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Marine Biological Laboratory Library

Woods Hole, Mass.

Presented by

ESTATE OF HERBERT W. RAND

January 9, 1964

COMPARATIVE ANATOMY

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COMPARATIVE ANATOMY

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BY
HERBERT V. NEAL

PROFESSOR OF ZOOLOGY, TUFTS COLLEGE

AND

HERBERT W. RAND

ASSOCIATE PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY

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WITH 540 ILLUSTRATIONS

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PHILADELPHIA
P. BLAKISTON'S SON & CO., Inc.

1012 WALNUT STREET

Copyright, 1936, by P. Blakiston's Son & Co., Inc.

PRINTED IN U. S. A.
BY THE MAPLE PRESS COMPANY, YORK, PA.

To
James F. Porter

IN ADMIRATION AND
AFFECTION

PREFACE

In one of the frescoes of the Sistine Chapel, Michelangelo portrays
Adam after his creation by Jahveh. The frontispiece of this book shows
only enough of this painting to suggest that Jahveh had a hand in the
event; but how Jahveh accomplished his creative work is left to biologists
who have tried to solve by scientific methods the problem of the genesis
of the human body. Such scientific investigation is not concerned with
the question of the ultimate origin of the universe nor of the presence or
absence of God in the cosmos, but with the secondary factors which
determine physical events in the physical world.

The scientific evidence which bears upon the problem of the genesis of
the human body is derived chiefly from the sciences of paleontology,
comparative anatomy, and embryology. These sciences provide the
factual support of the evolution theory, which in turn furnishes the clue
to the origin of species, including the human species. The remarkable
development of these sciences since Darwin's day may be attributed to
the conviction of biologists that by means of them light may be thrown
on the history of organisms in general and of the human body in particular.
Today such courses form standard constituents of the biological curricula
of universities and colleges.

American undergraduates study comparative anatomy not so much
from interest in lower animals as to gain the best approach to an under-
standing of human structure and function. The details of comparative
anatomy in themselves interest the average undergraduate slightly if at
all. To elicit the attention of the student the facts must be interpreted
for him and given meaning in terms of function or of genetic relations.
The earlier books in comparative anatomy served better as works of
reference than as college texts. The multiplicity of facts presented in
them tended to confuse the student, who consequently was unable to see
the woods for the trees.

The facts presented in this book have been selected chiefly because
they throw light upon the important problem of man's place in nature and
because they help the student to understand the major functions of his
body. If greater stress is laid upon morphological than upon physiological
matters, this is done, not because students are more interested in morpho-
logical problems, but because the authors are convinced that the central
problem of life is that of form. The best approach to this problem is
through the study of the changes which the body undergoes in ontogenesis
and phylogenesis. College and medical courses in physiology usually
assume that the student has a basis for them in a knowledge of the facts of
anatomy.

Vlll PREFACE

Among the difficulties which confront the teacher of comparative
anatomy is the reluctance of students to acquire a vocabulary of technical
terms. As far as possible in this text familiar, non-technical language is
used. Unfortunately, it is impossible to eliminate technical terms wholly
from an anatomical text. In defense of their use it should be emphasized
that they avoid much circumlocution and thus make for brevity.

This book is not intended to be used as a laboratory manual but as a
text to supplement, interpret, and integrate the facts acquired in the
laboratory. The foundations of a course in comparative anatomy should
be laid in the laboratory so that the student may have first-hand acquaint-
ance with at least a fish and a mammal — and if possible a tailed amphibian
also. For such laboratory work a suitable laboratory manual should be
used. Since this text deals comparatively with each of the various organ
systems in turn, a laboratory manual which follows this plan is desirable.

For some years this text in mimeographed form has had the benefit
of student criticism, with the consequent elimination of many obscurities
and inconsistences. The typescript copy has been read by Mr. Edwin
Tenney Brewster of Andover, Massachusetts, and by Professors A.
Brazier Howell and W. L. Straus, Jr. of the Johns Hopkins University.
For what these have done to improve the book the authors are deeply
grateful. The authors consider themselves most fortunate to have had
the efficient services of John Howell Neal who has drawn or redrawn most
of the text figures. All new drawings required for the chapters on repro-
duction and histology (chapters 2 and 3) were made by Mary B. Marks.
Of the text-books in comparative anatomy which have been consulted
those of Goodrich, Ihle, Kingsley, Plate, Stempell and Wilder have been
especially valuable. For embryological facts and figures, the authors
have leaned heavily upon Arey, Corning, Kellicott, MacBride, and Patten.
In human anatomy, the text-books of Braus and of Morris have been much
used. In histology, Bremer's text, and in neurology, the text-books of
Herri ck and of Ranson have been most helpful.

In this text-book the discussion of each organ or organ-system is
divided into three parts — phylogenesis, ontogenesis, and anatomy.
Since it may be assumed that the student has first-hand acquaintance
with the anatomy of such a mammal as the cat, and since he presumably is
more interested in the human species than in any other mammal, anatomi-
cal description in this text emphasizes human anatomy. The descriptions
of ontogenesis are also mostly based upon the human embryo. If, as
the authors believe, the main purpose of a course in comparative anatomy
is, to throw light upon the structure of the human body, to ignore it as
some text-books do seems like a performance of Hamlet with the Prince
left out.
Boston, Massachusetts

INTRODUCTION

" Grant a simple archetypal creature like the mudfish or Lepidosiren, with the five senses
and some vestiges of mind, and I believe natural selection will account for the production
of every vertebrate animal." Darwin to Lyell in 1859.

One of Gauguin's best-known paintings portrays a group of human
figures, some standing, some reclining, all in an attitude of melancholy
thoughtfulness. The picture might well serve as an illustration of
Tennyson's "Lotus Eaters." The painter, however, has entitled it
"Where do we come from? Where are we now? Where are we going?"
These persistent problems interest the biologist also as well as the poet and
artist. "The question of questions," says the elder Huxley, " the problem
which underlies all others and is more deeply interesting than any other, is
the ascertainment of the place which man occupies in nature and of his
relations to the universe of things."

The answer is to be found, if it is to be found at all, through scientific
methods of investigation and these seem to point inevitably to some
form of evolution. Evolution is the scientific theory that organisms
have arisen in nature by "continuous progressive change according to
certain laws and by means of resident forces." The theory assumes the
mutability of species, their blood relationship to one another, and their
origin in accordance with natural law by means of resident factors. For
its factual support evolutionists appeal to circumstantial evidence.
Geological evidence provides the strongest argument for evolution. For
the evidence from the rocks demonstrates that the earth has existed for
many millions of years, and that during this time the bodies of organisms
have progressively changed, so as to resemble, more and more, the forms
now living.

Many objections have been raised against the evolution theory, most
of them based upon misunderstanding. A few of these may be mentioned.
First, it is asserted that the foundations of the theory are weak, since it
depends upon circumstantial evidence. In reply to this objection it may
be pointed out that there is no more trustworthy evidence than circum-
stantial. Courts have found that eye witnesses are notoriously unreliable.

It is sometimes asked how it happens, if there has been evolution,
that there are any lower animals left. " Why haven't all monkeys turned
into men? " This supposed difficulty is evidently based upon the assump-

X INTRODUCTION

tion that evolutionary change is bound to occur, under whatsoever
circumstances. Evolution, however, is not the notion that organisms
are bound to change regardless of conditions. If an animal is adapted to
a particular environment, as the monkey is to the forest, and the forest
persists, we should not expect the monkey to change. It may be said
in this connexion, however, that evolution is not the theory that man
came from a monkey, but that the bodies of the two have had a common
animal origin.

Evolution is sometimes said to be "only a theory," as if it were no
more than an unfounded guess or pure assumption. On the contrary it
is doubtful if any other scientific theory has greater factual support.

A frequent objection raised against evolution is that "it cannot explain
the origin of life." It may candidly be admitted that we know nothing
about the origin of life. But the primary question of evolution is not how
life began but how organisms have changed since their origin. The
geological record leaves us in no doubt as to the fact of change. As to the
origin of life, L. J. Henderson is probably justified in saying that "any
theory about the origin of life is nothing but an unfounded guess."

Again, it is charged that the evolution theory "degrades" man by
making a monkey of him. In reply to this supposed objection it may be
said that the differences between man and monkeys obtain whatever
may have been their origin. "A man's a man for a' that." Values are
not determined by origins. The value of the Venus of Melos is not
afifected by the fact that the block of marble from which the statue was
carved came from a quarry.

Our ignorance of the causes of evolution has been considered an objec-
tion to the theory. It must be admitted that it has been found easier to
prove that evolution has taken place than to explain how it has been
brought about. Undoubtedly the nineteenth century belief that Darwin's
hypothesis of natural selection explained organic evolution led men to
accept this theory more readily than they otherwise would have done.
Nevertheless, the case for evolution does not depend upon the ability to
state its cause any more than the existence of light depends upon our
ability to explain how it reaches the earth.

Of the hypotheses advanced to explain evolution, three have best
survived criticism. Briefly stated they are:

I . The Lamarckian hypothesis assumes that organic evolution is due
to four factors:

1. The will to live

2. The use or disuse of organs

3. The influence of environment

4. The inheritance of the bodily modifications due to use or disuse or to the influence
of environment.

INTRODUCTION XI

The theory leaves unexplained how bodily modifications effect correspond-
ing changes in the germ cells which transmit inherited traits. On the
whole, experimental evidence does not support the assumption of the
inheritance of bodily modifications.

2. According the Darwin's hypothesis of natural selection, four
factors effect organic evolution :

1. Variation

2. Multiplication

3. Competition

4. The inheritance of useful variations

The theory asserts that no two individuals are precisely alike, and that
many more are born than can possibly live. The result is a struggle for
existence so severe that only those types survive whose variations favor
them in the struggle. These transmit their favorable traits through
heredity to their offspring. Carried on through many thousands of
generations, such changes would, it is assumed, eventually produce new
species.

3. During the twentieth century, Hugo De Vries has advanced a third
view, which he calls the Mutation Theory. According to the Mutation
Theory, new species arise suddenly, discontinuously, not, as Darwin
thought, by slow accumulation of slight differences. De Vries conceives
of an organism as a mosaic of traits. A new combination of characteristics
constitutes a new species. Breeding evening primroses in his Amsterdam
garden, De Vries discovered that mutations are inherited. In other
words, mutants breed true. After it arises, a mutant species is subject
to natural selection or elimination, but its origin, like that of the fluctuat-
ing variations of Darwin, is not dependent upon this struggle for existence.
The factors in evolution, therefore, according to De Vries are:

1. Mutation

2. Heredity

The laws of heredity are found to be in accord with the mutation theory.
The theory has, however, been criticised as failing to explain the origin
of adaptive mutations. Since the cause of adaptive change must affect
the germ-cell in order to be inherited, and since biologists are still searching
for the factors which determine adaptive mutation, it must be admitted
that the cause or causes of evolution are unknown. Of the three hypo-
theses mentioned, the Lamarckian is least acceptable to biologists.

At the present time two divergent conceptions of evolution are held by
biologists — mechanical and emergent.

According to the mechanical conception of evolution, the universe is
a machine operating in accordance with "immutable" laws. The entire
universe or any part of it consists of particles in motion grouped into

xii INTRODUCTION

various systems. Changes in these groupings constitute what we call
evolution. Random samplings of all parts of the universe — possible
through the spectroscope — prove that all consist of the same kinds of
particles moving in accordance with the same laws of motion. Hence
from a knowledge of any part of the universe it follows logically that we
can know about all the rest. Reduced to its lowest terms, the universe
consists simply of matter and energy. Since the total amount of matter
and energy is constant and evolution consists of changes in the distribu-
tion of matter, it would be theoretically possible, if we knew enough, to
calculate the future changes in arrangement, and predict the course of
evolution. Since living organisms consist of the same kinds of particles
as are found in the lifeless world and no form of energy peculiar to the
living has been discovered, prediction of what will happen in the living
world involves no new factors.

Against such a mechanical conception of evolution and of the universe
most minds revolt. To accept such a conception would mean the rejec-
tion of the most cherished beliefs of mankind. If the universe be a great
machine, and nothing more, there is obviously no place for freedom, moral
responsibility, or for values of any sort. Might and might alone prevails.
In a mechanical world, ideas, ideals, and aspirations, if they existed, could
have no more influence in the course of events than do shadows cast on a
summer day. But, it may be asked, if the universe is in reality a giant
machine in which all changes are only alterations of systems of moving
particles, how could there be any evolution at all? Mechanical changes
undoubtedly do occur in the universe as, for example, in the revolutions
of the planets around the sun. But changes like these are not truly
evolutionary at all. A mechanical universe which started with matter
and energy could consistently have only matter and energy in the end.
From such a point of view, the universe could contain only "eternally old
things." But evolution is above all else a process of novelty production.
Life and consciousness are such novelties. By no hocus pocus could a
magician with matter and energy in his hat conjure such entities as mind
or ethics. The mechanists, says Carrel, referring to living organisms,
have "built a machine, and like the vitaHsts, they were the engineers of
the machine. Then as Woodger pointed out, they forgot the existence of
that engineer." Mechanical evolutionists have made the same mistake.

While the mechanical conception of evolution may appear simple,
clear, and logical, it ignores too many facts to be true. The facts accord
better with the opposing emergent conception of evolution. According
to the doctrine of emergent evolution, evolution is above all else a process
of novelty-production. The differences which distinguish higher from
lower organisms are not simply quantitative but qualitative. The dififer-
ences are such that it is impossible to reduce the higher to terms of the

INTRODUCTION Xlll

lower. Biology is found to be not simply biochemistry and biophysics?
but a science in its own right. Mechanism is inadequate to life. The
notion that, from a knowledge of masses and motions, the future course
of organic evolution might be predicted is ridiculous.

The basis of the emergent conception of evolution is found in the
empirical fact that an organized whole — such as a living creature — has
characteristics which are qualitatively different from those of the elements
which enter into it. The properties of electrons give no clue to the
properties of the atoms which they form. The properties of atoms are
not found in the chemical compounds which they form. Add ten carbon
atoms together and they have the same properties which one atom has.
Add hydrogen atoms together and their properties remain the same.
But when carbon atoms are combined chemically with hydrogen atoms,
the hydrocarbons formed have entirely new properties. It seems increas-
ingly clear that "an organized whole is more and other than the additive
sum of its parts." For the appearance of such new properties science has
today no explanation to offer. Indeed, the concept of causation does not
seem to apply to such phenomena since cause and effect in the mechanical
sense involves a transfer of energy from the cause to the effect. Spaulding
has well called this process of formation of new characteristics through the
organization of parts into wholes "creative synthesis," since the properties
of the whole, or at least some of them, are new.

"From separate organic compounds to organized living protoplasm,"
says G. H. Parker, "we pass from one plane of organization to another
and consequently from one set of properties to another. The essential
properties of living protoplasm are at present no more to be understood
from its constituent compounds than are the properties of water from
those of hydrogen and oxygen. The properties of living protoplasm are
too manifold for description. They are those properties whereby living
protoplasm acts otherwise than its chemical constituents do."

Nineteenth century mechanism failed because it failed to consider
the factor of organization. But, as L. J. Henderson says, "there is that
which organizes matter in time and space." Consequently, if we are to
understand how new properties and capacities arise in nature, we must
add to the categories of matter and energy (which were considered suffi-
cient in the nineteenth century) a third category of organization. In a
strict sense this organizing factor is not "mechanistic." Certainly,
organization has up to the present not been recognized as a mechanistic
factor by physicists or chemists. But the evidence of such a factor in life
is indisputable. Without it, the evolutionary process is incomprehensible.

As mechanical evolution is a contradiction in terms, so emergent
evolution is a redundant expression. If there is no emergence there is no
evolution. The facts speak loudly in favor of emergent evolution.

XIV INTRODUCTION

"Things living," says Jennings, "behave themselves as if emergent
evolution were a true doctrine."

The acceptance of the doctrine of emergent evolution has greatly
relieved the minds of many who were depressed by "mechanical mythol-
ogy" as applied to man. According to emergent evolution, man, like
all other creatures, is a unique product. Consequently, man's capacities
and powers are what we find them to be in experience, and are not to be
logically deduced from the properties of lower animals. The doctrine of
emergent evolution also relieves the evolution theory of the charge of
materialism. If evolution be an emergent process, as it evidently is, we
can understand how the "strata" of reality assumed by pluralist philoso-
phers have arisen. Out of the lifeless has emerged the living, out of the
living the conscious, out of the conscious the ethical.

This text-book undertakes to answer the questions "Where do we
come from?" and "Where are we now?" on the assumption that the
human body has evolved. It seems unnecessary and undesirable to
present here the mass of evidence gathered by Darwin and his successors
in support of this opinion. Most of the facts stated in this book have a
bearing, either direct or indirect, upon it. If this book proves anything,
it is that the body of man is best understood in the light of its animal
origin.

No attempt is made, however, to convey the impression that evolu-
tionary change can be adequately explained at the present time. The
hypotheses of Lamarck, Darwin, and De Vries appear today less satis-
factory than they did a generation ago, and biologists are still searching
for the causes of adaptive evolution. To give students the impression
that we know the factors of evolution is to mislead them. A recent text-
book states that "it would seem that the immediate cause for the develop-
ment of dermal bones from tooth-bases . . . was the early need for teeth
and tooth supports in the young carnivorous larvae." Such an assertion
evidently raises more problems than it attempts to answer. As a causal
factor in morphology, need is probably about as effective as it is in eco-
nomic life in raising our balance at the bank. A future advantage or
possibility may influence human behavior, but teleology is ruled out of
scientific explanations.

To determine man's ancestry, three kinds of evidence are used —
paleontological, anatomical, and embryological. Except for skeletal
structures, paleontological evidence is generally incomplete. Con-
sequently, for the history of other organs morphologists have to depend
upon anatomical and embryological evidence. Unfortunately, evidence
from these two sources is sometimes equivocal or conflicting. Ontogenesis
does not always repeat the history of the race. There are too many
exceptions to the fundamental law of biogenesis. When embryological

INTRODUCTION XV

and comparative anatomical evidence conflict with one another, the
difficulties of interpretation are enhanced and morphological opinion is
likely to be divided. Where evidence conflicts, there is no criterion by
which the more reliable clues may be recognized. Anatomists tend to
value anatomical evidence more highly; embryologists favor ontogenetic
evidence. In such matters, personal opinion looms large. Frequent
differences of opinion among morphologists have given the impression
that phylogenetic conclusions are exceptionally speculative and uncertain.
Much of this divergence, however, is due to lack of sufficient evidence.
The history of morphology shows that with increasing knowledge there
has been an increase of agreement on controverted issues. As the recent
upheaval in theoretical physics has shown, speculation is not peculiar
to morphology. Even if it be admitted that the methods of the morpholo-
gist resemble those of Sherlock Holmes, this similarity does not invalidate
his conclusions.

CONTENTS

Preface . . .
Introduction

Page
vii

1. The Animal Kingdom i

The Linnaean System 2

The Animal Phyla 3

Summary of Classification 3°

Sequence of Organisms in Geologic Time 34

2. Reproduction 35

Differentiation of Sexes 35

The Germinal Bodies 36

Eggs of Fishes 3^

Eggs of Amphibians 39

Eggs of Reptiles and Birds 39

Eggs of Mammals 4^

Relative Size 42

Fertilization 4?

Developmental Potentialities 44

Means of Exit 44

Oviparity, Viviparity, Impregnation 45

Protection, Nutrition, and Respiration during Development 46

In Fishes 46

In Amphibians '• 49

In Reptiles and Birds 5i

In Mammals ... 53

Evolutionary Significance ... • • 55

Development -5°

Cleavage and Blastula 5^

In Amphioxus 59

In Amphibians 60

In Reptiles and Birds 62

Gastrula 63

In Amphioxus 63

Significance of the Gastrula 64

In Amphibians 66

In Reptiles and Birds 69

Comparisons 7o

Third Layer, Mesoderm 7i

In Amphioxus 7i

In Amphibians 74

In Reptiles and Birds 76

Early Development in Placental Mammals 77

Organogenesis ^°

Xviil CONTENTS

Page

Organogenesis in Amphioxus 8i

Organogenesis in Vertebrates 85

Neural Tube 85

Notochord 86

The Enteron 87

The Mesoderm 91

The Mesenchyme 102

Connective Tissue 102

Skeleton 103

Muscle 106

Circulatory Organs 106

Head, Neck, Diaphragm, Tail 107

Effect of Massive Yolk on Organogenesis 109

Embryonic and Fetal Membranes 113

Function of the Embryonic and Fetal Membranes 121

Summary and Interpretation 122

3. Histology 126

Epithelial Tissues 129

Simple Epithelium I3^^

Stratified Epithelium 132

Glands i39

Non-Epithelial Tissues 14°

Muscular Tissue 141

Nervous Tissue • ^45

Tissues Serving for Mechanical Support 15°

Connective Tissue iSi

Skeletal Tissues iS3

Notochord ■ • iS3

Cartilage ^54

Bone 158

Adipose Tissue ■ ^S8

Blood 158

Histological Specificity 162

4. The Integumentary System 164

Evolution of the Skin 164

Structure of the Human Skin 166

Development of the Skin 167

Finger-prints and Their Meaning 168

Appendages of the Integument 169

Horny Scales ^7°

Horns ^7i

Nails, Claws, and Hoofs i7i

Feathers ^72

Hairs ^73

Pigment ^7^

Cutaneous Glands ^79

5. Teeth ^^^

Evolution of Teeth 183

Evolution of Compound Teeth 187

CONTENTS XIX

Pagb

Teeth of Mammals 189

Teeth of Man i93

Development of Teeth iQS

6. The Skeletal System 201

The Axial Skeleton 203

Evolution of the Vertebral Column 203

The Vertebral Column in Man 206

Development of the Vertebral Column 209

The Ribs 210

The Sternum 214

The Cranium 216

The Visceral Skeleton 228

The Appendicular Skeleton 236

Evolution of Paired Appendages 236

Summary of Skeletal Evolution 244

The Appendicular Skeleton in Man 247

Development of the Appendicular Skeleton 253

7. The Muscular System 259

Evolution of the Muscular System 261

Muscles in Man 273

Development of the Muscles 284

8. The Digestive System 292

Evolution of the Digestive System 292

The Human Digestive System 294

Development of the Mouth 295

Evolution of the Mouth 295

Salivary Glands 3°i

The Tongue 3°3

The Pharynx 306

The Esophagus 3°?

The Stomach 308

The Intestine 3^2

Mesenteries and Omenta 3^9

The Liver 322

The Pancreas 326

9. The Respiratory System 328

Introduction 328

The Branchial System 329

The Development of Gills • 332

History of the Gills 333

Pharyngeal Derivatives 337

The Pulmonary System . 337

Development of the Lungs • • • 342

History of the Pulmonary System 344

10. The Vascular System 347

Evolution of the Blood Vessels 348

Evolution of the Heart 3S8

XX CONTENTS

Page

Evolution of the Aortic Arches 359

Evolution of Arteries 361

Evolution of Veins 361

Evolution of Lymphatics 362

Evolution of Hemopoietic Tissues 363

Development of the Vascular System 366

Changes in the Embryonic Circulation 376

Development of the Veins 379

Changes in Circulation at Birth 382

Development of the Lymphatic System 384

The Heart in Man 387

Pulmonary Circulation in Man 389

Systemic Circulation in Man 390

The Lymphatic System in Man 397

11. The Urogenital System 399

Evolution of the Urogenital System 399

The Urinary System 399

The Reproductive System 407

The Urogenital System of Man 415

Urinary Organs 4^5

Reproductive Organs 421

Development of the Urogenital System 43°

The Urinary System 43°

Reproductive Organs 439

12. The Endocrinal Organs 446

The Pancreas 447

The Gonads 448

The Suprarenal Glands 45 1

The Thyroid Gland 455

The Parathyroid Glands 4S8

The Ultimobranchial Bodies 460

The Thymus 460

The Pituitary Gland 462

13. The Nervous System 467

The Elements of the Nervous System 467

The Organization of the Nervous System 474

Nervous System of Chordates 477

Evolution of the Brain. . 495

Evolution of the Spinal Cord 5^4

Evolution of the Peripheral Nervous System 522

Evolution of the Cranial Nerves 524

Evolution of the Spinal Nerves 532

The Autonomic Nervous System 535

Evolution of the Autonomic System 539

Development of the Brain . 54°

Development of the Spinal Cord 549

Development of Peripheral Nerves 55 2

Meninges . 5oi

CONTENTS XXI

Page

14. The Sense Organs 5^3

Evolution and Development of Sense Cells 5^3

Evolution and Development of Cutaneous Senses 5^6

Evolution and Development of Lateral Line Organs 568

Evolution and Development of Muscle Spindles 57i

Evolution and Development of Olfactory Organs S7i

Evolution and Development of Taste Organs 577

Evolution and Development of Visual Organs S^o

Evolution and Development of Static and Auditory Organs 594

15. The Head Problem 607

Transcendental Phase 6°7

Anatomical Phase ^°9

Embryological Phase "^°

Summary of the Evidence ^26

16. The Ancestry of the Vertebrates 628

Annelid Theory ^3°

Delsman's Theory "3"

The Arthropod Theory 642

Gaskell's Arachnid Hypothesis 643

Patten's Arachnid Hypothesis 652

The Nemertean Hypothesis "57

The Balanoglossus Theory ^59

Appendicularia Theory ""^

Conclusion ^^'^

Glossary ^^7

Index 691

COMPARATIVE ANATOMY

CHAPTER I
THE ANIMAL KINGDOM

Since some of the so-called lower animals, li\dng or extinct, more or
less resemble hv-pothetical ancestors of man, some knowledge of them is
necessary for a proper understanding of the history of the human body.
Moreover, certain highly complex and obscure organs of man are most
easily understood in the light of the simpler conditions of lower forms.
Even the plants, so unUke us in outward appearance, contribute something
to our knowledge of ourselves.

But the organic world is so enormously complex that no human mind
can carry its detail adequately without some system by which facts are
classified and summarized. Most useful of such systems are those based
on natural relations, which, therefore, exhibit the course of evolution of
each species, and place it correctly in an evolutionary scheme. For
evolution, nowadays, is the key to all genetic animal relationships.

Such an evolutionary scheme begins by dividing all Hving things into
plants and animals. Plants are creatures which contain chlorophyl, and
therefore, can, produce or make their food directly out of inorganic mate-
rials, or else they are, ob\dously, such creatures as have lost their chloro-
phyl and adopted the feeding habits of the simpler animals. Animals
may or may not have descended from plants; only rarely do they contain
chlorophyl, hence all their structure and habits rest on other means of
obtaining food. There are, however, many simple organisms; for example,
the slime molds, which are as much one as the other, plants or animals
indifferently. Even some of the higher plants, Uke the venus fly-trap,
catch and devour insects; and some of the unicellular algae also feed like
animals.

The animal kingdom as a whole is commonly di^dded into about a
dozen phyla, the precise number and the precise definitions of which have
not yet been agreed upon by taxonomists. These phyla, in turn, are split
into classes, the classes into orders, the orders into genera, and the genera
into species. It is sometimes convenient, also to recognize sub-orders and
sub-classes, and to combine similar genera into families.

Scientific naming is by genera and species, a scheme devised by the
great naturahst Linnaeus, or Linne, about the middle of the eighteenth

COMPARATIVE ANATOMY

century, and called the Linnaean, or binomial, system. Thus all birches
are called by their Latin name Betula, and that is their genus. White,

A PHYLOGENETIC TREE OF THE ANIMAL KINGDOM.

Fig. I. — A phylogenetic tree of the animal kingdom, showing the dichotomy of
animals into Proterostomians and Deuterostomians. The former reach their evolu-
tionary climax in insects, the latter in chordates.

yellow, and black birches are therefore respectively, as species, Betula
alba, Betula lutea, and Betula nigra; but the American white birch is
Betula papyrifera, that is, the paper birch; and the common gray birch

THE ANIMAL KINGDOM 3

is populifolia, because it has leaves that twinkle in the wind like those of a
poplar tree.

The common cat is Felis domestica; the lion, Felis leo; the tiger,
Felis tigris; and there are, in all, some forty species more in the genus
Felis. Linne called us Homo sapiens. We belong to the family Homi-
nidae (of which we are the only living species) to the order Primates, the
class MammaUa, the phylum Chordata, and the animal kingdom. In a
general way, for the larger and more famiUar animals and plants, the
vernacular name, such as pine or elephant, refers to the genus.

Phylum I. Protozoa

The protozoa are the simplest of animals, their bodies usually con-
sisting of only a single cell. Since they reproduce by simple fission of the

^^If-^^ ^ - ^ ' ^ ~^FOOD VACUOLE

PSEUDOPODIUM-

AMOEBA.

Fig. 2. — Amoeba proteus. Amoeba illustrates the characteristic features of the

Protozoa.

parent body there is no natural death in the group and no protozoon has
ever lost an ancestor.

There are, at least, ten thousand species. Most protozoa are micro-
scopic; some are just visible to the unaided eye; a few colonies are milli-
meters in diameter; the slime molds, which are giant reticulated amoebas
and neither quite individuals nor quite colonies, are thin sheets some
centimeters in extent.

Among the simplest are the amoebas, which to the superficial observer
look like blobs of jelly with no particular shape. They are without
skeletons. But some protozoa, such as the not uncommon fresh water
diffiugia, though otherwise hke amoebas, have single-chambered shells of
chitin or sihca. The foraminifera have more elaborate shells, calcareous
in most forms, which have accumulated through geologic ages into beds
of present-day limestone. The White Chalk Cliffs of Dover, which are
composed mostly of protozoan skeletons, are four or five hundred feet
thick, and extend nearly to Paris.

More familiar are the forms with cilia or flagella — the sUpper animal-
cule, Paramecium, the trumpet animalcule, Stentor, and the host of

COMPARATWE ANATOMY

infusoria that swarm in water containing organic matter. Some are
parasitic, and produce sleeping sickness, sj'philis, and other diseases.
Some contain chlorophyl and might about as reasonably be counted as
plants. A few colonial forms, for example, the common green volvox,
have their individuals differentiated into vegetative and reproductive
cells, and thus take the first step toward the evolution of multicellular
organisms .

The protozoa, which are single-celled and, apparently, simple, make a
logical starting point for animal evolution. The amoeba, which is among
the simplest of the protozoa, is popularly assumed to be "the mother of
all living." This assumption is in no ^^nse unreasonable as there is no
direct evidence to the contrarv.

BlASTDPOR£CM<Xn>h»MJS>
A COeLENTIRATE.

BLASTOPOfiECMOUTM-WiiS)

B.FLATWORM.

aLAST::PCfl£CVOJTH)
2 MOLLUSC.

BCA5T0POSE CanUSJ

BLASTOPORE (ANUS

BlasTO<>0«C

C»CUR£NTCR1C

STOPOBE

€. EOHINOOCRM.

MOJTH
F. «MICH0ROATE.

CUKXHOPDATZ.

KCEPHAaXhORQME.

Fig. 3. — Diagrams of embr\-onic stages illustrating the contrast in the fate of the
blastopore in different groups of animals. The forms in which the embryonic blastopore
becomes the mouth were grouped together by Grobben as PROTEROSTOMIA. The
DEUTEROSTOMIA include those animals in which the blastopore becomes the anus
or lies in the anal region. The coelenterates, flatworms, annelids, and molluscs are
Proterostomians, while echinoderms and chordates are Deuterostomians.

Multicellular animals or Metazoa have bodies which consist of many
cells. Grobben, 1908, di\'ides the metazoa into two main subdivisions:

I. Proterostomia, in which the embryonic mouth or blastopore persists
as the adult mouth. Proterostomians include the Platyhelminthes,
Nemathelminthes, Rotifera, Molluscoida, Annelida, Mollusca, and
Arthropoda.

II. Deuterostomia, in which the blastopore forms the anus or Ues near
the anus. The deuterostomians include the remaining groups of animals,
— Echinodermata, Hemichordata, Urochordata, Cephalochordata and
Vertebrata.

Phylum 2. PORIFERA

The differentiation among individual cells, which began faintly in
certain colonizing protozoa, is carried somewhat further in the sponges,
which, though rated as multicellular animals, are hardly more than sHghtly

THE AXIM-\L KINGDOM 5

organized colonies of cells. They are so far independent of one another
that a sponge, even after being squeezed through cheese cloth, \\ill reor-
ganize itself and go on li\'ing.

Sponges vary from the common Grantia, five to ten millimeters in
height and simple in outline, to complex structures larger than a man's
head. Nearly all have skeletons, usually calcareous or horny, but siliceous
in the glass sponges. The familiar bath sponge is only a skeleton, from
which the flesh has been macerated away. Some dozen species of various
sizes and textures furnish the commercial product.

Most sponges are marine, but there are some fresh water species,
which develop in the form of thin sheets up to the size of one's hand.
There are branched forms, and also colonial.

WTiatever their size or complexity, most sponges consist essentially
of a body-wall that surrounds a central cavity, or cloaca, which communi-
cates with the exterior by, at least, one wide opening, the osculum. The
bodv-wall consists of an external covering (ectoderm) ; an iimer covering
(endoderm j ; and between the two, a thicker covering, or skin (the meso-
derm or mesogloea), which secretes the skeleton. There is no ner\'ous
system.

In the body-wall are two sets of radial canals. Those of one set open
into the cloaca and are lined with endodermal cells. The others open to
the exterior, are lined with ectoderm, and have muscle fibers at their
mouths. The two alternate, and are interconnected by smaller passages.
The endodermal cells bear flagella, which by their motion set up currents
of water inward through the canals to the cloaca, and outward through the
osculum.

Sexes are separate. The eggs are fertilized in the body of the female,
and there develop until they become free-s\\imming larvae, which finally
settle do-pvTi on the bottom and assume the sessile existence of the adult
sponge.

Since the sponges are a side branch of the phylogenetic tree of the
animal kingdom and seem not to have been the ancestors of any higher
group, they interest the student of the himian body chiefly as an example
of multicellular organization in ver>- simple terms, intermediate between
colonial protozoa and organized multicellular individuals.

Some 2500 li\dng species are recognized, and fossils are somewhat
common.

Phylum 3. COELENTERATA

The coelenterates include the familiar fresh water hydra and various
other pohi^s, solitary and colonial; corals; jellyfishes; and sea-anemones.
The Portuguese man-of-war is a colony in which there are several different
sorts of individuals, of which some are specialized for swimming and some

COMPARATIVE ANATOMY

for digestion; some become protective scales, some are reproductive,
some become thirty-foot tentacles armed with powerful nettle cells for
defense. Such a colony may contain a thousand individuals, whose differ-
ent kinds simulate the differentiated organs of higher forms and help to
explain how such organs arose.

Each polyp, jellyfish, sea-anemone, or coral, like one of the sponges,
possesses a central cavity, the enteron. The body wall is at least two-
layered, with an ectoderm on the outside and an endoderm lining the

C. SECT - BODY-WALL A. HYDRA- LONGIT. SECTION B. CROSS SECTION

Fig. 4. — Hydra is a typical genus of coelenterates. The body-wall consists of two
layers with a suggestion of an intermediate mesogloea. The single body cavity is
the enteron or digestive cavity.

enteron. In all except such simple forms as hydra, there is a thick
mesogloea which makes up the greater part of the body mass.

Like the sponges, the coelenterates have an enteron, no longer a mere
passage for water, from which individual cells pick up food, each for itself,
but a digestive organ — hence the name coelenterate. The single opening
into the enteron is both mouth and anus. Muscles, also, are more differ-
entiated; so that the polyp waves its tentacles vigorously about, seizes
food and conveys it to the mouth, or withdraws suddenly into its capsule.
The jellyfishes swim slowly by opening and closing the umbrella. There
is also a simple nervous system; and in the jellyfishes, sense organs, both
eyes and organs of equihbrium, are arranged along the edge of the
umbrella.

The symmetry of the body is generally radial, usually on four or six
axes, not on five as in the echinoderms. This radial symmetry may

THE ANIMAL KINGDOM 7

disguise a possibly primitive bilateral symmetry. But in some of the
ctenophores, which are the small jellyfishes whose enormous numbers
make the ocean phosphorescent at night, there are beginnings of bilateral
symmetry, especially in the tentacles.

The importance of the group for the student of animal evolution lies
in the fact that the two-layered gastrula stage of the embryo, of all higher
forms, duplicates in all essentials the two-layered adult body of the simpler
coelenterates. Coelenterates, therefore, are taken to lie on or close to
the main trunk of the ancestral tree of higher animals.

Taxonomists recognize some seven thousand species.

Phylimi 4. PLATYHELMINTHES

The platyhelminths or flatworms are well described by their name.
Included in the group are such diverse forms as liver flukes, planarians,

CNTERON

RHABOOCOELE "UJRBQJ-ARIAN

VENTRAL NERVE
OVARY.

•YOLK PRODUCING FOLLICLES

lOR INTESTINE
■PENIS

OVIDUCT

VAGINA
-GENITAL PORE
■EGG IN UTERUS

TESTI

VAS DEFERENS .. ,,™,„„

TRICLAD TURBELLARIAN.
Pig. 5. — Diagrams of Rhabdocoel and Triclad Turbellarians, types of the Phylum
Platyhelminths. The group illustrates the beginnings of head and of bilateral sym-
metry. (Redrawn from Van Cleave, after von Graff.)

tapeworms, and the nemerteans, which are nearly all marine. The
flattened bodies are bilaterally symmetrical, with a well-marked dorso-
ventral and antero-posterior differentiation. Nervous centers tend to
concentrate as a brain at the anterior end. There is no body-cavity, and
usually no anus except in nemerteans. The alimentary canal shows a
tendency to form a series of lateral diverticula, thus suggesting the begin-
nings of the coelomic pouches and gill-pouches of higher animals.

Muscle bundles extend in various directions within the thickened
mesogloea. Although most flatworms have neither true circulation nor
blood vessels with walls, lacunar spaces filled with liquid occur, and are

8

COMPARATIVE ANATOMY

thought by some morphologists to represent the beginnings of a circulation.
The nemerteans, however, have a true blood system in the form of dorsal
and lateral vessels with transverse connections.

The parasitic mode of Hfe which has been adopted by many flatworms
has led to more or less degeneration and to a great complication of life
histories.

Four thousand five hundred species are known.

Phylum 5. NEMATHELMENTHES

The nemathelminths or thread-worms are also well characterized by
their name. They differ from flatworms, not only in the shape of their

NEMATODE. C MONH YSTERA)

VENTRAL NERVE'

NEMATODE - CROSS SECTION.

Fig. 6. — Monhystera, a type of the Phylum Nemathelminths seen in lateral aspect.

(Redrawn from Ward and Whipple after Cobb.) Like M onhystera most nemathelminths

have an anal opening. As seen in the cross section the alimentary canal is separated

from the body-wall by a false body-cavity (pseudocoelom). (Redrawn after Stempell.)

bodies, but also in having a body cavity which, however, instead of lying
between two layers of mesoderm, separates the body wall from the
endodermal lining of the ahmentary canal, and is therefore a pseudocoelom.
Most of them have an anus.

Threadworms are much like annelids, but have not their metamerism.
Muscles are hmited to the body- wall, and consist exclusively of longitudinal
fibers, grouped in four bundles. The nervous system is a circumesophageal
ring and a dorsal, a ventral, and two lateral longitudinal nerves. Most

THE ANIMAL KINGDOM 9

have no true circulatory system, but the pseudocoelom distributes digested
food and collects wastes. A few possess a closed vascular system. Excre-
tion is effected by protonephridia devoid of flame-cells. The sexes are
usually separate.

Nemathelminths are subdivided into three classes, a. Nematodes or
round worms; b. Acanthocephala or hook-headed worms; c. Chaetognatha
or arrow-worms.

Sixteen hundred species are recognized, mostly parasitic. The thick-
ened cuticula on the outside of the body is probably an adaptation to
their parasitic mode of life.

Phylum 6. MOLLUSCOIDA

Molluscoids are animals of ancient ancestry, and the bivalve "lamp-
shells" or brachiopods are abundant in the older sedimentary rocks.
Living forms are mostly sessile and marine. The valves of the shell,
instead of being lateral and paired, as in the clam, are dorso-ventral.
The ventral valve is the more curved of the two, and encloses posteriorly
an opening for the stalk by which the animal is attached.

Molluscoids are coelomate, and the body cavities are bilaterally paired.
In them are enclosed the alimentary canal, the liver, and the gonads.
Transverse septa divide the coelom into three portions, as in Hemi-
chordates and in Sagitta. While some species lack an anus, most have
a U-shaped alimentary canal with the anus near the mouth. Paired
nephridia function both as excretory and as reproductive outlets. The
nervous system consists of a circumesophageal ring with dorsal and ventral
nerve chains. The heart is dorsal to the stomach. Some forms, such
as the bryozoa, are colonial.

The phylum is subdivided into three classes: Polyzoa, Brachiopoda,
and Phoronida.

The origin of molluscoids from free-swimming trochophore ancestors
is generally assumed. 1700 species have been identified.

Phylum 7. ROTIFERA

The rotifers are so named because the cilia that surround or encircle
the mouth appear under the microscope to rotate like a wheel. They are
little known, partly because of their small size, and partly because they
are often confused with ciliated protozoa which they superficially resemble.
They are, however, multicellular unsegmented worms with a pseudo-
coelom. The alimentary canal has both mouth and anus. Usually,
the protonephridia and gonadic ducts open into the posterior portion
of the intestine. The sexes are separate. Rotifers are frequently com-

lO

COMPARATIVE ANATOMY

pared with the trochophore larva of annelids, but their ontogenetic
development does not support this assumption.
Eight hundred and fifty species are recognized.

psajoococuM

DIAGRAM OF A ROTIFER.

Pig. 7. — Diagram of a Rotifer. The Rotifers are aberrant Proterostomians — and
one of the few invertebrate phyla which have not been suspected of being ancestors of
vertebrates. (Redrawn after Stempell.)

Phylum 8. ECHINODERMATA

The echinoderms include the star-fishes, sea-urchins, sea-cucumbers,
and sea-lilies. They are marine coelomate invertebrates with spiny
skins and a water- vascular system which they fill with sea-water. Super-
ficially radial in symmetry, the echinoderms are actually bilaterally
symmetrical animals which undergo metamorphosis from a bilaterally
symmetrical larva. Since the blastopore becomes the anal opening and
the mouth is formed secondarily, they are placed among the deuter-
ostomia, the group to which the chordates belong. The coelom is a true
enterocoel, that is, develops as an outpocketing of the enteron.

The nervous system consists of a circumesophageal nerve ring from
which five nerves radiate. In addition, there is an aboral nervous system.
While most echinoderms possess a lacunar blood system, some have radial
blood-vessels. Specialized excretory organs seem to be wanting. The
sexes are separate and the gonads are interradial in position, with external
apertures on the aboral side of the animal. Respiratory organs are
varied, among them being dermal papillae which contain outpocketings of
the coelom.

Some taxonomists recognize 10,000 species.

Phylum 9. Annelida

The annelids include earthworms, leeches, clam worms, and many
marine forms. All are metameric, coelomate, and without jointed legs.
True nephridia, segmental " and ectodermal, are present. In many, the
body-wall is beset with bristles or setae. The nervous system is meta-
meric and consists of supraesophageal and subesophageal ganglia, with
circumesophageal connectives and a ventral series of ganglia.

Circulation is through a closed system of blood vessels. Blood is
pumped forward through a contractile dorsal aorta, and thence around

THE ANIMAL KINGDOM

II

the pharynx to a ventral subintestinal blood vessel. Smaller branches
distribute blood to the body-wall and intestine. External respiration
takes place through the skin. External filamentous gills sometimes occur
in forms with parapodia. The muscles, both of the body-wall and of the
alimentary canal, are divided into circular and longitudinal sets.

The sexes, usually, are separate, and the gonads are limited to a few
segments of the body. Germ cells find their way to the exterior either

A. QJMBRICUS.

LATERAL ASPECT.

SOMITES.--

OPENING OF
SPERM-DUCT.

MUSCLES.
NEPHfiOSTOME
NEPHRIDIOPORE

GIANT FIBER

B EARTHWORM -CROSS SECTION

SUBNEURAU BUOOD VESSEL

Fig. 8. — Liimbricus, a type of the phylum Annelida. Figure A shows the com-
mon earthworm in left lateral aspect. (Redrawn after Vogt and Jung.) Figure B is
a cross section in the trunk region. (Redrawn after Chidester, Courtesy of D. Van

Nostrand.)

through pores in the body wall, or by way of ciUated tubules, the coelomo-
ducts. Many annelids undergo metamorphosis from a " trochophore "
larva.

Four thousand species are known.

Phylum 10. MOLLUSC A

Molluscs are coelomate and non-metameric. The body consists
of mantle, mantle-cavity, and foot without paired appendages. The
mantle cavity receives digestive wastes and the products of the excretory

12

COMPARATIVE ANATOMY

and reproductive organs, and in most forms contains gills. The foot is
muscular and usually is the chief organ of locomotion. The circulation
is not closed, but the blood passes through a system of lacunar spaces.
The dorsal heart is enclosed in a pericardium which is a part of the coelom.
Arteries and veins afford connexions with the gills. The excretory
organs bear a fundamental resemblance to the nephridia of annelids.
Respiration is by means of gills, which in air-breathing snails are enclosed
in a chamber with a narrow opening to form a sort of lung.

The alimentary canal has a muscular pharynx and a stomach enlarge-
ment. The anus is posterior or is displaced forward in the mantle cavity.

SALIVARY GL.

GONAD

HEART

FOOT
GANGLION

INT.GLAND INTESTINE

NEPHRIDIUM MANTLE CAV

MOLLUSC.

Fig. 9. — Diagrams of a Mollusc. Figure A shows a longitudinal section, while £ is a
cross section. (Redrawn from Stempell, after Kuhn and Sedgwick.)

The nervous system has a circumesophageal ring and a series of cerebral,
pedal and visceral ganglia with their commissures and connectives.
Sense organs are varied. Many molluscs are hermaphroditic, the gonads
opening into coelomic sacs. Development involves metamorphosis, and
the veliger larva resembles the trochophore larva of annelids.

Sixty-one thousand species are recognized.

The phylum contains a number of primitive or aberrant forms, among
which are the chitons with eight calcareous plates on the back instead
of a single shell, dentalium with a nearly straight tapering shell open at
both ends, and certain marine forms that are worm-like and without
shells.

Though the molluscs as a group are highly successful in the struggle
for existence, as witness the multitude of their species, and the enormous

THE ANIMAL KINGDOM 1 3

numbers of individuals of several, they do not lie near the main line of
animal evolution and have contributed nothing to the forms above them.
Three main classes are separated by differences in the structure and
functions of the foot.

Class Gastropoda

The gastropods are the snails. In them, the foot is the familiar
ventral surface on which the animal crawls. Most have a single shell
from which comes their other name, univalves. Commonly, the shell
is coiled; but the Chinaman's cap, patella, has an oval dome. Some are
earshaped, suggesting one shell of a bivalve. Some of the land snails,
which are air-breathing, like the common garden slug, have no shells
or only a minute rudiment. In many, the opening of the spiral shell
is closed by an operculum when the animal withdraws itself.

The enormous number of gastropod species, nearly 50,000, the variety
and beauty of the shells, and the ease with which they may be pre-
served and displayed, have long made conchology a favorite branch of
natural history.

Class Pelecjrpoda

The pelecypods or lamellibranchs are the common bivalves, such as
oysters and clams. They have no heads, and the foot, as their name
indicates, is hatchet-shaped. It functions as a burrowing tool. The
mantle, in addition to secreting the two shells, is often prolonged into a
siphon which, in the common clam, may be elongated to nearly a foot,
and reach the surface of the sand while the rest of the animal remains
safely buried. Commonly, the siphon has two tubes, through one of
which water is drawn into the mantle cavity for respiration and food,
while through the other are ejected the body wastes.

The pelecypods are, in general, less active than the gastropods; but the
fresh-water clams crawl about freely on the muscular foot, and the
scallops, or pectens, swim by flapping their shells together.

This group, also, with some ten thousand species, is sought after by
collectors. Pearls, and mother of pearl f-or buttons, are obtained from
both marine and fresh-water forms. The ship worm, teredo, which often
does great damage to ships and docks, is a pelecypod.

Class Cephalopoda

The cephalopods are the cuttlefishes, squids, and devilfish. In them,
the mantle is wrapped about the body to form a sac, which is nearly closed
except near the head, where openings admit water to the gills in the
mantle cavity. The foot is divided into long tentacles which surround
the powerful beak. The paired eyes are much like those of vertebrates.

14 COMPARATIVE ANATOMY

but the retina unlike that of vertebrates is not inverted. Altogether, the
cephalopods are much the most highly organized and active of the molluscs.

The class divides into two orders.

The tetrabranchiata, or four-gilled forms, have an external shell, coiled
in modern species like a snail's, but, unlike a snail's, divided into com-
partments, only the last and largest of which is occupied by the animal.
The chambered or pearly nautilus, of which there are only four species,
is the only survivor of a vast assemblage which flourished in the Paleozoic
and Mesozoic. Many of the earlier forms had straight shells. Alto-
gether, including the ammonites of the Mesozoic, some two thousand
fossil species are known.

The dibranchiates have an internal shell or none. The group includes
the squids and cuttlefishes, octopus, and devil-fish.

Phylum II. Arthropoda

The similar metamerism of their bodies prompted Cuvier to group
annelids and arthropods together as articulata.

Arthropods are metameric invertebrates with jointed legs, and covered
with a jointed chitinous exoskeleton usually beset with hairs. The
general organization of the body resembles, closely, that of annelids,
from which, it is believed, that arthropods have descended; but the
coelom is secondarily replaced by a pseudocoelom. The eyes are simple
and compound, both types often being present in the same individual.
The circulatory system is open, that is, the blood vessels open into the
lacunar spaces of the pseudocoelom. Comparative anatomy and embryol-
ogy suggest that, originally, each metamere possessed a pair of appendages,
which, however, in the course of phylogenesis have become greatly modi-
fied by specialization or by reduction.

Species number 400,000 or four-fifths of all known animals.

Owing to the differences in the method of respiration, arthropods are
divided into (i) Branchiata, which, having adapted themselves to aquatic
Hfe, breathe by means of gills; these gills, usually, being attached to their
appendages; and (2) into air breathing Tracheata, which use ectodermal
air-tubes that ramify throughout the tissues.

Class Crustacea

The crustaceans include all the water-breathing arthropods. Besides
the crabs, lobsters, crayfish, shrimps, and prawns, together with several
thousand species of smaller creatures allied to them, the group contains
the familiar sowbugs of damp woods, five hundred species of barnacles,
and some five thousand species of minute creatures, water fleas and the
Hke, in size from i cm. down to the Hmit of comfortable visibility, about
half of which are parasitic.

THE ANIMAL KINGDOM

15

Class Arachnoidea

This division of the arthropods, most of them air-breathing, but
without tracheae, in addition to a vast number of minute creatures,
contains the harvest men, the scorpions, and over ten thousand species
of spiders all with four pairs of walking legs.

The familiar horseshoe or king crab, Limulus, among the largest of
arthropods, in length up to 50 cm. is the only surviving relative of the
Paleozoic trilobites, which, in their day, dominated the sea. The newly-
hatched limulus is strikingly like a trilobite; and yet, the limulus is clearly
a much modified spider.

l.3j^it^»ll»!Sfe^\M^«IH^»^

LIMULUS -
AN ARACHNID

Fig. 10. — A young Limulus viewed from the dorsal side. William Patten and
Gaskell have attempted to demonstrate that this animal lies in the direct line of verte-
brate ancestry.

The Limulus has, moreover, a further interest, in that it also resembles,
superficially, certain of the Paleozoic ostracoderms, a group which, it is
maintained by Patten and Gaskell, contains the long-sought ancestor of
all the vertebrates. Limulus belongs to the order Xiphosura (Mero-
stomata). The Eurypterids are a closely related group of extinct paleozoic
arachnids of large size.

Class Tracheata

In addition to the insects, all of which as adults have three pairs of
walking legs, the group contains many difi^erent primitive forms which
link the arthropods to the annelids. Of these forms, the most primitive
is the genus Peripatus, with fourteen to forty-three pairs of legs, and so
strange a body that it has been taken both for a mollusc and for a worm.
The familiar centipedes and myriapods have from a dozen to nearly two

l6 COMPARATIXK ANATOMY

Iniiulred l>ody sogiiionts, mcisl oi thciu nearly alike, and m«>sl bearing
one or two pairs of legs. The common myriapods, an inch or so in length,
are popularly supposed to he worms.

The insects fairly rival the vertebrates as the dominant group in the
modern world. No vertebrate except man has carried social organizi\tion
nearly so far. No vertebrate groups and lew in\ertebrates at all approach
the insects in the number of individuals in a single species. In addition,
tlie number of species is enormous. In North America alone, there are
seven tliousand named species of butterllies and moths, ten thousand
beetles, and nearly as many tlies. Possibly, in the entire world, known
and unknown species together may number as many as a million.

Fortunately, the individual insect is small. The largest known are a
beetle six inches long, and a grasshopper with a wing spread of ten inches.
But there are fossil dragontlies with a wing spread of two feet. From
these sizes, the insects run down to tiny parasitic creatures that live out
their life-cycle inside the speck-like egg of the coddling moth.

We shall examine later the assumption that the two dominant groups,
the insects and the vertebrates, one of which contains the smallest of
highly organized metazoa and the other the largest, are genetically
related to one another.

Phylum 12. Chordata

The chordates are animals which, at least early in life, have a sup-
porting rod, the notochord or chorda dorsalis. between the alimentary
canal and the central nervous system. In lugher chordates the notochord
is replaced during ontogenesis by a cartilaginous or bony vertebral
column. All have a dorsal tubular nervous system. The heart is ventral,
and the pharynx has functional or embryonic gill slits. Most chordates
are metameric in structure, although the metamerism may become greatly
obscured in the adult. Segmental excretory organs are generally present.

Neiirly ^o.ooo species are known.

Four sub-phyla are included in the phylum.

Sub-Phylum HE^^CHORDA i^Exteropxeusta)

The hemichordates or enteropneusta hold a somewhat uncertain
position in the animal kingdom. Morphologists are by no means agreed
that their closest afhnities are wdth the chordates. Some associate them
with the annelids, while the resemblance of their larval stage to that of
echinoderms leacis others to place them near that group. Their inclusion
among the chordates rests on their possession of pharyngeal gill-slits,
enteric coelomic pouches, a notochord-like diverticulum of the fore-gut in

THE ANIMAL KINGDOM 1 7

the pre-oral lobe, and upon the relations of the blood vessels and nerves.
Segmental excretory organs are, however, absent.

There are possibly 50 species.

BALANOGLOSSUS, the best-known genus, may be taken as representa-
tive. The body of balanoglossus is worm-like, and is divided into five
regions, proboscis, collar, gill region, "liver" region, and intestinal region.
The proboscis is a hollow muscular organ with an opening, a pore, on the
left dorsal side of the neck. The mouth lies on the ventral side between
the proboscis and the collar. The collar, like the proboscis, contains a
division of the coelom, which opens to the exterior by a pair of pores
near the mid-dorsal line. Like the proboscis, the collar also is muscular,
and used by the organism as a means of burrowing in the sand where it
lives.

The pharynx is divided into a dorsal portion that contains the numer-
ous gill-apertures and a ventral portion which functions as the digestive

BALANOGLOSSUS. t^tOUTH GILLS
Fk;. II. — lialanoiilossiis, a ty7-)ical genus of the suVj-jjhylum Hemichorda. (Redrawn

after Bate.son.)

passage of the pharynx. Posterior to the pharynx, the body contains a
series of gonadic sacs, each of which has a pore-like opening to the exterior.
The sexes are separate.

In the so-called liver region, the intestine shows a series of paired
diverticula, each of which produces a corresponding bulging of the rela-
tively thin body- wall. These diverticula are glandular and supposed
to have a digestive function, hence their name. Behind the liver-region,
the intestine passes without convolution directly to the posteriorly
situated anus. The circulatory system resembles that of annelids, but is
supplemented by a lacunar system of lymph spaces. ''Fig. 532)

The nervous system consists of dorsal and ventral nerve strands
containing occasional giant nerve cells. There are no special sense organs.
The so-called notochord is a diverticulum of the intestine which extends
with a narrow lumen into the proboscis from a point just behind the
mouth.

The larva of Balanoglossus, known as Tornaria, shows rather striking
resemblances to the larva of echinoderms. As in echinoderms, the
blastopore becomes the anus. The sub-phylum, therefore, is included
in the group of Deuterostomia.

Cephalodiscus and Rhabdopleura are genera which show resemblances
to Balanoglossus but which have a U-shaped alimentary canal. Rhabdo-
pleura is without gill apertures. (Fig. 536)

1 8 comparative anatomy

Sub-Phylum Urochorda (Tunicata)

The urochordates, the tunicates or sea-squirts, are so named because
the notochord, absent in the sessile adult, is always limited to the tail
region. Another character common to the group is the presence of a
tunicin mantle which is secreted by the skin. Tunicin is a chemical
substance that resembles cellulose. A coelom is sometimes present, but is
limited to the region of the ventral heart. Nephridia or coelomoducts are
wanting. The body is unsegmented, and the alimentary canal is bent on
itself so that the anus lies near the mouth. The pharynx is perforated

TENTACLES
CIUATED GROOVE

NERVE CORD
ESOPHAGEAL

— STOMACH

ASCIDIA - A OROCHORDATE

Fig. 12. — Ascidia, a urochordate. The animal is viewed as if cut in median longitudinal
section and as seen from the right side. (Redrawn from Sewertzoff, after Boas.)

by gill-slits, the number of which varies greatly in the different species.
The nervous system consists of a nerve ganglion dorsal to the pharynx,
from which nerves extend to the various organs. In some forms both
sexual and asexual methods of reproduction occur. Individuals however,
are usually hermaphroditic. Development generally involves metamor-
phosis. The sexually-produced tailed larva bears certain striking resem-
blances to the larva of amphioxus.

Some systematists recognize 1400 species.

ClONA is a sessile tunicate, three or four inches in length, which is
attached by tunicin stolons to its substratum. A tunicin test or tunic,
which is secreted by the skin, encloses the entire animal as a sac. Beneath
the test and loosely connected with it, except in the region of the two
apertures of the body, lies the body-wall or mantle. This consists of an

THE ANIMAL KINGDOM

19

external simple epithelial ectoderm, and beneath this, connective tissue
containing a network of muscle fibers which are more abundant in the
region of the two apertures of the body, which they serve to close and
open.

Of the two external apertures, the more ventral is the inhalent or
oral siphon and the other the exhalent or atrial siphon. The former leads
directly to the mouth, which is surrounded by a circle of tentacles. The
mouth leads into a greatly enlarged pharynx, which is perforated by
numerous gill-sHts or stigmata. The action of the cilia on the bars of
these slits serves to maintain a current of water from the pharynx into
the surrounding peribranchial or atrial cavity. Such relations resemble

BRAIN
CILIATED FUNNEC
MOUTH
GILL SLITS

/ATRIAL CAVITY/ SPINAL CORD

\notochord
mntestine

NENDOSTYLE "^"^ B. METAMORPHOSIS.

Fig. 13. — Diagrams of stages in the metamorphosis of a urochordate larva. When
the larva settles down and becomes fixed by its adhesive papillae, the tail is lost and the
notochord disappears. Thus the chordate characters which are so evident in the larva
are partly lost in the mature organism. (Redrawn from Korscheldt and Heider, after
Seeliger.)

those of similar organs in amphioxus. In the floor of the pharynx extends
a longitudinal groove, the endostyle, which morphologists generally
homologize with the thyroid gland of vertebrates. A somewhat similar
groove extends also along the dorsal side of the pharynx. The alimentary
canal consists of a short esophagus, a spherical stomach, and an intestine
which leads to an anus situated well forward in the atrial chamber.

The heart lies ventral to the esophagus in the pericardial chamber.
There are no closed blood vessels, but the blood is pumped from the heart
forward to the pharynx in lacunar spaces the relations of which resemble
those of the afferent branchial vessels of vertebrates. The reproductive
organs lie in the loop of the intestine, posterior to the stomach. Their
ducts extend forward and open into the atrial cavity near the anus.
The gonads are hermaphroditic.

20 COMPARATIVE ANATOMY

The nervous system consists of a ganglion or brain, which lies in the
body-wall between the two apertures of the body. Ventral to the brain,
is a neural gland which has been compared with the neural part of the
hypophysis of vertebrates. The unpaired eye and static organ contained
in the brain vesicle of the larva degenerate in the metamorphosis.

Ciona during its ontogenesis, undergoes a striking metamorphosis,
which indicates that the animal is a degenerate descendant of a primitive
branch of the chordate tree.

Of the four orders of Urochordates the Larvacea are of special interest
since they develop without metamorphosis, and hence, show no sign of
degeneration. Their caudal appendage contains a notochord and spinal
cord. That they lie close to the main Hne of vertebrate ancestry seems
not unlikely.

Sub-Phylum Cephalochorda (Acrania)

The c'ephalochordates are those chordates in which the notochord
occurs not only in the head, as in hemichorda, or in the tail, as in the
urochorda, but throughout the entire length of the body. The group is
sometimes called the acrania because, as the name suggests, a brain case
is lacking. Metamerism is strikingly manifested in the muscles and
nerves, which form an unbroken series from the tip of the snout to the tip
of the tail. Segmental protonephridia are metamerically arranged, but
are limited to the gill region. As in urochordates, the gills open into a
peribranchial cavity. Development involves metamorphosis. (Fig. 539)

There are possibly 25 species.

AMPHIOXUS. The lancelet, amphioxus, the characteristic genus of
the group and the so-called connecting hnk between vertebrates and
invertebrates, interests morphologists because of its resemblance to the
hypothetical ancestor of vertebrates. If the amphioxus had not been
discovered, it must have been invented for theoretical reasons. Its
resemblance to the larva of cyclostomes is impressive.

The Amphioxus is a lance-shaped animal, not more than two inches
long, with a laterally compressed body and a median caudal fin. Its
external orifices are an anterior ventrally placed mouth, an anus to the
left of the caudal fin, and an atriopore somewhat behind the middle of the
body. The atrial chamber which surrounds the elongated pharynx is
formed by the union of paired lateral folds which meet in the mid-ventral
Une of the body. Such a structure seems to be an adaptation to the
sand-burrowing habit of the adult animal. The atrial cavity ends bUndly
in front, and opens externally by the atriopore just behind the pharynx.
In the region of the pharynx, a pair of ventro-lateral metapleural folds
extend as far back as the atriopore.

THE ANIMAL KINGDOM

21

The body is covered by a thin external cuticula secreted by the simple
epithelial epidermis. Beneath the skin and visible through it, are sixty
pairs of myotomes which alternate with one another along the two sides
of the body. As in the vertebrates generally, these myotomes are greatly
thickened along the dorsal side of the body. Each myotome is V-shaped
with the apex of the V pointed forward.

The mouth, surrounded by a circle of tentacles, leads directly into
the elongated pharynx, the walls of which are perforated by numerous
gill-slits. A ciliated groove, which is similar in function and in relations
with the endostyle of urochords, extends the entire length of the pharynx.
Opposite it, in the roof of the pharynx, is a somewhat similar epipharyn-
geal groove. The liver is a hollow tubular sac, which opens into the

CIRRI
ORAUHOOOI

Fig. 14. — Amphioxus, in ventral and side views. Metamerism, lacking in uro-
chordates, and scarcely evident in hemichordates, is strikingly shown by amphioxus.
Whether this metamerism is inherited from annelid-like ancestors or is a convergent
trait independently acquired, is a moot question in morphology. (Redrawn after
Kirkaldy.)

floor of the intestine just behind the pharynx and extends forward to the
left below the pharynx. The intestine is straight.

The coelom, considerably reduced in size in the region of the pharynx,
extends posteriorly to the region of the anus. Ninety pairs of nephridia,
Umited to the gill-region, open into the atrial cavity. The solenocytes
attached to the nephridia are specialized excretory cells which strikingly
resemble those of annelids. Nephrostomes are absent. (Fig. 331)

Sexes are separate. Two dozen or more gonadic sacs surrounded by
the peritoneum project into the atrial cavity. Except for the absence of
a heart, the circulatory system resembles that of fishes. The blood
contains few blood corpuscles.

The nervous system, as in vertebrates, is tubular and dorsal. The
brain is a simple vesicle, which may possibly be compared with the fore-
brain vesicle of vertebrates. The nerves are of two kinds, dorsal
(sensory and motor) and ventral (motor). The former pass directly to

22

COMPARATIVE ANATOMY

the skin and to visceral muscles by way of the myocommata. Dorsal
and ventral nerves do not unite. Sympathetic cells and fibers are not
segregated to form a sympathetic system.

Sense organs comparable with those of vertebrates are wanting.
A median dorsal pit at the anterior end of the brain is mistakenly spoken
of as the olfactory pit. A pigment spot on the brain is likewise somewhat
uncritically called the cerebral eye. Amphioxus is, however, very
sensitive to light. There is no ear.

LATERAL LINE

PERrrONEUM

Fig. 15. — Squalus acanthias, a typical Vertebrate, in left lateral aspect, with
cross sections in three typical regions. The figure illustrates the chief features of
vertebrate topographic anatomy. Section A-B is taken through the eyes; section C-D
through the spiracles; section E-F through the trunk immediately anterior to the
anterior dorsal fin. See also Fig. 465.

Sub-Phylum Vertebrata (Craniota)

The vertebrates or craniotes are chordates with a vertebral column
and a brain-case. The evolution and perfection of a light and strong
endoskeleton has been an important factor in making the vertebrates
masters of the world. Exoskeletal structures also appear, as among the
invertebrates, but only exceptionally are heavy enough to interfere with
the activity of the animal. A many-layered epidermis with various
appendages enables the vertebrates to withstand successfully the vicissi-
tudes of weather met by land animals. In correlation with their activity,
senses multiply and become acute and the brain is much enlarged. The
original metamerism characteristic of lower vertebrates becomes much

THE ANIMAL KINGDOM

23

obscured in the higher. The heart is ventral and may be either two,
, three, or four-chambered.

Some :>^,ooo species are known.

Vertebrates are divided into seven classes.

Class Cyclostomata

Cyclostomes are the round-mouthed lamprey eels and hag-fishes.
They have a persistent notochord, lack a biting jaw, and the beginnings
of vertebrae appear in the form of cartilaginous neural arches. In the
genus Bdellostoma there are often as many as fifteen pairs of gill-sUts.

BDELLOSTOMA

VGILL APERTURES (?)
■""''' " ■ C. PETR0MY2DN

Fig. 16. — Three characteristic genera of Cyclostomes — Bdellostoma, Myxine, and
Petromyzon. That they are the most primitive vertebrates is shown in many traits,
such as a permanent notochord, absence of paired appendages and jaws, etc. (Redrawn
after Dean.)

There are no scales in the skin, and the teeth are horny. Some species are
hermaphroditic. Paired appendages are absent.

The lamprey, Petromyzon, is a familiar genus which undergoes meta-
morphosis during its development. Its larval stage is known as ammo-
coetes. Other genera are Myxine and Bdellostoma.

Class Ostracodermi

The ostracoderms are fossil forms which, as Stensio and others have
shown, resemble cyclostomes in some striking respects. UnUke the latter,
however, their heads were covered by heavy bony armor. Like the lam-
preys they lacked jaws and paired appendages. As in cyclostomes the
nasal aperture was median and dorsal in position. It has been asserted
but not demonstrated that the ostracoderms are the ancestors of carti-
laginous fishes, which are consequently assumed to have lost their heavy
body exoskeletons. Most morphologists, however, consider ostracoderms

24

COMPARATIVE ANATOMY

rather highly specialized types and not primitive ancestral forms. Cepha-
laspis and Pterichthys are characteristic genera.

CEPHALASPIS -
AN OSTRACODERM

Fig. 17. — Cephalaspis, an Ostracoderm, appears to have affinities with cyclostomes and
has been thought by W. Patten to connect vertebrates with arachnids.

Fig. 18. — Types of three sub-classes of fishes — Heptanchus, an Elasmobranch;
Polypterus, a Crossopterygian Ganoid; and Scotnheromorus, a Teleost. (Redrawn
after Dean.)

Class Pisces

Fishes are vertebrates with usually scaly skins, permanent gills, and
paired fins. The heart is two- or three-chambered. The skeletons may
be cartilaginous or bony. Gill-apertures number four to seven pairs.

THE ANIMAL KINGDOM 25

Dorsal and ventral spinal nerves join to form mixed trunks. Sympathetic
ganglia are differentiated. The liver has at least two lobes.

Of special interest are the orders of fishes which are believed to be in
the line of ancestry of land animals. Cartilaginous forms Hke the Elasmo-
branchs (sharks and skates) were the common stock from which the remain-
ing orders of fishes were probably evolved.

The extinct Crossopterygians or lobe-finned fishes, which make their
first appearance in the Devonian, were air-breathers and possibly the
direct ancestors of land animals. Arthrodires are fossil forms possibly
related to the modern lung-fishes or Dipnoi. The Dipnoi have either one
or two lungs and are in many ways transitional between fishes and
amphibia. Ganoids and Teleosts are "ray-finned" forms which include
the greater number of living species of fish.

Class Amphibia

Amphibians bridge the gap between land and water vertebrates, since
some, like fishes, are lungless, and when lungs are present, either permanent

NECTURUS.

Fig. 19. — Necluriis, a urodele Amphibian, interests zoologists because more than any
other living amphibian it resembles the fossil Stegocephala, another "ancestral" group.

or temporary gills occur. Except in some fossil forms, scales are lacking
in the skin. The olfactory pits communicate with the mouth cavity by
means of narial passages. The paired appendages are toed. The heart
is three-chambered. A postcaval vein is present. The embryo develops
without an amnion.

Amphibia are subdivided into urodeles or tailed forms, the newts and
salamanders, annra or tailless forms, the frogs and toads, and the gym-
nophiona or limbless types. Besides these living orders of amphibia, the
fossil order stegocephala is important, since they appear to be the direct
ancestors of reptiles.

Fishes and Amphibia have been grouped together as Ichthyopsida in
contrast with Sauropsida which includes reptiles and birds. The embryos
of the latter are protected by fetal membranes, while those of the former
are without them.

Class Reptilia

Reptiles are horny-scaled vertebrates which breathe by lungs, the
embryos of which develop in a liquid-filled sac, the amnion. The skull

26 COMPARATIVE ANATOMY

articulates with the atlas vertebra by means of a single occipital condyle.
Arterial and venous blood are mixed in the dorsal aorta.

Living reptiles are divided into Rhynchocephalia, Lacertilia, Ophidia,
Chelonia, and Crocodilia. Among fossil orders, the Theromorpha are

SPHENODON

Fig. 20. — Sphenodon has been characterized as a "Uving fossil." As a "primi-
tive" type of reptile it interests the student of phylogenesis. It belongs to the Order
Rhynchocephalia.

important, since, especially in their dentition, they resemble mammals,
and the Dinosaurs because they are the ancestors of the Birds.

Class Aves

Birds differ from reptiles in having both feathers and scales, and in
having the anterior appendages modified as wings. The heart is four-
chambered, and the single aortic arch on the right. Teeth are wanting
in modern forms. The body temperature is higher than in other animals.

Two large divisions are recognized, the flying birds or Carinatae with
a keeled sternum, and the running birds or Ratitae which have no keel
on the sternum.

Class Mammalia

Mammals are vertebrates with hairs and mammary glands. A few,
the monotremes, lay eggs, but all the rest bring forth their young well
developed. Mammals have a pair of occipital condyles, a muscular
diaphragm, and a chain of three ear bones. The heart is four-chambered,
and the aortic arch is on the left. The jaw articulates between the
dentary and squamosal bones.

Two major divisions are recognized, placentals, the embryos of which
are attached to the mother by a vascular placenta; and the non-placentals,
the monotremes and marsupials, most of which lack a placenta.

Sub-Class Monotremes (Prototheria)

The monotremes or ornithodelphia are egg-laying mammals with a
cloaca. Teats are lacking.

THE ANIMAL KINGDOM

27

Ornithorhynchus, the duck-bill of Australia, is the best-known genus;
and there are two species of the spiny anteater, echidna. There seem to
be no more than a half-dozen species surviving for the entire sub-class.

ORNITHORHYNCHUS

Fig. 21. — Ornithorhynchus is a repre- Fig. 22. — Opossum, the typical genus of
sentative of the most primitive group of Didelphians. (Redrawn after Newman.)
mammals, the Monotremes. As an
egg-laying mammal it bridges over the
gulf separating reptiles and mammals.

Sub-Class Marsupials

The marsupials or didelphia give birth to their young in a most imma-
ture state and nourish them for some time in an external marsupial pouch
situated on the ventral side of the body of the
female. The brain has no corpus callosum.
A loose allantoic placenta occurs in some.
Dasyurus has a yolk-sac placenta.

Opossum and kangaroo are well-known
examples. All the indigenous mammals of
Australia are non-placental.

Sub-Class Placentalia

The placentals or monodelphia have a
placenta, a corpus callosum in the brain, and
no marsupial bones. Urogenital and digestive
outlets are separated.

Placentals are subdivided into at least ten
living orders.

Order I. Insectivora. The insectivores
include shrews, moles, and hedgehogs. They
are flat-footed and five-toed, and their denti-
tion is also unspecialized, so that they are
apparently nearest of surviving forms to the
original placental.

Order 2. Xenarthra. The xenarthra include part of the group
formerly included in the edentates such as the armadillos, sloths and

TUPAIA-AN INSECTIVORE.

Fig. 23. — Tupaia, the tree-
shrew, an insectivore.

28 COMPARATIVE ANATOMY

anteaters. The teeth of adults are either absent or lack enamel and
roots. Dentition is limited to a single set.

Order 3. Rodentia. The rodents are gnawing animals, such as
rats, rabbits, squirrels, guinea pigs, beavers, porcupines, gophers. Canine
teeth are absent, and the incisor teeth in both jaws grow continuously
throughout life. The cecum is very large.

Order 4. Carnivora. The carnivora include the fossil creodonts,
the cats, dogs, weasels, bears, racoons, and seals. Each foot has four or
five toes. The canine teeth are sharp and elongated. The clavicle is
reduced or absent.

LEMUR CATTA

Fig. 24. — Lemur, a primitive Primate. (Redrawn after Shipley and McBride.)

Order 5. Artigdactyla. Artiodactyls are such hoofed forms as
cattle, deer, swine, sheep, goats, and camels, llamas, hippopotamuses,
and giraffes which have an even number of toes on each foot. The third
and fourth toes are larger, and the second and fifth reduced or absent.
The stomach is complex and the cecum reduced.

Order 6. Perissodactyla. The perissodactyls are hoofed forms
usually with an uneven number of hoofs, such as horse, ass, zebra, tapir,
and the rhinoceros. The third toe is the largest and the only one func-
tional in the horse. The enamel of the back-teeth is complexly folded.

Order 7. Subungulata. Hoofed forms usually with plantigrade
feet. Subungulates are the elephants and mastodons, and the hyrax or
cony. The proboscideans such as the elephants have five toes on which
they walk. Their testes do not descend into a scrotum. Sirenians
(Manatee and Dugong) are a suborder of this group.

Order 8. Cetacea. The cetaceans include whales, porpoises and
dolphins. They are aquatic mammals with fish-like bodies. Hairs and

THE ANIMAL KINGDOM 29

pelvic extremities are absent in the adult. There are two abdominal
teats. Teeth may be replaced by whalebone.

Order 9. CmROPTERA. Chiroptera are the bats and flying foxes.
Their anterior Hmbs are modified as wings, the fingers are joined by a
web, and the sternum has a keel.

Order id. primates. The primates include lemurs, marmosets,
monkeys, baboons, apes, and men. They are mostly arboreal in habit.

YOUNG CHIMPANZEE

Fig. '25. — Young Chimpanzee, a t>pe of Anthropoid (From photograph by Fred

Johnson.)

Nearly all have five digits with flattened nails, and in all except the lowest
forms the thumb is freely opposable to the fingers. Mammary glands
are usually a single pair and thoracic.

Primates are divided into two sub-orders.

Sub-order Lemuroidea. The lemuroids include the lemurs and
tarsiers. They are arboreal and nocturnal, small and not especially
monkey-like. T>^ical lemurs have a claw on the second digit of the
hind-foot. Thumb and great toe are not completely opposable to the
other digits. The uterus is two-horned.

30 COMPARATIVE ANATOMY

Sub-order Anthropoidea. Anthropoids include the remainder of the
primates. Hands and feet are differentiated, and either the thumb or the
great toe is opposable. Finger and toe-nails are flat, except in the marmo-
sets which have claws.

Three chief sections of anthropoids are recognized:

Platyrrhina. The South American monkeys, with broad nasal septum,
three premolar teeth in each half-jaw (except the marmosets which, like
the Old World monkeys, have two), and a climbing foot.

Catarrhina. The Old World monkeys and the great apes, with a
narrow nasal septum, two premolar teeth, and a climbing foot.

Bimana. Also with narrow nasal septum and two premolars, but
with the great toe non-opposable and a walking foot.

CLASSIFICATION OF ANIMALS— SUMMARY
Animal Phyla

1. Protozoa. Unicellular. Reproduce by fission.

METAZOA, multicellular.

2. PoRiFERA. Multicellular. Acoelomate. Pores in body-wall.

3. CoELENTERATA. Multicellular. Acoelomate. Radial symmetry
may possibly disguise primitive bilateral symmetry. Two-layered body-
wall. Nettling-cells. Enteron with single opening.

4. Platyhelminths. Bilateral. Flat-bodied. Without coelom.
Anus in a few genera.

5. Nemathelminths. Pseudocoelomate. Cylindrical. Anus.

6. MoLLUSCOiDA. Usually coelomate. U-shaped alimentary canal.
Lophophore.

7. Rotifera. Pseudocoelomate. Trochophore-like worms. CiHa
around mouth.

8. Echinodermata. Coelomate. Spiny-skinned. Water-vascular
system. Bilateral symmetry disguised.

9. Annelida. Coelomate. Metameric. Appendages when present
without joints.

ID. MoLLUSCA. Coelomate. Non-metameric. Mantle, mantle-cav-
ity, foot.

11. Arthropoda. Pseudocoelomate. Metameric. Jointed append-
ages.

Classes; Crustacea, Arachnida, Onychophora, Myriapoda, Insecta.

12. Chordata. Notochord. Dorsal tubular nervous system.

Sub-Phyla of Chordates

Hemichorda. Notochord limited to oral and pre-oral region. Worm-
Hke. Body in three primary divisions. Balanoglossus.

THE ANIMAL KINGDOM 3 1

Urochorda. Notochord limited to tail-region. Body-wall covered
with cellulose sac. Hemichorda and urochorda together are often called
protochordates. Appendicularia. Ascidia.

CEPH.A.LOCHORDA. Xotochord in head, trunk, and tail throughout
life. Metameric. Amphioxus.

Vertebrata. Chordates with brain case and vertebrae. Squalus.

Classes of Vertebrates (Craniota)

1. Cyclostomata. Without paired appendages or biting jaw.
Hermaphroditic. Petromyzon, myxine, bdellostoma.

2. OsTRACODERMi, fossil monorhine fishes related to the cyclostomes.

3. Pisces. With paired appendages and movable lower jaw. Skin
scalv. Permanent gills. Fishes are subdi\'ided into five sub-classes:

Elasmobranchii, with naked gills. Skeleton cartilaginous. Sharks,
skates, and rays.

Crossopterygii, fossil forms related to the ganoids.

Ganoidei, with operculum. Cartilage skeleton largely replaced by
bone. Garpike, sturgeon.

Teleostei, with operculum and bony skeleton. Common bony fishes.

Dipnoi, with gills and one or two lungs.

All following are Tetrapods.

4. Amphibia. Living forms are without scales and usually have lungs.
Toed appendages instead of fins.

The Stegocephala are a group of fossil amphibia.
Fishes and Amphibia grouped together as Ichthyopsida.
All to this point are Anamnia. All that follow are Amniota, having the
embryo protected by an amnion.

5. Reptilia. Adults scaly. Lungs only. Aortic arch on both sides.
The Theromorpha are fossil reptiles.

6. A\t:s. Feathered. Modern forms toothless. Aortic arch on
the right side only.

Reptiles and birds grouped together as Sauropsida.

7. Mammalia. With mammary glands and hair.

Sub-Classes of Mammals

1. Monotremes. Egg-laying mammals.

2. Marsupials. Pouched mammals.

3. Placentalia. Mammals with a placenta.

Orders of Placentalia

1. Insectivora. Insect eaters.

2. Xenarthra. Toothless or teeth without enamel.

3. Rodentia. Incisors specialized for gnawing.

32 COMPARATIVE ANATOMY

4. Carnivora. Flesh eaters. *

5. Artiodactyla. Generally even number of hoofs.

6. Perissodactyla. Generally odd number of hoofs.

7. SuBUNGULATA. Proboscideans, hyrax, and sirenians.

8. Cetacea. Whales.

9. Chiroptera. Winged mammals.

10. Primates. A single pair of thoracic mammae. Thumb usually
opposable.

Lemuroidea. Half-apes. Thumb not fully opposable.

Anthropoidea-. Thumb or great toe, except in New World monkeys,
opposable.

Sub-Orders of Anthropoidea

Platyrhina. With broad nasal septum. New World. Ateles,
Cebus, Hapale.

Catarrhina. With narrow nasal septum. Old World. Pro-
pliopithecus, Dryopithecus, Australopithecus, Gorilla, Simla, Hylobates.

BiMANA. Great toe not opposable.

Species and Genera of Bim.\na

Pithecanthropus erectus, the Java man.
Eoanthropus Dawsoni, the Sussex man.
SiNANTHROPUS Pekinensis, Pekin man.
Homo Neanderthalensis, Neanderthal man.
Homo Heidelbergensis, the Heidelberg man.
Homo Rhodesiensis, the Rhodesian man.

Homo sapiens, . Cro-Magnon and modern man. (Negro, Mongolian,
etc.)

THE ANIMAL KINGDOM

33

34

COMPARATIVE ANATOMY

Sequence of Organisms in Geologic Time

Eras

Periods

Years
(Barrel!)

Characteristic Organisms

Recent

I , ooo , ooo to
1,500,000

Modern races of men. Recent plants and
animals.

Cenozoic

Pleistocene
(Glacial)

Early species of men and Primates. ]\Iam-
mals dominant life. "Age of man."

Tertiary

95 , 000 , 000 to
115, 000 , 000

"Age of mammals." Lemuroids and in-
sectivores appear. First placentals.

Cretaceous

116,000,000 to
136,000,000

jMammals mostly marsupials. Reptiles
highly specialized.

Mesozoic

C o m a n -
chean

120,000,000 to
150,000,000

Bony fishes abundant. Flowering plants
appear.

Jurassic

155,000,000 to
195,000,000

Diverse reptiles. Ganoid fishes. First
birds.

Triassic

190,000,000 to
240,000,000

Crocodiles and Dinosaurs. Reptiles dom-
inant. First mammals.

Permian

215, 000 , 000 to
280,000,000

ISIammal-like reptiles. Trilobites disappear.

Pennsyl-
vanian

250,000,000 to
330,000,000

Primitive amphibia and reptiles. Conifer-
ous plants.

Mississip-
pian

300,000,000 to
370,000,000

Earliest amphibian fossils. Horse-tails and
club-mosses.

Paleozoic

Devonian

360,000,000 to
420,000,000

Amphibian foot-prints. Lungfishes. Ear-
liest land plants.

Silurian

390,000,000 to
460,000,000

Ostracoderm (armored) fishes. Elasmo-
branchs. Land plants begin.

Ordovician

480,000,000 to
590,000,000

Vertebrates appear. First fishes. First
insects.

Cambrian

550,000,000 to
700,000,000

Invertebrate phyla abundant. First tri-
lobites.

CHAPTER 2
REPRODUCTION

Anatomy deals with the structure of the organs of an animal. Struc-
ture is described primarily in terms of form. Form is extension in space.
The animal, however, exists not only in space but in time. The animal's
anatomy is more than the form which it possesses at a particular instant.
It is form which passes through a series of definite and characteristic
changes beginning with the development of an egg and ending at death.
Therefore a description of the anatomy of an adult animal is to be
regarded as merely a cross section of its whole anatomy. Embryology
deals with the life history during that early period in which the form of
the organism is changing rapidly, progressing from the comparative
simplicity of the egg to the structural complexity of the adult.

In the anatomy of an adult vertebrate are found many things which
remain unintelligible so long as attention is confined to the adult stage of
the animal. Why does the chief artery emerging from the heart turn to
the right in a bird, but to the left in a mammal? Why are the major
blood vessels in the body of mammals, even among individuals of the
same species, highly variable in their arrangement? Why does the
human diaphragm receive its nerve supply from the neck region of the
spinal cord rather than from the neighboring trunk region of the cord?
Why does a male dogfish have an apparently useless structure which
looks like the anterior portion of the oviduct of the female? These
questions and many more of the same sort are answered by embryology.
Therefore it is essential that the study of the anatomy of any organ should
include the embryonic history of the organ.

DIFFERENTIATION OF SEXES

Reproduction in the vertebrates is always sexual. That is, it always
involves the differentiation of gonads of two types; the ovary which
produces eggs or ova, and the testis which produces sperm or spermatozoa.
However, in some members of the Urochorda (tunicates), which are
presumably remote chordate alUes of the vertebrates, occurs an alterna-
tion of sexually produced and asexually produced generations. The
sexual individual is hermaphrodite, that is, it possesses both ovary and
testis, but the eggs are usually, if not always, fertilized by sperm from
another individual. The fertiUzed egg develops into an adult which,
however, is devoid of gonads. Then this asexual individual produces

35

36 COMPARATIVE ANATOMY

external bud-like outgrowths each of which develops into a sexual and
hermaphrodite adult.

In the very great majority of vertebrates the individual animal is
differentiated as either a male or a female — the dioecious condition.
Among the lower vertebrates, however, there are a few animals which are
normally monoecious or hermaphrodite. The eel-like hag, Myxine,
of the class Cyclostomata, has a much elongated median gonad whose
anterior region may become a functional ovary while the posterior region
remains sexually inactive, or the anterior part may be inactive while the
posterior region differentiates as a testis. Self-fertilization is thus
impossible. That both regions become functional, but at different times,
in the life of the individual hag is not certainly known. It has been
claimed that the young hag is functionally male, but later in life becomes
female. Among the bony fishes (Teleostei) various hermaphrodite
conditions have been described. Serranus, a genus of perch, includes
fishes which are normally monoecious and self-fertilizing. Chrysophrys
aurata exhibits "successive hermaphroditism," producing eggs and sperm
alternately.

Among fishes which are normally dioecious, the hermaphrodite con-
dition may sometimes occur as an individual variation or abnormality.
Such cases have been reported for several fishes, including such common
ones as the cod, herring and mackerel. So far as known, no vertebrate
above the fishes is regularly and normally hermaphrodite, but many
abnormal cases have been reported, especially in amphibians, in which
germ cells of both sexes have been found in one individual.

THE GERMINAL BODIES

The production of a new individual vertebrate animal is the outcome
of the joint activity of a male reproductive body, the spermatozoon, and a
female reproductive body, the egg or ovum.

The spermatozoa are derived from cells in the walls of the dehcate
tubules which are the essential part of the testis (Fig. 27). The ova
come from primordial germ cells contained within the tissues of the usually
solid ovary (Fig. 28).

The spermatozoon and the ovum each results from the differentiation
of a single cell of the gonad. In many respects their differentiation pro-
ceeds in quite opposite directions. Nevertheless each retains the essential
features of organization of a single cell.

In the course of the differentiation of the spermatozoon (Fig. 29) the
cytoplasm (extra-nuclear protoplasm) becomes greatly reduced in volume,
very little remaining beyond what is required for the formation of a
locomotor apparatus which consists usually of a long filamentous "tail."
The "head" of the sperm cell consists of the compacted chromatin

REPRODUCTION

37

material of the nucleus. The "ripe" sperm cell is essentially a motile
nucleus.

Fig. 27. — Cross sections of testis tubules of a mouse. BV, blood vessel; F, fat
granules; IC, interstitial cells; S'-, spermatogonia; S^, spermatocytes; 5^, spermatids;
SP, spermatozoa; ST, sustentacttlar cells. X360. (Redrawn from Bremer, Text-book
of Histology.)

VESICULAR FOLLICLE^
OVUMn

(OVUM

/PRIMARY FOLLICLE

LLicuLAR'"'^^%J"^-v---;ii;^4^^i^^i'^^

LLs ^ ^ ^'>••fiH^^r^^■■'■■

/

FOLL
CELLS

Fig. 28. — Section through cortical region of the ovary of a mouse showing young
(primary) ovarian follicles and older (vesicular) follicles. X76.

The egg, in contrast to the spermatozoon, not only retains its original
cytoplasmic volume but, while still in the ovary, passes through a long

3^

COMPARATIVE ANATOMY

growth period, the result of which may be a very greatly increased body of
cytoplasm (Fig. 38). The volume of the extra-nuclear part of the egg is
further more or less increased by the acquisition of special food materials,
derived from ovarian sources, which are deposited in the cytoplasm in the
form of characteristic coarse granules or globules known collectively
as the deutoplasm or yolk of the egg.

Another point of contrast between the ovum and spermatozoon lies
in the fact that the latter, consistently with its motile nature, remains a
naked cell while the ovum may become invested by membranes or enve-
lopes of various sorts, either protective or nutritive. An egg that possesses
a bulky yolk-laden cytoplasm is covered by a delicate but tough vitelline

Fig. 29. — Spermatozoa of dogfish (Squalus), frog, (Rana), parrot (Psittacus), mouse
(Mus) and ape (Innus). H, head; M, middle piece; T, tail. The spermatozoon of the
frog is about o.i mm. long. (Redrawn from Retzius.)

membrane (Fig. 38). The bird's egg (Fig. 32) is protected by a hard
calcareous shell underlaid by a tough fibrous shell-membrane. Nutritive
substance also, such as the albumen or "white" of the egg of the common
fowl, may be deposited about the egg.

The spermatozoa of the various vertebrates are uniformly minute
motile bodies whose differences in size and structure are insignificant when
contrasted with the differences exhibited by various ova. Eggs differ
most remarkably as to the amount of yolk carried by the cytoplasm and
as to their outer investments. The microscopic egg of a mammal and the
gigantic ostrich egg encased in its hard shell would hardly be suspected
of being essentially similar objects.

Eggs of Fishes

Even within the group of fishes the eggs differ widely. In the great
majority of fishes the eggs are relatively small (less than 5 mm. in diam-
eter), contain a correspondingly small amount of yolk, and are invested

REPRODUCTION

39

by only thin and soft protective membranes. The eggs of sharks and

skates, which contain enormous yolk masses, rival in size the eggs of

birds, and are enclosed in cases or shells which are secreted by the wall

of the anterior portion of the oviduct. These shells consist of a substance

resembling chitin or horn. In viviparous

sharks and skates the shell is very thin and

soft. In those which are oviparous the

case (Fig. 30) becomes thick and stiff and

usually acquires a curious flattened

elongated quadrangular form, often with

long curling tendrils extending from each

corner. These tendrils serve to anchor the

egg by becoming entangled with seaweed

or other objects. In some sharks the

egg-cases are of even more compHcated

form.

Eggs of Amphibians

The eggs of amphibians, which always
contain a considerable amount of yolk, are
larger than the eggs of many fishes, but
smaller than the average for reptiles and
birds. Eggs of various species of frog range
in diameter from 1.5 to 3 mm. Eggs of
the large salamanders, Necturus and
Cryptobranchus (the hellbender), range be-
tween five and six millimeters in diameter.
The amphibian oviduct deposits upon the
egg a layer of gelatinous substance (Fig. 31)
which, after the egg has been extruded into
the water, swells to form a thick jelly-like
envelope. In some amphibians (e.g.,
Cryptobranchus, most toads) toughened
strands of the jelly pass from egg to egg,
thus tying many eggs together to form a
long string (Fig. 31C).

• )^ EGG-CASE

Fig. 30. — Egg-case of small
shark. T, tendrils coiled around
branches of a homy (gorgonian)
coral. About half actual size.
(Drawn from specimen in the
anatomi cal collection of the
Biological Laboratories. Harvard
University.)

Eggs of Reptiles and Birds

In accord with other evidences of the close relationship of reptiles
and birds, the eggs, as well as the methods of development, of the
animals of these two groups are closely similar. The eggs are large
both absolutely and in proportion to the size of the animal. There is

40

COMPARATIVE ANATOMY

some correlation between the size of the egg and the size of the animal.
Thus, among birds, the humming-bird's egg is smallest, and the largest
were produced by certain extinct relatives of the ostriches, the moas of
New Zealand and ^^pyornis of Madagascar.

G' 'OV

Fig. 31. — -Amphibian eggs. A, of frog, soon after laying; B, early larva of frog, just
before hatching; C, of the salamander, Cryptobranchus allegheniensis. A and C,
approximately actual size; B, enlarged. G, gelatinous layer; L, larva; OV, ovum.
(A and B, redrawn from Marshall, Vertebrate Embryology; C, after A. M. Reese.)

The great size of the eggs of reptiles and birds (Fig. 32) is due mainly
to the enormous amount of yolk present. In the comparatively small
eggs of amphibians (Fig. 31) and most fishes, the yolk substance is dis-

Dudeui of Pander

neck of latebra

dense albumen
Jess dense albumen
vitelline membrane

Fig. 32. — Diagram representing a section of a hen's egg cut in a plane including the
long axis of the egg and passing through the blastoderm. (From Patten, Embryology
of the Chick; after Lillie.)

persed throughout the cytoplasm, but there is a tendency — the more
marked the greater the quantity of yolk — for the yolk granules to become
aggregated at one side of the egg. Thus, a more or less definite polarity
of the egg results. The proportion of yolk to cytoplasm is least at the

REPRODUCTION 4 1

animal pole and increases to a maximum at the opposite vegetal pole

(Fig. 38). The nucleus lies in the animal hemisphere.

In the eggs of reptiles and birds the quantity of yolk is so enormously
greater than that of the protoplasm that the dispersion of the yolk granules
throughout the protoplasm would be comparable to dispersing a quart
of sand grains in a single drop of water. Accordingly the polarizing of
these eggs reaches its maximum in that the protoplasm is all aggregated
at one spot on the surface of the egg, thus marking the animal pole, while
the remainder of the egg is yolk devoid of protoplasm. The locaUzed
protoplasm constitutes the so-called germ disc (Fig. 40) which usually
appears as a small white fleck on the surface of the large yellow yolk mass.
Before the egg is fertilized the germ disc contains a single nucleus, the
original egg nucleus.

Such a large ovum, consisting, for the most part, of loosely coherent
yolk particles, is necessarily invested by a tough vitelline membrane.
External to the vitelline membrane are various additional envelopes.
Birds and most reptiles, the Uzards and snakes being the exceptions, have
a thick layer of albumen (the "white" of a hen's egg) which doubtless
afifords the egg some mechanical protection and, in the later stages of
development, may be utilized for the nutrition of the young animal. The
outermost shell consists of an organic matrix more or less impregnated
with salts of calcium. In most reptiles the shell is of a somewhat flexible
or leathery texture. In crocodiles and alligators and in birds the shell is
highly calcified and consequently hard and brittle. Against the inner
surface of the shell lies a thin fibrous shell-membrane.

It should be appreciated that the object referred to in kitchen and
market as an "egg" consists of the actual (in strict sense) egg or ovum,
plus various extraneous substances and structures. The hen's ovum,
comparable to some small fish egg, is merely the yellow sphere commonly
called the "yolk" of the "egg," enclosed in its vitelUne membrane.

Eggs of Mammals

In point of size and quantity of yolk, the eggs of birds lie at one
extreme. The eggs of mammals, with two exceptions, lie at the other.
The exceptions are the duck-bill (Ornithorhynchus) and the spiny anteater
(Echidna) of the Australian region. These mammals, presumably of
primitive tvpe, exhibit many reptihan features, conspicuous among which
is the method of reproduction. Their large eggs contain much yolk, are
encased in a tough shell, and are very similar to the eggs of reptiles. With
these exceptions, the eggs of mammals contain a minimum of yolk and are
of practically microscopic dimensions (0.06 to 0.3 mm. in diameter). The
egg (Fig. 33) is covered by a transparent membrane (zona pellucida)

42

COMPARATIVE ANATOMY

external to which may be a cellular membrane (corona radiata), both
contributed by the ovary.

Relative Size

Comparison of sizes of eggs with little yolk and eggs with much yolk
becomes more impressive if stated in terms of volume rather than diameter.
The following data illustrate this:

Egg of

Amphioxus

Some frogs

Domestic fowl ("yolk")

Approximate
diameter, mm.

o. I

2 .o

30.0

Relative
volumes

8,000
2 7 , 000 , 000

The volume of an ostrich ovum would be hundreds of millions of times
greater than that of a mouse egg whose diameter is about 0.06 mm.

The comparisons become more significant if every ovum, big or little,

is to be regarded as structurally a
single cell. There is, perhaps, a little
room for question as to whether one
of these immense yolk-laden ova, in its
entirety, can be properly regarded as
a cell. Such an ovum exhibits the
extreme limit of that process of
polarization (already well advanced in
amphibians) which results from in-
crease in the amount of egg yolk.
Therefore, viewed with reference to its
Fig. 33.^Human ovum surrounded supposed evolutionary history, it is

by follicular cells. Actual diameter of • i ^ ^ i m •

ovum about 0.25 mm. c, cytoplasm equivalent to an amphibian ovum
containing some yolk; CR, corona which is unquestionably a Cell. But,

radiata: F, follicular cells: N, nucleus; ^1 . 1 • /• n 1

ZP, zona peiiucida. (After Nagel.) thinking of a Cell as a dynamic proto-

plasmic unit, it is only the germ disc
which is organized ''living" protoplasm. In a physiological sense, it is
only the germ disc which is a cell. The whole ovum, however, can be
regarded as structurally analogous to a fat cell, a cell distended by a
relatively enormous globule of fatty material.

But the spermatozoon is also essentially a cell. A rabbit spermatozoon,
for example, with a "head" about 0.005 mm. long, possesses only an
exceedingly small fraction of the volume of a mammal ovum. Therefore,
to approach more closely the full range of cell size in the vertebrates, the

REPRODUCTION 43

volume of the smallest spermatozoon should be compared to that of the
ovum of a moa or ^Epyornis.

It has been stated above that among birds there is some correlation
between the size of the egg and the size of the bird. This is a necessary
consequence of the fact that in all birds the yolk must provide the material
needed to carry development up to the point of hatching, at which time
the young birds have attained roughly corresponding stages in develop-
ment and growth. When, however, animals of different classes and eggs
of different types are brought into comparison, there appears a striking
lack of any correlation between size of animal and size of egg. The eggs
of some small salamanders are vastly larger than the eggs of some large
fishes. From the reasonable assumption that the eggs of elephants and
whales are of the typical mammalian sort, it follows that the egg of a
humming-bird is tremendously larger than that of one of these gigantic
mammals. Egg size is correlated primarily with the method of develop-
ment. Correlation with body size appears only when the developmental
history of the animals is similar.

Fertilization

The event which immediately initiates the development of a new
individual is the "fertilization" of the egg by the spermatozoon. The
motile sperm cell penetrates the egg and the sperm "head," a compact
mass of nuclear substance (chromatin), becomes joined with the chromatin
of the egg nucleus. In consequence of the so-called " maturation" process
through which all germ cells pass, each mature ovum and spermatozoon
contains only the "haploid" complex of chromosomes. Therefore the
junction of the sperm chromatin and the egg chromatin provides the
fertiUzed egg with a nucleus possessing the "diploid" complex of chro-
matin— that is, the full complement characteristic of the nuclei of all
the body cells of the animal.

So far as known, the cytoplasmic part of the spermatozoon contributes
to the functional ovum nothing except, possibly, a single centrosome.
However that may be, it is certainly true that the fertilized egg, although
it is the product of two cells, possesses the complete mechanism character-
istic of a single cell, and it does not visibly possess anything more than
that. Its yolk is something characteristic of eggs, but yolk is an inert
food substance, not a mechanism. The fertilized egg contains no visible
structures which would adequately account for its development into a
large complex animal possessing the specific form and characteristics
of the parent animals. In fact, as compared with such specialized cells
as those of muscle and nervous tissue, the egg cell is strikingly devoid of
visible special mechanism.

44 comparative anatomy

Developmental Potentialities

The description of the germinal bodies up to the completion of the
fertilization process may very well give the impression that the motile
spermatozoon is the active and essentially "animal" body while the rela-
tively enormous egg with its burden of inert yolk is a passive and vegeta-
tive thing. It is true that, at first, the spermatozoon is the active and
aggressive body, but its activity as an independent reproductive agent is
very transitory, while the egg, as later events prove, is capable of becoming
highly dynamic and continuing so during a long period of constructive
development. In fact, the egg possesses this capacity quite independently
of any specific action of the spermatozoon. It has been proved that the
development of an unfertilized egg may be brought about by any one of a
large number of physical and chemical means, such as mere pricking with
a needle, or changing the osmotic pressure of the fluid external to the egg,
or changing the chemical constitution of the external medium. This
"artificial parthenogenesis" may result in the development of a perfectly
characteristic adult animal. Its individual peculiarities, however, are
only such as may have been derived from the female parent, since its
male "parent" may have been a mere needle prick. In normal develop-
ment the spermatozoon imparts the stimulus which initiates development
and provides for inheritance from a male parent, but the egg is fully
capable of producing a characteristic adult without the assistance of a
spermatozoon.

Means or Exit

The sperm is usually conveyed by way of ducts which lead from the
testis to the exterior, but in the cyclostome eels the sperm is discharged
from the testis into the body cavity and finds exit by way of abdominal
pores which pierce the body wall in the cloacal region. Also in some
Teleostei the sperm passes from the testes into the body cavity whence
it is discharged through genital pores. The sperm ducts are usually
closely associated in one way or another with the duct system of the
kidneys. (See Chapter ii.)

The ova are usually liberated from the surface of the solid ovary
(Fig. 28) into the body cavity whence they pass into oviducts which lead
to the exterior. In the cyclostome eels, however, there are no oviducts,
and the ova, like the sperm, find exit by abdominal pores. In some bony
fishes (Teleostei) the ovary is hollow, the eggs are liberated into its lumen
and pass to the exterior by way of a duct which is an extension of the
wall of the ovary. In other bony fishes (the salmon and others) there
are no oviducts and the eggs are discharged from the body cavity through
abdominal pores. In viviparous vertebrates the eggs develop usually

REPRODUCTION 45

in an enlarged and more or less specialized region (uterus) of the oviduct,
but in a few of the viviparous members of the Teleostei the young develop
within an ovarian cavity.

OVIPARITY, VIVIPARITY, IMPREGNATION

The means whereby ovum and spermatozoon are brought together
necessarily depends on whether the animal is oviparous or viviparous;
also on whether or not the outer investments of the egg can be penetrated
by the spermatozoon.

The great majority of fishes are oviparous and produce eggs whose
investing membranes can be penetrated by spermatozoa. In such cases
there is no copulation. The male discharges the sperm upon, or in the
vicinity of, the eggs after they are extruded into the water — "external
fertilization." Some oviparous fishes, however, produce eggs whose
outer covering can not be penetrated by a spermatozoon. This is true
of many skates and sharks. The tough thick egg-case is deposited on
the ovum, after it leaves the ovary, by the shell gland or oviducal gland.
This gland is a specialized anterior region of the oviduct. In these fishes
"internal fertilization" is necessary. There is copulation and the
spermatozoa must pass forward and effect fertilization before the egg has
arrived in the sheU gland, or at least before the deposition of the shell has
proceeded to any great extent. After such internal fertilization, the egg
may begin to develop and pass through the early stages before it is "laid."
Therefore the thing which is "laid" is not, in strict sense, an egg but an
early embryo.

Some sharks and skates and a few bony fishes (the guppy, Lebistes,
for example) are viviparous and therefore effect internal fertihzation.

Among Amphibia there is much diversity. In the great majority
of the Anura (frogs and toads) fertilization is external. At present only
a very few cases (certain African frogs) are known in which it is internal.
In the tailed amphibians (Urodela) fertilization is internal but in most
cases it is effected more or less indirectly by means of a spermatophore,
a mass of sperm agglutinated together by a secretion from the cloacal
glands of the male. The spermatophore may be inserted directly into
the female cloaca but more often is attached externally near the cloaca
or merely discharged and picked up later by activity of the female. In
caecilians (Gymnophiona) fertilization is internal.

In all reptiles and birds the nature of the egg-shell is such as to necessi-
tate copulation and internal fertihzation. The substance of the shell is
deposited upon the egg in the posterior uterine part of the oviduct.
Such eggs as possess a layer of albumen acquire it from a more anterior
glandular region of the oviduct. Therefore fertihzation must take place
before the egg arrives in the uterine part of the oviduct and presumably

46 COMPARATIVE ANATOMY

before any albumen is deposited around the ovum. If fertilization has
occurred, the ''eggs" which are "laid" contain not ova but embryos at
early stages of development.

Modern mammals, with two exceptions, are viviparous. The excep-
tions are Ornithorhynchus and Echidna, mammals of obviously primitive
t3^e and reptilian in many features. The large egg covered by a thick
tough shell makes necessary internal fertilization, as in reptiles. All
other mammals, by reason of their viviparity, require internal fertiliza-
tion. The copulatory apparatus becomes most highly specialized in
mammals.

PROTECTION, NUTRITION, AND RESPIRATION, DURING DEVELOPMENT

From the beginning of the development of the animal the building up
of new protoplasm and the elaboration of it into the organized parts of
the animal calls for food to serve as building material. The energy
expended in the constructive processes of development is derived from the
chemical breaking down and oxidation of organic materials. Therefore
food and oxygen are requisite throughout development. The chance that
the young animal will attain adulthood depends on the degree of immunity
from unfavorable conditions and inimical agencies in the environment.
A survey of the whole group of vertebrates reveals the most extraordinary
diversity as regards provision for protection, nutrition and respiration
during development. Comparing various vertebrate embryos at corre-
sponding stages, the most striking differences are those which relate to
protection, food and oxygen. The visible differences which identify one
embryo as potentially a fish and another a mammal are insignificant in
contrast to the spectacular difference occasioned by the elaborate but
merely temporary placental structures of the mammal embryo.

In Fishes. In the majority of fishes the eggs, fertilized after they are
discharged into the water, are exposed to the hazards of the environment.
Mortality is high. The eggs of some fishes (cod, mackerel, haddock, flat-
fishes and others) are of such low specific gravity that they float at or
near the top of the water ("pelagic" eggs). Others (herring, salmon,
trout, most fresh-water fishes, some shallow-water marine fishes) lie on
the bottom ("demersal" eggs) until the young hatch. Pelagic eggs do
not receive parental protection. Demersal eggs commonly have an
investing membrane which is adhesive so that the eggs stick together in
masses or may become attached to stones, shells, aquatic plants or other
submerged objects.

Many fishes, especially those of fresh water, arrange nests of a simple
sort. The eggs may be deposited in a hollow scooped out in gravel, sand or
mud. The female salmon covers her eggs with gravel. An Australian cat-
fish places the eggs in a hole dug out in the sand and then covers them with

REPRODUCTION 47

stones. In many nest-building fishes the nest, after deposit of the eggs,
is guarded for a time by one of the parents, usually the male. The eggs
of the stickleback are placed in a rude nest which the male builds of
weeds glued together by a substance secreted by the kidneys. After the
eggs are laid the male stands guard over the nest. In the sea-horse
(Hippocampus, Fig. 34) and the pipe-fish (Syngnathus) the male acts as
nurse by carrying the developing eggs in a "brood pouch" situated on the
ventral surface of the body or tail, an arrange-
ment suggestive of the marsupial pouch of the
female kangaroo. In some of the cat-fishes the
eggs, during development, are carried in the
mouth of one of the parents, usually the male.

Thick tough shells such as enclose the large
eggs of oviparous sharks and skates must be
highly protective. The curhng tendrils (Fig. 30),

, . 1 , , J i_^i BROOD-POUCH-

which such egg-cases commonly possess doubtless
serve to attach the egg in a favorable locality.
In the somewhat shark-like marine fish, Callo-
rhynchus (one of the Holocephali), the egg-case
may attain a length of 25 cm. Its external
appearance is suggestive of a strip of kelp. Fig. 34. — Sea-Horse

Most efficient of all is the protection afforded (Hippocampus); male.

^ with brood-pouch. (Re-

by viviparity, exceptional among fishes, but drawn after Bouienger in
occurring in many sharks and skates and in a J}}^ Cambridge Natural
few Teleostei.

The eggs of most oviparous fishes are so scantily endowed with yolk
as to be capable of only a small amount of growth until additional food
can in some way be obtained from the environment. Such eggs develop
rapidly and soon attain functional differentiation of the organs of all
systems except the reproductive, but with relatively little growth because
of the lack of building material. The resulting miniature fish at once
becomes actively free-living and self-supporting. It then enters upon a
long period of life concerned mainly with feeding and growth. The
bulk of the prospective adult may be hundreds of thousands or millions
of times that of the newly hatched young. During the brief develop-
mental period the embryo is directly exposed to the external water so
that, until specialized respiratory organs (gills) are developed, the neces-
sary oxygen must be obtained by absorption through such surfaces of the
embryo as are in contact with the water.

In many fishes the young differs from the adult in such respects as
possession of adhesive discs or long spines or other organs of merely
temporary significance, or in form of body or fins, or in gill structure and
various other features. These differences may be so marked as to compel

48 COMPARATIVE ANATOMY

recognition of a definite larval period which is followed by a period of
metamorphosis in course of which the adult characteristics are acquired.
A famous instance is that of the "glass-fish," a small transparent some-
what eel-like marine animal which was assigned to agenus,Leptocephalus,
and for many years failed of recognition as being the larva of common eels
of the teleost family, Anguillidae.

The young fish produced by an egg endowed with a large yolk mass
attains a relatively large size before it is compelled to obtain food from
an external source. Differentiation tends to take place more slowly.
Prolongation of this period of functional immaturity and dependence
increases the need of protection. In oviparous sharks and skates this
need is met by the shell within which the developmental period is passed
and from which the well-grown young fish emerges when ready to begin
active and independent life. These shells must be sufficiently permeable
to allow oxygen to pass from the water through to the enclosed embryo
or young fish.

The embryo and young of the viviparous fish not only receives maxi-
mum protection but may obtain from the mother some nutriment in
addition to the initial supply of yolk, and must somehow obtain necessary
oxygen from the maternal blood. In most viviparous sharks and skates
the developing young fish lies free in the posterior enlarged uterine region
of the oviduct. The walls of this part of the oviduct are highly vascular.
Even in the absence of any specialized mechanism for the purpose, it is
possible that nutritive materials may diffuse from the maternal blood and
pass to the embryo or young fish within, and there appears to be no other
way in which the developing fish can obtain its oxygen. In some sharks,
however, there is definite provision for transfer of material from mother
to young. From the inner surface of the wall of the oviduct develop
folds or processes (villi) which are richly vascular. Similar folds may
arise in the region of the abdominal wall of the embryo (in relation to the
yolk-sac: see page 53) and these two sets of projecting vascular structures,
maternal and embryonic, become closely approximated or interlocked in
such a way as to facilitate diffusion of materials from the blood of one to
that of the other. Respiration, a certain amount of nutrition, and
possibly to some extent the removal of excretion products from the
embryo are thus provided for. These sharks are sometimes referred to
as "placental" sharks because of the striking similarity of this arrange-
ment and the mammalian placenta.

Viviparity in fishes is certainly, in an evolutionary sense, a secondary
rather than a primitive condition. This is attested by the fact, among
others, that in some viviparous Elasmobranchii the large egg is encased
within a shell whose vestigial character is indicated not so much by the
extreme thinness and delicacy of the shell as by the fact that it possesses

REPRODUCTION 49

filamentous extensions which obviously correspond to the "curling
tendrils" (Fig. 30) which serve to anchor the deposited eggs of some other
elasmobranchs. It is quite certain, too, that there can be no direct genetic
relation between the placenta of the mammal and the placenta-like arrange-
ments in some sharks.

In Amphibians. The circumstances attending reproduction and devel-
opment are, if possible, even more diverse among amphibians than among
fishes. In point of initial yolk content, however, amphibian eggs are less
variable than fish eggs. The smallest amphibian eggs are much larger
than the smallest fish eggs and the largest amphibian eggs are small com-
pared to the eggs of sharks.

Viewing the Class Amphibia as a whole and remembering that it is a
very small group compared to fishes, it is probably fair to say that amphib-
ians show much more concern — in a figurative sense — for the survival of
their eggs than do fishes. The great majority of them lay their eggs in
water, and it is invariably fresh water except in the case of the large
toad (Bufo marinus) of the American tropics. The eggs are commonly
deposited in a locality which seems to be instinctively chosen as favorable.
They may be attached to the under surface of rocks or other submerged
inanimate objects or to the leaves of aquatic plants. Some tree frogs
(Hylidae) build a low wall of mud to form a rude hollow nest either at the
bottom of shallow water or on shore very near the edge of the water.
Jamaican tree frogs place their eggs in the small natural aquaria caused
by retention of rain water caught by the leaves of certain large-leaved
plants. Some frogs of the eastern hemisphere depart from water so far
as to deposit the eggs on leaves or stones or in mud but always near water
and the newly hatched young go immediately into the water.

Many salamanders deposit the eggs on land. They may be placed
in small hollows in soft moist earth under stones or logs, but usually very
near some body of water. Some salamanders are wholly terrestrial,
depositing the eggs in crevices in logs deep in the woods, or on moss and
remote from water. The females of some salamanders and of some
caecilians (Gymnophiona) protect their eggs by coiling the body around
the mass of eggs, the caecilian eggs being deposited in a burrow.

Certain frogs and toads have various specialized and peculiar ways
of caring for the eggs and young. The male of the European "obstetric
toad" (Alytes obstetricans) picks up the long strings of eggs extruded by
the female and winds them about his body and hind legs. They are
carried thus until the tadpoles emerge. The male of the South American
frog, Rhinoderma, carries the eggs in his vocal pouch, a capacious recess
which communicates with the mouth cavity.

In several instances the back of the female serves in one way or another
as a temporary "nest" for the young. The tadpoles of some anurans are

5© COMPARATIVE ANATOMY

for a time carried by the female parent to whose back they become
attached. The male of the toad, Pipa americana, places the eggs on the
back of the female where they sink into the soft skin and become com-
pletely enclosed, each egg in an individual pouch. Thus protected they
pass through the whole developmental period, omitting the development
of the gills which are characteristic of the very great majority of amphibian
larvae, and finally rupturing the skin pouches to hop forth as fully formed
miniature toads. The female of the South American "marsupial" tree-
frog, Gastrotheca (Nototrema), likewise temporarily houses the young
on her back, but en masse instead of in individual compartments. A fold
of skin opening posteriorly forms a capacious chamber into which the
eggs are put. In some species the young emerge from the "marsupial
pouch" as tadpoles and in others as fully formed frogs.

Fig. 35. — Necturus larva of about 25 mm. length. (After Eycleshymer.)

In a small minority of amphibians, including representatives of each
of the three orders, Urodela, Anura and Gymnophiona, the eggs are
retained in the oviduct where they are fertilized and pass through the
developmental period. This viviparity affords the maximum of protec-
tion during development. However, the enclosing of eggs and young in
a vocal pouch, in individual dermal sacs, or in a dorsal "marsupium"
must be regarded, so far as protection is concerned, as more or less efficient
substitutes for true viviparity.

The amphibian egg with its moderate endowment of yolk, whether
laid in the open or enclosed in some protective way, develops rapidly into
a highly characteristic larva, the tadpole or "polliwog" (Figs. 31^ and 35)
which, with its functional gills and well developed locomotor tail as well as
in many features of internal anatomy, is a distinctly fish-like animal and, if
its environment is external water, it lives essentially the life of a fish.
During the larval or tadpole period, the chief activities are feeding and
growth. The end of the larval period is marked by a metamorphosis
during which, and in relatively short time, various more or less profound
changes occur, thus transforming the animal to the adult type. The trans-
formation is most radical in frogs and toads, involving the development of
legs and lungs, complete absorption of tail and gills, closure of gill clefts,
and other changes. Certain frogs, however, produce especially large eggs
which are deposited on land and develop directly into the adult form
without passing through a tadpole stage. In the Urodela the changes

REPRODUCTION 5 I

are less marked, the tail and sometimes also the gills being retained.
In some urodeles the changes are comparatively insignificant and such
salamanders as adult Necturus are often referred to as "permanent
larvae."

The duration of the larval period ranges from a few weeks in some
salamanders to a year and more in some frogs. The amount of growth
attained during the period varies accordingly. Sexual maturity is ordi-
narily not attained during the larval period although exceptionally it may
be. The Mexican axolotyl, the larva of the salamander, Ambystoma
tigrinum, regularly breeds in the larval state.

It is evident, then, that in the development of most amphibians the
all-important source of food is the yolk deposited in the egg. This carries
development on to the beginning of the larval period or, in fact, the animal
may enter the larval period with an important remnant of the yolk still
available for further growth. If the larval stage is passed in the open
water, the tadpole proceeds to feed, thus augmenting the original food
supply. Given a prolonged larval period, a relatively great size may be
attained before metamorphosis. When the larval period is spent within
the oviduct, or within a dermal sac or a vocal pouch, there is obviously no
chance of feeding and less growth is possible before metamorphosis.
It should be noted, however, that the extraordinary development of the
gills in some of these confined tadpoles opens the possibility that some
nutriment as well as oxygen may be absorbed from the surrounding
maternal, or sometimes paternal, tissues or fluids. For example, the
tadpoles of the "marsupial" frog have enormously expanded bell-shaped
gills which expose a large and highly vascular larval surface to the interior
of the marsupial pouch. Such a surface might easily serve for absorption
of nutrient material as well as oxygen.

Prior to the development of the larval gills the respiration of the embryo
must be carried on by diffusion through the general outer surfaces whether
the environment be water or some contiguous parental tissues and fluids.

In Reptiles and Birds. In striking contrast to the reproductive
arrangements in fishes and amphibians, where the utmost diversity pre-
vails, the corresponding arrangements in reptiles and birds are marked
by a high degree of uniformity. The egg, always very large in proportion
to the size of the animal, contains a quantity of yolk sufficient to provide
for the whole developmental period, thus enabling the young animal to
attain a relatively large size (in contrast to the corresponding conditions
exhibited by most fishes and amphibians) before being cast upon its own
resources for obtaining food. A newly-hatched alligator is gigantic com-
pared to a newly-hatched salmon.

All birds and most reptiles are oviparous. Immediate protection for
the egg is provided by the strongly developed shell. Reptiles deposit the

52 COMPARATIVE ANATOMY

eggs on land, usually in some natural hollow or crevice which will afford
protection. Crocodiles and alligators build rude nests of sticks or any
available loose materials and the female may guard the eggs. Most
reptiles leave the eggs to be incubated by the warmth of the sun.

Many lizards and snakes are viviparous. The utmost protection
being afforded by viviparity, the importance of the protective shell is
reduced.

A prolonged developmental period in course of which the young
animal attains large bulk — all internal organs, except the reproductive,
fully formed and highly differentiated before the animal hatches — the
heart beating vigorously and the blood circulating rapidly — all of this
going on within a thick shell — here is a set of circumstances which make
respiration a problem. In warm-blooded birds the maintenance of high
temperature during development accentuates the need of suitable provi-
sion for supplying abundant oxygen. The outstanding feature of the
development of the reptile or bird appears when the embryo itself goes
about the business of constructing a complex system of membranes so
disposed and so equipped with blood-vessels as to serve very efficiently
not only for respiration but for some other and secondary functions.

Early in development, at a time when the main organs are in process
of formation (Fig. 84), the outer layer of the embryo, representing the
prospective body-wall of the animal, throws up a system of folds which
arch over and ultimately enclose the whole of the definitive embryo — much
as if an animal should enwrap itself in a highly exaggerated fold of its own
skin. Thus are formed the investing membranes known as the amnion
and the chorion. The amnion is derived from the inner layer of the fold,
the chorion from the outer. The amnion does not fit the embryo snugly.
The intervening space is occupied by a watery solution whose chemical
constitution resembles that of blood — and also resembles that of sea
water. Thus the embryo during its further development is bathed by a
fluid whose chemical nature is compatible with that of the embryonic
tissues. Further, immersion of the embryo in watery fluid affords the
best possible protection from externally caused mechanical pressures and
impacts.

Meanwhile the enormous yolk mass has been enclosed (Figs. 84
and 87) by cellular layers which are prospectively the wall of the digestive
tube. Then from the hinder region of the embryonic digestive tube a sac
bulges out ventrally and, like a great and growing hernia, pushes beyond
the ventral body wall. Having thus attained the exterior of the embryo
proper, it becomes vastly expanded (by growth) and eventually becomes
spread out so that the greater part of its outer surface is, in conjunction
with the chorion, in close relation to the inner surface of the egg-shell.
This sac is the allantois. It becomes highly vascular, its arteries and

REPRODUCTION 53

veins communicating with the main vessels of the embryo. A considerable
part of the blood of the embryo is diverted into the allantoic arteries
and circulates vigorously through a rich system of small vessels lying close
to the inner surface of the shell. The shell is porous. Thus ready inter-
change of respiratory gases between the blood and the external air is
provided for. The allantoic sac serves also as a receptacle for embryonic
waste. The ducts from the kidneys open into the extreme hind end of
the digestive tube whence the fluid excreted by the kidneys readily passes
into the cavity of the allantois.

The inner cellular layer (yolk-sac) immediately enclosing the yolk
mass is highly vascular and its vessels, like those of the allantois, com-
municate with the main arteries and veins of the embryo. The blood
circulating through these vitelline vessels picks up dissolved yolk
materials which are conveyed to all parts of the embryo, thus making the
yolk available everywhere for metabolism and growth.

In viviparous reptiles the amnion, the allantois with its vascular
system, and the yolk-sac circulation are developed as in the embryos of
oviparous reptiles. The oxygen obtained by the allantoic vessels, how-
ever, must be derived from the maternal blood in the wall of the oviduct.

In Mammals. Primitive mammals, as indicated by such surviving
examples as Ornithorhynchus and Echidna, must have retained reptilian
methods of reproduction. However that may be, the two animals just
mentioned are oviparous. In size and possession of a protective shell,
the eggs resemble those of reptiles. The duck-bill, a burrowing animal,
deposits the eggs (usually two) in the burrow. Echidna, producing
usually only one egg in a season, places the egg in a fold of abdominal
skin, a temporary marsupium, where it is carried and incubated by the
warmth of the body until the young hatches. The embryos of these two
mammals develop amnion, chorion, allantois, and allantoic and yolk-sac
circulations essentially as do reptiles. The material providing for develop-
ment and growth up to the time of hatching is all derived from the initial
yolk content of the egg. The one new thing which these animals do is
to provide the young with a convenient source of food to serve for a time
immediately after hatching. Milk produced by mammary glands (see
page i8i) developed in and by the abdominal skin serves to prolong the
period of dependence on the maternal food. The additional growth thus
made possible renders the young animal the more fit to succeed when it
finally starts out for itself.

