primates the platysma
myoides in the neck and head is the only muscle of the kind. It is in-
nervated by a facial nerve which in its primitive condition came from
the hyoid region. The platysma divides, giving rise to such muscles as
the orbiculares which close the lips and eyelids, and the muscles by
which orre lifts the lips, nose, lids, and by which some are able to move
the ears.

semimembranosus
gracilis

semimembranosu'i
semitendinosui

Fig. 469.

Cross-sections through the thigh of A, rabbit, and B, cat, to show the location
of the muscles. Black spots are nerves, small circles, blood vessels, a, greater
saphenous nerve, artery, and vein ; b, peroneal nerve ; c, tibial nerve ; d, sciatic
vein; e, femoral nerve, artery, and vein; /, sciatic nerve. (From Hyman after
Bensley.)

CHAPTER LVL

THE NERVOUS SYSTEM.

It will be remembered that the nervous system begins its career in
the embryonic state by the ectoderm of the gastrula becoming flat on the
dorsal surface of the embryo. This flat portion, called the neural plate,
extends practically the entire length of the embryo. It is slightly
broader at the head end than at the caudal end. The two edges of the
neural plate become raised slightly and finally meet in the mid-line of
the dorsal surface to form a tube. The closing of the tube begins at the
head end and gradually extends backward until the tube has become
completely closed. (Fig. 262).

The neural plate, consisting of ectoderm and folding as it does,
causes the interior lining of the tube to be ectoderm. This is a point
of considerable value in making it easier to understand various
structures, such as the development of the eye. It is also well to re-
member here that the various sense organs, or special organs of sense,
as they are often called, have to do with such things as touch, sound,
taste and light, whose stimuli come first to the exterior part of the body.
In the lower animal forms, such as the earthworms, there are no definite
eyes, and yet, light rays, when thrown upon any part of the earthworm's
body, cause it to move out of such light, showing that the animal is
sensitive to these light rays even though there be no organ developed
by which any one particular spot is specialized to receive more im-
pressions than another part.

Now, just as the complete digestive tract develops from a straight
tube by inpushings and outpushings, so the greater portion of the
nervous system has developed in a similar manner from the single nerve
tube that has just been discussed.

One of the explanations given as to why the nervous system de-
velops in the way it does from the ectoderm and on the dorsal surface of
the embryo, is that remote ancestors of the vertebrates may have spent
their years upon the ocean-bottom, causing the ventral surface of the
body to lie in contact with the ground substance and thus serve as a
protection from attack, while the upper part of the body came in contact
with substances and animals inimical to it. These vertebrate ancestors
thus needed a sense-perception organ for protective and nutritive pur-
poses. Interesting as this may be, it must be admitted that one of the
great difficulties with which biologists have had to deal is the fact that,
in the invertebrates, the nervous system lies upon the ventral surface. It

834 COMPARATIVE ANATOMY

is only in the higher forms that it lies upon the dorsal. Several in-
genious explanations have been attempted but none is satisfactory.
Students should appreciate the fact that our elaborate nervous system
which controls every movement of the body, is one of the most highly
and elaborately protective systems we could possibly possess.

The brain itself, the head-end of the nervous system, is enclosed in
a remarkable bony case, while the spinal cord (the continuation of the
brain, caudad), is encased within the slightly movable but nevertheless
well fitted vertebrae that make up the spinal column. The brain and
spinal, cord combined are called the central nervous system so as to dis-
tinguish it from the peripheral nervous system, which latter consists of
all those nerves arising from the brain and spinal cord.

As the central nervous system is composed of an infolding of the
outer portion of the body, the ectoderm thus infolded into the central
portion of the neural tube, becomes the sensitive part of the central
nervous system. This sensitive surface lines the lumen of the neural
lube, and v/hile this condition remains in all higher forms, including
man, we shall see in our study of the brain that larger or smaller masses
of gray matter may migrate to various parts of the brain.

It will be readily understood that a nervous system of this kind,
Avhich is well protected by a bony covering, has many advantages over
mere external tactile-sense spots, such as the earthworm possesses ;
•still, to be of any value whatever, any inner sensory portion must retain
its connection with the outer portion of the body. It is such connections
which, when they have definite cells and processes that unite with the
central nervous system and are grouped together, become special sense
organs. Such nerve fibres, together with their cells, are known as
sensory nerves. Sensory nerves must therefore carry impulses from
Outer portions to innermost regions, or in other words, from the external
portions of the body to the central nervous system.

The object of the nervous system is primarily to inform the animal
of the conditions, good and bad, in the environment, to correlate this in-
iformation, and to regulate the motion so that advantage may be had of
this knowledge. In those forms of animals which are segmented, that is,
in which metameres appear, especially when this metamerism is in the
mesothelium from which the myotomes develop into muscles, there are
usually one or more pairs of motor nerves going to these segments be-
cause each muscle must have its own nerve supply. The motor nerves
carry impulses from the central nervous system to the muscle or organ
in which they are placed.

The close association of sensory and motor nerves in the trunk
region of vertebrates has not been satisfactorily explained. In
Amphioxus the two kinds of nerves are independent of each other

NERVOUS SYSTEM

835

throughout their course which tends to show that the vertebrate condi-
tion is not primitive.

THE SPINAL CORD.

After the neural tube has formed by a joining in the dorsal midline
of the two folds of the neural plate, the cells on each side of the neural
tube proliferate very rapidly while those of the roof and floor do not.
Thi^s. causes an outgrowth of the two sides so that a fissure (Fig. 470) or

Fig. 470.
Cross section of spinal cord. A, "spider" cells, B, "mossy" cells.

groove is formed on the ventral surface running the whole length of the
cord. In fact, the cells on the side have already begun to proliferate be-
fore the closing of the tube. There is an ingrowth of connective tissue
and blood vessels on the dorsal midline which forms a posterior or dorsal
septum dividing the dorsal part of the cord into halves. The entire lining
of the central canal, composed of epithelial cells, is known as ependyma
and while no definite nervous cells can be seen, it is sensory, and remains
sensory throughout its entire career.

The remaining cells on each side develop into two kinds of cells, one
called neuroglia or simply "glia" cells, which are used to support the
true nerve cells ; the others form neuroblasts which develop true nervous
tissue. This latter type of cell must develop a fibre in order to connect
with other cells and with other portions of the body. These are formed

836 COMPARATIVE ANATOMY

by a cytoplasmic outgrowth from the neuroblast itself. Such processes
may be several feet in length or very short. Some of the little fibres
produced in this way may extend out from the cord as individual nerves,
while others run longitudinally within the cord. Others run on the out-
side of the cord longitudinally. Those that run along the outer portion
of the cord are often called the marginal layer because they form a sort
of envelope of fibers for trie neural cord itself. These fibers are medu-
lated or covered with a white substance and this white envelope is called
the white matter of the cord. That portion lying further toward the
lumen. and composed largely of cell bodies is what constitutes the gray
matter.

In cross-sections of the spinal cord of*a higher vertebrate there will
be seen a portion looking something like the capital letter "H" with a
central canal in the middle of the cross-bar. The entire substance which
looks like the letter "H" is the gray matter of the cord. The dorsal up-
right bars of the "H" form the posterior columns, while the ventral up-
rights form the anterior columns of the cord. Immediately lateral to the
crossbar on each side of the cord there is another column known as the
lateral column. This latter differs to a considerable extent not only in its
relation but also in its function from the dorsal and ventral column.
This H-shaped gray matter really divides the white matter into three
longitudinal tracts called funiculi, formerly also called columns. They
are known as the dorsal, ventral, and lateral funiculi.

It will be remembered th'at the white matter is composed of longi-
tudinal fibres. It is these longitudinal fibres which make up the various
funiculi which connect the different parts of the central nervous system
with each other. It is important to remember that these fibres are not
all alike, but that those in the dorsal funiculus carry impulses toward the
brain and' are therefore called ascending tracts; while the ventral funi-
culus is known as the descending tract in that it carries fibres from the
brain downward. The lateral funiculi have fibres of both kinds and carry
impulses in both directions.

The fibres in each of the funiculi are again grouped into smaller
oundles or fasciculi, each with its own name. Some of the fibres coming
from the brain are distributed at different levels along the cord, while
others, going to the brain, are added to the funiculi at different places.
The size of the funiculi thus decreases with the distance from the
brain. Some of the bundles may disappear in the more distal parts of
the cord.

The spinal cord is approximately cylindrical in the higher animal
forms, but in the lower it is flattened dorsoventrally, the flattening being
greatest in the cyclostomes. In the lower groups there is also a differ-
ence in the shape of the gray matter, the H shape being less distinct.
i; 'The cord tapers pretty regularly in fishes, from the brain to, the

NERVOUS SYSTEM 837

posterior end, but when legs have developed with an increase of muscu-
lature, the spinal cord becomes enlarged in the regions where the nerves
for the limbs are given off. Casts of the spinal canal in certain fossil
reptiles indicate that there was an accumulation of nervous matter near
the hind legs which exceeded the brain in size.

The nerves leave the spinal cord at nearly right angles to its axis
when development begins. Then there occurs an inequality in growth,
the body increasing more in length than does the cord. As a result the
more caudal nerves pursue a very oblique course, and in the hinder part
of the spinal canal of the higher vertebrates they form a bundle of
parallel nerves, the cauda equina (horse-tail). Often, too, another result
of the unequal growth may be the drawing out of the hinder end of the
cord into a slender, non-nervous thread, the filum terminale (Fig. 17).

Flexures (Fig. "288).

In the early stages of development, it will be remembered, the head
end of the developing spinal cord bends forward at almost right angles
to the main axis and this first bend is called the primary flexure. The
second bending occurs at the most caudal end of the medulla oblongata
and is called the nuchal flexure ; it bends in the same direction as does
the first or primary. The third bend is at a level with the cerebellum and
is known as the pontal flexure; it bends in the opposite direction of the
other two, thus drawing back the fore part of the entire brain to lie on
top of the more rearward portion.

The three flexures just mentioned remain throughout adult life in
all mammals, but even where one or more of the flexures appear in the
embryonic state in vertebrates lower than mammals, it is seldom that
more than one or two remain. In reptiles and birds the nuchal and
pontal flexures are weakly developed and entirely obliterated in the
adult.

Neuromeres (Fig 278).

There have been many interesting theories advanced in times past
as to whether or not skull and brain portion of animals were merely a
continued segmented portion of the spinal column and cord. There has
never been any satisfactory solving of the matter, however. This much
we know : that during its development the brain does show some traces
of segmentation in a linear arrangement. These segments are called
neuromeres, of which eight are well defined. Five lie in front of the ear,
one corresponds to the ear in position, and two lie behind the ear.

It is from the first of these segments (though some insist there are
two here) that the fore-brain arises, as well as the parts which in turn
arise from it. The second becomes the mid-brain. The third lies in the
region of the cerebellum. The fourth and fifth lie in the region of the

838 COMPARATIVE ANATOMY

more cephalic portion of the medulla oblongata where the trigeminal and
facial nerves arise. From the sixth the glossopharyngeal nerve arises,
while the vagus is directly connected with the remaining two.

Meninges.

In examining any brain, one finds, after the bony parts of the skull
have been carefully removed, a connective tissue envelope lying close to
the bone. This is called the endorhachis, but is really the periosteum or
perichondrium of the bony parts and not a true envelope of the brain.

In the ascending groups of vertebrates we find a more complex
arrangement of brain and spinal-cord-envelopes. It must be understood
that what is here said of the brain-coverings proper, must also be said
of the entire spinal cord.

In the fishes there is but a single covering envelope called the
meninx primativa. The blood vessels are carried within this meninx.
There is an open space between this meninx and the endorhachis called
the perimeningeal space filled, as are all such spaces, with the cerebro-
spinal fluid. Tiny strands of tissue pass between the two connective
tissue layers.

In the urodeles, and from there on upward in the various phyla, the
meninx has two layers, namely the pia mater, which bears the blood
supply and lies close to the neural cord, and the dura spinalis or dura
mater. A space between these two layers is called the subdural space,
while the perimeningeal space is then called peridural.

In mammals the outermost layer of the pia mater again separates
from the pia proper, becoming a delicate arachnoid layer, and the space
thus formed is called the subarachnoid space.

In man and some of the higher groups of.-animals the dura spinalis
unites with the endorhachis, obliterating the subdural space, and this
united sheet of covering is called the dura mater. This dura mater forms
two strong folds in the mammals, and to a small extent in birds, and
presses longitudinally into the longitudinal fissures separating the two
hemispheres of the brain. It is then known as the falx cerebri. The
other fold presses transversely between cerebrum and cerebellum form-
ing the tentorium. Sometimes these folds even ossify and unite with the
skull.

THE BRAIN.

The forepart of the spinal cord becomes constricted in two places
transversely, forming three divisions, each being hollow in the center
(Fig. 288.)

Starting with the cephalic end (Fig. 471), the first compartment thus
formed is known as the fore-brain or prosencephalon. The central por-
tion forms the mid-brain or mesencephalon while that portion extending
caudally is called the hind-brain or rhombencephalon.

NERVOUS SYSTEM 839

Cyclostomes are the only vertebrates whose brains remain in this
simple three-chambered state. In all other forms there are many modifi-
cations of the primitive brain, though no matter how many modifica-
tions there may be, they all form as ingrowths or outgrowths of this
primary type.

The prosencephalon divides into an end-brain or telencephalon con-
sisting of the cerebral hemispheres, and the twixt-brain or diencephalon
consisting of the thalamus and the hypothalamus. Each of these in turn
again divides, forming the parts enumerated in the accompanying table.
(Pages 840, 841).

The mesencephalon divides into four lobes (in mammals called
corpora quadrigemina and in lower forms of vertebrates, where these
bodies have not again divided transversely, corpora bigemina or optic
lobes), and cerebral peduncles.

The rhombencephalon is made up of the isthmus rhombencephali
(consisting of the superior cerebellar peduncles, the anterior medullary
velum, the trigonum lemnisci and the crura cerebri, the isthmus itself
connecting mesencephalon and rhombencephalon), the metencephalon
(consisting of cerebellum and pons), and the myelencephalon or medulla
oblongata.

THE CEREBRUM

The prosencephalon and the mesencephalon together are often called
the cerebrum.

* The greater part of the telencephalon is made up of the two hemi-
spheres which are divided by a longitudinal fissure. This fissure is not
well marked in fishes, but is very distinct in other groups of animals.
The lateral ventricles are contained one in each hemisphere, while a part
of the third ventricle (commonly called the foramen of Monro), (Fig.
303), lies between the two. The corpus striatum is a ganglion mass
lying upon the floor of the lateral ventricle, while the cortex of the hemi-
spheres is called the pallium.* The substance of the hemispheres varies-
to a considerable extent in the different types of animals. In fact, in.
the fishes it is practically all pallium, for there is merely a thin non-r
nervous covering to the ventricles. In reptiles and birds the gray matter
(nerve cells) is to be found on the ventricular surface, while the outer
surface is composed of white matter (fibers). In the reptiles there is the
beginning of a second layer of cells a little distance from the ventricular
surface. In birds this is still further increased, while in mammals there
is a complete layer called the cerebral cortex over almost the entire sur-
face of the hemisphere.

*The cortex merely means an outer portion, and, in the brain, is usually composed of grey matter,
while the pallium is merely the outermost covering of the hemispheres, whether composed of grey
matter or not.

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842

COMPARATIVE ANATOMY

As the brain grows in a bony case, it follows that as soon as the brain
has grown longitudinally the full length of this case, it must bend in the
various directions the case lays down for it. Therefore, in the mam-
mals, the posterior end of the hemispheres grow dorsally and downward,
and then forward again until that portion of the hemisphere which was
originally most posterior has now grown forward until it reaches, or at
least touches, the olfactory region. The part growing downward and then
forward grows over a part of the side-wall of the hemispheres which por-

Corpus pineale (Epiphysis)
Commissura post.

Commissura sup.

Lamina chor. cpithcJialis ventr. Ill

Stelle der Einstiilpung des
Plex. chorioideus

Tela chor. ventr. IV

Telencephalon
Diencephalon

esencepha/on
Metencepha/on

Corpus mamill

Hypophysis Chiasma
Nn. optic.

Hdhe des For. inter-
ventric. (Monroi)

Commissura ant.

Recessus optic. Lamina terminalis

Fig. 471.

7, Schematic median section of brain of a four month human foetus to show
the various changes caused by the developing hemisphere. (From Corning after
Burckhardt. )

II, Diagram of the development of the corpus callosum and septum pellucidum
in man. A shows the hemisphere in outline, ac, anterior commissure ; cc, corpus
callosum ; ep, epithelial roof of the third ventricle ; he, hippocampal commissure ;
It, lamina terminalis ; o, olfactory lobe ; oc, optic chiasma ; p, paraterminal body ;
s, septum pellucidum ; vh, vestigial precallosal and supracallosal hippocampus.
(From Kingsley after G. Elliott Smith.)

NERVOUS SYSTEM 843

tions of the side-wall thus form a little island in the depths of the longi-
tudinal fissure. This island is called the insula (of Reil).

The bending itself of the downward and forward growing parts has
caused a deep transverse fissure in each hemisphere known as the lateral
cerebral fissure or fissure of Sylvius.

All higher forms of mammals have the brain substance thrown into
many folds or convolutions known as gyri (Fig. 473).

The deeper grooves separating the gyri are called fissures, while
the lesser grooves are known as sulci. This folding permits a great
amount of cortex or gray matter to be provided for ; for, it will be noted
that not only do the tops of each convolution form cortex, but also the
entire sides of every sulcus.

The hemispheres are divided into various lobes : frontal, parietal,
temporal and occipital. The two hemispheres are connected by various
commissures, which likewise must be studied in the actual brain and
compared with the diagram.

Following are the chief commissures (Fig. 471) :

Anterior commissure, in all vertebrates.

Pallial commissure, dorsal to the anterior. This appears in verte-
brates from the amphibians upward.

Corpus callosum (Fig. 471), and

Hippocampal commissure. These last two are a variation in the
higher mammals of the pallial commissures in the lower. The corpus
callosum is developed to a greater extent in man than in other animals
(Fig. 472). This is explained by the fact that in no other animal does
mentality reach so high a state of development as in man, and, because
the cerebral hemispheres are the seat of mentality, it follows that much
greater connection between the cortex of the two hemispheres is needed
in man than in other animals. There is a thin translucent membrane
between the body of the corpus callosum and the fornix known as the
septum pellucidum, which leaves a slight cavity between the two septa
of each side. Formerly this cavity was called the fifth ventricle. It has
no connection whatever with any of the true ventricles.

Two tracts of nervous matter run back on the medial side of either
hemisphere, from the olfactory lobe to the hinder end of the cerebrum.
One of these is the hippocampus, which passes dorsad, and the other
is the olfactory tract, which goes ventral to the foramen of Monro. These
two and the associated olfactory substances make up practically all of
the so-called archipallium in the lower vertebrates, for in these the whole
cerebrum really is accessory to the sense of smell. In mammals, and
possibly as low as the reptiles, a part has been added to receive impres-
sions from other somatic senses. This is the neopallium which has
grown out lateral to the hippocampus and is especially large in the higher
mammals. In man it forms by far the* greater part of the cerebrum. Its
great development forces the olfactory parts to the medial and lower

844

COMPARATIVE ANATOMY

Comparison of Various Types of Brains. A-F (Edinger) are sagittal sections
showing structures lying in the median line and also paired structures (e.g.,
pallium) lying to one side of the median line. The cerebellum is black. It is
doubtful whether the membranous roof in A indicated as pallium is strictly
homologous with that structure in other forms. In B, Pallium indicates prepallial
s-tructures, Aq.Syl., Aquseductus Sylvii ; Basis mesen., basis mesencephali ; Bulb, olf.,
bulbus olfactorius ; Corp. striat., corpus striatum ; Epiph., epiphysis ; G.h., ganglion
habenulae ; Hyp., hypophysis ; Infund., infundibulum ; Lam.t., lamina terminalis ;
Lob. elect., lobus electricus ; L.yagi, lobus vagi ; L.opt., mid-brain roof ; Med.obl.,
medulla oblongata ; Opt., optic nerve ; Pl.chor., plexus chorioideus ; Rec.inf.,
recessus infundibuli ; Rec.mam., recessus mammillaris ; Saccus vase., saccus vas-
culosus ; Sp.c., spinal cord; vent., ventricle; v.m.a., velum medullare anterius ;
v.m.p., velum, medullare posterius. G and H show the mesial surface of the cerebral
hemispheres in a low (G) and high (H) Mammal. G Elliott Smith, Edinger,
slightly modified. The exposed gray matter of the olfactory regions is shaded, the
darker shade indicating the archipallium (preterminal area and hippocampal
formation), the lighter shade indicating the rhinencephalon, which consists of
the anterior and the posterior (principally pyriform) olfactory lobes, and a
central region made up of the hippocampus and the following gyri : fornicatus.
dentatus, uncinatus, introlimbicus, fasciolaris, and Andrae Retzii.

NERVOUS SYSTEM 845

surfaces so that they are exposed to view only by dissection. A part of
the original hippocampus is then vestigial.

"Beginning in the amphibia and reappearing in the reptiles is a tract
of fibers on either side, which connects the posterior part of the cerebrum
(where the hippocampus ends) with the hypothalmus. In the mam-
mals, by the flexure of the cerebrum, this same band of fibers, here called
the fornix, is obliged to take a circuitous course. Starting at the hippo-
campus on the medial side of the temporal lobe, the fornix runs up, then
forward, below the corpus callosum, and then down, in front of the in-
terventricular foramen to end in a protuberance, the corpus mammillare>
on the floor of the hypothalmic region."

Headward, on the dorsal side, the walls become somewhat thick-
ened, bulging out into a pair of prominences known as the optic lobes
or corpora bigemina in the lower forms of animals, while in the mam-
mals there are two such pairs of lobes which are therefore called corpora
quadrigemina (Fig. 473). The roof of this region remains comparatively
thin, but the floor becomes somewhat thicker and forms the cerebral
peduncle. Connecting the mid-brain with the hind-brain is a short con-
stricted area known as the isthmus. From here running caudad along
each lateral wall there is often a groove (seldom, if ever, seen in the
adult) called the limiting sulcus or the sulcus of Monro. This naturally
divides the brain and spinal cord from here to the tail-end into a dorsal
and a ventral half, a fact that is of considerable importance, because the
entire dorsal area is sensory while the ventral is motor in character.
Further, in the study of the central nervous system's development it is
the dorsal portion in which most of the changes come, comparatively
few developing on the ventral side.

The hind-brain is again divided. The part lying cephalad develops
Into the cerebellum or balancing brain (organ of coordination), while
the caudal end tapers rather gradually and is known as the myelencepha-
Ion or medulla oblongata. The cavity in the hind-brain, most of which
is located within the medulla, is known as the fourth ventricle, while the
small lumen which connects the third and fourth ventricle is called the
aqueductus cerebri or the aqueduct of Sylvius (Fig. 282).

It will, therefore, be noticed that from the earlier three compart-
ments of the head end of the brain and spinal cord there have developed
five brain divisions with four ventricles. All the ventricles form a con-
tinuous open space throughout the entire central nervous system.

The roof plate in the region of the cerebellum, which originally was
quite thick, forces the most cephalic portion of the two dorsal zones
far apart, so that they then become quite thin and broad, whereas the
floor plate becomes greatly thickened and constitutes the pyramids which
pass in front into the cerebral peduncles.

A comprehensive study of the brain is a tedious and difficult task
and requires a very thorough going over, and remembering the main

846

COMPARATIVE ANATOMY

Medulla oblongat*

H.

tuli drier, mamiltart

lf. frontal. Sulc. front. \Snlc. jrae- Sale, centralis Sale, pott
med. suft. centra' sn6. tKotnn<{i\ r,,ttm/;.

tral. sup. (Rola

rails StiU. in tc, fa rut

Sttlc. prnecentral.
inf

Sulci occipitaltt
— superior as et lattr.

Poltis occipitalif

AW™ ctretri Polus t,-m- Sulc. temp. Sutc. Snic. hcisura praeoccipHal.

lateral { R,,,,,. post.) for all, sup. f,;,,p. mfd. temp. inf.

Oyr Jrontalts ttiftr

' ' \

Cyr. /rctitiilis wfjitts

I Part ofrreal.
Pars trianfH-
lari,
Pan. ortit.,1. „

'yr. desceHttmt (Ectfrt

c.

Fig. 473.

A, Diagram to show development of five secondary brain vesicles. (After His.)
B, Median sagittal section through brain of man. C and D, dorso-lateral cerebral
surfaces. C to show fissures and sulci, and D to show gyri. (After Villiger.)

NERVOUS SYSTEM

847

points in the histology, general anatomy, and physiology of the frog.
And the task is made the more difficult on account of all the early studies
having been made upon the human brain before our better stains made
it possible to understand the finer structure of nerve cells and fibers.
The result is that the names of the various parts of the brain have been
derived from fanciful resemblances, often very confusing.

We shall attempt to study the entire central nervous system in
terms of function rather than in terms of structure, and the latter only
^in its development, as then, and then only, are we able to place a valid
interpretation upon our findings.

Following are several terms without which no progress in this study
can be made:

A center is any group of nerve-cells which performs a single func-
tion. (This does not imply, however, that all of this particular function
is located in this one center alone. There may be several, or many, per-
forming similar functions.)

It is these centers which form a sort of switch-board for the redis-
tribution of various nervous impulses.

Afferent fibers are those which conduct toward the centers.
Efferent fibers are those which conduct away from a center.
Peripheral nerves (those running from and toward the central sys-
tem) are naturally mixed
nerves in that they carry both
afferent and efferent fibers.

Inhibitory fibers are those
which check an action.

White matter (substantia
alba) is that portion where the
nerve fibers are covered with
white myelin sheaths.

Gray matter (substantia
grisea) is that portion where
there is a mass of nerve-cell-
bodies uncovered with myelin
sheaths.

Brain nuclei are the gray
centers within the brain, which
are divided in turn into:

Primary centers which are
those directly connected with
the peripheral nerves, either as
terminal nuclei of afferent
fibers or as nuclei of origin of efferent fibers, and

Correlation centers, which are those in which the impulse received is
redistributed after meeting with other impulses at a common center.

Fig. 474.
Five types of reflex arcs.

848 COMPARATIVE ANATOMY

Figure 474 shows the five ways in which impulses are and may be
distributed.

Ganglia are those centers similar to brain nuclei which lie outside
the brain, although some books still use this term interchangeably with
brain nuclei.

Brain stem (also called segmental apparatus, because it is supposed
that the primitive type of brain consisted of a mere tube of nerve-cells
with which the peripheral nerves were connected, a pair from each seg-
ment, similar to the spinal cord of the higher forms now), is that portion
of the cephalic end of the central nervous system upon which the enor-
mous cerebral and cerebellar hemispheres develop in all higher forms.
These latter are then called the suprasegmental apparatus.

Cerebrum consists of fore-brain and mid-brain, the most cephalic
part of which develops into the cerebral hemispheres which are again
divided as seen in the table.

The pallium in the highest animal forms is the cerebral cortex or
mantle (Fig. 472), but in the lower forms such as the fish, in which the
entire hemispheres are a part of its olfactory apparatus, the pallium con-
sists of this olfactory apparatus and the two tracts of nervous matter
connecting the olfactory lobe with the hinder portion of the cerebrum.
One of these tracts, the hippocampus, passes dorsal, and the other, the
olfactory tract, passes ventral to the foramen of Monro. They lie on the
medial side of each hemisphere.

Archipallium is the word now used to a considerable extent for the
pallium in the lower vertebrates where this mantle is concerned prac-
tically only with the olfactory apparatus.

Neopallium has therefore come into existence as a term to designate
the pallium of the vertebrates whose brain is not governed entirely by
its olfactory apparatus, but where impulses from the general somatic
senses may be adjusted and be redistributed in a great correlation region
— the cerebral cortex. In the table the pallium corresponds to this neo-
pallium which has grown out lateral to the hippocampus.

Rhmencephalon (nose-brain), the entire olfactory apparatus divides
into peripheral and central regions as in the table.

Corpus Striatum (Figs. 472, 473). This is the name given to the
entire mass of large nerve cells which connect the brain-stem with the
cerebral hemispheres. It is also called the basal ganglion. It will be
noted that the corpus striatum thus forms the main portion of the stem
of the end brain. It is called striated because it consists of masses of
gray matter separated by sheets of white matter, thus making it appear
striated.

In the lower forms of vertebrates (Fig. 473) some have this body
fairly well developed even though there be no cortex, while in reptiles
and birds in which there is a small amount of cortex it is quite highly

NERVOUS SYSTEM 849

developed. In these animals the corpus striatum seems to be a reflex
center of a higher order than the thalamus.

There is doubt as to the exact function of the corpus striatum.
Ramon y Cajal thinks that in mammals, at least, this body functions as
a reinforcement center of the descending motor impulses coming from
the cortex, as these fibers give off collateral branches when passing
through the corpus striatum, while the striatum itself sends important
descending tracts into the thalamus and cerebral peduncle.

The white matter consists of fibers that pass between the cortex
and deep parts of the brain-stem which have no functional connection
with the striatum itself. These are called projection-fibers, and are partly
ascending and descending fibers which pass between the thalamus and
the cortex, and partly descending motor projection-fibers of the cortico-
spinal or pyramidal tract, cortico-bulbar tract, and cortico-pontine tracts

The gray matter of the corpus striatum forms the two nuclei named
after their shape the caudate and the lentiform nucleus (Fig. 475). Most
of the projection-fibers pass between these two nuclei in a wide band
of white matter which is called the internal capsule. These same fibers
radiating from the internal capsule toward the capsule are called the
corona radiata.

The external capsule is formed of a thinner sheet of fibers external
to the lentiform nucleus.

As many cases of apoplexies and other cerebral diseases cause
hemorrhage and other injuries in the internal capsule, there destroying
some of the fibers, the study of the exact arrangement of sensory and
motor projection fibers within the internal capsule is of great clinical
importance.

Claustrum is the name given to the thin band of gray matter lying
between the external capsule and the cortex of the insula (Fig. 475, B).

Nucleus amygdalae is a small mass of sub-cortical gray matter under
the tip of the temporal lobe. It forms part of the nucleus olfactorius
lateralis.

Thalamus (Fig. 475). The middle and larger subdivision of the
diencephalon, sometimes even applied to the entire diencephaloii and
called the optic thalamus.

As all nervous impulses which reach the brain cortex, except those
that come from the olfactory organs, pass through the thalamus, this
organ serves as a sort of vestibule for the cortex and probably also as a
great relay station for the incoming and outgoing nerves.

It is to be remembered that the optic fibers which occupy the thala-
mus take up much of that organ, but it should not be called the optic
thalamus because all fibers to and from the cortex, regardless of whether
coming from the eye or not, pass through the thalamus.

Two parts of the thalamus are to be noted. The ventral portion,
contains chiefly motor coordination centers. In man this position is not

850

COMPARATIVE ANATOMY

Nucleus letitiformis

Capsula interna
(pars leaticulo- .
caudata)

Capsula interna (pars lenticulo-thalamica)
Nucleus caudatua

Nucleus amygdalae (cut)

Commissura anterior
Stria termmalis

Capsula interna (pars

sublenticularis)
^.Nucleus caudatus

•x ^Thalamus

Tractus. J^
olfactorius

Tractus opticus •'"_
Inf undibulum ''" "

Hypophy- (anterior lobe'"'
sis cerebri \ posterior lobe- —

Tubor cinereum///
Corpus mamillare/ /
N. oculomotorius /
Basis pedunculi'

Pons' ,

NCTVUS trigeminua (portio major)-xx
. Nervus trigcminus (portio minor)-'' ^_^-^

N. intermedius^'"''
N. acusticus^'"
N. abducens
N. glossopharyngeus^
Nervus vagus \

Oliva-
Fasciculus circumolivaria pyramidia

Corpus geniculatum

laterale

Corpus pineale
Cor. geniculaturn mediale
Colliculus superior
Colliculus inferior

s lateralis
Nervus trochlearis
^--Brachium conjunctivum

7—- — Brachium

pontis

— Fossa flocculi
— Crus flocculi
—Nucleus denta-
tus cerebelli

Corpus pouto-bulbare

' — Fasciculus spinocerebellaris
Nervus spinalis

B.

Fig. 475.

A, Left lateral aspect of a human brain from which the cerebral hemisphere
(with the exception of the corpus striatum, the olfactory bulb and tract, and a

small portion of the cortex adjacent to the latter) and the cerebellum (excepting
its nucleus dentatus) have been removed. The brain stem (segmental apparatus;
palseencephalon ) includes everything here shown with the exception of the strip
of cortex above the tractus olfactorius and the nucleus dentatus. Within its sub-
stance, however, are certain cortical dependencies (absent in the lowest verte-
brates), which have been developed to facilitate communication between the brain
stem and the cerebral cortex. The chief of these are found in the thalamus, basis
pedunculi, and pons. Compare this with the side view of an intact brain, Figure
473. (From Hsrrick after Cunningham.)

B, Horizontal section of human cerebral hemispheres. 1, 2, A, H, L, F, etc.,
Fiber systems.

NERVOUS SYSTEM 851

well developed and is there called the subthalamus, which is often con-
fused with the hypothalamus.

The dorsal portion of the thalamus is again divided into two por-
tions :

(1) The primitive sensory reflex centers, principally in the medial
group of thalamic nuclei.

(2) The regular cortical vestibule which forms the lateral nuclei.
These lateral nuclei are sometimes called the new thalamus (neothala-
mus) to distinguish them from all other portions of the thalamus, which
other portions are then called the old thalamus (palaeothalamus).

In man the new thalamus makes up by far the greater portion of
that organ. This portion includes "the lateral, ventral, and posterior
nuclei (for general cutaneous and deep sensibility) receiving the spinal,
trigeminal, and medial lemnisci ; the lateral geniculate body and pulvinar
(visual sensibility) receiving the optic tracts; the medial geniculate body
(auditory sensibility) receiving the lateral or acoustic lemniscus."

It will be noted that Professor Herrick, from whom this quotation
is taken, considers the two geniculate bodies as a part of the thalamus,
whereas our table calls them the metathalamus. The student will see
that all these parts are most intimately connected, and classification is
bound to be arbitrary no matter what pains may be taken to make such
classification as scientific as possible.

All the lateral nuclei are connected with the cerebral cortex by im-
portant systems of fibers running both to and from the cerebral cortex.
The fibers themselves are called sensory projection fibers and all of them
pass through or near the internal capsule of the corpus striatum.

While these lateral nuclei receive the impulses from the somatic
sensory fibers as well as the deeper sensibility impulses (such as touch,
temperature, pain, general proprioceptive sensibility, spatial localization,
etc., termed as a whole the somesthetic group), this latter group is prob-
ably separately represented in the thalamus although wre have not yet the
evidence to demonstrate it.

Each of the chief functional regions in the thalamus is connected
with a specific region in the cerebral cortex by its own projection fibers,
the tracts being known as radiations. For example, there are optic radia-
tions, auditory radiations, somesthetic radiations, etc.

The old thalamus, which comprises the more medial thalamic cen-
ters found in lower forms, has little or no cerebral cortex, such as fish, and
seems to retain its function in higher vertebrates. In other words, some
"awareness" of what is going on is carried by these medial centers, so
that the cerebral cortex is not absolutely necessary for the animal to be
aware of its own action or reaction. This means that the cerebral cortex
is not necessary for all, though it undoubtedly is for most conscious
purposes.

Professor Herrick says : "The thalamus can act independently of

852

COMPARATIVE ANATOMY

the cortex in the case of painful sensibility and the entire series of pleas-
urable and painful qualities ; for the thalamic centers when isolated from
their cortical connections are found to be concerned mainly with affective
experience, and destructive lesions which involve the cortex alone do not
disturb the painful and affective qualities of sensation."

Hypothalamus. That portion lying immediately beneath the thala-
mus. A small portion of the primitive neural tube to which the hemi-
spheres are attached has remained in a primitive state, not changing or
having any ingrowths or outgrowths. This unchanged portion is called
the pars optica hypothalami, and, as will be noticed by the table, is a
part of the end-brain and not the diencephalon. The hypothalamus is an
important correlation center for olfactory and various visual impulses,
including probably the sense of taste.

Tuber cinereum is the gray eminence forming the ventral portion of
the hypothalamus.

Infundibulum is a funnel-shaped extension of the third ventricle
passing through the hypothalamus to the end of the hypophysis
(pituitary body or gland which lies in the sella turcica).

Mammillary bodies are a pair of eminences at the posterior end of
the tuber cinereum. These bodies are olfactory centers.

Metathalamus. The posterior part of the thalamus consists of the
geniculate bodies. The lateral or external one is a visual center in the
thalamus and the medial or internal body is an auditory center.
Epithalamus. This is formed by the mem-
branous choroid plexus- which forms the roof of the
third ventricle, the habenula, and the stria medul-
laris, a fiber-tract which connects the olfactory cen-
ters of habenula and cerebral hemisphere. The
habenula itself is a center for the correlation of olfac-
tory sensory impulses with the various somatic
sensory centers of the dorsal part of the thalamus.
The pineal body, in a very few lower vertebrates,
is a sense organ, being called a "parietal eye" (Fig.
476). In the higher forms this sensory function has
been lost, though it is now supposed to be an organ
of internal secretion.

Fig. 476.

Anlage of the epiphysis
(pineal gland) and pari-
etal organ in the lizard
Iguana. A in a 9 day
embryo, and B in an 18
day embryo. Longitu-
dinal section, ep, epiphy-
sis ; pa, parietal organ ;
zw.h., wall of the ven-
tricle in the twixt-brain.
(After von Klinkow-
strom.)

TELENCEPHALON

In all comparative studies of animals one must
observe lower forms in order that the simpler ar-
rangement there found may furnish an understand-
ing of the more complex adjustment found in the
higher forms, as these latter, usually, possess parts
that the lower forms possess, plus something additional.

In the study of the nervous system the dogfish is a good laboratory

NERVOUS SYSTEM

853

example with which to work. It has no cerebral cortex developed into
immense hemispheres, as in man, and which make it so difficult to study
the underlying parts and note their relationship (Figs. 477, 478).

There is a regular system of small sensory canals widely distributed
in fishes containing sense-organs slightly similar to those in the semi-
circular canals of the internal ear. Their functions are supposed to be
somewhat between that of organs of touch in the skin and those of
equilibrium of the internal ear. The water vibrations of slow frequency
probably make it possible for the animal thus to orient itself. Their
innervation comes/from the VII, IX, and X pairs of cranial nerves. The
sensory canals just mentioned are called lateral line organs (Fig. 479)
and are absent in higher vertebrates.

If Figure 480 be studied carefully it will be seen that there is a
quite definite area or center for each group of impressions.

The acoustisco-lateral area is the terminal center of the lateral line
nerves as well as the acoustic nerve (VHI pair).

The general cutaneous area receives impressions from the remain-
ing general exterior of the body.

The nerves from the viscera (that is, from the gills, stomach, etc.)
enter a visceral area.

The eye is connected with the optic lobe.

•n. am-

pal. b.' / ^'' i

ing

Fig. 477.

A semi-diagrammatic drawing of a longitudinal section through a dogfish,
passing slightly to the right of the middle line, a.ch.p., anterior choroid plexus ;
a./., anterior fontanelle ; au., auricle ; au.v., auriculo-ventricular opening and
valve ; b.b., basibranchial cartilage ; b.h., basihyal cartilage ; c., centrum ; e.a.,
conus arteriosus ; cb., cerebellum ; cer., cerebrum ; cor., coracoid region of the
pectoral girdle ; gr., grooves in which the teeth are formed ; i.p., intercalary
plate ; inf., infundibulum ; lat.v., lateral ventricle ; M.c., Meckel's cartilage ; n.a.,
neural arch ; n.am., ampullary sense organs ; n.sp., neural spine ; nch., notochord ;
oes., oesophagus ; op.l., optic lobe ; p.ch.p., posterior choroid plexus ; p.st., pineal
stalk ; pal.b., palatine bar ; pm., pericardium ; pp.c., pericardio-peritoneal canal ;
s.au., sinu-auricular opening ; s.v., sinus venosus ; sp.c., spinal cord ; st., semilunar
valves ; tng., tongue ; v., ventricle ; v.ao., ventral aorta ; 3, third ventricle ; 4, fourth
ventricle. (From Borradaile.)

854

COMPARATIVE ANATOMY

The nose connects with the olfactory bulb and hemisphere. (Some
writers have considered the olfactory hemisphere an actual portion of the
brain equivalent to the cerebral hemisphere in man. These olfactory
hemispheres are, however, only portions of the olfactory apparatus.)

The important point is to note that definite brain regions are set
aside for sensory impressions, and to notice that they are all on the dor-
sal surface (except a part of the olfactory centers).

These regions which Professor Herrick understandably calls "nose
brains," "eye brains," "ear brains," "visceral brains," "skin brains," etc.,
show the simplest type of the pattern of functional localization of pri-
mary reflex centers. That is, all these special "brains" or centers show
that practically all of the parts of the brain (except the cerebellum) have
a very definite connection with some particular peripheral organs.

This means that this type of simple brain is concerned, in so far as
we can tell (with the exception of the cerebellum), only with simple
reflex action, there being no larg^e centers for the higher type of adjust-
ment found in the higher vertebrate brains. However, in the higher
vertebrates, even including man, there is this same type of simple con-

A. B.

Fig. 478.

•A, The brain of the dogfish, seen from above, cb., cerebellum; cer., cerebrum;
m.o., medulla oblongata ; olf.l., olfactory lobe ; olf.o., olfactory organ ; op..
ophthalmic branches of fifth and seventh nervas ; op.l., optic lobes ; p.st., pineal
stalk ; r.b., restiform body ; sp.c., spinal cord ; sp.n., spinal nerve ; thai., thala-
mencephalon ; S, 4, third and fourth ventricles ; II.-V., VIL-X., cranial nerves.

B, The brain of a dogfish, in ventral view, cer., cerebrum ; inf., return limb of
infundibulum, sometimes regarded as the pituitary body ; l.i., lobi inferiores ;
m.o., medulla oblongata ; olf.l., olfactory lobe ; olf.o., olfactory organ ; op.,
ophthalmic branches of fifth and seventh nerves ; sp.c., spinal cord ; s.v., lateral lobe
of saccus vasculosus ; s.v'., median lobe of the same; II. -X., cranial nerves.
(From Borrsdaile.)

NERVOUS SYSTEM

855

nection also, but it is obscured by the greatly enlarged correlation cen-
ters of which the cerebral cortex is the most important. The distinct
course in neurology given in the schools deals largely and primarily with
the histological structure and function of this cortex.

Because the cerebral cortex is found only in the higher forms of ver-
tebrates and, therefore, is supposed to have developed later in the evolu-
tionary scale than the simpler type such as the fish displays, it has been
called the neencephalon in contradistinction to the "fish type of brain
which is then known as the old-brain or palaeencephalon.

Another point to note is that the "ear brain," the "skin brain," and
the "visceral brain"" are all contained in the rhombsncephalon. In fact,
the "stem" of the rhombencephalon (also called the segmental portion)
is made up of these sensory "brains" and their corresponding motor
centers.

B

an. 2,

oph sup.

A and B, Schematic diagrams of sections of the skin. The sections pass
through the lateral line organs. A, of a Teleost, B, of a dogfish. N, lateral
line nerve ; S, sensory nerve ending ; the asterisk shows the cutaneous ori-
fice ; the arrows indicate the direction of the stimulus.

C, Lateral line nerve of a fish, anl, anastomosis between the anterior
and posterior portions of the lateral nerve ; an2, transverse anastomosis be-
tween the right and left lateral nerve ; buc., buccal branch of lateral nerve ;
g.l., lateral nerve ganglion ; mand.ex., mandibular branch of lateral nerve ;
m., spinal cord ; oph.sup., superficial ophthalmic branch of lateral nerve ; r.U.,
branch which follows the lateral line X, X cranial nerve (dotted) to show
partial fusion with lateral nerve. (From Vialleton, A and B after Dean.)

856 COMPARATIVE ANATOMY

In the higher forms this is also true, and the cerebellum (in man,
the pons also in a sense) are suprasegmental extensions.

In both lower and higher forms the "eye. brain" includes the retina
of the eye, the optic nerve, and a part of the roof of the midbrain. In fish
only a few fibers from the optic nerve pass to the thalamus, but in the
higher forms the number of fibers to this portion are many, in fact so
many, that the entire thalamus, as stated, is often called the "optic thala-
mus."

In the fish there are no true cerebral hemispheres, the seemingly
similar organs are hemispheres of the olfactory tract (with the exception
of the very small "somatic area" which becomes the corpus striatum and
cerebral cortex in the higher forms. The olfactory apparatus of the fish
also embraces the entire epithalamus and hypothalamus.

It follows from all that has been said, that no nervous impulses can
enter the cortex without passing through the reflex centers of the brain-
stem first. The brain-stem therefore must have all the fibers lying within
it which are to carry such impulses. The suprasegmental portions are
therefore correlation, coordination, and readjustment centers.

DIENCEPHALON

The twixt-brain or inter-brain lies directly in front of the posterior
commissure. Still further to the front it is bounded by the velum trans-
versum above and the lamina terminalis below (Fig. 282, A, C). The
cavity in the center is a portion of the third ventricle which extends to
the optic chiasma. The fiber tracts running from the cerebral hem-
isphere backward pass into the side walls. Those lying in the dorsal
region go through the thalamus ( ) where there is

a large nerve center. The ventral tracts are the cerebral peduncles al-
ready mentioned. Directly above and in front of the thalamus is the
epithalamus which also contains a nerve center known as the habenula
( ). The hypothalamus, lying as its name implies

below the thalamus, consists of the tuber cinereum in front and the mam-
millary bodies behind. Both the epithalamus and the hypothalamus bear
a relation to the sense of smell, and are, therefore, developed to a greater
extent in all lower animals in which this sense is highly developed. Di-
^ectly behind the velum transversum is the superior or habenular com-
missure.

EPIPHYSIAL STRUCTURES

It is customary to call various parts developed in the roof plate of
the primitive fore-brain epiphysial structures (Fig. 202). Just where the
cerebral hemispheres and the twixt-brain meet there is a little fold of
epithelium, already mentioned, called the velum transversum hanging
from the roof of the cerebrum. Directly in front of this is a little chorioid
plexus that secretes a fluid. This is called the paraphysis. The other

NERVOUS SYSTEM

857

€piphysial structures belong to the twixt-brain and consist of a parietal
organ and the pineal gland (Fig. 476). Both of these arise from the roof
of the twixt-brain between the habenular ganglion and the posterior com-
missure where twixt and mid-brains meet. Sometimes they develop as
a. single outgrowth and sometimes as distinct structures. The more
anterior of the two is the parietal organ or eye. The posterior is the
pineal gland, also known as the epiphysis. These two organs, although
varying in the different vertebrates, are usually always present. The
parietal organ in at least one group of lizards extends on a slender stalk
actually passing out of the skull and forming a sort of median eye on
the dorsal surface of the head. In those vertebrates in which the parietal
organ does not appear at all, the pineal gland seems to show tracts of
structure similar to the parietal organ when it does become an eye. It
will be remembered that the brow-spot seen on the frog is really the
spot where the pineal gland began growing toward the exterior of the
body but was cut off by the developing skull.

It is interesting to note that, notwithstanding the close relations of
the pineal and parietal organs, the former receives its nerve supply from
the posterior commissure while the parietal is connected with the supe-

Optic lobe
Epitha

n.II

Hypothalam

Ganglion V
Infra-orbital trunk

Acoustico-lateral area
, laterahs VII

n.E ai

r. hyomandibularis VII
Spiracle
. palatinua VII

.4.

anglion geniculi VII

B.

Fig. 480.

A, Side view of the brain of the dogfish Mustelus canis. (After Herrick.)

B, Longitudinal section of brain of Trout, aq, aqueduct ; bo, bulbus olfactorius ;
ca,, cK, ci, cp, anterior, horizontal, inferior, and posterior commissures ; cc, central
canal ; cl, cerebellum ; cs, corpus striatum ; h, hypophysis ; t, infundibulum ; iv,
trochlearis nerve ; cc, optic chiasma ; p. pallium ; pi, pinealis ; sv, saccus vasculosus ;
tl, torus longitudinalis ; to, tectum of optic lobes ; v, velum transversum ; «*, v*,
ventricles; vc, valvula cerebelli. (From Kingsley after Rabl-Riickhard. )

85b COMPARATIVE ANATOMY

rior commissure of the twixt-brain. All of these structures in the higher
vertebrates are completely covered by the cerebral hemispheres growing
backward over them. In many of the extinct reptiles there are large
parietal foramina and it is supposed that these animals therefore had
wrell developed parietal or pineal organs. Directly behind the lamina
terminalis there is a chorioid plexus located in the fourth ventricle. This
comes from the roof of the brain in this region and a part of it invades
the third ventricle, while another part, the inferior plexus, sends branches
through the interventricular foramina into the lateral ventricle. This
provides a blood supply to the interior portions of the cerebral hemi-
spheres.'

A funnel-shaped protrusion from the floor of the diencephalon may
be seen posterior and ventral to the optic chiasma known as the infundib-
ulum. This connects with the pituitary body or hypophysis, which latter
organ has developed from the mouth region. It is encased by the devel-
oping skull in a little bony case of its own called the sella turcica
(Turkish saddle). The epithelium of the mouth from which the hypo-
physis springs, remains connected for a time to that organ and the point
of ingrowth into the brain is called Rathke's pocket (Fig. 301, 1). It
will be noticed that the pituitary body grows upward from the oral
cavity just mentioned, while the infundibulum grows downward from
directly behind the optic chiasma to meet it. There are really two parts
to the pituitary body, both rich in blood and lymph vessels. The organ
is known as a gland of internal secretion. Its action is supposed to be
connected with the fat-storing powers of the animals ; sometimes there
is to be found a postoptic commissure connecting the ventral parts of the
brain in this region.

MESENCEPHALON

The mesencephalon or mid-brain, as has already been stated, does
not change very much from the way it appears in the embryo. On the
dorsal surface there are two lateral swellings, the optic lobes. In the
mammals these are transversely divided and are called the corpora quad-
rigemina. If they do not divide transversely they are called corpora
bigemina. Each optic lobe is connected with fibers from the eye on the
opposite side to which it itself is located. In the fishes the ventricle of
the mid-brain is quite large and extends into the optic lobes, but in the
higher groups of vertebrates the ventricle is reduced to a very small
opening or aqueduct. At the anterior end of the dorsal body of the mid-
brain a band of nerve fibers crosses from one side to the other. Any
such cross connections are called commissures. These connect the two
sides of the central nervous system. Cross fibers of this kind are very
numerous in the spinal cord and there are also several in the brain. This
particular one we have just mentioned is called the posterior commissure.

XERVOUS SYSTEM 859

RHOMBENCEPHALON
THE CEREBELLUM

The cerebellum or metencephalon (Figs. 472, 481) is the coordi-
nating organ growing behind the two cerebral hemispheres. The isthmus
which connects the mid-brain and hind-brain lies directly in front of the
cerebellum. The cephalic anterior wall of the cerebellum meets with the
isthmus to form a transverse fold known as the anterior medullary velum
(valve of Vieussens) which dips into the fourth ventricle. The median
ridge of the cerebellum is known as the vermis. This is the only part of
the cerebellum wnich the lower vertebrates possess. In some of the
higher reptiles and birds, however, a small outgrowth occurs on each
side called the flocculus and it is between the flocculus and the vermis
that the cerebellar hemispheres develop in the mammals. This pushes
the flocculus ventrad.

Quite a number of fibers grow from one side of the cerebellum to the
other on the ventral side of the brain stem. This forms a large transverse
band called the pons or bridge. The lower vertebrates have only a few
fibers of this kind so that the pons is very narrow in them. There is a
groove or tract running longitudinally from the cerebellum to the mid-
brain along the side of this pons and these lateral tracts are called an-
terior peduncles, while the central or median tract is called the middle
peduncle or brachium pontis. The origin in the cerebellum of the an-
terior peduncle is called the nucleus dentatus.

MEDULLA OBLONGATA

This is really a large swelling between the cephalic end of the spinal
cord and the brain proper. Various marrow-centers appear in the ven-
tral side of the floor serving as centers by which and through which
efferent or outgoing fibers are redistributed to other nerve cells. The
head end of the medulla, by being expanded, forces the various fiber
tracts of the dorsal funiculi, as well as of the dorsal part of the lateral
funiculi, over to the side of the cerebellum where they enter, bending ab-
ruptly inward and forming a cord called the corpus restiforme, also

Vtlum mednllare
tost.

Ncdulns

Leitulus gracilit
Lebvlvs semilvnaris
inftrior

=^-^

Fig. 481.
Human cerebellum viewed from below and in front. (After Villiger.)

$60 COMPARATIVE ANATOMY

known as the inferior cerebellar peduncle, on either side. The rest of
the fiber tract forms a pair of bands, called pyramids, on the ventral sur-
face of the medulla which extend cephalad beneath the mid-brain. These
extensions are called the cerebral peduncles or crura cerebri. They are
easily found in the lower vertebrates, but in the mammals the pons makes
them difficult or impossible to see.

TELAE CHORIOIDEAE

While the brain is supplied by blood vessels distributed over the
outer surface, extensions from, the outer vessels push the roof and floors
of most of the fore- and hind-brain behind them, into the ventricle of
these two regions, very much on the same principle as an outgrowth of
the digestive tract, such as the liver, pushes its peritoneum-covering be-
fore it. These foldings of the plates are called telae chorioideae or
choirioid plexuses, and it is through these that the nourishing blood
passes by osmosis into the ventricle and into the inner surfaces of the
brain. It is practically impossible to remove the brain and have the
fourth ventricle complete. Usually the chorioid plexus of this fourth
ventricle is torn away as it is very thin in this particular region. The
large open space or cavity that one sees when this has been torn away is
-called the fossa rhomboidalis.

SUMMARY OF THE BRAIN
AMPHIOXUS

The brain is extremely small, hardly as large in diameter as the rest
•of the neural tube. There are but two pairs of cranial nerves, which
have been called olfactory and optic, but in so reduced a brain, homolo-
gies are uncertain. The sense organs consist of a median olfactory fun-
nel opening into the neurocoele, a median eye-spot (not sensitive to
light) on the anterior end of the brain, representing probably a rudiment
of paired eyes. The notochord extends the entire length of the body,
projecting in front of the brain. This may mean that the brain has re-
treated from its primitive anterior position. There is no cranium.

CYCLOSTOMATA

The brain is small but typically vertebrate in structure. The vagus
nerve is not included in the cranial region. In the myxinoids, a groove
runs the entire length of the dorsal surface. There are four pairs of lobes
— •(!) olfactory, (2) cerebral hemispheres, (3) mid-brain, and (4) cere-
bellum. The nasal capsule is enormously developed. The eyes are de-
generate and without muscles or nerves. There is only one semi-circular
canal in the inner ear. In the lampreys the cerebral hemispheres are
distinct and a band-like cerebellum is recognizable. Eyes are well devel-
oped with both muscles and nerves. There are two semi-circular canals
in the ear, a condition intermediate between that seen in the myxinoids
and that in the true fishes, where three canals are always present.

NERVOUS SYSTEM

861

The flexures are never very well marked and disappear entirely in
the adult.

PISCES

The olfactory organs are paired and end blindly, not communicating:
with the pharynx as in terrestrial animals and hagfishes. The auditory
organs are entirely internal, and have no communication with the ex-
terior. They serve largely as organs of equilibration, though they also-
receive vibrations. The eyes are much like those of other vertebrates,
except that they are lidless and have spherical lenses of short range
vision in the water. The brain is small and shows no fissures. It never-
theless has all the characteristics of the vertebrate brain, though there
are but ten cranial nerves (Fig. 482). The spinal cord is like that in
other vertebrates.