All known existing mammals except the duck-bill and spiny ant-eater
are viviparous. The minute eggs contain so little yolk that they could
never pass beyond the very early stages of development unless additional
food material were somehow provided. In the great majority of mammals
this is done by means of an organ which is one of the most characteristic

54 COMPARATIVE ANATOMY

features of a mammal. The egg, liberated from the ovary and fertilized,
becomes caught and lodged in the superficial tissue of the uterine wall.
Here it passes into the early phases of development and very shortly
gives rise to an amnion, a chorion and an allantois essentially similar to
those structures as developed in reptiles and birds. Curiously, in spite
of the absence of any considerable yolk mass, a yolk-sac also, although
devoid of yolk, is formed. This is usually interpreted as a relic of reptilian
ancestors. The allantoic sac becomes greatly expanded, more or less
enwrapping itself around the embryo, and certain regions of it fuse with
the adjacent chorion and enter into a very peculiar relation to the uterine
wall (Fig. 89). From the conjoined allantoic and chorionic membranes
grow out slender extensions (villi) which penetrate more or less deeply
into the adjacent uterine wall. They may become more or less branched.
These villi are highly vascular, fetal blood circulating in them under the
drive of the fetal heart. The surrounding uterine tissue is likewise highly
vascular. There is, however, no open communication between the blood
vessels of the villi and those of the uterine wall. But the fetal and the
maternal blood vessels are so close together that materials readily diffuse
from one blood to the other. Dissolved food substances and oxygen pass
from the maternal to the fetal blood; waste materials and certain special
fetal substances of hormone nature pass from the fetal to the maternal
blood. By this placental relation between mother and young the nutrition
and respiration of the young animal are provided for through the usually
long period of intra-uterine development.

Mammals show many variations in the mode of origin and details of
structure of the placenta. The marsupial mammals (Metatheria; the
kangaroo and its alHes) produce only a weakly developed and briefly
temporary placenta or none at all. Accordingly the development of the
young cannot proceed beyond what is made possible by the initial small
yolk supply plus what nutritive material may be absorbed by the embryo
and its investing membranes directly from the neighboring uterine tissues
and fluids. The young marsupial is therefore necessarily born at an early
fetal stage and while very small. The deficiency of the intra-uterine
arrangements is compensated for by the marsupium, a pouch formed by
a fold of abdominal skin. The mammary glands are within this pouch.
The very immature and quite helpless new-born young (in the great
kangaroo, Macropus major, being only about one inch long) is transferred
to the marsupium by the mother. The young becomes attached to one
of the mammary nipples and feeds passively, the milk being pumped in
by contraction of muscle about the mammary gland. This "mammary
fetus" inhabits the marsupium for a time which is usually much longer
than its period of intra-uterine development. For example, in the great
kangaroo the period of intra-uterine gestation is between five and six

REPRODUCTION 55

weeks, but the young kangaroo is carried in the pouch and nourished by
mammary glands for about eight months.

In placental mammals, as compared to marsupials, the young are born
at a relatively advanced stage of development and growth. The mam-
mary organs, however, are in all cases an important post-natal provision
for bringing the young animal along to a degree of size and strength favora-
ble to ultimate success. They afford the great advantage, too, that the
young animal is not thrown upon the world abruptly but may acquire
independence gradually.

EVOLUTIONARY SIGNIFICANCE

Surveying the whole group of vertebrates, the great diversity in the
conditions and arrangements attending reproduction is most impressive.
It would be difficult to imagine any practicable reproductive expedient or
condition which is not exhibited by some animal. There are microscopic
eggs and there are ostrich eggs. The quantity of egg yolk may be vast
or it may be next to nothing. The primary food supply, yolk, may in
various ways be supplemented by secondary sources of nutriment — egg
albumen, maternal blood, mammary milk, pigeon "milk." One egg or
millions of them may be produced at a time. They may or may not have
shells. Parental care of eggs or young ranges from nothing to the human
maximum. Vertebrates may be oviparous or viviparous. A primary
oviparity may be succeeded by a secondary substitute for viviparity, as
when eggs develop within a fish's mouth, an amphibian vocal sac, or
integumentary pouches of various sorts. Differentiation of organs may
precede growth or it may be delayed until the embryo is relatively large.
The newly hatched larva of so large a fish as the Atlantic salmon is about
0.65 inch long; a new-born whalebone whale is about tw^enty feet long.
The embryo may develop directly to the adult form or there may be a
larval period terminated by a metamorphosis. The embryo may or may
not produce a complex set of temporarily functional membranes — amnion,
chorion, allantois.

The important point to be appreciated is that the association together
of any two or more of these various alternatives in a single animal is not
haphazard. If one circumstance is, in itself, inadequate for the success
of reproduction, it is supplemented by something else. If a large fish
were to produce one single microscopic egg annually and deposit it any-
where in the Pacific Ocean, the species would soon become extinct. On the
other hand, there is no unnecessary duplication of highly specialized
arrangements. A placental mammal does not produce a large yolky egg.
The entire complex of reproductive conditions occurring in any one
animal comprises a consistent grouping of alternatives which, as a whole,
is adequate. Despite the great differences in methods of reproduction.

56 COMPARATIVE ANATOMY

the net results are equally good, or nearly so, and generation after genera-
tion the life of the world goes on with, at most, only very slow change in
the general biological balance and scheme of things.

Among vertebrates may be recognized four fairly sharply defined
complexes of reproductive conditions. They may be briefly summarized
as follows:

I. Eggs small: very little yolk

1. As in many Teleostei
Oviparity

Very many eggs produced — thousands or millions

No parental care

Rapid development

Early differentiation

Early self-supporting

Long growth period after hatching

High mortality

2. As in higher mammals: minimum yolk
Viviparity

Few eggs produced
Placental development
Differentiation mainly pre-natal
Growth largely pre-natal
Post-natal supplementary nutrition
Parental care
Self-support deferred
Maximum survival
II. Eggs of medium size (1.5-6 mm.): moderate amount of yolk

3. As in many amphibians, especially .\nura
Oviparity usual

Moderate number of eggs — usually scores or hundreds
Parental protection in many cases
Free-living self-supporting larva
Much growth during larval stage
Metamorphosis
Considerable mortality
III. Eggs very large: much yolk

4. As in reptiles and birds
Oviparity usual

Eggs few — necessarily so because of size

Protective shell

Amnion and allantois

More or less parental care

Differentiation mainly before hatching

Growth largely before hatching

Self-support immediately or soon after hatching

Low mortality

There are, as already set forth, a great many modifications of these
four types of reproduction.

REPRODUCTION 57

Highly significant is the fact that so great a degree of diversity can
exist within a single class of vertebrates. The fishes and amphibians show
this diversity most markedly. Assuming a genetic series from fish to
bird and mammal, the evolution of reproduction has not been a direct
progress along one straight and narrow path. Instead, the animals within
each class, especially the lower, tried (so to speak) a variety of methods.
After the many reproductive "experiments" of the lower vertebrates
finally emerged two distinct types to which the higher vertebrates fairly
closely adhere. Reptiles and birds exhibit one of these types, mam-
mals the other. Yet certain distinctive features of these finally emergent
types of reproduction are anticipated by some lower vertebrates. The
enormous eggs of oviparous sharks and skates, encased in thick shells,
resemble eggs of reptiles. Some viviparous sharks produce vascular
uterine structures (see page 48) suggestive of the mammal placenta.
Certain viviparous Hzards (genus Seps) develop what is practically a
placenta. But there can hardly be any direct genetic connection between
these structures in sharks and the somewhat similar structures in reptiles
or mammals, nor between the "placenta" of a lizard and that of a higher
mammal. The exaggerated filamentous gills of the intra-uterine larvae
of some viviparous salamanders and the much expanded bell-shaped gills
of the larvae of the marsupial frog, Gastrotheca, suggest that the larva
may obtain nutriment as well as oxygen from neighboring maternal sources
— potentially a "branchial placenta."

The marsupial structures of vertebrates afford another example of
convergence in evolution — that is, the independent origin of functionally
similar but genetically unrelated things. Defining a marsupium as a
brood-pouch developed on the external surface of the body- wall, there are
marsupial fishes (sea-horse; pipe-fish), marsupial frogs and marsupial
mammals.

Viviparity is commonly thought of as something peculiarly mammalian.
Yet there are viviparous fishes, amphibians and reptiles. The only
vertebrate class which contains no viviparous members is Aves. In view
of the fact that all birds and the most primitive mammals that we know
are oviparous and the further fact that oviparity predominates among
the lower classes of vertebrates, it is highly probable that the earliest
vertebrates were oviparous and that the animals which constituted the
main trunk of the vertebrate genealogical tree were oviparous. But
viviparity has appeared on twigs of various lower branches of the tree as
well as at its mammalian top.

The chordate ancestors of vertebrates must have been small animals
and presumably produced small eggs with little yolk. It is likely that
primitive vertebrates had small eggs and that large yolk masses have
been secondarily acquired. But even within a small group of vertebrates

58 COMPARATWE ANATOMY

the yolk content of eggs may be highly variable, being apparently easily
susceptible to evolutionary change. In point of size and yolk content,
the vertebrate egg has evidently had many ups and downs.

In spite of the diversity of vertebrate methods of reproduction, an
evolutionary trend is clearly to be seen. There is a certain extravagance
about the primitive method — millions of eggs, perhaps, in a season, but
only a small percentage of survival. The evolutionary tendency has been,
by introduction of ef&cient protective, nutritive and respiratory arrange-
ments, together with parental care, to assure the survival of every potential
adult. This tendency bifurcates and culminates in two very differ-
ently specialized methods, one in birds, the other in mammals. With
increase in chance of survival there is reduction in number of eggs pro-
duced. This result has the appearance of achieving economy but there is
perhaps room for question as to just how and where the economy comes
in. Does it cost a cod any more to produce seven million eggs than it
costs a viviparous dogfish to bear four or five large "pups"? By either
method of reproduction the numerical status of the species may be main-
tained and so, as remarked above, the net results of the two methods are
equally good.

Unquestionably the high degree of efficiency which has been attained
by the sauropsidan method of reproduction and also by placental repro-
duction in mammals is somehow correlated with the necessity of adapta-
tion to the circumstances of living on land and in air. The primitive
fish methods would obviously be impracticable. An aquatic larval stage
in the development of a horse or an elephant can hardly be imagined —
although, developing as it does in the fluid-filled amnion, the terrestrial
descendant of ancient aquatic ancestors does spend its early life in a
fluid medium. But it is beyond question also that the specialized repro-
duction of the bird and the mammal emphasizes the importance of the
individual animal. The evolution of these highly specialized reproduc-
tive methods may very reasonably be regarded as somehow correlated
with the fact that the individual bird or mammal is more important.

DEVELOPMENT

Cleavage and Blastula

Development involves great protoplasmic activity. There must be a
building up of new protoplasm, rapid dividing of cells, movement and
change of form. All of this calls for rapid metabolism. Metabolism
requires inter-action of nuclear material and cytoplasm and exchange of
materials between the protoplasm and the external medium. The area
of the nuclear membrane and area of the external surface of the cell
therefore impose a limit on metabolic rate. Two cells are capable of more
rapid metabolism than one cell whose nuclear and cytoplasmic volumes

REPRODUCTION

59

are respectively equal to the combined volumes of the corresponding
parts of the two cells because the Hmiting membranes of the two cells
have greater total area than those of the single cell.

The smallest egg-cells are large compared to most tissue-cells of the
animal to which the egg belongs. The metabolic rate in an egg before
fertiUzation is relatively low. After fertiUzation the rate increases.
Before entering upon a prolonged ^^^^

period of activity at high metabolic
rate the bulky egg-cell increases its
surfaces by dividing into small cells.
The successive divisions of the original
egg-nucleus are, in fact, accompanied
by absolute increase in the quantity
of nuclear chromatin, a substance
which undoubtedly plays an impor-
tant part in determining the course
of development.

In Amphioxus. From the fore-
going it follows that the first necessary
process in development is the cleavage
of the fertilized egg (Fig. 36) into yeg.

many small cells. Figure 37 illus- Fig. 36. — Median section of a ferti-

, , ,, r , • lized egg of AMPHIOXUS. Diameter

trates the course of cleavage m of egg about o.i mm. ^iV, animal pole;

Amphioxus. Although sometimes A/', male and female pronuclei; P, polar

, .J- J 1 A 1- • • i. body; 5, remnant of spermatozoon;

classified as such, Amphioxus is not ^^c. vegetal pole; Y, region of cyto-

literally a vertebrate. But it is a plasm occupied by coarse granules of

, 1 , , . , , . yolk. (After Cerfontaine.)

chordate and in many respects obvi-
ously primitive. The adult is a slender fish-like animal about 5 cm. long
(see Fig. 14). The egg is correspondingly small, about o.i mm. in
diameter, and contains very little yolk.

The plane of the first cleavage of the egg corresponds to the definitive
median (sagittal) plane of the future adult. The two cells resulting from
the first cleavage therefore represent the right and left halves of the body.
The plane of the second cleavage is perpendicular to that of the first,
and the third cleavage plane is perpendicular to both the first and second.
The second and third cleavages each divide the egg slightly unequally.
Further cleavages follow one another in rapid succession, their planes
adhering to a fairly rigidly determined order. Meanwhile the cells
gradually shift their relative positions and surfaces of contact in such
a way that a space opens out at the center of the whole mass. At the
thirty-two cell stage the cells are disposed to form a hollow sphere whose
wall is everywhere one cell in thickness. Thus every cell of the thirty-
two is in direct relation to the exterior, a most favorable position for

6o

COMPARATIVE ANATOMY

respiration and excretion. This hollow spherical shape is retained as

NUCLEUS

Fig. 37. — Cleavage of egg of AMPHIOXUS. A, undivided egg; B, in process of first
cleavage; C, four-cell stage, lateral view; D, four-cell stage, polar view; E, eight-cell
stage, lateral view; F, sixteen-cell stage, lateral view; G, eighty-eight cells, lateral view;
H, same stage as G, median section; I, later stage, lateral view. P, polar body. (After
Hatschek.)

cleavage continues (Fig. 37, G-I) until between two hundred and three
hundred cells have been formed. This stage of the embryo is called

the blastula. The name, blasto-
coele, is applied to the cavity.

The second and third cleavages
introduce inequality of size among
the resulting cells. This inequality
persists as cleavage goes on. It is
correlated with the distribution of
yolk in the protoplasm, the larger
cells containing the more yolk.
The cells of the blastula grade from
minimum size at one pole (animal)
of the sphere to maximum size at
the opposite pole (vegetal). This

Fig. 38. — Ovarian egg of Frog; median , .^ . ,,1.1 1 • ,1

section. (Redrawn from Morgan, The Polanty IS established m the egg

Development of the Frog's Egg; The before cleavage begins.
Macmillan Co.) t * 1 ••l- o i •

In Amphibians. Some amphi-
bian eggs (not including the gelatinous envelope) are about 2 mm. in
diameter. Such an egg would possess a volume about eight thousand

YOUK

VEGETAL POLE

REPRODUCTION

6i

times that of an egg of Amphioxus. The greater part of the increased
bulk is yolk. The egg (Fig. 38) is strongly polarized with reference to
the distribution of the yolk in the protoplasm. From the animal pole
where yolk is at a minimum the quantity increases toward the opposite
vegetal pole where the maximum occurs.

Yolk is a non-living, quite inert substance. The active material in
development is protoplasm. The developmental behavior of eggs con-

ANIMAL POLE

BLASTOCOELE

Fig. 39. — Cleavage of the Frog's egg. A, first cleavage in process; B, two cells;
C, eight cells; D, fourth cleavage complete in animal hemisphere but just beginning in
the four cells at the vegetal pole; E, early blastula, median section; F, G, successively-
later stages, lateral view. {D, F, G, redrawn from Morgan, The Development of the
Frog's Egg; E, redrawn from Marshall, Vertebrate Embryology.)

taining much yolk shows quite clearly that the yolk is an impediment to
the free carrying out of developmental operations — just as the necessity
of carrying a heavy burden of food supplies may impede the progress of a
company of explorers. Figure 39 represents the cleavage stages of a
frog's egg. The successive divisions follow the same general order as in
Amphioxus. Cleavages succeed one another at intervals of about an hour,
but the period varies with temperature. The yolk, however, evidently
hinders cleavage, especially in the vegetal hemisphere. The second
cleavage begins at the animal pole before the first is completed at the
vegetal pole. In fact, the third cleavage may begin while both first and
second are still incomplete in the region of the vegetal pole. Further,

62

COMPARATIVE ANATOMY

the inequality in size of cells at animal and vegetal poles is much greater
than in Amphioxus, another consequence of the greater yolk mass.

After the third cleavage a cavity appears in the midst of the group
of eight cells. As cleavages proceed this cavity enlarges and the embryo,
as in Amphioxus, becomes a hollow sphere or blastula (Fig. ^gE). The
,„,., .,^,„,j^^^ frog blastula differs from that of

Amphioxus. Its cavity (blastocoele)
is excentric, occupying approxi-
mately the animal hemisphere only.
Its wall is more than one cell thick.
The great thickness of the wall of
the vegetal hemisphere and the
consequent excentricity of the blast-
ocoele are obviously due to the
yolk.

In Reptiles and Birds. In eggs,
such as those of reptiles and birds,
where the yolk mass greatly exceeds
that of the amphibian egg, the polari-
zation with respect to the distribu-
tion of yolk reaches its limit in that all
the protoplasm is segregated into a thin plate, the germ disc, lying on the
surface of the relatively enormous sphere of yolk (Fig. 40). In such an
egg, obviously, there is no mechanism for dividing the yolk. Cleavage is
confined to the protoplasm of the germ disc which, following fertiUzation

4,

#^^

I
I

L

I

Y«

Fig. 40. — Cleavage of the germ disc of
the egg of a Turtle (Glyptemys insculpta) ;
eight-cell stage. The egg-shell is not
shown. About twice natural size. A,
albumen; C, the eight-cell blastoderm;
F, yolk. (Redrawn from Louis Agassiz,
Embryology of the Turtle.)

lY' lY^

Fig. 41. — Early blastoderm of Chick; plane of section passes through center of
egg. B, blastocoele (subgerminal or cleavage cavity); C, cells of blastoderm; V, fluid-
filled vesicles; F', yellow yolk, F^, white yolk. Magnified nearly twenty diameters.
(Redrawn from Duval, Atlas d'Embryologie.)

of its nucleus, splits up rapidly and soon consists of hundreds of small
cells forming what is then called the blastoderm lying as a thin plate of
cells on the surface of the yolk (Figs. 40 and 41). But there is continuity
of blastoderm with yolk only around the periphery of the blastoderm.
Elsewhere a thin space, the subgerminal cavity, intervenes between
blastoderm and yolk (Fig. 41). Comparing this embryo with the blastula
stages of Amphioxus and frog, it seems reasonable to interpret it as a

REPRODUCTION 63

blastula whose blastocoele is the subgerminal cavity, while its blastoderm
is the animal much-less-than-" hemi "-sphere and the yolk mass is the
vegetal much-more-than-"hemi "-sphere. This recognition of a blastula
stage, comparable to that of Amphioxus, in the development of a reptile or
bird would hardly have been possible but for the intermediate condition
exhibited by the amphibian with its moderate yolk mass and total cleavage.

The blastula is to be conceived as an essentially one-layered stage of
the embryo, the "layer" being the wall of the blastula, whether one cell
thick or more than one cell thick. This stage has two-fold significance.
Its immediate importance is in the fact that it gives the embryonic mate-
rial increased superficial contact with the environment, thus favoring
metabolism. Its prospective significance lies in the fact that further
development is to consist, to a large extent, in the manipulation of layers
of embryonic material. The adult is hollow. It has a body cavity and
other cavities. Most of its organs are hollow. The walls of the hollow
structures are constituted of layers — skin, epithelium, endothehum,
peritoneum, muscle layers, connective tissue layers. For the construc-
tion of such a many-layered thing, the embryo naturally proceeds as early
as possible to dispose its building material in the form of layers.

Gastrula

In Amphioxus. The blastula stage is briefly transitory. At once
changes set in which transform it to a two-layered embryo. In Amphioxus
the two-layered gastrula form is attained in a very simple way (Fig. 42).
The vegetal hemisphere first flattens, then becomes curved inwards.
The infolding (invagination) continues until the material of the original
vegetal hemisphere comes into close relation with the inner surface of the
wall of the animal hemisphere. The spherical blastula thus becomes an
approximately hemispherical embryo whose wall is two layers thick
(Fig. 42C). As the process goes on the blastocoele is reduced and finally
obhterated. The gastrula is hollow. Its cavity, resulting from the
invagination process, at first opens widely to the exterior but the width
of the opening is rapidly diminished by inbending of the wall about it
and it is soon reduced to a narrow blastopore. In consequence of this
contraction of the wall around the blastopore, the form of the entire
gastrula tends at first to become spherical, but before the contraction is
completed the gastrula begins to elongate in the direction of the axis which
passes through the blastopore.

An important accessory activity attends this process of narrowing the
blastopore. The blastoporal rim is a region of transition from the outer
to the inner layer (Fig. 4 2D). This region is marked by very rapid pro-
liferation of cells, especially at the dorsal edge of the blastopore. Cells

64

COMPARATIVE ANATOMY

produced within this growth zone or "germ ring" are added, some to the
outer layer and some to the inner layer. This growth process, then, is
concerned both in the narrowing of the blastopore and the elongating of
the embryo. A direct consequence of it is that the material of a certain
region of the inner layer immediately adjoining the blastopore attained
its internal position not as a result of the primary invagination but by the
secondary growth process.

ANIMAL POLE
I

VEGETAL POLE

ECx

Fig. 42. — Gastrulation in AMPHIOXUS. The figures represent sections through
the polar axis of the embryo. A, blastula with vegetal region flattened; £ and C, earlier
and later stages of invagination of vegetal hemisphere; D, gastrulation completed; with
elongation of the gastrula, its long axis becomes the horizontal antero-posterior axis
of the embryo. A, archenteron; B, blastocoele; BP, blastopore; EC, ectoderm; EN,
endoderm; P, polar body. (After Cerfontaine.)

At the close of the gastrula period (Fig. 4.2D) the embryo is an elongated
ovoid, the slightly larger end being anterior while the now very narrow
blastopore marks the posterior end of the lang axis. So rapid is develop-
ment that this stage is attained about seven hours after fertilization.

Significance of the Gastrula. This gastrula form is of utmost signifi-
cance, certainly prospectively, and perhaps also retrospectively. Reduced
to lowest terms, what, essentially, is an organism? It is something which
carries on perpetual interchange of material, nutritive and respiratory,
with the environment, and it is capable of making such adjustments with

REPRODUCTION 65

the environment as favor its continuance. Having in mind the necessi-
ties.of some relatively large animal, nutrition — the digestion and absorp-
tion of food — is best effected at an internal surface. The structures and
mechanisms immediately necessary for maintaining favorable relations
to the exterior — passively protective structures and motor-reaction
mechanisms — must lie at the external surface or in relation to it. Respir-
ation can be provided for at either an external or an internal surface. The
essential and minimum animal, then, is a two-layered thing. The outer
layer is protective, sensory, and may (as it actually does in coelenterates)
give rise directly to motor mechanisms. The inner layer is primarily
nutritive. It may (as in some coelenterates) share in producing motor
structures. Conceivably, and in view of the fact that all the functions of
a Uving thing are carried on by an organism so minute that it is organized
as a single cell, one layer might suffice. Given a small hollow animal,
its cavity opening to the outside and its single wall being one cell thick
throughout, nutrition might be attended to at the inner surface of the
layer while the externally superficial protoplasm might provide the neces-
sary contacts and adjustments with the exterior. But the principle
of speciahzation, "division of labor," early became manifest in the
evolution of animals. Two layers of cells, each layer speciaUzed for a
particular kind of function, work better than one.

The gastrula is the animal in its bare essentials. The outer layer,
ectoderm, is potentially protective and nervous. It gives rise to the
essential part of the adult skin which produces so many important pro-
tective structures and to the whole nervous system, both peripheral and
central. The inner layer, endoderm, is nutritive. The cavity within it is
the primary digestive cavity or archenteron. It is significant that the
wall of the archenteron is derived from the vegetal hemisphere of the
blastula. Thus, appropriately, the greater quantity of yolk comes to
lie in the lining of the embryonic digestive cavity. In the vertebrates
the external aperture of the archenteron, the blastopore, never becomes
mouth and rarely becomes anus. The future motor mechanism, muscle,
is derived indirectly from the gastrula layers.

The gastrula is strongly suggestive of the two-layer body plan of a
coelenterate. A simple coelenterate such as Hydra, two-layered through-
out, including even the tentacles, can be regarded as a somewhat elabor-
ated gastrula, the Hydra "mouth" corresponding to the blastopore
(Fig. 43). This resemblance, together with the fact that a gastrula
stage, modified in one way or another, occurs nearly universally in the
development of metazoan animals, gave rise to Ernst Haeckel's "gas-
traea" theory which proposed that gastrula-hke animals (essentially
coelenterates) must have been the ancestors of all Metazoa. According
to this theorv, the occurrence of the gastrula form in the ontogeny of a

66 COMPARATIVE ANATOMY

vertebrate is a "repetition" of the coelenterate stage in phylogeny.
This may very well be true but it is not necessary to hold this view in order
to account for the gastrula stage in ontogeny for some such form as the
gastrula is the almost inevitable precursor of any adult metazoan which
has a skin (ectoderm) and a digestive tube (endoderm).

In Amphibians. Compared to gastrulation in Amphioxus, the corre-
sponding process in the amphibian is very complex. In Amphioxus the
entire wall of the blastula is thin. The wall of the vegetal half is only
slightly thicker than that of the animal half. The blastocoele is very
nearly spherical. In the frog the vegetal wall of the blastula (Fig. 39, E-G)
is so thick that the vegetal hemisphere is, in effect, solid. It consists of

A ^ B

Fig. 43. — Diagrams showing structural similarity of a coelenterate and a gastrula
.4, Hydra, longitudinal section; B, gastrula, axial section. A, archenteron, prospective
digestive cavity; BP, blastopore; E, enteric (digestive) cavity; EC, ectoderm; EN,
endoderm; M, mouth; T, tentacle.

large cells heavily laden with inert yolk. It is not to be expected that such
a wall could readily bend inward as does the corresponding thin and
labile layer of the Amphioxus blastula.

In the amphibian three processes going on simultaneously effect
gastrulation. The beginning of gastrulation is seen when a crescent-
shaped groove (Fig. 44.4 ; /) forms at a certain place on the surface of the
blastula. It lies just on the vegetal side of the equator determined by
the animal and vegetal poles and extends transversely to the median
plane determined by the first cleavage. The equator and a zone extending
superficially somewhat into the vegetal hemisphere are marked by espe-
cially rapid cell-proliferation. It is in this particularly active region,
the "germ ring," that the groove appears. Figure 44.4 'represents a section
in the median plane of an embryo at this stage. The groove (I) is the
result of an invagination which occurs near where the upper thin wall and
lower thick wall of the blastula join. The outer layer bounding the
invagination consists of smaller cells which have moved inward from

REPRODUCTION

67

the superficial germ-ring region; the deeper wall of the invagination
consists of yolk cells. The groove, initiated as a slight invagination,

AN

VEG

Fig. 44. — Gastrulation in the Frog. A, B, C, the whole embryo at successively later
stages; viewed toward the vegetal pole. A', B' and C represent, in somewhat diagram-
matic way, sections of corresponding stages cut in the plane including the polar axis
and bisecting the gastrular invagination, /; this plane corresponds to the median plane
of the adult. During the latter part of the period of gastrulation, as result of shifting
of the heavy yolk (compare B' and C), the embryo rotates so that the axis passing
through the blastopore (BP) becomes horizontal (see Fig. 48A). A, archenteron;
AN, animal pole; B, blastocoele; BP, blastopore; EC, ectoderm, EN, endoderm; /,
invagination; NP, neural plate; VEG, vegetal pole; Y, yolk; YP, yolk plug.

now rapidly deepens (Fig. 4.4B, B'), not by continued invagination, but by
active growth of that upper (for later events prove it to be dorsal) lip

68 COMPARATIVE ANATOMY

of the groove — that is, the lip resulting from the infolding of germ-ring
material. This growth process serves to build out the dorsal lip of the
original invagination so that the fold is caused to extend farther and
farther downward over the yolk cells. Meanwliile the groove, originally
a short crescent as seen on the surface of the blastula, lengthens laterally
or in the direction of the curve of the crescent (Fig. 44B) until it describes a
semicircle and, continuing, finally completes a circle. As the groove
progressively lengthens, the newly arisen region of its outer fold, con-
tinuous with the "dorsal lip" of the initial region of the groove, at once
begins to grow centripetally over the surface of the yolk cells. Therefore
the radius of the curve described by the groove is ever decreasing. The
groove is obviously deepest at the region where it began to form and
shallower in the successively newer parts of it. Having completed the
circle, the centripetal growth of the outer fold of the groove continues
until the original vegetal hemisphere is completely covered except for a
small aperture through which bulges a mass of yolk cells, the so-called
"yolk plug" (Fig. 44C,C').

If the processes just described can be visualized, it will be seen that
their result must be the formation of a new cavity in the embryo. This
cavity is bounded externally by the two layers of the overgrowing fold,
internally by the yolk cells. It potentially opens to the exterior but its
actual opening is blocked by the yolk plug. If no process other than
those already mentioned were involved the cavity would be exceedingly
thin. It is, in fact, greatly enlarged by another process. During the
progress of the overgrowth of the vegetal hemisphere, the large yolk
cells become extensively rearranged. They move into the blastocoele,
finally practically obliterating it. They carry out this movement in
such a way that the space left vacant by them is added to the cavity
formed by invagination and overgrowth.

Figure 44C' represents a median section of a frog embryo at the close
of gastrulation. The embryo is two-layered throughout. The outer
layer, ectoderm, is uniformly thin. The inner layer, endoderm, is very
thin over approximately the dorsal half of the embryo but thick in the
ventral region where the greater part of the original mass of yolk cells
persists. The endoderm surrounds a capacious cavity, the archenteron
whose external opening, the blastopore, is occupied by the yolk plug.
The blastopore marks the posterior end of the embryo. The greater
part of the original yolk is now in the endoderm.

This gastrula is essentially like that of Amphioxus. It is formed by
the cooperation of several processes of which the more important are
(i) invagination, which plays a minor part in the formation of the defini-
tive archenteron; (2) epiboly — that is, overgrowth of the yolk mass by
the fold resulting from invagination; (3) rearrangement of the yolk cells

REPRODUCTION 69

to extend and complete the endodermal sheet produced by the over-
growing fold and, at the expense of the blastocoele, to increase the archen-
teric space. The difference between gastrulation in Amphioxus and that
in the amphibian is essentially this: in Amphioxus the vegetal hemisphere
(prospective endoderm) of the blastula actively moves into the interior of
the embryo; in the amphibian the eventual interior position of the endo-
derm material is effected largely by the overgrowing activity of the
prospective ectoderm. In Amphioxus the endoderm goes inside; in
the amphibian it is put inside by being covered over. Quite clearly, the
difference is the necessary consequence of the presence of the great mass
of inert yolk in the amphibian blastula.

In Reptiles and Birds. A reptilian or avian embryo whose yolk mass
may be millions of times that of Amphioxus could hardly be expected to
carry out a process of gastrulation similar to that of Amphioxus — if,

Fig. 45. — Gastrulation in the Pigeon. Section approximately median, showing
formation of endoderm by invagination at posterior edge of blastoderm. A, archen-
teron; B, blastocoele (cleavage cavity); BP, blastopore; EC, ectoderm; EN, endo-
derm; V, vitelline membrane; Y, yolk. Magnified about 100 diameters. (After J.
T. Patterson.)

indeed, anything comparable to gastrulation were to be recognized at all.
Yet the original single layer of the blastoderm, formed by cleavage
(Figs. 40, 41), must somehow give rise to additional layers. The fact is
that the blastoderm does at an early period become two-layered. The
details of the mode of origin of the second layer differ considerably in vari-
ous members of the Sauropsida. In the present connexion the significant
fact is that, in all cases so far as known, the deeper layer (endoderm)
results, in part if not entirely, from an inward movement of blastoderm
cells at the median region of what proves to be the posterior edge of the
blastoderm (Fig. 45). This inward movement may consist in the forma-
tion of a small pit, an actual invagination, from whose bottom cells move
forward and laterally underneath the original blastodermic layer to become
the endoderm. In other cases there is merely an in-turning of the mid-
posterior edge of the blastoderm without formation of a complete pocket
or invagination. In either case the process is confined to the mid-posterior
region of the edge of the blastoderm. The endoderm, thus initiated,
rapidly spreads over the yolk mass and under the original layer which is
now identified as the ectoderm. The growth of the endoderm may be
augmented by cells which become detached from the under surface of the
outer laver.

70 COMPARATIVE ANATOMY

It is noteworthy that the place of origin of the endoderm in the
sauropsidan embryo is always at the edge of the blastoderm and it is
always the posterior edge. If the primary blastoderm is to be regarded
as corresponding to the animal hemisphere and the yolk mass to the
vegetal hemisphere of the amphibian embryo, then the formation of endo-
derm in the sauropsidan embryo begins at a point which corresponds very
closely to the position of the primary gastrular invagination in the amphib-
ian (Fig. 44.4', /). This fact, together with later events in the sauropsidan
embryo, justifies the application of the term, blastopore, to the aperture
of the Httle invagination or the slit formed by infolding of the hind edge
of the blastoderm.

In the development of the reptile and bird, therefore, there is a gastrula
stage which, in spite of the enormous mass of yolk, is essentially similar
to the gastrula of an amphibian. It seems fair to say that the process of
sauropsidan gastrulation is as nearly as possible — that is, as nearly as the
yolk will permit — like that of an embryo in which there is comparatively
little yolk (see Fig. 81).

Comparisons. Comparison of the early development of Amphioxus,
amphibian and reptile or bird compels the conclusion that, were it not for
difference in volume of yolk, the several embryos would be practically
alike in form, at least through the gastrula stage. It is as if the embryo
with the larger yolk mass " tried" to behave like the embryo of Amphioxus
but is compelled by the yolk to modify its behavior. Amphioxus with total
and nearly equal cleavage; the amphibian with total but very unequal
cleavage; the reptile or bird with partial cleavage; the several embryos at
corresponding stages exhibiting radical differences in the configuration
of their materials — yet analysis of the processes concerned in the develop-
ment of all these animals reveals a basic similarity.

The actual animal is the protoplasm. Developmental processes are
its dynamic expression. Yolk, although necessary, is mere inert luggage.
In all these animals its composition is essentially the same. The similari-
ties which exist in spite of variation in yolk volume are certainly much
more significant than the differences which exist because of variation in
yolk volume. The method whereby the sauropsidan embryo achieves a
two-layered condition is not the simplest imaginable. The easy and direct
way would consist in the splitting of the original blastoderm to form two
layers, an inner and an outer. Such splitting or " delamination " of layers
commonly occurs at other stages in development. The fact that the
sauropsidan embryo initiates endoderm formation by invagination or
infolding at the posterior edge of the blastoderm is open to no better
explanation than that there is some necessity of adhering as closely as
possible to the developmental methods employed by amphibians and
Amphioxus. Such necessity can come only through inheritance. The

REPRODUCTION 7 I

embryos thus far — that is, through the gastrula stage— are essentially
alike. They are all chordates. They merely carry different quantities
of food. Herein lies the reason why so much emphasis is laid on the study
of the development of Amphioxus. There is reason to believe that the
Amphioxus embryo with its nearly minimum encumbrance of yolk reveals,
at least in the early stages of development, the primitive and basic proc-
esses which in other chordates become more or less obscured by yolk.

The Third Layer, Mesoderm

The greater extent of the ectoderm of the embryo persists as the essen-
tial layer, epidermis, of the adult skin. The endoderm gives rise directly
to the lining epithelium of the adult digestive tube. But in the adult
animal a great complex of structures — muscle, skeleton, central nervous
organs, lungs, Hver, and the reproductive, excretory and circulatory organs,
making up the greater part of the bulk and weight of the animal — inter-
venes between the epidermis and the endodermal digestive epithelium.
Some of these intermediate organs take origin directly and independently
from the primary ectoderm or endoderm. For example, before the close
of the gastrula stage the central nervous organs begin to differentiate from
the dorsal ectoderm. Later, lungs, liver and pancreas arise as separate
localized outgrowths from the endoderm of the early digestive tube.
Others of the intermediate organs have an indirect relation to the primary
layers of the gastrula. The close of the gastrula stage is marked by the
formation of a layer, or system of layers, of embryonic material which
comes to be interpolated between the outer and inner layers of the gastrula.
This middle and third layer, the mesoderm, spreads extensively between
the primary layers and at first appears to be quite undifferentiated
throughout. Later it undergoes local differentiation to form muscle,
skeleton, kidneys, circulatory organs and various other structures.

Following cleavage the embryo became a one-layered blastula. This
form was succeeded by the two-layered gastrula. The next important
event is the development of a mesoderm whereby the embryo attains a
three-layered stage. The earlier period of development is concerned with
laying out the building materials, the embryonic or "germ" layers. In
the later and longer period these layers are shaped into organs. The
formation of the central nervous organs and the notochord may begin,
however, before the mesoderm is fully established.

In Amphioxus. At the close of the gastrula stage the Amphioxus
embryo is approximately ovoidal, the long axis antero-posterior with the
blastopore at its posterior end. The dorsal surface of the embryo is some-
what flattened. Figure 42D shows a sagittal section of the embryo at this
stage. Figure 46A shows a section cutting the embryo transversely and

72

COMPARATIVE ANATOMY

,NP

Fig. 46— AMPHIOXUS.

Transverse sections of embryos at successively later

FIG. 40.— Aivirni^^w^. ^'^f'^^^®^^!'^^?-' ^. „,p,oderm A. section somewhat

stages, showing origin of notochord Tan embryo sli^UoS^^^ than that represented

NP nlurSV^te-, NT, neural tube. (After Cerfontame.)

REPRODUCTION 73

within the anterior third of its length. Except for the dorsal flattening,
the configuration of layers is as simple as possible. Figures 46B-O show
transverse sections at stages successively later than that of Fig. 46 A.
Several things are happening simultaneously. A broad band of dorsal
ectoderm (NP), slightly thicker than the adjacent regions of the layer,
becomes separated, along its right and left edges, from the neighboring
ectoderm. This process involves the mid-dorsal ectoderm continuously
from the blastopore almost to the anterior end of the embryo. The
median ectoderm thus delimited from the lateral ectoderm is the material
of the prospective central nervous organ, the neural tube. In this initial
stage it is called the neural (or medullary) plate.

The dorsal endoderm is at first flattened in comformity with the neural
ectoderm but later (Fig. 46, D-F) it becomes convoluted along three fines
extending lengthwise of the embryo. Its median slightly thicker region
becomes sharply folded upward. Either side of this median fold a longi-
tudinal groove appears on the inner surface of the endoderm. Then the
endoderm in the region of each of these grooves assumes the form of a
fold extending outward dorso-laterally. Thus arise three folds, one
median and a lateral pair, all convex outward, and extending nearly the
whole length of the embryo. As time goes on these folds become more
emphasized, but soon a difference arises between the median fold and the
lateral folds. The median fold remains continuous throughout its entire
length. The lateral folds, however, become interrupted by the formation
of sharp deep transverse folds which cut from above downward through
each lateral fold. This process of subdivision or segmentation begins
near the anterior ends of the lateral folds. Its immediate result is a pair
of approximately globular pouches lying symmetrically either side of the
median fold, each pouch having a small central cavity opening by a narrow
passage into the archenteron. Later this passage is closed and then the
pouch becomes detached (Fig. 46F) from the archenteric wall which, at
the place where the pouch had formed, closes so that nothing is left to
mark the spot (Fig. 46G).

Immediately behind each pouch of the first pair another similar pouch
forms exactly as the first did. At this stage of development, marked by
the presence of two pairs of these pouches, the embryo escapes from the
egg membrane ("hatches"). The period between fertilization and hatch-
ing varies considerably, its average being probably not far from twelve
hours.

These two pairs of pouches derived from the dorso-lateral endodermal
wall of the archenteron constitute the first definitely delimited mesodermal
material. The remainder of the dorso-lateral folds, extending back to
the blastoporal region, is destined to give rise, after hatching, to additional
mesodermal pouches. The median endodermal fold, which has remained

74

COMPARATIVE ANATOMY

intact during this process of segmentation of the lateral folds, is the mate-
rial of the future notochord (Fig. 46, NC).

At the time of hatching, then, the embryo has made important progress
beyond the gastrula stage. Not only has the segregation of mesoderm
begun but two important organs, the central nerve tube and the notochord,
are indicated.

After hatching, additional pairs of mesodermal segments are cut off
from the lateral mesodermal folds, the addition taking place progressively
from anterior to posterior, until a total of ordinarily fourteen pairs have
been produced. In several of the more posterior segments cavities do not
occur, the mesodermal folds merely breaking up into a succession of soHd
blocks of cells.

^MES 1-6^ e'c

Fig. 47. — AMPHIOXUS. Frontal (horizontal) section of an embryo having
six pairs of mesodermal somites. The section is through the notochord and just below
the blastopore. At the posterior end of the section may be seen a region where the
notochord, endoderm and mesoderm merge indistinguishably. A, archenteron near
the blastopore; EC, ectoderm; EN, endoderm; MES 1-6, mesodermal somites; NC,
notochord. (After Cerfontaine.)

In the formation of these fourteen pairs of mesodermal pouches the
material of the original mesodermal folds is completely utilized. These
folds, however, terminated posteriorly in the close vicinity of the blasto-
pore (Fig. 47) a region where, since an early gastrula stage, very rapid
cell proliferation has been going on. Following, then, the formation of
the fourteen pairs of mesodermal segments from the lateral folds, the series
of segments is extended backward by addition of successive soHd blocks
of cells which become detached from the growth zone encirchng the blasto-
pore. By this means the number of pairs of mesodermal units, the earlier
and more anterior developing as hollow endodermal outgrowths, the later
and more posterior as solid blocks derived by ingrowth from the blastoporal
rim, is increased to the adult total, usually sixty-one.

In Amphibians. About certain details of mesoderm formation in
amphibians there is more or less obscurity. In some respects the process
differs conspicuously from that in Amphioxus. On the whole, however.

REPRODUCTION

75

the similarities seem more significant than the differences. The point of
unmistakable correspondence is to be seen in the important part played
by the proliferation zone of the blastoporal rim or "germ ring." The
amphibian gives little evidence of anything comparable to the paired
mesodermal pouches which push out from the dorso-lateral endoderm of
Amphioxus. However, about three quarters of the ultimate number of
mesodermal segments in Amphioxus have origin in the blastoporal ring.
In amphibians the germ ring is the all-important source of mesoderm.

During the process of gastrulation in the amphibian the material
destined to become mesoderm lies within the advancing edge of the over-
growing fold (Fig. 44) which is the chief agency in the enclosing of the
yolk. As the edge of this fold, the narrowing blastoporal rim, advances,

NP\ ,NC

Fig. 48. — Sections of an amphibian embryo at an early stage in the development
of the notochord and mesoderm. Semi-diagrammatic. A , median longitudinal section.
B, transverse section near the middle of the longitudinal axis. .4, archenteron; BP,
blastopore; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural
plate.

it (in effect) leaves behind it — "behind" being anterior because the fold
advances posteriorly — a trail of potential mesoderm which, however, is
at first in no way distinguishable from other material destined to be
permanently endoderm (Fig. 44C', EN). That is, the two materials
together and in no way delimited from one another constitute the deeper
layer of the overgrowing fold. Later this layer virtually splits (the process
called delamination) to form two layers, an inner one abutting on the
archenteric cavity and an outer one which is then recognizable as a definite
mesoderm (Fig. 485). This layer, although now distinct from the endo-
derm which parallels it, for a time retains continuity with its source, the
proliferation zone about the blastopore (Fig. 48.4). Initiated in this way,
the mesoderm extends into the lateral and anterior regions of the embryo
partly by growth within itself, partly by continued contributions from
the blastoporal growth zone and possibly augmented by the detachment
of cells from neighboring surfaces of the endoderm.

76

COMPARATIVE ANATOMY

-o

The mesoderm of Amphioxus is segmented at the time of its detach-
ment from the primary gastrular layers and some of the more anterior
segments are hollow. The amphibian mesoderm is primarily unseg-
mented and solid. In view of the fact that it later acquires segmentation

and hollowness, these initial differences are
outweighed by the essential similarity in
the relations to the blastoporal region.

In Reptiles and Birds. In reptiles and
birds the formation of an endoderm is
initiated by a small invagination or infolding
at the posterior edge of the early blastoderm
(see page 69). The abortive blastopore
thus produced exhibits the usual feature of
a blastopore in that, in terms of germ layers,
it is an indifferent region where ectoderm
and endoderm merge together without sharp
of demarcation (Fig. 45). Following gastrula-
tion the blastodermal layers continue to

-PS

Fig. 49. — Surface view
blastoderm of Chick after 15 hours
incubation. C, "anterior cres-
cent," occasioned by an irregular Spread rapidly over the surface of the yolk.
fold of underlying endoderm; M. j^ go doing, the growth posteriorly causes

region occupied by mesoderm; , .

O, area opaca whose opacity is the somewhat thickened region of the
caused by adherence of yolk to blastoporal rim to become drawn out into a

the blastoderm; P, area pellucida

long Streak, the "primitive streak," lying
in the median line of the blastoderm (Fig.
49). Along the whole extent of this modi-
fied blastoporal region the ectoderm and

endoderm merge without sharp demarcation just as they did in the

earlier blastoporal wall (Fig. 50).

This primitive streak is the primary seat of mesoderm formation.

Rapid cell proliferation within the substance of the thickened "streak"

— transparent in absence of adher-
ing yolk (see Fig. 45); PS,
primitive streak. X14. (After
Duval, Atlas d'Embryologie.)

y--^:^~^t

Fig. 50. — Section transverse to the primitive streak of a chick embryo of about 15
hours incubation. The section is taken near the middle of the length of the streak. EC,
ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak;
Y, yolk at inner margin of area opaca. Xioo. (After Duval, Atlas d'Embryologie.)

gives rise to masses of cells which move out into the space between ecto-
derm and endoderm (Fig. 50, MES). These masses of cells increase by
continued contribution from the "streak" and by growth within them-
selves and soon become arranged in a layer which rapidly grows laterally

REPRODUCTION

77

and forward from the primitive streak and always in the space between
ectoderm and endoderm. This layer, like the early mesoderm of amphib-
ians, is unsegmented and devoid of cavity.

In the sauropsidan embryo, then, as in the amphibian, rapid growth
and cell proliferation within the blastoporal rim is the primary source of
mesoderm.

Early Development in Placental Mammals

The early development of placental mammals exhibits features peculiar
to the group and more or less difficult of comparison with anything in the
development of lower vertebrates. The minute egg (Figs. 28 and S3)
contains a bare minimum of yolk. Cleavage is total, more or less unequal
and often very irregular in respect of planes and sizes of cells (Fig. 51.4).

CV-

A. B.

Fig. 51. — Early stages in development of a rabbit. A, morula stage, 47 hours after
coitus; B, early blastodermic vesicle, 80 hours; C, blastodermic vesicle at 83 hours.
The investing layers of the embryo are not shown. CV, cavity of blastodermic vesicle;
7, inner cell-mass; T, trophoblast. Magnified about 285 diameters. (After Assheton.)

The cells resulting from cleavage remain in a sohd cluster, the "morula,"
until as many as sixty or seventy cells are present. Then, as the number
increases further, a cavity appears within the morula (Fig. 51^, C).
Most of the cells remain in a solid group at one side of the cavity whose
wall elsewhere is only one cell thick. At this stage the embryo looks like a
blastula, but further development proves that the stage is not the equiva-
lent of a blastula of a lower vertebrate. The term, blastodermic vesicle,
is applied to this stage of the mammalian embryo. The definitive embryo
is developed entirely from the thick cell-mass of the vesicle. The thin
region (trophoblast. Fig. 51, T) of the wall of the vesicle becomes concerned
with the early attachment of the embryo to the wall of the uterus.

The fluid-filled cavity of the blastodermic vesicle rapidly enlarges
and meanwhile the thick cell-mass sphts off a thin layer adjoining the
cavity (Fig. 52). This inner sheet of the thick mass then extends over the
inner surface of the thin wall of the vesicle and ordinarily completely
fines it. The vesicle as a whole thereby becomes two-layered throughout,
a condition which characterizes a gastrula stage. The further history
of the two layers identifies them as embryonic ectoderm and endoderm.

78

COMPARATIVE ANATOMY

However, both in mode of origin and in further history the mammalian
embryo at this stage shows perplexing discrepancies as compared to the
gastrula of a lower vertebrate.

As stated above, the material which constitutes the definitive embryo
is within the thick and solid cell-mass (Fig. 51, /) of the early blastodermic
vesicle. As development proceeds, the behavior of this cell-mass is very
much like that of the blastoderm of the embryo of a reptile or bird. If the

cavity of the vesicle were occupied by yolk
instead of by a watery fluid the whole
embryonic complex would resemble closely
an early reptilian embryo. The thick cell-
mass, lying in relation to the vesicular

Fig. 52. — Early stage of the Fig. 53. — Embryonic area or "shield" of the

blastodermic vesicle of the Hedge- blastodermic vesicle of the Rabbit after about 172

hog. EC, ectoderm; EN, endo- hours development. PS, primitive streak; 5-5,

derm; L, lacunae, spaces occupied position of section represented in Fig, 54. (After

by maternal blood; T, trophoblast Assheton.)
(trophoderm). (After Hubrecht.)

cavity much as the reptilian blastoderm lies upon the surface of the yolk,
flattens and thins out to form the "embryonic shield" (Fig. 53) in the
axis of which appears an elongated thickening similar to the primitive
streak of a sauropsidan embryo. At the anterior end of this mammahan
streak is usually found a small pit or even a perforation extending through
the "shield" into the cavity of the blastodermic vesicle — very suggestive
of an abortive blastopore. It is along this mammalian primitive streak,
as in the similar sauropsidan structure, that rapid cell-prohferation pro-
duces a mesoderm (Fig. 54) which progressively interpolates itself
between the already separated ectoderm and endoderm and spreads
eventually into all regions of the embryo. The mesoderm is at first a
continuous layer — unsegmented — and devoid of cavity.

In a rabbit embryo the embryonic shield is established ordinarily by
the fifth day of development, the entire blastodermic vesicle then having
a diameter of about 1.5 mm.

REPRODUCTION

79

The early development of the placental mammal presents many
perplexing features. It could be expected that the minute egg, unem-
barrassed by yolk, would revert to the relatively simple and direct methods
of early development which, for the most part, characterize Amphioxus.
But it does not. Mammalian stages precisely comparable to the blastula
and gastrula of Amphioxus or amphibians cannot be recognized. When
it comes to the formation of mesoderm, the laying out of the germ layers,
and the early shaping up of the embryo, the behavior of the mammal is
closely similar to that of a reptile or bird. This similarity exists in spite
of the absence of a large yolk mass in the mammal. These facts, taken
together with the fact that certain mammals which are in most respects

lEC

@c

i£&"!SSs«l4^^

Vmes 'en

Fig. 54. — Transverse section of the embryonic shield of a Rabbit at the stage
represented in Fig. 53. The section is taken at the position indicated by the line
6^-5 in Fig. 53. EC, ectoderm; EN , endoderm; MES, mesoderm; PG, primitive groove
of primitive streak. X175. (After Assheton.)

obviously of primitive type produce large yolk-laden eggs whose mode of
development is essentially reptilian, point to what may be regarded as a
fairly safe interpretation of the conditions in placental mammals.

In connection with the attainment of viviparity the yolk content of
the mammalian egg became reduced to almost nothing. Apparently,
however, the developmental behavior of the reptilian embryo had become
so strongly established in the protoplasm of ancestral reptiles and primi-
tive mammals that it persisted even though the reduction of yolk had
removed the immediate necessity for many of its peculiarities. The
many millions of years of primitive mammalian and of reptilian lineage
constituted a barrier quite impassable by any tendency for reversion to the
indefinitely more remote developmental methods of primitive Amphioxus-
like chordates. Yet something from these primitive chordates came on
down to mammals. If, as seems fairly certain, the reptilian primitive
streak is a modified amphibian blastopore and, further, if the axial thick-
ening in the early embryonic shield of a mammal is, as seems likely, a
modified reptilian primitive streak — even to the inclusion of a blastopore
so belated and so atrophied as to have completely lost its original sig-
nificance as the aperture of a gastrular invagination — then a heritage
from primitive chordates is to be recognized in the embryo of a modern
mammal.

Unquestionably the yolk content of the chordate egg is much more
readily subject to evolutionary change than is the developmental mecha-

So COMPARATIVE ANATOMY

nism of the germinal protoplasm. That mechanism can be changed, but
there is a high degree of inertia about it. As the vertebrate egg acquired
a bulky yolk mass certain adaptive changes in developmental methods
were inevitable. But the primitive methods were so far as possible
retained even at the expense of compelling the embryo to adhere to some
processes which, under the new circumstances, are indirect and unneces-
sarily complicated — as when the sauropsidan endoderm arises by infolding
at the edge of the blastoderm rather than by some simple and direct
process such as delamination from the entire blastoderm. Again, in
connexion with the reduction of yolk volume in eggs of recent mammals
the developmental mechanism inherited from reptiles has undergone
relatively little change beyond what has been immediately forced upon
it by absence of the yolk mass and, secondly, by early attachment to the
uterine wall in connection with the estabhshing of the placenta.

The initiation of evolutionary change is evidently not within the
embryo itself. The embryo — that is, the living protoplasmic thing,
exclusive of inert yolk — is highly conservative. Its inertia is such that
it tends always to follow the old methods and it changes only as it must.

In striking contrast to this evolutionary conservatism of the embryo
is its capacity for prompt adjustment to immediate emergencies. If
one cell of the two-cell cleavage stage of Amphioxus or an amphibian is
removed or so damaged as to prevent its development, the remaining
cell may produce a complete embryo nearly or quite normal in every
particular except size. A large portion of the early" blastoderm of a fish
egg may be destroyed and yet the remaining fraction of it will develop
into a normal young fish.

Organogenesis

Appreciation of the processes whereby a relatively small spherical
body of not visibly differentiated protoplasm becomes a large animal
having the complex body-plan of a vertebrate requires a knowledge of
the early embryonic stages, and especially the origin of germ layers and
their characteristic configuration at the time when the shaping up of
special organs is about to set in. Given the embryos of vertebrates of the
several classes, the three germ layers having been fully established, the
development of any major organ proceeds with a high degree of uniformity
in all the classes. In view of these facts, the foregoing account of early
development has been somewhat detailed while the following description
of the embryonic origin of the more important organs can be relatively
brief and general. These descriptions will be supplemented, too, in the
later chapters dealing with the comparative anatomy of the several
systems of organs.

REPRODUCTION

8l

As an introduction to the account of the development of organs in
vertebrates reference to Amphioxus will be helpful. Amphioxus, partly
because it is so small and partly because it is in so many respects primitive,
affords what may be regarded as a simplified and diagrammatic view
of the early relations of organs.

Organogenesis in Amphioxus

In the preceding account of the early development of Amphioxus the
embryo has been followed to a stage where the mid-dorsal ectoderm has
become delimited from the lateral ectoderm to form the neural plate,
the mid-dorsal endoderm has given rise to a sharp thick upward fold which
is the prospective notochord, and paired mesodermal pouches are in pro-

,NP ,NC

Fig. 55. — AMPHIOXUS. Median longitudinal section of an embryo having
two mesodermal pouches, a stage approximately like that of the transverse section in
Fig. 46 E. The blastopore, roofed over by ectoderm, has become the neurenteric
canal. A, archenteron; EC, ectoderm; EN, endoderm; NC, endoderm destined to
become notochord; NE, neurenteric canal; NP, neural plate; P, neuropore. X350.
(Based on a figure by Hatschek.)

cess of formation from the dorsal endoderm either side of the notochordal
fold, the pouches increasing in number by addition of new pouches succes-
sively from before backward (Figs. 46 and 47).

In the course of further development the thickened ectodermal neural
plate becomes depressed slightly below the level of the neighboring lateral
ectoderm (Fig. 46, B-D). Along the line of demarcation between neural
plate and lateral ectoderm separation occurs following which the lateral
ectoderm extends progressively over toward the median plane and external
to the neural plate. Eventually the edges of the right and left sheets of
ectoderm meet in the median plane and coalesce to form a continuous
layer above the neural plate (Fig. 46E). Meanwhile the neural plate
transforms itself into a tube by bending its lateral regions upward and
inward until the edges meet in the median plane where they become joined
(Fig. 46 F, G). The neural plate originally extends back to the blastopore.
The overarching process whereby the neural plate is covered proceeds

82 COMPARATIVE ANATOMY

backward and around the posterior margin of the blastopore. Thus neural
plate and blastopore come to lie under a common roof of ectoderm and the
blastopore, no longer opening directly to the exterior, opens into the small
space between the neural plate and its newly acquired ectodermal roof.
The resulting relations of layers and cavities are shown in Fig. 55, a
sagittal section of an embryo at this stage. Upon conversion of the plate
into a tube, the blastopore is left in communication with the lumen of the
tube. At its anterior end the closure of the neural tube is delayed so
that for a time its lumen is open to the exterior by a small aperture, the
neuropore. The extraordinary result of these changes is an embryo
whose prospective digestive cavity, still devoid of definitive mouth and
anus, communicates via the neurenteric canal (the former blastopore)
with the hind end of the cavity of the prospective spinal cord and brain
and thence to the outside by the anterior neuropore (Fig. 55, P). These
relations, however, are merely temporary. Eventually neuropore and
neurenteric canal close. The definitive enteric apertures, mouth, gill
clefts and anus arise by very similar processes. At the appropriate
locality enteric endoderm and superficial ectoderm approach one another
and coalesce. The resulting double layer then thins out until perforation
occurs.

The notochord, whose development is initiated by an upward folding of
mid-dorsal endoderm (Fig. 46D-F), early becomes detached from the
enteric endoderm and acquires its characteristic cylindrical form. The
enteric endoderm meanwhile closes in beneath the notochord and restores
the integrity of the dorsal wall of the enteron (Fig. 46G). As the embryo
increases in length the notochord grows within itself and receives acces-
sions from the active blastoporal region with which its posterior end
remains for some time connected (see Fig. 47).

The more anterior mesodermal pouches (or somites), soon after their
formation and long before the more posterior somites have been developed,
begin to acquire their characteristic differentiation. The pouch expands,
especially ventralwards, and its cavity is correspondingly enlarged. That
part of its wall lying against the notochord becomes much thickened (Fig.
56) while elsewhere the wall remains relatively thin. The expansion of
the pouches continues until the walls of right and left pouches meet in
the median plane beneath the enteric endoderm. At this stage three
regions of the mesoderm may be distinguished: the thickened part lying
alongside the notochord; an outer thin layer contiguous with the ecto-
derm; and an inner thin layer similarly contiguous with the endoderm.
The thick part is destined to form a segment of body muscle and is there-
fore called the myotome (Fig. 56, M). The outer layer — being, in con-
junction with the ectoderm, the body-wall of the embryo — is called the
somatic or parietal layer. The inner layer, associated with the wall of the

REPRODUCTION

83

enteron, is called visceral or splanchnic. The now capacious cavity
resulting from expansion of the pouch is a segment of the embryonic body-
cavity or coelom.

The myotome rapidly thickens and also increases its dorso-ventral
extent. As it thickens, the adjacent upper portion of the coelomic space
is correspondingly reduced. Eventually the somatic and visceral layers
become joined by a horizontal septum formed just below the myotome
(Fig. 56). Consequently a lower major part
of the original coelomic space is separated
from an upper remnant of it, the myocoele
(MC), which, with continued expansion of the
myotome, is finally obliterated, while only the
lower cavity participates in forming the
definitive coelom (C). The thin portion of the
wall of the myocoele later gives rise to connec-
tive tissue including the myocomnias which
intervene between and tie together successive
muscle segments.

As a result of the general expansion of the
mesodermal layers, not only, as stated above,
are the walls of right and left pouches brought
together in the mid-ventral region, but the
adjacent walls of successive pouches on the
same side of the embryo become closely pressed
together. At this stage, then, the paired
coelomic spaces of the several pouches are
separated from one another by thin partitions,
some transverse and others median, each con-
sisting of two layers of cells. These partitions
become progressively thinner until they perforate and finally completely
disappear except that remnants of the median ventral wall may persist in
connection with the development of blood vessels. With the oblitera-
tion of these partitions, the several segmentally developed coelomic
cavities are all thrown into free communication to form one large space,
the definitive coelom, which finally shows no trace of its segmental origin.

Consistently with the fact that the mesodermal pouches arise not
simultaneously but in sequence from anterior to posterior, differentiation
begins in the most anterior or oldest pouches and proceeds backward at a
rate depending on the age of the successively formed pouches. Thus,
the myotomes of the first pair of pouches may have differentiated so far
as to show the beginning of the development of muscle tissue while as
yet not more than ten of the eventual sixty-one pairs of somites have been
definitelv formed.

Fig. 56.— AMPHIOXUS.
Transverse section midway of
the length of the body of a
larva with five gill clefts.
C, coelom; EC, ectoderm;
EN, endoderm; I, intestine;
M, myotome; MC, myocoel;
NC, notochord; AT, neural
tube; V, subintestinal vein.
(Modified from a figure by
Hatschek.)

84

COMPARATIVE ANATOMY

An emliyo of Amphioxus, at a stage when fourteen or fifteen pairs of
mesodermal pouches are present, is a delicate colorless transparent
animal having a length of about one millimeter and a diameter of one-
eighth that except at the somewhat enlarged head end (Fig. 57). It has
a straight digestive tube (enteron) /, extending from an anterior mouth to
a posterior anus. There is a single gill cleft, opening from the right side
of the anterior region of the digestive tube. The mouth also is unsym-
metrical at this stage, opening on the left side. Later, as numerous
additional gill clefts are formed, they shift their positions so as to become
ultimately a series of symmetrically placed paired apertures. Meanwhile
the mouth shifts from its original left to a median position. Just above
the digestive tube lies the median rod-like notochord {NC) extending the
entire length of the animal. Immediately above the notochord is the

Fig. 57. — Amphio.xus at beginning of larval period; 14 or 15 pairs of mesodermal
somites. Actual length of larva about i.o mm. CG, club-shaped gland; /, intestine;
MES, mesodermal somites; NC, notochord; NE, neurenteric canal; NP, neuropore;
NT, neural tube; P, pigment spot in neural tube. (After Hatschek.)

neural tube (NT), its somewhat enlarged anterior region suggesting a brain.
At the anterior end of the neural tube the dorsal neuropore (NP) is still
open. The neurenteric canal (NE), at this stage, has ordinarily become
closed. In the anterior region, where the differentiation of the mesoderm
is most advanced, a coelom intervenes between the enteric tube and the
outer body- wall (Fig. 56, C). The body wall (somatopleure) consists of
the ectoderm and the somatic layer of mesoderm. The enteric endoderm
together with the contiguous visceral or splanchnic layer of mesoderm
constitute the wall (splanchnopleure) of the digestive tube. The somatic
and visceral sheets of mesoderm provide the coelom with a continuous
and complete lining, the peritoneum. The superficial ectoderm is a skin.
The more anterior myotomes contain partially differentiated muscle tissue
capable of feeble contraction. The animal is free-swimming but the
locomotor mechanism consists merely of long cilia produced by the ecto-
dermal layer.

In its main features this young Amphioxus is like a vertebrate. If
its true origin and nature were not known, it might reasonably be expected
to proceed to develop directly into a typical vertebrate. But it does not.
It acquires no vertebral column; the notochord serves as definitive axial

REPRODUCTION 85

skeleton. It develops no structures morphologically similar to the heart,
kidneys, specialized sense organs, or paired appendages of a vertebrate.
Further, in later development it acquires, especially in the head region,
a variety of unique structures which adapt the adult to its peculiar mode
of Uving but make it conspicuously unlike any adult vertebrate. Never-
theless Amphioxus is "vertebrate" in too many features to make it
credible that they could have arisen otherwise than in genetic relationship
with those of the vertebrates. Herein, then, lies in part the justification
for describing the early development of Amphioxus to illustrate the main
features of the corresponding stages of vertebrates. Further justification
is derived, as already stated, from the fact that the paucity of yolk in the
egg of Amphioxus relieves the embryo of the factor which introduces
varying degrees of complication into the development of vertebrates and
occasions much difficulty and obscurity in the study and interpretation of
the processes.

Organogenesis in the Vertebrates

In the late embryo of Amphioxus the main lines of the body-plan of a
vertebrate are drawn. There are, however, some noteworthy differences
in the mode of origin of corresponding organs of Amphioxus and
vertebrates.

Neural Tube. In Amphioxus the neural plate becomes detached from
the adjacent lateral ectoderm (Fig. 46) and transforms itself into a
tube not until after it has been covered by the lateral ectoderm. In
vertebrates a longitudinal folding of the neural plate and adjoining ecto-
derm occurs in such a way that the movement of the neural material
into a deep position, its conversion into a tube, and the covering of it by
lateral ectoderm take place simultaneously (Fig. 58). Not until the
tubular form is attained does the neural ectoderm of vertebrates become
detached from the overlying superficial ectoderm. Figure 59 shows,
in diagrammatic way, the characteristic appearance of a recently formed
neural tube with its neural crests, dorsolateral extensions of ectodermal
material on each side of the tube. Later the neural crest becomes detached
from the tube, undergoes segmentation corresponding to that of the
myotomes, and gives rise to spinal ganglia. Cells of the crest become
ganglion cells whence grow out nerve fibers which constitute the dorsal
sensory root of a spinal nerve. The fibers of the other constituent root of
a spinal nerve, the ventral motor root, grow out from cells within the
neural tube. Some cells of the neural crests migrate into various visceral
localities and give rise to ganglia and nerves of the autonomic system.

The anterior region of the tube expands to form the brain. Three
enlargements, one behind another, the primary brain vesicles — fore-brain,
mid-brain and hind brain (Figs. 61, 63, 85) — characterize the cephalic

86

COMPARATIVE ANATOMY

-NP-^_,

part of the tube in all vertebrate embryos. Later subdivision of the first
and third vesicles results in the live brain regions universally characteristic
of adult vertebrates. The nervous structures (retina and optic nerve)
of the paired eye grow out from the second (numbered from in front)

region but the lens of the eye is
derived from neighboring super-
ficial ectoderm (Fig. 60). The
receptor (that is, stimulus-receiv-
ing) nervous structures of the ear
and olfactory organ originate not
from the neural tube but from
superficial ectoderm.

Notochord. In contrast to the
quite definite manner of origin of
the notochord in Amphioxus, there
is considerable obscurity as to the
precise nature of the process in
many vertebrates and it must be
admitted that the process differs
markedly in the several classes.
In amphibians the notochord is in
part and to greater or less extent
folded off or split off from the mid-
dorsal endoderm and in part built
forward from the actively growing
blastoporal region. In amniotes
the origin of the notochord is closely
related to that of the mesoderm —
so much so that the notochord has
sometimes been described as of
mesodermal nature. Its material,
like that of the mesoderm, usually
seems to be derived from the
primitive streak (see pages 76, 78),

As

V

Fig. 58. — Diagrams illustrating method
of origin of the neural tube of vertebrates.
Transverse sections in the mid-trunk region
of embryos at successively (A to D) later
stages. C, neural crest; CC, canalis
centralis of neural tube; EC, ectoderm;
EN, endoderm; MES, mesoderm; NC,

notochord; NG, neural groove; NP, neural a region where ectoderm and endo

plate; NT, neural tube; F, blood vessel j . ,. . • u ui

(paired dorsal aorta). derm merge mdistmguishably

cells proliferated from the streak
laterally give rise to mesoderm, so proliferation forward from the
anterior end of the streak produces a median cord of cells which form
the notochord. It may, however, receive accessions from the endoderm
with which it is usually in close relation.

If it be granted that Amphioxus is a near relative of immediate ances-
tors of vertebrates, the mode of origin of its notochord does much to

REPRODUCTION

87

relieve the perplexity occasioned by the variability in the development

of the organ in vertebrates. In Amphioxus the anterior part of the

notochord is folded up from the endo-

derm but a considerable posterior extent

of it is built on by proliferation at

the blastoporal region. The primitive

streak of the amniote embryo is open to

interpretation as being a modified

blastoporal region. The streak is then

the equivalent of the blastoporal rim,

both actively growing regions. If all

this be granted, then the lower verte-

(From Kingsley, Comparative Anat-
omy of Vertebrates.)

Fig. 59. — Stereogram of embryonic
neural tube showing the segmenting
neural crest, e, superficial ectoderm;
brate (amphibian) retains more of the nc, neural crest; s, central canal.

Amphioxus method of forming noto-
chord while in higher vertebrates there
is less of direct and obvious origin from endoderm, and proliferation
from the blastoporal streak plays the main role — all of which seems

quite reasonable.

The Enteron. Gastrulation produces
a two-layered embryo whose endoderm
surrounds a cavity opening to the exterior
by the blastopore. This archenteric
cavity is the prospective digestive cavity.
As the embryo elongates, the cavity is
correspondingly elongated and in later
development the enteric tube increases in
length faster than the embryo with result'
that the tube becomes bent or even coiled
to adapt itself to the coelomic space.

In the early embryo the ectoderm at a
median antero-ventral position gives rise
to a shallow depression or pit, the
stomodeum, whose deeper wall meets the
forward-growing endoderm to form tem-
porarily a two-layered oral membrane
separating the external stomodeal cavity
from the enteric cavity. Soon a perfora-
tion appears at the center of the mem-
brane (Fig. 61, 0) and its peripheral
remnant is rapidly obliterated. The
perforation and obliteration of the membrane apparently result from pro-
gressive centrifugal flow or movement of its cellular substance. Thus
is formed the mouth. The posterior enteric aperture or embryonic

Fig. 60.- — Stereogram of the
developing eye. The head of
the embryo is cut transversely
in the region of the fore-brain.
cf, choroid fissure; fb, wall of
fore-brain; /, ectodermal thicken-
ing which invaginates to form
lens; oc, optic cup; os, optic stalk;
p, outer thin wall of optic cup,
becoraing the pigmented epithe-
lium which lies behind the defini-
tive retina; r, inner thick wall of
optic cup, becoming the sensory
retina of the eye. (From Kings-
ley, Comparative Anatomy of
Vertebrates.)

66 COMPARATIVE ANATOMY

"anus" develops usually by a similar process. The blastopore rarely
persists as a definitive posterior aperture although it does so in cyclostomes
and possibly in some urodele amphibians. Otherwise, exactly as in
Amphioxus, it becomes roofed over by the neural folds and thus converted
temporarily into a neurenteric canal (Fig. 6i) connecting the hind ends
of neural tube and enteric cavity. In reptiles and birds the formation of
an actual neurenteric passage is more or less obscured by the presence

NT NC EN

Pig. 6i.- — Frog: median longitudinal sections of embryos; A, just before conversion
of blastopore into neurenteric canal; B, just after formation of neurenteric canal and
perforation of proctodeum to form cloacal aperture. B, brain; BP, blastopore; C,
cloacal aperture; EC, ectoderm; EN, endoderm; H, hypophysis; HT, heart; MES,
mesoderm; NC, notochord; NE, neurenteric canal; NT, neural tube; O, region where
mouth will perforate; P, proctodeum; PH, pharynx; R, rectal region of enteron; Y,
yolk cells of endoderm; i, fore-brain; 2, mid-brain; 3, hind-brain. A, X24; B, X19.
(Redrawn from Marshall, Vertebrate Embryology.)

of the massive yolk but definite traces of the canal may be recognized.
An ectodermal pit, the proctodeum, situated just below the neurenteric
canal, perforates into the hind end of the enteric cavity to form the
definitive hind aperture, either anal or cloacal (Fig. 61). As result of
the mode of development of the enteric apertures, the lining of more or
less of the mouth cavity is derived from stomodeal ectoderm and that of
the posterior region from proctodeal ectoderm. The remaining and
by far greater part of the adult enteric tube is lined by endoderm which

REPRODUCTION 89

constitutes the essential secreting and absorbing layer of the tube, the
digestive epithelium.

It is a noteworthy fact that various organs which have nothing directly
to do with digestion have their origin in the enteric endoderm. The
anterior region of the embryonic enteron — the part becoming the pharynx
of the adult — is concerned particularly with the organs of respiration.
Gills of fishes and amphibians develop in relation to paired apertures,
the pharyngeal or visceral clefts, which pierce the lateral walls of the
enteron and the ectoderm and open to the exterior. A pharyngeal cleft
is developed as follows. A deep lateral pouch or furrow of the endoderm
bulges outward and meets a similar but shallower pouch or furrow which
the ectoderm pushes inward. The resulting two-layered membrane is
then obliterated by the same process which removes the oral membrane,
leaving a free passage between the pharynx cavity and the exterior.
Vascular complications of the endodermal lining of these clefts produce
internal gills — although it is possible that some so-called internal gills
are derived from ingrowing ectoderm. External gills are ectodermal
structures developed in close relation to the external apertures of pharyn-
geal clefts.

The number of pairs of pharyngeal pouches varies from a maximum
of fourteen in some cyclostpmes to, in birds and mammals, four or five
which are rudimentary in the sense that they exist only temporarily in the
embryo and even then may not perforate to the exterior. The clefts of
the first pair do not ordinarily produce functional gills in fishes but
become the spiracles of sharks and some ganoids. In all amniotes the
pharyngeal pouches are merely temporary embryonic features except
as those of the first pair are, in a modified way, represented in the auditory
passages.

Lungs develop by formation of a mid-ventral pouch in the floor of the
pharynx. As the pouch enlarges, it bifurcates forming right and left
lungs. The entire epithelial lining, being the essential respiratory mem-
brane, of the adult lung is endodermal and continuous, by way of the
lining of bronchi and trachea, with the lining of the digestive tube. In
birds the respiratory cavities are continued beyond the lungs into a system
of air sacs, branches of which extend into various parts of the body
including even the cavities of some of the bones. It seems an extra-
ordinary fact that a bird's humerus — less often the femur — may contain
an air tube whose wall is derived from and is still continuous with the
enteric endoderm.

The air-bladders (swim-bladders) of fishes are endodermal sacs
which grow out from an anterior region of the embryonic enteron.
They are usually dorsal, rarely lateral, or ventral as in the ganoid
Polypterus.

QO COMPARATIVE ANATOMY

The important endocrine glands, thyroid, parathyroid and thymus,
and various gland-Uke bodies mostly of dubious nature and function,
arise as outgrowths of the endoderm of the pharyngeal pouches or the wall
of the pharynx.

More posterior regions of the enteric endoderm give rise to various
accessory digestive organs, most important of which are the liver and
pancreas. The liver, uniformly in all vertebrates, develops as a mid-
ventral outgrowth, or sometimes more than one, from the anterior region
of the prospective intestinal portion of the enteron. Vascular and con-
nective tissues make up a large part of the bulk of the adult liver but the
essential liver substance — the hepatic cells which carry on the specific
functions of the liver — are endodermal. The position of the opening of the
bile duct into the intestine marks the point of origin of the embryonic liver.

The pancreas and liver arise in close relation to one another, but the
pancreas commonly has more than one point of outgrowth from the
enteron. The secretory tissue of the pancreas is endodermal.

The cloaca of the adult vertebrate is a superficial chamber situated
at the hind end of the body cavity and opening ventrally to the exterior.
Into it open the intestine and the ducts of the kidneys and genital organs.
It is commonly present in vertebrates below mammals except in Teleostei.
Mammalian embryos develop a cloaca but only those primitive mammals,
Ornithorhynchus and Echidna, retain it in the adult. In other mammals
the embryonic cloaca becomes subdivided into a dorsal part connected
with the intestine and a ventral part which receives the urinogenital ducts.
In course of further development these two divisions of the cloaca are
separated and carried apart and acquire independent openings to the
exterior so that eventually the anus and urinogenital aperture are far
removed from one another, the latter being the more ventral in position.
Therefore the more distal portion of the urinogenital passage of the adult,
both male and female, is a remnant of the cloaca while another remnant
of it persists in the posterior region of the rectum.

In the great majority of vertebrates the cloaca is derived from the
extreme hind end of the embryonic enteron. As the kidneys and gonads
develop, their ducts acquire connexion into this part of the enteron.
It becomes more or less enlarged in diameter as compared to the intestinal
region of the enteric tube. The proctodeal perforation provides it with
an exit to the exterior and it becomes the definitive cloacal chamber.
It has no digestive function other than serving as an avenue for passage of
digestive waste as well as the renal and genital products. This cloaca is
lined with endoderm. But the cloaca of the frog has been described as
developing from the proctodeum, an ectodermal invagination. Such a
cloaca would be lined by ectoderm. It is therefore possible that cloacal
chambers are not all precisely homologous.

REPRODUCTION

91

If "anus" is to be defined as the posterior aperture of the digestive
tube — digestive in strict sense — then the term should not be applied to
the external opening of the cloaca but rather to the internal aperture
between rectum and cloaca. By this definition the proctodeal aperture
would ordinarily become the external cloacal opening but in some cases
would become an external anus (Teleostei) or an internal anus (frog).
If the posterior extremity of the adult mammalian digestive tube includes
a remnant of the embryonic cloaca, then the mammalian "anus" is in
reality a remnant of an external cloacal aperture. There is much con-
fusion and doubt about homologies in this region of the vertebrate and,
among authors, there is corresponding confusion of terms.

The Mesoderm. In contrast to the early condition of the mesoderm
in Amphioxus, the vertebrate mesoderm is at first devoid of segmentation

Fig. 62. — Transverse sections of embryos of the amphibian Ambystoma; A, earlier
stage; B, later and more posterior than A. a, archenteron; e, otic pit; en, endoderm;
m, mesoderm; n, notochord; nc, neural crest; ng, neural groove; np, neural plate; p,
primitive groove. (From Kingsley, Comparative Anatomy of Vertebrates.)

and ordinarily contains no definite cavity. At an early stage it appears
as a somewhat compactly arranged but indefinitely delimited sheet of
cells rapidly spreading from its region of origin into the space between
ectoderm and endoderm. The layer becomes thickest dorsally alongside
the neural tube and notochord (Fig. 62). At an early embryonic stage the
mesoderm upon either side spHts into two layers; an outer, lying against
the ectoderm, and an inner lying against the endoderm. The two layers
remain connected, however, at the upper edge of the original sheet (Figs.
63, 64A). At about the same time the dorsal and thicker part of the
mesoderm develops transverse fissures which divide it into a series of
paired blocks (somites) lying symmetrically either side of the neural tube
(Fig. 63). This segmentation begins in the anterior part of the embryo
and progresses backwards just as, in Amphioxus, the mesodermal pouches
are formed successively from anterior to posterior. Figure 64B shows a
diagrammatic section of an amphibian embryo at this stage. Figure 65

92

COMPARATIVE ANATOMY

represents a chick embryo after about t,:^, hours incubation. The meso-
dermal somites, easily seen through the very thin ectoderm, appear as
pairs of denser or darker squarish blocks.

Fig. 63.- — Stereogram of the anterior region of a vertebrate embryo showing the
segmentation of the mesoderm. The ectoderm has been removed from the left side
of the embryo, al, endoderm of aUmentary tube; c, coelom; em, epimere;/6, fore-brain;
hb, hind-brain; hm, hypomere; m, myotome; mb, mid-brain; mm, mesomere; n, neural
tube; nc, notochord; s, stomodeal region; sk, sclerotome; so, sp, somatic and splanchnic
walls of coelom. (From Kingsley.)

The process of segmentation involves only the upper part of the
mesoderm. As segmentation goes on, the space between the two thin
and unsegmented layers becomes wider — a space already recognizable

Fig. 64. — Diagrams showing embryonic origin of pronephric tubules. A, earlier
stage; B, later, c, coelom; d, pronephric tubule and duct; e, epimere; h, hypomere;
w, mesomere (cross-lined); my, myotome; «, nephrostome; so, somatic layer of hypo-
mere; sp, visceral (splanchnic) layer of hypomere. (From Kingsley, after Felix.)

as the coelom bounded externally by a somatopleure consisting of ecto-
derm and the outer sheet of mesoderm, and internally by a splanch-
nopleure consisting of endoderm and the adjacent layer of mesoderm.
The mesodermal layers upon either side grow down to the mid-ventral

REPRODUCTION

93

region, carrying with them the coelom, and meet mid-ventrally to form a
double vertical layer, a ventral mesentery, extending from the enteron

prosencephalon

proamnion

anterior neuropore

optic vesicle

■ mesencephalon
border of fore-gut

omphalomesenteric vein

rbombencephalon

lateral mesodean

notochord

sinus rhomboidalis primitive streak

Pig. 65. — Dorsal view of an entire chick embryo having 12 pairs of mesodermal
somites; after about :is hours incubation. X14. (From Patten, Embryology of the
Chick.)

to the outer body wall and separating right and left coelomic cavities
(Fig. 63).

The splitting of the original sheet of mesoderm extends so far dorsally
as to involve the somite which accordingly contains a more or less definite
cavity, the myocoele — "myo" because the somite is mainlv muscle-

94

COMPARATIVE ANATOMY

forming. Shortly the somites become detached from the lower somatic
and visceral sheets of mesoderm and the myocoeles lose continuity with
the permanent coelom (Fig. 64B) . Eventually, as the somite differentiates,
the myocoele is obliterated.

The differentiation of the mesoderm at the stage just described —
dorsally the pairs of quite detached hollow somites; ventrally the unseg-
mented somatic and visceral layers enclosing between them the coelom —
has its exact counterpart in Amphioxus. But in Amphioxus segmentation

and hollowness of the mesoderm are
primary and continuity of the coelomic
space is secondarily acquired. In verte-
brates segmentation is secondary and so
isp also is hollowness except that in cyclo-
stomes and some urodele amphibians the
early mesoderm appears to have folded
outward from the dorsal wall of the
enteron and exhibits a thin cavity which
Fig. 66. — Stereogram of the may be continuous with the enteric cavity

anterior region of an amphibian .p. ^^^ ^j^j^ exceptional Condition,

(urodele) embryo, a, anterior end; \ b -> f

ar, archenteroh; c, coelom; ch, noto- although it involves uo primary segmen-

chordal cells; e, yolk cells; ec,ecto- . ^^ ^g ^he mode of Origin of the

derm; mp, mesodermal pouch; ng, . .

primitive groove; np, neural plate; mesoderm in AmphioxuS.

nr, neural folds; sc, segmentation j • f ^^er history, however, the

cavity (blastocoele) ; so, somatic wall -' '

of coelom; 5;^, splanchnic (visceral) differentiation of the vertebrate mesoderm
wall of coelom. (From Kingsiey ) .^ ^^^^ elaborate than that of Amphioxus,

especially in the prospective trunk region. Here a region of mesoderm
intermediate between the upper somite or muscle-forming part and the
lower peritoneum-forming layers becomes early designated to play a very
important role, the formation of the essential structures of the kidneys.
In the trunk mesoderm, then, there are upon each side three zones of
differentiation: the epimere, a dorsal muscle-forming part; the mesomere,
a kidney-forming zone situated just below the epimere; and the hypomere,
the most ventral zone, constituting the somatic and visceral layers of
peritoneum (Figs. 63 and 64^).

The epimere undergoes three kinds of differentiation. Its heavier
inner wall is mainly converted into striated body-muscle. The embryonic
myotome gives rise not only to the dorsal but to the ventral body-muscle.
The myotome material grows ventral-wards, pushing its way between
the ectoderm and the somatic mesoderm, until it reaches the mid-ventral
plane (compare Figs. 67 and 68). From the medial surface of the inner
or myotomic wall of the epimere numerous cells become detached and
give rise to loosely aggregated cellular masses surrounding the notochord
and neural tube (Fig. 69). Most of this material enters into the forma-

REPRODUCTION 95

tion of the supporting structures — connective tissue, cartilage and bone —

Fig. 67. — Diagrammatic transverse section of the body of a vertebrate embryo at an
advanced stage. The muscle-forming myotome is beginning to extend into the ventral
body-wall of the embryo, c, coelotn; g, genital ridge; m, muscle derived from myotome;
mc, myocoele; p, peritoneum; pd, pronephric duct; so, somatic layer of somite; v, advanc-
ing ventral border of myotome; the finely dotted areas are occupied by mesenchyme.
(From Kingsley.)

which are eventually developed around these two axial organs. The
outer or somatic wall of the somite
breaks up to form loose cellular
masses which spread out under-
neath the ectoderm. As the ecto-
derm differentiates into the
definitive epidermis of the skin, the
underlying mesodermal material
forms the dermis (or coritim), the
deeper fibrous and vascular layer
of the skin. As the muscle-form-
ing material of the somite is called
the myotome, so the medial
skeleton-forming region, or the
cell-mass derived from it, is con-
veniently called the sclerotome, and
the somatic dermis-forming layer
is called the dermatome (Figs. 63
and 67).

The mesomere in lower verte-
brates undergoes a segmentation
corresponding to that of the
epimeres (Fig. 63), but in amniotes
segmentation of the mesomeres is obscured or lacking. In anamnia the

Fig. 68. — Diagrammatic transverse
section of the body of a vertebrate.
av, aorta; c, coelom; e, ectoderm; ep,
epaxial (dorsal) muscle; g, gonad; ha,
hemal rib; hp, hypaxial (ventral) muscle;
i, intestine; mes, mesentery; n, nephrid-
ium; o, omentum; r, rib; p, somatopleure;
sp, splanchnopleure; v, centrum of verte-
bra and, above it, neural arch containing
spinal cord. (From Kingsley.)

96

COMPARATIVE ANATOMY

mesomeres become transformed into kidney tubules. The process begins
in the more anterior mesomeres and progresses posteriorly. Certain
differences in mode of development and in eventual structure compel the
distinction between an earlier and more anterior system of tubules, the
pronephros, and a later and more posterior system, the mesonephros. In
Anamnia the mesonephros becomes the adult kidney and the pronephros
disappears except that in a few fishes it is the definitive and only kidney.
In amniotes, following development of a pronephros and a mesonephros,

the tubule-forming process continues
backward, but with some modifi-
cations and compKcations, to form
a third kidney, the metanephros,
which becomes the adult kidney.
The tubular epididymis, associated
with the testis of the adult amniote,
is a part of the embryonic mesone-
phros which otherwise disappears
except for certain vestiges which are
apparently of little functional
importance.

In Anamnia each mesomere at
first forms a single renal tubule. In
what is conceived to be the more
primitive method of development of
the tubules, the cavity is the original
cavity of the mesomere and is there-
fore a part of the embryonic coelom
and continuous with the coelomic
space of the hypomere. In amniotes,
however, the mesomere material may be not only unsegmented but
devoid of any original cavity. In such case the tubule shapes up as a
solid cord, later hollowing out.

Meanwhile, as the pronephric tubules form, the mesomere material
on each side of the embryo gives rise to a longitudinal tube (Fig. 64) which
extends backward from the pronephric region to the wall of the cloaca into
which the cavity of the duct finally opens. The pronephric tubules of
each side are joined to the corresponding pronephric duct (Fig. 70) thus
putting the coelom into communication with the exterior by way of the
cloaca. The coelomic openings or nephrostomes (Figs. 64B, n, and
70, 7zs) of the pronephros are ciliated. The arrangement apparently
serves for drainage from the coelom to the exterior.

The mesonephric tubules develop in many more segments than do the
pronephric (Figs. 70 and 71). They acquire connexion with the already-

PiG. 60. — Transverse section of the
embryo of a lizard (Lacerta muralis)
having twenty-eight pairs of mesodermal
somites; cut through the tenth pair;
much enlarged, a, dorsal aorta; m,
myotome — muscle-forming part of meso-
dermal somite; n, notochord; o, omphalo-
mesenteric vein; sc, neural tube (spinal
cord) ; sk, mesenchyme derived from
sclerotomic part of sorr;ite. (From
Kingsley.)

REPRODUCTION

97

SPINAL CORD

SPINAL
GANGLION

NEPHROSTOME

GLOMERULUS j
POSTCARDINAL VEIN

,^- ' PERITONEUM
MESENTERY PRONEPHRIC TUBULES

I MESONEPHRIC TUBULES

PRIMITIVE DUCT

Fig. 70. — Stereogram of the developing pronephros and mesonephros. (After Kingsley

modified.)

MESONEPHRIC TUBULE

MYOTOME
DERMATOME

MESONEPHRIC TUBULE^
hJEURAL TUBE \

fHORDA /;

AORTA

GLOMERULUS NEPHROSTOME I | GENITALRIDGE | PRIMITIVE DUCT

PERITONEUM POSTCARDINAL VEIN MESENTERY COELOM

Fig. 7 1 . — Stereogram of the developing mesonephros at a stage later than that of Fig. 70.
(After Kingsley modified.)

98

COMPARATIVE ANATOMY

formed longitudinal duct which, as the pronephros degenerates, then
serves at least in part as the mesonephric or "Wolffian duct. In Anamnia
each mesonephric tubule has a ciliated nephrostome opening into the
coelom (Fig. 72). Near the nephrostome the tubule gives rise to a cup-
shaped expansion (Bowman's capsule). The hollow of the cup is occa-
sioned by ingrowth of a dense network of fine blood vessels, the glomerulus.
The capsule and glomerulus together constitute a renal (or Malpighian)
body or corpuscle. The part of the tubule between the renal body and
the mesonephric duct eventually becomes much elongated, coiled and
locally differentiated. At first there is but one pair of tubules per seg-
ment. Later additional tubules are ordinarily formed in each segment
by a process of branching from the primary tubules.

Fig. 72. — Diagram of renal (Malpighian) corpuscle, a, artery; b. Bowman's
capsule; gl, glomerulus; n, nephrostome; t, nephridial tubule; v, vein. (From Kingsley,
Comparative Anatomy of Vertebrates.)

In amniote vertebrates, in which the mesomere material ("inter-
mediate mesoderm") is not segmented and not hollow, the pronephric
and mesonephric tubules ordinarily do not possess nephrostomes or, at
most, exhibit merely vestiges of them.

The amniote metanephros has outlet by way of a duct, the ureter,
which develops as a forward-growing branch from the cloaca! end of the
mesonephric duct of the same side of the embryo. The tubular structures
of the metanephros are formed partly from mesomere material lying
posterior to the mesonephros and largely by outgrowth from the anterior
end of the ureter. These tubules produce renal corpuscles similar to
those of the mesonephros but do not possess nephrostomes. In the
absence of nephrostomes drainage of waste from the coelom does not
occur and the function of excretion must be confined to the renal cor-
puscle, where the glomerulus brings blood vessels into close relation to
the lumen of a kidney tubule, and to other vascular regions of the tubule.

The adult kidney consists of the entire system of tubules — meso-
nephric or metanephric — of one side of the embryo, increased to great
number by the branching process, each tubule tremendously elongated
and much coiled, the tubules bound together by connective tissue with

REPRODUCTION 99

blood vessels richly interspersed, and the whole complex ensheathed by
connective tissue and thereby dehmited from adjacent tissues of the
body-wall.

The hypomere mesoderm, later backed up by a layer of connective
tissue, becomes the definitive peritoneum. Its somatic layer completely
lines the body-wall. Its visceral layer covers the coelomic surfaces of the
digestive tube, of such derivatives of the tube as the liver and pancreas,
and of all other organs which occupy the coelom. As the originally
bilateral coelomic spaces expand to accommodate the developing viscera,
the right and left visceral (splanchnic) layers of the hypomere meet one
another in the median plane at all regions not occupied by median organs
(Fig. 63). Here the two layers coalesce and, later reinforced by formation
of connective tissue between them, become the system of membranes or
mesenteries which connect and support the viscera. These median mes-
enteries are usually much more extensive in the embryo than in the adult.
In later stages of development they undergo considerable reduction, espe-
cially those between the digestive tube and the ventral body wall (Fig. 67).

The mesenteries not only support the major visceral organs but they
are important as bridges across which nerves and blood vessels pass from
the body wall to organs which are suspended within the coelom, or they
may support ducts which traverse coelomic space in passing from one
organ to another — the bile duct, for example. In the earHer embryo
the establishing of necessary relations of nerves, blood vessels and ducts
to their appropriate organs is facilitated by the extensive system of
mesenteric bridges. After proper connexions have been made, consider-
able regions of the mesenteries may be obliterated. A blood vessel which
may appear to extend across coelomic space freely and without support is
always encased in peritoneum which was left surrounding the vessel after
the original supporting mesentery degenerated. The mesenteries of the
early embryo to some extent serve as scaffolding, necessary during the
period of construction, but removed after the work is finished.

Many complications arise in consequence of the secondary elongation,
bending and twisting of the digestive tube. For example, at a given
transverse section of the animal, the digestive tube may be cut two or more
times (Figs. 67 and 68), one section of it lying above or alongside another
and mesentery extending between adjacent regions. Lateral branches of
the median mesentery may be developed in relation to lateral organs such
as gonads.

Consideration of the relations of the peritoneum to the viscera makes
it clear (Fig. 73) that no organ can be said to lie in the coelom except
as the peritoneum investing that organ is regarded as a part of the organ.
In strict morphological sense, no organ lies in the coelom. The essential
organ — that is, the structure constituted of tissues specialized for a

lOO

COMPARATIVE ANATOMY

particular function such as digestion or secretion — lies between the
peritoneal sheets of the right and left halves of the body. The peri-
toneum is not so much the property of the organ which it invests as it is
specifically the immediate wall or lining of the coelom.

The peritoneum plays a part in the development of the gonads
although it is not necessarily the source of the germ cells. At the earliest

MVJ

Fig. 73. — Diagrammatic transverse section of the body of a vertebrate showing
relations of organs to the peritoneum and coelom. .4, dorsal aorta; C, coelom; EN,
endodermal epithelium of digestive tube; G, gonad; /, integument; K, kidney; L, liver;
M, muscle layer of digestive tube; MD, dorsal muscle of body-wall; MV, ventral muscle
of body- wall; NC, position of embryonic notochord; NT, neural tube (spinal cord);
PP, parietal peritoneum; PV, visceral peritoneum; R, rib; VC, vertebral column.

stage when they are easily recognizable, the prospective gonads appear as
longitudinal thickenings or genital ridges in the dorsal peritoneum, one
on each side and between the dorsal mesentery and the mesonephros
(Figs. 67, 68 and 74). The earlier belief that the germ cells are derived
from the peritoneal layer has been shaken by evidence that they are of
endodermal origin. The deeper substance of the definitive gonad is
derived either from the thickened peritoneum of the genital ridge or,
especially in the male, from the mesoderm of the closely adjacent meso-
nephros (Fig. 75). It has been claimed, however, that the earliest recog-
nizable primordial germ cells, in embryos of various vertebrates, lie in the
mid-dorsal enteric endoderm. From this location they are said to migrate

REPRODUCTION lOI

upward between the two peritoneal layers of the dorsal mesentery and

Fig. 74. — Section of genital ridge of a chick of five days incubation, e, peritoneal
epithelium of ridge; c, genital cords; o, primordial germ cells. (From Kingsley, after
Semon.)

then laterally outward to enter the genital ridge. As development pro-
gresses, the genital ridges bulge more and more
into the coelom and eventually the substance of
the gonad becomes entirely detached from the
body-wall except as the peritoneal investment
of the gonad remains in continuity with the
parietal peritoneum forming a two-layered
supporting membrane, the mesovarium for the
ovary and the mesorchimn for the testis.

The gonads find outlet by way of ducts
which arise in relation to the kidneys. The
seminiferous tubules of the testis acquire connex-
ion with the neighboring mesonephric tubules
and thereby gain exit by way of the Wolffian
duct which therefore, in adult vertebrates
which retain the mesonephros, serves as a
urinogenital duct. In amniotes the adult male
retains, in the epididymis, that part of the
embryonic mesonephros which provided connex-
ion between the testis and the Wolffian duct.
With metanephros and ureter serving the
urinary function, the Wolffian duct is left as a Waideyer.)
vas deferens or sperm duct only.

The oviducts of Anamnia and Amniota are perhaps not exactly homo-
logous. In elasmobranchs and probably some amphibians the pronephric

Fig. 75. — Transverse sec-
tion through the urinogeni-
tal region of a four-day
chick embryo. g, mesoder-
mal epithelium (peritoneum)
of genital ridge; m, infolding
of peritoneum to form Miil-
lerian duct; ms, mesentery; s,
mesenchyme cells which give
rise to the stroma (non-geni-
tal tissue) of gonad; t, meso-
nephric tubules; W , Wolffian
duct. (From Kingsley, after

I02 COMPARATIVE ANATOMY

duct splits longitudinally, one portion of it serving thereafter as the
Wolffian duct in connexion with the mesonephros while the other portion
acquires, by fusion of several pronephric nephrostomes, a wide anterior
opening into the coelom in the vicinity of the ovary. The oviduct
(Miillerian duct) thus formed has no connection with the kidney in the
adult. In other vertebrates, however, the oviduct develops as a fold of
peritoneum (Fig. 75, w) closely parallel to the Wolffian duct but inde-
pendent of it. The Miillerian duct, functional only in the female, develops
alike for a time in both male and female embryos but only vestiges of it
persist in the adult male.

The Mesenchyme. Reference has been made (pages 94, 95) to the
fact that certain regions of the mesodermal somite, the sclerotome and the
dermatome, are the source of cellular material which becomes detached
from the somite and, becoming eventually more or less densely aggregated
in the spaces between the somite and neighboring organs or layers, plays
an important part in producing skeletal, connective and integumentary
tissues. This secondary mesoderm ("derm" implying a sheet or layer),
being usually not disposed in definite layers, is called mesenchyme.
But the somite is not the only source of mesenchyme. Quantities of it
are produced in all regions of the embryo and from various layers other
than the somites.

Beyond question, most of the mesenchyme comes from the mesoderm.
The parietal and visceral layers of the hypomere are a prohfic source of it,
numerous cells becoming detached from the outer (next the ectoderm)
surface of the parietal layer and from the inner (next the endoderm)
surface of the visceral layer. Also the endoderm contributes to the mass
of mesenchyme which accumulates between the enteric wall and the
adjacent layers of mesoderm. In fact, any mesodermal surface which
faces toward endoderm or ectoderm and any endodermal surface which
faces toward mesoderm is potentially a seat of mesenchyme formation.
The ectoderm plays a minor part but, especially in the anterior region of
the embryo, cells may be detached from the inner surface of the ectoderm.
Evidence has been found indicating that mesenchyme of ectodermal origin
participates in the development of the skeleton of the pharyngeal region.

But no mesenchyme is produced at the coelomic surface of the meso-
derm or at the inner surface of the enteric endoderm. Mesenchyme in
the coelom or in the enteric cavity could have no constructive significance.

Mesenchyme spreads from its place of origin and eventually is found
in all parts of the embryo. Although late in origin, the importance of the
part which it plays is by no means secondary. Chief among its deriva-
tives are the following materials and structures.

Comiective Tissue. Fibrous connective tissue is omnipresent in the
adult vertebrate. It invests, supports, connects, separates or cushions

REPRODUCTION IO3

parts of the body. Its microscopically delicate ramifications penetrate
throughout such massive structures as a liver or kidney, binding together
the exceedingly tenuous tubules of those organs and serving as a basis
for entrance and distribution of blood vessels. In a modified form it
becomes bulky masses of subcutaneous fat. Whale blubber is pre-
sumably a derivative of mesenchyme.

Skeleton. Every location where cartilage or bone is destined to
develop is occupied by mesenchyme. The deeper parts of the skull, the
vertebral column, ribs, sternum and the skeleton of the paired appendages
are first constructed of cartilage. The entire endoskeleton is permanently

Cart

Mes

Pre Cart

•V

. ?-.--/'—

^

^ :^.^^^

i

^^^ :-''

A

r^ Q

Fig. 76. — Diagrams illustrating formation of cartilage by mesenchyme. A, in fishes,
according to Studnicka; B, in mammals, according to Mall. Cart., cartilage; Mes.,
mesenchyme; Pre. Cart., precartilage. (From Bremer, Text-book of Histology.)

cartilaginous in elasmobranchs. Cartilage is a direct product of mesen-
chyme. Cells of the mesenchyme become cartilage cells (Fig. 76) and
deposit the ground substance or matrix of the cartilage. In the great
majority of vertebrates the primary cartilaginous skeletal structures are,
in later development, more or less completely replaced by bone. The
process of replacement involves the destruction of the greater part of the
cartilage. The remnants of the cartilage are in form of a spongy mesh-
work whose strands become calcified and serve as a framework upon which
bone is deposited (Fig. 77). The bone-producing cells, osteoblasts,
seem to be derived mainly, if not entirely, from the perichondrium, the
membrane which invests the surface of any cartilage. In the develop-
ment of one of the long bones of an appendage, the perichondrium
produces a shell of bone upon the external surface of the cartilage (peri-
chondral bone) while extensions of the perichondrium burrow into the
degenerating cartilage and provide the osteoblasts which build up endo-
chondral bone on the calcified remnants of the cartilage. The process of
ossification involves at first only the middle region or shaft of the devel-
oping long bone. The terminal regions (epiphyses) remain for some time
cartilaginous and do not become wholly ossified until growth is completed.

I04

COMPARATIVE ANATOMY

Osteogenic
tissue.

Hyaline carti-
lage (cells in
groups).

Hyaline car-
tilage (cells
enlarged)

Osteoblasts.

Endochondral bone.

Fig. 77. — Development of cartilage bone (primary bone). Part of a longitudinal
section of a phalanx of the first finger of a human embryo of the fourth month. Ossifica-
tion within the cartilage produces endochondral bone; the perichondrium deposits bone
superficially to the cartilage, perichondral bone, and thereby becomes the periosteum.
X220. (From Bremer, Text-book of Histology.)

REPRODUCTION

105

Since the perichondrium is produced by mesenchyme, the bone is indirectly
a derivative of mesenchyme.

In the development of certain of the more superficial bones of the
cranium, the outer bones of the jaw skeleton and some parts of the shoulder
girdle, no cartilage is formed. At the site of the prospective bone,
mesenchyme produces a fibrous tissue whose fibers become calcified
(Fig. 78). Mesenchyme cells, becoming osteoblasts, congregate on the
surfaces of the strands or spicules of this calcified spongy matrix and,
using it as a foundation, build up successive delicate lamellae of the
calcareous material which constitutes bone. Occasionally an osteoblast
will become enclosed in a minute space or lacuna between bone lamellae
and persists there as a bone cell.

Osteoblasts. Calcifying connective tissvie bundles. Bone matrix.

Bone cells .

Fig. 78. — Development of dermal (secondary) bone from mesenchyme. From a
section of the mandible of a human embryo of four months. X240. (From Bremer,
Text-book of Histology.)

Most of the bones which develop in the manner last described are
derived from the embryonic mesenchyme of that same general superficial
layer which otherwise gives rise to the dermis (or corium) of the skin.
They are accordingly called dermal bones. In course of evolution they
apparently originate by expansion and fusion of the basal plates of
calcareous placoid scales of elasmobranchs. Certain bones — for example,
the clavicle of the shoulder skeleton — which develop without intervention
of cartilage occur in positions so deep that the bones are not obviously and
literally "dermal." They are called membrane bones, a term which is,
however, often expanded to include all bones which arise directly from
mesenchyme. Presumably deep "membrane" bones had their phyloge-
netic origin in dermal bones. Assuming this, all bones developed directly
from mesenchymatous fibrous tissue may be called dermal, as is commonly
done. It follows from all of this that no distinction can be maintained

Io6 COMPARATIVE ANATOMY

between "dermal" and "membrane" bone. "Dermal" seems the more
significant descriptive term.

Bone resulting from replacement of cartilage is called cartilage bone.
It is sometimes called primary bone because its cartilage precedes dermal
ossification whose product, therefore, may be called secondary bone.

Muscle. Mesenchyme is the source of nearly all unstriated or
"smooth" muscle whether in the walls of viscera or in the body wall.
Most visceral organs are hollow. In their early embryonic stages their
primary and essential walls are either endoderm as in the case of the
digestive tube, lung or urinary bladder; or mesoderm as in the urino-
genital ducts. The outer surfaces of these primary walls are always
adjacent to regions occupied by mesenchyme. The unstriated muscle
fibers which constitute the muscle layers of the walls of these hollow organs
are differentiated from cells of the adjacent mesenchyme. In such organs
as the stomach and uterus the muscle layers become thick and strong.
They are relatively thin in a lung, air bladder or urinary bladder. The
walls of the larger blood vessels in all parts of the body contain unstriated
muscle whose fibers lie in planes transverse to the axis of the vessel so that
their contraction decreases the diameter of the vessel. Unstriated muscle
occurs in the walls of some integumentary glands, serving to expel the
contents of the gland. Hairs and feathers are erected by contraction
of delicate muscles attached to the follicles in which the organs are inserted.
These muscles are unstriated or possibly sometimes striated in case of
feathers. Most unstriated muscle of integumentary organs is derived
from mesenchyme. The changes in diameter of the pupil of the eye are
effected by unstriated muscle in the iris. The dilator fibers in the human
iris, however, are apparently of ectodermal origin. Mesenchymatous
unstriated muscle, wherever situated, is innervated by fibers from the
autonomic system.

Circulatory Organs. The general statement that the circulatory
vessels are derived from mesenchyme is probably admissible although
some vessels seem to arise fairly directly from the mesoderm. There is
apparently much diversity in the manner of origin of the vessels. They
may arise as soHd cords of cells, later becoming hollow, or they may be
hollow from the beginning. The essential wall or endotheliiun having
been established, the outer layers of connective tissue and unstriated
muscle are provided by adjacent mesenchyme. The heart develops as a
large blood vessel. In the region just behind that where the pharyngeal
clefts are forming, the right and left hypomeres of the mesoderm push
ventralwards and toward one another just as they do in a more posterior
region of the body. But instead of meeting to form a median ventral
mesentery, they bulge away from one another (Fig. 79) so as to give
rise to a median ventral space between them. In this space accumulate

REPRODUCTION

107

cells derived from the adjacent mesodermal layers, therefore essentially
mesenchymatous in nature. These cells arrange themselves to form
the very thin wall of a relatively large vessel. The wall is the endothehal
Uning or endocardium of the prospective heart. In some cases, at first
two endothehal tubes are formed, lying side by side, later coalescing into
one. The thick muscular layer (myocardium) of the wall of the heart is
derived from the adjacent hj'pomeric mesoderm, but this muscle, unlike
that of blood vessels, is striated. In details of structure it differs from
the striated muscle of the body walls. The coelomic space on either side
of the developing heart remains as the pericardial cavity. The visceral
layer of the hypomere, not wholly consumed in giving rise to the myo-

FiG. 79. — -Diagrammatic transverse sections of developing heart. In A the descend-
ing right and left mesodermal hypomeres have nearly met; mesenchyme cells appear
between them. In B the layers have met ventrally forming the ventral mesocardium;
the enclosed mesenchyme has formed the endocardium. In C the layers have met
dorsally to form a dorsal mesocardium; meanwhile the ventral mesocardium has dis-
appeared and the right and left coelomic spaces have become the pericardial cavity.
c, coelom; ec, ectoderm; en, endoderm; end, endocardium; m, ventral wall of hypomere;
p, pericardial cavity; v, mesenchyme cells. (From Kingsley, Comparative Anatomy
of Vertebrates.)

cardium, forms an outer layer on the wall of the heart, the epicardiimi,
which remains in continuity with the somatic layer of the hypomere. The
latter constitutes the Hning or pericarditmi of the pericardial cavity.
The pericardium and epicardium evidently correspond to the peritoneum
of the more posterior region of the coelom. The right and left hypomeric
layers, meeting above and below the heart, produce mesentery-like con-
nexions (dorsal and ventral mesocardium) between the heart and peri-
cardial walls. Later these mesocardia are largely, or usually entirely,
obliterated leaving the heart in an undivided pericardial cavity. Later a
transverse septvmi (Fig. 80) arises to wall off the pericardial cavity from
the more posterior coelomic cavity.

Concerning the origin of the earliest blood cells or corpuscles in the
embryo, there is some uncertainty but it seems probable that they have
common origin with the walls of the vessels.

Head, Neck, Diaphragm, Tail. The development of the head region
presents many problems of which the most perplexing is that of the
segmentation of the head. This problem is discussed in other chapters of
this book. The mesoderm of the head is less definitely organized than
that of the trunk. It presents some obscure evidences of segmentation,
but it is difficult to ascertain how many segments are present and their

io8

COMPARATIVE ANATOMY

fate. The six muscles, consisting of striated fibers, which effect the
movements of the eyeball in its orbit are developed from head mesoderm
which is probably the equivalent of three somites or epimeres of the trunk.
But there is nothing corresponding to the mesomere of trunk mesoderm.
The neck region, whether or not differentiated externally, corresponds
approximately to that of the embryonic pharyngeal pouches. In this
region there is, on each side, a lateral sheet of mesoderm corresponding,
except as to its derivatives, to the hypomere of the trunk. Whereas the
trunk hypomere forms no muscle other than (indirectly by way of mesen-
chyme) the unstriated muscle of the digestive tube and other visceral
parts, the pharyngeal hypomeric mesoderm produces striated muscle

Fig. 8o. — Diagrams showing the relations of the coelomic cavities (black) in fishes
(A), amphibians and savxropsida (B), and mammals (C). L, liver; P, lungs; 5, septum
transversum; D, diaphragm. In B the lungs lie in the peritoneal (or pleuroperitoneal)
cavity; in C they occupy special pleural subdivisions of the coelom. (From Kingsley.)

which constitutes an elaborate system of muscles related to the skeleton
of the jaws and to the visceral skeleton which supports the gill region
of the enteron. These muscles effect the movements of the jaws and of
the branchial apparatus. They are commonly referred to as "visceral
muscles," a designation which is, however, unfortunately misleading,
for "visceral" implies a position much deeper than that occupied by
certain of these muscles which he quite superficially and just under the
skin of a fish. In amniotes various muscles of the neck and facial region,
including in man the very deUcate superficial muscles which control
facial "expression," can be recognized as derivatives of certain of the
"visceral" muscles of a fish — which makes the term the more inappro-
priate. To call the muscles of this system "branchiomeric," as urged by
H. H. Wilder, would obviate confusion.

The diaphragm of the mammal is not the exact equivalent of the
septum transversum of other vertebrates. The latter arises as merely a
fold of peritoneum. The diaphragm is muscular. The muscles are
striated. The muscular part of the diaphragm is formed by ingrowth

REPRODUCTION IO9

from the body wall so that it is derived actually from epimere mesoderm.
The somatic folds which are involved in forming the diaphragm arise far
forward and shift back to the definitive position of the organ. This fact
accounts for the innervation of the diaphragm by cervical spinal nerves.

In vertebrates other than mammals there are two divisions of the
coelom, the pericardial and the peritoneal (abdominal or pleuroperitoneal) ,
and the lungs he in the posterior division (Fig. 80). In mammals that
part of the coelomic space lying on the cephaHc side of the diaphragm is
subdivided into three cavities, the pericardial and the right and left
pleural cavity containing the corresponding lobes of the lungs.

The tail is produced by growth of ectodermal and mesodermal parts
backward from the region of the blastopore or neurenteric canal. Growth
of the mesoderm keeps pace with that of the neural tube and notochord.
The mesoderm forms somites, more or less numerous depending on the
length of the tail, but these produce only the segmental striated caudal
muscle and the mesenchyme which goes into the formation of skeletal,
vascular and connective tissue structures of the tail.

Effect of Massive Yolk on Organogenesis

In the description of the earher embryonic stages of reptiles and birds
attention has been given to the peculiarities which are imposed upon the
earher developmental processes by the massive yolk of the egg. Elasmd-
branchs, with their large yolk-laden eggs, exhibit similar peculiarities.
Cleavage, gastrulation and the mode of origin of the mesoderm and the
notochord are necessarily much affected by the presence of the bulky
and inert yolk (Fig. 81). Once the germ layers have been established,
however, the development of organs proceeds in vertebrates of all classes
with only minor differences in details of the processes. Apparently each
germ layer is capable of producing certain structures and no others, and
those particular structures arise from that layer in all vertebrates, whether
fish or man. Indeed, there is so much evidence of rigid necessity in the
germ-layer origin of organs that, in cases where homology of adult organs
is in question, the germ-layer relations of those organs are generally
regarded as the safest criterion for homology. Yet at early stages of
development the embryonic material may not be so rigidly determined.
By appropriate operations at sufficiently early stages of embryos, both
vertebrate and invertebrate, it has been proved that a certain region of
germ material may be caused to produce structures other than those
which it would have produced normally.

While a particular organ develops in essentially the same way in
vertebrates of all classes, regardless of the quantity of yolk, yet the
presence of a large yolk mass does profoundly affect the general configura-
tion and topography of the layers of the embryo. Sections of embryos of

no

COMPARATIVE ANATOMY

amphibian and bird at similar stages and at corresponding planes may
appear to be so unlike as to be hardly comparable. Yet careful study
of them reveals that the differences are mainly quantitative. The}^ are

GASTRULATION IN FORM WITH ISOLECITHAL EGG HAVING ALMOST NO YOLK— AMPHIOXUS.

GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE
AMOUNT OF YOLK— AMPHIBIA.

gastrocoele

GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE
AMOUNT OF YOLK— BIRDS.

Fig. 8i. — Diagrams illustrating the differences in gastrulation depending on whether
the egg is isolecithal (having little yolk uniformly distributed) or telolecithal (having
much yolk more or less concentrated toward one pole of the egg), blc, blastocoele;
bid., blastoderm; blp., blastopore; ect., ectoderm; enl., entoderm; mil., cell undergoing
mitosis; yk., yolk; yk.g., cells containing yolk granules; yk.p., yolk plug. (From Patten,
Embryology of the Chick.)

differences in proportions and extent of layers. The essential relations
of layers and developing organs are the same. View a mid-trunk trans-
verse section of an early frog embryo and imagine that the mass of yolk

REPRODUCTION

III

cells in the wall of the enteron is increased in diameter thirty or more
times, meanwhile the layers of the splanchnopleure and somatopleure

ectoderm of neural plate
ectoderm of blastoderm

yolk

primitive pit

primitive streak

open neural groove
primitive pit

Rathke's pocket

allantoic bud
yolk- stalk

Fig. 82. — Diagrams representing median longitudinal sections of chick embryos
after incubation for approximately one day, .4 ; two days, B; three days, C; four days, D.
The four stages show progressive differentiation of the regions of the enteron and pro-
gressive constriction between the yolk-sac and the shaping body of the embryo. (From
Patten, Embryology of the Chick.)

Stretching to accomodate themselves to the swollen enteron. The result

would be a section essentially like the corresponding section of some bird.

Yolk is food. In an animal having an enteron, the appropriate place

for food is in the enteron. In an amphibian embryo the relatively small

112

COMPARATIVE ANATOMY

amount of yolk is contained within cells which, although more or less
encumbered by the yolk, are yet capable of developmental activity.
Gastrulation having established the enteron, the greater part of the
embryonic food is then present, not in the enteric cavity but, even better
than that, within the cells which constitute the wall of the enteron where
it may be directly acted upon by the endodermal protoplasm and made
available, as the blood system develops, for transportation to all parts of
the growing embryo.

The enormous yolk of the egg of a reptile or bird is morphologically
a part of the original ovum. But by the time cleavage of the germ disc
has progressed so far as to produce a many-celled blastoderm spreading
out thin and flat on the surface of the volk, the cells of the blastoderm can

Fig. 83.

-Young dogfish shortly before birth. The yolk-sac, containing a remnant
of the yolk of the egg, protrudes from the ventral body-wall.

be regarded as, at most, merely joint proprietors of the food supply and the
yolk is essentially extra-cellular. As development proceeds, the blasto-
derm differentiates into the typical germ layers, the mesoderm splits to
form somatic and visceral sheets with coelomic space between them, and
all of these layers progressively spread over and around the non-living
yolk (Figs. 86 and 87) until eventually it is entirely enclosed by splanch-
nopleure and somatopleure with coelom between them.

In the amphibian the food supply is, from the beginning, not only
inside the embryo but, for the most part, inside the prospective wall of the
enteron. In reptile and bird the food is external to the early embryo.
Therefore the embryo is put to the necessity of building not only its
enteron but its body wall around its prospective food.

As development proceeds the yolk is assimilated and utilized in the
building of new protoplasm. It therefore steadily decreases in bulk both
relatively and absolutely. As the body of the embryo begins to take
form, a constriction, involving both somatopleure and splanchnopleure,
appears between the yolk-sac and the remainder of the embryo (Fig. 82).
The constriction deepens until the embryo presents the appearance of a

REPRODUCTION 11,3

small animal having a narrow-necked globular sac suspended from the
under side of the body (Fig. 83). In amniotes the amnion is concerned in
this constriction (Fig. 84). As the embryo increases in size the shrinking
yolk-sac is drawn up into the body. The inner wall (splanchnopleurej
of the sac finally constitutes a small region of the wall of the intestine.
In elasmobranchs the somatopleure of the yolk-sac finally flattens out and
persists as a part of the abdominal wall. In reptiles and birds the corre-
sponding region of the somatopleure is concerned in the formation of the
amnion and chorion. At the time of hatching the somatopleure is rup-
tured at the constriction between the definitive body and the extra-
embryonic structures and everything external to the rupture is abandoned.

Embryonic and Fetal Membranes

In the general account (pages 51-55) of the reproductive arrangements
in vertebrates mention was made of the embryonic membranes, amnion,
chorion and allantois, which occur in reptiles, birds and mammals. The
foregoing account of the origin of the germ layers and the shaping up of
the embryonic body now makes possible more explicit statements con-
cerning the manner of formation of the embryonic membranes.

All eggs are invested by protective membranes or coverings which are
either produced by the egg-cell itself or are deposited about the egg by
secretory activity of the walls of the oviduct. Such membranes consist
of material which is not cellular and not in any^ sense living. They have
merely passive functions. The unique thing about these other mem-
branes, the amnion, chorion and allantois, is that they are produced by
the germ layers at a relatively advanced stage of the embryo, they are
therefore constituted of living cellular material and they are actively
concerned with such important functions as nutrition, respiration, excre-
tion and circulation.

The amnion and chorion of reptiles and birds are simultaneously
produced by an up-rising fold of the embryonic body wall or somato-
pleure (Fig. 84), A chick embryo in the second day of incubation shows
a crescentic transverse fold of somatopleure just beyond the head (Fig.
85). As development goes on, the head grows forward into the fold
and at the same time the fold grows backward over the head. Meanwhile
the fold extends backward along either side of the embryo. On the third
day a similar fold arises just beyond the tail and begins to grow forward
over the caudal part of the embryo. Head-fold and tail-fold having
become joined by lateral folds, the embryo is then surrounded by one
continuous fold which grows in centripetally from all directions and
finally completely encloses the embryo. Where opposite edges of the
fold meet above the embryo they become coalesced. Reference to

114

COMPAEATIVE ANATOMY

SOMATOPLEURE
AMNION CAVITY\

EXTRA-EMBRYON
COELOM

SEROSA fcHORION

AMNION
COELOM

A.

AMNION CAVITY
AMNION

SEROSA

FOETAL PLACEf^JTA
ALLANTOIS

ENTERON
BRAIN

COELOM
YOLK-STALK

YOLK

ALLANTOIC
STALK

SPLANCHNOPLEURE

Fig. 84. — Diagrams illustrating the development of the amnion and allantois.
The upper Figure (.4), a cross section of an earlier stage, shows the amniotic folds rising
above the embryo. The lower Figure (B), a longitudinal section of a later stage, shows
the allantois as well as the completely closed amnion. In most mammals the fused
allantois and serosa (chorion) form the foetal placenta. Ectoderm in diagram is striped,
mesoderm stippled in black, endoderm stippled in white.

REPRODUCTION

115

Figs. 86 and 87 will serve better than description to make clear the rela-
tions of layers and spaces resulting from the development of these folds.
The somatopleural folds which give rise to the amnion and chorion are,
at the time of their formation, a living part of the embryo. The state-
ment that the folds eventually enclose the embryo anticipates the fact
that the amnion and chorion do not become any part of the adult. There-
fore "the embryo" which the folds enclose is the definitive body

zdJX'-

Fig. 85. — A, dorsal view of head of a chick embryo after about 33 hours incubation.
The head-fold of the amnion covers a small anterior portion of the embryo's head.
X32. B, section of head of an embryo at a similar stage, the plane of section indicated
by 5-5 in A. A, auditory invagination of ectoderm; AM, head-fold of amnion; EC,
ectoderm; EC, superficial ectoderm of head; EN, endoderm; H, heart; HE, endotheUal
cavity of heart; MH, hypomere mesoderm which gives rise to the myocardium, epicar-
dium and pericardium (see page 107); MS, mesodermal somites; NC, notochord; AT,
neural tube; O, optic vesicle; P, pharynx; i, fore-brain; 2, mid-brain; 3, hind-brain.
(Modified from figures by Duval, Atlas d'Embryologie.)

region of the embryo. Everything else is conveniently referred to as
extra-embryonic.

Since it is a fold of somatopleure which covers the embryo, there are
necessarily, at the completion of the process, two layers of somatopleure
external to the embryo. The inner one, the amnion, has its ectodermal
surface toward the embryo. The outer one, the chorion or serosa, has its
mesodermal surface toward the embryo. Between outer and inner
layers of the fold the mesoderm-lined space is obviously a part of the
coelomic cavity but, because it becomes no part of the adult coelom, it is
described as "extra-embryonic."

The allantois is a sac formed as a mid-ventral outgrowth from the
hind or cloacal region of the embryonic enteron (Figs. 84, 86-88). It is

ii6

COMPARATIVE ANATOMY

therefore a product of the splanchnopleure and is lined by endoderm. It
grows rapidly, swelling out into the space between the yolk-sac and the
chorion and eventually acquires more or less extensive and intimate fusion

allantois

allantoic cavity

extra-embryonic
coelom

somatopleure

yolk sac
(splanchno-
pleure]

albumen

allantoic cavity

allantois
serosa

shell

sero-amniotic
cavity

yolk-sac

albumen

vitelline
membrane

belly stalk

Fig. 86. — Diagrams showing the extra-embryonic membranes of the chick. The
diagrams represent longitudinal sections of the entire egg, the body of the embryo being
cut transversely. C, embryo of about five days incubation; D, embryo of about fourteen
days incubation. (From Patten, Embryology of the Chick; after Duval.)

with the chorion. In the apposed mesodermal layers of the allantois and
chorion is developed a rich network of fine blood vessels which are con-
nected by the allantoic arteries and veins to the main blood vessels of the
definitive body of the embryo. In this way an extensive circulation is

REPRODUCTION

117

established in close relation to the inner surface of the shell of the egg of a
reptile or bird, providing for exchange of respiratory gases between the
blood and the external medium. As development proceeds the albumen

Q

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6 S

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becomes much reduced in bulk. The expanding allantois apparently
pushes the remainder of it out of most of the peripheral territory within
the egg shell and causes it to become concentrated at one spot (Figs. 86

ii8

COMPARATR'E ANATOMY

and 87) thus leaving the greater part of the inner surface of the shell
membrane free to make close contact with the vascular and respiratory
chorio-allantoic membrane. The now localized remnant of albumen is
gradually absorbed by the embryonic layers adjacent to it.

ectoderm
neural tube

sero-amniotic cavity
hind- gut

proctodaeum

amniotic cavity

netiral tube
notochord

post-anal
gut

Fig. 88. — Diagrams representing median longitudinal sections of the caudal region
of chick embryos. A, after about three days incubation; B, after about four days
incubation. (From Patten, Embryology of the Chick.)

Before the time of hatching the shrinking yolk-sac is drawn up into
the growing body. The umbilical stalk — that is, the whole complex of
connections between the definitive body of the embryo and the extra-
embryonic membranes — becomes narrowly constricted. At time of

REPRODUCTION

119

hatching the amnion and the slender neck of the allantois are ruptured at
the umbilicus. As the young animal emerges into free life the amnion
and chorion and the extra-embryonic part of the allantois are abandoned.
The proximal portion of the allantois, remaining within the body, becomes
enlarged and serves as the urinary bladder of such adult reptiles as
possess that organ. In birds, the adult having no urinary bladder, the
proximal remnant of the allantois degenerates.

Among mammals there is considerable diversity as to the manner of
origin of the amnion and chorion. Once established, however, these

Fig. 89. — Diagram of the fetal structures of a Tnammal. (The broken lines represent
mesoderm.) A, amnion; AL, cavity of allantois; B, brain; C, chorion; E, enteron;
EX, e.xtra-embryonic coelom; H, heart; NC, notochord; NT, neural tube; P, placental
region of allantois and chorion; SM, somatopleure; SP, splanchnopleure; V, chorionic
villi; YS, cavity of yolk-sac.

membranes possess the same relations to the germ layers and to the
definitive body of the embryo as in reptiles. The allantois develops,
as in reptiles, from the cloacal region of the enteron. An abortive yolk-
sac is present but devoid of yolk except in those presumably primitive
mammals, Ornithorhynchus and Echidna, whose manner of development
is essentially reptilian.

In most eutherian mammals the allantois expands until it becomes
apposed to the inner surface of the chorion and the two membranes undergo
more or less extensive fusion of their contiguous mesodermal layers.

I20 COMPARATIVE ANATOMY

The main facts concerning the development of a placenta by the chorio-
allantoic membrane have already been stated (see page 54). The
essential structures of the placenta are the highly vascular villi
produced by the chorio-allantoic membrane (Fig. 89). These villi may
be merely lodged in depressions in the uterine wall or they may pierce
more or less deeply into its tissues. Their terminal regions may lie
within a vascular connective-tissue layer of the uterus and the surface
of a villus may be in close contact with the very thin wall of a blood
vessel. These deeper penetrations of the villi are attained at the expense
of actual eroding or destruction of uterine tissue. In extreme cases there
is destruction of walls of uterine blood vessels and the extravasated blood
fills large spaces or sinuses in the uterine wall. The branched ends of villi
project into these sinuses so that the villous surfaces are directly bathed

Fig. 90. — Fetus of Cat, removed from uterus without rupturing chorionic sac (C),
showing zonary distribution of placental villi.

by maternal blood, an arrangement providing maximum efficiency in the
exchange of materials between fetal and maternal blood. In general
the extent of penetration of the villi and the degree of intimacy of their
relation to the maternal blood is an index of the degree of specialization
of the placenta. In the human placenta the villi are long, much branched,
and exposed directly to maternal blood which fills capacious sinuses in
the uterine wall.

Mammals exhibit various types of placenta, depending on the dis-
tribution of villi in the chorionic surface. When the villi are uniformly
distributed over the chorion, as in the horse, pig and many other ungulates,
the placenta is called diffuse. In many ruminant ungulates, such as
cattle, the villi are localized in numerous patches or clusters of varying
sizes — the cotyledonary placenta. In carnivores the placenta usually
takes the form of a broad band or zone encircling the chorion at a position
about midway between head and tail of fetus — the zonary placenta
(Fig. 90). A discoidal placenta, in which villi are restricted to a single
relatively large area of the chorion, occurs in insectivores, bats, rodents
and higher primates including man.

REPRODUCTION 1 21

The terms deciduate and non-deciduate, as applied to the placenta,
refer to conditions which obtain at time of birth. A fetal placenta whose
villi do not penetrate deeply into the uterine wall separates from it readily
and without loss or destruction of uterine material. Such a placenta,
called non-deciduate, occurs in most ungulates, in the whale and dugong,
and in lemurs. When, however, the fetal villi are deeply imbedded in the
uterine wall, at time of birth the involved layer of the uterus is split off
and discharged with the fetal placenta. This deciduate condition of the
placenta occurs in the carnivores, in the elephant, and commonly in
animals having a placenta of the discoidal form.

In marsupial mammals few or no villi are produced on the chorion
and there is merely contact of chorion and uterine wall without actual
connection. Usually, too, the chorion does not become vascular. It is a
noteworthy fact, however, that in some marsupials (Perameles) the
allantois, in conjunction with the chorion, forms a limited vascular area
which becomes closely related to the uterine wall, while in others (Dasy-
urus) it is the splanchnopleure of the yolk-sac which joins the chorion and
forms a placenta-like vascular area which is apposed against the uterine
wall. These facts suggest that in early mammals both the yolk-sac
and the allantois were potentially placenta-forming. In higher Primates,
however, the allantoic sac is rudimentary and the fetal portion of the
placenta is of chorionic origin only; yet the allantois develops far enough
to bring its blood vessels into connection with the chorionic vessels of
the placenta.

At time of birth the amnion and chorion are ruptured and the young
mammal is expelled, along with the amnionic fluid, by muscular contrac-
tion of the uterine walls. The amnion, chorion, allantois, fetal placenta,
and more or less uterine tissue in a placenta of the deciduous type are
discharged later as the "after-birth." The umbilical cord is the much
elongated and attenuated connexion between the body of the fetus and
the extra-fetal membranes. It includes somatopleure, extending from
the fetal body wall to the amnion, and the stalks of the yolk-sac and
allantois with their vitelline and allantoic blood vessels. During later
fetal life the yolk-sac degenerates. Its cavity is obliterated, its blood
vessels close and it becomes reduced to a slender solid cord contained
within the umbilical cord. After birth the umbilical cord is, in one way
or another, severed and such remnant of it as remains attached to the
body shrivels up and is sloughed off at the navel. That portion of the
allantois remaining within the body becomes the urinary bladder.

Functions of the Embryonic and Fetal Membranes. The amnion
is protective in mechanical ways. It provides a fluid medium in which
the embryo is suspended and so protected from deforming pressures and
impacts. The amnionic mesoderm produces unstriated muscle fibers

122 COMPARATIVE ANATOMY

which impart contractility to the amnion. Its gentle undulations keep
the amnionic fluid moving, thus preventing local stagnation in the fluid
and possible adhesions of embryonic parts.

The main importance of the chorion is in its serving, in conjunction
with the allantois (rarely the yolk-sac), to form respiratory, excretory
and nutritive membranes.

The allantois in reptiles and birds is the embryonic respiratory organ.
In viviparous reptiles its circulation may pick up some nutrient material
from the uterine wall or fluids. The cavity of the allantois serves as
receptacle for waste from the embryonic kidneys. Consistently with
this function its proximal portion may persist, after hatching, as the
urinary bladder.

In mammals, although primitively the yolk-sac may have had placental
possibilities, the allantois came to be the all-important agency in estab-
lishing the placenta. The placental villi are primarily chorionic but their
vascular structures are usually derived from the allantoic circulatory
system. In the placenta, then, are concentrated the vital functions,
nutrition, respiration and, at least in part, excretion.

The vascular splanchnopleural wall of the yolk-sac is the essential
nutritive membrane in reptiles, birds and primitive mammals, but
becomes rudimentary in later mammals.

SUMMARY AND INTERPRETATION

A backward glance over the foregoing account of vertebrate develop-
ment suggests emphasis on the following points.

Cleavage may be described as a process of mobilizing- the agencies
within the fertilized egg. The smallest egg is a large cell. Nutrition,
respiration and excretion involve the limiting surfaces of cells. Metabolic
rate is high during development. Cleavage increases cell surface.
Nuclear material is certainly somehow important. Cleavage increases
the quantity of nuclear material both absolutely and relatively to the
amount of cytoplasm.

In the blastula stage the embryonic material becomes arranged in
form of a layer. The increased exposure of surface to the external medium
favors metabolism. The disposition of cells in a layer is especially
significant in view of the fact that the adult animal is constituted essen-
tially of layers of cells.

Gastrulation transforms the one layer of the blastula into two, an
outer and an inner. The cavity of the blastula has no permanent signifi-
cance. The new cavity produced by gastrulation is the definitive enteron.
The ectoderm, potentially protective and nervous, provides for necessary
relations of an animal to its environment; the endoderm is nutritive. At
the gastrula stage, therefore, a minimum metazoan animal is estabhshed.

REPRODUCTION 1 23

Respiration and excretion may be carried on at all surfaces, without
specialized organs, as in Coelenterates.

Ectoderm and endoderm provide for the bare necessities of animal
life. The addition of a mesoderm provides for some of the luxuries,
especially that of being large and powerful. Swift movement of massive
bodies demands powerful motors. Muscles are necessarily bulky. To
meet the metabolic requirements of cells which are deep within some bulky
mass of tissue such as a muscle, a circulatory system is necessary. Blood
carries to deep-seated tissues the food and oxygen which are otherwise
not directly accessible to them, and removes wastes from them. In order
that the blood stream of a large animal may be properly supplied with
oxygen and freed of waste materials, it must be related to efficiently
specialized respiratory organs — gills or lungs, and excretory organs,
kidneys. With increase in bulk and in structural complexity provision
for mechanical support becomes necessary. There must be tensile
connective tissues and rigid supporting structures — skeleton.

Mesoderm is the main source, directly or indirectly, of these organs
which are of accessory nature. There are, admittedly, small animals
which possess a mesoderm, but there are no very large animals which do
not have mesodermal structures.

Mesoderm produces directly the striated musculature of the body wall
and the essential tubules of the kidney. Indirectly, by way of the
mesenchyme (receiving some very minor contributions from endoderm
and ectoderm), arise circulatory organs, connective tissues, and skeleton.
Respiratory organs are essentially vascular and insofar derived from
mesenchyme, but aeration of the respiratory blood demands the inter-
mediation of a layer which is directly or indirectly in relation to the
external medium. The endoderm plays this role for internal gills and
lungs, the ectoderm for external gills. It is particularly the pharyngeal
endoderm which is the seat of development of specialized respiratory
organs, gills and lungs, in vertebrates. In general, therefore, the verte-
brate endoderm is concerned with the business of dealing with those
materials, food and oxygen, which the animal must receive from the
environment.

There may seem to be some inconsistency between the fact that the
notochord, that most fundamental structure of the vertebrate skeleton,
is derived wholly or in part from endoderm and the fact that cartilage and
bone, the definitive skeletal materials of vertebrates, are derived from
mesenchyme. But the notochord is very ancient. Its presence and
condition in such animals as Amphioxus and the urochordates makes that
certain. While the notochord is not derived from already-formed meso-
derm, it develops, not only in Amphioxus but in all vertebrates, simul-
taneously with the mesoderm and the two are very closely similar in their

124 COMPARATIVE ANATOMY

manner of origin in the embryo. But Amphioxus has very little that can
be regarded as corresponding to the mesenchyme of vertebrates. Unques-
tionably cartilage and bone are phylogenetically new skeletal materials
as compared to the notochord. Consistently with this assumed fact,
the mesenchyme, which is the embryonic source of cartilage and bone, is
the last of the germinal building materials to be differentiated in the
embryo.

The principle of "recapitulation" was formulated by Ernst Haeckel
(1874) in the phrase "Ontogeny is a brief repetition of Phylogeny."
Nineteenth century biology was perhaps inclined to over-work the idea.
Nevertheless many facts of vertebrate development justify, or even
compel, the inference that there is some direct relation between the
phylogenetic and the ontogenetic order.

Early embryos of reptiles, birds and mammals develop briefly transi-
tory pharyngeal clefts (or pouches). These clefts are not accompanied
by the formation of functional gill structures such as occur in connexion
with the pharyngeal clefts of adult fishes. But they are closely similar
to those embryonic pharyngeal clefts which in fishes come to be associated
with vascular filaments and the other structures which, altogether, con-
stitute functional gills.

The notochord is developed in the early embryo of all vertebrates.
In shark-like fishes (Elasmobranchii) wholly cartilaginous vertebrae are
developed around the notochord of which traces persist in the adult.
In reptiles, birds and mammals the embryonic notochord becomes sur-
rounded by cartilaginous vertebral structures. In later development the
cartilage is replaced by bone. The adult vertebrae consist of solid bone.
Ordinarily no traces of notochord remain. Nowhere else in the body is
bone preceded by any such structure or tissue as notochord. Many
bones other than vertebrae are produced directly from mesenchyme with-
out the intervention of cartilage. It seems clear, then, that in general
the formation of bone is not inherently dependent on preexisting cartilage
or notochord.

Elasmobranch fishes have an all-cartilaginous skull. The skull of
adult amniotes is nearly all bone. But the amniote embryo develops
first a cartilaginous skull whose general plan resembles that of an elasmo-
branch. In later development the cartilage is almost wholly replaced by
bone and numerous dermal bones are developed superficially to the
cartilage. The-geologkxecord shows that elasmobranchs existedjong
>^re any^rtebrartgswith bo,By^eleton^^c^e upon th^cene^.^"''^

In the phylogenetic history of the axial skeleton of vertebrates,
notochord is the oldest structure and bone is most recent. In the develop-
ment of the bony vertebral column of a modern animal, notochord,
cartilage and bone succeed one another in phylogenetic order. In all

REPRODUCTION 12$

vertebrate embryos the notochord is one of the first three organs to be
estabUshed, the two others being the enteron and neural tube. It appears
simultaneously with the mesoderm. But it is not until long afterward
that mesenchyme, the source of cartilage and bone is produced. The
early formation of the notochord in the vertebrate embryo and the
relatively late appearance of mesenchyme, followed by cartilage and bone,
is in accord with the phylogenetic order.

It is, however, not safe to assume that the embryo always adheres
to the phylogenetic order. Cartilage is a very ancient skeletal material.
The amnion is phylogenetically recent. Yet in an amniote embryo
the amnion develops long before any cartilage is formed. Some of the
departures from phylogenetic order are so glaring as to shake faith in
the principle of recapitulation. Yet, on the whole, it would seem that
the instances of adherence to the phylogenetic order are more striking and
more significant than the departures from it.

It cannot be too much emphasized that the embryo of a later verte-
brate resembles or "recapitulates" not the adult stage of an ancestor but
the embryo of that ancestor. The mammal embryo does not have gill
pouches; it has pharyngeal pouches similar to those which, in a fish
embryo, develop into gill clefts. Karl Ernst von Baer in the first quarter
of the nineteenth century recognized the fact that the early embryos of
vertebrates of all classes are fundamentally similar and that the special
features which identify the animal as a member of a particular class
appear relatively late in ontogeny.

The embryo whose development may be observed in the laboratory
today has had unbroken protoplasmic continuity with ancestors hundreds
of millions of years remote. Herein is the physical basis for recapitulation.
Nevertheless protoplasm seems such a fragile and labile thing that it is
almost incredible that it should be able to retain the impress of structures
which, long ages ago, lost all functional significance for the adult animal.

CHAPTER 3
HISTOLOGY

The material, source and basis of everything that an animal is or does
is the chemically specific and complex "living substance," protoplasm.
In such very small animals as Protozoa the protoplasm of the minute body
exhibits an organization similar to that of a single one of the structural
subdivisions or "cells" of the body of a large animal. Within the body
of one of these so-called "unicellular" animals there is more or less local-
izing of functions and a corresponding structural differentiation of regions.
There may be contractile fibrils, digestive vacuoles, excretory vacuoles
and other localized intracellular organs. Similarly, the body of a large
animal is locally differentiated for the carrying on of various functions.
The specialized regions, more or less definitely dehmited from one another
and each characterized by a configuration which is consistent with its
special function, we call organs. But these organs, in contrast to the
intracellular organs of a protozoan, comprise many cells, and the cells of
any one organ, so far as they are concerned in carrying on one common
function, all exhibit intracellular differentiation of the same kind. Such
a group or system of cells, coordinated in one common function and alike
in their internal differentiation, constitutes a tissue.

An ideally simple organ would consist of only one tissue. As a matter
of fact, nearly all organs are concerned with more than one function.
An organ's primary function usually demands certain accessory functions
and a corresponding diversity of tissues enters into the constitution of the
organ. The primary function of a liver is secretion of bile. But secretion
calls for blood supply, and a complex of hepatic tubules and blood vessels
requires mechanical support. Therefore vascular tissue and connective
tissue make up a large part of the substance of the liver. In a stomach,
the primary tissue is the digestive epithelium. Muscular, nervous,
vascular and connective tissues play accessory but nevertheless necessary
roles. Vascular and connective tissues enter into the constitution of all
major organs. A certain tissue may be the primary and essential tissue
in one organ and occur as an accessory tissue in another organ. Nervous
tissue is the essential tissue in a brain or spinal cord but it appears as an
accessory tissue in nearly all other organs.

Anatomy deals with organs as such. It considers their form, relations
and all those grosser features of structure which are external to tissue
structure or cell structure.

126

HISTOLOGY

127

Histology concerns itself with the internal and specific structure and
organization of tissues. Since the tissue is constituted of cells, histology
is necessarily concerned with those specific intracellular differentiations
which collectively create a tissue.

C3rtology, narrowly defined, deals with cells as such — that is, with
that fundamental cell mechanism which is common to all cells and inde-
pendent of tissue specialization. In practice, however, it is impossible
to maintain a sharp distinction between cytology and histology. The
anatomist, too, never hesitates to become, upon occasion and temporarily,
a histologist or a cytologist.

Most vital functions involve the surface between protoplasm and the
medium external to it. Food enters from without. Respiratory gases pass
in and out. Waste is expelled from the surface. Special secretions are
produced at the surface. External forces impinge upon the surface and
give rise to internal progressive changes (nerve conduction) which may
result in a reaction directed toward the exterior. Consistently with these
facts, as the size of animals increases above that of the unicellular proto-
zoans the substance of these larger animals is so disposed as to present,
directly or indirectly, an adequate amount of surface to the exterior.
Indefinite increase in size of cells and nuclei is rendered impossible by
the necessity of maintaining appropriate relations of cell surface to cell
volume as well as of nuclear surface to nuclear volume, for the necessary
interaction of nucleus and cytoplasm must take place through the nuclear
membrane. Therefore the protoplasm of the larger animal must be
subdivided into many cells.

In the preceding chapter on embryology it was pointed out (page 123)
that a large and powerful animal necessarily possesses various specialized
organs. To say that an animal is large because it has many organs and
is complex is the reverse of the truth. Attainment of relatively large
size together with capacity for powerful movement and diversified activi-
ties compels the differentiation of various organs and results in a high
degree of anatomical and histological complexity. Protoplasm being
what it is and under necessity of maintaining certain surface relations with
the exterior, larger animals must be multicellular and must be complex.

Among the coelenterates are found the simplest of multicellular
animals, consisting of two layers of cells, an outer which is essentially
protective and an inner which is nutritive. The double wall encloses a
cavity which has a single external opening for admission of food and
expulsion of waste.

The gastrula is a form attained by the early embryo of nearly all
metazoan animals, both invertebrate and vertebrate. At the gastrula
stage all of the embryonic material is arranged in two layers, an ectoderm,
prospectively protective and nervous, and an endoderm, nutritive. The

128 COMPAEATIVE ANATOMY

double wall encloses a cavity which is the primitive enteron. Very
soon the gastrula adds a mesoderm, a third layer, and for some time
thereafter all of the embryonic material continues to be disposed in layers
of cells more or less widely separated by spaces. There is no important
departure from this arrangement until the myotomes begin to thicken,
preparatory to forming muscle, and mesenchyme cells begin to aggregate
around the notochord and neural tube preparatory to forming skeleton
— and by this time the circulatory system is beginning to develop.

Most of the organs of the adult animal are hollow. They contain
something or they convey something — food, air, blood. Even such
organs as the liver and pancreas, upon casual inspection apparently quite
solid, are minutely hollow. The essential liver tissue consists of tubules
whose bore is, however, exceedingly small. Upon the other hand, muscles
are solid. Connective and skeletal tissues may form bulky solid masses —
solid, that is, except insofar as they are penetrated by blood vessels.
Bone may, indeed, contain cavities, but these cavities have not the same
significance as those of such organs as, for example, blood vessels whose
cavities are essential to the functioning of an organ whose tissues are
actively alive. Fully developed bone consists mainly of non-living
material and the cavities within it have a merely passive mechanical
significance. The occupation of bone cavities by a blood-forming marrow
makes advantageous use of what might otherwise be mere waste space
in the animal, but this marrow tissue has no direct relation to the skeletal
function of the bone.

Such nervous organs as brains, ganglia, central nerve cords, and nerves
need not be hollow and ordinarily are not. The hollo wness of the brain
and spinal cord of the adult vertebrate is an incident of the development
of those organs from an embryonic neural tube. The cavities of the
brain and cord serve as a channel for a cerebro-spinal fluid which has some
metabolic significance, but in the absence of such cavities the metabolic
needs of the nervous tissue could doubtless be provided for, as in other
massive tissues, by the usual blood and lymph channels.

Every surface of the animal, whether apposed directly to the external
medium or to some internal cavity, is a critical region of the organism.
It is a surface on the one side of which is living substance while on the
other side of it may be food, water, air, blood or something else between
which and the protoplasm is being carried on some vitally necessary
activity — digestion, respiration, absorption, secretion, excretion, diffusion.
Or it may be a surface at which the underlying protoplasm deposits a
non-living substance which serves as a passive mechanically protective
barrier between the living substance and the adjacent space, as when skin
produces an external cuticula or a horny layer for protection against
external agencies, or as when the lining of a bird's gizzard deposits a tough

HISTOLOGY 129

horn-like layer which prevents abrasion of the walls of the organ when its
powerful muscles grind up hard seeds with the aid of ingested pebbles.

Provision for the adequate carrying on of these diverse and important
surface activities can be afforded only by the presence of a superficial
membrane constituted of living material and specialized appropriately
for the functional requirements of the particular surface. Tissue highly
specialized for motor, skeletal or nervous functions could not at the same
time meet the requirements of a surface layer. Consequently, with very
rare exceptions, every free surface of an animal, external or internal, is the
surface of a more or less specialized cellular layer, an Epithelium.

EPITHELIAL TISSUES

In view of the foregoing considerations, it is clear that epithelia are
tissues of primary importance. They are the immediate agencies con-
cerned in the necessary interactions between protoplasm and the materials
and agencies external to it. They provide a barrier against invasion by
disease-producing organisms. They are important also in serving to seal
the surfaces of the animal against leakage of the omnipresent intercellular
lymph of deeper tissues. If human skin is abraded so slightly as not to
cause bleeding, nevertheless a clear watery fluid, lymph, exudes. The
outer portion of an intact epidermis prevents such leakage.

Epithelia are, in double sense, the most primitive of tissues. The
smaller simpler coelenterates consist merely of an outer and an inner
epithelium. There is much reason for believing that all other metazoans
are descended from some ancient and primitive coelenterate. The gastrula
of animal embryos consists of two epithelia. All tissues of the adult are
derived from the primary ectoderm and endoderm. Hence, presumably in
phylogeny and certainly in ontogeny, all tissues are derived from epithelia.

It is evident, therefore, that epithelium provides for all animal needs,
and therefore all-epithelial animals may and do exist. But there are no
non-epithelial animals — unless protozoans be considered as such, but even
a unicellular protozoan may be regarded as a minimum epithelium, being
one cell thick and one cell in extent !

The outer layer of the gastrula, while it is the source of various gland-
ular and other structures which attain a deeper position, otherwise persists
as the epidermis which is the external epithelium of the adult body. The
inner layer of the gastrula, which gives rise to various organs such as the
liver, pancreas and lungs which grow outward from the enteron, otherwise
persists as the lining of the digestive tube, the digestive epitheliimi, which
is the innermost epithehum of the adult body. These two layers, then,
the very thin epidermis and the even thinner digestive epithelium, which
together constitute only an extremely small fraction of the bulk of the

I30

COMPARATIVE ANATOMY

animal are the gastrular layers persisting in their original relations to
space. By far the greater part of the massive adult has been inserted
between the two primary layers.

In view of the foregoing facts, the significance of the outer and inner-
most epithelia of the body and their immediate derivatives is such that
attempts have been made to recognize it in terminology. The embryolo-
gist, Wilhelm His, in 1865, proposed to apply the term endothelium to all
mesodermal limiting layers, thus restricting "epithelium" to the ectoder-
mal and endodermal epithelia and their immediate derivatives. At
present, however, the term endothelium is rarely used except to designate
the lining layer of blood vessels and lymphatic vessels. To designate the
mesodermal layer lining the coelom, that is, the peritoneal epithelium,
the term mesothelium has been proposed.

In present usage, the term endothelium may be correctly appHed to the
lining layer of blood vessels and
lymphatics. "Mesothelium" may
be used for the peritoneal
epithelium.

In certain localities are found
tissues whose cells, in their appear-
ance and arrangement, resemble

Fig. 91. — Types of epithelia. B, simple squamous; C, simple columnar; D, strati-
fied columnar, ciliated at E\ F, stratified polyhedral, upper cells squamous. (From
Kingsley.)

epithelial cells, but they do not abut upon a cavity. The insuhn-
producing ''islands" or "islets" in the pancreas, the deeper tissue or
medulla of the suprarenal gland, and the tissues of some other endocrine
glands are of this nature. These are glands whose secretion is not
collected in a cavity but transmitted directly to the blood in closely
adjacent vessels. To such tissues, which are not typically epithelial, is
applied the adjective epithelioid.

The most notable exception to the rule that all free surfaces of the-
animal are epithelial is found in lymph glands whose irregular internal
spaces do not have a continuous endothelial covering. Its absence is
intelligible. Cells proliferated from the spongy deep tissue of these glands
float away in the lymph stream to become new white blood cells. An

HISTOLOGY 131

endothelium would interfere with easy release of these cells. Also in the
spleen and smaller blood glands — that is, glands whose spaces are occupied
by blood instead of lymph — there is more or less interruption of the
endothelial lining.

Epithelia carry on functions of most diverse kinds. The diversity is
reflected in the structure of epitheha. Only a few of the more general
features of structure can be mentioned here.

Simple Epithelium. Most epithelia are only one cell in thickness.
Such layers are termed simple. There is, however, great variation in the

Term. bar. ^ Top plate. q^U ^^H_

^ (^ ^ r-s ^^V.

^^ \ ^TS 6 3 "^

Conn, tissue. Squam. epith.

Fig. 92. — At left, section of the allantois and amnion of a pig embryo at a region
where the mesodermal layers of the two membranes have coalesced. The section is
perpendicular to the surfaces of the allantois (above) and the amnion (below). At
right, surface view of allantois. The allantoic epithelium is cuboidal, the amnionic
epithelium is squamous. The "top plate" is a superficial denser layer of the cell;
"terminal bars" are thickenings of intercellular substance just beneath the surface
of the epithelium. (After Bremer, Text-book of Histology.)

thickness of simple epithelia. The cells may be of such form that, seen in
sections perpendicular to the surface, their outline is approximately square.
Such an epithehum is commonly called cuboidal (Fig. 92), but unfor-
tunately so, for the cells are not cubes. They are prisms of usually hexa-
gonal form and with bases at opposite surfaces of the layer. Or a simple
epithelium may consist of greatly elongated cells with long axes extending
from base to free surface of the layer (Fig. 91C). Such a layer is called
colimmar — but incorrectly called cyhndrical for the cells are elongated
prisms, approximately hexagonal. At the extreme of thinness are epithelia
each of whose cells is a broad flat plate, hexagonal in outhne (Figs. 91-S
and 92), but so thin that, as seen in section perpendicular to the surface,
even under high power of a microscope it has scarcely perceptible thick-
ness except where the nucleus, itself much flattened, occasions a bulge in
the outline. Such a layer is called flat or squam.ous.

Many columnar epithelia depart from being strictly "simple" in
having relatively small or short basal cells interposed among the basal ends
of the taller columnar cells (Fig. 93).

Most of the ectodermal and endodermal epithelia are relatively thick,
ranging from taU to low columnar. The squamous form occurs commonly

132

COMPARATIVE ANATOMY

among the mesodermal epithelia. The endothelium of blood vessels is
one of the thinnest of layers. In studying sections of a tissue it is often
difficult to ascertain whether or not a certain space in the tissue is lined by
endothelium. The peritoneal epithelium of a frog tadpole is of utmost
thinness.

In such organs as the digestive tube and urinary bladder the Hning
epithelium is stretched as the wall of the organ becomes distended and the
form of the individual epithelial cell varies accordingly. A cell which is
tall-columnar when the organ is contracted may be cuboidal or even flat
when the wall is stretched.

■^nt'ttfflaa&Muw

'^*fl**"ini-""iiiMttfoi.i. w

n

Fig. 93. — Columnar ciliated epithelium from human trachea. Most of the cells
are slender, with axes more or less curved, and extend from the basement membrane
to the free surface of the epithelium. Occasional short cells, basal cells, lie at or near
the basement membrane and do not extend to the free surface. Several swollen mucous
cells ("goblet" cells) are shown. (From Bremer, Text-book of Histology.)

Stratified Epithelium. On Amphioxus, a slender marine animal only
four or five centimeters long, an epidermis one cell thick affords adequate
protection. On an elephant it would not. In such large arthropods as
crabs and lobsters the epidermal epithehum is simple but it attains con-
siderable thickness by virtue of the fact that its columnar cells are exceed-
ingly tall. These cells secrete the characteristic "shell" consisting of
organic chitinous material hardened by deposit of calcium salts. As the
animal grows, the shell is periodically cast and, until the new shell becomes
hardened, the animal is in a somewhat precarious " soft-shelled " state.
While this arrangement serves the crab passably well, it would have obvious
disadvantages for, again, the elephant. Certain surfaces of large heavy
animals are exposed to excessive mechanical friction and impact. The
inevitable loss of material at the surface is best compensated for by a

HISTOLOGY

133

Stratified epithelium whose lower layers persistently grow to replace the

loss.

A stratified epithelium may be two or several or many cells in thickness
(Fig. 91, D-F). In all vertebrates the epidermis is stratified (Fig. 94).
Its thickness varies with the size and habits of the animal, and in a
particular animal it varies locally depending upon the degree of exposure
to mechanical wear. The epithelial lining of the vertebrate mouth,
pharynx and esophagus is stratified. Sebaceous glands, associated with
the roots of hairs, have walls several cells in thickness. The deeper cells,
becoming quite filled with the oily
secretion, break down and pass out with
the discharged product of the gland.
The outer cells replace those lost. In
general, an epithelium which, for any
cause, is subject to loss of cells at the
surface, is likely to be stratified. The
fact remains, however, that in fishes
there is no constant or periodic shedding
of epidermal material and yet the
epidermis is stratified.

The cells of the bottom tier in a o, • . , « 1, d .

Fig. 94. — Skin of lung-fish, Proto

stratified epithelium are usually columnar ptems; section perpendicular to sur-

(Fiff qO their deeper ends resting on face; much enlarged, c, dermis
^ o ^J ' i- (corium); e, epidermis; g, multi-

a non-protoplasmic basement memorane cellular gland; u, unicellular gland.
which gives the layer a flat and smooth (From Kingsley.)
surface. At the free surface of the

epithelium the cells are more likely to be flat or squamous. In a
thick stratified epithelium the cells of several or many of the outer
layers may be more or less flaUened (Fig. 95). The form of cells in
the intermediate level of the epithelium is usually such as would
result from crowding tightly together a mass of compressible spheres, that
is, polyhedral. And yet the cells are not actually packed tightly together.
They are separated by excessively thin intercellular lymph spaces through
which seeps lymph derived from underlying blood vessels and serving to
provide for the metaboHc needs of the individual cells. CeUs on opposite
sides of the intercellular space are connected by most delicate strands of
solid, or at least dense, substance. Presumably protoplasmic, the strands
are called protoplasmic bridges or plasmodesms.

Epithehum, even when stratified, is very rarely vascular. Nutrition,
respiration and excretion of its individual cells depend on movement of
intercellular lymph derived from blood vessels of some adjacent vascular
connective tissue. In amphibians, however, the skin is more or less impor-
tant as a breathing surface. In the lung-less salamanders, also gill-less

134

COMPARATIVE ANATOMY

in the adult stage, the integument and the pharyngeal hning carry on all
of the respiratory interchange. Accordingly capillaries penetrate to some
extent into the epidermis and pharyngeal epithehum, thereby bringing
blood into closer relation to the external medium.

Many epithelia, although "simple" in the sense of being only one cell
thick, are not the ideally simple tissues of the definition (page 126), consti-
tuted of cells all "aUke in their internal differentiation." Among the

Stratum
comeum.

Stratum
lucidum.

Stratum
granulosum.

Stratum
germinativuiti.

Corium
(Tunica
propria.)

Fig. 95. — Epidermis from the sole of the foot of an adult man. Section perpendicu-
lar to surface of skin. External to the stratum germinativum, the strata show successive
stages in the production of the stratum comeum. X360. (From Bremer, Text-book
of Histology.)

special functions of an epithehum are the following: (i) production of a
superficial covering of protective, non-living, mechanically resistant sub-
stance; (2) production of special secretions such as mucus; (3) reception of
external stimuH; (4) provision for motile activity. Two or more of these
functions may be carried on by one "simple" epithehum or by a stratified
epithehum. Within the epithelium, then, cells will exhibit differentiation
of as many types as there are functions.

(i) Most epitheha produce at the free surface a protective covering
which ranges from a very thin and dehcate cuticula, such as occurs on the
epidermis of a small fish, to the massive horny layer on a large reptile or
the hoof of a horse.

HISTOLOGY

135

A cuticula is a dense, tough or hard material formed at the exposed
surface of an epithelial cell. The cuticular products of adjacent cells are
perfectly continuous thus giving rise to an uninterrupted layer over the
epithelium. Such a layer occurs, as stated above, on the stratified epi-
dermis of fishes (Fig. 94), and is found also, in varying degrees of develop-
ment, on many internal epithelia.

Keratin is a nitrogenous organic substance which may be formed by
epithelial cells. It is the basis of homy structures. Its most character-
istic development is seen in the epidermis of vertebrates. Produced within
the cell, the keratin is deposited in the peripheral region of the cell and at
the expense of the cytoplasm. As the process reaches its limit, the nucleus
and remnant of cytoplasm die and dry up. What was a living cell is then
merely a minute horny scale — in contrast to the fact that cells which
produce a cuticula remain alive. As the keratin is deposited, adjacent
cells somehow become strongly adherent so that the entire keratinized or
"horny" layer (stratum comeimi) acquires a high degree of mechanical
resistance. The process may involve only the outermost tier of cells of
the epidermis, as in some amphibians, or, as in reptiles, several or many
of the upper layers of cells become horny. On the human body the stratum
corneum varies from a thin and flexible layer, as on the back of the hand,
to a thick hard and tough layer, as in the callosities of the palm and sole
(Fig. 95).

The stratum corneum is one of the most important epithelial products
of a vertebrate. Fishes have merely a cuticular outer layer on the epi-
dermis. Apparently amphibians introduced the stratum corneum. The
characteristic superficial scales of reptiles, and feathers, hair, claws, hoofs,
nails and the hollow horns of ruminant ungulates are all differentiations
of the stratum corneum — they are epithelial products.

In amphibians and reptiles the horny layer is shed periodically and
either entire or in large fragments. In birds and mammals minute par-
ticles of the layer are constantly sloughing off. The material thus lost is
replaced by growth in the deeper part of the epidermis. In animals
which shed periodically, a new horny layer is well established beneath the
old before the old is shed. The animal therefore passes through no such
critical period as the "soft-shelled" stage of a crab. It is this ease of
repair and replacement of the outermost layer of the body which makes
the stratum corneum incomparably superior to a cuticular layer for the
uses of large heavy land animals.

Calcified structures may be formed by an epithelium. The shells of
mollusks are epidermal products. In vertebrates the enamel which caps
the spine of a placoid scale of a shark and covers the crown of a tooth is an
epithelial product. But, in contrast to cuticular and horny layers which
are produced at the free surface of an epithelium, enamel is deposited at

136

COMPARATIVE ANATOMY

the basal surface and in coordination with the building of the dentine of
the scale or tooth by the adjacent dermal or mesenchymal layer (Fig. 96).
(2) A glandular epithelium is one in which certain of the cells are
specialized for secretion. Single glandular cells may be scattered more or
less abundantly throughout an epithelium, either simple or stratified. In

Fig. 96. — Developing scales of dogfish, Squalus; sections perpendicular to surface
of skin; much enlarged, c, upper layers of epidermis; d, dentine of scale, deposited by
dermal cells beneath it; ee, enamel-forming organ of scale — a specialized region of the
germinative layer (m) of the epidermis; -p, "pulp", the dentine-forming organ. (From
Kingsley.)

stratified epithelium such cells are at or near the surface (Fig. 94, u) and
open upon it, usually a prolonged and narrow extension of the cell serving
as a ductule. The mucus which coats the surface of an earthworm or fish

is produced by such unicellular
epidermal glands. Many internal
epithelia are mucous.

(3) A sensory (or neuro-) epi-
thelium is one in which certain cells
are specialized for reception of
stimulation by some agency in the
cell's environment. Such a cell is
usually elongated and slender and
characterized by one or more ex-
ceedingly delicate "hairs" or
"bristles" at its distal end (Fig.
97). The hair, which is evidently
an important part of the receptor mechanism, extends through the
cuticula (if present) or is otherwise disposed so as to be readily accessible
to the appropriate stimulating agency.

Sensory cells may occur in either simple or stratified epithelia. They
may be scattered singly throughout an epithelium as in the epidermis of
various invertebrates. In the olfactory epithelium of the nasal cavity of
mammals the non-nervous cells are of very elongated form and interspersed
among them are the even more attenuated olfactory sensory cells (Fig.

Fig. 97. — Sensory cells. A, cell from
the sense organ (crista acustica) of an
ampulla of the ear; B, rod cell from the
retina; C, cell from the olfactory epithelium.
(From Kingsley, After Fiirbringer.)

HISTOLOGY

137

97C). Epithelial sensory cells may be grouped in small clusters forming
specialized sense organs. Such are the sense organs of the lateral-Hne
system of fishes and amphibians (Fig. 98)
and the "taste buds" in the mouth of higher
vertebrates (Fig. 99).

The essential layer of the internal ear
is derived from embryonic ectoderm. This
auditory epithehum produces sensory cells
in localized groups which become the special
sensory organs of the ear, the maculae and
cristae acusticae (Fig. gjA). The organ of
Corti in the spiral cochlea of the mammalian
ear is a highly specialized sensory epithelium.

The retina of the vertebrate eye is pro-
duced by outgrowth from the wall of the
embryonic forebrain and consists of two
ectodermal epithelia (Fig. 60). The outer
is a simple pigmented and non-nervous epithelium. The inner is
the most complex of all sensory epithelia, all the cells at its outer
surface becoming the sensory rods and cones, while the deeper cells
become speciahzed as nerve cells arranged in two layers intervening

Fig. 98. — Sense organ of
the lateral line of the amphib-
ian, Triturus (Diemyctylus).
c, cone cells; s, spindle cells.
(From Kingsley, Comparative
Anatomy of Vertebrates; after
Kingsbury.)

Sustentacular Pore Gustatory
cell canal cell

Gustatory Pore Sustentacular
cell canal cell

Connective tissue
of tunica propria

Fig. 99. — Taste-buds from a vallate papilla of the human tongue; as seen in section
perpendicular to the surface of the epithelium. 5 is a diagrammatic representation
of the structure of one "bud." X47S. (From Morris, Human Anatomy.)

between the rod and cone layer and the optic nerve (Fig. loo). The epithe-
lial nature of the adult retina is obscured by the fact that the original open
space between the outer pigmented epithehum and inner sensory epithe-
lium is obliterated, thus bringing the two epithelial surfaces into close

138 COMPARATIVE ANATOMY

contact. In the adult state, then, they are epithelioid, but in origin and
essential nature they are truly epithehal.

Most receptors of external stimuli — that is, the exteroceptors of the
body — are more or less specialized sensory epithelia.

(4) Cilia are extremely delicate motile filaments borne by the free end
of an epithelial cell (Fig. 93). A single cell may carry from one (called
flagellum if especially long) to over a hundred cilia.

The vibratile beating of a cilium is caused by a motor mechanism which
may be either at the base of the cilium or within the filament itself. As in

Fig. 100. — Retina of a mammal. The upper part of the figure is a simpHfied repre-
sentation of a section through the retina; much enlarged. Below are shown the relations
of individual elements of the several layers. The cavity of the eye-ball is toward the
left, c, cone; cc, cone cell; g, nerve cells; ig, inner "granular" layer, the granular
appearance being due to numerous small nerve cells; im, inner "molecular" layer
consisting of the processes of the nerve cells of adjoining layers; in, basal membrane;
nf, nerve fibers of optic nerve; og, outer "granular" layer, the "granules" being the
deeper nucleated portions of the rod cells and cone cells; om, outer "molecular" layer;
r, rod; re, rod cell. (From Kingsley.)

rowing a boat, the stroke of a cilium is effective in one direction only. The
beat of all cilia in a region is not simultaneous, but progressive waves of
ciliary motion pass along the epithelial surface.

Beating of cilia may cause motion of the body on which the cilia are
carried, or of an external fluid medium within which the cilia beat, or it
may transport small solid bodies along the cihated surface. The gastrula
of Amphioxus is propelled through the sea water by beating of ectodermal
ciHa. The ciliated nephrostomes of the mesonephros (page 98) cause a
flow of coelomic fluid toward the mesonephric duct. Certain regions of
kidney tubules are ciliated. The cilia on the lining of the trachea and
bronchi propel inhaled foreign particles upward and outward. Oviducts,
efferent sperm ducts and many other hollow organs are lined by ciliated
epithelium.

HISTOLOGY 139

The simple external epithelium of the earthworm and the stratified
epidermis of a fish combine cuticular, glandular and sensory speciahzations.
CiHa and mucous glands commonly occur together in the same epithehum.
An epithelium, either simple or stratified, may be rendered sensory by the
presence of free nerve terminations, that is, the terminal twigs of a nerve
fiber ramifying amongst the epithelial cells (Fig. loi). These nervous
structures, however, are not produced by the epithehum itself but invade it
from adjacent tissue. The term "sensory epithehum " should be restricted
to those whose speciahzed sensory structures
are certain of the cells of the epithehum. ''^^^^'^ — ^^^^^^^^?^^^^^^^^~"^=^__^

The name "gland" is apphed to various \ }r\ y i r^h{f'''f\)jj^

organs which have Httle in common beyond rS^T^f^^^

a superficial similarity which may cause a S^-I^.y'^ ' ' ' ; .- ] \ \\

lymph gland to be mistaken for a salivary ( JH^ ' \ i

gland. In general, glands produce or dis- \ '
charge something. But organs whose prod- Fig. ioi.— Free nerve ter-

,... . J • •£: mination in the epidermis of

ucts are as different m nature and signifi- saiamandra. (From Kingsiey,
cance as are sweat, eggs and blood cells after Retzius.)
hardly merit the same name. Accepting

the name, it is at once necessary to distinguish the following funda-
mentally different types of glands: (i) secretory glands whose prod-
ucts are retained, at least temporarily, and serve the animal in some
useful way — e.g., mucous, sahvary and thyroid glands; (2) excretory
glands which ehminate waste from the body — the kidneys; (3) cjrtogenic
glands whose products are Hving cells — the reproductive glands which
produce sperm cells and egg cells, and the various lymph and hemal
glands, including the spleen, which produce white blood cells. The prod-
ucts of these cell-producing glands may either be retained to serve useful
purposes as are secretions, or discharged as are excretions. Blood cells
are retained, but reproductive cells are discharged by means of ducts which
are identical with or closely related to those which discharge waste from
the kidneys.

Secretory glands are of most diverse sorts. The simplest and presum-
ably primitive type of gland is the unicellular gland of a glandular
epithelium. The association of numerous secreting cells together to
form a glandular organ provides for a more efificient carrying on of
the secreting process. Such an organ is called a multicellular gland
(Fig. 94, g).

Nearly all multiceUular secretory glands arise directly from epitheha
and retain their epithelial character. Certain of the endocrine glands,
whether arising directly from epithelium or not, are epithelioid.

I40

COMPARATIVE ANATOMY

The great majority of secretory glands arise directly from the ectoder-
mal and endodermal epithelia and discharge at the surface of their native
epithelium. Such are the many kinds of integumentary glands and the
digestive glands. The mesoderm gives rise to some secretory glands,
especially in connection with the reproductive system— e.g., the albumen
glands and shell glands of oviducts and the mucous glands of the mam-
malian uterus.

A multicellular secretory gland, other than endocrine, is an invagination
of an epithelium into adjacent tissue (Fig. 102). The secretory activity
is usually Hmited to cells of the deeper part of the gland, the more super-
ficial part of it serving merely as duct.

Fig. 102. — Types of multicellular glands. A-D, tubular; E, F, alveolar or acinous.
A, simple; B, coiled; C-F, branched. The duct pierces the epithelium from which the
gland has been produced. (From Kingsley.)

Varying in the form which the invagination assumes, two main types
of gland are recognized. A tubular gland (Fig. 102, A-D) is one in which
the secretory portion and the duct are of approximately the same diameter.
An alveolar or acinous gland (Fig. io2£, F) has an enlarged and more or
less nearly globular secretory region. Glands of either type, complicated
by branching, are called compound.

The larger multicellular glands, and especially those which are com-
pound, require certain accessory structures. A good blood supply must
be provided. Therefore the gland may have an outer investment of con-
nective tissue containing blood vessels and lymphatics. A thin layer of
unstriated muscle fibers may be present on the wall of a gland which dis-
charges its contents abruptly. The muscle would be accompanied by
nerve fibers and in some glands nerves may be traced to the secretory cells.

Secretory glands in vertebrates range from unicellular mucous glands
in the skin of fishes and amphibians and in the digestive epithelium of all
vertebrates to such massive compound multicellular glands as the mam-
mary glands and the liver.

NON-EPITHELIAL TISSUES

The primarily essential parts of a metazoan animal are the epidermal
epithelium and the enteric epithehum. Certain of the organs which, in

HISTOLOGY

141

the adult, lie between these two layers consist of tissues which do not
retain the epithelial character of the embryonic tissues from which they are
derived, but give rise to more or less bulky and solid masses of material.
Non-epithelial tissues also play an important part in providing structures
accessory to the primary epithelia, as when connective tissue forms a
dermal layer of the skin, or muscular and connective tissues become associ-
ated with the enteric epithelium to form the wall of the digestive tube.

The important types of adult non-epithelial tissues are the following:
(i) muscular; (2) nervous, exclusive of neuro-epithelial structures; (3)
tissues serving for mechanical support — the connective and skeletal tissues;
(4) adipose tissue or fat; (5) blood.

Muscular Tissue

Locomotion in some protozoans is effected by beating of cilia. The
movements of large animals depend on contractile mechanisms. Con-
tractility is inherent in protoplasm. The least specialized protoplasm is

Fig. 103. — A, unstriated ("smooth") muscle cell with single nucleus; B, shows a
small portion of the length of a multinucleate striated fiber. (From Kingsley.)

apparently able to contract in the direction of any of its axes. When
protoplasmic mechanism for effecting vigorous, quick or long continued
contracting is established, the abihty to contract becomes restricted to one
axis. The protoplasmic structures which seem to be somehow imme-
diately concerned with contraction are exceedingly fine fibrils, the myofi-
brils, which extend through the cell parallel to the axis of contraction.
Some unicellular animals exhibit fibrils which are apparently of the nature
of myofibrils and serve as intracellular motor elements. In metazoa cer-
tain cells become more or less elongated and differentiate myofibrils
extending lengthwise of the cell — a muscle cell (Fig. 103). In the simple
small coelenterate Hydra, however, the contractile fibers which effect the
movements of the animal are not independent muscle cells but are merely
long processes from the basal ends of the epithelial cells. It is significant
that, in the absence of a mesoderm, the primary epithelia are able to
provide a motor mechanism. Such epithelial motor processes may have
been the forerunners of muscle cells.

Among invertebrates the usual type of muscle element is a much
elongated cell having a single nucleus, more or less numerous myofibrils

142 COMPARATIVE ANATOMY

extending through the protoplasm lengthwise of the cell, and having the
usual cell wall devoid of any special membranous covering. Such cells,
associated together to form layers, bundles or masses, constitute the
muscles of the body wall and the viscera. Certain invertebrates, however,
whose muscles are, in one way or another, especially efficient have muscle
cells of much more complex sort. The myofibrils become strongly
developed and each fibril exhibits an alternation of darker and lighter
zones. The zones of either type lie exactly alongside one another on
adjacent fibrils so that they give the impression of transverse bands or
striations extending continuously across the cell. Muscle cells of this sort
are called striated. Uninucleate striated fibers occur in the heart of some
mollusks. In arthropods, especially insects, striated fibers attain great
length, are multinucleate, and exhibit a most complex system of trans-
verse striations.

Vertebrates possess both striated and unstriated (or "smooth")
muscle (Fig. 103). In general the muscle of the body wall is striated and
visceral muscle is unstriated. But unstriated muscle occurs in the walls
of blood vessels which lie in the body wall, in connection with some skin
structures such as hair and certain glands, and also in the iris of the eye.
The muscles in the walls of the mouth, pharynx and at least the upper part
of the esophagus are striated, and it is said that striated muscle occurs in the
wall of the stomach of some fishes. Also the external anal muscle is
striated. The muscular part of the diaphragm is derived from the
embryonic body wall (see page 108) and its muscle is accordingly striated.
And in all vertebrates all of the muscle of the wall of the heart is striated.

In general, striated muscle is innervated by nerves of the cerebro-spinal
system and unstriated muscle, even in the body wall and skin, is inner-
vated by autonomic nerves. The old distinction to the effect that
striated muscle is "voluntary" and unstriated "involuntary" is inaccurate.
The striated muscles of the heart, diaphragm and esophagus are involun-
tary. The striated muscles of the vertebrate body wall and appendages
are capable of quick and powerful contraction. Heart muscle is notable
for its capacity for long-sustained rhythmic action.

Unstriated muscle fibers in vertebrates are much like those of inverte-
brates. They are ordinarily not over a fraction of a millimeter in length
and, in man, much less than a hundredth of a millimeter in diameter.
They are usually spindle-shaped (Fig. lo^A), lying in the tissue with their
tapering ends overlapping.

The somatic striated fibers of vertebrates are enormously larger than
unstriated fibers. Their diameter may approach a millimeter and their
length, not accurately known, doubtless reaches several or many milli-
meters. But these great fibers are not, in strict sense, single cells. They
contain scores or hundreds of nuclei. In lower vertebrates the nuclei are

HISTOLOGY

143

ordinarily scattered throughout the interior of the fiber, but in higher
vertebrates the nuclei are at the surface of the fiber as if crowded out of the
deeper regions by the closely packed myofibrils (Fig. 104). Probably

a ■

■>^'

^,^

I'

V'-'

' i

r'

A

\^

^

\

"^-

port!S;r;;;!ra11bt"Tet4^""t"- .^^°"'' '" '°^^^^''^^-' action, showing small
He at the su^aTiAt^^tl^V^or^^^^^^ Nuclei

however, the fiber is the product of a single mesoderm cell of the embryo
and many nuclei are derived, by repeated divisions, from the original
mesoderm nucleus. The fiber, therefore, is a syncytium rather than a cell.

. ! Tf °^ '^'^^'"^ ^^^'^ ^'^ "^"^h coarser than those of

unstriated fibers. They are imbedded in a peculiar fluid sarcoplasm

144

COMPARATIVE ANATOMY

||S*ia»fe!:-.=

B

A

Fig. 105. — Motor plates. A, from
guinea-pig; surface view of muscle fiber:
B, from hedgehog; section perpendicular
to surface of muscle fiber, g, granular
substance of the motor plate; w, striated
muscle; n, nerve fiber; t.r., terminal ramifi-
cation of the nerve fiber. (From Bremer,
Text-book of Histology; after Bohm and
Davidoff.)

which is probably a nutrient medium rather than ordinary cyto-
plasm. The wall of the fiber, much more prominent than an ordinary

cell-wall, is called the sarcolemma.
Some investigators maintain that
extraneous connective-tissue cells
may give rise to a second investing
membrane reinforcing the inner
sarcolemma which is a product of
the fiber itself.

The alternate dark and light
bands on the individual fibril are due
to physical differences such that,
in polarized light, the dark bands
are doubly refractive (anisotropic)
while the lighter bands are singly refractive (isotropic). Both the dark
and the light bands are traversed by finer markings, as seen under high

magnification, so that altogether a Nucleus. Sarcoplasm. Fibrils. Lateral branch.

very complex structure in the fibril
is indicated.

In the act of contracting, pro-
found changes occur in the appear-
ance and relations of the striations.
Undoubtedly the contraction of a
fibril is due to specific chemical
and physical differentiation within
the fibril; the contraction of a fiber
is the collective contraction of its
fibrils; and the contraction of a
muscle is the resultant of the action
of its numerous fibers.

The relation of an unstriated
fiber to its nerve is apparently of
the simplest sort. A terminal twig
of nerve merely attaches to the
surface of the fiber, the end of the
nerve often showing a knob-like
enlargement. Presumably every
striated fiber has a nerve connected
to it. The nerve, however, enters
a small flat plate of nucleated proto-
plasm lying superficially on the
muscle fiber. Within this motor plate (Fig. 105) the nerve ramifies into
fine twigs which seem to terminate in the substance of the plate.

X Conn, tissue. Capillaries.
Fig. 106. — Longitudinal section of a
papillary muscle from the human heart.
"x" marks the position of intercalated
discs. X240. (From Bremer, Text-book
of Histology.)

HISTOLOGY

145

Striated fibers are bound together in bundles enwrapped by a con-
nective-tissue perimysium. Thick muscles consist of several or many
such bundles wrapped together.

Cardiac muscle has striations which resemble those of somatic muscle
but the fibers are relatively short and they are branched. The sarcolemma
is less strongly developed than in somatic fibers. A peculiar feature of the
cardiac fiber is the presence of conspicuous transverse bands, the inter-
calated discs (Figs. 106 and 107), which are quite distinct from the ordinary
striations. Their significance is not known.
A cardiac fiber develops from a syncytium of
mesenchyme cells so that the adult fiber is the
product of several mesenchyme cells.

Human car-
. very small

Nervous Tissue
The proper relating of an animal to its
environment depends upon a nervous system
whose outposts must be at the surface of the
animal, therefore in, or at least very near,
the external epithelium. Central nervous
organs, presumably in evolution and certainly pj^. 107.-
in ontogeny derived from ectoderm, retreat to diac muscle;

. . o • 1* 1, portion seen under much

a protected deep position. Stimuh set up by higher magnification than

impact of external agencies must be carried to that used in Fig. 106. d,

1 . 1 • intercalated disc; Z, Krause s

deep central organs which must, m turn, cause membranes which lie trans-
appropriate muscular or other reaction. For versely at regular intervals
.. . r ^^ r ^ along each myofibril, bisect-

complete coordination of all parts of a large -^^^ ga,ch light band. The
complex animal it is necessarv that deep-seated distinction between light and

.... 11' 1 • J • 1. dark bands does not appear in

visceral activities be controlled in accord with ^^e figure. (From Bremer,
the external situation, and equally necessary Text-bookof Histology; after
that external actions fit the internal state.

Therefore the animal must have a system consisting of receptors
connected by conductors (nerves) to a coordinating central nervous
organ whence other conductors extend outward to the effectors which
are such organs as muscles, glands and the electric organs of fishes.
Receptors are distinguished as exteroceptors which are stimulated by
external agencies, interoceptors which pick up visceral stimuli, and pro-
prioceptors which lie within the motor mechanism of the body-wall — the
muscles, tendons and joints. The central nervous organ of an automobile
is the driver. His own eyes and ears serve as exteroceptors for the moving
car. Certain interoceptors register on the instrument board the condition
of the gasoline and water which are concerned with the metabolism of the
car. Then there are proprioceptors which, registering in the speedometer
and oil gauge, give information concerning the performance and intrinsic
condition of the motor mechanism.

146

COMPARATIVE ANATOMY

Fig. 108. — Types of nerve cells. A, multipolar cell; B, portion of nerve fiber with
sheaths; C, unipolar cell (such a cell may arise by modification of a bipolar cell as shown
in Fig. no); D, pyramidal cell (from cerebral cortex); a, axon; c, collateral; d, dendrites;
cb, cell body; m, medullary sheath; n, nucleus of cell of Schwann's sheath; r, node of
Ranvier; s, sheath of Schwann; t, telodendron. (From Kingsley.)

A

B

Fig. 109. — Cell-bodies of neurons showing arrangement of neurofibrils. A, from
human spinal ganglion; two cut fragments of the neuraxon lie near the cell-body. B,
"giant pyramidal cell" from human cerebral cortex. Highly magnified, a, neuraxon.
(From Morris, Human Anatomy.)

HISTOLOGY 147

Nerves which conduct impulses from receptors toward the central
nervous organ are called sensory or afferent. Those which conduct from
a central organ to some effector are called motor or efferent.

Nerves conducting impulses from exteroceptors or proprioceptors to
the central organ are somatic sensory (afferent); those conducting from
the central organ to the striated muscle of the body wall are somatic motor
(efferent). Similarly visceral sensory and visceral motor nerves are
distinguished.

All nervous functions are carried on by protoplasm organized, as
always, in cells. To say, as is often done, that nervous tissues consist of
nerve cells and nerve fibers is inaccurate. So far as known, every fiber
which conducts nervous impulses is developed as an outgrowth from a cell
and can function and survive only so long as it remains in physical and
physiological continuity with the nucleated Bipolar cells. T-ceii.

region of the cell of which it is an integral I I _,„.^-^V'/

part. Any cell engaged in nervous opera- Lr-^^f^^^^^-^^^'"''^ J^''^^

tions, together with all conducting fibers ^/^ ^^ ^.—/^

which have grown out from it, is called a /^ ^^ \^i

neuron. ^ mL

A central nervous organ is a more or w

less complex system^ of physiologically ^ J- :^X;;SS'^'7^

related neurons serving for the proper cell is transformed into a unipolar

association, coordination and integration of '^^^-Tlrt^x °r» 5

nervous impulses. A ganglion is a minor spinal nerves. (From Bremer,

locaHzed nerve center consisting of the cell- Text-book of Histology.)
bodies of neurons together with the adjacent regions of their nerve
processes. The neurons of such gangha as the dorsal or spinal gangHa of
vertebrates apparently serve only for conduction — that is they do not
initiate nerve impulses.

Neurons are of various types depending on the form of the cell-body
and the number of nerve processes (Figs. io8 and 109). Unipolar cells,
of comparatively rare occurrence, have a single process ; bipolar neurons are
usually spindle-shaped and have a process at each end ; multipolar cells have
several processes of which one, the neuraxon (axon or neurite), is rela-
tively long, while the short dendrites branch out into fine twigs which end
within a short distance of the cell-body. The dendrites seem to be proto-
plasmic but the neuraxon is devoid of the granules which characterize the
protoplasm of the cell-body. The neuraxon may give off lateral branches
(collaterals) and its distal extremity breaks up into fine branches forming
the terminal arborization.

The unipolar cell characteristic of the dorsal gangha of adult vertebrates
is apparently a "disguised bipolar" cell. At least in some animals it
develops from an embryonic bipolar cell which, in effect, becomes bent

148 COMPARATIVE ANATOMY

upon itself so that the two processes are joined into one which divides
into two near the cell-body (Fig. no).

Most types of receptor neurons are epithehal. In some of these the
receptor cell itself produces a nerve fiber which conducts to the central
organ (e.g., an olfactory cell and its fiber). In such case, one neuron serves
as both receptor and conductor. In other cases, as in the auditory organ,
the epithehal receptors do not produce nerve fibers but are intimately
related to the terminal twigs of afferent nerve fibers whose cell-bodies lie
in some deep ganghon such as the acoustic ganghon or a spinal ganghon.
Between the receptor neuron and an ultimate motor neuron may
intervene a chain of several neurons, the nerve impulse being relayed from
one member of the chain to another.

Nerve cells vary greatly in size, but in general are relatively large.
They are often the largest cells in the body exclusive of eggs. When the
possible length of a neuraxon is considered, the dimensions of some neurons

become impressive as contrasted with
the usual microscopic size of cells.
A neuron whose body lies in a spinal
ganglion of the giraffe presumably has
a nerve process extending to the
extremity of a front leg.

Some of these very large nerve
cells have two or more nuclei. Their
cytoplasm sometimes exhibits a sys-
FiG. iii.-Nerve cell, with processes ^^^ ^^ ^^^^ narrow branching canals

cut short; from human spinal cord. ^ _ o _

X430. (From Bremer, Text-book of apparently Serving for circulation of
Histology.) lymph which enters from the adjacent

intercellular lymph space. The cell may be supported by a delicate close-
fitting membrane of connective tissue, strands of which may enter, the
cytoplasm.

The most striking characteristic of the body of a neuron is the presence
of large masses of a granular substance which has a strong affinity for
the anilin dye, methylen blue. These Nissl's bodies (Fig. in) have been
shown to become reduced in neurons which have been excessively active,
indicating that the bodies contain something which is a source of energy
for nervous activity. Less conspicuous are the neurofibrils (Fig. 109),
extremely fine fibrils which are ordinarily seen only after use of special
staining methods. Such neurofibrils may form an elaborate system within
the body of the neuron and may be traced into the neuraxon and larger
dendrites. The appearance and arrangement of these neurofibrils strongly
suggest that they are speciahzed avenues for conduction of impulses.
The objection has been made, however, that they are artificial products of
the special preserving and staining technique which is employed.

HISTOLOGY 149

The neuraxon is a delicate tliread consisting of a probably modified
protoplasm in which, as just mentioned, neurofibrils may be demonstrated.
It may be surrounded by one or even two special ensheathing layers.
The medullary or myelin sheath is a relatively thick layer of fat-like
substance, myeHn, fitting the neuraxon closely. The neurolemma or
sheath of Schwann is a cellular layer wrapped around the neuraxon. Its
cells are so exceedingly thin that the layer could hardly be detected were it
not for the nuclei which at intervals occasion roundish bulges in the con-
tour of the nerve fiber.

A neuraxon may possess either, both, or neither of these two sheaths.
When both are present the myelin sheath is always next the nerve fiber
and, at fairly regular intervals (in man averaging about 0.5 mm.) along
the fiber, it seems to be nearly or quite interrupted so that the neurolemma
there comes into close relation with the nerve fiber (Fig. loSB). The
neuraxon therefore presents a segmented appearance due to these nodes
of Ranvier. Usually only one neurolemma nucleus can be found between
successive nodes. A single internodal segment therefore probably repre-
sents the territory covered by one sheath cell.

Nerves whose individual fibers possess the myehn sheath appear more
nearly white than do non-medullated nerves. The so-called "white"
parts of the brain and spinal cord consist mainly of meduUated nerves.
Non-medullated fibers and the cell-bodies of neurons are the chief con-
stituents of "gray matter."

On meduUated nerves within the brain and spinal cord no neurolemma
can ordinarily be found, but meduUated fibers in nerves external to the
brain and cord commonly have both sheaths except in the vicinity of the
terminal arborization of a neuraxon. The myelin sheath stops short of
the arborization. The neurolemma may continue alone somewhat further,
but the terminal twigs of the neuraxon are always bare.

Most autonomic fibers and the fibers of the olfactory nerve are non-
medullated. A cellular neurolemma may occur on a non-medullated fiber.

The sheaths doubtless serve for the protection, insulation and nutrition
of the nerve fiber. The source of the myehn is not definitely known.

A n-ervous organ is constituted of neurons supported by connective
tissues accompanied by vascular tissues. In the brain and spinal cord
of vertebrates occurs not only the usual mesenchymal connective tissue
but another which is unique in that its cells have ectodermal origin in
common with the nerve cells. Some of the cells of this neuroglia possess
branched processes which make them confusingly similar in appearance to
nerve cells. The neuroglia cells form, by means of their processes, a
supporting network for the nerve cells.

A nerve is a bundle of neuraxons, each of which may be ensheathed as
described above, and all wrapped together within a sheet of connective

I50

COMPARATIVE ANATOMY

tissue, the perineurium (Fig. 112), extensions of which (endoneurium)
may penetrate into the bundle. Larger nerves consist of several or many
bundles all tied together by connective tissue and enwrapped by a rela-
tively thick epineurium. Small blood vessels traverse the connective-
tissue layers of the nerve.

The component neuraxons of a nerve may include one or more of the
four types of conductor, somatic afferent and efferent and visceral afferent
and efferent. All four kinds of "nerve components" occur in a spinal
nerve. In cranial nerves fibers which are connected with the special
sensory mechanisms of the head are designated as "special" components.

Fat cells. ;:r

Artery.

Bundles of nerve fibers

Epineurium.

Perineurium.

Endoneurium.

Fig. 112. — Structure of a nerve. The figure represents a small part of a transverse
section of a large nerve constituted of many bundles of medullated fibers. X20.
(From Bremer, Text-book of Histology.)

Tissues Serving for Mechanical Support

Protoplasm is a substance of semi-fluid or gelatinous consistency.
An elephant constituted of protoplasm only would be a mechanical
impossibility. Large animals, especially if they are land animals, require
mechanical support. That essentially protoplasmic and socially organized
being which is mankind provides for certain of its physical needs, both
individual and collective, by use of various mechanical, architectural and
engineering contrivances which are external to the human body and con-
structed of non-hving materials derived from the environment. Similarly
the organized cellular protoplasm which is an animal appropriates various
materials from the environment and builds them into non-hving structures
which are external to the cells and physically adapted to the mechanical
needs of the animal as a whole and of its parts. Just as man uses steel
wires, various cements and masonries, so the animal has its connective-
tissue fibers, intercellular cements and that most delicate masonry, bone.

HISTOLOGY

151

The basis of the material of these supporting structures consists of
various nitrogenous or protein substances. By impregnation of the mate-
rial with inorganic salts, chiefly those of calcium, hard or rigid supporting
structures are produced. The protoplasmic or cellular agencies concerned
in building the supporting tissues are mesenchyme cells, except in the
cases of the notochord and the pecuHar connective tissue of nervous organs,
the neuroglia (page 149) which is ectodermal.

The embryonic precursor of supporting tissues other than the excep-
tions mentioned is a more or less spongy mesenchyme (Figs. 69 and 113^)
whose individual cells have branching processes by means of which the

^-^7^

-^.S-"

v

%%

Tk

5 ^

■ -■■■r'f

A

A B

Fig. 113. — A, mesenchymal tissue from embryo of the amphibian, Ambystoma.
B, pigment cells from Ambystoma; below, a cell with pigment dispersed in numerous
branched processes of the cell; above, a "contracted" cell with pigment concentrated,
the transparent processes not shown. C, part of a tendon in longitudinal section, highly
magnified; nucleated cells are seen among the tendon fibers. (From Kingsley.)

cells are joined together. The spaces within the meshwork of cells is filled
by a homogeneous fluid substance, the matrix. Presumably the cells are
the source of the matrix.

CoNNECTrv'E Tissue

Connective tissue consists essentially of fibers. The source of the
fibers has long been a matter of controversy. The earlier behef that they
are formed in the protoplasm of cells has largely given way in view of much
evidence that they arise in the intercellular matrix by some process
resembling precipitation, the cells probably serving merely to bring about
the reaction in the matrix.

The ordinary connective tissue which is present throughout the body
of a vertebrate serves to bind structures together or to ensheath them,
provides internal support for massive soft structures or, in general, affords

152

COMPARATIVE ANATOMY

resistance to tensile strain. The essential mechanical structures in such
tissue are relatively coarse white fibers (Fig. 114) consisting of an albumi-
noid substance, collagen, the source of gelatin and glue. These collagenous

Fig. 114. — Subcutaneous connecuve libsue from a cat. Highly magnified. The
fibers were artificially separated and examined, unstained, in water, except the fiber
marked "a" which was treated with dilute acetic acid, a, c, white fibers; b, fat cell;
d, connective-tissue cell; e, elastic fibers. (From Bremer, Text-book of Histology.)

fibers are only slightly elastic. They may be branched. Each fiber is a
bundle of very delicate fibrils. Exceedingly flattened cells with flat

nuclei appear as if clinging closely to the
surface of a fiber. These connective-tissue
cells or fibrocytes are presumably the
agencies which have brought about the
production of the fiber.

Elastic fibers (Fig. 114, e) are much
finer than collagenous fibers and differ from
them chemically in being composed of
elastin which is not a source of gelatin. An
occasional elongated fibrocyte may be seen
stretching along the surface of a fiber
(Fig. 115). Elastic fibers commonly occur
intermingled with collagenous fibers. The
elasticity of a connective tissue as a whole
depends upon the relative abundance of
elastic fibers in it.

Connective tissue forming a loose open
mesh-work, as does the subcutaneous tissue
lying between the skin and the muscle of
the body, is called areolar tissue.

Reticular tissue is composed of cells whose branched processes join
to form a network. Associated with the processes are compara-
tively short fibers which seem to be similar to collagenous fibers.

Fig. 115. — A, elastic fibers of
the subcutaneous areolar tissue of
a rabbit. B, cells related to
elastic fibers, as seen after treat-
ment with acetic acid; from sub-
cutaneous tissue of a pig embryo.
(From Bremer, Text-book of
Histology: A, after Schafer; B,
after Mall.)

HISTOLOGY 153

This tissue forms the supporting framework of such organs as lymph
glands and the spleen. In form and arrangement its cells resemble
embryonic mesenchyme cells. The spaces within the network are occu-
pied by a fluid substance in which are numerous lymphocytes and other
cells.

Tendons and ligaments are connective-tissue structures highly adapted
to resisting tensile strain. Tendons (Fig. 113C) consist of coarse collage-
nous fibers lying parallel to one another in compact bundles. Tendons are
inelastic. The elasticity necessary to a ligament is due partly to a certain
degree of elasticity in the individual fiber and partly to the fact that the
fibers are wavy in contour when not under strain and are more or less
interwoven.

Chromatophores, pigment cells (Fig. 1135), may occur in connective
tissue, especially in the dermal layer of the skin. The specific pigment
appears as granules lying in the cytoplasm. Black pigment (melanin) is
most common and cells containing it are called melanophores. Chroma-
tophores are usually richly branched. The pigment may at one time be
distributed throughout the processes ("expanded" phase), at another
time densely massed in the central part of the cell ("contracted" phase).
Some pigment cells are migratory.

Skeletal Tissues

Notochord. Among skeletal structures the notochord is histologically
unique in that its tissue is for the most part neither hard nor even solid;
it is fluid. It is exceptional also in its embryonic origin. It is not a
product of mesenchyme but arises simultaneously with the mesoderm and
in much the same way although independently of it (see page 86).

The essential notochord material consists of cells each of which contains
a relatively enormous vacuole occupied by a substance of fluid or possibly
gelatinous consistency. The cytoplasm of the distended cell is so
stretched that it appears as the thinnest possible layer surrounding the
vacuole. The very flat nucleus occasions a bulge in the contour of one
side of the cell (Fig. 116). The outer cell-membrane, while very thin, is
probably of semi-rigid consistency. Seen under the microscope, this
tissue looks hke a mass of soap bubbles crowded closely together, the cyto-
plasm and ceU-membrane of each cefl being the waU of a bubble.

The vacuolated notochord tissue is enclosed by sheaths which differ in
number and nature in various animals. There is commonly an inner
elastic sheath (Fig. 116, ei) composed of material secreted by an outer
epithelioid layer of the notochord tissue, and a thick outer sheath of dense
fibrous connective tissue.

The capacity of the notochord as a whole to resist deforming strains is
doubtless due in part to the cell walls of the vacuolated tissue, these walls

154

COMPARATIVE ANATOMY

collectively forming an alveolar framework for the inner tissue. It must,
however, be due largely to the enclosing sheaths which, taken all together,

Fig. ii6. — Developing vertebrae of the amphibian, Ambystoma; I, earlier; II, later.
Longitudinal sections. Cartilage and bone are forming around the notochord. cc.
cartilage in center of vertebra; ei, epithelioid internal elastic sheath of notochord; i,
incisure cutting through ic, intercentral (intervertebral) cartilage; w, notochord; ns, outer
notochordal sheath; v, developing bone (black) of centrum of a vertebra. (From
Kingsley.)

are tough and not highly elastic. Mechanically, the notochord resembles a

length of rubber tubing, closed at the
ends, and filled with liquid under
some degree of pressure.

Cartilage. In development of

cartilage, mesenchyme cells become

densely massed and then produce

an abundant intercellular substance

whose accumulation causes the cells

to become more or less widely

separated from one another (Figs.

76, 117, 118). The intercellular

substance, the matrix, becomes

solid and acquires a firm or even

hard consistency. Chemically it is a

complex of collagenous, albuminoid and other protein substances. The

cartilage cells remain imbedded in the matrix, each occupying a

close-fitting space, a lacuna. In some cartilages have been described

Fig. 117. — Hyaline cartilage with
cartilage cells lying in lacunae enclosed
by the matrix. Thin section highly
magnified. (From Kingsley.)

HISTOLOGY

155

exceedingly fine canals penetrating the matrix and putting any one
lacuna into communication with neighboring lacunae.

The external surface of cartilage is invested by a membrane, the peri-
chondrium, consisting of collagenous connective-tissue fibers, densely
arranged and for the most part parallel to the surface of the cartilage.

Capsule.

Perichond

Matnx.

Bl.v.

Fig. 118. — Hyaline cartilage, with perichondrium; from human trachea. Bl.v.,
blood vessel; x, cartilage cell whose nucleus is not in section; y, new matrix forming
between two cells resulting from a recent division of a cartilage cell. (From Bremer,
Text-book of Histology.)

The perichondrium contains blood vessels but they do not penetrate into
the cartilage. Hence cartilage cannot occur in thick masses.

In growing cartilage, cells from the adjacent surface of the peri-
chondrium become cartilage cells and add cartilage to the exterior of the
mass already formed. At the same time growth may take place within
the mass. A deep cartilage cell divides. The resulting two cells secrete
matrix substance whereby they become separated, each to lie in a lacuna
of its own. In growing cartilage, therefore, the matrix substance is not
so firmly "set" that it may not expand as a result of such internal or
interstitial growth.

156

COMPARATIVE ANATOMY

Several types of cartilage are distinguished. Hyaline cartilage (Fig.
118), appearing bluish and clear or translucent, has a matrix which is quite
or nearly devoid of fibrous material. Fibre -cartilage is, so to speak, a
hybrid of cartilage and connective tissue. Its matrix consists partly of
hyaline material but largely of fibers similar to those of ordinary connec-
tive tissue. Elastic cartilage contains numerous elastic fibers which impart
to it a yellowish color and a more or less elastic texture. Calcified
cartilage is rendered white and relatively hard by deposit of calcium salts
in the matrix.

Bone. Cartilage and bone are similar in that their essential skeletal
material is an intercellular matrix within which are imbedded the cells
which produced it. Bone differs from cartilage in that the matrix is

Fig. 119. — A, stereogram representing a sector of the shaft of a long bone. B,
transverse section, much more enlarged, showing part of one Haversian system, bl,
bone lamellae; c, canaliculi; h. Haversian canal; I, lacuna. (From Kingsley.)

highly calcified and correspondingly. hard and also in that it never exhibits
the apparent homogeneity of the matrix of hyahne cartilage but is disposed
in very thin parallel layers. Usually the deeper substance of a bone is of a
porous or spongy texture (cancellous bone) while the outer region is dense
or solid (compact bone).

A section of fully developed compact bone, seen under high magnifica-
tion, shows the matrix layers or lamellae arranged in parallel or concentric
order (Figs. 119^ and 120). Between adjacent lamellae are minute cavi-
ties, the lacunae. A lacuna is broad in any direction parallel to the surface
of a lamella but very thin in the direction perpendicular to that surface.
Exceedingly fine canals, the canaliculi, extend between each lacuna and
neighboring lacunae, piercing the intervening lamellae. In bone of a
hving animal each lacuna is occupied by a living bone cell (osteoblast)
from which processes extend into the adjoining canaliculi and may even
unite with similar processes from the occupants of neighboring lacunae.
It follows, therefore, that the bone is penetrated by a system of continuous
spaces and it is probable that in young bone the bone cells, joined by their
processes, constitute a continuous network of protoplasm throughout the
bone.

HISTOLOGY

157

All external surfaces of bone are covered by a membrane, the peri-
osteum (Fig. 77), of dense fibrous connective tissue well supplied with
blood vessels. Most bones, notably the long bones of the appendages,
have internal cavities (Fig. 208) occupied by a more or less vascular soft
tissue the marrow. The ''yellow marrow" of long bones contains much
fat. "Red marrow" is very highly vascular and contains little fat.
Marrow has a framework of reticular tissue in whose spaces occur marrow
cells (myelocytes) and various types of blood cells.

w

Fig. 120. — Section, highly magnified, of compact bone from the shaft of the human
humerus. The section, cut transversely to the long axis of the bone, shows four
Haversian systems with their central canals, concentric lamellae of bone, lacunae between
adjacent lamellae, and canaliculi extending between lacunae. (From Bremer, Text-
book of Histology; after Sharpey.)

Blood vessels from both the periosteum and the marrow enter and
branch throughout the bone. From these vessels substances necessary
for the metabolism of the bone cells diffuse through the system of con-
nected lacunar spaces.

In long bones the larger blood vessels lie approximately parallel to the
long axis of the bone. Around such a vessel the bone lamellae are arranged
in concentric order (Figs. 119 and 120) forming a so-called Haversian
system. The central space, occupied by the blood vessel, is called a
Haversian canal. These concentric systems are much less prominently
developed in flat bones.

The matrix of bone consists of commingled organic and inorganic
materials. Collagenous and other protein substances constitute the

158 COMPARATIVE ANATOMY

organic part while various salts of calcium, mostly the phosphate and
carbonate, are the most important inorganic ingredients. The stratified
structure of the matrix is primarily due to the arrangement of collagenous
fibers in the organic basis of the matrix. The subsequent depositing of
calcareous substance in relation to the stratified fibrous material produces
the laminated structure of the finished bone.

The development of bone has been briefly described (pages 103-106)
and the distinction between cartilage bone and dermal or membrane bone
has been made. Differing in mode of development, bones of the two kinds,
when fully developed, are histologically ahke. Just as the perichondrium
adds cartilage at the surface of an already-formed mass, so the periosteum
builds lamellae at the surface of a bone. But bone, because of the rigidity
of its calcified matrix, is incapable of such interstitial growth as occurs in
cartilage, unless it be at stages of bone formation so early that the matrix
is not yet fully calcified. A further difference between cartilage and bone
lies in the fact that the cartilage cell produces matrix in all directions and
thus surrounds itself by its own product, whereas the osteoblast produces
matrix only at such part of its surface as is adjacent to the already-formed
bone. A layer of bone cells building up lamella upon lamella of bone may
be likened to a group of masons laying course upon course of stone at the
unfinished top of a wall. But, in the case of the bone, every now and then
one of the masons, an osteoblast, is left behind and buried between succes-
sive courses of the wall, remaining there in his Httle lacuna as a permanent
bone cell.

Bone once formed may be broken down or "resorbed." By this
means the marrow cavity of a long bone is enlarged as the bone grows.
The nature of the resorption process is not definitely known, although it
has been attributed to action of cells called osteoclasts (Fig. 77).

Adipose Tissue

Adipose tissue or "fat" consists of cells each of which contains a
globule or vacuole of oil so large that the cytoplasm appears as merely
an exceedingly thin layer surrounding the vacuole (Fig. 121). The flat
nucleus Kes in the peripheral layer of cytoplasm. The irregular polyhedral
form of the cells is doubtless the result of their mutual pressures. As seen
in sections, fat looks very much Hke notochord tissue, the individual cell
of each being greatly distended by its fluid vacuole.

Fat develops in richly vascular regions, as along the course of large
blood vessels and in the subcutaneous connective tissue.

Blood

Blood is the circulating medium which provides for the metaboUc
needs of tissue cells which are remote from the source of food and oxygen

HISTOLOGY

159

and must be relieved of their waste products. Its circulatory function
requires that it be fluid but various special services are rendered by cells
suspended in the fluid, some of them passively carried by it, others capable
of independent motion somewhat Hke that exhibited by an ameba.

The fluid part of blood, the plasma, is water containing all the other
substances which enter into the constitution of protoplasm. In its
inorganic chemical constitution, the plasma resembles sea water. Derived
from the food are various proteins and carbohydrate substance (sugar),

c.t.-

-c.t.

bl.v. i bl.v.

Fig. 121. — Fat cells in subcutaneous tissue of a human embryo of four months.
U.V., blood vessel; c.t., white connective-tissue fibers; ^6., young fibrocyte; mes., mesen-
chymal cell; X, young fat cell, nucleus not in section; i, 2, 3, developing fat cells. (From
Bremer, Text-book of Histology.)

while fat may be carried as minute droplets suspended in the plasma.
Other important constituents of the plasma are hormones produced by
such endocrine glands as the pituitary, thyroid and suprarenal, and the
various wastes of metabolism.

In the coagulation of blood, on exposure to air or under some other
circumstances, a nitrogenous substance, fibrinogen, carried by the plasma
in solution, becomes transformed into fine solid filaments of fibrin (Fig.
122). The uncoagulated portion of the plasma is called serum. The
"clot" is a mass of fibrin with blood cells caught in its meshes.

Blood cells are of two main kinds, red corpuscles or erythrocytes and
white corpuscles or leucocytes. The red cells are much more numerous.

i6o

COMPARATIVE ANATOMY

In human blood the red cells outnumber the white in the ratio of five or
six hundred to one.

Erythrocytes (Figs. 122 and 123) are relatively small and usually have
the form of flat discs with elliptical outlines. These blood cells are heavily
loaded with hemoglobin, a very complex protein substance containing iron
and having a strong affinity for oxygen which the cells pick up at the
respiratory surfaces of the animal. The red cells are therefore the oxygen-
carriers. Their color is due to the hemoglobin. The mature erythrocytes
of all vertebrates except mammals are nucleated, although the nucleus

Fig. 122. — Coagulated blood. Biconcave red corpuscles arranged in "rouleaux";
filaments of fibrin radiating from minute blood plates. (From Bremer, Text-book of
Histology; after Da Costa.)

seems to be in a more or less degenerate condition. But in adult
mammals the red cells in course of their differentiation lose their nuclei,
thereby acquiring the form of concavo-convex discs (Fig. 122).

Erythrocytes are produced in mesenchymal tissue in the liver and in
the spleen of embryos, but in the adult their chief source is probably the
red bone-marrow. They serve as oxygen-carriers for a limited time and

Fig. 123. — Cells from smear preparation of normal human blood, Wright's stain.
In the center: adult red blood corpuscles, blood platelets and a polymorphonuclear
neutrophile. At left above: two polymorphonuclear basophiles and two polymorphonu-
clear eosinophiles. At right above: three large and four small lymphocytes. At left
below: polymorphonuclear neutrophiles; two of these cells, the uppermost and lower-
most of the group, are young, with merely crooked nuclei; the mature cells have multi-
lobed nuclei. At right below: six monocytes; in the younger cells the nuclei tend to be
rounded, in the adult cells they are horseshoe-shaped, indented or lobed. (From
Bremer, Text-book of Histology.)

HISTOLOGY

l6l

'• .,• _.*-■•- v•

•»•;Cv

•^^' <»V

•v.

••'-Ai^'

%

/»'/

/'^r.

5> fe

aP^^^"'

-m^^

.^

e:- 7-;>,77r"

Fig. 123. — (See page 160 for description).

1 62 COMPARATIVE ANATOMY

then break down and are removed from the blood by action of phagocytic
cells in the hemal glands and spleen.

Leucocytes are permanently nucleated and do not carry hemoglobin.
Several types of leucocyte are recognized (Fig. 123):

Lymphocyte; usually small (occasional larger ones), cytoplasm
scanty and usually non-granular, nucleus spherical and relatively
large.

Large mononuclear leucocyte (monocyte); larger than lym-
phocyte, non-granular cytoplasm, nucleus excentrically placed in
the cytoplasm and spherical, or indented on the side adjacent to
the larger cytoplasmic mass.

Polymorphonuclear leucocyte; large, with conspicuous gran-
ules in cytoplasm, nucleus indented, lobulated, irregular or
separated into two or more parts. Several kinds of polymorpho-
nuclear leucocytes are distinguished on the basis of the reaction
of their granules to anilin dyes. Basophiles have granules which
take basic stains; eosinophiles have an affinity for eosin, an acid
dye; the granules of neutrophiles take both basic and acid dyes.
Most leucocytes are capable of active ameboid motion and many are
phagocytic. Some of them are apparently capable of penetrating the wall
of a blood vessel and emerging into intercellular spaces in neighboring
tissue. They are produced in the lymph glands, in bone marrow, and in
lymphoid tissue variously situated throughout the body.

Blood Plates. In addition to the red and white blood cells, blood
contains minute bodies which seem to be protoplasmic and yet are not
nucleated. These blood plates, much smaller than the smallest blood
cells, probably result from fragmentation of cells in bone marrow or else-
where. Their function is not certainly known but they seem to have some
relation to the clotting of blood as indicated by the fact that the filaments
of fibrin tend to radiate from blood plates.

Lymph, as found in the lymphatic vessels, resembles blood but differs
from it chiefly in lacking erythrocytes and therefore being colorless. The
fluids occupying the several coelomic spaces and the cavities of brain and
spinal cord, the aqueous humor of the eye and the amnionic fluid are all
of the general nature of lymph but contain relatively few cells and differ
from one another in details of chemical constitution.

HISTOLOGICAL SPECIFICITY

In general, histological differences are less conspicuous than the corre-
sponding anatomical differences— that is, tissue of a particular sort may
enter into the constitution of most unUke organs. Bricks from the same
kiln may be used in the construction of either a church or a brewery.

HISTOLOGY 163

Unstriated muscle fibers appear much the same whether they are in the
wall of a stomach or of a lung.

Nevertheless tissues and cells may exhibit characteristics which mark
them as belonging to a particular organ or animal. The nerve cells of a
spinal gangUon differ from the motor nerve cells in the spinal cord of the
same animal. The striated muscle of the vertebrate heart differs from
that of the body wall. Further, vertebrate cardiac muscle differs from
cardiac muscle of a lobster. Epidermal tissue of a fish differs from that of
a reptile. Anyone familiar with the nervous tissues of annelids could
easily distinguish gangHonic tissue of an earthworm from that of a leech.

It follows, therefore, that the individual tissue cell may, in its visible
structure, exhibit characteristics reflecting as many as four grades of
organization. First there are those cell organs, such as nucleus and
chromatin bodies, which are common to all cells and which represent the
fundamental organization of protoplasm as cells. Then there are those
intracellular structures such as myofibrils or neurofibrils which mark
the cell as belonging to a particular tissue — muscular or nervous. Thirdly,
there may be features which identify the tissue as that of a certain organ ;
for example, the intercalated discs in the heart muscle of vertebrates.
Finally, the individual tissue element may have peculiarities which are
specific for animals of a certain group; for example, the striated muscle
fiber of an insect differs in details of structure from that of a vertebrate.

Beneath all of this structural differentiation there is chemical speci-
ficity. Although it does not necessarily always manifest itself in visible
structural differentiation, there must exist from top to bottom of this series
of levels of protoplasmic organization a chemical specificity corresponding
to a specific genetic group of animals — or, indeed, corresponding even to
the individual animal.

CHAPTER 4

THE INTEGUMENTARY SYSTEM
EVOLUTION OF THE INTEGUMENT

Since all life involves continual adjustment of processes within the
organism to conditions outside, the skin and its appendages which mediate
this relation are highly important organs.

Even among the protozoa, an external semipermeable membrane
separates the living protoplasm from the surrounding medium. Most
protozoa have in addition an outer differentiated layer of clearer cytoplasm,
the ectosarc, analogous in function to the skin of the higher animals
though without genetic relation.

A true multicellular skin appears first in sponges and coelenterates,
the ectodermal layer of hydra being a familiar example.

Even in so simple a skin as this, there is some differentiation among
cells. Most are epithelial covering cells, each commonly prolonged at its
base into a contractile thread. But among these are gland cells, which
by their different secretions in different coelenterates indicate a wide
difference in metabolic processes. The secretion of lime salts by the skin
of coelenterates may be regarded as the beginnings of the exoskeleton of
many higher invertebrates.

Most invertebrates retain essentially unaltered the simple epithelial
ectoderm of coelenterates. Some have a ciliated epidermis which aids
locomotion. Many secrete an external cuticula, in which lime may or
may not be present.

The evolution of a simple epithelium into a stratified epidermis, such
as occurs in vertebrates, results, presumably, from a change in the direction
of cleavage planes during cell multiplication. So long as cell walls form
perpendicular to the surface, a simple epithelium results. When, however,
cleavage planes form parallel to the surface, the membrane becomes
stratified. The outer layers of cells serve to protect the lower layer where
growth and cell multiplication take place. In animals exposed to dry
air, an outer layer of dead cells is obviously adaptive. Yet the beginnings
of a protective outer layer appear in the exoskeletons of water-dwelling
invertebrates. Among invertebrates appears also, though exceptionally,
a connective tissue layer or corium beneath the epidermis.

The lowest chordates (balanoglossus, ciona, amphioxus), have both an
outer epidermis and an inner corium; but the epidermis is only a single

164

THE INTEGUMENTARY SYSTEM

l6:

layer of cells. Gland cells are numerous in the epidermis of amphioxus,
and they secrete a thin cuticle like that of annelids. The corium in
amphioxus is gelatinous.

Although the epidermis is stratified in all vertebrates, such low forms
as cyclostomes do not have the outer dead horny layer, and they do have
the thin cuticular layer of the invertebrates and amphioxus. The skin
of fishes is like that of cyclostomes, except for differences in gland
secretions. See Fig. 1 24.

The outer dead horny layer of the epidermis, the corneum, appears
first in amphibia, correlated apparently with the land habit, since most

A. AMPHIOXUS.

a PETROMYZON.

DUCT
EPIDERMIS- —
L
STRATUM CCTMINATIVUM.

MUOJS GLANDS

CORIUM

LOOSE 03MCCTIVE TISSUE'

COMPACT CONNECTWE TISSUE-

C.SQUALUS. D.RANA.

Fig. 124. — Cross sections of the skin of four chordates, amphioxus, petromyzon,
squalus, and rana, showing the fundamental differentiation of the skin into corium and
epidermis. The differentiation of the epidermis into a dead outer layer and an inner liv-
ing layer began in aquatic animals. (Redrawn mainly after Plate and Schimkewitsch.)

land animals have it. As the amphibian skin is fundamentally like that
of higher vertebrates, the evolution of the skin beyond the amphibia
presents no serious difficulties. The striking differences are in the secre-
tions of the glands. It is indeed difficult to imagine how the skin mucus
of amphibians could have evolved into the milk of mammals. It should,
however, be remembered that slight chemical differences often result in
striking differences in properties; so that we should not be surprised to find
chemical differences in the skin secretions of vertebrates that are far greater
than any morphological differences in the glands themselves.

i66

COMPARATIVE ANATOMY

STRUCTURE OF THE HUMAN SKIN

The skin of man, together with its appendages, hair, nails, teeth, mem-
brane bones, and glands, is only about four per cent of the body weight.
Like that of other mammals, it consists of two tissues, an outer epidermis
and an inner connective tissue corium.

:\ty-

SWEAT gland''-'

Fig. 125. — A cross section of the thickened skin of the sole. The stratum corneum is
especially thickened on the sole and on the palm of the hand.

A cross section of the epidermis shows under the microscope a many-
layered epithelium, which varies greatly in thickness in different parts of
the body. Even where it is thinnest, as for example on the back, at least
two layers of cells are distinguishable, an inner, growing stratum germina-
tivum and an outer, horny stratum comeum. The cells of the stratum
germinativum are columnar in shape; those of the stratum comeum are
flattened and scale-like. The former are alive, and by their constant
proliferation on division planes parallel to the surface of the skin, they
make continual additions to the strattun comeimi. The living cells in

THE INTEGUMENTARY SYSTEM 1 67

their turn, as by the wearing off of the outer layers they come nearer and
nearer the surface, alter their living protoplasm into keratin, and become
the horny scales of the outer epidermis. In man as in most mammals, the
stratum corneum wears away as rapidly as it is formed and never becomes
greatly thickened on most parts of the body. Amphibia, however, shed
the stratum corneum in sheets, sometimes sloughing off the entire covering
of the body at once. Serpents do the same thing, scales and all.

Sections of the thick epidermis of the palms and soles show between
the stratum germinativum and the stratum comeimi, two intermediate
layers, a stratum granulosum and a stratum lucidum. These, however,
are merely transitions between the inner growing layer and the outer
lifeless horn. See Fig. 125.

Cerium. The deeper layer of the skin, the corium, cutis, or dermis, is
connective tissue, with a much greater variety of cell elements than the
epidermis, and, unlike the epidermis, richly supplied with blood vessels.
Where it touches the epidermis, especially on the palms and soles, the
corium is thrown up into many fine papillae, the capillaries of which feed
the cells of the stratum germinativum. In some of these papillae, are
tactile corpuscles and other nerve terminations. Cutaneous glands and
the roots of hairs, both derived from the epidermis, become embedded in
the corium, and from it they are fed. Fat cells are numerous especially
in the lower laj'ers.

The greater portion of the corium is made up of connective tissue
fibers, both elastic and non-elastic. Most of these He parallel to the
surface, interwoven like the fibers of felt ; but some bundles are perpendicu-
lar to the surface. The fibers are more compactly set in the outer parts
of the corium than in the inner. The deepest layer is the loose or areolar
connective tissue by which the entire skin is attached to the underlying
muscle or bone. Skin muscles are few, and are mostly connected with the
bases of the hairs. The elasticity of the skin decreases with age.

Leather is made from the outer, compact layer of the corium of animals.
The epidermis is removed by maceration, and the connective tissue
fibers are toughened by tanning.

DEVELOPMENT OF THE SKIN

Notwithstanding the close connexion between the two main layers
of the skin, their origin in the embryo is diverse, the epidermis developing
from the ectoderm, the corium from mesenchyma. Since the mesen-
chyma is derived chiefly from the mesoderm, this contrast in origin is
fundamental.

Epidermis. The embryonic epidermis arises directly from the
ectoderm, and is at first a simple cuboidal epithelium. By the end of the
first month, as the result of cell divisions in a plane parallel to the surface,

1 68 COMPARATIVE ANATOMY

this epithelium becomes two-layered, the outer and thinner layer being
the periderm.

By the continued multiplication of the basal cells, the number of
layers gradually increases, until by the fourth month, all four layers of
the thicker parts of the adult skin have appeared. The cells of the
stratum comeum contain a fatty or waxy substance, which helps to form
the pasty vemix caseosa which covers the body of the new-born infant.
Developing hairs, instead of penetrating this layer, lift it as a continuous
sheet, the epitrichial layer.

Cerium. In most parts of the body, the mesenchyma which produces
the corium is derived from cells which have migrated from the parietal
layer of the mesoderm. For this reason, that part of the epimere which
forms the corium is called the dermatome. Some observers, however,
assert that in mammals the entire epimere forms muscle and there is no
dermatome in embryos of the group. Moreover, in some vertebrate
embryos, if not in all, the ectoderm also contributes to the mesenchyma
of the head and possibly, therefore, to the corium.

Embryonic mesenchyma consists of scattered, stellate cells, separated
by wide spaces. It becomes the connective tissue of the corium by secret-
ing intercellular fibers, both elastic and non-elastic. By the fourth
month, the compact fibrous layer of the corium is distinguishable from
the loose areolar tissue under it. Blood vessels and nerves invade the
corium from below, hairs and glands grow into it from the epidermis.
Abnormalities in the distribution of blood vessels cause birthmarks.

FINGER-PRINTS

In all primates, the entire surface of the palms and soles, but no other
portion of the body, is marked with fine parallel ridges separated by
equally fine grooves.

At definite places on hands and feet, these ridges form concentric
lines or loops. Eleven distinct "friction-ridge patterns" have been
distinguished, five on the finger tips, four at the base of the fingers, two
near the wrist or ankle. Those of the finger tips are most familiar,
since they are used for identification. Since no function has ever been
proved for these designs, their presence and their constant position has
stimulated much interest and discussion. To useless organs, the hypothe-
sis of special creation gives no clue. Are they, then, rudiments of struc-
tures functional in the lower animals? See Fig. 126.

The significant fact about these patterns is that they match precisely,
both in number and position, the concentric rows of horny scales on the
foot-pads of insectivores, a group which, for various reasons, is thought
to be near the direct line of man's ancestry. In the insectivores, the
position and arrangement of the scaly ridges is clearly adaptive and

THE INTEGUMENTARY SYSTEM

169

the best possible one to prevent slipping in any direction. These finger-
print patterns, therefore, serve to convict men of animal ancestry, as
they have on occasion served to convict them of crime.

_ FRICTION
::»DIGITAL PADS. .^S^RIDGES

^TRIRADII

TRIRADII

THENAR

PAD
HYPOTHENAR
PAD HYPOTHENAR

PAD

A. INSECTIVORE B. MONKEY C MAN

Fig. 126. — Friction ridge patterns in three mammals, — insectivore, monkey, and
man. The presence of such useless and rudimentary concentrically arranged ridges
in the human hand receives its only reasonable interpretation in the light of the evolution
theory. (Redrawn after Wilder.)

On the sides of the fingers, the friction ridges merge into rows of wart-
like elevations. This has been interpreted as confirming the opinion that
the ridges are remnants of rows of horny scales. That the ancestors
of the mammals were scaly is, however, supported by more convincing
evidence than this.

COMPACT

CONNECTIVE

TISSUE

LOOSE

CONNECTIVE

TISSUE

BASALPLATE
C TISSUE E. A. TISSUE q p

Fig. 127. — A vertical section of the skin of an elasmobranch, showing five stages
in the development of a placoid scale. The development of a placoid scale is essentially
like that of a tooth. This fact taken together with the similarity of their structure sug-
gests that teeth may have evolved from placoid scales. (Redrawn after Schimkewitsch,
modified.)

APPENDAGES OF THE INTEGUMENT

Throughout almost the entire animal kingdom the skin tissues form
various calcareous, chitinous, or horny structures — shells, spines, teeth,
bones, scales, hair, feathers, horns — which serve for defence, support of
tissues, or attachment of muscles. The limy shells of molluscs and the
chitinous exoskeletons of arthropods serve all three purposes.

170

COMPARATIVE ANATOMY

Among vertebrates, the placoid scales, which first appear in certain
sharks of the Upper Devonian, are especially important because of their
further evolution. Each of these scales has a flat basal plate of dentine
embedded in the skin, and each has commonly also a projecting spine
coated with hard enamel like a tooth. From these minute placoid scales
of ancient sharks have evolved all the multiform teeth of all the higher
vertebrates.

From these and other types of scale have evolved also, by simple
enlargement, the heavy continuous dermal armor of ganoids and other

^^^^

•ENAMEL SPINE.

Fig. 128. — The imbricated pattern of placoid scale arrangement in elasmobranchs.
The scales are arranged in rows and usually each scale is in line with the interval between
scales of the lines in front and behind. (Redrawn after Klaatsch.)

fishes. These same bony plates survive also in man and the higher
vertebrates as "membrane bones" which, unlike most parts of the skeleton
are not pre-formed in cartilage but develop directly in connective tissue,

HORNY SCALES

Vertebrates, besides bony scales, have also horny; but these have
played a much less important part in evolution, and are confined to
amniotes, more especially, reptiles.

In reptiles, the stratum corneum forms a continuous scaly layer over
the entire body, the separate scales being local thickenings which con-
tinue to grow by the addition of new keratin from underneath. Serpents
commonly shed this scaly coat twice a year. But the rattlesnake retains

THE INTEGUMENTARY SYSTEM

171

bits of the old skin at the tip of the tail. These become the rattle, which
therefore grows two rings a year.

Most reptiles have substituted horny scales for the bony scales charac-
teristic of fishes. But crocodiles have both sorts on the same individual.
On the ventral side of some snakes, large scales are attached to muscles
and become organs of locomotion.

The largest reptilian scales are those of chelonia, in which horny scales
fuse with the bony carapace and plastron. In birds horny scales cover
the feet.

Among mammals, the East Indian manis, and the tails of rats and
mice are scaled.

Fig. 129. — Section of developing scales of lizard, Sceloporiis. c, papilla of corium;
e, outer layer of epidermis which later becomes cornified; /, fibrous layer of skin; m,
Malpighian (stratum germinativum) layer; p, periderm; ts, tela subjunctiva. (From
Kingsley's "Comparative Anatomy of Vertebrates.")

It is a curious fact that while horny scales are purely epidermal
structures, their development is initiated, like that of bony scales, by the
corium.

HORNS

To produce such diverse structures as hairs, feathers, scales, nails,
and hoofs, demands most exceptional evolutionary potentialities on the
part of the horny layer of the skin. Among the surprising developments
of keratin-forming tissues are the horns of ruminants and rhinoceros.
Those of rhinoceros are formed wholly of keratin produced by the stratum
corneum on the snout. The hollow horns of cattle have, in addition to
external keratin, a bony base and core, which grows from the frontal bone
into the cavity of the horn. The antlers of the deer tribe are bony out-
growths with no covering of horn, but only the skin or "velvet" which
is soon lost.

Horns are best interpreted as weapons of defense and offense.

NAILS, CLAWS, AND HOOFS

Nails are scale-like thickenings of the stratum lucidum at the ends
of the fingers and toes, formed of homogeneous keratin identical with that
of the stratum lucidum from which they develop.

Nails and claws are strikingly alike except in form. Both develop
from a matrix at the base, which in man appears as the whitish "lunula."

172

COMPARATIVE ANATOMY

Both have their bases surrounded by a fold of skin, the "nail wall."
In both, a convex outer plate on the upper side of the digit may be dis-
tinguished from a concave "ventral plate" on the under side, each being
morphologically a reptilian scale. The ventral plate in man is reduced
to a narrow fold of skin between the nail and the finger pad.

Claws appear first in urodele amphibia. In some Anura, they are
limited to certain hind toes. But certain male frogs, at mating time,
develop horny papillae on their thumbs, which serve to hold a slippery
female. Reptiles have claws on all toes. Those of mammals are like
those of reptiles, except where the mammalian claw has altered into a
hoof, or become retractile, as in cats, which walk on foot-pads and keep
their claws sharp by raising them off the ground. Claws of mammals
intergrade with nails, so that it is difficult to draw a line between the two.

Fig. 130. — Diagrams of (A) nails, {B) claws, and (C) hoofs, e, unmodified epidermis;
M, unguis; 5, subunguis. (From Kingsley, after Boas.)

Some primates have both claws and nails on the same foot. Nails are
then rudimentary claws, modified to correlate with the increased sensi-
bility of the ends of the digits and their use as organs of touch.

Some mammals, such as the horse and deer, which run on their toes,
have hoofs instead of claws. The structure, development, and relations
of hoofs, however, prove that they are nothing more than enlarged and
modified claws. Both have dorsal and ventral plates. The attempt to
divide mammals into hoofed and clawed tj^jes encounters the difficulty
that at least one animal, Hyrax, has both claws and hoofs.

FEATHERS

Feathers, which are characteristic of birds, are modified scales, and
their early development is the same. A corium papilla initiates both;
but the feather anlage, instead of flattening to a scale, becomes an elon-
gated cylinder, which spHts into the plumes, barbs, and barbules of the
developed feather. A down feather, in fact, suggests an elongated and
frayed out scale.

THE INTEGUMENTARY SYSTEM

173

Birds, which have descended from reptihan ancestors, still have
scales on their feet, and even their bills and claws are presumably enlarged
and modified scales.

HAIRS

Hairs, which are characteristic of mammals, are not comparable
morphologically with either feathers or scales, since their development is

CORIUM
PAPILLAV

BARBULCS

PULP^^^==7Z/^

C. rOLLICLE'

Fig. 131. — Four stages in the development of a feather. A, B, and C represent
stages in the development of a down feather. D shows a contour feather in the feather-
sheath. A-B and C-D are sections of a young contour feather at the levels indicated
in D. In contrast with a hair the development of a feather is initiated by a corium
papilla. (Redrawn from Ihle, after Blitschli.)

initiated by the epidermis and not by the corium. When, as in Manis,
hairs and scales occur together, the hairs are at the apices of the scales.
That scales are older phylogenetically than hairs, is indicated by the
fact that scales develop earher in the embryo; and fossil evidence demon-
strates that mammals have evolved from some scaly Stegocephalan-Hke
Cotylosaurian. But since neither the skin nor its non-bony appendages
are commonly fossilized, their history has to be made out chiefly from
embryology and comparative anatomy.

All mammals have hair; and man's relative hairlessness is by no
means a distinctive human trait, since some mammals, for example the
whales, have less hair than man. It is well known that changes in the
secretion of the endocrine glands affect profoundly the growth of hair,
and man's loss of hair may have been thus brought about.

174

COMPARATIVE ANATOMY

Hair Structure. The hair of all mammals is essentially similar. There
are, however, such differences of detail as enable an expert to identify
different species.

Each hair consists of a "root" buried in the skin, and an external
shaft. Microscopic examination shows a multicellular structure, with
the cells in three layers, an outer cuticle, a cortex, and a central medulla.
The cells of the cuticle are scale-like, overlapping one another like shingles
on a roof. Cortex cells, greatly elongated, make up the greater portion
of each hair. The medulla occurs only in the "contour hairs" and is

Fig. 132. — Diagram of structure of hair, h, blood-vessels; cl, cuticle of hair; ex,
cortex; g, glaiid; h, hair; he, Henle's layer; hf, hair follicle; hx, Huxley's layer; m, medulla;
p, papilla; sg, stratum germinativum of epidermis. (From Kingsley's "Comparative
Anatomy of Vertebrates.")

wanting in the finer. It is made up of cuboidal cells usually in a double
row.

The root is surrounded by epithelial and connective tissue sheaths.
It ends in a swollen "bulb," from which it grows and which contains a
connective tissue papilla, with capillaries which feed the hair.

Hairs of different human races differ in cross section. In general, the
rounder the hair, the straighter it is; the more compressed, the curlier.
It has not been shown that these differences have been developed either
by natural or by sexual selection.

Hair Direction. Hairs, instead of projecting vertically from the skin,
emerge at an acute angle, have a slant in some special direction, and thus
form streams in various parts of the body. Where such currents meet,
either "rhomboids" or "vortices" may form, the latter being commonly
called "cowlicks." The fact that such rhomboids and vortices appear
on the human body in regions where the hair is short, has been interpreted
to mean that man's hairy covering was once longer than at present.

THE INTEGUMENTARY SYSTEM

175

Although in general the direction of hair growth is such as to make
gravity the determining influence, it is a curious fact that the hair on
the human forearm suggests his animal ancestry. The hair of the fore-
arm slants from the wrist toward the elbow, in the reverse direction to
the slant on the upper arm. Man shares this pecuHarity with the apes
alone. All other mammals have the same hair direction on both parts
of the limb. Why this resemblance of man to the apes unless they share
a common ancestry? The pecuHarity is not adaptive, and it is not easy
to see why, if man and apes were
independently created, they should re-
semble one another in this detail.

Hair Arrangement. That the
arrangement of hairs on the human body
has any evolutionary meaning is, to say
the least, surprising. Indeed, since such
patterns can have no use, we should
hardly expect to find them at all. No
less surprising is an arrangement of hair
in mammals that indicates descent from
scaly ancestors.

In most mammals, the hairs occur in
groups of three or more. These groups
are arranged in parallel rows in such wise ^^^ ,33. -Arrangement of the

that each cluster lies opposite an interval hairs in groups of threes and fives

in the rows in front and behind. In ^^ the human embryo, with the

probable ancestral arrangement oi

short, the arrangement is imbricated, the scales. (From Kingsiey, after
like the universal arrangement of scales. Stohr.)

This arrangement, though quite useless, is precisely what we should
expect if mammals have descended from scaly ancestors.

Histogenesis of Hairs. Hairs are, in origin, epidermal, and therefore
ectodermal. Each begins as a minute epidermal papilla, which has
arisen by local cell proliferation in the stratum germinativum. Continued
proUferation gradually converts this papilla into a cellular column, which
extends obhquely downward into the underlying mesenchyma which is
to become the corium. The growing end swells into a bulb, in which
later develops the corium papilla from which the hair is to grow. Cellular
differentiation of the hair column results in an inner sheath and the
hair-shaft, all surrounded by an outer sheath. From the bulb to the
point in the hair column where the sebaceous gland develops, the cells
of the hair-shaft become cornified. Above this point the central cells
degenerate to form a canal in which the hairshaft grows towards the
surface. Continued cell multipUcation of the stratimi germinativum
of the papilla elongates the central hair shaft to extend beyond the skin.

176

COMPARATIVE ANATOMY

Each hair thus formed continues to elongate throughout its Ufa of
some months or years, the rate of growth varying greatly in different
parts of the body. But finally, growth ceases, the hair dies, and is shed.
If the hair papilla retains its stratum germinativum, a new hair grows.

Each hair column, in addition to producing a hair, may form as lateral
outgrowths one or more sweat or sebaceous glands. Muscle cells devel-
oped from the mesenchyma of the corium attach themselves to the hair-
roots and become arrectores pilorum.

MESENCHYMA

HAIR COLUMN

CORN in ED
INNER SHEATH

.- ARRBCTOR MUSCLE

SEBACEOUS GLAND
EPITHELIAL BED

■ROOT OF HAIR

MESENCHYMA

Fig. 134. — A vertical section of skin of a five month human embryo, showing four
early stages in the development of a hair. The growth of a hair is initiated by the forma-
tion of an epidermal papilla projecting (down) into the underlying corium. (Redrawn
from Bremer after Stohr.)

The human foetus has before birth a hairy covering, the "lanugo,"
which is shed shortly before or soon after birth. The coat persists,
however, in certain types of "hairy men." The evolution theory affords
the only rational explanation of the lanugo.

PIGMENT

Skin color in man is due in part to the blood in the capillaries of the
corium. In addition, there are two pigments in the skin and hair, a
brown, sometimes darkened to a black, both in granules, and a yellow,
that may strengthen to a red, both diffused in the tissues. All are prod-
ucts of cell metabolism.

The pigments of the hair are confined to the cortex. The epidermis
and the outer parts of the corium are both pigmented. Not until shortly
after birth do pigment granules appear in the stratum germinativum,
so that even negroes are born white.

Moles and freckles involve excessive local pigmentation. Freckles
are small local patches of excess pigmentation, which are more Hkely to
occur in light-skinned individuals who have been exposed to strong sun-

THE INTEGUMENTARY SYSTEM

177

light. A mole or nevus is an elevation of the skin due to local prolifera-
tion of epidermis and corium, and is usually excessively pigmented. When
a mole is congenital and involves blood capillaries, it forms a "birthmark."
Since pigments like those of vertebrates are found also in invertebrates,
there is no reason to question their common origin. Many animals
below the mammals have their pigments in special cells, the chromato-
phores, which expand or contract under the influence of hormones and
thus alter the color of the skin. The colors of lizards, which are often

STRATUM CORNEUM---;

mn

CORIUM PAPILLA
/

mmiiiinMjiniK-

,PILLA--^**S1*''

STRATUM GERMINATIVUM

EVOLUTION OF
FEATHER

EVOUUTION OF
HAIR

PAPILLA--^

A. EVOULTTION OF PLACOID
SCALE

Fig. 135. — A series of hypothetical stages in the phylogenesis of cutaneous append-
ages of vertebrates. A common origin of placoid scale, horny scale, feather, and hair
from a corium papilla is assumed. (Redrawn after MatviefT.)

brilUant, are not in their scales, but in chromatophores of the underlying
corium.

Widely among vertebrates, pigments of scales, skin, hair, or feathers
often show striking and elaborate patterns that serve for protection,
warning, recognition, or sexual allure; but in man chiefly the region of the
nipples and the external genitals are slightly darker than the rest of the
body. In man and some other hairless mammals, such as the elephant,
the function of the skin pigment is to check ultraviolet light before it
penetrates to Uving cells. Everyone has observed the effect of the
sun's rays upon unwonted skin, and the promptness with which the skin
responds by tanning. Lacking skin pigment, men could not live in
some parts of the earth.

178 COMPARATIVE ANATOMY

Color in Races and Individuals. The blue of the iris of human children
and new-born kittens is an interference color, like the blue of the sky or
the eyes of a peacock's tail. Later, as the iris fibers thicken, the inter-
ference is less perfect, and the eye is gray. Brown pigment in some fibers
only, gives hazel. Brown eyes are evenly pigmented. Dark brown eyes
are called black.

There is also the yellow pigment, which, nearly free from brown, gives
the amber eyes of some blondes. The same color intensified, makes the
red iris that sometimes accompanies red hair. The interference blue
slightly masked by yellow, gives that rarest of all eye colors, green.

In general, among Europeans, the eyes are less pigmented than the
hair, so that dark hair with gray-blue eyes is common. But some blondes
have a striking color scheme, eyes darker than the hair.

Hair is colored by the same two pigments, both usually present, with
the brown-black, masking the red-yellow, except in strong light. But
some dark hair lacks the red and is blue-black.

Some blondes have no brown pigment, and little yellow. Most have
brown also, along with varying amounts of yellow. The tow head with
a touch of dark, is the ash blond. Yellow with some red is golden; and
starting from this, the red may strengthen to a rather unadmired carrot
or orange. More brown carries the red over into auburn; still more gives
bronze.

Hair that has lost its color is white, for the same reason that snow is,
the crystal faces of the one and the cell walls of the other scatter the
light.

Skin color is like hair color except that the blood color below the
pigment may show through, and that sunHght, which fades the lifeless
hair, stimulates the living skin to turn dark.

Primitive man was dark as the ape ancestor was, and as most races
are still. Reducing the black pigment with the yellow retained, gives
the Mongolian skin color. The stronger yellow, together with a good
deal of brown, is the traditional hue of the Red Man, though as a matter
of fact, most Indians are brown, like most White Men.

The blond White Man is a local race that originated in some region
near the Baltic, apparently since the last Ice Age. Being highly energetic
and uncommonly well endowed, the descendents of these blond giants
have made their way all over the world, and, much diluted with darker
blood, still appear in most civilized countries of the world.

Why their blondness, nobody knows. It cannot be due to climate,
for the Eskimos, Samoyads of Siberia, the Patagonians, and the people
of northern China are all dark. Naturally, a blond race could hardly
survive in the tropics; but a white skin is no obvious advantage anywhere.
Yet certain studies of Chicago children show that the highly pigmented

THE INTEGUMENTARY SYSTEM

179

Italians are more liable than lighter stocks to rickets, which is a sunlight
deficiency disease. So it may be that in high latitudes, in a wooded coun-
try or one that has much cloud and fog, a fair skin that is still able to tan
may have a selective value and be accounted for on Darwinian principles.
Skin color plays queer tricks. Any two parents, even two Negroes,
may have an albino child; two dark-haired parents may somehow miss
with the brown pigment and have red-haired offspring. The most that
anyone can say is that "Nordic" man probably began as a mutant from
a dark stock. Possibly, after the mutation appeared, it was admired and
selective mating kept it to the fore.

ASSOC I ATI OM
NEURONE

EFFERENT NERVE

• SWEAT GLAND

iCAPILI-ARIES

Fig. 136. — A diagram illustrating the nervous mechanism of temperature regulation
in man. The quantity of secretion of tubular glands (and consequently the amount
of sweat which may evaporate to cool the body) depends upon the quantity of blood in
the capillaries associated with the glands and dermal papillae. Through a reflex arc the
circulation is regulated by the temperature of the skin. (Redrawn after Hough and
Sedgwick.)

CUTANEOUS GLANDS

Since among invertebrates most glands are unicellular, it has generally
been assumed that the multicellular glands of vertebrates have evolved
from such beginnings, an increase in the size of the secreting cells tending
to carry them into the underlying corium, where together with other
epidermal cells, they become multicellular organs.

Be this as it may, cutaneous glands develop, much as hairs do, from
solid cords, which are proliferated from the stratum germinativum and
grow downward into the underlying corium. The lumen of the gland
forms later, to connect with the exterior, and the gland anlage differen-

i8o

COMPARATIVE ANATOMY

tiates into a secretory portion and a lining for the duct. The secretory
cells become intimately associated with blood vessels and nerves.

Sweat Glands. In man sweat glands occur in most regions of the
body, and are especially abundant on the palms and soles. They are,
for the most part, of the simple tubular type, much coiled to increase the
secreting area; but those of the axillae are branched and greatly enlarged.
They are of the "vitally secretory" type, that is, the cell protoplasm
merely produces the secretion, but is not converted into it, and the cell
continues ahve indefinitely. The sweat is usually oily, but becomes
watery under the influence of the nerves. See Fig. 136.

Fig. 137. — Scheme of different kinds of nipples. Single line, ordinary integument,
double line, that of primary mammary pocket. A, primitive condition, found in
Echidna; B, human nipple; D, Didelphys before lactation; C, same at lactation E,
embryonic, F adult conditions in cow. B and C are true nipples, F a pseudo-nipple
(teat). (Based on figures by Weber from Kingsley.)

Sebaceous Glands. Sebaceous glands in man also occur on most
parts of the body, but are wanting on the palms and soles. Most hairs
have sebaceous glands connected with their follicles.

They are of the acinous type, and necrobiotic, that is, their protoplasm
forms the fatty secretion, which the cell extrudes, and then dies.

Other Glands. Besides the sebaceous and sweat glands, there are
other highly speciaHzed cutaneous glands, the lacrimal glands of the eye
and the Meibomian of the eyelids, the wax glands of the auditory meatus,
besides preputial, vaginal, anal, and mammary glands, which occur in
most mammals.

Of the organs which have evolved from glands, none are more sur-
prising than the luminous organs or photophores of deep sea fishes. These
are true dark lanterns since they have a condensing lens and a reflecting

THE INTEGUMENTARY SYSTEM

l8l

membrane. The light is produced by the oxidation of luciferin secreted
by the gland. No carbonic acid and little heat are evolved in the process.

Fig. 138. — The presence of supernumerary teats (polymastism) in man supports
the theory of the animal origin of the human body. Their repeated occurrence has
received no other rational explanation. They are reversions or atavisms. (Redrawn
after Wiedersheim.)

CUT LIMB BUD

POSITION OF DEFINITIVE GLAND

GLAND ANLAGE

NIPPLE

EPIDERMIS ^i J

SMOOTH MUSCLE. ttLa. ci^^i

rCORIUM —

laSMM. HUMAN EMBRYQ B MUSCLE C FAT

Fig. 139. — A figure illustrating the development of the mammary gland in man.
A 13.5 mm. embryo shows the "milk line", a ridge which extends from the axillary
region to the groin. The definitive gland develops only from the anterior portion
of this line. Taken with the evidence of supernumerary teats in man, the line is
interpreted as proof that the ancestors of man had more than a single pair of mammary
glands. (Redrawn from Arey, after Kollmann.)

A, B, and C are sections of the definitive mammary gland in successive stages of
ontogenesis. A is from a two-months embryo, B from a four-months embryo, and
C from a seven-months embryo. From its development the mammary gland is
seen to be a compound tubular gland. (Redrawn from Arey, after Tourneux.)

Mammary Glands. Mammary glands first appear in monotremes,
as a pair of milk-secreting organs on the ventral side of the body. They
are without nipples, and in Echidna they pour their secretion into a

1 82 COMPARATIVE ANATOMY

depression, the "mammary pocket," surrounded by a fold of skin. From
this condition in monotremes, the teats of the higher mammals have
evolved, either by elevating the milk-field at the bottom of the pocket
into a "true teat," as in man, or else by elevating the surrounding ridge
to form the "false teat" of ruminants. The number of teats corresponds
roughly to the number of young in a litter.

Functional differences among the glands of vertebrates are much
greater than morphological, and their physiological evolution is a difficult
problem in biochemistry.

Certain abnormalities in the milk glands of man, however, confirm
strongly the theory of the animal origin of the human body. Super-
numerary nipples appear in man with a certain statistical frequency.
But these extra teats, instead of being placed at random, are usually set
in two ventral rows, precisely like two rows of nipples which form the
milk lines of lower animals. They are, then, best interpreted as rever-
sions to an animal ancestor. The theory of special creation gives no
clue whatever to their occurrence in human beings. See Fig. 138.

CHAPTER 5
TEETH

No invertebrate has teeth at all comparable, save in function, position,
and material, with the teeth of vertebrates. Some annelids, hke Nereis,
have horny pharyngeal teeth that act Hke pincers. A circle of calcareous
teeth surrounds the mouth of the vegetarian sea-urchin to form the
lantern of Aristotle, but each tooth has its own muscles and there is no
jaw. Some snails have a radula with which they rasp their food. Many
arthropods, notably the biting insects, have their appendages modified
into hard mouth-parts that are both jaws and teeth. But a series of
independent teeth operated by a movable jaw is pecuHar to chordates.

Nor do all chordates possess such teeth. The protochordates have no
teeth of any sort. Cyclostomes have within the oral hood horny ecto-
dermal teeth, with which they cling to their prey or bore their way into
its flesh. In this absence of calcareous teeth attached to jaws, as in so
many other characteristics, the cyclostomes exhibit their primitive
nature.

The larvae of some amphibia have upon their jaws horny teeth in
the form of papillae. Most reptiles have true teeth; but the chelonia
have replaced those of their ancestors with horny beaks. So, too, have
all modern birds, although their ancestors of the Jurassic and Cretaceous
had typical reptihan teeth. The embryo of the duck-billed platypus has
rudimentary teeth which it does not use. Even among the placental
mammals, the edentates either have no teeth or have them without enamel.
In the toothless whales, teeth are present in the embryo, but the adult has
only whalebone strainers.

EVOLUTION OF TEETH

Since the protochordates are without teeth, the cyclostomes have
horny ones, and all attempts to discover any sort of rudimentary cal-
careous teeth in cyclostomes have proved unsuccessful, it seems clear that
teeth of the vertebrate type are a new acquisition with no homologs
anywhere among invertebrates.