DOGFISH

Although the brain is very small and compact, it is larger in propor-
tion to body size than that of the cyclostomes. The most striking feature
is the large size of the olfactory lobes, and the slight development of the
intercerebral fissure. The cerebral hemispheres are well defined, the
cerebellum is large, and overlaps anteriorly a part of the optic lobe, and
posteriorly a part of the medulla oblongata. The corpora restiformia
are large folds on each side of the cerebellum in front and lateral to the
rhomboid fossa. The region of the thalamencephalon from which the
optic nerve springs is comparatively small and slender. The spinal cord is
typical and enclosed within cartilaginous neural arches. The dominant
sense of the dogfish is olfactory, the sense organs consisting of large con-
voluted invaginations in close contact with the olfactory lobes of the
brain. The eyes, although small and probably not especially keen-
sighted, are well developed and connected within the brain by a rather
slender optic nerve. The auditory organs are enclosed in cartilaginous

Zphpr

Fig. 482.

Cranial Nerves of the Fish. (Schematic.) ev., spiracle; mand.,
mandibular branch of the V; max., maxillary branch of the V; m.t.,
masticator branch of V ; m., neural cord ; oph.pr., deep ophthalmic branch
of the V; ph., pharyngeal branches of branchial nerves ; pot., post-
trematic branch ; pt., pretrematic branch ; pl.c., cervico-branchial plexus ;
r.pal. VII, palatine branch of the VII; r.pal. IX, palatine branch of the
IX; sp, spinal nerves; sp.o., spinal-occipital nerves; V to X, pairs of the
corresponding cranial nerves; 1 to 4, branchial slits. (From Vialleton.)

862

COMPARATIVE ANATOMY

capsules and consist of three semi-circular canals, a utriculus, and a
small simple sacculus. The lateral line sense organs are in grooves of
the skin not completely closed. They divide into several branches in
the head region, one above and one below the eye, and some in the hyo-
mandibular region.

TELEOSTS

The vertebral column is not very compact, the vertebrae being often
without a centrum. If the latter is present, it is an arch-centrum. The
nasal tract has no naso-oral groove. It opens by separate nares. The
brain Has a much reduced cerebrum with all olfactory lobes. The pallium
is usually non-nervous, causing the cerebrum to consist largely of the
corpus striatum. The cerebellum is larger than a surface view shows be-
cause a great portion projects into the ventricle.

AMPHIBIA

The cerebrum is larger than the optic lobes, while the olfactory bull)
is separated from the cerebrum by a long tract. The various brain parts
are quite distinct. In the tailless amphibia the two halves of the cere-
brum are secondarily connected by a transverse band behind the olfac-

tory lobes so that a gap is left farther
back.

The telencephalon is larger than
in fishes on account of the pallium
being invaded with nervous matter
on the inner side. There is no true
cortex.

The optic lobes are large and
the pineal gland reaches the cranial
roof in the tailless amphibia. In the
gymnophiones there is a pontal
flexure which brings the pituitary
gland beneath the medulla ob-
longata.

REPTILIA
\ '--• -"ii\7

There is an advance in the nerv-

ous system beyond the amphibia.
The cerebral hemispheres are larger
and the cerebellum more complete
and a cortex is developed. Some-
thing of both pontal and nuchal flex-
ures is retained. There may be a be-
ginning of a temporal lobe. A parietal
It is rudimentary in other groups.
The olfactory lobes are merged in the hemispheres. The eyes are small,
the pupil round, and the iris unusually dark in color. The thalamt de-

n

: '

'

Fig. 483.

Side and dorsal views of young Alligator.
<?, cerebrum ; cl, cerebellum ; e, epiphysial
structures ; h, hypophysis ; i, inf undibulum ; ol,
optic lobe^ ; 1I-XII, cranial nerves. (From
Kingsley after Herrick.)

eye is well developed in lacertilia.

NERVOUS SYSTEM

863

velop so far as to reduce the third ventricle to a narrow slit, even causing
the two edges to unite. This forms the soft commissure or intermediate
mass of the mammalian brain. The sense of hearing is not very acute.

-ol.b.

'Cr,t.

x a.ch. p.
' ,> l.v.

op. t*.

A. B.

Fig. 484.

A. The brain of a rabbit, seen from above with part of the right cerebral hemisphere cut
away, a.c.q., Anterior corpus quadrigeminum ; a.ch.p., anterior choroid plexus ; cb., cerebellum ;
cer.h., cerebral hemisphere ; crt., cortex ; ft., flocculus ; fr.l., frontal lobe of cerebral hemisphere ;
l.v., lateral ventricle ; lat.l., lateral lobe of cerebellum ; m.o ., medulla oblongata ; occ.L, occipital
lobe of cerebral hemisphere ; ol b., olfactory bulb ; op.th., optic thalamus ; p. b., pineal body ;
p.c.q., posterior corpus o.uadrigeminum ; par.L, parietal lobe of cerebral hemisphere ; r.3, roof of
third ventricle ; sp.c., spinal cord ; Sy.f., Sylvian fissure ; tp.l., temporal lobe of cerebral hemi-
sphere ; ver., vermis.

. B. The brain of a rabbit from below, c.al., corpus albicans ; fl., flocculus ; fr.l., frontal
lobe of the cerebral hemisphere ; hip.L, hippocampal lobe ; m.ob., medulla oblongata ; ol.b., olfactory
bulb; ol.t., olfactory tract; p.V., pons Varolii ; pit., pituitary body; rh f.. rhinal fissure;
Sy.f., Sylvian fissure ; tp.l.* temporal lobe of the cerebral hemisphere ; II.-XII.^ roots of the
cranial nerves. (From Borradaile. )

The tympanic membrane is thin and exposed, and is connected with the
auditory organ by a slender columellar bone. The sense of smell is the
keenest of the senses in the turtle, both in the air and in the water. In
correlation with the keen olfactory sense, the olfactory lobes of the brain
arc highly developed. In the crocodile (Fig. 483), the brain is decidedly
advanced in structure for a reptilian brain. The large cerebral hemi-
spheres are especially noteworthy. The tympanic membrane is sunk in
a pit. This is a tendency that is carried much further in the birds and
mammals.

AVES

The brain is very short and broad; the cerebrum is large but not
convoluted. The cerebellum is very large and complex. All three
flexures are partially retained throughout life. The optic lobes are well
developed. The olfactory lobes are rudimentary, indicating a poor sense
of smell. The olfactory epithelium is poorly developed, and sense of

864

COMPARATIVE ANATOMY

taste is almost as poorly developed as the olfactory sense. The inner ear,,
especially the cochlea, is more complex than in reptiles. The eye of
birds is large and highly organized, probably keener than that of any
other animal. Sclerotic plates cover the eyeball. A fan-shaped pecten
(Fig. 490) of unknown function is inserted in the vitreous humor.

A. c.

Fig. 485.

Nerve end-organs. A, longitudinal section of tactile papilla, containing a Meissner'*
corpuscle. B, Section through a terminal corpuscle (end-bulb of Krause) from the conjunctiva,
C, Section of a Pacinian corpuscle. The nerve fiber, n,m, enters the capsule through the channel
/, and has its terminal branches at a. (A, C, from Ranvier, B, from Dogiel.)

MAMMALIA

It seems that some of the archaic mammals did not have a more
highly developed brain than reptiles. Modern mammals, however,
especially the higher groups, have a brain that is much more highly de-
veloped than that of all other forms.

In these higher groups the brain is relatively large (Figs. 472, 484) r
the cerebral hemispheres showing the greatest increase. The increase
is practically confined to the pallium (neopallium).

There is an elaborate system of commissures to connect the two
sides of the brain, the corpus callosum being the most important. In
fact, the corpus callosum is largest in the highest groups.

In the lower animals the olfactory lobes lie at the tip of the cere-
brum, but in the higher forms the pallial increase pushes the frontal
lobes forward so that the olfactory lobes are brought to the lower sur-
face and are separated from the cerebrum proper by a rhinal fissure on
each side.

The olfactory tract and the hippocampal tract connect the olfactory
lobes with regions farther back, but in man the hippocampal tract is
largely rudimentary, the corpus callosum acting as the great connecting
region.

The great numbers of fibers from the increased pallium form the
corona radiata which connects the cortex with the more posterior por-
tions of the brain. And, as connection is made through the thalami, the
thalamic regions become greatly enlarged, extend into the third ven-
tricle and reduce that to a mere slit. Where the two walls come in con-
tact the intermediate mass is developed.

The mesencephalic lobes are four in number, now called the corpora

NERVOUS SYSTEM 865

quadrigemina, only the anterior pair being connected with the optic
nerves, the posterior pair being connected with the sense of hearing. ,

An important point in the understanding of certain brain structures
is the knowledge that the pallium causes a folding so that the original
postero-ventral end of the cerebrum, lateral to the pyriform lobe, is
pushed below and to the outside of the lateral parts of the hemispheres,
the fissure of Sylvius marking the, place of folding. It is at the bottom
of this fissure that the island of Reil is found, which is only the covered
part of the sides of the hemispheres.

All higher forms of mammals have the hemispheres arranged in
many convolutions. This permits an increase in surface without neces-
sitating a great bulk increase. However, some animals with less mental
ability apparently have more convolutions than the more highly organ-
ized, so that it cannot definitely be said that greater convolutions neces-
sarily carry greater mental power.*

The mammalian cerebellum, while better developed than that of
reptiles, is not as highly developed as that of birds.

Ornithorhynchus (Fig. 472) has the most primitive brain of all
mammals. It is small, the cerebral hemispheres are smooth and lack all
convolutions. This animal is aquatic in its habits, living in stagnant
water and feeding chiefly on mollusks, crustaceans, and worms which it
secured by scooping up the muddy bottom with its bird-like bill.
THE ORGANS OF SPECIAL SENSE

It will be remembered that one of the outstanding characteristics
of living matter is its irritability. Contractility is usually added to
irritability when living protoplasm is discussed. It has already been
shown that various functions in the body may be carried on when the
entire nerve supply to that portion has been destroyed. We may there-
fore say, that while irritability and contractility are essential parts of
living matter, the irritability which causes contractility need not be
brought about by a definite system of nerves, although the nerves do
carry the stimulus from one part of the body to another, thus coordi-
nating the various parts and permitting them to work together for some
common end.

In all higher forms, there are external organs of special sense, such
as the nose, the eye, and the ear. In some of the lower forms such as
the earthworm, we found, that while there are no definite eyes, the earth-
worm nevertheless responded to light thrown upon its body by moving
out of -the way as rapidly as it could. We. know from this experiment
that the earthworm is sensitive to light, and that therefore there are
definite sensory regions, more or less specialized, in its skin by which
it can receive a stimulus from light.

.It -w.ould profit an animal little to be. able to receive a stimulus if
it could not in some way move itself .toward or away from such stimu-
lus. The muscles., by -which an animal may move out of harm's way or

* Echidna has more brain convolutions per body- weight than man.

866

COMPARATIVE ANATOMY

toward a food stimulus, and the glands which can secrete substances
that will repel an enemy serve such a purpose.

In order, then, that an animal may profit by the various stimuli it
encounters, it must have (1), a sensory region or surface of some kind
which such stimuli may effect; (2), it must have an organ, such as mus-
cles or glands, which will permit a reaction to the stimuli, (3, it must
have a conducting mechanism by which the stimulus may be sent to the
reacting organs.

The nerve cells become specialized in structure (or in their man-
ner of connection) in three different ways, namely: (1) they may de-
velop sensitivity and form organs of special sense. These nerve cells
then receive specific stimuli. (2) If the nerve cell develops conductivity,
it can transmit impulses, such as sensory, to the central nervous system,
or motor from the central nervous system. The conducting parts formed
by this second group of specialized neurons form nerve tracts. (3) The
third type of specialization of nerve cells is found in the central nervous
system itself. This type forms what are called correlation and associa-
tion fibers in the sensory field, and coordination fibers in the motor field.
In practically all parts of the skin, there are tiny nerve endings,
commonly called free nerve terminations, by which we recognize sub-
stances when we come in con-
tact with them. Then there are
certain parts of the tips of the
fingers where definite end or-
gans are found, and where the
sense of touch is quite highly
developed. The nerve endings
in such special tactile regions
are much more complicated
than in the simple free nerve
terminations. Figure 484 shows
some of the various types of
these tactile corpuscles.

In our embryological study
we have already discussed the
lateral line organs (Figs. 340,

B, Section "through the cochlea of a guinea pig. 479), which are in all probabil-

a., ampullus ; etc., anterior canal; c., cochlea; cr., . , , ,

crus; de., endolymph duct; Is., spiral ligament; ity tactile, and probably

nc.f cochlear nerve; r., Reissner's or vestibular

membrane ; s., sacculus ; se., endolymph sac ; sg ,

spiral ganglion; sm,st,8v., scalae media (ductus

cochlearis), tympani and vestibuli ; t, tunnel; u.,

utriculus; v vestibular nerve (From Kingsley, three great divisions of the ear,

A after Streeter and B after Schneider.)

namely, an external, internal,

and middle ear. Of these three portions, the inner ear is the most primi-
tive. All lower vertebrates that develop a definite ear organ at all begin
by having simply an inner ear (Fig. 19). To this the next succeeding

Fig. 486.
A, Labyrinth of human embryo, 30 mm. long.

sound-perceiving organs. In
the higher vertebrates there are

NERVOUS SYSTEM

867

higher forms add the middle ear or tympanum, while the highest forms
add the outer ear.

The Inner Ear. This consists of a mere area of thickened ectoderm
on each side of the head, between the seventh and ninth cranial nerves.
A review of the embryology of the ear will recall the cup-shaped auditory
vesicle. In the dogfish, the cavity of this remains connected with the
exterior by a slender tube known as the endolymph duct (Fig. 486). In
the frog and in higher forms, there is no open auditory cup. There are
two layers of ectoderm, the outer one forming an unbroken sheet across
the cup. In the dogfish, these endolymph ducts can be seen as external
portions on the top of the head.

The distal end of this endolymph duct becomes enlarged, the enlarge-
ment being called the sacculus endolymphaticus. In the frog and other
amphibia, the ducts of both sides often unite dorsal to the brain, while
the other parts branch and extend into the spinal canal in a root-like
manner. In the frog definite diverticula are sent into the so-called cal-
careous glands, surrounding the basal portion of the spinal nerves.

The auditory vesicle constricts into
two chambers, an upper vestibule or utric-
ulus and a lower sacculus, connected by
a narrow sacculo-utricular canal. Three
outgrowths now take place, one each from
the outer, posterior, and lateral walls of
the utriculus ; the one from the lateral
svall lies in a horizontal plane, the others
in vertical planes. These outgrowths form
tubes, and as they are circular in outline,

they are called the semi-circular canals.

Some of the sensory epithelium has

spread into all of these regions, but a defi-

nite patch of this sensory epithelium can variation of ear.boneg in Mammalia.

be seen in each of the semi-circular canals,

A *

Fig. 487.
Four diagrams to illustrate the

and it is around this patch that the Wrall Of the inner side. The tympanic mem-

f , , r brane is cross hatched and cartilage

the Canal expands tO form an ampulla, bones are covered with small circles,

-•-.• A&£ -11 1 1. j- j i whilst membrance bones are left un-

Figure 486 will have to be studied and shaded.

the model of the ear seen or an ear defi- phou

dition in later Theromorphous Reptile.
The dentary has met the squamosal,

nitely worked out in one of the animals •*

to make this Clear.

In forms higher than fish and am- an.d the. Qu.adrate and articular are

0 reduced in size. C. Condition in hypo-

phibia, there is a little pOCket Called the thetical form, the link between Thero-

. . morpha and Mammalia. The supra-

lagena, given Oil from the posterior Side angular has begun to extend along the

f . , f*> . , ... border of the tympanic membrane.

Of the SaCCUluS. Sensory epithelium ex- D. Condition in primitive Mammalia

tends into this pocket, and in the higher tS*°*1S^^

forms the lagena becomes a peculiar struc- itSSfr Jfc
ture called the cochlear duct.

<Fr°m Shipley and

868

COMPARATIVE ANATOMY

The structures of the internal ear just mentioned form the mem-
branous labyrinth, which is filled with a fluid called endolymph, in which
a number of solid particles, called otoliths, are found. The otoliths are
very tiny crystals of calcium carbonate which cause the endolymph to
have a milky appearance. In the true fishes these tiny crystals have
become aggregated into one or more ear stones of considerable size. In
the dogfish, as the endolymph duct has an opening to the outside, sand
from the exterior may also form part of the otoliths. A:

Fig. 488.

Diagrams of ethmoturbinals in Mam-
mals. A, Type showing endoturbinalia
alone. B, Type with endoturbinalia (heavy
lines) and two ranks of ectoturbinalia.
C, Turbinals with pneumatic cavities in
the ox. D,E,F, three actual cases in man,
showing individual variation. (From Wilder
after Paulli.)

Fig. 489.

Lateral wall of nasal cavity of man.
eg., .crista galli ; ci.cin.cs., inferior, middle
and superior conchae ; fpn., foramen pala-
tinum majus ; fsp., sphenopalatine fora-
me*!! ; ic., incisive canal ; osm., opening of
maxillary sinus ; af, frontal sinus ; ss.,
sphenoidal sinus. (After Corning.)

Cartilage then appears, practically covering the membranous laby-
rinth. This becomes the skeletal (in higher forms the bony) labyrinth,
and is separated from the membranous labyrinth by a small space filled
with a fluid called perilymph. As ossification then takes place in the
cartilage* the ear bones are formed (Fig. 487). There are two openings
into the middle ear, the fenestra tympani, also called the fenestra rotunda,
which is closed by a membrane, and the upper opening, known as the
fenestra ovale or the vestibule. The stapes (Fig. 487), a small piece of
cartilage or bone, is held in place by the membrane.

The lagena grows quite rapidly and coils up into a spiral, in fact,
it extends to the skeletal labyrinth, thus dividing the perilymph space
ihtb two spiral tubes, called scalae (Fig. 486). The upper one is. called
the scala vestibuli, and the lower the scala tympani, while the lagena
itself forms the scala media, also called the cochlear duct. The bony
labyrinth which contains the scala is named the cochlea. >

As the cochlea only develops in the higher forms,; and as our own

NERVOUS SYSTEM 869

sense of hearing is very incomplete without this structure, we must come
to the conclusion that the lower forms of life which do not possess such
an organ must either hear entirely different sounds from those that we
hear, or some other part of the structures they do have must do work
other than that done in man. A very complicated organ of Corti arises
within the scala media. Fine hair-like structures develop in the organ
of Corti which can only be worked out microscopically and with great
difficulty. There is a membrane extending out from the middle wall
over some of the hair cells in the organ of Corti, called the membrana
tectoria. Various functions have- been assigned to this membrane, one
of them being the ability to recognize pitch in sound. Birds, however,
have no organ of Corti at all, and the evidence is quite conclusive that
they can distinguish pitch.

The Middle Ear or Tympanic Cavity. This first appears in the
Anura, that is, in the tailless amphibians such as the frog. We have
already seen from our dissection of the frog that a tympanic cavity con-
nects with the pharynx by a slender duct, the Eustachian or auditory
tube. Externally there was a tympanic membrane extending across the
fenestra ovale, through which sounds were transmitted to the inner ear.
The Eustachian tube is usually considered the homologue of the narrow
internal end of the spiracle in the dogfish. Frogs, birds, and reptiles
have a chain of ear bones consisting of a columella and stapes, while in
the mammals the incus and malleus replace the columella (Fig. 487).

The External Ear. It will be remembered that in the frog the tym-
panic membrane lies on a level with the surface of the head. In higher
forms the tympanic membrane lies at the bottom of a canal called the
external auditory meatus. In most mammals, with the exception of
those that live in water such as the whales and seals, there is even an
external conch developed behind the meatus, so as to assist in collecting
the sound waves and directing them internally. In some of the birds,
feathers are arranged about the external meatus to function as does this
conch.

It is to be remembered that the ear not only serves the purpose of
taking in and interpreting sounds, but that the semi-circular canals, lying
as they do in three dimensions of space and filled with the endolymph,
function somewhat similar to a carpenter's level, sending sensations to
the brain by which the animal recognizes the position of its own body
relative to its surrounding environment. We may therefore think of the
semi-circular canals as an organ of equilibration.

THE NOSE

The real sensory part of the smelling apparatus is always restricted
to one or two small patches of olfactory epithelium near the end of the
head. As the olfactory sac sinks beneath the surface of the ectoderm,
it remains connected by a pair of external openings called the nares.

870 COMPARATIVE ANATOMY

As the dorsal portion of the head then continues growing, the nares are
carried toward the tip of the snout or, in the elasmobranchs, toward the
ventral side of the head. We have already discussed the forms of the
nasal capsules with the skeletal system. In the higher groups, glands
form which keep the epithelium moist.

Beginning with the amphibia and appearing in the higher groups
of animals, there is an accessory olfactory organ known as the organ of
Jacobson (Fig. 339), which is probably used to test the character of food
while it is in the mouth. The first and fifth cranial nerves supply the
organ of Jacobson, which lies near the' internal nostrils.

Cyclostomes have but a single nostril. In all other vertebrates there
are paired nostrils called nares. In the cyclostomes, the nostril is directly
connected with the hypophysis, a condition not found in any other ver-
tebrates. In animals living within the water there are folds formed on
one side of each naris which practically divides it into two, in fact, in
many of the teleosts, each primitive nostril is actually divided into two ;
this permits water to circulate through the olfactory sac, thus carrying
various sensations of odors to the extensive sensory surface of the sac.

In air-breathing vertebrates, including lung fishes, contrary to the
water-breathing groups, the nasal cavity has a connection with the ali-
mentary tract. In some elasmobranchs the first traces are seen of an ar-
rangement for drawing air over the sensory surface. This arrangement
is an oro-nasal groove which leads from each nostril to the angle of the
mouth (Fig. 296, nas. gr) ; in fact, this groove may, in some species,
form a definite tube. From the lung fishes upward a similar groove is
formed on each side before the skeletal parts form. As this closes, the
edges of each groove unite to form a duct leading from the nostril into
the oral cavity where an internal naris or choana is thus formed. The
position of these choanae varies in the different groups of animals.
Maxillary and pre-maxillary bones arise ventral to the nasal passage,
causing the ducts to appear as though running through the skull.

If the oro-nasal groove does not close, harelip results, just as the
failure of the palatines to come together causes a cleft palate.

In some of the urodeles a projection occurs from the lateral wall.
This is one of the first indications in the rising groups of vertebrates
of the conchae, well known in all higher groups. Often the ventral por-
tion of the nasal passage is lined only with ordinary epithelium. This
portion is then called the respiratory duct. The more dorsal portion
lining the tract with sensory epithelium is known as the olfactory duct.
The organ of Jacobson (Fig. 339) is on the medial side of the nasal
cavity in the lower urodeles. In the higher urodeles it is ventral, while
in the highest it has rotated to the lateral side.

The external nares are closed by the smooth muscles in some of
the amphibia. There is little change in the choanae between amphibia
and reptiles. The reptiles, however, show a tendency to have a differ-

NERVOUS SYSTEM 871

entiated anterior region known as the atrium or vestibule, a middle
region connected with the original region, and a posterior region called
the naso-pharyngeal duct. The naso-pharyngeal duct may vary in length
according to whether the choanae are anterior or posterior in position.
The crocodiles show an extreme elongation due to the palatines and
pterygoids growing inwardly, which causes the internal nares to be
pushed to the hinder end of the skull. There is a single concha, which
is supported by bone in the lateral wall of the nose of reptiles. This is
rather weak, however, in turtles and in the crocodiles. It is divided in
front, while a "pseudobranch" develops above and behind the true
concha. It is only in the snakes and lizards that Jacobson's organ occurs
(Fig. 339), and that only as a simple pocket ventral and medial to the
nasal cavity near the nasal septum.

In birds there is an anterior and inferior concha vestibuli, and also
a middle and a superior fold, formed by three folds on the wall of each
nasal cavity. There is no olfactory epithelium formed on the vestibular
conch; it is present on the middle conch immediately before hatching,
and disappears immediately after hatching, so that the upper conch is
the sole seat of smell in the adult. Jacobson's organ is only found in
embryos.

In mammals much greater complication sets in. The naso-pharyn-
geal duct becomes elongated, while the olfactory area lies directly below
a portion of the brain cavity. The interior arrangement of the bones
formed within the nose forms what is called the nasal labyrinth. The
ethmo-turbinals, the naso-turbinals, and the maxillo-turbinals are the
supporting bones or cartilage of the folds of the labyrinth. This arrange-
ment permits a great expansion of sensory surface, while the supporting
structures keep the folds from touching each other.

In those animals whose sense of smell is very low, not only the folds,
but the bones themselves, may be likewise reduced.

The maxillo-turbinals and the naso-turbinals arise from the lateral
wall of the nasal cavity. The ethmo-turbinals are outgrowths fronr the
ethmoid bone, growing out from the upper hinder part of the septum
and extending to the lateral wall. This causes the ethmo-turbinals to
insinuate themselves between the hinder ends of the other two. Any
of these turbinals may divide in turn. The subdivision of the ethmo-
turbinals may be of varying heights so that to form the ecto and ento-
turbinals the naso-turbinals may disappear in the adult. The epithelium
of the maxillo-turbinals is not sensory, so that it is assumed that this
portion of epithelium serves only to warm and moisten the air in its
passage to the lungs. The various forms which the ethmo-turbinals
assume in mammals may be seen from Figure 488. Peter gives the fol-
lowing table of homologies of the nasal labyrinth in the amniotes :
I. Concha of the anterior epithelium: concha vestibuli (birds).

$72 COMPARATIVE ANATOMY

II. Conchae of the primitive sensory epithelium :

• 1. Arising from the lateral wall (conchae laterales).

A. Anterior:

(a) Primary, ventral : concha of reptiles ; middle concha

of birds ; maxillo-turbinals of mammals.

(b) Secondary, dorsal: Upper or posterior of birds; naso-

turbinals of mammals (Pseudoconch of crocodiles).

B. Arising from the posterior part: conchae obtectae of

mammals.

2. Arising from the primitively median wall : ethmo-turbinals
of mammals, numbered from in front backward.

Jacobson's organ (Fig. 339), also called the vomero-nasal organ,
can be seen in the embryo of most mammals as a pocket on the lower
middle side of each nasal cavity. It opens near the duct from Stenson's
gland in rodents and in man, while in other mammals its duct is appar-
ently cut off from the nasal cavity and opens into the naso-palatal canal.
Its middle wall is covered with a sensory epithelium to which branches
of the olfactory nerve extend. The organ degenerates in adult primates.

There are two kinds of glands in the nasal cavity, known as Bow-
man's glands, which are the smaller, and Stenson's gland, which lies in
the lateral ventral wall and opens into the vestibule. There are usually
sinuses in the bones of the skull connected with the nasal cavity by
various openings. Figure 489 shows several of the principal sinuses
in the bones, such as the maxillary, frontal, and sphenoidal.

All mammals have an external fleshy nose supported by nasal bones
and cartilage, but in swine the fleshy portion forms a proboscis of con-
siderable size, and in the elephant this fleshy proboscis extends tremen-
dously to form the trunk.

It is interesting that while most mammals have a well developed
sense of smell, in seals, whale-bone whales, and primates, it is not very
great, while it is practically absent in the toothed-whales. Often the
olfactory nerve disappears entirely.

THE EYE (Figs. 18, 289, 338)

A review of the embryology of the eye in both frog and chick will
make clear how the eye cup, vesicle, stalk, and lens are formed.

A detailed description of the eye must be sought in text-books of
histology. Here we can give but a general outline so as to make intel-
ligible the dissection and comparison of the eye in the various groups of
animals studied. The adult eyeball is made up of three tunics, tabulated
as follows :

I. Tunica Externa.

1. Sclera.

2. Cornea. i

NERVOUS SYSTEM 873

II. Tunica Media.

•

1. Choroid coat.

2. Iris.

3. Ciliary body.

III. Tunica Interna.

1. Retina.

2. Pigment membrane.

The refracting media, or transparent media of the eye traversed by
a ray of light are :

1. The cornea.

2. Aqueous humor.

3. Lens.

4. Vitreous humor.

Each of these layers is made up of other layers in turn and can be
best understood from a careful observation of Figure 18. The retina
is made up of several layers of ganglion and sensory cells. The sensory
cells lie toward the outside of the eyeball and have a rod or cone toward
their outer end. This is the real seeing-portion of the cell. The cells
themselves are called rod and cone cells (Fig. 490, C).

The yellow spot at the center of the retina where vision is the most
distinct is called the macula lutea or fovea centralis.

From what has been said in our general discussion of the central
nervous system, we have seen that the surface lining of the central canal
of brain and spinal cord is the sensory portion. This is called the
ependyma. Originally the rods and cones are on the primitive outer
surface, and the ganglion cells and nerve fibers are on the ventral sur-
face, of the ectoderm. The rods and cones, therefore, correspond to
other sensory organs, such as the organs of the lateral line, taste buds,
etc. Now for the light to get to the rods and cones, it is necessary that
such light traverse the whole retina and then the nervous impulses have
to travel back through the same layer to reach the optic nerve.

It is well at this point to compare the vertebrate eye which we "are
now studying with the parietal eye of the reptiles.

The space between lens and retina is filled with a semi-solid sub-
stance called the vitreous humor. What this vitreous substance is and
how it arises is still in dispute.

The outer wall of the optic cup forms the pigmented epithelium of
the eye ; the black pigment developed in this region ultimately surrounds
and isolates the rods and cones, so that only light which falls directly
upon them can affect them. It is from the outer portion of this pig-
mented layer that the various tunics of the eye develop. These tunics
of the eye are mesenchynial in origin. The tunica vasculosa which
surrounds the retina is divided into a choroid and a ciliary portion. The

874

COM PAR ATI VE A NATO M Y

set

pet

Fig. 490.

A and B, The eye of Columba livia.

A, in sagittal section ; B, entire organ, external aspect, en., cornea ; ch., choroid ; cl.pr.,
ciliary processes ; ir., iris ; 1., lens ; opt.nv., optic nerve ; pet., pecten ; rt., retina ; scl.ps., sclerotic
plates. (From Parker and Haswell after Vogt and Yung.)

C. Schematic representation of the sensory apparatus in the retina of the human eye.

1. layer of pigment cells next to the choroid ; 2, processes of the pigment cells ; 3, rods ;
4, bodies of rod-cells ; 5, cones ; 6, axones of cone cells ; 7, cone-bipolar cells ; 8, 9, ganglion cells ;
10, optic nerve fibers (axones of ganglion cells) ; 11, 12, horizontal cells ; 13, 14, 15, 16y cells of
different type; functions unknown; 17, fibres (probably axones) of cells having bodies in the
brain; 18, neuroglia cells; 19, radial fiber (Miiller's fiber: part of the sustentacular syncytial
framework of modified neuroglia). (From Dunlap after Merkle-Henle).

choroid contains a great many blood vessels and covers the greater por-
tion of the eyeball. It meets the front of the eyeball with the circular
ciliary process, in which there are various ciliary muscles by which the
lens is moved toward or away from the retina so as to alter its shape.
This changes its focal point. The change of focal point is known as
accommodation of the eye. The center of the ciliary process is the iris,
a sort of circular curtain, with a central opening. The opening is known
as the pupil. There are circular muscles called sphincter pupillae and
dilator pupillae, which contract and enlarge the pupil respectively.

NERVOUS SYSTEM 875

The outer capsule surrounding the various layers just discussed,
consists of a sclera, which covers the proximal side of the eye, and the
cornea which is transparent, and through which light first passes before
reaching the lens. The sclera is usually white, and is covered externally
in part by the conjunctiva, which is modified epithelium. In some of the
extinct amphibia, and in many of the modern reptiles and birds, portions
of the sclera ossify and form a ring of sclerotic bones. Snakes and croco-
diles, however, do not develop sclerotic bones. In the sturgeon, and in
many teleosts, there are two or more dermal bones developed on the
sclera, and in some of the sharks and teleosts there are calcifications to
be found also, but these are not true sclerotic bones. Between the lens
and the cornea there is an opening, partly divided by the iris into an-
terior and posterior chambers. The two chambers are in direct connec-
tion through the pupil and are filled with a liquid, called the aqueous
humor.

All that has been discussed so far in connection with the eye forms
the eyeball proper, also called the bulbus oculi. The eyeball is moved
in its socket by six muscles, best understood by studying Figure 466.

Amphibia possess a distinct muscle which draws the eyeball back
into its socket. This is known as the retractor bulbi. Even some of the
jaw muscles may assist in elevating and depressing the ball. In the dog-
fish there is a cartilaginous rod, called the optic pedicle extending from
the eyeball to the skull, this being replaced in the bony fishes by a fibrous
band, the tenaculum.

The eyelid varies in different groups. The upper and lower, as in
the higher vertebrates, and the third lid, called the nictitating membrane,
usually drawn horizontally across the front from the inner angle of the
eye, all form beneath the lower lid. The eyelids themselves have a lining
which lies next to the eye and which is a continuation of the conjunctiva
already mentioned. In the higher mammals the nictitating membrane
appears as a rudimentary fold called the plica semilunaris in the inner
angle of the eye (Fig. 4).

There are no glands connected with the eye in cyclostomes or fishes.
In amphibia they are of the rudimentary type, but in both reptiles and
birds they are divided into two groups, Harder's glands (nictitating
glands), lying near the inner angle and the true lachrymal or tear glands,
lying in the outer angle.

The tear glands in mammals ultimately come to lie beneath the
upper lid and lead, by many ducts, into the conjunctival sac, while
Harder's glands degenerate. The tears secreted by the lachrymal glands
pass over the conjunctiva and are collected at the inner angle of the eye,
where they then pass through the lachrymal duct into the cavity of the
nose.

The eyes of the cyclostomes are of a very degenerate type. In the

876 COMPARATIVE ANATOMY

next higher group, namely, the myxinoids, the lens and the eye muscles
are lacking, while the iris, cornea, and sclera seem quite as one layer.

Fishes have a flattened cornea and a spherical lens, and long rods in
the retina. There is also a peculiar falciform process of vascular and
muscular structure that enters the retinal cup through the choroid fissure,
where it expands (Fig. 282, B). This expansion is called the campanula
Halleri. As there are no ciliary muscles it may be that this process
serves as a means of accommodation. In the flat fishes one of the eyes
migrates during the embryological development so that both eyes are
found on one side of the head (Fig. 371).

In birds and reptiles there is usually a process developed from the
inner surface of the retina forming the pecten of birds, already described,
while in reptiles it is merely a small cone-shaped process at the point of
entrance of the optic nerve. In birds it is quite fan-shaped. Its function
is not known, although it is rich in sensory cells.

In mammals the pupil varies from a vertical slit in cats to a hori-
zontal opening in whales and many ungulates.

The lids may fuse together during embryological development and
separate again some time after birth. At the edges of the lids, there are
eyelashes or cilia, and immediately interior to these are the ducts of
sebaceous glands called Meibomian or tarsal glands, the glands them-
selves lying in the substance of the lids.

There is a retractor muscle of tKe eyeball in the ungulates. In most
mammals the superior oblique muscle of the eye passes through a loop
known as a trochlea before becoming attached to the eyeball proper.

Eyes may be of various sizes even in mammals. The blind mar-
supial Notoryctes of Australia have neither lens nor differentiation in
the cornea, sclera, or choroid, while the retina is lacking in rod and cone
cells. In the mole the eye is quite similar to that of other mammals, but
the lids remain fused in the adult.

THE PERIPHERAL NERVOUS SYSTEM

,: :,A11 nerves running to and from the central nervous system consti-
tute the peripheral nervous system. We have already seen in our em-
bryological study how the spinal nerves have their origin in the neural
crests, which have been left on each side as the neural tube closed. Fig-
ures 470 and 492 show how the dorsal and ventral roots come together
before sending out dorsal, ventral, and visceral rami. Not only is the
dorsal root of the spinal nerve sensory and the ventral root motor in
action, but each root has two types of nerve fibres within it. These are
the somatic sensory and the somatic motor fibers, which are distributed
to the skin as well as to the external sense organs and voluntary muscles.
Then there are the visceral sensory and visceral motor fibers which sup-

NERVOUS SYSTEM 877

.

ply the viscera and circulatory system (Fig. 491). The dorsal and ven-
tral rami, after leaving the connection formed by dorsal and ventral roots,
contain mostly somatic fibers with just a few of the visceral type. The

visceral ramus contains visceral fibers
alone.

•In the lower vertebrates, some of
the visceral motor fibers actually pass
through the dorsal root, so that one
cannot in all strictness say that the

sh'Fig 491M dorsal root is exclusively sensory, but

of cross section of spinal cord in mammals, dorsal roots are purely

and ventral roots

half of figure is that of a fish and right rnotnr
half that of the spinal cord of man. Note

the relatively greater size of the dorsal TVi^ Vricr^ral fiK^rc '" nf ' fTi*» eninol

gray columns and dorsal funiculi in man. *• ne VlSCCral hDCrS Ot tne Spinal

This is correlated with the greater im- rtf*r,rf>~ ni-« t-k-n^f i^o IKr nil »ff*»t-»ti4- r\r

portance in man of the assembling con- nerves are poetically all ctterent or

nections between the cord and the frame. motor jn function. They ido'.IlOt paSS

to their terminal organs, such as the smooth muscles, glands, etc.,
directly, but always end in some sympathetic ganglion where there is
a functional connection made. From this ganglion the impulse is then
carried to a peripheral organ by axones from the sympathetic nerve cells.

In the lower vertebrates, in the regions where the limbs form, there
are usually networks or plexuses formed. These are paired, as "cervico-
brachial" for the fore-limbs, and "lumbo-sacral" for the hind-limbs,
while in the higher vertebrates these have separated and there are defi-
nate cervical, brachial, lumbar and sacral plexuses. There are usually a
great number of nerves (up to about tweny-five) going to form a plexus,
in fact, twenty-five is the largest number known, and this occurs in the
pectoral fin of skates (Fig. 336).

. As there is also a plexus in snakes and limbless lizards in the regions
where limbs usually develop, it is often thought that these animals de-
scended from limbed ancestors, although no trace of limbs occur during
their embryological development. It is from these plexuses that dorsal
and ventral branches of nerves pass out to the two sides of the limbs. In
all four-footed animals there is this simple arrangement of a single main
nerve trunk on the dorsal and ventral side of the limb, but in mammals
there are two nerve trunks on the ventral side of each limb.

In fishes a connecting nerve, which joins some Of the anterior seg-
mental nerves with the limb-stem, passes to the lumbo-sacral plexus.
In some fishes, however, not only is there no connecting nerve, but even
a plexus is wanting, the spinal nerve entering directly into the limb.
The spinal nerves pass to and from the central nervous system, a pair
between each two vertebrae. Thev* receive their names from those of

878 COMPARATIVE ANATOMY

the vertebrae immediately anterior to the nerve, with the single excep-
tion of the first cervical nerve, which lies between the skull and atlas.
We therefore find eight cervical nerves in the neck region, although there
are only seven cervical vertebrae. The spinal nerves are then named
according to the regions of the spinal column. In man there are twelve
thoracic spinal nerves, five lumbar, five sacral, and three to five coccy-
geal.

THE SYMPATHETIC NERVOUS SYSTEM

There is a great variation in the sympathetic system of various ver-
tebrates. The reason for such variation may be accounted for by the
differences of the action and function in the different groups of animals,
for it is the sympathetic nervous system which is not under the control
of the will, but whose work is the regulation or control, either direct
or indirect, of the internal organs, glands, blood vessels, respiratory and
reproductive organs. This control is brought about by either stimulating
or inhibiting the smooth muscle cells in the walls of the blood vessels,
so that by the enlargement of the contraction of the blood vessels a
greater or a lesser amount of blood may be supplied to any part. It is
also of interest to know that the sympathetic nervous system contains
sensory fibers, although when these are stimulated, consciousness of
such stimulation does not result.

The sympathetic nervous system is connected with the spinal nerves
by the visceral rami. The method of their development has already
been discussed in embryology, which should be reviewed at this point
(Fig. 337).

In some of the higher groups of animals, large plexuses are formed
in the more important and vital body regions. These have received the
special names of cardiac, pelvic, and hypogastric ganglia, while the large
one in the abdominal region is known as the solar plexus.

There is usually a longitudinal sympathetic trunk connecting the
chain ganglia of each side, though in the lampreys the chain ganglia are
not connected with each other at all, and the sympathetic system is con-
fined entirely to the body cavity, while in slightly higher groups there
may be extensions from one to the other ganglion.

Besides the visceral sensory and motor elements which we have
been discussing in the sympathetic system, the visceral rami also carry
fibers which arise in the ganglion cells of the dorsal ganglion or in the
lateral column of the cord itself (Fig. 492). As their axones and
dendrites develop, they interlace with both motor and sensory ganglion
cells lying in the chain ganglia. Nerve fibers from these then extend
out to the viscera, while others run backward in the dorsal and ventral
rami of the spinal nerves to reach blood vessels and smooth muscle
fibers in the more peripheral regions of the body. These are purely

NERVOUS SYSTEM

879

Fig. 492.

Transverse section through
the body of a typical Vertebrate,
showing the peripheral (seg-
mental) nervous apparatus.
Small dots, afferent visceral
neurones ; coarse dots, afferent
somatic neurones ; dashes, ef-
ferent visceral (ventral root
and sympathetic) neurones ;
lines, efferent somatic neurones.
Darm, gut ; Ggl. spin., spinal
ganglion ; Ggl. vert., vertebral
sympathetic ganglion ; Ggl. mes-
ent., mesenteric sympathetic
ganglion. The peripheral sym-
pathetic ganglionic plexuses
(Auerbach and Meissner) are
not shown. Muse., muscle ; Rod.
dors., dorsal root ; Rad. vent.,
ventral root ; R. comm., whit«
ramus communicans. Two sym-
pathetic neurones are repre-
sented as intercalated in the
visceral efferent pathway. It is
doubtful if there should be more
than one. (After Froriep.)

sympathetic fibers, and, as they are non-medul-
lated, they are gray in color, so that the trunk
carrying these gray fibers from the chain
ganglion to the dorsal and ventral branches is
called the gray ramus.

It is important for future work to know
that some of the cells from the spinal cord or
spinal ganglia of the dorsal root migrate to
various parts of the body, and are usually quite
closely associated with the glands of internal
secretion, such as the hypophysis, carotid
gland, suprarenals, etc. These cells have a
peculiar affinity for chromic acid salts, and are
therefore often called chromaffine cells, though
their function is yet unknown.

THE CRANIAL NERVES

In the early part of our work, we found
that the frog has ten cranial nerves, and that
mammals have twelve. The cranial nerves
differ from the spinal in not being truly seg-
mental nerves (Fig. 493), and in some of them
being purely sensory, some purely motor, and
still others of a mixed nature. Besides, the
cranial nerves which carry sensory fibers all
have a ganglion near the root, while those of
a pure motor nature do not have.

They are like the spinal nerves in so far
as some of them have somatic sensory, somatic motor, visceral sensory,
and visceral motor fibers, but are unlike the spinal nerves in that two
additional components occur in connection with the cranial nerves.
These are the nerves of special sense, and in the fishes, the nerves of the
lateral line (Figs. 340, 479).

The somatic sensory nerves in the head are called general cutaneous
nerves. They terminate in the skin either as free nerve ends or as special
sense organs of touch. The visceral sensory fibers end in taste organs,
usually inside the mouth, but in some of the teleostomes they are dis-
tributed over the surface of the body. The termination of the elements
going to make up the lateral line arrangements are sensory, and termi-
nate in little collections of sense cells commonly called "hillocks" or
neuromasts in the ear and in the lateral line organs of certain groups
of fishes and amphibians.

The lateral line organs have been thoroughly discussed in the em-
bryology of the frog.

880

COMPARATIVE ANATOMY

vspma. nerve

deep ophthalmic

maxillary '
mant^bular

gill slits

pretrematic branch
posttrematic branch

Fig. 493.

Diagram to illustrate the segmentation of the vertebrate head and the relation
of the cranial nerves to the segmentation. The numbers above the figure designate
the cranial nerves ; the numbers in the figure are situated on the head myo£omes ;
the sensory part of the nerves is represented by heavy continuous lines ; the motor
part by broken lines. The anterior head cavity is the first myotome and therefore
the myotome which is numbered 1 is really the second myotome, and so on; But as
the myotomes were numbered before the anterior head cavity was discovered, the old
numbers are generally retained. The myotomes numbered 1, 2, and 3 produce the eye
muscle ; those numbered 4, 5, and 6 degenerate in the majority of vertebrates ; those
from 7 on probably contribute to the tongue musculature but never from typical
parietal muscles, such as occur in the trunk. It is seen from the figure that the third
cranial nerve and, the deep ophthalmic branch of the fifth belong to the first (really
second) head segment; the fourth and remainder of the fifth to the second (third)
segment; the sixth and seventh to the third (fourth) segment ; the ninth to the
fourth (fifth) segment; and the tenth to the fifth to eighth (sixth to ninth) segments.
The gill slits are intersegmental in location. The relation of the cranial nerves to
the gill slits should also be noted. (From Hyman after Goodrich.)

From their functions the cranial nerves may be divided into four
groups :

I. Nerves of special sense, namely, the olfactory and optic. These
arise in the primitive fore-brain, the olfactory passing to the nose and
the optic to the eye.

II. Nerves of the eye muscles (Fig. 494), namely, the oculomotor,
trochlear, and abducens. These are of the somatic motor type, with a
few visceral motor and sensory fibers in the oculomotor. They control
the muscles of the eye. It is well to mention the hypoglossus at this
point, as it belongs to the purely somatic motor group, the spinal acces-
sory being of the visceral motor type.

III. The acustico-lateralis system, namely, the acoustic nerve, and
those portions of the seventh, ninth, and tenth cranial nerves which are
connected with the sense organs of the lateral line in the gill-breathing
fish and amphibians. These nerves have a separate center in the upper
anterior end of the medulla oblongata.

IV. The fifth, seventh, ninth and tenth cranial nerves are more like
the spinal nerves than any of the preceding. They all arise in the me-
dulla, each having a dorsal root and a ganglion, and each containing
somatic sensory (general cutaneous) and visceral sensory and visceral
motor fibers. The seventh and tenth may also include some of the fibers
running to the lateral line. The ninth nerve is the simplest of these, and
arises from the dorsal side of the medulla, dividing just behind its
ganglion into two branches, one, the pre-trematic (Fig. 482), which

NERVOUS SYSTEM

881

passes in front of the gill cleft, and the other, the post-trematic, which
passes behind that opening. The seventh nerve is arranged in a manner
quite similar to that of the ninth, the spiracle being a reduced gill cleft.
The fifth nerve, which is also divided in the same way, has its post-
trematic, called the maxillary, and the pre-trematic branch, the mandibu-
lar, the mouth forming the opening about which this nerve divides. The

Fifir. 494.

Diagram showing: cranial nerves of a cat with the lower
jaw reflected. Il-XII, cranial nerves ; ct., chorda tympani ; d,
dentary nerve ; g, Gasserian ganglion ; to., infraorbital nerve ;
I, lingual nerve ; li.la., laryngeus inferior and superior ; md,
mandibularis nerve ; nix., maxillaris nerve ; o., ophthalmic nerve ;
£., tongue. (From Kingsley after Mivart).

tenth or vagus nerve supplies all of the remaining gill clefts and is there-
fore supposed to be complex, composed of as many nerves as there are
clefts behind the first; however, there is no embryological evidence of
distinct roots and ganglia.

The cranial nerves are of considerable importance in all physiologi-
cal, neurological and pathological work, so it is important that they be
thoroughly studied.

I. The Olfactory Nerve connects the olfactory lobe of the brain
with the sensory epithelium of the nose. It is different from all other
cranial nerves in that it consists of many tiny prolongations of the
sensory cells themselves, and in having no ganglion separate from these
cells. The true olfactory nerve consists of these tiny fibers or threads,
extending from the olfactory epithelium in the nose to the so-called
mitral cells in the olfactory lobe. The places where the dendrites of
the mitral cells meet with the terminations of the olfactory fibers from
the olfactory epithelium are known as glomeruli. In the dogfish, for
example, and in many fish, snakes, lizards, and mammals, the true olfac-

882

COMPARATIVE ANATOMY

tory nerve is very short, but the olfactory lobe is drawn out (Fig. 478),
while the distal end of the lobe is enlarged into an olfactory bulb con-
taining the glomeruli, extending proximally from this slender olfactory
tract. In these cases, the olfactory bulb lies very close to the olfactory
epithelium.

In some fishes, amphibia, lizards, and turtles, the nerve is long,
while the olfactory lobe is shortened.

It has recently also been found
that in all vertebrates there is a termi-
nal nerve, sometimes called the pre-
optic nerve, which leaves the brain near
the base of the olfactory nerve. It has
a ganglion upon it but its functions and
connections are as yet unknown.

II. The Optic Nerve. This arises
in the eye and extends to the floor of
the diencephalon. Branches from it
are distributed over the entire inner
surface of the retina, the ganglion cells
lying in the inner layer. A fiber from
the right eye passes to the left side of
the brain, and those from the left eye
to the right side of the brain (Fig. 495).
The crossing of the fibers forms what
is called the optic chiasma. The fibers,
after crossing, extend dorsally and
backward into the optic lobe. The optic
chiasma is imbedded in the brain of
cyclostomes. In other vertebrates it
may be plainly seen from the outside.
In mammals the crossing in the
chiasma is incomplete, some of the
fibers not crossing. A clear understanding of the growth and develop-
ment of both olfactory and optic nerves can only be had from an under-
standing of their embryological development.

There is also a small thalamic nerve which arises between the
diencephalon and the mesencephalon. This has so far, however, been
seen only in some embryonic fishes. It shortly disappears, and little is
known of its function.

The third, fourth, and sixth nerves are the oculo-motor, trochlear,
and abducens nerves, or the eye-muscle-nerves, all of which assist in
moving the eye in its socket (Fig. 494). The oculo-motor nerve arises
from the ventral surface of the midbrain. It supplies the superior, mid-
<ile and inferior rectus, and the inferior oblique muscles (Fig. 466). The
abducens arises from the inferior surface of the medulla and supplies the

Fig. 495.

Diagram of the central connections of
the optic nerve and optic tract to show
crossing of the fibers. (From Cunningham's
Anatomy. )

NERVOUS SYSTEM »83

lateral rectus muscle. The abducens is often united with the fifth nerve
close to its origin. In some very few forms it is absent.

An understanding of how these nerves come to supply the muscles
they do in the way that they do can be had only from our knowledge
of the manner in which these muscles originally develop. The eye mus-
cles arise from three myotomes (Fig. 493). One of the somites forms
three of the rectus muscles as well as the inferior oblique muscle, while
the other two somites form only one muscle each. As the myotome
therefore divides into four parts, the oculo-motor nerve is carried into
each part.

The trochlear nerve is the only motor nerve that leaves the dorsal
surface of the central nervous system. It has another peculiarity, in that
it has a chiasma in the dorsal surface of the midbrain, from which its
fibers extend downward to the ganglion cells of the floor of the mid-
brain.

The nerves of the eye muscles are the only somatic motor nerves in
the head of the lower vertebrates.

V, or Trigeminal Nerve. The trigeminal nerve is one of the largest
of the cranial nerves, and arises from the antero-lateral angle of the
myelencephalon. The trigeminal nerve bears a large semilunar or
Gasserian ganglion (Fig. 494), near its origin. This ganglion may be
either just inside or just outside the skull. In all higher vertebrates the
nerve divides into three main branches, known as the ophthalmic, maxil-
lary, and mandibular branches (Fig. 494). In the lower vertebrates the
maxillary and mandibular nerves may be united for some distance: In
the fishes, the ophthalmic nerve branches, forming a superficial and a
profound ophthalmic nerve, both of which are purely sensory. In the
higher vertebrates, it is the profound ophthalmic nerve which alone
persists.

The superficial ophthalmic is distributed to the skin, the top of the
head, and the tip of the snout. The profound ophthalmic passes between
the eye muscles to send branches to the eyelids and conjunctiva, and
then extends to the mucous membrane of the nose. It is connected with
the ciliary ganglion to which sympathetic fibers also run. Nerves from
this ganglion are sent to the iris and to the ciliary muscles of the eye.
The ciliary ganglion controls the smooth muscles of the eye.

The superficial maxillary nerve passes along the margin of the upper
jaw, and supplies the face and the teeth. If, however, the profound
ophthalmic is 'reduced, the maxillary nerve extends into the region which
the profound ophthalmic usually supplies. In the higher vertebrates,
the superior maxillary unites with the spheno-palatine ganglion, to which
sympathetic fibers also pass.

The mandibular nerve goes to the lower jaw, passing on the outer
side of. Meckel's cartilage (Fig. 494). In all higher forms, where the
jaws become ossified, the nerve lies within the bone. This nerve carries

884 COMPARATIVE ANATOMY

the motor fibers which innervate the muscles of the jaw as well as the
sensory fibers supplying the lips and teeth. In the mammals it also
supplies part of the face, and in some of the reptiles a branch called the
lingual nerve is sent into the tongue. In mammals the mandibular nerve
is connected with both an otic and a superior maxillary ganglion, while
fibers from the sympathetic system also connect.

It is now thought that the ophthalmic branches of the trigeminal
really form a distinct nerve, and are not parts of the trigeminus at all.
The reason for this belief is that the trigeminal proper arises in the
medulla, while certain fibers of the ophthalmic seem to arise in the
midbrain.

The trigeminus nerve may also send a twig backward. This is
called the recurrent, or lateralis, of the fifth nerve. This recurrent nerve
from the fifth is only found in teleosts and passes to the dorsal side of
the jaw near the dorsal fin, or, in a few fishes, it goes into the paired fins.

VII. The Facial Nerve. This arises just in front of the ear, in the
medulla. In all higher forms than those which have lateral line organs,
there is a single geniculate ganglion which is closely associated with the
semilunar (Gasserian) ganglion of the fifth in the fishes and the amphib-
ians. The facial nerve gives off a palatine branch running to me root
of the mouth, where it divides into a pre-trematic nerve, as already
stated, which enters the lower jaw, and a post-trematic branch called
the hyoid nerve. The hyoid in man is called the facial nerve, and in all
the higher vertebrates a small twig Called the chorda tympani (Fig. 494)
leaves the hyoid and unites with the mandibular branch of the fifth
nerve, after which its fibers enter the lingual branch of the fifth nerve,
and go to the taste organs at the tip of the tongue. The remaining por-
tion of the facial nerve is almost entirely motor in function, its fibers
controlling the muscles of the neck and the muscles which open the
mouth. In mammals that have a large facial musculature the facial
nerve is distributed to more regions of the face.

In animals having organs of the lateral line (Fig. 479), the seventh
nerve has an additional ganglion immediately beyond which it divi4es
into three branches, namely, the superficial ophthalmic, which usually
unites with the superficial ophthalmic of the fifth cranial nerve and then
supplies the lateral line organs and related structures on the top of the
head dorsal to the eyes. The buccal nerve, which supplies the organs
below the eye and along the line of the upper jaw, and the external
mandibular nerve, complete the three branches. The latter is connected
to the lateral line organs, of the operculum when this is present, or with
the lower jaw.

The buccal nerve is usually quite closely associated with the maxil-
lary nerve, while the external mandibular is closely associated with the
mandibular ramus of the fifth nerve.

In all higher vertebrates, where no lateral line organs appear, there

NERVOUS SYSTEM 885

is no trace of these nerves, even in the embryo, although in the frog and
other amphibia which pass through a gilled larval stage, these nerves are
outlined in the development, but are lost during metamorphosis.

VIII. Acusticus (Auditory) Nerve. This nerve is quite closely
related to the facial (Fig. 494), the two ganglia being fused, although
the roots are quite distinct. The auditory nerve is entirely sensory, and
has two branches, the cochlear and the vestibular. Both of these are
distributed to the sensory structures of the inner ear. Because of the
peculiar relations of this nerve to the ear, it is often assumed that, in
higher vertebrates, this is the remains of the original lateral line system.