Typical or "true" vertebrate teeth have their beginning in the innu-
merable, minute placoid scales, which so roughen the skin of sharks that
in former time shark skin, under the name shagreen, was widely used as
an abrasive.

183

i84

COMPARATIVE ANATOMY

In elasmobranchs, on the edges of the jaws, these minute scales become
enlarged into formidable biting teeth or sometimes inside the mouth
into bony pavements that are used for grinding the food.

That the biting tooth of elasmobranchs is a modified placoid scale is
obvious from inspection, since the two look aUke, and there is a transition
in size and form between them. This identity is further borne out by

OLFACTORY PIT-

TASTE PAPILLA

PLACOID SCALE

ESOPHAGEAL VILLUS

Fig. 140. — The pharynx of an elasmobranch (Squalus) laid open to show the double
row of teeth in both upper and lower jaws. Such teeth differ only in size from the
placoid scales of the pharynx and skin. Elasmobranch teeth, like scales, are fastened
in the skin and are not attached to the jaw cartilages. (After Cook.)

other evidence. Like true teeth, placoid scales have a base of dentine,
which contains a pulp-cavity filled with connective tissue. Both scales
and teeth have a spinous process, covered wath enamel, which protrudes
through the skin. Moreover, their development is similar, in that, in
both, the enamel is secreted by the ectoderm and the dentine by mes-
enchyme, and both arise in that portion of the mouth where the ectoderm
has invaginated to line the digestive tube. See Fig. 141.

TEETH 155

This evidence for the identity of teeth and placoid scales is not invali-
dated by the fact that the endodermal lining of the pharynx of some
elasmobranchs is also beset with placoid scales like those of the external
skin. This simply shows that the potentiahties of the germ-layers are
greater than was formerly supposed. The hning of the pharynx has also
taste-buds, although sense organs usually develop from the ectoderm.
The presence of scales and of taste-buds in the pharynx has led some
morphologists on a priori grounds to assume that ectoderm has invaded
mouth, pharynx, and even the esophagus. There is no question that
the hning of the pharynx is primarily endodermal, and no one has ever
been able to discover evidence that this primary hning degenerates and

Fig. 141. — Comparison in development and structure between a placoid scale and a
tooth, a, b, and c represent the scale; d, e, and / the tooth. In all the figures
the stratum corneum is dotted, the stratum germinativum is represented by a layer of
large cells with nuclei; and the cutis is presented as composed of fibers with scattered
cells. X, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity.
(From Wilder's "History of the Human Body," Henry Holt & Co.)

is secondarily replaced by ectoderm. Until such evidence is presented,
we have no alternative to the conclusion that the endodermal hning
persists and produces both taste-buds and enamel. Whatever the origin
of the enamel of these pharyngeal scales, there is no doubt that the
enamel of the teeth, like that of dermal scales, is of ectodermal origin.

Originally, in vertebrates, the teeth were for seizing and holding
prey. Grinding and cutting teeth, tusks, and fangs, are all modifications
of the primitive mouth trap.

The number of these holding teeth is indefinite in elasmobranchs,
which may have as many as one hundred. They are not attached to
the jaws, but merely imbedded in the skin of the mouth. They are all
about alike; and when one is lost, another moves forward into its place.

1 86 COMPARATIVE ANATOMY

In teleosts, the number of teeth is somewhat reduced, although all
parts of the mouth and even the pharynx may carry them. The special
advance made by the teleosts is to set the teeth more firmly by fusing
their bases with the membrane bones of the mouth.

Amphibia still farther reduce the number of teeth ; but retain them on
premaxilla, maxilla, mandible, vomer, and palatine bones, and more
rarely on the parasphenoid. But toads have no teeth whatever. A strik-
ing feature of certain ancient and long extinct amphibians, the labyrin-
thodonts, which arose in the Coal Period and survived into the Triassic,
was the enormously comphcated folding of the tooth enamel and dentine,
which anticipated, yet went far beyond the similar arrangement in some
mammals.

Reptiles make two important advances toward the condition in
mammals. Some of them, like some of the amphibia, have their teeth
set on a ledge on the inner side of the jaw — pleurodont dentition. Or they
may have the tooth set directly on the bone, acrodont dentition. But the
crocodiles and some fossil reptiles attain to a thecodont dentition, in
which each tooth is fixed in a separate socket, as in mammals. In addi-
tion, some hzards and numerous fossil reptiles abandon the original
homodont dentition, with all teeth about alike, and have their teeth
more or less differentiated into incisors, canines, and molars, as in mam-
mals, a heterodont dentition. The differentiation of teeth is obviously
an adaptive division of labor, the incisors acting as cutters or chisels,
the canines as daggers, and the molars as grinders. The reptiles, more-
over, hmit their teeth to the two jaws.

An especially elongated tooth occurs in the lower jaw of Hzard and
snake embryos, which is used to break through the tough membranous
shell. A hardened tip of the horny beak of birds is used for the same pur-
pose. The two structures are, however, morphologically quite different.

Especially remarkable in reptiles are the highly specialized poison
fangs of certain snakes. These are modified from the ordinary conical
tooth, first by a folding of the tooth to form a groove along which the
venom from a modified salivary gland flows into the wound. In other
snakes, by a still further folding the edges of the groove unite, and the
tooth becomes a hollow needle. One pair only is functional at any one
time, others up to nearly a dozen pair being held in reserve to take the
place of the large fangs when these are lost. All the vipers including the
rattlesnakes fold back the functional pair of fangs when the mouth is
closed, and only in the act of striking pull them erect by special muscles.
Mammals, besides having nearly always a heterodont dentition, with
incisors, canines, and molars well differentiated, have adopted also a
distinction between a milk and a permanent set, as a means of adjusting
to an enlarging mouth solid teeth of limited size.

TEETH

187

The elasmobranchs, indeed, do have a certain succession of teeth,
Init only one at a time as single teeth are lost. Lower vertebrates,
reptiles conspicuously, have a somewhat indefinite number of sets, and
are said therefore to be polyphyodont. Only the mammals have two
definite sets, and are therefore diphyodont. But monotremes, sirenians,
and toothed whales retain their milk teeth throughout life, have no second
set, and are said to be monophyodont.

In general, then, the course of evolution has been from a large and
indefinite number of simple teeth all alike, not fixed firmly in place,
and borne by any part of the mouth, to a reduced and definite number, set
firmly in alveoli, confined to the jaws only, and differentiated into three

A C

Fig. 142. — Jaws of some primitive Jurassic mammals. The resemblance of these
jaws and teeth to those of the theriodont reptiles of the same period suggests a similar
genetic origin. (Redrawn from Romer, after Simpson.)

sorts. Along with this, has gone a shortening of the jaws and a change
of food habits. But whether the change in diet caused the change in
teeth, or the change in teeth made possible the change in foods, is still
an unsolved problem.

EVOLUTION OF COMPOUND TEETH

Compound teeth resembling the molars of mammals first appear in
certain late Permian and early Triassic reptiles, the theromorphs. Since
amphibia and the earlier reptiles had simple conical teeth, the conclusion
has been drawn that compound teeth are derived from conical teeth, and
morphologists have advanced two theories as to how this evolution came
about.

The Differentiation Theory of Cope and Osborn assumes that the
teeth of vertebrates were originally of the simple conical type found in
most Reptiles. Such were the teeth of the premammalian Stegocephala
and of primitive Theromorph reptiles.

The first multitubercular type of molars of modern mammals appears
in such a Triassic mammal as Dromatherium, the teeth of which had a
large median cone or protocone in line with two smaller cones, a paracone
in front and a metacone behind. Corresponding parts in the teeth of

COMPARATIVE ANATOMY

the lower jaw are called protoconid, paraconid and metaconid. Teeth of
this sort are known as triconodont. Besides the three cones, triconodont
teeth have a basal rim, the cingulmn, which forms part of the crown.
Marsupial-like mammals of the Tertiary had teeth of this triconodont
sort. See Fig. 143.

The secondary tubercles of such teeth show a tendency to enlarge
to the size of the protocone. A further advance occurs when the three
cones assume a triangular relation to one another, the secondary cones
of the upper jaw migrating inwards, those of the lower jaw outwards.
Teeth of this tritubercular sort occur in Amphitherium of the Jurassic
period.

Later, in mammals, appeared a posterior projection or talon, and a
fourth tubercle, the hypocone and hypoconid. With these additions,
the molar teeth assumed more and more the modern form with six cusps.

Fig. 143. — A, triconodont tooth of Dromalheriiun; B, tritubercular tooth of Spalaco-
therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth
(after Osborn) ; D, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m,
metacone (metaconid) ; pa, paracone (paraconid) ; pr, protocone (protoconid) ; t, talon.
(From Kingsley's "Comparative Anatomy of Vertebrates.")

It took many million years to accomplish these changes, which were
naturally based upon change in the form of the tooth-germ and involved
budding of that organ.

The concrescence theory accounts for the multitubercular molar
teeth of mammals by supposing a fusion of the anlagen of conical teeth,
the number of cusps corresponding with the number of conical teeth
involved. Some observers claim to have found evidence of fusion of
tooth-germs in vertebrate embryos, but most investigators are sceptical.
It must be said, however, that tooth fusion is known to occur in the case
of the massive pavement teeth of dipnoi. At the present time the
concrescence theory seems to have less factual support than does the
differentiation theory.

According to Bolk, in a modified form of the concrescence theory,
compound teeth are formed by the fusion of the germs of successive sets.
His theory assumes that the ancestors of mammals had more than two
generations of teeth like the milk and permanent sets, that is, their
dentition was polyphyodont. Under these conditions, the germs of

TEETH 189

successive sets might fuse with one another. The factual foundations
of the theory, however, are weak.

TEETH OF MAMMALS

Teeth of mammals are especially important for the paleontologist,
partly because they are hard and therefore likely to be preserved, but
more because mammalian teeth are closely correlated with feeding habits.
But feeding habits, in their turn, are correlated with the entire bodily
structure, so that teeth are a key to the whole organism. Moreover,
mammalian teeth are so highly specialized and so diverse in size and
structure, that a single one is often sufficient to identify a species.

In general, the tendency has been to reduce the number and to do
away with the division into two sets, and at the same time to specialize
and elaborate individual teeth. An ideally complete set for a placental
mammal would consist of three incisors, one canine, four premolars, and
four molars in each half-jaw. The distinction between premolars and
molars is that the premolars replace milk teeth, and are therefore of the
second set, like the teeth in front of them; but the molars have no predeces-
sors, and are therefore really of the first set. Functionally, however, and
often in size and shape, there is little difference, and the two groups are
conveniently lumped together as cheek teeth.

But while 3-1-4-4, 48 teeth in all, is ideal, no placental mammal
conforms to it, 44 being the usual limit in any actual animal and 3-1-4-3 a
common formula. Nevertheless, it is convenient to think of each actual
tooth as one particular member of the ideal set. Thus can the history
of each tooth be followed throughout all the placental mammals, and
each be identified wherever it occurs. But the marsupials are aberrant,
opossums, for example, having four and five incisors, so that their teeth
cannot always be homologized with those of placentals.

Starting, then, with the ideal dental formula: i-l, c\, pi, ml, the httle
hyrax or cony allied to the elephants is one of the few mammals to retain
the full eight cheek teeth. But its incisors are reduced to one in each
half -jaw and it has no canines. On the other hand, the ungulates tend
to have the typical four front teeth, but to lack one molar and sometimes a
premolar also. They usually, have the canine like the incisors, and so
have practically, though not morphologically, four incisors. But all
hollow-horned ungulates lack upper canines, and many, like domesticated
sheep, have lost all four incisorform teeth from the upper jaw. Their

dental formula is, therefore, in brief form: ^-^•

The pigs, with forty-four teeth in all, are peculiar in having the
canines in both jaws grow throughout life as fast as they wear away, and
are kept sharp by whetting against one another. The walrus makes

igo COMPARATIVE ANATOMY

tusks of the upper pair only, which also are unrooted. The narwhal,
for a Hke purpose, uses one incisor, its mate growing indefinitely in size,
and has no other teeth.

The carnivora make the canines, especially the upper pair, into long
curved daggers, which reach their extreme development in the extinct
saber-toothed tiger of the Pliocene but are noteworthy even in the domestic
cat. With each canine, in the flesh eaters, goes a "carnassial" tooth,
especially developed in the cats, a premolar above and a molar below.
Other cheek teeth, especially in the cats, tend to be reduced almost to
rudiments.

Moreover, the carnivora, though uniform as to incisors and canines,
differ somewhat widely in the cheek teeth. Thus, while the dog is

^'^''^'^ the cat is reduced to - — - — The lynx is made a separate genus

3-1-4-3 3-I-2-I

from the cats because it has lost the minute first premolar of the upper jaw

and brought its dentition down to On the other hand, some of

the whales have gone back to primitive conical teeth used only for holding,
are virtually or quite homodont, and have fifty or more pegs in each jaw.

Characteristic of rodents is the complete absence of canines, and the
reduction of the incisors to one functional pair in each jaw. The single
pair, however, is a remarkable tool. Each tooth grows from a permanent
germ, that is set far back in the jaw, so that each passes under all the
cheek teeth, before it emerges at the front of the mouth. Enamel coats
the front surface only, so that as the tooth wears, the dentine wears most,
and the thin plate of enamel remains always sharp. Since these teeth
grow throughout life, if they are not worn away by gnawing, they become
too long and the animal cannot feed.

No rodent has more than six cheek teeth, many have only four, and
an AustraHan mouse so far depends on its incisors, that it has brought its

dentition down to ■ ■• But the hares and rabbits, and some other

I-0-0-2

rodents, as if to exhibit their affinities with other mammals, have two
more incisors, very minute, behind the large pair in the upper jaw.

Probosidians. The most specialized of all teeth are those of elephants.
Incisors and canines are completely lacking in the lower jaw. In
the upper jaw, one pair only of incisors become the tusks, but the other
two pairs have so completely vanished that it is not known certainly which
pair remains. The tusks are rootless, and grow from far up in the skull.
They elongate throughout life, growing faster than they wear away, until
in some instances, they have reached a length of eight feet and a weight
of more than 150 pounds each. Certain extinct elephants had tusks even
larger, up to twelve feet and two hundred pounds. In the Indian ele-
phant, only the males have tusks. But the larger African species, which

TEETH 191

uses the tusks for digging roots, has them in both sexes. The famous
African elephant Jumbo, in a fit of rage, broke off both tusks inside his
cheeks. When they grew out again, they made new holes through the
flesh, but the original holes remained for the rest of the animal's life.

Two extinct proboscidians, tetrabelodon and dinotherium, had
tusks on the lower jaw also, those of tetrabelodon nearly parallel with
the upper pair, those of the dinotherium turned downward like those of a
walrus.

Curiously, the small milk tusks of the young elephants, which are
shed early, are rooted like ordinary teeth — another illustration of Von
Baer's law that the young in a specialized group tend to resemble general-
ized ancestors.

The cheek teeth of proboscidians, less conspicuous than the tusks,
are even more remarkable. There are six in each half jaw, i.e., twenty-
eight teeth in all including the tusks and ferUf evanescent incisors. But
of the six grinders not all are in use at one time. As the foremost wears
down and is shed, a second and larger moves into its place, only to be
followed by the remaining teeth in succession. Thus an old animal,
since there are no canines, may have only the two tusks and four grinders.

The grinders themselves are remarkable for their enormous size,
the largest being more than a foot from front to rear and four inches wide.
Each tooth is highly complex, with intricate folding of enamel and dentine,
so that as the softer dentine wears away faster, the tooth keeps always
its sharp grinding ridges. The same arrangement on a smaller scale
appears in various other vegetarian mammals, notably in the horse. The
huge single teeth coming successively into use are a device for prolonging
the life of the animal long after any set of teeth all functional at once
would wear away. Apparently as a result, the elephants are among the
longest lived of mammals.

Primates. The primates, except for their hands, feet, and brains,
are a somewhat unspecialized group, and their teeth, though reduced in
number to conform to the shortened jaws, are little differentiated and
the enamel is not folded. The dental formula for the Old World monkeys
is 2-1-2-3 in both jaws. But the New World monkeys have another
premolar, and sometimes lack one of the three molars. It is a curious
fact, which no special creationist has attempted to explain, that man,
also an Old World primate, has exactly the dental formula of the others.

The canines in monkeys are somewhat longer than the other teeth,
and in the male gorilla are much like those of the less specialized carnivores.
Significantly, in man, although even the upper canines are hardly larger
than incisors, they have nevertheless the long roots of the animal tusk.

That general tendency to shorten the mammalian face which has
brought down the cats to three and four cheek teeth and the higher

192

COMPARATIVE ANATOMY

primates to five, continues in man by a general reduction in size of all
the teeth and by closing the diastema, the open space next the canines.

""^PALATINE

palatine'

A. CAUCASIAN B. NEGRO C. ORANG OUTAN

Fig. 144. — Dental arcades of ape. Negro and Caucasian. The form of the Negro
arcade is transitional between that of the ape and white man. "With the shortening
of the human jaw the diastema between incisor and canine teeth seen in the ape jaw
is lacking. The refinement of the face is one of the most striking results of primate
(Redrawn after Wiedersheim.)

evolution.

INCISORS

iCAN I NE
I

PREMOLARS A^^^J^^l—

'MOLARS

/ \ ^PREMOLARS

INCISORS ^CANINE
Fig. 145. — Human teeth viewed from the left side. The human dental formula is:
il c\, pml mi As a result of the shortening of the hvmian jaws the third molars fre-
quently do not erupt. The elongated root of the canine tooth suggests that as in lower
primates the ancestors of man may have had fangs. (Redrawn after Braus.)

Consequently, human teeth are a continuous series and no tooth is very
much larger than another, for the canines ceased to be tusks at the begin-

TEETH 193

ning of human evolution. Along with these, has gone a change in the
direction of the incisors, correlated with the appearance of a chin. In
the apes, the incisors protrude, in man, they stand upright. Incidentally,
the human bite becomes horseshoe-shaped, with the rows of cheek teeth
no longer parallel, as in all lower forms, even the apes. In addition, the
triangular upper molars of the apes, with three cusps, become in man
quadrangular with four, and, correlated with the reduced size of the single
teeth, their pulp cavities become relatively still farther reduced, not to
sacrifice unduly the thickness of the tooth wall. x\pe teeth are, then,
taurodont, so that the X-ray sometimes identifies a fossil human tooth
when other tests fail.

All these differences between apes and men are, however, bridged by
various fossil creatures, on the whole human, some of the genus Homo
but not our species, others quite outside the genus, but still within the
family.

TEETH OF MAN

Human teeth are in structure substantially like those of most other
mammals, and very like indeed to those of other primates.

In each tooth three parts are distinguishable, an external enamel-
capped crown, — a root buried in a bony socket or alveolus, — and a neck
or constricted region between root and crown. The number of cusps or
tubercles on the crown varies in the different teeth. The incisor and
canine teeth have a single cusp, the premolars have two, and hence are
known as bicuspids, and the molar teeth may have as many as five. The
number of roots also varies in the different teeth. Incisors, canines, and
premolars have but one, although the roots of the premolars are some-
times divided into two. The lower molars have two roots, and the upper
molars three.

The finer structure of a tooth may be best seen in a thin longitudinal
section (See Fig. 146). The central portion, the pulp-cavity, is filled
with connective-tissue containing blood capillaries and nerve-fibers, which
enter the tooth through a minute foramen at the end of the root. The
larger mass of the tooth is formed by a bone-like substance, the dentine
or ivory. Unlike bone, however, dentine is devoid of cells. In section,
the dentine takes on a somewhat prismatic appearance from the presence
of parallel tubes, the dental canaliculi, which radiate from the pulp-
cavity through the dentine. At their peripheral terminations in the
dentine, the canaliculi branch profusely. The sensitivity of the dentine
to the dentist's drill is probably due to the living protoplasm in these
canaliculi, which acts in the manner of nerve fibers. The larger part
of the dentine, approximately 75%, consists of inorganic mineral salts
such as calcium phosphate and calcium carbonate. The remaining 25%

194

COMPARATIVE ANATOMY

is organic material. At no place on the tooth does the dentine reach the
surface, since the crown and neck are covered with enamel, while the
rt)ot is surrounded Ijy a heavy cement.

Enamel is the hardest substance in the human body, since it contains
only three and a half per cent of organic substance. It is thickest at the

growth lines

in enamel

pulp chamber

growth lines

in dentine

root canal

oral epithelium

osteoblasts

of periosteum
of alveolus

connective
tissue fibers

cementoblasts
( from
dental sac)

bone of
mandible

blood vessels
and nerves

Fig. 146. — Schematic diagram showing the topography of a tooth and its relations
to the bone of the jaw. The numbered zones indicate empirically the sequence of
deposition of the dentine and enamel. The so-called growth lines in the dentine and
enamel follow the general contours indicated by the dotted lines in the figure but are
much more numerous. (From Patten's "Embryology of the Pig.")

apex of the crown, and thins out towards the neck and root. High
magnification shows that the enamel consists of minute parallel hexagonal
prisms which rest on the dentine and extend to the outer surface of the
crown. Increase in the amount of enamel toward the outside of the
crown is effected by means of increase in the number of enamel prisms
and not by their enlargement or branching. In this way the soHdity

TEETH

The

of the enamel is maintained throughout the crown of the tooth
mineral constituents of enamel are identical with those of dentine.

Cement is a bone-like substance covering the root of the tooth as a
thin layer which becomes thickest at the apex. Like other bone, the
cement contains lacunae connected with one another by canalicuH.
The mineral constituents are identical with those of bone. Surrounding

ODONTOBLASTS'

Fig. 147. — Diagrams illustrating the difference in the secretion of dentine A and
of bone B. The functional polarization of the odontoblasts and osteoblasts is, how-
ever, similar. (Redrawn after Braus.)

the cement, is a connective tissue dental sac or membrane continuous
with the periosteum of the alveolus and at the neck connected with the
covering of the gum, gingiva.

Development of Teeth

When the human embryo has attained a length of about 11 mm.,
that is, by the end of the sixth week, the ectodermal epitheHum covering

ERDERMIS

DENTAL RIDGED

DENTAL
RIDGE

CORIUM

BONY ALVEOLUi

LINGUAL LAMINA

ENAMEL ORGAN OF PERMANENT TOOTH
Fig. 148. — Diagrams of three stages in the development of a mammalian tooth as
seen in'sections of the jaw. The anlage of the permanent tooth lies on the lingual side
of that of the milk-tooth. (Redrawn after O. Hertwig and Arey.)

the upper and lower jaws grows rapidly down into the underlying con-
nective tissue to form a horseshoe-shaped ridge or lamina extending
along the edge of the jaw. As growth continues, the lamina divides into
an outer labial lamina and an inner lingual lamina. The two ingrowths.

196

COMPARATIVE ANATOMY

however, soon separate, one growing in labially, the other lingually.
The latter forms the dental ridge or lamina. As in development of a

hair, the dental ridge is formed by cell multiplication in the stratum

germinativum of the epidermis.

Early in the development of the dental lamina, a series of bell-shaped

enlargements, ten in each jaw, appear along its labial border (Fig. 148).

These are known as enamel organs
since they secrete the enamel covering
of the crowns of the teeth. Each of
the twenty milk teeth has a separate
enamel organ, and all of them are
present in a 2}/^ months embryo.
Each enamel organ contains a mesen-
chymatous dental papilla, the outer
cells of which, the odontoblasts,
secrete the dentine of the tooth.
The remaining cells of the papilla
become the pulp of the tooth. As
development proceeds, each enamel
organ recedes from the dental lamina
with which it retains a transient con-
nexion by means of a "neck" or cord
of cells.

^

~"">3

#te

The free edge of the dental lamina,
Fig. 149. — Diagrams representing losing connexion with the anlagen

stages in the phylogenesis of the dental ^^ ^^^ ^j^j^ ^^^^. ^^^^^ ^ SeCOnd Set
lamina. In elasmobranchs, the placoid ' _ i- i

scales which become teeth are lodged in of enamel organs lying on the Imgual

a groove on the edge of the jaw. In the .^^ ^^ ^j^^ primary Set. In this

higher animals, the groove becomes '^ ■'

filled with indifferent cells and converted way, the anlagen of the thirty-two

into the dent^al lamina. A primarily permanent teeth come to He embedded

indefinite number of sets of teeth formed ^ _ ^ r i •

in the lamina finally becomes limited in the Connective tissue of the jaws

to two in man C, A £. and f represent ^^^ jj^ j gj^^ ^f ^j^^ primary

stages of tooth development m reptiles; » i r

G and H in mammals. (Redrawn from set. The permanent teeth are, how-
Ihie, after Boik.) &v^x, relatively slow in development,

the third molar usually not forming in the jaw before the fifth year.

Soon after the enamel organs emerge from the dental lamina, they
become differentiated into three layers, an inner ameloblast layer which
secretes the enamel, a mesenchymatous enamel pulp, and a layer of outer
enamel cells. The ameloblast cells which line the enamel organ are
columnar epithelial cells derived directly from the stratum germinativum
of the epidermis. Viewed from the inner surface, each ameloblast cell is
hexagonal and each secretes a simple hexagonal prism of enamel. As
the enamel increases in thickness, the multiplication of ameloblast cells

TEETH

197

results in an increase in the number of enamel prisms. The twisting
and curvature of the prisms in the developed tooth is a consequence of
the torsion of the ameloblast layer during active secretion. While the
enamel grows by addition from the outside, the dentine increases in
thickness from within. Consequently as the tooth is formed the amelo-
blast and odontoblast layers are pushed farther and farther apart. Dur-
ing the secretion of the dentine, protoplasmic strands from the odonto-
blasts are retained within the dentine to form the dental canaliculi.
The odontoblast cells persist throughout life, and by their continued
secretion may in old age entirely obliterate the pulp cavity of the tooth.

blood vessel
in pulp

dentine

enamel

blood vessel in
mesenchyme

odontoblast ' dentinal Time's ameloblast outer epithelium

layer of enamel organ

Fig. 150. — Projection drawing of small segment of developing incisor from 130 mm.
pig embryo to show formation of enamel and dentine. X3S0. (From Patten's "Em-
bryology of the Pig.")

The crown of the tooth is the first to develop, and for a while the
tooth resembles a silver-plated thimble, the thin enamel coating cor-
responding to the silver plate covering, the dentine to the bare metal.
As the tooth grows, it increases in length as well as in thickness, adding
first a neck and later a root. The opening into the inner pulp cavity
becomes more and more restricted as the root elongates until finally only a
minute foramen remains to admit blood-vessels and nerves. The nerves
grow into the pulp and acquire free terminations among the odontoblast
cells. The cement layer is the last to be added. Cement is secreted by
bone-cells which penetrate the connective tissue sac enclosing the tooth.
Membrane bone is formed around the root of the teeth to form the alveoli
of the jaw-bone and to hold the teeth firmly in place.

The mechanics of the eruption of teeth is a problem which needs
further elucidation. Among the factors which operate is the elongation

198

COMPARATIVE ANATOMY

of the root, although teeth erupt before the root has completed its growth.
The eruption of the deciduous teeth begins during the seventh month
after birth, and is usually completed by the end of the second year. Of

Dental sac.

Outer layer. Inner layer.

Outer enamel cells.

Enamel pulp.

Inner enamel cells.

Dentine

Odontoblasts

Dental papilla
(future pulp).

Blood vessel
Bony trabecula of the lower jaw

Fig. 151. — Longitudinal section of a deciduous tooth of a newborn dog. The white
spaces between the inner enamel cells and the enamel are artificial, and due to shrinkage.
X42. (From Bremer's "Textbook of Histology.")

the permanent set, the first to erupt are the first molars which appear
during the sixth year. The last to erupt are the third molars, which
frequently become impacted in the jaw-bone so that eruption is impossible.

TEETH

199

ENAMEL PULP

ENAMEL
j«^- DENTINE

—ODONTOBLAST LAYER
-- PULP CAVITY
f* — ^^ TEMPORARY TOOTH

^''JAW-BONE

Fig. 152. — A section of the jaw of a nine-months human embryo, showing the anlage
of a canine tooth. The enamel organ of the permanent incisor is seen on the lingual
side of the milk-tooth. (Redrawn after Corning.)

SECOND
PERMANENT-
MOLAR.

PERMANENT-

PERMANENT-
CANINE.

/ « PERMANENT
'/ Jj MOLAR.
2^=^ PERMANENT
INCISORS.

Fig. 153.— The teeth of a five-year-old child. Portions of the jaws have been
removed so as to expose the roots of the milk teeth and the anlagen of the permanent
teeth. The latter are stippled in the figure. (Redrawn after Sobotta.)

200 COMPARATIVE ANATOMY

The shape of a tooth is determined by that of the tooth-germ. If
the layer of ameloblasts is folded, the enamel is correspondingly modified,
and teeth such as those of ruminants and elephants, which become
ridged by wear as the result of the difference in hardness of enamel and
dentine, owe this adaptive characteristic to the folding of the ameloblast
and odontoblast layers. The multiplication of roots as in molar teeth is
produced by the budding of the odontoblast layer of the dental papilla.

CHAPTER 6 ■

THE SKELETAL SYSTEM

Some creatures, jelly fish for example, have no skeleton. In some,
as in many molluscs and more conspicuously the corals, the skeleton is
heavier than all the soft parts combined. In man, the bones make up
about a fifth of the entire weight of the body, and this is not far from
the average for active air-breathing vertebrates that do not have dermal
armor.

All skeletons support or protect the softer parts. Supporting skeletons
occur even in such lowly creatures as protozoa and sponges. Protective
skeletons are conspicuous in echinoderms and molluscs, are universal
among the arthropods, and are found among vertebrates in such diverse
groups as the Paleozoic ostracoderms, ancient and modern ganoids,
dinosaurs, turtles, and armadillos.

Skeletal parts, which are also jointed levers used in locomotion, occur
in arthropods and vertebrates alone.

Arthropods solve the problem of locomotion by means of a chitinous
exoskeleton with the muscles inside it. Such a skeleton is highly efficient
as attachments for muscles, and it has the further advantage of providing
armor at the same time. Its disadvantage is that it cannot grow, so that
all the arthropods, by one device or another, shed their exoskeletons as
their bodies enlarge. This leaves them for a time helpless. Furthermore,
since the tissues of the molting arthropod are unsupported, no arthropod
can attain any considerable size. Among arthropods the largest air-
dwellers are foot-long centipedes; and although among water-dwellers
the euripterids of the lower Paleozoic and earlier were more than a yard
long, a twenty-pound lobster is about the limit for a modern form. The
typical arthropod is a tiny insect.

The endoskeleton of vertebrates, light and strong, and capable of
indefinite growth, has the single disadvantage that skeletal armor must
be developed independently. But vertebrates have for the most part
abandoned armor. Their success as a group has depended not a little
on their admirable endoskeleton. To its usual functions, the vertebrates
add the production of blood cells by the marrow, especially of the long
bones.

The Two Parts of the Skeleton. Historically, the vertebrate skeleton
consists of two parts, which begin independently, have evolved separately,

202

COMPARATIVE ANATOMY

and not even in the higher forms have become completely integrated.
These are the appendicular skeleton of the four limbs with their girdles;
and the axial skeleton, which includes the skull with the jaws, and the

Fig. 154. — Diagrams illustrating two theories of the origin of chondrin in the
formation of cartilage. According to one opinion (A) chondrin is of intracellular origin.
According to the other view (B) chondrin is of intercellular origin. (Redrawn after
Bremer.)

vertebral column, the sternum, and the ribs. The individual bones
number in man, sixty-four for the shoulder girdle and the arms, sixty-two

_J . /SUPI

SUPRAOCCIPITAU
EXOCCIPITAL

METATARSALS
PHALANGES

Pig. 155. — A diagram of the vertebrate skeleton, showing the division of the skeleton
into axial, visceral, and appendicular. Membrane bones are shown in black, cartilage
bones stippled.

in the pelvic girdle and the legs, twenty-three in the skull, twenty-six
in the backbone, and twenty-five for ribs and sternum, with six ear bones
besides, over two hundred in all.

THE SKELETAL SYSTEM

203

THE AXIAL SKELETON
Evolution of the Vertebral Coltunn. Nothing like a vertebral column
appears in any invertebrate, so that the earlier portions of its history are

Fig. 156. — Diagram of the skeletogenous tissue in the caudal region of a vertebrate.
bv, blood-vessels; d, corium; epmu, epaxial muscles; lis, horizontal septum; hymy,
hypaxial muscles; 7nsd, msv, dorsal and ventral median septa; tnys, myosepta; n, spinal
cord; nc, notochord. (From Kingsley's "Comparative Anatomy of Vertebrates.")

/NEURAL PROCESS

B. CESTRACION, A SHARK.

Fig. 157. — A, The skeleton of a cyclostome, Petromyzon; B, The skeleton of an
elasmobranch, Cestracion. Elasmobranchs were the first animals to invent paired
appendages and the skeletal elements to support them. Marked differences in the axial
and branchial skeletons of cyclostomes and elasmobranchs also appear. (Redrawn
after Dean.)

unknown; though, if amphioxus gives the clue, it was once no more than a
medial dorsal fold of the alimentary canal. Its first certain beginnings are

204

COMPARATIVE ANATOMY

the notochord of the lower chordates, the hemichorda, urochorda, and
cephalochorda. In the cyclostomes, the notochord is still the main
part of the axial skeleton. Since the cyclostomes have cartilaginous neural
arches, it is probable that neural arches are the earliest vertebral elements.
Elasmobranchs, both fossil and modern, show a considerable advance
over the cyclostomes. Cartilaginous haemal arches and centra appear,
with both neural and haemal spinous processes. The anterior trunk
vertebrae of elasmobranchs have short lateral or "costal" processes which
extend between the myotomes and which suggest the future ribs of
mammals. Since in fossil and living forms two centra may occur in each
body segment, and since each centrum usually develops in ontogenesis
by the fusion of antero-posterior anlagen, it is possible that two centra

in each segment (diplospondyly)
may have been the original arrange-
ment in vertebrates. Elasmo-
branchs, moreover, begin the long
process of vertebral differentiation,
the vertebrae of the tail being
unlike those of the trunk, the
difference correlated with a differ-
FiG. 158. — Sagittal section of Acanthias ence in the relation of the coelom

vertebrae, cut surfaces obliquely lined. ^^ ^^^ vertebrae. In the trunk
c, calcifications of centra; cd, caudmeurals;

cdh, caudihemals; cr (i), cranineurais region, where the body cavity lies,

(intercalaria); d exits of dorsal nerve roots; ^^^ haemal arch of each vertebra is

crh, cranihemals; n, notochord; v, exits 01 ^

ventral nerve roots. (From Kingsley's incomplete, while in the caudal

" Comparative Anatomy of Vertebrates.") ^^^j^^ ^^^^ ^^^^ -^ complete with a

median spinous process. The notochord persists intervertebrally and the
centra are biconcave. The skeleton is still cartilaginous, but the
cartilage is often hardened with lime.

Bony vertebrae make their appearance in Ganoid fishes, some of
which however retain a cartilaginous vertebral column. Ball and socket
joints between the centra are developed in Lepidosteus (Gar-pike) as
in some Amphibia. Amphicoelous or biconcave vertebrae, however,
predominate in all groups of fishes. Centra are wanting in the Dipnoi.

With the amphibia, bone succeeds cartilage; and the vertebrae are
differentiated into cervical, trunk, sacral, and caudal. The single sacral
vertebra is but slightly modified for attachment to the pelvic girdle. A
single atlas represents the cervical series of higher forms. Zygapophyses,
for articvdating each vertebra with its two neighbors, first appear in this
group. Articulation with the ribs is effected by two sorts of processes,—
diapophyses from the neural arches and parapophyses from the centra.

Lumbar vertebrae are first differentiated in reptiles, which also have
two sacral vertebrae. Here also appear vertebrae with centra flattened

THE SKELETAL SYSTEM

205

on both anterior and posterior sides and with the centrum of the atlas
fused with the axis, as in mammals.

The vertebral column of mammals shows little advance beyond that
of reptiles. A few Insectivora have intercentra in the lumbar region, — -

CARTIU\GE
CELLS

^d^-^ IW^ DEGENERATION

OSTEOCLASTS

OSTEOBLAST

BONE

Fig. 159. — Endochondral bone formation at the end of a long bone. Destruction of
cartilage is followed by the secretion of lime in the form of thin lamellae. Osteoblasts
then lay down bone upon these lamellae. In this way cancellous bone replaces cartilage.
(Redrawn after Dahlgren and Kepner.)

a diplospondylous condition reminiscent of elasmobranchs. Parapophyses
are reduced to shallow pits for articulating the heads of the ribs.

The human spine differs little from that of other mammals, except
that the tail is reduced to a coccyx with a few variable muscles attached.
Man's only distinctive feature is the sigmoidal curve, which bends his
spine in two directions, instead of one only as in other creatures. In

2o6

COMPARATIVE ANATOMY

addition to the two main spinal curvatures, thoracic and lumbar, man
also has two lesser curvatures, cervical and sacral, in the region of the
neck and sacrum respectively.

Fig. i6o. Fig. i6i.

Fig. i6o. — Diagrams of [A and B) fish vertebrae and (C) vertebra from higher
groups, b, basal stumps; c, centrum; ch, capitular head of rib; d, diapophysis; ha, hemal
arch; hr, hemal rib; n, notochord; na, neural arch; p, parapophysis; r, rib; t, tubercular
head. (From Kingsley's "Comparative Anatomy of Vertebrates.")

Fig. i6i.— Two caudal vertebrae of alligator, c, centrum; ha, hemapophysis; hs,
hemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses;
t, transverse process. The arrow passes through the neural arch. (From Kingsley's
"Comparative Anatomy of Vertebrates.")

The Vertebral Column in Man. In the backbone of a child there are
thirty-three vertebral elements. During growth the last nine fuse to form
two adult bones, the sacrum and the coccyx. The other twenty-four

c D

ARTICULAR PROCESS INTERVERTEBRAL CARTILAGE
i CENTRUM I SPINOUS PROCES^

Fig. 162. — Diagrammatic sagittal sections of (A), amphicoelous; (B), procoelous;
(c), opisthocoelous; and (D), amphiplatyan vertebrae; the head is supposed to be at the
left. Cut surfaces obliquely lined. (After Kingsley modified.)

vertebrae remain separate throughout life and become differentiated into
seven cervical vertebrae, twelve dorsal or thoracic, and five lumbar.
These are sometimes called "true" vertebrae in contra-distinction from
those of the sacrum and coccyx which are called "false" vertebrae.
Although the vertebrae are separate bones, they are nevertheless so firmly
fastened together by ligaments and fibrous cartilages as to make the

THE SKELETAL SYSTEM

207

EPISTROPHEUS:-?

CURVATURES

"zni,'

(

backbone a fairly rigid column. Four curvatures appear in the adult —
cervical, thoracic, lumbar, and sacral.

The Structure of a Vertebra. A typical vertebra consists of a cylin-
drical body, the centrum, which is flattened on its superior (cranial) and
inferior (caudal) surfaces. A neural arch arises from the dorsal side of
the centrum and surrounds a verte-
bral canal. That part of the neural
arch which connects with the
centrum is the pedicle. A spinous
process extends backwards and
downwards from the mid-dorsal side
of the neural arch. That part of
the neural arch between the spinous
process and the pedicle is the lamina.
Anterior and posterior notches or
incisures constrict the pedicles so
that the incisures of two successive
vertebrae form the foramina for the
spinal nerves which pass out between
the vertebrae. Articular processes
or zygapophyses project forwards
and backwards from the neural
arches. A postzygapophysis of one
vertebra overlaps a prezygapophysis
of the next vertebra and the two are
bound together by ligaments: thus
the backbone is strengthened, but
at the same time made less flexible.
On each side a transverse process
projects from the neural arch
laterally into the muscles of the body
wall. See Fig. 164.

The Kinds of Vertebra. There
are five kinds of vertebrae, cervical,
thoracic, lumbar, sacral, and caudal
or coccygeal. A distinguishing
feature of cervical vertebrae is a

transverse foramen which in the upper six vertebrae transmits the
vertebral artery. The lateral border of this foramen is formed by
the fusion of a rudimentary rib with the vertebra. The first two
cervical vertebrae are the atlas and the axis or epistropheus. A peculi-
arity of the two is that the centrum of the atlas fuses with that of
the axis to form the odontoid process upon which the atlas rotates. The

f

_.jr>tf>) I \ "--^SPINOUS

r

^^«S-~JVj J.-- PROCESSES.

' /

' ^' .'f^F^-^ ^"^ '

( '5

=^J5l^ °'!

LUMBAR J
CURVATURE.'. JH

Jjgf^f^^ ™yuMBAR.

i ' T

^i'^V""^ E.'i

/j|^^^C^J \ INTERVERTEBRAL

rNTERVERTEBRALj IT
FIBROCARTILAGE . ', K

„^^J^/t^?^^-7^'"^"\>0RAMEN.

\ \

^>%IC^xj \

\^

*«/^\!^^l^ " \

l^^^^-'^^^VinVACRAL

^-T^OT'^A

^\ ^rafj Y. i

SACRAL L Ur 1 I
CURVATURE>, % 7 y

K

Coccygeal.

Fig. 163. — The human vertebral col-
umn viewed from the left side. (Redrawn
after Sobotta.)

208

COMPARATIVE ANATOMY

forms and arrangement of the cervical vertebrae permit greater freedom
of movement than is possible in other parts of the column. The spinous
process of each cervical vertebra except the last is forked or bifid.

Only the twelve thoracic vertebrae carry ribs. A pit in the centrum
articulates with the head of the rib and a similar pit at the extremity of the

POSTERIOR

•

TUBERCLE <^

SPINOUS PROCESS-/?
VERTEBRAL |1 ,

^

NEURAL ^.^^-^FS

^'FORAMEN NEURAL ARCH,. ^ *5%l^

I

ARCH i>"'^-3;a3:t '

L

yfTU-^^^^0l^^i£

^^Ss., SUPERIOR \ ">^jJ

% VERTEBRAL

TRAMSVERSE J^ \ JT^ ,

"Xife-,2^ ARTICULAR PROCESS • f /^

■^^ ^'FORAMEN

PROCESS 1 Jma^J^ /

V^^Sf' ^jL^rfn /

>^

INFERIOR
VERTEBRAL NOTCH

\MNFERIOR
^ARTICULAR
PROCESS

Fig. 164. — Various human vertebrae A-D. A represents an atlas vertebra, superior
surface; 5 is a cervical vertebra, superior surface; C-C a thoracic vertebra in lateral and
superior aspect; D-D a lumbar vertebra in lateral and superior aspect. (Redrawn
after Sobotta.)

transverse process articulates with the tubercle of the rib. The head of
most ribs articulates with two adjacent centra.

The five lumbar vertebrae are the largest. Short ribs fuse with them
to form conspicuous transverse processes. The neural arches of these
vertebrae have mammillary and accessory processes in addition to
articular.

THE SKELETAL SYSTEM

209

The sacnim is a spade -shaped bone formed by the fusion of five
vertebrae. Its lateral wings are modified ribs fused together and also
articulated with the hip bones. Spinous proc-
esses are much reduced. Between the costal
processes four pairs of sacral foramina provide
exit for nerves and blood vessels. The sacral
canal is the continuation of the vertebral canal.

The coccyx consists of four fused centra
which lack neural arches and processes.
Frequently the first of these vertebrae fuses
with the sacrum, and only the last three form
the coccyx.

Successive vertebrae are connected to form
a continuous column by intervertebral discs of
fibrous cartilage. Interconnexions are further vertebrate embryo at begin-

.... ning of scleroblast migration.

Strengthened by numerous vertebral ligaments, showing grooves (g) between
Development of the Vertebral Column. In notochord and spinal cord

. . . , and blood vessels, a, artery;

man as m other vertebrates the primary axiai

Fig. 165. — Diagram

tn, myotome; tns,
Around this the ing mesenchyme;

migrat-
, noto-
chord; s, spinal cord; v,
vein. (From Kingsley's

skeleton is the notochord

definitive axial skeleton is built; and the

notochord disappears, slight traces only being "Comparative Anatomy of

left as nuclei pulposi of the intervertebral

cartilages. The processes involved in this replacement are complicated,

beginning with the appearance of mesenchyma cells around the notochord

__^^^^^^^^^^^s§«:ss2Sii:^^^^^S5EES — EPIDERMIS

I— SCLEROTOME

NOTOCHORD

— MYOTOME
— DERMATOME

Fig. 166. — A horizontal section of an elasmobranch embryo, showing the differentia-
tion of the mesoderm (epimere) into sclerotome, myotome and dermatome. The
sclerotome surrounding the notochord gives rise to the centrum of the vertebra.

and the neural tube. In this mesenchymal matrix, cartilage develops
only to be destroyed in its turn and replaced by bony vertebrae.

The mesenchyma from which the vertebrae arise is produced by
proliferation of the sclerotome, the cells of which migrate into the space

2IO

COMPARATIVE ANATOMY

between the mesoderm and the notochord. Later, by a continuation of
the same migration, the neural tube becomes completely surrounded by
mesenchyma.

Before cartilage is secreted in the mesenchyma, the sclerotome median
to each myotome becomes differentiated into a denser posterior portion
and a less dense anterior half. As the definitive vertebrae are formed,
the posterior half of each vertebral anlage fuses with the anterior half
of the following one. By this process the definitive vertebrae come to lie
intersegmentally, alternating with the myotomes. The result is obvi-
ously adaptive, since only by this arrangement could each myotome
become connected with two vertebrae and with two successive ribs.

EPIDERMIS

DERMATOME

MYOTOME

NOTOCHORD

The chondrification of each vertebra starts in six centres, two in each
centrum, two in each neural arch, and two in the costal processes. During
the third month ossification of the vertebrae begins; but it is not com-
pleted until several years after birth. At about the seventeenth year,
bony epiphyses arise as separate ossifications of the anterior and posterior
surfaces of the centra. In the twenty-first year, these fuse with the
centra and elongation of the vertebral column ceases. Of the original
cartilaginous matrix only the intervertebral regions persist. The fusion
of the sacral vertebrae is deferred until the twenty-fifth year.

The Ribs. Man has twelve pairs of ribs, which form a basket sur-
rounding the thoracic cavity. Each rib is a curved flat bone ending
ventrally in a costal cartilage. By means of these costal cartilages the

THE SKELETAL SYSTEM

211

first seven pairs connect directly with, the sternum and are therefore
called sternal or true ribs while the five remaining pairs are distinguished

PHALANGES .

-METACARPALS.
CARPALS.

PHALANGES

TARSALS.

METATARSALS.

Pj(, j68 —Human skeleton viewed from behind. (Reproduced in modified form
from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, I930 by Alfred
A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, inc.,
authorized publishers.)

as false ribs. The last two pairs, the eleventh and twelfth, do not connect
with others, and are known as floating ribs. Each rib has a head or
capitulum, which articulates with the vertebral centrum, and a tuber-

212

comparativp: anatomy

culum, which articulates with the transverse process. As the rib basket

rises and falls in breathing, each rib rotates on an axis running through

the tuberculum and the capitulum.

Each rib has a costal groove extending along its lower or posterior

border. To the ridges which border this groove are attached the external

and internal intercostal muscles.

The Development of Ribs. Ribs develop in the embryo as costal

processes of the vertebrae in the intermuscular septa or myocommata.

Primarily, the cartilaginous anlagen
of the ribs are continuous with the
cartilaginous vertebrae. The short
costal processes in the cervical,
lumbar, and sacral regions unite
with the transverse processes and
are indistinguishable from them in
the adult. In the thoracic region,
separate centers of ossification in the
ribs are formed and articulations
with the vertebrae develop. Epi-
physes at the capitulum and tuber-
culum make possible the elongation
of thoracic ribs. The ventral extrem-
ities of the ribs do not ossify but
remain throughout life as the costal

-Diagrammatic section of a
vertebrate to show the relation of ribs CartilagCS
to the muscles of the body wall, av,
aorta; c, coelom; e, ectoderm; ep, epaxial
muscles; g, gonads; ha, hemal rib; hp,
hypaxial muscles; /, intestine; mes,
mesentery; n, nephridium; o, omentum;
r, true rib; p, somatopleure; sp, splanchno-
pleure; v, vertebra. (From Kingsley's
"Comparative Anatomy of Vertebrates.")

The Evolution of Ribs. Ribs are
wanting in chordates below the
elasmobranchs, and even in elasmo-
branchs they occur only in the
anterior trunk region as short carti-
laginous processes lying in the
horizontal septum separating epaxial and hypaxial muscles. Such true
ribs should not be confused with the hemal arches of fishes which are
median to the lateral trunk muscles and adjacent to the peritoneal lining
of the body cavity. See Fig. 169.

The ribs of modern amphibia show little advance above those of the
elasmobranchs, and in many Anura continue as short cartilaginous
processes of the vertebrae. But bony ribs are present in urodeles such as
Necturus and the attachment to the vertebrae is, as in the higher verte-
brates, by means of tubercular and capitular processes. In some fossil
amphibia the ribs were elongated and extended around the body to the
ventral side. Abdominal ribs were also present, as in some modern
reptiles. ,

THE SKELETAL SYSTEM

213

In reptiles, ribs increase in number, and in some forms encircle the
abdominal cavity. Abdominal ribs are common. The ribs of snakes are

PHALANGES

METACARPALS

MANDIBLE.

/CLAVICLE.

ACROMION PROCESS.

-CORACOID.

-MANUBRIUM.
STERNUM.
XIPHOID PROCESS.

^-CARPALS.

PHALANGES.

Fig. 170.— Human skeleton viewed from in front. (Reproduced in modified form
from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, 1930 by Alfred
A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc.,
authorized publishers.)

especially numerous. In mammals and man, ribs which articulate with
the vertebrae and extend around the body cavity are limited to the thoracic
region.

214

COMPARATIVE ANATOMY

The Sternum. The sternum is a flat, dagger-shaped bone lying
mid- ventral ly of the chest.

Three parts are distinguished, i. The manubrium or presternum,
triangular, the widest portion and the most anterior. It articulates with
the clavicle. 2. The gladiolus or mesosternum, the longest portion,
formed by the fusion of four sternal elements or sternebrae, 3. The
posterior metasternum, xiphoid or ensiform process. The xiphoid
process is sometimes perforated by a foramen and is sometimes forked
posteriorly.

Development of the Sternum. The sternum arises from connective
tissue which is afterwards chondrified to become a pair of cartilaginous

c p

X

1

A

Fig. 171. — Scheme of development of mammalian sternum. A, early stage; B,
cartilage, the halves uniting; c, Pcoracoid precartilage; d, clavicle; co, centers of ossifica-
tion; ni, mesosternal parts; ?««, manubrium; p, presternum; st, sternebrae; x, xiphister-
num. (From Kingsley's "Comparative Anatomy of Vertebrates.")

bars, which secondarily unite in the mid-ventral line and only later
connect with the costal cartilages. Ossification begins in a series of paired
centers, but the manubrium usually has one center only. Ossification of
the metasternum or xiphoid process remains incomplete until very late in
life.

Evolution of the Stemtmi. Opinion is divided as to the beginnings of
the sternum. Some morphologists take the median portion of the
elasmobranch pectoral girdle to be the homologue of the mammalian
presternum, notwithstanding the fact that in some urodeles the sternum
is a midventral plate of cartilage quite unconnected with the pectoral
girdle. Since, however, the median ventral portion of the elasmobranch
pectoral girdle is limited to a single intersegment, while the sternum of
higher vertebrates extends through several segments and in mammals is
clearly metameric, this hypothesis leaves the metamerism of the sternum
unexplained. To meet this difficulty, it would be necessary to assume an

THE SKELETAL SYSTEM

215

antero-posterior extension of the sternum along the mid-ventral line,
and a secondary segmentation. But the fact that in urodeles, where the
sternum makes its first appearance in the vertebrate series, the sternum
is independent of the pectoral girdle, and the additional fact that the
sternum develops in ontogenesis independently of the pectoral girdle,
makes it difficult to accept this hypothesis. See Fig. 172.

A second more plausible hypothesis assumes that the sternum arose
by the fusion of the ventral ends of a series of ribs. In favor of this opinion
it is pointed out that in such a primitive amphibian as Necturus the

SUPRASCAPULA,

INTERCLAVICLE

XIPHOIO PROCESS

Fig. 172. — Types of vertebrate sterna. A. Squalus; B. Salamandra ; C. Necturus;
D . Rana; E. Felis;F. Crocodilus;G. Homo. While there is no doubt of the homology
of the various amniote sterna, their homology with those of anamnia is in dispute.

Sternum is represented by a series of four or five pairs of cartilages near
the mid-ventral line. Like ribs these cartilages are intermyotomic.
While in Necturus ribs do not extend from the vertebrae to the ventral
side of the body, it is believed that there were primitive fossil amphibia
in which the ribs were so extensive. The hypothesis that the sternum
is a rib-sternum has at least this much foundation.

The facts of mammalian ontogenesis, however, do not appear to
support this view. As stated above, the mammalian sternum arises
independently of the ribs by the union of a pair of longitudinal cartilages
which arise near the mid-ventral line. The connexion of these cartilages
with the ribs is secondarv.

2l6

COMPARATIVE ANATOMY

If an opinion were to be based upon the relation of the sternum in
Necturus and of the ontogenesis of the mammalian sternum alone, we
should have to conclude that the sternum arose from paired segmented
cartilages formed near the mid-ventral line independently both of the
girdle and of the ribs. Under the circumstances, and until more decisive
evidence is discovered, suspension of judgment is necessary.

In the reptiles the sternum is converted into a metameric structure
composed of a series of sternebrae and connected with the ribs as in
mammals.

Fig. 173. — Diagrams showing the development of the primordial skull. Since this
organ develops primarily beneath the brain as a support the figures represent the ventral
aspect. (A) Early stage, before the appearance of cartilage. The notochord is seen
lying along the nerve cord as far forward as the hypophysis. The three sense-organs,
nose, eye, and ear, have already appeared. (B) This stage shows the trabeculae (l),
the parachordals (p), and the capsules around the sense-organs. (C) In this the
trabeculae, the parachordals, and the nasal and otic capsules have fused into a single
mass, the primordial skull, or chondrocranium. The anterior end of the notochord is
imbedded in this. The cartilaginous capsule of the eye remains free to allow the
necessary movements of the eyeball. (From Wilder's "History of the Human Body,"
Courtesy of Henry Holt and Co.)

The mammalian sternum differs little from that of reptiles. It is
divided into the same three elements as those of reptiles and man, pro-
meso-, and meta-sternum.

The Skull. There are two chief parts to the skull, which have different
origins and a different history. One of these is the cranium or brain-case,
together with the bones of the face except the two jaws. The other is
the visceral skeleton, that is to say, the two jaws, the hyoid bone, the
ear bones, and the cartilages of the larynx.

The Evolution of the Cranium. In the early part of the nineteenth
century it was generally assumed by morphologists that the skull consists
of four or five enlarged vertebrae. Originally suggested by the poet

THE SKELETAL SYSTEM

217

Goethe, this "vertebral theory" of the skull was developed by Oken in
Germany and by Owen in England. The basis of this theory may be
seen in any mammalian skull, which consists of four bony rings beginning
with the nasal region and ending with the occipital. Owen pictured an
archet>'pal vertebrate, the axial skeleton of which consisted of a series of
typical vertebrae, the anterior four being enlarged to form the skull.
The vertebral theory received its death blow, however, when Huxley
called attention to the fact that in the skull of such lower fishes as
the elasmobranchs there is nothing remotely resembling a vertebra. The
absence of vertebrae where they should be most evident, together with the
lack of cranial vertebrae in vertebrate embryos except in the occipital

Fig. 174. — Ventral (F) and dorsal (D) views of typical chondrocranium showing
the cartilage bones derived from it. als, alisphenoid; ch, choana; eo, exoccipital; fl,
foramen lacerum; hf, hypophysial fossa; of, orbital fissure; os, orbitosphenoid; qu,
quadrate. (From Kingsley's "Comparative Anatomy of Vertebrates.")

region, led morphologists to abandon the theory. Failure to demonstrate
vertebrae in the skull has not, however, altered the opinion that head and
trunk had at one time a similar metameric structure. See Fig. 174.

The notochord forms in chordates the primary skeleton of the head
as well as of the trunk. Evidence from comparative anatomy and
embryology also indicates that the next step in evolution was the appear-
ance of the parachordal and trabecular cartilages. The former as their
name suggests parallel the anterior end of the notochord while trabecular
cartilages lie anterior to the notochord beneath the forebrain vesicle.
Enlarging these cartilages and fusing them with the cartilaginous nasal
and otic capsules formed the primordial chondrocranium. The loosely
constructed cartilaginous skull of cyclostomes represents roughly this
stage of evolutionary development. In the cranium of cyclostomes,
however, in addition to the parachordal and trabecular cartilages there is
an ethmoid plate anterior to the trabeculae, and the beginnings of a tectum
covering the brain in the region between the otic capsules. (Fig. 1 73)

2l8

COMPARATIVE ANATOMY

A further advance towards the skull of higher vertebrates is presented
in elasmobranchs, where the fusion and extension of cranial cartilages has
produced a brain case which covers the brain except for an anterior and
a posterior fontanelle. Among novelties in the skull of elasmobranchs are
cartilages homologous with those which give rise to the alisphenoid bones
of higher vertebrates. The beginnings of a dermal skeleton appear in
this group in the form of placoid scales. The bone-like basal plates of

OLFACTORY PIT

5^,-PREFRONTAL

CHONOROCRANIUM
C5TIPPLE0)

SQUAMOSA It

POSTFRONTAL

EXOCCIPITAL

SUPRAOCCIPITAL

Fig. 175. — The head of a sturgeon, viewed from above as a translucent object.
Membrane bones (scutes) are outlined and the inner cartilaginous cranium stippled.
By means of comparative anatomy it is possible to identify certain scutes as homologues
of bones in the mammalian skull. (Redrawn after Gegenbaur.)

these scales are considered as the beginnings of the dermal skeleton and of
the membrane bones of the higher vertebrates.

In ganoid fishes the dermal scales of the head fuse into bony scutes, a
number of which (nasal, frontal, parietal, squamosal, etc.) may be traced
directly into the membrane bones of man and mammal. The cartilaginous
brain case within these dermal plates differs in no essentials from that
of elasmobranchs; but a progressive integration and fusion of cartilaginous
and dermal constituents explains the two modes of development of bones
in the mammalian skull. Intermediate stages in this evolution appear in
living and fossil vertebrates. (Fig. 175)

THE SKELETAL SYSTEM

219

Among the noteworthy changes in the skull during its evolution from
fishes to man is the considerable reduction in the number of bony elements.
Professor W. K. Gregory points out that the primitive fishes have as many
as 180 skull bones while higher fishes have only about 100. In amphibia
they number from 95 to 50. The
skulls of earlier fossil reptiles had
80 bones while those of the highly
specialized modern snakes have
. only 50. There were 70 bones in
pidSS' Ahe skulls of (tertiary )reptiles from
which mammals evolved; but
mammals in general have half that
number and the skulls of primates
do not have more than thirty
bones. Pre- and postfrontals are
present in the reptilian skull but
are wanting in mammals. Curio-
usly the peccaries, like the birds,
have the entire skull fused to a
continuum and only the lower jaw
a separate bone.

Thi\ rpdiirl-inn k f-<:;npriallv ^'^- 1 76.— Skull of a stegocephalan

iniS ^reauction is especially (^Capitosaurus) . eo, exoccipital; «/., tabu-

Striking in the evolution of the lare; /, frontal; ju, zygomatic (jugal); la,

Hprmnl <;kplptnn Aq thp Hprmal lacrimal; w*, maxilla; «a., nasal; o, orbit; ^a.

dermal skeleton. As tlie dermal parietal; pmx, premaxilla; por, postorbital;

scales sink into the deeper layers prf, prefrontal; ptf, postfrontal; qj, quad-

of the skin, they unite with the T^^':Sn^ tilT^T's

bony elements which are preformed "Comparative Anatomy of Vertebrates,"

in cartilage so that there frequently ^ ^^ 1 e .;

results a complex bone that has both cartilaginous and membranous

elements.

The tendency of the primitive cartilaginous brain case to become bony
begins in the ganoids, advances in amphibia and reptiles and is nearly
completed in birds and mammals. Although the advantages of a bony
skeleton for land animals is obvious, it is difficult to explain by any hypo-
thesis of Lamarck, Darwin or DeVries the substitution of bone for
cartilage in aquatic animals. Elasmobranchs such as the dogfishes seem
as successful in the struggle for existence as are the teleosts. The former
have a cartilaginous and the latter a bony skeleton. Cartilage is not a
prerequisite condition for the appearance of bone since membrane bones
develop directly from mesenchyma without an intermediate cartilaginous
stage. The difficulty of explanation of the emergence of bony skeletons
from cartilaginous beginnings is greatly increased by the complexity of
the processes by which in ontogenesis, and therefore presumably in

220

COMPARATIVE ANATOMY

Fig. 177. — Membrane bones of typical tetrapod; chondrocranium in dotted outline.
inter p, interparietal; pftix, premaxilla; pof, postf rental; porb, postorbital; prf, prefrontal;
qj, quadratojugal; zyg, zygomatic. (From Kingsley's "Comparative Anatomy of
Vertebrates.")

sDENTARY
BONE

C. . D.

Fig. 178. — The diagrams A-D illustrate the growth and enlargement of the mem-
brane bones of the skull and their encroachment upon the chondrocranium. Cartilage
and pro-cartilage bones are stippled, membrane bones black. Membrane bone is
represented in the basal plates of the placoid scales of elasmobranchs (A). In the
ganoid (B) the scales have fused and enlarged to form bony skutes, but the chondro-
cranium remains cartilaginous. In amphibia (C) the cartilage is largely changed to
bone and the two kinds of bone become fused together. In mammals (D) very little
cartilage is left and the two kinds of bone unite to form bone complexes. Most of the
covering bones of the mammalian cranium are membrane bones.

THE SKELETAL SYSTEM
X

221

Fig. 179. — Dorsal and ventral views of the skull of turtle, Trionyx. ho, basioccipital;
bs, basisphenoid; eo, cs, exoccipital; /, frontal; j. zygomatic (jugal); m, mx, maxilla; «,
prefrontal; opis, opisthotic; p, (behind orbit) postfrontal, (others) parietal; pal, palatine;
pmx, premaxilla; pno, prootic; pt, pterygoid; q, quadrate; s, supraoccipital; v, vomer.
(From Kingsley's "Comparative Anatomy of Vertebrates.")

SUPRAOCCIPITAH
BASlOCCIPITAl;
HYOMANDIBULARn
SQUAMOSALS^
PAR I ETAL-
PARASPHENOID^

1ST VERTEBRA

NEURAL SPINE

PTERYGOIDS^,
FRONTAL^~^_;

ECTETHMOID^^O'^^''
MESETHMOID
NASAL-
PALATINE

-OPERCU.AR
— y-^ — PREOPERCULAR
■^^^^SUBOPERCULAR
■SYMPLECTIC

QUADRATE
/2_.|MTER0PERCULAR

Fig. 180. — Skull of hake in left lateral aspect. Membrane bones are cross-hatched,
pro-cartilage bones stippled. Compared with the skull of ganoids, that of teleosts
shows an increasing dominance of membrane bones over those preformed in cartilage.

222

COMPARATIVE ANATOMY

Fig i8i —Diagram of the bones of the mammalian skull. Cartilage bones dotted,
membrane bones lined; 2-12, nerve exits. (Altered from Flower, from Kingsley's
"Comparative Anatomy of Vertebrates.")

FRONTAL,
PREFFONTAL'
LACRIMAL'

SEP70MAXILLARY.
MAXILLA
PRFMAXILLARY-

LACRIMALv
NASAL-
MAX I LLAs

SEPTOMAXILLARY-
PREMAXILLARY-

MAXILLA
SEPTOMAXILLARi'

PREMAXILLAR'

NASAL-
PREMAXILLjARY-

OCClPlTAL
-TEMPORAL

(mandible)

Fig. 182.— a series of fossil skulls (A-G) which are believed to represent fairly well
the phylogenetic changes of the human skull. (Redrawn after Romer's "Man and the
Vertebrates," University of Chicago Press.)

THE SKELETAL SYSTEM

223

phylogenesis, cartilage is gradually destroyed and later replaced by
bone.

Since reptiles have only a single occipital condyle to articulate the
skull with the atlas vertebra, while mammals have two, the descent of
mammals from reptiles has on this account been questioned. Also for
this reason, the attempt has been made to prove that mammals have
evolved directly from amphibia, which also have two occipital condyles.
But some reptiles have a tripartite condyle, and mammals may have

iPARASPHENOID

PARASPHENO

'PREMAXILLARY

PALATINE

MAXILLA- -
ISPHENOID

OCCIPITAL

■STAPES

pfemaxillary/

int.nares'

DIMETROOON.

PALATI NE'

PTERYGOID^-''
QUADRATE +
QUADRATOJUGAL

CYNOGNATHUS.

j;emporal aOCCipital

PRESPI-CNOIO
PALATINE
PREMAXILLARY

IPITAL PALATINE

-BASIOCCIPITAL

INTERNAL NARES

^INTERNAL NARES p-jL

C. DOG. Varietal-'

D MAN

Fig. 183. — Stages in the recession of the internal nares (choanae) are represented in
the Tertiary reptiles, Dimetrodon and Cynognathus. By the growth of a horizontal
septum from the maxillary and palatine bones the narial passages become separated
from the mouth cavity in higher reptiles and mammals. Thus the internal nares or
choanae finally open, not into the mouth cavity directly as in amphibia, but into the
pharynx. (Redrawn from Romer's " Man and the Vertebrates," University of Chicago
Press.)

derived their two condyles from this tripartite reptilian condyle by the
disappearance of its median basioccipital element.

One of the most conspicuous evolutionary developments of the skull
has been the relative enlargement of the membrane bones and the reduc-
tion of the cartilage elements. The only cartilage bones which persist
are those which support the brain, those which cover the brain being
exclusively dermal. This change is correlated with the increased size
of the brain. See Fig. 178.

Except in proportion, there is little difference between the skull of man
and that of other mammals. The enlargement of the brain and the
correlated enlargement of the roofing bones of the skull carries the olfactory
lobes, the foramen magnum and the otic capsules to the floor of the

224

COMPARATIVE ANAT(3MY

AUSTRALOPITHECUS

SINANTHROPUS

CRO-MAGNON

Fig. 184. — Skulls of fossil men restored, left lateral aspect. Australopithecus, how-
ever, as its name implies, is an ape rather than a man. Opinions differ as to whether
pithecanthropus is man or ape. Sinanthropus is indisputably human. While none of
these fossil types is believed to be in the direct line of ancestry of modern man (Homo
Sapiens), their discovery proves that more than one species of man has inhabited the
earth. It is also significant that, with the exception of Cro-Magnon man, the earlier
species of men were more ape-like than is modern man, as would be expected if apes and
men have had a common ancestry. (Redrawn after Romer's " Man and the Verte-
brates," University of Chicago Press.)

THE SKELETAL SYSTEM 225

cranium. Among other changes is an increase in the facial angle from
an acute to a right angle. The facial angle is the angle between a line
from the frontal bone to the maxilla and one from the basioccipital to the
base of the nasal septum. Apes and fossil species of men, however, help
to bridge this contrast. See Fig. 184.

The heavy superciliary crests characteristic of the chimpanzee and
gorilla, but lacking in modern species of men, are present in fossil Neander-
thal and Rhodesian man. Furthermore, the usual contrast between apes
and man disappears in the orang-utan which, like modern man, has rudi-
mentary superciliary ridges. Furthermore, the contrast between the
skulls of apes and man holds for the adult only, not for the young, the
differences increasing with age.

Against the evolution theory as applied to the human species it used
to be urged that there are no connecting links between man and apes,
contrary to expectation if man and apes have evolved from a common
ancestry. The contrast in brain size between man and apes is especially
striking. The brain of the gorilla is never larger than 600 cc, while the
smallest human brain is not less than 1000 cc. and the normal male brain is
1400. The discovery of the cranium of the Java man with a brain
capacity of between 800 and 900 cc. helps to reduce this contrast. The
striking fact revealed by fossil human skulls is that the characteristics
which distinguish them from the skulls of modern man tend to bridge
over the gap between man and apes. In other words, all fossil skulls,
except that of the Cro-Magnons which is like that of modern man, are
more ape-like than those of modern races. The dental arch of Negroid
races is intermediate between that of apes and Europeans. The chin
which is such a striking feature of the modern human jaw is lacking in
some fossil men as in the great apes.

Another contrast between the skull of man and apes is in the relation
of the skull to the backbone. The skull of modern man is poised on the
occipital condyles at about its center of gravity, but the condyles of apes
lies far behind the center of gravity of the head. Therefore are the neck
muscles of modern man relatively weak, those of the ape massive. It is
an interesting fact from the evolutionary point of view that the skull of
Neanderthal man shows an intermediate condition.

Bones of the Cranium. Eight bones in man enclose the brain, frontal,
occipital, ethmoid, and sphenoid and the paired parietals and temporals.
The facial skeleton includes twelve bones, the lower jaw or mandible,
the vomer, and the paired maxillaries, zygomatics, nasals, lacrimals, and
palatines. Since the turbinal bones of the nose are extensions of the
ethmoid they are not counted as separate bones.

The frontal bone forms the front of the cranium. Ridges of this bone
above the orbits of the eyes are the superciliary crests. Within the

226

COMPARATIVE ANATOMY

frontal bone are the large frontal sinuses. Articulating with the frontals
and forming the greater part of the roof and sides of the cranium are the
paired parietals. Curved ridges on the sides of the parietals mark the
origin of the temporalis muscle. See Fig. 185.

The occipital bone articulates with the parietals and forms the most
posterior of the roofing bones of the skull. Through a large median

NASAL

LACRIMAL

MAXILLA

ZYGOMATIC

MECKEL'S CART'^

MANDIBLE

STYLOID PROCESS

MAXILLA,
PALATINE
ZYGOMATIC

STYLOID PROCESS
AUDITORY MEATUS -

PARIETAL

FORAMEN LACERUM
TEMPORAL. OCCIPITAL CCTCYLE

PARIETAL
CHONDRDCRANIUM

FORAMEN MAGNUM

OCCIPITAL'

NASAL CONCHA

OCCIPITAL
TEMPORAL
MASTOID PROCESS
'AUDITORY MEATUS
ZYGOMATIC ARCH

Fig. 185. — The human skull, embryonic (A) and adult {B-D). In the fetal skull
(14 weeks) membrane bones are black, and the cartilage cranium (chondrocranium)
stippled. Figure B shows the adult skull in basal aspect, figure C in frontal aspect, and
figure D in left lateral aspect, approximately one quarter natural size. (Redrawn after
Sobotta.)

aperture, the foramen magnum, the spinal cord passes from the vertebral
canal into the cranium to connect with the brain. Many of the neck
muscles are inserted on the basal portion of the occipital. On each side
of the foramen magnum the occipital condyles form the articulation of
the cranium with the atlas vertebra.

The sphenoid bone, at the base and sides of the cranium in front of the
occipital, is in form the most complex of the cranial bones. It has a
median body and two pairs of wings, the lesser and the greater. Paired
pterygoid processes project downward and backward toward the pharynx.

THE SKELETAL SYSTEM 227

The pituitary gland or hypophysis occupies a median depression of the
sphenoid, the sella turcica. A number of nerves and blood vessels have
their foramina of exit through the sphenoid, and a large foramen, the
foramen lacerum, marks its posterior boundary.

The temporal bone is conspicuous on the sides of the skull, articulating
with the sphenoid in front, the parietal above, and the occipital behind.
Among its varied functions, it provides an articulation for the mandible
and a protection for the sensory ear. Its three divisions are the squamous,
mastoid, and petrosal. The squamous element covers the temporal
lobe of the brain and contains a fossa for the articulation of the mandible.
Several neck muscles, including the sterno-cleido-mastoid, are inserted on
the mastoid process of the temporal. The inner cavity or antrum of the
mastoid connects with the Eustachian tube and is consequently liable to
infection from the throat. Within the petrous portion of the temporal
are contained both the sensory and the bony portions of the inner ear.
The middle ear, lying between petrous and squamosal elements contain
the three ear bones, malleus, incus, and stapes, which belong to the
visceral skeleton.

The ethmoid is a bone of delicate texture lying at the anterior end of
the brain case and articulating with the frontal, sphenoid, and other
bones. It consists of a dorsal, horizontal perforated plate, the cribriform
plate, a vertical mesethmoid plate which divides the nasal passages, and
paired lateral plates from which are derived the turbinal bones or conchae
of the nose. The inferior conchae, although fused with the maxilla in
the adult, are believed to be derived from the ethmoid.

The vomer or "ploughshare" bone derives its name from its resem-
blance to a plough. It articulates with the mesethmoid and forms the
lower portion of the nasal septum. The lacrimal bones lie on the medial
surface of the orbit and are so named because the naso-lacrimal ducts pierce
them. They are the smallest bones of the cranium. The two nasal
bones form the bridge of the nose. They articulate with the frontal
above and the maxilla on the sides.

The maxilla, in addition to its function as the upper jaw, helps also to
form the orbits, the palate, and the narial passage. As a part of the
visceral skeleton (splanchnocranium) its description will be taken up later.

The posterior part of the hard palate is formed by the palatine, an
L-shaped bone, consisting of a vertical and a horizontal member, the
sphenopalatine notch dividing it into dorsal and ventral portions. It
articulates with sphenoid, maxilla, vomer, and ethmoid.

The zygomatic or malar bones are quadrangular, with processes
articulating with the frontal, maxillary, and temporal bones.

Development of the Cranium. The history of the vertebrate skull
revealed by the study of its comparative anatomy is amply supported

228 COMPARATIVE ANATOMY

by its embryology. Both prove it to be a complex formed by the union
of diverse elements, capsules that contain the sense organs, and supports
for the gills, underpinning and protective covering for the brain, while
in the occipital region vertebrae appear to have fused with the brain
case. The primordial cartilaginous cranium of the human embryo arises
as a pair of parachordal cartilages and a pair of prechordal cartilages or
trabeculae. These fuse together; later they combine with two pairs of
sensory capsules, the olfactory and the auditory. This formation of the
cartilaginous basis of the cranium, the chondrocranium, takes place
during the second month of intra-uterine life. Ossification from separate
centers begins with the third. See Fig. 173.

Evidence from comparative anatomy proves that the bones of the
human skull correspond to a much larger number of separate bones which
appear in the fishes and have been progressively reduced by fusion with
one another during the course of evolution. This also is borne out by
the development of the human cranium.

Ossification of the occipital bone, for example, begins in four centers
corresponding with the basioccipital, the paired exoccipitals with their
condyles, and a supraoccipital of lower vertebrates. To these are later
added a membranous interparietal. Ossification of the occipital begins
in the third month but is not completed until the seventh year.

The development of the sphenoid bone is even more complex, no fewer
than ten centers of ossification are recognized. Six of these arise in the
body of the bone and four more in the two paired wings. Membrane bone
is added both to the pterygoid processes and to the great wings. Fusion
of the separate elements is completed before the second year.

Ossification of the ethmoid remains throughout life incomplete.
Three centers of ossification corresponding to the pro-, epi-, and opisthotic
bones of lower vertebrates develop in the otic capsule and help to form
the petrosal and mastoid portions of the temporal bone. The styloid
process of the temporal is an ossified portion of the hyoid cartilage which
fuses with the temporal. The squamous portion of the temporal is mem-
branous in origin. An outgrowth of the epithelium of the middle ear
penetrates the mastoid process to form a cavity or antrum.

The rest of the cranium is membrane bone. Because the roofing
bones of the brain case ossify slowly and expand from centers, uncovered
regions or fontanelles persist for some months after birth as "soft spots"
between the frontal, parietal, and occipital bones.

Evolution of the Visceral Skeleton. Evidence from both comparative
anatomy and embryology indicates that the upper and lower jaws, the
hyoid bone, the ear bones, and the laryngeal cartilages of man have evolved
from the skeletal gill supports of primitive fishes.

THE SKELETAL SYSTEM

229

Cartilaginous supports for the respiratory system are lacking in the
lowest chordates. In amphioxus, the velum in which the mouth lies,
and the gill arches are supported by slender rods of a material which from
its resemblance to cartilage is called pro-cartilage. A truly cartilaginous
visceral skeleton first appears in cyclostomes in the form of a gill-basket in
which the number of cartilage rods corresponds with the number of gill-
arches. Bdellostoma may have as many as fifteen of these, other cyclo-
stomes eight or nine.

In elasmobranchs the number of visceral cartilages is reduced to corre-
spond with the reduced number of gill apertures. The maximum number

h Idrn ^° ^9 9c bb J^

Fig. 186. — Ventral and lateral views of the skull of lamprey {Petromyzon marinus).
ad, anterior dorsal cartilage, bb, branchial basket; gc, gill cleft; Ic, labial cartilage;
Idni, lateral distal mandibular; Lg, lingual cartilage; nc, nasal capsule; oc, otic capsule; on,
optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage. (After Parker from
Kingsley's "Comparative Anatomy of Vertebrates.")

is nine, in heptanchus, of which the first, the mandibular, is modified to
become the cartilages of the upper and lower jaws, while the second arch,
the hyoid, functions slightly as a gill arch, and its dorsal division, the
hyomandibula cartilage, acts as a suspensory apparatus for the lower jaw.
The dorsal division of the first visceral arch, which forms the cartilage
of the upper jaw, is called the palato-pterygo-quadrate cartilage, since
palatal, pterygoid, and quadrate bones develop from it in amphibia and
reptiles. The lower half of the first visceral arch forms cartilage of the
lower jaw, Meckel's cartilage. An articulation is formed between the two,
so that in elasmobranchs a biting mouth replaces the sucking mouth of
cyclostomes. Nevertheless, the upper jaw does not fuse with the cranium

230

COMPARATWE ANATOMY

in elasmobranchs but remains independent. The suggestion that the
labial cartilages of elasmobranchs are rudiments of cartilages which sup-
ported pre-oral gill arches in the ancestors of elasmobranchs has too slender
a foundation to be taken seriously. This, however, is not to deny the
possibility that there were preoral segments in such ancestors.

Posterior to the hyoid arch, the visceral cartilages persist in elas-
mobranchs as supports of the branchial or gill arches. The number is
commonly reduced to five, but whether the loss takes place at the end or
in the middle of the series is a disputed point. Most teleosts have only
four functional branchial arches, and in some the number is reduced to
two. Perennibranch amphibia have either two or three. Land animals

Fig. 187. — Branchial arches of {A) Heptanchus; (B), Chlamydoselachus; and (C)
Cestracion. c, ceratobranchial; e, epibranchial; h, hyoid; hb. hyobranchial; he, hyoid
copula; cbr, cardiobranchial (posterior copula); p, pharyngobranchial; 1-7, branchial
arches; m, Meckel's cartilage. {A and C after Gegenbaur, B after Garman from
Kingsley's "Comparative Anatomy of Vertebrates.")

have lost their gills entirely, but cartilaginous and bony skeletal supports
persist and are turned to new and diverse uses.

In amphibia, both upper and lower jaw cartilages are at least in part
converted into bone, and the upper jaw becomes firmly fastened to the
cranium. The hyomandibular in this group ceasing to function as a
suspensory apparatus of the jaw, slips into the tympanic cavity to form a
connexion between the ear-drum and the inner sensory ear, the columella
or stapes. The hyoid cartilage forms the basis of attachment of the
tongue muscles. With the disappearance of the gills as functional organs
in land amphibia, the remaining visceral cartilages become further
reduced and associated with the larynx.

Some advance towards the mammalian visceral skeleton appears in
hving reptiles. The teeth both in upper and lower jaws become lodged
in alveolar sockets in two membrane bones, the premaxillary and maxillary

THE SKELETAL SYSTEM

231

of the upper jaw and the dentary or mandibula of the lower. These
membrane bones, however, do not extend as far as the articulation of the
jaw which, as in amphibia, is between the quadrate of the upper jaw and
the articulare of the lower.

In mammals a new articulation of the jaw is formed between two
dermal bones, the dentary of the lower jaw and the squamosal of the
brain-case. The evidence for this change in the articulation of the jaw is
too strong to permit of any reasonable doubt. How such a change could

OUTER ASPECT.

Fig. 188. — Jaws of Tertiary reptiles viewed from their inner and outer aspects.
Dimetrodon belongs to the lower Tertiary, while Scymnosuchus and Cynognathus
belong respectively to the middle and upper Tertiary. A notable increase in the size
of the dentary and relative diminution in the size of the articular element is seen. Such
changes suggest how in the ancestors of mammals the articulation of the lower jaw
shifted from the articulare to the dentary. Thus the articular was set free to become
the malleus of the ear. By similar changes in the upper jaw the squamosal bone replaced
the quadrate, which then became the incus. The teeth of the three jaws also show a
change in the dentition in the direction of that of mammals. (Redrawn after D. M. S.
Watson.)

occur while the jaw was still functioning has been one of the vexed ques-
tions in vertebrate morphology. Thanks, however, to recent discoveries
of fossil reptiles which bridge the gulf between reptiles and mammals, the
way in which the shift was effected is now fairly clear. (Fig. 188)

D. M. S. Watson and others have found that in Tertiary reptiles
changes in the articulation of the jaw occurred which involved reduction
of the articulare element and a corresponding enlargement of the dentary.
The relation of the two bones is such that the lower jaw at one time was
probably hinged by both. The enlargement of the dentary and the
diminution of the articulare set free the latter to pass into the tympanic

232 COMPARATIVE ANATOMY

cavity and join the chain of earbones as the malleus. By a similar piracy
in the upper jaw, the squamosal replaced the quadrate which also was freed
to pass into the tympanic cavity and become the incus. (Fig. 190)

The evidence, therefore, compels us to believe that the skeletal ele-
ments which primarily functioned as supports of the visceral arches in
connexion with respiration have been converted through evolutionary
change into organs of mastication, sound-conduction, speech, and support
of tongue muscles. That the cartilage of the external pinna of the ear
is a derivative of the hyoid arch has been asserted on the basis of doubtful
evidence.

The Visceral Skeleton in Man. The visceral skeleton is that part of
the axial skeleton which is related to the mouth and pharynx. In man it
includes upper and lower jaws, the hyoid bone, the cartilages of the voice
box, and the ear bones, malleus, incus, and stapes. The maxilla or upper
jaw is really a pair of bones fused together along the middle line. It is
nearly cubical in shape, has a body and four processes and encloses the
maxillary sinus. The upper surface of the maxilla forms the lower surface
of the orbit of the eye. From its inner surface the inferior concha projects
into the nasal passage.

Of its four processes, the frontal articulates with the frontal bone, the
zygomatic with the zygomatic bone, the palatine with the palatine bone,
while the alveolar process bears the sockets for the teeth of the upper jaw.
Near the incisor teeth each half of the maxilla is perforated by an incisive
canal through which the nose communicates with the mouth. The
maxilla also contains a large sinus, the antrum of Highmore, which opens
into the narial passage.

The mandible or lower jaw consists of a body and two rami, which
extend upwards nearly at right angles with the body. That portion of the
jaw in which the teeth are set is the alveolar limbus. One of the dis-
tinguishing characteristics of the mandible of modern man, Homo sapiens,
is the presence of a chin or mentum, which is lacking in most fossil types.
Near the chin on the side of the jaw is a mental foramen through which
nerves and blood vessels penetrate the jaw. See Fig. 189.

Each flattened ramus of the mandible has two prominent processes,
the condyloid and coronoid. The head, capitulum, of the condyloid fits
into the mandibular fossa of the temporal bone to form the articulation of
the jaw.

The hyoid is a small, flattened, U-shaped bone located in the throat
between the larynx and the base of the tongue. Like the mandible,
the hyoid consists of a body, corpus, and two paired "horns." The
anterior horns are much smaller than the posterior and frequently remain
cartilaginous. Each lesser horn is connected by a stylohyoid ligament
with the styloid process of the temporal bone.

THE SKELETAL SYSTEM

233

Included in the visceral skeleton of man are the cartilages of the larynx
or "voice-box." The largest and most prominent of the laryngeal
cartilages is the thyroid or shield-shaped cartilage. The right and left
halves of this cartilage stand at approximately right angles to one another.
From its dorsal borders extend paired superior and inferior horns.

The cartilage immediately below the thyreoid has the shape of a seal-
ring and consequently is called the cricoid cartilage. The cricoid expands
into a broad lamina on its dorsal or posterior surface. Articulating with

Fig. 189. — The Heidelberg jaw. (Drawn with the camera lucida from a cast and
photographs.) (From Hooton's "Up from the Ape," Courtesy of the Macmillan
Company.)

the cricoid on its anterior surface is a pair of pyramid-shaped arytenoid
cartilages, so-called because of their resemblance in section to a funnel.
A fifth cartilage supports the epiglottis.

In addition to these, two small paired cartilages usually occur in the
larynx, the comiculate and cuneiform cartilages. The corniculate
cartilages lie at the apices of the arytenoids. The cuneiform cartilages
are of variable occurrence. The cartilages which support the trachea may
also doubtfully belong to the visceral skeleton. They form a series of
incomplete rings extending along the trachea.

The three ear bones are named malleus, incus, and stapes from their
fancied resemblance to hammer, anvil, and stirrup. Within the cavity

234

COMPARATIVE ANATOMY

of the middle ear they extend in the order given from the ear drum or
tympanum to the oval window or fenestra vestibuli of the internal ear.
Thus they serve to carry vibrations from the ear drum to the liquids of
the internal ear. Their arrangement is such as to reduce the amplitude of
vibrations and to increase their intensity. See Fig. 496.

quadrate:
palato-pterygoid '

^ HYOMANDIBULAR

ARTICULAR- ~

DENTARY

MANDIBULAR
A. ELASMOBRANCH

HYOMANDIBULAR (COLUMELLA >

QUADRATE

ARTICULAR_._
DENTARY^
MANDIBULAR.

2ND
MANDIBULAR

B TELEOST

STYLOID PROCESS

(HYOID) 1

STAPES CHY0MAN0IBULAR3

INCUS CQUADRATEJ
MALLEUS CARTICULAR)

MECKEL'S

CARTILAGE CMANDIBULAR)

HYOID

DENTARY-
STYLOHYOD LIGAMENT CHY0ID3 ■

C AMPHIBIAN AND REPTILE. D. MAMMAJL.

Fig. 190. — Diagrams of the first and second visceral arches in .4, Elasmobranch, B,
Teleost, C, Amphibian and Reptile, and D, Mammal, illustrating the transformation
of the hinge of the jaw of lower vertebrates into the malleus and incus of the mammal.
The third earbone, the stapes, comes from the hyomandibular. (Redrawn after
Gegenbaur and Stempell.)

Development of the Visceral Skeleton. In the human embryo a series
of visceral arches separated by pharyngeal pouches appear in relations
corresponding to those of aquatic vertebrates. In the first of those arches
the maxilla of the upper jaw and the mandible of the lower jaw develop
as membrane bones. The mandible, however, surrounds a cartilage,
Meckel's cartilage, which corresponds to the mandibula or cartilage of the
lower jaw of elasmobranchs. While most of Meckel's cartilage disappears
during ontogenesis, that portion which extends into the cavity of the

THE SKELETAL SYSTEM

235

ear ossifies in two centers one of which forms the malleus and the other

the incus.

The so-called Meckel's cartilage of the mammalian embryo appears
therefore to correspond not only to the mandibular or Meckel's cartilage

Fig. 191. — Early chondrocranium of Acanthias. (The brain in outline.) als,
alisphenoid cartilage; ch, anterior end of notochord; h, hyoid arch; ma, mandibular arch,
not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; t, trabecula;
r-5, branchial arches; cartilages dotted. (From Kingsley's " Comparative Anatomy of
Vertebrates," after Sewertzoff.)

of lower vertebrates, but with their quadrate element also. The quadrate
element develops into the incus while the articular portion or articulare
element of the mandibula ossifies as the malleus. All this agrees com-
pletely with the evidence from comparative anatomy, which makes the

Fig. 192. — Diagram of early elasmobranch skull, bp, basal plate; c, trabecular
cornu; Jl, foramen lacerum; ga'-^ gill arches; gc, gill cleft; h, hyale; hm, hyomandibula;
i>, 1 2, upper labials; II, lower labials; m, Meckel's cartilage; nc, nasal capsule; oc, otic
capsule; of, orbital foramen; ov, occipital vertebrae; pq, pterygoquadrate; s, suspensor
ligament; sp, spiracle; si, sphenolateral; t, trabecula; v, vertebrae; I-VII, visceral arches;
i-io, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates.")

malleus homologous with the articulare bone of the lower jaw, and
the incus the modified quadrate of the upper jaw of reptiles, so that the
articulations of the reptilian jaw become in man and mammals a part of a
sound-conducting apparatus. See Fig. 190.

The cartilages of the remaining visceral arches in the human embryo
have a diversified fate. The dorsal part of the second, the hyoid, ossifies

236 COMPARATIVE ANATOMY

to form the stapes of the middle ear, while the ventral portion forms the
lesser horn and a part of the body of the hyoid bone. The intermediate
portion of the hyoid cartilage forms the stylohyoid ligament by which the
hyoid bone is suspended from the petrosal bone of the cranium. The
cartilage of the third visceral arch fuses with that of the second, and later
ossifies to form the greater horn and part of the body of the hyoid bone.
The cartilages of the fourth and fifth arch persist as the thyroid and
arytenoid cartilages of the larynx, and also form the cuneiform and
corniculate cartilages. The cartilage of the fifth arch is said to form the
cricoid cartilage also. Other observers claim that the cartilage of the
sixth arch also contributes to the formation of the cricoid. That cartilages
of posterior visceral arches form the ring cartilages of the trachea has been
asserted on insufficient evidence.

Just as no theory of special creation can explain why there should be
both cartilage and dermal bones in the same human skull, so no theory
except evolution can explain why a bone in man should ossify from several
centers, each of which corresponds to a separate bone in a lower form.

n. THE APPENDICULAR SKELETON

Evolution of Paired Appendages. The cyclostomes have no paired
appendages and, so far as the evidence goes, never have had them. We
are, consequently, forced to conclude that the paired appendages of
vertebrates have no genetic connexion with those of invertebrates, but
have arisen independently as vertebrate novelties. Unfortunately for
the speculative morphologist, the beginnings of these appendages are
obscure. Those of elasmobranchs are the simplest in living vertebrates,
but even these are highly differentiated.

The most promising attempt to solve the problem of the origin of
paired fins is the so-called fin-fold theory. According to this, paired fins
began as paired folds of skin extending from the region posterior to the
gills back to the anus. The paired metapleural folds of amphioxus are
often mentioned, with dubious justification, as structures which suggest
how the fin-folds may have had their origin. Pectoral and pelvic fins
are supposed to be formed by enlarging the end portions of these folds
and suppressing the intermediate region. In favor of this hypothesis is
the presence of longitudinal paired " Wolfiian " folds in vertebrate embryos,
and the fact that the anlagen of the appendages extend through more
segments of the embryonic body than do the appendages of the adult,
the base of the appendages becoming constricted during ontogenesis.
Some morphologists have used the continuous fin-folds of skates as evi-
dence supporting the fin-fold theory, while others doubt their significance.
The continuity of the folds, as Goodrich has suggested, is probably not an
essential element in the theory. See Fig. 193.

THE SKELETAL SYSTEM

237

The alternative theory of Gegenbaur that the paired fins of fishes
arose from modified gill arches and their associated rays has too little
evidence in its support to make it worthy of serious consideration.

The second step in this evolution was the invasion of connective tissue
and muscle into the fin-folds. A similar migration actually occurs in

MEDIAN FIN-FOLD

PAIRED FIN-FOLDS

MEDIAN FIN

ANUS

PECTORAL FIN
PRIMITIVE RADIALS

T PELVIC FIN
ANUS

PELVIC GIRDLf

Fig. 193. — Diagram illustrating the hypothetical evolution of the paired fins and
their skeletal supports. A represents the primitive stage of continuous fin-folds. The
dorsal fin and the ventral fin posterior to the anus are median and unpaired. B is the
definitive Elasmobranch stage. The paired fin folds persist only in the region of pectoral
and pelvic fins. The median fins also become discontinuous. C~R illustrate hypo-
thetical stages in the evolution of the skeleton of the pelvic fins of Elasmobranchs.
The right side of C and E represents a later stage in phylogenesis. In £ the skeletons
of the girdle and extremity are differentiated. (After Wiedersheim.)

ontogenesis. In elasmobranchs the muscle buds which invade the fin-
folds are metamerically arranged.

The third step in this evolution was the appearance of a series of inter-
myotomic cartilage rays like those which support both median and paired
fins of elasmobranchs. The universal occurrence of skeletal material in
connexion with muscle, and indeed wherever in an organism stresses occur,
may possibly be taken as explaining these radial cartilages. The position
between the myotomes is obviously adaptive, as is also their position

238

COMPARATIVE ANATOMY

between the dorsal extensor muscles of the appendage and the ventral
flexor muscles.

Further steps in the evolution of the appendicular skeleton involve
the thickening and fusion of the basal or proximal portions of the radial
cartilages and the extension of the basal cartilages thus formed into the
body wall and towards the mid-ventral line. The result of this appears
today in the pelvic fin of elasmobranchs. The beginnings of a girdle are
seen in a ventral cartilaginous plate, the ischio-pubis. A doubtful
beginning of the ilium may be seen in the so-called iliac process. The

PREAXIAL BORDER TO RIGHT

ABCD HYPOTHETICAL STAGES

Fig. 194. — Diagrams illustrating the hypothetical evolution of the extremities of
Dipnoan (7), Ganoid (//), and Elasmobranch (G) from a fin-fold supported by a series
of similar radial cartilages. By fusion basal elements are differentiated. The skeletal
supports of fins eventually differ in the relations of the basal elements to the radialia.
(Redrawn after A. Brazier Howell.)

evolution in the pectoral girdle seems to have been more rapid than in the
pelvic girdle, if we may base our conclusion on the fact that in elasmo-
branchs the scapula or dorsal arm of the pectoral girdle is already well
developed when there is little, if any, indication of a dorsal arm, the ilium,
of the pelvic girdle. In both girdles in the elasmobranch, however,
a ball-and-socket articulation between girdle and free extremity has
already made its appearance.

An advance towards the pectoral girdle of higher vertebrates appears
in living and fossil ganoids in which a membrane bone, the clavicle, is
added to the pectoral girdle. There is no structure in the pelvic girdle
homologous with the clavicle.

A tripartite pectoral girdle makes its first appearance in amphibia.
The ventral arm, which in fishes was single and undivided, becomes in

THE SKELETAL SYSTEM

239

amphibia differentiated into posterior and anterior moieties, the coracoid
and precoracoid. The dermal clavicle becomes closely apposed to the
precoracoid. The dorsal scapula
and suprascapula remain undivided
as in fishes.

The dorsal arm of the pelvic
girdle, the ilium, articulates with
the transverse process of a single
sacral vertebra. In its most primi-
tive form in amphibia, the ventral
portion of the pelvic girdle re-
sembles that of ganoid fishes, and
consists of a broad cartilaginous
plate with which the femur
articulates. Centers of ossification
corresponding with ischium and
pubis arise successively in this
plate. The attempt of morphol-
ogists to discover in ganoid and
dipnoan fishes transitional stages
leading from the pelvic girdle of
eiasmobranchs to that of amphibia
has had little success. Fossil
remains also throw little light on
the problem. See Fig. 197.

The girdles of reptiles are
essentially like those of amphibia.
In the turtle they become definitely
Y-shaped. The clavicle fuses with
the precoracoid and becomes
indistinguishable from it. The
ilium connects with two sacral
vertebrae. In pythons, a rudimen-
tary hip girdle connects with a pair _ -,r . , r r ^, j

, . Fig. 195. — Ventral surface of Cladose-

of rudimentary claws in the anal lache. In this fish the skeletons of both

region. Both are useless; both go pectoral and pelvic fins are extremely

° ' ° primitive. (After Jaeckel from Kingsley s

to prove descent of serpents from "Comparative Anatomy of Vertebrates.")

tetrapod ancestors.

In mammals, the coracoid is reduced to a process of the scapula. In
man in addition to the coracoid process a remnant of the coracoid bone
survives in the coracoid ligament which extends from the coracoid
process to the sternum, and in which occasional pieces of cartilage
are found as rudiments of the coracoid. The clavicle has supplanted

240

COMPARATIVE ANATOMY

the precoracoid, remnants of which, however, usually occur within the
clavicle. See Fig. 196.

-+-01V10STERNUM

SUPRASCAPULAi
SCAPULA'

CLAVICLEv

EPIPHYSIS
OF CLAVICLEi

INTER-
CLAVICULAR
LIGAMENT

Fig. 196. — Diagrams illustrating the fundamental similarity of the human (B) and
amphibian (A) pectoral girdle. In man the coracoid element has degenerated into a
process (coracoid) and a connective tissue ligament containing occasional cartilage
nodules. (Redrawn after Huntington.)

The mammalian hip bone differs little from that of reptiles. The
number of sacral vertebrae to which the coxal bone is attached increases in

_UO-PUBOIsaflADtC CAflT, , _

ILIO-PUBO-ISCHAOtC CAfiTIUAO€_

D.NECTUBUS. E. OACTYLETHRA T CHELYDRA.

Pig. 197. — A series of six appendicular skeletons illustrating the gradual emergence
of the elements of the pelvic girdle found in reptiles and mammals. They probably
represent fairly well stages in the evolution of the human pelvis. First came the separa-
tion of girdle and extremity {A and B) ; then the fusion of the paired elements of the
girdle into a median ventral cartilaginous plate (C and D) ; the difTerentiation of bony
ischium, pubis, and ilium (D and E); and finally the appearance of the obturator
foramen (F). There is no essential difference between the reptile and mammal girdle.
(Redrawn after Wilder, "History of the Human Body"; Henry Holt & Co.)

mammals. In man there are five sacral vertebrae, to three of which the
hip bone is attached.

THE SKELETAL SYSTEM

241

Evolution of the Free Extremities. Two contrasting types of free
extremity appear in vertebrates, the fins characteristic of fishes and the
toed appendages such as are found in the remaining classes from amphibia
to man. The conversion of the one into the other continues to be a vexed

VERTEBRAL COLUMN

Fig. 198. — The presence of rudimentary hind-limbs in the Python is evidence that
snakes have evolved from limbed ancestors. The presence of thoracic and lumbar
plexuses of spinal nerves points in the same direction. A, Ventral external aspect of
anal region of python. B, Skeleton in pelvic region, showing rudimentary femur and
ilium. (Redrawn after Romanes.)

question of vertebrate morphology. Technically stated, the problem
has been to determine how the evolution of the ichthyopterygium into the
cheiropterygium has occurred. Interest has centered especially in the
transformation of the skeleton.

VERTEBRAL MARGINS

STERNAL EXTREMITY

Fig. 199. — Human pelvic and pectoral girdles in lateral aspect. A is the pelvic girdle
of the right side and B-C the pectoral girdle of the same side.

Primarily the fish fin like that of the fossil shark Cladoselache was
supported by radial cartilages which articulated with basalia, of which
one or more articulated with the girdle. In the pectoral fin of modern
elasmobranchs three basalia, propterygium, mesopterygium, and meta-
pterygium, connect the fin with the girdle. Morphologists, however,

242

COMPARATIVE ANATOMY

/m///y.

Fig. 200. — Diagrams illustrating theories of origin of appendages. A, B, C, origin
of biserial appendage (C) from gill arch {A)\ D, biserial appendage (archipterygium) ;
E, F, evolution of elasmobranch fin; G, dotted lines indicate parts involved in origin of
leg from fin; H, dotted parts show another view of origin of elements of leg. (From
Kingsley's "Comparative Anatomy of Vertebrates.")

PHALANGES

METACARPALS

BCHElROPTERYGIUM,

Fig. 20I. — Diagrams of the pectoral appendages of lower and higher vertebrates.
A persistent problem of morphology is how the fish extremity or ichthyopterygium was
transformed into the fingered extremity (cheiropterygium) of land vertebrates.

THE SKELETAL SYSTEM

243

disagree as to the skeleton of the primitive extremity, the archipterygium.
While some suppose it to have been uniserial, i.e., the radial cartilages
were limited to one side of the basal cartilages or axis as in elasmobranchs,
other morphologists regard the biserial fin skeleton of dipnoi as the more
primitive. Conclusions in regard to the evolution of the skeleton of the
extremity differ, therefore, as one or other of these two types of fish-fin
skeleton is assumed as more primitive.

PRE AXIAL
DIGIT

Fig. 202. — Goodrich's solution of the problem of the transformation of the ichthy-
opterygium into the Cheiropterygium. The diagram (B) of the ichthyopterygium is
based upon the fossil fish Sauripterus. The main axis of the appendages is indicated
by a dotted line. A and C are diagrams of cross sections showing the relations of girdle
and extremity. D represents the appendicular skeleton of Tetrapods. (Redrawn after
Goodrich.)

There is also extreme divergence of opinion as to the exact homologies
between the elements of the ichthyopterygium and of the cheiropterygium.
The skeleton of the latter invariably articulates in a socket of the girdle
by a single skeletal element, the humerus or the femur. The fish extrem-
ity, on the contrary; usually articulates by more than a single skeletal
element. While the forearm and shank of tetrapods have respectively
two elements, radius and ulna and tibia and fibula, the number in the
ichthyopterygium is usually considerably greater than this. Obviously,
in the evolution of the skeleton of the fish extremity into that of land
animals there has been a loss of skeletal elements as well as a great elonga-
tion of the appendage. It is difficult to determine which elements have
persisted and which have disappeared. See Fig. 200.

244

COMPARATIVE ANATOMY

Summary of Skeletal Evolution. Most animal phyla, even the
protozoa, have some sort of skeletal structures. But there seems to be no
genetic connexion between the skeletons of invertebrates and those of
vertebrates. In the evolution of a skeleton, vertebrates have been given
carte blanche. In the process of acquiring a skeleton, chordates first
converted the roof of their alimentary canal into a supporting rod, and
later used this as a foundation upon which to build a vertebral column
of cartilage. The replacement of cartilage by bone in higher vertebrates
is an astonishing engineering feat for which our current theories of adapta-
tion seem quite inadequate.

To the notochord, cyclostomes added neural arches and the rough
beginnings of brain case and visceral skeleton. Elasmobranchs show a

CLAVICLE

Fig. 203. — The pectoral girdle and fin of Sauripterus, an upper Devonian Crossop-
terygian fish. Interest in this type of fish fin lies in the similarity of relations of the
proximal elements of the extremity to those found in the pectoral extremity of tetrapods.
(Redrawn after Broom.)

marked advance towards a more elaborate skeleton. To the vertebrae
they added hemal arches, centra and spinous processes. They converted
a gill arch into a biting jaw, and invented paired fins and a scaly dermal
skeleton. Ganoids went to extremes in dermal armor, but made a perma-
nent contribution to our skulls. In this group, bone began to replace
cartilage.

Crossopterygian ganoids of the Devonian suggest the beginnings of
the tetrapod extremity. The extremities of two of these, Sauripterus and
Eusthenopteron, are especially significant. The skeletal elements of the
pectoral extremity of these forms consisted of a single proximal element,
and articulating with this two distal elements. Furthermore, the proximal
element, interpreted as a humerus, articulates in a socket of the pectoral
girdle. From the evolutionary standpoint this evidence is most important
since it shows that in fishes even before locomotion on land was adopted
the arrangement of the proximal bony elements in the extremity had
already come to resemble that of land animals. Thus Sauripterus and

THE SKELETAL SYSTEM

245

TARSALS. -
METATARSALS

PHALANGES.

Fig. 204.— Skeleton of gorilla viewed from in front. The bones of the gorilla are
identical with those of man. Differences of proportion, however, are seen. Among
these dififerences, that of the relative length of arm and leg bones is most striking. The
limb proportions of the human infant, however, tend to resemble those of the ape.
(Redrawn after Brehm.)

246 COMPARATIVE ANATOMY

Eusthenopteron help to bridge over the structural gulf separating the
ichthyopterygium and the cheiropterygium.

The emergence of animals from the water to the land was accompanied
by momentous changes in their skeletons. Somehow or other fins were
changed into fingered appendages capable of supporting the weight of the
body. This transition was probably accomplished by amphibians, which
because they had lungs and could breathe air, were able to meet this
crisis in animal life. Legs and not tails became the locomotor organs.
Even the vertebral column was affected by these developments.
The pelvic girdle became attached to the vertebral column and sacral
vertebrae were differentiated. Speed was of life-saving value and legs
elongated. All organs, — heart, kidneys, brains, and all the others — were
improved.

Profound changes in the visceral skeleton also occurred. With the
replacement of gills by lungs as organs of respiration, the skeletal supports
of the gills were set free for other uses. Fishes had already demonstrated
that a cartilaginous gill arch could be used for seizing food. Land animals
turned the remaining arches to other divergent uses. The tongue became
attached to the hyoid arch. The hyomandibular element in amphibia
became a sound-conducting apparatus. Some of the arches were used to
support the vocal cords and the voice box.

Amidst all the many adaptive changes which affect the skeleton of the
appendages in land animals, both pectoral and pelvic extremities retain
their fundamental similarity to one another. Even the differentiation
of hand and foot does not obscure this fact. The diversities of form and
function of the tetrapod appendage do not concern us in this discussion.
It is, however, interesting to note that notwithstanding the high degree
of specialization of the extremities of man they differ little in fundamental
structure from those of amphibia.

To assist their hind legs, amphibia connected the pelvic girdle with the
sacrum; to assist their hearing, they converted the suspensorium of the
jaw into an earbone, the columella. Tripartite girdles made their appear-
ance in amphibia. The sternum also is a novelty in this group. In the
theromorph reptiles, changes in the articulation of the jaw began to take
place, so that when mammals appeared, the old hinge between articulare
and quadrate had been lost, and a new hinge between dentary and
squamosal had taken its place. Thus were articulare and quadrate set
free to become malleus and incus of the ear. The beginnings of hands
and feet appear in arboreal mammals in adaptation to life in the trees.
In man, the backbone becomes vertical and the skull is balanced on the
occipital condyles. The facial angle, in correlation with the great enlarge-
ment of the brain, increases to a right angle; and a sigmoid flexure makes
its appearance in man's spine.

THE SKELETAL SYSTEM

247

The Appendicular Skeleton in Man

I. Upper Limb. The upper limb is divided into four regions, pectoral
girdle, upper arm, forearm, and hand. The pectoral girdle consists of
two bones, the scapula and clavicle or collar bone.

The Scapula. The scapula or shoulder blade is a triangular bone
embedded in the muscles of the back and forming the chief skeletal support
of the shoulder. It consists of a flattened body with two processes, the
coracoid and the acromion. The dorsal surface of the scapula is divided

PRIMITIVE
REPTILES

MAMMAL-LI KE
REPTILE

Fig. 205. — A series of skeletons of hands and feet of tetrapods showing the con-
jectured evolution of the human hand and foot. The human hand is evidently less
specialized than is the foot. (Redrawn after Romer's "Man and the Vertebrates,"
University of Chicago Press.)

unequally by the spine into a smaller upper supraspinous fossa containing
the supraspinatus muscle, and a larger lower infraspinous fossa in which
is the infraspinatus muscle. The concave inner or costal surface of the
scapula lodges the subscapularis muscle. The three borders of the
scapula are the superior, vertebral, and axillary.

At the outer or lateral angle of the scapula is a shallow depression,
the glenoid fossa, which provides the articulating surface for the "bend"
of the humerus. The spine of the scapula terminates laterally in the
acromion process, where it forms the summit of the shoulder and serves as
the origin of parts of the deltoid and trapezius muscles. The clavicle
articulates in a concavity in the acromion process. The coracoid process,
shaped like a bent finger, projects laterally and forward from the "neck"
of the scapula.

248

COMPARATIVE ANATOMY

The Clavicle. The ventral element of the pectoral girdle is the
clavicle or collar bone. Laterally, the clavicle articulates with the
acromion process of the scapula; medially, with the manubrium just above

iTROCHLEA

,MEDIAL EP I CONDYLE

/SHAFT

tatauai&i

iaHiii

HUMERUS

LESSER TUBERCLE-'

GREATER TUBERCLE'

SEMILUNAR NOTCH,
CORONOID PROCESS -,<<CViLj«^w?\
SHAFT ^^Z^ '7"--^^y?r ^

ULNA TUBEROSITY- — > oleCRANON\
RADIAL NOTCH/ ^^^'|'|9^°''^

TUBEROSITY-

.1^ ■lujjjijmij*/^

^STYLOID PROCESS

C. RADIUS

GREATER
TROCHANTER!

-MEDIAL MALLEOLUS -tidia

I _ r. T I D I A

(INFERIOR

ARTICULAR SURFACE ,^^,^^,

INTERCONDYLOID EMINENCE'

Fig. 206. — The long bones A~F of the human arm and leg. Compare with those shown
in Fig. 204. (Redrawn from Sobotta.)

the first costal cartilage. The clavicle, with its two curvatures, roughly
resembles the italic letter /. Several muscles have their origin at least
in part on the clavicle, among them the trapezius, deltoid, pectoralis,
sterno-mastoid, and sterno-hyoid.

THE SKELETAL SYSTEM 249

The Humerus. The humerus, which forms the single skeletal element
of the upper arm, is the longest and largest of the bones of the upper
extremity. It articulates in a ball-and-socket joint with the scapula and
in a hinge joint with the radius and the ulna of the forearm. Like other
long bones, the humerus consists of a shaft and two extremities. The
upper or proximal extremity has a hemispherical head and two tuberosities,
greater and lesser, of which the lesser, although smaller, is the more
prominent. Shoulder muscles are inserted on these tuberosities.

The distal end of the humerus is somewhat flattened and divided by a
ridge into a trochlea and a capitulimi. The trochlea provides the artic-
ulating surface for the ulna while the capitulum articulates with the
radius. On the dorsal surface just above the trochlea is a deep pit, the
olecranon fossa, which receives the olecranon process of the ulna when
the arm is extended. Similar depressions on the ventral surface receive
the coronoid process of the ulna and the head of the radius when the
forearm is completely flexed.

Radius and Ulna. The skeletal elements of the forearm are the radius
and the ulna. The radius is lateral in position and smaller, the ulna
median and larger. The radius is smaller at the elbow and larger at the
wrist, the ulna is larger at the elbow and smaller at the wrist.

The proximal end of the radius is the head. It articulates both with
the capitulum of the humerus and with the radial notch of the ulna. The
head of the radius is connected with the shaft by the "neck." Between
the two, projects the tuberosity for the insertion of the biceps muscle.
Distally the radius articulates with the navicular and lunate wrist bones
and with the extremity of the ulna. From the extremities of both the
radius and ulna project styloid processes which afford attachment for
wrist ligaments.

The proximal end of the ulna is distinguished by the olecranon process
to which the tendon of the triceps muscle is attached. By means of a
concave semilimar notch the ulna articulates with the trochlea of the
humerus. The lower border of this notch is formed by the coronoid
process. An adjacent fossa, the radial notch, serves for articulation with
the radius. Radius and ulna are so related that the palm may be turned
either up or down. In supination of the hand, the palm is up, and the
two bones are parallel. In pronation of the hand, the palm is down, the
radius crosses the ulna, and the position of their distal ends is reversed.

The Carpus. The wrist has eight bones arranged in two rows, a
proximal and a distal. Beginning on the radial side of the wrist, the four
proximal carpals are the navicular, lunate, triquetrum, and pisiform. The
four distal carpals are the greater multangular, lesser multangular,
capitate, and hamate. Except the pisiform, which is inside a tendon,
each of the carpals has six surfaces. Articulating surfaces with the bones

250

COMPARATIVE ANATOMY

of the forearm are provided by the navicular and lunate. The triquetrum
is separated from the head of the ulna by an interarticular cartilage.

Metacarpus and Phalanges. The metacarpus consists of five cylin-
drical bones which articulate with the distal carpals on one side and with
the proximal phalanges on the other. In form the metacarpals are
miniature long bones.

All the digits except the thumb have three phalanges each.

2. The Lower Limb. The bones of the lower extremity are also
divided into four groups, hip, thigh, leg or shank and foot. The skeletal

>METATARSALS

PHALANGES<^

,PHALA^X3ES
iPOLLICIS

METACARPALS<v

CARPALS^
PISIFORM ;-

TRlQUETI^OvH"

LUNATE- 'i

ULNA-

MULTANGULUM
'MAJUS
_MULTANGULUM
MINUS
■CAPITATE

^ NAVICULAR

Fig. 207. — Human right foot (A) and left hand (B) viewed from the dorsal side.
Their fundamental similarity in structure notwithstanding their divergence in function
is striking. (Redrawn from Sobotta.)

elements corresponding are the coxal or hip bone, femur, tibia and fibula,
tarsus, metatarsus, and phalanges.

The Coxal Bone. The hip or coxal bone consists of three parts, a
dorsal ilium, and two ventral elements, pubis and ischium. The ilium
articulates dorsally with the sacrum, the two pubic bones meet in the
mid- ventral line in the pubic sjnnphysis. The pubis and ischium articulate
with one another above and below the large obturator foramen. Laterally,
in the region where the three elements of the coxal bone meet, is a cup-like
depression, the acetabulum, with which the head of the femur articulates.
The concavity of the acetabulum is divided into a smooth articular portion
and an acetabular notch to which the head of the femur is attached by the

THE SKELETAL SYSTEM

251

teres ligament. Many muscles of the thigh and body wall are attached
to the coxal bone, which also serves to support the abdominal viscera.
The two coxal bones together with the sacrum and coccyx form the pelvis,
the shape of which differs somewhat in the two sexes.

Femur. The femur or thighbone, which is the largest bone of the
body, has a shaft and two extremities. The upper extremity includes a
hemispherical head, a neck, and two trochanters, the greater and lesser.
The large gluteus muscle of the buttocks is inserted on the greater trochan-
ter while the psoas muscle is connected with the lesser trochanter.
Between the two extends the ridge of the intertrochanteric crest.
On the anterior side of the femur, the tendonous capsule of the hipjoint
is attached to the intertrochanteric line which separates neck and shaft.

Jblood vessel

^COMPACT BONE

ARTICULAR LIGAMENT
Fig. 208. — Diagram of the structure of a long bone. (Redrawn after Kahn's "Das
Leben des Menschen," W. Keller & Co.)

Among the remarkable adaptations manifested in the femur none is
more amazing than the arrangement of bony lamellae within the calcellous
bone of the upper extremity in such wise as to withstand the stresses and
strains of supporting the weight of the body with a maximum of strength
and minimum weight of material. The relations of the parts resemble
those of a Fairbairn's crane. In addition, the femurs together with the
coxal bone and sacrum form a natural arch of which the sacrum is the
keystone.

The shaft of the femur is nearly cylindrical but on its dorsal side the
linea aspera extends as a ridge throughout its entire length. Several
muscles have their insertion along this line. The lower extremity of the
femur is distinguished by two articulating condyles, the median and the
lateral, separated by an intercondylar fossa.

Patella. The knee-pan is a flattened bone which develops in the
tendon of the quadriceps femoris muscle at the lower extremity of the
femur. It has median and lateral facets, which articulate with corre-
sponding facets in the lower end of the femur.

The Tibia. The tibia is considerably the larger of the two bones of
the shank. Its function is to transfer the weight of the body from the

252

COMPARATIVE ANATOMY

femur to the foot. Two flattened condyles articulate with the femur.
The ligament of the patella is attached to the ventral surface of the upper
extremity of the tibia. The shaft of the tibia is triangular having an
anterior sharp crest from which the tibialis anterior muscle has its origin.
The lower extremity of the tibia is smaller than the upper. Articulation

Fig. 209. — Stages in the development of the extremities in mammals. (Redrawn
after Bardeen, Lewis and Corning.) A is a cross section of a monkey embryo showing
an arm bud. (Redrawn from Arey after KoUman.) 5 is a seven-week human embryo
in left lateral aspect. (Redrawn after Corning.) C is an eight-week human embryo in
left lateral aspect. (Redrawn after Corning.) Z? is a nine-week human embryo in left
lateral view.

is with the talus bone of the ankle. The large process of the medial
malleolus extends from its lower outer border. (Fig. 206)

The Fibula. The fibula is the slenderest of the long bones. It func-
tions chiefly as a basis of attachment of leg muscles which serve as
extensors and flexors of the toes. Its upper extremity articulates with
the tibia, its lower is prolonged into a conspicuous lateral malleolus.

The Tarsus. The tarsal or ankle bones number seven. The proximal
tarsals are the talus and calcaneus. Three ctmeiform bones and a cuboid
form the distal tarsals. Between the two on the inner side of the instep

THE SKELETAL SYSTEM

253

is the navicular. Each of the seven tarsals has six surfaces. The tibia
rests upon and articulates with the talus. The largest bone of the foot is
the calcaneus, which articulates with the talus and cuboid. To the
posterior tuberosity of the calcaneus is attached the tendon of Achilles.
Since the calcaneus acts as a lever with the talus as a fulcrum, it is not
surprising that the size of the calf muscle which provides the power varies
inversely with the length of the heel bone.

Metatarsus and Phalanges. The metatarsal and phalanx bones
differ little from the corresponding bones of the hand, except that the

NEURAL PROCESS!

Fig. 210. — Stages in the development of the appendicular skeleton of man. A, Left
lateral aspect of arm in 11 mm. embryo; B, left lateral aspect of arm in 16 mm. embryo;
C, left lateral aspect of arm in 20 mm. embryo; D, left lateral aspect of leg in 11 mm.
embryo; E, left lateral aspect of leg in 14 mm. embryo; F, left lateral aspect of leg in
20 mm. embryo. (Redrawn after Bardeen and W. H. Lewis from Keibel and Mall.)

phalanges are considerably shorter in the foot and the great toe has much
less freedom of movement than the thumb.

Homologies of the Limb Bones. The striking similarity of the bones
of the upper and lower limbs, notwithstanding their great diversity of
function, is interpreted by morphologists as indicating a primary similarity
in use. Their present differences in form, size, and function have arisen
secondarily and adaptively.

Development of the Appendicular Skeleton. The paired appendages
of vertebrates arise from two Wolffian folds, which extend along the
sides of the embryo at approximately the level where the hypomere
connects with the mesomere. Only the end portions of these folds,

254

COMPARATIVE ANATOMY

however, go to form the limbs; the intermediate region atrophies and
disappears.

The Wolffian folds consist of an external covering of ectoderm and a
core of mesenchyme, which in man is of uncertain origin. Although in
elasmobranchs myotomic buds grow into the fin-folds, no such buds occur
in mammalian embryos. So far as can be seen, the mesoderm of the
paired appendages in man and mammals is proliferated as loose mesen-
chyma from the hypomere and not from the epimere as in the lower
vertebrates. However the somatic motor innervation of appendicular
muscles throughout the vertebrate series suggests that they are homo-
logous. (See p. 289.)

''--...^PHALANGES

''^^ METACARRiiLS /HAMATE

/ /TRIQUETRAl'JCARPALS

/PISIFORM

/ULNA

■ POLLEX

RADIUS
LUNATE ')

NAVICULAR' .„„.. ^
. > CARPALS

\MULTANGULUKV
MAJUS „/

Fig. 211 . — The bones of the human right hand and wrist viewed from above, showing
the regions of growth of the long bones. Growth (elongation) continues as long as a
disc of cartilage separates the diaphysis from the epiphysis.

In their early development, arms and legs take the form of shovel-
shaped outgrowths. At this stage two surfaces may be distinguished,
of which the dorsal forms the extensor surface of the appendage while
the ventral side becomes the flexor surface. During development, how-
ever, each appendage is rotated on its long axis. The rotation of the arm
is, however, the reverse of that of the leg, with the result that the flexor
muscles of the arm are in front and the elbow bends back, but the extensor
side of the thigh is in front and the knee bends forward. In the mean-
while, the elongation of the appendages is correlated with division into
proximal and distal portions. Fingers and toes appear as early as the
second month. Up to this point there has been little difference in the
skeletons of the two appendages. (Fig. 210)

THE SKELETAL SYSTEM

255

Development of the Upper Limb. The anlagen of the arms extend
through five myotomes of the cervical region. During the second month
indications of an appendicular skeleton appear as axial thickenings of the
mesenchyme. In this compact mesenchyme, as early as the sixth week
cartilage forms between the cell masses which are to become the muscles
on the upper and lower sides of the limb. By the end of the seventh week
the entire cartilaginous matrix of the skeleton of the arm is formed except

GROWTH DISC

GROWTH DISC

DIAPHYSIS— ^

wP perichondrialF

"^ BONE ^""^

CARTILAGE

DIAPHYSIS-

--MARR0W

PERICHONDRIAL
BONE

ENDOCHONDRAL
BONE ^

GROWTH DISC

Fig. 212. — Diagrams illustrating four stages in the development of a long bone.
Perichondral bone cross-hatched; endochondral bone stippled; marrow black; cartilage
unshaded. (Redrawn from Coming's "Human Embryology," after Duval.)

the third phalanx of the fingers. At this stage, however, it is impossible
to distinguish sharply individual elements, and the cartilage of the entire
appendicular skeleton is virtually continuous.

Ossification begins in the seventh week but is not completed in all
bones of the body until the twenty-fifth year. Ossification of the clavicle
begins before the third month, but in the coracoid process not before the
end of the first year, and of the acromium process only at puberty. The
shaft or diaphysis of the humerus is usually the only portion ossified at
birth. The distal epiphysis fuses with the diaphysis during the sixteenth
year and the proximal epiphysis during the twenty-fifth.

256

COMPARATIVE ANATOMY

The shafts of the radius and ulna begin to ossify about the middle of
the second month. The bony epiphysis of the olecranon process appears
at ten years and unites with the shaft at sixteen years. The epiphyses of
the radius appear earlier and fuse with the diaphysis later. (Fig. 211)

The wrist bones are cartilaginous at birth but begin to ossify during
the first year, the pisiform last of all during the tenth to twelfth years.
Metacarpals and phalanges ossify relatively early, beginning with the
third month. Diaphysis and epiphyses are formed as in all long bones.

VESSELS

UOWER END AT BIRTH

Fig. 213. — Diagrams illustrating the development of the thigh-bone from birth to
the twenty-fifth year. Since the marrow cavity of the adult bone is large enough to
contain the entire bone of the new-born infant, it is obvious that the growth of the bone
involves continuous destruction of the bone from within as well as additions to the
outside. (Redrawn after Keith.)

Development of the Lower Limb. The concentration of connective
tissue to form the anlage of the pubis and femur begins in the fifth week
of fetal life. At the time when cartilage first appears, the axis of the coxal
bone, which primarily stood at right angles to the vertebral column, shifts
backward and comes to lie parallel with the backbone. Connexion with
the sacrum begins with the sixth week. By the end of the second month,
all of the elements of the pelvic appendage including the coxal are per-
formed in cartilage. Ossification of the shaft of the femur usually occurs
bv the end of the second month.

THE SKELETAL SYSTEM

257

Beginning with the ninth week, the coxal bone ossifies from three
centers corresponding with ilium, pubis, and ischium. In the region of
the acetabulum, ossification is not completed before the tenth year, and
ossification of the coxal bone continues until the twenty-fifth year. The
bones of the leg and foot develop in a manner resembling the manner of
growth of bones of the arm and hand. The bones are mostly long bones
and develop from the usual three centers, a diaphysis and two epiphyses.
Separate epiphyses are formed for the head of the femur and the greater
and lesser trochanters. Ossification of the patella begins in the third year.

The tarsal bones begin to ossify before birth, but the epiphysis of the
calcaneus not until the ninth year. Metatarsals and phalanges begin
to ossify in the third month.

Fig. 214. — Diagrams illustrating the formation of a joint (diarthrosis). A single
or double cavity may be formed. In the latter case the intermediate cartilage dis-
appears later. (Redrawn from Coming's "Human Embryology", after Testut.)

Malformations of the hand and foot are not uncommon. Most of
them result from a duplication or a fusion of parts. They include fusion
of digits, extra digits, and even duplication of entire hands or feet.

The Development of Joints. While the precartilaginous anlage of the
skeleton is usually described as a histological continuum, the statement
needs qualification. For in the regions where joints appear, the connective
tissue does not assume the characteristics of cartilage. Such undifferen-
tiated tissue resembles histologically, and is continuous with, the peri-
chondral sheath which surrounds each embryonic cartilage. When such
a joint persists throughout life without motion, the condition is known as
synarthrosis, and the connective tissue of the joint may change little
throughout life. With increasing age, however, such immovable joints
tend, as in the case of the sutures of the cranium, to close up or ankylose,
so that the bone becomes a continuum.

When, however, a movable joint is formed between cartilages such as
those which form the phalanges, the connective tissue of the joint first
becomes gelatinous and then disappears leaving a liquid-filled cavity
or cleft between the phalanges. The connective tissue which surrounds
the joint, and which is continuous with the perichondrium of the phalanges,
persists and becomes differentiated as the ligamentous capsule of the
joint. Most of each phalanx is eventually converted into bone, but the
articulating surface of the joint remains cartilaginous throughout life.

258 COMPARATIVE ANATOMY

This not only insures some degree of elasticity at the joint but also pro-
vides a means of lubrication. As the cartilage wears down through use,
the cartilage cells are converted into one of the best lubricants known to

man.

In the development of some joints, a cartilaginous disc primarily
divides the liquid-filled cavity in two. Later this disc disappears and a
single cavity is formed.

CHAPTER 7
THE MUSCULAR SYSTEM

The muscular system of an active vertebrate makes up nearly half
the entire body -weight, in man sHghtly more than forty per cent.

A muscle can do only one thing — contract. It cannot expand; and
having once contracted, must be pulled out to its resting length by one or
more antagonistic muscles. Each skeletal muscle consists of a fleshy part

TENDON-

BELLY- -

"ARTERY

VEIN

Fig.

NERVE

TENDON
215. — A diagram of the biceps muscle taken as a typical muscle, showing its
nervous and vascular relations. Each skeletal muscle is attached to a bone either
directly to the periosteum or indirectly — as in the case of the biceps — by means of
tendons. (Redrawn after Keith.)

or belly, each end of which if attached to a bone or cartilage, either directly
to the periosteum or indirectly by means of a tendon. The attachment
which moves most when the muscle contracts is its insertion; the other
is its origin. Each muscle is surrounded by a connective tissue membrane
or perimysium, from which septa may grow into the muscle and divide
it into several muscle slips, each of which has a separate function.

Muscles vary greatly in shape according to the arrangement of their
fibers and the way these are attached. Muscles may be segmented into
a series of similar units such as appear in the body muscles of fishes.

259

26o

COMPARATIVE ANATOMY

Fig. 2 1 6. — Diagrammatic outlines to illustrate various types of muscle architecture,
and the relations of the main nerve branches to the fiber-bundles of the muscle, a. Two
segments of the rectus abdominis muscle of a small mammal, b, Portion of sheet-like
muscle with two nerve-branches and intramuscular nerve plexus, c. Typical quadri-
lateral muscle with nerve passing across the muscle about midway between the tendons.
d and e, Two triangular muscles with different types of innervation. /, Long ribbon-like
muscle with interdigitating fiber-bundles, g, Unipenniform muscle, h, Bipenniforra
muscle, i. Typical fusiform muscle. The main intramuscular nerve-branches are
distributed to the fiber-bundles about midway between their origins and insertions.
(From Morris' "Human Anatomy.")

THE MUSCULAR SYSTEM 261

They may spread out in thin sheets that are ribbon-like, triangular,
pinnate, or fan-like. Appendicular muscles are more frequently spindle-
shaped and massive.

Each muscle is well supplied with capillaries and with both motor and
sensory nerves.

As to origin, muscles are sharply divided into two kinds; i. Skeletal
(epimeric or myotomic), derived from the dorsal or epimeric portion of the
mesoderm; and 2. Visceral (hypomeric) derived from the hypomere.
In the trunk region in contrast with the head visceral muscles arise from
the splanchnic layer of mesoderm only.

Skeletal muscles are composed of striped fibers which respond to
stimulation by rapid contraction. Most visceral muscles, on the other
hand, consist of slow-acting smooth or non-striped fibers. The former
are voluntary and under control of the will, the latter are usually involun-
tary. Exceptions are found in the heart muscle which is visceral and
involuntary, but formed of striped fibers, and in the chewing and facial
muscles which are visceral and at the same time striped and voluntary.

EVOLUTION OF THE MUSCULAR SYSTEM

The muscles of man and mammals are the last term in the series of
transformations of the mechanism of contraction, the evolution of which
it is now possible to sketch in fairly firm outlines.

Contractility appears to be one of the original properties of living cells.
Touch an amoeba, and it responds by drawing together into a sphere.
There is no single axis, but contraction takes place from all directions
towards a center. In some protozoa, however, progressive advance in
the function appears in the differentiation of contractile fibrils. A cluster
of such fibrils in the stalk of vorticella is so arranged as to contract in one
direction only, like a muscle fiber.

True muscle cells first appear in the animal series in the sponges.
The primary independence of muscle and nerve is indicated by the
presence of muscle cells in this group which lacks nerves altogether. The
epithelio-muscular cells of coelenterates are essentially similar to those
of sponges.

The next step in the evolution of muscles appears in the flatworms,
in which muscle cells are aggregated into clusters. The bilateral sym-
metry characteristic of the muscles of higher animals also appears in
this group.

Transitional evolutionary stages between flatworms and chordates are,
it must be admitted, highly speculative. Even if we accept the assump-
tion that annelids resemble the ancestors of vertebrates, there still remains
a wide gulf to be filled between flatworms and annelids. In certain
characteristics, the muscles of annelids, it is true, strikingly resemble those

262

COMPARATIVE ANATOMY

of vertebrates. Among these are the segmentation of the muscles, and
their separation by a body cavity into somatic and visceral divisions.
It is impossible, however, to be sure that these similarities are not cases
of convergence. The eyes of cuttle fish and of man are similar in many
respects, but this does not prove a genetic connexion. Moreover, there
are striking dissimilarities between annelids and vertebrates, such as the
circular trunk muscles of annelids which have no homologues in verte-
brates, and which consequently make it difficult to accept the hypothesis
of the annelid ancestry of vertebrates.

METAPLEURAL FOLD \/ LATERAL TRUNK CSOMATIC) MUSCLES

A AMPHIOXUS.

OLL APERTURES

BPETROMYZON.

HYPOBRANCHIAL MUSCLE

LATERAL TRUNK MUSCLES ANUS

^RSAL CONSTRICTORS
SPIRACLE

pAXIAL TRUNK MUSCLES

MOUTH
C.SQUALUS

HYPAXIAL MUSCLES

Fig. 217. — The lateral trunk muscles of a cephalochordate, a cyclostome, and an
elasmobranch, showing their striking metamerism, and fundamental similarity. A,
Amphioxus; B, Petromyzon; C, Squalus.

While the pre-chordate history of muscles is obscure, the evolutionary
changes of muscles in chordates are fairly clear. Since the lower chor-
dates, the hemichorda and urochorda, are non-metameric, we must
assume that the metamerism of amphioxus and vertebrates is a new
acquisition in the group. The trunk muscles of Amphioxus form an
unbroken series of segments extending throughout the entire length of the
animal. Each muscle segment or myotome is a mass of muscle fibers
which extends around the sides of the body nearly to the mid-dorsal
and mid-ventral line. Each myotome terminates anteriorly and poste-
riorly in connective tissue septa, the myocommata, which separate suc-
cessive myotomes. A sharp bend near the middle of each myotome

THE MUSCULAR SYSTEM

263

gives it iii side view the shape of a letter V. All alike are innervated by
somatic motor nerves.

MYOTOME I

MYOTOME 10

ANTERIOR/;
CAVITIES

ENDOSTYLE

A. AMPHIOXUS EMBRYO, ist perm. gill- slit

FACIALIS GANGLION

OTIC CAPSULE
MYOTOME 4-

MYOTOME 10

SPIRACULAR POUCH 1ST GILL-SUT

B. CYCLOSTOME EMBRYO.

HYPOBRANCHIAL
MUSCLE

OTIC CAPSULE LATERAL TRUNK MUSCLE

MYOTOME 4

IST GILL-SLIT HYPOBRANCHIAL MUSCLE

C. ADULT CYCLOSTOME.

Fig. 218. — Diagrams illustrating the origin of the hypobranchial muscles of verte-
brates. Lacking in cephalochordates (amphioxus), hypobranchial muscles make their
first appearance in cyclostomes in the form of muscle buds from post-branchial myo-
tomes. They become the tongue muscles of tetrapods and are innervated by the
hypoglossal (XII) nerve. In cyclostomes as in higher vertebrates myotomes i, 2, and
3 form eye muscles.

The visceral muscles connected with the intestine are non-metameric,
and are differentiated into an inner circular and an outer longitudinal
group. In the region of the gills, the visceral muscles are connected with

264 COMPARATIVE ANATOMY

the gill cartilages, and are differentiated into levators, depressors, and
constrictors of the gills.

The lateral trunk muscles of cyclostomes strikingly resemble those of
amphioxus. In the region of the body cavity, on the ventral side, an
external layer of oblique muscles is differentiated. The most important
evolutionary advance, however, appears in the differentiation of six
eye muscles. Paired eyes first appear in this group, and with them the
same six eye muscles as in all vertebrates up to man. All six are formed
from the first three embryonic myotomes. It is generally assumed that,
like the eye muscles of higher vertebrates, they are innervated by the
3rd, 4th, and 6th cranial nerves. Since in cyclostomes the fourth myo-
tome of the embryo forms the first permanent trunk myotome, all the
myotomes of the embryo persist in the adult. Of none of the higher
vertebrates is this true. (Figs. 218, 219, 220.)

Sup. rectus^ ^^_ oculomotor k

Tf«OCMLEABIS^_
M. OSLIQUUS SUR.

EXT. RECTUS— —

Fig. 219. — Diagrams of the eye muscles of man. A shows the left eye-ball and
associated muscles viewed from the outer side. B is the left eye-ball with associated
muscles and nerves viewed from the median side. (Redrawn after "Warren and
Carmichael, Coiirtesy of Houghton Mifflin & Co.)

Hypobranchial muscles, lacking in amphioxus, first appear in cyclo-
stomes. They arise from postbranchial myotomes which send myotomic
buds ventrally and anteriorly below the gills as far forward as the mouth.
The development and nerve relations of this hypobranchial musculature
prove that it is the homologue of the tongue and throat muscles, which
in higher vertebrates, are innervated by the twelfth nerve, the hypoglossal.
Cyclostomes, however, have no true tongue. The hypobranchial muscles
function as a part of the lateral trunk muscles. (Fig. 218.)

A further evolution of muscles also appears in elasmobranchs. The
embryos of this group provide a clue to the history of the eye muscles, by
demonstrating that the differentiation of the three anterior myotomes into
the six eye muscles involves primarily a longitudinal sphtting of the
myotomes into dorsal and ventral moieties such as happens also in the
first and second post-otic myotomes of cyclostomes. The facts suggest
that the splitting occurred along the series of lateral line sense organs,
which at one time may have included the lens of the eye and the ear
capsule. Each of the two divisions of the first myotome spHts again
lengthwise, thus making the four eye muscles innervated by the oculo-

THE MUSCULAR SYSTEM 265

motor nerve. The dorsal of the two moieties of the second myotome
forms the superior obUque muscle innervated by the trochlearis nerve,
while the ventral portion unites with the third myotome to form the
external rectus muscle innervated by the abducens nerve. The myotomes
of the fourth, fifth, and sixth somites break up into connective tissue,
so that the first persistent trunk myotome is the seventh. In this way,
a hiatus is left in the series of myotomes, and the eye muscles are left as
an isolated group which owe their persistence to the fact that they become
functionally connected with the eyeball. (Fig. 220*)

If we may draw phylogenetic conclusions from these facts of onto-
genesis, we must consider the eye muscles not as relatively young muscles
or as post-otic muscles which have migrated into the pre-otic region, but
as the first three myotomes of the vertebrate body. Their present isola-
tion may be interpreted as a consequence of the enlargement of the otic
capsules. The ontogenesis of cyclostomes and elasmobranchs supports
the assumption that in the ancestors of vertebrates, as in amphioxus
today, the myotomes formed an unbroken series extending throughout the
entire length of the body.

Are the three myotomes which form the eye muscles exactly homo-
logous with the first three myotomes of amphioxus? On account of the
absence of eyes, ears, and brain vesicles in amphioxus, it is difficult to
answer this question categorically. It is not even agreed that the mouth
of amphioxus is the homologue of the vertebrate mouth.

That the first myotome of amphioxus may be compared with the first
myotome of elasmobranchs is supported by the evidence that these
myotomes have the same relation to the anterior endodermic diverticula
of amphioxus that they have to the anterior head cavities of ganoids and
elasmobranchs. In ganoids the anterior head cavities, hke the anterior
endodermic diverticula of amphioxus, are paired outpocketings of the
anterior blind end of the enteric cavity and only secondarily acquire
openings to the exterior, as also do the proboscis cavities of the hemi-
chorda and the left cavity of amphioxus. Since, therefore, these cavities
are the most anterior in the chordate body and share the peculiarity of
opening by pores to the exterior, and since they have similar topographic
relations to the first permanent myotomes and to the mouth, the assump-
tion of their exact homology seems justified.

If, then, we make this assumption, the history of the eye muscles sums
up as the transformation of the first three myotomes of an amphioxus-like
ancestor into the six eye-muscles of the vertebrates. Primarily, as in
amphioxus, the three anterior myotomes were members of an unbroken
series of segmented muscles extending throughout the entire length of
the body. When lateral line organs and enlarged cranial ganglia asso-
ciated with them evolved, the anterior myotomes became split lengthwise

266

COMPARATIVE ANATOMY

FACIALIS GANGLION qTIC CAPSUL f

MYOTOME 3 ^ tAPSULt
MYOTOME 2
MYOTOME I.

MYOTOME

MYOTOME 2

A. PETROMYZON.

FACIALIS GANGLION
MYOTOME JV
MYOTOME 2
MYOTOME

HYPOBRANCHIAL MUSCLE
1ST PERM. GILL-SLIT

OTIC CAPSULE

ST PERM. META-OTIC MYOTOME

THYROID
ANTERIOR CAV.

HYPOBRANCHIAL MUSCLE

MYOTOME

B. SQUALUS.

SUP. RECTUS
SUP. OBLIQUE

INT. RECTUS

EXT. RECTUS

1ST META-OTIC MYOTOME

OLF. PIT

INF. OBLIQUE

INF. RECTUS

C. ADULT SQUALUS. hypobranchial muscle

Fig. 220. — Diagrams based upon cyclostome and elasmobranch embryos illustrating
the phylogenesis of the six eye muscles. Myotomes are cross-hatched. Those which
degenerate in ontogenesis are cross-hatched with broken lines. In C the eye muscles
are shown as if viewed from the median side of the eye.

THE MUSCULAR SYSTEM

267

into dotsal and ventral moieties. Further separation and displacement
followed the enlargement of the optic and otic vesicles. In this way,
eventually, two sets of muscles, one dorsal and one ventral, were brought
into close proximity to the enlarging optic vesicles with which they finally
became functionally associated.

During the phylogenetic transformation of myotomes into eye muscles
noteworthy changes come about in their nerve relations. While the first
myotome retains throughout phylogenesis its primary connexion with
the oculomotor nerve, the nerve of the second myotome, the trochlearis,
acquires a dorsal chiasma and retains connexion only with the dorsal
moiety of the myotome which becomes the superior oblique muscle.
There is no satisfactory explanation of the appearance of this chiasma.
The cells which by their outgrowth produce the fibers of the trochlear

HINOBRAIN'

'3RD SOMITE

Fig. 221. — The mesodermal somites 1-3 from which the eye muscles develop, as
seen in a wax reconstruction of a 5-6 mm. Squalus (elasmobranch) embryo made by
Alton F. Chase. The somites are viewed from the left side. The anterior somite of
Miss Piatt later breaks up into mesenchyme.

nerve are located in the base of the brain posterior to those which form
the oculomotor nerve. But the fibers of the trochlear nerve instead of
growing directly to the muscle of the same side, as do those of the oculo-
motor nerve, grow around the wall of the brain and cross above the brain
to connect with the superior obHque muscle of the opposite side. In the
days when it was assumed that there is a primary connexion between
nerve and muscle, some morphologists thought it necessary to assume that
not only the nerves but the muscles had interchanged sides — a sort of
internal leap-frog for which there is not a particle of evidence. But even
our present knowledge that muscle and nerve become connected with
one another secondarily gives us no clue as to how the chiasma was formed
during phylogenesis.

Another difficult problem is presented in the nerve relations of the
external rectus muscle, which is formed from the ventral moieties of the
second and third mvotomes. The nerve of the external rectus muscle is

268

COMPARATIVE ANATOMY

not the trochlear, which is the nerve of the second myotome, but the
abducens, the sixth nerve. We appear to have here to do with a case of
nerve piracy, in which a nerve of one myotome has invaded the territory
and usurped the place of another nerve. Similar cases of nerve substitu-
tion are not unknown, and the innervation of a myotome by nerves of
more than one segment is the rule rather than the exception. A difhculty
which is perhaps more serious is that the abducens nerve appears not to
be the original nerve of the third myotome, but to be a post-otic nerve
which has invaded pre-otic territory. The causes of such a nerve sub-
stitution are as obscure as those of the trochlearis chiasma (Fig. 505).

Muscles in Elasmobranchs. The metamerism which is such a char-
acteristic feature of the musculature of cyclostomes is retained with slight

LATERAL LINE

CONSTRICTORS OF GILLS
SPIRACLEv

ADDUCTOR OF MANDIBLE'

Fig. 222. — The superficial muscles in the shoulder region of Squalus. From such
relatively simple beginnings have evolved the complex muscles of the arm and shoulder
of man. The flexor protractor muscle which corresponds to the deltoid muscle in
mammals is covered in the figure by the posterior gill constrictor. (Redrawn after
A. Brazier Howell.)

modification in elasmobranchs. A more elaborate folding of the myo-
tomes of elasmobranchs, however, greatly complicates their form. The
cause of this folding is unknown. The total amount of muscle remains
the same; and although the myocommata are folded along with the
myotomes, so that the surface of attachment of the muscles is increased,
it has not been proved that this increase is adaptive.

A novelty first appearing in this group, is the division of the lateral
trunk myotomes by a horizontal connective tissue septum into epaxial
and hypaxial groups of muscles. Five post-branchial myotomes send
buds anteriorly into the floor of the pharynx to form the hypobranchial
musculature innervated by the hypoglossal nerve.

The most important advance, however, made by the elasmobranchs
is the first appearance in vertebrates of the muscles of pectoral and pelvic
fins. As the myotomes extend ventrally in the body-wall, hollow epithe-
lial buds branch off laterally into the fin anlagen. The appendicular

THE MUSCULAR SYSTEM

269

muscles are thus seen to be derivatives of lateral trunk muscles. Differen-
tiation of the muscles thus formed takes place in two directions in elasmo-
branchs and higher animals. First, the appendicular muscles are
subdivided into intrinsic muscles which lie within the fin and extrinsic
muscles which are connected with the fin but lie within the body-wall.

LEVATORES ARCUORUM Cl-7)

A.

VISCERAL SKELETAL ARCHES Cl -7) \

DEPRESSORES ARCUORUM Cl-7)

LEVATORES f-4

DIGASTRICUS
MASSETER
TEMPORALIS

DORSO-LARYNGIS AND
DORSO-TRACHEALIS

B
IMTERMANDIBULARES |/ RYO-LARYNGEI

HYO-PHARYNGEI

Fig. 223. — Diagrams illustrating the hypothetical evolution of the branchiomeric
muscles. A. Hypothetical ancestral form. B. Branchiomeric muscles in urodele
amphibian. (Redrawn after Wilder's "History of the Human Body," Henry Holt
& Co.)

Both groups are subdivided into levators and depressors. On the anterior
side of the fin, a muscle is formed which pulls the fin forward towards the
head. No special antagonistic muscle is differentiated in elasmobranchs,
the adduction of the fin being effected by the combined action of the
posterior part of the levator and depressor groups acting together.
The extension of the extrinsic muscles of the fins in fan-Hke form over the
lateral trunk muscles tends to obscure the metamerism of these in the
region of the appendages. The trapezius muscle which extends from

270

COMPARATIVE ANATOMY

the scapula anteriorly above the gills makes its first appearance in this
group.

In the head region, the visceral muscles become specialized in relation
to the jaws. The levators of the first two visceral arches form the jaw

^•^BOCORACOHUMEnALI.
iNTESKWNCHayLARIS

TRJC£;PS-A^4C0NE^JS
BICEP:

D NECTURUS.

EB^XIAL TRUNK MUSCLES

3BLIQU I - EXTERNUS AND JNTERNUS

DCPRESSOft UAhJWBULAE
SPHINCTER COLLI
TRAPEZIUS

OOflSAUS SCAPULAE
LATISSIMU;

E.SPHENODON.

TRANSVEBSOSPINALIJ
LONGISSIMUS
LE0C04TAL1S

UNCINATE Process

:U1. MUSCLES

KTKUUAL MUSCLU

SARTORIUS

TENSOR VACtUE FDMRtS
0LUTEU5 MAXIMUS

Fig. 224. — Superficial lateral trunk muscles in an amphibian, a reptile, and a mam-
mal. D. Necturus, E. Sphenodon, F. Felis. The metamerism of the lateral trunk
muscles which is such a striking feature of the lower vertebrates is retained in urodeles
and reptiles, but disappears in inammals. The factors in this change are chiefly the
increasing dominance of the appendicular muscles and the fusion of the primarily
metameric embryonic trunk muscles. The primitive metamerism, however, appears in
mammalian einbryos.

muscles, including the masseter, temporalis, pterygoids, while the depres-
sors of these arches become the intermandibularis muscles. The muscles
of the remaining visceral arches remain relatively unmodified. (Fig. 223.)

THE MUSCULAR SYSTEM

271

In the urodeles, the metamerism of the lateral trunk musculature
persists as a striking characteristic. The extrinsic muscles of the append-
ages, however, become widely extended anterior and posterior to the legs.
Such definitive muscles as the pectoraUs and the latissimus dorsi now
appear, and the intrinsic muscles subdivide into those of the arm and
thigh, the forearm and shank, and the feet. By further sphtting of the
original muscle mass within the limb, many new muscles arise, some of
which may be homologized with those of man. On the sides of the body,
the lateral trunk muscles become delaminated into layers, some amphibia
having as many as four. The*epaxial muscles of the trunk divide into
longitudinal bundles connected with the head.

Fig. 225. — Superficial muscles of dog. b, brachialis. be, brachiocephalic; bf, biceps
femoris; cc. cleidocervical; d, digastric; di, deltoid; g. gluteus; i, intercostals; Id,
latissimus dorsi; m, masseter; mh, mylohyoid; oe, external oblique abdominal; ol,
omotransversarius; p, pectoralis; pg, parotid gland; ra, rectus abdominis; 5, sartorius;
sc, sternocephalicus; sg, submaxillary gland; sh, sternohyoid; sm, semimembranosus;
St, semitendinosus; t, trapezius; lb, triceps brachialis. (From Kingsley's "Comparative
Anatomy of Vertebrates," after Ellenberger and Baum.)

A further novelty in amphibia is a movable tongue. Its intrinsic
muscles are those which, as we have seen, grow from occipital myotomes
into the floor of the throat and are innervated by the hypoglossal nerve.
In this group also we find sternohyoid and geniohyoid muscles, which
connect sternum and lower jaw respectively with the hyoid, differentiated.

No very striking developments affect the muscles of reptiles. The
three sets of epaxial muscles of the trunk, — transverso-spinalis, lumbo-
costaHs, and iUo-costaUs, appear. The fusion of the lateral trunk
myotomes and the consequent loss of metamerism leads towards the con-
ditions in mammals. An extreme degree of delamination affects the
lateral trunk muscles, some reptiles having as many as eight layers in the
body- wall.

With the great enlargement of the appendages of mammals, there
appears a corresponding increase in the appendicular musculature and

272

COMPARATIVE ANATOMY

trunk muscles become relatively reduced. Subdivision and migration of
muscles increases. Caudal muscles dwindle with the reduction of the tail.
In the trunk region, metamerism is preserved only in the intercostals,
the rectus abdominis, and the intervertebral muscles.

Integumental (Dermal or Cutaneous) muscles in the form of a panni-
culus carnosus group appear suddenly in monotremes and marsupials
only to disappear in the higher primates except as rudiments. In the

-M. PIRIFORMIS

ABDUCTOR CAUDAE VENT.

M. EXTENSOR CAUDAE
LATERALIS

M. ABDUCTOR CAUDAE
DORSALIS

B. DORSAL

Fig. 226. — Human caudal muscles viewed from A. ventral and B. dorsal side.
These rudimentary muscles are the last remnants of the powerful caudal muscles of the
lower vertebrates. The presence of such useless rudiments receives its best interpreta-
tion in the evolution theory. (Redrawn from Wilder's " History of the Human Body,"
Henry Holt & Co.; after Lartschneider.)

head and neck region, however, the platysma and facial muscles persist
in man and apes. In the trunk region, these integumental muscles are
outgrowths of the pectoralis minor complex. In the head region, however,
they are visceral in origin.

The most important muscular novelty contributed by mammals is the
diaphragm. Its innervation by branches of cervical spinal nerves proves
that it is a derivative of cervical myotomes.

There is no essential difference between the muscles of man and those
of other mammals. The presence in man of such useless muscle rudi-
ments as the sacro-coccygeal and ear muscles suggest a mammalian

THE MUSCULAR SYSTEM

273

derivation. The evolutionary process of subdivision, fusion, migration,
and splitting of muscles reaches its cUmax in primates, forearm and hand
being especially noteworthy.

FRONTAL-
M. ORBICULARIS OCULl

NASAL-
ZYGOMATIC
AURICULO-LABIAL SUP.
AURICULO-LABIAL INF.

TRIANGULAR

A. ATELES.

M. PLATYSMA

SUR AURICULAR
OCCIPITAL
POST. AURICULAR

-J- M. ANTITRAGICUS
ANT. AURICULAR

FRONTAL-

M. ORBICULARIS OCULl
NASAU

M. QUADRATUS LAB 1 1 SUR

CANINE-

M. ORBICULARIS OR

M. RISORIUS

M. QUADRATUS LAB II INF

M. MENTAL IS

ANT. AURICULAR

SUP AURICULAR

OCCIPITAL
POST. AURICULAR

YGOMATIC

"^. PLATYSMA

TR lANGULARl

B.HOMO.

Fig. 227. — Mimetic muscles in monkey (ateles) and man. A. Ateles (redrawn from
Wilder after Ruge) and B. Homo. The similarity of these muscles both in function
and relations attests their similar genetic derivation.

Muscles in Man

In the human body are nearly four hundred paired or bilaterally
symmetrical muscles, of which forty-seven pairs are visceral and the rest
skeletal. In addition to these, four unpaired muscles are recognized.

Muscles of the Head. One of the most distinctive features of the
head is the presence of numerous integumentary muscles, visceral in
origin, inserted in the skin of the head and neck, innervated by the facial

274

COMPARATIVE ANATOMY

ANTERIOR
AURICULAR'

nerve, and used chiefly for the expression of the emotions. The facial
muscles arise chiefly, if not exclusively, from the mesoderm of the hyoid
arch, from which they spread to the face and scalp. Although most
visceral muscles are involuntary, those of the face are under the control
of the will.

Among the best developed of the facial muscles are those connected
with the lips. The platysma is a broad sheet extending from the corners

of the mouth along the sides of the
neck. When it contracts it de-
presses the corners of the mouth as
in grief. The orbicularis oris
encircles and closes the mouth. The
risorius, connected with the corners
of the mouth, is not a laughing
muscle as its name suggests, but is
used in drawing back the corners as
in pain or grief. The buccinator,
which also radiates from the corners
of the mouth, is used in compressing
the cheeks to keep food between
the teeth. The caninus raises the
corners of the mouth as in sneering.
The zygomaticus runs from the
corners of the mouth to the zygo-
matic bone and is the true smihng
muscle. The triangularis depresses
the corners of the mouth as in grief.
A few weak slips of muscle surround the nasal orifices, among which the
nasalis functions both as constrictor and dilator of the nares.

One of the facial muscles, the orbicularis oculi, closes the eyeUds.
But the muscle' which raises the upper lid, the levator palpebrae superioris,
is innervated by the oculomotor nerve and is therefore not regarded as
one of the facial muscles. The corrugator muscle, however, which serves
to wrinkle the forehead in frowning, is a facial muscle.

Of the muscles of the scalp, the frontalis draws the scalp forward and
wrinkles the forehead, the occipitalis draws the scalp back. A number
of auricularis muscles retract or raise the ears. The six eye muscles, —
the superior, inferior, internal, and external rectus, and the superior and
inferior oblique, do not belong to the integumentary group. The function
of the obhque muscles is thought to be to prevent rotation of the eyeball
when the rectus muscles contract.

A number of visceral muscles associated with the jaw and innervated
by the trigeminal nerve are used in mastication. These include the

ANTITRAGICUS/ AURICULAR

Fig. 228. — The auricular muscles in
man. The presence of such useless
muscles in man and their similarity to
those of other mammals has an evolu-
tionary significance and interest.

THE MUSCULAR SYSTEM

27:

masseter, which raises the mandible; the temporalis, which raises the
mandible and draws it back. The chief function of the pterygoids,
internal and external, is in moving the jaw from side to side, as in chewing.
The digastric lowers the jaw.

Some of the tongue muscles are intrinsic but most are extrinsic. The
intrinsic lingualis forms an interwoven complex extending longitudinally,
vertically, and transversely. Of the extrinsic group, the hyoglossus and
styloglossus retract the tongue, while the genioglossus, which forms most

SPLENIUS-

SEMISPINALIS
CAPITIS.

LEVATOR
SCAPULAE-

SCALENUS

MEDIUS

SCALENUS
ANTERIOR-

LONGUS
CAPITIS

ESOPHAGUS
ARYTENOIDEUS
TRANSVERSU

THYREOARYTENOID

TRANSVERSUS
r-SPINALIS.

LONGISSIMUS
CAPITIS.

LONGISSIMUS
ERVICIS.

CONSTRICTOR

PHARYNGIS

INFERIOR.

■STERNOCLEIDO-
MASTOID.

PLATYSMA-
STERNOHYOID

MOHYOID.

STERNOTHYROID-

■THVREOHYOID-

ICAVITY OF LARYNX .

Fig. 229. — Human neck muscles in cross section. Muscles stippled and skeletal
elements cross-hatched. (Redrawn after Morris modified.)

of, the body of the tongue, both extends and retracts it, depending upon
which part of the muscle contracts.

Another group of muscles lies in the floor of the throat between the
mandibles and thehyoid bone, among them the anterior belly of the digas-
tric, the geniohyoid, and the mylohyoid. The stylohyoid and the posterior
belly of the digastric extend from the hyoid to the styloid process. Food
is forced into the esophagus by a group of constrictor muscles of the
pharynx.

Muscles of the Neck. In addition to the cutaneous platysma, the
neck contains sixteen paired muscles. Underneath the platysma on the
side of the neck, the conspicuous stemocleido-mastoid bends and rotates
the head. The trapezius, strictly a muscle of the shoulder, extends well
up on the back of the neck. Another shoulder muscle, the levator
scapulae, draws the shoulder blade towards the head. (Fig. 229.)

276

COMPARATIVE ANATOMY

The dorsal muscles of the neck extend the head and bend the neck.
Included in this group are the splenius, longissimus, semispinalis,

TEMPORALrf---
OCCIPITALf
STERNOCLEIDOMASTOID.

izius^r^

SPINE OF SCAPULAi,„*^'\ \
DELTOID-^- ^

INFRASPINATUS
TRICEPS
BICEPS

/EXr.POLUCIS BREVIS.
.ABD.POLLICIS LONGUS.
/ FLEX . DIGITORUM PROF.

SUPERFICIAL
■r-^EXTENSORS
AX OF FOREARM.

^riV- ANCONEUS.

,___i^»'**'VEXT CARPI RADIALIS.
BRACHIALIS.\^°^<=^S

'brachioradial.

BRACHIORADIAL

FLEX. CARPI ULNy

EXT. CARPI ULN -/*

PALMARIS LONG

FLEX CARPI RAD

FLEX DIGITORUM-
PROFUNDUS.

VASTUS LATERALIS

PERONEUS LONGUS.

/CRUCIATE LIGAMENT.

ACHILLES TENDONr

FlG. 230. — Superficial muscles of human body back view. (Reproduced in modified
form from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, 1930 by
Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf,
Inc., authorized publishers.)

multifidus, and the long and short rotators, the most powerful being the
semi-spinalis. Also on the back of the neck, connecting the occipital
bone with the first two cervical vertebrae, are the rectus and obliquus

THE MUSCULAR SYSTEM 277

muscles of the head. Between the spinous and the transverse processes
of the neck vertebrae, are inter-spinales and intertransversarii muscles.

On the ventral side of the throat, between the sternum and hyoid
bones, are the sternohyoid, stemo-th5rroid, thyrohyoid, and omohyoid
muscles, which pull the hyoid toward the sternum. The longus capitis,
longus cervicis, and the scaleni, anterior, middle, and posterior, are a
deeper group, ventral to the vertebrae, which bend the neck.

Muscles of the Back. The dorsal muscles of the back extend from
the sacrum to the neck. Starting posteriorly as the sacrospinalis muscle,
the mass divides into a medial longissimus and a lateral ilio-costalis, both
of which as they extend into the neck region give off slips to all the ribs.
Closely connected with the ilio-costalis, is a more medial muscle, the
spinalis, which also extends along the backbone. In the groove between
the spinous processes of the vertebrae and the transverse processes, is a
group of muscles which have already been mentioned as the semi-spinalis
group in the neck. In the lumbar region this group is known as the
transversospinalis muscle. A deep subdivision of this group is the
multifidus. Between the vertebrae, connecting spinous and transverse
processes, are the inter spinales and intertransversarii muscles which
stiffen the spine.

The prevertebral group of muscles of the trunk, immediately ventral
to the vertebral column, are less developed than those just mentioned,
so that the backbone is bent chiefly by other muscles not closely related
to the vertebrae. Prevertebral muscles are wanting in the thoracic
region. In the lumbar region, this group is represented by the major
and minor psoas. The psoas major is, however, more concerned with
bending the thigh than with flexing the back. The quadratus lumbonmi
bends the back sidewise.

In the coccygeal region, the rudimentary and variable sacrococcygeus
muscles appear to be f unctionless but interest the morphologist as remnants
of the powerful caudal muscles of man's ancestors.

Abdominal Muscles. The muscles of the lateral abdominal wall are
spHt into three layers, the external and internal obliqui, and the trans-
versus. The contraction of these muscles lowers the ribs, compresses
the abdomen, and by pushing the viscera against the diaphragm, expels
air from the lungs. Extending from the ribs to the pubic bone along
the ventral wall of the abdomen, is the segmented rectus abdominis, the
function of which is to lower the thorax and to flex the backbone.
Although this muscle is innervated by six or seven spinal nerves, it is
divided by inscriptiones tendineae into only four or five segments, which
are interpreted as remnants of the primitive metamerism of the trunk
musculature. The lateral halves of the rectus abdominis are separated
in the medial hne by a connective tissue linea alba. In the region where

278

COMPARATIVE ANATOMY

the linea alba is attached to the pubis, a small muscle, the pjrramidalis,
is usually present. When well-developed its length is 7 to 8 cm., but

FLEX. CARPI ULNARIS.
FLEX. CARPI RAD. LONG
BRACHIORADIAL

EXT CARPI RAD
UONGUS.

PALMARIS_

LONGUS.

FRONTAL .

ORBICULARIS OCULI.
Ifl^-AURICULAR .■
^ZYGOMATIC.
-MASSETER.
OMOHYOID.

STERNOCLEIDOMASTOID.
SCALENES .
^ilSi- -T RA PEZ I US .

BRACHIALIS.-

TRICEPS <-

CORACO BRACHIALIS

TERES major'
LATISSIMUS DORS I
SERRATUS ANTERIOR

LINEA ALBA
RECTUS ABDOMINIS -
EXTERNAL OBLIQUE -
ILIUM

TENSOR FASCIA LATA
ILIOPSOAS
RECTI NEUS
PUBIS
RECTUS FEMORIS-
SARTORIUS -
ADDUCTOR LONGUS
ADDUCTOR MAGNUS -

FLEXOR CARPI ULNARIS

VASTUS LATERALIS

PERONEUS LONGUS

GASTROCNEMIUS^-^
PERONEUS BREVIS.

EXT. HALLUCIS LONG.-
TRANS. CRURAL LIGAMENT. -

SARTOR 1 US.
GRACILIS.
VASTUS MEDIALIS
ILIOTIBIALTRACT.

-PATELLAR LIGAMENT.

INSERTION OF SARTORIUS.
TIBIALIS ANTERIOR.
•SOLEUS.
TIBIA.
-EXT. OIGITORUM COM. LONG.
FLEX. DIGITORUM COM. LONG.

CRUCIATE LIGAMENT.

Fig. 231. — Superficial muscles of man front view. (Reproduced in modified form
from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, 1930 by Alfred
A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc.,
authorized publishers.)

rarely it may extend craniad to the sternum. Its relations and variable
size suggest that the pyramidalis is a remnant of the muscle attached

THE MUSCULAR SYSTEM

279

to the marsupial pouch of didelphians. On the sides of the abdomen,
the quadratus liunbonmi muscle extends from the last rib to the

-^'STERNOCLEIDOMASTOID.
GENI0HY0ID7-''=''^!i^^^a^^Bi^aHS^<J^^^^'^aP^-SPLENIUS CAPITIS.
STYLOHYOIDt
LONGUS CAPITISr-^
THYROHYOID-

.-LEVATOR SCAPULAE
-SCALENUS ANTERIOR.

SCALENUS ANTERIOR-.
STERNOTHYROIDt-

SERRATUS ANTERIOR^
SUPERIOR PART.

SERRATUS ANTERIOR- si^-
MIDDLE PART.

-OMOHYOID.
TRAPEZIUS.
-CLAVICLE

PECTORALIS MINOR

.7DELTOID.

PECTORALIS MAJORr.r'

PECTORALIS /
MINOR.

SERRATUS x«sr VA

ANTERIORi=^^-- -"
INFERIOR

INTERNAL'-
OBLIQUE . "

Fig. 232. — Deeper trunk and neck muscles of man, viewed from the left side. (Redrawn

after Sobotta.)

ilium. Its function is to depress the thorax and bend the backbone
backwards.

28o

COMPARATIVE ANATOMY

Thoracic Muscles. The intrinsic muscles of the thorax are covered
by the extensive muscles of the shoulder and chest. When these are
removed, the same three muscles found in the abdomen are exposed.
The outer of these, corresponding with the external oblique of the abdo-
men, is the external intercostal ; the internal intercostal is the homologue
of the internal oblique of the abdomen; the m. transversus thoracis corre-
sponds to the transversus abdominis. In addition to these, an external
sheet of muscle which is continuous in many marsupials is represented in

1ST LUMBAR VERTEBRA-

..^Sasan^^®" PSOAS

POSTCAVA

TENDINOUS
DIAPHRAGM

MUSCULAR
DIAPHRAGM

STERNUM. _T^:^_

Fig. 233. — The human diaphragm viewed from above. Only the peripheral region
of the diaphragm is muscular. The central portion is tendinous. The diaphragm is
dome-shaped with the convex side uppermost. (Redrawn from Braus.)

man by the serratus posterior, which is divided into superior and inferior
moieties. When certain parts of the intercostal muscles contract, the
rib-basket is raised, the thoracic cavity enlarged, and air is inspired to
fill the space thus created. The levators of the ribs are twelve pairs of
triangular muscles which connect the ribs with the transverse processes
of the vertebrae and are used to raise the ribs. (Fig. 232)

Diaphragm. The diaphragm is a dome-shaped muscle separating
thoracic and abdominal cavities. The central portion is tendinous and
only the outer portion which is attached to the ribs is muscular. When
the muscles of the diaphragm contract, the dome is flattened and the

THE MUSCULAR SYSTEM

2«I

thoracic cavity is enlarged. The curvature of the diaphragm is restored
when the intercostal muscles relax and the abdominal muscles push the
viscera against the diaphragm.

Shoulder and Chest Muscles. The muscles of the shoulder and chest
function chiefly in relation to the arms. The most superficial of these
on the back is the trapezius which elevates the shoulder and in conjunction
with the rhomboideus draws the shoulder blade towards the backbone.
Underneath the trapezius, the rhomboideus major and minor also draw
the shoulder blade towards the backbone. The deltoideus is the shield-
shaped muscle which covers the shoulder and extends from the scapula to

M. TRANSVERSOSPINALIS.
TRANSVERSE PROCESS.
M QUADRATUS
LUMBORLM

,-M LDNGISSIMUS OORSI.

M RHOMBOIDEUS.

•M. SERRATUS POSTERIOR.
^SCAPULA.

M. LATISSIMUS DORSI.
GLENOID CAVITY.

M. EXTER^4AL
OBLIQUE.
M. RECTUS ABDOMINIS^

'ACROMION.
tORACDID.
-■M. SUBCLAVIUS.

M. BRACHIALIS'
PECTORALIS MAJOR. INFERIOR.

M. TRANSVERSUS THORACIS.

MANLBRIUM STERNi;

CLAVICLE-;

Fig. 234. — Thoracic and lumbar muscles of man as seen in cross section. Thoracic
muscles on the right, lumbar on the left. (Redrawn after Braus.)

the humerus. Its chief function is to abduct the arm. The supra-
spinatus and infraspinatus muscles on the dorsal side of the scapula
together with the teres minor help to rotate the arm. On the under
surface of the scapula the subscapularis is an inward rotator. The teres
major muscle adducts the arm towards the body. The latissimus dorsi
is a broad muscle, inserted on the humerus, which draws the arm down
and backward. In the region of the chest, the pectoralis major and minor
are the chief muscles. They adduct the arm. The subclavius is a small
muscle connecting the clavicle with the first rib, used in depressing the
clavicle. The serratus anterior extends from the scapula to the first nine
ribs and draws the shoulder forward.

Muscles of the Upper Arm. The intrinsic rnuscles of the arm are
divided into dorsal or extensor muscles and ventral or flexor muscles.
Included in the dorsal group are the triceps and anconeus which extend
the forearm. The biceps, coraco-brachialis, brachiaUs constitute the
ventral group. The coraco-brachialis flexes the arm, the biceps and

282

COMPARATIVE ANATOMY

brachialis bend the elbow. The biceps brachii is also the most powerful
supinator of the forearm.

Muscles of the Forearm and Hand. There are many more muscles
in the forearm than in the upper arm, and the diversity of function is also
much greater. Two groups are distinguished, a radio-dorsal which
straightens the wrist and supinates the forearm, and an ulno-volar group

GASTROCNEMIUS
FLEX. OIGITORUM,
LONGUS. ^

EXT. CARPI ULNARIS ^
SUPINATORr-^^Ci

FLEX. DIGITORUM. /
PROFUNDUS . -

EXT. CARPI
^r-'rPAOIALlS.
£^lA-BRACHiORADIAL
iilLJ-RADIUS.

SUBLIMIS

FOREARM

pronator teres .>^;^;,^
Vlex carpi RADIALIS
palmaris udngus. tibialis anterior
iflexor digitorum ext. digitorum long

peroneus longus

SOLEUS.
FIBULA.

GRACILIS.

, ADDUCTOR MAGNUS.

SEMIMEMBRANOSUS.

SEM TENDINOSUS.

HUMERUS'
C. UPPER ARM

Fig. 235. — Muscles of arm and leg in cross section. A, cross section of forearm;
B, cross section of lower leg; C, cross section of upper arm; D, cross section of thigh.
(Redrawn after Braus.)

which flexes the wrist and pronates the forearm. On the medial side
of the arm, the two groups are separated by the ulna bone.

Among the extensor muscles of the forearm are the extensor carpi
radialis longus, and the extensor carpi radialis brevis, which lie on the
radial side, the extensor carpi ulnaris of the ulnar side, and intermediate
between these, the extensor digitorum communis and extensor digiti
quinti proprius. Deeper muscles beneath these include the supinator,
abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis,
and extensor indicis proprius.

Included in the flexor-pronator group of muscles are the pronator
teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor
digitorum sublimis, flexor digitorum profundus, flexor pollicis longus, and

THE MUSCULAR SYSTEM 283

the pronator quadratus. An additional muscle is the brachioradialis,
which, although belonging with the extensor group, has come to have
flexor action. Altogether there are nineteen muscles in the forearm.

Besides these muscles of the forearm most of which act upon the hand,
there are nineteen intrinsic to the hand. Four of these are inserted on
the thumb, the abductor pollicis brevis, opponens pollicis, flexor pollicis
brevis, and adductor pollicis. Three muscles are inserted on the little
finger, abductor, flexor brevis, and opponens digiti minimi. In the palm
of the hand are four lumbrical and seven interosseus muscles. The
palmaris brevis is usually considered an integumentary muscle like those
of the face.

Hip Muscles. The muscles of the hip, like those of the shoulder,
may be divided into dorsal and ventral sets. The dorsal muscles have
their origin on the vertebral column and the ilium. Of these, the ilio-
psoas protracts the thigh, and the gluteus maximus extends it. Other
dorsal muscles, are the gluteus medius, gluteus minimus, piriformis,
and tensor fasciae latae, all of which are inserted on the femur.

The ventral muscles of the hip have their origin on the pubis and the
ischium, and their insertion on the femur. Among them are the obturator
extemus, obturator intemus, quadratus femoris, and the gemelli, all of
which both adduct and rotate the thigh. The obturator externus, here
included in the hip muscles is sometimes regarded as a thigh muscle.

Muscles of the Thigh. There are three groups of thigh muscles,
anterior — extensor, medial — adductor, and posterior — flexor muscles.
Their relations are such that they act upon both the thigh and the lower
leg.

The anterior extensor group includes five muscles, the sartorius or
"tailor muscle," and the four divisions of the quadriceps femoris, the
rectus femoris, vastus lateralis, vastus medialis, and vastus intermedins.
In the medial adductor group are five muscles, gracilis, pectineus, adductor
brevis, adductor longus, and adductor magnus. The posterior or "ham-
string" muscles which bend the leg and extend the thigh, are the semi-
tendinosus, semimembranosus, and biceps femoris.

Muscles of the Lower Leg and Foot. There are thirteen muscles
in the lower leg, four of which form an anterior group which bends the
ankle and hfts the toes, the tibialis anterior, extensor digitorum longus,
peroneus tertius, and extensor hallucis longus. Two lateral muscles,
the peroneus longus and brevis, extend the foot. Three superficial calf
muscles, the gastrocnemius, soleus, and plantaris, connect with the
calcaneus by the tendon of Achilles, and extend the foot. Beneath them
are four deeper muscles, the popliteus, which rotates the leg medially,
and the flexor digitorum longus, flexor hallucis longus, which bend the
toes, and tibialis posterior, which extends the foot.

284 COMPARATIVE ANATOMY

The muscles of the foot strongly resemble those of the hand. The
extensor digitorum brevis, which extends across the top of the foot from
the heelbone to the four lateral toes, has, however, no homologue in the
hand. On the top of the foot also are four dorsal interosseus muscles.

The most superficial muscle of the sole is the flexor digitorum brevis
connected with the four lateral toes. The quadratus plantae is inserted
upon the tendons of the flexor digitorum longus. The four limibricales
are flexors of the four lateral toes. Three muscles, abductor, flexor, and
adductor, connect with the great toe. The little toe also has three special
muscles, abductor, flexor, and opponens. The three ventral interossei
lie between the metatarsals.

Pelvic Muscles. Muscles of the pelvic diaphragm, of the urogenital
diaphragm, and of the external genitals are, from their position, regarded
as pelvic.

Those of the pelvic diaphragm are the coccygeus, which connects the
coccyx with the ischium, the levator ani, a muscle with three divisions
which lifts the anus, and the sphincter ani extemus, consisting of three
muscular rings surrounding the anus. All are supplied with branches of
sacral nerves. The levator ani complex represents the caudal flexor-
abductor musculature of tailed mammals, which in man, as in the anthro-
poid apes, with the reduction of the tail, has gained new relationships
with the pelvic viscera. The coccygeus represents the proximal ventral
caudal abductor of tailed mammals.

The urogenital diaphragm has two muscles, a transversus perinei
and a sphincter urethrae. The latter closes the urethra and is under
voluntary control. In the male, the sphincter also compresses the prostate
and bulbo-urethral glands. In the female, it compresses the vagina and
the glands of Bartholin.

The external genital muscles include the bulbo-cavemosus, which in
the female compresses the vagina and in the male increases the turgescence
of the penis, and the ischio-cavemosus, which serves to maintain the
turgescence of the penis or clitoris.

Development of the Muscles

The classification of muscles into striped and smooth upon the basis
of their finer structure, like that into voluntary and involuntary, does not
correspond with a classification into skeletal and visceral based upon their
embryonic development.

Somatic Muscles, Derived from the Epimere. The Eye Muscles.
The details of the development of the six eye muscles in man are not fully
known. The fact, however, that they are innervated by the same somatic
motor nerves, III, IV, and VI, as in the lower vertebrates has led morphol-

THE MUSCULAR SYSTEM

285

ogists to assume that their development corresponds in essentials through-
out the vertebrate series.

lEUfWL PLATt
NOTOCMORO

URAL fl>TE
McSOOCRM

rCURAL
SOMATIC

ECTOOERI

NCTTOCHOflO
NOTOCHORO

COELOM
ENDODERM

EHX}0£RM
ENTERON

SPU^NCMN
ECTODERM MESODERM

EhfTEHOI

SOMATIC MOTOR
NERVE
iTIC MOTOR tCR>t
MYOTOME
EPIMERE MYCTTOME

DERMATOME
NCSOhCRE

NOTOCHORD
COELOM
HYPOMERE
MESENCWME ^O^TA'

IDERM ENTE

ITERON keSEN

ICTOOERM ENtXDOi

(SPINAL CORD

SPINAL
DORSAL ROOT GANCJJONi

EPAXIAL MUSCLI
"^ NOTOCHORI

GONAD

NTESDNE

RITONEUM

SCERAL MUSCLE

HEMAL ARCH
DORSAL AORTA-
LATERAL ur
CARDINAL VE
PRIMITIVE DUCT-
MESOrCPWIC TUBULE
HYPAXIAL MUSCLE-
CONA

Fig. 236.

-A series of diagrams A-F illustrating the ontogenesis of muscles in vertebrates.
(Based upon figures by Hatschek and Kingsley modified.)

Their development has been most thoroughly investigated in elasmo-
branchs. In embryos of this group, as well as in higher vertebrates to

hyp.n

Fig. 237. — Diagram of muscle segments in head of embryo vertebrate, based upon a
shark. The anterior myotomes tend to divide into dorsal and ventral moieties;
persistent myotomes lined, transient with broken lines; central nervous system dotted,
nerves black, a, premandibular somite; abd, abducens nerve, hyp, hypoglossal muscula-
ture; hypn, hypoglossal nerves; om, oculomotor nerve; sp, spiracle; 1-6, first six somites
(4, 5, 6, functional in Petromyzon) ; I-VIII neuromeres. (From Kingsley's "Com-
parative Anatomy of Vertebrates," after Neal.)

mammals, the eye muscles develop from somites or "head cavities."
The first head cavity (somite i) gives rise to the four muscles innervated
by the oculomotor nerve, the superior oblique muscle comes from the

286

COMPARATRE ANATOMY

second head cavity, while the third head cavity combining with the
second forms the external rectus muscles. (Figs. 237, 238.)

The serial homology of the head cavities or somites with trunk somites,
which for many years was a controverted problem, has now been demon-
strated bv the fact that in the embryos of lower vertebrates the head
somites form a series of mesodermal segments continuous with the trunk
somites. Like the latter, they become differentiated into myotome and
and sclerotome, are innervated by somatic motor nerves, and are dorsal

MESCNCHYMA

TRIGEUtNUS

Fig. 238. — A camera drawing of a section of the anlage of the posterior (external)
rectus muscle — right side — in a 29 mm. Squalus embryo, showing its two constituents
derived from the second and third myotomes. Although the two constituents persist
in the adult muscle, the limiting membrane which separates them in the embryo dis-
appears in ontogenesis. The section from which the figure was drawn is a parasagittal
one. The embryo was preserved and stained by the vom Rath method.

to notochord and dorsal aorta. Furthermore, their segmentation is
independent of that of the visceral arches. Another point of resemblance
is that the first and second head cavities divide during ontogenesis into
dorsal and ventral moieties precisely as do the first and second post-otic
myotomes in petromyzon. The fusion of portions of two myotomes, the
second and the third, to form the external rectus muscle of the eye resem-
bles the fusion of trunk myotomes such as occurs in the formation of the
tongue muscles. (Fig. 238)

Lateral Trunk Muscles. The lateral trunk muscles of man develop
from myotomic segments which first appear in the fourth week of onto-
genesis, and by the end of the second month, have increased to nearly
fortv pairs. The original metamerism of the myotomes, which persists

THE MUSCULAR SYSTEM

287

even in the adults of the lower vertebrates, becomes largely lost in adult
man and mammals as the result of a number of processes among which
fusion is the most important. As the myotomes grow in size and thick-
ness through cell multiplication, the connective tissue septa between them
disappear. In this way are formed such elongated muscles as the spinalis
and iliocostalis. Among the processes tending to obscure the original
metamerism is the degeneration of myotomes into connective tissue fasciae
and aponeuroses which may be very extensive. Migration of muscles
may accompany their fusion, as in the case of the tongue and throat
muscles which develop from four or five occipital myotomes. In man,

SOMATIC MOTOR NERVE
SPINAL CORD
SPINAL GANGLION
RAMUS DORSALIS,/^
NOTOCHORO
CENTRUM
NEPHROTOME
CORIUM
VISCERAL MUSCLE
RAMUS VENTRAL IS
EVTREMITY

SOMATIC MUSCLE

INTESTINE
ENDODERM
COELOM

NEURAL CREST

/ DERMATOME
MYOTOME
SCLEROTOME
PARIETAL MESODERM

VISCERAL MESODERM

Fig. 239. — A stereogram of the trunk region of a vertebrate embryo, based upon
elasmobranch embryos. The figure shows an earlier stage of development on the right
side — a later stage on the left. The extension of the myotome to form the lateral trunk
musculature is shown. The lateral trunk musculature of the ventral half of the body-
wall thus arises as a secondary invasion. (Redrawn after Braus.)

the number is reduced to three. Among the other ontogenetic changes in
trunk myotomes, is tangential spHtting of muscles into sheets. One of
the most characteristic ontogenetic processes affecting the trunk muscles
is the subdivision of a muscle mass into a number of bellies each of which
acquires an independent origin or insertion, or both. The original seg-
mentation of the trunk myotomes is, however, retained in such muscles as
the transversospinalis, intercostal, and rectus abdominis. By the growth
of a horizontal connective tissue septum which extends laterally from
the transverse processes of the vertebrae, the lateral trunk muscles
become divided into epaxial and hypaxial portions, of which the former
is innervated by dorsal rami of the spinal nerves, the later by ventral rami.
Hypaxial muscles are further divided into a prevertebral and a lateral

288

COMPARATIVE ANATOMY

group including obliquus and transversus sets. The muscles of the
diaphragm, which are peculiar to man and mammals, migrate into the
chest from the neck, as is evidenced by the fact that they are innervated
by branches of the third, fourth, and fifth cervical nerves.

Tongue Muscles. The origin of the hypoglossal muscles in the human
embryo is somewhat uncertain. Since, however, they have the same
innervation as in lower vertebrates, it is generally assumed that their
development is essentially similar. In all vertebrates below mammals.

SOMITES 10-14

SOMITES 7-11

IWTOMIC BUDS
VISCERAL ARCH

A CYCLOSTO^E

SOMITE \ MUSCLE

'OLF/CTORY PIT VlSCERAU ARCH 4
B- ELASMOBRANCH

HYPOGLOSSUS

C^ REPTILE D. MAMMAL

Fig. 240. — Diagrams illustrating the mode of origin of hypoglossal (hypobranchial)
muscles in A. Cyclostome, B. Elasmobranch, C. Reptile, and D. Mammal. In A, B, and
C cervical myotomes send myotomic buds into the hypobranchial region. In mammals
such buds are not formed but a migration of mesenchyme cells from cervical myotomes
provides material for these muscles. The number of myotomes which participate is
usually four or five.

muscle buds grow from four to five occipital myotomes ventrally into
the floor of the throat. From the mass of cells thus formed, arise the
intrinsic muscles of the tongue, innervated by the twelfth nerve, the
hypoglossus. In man and mammals evidence is lacking of muscle buds
in the formation of the hypoglossus muscles. It may be assumed that
cell migration takes the place of bud formation and extension.

Appendicular Muscles. In the embryos of lower vertebrates, elasmo-
branchs to reptiles, as the myotomes grow ventrally in the body-wall
and reach the level of the lateral folds from which the appendages develop,
they give off lateral buds into the appendicular folds. After they have
entered the folds, these buds lose their connexion with the trunk muscles,
although they still retain their epithelial character. Within the anlage of
the appendage, the appendicular muscle buds subdivide into dorsal and

THE MUSCULAR SYSTEM

289

ventral moieties, from which develop respectively the levator and depressor
muscles of the appendage. (Figs. 241, 242)

The appendicular muscles of man and mammals, on the contrary,
do not develop from myotomic buds, but arise by cell migration. The

SPINAL CORD
DERMATOME
SOMATIC MOTOR NERVE
MYOTOMr

SENSORY GANGUON

NOTOCHORD

HYPOCHORDA
"DORSAL AORTA

POSTCARD! NAL VEIN

SPINAL NERVE

FIN- BUD

VITELUNE
VEIN

MESENCHYME

Fig. 241. — A cross section of a 17 mm. elasmobranch (squalus) embryo in the
trunk region, showing an early stage in the growth of a myotomic bud into the fin-fold.
The yolk-sac to which the embryo is attached has been removed.

two methods are after all not radically different. In fishes, for example,
where most of the appendicular muscles arise from myotomic buds, some
muscles which develop later than the others come from migrant mesen-
chymatous cells as they do in mammals. Similarity of innervation,

3ny

my

Fig. 242. — Budding of muscles of appendage from myotomes in PrisHurus. b,
muscle buds; my, myotomes. (From Kingsley's "Comparative Anatomy of
Vertebrates," after Rabl.)

however, attests the homology of the appendicular muscles throughout
the vertebrate series.

The fact that the arm muscles of man are innervated by the last four
cervical and the first thoracic nerves further justifies the assumption that

290 COMPARATIVE ANATOMY

they are derived from the myotomes of these segments. To the group
of muscles derived from this source, are added other, such as the trapezius,
sterno-cleido-mastoid, and levator scapulae. The pectoralis and latissi-
mus dorsi muscles spread out from the arm. Most of the muscles of the
shoulder, chest, and arm appear early in the second month, and are
differentiated by the beginning of the third.

From the connexion of the muscles of the lower leg with spinal nerves,
including the last four lumbar and first three sacral, it may be assumed
that their cellular anlagen are derived from the corresponding myotomes.
In all essentials their development resembles that of the muscles of the
arm. A common mass of cells within the limb-bud differentiates into
dorsal and ventral muscle anlagen. The muscles from the ventral group
become innervated by the femoral nerve while the dorsal group are con-
nected with the obturator. The subdivision of the primary muscle
mass into the separate muscles of the adult limb is mostly completed by
the end of the second month.

Visceral Muscles, Derived from the Hypomere. The visceral or
hypomeric include those of the heart and main blood vessels as well as
those associated with the alimentary canal. While most of them consist of
smooth muscle fibers, the visceral muscles of the head and heart are striped.

The muscles of the wall of the alimentary canal are formed from
mesenchymatous cells proliferated from the visceral layer of the hypomere.
Such cells fill the space between the mucous epithelium lining of the
alimentary canal and the adjacent hypomere. They also differentiate
into both the connective tissues and the blood vessels of the wall of
the alimentary canal and into its circular and longitudinal muscles. The
circular layer of muscles is formed before the longitudinal layer. The
histogenesis of smooth muscle in some respects resembles that of striped
muscle. The mesenchyma cells elongate and within them rows of cyto-
plasmic granules are converted into myofibrils. Unlike the fibrils of
striped muscle, however, light and dark bands are not formed. As the
fibers elongate into spindle-shaped cells there is no nuclear division and
the fibers remain mono-nucleate. The protoplasmic bridges (plas-
modesms) by which the mesenchymatous cells were connected with one
another persist and bind the muscle fibers together.

The fate of the hypomere in the head is much more complex than in
the trunk. Besides forming the heart and pericardium, the head hypo-
mere gives rise to the chewing muscles, the muscles of expression, and the
pharyngeal and laryngeal muscles. In general, the processes involved
are similar in lower and higher vertebrates.

In embryos of lower vertebrates, e.g., elasmobranchs, the coelom
extends throughout head and trunk. In the head region, as a result of
the outpocketing of pharyngeal pouches, the hypomere becomes divided

THE MUSCULAR SYSTEM

291

into a series of cavities each of which lies in a visceral arch. This hypo-
meric segmentation (branchiomerism) is independent of the segmentation
of the epimere (mesomerism), and should not be confused with this,
although it is possible that the two types of segmentation may originally
have coincided. From the mesoderm of the visceral arches arise the
muscles, connective tissues, and blood vessels of the arches. In the
fishes, these muscles are differentiated into levators, depressors, and
constrictors of the gills. In the process of conversion the epithelium of
the hypomere breaks up into mesenchyme and the coelomic cavity
disappears.

.^MYOTOMES 1-4

ANL. M. TRAPEZIUS +
"M. STERNOCLEIDO-
MASTOID

MANDIBULAR MUSCLES/ ■-•^--^^-^ ^^C^ ^::^^ ^'ANLACE DIAPHRAGM

1ST THORACIC MYOTOME''' ^^

Fig. 243. — The anlagen of the cranial muscles with their nerve relations as seen in a
7 mni. human embryo. (Redrawn from Keibel and Mall, after W. H. Lewis.)

In mammals and man, the coelom is absent in the visceral arches and
the muscles are formed from masses of mesenchymatous cells. From
the first visceral arch arise the muscles innervated by the mandibular
branch of the fifth nerve, the masseter, temporalis, pterygoid, mylohyoid,
and tensor veli palatini. From the same source come the tensor tympani
of the ear and the anterior belly of the digastricus. The muscles inner-
vated by the facial nerve are derived from the second visceral arch, the
hyoid. They include the muscles of expression, the stylo-hyoid, stapedius,
and the posterior belly of the digastricus. From the third visceral arch
arise the stylopharyngeus muscle innervated by the glossopharyngeus
nerve, and the constrictors of the pharynx innervated by the vagus nerve.
The laryngeal muscles innervated by the vago-accessory nerve originate
from the fourth and fifth visceral arches. As already explained the
muscles of the tongue and throat innervated by the hypoglossal nerve
are myotomic, not visceral, in origin.

CHAPTER 8
THE DIGESTIVE SYSTEM

Life depends upon an unceasing intake and outgo of matter. Each
living thing takes in food or the raw materials for food, assimilates this
into its own peculiar sorts of protoplasm, and after forming these chemical
substances, promptly burns them up into simpler chemical substances,
which finally leave the body as the wastes and ashes of Hfe. Upon this
fundamental chemical process of metabolism, all other vital functions
depend. The foundations of life are chemical.

The products of plant metabolism, on their way back to the inorganic
world, become, directly or indirectly, the food of animals. Thus all
animals are parasites on the green plants. But their feeding habits are
varied. Some marine organisms live on the mud as well as in it; earth-
worms pass through their digestive tract enormous quantities of soil for
the sake of the organic matter which they extract from it. But leeches
live chiefly on blood. Oysters sweep bacteria into their mouths by ciliary
action. Barnacles kick food into their mouths by means of their six
pairs of legs. Some insect larvae feed on cellulose, some on fur and wool.
Some whales eat minute swimming Crustacea, which they strain out by
means of the whalebone. Others live chiefly on gigantic cuttle-fish.
Some mammals are herbivorous; some are carnivorous; others, Hke man,
are omnivorous. Man alone cooks his food.

Digestion. The first chemical change which ingested foods undergo
is a process by which insoluble substances are made soluble, so that they
may be absorbed through the lining membranes of the small intestine.
The agents in this chemical process are certain remarkable enzjrmes which,
like other and inorganic catalyzers, are able to bring about chemical
changes without appreciable effect on themselves. During digestion,
these enzymes split up the huge molecules of colloids into simpler mole-
cules, small enough to pass through animal membranes. Their composi-
tion is unknown; but they are thought to be rather simple colloids derived
from proteins. The specificity of their action is remarkable, each enzyme
affecting only one food substance. All are secreted by glands connected
with the alimentary canal.

EVOLUTION OF THE DIGESTIVE SYSTEM

The protozoa have no digestive system. The single cell merely
engulfs the food particle, surrounds it with protoplasm, digests and assim-

292

THE DIGESTIVE SYSTEM

293

ilates it, and extrudes what remains. Tlie porifera, though they have a
cloacal cavity, do their feeding essentially like protozoa, each cell for
itself.

The first real step in evolving a proper digestive system is taken by
the coelenterates. These, as their name affirms, have a cavity or enteron
which is also the digestive tract. This has but one opening to the exterior,
which serves both as mouth and anus.

Fig. 244. — Diagram of a vertebrate, a, anus; b, brain; c, coelom; da, dorsal aorta;
df, dorsal fin; g, gonad; gd, genital duct; h, heart; i, intestine; I, liver; m, mouth; n,
nephridia; o, oviduct; p, pancreas; pc, pericardium; pf, pectoral fin; ph, pharynx, with
gill clefts; s, stomach; sc, spinal cord; sp, spleen; u, ureter; vf, ventral fin. (From
Kingsley's "Comparative Anatomy of Vertebrates.")

Most flatworms, like coelenterates, have a single opening to the
digestive cavity (enteron), and this opening serves as both mouth and
anus. A few species of flatworms, however, possess an anus — some indeed
have two ani — the invention of which therefore should be credited to

Fig. 245. — Spiral valve of Raia. Cartilaginous fishes increase the absorbing surface
of their intestine not by elongation, as is done by higher animals, but by a spiral fold in
the intestine. (From Kinglsey's " Comparative Anatomy of Vertebrates," after Mayer.)

flatworms. Threadworms, with few exceptions, have both mouth and
anus, and their alimentary canal is separated from the muscular body
wall by a space, a false body-cavity or pseudocoelom. The digestive
tube in threadworms is purely epithelial and non-muscular.

A muscular digestive tube, one of the important steps in animal evolu-
tion, is contributed by the annelids. In these for the first time in the
phylogenesis of animals an epithelium-lined coelom or ''body cavity"

294

COMPARATIVE ANATOMY

NASAL ocvmr

PALATE
MOUTH

TONGUE

NASAL PHARYNX

proper separates the alimentary canal from the body wall. In annelids
as in all the higher animals there is no connexion between the two cavities,
enteron and coelom. The single tube that forms the body of lower forms
has become double, and the muscular activities of the alimentary canal
are carried on independently of those of the body wall.

Among the forms which He near the main line of human ancestry,
pharynx, esophagus, and stomach are first differentiated in urochordates.

A liver arises in the cephalo-
chordates. What is possibly a new
mouth, not homologous with the
mouth of amphioxus, appears in
cyclostomes. But this mouth is
still a sucking one, without jaws.
The cyclostomes contribute also a
pancreas and a bilobed liver.

Elasmobranchs, utilizing dermal
scales as teeth and transforming a
visceral arch into a jaw, convert
the sucking mouth into a biting
one. They develop also a new
cavity, the cloaca, to receive the
wastes and secretions of the
urogenital and digestive systems.

The amphibia fasten their teeth
in a groove in the jaw bone, invent
salivary glands, utilize hypo-
branchial muscles to make a mobile
tongue, and differentiate small from
large intestine.

Mammals greatly elongate the
intestine, and by suppressing the
cloaca separate the rectum from the urogenital sinus. The result is a
muscular, epithelium-lined alimentary canal, differentiated into nearly a
dozen different organs, and having about the same number of different
glands associated with it.

Fig. 246. — Diagram of the alimsntary canal
(From Morris' " Human Anatomy.")

THE HUMAN DIGESTIVE SYSTEM

Mouth

The mouth cavity is divided into an anterior vestibule or labial cavity
lying between the lips and the teeth, and a posterior mouth cavity proper
or buccal cavity underlaid by the tongue and extending to the posterior
margin of the soft palate. The roof of the mouth cavity proper is formed

THE DIGESTIVE SYSTEM

205

by the hard and soft palates, which separate the mouth cavity from the
narial passage above.

Development of the Mouth

At a relatively late stage of ontogenesis, at the anterior end of the
fore-gut where the mouth is to break through, the ectoderm invaginates to
form the stomodeum, the depth of which, in amniotes, is considerably
increased by the expansion of the fore-brain in front and the heart behind.
At the bottom of the stomodeum, ectoderm and endoderm are in contact
as a two-layered membrane, which ruptures and disappears leaving no

dicnccphalon

epiphysis

lateral telencephalic
vesicle

hyomandibolar cleft

hyoid arch
visceral arch in

Fig. 247. — Drawing to show the external appearance of the structures in the oral
region of a four-day chick. Ventral aspect. (From Patten's " Embrvology of the
Chick.")

trace in the adult. Thus, in forming the mouth, the ectoderm takes the
initiative, whereas the gill-slits are primarily outpocketings of the
endoderm.

The covering of the lips and gums is derived from the ectodermal
stomodeum, while that of the rest of the mouth is endodermal. The
salivary glands arise from the epithelial lining of the mouth and are
generally supposed to be ectodermal in origin. The enamel of the teeth
is a product of the stomodeal wall, and therefore, although within the
digestive cavity, ectodermal.

Evolution of the Mouth

There is no doubt that the mouths of all vertebrates are homologous,
the sucking mouth of cyclostomes being no exception. Cyclostome and
gnathostome mouths have the same fundamental structure, development,
and relations to other parts, and must therefore be considered homologous.

296

COMPARATIVE ANATOMY

The ectodermal horny teeth of cyclostomes Hke the movable lips and
cheeks in mammals, are merely details which do not affect the general

issue.

But concerning the question of the homology of the mouth of verte-
brates with that of the lower chordates, opinion is divided. Van Wijhe

OLFACTORY PIT

^OLFACTORY PIT

HYPOPHYSIS

OLFACTORY

PIT

HYPOPHYSIS,

NEOSTOMA'

lUllllliiiiiiiiiiiiriiim
PHARYNX

iriiiii'i'"!

jHYPOPHYSIS

PHARYNX

NEOSTOMA

I NEOSTOMA

AMPHIOXUS. PETROMYZON.

Fig. 248. — Diagrams illustrating the paleostoma theory. A-D show stages in the
development of a pre-oral (hypophysial) opening as in myxinoids and, according to
Legros, as in Amphioxus. E-H show stages in the development of the hypophysis in
petromyzon, in which the neostoma supplants the paleostoma. In the higher verte-
brates the hypophysis is converted into an important endocrinal organ. (Redrawn
from Johnston after Legros.)

(1914) suggested that the mouth of Amphioxus is not homologous
with that of vertebrates, but is comparable with the left spiracle of
elasmobranchs.

The foundations of this assumption, however, are weak. The enor-
mous enlargement of the larval mouth of amphioxus, as well as its asym-
metry are best interpreted as larval adaptations.

THE DIGESTIVE SYSTEM 297

Beard and Von Kupffer are persuaded that vertebrates have had two
mouths — an old paleostoma, and new neostoma. The paleostoma, in
their opinion, may be represented by the hypophysis, which in some
Cyclostomes, e.g. Bdellostoma, opens directly into the pharynx. Further-
more, the presence of a pre-oral gut in vertebrate embryos is interpreted
as a support of this hypothesis. Von Kupffer thinks that the hypophysial
mouth may be homologous with the mouth of Urochords, while Dohrn
assumes that it is comparable with the mouth of annehds. Such views
have seemed to morphologists to have a rather insecure support, and have
therefore been regarded as speculations. (Figs. 248, 250)

That the chordate mouth is not homologous with the mouth of inverte-
brates has generally been assumed, especially by those who have upheld
either the annelid or the arachnid hypothesis of vertebrate ancestry.
Each of these theories seems to need to assume a reversal of dorsal and
ventral surfaces. But were such a reversal to occur, the ventrally situated
mouth of the invertebrate would take a dorsal position, and would con-
sequently He dorsal to the central nerve cord, whereas in all chordates
the mouth is ventral to the central nerve cord. Hence the assumption
is made that a new ventral mouth must have arisen when the worm turned
over and became a chordate. Minot has sought to avoid this theoretical
necessity by assuming that the lateral halves of the supra-esophageal
ganghon of the annelid became separated and converted into the eyes
of vertebrates, while the old mouth migrated to its present position on
the ventral side of the vertebrate, which is the former dorsal side of the
invertebrate.

Whether we accept or reject either the annehd or the arachnid theory
of the origin of vertebrates, we must believe that there have been at least
two mouths in the course of vertebrate phylogenesis. The reason for this
conclusion is that the original coelenterate mouth becomes the mouth in
no vertebrate, while only in Cyclostomes, dipnoi, and possibly some
amphibia does it become the anus. But the coelenterate mouth becomes
the blastopore of chordate embryos. And the blastopore of chordates
lies at the posterior end of the body and forms the neurenteric canal,
which connects the neural tube with the enteron, while the chordate
mouth develops at the anterior end of the enteron. Consequently, it
seems indisputable that there have been at least two mouths in the history
of vertebrates.

While, however, all agree that the vertebrate mouth is not the primary
animal mouth, and that at least two mouths have successively appeared,
some morphologists believe that there have been at least three mouths.
Delsman (1922) reviving an earlier suggestion of Kowalevsky (1877)
claims that "in the ontogeny of vertebrates we see three successive
mouths appear in the same succession as they appeared in phylogeny,

298

COMPARATIVE ANATOMY

VIZ., the blastopore (Urmund), the neuropore (the anneUdan mouth),
and finally the definitive mouth." According to this view, the neural
tube was formerly a part of the digestive system, and its anterior embryonic
external opening, the neuropore, once functioned as a mouth. For a part
of the digestive system to become nervous in function is indeed a sur-

BLASTOPORE

ENDODERM

A.GASTRULA

GILL POUCHES

B.AMPHIOXUS EMBRYO

NOTOCHORD

BLASTOPORE

ENTERON

MOUTI

NEUROPORE

^PINALCORD ^^HOPj,

GILL POUCHES

C.UROCHORDATE LARVA

ENOODERM STRAND

NEUROPORE

OTIC CAPSULE

NEUR-

NOTOCHORD

ENTE

RIC CANAL ,
(BLASTOPORE)

POST-ANAL GUT

ANUS

GILL POUCHES
DEFINITIVE MOUTH
HYPOPHYSIS

D. VERTEBRATE

Fig. 249. — Diagrams illustrating the hypothetical phylogenesis of the vertebrate
mouth. The primitive animal mouth, the blastopore, is converted in vertebrates either
into an anus or a neurenteric canal. The definitive mouth of vertebrates therefore is a
secondary mouth. But the relations of the neuropore are such that at one time in the
ancestry of chordates this may have served as a mouth and the neural tube as a foregut.
It is also possible that the mouth of urochordates is not homologous with the definitive
mouth of vertebrates. The evidence of a paleostoma or hypophysial opening suggests
that this may once have been a functional mouth. Thus the definitive mouth may have
been the last in a series of four mouths.

prising assumption, yet scarcely more so than many other transformations
assumed by evolutionists. The digestive apparatus postulated by
Delsman on the basis of the relations presented in the Amphioxus embryo
has seemed to zoologists too impractical for daily use by any adult animal.
The notion that the neuropore of chordate embryos represents a former

THE DIGESTIVE SYSTEM

299

mouth or that it is homologous with the annelid mouth lacks sufficient
evidence to convince most zoologists that it is more than an interesting
speculation.

According to Von Kupffer, the hypophysis of vertebrates represents a
paleostoma which functioned as a mouth in prechordates, following their
abandonment of the blastoporic mouth. In support of this assumption,
he points out that the definitive mouth of vertebrates arises late in onto-
genesis in such relation to the series of gill-slits that it might have been
formed from a pair of coalesced gill-sHts; that the presence of a pre-oral

HYPOPHYSIAL
DUCT

A.BDELLOSTOMA. pharynx/

HYPOBRANCHIAL MUSCLE

(RESPIRATORY TUBE

HYPOPHYSIAL ^

DUCT ^=-^

NOTOCHORO

I ST GILL APERTURE
B. PETROMYZON.

HYPOBRANCHIAL MUSCLE

Fig. 250. — Diagrams of median longitudinal sections of the heads of Bdellostoma and
Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the
former the hypophysial duct opens posteriorly into the pharynx, suggesting the possibil-
ity that it may once have served as a mouth (paleostoma). In petromyzon the hypo-
physis fails to open into the pharynx and is converted into a pipette-like organ into
which the olfactory pits open. On the basis of this difference cyclostomes are divided
into two sub-classes, Hyperotreta and Hyperoartia.

gut in vertebrate embryos suggests that the alimentary canal formerly
extended anterior to the present mouth; and, finally, that in the Myxinoids
and the embryonic sturgeon the hypophysis actually opens into the
pharynx, and like the mouth of urochordate larvae, has a dorsal external
opening. Kupffer, however, does not waste time guessing as to the cause
of this substitution of a new mouth for an old one. It is possible that
the development and enlargement of the pre-oral lobe was a factor
in effecting the displacement of the mouth from a dorsal to a ventral
position. Diagrams showing the position of the four mouths mentioned
in this discussion are shown in Fig. 249. The objection to the idea that
there have been a series of mouths in the course of animal phylogenesis,
on the ground that the chances are against the appearance of more than
one ingestive opening into the enteron, loses much of its weight in view

300

COMPARATIVE ANATOMY

of the fact that many openings such as the gill-slits have made their
appearance in the course of phylogenesis.

The fate of the original blastoporic mouth of Coelenterates, has been
mentioned in connection with the classification of animals. In most
invertebrates, the blastopore becomes the mouth, while in echinoderms

and chordates it either becomes the
anus or lies in the anal region. Ani-
mals in which the coelenterate mouth
persists as the definitive mouth are
known as Protostomians, while those
in which a new mouth is formed are
called Deuterostomians. The two
main branches of the animal king-
dom shown in Fig. i are separated
on the basis of this distinction.

According to the protostoma
theory of Adam Sedgwick (1884)
the oral surface of coelenterates
with its encircling nerve ring
becomes the ventral, and neural,
surface of non-chordates, while in chordates it forms the dorsal,
and neural surface. Thus the theory assumes that the non-chordates
are the descendents of coelenterates which moved with the oral
side down, while the chordates have come from coelenterates which

ec

Fig. 251. — Schematic section of the
hinder end of an amphibian embryo, show-
ing the relations of the neurenteric canal.
ac, alimentary canal; ec, ectoderm (black);
n, notochord; ne, neurenteric canal; nt,
neural tube; p, proctodeum; pa, post-anal
gut; y, yolk. (From Kingsley's "Com-
parative Anatomy of Vertebrates.")

.BLASTOPORE

NEUROPOREi

CBLASTOPORE)
,'NEURENTERIC CANAL

'^'"\^^^'

A B. C.

Fig. 252. — A diagram illustrating the way in which according to Delsman the blasto-
poric mouth of coelenterates is in chordates converted into the neurenteric canal.
Delsman homologizes the chordate neural tube with the ectodermal foregut of annelids.

moved around with the oral side up. Sedgwick holds that the two sides
of the elongated slit-like mouth of the coelenterate (such as an actinian)
have become the right and left sides of the bilaterally-symmetrical bodies
of higher animals. He finds that in the primitive worm-like arthropod
Peripatus the elongated gastrula mouth closes along the median line of
the embryo, but anterior and posterior dilatations persist as the definitive
mouth and anus. In this way the alimentary canal is formed with two
external openings. The concrescence of the right and left halves of the
vertebrate embryo is taken as a confirmation of the theory. It seems

THE DIGESTIVE SYSTEM 3OI

impossible, however, to compare the definitive mouth and anus of verte-
brates with the mouth and anus of Peripatus, since the former He outside
of and ventral to the ring of nervous tissue which forms the central nervous
system, while the latter lie within the nerve-ring. Only the neuropore
and neurenteric canal of vertebrates could be compared with the mouth
and anus of Peripatus.

The phylogenesis of the vertebrate mouth remains, therefore, an
unsolved problem. That there have been at least two mouths in the
course of animal evolution, all morphologists agree. These are the
coelenterate mouth, which is the blastopore, and the definitive vertebrate
mouth. Evidence is however not wanting that the embryonic neuropore
and the hypophysis may have served as mouths. Such assumptions are
considered to have a relatively insecure foundation.

The Lips. The lips are fleshy folds of skin surrounding the mouth of
tetrapods. They are immovable in amphibia and reptiles. In mammals,
however, slips of muscle derived from the sphincter colli migrate into
the lips, which as a result become movable, and useful in sucking and in
the prehension of food. Their increased size in mammals is correlated
with the habit of mastication; and they, together with the cheeks, restrict
the opening of the mouth. In man, the lips become an important aid
to articulate speech.

The Salivary Glands in Man

As food enters the mouth, it is moistened by the secretion of a number
of saHvary glands, in addition to which are hngual, labial, buccal, palatine
and molar mucus-secreting glands. Besides moistening the food, the
chief saHvary glands contain serous cells which secrete the starch-spHtting
enzyme ptyalin and the sugar-spHtting enzyme maltase. The sublingual
and submaxillary glands secrete mucus also.

The largest of the saHvary glands is the parotid, which lies below the
ear. It is a serous tubulo-acinous gland, and empties by Stenon's duct
into the vestibule of the mouth opposite the second upper molar tooth.
It is innervated by fibers from the otic gangHon and has nervous connexion
both with the fifth and ninth cranial nerves. Inflammation of the parotid
is the cause of mumps.

The submaxillary is a mixed (mucous and serous) tubuloacinous gland
located in the floor of the mouth near the angle of the lower jaw. Its
secretions are carried by Wharton's duct to the frenulum at the front
margin of the tongue near its median Hne. The cells of the gland are
mostly serous, mucus-secreting cells occurring irregularly and constituting
only a minor portion. The slender intercalated ducts present in the
parotid are absent in the submaxillary.

302

COMPARATIVE ANATOMY

The sublingual is also a mixed tubulo-acinous gland lying below the
tongue in the front of the mouth near the median line. Mucus and serous
cells are about evenly distributed. In each acinus, the mucus cells are
central and the serous peripheral so that the serous tend to form demi-
lunes. The openings of the sublingual ducts lie in front of the tongue near
those of Wharton's ducts.

Development. From their position and the relations of their ducts,
it is generally assumed that the chief salivary glands are of ectodermal
origin. This is probably not true, however, of the numerous glands of

A crescent consisting of
eight serous cells.

Part of an excretory duct

<4'

Lumen,

Tangential
section of serous
cells.

Mucous cells and

thick membrana

propria .

Fig. 253.

-Section of a human sublingual gland X252.
Histology.")

(From Bremer's "Text Book of

the tongue, all of which are formed by the local prolification of the stratum
germinativum of the mucous lining of the mouth.

History of Salivary Glands. Salivary glands are not unknown among
the invertebrates. Multicellular mucus glands connected with the
mouth are present in molluscs. Malaria is transmitted by the saliva of
mosquitoes. It is doubtful, however, if the salivary glands of inverte-
brates have any genetic relation with those of vertebrates.

Lower chordates have no salivary glands, and fishes only unicellular
mucus glands. It has generally been assumed that the multicellular
glands of the higher vertebrates have their beginnings in such unicellular
glands.

Multicellular oral glands appear in amphibia. Besides the mucus-
secreting cells of the tongue, most amphibia have an intermaxillary gland,

THE DIGESTIVE SYSTEM

303

the duct of which opens between the intermaxillary bones. In some
amphibia, e.g., Rana, mucus glands are also located in the posterior
narial passages. That enzymes are secreted by the mucus cells of fishes
and amphibia has, however, not been demonstrated.

In the reptiles, there are serous cells in the oral glands, and lingual,
sublingual, and palatine glands occur. Glands connected with the teeth
are dififerentiated as the poison glands of some snakes.

True salivary glands secreting enzymes are limited to mammals.

There seems no good reason to doubt, however, that the salivary
glands of mammals are derived from the oral glands of reptiles. Labial
and buccal glands become abundant, and possibly the parotid is an

.COMPOUND
TUBULAR GLANO

. /emus OR

Fig. 254. — Various types of digestive and endocrinal glands which develop from the
endodermal (mucous) lining of the alimentary canal. The endocrine glands are ductless.
The digestive glands may be simple or compound, tubular or alveolar (acinous).
(Redrawn after Braus.)

enlarged buccal gland. In addition to the lingual and palatine glands,
the sublingual and submaxillary glands are present; and in general, the
glands of man resemble those of other primates.

The Tongue

The tongue is a muscular organ of miscellaneous functions — digestive,
sensory, conversational — lying in the floor of the mouth cavity and
attached to the hyoid bone. It consists of an apex or body directed
towards the teeth of the lower jaw, a root of muscular attachment, a
dorsum divided by the sulcus terminalis into an anterior papillated por-
tion and a posterior tonsillar and glandular portion, and an inferior surface
below the apex. The sulcus terminalis is a V-shaped groove with the
apex of the V pointing backwards and marking the position of the foramen
coecum. Fig. 255.

The dorsum of the tongue anterior to the sulcus is covered with
numerous papillae which give the tongue its characteristic rough appear-
ance. Four kinds of papillae are distinguished, vallate, filiform, foliate,
and fungiform. . The vallate papillae are the largest, and are distinguished
also by the deep depression or fossa which surrounds each of them. On

304

COMPARATIVE ANATOMY

their sides they bear numerous taste-buds. Their number varies from six
to twelve, and they occur in a V-shaped row just in front of the sulcus
terminalis. Of the various forms of papillae on the tongue the filiform
papillae are the most numerous. Each filiform papilla is covered with
filamentous processes. Foliate papillae are three to eight parallel folds
on each side of the tongue. Like the vallate papillae, the foliate papillae
have taste-buds. The fungiform papillae are scattered over the entire
dorsum of the tongue, and are distinguished by their reddish color and

EPIGLOTTIS.

FOLIATE
PAPILLAE

Fig. 255. — The dorsal surface of the tongue. The sulcus terminalis divides the
body or apex of the tongue from the root. The two regions have a different embryonic
origin. (Redrawn after Sobotta.)

their globular mushroom shape. They also bear taste-buds. No papillae
occur on the posterior and inferior surfaces of the tongue. (Fig. 255)

Most of the mass of the tongue consists of striated muscle. In the
connective tissue corium of the tongue, both mucus and serous glands are
abundant. The lingual tonsils lie on the posterior dorsum.

Development of the Tongue. The apex and root of the tongue, which
develop from separate anlagen, remain throughout life divided by the
sulcus terminahs. The apex of the tongue is formed by the union of a
median tuberculum impar with the basal portions of the two halves of the

THE DIGESTIVE SYSTEM

305

mandibular arch. The root of the tongue arises from portions of the
second, third and fourth visceral arches. The tongue muscles, however,
are not formed from those of the visceral arches, but from post-occipital
myotomes which send buds downward and forwards into the tongue.

History of the Tongue. None of the lower chordates has a tongue,
so that the vertebrate tongue seems to be an emergent organ like the
notochord. The so-called tongue of cyclostomes is a muscular piston
associated with the sucking mouth and cannot be compared with the
tongue of higher vertebrates since the hypobranchial muscles which form
the mass of tongue muscles in higher vertebrates, though present in
cyclostomes, have no connection with the so-called tongue.

BODY

"^ FORAMEN
CAECUM

Fig. 256. — Two stages in the development of tongue and pharyngeal floor of man.
The body of the tongue comes from paired and unpaired anlagen of the mandibular arch;
the root from second and third visceral arches. That the fourth arch is involved is
doubtful. (After Kallius.)

Gnathostome fishes have an immovable tongue, which forms a swelling
in the floor of the mouth and is supported by the basihyal, or os ento-
glossum. Although it lacks muscles, this fish tongue is generally regarded
as homologous with the root of the tongue of tetrapods.

The tongue of tetrapods beginning with amphibia consists of an apex
and root as in man. While the root is derived from the tongue of fishes,
the body is a new formation derived from the mandibular arch united
with a median outgrowth from the floor of the mouth.

The tetrapod tongue is further modified by the ingrowth of hypo-
branchial muscles by which it attains a high degree of mobility. Con-
sequently, in addition to its other functions of moving food in mouth
and swallowing, it serves as a means of capturing food. Its gustatory
function continues throughout the entire vertebrate series. Some have
assumed that the primary function of the tongue muscles was that of
squeezing secretions out of the lingual glands. Papillae appear first in
amphibia, but become more highly differentiated in mammals.

3o6

COMPARATIVE ANATOMY

The Pharynx

The pharynx is that part of the ahmentary canal where the respiratory
and digestive passages cross one another. The constricted opening
through which the mouth cavity communicates with the pharynx is the
isthmus faucium. It is bordered by the soft palate above, the tongue
below, and the glossopalatine arch on each side. The glossopalatine arch
partially covers the palatine tonsil, a mass of adenoid tissue pitted with

PARIETAL BONF
GYRUS CINGULI
PITUITARY,

RIGHT CEREBRAL HEMISPHERE

RPUS CALLDSUM
FORNIX

PINEAL GLAND

SUBDURAL CAVITY-
FRONTAL LOBE-
FRONTAL BONE

DURA MATER

NASAL ^^^^—^^m^S^^^iai^ffi Iwaai^B^Si. ' J \y/'^w'~°^'^^^"'^^ ^°^^

SPHENOID BONE- ' j • ^

NASAL C0NCHAE-="=*^^^^^^^^^^^^^^3^"^^^^--^ ^"^^^^^^Sa^^^S^^^^^EREBELUW

EUSTACHIAN TUBE
MAXILLA-
MOUTH CAVITY-
PALATINE BONE-
VESTIBULE-
SOFT PALATE
M. GENIOCLOSSUS
MANDIBLE-
M. GENIOHYOID--^'^^^^

M. MYLOHYOID

PHARYNX^

EPIGLOTTIS''

LARYNX ^\?U,^-

ESOPHAGUS' XENTRA

Fig. 257. — A median longitudinal section of the human head showing the rela-
tions between digestive and respiratory passages in the pharyngeal region. (Redrawn
after Braus.)

numerous crypts which tend to be a source of infection. The isthmus is
surrounded on all sides by adenoid tissue, the lingual tonsil being included
in the ring. The hypertrophy of adenoid tissue, especially that of the
soft palate in childhood, interferes with breathing and often requires
surgical treatment. (Fig. 257)

The soft palate is a muscular partition separating digestive and
respiratory portions of the pharynx. From its posterior border hangs
the uvula. By the action of the palatine muscles, the food pushed by
the tongue into the pharynx is forced backwards towards the esophagus
and prevented from entering the nasal portion of the pharynx.

THE DIGESTIVE SYSTEM

307

Seven cavities open into the pharynx, the mouth, the two nasal
passages, the two Eustachian tubes, the larynx, and the esophagus.
Three divisions may be distinguished, oral, nasal, and laryngeal. The
palatine tonsils lie in the oral portion, the nasal passages and Eustachian
tubes open into the nasal portion, while the larynx opens into the laryngeal
portion. When food enters the pharynx the entire pharynx is raised by
the contraction of the stylo-pharyngeal muscles while the constrictor
muscles of the pharynx squeeze the bolus towards the esophagus. The
nerve supply of the pharynx comes chiefly from the glossopharyngeal.
Like the mouth cavity, most of the pharynx is Hned by stratified squamous
epitheHum. The nasal portion, however, is lined with ciliated columnar
epithelium.

Since the pharyngeal region is closely associated with the respiratory
organs of vertebrates, the description of the evolution and development
of the pharynx is omitted here and will be found in the following chapter.

The Esophagus

The esophagus is that portion of the alimentary canal which extends
from the pharynx to the stomach. It is nearly ten inches in length and is
the narrowest part of the digestive tract. From the pharynx it passes
just beneath the back bone through the mediastinum and diaphragm
to the cardiac region of the stomach.

The wall of the esophagus consists of the four layers characteristic
of the digestive tract, tunica mucosa, tunica submucosa, tunica muscularis,
and tunica adventitia ; but the serous layer which covers the stomach and
intestine is wanting in the esophagus, since the body cavity lined by the
serosa does not extend into the neck. The tunica mucosa includes not
only the stratified squamous epithelium which fines the esophagus, but
also a connective tissue tunica propria and a muscularis mucosae, a thin
layer of longitudinal muscle fibers. The muscular coat of the esophagus
consists of striped fibers in the upper third, while those of the lower two-
thirds are smooth. In the contracted state of the esophagus, the mucosa
is thrown into longitudinal folds fike those of the stomach. (Fig. 258)

The submucosa is a layer of loose connective tissue containing glands
and many blood and lymph vessels. The tunica muscularis consists of an
inner layer of circular muscles and an outer longitudinal layer. The
connective tissue between them contains a plexus of sympathetic nerve
fibers. By the wave-fike peristalsis of the circular muscles food is con-
veyed from the pharynx to the stomach.

Development of the Esophagus. Beginning with the fourth week,
the esophagus develops as an elongation of the fore-gut between pharynx
and stomach. Its single-layered columnar epithelium becomes gradually
converted into a stratified squamous epitheHum like that which lines

3o8

COMPARATIVE ANATOMY

the pharynx. During this change, its lumen becomes occluded tem-
porarily, a new lumen arising secondarily through the union of a number
of vacuolar spaces. Ciliated epithelial cells occur in the esophagus until
relatively late (loth week) stages of development.

History of the Esophagus. There is little to distinguish the esophagus
of a fish from its stomach, except the relative scarcity of glands, and the
fact that its muscle fibers, Hke those of the pharynx, are striated, while

EPITHELIUM

rUNICA PROPRIA

MUSCULARIS MUCOSAE

MUCOUS GLAND
SUBMUCOSA

•IJl-^'-t^ .tJ%" ^tS/*? .'CIRCULAR MUSCLES

LONGITUDINAL
MUSCLES

i ADVENTITIA

VAGUS NERVE

Fig. 258. — The esophagus as seen in cross section. A is a section of the entire
esophagus. 5 is a small portion much enlarged. The five layers of tissue characteristic
of the entire alimentary canal are found in the esophagus. (Redrawn after Braus.)

those of the stomach are smooth. In amphibia, the esophagus becomes
slightly elongated. Its considerable elongation in reptiles and mammals
is correlated with the elongation of the neck. In these groups, it becomes
constricted in diameter and most of its muscle fibers become smooth.

The Stomach

The stomach, lying between the esophagus and small intestine, is the
most expanded part of the alimentary canal. Its shape in man varies
greatly, depending upon the quantity of food contained. The human
stomach lies almost transversely across the abdominal cavity with a

THE DIGESTIVE SYSTEM

309

greater curvature on the left side of the body and a lesser curvature to
the right. The opening of the esophagus into the stomach is the cardiac
orifice, that into the small intestine is the pylorus.

The anterior more enlarged portion of the stomach is the cardiac
portion, the posterior more constricted portion is the pyloric portion.
In the cardiac division, may be distinguished a main body and a blind
pouch, the fundus. The pyloric portion of the stomach diminishes in
size towards the pylorus, which is reduced to a small aperture by a local
ring-like thickening of the mucous lining and of the layer of circular

GREATER CURVATURE

PYLORIC STOMACH

Fig. 259. — The right half of the human stomach, viewed from within.

Braus, after Elze.)

(Redrawn from

muscle. By this mechanism, only finely divided material is allowed to
pass in jets into the duodenum, forced by the peristalsis of the stomach.

The wall of the stomach contains the same four layers of tissue as are
seen in the esophagus, plus an external serous layer. These beginning
with the outermost are the serosa, including the adventitia, the tunica
muscularis, tunica submucosa, and the tunica mucosa. The tunica
serosa is a connective tissue layer covered with the peritoneal epithelium.
In it are many blood vessels, with branches of the vagus nerve and the
celiac plexus of sympathetic nerve fibers.

The tunica muscularis contains three layers of muscle, longitudinal,
circular, and obHque. By their combined action under the stimulus of
the sympathetic nerves, the stomach maintains a peristaltic churning
action as long as food is present. The tunica submucosa consists of loose
areolar connective tissue richly supplied with blood vessels and with a

3IO

COMPARATIVE ANATOMY

plexus of sympathetic nerve fibers. The mucosa is divided into a mucous
columnar epithelium, a tunica propria, and a muscularis mucosae contain-
ing circular and longitudinal muscle fibers. The tunica propria includes
considerable adenoid tissue.

The simple mucous epithehum which lines the stomach joins abruptly
the stratified epithehum of the esophagus. Viewed with a hand-lens,
the inner surface of the stomach appears to be filled with minute pores,
which are the apertures of the ducts of the gastric glands. Three kinds of
stomach glands are distinguished, cardiac, gastric, and pyloric. The

"MUCOUS EPITHEUUM-'

"GASTRIC P1T<

HIEF CELLS

.PARIETAL CELLS

MUSCULARIS'.

'Fig. 260. — Cross sections of the wall of the human stomach, showing A, the struc-
ture of the gastric (fundus) glands, and B, that of the pyloric glands. While the secre-
tions of gastric glands are chiefly digestive (gastric juice), the pyloric glands secrete
mucus chiefly. (Redrawn after Braus.)

cardiac glands occupy a relatively small area near the cardiac orifice
and resemble closely the glands of the esophagus. Each cardiac gland
consists of a group of parallel tubules opening into a single duct or pit.
The walls of the tubules are formed of cells which secrete zymogen or
pepsinogen granules, of parietal cells which secrete the chemical precursor
of hydrochloric acid, and of mucus-secreting cells.

Most of the glands of the stomach are gastric each of which, Uke the
cardiac glands, consists of a duct or pit connected with a group of straight
or slightly curved tubules. During life, the number of pits is multipUed,
while the number of tubules connected with each crypt is correspondingly
reduced. The pits are relatively short and are lined with mucous gland
cells hke those which cover the inner surface of the stomach, while the

THE DIGESTIVE SYSTEM

311

tubular glands are relatively elongated and their walls are formed of
granular chief cells and of peripheral parietal cells. The chief cells
secrete two kinds of zymogen granules — pepsinogen and prochymosin.
When mixed with hydrochloric acid secreted by the parietal cells, pep-
sinogen becomes pepsin, which splits the molecules of albumen into
peptones, and the prochymosin becomes chymosin or rennin, which
changes casein into paracasein. It is also asserted that the gastric glands
secrete lipase, a fat-splitting enzyme.

^.eOCFr STALK

STOMOOEUM''' \\

YOLK STALK>

'STOMACH

pORSAL PANCflEAS

" k -vtr/TRAL PANCREAS

-SMALL INTESTINE
, -CAECUM PHARYNX'

?ITDNEAL CAVITY
^MESONEPHRIC

ESOPHACUS
TRACHEA-

DORSAL PANCREAS
VENTRAL PANCREAS
RO SINUS

rCLK STALK'

ANTOIS
PR0CT00€Um\

UMSIUCAL CORD'

Fig. 261. — Stages A-F in the ontogenesis of the alimentary canal and associated
structures in the human embryo. A, Early embryo; B, Three- weeks embryo; C, Three
to four weeks embryo; D, Four weeks embryo; E, Five weeks embryo; F, Seven weeks
embryo. Notable among the changes represented are the great elongation of the canal,
the outgrowth of numerous appendages, and in the cloacal region the separation of the
organs of excretion and digestion. (Redrawn after Thompson, Ingalls, P. T. Lewis and
Arey.)

The pyloric glands are limited to the pyloric portion of the stomach.
Their chief secretion is mucus, but the presence of some chief and parietal
cells suggests that they may also secrete some gastric juice. They differ
from gastric glands also in having relatively long pits and short, branched
and twisted tubules. Thus they resemble duodenal glands.

Development of the Stomach. During ontogenesis, beginning with
the fifth week, the stomach arises as a local enlargement of the fore-gut.
Its lining, therefore, together with the glands derived from it, is endo-
dermal. The external peritoneal membrane is mesodermal; the remainder

312 COMPARATIVE ANATOMY

of the stomach wall, including the submucous and muscularis layers, is
mesenchymatous. The mesentery which attaches the stomach to the
dorsal body wall becomes the greater omentum of the adult, while the
ventral mesentery between the stomach and liver becomes the lesser
omentum. The more rapid growth of the dorsal wall produces the
greater curvature of the stomach and results in its left-sided displacement.
The lesser curvature develops from the ventral side. In this way the
original dorsal side shifts to the left side of the body, while the primitive
ventral side comes to lie towards the right. Gastric glands begin to
appear as local proliferations of the lining epithelium during the seventh
week.

History of the Stomach. Since stomachs are not unknown among
invertebrates, it might be assumed that the stomach of vertebrates is
derived directly from that of invertebrates. However, among the proto-
chordates, the hemichordates and some urochordates possess a stomach,
while the cephalochordates do not, the pharynx passing immediately into
the intestine. The liver of amphioxus develops as a ventral outgrowth
a short distance behind the pharynx. Consequently, if we consider
amphioxus as an ancestral type, the stomach of vertebrates must have
arisen from the short portion of the alimentary canal which in cephalo-
chordates lies between pharynx and liver. The esophagus must likewise
have developed from this region. Furthermore, the innervation of the
stomach by a cranial nerve, the vagus, is regarded by some as a proof of the
derivation of the stomach from the anterior portion of the alimentary
canal.

In the cyclostomes, the stomach is a slight enlargement of the ali-
mentary canal. As in the dipnoi, there is no flexure. In most fishes,
however, the stomach becomes J-shaped by the bending of the pyloric
region, and this curvature persists throughout the vertebrate series.
The complications of stomachs such as are found in ruminants are of
considerable importance and interest. The stomach of the cow, for
example, is divided into four functional divisions, rumen, reticulum,
omasum (psalterium) , and abomasum. Since, however, such adaptations
to a special diet throw no light on the problem of human phylogenesis,
detailed description is omitted.

The Intestine in Man

The intestine is the portion of the alimentary canal from the pylorus
to the anus. Its length averages about thirty feet, of which five feet
are included in the large intestine and the remainder in the small intestine.

Small Intestine. The small intestine extends, gradually diminishing
in diameter, from the pylorus to the ileocolic valve of the colon. The
small intestine is distinguished not only by its smaller diameter but also

THE DIGESTIVE SYSTEM

313

by the presence of numerous villi which cover its inner surface and give
it a velvety appearance. Somewhat arbitrarily thiee regions are dis-
tinguished, duodenun, jejunum, and ileum. The duodenum, the anterior
portion of the small intestine, averages about nine inches in length,, and
is characterized by the presence of tubulo-acinous glands located in the
submucosa and known as duodenal or Brunner's glands. The duodenal
glands secrete an alkaline mucus which neutralizes the acidity of the
food which enters the duodenum from the stomach. Zymogenic cells
are also found in the duodenal mucosa.

PLEXUS OF AUERBACH^

PLEXUS OF MEISSNERIk

PERITONEUM

Fig. 262. — A stereogram of a portion of the small intestine, showing the arrangement
of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are
shown in black, sensory cells with white nuclei. (Redrawn after Kahn's "Das Leben
Des Menschen," W. Keller & Co.)

The jejtmimi, which forms two-fifths of the remainder of the small
intestine, contains numerous transverse crescentic folds, the valvulae
conniventes, covered with large villi. These serve to retard the passage
of food and also to increase the absorptive surface. In the ileum, the
crescentic folds disappear, and villi become smaller and more scattered.
Lymph nodules are abundant in the tunica propria.

The four layers of tissue characteristic of the alimentary canal are
present in the small intestine. The mucous epitheHum is of the simple
columnar sort, and each cell has a striated border on its inner free surface.
Throughout the entire length of the intestine are numerous tubular
mucus-secreting glands, perpendicular to the surface of the intestine,
the intestinal glands or crjrpts of Lieberkuehn. Goblet-shaped cells
distended with mucus are abundant in the walls of these glands. The
secretions of these glands are said to stimulate peristalsis of the intestine
as well as lubricate its surface.

314

COMPARATIVE ANATOMY

Each villus is covered with a mucus epithelium containing numerous
goblet-cells, and each villus has a core of connective tissue filled with
capillaries and lymph vessels. A single lymphatic or lacteal occupies the
center of each villus, and a network of capillaries lies just below the base-
ment membrane of the mucous epithehum. Each villus is therefore a
mechanism admirably adapted for absorbing the digested food which

ViUi.

Plica circularis.

Intestinal gland:

Submucosa.

Submucosa. A" ^

Circular muscle

Longitudin
muscle.
Serosa

Fig. 263. — Vertical longitudinal section of the jejunum of an adult man. The
plica circularis on the right supports two small solitary nodules, which do not extend
into the submucosa; one of them exhibits a germinal center, x. The epithelium is
slightly loosened from the connective tissue core of many of the villi, so that a clear
space, XX, exists between the two. The isolated bodies lying near the villi (more
numerous to the left of the plicae circulares) are sections of villi that were bent, so that
their ends were cut off in sectioning. X16. (From Bremer's "Text Book of Histology.")

bathes it. Besides the peristaltic waves which pass along the intestine
squeezing the food backwards towards the large intestine, divisive or
churning movements are also carried on, bringing the digested food into
contact with the villi.

Absorption takes place in the small intestine, in accordance with the
law of osmosis. The dissolved foods pass through the lining membranes,
are taken up by the blood capillaries and the lymphatics, enter the
general circulation, and are absorbed into the cells of the various tissues.

The Large Intestine. The large intestine or colon differs from the
small not only in its great diameter but also in the absence of villi in the

THE DIGESTIVE SYSTEM

315

adult. The walls of the large intestine are sacculated, and they bear
externally numerous fatty appendages, the appendices epiploicae. The
longitudinal muscles do not form a continuous layer as in the small
intestine, but are arranged in three longitudinal bands, the teniae. Trans-
verse crescentic folds, the plicae semilunares, are abundant. Between
these the wall of the colon bulges out to form haustra.

The large intestine is divided into cecum, vermiform appendix, colon,
rectum, and anus. The cecum is a bUnd sac, about two and a half inches

Glands. ?

Muscularis
mucosa.

Submu-
cosa.

Solitary nodule with germinal center.
— Vertical section of the mucous membrane of the descending colon of an
adult man. The fat has been blackened with osmic acid. (From Bremer's "Text Book
of Histology.")

Fat cells.

Fig. 264.

in length, lying near the ileocolic valve in the right iUac fossa. The
vermiform appendix of the cecum is an elongated worm-shaped tube
between three and four inches in length, attached to the apex of the cecum.
The structure of the appendix is similar to that of the large intestine in
having numerous Lieberkuehn's glands and lymph nodules. In the
majority of persons, the lumen becomes occluded in later life. The
appendix appears to be a rudiment of a more extended cecum functional
in the ancestors of man.

The colon is divided into four regions, ascending, transverse,
descending and sigmoid colon. The ascending colon passes up the right
side of the abdominal cavity as far as the liver, where it bends to the left

3l6 COMPARATIVE ANATOMY

to form the transverse colon. Reaching the lower end of the spleen on
the left side, it curves sharply downwards, to become the descending
colon. Passing down the left side to a point below the kidney, th^
descending colon bends towards the median plane of the body and enters
the pelvic cavity, where it forms the sigmoid flexure. The rectum is
continuous with the sigmoid colon and extends to the anus. In the
rectum, a number of transverse folds of the wall tend to prevent fecal

Fig. 265. — Transverse section of the human vermiform process. Note the absence
of villi and the abundance of nodules. Clear spaces in the submucosa are fat cells.
Only a part of the circular layer of the muscularis has been drawn. X20. (From
Bremer's "Text Book of Histology," after Sobotta.)

matter from pressing into the anal canal. In the anal region, the layer of
circular muscles is thickened to form the sphincter ani, which, unlike that
of the lower rectum, is non-striated and not under control of the will.
The external sphincter of the anus, however, is striated and voluntary.

Development of the Intestine. Except in the mouth and anal regions,
the mucous lining of the alimentary canal and the secretory epithelium of
the glands connected with it develop from the endoderm. Primarily, the
endoderm of the embryonic area is continuous with that which lines
the yolk-sac. In correlation with the development of head fold and tail
fold, a fore-gut and hind-gut are formed in connexion with the yolk-sac by

THE DIGESTIVE SYSTEM

317

means of anterior and posterior intestinal portals. From the fore-gut
develop pharynx, esophagus, stomach, and the anterior part of the small
intestine; from the hind-gut the remainder of the intestine. Early in
development, an allantois arises as a ventral outpocketing of the hind-

NEURAL PLATE ^.,.,.,_— =>-;,/NOTOCHORD

NEURAb
GROCVE

NOTDCHORD

DORSAL ROOT
GANGUON

MATIC MESODERM

DERMATOME

\\_MUSCLE
\ BUNDLE
MYOTOME

VISCERAL OR

.SPLANCHNIC
MESODERM

KIDNEY
GENITAL RIDGE
MESENCHYME-

DIGESTIVE TRACT-
COELOI

PERITONEUM

SPINAL CORD

NOTOCHORD

NEURENTERIC
"CANAL

ANAL PIT

^ , "'-DIGESTIVE TRACT

MOUTH PIT — ^yy ^

GILL CLEFT&^\S;lo>:=^5^,g;3fc^^,,;:;,j^^ p

'^ •^UVER OUTGROWTH

Fig. 266. — Stereograms of stages in the ontogenesis of the chordate showing early
stages in the development of the digestive system. A, B, and C are based upon early
stages in the development of Amphioxus; D and E upon elasmobranch embryos; F upon
an amphibian embryo. F represents the right half of an embryo cut in a median longi-
tudinal section. The blastopore has been converted into the neurenteric canal.
(From "Being Well Born," by M. F. Guyer, Copyright 1916, 1917, used by special
permission of the publishers, The Bobbs- Merrill Company.)

gut, with which it retains connexion by an allantoic stalk. The cloaca is
the posterior portion of the hind-gut into which allantois and intestine
open, and which is closed to the exterior by the cloacal membrane.

The later development of the intestine involves its elongation and
twisting. The opening into the yolk-sac becomes reduced to a slender

3l8 COMPARATIVE ANATOMY

vitelline duct, which disappears during the second month. Becoming at
first too long for the body cavity, a loop of the intestine pushes down into
the umbilical cord. In a six weeks' embryo, the beginning of a cecum. is
indicated by a swelling posterior to the vitelline duct. Later a horizontal
septum grows backward to divide the cloaca into a dorsal rectum and a
ventral urogenital sinus. Thus an important evolutionary change is
briefly repeated in ontogenesis. The septum forms the perineum of the
adult. During the second month, an anal canal is formed by the invagina-
tion of an ectodermal proctodeum and the rupture of the cloacal mem-
brane. The elongation and torsion of the intestine continue, and result in
the many convolutions of the small intestine and the ascending, trans-
verse, and descending portions of the colon. (Figs. 261, 266)

The four layers of the intestinal wall develop as has been described for
the stomach. During the second month, the lumen of the small intestine
become temporarily occluded. Muscle fibers first appear in the second
month, and intestinal glands during the third month.

History of the Intestine. The intestine as a region for the digestion
and absorption of food is present in the great majority of animals from
flatworms to man. An anal aperture makes its first appearance in flat-
worms, some of which, indeed, have two ani. Can the intestines and
anal apertures of invertebrates be homologized with structures having the
same name in vertebrates? Is an anus which Hes near the mouth, as in
cephalodiscus, rhabdopleura, and in mofluscoids, homologous with that
which, as in vertebrates, lies near the posterior end of the body? In view
of the present uncertainty in regard to the ancestry of vertebrates, it is
obviously hazardous to-day to attempt to homologize any vertebrate
structure with any organ of invertebrates.

We may take as a test case the question of the origin of the vertebrate
anus. Is the vertebrate anus derived directly from that of any inverte-
brate type? Can we homologize the anus of cyclostomes and some
urodeles, which develops directly from the embryonic blastopore, with
that of an elasmobranch the anus of which does not develop from the
blastopore? Ignoring this difference in origin, Dohrn suggested that the
vertebrate anus, like the vertebrate mouth, may have been a new structure
formed by the coalescence of a pair of gill shts. The suggestion was made
notwithstanding the fact that in no chordate do gifl shts extend as far
back as the anus. The fact that, assuming amphioxus as a primitive
form, the lower the chordate, the larger the number of gill shts, seemed to
Dohrn a sufficient foundation for the hypothesis. That the vertebrate
anus is secondary has seemed further attested by the fact that there is a
postanal gut in vertebrate embryos and that the definitive anus arises
relatively late in ontogenesis. Vertebrate morphologists generally
regard the anus of vertebrates as homologous throughout the group not-

THE DIGESXrV'E SYSTEM 319

withstanding differences in ontogenetic development in different groups.
The post-anal gut may be interpreted as a special modification correlated
with the elongation of the tail, and not as a primitive trait. The assump-
tion of a partial homology of the vertebrate anus with the blastoporic
mouth of invertebrates seems to be in harmony with all known facts.

The uncertainty of pre-chordate homologies will explain why most
vertebrate morphologists take the intestine of amphioxus as the starting
point for intestinal evolution. Yet it is obvious that the intestine of
amphioxus is an inheritance from some prechordate ancestor of whatso-
ever sort. The intestine of amphioxus extends as a straight tube from
the region of the liver directly to the left-sided anus. The intestine of
cyclostomes is almost as simple. A spiral fold projecting into the cyclo-
stome intestine, however, suggests the beginning of a small intestine, since
the small intestine of elasmobranchs contains a spiral valve. Intestinal
elongation has its inception in the sigmoid flexure of elasmobranchs. A
similar functional result is effected in elasmobranchs and ganoids through
the development of a spiral valve in their small intestine. A finger-like
rectal gland makes its appearance in elasmobranchs near the anus. The
rectal gland is sometimes homologized with the cecum of higher verte-
brates, and its position consequently is held to mark the boundary between
large and small intestines. A cloaca also makes its first appearance in
this group. A further step in advance is seen in the teleosts, which have a
convoluted small intestine, intestinal ceca, and a somewhat enlarged
colon. Most amphibia except gymnophiona have small and large intes-
tines. All have a cloaca. Some have in their small intestine intestinal
glands, valvulae conniventes, and villi.

The intestine of reptiles is relatively short. Their large intestine is
short, and they retain a cloaca.

In mammals, the small intestine becomes greatly elongated and differ-
entiated into duodenum, jejunum, and ileum. Valvulae conniventes^
viUi, and intestinal glands become very numerous. Duodenal glands
make their appearance. Colon and rectum are differentiated. In many
mammals, especially herbivorous forms, the cecum becomes much elon-
gated and forms an important organ of absorption. In others, as in man,
it degenerates in size and serves as an adenoid organ.

Mesenteries and Omenta

The peritoneum lining the abdominal cavity is a serous membrane
formed from the embryonic hypomere. It not only lines the body wall,
but is reflected over the viscera, so that parietal and visceral portions are
distinguishable. The complex relations of the peritoneum are due
chiefly to the complications of the alimentary canal with which it is
connected. These are best understood by tracing their development in

320

COMPARATIVE ANATOMY

the embryo. In the region of the pharynx the splanchnic layers of meso-
derm unite in the median plane to form the tubular heart and the meso-
cardial membranes in which the embryonic heart is suspended. In the

dorsal mesoderm

diate mesodenn
lateral mcsade

ighl and left
coelom confluent

Fig. 267.- — Schematic diagrams of cross sections at various stages to show the
establishment of the coelom and mesenteries. (From Patten's "Embryology of the
Chick.")

abdominal part of the coelom the splanchnic layers of mesoderm unite
above and below the aUmentary canal to form dorsal and ventral mesen-
teries. The dorsal mesentery persists throughout life, but the greater

THE DIGESTIVE SYSTEM

321

part of the ventral mesentery disappears in ontogenesis. The anterior
portion which connects stomach, Hver and ventral body wall only is
retained. With the differentiation of the successive regions of the ahmen-
tary canal corresponding portions of the dorsal mesentery are recognized
as mesogaster, mesentery, mesocolon, and mesorectum. The mesenteries
serve not only as means of attachment of the intestine to the body wall,
but also as a passage for the blood vessels of the alimentary canal. In
the adult the mesenteries become very complex in relations as the result
of the elongation of the intestine, formation of omenta, and local adhesions.
As the stomach develops its greater curvature it rotates on its long
axis so that its left side becomes ventral and the right side dorsal. As a

MESOGASTER

GREATER OMENTUM ,'

STOMACH

DUODENUM--

-CAECUM —
APPENDIX-
SMALL INTESTINE-
COLON

YOLK STALK

Fig. 268. — Diagrams illustrating the development of the mesenteries and omentum
in the human embryo. An arrow marks the opening (foramen of Winslow) of the
greater omentLun. (Redrawn after Hertwig.)

result the dorsal mesogaster is stretched to the left and a pouch or bursa
between the mesogaster and the right side of the stomach is formed.
The pancreas and spleen which lie in the dorsal mesogaster are also car-
ried over to the left side of the body where they form adhesions with the
dorsal body wall. As the sacculation of the mesogaster progresses, dorsal
and ventral layers become distinguishable. The two-layered sac thus
formed grows ventrally and posteriorly between the viscera and the ventral
wall of the abdomen as an apron-like membrane, the greater omentum.
Much of the original cavity of the omentum is lost through the fusion of
dorsal and ventral layers. In the region of the stomach, however, the
cavity persists as the bursa omentalis, which opens by the foramen
epiploicimi into the coelom of the right side. The omentum becomes the
seat of deposit of considerable fat and serves as a blanket to keep the
viscera warm.

322

COMPARATIVE ANATOMY

As the intestine elongates the mesentery becomes pleated and several
adhesions are formed. With the growth of the colon the mesocolon twists
around the superior mesenteric artery as an axis of rotation.

DIAPHRAGM

ESOPHAGUS
■SPINAL CORD
DORSAL AORTA

LIVER

rORAMEN EPIPLOICUM
PANCREAS

--J._I_. STOMACH
, ^^sJ-j-SUR MESENTERIC ART.

i_tri_(_TRANSVERSE COLON

iiVlLiJ.INF. MESENTERIC ART.
fyf^-l — MESENTERY

rV->— — i-A-OMENTAL BURSA

UTERUS

RECTUM

Fig. 269. — A median section of the abdominal cavity, showing the relations of
the omental bursa. The diaphragm is cross-hatched. The course of the duodenum-
jejunum and of the colon is shown by means of arrows. (Redrawn after Braus.)

The Lr\ er

The functions of the liver are diverse. During early ontogenesis, it
forms red blood corpuscles. Later in life, it becomes an agent in the
elimination of blood-cells. It transforms both sugar and protein into a
polysaccharid, glycogen, which it stores in its cells for later use. It also
secretes bile, which aids in the emulsification of fats, in the activation of
lipase secreted by the pancreas, and in the stimulation of peristalsis of the
intestine.

THE DIGESTIVE SYSTEM

323

The liver is a reddish brown organ lying between the stomach and the
diaphragm and is the largest gland in the body, weighing between two and
three pounds. It is wedge-shaped, and divided into a smaller left lobe
and a larger right lobe. The two lobes are separated by the falciform
ligament, which is developed from the ventral mesentery and attaches the
Hver to the diaphragm and the ventral body-wall. Two smaller lobes,
the caudate and quadrate. He between the right and left lobes on their
inferior surface. Impressions are made on the inferior surface of the
hver by the pressure of the stomach, colon, kidney, suprarenal, and
duodenum. The gall bladder Hes below the right lobe near the duodenum.
The postcaval vein passes through the right lobe.

POST CAVA VEIN

CUT EDGE OF
PERITONEUM

HEPATIC
ARTERY

LEFT LOBE

QUADRATE LOBE

GALL BLADDER/

Fig. 270. — The human Hver viewed from below.

RIGHT LOBE

BILE DUCT

(Redrawn after Sobotta.)

Secretions pass from each lateral lobe by a single duct, the two uniting
to form the hepatic duct. Nearer the intestine, the hepatic duct joins the
cystic duct from the gall bladder to form the common bile duct or ductus
choledochus, which opens into the duodenum at a point about three or
four inches from the pylorus.

The liver is a compound tubular gland, the tubules of which are
arranged radially around branches of the hepatic vein. Each cluster of
tubules around a central intralobular vein forms a lobule. The numerous
lobules of the liver are bound together by interlobular connective tissue
containing interlobular veins, which are branches of the portal vein,
interlobular ducts carrying bile, and branches of the hepatic artery.
Connexions between intralobular and interlobular veins are effected by
means of intralobular capillaries or sinusoids, which bathe the liver
tubules and supply them with the materials for secreting the bile. The
relations may best be understood by the examination of the diagram

324

COMPARATIVE ANATOMY

(Fig. 272). While branches of the vagus nerve reach the liver, most of its
nerves belong to the sympathetic system.