IX. Glossopharyng-eal Nerve. This is the first of the cranial nerves
lying posterior to the ear. It arises from the medulla close to the tenth
nerve, and in amphibia, both its roots and its ganglion, called the petro-
sal ganglion, fuse with those of the vagus. In gilled vertebrates the
nerve goes to the second gill cleft, there dividing into a pre-trematic
and a post-trematic branch. The pre-trematic passes to the region of the
hyoid arch as well as to the oral cavity. In teleosts it also passes to
the pseudobranch. The larger of the two branches, the post-trematic
supplies the muscles of the first cleft and sends a single branch to the
taste organs, called the lingual branch. In amniotes, the post-trematic
branch is called the pharyngeal nerve, and while there are no gill clefts,
the distribution is practically the same as though there were.

There may be a dorsal nerve given off from the glossopharyngeal
near the petrosal ganglion, which is somatic sensory in function, and
supplies the skin on the upper side of the head. This is quite similar to
the auricular nerve in mammals. In fishes and amphibia the glosso-
pharyngeal has a connection with the fifth nerve ; the connection is
called Jacobson's commissure.

X. Vagus or Pneumogastric Nerve. This is made up of several
metameric nerves. In comparative anatomy it is important to remem-
ber that in fish and amphibia this nerve differs considerably from that
of the higher forms.

In all gilled vertebrates the vagus arises by a number of tiny roots,
while it has two closely associated ganglia, the anterior being called the
lateralis, and the posterior being called the jugular. The jugular
ganglion contains both somatic and visceral sensory cells. It is from
the jugular ganglion that a branchio-intestinal nerve arises, which sends
branches to each gill cleft behind the first. In the dogfish there is an
epibranchial ganglion on each of these branchial nerves. In the higher
fishes these ganglia are fused in the main trunk. Beyond the ganglion,
each branchial nerve divides into a pre-trematic and a post-trematic
branch, just as does the ninth. Beyond the last cleft, the nerve-trunk
continues as the intestinal nerve, going to the heart, stomach, and air
bladder when that is present. It is for this reason that the nerve is called
pneumogastric in human anatomy. The lateralis part of the nerve fol-

886 COMPARATIVE ANATOMY

lows the lateral line organs of the body back to the tail sometimes close
underneath the skin and sometimes close to the vertebral column.

The lateralis part of the vagus nerve naturally only occurs in gilled
animals or those which, like the frog, have gills in their larval stage. In
these latter cases the lateral part of the vagus disappears in metamor-
phosis.

There is a nodose (knot-like) ganglion lying on the intestinal nerve.

In animals possessing lungs, the musculature of the lungs is also
supplied by the vagus. The dorsal ramus of the lower fishes is returned
as the auricular nerve, and this unites with the auricular branch of the
ninth nerve, while sympathetic connections are made at various points
with the vagus.

It is also important to remember that the nerve supply of an organ
is considered the best test of its homology, well shown in the instance
of the vagus nerve, which arises in the head region but is, nevertheless,
distributed to the heart, stomach, and lungs, it being recalled, from our
study of embryology that the heart, stomach, and lungs, originally form
in the head region of the. embryo.

XI. The Spinal Accessory Nerve. The e1eventh and twelfth cranial
nerves occur only in the amniotes, although in the dogfish there are
twigs from the vagus which innervate the trapezius muscle, while simi-
lar conditions are found in amphibia. In the dogfish and the frog, the
brain-center for the eleventh nerve is in the medulla oblongata, while
in amniotes the center for this nerve lies both in the medulla and in the
spinal cord'; in fact, some of the more posterior rootlets that go to form
the spinal accessory nerve leave the cord as far down as the seventh
cervical nerve. These rootlets unite into a trunk that runs cephalad
into the cranium between the dorsal and ventral roots of the spinal nerve
and then leaves the skull close to the vagus.

The spinal accessory is motor in function, and supplies the trapezius
and sternocleidomastoideus muscles which move the shoulder girdle.

XII. The Hypoglossal Nerve. This is also a purely motor nerve,
although during embryological development, ganglia do appear on the
dorsal roots in some mammals, but all these disappear again. The root-
lets of the nerve are only two or three in number, although there may
be more. These unite to form the hypoglossal nerve. The hypoglossal
nerve and the more anterior nerves then unite to form the cervical
plexus. It is from this plexus that the main trunk goes to the hypo-
glossal muscles as well as to the retractors of the tongue, and, in birds,
to the syrinx.

Small occipital nerves leave the skull immediately behind the vagus
in many fishes. They pass backward and dorsal to the gill clefts and
go forward to supply the muscles at the posterior portion of the head,
and the muscles of the pectoral fin. It may be that these occipital nerves
are homologues of the amniote hypoglossus.

PRONOUNCING - INDEX - GLOSSARY

TABLE OF PREFIXES AND SUFFIXES
(To be Memorized)

The object of this Index-Glossary is not to furnish a detailed explan-
ation. This must be sought in the text on the page assigned. The
object is to give the student such knowledge of the technical words used
in Biology as will enable him to take apart the words he finds in his
scientific reading and analyze them. Therefore, he must learn all of the
immediately following prefixes and suffixes :

A or an (G. prefix, without) e.g. apoda,
i.e. without feet.

Ab (L. prefix, away from) e.g. aboral,
i.e. away from the mouth.

Ad (L. prefix, toward, upon) e.g. adrenal,
i.e. upon the renal gland.

ae plural-ending for Latin singular nouns
ending in A.

Ambi (L. prefix, both) e.g. ambidextrous,
i.e. ability to use both hands.

Amphi (G. prefix, on both sides) e.g. am-
phibia, i.e. to live on land and in
water.

Ante (L. prefix, before in place or time)
e.g. antebrachium, i.e. placed before
the arm.

Anti (G. prefix, opposite, or opposed to)
e.g. antitoxin, i.e. opposing or neu-
tralizing a toxin.

Arch (G. prefix, chief or early) e.g. arch-
enteron, i.e. the earliest enteron or
digestive tract.

Auto (G. prefix, self) e.g. auto-intoxica-
tion, i.e. poisoning produced within
one's own body.

Bi (L. prefix, double) e.g. bilateral, i.e.
same on both sides.

Blast (G. either prefix or suffix, a sprout,
or bud), e.g. blastoderm, and neuro-
blast, i.e. a primitive germ-layer and
a primitive nerve cell.

Brevis (L. short) e.g. adductor brevis,
i.e. the short adductor.

Caudad (L. tail) used only in an ad-
verbial sense, as growing caudad, i.e.
tailward, or toward the tail.

Cephalad (G. head) used only in an ad-
verbial sense, as growing toward the
head.

Chondro (G. gristle or cartilage) e.g.
chondrocranium, i.e. that part of the
cranium developing from cartilage.

Circum (L. prefix, round-about) e.g. cir-

cumoesophageal, i.e. running around

the oesophagus.
Cleido (L. clavicle — key) e.g. sterno-

cleido-mastoid muscle, i.e. the muscle

attached to the sternum, clavicle and

mastoid bones.
De (L. prefix, off) e.g. degenerate, i.e. to

become inferior — to lose generative

ability.
Di (G. prefix, twice) e.g. diploblastic, i.e.

to remain in the two-germ-layer state.
Dorsad (L. back) used only in an ad-
verbial sense, as toward the back.
Ecto (G. prefix, outside) e.g. ectoderm,

i.e. the germ-layer lying toward the

outside.
En (G. prefix, within), e.g. encephalon, i.e>

brain — within the cephalon or head.
Endo (G. prefix, within), e.g. endoderm,

i.e. the germ-layer lying toward the

inside.

Ento — Same as Endo.
Epi (G. prefix, upon) e.g. epinephros, I.e.

same as adrenal, namely, lying upon

the nephridic organ.
Ex (G. prefix, without or outside) e.g.

exoskeleton, i.e. having a skeleton on

the outside.

form (L. suffix, shape) e.g. fusiform, i.e.
shaped like a spindle.

Genetic (G. to produce) e.g. pathogenetie,

i.e. to produce disease.
Ilemi (G. prefix, half) e.g. hemisphere,

i.e. half a sphere.
Hyper (G. prefix, above or beyond) e.g.

hypertrophy, i.e. an overgrowth.
Hypo (G. prefix, under) e.g. hypoglossal,

i.e. under the tongue.
Infra (L. prefix, below) e.g. infraorbital,

i.e. beneath the orbit.

888

INDEX

Inter (L. prefix, between) e.g. intercellu-
lar, i.e. between the cells.

Intra (L. prefix, within) e.g. intracellu-
lar, i.e. within the cell.

Laterad (L. side), used only in an ad-
verbial sense, as "toward a side."

lysin (G. suffix, a loosing or dissolving)
e.g. bacteriolysin, i.e. a substance
which dissolves bacteria.

Macro (G. prefix, large) e.g. macro-
cephalon, i.e. a large head.

Major (L. greater) e.g. pectoralis major,
i.e. the greater of the pectoral mus-
cles.

Mega (G. great) e.g. megaspore, i.e. the
larger of the spores.

Mesiad (G. middle), used only in an ad-
verbial sense, as "to grow mesiad" or
toward the center of the body.

Meso (G. prefix, middle) e.g. mesoderm,
i.e. the middle germ-layer.

Meta (G. prefix, after) e.g. metaphase, i.e.
the phase in mitosis coming after the
prophases.

Micro (G. prefix, small) e.g. micro-organ-
isms, i.e. organisms not seen by the
naked eye.

Minor (L. lesser) e.g. pectoralis minor
muscle, i.e. the lesser pectoral mus-
cle.

Mono (G. prefix, alone) e.g. monogamy,
i.e. marrying but one spouse.

Multi (L. prefix, many) e.g. multicolored,
i.e. many-colored.

Myxo (G. prefix, slime) e.g. myxophyceae,
i.e. slime-algae.

oid (G. suffix), to be added to make an

adjective, e.g. odontoid, i.e. like a

tooth.
Para (G. prefix, beside) e.g. parachordal,

i.e. lying beside the notochord.
Peri (G. prefix, around) e.g. pericardium,

i.e. around the heart.
Poly (G. prefix, many) e.g. polymorphic,

i.e. many-formed.
Post (L. prefix, after) e.g. postbranchial,

i.e. behind the gills.
Pre (L. prefix, before) e.g. pre-oral, i.e.

before the mouth.
Pro (G. prefix, first, or early) e.g. pro-

branchia, i.e. the first gills that form.
Pseudo (G. prefix, false) e.g. pseudopods,

i.e. false feet.

Retro (L. prefix, backward) e.g. retro-
lingual, i.e. backward from the

tongue.
Semi (L. prefix, half) e.g. semicircular,

i.e. half circle.
Sub (L. prefix, under) e.g. submandibular,

i.e. under the mandible.
Supra (L. prefix, above) e.g. supratem-

poral, i.e. above the temporal bone.
Sur (same as supra) e.g. surangulare, i.e.

above the angulare bone.
Tera (G. prefix, monster) e.g. teratology,

i.e. the study of monstrosities.
Tetra (G. prefix, four) e.g. tetrapoda, i.e.

four-footed animals.

Toxic (G. poison) e.g. toxemia, i.e. toxic -f-
haemia, blood-poison.

Uni (L. prefix, one) e.g. uniramous, i.e.
single branch.

Ventrad (L. belly), used only as an adverb
of direction, as "to grow ventrad."

INDEX - GLOSSARY *

In all probability some of the pronunciations, as well as some of the
derivations, will not meet with the approval of those who are specialists
in Latin and Greek, for, often various forms of words have been used to
show the student the varying forms of the same word that he will meet
in scientific literature, rather than the same form throughout. Thus, for
example, meros, thigh, and meros, a segment, have both been translated
as though they were spelled alike. In Greek, the former has a long "e"
and the latter a short "e", which really makes the words totally different.

Then too, in pronunciation, those who have learned and know a
foreign language will always (and rightfully so) pronounce the word as
it is pronounced in that language. This makes any definite pronuncia-
tion impossible, at least to the exclusion of other pronunciations.

Englishmen learn an anglicised Latin pronunciation, while Conti-
nental Europeans and Americans pronounce their "a" as in lark, "e" as
the "a" in lake, and "i" as the "e" in see.

Dictionaries have sometimes used one method and sometimes
another. The European is likely to pronounce "c" in such words as
cephalon as "k", though in America this is not customary, but is some-
times heard.

Not each and every artery, vein, and nerve has been listed separately,
as these appear under the more 'general headings of "Circulatory System"
and "Nervous System," but one of a name has been listed so as to show
the manner of usage of the definitive word. The same word is often
used in different senses. The references cited have been chosen to make
these different meanings clear.

If the page number is in italics, the word indexed is to be looked for
under the illustration on the page assigned.

Lastly, as this book is written solely for the student, we have used
everything which would make matters clearer to him. Therefore,
although generally using a consistent marking for the pronunciation, we
have also brought in a type of marking which he will find in some of the
books, and it is well that he be familiar with it. Such is the case, for
instance, in tiu'ni kay'tah, for tu'ni ka'ta, as it is generally given.

KEY TO PRONUNCIATION.

n — as in fate. 6 — as in hen. o — as in go. ow — as in cow.

ii — as in fat. 6 — as in her. 6 — as in not. u — as in pure,

a — as in far. I — as in pine. 0 — as in form. u — as in nut.

e — as in he. I — as in pin. oi — as in toy. u — as in French u.

KEY TO DERIVATIONS.

Ar. — Arabian. G. — Greek. L. L. — Late Latin. 0. F. — Old French.

A. S— Anglo-Saxon. Hind.— Hindustani. M. D.— Middle Dutch. °- H- G.— Old High

F.— French. Icel.— Icelandic. M. E.— Middle English. p _Portu ^sl™

Gael. — Gaelic. It. — Italian. M. L. — Middle Latin. gp Spanish.

Ger. — German. L. — Latin. N. L. — New Latin. Sw. — Swedish.

* As there is considerable variation in usage when foreign words are pronounced, such pro-
nunciation has been chosen as seemed consistent with the best usage of the language from which
the word was taken, as well as from international usage. Consequently, many words are as yet
not authoritatively defined as to pronunciation and exact derivation.

It is hoped these may be added in a future edition of this book. The author will, therefore,
consider it a favor to receive any and all suggestions which may be of help.

390

INDEX

ABDOMEN, abdo'men (L. abdomen,

belly).

Abdominal cavity 47

Abdominal vein 48

ABDUCENS, abdu'sens (L. abducere,

to draw away from ) 68

ABDUCTOR, abduc'tor (L. abducere,

to draw away from), definition of.. 78

ABIOGENESIS, a"biogen'esis (G. a,
without -\-bios, lit e>-\- genesis, begin-
ning). The production of life from
non-living matter 406

ABOMASUM, aboma'sum (L. ab, with-
out+ omasum, bullock's tripe), defi-
nition of 745

ABORAL, abo'ral L. ab, without + os,

mouth ) 254

Situated at the end opposite the
mouth.

ABSORPTION, absorp'shun (L. a,

from -\-sorbere, to suck up) 30

Condensation of gases on the surface
of solids.

Tissue, definition of 234

ABSTRACT IDEAS, ab'stract Ide'as
(L. ab, from + traho, to draw
away ) , definition of 182

A CANTHOCEPHALA, akanthosef 'ala
(G. akantha, a spine + kephale. the

head) 424, 432

An order of worm-like parasites with
no mouth or alimentary canal, which
attach themselves to their host by
means of a hooked proboscis.

ACANTHOCEPHALUS (singular of
Acanthocephala ) , definition of ..... 306

ACANTHOCHEILONEMA PER-
8TAN8, akan'thokilo'nema perstans
(G. akantha, spine -f- cheilos, Lip. -f-
L. perstare, persist) 303

ACANTHOPTERYGII, . . akan"thopte-
rij'ii (G. akantha, a spine -f- pteros,

wing or fin ) 645

A group of fishes, including the
perch, bass, swordfish, etc.

ACARINA, akari'na (G. akari, a kind

of mite bred in wax) 350, 422

An order of Arachnida, including the
mites, ticks, etc., the head, thorax,
and abdomen appearing to be one.

ACCESSORY CHROMOSOMES, akses'-
ory krd'mosomes, definition of. 100, 168

ACCESSORY CLEAVAGE, definition
°f -.448

ACCESSORY REPRODUCTIVE OR-
GANS 439

ACCOMMODATION OF EYE, defini-
tion of 874

ACETABULUM, asetab'ulum (L. ace-
tabulum, a vinegar cup ) . the socket
for the head of the femur 76

ACETABULUM, homologue of 704

ACETABULUM OF WORMS 292

ACHILLES, tendon of 80, 85

ACICULAE, asik'ula (L. acus, a nee-
dle*) , definition of'. 283

ACID GLAND 363

ACINOUS GLAND, as'inos (L. acinus,
a grape) 673

ACIPEN8ER STURIO, asipen'ser
stu'rio (G. akkipesios, the sturgeon.
( See sturgeon ) 6Jj3

A C 0 E L 0 M A T A, fise'lomata (G. a
without -f- koilos, a hollow tube ) ,
definition of 258

ACONITE, ako'nit (L. aconitum, the
poisonous plant Wolf's-bane) . The
Wolfs bane 237

ACOUSTIC, akoo'stic (G. akoustikon,

pertaining to hearing) 885

Pertaining to sound.

ACQUIRED CHARACTERISTICS ...403
Characters produced during the life-
time of an individual.

ACRIDIIDAE, akridi'ide (G. akris, a
locust), definition of 351

ACRODONT, ak'rodont (G. akros, at
the edge + odont, tooth), definition
of 733

ACROMION, akro'mion (G. akromia,

the point of the shoulder blade) . . . .701
• The distal end of the spine of the
scapula or shoulder blade.

ACTINIARIA, aktinia'ria (G. actin, a

ray) .257, 420

A division of Actinozoa, including
the sea-anemones; approximately the
Malacodermata.

ACTINOMMA, Aktinom'a, (a ray +
omma, the eye), definition of 148

AGTINOMYCE8 BOVIS, aktinomisez
bo'vis (G. actin, a ray + mykes, a
mushroom; L. Bos, bovis, an ox),
definition of 214

ACTINOPHRY8, aktinof'rfs (G. actin,
a ray + ophrys, brow), definition of

INDEX

891

ACTIVATOR, fik'ttvator (L. ago, agere,..
to drive). From active — producing

action 823

ACTIVE POLE, definition of 105

ACUTIFOLIUM, acutifol'ium (L. acuo,

sharpen -f- folium, leaves) 216

A species of Sphagnum.
ADAPTATION, adapta'shun (L. adap-
tare, to fit to). The adjustment to

circumstances 45, 316

ADAPTIVE SPECIALIZATION 716

ADELOCHORDA, adelokor'da (G.
adelos, not evident + chorde, a

cord ) , definition of . . '. 639

ADENOID, Ad'enoid (G. aden, gland

-f- eidos, shape), definition of Ill

ADEQUAL, ade'qual (L. ad, to +

aequus, equal ) 447

Pertaining to the addition of sim-
ilar forms.

ADHERENCE THEORY, adhe'rens the'-
ory (L. ad, to -f naerere, to stick),

definition of 122

ADIPOSE, ad'ipos (L. adeps, fat),

definition of Ill

ADOLESCENCE, adoles'ens (L. ad

ulescens, young), definition of 620

ADORAL, ado'ral (L. ad, to -f os, oris,

mouth ) 155

Situated at or near the mouth.
ADRENAL BODIES, addree'nal (L.

ad, upon ; renes, kidneys ) 53

ADRENALIN, addren'al'yn 53

A crystalline substance obtained
from suprarenal extract.
ADSORPTION, adsorp'shon (L. ad, to

+ sorptio, sucking up)

ADVEHENT, ad'vehent (L. ad, to +

vehens, carrying) 601, 798

AEPYORNITHIFORMES, epiorntth'i-
formez (G. aipys, high, -f- ornis,

bird + forma, form) 425

AERATED, a'erat (G. aer, air) 321

To expose to the air.
AEROBIC, aero'bik (G. aer, air +

fiio.9, life)

AESTIVATION, estiva'shon (L. aes-

tivare, to pass the summer ) 262

AESTIVO-AUTUMNAL, este'voatum'-
nal (L.- aestivus, summery -f- autum-

nalis, belonging to autumn) 133

AFFECTIONS, affekshons (L. affectio,

a state of mind), definition of 174

AFFECTOR, aff'ekter (L. afficere, to

act upon), definition of 177

AFFERENT, aff'errent (L. ad, to; fero,
bear )

AGASSIZ, a'gasee', Jean Louis Ro-
dolphe 380, 415

AGGLUTINATION, aglutina'shon (L.
agglutinare, paste to), definition of.200

AGLOSSA, aglo'sa (G. aglossos,

tongueless ) , definition of 650

AIR-SACS OF INSECTS 329, 359

ALAE TEMPORALES, aletempora'les
(L. ala, the wing, pi. alae + tem-

pora, temples ) 687

ALBERTUS MAGNUS 377

ALCYONACEA, al'sldna'sea 420

An order of Alcyonaria.
ALCYONARIA, al"siona'ria ( G.

alky on, a kingfisher) 256, 420

ALEXIN, alek'sm (G. alexein, to ward

off ) , definition of : ... 199

ALGAE, al'je (L. alga, seaweed; pi.

algae) , definition of 203

ALIMENTARY CANAL, alimen'tary
kanal' (L. alimentarius, pertaining

to food; canalis, channel) 267

A LI 8 M A PL A A7 TA GO A Q UA TIC A ,
alis'ma planta'goakwat'ika (G.
alisma, a water plant + L. plantago,
plantain -f- aquations, pertaining to

water ) 245

ALISPHENOID, alisfe'noid (L. ala,
wing + G. sphenoeides, wedge-
shaped) 695

ALKALINE GLAND, al'kalin or Hn

(F. alcalin) 361

ALLANTOIC VESICLE, alanto'ik ves-
ik'l (see allantois -f- L. vesicula, lit-
tle blister ) , definition of 488

ALLANTOIDA, alantoi'da (G. allan-

toeides, sausage shaped) 663

ALLANTOIS, alan'tois (G. alias, sau-
sage; eidos, form) 65

ALLIGATOR, brain of, al'ligater (Sp.
El lagarto, lizard; L. Lacertus,

lizard) 862

ALLOGROMIA, alogro'mea 148

An order of the Foraminifera.
ALLOLOBOPHORA FOETIDA, alolo-
bof'ora feti'da (G. allos, other +
lolos, pool + phora, fruit crop, +
L. foetidus, stinking), definition of. 281
ALTERNATION OF GENERATIONS

IN HYDRA 251

ALTERNATION OF GENERATIONS

IN INSECTS 346

ALTERNATIONS OF GENERATIONS

IN PLANTS 225

ALVEOLI OF LUNG, alvee'ohlie (L.

a little cavity ) ! : . . . . 63

ALVEOLUS OF TOOTH, alve'olus (L.
alveolus, a small hollow), definition

of 733

ALYTES OBSTETRICANS, al'itez
(G. alytos, continuous, -f o"bstetri-

care, to be a midwife) 650

AMARYLLIS, amaril'is (G. amarys-
sein, to sparkle, or L. + G. amaryc-
lis, name of shepherdess ) 24$

892

INDEX

AMBLYCORYPHA OB LONGI FOLIA,
amblikor'ifa (katydid), (G. amblys,
blunt + koryphe, head, top, + L.
oblongus, rather long + folium (pi.
folia), a leaf 342

AMBLYS TOM A, amblis'tomti (G.
amblys, blunt + stoma, mouth) 55,9

AMBLYSTOMA TIGRINUM, tigre'num
(G. ambly stoma + L. tigrinus, tiger
like) 559, 61,8

AMBOCEPTOR, ambosep'tdr (L. ambo,
both + (re-)ceptor, a receiver),
definition of 199

AMEWRUS, fimiii'rus (G. A, without
+ meiouros, curtailed) 64 4

AMETABOLOUS, ametab'dlus (G.
ametabolos, unchangeable) 331

AMI A CALVA, am'ia (G. amia, a kind
of tunny) . . 643

AMITOSIS, amito'siz (G. a, without

-f- mitosis)

Direct cell-division.

AMMOCOETES, amose'tez (G. ammos,

sand + koite, bed) 461, 543, 578

The larvae of the Lamprey.

AMNIO-CARDIAC VESICLES, amnto-
kar'diak, definition of 469

AMNION OF INSECT, am'neeon (G. a
membrane of the embryo), definition
of 346

AMNIOTA ( AMNIOTES ) , amnto'ta
(see amnion), definition of 663

AMNIOTIC CAVITY, amniot'ik (G.
amnion, membrane around the fetus) 623

AMNIOTIC CAVITY, false, definition
of 623

AMOEBA, fime'ba (G. amoibe, change) 121

AMPHIBIA, amfib'eeah (G. amphi,
double; bios, life) ... 43, 425, 431
Classification of 646

AMPHIBIA—

Muscles of 830

Skull of 694

AMPHICOELOUS, amfeesee'lous (G.
amphi, double + koilos, hollow),
definition of 683

AMPHICYON, amfis'ion (G. amphi +
kuon, dog) 73g

AMPHINEURA, amfinu'ra (G. amphi,
around + neuron, sinew, nerve) .421, 429

AMPHIOXUS, cleavage of, am'flox'us
(G. amphi, both; oxys, sharp) W

AMPHIPLATYAN, am'feepla'teean
(G. amphi, both; platys, gat), defini-
tion of 683

AMPHIPOD, am'flpod (G. amphi +
pous (pod) foot) 316

AAfPHISBAENIDAE, amftsbe'nfde (G.
amphisbaina, serpent, believed to

move with either end first) 417

The family of snake-like lizards.

AMPHISTYLIC, fimfistl'llk (G. amphi
+ stulos, pillar), definition of 698

AMPHIUMA MEANS, amfiu'ma (G.
amphi, on both sides + pneuma,
breath) 648

AMPHIUMIDAE, amflu'mide, defini-
tion of . (See amphiuma) 647

AMPULLA, ampull'ah (L. flask), defi-
nition of 867

AMYGDALAE, amig'dale (L. amyg-
dala, almond), definition of 849

AMYLOPSIN, amylop'sin (G. amylin,
starch + pepsis, cooking) 50

AN ABAS SOANDENS, an'abas (climb-
ing perch) ; (G. anabainein, to go
up) 645

ANACANTHINI, an'akanthl'm (G.
ana'kanthos, spineless ) 644

ANAEMIA, ane'miti (G. anaimia, want
of blood), definition of 133, 283

ANAEROBIC, anaero'bik (G. an=
without + aer = air + bios = life) ,
definition of 189

ANAL GLANDS, a'nal (L. anus =
ring + gland), definition of 673

ANALLANTOIDA, analantoi'da (G.
an, without + allantois), definition
of 663

ANALOGOUS, anal'ogus (G. ana, ac-
cording to + logos, a ratio), defini-

. tion of 118

ANAMNIA, anam'neeah (G. an, with-
out; amnion, embryonic membrane),
definition of 663

ANAMNIOTA. (Same as anamnia) . .663

ANAPHASE, an'tifaz (G. ana, up +
phasis, appearance) . .,» ,97, 98

ANAPHYLACTIC SHOCK, anafilak'tik
(G. ana, up + phylaktikos, fit for
preserving), definition of 201

ANAPHYLAXIS, anafilaksis (G. ana,
up + phylax, guard) 201

ANAS BOSCH AS, a'nas bosk'as (L.

anas, duck ) If 01

The mallard.

AN AS A TRITIS (squash-bug), dn'dsd

ANASTOMOSIS, anastomosis (G. an
opening) 1 12, 77//

ANATOMY, ana'tom! (G. ana, up +
tome, cutting ) 30, 31

ANCHYLOSE, ang'klloz (G. angkylo-...
sis, stiffness ) 733

ANCONEUS, an'konee'us (L. ancon,
the bend of the arm) 80, 81

ANDRAE RETZII, gyrus of 844

ANDRE AE A PETROPHILA, andree'a
(after the German botanist, An-
dreas) 215

ANDREAEALES, definition of 216

ANDROECIUM (stamens), andre'-
shium (G. aner, man -f- oikos,
house ) 203

INDEX

893

ANGIOSPERMS, an'jiosperms (G.

anggeion, vessel -f- sperma, seed),

definition of 227

The slow worm of Europe.

ANGLER 645

ANGUILLA GHRYSYPA, angwil'a (L.

eel) 644

ANGUILLULA ACETI, angwil'ula (L.

diminutive of eel ) 306

AN GUIS FRAGILIS, ang'gwis (L.

anguis, snake ) 655

ANGULAR CARTILAGES 617

ANGULI 79

A genus of mollusks.
ANGULO-SPLENIAL, anggulosple'nial

(L. angulus, corner -f- splenium, a

patch) 72

ANIMAL POLE, definition of . . . .105, 439

ANIMAL PSYCHOLOGY 172-184

ANKYLOSTOMA DUODENALE,

ankflos'toma (G. ankulos, crooked -j-

stoma, mouth ) 303

ANKYLOSTOMIASIS, ank'ilostomi'a-

sis 303

The disease caused by Ankylostoma.
ANLAGE, anla'ge (Ger. anliegen, to

lie on) 458

ANNELIDA, anel'ida (L. annulus, ring

-f G. eidos, resemblance) 264, 528

ANNUAL PLANTS, an'fial plants (L.

annus, year -f- planta, a plant) . . . .232
ANNULAR TUBES, anular tubs, (L.

annulus, ring + tubus, water pipe),

definition of 234

ANNUL AT A, anula'ta (G. anulus, a

ring), definition of .264

ANNULI, definition of 264

ANNULUS FINGER, definition of.... 710

ANNULUS OF VIEUSSENS 65

ANNULUS TYMPANICUS, definition

of ......>«., 616

ANOLIS PRINOIPALUS, ano'lis (a

native name in the Antilles ) 655

ANOPHELES, anof'eles (G. anophe-
les, hurtful) 133, 137

ANOPHELES MACULIPENNIS 335

AN8ERIFORMES, an'sertfoVmez (L.

anser, goose + form) 425

The goose-like birds. Aquatic

birds with beaks covered with a soft,

sensitive membrane and edged with

horny lammellae.

ANTAGONISTIC MUSCLES 762, 828

ANTARTICULAR 697

ANT-EATERS 657

ANTEBRACHIUM, nn'tebrakium (L.

ante, before + brachium, arm),

definition of 708

ANTENNA, anten'a (L. antenna, a

sail-yard 3jf3

ANTENNULE, anten'ule (L. dim, from

antenna ) 813

ANTERIOR INTESTINAL PORTAL,
anter'ior (L. anterior, preceding),

definition of 467

ANTERIOR-POSTERIOR DIFFEREN-
TIATION 265

ANTHER, an'ther (G. anthos, flower)
The part of the stamen which
bears the pollen.

ANTHERIDIUM, antheri'dium (plural,
antheridia) ; (G. antheros, flowering
-f- idios, one's own, personal), defi-
nition of 181, 208, 217

ANTHOMEDUSAE, fm'thomedii'sa (G.

anthos, a flower -f- medusa) 419

A typical genus of the family An-
thomedusidae.

ANTHOZOA, anthozo'a (G. anthos,
flower + zoon, an animal) .256, 420, 427

Animal plants. A former class or

large group of zoophytes somewhat

like modern actinozoa.

A NTHROPITHECUS, anthropf th'ecus

(G. anthropos, man + pithekos, one

who plays tricks) 660

Chimpanzee.

ANTHROPOMORPHISM, an'thropo-
mor'fism (G. anthropos, man -4-

morphe, form), definition of 172

ANT I ARC HI, antiar'ki (G. anti,

against + archos, rectum) 646

A group of extinct mailed fishes.
ANTIBODY, an'tibod'i (G. anti,
against + A. S. bodig, body), defi-
nition of 197

ANTIGEN, an'tfjen (G. anti, against

-f qenos, birth), definition of 199

A NTIPA THIDEA , antipath'ide ( G.
antipathes, of opposite feeling or

property) 420

A family of black, sclerobasic

corals, with branched fibrous axis,

and soft friable coenenchyma.

ANTIPODAL CELLS, anti'podal (G.

anti, against -f- pous (podos) foot),

definition of 244

ANTISEPTIC, antlsep'tik (G. anti,
against -f- sepsis, putrefaction).

definition of 192

ANTITOXIN, an'tftok'sin (G. anti,
against 4- toocikon, poison for

arrows ) , definition of 197

ANURA, anew'rah (G. an, without;

oura, tail ) 425, 650

Tailless amphibia.

ANUS OF FROG, ay'nus (L. ring) ... 45
AORTA, aor'ta (G. aorte, to lift) . . .

54, 775

AORTIC ARCHES, aor'tik (G. aorte,

the Great Artery) 5.9

APE ..659

894

INDEX

APHIS MAIDIRADIC1S, afis (doubt-
ful origin ) 372

Plant-sucking insects.

API DAE, ap'ide (L, a bee) 367

The bee family.

APIS MELLIFERA 35 J,

APIS NOSEMA, a'pis (L. a bee) 153

APLACOPUORA, aplakof'ora (G. a,
without -f- plaxma, flat part +

phoros, to bear) 421

A suborder of Amphineura, having
a spiculed, vermiform body, with
foot absent or merely a groove.
APQCYM UM AN.DROSAEMIFOLI UM,

apos'inum (G. apokunon, a plant) . .235
APOD A, ap'oda (G. a, without + pous,

foot) 425, 643, 647

APONEUROSIS, apoh'newrow'sis (G.
apo, from + neuron, tendon ) , defini-
tion of 78, 828

APOTHECIA, apothe'sia (G. apo,

away + theke, cup) 205

APPENDICITIS, apendisi'tes (L. ad,

to + pendere, to hang) 302

APPENDIGULARIA, iipendikiila'ria
(L. ad, to -j- pendere, to hang), defi-
nition of . .639

APPENDICULAR SKELETON, fipfm-
dik'ular (see Appendix), definition

of 73

APPENDIX VERMIFORMIS, apendtks
(L. ad -f- pendere, to hang), defi-
nition of . .' 728, ?'//?'

APPLICATIONS VS. PRINCIPLES. 25, 32
APPOSITION IMAGE, ap'ozish'un (L.
ad, to -f- ponere, to place) , definition

of 322

APTERIUM, ap'te'rium (G. a, without

+ pteron, wing) 069

APTERYGIFORMES, apteri'j if orm'ez

(G. a, + pteron) 425

AQUATIC, akwa'tik (L. aqua, water.).

223, 654

AQUATIC VERTEBRATES 663

AQUEDUCT OF SYLVIUS, ak'weduct

(L. aqua, water; duco, lead) . . .511, 845
AQUEDUCTUS CEREBRI, akweduk'-

tiis 845

AQUEOUS HUMOR, a'kweus (L. aqua,. .

water) 7.0, 873

AQUINAS, THOMAS, akwi'nas 378

ARACHNID A, arak'mda (G. arachne,

spider) 429

ARACHNOID, arack'noid (G. arachne,

spider) 838

A R A CHNOI DEA, firaknoid'ea (G.
arachne, spider -f- eidos, form. 422, 430

Spiders.
ARANEAE, ara'nea (G. arachne,

ppifler) 422

An old genus of spiders.
ARANTII, duct of.. .795

ARBOR]'] AL, arbf/real (L. arbor, tree) 654
ARC ELL A, arsera (N. L. diminutive

of area, little box), definition of. . . .147
ARCHAEN, arke'an (G. archaios, an-
cient) i 395

ARCHAEOPTERYX, arkeop'teriks (G,

archaios + pteryx, wing) 671

Oldest fossil bird, having some
characteristics of a reptile.
A R C II A E ORNITHE8, arkeor'nlthez
(G. archaios, ancient -j- ornis, a

bird) 425

Fossil birds of the genus Arch-
aeopteryx.

ARCHEGONIA, arkego'nia (G. arche,
beginning -f- gonos, offspring), defi-
nition of 215

ARCHEGONIOPHORES, arkegoniof-
orez (G. arche, beginning -f- gonos,
offspring + pherein, bear), defini-
tion of 218

ARCHENTERON, arken'teron (G.
archos, chief, first; enteron, intes-
tine) , definition of 107

ARCHEOLOGY 32

ARCHES, VISCERAL, definition of.. 585
ARCHESPORE, ar'kespor (G. arche,

beginning -(- sporos, seed 239, 243

( Spore-mother-cell. )

ARCHIANNELIDA, arkianel'ida (G.
. archi, first + annulus, a ring) ....

283, 420, 428

A subclass of annelida, supposed
to be the nearest living relatives of
the archetypal segmented worms.

ARCHIMEDES, arkime'des 24, 25

ARCHINEPHRIC DUCT, arkinefrtk
(G. archi, first -f- nephros, the kid-
ney) 207, 208

ARCHIPALLIUM, ar'kipal'ium (G.
archi, first -f- L. pallium, mantle),

definition of 843, 848

ARCIIOPLASMIC, arkoplns'mik (G.
archon, ruler + plasma, something

moulded) , definition of 91

ARCUALIA, ar'kiuale'eeah (L. arcus,

bow) .716

AREA OPACA, a'rea opfi'ka (L. area,

ground space), definition of 439

AREA PELLUCIDA, definition of.... 439
AREA VASCULOSA, definition of.... 467
AREA VITELLINA, definition of .... 467
AREOLAR, are'olar (L. areola,

small space) 110

ART8TOLOCHIA SIPHO, rir'istdlo'kta
(G. aristos, best + locheia, child-
birth) 23t

The common Dutchman's pipe.

ARISTOTLE, fmstot'al 376, 388

ARMADILLO, nrmadil'o (from armed) 659
1RMOURED FISHES, definition of.. 676

INDEX

805

AROMATIC COMPOUNDS (L. aro-

maticus, having a spicy odor) 1)5

ARKOU -WORM 308

ARTERIES OF FROG, fir'terez (L.

arteria, artery ) 53-60

ARTERIES, SOMATIC 790

ARTERIES, SPLANCHNIC 790

ARTERIES, VISCERAL 790

ARTERIOLES, arterio'ls (L. arteri-

ola, small artery), definition of.... 799
ARTHROBR ANCHES, fi r'throbrfm'-
kez (G. arthron, a joint), defini-
tion of 321

A R THRO PL EON A, ilrthrdple'ona ( G.

arthron + pleon, full) 422

An order of the class Collembola.
A R T H R 0 P O D A, firth ro'poda ( G.

arthron, joint + pons, foot)

312, 374, 424

Crayfish 312-327

Fly . . . 367

Grasshopper 332

Honeybee 353

Insects at Large 328

Plankton 325

Terrestrial crustaceans .... 326

ARTICULAR PROCESSES, artik'ulfir

(L. articulus, joint) 681

ARTICULARE, hrtik'iilfirfi 697

ARTICULATA, arttkfllfita 429

ARTICULATE, artik'ulat (L. articu-

litfi. joint) 73

ARTIFACT, ar'tifakt (L. ars, ' art +

facere, to make) ; ,7,75

An artificial product.

ARTIFICIAL FERTILIZATION 205

ARTIODACT YLA, firtiodak'tilfi ( G.
artios, equal + daktylos, finger)..

'. 426, 660

ARYTENOID, fir'ite'noid (G. arytaina,

a pitcher ) 762

A8CARI8 LUMBRWOIDE8, iis'karis

(ft. an intestinal worm) 298

A8CELLUB AQUATICU8, nsel'us (G.

diminutive of askus, a wine skin) . .326
A&CIDIACEA, asidia'sea (G. askidioy.

a wine-skin ) 424, 639

An order of tunicates.
A8COMYCETE8, fts'komTse'tez (G.
askos, bag + mukus, a mushroom) .

207, 209

ASCOSPORE. fis'kospor (G. askos, bag

4- sporos, seed ) 188

ASCUS, as'kfis (G. askos, bladder).

definition of /§#

A S E X IT A L REPRODUCTION IN

VA UCTTKRIA 207

ASTATIC LUNG-FLUKE 291

ASPERA. fis'porA (L. aspera, rough) .149
AKPERGILLU8 FUMIGATU8, Hsper-
jil'us (L. aspergere, sprinkle), defi-
nition of 211 , 212

ASPIDIUM, aspid'ium (G. aspidion, a

little shield ) 22 //

A species of fern.

ASPLDOBRANCHIA, us'pidobrang'kia
(G. aspis, shield + branchia, gills). 421
A group of Gastropods.

ASSIMILATING TISSUES, ashn'ilat-

ing (L. ad, to + similis, like) 236

Conversion into protoplasm of in-
gested nutrient material.

ASSIMILATION 30, 63

ASSOCIATION MEMORY, definition
of 66, 178

AST AC UN FLUVIATALIS, fis'takus
(G. a crayfish) 313

ASTEROIDEA, asteroi'dea (G. aste-

roeides, star-like) 420, 428

An order of Echinoderms, the Star-
fishes.

ASTRAGALUS, asstrag'alus (G. an
ankle bone) ?#, 709

ATAVISM, at'fivlzm (L. atavus, ances-
tor)

Reversion to an ancestral type.
Recurrence of a characteristic of a
remote ancestor.

ATHECAE, athe'ke (G. without a
carapace) , definition of 652

ATLAS (G. atlas, a giant) 72, 684

ATRIAL, a' trial (L. atrium, a central
room ) , definition of 530

ATRIOPORE, a'triopore (L. atrium +

poru^t, channel ) 752

The opening from the atrial cavity
to the exterior in Cephalopods.

ATRIO-VENTRICULAR CANAL, defi-
nition of 530

ATRIUM, ay'treeum (L. a court) —

Of Amphioxus 752

Of heart 773

Of lungs 765

Of nose 871

ATROPHY, at'rofi (G. a, without,

trephein, to nourish) 316

ATTRACTION SPHERE, definition of 90
AUDITORY, aw'ditoery (L. auditorius,

pertaining to hearing) 68

AUDITORY CAPSULES 540

Organs of insect 337

Pits .^480

Placodes 480

Sac 575, 581

Tube 869

AUERBACH'S PLEXUS 87.9

AURICLE, aw'reekal (L. auricula, a

little ear ) 55, 778

AUTOGENOUS, oto'jenus (G. autos,

self -j- (*en,esis. birth), definition of. 140
AUTOMATIC MUSCLES, definition of 112
AUTOSTYLIC, ot'Osti'lik (G. autos,
self + stylos, pillar), definition of. 698

896

INDEX

AUTOTROPH1C, Ot'otrof'ik (G. autos,

self + trephein, to nourish)

AVE8, ay'veez (birds) 425, 431, 656

AVICULARIA, fivikiilfi'ria (G. avicula,

diminutive of bird ) 309

AWARENESS, definition of 174

AXIALITY

The growth along a definite axis.

AXIAL SKELETON, definition of 73

AXIAL THICKENING, definition of.. 538
AXIL, ak'sil (L. axilla, arm-pit) .203, 217
AXILLA, axill'ah (L. axilla, a little

axis ) ' 831

AXILLARY, ak'silari (L. axilla) .203, 792

Pertaining to axil; growing in the
axil, or pertaining to the arm-pit.
AXIOM (L. aucioma, a self-evident

proposition) 88

AXIS, definition of 684

AXIS CYLINDER 114

The central tract of a nerve fiber.

AXIS OF EGG 553

AXOLOTL, ak'solotl 554, 648

A Mexican tailed amphibian which
remains in the larval state.

AXON, ak'son (G. axon, axle) 68

The axis-cylinder process of a
multipolar nerve-cell.
AZYGOUS, az'eegos (G. a, without;
zygon, yoke) 698, 797

BACILLUS, basfl'us (L. bacillum, a

small stick) 190

Rod-shaped bacteria.'

BACKGROUNDS 157

BACON, ROGER 378

BACTERIA, bakte'ria (G. bacterion,

rod ) , definition of 190

rate of increase 192

BACTERIOTROPIN, bfikter'iotro'ptn,
definition of . . 200

BACTERIUM ACTINOCLADOTHRIX,

bakte'rlum ak'tinoklfid'othriks (G.
aktis, ray -j- klados, branch + thrix,
hair) 214

BALAENA MYSTECETUS, bule'na

(L. balaena, whale) 662

BALANCING ORGAN, definition of.. 323

B A L A N 0 GLOSSUS CLAVIGERUS,

bahl'anohgloss'us ( G. balanos,
acorn -f- ylossa, tongue) 546

BALANTIDIUM COLT, balantid'ium
(G. balantidium, a little bag) 144

BALEEN, bnlon' (L. balaena, whale),
definition of . .737

BALEEN WHALES 663

BARBERRY, biir'ber'i (M. L. barbaris,

berry ) 243

A shrub whose ill-smelling flowers

produce red berries of a pleasantly

acid flavor.

BARBS (L. barba, beard) 671

Thread-like structures.
BARBULES, bfir'buls) (L. diminutive

of barba ) 670

BARK, definition of 231

BASAL, ba'sfil, at or near the base. . .218

BASAL DISC 248

BASALE, basfi'le, bfisa/lfi (L. las-is,

base ) 690

BASAL GANGLION, definition of .... 848

BASAL PLATE, definition of 615

BASCANION ANTHONYI STEJNE-

GER, baska'nion .. . , 416

A genus of snakes, such as the

blacksnake.

BASIDIOMYCETES, bfistd'tomlce'tes
(G. basis + mukus, fungus) . .201, 209

BASIDIUM, bnstd'ium (G. basis,

base) 207

A special cell of certain fungi
forming spores by abstriction.

BASI-HYAL, ba'sihl'al (G. basis +
hyoeides, y-shaped ) 738

BASILAR PLATE, bas'ilar, definition
of 540

BASIOCCIPITAL, bfi'siokslp'itfil (L.
basis + occiput, back of head) 73

BASIPODITE, bnsip'odite (G. basis +
pous, foot), definition of 314

BASLE NOMENCLATURA ANATOM-

ICA, bal nomenklatu'rfi 416

BAST (A. S. baest, bast), definition
of 230

BATESON, PROFESSOR 388, 679

BATOIDEA, ba'toi'de'fi (L. batus, a

genus of fishes ) 424, 642

A sub-order of Plagiostomes hav-
ing ventral gill-openings.

BDELLOSTOMA DOMBEYI, de'los'to-
mfi (G. bdella, a leech + stoma,
mouth) ; named for J. Dombey,

French botanist 641

• Cyelostomes having suctorial
mouths resembling those of leeches.

BDELLOSTOMA POLYTREMA (G.

poly, many + trema, hole) 7/3

Myzonts having many pairs of
branchial apertures.

INDEX

897

BEE 353

Behavior 363

Circulatory system 357

Classification of 367

Digestive system 357

Embryology of 362

Enemies of 365

Excretory system 359

External appearance 354

Gynandromorphs 366

Internal anatomy 357

Metamorphosis 362

Moth 365

Muscular system . . . 360

Nervous system . . . : 359

Reproductive system 361

Respiratory system 358

BEHAVIORISTS, definition of 173

BELON 379

BESTIARIES, bes'ti'nr'es (L. bestia-
ries, pertaining to wild beasts) . . . .377

Books treating of animals.
BICEPS, buy'seps (L. bi, two; caput,

head) 81

BICHAT, besha' 388

BICORNUUS UTERUS, bikor'nuus (L.
bis + comu, horn) 819

BICUSPID, blkus'pid (L. bis, twice +

cuspis, point) 1/36

BICUSPID VALVE, definition of 786

BIDDER'S ORGAN 610, 813, 822

BIFARIA, blfa'ria (L. bis, twice +
fari, to speak ) 155

BILATERAL SYMMETRY (L. bis +

latus, side) 247

BILE (L. bills, bile) 50, 750

BILE CAPILLARIES 50

BILE DUCT, development of 589

BILHARZIA HAEMATOBIUM, bil-
har'zea hematobium, definition of. .291

(After T. Bilharz.) A genus of
flukes or trematodes.
BINARY, bl'nary (L. bina^ius, of two),

definition of 125, 120

BIOGENESIS, bi'ojen'esis (G. bios +

life + genesis, birth) 406

BIOLOGICAL UNIT, definition of .... 88
BIOLOGY (G. I'ios, life + logos, dis-
course)—

Chronological table of 389

History of 375-392

Notable men in 386

BIOMETRICS, biomet'riks (G. bios +
mch-on, measure) 32

BIOPLASTS bT'oplfists (G. bios +
plasma, something moulded) 91

BIPARTIBUS UTERUS, bipart'ibus

(L. having two parts) 819

BIRAMOUS, blra'mos (L. bis +

ramus, branch) 315

BIRD, definition of 656

BIRD, circulation of . 785

Skeleton of 103

Urogenital organs of 821

BISERIAL, blse'rial (L. bis -f series,

series), definition of . .707

BISEXUAL, blsek'sual (L. bis +

sexus, sex)

BLACK-MOSSES, definition of 216

BLADDER 48

How formed 590

BLADE 203

BLASTOCOELE, blas'toeseal (G.

blast os, germ + koilos, hollow) .1 06, 446
BLASTOCYST, blas'tosist (G. blastos

+ kystis, bladder) 626

BLASTODERM, blas'toederm (G.

blastor, germ; derma, skin)

438, 439, 449

BLASTODERMIC VESICLE, defini-
tion of 621

BLASTOMERE, blas'tSmer (G. blastos
+ meros, part), definition of.. 105, 449

BLASTOMYCES, blastom'isez (G.
blastos + mukus, fungus) 210

BLASTOMYCETES, blastomise'tez,

definition of 210

Plural of blastomyces.

BLASTOMYCOSIS, blnstomico'sis, defi-
nition of 211

The disease produced by Blastomy-

cetes.
BLASTOPORE, blas'topor (G. blastos,

bud + poros, passage) .107, 278, 452, 554
BLASTOSTYLE, blas'tostil (G. blastos

+ stylos, pillar) ' '252

BLASTULA, blas'tiulah (L. a little

germ) 106, W

Of Amphioxus .44>

Of Chick 447

Of Frog 447, 554

BLATTAEFORMIA, blat'ta'formea (L.

blatta, cockroach ) 423

A subclass of Pterygogenea hav-
ing the form of a cockroach.
BLATTOIDES (L. blatta + oid) , re-
sembling a cockroach 423

An order of Blattaeformia.
BLIND-WORMS, definition of 647

BLOOD CORPUSCLES OF GRASS-
HOPPER . . . 339

898

INDEX

BLOOD-ISLANDS, definition of 467

BLOW-HOLE, definition of 729

B. N. A. (Basle Nomenclatura Ana-

tomica ) 416

BODY-STALK, definition of: 627

BOMBINATOR IGNEUS (M. L. bom-
binare, to buzz + igneus, fiery.) A
species of European frogs 100

BONE, how it grows 679

BONNET 382

BORDERING CELLS 236

BORER, definition of 641

BORK. (periderm) 232

BORRADAILE 405

BOTALLI, DUCTUS 596

BOTALLI'S LIGAMENT 785

BOTALLUS, duct of, definition of 531

BOTANY, bot'any (G. botane, pas-
ture) 185-193, 202-246

Bacteria 190

Bryophytes 215

Economic 32

Flowers 242

Fungi 208

Pathogenic fungi 209

Photosynthesis 186

Plant histology 228

Pollination 238

Pteridophytes . 223

Spermatophytes 225

Thallophytes 204

Three higher groupings .215

Vaucheria 207

Yeasts 188

BOT-FLY LARVAE 369

BOTHRIOCEPHALUS LATUS, both'-
reosef'lus la'tus (G. bothrion, pit +
kephale^ -head). A genus of tape-
worms also called Dibothriocephalus.2$?>

BOTRYOIDAL TISSUE, bdt'rioidal
<G. botrys, bunch of grapes), defini-
tion of \ 284

BOUSINGAULT 381

BOUTON, boo' ton (F. bouton, button),

definition of 355

BOVIDAE, bo'vt'de (L. bos (bov)

ox) , definition of 660

BOW-FIN 643

BOWMAN'S CAPSULE . . . 64

BOWMAN'S GLAND, definition of... 872

BOYLE 380

BRACHETI 552

BRACHIAL, bray'keeal (L. brachium,
arm)

BRACHIOPODA, brak'iop'oda (G.
brachium, -\- pous, foot), definition
of 310, 424, 432

BRACHIUM, brak'iiim (L. brachium,
arm) 67, 707

BRACHIUM PONTIS, definition of . .
841, 859

BRACHIUM QUADRIGEMINUM, defi-
nition of 841

BRACT, brakt (L. bractea, thin plate
of metal ) 239

BRAIN, bran (M. E. brayne, brain) —

Center .... 847

Ganglia 848

Longitudinal section of brain.... 842

Nuclei 847

Of Alligator 862

Of Man 846, 850

Stem; 848

Table 840-841

Types of 844

BRANCHIA, bran'keeah (G. branchia,
gills ) , definition of 757

BRANCHIAL 724

Clefts 498

Rays 758

BRANCHIATA, brangkea'ta, definition

"of 312

(L. branchiatus, having gills.)

BRANCHIOMERIC (G. branchia +
meros, apart), definition of 573

BRANCHIOPODA, brfmgkeop'oda ...421

BRANCHIOS AURUS SALAMAN-
DROIDES (G. branchia + saurus,

lizard) 691

A species of gill-breathing lizard.

BRANCHIOSTEGAL M E M B R ANE,
bran'keeoss'tegal (G. branchia, gills;
stego, to cover ) 759

BRANCHIOSTEGITE, brangkeos'tejit
(G. branchia + stege, roof) . . .313, 315

BRANCH ROOT . . . 203

BRAUN'S QUILLWORT ..
One of the Fern Allies.

BREAD MOLD

BREVICEPS GIBBOSUS,

.208

brevtseps
gib'bd'sus (L. brevis, short -f- caput,

head + gibbosus, humped) 101

Genus of tailless amphibians.

BROAD LIGAMENT, definition of. . . .817

BRONCHIA, bron'keea (G.
windpipe)

BRO1

INDEX

899

BRONCHUS, brong'kus (L. bronchos,

windpipe ) 757

BROUN 381

BROW-SPOT OF FROG 45

BRUES 418

BRUNNER 380

BRYALES, bri'ales (G. bryon, moss),

definition of 216

BRYOPHYTES, brl'oflts (G. bryon +

phyton, plant), definition of... 203, 215
BRYOZOA, brl'ozo'a (G. bryon -f zoon,

animal), definition of 310, 424, 432

BUBONIC PLAGUE, bubon'ik (L. bu-

bonicus, buboes, i.e/; swellings) . . . .350
BUCCAL, buck'al (L. bucca, cheek or
mouth ) —

Cavity 48

Glands 742

Pouch 267

BUDDING—

In Hydra 251

In plants 211

BUFFON, boofon' 384

BUFFONIDAE, bufon'ide, definition

of 651

The toads.

BUFO VULGARIS, bfifo 5J,S, 'TOO

A genus of toads.

BUGULA AVICULARI8, bu'gula (G.
diminutive of ox tongue, bos, ox -f-

glossa, tongue) 309

BULBUS AORTAE, (L. bulbus, globu-
lar root), definition of 595

Arteriosus 58, 471, 481

Cordis 778

Oculi 875

BULLER 327

BULLHEAD 644

BUMBLE BEE 366

BUNODONT, bu'nodont (G. bounos,

mound -f- odous, tooth) 734

BURSA, burr'sah (L. a purse) —

Copulatrix, a genital pouch. 330, 344

Entiana 747

Ingiiinalis

BURS ARIA, ber'sa'ri'a (L. bursa,

pouch) 154

A genus of infusoria.
BUTTERCUP 246

CACEPU8 8YSTOMA If 01

CAECA, see'kah (L. caecus, blind) . . .329

Colic 728

Pyloric 728

CABCILIAN8, sesil'Ians (L. caecus

serpens, a kind of lizard), definition

of 647

CAECUM, see'kum (L. caecus, blind),

definition of 747

CALAMICHTHYS 642

CALCANEUM, kalkay'neum (L. calx,

heel) . 76

C'ALCANEUS, kalkay'neeus (L. calx,

heel) 103, 709

CALCAR, ksil'kar (L. calcar, a spur). 77
CALCAREA, kal'ka're'a (L. calcarius,

pertaining to lime) 419, 427

A sub-class of 8pongiae; the chalk
sponges, whose skeletons are made
up chiefly of calcium carbonate.
CALCIFEROUS GLANDS, kalsif'erus
(L. calx, lime + ferre, to carry) . . .268

CALCIFIED, definition of Ill

CALCULUS 39

CALOPTENUS FEMORA RUB RUM,

the common red-legged grasshopper .330
CALOPTENU8 ITALICUS, kfil'op'te'-
niis I'tal'icus (G. kalos, beautiful +
pteros, winged). A species of grass-
hopper 336

CALOPTENUS SPRETUS, the Rocky
Mountain locust, which does incal-
culable damage to vegetation. . . 329, 341
CALYPTRA, kalip'tra (G. kalyptra, a

covering) 216, 219

CALYX, ka'liks (L. calyx, a calyx), a

cup-like portion

Of flower 203, 242

Of kidney 8*7

CAMBARUS AFFINIS, kam'barus af-
fin'is (L. camarus, a sea-crab +
affinis, related to ) . A species of

crayfish 313

Virilis 313

CAMBIUM, kam'bium (L. cambium,

change) 230, 231

The soft tissue from which new
root and bark are formed in stems
and roots of shrubs and trees.

CAMPANULA HALLERI 876

CAMPER 380

C AM POD EOIDEA, kampo'de'oi'dea
(G. kampe, a caterpillar + eidos,
like). A family of elongated in-
sects 422, 430

CANALICULI, kanalik'ul! (L. canalic-

ulus, a small channel), definition of. 112
CANCELLOUS, kan'seliis (L. cancel-

losus, chambered), definition of.... Ill
CAN I DAE, kan'fde (L. canis, dog +

idae, like ) 658

The dog-tribe, as dogs, wolves,
foxes.
CANINE TEETH, kaynine' (L. canis,

dog) 734

CANIS FAMILTARI8, kan'is 701

CAPILLARY, kap'eelay'ry (L. capil-

lus, hair) 60

Attraction 37

Definition of 849

CAPTT08AURU8, kapitosau'rus (L.

capito, large head -f- saurus, lizard) .694
CAPSULE, INTERNAL, of brain, defi-
nition of . . 849

900

INDEX

CARAPACE, kair'apace (F. probably
L. capa, hood) 313

CARBOHYDRATE, kar'bohi'drat (L.
carbo, coal -f- G. hydros, water) .51, 96

CARBON DIOXIDE, kar'bon diok'sid
( L. carbo + di, two -f oxys, sharp ) . 63

CARCHESIUM, kar'ke'smm (G.
karchesion, a drinking-cup, a mast-
head), definition of 155

A genus of infusorians; the zOoids
are associated in dendriform colo-
nies.

CARCINUS MAENU8, kar'si'mis me'-
nfis (G. karkinos, crab -f- maenas,
sea-fish). The edible green crab... 327

CARDIAC, kAr'deeak (G. kardia,

heart) 318

End of stomach 49

CARDINAL VEINS (L. cardo, a
hinge) 482

CARDO, kar'do (L. cardo, a hinge),
definition of 335

CAREY, E. J 409, 685

CARINATAE, kar'inn'to (L. carinatus,

keel-shaped) 656

A sub-class of Aves, having a
keel-shaped sternum.

CARNASSIAL TEETH, karnas'ifil (L.

caro, flesh ) 736

Cutting teeth of carnivores.

CARNIVORA, karniv'ora (L. caro,

flesh + vorare, to devour) .426, 658

Teeth of 73.}, 735

CAROTID ARCH, karot'id (G. karos,

stupor ) 55

Gland 55, 588, 743

CARPAL, kar'pal (L. carpus, wrist) . . 76

CARPELLATE CONES, definition of. 238

CARPELS, karpelz (G. karpos, fruit)
227, 242

CARPOMYCETES, karpomiset'ez (G.
karpos -f- mukus, fungus) . 208

CARPOPODITE, karpop'odite (L.
karpus + G. pous, foot) 314

CARPOTROPIC, karpo'tropic (G. kar-
pos + tropikos, tropic, change) .. .239
Pert to tropical fruit.

CARPUS, kar'pus {L. carpus, wrist),
definition of 708

CARYOGAMY, kari'ogame (G. karyon,
nut + gametes, spouse), definition
of 147

CAST, in paleontology 398

CASTLE 388

CASUARIIFORMES 425

Flightless terrestrial birds with
very small wings and feathers with
large aftershaft, such as the Casso-
wary or Emu.

CAT—

Cranial nerves of 881

Cross section of 832

Muscles of 831

CATARRHINE, ka'tar'in (G. kata,
near -f- ris, nose), definition of.... 659

CATFISH, definition of 644

CATHARINE A UNDULATA 216

A species of moss.

CA T08TEOMI 643

GAUD AD, kaw'dad (L. cauda, tail).. 64
CAUDA EQUINA, kawd'a ekwi'nA
(L. cauda + equus, horse), defini-
tion of 837

CAUDAL, kaw'dal (L. cauda, tail) . . .684

CAUDATE NUCLEUS 849

CAUDATUM, kawdfi'tiim 158

CAYLUXI 73//

CEBU8 HYPOLENCUS 6~>9

CECIDOMYIA DESTRUCTOR, sesid'-
ome'a (G. kekis, a gall-nut -f- muia,

a fly) '3>,:>

A genus of flies.
CECIDOMYID, sesid'omid (G. kekis

+ muia} 345

CECUM (same as caecum) 73.0

CELL (L. cella, a compartment) . .88-116

Chemistry of 94

Division of .96-100

Fertilization of 104

Inclusions and products 90

Maturation of 100

Membrane 90

Mitosis, meaning of 99

CELLI 134

CELLULOSE, seTulos (L. cellula,

small cell ) .l 93

CEMENT GLAND, sement' (L. cac-

mentum, mortar ) 3^0

CEMENT OF TOOTH 732

CENOZOIC, senozo'ik (G. kainos, new
-f zoe, life). Geologic stratum con-
taining recent forms of life 394

CENTER—

Brain 847

Correlation of 847

Nerve 115

Of ossification ,777

Primary 847

Reflex 851

CENTRAL CANAL 67, 513

Cells 447

Nervous system of crayfish 322

Nervous system of frog 65, 66

Nervous system of honeybee 359

Nervous system of insect 330

CENTR080ME, sen'trdsom (G. kcn-

tron, a center -}- soma, body) 90

CENTRO SPHERE, sen'trosfer (G.

Jcentron -f- sphaira, a ball) 90

CENTRUM, sen'trum (L. center) ... 72

INDEX

901

CKI'HALAD, se'phalad, toward the

head

CEPHALIC FLEXURE, sephal'ik (G.

kepliale. head) ^78

CEPHALOCHORDA, sef'alakor'da

424, 430

CEPIIA LOCIIORDATA, seph'alowchor-
day'tah (G. kephalc, head; chorde,

string) 639

CEPIIA L ODI8CU8 DO DEC A L OPH US,

sef alodtsk'us 546

C E P H ALO P O.DA, sef'fildp'oda (G.

kephale + pous, foot) 421, 429

A class of Mollusca.

CEPHALOTHORAX, sef'alothdr'ax ...313
CERATIUM, serfi'shiiim (G. keration,

little horn), definition of 151

CERATODU8 765

CERATOHYAL, ser'atohl'al (G. keros,

horn + uoides, U-shaped) 738

CERATOHYAL CARTILAGE 616

CERCARIA, serka'ria (G. kerkos,

tail ) , definition of 290

CERCOMOXAS, serkom'onas (G.
kerkos, tail + monas, monad) 145
A genus of protozoa.

CERCUS, ser'kiis (G. kerkos, tail)

, .-'. 329, 337

CEREBELLUM, ser'ebeTum (L. cere-
brum, brain), definition of. .66, 845. 859
CEREBRAL CORTEX, ser'ebrsil, defi-
nition of 848

CEREBRATULU8 FU8CU8 301

Lacteus 301

CEREBRUM, ser'ebrum (L. brain)..

. ; . .;.<. 66, 839, 848

CERIANTBIDEA, sertfm'thide'a (G.
Tceras, a horn + anthos, a flower) . .420

An order of Zoantharia.
CERVICAL, ser'vikal ( L. cervix,

neck ) 684

CERVIDAE, ser'vlde (L. cervus, a

stag ) 660

A family of solid-horned rumi-
nants, as stag.
CERVIX, ser'viks (L. cervix, neck)..

CESALPINO, sesalpl'no 381

CEftTODA, sesto'da (G. kestos, girdle

4- eidos, resemblance) . . . .285, 420, 428
CETACEA, seta'sea (G. ketos, whale)

; 426. 662

CHAETAE, GENITAL, ke'tfi (G.

chaite, hair) 265

CHAETOGNATHA, ketog'natha (G.
chaite, bristle -+- gnathvs, jaw) ....

308, 424, 432

A group of invertebrates of uncer-
tain position.
CHAETOPODA, ketop'oda (G. chaite

+ pous, foot) 283, 420, 428, 429

A class of Annelida with setae.

CHAIN REFLEX ACTION, definition

of 181

CHALAZA, kala'za (G. chalaza, hail,

tubercle) 244, 438, 439

CHALC1D, kal'sid (G. chalkis, per-
taining to Chalcis, Greek city).... 345
CHALONE, kal'on (G. chaldo, I relax)

An anti-hormone 52

CHAMAELONTE8 654

CHAMELEONS, ka'me'leonz (G. cha-

maileon, ground or dwarf lion ) . . . . 654
CHAPELLE-AUX-SAINTS SKULL. . .1,00
VHARADRIIFORM&8, karad'rifor'mez
(L. charadra, ravine + forma,

form ) 425

Terrestrial, arboreal, or marine
plover-like birds.

CHARYBDEA MARSUPIALIS, karib'-
dia (G. charybdis, Homeric figure, -f-

L. marsupium, a pouch ) 256

CHELA, ke'la (G. chele, claw) 313

CHELIPEDS, ke'lipeds (L. chela, a

claw + pes, foot) 314

CHELONE IMBRWATA (G. chelone

-|- L. imbricatus, covered with tiles). 653
CHELO^ETI, kelon'eti (G. chelone, a

tortoise ) 422

An order of Lipoctena.
CHELOXIA, keelow'neeah (G. chelone,

turtle) 425, 652

Caouana 554

Longicollis 119

My das 653

CHELOXIDAE, kelon'ida (G. chelone

+ eidos, resemblance) 652

CHEfjYDRA SERPENTINA, kel'idra
(G. chelydros, amphibious serpent) .

,...653, 803

CHELYDRIDAE, kelid'ride (G. chely-
dros + eidos} 652

CHELY8 FIMBRIATA, kel'is (G.
chelys, a tortoise -f- L. fimbria,

fringe) 683

CHEMISTRY 34

Analytical 36

Of living matter 94-96

Synthetic 36

CHEMOTROPISM, kemot'ropizm (G.

chemos, juice + trope, turning) . . . . 126
CHIASMA, kaias'ma (G. cross-mark). 66
CHICK—

Embryology of 433-543

DEVELOPMENT BEFORE THE

EGG IS LAID 435

Blastulation 450

Gastrulation 451

Morula stage 449

Oogenesis 442

Reproductive organs of fowl 440

INDEX

PRIMITIVE STREAK AND ORI-
GIN of MESODERM 455

Neural plate 459

Notochord 458

FOUR TO SIX SOMITE STAGE . . . 465

FIRST HALF DAY 470

Brain differentiation 472

Lengthening of fore-gut 475

SECOND HALF OF SECOND DAY.

Brain 476

Blood-corpuscle path 483

Circulatory system 480

Excretory system 484

Torsion 480

E X T R A-E MBRYONIC MEM-
BRANES 485

Allantois 487

Amnion 487

Chorion 488

Serosa 487

Yolk-sac . . . 485

DEVELOPMENT OF THE THIRD

DAY ..480

Circulatory system 500

Digestive tract 492

Liver 496

Lungs 495

Nervous system 490

Optic vesicles 490

Pancreas 497

Thymus gland 497

Thyroid gland 497

Visceral arches 498

Visceral clefts 498

DIFFERENTIATION OF SOMITES.502

Excretory system 503

DEVELOPMENT OF THE FOURTH
DAY—

Adrenal, bodies 520

Allantoic circulation 524

Circulatory system 520

Excretory system 518

Ganglia of cranial nerves 513

Heart 529

Intra-embryonic circulation 526

Nervous system 511

Organs of special sense 514

Reproductive system 519

Skeletal structure 516

Spinal cord 513

Veins 531

COELOM AND MESENTERIES 536

DEVELOPMENT OF THE FIFTH

DAY 538

Heart 540

Limbs 538

Skull 538-540

CHIGGERS, chig'ers (F. chique, lo-
cal) 350

CHILOMONA8, kilom'onas (G. cheilos,
lips + monas, a unit) .... . . 150

CHILOPODA, kilop'oda (G. cheilos,

lip + pous, foot) 422

An order of Opisthogoneata.
CHI M AE RA MONSTROSA, kime'ra

(G. chimara, a fabled monster) 6-'i2

CHIMAPHILA, kimiif'O'lali G. cheima,

winter + philos, loving) 23.'i

CHIMPANZEE, chimpan'ze (Sp. chim-

pance, a native name) 660

CHIRONOMIDAE, kironom'ide, (G.

cheir, hand -j- namenin, to manage). 345
CHIRONOMUS (G. cheironomos, one

who moves the hands ) 330

CHIROPTERA, kirop'tera (L. chirop-

teras, wing-handed) 426. 658

CHIROPTERYGIA, kirop'terlg'ta (G.

cheir, hand -f- pteron, a wing) 707

CHITIN, kl'tin (G. chiton, a tunic) . . 93

A carbohydrate derivative forming
the skeletons of Arthropods.

CHLORAGOGEN CELLS 259

CHLOROPHYCEAE, kldro'fl'see (G.
kloros, yellowish-green + phykos,

sea-weed) 204

CHLOROPHYL, klo'rofil (G. chloros,

grass green + phyllon, leaf) 185

Chemical formula of 186

CHLOROPLAST, klo'roplast (G.
. chloros + plastos, moulded) .. 185, 204
'CHLOROSIS, kloro'sis (G. chloros,

grass green ) 303

A peculiar form of anaemia,
characterized by a green-colored
skin.

CHOANA, koh'anah (G. funnel)

515, 582, 730

CHOA NAFLA GELLA TA , ko'anofla jela-
ta (G. choane, funnel + flagellum,

whip) 151, 418

Mastigophora with a contractile
collar from which the single flagel-
lum extends.

CHOLAEPUS DIDACTYLU8 659

CHOLEDOCHUS, DUCTUS, koled'okus

(G. choledochos, containing bile)... 497
CHONDRICONTS, kond'rikont's (G.
vhondros, cartilage + kontos, pole). 91

Rod-shaped chondriosomes.
CHONDRIOMITES, kond'riomites' ... 91

Granular chondriosomes.
CHONDRIOSOMES, kondriosomz' (G.

chondros + soma, body) 91

CHONDROCRANIUM, kon'drocray'-
neeum (G. chondros, cartilage; kra-

nion, skull ) 614. 687

CHONDROSTEI, kondross'tee'eye (G.
chondros, cartilage; osteon, bone) . .

425, 642

CHORDATA, classification of

430, 543, 639-663

Definition of .... . .638

INDEX

903

CHORDA TYMPANI (G. chorde, a

string) 884

Tendineae 55, 778

CHORION FRONDOSUM, ko'rion
fron'dosum (G. chorion, membrane

+ frondosus, richly foliated) 030

Laeve 630

In insects 346

CHOROID, koh'roid (G. chorion, mem-
brane ) 70

Coat of eye 873

Plexus 512, 571

CHROMAFFINE CELLS, kromaf'in
(G. chroma, color -(- L. affinis, re-
lated) . . 879

Cells which turn yellow after
treatment with chromic salts.
CHROMAPHILE CELLS, kromofil (G.

chroma + philein, to love ) 822

Same as chromaffine.
CHROMATIX. kro'matin (G. chroma),

definition of 90

C H R 0 M ATOPHORES, kro'matoforz
( G. chroma + pherein, to bear ) . . . .

. 128, 204, 667

CHROMOMERES, kro'momerz (G.

chroma + meros, part) 89

The chromatin granules which
form chromosomes.
CHROMOSOME, kro'mosom ( G.

chroma + soma, body) 97, 159

Accessory 168

Double 168

Hetertropic 168

X 168

Y 168

C H R 0 M 0 TROPISM, kromot'ropizm

(G. chroma -f- trepein, turn) 127

CHRONOLOGICAL TABLE OF BIO-
LOGICAL EVENTS 389

CHRYSOHYIA MACELLARIA 368

CHRYSOPHRYS 824

CICATRIX, sik'atriks (L. cicatrix, a

wound) 440, 443

In botany, the mark left after a
wound is healed.

In embryology, the blastoderm in
bird and reptile eggs.

In zoology, a small scar in place
of previously attached organ.
CICONIIFORMES, sikon'nffor'mez (L.

ciconia, stork ) 425

Stork-like birds.

CILIA, sil'ifi (L. cilium, an eyelid) . . .109
CILIARY BODY OF EYE, sill'eeary

(L. tilium, eyelid) 873

CILIATA, sil'm'ta (L. cilinm) .. . 153, 419

A sub-class of Infusoria.
CINCHONA CALI8AYA, sinko'na
kalisa'oa (Sp. Chinch on, town in
Spain + calisaya, name of a bark) .231

CINGULUM, sing'gulum (L. cingulum,

a girdle) 276

CINOSTERNIDAE, sinoster'nide (G.
kinein, move + sternon, breast-bone

-4- eidos, resemblance) 652

CINOSTERNUM ODORATUM 119

CIONA INTESTINALIS, si'ona (G.

kiwn, pillar ) 554

CIRCLE OF WILLIS, definition of... 792
CIRCULATION, ser'kula'shun (L. cir-

culatio, act of circulating) 30, 53

Of bird 785

Closed 27O

Crocodile 185

Double 535

Frog 54

Open 270

Pulmonary 54, 56

Systemic 53, 56

CIRCUMFERENTIAL CANAL, ser'-
kiimferen'shal (L. circum, around

+ ferre, to bear ) 254

CIRCUMVALLATE, ser'kumvarat (L.

circum, around -f valla, wall ) 739

CIRRIPEDIA, siripe'difi (L. cirrus, a

curl + pes, foot) 421

A sub-class of Crustacea.
CIRRUS, sir'rus (L. cirrus, tuft, lock

of hair) 287

CI8TUDO LUTARIA, sis'tudo luta'roa
(L. sista, box -f- testudo, a tortoise
-\-lutarius, belonging to mud) . . . .702
CIVET, siv'et (P. zabad, froth of milk

or water ) 734

CLADISTA, kladis'ta (G. klados, a

branch + histion, a web) 642

CLADOPHORA, kladof'ora (G. klados

+ phoros, bearing) 205

CLADOT HRIX ACTINOMYC08E8,
klnd'othrlks (G. klados, branch -{-

thrix, hair ) 2 12

CL ARIAS 767

CLASPER, klas'per (M. E. claspen, to

hold) 707

CLASS, in classification 415

CLASSIFICATION. .31, 32, 414. 418, 432
Of Chordata 543, 639-663

CLAUSTRUM, klfis'trum (L. claus-...

trum, a bar), definition of 849

CLAVICLE, klav'ikel (L. clavicula,

little key) 76

CLAVICULA, klav'ik'ula (L. clavic-
ula) 70;

CLEAVAGE, kle'vaj (A. S. cleofan, to

cut) 105

Accessory 448

Amphioxus 446

Fro.sr 553

Gastrula 556

Spindle 444

904

INDEX

CLEFTS, BRANCHIAL, definition of. 41)8

Gill 498

Visceral 498

CLEITHRUM, klyth'rum (G. kleith-
ron,, bar, gate) 1101

CLEP8IDRIXA BLATTARUM, klep'-
sid'rina (G. klepsia, theft 4- hydor,
water + L. blatta, a cockroach) . . . . 15 1

CLIMBING PERCH 645

CLITELLUM, klitel'iim (L. clitellae,
a pack-saddle ) 205

CLITORIS, kly'toriss (G. kleio, to
close) 820

CLOACA, klo-ay'kah (L. cloaca,

sewer )' 49

Glands of 67.'}

CLOCK, the marvelous 34

CLONORCHI8 SINENSIS, klonor'kis
slnen'sis (G. klon, branch -f- orchis,

testicle) 801

A species of fluke having ramified
testicles.

CLOSED CIRCULATION, definition
of 60, 270, 781

CLOSSENDROMORPHA 422

CLUB-MOSSES (one of the Fern
Allies ) 224

CLYPEUS, klip'eus (L. clypeus, a
shield) 335

CLYTRA, kllt'ra (word of no mean-
ing) 362

CNEMIAL CREST, nem'ial (G.
knemis, shin ) 103

CNIDOBLAST, ni'doblfist (G. knide, a
nettle -f blastos, bud) 248

CNIDOCIL, ni'dosil (G. knide -f
cilium, an eyelid) 248

COAGULATION, kofigiila'shun fL.
cum, with -f agere, to drive) . . .39, 325

COBITIS, kobi'tis (G. kobites, gudgeon-
like) 770

COCCI, kok'si (plural of coccus)

COGCIDIIDEA, kocksidiid'ea (G. coc-
cus + idea, a picture of) 153, 419

An order of ftporozoa.

COOCIDIUM 8HUBERGI, koksid'ium
152, 153

COCCUS, kok'iis (G. kokkos, a seed),
definition of 190

COCCYGEO-ILIACUS 81

COCHLEA, kock'leah (G. kochlias, a
snail ) 866

COCHLEARIS DUCTUS, kocklea'ris . .866

COCOON OF EARTHWORM, kokoon'
(F. cocon, a shell) 276

COD 64.',

CODES FOR CLASSIFICATION.. ..416

COELENTERATA, sGlentera'ta (G.
koilos, hollow -f enteron, intestine)

247, 257, 427

Classification of 255-258

Gonionemus 254

Hydra fusca . .' 247

Obelia - . .252

Polymorphism in 255

COELENTERON, solen'teron (G. koilos

+ enteron ) 252

COEL1AC, see'leeak (G. koilia, stom-
ach )

Artery 791

Axis 791

Plexus 68

COELOM, see'loam (G. koilos, hollow)

47, 247

COELOM AT A, animals possessing a

coelom 258

Introduction to 258-261

COELOMIC CIRCULATION OF

EARTHWORM, selom'ic 271

COENENCHYMA, scnen'kima (G.

koinos, common + enchein, infuse) .

The calcified tissue of the coeno-

sarc of Actinozoans.

COENOCYTE, se'nosit (G. koinos,

. common + kytos, hollow) 78, 207

COENOSARC, se'nosark (G. koinos,

common + sarac, flesh ) 252

COENURUS, senu'rus (G. koinos +

onra, tail) 294

A metacestode with a large blad-
der, from the walls of which a large
number of heads are formed.
COHNHEIM'S FIELDS, definition of. 114
COITION, koi'shun (L. coitio, a going

together )

Sexual copulation.

COLCHWUM, kol'chikum (G. kolchi-
kon, plant with poisonous bulbous

roots ) 235

COLEOPTERA, koleop'tera (G. koleos,

sheath -4- pteros, wing) 423

An order of Coleopteroidea.
COLEOPTEROIDEA, koleopteroid'ea . .423

A sub-class of Pterygogenea.
COLEPS HIRTUS, kol'eps hir'tus (G.
koleps, bend or hollow of knee + L.

liirtus, stubby) 154

C O L L A T ERAL FIBRO VASCULAR
BUNDLES (L. cum, with + latera,

sides) 236

COLLATERAL ORGANS OF INSECT.337
COLLATERIAL GLAND, kolete'rial
(G. kolletos, from kollan, glue to-
gether) 340

COLLECTING TUBULES 64, 505

COLLEMBOLA, kolem'bola (G. kolla,

glue -f- embolan, peg or wedge) .422, 430
COLLENCHYMA, koleng'kima (G.
kolla,, glue -f- enghyma, infusion) . .23^

INDEX

906

COLL1CULI, kulik'uli (L. colliculus,

a little hill) 840

COLLOID, kol'oid (G. kolla, glue +

eidos, form), definition of 94

COLON, koh'lon (G. kolon, member) .338
COLONIAL FORMS, living in colonies.129

GOLPIDIUM COL POD A 15 J,

COLPODA, kolpo'da (G. kolpodes,

winding sinews) 154

COLUBER ANTHONYI, kol'uber (L.

coluber, serpent) 416

COLUMBUS 378

COLUMELLA, kol'yumell'ah (L. little

column) 71, 72, 215

Of ear 867

Of Frog 45

COLUMNAE CARNEAE, kolum'nfi
carnea (L. columnae + carnis,

flesh) 778

COLUMNAR, definition of . . 109

COL YMBIFORMES, kolim'bif or'mez

(G. kolymbun, divert + form) 425

Aquatic birds with webbed or lobed
toes, feet far back and body carried
upright, such as the loons and
grebes.

COMMISSURE, kom'isur (L. cum, to-
gether + mittere, to send) 65

Of brain 843, 858

COMPARATIVE ANATOMY ....637-886

INTRODUCTION TO 637

CLASSIFICATION OF CHORDATA

639-663

INTEGUMENT 664

Of amphibia 668

Of birds 669

Of fishes 668

Of mammals 671

Of reptiles 669

Glands 672

Hair 672

Scales 675

ENDOSKELETON 681

Appendicular skeleton 698

Free appendages 707

Hip girdle 704

Limbs 707

Paired appendages 699

Regions of vertebral column 684

Skull 685

Summary of cranium 714

Summary of skeletal system 716

Vertebral column . . . . " 681

DIGESTIVE SYSTEM 722

Detail study 728

Glands 741

Intestine 746

Liver 749

Oesophagus 744

Pancreas 751

Pharynx 742

Stomach . . 745

Summary .-. 751

Teeth 731-737

Tongue 738

RESPIRATORY SYSTEM 757

Air-bladder and accessory or-
gans 767, 770

Lungs and air-ducts 762

Summary 766

Swim-bladder 761

CIRCULATORY SYSTEM .772

Aorta and aortic arches 786

Arteries 78«

Arteries of dorsal aorta 790

Detail studies 776

Development 781

Heart 776

Lymphatic system 799

Somatic arteries 791

Summary 800

Vascular system 779

Veins 793

Visceral arteries 791

UROGENITAL SYSTEM 806

Adrenal organs 822

Mesonephric ducts 809

Mesonephros 807

Metanephros 810

Organs of copulation 821

Oviducts , . . 814

Pronephros 807

Reproductive ducts 811

Summary 823

Urinary bladder 810

MUSCULAR SYSTEM 826

Visceral muscles 829

NERVOUS SYSTEM 833

Brain 838

Brain table 840-841

Cerebellum 859

Cerebrum 839

Cranial nerves 879

Diencephalon 856

Epiphysial structures 856

Medulla oblongata 859

Meninges 838

Mesencephalon 858

Neuromeres 837

Organs of special sense 865

Peripheral 876

Rhombencephalon 859

Spinal cord 835

Summary 860

Sympathetic . 878

Telae chorioideae 860

COMPLEMENT, definition of 199

COMPLEMENTAL MALES 327

A purely male form, usually small,
living in close proximity to hermaph-
roditic forms.

COMPLETE FIBROVASCULAR BUN-
DLE 232

COMPLETE FLOWER . . . 228

906

INDEX

COMPLEX REFLEX ACTION 178

CONCENT RIG FIBRO VASCULAR

BUNDLES 236

CONCH, konk (G. kongche, shell) ... .869
CONCHA, NASAL, kon'ka (L. shell) .515

CONCHOLOGIST, konkol'ojist 418

CONCLUSIONS, definition of 182

CONCRESCENCE, konkres'ens (L.
cum, together + crescere, to grow) .452

CONDENSATION 38

CONDUCTING TISSUES, definition

of 234

Mechanism 866

CONDUCTIVITY, kon'duktiv'iti (L.

conducere, to lead together) 38

CONDYLE, kon'dill (L. condylus,

knuckle) 72, 70S

CONE (G. konos, a cone) 216, 223

CONEY 662

CONFLUENCE, kon'flooens (L. cum,

with + fluere, to flow) 452

CONGO SNAKE 648

CONIDOSPORES, komd'io'spore (G.

konidion, particle of dust -f spore } .211
CONJUGATION, kon'joogashun (L.

cum, together with + jugare, yoke) .158
CONJUNCTIVA, kon'junktie'va (L.

conjunctus, join together) 875

CONNECTIVE TISSUE (L. cum +

nectere, to bind) 49, 109

CONSCIOUSNESS, stream of 174

CONSPICUOUSNESS VS. IMPOR-
TANCE 32, 375

CONTINUITY, law of 376, 406

Physiological 150

CONTRACTILE, kontrak'til (L. cum

-f trachere, to draw ) 93, 865

Fibrils 248

Theory 122

CONUS ARTERIOSUS (G. konus,

cone) 790

CONVALLARIA, konvala'rea (L. con-

valis, a valley enclosed on all sides ) .234
CONVECTION, konvek'shun (L. con-

vehere, convey ) 38

CONVERGENT EVOLUTION, konver'-
gent (L. convergere, to incline to-
gether) 409

CONVOLUTIONS OF BRAIN, kon'vo-
hi'shun (L. cum, together + volvere,

to wind ) 843

COORDINATING FACTS 40

COORDINATION OF SUBJECTS

STUDIED 34

COOTS, koots (M. E. coote, coot) 417

COPE 388

COPEPODA, kopep'oda (G. kote, oar

+ pous, foot) 421

COPULA, kop'yulah (L. cum, to-
gether; apo, bind) 616, 740

COPULATION PATH . ..551

CO RAG 1 1 FORMES, kora'seef or'mez
(G. korakias, a kind of raven -f- L.

forma, shape ) 425

Arboreal roller-like birds such as
swifts, kingfishers, owls, etc.
CORACO-ARCUALES, kor'ako arkiifi-
lez (G. korakinas, beak like a raven

-f- L. arcus, a bow ) 831

Radiales 79

CORACOID, kor'akoid (G. korakoeides,

like a crow's beak) 75

CORALLIUM, koral'ium (G. korallion,

red coral ) 256

CORDS, MEDULLARY, definition of . .520

Ovarial 520

CORIUM, koh'reeum (L. leather) 665

CORK CAMBIUM (Sp. alcorque,

cork) , definition of 231

CORM, k6rm (G. kormus, a trunk) . .255
CORNEA, kor'neeah (L. corneus,

horny) 69, 872

CORNEUM, kdr'neeum (L. corneus,

horny ) 665

CORNICLES, kor'nlklez (L. cornicu-

lum, little horn) 372

CORNIFICATION, kor'nifika'shun (L.
cornu, horn -f- facere, to make) .667, 737

CORNUA, kor'niuah (L. horn) 72

COROLLA, korSl'a (L. corona, a

crown) 203, 2J,2

The petals of a flower.
CORONA RADIATA OF BRAIN, koro'-

na ra'dmta 849

CORONARY VESSELS, kor'onftrl (L.

. corona, crown) 780

Crown-like vessels.
CORONOID, kor'ohnoid (G. korone,

crow) 697

CORPORA BIGEMINA, kdr'pfml

blgem'ina 839

Quadrigemina 512

CORPUS ALBICANS, kor'pus al'bi-
knns (L. corpus, body -{- alba,

white) 863

Callosum 842

Cavernosum 820

Luteum 811

Mammilare 845

Restiforme 859

Striatum 839. 848

CORPUSCLES, BLOOD OF GRASS-
HOPPER, kor'piials (L. corpusculus,

a small body ) 339

CORRELATION CENTERS, definition

of 847

CORRODENTIA, koroden'shia (L.

com, together + rodere, gnaw) 423

An order of Blattaeformia.
CORTEX, kor'teks (L. cortex, bark) . .216
The outer portion of an organ.

Of brain 839

Primarv . . 230

INDEX

907

CORTI, ORGAN OF 869

CORTICAL PARENCHYMA, definition

of 230

COSTA, kos'tah (L. costa, a rib) 335

COTYLEDON, kot'ile'don (G. kotyle,

a cup ) 203

COTYLOID BONE, kot'Iloid (G. kotyle

+ eidos, form ) , definition of 707

COTYLOSAURIA, kot'ilosa'ria (G.

kotyle + sauros, lizard) 694

COVERING TISSUES, definition of.. 234

COWPER'S GLAND 820

COXA, kok'sfi (L. coxa, hip) 328

COXOPODITE, koksop'odit (L. coxa

+ G. pous, foot) 31 ft

CRANIAL NERVES (G. kranion,

head) 67, 68, 861

CRANIATA, krania'ta (G. kranion, the

skull) 424, 431, 543, 640

Definition of '. . .431

CRANIUM, kra'ninm (G. kranion,

head) 73, 686

CRAYFISH 312-327

Adaptations 316

Autotomy 324

Circulatory system 318

Crustacea 325, 326

Digestive system 318

Excretory system 321

External appearance 313

Muscular system 323

Nervous system 321

Parasitic Crustacea 325

Regeneration 324

Reproductive system 323

Respiratory system 320

Sense organs 321

Serial homologies 316

CREMASTER MUSCLE, kremas'ter

(G. kremannunai, to hang) 818

CREODONT, kre'odont (G. kreas, flesh

+odous, tooth ) 73//

CRESCENT, GRAY, definition of 552

CREST (L. cresta, a crest) 77

CRETINISM, kre'tinizm (F. cre-

tinisme, probably Christian) 53

CRICKET, krik'et (M. D. kriecker,

creaker ) Sj2

CRICOID, krik'old (G. krikos, a ring

+ eidos, form ) 762

CRIXOIDEA. krinoi'de (G. krinon, a

lily -f- eidos, resemblance) 420, 428

A class of Echinodermata, the sea
lilies or feather stars.
CRISTA GALLI (L. crista, a crest -j-

fjalhis, cock ) 868

CRITERIA FOR A SATISFACTORY

EVOLUTIONARY THEORY 413

CROCIDURA COERULEA, krosidii'ra
seru'lea (G. krokys, piece of woolen
cloth -f- L. coeruleus, sky-colored) .678

CROCODILE HEART 785

CROCODILfA, krok'ohdill'eeah (L.

crocodile) 425, 651, 653

CRO-MAGNON MAN (Gael.-Ir. cro,

blood, death ) , definition of 400

CROP (M. E. croppe, top of plant)

329, 744

CROSSOPTERYGII, cross sop'teryg'ee-

eye (G. krossoi, fringe; pteron,

wing) 425, 642

CROTALINE SNAKES (G. krotalon,

a rattle) 733

CROWN OF TOOTH 735

CRURA CEREBRI, kroo'ra cg'rebri

(L. cms, leg + cerebrum, brain)

572, 839, 860

CRUREUS, kroo'reus (L. crus, leg) . . 82
The vastus internus muscle of the

thigh.

CRUS, kriis (L. crus, leg) 45, 708

CRUSTACEA, krusta'shia (L. crusta,

a crust) 421, 429

A class of Arthropoda, mostly
aquatic, that breathe by gills.

CRYPTOBRAXCHUS ALLEGHENI-
EN8I8, kriptobrang'kus (G. kryptus,
hidden + brangcJtos, gill) 648

CRYPTOCEPHALA, kriptdsefalfi (G.
kryptos, hidden -f kephale, head )'.'. 420
An order of Polychaeta.

CRYPTODIRA, kriptodi'ra (G. kryptos
+ deire, neck) 652

CRYPTOGAMS, krip'togam (G. kryp-
tos, hidden -j- gamos, union ) 225

CRYPTONISCUS, kriptonis'kus (G.
kryptos + oniskos, a wood louse ) . . 327

CRYPTURJFORMES, kriptn"ifor'mez
(G. kryptos, hidden + oura, tail -f-

forma, form) 425

Flying terrestrial birds with short
tail and no pygostyle, as Stinamus.

CRYSTALLINE LENS, kris'talln (G.

krystallinos, crystalline) 70

Transparent. "

CRYSTALLOID, kris'taloid (G. krystal-
los + eidos, form) 94

CTEIPHORA, tl'fora (G. kteinein, to

kill -f pherein, to bring) 422

A sub-class of Arachnoidea.

CTENOID, ten'oid (G. ktein, comb).. 676

CTEXOPHORA, tendf'ora (G. ktenos,

a comb + pnoreo, I bear) 427

Transparent, free-swimming ma-
rine animals, the Sea Walnuts or
Comb Jellies.

CUBITIS, kii'bitus (L. cuUtus, elbow). 335

CUBOID, ku'boid (G. kuboeides, cube-
like) 109

908

INDEX

VUBQMEDU8A.E, ku'bomf'du'se (L.
cubus, a cube + medusae, medusa i .

256, 419

An order of Scyphozoa.
CUCULIFORME8, ku'kulifor'niez (L.
cuculus, a cuckoo + forma, form) . .425

Arboreal cuckoo-like birds with
first and fourth toes directed back-
wards. The fourth toe may be re-
versible.
CULEX, ku'leks (L. culex, a gnat) . . . 137

CULTURED MAN, definition of 27

CUNEIFORM, kiu'neeiform (L. cuneus,

wedge ) 709

CURSORIAL, kerso'rial (L. cursorius,

pertaining to running) 654

CUSP (L. cuspis, a point) 675, 736

A prominence, as on a tooth.
CV 8 PI DATUM, kus'pidatum (L. cus-

pidatus, made pointed) 220

A species of Sphagnum.
CUTANEOUS, kiutay'neeus (L. cutis,

skin) 793

CUTICLE, ku'tikl (L. cutis, skin). 93, 267

An outer skin or pellicle in zool-
ogy.

The epidermis in botany.

CUTIS PLATES, definition of 591

CUVIER 379, 384, 388, 386, 415

CUVIER, DUCT OF, definition of.... 501
CYCLOID, sigh'kloid (G. kyklos, cir-
cle), definition of 676

CYCLOPS, si'klops (G. kyklops, round-
eyed) 305

CYCLOPTERUS, siklop'terus (G.

kyklos, a circle -f pteron, wing)... 816
CYCLOSIS, siklo'sis (G. kyklosis, a

whirling round) 139

The circulation or movement of

protoplasm within a 'cell.
CYCLOSTOMATA, sigh'klostow'matah,

(G. kyklos, circle; stoma, mouth) . . .

424, 431, 640, 729

CYMOTHOIDAE, simotho'ide (G.

kyma, anything swollen -f- thoos,

quick or pointed + eidos, resem-
blance ) 327

CYNOHYAEDON, smohle'don (G.

kynos, dog + hyaina, hyena ) -. 73.)

CYNTHIA BI PARTI A, sm'thia (G.

kynthos, mountain in Greece) 554

CYPRINOIDS, sip'rinoids (G. kyprinos,

a carp -f eidos) 761

CYPRIS, si'pris (&• kypris, Venus)... 327
CYST, sist (G. kystis, a bladder) ... . .151
CYSTIC ERCUS. sls'tiser'kus (G.

kystis + kerkos, tail ) 294

The larval form or bladder-worm

stage of certain tapeworms.

CYtiTOFLA UKLLATA , sis'toflajelsVta
(G. kystis, bladder + L. flagellum,

whip) 151, 419

An order of Mastigophora.

CYSTOGENOUS CELLS, sistoj'enus

(G. kystis + genos, offspring) 28i)

Large nucleated cells in the cer-
caria of distomum which secrete the
cyst.

CYTOFACTS, si'tofakts (G. kytos,
hollow -f- L. facere, to make) 91

CYTOLOGY, sltol'oji (G. kytos, hol-
low -|- logos, discourse) 31

CYTOLYTIC, sitolit'ic (G. kytos -f
lysis, loosing), definition of 200

CYTOPLASM, definition of 90

CYTOSTOME, sl'tostom (G. kytos +
stoma, mouth ) 1 50

CYTOTROPIN, si'totro'pin (G. kytos
+• trophe, nourishment) 200

DACTYLOPOPODITE, dak'tilop'odit

(G. ddctylos, finger; pous, foot)... 31 4
DACTYLOZOOID, dak'tilozo'oid (G.

dactylos, finger; zoon, animal) 255

DARBISHIRE, A. D 164

DARWIN, CHARLES

262, 385, 388, 397, 403, 749

DARWIN, ERASMUS 384

DARWINIAN THEORY UNSATIS-
FACTORY 397

DASYPU8 SEXCINCTUS, das'ipiis (G.
dasus, hairy + pous, foot + L. sex,

six, cingo, to bind around ) 659

DAWSON 370

DECAPOD, dek'apod (G. deka, ten;

pous, foot) 316

DECAPOD A, dekap'oda 326

DECIDUA BASALIS, desid'ua (L.

de, away ; cadere, to fall ) 629

Capsularis 629

Reflexa 629

Serotina 629

Vera 629

DECIDUOUS TEETH, definition of.. 732

DEFERRED, definition of 178

DEGENERATION, dejenera'shun (L.

degener, base) 325

DE GRAAF 380

DELAMINATION, delam'inashun (L.
de, down; lamina, a layer), defini-
tion of 107, 557, 561

DELTOID, dell'toid (G. delta, fourth
letter of the Greek alphabet, triangu-
lar in form) 80

DEMIBRANCH, de'mibrank (G. demi,

half + branchia, gill) 758

DEM08PONGIAE, demospon'jie (G.
demos, the people + spongos,

sponge ) 419, 427

A class of porifera to which most
sponges belong.

INDEX

909

DEXDR1TES (G. dendron, tree) 68

DEXDROCOtiLUM GRAFFI, dend'ro-

se'liim (G. dendron, tree -f- koilia,

bowels ) 286

Lacteum 285

DENSITY 37

DENTAL FORMULA, den'tal (L. dens,

tooth ) 739

Ridge 732

DENTARY, den'taree (L. dens, tooth).

12, 617, 697

DENTATUS GYRUS, denta'tus jlrus
(L. dentatus, toothed + guros, turn)

884

Nucleus 859

DENTINE, den'tin (or teen), (L. dens,

tooth), definition of 732

DENTITION, dentish'un 732

DEPRESSOR MUSCLES, depres'or

(L. deprimere, to lower) 78, 829

Nerve 755

DERMAL BONES, der'mal (G. derma,

skin ) , definition of 677

DERMAL MUSCLES, definition of . . . .826
DERM AFTER A, dermap'tera (G.

derma, skin -f- pteron, wing) 426

An order of Orthopteroidea.
DERMARTICULARE, der'marticula'-

re ( G. derma, skin ; L. articulare, to

divide into joints) 697

DERM A TO GEN, der'matojen (G.

derma, skin; qiqnesthai, to produce). 228
DERMA TOM E, derr'matome ( G.

derma, skin; tomos, cutting), defini-
tion of 502

DERMIS, der'mis (G. derma, skin)... 46

Component parts of 665

DERMOCHELYS CORLACEA, der-

mok'ells (G. derma, skin + chelus,

tortoise) 652

DERMOPTERA, dermop'tera (G.

derma, skin + pteron, wing) 423

DERO, de'ro (G. dero, neck) 286

A sub-class of Oligochaeta.
DEROTREME8, derotrems (G. dero,

neck + trema, a hole) 760

DE SAUSSURE 381

DESCEMETS MEMBRANE, definition

of 579

DESCRIPTION VS. EXPLANATION. 40
DE8MOGNATHUB FUSCUS, desmoj?'-

nfithus (G. desmos, a band, -f- G.

(jnathos, jaw) 6.'t8

DESSICATION (L. desiccare, to dry

np) 147

DEUTOMERITE, dutom'erit (G. deu-

teros, second; meros, part) 151

DEUTOPLASM, dii'toplasm (G. deute-

ros, second + plasma, something

formed), definition of 550

DEVELOPMENT, definition of 31

Of digestive canal 723

DE VRIES, HUGO 388, 403, 679

DEXTROSE (L. dexter, right +

glukos, sweet) 187

DIAGNOSIS, di'agno'sis (G. dia,

through; gignoskein, to know), cor-
rectness of 20

DIAPHRAGM, dye'aframm (G. dia,

between; phragnymi, to inclose). 755, 756
DIASTASE, di'astas (G. dia, through;

histanai, to set), definition of 268

DIASTOLE, di'as'tole (G. diastole,

difference) 58

DIATOM, diatom (G. dia, through;

temnein, to cut ) . 124

DIBOTHRIO CEPHALUS LATU8,

diboth'reosef 'alus 296, 301

Same as Bothriocephalus.
DIBRANCHIA, dibrang'kia (G. di,

two + branchia, gills ) 42 1

An order of Cephalopoda with two

gills, two kidneys, and two auricles.

Shell enveloped by a mantle.
DICELLURA, • dlsel'ura (G. dis -f

kulus, hollow) 422

An order of Campodeoidea.
DICOTOPHYME RENALE (G. diko-

tyledon -f- phyme, to grow in the

body) 306

DICOTYLEDON, dlkotlle'don (G. di,

two; kotyledon, cup-shaped hollow). 221
.DICYEMA PARADOXUM, disi'ema

( G. dis, twice + kyema, embrvo ) . . 307
DWYEMIDAE, disiem'ide (G. di, two

+ kyema, embryo ) 306

DIDELPHIA, didel'fia (G. di, two +

delphis, womb) 426, 431. «57

Definition of 431

DIDELPHY8 DORSIGERA, didel'fTs

dorsij'era 057

DIDINIUM, cli'dinium 73.9

DIEM ICTY LU-S VIRLDESCENS,

dlemik'tilos (G. diameno, to perse-
vere + ichthys, fish ) 649

DIENCEPHALON, diensef'alon (G. dia,

between ; engkephalon, brain )

480, 840, 8J,2

DIFFERENTIATION, dif'eren'shia-

shun (L. differe, to differ)

Anterior -posterior 265

Dorso- ventral 265

DIFFLUGIA, difloo'ji'a (L. diffluere,

to flow apart) 148

A genus of Rhizopods.
DIFFUSION (L. dis, apart; fundere,

to pour) . 37, 38

Becoming widely spread.
IUGENEA, dljen'ea (G. di, two +

genes, sexes ) 420

Trematod-a which pass through

several different stages in their life

history.

910

INDEX

DIGENETIC, dljenet'Ik (G. dis, twice;

gignesthai, to produce) 288

DIGESTION, dijes'chun (L. digestio,

digestion) 30, 03

DIGESTIVE CANAL 47

Glands 49

System 47

Tract, development of 494

DIGIT, dij'it (L. digitus, finger) ..45, 708
DIGITATUM, dij'itfitiim (L. digitatus, .

having fingers or toes ) 256

Applied to leaves with finger-like

divisions or processes.
DIGITIGRADE, dij'itigrade (L. digi-
tus, finger, toe; gradus, step, walk).714
DILATOR, dil'fit'or (L. dilitare, to

expand ) 7tf,°f

DIMORPHIC, dlmor'fic (G. dis, twice;

morphe, form ) 255

DINOFLAGELLATA, dmdfla'jelata (G.

dinos, a round area, -f- L. flagellum,

a whip) 151, 41!)

An order of Mastigophora.
DINORNITHIFORMES, dmornithifor'-

mez (G. deinos, terrible, + ornis,

bird -|- L. forma, form ) 425

An order of extinct Aves, that

were flightless^, with enormous hind

limbs, and no wing bones. The Moas.
DINOSAUR, di'nosawr (G. deinos -f

sauros, lizard) 05 1

DIOCOELE, dio'sel, opening in dien-

cephalon, definition of 511

DTODON MACULATU8, di'odon (G.

dis, twice + odous, tooth) tf.J5

DIOECIOUS, die'shiis (G. dis, twice +

oikia, house) 217

DIOSCORIDES, dloskor'Idez (Name of

Greek physician) 376

DIPHYCERCAL, diff'eesir'kel (G.

diplii/es, twofold; kerkos, tail) 084

DIPHYODONT, dif'oodont (G. di, two

+ phein, to produce -f- odous, tooth). 733

Having two sets of teeth.
DIPLOBLASTIC, dip'ldblas'tic (G.

diploos, double; blastos, bud); con-
sisting of two germ-layers 247

DTPLOCOCCI, diplokok'si (G. diploos,

double + kokkos, berry) 100

DIPLOGLOSSATA, diploglosa'ta (G.

diploos, double + glossa, tongue) . .423

An order of Orthopteroidea.
DIPLOID, dip'loid (G. diploos, double

+ eidos}. definition of 101

DIPLOPQ.DA, diplop'ddfi. (G. diploos,

double -+- pous, foot) 422

An order of Progoneria.
DIPKEU8TI, dipniis'ti (G. di, two +

pneustos; f. pnein, to breathe) .425,045
DIPNOAN. dip'noan (G. dis, twice;

pncin , to breathe ) 084

l)ir\Ol, dip'noi (L. dipnous, double

breathing) 425, 045

DIPTERA, dip'tera (G. dis, twice;

pteron, wing) 330, 345, 424

DIPYLIDIUM CAN1NUM, dipeli'dium

( G. dipylos, with two gates ) 296

Latum -301

DISCOGL08SU8, diskoglos'us (G.

diskos, disk + glossa, tongue) 813

DISCO1DAL, SEGMENTATION, defini-
tion of 1 00

DISCOIDES, diskoi'des (G. diskoeides,

disk-shaped) U,8

W8VQWEDU8AM, discomcdu'sa (G.
diskos, a disk -(- L. medusae, me-
dusa) 419

An order of Scyphozoa, in which
umbrella is disk shaped.

DISCRIMINATION, definition of 175

DISSECTION OF FROG 7/8

DISSEPIMENTS, disep'iments (L. dis,

apart; saepire, to hedge in) 200

DISSIMILATION, definition of 125

DISTAL, dis'tal (L. dis, apart; stare,

to stand), definition of. 70, 248

DISTENSIBLE 05

DISTILLATION 38

DISTOMA BU8KI J(G. dis, two +

stoma, mouth) 293

A name for the various genera of
Trematoda.

Felineus 292

Sinensis 293

.DI8TOMUM CONJUNCTIVUM 292

.DISTRIBUTION, dis'trTbu'shun (L.
dis, apart; tribuere, to allot), geo-
graphical, definition of 32

DIVERTICULA, di'vertik'tila (L. de,

away ; rertere, to turn ) 722

DIVISION OF LABOR, divizh'fm (L.
dimdere, to divide), definition of.. 120

DOCILITY, definition of 175

DOG—

Skull 69',

S\veat-glands of 074

DOGFISH—

Brain 85.), 857

Dissected 153, 802

Eye muscles of 827

Longitudinal section of 853

Urogenital organs 820

DOG-SHARK 041

nOLICHOGLOSSVtt KOWALEVSKI.
ddlikoglo'siis (G. dolichos, long +

glossa, tongue ) $46

DOLPHIN 662

DOMINANCE, definition of 103

DORSAD, definition of. (See prefixes
and suffixes at top of Index-Glos-
sary,)

DORSAL AORTA, dor'sal (L. dorsum,
back) J8

INDEX

911

DORSU YEXTKALITY. definition of. .:>(!f>
DOUBLE CIRCULATION, definition

of 535

DRACUN CU LU S MEDINENSIS,

drakung'kuliis (L. little dragon )... 305

A genus of Nematode parasites.
DREPANIDOTAENIA SETIGERA ... 298

DRONE 353

DUALIST IN PHILOSOPHY 174

DU BOIS-REYMOND 380

DUCK-BILL 65IJ

DUCT OF BOTALLUS, dukt (L.

ducere, to lead), definition of 531

of gall bladder and pancreas. ... ^8

DUCTLESS GLANDS 751

DUCTUS BOTALLI 531, 596

Cuvieri 501

Venosus 501

DUGUNG '...661

DUMAS 380

DUODENAL LOOP, definition of 590

DUODENUM, diu'ohdee'num (L. duo-

deni, twelve ) !f8, 49

DURA MATER, diu'rah mah'ter (L.

dura, hard; mater, mother) ... .67, 838
DYAD, diad (G. dyas, two), definition

of 102

EAR—

External 869

Inner 867

Middle 869

Of locust 336

Stones 868

EAR BONES OF MAMMALS 807

EAR DRUM OF FROG 45

EARTHWORM 262-284

Behavior 281

Circulatory system ' 209

Digestive system 267

Embryology 279

Excretory system 272

External appearance 264

Grafting 283

Internal structure 266

Nervous system . 273

Oogenesis 279

Regeneration 282

Reproductive system 275

Respiration 272

Sense organs 275

ECDYSIS, gkdi'sis (G. ek, out; dyein,
to enter ) , definition of 330

ECHIDNA ACULEATA, ekid'nfi (L.

echidna, an adder ) 656

Hy striae 656

ECHIDNIDAE, ekid'nide (L. echidna
+ G. eidos) 657

ECHWNODERMATA, ekmoder'mata
(G. echinos, a sea hedge-hog -(-
derma, skin) . . 428

Radially symmetrical, spiny-
skinned sea animals.

UCHINOIDEA, ekinoi'dea (G. echinos,
a hedge-hog + eidos, form) . . .420, 428

A class of Echinodermata such as
the sea urchin and sand dollar.

HCHINORHYNCHUS, ekmoring'kus
(G. echinos + hynchos, snout) 306

ECHIUROIDEA, ekiuroi'dea (G. echis,
adder + oura, tail + eidos) 310

EGHIURUS PALLA8I, ekiu'rus (G.
echis -\- oura ) 31 1

ECOLOGY, ekol'oji (G. oikos, house;

loyos, discourse) 31

The study which deals with the
relationship between organisms and
their surroundings.

ECONOMIC BOTANY, definition of . . 32
Zoologv 32

ECTOBLAST, ek'tdblast (G. ektos,
without; blastos, bud)

ECTOCHONDROSTOSIS, ek'tokondros-
to'sis (G. ektos, without; chondros,
cartilage; osteon, bone), definition
of 681

ECTODERM, ek'toderm (G. ektos, out-
side; derma, skin), definition of.... 106

ECTOPLASM, ek'toplazm (G. ektos,
outside ; plasma, something moulded ) .

310

The external layer of protoplasm
in a cell, usually slightly modified.

ECTOPROCTA, ektoprok'ta (G. ektos,
-\- proktos, anus), definition of 310

ECTOSARC, ek'tosark (G. ektos, out-
side; sarx, flesh), definition of 121

EDENTATA, ee'dentay'tah (L. e, with-
out; dens, tooth) 426, 658

EDUCATION 32

EDWARD8IIDEA, edwardzl'ide (named
after Henry Milne-Edwards, a French

naturalist ) 420

An order of Zoantharia.

EEL '.(/.'/.$

EFFERENT DUCTS, definition of . . . .610

EFT 6J8

EGESTION, ejes'chun (L. ex, out;
gerere, to carry) 30, 63

EGG AXIS (Icel. egg), definition of.. 553

Tooth, definition of 737

EGGS, OF PARASITIC WORMS 301

Frogs 44

EIMER 404

EJACULATORY DUCT IN EARTH-
WORM, ejak'ulatory (L. ex, out;

jacere, to throw) 272

ELASMORRANCHII, eelas'mowbran'-
kee-eye (G. elasmos, plate; branchia,

gills) 424, 641

ELASTIC SHEATHS, definition of... 612

912

INDEX

ELASTOIDIN, elastol'din (G. elastikos,

elastic) 699

ELECTROTROPISM, elektrotro'pizm
(G. electron, amber + trope, turn-
ing ) , definition of 127

ELEMENTARY EMBRYOLOGY 100

ELEPHANTIASIS, elefantl'asis (G. for

elephant disease) 304

ELLIOT 592

ELYTRA, ellt'ra (G. elytron, sheath) .336

The anterior wing of certain in-
sects, hard and case-like; one of the
scales or shield-like plates found on
the dorsal surface of some worms.