In most microscopic preparations of the liver the branching tubules
appear as solid cords of epithelial cells. In microscopic preparations
made by the Golgi method, however, the bile capillaries which carry the
secretions of the liver cells are stained black and contrast strongly with
the lightly colored cells. In such preparations, the bile capillaries are
seen to be minute passages between the gland cells leading to the inter-

FiG. 271. — Liver of a pig. The lobules have artificially shrunken from the inter-
lobular tissue, a; b, bile duct; c, hepatic artery; d, interlobular vein (a branch of the
portal); e, trabeculae ; /, central vein. (From Bremer's "Text Book of Histology," after
Radasch.)

lobular ducts and the hepatic ducts, which carry the secretions to the gall
bladder where they are temporarily stored. (Fig. 270)

The gall bladder is a pear-shaped muscular sac between three and four
inches in length, holding about 30 cc. Its inner surface is lined by a
mucous epitheUum which is thrown into folds. Crescentic folds in the
neck of the bladder and in the common bile duct form a sort of spiral
valve. When food enters the duodenum from the stomach, the muscles of
the gall bladder squeeze the bile into the intestine. This action is some-
times impeded by gall stones formed by constituents of the bile such as
cholesterol. Pressure from the intestine caused by constipation may
have the same effect, and result in "bilious" disorders.

THE DIGESTIVE SYSTEM

325

Development of the Liver. The anlage of the liver appears in a
2.5 mm. human embryo as a ventral outpocketing of the foregut near the
anterior intestinal portal, between the two vitelline veins. The liver
diverticulum projects into the ventral mesentery and the mesoderm of
the septum transversvmi which separates the pericardial cavity from the
abdominal cavity. The outgrowth soon becomes differentiated into an
anterior mass of branching tubules surrounded by branches of the vitel-
line veins, and a posterior hollow sac which later becomes the gall bladder.
The multiplication of the tubules correlated with that of the blood capil-
laries associated with them produces the lobules. Mesenchyme cells

fev.w4 ft:-'''

^- —PORTAL VEIN"

LOBULE.

HEPATIC VEIN

B C

and C — diagrams of successive stages in the development of the
The subdivision of the Hver anlage into lobules is correlated with
branching of the portal and hepatic veins. The branches of the hepatic vein are
intralobular, and those of the hepatic portal vein — shown in black — are interlobular.
(Redrawn after Mall.)

Fig. 272. — A, B
lobules of the liver.

form the interlobular connective tissue which binds the lobules together.
Bile capillaries appear within the cell cords, and the blood capillaries
acquire endothelial walls. As a result of this, the lumen of each bile
capillary is separated from that of each blood capillary by a layer of
gland cells and a layer of endotheHal cells. (Figs. 261, 267, 272)

The multiplication of tubular cords and of blood spaces results in a
rapid enlargement of the liver, which begins to bulge out from the septum
transversum and the ventral mesentery and to push into the abdominal
cavity between the septum and the stomach. In this way, the liver
becomes covered by the peritoneum and acquires its two chief lobes.
The ventral mesentery into which it originally grew forms the falciform
ligament. (Fig. 267)

326 COMPARATIVE ANATOMY

History of the Liver. The vertebrate liver has no homolog among
invertebrates, though many of these have organs which are called livers.
That the so-called liver or digestive gland of the urochordates is the
homolog of the vertebrate liver has not been demonstrated. The liver
of amphioxus is generally regarded as representing the beginning of that
of vertebrates. This is a ventral outpocketing of the intestine immedi-
ately behind the pharynx, which grows ventrally and forwards beneath
the pharynx, and remains a hollow sac throughout life. Its relations to
the blood vessels resemble those of the liver of vertebrates.

The liver becomes bilobed in cyclostomes and elasmobranchs, and a
gall bladder is differentiated. In the higher vertebrates and man no
important morphological changes occur. The form, however, varies
with the shape of the abdominal cavity and the pressure of surrounding
organs. In snakes, for example, it becomes reduced again to a single lobe.

The Pancreas

The pancreas is a light pinkish organ about five inches in length,
extending across the abdominal cavity from a loop of the duodenum on
the right side to the left cohc flexure. Three regions are distinguished, a
head lying in the intestinal loop, a body, and a tail. In man, the pan-
creas usually has two functional ducts. One of these, the pancreatic or
Wirsung's duct, generally opens into the common bile duct; the other, the
accessory or Santorini's duct, opens into the duodenum about an inch
above the opening of the bile duct.

The pancreas secretes tr3rpsinogen, which is converted into trypsin
through the action of enterokinase secreted by the intestinal glands.
Trypsin splits proteins into amino-acids. The enzyme amylopsin secreted
by the pancreas splits starch into monosaccharids. Another enzyme,
lipase or steapsin when activated by enterokinase breaks fats into fatty
acids and glycerine. Another enzyme, ereptose or erepsin, spHts pro-
teoses and peptones. The digestive activity of the pancreas is stimulated
through the endocrinal effect of secretions poured into the blood by the
intestinal glands when the acid chjmie enters the intestine from the
stomach.

Besides this digestive function, the pancreas acting as an endocrine
gland regulates the sugar metabolism of the body by means of the endo-
crine insulin.

The histological structure of the pancreas strikingly resembles that of
the parotid gland, both being compound acinous glands divided into lobes
and lobules by connective tissue septa which contain interlobular ducts,
blood vessels, and nerves. The acini of the pancreas, instead of being
hollow, contain central cells. The secretory pancreatic cells of the acini
are wedge-shaped in section and contain zymogen granules largely con-

THE DIGESTIVE SYSTEM

327

centrated near the lumen of the acinus. Secretions pass from the lumina
of the glands into fine intercalated ducts, and from these into secretory
ducts like those of the parotid.

Scattered irregularly among the acini of the pancreas are clusters of
lightly-staining cells. The area of these clusters in section is considerably
greater than that of a single acinus. These are the islands of Langerhans,
endocrinal organs which secrete insulin.

Development of the Pancreas. Like the Hver, the pancreas develops
from the endoderm. It is formed by the fusion of two separate out-
growths of the intestine, a ventral bilobed outpocketing from the bile-
duct, and a dorsal evagination of the intestine slightly anterior to that of
the Uver. By the proliferation of the cells of these anlagen, two pan-
creases are formed, which secondarily unite, but retain usually the two

^-STOMACH

DUCT OF SANTORIN
DORSAL RAiNCREAS

DORSAL PANCREAS

—VENTRAL RfiiNCREAS'
DUODENUM

BILE DUCT

WIRSUNG'S DUCT

A EARLIER STAGE. B. LATER STAGE.

Fig. 273. — A and B, two stages in the development of the pancreas. The duct of
the dorsal pancreas, Santorini's duct, may degenerate in ontogenesis. The two gland
anlagen unite into a single organ in the adult. (Redrawn after Broman.)

primary connexions with the intestine, the ventral becoming Wirsung's
duct and the dorsal Santorini's, the two connecting wathin the body of the
gland. The dorsal pancreas grows much faster than the ventral, and
forms the body and tail of the gland and part of the head. The connec-
tive tissue of the gland comes from mesenchymatous cells which penetrate
between the acini and lobules.

History of the Pancreas. The pancreas seems to be an emergent trait
of vertebrates, since no comparable structure is found in the invertebrates
or even in the lower chordates. In cyclostomes, the pancreatic tissue
remains buried in the substance of the hver or in the wall of the small
intestine. Since no duct appears in these forms, it is assumed that
the pancreas was primarily endocrinal and not digestive. Higher verte-
brates, beginning with the elasmobranchs, have both dorsal and ventral
pancreases.

CHAPTER 9
THE RESPIRATORY SYSTEM

Introduction. Living protoplasm, that is to say a living organism,
burns slowly and continually. When oxidation ceases, life ceases also.
Galen in the second century saw the similarity between respiration and
burning. But it was many centuries before Lavoisier (1771-1780)
proved its chemical nature. Breathing is but a subordinate part of
respiration. Respiration is the process of gaseous exchange which occurs
in a living body through the oxidation of carbon compounds. This
exchange involves an intake of oxygen and an outgo of carbon dioxide.
This process requires uncombined oxygen, which forms one fifth of the air.
Aquatic organisms obtain their oxygen from air dissolved in water.

Two kinds of respiration may be distinguished, external and internal.
In external respiration, animals make use either of a moist skin or of
specialized respiratory organs such as lungs and gills in which blood
capillaries are brought into intimate relation with moist membranes.
Under these conditions, respiration goes on in accordance with the law of
diffusion of gases separated by semipermeable membranes. In internal
respiration, in accordance with the same law, gaseous exchange takes
place within all the tissues of the body which are bathed with blood or
lymph. Cells draw on the oxygen in these just as a burning match gets
its oxygen from the air. The living cell, however, unlike the match, is
the master of the oxidative process and not its servant.

The necessity for two kinds of respiratory organs, one adapted to
aquatic and the other to land and aerial life has produced in chordates two
distinct but possibly not entirely independent respiratory systems to
compHcate evolutionary history. These are the pharyngeal gills of the
lower and the lungs of the higher classes. Chordates have not inherited
their respiratory system from their invertebrate forbears, but have
invented new ones of their own.

Fortunately for the land vertebrates, their fish ancestors were already
prepared for the transition from water to land life before the event occurred.
By a change of function and some modifications of structure and relation,
the bilobed air bladder of the crossopterygian fishes was made to serve as
a lung. Furthermore the advantage of nasal passages in air breathing
was probably already anticipated by the fish ancestors of amphibia.
This assumption seems justified by the fact that some fishes, such as the

328

THE RESPIRATORY SYSTEM 329

Dipnoi, have narial passages. But it is not generally believed that the
Dipnoi are in the direct line of amphibian ancestry.

The story of gills is one of great multiplication in number in forms like
the protochordates which use the pharynx both for obtaining food and for
gaseous exchange. In the fishes and amphibia, however, the gills are
considerably modified, are reduced in number and finally in higher verte-
brates disappear. Startling changes of function occur. Supporting
skeletal elements are converted into a sound-conducting apparatus.
Gill-slits degenerate into blind pharyngeal pouches, which in turn become
endocrinal glands.

The transformation of a ventral air bladder into lungs is sufficiently
well attested to be plausible. The chief evolutionary change which lungs
undergo is an enormous increase of respiratory surface so that, even within
the hmits of the mammalian chest, they expose many square yards of
moist surface for gaseous exchange.

To meet respiratory needs, two sorts of organs have emerged in
animals, branchial organs or gills found in aquatic animals and pulmonary
organs characteristic of land forms.

A. The Branchial System. The fact that lungs are wanting in all
classes of protochordates as well as in the more primitive groups of verte-
brates, proves that the primary respiratory system is that series of paired
pharyngeal gills which form the branchial system of chordates. Rem-
nants of this system persist in all higher vertebrates. The transition
between gilled and lunged forms occurs in the amphibia most of which, at
least at some time in their individual development, have both gills and
lungs and which thus bridge the gap between aquatic and terrestrial life.

Gills, like lungs, function as respiratory organs by bringing a network
of blood capillaries in close contact with moistened membranes through
which gaseous exchange takes place. Their efficiency is increased either
by the activity of cilia which cover the surface of the gills or by the con-
traction of muscles which pump a stream of water through the pharynx,
or by waving the gills to and fro, as in Necturus.

It may be doubted whether we have any adequate explanation of the
substitution of lungs for gills as respiratory organs. The fact that lungs
are much better adapted to the needs of land animals than gills which tend
to dry in air does not explain their origin. It is to be noted however
that in this change of life animals have played safe. Even before they
abandoned the water for a land Hfe, they had acquired an organ, the
air bladder, which would serve as a substitute for gills.

Gills are not the pharyngeal openings through which water passes in
respiration; these are gill sUts or gill clefts. Two sorts may be distin-
guished, internal gills within the body wall and external gills. Those of
most animals are internal; a few fishes and amphibians have external.

330

COMPARATIVE ANATOMY

The gills of elasmobranchs may be taken as typical. They are modifica-
tions of the branchial bars or arches which alternate with the gill-sHts
and serve to keep them open. Each branchial arch consists of an inter-
branchial septum of connective tissue which is covered on the surface of
the body by skin, and which includes near the pharyngeal lining a car-
tilaginous arch as a support. Within the septum are branches of the
dorsal and ventral aortae which supply the gills with blood. The septa
are further supported by skeletal gill-rays extending from skeletal bran-
chial arch laterally towards the skin.

Fig. 274. — Diagram of gill clefts in (A) elasmobranchs and (B) teleosts. A' and
B', a single gill of each, a, artery; ba, branchial arch; br, branchial ray; d, demibranchs ;
gc, atrial chamber; gr, gill raker; o, operculum; oe, esophagus; 00, opercular opening;
s, spiracle, in A', septum; v, veins. (From Kingsley's "Comparative Anatomy of
Vertebrates.")

Each interbranchial septum bears on each surface a half-gill or hemi-
branch, which together constitute a holobranch. Each hemibranch is a
mucous membrane folded into minute parallel lamellae or branchial
filaments, each of which has parallel secondary folds containing a capillary
network. Between the capillaries and separating them are pilaster cells
pecuHar to the gill filaments. In the ganoids and teleosts the inter-
branchial septum becomes reduced and tends to disappear, leaving only
the portion containing the skeletal arch and branchial blood vessels. In
these forms, the gill slits do not open separately to the exterior as in
elasmobranchs but are covered by an operculum formed by the backward
growth of the septum of the hyoid arch. (Fig. 274)

THE RESPIRATORY SYSTEM

331

The mechanism of breathing differs considerably in fishes which, Hke
the elasmobranchs, have modified the first gill shts into spiracles, and
those which have not. In all fishes, through the action of antagonistic
pharyngeal muscles, the cavity of the pharynx is alternately expanded
and contracted, so that water is sucked in through the mouth or the

Fig. 275. — Diagram of the relations nt external and internal gills in the anuran
tadpole, ab, eb, afferent and efferent branchial arteries; h, heart; o, ear cavity; ph,
pharynx; ra, radix aortae. (From Kingsley's "Comparative Anatomy of Vertebrates,"
after Maurer.)

spiracles and forced out through the gill slits. In forms with an opercu-
lum, this functions as a valve, and prevents the entrance of water through
the gill slits. Gaseous exchange takes place through the thin mucous
epithelium which covers the gill lamellae. The gills of fishes function
also as excretory organs, excreting nitrogenous waste as do the kidneys.

PIENCEPHALON
LENS

COELOM

ESOPHAGUS

ECTODERM

SPINAL CORC

PRONEPHROS

NOTOCHORD
MYOTOMES
DORSAL AORTA

PETROMYZON. 16-DAY EMBRYO -FRONTAL SECTION.

Fig. 276. — Frontal (horizontal) section of a 16-day petromyzon embryo, showing
seven pairs of gill-pouches (1-7) formed as lateral diverticula of the pharynx. Slight
invaginations of the ectoderm to meet the gill-pouches are seen. Between the successive
gill-pouches the mesoderm is divided into a series of branchiomeric segments, from
which the muscles and skeletal arches of the gills develop.

External Gills. External gills are of two sorts, external gill filaments
such as occur in elasmobranch embryos as prolongations of the posterior
gill lamellae, and external gills which characterize some adult urodeles
and the larvae of some fishes and amphibians. The evidence on the
whole supports the opinion that they are secondary derivatives of the

332

COMPARATIVE ANATOMY

internal gills developed in adaptation to special conditions. They have
no genetic relation to any human structure.

Development of Gills. Gill sUts develop from a series of paired endo-

dermic diverticula of the pharynx which
meet corresponding invaginations of the
ectoderm. By the disappearance of the
double membrane thus formed the
pouches are converted into gill sUts.
The branchial arches develop from the
regions between the gill sUts. Each arch
has an endodermal pharyngeal lining
and an external ectodermal covering.
The core of each arch is mesodermal.
The difhculty of distinguishing the
boundary between ectoderm and endo-
derm of each branchial arch has re-
sulted in a difference of opinion in
regard to the origin of the gill lamellae.
Some embryologists assert that these
are ectodermal, while others claim that
they are endodermal. Sewertzoff (1916)
distinguishes " endobranchiate " cyclo-
stomes from " ectobranchiate " gnatho-
stomes. The issue does not appear to be
important since Cook (192 1) has found
that the pharyngeal endoderm of fishes
produces such characteristic ectodermal
structures as taste-buds and placoid
scales. Consequently we are forced to
conclude that the contrast in the
potencies of the two germ layers is not as
great as has sometimes been assumed.

The levator and depressor muscles of
the gills are developed from the hypo-
meric mesoderm enclosed in each
branchial arch. The connective tissue,
the cartilage or bone, and the blood
vessels of each arch are derived from the
mesenchyma. The origin of this
mesenchyma has been a subject of controversy, but the evidence favors
the opinion that the mesenchyma of the branchial arches is proliferated
from both mesoderm and ectoderm, not exclusively from the mesoderm as
was once supposed.

Fig. 277. — Pharyngeal region of
a young Squalus embryo, b,
bloodvessels; c, coelomic cavities of
gill arches; g, developing gills; gc,
gill clefts; h, hypophysis; m, mouth;
n, notochord; o, oculomotor nerve;
oe, esophagus; p, peritoneal cavity;
s, spiracular cleft; I— III, first to
third head cavities (I and III reversed
in figure). (From Kingsley's " Com-
parative Anatomy of Vertebrates.")

THE RESPIRATORY SYSTEM

333

History of the Gills. Pharyngeal gills are peculiar to chordates and
are one of the most constant characteristics of the group. This should
not be understood to imply that invertebrates are without structures
from which gills might have evolved. The origin of gills from endodermic
diverticula suggests the possibility that their beginnings may be seen in

NEUROPORE BRAIN
MOUTH t -I.

SPINAL CORD

NOTOCHORD

ENCXX>ERMIC STRAND

A. LARVAL UROCHORDATE.

NEUROPORE MYOTOME I

NOTOCHORD

NEURENTERIC CANAL

M
ENDOSTYLE
CLUB-SHAPED GLAND

B. LARVAL AMPHIOXUS.

ENTERON
STCTRANSIENT) CILL-SUT

ANTERIOR CAVITY
NEUROPORE

NOTOCHORD
BLASTOPORE

NEUROPORE

hfYOTOME. I

NEURAL TUBE

NOTOCHORD

ENDOSTYLE
CLUB-SHAPED GLAND

ENTERON

C. AMPHIOXUS EMBRYO.

ANTERIOR .
CAVITY
ENDOSTYLE'
CLUB-SHAPED GLAND

1ST (TRANSIENT) GILL-SLIT

D. AMPHIOXUS OLDER EMBRYO.
Fig. 278. — While gill-slits occur both in urochordates and in cephalochordates
(Amphioxus), evidence is lacking that the anteriormost gill-slits of the two groups can
behomologized. Those of the urochordate are permanent, while those of amphioxus
do not persist as functional gills. The first pair of pouches in amphioxus become the
endostyle, the second pair form the transient larval club-shaped gland, the left third gill
pouch disappears early in ontogenesis. As shown in D the anterior endodermic diver-
ticula (anterior cavities) resemble gill pouches in relations and in mode of origin.

the intestinal diverticula of flatworms. Were these diverticula to meet
the skin and become perforate, apertures similar to gill sUts would be
formed.

Gill slits first appear in the hemichordates. Rhabdopleura has none,
but cephalodiscus has a single pair. In most hemichordates the number
is considerable and increases throughout Hfe. Early in their development,
their number is doubled by the growth of "tongue-bars" which extend
from the dorsal side of the gill aperture to the ventral side. Later the

334

COMPARATIVE ANATOMY

gill bars thus formed become interconnected by cross rods or synapticulae
such as occur also in urochordates and cephalochordates.

In urochordates, the number of gill slits varies from a single pair in
Appendicularia to the many characteristic of most genera. The gill slits
of this group open into an atrial cavity developed by an ectodermal
ingrowth along the dorsal side of the body.

ANTERIOR CAVITY

MYOTOME 2

CHORDA

ENTERON

MOUT
ENDOSTYLE

LEFT 1ST (TRANSIENT)GILL-POUCH
CLUB-SHAPED GLAND

A. EARLIER LARVA.

ENDOSTYLE

RIGHT 1ST PERMANENT GILL-SLIT

PREORAL PIT

CLUB-SHAPED GLAND
TRANSIENT GILL-SLIT

LE FT 1ST PERMANENT GILL-SLIT
MEDIAN VENTRAL BLOOD-VESSEL

B.LATER LARVA.

Fig. 279. — A, young amphioxus larva viewed from the left side as a translucent
object redrawn after Hatschek. B, later larva. (Redrawn after van Wijhe.) The
mouth of Amphioxus becomes enormously enlarged and, by its growth backward on the
left side interferes with the symmetry of development of the gill-slits. The gill-slits
of the left side of the body develop before those of the right side. The median line of
the ventral side of the pharynx is indicated by the median ventral blood-vessel. Modifi-
cation of function and degeneration affect the anterior three pairs of gill-slits. The
first pair become the endostyle. The second pair form the transient larval club-shaped
gland. The left third slit has no mate and soon disappears. The fourth pair of slits
form the first permanent pair.

Amphioxus, the typical genus of cephalochordates, has as many as
one hundred and eighty paired openings or stigmata. As in hemi-
chordates, the number is doubled by the growth of secondary gill slits.
Before metamorphosis the number of primary gill slits in the larva of
amphioxus is nineteen pairs. The large number of gill slits in the proto-
chordates is apparently an adaptation, since these organisms use their
gills not only for respiration but also as a mechanism for obtaining food
by cihary action. The multipUcation of branchial bars means a corre-
sponding increase in the number and efficiency of the cilia which cover

THE RESPIRATORY SYSTEM

335

their surface. The fact that the peribranchial cavity of amphioxus
develops from the ventral side of the body, makes it seem impossible to
compare this with the atrium of the urochordates which arises on the
dorsal side. See p. 66i, Fig. 535.

The history of gills in vertebrates is one of continuous reduction in
number and modification in function. The transformation of their
skeletal supports has been described in the history of the skeletal system.
Even in amphioxus, some of the gill pouches of the embryo are modified
or lost. The first pair become the endostyle while the second pair form
the larval club-shaped gland. The third left slit never has a correspond-
ing right sUt and disappears early in ontogenesis. The first permanent
gill sUt of amphioxus is therefore the fourth of the ontogenetic series.

ENDOSTYLE
_3RD RWRED
/GILL POUOCS

5— VENT. AORTA

(s) ^^

AJONGUE-BAR

S*->CILL POUCHES CILL SUTS'

C. D

Fig. 280. — Four stages in the development of the endostyle of amphioxus. The
evidence indicates that the endostyle is developed from a pair of modified gill-pouches.
The evidence is equally strong that the endostyle is the homolog of the thyroid gland
of vertebrates. The conclusion is drawn that the thyroid has evolved from a pair of
gill-pouches. There is no evidence, however, that endostyle and thyroid are functionally
similar. (Redrawn after van Wijhe.)

There are cogent reasons for homologizing this slit with the spiracle of
elasmobranchs, but there is Uttle agreement among morphologists in
regard to the exact homology of serial organs in chordates.

The popular belief among morphologists that the vertebrate mouth
has been formed by the coalescence of a pair of gill slits is supported by
the mode of development of the mouth in amphioxus. The endoderm
takes the initiative in the development of the mouth of amphioxus, as
would be the case if it were a gill slit. In this respect, the mouth of
amphioxus differs from that of vertebrates, in which the ectoderm initiates
development. Later the mouth of amphioxus assumes a left-sided posi-
tion and becomes enormously enlarged. Partly from the fact of its
left-sided position, van Wijhe (1893) homologized the mouth of amphioxus
not with that of vertebrates, but with the left spiracle of elasmobranchs.
Other morphologists have not accepted this homology, but are not wholly
agreed that the mouth of amphioxus is homologous with that of verte-
brates. At metamorphosis, the mouth of amphioxus assumes a position

?j;6 COMPARATIVE ANATOMY

in the center of the velum and becomes greatly reduced in size. The asym-
metr". ' - ' :he hyjK^rtrv>phy aj>j>ear to be larval adaptations,

I - 'XT or not gilb are metameric structures has l>een

an open one. rhe metamerism of chordates b manifestetl primarily in
the r V .il s^>mites, Sim-e there are none of these in hemichordates
and . ues. it is impcT>ssible to demonstrate in these forms a v-or-

respondence between mesomerisana and branchiomerism. and thus to
establish the metamerism of the latter. The case is different. howe\-er,
in amphioxus, where the mesodermal segmentation is one of the most
striking features. In the adult animal, there is no correspondence
between gills and myotomes. But in the larva, the gill slits not only
take an internvetameric position in relation to the myv>tomes, but also are

"^

aiwJ ,\mTOocoet«*. and (If" - ^ c»->j»h8vsvis;

«, thyroid gUuad. (.From Kxacsicy s Cocnporanvv Anauwoy ot V«wtorat«s.")

innervated by metameric nerves, .\ similar metameric correspondence is
s ' ' \^stomes. The conclusion drawn

> corresjxmd.

rhe number oi gills varies greatly in different c>xlostomes. In the

' * " - ., the numt>er ranges fK>m fourteen to six pairs. The

- and petrv>myzon is res^^ev lively si\ and seven, or one

more counting the spiracular pouch which does iK>t become perforate.

By the Ivickwan^i grv)wth of the hyoid septum, the external apertures in

myxine l>ev\>mes reduced to a single pair, a condition not unlike that in

bony fishes. The posterior displacement of the gills in bdellostoma is an

' - i>arasitic habit oi this animal, which burrows with its

: - - of its host.

Among elasmobranchs, Heptanchus has seven pairs of gill slits in
.^ ' ' ^ \s, which are evidently mw' " ' : ' slits since they

:>ranchs, Hexanchus an*.: .i have six pairs

of gill shts. Most eiasmobraiKhs ha\-e nve pairs of gili slits plus spiracles,
In bony fishes the number is reduced to four pairs and the spiracle is
absent.

Gill slits disappear in adult tailless amphibia, but are present in
some aquatic urodeles. The number however is reduced. Some adult
urvxleles have three pairs oi gill slits, some two. and some only one.
In the newts they disapj>ear entirely. Cutaneous respiration b common

THE RESPIRATORY SYSTEM 337

in the group, and some respire by means of a highly vascular pharynx.
Nevertheless, even in those adult forms which are devoid of functional
gills, gill pouches occur in the embryo, and the embryos of gymnophiona
may have as many as six such pouches, suggesting a corresponding number
of functional gills in their ancestors. Most amphibian larvae have
functional gills.

Functional gills are lacking in amniotes, but rudiments of gills are
represented by transient embryonic gill pouches and their intermediate
visceral arches. In the embryos of reptiles, some of the gill slits usually
become perforate and later close. The perforation of gill slits in mam-
mals is aijnormal. Pharyngeal pouches are, however, always formed in
the human embryo, and when these become perforate listulae in the throat,
they may persist and require surgical treatment. The presence of five
pharyngeal pouches and six visceral arches alternating with them receives
its only reasonable interpretation in the evolution theory.

As has already been explained, the disappearance of the visceral
arches in man and mammals is incomplete. The skeletal elements are
converted into ear bones, attachment for the tongue, and support of the
larynx. Three of the aortic arches also persist, as will be shown in the
next chapter. Moreover, in addition to these rudiments th^re are
certain derivatives of the gill pouches which require special discussion.

Pharyngeal Derivatives. From the epithelial lining of the embryonic
pharyngeal i)ouches arise some of the important endocrinal glands,
thyroid, parathyroid, thymus, and the ultimobranchial bodies. In
addition to these which occur in man, some vertebrates have also epithelial
bodies and suprapericardial bodies. From the second pair of {pharyngeal
pouches come the palatine tonsils. The history and development of these
pharyngeal derivatives will be taken up in the chapter on endocrinal
organs.

B. The Puhnonary System. The respiratory system of man and
mammals includes lungs, larynx, trachea, bronchial tubes, narial passages,
and diaphragm.

Lungs. Lungs are the essential respiratory organs of land vertebrates.
Man, like virtually all land animals except snakes, has two, the left
having two lobes and the right three. The absence of a middle lobe on
the left side appears to be an adaptation to the presence of a left-sided
aorta. The lungs lie within the rib basket, and when expanded obliterate
the potential pleural cavities. They are separated from one another
by the mediastinum or interpleural space, which contains the heart,
esophagus, and the great blood vessels which leave the heart. In child-
hood, the color of the lungs is pinkish, but becomes slaty grey in the adult
as the result of the accumulation of soot. Each lung is conical with an
apex and base. However, two surfaces, costal and mediastinal, and three

33^

COMPARATIVE ANATOMY

borders are distinguished. The heart and pericardial cavity produce a
deep concavity on the mediastinal surface of each lung, while the costal
surface is convex in conformity with the inner surface of the chest. The
base of each lung is concave to correspond with the diaphragm with
which it is in contact.

The structure of the lungs is admirably adapted to the need of exposing
to the air a large amount of surface, estimated to equal that of a balloon

HYPOPHYSIS

■MOUTH

-,POUCH 1 ,

i (EUSTACHIAN TUBE)

-THYROID GLAND

-PALATINE TONSIL

- POUCH 2

ijfc-PARATHYROID 1
K"~~POUCH 3
m-THYMUS 1
JBF-PARATHYROID 2

a ^"THYMUS 2

POSTBRANCHIAL BODY

LUNG LOBE

ESOPHAGUS

Fig. 282. — Ventral view of pharyngeal region of a human embryo showing the pharyngeal
pouches and their glandular derivatives — semidiagrammatic.

ten feet in diameter, and a section of the lungs shows that the volume of
air space greatly exceeds that of soUd tissue. The required moisture is
supplied by mucous glands.

The trachea or wind-pipe subdivides into bronchi, both structures
having cartilaginous supports. The bronchi divide into bronchioU,
the bronchioU into alveolar ducts, the alveolar ducts into atria, alveolar
sacs, and alveoli, which form the ultimate subdivisions. Exchange of
gases occurs chiefly in the alveoU, although the thin respiratory epithelium
is found also in the atria and alveolar sacs and mav extend even into the

POUCHY-
POUCH 5""^
TRACHEA-

THE RESPIRATORY SYSTEM

339

bronchioli, which in general are lined with a simple cuboidal non-respira-
tory epithelium. There is an elaborate network of capillaries in the
walls of the alveoU, so that only two extremely thin membranes separate
the blood in the capillaries from the air in the alveoU. J. S. Haldane has

ALVEOLAR SAC

Fig. 283. — Diagram of a lung lobule showing the subdivision of a bronchiolus into
alveolar ducts, sacs and alveoli. Respiratory epithelium may extend into the bronchioli.
(Redrawn after Bremer.)

shown that during this exchange a constant quantity of carbon dioxide
is maintained in the alveoli and that the rhythm of breathing is dependent
upon this factor.

Lungs are very elastic, and their elasticity is increased by the smooth
muscle fibers which extend into the connective tissue of the lungs as far
as the alveolar sacs but not into the walls of the alveoli.

340

COMPARATIVE ANATOMY

The respiratory blood vessels of the lung are branches of the pulmonary
arteries and veins. The bronchial artery and vein supply the connective
tissues of the lungs. The innervation of the lung is through branches
of the vagus and of the sympathetic.

On the outside of the lung the pleura, corresponding to the peritoneal
lining of the abdominal cavity, consists of a subserous connective tissue

Epithelium.

Tunica Circular

propria. muscle fibers, i.

■:^<w:-i-f*s!».-..,. 4 X, ^^

s-^^M...ill^%^

— AlveolL

Fat cells.

Cartilage.

Connective tissue.
Bronchial gland.

Duct of gland.

Fig. 284. — Cross section of a bronchus 2 mm. in diameter, from a child. (From
Bremer's "Text-book of Histology.")

which extends into the lobules of the lung, and of an external epithelial
serosa. The pulmonary pleura is reflected back on the inside of the
chest as the parietal pleura.

Larynx. The larynx or voice-box hes between the root of the tongue
and the trachea, and opens into the pharynx by the glottis. Nine carti-
lages support it, the unpaired epiglottic, thyroid, and cricoid cartilages,
and the paired arytenoids, corniculate, and cuneiform cartilages. Small
paired triticeous cartilages also sometimes are found. Numerous muscles
are attached, some extrinsic and some intrinsic. The extrinsic muscles
are chiefly to lift the larynx in swallowing. Among the intrinsic

THE RESPIRATORY SYSTEM

341

muscles are the thyro-arytenoid or vocalis and the cricothyroid, which
affect the pitch of the voice. At puberty in the male, the larynx becomes
enlarged and the vocal cords within it elongated so that the voice is
deepened. The epiglottis and vocal cords are covered with the same
kind of squamous stratified epithelium as that which Hues the pharynx,
but the rest of the larynx is fined with cifiated columnar epithelium similar
to that of the trachea. The action of the cilia is such as to carry the
secretions of the mucous glands of the lungs together with particles of

PARIETAL BONE

GYRUS CINGULI

PITUITARY,

SUBDURAL CAVITY

FRONTAL LOBE

FRONTAL BONE

FRONTA
SINUS

^RICHT CEREBRAL HEMISPHERE

iRPUS CALLOSUM

•FORNIX

PINEAL GLAND

CORPORA
QUADRIGEMINA

NASAL BONE
SPHENOID BONE
NASAL CONCHAE
EUSTACHIAN TUBE
MAXILLA
MOUTH CAVITY
PALATINE BONE
VESTIBULE
SOFT PALATE
M. GENIOGLOSSUS
MANDIBLE
M. GENIOHYOID

M. MYLOHYOID

.OCCIPITAL
LOBE

dura kmter
-occipital bone
:erebelljum

SEMISPINALIS
CERVICIS

esophagus' '^:ENTRA

Fig. 285. — A median longitudinal section of the human head showing the relations
between digestive and respiratory passages in the pharyngeal region. (Redrawn
after Braus.)

dust out into the pharynx. Mucous glands are numerous. The nerve
supply is from the vagus and the sympathetic.

Trachea and Bronchi. The trachea or windpipe is a membranous
tube, four to five inches long, supported by fibrous connective tissue and
incomplete U-shaped rings of cartilage. It carries air to and from the
lungs. The cartilages vary in number from sixteen to twenty and are
incomplete on the side next to the esophagus. The trachea divides to
form the right and left bronchi. The lining of the trachea is a mucous
ciUated stratified columnar epitheUum. Below this is a submucous
connective tissue containing many mucous glands derived from the

342

COMPARATIVE ANATOMY

mucous layer. Between the cartilage and the mucosa is a layer of
circular muscle fibers.

Nasal Passages. Air is taken in and expired through the nasal
passages. The external orifices are the external nares and the openings
into the pharynx are the choanae. The paired nasal passages are sepa-
rated from one another by the nasal septum and the median plates of the
maxillary and vomer bones, and from the cavity of the mouth by maxillary
and palatine bones. They are lined with a ciHated columnar epithelium
containing many mucus-secreting goblet cells.

Fig. 286. — Stages in the development of the trachea, bronchi and lungs in the pig.
The pulmonary arteries are shown in black; the veins are cross hatched. Ep, bud of
eparterial bronchus. (From Patten's "Embryology of the Pig," after Flint.)

Diaphragm. Air is drawn into the lungs under atmospheric pressure
as the result of the contraction of the muscles of the diaphragm and ribs.
Their contraction raises the rib-basket and flattens the dome-shaped dia-
phragm. As a result, the size of the pleuroperitoneal cavity is increased.
To fill the enlarged space thus formed air enters the lungs and inflates
them to the size of the chest cavity. The diaphragm is a muscular
partition which divides the cavity of the chest from that of the abdomen
and which occurs only in man and other mammals. Lacking a diaphragm
the amphibia must swallow their air. The phrenic nerve, a branch of
the cervical plexus of nerves, innervates the diaphragm.

Development of the Ltmgs. During the fourth week of development
a laryngo-tracheal groove is formed in the floor of the pharynx immediately
behind the fourth gill-pouch. Externally this groove appears as a ridge
which is bordered on either side by a groove or furrow. By the approxima-
tion of these paired lateral grooves and their union in the median plane,
the lung anlage is separated from the pharynx, except anteriorly where
cormexion with the pharynx is retained. The posterior blind end of the

THE RESPIRATORY SYSTEM

343

diverticulum swells to form the lung anlage while the less expanded
anterior portion becomes the larynx and trachea. The lung anlage later
divides into two lateral buds which, by successive subdivision, gradually
assume the adult structure. (Fig. 286)

The cartilages which support the larynx correspond exactly with
those which in aquatic vertebrates support the fourth and fifth branchial
arches. The muscles of these arches form the laryngeal muscles. Vocal
cords appear during the eleventh week.

Beginning with the fifth week, the paired lung-buds branch in the
manner of a compound tubular gland. In this way, the entire lining of

MOUTH INVAGINATION
lU. POUCHES

TRACHEA
BRONCHUS-

ANLASE OF UUNG
PHARYNX

PRONEPHROS

Fig. 287. — A-D Stages in the development of lungs in vertebrates. A is a hori-
zontal section of a salamander embryo showing the series of paired pouches which form
the gill slits after Goette. The last pair of pharyngeal pouches are the anlagen of the
lungs. Such evidence suggests that lungs may have arisen in phylogenesis from a pair
of gill pouches which failed to reach the surface. B and C are earlier and later stages
in the development of the lungs in an amphibian. D is a. cross section of the lung anlage
in a reptile, after Wiedersheim. (Redrawn from Ihle.)

the lungs is derived from the pharyngeal endoderm. The connective
tissue develops from the surrounding mesenchyma. The splanchnic
mesoderm forms the serosa, which covers the lungs and Hues the chest
cavity. As the two lungs enlarge, they push laterally into the body cavity
and by their ventral extension nearly surround the heart, from which they
remain separated by the pericardium. With the development of the
diaphragm, the pleural cavity containing the lungs becomes separated
from the more posterior peritoneal cavity. According to Broman the
diaphragm arises from four sources, the septum transversum anterior to
the liver, the pleuroperitoneal membranes, the body-wall, and the dorsal
mesentery.

The nasal passages of lower vertebrates, such as the dipnoi and
amphibia, develop from nasobuccal grooves similar to those seen in some
adult elasmobranchs. In the embryo an ectodermal groove extends from

344

COMPARATIVE ANATOMY

each olfactory pit to the corner of the mouth. Later the groove deepens,
its edges meet and fuse together and convert the groove into a tubular
passage which connects the pit with the mouth cavity (Fig. 471).

The development of the narial passage in the human embryo is
slightly different. In the month-old embryo a similar nasobuccal groove
makes its appearance. The narial passage, however, is not formed by

the closure of this groove, but by the
backward extension of the epithelium
of the olfactory pit, which thus
acquires a secondary connexion with
the mouth (Fig. 473).

That this method of formation
of the narial passage is a modification
of the method found in lower verte-
brates is indicated by the not infre-
quent occurrence of hare-lip in
infants. Hare-lip is best explained
as a reversion to the more primitive
mode of development of the narial
passage from a groove. The hare-
lip arises when the groove fails to
close over. In some infants the
defect in growth is so extensive as to
cause a perforate or "cleft" palate
and to necessitate surgical operation.
History of the Pulmonary
System. Invertebrates have no
organs comparable with the human
pulmonary system, the so-called
lungs of pulmonate molluscs being
modifications of the mantle and not
outgrowths from the aUmentary
canal. Opinions are divided as to
the origin of the lungs. According to some, a pair of gill pouches which
failed to reach the skin have been converted into lungs. Others suppose
that lungs have evolved from the air bladder of fishes. Some seek to
reconcile these two divergent opinions by asserting that the air bladder is
itself derived from a pair of modified gill pouches.

Goette (1875) was the first to suggest that lungs are modified gill
pouches, on the ground that in some amphibian embryos the lungs develop
from a pair of posterior endodermal pouches in series with the gill pouches.
A number of observers have confirmed this observation and reached the
same conclusion. In support of Goette's hypothesis is the fact that the
pulmonary arteries develop from the sixth pair of aortic arches. Further-

FiG. 288. — Diagrams of air bladder
in fishes. A, Physostomous fishes; B,
Lepidosteus and Amia; C, Erythrinus;
D, Ceratodus. The air bladder of the
Crossopterygian fish, Polypterus, is, like
the lungs of amphibians, bilobed and con-
nected with the floor of the pharynx.
(From Kingsley's "Comparative Anat-
omy of Vertebrates," after Dean.)

THE RESPIRATORY SYSTEM

345

more, it is obvious that if a gill pouch were to fail to reach the skin and
were to grow backwards into the body cavity it would assume the relations
of a lung.

On the other hand, supporters of the air bladder hypothesis emphasize
the fact that the air bladder of such a fish as the Nile bichir (Polypterus)
develops like the lung as a median ventral outgrowth of the pharynx.

Fig. 289. — Diagrams of stages in the phylogenesis of the lungs. The respiratory
surfaces are stippled, and conductory passages cross-hatched. Embryological stages
corresponding with the comparative anatomical series shown in A-E occur in the
ontogenesis of lungs in mammals. (Redrawn after Huntington.)

Its bilobed adult form is secondary, as is also its vascular connexion with
the sixth aortic arch. Basing the homology of air bladder and lung upon
their similar development as median ventral outgrowths from the pharynx,
the supporters of this view are skeptical of the attempt to compare a.
median organ with paired structures such as gill pouches.

To meet this difficulty, it may be pointed out that the transformation
of a paired organ into a median one is not unknown. For example the
thyroid gland in all vertebrates develops as a median ventral outpocketing
of the pharynx, yet all morphologists agree in homologizing the thyroid
with the endostyle of amphioxus. The endostyle, however, in amphioxus
develops from a pair of gill pouches. Likewise it is sometimes assumed
that the median and unpaired pineal organ of the brain arose by the
union of paired diverticula of the brain.

346

COMPARATIVE ANATOMY

While some uncertainty remains in regard to the origin of the lungs,
the facts on the whole seem to accord with the gill pouch hypothesis.
If it is assumed that the crossopterygian air bladder is a pair of modified
gill pouches, the rest of the problem of the history of the lungs is easily

Fig. 290. — Air sacs and canals of pigeon, c^—c^, intertransverse canals; da^-da-,
axillary sac and its ventral diverticulum; dc, canal for ribs; dot, infraclavicular canal;
ds, subscapular sac; dsl, sternal canal; pc, preacetabular canal; sad. sas, right and left
abdominal sacs; sc, cervical sac; sia, sip, anterior and posterior intermediate sacs.
(From Kingsley's "Comparative Anatomy of Vertebrates," after Bruno Miiller.)

solved, since there are among living vertebrates all intergradations in
complexity between the simple air bladder of polypterus and the mam-
malian lung. The evolutionary changes which occur involve chiefly a
great increase in the lung surface effected through the branching and
subdivision of the primary lobes. The facts of embryology and com-
parative anatomy are in complete agreement. An evolutionary series
based upon evidence from comparative anatomy is shown in Fig. 289.

CHAPTER lo

THE VASCULAR SYSTEM

The Vascular System. While the blood of many invertebrates fills
intercellular spaces without specialized walls, the circulation in verte-
brates is a closed system, the essential components of which are a circulat-
ing fluid, a heart with receiving and propulsive chambers and valves so
arranged as to permit the blood to flow in one direction only, arteries to
carry blood away from the heart, veins to bring blood back again, and
microscopic capillaries to connect arteries and veins. The walls of the
capillaries are so thin that they permit passage of plasma from the blood

DORSAL AORTA
PRECARDINAL V | POSTCARDINAL V MESENTERIC

CAUDAL ARTERV

CAUDAL VEIN

MOUTH VENTRAL AORTA I L.COMMON CARDINAL V. 1 SUBINTESTINALV.

AORTIC ARCH 6 ABDOMINAL V

Fig. 291. — Diagram of the primitive chordate circulation. The arteries are shown
in black, the veins are stippled. The similarity of this circulatory system to that of
annelids has suggested to morphologists a common genetic origin. (After Kingsley
modified.)

into the tissues. This fluid in the form of lymph is restored again to the
veins by way of special vessels, the lymphatics, which like veins, permit
flow in only one direction.

It was William Harvey (1616) who first demonstrated the circulation
of the blood. Before Harvey's day it had been assumed that the blood
ebbs and flows in the arteries and veins Hke water in tidal streams. Har-
vey was able to demonstrate that the valves in the heart and blood vessels
permit a one-way movement only; that a cut artery spurts blood from the
cut end nearer the heart, while a cut vein bleeds most from the end farther
from the heart; that pressure of a finger on a vein results in distension on
the side farther from the heart; and that in a dead body Uquid injected into
an artery will return to the heart by a vein, while the Hquid injected into a
vein wiU not return to the heart by an artery. Later, following the inven-
tion of the microscope, Malpighi (1661) discovered the interconnexion of
arteries and veins by way of the capillaries.

Blood. The human body contains about one gallon of blood, one
twentieth of the weight of the body. Blood has two constituents, a fluid

347

348 COMPARATIVE ANATOMY

plasma and blood corpuscles which are cells. Blood is, therefore, regarded
as a tissue composed of cells (corpuscles) and of a liquid intercellular mate-
rial. The plasma consists of a liquid serum and a coagulable material,
fibrinogen. By stirring blood it is possible to separate these two elements.
Blood corpuscles are of two sorts, red er3rthroc3rtes and white leucocytes.
To every cubic millimeter of blood there are from four and one-half to five
million erythrocytes, and from five to seven thousand white corpuscles.
Variations from this proportion are of diagnostic value in disease.

Functions of the Blood. The blood vessels and blood constitute an
organic transportation system. Blood is a common carrier of foods and
wastes to and from all parts of the body. Among its numerous functions
are equalizing the temperature of the body, regulating the water content

Fig. 292. — Diagram of the circulation in an early stage of a small yolked vertebrate
(amphibian), a, anus; ca, cv, caudal artery and vein; da, dorsal aorta; dc, Cuvierian
duct; ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal
carotid; ij, inferior jugular; j, superior jugular; /, liver; m, mouth; oma, omv, omphalo-
mesenteric artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic
arches. (From Kingsley's "Comparative Anatomy of Vertebrates.")

of the various tissues, distributing endocrines and thus assisting in the
integration of the body. The red corpuscles carry oxygen, the white act
as scavengers of the blood. The color of red corpuscles is due to hemo-
globin, a nitrogenous substance with an affinity for oxygen. Most, if not
all, white corpuscles are amoeboid, and like amoebae surround and engulf
bacteria. Foods and wastes are carried in the plasma and not by the
corpuscles. The plasma also contains mineral salts of the same sorts and
in nearly the same proportions as those of somewhat dilute sea water.
Among other constituents of the blood are enzymes, antibodies, antitoxins,
antithrombin, etc. Antibodies are substances produced in the tissues in
response to the poisons caused by bacteria. They are regulatory in their
action and help to preserve the normal chemical balance of the blood.
Antithrombin is a substance, normally present in blood, which prevents the
clotting of the blood by preventing the action of thrombin on fibrinogen.
Evolution of the Blood Vessels. Two types of circulation may be dis-
tinguished in animals, intracellular and intercellular. The former occurs
in all cells alike as foods enter the cell and are distributed to the various
cell-organs and as the cell wastes are excreted. Intracellular circulation is
similar in protozoa and metazoa. A true vascular system, however, is
multicellular and therefore limited to the metazoa.

THE VASCULAR SYSTEM

349

The simpler metazoa, such as the sponges and coelenterates, are devoid
of a vascular system. In an animal such as hydra in which the body-wall
forms the aUmentary canal, a vascular system is unnecessary. With only
two layers of cells in the body-wall, the diffusion of food stuffs from the
digestive cavity into the cells may take place by osmosis. The excretion
of wastes is likewise direct and requires no special system of transportation.

A circulatory system is necessary and is present in all animals in which
the body-wall is separated from the lining of the aUmentary canal either
by a mass of mesoglea or by a body-cavity. In other words, the emer-
gence of a circulatory system in animals is conditioned by increase in size of
body and of mass of tissue, as well as by the separation of the body-wall

Fig. 293. — Diagram of embryonic circulation in a large-yolked vertebrate, aa,
aortic arches; al, allantois; an, anus; ca, cv, caudal artery and vein; da, dorsal aorta;
dc, Cuvierian duct; h, heart; ha, hypogastric (allantoic) artery; j, jugular vein; I, liver;
oma, om, omphalomesenteric artery and vein; pc, postcardinal vein; si, subintestinal
vein; st, sinus terminalis; va, ventral aorta; y, yolk; ys, yolk stalk. (From Kingsley's
"Comparative Anatomy of Vertebrates.")

from the alimentary canal by a coelom. Stages in the evolution of blood
vessels are represented in living invertebrates.

Metazoa have two kinds of vascular systems, an open lacunar system
such as occurs in most invertebrates and a closed system like that of
vertebrates. The facts support the assumption that the lacunar system
is the more primitive. A lacunar system is well represented in flatworms,
in which a fluid plasma fills the spaces between loose mesenchymatous
cells. No heart is present and no true circulation occurs. The contraction
of the muscles of the body wall and the movements of the worm bring
about more or less irregular currents in the plasma. In many flatworms,
numerous diverticula of the intestine bring digested food near most parts
of the body so that a vascular system is unnecessary. The beginnings of
blood vessels, however, make their appearance in nemerteans which are
sometimes classified with flatworms. Nemerteans have in addition to
lacunar spaces in the mesenchyma three longitudinal blood vessels, two
lateral and one dorsal. Interconnexions between these vessels occur at
the anterior end of the worm. The fluid contained in these vessels is a

350

COMPARATIVE ANATOMY

sort of lymph, without blood corpuscles and without hemoglobin. It may
he assumed that the walls of these vessels are formed directly from the
surrounding mesenchyma and that the vessels therefore are evolved from
lacunar spaces. In the nematodes, a pseudocoelom provides an adequate
mechanism of circulation in animals which have no thick masses of tissue
to nourish.

Most of the invertebrate phyla above the nematodes have composite
circulatory systems, partly lacunar and partly closed. Before Malpighi
(1661) discovered the capillary circulation in vertebrates and thereby
demonstrated in them a closed circulation, it was assumed that the verte-
brate circulatory system was likewise partly lacunar and partly closed.
That the lacunar system of invertebrates is comparable with the lymphatic
system of vertebrates has been more recently suggested. Such a sug-
gestion however is obviously based upon erroneous assumptions. The
facts accord better with the assumption that both blood-vascular and
lymphatic systems have had a common origin from the primitive lacunar
systems of invertebrates. While in the invertebrates the circulatory
system has remained partly open, in the vertebrates on the other hand the
circulatory system, both blood-vascular and lymphatic, has become
wholly closed.

The vascular system of annelids is fundamentally like that of chordates.
In both groups occur two main longitudinal vascular trunks, one above
and one below the alimentary canal, connected with one another by aortic
arches around the pharynx. In the earthworm, there are five pairs of
aortic arches. It is true that the direction of flow of blood in annelids is
the reverse of that in vertebrates. But this difference in blood flow is
nullified if the dorsal and ventral sides of the worm are reversed. Reversal
of the annelid is also necessary in order to bring the central nervous sys-
tems of annelids and chordates into a similar position, dorsal to the aU-
mentary canal. It is therefore not surprising that proponents of the
annelid hypothesis of vertebrate ancestry have stressed the similarity of
the vascular systems of the two groups as a strong support of their views.
The absence of a heart in anneUds and its presence in vertebrates is not a
serious objection to this view, since the dorsal blood vessel of annelids is
contractile throughout its length and it may be reasonably assumed that
this contractile function is concentrated and localized in the vertebrate
heart.

Such a diagram of the hypothetical primitive vertebrate blood-vascular
system as is shown in Fig. 291 is based, however, not on the assumption
of an anneUd ancestry of chordates but upon evidence from comparative
anatomy and embryology. While the circulation of blood in the ancestral
chordate was probably due, as in annelids, to the contractility of the walls
of the blood vessels, a contractile heart such as is found in all vertebrates

THE VASCULAR SYSTEM 35^

is added to the diagram. Blood is pumped by the heart towards the head
through a median ventral truncus arteriosus from which pass the series of
aortic arches, which connect around the pharynx with the median dorsal
aorta. In lishes and in amphioxus, the aortic arches are divided by a
network of capillaries in the gills into ventral afferent and dorsal efferent
arteries. The carotid arteries carry blood forward to the head while the
dorsal aorta carries it posteriorly to the trunk and tail, giving off metameric
intersegmental arteries to the body-wall and median unpaired splanchnic
vessels to the alimentary canal. In the tail, venous blood is carried
towards the heart by the caudal vein. In the primitive circulation, the
caudal vein is assumed to be connected by intersegmental vessels with the
caudal artery. When the caudal vein reaches the region of the anus it
encircles the aUmentary canal. From this point blood may return to the
heart either by a subintestinal vein (which also collects blood from the
intestine) or by an abdominal vein which extends along the median ventral
body-wall. These two vessels parallel one another and both connect
anteriorly with the heart. Venous blood from the body-wall is returned
to the heart by the cardinal veins, anterior and posterior.

Amphioxus. In most details the vascular system of amphioxus
resembles the hypothetical ancestral system just described. But amphi-
oxus is heartless, and the circulation of its blood is dependent upon the
contractility of the walls of its main blood trunks. Blood is carried for-
ward beneath the pharynx by a contractile ventral blood vessel, the
truncus arteriosus, and distributed to the numerous gills by a series of
paired afferent vessels, the aortic arches. Contractile enlargements or
bulbils of these vessels aid in the propulsion of blood to the gills. Neph-
ridia similar to those of annelids are associated with the gills and pre-
sumably assist in the^ elimination of nitrogenous wastes. From the
efferent vessels the blood passes to the paired dorsal aortae, the anterior
extensions of which correspond to the internal carotid arteries of verte-
brates. Posterior to the gills the paired aortae unite to form the median
dorsal aorta of the trunk region. From the dorsal aorta paired inter-
segmental vessels are given off to the body-wall and a series of median
unpaired vessels to the alimentary canal. In the tail region the caudal
artery has intersegmental connexions with the caudal vein. Beginning
at the anal region blood from the caudal vein may be returned to the heart
either through the right postcardinal vein or by the subintestinal vein.
Like the hepatic portal vein of vertebrates, the subintestinal vein of amphi-
oxus breaks up in capillaries within the Uver. Anteriorly the hepatic
capillaries unite to form the hepatic vein which carries blood to the sinus
venosus and the truncus arteriosus. Amphioxus has no renal portal
system such as occurs in fishes and amphibians. Venous blood is brought
from the anterior part of the body by the precardinal veins and from the

352

COMPARATIVE ANATOMY

posterior body-wall by the postcardinal veins. The two unite with the
sinus venosus by means of paired common cardinal veins or ductus

PRECARDINAL V| 0O'>S»- AORTA| .COMMON CARDINAL V posrCARDINAL V

l^^■ lUAC V

SUP VESICAL A.

HEPATIC V

CORONARY A'
HEPATIC PORTAL

DUODENAL a:

Pig. 294. — Diagrams of the circulatory system of Amphioxus, Squalus, Necturus,
and Felis. Arteries black, veins stippled. The vessels are shown as if projected upon
the median plane. Arrows indicate the direction of flow of blood. A-D are believed
to represent an evolutionary series. In the mammal three aortic arches persist, viz.,
carotid (3), aorta (4), and pulmonary (6 in part). (Redrawn after Wierstratz, Woodruff,
and Kingsley. Felis original.)

Cuvieri. The vascular system of amphioxus thus resembles that of cyclos-
tomes and elasmobranchs. The blood however lacks hemoglobin and is
colorless. Blood cells are scarce. (Fig. 294, A)

THE VASCULAR SYSTEM

353

Cyclostomes. The vascular system of cyclostomes shows little
advance above that of amphioxus. An S-shaped heart with three cham-
bers, sinus venosus, atriiim, and ventricle, is a novelty in this group. A
conus with valves makes its appearance at the root of the truncus arterio-
sus. The common cardinals drain into a thin-walled sinus venosus. In
the adult the left common cardinal degenerates and both precardinals
connect with the right common cardinal or precava as in some mammals.
The two posterior cardinals unite into a common cardinal which passes to
the left side of the body and unites with the sinus venosus. In the heart
atrio-ventricular valves prevent a reverse flow of blood. With the
development of a septum transversum the coelom becomes divided into

SINUS VENOSUS

SEMILUNAR)
VALVES

Fig. 295. — A diagram of the primitive (fish) heart, as seen in a median longitudinal
section. Anterior is to the right. The course of blood in the heart — ^indicated by
arrows — takes the form of a letter S. (Redrawn after Keith.)

an anterior pericardial cavity and a posterior abdominal cavity. Red
blood corpuscles make their appearance in this phylum and the bloodTs
consequently red. A renal portal system is wanting in cyclostomes, the
caudal vein draining directly into the postcardinal veins.

Elasmobranchs. The blood vascular system of elasmobranchs differs
little from that of cyclostomes and with slight changes may easily be
derived from the latter. Associated with the appearance of paired fins
the subclavian and iliac arteries and veins are present. The arteries are
connected with the dorsal aorta. The subclavian vein is a branch of the
precardinal, while the iliac vein drains into the lateral abdominal vein.
The latter is believed to have been developed from the median ventral
abdominal vein of the primitive chordates. Internal jugular veins paral-
leling the precardinal veins are a novelty in this group. The caudal

354

COMPARATIVE ANATOMY

vein of Elasmobranchs divides anteriorly into the paired renal portal
veins which break up in capillaries within the mesonephroi. The heart
resembles that of cyclostomes. The branches of the dorsal aorta are
subclavians, intersegmentals, celiac, anterior and posterior mesenteries,
spermatics and ovarians, and renals. (Fig. 294, B)

Dipnoi. In the dipnoi with the emergence of lungs some advances
towards the mammalian circulation are seen. In the heart of lung fishes
both atrium and sinus venosus become partly divided by incomplete
longitudinal septa. Impure blood from the veins enters the right atrium
while aerated blood from the lungs flows into the left atrium. Atrio-
ventricular valves are lacking but the conus contains a series of valves.

Fig. 296. — Diagram of vertebrate circulation based on a urodele. Arteries cross-
lined; veins black except the pulmonary vein, white, av, abdominal vein; c, celiac
artery; ca, cv, caudal artery and vein; d, dorsal aorta; ec, external carotid; g, gonad; h,
hepatic vein; ha, hepatic artery; hy, hypogastric artery; ic, internal carotid; il, iliac
artery and vein; j, jugular; Iv, liver; m, mv, mesenteric artery and vein; pa, pulmonary
artery; pcd, postcardinal; pcv, postcava; pv, hepatic portal vein; r, rectal artery; ra,
renal advehent (portal) vein; sc, subclavian artery and vein. (From Kingsley's
"Comparative Anatomy of Vertebrates.")

Immediately in front of the conus the truncus divides into four pairs of
aortic arches, the third to the sixth of the original series. In the dipnoi
pulmonary arteries make their first appearance in the vertebrate series as
posterior branches of the last pair of aortic arches. As another novelty
in fishes, the right postcardinal vein degenerates and a new vein, the
postcava, drains most of the posterior part of the body. The caudal vein
in this group bifurcates into the left postcardinal and the postcaval veins.
The iliac veins as they leave the pelvic fins divide into pelvic and renal
portal veins. The two pelvic veins unite to form a median abdominal
vein. Venous blood from the fins may thus reach the heart either by
way of a capillary network in the mesonephroi or by the abdominal vein.
The efferent renal veins drain into the postcava and into the left post-
cardinal veins. Thus in the appearance of an atrial septum, of pulmonary
arteries and veins, and of a postcaval vein the dipnoi make notable
advances towards the circulatory system of the higher vertebrates. The
differences between the dipnoan and amphibian circulation are slight.

THE VASCULAR SYSTEM 355

Amphibia. In the amphibians the connexion of the sinus venosus is
shifted to the right atrium while the pulmonary veins connect with the
left atrium. The two atria are divided by a septum which is usually
perforate in urodeles. There is however little mixing of impure and
pure blood in the atria. In the undivided ventricle some mixing of the
two kinds of blood does occur. A spiral septum in the truncus arteriosus
shunts the venous blood from the right side of the ventricle chiefly into
the pulmonary arteries while that which passes to the dorsal aorta and
systemic arteries is mostly aerated blood. Of the six original aortic arches
of the embryo the last four persist in some adult amphibia while in others
only the third and fourth arches persist. One of the most important
changes in circulation which occurs within the group is the abandonment
by the higher amphibia of the capillary branchial network character-
istic of fishes. In the perennibranch amphibia most of the blood in the
aortic arches short-circuits the gills, and with the loss of gills in the anura
the aortic arches form direct connexions between ventral and dorsal
aortae. In the anura as in most amniotes that portion of the dorsal
aortae between the carotid (third) and systemic (fourth) arches degener-
ates. In urodeles as in dipnoi the pulmonary arteries form posterior
branches of the sixth aortic arch while in the anura the connexion by a
ductus arteriosus with the dorsal aortae is lost as in mammals. Several
splanchnic arteries convey blood from the dorsal aorta to the intestine.
In anura however they are reduced to three, celiac, anterior, and posterior
mesenteric arteries. In urodeles three veins drain the mesonephroi, the
right and left postcardinal veins and the postcava. The connexion of the
iliac veins with the renal portal or advehent veins which made its appear-
ance in dipnoi is also present in amphibia. Impure blood from the hind-
legs may thus return to the heart either through the mesonephroi, or by
the abdominal vein. The increased flexure of the heart brings the atria
anterior as well as dorsal to the ventricle. (Fig. 296)

Reptiles. The reptilian vascular system strikingly resembles that
of amphibia. The main arteries and veins are homologous in the two
groups. The chief differences appear in the heart and truncus arteriosus.
The ventricle is partly divided by a septum in lower reptiles and more
or less completely divided into the crocodiles and alligators. Conse-
quently pure and impure blood are separated in the two sides of the heart
as in mammals. A peculiarity of the reptilian circulation, however, is
manifested in the triple spHtting of the truncus arteriosus. Three
arteries instead of the two characteristic of mammals leave the heart.
One of these is the pulmonary artery carrying venous blood from the
right ventricle to the lungs. The remaining two vessels are the systemic
arteries, one of which comes from the right, and the other from the left,
ventricle. Soon after leaving the heart each artery crosses to the opposite

356

COMPARATIVE ANATOMY

side of the body. Thus the right aortic arch comes from the left ventricle
and conveys pure blood to the dorsal aorta and the head. The left aortic
arch comes from the right ventricle and carries mixed blood into the
dorsal aorta. Consequently, the dorsal aorta of reptiles contains mixed,

P^CATOINAL

EXT CABCTTID A.

■JUGULAR «

■mjNC.AKT.

Fig. 297. — Diagrams of the circulatory system in fish, urodele, anuran, reptile, and
mammal. Arteries black, veins stippled. In general features the series is believed to
represent stages in the phylogenesis of the mammalian circulatory system. It will be
noted that some blood vessels persist throughout the entire series. (Redrawn after
Stempell.)

and not pure, blood. Since both celiac and mesenteric arteries are given
off from the left aortic arch which carries mixed blood, they carry mixed
blood to the stomach and intestine. In some reptiles a foramen Panissae
connects the blood streams in the two ventral aortae so that some mixing
of the blood in the two vessels may take place. In the lower reptiles

THE VASCULAR SYSTEM 357

systemic and carotid arches are connected with one another, as in urodeles,
by the dorsal aortae. In the crocodiles this connexion is lost, as in
mammals. The connexion between the postcava and the postcardinals
is lost in reptiles and the blood from the kidneys returns to the heart by
the postcava as in mammals. Both right and left common cardinals
(ductus Cuvieri) persist and bring blood from the head and anterior
limbs into the sinus venosus. Thence it passes to the right atrium. Blood
from the hind legs as in amphibia may return to the heart either by the
renal portal veins or by the lateral abdominal veins.

Mammals. The complete division of the heart into a right venous
half and a left arterial half which was attained by reptiles is retained by"
mammals. In mammals, however, the sinus venosus merges into the
right atrium. In this region is located the sino -auricular node, a bundle
of muscular and connective tissue richly supplied with nerve fibers, which
is said to be the "pace-maker" of the heart-beat. Mammals have a
single ventral aorta. Of the paired systemic arches of amphibians and
reptiles only the left one persists. The renal portal system has disappeared
and with it the abdominal veins. The latter however form the transient
umbilical veins of the fetal circulation. The right and left iliac veins
establish connexions with the postcava by way of the posterior cardinals
and of their transverse anastomosis in the lumbar region.

The postcava of mammals appears to be only in part homologous with
that of lower vertebrates. As is shown in Fig. 320, four distinct embryonic
vessels unite to form the mammalian postcava. These are the anterior
hepatic portion, the subcardinal anastomosis, the supracardinal veins
(in part), and the posterior portion of the right postcardinal vein. The
supracardinal veins seem to be mammalian novelties, arising in the embryo
dorsal to the post- and subcardinal veins. There is difference of opinion
in regard to the first appearance of subcardinal veins, whether they are
new in mammals or present in vertebrates from amphibia to man. Accord-
ing to McClure subcardinal veins are present in all vertebrates.

The origin of the azygos and hemiazygos veins is also in doubt. Most
textbooks regard these vessels as persistent remnants of the postcardinal
veins together with their transverse anastomosis. The researches of
Huntington and McClure however indicate that the azygos and hemi-
azygos veins are limited to mammals and that they are in chief part
persistent portions of the supracardinal veins which appear to be mammal-
ian novelties. The renal veins of mammals are not the homologs of the
renal veins of lower vertebrates but are new vessels formed from the
intersubcardinal anastomosis.

In the lower mammals both common cardinals occur as in lower
vertebrates. In the higher mammals and man, however, a transverse
anastomosis between the precardinals or jugulars is converted into the

358

COMPARATIVE ANATOMY

left innominate vein which brings venous blood from the left arm and
left side of the head across to the right jugular vein. The left common
cardinal consequently degenerates but persists in part as the coronary vein.
Evolution of the Heart. The chief changes which the heart has under-
gone in phylogenesis may be briefly summarized as follows: The verte-
brate heart is a differentiated portion of a median ventral blood vessel.
The contractile function which originally extended throughout the length
of this vessel became localized and concentrated in the subpharyngeal
region. Primarily the heart had neither valves nor chambers but con-
sisted of a two-layered tube with a muscular wall and an endothelial
lining. The first subdivision of the heart was into a receiving chamber

^ic -^;c y^' "'^p'^n'.PJ^ '>y^

Fig. 298. — Different stages in the differentiation of the parts of the heart, ventral
view. A, elasmobranch; B, teleosts; C, amphibia; D, lower reptiles; E, alligator; F,
birds and mammals, a, atrium; ao, aorta; b, bulbus arteriosus; c, conus; cd, Cuvierian
duct; h, hepatic veins; pa, pulmonary artery; pc, pre- and postcaval veins; pv, pulmonary
vein; s, sinus venosus; sa, septum atriorum; v, ventricles. (From Kingsley's "Com-
parative Anatomy of Vertebrates.")

or atrium and an anterior propulsive division, the ventricle. Later were
added a posterior sinus venosus and an anterior conus. Atrioventricular
and semilunar valves in turn made their appearance, thus ensuring a
one-way flow of blood. With the elongation of the heart in confined space
a sigmoid flexure was formed and the atrium consequently came to lie
dorsal to the ventricle. Fishes added a muscular bulbus anterior to the
conus. In the dipnoi and amphibia the connexion of the sinus venosus
was shifted to the right atrium while aerated blood from the lungs entered
the left atrium. In the dipnoi and amphibia the atrium became divided
by an incomplete septum into right and left atria while the ventricle
remained undivided, so that some mixing of aerated and impure blood
occurs. The increased flexure of the heart brings the atria in amphibia
anterior to the ventricle. In the crocodilian reptiles the complete division
of the heart into arterial and venous halves is effected, but the beneficial
effects of this separation are partly neutralized by the mixing of the two

THE VASCULAR SYSTEM

359

kinds of blood in the dorsal aorta. In mammals the sinus venosus
becomes merged with the walls of the right auricle.

Evolution of the Aortic Arches. The device of oxygenating blood in
pharyngeal gills is peculiar to chordates. Nevertheless aortic arches
connecting ventral and dorsal aortae in the pharyngeal region occur in

Fig. 299.— Modifications of the aortic arches in different vertebrates. A, primi-
tive scheme; B, dipnoan; C, urodele; D, frog; E, snake; F, Uzard; G, bird; H, mam-
mal, c, celiac artery; da, dorsal aorta; dh, ductus Botalli; ec, ic, external and internal
carotids; p, pulmonary artery; 5, subclavian; va, ventral aorta. Vessels carrying venous
blood black; those with mixed blood shaded; those which disappear, dotted outlines.
(From Kingsley's "Comparative Anatomy of Vertebrates," after Boas.)

annelids. It is a matter of opinion whether this point of resemblance
between annelids and chordates has a phylogenetic significance or is
simply a case of convergence. In chordates the number of aortic arches
is correlated with the number of visceral arches. Amphioxus has the
largest number, nineteen pairs, of primary visceral arches among chor-
dates. The aortic arches are correspondingly numerous. The largest
number of aortic arches in vertebrates occur in some species of cyclostomes,
fifteen pairs in bdellostoma stouti. Although some elasmobranchs have

36o

COMPARATIVE ANATOMY

more, it is customary to assume six pairs as the primitive number of
aortic arches in gnathostomes. Of these the first two pairs belonging to
the mandibular and hyoid arches partly lose their respiratory function
and, consequently, the associated aortic arches. In fishes the persistent
aortic arches are broken up into a capillary net-work in the gills. In
urodeles the last four pairs of aortic arches persist, the blood from the

Artery

r Endothelium.
Intima i Inner elastic
1 membrane.

Media ■

Muscle fibers.

. Conn, tissue.
Outer elastic
membrane.

Externa \ Conn, tissue s

For orientation.

Vasa
k vasorum

Fat cells '

^yfe?^

^^^^

Fig. 300. — A section through a human ulnar artery and vein, showing the wall of
the artery on the left and of the vein on the right. The upper part of the figure {a-d) is
from a section of the same vessels stained with resorcin-fuchsin, an elastic tissue stain.

a. Circular, and b, radial elastic fibers of the media of the artery; c, external elastic
membrane; d, elastic fibers in the media of the vein; e, circular, and g, longitudinal
muscle fibers of the media; /, endothelium. XS50. (From Bremer's "Text Book of
Histology. ")

ventral aorta largely short-circuiting the gills. In anura gills are lost
and blood passes directly through the aortic arches to the dorsal aorta.
In the anura the fifth pair of aortic arches degenerate. That portion, the
ductus arteriosus, which connects the pulmonary branches of the sixth
aortic arch with the dorsal aorta also atrophies. The third pair of arches
persist as the roots of the carotid arteries. In reptiles as in anura three
pairs of aortic arches persist, the 3rd, 4th, and 6th in part as pulmonary.
In reptiles the common carotid arteries connect with the right and not
with the left systemic or fourth aortic arch. In mammals portions of

TTTE VASCULAR SYSTEM 36 1

three aortic arches persist in the adult, 3rd, 4th, and 6th as in reptiles.
The systemic arch of the right side, however, forms the right subclavian
artery while that of the left side becomes the arch of the aorta. Since
this is connected directly with the left ventricle it carries only aerated
blood to the dorsal aorta. The aortic arches in man resemble those of
other mammals. (Fig. 299)

The Evolution of Arteries. Aside from the transformation of the
aortic arches the phylogenetic changes in the arterial portion of the
vascular system have not been profound. The main trunks persist
throughout the series proving that nature holds fast to that which is good.
The appearance of the subclavians and iliacs is correlated with that of the
paired fins, but once invented these vessels persist in man to nourish
the arms and legs. In the dipnoi pulmonary arteries make their debut
as branches of the sixth pair of aortic arches when the lungs which they
supply emerge from air bladders. The connexion of the pulmonary
arteries with lungs persists throughout the vertebrate series. With the
substitution of a definitive kidney for the mesonephroi of lower vertebrates
new renal arteries are formed. The history of the caudal artery is one of
degeneration until in man it becomes the rudimentary median sacral
artery. The number of splanchnic arteries supplying the intestine in
man remains the same — three — as in elasmobranchs. (Fig. 297)

The Evolution of Veins. The phylogenetic alterations of the veins
are much more radical than those of the arteries just described. Few
veins persist throughout the entire chordate series. Included among
sucn persistent veins are the jugulars, the precava (the right common
cardinal or right ductus Cuvieri) and subintestinal, a part of the por-
tal vein. The usual assumption that the postcardinals persist as the
azygos and hemiazygos, except in part, is not supported by the evidence
from embryology.

The primary veins are paired, e.g., the precardinals, postcardinals,
lateral abdominals or umbiHcals, vitelUnes, and even the subintestinal
vein which is paired at the time of its first appearance in vertebrates.
The portal vein is one of the primitive veins of amphioxus and the modi-
fications of its development in connexion with the right vitelline vein
appear to be recent adaptations. The postcava appears as a new vein
in dipnoi and undergoes considerable reconstruction in mammals through
the addition of parts of the subcardinal anastomosis, right postcardinal,
and supracardinal veins. The views of investigators in regard to the
origin of the subcardinal veins are divergent. The evidence seems to
support the opinion that the subcardinals make their appearance with
that of the renal portal veins and that they persist throughout the verte-
brate series and in part are incorporated in the postcava of mammals.
In man, and to a lesser degree in other mammals, veins become differenti-

362 COMPARATIVE ANATOrV

ated into a superficial set which drain the skin and outer organs, and a
deep set which carry blood away from the deeper organs of the body.

Evolution of the Lymphatic System. Little is known of the relations
of chordate lymphatics to those of prechordates. The contrast between
lymphatics and blood vessels is less marked in invertebrates than in
vertebrates. It seems not unreasonable to assume that primarily there
was no distinction between blood vessels and lymphatics and that the

Fig. 301. — The lymphatics of the scrotum. Showing the transition of the capillaries to

the vessels with valves ((7, a, a). (From Morris, after Teichmann.)

two systems have had a common origin. As in the case of most blood
vessels it is impossible at the present time to homologize particular
lymphatic vessels in chordates and pre-chordates.

In amphioxus the lymphatics surround the blood vessels and occur
also in the metapleural folds, dorsal fin, and around the central nervous
system. In vertebrates the distribution of lymphatics corresponds
roughly with that of veins, although lymphatics are far more variable in
position than are veins. Like the veins the lymphatics are divided into
superficial and deep systems. The deep system develops in close relation
to the cardinal veins and acquires connexions with them and with the
superficial system.

Lymphatic vessels occur in cyclostomes and fishes. They surround
the veins in elasmobranchs, while their relations to the veins are less
intimate in other fishes. One or two main trunks may parallel the dorsal
aorta in this group and therefore be compared with the paired thoracic

THE VASCULAR SYSTEM

363

ducts of mammals. Lymph sinuses surround ihe heart and nervous
system. Some fishes have lymph hearts serving to assist the circulation
of the lymph. But fishes do not have lymph glands.

A thoracic duct is present in amphibia. Larger subcutaneous sinuses
occur in this group possibly as an adaptive arrangement which prevents
dessication. Lymph hearts may occur in various parts of the body.
Reptiles have large paired lymphatic trunks. Lymph hearts are also
found in this group.

Most mammals have paired thoracic ducts. In man the left duct
persists throughout life in connexion with the left subclavian vein, while

EXT. JUGULAR

EXT. JUGULAI?.

UBCLAVIAN

INTERCOSTAL

GLANDS-

THORACIC DUCT.

AZYGOS V. ^"i

POSTCAVAL-
RENAL
COMMUNICA-
TIONS

RECEPTACULUM
CHYLI

LYMPH GLANDS\

A ^-B - C D

Fig. 302. — Diagrams illustrating the chief lymphatic trunks and their relations to the
veins in mammals and man. A, South American monkeys; B, Mammals (Lepus) in
which postcaval-renal communications are wanting; C, Mammals in general; D, Man.
In all mammals lymph enters the veins at the point of junction between the jugular
and the subclavian veins. In most mammals there is also communication between
the lymph vessels and the postcaval and renal veins. In man the right thoracic duct
degenerates in part and the only communication with veins is at the root of the jugulars.

the right duct is rudimentary having a length of only a few centimeters.
One lymphatic sinus, the cistema chyli, also persists in man. The
lymph is returned to the veins at the point of least pressure, where sub-
clavian and jugular veins meet. Lymph glands are numerous in man and
mammals and generally occur in clusters in the axillary region and in the
groin and neck. Lymph hearts are wanting in mammals and man.
Lymphoid or adenoid tissue is found in all vertebrates. Lymph-nodes
however make their first appearance in reptiles.

Evolution of Hemopoietic Tissues. Hemopoietic or blood-forming
tissues are found in several phyla of invertebrates. Blood-cell formation
consequently appears not to be a novelty in chordates. Identical highly

3^4

COMPARATIVE ANATOIVrY

■4 • V^'i.,;'

I \

*^

•?

^
% ^

.■f^

I'

«

-.^

f

(f «

JS2) "^ g

t

Fig. 303. — Human blood corpuscles. Left hand column, as seen after Zenker's
fixation and haematoxylin and eosin stain; at the right, as seen in smears stained by
Wright's triple stain, i, neutrophiles; 2, mononuclear leucocytes; 3, lymphocytes; 4,
eosinophiles; 5, basophiles. (From Bremer's "Text Book of Histology," after Mallory.)

THE VASCULAR SYSTEM

365

specialized blood cells occur both in pre-chordates and chordates. The
lymphocytes of cyclostomes resemble those of some annelids. The
erythrocytes of gephyreans are similar to those of vertebrates and some
echiuroids have enucleated red blood corpuscles Hke those of mammals.

The primordial blood cell of animals appears to have been a lymphocyte
from which the various types of blood cells— erythrocytes, granulocytes,
monocytes, etc.— are differentiated. The original lymphocytes in
vertebrate embryos are derived from the splanchnic mesoderm. All
blood cells of vertebrates are therefore believed to be mesodermal. That

iHIND GUT

•ALiAN-ro I S
t PANCREAS

SMALL INT.

•',' / ^vjCAECUM

ECTUM

lUROGENlTAL
SINUS

'ALLANTOIS

Fig. 304. — Diagrams of early stages of development of the human embryo. A is an
early human embryo, B an embryo of four weeks and C one of five weeks. Blood cells
make their first appearance in the wall of the yolk-sac. Since the yolk-sac is to be
regarded as a herniated portion of the alimentary canal, the conclusion is drawn that in
the phylogenesis of the vascular system blood made its first appearance in the wall of the
alimentary canal. (Redrawn after Mall and Romer.)

the first blood cells of vertebrate embryos appear in the wall of the yolk-sac
is not surprising since the yolk-sac is a herniated portion of the alimentary
canal and its mesodermal wall therefore splanchnic.

The spleen is the fundamental blood-forming organ. In cyclostomes,
the splenic tissue lies in the submucosa of the stomach and intestine,
in the lamprey within the spiral valve. In the dipnoi, splenic blood-
forming tissue is concentrated in the stomach wall. In elasmobranchs and
ganoids, the spleen becomes attached to the mesentery and in amphibia
remains the chief erythrocytopoietic organ. In some amphibian larvae,
however, the mesonephros also functions as a blood-forming organ.

With the appearance of hollow bones in the anura, the spleen begins
to lose its importance as an erythrocytopoietic organ. In mammals,

366

COMPARATIVE ANATOMY

this function is vestigial in the spleen, and bone-marrow has usurped the
function. Bone-marrow is well-adapted for erythrocyte formation, since
the blood there is relatively static and the carbon dioxide tension is high.
Blood-cells, however, whether derived from spleen or bone-marrow,
become vascular elements only following migration. Hemoblasts are
transformed into erythrocytes in the venous sinuses of the bone marrow.
Development of the Vascular System. It is now agreed that the
blood and blood vessels are mesenchymatous in origin. The heart,
however, with possibly the exception of its endothelial lining, develops
from the visceral layer of mesoderm and is therefore of epithelial origin.

ectoderm
yolk granule

mesoderm
blood island

ectoderm

central cells of
blood island

peripheral cell
of blood island

somatic
mesoderm
coelom

endothelial cell
lumen

yolk

Fig. 305. — Drawings to show the cellular organization of blood islands at three
stages in their differentiation. -4, from blastoderm of 18-hour chick; B, from blastoderm
of 24-hour chick; C, from blastoderm of 33-hour chick. (From Patten's "Embryology
of the Chick.")

Development of Blood and Endothelium. Blood makes its first
appearance in the form of solid masses of mesenchymatous cells known as
blood islands in the visceral layer of the mesoderm adjacent to the endo-
derm in the extra-embryonic yolk-sac. These blood-islands consist of cell
masses which secondarily unite to form a network of solid cords. In this
condition, they constitute the angioblast or source of the primary blood
cells. The central cells of the cords are transformed into both red and
white blood corpuscles, while the peripheral cells become the endothelial
Hning of the blood-vessels. The first blood vessels are, therefore, extra-
embryonic, and those within the embryo are secondary.

THE VASCULAR SYSTEM 367

The differentiation of blood corpuscles is correlated with the appear-
ance of an intercellular fluid plasma. Thus the blood becomes a Hquid
tissue. Connexions of the vitelline vessels are soon established with those
formed within the embryo and similar vessels extend into the body-stalk
and chorion. Blood is already present in a human embryo of three to
four weeks. The extension of the angioblast and the formation of blood
channels is believed to occur, not by the sprouting and elongation of the
primary trunks, but by the confluence of separate cords of cells, as a
drain is built of separate tiles placed end to end. Some blood vessels,
however, appear to be formed out of capillary networks by the enlarge-
ment of a single channel and the disappearance of the others. Primarily,
the walls of the blood vessels consist of a single layer of endothelium.
The muscular and connective tissue layers are added from the surrounding
mesenchyme.

There is a divergence of opinion as to the origin of the various types of
blood cells of the adult blood. According to the monophyletic theory,
both leucocytes and erythrocytes come from a primitive hemoblastic
cell, and thus have a common origin. According to the alternative
poljrphyletic theory, red and white corpuscles have a diverse origin.
Difference of opinion has also arisen as to whether or not the blood
corpuscles of the adult are the direct descendants of the primary blood
cells of the embryo which have migrated to the various hemopoietic
organs and centers of proliferation. The evidence on the whole favors
the conclusion that the blood cells of the adult have diverse sources of
origin (red marrow, endothelium, lymph nodes, spleen, etc.) and are
not the direct descendants of the primitive angioblast cells.

In the embryo, blood cells arise successively from diverse centers,
including the blood islands, vitelline blood vessels, the adenoids, liver,
spleen, and finally from bone-marrow. Proliferation of blood cells
continues throughout life in the bone-marrow, lymph glands and spleen,
but ceases early in other organs. It has been calculated that erythrocytes
are manufactured in the adult body at the rate of ten to thirty thousand
every second.

The primary type of blood cell is the hemoblast cell, which contains a
relatively large vesicular nucleus and relatively small surrounding cyto-
plasm. During its histogenesis, the nucleus of the hemoblast shrinks,
while the cytoplasm increases in volume. In this way, the hemoblast is
converted into a normoblast, if the cell is to become an erythrocyte.
The nucleus of the normoblast, in correlation with the concentration of its
chromatin, stains intensely. Eventually, the nucleus of the erythrocyte
is extruded bodily, though whether by active migration from the cyto-
plasm or by being abandoned by the cytoplasm is uncertain. As a
consequence of its loss of a nucleus, the life of an erythrocyte is relatively

368

COMPARATIVE ANATOMY

short — possibly a month. The first enucleated erythrocytes appear
during the second month, mostly from the liver. Up until that time all
embryonic blood cells are nucleated, as in the adults of lower vertebrates.

neural plate ectoderm
dorsal mesoderm

superficial ectoderm
neural groove

] — somatopleure
splanchnopleure

splanchnic mesoderm

gut imrnedia&ely caudal t
anterior intestinal portal

dorsal mesoderm

epi-myocardium

endocardium ^ g^t immediately caudal to
anterior intestinal portal

> somatopleure

y splanchnopleure

epi-myocardium
endocardium

dorsal mesoderm

epi-myocard:

endocardium

I — somatopleure
J- splanchnopleure

Ime offusionof lateral margins of
anterior intestinal portal

dorsal mesoderm

D

fore-gut

dorsal mesocardium

J— somatopleure

.J- splanchnopleure
epi-myocardium

ventral mesocardium

endocardium

E

dorsal mesoderm
notochord

dorsal mesocard:

somatopleure

J — splanchnopleure

epi-myocardium

Fig. 306. — Diagrams of transverse sections through the pericardial region of chicks
at various stages to show the formation of the heart. A, at 25 hours; B. at 26 hours; C,
at 27 hours; D, at 28 hours; £, at 29 hours. (From Patten's ** Embryology of the
Chick.")

White blood corpuscles are of two types, granular and non-granular.
Most of the non-granular are lymphocytes which have about the same

THE VASCULAR SYSTEM

369

diameter as the erythrocytes. Their source is both in the bone-marrow
and the lymph glands. The granular leucocytes arise in the bone marrow
as myelocytes. They become differentiated mostly into neutrophiles,
the nuclei of which have diverse shapes while the cytoplasm is neutral
in its staining reactions. A small proportion of the myelocytes become
eosinophiles, which have a coarsely granular cytoplasm with a strong
affinity for eosin and other acid dyes. The cytoplasm of other leucocytes
stains with basic dyes. They are, consequently, known as basophiles.

ural plate

C D

Fig. 307. — Sections cut transversely through the cardiac region of pig embryos of
various ages to show the origin of the heart from paired primordia. A, 5-somite embryo;
B, 7-somite embryo; C, lo-somite embryo; D, 13 somite embryo. (Projection diagrams
X50, from series in the Carnegie Collection.) (From Patten's "Embryology of the
Pig.")

Development of the Heart. The heart makes its appearance as a
two-layered tube ventral to the pharynx, so that the early embryo has its
heart in its throat. Of the two layers, the inner becomes the endothelial
lining of the heart, while the outer forms the epicardium and the muscular
myocardium. The right and left halves of the heart begin as longitudinal
folds of the splanchnic mesoderm. Between these mesodermal folds
and the adjacent endoderm, scattered mesenchyme cells appear, and soon
become arranged as a thin-walled endothelial tube in each of the folds.

The paired mesodermal folds with their enclosed endothelial tubes
arise before the ventral wall of the pharynx is formed. The union of the
two halves occurs in correlation with the formation of the floor of the

370

COMPARATIVE ANATOMY

pharynx from the endoderm. Successive stages in the process are shown
in Figs. 306 and 307.

Soon after the tubular heart forms below the pharynx, its wall becomes
three layered by proliferation of cells from the outer or epimyocardial
layer. In this way, a thick muscular layer is formed between the endo-

B30 HOURS O 32 HOURS T^ 38 HOURS T7' 40 HOUF

10 somites ^ 12 somites -'--' '6 som.tes JJ/ 1! somit,

rL 44 HOURS

H

K

76 HOURS
M somites

100 HOURS
4S somites

Fig. 308. — Ventral views of the heart of chick embryos at successive stages to show
its changes of shape and its regional differentiation. Abbreviations: a. v., constriction
between atrium and ventricle; i.v., interventricular groove. (Prom Patten's " Embryol-
ogy of the Chick.")

thelial lining and the outer serosa. By the union of the visceral layer
of mesoderm above and below the heart, dorsal and ventral mesocardia
are formed in a manner resembling the formation of mesenteries in relation
to the intestine. By the time the embryo is a month old, the ventral
mesocardial membrane disappears and the right and left halves of the
pericardial cavity are thus brought into direct connexion with one another.
By the formation of septum transversum and diaphragm, the pericardial
cavity becomes separated from the abdominal cavity. (Fig. 311)

THE VASCULAR SYSTEM

371

Soon after the two halves of the heart are united in the mid-ventral
line, the heart itself becomes S-shaped as a consequence of its elongation
in a confined space. The dorsal curve is posterior and connects directly
with the paired vitelline (omphalomesenteric) and umbilical veins.
The ventral curve is anterior and extends forward beneath the pharynx
as the truncus arteriosus. Circulation has already begun when the heart
is in this tubular condition. By the time the embryo is two months old,
the heart, although its size is minute, has reached its adult form and
structure. (Fig. 308)

The processes involved in converting a tubular heart into a four-
chambered one include: i. The increased flexion of the heart so that the

PRECARDINAL VEIN
AORTIC ARCHII
AORTIC ARCHlv,^^^ \,,ii-'rT7T7-*r-^

,SINO-ATRIAL REGION OF HEART

^POSTCARDINAL VEIN

'INTERSEGMENTAL ART.

■MESONEPHRIC VESSELS

ALLANTOIC

VASCULAR

PLEXUS

YOLK SAC '^---.:r:rr^::i—'' ^allantois

Fig. 309. — The circulatory system of a young swine embryo. The arteries are shown
in black, veins are stippled. All blood vessels at this stage are paired, but those of the
left side only are shown in the figure. (Redrawn after Patten.)

posterior atrial portion becomes anterior, while the morphologically
anterior ventricular portion lies posteriorly. 2. The formation of a
longitudinal septum which divides the heart into right and left chambers.
3. The relative hypertrophy of the two atria, that of the right side enlarg-
ing the more rapidly. 4. The separation of atria and ventricles by the
growth of the atrio- ventricular valves. 5. The inclusion of the posterior
division of the heart, the sinus venosus, within the right atrium. 6. The
division of the anterior portion of the heart, the conus, into aorta and
pulmonary artery. The changes thus briefly summarized are best
understood from diagrams of the successive stages, (Figures 298, 308.)

During intra-uterine life, the septa which divide the heart into right
and left halves are perforated to permit blood from the right side to pass
through foramina into the left side, and thus to make possible a systemic
circulation even when the pulmonary circulation is small. In the atrium,
two longitudinal septa arise, the one first formed (Septum I) being on
the left. Since relatively little blood comes from the lungs, the blood

372

COMPARATIVE ANATOMY

pressure in the right atrium is greater than in the left atrium and conse-
quently the septum on the left is pushed back like a swinging door into
the left atrium, and blood enters freely from the right atrium through
the foramen ovale in Septum II. (Fig. 321)

When, however, the child is born and the pulmonary circulation is
augmented, the increase in the amount of blood in the left atrium equalizes

right duct

left duct
of Cuvier

ndocardial cusViion
of
'fS!Sig5S\ atrioventricular
canal

- trabecular
carneae

Fig. 310. — Section through the heart of a 9.4 mm. pig embryo, at the level of the atrio-
ventricular canals. X45. (From Patten's "Embryology of the Pig.")

the pressure in the two atria, the left septum (Septum I) is forced back
against the right (Septum II), and the foramen ovale is closed. The blood
in the two atria is therefore no longer mixed. After a few months or
years the two septa normally unite to form an imperforate septum.

Development of Valves. The processes involved in the development
of the atrio-ventricular valves are complicated. The valves arise from
thickened folds of the heart- wall lying in the constricted passage between
atrium and ventricle. The folds from which the valves develop are lip-

THE VASCULAR SYSTEM

373

like, one dorsal to the other. Both are known as endocardial cushions.
The extension of the interventricular septum described above results in
its fusion with the endocardial cushion and in the division of the single
atrio-ventricular foramen into right and left atrio-ventricular foramina.
The endocardial cushions guarding the right atrio-ventricular foramen
become the tricuspid valve, while those of the left foramen form the mitral
valve. The chordae tendineae and the papillary muscles which are con-

sophagus

pharynx

epi-myo-

cardium

C. D.

Fig. 311. — Schematic diagrams showing the manner in which the pleural and
pericardial region of the coelom become separated. (From Patten's "Embryology of
the Pig.")

nected with the valves are derived from the inner walls of the ventricles,
which primarily are spongy masses of muscular trabeculae. As the walls
of the ventricles are converted into the compact muscle of the outer heart-
wall, the inner trabeculae become papillary muscles and chordae tendineae.

Descensus Cordis. When the heart begins its development, it lies well
forward beneath the pharynx. As it takes on adult form and structure,
it shifts backward into the thorax. During its migration the atria which
were primarily posterior to the ventricles come to lie anterior to them.

Development of Aorta and Pulmonary Artery. Extending anteriorly
from the ventricle, the bulbus and truncus arteriosus carry blood from

374

COMPARATIVE ANATOMY

the heart to the aortic arches. The liulbus and truncus are primarily a
single vessel. During the second month of foetal life, however, a spiral

BULB

INTERNAL CAROTID A

■A3RTIC ARCH I
ILL POUCH I
•AORTIC ARCH H
POUCH 2

DORSAL AORTA.
ATRIUM

ARCH I

POUCH 4+5-^

INT CAROTID A.
POUCH I
ARCH n
ARCH HI
^^POUCH 3

ARCH W+
PULMONARY

DORS. AORTA

INT. CAROTID A;

POUCH 2

POUCH 3
POUCH 4+5-
PULM. ARCH
PULMONARY A:
LUNG BUD

VENTRICLE

INT CAROTID A.

POUCH

POUCH 2

ARCH ni
POUCH 3

ARCH W

POUCH 4+5 PULMONARY

PULM. ARCH VI

LUNG
ES0PHA3US

INT CAROTID A.

ARCH 17=
AORTIC ARCH

DORS.
"AORTA

ARCH

PULMONARY A"

DUCTUS
ARTERIOSUS

LUNG

'^LUNGS \ PULMONARY A.

D. E. F

Fig. 312. — A~F show successive stages in the development of the aortic arches in
human embryos. The arteries are shown in black, the pharyn.x and lung anlagen are
stippled. A is an embryo of four weeks, B of four and a half weeks, C of five weeks, D of
six weeks, E of six and a half weeks, and F at birth. (Redrawn from Romer, after
Streeter.)

LUNG ANLAGt
PHARYNGEAL POUCHES
MYELENCEPHAlON ^ ^<^\'

ECTODERM

SPINAL CORD

TOMACH

NOTOCHORO

DORSAL AORTA

OPTIC VESICLE

Fig. 313. — A diagram of a 4.2 mm. human embryo showing five aortic arches.
(Redrawn after W. His.)

septum is formed, dividing them lengthwise into two trunks, the aorta
and the pulmonary artery. At the time when this occurs, four thickenings
of the wall of the bulbus are converted into the six semilunar valves, three

THE VASCULAR SYSTEM

375

in the aorta and three in the pulmonary artery. As a result of the spiral
splitting of the bulbus, the pulmonary artery is made to connect with the
right ventricle, and the aorta with the left.

Development of the Aortic Arches. In very early stages of ontogenesis,
in 1.5 mm. to 2.00 mm. embryos, before the two halves of the heart are

arterial circle
(of Willis;

hypophysis

aortic root ophth. a.
ventral to
pharynx

ant. cerebral a.
mid. cer. a.

post. cer. a.

int. carotid
artery

subclavian a.
(left)
thoracic
intersegmental
arteries

vertebral a.

subclavian a

internal
mammary

dorsal aorta

Fig. 314. — Diagrams illustrating the changes which occur in the aortic arches of
mammalian embryos. A, ground plan of complete set of aortic arches; B, early stage in
modification of arches; C, derivatives of aortic arches. Abbreviations: br. ceph.,
brachiocephalic a,TteTy; cer. a., cerebral artery; lin., lingual artery; max., maxillary
artery; ophth. a., ophthalmic artery; stap. a., stapedial artery; thy., thyroid artery.
(From Patten's "Embryology of the Pig," adapted from several sources.)

completely united in the mid-ventral line, connexion between ventral and
dorsal aortae is established around the pharynx by means of a single aortic
arch, the first. In a 2.6 mm. embryo, a second aortic arch, the hyoid, is
added. Others are added in succession until in a 4.2 embryo there are five
aortic arches. But the fifth or last of these is really the sixth arch, the
true fifth aortic arch being a rudimentary vessel which appears only
transiently in the 7.0 mm. embryo. By this stage, however, the first two

,S76

COMPARATIVE ANATOMY

aortic arches have degenerated. Consequently, while all six aortic arches
arise, they are not present simultaneously in the human embryo.

Only three of the six embryonic aortic arches are represented in the
adult, the third, fourth, and in part sixth. The right and left third aortic
arches become respectively the root of the right and left internal carotid
arteries, the external carotids coming from the ventral aorta. The left
fourth aortic arch becomes the arch of the aorta of the adult, while the
right forms the right subclavian artery. The dorsal aorta between the
third and fourth arches degenerates and disappears. Posterior branches
of the sixth pair of aortic arches connect with the lungs and form the
pulmonary arteries. Until birth, the remainder of the left sixth aortic
arch persists as the ductus arteriosus connecting the pulmonary artery

X- RAMUS OORSALIS

\\ ARTERY---^.

-VX - SEGMENTAL SOMATIC / /

INTERMEDIATE ART

VISCERAL ARTERY"-

" RAMUS VENTR AUS"

A DIAGRAM OF ARTERIES IN HEAD REGION. B ARTERIES IN TRUNK REGION.

Fig. 315. — Diagrams illustrating the relations of somatic and visceral arteries in A,
Head and B, Trunk. The coelom in the head region is embryonic and transient.
(Redrawn from Corning, after Young and Robinson in Cunningham's Anatomy.)

with the dorsal aorta. The vessel which connects the right pulmonary
artery with the right dorsal aorta degenerates. The fifth pair of arches
degenerate soon after their appearance.

Changes in the Embryonic Circulation. The vascular system of a
1.5 mm. to 2.0 mm. embryo includes paired dorsal aortae from which are
given off intersegmental vessels to the body-wall, and a series of vitelline
arteries to the yolk-sac. Blood is returned to the sinus venosus of the
heart through vitelline veins from the yolk-sac and paired umbilical veins
from the placenta.

In embryos of 2.0 mm. to 2.5 mm., blood is returned from the head to
the heart by the paired precardinal veins. At this stage, six pairs of
vitelline arteries carry blood to the yolk-sac. In embryos of 2.6 mm.,
postcardinal veins join the ptecardinal veins to form the common cardinal
veins, the ductus Cuvieri. These, together with the umbilical and vitel-
line veins, return blood to the sinus venosus. Also, at this stage, the
paired dorsal aortae begin to unite into a median unpaired trunk. In the
head region, however, this fusion does not occur, and the paired dorsal
aortae persist as the internal carotids of the adult body. The vitelline

THE VASCULAR SYSTEM

377

arteries become reduced in number to a single vessel formed by the
coalescence of a pair of arteries. This vessel persists as the superior
mesenteric artery of the adult. In a 2.6 mm. embryo, there are two pairs
of aortic arches, and by the time the embryo is 7 mm. long, five pairs of
aortic arches are present. A series of twenty-seven branches of the
dorsal aorta form the intersegmental arteries of the body-wall. Celiac,
superior mesenteric, and inferior mesenteric arteries develop from the
vitelline arteries. Three types of branches develop from the dorsal aorta,
somatic or intersegmental, lateral or intermediate, and ventral or visceral
branches. Lateral or intermediate branches are limited to the trunk
region. From the dorsal branches arise the metameric vertebral, costal,
and lumbar arteries. The vertebral arteries below the brain combine as
the basilar artery. The subclavian arteries develop from the seventh
cervical intersegmental arteries of the embryo.

Fig. 316. — Three stages in the development of the arteries of the fore limb of the
white mouse. A, 8 days; B; 9 days; C, 10 days; a, aorta; b, brachial plexus. (The
vessels are extremely variable, not agreeing even on the two sides of a single individual.)
(From Kingsley's " Comparative Anatomy of Vertebrates," after Goppert.)

From the intermediate branches of the dorsal aorta come the renal,
adrenal, phrenic, and spermatic or ovarian arteries. The vessels which
supply the intestine, the celiac and mesenteric, are derived from the
ventral visceral arteries. The aortic arches, limited to the head region,
are also visceral. The umbilical arteries during ontogenesis change greatly
in their relations. They arise in a 1.5 mm. embryo opposite the fourth
cervical myotome. In a 5 mm. embryo, they have been shifted backward
to a connexion opposite the fourth lumbar vertebra. In the adult," they
persist as the iliac arteries. The median sacral artery is the continuation
of the dorsal aorta into the sacrum and coccyx. (Fig. 315)

The arteries of the upper and lower extremities grow into them from
the dorsal aorta. The brachial artery of the arm is formed by the union
of three segmental vessels, the middle of which belongs to the last cervical
segment. Extending into the forearm, this artery becomes the interos-
seous artery. In the hand region five branches are given off to the five
fingers. Through the growth of a median artery paralleling the interosse-
ous artery, the latter loses its connexion with the digital arteries. The
ulnar artery is added as a third vessel in the forearm, and in the hand

378

COMPARATIVE ANATOMY

becomes connected with the median artery by a volar arch. A fourth
vessel, the radialis, grows from the brachial artery into the hand. Dia-
grams of these stages (Fig. 317) summarize these changes.

<^^DIGITAL ART.

33P32E

INTEROSSEOUS ART BRACHIAL ARTER^f

DIGITAL

1 INTEROS

SUR BRACHIAL 1

Median i brachial

ULNAR

ischiadic
"post tibial

Fig. 317. — Diagrams of the development of the arteries in arm and leg. A—E,
stages in the development of the arteries in the arm; F-I, stages in the development
of the arteries of the leg. Proximal is to the right, distal to the left. (From Corning,
after McMurrich in Piersol's "Anatomy.")

ro loA

Fig. 318. — Three stages in the development of the hepatic portal system. A,
primitive; B, liver tubules beginning to develop, right omphalomesenteric interrupted;
C, definitive condition, liver not indicated, dc, Cuvierian ducts, hp, hepatic portal
vein; hv, hepatic vein; /, liver; lo, ro, left and right omphalomesenteric veins; si, sub-
intestinal veins; sv, sinus venosus. (From Kingsley's "Comparative Anatomy of
Vertebrates.")

The arteries of the lower extremity differ somewhat in their develop-
ment from those of the upper. Two primary arteries make their appear-
ance in the thigh, the femoral and ischiadic arteries. The latter extends

THE VASCULAR SYSTEM

379

into the foot and ends in the five digital arteries of the toes. From the
femoral artery, a saphenous artery grows diagonally across the leg and by
making connexion with the digital vessels supplants the ischiadic vessel,
which now becomes the peroneal artery of the calf. As a consequence of
the union of the saphenous artery with the ischiadic, the latter partly
atrophies. In the knee region, the saphenous becomes the popliteal

duct of Cuviep

Fig. 319. — Diagrams showing the development of the hepatic portal circulation from
the omphalomesenteric veins, and the relations of the umbilical veins to the liver.
A, is based on conditions in pig embryos of 3-4 mm.; B, on embryos of about 6 mm.;
C, on embryos of 8-9 mm.; D, on embryos of 20 mm. and older. (From Patten's
"Embryology of the Pig," adapted from several sources.)

artery, and in the leg, the tibialis posterior artery. By an outgrowth of
the popHteal artery, the tibialis anterior artery is formed.

Development of the Veins. At the time of its first appearance in a
2 mm. embryo, the sinus venosus receives blood from two pairs of veins,
the vitelline veins from the yolk-sac and the umbilical veins from the
chorionic villi of the placenta. A third pair of veins connected with the
sinus appear in a 2.6 mm. embryo. These veins are the common cardinals,

38o

COMPARATIVE ANATOMY

Heart

- Ant. Cardinal

Developing

Subcardinal

Plexus in

fleson.

Right—
Subcord

Fbat Card.
Iliac -

Fig. 320. — Diagrams illustrating stages in the development of the systemic veins
of the pig. The cardinal and omphalomesenteric veins are shown in black, the sub-
cardinal system is stippled, the supracardinals are horizontally hatched, and vessels
arising independently of these three systems are indicated by small crosses. A , Ground
plan of the veins of a young mammalian embryo; B, Cross section (at level of arrow in A)
showing dorso- ventral relations of the various veins; C, Diagrammatic plot of veins of

THE VASCULAR SYSTEM

381

Int Jugular
(Ant. Cardinal

lit Jugular

Subclovian

Fiost Cardinal
Supracardinol

5ubcardinal
Anastomosis

Supracerdina

Ext Iliac
Int. Iliac.

Gonadial-

'Int Jugular
-Ext. Jugular

left
Subclavian

Coronari^
5inu5

Int. Iliac

(Fig. 320.)
5-6 mm. pig embryos; D, Arrangement of veins in 6-7 mm. pig embryos; E, Cross sec-
tion (at level of arrow in D) showing dorso- ventral relations of vessels; F, Veins in
12-13 mm. embryos; G, Veins in 16-19 mm. embryos; H, Veins in 22-24 mm. embryos;
/, veins in 30-35 mm. embryos; J, Cross section of 17 mm. embryo at level of arrow in
G; K, Plan of veins in adult pig. (From Patten's "Embryology of the Pig," after
Butler.)

382 COMPARATIVE ANATOMY

the ductus Cuvieri. The blood from the head is returned by the pre-
cardinals or jugulars and that from the trunk by the postcardinals. The
subclavian, renal, and external iliac branches of the cardinals soon make
their appearance. At a later stage, transverse anastomosing vessels
connect right and left pre- and postcardinal veins. Another transverse
anastomosis is formed in the region of the iliac veins.

The rapid growth of the liver effects radical changes in the vitelline
and umbilical veins which pass through it, both breaking up into capillary
networks within the organ. Three transverse connexions between the
parallel vitelHne veins are formed. Of these the first and last are dorsal
to the duodenum while the middle anastomosis is ventral to the intestine.
The united veins thus form a figure 8; but the left half of the anterior loop
and the right half of the posterior loop atrophy, and the resultant portal
vein assumes the shape of a letter S. With the atrophy of the yolk-sac
and its associated vitelline veins, and with the appearance of splenic
(Henal) and superior mesenteric branches from the intestine, the portal
vein attains its adult relations. The right umbilical vein disappears,
while the left umbilical acquires direct connexion with the hepatic portal
vein. In this way, a ductus venosus is formed carrying blood directly
from the intestine and placenta to the postcava and the heart. (Fig. 319)

The precardinal veins, which drain the blood from the head, are
primarily paired and symmetrical. In an eight week embryo, a transverse
anastomosis is established and later converted into the left innominate
vein. Eventually the left common cardinal atrophies, but occasionally
persists as the oblique vein of Marshall. The left common cardinal
persists also as part of the coronary sinus.

In the older texts it is stated that the posterior cardinals persist as the
azygos and hemiazygos veins. The more recent researches of Huntington
and McClure, however, indicate that the ontogenesis of the veins below
the heart is extremely compHcated. The changes may be more clearly
expressed in a series of diagrams of successive stages than from a descrip-
tion (see Fig. 320). Postcardinal, subcardinal, and supracardinal
veins make their appearances successively in the embryo and acquire
complex connexions with one another. The adult postcava is formed by
the coalescence and anastomosis of four primarily separate veins, the
hepatic, subcardinal, postcardinal, and supracardinal. In addition to
these, a sub-supracardinal anastomosis is built into the wall of the post-
cava. These changes are seen in the stages represented in Fig. 320.

Changes in Circulation at Birth. In the fetus oxygenated blood is
returned to the heart by the umbilical vein, ductus venosus and the post-
cava. In the postcava, venous blood from the lower part of the body
mingles with the pure blood from the placenta. Further dilution of the
oxygenated blood occurs in the heart by the addition of venous blood from

THE VASCULAR SYSTEM

383

right subclavian artery

innominate artery
jugular veins

innominate vein
subclavian vein

anterior vena caVa

through foramen ovale
right atrium

posterior vena cava

to pulmonary orifice
from

hepatic
sinusoids

umbilicus

umbilical
artery

coeliac artery

left common carotid artery
left subclavian artery

ductus arteriosus
left pulmonary artery
pulmonary veins

left atrium

mitral valve

to aortic orifice
from left ventricle

anterior mesenteric a.

dorsal aorta

posterior mesenteric a.

common iliac artery
external iliac artery
internal iliac artery

hypogastric artery

Fig. 321. — Diagrammatic plan of circulation of mammalian foetus just before birth.
(From Patten's "Embryology of the Pig.")

384 COMPARATIVE ANATOMY

the head. Some of the mixed blood in the right auricle finds its way
through the foramen ovale into the left auricle and thence to the left
ventricle from which it is pumped to all parts of the body. The blood
which is pumped from the right ventricle into the pulmonary artery also
finds its way mostly into the dorsal aorta by way of the ductus arteriosus.
The pulmonary circulation, it is assumed, is reduced in amount, since the
lung sacs are not fully inflated while the embryo is in utero.

At birth, when the child draws its first breath the lungs become filled
with air and expanded, with consequent increase in their capillary circu-
lation. As a result, the blood which had been flowing through the ductus
arteriosus into the aorta passes to the lungs and returns to the heart by
the pulmonary veins. The consequent increase in pressure in the left
auricle forces septum I of the auricle against septum II and closes the
foramen ovale. (See Fig. 321 and description on page 372.) Thereafter
the blood in the left side of the heart is aerated blood, while that of the
right side is venous blood. As a result of these changes in circulation
the lumen of the ductus arteriosus usually disappears. The walls of
the vessel, however, persist as a connective tissue cord, the ligamentum
arteriosum of adult anatomy.

Following the birth of the child the placental circulation stops. That
portion of the umbilical vein which extends from the navel to the liver
loses its lumen and becomes the ligamentum teres of the liver. The two
umbilical arteries are reduced to the lateral umbilical ligaments of the
adult body. The ductus venosus of the liver becomes the ligamentum
venosum. Some of these changes occur slowly so that in most infants
the ductus arteriosus becomes imperforate only at three months. The
fusion of the interauricular septa occurs still more slowly, and is complete
at three months after birth in only one third of all individuals, and even
at maturity is incomplete in a quarter of the population. The failure
of the septa to close the foramen ovale results in a "blue" baby, for which
the chances of life are small.

Development of the Lymphatic System. Lymphatic vessels are
thin- walled channels which, like arteries and veins, have an endothelial
lining and terminate in fine capillaries. In addition to vessels and capil-
laries lymphatics have nodular swellings or lymph-nodes which act as
"filters of lymph" and centers of lymphocyte formation. Since, how-
ever, lymph nodes or glands are supplied with blood capillaries, it cannot
be said that they are peculiar to the lymphatic system.

The origin and differentiation of lymphatics have long been matters of
dispute, and some of the issues still remain unsettled. Although lym-
phatics at the time of their first appearance as well as in later life are
closely associated with veins, it seems to be increasingly evident that they
are not formed as outgrowths of the veins. They arise as independent

THE VASCULAR SYSTEM

385

spaces in the mesenchyma, and the mesenchymal cells surrounding them
are converted into the endothelial lining of the lymphatic vessel. Since
connexions with veins appear very early and the number of connexions

Fig. 322. — Lymphatic vessels and veins in a rabbit of fourteen days, eighteen hours;
14.5 mm. The lymphatics are heavily shaded, x being a vessel along the left vagus
nerve and y along the aorta. The large jugular lymph sac is in contact with the internal
jugular vein, In.J.; it passes to the junction of the external jugular {Ex. J.) and sub-
clavian veins, the latter being formed by the union of the primitive ulnar, Pr.UL, and
external mammary veins, Ex.M. The mesenteric sac is in front of the vena cava
inferior {V.C.I.) and below the renal anastomosis {R.A.). Other veins include AZ.,
azygos, v., vitelline; C, gastric; S.M., superior mesenteric; etc. The figures indicate
the position of the corresponding cervical nerves. X11.5. (From Bremer" s "Text
Book of Histology.")

diminishes in later stages, it was formerly thought that lymphatics are
outgrowths of veins. Today, however, this evidence seems less con-
vincing than it did to earlier investigators.

386

COMPARATWE ANATOMY

The separate mesenchymal spaces which form the anlagen of lym-
phatic vessels secondarily unite with one another to form the continuous
channels of the adult lymphatics. Later in their development, however,
there is good evidence that the extension of the lymphatic system involves
branching and sprouting of the primary lymphatic trunks.

In the two-months human embryo paired thoracic ducts already
appear near and parallel to the posterior cardinal veins. They arise in
connexion with the paired jugular lymph sacs which lie lateral to the
internal jugular veins. Later the thoracic ducts become connected with
posterior lymph sacs and with the cistema chyli in the pelvic region.
From these main trunks as a basis lymphatic vessels grow into all parts

May 19.
1 1 A. M.

May 19,
II p. M.

May 16, May 16, May 17. May 18,

11.30 A. M. 11.30 P.M. II. A. M, 12.30 P. M

Fig. 323. — Successive stages in the growth o£ a lymphatic vessel (lym.) in the tail of a
tadpole (Rana palustris). b.v.. Blood vessel; n., nucleus of the lymphatic vessel.
X 135. (From Bremer's " Text Book of Histology," after Clark.)

of the body — head, trunk, and appendages. The jugular lymph sacs
acquire direct connexion with the jugular veins and retain it throughout
life. Although blood cells are abundant in the anlige of the lymphatic
vessels these cells largely disappear during ontogenesis. The lymph sacs
of the embryo are later replaced by lymph glands or nodes.

During the third month of development paired lymph glands make
their appearance in the axillary, iliac and maxillary regions. In their
formation the cells of the lymph vessels or sacs do not appear to partici-
pate. Adenoid tissue becomes intercalated in the course of the lymphatic
vessel and after its envelopment by the lymphatic to form a peripheral
sinus, the gland becomes differentiated into an outer nodular cortex and
an inner mass of medullary cords. The development involves an ingrowth
both of lymphatic capillaries and blood capillaries, which are thus brought
into close relation with the adenoid, lymphocyte-forming cells. The
ingrowth of capillaries takes place by a process of sprouting.

THE VASCULAR SYSTEM

387

The Heart in Man. The human heart is a hollow muscular organ
about the size of the closed fist, weighing from nine to eleven ounces.
It is shaped like a truncated cone with its apex pointing downwards to
the left. It is placed asymmetrically behind the sternum, with its apex
just above the left fifth rib. A muscular partition extending from apex
to base divides it into right and left cavities, each of which is subdivided
into an anterior atrium or auricle and a posterior ventricle. Externally
the division of the heart into atria and ventricles is indicated by a groove,
the sulcus coronarius, and the two are separated internally by atrio-
ventricular valves. Two large veins, the precava and postcava, enter
the right atrium, while four pulmonary veins return blood from the lungs

Afferent lymphatic vessels.

Peripheral

sinus.,

Lymph sinus.

Capsule,

Fig.

Efierent lymphatic vessels.
324. — Diagram representing two stages in the development of a lymph gland.

The left half of the diagram is an earlier stage than the right half.
"Text Book of Histology.")

(From Bremer's

to the left atrium. The pulmonary artery connects with the right, the
aorta with the left ventricle.

The walls of the ventricles are thicker than those of the atria, and
the wall of the left ventricle is thicker than that of the right, for the right
ventricle pumps blood only to the lungs, while the left ventricle forces
blood to all other parts of the body and is correspondingly more muscular.

The heart-wall has three layers, endocardium, myocardium, and
epicardium. The endocardium consists of a thin layer of connective
tissue and an endothelial layer continuous with that of the blood vessels.
The myocardium is the thick muscular layer. The fibers of cardiac
muscle are striped, and are peculiar in having anastomosing connexions
with one another. The epicardium is a thin layer of connective tissue
covered with the serous membrane which lines the pericardial cavity.

388

COMPARATWE ANATOMY

A similar serous epithelium is reflected on the outer side of the pericardial
cavity. The cavity between contains a small amount of fluid.

The muscles of the myocardium are wound circularly around the
heart and arranged in layers. The fibers of the outer layers run at
approximately right angles to those of the inner layers, thus insuring a
maximum contraction of the heart cavities during contraction or systole.

ijt-LEFT SUBCLAVIAN ART.
ARCH OF AORTA

""^-'PULMONARY ARTERY

PULMONARY
VEINS

'post cava

tricuspid'

VALVE

Fig. 325. — A diagram of the chambers of the mammalian heart and their associated
vessels and valves. The walls of the ventricles are shown in black, those of the auricles
are stippled. The direction of flow of blood is indicated by arrows. (Redrawn after
Jammes.)

The muscle of the atria is mostly independent of that of the ventricles
and the two are separated by a connective tissue septum. There is,
however, an atrioventricular bundle of specialized muscle fibers which
extends from the atrial septum to the ventricular septum and serves to
convey to the ventricles the rhythm of contraction of the atria.

The atrio-ventricular valves are attached by chordae tendineae to
the walls of the ventricles in such a manner as to open freely into the
ventricles but to prevent the return of blood when the ventricles contract.
The attachment of the chordae tendineae to the heart-wall is by means of

THE VASCULAR SYSTEM 389

special papillary muscles, anterior and posterior, in each ventricle. The
right valve is partially divided into three ''cusps" and the left into two.
Hence they are known respectively as tricuspid and bicuspid or mitral

valves.

At the opening of the aorta and of the pulmonary artery crescentic
semilunar valves prevent the return of blood into the ventricles. Each
artery contains three of these valves so arranged that under pressure of
the blood they meet together and occlude the lumen completely. Near
the semilunar valves lie the openings of the coronary arteries which
supply blood to the wall of the heart. This blood is returned to the
coronary sinus of the right atrium by coronary veins which parallel the
coronary arteries.

Course of Blood in the Heart. The blood from the systemic veins,
the precava and the postcava, enters the right atrium and by the con-
traction of the atrium is pumped through the tricuspid valve into the
right ventricle. From the right ventricle the blood is carried by the
pulmonary artery to the lungs, returning from the lungs by the four
pulmonary veins to the left atrium. Forced by the contraction of the
atrium through the bicuspid valve into the left ventricle it is pumped into
the aorta and to all parts of the body.

PULMONARY CIRCULATION

The pulmonary artery carries impure or venous blood from the right
ventricle to the lungs. Near the heart it divides into right and left
branches which connect with the corresponding lungs. At the point of
separation of right and left pulmonary arteries the ligamentum arteriosum
connects them with the aorta.

From the lungs, blood is returned to the left atrium by pulmonary
veins which, unlike other veins, convey aerated blood. While there are
usually four pulmonary veins, occasionally there are five and the blood
from the middle right lobe enters the atrium independently. Within
the lung lobes the pulmonary veins parallel the arteries.

Arteries. Arteries convey blood away from the heart, and because
they are subjected to considerable pressure when the heart contracts,
their walls are correspondingly thick and elastic. A cross section shows
three layers, an intima, a relatively thin layer consisting of the lining
endothelium and a connective tissue layer with elastic fibers, a media, a
relatively thick layer of muscle and elastic fibers, and an externa, a.
layer of loose connective tissue consisting largely of white inelastic fibers.
Because of the strength and elasticity of their walls, arteries usually do not
collapse after death. (Fig. 300)

Arteries have their own circulatory and nervous supply. The capil-
laries of the walls are the vasa vasorum. The nerves are branches of the

390 COMPARATIVE ANATOMY

autonomic system, and are of two sorts, vasoconstrictor nerves which
stimulate the contraction of the blood vessels and thus check the flow of
blood, and vasodilator nerves which act as inhibitors and thus permit the
dilation of the blood vessels.

SYSTEMIC CIRCULATION

The Arteries. All of the blood of the head, trunk, and limbs leaves
the heart by way of the aorta, the largest artery of the body, with a
diameter of over an inch where it leaves the heart. Beginning at the
left ventricle, the aorta ascends to the level of the second rib, curves to
the left over the bronchial tube, and then descends along the backbone
to the fourth lumbar vertebra, where it divides into right and left common
iliac arteries. The divisions of the aorta near the heart are known
successively as the ascending aorta, aortic arch, and descending or dorsal
aorta. From the ascending aorta in the region of the semilunar valves,
two small arteries, the right and left coronary arteries, are given ofif to
the walls of the heart. From the upper side of the aortic arch three
large arteries arise. The first is the innominate artery, which extends
towards the head for a short distance and then divides into right sub-
clavian and right common carotid arteries. Near the origin of the
innominate the left common carotid artery leaves the aorta and passes
to the head. Immediately to the left of the origin of the left common
carotid the left subclavian artery extends from the aorta into the left
arm. From the lower side of the aortic arch near the ligamentum arteri-
osum the upper and lower bronchial arteries pass to the right and left
bronchial tubes. (Fig. 326)

The two common carotid arteries extend towards the head to about
the level of the larynx and there each divides into internal and external
carotids. The internal carotid supplies chiefly the brain, while the
external carotid carries blood to the remaining parts of the head and face.
Among the many branches of the external carotid artery are superior
thyroid, ascending pharyngeal, lingual, external and internal maxillary,
sterno-cleido-mastoid, occipital, posterior auricular, and superficial
temporal arteries. The names of these vessels suggest their distribution.

The subclavian artery extends beneath the clavicle giving off as
branches vertebral, internal mammary, superficial cervical, thyroid axis,
and costo-cervical arteries. The vertebral arteries, after passing through
the costo-transverse foramina of the cervical vertebrae, unite beneath the
brain to form the basilar artery which, with the internal carotids, supplies
the brain. The internal mammary artery conveys blood to the sternum,
diaphragm, and intercostal muscles. The thyreo-cervical or thyroid axis
artery supplies thyroid gland, esophagus, trachea, and muscles of the
shoulder blade and neck.

THE VASCULAR SYSTEM

391

SUPERFICIAL VOLAR ARCH
DEEP VOLAR ARCH

VOLAR INTEROSSEOUS
ULNAR
COM. INTEROSSEOUS
RADIAL RECURRENT

SUPERIOR

ULNAR COLLATERAL

DEEP BRACHIAL

THORACODORSALIS
SUBSCAPULAR
LATERAL THORAC IC
INNOMINATE
VENA CAVA
RIGHT AURICLE
RIGHT GASTRIC
GASTRODUODENAL ■^

RAM. FRONTAL
•TEMPORAL
TRANSVERSE FACIAL
OCCIPITAL
INFERIOR ALVEOLAR
EXTERNAL CAROTID
LINGUAL
THYREOCERVICAL

TRANSVERSE COLLI
AXILLARY

THORACOACROM I AL
SUBCLAVIAN
LEFT COMMON CAROTID

-.». ^-INTERNAL MAMMARY

•s^f-^'V V-C'"'^^'^'^'-^ HUMERAL

\ ■; T- ARCH OF AORTA

4— BRACHIAL
■^ — PULMONARY

LEFT AURICLE
DEEP BRACHIAL
VENTRICLE
SPLENIC

SUPERIOR MESENTERIC
I NTERCOSTAL
RENAL
SPERMATIC COVARIAN)
RADIAL RECURRENT
ULNAR RECURRENT
ULNAR
DORSAL INTEROSSEOUS

RADIAL
VOLAR INTEROSSEOUS
V-.-DORSAL RETE

METACARPAL

iA — PRINCEPS POLLICIS

LATERAL CIRCUMFLEX

RAM. MUSCULAR
GENU SUPERIOR LAT. •
POPLITEAL

RAM. PERFORATING PERONEAL
TARSAL (lateral)
ARCUATE
PLANTAR ARCH

ANTERIOR TIBIAL
PERONEAL
POSTERIOR TIBIAL

ANT. LATERAL MALLEOLAR
LATERAL TARSAL
ARCUATE
METATARSAL

DIGITAL (plantar)

Fig. 326. — The chief arteries of the human body viewed from in front.

392 COMPARATIVE ANATOMY

As the subclavian artery passes out from under the clavicle towards
the arm it becomes the axillary artery. In the five inches of its length
the axillary artery gives off the superior thoracic, thoraco-acromial,
lateral thoracic, subscapular, anterior circumflex humeral, and posterior
circumflex humeral arteries to the muscles of the shoulder and chest.
Continuing into the arm the axillary artery becomes the brachial artery
which extends to the elbow and there divides into radial and ulnar arteries.
Its branches supply the bone muscles and skin of the upper arm. The
radial and ulnar arteries are the chief arteries of the forearm; those of
the hand are the volar and digital arteries, their branches supplying chiefly
muscles and skin. The pulse is usually taken from the radial artery in the
wrist.