EMBIDARIA 423

A sub-class of Siphunculata.

EMBIIDINA 423

An order of Embidaria.
EMBRYO, em'brio (G. embryon, em-
bryo ) 624

EMBRYOLOGICAL AREA, definition

of 460

EMBRYOLOGY 31

Elementary 100

Of bee 362

Of chick • 433-543

Of earthworm 279

Of frog 543-618

Of insects 346

Of mammal 619-633

EMBRYONIC DISC OF CHICK 456

Of lizard 458

EMBRYOMC SHIELD, definition of.. 624

Sac 244, 464

EMERIA STTEDAE 152

EMOTIONS, definition of 174

EMPIS, em'pis (G. empis, a mosquito

or gnat) 330

EMULSION, definition of 50, 94

ENAMEL ORGAN, enam'el (O. F.
esmaillier, to coat with enamel),

definition of 732

ENCYRTU8, enser'tus (G. enkyrtos,

curved in ) 345

ENCYST, ensist' (G. en, in; kystis,

bladder) 127, 129

For a small animal or a cell to
surround itself with an outer coat.
ENDOCARDIUM, en"d6kfir'(lium (G.

endon, within; kardia, heart) 470

ENDOCRINE GLANDS, endokrm (G.
endon, within ; krinein, to separate ) . 52

ENDOCRINOLOGY , . 744

ENDODERM or ENDODERMIS, en'do-
derm (G. endon, within; derma,

skin), definition of 106, 229, 230

ENDOLYMPHATIC DUCT, definition

of 581, 867

ENDOLYMPH SAC, endolimf (G.
endon, within ; lymph, water ) 866

E N D O P A RAS1T1C, en'dopfir'fislt'ik
(G. endon, within; para, beside;
sitos, food), definition of 288

ENDOPLASM, en'dopliizm (G. endon,
within; plasma, something moulded)
See Endosarc.

ENDOPODITE, endop'odlt (G. endon,

within ; pous, foot ) 31 '/

The inner or mesial branch of a
biramous crustacean limb, or the
only part of a biramous limb re-
maining.

END ORGANS 864

ENDORHACHIS, gndor'akis (G. endon,
within -f- rachis, spine) 838

ENDOSARC, en'dosark (G. endon,

within ; sarx, flesh) 121

The inner portion (or endoplasm)
of the protoplasm in a cell.

ENDOSKELETON, en'doskel'eton (G.

endon, within ; skelletos, hard ) 46

The inner skeleton as opposed to
the exoskeleton.

ENDOSPERM, Sn'dosperm (G. endon,
within; sperma, seed), definition
of 240, 245

ENDOSPORE, Sn'dospor (G. endon,
within; sporos, seed), definition of.. 191

ENDOSTYLE, en'dough'style (G. endo,
within; stylos, column), definition
of 544, 744

ENDOTHELIUM, en'dotheTium (G.
endon, within; thele, nipple), defini-
tion of 271, 470

ENERGY 30

ENGLISH LANGUAGE, need of 40

E NT AMOEBA BUCOALIS (G. entos,

within -j- amoeba ) 142

Coli ' U4

Dentalis 142

Gingivalis - 142

Hystolytica 142

PJNTEROBIUS VERMWULARIS ....301

ENTERON, en'teron (G. enteron,

gut) 280

The alimentary tract.

ENTEROPNEU8TA 424, 640

E N T O C H ONDROSTOSIS, en'tokon-
drosto'sis (G. entos, within ; chon-
dros, cartilage; osteon, bone), defini-
tion of 681

ENTODERM, en'toederm (G. endo +
derma, skin ) 106

ENTOMESODERM, definition of 362

ENTOMOLOGIST, entomol'ojist (G.
entomon, insect; logos, discourse),
definition of .418

ENTONISCLDAE, entonis'kidr- (G.
entos, within + oniskos, a wood
louse ) 327

ENTOPROCTA, entoprok'ta (G. entos
-(- proktos, anus), definition of 310

INDEX

913

ENTOVARIAL CANAL, en'tova'rial
(G. entos, within; L. ovum, egg),
definition of 816

ENVELOPES, FLORAL, en'velups (F.
enveloppe, covering) 242

ENVIRONMENT, environment (F.

environ, about) 32, 157

The sum total of the external in-
fluences acting on an organism.

ENZYME, en'zim (G. en, in; zyme,

leaven) 95, 189

A chemical or unorganized, soluble
ferment.

EPARTERIAL BRONCHI, ep'ftrterial
bronkl (G. epi, upon; L. arteria,
artery + G. bronchus, a windpipe) .770

The first branch of the right
bronchus.

EPAXIAL MUSCLES, epak'sifil (G.
epi, upon; L. axis, axis), definition
of 826

EPENDYMA, epen'dima (G. ependyma,
outer garment), definition of 572

EPHEMEROIDEA 423

A sub-class of Pterygogenea.

EPIBLAST, ep'Iblast (G. epi, upon;
blastos, bud) , definition of 106

EPIBOLE or EPIBOLY, epib'oli (G.
epi, upon; lallein, to throw), defini-
tion of 107, 557

EPICARDIUM, ep'ikar'dmm (G. epi,
upon ; kardia, heart ) , definition of . 520

EPICOELE, ep'Icel (G. epi, upon;
koilos, hollow), definition of 461

EPICORACOID, ep'ikor'akoid (G. epi,
upon ; korax, crow ; eidos, resem-
blance) 76

Pertaining to an element, usually
cartilaginous, at the sternal end of
the coracoid in amphibians, reptiles,
and monotremes.

EPICRANIUM, ep'ikranmm (G. epi,

upon ; kranion, skull ) 334

The region between and behind the
eyes in an insect's head.

EPIDERMIS, ep'ider'mis . (G. epi,

upon ; derma, skin ) 46, 229

Component parts of 665

EPIDIDYMIS, ep'idid'imls (G. epi,
upon; didymos, testicle), definition
of 284, 519, 814

EPIGENESIS, ep'ljen'esls (G. epi,
upon ; gignesthai, to be born ) , defini-
tion of 382

EPI-HYAL, ep'lhi'al (G. epi, upon;

hyoeides, Y-shaped) 738

Pertaining to the upper portion
of the ventral portion of the hyoid
arch. As a noun, the upper element
of the ventral portion.

EP1MERA, epi'mera (G. epi -f meros,
thigh), definition of 315

EPIMERE, ep'imer (G. epi, upon;
meros, part) 460

EPIMYOCARDIUM, epl'mid'kardium
(G. epi -f- mys, muscle -f- kardia, . .
heart) , definition of 529

EPINEPHRIN, epinef'rin (G. epi -f
nephros, kidney ) 823

EPIPHARYNX, definition of 337

EPIPHYSIS, epif'isls (G. epi, upon;

phyein, to grow) 66, 490, 681, 852

Of frog 45

EPIPLOIC FORAMEN, ep'iplo'ik fora-
men (G. epiploon, the caul -f- L.

foramen, an opening )

The opening between the bursa
omentalis and the large sac of the
peritoneum; the foramen of Winslow.

EPIPODITE, eplp'odit (G. epi, upon;

pous, foot) 314

A process arising from the basal
joint of the crustacean limb and
usually extending into the gill cham-
ber.

EPISTERNUM, ep'ister'nfim (G. epi,
upon ; sternum, breast bone ) 76

EPISTROPHEUS, ep'Istro'feus (G.

epistropheus, turning ) 684

The axis vertebra.

EPITHALAMUS, ep'ithal'amus (G.
epi, upon; thalamos, chamber), defi-
nition of 852

EPITHELIOID BODIES, definition of .587

EPITHELIUM, ep'ithe'lmm (G. epi,
upon; thele, nipple) ; surface tissue. 108

EPITRICHIUM, ep'itrlk'ium (G. epi,

upon ; thrix, hair ) 657

An outer layer of the epidermis in
the foetus of many mammals, usually
shed before birth.

EQUAL SEGMENTATION, ekwal (L.
aequalis, equal), definition of 106

EQUATORIAL PLANE, e'kwator'ial
(L. aequalis, equal) 98

EQUIDAE, ek'wide (L. equus + G.
eidos, like) .662

EQUILIBRIUM, ORGAN OF 323, 809

EQUISETALES, ekwiseta'lez (L. equus

4- seta + o,les ) 223

One of the fern allies.

EQUI8ETUM FUNSTONT, ekwise'tum

(L. equus -f- seta, bristle) 224

One of the fern allies.

ERROR, PROBABILITY OF, CHART. 20

ERYOP8, er'iops (G. eryein, to draw
out -f ops, face) 082

ERYTHROCYTES, erith'roslts (G.

erythros, red; kytos, hollow) 56

The red blood cells.

ESSENTIAL ORGANS, definition of.. 228

ETHICS . . 40

I M)i.\

ETHMOID, eili'moid (<!. , //M/M..S. RifVtj

, 1,/o.V. shape I

Plate -HI

Where formed .>01f>

KT10GENOIS

EUGENICS, ujen'iks (G. «-M, well |

,;<no«, birth i ' 1«2

I)., letalOC dealing with the im-
provement ..1 stoek, usually referred
to tin- betterment «•!' tin- human race.
I I '.LENA, ugle'nii (G. tut, well; gl. „. .
pupil of the- eye) 127

A species of Mattiffophom.
i: II LAM /•; I LI BRANCH! A, nlameli-
hning'kia (L. lnnifllti, a thin plate

I (I. l.ran.-lnat; gills) 421

AII order cl 1'iltcypoda.
EHSTAC11IAN TUBE, usta'klan (I.
l.n.shirlii. Italian physician ) , defini-
tion of ' 71, 490, 8M

Of frog 4f>

/ I ////.'/»•/,!, uthe'ria (G. n,, well -f-

/Arnon, a beaut) 42tt. 4:11. <>a7

I). Unit ion of 4.'il

/ / nn A/./7M, uthinu'ra (G. ri///n/«.

straight + neuron, nerve) i ' I

A sub-class of Gastropoda.
IMAGINATION, pvfiji»iVshun ( L. <>,

tmt; vagina, sheath) . 2!>'i

The proeews of unsheathing, or
|n«)(luct of this process; an out
growth.

KVAI'OKATION MS

I-VIDKNCE FOR EVOLUTION 40(J

EVOLUTION, ^volu'shftn (L. t'n>h,,,.

to unroll i 402-413

Cause of , 412

* 'onvrr^vnt 412

* <ntiM> of 412

Criteria for a sat isfuci.u \ llu-orv

of ' '.413

Divergent 4 I'J

KxiiU'iiiT for KM;

Evidence against -Ins

riulivid\ial 31

i;:u>ial 31

Summary oi «-\ iili-nci- H-.'

I \CKET1ON, ekskn-'Nliun ( L. «.* , out;

. . nn a, to shift) 30

Act of eliminating waste material,
or the product of the process

I M'lJETOHY SYSTEM tl3

I -Aui'CIPITAL, eksokaip'ital (L. e-jr -f

nt'fiput, back of head) 73

i \tu oi !.<>M. ,-x ..so'lom (L. r.r -f G.

/,.u7ow, hollow), definition of 461

i Mu'IUNE GLANDS, eks'okrin (L.

G. krino, I separate) 52

KXONAUTE8 GILBKRTI, eks5nft't6z
(G. rxo, outside + Hunt*-*, sailor) .<; '/ ',

KXOI'oDITK. .'k^op'odit. (U. evo,

without ; IKHIH. foot I ,>'/ )

Tin- oiitrr brani'h of a tvp'cal hira

mous crustacean limb.
K \OSKELETON, ek'sosk^h-ton ( (!. , 9O,

without ; ,s7i» 'It-ton, hard) 347

A hard supporting struct HIT

.secreted by tlic ectoderm or by t he

•kin.

I-. \riKATlON, .'kspira'shfui (L. , -.,-.

out; spirare, to breathe) 30

BXTENSILlS TONGUE 47

i:\TENSION, definition of 43. 7H

EXTENSORS si'it

EXTERNAL CAPSULE Ol«' HKAIN st!»

Gills 549

INTERNAL EAR OF FROG, ekster'-

nal (L. externus, outside) J ..

EXTRA EMBRYONAL AREA. .Idini

tion of I t'.n

lAIKA EMBRYONIC MEM BRA MIS

<H CHICK, ek'stra-ern'brion'ik ((J.

•exter, outside; embn/on, foetusi /r/,{
EXTRINSIC MUSCLES, okstm. \ik

(L. extrinsecua, on the outside) s-J(.>
KXUMBRELLA, ekaumbreTa (L. -

out; umbru, -hade), dclinitioii of. .'..I

KYE MUSCLES (M. E. ighe)

Lens 4!H

Birds ,S?'/

FABELLA, fabel'fi (L. faMella, a small

bean) 710

FABRIC1US, fabre'shus (L. fabric*.

a German entomologist) .'IS I

FACIAL, fA'shal (L. farics, face) C.s

FA.CIOLOP818 BUSKl !!> :?

FACTS, COORDINATING 40

PACTS VS. INTERPRETATIONS 32

KAKCKS, fe'sez (L. faeces, dregs) .... 50

The excrement or waste matter

from the bowels.
I •' \LCIFORM, fftl'siffirm (L. fair,

sickle) ;,37

Sickle-shaped or scythe-shaped.

Process of eye 87<i

I \LCONIFORMES, fftl'e«>ni forme/ ( L.

/a/ro, a falcon -f forma, form) .... 426
An order of falcon-like Ares with

curved beak, hooked at end, and

sharp, strong claws.
FALLOPIAN, falo'pian (after Kallo

plus, an anatomist). See Oviduct.
KALLOl'll S, ITALIAN ANATOM1ST.37S
!• \IA ( KKi:i;Kl. fAlks cerebri (L.

fair, a sickle) S.1S

FAMILY, famili (L. familia, the

household ) 4 1 f>

/ I \ \/ I c\ MCVLAR18 . . .368

FASCIA, fAi-li'isi (L. fam-ia, u hand or

bandage) 78. 838

An erishea then ing band of connec-
tive 1.i--u<-.

1CULUS, FASCICULI, fa'siku-
IHH uli (L. a little bundle) . . . .828, 836

The direct pyramidal tract or
tracts.
FASCIOLA HEPATICA, faseola (L. a

-trip of cloth) %m

The liver fluke, also railed Dig-
toma.

FASCIOLAR1S GYRUS, fas'Iola'rls
jl'rus (L. fascwla, a small bandage). 8^4

OIOJ*LOP8I8 ItLHKI 292, 301

i AT BODY (A. S. faett, fat ) 48

FATS, definition of ,'..;*. ,'-. . 90

FAUCES, fawV'K (L. throat) .739

FEATHER—

Development of .-....- 057

Structure of ,<.H /. y. ' . .670

Tracts .;-. . ,v./.rf, * , ; ..1C,:.?. 069

FEELINGS, definition of 174

FELIDAE ^ ,f >. , , 058

I I. LI 8 LEO ,, ..,..; . , ; , . ^ . . 73/,

FEMORAL, FEMORIS, fem'oral, or is

( L. femur, thigh ) . 792

Pertaining to the thigh.
I J. MORAL PORES, definition of. 009, 073
FEMUR, fe'mur (L. thigh), definition

of ......-;".,. ....77, 707

I-KNESTRA OVALK .808

FENESTRA OVAL16 715

Rotunda 808

Tympani 808

i KKUGREK y.;

FKRMEKT, fer'rnent ( L. ft-rmentum,

ferment) 50, 92, 188

FERN ALLIES 22',

FERTILIZATION, fer'^ilixa'shun (L.

fvrtilis, fertile) 97, 104

Artificial „ 205

Meridian 550

FfltRILLAE, fibrll'e (L. fibrilta, nmall

fib-r) ..../ -.112

The thread-like branches of roots;
minute elastic fibers secreted within
l<< rigin cells; minute muscle-like
ili reads found in various Infusoria,
(HO called from being found in hay
infusions) .

: 1 1 ."i

FIBULA, fib'ul;, (L. .-lanp or buH.I,

77, 703

The outer ami smaller bone of tin
ittlri.

. 1 98
.617

FIJUiOBLASTS

F I BROS SHEATH

FIMROVASCULAK BUNDLE 210

Bi-collateral 230

Closed collateral 230

Collateral 230

Complete 232

Concentric . . 230

Incomplete 232

Radial ...23<i

FIEHAHTHH ACUH . .644

FILAMENT, fil'ament (L. filum,

thread) 75, 243

The stalk of the anther; the stalk
of the down feather.
I 1 1. \ltlA BANCROFTI (L. filum,

thread ) 303

Mcdinen»ig , 30.J

Oxardi , 303

/'ILARIIDAE .-304

I I LAR THEORY .;,,,,. 95

FILIAL GENERATION, definition of 168
VILIBRANCHIA, filibrang'kia (L,
filum, thread -f- branchiae, gills) . . .421

An order of Pelecypoda.
I 1 1. 1 FORM, fl'lifurm (L. filum, thread'

-f- forma, form) , , *, ,7.730

Thread-like.
F1LOPLUME, fil'oplum (L. filum,

thread -4- pluma, feather) .670

A delicate, hair-like feather with
long axis, and a few barbs at the
apex.

FILUM TERMINALS 67, 837

FIMBRIA, fim'bria (L. fringe).. ..470
Any fringe-like structure.

FINLAY, DR. CHARLES 130

FINS—

Undifferentiated MM

FISCHER , ,« ;.19I

FISHES—

Cranial nerves of Hfil

Development of skull ~t3!t

lOX/finh'un (L. fiattuK, a cleft) . .250
Cleavage of cells; division of a
unicellular organism into two or
more parts, thereby reproducing its
kind.
FISSURE, fish'ur (L. figgug, cleft). . 07

A cleft or deep groove, or furrow,
dividing an organ into lobes.

Of brain 843

FISTULA, fts'tula (L. fistula, a pipe,

tube) 729

FIXED NERVE ARCS, definition of.. 180
I J.ACKLLATA, flajebVta (L. fiagel-

lum, whip) 143, 149, 418

An order of Masliyophctra.
FLAGELLUM, flajgl'um (L. a whip),

definition of ,,, 127

FLAME CELLS, definition of 280

FLAT WORMS AND THREAD

WORMS , 285-311

S<><» Platyhelminthes and Nema-
t helrninthes.
FLEAS - .MU

916

INDEX

FLEXION, flek'shun (L. flexus, bent) .

43, 78

FLEXORS, definition of 829

FLOCCULUS, flok'ulus (L. a little

piece of wool ) 859

A small accessory lobe on each
lateral lobe of the cerebellum.
FLORAL ENVELOPES, flo'ral (L.

//os, flower ) 2.}2

Organs 2j2

FLOUNDER 645

FLOWER CL..flos, flower), purpose of. 242

FLOWERING PLANTS MM

FLOWERS (L. flos, flower) 242

The blossom of a plant, compris-
ing generally sepals, stamens, and
pistils.

A leafy shoot adapted for repro-
ductive purposes.

FLUKE, LIVER 289-293

. Asiatic lung 291

Bronchial 291

FLY 307-371

Foot of 333

Head of 333

Killers . 371

Life history 3(59

FLYING FiiSH 044

FOETUS, or FETUS, fetus (L. foetus,

offspring ) 624

An embryo in the egg or in the
uterus.
FOLIACEOUS, folifi'shiis (L. folium,

a leaf) 315

Leafy.

FOL1OSE LICHEN, fo'lios li'ken (L.

folium, leaf; G. leichen, liver wort).

FOLLICLE, fol'ikl (L. folliculus, a

little bag) 442

A capsular fruit which opens on
one side only.

FOLLICLE, GRAAFIAN . .443

FONTANELLE, fontaneT (F. a little

fountain ) 72, 684

A gap or space between bones of
the cranium, closed only by a mem-
brane.

FOOT CELLS OF SERTOLI . . . . 520, 811
FORAMEN, FORAMINA, foramen, rfi'-

mena

Any opening.

FORAMENIFERA, loraminifera (L.
foramen, a hole; ferre, to bear) .148, 418
An order of Rhizopoda.

FORAMEN MAGNUM 72

Of Panizza 786

Ovale, definition of 534

FORE BRAIN* 472

Gut 455-484

FOREIGN LANGUAGES, value of. ... 41
FORMATION OF GERM LAYERS IN
MAMMAL . . 622

FORMATIVE POLE, definition of.... 105
FORN1CATUS GYRUS, fornikfitus

jirus (L. fornicatus, arched) 844

FORNIX, fOr'niks (L. an arch or

vault) 845

An arched sheet of longitudinal
fibers beneath the corpus callosum.

FOSSA, fOs'fi (L. a pit or cavity) 72

A ditch or trench-like impression.

Hypophyseos 687

Rhomboidalis 800

FOSSIL, fos'il ( L. fossilis, dug up ) . .

Remains 399

Kndex 398

FOSSORIAL 654

FOVEA CENTRALIS, fo'veft (L. fovea,

depression ) 873

FRENULUM, fren'ulum (L. a bridle

or bit)

.A fold of membrane as of tongue
or clitoris.

FRENUM, fre'num (L. bridle) 73/>

A frenulum.

FRICTION RIDGES ....678

FROG—

Arteries 59

Brow spot 45

Circulatory system 53

Columella 45

Digestive system 47

Ear 45, 71

Egg 44

Embryology —

Comparison of Tadpole and Chick. 543

Blastulation 553

Classification of chordata 543

Embryo formation 564

Fertilization 550

Gastrulation 550

Later development of tadpole . . . 569

Lateral line organs 582

Maturation 552

Medullary plate . 5f!2

Nervous system 570

Somites . 568

Digestive Tract 585

Midgut derivatives 589

Hindgut derivatives 590

Mesodermal Somites 591

Table of Somites 592

Circulatory System 594

Arterial system 590

Heart 594

Origin of —

Circulatory system 599

Venous system 599

Lymphatic system 602

Septum transversum 604

Urogenital System 605

Adrenal bodies 610

Epinephroi 610>

Mesonephros 608;

INDEX

917

Wolffian body 608

Skeletal system 612

Skull 613

Epiphysis 45

Eustachian tube 45

Excretory system 63

External features 44

Eye 69

Fat bodies 86

Glands , 52

Heart ~*

Histology lOv.

Internal structure . . 46

Iris '... 44

Medial eye 45

Muscular system 77

Names of 45

Nervous system 65

Olfactory organ 71

Orbft 44

Rep\ >ductive system 85

Resp, ration 62

Sense organs 68

Skeleton 73

Summary 117

Tongue 71

Touch and pressure 71

Tympanic membrane 44

Veins . 60

FROG-SPIT, definition of 204

FROND, frond (L. frons, leafy

branch ) 223

A leaf, especially of fern.
FRONTAL, frun'tal (L. frons, brow) . . 73
In the region of the forehead.

FRONTALS 617

FRONTIGENA 335

FRUIT, frfit (L. fructus, fruit), defini-
tion of 227, 242

FUCUS, fukus, experiment on eggs of,

(G. fykos, sea-weed) 206

FUN ARIA, fumYria (L. funarius, be-
longing to a rope) 211, 220

FUNCTIONAL PSYCHOLOGY, fungk'-
shunal (L. functus, performed), defi-
nition of 175

FUNDAMENTAL TISSUES, definition

of 229

FUNDUS, fun'dus (L. the bottom; the

base of an organ )

FUNGI, fiin'ji (L. fungus, a mush-
room) 190, 203, 208

FUNGIFORM, fun'jiform (L. fungus,

a mushroom ) 739

Shaped like a fungus.

FUNGI TUBE 208

FUNICULUS, fiunik'yulus (L. a small

rope or cord) 244, 836

A small cord or band of fibers,
especially in the brain.

FURCULA, fur'kiulah (L. a little

fork) 329, 704

A forked process or structure, the
merry-thought bone.

FUSCA, fus'ka (L. fuscus, dark,
dusky) 249

GADOW 652

GADUS MORRHUA, ga'dus morua (L.

gadus, cod-fish ) 644

GALEN 376, 388

GALL 750

GALL-BLADDER, gol (A. S. gealla,

gall) 48, 50

GALLERIA MELLON ELL A 365

GALL-FLY 345

GALLIFORMES, galifor'mez (L.

gallus, a cock -+- forma, form) 425

An order of fowl-like birds with

feet adapted for perching.

GALLS 192

GALTON 384

GAMBUSA, gambu'sfi (Cu. gambasina,

nothing ) 138

GAMETE, ga'met (G. gametes, spouse) 85
GAMETOCYTE, game'toslt (G. gametes

+ kytos, hollow) 132

GAMETOGENESIS, gametojen'esis (G.

gametes -+- genesis, birth) 100

GAMETOPHYTE,g!\me't6flt (G.

gametes -f phyton, plant) 215, 220

GANGLIA, gang'glia (G. ganglion,

little tumor ) 68

Brain 847

GANGLION CELLS, gan'gleeon (G. a

tumor ) 115

Gasserian 65

GANOID, gan'oid (G. ganos, bright) .643

GARPIKE 643

GASSERIAN GANGLION, gasee'rian

(from a physician, Gasser) . .65, 513, 884

Same as semilunar ganglion.
GASTRIC, gas'trik (G. gaster, stom-
ach) 317

Of birds 754

GASTRIC MILL, diagram of 317

GASTROCNEMIUS, gas'troknee'meus

( G. gaster, stomach ; kneme, shank ) 80
GASTROCOEL, gas'trohseal (G. gaster,

stomach ; koilos, hollow) 722

GASTRO-HEPATIC OMENTUM 537

GASTROPODA, gastrop'oda (G. gaster

+ pous, foot ) 421, 424

A class of Molhisca such as snails,

slugs, etc.
GASTROVASCULAR CAVITY, gastro-

vaskular (G. gaster -f- L. vasculum,

a small vessel), definition of 247

GASTROZOOID, gas'trozo'oid (G.

gaster -f- zoon, animal) £55

918

INDEX

OASTIU'LA, .gas'tre\vlah 0- »

- stomach ) : .

Of Amphibian ... . 451

Of Amphioxus . //5/

.Of Bird , '/50, 45 1

Of Frog 555

GASTRULAR CLEAVAGE, definition

; of 55(5

Groove 556

GASTRULATION, definition of ...... 1G6

Of frog ; ...558

GAULE, ALICE 51

G AVI AH DAE. GANGETICU8 .653

GECKONES, gek'onez (Malay, gekok.

imitator ) 654

GEGENBAUR , ,380

GELATIN, jeTatin (L. gelare, to con-
geal) . . .' '. v;',.. 93

GEMMAE, jem'a (L. plu. of gemma,

bud ) 222

GEMMATION, jemfi'shon (L. gemma)..l8V
GENAE, jc'nf> (L. plu. of gena, cheek). 334
GENE, jf'n (G. gene, descent) . . . . . i . .16(5
GENETICS, jenet'iks (G. gignestluii,

to produce) 32, 165, 171

GENICULATE BODIES, jenik'ulat (L.

• geniculum, little knee). 852

GEN10HY01D. jinye'ohigh'oid (G.
geneion. chin; upsilon, a Y-shaped
letter of the Greek alphabet) ...... 81

GENITAL PORE, jen'ital (L. gigno,

to reproduce) . . . .284

'TENU, jee'new (L. knee) ,

GENUS, je'nus (L. genus, race) . . 415
GEOGRAPHICAL DISTRIBUTION... 32

GEOLOGIC CHART 394-305

GEOMETRY, gOom'etri (G. geometria,

geometry, land measuring) . . . 39

GEOTROPISM. jeot'rdpizm (G. ge,

earth 4- trepein, to turn ) 127

GEPHYRKA, jefire'a (G. gephura, a

mound )•....'.. '.310. 424. 432

Worm-like animals of uncertain
position.
GERM, ierm (L. qermen, Inid ) 190

GERM-BAND OF INSECT 3fi2

layers .280

Plasm .' 99

Ring 556

Wall 451

GERMINAL CELLS. jorm'Tnal (L. r/er-

mrn. bud ) 572

Disc 443

Emthelimn /^2

Vesicle .443

GERMINATE 220, 224

GERM! NATI VUM, jerr'minayti vum

(L. (tcrtnino, to sprout) .665

QKRYOXIA, jerlo'nia (G. Geryon,
Geryon, a three-bodied giant) ..... .254

GERYOXIIDAE, jer'Tonide (G. ger-

i/on } 25$

A family of Coelenteratefi.

(JKSTATION. josta'shnn ( L. yerere, to

. carry) ....<>!!>

GIANT FIBERS, definition of... 374

GIBBON MHi

GIGANTORHYNCHUS GIGAS...305, 30(i
tUGANTOSTRACA, gigantos'traka (G.
gigos, giant + ostrakon, a shell) . . .421
An order of Merostomn.

GIGARTINA 205

GIL A MONSTER, Hela . .654

GILL-ARCH, gil (M. E. gille, gill).

Theory of . ... . : . . . 69!)

Clefts [.«. .4<)S

Pouches ,. . . . .49S

Septa ... . . .775

: Slits 72!)

GIRAFFIDAE, jiraf'idc (At. • -saraf,

giraffe + eidos) .-. . . 660

GIZZARD, giz'ard (M. E. gizer, gi/

. zard) . . : .'. ,. . .267, 745

GL.WDIVEPti HA ryA'*S7 . . 7 ',H

GLANDS, glandz (L. gln-M, an acorn) 52

Acid ' -. . ,. ; . .Ml

Acinous -. . . . .m.t

Adrenal 53

Alkaline . . . Ml

Anal . ; . ., r. . .V.;. . : . . . .157:*

Bowman's - .... 872

Buccal . .. . . 74-2

Calciferous . .- . :"* ......... 2iis

Carotid '>'», oi), 588. 74:;

Cement . ., -i'l"

Cloaca! . , ; «»7-'J

Colieterial . ^.;. > . . •>>!il>

Cowper's 820

Digestive 4!i

Ductless 751

Endocrine 52

Exocrine . . . 52

Green 321

Harder's 875

Intermaxillary 741

Internasal 741

Of internal secretion 75 1

Labial 741

Lachrymal 5S2

Lymph $<K>

Mammary (57. 'I

Meibomian (>7.">

Molar 742

Mucous 4(5

Necrobiotic (!7:>

Orbital ' 74-2

Palatal 742

Pineal 490

Pituitary 490

Poison Mf

Prostate : . 819

Prostate of Hit inline -'*..'

Pterygopodial (5(58

-'Red'. 7>i/

Retrol:ngual 742

Saliyary . .741, 742

.INDEX

Si-bae;>ous 673

Sebitie . ..... ..... .344

Serous • • - - 742

Spleen (504

Stenson's 872

Sub mandibular 742

Sweat .'. 672

Sweat in amphibia 63

Tarsal . . 673

Thymus 53, 587, 77/3

Thyroid 53, 7//S

Tubular . . . 40

Uropygial 670

Vitally secretory - . 673

Vitelline '. 292

Wax 1 364

pLENOID. glee'noid (G. glene, a

socket ) 76

Cavity VS«*k*fc« • 77y>;

GLIA CELLS, gll'A. gle'a (G. glia,

glue) 572, 835

G LOU I G E R 1 N A, globij'erl'na (L.

globus, globe ) 1 48

An ooze largely composed of
Foraminifera shells.

GLOMERULI OF NERVES SS I

GLOMERULUS, glowmer'yulus (L.

glomus, a ball of yarn) (i'i

GLO&SV.Y.1 PALPALIS, glosi'na (G.

glossa, tongue) : 144

( ! LOSSC ) PE1A R YNGEA L. gloss'ohfa-
rin'jeeal (or far'inje'al) , (G.
glossa, tongue; pharynx,' pharynx) . . 68
GLOTTIS, glolt'iss (G. glol-ta, tongue)

' 48, 588

GLUTEl'S, glutee'us ((J. gloutos.

rump ) 80

GLYCOGEN, gl!'kojen(G. glykox, «*-eet

-(- lyein, to loosen ) 51

GNA THOSTOMA TA , nat h'oli stow'ma -
ta (G. gnat h os, jaw; stonia, mouth)

424. 431, «',/, 720

Definition of 431

GOBLET CELLS, gob'let (L. rupcllus,

a little cup ) 40, 10!)

GOEHLICH .277

GOETHE 3S4

GOLGT 134

Apparatus 01

GONAD. gonn'ad (G. gonos. seed) .... 85

GONANGIA, plu. of gonanghnn 252

GONANGIUM. gonan'jium' (G. gonr.

seed + anqyeion, vessel) 253

GONIALE, gori'iale, gonmla (G. gonid.

corner ) 607

G 0 N I 0 A7 E M U 8, gonio'nemus ( G.

gonia, an angle, corner ) !?;7.1, 254

A Coelenterate.
GONOTHECA. gon'othe'ka (G. gone +

theke, cup) '. 2~,2

GONOTOMES. gon'otxlmz (G. gone +

hi, to cut ) ^.

GOR.DIIDAE, gordl'idr- ((;.

king of Phergia) . . 308

GORDIU8 AQUATICUN ....... . . . . .308

Lineatus 308

GO RGON AC E A, gorgon'acea ( G.

gorgos, grim, fierce) ,- • • • 420

An order of Anthozoa.
GORILLA EN GEN A, goril'a (G. goril-
la, native name of the wild crea-
ture) . .^'.660

G088YPARIA MANNIFER8 (L. gos-

sypion, a cotton-tree ) 340

GRAAFIAN FOLLICLE, gralif'ean
(after de Graaf, a Dutch physician)..,

440, >,',3, 620

GRACILIti, grass'ilis (L. slnuier) .82. 156

GRAIN LOUSE 350

GRANULES, grfm'ul/ ( L. granti-lum,

small grain) 89

GRANULOMA CQQQIDlQWm 212

GRANULOSUM "'. 665

GRASSHOPPER . 332

Behavior : 348

, Circulatory system 338

i, ,;. Digestive system 337

Excretory system . , 330

External appearance . .334

Internal anatomy 337

v Muscular system .343

Nervous system 340

Paedogenesis . .345

Polyembryony 345

Reproductive system 343

Respiratory system 330

Senses 341

GRASSI 132

GRAY 385

GRAY CRESCENT, definition of 552

Matter of nervous system . . . .67, 836

GREIttiti ". 417

GREEN FELT, definition of 207

Glands 321

GREEK, value of 30

GREGARINA 143, 1 53

GREGARJNIDIA, «,nvgfmn'Idfa (L.

gregarius, gresrarious) 152, 410

An order of Telospnridiu.

GROWTH 30

Zone 556

GRUB 348

('JRriFORMEM, gruifor'mex (L. grns,
a crane -|- formia, formed like) . . . .425

An order of crane-like birds such
as cranes, rails, etc.

GR YLLOBLA TTOIDE. \ ( L. gryllutt, a
cricket + blatta, cockroach or

moth ) 423

An order of Ortliopteroidea.

<}RYIJ,l'tf PENNRYLYANICUR 3/,2

A cricket.
GUARD CELLS . .236

920

INDEX

GUBERNACULUM, giu'bernak'yulum

( L. a rudder ) 818

GUDERNATSCH 52

GULEA 335

GULLET, giil'et (L. gula, gullet) 127

GYM NOPHIONA, jlmnofl'ona (G.
gymnos, naked + ophioneos, serpent-
like) 425, 504, 647

GYNAECOPHOROUS CANAL, jine'ko-
f orus ( G. gyne, woman + pherein, to

carry) 291

GYNANDROMORPHS, jinfm'dromorfs
(G. gynandros, of doubtful sex -f

morphe, form ) 366

GYNOECIUM, jme'smm (G. gyne -f

oikos, house) 203

Same as pistil.

GYRUS, jl'rus (plural, gyri), (G.
gyros, round ) 843

HABENULA, haben'yula (L. a little

band) 856, 571

HABITS, definition of 180

HAECKEL, ERNST 385

HAECKEL'S LAW OF BIOGENESIS. 407

Invalidity of 161, 409

HAEMAL ARCH, hem'al arch (G.

haima, blood ) 681

HAEMATOBIA 8 ERR AT A, hemato'bia

(G. haima + Mos) 369

H A E M A T O CHROME, hem'atokrom
(G. haima, blood -f- chromos, color). 128

HAEMATOCOCCUS 185

HAEMATOZOIN, hem'atozd'm (G.

haima -f- zoon) 132

HAEMATUM (G. Jiaimatos, of blood). 709
HAEMOCYANIN, hem'osi'fmm (G.
haim-a -f- kyanos, dark blue sub-
stance) 318

HAEMOGLOBIN, hem'dgld'bm (G

haima -f- globos, sphere) 56, 269

HAEMOLYTIC, hemoli'tik (G. haima

+ lyein, to dissolve) 200

HAEMOSPORIDIA, hemo'sporid'ia (G.

haima -f- spora, seed) 143, 153, 419

HAG FISHES, definition of 641

HALE 380

HALF-BREEDS 166

HALICORE DUGUXG, hfilik'ore, doo-
gung (G. hals, sea -f- kore, made) .661

HALL 380

HALLER 380

ItALLERl CAMPANULA 876

HALLUX, hal'luks (G. hallux, toe)..

77, 710

HALM ATU RU S, halmatu'rus ( G.

haima, spring -\- oura, tail) 672

HALZOUN, hal'zun 292

A disease caused by the liver fluke.

HANDLIRSCH 418

HAPLOID, hap'loid (G. haploos, sim-
ple + eidos, like) '. . 101

BAP LOU I 643

HARDER'S GLAND 875

HARE-LIP, definition of 500

HARVEST MITES 350

HARVEY 378, 386, 388

HAVERSIAN (Havers, English physi-
cian) 110

HEAD KIDNEY 504

HEART—

Beat, when it begins 522

Development of ^77

Development of, in chick 468

Of frog 55, 57

Synangium 55

Ventricle 55

HEATING 38

HEAT-REGULATING APPARATUS.. 38

HEGNER, R. W 418

HEIDELBERG JAW 39!)

HELIOZOA, he'liozo'a (G. helios, sun

H- zoon} .148, 418

An order of Rhizopoda.

HELLBENDER 6J,8

HELMHOLTZ 380

HELO.DERMA HORRIDUM 654

HEMIARCYRIA ' 205

HEMIAZYGOUS VEIN, hemiaz'igos

(G.hemi, half + azygos, unyoked). 797
H E MI CHORD ATA, hem'ikorda'tfi

( L. hemi, half ; chorda, cord )

424, 431, 640

HEMIDAGTYLU8 TURICUS, hemi-

dak'tilus (G. hemi + daktylos, a

finger ) 655

HEMIPTERA, hemip'terfi (G. hemi +

pteron, wing) 424

An order of RhyncJwta.

HEMISPHERES OF BRAIN 843

HEMI8U8 GUTTATUM 101

HENBANE, heii'ban (A. S. hennelell,

lit. hen-bell) 234

HENLE'S LOOP, definition of... 505, 806
HENSEN, LINE OF, definition of.... 114

HENSEN'S NODE, definition of 453

HEPAR, he'pfir (L. hepar, liver)

HEPATIC (L. hepar, liver) 234

HEPATICA (L. hepar, liver) 234

HEPATIC PORTAL SYSTEM 50

HEPATOLYTIC, hep'sito'lltik (L.

hepar + lyein ) 200

ITEPTANCHUS, heptang'ktis (G.

hepta, seven + anfjchein, to choke) .766
HERBIVEROUS, herbiv'orus (L.

hcrba, grass -j- vorare, eat) 660

HEREDITY, hered'Iti (L. hereditas,

heirship) 32

HERMAPHRODITE, hermaff'rowdite

(G. Hermes, mercury; Aphrodite,

Venus )

Frog 822

Moth 366

True, definition of . . . 822

INDEX

921

HERRICK 851, 854

HESPERONITHIFORMES, hesperom-

theform'ez (G. esperos, west -\- ornis,

bird + form ) 425

An order of fossil, toothed birds

with teeth in a groove.

HESSIAN FLY &7//

HKTEROCOELA, het'erohsee'lah (G.

heteros, different; koilos, hollow).. 419
HETEROCERCAL, het'eroser'kal (G.

heteros + kerkos, tail ) 685

HETEROCYEMIDAE 306

HETERODONT, het'erohdahnt (G.

heteros, different ; odon, tooth ) 733

HETEROGENESIS, het'erojen'Ssis (C.

hetcro -\- genesis) 403

HETEROMETABOLOUS, het'eromet-

ab'olus (G. heteros + metabole,

change) 331

HETEROMl 643

HETEROMORPHOSIS, het'eromor'fo-

sis (G. heteros -\- morphe, shape).. 282

HETEROPHYES 292

HETEROTRICHA, heterot'rlka (G.

hetros, other + thrix, hair).. 154, 419

An order of Ciliata.
HETEROTROPHIC, het'erotrof'ik (G.

Jutcros 4- trephein, to nourish) . . . .186
II E T E R O T ROPIC CHROMOSOME,

definition of 168

HETEROZYGOUS, heterozl'gus (G.

hetero -\- zygein, to yoke) 167

HEXACTINELLIDA, heksaktmel'ida

(G. hex, six -f actis, ray) 419, 427

An order of Porifera, such as the

deep sea-sponges.
IIEXANCHUS, heksang'kus (G. hex +

agchein, to choke) 766

HEXAPODA, heksap'oda (G. hex, six

-f pous, foot) 328

HIND BRAIN 66, 472

Gut 486

HIPPOCAMPUS, hip'pohkam'pus (G.

hippos, horse; kampos, sea -monster)

Barbouri 6^.)

Of brain 843

HIPPOCRATES 376

HIRUDINEA, hiriidin'ea (G. hirudo,

leech ) 283, 421, 428

A class of Annelida without setae

or parapodia, but with suckers.
HIRUDO MEDICIXALI8, hiru'do (L.

hiriido, leech) 282

HISTOLOGY, histol'oji (G. histos,

tissue + logos, discou-Fse) 31

Of frog 108

Plant 228

HISTORIA ANIMALTUM 378

HISTORY OF BIOLOGY 375-392

HODGE 370

HOFER 124

HOLDFAST . ..187

HOLOBLASTIC, hol'oblas'tik (G.
holos, whole; blastos, germ) 279

IIOLOCEPHALI, holosef'ale (G. holos
+ kephale, head) 424, 642

HOLOMETABOLOUS, hol'ometab'olus
(G. holos + metabole, change) 331

HOLOPHYTIC, holofit'ic (G. holos +
phyton, plant) 128

HOL08TEI, holoss'tee'eye (G. holos,
whole; osteon, bone) 425, 643

HOLOTHURIA, holothu'ria (G. holo-
thourion, a kind of zoophyte) (?-).}

HO LOTH UROIDEA, holothuroi'dea
(G. holos, whole + thourios, rush-
ing) 420, 428

A class of Echinodermata with
ovoid, muscular body -wall and ten-
tacles around mouth. Example: sea-
cucumbers.

HOLOTRICHA, holot'rika (G. holos,

whole + thrix, hair) 153, 419

An order of Ciliata.

HOLOZOIC, hol'ozo'ik (G. holos +
zoon, animal ) 129

H O M 0 C E R C A L, hom'oser'kfil (G.
homos, the same; kerkos, tail) 685

HOMOCOELA, homose'la (G. homos,

same + koilos, hollow) 419

An order of Porifera.

HOMODONT, hom'odont (G. homos +
odous, tooth) 733

HOMOGENEOUS, homoje'neus (G.
homos + je'nos, race or family ) . . .111

HOMOLOGIES 119

Serial 316

HOMOLOGUE, hom'olug (G. homos +
logos, speech ) 65

HOMOPTERA, homop'tera (G. homos

-f- pteron, wing) 424

An order of Rhynchota.

HOMOZYGOTE, hom'ozi'got (G. homos
+ zygein, to yoke) 167

HOMOZYGOUS, homo'zigus (G. homos
+ zygein, to yoke) 167

HONEY-COMB OF STOMACH 745

HOOKE, ROBERT 378

HOOKER 385

HOOKWORM EGGS 301

HORMONE, hor'mon (G. hormao, to
excite) 52, 823

HORSE-HAIR SNAKES 30S

HORSETAILS 224

HORTUS 8ANITATI8, hor'tus sanita'-
tis (L. hortus, garden + sanitas,
health) 377

HOSTS OF TAPEWORMS 297

HOW TO STUDY.... 27

HUMERUS, hiu'merus (L. the bone of
the upper arm ) 76, 708

HUMUS, hu'miis (L. humus, earth) . .263

jTT^rp^ JOHN 380

HUXLEY, THOMAS 365, 380, 385

INDEX

VKOWTA (Xi. /,//.«///«, a

hyena ) . . ?#)

HYALINE, hi'alin (G. hyalos, glass). Ill
HYALOPLASM, hi'aloplasm (G. hyalos

+ plasma, something moulded )•.... 90
HYDATINA, hidat'ma (G, hydntis,

watery vesicle ) 308

HYDRA FU8CA, hi'dra fus/ka (G.

hydra, water snake ; fuscus, dark

brown ) 247

HYDRANTH, hi'dranth (G. hydor,

water + anthos, flower) ,„ . . .252

HYDRAZOA, hl'dra/d'a (G. hydor +

zoon) . .256, 419^427

A class of (Joelenlerata.
HYD#OCAULI, hl'drd'koli (G. hydor .

-b kaulos, stalk ) . 252

// VltROCORA LLI\AK, hldrdkorali'nf'

(G. hydra, watiM' -f- L. cpralliniis,

coralline} 410

An order of colonial Hydrosoa.
HYDROFUGE PLATES, hi'droffig (G.

hydor -f L. fuqare, to put to flight). 137
HYDROMETERS, hidrom'eter (G.

hydrometrion, a vessel for measur-
ing hydrostically ) 38

IfYDROPHYL LI UM, hldrdf ll'iiim ( G.

hydor + phyllon, leaf ) 2 5, 5

UYDROPHYLLUM, hl'drofil'um (G.

hvdor 4- phyllon, a leaf) 2.}2

HYDRORHIZA, hi'drori'za (G. hydor

+ rhisa, root) ' 252

H Y D R 0 T H E C A, hl'drothe'ka ( G.

hydor -\- thekc, cup ) 2~)2

HYGROMETERS, higrom'eter (G.

hygros + nietron, a measure) 38

HYGROSCO PTC, hi'groskop'Tk ( (\ .

hyarof, wet + akopein, to regard ) .

definition of 217

HYLOBATE8 EXTELLOIDE8, liilobTi-

tez (G. hylolxiicfi, one who haunts

the woods) (>CtO

HYMENIUM, InniO'iM-fiiji (G. hymen,

skin) . : . . ... . . . . . /; ' 201

HYMENOCHIRUR 650

JfYMEXOLEPffil DntIVl7T\, l.Tn.rM.ol'-

epis 2J)S, 30 /

Nana (G. uanux. dwarf) 298

// y.WNNOPTERA , liT'mfMinp'tfM-ri ( G.

hymen -j- pteron-, win'?) -3J-7. 423

A sub-class of fliphuncitlahi.
HYOBRANCH1AL APPARATUS, hio-

brang'kial (G. Y + branflchia,

gills) ' 617

Support 738.

HYOGLOSSUS, hi'oghVus (G. F +

glossa, tongue ) 81

HYOID ARCH, hi'oid (G. hyoeides,

Y-shaped) . 73

HYOSTYLIC, hl'dstillk (G. Y + stylos,

pillar) '. . . .698

HYPAKTKK1AL JJRONCHI. liTp7irt<V-

rial (G. hypo, under -f-'L, arteria,

channel ) 770

HYPAXIAL Ml'SCLKS. hipax'eeal (G.

hypo, below; axis, axis) 82<>

HYPERMASYISM, definition of ........ tf7:>

HYPERTHELISM, definition of 675

HYPERTROPHY, hiper'trofl (G.

hyper, above -4- trophe, nourish-
ment) ..... ... . 07!>

HYPHAE, hife (G. hyplu\ web) . .2<)s

HYPHOMYCETES, hl'foinise'tez (G.

hyphe, web -J- mukus, fungus) 210

HYPOBLAST, hl'pobltist (G. hypo -f

blastos, bud) 106, '»(!.'>

HYPOBRAXCHIAL PLATE, hi'po-

brang'kial (G. hypo -f. brangchia,

gills) . s .617

HYPOCHORDAL ROD, hl'pokordal
?;t4i(G. hypo -4- chorde, string) ....... .59(1

HYPOCOELE, hl'pd^l, (G. hypo +

koilos, hollow) ., . , -..{i. . , . ... ,459

HYPOCON1D, hlpoko'nid (G. hypo -f .

konos, cone) 7:5(1

HYPOCOTYL, hipdkdt'ol (G. hypo +

kotyle, hollow ) >,?.'/

HYPODERMIS, hi'pdder'mts (G. hypo

+ derma, skin), definition of >2H

HYPOGASTRIC, hipdgas'lrik (G. liypn

+ g aster, belly )

HYPOGLOSSAL, high'pohgloss'al (G.

hypo; glossa, tongue)

HYPO-ISCHIUM, hl'pois'kium ((J.

hypo + ischioti, hip) 70(>

HYPOMERE, high'poh'mere (G. hypo,

below; meros, part) 4fift

HYPONOMEUTA 345

HYPOPHARYNX, hipdfar'ingks (G.

hypo + pharyny, gullet) 337

HYPOPHYSIS, hipf.fdse/ (G. hypo,

below; phyo, to cause to grow) .66, 490
HYPOSTOME, hl'pdstdm (G. hypo +

stoma, mouth ) . 253

HYPOTHALAMUS, hi'pdthril'amus (G.

hypo + lhalamos, chamber) .. .839, 851
IIYPOTRICHA, hl'pot'rika (G. hypo

+ thrix, hair) 154, 41!)

An order of Ciliata.
11YRACOIDEA, h!rakoi'df>a ((J. hyra,,\

shrew-mouse + cidos, like) . . . .426, 662
HYRAX ABY88IXWU8 (iG?

TATRO-CHEMICAL SCHOOLS, iatro-
kem'ikal (G. iatros, a physician +...

E. chemical ) 380

Mechanical schools 380

ICHNEUMON FLY, ikiiii'mon (G. ich-
neumon, the tracker) 37,?

irHTTTYOPHfK dLVTJ'KOftA C, '/?

] XDKX

IpBTHYOPSWA, ik'thyop'sidah {G.
irhthys, fish; o^.v/.s. appearance i .

definition of «63

WHTHYOPTER YGIA , ik'thlopteri j 'la
__ - ;-(G. ichthys, fish -f- pteryx, whig).

definition of .... 707

I V HT HY OR \ I T HI FOR ME* (G,
. ichthys, fish -f ornis, bird) ... , . . . .425

An order of fossil, toothed-bird-
\vith teeth in .separate sockets.
ICHTHYO tiAURti, ik'thiosars (L.

ichthyosaurus, fish-like lizards) . . -. .690
IGNEOUS FORMATIONS, ig'nf>us ( L.

iV/nta, fire) . definition of . ,393

IGUAXA TCni-RCJLATA. igwa'iia •
(Sp. iguana, from native Carib

name) . 655

ILEUM. iU'eum ((J. filo. twist) .. . .)tS, J,9

Definition of 338, 748

ILIACUS, ili'akiis (L. iliaciis. relating

to the colic J .,'». 80

ILIO-COSTALIS MUSCLE, lll'Iu-kos'-
talis '(<}.. eilo -f L. costa, rib) .... ..829

ILIOPSOAS, iliop'soas (L. ilium, the
flank -f- G. psoa, a muscle of the

loins), definition of . 83

ILIUM, ill'eeum (L. flank) . . ' . ?£

llumologiie .. . .704

IMAGE, im'aje, APPOSITION/ (T,.

/mar/o, _ image), definition of 322

Mosaic, definition of .321

Stipple, definition of ............ 321

Superposition, definition of 322

I .NT AGO, mia'go (L. imago,' image) . . .331

Definition of 348

IMMUNITY, imii'niti (L. im MIDI is,
exempt from public service) .. .194. 201

Active '.197

Artificial 197

familial 190

Individual 19(5

Natural ........: 197

Passive 197

IMPERFECT FLOWER, definition of.22S
IMPLANTATION, implanta'slum (L.

in + plantare, to plant).. . 624

IMPORTANCE VS. ( oXspiCUOUS-

NESS :... 32. 375

IMPREGNATION, Im'pregna'slmn (L.
impraegnare, to fertilize), definition

°f .105

(See also fecundation, fertiliza-
tion, etc.)

IMPULSE, im'puls (L. impnJsiift. im-
pelled) 17s

INCISOR, insi'sor (L. incido. cut

into) 70^

Teeth 73 /;

INCISURES OF SCHMIDT, definition

of 114

INCOME, LOSS OF BY INSECTS. . . .332
INCOMPLETE Fl BRO - VA Rf ' ULA R

;, BUNDLES, drtiniuon of., .232

I. \CHKASK IN FLIES 370

INCUS, inn'kuss (L. anvil) ,%T

J X I ) ENTATION. indenta'shun (L. "in-
. dentare, to indent), definition of/. .106
INDEX FINGER, in'dex (L. indicare,

,t to indicate), definition of 710

Fossils 39S

MDUSIUM, indu'zium (L. induere, to

put on) 225, 242

INFECTIONS, BY CESTODES, infek'-
shun (L. infic$re, to infect) ....... .295

, By nematodcs 302

Mixed . . 196

Trematode .....' "... .29 1

I X FECTIVE TREMATODES [".... 2.'» >

INFRAORBITAL LINE, in'fraor'bltal

(L. infra, below + orbis, circle) .. .582
INFUNDIBULUM, inn'fundib'yulum

( L. funnel ) 0.7

(See also hypophysis, pituitary
body. )

Definition of 66, 852

Of oviduct j/fO

fXFVSORIA, in'ffizo'riu (L. infusus,
poured into + forma, shape) . .419, 427

definition of 153

Why so named 138

1NGEN-HOUSZ 381

INGESTED (L. ingestus, taken in).. 63
TNGESTION (L. ingestus, taken in) . . 30
INGLUVIES, Tngloov'Jez (L. ingluvies,

crop ) 338, 744

INHERITANCE, inher'itiins (L. in +

heres, heir ) 157

INHIBITION, m'hibish'on (L. ink-Mere,

to prohibit) 52

INHIBITORY FIBERS, definition of. .847
INITIATIVE, inish'iativ ( L. initiare,

to begin), definition of 17£

INNER CELL MASS, m'er (A. S. inra,

inside), definition of 621

INNER EAR—

Of frog 45

INNER STIMULUS, definition of.... 178
INNER VATION, m'nervfi'shun (L. in,

into + nervus, sinew) 46, 77

INNOMINATE, inn'nomm'inate (L.

innominatus, without a name)

Artery 755

INNOVATION BRANCHES, in'ovfi'-

shun (L. innoi'are. to renew) 220

INORGANIC, inorgan'ik (F. inor-

ganique, inorganic)' , 95

I \NKCTA, insek'tn (L. insectus, cut

«ff) 328-374, 429

R*e 353

Fly 367

Grasshopper 332

Alternation of generations in 346

Central nervous system of. . .330, 359
Embryology of 346, 362

924

INDEX

Excretory system 359

Gynandromorphs 366

Metamorphosis 362

Muscular system 360

Paedogenesis 345

Parasitic forms 372

Polyembryony 345

Senses of 341

Sympathetic nervous system 359

Vermin 350

IN8ECTIVORA, m'sektiv'Ora (L. in-

sectus + vorare, to devour) . . .426, 658
INSERTION, OF MUSCLES, mser'shun

(L. in$ertus, joined) 78

INSPIRATION, mspira'shun (L. in-

spirare, to inhale) 30

INSTARS, m'starz (L. instar, form). 330
INSTINCT, in'stingt (L. instinctus,

impulse) 126

INSTINCTS, definition of 177

INSTRUCTION, instruk'shon (L. in-

struere, to build ) 32

IN8ULA, in'sula (L. insula, island).. 843
INTEGUMENT, integ'ument (L.
intego, to cover), component parts

of 665

INTELLIGENCE, intel'ijens (L. intel-
ligentia, discernment), definition of. 182

iNTERACTiONisT, definition of 175

INTERAURICULAR SEPTUM, in'ter-
orik'ulnr (L. inter, between +
auricula, little ear + septum, parti-
tion) 541

INTERBRANCHIAL SEPTUM, m'ter-
brang'kifil (L. inter + (*• branchia,

gills) 758

INTERCOSTAL ARTERIES, m'terkos'-

tal (L. inter + costa, rib) 791

INTERMAXILLARY GLANDS (L.

maxilla-, jaw ) 741

INTERMEDIATE CELL MASS (L.

medius, middle) . 593

INTERMEDIATE MASS OF BRAIN,

definition of 863

INTERMEDIATE ORGANISMS. . 185, 193

Bacteria 190

Yeasts 188

INTERMEDIUM 76

INTERNAL CAPSULE OF BRAIN,

definition of 849

INTERNAL GILLS 586

INTERNAL SECRETIONS 52

INTERNAL SECRETIONS, glands of. 52

INTERNAL STRUCTURE 46

INTERNASAL GLANDS (L, nasus,

nose) . .741

Plate, definition of 613

INTERNODE (L. nodus, knot) 203

INTERORBITAL SEPTUM (L. orUs,

circle) , definition of 540

INTEROSSEOUS (L. os, bone) 85

INTERPRETATIONS VS. FACTS.. . 32

Of facts 157

INTERRENAL (L. renes, kidneys),

definition of 822

INTERRENAL VEINS 798

INTERSEGMENTAL 602

INTERSTITIAL, stish'al (L. sistere,

to set) 248

INTERSTITIAL CELL 819

INTESTINAL LAYERS 49

INTESTINE, CROSS SECTION OF,

intes'tin (L. intestinus, inside).... W

INTRALIMBRICUS 844

INTRINSIC MUSCLES, mtrm'sik (L.

intrinsecus, inwards ) 829

I N T R 0 S PECTIONISTS, intrdspek'-

shonists (L. introspectare, to look

into) , definition of 173

INTUSSUSCEPTION, m'tussusep'shun

(L. intus, within -f- suscipere, to re-
ceive ) .

In Zoology, growth from within

outwards ; in Medicine, one portion

of the intestine being pushed into

another portion.
INVAGINATE, mvaj'inet (L. in, into

+ vagina, sheath ) 259

INVAGINATION, innvaj'inay'shun (L.

in, in; vagina, sheath), definition of. 106

INVENTORS VS. SCIENTISTS 25

INVERSION, OF GERM LAYERS, in-

ver'shon (L. invertere, to turn

about) C>23

INVERTEBRATES, mver'tebrats (L.

in, not -f- vertebra, joint)

Of uncertain position 431

INVOLUTION, m'volu'shun (L. in-

volvere, to roll up) 557

IRIS, eye'riss (G. rainbow), definition

of 70

Of eye 873

IRRITABILITY, Ir'itabinti (L. irri-

tare, to provoke) 65

ISCHIADIC ARTERY, is'klfid'ik (G.

ischion, hip ) 792

ISCHIOPODITE, la'kiop'odlt ( G.

ischion -f- pous, foot) 31 'i

ISCHIUM, is'keum (G. ischion, hip)..7tf

Homologue 704

I8HNOP8YLLUS 350

ISLE, ISLET, OR ISLAND OF REIL.843
ISOETALES, fern allies, Tsoetfi'le/ (G.

isoetes, equal in years) 22-'i

I80POD, I'sopod (G. isos, equal +

pous) 316

ISOPODA , definition of 326

I80PTERA, IsOp'tera (G. isos +

pteron, wing) 423

ISTHMUS, OF BRAIN, is'mds (G.

isthmos, neck), definition of 845

Of oviduct //.'/0

Of pharynx 759

Rhombencephali 839

INDEX

925

IVORY, of tooth, i'vori (L. ebur, ivory, LACAZE-DUTHIERS 380

f. French) .732 LACERTAE, Ifiser'tfi (L. lacerta, liz-
ard ) , definition of 654

JACOBSON'S COMMISSURE, defini- LACERTILIA, las'ertil'ia (L. lacerta,

tion of 885 lizard ) 425, 653

JACOBSON'S ORGAN, definition of . . . LACHRYMAL GLANDS, lak'remal (L.

582, 870 lacrima, tear ) 582

A diverticuhim of olfactory organ LACINIA, lasm'ia (L. lacinia, flap),

of many vertebrates, often develop- definition of 335-

ing into an epithelium-lined sac LACTEALS, lak'tealz (L. lac, milk) . .

opening into the mouth. ^.9, 800

JEJUNUM, jee'jew'num (L. hungry), LACUNAE, laku'na (L. lacuna, cav-

definition of 748 ity), definition of 57, 111, 467

JUGAL, jew'gal (L. jugum, yoke), L A G E N A, lajee'nah (G. lagynos,

Malar bone 697 flask ) , definition of 867

JUGULAR, jew'giular (L. jugulum, LAMARCK 384,386,388,397,415-

the collar bone) L AMELIA, definition of 14$

JUNG 381 LAMELLAE, laimell'ee (L. lamina,

a thin sheet), definition of.... 112, 320

KALA-AZAR, kfi'lfifizar (Hind, kala, LAMINA, lamm'eenah (L. a thin

black -f azar, sickness) 144 plate) 466, 85ft

KAMMERER 649 LAMINA TERMINALIS, definition of .570

KARYOPLASM, kar'ioplazm (G. kar- LANCELET, definition of 639

yon, nucleus -f- plasma, something LARVA, lar'va (L. larva, ghost)

moulded), definition of 90 43, 331, 547

KATABOLISM, kutfib'ollzm (G. kata, LARVACEA 424. 630

down -f ballein, throw), definition LARYNGEAL CHAMBER, larin'jeal

of 125 ( G. larynx) , definition of 588"

RATAL YSTS 96 LARYNX, lar'inks (G. larynx, gullet)

KATYDID 342 62, 74

KELLICOTT 592 Definition of 72ft

KELLOGG, VERNON . . 179 LATEBRA, lat'ebrfi (L. latebra, hiding-

KERATIN, ker'atin (G. keras, horn) . 93 place) /,##

The basis of epidermal structures, LATERAD

as horn, nails, hoof, etc. LATERAL, Ifit'erfil' (L. latus, side)... 269

KERAT08A, ker'atos'a (G. keras) ... 419 Folds 48£

An order of Porifera with main Limiting sulci 485

skeleton of spongin. Line organs 853, 855~

The regular bath-sponge, Enspon- Nuclei 851

gia. Plate 461

KIDNEY, kid'm (M. E. kidnere, kid- Ventricles 511

ney ) 47, 48 LATEX TUBES, lat'eks (L. latex,

Section of 8/7 fluid ) , definition of 23S

KING, A. F. A 134 LATTSSIMUS DORSI, latiss'imus

KINGSLEY 687, 806 dor'si (L. the broadest) 79, 829-

KOCH. ROBERT 389 LATREILLE, PIERRE . 415

KOELLIKER, VON 389 LATTER 262, 276, 321

KORSCHINSKY 403 LAVERAN 134

KRAUSE, END-BULB OF 864 LAVOISIER ' 390

KRAUSE'S MEMBRANE, definition LAW OF CONTINUITY 376

of ' 114 LAYERS OF INTESTINAL TRACT^ . //,9-

LEAF AXIL 203

LABIAL GLANDS, lay'beeal (L. Seed 203

labium, lip ) 741 LEARNING, definition of . ' ' 179

LARIATAE 866 LEATHER, definition of 667

LABIUM, definition of 334 LEATHER!! AOK TURTLE 652

LABRUM, la'brum (L. labium) LEI 8HMANI A DONOVAN! . . I',.',

LABYRINTH, lab'irmth (L. labyrin- Infantum 144

thus, labyrinth) 749 Jovanani 144

Membranes 868 Tropica U,rf

Nasal 871 LEMNISCI, lemnis'ki (G. lemniscus,

Of ear 866 fillet ) . . 851

INDEX

LtiNS OF EYE, leu/ (:L.'taw», lentil). 491
LENTICEL, len'tisel (L. lens) ; . . '. :': . .236
LENTIFORM NUCLEUS, len'tiform

(L. lens1-}- forma, shape) . . .849

LEPtbOPT ERA, lep'idop'tera (G,

lepis, scale + pteron, wing) . 423

An order of Panorpoidea.

LKPID08IREN . . 646

LEPID08TEU8 088EU8, lepp'idoss'-

teeus (G. lepis, scale; osteon, bone).6.)3

LHP18MATOIDEA .422

An order of Thysanura.

LEPTODISOUS . . . '. ..:...... 151

LKPTOMEDU8AE, lep'tomedu'se (G.

leptos, thin -f- L. medusa) . 419

An order of Hydrozoa.

JjEPTOSTRACA . 421

LKPTU8 IRRITAXN, lop'tus (G. lep-

tos, small) 350

LESION 209

LEUKOCYTE, lii'kosit, loo (G. leukos,

white -f- kytos, hollow) .49, 57

LEUKOPLAST, lilkoplast, loo (G.

leukos -\-plastos, formed), definition

uf :..... 237

LEVATOR, levay'tor (L. a lifter), defi-
nition of ' 78, 829

LEVERS, TYPES OF ... 36

LEYDIG'S DUCT, definition of 809

LfliELLULOIDEA 423

A sub-class of Pterygogenea.

LICE ,.. . .350

LICHEN, ll'ken (G. leichen, liver-
wort) .205

L1EBIG .' 380

LTEN, le'en (L. lien, spleen).

I J FE CYCLE 132

Of Angiosperm 244

Of Hookworm .303

Of Liver-fluke 289

Of Plasmodium malariea. 131

Of Pine . ..." 241

Of Tapeworm 293

Of Trichinae 302

LIFE HISTORY OF SPHAGNUM. . .221
LIGAMENT, lig'ament (L. ligamen-

tum, bandage) 74

L1GNIN, Hg'non, ITg'nin (L. lignum,

wood ) 93

L1GULA, Hg'ula (L. ligula, little

tongue ) 335

LIMITING SULCUS, definition of 845

LIMNAGA 28.9

T.I NEA ALBA, lin'ea Al'ba (L. linea,

line + alba, white) 79, 827

L1NGUATULIDA, ling'gwatu'lida (L.

lingua, tongue) 422, 430

A class of Arthropoda.
LINQULA, Hng'gula (L. lingula, little

tongue ) 310

LINNAEUS 386, 388, 414

LINNE; CARL VOX (same as Lin-
naeus)

LJN1N, li'nin (L. linum, flax) 90

Achromatic network of cell
nucleus.
JJPOCTENA . . . 422

A sub-class of Arachnoidea.

L1POIDS . .:. 95

LIRIOPE EXIGUA .253

A coelenterate.

L I 8 8 A M P H I RIA , lis'amf ib'ia ( G.
Ussos, smooth; amphibios, double !

life) ..425, 047

LISTER -. 3«

LIVER, liv'or (A. S. lifer, liver) ..... ..'/.S

Development of ................. 589

Lobes of 47

LIZARDS 651

Pineal eye of /i(>

LOA LOA . 303'

LOB08A, lo'bos'a (G. lobos, lobe) .147, 418

An order of Rhizopoda.

LOBUS VAGI .........: ,SV/ '/

LOCALIZATION OF BHAIN CEN-
TERS (L. locus, place) 85 1

LOCU8TIDAE . . 351

LOCY, WM 385

LOEB, JAQUES 20(i

LOEMANCTUS LONGPIPE8 101

LOGIC (G. logos, discourse, word)... 40
LOXGATA .205

A species of spirogyra.
LONG-HORNED GRASSHOPPER. . . .3-73

LONGISSIMUS CAPITIS, lonjiss'ee-
mus cap'itis (L. the longest of the

head) 829

Dorsi 829

LOPHIU8 PI8CATORIU8 6//J

LOPHODONT, 16'fodont (G. lophos,

crest + odous, tooth) 734

Having transverse ridges on the
cheek-teeth grinding surface.
LOPHOPHORE, 16'fofdr (G. lophos -f
pherein, to carry). An oval tentacle-
supporting organ in Polyzoa and

Brachiopods 309

LOW, DR '. 136

LOXOPHYLLUM, R08TRATUM U'i

LUC1DUM, loo'sidum (L. lucidum,

clear) 665

LUDWIG 380

LUMBRICU8, lum'brikus (L. lumbri-

cus, earth-worm) 250

Herculeus 270

Terrestris 268

LUMPY-JAW, definition of 213

LUNATUM, luna'tum (L. luna, moon). 709

INDEX

927

IJ'NG, lung (A. S. hniyc, lung) . . , . . ' j&

Alveoli .' . . 68

Development of -MS 588

Fishes '.(540

Fluke 291

L } COPODIA /,/•>'•; 22',

One of the Fern Allies.

/. Y(i()PO.DILM : . . 2> ',

A Fern Ally.

LYKLL ." 384

LYMPH, liinnif" ( L. li/mpha. clear

water) 6'2

Duct 780

Gland ; 800

Heart . . .*, . :-.- .5j, 5?

Vessels 57, 800.

LYMPHOCYTES, lim'fosits (L. lympha
+ G. kytos, hollow), definition of.. 800

MAVACU8, makfi'kus (Sp. macaco,
word of African origin), definition

Of ... . ...:.... ..;.... ...C7N

MCGREGOR, j. H 400

MclNDOO 342

MACHAIRODUK CVLTR-l DENS' 734

MACHILOIDEA 422

An order of Thi/sanura. . .*....

M ACROCLEMMY8 TEMMINCKI 719-

MACRODRILI .421

An order of Oligochaeta.
MACROGAMETES, makroga'mets (G.
makros, large -f- gametes, spouse),

definition of 130

MACRO MERES, mak'romers (G.

makros -\-meros, part) 279

MACULA LUTE A, mak'ula (L. mac-
ula, spot -f luteum, yellow), defini-
tion of ...873

M ADREPORAR1A, mad'reporfi'ria (L.
mater, mother + G. poros, a light

friable stone) -. . . 2J7, 420

An order of Zoantharia.

MADURA DISEASE, definition of 214

MAD URELLA MYCETOMA ? / ',

MAGELLAN I A FLAVE8CENS till

MAGENDIE 380

MAGGOTS, mag'otz (M. E. magot, a
grub). The most lowly organized,
completely worm-like insect larva
without insect appendages or distinct

head 370

MAILED FISHES, definition of 676

MAJOR GROUPS OF ANIMALS, may'-

jor (L. the greater) ..426

MA LACOBDELLA GRO88A 301

M A 1<A COPTERYGII 643

.V.4 LA CO8TRA CA , ma 1 'a kos'traka ( G.
malakos, soft + ostrakon, shell)

316, 421

Definition of 326

MALAR, may'lar (L. mala, cheek)... 607
MALARIA, definition of . ..134

MALAR1AH. 1>LAN.\JO DICM. defini-
tion of .130

MALE TO FEMALE, development of

in Cymothoidae 327

MALL EUS, mahl'eeus ( L. hammer ) . .

716, 861

MALLOPHAGE, malof'aga (G. mallos,
a lock of wrool + phagein, eat) . . . .423

An order of Blattaeformia.
MALP1GHI. malpig'f..37». 382. 386, 388
MALP1GHIAN, malpig'ean (after
Malpighi, an Italian biologist).

Body or corpuscle ... .. 6!i

Layer .....665

MAMMAL, mam'mal (L. mamma, >

breast ) . . . * . . . . \ .- •.•-£_ ,.

Cranial nerves of , .... .881

Skull of ..694

M . { M M ALI A, mamay 'leeah ( L.

mamma, breast) 426, 431

Definition of 656

MAMMALIAN EMBRYOLOGY ..619-633

Allantois ..628

Attachment of blastodermic ves-
icle .. 7. . . 623

Blastoderm 62 1

Decidual membranes 629

Embryonic membranes '..'......". .625

Fertilization 620

Formation of germ-layers 622

Implantation 624

Placenta 626

Umbilical cord 631

Yolk-sac 627

MAMMARY GLANDS, mam'aree (L.

mamma, breast), definition of 673

MAMMILLARY BODIES OF BRAIN,

definition of 852

MAMMOTH :..... .398.

MANATEE ... .; .66J

MAXATUS- 661

MANDIBLE, MANDIBULAR, man'di-
bl, mandib'yular (L. mandibula,

jaw) " 661

MANDIBULAR ARCH, definition of.. 74
MANNA, manTi (G. manna, manna),

definition of 349

The hardened exudation of the
bark of certain trees (hot.) ; honey-
dew secreted by certain Coccidae
( zool. )

MANSON, SIR PATRICK 135

MANTOIDEA 423

An order of Blattaeformia.
MANUBRIAL CARTILAGE, manu'br'al

(L. manubrium, handle) 617

MANUBRIUM, maniu'breeum (L.

handle) 251

MANUS, may'nuss (L. hand), defini-
tion of 708

Of frog 45

MANYPLIES, definition of 745

928

INDEX

MARCHAL 345

MARCHIAFAVA 134

MARINE 654

MARROW, definition of 112

MARSH 380

M A R S U P I ALT A, marsu'pea'lia ( L.

marsupium, a pouch ) 426

Definition of 657

MARSUPIUM, definition of 619

MASSA INTERMEDIA 863

MASSETER, masee'ter (G. maseter, a

chew ) 81

MA8TIGAMOEBA, mas'tigame'ba (G.
mastix, a whip + amoibe, change),

definition of 149

MASTIGOPHORA, mastigof'ora (G.
mastix, a whip + plioros, to bear) .

127, 143, 149, 418, 427

MASTOID, mas'toid (G. mastos,

breast)

The name of the protruding bone
directly behind the ear.

MATERIALIST, definition of 175

MATHEMATICAL SCIENCES 39

MATRIX, ma'triks (L. mater, mother)
The ground substance of tissues. . . .110

MATTER AND MIND 174

MATURATION, mfit'urfi'shun (L.

maturus, ripe), definition of 100

MAXILLA, maxill'fi (L. maxilla, the

jaw bone ) 48, 73

MAXILLAE, maxill'a 313

MAXILLIPEDS, maxll'ipedz (L. max-
illa + pes, foot) 313

An appendage in Arthropods pos-
terior to the Maxillae.

MEANDRINA 257

MEATUS, mea'tus (L. passage), defi-
nition of 869

MEATUS VENOSUS, definition of.... 501
MECHANICAL TISSUES, definition

of 234

MECHANICS 37

MECKEL ..380

MECKEL'S CARTILAGE, definition of ,61 6

Diverticulum, definition of 628

MEDIA, me'dia (L. medius, middle) . .335

MEDIAL EYE 45, 66

MEDIASTINUM, me'diiis'tinum, astl-

nfim (L. mediastinum, servant) ... .755
MEDULLA OBLONGATA, medfil'a ob'-
long'gfita (L. medulla, marrow, pith

+ oblongatus, oblong) 66, 839

Definition of 845

MEDULLARY, medd'ulayree (L. med-
ulla, marrow, pith) 812

MEDULLARY CORDS, definition of.. 520

MEDULLARY FOLDS, definition of.. 460

Groove, definition of 460

Plate, definition of 460

Rays, definition of 231, 235

Sheath, definition of 114

MEDULLATED NERVE FIBERS,

definition of 836

MEDUSA, medu'sa (G. medousa, one

who rules ) 251

A jelly-fish.
MEGALOPS ATLANTICUS, meg'filops

(G. megalon, great + ops, eye) ff.^

Larval stage of certain crusta-
ceans, conspicuous by large stalked
eyes. Habitat — Atlantic ocean.
MEGALOPTERA, megalop'tera (G.

megas, great + pteron, wing 423

An order of N europteroidea.
MEGASPORANGIA, megasporan'ji'a
(G. megas, large; sporos, seed;

anggeion, vessel) . 238

MEGASPORE CONES, definition of.. 238
MEGASPOROPHYLS, meg'aspo'rofilz
(G. megas -f- sporos + phyllon,

leaf ) , definition of 238

MEIBOMIAN GLANDS, imbo'mian,

definition of 673

MEISSNER'S CORPUSCLE S6V,

Plexus, a gangliated plexus of
nerve fibers in the submucous coat

of the small intestine 87.9

MELANDER 418

MELANIN, mel'anin (G. melas, black),

definition of 132

MELANOPLVS DIFFERENTIALS. . .335

Femur-rubrum 352

MEMBRANOCRANIUM, definition of. 687

MENDEL 162, 384, 388

MENDELIAN THEORY 163

MENTAL, men'tal (L. mentum, chin). 616

MENTAL PHENOMENA 173

MENTO-MECKELIAN, men'tomeke'-
Han (L. mentum, chin + Meckel) . . 72

A cartilage bone present in a few
lower vertebrates at either side of
the union of the two halves of the
lower jaw.

MENTUM, definition of 335

MERIDIAN, FERTILIZATION, defini-
tion of 550

HERIDOGASTRES 422

An order of Lipoctena.

MERISTEM, definition of 228

MEROPODITE, merop'fxllt' (G. meros,

thigh + pous, foot) 31 ff

MER08TOMATA, merostdm'mata (G.
meros, thigh -f- stoma, mouth) .421, 42J)

A class of Arthropoda.
MEROZOITES, merozo'Its (G. meros

+ soon ) 152

Definition of . . .132

INDEX

929

:>I ESENCEPH ALON, mes'ensef 'alon,
(G. m,esos, middle -f- en, in -(- kep-
hale, head) 472

M K S E N C H Y M E, mess'enkime (G.
mcsos, middle; enchyma, in a fluid). 247

Definition of 259

Formation of 461

M E SEN T E RIC, mess'entare'ik (G.
mcsos, middle; enteron, gut) 532

MESENTERIES, definition of 537

MESENTERIES IN HYDRA—

Definition of 256

MESENTERON 320

MESENTERY, mes'entarey (G. mesos,
middle; enteron, gut) 47, ^9

MESOBLASTIC BANDS, mesoblas'tik
(G. mesos + blastos, bud), defini-
tion of 279

MESOCARDIA, definition of 529, 604

MESOCARDIUM, meVokar'dium (G.
mesos -f- kardia, heart ) , definition
of 470, 776

MESOCOELE, mes'osel (G. mesos +
koilos, hollow), definition of 511

MESOCOLON, mes'oko'ldn (G. mesos
+ kolon, large intestine), defini-
tion of 537

MESODERM, mess'ohderm (G. mesos,
middle ; derma, skin ) , definition of .

107, 724

Origin of in chick 456

MESOGASTER, mess'ohgas'ter ( G.
mesos, middle ; gaster, stomach ) ,
definition of 537

MESOGASTRIUM, definition of 745

MESOGLEA, mes'ogle'a (G. mesos +
gloia, glue), definition of 247

MEKOGONIMU8 HETEROPHYES 293

MESOHEPATICUM (G. mesos -f L.
hepar, liver), definition of 604

MESOMERE, mess'ohmere (G. mesos,
middle; meros, part), definition of. .
460, 503, 504

MESONEPHROS (G. mesos + nephros,
kidney ) , development of 503

MESOR€HIUM, messor'keeum (G".
mesos, middle -f- orchis, testis ) , defi-
nition of 609, 811

MESOSTERNUM, mes'oster'num (G.
mesos + L. sternum, breast bone) . .335

MESOTHORAX, mes'otho'raks (G.

mesos + tlioraa;, chest) 614

The middle segment of the thoracic
region of insects.

MESOTIC CARTILAGE 609

MESOVARIA, definition of MO, 811

MESOVARIUM, messohvay'reeum (G.
mesos, middle; L. ovum, egg) . .424, 431

MESOZOA (G. mesos -f- zoe, life) 306

Three families of parasites of un-
certain position.

MESOZ01C, mes'ozo'ik (G. mesos +

zoe) 394

The middle or secondary group of

rock-systems.
METABOLISM, metab'olizm ( G.

metabole, change) 95

METACARPAL, met'akar'pal ( G. meta, . .

after; karpos, wrist) 76

METACARPUS, definition of 708

METACONE, met'fikon (G. meta, after

+ konos. cone ) . 736

The posterio-external cusp of an

upper molar.
METACON1D, metTiko'nid (G. meta +

konos + cidos, resembling) 254

The posterio-internal cusp of a

lower molar.

METAGENESIS, definition of 254

&ETAQONIMUS YOKOGAWAI 301

METAMERE, met'amere (G. meta,

after ; meros, part ) , definition of ...

264, 460

METAMORPHOSIS, met'fimor'fosTs (G.

meta, beyond + morphe, form),

definition of 330, 504

Of bee 362

Of frog 547

METAPHASE, met'affiz (G. meta +

phainein, to appear) i)7

Definition of 98

METAPLASM (G. meta -f plasm,

something moulded), definition of.. 91
METAPODIUM, metTipo'dium (G.

meta + pous, foot), definition of. . .708
METATARSUS, met'atnr'sus (G. meta

+ L. tarsus, ankle), definition of. . .708
METATHALAMUS, met'sithal'amus (G.

meta + thalamos, chamber), defini-
tion of 852

METATHERIA, metathe'ria (G. meta,

beyond + therion, a wild beast) ... .431

Definition of 657

METATHORAX (G. meta + thorax,

chest) 335

Posterior segment of the thorax of

insects.
METATROPHIC, met'atrofik (G.

meta + trophe, nourishment) 191

Definition of 258

METAZOA, metazo'a (G. meta + zoon)

Many-celled animals. Definition of. 96
METRATERM, me'traterm (G. metra,

uterus -f- L. terminus, end) 296

METRIOR1IYXCTIU8 6S2

MIASTORCA, mlas'torka (G. miastor,

a guilty wretch, also avenger ) 3fj5

TUWROCENTRVM RETINERVE 352

MICRODRILI 421

An order of

yau

INDEX

MiQ&OFlLARIA

Diurna 303

Nocturna 303

MICROGAM?]TES, ml'kroga'metz (G.

mikros, small + gametes, spouse).. 130
MICROMERES, mT'kromers (G. mikros,

-f- meros, part) 279

A cell of the upper pole in mero-

blastic eggs.
MICRON, mi'kron (G. mikros, small).

One thousandth part of a millimeter. 1 90

Definition of »

MICROPYLE, mi'kropil (G. mikros +

pyle,. gate) , definition of 10f>

MICROSPORES (pollen) , 243

MTCROSPOROPHYTES (G. mikros +

sporos, seed -(- p'liifta. plant), defini-
tion of ' 238

M1CROTOMK 30

MICRURA LE1DY 307

MID-BRAIN 60

Definition of 472

M1DGUT, definition of 480

MIGRATIONS, mignVshunz (L.

migrare, to transfer), of plants and

animals 390

MIGRATORY CELLS, ameboid cells

of the blood 247

MIGULA 190

MILK DENTITION, definition of.... 732

MILNE-EDWARDS 380

MINCHIN'S CLASSIFICATION 151

MIND, definition of 174

MIND AND MATTER : 174

MINIMUS, min'Imus ( L. minimus,

least). The fifth digit, definition of.710
MIRAC1D1A, mir'fisid'm (G. dim. of

meirakion, a stripling), definition

of 280

MIRIENTOMA TA 422, 430

MITE, ITCH ..350

MITOCHONDRIA, mltokon'drif. (G.

mitos, thread + chondros. grain).

definition of 91

M I T O S I S, mit.Ysis (G. mito*. n.

thread) , definition of 97

Meaning of 99

MITRAL VALVE, mi'tral ( K. mitre. a

peaked cap), definition of 780

MIXED NERVUS. definition of 576

MIX1PTERYGIUM, mTk'sipterij'iiim
(G. mixis, mixing -}- ptcn/gion, little

wing) 707

MODERN LANGUAGES 41

MOLAR GLAND, mf/lar (L. mold,

millstone) 742

MOLAR TEETH, mu'lfir (L. wioZrrr, to

grind) 734

MOLDS, in paleontology, definition of. 398
MOLE . . . 6X8

MOLLi'tfVA, molus'kfi ( L. m-ottimvus,

soft) 259, 429

Unsegmented triploblastic animals
with characteristic muscular foot.
Examples, clams, snails, squids,
octopi.

M ON AXON ID A , monakson'ida ( G .

monos, alone -f axon, axis ) 419

An order of Porifera with mon
axon but no tetraxon spicules.

MOXILIFORM, monil'iform (L. monile,..

necklace -(- forma, shape) 77 /

Constricted at regular intervals,
giving the appearance of a chain of
beads.

MONIST, ino'nist (G. monos, alone),
definition of 1 7.1

MONITOR, mo'nitor (L. monitor, one
who reminds ) , 654

MONKEYS, mimg'kis (origin unknown;
prob. Fr. G. mimo, an ape ) 659

MONOCOTYLEDONS, monokot'iledonz
(G. monos, alone + kotyledon, a cup-
shaped hollow) 227

Plants having one embryo lobe.

MONOCYST, mon'osist (G. monos, sin-
gle + kystos, a bag), definition of.. 1 5:5

I/ 0 N 0 D E L P HI A , monodel 'f TA ( G .
monos, single + delphis, womb) ....
426, 43 1 . 658

MONODINE (G. monodia, a solo, la-
ment ) 143

MONODON MONOCEROS, mon'odon
mono'serus (G. monodous, one-
toothed + monokeros, a unicorn) . . .662

MONOECIOUS, mone'shus (G. monos,
single + oikos, house), definition of. 217

MOXOGENEA, monoje'nia (G. monos,

single + genos, kind) 420

An order of Trematodes which de-
velop direct from the egg.

MONOGENETIC, mon'ojenet'ik (G.

monos -\- genesis, descent), 288

Definition of 403

Direct asexual reproduction.

MONOPHYLETIC, mon'ofilet'ik (G.
monos + phyle, tribe), definition of. 403

MONOPHYODONT, monofi'odont (G.
monos -f- phyein, to produce -f- odons,
tooth) , definition of 733

MONOS ACCHARIDE, monosfik'arid
(G. monos, alone -f- L. saccharum,
suornr) ]87

VONOSIGA, definition of 15 i

MO A7 0 TREMA TA , mon'ohtremm'atah
(G. monos, one; trema, opening) .. .426
Definition of ... . 656

MONRO—

Foramen of 839

Sulcus of, definition of 845

MORAL SENSE, definition of.. 183

MORGAGNT'S SINUS, definition of.. 764

INDEX

MORPHOLOGY, niorfol'ojl (G. morphe,

form -j- logon, discourse) 31

Science of form and structure of
animals and plants.
MORULA, mor'iila (L. moriun. a mul-

.berry), definition of 106, 447

Definition of, in protozoans 132

Of rabbit 621

MOSAIC IMAGE, definition of 321

MOSQUITOES AND MALARIA 134

MOSS-ANIMALS, definition of 310

MOSSES ..215

MOSS PLANTS, definition of 203

MOTILE, mo'til (L. motus, moved) . .225
MOULT, molt (L. tnutare, to change). 667

Definition of 330

MUCOSA, miikf/sfi (L. mucus, mucus) 49
A membra no secreting mucus.

MTCOSUM ...665

MUCOUS GLANDS 46

MUD-EEL G#)

MUD-PUPPIES 641

Definition of 649

MuLLER, JOHANNES. 380, 386, 388. 415
MULTANGULUM, inultang'giHum (L.
multus, many -j- angitlus, angle) .. .701)

Two carpal bones, the trapezium
and the trapezoid.

]/r,s'CA DOMESTICA ' ,%'.<)

MUSCI, mus'ki (L. muscus, moss) ... .215
MUSCLE, mus'l (L. musculus, muscle)

Circular ',9

Longitudinal ,).<>

Plate, definition of 5 Hi, 591

MI'NTELIDN, mus'teUds (L. mustcln.
a weasel. als(» kind of fish), defini-
tion of ?,f.7

Mt/KTRfj/N, definition of 641

MUTATION THEORY, muta'shiin (L.
niutare, to change). definition

of 164, 404

MUTTKONVSKI 138

MYCELIUM, nn-e'liiini (G. W//AVK.

fungus), definition of 208

MYVETE8. mlstVtt1/ (G. myketes. a

bellower ) . definition of 190

MYCETOMA. mTseto'ma (G. mi/ken.

fungus -f owirt). definition of 214

MYCOSIS, mlkfi'sis (G. mt/kcs. fungus

-f osis], definition of ...214

MYELENCEPHALON, nil'elensef'alon,
(G. myelos, marrow -j- en, in -f-

kephale, head) . definition of . . 845

MYELIN. mi'elln (G. my clou, mar-
row ) , definition of 114

MYELOCOELE, ml'closel (G. myelos.
marrow -+- A-O//ON. liollow). definition

<*f 511

MYLOHYO1I). m.y'lowhjgh'oid (G.
myle. mill: npftilon. the letter »/) . . .
'.80. 831

YOCARDIUM, mi'wk

iniisclo -f- kardia. heart ) . .469

Definition of 529

MYOCOELE, mi'osel (O. my* -\-

koilos) , definition of 502

M Y O C O M M ATA , mi "okfnn'atn ( G .

mys, muscle; komma. that which is

cut off), definition of . . 592

MYOEPICARDIAL MANTLK, ml'oepT-

kar'dial (G. mys + cpi, upon +

kardia, heart), definition of 777

MYOSITIS, miosl'tis (G. W.//K. a muscle

+ itis), definition of . . . 303

MYOTOME, my'ohtome (G. mys, mus-
cle; tome, cutting), definitions of..

.502, 591

MYRIOPODA, mlrlop'dda (G. myrioi,

10,000 -|- pous, foot) 422, 429, 430

.17 Y X T A COC ETl, mis'tacose'tT (G.

mystoN. mustache -)-- /rr/o.s. whale) . .

;-.. 426, 663

An order of cetaceans, including

whahbone whales.

They are toothless but have

baleen in upper jaw.
MYTILrN. mit'ilus ((J. niytUon. a sea-
mussel ) 155

.l/r.Y/.YK GLrrr\-OH.\. miksT'iu' glut!

no'sa ((i. myxa, slime -f- L. (flutino-

.V//-S, gluey ) 6fi/

M Y \ I \ 01 I) /•;.!. n.ikVinoid'ia ((J.

ini/.fd. slirne -\- ciiloH. resemblance i . 424

MYX1PTERYGIUM 707

MYXOPlIYCIvAE. miksofi'sr-e ((i.

tni/s<i. mucus -(- phytpoft, sea weed).

definition of 204

MYXOSPOHIDIA. inik'sospr.-riil'in (G.

nit/.m, mucus + npor<i. s;««'d ) 419

An order of Heosporifaa.

Definition of 15:5

NAGKLI ' 404

NAIL, ufil (A. S. iiuffft'l. nail ) MM

The terminal horny plate of finger
or tot1.

Y.1/.S'- 283

A sub-class of Oligochpeta,

XAPLIUS ' 327

AM R<1OM KIH'NA /•;. nar'kAmedu'sr ( G.
na-rke, numbness -(- L. iHwhitm ) . . . .419

An order of Ifydrazoa.

NARES. nfi'ies (L. nostril) . .62. 582. 869
\ARWHAL, nar'hwal (Icel. iifllmilr,

a corpse-whale ) (HiJ

NASAL, iw'zal (L. //««»/«. nose ) . . 73, 617
NAVEL, nsVvel (A. S. naf.elc, navel). 632

Place of attachment of umbilical
cord.
NAVICITLARE. navi'k'ulare ( L. »«/•/«.

ship) 709

The scaphoid radiale of mam-
malian carpus.

932

INDEX

NEANDERTHAL MAN 399, 1,00

NEBULIFERA, nebfilif'era (L. nebula,
cloud 4- ferre, bear) 156

NEGATOR AMERICANUS 303

tfECROBIOTIC GLANDS, nek'robiot'ik
(G. nekros, a dead body -f- bios,
life) , definition of 673

NECTAR, nek' tar (G. nektar, nectar). 364

Sweet substance secreted by special
glands, the nectaries, in plants.

NECTONEMA AGILE (G. nektos,
swimming) 308

NECTOPHORES, nek'tofors (G. nektos,
swimming 4- pherein, to carry ) . . . . 255

NECTURUS MACULATUS, nectur'us
(G. nektos, swimming; oura, tail) .649

NEENCEPHALON 855

N EM ATE ELMINTHE8, nem'athel-
min'thez (G. nema, thread 4- thel-
mins, a worm) 265, 285, 428

NEMATOCYST, nem'atosist (G. nema,

thread 4- kystis, bladder) 348

A stinging cell.

NEMATOMORPHA (G. nema, thread
+ morphe, form ) 306, 308, 424, 432

NEMERTINAE, nemertl'ni (G. nemer-

tes, true) 306, 424, 432

A group of worms of uncertain
position.

NEOCERATODUS FOKTERl GJtf

NEOPALLIUM, ne'opsil'ium (G. neos,
young 4- pallium, cloak), definition
of 843, 848

NEORNITHEB, neor'nithez (G. neos,

new + ornis, bird ) 425

A sub-class of Aves. Recent.

NEO8PORFDIA, neosporid'ia (G. neos

4- spora, seed) 143, 153, 140

A sub-class of Sporozoa.

NEOTHALAMUS, neothalTimiis (G.
neos 4- tJialamos, a chamber), defini-
tion of g51

NEOTONY, definition of ...648

NEPHRIDIA, definition of 266

Views as to relations 807

NEPHR1DIOPORE, nefrid'iopor (G.
nrwhros 4- woros. a passage) . . . 259

NEPHRIDTUM, nefrld'ium (G. nekros.
kidnov) 259 213

NEPHROSTOME, neff'rowstome (G.
nephros, kidney; stoma, mouth)...
64, 259. 505

NEPHROTOME, neff'rowtome (G.
nephros, kidney; tome, cutting), defi-
nition of 503, 593

NEREIS PELAGIA, ne'reis (G. nereis,
a sea-nymph ) 280

NERVE ARC, nerv (L. nervus, sinew),
definition of 177

NERVE, artery and vein 58

Centers 115

Vagus . 65

NERVOUS ACTION, experiment of.. 52 1
NERVOUS SYSTEM OF FROG .... 65, 66

Of amphibia ]

Of aves

Of fishes .>..

Of insects 1-360

Of mammals

Of reptiles ...

NERVURES, ner'vur (L. nervus,

sinew) .335

One of the rib-like structures
which support the membranous
wings of insects.
NEURAL, nu'ral (G. neuron, nerve) . .

Arch 72, 68 1

Canal 6(5

Crests 467

Folds 460, 563

Groove 460, 563

Plate 45!)

Ridge 562

Tube 466

NEURENTERIC CANAL, nur'enter'ik
(G. neuron, nerve -f- enteron, gut),

definition of 475

NEUROBLASTS, nu'roblfistz (G.

neuron 4- blast os, bud) 280

Primitive nerve cells.
NEUROCHORD, nfi'rokord (G. neuron,
nerve 4- chorde, string of gut), defi-
nition of 274

NEUROGLIA, rnVrogltVii, nurog'llfi (G.

neuron 4- glia, glue) 830

Supporting tissue of nerve cells
and nerve fibers.
NEUROLOGY (G. neuron 4- logos).

definition of 31, 40

NEUROMASTS, mVromasts (G. neuron

4- mastos, knoll) 870

Groups of sensory cells in the lat-
eral line of fishes.
NEUROMERE (G. neuron 4- meros, a

4- meros, part) 473, 478

A spinal segment.

NEURON (G. neuron, nerve), defini-
tion of 114

NEUROPORE, mVropor (G. neuron 4-

poros) 466, 474, 565

The anterior opening of the neu-
rocoele to the exterior.
NEUROPTERA, mVrop'terfi (G. neuron

4- pteron, wing) 423

Animals having wings with a net-
work of nervures.

An order of Neuropteroidea.
NEUROPTEROIDEA, nurdpteroi'df'fi
(G. neuron, nerve 4- pteron, wing) .423
A sub-class of Pterygogenea.

NEWMAN, H. H 397, 543

NEWTS 64S

NICTITATING MEMBRANE, nik'ti-
toy'ting (L. nicto, wink) ^5

INDEX

933

NIDAMENTAL GLANDS, iiul'fimC'ii'tfil

(L. nidus, a nest) 86, 816

Glands which secrete material for
an egg-covering.
XIGHTSHADE, nit'slifid (A. S. niht

scada, nightshade) g.ffl

30CARDIA BOVIS 21',

Actinoniyces 2l.fi

NOCARDIOSIS 214

NOCTILUCA, nok'tiloo'kfi (L. vox,

night + lux, light) 150, 151

Phosphorescent forms.

NODE, nod (L. nodus, knoh) 203

Of Ranvier 114

NODOSE GANGLION, nodos (L.

nodus) . definition of 886

K08EMA APIS, no'sema (a new Latin

word) l.W

Bombycis 1 53

NOTABLE MEN IN BIOLOGY 38fi

NOTIDANID, ndtid'amd (G. notidanos.

with sharp-pointed dorsal fin) 786

A sub-order of sharks with more
than five gill-clefts.
NOTOCHORD, no'toekord (G. notos,

back; cJiorde, string) 562, 639

N 0 T O G E N E S IS, no'togen'esis (G.

notos + genesis) 563

XOTORYCTE8, notdriktez (N. L.

same word, a generic name) 876

XOTOTREMA, notdtre'ma (G. notos

+ trema, perforation ) 816

A genus of South American toads
with dorsal brood-sac.
NOTUM, no'tnm (L. no him, back)... 336

The dorsal portion of an insect
segment.
NUCELLUS, niisel'iis (L. dim. of mix,

nut) 239

The central region and chief part
of an ovule.
NUCHAL, new'kal (L. micha, nape of

the neck) "02

Brain flexure 837

NUCLEAR SAP, definition of 90

NUCLEI, OF BRAIN 847

NUCLEOLUS, nrtkle'oliis (L. dim. of

nucleus, kernel ) 89

A rounded mass in nucleus.
NUCLEOPLASM, nu'kleopl-izm, defini-
tion of 89

NUCLEUS, nu'kleiis (L. nucleus, ker-
nel) 89

A complex spheroidal mass essen-
tial to the life of a cell.

NURSE CELLS, definition of 443

NUTRIENT 03

NUTRITIVE CELLS, definition of . . . .811

Pole, definition of 105

XUX VOMIOA ..285

X YCTERIDIPHIL U8 HEX A CTEXUS,
nlk'teridif'ilus (G. nykteros, by

night -f- philos, friend) 350

NYMPH, nimf (G. nymphe, bride) 331, 348

A stage following the larval in in-
sect metamorphosis.

NYMPHOMORPHA 422

An order of Pantopoda.

OBELIA, obe'lia (G. obelos, a spit).. 252
A genus of polyps.

OBJECTIVE, definition of 173

OBJECT OF SCIENCE 35

OBLIQUE SEPTUM, definition of . . . .757
OBLIQUUS, obll'kwus (L. oblique) ... 79
OBSTETRICAL TOAD, obstet'rikal (L.

obstetricus, pertaining to midwifery,

from L. ob, before, and stare, to

stand ) 650

OBTURATOR, ob'ttirator (L. obturare,..

to stop up ) 83

Foramen 703

OCCIPITAL CARTILAGE 615

OCCIPUT, ok'siput (L. occiput, the

back of the head, from ob, over

against + caput, head) 615

OCELLI, plural of ocellus, definition

of 328

OCELLUS, osel'us (L. ocellus, a bulb

or little eye) S',1

OCULINA SPEC10SA, ok'ull'na (L.

oculus, an eye) 257

The typical genus of the family

Oculinidae.
OCULOMOTOR, ok'Tilomf/tor (L.

oculus, eye + moveo, to move) 68

Moving the eyeball.
O.DOXATA, odona'ta (G. odous, tooth) 423

An order of Libclluloidea.
ODONTOBL ASTS, odfmt'oblasts ( G.

odous, tooth -f blastos, a germ) . . . .732
Embryonic cells to form future

teeth.

O.DONTOCETI, odontose'tT (G. odous,

tooth + ketos, a whale) 426. 662

A sub-order of Cetacea.
ODONTOID, odont'oid (G. odous +

eidos, resemblance, tooth-like) 517

Process 684

OESOPHAGUS, esof'agus (G. oesopha-
gus, gullet ) 49

The canal through which food
passes to the stomach.

OIDIA, oid'ia. See oidium 210

OIDIOMYCOSIS, oid'iomyko'sis (G.
oidium + mykos, a fungous + osts/.212

The presence in the body of fungi
as parasites.
OIDIUM, oid'ium (G. oon, egg -f-

dim. suffix, idion) ,211

A genus of parasitic fungi.

934

INDEX

OIKOPL&URA, oikoplii'ra (G. oikos,

house -+- pleura, side) 544

One of the order JAirvacea.

OKEN, o'ken 415

A German naturalist, 1779-1851.
OLECRANON, olekra'non (G. olekra-

non, point of the elbow ) 711

Pertaining to a process of the
ulna; the elbow.

OLFACTORIUS LATERAL/IS NU-
CLEUS 841)

OLFACTORY, olfak'tori (L. olere,

smell ) 62, 68

Capsules 540

Ditot s 870

Pits 581

Placodes 581

Recess 570

OLIGOCHAETA, ol'igdke'ta (G. oligos,

few + cMite, hair) 283. 421

A class of Annelida.

OLIVE OF SPINAL CORD 841

OLMS (German name for the white,

blind, cave mud-puppies) 649

0 M A S U S, om.Vsfts (L. omasum,

paunch ) 745

OMENTUM, oment'iim (L. omentum,

a fold) 537, 726, 746

OMMATID1UM, dm'atid'ium (G. omma.

eye) 322

OMNIVOROUS, omniv'orus (L. omnis,

all -f voro, eat ) ISO

Eating both plant and animal food.

OMOSTERNUM, 6'moster'num ( G.

omos, shoulder -f- sternon, breast) . .

. . .76, 101

OMPHALOMESENTER1C VEIN, om'-
falomes'enter'ic (G. omphalos, navel

+ mesenteron, mid-gut) J/Oi)

OMPHALOMESENTERIC VS. VITEL-

LINE, vitel'm (L. vitellus, yolk) . . .483
O\7I8CU8 A8CELLV8, onis'kus asel'us

(G, oniskos, a wood-louse) .82(>

Typical genus of Oniscidae.
ONYCOPHORA, onikof'ora (G. OMOS, a
claw or nail + pherein, to bear) . .

421, 428

OOCYST, 6'dsist (G. oo», egg + kustis,

bladder) 132

OOCYTE, 6'oslt (G. oow + kytos. cell)

'. . . .100, >,!,X

OOECIUM, de'shium (G. oon, egg +

oikos, house) 310

A brood-pouch.

OOGENESTS IN EARTHWORM,
oojen'esis (G. oon + genesis, de-
scent) 279

OOGONIIA, plural of oogonium/ 10<)

OOGONIUM, dogo'nium (G. oon -4-

yonos, offspring) 298

OOKINETE, ookinet' (G. oon +

kinein, to move) ,' 132

OOKPORA HOVIN. o'dspora (G. oon -f'

sporos, seed ) £1 /

OOSPORE (same as oospora) . . . 151, /<S7
OOTYPE, d'otype (G. oon -f type,

place) . '. '#.%'

Place where the sliell is formed.

OPALINA, opall'na (like opal) 15 .',

OPAQUE, opak' (L. opart/ «, darkened) 437

Impervious to light.

OPEN CIRCULATION, definition of.. 270
. OPERCULAR TUBE, definition of . . . ,586
OPERCULUM, oper'kulum (G. opercn-

lum, a lid) 21t>

Of plants 220

Of tadpole 549, 580

OPIIIDIA, ofid'ia (G. ophis,- serpent

+ eidos, form) . .425, 654, 704

OPHIU ROI DE A, of 'iuroi'dea ( G.

ophiouros, serpent-tailed) .... .420, 428
OPHTHALMIC, ofthal'mic (G. opthal-

mikos, of or for the eyes ) 31!)

OPILIONE8, opilio'nez (L. opilio, a

shepard) 422

An order of Arachnids.
OriSTHOBRANCHIA, opisthobrang'-
kia (G. opisthe, behind -f- branch,

gills) 421

OPISTHOCOELOUS, opisthose'lus (G.
opisthe + koilos, hollow), definition

of . 683

OPISTHODELPHYS, opisthodel'fis (G.

opisthen, behind +• delphis, dolphin ) 81 6
OPI8THOGONEATA (G. opisthe +

gone, genitalia) . .422

A subclass of Myriapoda.
OPISTHOMI, opisthd'mi (G. opisthen.

behind + omos, shoulder) <)4.">

OPI8THORCI8 FELINEUS, opisthor'
kis felin'eus (G. opisthe + orchis.

testicle) 2!>2

Noverca 2!*.'

Sinensis , 2!>:{

OPISTHOTIC, dp'isthdt'ik (G. opisthe

+ oiut, ear) (H>^

OPOSSUM, opos'um f>.7?

OPOTERODONT8, opot'erddonts (G.

opoteros, either -j- odoirs, tooth) . . . .70(5
OPSONIN, dp'sdnin (G. opsonein, to

cater) . definition of . 200

OPTIC CHIASMA (G. opsis, sight).. 571

Tjob.es 512

Nerve , 70

Pe.dicle 875

Stalks 478

Vesicles 474

OPTICAL SECTION 2?/'

OPTIMUM, dp'timum (L. optimus,

best ) , definition of 1 29

OPTOCOELE, op'tosel (G. opsis, sight

+ koilos, hollow ) 490

ORAL OPENING (L. os. mouth) ..... 48
Plate . . .493, 58.1

INDEX

935

OK.V.YUK BLOSSOM '..'i>

OHAXG-UTAX. drang'otan (Malay,

orany. man -f titan, woods) 660

ORBICULAR] S MUSCLES, orbik'u-

laris (L. orbits, an orb) 832

ORBITAL GLAND, or'bital (L. orbita,

circuit.) 742

ORDERS IN LIVING THINGS 415

ORGAN (L. organnni. an implement). 40

Balancing 323

Of equilibrium 323

Floral ?.'/.?

ORGANIC, definition of 04

Pertaining to living organisms.

ORGANIISM, definition of 108. 1J!»

ORIGIN OF MUSOLE. 78

Of species 403

ORNITHOLOGIST, Or'nithol'ojist (G.

ornis, bird -4- logos, discourse) 418

A student of birds.

O K N I T HORHYNCHIDAE, or'nitho-
ring'kide (G. ornis, bird -4- rugxus,

beak ) 657

ORNITHORU YNGHV8 A NA TI A I T8.. 65<>
ORTHOGENESIS, or'thojen'esis (G.
orthos, straight; genesis, descent),

definition of 404

ORTHO A ECTIDAE, or'thdnek'tiday
(G. orthos, straight -4- nektys, dead

body ) 306

ORTHOPTERA, orthop'tera (G. orthos,

straight; pteron, wing) 334, 423

An order of Ortlwptcroidea.
ORTHOPTEROIDEA , orthop'teroid'ea.423

A subclass of Pterygogc-tiea.
OS CLOACAE, Os kloa'ka (L. cloaca.

a sewer ) 706

Entoglossum, entoglos'sum (G.

entos, within; glossa, tongue) . .740
OSMOSIS, osmo'sls (G. otheiii. to

push) 37

OSSICLES., os'sicles (L. diiuin. of os,

bone) 318

OSSIFICATION, os'sifika'shiin (L. os

4- fio, to become) 74

Centers .-7/7

OSTARIOPHYSI, ostii'riofl'si (G.
ostarion, a little l>one -4- phusa.

bladder) 643

OSTEOBLASTS, ds'teoblasts (G. osteon.

bone -4- blastos, a bud) 1 1'2

Primitive bone cells.
O8TEOLEPIDA, osteolep'idfi ( G .

osteon + lepis, a scale) 642

OSTIA, ostia (L. ostium, a door) 339

OSTIUM OF OVIDUCT W

Tubae ( L. tuba,, a horn ) 824

Tubae abdominale 808

O8TRACODA, ostrakd'da (G. ostrako-

das, like a shell ) 42 1

A sub-class of Crustacea.

ONTKACOLtKKMI, os'trakoder'mi (G.

ostrakodei'mos, bony skin) 64G

OTIC, d'tik (G. ous, ear) 73

OTOCYST, o'tosist (G. kystis, bladder)

definition of 323, 581

OTOLITHS, otoliths (G. ous, ear +

lithos, stone), definition of 868

OVARIAL CORDS, ovar'ial (L. ova-

rium, an ovary ) 520

OVAR1OLES, ovar'iols ( L. ovarium) ,

•lefinition of 344

OVARY, dvari (L. ovarium} 47

OVEHTON 20(i

OVIDUCTS, 6'vidukts (L. or-um, egg

+ duco, to lead) 47, SO

OVIPOSITOR, d'vipoz'itor (L. ovum,

egg H- pono, to place) 337

OVULE, ov'nle (L. dim. of ovum, egg)

226, 2J2

OVUL1FEROUS, ovuli'ferus (L. ovum,

egg + fero, to bear ) 23'.)

OVUM AS USED IN MAMMALIAN

EMBRYOLOGY (G. oon, egg) 624

OWEN 380

OXIDATION, oksida'shun (G. oxys,

acid) 30, 38, 12i>

OXYTRICHA, oksit'rika (G. oksus,

sharp -j- trix, hair ) 1 54

OXYURI8 INCOGNITA, oksiu'ris (G.

oksus -f- our a, tail) 30 L

fncognitus, unknown.

Vermicularis (L. vermis, a worm ).$()[

1» ACINI AX CORPUSCLE, jifi.sTn'Ian
cor'pusl (L. corpusculus, a small
body) 86}

PADDLE FISHES fifrt

PAD, SUBARTICULAR. suh'artik'ular
(L. sub, below -f artioulus, joint) . . 4(>

PAEDOGENESIS, pe'dogen'esis (G.

pais, child; genesis, descent)

'. 345, 648, 100

PALAEENCEPHALON, pa'leensef 'a -
Ion (G. palaios. ancient -f- en, within
-f- keplialos. head, i.e., primitive
brain ) 85f>

PALAEMON, pnlo'mon (G. palaimon,
a sea god ) 32.",

PALAEONTOLOGY, pa'leontdl'ogy (G.
palaios, ancient + ons, being +

logos, discourse) 32, 393-401

Chart 394, 395

PALAEOSPONDYLI DAE, paleospondi'-
liday (G. palaios, ancient + spondu-
los, vertebra ) «4fi

PALAEOTHALAMUS, pa'ledthal'amus
(G. palaios, ancient 4- thalamos, a
receptacle) 851

PALAEOZOIC, pa'leozo'ik (G. palaios,
ancient; zoon, animal) S95

PALAMEDEIDAE, pal'amede'ide (G.
paints, palm-webbing of foot) . . . 719

INDEX

PALATAL GLANDS, pal'atal (L. pala-

tum, palate ) 742

PALATE, paint (L. palatum, palate). 730
PALATINE, pal'aten (L. palatum,

palate) 12, 610, 617

PALATO-QUADRATE, paTsito-quaw-

drate (L. palatum + quadratus,

squared )

Where formed 616

PALLIUM, pahl'eeum (L. cloak.) 839, 848
PALPUS, palpus (L. palpare, to feel). 335
PALU8TRI8, paliis'tris (L. palus, a

swamp) 101

PANCREAS, pan'kreeass (G. pan, all;

kreas, flesh ) 4H

Development of 58!)

Duct of '/*

Juice of 50

PANDER, NUCLEUS OF 4315

PANIZZA'S FORAMEN, definition of. 78(5
PANORPATAE, panorpfi'te (G. pan,

all + arpe, sickle) .423

An order of Panorpoidea.
PANORPOIDEA, panorpoi'dea (G.

pan, all + arpe, sickle) 423

A sub-class of Pterygogena.
PANTOPODA, pantop'oda (G. pan, all

-f pous, foot) . 422, 430

PAN-TROGLODYTES, pantrog'lodites

(G. pan, all + troglodytes, cave

dweller) (H>0

PAPILLARY LAYER, pap'illari (L.

papilla, a pimple)

PARA, par'ah (G. prefix, beside,

near).