The thoracic portion of the dorsal aorta extends from the aortic arch
at the level of the fourth thoracic vertebra to the level of the twelfth
thoracic vertebra, where it passes through the diaphragm and becomes
the abdominal aorta. The chief branches of the thoracic aorta are ten
pairs of intercostal arteries, which not only supply the intercostal muscles
but also the vertebral canal and the skin of the thoracic region. Small
arteries are given off to the bronchi, lungs, esophagus, and mediastinum.

The abdominal aorta extends from the diaphragm to the fourth lumbar
vertebra. As it passes downwards it gives off in succession right and
left inferior phrenics, celiac, right and left middle adrenal, right and left
first lumbar, superior mesenteric, right and left renal, right and left
internal spermatic, right and left second lumbar, inferior mesenteric,
right and left third and fourth lumbar, right and left common iliac, and
middle sacral arteries. Of these the phrenics and lumbars are parietal
in distribution while the remainder, except the iliac and sacral, are
visceral.

The right and left inferior phrenic arteries supply the diaphragm and
adrenals. The lumbar arteries supply the lumbar vertebrae and the
lumbar muscles. One branch anastomoses with the renal artery. The
celiac artery divides into three chief branches, the left gastric, the hepatic,
and splenic (lienal) arteries. The left gastric artery supplies the lesser
curvature of the stomach and the lower part of the esophagus. The
hepatic artery supplies the liver, duodenum, pancreas, and the right
half of the greater curvature of the stomach. The third branch of the
celiac is the splenic which, as its name implies, supplies the spleen and also
the pancreas and the stomach wall.

The superior mesenteric artery extends for most of its course in the
mesentery and gives off five chief branches, inferior pancreatico-duodenal,
intestinal, ileo-colic, right colic, and middle colic. The inferior pan-
creatico-duodenal artery anastomoses with the superior pancreatico-
duodenal artery, a branch of the hepatic. The intestinal arteries supply

THE VASCULAR SYSTEM 393

the jejunum and ileum as well as the mesentery. The colic arteries carry
blood to the colon.

The renal arteries supply kidneys, adrenals, and the upper portion
of the ureters. Thus the adrenals receive blood from three sources,
phrenic, renal, and middle adrenal arteries. The internal spermatic
arteries are given off just below the renals and pass along the spermatic
cord to the testes. From them branches are given off to the ureters,
cremaster muscle, and epididymis. The ovarian arteries in the female
correspond to the spermatic arteries of the male. Branches from the
ovarian arteries pass to the ureters, ovaries, uterine tubes, uterus, and
to the round ligaments of the uterus.

The inferior mesenteric artery leaves the abdominal aorta about an
inch and a half above its division into the iliacs. It is distributed chiefly
to the colon and rectum. Arising just above the bifurcation of the
abdominal aorta, a median sacral artery supplies the last lumbar vertebra,
sacrum, and coccyx.

The common iliac arteries are formed by the bifurcation of the abdom-
inal aorta when it reaches the level of the fourth lumbar vertebra. At
the level of the lumbo-sacral articulation, the right and left common
iliacs divide into internal or hypogastric and external iliac arteries.
From the internal iliac arteries both visceral and parietal branches are
given off. The visceral branches are the umbilical, inferior vesical,
middle hemorrhoidal, uterine, and internal pudendal arteries. The
parietal branches are the ilio-lumbar, lateral sacral, obturator, and gluteal
arteries. The ilio-lumbar arteries are serially homologous with the
lumbar arteries and have a similar distribution to the vertebrae and
lumbar muscles.

The lateral sacral arteries, as their name suggests, supply chiefly
the sacrum. The obturator artery supplies the bladder, iliac and pubic
bones, and obturator and adjacent muscles. The gluteal arteries supply
gluteal and neighboring muscles.

Of the visceral branches of the hypogastric artery the umbilical
artery supplies chiefly the bladder. The inferior vesical artery sends
branches to the bladder and prostate gland. The middle hemorrhoidal
artery carries blood to the rectum and, in the female, to the vagina.
The uterine artery after a tortuous course supplies the uterus and vagina.
The internal pudendal artery is distributed to the muscles of the perineum,
penis or clitoris, and to the scrotum or labia majora.

The external iliac artery is the larger of the two vessels into which
the common iliac artery divides. In its course it gives off inferior epi-
gastric, deep circumflex, iliac, as well as smaller branches to adjacent
muscles. As the external iliac passes into the thigh it becomes the
femoral artery. Continuing to the knee it becomes the popliteal artery.

394

COMPARATIVE ANATOMY

SUP. OPTHALMIC

INTERCAPITULAR'

^^'^K^ ^^^ CEREBRAL
^^J^SJP. SAGITTAL SIN.

BASILIC
BRACHIAL^

RIGHT INNOMINATE

SUPERIOR CAVA— T' .
INTERCOSTAL—'^
PULMONARY-"^
RIGHT AURICLE
HEPATIC

HEPATIC PORTAL ,~ . ■-

SUPERIOR MESENTERIC — ^^)\|( t' TSlP
INFERIOR CAVA— -raTfTT^^

I NTESTI NALS \\Va^^

RENAL ^232113-.'

(OVARIAN) SPERMATI- "^ ^-'

ILIOLUMBAR

HyPOGASTR IC — V ^\j^^' f

EXTERNAL lUAC *"" J'^

GLUTEAL —
SUPERIOR EPIGASTRIC
CIRCUMFLEX ILIAC

IXTERNAL PUDENDAL

INF SAGITTAL SIN.
CEREBRI MAGNA
TRANS. SINUS
MASTOID ANGULAR'

SUP BULB INT. JUG.
COM. FACIAL

MENTAL

^-INT. JUGULAR

TRANS COLLI
— TRANS. CERVICAL
, . AXILLARY
^^g:::^^. LATERAL THORACIC
^'^^^'^ CIRCUM. HUMERUS POST.
CEPHALIC
RACH lAL
THORACODORSALIS

DEEP BRACHIAL
THORACO-EP I GASTR I C
SUP. ULNAR COLLATERAL

— SPLENIC
• - -AZYGOS
-y- HEMIAZYGOS

SUPER. TEMP
■OCCIPITAL
POST. FACIAL
ANT FACIAL
EXT. JUGULAR

SUPER- CERVICAL
TRANS. COLLI

INF THYROID

"YI^^T

DORSAL RETE
DORSAL ARCH
DIGITAL
INTERCAPITULAR

rjtua: .-y- basilic

- INFERIOR mesenteric
ULNAR
WVt \ " DORSAL interosseous

«jSvA — COMMON interosseous

RADIAL

deep circumflex IUUM

deep volar arch
superficial volar arch
metacarpal
volj^r digital

POSTERIOR TIBIAL

ANTERIOR TIBIAL

•PLANTAR ARCH
INTEROIGITAL

Fig. 327. — The chief veins of the human body viewed from in front. Superficial veins
are shown on the right side, deeper vessels on the left side of the body.

THE VASCULAR SYSTEM 395

Below the knee the popliteal artery terminates in the posterior and
anterior tibial arteries which are deeply situated beneath the calf
muscles.

The posterior tibial artery extends to the foot. In the leg its branches
supply the leg muscles and the skin of the lower leg. One of its branches,
the peroneal artery, parallels the posterior tibial artery as far as the ankle.
In the foot the latter terminates as the lateral and medial plantar arteries.
The anterior tibial artery extends along the front of the leg to the foot
where it terminates as the dorsalis pedis artery with its metatarsal and
other branches.

The Systemic Veins. Blood is returned to the heart from the head,
the upper part of the body, and from the arms by the precaval vein or
vena cava superior. The precava is formed by the union of the right and
left innominate veins. From the thorax blood is returned to the precava
by the azygos vein. (Fig. 327)

Each of the innominate or brachiocephalic veins is formed by the
confluence of the internal jugular and subclavian veins. The left iimomi-
nate crosses the median plane of the body just above the arch of the
aorta, and unites with the right innominate vein immediately to the left
of the aortic arch.

Blood from the anterior part of the scalp and face is returned to the
facial vein, a branch of the internal jugular. Venous blood from the
back of the scalp and neck enters the external jugular veins, and thus is
carried to the subclavian vein. The two chief veins tributary to the
common facial vein are the anterior and posterior facial veins.

The external jugular vein is formed by the union of the posterior
auricular vein and the communicating branch of the posterior facial
vein. Other tributaries of the external jugular are the occipital, posterior
external jugular, transverse scapular, and anterior jugular vein. For
purposes of description, it is customary to divide veins into two classes,
superficial and deep. Most of those of the head and neck mentioned
belong to the group of superficial veins.

The deep veins of the head include the veins of the diploe, which
drain the cancellous bones of the skull, the venous sinuses of the brain
membranes, and the veins of the brain, nose, eye, ear, pharynx, and larynx.
The chief deep veins of the neck are the internal jugular veins, which have
as tributaries the lingual from the tongue and the superior th3n:oid from
the thyroid gland. The vertebral veins which bring blood from the
vertebrae unite with the innominate veins, which also receive blood from
the deeper neck muscles through the deep cervical veins. From the
thyroid gland come the inferior thyroid veins and the thyroidea ima vein
which is usually single and median. Veins from the thymus gland,
trachea, and esophagus also open into the left innominate vein.

396 COMPARATIVE ANATOMY

In the thoracic region are the azygos, hemiazygos, accessory hemi-
azygos, ascending lumbar, vertebral, and internal mammary veins. The
azygos vein extends along the thoracic vertebral column, receiving blood
from the right intercostal veins, from the hemiazygos vein and from the
accessory hemiazygos vein. It opens anteriorly into the precava. The
hemiazygos vein receives blood from the left intercostal veins, and joins the
azygos at about the level of the eighth rib. The accessory hemiazygos vein
crosses the backbone from left to right just above the hemiazygos
vein. In the region below the diaphragm, the azygos and hemiazygos
veins continue as right and left ascending lumbar veins. The vertebral
column in the thoracic and lumbar regions is drained by the vertebral,
intercostal, lumbar, sacral, and intervertebral veins. The internal
mammary veins drain the superficial veins of the thorax as well as the
anterior intercostal, bronchial, and pericardiac veins and open into the
right and left innominate veins.

The chief superficial veins of the upper extremity are the cephalic
and basilic veins. The cephalic vein joins the axillary vein below the
clavicle. The basilic vein unites with the brachial vein at about the
middle of the upper arm. The deeper veins of the arm, forearm, and
hand parallel the arteries of these regions. Each artery, however, is
accompanied by a pair of veins, venae comitantes. Beginning with the
volar veins of the hands, the veins of the upper extremity, in order, are the
metacarpal, ulnar and radial, brachial, axillary, and subclavian. The trib-
utaries of these veins correspond closely in number and relations with
arteries of the same name.

Most of the blood from the legs, abdomen, and viscera is returned
to the right auricle by the postcava. The tributaries of the postcava
are the right and left common iliac veins, which return blood from the legs,
the lumbar veins from the lumbar body-wall, the right spermatic or
ovarian vein from the gonad, the renal veins from the kidneys, the right
adrenal vein (the adrenal vein of the left side and the left spermatic or
ovarian veins are tributaries of the left renal vein), the inferior phrenic
veins, of which the left usually opens into the left renal vein, and the
hepatic veins which are largest of all.

The portal vein is unique in having capillaries at both of its ends.
The portal collects blood from the intestine, stomach, and spleen and
deUvers it into the capillaries (sinusoids) of the liver. Its branches
parallel those of the mesenteric arteries and some of those of the celiac
artery. The chief tributaries of the portal vein are the superior mesenteric
vein, the splenic vein which drains the spleen, stomach, and pancreas,
the inferior mesenteric vein (the relations of which correspond to those
of the artery of the same name), the coronary or gastric vein from the

THE VASCULAR SYSTEM 397

Stomach, the pyloric vein from the pylorus, pancreas, and duodenum,
and the cystic vein from the gall-bladder.

The common iliac veins parallel the arteries of the same name and are
formed by the union of the external and hypogastric or internal iliac veins.
The middle sacral vein, which is thought to be a rudiment of the caudal
vein of lower vertebrates, returns blood from the sacrum and lower pelvis
into the left common iliac vein. The hypogastric vein parallels the artery
of the same name. Its tributaries are the hemorrhoidal vein from the
rectum, uterovaginal vein, vesical from the bladder, and the pudendal
vein from the external genitals. The hypogastric also receives blood
from the inferior and superior gluteal, obturator, lateral sacral, and
ilio-lumbar veins, the distribution of which is similar to that of the arteries
of the same names.

In the leg the chief superficial vein is the great saphenous which
returns to the femoral vein most of the blood from the surface of the
thigh, leg, and foot. The veins of the skin beginning with the digital
veins of the foot form plantar and dorsal networks which combine on
the inner side of the leg into the great saphenous vein. The small
saphenous vein extends from the foot along the back of the calf to the
knee where it combines with the popliteal vein.

The deep veins of the leg parallel the arteries and are given correspond-
ing names. In the region of the shank there are two veins to each artery.
Beginning with the plantar veins in the foot, the more important of the
deep veins of the leg are the anterior and posterior tibial veins, which
unite in the knee region to form the popliteal vein. In the upper part
of the thigh, the popliteal vein in turn becomes the femoral vein which
directly continues as the external iliac.

The Lymphatic System. The circulatory system thus far described
is closed, in contrast with the open or lacunar systems of invertebrates.
That is to say, the blood vessels of vertebrates form a system of tubes the
walls pf which are continuous and without openings. But to nourish
the cells in the tissues the blood vessels must be permeable. Loss of
fluid plasma from the blood takes place in the capillaries, which are
formed of a single layer of endothehum. It is generally thought to be
due to three factors, blood pressure, osmosis, and diffusion; but active
secretion by the endothelial cells may also be involved.

The fluid which passes from the capillaries forms lymph. Lymph
is essentially blood plasma from which the corpuscles, except a few
leucocytes, have been removed. The lymph lost from the blood capil-
laries is taken up by the lymphatic capillaries which like the blood capil-
laries are microscopic vessels whose walls consist of a single layer of
endothehum. Unlike blood capillaries, however, the lymphatics vary

398 COMPARATIVE ANATOMY

greatly in size, and in the translucent mesentery are visible to the naked
eye when distended with the milky lymph absorbed from the intestine
after a meal. (Fig. 301)

The lymphatic system includes capillaries, collecting vessels, and
lymph nodes. Like blood capillaries, lymph capillaries exchange dissolved
substances with the surrounding tissues. They form complicated
plexuses, of which few parts of the body are devoid.

From the lymph capillaries, the lymph passes into lymph vessels or
ducts by which it is conveyed to the innominate veins. The largest
of the lymph trunks is the thoracic duct, which extends along the backbone
and enters the left innominate vein. The wall of a lymph vessel resembles
that of a vein, having three coats, intima, media, and adventitia. The
lymph vessels, however, have more valves than the veins to insure flow
in one direction only.

On its way through the lymph vessels, lymph before it enters the vein
passes through one or more lymph nodes consisting chiefly of adenoid
tissue. Lymph vessels enter as vasa afferentia into the cortex of a node,
where they form lymph sinuses surrounding lymph nodules. Within the
lymph node, the lymph vessels pass from the outer cortical sinuses into
the medulla of the node, where they again form sinuses surrounding
medullary cords of adenoid tissue. The vasa efferentia which convey
lymph away from a node are relatively large vessels. Each lymph node
has on one side an indentation or hilum where arteries enter and veins and
efferent lymphatic vessels emerge. Lymph nodes have a double function.
They are centers of proliferation of lymphocytes, and they serve as lymph
filters. The gland cells have the capacity, it is believed, to ingest bacteria
and to neutralize the action of foreign substances in the lymph. Fig. 324.

The thoracic duct extends from the second lumbar vertebra along the
spinal column to the point where the jugular and subclavian veins unite to
form the left innominate vein. Here the lymph is restored to the veins
in the region of least pressure. Into the thoracic duct flows lymph from
the lower part of the body as well as the lymph brought by the left jugular
and subclavian lymph vessels. At its lower extremity the thoracic duct
expands into a cistema chyli or receptaculum. (Fig. 302)

CHAPTER II

THE UROGENITAL SYSTEM

EVOLUTION OF THE UROGENITAL SYSTEM

The evolution of urinary and reproductive systems may be conveniently
discussed as if the two were independent.

The Urinary System

Metabolism furnishes the necessary chemical foundation of life. But
the oxidation of nitrogenous compounds, essential to the process of
metabolism, results also in poisonous nitrogenous wastes, which must
be removed. It is, therefore, not surprising that all animals have mecha-
nisms for removing the ashes of life, intra-cellular mechanisms in the
protozoa, porifera, and coelenterates, multicellular in higher animals.

FLAME CELLS

PROTOPLASMIC
PROCESSES >^---

A.CANAL SYSTEM B.FLAME CELLS

Fig. 328. — Excretory organs in flatworms. A shows the branching excretory canals
terminating in flame-cells. B shows three flame cells enlarged. By the action of the
cluster of cilia (flame) a current carries wastes away from the excretory cell into the
canals. (Redrawn after Benham.)

Protozoa. The characteristic excretory organ of protozoa is the con-
tractile vacuole. Usually each cell contains a single vacuole, but there
may be two in protozoa with elongated bodies. The position of the
vacuole just beneath the ectosarc is constant and its external orifice
permanent. Liquid wastes formed within the cytoplasm stream toward
the vacuole from all directions. When the tension of liquid within the
vacuole reaches a maximum, the external orifice is forced open and
the liquid passes to the outside. The rate of contraction is a function of
the temperature, warmth increasing and cold retarding excretion. There
is, of course, no genetic connexion between the intracellular organs of
protozoa and the multicellular organs of metazoa.

399

400

COMPARATIVE ANATOMY

Lower Metazoa. While specialized excretory organs are not present
in coelenterates, they do occur in flatworms, where they assume the form
of branched tubules which carry liquid wastes to the exterior. In many
flatworms, their apertures, usually paired, open on the dorsal side of the
body near its anterior end. One pecuUarity of the excretory system of
flatworms is the flame cells located at the terminations of the branched
tubules. These are relatively large hollow cells containing numerous
long cilia which extend into the cavity of the cell and which sometimes
unite to form a single long flagellum. The function of the cilia appears
to be to drive the secretions of the cell towards the excurrent canal and
the exterior. Many invertebrates besides flatworms possess flame cells
in connexion with their excretory systems. The flame cells of flatworms

^y~- — J-PROTONEPHRIDIUM

NEPHRIDIOPORE

COEUDMOSTOME
.COELX)M0DUCT
■ORE

COE1jOMOSTO«
;OELOMODUCT

&4— PORE

i-NEPHRIDIOPORE

NEPHRIDIUM

COELOMOSTOME

COELOMODUCT

NEPHRIDIOPORE

COEUDMOSTOME

DEGENERATE
COELOMODUCT
COELOMODUCT

NEPHROSTOME

NEPHRIDIUM
lEPHRIDIOPORE

"\NEPHROSTOME NEPHRIDIOPORE'

Fig. 329. — Diagrams illustrating the varying relations of coelomoducts and nephridia.
in different annelids. Nephridia like those seen in annelids also occur in cephalo-
chordates. The coelomoducts are homologized with the renal tubules of vertebrates,
although the latter are mostly excretory and not — as in annelids — reproductive in
function. (Redrawn after Goodrich.)

are probably the prototypes of those of the highly specialized solenocytes
of annelids and amphioxus. (Fig. 328)

Annelids. Two sorts of excretory organs occur in annelids, proto-
nephridia and metanephridia, the former, as their name suggests, con-
sidered the more primitive. (Fig. 329)

Protonephridia. The protonephridia of annelids and molluscs are
branched or unbranched excretory tubules which end bhndly within the
body cavity or in the connective tissues. In the annelids they are seg-
mentally arranged and are ectodermal in origin. Their inner blind ends
are beset with numerous solenocytes. Each solenocyte is prolonged into
a tubular capillary containing an elongated flagellum, which extends
throughout the length of the capillary into the cavity of the nephridial
tubules. The solenocytes gather liquid wastes from the surrounding
tissues, and pass them to the nephridium and thus to the outside. Thus

THE UROGENITAL SYSTEM

401

protonephridia resemble the branched tubules of flatworms not only in
structure but also in function. (Fig, 331)

Metanephridia. Many annelids also have metanephridia, which are
nephridia devoid of solenocytes and opening not only to the exterior
but also into the coelom. The internal aperture is the nephrostome. In
oligochaetes the metanephridia are much convoluted, and the nephro-
stome opens into the coelom of the segment anterior to the one which
contains the external aperture. The metanephridia are paired and occur
in most segments of the body throughout its length. Cilia surrounding
the nephrostome sweep wastes from the body cavity into the nephridium

lORSAL BLOOD VESSEL

'CIRCULAR MUSCLES

Fig. 330. — A diagrammatic cross section of an annelid, showing the coelomoducts.
With a ciliated opening into the coelom and an external aperture, each coelomoduct
serves as an outlet of the reproductive cells — eggs or spermatozoa. They are homolo-
gized with the renal tubules of chordates. (Redrawn after Lang.)

and thus to the exterior. Like the protonephridia, the metanephridia are
ectodermal in origin. It is assumed that they are genetically related to
the protonephridia.

Coelomoducts. In addition to the two kinds of nephridia just described,
many invertebrates have a third type of tubules, coelomoducts. These in
annelids, however, are not excretory in function but serve as repro-
ductive ducts. Like the metanephridia, they have coelomostomes
which are ciliated and open into the coelom. Like nephridia, also, they
are paired and metameric in arrangement. Their mesodermal origin,
however, makes it impossible to compare them with nephridia. Conse-
quently, they must be considered as novelties first appearing in annelids.
Their special interest to morphologists lies in the fact that they resemble
the kidney tubules of vertebrates and are consequently regarded as proto-
types of the latter. If this comparison is valid, we must assume that, in
the course of phylogenesis, they have changed from a reproductive to an

402

COMPARATIVE ANATOMY

excretory function. In this connexion it is interesting to note that the
coelomoducts of molluscs are excretory in function, and that in male
vertebrates some of the anterior tubules of the mesonephros serve as
reproductive outlets.

Acrania. Among the protochordates, the urochordates appear to
have no organs comparable with either nephridia or coelomoducts. The
hemichordates, with the possible exception of rhabdopleura, have no
paired excretory organs. The proboscis cavity of rhabdopleura contains
a couple of ciliated tubules which are usually denominated nephridia.
We find, however, in acrania, protonephridia strikingly similar to those of
annelids. Their presence in chordates is frequently interpreted as sup-

SOLENOCYTES

...DORSAL

f AORTA CAPILLARY,

— PROTONEPHRIDIUM

PHARYNX

PERI BRANCHIAL

'NEPHROPORE

Fig. 331. — Diagrams of the protonephridia of Amphioxus. A is a section in the
pharyngeal region — through a gill septum on the left and through a gill slit on the right.
B is an enlarged section of a protonephridium showing the attached solenocytes. Such
a protonephridium is practically identical in structure with a protonephridium of
annelids. (Redrawn after Boveri and Goodrich.)

porting the assumption that chordates have an annelid ancestry. It
seems more likely however that we have here another case of convergence.

The nephridia of amphioxus are protonephridia associated with the
secondary gill-arches. They are short branched tubules which open into
the peribranchial cavity, sometimes as many as ninety pairs. The fact
that they are ectodermal in origin and are limited to the gill-region, while
the excretory tubules of vertebrates are mesodermal and postbranchial, is
sufficient to show that the two are not homologous. That the nephridia of
amphioxus are protonephridia comparable with those of annelids is
evidenced by the fact that they are ectodermal and metamerically arranged
and have their blind terminations beset with solenocytes almost identical
in structure with those of annelids.

Craniota. Three successive kidneys make their appearance in verte-
brates, pronephroi, mesonephroi, and metanephroi. That the tubules
of these three kidneys are comparable with coelomoducts is sufficiently

THE UROGENITAL SYSTEM 403

attested by their mesodermal origin and with the exception of the tubules
of the metanephros, their metameric arrangement. The pronephros in
vertebrates is an embryonic structure which persists only in the adults of
some cyclostomes, teleosts, and dipnoi. The mesonephros becomes the
functional kidney of adult anamnia, the metanephros of adult amniotes.

Comparative embryology and anatomy suggest therefore, that the
function of excretion has phylogenetically migrated from the anterior
to the posterior part of the body. The possibility, however, cannot be
excluded, that originally every metamere in chordates, as in annelids,
contained a pair of excretory organs. But this assumption is weakened
by the evidence that the excretory tubules of acrania are not serially
homologous with those of craniotes. The possibility that the gonadic
sacs of amphioxus may be homologous with coelomoducts and kidney
tubules has, however, been suggested somewhat plausibly.

PROTONEPHRIDIUM PRONEPHROS. MES0NEO"«0S METANEPHROS.

PROTONEPHRICHA RENAL TUBULES GLOMERUU WOLFFIAN DUCT

SECRETORY TUBULES.

COLLECTING TUBULES'

CHORDATE KIDNEYS. ureter'

Fig. 332. — A diagram illustrating the four types of kidneys which occur in chordates.
The excretory tubules of amphioxus are ectodermal in origin like the protonephridia of
annelids, while they are mesodermal in vertebrates. In the course of phylogenesis the
excretory organs of chordates have migrated farther and farther back in the body, as is
shown in the diagram.

Pronephros. The first vertebrate kidney to develop in ontogenesis,
and possibly also the oldest in phylogenesis, is the pronephros or head
kidney. This consists of three to fifteen pairs of segmental tubules, each
of which opens by a nephrostome into the body cavity. Each tubule is
connected laterally with the primitive pronephric duct which carries excre-
tions posteriorly to the cloaca. It is generally assumed that originally
each pronephric tubule, like the coelomoducts of annelids, had its own
aperture. The eHmination of liquid wastes by a common pronephric
duct must be considered as a secondary condition. The factors in this
phylogenetic development are obscure. Similar relations, however, are
found in some annelids. In some oligochaetes and hirudinea, a longi-
tudinal duct conveys the products of the testes to the outside. In others,
the nephridia may join a longitudinal duct which opens into the cloaca.
If, as is generally believed, the nephridia are ectodermal in origin, we
have here another case of convergence of structures which have a diverse
origin — the nephridia from the ectoderm, pronephros and duct from the
mesoderm.

The pronephros functions in few adult vertebrates, but it appears to
function in the early ontogenesis of those craniotes which have little yolk

404

COMPARATIVE ANATOMY

Fig. 333. — Diagram of possible connexions of coelom with the exterior, c, coelonx;
cU cloaca; g, glomerulus of kidney; i, intestine; n, nephrostome; pa, porus abdominalis.
(From Kingsley's "Comparative Anatomy of Vertebrates," modified after Bles.)

•GLOMERULU:

NEPHROSTOME'

.PRONEPHRIC
TUBULES

A. PRONEPHROS

WOLFFIAN DUCT.

B. MESONEPHROS

K«;SONEPHR IC
TUBULES

AFFERENT ARTERIOLE
EFFERENT ARTERIOLE

GLOMERULUS

NEPHROSTOME

MESONEPHRIC
TUBULE

-'^'--"■-^■"■■^-'••■"'''^

■WOLFFIAN DUCT

^aa^l!^.-R^^...j^ai.^-,.^-.^-...,>^^„-...„^u-U;.t.-.. .>»,..;. :...■„.;, u:;,^.jj;v.i^>vi

C MESONEPHROS D. METANEPHROS

Fig. 334. — Diagrams illustrating the relations of A, pro-, B-C meso-, and D,
metanephridial tubules. The primary or pronephric tubules remove wastes from the
body -cavity, while the mesonephric tubules get their wastes directly from the blood;
many of them, however, retain the original connexion with the coelom. The meta-
nephric tubules lose all connexion with the coelom. (Redrawn after Corning)

THE UROGENITAL SYSTEM 405

in their eggs and which consequently have a prolonged larval period.
Usually pronephric tubules are not connected with glomeruli. The
associated glomeruli, instead of connecting directly with the tubules
through a Bowman's capsule, project into the body cavity in the neigh-
borhood of the nephrostomes. In this way wastes excreted into the body
cavity find their way indirectly to the tubules of the pronephros and
thus to the cloaca. In some cyclostomes and bony fishes, however, the
pronephros includes a segregated portion of the body cavity. This
becomes a pronephric chamber into which an inner glomerulus projects.

It has been suggested that the glomeruli of the pronephros originally
belonged to the gills and not to the urinary system, and only with a reduc-
tion in the number of gills came to serve the pronephros. Such a sugges-
tion is quite consistent with the fact that the gills of fishes supplement the
kidneys as excretory organs.

Mesonephros. The fact that the mesonephric tubules arise later
than the pronephric tubules during ontogenesis by no means proves that
they are phylogenetically younger, since the difference in time of appear-
ance in the embryo may be only an instance of the law of antero-posterior
differentiation in accordance with which anterior structures in the embryo
develop earlier than posterior ones. The chief reason for thinking that
the pronephros is phylogenetically older than the mesonephros is the fact
that the pronephros is more conspicuous in cyclostomes than in fishes and
amphibia.

The mesonephros is the functional kidney of anamnia. Its tubules,
like those of the pronephros, are mesodermal and derived from the inter-
mediate cell mass, the nephrotomy They utilize the primitive (pro-
nephric) duct as an outlet. From their structure, development, and
relations, they are seen to be serially homologous with the pronephric
tubules. This is one of the facts which has led morphologists to postulate
an archinephros which consisted of a series of homologous tubules extend-
ing throughout the length of the trunk, and possibly, as in annelids,
throughout the entire length of the body. If the latter assumption is
made, it is also necessary to assume a pair of nephridia and a pair of
coelomoducts in each metamere, as in some annelids, and that in acrania
coelomoducts have degenerated while in the gill region nephridia have
persisted.

It may be reasonably assumed that, like the pronephric tubules, those
of the mesonephros were metameric in origin. Indeed, they are metameric
in the embryos of elasmobranchs. In amniotes, however, the mesonephric
tubules branch repeatedly and lose their primary metamerism. In the
posterior portion of the mesonephros, the intermediate cell mass, nephro-
tome or mesomere, does not divide into segments, but forms its tubules
from an unsegmented mass. The mesonephric tubules differ from pro-

4o6

COMPARATIVE ANATOMY

nephric tubules also in the fact that they acquire connexions with glomeruli
and thus are enabled to eliminate liquid wastes directly from the blood
vessels and not indirectly by way of the body cavity.

In the male the anterior portion of the mesonephros changes its
function from that of an excretory organ to that of a reproductive organ.
In other words, the tubules of the anterior part of the mesonephros

Fig. 335.- — Diagrams of urogenital structures in (.4) indifferent and in female
elasmobranchs and amphibians; {B) male elasmobranchs and amphibians; (C) male
amniote (mammal); (Z?) femal'e amniote (mammal), b, urinary bladder; c, cloaca;
e, epididymis;/, Fallopian tube ; g, gonad; /?, " stalked hydatid "; ^, kidney (metanephros) ;
I, longitudinal tubule; m, MuUerian duct (oviduct), rudimentary in B and C; mn,
mesonephros; o, ovary; ot, ostium tubae abdominale; pd, paradidymis; po, paroophoron;
pv, parovarium; r, rectum; ti, testis; u, uterus; iia, urethra; nr, ureter; va, vas aberrans;
vd, vas (ductus) deferens; ve, vasa efferentia; w. Wolffian duct, urinary in A, urogenital
in B, genital in C and rudimentary in D. (From Kingsley's " Comparative Anatomy of
Vertebrates.")

return to the original function of the coelomoducts from which, assuming
an annelid ancestry, they are derived. (Fig. 335)

Metanephros. The third kidney or metanephros appears not to be an
amniote novelty, since transitional conditions between mesonephros and
metanephros occur in some amphibia in the form of a combined meso-
nephros-metanephros, the opisthonephros. In the amniote embryo, the
nephrogenic tissue from which the excretory tubules of the metanephric
kidney arise resembles that which forms the mesonephros. The tubules
. of the metanephros, like those of the posterior portion of the mesonephros,
arise from the nephrotome and are non-metameric in origin.

THE UROGENITAL SYSTEM 407

Even the ureter of amniotes seems not to be a novelty in the group.
In some elasmobranchs, a secondary renal duct arises as an outgrowth
from the primitive duct in a manner similar to that of the ureter. A
urinary duct, which in the embryos of some reptiles drains the posterior
portion of the mesonephros, develops like the ureter of amniotes as an
outgrowth of the mesonephric duct. The fact that in some mammals
and in man a number of ureters may develop, suggests the possibility
that ureters were primitively multiple but that they have become reduced
in number in mammals. The present ureter has been regarded also as
an elongated renal tubule which secondarily acquires functional con-
nexions with the numerous tubules of the metanephros.

In the female amniote, the mesonephros disappears except in the form
of functionless rudiments, the epoophoron, the paroophoron, and the duct
of Gartner. In the male, the primitive mesonephric duct is utilized as a
ductus deferens. The anterior part of the mesonephros becomes the
epidid)nnis. Such relations are inherited from those of anamnia. Rem-
nants of the posterior portion of the mesonephros may persist in the adult
as the rudimentary paradidymis and ductus aberrans.

In vertebrates except most mammals, the excretions are poured into
the cloaca. This is also true of monotremes. A bladder comparable to
that of mammals makes its first appearance in amphibia as a hollow out-
growth from the floor of the cloaca. In neither the amphibians nor
reptiles, however, is the bladder directly connected with the excretory
ducts, so that excretions reach it only by way of the cloaca. In those
mammals which are without a cloaca, the ureters acquire direct connexions
with the bladder and open upon its dorsal surface.

The Reproductive System

Cells speciahzed as reproductive elements make their first appearance
in colonial protozoa. Sexual reproduction, however, involving the
union of two gametes occurs in all classes of protozoa. Protozoa conju-
gate periodically. The beginnings of differentiation of gametes appear
also in protozoa. In unicellular organisms transitional stages between
the union of similar gametes, isogamy, and that of speciahzed eggs and
spermatozoa, heterogamy, may be recognized. Such a differentiation of
gametes is generally interpreted as adaptive. The ovum contains the
food supply for the developing embryo and consequently loses the motility
which is retained by the spermatozoon as a way of insuring union of
gametes. Biologists are not agreed in regard to the meaning of sexual
reproduction. Some hold that it increases variability, while others
assume that it increases stability of species by checking extreme variation.

Coelenterates. Among coelenterates the individual may be hermaph-
roditic as in the case of hydra, or sexually differentiated as in the case of

4o8

COMPARATIVE ANATOMY
FLATWORM.

NEMERTEAN.

ANNELID.

VERTEBRATE EMBRYO.

PRIMITIVE VERTEBRATE.

Fig. 336. — Diagram illustrating the hypothetical evolution of the urogenital system
of vertebrates, beginning with the gonadic sacs of fiatworms. The diagram assumes
that the gonadic sacs of flatworms have become the coelomic pouches of chordate
embryos. The latter in turn unite to form the extended coelom of vertebrates. Coe-
lomoducts appear first in annelids and become in vertebrates the renal tubules. Verte-
brates "invent" a longitudinal (primitive) excretory duct. During phylogenesis the
region of proliferation of germ-cells becomes greatly restricted.

THE UROGENITAL SYSTEM 409

most jelly-fishes. Vegetative methods of multiplication are common in
the group. Some have a regular alternation of sexual and asexual
methods of reproduction. In the jelly-fishes, eggs and spermatozoa
arise in the endoderm and are discharged to the outside through the
mouth. In vertebrates a similar endodermal origin of primordial germ
cells has been asserted.

Flatworms. An advance towards the reproductive organs of chordates
is made by the flatworms, in which the gonads take the form of a series
of paired gonadic sacs. Sexes in the nemerteans are separate and the
gonadic sacs contain either ova or spermatozoa. Germ-cells are prolifer-
ated from the epithelial fining of the gonadic sacs, and are discharged to the
exterior through paired apertures. It is possible that the metamerism
characteristic of vertebrates had its origin in such a series of paired
gonadic sacs. (Fig. 336)

Annelids. The number of metameres in which germ-cells develop
is much reduced in annelids. The region of proliferation of eggs and
spermatozoa is also limited to restricted areas of the peritoneal lining
of the coelomic cavities. Paired and metamerically arranged coelomo-
ducts provided with ciliated internal apertures convey the germ-cells
to the exterior. In some species, the nephridia join longitudinal paired
ducts which in their development and relations resemble the primitive
kidney ducts of vertebrates. Their mesodermal origin, however, seems
doubtful. Consequently their presence may not be used as evidence to
support the annefid theory of vertebrate ancestry.

Protochordates. The gonads of balanoglossus have a striking resem-
blance to those of flatworms. Like the latter, they form a series of
paired sacs each of which opens to the exterior by an external aperture.
The gonads of amphioxus also are metamerically arranged in segments
10 to 35. From them the germ-cells escape to the peribranchial cavity
and through the posterior atriopore to the exterior. It has been suggested
that the relations of the gonads of amphioxus resemble those of the
coelomoducts of annelids as well as those of the pronephric tubules of
vertebrates.

Cyclostomes. The gonad of cyclostomes is peculiar in being a median
and unpaired organ which extends through nearly the entire length
of the body cavity. In myxine the right gonad alone persists in the adult.
Metamerism is, however, not evident. That vertebrates have metameric
gonotomes as well as myotomes has not been demonstrated. It is,
however, possible to believe that the elongated gonads of cyclostomes and
of some fishes have been formed by the fusion of primarily separate
metameric gonadic sacs of invertebrates. The early gonad of cyclostomes
appears to be hermaphroditic, but during ontogenesis changes into either
an ovarv or a testis. The factors which determine which of the two shall

4IO

COMPARATIVE ANATOMY

arise are obscure. Sex in cyclostomes, however, appears not to be as
definitely predetermined in the chromatin constitution of the fertiHzed
egg as it is assumed to be in higher vertebrates.

No special reproductive ducts are found in cyclostomes. The eggs
collect in the body cavity and pass to the outside by way of paired abdom-
inal pores, which in structure and relations resemble the paired pores
of the gonadic sacs of flatworms. The elongated body cavity of verte-
brates may be considered as formed by the fusion of the cavities of a
similar series of paired coelomic sacs. Indeed, evidence that this assump-
tion is correct is furnished by the ontogenesis of the body cavity in
amphioxus. In the embryos of this animal each mesodermal somite

-MESORCHIUM

A UROGENITAL SYSTEM - RANA B SECTION A-A' - RIGHT GONAD. C SECTION B-B' - LEFT GONAD.

Fig. 337. — The urogenital system of a young leopard frog showing a stage in the
transformation of the primitive gonad into a testis. In the left gonad the metamorphosis
is nearly completed. Only the small posterior lobe contains eggs. In the right gonad,
which appears like a young unpigmented ovary, some crypts containing spermatozoa
have already made their appearance. Most of the lobules, however, are filled with ova
as if the gonad were to become an ovary.

contains a separate cavity (coelom). As the somite extends around the
side of the body its ventral wall meets that of the somite opposite below
the intestine. By the disappearance of the double membrane thus
formed the coelomic cavities of the two somites become continuous.
Later, on the ventral side of the body in the trunk region, the series of
partitions which separate the cavities of successive somites disappear and
a continuous body cavity or coelom extending through many metameres
is formed. The extended body cavity of vertebrates is believed to have
had a similar phylogenetic origin through the fusion of such serial gonadic
sacs as occur in flatworms and nemerteans. This assumption is further
justified by the fact that the relations of the abdominal pores in verte-
brates resemble those of the gonadic pores of flatworms. Moreover, their
function is similar. In petromyzon both the primitive prcnephric ducts

THE UROGENITAL SYSTEM

411

and the abdominal pores open posteriorly into the urogenital sinus and
to the outside through a urogenital papilla.

Elasmobranchs. The gonads of elasmobranchs are paired, and in
some species greatly elongated, especially in the male. The testes
acquire secondary connexions with the anterior part of the mesonephros,
and use the mesonephric ducts as an outlet for the sperm. Elasmobranchs
retain abdominal pores, but they appear to be functionless. In this
group, the primitive pronephric duct splits longitudinally to form Wolffian
and Muellerian ducts. In the female the Wolffian ducts are purely

OVARIAN OVUM

DEGENERATING
OVUM

.-fby®5^j^0

SPERMATOZOA

INTERSTITIAL TISSUE

HISTOGENESIS OF TESTIS - RANA.
PORTION OF SECTION C-C MUCH ENLARGED.

Fig. 338. — An enlarged portion of a section of the posterior lobule shown in
337, A. The ovarian sacs are being changed into crypts containing spermatozoa,
degenerating sac on the left spermatozoa have already made their appearance.

Fig.
In a

excretory, while the Muellerian ducts form the oviducts. In the male,
the Wolffian ducts combine both urinary and reproductive functions, while
Mueller's ducts atrophy to form the uterus masculinus. Many elasmo-
branchs are viviparous, and the eggs are retained until hatched within
uterus-like enlargements of the oviducts. In some species, the embryonic
yolk-sac, which is richly supplied with blood-vessels, unites with the wall
of the oviduct to form a yolk-sac placenta.

Amphibia. The gonads of amphibia resemble those of elasmobranchs.
As in the latter, the testis becomes connected by means of vasa efferentia
with anterior tubules of the mesonephros, and this part of the mesonephros
tends to atrophy and lose its excretory function. The Wolffian duct
serves as a urogenital outlet in the male. The Muellerian ducts, which
persist as rudiments in the male, become the oviducts of the female.

412 COMPARATIVE ANATOMY

As in elasmobranchs, the oviducts open far forward in the body cavity
near the liver. The primary gonads show the hermaphroditic potencies
manifest in cyclostomes. Ovaries and testes at first resemble one another
and have the appearance of ovaries, but either before or after meta-
morphosis of the larva, the ovary-like gonad of the male is transformed
into a testis. In exceptional instances, this metamorphosis is retarded
and may take place in a full-grown individual. Frequently the trans-
formation of an ovary into a testis may occur on one side before it has
begun on the other. Such an individual may appear superficially as a
unilateral gynandromorph. The so-called fat-bodies of the frog are
formed from the anterior portion of the genital ridges and appear to serve
as a reserve food-supply for the germ-cells. (Figs. 337, 338)

Reptiles. The differences between the reproductive systems of
amphibia and reptiles are relatively slight. In the reptiles the gonads
of both sexes have shifted posteriorly. Also the beginnings appear of a
division of the cloaca into a dorsal rectal, and a ventral urogenital moiety;
but these affect only the anterior part of the cloaca, which remains
undivided posteriorly. The excretory and reproductive ducts which in
amphibians open on the dorsal side of the cloaca, shift their connexions
ventrally in reptiles so as to open into the new urogenital passage.

A phallus comparable with that of mammals also appears first in
reptiles, notably in chelonians and crocodiles, in the form of a shallow
"seminal" groove in the floor of the cloaca. This groove is bordered by
paired "seminal" ridges, each of which contains erectile tissue. Pos-
teriorly the groove terminates in a free swelling or glans which also con-
tains erectile tissue. When the erectile tissue of the seminal ridges is
distended, the groove between them is converted into a tubular passage
which serves as an intromittent organ to convey the semen to the cloacal
cavity of the female.

Mammals. The genital system of monotremes differs little from that
of reptiles and is readily derived from it. In the male, the genital portion
of the mesonephros is converted into an epididymis, while the mesonephric
duct becomes a ductus deferens. The remainder of the mesonephros
atrophies, although remnants persist as a paradidymis. In higher
mammals, the testes migrate into a scrotal sac.

In the female monotreme, the gonad produces large yolk-laden ova
similar to those of reptiles. In the placental mammals, however, the
ova are reduced in size, and the embryo depends for its nourishment
upon the mother. The proliferation of oocytes ceases early in the life
of the mammal, and the number of mature eggs produced by the ovary is
greatly reduced as compared with the number in anamnia. Like the
testis, the ovary acquires connexions with the mesonephros; but the
connexion never becomes functional, and the mesonephros persists only

THE UROGENITAL SYSTEM

413

in the form of a rudimentary epoophoron and paroophoron. The meso-
nephric duct in the female becomes the functionless Gartner's duct.
The descensus of the ovaries is slight compared with that of the testes.

With the development of a horizontal longitudinal septum, the
cloaca disappears in placental mammals, and two cavities, the urogenital
and rectal, take its place. As this change occurs the ureters shift their
connexions from the cloaca to the bladder while the ductus deferentes
open into the urogenital cavity or urethra.

Oviduc-t
~~ Uterus -

Vagi

na

Fig. 339. — Four types of uteri occurring in different groups of mammals. A, duplex,
the type found in rodents; B, bipartite, the type found in certain carnivores; C, bicor-
nate, the type found in most insectivores and prosimians; D. simplex, the type
characteristic of the primates. (From Patten's "Embryology of the Pig," after
Wiedersheim.)

In female mammals a tendency of the Muellerian ducts — which
parallel the Wolffian ducts and lie medial to them — to fuse in the median
line is evident. Beginning with the monotremes the posterior part of
the Muellerian ducts fuse into a single uterus, while their anterior portions
remain separate as the paired uterine or Fallopian tubes. The monotreme
uterus opens into the urogenital sinus. In the higher mammals a vagina
for the reception of the penis is differentiated between the uterus and
the urogenital sinus. The vagina is therefore seen to be a portion of the
united Muellerian ducts. The duplex character of the vagina appears
in the marsupials (didelphians) which have two vaginae. In placental
mammals, however, the vagina is single. Four stages in the increasing

414

COIkCPARATIVE ANATOMY

fusion of the uteri are represented in the placental mammals — uterus
duplex, bipartitus, bicornis, and, in primates, uterus simplex. Even
in the primates, however, the original duplex character of the Muellerian
ducts is retained in the paired uterine tubes. (Fig. 339)

External genital organs also make their appearance in mammals.
Copulatory organs are, however, not wholly new in this group. Some
of the flatworms have an intromittent organ by which sperm is conveyed

Fig. 340. — Diagrams of male urogenitalia in I, monotreme; II, marsupials; and III,
monodelphs. a, anus; b, bladder; c, cloaca; cm, corpus cavernosum urethrae; c/), corp.
cav. penis; cd, Cowper's gland; p, perineum; pg, prostate gland; r, rectum; s, symphysis
pubis; t, testis; u, ureter; v, vas deferens; vg, vesicular gland; vm, ventral muscles.
(Prom Kingsley's "Comparative Anatomy of Vertebrates," after Weber.)

to the seminal receptacle of another individual. In elasmobranchs,
the pelvic fins of the male are modified as claspers which in copulation are
inserted into the cloaca of the female. None of these structures however
are morphologically comparable with the phallus of mammals.

The free posterior extremity of the paired erectile folds in the floor
of the cloaca of chelonia forms a glans.

In monotremes likewise, the phallus points posteriorly and is contained
in the floor of the cloaca. In the walls of the monotreme cloaca, the
seminal groove of the reptile has become a tubular canal surrounded by
erectile tissue, the corpus cavemosum urethrae. This canal is used in

THE UROGENITAL SYSTEM 4I 5

monotremes for the passage of sperm only, while urine passes from the
bladder to the cloaca by way of a urinary canal. In addition to the
erectile tissue of the corpus cavemosum urethrae, paired masses of
erectile tissue, the corpora cavernosa penis, make their appearance in
the phallus of monotremes. The posterior free portion of the phallus
of monotremes becomes more elongated than in reptiles, and is surrounded
by an integumentary fold, the preputial sac, which disappears during
erection. (Fig. 340)

In marsupials and placental mammals, with the disappearance of
the cloaca the phallus becomes more independent as an external penis.
The urinary canal of monotremes disappears, and the urine is conveyed
to the outside through the urogenital canal or urethra. The glans penis
with an enlarged corpus cavernosum urethrae persists. In the female, it
forms the rudimentary glans clitoridis. In the marsupials, in correlation
with the presence of a double vagina, the penis has a forked termination.

While the preputial sac of the phallus of monotremes, marsupials,
and lower placental mammals is directed posteriorly, in most higher
mammals, the sac shifts its direction so as to point anteriorly. Finally,
in primates the penis is released from the skin of the abdominal wall,
the organ becomes pendulous, and the preputial sac opens downwards.

In the female, the external genitals correspond with those of the male
but in a rudimentary form. The clitoris is the homolog of the glans penis
of the male. The remainder of the male phallus is represented in the
female by the labia minora. A corpus cavernosum urethrae is lacking
in the female. The labia majora correspond to the scrotum of the male.

THE UROGENITAL SYSTEM OF MAN

Although the excretory and reproductive systems of mammals are
so closely associated that it is difficult to describe them separately,
their wide divergence in function makes this desirable.

Urinary Organs

The urinary apparatus of man and all placental mammals consists
of four parts: excretory glands, the kidneys; urinary ducts, the ureters;
a urinary reservoir, the bladder; and the external outlet of the bladder, the
urethra.

The Kidneys. The kidneys in man are bean-shaped organs, lying
in the lumbar region, closely pressed against the dorsal body- wall, and on
account of the large amount of blood in them, of a deep reddish color.
Each is approximately four inches long, two inches wide, and an inch
and a half thick. The upper end of each kidney overlaps the twelfth
rib, the left kidney being usually somewhat higher than the right. The
lateral border of each kidney is convex; the medial border is concave,

4i6

COMPARATIVE ANATOMY

with a slit-like aperture, the hilum. The kidneys are held in position
])oth by the peritoneum and by a connective tissue renal fascia. Only
upper (anterior) and lower (posterior) surfaces of the kidneys are covered
with peritoneum, the middle of the right kidney being crossed by the
colon, that of the left by the pancreas. These relations, however, are
subject to considerable variation.

Structiire. A cross section of the human kidney in the region of the
hilum shows that under its peritoneal and fatty investments the kidney

PROXIMAL CONVOLUTED TUBULE,
DISTAL CONVOLUTED TUBULE,

ARCUATE ARTERY-
ARCUATE VEIN

COLLECTING TUBULE -rrrt-r
HENLE's LOOP-^j)

COLLECTING TUBULE PAPILLARY DUCT

EXCRETORY TUBULE

CALYX— T •

Fig. 341. — A diagram of the finer structure of a kidney. ^ is a section of the entire
kidney showing the contrast between cortical and medullary regions. The relations
of the chief arteries and veins are indicated. 5 is a reconstruction of a single tubule,
showing its relations to the blood and the pelvis of the kidney. (Reproduced in modified
form from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, 1930 by
Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf,
Inc., authorized publishers.)

is covered with a thin but tough fibrous capsule, which in a young kidney
may be pulled off like the skin of an orange.

The substance of a kidney consists of an outer cortex and an inner
medulla. A variable number, three to twenty, of renal pyramids form
the medulla. Straus has shown that among Primates man and the
spider monkey are peculiar in having a multipyramidal kidney, while
the anthropoid apes have monopyramidal kidneys. Each pyramid has its
base upon the cortex, and a cone-shaped apex which projects as a papilla
into the renal sinus. The cortex, which is about half an inch thick,
appears striated from the presence of cortical rays. Between the renal
pyramids, the cortex extends to the renal sinus in the form of renal col-
umns in which blood is conveyed to and from the cortex. The renal
sinus is filled by the expanded termination of the ureter, the pelvis.

THE UROGENITAL SYSTEM

417

Renal Tubules. The structure of the kidney as just described is
determined by the arrangement of the kidney tubules, which are the
functional units of the kidney. In the cortex, the renal tubules are
convoluted and connected with knots of blood capillaries, the glomeruli.
The pyramids, on the other hand, consist chiefly of straight collecting

Renal corpuscle. Convoluted tubules. Cortical ray.

Interlobular vein.

/

Henle's loop. Arciform vein. Arciform vein.

Fig. 342. — Part of a radial section of a human kidney. At x a renal corpuscle has
dropped out. X5. (From Bremer's "Text Book of Histology.")

tubules, which open into the renal pelvis by numerous apertures lying
at the ends of the papillae.

Each renal tubule begins in a spherical capsule surrounding a glomeru-
lus, the combined structure being known as a renal corpuscle. The
swollen termination of a tubule is a Bowman's capsule. The portion
of the tubule adjacent to the corpuscle is convoluted and thickened, and
is known as the proximal convoluted part of the tubule. Passing into
one of the pyramids, the tubule becomes slender and straight, and is

4i8

COMPARATIVE ANATOMY

known as the descending limb of Henle's loop. Bending sharply, it
forms the ascending limb of Henle's loop, and returns to the cortex, where
it becomes convoluted again, the distal convoluted portion, and unites
with a collecting tubule. As an element in a cortical ray, each collecting
tubule passes from the cortex into a pyramid. Usually a group of col-
lecting tubules unite and open by a common aperture into the pelvis
of the ureter. There is evidence that the secretion of urine takes place
chiefly in the renal corpuscles and in the convoluted portions of the
tubules, while the limbs of Henle's loop are chiefly absorptive in function.
The high degree of concentration of the constituents of urine leads physi-
ologists to assume that the quantity of water excreted by the renal
corpuscles is many times greater than that which finds its way through

PROXIMAL -CONVOLUTED TUBUL;

COLLECTING TUBULE

CAPILLARY ^,

GLOMERULUS

A. CORTEX B. MEDULLA

Fig. 343.- — Sections — A and B — of the human kidney. Section A is taken from the
region of the cortex, B from the medulla. Glomeruli which are abundant in the cortex
are lacking in the medulla. (Redrawn from Bremer after Schaper.)

the collecting tubules into the ureter, and consequently to conclude that
most of this water is reabsorbed by the tubules. Their great length is
assumed to be an adaptation to this absorptive function. (Fig. 341)

Blood Supply. Arteries enter and veins leave the kidney by way of
the hilum on the median side of each kidney. Within the renal columns,
branches of the renal artery form interlobar arteries. Passing between the
medulla and cortex, these, in turn, become the arcuate arteries, and
give off branches both to the medulla and the cortex. In the cortex,
they are known as interlobular arteries, from which branches supply the
glomeruli. The arteriole which carries blood to a glomerulus is larger
than that which leaves the glomerulus. By this arrangement, blood
pressure in the glomerulus is considerably greater than that in capil-
laries generally, so that filtration through the glomerular capillaries is
increased. The renal veins parallel the arteries and receive the same
names. Lymphatics and sympathetic nerves are abundant.

THE UROGENITAL SYSTEM 419

Ureters. The renal ducts or ureters convey the secretions of the
kidneys to the bladder. Each ureter is a tube about a quarter of an
inch in diameter and about twelve inches long. Within the sinus of
the kidney, each ureter enlarges into a renal pelvis, which fills the sinus
and branches into the renal calyces. The human kidney has two major
calyces, superior and inferior, and each of these subdivides again in three
to five minor calyces. Each of the minor calyces is connected with one
or more renal papillae and in this way receives the urine from the col-
lecting tubules. The two ureters open into the bladder on its posterior
surface, about an inch from the beginning of the urethra. Three layers
appear in a cross section of an ureter, an inner mucosa of transitional
epithelium, a muscularis coat of longitudinal and circular muscles, and an
outer adventitia of loose connective tissue.

The Bladder. The bladder is a muscular sac, the shape and size of
which vary greatly since, when distended with urine, it may increase
twenty to thirty times its size when empty. A median connective tissue
ligament, the urachus, a rudiment of the embryonic allantoic stalk,
extends from the bladder to the umbilicus. The hypogastric arteries
of the embryo degenerate to form lateral imibilical ligaments connected
with the bladder. Other ligaments in addition to these serve to hold
the bladder in position. Viewed from within, the dorsal wall of the
bladder shows a triradiate figure called the trigone, formed by three
ridges extending from the two openings of the ureters and from the
urethra.

The lining of the bladder is a layer of transitional epithelium capable
of greater distension. Next to this is a layer of loose connective tissue
with many elastic fibers. The greater portion of the thickness of the
wall of the bladder is, however, a coat of smooth muscle fibers arranged
in three layers, to which nerve fibers are distributed from both the spinal
cord and the sympathetic system. At the origin of the urethra, a thick-
ened ring of muscle forms a sphincter. In the act of urination, nerves
stimulate the muscles of the bladder to contract, and inhibit the contrac-
tion of the sphincter muscle of the urethra.

Urethra. The passage from the bladder to the external orifice is
the urethra. Its length differs in the two sexes, being about four inches
long in the male and an inch to an inch and a half in the female. In the
male urethra, three portions are distinguished, a prostatic portion, which
is surrounded by the mass of the prostate gland and into which the
ejaculatory ductus deferentes open; a membranous portion, which is
constricted where the urethra passes through the urogenital trigone or
diaphragm; and a cavernous portion surrounded by the corpus cavernosum
urethrae. The diameter of the urethra differs considerably among these
regions.

420

COMPARATIVE ANATOMY

THE UROGENITAL SYSTEM 421

Reproductive Organs: Female

Ovaries. The ovaries are paired whitish almond-shaped organs from
one to two inches in length lying on the sides of the pelvis, usually with
the long axis parallel to that of the body. Each ovary is attached by a
thick membranous mesovarium to the broad ligament which supports
the uterus. The ovary is also attached to the side of the uterus by an
ovarian ligament. Another suspensory ligament carrying blood vessels
and nerves extends upwards from the ovary along the wall of the pelvis.

youno ovum

Miron* j . .

railattt-v i ^jnr>e.solhel»u.Tn

V J ^^^
--C:^ V-" "^""^ - — -f^^r^ ..— ^' <''' ^ stra\iim oranatosum

"iunica, interna, oi thwa

coaoulkted * i n\ "^^

Vvaoftp JolUcuVi' viteWvne "^vasa ^tuwiea txUrna <A Ihcta

\n anUum membrtine

Fig. 345. — Drawing of an ovarian (Graafian) follicle approaching maturity, show-
ing details of its structure and relations. (From Patten's "Embryology of the
Pig.")

The ovary is covered by a layer of columnar epithehum, which is
regarded as a special modification of the peritoneum. Most of the
substance of the ovary is a connective-tissue stroma containing some
smooth muscle libers. In the cortex are ova in various stages of develop-
ment, surrounded by follicle cells. As an ovum approaches maturity,
the follicular layer of cells which surrounds it increases in thickness.
Eventually, a liquid-filled cavity appears in the follicle and the ovum is
crowded to one side. As the folHcular liquid increases, the follicle migrates
toward the surface of the ovary. The pressure is finally sufficient to
burst the follicle, and the ovum escapes into the uterine tube, the coelomic
opening of which, the ostiimi tubae, almost completely surrounds the
ovary. Sections of mature ovaries show their Graafian follicles in various

422

COMPARATIVE ANATOMY

Stages of growth. That portion of the follicle which remains in the
ovary is converted into an endocrinal tissue the corpus luteum. If
the ovum is not fertilized and therefore does not become implanted in the
uterus, the corpus luteum soon degenerates to form a corpus albicans,
which eventually disappears. (Figs. 344, 345)

Remnants of the embryonic mesonephros remain associated with
the ovary throughout life, as the epoophoron, paroophoron, and the
stalked hydatid of Morgagni. So far as is known, these are functionless

Serosa.

\ Longitudinal muscles.
^\ Blood vessels.
Circular muscles.

Mucosa.
Fig. 346. — Cross section, near the ampulla, of a uterine tube from an adult woman.
(From Bremer's "Text Book of Histology.")

rudiments of organs functional in lower vertebrates. Ovarian arteries
and veins enter the ovary besides a branch of the uterine artery. The
nervous supply is sympathetic. In addition to the reproductive function,
it is generally assumed that the ovaries serve as endocrinal organs which
in ontogenesis determine the secondary sexual characters of the female.

Uterine Tubes. The uterine tubes, which convey the ova to the
uterus, vary in length from three to five inches. Near the uterus each
tube is straight, but as it approaches the ovary it becomes sinuous and
enlarged in diameter. The termination of each tube is a funnel-shaped
structure, the infundibulum, which opens into the body cavity and

THE UROGENITAL SYSTEM

423

partially surrounds the ovary. The margin of this opening is prolonged
into numerous fimbriae which surround the ovary and convey the ova

^rUNOUS UTERI

NUCLEAR OIVrSION

IMPLAfTTATlON

-CORPUS UUTEUM

UTERINE TUBE

SPERMATOZOON

OVULATION

ST POLAR BODY STAGE

UTERINE GLANDS

Fig. 347. — Diagram illustrating the passage of the fertilized egg from the ovary to
the uterus. The ovum is a week in the tube and three days in the uterus before implan-
tation takes place. (Redrawn after Dickinson's "Sex Anatomy," Williams & Wilkins Co.)

into the uterine tube. A ciliated columnar epithelium Hnes the tubes,
the cilia beating towards the uterus and carrying the ova in that direction.

Fig. 348. — Diagram of uterus simplex. /, opening of Fallopian tube; g. uterine
glands; m, muscular layer (no attempt to indicate direction of fibres); tnu, mucosa;
o, early implanted ovum. (From Kingsley's "Comparative Anatomy of Vertebrates.")

The number of cilia is greatly increased by the complex folding of the
lining of the tubes. It has long been a question how spermatozoa make

424

COMPARATIVE ANATOMY

their way up the tubes against the action of these ciHa; but the walls
of the tubes have strong circular and longitudinal muscular layers, so
that it is possible that spermatozoa are carried up the tube by peristaltic
waves. The blood supply of the tubes comes from ovarian and uterine
vessels. The nerves are sympathetic. (Fig. 347)

Disinteg^ating._l^^l^!Kl. f iiJ.
epithelium. Va-V-;,'?^' vj v,' •<■' ■*

Blood vessel,

-4^^S

£V^''\-'

¥'^T

"Surface epithelium.

"0 6;;^ •■••'■ 5 • ='!■•• '■•■"■"« a f-t«' :3?T'^'i^^^''^-^" Disintegrating

Uterine gland. '-•'■^'^':-)(^i>7^%^^^^ P'|-^ i^V^'-t^''*""" ^P'^'^s^^"™-

Cystic tubule.

Blood vessel.— ".

, Pit-like depression.

Bifurcating tubule.

— Coiled tubule.

-1%-yjf^^ Blood vessel.

Muscularis.

Fig. 349. — Mucous membrane of a virgin uterus during the first day of menstruation.
X30. (From Bremer's "Text Book of Histology," after Schaper.)

The Uterus. The uterus is a median pear-shaped organ suspended
in the broad ligament. Round and uterosacral ligaments form additional
supports. The body of the uterus is normally bent forward so that it
rests upon the bladder. By a constriction, the isthmus, the uterus is
divided into an upper body and a lower cervix. Its cavity is connected
with that of the vagina below and those of the paired uterine tubes above.

The cavity of the uterus is lined with a simple ciliated columnar
epithelium, from which numerous tubular glands of unknown function

THE UROGENITAL SYSTEM

425

grow down into the underlying connective tissue. This mucous hning,
the endometrium, is partly lost during menstruation with some extravasa-
tion of blood. The thick muscular wall of the uterus consists of inter-
woven masses of^ smooth muscle fibers. Uterine and ovarian vessels
provide the blood supply. The nerve supply is sympathetic.

The Vagina. The vagina provides the outlet of the reproductive
apparatus. It is a highly distensible muscular canal between two and
three inches in length, extending from the lower end of the uterus, the
OS uteri, to the vestibule. Previous to sexual congress its external opening
is more or less completely occluded by a membranous hymen. The vagina

^-OVARIAN LIGAMENT

PUBIC-
SYMPHYSIS

'UROGENITAL
DIAPHRAGM.

Fig. 350. — The urogenital system — female. (Sobotta.)

is Hned with squamous stratified epithehum free from glands. Its
muscles are divided into circular and longitudinal layers of smooth fibers.
Near its orifice the bulbo-cavernosus muscle acts as a sphincter.

External Genitals. The external genital organs in the female form
a median longitudinal groove extending from the mens pubis in front
nearly to the anus behind. This groove is bordered by two fleshy folds,
the labia majora, which contain considerable fatty tissue, and which are
somewhat beset with hairs both on their inner and outer surfaces. Usu-
ally, the median edges of the labia majora are in contact with one another,
thus conceaHng the remaining parts of the external genitals. Sebaceous
and sweat glands are numerous on the median surfaces. (Fig. 350)

Between and more or less concealed by the labia majora is a second
pair of integumentary folds, the labia minora, which are devoid of fat

426

COMPARATIVE ANATOMY

and of hairs. The space between the two labia minora is the vestibule,
at the ventral end of which projects an erectile organ, the clitoris. Both
urethra and vagina open into the vestibule. On the sides of the vestibule,
are the openings of the greater vestibular glands or glands of Bartholin,
which correspond with the bulbo-urethral glands of the male.

Male Reproductive Organs

Testes. The male reproductive glands are lodged in an integumentary
pouch, the scrotum, which is divided by a median septum into two

compartments. This septum de-
velops through the union of two
fleshy folds of skin comparable with
the labia majora of the female, the
line of fusion being marked exter-
nally by a ridge, the raphe.
Included in the wall of the scrotum,
is a cremaster muscle, the fibers of
which are striped, and by contraction
lift the scrotum and testes.

The testes are oval glands which
have migrated into the scrotum from
the body cavity through the paired
inguinal canals, and which have
both reproductive and endocrinal
functions. Attached to the posterior
border of each testis is a mass of
efferent tubules forming the
epidid3miis. Each testis is enclosed
in a tough connective tissue capsule,

capitis; «. nucleus; nk., neck; p., proto- ^he tuuica albugiuea, which pene-
plasm; ^.c, posterior centrosome. (From trates into the Substance of the

C E F

Fig. 351. — Diagrams of the develop-
ment of spermatozoa, a.c, anterior
centrosome; a./., axial filament; c.p., con-
necting piece; ch.p., chief piece; g.c, galea

Bremer's "Text Book of Histology," after
Meves.)

testis as septula and divides the

testis into lobules.
The testis is a compound tubular gland composed of convoluted
seminiferous tubules which are held together by interstitial connective
tissue. In each lobule several of the seminiferous tubules unite into a
common efferent tubular outlet. These efferent tubules, the tubuli
recti, in turn combine into a network of efferent tubules, the rete testis.
From the rete testis pass the efferent ducts which convey the sperm from
the testis into the ductus epididymidis. These relations are shown in

Fig. 352.

The seminiferous tubules are lined with an irregular many-layered
epithelium from which the spermatozoa are proliferated, the multiplica-

THE UROGENITAL SYSTEM

427

tion of cells beginning in the basal layers. During this process a
reductional or maturation division takes place previous to the meta-
morphosis of the cells into mature spermatozoa. All transitional stages
in the conversion of epithelial cells into spermatozoa may be seen in a
cross section of the seminiferous tubules. The Sertoli cells which occur
among the germ-cells are usually assumed to have a nutritive function.
As the spermatozoa lose connexion with the epithelium, they pass into
the lumen of the tubules, and thus find their way to the epididymis,
in which they may be retained for some time. They are contained in
a mucous alkaline liquid, also secreted by the epithelium of the seminifer-

SPERMATIC-
CORD.

PUBIC
SYMPHYSIS
CORPUS
CAVERNOSUM
PENIS.

CORPUS-
CAVERNOSUM
URETHRAE

UROGEN ITAL
"DIAPHRAGM.
•BULBOURETHRAL
GLAND.
BULBUS.

•RECTUM-
EXTERNAL
-SPHINCTER
MUSCLE.
-ANUS.

Fig. 352. — The urogenital system — male. The glandular complications of the male
urogenital system appear to be in part an adaptation to the double function of the
urethra — excretory and reproductive. The alkaline secretions of the glands serve to
neutralize the acidity of the urethra caused by the acid urine. (Sobotta.)

ous tubules. Diminution in the activity of the tubules is seen in old age,
and may begin as early as thirty-five years.

The interstitial tissue between the seminiferous tubules is believed to
have an endocrinal function and to influence the development of secondary
sex traits and the vigor of the individual.

Also associated with the testes are certain rudimentary organs,
ductuli aberrantes, paradidymis, hydatid of Morgagni, the significance
of which will be better understood after the description of their develop-
ment. Each testis is attached to the scrotum by a connective tissue cord,
the gubemaculum.

Ductus Deferens. By the efferent ducts of the testis, sperm is carried
to the ductus epididymidis, a much convoluted tube twenty or more feet

428 COMPARATIVE ANATOMY

in length, which together with the efferent ducts forms the head of the
epididymis. The ductus epididymidis is the beginning of the ductus
deferens and, like it, lined with ciliated columnar epithelium. The
cilia beat towards the urethra and carry the spermatozoa to the seminal
vesicles where they may be temporarily stored. Layers of circular and
longitudinal muscles are present in the wall of the duct. At the lower
end of the testis the ductus loops back along the tail or cauda of the
epididymis, and then leaves the epididymis to join the spermatic cord.
As a component of the spermatic cord it passes through the inguinal canal.
Entering the body cavity, the ductus leaves the spermatic cord and
passes medially to the ureter to enter the prostate gland where it becomes
the ductus ejaculatorius. As it approaches the prostate, it enlarges into
an ampulla and is joined by the duct of the seminal vesicle. Each
seminal vesicle is about two inches in length and three quarters of an
inch in diameter, formed by an elongated tube four to five inches long
coiled within a connective tissue capsule. It secretes continuously an
alkaline mucous fluid. (Fig. 352)

Prostate and Bulbo-urethral Glands. At the point where the ductus
ejaculatorius opens into the urethra, this passage is surrounded by a
conical mass of glandular and muscular tissue, the prostate gland. The
glandular portion of the prostate is formed by fifteen to thirty branched
tubular glands. These are embedded in connective tissue containing
compact masses of smooth muscle fibers. The development of the
prostate shows that it is a modified portion of the wall of the urethra.
Its alkaline mucous secretion, produced at times of sexual excitement,
has a stimulating effect upon the movement of spermatozoa. Character-
istic albuminoid concretions are formed in the alveoli of the gland. In
later years of life these concretions increase in size and number and
become calcified, so that the lumen of the urethra tends to become occluded
by the pressure of the prostate. Embedded in the prostate is a median
pouch, the prostatic utriculus or uterus masculinus, which opens by a
median aperture near the openings of the ejaculatory ducts. The
utriculus is a rudiment of the embryonic Muellerian ducts which in the
female become the uterus.

The bulbo-urethral glands or Cowper's glands are tubulo-alveolar
glands less than half an inch in diameter, embedded in the connective
tissue of the urogenital diaphragm near the bulbus urethrae. Their
ducts open into the cavernous portion of the urethra. At times of
sexual excitement they secrete an alkaline mucous liquid. They appear
to be the first portion of the urogenital apparatus which ceases to function
in old age.

The Penis. The male urethra extends into the intromittent organ,
the penis. Thus in the male the urethra serves both as an excretory and

THE UROGENITAL SYSTEM

429

as a reproductive outlet. Three portions of the urethra are recognized,
prostatic, membranous, and cavernous portions. (Fig. 352)

The body of the penis consists of three masses of erectile tissue,
paired corpora cavernosa penis and the unpaired corpus cavemosum
urethrae. The latter enlarges at the root of the penis into a bulbus
urethrae and terminates at the extremity of the penis as a swollen mass
of erectile tissue, the glans penis. In its flaccid condition the glans is
covered by the foreskin or prepuce. The paired corpora cavernosa are
prolonged into the peritoneal region as far as the tuberosity of the ischium.

Connective tissue,

Epithelium.

Red corpuscles in a
blood vessel.

Smooth
muicle fibers.

Fig. 353. — From a section of the prostate of a man twenty-three years old. The
epithelium is cut obliquely at x, and has artificially separated from the connective tissue
at XX. X360. (From Bremer's "Text Book of Histology.")

In this way they form the fixed portion of the penis, the crura penis.
To each crus is attached an erector muscle, the ischio-cavemosus.

The nerves of the penis are several. Branches of the second, third
and fourth sacral (spinal) nerves are known as erector nerves since their
stimulation causes the erection of the penis. Pressure upon the sym-
pathetic centers of the hypogastric plexus also stimulates erection. In a
flaccid penis the arterial blood supply is reduced through occlusion of the
lumen of the vessels by the contraction of local thickenings of their walls.
When, however, the artery is dilated, a free flow of blood into the venous
spaces of the corpora cavernosa causes them to become engorged with
blood and the penis consequently erected.

430

COMPARATIVE ANATOMY

DEVELOPMENT OF THE UROGENITAL SYSTEM
The Urinary System

The urinary organs, except the lining of the bladder, are mesodermal in
origin, both urinary tubules and ureter being formed from the nephrotome.
Strangely enough, three renal organs, pronephros, mesonephros, and
metanephros, develop in succession in the human embryo as well as in all
amniote embryos. The pronephros in man is a f unctionless rudiment.

NAL CORD

MVOCOEL

^^/MYOTOME (EPIMERE)

-PRIMITIVE DUCT

A.

MESOMERE

MYOTOME

NOTOCHORD
HYPOMfRE

INNER GLOMERULUS

AOR

NEPHROSTOMY

PRINCIPAL DIVISION

PRIMITIVE DUCT

SUPPLEMENTARY DIVISION
NEPHROSTOME
HYPOMERE

^OUTER GLOMERULUS
Fig. 354. — A diagram showing three stages A-C in the development of the primitive
duct. The duct and each tubule connected with it arises from the mesomere. Two
types of glomeruli — outer and inner — become associated with the pronephric tubules.
(Redrawn after Felix.)

The mesonephros probably functions during foetal life. The metanephros
is the definitive kidney. The occurrence of three kidneys in the embryos
of amniotes is best interpreted by the evolution theory.

Pronephros. The pronephros or "head kidney" is the anterior-most
of the three, and in the human embryo develops from the nephrotomes of
segments seven to fourteen. The first of the pronephric tubules makes its
appearance in a 1.7 mm. embryo, and all eight tubules are formed by the
time the embryo has reached a length of 2.5 mm. Degeneration begins
soon and the anterior tubules disappear before the posterior ones are
differentiated.

The development of pronephric tubules is initiated by the parietal
layer of the nephrotome or intermediate cell mass from which cells are

THE UROGENITAL SYSTEM

431

SPINAL CORD^

SPINAL
GANGLION _/.. J

1 MESONEPHRIC TUBULES

NEPHROSTOME

GLOMERULUS
POSTCARDINAL VEIN

PERITONEUM '\v PR'MiTIVE DUCT

MESENTERY PRONEPHRIC TUBULES

Fig. 355.— StcreLiferani ot the developing pronephros and mesonephros. (After Kingsley

modified.)

post, cardinal
vein

nephrostome

glomus

somite

dorsal aorta

D

nephrostome
' glomerulus

Fig. 356. — Drawings to show structure of nephric tubules. A. Pronephric tubule
from section through 12th somite of a i6-somite chick embryo. (After Lillie.) B.
Diagram of functional pronephric tubule. (After Wiedersheim.) C. Primitive meso-
nephric tubule with rudimentary nephrostome, from section through 17th somite of
30-somite chick embrj'o. D. Schematic diagram of functional mesonephric tubule of
the primitive type which retains the nephrostome. (After Wiedersheim.) (From
Patten's "Embryology of the Chick.")

432

COMPARATIVE ANATOMY

proliferated towards the ectoderm. The nephrotome loses its connexion
with the epimere above, and together with the lateral outgrowth just
mentioned forms a pronephric tubule. In most vertebrate embryos,
pronephric tubules are primarily solid, but become hollow later. As they
grow laterally towards the ectoderm, the pronephric tubules also grow
posteriorly and unite to form a mass of rapidly dividing cells which con-
tinue to extend posteriorly as a cellular rod until they reach the cloaca.
Connexion with the cloaca is established and a lumen forms. In this
way is produced the primitive or pronephric duct. Although the proneph-

MESONEPHRIC TUBULE

MYOTOME
DERMATOME X^iik

MESONEPHRIC TUBULES
NEURAL TUBE {

<PHORDA
««i-js -J /^ORTA

! GLOMERULUS NEPHROSTOME | GEMITALRIDGE I PRIMITIVE DUCT

PERITONEUM POSTCARDINAL VEIN MESENTERY COELOM

Fig. 357. — Stereogram of the developing mesonephros at a stage later than that of
Fig- 355- (After Kingsley modified.)

ric tubules which produced the primitive duct degenerate soon after their
appearance, the duct itself persists as the Wolffian or mesonephric duct,
so-called because it forms the outlet of the tubules of the mesonephros.
In some vertebrates, but apparently not in the human embryo, the
pronephric tubules open into the body cavity and persist, at least in part,
in the adult as the ostium tubae (the anterior opening of the oviduct).

Mesonephros. The tubules of the mesonephros are formed from
nephrotomes posterior to those which form the pronephros. At its
full development in the human embryo, the mesonephros extends from
the sixth cervical to the third lumbar segment. Since as many as eighty-

THE UROGENITAL SYSTEM

433

three pairs of mesonephric tubules occur in
human embryos, it is obvious that, unUke
the pronephros, the mesonephros is not
metameric in its origin. While the anterior
nephrotomes are segmented, the most pos-
terior mesonephric tubules are derived from
a continuous unsegmented nephrogenic cord
or intermediate cell mass. At their first
appearance, the anlagen of the mesonephric
tubules are budded off as solid spherical
cell masses, which secondarily attain con-
nexion with the primitive duct, now become
the mesonephric duct.

The differentiation of mesonephric
tubules takes place thus: The spherical
cell-masses elongate towards the meso-
nephric duct and at the same time acquire
a lumen. While the medial portion of the
anlage enlarges into a vesicle, the Bowman's
capsule, the lateral end unites with the
mesonephric duct and finally opens into it.
Each vesiculated enlargement is converted
into a renal corpuscle by the ingrowth of
a knot of capillaries derived from the dorsal
aorta. A sharp bend of the mesonephric
tubule near the Bowman's capsule divides
it into a distal secretory portion and a
collecting portion proximal to the duct.
These changes, in conjunction with the
growth of capillary networks derived from
postcardinal and subcardinal veins and
the associated enlargement of the genital
folds, cause the combined organs to bulge
into the body cavity as urogenital folds.

Most of the mesonephric tubules de-
generate, only twenty-six pairs remaining in
a 20 mm. embryo. In the male, some of
these are converted into the efferent ductules
of the testes, and in this sex the mesonephric

Fig. 358. — Drawings showing the development
of the mesonephric tubules in the pig. d, meso-
nephric duct; t, mesonephric tubule; cap., glomerular
(Bowman's) capsule. (From Patten's "Embryol-
ogy of the Pig," based upon figures by McCallum
and Lewis.)

^WO-i

leteraU

-t >mesiat

ventral

434

COMPARATIVE ANATOMY

or Wolffian ducts become the ductus deferentes. It is generally assumed,
but not demonstrated, that the mesonephros functions as an excretory
organ in the embryo.

Metanephros. The definitive kidney of man and other amniotes,
the metanephros, is the last to appear in ontogenesis. Like the meso-
nephros, it has a double origin. The collecting tubules and the ureter
are derived from an outgrowth of the mesonephric duct. The cortex
of the kidney, on the other hand, arises from the posterior portion of

neural tube

notochord

dorsal aorta
afferent vessel
glomerulus

efferent vessel

somite

posterior

cardinal vein

collecting vein
connecting
sub- and post,
cardinals

capsule

mesonephric
duct
subcardinal
vein

A, B.

Fig. 359. — Diagrams showing the relations of the blood vessels to a mesonephric tubule.
(From Patten's "Embryology of the Pig," based on figures by McCallum.)

the nephrogenic cord in the lumbar region. In a 6 mm. human embryo,
the ureter appears in the form of a hollow outgrowth from the mesonephric
duct near its posterior end. At its anterior end, this outgrowth expands
into a vesicular enlargement. Growing dorsally, the vesicle comes in
contact with the nephrogenic cord which covers it as a cap. As the ureter
elongates, the nephrogenic cap is pushed anteriorly, and takes a position
dorsal to the posterior portion of the mesonephros. (Fig. 360)

The vesicular enlargement of the ureter becomes the pelvis. Two
outgrowths from it, one anterior and one posterior, form two major
calyces. Two more are added later between the first two. Minor caly-
ces arise by the continued subdivision and branching of the major
calyces. Further branching to the twelfth generation produces the
collecting tubules and the medullary portion of the kidney. Those of the
fifth generation to the number of twenty to eighty for each renal calyx

THE UROGENITAL SYSTEM

435

become the papillary ducts, which convey urine from the collecting tubules
and which open directly into each of the renal calyces. (Fig. 362)

Each of the collecting tubules terminates in a slight swelling or vesicu-
lated enlargement, upon which a mass of nephrogenic tissue rests as a cap.
,^$^g*^? FIRST CERVICAL

V- PRONEPHROS

--FIRST THORACIC
-GLOMERULUS

NEPHROGENIC,
TISSUE

METANEPHROS
PRIMITIVE DUCT/

CLOACA

NTESTINE

-^^^'^'^---FIP

URETER
RST LUMBAR

Fig. 360. — A diagram of pro-, meso-, and metanephroi in a 5-6 mm. human embryo.
Somites are cross-hatched. (Redrawn after Braus.)

From this mass arise excretory tubules, which subsequently acquire
connexion with the collecting tubule. Differentiation of the excretory
tubules begins with the formation of vesicular cell-clusters which separate
from the remaining nephrogenic tissue. These vesicles elongate into
excretory tubules; and their ends enlarge to form Bowman's capsules.
Subsequent changes involve the attachment of the tubule to the adjacent
collecting tubule, the elongation of the excretory tubule, and the ingrowth

436

COMPARATIVE ANATOMY

of a glomerulus into the Bowman's capsule. Arterial and venous con-
nexions are subsequently established similar to those of the mesonephros.
These changes occur in the later months of intra-uterine life. (Fig. 365)

mth

Fig. 361. — Profile reconstructions of lizard {Lacerta agilis) {A) i6 mm. long; {B)
20 mm. long; and (C) huinan embryo 115 mm. long, a, allantoic stalk; c, cloaca; cc,
cranial collecting tubule; cd, caudal collecting tubule; k, permanent kidney (meta-
nephros) ; met, median collecting tubule; ms, mesonephros; ml, metanephric (nephro-
genic) tissue; mtb, mesonephric tubules; pet, primary collecting tubule; pii. Wolffian
duct (primitive ureter); r, rectum; s, secondary collecting tubule; u, ureter; cm, u and
pu, common portion of primitive and permanent ureters. (From Kingsley's " Com-
parative Anatomy of Vertebrates," after Schreiner.)

TERTIARY COLLECTING TUBULES

SECONDARY
COLLECm NG
TUBULES '^'^

'PELVIS

Fig. 362. — The anterior expanded portion of the ureter in a 20 mm. embryo showing
the beginning of formation of collecting tubules. Anterior is to the right. (Redrawn
after Huber.)

Connexions with the Bladder. During the earlier stages of its devel-
opment, each ureter shares with a mesonephric duct a common lateral
opening into the cloaca. Between lo mm. and 17 mm. stages, the cloaca
becomes divided by a septum into a dorsal rectum and a ventral urogenital

THE UROGENITAL SYSTEM

437

sinus. The septum, which is completed during the seventh week, becomes
the perineum of the adult. After the septum is formed the mesonephric
ducts and ureters retain their connexion with the urogenital sinus. Even

NEPHROGENIC TISSUE

Fig. 363. — A diagram illustrating the repeated branching of the collecting tubules
in a nine weeks (30 mm.) human embryo. The diagram also shows the origin of
secretory tubules from the nephrogenic tissue. (Redrawn from Braus, after
Kampmeier.)

before the development of the septum is completed, in an 11 mm. embryo,
the urogenital sinus becomes subdivided into a vesico-urethral portion
into which the ureter and mesonephric ducts enter, and a phallic portion

CONNECTIVE TISSUE CAPSULE
NEPHROGENIC TISSUE

-COLLECTING TUBULE

GLOMERULUS
BLOOD VESSEL

Fig. 364. Section of the kidney of a 38 mm. human embryo. The section shows the
branching of the collecting tubules and the relation of their terminations to the nephro-
genic tissue from which the secretory tubules are derived. (Redrawn after Coming's
"Htunan Embryology.")

which extends into the genital tubercle. By the time the embryo attains
a length of 25 mm. (2 months), the ureters and mesonephric ducts are
separated, the ureters open into the bladder, and the mesonephric ducts
into the urethra. The bladder, therefore, arises not from the allantois

438

COMPARATIVE ANATOMY

NEPHROGENIC ':*»;
TISSUE ~'^.

ANLAGE OF "
EXCRETORY-
TUBULE (■■■>(

COLLECTING TUBULE-

B.

COLLECTING^
TUBULE

:.:m^ -■ --'

r^NEPHROGENIC TISSUE— ^lai

ANLAGE OF EXCRETORY ^r^-r
-^ TUBULE " '

;r COLLECTING TUBULE-
' ^^ GLOMERULUS'^

■J-- EXCRETORY TUBULE-,

Fig. 365. — Diagrams A-E illustrating the development of collecting and secretory
tubules of the kidney. Vesicles derived from nephrogenic tissue become attached to the
collecting tubules and thus functionally connected with them. The swollen termination
of each secretory tubule encloses a glomerukis and forms a renal corpuscle. E repre-
sents an earlier, D a later stage of development. (Redrawn after Coming's " Hiunan
Embryology.") ,

■ARCH OF COLLECTING
TUBULE

NAL CORF=USCLE

ARCH OF COLLECTING
TUBULE

distal convoluted tub.
■stdcrck's loop,
proximal convoluted

TUBULE

GLOMERULUS
BCWMAN's CAPSULE

0ESCEh40ING LIMB

GLOMERULUS

B- ^ STOERCK'S UDOP

Fig. 366. — Diagrams of three stages A-C in the growth and differentiation of a

uriniferous tubule. The collecting tubule is shown unshaded,
after Huber and Stoerck.)

(Redrawn from Arey,

THE UROGENITAL SYSTEM

439

but from the cloaca. The bladder anlage, however, is continued ventrally
as the allantoic stalk, which subsequently atrophies to form the middle
umbilical ligament.

ventral
mesentery

D.

ght and left
coelom confluent

-Diagrams illustrating the development of the mesonephros and gonadic
ridge. (From Patten's " Embryology of the Pig.")

Reproductive Organs

The human embryo is usually characterized as sexually indifferent.
This popular view is based upon the similarity of the anlagen of male and
female reproductive glands in the early embryo, and upon the occasional
appearance of hermaphroditic adult individuals. According to modern
genetical opinion, however, sex is definitely predetermined in the fertilized

440 COMPARATIVE ANATOMY

egg and is only exceptionally modifiable. Nevertheless, ovaries and
testes develop from morphologically similar genital folds, located between
the mesonephroi and the mesentery. At their first appearance, the genital
folds are elongated masses of epithelial cells, which become differentiated
into an external many-layered epithelium and an inner epithelial mass
derived from the peritoneum. Together with the mesonephros, the
gonad forms a urogenital ridge, the prominence of which is increased by
the growth of underlying adrenal tissue. (Fig. 367)

As the genital folds increase in size, longitudinal grooves develop
separating them from the lateral mesonephros and the median mesentery.
Connexion of the gonad with the mesonephros and with the body- wall
is finally reduced to a thin mesentery-like membrane. In the male, this
membrane forms the mesorchium and in the female the mesovarium
through which blood and nervous connexions are retained. The gonads
remain undifferentiated in a ten to twelve millimeter embryo.

Histogenesis of the Testis. By the time the male embryo has reached
a length of 13 mm., the gonad assumes characteristics which differentiate
it as a testis. One of these is the connective tissue tunica albuglnea
beneath the peritoneal epithelium. Within the medullary region of
the gonad, branched and anastomosing testis cords appear, separated by
less compact intermediate cords. Two kinds of cells may soon be seen
within the testis cords, primordial germ-cells and indifferent cells.
Opinion is divided whether the testis cords are formed by ingrowths from
the peritoneum or by concentration of epithelial cells within the medulla.
Uncertainty also prevails as to the source of the primordial germ-cells.
Some assert that they are derived from cells which migrate into the
gonad from the intestine. Others claim that they are peritoneal cells
which have entered the medulla. The latter opinion seems to have the
greater support. The fate of these cells is equally in doubt. Some
assume that they are the progenitors of the definitive germ-cells, the
spermatogonia, while others believe that they disappear and that the
indifferent cells become spermatogonia.

The testis cords converge towards the mesorchium as a center. At
the base of the mesorchium, they combine to form the tubules of the
rete testis. These in turn unite with tubules of the mesonephros. Finally
the testis cords become hollow and differentiated as seminiferous tubules.
Epithelial cells within the intermediate cords form interstitial cells
which are believed to have an endocrinal function. (Fig. 368)

Histogenesis of the Ovary. Differentiation of the ovary is slower
than that of the testis. The female embryo is from ten to eleven weeks
old (50 mm.) before ovarian characteristics appear. In contrast with
the testis, the ovary lacks a tunica albuginea and epithelial cords. At
three to five months, the primordial germ-cells in the medullary region

THE UROGENITAL SYSTEM

441

begin to disappear, and a new cortical zone is formed, probably derived
from the peritoneum. Rows of primordial ova known as Pflueger's
egg tubes extend through the cortex to the medulla. Their arrangement
suggests that the egg-tubes are proliferated from the peritoneal epithelium,
which therefore is known as germinal epithelium. In late fetal life the
primordial ova become surrounded by indifferent follicular cells and
differentiated as the definitive ova. The changes in the Graafian follicles
have already been described (p. 421). Medullary cords and a rete ovarii
comparable with the testis cords and rete testis appear as transient
structures in the ovary.

Reproductive Ducts. The mesonephric or Wolffian ducts are utilized
by the male as reproductive ducts, the ductus deferentes. Mueller's

CORTNER'S DUCT)

Fig. 368.

-Diagrams of the testis (A) and ovary (B) showing the homologies of their
components. (Redrawn after Mall modified.)

ducts are also developed in both sexes. Each Mueller's duct arises from
a longitudinal groove on the lateral side of the mesonephros. The
peritoneal epithelium sinks into the underlying mesenchyma. Except
at its anterior end, where it remains open as the ostium tubae, the groove
closes over and grows posteriorly as far as the cloaca into which it acquires
an opening.

Male Reproductive Ducts. In a three month embryo, the anterior
and posterior regions of the mesonephros differ. In the anterior region,
which consists of five to twelve renal tubules, the collecting portions
of the tubules separate from the excretory portions and acquire connexion
with the tubules of the rete testis. In this manner, anterior mesonephric
tubules form the ductuli deferentes and serve as outlets for the secretion
of the testis. The remaining, posterior, portion of the mesonephros
mostly degenerates, remnants persisting as the paradidymis and ductuli
aberrantes. The cranial portion of the mesonephric duct increases
greatly in length to form the ductus epididymidis. The posterior portion
becomes the ductus deferens.

In the male, Mueller's ducts begin to atrophy in the third month.
In the adult, remnants of the anterior and posterior extremities may

442

COMPARATIVE ANATOMY

persist as functionless rudiments. The former is the appendix testis
and the latter the uterus masculinus.

Remnants of the mesonephros also persist in the female. Some
anterior mesonephric tubules unite with the rete ovarii to form the
epoophoron. The posterior part of the mesonephros becomes the rudi-

ly

pronephric

tubules

pronephric tubules
(degenerating)

mesonephric J

pronephric
duct

tubules with 1
nephrostomes

S

i

— mesonephric
tubules

mesonephric

tubules without

nephrostom

i|

\ /

I
(

[

c

f

„,^,

w

pronephric

ef -" tubule

[:■ .^y~^"| mesonephric
■ C^^, >— tubules with

nephrosto

allantois

phric duct
phric duct

Fig. 369. — Schematic diagrams to show the relations of pronephros, mesonephros,
and metanephros at various stages of development. In the adult male the mesonephric
("Wolffian) duct is retained as the ductus deferens. (From Patten's " Embryology of the
Chick.")

mentary paroophoron. The functionless remnant of the Wolffian duct
in the adult is known as Gartner's duct.

Female Reproductive Ducts. Mueller's ducts parallel the meso-
nephric ducts and open into the urogenital sinus median to them. The
position of the primary opening marks the place where later the hymen is
located. During the fourth month, the posterior portions of the paired
Muellerian ducts unite to form uterus and vagina. The anterior portions
form the uterine tubes.

THE UROGENITAL SYSTEM

443

Descensus of Gonads. A comparison of earlier and later stages
reveals the fact that the gonads shift their position posteriorly in the body
cavity. The prime factor in this backward migration is the continued
growth of the posterior portion of the gonads and the associated atrophy
of the anterior portion. These processes result in the change of the
gonads from an abdominal to a pelvic position. The ovaries retain this
position throughout life but the testes migrate into the scrotal sac.

The testis is originally an abdominal organ like the ovary, and its
position in the scrotum the result of a migration or descensus in which it
drags with it blood vessels, lymphatics, nerves, and the cremaster externus
and internus muscles, which, together with the ductus, constitute the
spermatic cord.

During the third to the sixth month of development, paired out-
pocketings of the body cavity, vaginal sacs, extend ventral to the pubic

Fig. 370. — Schematic diagrams illustrating the descent of the testis as seen from the
side. d. def., ductus deferens; Proc. Vag., processus vaginalis (the diverticulum
of the peritoneum pushed into the scrotal sac). (From Patten's "Embryology of the
Pig.")

bones into the scrotal sacs. During the seventh to the ninth month,
the testes descend into the scrotal sacs. This "descensus" occurs not
into the vaginal sacs but beneath the peritoneum dorsal to the vaginal
sacs. Normally, the passage between the body cavity and the vaginal
sac is obliterated soon after the migration of the testis (7th to 9th month).
Failure to close results in liability to inguinal hernia. The condition of
undescended testes is known as cryptorchism and is accompanied by
sterility since spermatozoa are unable to survive the normal temperature
of the body. The scrotal sac appears to act as a thermoregulator.

The factors involved in the descensus of the testes are complex.
Chief among them appears to be the contraction of the connective tissue
gubemaculum testis which extends from the testis to the posterior wall
of the scrotum. The gubernaculum contracts to one quarter of its
original length and after the descensus almost completely atrophies.
During the course of its descent, each testis rotates through an arc of
180° so that its anterior and posterior ends are reversed.

Rarely, the ovaries undergo a similar descent into the labia majora.
Normally, however, the enlargement and relations of the uterus prevent
this migration.

444

COMPARATIVE ANATOMY

External Genitals. The external genitals of the two sexes, like the
gonads, have similar beginnings. Slight differences, however, quickly
make their appearance. In an 8 mm. embryo, a rounded eminence, the
genital tubercle, develops between the tail and the umbilical cord. Along
its caudal surface extends a shallow urethral groove bordered by urethral

A B

Fig. 371. — Development of the male external genitalia of man. A, indififerent stage
from which either sex may develop; B, early, and C, later stages; a, anus;/, genital folds;
g, urogenital groove; p, genital prominence; r, genital ridge (outer genital folds); s,
scrotum. (From Kingsley's " Comparative Anatomy of Vertebrates," after O. Hertwig.)

folds, the inner genital folds. Labial or scrotal swellings, the outer
genital folds, border the urethral folds laterally. When the embryo
has reached a length of 15 mm., the urethral groove is perceptibly longer
in the male. In both sexes the tubercle elongates to form a phallus,
the termination of which enlarges as the glans penis or the glans clitoridis.

PERITONEUM PERlTONaJM

URETHRAL GROOVE

URETHRAL GROOVE

a c. 0.

Fig. 372. — Diagram showing four stages in the development of bladder and rectum
in the human embryo. As a result of the backward growth of a septum the cloacal
cavity becomes divided into a more dorsal rectal cavity and a ventral urogenital sinus.
Since the cloaca is lined by endoderm the lining of the bladder is chiefly endodermal.
(Redrawn after Coming's "Human Embryology.")

In the male the urethral folds unite along the median line to form an
enclosed tubular urethra, the line of fusion persisting as the raphe of the
adult penis. Closure takes place last in the region of the glans and is
completed during the fourth month. Some observers state that the
scrotal swellings subsequently migrate posteriorly and unite behind the
penis, but this is denied by others. An external raphe and an internal
septum persist along the line of union of the scrotal swellings. At the end

THE UROGENITAL SYSTEM

445

of the phallus the prepuce or foreskin, which encloses the glans, is formed
by an ingrowth of epidermis around the glans. By the degeneration of
the central cells of this ingrowth the prepuce is separated from the glans
except on its anal side, where the glans and the prepuce remain connected
by the frenulum of the prepuce.

In the female, changes occur more slowly. Instead of closing as in
the male, the urethral groove remains permanently open as the vestibule,
and the urethral folds persist as the labia minora. The labial swellings
elongate and become in part the labia majora. The elongation of the
phallus characteristic of the male does not occur in the female. The
glans however persists as the clitoris. Consequently, in respect to
external genitals, the female is the undeveloped male.

Table of Homologies

Indifferent stage

Male

Female

Internal Genitalia

Genital fold.

Mesonephric tubules.

Mesonephric duct.

Mullerian duct.

Urogenital sinus.

Testis

Mesorchium
Ligamentum testis
Ductuli efferentes

Paradidymis

Ductuli aberrantes

Appendix epididymidis
(stalked hydatid)

Corpus and Cauda epi-
didymidis

Ductus deferens

Seminal vesicles

Ejaculatory duct

Appendi.x testis (sessile
hydatid)

Sinus pocularis or
uterus masculinus

Prostatic and mem-
branous urethra
Prostate gland
Bulbo-urethral glands

Ovary

Mesovarium
Ligamentum ovarii
Epoophoron (hydatid
of Morgagni)
Paroophoron

Appendix fimbriae
Collecting duct of epo-
ophoron (canal of
Gartner)

Appendix ovarii

Uterine tube

Uterus

Vagina

Urethra

Vestibule

Para-urethral glands

Vestibular glands

External Genitalia

Genital tubercle

Glans penis

Glans clitoridis

Corpus penis

Corpus clitoridis

Urethral folds

Raphe

Labia minora

Labioscrotal swellings

Scrotum

Labia majora

^S?gX

CHAPTER 12
THE ENDOCRINAL ORGANS

One problem of a living organism is to get its organs to work together.
In fact, the difference between a living creature and a dead one is that
the living organism is integrated and the dead is not. Moreover, the
more complex an organism, the more difficult it is to secure functional
correlation among its different but interdependent parts, and the more
complicated is the mechanism which accomplishes this end.

PITUITARY

-PARATHYROIDS

, — ^r^l THYMUS

PANCREATIC
I SLANDS

GONAD -■— V

Fig. 373. — The endocrinal glands in man.

This functional integration of the animal body is brought about by
two means, one nervous, the other chemical. In general, quick adjust-
ments, muscular activities and correlations, and responses to the outward
environment are controlled by the nervous system. But growth, for
example, metabolism, and in general the adjustment of the several tissues
to one another, is largely taken care of by the ductless or endocrine glands,
which secrete into the blood stream minute quantities of so-called hor-
mones that speed up ordinary chemical reactions. With the discovery

446

THE ENDOCRINAL ORGANS 447

that nerves produce neurohumors which act in the manner of hormones,
the contrast between nervous and endocrinal action does not appear as
great as was formerly thought.

In a sense, every organ acts as an endocrine gland toward every other.
All, for example, as a result of their metabolism, form CO2, which diffuses
into the capillaries, and is carried by the blood stream to other parts of
the body. But any increase in the CO2 content of the blood affects the
respiratory center in the medulla and increases the activity of the breath-
ing muscles. Strictly, however, the term endocrine organ is restricted to
those only which secrete hormones that act somewhat specifically and
usually on particular tissues. But there is no hard and fast line to be
drawn.

The word "hormone" was first used by Bayliss and Starling in their
description of the chemical regulation of the secretion of the pancreatic
digestive juices. They discovered that when hydrochloric acid enters
the intestine from the stomach, the digestive secretion of the pancreas is
poured into the intestine. But the same effect was produced even when
all nervous connexions had been previously cut. Therefore, they con-
cluded, the connexion must be chemical, by way of the blood; and they
were led to postulate a chemical messenger or hormone produced by the
epithelial cells lining the duodenum. They gave to this substance the
name secretin. Their results have been repeatedly confirmed, and
endocrinologists include the duodenum in the list of endocrine organs.

THE PANCREAS

General interest in endocrine organs was increased by the important
discovery that the pancreas has an endocrinal as well as a digestive func-
tion. Scattered among lobules of the pancreas, are aggregations of
lightly-staining cells, the pancreatic islands or islands of Langerhans.
It has been shown that these islands secrete a hormone, named by its
discoverers insuUn. While its chemical formula remains uncertain, the
substance has been extracted from the pancreas of animals, and is used
in the treatment of diabetes. Insulin poured into the blood has the
function of regulating the oxidation of carbohydrates in the tissues and
the storage of glycogen in the liver. How this is done is an unsolved
problem.

Both deficiency and excess of secretion of insulin is dangerous, and
often fatal. Diabetes results from too little insulin. The liver fails
to convert sugar into glycogen, and the excess of sugar in the blood is
eliminated by the kidneys, so that sugar in the urine and excessive urina-
tion are among the symptoms. As a result of this loss of sugar, the
tissues are obliged to burn their reserves of fat and protein, and the
patient becomes emaciated. Acidosis and death follow in the absence of

448

COMPARATIVE ANATOMY

proper medical treatment. Today, however, as a result of the discovery
of insulin and the part it plays in life, thousands of people who a generation
ago were destined to prolonged suffering and premature death are able to
live a fairly normal life.

Pancreatic islands occur in all classes of vertebrates, usually in con-
nexion with the pancreas, though in some bony fishes the two glands are

Blood vessel

Al\ eoli^

Fig. 374. — An island of the pancreas with the stirrounding alveoli, from an adult.
X400. (From Bremer's "Text Book of Histology.")

independent. Both arise from the endoderm, but there is no evidence
that pancreatic gland cells are ever converted into cells of the blood-islands.

THE GONADS

Male Sex Glands. The structure and development of the male gonads
have already been described in detail. We are now concerned with their
endocrinal function. It has long been known that removal of the gonads
in childhood prevents the appearance of secondary sex characters of the
male, such as beard, deeper voice, broadened shoulders, etc., the castrated
individual becoming a eunuch and lacking the virile traits of the normal
male. Evidently, then, the appearance of those traits which distinguish
male from female depends upon some physiological action of the testes.
That this is an endocrinal effect of the interstitial Leydig cells of the
testes and not of the seminiferous tubules which produce the spermatozoa,
may be inferred from the fact that male traits develop in the absence of
the seminiferous tubules, provided only that the interstitial tissue is

THE ENDOCRINAL ORGANS 449

present. The individual without seminiferous tubules must, indeed,
be sterile, but he has the secondary traits of the male. This condition
occurs when the testes fail to descend into the scrotal sac.

Although, according to present genetical opinion, primary sex differ-
ences are already determined in the fertilized egg by the chromosome
complex, there is, nevertheless, plenty of evidence that endocrines influ-
ence the secondary sex traits, the first striking evidence appearing at
puberty when the boy assumes some of his adult male characteristics.
That the chromatin constitution of the body cells has little or no influence
on sex is proved by the fact that, if ovaries are grafted in place of testes,
the individual takes on female characteristics. Crew cites the case of a
fowl which was successively a mother and a father as a result of the
destruction of the ovary by disease and subsequent growth of a testis in
place of the ovary. Further evidence of the influence of endocrines early
in development is afforded by the "freemartin." When a cow gives
birth to twin calves, one male and one female, the latter is sterile and
shows some male characteristics. In this case, the primary sex is prob-
ably determined in the fertilized eggs. But since the twin calves are
attached to a single placenta and share a common blood stream, the
male endocrine not only sterilizes the female, but also gives her masculine
traits from which she never recovers. Precisely how the male endocrine
dominates over the female is as yet an unsolved problem.

THE FEMALE SEX GLANDS

In the ovary, as in the testis, interstitial and germ cells may be dis-
tinguished. That the interstitial cells have an endocrinal function, is
suggested by the fact that they increase during pregnancy. The argu-
ment from analogy with the testis has less weight.

There is also the following evidence that the Graafian follicles have an
endocrinal as well as a reproductive function. Before puberty, the
follicles mature in the ovary, but are not discharged. Instead, they
atrophy and the follicular liquid is reabsorbed into the blood, so that any
endocrine which the liquid may possibly contain enters the blood stream
to influence growth and development.

At puberty, however, when a follicle, instead of degenerating, dis-
charges its egg, the follicular liquid stimulates changes in the uterus,
which prepare it for the implantation of the fertilized egg. At the same
time, correlated changes take place in the ovary. Of the follicular cells
which are left behind in the ovary when the ovum is discharged, some
undergo rapid multiplication and become converted into the fatty cells
of a corpus lutemn. (Fig. 344)

The fate of the corpus luteum depends upon that of the ovum dis-
charged. If the ovum is fertilized and pregnancy occurs, the corpus

450

COMPARATIVE ANATOMY

luteum persists throughout pregnancy and for some time subsequently.
If, on the other hand, the ovum is not fertilized, the corpus luteum degen-
erates in a few months and forms a transient structure, the corpus albicans.
Observed facts convince physiologists that at least two hormones,
the follicular and the luteal, are secreted by the ovary. The causal
relation between follicular secretion and menstruation is demonstrated
by the fact that menstruation follows the discharge of the follicular
secretion and ceases when the ovaries are removed. Furthermore, the

ANTERIOR

/ >

PITUITARY

I THYROID j

/

/i\

o^ V^:>'

FOLLICLE ^ //i--^:— »- ^«=ra=i^»Vw2 LUTEUM

Fig. 375. — A diagram of the interactions of pituitary, follicular and luteal hormones.
(Redrawn after Dickinson's "Sex Anatomy," Williams & Wilkins Co.)

follicular hormone, theelin, is used in cases of delayed menstruation and
puberty. Definite physical changes in a woman follow the menopause
when ovulation stops. Facial hairs tend to grow. Fat is deposited on
parts of the body. The breasts tend to atrophy. Nervous and mental
disturbances are likely to arise.

A mature animal from which the ovaries are removed fails to manifest
the normal reproductive instincts, for example, estrus or "heat." An
immature animal which is ovariotomized fails to develop secondary
sexual traits, and remains throughout life infantile and sexually
neutral. Experiments indicate that estrus in animals depends upon
the follicular hormone. Follicular liquid injected into young female
rats brings them to sexual maturity in a few days. Injected into adult

THE ENDOCRINAL ORGANS

451

females from which the ovaries have been removed, the estrus which had
been lost recurs.

The follicular hormone, theelin, has the chemical formula C18H2.1O3
and is a fatty substance, soluble in alcohol and other lipoid solvents.
It is used h}TDodermically to induce puberty and menstruation. It is
now sold as Theelin, Progynon, and under other names. Its use may
cause abortion.

One of the names for the corpus luteum hormone is progestin. Its
action is antagonistic in some effects to that of theelin, for, while theelin
acts as a sex stimulant, progestin prepares the uterus for the reception
of the ovum. In animals, experimental removal of the corpus luteum
early in pregnancy causes abortion. Progestin prevents ovulation and
menstruation in pregnancy.

HEP. PORTAL V

BILE DUCT

HEPATIC A —
R. SUPRARENAL VES^

ESOPHAGUS

L INF PI-RENIC A.

DORSAL AORTA

R. RENAL VES

VENA CAVA

LEFT SUPRARENAL GLANO

L. KIDNEY
L. SUPRARENAL VES.
RENAL VESSELS

SUP MESENTERIC A.

;\-Y\r-^"^ — URETER
SPERMATIC COVARIAN) VES(^ INF. MESENTERIC A

Fig. 376. — Shows the suprarenals in relation to the kidneys as seen when the fat
which normally encloses them is removed. The relations of the blood vessels which
supply them are also shown.

THE SUPRARENAL GLANDS

Anatomy. The suprarenals or adrenals of man are two small glands,
each averaging only four to five grams in weight, and each, as the names
suggest, lying like a cap upon the upper end of a kidney, embedded usually
in the same mass of fat. Accessory adrenals also occur not infrequently
near the kidneys or the gonads. From their relation to kidneys, the
adrenals were once assumed to be also excretory. Later discovery that
their removal or destruction results in death has made them objects of
much interest and research.

The adrenal gland has a rich blood supply. Arterial blood comes
from three sources, phrenic, aortic, and renal vessels, which give off
respectively the superior, middle, and inferior suprarenal arteries. The
gland is drained by the right and left suprarenal veins, the former con-
nected with the postcava, the latter with the left renal. Lymphatic
vessels are abundant.

452

COMPARATIVE ANATOMY

Histology. An adrenal has two kinds of tissue, an outer yellowish
cortex and an inner brownish medulla. The cortex is subdivided into
three layers, an outer glomerular layer composed of rounded clusters of
epithelial cells, a middle fasciculate layer of rounded cells arranged in
cords perpendicular to the surface, and an innermost reticular layer in
which the cellular cords unite in a network. The central or medullary
portion of the adrenal is characterized by relatively large "chromaffin"
cells, so-called because they have a strong affinity for chromic salts, which

■ZONA FASCtCUUVTA.

Fig. 377. — A portion of a section of the adrenal gland as seen under low power
microscope, showing the differentiation into cortical and medullary regions. Different
endocrines are secreted by these two tissues, which have a different embryonic origin.

Stain them brown. They are arranged in clusters separated by numerous
lacunar blood spaces. A compact connective tissue capsule encloses the
glandular tissues. (Figs. 377, 378)

Physiology. The adrenals have a double function corresponding with
their histological differentiation into two tissues. The cortex secretes an
endocrine, cortin, of unknown chemical composition, which is essential
to life. Destruction of the cortex by tuberculosis or tumor is followed
by Addison's disease, which is characterized by a deep pigmentation of the
skin and great weakness. Death comes rapidly unless cortin is adminis-
tered. The effect of cortin is, however, immediate, and the patient
suffering from adrenal failure promptly arises as though from the dead.

The cortex of the adrenal is, relatively to the medulla, largest during
foetal life, and there is little doubt that its secretions have a strong influ-
ence upon growth. Excessive activity of the gland induces precocious
development. The stages of life are so speeded up that a child of four
or five may look like an old person. Hyperactivity of the cortex also

THE ENDOCRINAL ORGANS

453

intensifies masculine traits, so that a woman may acquire the beard and
deep voice of the male. The " bearded lady " of the side show is a familiar
example.

The endocrine secreted by the medullary tissue of the adrenal is
epinephrine or adrenin, its empirical chemical formula being C9H13O3N.

ENDOCRINE INTERRELATIONS

POSTERIOR LOBE ^PITRESSIN AND PITOCIN) PANCREATOTROPIC, KETOCENIC, PARATHYREOTROPIC

4TH DAYOF MENSTRUAL CYCLE

Fig. 378. — A diagram illustrating the complex interrelations of the endocrine
organs. The multiplicity of pituitary influences is only partially indicated. For the
sake of clearness the diagram over simplifies the relations. The pancreatropic, keto-
genic, and parathyreotropic factors are not thoroughly understood. (Adapted from
Therapeutic Notes through courtesy of Parke, Davis & Company.)

Most commercial adrenin is now made synthetically and not derived as an
extract from animal tissue. Like the sympathetic nerves, adrenin has a
stimulating effect upon smooth muscle. Since sympathetic nerves and
the chromaflEin cells of the medulla have a common origin in the embryo,
this similarity of function might be expected. It is so potent a drug that
its physiological effects appear even when it is diluted to one part in
400,000,000 of blood.

454 COMPARATIVE ANATOMY

Cannon's well-known experiments upon animals led him to his emer-
gency theory of adrenal action. He found that in a quiet undisturbed
animal adrenin is absent from the blood. When, however, the animal
is excited by pain, fear, or anger, adrenin increases. As a result, the
heart beats more strongly, breathing becomes deeper and more rapid,
intestinal action ceases, the liver releases sugar more rapidly, the muscles
respond more quickly to stimulation, the tonus of the blood vessels is
raised, the coagulability of the blood increases, and so the animal is pre-
pared either to run or fight. Thus the medullary secretion reinforces the
action of the sympathetic nerves.

Cannon has also shown that a substance which resembles adrenin in
its effects is given off by organs stimulated by the sympathetic nervous
system. This substance, the chemical nature of which is unknown, has
been called sjrmpathin.

Development. Corresponding with its double function and its
division into cortex and medulla, each adrenal has a double origin. The
cortex develops from mesoderm, while the medulla is derived from sympa-
thetic ganglia and is therefore ectodermal in origin. In the human
embryo of 6 mm., the cortical substance makes its first appearance as a
proliferation of the coelomic epithelium near the root of the mesentery.
The cells of the anlage lose their connexion with the epitheHum and take
positions at the sides of the abdominal aorta. There is no evidence of a
segmental origin.

The cells of the medulla, on the other hand, come from the chain
of sympathetic ganglia. In a 6 mm. human embryo, the cells which later
form the adrenal medulla may be distinguished in the ganglia by their
affinity for chromic acid which stains them brown while other cells of the
ganglia remain unstained. The migration of chromafl&n cells into the
adrenal cortex begins in embryos of seven weeks (17 mm.) and may
continue until after birth. In embryos of the third month, the adrenals
are more conspicuous organs than the kidneys, but as development pro-
ceeds, they fail to keep pace with the enlargement of the body.

Evolution. Adrenal glands are limited to vertebrates. Homologous
organs are lacking in invertebrates and protochordates. In Cyclostomes,
both the cortical layer and the chromaffin are present, but are spatially
separated. The cortical substance is represented by clusters of cells,
the interrenal bodies, which lie throughout the length of the body cavity
near the post-cardinal veins. The chromaffin cells, on the other hand,
are arranged as strands along the dorsal aorta. In the Elasmobranchs,
the interrenal bodies tend to aggregate in the posterior part of the body
cavity, while the chromaffin cells are arranged in metameric groups near
the sympathetic ganglia.

THE ENDOCRINAL ORGANS 455

In the amphibia the organization of the adrenals is intermediate
between that of elasmobranchs and that of amniotes. Chromaffin and
interrenal cells lie in close proximity to one another. The two kinds of
tissue are usually interspersed and extend along the surface of the meso-
nephroi. In some amphibia the chromafiEin tissue surrounds the interrenal
cells. The relations of the two tissues vary considerably in reptiles.
In some, though not in all, the chromafhn cells, as in fishes, are separate
from the interrenal bodies. In crocodiles and tortoises, however, the
two tissues are mixed as in amphibia. Strands and clusters of chromaffin
cells are embedded within the interrenal tissue. In birds also the two
kinds of tissue are intermingled. Finally, in mammals, the interrenal
tissue forms a cortex which encloses the chromaffin cells as a medulla.
The quantity of cortical tissue in mammals greatly exceeds that of medul-
lary (chromaffin) tissue.

THE THYROID GLAND

Anatomy. The thyroid gland of man is a bilobed brownish organ
closely apposed to the trachea just below the larynx. The lateral lobes
are generally connected by an isthmus across the trachea, so that the
shape of the gland varies from a "U" to an "H," depending upon the
relative position of the isthmus. Not infrequently, especially in younger
persons, there is a median pyramidal lobe. The accessory thyroids which
sometimes occur are usually remnants of this pyramidal lobe. There is,
also, occasionally another accessory gland beneath the upper end of the
sternum.

Although the size of the thyroid varies greatly in different individuals,
the average weight is thirty-four grams. The connective tissue capsule
surrounding the gland is a double membrane. Suspensory ligaments of
connective tissue attach the thyroid to the tracheal and laryngeal
cartilages.

The blood supply of the thyroid, like that of the adrenals, is exception-
ally abundant. Four, and frequently five, arteries connect with the
organ; seven veins drain the blood away. The arteries are the paired
superior and inferior thyroid, and the ocasional thyroidea ima. On the
surface of the gland, a plexus of veins gives rise to paired, middle, and
inferior thyroid veins and to the unpaired thyroidea ima. Lymphatics
are abundant. The nerve supply is sympathetic.

Histology. The thyroid is formed of numerous spherical masses of
glandular tissue, separated from one another by connective tissue parti-
tions. The glandular tissue consists of rounded follicles, each enclosed
by a single layer of cuboidal epithelium. Loose connective tissue filled
with blood vessels and lymphatics binds the follicles together. Each
follicle is filled with jelly-like colloid material which has a strong affinity

456

COMPARATIVE ANATOMY

for acid dyes such as eosin. The colloid is evidently a secretion of the
cuboidal epithelium and is generally assumed to be a store of the hormone.

Physiology. The most important, if not the exclusive, endocrinal
secretion of the thyroid gland is an iodine compound thyroxine, the
empirical chemical formula for which is C15H11O4NI4. A substance with
identical properties has been made synthetically, and it is generally the
synthetic drug which is used in medical practice.

One function of thyroxine is to control metabolism, especially of the
carbohydrates, a thirtieth of a grain increasing oxidation by one percent.
Many commercial flesh-reducing preparations, therefore, contain thyrox-
ine, often in dangerous quantity. It regulates also growth before birth,

iLOOD VESSEL

Fig. 379. — A portion of a section of the thyroid gland enlarged, showing the secretory-
epithelium and the colloid-filled follicles.

through infancy, and at puberty, so that, in no small measure,
we are what we are in virtue of our thyroid glands. Tadpoles fed with
thyroid may become frogs no larger than flies.

Excessive activity of the gland is a common malady, manifested by
extreme nervousness, rapid pulse, insomnia, and basic metabolism above
normal, so that the patient loses weight. In man, exophthalmic goitre
is one of the common manifestations.

As too much thyroxine speeds up the life processes, too little slows them
down. Basal metabolism is low, so that the tendency is to put on fat.
In the young, both growth and development are retarded; and if the
deficiency is very great, a child, unless given thyroid artificially, may
become an idiotic dwarf not unlike a cretin.

Iodine is needed for the production of thyroxine, and in regions where
iodine is lacking in the food, as in the central states of America and in the
Alps, the glands may attempt to make up the deficiency by enlargement
and increased secretion. This results in the more usual form of goiter.
The remedy is to add potassium iodide to the diet.

Development. In man, as in other Vertebrates, the thyroid gland
arises as a median ventral outgrowth from the floor of the pharynx at the

THE ENDOCRINAL ORGANS

457

level of the first visceral pouch. For a while the anlage of the thyroid
retains connexion with the floor of the pharynx by means of the thyro-
glossal duct. When, at a considerably later stage of the development,
the tongue forms, the point of connexion of the thyroglossal duct is indi-
cated by a pit, the foramen cecum, near its posterior border. The thyro-
glossal duct persists for some time in the embryo, and becomes gradually
elongated as the gland anlage grows backward to take its definitive
position on the sides of the trachea. Eventually, however, connexion
with the tongue is lost. The median lobe of the gland, if present, develops
from the thyroglossal duct, and the accessory thyroids frequently come
from the same source. The lateral lobes increase in size more rapidly
than the median portion which persists as the isthmus. (Fig. 380)

aaaauatffi^^B^S^^i^^

Fig. 380. — Longitudinal section of head of 19-day Petromyzon embryo, ch,
optic chiasma; ep, epiphysial outgrowth; h, hypophysial ingrowth; mes, mesenteron; n,
nasal epithelium; nc, notochord; oc, oral cavity; op, oral plate; sc, canal of spinal cord;
th, thyroid. (From Kingsley's "Comparative Anatomy of Vertebrates.")

Evolution. The thyroid gland is a peculiarity of chordates, and is
found in all classes of this phylum. Since no homologous structure is
known in non-chordates, we must assume that the gland, like the noto-
chord, emerged with the phylum from unknown non-chordate ancestors.
In the hemichordates, the only possible homologue of the thyroid is a
groove apparently functionless in the floor of the pharynx. The uro-
chordates and cephalochordates have a ciliated groove, the endostyle,
in the floor of the pharynx. This groove is morphologically, if not
physiologically, comparable with the thyroid.

In Amphioxus, a typical cephalochordate, the endostyle is lined by
columnar epithelial cells of two sorts, glandular and ciliated. Four rows
of gland cells alternate with rows of ciliated cells which extend the entire
length of the pharyngeal floor. Particles of food caught up in the mucus
are swept forward towards the mouth, and are carried by a ciliated cir-
cumpharyngeal groove to a median epibranchial groove, which carries
them posteriorly to the intestine. It has been suggested that the epi-
branchial groove is represented in vertebrate embryos by the hjrpochorda,
a transient embryonic structure below the notochord, which disappears
during ontogenesis.

458 COMPARATIVE ANATOMY

The endostyle of petromyzon larvae, like that of amphioxus, is a
mucus-secreting organ with four rows of mucus-secreting cells alternating
with rows of ciliated cells. This larval endostyle, however, is a transient
structure which develops into the thyroid gland of the adult animal.
Connexion of the anlage with the pharynx is eventually lost, and the organ
becomes vesiculated like the thyroid of higher vertebrates. The vesicles
secrete colloid, and the function is evidently endocrinal. The homology
of the thyroid gland and endostyle is further attested by the fact
that in the cyclostome Bdellostoma the median groove from which
the thyroid gland develops extends the entire length of the floor of the
pharynx, precisely as does the endostyle of Amphioxus. In fishes, as in
cyclostomes, the thyroid is usually unpaired. A subdivision into two
lobes is, however, characteristic of Amphibia.

In reptiles, the gland is again unpaired and remains unpaired in most
mammals, with a tendency to form lateral lobes as in man. The position of
the gland is fairly constant, ventral to the trachea and just below the larynx.

Evidence has been given (p. 335) that the endostyle is a modified
gill pouch. The history of the thyroid therefore reveals a complete
change of function such as we have already seen to be a general character-
istic of pharyngeal structures.

THE PARATHYROID GLANDS

Anatomy. The parathyroid glands in appearance resemble lymph
nodes, and are usually four in number in man. Their diameter varies
from three to thirteen millimeters. Their position is also variable. They
generally lie dorsal to the thyroid, but may be occasionally enclosed
within its connective tissue capsule. This relation, however, is purely
topographical; there is no functional similarity. Their blood supply is
from the inferior thyroid arteries.

Histology. Unlike the thyroids, the parathyroids are formed of
masses and cords of polygonal epithelial cells, among which numerous
blood vessels are interspersed. Colloid-filled follicles are rare; but they
multiply in number when the thyroid is removed, and may take over the
function of the thyroid in an emergency.

Physiology. While, therefore, an animal may lose its thyroid glands
without necessarily fatal consequences, the removal of the parathyroids
is followed by convulsions and death unless parathyroid extract is adminis-
tered intravenously. The effects of loss of parathyroids resemble those
which follow the use of strychnia or the bite of a mad-dog. There is a
marked fall in the calcium content of the blood, followed by cramps and
muscular tetany. Apparently calcium salts act as a nerve sedative to
prevent acute stimulation of muscular contraction, so that hypopara-
thyroidism has been thought to be one of the causes of extreme irrita-

THE ENDOCRINAL ORGANS

459

bility and "touchiness." The injection of parathyroid extract into the
blood is followed by a rapid increase in blood calcium, so rapid that a slight
overdose may cause death, for the chemical balance of the blood must be
delicately regulated to maintain normal health. The chemical nature of
the parathyroid endocrine is not yet known.