PARABRONCHI, pa'rabron'ki (G. para,
near -f- brongchos, windpipe) 700

PARACHORDAL PLAT10S, parakor'-
dal (G. para, near -f- chorde, a
cord ) '. . . . 53!)

PARACONE, psVrakon (G. para, near
+ konos, a cone ) 13(1

PARACORDAL (same as parachordal )«14

PARAGLOSSAE, paraglos'a (G. para,
near -f glossa, tongue) 35.), 740

P A RAG ONIMU8 WK8TKRMA.WNI,
parigon'imus (G. para + gonimus,
with generative power) •. 291

PARAGORDIU8 VARIUS (G. para -f
L. gordius, complex from Gordian
knot ) 308

PARALLEL INDUCTION THEORY,
definition of 397

PARALLELIST, definition of 175

PARAMOECIUM, parame'sium (G.
paramekes, of longish shape) 138

PAR.AMYLUM, parfi'myliim (G. para,
beside -j- amylum, starch) 128

PARAPHYSIS, paraff'eesis (G. para,
beside; phyo, produce) . . . .211, 571, 850

PARAPLASM1C, paraplas'mic (G.
para, beside + plasma, something
moulded ) 92

PARAPODIA, parupo'dia (G. para,
beside + pous, foot) 283

PARASITES, par'aslt's (G. para, be-
side + sitos, food) 37, 208

PARASITIC CRUSTACEA, krus'ta-

shya (L. crusta, a shell) 325

Nemalodcs 305

Worms 303

PARASPHENOID, parasfen'oid (G.
para, beside -f- sphcn, wedge -f- eides,
like) 73, 617

PARATERMINAL BODY, parater'mi
nal (G. para, beside -f- terminus, the
end) 81,2

PARATHYROID BODY, parathl'roid
(G. para, beside + thyreos, a shield) 744

PARATROPHIC, paratrd'fic (G. para,
beside + trophe, nourishment) 191

PARENCHYMA, paren'kima. See page

757 258

Cortical 230

Porous 235

Whore found 233

PARENCHYMATOUS ORGANS, def-
inition of 757

PARIETAL, parl'etal (L. paries,

wall) 73, 269, 617

Foramina 858

Muscles 820

Organs 852

P ARM ELI A, parme'lia (G. parme, a

small shield) 205

A species of lichen.

PAROTID, parot'id (G. para, beside;
ous, ear ) .

PAROVARIAL CANAL, par'ovar'ial
(G. para + L. ovarium, an ovary) .816

PAROVAR1UM (G. para + L. ova-
rium ) 441

PARS OPTICA, pahrs op'tika (L. pars,
part -f- options, pertaining to vision) 852

PA RTIIENOGENESI8, pahr'thenojen'-
esis (G. parlhenos, a virgin + gen-
esis, descent) 100, 344

P A R T H ENOGONADIA, pahr'theno-
gonad'ia (G. parthenos, virgin ,+
gonos, offsprins:) 130, 150.

PARTIAL SEGMENTATION, defini-
tion of 106

PARTURITION, pahr'tfirfshun (L.
parturire, to be in labor) .620

I NDEX

937

PAMERIFORMEti, pas'erifor'mez (L.
passer, sparrow -f- formes, form)... 425

Sparrow-like birds. More than one-
half of all birds belong to this group.
There are 64 families.

PASSIVE POLE, definition of.. 105

PASTEUR, LOUIS 30, 382, 388

PASTEUR'S SOLUTION 189

A bacteriologic culture-fluid con-
sisting of 100 parts water, 10 parts
sugar, and 1 part each ashes of yeast
and ammonium carbonate.
PATELLA, patell'ah (L. a small pan

or dish) 710

PAT ELLIN A (L. patellae, a small

dish) 156

A species of Vorticella.
PATENS (L. pateo, to lie exposed) .. .155

A species of Condulostoma.
PATHOGENIC, path'oge'nic (G. pathos,

suffering + genos, offspring) 141

Fungi 209

Protozoa 142

PATHOLOGY, puthol'oji (G. pathos,

suffering -f- genos, offspring) 141

PATTERSON! 460

PATTON 638

PAUNCH, punch (L. pantex, belly) . .745
PAUROPODA, parop'oda (G. pauros,

small + pous, foot) 422

An order of Progoneala.

PEA FRUIT DEVELOPMENT 21,5

PEAT- MOSSES, definition of 215

PEBRINE, peb'rin (F. pebrine, dis-
ease of silkworm) 153, 333

PECTEN, pak'ten (L. comb) 874

PECTINEAL, pek'tine^'al (L. pecten,

comb ) 101

Process 705

PECTINEUS, pektine'us (L. pecten,

comb) 83, 829

PECTIN I BRANCH I A , pek'timbran^'-
kia (G. pecten, comb + brdnchia,

gills) 421

An order of Gastropoda.
PECTORAL, pek'toeral (L. pectoralis,

referring to the chest) 73, 531

Girdles 6!)9, 100

PECTORALIS, pektoeral'is (same as

pectoral ) 79, 829

PEDICEL, ped'isel (L. pediculus, a

small foot) 218

PEDICELLINA, pedisele'na (N. L.

pedicellus, pedicel ) 310

PEDICLE, OPTIC, ped'ikle (L. pedic-
ulus) 875

PEDICULATI, pedi'ciila'ti (L. pedicu-
lus, a little foot) 645

PEDIGU L OIDE8 VPJNTRICOSU8,
pedikilloid'ez (L. pediculus, louse). 350

PEDI PALPI, pedipfil'pi (L. pes, foot

+ palpus, palp ) 422

An order of Lipoctena.
PEDUNCLE, pedung'kle (L. peduncu-

lus, a small foot) 310

Anterior 85!)

Cerebellar 512

Cerebral 512, 839, 845

PEL ARGON ILAI, pelargo'nium (G.
pelargos, stork, from the resem-
blance to a stork's bill ) 23 '/

A species of Geranium.
PELECYPODA, peleslp'oda ( G.

pelekus, ax + pous) 421, 429

PELLICLE, peTikle (L. pellicula, a

small skin ) 1 38

PELLUCID, pelusid (L. pellucida,

transparent) 439

PELVIS, pel'vls (L. pelvis, basin, the

pelvis) 73, 701

PELVIC GIRDLE 705

PENETRATION PATH, definition of. 551
PENIS, pee'nis (L. male copula tory

organ) .820

PENNATULA SULCATA, penfit'fila

(L. pennatus, winged) 256

PENNATULACEA, penatula'sea (L.

penndtus, winged ) 420

An order of Alcyontiriu.
PENTADACTYL, pen'tfidfik'tll (G.

pente, five + daktylos, finger) 646

PENTASTOMOIDEA , pen'tastomoi'defi
(G. pente, five -f- sloma, mouth) . . . .422
An order of Linguatulida. -

PENTRICHA 154

PEPPERMINT 235

PEPSIN, pep'sin (G. pepsis, a digest-
ing) . 50

PEPTONES, pep'tons ..50, 268

PERCEPTION (L. per, through +
capio, to grasp) , definition of 177

PERCESOCE8, perses'osez (G. perke,
perch + esox, a kind of pike) 64:5

PERENNIALS, peren'nials (L. per,
through -4- annus, year) 232

PERENNIBRA NCHII, percn'nibra ng'-
kei (L. per + annus + G. brangchia,
gills) ..649

PER ENNIBRA NCH8, peren 'n ibranks
(L. per + annus + G. brangchia) .760

PERFECT FLOWERS, definition of . .228

PERIANTH, per'ianth (G. peri, around
+ anthos, flower) 203, 2 ',2

PERIBLAST, per'iblast (G. peri,
around -f- btastos, a bud), definition
of .445

PERIBLEM, per'Iblem (G. peri, around
+ blcma, a coverlet) 228

<J38

INDEX

1'ERlCARDiAL CAVITY, per'ekar'- PERONEAL ARTERY, perone'al (G.

deal (G. peri, around -f- kardia, perone, pin of a brooch, referring to

heart ) 470 the fibula ) 792

Region 530 PERONEUS, peronc'us (G. perone,

Sinus

319

fibula )

84

PERICARDIUM, pere'kar'deuin.47, .7.7, 520 PERSONALITY, definition of ........ 174

PERWHAET1UM, perike'teum ((!. PES, pez (L. pea, foot) .......... 45, 708

peri, around -f- ehaite, loose hair).. PETAL, pet'al (G. petalon, a petal) .242

............................. ;2/C, 218 PETER ............................ 871

I'ERICHONDRIUM, perikon'drf'urn (G. PETIOLE, pe'teol ( L. petiolus, a little

peri, around -f- ehondros, cartilage) .

^t )

203

1 1 1 . 08 1 PETRIFACTION, petnfak'shun (L.

PERI DERM, per'iderm (G. peri -j- /,\ 7ro.s7/s, stony -f- fado, to make),

definition of .

398

, petrog'ale
((». petra, rock + </ale, a weasel) . .6'.7?
PETROHYOID, ])etroiii'oid (G. petroa.
stone -f- hyoides, Y-shaped) 81

derma, skin i 000

/' K R I /U \ I T I/, peridin'ium (G.

peridiriea, whirled around ) 151

PERIDTRAL. peridur'al (G. peri,

around -f L. dura, hard), definition

of 838 PET ROM YZO\ MA RINU8, petro'mi-

PERI \j\ M Pll. per'ilymph (G. peri, /on marenus (G. petros -f- myzein,

around -f- L. li/rnpha. water ) . deiini- suck ) 6'il

tion of 808 PETROM YZOXT/A , potronmon'tia . . .

PER1MY81TM, perimys'ium ((i. pr-ri, - 424, 041

around -(- mys, muscle) 828 PETROSAL. petrow'sal I L. petrosux,

PERINEAL FOLD, per'ine'al (G.

, region between anus and

PEYHJR, pl'er
'

(«)(i

3SD

scrotum)

PERIOSTEUM, pfM-ios'tr-mu (G. pen,
around -f- on I eon, hone), definition
of ...112

PERI PATH X, peripfi'tfis ((!. peripii-
tos, walk about) 429

PERIPHERAL, perilVeral ((i. peri

758 PH'A EOCHROME CELLS, te'okrom (G.

phaeos, dusky -f- <-ltroma, color).

definition of 822

Tissues, definition of 010

PHAGOCYTE, fagV.sit (G. phaaein.
' to eat -f- Ici/los. hollow), definition

of . .199, 21 1

around; phero, bear) 274 PHALANGES, faylan'geefl |(J. pha,l<in.r.

Nervous system (it;. (57 ha.ttle-line) 70, 70S

Segmentation ..100

PURIPHYLLA //rir/.YV1/// I. |,ei i

ftl'a. (G. peri, around -j- pliijUoii.

leaf) '. . . . >~>{;

PERISARC. per'ysahrk (G. peri -f

sar.v, flesh ) 252

PERISSODACTYLA, perisodak't ila (G.

perissodaktuloa, more than regular

number of fingers or toes) 420, 662

PERISTALSIS, per'istal'sis (G. peri

-I- stellein, to place), definition of. . .270
PERITONEAL, perr'itoenee'al, -urn

(G. peri, around; teino, stretch ) . . .536
PERITONEUM, perr'itoenep'um. (See

above) 47. 727

PKRITRWHA, perrit'rika (G. peri +

thrift, h-iir) 419

PERIVITELLINE SPACE, pei -i'vitel'-

y»S', fular'opes (G. phala-

rz'.s1, a coot + pous. foot) 417

PUA NEROCEPHA LA . fan'erosef'a la

(G. plMneros, visible -}- kephale.

head ) : 420

PHANEROGA J/X. fa ner'oga ins ( ( J .

phaneros + yatnos. union) 225

PHANEROGLORRA, fan'eroglos'a ((J.

phaneros -4- L. qlossa, tongue) 651

l^ARYNGEAL POUCHES, definitions

of 728

Pockets 729

Teeth 73:!

PHARYNX, far'inks (G. pharynx, the

throat) '. . .267, 728

PHASCOLARCTU8, faskolark'tos (G.

phaskolos, a leather bag -f- arktos,

bear) 748

In (G. peri + mtellus, volk) 551 PHASES OF A CHEMFCAL SYSTEM, iif,

PERLOIDEA, perloi^ea (N. L. Perla, PHASMOIDEA. fasmoid'ea (G.

phdsma, apparition

eidos, resem

blnnce) . . 423

PHELLOGEN, fel'ojen (G. phelloit,

cork + gene, production ) .-.231

eastern Russia) 397 PHENOMENA, MENTAL, fenom'ena

PEROMEDUSAE, pe'romedu'se (G. - (G- Phenomenon, phenomenon) 173

pera, a pouch -f L. medusa,} . .256, 419 PHILIDIUM, filid'Ium .W$

a proper name + G. eidos, resem-
blance) 423

A sub-class of Pteryaoaenea.
PERMIAN PERIOD (after Perm in

l.NDKX

PHILODIXA, filodi'na (new Latin
word) 308

PHILOSOPHY, tilos'ofi (G. philein, to
love; sophia, wisdom) 40

PHILTRUM, fil'trum (G. philltron, a
love charm ) 731

PHLOEM, floem (G. phloios, smooth
bark) 230

PHOLLDOTA. folido'ta (G. pholis,

scale) > .426

The scaly ant-eater.

PHORONIDIA, for'onid'ea . . 424, 432, 640

PHORONI8 ARCHITECT A, foro'nis
(G. Phoroneus, name of king of
Argos ) 309

PHORONI8, foro'nis (G. Phoroneus) .310

PHOTOPHORES, fo'tofors (G. pJws,
light + pherein, to bear ) 668

PHOTOSYNTHESIS, fotosin'thesis (G.
phos, light -f- synthesis, putting to-
gether), definition of 128. 186

PHOTOTROPISM, fotot'ropizm (G.
phos + trope, a turning), definition
of 127

PHRENIC NERVE, fren'ik (G. phren,
diaphragm ) ?.>;>

PHYCOMYCETES, fikomlse'tez (G.
phykos, sea -weed + mylces, a
fungus) 201, 208, 209

PHYLLAPHIA COW EN I 169

PHYLLOXERA VASTATRIX, filokse'-
ra (G. phylon, leaf + xeros, dry) . .350

PHYLOGENY, filo'jeni (G. phylon,
race + genea, birth ) . . . 3*1

PHYLUM, fye'lum (G. phylon, race,
tribe) 415

PffYSALIA, fisfi'lia (G. physalis, a
bladder) 255

PHYSETER MACROCEPHALV8, fise'-
ter (G. physeter, a blow-pipe).. ..662

PHYSICS 35

PHYSIOLOGICAL CONTINUITY 150

PHYSIOLOGUS, fisiol'ogus (G. phi/si*.
nature; logos, discourse) .... 377

PHYSIOLOGY, fiziol'oji (G. physis +
logos] , definition of 30, 31

PHY808TOMT, flsos'tomi (G. physa,
bladder + stoma, mouth) 761, 824

PIA MATER, pe'amahter (L. pia ma-
ter, kind mother ) 67. 838

PIGEON HORNTAIL, definition of. . .373

PIGEON SKELETON . ?fl.f

PIGMENT, pig'ment (L. pingere, to
paint ) 90

PIGMENT LAYER OF EYE 579

Spots 46

P1LIDTUM, pilid'ium (G. pilidioti, a
small cap ) 307

PINEAL EYE OF LIZARD, pin'eeal

(L. pinea, pine cone) J^fi

Gland or epiphysis 45, 400, 852

/'/.YCN /.\>'7<7'A7,S. piiuis insignia (L.
pin us, a fir + insign-in, notable) . . .231
tiilvestriti. sllves'tris, pertaining to

a forest 238, 240

Pl'M'N 8TROHUK. pinus ( L. pinus, a

pine tree ) 2^0

Virginiana 231

PINWORMS, definition of 302

PI PA AMERICANA, pipa amer'ika'na.650
PISCES, piss'ees (L. fish) . . .242, 431, 641
PISIFORM BONE, pi'slform ( L. pisum, . .

pea ; forma, shape ) 703

PISTIL, pis'til (L. pifttillum, a pestle)

.203, 242

PITH, pith (A. S. pitha. pith), defini

tion of 231, 233

PITHECANTHROPUS E It U C T U 8,
pithekan'thropus (G. pithekos, an

ape + anthropos, man ) 399, 400

PITS, AUDITORY 480

PITTED TUBES, definition of . . .234

PITUITARY BODY, pitti'itayree (L.

pituita. mucus) 66, 490

PLACENTA, plasen'tfi (L. placenta,

<'ake) 626

Embryonic maternal . ... .629

Foetal 631

Function of 627

PLACENTAL ANIMALS 626, 658

PLACODES, AUDITORY, plak'ods (G.

plax, a plate) 480, 574

PLACOID, plak'oid (G. plaa. plate).. 675
PLAGIOSTOMI, plfijios'tomi (G.

plagios, oblique; stoma, mouth.) 424, 642
PL AN ARIA, planar'ia (L. plants,

flat) 28<>

Maculata (L. totacM&ites, spotted) .286

Polychroa 28(>

PLANKTON, plangk'ton ((J. plangktos,

wandering ) 325

PLAXORBI8, planor'bis (L. planus,

flat + orbis, circle) 259

A mollusk.
PLANTARIS, plantAr'is ( L. planta,

sole of the foot ) 85

PLANT BODY' 202

PLANT, FLOWERING 20,1

Histology 22.8

Parts .'.... 202

Seed 203

PLANTIGRADE, plan'tigrade (L.

planta, sole; gradits. walk) 713

PLANT WORLD—

Rryophytes 21 .1

Flowers 242

Fungi 208

Histology , . . . 228

Pathogenic Fun"! 209

Pollination .238

Pteridophi/tes 223

Simple plants 202

Spermatophylc^ 225

Thallophytes 204

940

INDEX

Three higher groupings 215

Yauchena 207

PLANULA, plfm'ula (L. planus, flat) .254
PLASMA, BLOOD, plas'ma (G. plasma,

something formed), definition of... 62
PLASMODIUM IHMACULATUM
LAVERANIA, plazmo'deum imfiku-
lat'um laveran'Ia (G. plasma, a

moulded figure + eidos, form)

Malariae 130

Vivax 133

PLASMOGAMY, plasmog'amy (G.
plasma, a mould -+- gamos, mar-
riage), definition of 147

PLASMOSOMES, plas'mosoms (G.

plasma + soma, body), definition of. 91
PLASTIDS, plas'tids (G. plastos,

formed) 89, 92, 181?

PLASTIDULES, plast'iduls (G. plastos,

formed [diminutive ending] ) 91

PLASTOCHONDRIA, plast'okon'drea

(G. plastos + chondros, cartilage) . 91
PLASTOCONTS, plas'toconts ( G.

plastos, formed) 91

PLASTOSOMES, plast'osoms (G. plas-
tos -f- soma) 91

PLASTRON, plas'tron (F. plastron, a

breast plate) .1102, 719

PLATEMY8 683

PLATO, pla'to 376

PLATOPHRY8 LUNATUS, plat'ofris

(G. platys, flat + ophris, brow)...6'^5
PLATYHELMINTHES, plat'yhelmin'-
thez (G. platys, flat + helmins, a

worm) 265, 285, 427

PLATYHELMINTHES AND NEMA-
THELMIN THES, plat'ihelmin'thez
(G. platys, flat -j- helmins, a worm)

285, 311

Flat worms 285

Turbellaria 285

Excretory system 280

External appearance 280

Muscular system 287

Nervous system 287

Regeneration 288

Reproductive system 287

Trematoda 289

Infections by 291

Life cycle of 289

Cestoda 293

Eggs of 301

Infections by 295

Life cycle of 293

Types of £.%*

"Nematoda (threadworms) 298

Ascaris 299

Digestive system 300

Eggs of 301

Excretory system 300

Infections by 302

Intermediate and Uncertain Forms

..306

Nervous system .300

Parasitic '. 303, 305

Reproductive svstem . .300

PLATYRRHINE, plat/inn (G. platys,

flat + rhis, nose) . 659

PLATYSMA MYOIDES, platis'ma (G.

platysma, a flat piece) 832

PLECOPTERA, plekop'tera (G. plekein

twisted + pteron, wing) 423

An order of Persoidea.
PLECTOGNATHI, plektog'nathi (G.

plektos, plaited -f~ gnathos, jaw) . . .04")
PLECTOPTERA, plektop'tera (G.

plekein -f- pteron) 423

An order of Ephemeroidea.
PLEROME, pler'om (G. pleroma, a fill-
ing up ) 228

PLESI08AURS, ple'siosars (G. plesios,

near -4- sauros, lizard) 651

PLEURA, plew'rah (G. pleura, rib,

side) 315

PLEURAL REGION 536

PLEURITES 336

PLEUROBRANCHS, plor'obranks (G.

pleura, a side + brarigchia, gills) . .321
PLEUROCOOCUS, pliiro'kok'us (G.

pleura; L. coccus, a berry) 18i>

PLEURODIRA, plorodi'ra (G. pleura,

a side -}- deire, the neck ) 652

PLEURODONT, plu'rodfmt (G. pleura

+ odontos, a tooth ) 733

PLEXUS, pleks'us (L. intern -caving) . 07
Choroid, kor'oid (G. chorion, skin
-f eidos, resemblance) ... .512, 57 /

Nerve (58

PLICA SEMILUNARIS, plik'asemilu-
nfiris (L. plica, a fold + semilunaris,
half-moon shaped ) 875

PLICAE 40

F i m b r i a t a e ( L. fimbriatus,

fringed) 741

PLINY THE ELDER 370

PLUMAE, plum'ay (L. pluma, a

feather) 670

PLUMULAE, plew'miulee (L. little

feather), definition of 070

PNEUMATOPHORE, numat'ofor (G.

pneuma, air + phcrein, to 'hear) . . . .2J.7
PNEUMOGASTRIC, nfimogas'trik (G.

pneuma -f- L. gaster, stomach ) . . 68, 885
PODICAL PLATE, pod'ikl (L. podex,

rump) , definition of 337

PODIUM, po'deum ( G. po-u-s, foot ) ,

definition of 708

PODOBRANCHS, po'dobranks (G. .

pous + brangchia, gills), definition

of ....321

PODOPHYRA, podo'fira (G. pous,

foot + phyro, to mix) 1 55

INDEX

941

POECIfjOPQDA, pesilup'oda (G. poeki-

los, many-colored -f- pous, foot) .421, 429

POISON FANG 733

Gland 46, 361

POLAR BODIES, pol'ar (G. polas,

pivot), definition of 101, 444

POLARITY, definition of 288

POLAR NUCLEI, definition of 244

Spindle 444

POLE-CELLS, definition of 259

POLLACK 58$

POLLEN, pol'len (L. pollen, fine flour) 226

Sac, definition of 243

Tube 239

POLLEX, pol'leks (L. pcllex, thumb).

76, 710

POLLINATION (L. Pollen, fine flour),

definition of 238

POLYCHAETA, polike'ta (G. polys,

many + chaite, mane) 283, 420

A subclass of Annelida.
POLYCLADWA, poliklad'ida (G.

polyclados, many branched) 420

An order of Turltellaria.

POLYDON SPATHULA 643

POLYEMBRYONY, pol'iem'briuni (G.

polys, many + embryon, a fetus) . .345
POLYGENETIC, polljene'tik), defini-
tion of ; . 403

POLYGNOTUS, (G. polys, many +

notus, known ) 345

POLYGORDIUS, poligor'dius (G.

polys, many -f- gordios, gordius) . . .283

Appendiculatits 280

POLYMORPHIC, polymor'fic (G. polys,

many + morphe, form), definition

of 255

POLYMORPHISM, pdlimor'fizm (G.

polys -f morphe) 255

POLYP, pol'ip (L. polypus, a polyp) . .251
POLYPEDE, pol'yped (G. polys^many

+ pes, foot), definition of 310

POLYPHYLETIC, polifilet'ik (G. polys,

many -f phylon, race), definition of. 403

POLYPINUM 156

POLYPLACOPHORA, poliplakof'ora

(G. polys, many -f plax, a tablet) . .421

An order of Amphineura.
POLYPTERUS, polip'terus (G. polys,

many; pteron, wing) ...642, 6j3

P O L Y S P E R M Y IN FOWLS, pol'-

ispermy 444

POLYTRWHUM COMMUNE, pol'ytri-
kum comunay (G. polys, many -f-

thrix, hair ) 217

POND SCUM, definition of 204

POND SNAIL 289

PONS, ponz (L. bridge) definition of.

512, 839, 859

PONS VAROLIL pens' varo'lei (L.
pons Yarolii, bridge of Varolius) . . .863

POSTAL BRAIN FLEXURE, pon'tl

(L. pons, a bridge) 837

POPLITEAL ARTERY, pop'lite'al (L.

poples, the ham) 792

PORCUPINE FISH $1,5

PORES, EXCRETORY (G. poros) 266

PORIFERA, porifera (G. poros, chan-
nel + L. ferre, to bear ) 427

POROtiPORA GIGANTE8, por'ospor'a
jigfin'tes (G. poros + L. sporum,
seed -}- gigans, large), definition of. 153
PORTAL SYSTEM, por'tal (L. porta,

gate) 50, 198

PORTUGESE MAN-OF-WAR 255

POST AXIAL, postak'sial (L. post,

behind ; axis, axle) 82

POST-ANAL GUT, post'anal (L. post

+ anus, vent) 590

Branchial bodies (G. brangchia,

gills) 588,. 744

Caval, kavl (L. cavus, hollow) ....
Same as vena cava inferior.

Optic commissure 858

Otic, otik (G. cms, ear) 739

Trematic nerves, tremat'ik (G.

trema, a hole ) 881

Zygapophysis, zlgapof 'isis ( G.
zygon, yoke + apo, from +

physis, growth) ?'#

POUCHES, GILL, definition of 498

Visceral, vis'eral (L. vi^cus, an in-
ternal organ), definition of.... 498

POULTON, PROFESSOR 385

PRACTICAL VS. THEORETICAL. ... 22

PRAWN 325

PRE AXIAL, pre'ak'seal (L. prae, be-
fore + axis, an axle) 82

PRECAVAL, pre'ka'vl (L. prae -f-

cavus, hollow )

Same as vena cava superior.
PRECHORDAL, pre'kor'dl (L. prae +

G. chorde, a cord) 474

PRECORACOID, pre'kor'akoid (L. prae

-f- G. korax, crow) 75

PREFORMATIONISTS, definition of. 381
PREHALLUX, pre'hnl'uks (L. prae +

hallux, the thumb) . . 46

PREHENSILE, pre'hen'sil (L. prae-

hendere, to grasp ) 659

PREMAX1LLA, pre'maksil'la (L.

prae + maxilla, jaw) 48, 73

PREMOLAR TEETH, pre'mol'r (L.

prae mola, a hill ) 73 )

PRE-ORAL GUT, pre'or'al (L. prae +

os, mouth ) 493

PRETREMATIC NERVES, pre'tre-

mat'ik (L. prae + G. trema, a hole). 880
PREZYGAPOPHYSIS, prezygapofisis
(G. zygon, yoke + apo, from +

physis, growth ) 7 3

PRIAPULOIDEA, prlap'uloid'ea (G.
priapos, priapus + eidos, like) 311

INDEX

hPlim (M/ /M77X 31.1

PRIESTLY, pnVtlC? 380

PRIMARY CORTK.X. definition of 230

Vesicle? 4~<S'

. Tissues 228

PRIMATE, pri'malc ( L. primus, first)

227, 426, 658

PRIMITIVE (L. primus, first)

Groove, definition of '/•>•>, 500

Gut 457

Pit .45')

PRIMORDIAL, prnnor'doal (L. primus,

first -f- ordo. order or rank) 100

PRINCIPLES VS. APPLICATIONS.

. .25, 32

PROAMNION. pro'fim'neon (G. pro.

before; amnion, a vessel for receiving

blood) , definition of 457

PROARTHROPODA , pnVarthrop'oda

(G, pro, before; arthros, joint; pous,

foot) .' 425)

PROATLAS, pro'atlas (G. pro + Atlas,

of Greek mythology), definition of . .684
PROBABILITY OF 'ERROR CHART. 20
PROBOSCIDEA. pro'bosld'ea (G. pro

-f- boscein, to graze) 420, 062

PROBOSCIS. prf/lxVTs (G. pro +

boseein ) 285

PROCELLARI1 FORMES, pro'selfirel-

for'mez (L. proc-clla, a storm -\-

forma, form ) 425

PROCESS, TRANSVERSE (L. pro-
cedere, to go forth ) 7.?

PRO-CHORD ATE. pro'kor'dfit (G. pro
-f- chorda, a cord), definition of . . . .638

PROCOELOUS. pro'sclous (G. pro, be-
fore; koilos, hollow), definition of . .683

PROCTODEUM. prok'todfMim (G. pror-
t-os, anus; daio. divide), definition
of 260

J'ROCTOTRYPU). proktotrlp'id (G.
proktos, anus 4- trypan, to bore
through ) 345

PROGLOTTIDS. pro'glot'T'K (<j. pro
4- nlotta,, tongue), definition of.... 293

PROG ON EAT A, progonea'ta (G. pro
4- <ione, a generative organ ) 422

PROJECTION FIBERS (L. proioere,
to throw forward, project), defini-
tion of 840

PROMORPHOLOGY. j.v-.w-fol'oji (G.
pro -(- morpiir. form -f~ logos, dis-
course) 31

PRONEPHRTC CAPSULE, pro'nef'rik
(G. pro -f nephroa, kidney), defini-
tion of ' 606

Chamber . 600

Duct 005

PRONEPHRIDIITM, pro'nefrid'ium (G.
pro -f- L. nephredium, a little kid-
ney ) 2SO

PRONEPHROS. pronef'ros (G. pro +

nephros ) 504

I'RO-NUCLEUS, pronuc'leus (G. pro
nucleus, kernel), definition of. .....

.102, 104, 150

Otic (G. ous, ear) 72, 615

Phase (G. phasis, appearance) . . .,97, 98
PROPOLIS, prop'olis (G. propolis, sub-
stance with which l>ees line their

hives) , definition of W*

PROSENCEPHALON, ])r6s'ensef'alou

(G. pro + en, within 4- kephalos,

head) -. 472

PROSENCHYMA, proseng'kTma (G.

pros, near + engchymft, infusion)

definition of 230

Where found 233

PROSOCOELE, pro'sosel (G. pros, near

+ koilos, hollow), definition of.... 490
PROSTATE GLAND, pross'tayte (G.

prostates, in the front rank).. .282, 819
PROSTOMIUM, pro'stom'ium (G. pro,

before; stoma, mouth) 214

PROTECTIVE, definition of 178

Tissues 234

PROTEIDAE, protc-'ido (G. Proteus, a

sea-god -f- eidos, shape) 649

PROTEINS, pro'tenz (G. -prolos, first),

definition of 96

PROTEROSPONGIA , prot'erospon'jia

(G. proteros, fore 4- spogyia, a

sponge) 151

PROTEU8 ANGUINEU8, prof ens (G.

Proteus, a sea-god ) 649

PROTHALLIUM, pro'thal'f-um (G.

pro, before + 1 hallos, a young

shoot) , definition of 225

PROTHALLUS, pro'thal'us (G. pro +

thallos) 216

PROTHORAX, pro'thor'aks (G. pro,

before + thorax, breast) . . 335

PROTOBRANCHIA, pro'tobrank'ia (G.

protos. first -f branrjchia. yfills) • • • -421
PROTOCONE, pro'tokon (G. protos,

first + konos, a cone), definition of. 736
PRO TOME RITE, prf/tom'erit (G.

protos, first -f- meros, part) 151

PROTONEMA, pro'tdne'ma (G. promos,

first + nema, thread ) 220

PROTONEMATA, pro'tonoma'ta (G.

protos + nema) 219

PROTOPLASM, pro'toplazm (G. protos,

first + plasma, form) 50, 89, 94

PROTOPODITE, pro'top'odit (G.

protos, first; pous, foot), definition

of ..314

PROTOPHYTES, PATHOGENIC, pro'-

toflts (G. protos, first; phyton,

plant) 210

PROTOPTERU8, pro'top'terus (G.

protos, first; pteron, wing) 6Jf6

INDEX

94:3

P If OTOT U E RIA, pro'tother'ia (G.
protos, first; Iher, a wild beast) . .'. .

424, 431, 656, 867

PUOTOTROPHIC. pro'totro'phik (G.
protos, first; Irophe, nourishment),

definition of 1<H

PROTOZOA, protozo'a (G. protos,
first; zoon, animal )..:... 121, 164, 426

Amoeba 121

Behavior of .... 126

Movement of 122

Kuglena 127

Behavior of 129

Encystment 129

External features . 127

Internal features 127

Locomotion 128

Nutrition 128

Reproduction 129"

Flagellates of Uncertain Position. .. 144

l-'aramoecium 138

Pathogenic Protozoa 142

Plasm-odium Mala fine 130

Life cycle 131

How discovered 134

Summary of Protozoa 147, 156

PROTRACTOR, prf/trak'tflr (L. pro,
before; tractus, drawn out), defini

tion of 739, 829

PKOTURA, protu'ra (G. protos, first

+ ura, a tail ) 422

An order of Mirientomata.
PROVENTRICULUS, proh'ventrick'-
yulus) (L. pro, before; venter,

belly), definition of 276, 745

J'KOVIVERRA, provive'ra 73 /,

PROXIMAL, prok'simal (L. proximus,

nearest) 75

PSALTERIUM, solte'rium (L. psalte-

rium, a psalter), definition of 745

PSEUDOBRANCHS. sii'dobranks (G.

pseudes, false; branf/chia, gills) . . . .760
P X EU DOBR A XOHU8 8TRIATU8,

strla'tus (L. stria-tus, striped) 650

PSEUDOPODIA. sii'dopo'dia (G.
pseudes, false: poiis. foot), definition

<>f 172, 215, 219

PSEUDO-REDUCTION, definition of.. 102
PSEUDOTHYROID BODY, su'dothi'-

roid, definition of 588

PSYCHOLOGY, sikol'oji (G. psychos,

the soul -f- logos, discourse) 32, 40

Animal 172-184

Functional 175

Structural 175

PSYCHOZOIC, slkozo'ik (G. psychus

-j- zoon, animal ) $94

PTKRIDOPHYTE8, terld'oflts (G.

pteris, fern; phyton, plant) . . . .203, 223
PTERIS, teris (G. pteris, fern).. . .219

I'TEROtiRA A CHI ATA, tero'lminkia'tfi
(G. pteron, wing -p- brangchia, gills)

424, 640

PTEROSAURS, ter'osors (G. pteron,

wing + saurus, lizard) 651

PTEROTIC, tero'tik (G. pteron, wing

+ oys, ear ) 696

PTERYGOGENEA, ter'igoje'nea (G.

pteryx, wing ; genos, kind ) . . . . 423, 430
FTERYGOI'D, ter'igoid (G. pteryx,

wing, eidos, resemblance)..?^, 616, 692
PTERVGOIDEUS MUSCLES, ter'-

igoid'eus (G. pteryx + eidos}. 832

PTERYGOPODIAL GLANDS, ter'igo-

po'deal (G. pteryx + pous, foot).. 668
PTERYGOQUADRATE, ter'igoquahd'-

rut (G. .pteryx -\- L. .quadratus,

s(juarod ) 690

PTERYLA, ter'ila (G. pteron, feather;

ylc, a wood ) 669

PUBIS, pu'bis (L. pubes, mature) . . . : 77

Homologue of 704

PUBOFEMORAL1S, pubofemoral'is ..829
PULEX IRRITAN8, pew'leks ir'ri-

tahns (L. pulex, a flea) 350

Senaticcps $50

PULMONARY CIRCULATION, pull'-

mownay'ree (L. pulmo, lung) 54

PULMONATA, pulmona'ta (L. pulmo,

lung ) 42 1

PULSATING 93

PULV1LLUS, pulvil'lus (L. pulmllus,

a small cushion ) 337

PUPA, pewpfi (L. pupa, a puppet) . . .

332, 345, 363

PUP ARIA, pewpu'rea (L. pupa) 370

PUPIL, pevvpil (L. pupilla, the pupil

of the eye), definition of 70, 874

PURPOSE OF FLOWER 242

PURPOSITIVENESS 405

PUMILLA, pusil'la (L. pusillus, very

little, pretty) .1^9

I-YCONOGOMORPHA, pikmnronior'fa

(G. pyknos, thick + yony, the knee

-f morphe, form ) 422

An order of Pantopodn.

PYGAL. pl'gal (G. pyqe, rump) ?0£

PYLORI C CAECA, pllor'ik se'ka (G.

pyloros, a gate-keeper; L. caecum,

blind) 747

PYLORIS, pilor'is (G. pyloros, a gate-
keeper) ' 49, 318

PYLORUS, pyeloh'rus (G. pyloros,

gate-keeper) 747

PYORRHEA, pi'ore'fl (G. pyon, pus +

rhoia, a flow) 141

PVR AMI DAL, pyram'idsil (L. pyramis,

a pyramid ) 320

PYRAMIDS (L. pyramis, a pyramid),

in spinal cord, definition of . .845

PYRENOIDS, pi'rgnoids (G. piren, a

frnit stone), definition of..... 128, 204

INDEX

PYRIFORMIS, pirifor'mis (L. pyrum,

pear ; forma, shape ) 83

QUADRANGULAR, quadrang'ular (L.

quadratus, squared ) 73

QUADRATOJUGAL, quadra'tojug'al
(L. quadratus + jugum, yoke),

where found 615

QUADRATUS, quadra'tus (L. quad-
ratus) .... 84

QUARTAN FEVER, kwor'tan (L.
quartanus pertaining to the fourth). 132

QUILLWORTS, kwil'worts 22 ft

A plant of the genus Isoetes with
quill like leaves. One of the Fern
Allies.

QUINCUNX, kwin'kiinjrks (L. quinqne,
five + uncia, twelfth part), defini-
tion of 672

QUININA, kwme'na (L. quinina, qui-
nine) 205

A species of Splrogyra.
RABBIT, rab'et (Fr. dial, robot te,
rabbit) —

Brain of 86,1

Cross section of 832

Dissection of 755

Muscles of 830

Skeleton of 702

Urogenital organs of 820

RADIAL CANALS, ray'deeal (L.

radius, ray ) 253

Fibrovascular bundles 230

Symmetry 247

RADIALE, ra'difi'lf? (L. radius, ray) .

76, 699

RADIATA 255

RADIATION 38, 851

RADICES AORTAE 780

RADIOLARIA 148, 418

RADIOULNA, radioul'na (L. radius,

ray -\- ulna, elbow) 76

RADIUS, rfi'diiis (L. radius, ray) .335, If 03
RADIX, rad'iks or rfi'diks (L. radix,

root) 791

RAIA ERIN ACE A, ra'a (L. raia, a
ray) 61,2

RAINEY'S TUBULES, definifon of . . . Uf3
RAMUS COMMUNICANS, ray'mus (L.

a branch) 65, 513

RAN A ESCULENT A, rfi'na (L. rana,

frog) 51, 571

Fusca 551

Pipiens 43

Sylvatica 583

RANA TEMPORARIA 5J,9

Virescens 583

RANIDAE, ran'ide (L. rana, frog +

idae) 651

RANVI-ER, NODES OF 114

RAPHE, ray'fee (G. seam) 485

RAPHIDOIDEA . ..423

aso'rf'z (L. rasor, a scra-
per) 670

RATITAE, .riitj'te (L. ratitus, marked

with the figure of a raft) 656

Flightless birds.

RATHKE 380

RATHKE'S POCKET 490

RATTLESNAKE'S BITING MECH-
ANISM 13.!

RAY, JOHN 414

REASONING, definition of 182

KEAUMUR 380

RECAPITULATION THEORY (same

as Haeckel's Law of Biogenesis) . .
RECEPTACLE, resep'takl (L. recipere,

to receive) 203

RECEPTACULUM, reseptak'iilum (L.

recipere, to receive ) 344

Chyli 800

RECEPTOR, resep'tdr (L. recipere, to

receive) 177

l>n immunity ,- • • • 198

RECESSIVE, reses'iv (L. recessus,

withdrawn ) 1 (>:>

RECESSUS MAMMILLAR1S .S.'/.fr

Options , 512

RECTUM, rek'tum (L. rcctus,

straight) >tS, 33&

RECTUS, rek'tus (L. straight) 79

RECURRENT NERVE, rekur'ent (L.

re, back -f- currere, to run ) 884

RED GLAND 7(il

REDI, Italian scientist 382

REDIA, rodia (after Italian scientist,

•Redi) 290

REDUCTION DIVISION, reduk'shun

(L. reduotus, reduced), definition of. 102

REDUCTION KNT LIVER 51

REED, MAJOR WALTER 130

REESE 4(53, 500

REFLEX, re'fleks (L. re^ectere, to

turn back), definition of 177

Arcs 8J,7

Centers 851

REFRACTION, refrak'shun (L. re,

l>ack + frangere, to break ) 39

REGENERATION, rejen'era^hun (L.

re, again + generare, to beget) . . . .124

In Hydra 251

REIL, ISLAND OF, definition of 843

REISSNER'S MEMBRANE 86'tf

REMORA BRACHYPTERA (itf

RENAISSANCE, Rennsans' (L. renus-

cor, to be born again ) 377

RENAL COLLAR, ree'nal (L. rem-s,

kidneys) .7.9.}

Portal system 7#,S

Tubules G'I

REPRODUCTION, re'produk'shun (L.

re -\- pro, forth + ducere, to lead ) . 30
REPRODUCTIVE TISSUE . ..238

INDEX

KKPTILK. rep'til (L. repere, 1o crawl)

Brain of M2

Skull of W

Medial eye of 45

REPTILIA, Yeptill'eeah (L. reptilus,

reptile, from repo, creep) .425, 431, 651
RESIDUAL BODY, rezid'ual (L. re-
siduum, residue) 152

RESISTANCE IN IMMUNITY, rezis'-

tans (L. resistere, to resist) 196

RESONATOR, rez'onator (L. resonare,

resound ) 63

RESPIRATION, res'pira'shun (L. re

-f- spirare, to breathe) 62

RESPIRATORY DUCT 870

RESTING STAGE (Ger. .rast, repose). 98
RETE MIRABILE, ree'tee (L. intricate

net) 701

RETICULAR LAYER, retik'ular (L.

same as Reticulate) 665

Theory 95

RETICULATE TUBES, retik'iilat (L.

reticuluni, a small net ) 234

RETICULUM, retik'ulum (L. a small

. net) 85

Of stomach 745

RETINA, ret'inah (L. rete, net) . . .70 ,873
RETRACTILE, retrak'til (L. retractus,. .

withdrawn ) 740

RETRACTOR, retrak'tor ( L. retrahere, . .

to draw back ) 739, 829

Bulbi 828

Muscles 257

RETROLINGUAL GLAND, ret'roling'-

gwal (L. retro, backward + lingua.

tongue ) 742

REVERENT, reve'hent or rev'ehent

(L. revehens, carrying back) 601

Veins .,. 79!>

REVERSION, rever'shun (L. re +

vertere, to turn ) 675

RHABDITE, rab'dit (L. rhabdos, a

rod) 286

RHABDOCOELIDA, rabdose'lkla (G.

rhabdos, a rod -\- koilos, hollow) . . .420
RHABDOME, rab'dom (G. rhabdos, a

rod) 322

RHABDOPLEURA, rabdoplu'ra (G.

rhabdos + pleuron, a rib) SJtf

RHABDURA, rabdu'ra (G. rhabdos, a

rod) 422

RHACOPHORU 8, rakof 'orus ( G.

rhakos, a rag -+- pherein, to bear ) . . 650
RHEIFORMES, re'ifor'mez (G. rea,

rhea -f L- forma, form) 425

Flightless terrestrial birds with

partially feathered head and neck.
R H E O T R O P I S M, reot'ropizm ( G.

rhein, to flowr + trope, a turning) . .127
RHINE NCEPHALON, rl'nensef'alon
(G. rhis. nose + engkephalon,

brain ) . . 848

KHI \OCEROTOWE A, rluOs'erotoi'dea

(G. rhinokeros, nose, horned) 662

RHIZO1DS, ri'zoidz (G. rhiza, root -f

eidos, like ) 208, 220

RHIZOMES, ri'zomz (rhizoma, a root) 223
RHIZOPODA, rizop'oda (G. rhiza,

root H- pous, foot).. 142, 147, 418, 426

Definition of 426

RHIZOPODS 122

RUODILE8 ROSAE, rodl'lez (G. rho-

dites, pertaining to a rose) ,3^5.

RHODOPHYCEAE, rodofl'see (G. rho-

don, rose + phykos, sea-weed) 204

RHOMB ENCEPHALON, romb'ensef-

alon (G. rhombus, magic wheel -\-

engkephalon, brain ) 472

RHOMBOID, rom'boid (G. rhombus +

eidos, like) 475, 67T

RHOPALURA GIERDII, ropal'ura (G.

rhopalon, a club + ura) 307

RHYNCHOTA, ringko'tfi (G. rhynchos,

snout ) 424

A subclass of Pterygogenea.

RIDGE, DENTAL . . .' 732

Friction . . 678

RIGHT, being right one-half one time,

19-26

RIZOPU8 NIGRWAX8 207

ROCK WALLABY ,651

RODENT! A, ro'den'shia (L. rodent,

gnawing) 426, 658

RODS AND CONES 57.9, 873

ROOT BRANCH, root (A. S. wyri,

root) 20H

Cap 22!>

Gall louse 350

Stocks 223

Tap 20$

ROSS, MAJOR RONALD. ....... 135

ROSTRUM, ross'trum (L. rostrum,

beak) .313

ROT ARIES, rotar'es (L. rota, a wheel) 78

ROTATORIA 309

ROTATORS (same as rotaries) 78

ROTIFER A, rotif'era (L. rota, wheel

+ ferre, to bear ) 308, 424, 429, 432

Definition of 432

RUCKERT 466

RUDIMENT, rood'imeiit (L. rudimen-
tum, first attempt) 45

RUDIMENTARY STRUCTURE. . 435, 43G
RUGAE, ru'je (L. ruga, a wrinkle) . .73.9

RUMEN, room'en (L. rumen, the

throat) ...745, 7^

The first cavity of a ruminant's

stomach.
KUMINANTIA, room'inantm ( L.

rumen, the throat) 661

8ABAL, sa'bal (Said to be from a

South American or Mexican name) .235

INDEX

XAVCUAROMYCETKti, sak'aromlset'ez
(N. L. saccharum. sugar -f- G.
mykes, a mushroom), definition of . .210
8A CCHAROMYCO8I8, sak'aromlkd'sis
(G. sakcharon - + X. L. mycosis, defi-
nition of 211

SACCULINA, definition of .325

MAVCULINA CARCINUtt, sakull'na

(L. sacculus, a little sack) .127

SACCULUS, sak'yulus (L. little sac.) 8(57
SACCUS VASCULOSUS, sak'us (L.

saccus, a sac) 8-f)

SAG-LIKE FUNGI 207

SACRAL, say 'era 1 (L. saver, sacred).

definition of • 684

tiAGGITA HEXAPTERA, saj'ita hex
ap'tera (L. saggita, arrow; G. /J.P.TYI,

six; pteron, wing) 308

ST. AUGUSTINE 384

SAINT-HILARE 384

8 A LAM AN DR A, salaman'drA (L. ««/.-

amandra, salamandria)

Atra, atra (L. black) (549

Maculosa, makulosa (L. spotted) .649

8ALAMANDRIDAE, definition of 648

SALIVARY GLANDS, sAl'Ivary (L.

saliva, spit) 131

Definition of 74 1 , 742

8ALIX, sa'liks (L. a willow) ; (species

of willow) - 203

' 8ALMO FAR10, sal'mo (L. salmon) .GJ,.',

The Brook Trout.

SALMON, sam'un (L. satire, to leap). 644
' 8ALPIAK8, sal'pianz (N. L. salpa, a

kind of stock fish), definition of . . . .639
SALTATION THEORY, salta'shon (L.

saltare. to jump), definition of 403

SAL VIA, aar via (L. sage) 3(><>

SALVINIA NAT Ay 8, salvin'ia (after

SaJvini, a Florence professor) 227i

A species of fern ally.

SALVINIALE8 (a Fern Ally) 22 .'/

SAMBONI, DR 13(!

SANTORINI, DUCT OF (named after
•the Venetian anatomist, Santorini),

definition of 75.1

'SAPAJOU, sap'aju (F. sapajoti. word

of Tubi origin)' 659

SAPROLEGNIA, saproleg'nia (G.

sapros H- leffnwi, hem.) .iOl'

SAPROPHYTE, sap'rofit (G. sapros,
rotten -4- phyton, plant), definition

of . 129, 208

8ARCOCY8TI8, sarkosis'tis (G. sarcos,

flesh -f kystis, bladder) 153

Lindemawni 1 53

Afiesctieriana 143, 1 53

Muris 153

SARCODE, sar'kdd (G. sane, flesh) . . . 147
13ARCOLEMMA, sar'kolem'a (G. sarx,
flesh + lemma, skin), definition of. 113

SARCOMKRES, sar'kfmuVr/ (G. same,
flesh -f- meros, part), definition of.. Ill

8ARCOPHAGA, sarko'faga (G. sarx,
flesh -f- phagein, to eat) 850

SARCOPLASM, sar'koplasm (G. sarx,
flesh + plasma, something moulded),
definition of 113

NARCOPTEN SCAB El, sarkop'tez (G.
sarcos, flesh + koptei-n, to cut) . . . .350

KARCONPORIDIA 143, 419

Definition of 153

SARCOSTYLE, sar'kostll (G. sarx,
flesh + stylus, pillar), definition of.113

&AR8APARILLA, sarsaparil'a (Basque

sarizia, a bramble) :V.i?

SARTORll^S. sartoe'reeus (L. sartor,
tailor ) fit)

SAJJRIA, so'ria (G. sauros, li/ard),
definition of S,2i>, 653

SAUROPSIDA, sahropp'sidah (G.
sauros, lizard; opsis, appearance),
definition of . '. 638, 663

SAVIGN Y'S LAW 313

SAW-FISHES, ssl fish'es (M. E. s<i<ih<:
a saw ) 642

SGALA MEDIA, skfila media ( L. scala,

ladder -j- medium, middle) 86(i

Definition of 868

Tyampani, definition of 868

Vestibuli, definition of 868

SCALARIFORM TUBES, skala'rifOrm
tubs ( L. scala, ladder -+- forma,
form -\-tubns (water) pipe), defini-
tion of 234

SCALOPti AQVATICUS, skfi'lops (G.
skalops, a mole ) 658

UCAPHO GNA THITE, skafogmith'it,
skafog'nathit (G. skaphe, boat -+-
gnathos, jaw ) 31 )

SCAPHOID, skaf'oid (G. skaphe, boat
+ eidos, form ) 709

SCAPHOPODA, skafo'poda (G. skaphe,

boat -f- pous, foot) 421

Definition of 429

SCAPULA, skap'yulah (L. shoulder
blade) 75, 703

8CHI8TO8OMUM JAPONICUM, skis- .
to'somum (G. schistos, cloven -4-

soma, body) 291

HaematoUum 290, SOI

Japonicum vel Oattoi. definition

of 291

Japonicum 301

Mansoni 301

KCHIZOCARDIUM li If A Z /L1EN8U,
ski'zdkar'dium (G. schizein, to cleave
-f kardia, heart) 546

S( IIIZOMYCETES, skT/'onilsf-'tez (G.
schizein -j- mykes, fungus) .... 190, 210

SCHIZONTS, skiz'ont's (G. schizein, to
cleave + ons, being), definition of.. 132

SCHLEIDEN ..381

INDEX

947

ISCHLERENCHYMA (same sis Scle-

renchyma, which .see ) .
SCHMIDT. INCIST RES OK. definition

of ' ;.'". iu

SCHULL 418

SCHULTZE 381, 386, 388

SGHWANN (see also Schleiden)

381, 386, 388

.SCIATIC ARTERY, sighat'ik (L.
sciaticu-s, originally ischiadicus, from
(Jr. ischion, hip) . 792

SCIENCE, si'ens ( L. tscientia, knowl-
edge), definitions of . . . 25

Object of 26, 35

SCIENTISTS VS INVENTORS 25

SCLERA, OF EYE, sklay'rah (G.
skleros, hard) 87 '2

S< LKRENCHYMA, skelreng'kima (G.
xklcros, hard -f- engehyma, an infu-
sion ) , definition of . . 234

S( WRITES, definition of 336

SCLEROTIC BONES OF EYE, skle-

rot'ik (G. skleros. hard) 875

Cells 235

Coat 70, 687

SCLEROTOME, skle'rohtome (G.
skleros, hard; tome, cutting) ,}6'/

SCOLEX, sko'leks (G. skolex, worm),
definition of 293

XCORPIONE8, skorpio'ne/ (G. skor-

pios, scorpion ) 422

An order of Arachiwidea.

SCOURING RUSHES, skour'ing (M.
L. saurare, to rub ) 22$

SCREAMERS, skre'mers (Sw. skria,
cry aloud ) 719

SCROTUM, scro'tum (L. uncertain
origin ) , definition of 811

SCUTE, skiut (L. wtliun. shield),
definition of 67<>

8CYPHOZOA, slfozo'a (G. skyphos, a
cup + zoon, animal) 256. 419, 427

SEA ANEMONE* 251

SEA-COW 661

XEA-HORSE 644

SEA-SQUIRTS, definition of 63')

fit E A - U R C H I A. EGOS. EXPERI-
MENTS ON 206

SEBACEOUS GLANDS, seba'shus (L.
sebum, tallow), definition of 673

SEBIFIC GLANDS, sebif'ik (L. sebum,
tallow) 344

SECODONT, sek'odont (L. secare, to
cut -f G. odous, tooth) 734

SECONDARY TISSUES, sek'ondari
ti'shus (L. sccondus, second -f- F.
tissu, woven ) 231

SECRETION, sokreshun (L. secernere,

to separate) . . . 30, 63

SECRETION CAVITY", definition of. . .238

SECRETING CELLS 237

SECRETIONS.

Intestinal 52

SEED LEAF, sed lef (A. S. saed, seed
-h leaf, leaf ) 203

SEED PLANTS, sed plants (A. S.
saed, seed + L. planta, plant), defi-
nition of 203

SEED SCALE, sed skill (seed -f- L.
scald, ladder) . . .... . . . .239

SEESSEL'S POCKET, definition of. . .493

SEGMENTAL APPARATUS, segmeri':
tal apara'tus (L. segmentum, a seg-
ment + apparatus, an apparatus),
definition of 848

SEGMENTAL DUCT, definition of.... 605

Plate, definition of .' . . .569

Zones, definition of 458

SEGMENTATION, seg'mentfishun (L.

segmentum, a segment) 105, 106

Cavity, definition of . .448

SELACHH, sehVke! (G. selavhos, a

cartilaginous fish ) 424

Definition of 642

SELECTIVE FACTOR, selektiv (L.
seligere, to choose), definition of... 404

SELENODONT, sele'nodont (G. selene,
moon + odous, tooth) 734

SELLA TURCICA, sel'a tur'sika (L.
sella, a seat + turcicus, Turkish;
also called fossa hypophyseos ) .... 687

SEMICIRCULAR CANALS, sem'iser'-
kular (L. semi, half + circulus, cir-
cle), definition of 867

SEMILUNAR GANGLION, seinilfi'mir
gang'glion (L. semi, half -f- luna,
moon). (See trigeminal ganglion) . .884

SEMILUNAR VALVES, semm'iliu'nar
(L. semi, half; luna, moon), defini-
tion of 541

SEMIMEMBRANOSUS, semi in e m'-
brano'sus (L. semi, half + mem-
branosus, membranous) definition of. 83

SEMINAL RECEPTACLES, sem'Inal
(L. semen, seed) 266

SEMINAL VESICLE . . . .86.819

SEMINIFEROUS TUBULE, semm'i-
niff'erous ( L. semen, seed : fero, to
bear), definition of 520

SEMITENDINOSUS semiten'dinosus
(L. semi, half -f- tendere, to stretch),
definition of 83

SENNA, sen'a (Hind, sena, a name) . .23%

SENSORY MEMORY, sen'sori mem'ori
(L. sensus, sense + memoria, mem-
ory) , definition of . . . 178

SENSU STRICTO 423

SEPAL, se'pal (G. sepalon, a sepal) . .242

SEPTA, sep'ta (L. septum, partition),
definition of . .266

948

1 NDEX

8&PTW&ANOHIA, sep'tibrang'kia (L.
septum,, partition -j- G. bra-ngchia,

gills) . 421

An order of Pelecypoda.

SEPTIC, definition of 192

SEPTUM, sepp'tum (L. fence, wall)

Interbranchial 758

Oblique 756

Pellucidum Xl,2

Transversum, definition of 756

SERIAL HOMOLOGY, se'rial (F.

seriel, serial ) 316

SEROSA IN INSECTA, seerow'sah (L.

serum, serum ) , definition of 346

SEROUS GLANDS, se'rus gland (L.
serum,, serum + glans, an acorn ) ,

definition of 742

8 E RRAN U 8, sera'nus (L. serra, a

saw ) 824

SERRATUS, sera'tiis (L. serra, saw),

80. 829

SERTOLI

Foot or sustentacular cells of. 520. 811
(See also testes.)

SERUM, se'rum (L. serum, serum) . . 62
SESAMOLD BONES, sess'amoid (G.
sesamon, a plant, referring to the

shape of the seeds ) 828

SESSILE, ses'il (L. sedere, to sit)... 205
SETAE, se'te (L. seta, bristle) 259, 283

SEVERINUS 371)

SEX-LINKAGE, seks ling'kaj (L.
sexus, sex -f lingula, a small tongue
from ligare to bind), definition of . . . 168
SEXUAL REPRODUCTION, sek'sual
reproduk'shun, in Vaucheria (L.
sexus. sex -f- re, again -f- producere,

to bring forth) 207

SEYMOUR! A 694

SHANK, shangk (A. S. scanca, bone

of leg), definition of 45, 84, 708

SHARK-SUCKER 645

SHELL-GLANDS, shel glands (A. S.

schell, shell + L, glans, an acorn) .816
SHOCK, ANAPHYLACTIC, definition

of, see anaphylactic 201

SHORT-HORNED GRASSHOPPER. . .352
SHREW, shru (A. S. scrcwa, a wicked

person, a shrew-mouse) 658

SIEVE TUBES, siv (A. S. siefe), defi-
nition of 230, 235

SIGMOID FLEXURE, sig'moid flek-
shur (G. E. sigma, -f- eidos, shape,

-f L. flexus, the bending) 748

SILICA, si'lika (L. silex, flint) 93

SILUROIDS, silu'roids (G. silouros,
a river fish) 761

SIMIA SATYRUS, sim'ia (L. simia,
an ape ) 660

SIMPLEX UTERUS, sim'pleks (L.

, simple), definition of 819

SINUS, sigh'nus (L. sinus, curve) . . . .284

Rhomboid, definition of 475

Terminalis, definition of 467

Venosus 55, 58, 501

SINUSOID, sm'usoid (L. sinus, curve
+ eidos, shape ) 795

S1PHONOGLYPHAE, sifonoglif'e (G.
siphon, tube + ylyphein, to en-
grave) 256

SIPHONOPHORA 255, 419

8IPHUNCULATA, slfungkula'ta (L.

siphunculus, a small tube) 423

An order of Pterygoge-nea.

SIPHUNGULOIDEA, sifunkuloid'ia
(L. siphunculus, a small tube -f- G.
eidos, shape), definition of 311

SIPHUNCULUS NUDUS, sifun'kulus
niidus (L. siphunculus, a small tube
+ nudus, naked) 311

SIREN LACERTINA 6S,9

SIRENIA, sire'nia (L. siren, a siren

or mermaid) 426

Herbivorous, educabilian, placen-
ta! mammals, having the body fish-
like in form with the hind limbs and
pelvis more or less atrophied, and the
body ending in a horizontal expan
sive tail.

Definition of 662

SIRENIDAE 650

SKATES, skats (probably L. squatus,
a kind of shark) .641, 642

SKEINS, skans (Ir. Gael, again,
split) ? :. 99

SKELETOGENOUS SHEATH, skell'
etoj'eenous (G. skeleton, from skello,
make dry ) 612

SKELETON, INTERNAL, skel'eton
(G. skeletos, dried, hard) 4(i

SKULL, skill (M. K. xkullr, the era
nium) 73

SKULL—

Development of Wi)

Of reptile 692

Representative types of 694

SLOTH, sloth (A. S. slaw, slow) 659

SNAKES, snaks (A. S. snaca, creeper). 651

Definition of 654

Skull of 692

SOCIOLOGY, sosiol'Oji ( L. socius,
companion -}- G. logos, discourse) ... 32

SOL 148

SOLAR PLEXUS, soh'lar (L. sol, the

sun; plexus, interweaving), definition
of 878

SOLIFUGAE, solif'uje (L. sol, sun +

fugere, to flee ) 422

An order of Lipoctena.

SOMATIC LAYER, sohmat'ik (G.

som®, body), definition of . .259

INDEX

949

,StOJ/17'O/i//-.4N7'.S. som'atoblasts (G.
soma, body -f- blastos, bud ) , defini-
tion of . .' 281

SOMATOPLASM. som'atoplasm ( G.
soma, body -(- plasma,, something
moulded) , definition of 99

SOMATOPLEURE. soh'matoeplure (G.
tsoma, body; pleura, side), definition
of 25$)

SOMITE, sob/might (G. soma, body),

definition of ! . .264

Metameric, definition of 460

SOREX VULGARI8, sor'eks (G.
hyrax, a shrew-mouse ) 658

SORI, definition of : . : 224

SORUS, so'rus (G. soros, a pile) 225

SOUL, sol (Goth, saiwala, soul), defi-
nition of 174

SPALLANZANI 200, 380

SPECIES, spe'shiox (L. species, a par-
ticular kind) 164

Definition of 402

Origin of new 408

SPECIFIC, spesif'ik (L. species, a par-
ticular kind -f- facere, to make ) , defi-
tion of 95

SPELERPES FUSCUS, speler'pez fus'-
kus (G. spelaion, a cave + herpein,
creep) 738

SPERM, sperm (G. sperma, seed) 8f>

S P E R M A R T K S, sper'maries ( G.

sperma) 86

Of Hydra 2J8

SPERMATHECA. sper'mathe'ka (G.

sperma, seed + theke, a case ) 7-7 /

Of insect, definition of Ml

SPERMATIC, spermat'ik (G. sperma,
sperm).

Artery 793

# PERM A T 0 CYTE8, sper'matosits
(G. sperma* seed -f- kytos, hollow),
definition of ". 100

SPERMATOGON1A. sper'm}»to!/f/nia
(G. sperma, seed -+- yonos, offspring),
definition of '. 1 00

S P E R MATOPHORKS, sper'matr.fr.rs
(G. sperma, seed -f- pherein, to

bear) 276

Definition of 344, 825

8PERMATOPHYTEK. aper'matoflt*/
(G. sperma -f- phi/ 1 on, plant), defi-
nition of 203. 225

SPERMATOZOA. sptVinatozf/a (G.

. sperma -f- zoon. animal), definition
of 101

SPERM-SAC 218

SPHAEROPH YRA, sforof'ira ( G.
sphaira, a ball -)- phyro, to mingle). 155

SPffAGNALES. sfagna'lez (G. sphaq-
nos, a kind of moss), definition of. .215

ftPffARGIS CORLACEA. sfar'jis kor'-
lasea . . .652

SPHENETHMOID, speneth'moid (G.
sphen, wedge -j- cthmos, sieve -j-
eidos, form) 72

UPHENISCIFORMES, sfenis'kifdrm'ez
( G. spheniskos, a wedge -f L. forma ) 425

Penguins. Flightless marine birds
with small, scale-like feathers; wings
modified as paddles for swimming;
one family.

8PSENODON PUXCTATUM, afee'noh-
don (G. sphen, wedge; odous, tooth ).65t

SPHENOID, sfee'noid (G. sphen,
wedge ) 692

SPHINCTER MUSCLES, sfing'kter (G.

sphinggein, to bind tightly) 763

Definition of 828

XPIHERWORT, splderwert (name of
plant) 22!)

SPINAL CORD, spl'nal kord ( L. spina,

+ G. chorde, a cord ) 67

Cord, cross section of 877

SPINAL NERVES, spi'nal nervs (L.
spina, spine -f- nervus, sinew) ...... 67

SPINA SCAPULAE, spe'na skapule (L.
spina, spine + scapula, shoulder-
blade) 704

SPINDLES, spin'dls (A. S. spinnan, to
spin ) 99

SPINE, spin (L. spina, spine) ... .67. 681

SPINE, DORSAL, spin' dor'sal (L.
spina, spine -f- dorsitm, back) 74

SPIRACLE, spear'ahkel (or spire'ah-
kel) (L. spir<icnluiii, air hole), def-
inition of : 329

Of fishes 766

Of in.sect t-a r 336

Of tadpole .549

SP1RACULUM, spfrak'nlfim ( L. spi-
raculum, an airhole), definition of. 729

SPIRAL TUBES, spi'ral tubs (L.
spira, a coil -\- Ititnis, a pipe), defi
tion of 234

SPIRAL VALVES, spT'ral valvs (L.
spira, a coil + ralca, a fold), defi-
tion of 754

S P I R E M E STAG K, spl'ri-m ( G .

speirema, a coil) .97, 99

Definition of 98

SPIRILLUM, splril'um (L. spirillum.
a small coil) , definition of 190

SPIRITUALIST, spl'ritiiallat (L. spir-
itualis, spiritual), definition of.... 174

8PIROCHAETA RECURRENTlti, def-
inition of 1/f6

Refringens Uj6

8PIROCHAETK .PENTIUM. spTroke'ta
(G. speira, a coil -f- chaite, a

bristle) tJ,6

Plicatilis ////?

HPIROQYRA, splroji'ra (G. speira -f
ffyros, a circle), definition of 20/i

D50

INDEX

f*PIR08rOMl!Al, spiros'toinum (G.
speira + stoma, mouth) .154

SPLANCHNOLOGY, splangknol'ojT (G.
splanchnon, an entrail + logos, a dis-
course ) 31

SPLANCHNOPLEURE, splank'nob-
pluro (G. splanchnon -j- pleura, side),
definition of 259

SPLEEN, splen (G. splen, spleen). 48, 53
Development of 604

SPLENIAL, splee'neeal (G. splen,
spleen ) 097

SPONGIOPLASM, spon'jioplasm (L.
spongid, a sponge + G. plasma,
something moulded), definition of.. 90

SPOONBILL fi',3

SPORANGIA, sporan'jia (G. sporos,
seed 4- anggeion, vessel), definition
of . 243

SPORANG10PHORES, sporan'jiofors
(G. sporos, seed 4- anggeion, vessel
-f- pherein, bear), definition of .209

SPORANGIUM, sporfm'jiiim (G.
sporos, seed 4- anggeion, vessel ) ,
definition of 209

SPORE-CAPSULE, spor-kap'sul (G.
sporos, seed -f L. capsula, capsel) 218

SPORE-CASE, spor-kas (G. sporos,
seed 4- L. capsa, chest, box), defini-
tion of 209. 219

SPORE-MOTHER-CELL ( a rcheospore )
- 243

SPORE-MOTHER CELLS 219

SPOROBLASTS, spo'roblasts (G.

sporos, seed 4- Wastos, bud) 152

Definition of 132

SPOROCYST, spo'rosist . (G. sporos,
seed 4- kystis, bladder), definition
of 290

SPOROGENESIS, spo'roje'nesis - (G.
sporos, seed + genos, offspring) . . . . 13 1

SPOROGENOUS, sporSj'enus (see
Sporogenesis ) , definition of 219

SPOROPHYTE, spo'rofit (G. sporos,

seed + phyton, plant) . 215

Definition of 219

HPOROTRWHOSES, sporotrlko'ses '(G.
sporos 4- thrix, hair), definition of). 21 3

VPOROTRICHTJM, Beurmanni 21S

8POROZOA, spo'rozo'a (G. sporos,

seed 4- zoon, animal) 143, 410

Definition of 151 427

SPOROZOITES, sp&Yozd'Its" (see
Sporozoa). definition of 132

SPORT IN NATURE ] 04

SPORULATION, spor'iilashun (L.
sporula, a small seed), definition of. 126

S Q U A L U 8 ACANTHI AS, skwa'liis
a'kanthi'as (L. squalus, a kind of
sea-fish ) ^42

SQUAMATA, skwammay'tah (L. squa-
ma, scale) . . 425

S Q U A M O S A L. skwamf/sal ( L.
squama, scale ) 72. 74

SQUAMOSALS 618

SQUAMOUS, skwa'mus ( L. squama,
scale) , definition of 109

8QUARROSUM, skwaro'sum (L. squar-
rosus, scurfy). (Species of Sphag-
num) 216

STAMENS, sta'mens ( L. stare, to
stand) 20.3, 2><2

STAPES, stay 'pee/ (L. stirrup) .716, <S6'7

S T A P H Y LOCOCCUS, staf 'Ilokok'us
(G. staphyle, a bunch of grapes +
kokkos, a berry), definition of 191

STARCH, startch (Ger. starke, starch) 50

STATOCYST, statoslst (G. statos, sta-
tionery + kystis, bladder) ... .255, 323

STAURO MEDUSAE, staromeduse (G.
stauros, a cross -f L. Medusa, femi-
nine name) 256, 419

8TEAP8IN, stofip'sln (G. stear, tal-
low + pepsis, digestion ) 50, 268

8TEGOCEPHA LA , steg'ohseff'ahlah
(G. stego, cover; kephale, head) ....
.425, 646, 69 'f

8TBMONITI8, sttmioni'tis (G. stemon,
warp in loom, a thread -f itis, in-
flammation ) 205

STENOBOTHRUS. stenobuth'rus (G.
stenos, narrow + ItoUiros, a hole) . ..{.in

STENOPS, sten'oj)s ((!. xlcnos. narrow
+ opsis, to look) 711

STENSON 3sn

Duct of 742

Gland of, definition S72

MTENTOR, sten'tor (G. Stentor, a
Greek hero in the Trojan War),
definition of * | f> I

STERIGMATA, sterig'mata (G. *te-
rigma, a support) >dl

STERILIZATION, sterTliza'shun (L.
sterilis, barren), method of ...... 101

STERILIZING, sterlll'/ing (L. uteri Ha,
barren) 3$

STERNAL, ster'nal ((;. xtcrnon,
breast) 3i<>

STERNEHHA. 'slir'nebrali (G. sternon
-j- L. rrrf.ebra, joint) .101

STERN1TES, ster'nlts (<;. stemon.
breast), definition of . .'.330

S T E R N OCLE1DOMASTOI 1). stiVnf.
(G. strrnon, breast-plate + kiris, key
4- was/o.s,. mastoid 4- eidos, resem-

blance) S29

Also Sternomastoid.

STERNOHYOID, sternohi'oid ( G .
sternon .4- hyoides, Y-shaped) 81

VTERNOTHA KRUfH NIGRJOA N8, stor-
notho'rus (G. sternon 4- thairos,
hin^e of a door) 7/,9

STERNUM', sfciT'numm (G. sternon,
breastbone). defiTiition of.. .3/5

I XDEX

STIGMA, stlg'ma (G. stigma, a pricked

mark ) . . . '. 121, 242

Of insect ear .'f-W

STIGMATA, stig'mata (plural of

stigma ) , definition of 334

STIMULATION, sti'mula'shun (L.

stimula-re, to incite) 52

STIMULI, stl'mtill (plural of stimu-
lus) 30

STIMULUS, sti'mulus (L. stimulare,

to incite) 77

STING, OF BEE, sting (A. S. stingan,

to sting ) 361

STIPES, sti'pez (L. stipes, stalk),

definition of 335

STIPPLE IMAGE, stip'l (Ger. stippen,

to prick ) , definition of 321

8TOLOXIFE&A, stti'lonlf'era (L. stolo,

a shoot + ferre, to carry ) 420

Definition of 256

STOMA, sto'ma (G. stoma, mouth) . . .220
STOMACH, stu'mak (G. stomacJtos,

throat, gullet ) ; . . //8

S TOM AT A, std'matfi (G. stoma,

mouth ) 222

STOMODEUM, stdw'mohdeeuin (G.
stoma-, mouth : da-io, divide ) , defini-
tion of 2<i()

8TOUOXY8 CALC1TRAH8, stomok'-
sis kal'sitrans (G. stoma + oxys,
sharp ) 3(50

STORAGE TISSUES, stor'aj (L.
staurum. to store ) 237

STRATIFIED, stra'tlfld (L. stratum,
laver -4- facere, to make), definition
of* . .109

STRATUM, stray'tum (L. a spread or
cover ) 605

STREAMING MOVEMENT ... 130

STREAMING OF PROTOPLASM 552

STREAM OF CONSCIOUSNESS, strem

(Ger. strom, a stream), definition

of 174

8TREP81l'TERA, strepsip'tera (G.

strephein, twist -4- pteron, wing) . . .423

An order of Coleopteroidea.
STREPTOCOCCUS, streptokok'us (G.

streptos. twisted 4- kokkos, a berry). 100

STREPTOXEURA, streptonu'ra (G.
strephein, twist -4- pteron, wing) . . .423

STREPTOTH RL\ A OT1 KOMYCEH,

strepto-thriks( G. streptos -f- thrix,

hair) .21.',

Israeli 21',

Madurae , 214

STRIATED, Rtrlfi'ted (L. stria,' A chan-
nel) 77

S T R O B IL1ZATION. stro'hiliza'shun
(G. strohilos. a fir cone) . . . .293

LOIDKti J \TKNTINALI8,

stron jiloi'df'z (G. strongylus. glohu-

lar -f- eidos, resemblance) 30ft

8tercoralis M).i

STRUCTURE DKTERMIXKS FUNC-
TION, strflk'tsher (L. struere, to

build) 176

STRUCTURE, EXTERNAL, struk'tsher 44

Internal 4ft

STRUGGLE FOR EXISTENCE 403

STRUTHIONIFORMES, struthion'ifor-
mez (L. struthio, ostrich 4- forma,

form ) 425

The ostrich. Flightless, terres-
trial, two-toed birds.
STURGEONS, ster'joris (F. wlurfleon,

a sturgeon ) (542, H^.i

STYLE, stil (L. stilus, a pricker) 3//f

STYLO-HYAL, sti'lo-hf'nl (G. stylos,

column -f- Tiyoides, Y-shiped) 738

STYLOID PROCESS, stTl'oid (G.
stylos, pillar -f- eidos, resemblance),

definition of 738

8TYLONIOHIA 154

STYLORHYNCHUR 151

SUB ARACHNOID, subarak'noid (L.
sub, under ; G. aravhnc, sj>idor; eidos,

resemblance ) 838

SUBARTTCULAR PADS 40

SUBCEPHALIC POCKET. <lelinit:on

. of 464

SUBDURAL, subdu'ral (L. sub, under;

durus, hard) .838

SUBGENITAL PLATE, siibje'nital (L.

sub, under -4- gendtalis, genital) . . . .337
SUBINTESTINAL VEINS, subintes'-
ti'nal (L. sub, under 4- intestinus,

internal) 704

SUBJECTIVE, subjek'tiv (L. subiicere,

to throw under) , definition of 173

SUBLTNGUA, sublTii'giin (L. sub, un-

. der + lingua, tongue) 741

SUBMANDIBULAR GLAND, snbmfin
dib'iilar (L. sub. under -4- mandib-

ula,, a jaw) 742

SUBMENTUM, submen'tum (L. sub.
under + mentum. chin), definition

of . . . : 335

SUBMUCOSA, submukr/sa (L. .sub,

under + mucosus, mucous) 4(1>

SI^BSTANTIA ALBA, substan'sia al'ba
(L. substantia, substance -f- alba,

white) , definition of -. 847

Grisea, definition of 847

Propria o 1 4

SUBTHALAMUS, siibthfi'Iaiuiis (L.
sub. under 4- G. thalamos, bridal

chamber) , definition of 851

SUBUMBRELLA, sfibumbreria (L.
sub, under + It. ombrella, from
ombra. shade), definition of.. ..254.

INDEX

SUBI'NGUIS. ftiibfing'gwis (L. sub,
under -f- ungnis, nail), definition of. 667

SUBZONAL LAYER, subzo'nal (L.
sub, under -+- zona, belt) definition
of 621

SUCKERS, OF TADPOLE, siik'ers (A.
S. sucan, to suck ) 548

SUCTORIA, sukto'ria (L. sugere, to
suck), definition of.. 153, 155, 410, 424

SUCTORIAL, sukto'rial (see Suctoria)730

SUCTORIAL PADS 254

SUGAR, shii'gar 50

8TJIDAE, su'ide (L. SMS, swine + idae,
like) , definition of 660

'-SUINA, sui'na (L. sus, swine), defini-
tion of 661

SULCI, sul'si (L. sulcus, furrow) ....

Of brain : 843

Lateral limiting, definition of 4«n

SULCUS, sull'kuss (L. a furrow or
groove) 730

SULZE (jelly) 632

SUPERPOSITION IMAGE, su'perposi'-
shun (L. super, above; ponere, to
place) 322

SUPPORTING TISSUE, supor'ting (L.
supportare, to bring up to), defini
tion of • 234

SUPPORTING TISSUE CELLS OF
VERTEBRATES 402

SUPRAOCCIPITAL, su'praoksip'ital
(L. supra, above -4- occiput, back
part of head) 73

SUPRAORBITAL LINE, su'praor'bital
(L. supra, above + orbis, a circle) .582

SUPRA-PERICARDIAL BODIES, sii-
praperikar'dial (L. supra, above -f-
G. peri, around + G. kardia, heart),
definition of 588

SUPRA-RENAL, supra-re'nal ( L.
supra, above -f renes," kidneys), defi-
nition of 822

( See also adrenal, epinephric. )

:SUPRAROSTRAL CARTILAGES, su
ipraros'tral (L. supra, above -4- ros-
trum, beak ) 613

SUPRASEGMENTAL APPARATUS,
suprasegmen'tal (L. supra, above -f
segmentum, segment), definition of. 848

SUPRATEMPORAL LINE, siipratem'-
poral (L. supra, above -f tempus,
time) .584

SURANAL, siirfmal (L. supra, above
-4- anus, anus), definition of 336

SURANGULARE, siirang'ulare (L.
supra, above + angulus, an amjle) .692

SURANIM TOAD, suri'nam (South

American name ) 650

SURFACE TENSION THEORY, sur'
fas ten'shun (L. super, upon +

fades, face), definition of. 122

SURVIVAL OF THE FITTEST. . . .403

SURVIVAL VALUE, servl'val ( L.
super, over -f- vivere, to live), defi-
nition of 403

SUSPENSOR1UM, suspensor'ium (L.
suspendere, to hang down), defini-
tion of 74

SUTURE, su'tur (L. suo, to sew) 336

SUTURES, sfi'turz (L. sutura, a

seam, in insects), definition of 336

SWAMMERDAM 370

SWAN, CIRCULATION IN, swan

( Ger. schu-an } 785

SWEAT GLANDS, swet (A. S. swat),

definition of 672

In amphibia 63

S\\ '1MMERET, svvim'eret ( Dim. of

sicimmer) 313

SYLVIUS, JACOBUS 378

SYrLVIUS 380

Aqueduct of, definition of... 511, 845

Fissure of 843

SYMBIOSIS, simbio'sis (G. symbioun,

to live together ) 208

# Y M KRANCHIL simbrang'kii (G.

syn + brangchia) 043

SYMPATHETIC CHAIN, simpAthet'ik
(G. syn, with + pathos, feeling) .... 65

Ganglion 65

Ganglia 514

Nervous .system -77.S

Of insect 35!)

Nervous system, definition of 00

8YMPHOPLEONA, simfople'ona (G.
.symphorein, to bear together -}-

pleion, more) 422

An order of Collevibola.
8YMPHYLA, sim'fila (G. *i/n +

phylon, tribe) 422

An order of Proganeata.
SYMPHYSIS, simm'feesis ((.». union)

77, 097

SYNAPSIS, sinap'sis (G. synapsis,

union) , definition of 101

SYNCYTIUM, slnsit'ium (G. syn. with

+ kytos, hollow), definition of. 11 4, 207
SYNERGID, siner'jid (G. synergos. co-
operating), definition of 244

SYNOTIC TECTUM, synot'ik (G. *//»,
Together, ous, ear), definition of 087

SYNSACRUM, sinsa'krum (G. syn,
with + L. sacrum, holy) 706

SYNTHESIS TISSUES, sm'thesis (G.
syn, with + thesis, a setting, an ar-
rangement), definition of 236

SYPHILIS, sif'ilis (N. L. syphilis,
from poem "8yph*>lu&"} 140, 147

SYRINX, sir'inks (G. pipe), definition
of ! Iffi't

NYRPHUN FL1K8, ser'fus (G. syrphos,
a gnat) 868

SYSTEMATIC ARCH . . 55

INDEX

953

SYS T E M AT1STS, sis'tematists (G.
systema.. an arrangement) 370

SYSTEMIC ARCH 531

Circulation 33

SYSTOLE, sis'tolo (G. systole, a draw-
ing together ) 56

TAB ANUS, tabfi'niis (L. tabanus, a

gad fly ) 330

TABLE OP ANIMALS 418, 432

TACHINA FLY. takl'na (G. tachus,

swift) 368

TACTILE, tak'til (L. tangere, to

touch) '. 254

TADPOLE, tad'pol (M. E. fade, toad

H- polle, head ) 547

Eggs and adult stage of frog J^Jt

TAENIA CONFUSA. te'ma konffi'sa

( L. taenid, ribbon ) 298

Cucumerina 297

Same as dipyl (Hum caninum.

EohinocoGCns 295

Elliptica 297

Flavo-punctata 297

Mediocanellata 295

Multilocularifi 297

Nana, 298

Saginata 295

Solium •. 293

TALON", ta'lon (L. talus, ankle) 736

TALUS, ta'lus (L. talus, ankle) 709

TAMANDUA TETRADACTYLE, ta-
man'dM (native name for four-toed

ant-bear) (;,7.9

T AM AN IX GALLIC A .... ..349

TAPEWORMS 2<)J,

TAPIRIDAE, tapir'ide (native name). 662

Family of tapirs.
TAP ROOT, tap'root (M. E. iappe, a

short pipe + A. S. wyrt, root) 203

TARDTGRADA, tardig'rada (L. tar-

digradus, slow-going) 429

T A REN TO L A . taren'tola ( nati ve

name) 5,55

Mauritanica —

A genus of deck OUCH.

TARPON, tar'pon (native name) . .'. .fi.J.f
TARSAL, tar'sal (G. Inrwm. a flat sur-
face) 77

Glands 073

TARSUS, tar'sus (G. torsos, sole of

the foot) 328, 708

TASTE BUDS 741

TAXIS, tak'sis (G. taxis, arrange-
ment), definition of 126

TAXONOMY, takson'omi (G. taxis, ar-
rangement -h nomos, law ) 32

TECTUM SYNOTICUM, tek'tum (L.

tego, to cover) 539, 687

TEETH ALVEOLI, teth (A. S. toth, a

tooth) , definition of 733

Classification of . ..732, 737

Development of 731

Egg 737

Of fishes 731

Names according to usage and

shape 733

Pharyngeal 733

TEGMEN CRAN1I, teg'men 687

TEGMENTUM, tegmen'tum (L. tego, to

cover ) 840

TEGMINA, teg'mina (L. tegmen,

covering) 336

TELAE CHOROIDEAE, te'le 860

TELEOSTEI, tell'eeos'tee'eye (G.
teleos. whole, perfect, osteon, bone)

425, 643

TELEOSTOMI, tell'eeoss'toemy (G.
teleos, perfect, stoma, mouth) . .425, 642

TELOCOELE, te'losel ... 511

The vesicle in the Telencephalon.
TELOLEC1THAL, tell'ohless'ithal (G.

telos, end, tekithos, yolk) 550

TELOPHASE, teTofaz (G. telos. end

-h phasis, aspect ) 57, 98

TELOXPORIDJA, telosporid'ea (G.
telos, end + spora. seed) .143, 152, 419

Bporozoa in which the life of the
individual ends with spore formation.
TELSON. tel'son (G. telson. extrem-
ity) 313

The unpaired terminal abdominal
segment of Crustaceans.
TEMNOSPONDYLOUS, tem'nospon7-
dilus (G. temnein, to cut; sphondy-

los, a vertebra) 682

With vertebrae not fused, but in
articulated pieces.
TEMPORAL, tem'pohral (L. tempu*.

temple) 697. 843

Pertaining to, or in the region of
the temples.
TEMPORAL/^, temporal'is (L. tempus,

time) 81

TENACULUM. tenak'ulfim ( L. tertax,

holding) 875

In teleosts, a fibrous band extend-
ing from eyeball to skull.
TENDON, ten'don (L. tcndo. to

stretch) 78

A white, glistening, fibrous cord
connecting a muscle with a movable
structure.

TENSOR, ten'sor (L. tensus. stretch). 336
TENTACLES, ten'taklz (L. L. tent tin, -

liim. a feeler) 247

Slender flexible organs on the head
of many small animals, used for
feeling, exploration, etc.. as in snails,
insects, crabs.

TENTACULOCYSTS, tentak'ulosi>t>
(L. L. tentaculum. a feeler -h G.

ki/stis. a bladder ) 256

Club-shaped bodies on the umbrella
margin.

INDEX

TENTOR1UM, tentoli'rceum ( L. Icmio.

to stretch) 838

TERATOLOGY, ter'atol'oji (G. teras,

a monster -f logos, discourse) 31

TERES, tee'reez (L. round) . . . 155

Ligament 797

TERGITE, ter'jlt (L. tergum, back) .336

The dorsal chitinous plate on each
segment of most Arthropoda.
TERGUM, ter'gum (L. tergum, back) .315
T E R R E S T R 1AL VERTEBRATES,

te.res'trial (L. terra, earth) 663

Vertebrates living on the ground
as opposed to aerial, aquatic.
TERTIAN FEVER, ter'shan (L.
tertianus, of the third day), defini-
tion of ' 132

TESSERA PRTNCEPS, tes'§ra (G.

tessarec, four ) 256

TESTES, tess'teez (L. testis) 47. .'/«S

Paired male reproductive org;ms
producing spermatozoa.

Descent of 818

TESTUDINIDAE, testudin'ide (N. L.

testudo, tortoise) 652

TESTUDO ELEPHANTQPU8 720

Ibera 7.W

TETRABRANCHIA. tetrabrang'kia (G.

tetra, four -f- branchia, gills) 421

Cephalopoda with four gills, four
kidneys, and four auricles, and large
external shell, no suckers, and short
arms. Example — Nautilus.
TETRAD, tet'rad (G. tetras. four)-.. 102

A group of four.
TKTRAPO.DA, tetrap'dda (G. hlra,

four + pous, foot ) 663

TETRAXONIDA, tetrakson'ida (G.

tetra -f- axon, axis) 419

Sponges with tetraxon spieules.
TETRONERYTHR1 N, tet'ronerith'rin
(G. tetra + on -f erythros, red).. 31.8

TETROPHI8 822

A species of worms.
TH AL AMENCEPH ALON. th a 1 7i men -
sef'alon (G. thalamos, a receptacle.;

engkepttalon, the brain) .,..." 66

THALAMUS, thal'amus (L. chamber)
571. 839. 849

TffALASSTCOLA, thalasikol'a (G.
thalassa, the sea -f- kolla, glue) .... 148

The typical genus of Thfihixwi-
coUdae.

THALESSA, thales'a SI 2

(Native name, subgenus of pnr-
pora. )
THALIAOEA, thalia'sea (G. ihaliea,

blooming ) 424, 639

Free swimming Urochordata with-
out tail in adult.

Example — Salpians.
THALLOPHYTE, thal'lofit (G. 1 halloa,

a young shoot -4- phytots, a plant) ..,, ,.

203, 204

THALLUS, thal'us (G. t hallos, a young

shoot) 220

THECODONT, thee'kohdont (G. theke,

sheath; odous, tooth), definition of. 733
THECOPHORA, theko'fora (G. theke,

sheath ; pherein, to bear ) 652

THEOPHRASTUS, theofras'tus (Theo-

phrastus, a Greek philosopher) 376

THEORETICAL VS. PRACTICAL.. . . 22

THEORIES OF LIFE 1 57

THE PLANT-WORLD. .185, 193, 202. 240
THERAPEUTICS, therapu'tiks (G.

therapon, an attendant) lit 7

THERMOMETERS 38

THERMOTROPISM, thermot'tropizm
(G. therme, heat + trope, a turn-
ing) ' 127

THEROMORPHOUS. theromor'fus (G.
ther, a wild beast -f- morphe, form ) .

: 45, 77

T H I G M OTROPISM, thigmot'rOpizm
(G. thigma, touch + trope, a turn-
ing) 126

THORACIC, thohras'ik (L. thorax,

thorax) .319. 364

Duct 800

THOUGHT, definition of 182

THREADWORMS 285, 302

THYMUS, thy'mus (G. thymon, thyme)

* - .53, 77,:?

Development of 587

Extracts of ^'L

THYROHYALS, thirohialz (G. thyra,

door + hyoeidrs, Y-shaped) 73S

The greater corn u a of the hyoid
bone.

rrHYROID (or THYREQID), thy'roid.
thy'reeoid (G. 1hj/rco*. shield) . .53. ?//,?

Cartilages .'.' 763

Development of 588

Extracts of 52

THYSANOPTERA, thisanop'tera (G.
thyranos, a tassel -\- pteron, wing) .423

Insects with four narrow, mem-
branous wings fringed with long
hairs as the 77/n'/;.s.

THY8ANURA 422, 430

TIBIA, tibb'eeah (L. tibia, the shin

bone) 77, ?'".*

TIBIALIS, tibial'Ts (L. tibia, the shin

bone) 80

Tf BIO-FIBULA , tib'iofib'ula ( L. tibia,

a flute -f fibula, which see) 7e?

TICKS, definition of 373

TISSUE, tis'u or tish'u ( F. tissu,

Avoven ) 46

Absorption 234 "

Assimilating 236

Botryoidal 284

Conducting 234

Covering 234

I.XDKX

Fundamental 229

Mechanical 234

Primary 228

Protective 234

Reproductive 238

Secondary 231

Storage ' 237

Supporting 234

Synthesis 236

TOLERANCE 190

TONGUE, tiing (A. S. lunge, tongue) .

Development of 588

Extensile 47

TONSIL, ton'sil (L. tonsilla. tonsil)

731, 73it-

TOOTH OF RATTLESNAKE ' .733

TOPOGRAPHICAL, topografikal (G,
tophos, place -4- graphein, write) . . . 4H4

TORPEDOES (542 .

TORRENS 38S

TORSION, tor'shun (L. torquerv: to

twist) 480 .

The twisting round of a gastropod
body as it develops.
TORUS TRANSVERSUS, to'rus (L.

torus, a swelling ) 571

TOXIC, tox'ik (G. toxikon, poison) ... 95
TRABECULAE ,trabek'yulee (L. little

beam) 5i;

Carnae 530. 541

Cranii 540

TRACHEAE. trfikf-f- (G. tracln/s.

rough ) }.'>/>

TRACHEATA. trakeat'a (G. trachys,

rou?h) 312

TRACHEIDS. trak'eids (L. trachia,

windpipe) 231, 235

TRACHYMEDUSAE, trak'imedn'se (G.
trachys, rough -f- L. medusa,

medusae ) 419

Hydrozoa without alternation of
generations.

TRAGER 623

TRAINING. MEANING OF 32. 157

TRANSITIONAL CELLS, tranzish'onal

(L. transire, to go across) 557

TRANSVERSE PROCESS, tranz'vers

(L. trans, across -+- vertere, to turn) .72
TRAPEZIUS, trapee'zeeus (G. trapcza.

table) 829

TRAPEZOID, tra'pezoid (G. lr<i)»;<i.

table) 700

TREMATODA, tremato'da (G. trema,

a pore + eidos, resemblance)

285. 420. 427

The liver flukes.
TREMEX COLVMKA, tre'meks (G.

trema, a hole) 373

TREPOKEMA PALLIDUM, trepone'ma
(G. trepein, turn + nema, thread) . Ut6

The spirochete of syphilis.
TR-ML AND ERROR. . . . 1 40. 181

TRICEPS, try'seps ( L. trtat. three ;

oaput, head j 81

TRICHIA, trik'ia (G. thrix, hair) .. .205 ,
TRICHINA, trikl'na (G. thrix, a hair) 302

A parasite nematode worm.
TRICHINELLA SPIRALIS, trikinel'la.gy.9

TR1CHIMASIS (see trichina) 302

TRICHOCEPHALUS DISPAR, triko-

sef'alus (G. thrix, a hair -4-. kephale,

the head) 30$

TRICHOCYSTS, trik'osists (G. trix,

hair + kystis, bladder ) 138-

TRICHOMONA8, tnkOm'onas (G.

thrix, hair + monos, single) 7^/>

TRICHOPTERA, trikop'tera (G. thrix,

hair + pteron, wing) 423

Insects with four membraneous

wings, longitudinally veined and

covered with hairs. Example — Cad-
dice Flies.
TRICE OSTRON C, YL US O RJ UN TA LI S

301

TRICHURIS TRICHirRA, trlkur'is

triku'ra (G. thrix, a hair 4- oura,

tail) 301

A genus of Trema toda.

TRICLADIDA 420

Turbellaria with three main

branches to intestine.
TRICONODONT, tricun'odunt (L. trett,

three + conits, a cone + G. odontos,

a tooth) 736'

TRICUSPID VALVES, trykus'pid (L.

tres, three ; cuspis, point ) 804

TRIFID NUCLEUS, tri'fid (L. tres,

three + findere, to cleave) US

TRIGEMINAL GANGLION, trijem'i

nal (L. trigeminus) 575

TRTGEMINUS, tryjem'inus (L. three

at a birth, triple) (K8

TRIGODONT, trigodont (L. tres +

odontos, a tooth) 736

TRIGONUM LEMNISCI. trigo'num (L.

trigonum. a triangle) 839, 841

TRIONYX GANGETICTW, tri'oniks

(L. tres, three -f- onyx, a nail) . . . .683
TRIONYX HURUM (see above) t L.

tres + G. onyx) 683

T RI PLOBLA ST, trip'loblast ( G.

triplax, triple + blastos, a bud). 107, 247
• T R I Q U E T R U M, trikwet'rum ( L.

'triquetrus, three-cornered ) 709

The cuneiform carpal bone; a

wormian bone.
TRI.RADIAL, trlra'dial (L. tres. three;

radius, a ray ) 678

TRITICUM. trit'icum (L. tri-tioum,,

wheat) ?•''•'•

A genus of grasses.
TRITON CRISTATVH, trl'ton erista'-

tus (G. Triton, a sea god) . . . .584, €48

1)56

INDEX

TROCHANTEK, tmlikan'ter (G.

trochos, wheel) 328, 101, 71 1

T KOCH L E A R I S, trok'lear'is ( G.

trochilia, a pulley) 68

TROCHOPHORE, trok'ofor (G. trochos,

wheel + pherein, to bear) .309

TROCHOSPHERE, trok'osfer (G.

trochos, wheel -4- sphaira, globe) . . .280

TROPHECTODERM 02!)

TRO PRO BLAST, trof'oblast (G.

trophe, nourishment + blastos, bud). 622
T ROPHO DERM, trof 'oderm ( G.

trophe, nourishment + derma,

skin) . .' 625, 629

TROPICAL BOIL 144

TROPISM, tro'pizm (G. trope, a turn-
ing) 126

TRUNCATELLA , trunk'atel'la ( L.

truncare, cut off) 155

TRUNCUS ARTERIOSUS, trunk'us

arter'id'sus (L. truncits, a stock) . . .

54, 55, 58, 595

TRY PANG SO MA GAMBIENSE, tri-

pan'oso'mah (G. trypanos, borer +

soma, body ) 143

TRYPSIN, trip'sin (G. tryein, to rub

down -f- pepsin, a digestion ) . . . . 50, 268

TSCHERMAK .388

TSETSE FLY 144. 369

TUBE FUNGI, tub (L. tuba, a pipe) .208
TUBER CTNERUM, tiu'bur (L. a

swelling) 852

TUBERCLES, tii'berkl (L. tuberculum,

a small hump) ....192

TUBERCULA PUPERTATIS 277

TUBERCULUM POSTERIUS, tiuburr'

kiulum ( L. tuberculum, a little

swelling) 512, 570

Impar 739

TUBEROSITTES OF TEETH 735

TUBES—

Annular 234

Latex 235

Pitted 234

Reticulate 234

Scalarifonn 234

Sieve 235

Spiral 234

TUBIFEX, tub'ifeks (L. tubus -f

fncere, to make ) 283

A sub-class of Oligochaeta.
TUBIPORA MU8WA, tnbT'— ra (L. •

tubus, a. tube + pora. a pore) 256

Pipe-orsfan coral.

TUBO-TYMPA'NUC CAVITY 586

T 11 K V L I DENT AT A, tiVbulidentA'ta

(G. t it I > a lu H. a tube + dentatus,

toothed ) . 420

Burrowinir mammals of Africa

known as Cape ant-eaters.
TUNIC, tu'nik (L. tunica, a coating) .752
TUNICA OF EYE (See tunicata) .872.873

TUN1CATA, tiii'nikay'tah (L. tunica,

tunic) '. .424, 430, 5J3, 639

Fixed forms of Urochordata with-
out tail in adult. Example — sea
squirt.

TURBELLARIA, terbela'rifi (L. turbo,

disturb) 285, 420, 427

Free-living Platyhelminthes with
ciliated ectoderm.

TURBINAL, turr'binal (L. turbo, any-
thing that whirls ) 868

TURBINATE, tur'binat (L. turbo, a
whirl ) 696

TURTLE 651

Skull of 692

Urogenital system of 825

TYMPANIC, TYMPANUM, timpan'ik,
tim'panum ( L. tympanum, drum ) .45, 7 1

TYMPANOHYAL, tim'panohial (G.
tympa.non, a drum -f- hyoeides,
Y-shaped ) 738

TYMPANUM, tim'panum (G. tympa-

non, a drum ) 71

Of bird trachea 764

Of Insect M(i

TYPES OF CESTODA, tip (L. typus,
an image) 296

TYPHLOMOLGE RATHBUXI, tiflo-
mol'je (N. L. A genus of tailed
amphibia ) 650

TYPHLOPIDE, tiflop'ide ( N. L.
typhlops -\- G. eidos) 417

TYPHLOSOLE, tlf'losol (G. typhlos,
•blind -f solen, a channel ) 259

TYSON 379

ULNA, ull'nah (L. ulna, elbow) JOS

ULNARE, ulnsVre, or ulnarfi (L. ulna,

elbow) ; . 76

ULTIMO BRANCHIAL BODIES, defini-
tion of 588

UMBILICAL GROOVE OF FEATHER,
umbilical, umbill'ikel (L. umbilicus,

navel) 671

UMBILICUS, umbllT'cus (L. navel).. 632

UNCINARIASIS 303

UNCINATE PROCESS. iinn'seenat.f

( L. uncus, hook ) 719

UNCINATUS GYRUS 8',',

UNDULATING MEMBRANE, defini-
tion of 139

UNEQUAL SEGMENTATION, defini-
tion of 106

UNGUES, definition of 337

UNGUWULATA, unggwikula'ta (L.

unguiculus, a nail) 426, 658

Clawed Mammals.

UNGUIS, ung'wis (L. unguis, claw),
definition of 667

UNGULATA, ung'gfila'ta ( L. ungula,

hoof) 426, 660

Mammals witli hoofs.

I NDKX

UNGUL1GRADE, unngiu'ligrade (L.

ungulii, hoof: gradu-s, walk), defini-
tion of 714

UNIRAMOUS 315

UNISERIAL, u'nise'rial (L. unus, one

+ series, rank ) , definition of 707

UNIT, biological 88

Characters 165

URACHUS, u'rakus (G. ouroti, urine

+ ecfcew, to hold) 629

URCEOLATA, ur'seola'ta (L. urceolus,

small pitcher) 1^8

UREA, ure'a (G. ouron, urine) 64

URETER, yuree'ter {G. ouron, urine)

.64, 85, 518, 810

URETHRA, yuree'thrah (G. ouron,

urine )

The canal from the bladder

through which urine is discharged.

URIC ACID 339

URINIFEROUS TUBULES 6'.',

URINOMETERS 38

URNATELLA 310

URO CHORD ATA, urokorda'ta (G.

oura, tail + chorde, cord) .424, 430, 630
The group of pro-vertebrates in

which the notochord is confined to

the caudal region.
UROCYST, u'rosist (G. ouron, urine

-(- kystis, a hollow), definition of. ..810
VRODELA, yu'rowdee'lah (G. oura,

tail; delos, evident) 425, 647

Amphibians with persistent tails.
UROGENITAL PLEXUS, urojen'ital

(G. ouron, urine; L. genitalis, geni-
tal) 68

Sinus 629. 824

Systems 812. 8 J.I

UROGLENA 150

UROHYAL, urOhi'al (G. oura, tail +

hyoeides, Y-shaped), definition of.. 740
UROPATAGIUM, fi'ropata'jiimi (G.

oura, tail -+- patagium, border) . . . .320
UROPODS, u'ropods (G. oura, tail +

pous, foot), definition of 314

UROPYGIAL GLAND, uropij'ial (G.

oura, tail; pyge, rump) 670

UROSTYLE, yu'rowstyle (G. oura,

tail ; stylos, column) 67, 72

URSU8 ARCTU8, iir'sus firkins (L.

iirsus, a bear) 7S//

UTERUS, yu'terus (L. utrnix. womb)
.' 86. 8 1 8

Masculinus 81 9

UTRICULUS, yutrik'yuliis (L. a little

bag) * ' 866, 807

VACUOLES, vak'uolz (L. vacuus,

empty ) , definition of 8P

VAGINA, vajye'nah (L. a sheath) ....

VAGUS, vay'gus (L. wandering) 68

Ganglion . 65

VALVK, vsilv (L. valva. fold) . . .

Auriculo-ventricular 55

Bicuspid, definition of 786

Mitral, definition of.. .-786

Spiral 55

Spiral, definition of 754

VALVES—

Of heart, how formed

Semilunar

Sinu-auricular

Tricuspid 804

VALVULA CEREBELLI, vfil'vula (L.

valvula, a small fold) 857

VAN HELMONT 380, 381

VAX LEEUWENHOEK

VARANUS

Malvator 65*

\'ARIATIOKH, vfi'riri'shun/ (L. va-

riare, to change) 164, 402

VAROLII, PONS 863

\ ASA EFFERENTIA, va'sa efferen'
shefi (L. ras, a vessel; ex, from +

ferre, to carry), definition of 610

Vasorum 781

VASCULARIZATION 629

VASCULAR SYSTEM, vas'kular (L.

vasculum. a small vessel ) 223

VAS DEFEREN1S (L. vas -h deferre,

to carry from), definition of 610

VASTUS, vass'tus ( L. vast) 8ft

VAUGHERIA 187, 207

VEGETABLE POLE, definition of 10f>

VrEIN, van (L. vena, vein) 53, 60

Abdominal

Ant. abdominal 5.J

Artery and nerve 5£

Brachial 61

Dorso-lumba r . 61

Femoral '/8, 61

Hepatic 61

Hepatic portal 54, 61

Iliac 61

Musculocuta neons 5), 61

Ovarian 61

Pelvic 48, 61

Postcaval 54, 61

Precaval 54, 61

Pulmo-cutaneous 5//

Pulmonary 5-), 55, 61

Renal ,., 61

Renal portal '/8, 54, 61

Sciatic 61

Sinus venosus 5)

Spermatic 61

Transverse iliac 61

Vesical 48, 61

VELUM, vee'lum (L. veil), definition

of .. .254

Anterior medullarv, definition of. 85!)

Of Hydra 251

Medullare, definition of 841

Palati, definition of 730

Transversum. definition of. . .512, 85fi

INDEX

VKNA CAVA, vc'na ca'va (L. vena +

cava, hollow), definition of 775

VENTER, ven'ter (L. renter, belly),

definition of . . . . 218

VENTILATION 38

VENTRAL, ven'tral (L. venter, belly). 47
VENTRICLE, ven'treekel (L. venter,

belly) . .

Laryngoal. definition of 7(54

Of heart, definition of 479. 778

VENTRICLES—

Of brain, where located 840-841

Lateral, definition of .511

VENTRIGULUS, ventrik'ulus (L. ven-
triculus, dim. of venter, belly), defi-
nition of 329

Impar , 57 '1

VENULE8, ven'fil/ (L. renula, dim. of

vena,, vein I

VERMIN 350

VERMIS, ver'mis (L. worm), defini-
tion of -.-. 859

VERTEBRAL PLATE, ver'tebral (L.

vertebra, joint), definition of 401

VERTEBRATA, verretebray'tali (L.

vertebra, joint) 424, 431

Definition of 040

VERTEBRATE, ver'tebrate (L. verte-
bra, joint ) 43

VERTEX, ver'teks (L. vertex, top) . .33!)

Breech measurement 6.W

VESALIUS 378. 38(1, 388

VESICAL, vess'ikal (L. vesica., blad-
der) •

Allantoic, definition of 488

Blastodermic ( see blastoderm ) .

Germinal, definition of. 443

Of nucleus 55 1

Seminal <%'

VESICLES, ves'ikls (L. vesicn-la. a

vesicle) 93

VESICULA SEMINALIS, sem'inal'is
(L. vesicula -\- seminalis, pertaining

to seed; by usage, sperm) '/8

VESTIBULE, ves'tllml (L. vestibulum,

passage) ; 820

Of ear 866

Of ear, definition of 868

Of nose, definition of 87 1

VESTIGIAL, vestij'lal (L. vestigium,

trace) 316, 574

VICQ D' AZYR 380

V-ILLI, vill (L. villus, shaggy hair) ..627

Definition of 725

VINEGAR EEL 306

VIRCHOW 389

VIRULENCE, vir'ulens (L. virulentus,

full of poison ) 199

VISCERA, viss'errah (L. viscus, inter-
nal organ ) 47

VISCERAL ARCHES, vis'eral (L.

risous, internal organ) 74

Definition of . . . 585

VISCERAL CLKFTS—

Definition of . . . . . .<. .498, 680

Muscles, definition of. . 820

Pouches, definition of 498

Skeleton 73

Skull, definition of 538

VITALISM, vl'talizm (L. vita, life).. 405
VITALLY SECRETORY GLANDS,

definition of 07;{

VITELLINE ARTERIES, vi (or vi)

tell'inn (L. vitellus, yolk) 471

Glands .. . .. 292

VITELLINE ( pertaining to yolk ) . . . 438
VITELLINE VS. OMPHALOMESEX-

TERIC, definition of 48:!

VITELLUM. definition of 438

VITELLUS, vlteTus (L. vitellus, yolk) 438
VITREOUS HUMOR, vit'reeous (L.

glassy ) 57.0

Definition of 70

Of eye 873

VIVERRA, vlver'ra (L. viverra, a fer-
ret) 134

VIVERRA IN DIC A, teeth of .735

VIVIPAROUS, vivip'arus (L. vivus,

alive + parere, to beget) 619

VOCAL CORDS, vo'kal (L. vox, voice) 43

How formed . . 762

VOICE BOX 763

VOLANT, vf/lant (L. rolatis, flying) .654
VOLAR, vo'lar (L. vola, palm of hand) 69
VOLVOCA CEAE, vol'vokfi'soe (L.

volvere, to turn around) 201

VOLVOX, vol'voks (L. volvere) 129

Definition of 150

Globator 20'f

VOMER, voh'mer (L. ploughshare) .48, 73
VOMERO-NASAL ORGAN (L. vomer) ,

definition of 872

VOMERS, vomers (L. vomer) 617

VON BAER 382, 386, 388

VORTEX, vor'teks (L. vortex, vortex). 522
VORTICELLA, vorticel'la (L. vortex,
a whirl), definition of 155

WALDHKIMIA AUSTRALIA ,il I

WALLACE, ALFRED RUSSELL. 385, 400

WARBLE FLY . . 348

WAX GLANDS, waks (A. S. weax,

wax) 364

WEBER 380

WEIGHT OF HUMAN EMBRYO .... 632

WEISMANN, AUGUST 161, 384

WEISMANMAN BRIDGE 162

WHALEBONE 737

WHALES 662

WHARTON'S DUCT, definition of. . . .742
Jelly 632

WHAT TO OBSERVE 28

WHEELER 418

WHIPWORM 306

WHITE MATTER . ..07. 836

INDEX

959

WHORLS, h*.,rljB (A. ,>. hvcorfa, a

wheel) 242

WHY TO STUDY 1«)

WIDAL REACTION, definition of.... 200

WILDER 744, 748

WILLIS, CIRCLE OF. definition of.. 792
\VIXG MUSCLES OF INSECT, wing

( M. E. winge, wing ) 340

WING OF INSECT 335

Wi;N!SLOW, FORAMEN OF. definition

of 746

^\l^THEMIA 4-PU8TULATA 368

WIRSUNG'S DUCT, definition of.... 751

WISHBONE 704

\\6HLER ..".', 380

W< )LFF 38 1 . 386, 388

WOLFFIAN BODY, wolf 'eean (after
K. F. Wolff. German anatomist), def-
inition of 482, 504, 808

Duct 64

Ridge ..510

WOOD, wood (A. S. icudu, wood),

definition of 230

WUNDT, WM 182

X CHROMOSOME, definition of. 100, 168

-\K\OPU8 650

X I PHISTBRNUM, /insteT'num (G.

.ziphos, sword + L. sternum, breast) 701
XIPHOID, zif'oid (G. .viphos, sword

-f- eidos, shape ) 80, 101

MPHO8URA, zifosfmi (G. xiphos,

sword + o?/ra, tail ) 421

King-crabs.

Y-CHROMOSOME. definition of 168

YEASTS 211

YELLOW FEVER-
HOW caused 130

YOLK PLUG, yok (A. S. geoloca, the
yellow part), definition of 451, 554

YOLK STALK 462, 48<*

YPSILOID CARTILAGE, ip'si'loid (G.
•tipsilon -f- eidos, resemblance) ..... .706

ZI'ITEL 388

ZOANTHARIA, zdontluVria (G. zoon,

animal -\- anthos, flower) 251, 420

A subclass of (Joelenl crates.
ZOANTHIDEA, zoanthi'dea (G. zoon

+ anthos) 420

An order of Zoanthar.'a.

ZONA RADIATA, definition of 443

ZONE OF JUNCTION, zon (G. zone, a

girdle) , definition of 450

ZOOCHLORELLAE, zo'okloreTe (G.

zoon, animal + rhloreos, green) . . .286
ZOOECIUM, zoe'sium (G. zoon. +

oikos, house ) 310

ZOOGEOGRAPHY, zoogeog'rafl (G.

zoon -\- ge, earth -f- graphein, to

write) , definition of 32

ZOOLOGY", zodl'Ogi (G. zoon + logos, a

discourse)

Economic 32

ZOOPHYTA, zo'ofl'ta (G. zoon +

phyton, plant), definition of 255

ZOOSPORE, zoospor' (G. zoon +

spora, seed ) , definition of .... 150, 207

ZOOTHAMNION 155

ZORAPTERA 423

An order of Blaltaeformia.
Z YT G A P O P H Y S I S, zlgapof't-sis or

zigapof'esis (G. zygon, yoke; apo-

physis, process ) 682

ZYGOMATIC, zigomat'ik or zigomat'ik

( G. zygoma, yoke ) 697

ZYGOSPORE, zl'gospor (G. zygon +

sporos, seed ) , definition of 205

ZYGOTK, zl'got (G. zygotes, yoked).. 101

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