On the other hand, even a slight over activity of the parathyroids may
have serious consequences. The reserve supply of calcium for bodily use
is limited, and excess of parathyroid endocrine in the blood may cause
the withdrawal of calcium from the bones and teeth, which lose their
hardness and become fibrous.

Fig. 381. — Diagram to show the derivatives of the branchial pouches. le, He, Hie.
IVe, Ve, external branchial grooves; li. Hi, Hit, IVi, internal branchial pouches;
TA, auditory tube and tympanic cavity; Tons., palatine tonsil; EpIII, EpIV, para-
thyroid glands; Ub, ultimobranchial body; Th., thyroid gland. D.th.gl., ductus
thyroglossus. (From Morris, after Keibel and Mall.)

Since calcium and phosphorus metabolism are also dependent upon
the presence of vitamin D in the food, the problem of calcium balance is
not simply a problem of endocrinology.

In brief, the action of the parathyroid endocrine resembles that of a
catalyst which facilitates the manufacture of calcium salts in the body
and thus helps to regulate the irritability of the nerves.

Development. In the human embryo, the parathyroid glands are
formed from the epithelium of the third and fourth pharyngeal pouches.
By proliferation from the dorso-lateral wall of these pouches are formed
masses of cells which soon lose connexion with the pouches and migrate
caudally. In this migration they become closely associated with the
anlagen of the thymus gland. The cells of the third pouch are carried

460 COMPARATIVE ANATOMY

back as far as the posterior border of the thyroid gland. The migration
of the cells of the fourth pouch is less extensive, and they usually come to
lie near the anterior border of the thyroid. The epithelial character of
the cells of the parathyroid is retained during the histogenesis of the
gland. Mesenchyme cells work their way into the anlage, forming blood
vessels and breaking up the cellular mass into cords. (Fig. 381)

Evolution. Parathyroid glands occur in all vertebrates except fishes,
making their first appearance in Amphibia in which the gills have begun
to degenerate. The factors involved in the emergence of their endocrinal
function are at present speculative, since to say that they owe their origin
to gill degeneration means little. Comparison of lower with higher
vertebrates reveals a progressive reduction in number but apparent
increase in endocrinal importance.

THE ULTIMOBRANCHIAL BODIES

Among the pharyngeal derivatives which may have an embryonic
endocrinal function are the ultimobranchial bodies, called also post-
branchial and suprapostcardial bodies. These are paired glands which
arise from or near the fifth pair of gill-pouches. They come to lie near
the posterior border of the thyroid and attain a vesicular structure.

Evolution. Ultimobranchial bodies occur in all gnathostomes except
possibly teleostome fishes. In Amphibia, the gland may be paired or
unpaired. It is not unlikely that, as its name suggests, it represents a
posteriormost pair of gill-pouches modified as an endocrinal organ. In
man it persists only as a transient embryonic structure the function of
which has become that of an endocrinal organ. (Fig. 381)

THE THYMUS GLAND

Anatomy. The thymus glands or throat sweetbreads are paired
organs located where the throat joins the chest. They are pinkish in
color in childhood, but become yellow in old age from fatty degeneration.
They enlarge up to puberty and thereafter gradually shrink. Contrary
to earlier opinion, the gland does not completely disappear in the adult,
but persists as a shrunken remnant within the mediastinum.

Branches of the subclavian or of the internal mammary artery supply
the gland with blood. Its numerous veins drain into the internal mam-
mary or the inferior thyroid. Lymph vessels are abundant. The nerves
are sympathetic nerves derived from the vagus.

Histology. From the connective tissue capsule of the gland, partitions
extend inwards and divide it into lobes and lobules. In each lobule a
cortex and medulla are distinguishable as in a lymph node, but the large
lymph sinuses in the medulla, and the germinal centers which are present
in lymph nodes, are lacking in the thymus. The resemblance of the

THE ENDOCRINAL ORGANS

461

thymus tissue to adenoid tissue makes it probable that it is a lymph-
forming organ, whatever other functions it may have. The most striking
histological characteristic of the thymus is the presence of thymic or
Hassall's corpuscles, relatively small clusters of concentrically arranged
epithelial cells, usually assumed to be degenerate and non-functional.

Physiology. The endocrine function of the thymus is doubtful,
since removal of the gland is not followed by disturbances of normal

Fig. 382. — A portion of a section of the thymus gland as seen under low power
microscope, showing its subdivision into lobules and their differentiation into cortical
and medullary portions. Thymic corpuscles which are peculiar to the thymus, occur in
the medullary regions. Two of these much enlarged are shown in the lower figure.

functions. The enlargement of the gland in early life and its later
atrophy suggest that its functional activity may be limited to stages of
growth and differentiation.

Hypertrophy of the thymus sometimes accompanies that of other
lymphatic organs, the condition being known as status thymicoljrmpha-
ticus. It results in serious metabolic disturbances and sometimes even
death. Adults in whom the thymus does not atrophy may die suddenly
under ether.

462 COMPARATIVE ANATOMY

Development. Thymus glands first appear in a six-weeks embryo
as hollow tubular outpocketings of the third pair of pharyngeal pouches,
dorsal to those of the parathyroid glands. As they grow caudally,
the anlagen become solid and considerably elongated, the anterior part
atrophies, and the posterior enlarges. In the tenth week, after the gland
attains its definitive position in the mediastinum, thymic corpuscles
appear. By the third month, cortex and medulla are differentiated, and
the gland assumes the appearance of a lymph gland. Enlargement
continues until puberty. (Fig. 382)

Evolution. Thymus glands are peculiar to vertebrates but occur
in all members of that sub-phylum. In cyclostomes all gill-pouches
give rise to thymic tissue and the gland persists dorsal to the gills through-
out life. Ventral anlagen have also been described. In fishes there is a
reduction in the number of thymus anlagen, since some of the gill-pouches
make no contribution to the thymus. In amphibia the thymus is further
reduced, and comes to lie at the angle of the jaw. The thymus glands
of reptiles are lobular organs located in the sides of the neck. In mam-
mals the third pair of pharyngeal pouches, or sometimes the third and
fourth, produce the cells which form the thymus. The gland assumes
its definitive position in the mediastinum at the base of the neck. The
history of the thymus is thus one of reduction in the number of gill-pouches
which contribute to the gland. However, in some orders of mammals,
such as marsupials, rodents, ungulates and prosimians, a new t>^e of
thymus gland appears, the cervical thymus, which is said to be of ecto-
dermal origin. It is of relatively small size and of unknown function.

THE PITUITARY GLAND

Anatomy. The pituitary gland is a flattened oval body, its longest
diameter averaging 10 to 12 mm., located at the base of the brain near
the optic chiasma, where it is attached by a stalk to the lower end of the
inf undibulum. Lodged in a cavity, the sella turcica, in the sphenoid bone,
the pituitary is as safe from injury as possible, (Figs. 285-383)

Like the suprarenals, the pituitary is formed of two elements of diverse
origin and function. The chief parts are a larger anterior glandular
lobe, which develops from the ectoderm of Rathke's pouch, and a smaller
posterior nervous lobe derived from the floor of the brain and partly
surrounded by the anterior lobe. Between the two major lobes are two
smaller glandular masses, the pars intermedia and the pars tuberalis.
Blood is richly supplied from the adjacent arterial circle, while numerous
veins drain the blood into the venous circle and the basilar plexus of
veins. Nerves are supplied from the carotid plexus and the infundibulum.

Histology. The anterior lobe consists of cords of epithelial cells
interspersed among numerous blood and lymph spaces. In their staining

THE ENDOCRINAL ORGANS

463

properties, these cells vary from cells with strong affinity for dyes to
those with weak affinity. Some are acidophilic, some basophilic. The
acidophilic cells are conspicuous in cases of gigantism, while sex derange-
ment is associated \^'ith changes in the basophilic. In the pars intermedia
some colloid-filled folHcles resembling those of the thyroid occur among
the cell cords. The cells of the posterior lobe are chiefly neuroglia cells
with numerous connective tissue fibers. (Fig. 384, F)

Physiology. One of the important functions of the anterior lobe is
the stimulation of growth. Enlarged pituitary in early life is accom-

MASSA INTERMEDIA THAJ.AM
SULCUS ClMGJ_
GYRUS CINGULl

SUPERIOR
FRONfTAL GYRUS

HALAMUS

•CENTRAL SULCUS

CHORIOIDEA C3 RD VENT.)
CORPUS CAU-OSUM

POSTERIOR COMMISSURE
PINEAL BOW

SUBPARIETAL SULCUS

FORN

FRONTAL POLE '
ANTERIOR COMMISSURE'

TEJ»/(INAL LMMI-^

OPTIC NERVE—'
OPTIC CHIASMA"
IhfUNDIBULUM •

hypophysis'
mammillary boo
oculomotor nzr/e

PONS'
FOURTH VENTRICLE'

MEDULLA'
CENTRAL CANAL'

LAMINA QUADRIGEMINA

OCCIPITAL LOBE

CAlCARINA FISSURE

UNGUAL GYRUS
OCCIPITAL POLE

'CEREBELLUM
CHORIOIDEA C4TH VENT.)

-SPINAL CORD

Fig. 383. — The human brain in median section and aspect. The pituitary gland
(hypophysis) is attached to the infundibulum of the brain and embedded in the sella
turcica of the sphenoid bone. (Redrawn after Sobotta.)

panied by excessive bone growth. Deficient secretion, on the other
hand, produces a certain t>'pe of dwarf, the ateliotic dwarf or "midget."
Administration of anterior lobe extract has in some instances increased
the height of such dwarfs.

After the growth of the long bones has ceased, as in the normal adult,
it is of course impossible to stimulate further growth in height. But
when, as the result of tumorous enlargement, anterior lobe secretion is
increased, a local growth of bone, sometimes asymmetrical, may occur.
Such cases of acromegaly are not infrequent, .\nother effect of excessive
pituitary secretion is increased hairiness, like that caused by h}-pertrophy
of the adrenal cortex.

Removal of the anterior lobe in young animals is followed by cessation
of growth and by increase in fatty tissue. In this way it is possible
to produce ateliotic dwarfs experimentally. Experiments also demon-
strate that deficiency of anterior lobe hormone in young animals results

464 COMPARATIVE ANATOMY

in failure of sexual development and atrophy of the sex glands. Another
effect is reduced secretion by the suprarenal and thyroid glands, so that
it is quite possible that the effect of the pituitary upon sexual development
is indirectly through the effect upon suprarenals and thyroid.

While the histological structure of the posterior lobe does not suggest
a glandular function, treatment with extract of the posterior lobe causes
a marked rise in blood pressure. Two kinds of substances which act as
endocrines have been obtained from the posterior lobe of the pituitary.
One of them, vasopressin, increases blood pressure; the other, oxytocin,
stimulates the contraction of the muscles of the uterus and is now used
following childbirth to bring the uterus back to normal size. Posterior
lobe extract is found to reduce obesity and certain types of obesity are
now ascribed to deficiency of this hormone. Increase of posterior lobe
secretion causes decreased urination, possibly brought about indirectly
through the nervous system.

The wide-spread and contrasting effects of the secretion of the pitui-
tary, which may have as many as eight different hormones, taken together
with the experimental evidence of the mutual interaction of endocrinal
organs, demonstrates the enormous complexity of the problem of the
biology of endocrines. Under the circumstances, we may follow Hoskins
in suggesting that "To those who, in the present state of our knowledge,
would glibly re-write physiology and psychology in terms of pituitary
functions the timorousness of the proverbial angel is commended."

Development. The pituitary gland has a double origin. The anterior
lobe comes from an ectodermal sac, Rathke's pouch, in the roof of the
mouth, while the posterior lobe is formed as an outgrowth of the base of
the diencephalon. Rathke's pouch makes its appearance in a 3 mm.
embryo. The posterior end of this sac comes into contact with the
infundibular outgrowth from the base of the brain. As development
proceeds, Rathke's pouch loses connexion with the ectoderm and breaks
up into vesicles, the cavities of which are remnants of the pouch cavity.
In this way, the anlage of the anterior lobe assumes its definitive position
anterior to the infundibulum. The ventral end of the infundibulum
becomes thickened as the anlage of the posterior lobe. The anterior
lobe enlarges and partly surrounds the posterior lobe. The cells which
later become differentiated as pars intermedia and pars tuberalis are
derived from the anlage of the anterior lobe. By the end of the fourth
month, the gland attains its adult form and appearance. Blood vessels
and connective tissue capsule are of mesenchymatous origin.

Evolution. The pituitary gland occurs only in vertebrates. No
comparable structure has been identified in the protochordates. The
elements of the pituitary make their first appearance in cyclostomes.
In myxinoids (Myxine, Bdellostoma) the neural (posterior) lobe is repre-

THE ENDOCRINAL ORGANS

46 =

sented by the epithelial ventral termination of the infundibulum. The
anterior lobe is represented by the hypophysial duct, an ectoderm-Uned
tube, which opens in front of the mouth anteriorly and into the pharynx
posteriorly. Into this duct the paired olfactory pits open. The only
element of the myxinoid pituitary which is glandular is the intermediate
lobe, represented by clusters of cells lying between the infundibulum and
the hypophysial duct. These cells are proliferated from the ectoderm
of the hypophysial duct, just as in embryos of the higher vertebrates
the cells of the intermediate lobe are proliferated from the hypophysis
(Rathke's pouch). The scarcity of blood-vessels in the region of

INFUNDIBULUM

HYPOPHYSIAL DUCT
B. PE.TROMYZON

C. HEPTANCHUS

PARS TUBERALISV

INFUNDIBULUM

— ANTERIOR LOBE-

POSTERIOR

•-"^^ ANTERIOR LOBE-

INFUNDIBULUM

POSTERIOR LOBE

0. RANA E. REPTILE F. HOMO

Fig. 384. — A series of diagrams showing conditions in six different vertebrates, which
are beUeved to represent stages in the evolution of the pituitary gland. The complexity
of origin of this gland is correlated with the complexity of its endocrinal functions. The
posterior lobe is cross-hatched, intermediate lobe stippled, anterior (hypophysial) lobe
piebald, and the pars tuberalis solid black. (Redrawn from Oppel, after Stendell.)

the intermediate lobe of the pituitary in cyclostomes makes it seem likely
that, if it has an endocrinal function in this group, its secretions are
poured into the infundibulum. Since the epithelium of the infundibulum
and of the hypophysial duct is not thickened but remains single-layered
in myxinoids, there is no evidence that these elements in this group have
an endocrinal function. Stendell, therefore, seems justified in the
conclusion that the intermediate lobe is the first part of the vertebrate
pituitary which is differentiated as an endocrinal organ. (Fig. 384)

An advance in the evolution of the pituitary is found in Petromyzon.
In this animal, as in all higher vertebrates, connexion of the hypophysial
duct with the pharynx is lost and the organ ends blindly at its posterior
end. From it, however, are proliferated cells into the region between
the duct and the infundibulum. The cells which take a position next

466 COMPARATIVE ANATOMY

to the infundibulum form the intermediate lobe. Other clusters of
cells nearer the hypophysial duct produce hollow vesicles which are
believed to represent the beginning of a glandular anterior lobe. Since
the remainder of the embryonic hypophysis persists in the adult as a
blind pouch and is not, as in higher vertebrates, converted into anterior
lobe, it is evident that the anterior lobe of the pituitary of Petromyzon
is only partially homologous with that of the higher vertebrates. The
nervous lobe of the pituitary can hardly be said to exist in Petromyzon.
In this animal where the infundibulum is in contact with the cells of the
intermediate lobe, the epithelium is slightly thickened and the infundibu-
lum shows a well-marked hypophysial recess.

The evolution of these elements which have their inception in cyclo-
stomes may be briefly summarized. As one passes through the verte-
brate series from fishes to man, all three elements seen in cyclostomes are
present. The anterior lobe steadily increases in relative size while the
intermediate lobe shrinks. An increase takes place in size and dififerentia-
tion of the posterior lobe. The presence of colloidal material in the
pituitary in all vertebrate groups justifies the assumption that the gland
has an endocrinal function throughout the series. In elasmobranchs
the nervous portion of the gland is only slightly indicated. The region
of the infundibulum to which the intermediate lobe is attached is thickened
in ganoids. The posterior lobe becomes more conspicuous as we pass
up the vertebrate series to mammals.

According to the view just expressed, the evolution of the hypophysis
involves the metamorphosis of a tubular hypophysial duct into an endo-
crine organ. Another view is that the hypophysis was in the beginning a
gland which opened into the mouth, but for this opinion there seems to be
less evidence.

CHAPTER 13
THE NERVOUS SYSTEM

Of the two agencies which integrate the various functions of the body
the nervous system is the more important. In addition, however, to this
integrative function the nervous system, with the intermediation of
the sense organs, serves to bring the organism into relation with its
environment.

The general protoplasmic properties upon which the actions of nerves
depend are merely the irritability and conduction which are characteristic
of all cells. An amoeba, for example, responds to a stimulus by contract-
ing. If one of its pseudopodia is touched, all pseudopodia withdraw.
Obviously, both irritability and conduction are involved in this reaction.
All cells of higher animals presumably retain these two powers, but they
become the special functions of nerve and sense cells.

ELEMENTS OF THE NERVOUS SYSTEM

In simple colonial animals such, as volvox nerves are wanting, and
impulses are transmitted from cell to cell by means of intercellular bridges
or plasmodesms. Special nervous cells first appear in coelenterates, in
the form of netirosensory cells located in the skin. Each neurosensory or
receptor cell is connected with deeper tissues, such as muscle fibers, by an
elongated process or neurite, which carries nervous impulses to the effector
cell. In a characteristic reflex action in worms, a ganglion or transmitter
cell, comparable with the motor cell of vertebrates, is interpolated between
the receptor and effector cells. A similar reaction in vertebrates usually
involves four cells: — i. a receptor cell in the skin or sense organ; 2. an
afferent or sensory cell; 3. an efferent or motor nerve cell, which is con-
nected with 4. an effector cell by a neurite. Further complications arise
by the chaining together of additional nervous units within a central
nervous system, until, in vertebrates, so few cells are devoid of nervous
connexions that, if all were destroyed except the nervous tissues, the
general form of the body would still be preserved.

The steps in the evolution of a reflex nervous system such as that of
worms and vertebrates involve, first, the differentiation of a neurosensory
cell in the ectoderm. The body of such a cell remains in the ectoderm,
and one or more protoplasmic hairs may extend above the surface. The
most characteristic feature of such a cell, however, is the neurite, which

467

468

COMPARATIVE ANATOMY

CEREBRUM

CEREBELLUM-
CERVICAL PLEXUS

BRACHIAL PLEXUS

SYMPATHETIC GANGLION

Fig. 385. — A general view of the nervous system of man as seen from behind.

THE NERVOUS SYSTEM

469

grows away from the surface towards the underlying muscles. By branch-
ing into terminal telodendria, such a neurite may increase the number of
its connexions.

A second evolutionary step is taken when the body of the neurosensory
cell sinks below the surface into the underlying connective tissue, but
retains connexion with the superficial epithelium by means of a process
with branched terminations or dendrites attached to the skin.

Further advance appears when the dendrites, instead of ending freely
among the epithelial cells, become connected with special receptor or

EPITHELIAL CELL

rail I jj^^rnm

^MUSCLI

OMIEEEEllEEBmEM

.NEUROMUSCULAR SECONDARY

NEUROSENSORY CELL CELL 1 SENSORY CELL

SENSORY NEURONE.

MUSCLE CELL

V"-SENSORY CELL—-

'muMnm

SECONDARY SENSORY CELL T _SENSORY NEURONE

miidcieiiiHiiiim

SECONDARY

SENSORY MOTOR

NEURONE

Fig. 386. — A diagram showing hypothetical stages in the evolution of the reflex arc of
the higher animals. In A the series begins with the neuromuscular cell of coelenterates.
In B the neurosensory cell becomes differentiated from the muscle fiber. In C the
body of the neurosensory cell recedes from the surface. In D the neurosensory cell
becomes a sensory neuron and a secondary sensory cell conveys external stimuli to the
sensory neuron. In £ a motor neuron is interpolated between the sensory neuron and
the muscle fiber. Finally, in F, by means of several motor neurons, the connexion of a
sensory neuron with several muscle fibers is effected. (Redrawn after Kahn's "Das
Leben Des Menschen," W. Keller & Co.)

secondary sense cells in the skin. The somatic sensory cells of verte-
brates are at this evolutionary stage.

The so-called primitive ganglion cells of coelenterates exhibit another
line of differentiation. That these cells derive from neurosensory cells
is a conclusion supported by considerable evidence. That they are more
differentiated than neurosensory cells is indicated by the fact that they
contain tigroid substance and neurofibrillae characteristic of the nerve
cells or neurons of the higher animals. Physiologically, however, they
are simpler than neurosensory cells, since they transmit nervous impulses
in any direction, while neurosensory cells are definitely polarized and
transmit impulses in a single direction only. They are, therefore, inter-
preted as neurosensory cells which have lost both their primary connexion
with the skin and their functional polarity. (Fig. 388)

470

COMPARATIVE ANATOMY

In some coelenterates, the primitive ganglion cells form a loose subcu-
taneous network or plexus in which, as experiments show, nervous impulses
may be carried in any direction. Morphologists incline to the opinion
that the nerve net of coelenterates becomes the central nervous system of
higher animals.

A final step in the evolution of the neuron is taken by the flatworms,
in which the neurons, like the neurosensory cells of coelenterates, trans-

NEUROSENSORY CELL
I BIPOLAR NEURONE

UNIPOLAR NEURONE

Fig. 387. — A diagram illustrating hypothetical stages in the phylogenesis of the
characteristic sensory (afferent) neuron of vertebrates. Earlier stages at the left, final
stage at the right. Arrows suggest that the direction of growth of the neurite is away
from the source of stimulus. The diagram assumes that the primitive neurosensory cell
becomes the definitive sensory neuron and that the definitive sense receptor is secondary.
It is quite possible, however, that the neuron is secondary and that the neurosensory
cell becomes the definitive receptor. (Redrawn after Ariens-Kappers modified.)

mit nervous impulses in only one direction. In contrast with neuro-
sensory cells, however, a neuron has at least two nervous processes, of
which the dendrite carries impulses toward the cell body, while the
neurite (neuraxon) carries impulses away from the cell body. Each
neuron has but one neurite, and may have one or many dendrites. In
ontogenesis the neurite grows away from the source of stimulation and
the dendrites towards the stimulus.

THE NERVOUS SYSTEM

471

The differentiation of the neuron is accompanied by the appearance
of two sheaths, neurolemma and medullary or myelin, which cover the
nervous processes and serve to insulate and nourish them. Each neurite
may be covered by a chain of neurolemma cells, or it may not; each may

■NERVE FIBERS

■^ ^ WNCLION CELLS

Fig. 388. — Plexus of ganglion cells and fibers in the tentacle of a coelenterate.
From such a nerve-net as this it is believed that the nervous systems of higher animals
have evolved. (Redrawn from Ariens-Kappers, after Wolff.)

or may not have a fatty medullary sheath. The familiar distinction
between white and gray matter in the nervous system rests on the presence
or absence of medullary sheaths. Ganglia and non-medullated fibers
form gray matter; meduUated fibers appear white. That the medullary

Piii! ['||'iWll''!|l|IIH EPINEURIUM

. NODE OF RANVIER

NEURILEMMA
MYELIN SHEATH

NEURITE (AXONE)
ENDONEURIUM
NUCLEUS OF NEURILEMMA'^'

A LONGIT SECTION B. CROSS SECTION

Fig. 389. — A portion of A, longitudinal and B, cross section of a nerve prepared by
the vom Rath method. A nerve is a bundle of axons (neurites) covered by an epi-
neurium. Each axon is surrounded by an inner myelin (fatty) sheath and an outer
cellular neurolemma sheath.

sheath serves at least for insulation is indicated by the fact that nervous
impulses are conveyed more rapidly in medullated than in non-medullated
nerves.

Within the central nervous system of vertebrates, neurites lack the
neurolemma sheath, but are usually medullated. The presence of a
neurolemma is, therefore, not essential to the secretion of this fatty myelin
sheath. Most peripheral nerves are medullated, but the medullary sheath

472

COMPARATWE ANATOMY

does not appear until after the neurolemma is differentiated. The nerves
of amphioxus and of cyclostomes are not medullated. This primitive
condition is retained by the sympathetic nerves and plexuses of higher
forms. Primitive ganglion cells like those of coelenterates occur in
vertebrates only in the parasympathetic plexuses associated with the
alimentary canal.

The forms assumed by neurons in vertebrates are varied; and in
general, the more complex the animal the more complex its neurons.
Complication in form usually involves an increase in the number of
dendrites, and denotes a multiplication in the number of possible func-

A. DIAGRAM OF A REFLEX ARC

Fig. 390. — Diagram of a nervous arc. In A three neurons — afferent, intercalary,
and efferent — are shown in their relations to one another and to the skin and muscle.
The intercalary neuron is located in the gray matter of the spinal cord. B is an enlarged
section of a nerve fiber.

tional relations. In ontogenesis, as in phylogenesis, all the processes of
neurons are formed as processes of primarily simple neuroblasts.

Nerve cells manifest a tendency not only to spin out elongated proto-
plasmic processes so as to connect with various parts of the body and with
one another, but also to form- plexuses and ganglionic masses. In this
way, the complex nervous systems of higher animals have been built up.
The first nervous connexions appear to have been between skin and
muscle, by means of neurosensory cells, so that a motor response to
external stimulus is made possible. So simple an arrangement as this is,
however, rarely found, even in worms and molluscs. Usually at least
two nerve cells are involved in a reflex action, a neurosensory receptor and
a motor ganglion cell which connects with muscle or gland. Even more
frequently, a third or association cell is interpolated between the receptor
and the motor cells. These association cells may multiply to form a chain
of neurons within the central nervous system. Such complications were

THE NERVOUS SYSTEM

473

made possible by the genesis of the central nervous systems of higher
animals from the nerve net of coelenterates. (Fig. 390)

Dorsal root.

Spinal nerve

Cell of a
sympathetic
ganglion. Sympathetic
ramus.

Spinal
ganglion.

End disc

Capstile.

Fig. 391. — Diagram of the nervous elements in a human spinal ganglion. (From
Bremer's "Text Book of Histology.")

In the primitive nerve plexuses of lower animals such as the coelen-
terates, and also in those of the vertebrate alimentary canal, the ganglion

474 COMPARATIVE ANATOMY

cells are in protoplasmic continuity with one another, and nerve impulses
are carried directly from cell to cell. The differentiated neurons of
higher animals appear not to be so interconnected, but have greater
individuality, for the telodendria of one neuron are brought into relation
with the dendrites of another only indirectly through the so-called synapse.
A synapse is the region where the fine telodendria of one neuron are
brought into physiological relation with the dendrites of another neuron.
Nerve impulses which involve the activity of two neurons must pass
through such a synapse. The transmission of a nervous impulse through
the synapse is believed to involve a semipermeable membrane through
which impulses pass when conveyed from one neuron to another. The
physical process by which such a transfer is effected has been compared
to the jump-spark action of a gasoline engine.

It should not, however, be understood that this synaptic membrane
has been demonstrated beyond a reasonable doubt. Its presence is
inferred chiefly because refined neurological technique has not been able
to demonstrate the continuity of the neurofibrillae of adjacent neurons.
The fact that each neuron arises from a neuroblastic cell which is primarily
independent of other cells and that the termination of a growing neurite
is free further strengthens this conclusion. Some physiological experi-
mental evidence points in the same direction.

With the differentiation of the neuron, with neurite and dendrites
which normally convey nerve impulses in one direction only, and with
tigroid bodies and neurofibrillae in its cytoplasm, the evolution of the
nervous unit or neuron reaches its climax.

The steps in the phylogenesis of the nervous system are, therefore, the
differentiation of the neurosensory cell, the attainment of functional con-
nexion with muscle or gland cells, the recession of the neurosensory cell
from the external epithelium to form a primitive ganglion cell, the forma-
tion of an interconnected nerve-net containing association cells, the union
of afferent and efferent neurites into bundles or nerves, and the concen-
tration of nerve-cell bodies to form ganglia.

ORGANIZATION OF THE NERVOUS SYSTEM

When nervous units convey impulses towards and away from a subcu-
taneous nerve-net, as in the coelenterates, there are the beginnings of a
nervous system. The nerve net forms the central nervous system, the
afferent and efferent neurites the peripheral nervous system. The primi-
tive and characteristic function of such a system is the nervous reflex.
A nervous reflex or reflex action is a simple motor response to stimulus
involving sensory and motor neurones and their interconnexions within a
nerve-net or nerve center.

THE NERVOUS SYSTEM

475

If we take the subcutaneous nerve-net of coelenterates as an early
stage in the evolution of the central nervous system and the neurosensory
cells as the beginnings of sensory nerves, the primitive ganglion cells are
the original association and motor cells. From such beginnings it is not
difficult to derive the complex nervous systems of the higher animals.

The fiatworms show a distinct advance above the coelenterate stage.
In them, the nerve net is partly aggregated into two or more paired longi-
tudinal cords or connectives, which unite at the anterior end of the worm,
in close association with pigmented eye-spots, to form the beginnings of a
brain. (Fig. 392)

Morphologists are inclined to derive the paired lateral nerve cords of
fiatworms directly from the subumbrellar ganglionic ring of a medusa.

A. tOWER FLATWORM B. HIGHER FLATWORM C. ARTHROPOD D. VERTEBRATE

Fig. 392. — Diagrams of the nervous system of A, Lower Flatworm, B, Higher
Flatworm, C, Arthropod, and D, Vertebrates. The higher nerve centers are cross-
hatched. The sympathetic cords are indicated by dotted lines. Many morphologists
assume that the figures represent a phylogenetic series. Such a view, however, which
assumes that annelids are ancestors of vertebrates, meets with very serious, if not
insuperable, diffictilties. See Chapter 15. (Redrawn after Stempell.)

Like the subumbrellar ring, the nerve cords of fiatworms consist of nerve
fibers associated with clusters of primitive ganglion cells. Kappers
explains the concentration of nervous material in the anterior brain as a
result of the great exposure of the head to stimuli. Unfortunately, such
an hypothesis is unsupported by experimental evidence. (Fig. 393)

In the simple flatworm Planocera, there is a single pair of nerve cords.
The number increases in other forms, and the cords may be dorsal and
ventral as well as lateral. This fact is important in its bearing upon the
development of the nervous systems of higher animals, which in general
are assumed to have evolved from fiatworm-like ancestors. For the
presence of both dorsal and ventral nerve cords in fiatworms makes it
possible to derive annelids and arthropods from fiatworms in which the
ventral cords become the dominant nervous centers, and to derive verte-
brates from fiatworms in which the dorsal cords become predominant.
The dorsal nerve cord of vertebrates therefore need not have been derived
from the ventral cord of annelids by the inversion of the worm. It has

476

COMPARATIVE ANATOMY

been asserted that the ventral nerve cords are larger in flatworms which
crawl, while the dorsal cords are larger in free-swimming types.

As we pass from the flatworms to higher groups, two contrasting trends
are noticeable. In annelids and arthropods the nerve cords become
markedly metameric and are non-tubular, while that of chordates is
tubular and primarily non-metameric, as in Protochordates.

NERVE CORD

FLATWORM

Fig. 393. — A diagram illustrating the hypothetical evolution of the nerve-net of a
Hydra into the sub-unibrellar nerve-ring of a medusa on the one hand and into the
paired nerve cords of flatworms and annelids on the other. The nerve cords are formed,
it is assumed, by the concentration of the fibrillar and cellular elements of the nerve-net.
(Redrawn after Kahn's "Das Leben Des Menschen," W. Keller & Co.)

The nerve cord of the primitive annelids consists of a chain of paired
ganglia linked together both by longitudinal connectives and by trans-
verse commissures. The longitudinal connectives pass around the eso-
phagus to connect the supra- and sub-esophageal ganglia. In the higher
annelids the paired ganglia tend to unite in the mid-ventral line and to
lose the primitive rope-ladder arrangement. Concentration and fusion
greatly reduce the number of ganglia, especially in arthropods. (Fig. 394)

These profound changes in the form of the nervous system of articu-
lates are accompanied by histological and physiological differences. Most
of the nerve cells become definitely polarized to transmit impulses in one
direction only, either towards or away from the central nerve cord. Thus

THE NERVOUS SYSTEM

477

the neurons become in all essentials like those of vertebrates, being
differentiated either as sensory or motor, while those within the nerve-cord
become association-cells. A reflex-arc pattern is thus established, in
which the motor and sensory cells involved in the reflex may be either
homolateral or heterolateral according as the neurons involved belong to
one or to both sides of the body. In insects separate motor and sensory
nerve roots appear, reversed in position as compared with vertebrates.
Motor roots are dorsal and the sensory roots ventral, while in vertebrates
motor roots are ventral and sensory roots dorsal. It will be noted, how-
ever, that if the insect were turned over on its back so as to bring the nerve
cord on the dorsal side of the body as in vertebrates, the relations of the
nerves would be similar to those of vertebrates. Giant ganglion cells and
fibers, resembling those of fishes, appear in the nerve cord of annelids.

-;- GANGLION CELLS.

NERVE FIBERS.

'EPITHELIUM. NmUSCLE.

Fig. 394. — Invertebrate (annelid) and vertebrate nervous systems compared. In
both sub-kingdoms the nervous system is derived from the ectoderm. The central
nervous system of Sigalion (A) retains its original connexion with the skin. In
Allolobophora (B), however, as in most annelids and in vertebrates (C) the nerve cord
separates from the skin. In figure C the vertebrate spinal cord is shown in reversed
position with dorsal side down. In A, B, and C the axon processes of the ganglion
cells within the nerve cord are similarly directed away from the surface. D and E
show the relations of afferent and efferent neurons in an anneUd (L>) and a vertebrate (£).
(Redrawn after Parker.)

NERVOUS SYSTEM OF CHORDATES
Hemichordates. Hemichorda.tes have a dorsal tubular nerve cord
limited to the collar region, but a ventral invertebrate type of nerve
cord in the gill and trunk region. The tubular portion of the dorsal cord
remains open at both ends throughout life. It contains neurosensory
cells of a primitive type and some giant ganglion cells, is surrounded by
an outer fibrous layer as in vertebrates, and continues forward into the
proboscis and backwards into the body as a non-tubular strand of nervous
tissue. A circumesophageal ring connects this dorsal cord with a ventral
median strand. The ventral nerve strand is simply a local thickening
of a layer of nerve fibers which forms continuous network beneath the skin.

478 COMPARATIVE ANATOMY

The association in hemichordates of invertebrate and vertebrate
types of nervous system helps to bridge over the gulf between the two
groups. (Fig. 532)

Urochordates. The nervous system of larval urochordates shows an
advance towards that of vertebrates, for the ventral invertebrate nervous
system has. disappeared, and the nerve cord is tubular throughout.
Three divisions may be distinguished, an expanded anterior brain or
sense vesicle which encloses an unpaired eye and static organ; a short
trunk portion; and, behind, a slender cord which in the larva extends
into the tail. Paired nerves connect the cord with the caudal muscle.
The brain is anterior to the notochord, and has therefore been considered
as the homologue of the forebrain of vertebrates. (Fig. 538, A)

In most urochordates, the tail is lost during metamorphosis, and with
it the associated nerves, so that only in Appendicularia do these persist
throughout life.

Cephalochordates. Cephalochordates have a nervous system with
many vertebrate characteristics. The cord extends through the entire
length of the body as a tube with a slit-like lumen, which is expanded
anteriorly in the region of the so-called brain. It also resembles that of
vertebrates in its origin from a thickened placode of ectoderm on the
dorsal side of the embryo. The neuropore persists in the larva, but
closes in the adult animal to form the so-called olfactory pit. (Fig. 538,5)

Two divisions of the brain are recognized, an anterior prosencephalon
and a posterior deuterencephalon. The prosencephalon is lined with
ciliated columnar epithelium which shows little if any nervous differentia-
tion. In its anterior wall is a pigment spot which, with scant justification,
is called an eye. From the prosencephalon a pair of sensory nerves, the
terminal nerves, extend forward towards the snout. The posterior
boundarv of the brain is marked by a cluster of ciliated sense cells, the
infundibular organ.

The deuterencephalon is possibly homologous with the mid- and
hindbrain of vertebrates. It differs from the spinal cord in having in its
dorsal wall large neurosensory cells known as cells of Joseph. Two
paired dorsal sensory nerves — numbered II and III — connect with the
deuterencephalon. The first pair of motor nerves connect with its
ventral wall. (Fig. 436)

In amphioxus dorsal and ventral nerves alternate with one another
throughout the length of the body. Except the two anterior pairs,
which are wholly sensory, the dorsal nerves of Amphioxus are mixed in
function. They extend between the myotomes to the skin, where they
divide into dorsal and ventral rami. Ventral nerves, on the other hand,
pass from the cord directly to the myotomes opposite. Consequently,
in cephalochordates, dorsal and ventral nerves do not unite. The

THE NERVOUS SYSTEM

479

ganglion cells of the dorsal nerves lie either in the dorsal wall of the cord
or are embedded in the nerves. The motor ganglion cells, as in verte-
brates, lie within the ventro-lateral wall of the cord. (Fig. 429, .4)

On the basis of their peripheral distribution, four kinds of nerve fibers
are distinguished, somatic motor and somatic sensory, visceral motor and
visceral sensory. Each somatic
motor nerve innervates three
successive myotomes, but most of
its fibers pass to the middle one.
Giant ganglion cells occur in the
mid-dorsal line of the cord at the
anterior and posterior parts of the
body, but are wanting in the
intermediate region. Since these
connect with the sensory nerves,
they are probably elements in a
reflex system. Sympathetic nerve
fibers connect with the blood vessels
and the viscera, but there is no
chain of sympathetic ganglia.

Cyclostomes. Compared with
cephalochordates, the cyclostomes
show a marked advance in the
complexity of their nervous system.
Instead of only two brain divisions,
cyclostomes have five, telence-
phalon, diencephalon, mesencepha-

1 , 1, „ 1 „ r, r^ A Fig. 30 S- — Dorsal and lateral views of

Ion, metencephalon, and ^^^ ^,^^^1^ Petromyzon planeri. Thetelae

myelencephalon. Ontogenesis re- chorioideae removed, and the epiphysial

1 1 „ i-Ur^t- fVi-ic-o f,-irc^ structures not shown in the side view, c,

veals, however, that these five ^^^^^^^^^ ^^^^^. ^_ epiphysial structures;

vesicles of cyclostomes and higher j^, saccular part of infundibulum-, /a, acoustic
, , , 1 1 r ,^ -v,^ ,^r■^r,^ lobc: Ik, left habenular ganglion; li, lobular

vertebrates develop from the origi- p^^'^f -^f^^dibulum; iv, lobe of vagus; m,

nal three which are correlated with mid-brain; of, olfactory bulb; pc. posterior
thp thrpp maior senses smell sieht commissure; rh, right habenular ganglion;

tne tnree major senses, bmeii, bigiiL, ^^ ^^^^ ^^.^^^ ^^^^^. ^^^^ thalamus; 1-12,

and hearing. The primitive fore- cranial nerves. (From Kingsley's "Com-
brain, which in the opinion of ^Xm.) ^"^'"""^ °^ Vertebrates." after

most morphologists corresponds to

the prosencephalon of amphioxus, becomes the telencephalon and dien-
cephalon, the midbrain continuing as the mesencephalon, and the original
hindbrain divides into metencephalon and myelencephalon. Since the
cyclostome brain may be taken as the complete prototype of that of all
vertebrates, and since most of its features persist in higher forms, these
are worthy of mention in some detail. (Fig. 395)

480 COMPARATIVE ANATOMY

The telencephalon is paired, in correlation with the development of
paired olfactory lobes. Although paired eyes develop from the forebrain,
they acquire sensory centers within the optic lobes of the mesencephalon.
Olfactory centers arise in the diencephalon, and effect connexion with
spinal motor centers to form a mechanism for olfactory reflexes. Also
involved in these reflexes are paired habenular ganglia in the roof of
the diencephalon, and the interpeduncular nucleus in the base of the
mesencephalon. Paired epiphyses project from the roof of the diencepha-
lon, and its lateral walls are thickened as the ganglionic masses of the
thalami. A funnel-shaped infundibuliim projects from the floor towards
the roof of the mouth. (Fig. 396)

The roof of the mesencephalon of cyclostomes is peculiar in having a
chorioid plexus, which serves as a means of nourishing the brain. Lateral
to this plexus are the conspicuous paired swellings of the optic lobes,
the centers of vision. The thickened lateral wall of the midbrain is
largely fibrous and is known as the tegmentum. The floor of the mid-
brain contains the motor center of the oculomotor nerve, which innervates
four of the eye muscles. The functions of the midbrain are predominantly
locomotor and somatic rather than visceral.

From the roof of the metencephalon arises an inconspicuous cerebel-
lum, which is the anterior continuation of the lateral line centers of the
myelencephalon. In it are located the complex Purkinje cells, which
have fiber connexions with motor cells in the medulla and cord, and are
found in the cerebellum of all vertebrates. The lateral walls and base
of the metencephalon consist largely of fiber tracts, most of which are
ascending and descending fibers which connect brain and spinal cord.
No pons, which is so prominent a feature of the base of the mammalian
metencephalon, is present in cyclostomes or in any of the lower
vertebrates.

The myelencephalon or medulla oblongata is a transitional region
between brain and spinal cord. Its roof is largely differentiated as a
chorioid plexus. The lateral walls contain the sensory centers of the
lateral line nerves as well as those of other cranial nerves. The motor
centers of the trigeminal, facial, glossopharyngeal, and vagus nerves are
also located in the medulla.

The lumen of the brain expands into four large ventricles, two of
which lie in the paired divisions of the telencephalon, the third is in
the diencephalon, and the last in the myelencephalon. Three fiber
tracts or commissures connect the right and left halves of the brain,
the anterior in the wall of the telencephalon, the habenular in the roof
of the diencephalon and the posterior in the roof of the mesencephalon.
All three persist throughout the vertebrate series to man, and serve as
important landmarks by which to determine homologies.

THE NERVOUS SYSTEM

481

In addition to the numerous nervous structures which emerge in
the cyclostome brain, the hypophysis acquires intimate relations with
the infundibulum, and shows the first stages in the formation of the
pituitary gland, which is represented by a cluster of vesicles derived from
the hypophysis.

TELEN-

DIEN-

epiphysis'
parietal organ,
habenular ganglion
dorsal sac
paraphysis
Monroe's foramen
hemisphere-

MESEN- METEN-
---" V ^ V

.POST. COMMISSURE

/TECTUM OPTICUM

MYELENCEPHALDN

SPINAL
[GANGLION

BASAL GANGLION
ANT COMMISSURE
CHORIOID PLEXUS

^ Y" uS?^ I NFUND I BULUM

(OPTIC \ \hypophysis

CHIASMA \qp^,(. thalamus

LATERAL VENTRICLE/

BASAL GANGLION

HEMISPHEREv

OLFACTORY
LOBE I

OPTIC THALAMUS
EPIPHYSIS
VENT HI I I /HABENULAR GANG.

ANTPALUI
COMMISSURE

ANT COMMISSU

Fig. 396. — Diagrams of the vertebrate brain, based upon the brain of a cyclostome.
A shows the brain in median longitudinal section, with nerves as if projected upon the
median plane. B is the brain viewed from above. (Redrawn from Plate, after
Biitschli.)

The nourishment of the brain is effected chiefly through the three
chorioid plexuses in the roof of diencephalon, mesencephalon, and
myelencephalon.

The spinal cord in cyclostomes has become a thick-walled tube in
which three layers are differentiated, an outer marginal layer of fibers,
a middle mantle layer of gray matter, and a central ependymal layer
which lines the central canal. The gray matter has only two lateral
wings or columns instead of the four characteristic of higher vertebrates.
A number of giant nerve fibers like those of amphioxus extend along
the spinal cord, but they do not decussate (i.e., cross to the opposite

482

COMPARATIVE ANATOMY

side of the cord), and they carry impulses caudad only, an indication of
the growing dominance of the anterior portion of the nervous system.
A primitive trait appears in the location of sensory ganglion cells within
the wall of the neural tube.

The anterior ten pairs of nerves in Cyclostomes have their exit through
foramina in the cranium and hence are known as cranial nerves. It is
not unlikely that all correspond to anterior nerves" of amphioxus, except
the optic which is a fiber tract of the brain and not a true peripheral
nerve. The ten cranial nerves are the olfactory (I), optic (II), oculo-
motor (III), trochlearis (IV), trigeminal (V), abducens (VI), facialis (VII),

Fig. 397. — Diagram of cranial nerves of lower vertebrate. Eye-muscle nerves
omitted; central nervous system dotted, fifth nerve represented as composed of two
nerves; lateralis nerves separated from the ninth and tenth nerves. I-X, cranial
nerves; 1-5, gill clefts; h, buccalis nerves; c, chorda tympani; g, geniculate ganglion;
h, hyoid nerve; i, intestinal (pneumogastric) nerve; i, jugal ganglion, I, lateral line nerve
of X; m, mouth; md, mandibular nerve; ml, mentalis nerve; 7nx, maxillary nerve; op,
ophthalmicus profundus nerve; opV, opVII, superficial ophthalmic nerves of V and
VII; p, palatine nerve; po, posttrematic nerves; pr, pretrematic nerves; pi, petrosal
ganglion; 5, semilunar (Gasserian) ganglion; sp, spiracle. (From Kingsley's "Com-
parative Anatomy of Vertebrates.")

auditory (VIII), glossopharyngeus (IX), and vagus (X). In lower
vertebrates the hypoglossus and spinal accessory nerves are not cranial
but spinal. Of the ten cranial nerves, I, II, and VIII are sensory. III,
IV, and VI somatic motor, and the others mixed sensory and motor.
The so-called nervus terminalis appears not to be an independent nerve,
but a component of the olfactory. The remaining neurites of the olfactory
are processes of neurosensory cells in the olfactory epithelium.

The optic nerve develops in correlation with the eye, the retina, from
which the optic nerve fibers arise, being a segregated part of the wall
of the telencephalon. Sorrie of its fibers cross below the brain in front of
the infundibulum to form the Optic chiasma. The optic nerves, after
entering the wall of the diencephalon, pass by way of the optic thalami
to their reflex centers in the roof of the mesencephalon.

The oculomotor, a somatic motor nerve with its nucleus or motor
center in the base of the midbrain, innervates four eye muscles, the

THE NERVOUS SYSTEM 483

superior, inferior, and anterior recti and the inferior oblique. Sympathetic
non-medullated fibers presumably occur in the oculomotor, but no distinct
sympathetic ganglion is formed.

The trochlearis arises from a motor nucleus in the floor of the meten-
cephalon posterior to that of the oculomotor and supplies the superior
oblique eye muscle. Its fibers emerge from the medulla near the root of
the profundus. The dorsal chiasma of the trochlearis appears to be
absent in cyclostomes.

The trigeminus is so named because, in all vertebrates, it has three
chief branches, a sensory ophthalmicus profundus which extends above
the eye to the skin on the upper side of the snout, a sensory maxillaris
branch to the skin on the side of the snout and the region which cor-
responds to the upper jaw of fishes, and a mixed mandibularis branch
which supplies the skin and muscles of the first visceral arch.

Cyclostomes are the only vertebrates in which the profundus branch
arises by an independent root. This fact supports the conclusion that
the profundus was once an independent segmental nerve and that its union
with the trigeminus in all vertebrates above cyclostomes is secondary.

The motor center of the trigeminus is in the lateral column of the
medulla. A unique feature of the trigeminus is a sensory nucleus in
the roof of the mesencephalon, the fibers of which bring nerve impulses
from mandibular muscles. Most of the sensory fibers of the trigeminus
arise from ganglion cells in the large Gasserian ganglion near its root of
origin from the medulla. With few exceptions, the sensory nerves of all
vertebrates have similar ganglia near their roots of origin.

The abducens is a somatic motor nerve which emerges from the
medulla ventral and posterior to the root of the trigeminus and innervates
the posterior or external rectus eye muscle.

The facialis carries both special and general somatic sensory fibers,
and also visceral sensory and motor fibers. The motor fibers arise from
an elongated nucleus in the lateral column of the medulla, and supply
muscles of the hyoid arch. There are four major branches. The sensory
ramus ophthalmicus superficialis innervates the supraorbital series of
lateral line organs. The buccalis supplies the infraorbital series. The
deep palatine branch is distributed to the skin of the roof of the mouth.
The hyomandibular innervates the muscles of the hyoid arch and the skin
of the mandibular region.

The auditory supplies the otic capsule which in cyclostomes is chiefly
an organ of equilibration. As might be expected from its origin as a
branch of the facial nerve, its roots are closely associated with those
of the facial. Since the otic capsule is a modified lateral line organ, the
fibers of the auditory nerve belong to the group of lateralis or special
somatic sensory components.

484

COMPARATIVE ANATOMY

■ \aleTaV\s
: \libctral moloi:
: visccTol seasoivj

Fig. 398. — Diagrams of the branches and components of (A) the trigeminal, (5)
facial, and (C) glossopharyngeal and vagus nerves of a lower vertebrate, b, buccalis
nerve; ct, chorda tympani; d, dorsal rami of IX and X; g, gastric nerve; gg, in A, Gas-
serian ganglion, in B, geniculate ganglion; h, hyoid nerve; hm, hyomandibular trunk;
j, Jacobson's connective; jg, jugular ganglion; I, lateralis nerve; 7nxe, maxillaris
externus nerve; op, ophthalmicus profundus nerve; os, superficial ophthalmic nerve;
pal, palatine nerve; po, pr, post- and pretrematic rami; sp, spiracle; st, nerve to supra-
temporal lateral line organs; 1-5, gill clefts. (From Kingsley's " Comparative Anatomy
of Vertebrates.")

THE NERVOUS SYSTEM

485

The glossopharyngeal is the mixed nerve which supplies the third
visceral arch. It forks over the first gill-slit and a pretrematic branch is
distributed to the posterior wall of the hyoid arch. The post-trematic
branch contains sensory fibers from the floor of the pharynx and motor
fibers which innervate the muscles of the third arch.

The vagus is a mixed nerve formed of fibers which are distributed
to the muscles and skin of the posterior visceral arches. This suggests
that a number of segmental nerves are united in the vagus. A lateralis
branch is the nerve of the posterior series of lateral line organs. A
visceral branch goes to heart, stomach, and intestine, and carries sym-

I ST. SPINAL GANGLION
VAGUS

SPINAL GANGLIA

N . GLOSSOPHARYNGEUS
OTIC CAPSULE^^
N. FACIAL
EYE

^HYPOBRANCHIAL N. HYPOGLOSSUS

MUSCLE.

Fig. 399. — The head of Ammocoetes larva of Petromyzon in left lateral aspect,
showing that the hypobranchial muscle is innervated by the somatic motor fibers of
seven metaotic nerves which in higher vertebrates become the hypoglossal nerve.

pathetic fibers connected with these organs. Each branchial branch
divides into pre- and post-trematic rami, which contain both visceral
sensory and visceral motor fibers.

In the trunk region there is a general correspondence between the
number of myotomes and of spinal nerves, since for each myotome there
are usually a sensory and a motor nerve. In Petromyzon these are not
united; but in Myxinoids they join to form a mixed spinal nerve with two
roots, a dorsal ganglionated sensory root and a ventral motor root, each
neurite of which arises from a multipolar ganglion cell located in the gray
matter of the cord. Synaptic connexions between sensory and motor
neurones take place within the gray matter. Peripherally, each spinal
nerve divides into dorsal and ventral rami which supply skin and muscles.

This simple one-to-one metameric correspondence of spinal nerves
and myotomes is, however, somewhat modified in the occipital region of
petromyzon where the first five post-otic myotomes are innervated by the
nerves of the fourth and fifth myotomes, and the nerves of the three
anterior myotomes have disappeared, at least as independent roots.
The nerves of post-otic myotomes 6 to 12 unite to form the hypoglossal
nerve which supplies the hypobranchial muscles. In this fusion of

486

COMPARATIVE ANATOMY

occipital nerves, may be seen the beginnings of the cervical plexus which
persists throughout the vertebrate series. Since paired appendages are
wanting in cyclostomes, no thoracic or lumbar plexuses are formed.

Cerebellum

Optic lobe

Thalamus

Cerebral hemisphere

Olfactory bulb

Olfactory tract

Vagus nerve N. X J /

Glossopharyngeal nerve N. IX ' ,

Acoustic nerve N. VIII /

Abducens nerve N. VI

Optic nerve N. II

Inferior lobe

Oculomotor nerve N. Ill
i Saccus vasculosus
Trochlear nerve N. IV

Trigeminal and facial nerves Nn. V, VII
Fig. 400. — The brain of the dogfish, Squalus acanthias, lateral view. (From Ranson's
"The Anatomy of the Nervous System," courtesy of W. B. Saunders Company.)

Little can be said concerning the sympathetic nervous system of
cyclostomes. The nerves of cyclostomes like those of Amphioxus, are
not medullated, so that this means of distinguishing sympathetic from
other fibers cannot be used. A number of observers claim to have

Epiphysis
I
Paraphysis
Cerebral hemisphere \
Olfactory tract , .''n, !

Olfactory bulb

Optic lobe

Mesoccele
! Cerehellurrt

Metacoele

Tiiberciilum acusticum
Tela chorioidea
I Fourth ventricle
Visceral lobe

Telencephalon

Preoptic recess
Velum transversum

Optic chiasma

i i \ Metencephalon Myelencephalon

I i Mesencephalon

1 Saccus vasculosus

Third ventricle

Fig. 401. — The brain of the dogfish, Squalus acanthias, medial sagittal section.
(From Ranson's "The Anatomy of the Nervous System," courtesy of W. B. Saunders
Company.)

found clusters of sympathetic ganglion cells associated with the vagus.
Also a plexus comparable to Auerbach's plexus of higher vertebrates is
found in the intestinal wall. Chromaffin cells, which have an origin in
common with sympathetic cells, are found in the trunk region and have a
segmental distribution as in mammals.

THE NERVOUS SYSTEM

487

Elasmobranchs. The nervous system of elasmobranchs shows some
advances above that of cyclostomes. The roof or pallium of the telen-
cephalon has thickened and expanded. To the corpus striatum is added
an epistriatum, both connected with olfactory fibers. The telencephalon
remains predominantly an olfactory center. Elongated olfactory tracts

Nasal capsule

/Olfactory rrerve N. I
' / Rhinoccele

Lateral ventricle

" Olfactory bulb
Nervus terminalis
->-- Olfactory tract-

~ Cerebral hemisphere-

I ntervenlricular for.

Y Epiphysis

Optic nerve N. II
' -- - Thalamus

-Optic lobes

*^ Trochlear nerve N. I\'

-Cerebellum

■Lcbus linecB lateralis.
Facial nerve N. VII

Acoustic nerve N. VI IT..,

Tuberculum acusticum

Medulla oblongata

Glossopharyngeal nerve N. IX
■—Medial longitudinal fasc. ...
-Visceral lobe

Vagus nerve N. X

Spinal cord

Telencephalon

\ Third ventricle
> Diencephalon

'-Mesoccde
^Mesencephalon

y ^Fourth ventricle

Metencephalon

'.^■Cerebellum

(caudal part)

4 Fourth ventricle
\ Myelencephalon
>

Fig. 402. — The brain of the dogfish,
Squalus acanthias, dorsal view. (From
Ranson's "The Anatomy of the Nervous
System," courtesy of W. B. Saunders
Company.)

Fig. 403. — The brain of the dogfish,
Squalus acanthias, with the ventricles
opened, dorsal view. (From Ranson's
"The Anatomy of the Nervous System,"
courtesy W. B. Saunders Company.)

are differentiated. A saccus vasculosus, which possibly functions as a
pressure organ, is appended to the infundibulum. The midbrain has
lost its chorioid plexus and its roof has become thickened and wholly
nervous. With the increased importance of the lateral line organs, their
centers in the lateral lobes of the medulla are more developed. Possibly
for the same reason, the cerebellum, which is a static center, is greatly
enlarged.

488

COMPARATIVE ANATOMY

Fig. 404. — Brain and cranial nerves of Carcharias littoralis (Princeton 310), natural
size, bo, olfactory bulb; br^~*, branchial nerves; cr, auricle of medulla; d, diencephalon;
e, epiphysial staUc, the pineal organ lacking; ec, external canal of ear; er, external
rectus; eo, external oblique; hm, hyomandibular nerve; io, inferior oblique; I, lateralis
nerve; tnd, mandibularis nerve; ml, myelencephalon; ms, mesencephalon, also, maxillaris
superior; ml, metencephalon; 00, olfactory organ; os, ophthalmicus superficialis nerve;
ol, olfactory tract; pal, palatine nerve; pc, posterior canal; po, posttrematic branch; pr,
pretrematic branch; so, superior oblique; sr, superior rectus; I, telencephalon; u,
utriculus; V, visceral branch of X; I-X, cranial nerves. (From Kingsley's "Compara-
tive Anatomy of Vertebrates.")

THE NERVOUS SYSTEM

489

In the spinal cord, dorsal and ventral columns of gray matter are
differentiated. The dorsal column, however, remains unpaired.

The cranial nerves are identical with those of cyclostomes but rela-
tively enlarged both in correlation with the increased size of sense organs
and muscles, and by the addition to the nerve fibers of medullary and
neurolemma sheaths. A cranial sympathetic ganglion, the ciliary, has
developed in association with the oculomotor and profundus nerves.
The profundus has now become a branch of the trigeminal. Some of
the supraorbital series of lateral line organs are innervated by fibers of the
superficial ophthalmic branch of the fifth nerve, while the remainder are
supplied by the superficial ophthalmic of the facialis.

The somatic motor nerves of five post-otic myotomes unite to form
the h>'poglossal, which supplies hypobranchial muscles. A thoracic

Fig. 405. — Brain of Prolopterus, a Dipnoan fish, ch, cerebellum; e, epiphysial
structures; h, hypophysis; i, inf undibulum ; m, mid-brain; se, saccus endolymphaticus;
sp, spinal nerves; t, cerebral hemisphere; 1-12, cranial nerves. (From Kingsley's
"Comparative Anatomy of Vertebrates," after Burckhardt.)

plexus is formed by the union of the nerves immediately posterior to those
of the cervical plexus. But the number of nerves which participate
varies greatly in different elasmobranchs. In many species the fibers
of the cervical and thoracic plexuses unite as a cervico-thoracic plexus.
In the region of the pelvic fin is a similar but smaller lumbo-sacral plexus.

Well developed sympathetic ganglia appear in the trunk region,
in the vicinity of the dorsal aorta. Their arrangement is metameric,
but the anterior and largest is formed by the union of primarily separate
ganglia. Each is connected by a ramus communicans with a spinal
nerve. A longitudinal sympathetic cord or connective is only imper-
fectly developed. An intestinal plexus occurs, as in all vertebrates.

Amphibia. Amphibia have a relatively simple brain like that of
cyclostomes and dipnoi. Olfactory lobes are relatively large, and merge
without constriction into the cerebral hemispheres. The pallium is
thick, and cells have migrated from the gray matter into the external
marginal zone of white matter. The lumen of each hemisphere is reduced
by the thickening of its median and lateral walls. An inner longitudinal
sulcus or groove divides these walls into dorsal and ventral halves. The
dorsal half of the lateral wall is the paleocortex, the ventral half is the epis-
triatum, and below the epistriatum is the paleostriatum. In the median

490

COMPARATIVE ANATOMY

wall, the dorsal half is the archicortex or primordium hippocampi. The
medio- ventral wall forms the septum by which fibers pass to and from the
hippocampus. The hemispheres are interconnected by anterior, anterior
pallial and posterior pallial commissures located in the lamina terminalis.

Habenular and posterior commissures persist in the roof of the dien-
cephalon. The epiphysis forms a pineal gland. A chorioid plexus

Fig. 406. — Side and dorsal views of brain of young alligator, c, cerebrum; cl,
cerebellum; e, epiphysial structures; h, hypophysis; i, infundibulum; ol, optic lobes;
II-XII, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates,"
after Herrick.)

invaginates into the third ventricle. The saccus vasculosus of fishes has
disappeared. The thickened walls of the midbrain reduce the lumen
to a narrow passage, the aqueduct. The cerebellum is rudimentary like
that of cyclostomes. (Fig. 414)

The ten cranial nerves of fishes persist in amphibia. Urodeles have
lateralis nerves, but these disappear in the anura in correlation with
the loss of lateral line organs. With the loss of gills, the number of
branches of the vagus is reduced.

Cervical and lumbar enlargements of the spinal cord appear in cor-
relation with the enlargement of the appendages.

THE NERVOUS SYSTEM

491

Sympathetic nerve cords or connectives unite the series of sympathetic
ganglia.

Reptiles. The cerebral hemispheres of reptiles are larger than those
of amphibia and by extension caudad have partially overgrown the
diencephalon. The paired ventricles are nearly obliterated by the enlarge-
ment of the striate bodies, archistriatum and neostriatum. For the
first time in the vertebrate series, a cortical layer of pyramidal cells
appears in the pallium, having nervous connexions with fibers of the

Fig. 407. — Brain of goose, ac, anterior commissure; cb, cerebellum; e, epiphysis;
/, flocculus; h, hypophysis; hs, hyperstriatum; i, infundibulum; I, lateral ventricle; m,
medulla oblongata; 7ns, mesostriatum; ob, olfactory bulb; ol, optic lobe; s, striatum;
I, temporal lobe; III, third ventricle; x, plane of section E. (From Kingsley's "Com-
parative Anatomy of Vertebrates," after Biitschli.)

oKactory tract. Septum and hippocampus appear in the medial wall
much as in amphibia. The dorsal wall of each hemisphere is homologized
with the gyrus dentatus of the mammalian brain. The transitional
region between this dorsal pallium and the neostriatum has important
potentialities, since it is unconnected with olfactory fibers, and since in
mammals it becomes the neopallium from which develops the greater part
of the cerebral cortex on which the higher psychical activities of man
depend. (Figs. 406, 414)

In the region of the diencephalon of lizards the anterior epiphysial
outgrowth, the parietal organ, develops a lens, retinal and pigment layers,
and nerve fibers which are connected with centers in the brain wall.

492

COMPARATIVE ANATOMY

Pig. 408. — Ventral view and median section of the brain of Ornilhorhynchus . AC,
anterior commissure; bo, olfactory bulb; c, first cervical nerve; cl, cerebellum; CM,
massa intermedia; e, epiphysis; /, flocculus; fd, fasciculus dentatus (anterior part of
hippocampal tract); fi, interventricular foramen; h, hypophysis; he, habenular com-
missure; HC, pallial (hippocampal) commissure; Ip, piriform (hippocampal) lobe;
mc, mammillary bodies; mo, medulla oblongata; n, nodulus; ol, olfactory lobe; at,
olfactory tubercle; pal, pallium (temporal lobe); pc, posterior commissure; rf, rhinal
fissure; tc, tuber cinereum: tV, tuberculum quinti; v, velum medullare anterius; vm,
motor root of V nerve; vmd, mandibularis root of V; vtnx, maxillary root of V. (From
Kingsley's "Comparative Anatomy of Vertebrates," after G. Elliot Smith.)

THE NERVOUS SYSTEM

493

It is therefore, so far as its structure goes, an eye. The thalamic thicken-
ings of the lateral wall divide the third ventricle into a dorsal and ventral
cavity connected by a narrow slit-like passage. In snakes the optic lobes
become corpora quadrigemina by the division of each optic lobe into
anterior and posterior moieties.

The cerebellum is slightly larger in reptiles than in amphibia. The
gray matter of the spinal cord as seen in cross section assumes the form of a
capital H with dorsal and ventral columns, as in mammals.

In reptiles occipital vertebrae fuse with the cranium. Consequently,
two nerves, the spinal accessory which innervates shoulder and neck

Fig. 409. — Median section of brain of calf, a, aqueduct; ac, anterior commissure;
cc, corpus callosum;/, fornix; h, habenula; hy, hypophysis; i, infundibulum; im, inter-
mediate mass ("soft commissure"); mb, mammillary body; ob, olfactory bulb; oc, optic
chiasma; ol, optic lobes; p, pinealis; pc, posterior commissure; r, recessus suprapinealis;
s, septum pellucidum; III. IV, third and fourth ventricles. (From Kingsley's "Com-
parative Anatomy of Vertebrates," based on a figure by Butschli.)

muscles and the hjrpoglossus which supplies the tongue, which are spinal
in lower vertebrates, now become cranial. Each arises by a series of
segmentally arranged roots, and is therefore believed to be formed by
the union of a number of spinal nerves. Other changes in the nervous
system in reptiles are relatively unimportant.

Mammals. The brains of lower mammals differ little from those of
reptiles. Within the mammalian group, from monotremes to man,
there is an enormous enlargement of the cerebral hemispheres and of the
cerebellum. The expansion of the hemispheres affects chiefly the neopal-
lium, the beginnings of which were noted in reptiles. The archipallium
of reptiles, which serves chiefly as an olfactory center, becomes in mam-
mals the hippocampal lobe. As a result of the growth of the neopallium,
the hippocampus is crowded to the lower part of the brain.

494

COMPARATIVE ANATOMY

Increase in the size of mammalian brains is accompanied by com-
plication in form and structure. The cortex becomes cellular, and
consequently gray in color. The amount of cortical material increases
manyfold, so that if the human cortex were spread out fiat it would cover

/TELA CHORIOIDEA

xCEREBOJJJM /TELA CHORIOIDEA

/■MEDULLA OBLONGATAX

DIENCEPHALON

CHORIOID PLEXUS,
PARAPHYSISi
MONROE'S FORAMEN

TELENCEPHALON,
OLFACTORY

OLFACTORY/
LDBE

LAT VENTRICLE'

HIPPOCAMPUS'
HEMISPHERE'

'^ — ^^\-

VENTRICLE©
VtELA CHORIOIDEA

Fig. 410. — Brains of Petromyzon (A), Scymnus {B), and Rana (C) in dorsal aspect.

(Redrawn after Plate.)

a surface eighteen inches square. The number of neurons runs into the
billions, and five cell layers may be distinguished. The increase in the
mass of the cerebral hemispheres as we pass from lower to higher mammals
is the result, not of multiplication of layers of neurons in the cerebral
cortex, but of folding of the cortex.

THE NERVOUS SYSTEM

495

A notable development is that of the anterior pallial commissure,
which enlarges enormously to form the corpus callosum, interconnecting
the two hemispheres. The olfactory lobe degenerates and is covered
by the hemispheres. The corpora striata elongate caudally and rest
upon the thalami. The epiphysis forms a gland, the pineal. Cerebellum
and pons enlarge. The pons is a bridge of nerve fibers, present only in
birds and mammals, which extends around the brain-stem ventral to
the cerebellum, and which connects the two halves of the cerebellum.

Following this brief outline of the main advances in the nervous system
as represented in living chordates, it seems desirable at the risk of some
repetition to present separately and in greater detail the evolution of
the three major divisions of the nervous system: i. The evolution of the
Central Nervous System (Brain and Spinal Cord). 2. The Evolution
of the peripheral nerves (cranial and spinal). 3. The Evolution of the
Autonomic or Sympathetic System.

Evolution of the Brain

Comparison of vertebrate brains from cyclostomes to man reveals a
gradual and progressive change such as would be expected if the higher
forms have evolved from the lower. The cerebral hemispheres are the
least conservative regions. Although the hemispheres are enormously
enlarged in man, the differences between man and apes are quantitative
rather than qualitative. Even the speech center in the frontal lobes,
which is said to be peculiar to man, is but an enlargement of regions
already developed in apes. The brains of such fossil types as the Java
and Peking man are transitional between those of modern man and apes.

Nor is the gap between the brain size of mammals and reptiles formid-
able. In a dorsal view, all five divisions of the primitive brain are visible
alike in monotremes and alligators. The overgrowth of the hemispheres,
begun in reptiles, reaches its climax in man, whose domination in the
animal world may be ascribed to the enlargement of his conscious control
centers in the hemispheres.

The cortical enlargement in mammals, however, involves more than
an increase of gray matter. Correlated with the multiplication of cells
is an increase in the number of nervous interconnexions. Association
fiber tracts connect all parts of the enlarging brain so that all regions,
however remote from one another, are interconnected. The brain
produced by these evolutionary changes is an organized and integrated
whole, no part of which appears to function independently of the rest.

In primates a marked retrogression of the olfactory lobes accompanies
the enlargement of the hemispheres. The olfactory centers in the hip-
pocampus persist, but other regions of the cortex enlarge dispropor-

496

COMPARATIVE ANATOMY

EPIPHYSIS] .PARENCEPHALON /CHORIOID PLEXUS
PARIETAL I / HABENLLftR / POST COM /OPTIC

CEREBEUJJM

\ CHORIOlOeA

IPITAL NERVE
SPINAL

ACCESSORY^ LOBE

PIRIFORM!

ANT COM,'*-^
VENTRICLE ID'

'INFUNDIBULUM

Fig. All. — Brains of A, a cyclostome (Petromyzon); B, an Elasmobranch (Scymnus); C. a
teleost (Cyprinus); and D, an amphibian (Rana) as seen in left lateral aspect. The main
fiber tracts and nerves are shown as if projected upon the median plane. Homologous fiber
tracts in all figures are given identical numbers. Nervous Pathways of the Brain (fasciculi or
tracts). I, olfactory; 2, olfactohabenularis; 3. olfactohypothalamicus; 4, olfactocorticalis; 5,
olfactoepistrialis ; 6, olfactopeduncularis; 7. olfactoammonicus; 8, olfactomammillaris; 9, olfactq-
amygdalinus; 10, parolfactohabenularis; 11, optic; 12, opticotectalis; 13, isthmoopticus; 14, preopti-
cohabenularis; 15, striohypothalamicus; 16, striothalamicus; 17, striomesencephalicus; 18, fasciculus
retrofiexus; 19, pallial; 20, hippocampothalamicus; 21, hippocampomammillaris; 22, bulbothalami-
cus; 23, bulbocorticalis; 24, frontobulbaris; 25, tectobulbaris; 26, tectospinalis; 27, tectothalamoocci-
pitalis; 28, tectocerebellaris; 29, frontothalamicus; 30, frontoepistrialis; 31. corticohabenularis;
32, corticothalamicus; 33, corticomammillaris (fornix); 34. corticobulbaris; 35. lobopedunculo-
spinalis; 36, habenulointerpeduncularis; 37, habenulocorticalis; 38, mammillotegmentalis; 39,
mammillothalamicus; 40, rubrothalamicus; 41. rubrocerebellaris; 42, rubrospinalis; 43, thalamico-
spinalis; 44, tegmentocerebellaris; 45, trigemmocerebellaris; 46, quintocerebellaris; 47, septomesen-
cephalicus; 48, octavocerebellaris; 49, olivocerebellaris; so, deiterocerebellaris; 51, diencephalo-
cerebellaris; 52, mesencephalocerebellaris; S3, vestibulocerebellaris; S4. lateralicerebellans; SS.
facialicerebellaris; 56, spinocerebellaris; S7, cerebellotegmentalis; 58, cerebellodiencephahcus;
59, spinothalamicus; 60, spinohypothalamicus; 61, octavomesencephalicus; 62, mammdlopedun-
cularis; 63, frontobulbaris; 64, lobotectahs. (Redrawn after Plate.)

THE NERVOUS SYSTEM

497

CHORIOID PUEXLS
HIPPOCAMPUS,

PARIETAL ORGAN;

POST <XIM ylSTHvtlOjgc

/Sp^laX././^'"'^^^^

CHORIOID PLEXUS'
EPISTRIATUM
ECTOSTRIATUM
HVTERSTRIAT
PALLIUMx/

olfactory'

BULB

mesostriatumI

SPIM
f^DULLA CORD
OBLONGATA

HIPPOCAMPUS
FORNIX
CORPUS CALLOSUM
HABENUUW GANG. _
NEOPALL IUM-. /^(\ /
ANT COM,_^ *

PULVINAR NUC.

IPORA QUADRIGEMINA
■CEREBELLUM

DENTATE NUCLEUS

-p _^. — v.^/v.ni.t«rs riui... NUCLLUS OBLONGATA

(generali^edra?S?n &\l^eltlt^t ^ '^^'''' ^Lacerta) ; F, a. bird (anas ) ; and G. a mammal
projected upon the median plane HoXw^^ fiber, tracts are shown as if

Nervous Pathways of the Brain (fasc?r^Hn?t^^ntcf ^^^""^^"^ fll figures are given the same numbers,
hypothalamicus; 4. olfactocoXalfs .o f/rto^^ t ■ r' °'^^^,^°ry; 2. olfactohabenularis; 3, olfacto-
cus; 8, olfactom'amminarisrg olfactoamvcfalfn^' *T^^ ^' o"^^topeduncularis; 7. olfac'toammoni-
tecta s; 13, isthmooDticuq- t , °^^^ctoamygclalinus, 10, parolfactohabenulans; 11, optic; 12 ODtico-
17. striomisetephafiS -iS^S^u^uf rttrofl Vul'- '/o ^t"°h>-P°^halamicus ; 16. s^triothalaS^
hippocampomammillaris; 22 bKhakn^fcus 2. h,,lh P^"-'^';. ^O- hippocampothalamicus; 21.
bulbaris; 26, tectospinalis- 27 tertntht t^ '■ ^^' ,• '''2''°'"*''=^''^: 24. frontobulbaris; 25, tecto-
30. frontoepistrial°sr3i corticoSnufIri? ?f V''orf''';>?^^^ tectocerebellaris; 29, frontothalamicus;
34.. corticobulbaris; 35 lobopeduncTosn n.bV =°/t'^°thalamicus; 33. corticomammillaris (fornix)
cals; 38, mamminitegmentai^^ To maC^i Wh\l l^^benulomterpeduncularis; 37. habenulocorti-
lans; 42. rubrospinalif;43 thalamic^nh^^ I ,. ^"'"''"'V^^' ^brothalamicus; 41. rubrocerebel-
46. quintocerebellaris;' 47.' septomesenceDhalir,^- fr''"*?^"^^''^"^^?^ ^5, tngeminocerebellaris;
SO. deiterocerebellaris 51 dfencepra1Sce?ehe Wk- ^h °^tavocerebellans; 49, olivocerebellaris;
cerebellaris; S4, lateraHcerebeUrris 55 fadSrLb^ 53. vestibulo-

mentahs; 58. cerebellodiencephalicus ^O sntnoth^tJil^ ' ^^' sP'Pocerebellans; S7, cerebelloteg-
me.ncepha,icus; 62. mammifloperuncu^aris^'^I^'f^^^^^^^^^^

498

COMPARATIVE ANATOMY

tionately. Vision in primates is more important than smell, and brain
changes express relative functional values.

Cortex. Two parts of the primitive vertebrate brain participate
specially in the great enlargement of the cerebral hemispheres, the striate
body and the neopallium. These two regions are not clearly differentiated
in cyclostomes, but are distinguishable in fishes. Of the two, the pallium
changes more. The hemispheres of teleost fishes have a thin epithelial
pallium or mantle. Compared with the mantle' of teleosts, that of
elasmobranchs and dipnoi, which are more directly in the line of mam-

B C ' ' D

Fig. 413. — Horizontal diagrams of ichthyopsid brains. A, sturgeon; B, elasmo-
branch; C, teleost; D, amphibian. Primitive forebrain wall stippled; telencephalic
evaginations horizontally lined; thalamus vertical lines, c, common ventricle, /,
interventricular foramen; I, lateral ventricle; m, midbrain; o, olfactory bulb; t, terminal
lamina; II, optic nerve; 3, third ventricle. (From Kingsley's "Comparative Anatomy
of Vertebrates," after Herrick.)

malian ancestry, is relatively thick. Homologies with the pallium of
higher vertebrates are difficult on account of the lack of differentiation.
In the pallium of fishes the cellular gray matter is adjacent to the ventricle,
while the external layer is fibrillar. Even in the pallium of fishes, how-
ever, some cells migrate from the gray into the fibrillar zone. (Fig. 413)

The pallium of amphibia, taking Rana as a type, is thick, and is
differentiated into a median archicortex and a lateral paleocortex, both
associated with olfactory fibers. In reptiles the number of cell layers
in the pallium increases to three. The medio-dorsal region of each
hemisphere forms an archicortex or hippocampus. In the lateral pallium,
dorsal to the striate body, are possibly the beginnings of a neocortex.

A true many-layered neocortex appears in all mammals and enlarges
so much that the paleocortex is crowded into a ventral position and
the archicortex pushed dorsally toward the median plane. The number
of cell layers has increased until five are distinguished in most, if not all,
mammals. (Fig. 414)

The evolution of the cortex is accompanied by cellular changes.
In the pallium of lower vertebrates, cell bodies lie close together. The

THE NERVOUS SYSTEM

499

thickening of the cortex in mammals is correlated with separation of the
cells, which, however, retain connexion with one another by means of
elongated dendritic processes, the number of interconnexions with adjacent
neurons increasing with the multiplication of dendrites. In general, the
higher the animal, the longer and more numerous the dendrites, and

NEOCORTEX -

MEDIAL AREA

EPISTRIATuM

A LEPIDOSIREN.

A IXPIDOSIREN

HIPPOCAMPUS

EPISTRIATUM

B. RANA. VENTRAL C. LACERTA.

Fig. 414. — Cross sections of the left cerebral hemisphere of A-A ', a fish (Lepidosiren),
B, an amphibian (Rana), and C, a reptile (Lacerta), showing the increasing relative
importance of the epistriatum. The epistriatum arises dorsal to the striatum as a local
thickening of the ventro-lateral wall of the hemisphere. (Redrawn from Plate, after
Kuhlenbeck.)

consequently the greater the possible number of interrelationships
between cellular elements. A correlated increase appears in the number
of associational fiber tracts which connect the gyri or folds of the cortex.
In the milHons of years which elapsed during the Tertiary period,
there was a marked increase in the size of mammalian brains. This
increase affected all parts, and was accompanied by a corresponding
increase in the size of the cranium. The growth of the cerebral cortex

500

COMPARATIVE ANATOMY

was, however, out of all proportion to the enlargement of the rest of the
brain. This increase was made possible by the complex folding of the
outer layers of the brain and resulted in the formation of the gyri and
sulci which are such a characteristic feature of the surface of the human
brain. (Figs. 418, 419)

rRlM'NEOPALUI

"\ AREA

PVRIFOftMiS

CLAUSTOUM

T STRiATt AKteltf

Tu^Elt. OUF.

Fig. 415. — Diagrams of transverse sections of the right cerebral hemisphere. To
indicate how the primitive reptiHan arrangement (left; above) becomes transformed in
the evolution of the mammal, and especially the way in which the hypopallium of the
reptile {H. i, 2, 3, 4, 5) becomes converted into the upper part of the nucleus caudatus
{N.C), putamen (F) of the nucleus lentiformis, and claustrum. (From Wilder's
"History of the Human Body," after G. Elliot Smith; courtesy of Henry Holt and
Company.)

Hemispheres in Man. The human cerebral hemispheres are sepa-
rated in the median plane by a longitudinal fissure. Viewed from the
surface, four topographic but not functional divisions are recognized,
the frontal, parietal, occipital, and temporal lobes. Each is subdivided
by sulci or fissures into convolutions or gyri. The gyri of the frontal
lobe are the superior, middle, inferior, rectus, and orbital. Those of the

THE NERVOUS SYSTEM

501

parietal lobe are the anterior and posterior central, the superior and
inferior parietal, the submarginal, and the angular. The occipital
lobe has lingual, fusiform in part, and lateral gyri; the temporal lobe
superior, middle and inferior, hippocampal, and uncus. The g3n-us
cinguli on the median surface of the hemispheres extends through frontal
and parietal lobes. Covered by the parietal, frontal, and temporal
lobes on the lateral side of the hemisphere is an insula, the surface of which
is subdivided into short and long gyri. (Figs. 418, 419)

VISUAL

'OLFACTORY/ ^^/-^ — ^^ ^ >f ACOUSTIC -

ACOUSTIC

C. TARSIER. D. MARMOSET.

Fig. 416. — Diagrams of the brains of insectivores and of lower primates viewed
from the left side. The figures show the increasing dominance of the centers of vision
over those of smell. A. Brain of Jumping Shrew. B. Brain of Tree Shrew. C. Brain
of the primate Tarsier. D. Brain of the Marmoset. (Redrawn after G. Elliot Smith.)

Each lobe contains a restricted portion of a lateral ventricle. Con-
nexion with the third ventricle is effected by an interventricular foramen
or foramen of Monro.

Corpus Striatum. The second portion of the telencephalon which
undergoes striking changes and enlargement in the course of phylogenesis
is the corpus striatum, so-called because of its striped appearance in
sections. This striate body or basal ganglion arises as a local thickening
of the ventro-lateral wall of the telencephalon. Like other parts of this
division of the brain, the striate body is connected primarily with olfactory
fibers and is an olfactory reflex center. The organ is poorly developed
in cyclostomes; but is a well-marked swelling in elasmobranchs, with a
paleostriatum and an epistriatum. (Figs. 414, 415)

The amphibian brain, however, may be taken as the prototype of
the vertebrate. Each hemisphere of the amphibian brain is divided

502

COMPARATIVE ANATOMY

by a longitudinal groove into a dorsal and ventral half, and by the fissure-
like ventricle into median and lateral walls. The septum is the ventral
division of the median wall, and the primary hippocampus or archipallium
the dorsal. In the lateral wall, the striatum is ventral and the paleopal-

GOLGI CELLS

TYPE n _

ASSOCIATION / J

NEURONES

CELL OF
MARTI NOTTI

NEURITES

CENTR I PETAL
FIBER

COLLATERALS-^

A B C

Fig. 417. — The histological structure of the motor area of the cerebral cortex,
showing the different effects of three methods of staining (A, B, C). In A is shown
the effect of the use of the Golgi method; in B that of the Weigert method; in C that of
hematein and eosin. I to V indicate the five layers of neurons characteristic of mammals
in general. (Redrawn from Jordan and Ferguson, after Berkley.)

lium dorsal. Functionally, the septum is the intermediate station of
fibers which pass to or leave the hippocampus. In the amphibian as
in the elasmobranch striatum, a ventral and median paleostriatimi and
a dorsolateral epistriatimi are differentiated. It is not believed that the

THE NERVOUS SYSTEM

503

Striate body is directly connected with the olfactory organ. A large
fiber tract, the ventral peduncle, connects the striatum with the thalamus.

,FROt^AL POLE

SUPERIOR FRONTAL GYRUS

MIDDLE FRONTAL GYRUS

FRONTAL LOBE

-CENTRAL SULCUS

ANGULAR GYRUS

C-PARIETO-OCCIPITAL FISSURE

OCCIPITAL POut' \L0NGITUD1NAL FISSURE

Fig. 418. — The human brain in dorsal aspect. (Redrawn after Sobotta.)

In reptiles, the striatum becomes so greatly enlarged as nearly to
obliterate the ventricle of the hemisphere. Both paleostriatum and

INFERIOR FRONTAL GYRUS^

FRONTAL LOBE

FRONTAL POLE
ORBITAL GYRI

/ANTERIOR CENTRAL GYRUS
'CENTRAL SULCUS

•POSTERIOR CENTRAL GYRUS
lETAL LOBE

SUPRAMARGINAL GYRUS

- ANGULAR' GYROS

FISSURE OF SYLVIUS

SUPERIOR TEMPORAL GWUS

MIDDLE TEMPORAL GYRUS

TEMPORAL LOBE

PARIETO-

-OCCIPITAL

FISSURE

OCCIPITAL LOBE
'OCCIPITAL POLE

TEMPORAL POLE' "^TnFERIOR TEMPORAL GYRUS

Fig. 419. — Human brain in left lateral aspect. (Redrawn after Sobotta.)

epistriatum persist, but an archistriatum is added dorsal and lateral
to the paleostriatum. Anterior to the archistriatum a neostriatum

504

COMPARATIVE ANATOMY

arises from the basal wall of the hemisphere. The archistriatum is
presumably chiefly olfactory in function, but the neostriatum is a receptory
center for fibers from the thalamus.

These divisions of the striate body persist in mammals in which the
archistriatum becomes the amygdaloid nucleus, the paleostriatum is the

Insula

Olfactory tract
Hypophysis

Anterior
perforated
substance

Mamillary
bodies

Cerebral
peduncle

Semilunar

(Gasserian)

ganglion

Hypoglossal nerve (XH)

Pyramid

Decussation of pyramids ''

Fig. 420. — Ventral view of brain stem.

1 Optic nerve (11)

Optic tract
/

Tuber cinereum

^"*' ^^ Oculomoto'-

^,.-. nerve (HI)

Lateral geni-
culate body

~ Trochlear nerve

■'■^^"--^M.SLstica.toT
nerve

■-■jTrigeminus

Abducens
— (VI)
__Brachium of
pons

Facialis (VII)

~~^_Glossopalatine

nerve
'^N^ Cochlear and
^vestibular nerves
(Vni)
» Glossopharyngeal
nerve (IX)

'Vagus nerve (X)

^^N Accessory nerve (XI)
(spinal accessory)

"^v Cervical I

Cervical II

(From Morris, after Allen Thomson.)

globus pallidus, and the neostriatum divides into caudate nucleus and
putamen. With the elongation of the hemispheres, the striatum is
lengthened and rests upon the thalamus like a sac of meal on a packhorse.
The division of the striate body is a result of the growth of fiber tracts
such as the internal capsule which conveys ascending and descending

THE NERVOUS SYSTEM

505

fibers to and from the cortex. The corpus striatum may be considered
as a front porch or main entrance to the higher centers of the brain.
Sympathetic functions are ascribed to it.

Evolution of the Diencephalon. The evolutionary potential of the
diencephalon is low, and it changes little from fish to man. The founda-

TURGENIEFF,

7 2012 C

TAGUCHI
l«0 G

-

BYRON
I007G

-

THACKEBY

lesoo.

HACKEL
1575 G

AGASSIZ
1495 G

—

NORMAL

SCHUBERT
1420 G.

-

LIEBIG
I3ff2 G

WHITMAN
1202 G.

DOLUNGER
iiai G

AMATOLE
FRANCE.
rOI7G.

WEIGHT
IN GRAMS.

'200-1000.

ORANG OUTAN. 350 G

Fig. 421. — Variation in human brain size. The range is from 369 grams to 2400
grams, both of these extremes being the brains of idiots. The average male brain
weighs 1400 grams. (Redrawn after Kahn's "Das Leben Des Mensschen," W. Keller
& Co.)

tions of the human brain are laid in that of cyclostomes. Three regions
are distinguishable in the primitive diencephalon of cyclostomes, a dorsal
epithalamus, a lateral thalamus, and a ventral hypothalamus. The
epithelial roof remains thin, and there is no velum transversum to divide
the diencephalon from the telencephalon. Three dorsal outgrowths arise
from the epithalamus, an anterior paraphysis, a parietal outgrowth or

5o6

COMPARATIVE ANATOMY

eye, and a posterior epiphysis which in mammals becomes the pineal
gland. A pair of habenular ganglia, connected by an habenular com-

-MODERN MAN

\ — NEANDERTHAL

--V i--SINANTHROPUS

-PITHECANTHROPUS

■CHIMPANZEE

Fig. 422. — Outlines of the brain of modern man, fossil men, and ape showing relative
sizes. As the evolution theory would lead us to expect, the brains of fossil men are
intermediate in size between ape and modern man.

CAUDATE NUCLEUS''

FRONTO-

OCCIPITAL

FASCICLE

FRONTO-

TEMPORAL

FASCICLE / i

CUNCINATE) * ^

OCCIPITO- /

TEMPORAL FASCICLE FRONTO- /

TEMPORAL FASCICLE

(CINGULUM>

Fig. 423. — A diagram of the association fiber tracts of the cerebral hemisphere as
seen in lateral aspect. Through the agency of these tracts all parts of the cerebral
cortex are brought into relation with one another. The integration of the activities
of the various parts of the brain is, it may be assumed, effected in this way. (Redrawn
from W. Howell, after Starr.)

missure, develops at the anterior border of the epithalamus. The thick-
ened lateral walls of the cyclostome third ventricle form the thalami,

THE NERVOUS SYSTEHi

507

which become a pathway for fibers connecting anterior and posterior
parts of the brain. From the hypothalamus, the infundibulum is formed
as a ventral funnel-like outgrowth; and immediately behind this is a
second depression, the saccus vasculosus, limited to fishes and supposed
to be an hydrostatic pressure organ. Below the infundibulum, the
hypophysis proliferates vesicular masses which are believed to be homolo-
gous with the intermediate lobe of the pituitary gland.

In elasmobranchs geniculate nuclei connected with fibers of the optic
nerves develop in the lateral portion of the thalami; hence these are often
called optic thalami.

, PYRAMIDAL TRACT

CORTICO-PONTAL TRACT

CENTRAL GYRUS

'LEMNISCUS TRACT
THALAMUS
r' PARIETAL LOBE

VISUAL TRACT
-AUDITORY TRACT

CAUDATE V_

NUCLEUS

TEMPORAL LOBE— V--

OCCIPITAL U)BE

-CEREBELLUM

^MIDDLE PEDUNCLE
MNFERIOR PEDUNCLE

Fig. 424. — A diagram showing the relations of the "projection" fiber tracts of
the human brain. The cranial nerves are numbered with Roman numerals. (Redrawn
after W. H. Howell.)

The diencephalon of Amphibia differs Httle from that of elasmobranchs,
except that some amphibia have a rudimentary parietal eye which pierces
the skin or lies just beneath it. A chorioid plexus projects into the third
ventricle. The pituitary gland has the three lobes found in man.

In reptiles, the chorioid plexus of the diencephalon grows forward
into the lateral ventricles, also. The epiphysis is glandular. The thick-
ened thalami nearly meet in the median plane and divide the ventricle
into dorsal and ventral chambers.

The chorioid plexus of the diencephalon persists in mammals. The
epiphysis becomes the pineal gland, of problematic function, and there
is no parietal organ. The habenular ganglia are rudimentary. The
striate body of the telencephalon rests upon the thalamus, and the two

5o8

COMPARATIVE ANATOMY

form a pathway for fibers to and from the cerebral cortex. From the
posterior walls of the infundibulum are differentiated a tuber cinereum,
and also paired mammillary bodies, which have fiber connexion with
the olfactory organ. Fibers are also received from the cerebral cortex
and sent to the thalami. (Fig. 420)

OLFACTORY
LOBE

U=-J TELENCEPHALON
DIENCEPHALON
MESENCEPHALON
METENCEPHALON

MYELENCEPHALDN

Fig. 425. — A diagram of the brain of a four-months fetus as seen in median longi-
tudinal section. The figure shows the location of the more important brain commissures.
(Redrawn from Coming's "Human Embryology," after Burckhardt.)

Brain Commissures. Commissures are fiber tracts which cross the
median plane of the body and bring lateral halves of the nervous system
into relation with one another. Some of those in the brain persist through-
out the vertebrate series, and serve as important topographic landmarks
to determine homologous regions. (Fig. 425)

Commissures have not been demonstrated in Amphioxus. In the
terminal lamina of the telencephalon of cyclostomes are two, a more
ventral anterior and a more dorsal pallial. Both connect the olfactory
lobes with the hippocampi of the opposite side. Two habenular ganglia
in the roof of the diencephalon are connected by the habenular commis-
sure. Fibers from the hemispheres and from the hypothalamus also
are contained in this commissure. Another commissure in the roof of

THE NERVOUS SYSTEM

509

CEREBELUDM

ANAL3 /O-/
COCHLEA

•Sensory
Patla

tioioc
Paths

Motor Fibers from

Higher Arc3 to Alt

4 tegmental Levels

Long Sensory Fibers

to Higher Arcs
via. .Sensory Paths

Sensory

Nerve Endings

in Shin

Fig. 426.— Schematic diagram showing the nature of the activities carried out in various parts
of the central nervous system. Arc 7. Voluntary and Inhibitory Control. Choice ot response
based on memory of past experience. (Via pyramidal tract.) Arc 6. Automatic associated control
of complex muscular actions. (Via striato-rubro-spinal tract.) Arc 5- a. Auditory Reflexes, e.g..
Automatic response to sudden noise. 6, Visual reflexes, e.g., Automatic response to blinding Hash ot
light. (Both via tecto-spinal tract.) Arc 4. Synergic Control. Automatic coordinating control ot
muscular actions. (Via rubro-spinal tract.) Arc 3- Equilibratory Control. Automatic balancing
reactions. (Via vestibulo-spinal tract.) Arc 2. Intersegmental Reflex. Impulse carried by
association neurones to neighboring segments causing coordinated response of muscles m several
segments. Arc i. Intrasegmental Reflex. Response limited to segment stimulated, (from
Patten's "Embryology of the Pig.")

5IO

COMPARATIVE ANATOMY

the brain, the posterior commissure, marks the boundary between dien-
cephalon and mesencephalon. A dorsal commissure in the roof of the
midbrain connects the optic lobes.

In addition to the commissures, the fibers of two cranial nerves, to
optic and the trochlearis, cross the median plane to form chiasmas.
The optic chiasma is ventral and just anterior to the infundibulum. The
trochlearis chiasma, which occurs in all vertebrates except cyclostomes,
lies in the dorsal constriction which separates mes- and metencephalon.

CENTRAL SULCUS

LENTIFORM NUC.-;<^

^i^^^Ovj THALAMUS

^-OCCIPITAL POLE

_i^ DENTATE NUC.

NUC. VESTIBULARIS
y^NUC. GRACILIS AND CUNEATUS

STRIATED

MUSCLE ■

FIBER

SOMATIC MOTOR NEURONE

J r/*- 1 IN I « Pi

!j CORPUSCLE

SKIN
MUSCLE SPINDLE
■ASSOCIATION NEURONES

Fig. 427. — A diagram of the chief efferent tracts of the brain in relation to the spinal
cord. (Redrawn after Rasmussen s "Principal Nervous Pathways," The Macmillan
Co.)

The dorsal commissure is lacking in fishes. In amphibia and reptiles
a dorsal pallial or hippocampal commissure, connecting right and left
hippocampi, adds a third to those located in the terminal lamina of the
brain. In monotremes and marsupials both anterior and posterior
pallial or hippocampal commissures occur. The corpus callosum of
placental mammals is a new commissure — possibly derived from the
anterior pallial — which connects the two halves of the neo-cortex. Its
enlargement is correlated with that of the cerebral cortex.

Mesencephalon. The midbrain vesicle changes in phylogenesis
less than any other division.

The mesencephalon of cyclostomes is peculiar in having a chorioid
plexus. In cyclostomes, as in fishes, the optic lobes of the midbrain are
the reflex centers of the optic nerve, such reflexes being mediated by

THE NERVOUS SYSTEM

511

eflferent fibers which pass as a bundle from the tegmentum or lateral wall
of the midbrain to the floor of the medulla and cross to the other side by
way of the ansulate cormnissure. In the tegmentum are located also
a number of giant cells, the large neurites of which extend along the spinal
cord to tail muscles and thus form a physiological through-way. In the
floor of the midbrain lies the oculomotor nucleus and the interpeduncular
ganglion. In cyclostomes, as in higher vertebrates, the floor of the mid-
brain serves as a pathway for fiber tracts leading to and from the hemi-
spheres. One of the ascending fiber tracts is the spinothalamicus.

The thickening of the tegmentum in amphibia constricts the lumen
to a slender tube, the aqueduct, which connects third and fourth ventricles.

In reptiles some of the optic nerve fibers, instead of connecting with
the optic lobes as in lower vertebrates, become connected, indirectly by
way of synapses within the thalami, with the posterior part of the cerebral
hemispheres. This shift in brain connexions begins in reptiles and
reaches its climax in mammals. (Fig. 484) A new nucleus, the nucleus
ruber, so-called because of red granules associated with it, first appears in
reptiles. It is a way station or association center for fibers which carry
impulses from the cerebellum to the cerebral cortex. In snakes the optic
lobes or corpora bigemina become subdivided into corpora quadrigemina.

The midbrain of mammals differs little from that of reptiles. Corpora
quadrigemina are present in all mammals. The sensory centers of vision
shift to the occipital lobes of the hemispheres. The upper swellings or
superior colliculi of the corpora quadrigemina are optic reflex centers.
The inferior colliculi are auditory reflex centers. They correspond to the
tori of reptiles, which are covered by the optic lobes and are thought to
be derived from the roof of the brain posterior to the optic lobes of fishes.

Metencephalon. The metencephalon is hardly distinguishable in
cyclostomes, but is represented by a rudimentary cerebellum and a
transitional region connecting mesencephalon with the widely expanded
medulla. The cerebellum appears to be the anterior continuation of the
equilibratory centers of the medulla. Since the cerebellum receives
fibers from the medulla, thalami, and optic lobes, it may correlate olfac-
tory, visual, and static functions. A decussation of fibers in its floor
may possibly be interpreted as the beginnings of a pons. The floor,
however, serves chiefly as a pathway for ascending and descending fiber
tracts which connect brain and spinal cord.

In fishes there is great variation in the size of the cerebellum. In the
dipnoi it is small and covered by the optic lobes. In some elasmobranchs
it becomes conspicuous and even convoluted. In teleosts it invaginates
into the cavity of the midbrain to form a valvula. The peculiar Purkinje
cells, which are such conspicuous elements in the cortex of the cerebellum
of higher vertebrates, occur in fishes. The numerous sensory and motor

512

COMPARATIVE ANATOMY

connexions of the cerebellum of fishes indicate that it is an important
correlating and coordinating center. (Fig. 41 iC)

Amphibia, compared with fishes, have a degenerate cerebellum, this
reduction being correlated with a loss of physical activity.

The cerebellum of reptiles differs only slightly from that of amphibia.

In mammals, the cerebellum is much enlarged. Only its median
lobe or vermis corresponds to the cerebellum of lower vertebrates. The

Dorsal

Entrance median

zone. septum.

Median. . . ) Portion of
> dorsal
Lateral ) root. Dorsal root.

Zona
ferminalis

Zona

spongiosa.'

Substantia
gelatinosa.

Pormatio (ti
reticularis.

and

Ventrn- media
Groups of nerve cells

Central canal.

Ventral root. White Ventral Ventral funiculus,

commissure, median
fissure.

Pig. 428. — Cross section of the Itimbar enlargement of the human spinal cord. X8.
(From Bremer's "Text Book of Histology.")

two lateral lobes or hemispheres are mammalian novelties. Increase
of the cerebellar cortex by folding results in so complicated a structure
that the term arbor vitae applied to its white fiber tracts seems not inap-
propriate. Three paired bundles of fibers, the superior, middle and
inferior peduncles, connect it with adjacent parts of the brain. The
superior peduncles or brachia conjunctiva connect the cerebellum with
the midbrain; the middle peduncles or brachia pontis connect with the
pons: the inferior peduncles or restiform bodies connect with the medulla

THE NERVOUS SYSTEM • 513

oblongata. Where the restiform bodies join the medulla, its wall projects
laterally to form the cerebellar flocculi. (Fig. 424)

The cerebellar cortex consists of gray matter. Embedded in its
central core of white fibrous matter is a folded layer of gray matter, the
dentate nucleus. The Purkinje cells of the cerebellar cortex have synaptic
connexions with ascending sensory fiber tracts. From them impulses
pass to the cells of the dentate nucleus and thence by way of the brachia
conjunctiva into the tegmentum of the midbrain.

A conspicuous feature of the ventral wall of the metencephalon of
mammals is the pons (Varoli), which has already been described as a
bridge of nerve fibers connecting the two hemispheres of the cerebellum.
Each lateral half of the pons becomes a middle cerebral peduncle or
brachium pontis. The fibers of the pons come largely from the cerebral
hemispheres, which decussate or cross to the opposite side as they pass
to the cerebellar hemispheres. The posterior boundary of the pons marks
the anterior limit of the myelencephalon or medulla oblongata.

Medulla Oblongata. The non-nervous roof of the myelencephalon
or medulla oblongata of cyclostomes is modified as a chorioid plexus and
serves as a mechanism for nourishment of the brain. A similar plexus
appears in the medulla oblongata of all vertebrates. Cranial nerves V to
X have their roots and their motor centers in the lateral walls of the
medulla. A number of giant ganghon cells occur in the medulla of
cyclostomes, and, by their neurites or Mueller's fibers, connect with the
tail muscles. The main mass of the lateral and ventral wall of the medulla
consists of longitudinal fiber tracts of both ascending and descending
fibers, which connect brain and spinal cord.

In Elasmobranchs, Ganoids, and Dipnoi the lateral walls of the
anterior part of the medulla form conspicuous "ears," the restiform
bodies, in which are equiUbratory centers, which correlate the sensory
nerves of the semicircular canals with trunk muscles. With the develop-
ment of lateral line organs, their nervous centers in the lateral walls
of the medulla form the conspicuous paired longitudinal vagus lobes.
But in amphibia and reptiles these vagus lobes disappear, in correlation
with the loss of the lateral line organs.

The beginning of the medulla oblongata at the posterior boundary
of the pons is marked in the human brain by the origin of the abducens
nerve. Extending along the ventral surface of the medulla are the paired
P3n-amids, formed by bundles of fibers which connect the cerebral hemi-
spheres with the trunk. Lateral to the pyramids are two shorter swellings,
the oliva, which mark the position of the inferior olivary nucleus.

Sections of the olivary nucleus show that, like the dentate nucleus
of the cerebellum, it is a horse-shoe shaped and crenated mass of gray
matter. Fiber tracts connect the olivary nucleus with the cerebellum.

514

COMPARATIVE ANATOMY

In the human brain the medulla is largely covered by the cerebellum.
To the cranial nerves connected with the medulla in the anamnia are
added the spinal accessory (XI) and the hypoglossal (XII). (Fig. 437)

EVOLUTION OF THE SPINAL CORD

The spinal cord is a much more conservative portion of the central
nervous system than the brain. Consequently, although the contrast
between the so-called brain of amphioxus and that of man is so very great
that their homology may be doubted, the spinal cords of these chordate

AMPHIOXUS

;a,.^

CeNfTRAL CANAL

SOMATIC MOTOft

Pig. 429. — Cross sections of the spinal cord of various vertebrates: — A, Amphioxus;
B, Petromyzon; C, Stjualus; £>, Rana; £, Alligator and F, Homo. The magnification
is not to the same scale. The section of the cord of Amphioxus is enlarged four times
as much as that of Petromyzon. In these two forms the axons are non-meduUated.
The striking differences between the cord of Amphioxus and that of man are bridged
over by intermediate conditions in lower vertebrates.

extremes are recognizably similar. Both are tubular and both have a
central mass of gray matter and an external layer of fibrous tissue. The
relations of dorsal and ventral nerve roots are similar. The differences
are bridged over by intermediate conditions in the vertebrate series.

The fact that the brain of am.phioxus differs from its spinal cord, not in
greater size but chiefly in the expansion of its lumen, appears to support
the inference that spinal cord and brain were originally undifferentiated
from one another. The same columns of gray matter are recognizable
in both, although because of the absence of myelin nerve sheaths, amphi-
oxus lacks the color contrast between white and gray matter. This
conclusion is substantiated by evidence from ontogenesis, which shows

THE NERVOUS SYSTEM

515

that the distribution of nervous matter is essentially similar in cord and

brain.

The spinal cord of amphioxus is somewhat triangular in cross section,
with the apex of the triangle dorsal, the base resting upon the notochord.
The small amount of gray matter lies close to the sUtlike central canal.
The lateral walls are much thicker than the dorsal and ventral, since
fibrillar material which makes up most of the substance of the cord is
wholly lateral in position. Some cellular differentiation into sensory and
motor ganghon cells is visible in the gray matter, ependymal cells being
the most abundant. Sensory ganglion cells connect with the dorsal
nerves, and giant neurochord cells extend across the central canal. The
fibers of the giant nerve cells, after decussation, extend lengthwise of the
cord, some caudad and some cephalad. Since these do not form nerve

DORSAL .NUCLEUS

CCLARKES COLUMN)^ GROUND Bl*IDLD

FASC. GRACILIS

FASC CUNCATUS

DORSAL ROOT

SUBSTANTIA GELATINOSA

POSTERIOR COLUMN

LATERAL CELLS

LAT COLUMN CELLS -

MIDDLE CELLS -I

•'OOHSO- LATERAL-

INTERMEDIATE

^yENTRO- LATERAL

ANT \ VENTROMEDIAL

COLUMN i~^

CELLS VENTRAL ROOT/!

■sensory tract
lissauer's marginal zone
~fasc. cerebello-spinalis
fasc. cerebro -spinalis lat.

:. ANTEROLATERAL SUPEBFIC
-THALAMIC TRACT
HEUVEG'S BUNDLE

AlsTTERIOR PR0PRIU3
ANTERIOR MARGINAL BUNDLE

'^^^ DORSOMEDIAL' MARQINALia

Fig. 430. — A diagram 01 a cross section of the spinal cord, showing the fiber tracts
or fascicuU, and the arrangement of nuclei in the gray matter. (Redrawn after
Sobotta.)

fibers which leave the cord, it is supposed that the giant fibers give off
collaterals to the motor centers along the cord, and serve to correlate their
activities. (Fig. 429)

The spinal cord of cyclostomes is much flattened, with the cellular
matter distributed in a pair of lateral wings. Ventral to the central
lumen, a number of giant or Mueller's fibers extend lengthwise and carry
from the brain impulses thought to be chiefly static. Dorsal to the lumen,
are sensory ganghon cells Hke those of Amphioxus. The outer fibrous or
marginal layer of the cord is divided into longitudinal bundles of fibers,
the funiculi. Medullary sheaths are lacking, so that sympathetic fibers
can not be distinguished from others. Neurone relations in the cord
indicate that it is a reflex center and a pathway for intersegmental nervous
connexions. Increase in descending fiber tracts demonstrates the increas-
ing dominance of the brain. Polynuclear gland cells possibly of endocrinal
function, occur in the caudal region of the cord. (Fig. 429)

5l6 COMPARATIVE ANATOMY

In elasmobranchs nerve fibers are myelinated, so that white and gray
matter show the same contrast as in the cord of higher vertebrates.
Dorsal and ventral columns of gray matter are differentiated, but the
dorsal columns merge together in the median plane. Somatic motor
cells of the ventral column are very large, as in other fishes, amphibia,
and reptiles; and the dendrites extend into the dorsal column. Sensory
ganglion cells, except the embryonic and transient Rohon-Beard cells,
have migrated into the spinal ganglia. Dorsal, lateral, and ventral
funiculi have relations similar to those of higher vertebrates. The enlarge-
ment of the lateral walls of the cord results in the formation of a deep
ventral fissure. The suggestions of the formation of a dorsal septum are,
however, slight, (Fig. 429)

The spinal cord of amphibia resembles in fundamental characters
that of elasmobranchs. The dorsal columns of gray matter become more
distinctly paired, so that the gray matter assumes in cross section the
form of a capital H characteristic of all higher animals. In the gray
matter the nerve cells retain their central position surrounded by a
network of fibers and their synaptic connexions. A dorsal septum has
developed as a result of the increased thickness of the dorsal portion
of the lateral wall of the cord. In the cervical and lumbar regions the
diameter of the cord is considerably increased in correlation with the
enlargement of the appendages. (Fig. 429)

The spinal cord of reptiles differs in no essentials from that of mammals.
The increase in thickness of the marginal layer of longitudinal fibers
indicates an increased integration of the body. The fibers which ramify
through the gray matter are non-medullated, and their color is gray in
contrast with the white color of the medullated fibers of the marginal zone.

Within the gray matter of the cord, sensory and motor nerves of
reflex arcs usually effect their synaptic connexions by the intermediation
of association neurones located in the gray matter. The intermediolateral
column, which throughout the vertebrate series contains the ganglion
cells of the motor nerves which supply visceral muscles, becomes more
distinctly demarked than in lower vertebrates. The gray matter crosses
the median plane of the cord as the gray coniinissure which surrounds
the central canal.

A considerable increase in the amount of white matter in mammals
indicates a further dominance of the brain and a greater integration
of the body. The relative amount of white matter diminishes from the
medulla to the filum terminale in which the spinal cord ends. In the
region of the arms and legs, at the cervical and lumbar enlargements, both
gray and white matter increase in quantity. The division of the white
matter into funicuH, begun in cyclostomes, reaches its cUmax in man.
Dorsal, lateral, and ventral funicuU are separated from one another not

THE NERVOUS SYSTEM

517

only by dorsal and ventral nerve roots, but also by external dorso-lateral
and ventro-Iateral grooves or sulci, which extend lengthwise of the cord.

The medullary sheaths of the fibers which compose the funiculi
develop at different times in ontogenesis as the fibers come into functional
activity. By the study of the time of myelination of fibers and their
degeneration after they are cut, it has been learned that fibers of similar
origin and function occur in bundles or tracts. Each funiculus consists of
a number of such tracts, together with groups of tracts or fasciculi.
A tract or a fasciculus may contain either ascending sensory fibers or

PARIETAL LOBE-^>;

INTERNAL CAPSULE

CEREBRAL PEDUNCLE

TEMPORAL LOBE

CORTICAL MOTOR AREA

CAUDATE NUCLEUS

LENTICULAR NUCLEUS

VENTRAL CEREBRO-SPINAL TRACT

LATERAL CEREBRO-SPINAL TRACT.

't-VENTRAL ROOT OF SPINAL NERVE

SPINAL CORD

Fig. 431. — Diagram of the descending (pyramidal) conduction paths. (Redrawn after

Morris.)

descending motor fibers. A fasciculus may contain both kinds. Fibers
may have their origin or termination at any level and relatively few
extend the entire length of the cord. (Fig. 430)

Among the more important ascending fiber tracts are:
The fasciculi gracilis and cuneatus, which consist chiefly of sensory
fibers from the dorsal roots. Most of these sensory neurites connect
with neurones in the spinal cord, chiefly with association neurones of
the Golgi type II. Those which reach the brain have synaptic relations
with secondary neurones in the wall of the medulla oblongata.

The fasciculus spinocerebellaris dorsalis, formed by neurites of
ganglion cells in the nucleus dorsalis or Clark's column of the cord. These
fibers reach the cerebellum by way of the restiform body.

5i8

COMPARATIVE ANATOMY

The ventral spinocerebellar tract, which consists of fibers from cells
of the dorsal column. These fibers reach the cerebellum by way of the
pons.

The lateral and ventral spinothalamic tracts which include fibers
from the dorsal column which cross to the opposite side and finally reach
the thalamus.

Bocsat Root

Ventral Rod

Ramus
dorjetis

Ramus
comm.

amus
ventralis

Fig. 432. — Diagram of the nerve components of a spinal nerve. Somatic motor
fibers are indicated by continuous lines; visceral motor in long broken lines; somatic
sensory in short broken lines; visceral sensory by fine dotted lines. (From B. Patten,
after Froriep.)

The more important descending fiber tracts are:

The lateral corticospinal or crossed pyramidal tract, which contains
fibers from pyramidal cells in the cerebral cortex of the opposite side.
The telodendria end in the gray matter of the cord in relation to the
somatic motor neurons, and form part of the mechanism of voluntary
control.

THE NERVOUS SYSTEM

519

520

COMPARATIVE ANATOMY

The ventral corticospinal or direct pyramidal tract, formed of fibers
from the cerebral cortex of the same side which have synaptic connexions
with somatic motor cells. The size of this fasciculus is proportionate
to the use of the forelimbs and to the degree of intelligence.

The rubrospinal tract, carries fibers from the red nucleus of the mid-
brain which cross the median plane and connect with somatic motor
cells in the ventral column of the spinal cord.

Neuron Relations in the Cord. The spinal cord is both the center
for reflexes and the pathway for impulses towards and away from the
brain.

In the simplest possible reflex action where two neurons only are
involved, the synaptic connexions between the two He in the gray matter
of the cord. The cell body of the afferent neurone is in the sensory
ganghon of the afferent nerve. That of the effector neuron is in the
ventral column of the spinal cord, and its neurite extends by way of the
ventral root to a muscle fiber or a gland.

Usually, however, more than two neurons are chained together in a
reflex act, for there may be one or more association neurons located
in the gray matter of the cord, which carry the impulse from the receptor
neuron to the effector neuron. But it should not be understood that a
somatic motor nerve cell located in the ventral column has synaptic
connexions with the telodendria of only a single neurite. On the contrary,
many neurites may have synaptic relations with the dendrites of each
somatic motor neuron. The motor neuron is simply ''the final common

path."

Within the gray matter of the spinal cord, the central connexions
of the neurites of an afferent neuron may be of various kinds. The
telodendria may connect directly with the dendrites of a somatic motor
cell. This is the simplest relation. They may connect with a neuron
in Clark's column near the median line at the base of the dorsal column,
and the nervous impulse be carried in the dorsal cerebellar tract; or they
may pass to a commissural neuron and the impulse be carried to the
opposite side of the spinal cord. There are also connexions with sym-
pathetic neurons.

The neurites of receptor neurons, having entered the spinal cord,
immediately dichotomize to give off long ascending and short descending
branches and thus add to the fibers of the fasciculus cuneatus. As these
fibers pass towards the brain, they are displaced inwards by the fibers
which are added from higher levels. The result of this process is that,
in the neck region, the neurites which enter the cord in the lower trunk
region come to lie in a median fasciculus, the fasciculus gracilis. Most of
the descending short fibers end in the gray matter of the cord. From
both ascending and descending neurites, fine collateral branches pass

THE NERVOUS SYSTEM

521

into the dorsal column of gray matter, and come into synaptic connexions
with dendrites of the same or of the opposite side.

The fibers which enter the spinal cord by the dorsal root are somatic
afferent or visceral afferent, depending upon their peripheral connexions.

Fig. 434. — Bifurcation of the dorsal root fibers within the spinal cord into ascending
and descending branches, which in turn give off collaterals; the termination of some of
these collaterals in synaptic relation to cells of the posterior gray column. (From
Ranson's "The Anatomy of the Nervous System," Courtesy of W. B. Saunders
Company.)

The visceral motor fibers, which appear in the dorsal roots of lower
vertebrates, are not found in the dorsal spinal nerves of mammals.

The neurites of the ventral roots are purely efferent. Of these there
are two kinds, somatic motor and visceral motor. The somatic innervate
skeletal muscles derived from the mesodermal somites, the visceral are
connected through the sympathetic ganglia with visceral involuntary

522

COMPARATIVE ANATOMY

muscles or with glands. The ganglion cells of both types lie within the
gray matter of the spinal cord; but those of the somatic motor nerves
lie in the ventral column, while the visceral motor fibers come from
ganglion cells in the intermediolateral column.

EVOLUTION OF THE PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system consists of two main divisions, the
cerebrospinal nerves and the autonomic system. The cerebrospinal
nerves are also of two kinds; cranial, which have their foramina of exit
from the cranium, and spinal, which are connected with the cord and leave
the vertebral column by way of intervertebral foramina.

MUCOUS EPITHELIUM

SOMATIC MUSCLE

SYMPATHETIC GANGLION
VISCERAL SMOOTH MUSCLE

Fig. 435- — Neuron relations of motor and sensory nerves which have their centers in the
medulla oblongata. (Redrawn from Patten, after Ranson.)

Nerve Components. In addition to the four kinds of nerve fibers
found in spinal nerves and distinguished by differences in function and
distribution, some of the cranial nerves may have special fibers which
supply sense organs or specialized muscles. Four special groups of nerves
must therefore be added to the four general kinds which have already been
mentioned, making a total of eight kinds of nerve components. (Fig. 435)

I. General somatic efferent fibers are relatively large neurites, formed
as outgrowths from nerve cells located in the ventral column of the spinal
cord and brain. Somatic motor cells are multipolar and contain numerous
Nissl bodies. The neurite arises from a hillock or implantation core
free from Nissl granules. As the neurite emerges from the cord or brain
it is surrounded bv mvelin and neurilemma sheaths. Both sheaths are

THE NERVOUS SYSTEM 523

interrupted at intervals by nodes of Ranvier, but the neurite continues
until it terminates on the surface of one or more muscle fibers in the form
of a motor end-plate. At this end-plate the neurolemma becomes con-
tinuous with the sarcolemma sheath of the muscle fiber. The granular
protoplasm of the end-plate contains many nuclei as well as pad-like
terminations, the telodendria of the neurite. General somatic efferent
fibers supply most of the skeletal muscles of the body. They are absent
from cranial nerves with the possible exception of the spinal accessory
of amniotes. But this nerve is a spinal nerve in the anamnia.

2. Special somatic efferent fibers innervate specialized somatic
muscles of the head. These muscles are the six eye muscles and some
of the muscles of the tongue. The eye muscles are innervated by the
oculomotor, trochlearis, and the abducens nerves. The hypoglossus
nerve supplies the tongue muscles. The hypoglossus is, however, a
spinal nerve in the anamnia.

3. General somatic afferent nerves supply the skin and skeletal
muscles. They are the nerves of general exteroceptive sensibility and of
deep proprioceptive sensibility. With a few exceptions already noted
the cell-bodies of somatic sensory nerves lie, not in the spinal cord or
brain, but in the ganglia of the dorsal roots. Two kinds of ganglion
cells may be distinguished, large and small, both usually of the unipolar
(disguised bipolar) type. Their fibers are mostly myelinated, and the neu-
rilemma sheaths cover both the cell-bodies and the neurites. (Fig. 390)

As the neurite leaves a ganglion cell, it is twisted into a glomerulus.
It soon divides into two branches, one of which passes into the cord,
and the other to the skin or to a muscle fiber, where it terminates as a
muscle spindle. The ganglion cells are frequently surrounded by a pericel-
lular network of nerve fibers which are possibly of sympathetic origin.

Of the cranial nerves, the trigeminal, glossopharyngeal and vagus
contain general somatic afferent fibers from the skin. The dorsal roots
of spinal nerves consist largely of such general somatic fibers. The
oculomotor, trochlearis, abducens and trigeminal nerves carry proprio-
ceptive impulses from the muscles.

4. Special somatic afferent components are limited to some of the
cranial nerves and are not found in spinal nerves at all. The facial,
acoustic, vagus and (?) glossopharyngeus nerves contain them. These
are nerves of special sense organs, the ear and lateral line organs. The
fibers of the cochlear branch of the acoustic nerve carry exteroceptive
impulses while those of the vestibular branch are proprioceptive. Latera-
lis fibers are contained in the facial and vagus nerves, and, possibly in
some fishes, in the glossopharyngeus nerve. If the optic nerve were not a
specialized fiber tract of the brain, it might be classed as a special somatic
afferent nerve.

524 COMPARATIVE ANATOMY

5. General visceral efferent nerves supply smooth muscles of the
viscera, heart muscle and digestive glands. They are relatively small
medullated fibers which connect by way of the white rami communicantes
with the sympathetic. Their ganglion cells, located in the lateral motor
column of the brain and cord, are small. Of the cranial nerves the oculo-
motor, facial, glossopharyngeal and vagus nerves contain these fibers.
General visceral efferent nerves form the preganglionic fibers of the
sympathetic system. Among peripheral nerves they are peculiar in
relaying their impulses to a second set of neurones, the cells of which are
located in sympathetic ganglia. In amniotes they leave the spinal cord
by way of ventral roots only, but in Anamnia some of them are contained
in the dorsal roots. (Fig. 442)

6. Special visceral efferent components are limited to the head where
they innervate special striped muscles which develop from the mesoderm
of the visceral arches. Their motor centers lie in the ventro-lateral
motor column of the medulla between the somatic motor column and
the general visceral efferent column. Unlike the general visceral efferent
fibers they have no connexion with sympathetic ganglia. They are
among the constitutents of the trigeminal, facial, glossopharyngeal,
vagus and accessory nerves.

7. General visceral afferent nerves carry impulses from the mucous
lining of the alimentary canal, larynx and trachea. They use the sympa-
thetic nerves as pathways but do not have nervous connexion with the
cells of the sympathetic ganglia. Some of the fibers reach the medulla
oblongata by way of the facial, glossopharyngeal and vagus nerves.
Their sensory centers lie in the extended nucleus of the tractus solitarius
within the medulla.

8. Special visceral afferent fibers are found only in cranial nerves in
connexion with the senses of smell and taste. The olfactory nerve, and
with it possibly the terminal nerve, has been regarded by some as a highly
specialized visceral afferent nerve. Gustatory fibers pass to the medulla
by way of the facial, glossopharyngeal, and vagus nerves. Like general
visceral afferent components their fibers are connected with the nucleus
of the tractus solitarius. The centers in the cerebral hemispheres associ-
ated with sensations of smell and taste are at present uncertain, but
are usually ascribed to the gyrus cinguli.

EVOLUTION OF THE CRAOTAL NERVES
The two anterior pairs of nerves of amphioxus are purely sensory,
while all posterior dorsal nerves are mixed. It is customary to compare
the first pair of nerves of amphioxus with the olfactory nerves, or with the
"terminal" nerves of vertebrates. Since amphioxus has no eyes, no
nerves are present comparable with the optic nerves of vertebrates, and

THE NERVOUS SYSTEM

525

the second pair of sensory nerves of amphioxus may be the homologues
of the ophthalmic nerves of craniates. The four mixed nerves posterior
to the ophthalmic have been compared respectively with the trigeminal,
facial, glossopharyngeal, and vagus. It is probable, however, that at
least three segmental nerves are represented in the vagus nerve of verte-
brates. Of the somatic motor nerves of vertebrates, the oculomotor is
possibly represented in the first ventral root of amphioxus.

CIRRI

AMPHIOXUS.

VELUM

Fig. 436. — A diagram showing the distribution of the anterior nerves of amphioxus.
The two anteriormost nerves of amphioxus lie anterior to the myotomes and are purely
sensory. The remaining nerves are either mixed like those shown in the figure or are
purely motor. The former alternate, the diagram shows, with the myotomes, while the
latter are metameric in position. (Redrawn after Hatschek slightly modified.)

The number of cranial nerves varies in craniotes. Fishes and amphibia
have ten pairs; reptiles, birds, and mammals have twelve. These,
beginning with the anteriormost, are:

I. Olfactory. Special visceral afferent.

II. Optic. Sensory. A specialized fiber tract of the brain.

m. Oculomotor. Somatic efferent, with some visceral efferent fibers.

IV. Trochlearis. Somatic efferent.

V. Trigeminus. General somatic afferent and visceral efferent.

VI. Abducens. Somatic efferent.

VII. Facialis. Mixed visceral afferent and efferent. Special somatic afferent.
Vm. Acoustic. Special somatic atTerent.

IX. Glossopharyngeal. General and special visceral mixed. (?) Somatic sensory.

X. Vagus. General and special visceral mixed. Special somatic afferent.

XI. Accessory. (?) Somatic efferent, and general and special visceral efferent.
Xn. Hypoglossal. Somatic efferent.

526

COMPARATIVE ANATOMY

Striking differences in function and distribution distinguish these
twelve and set them apart from spinal nerves. These differences may be
briefly summarized:

I. Olfactorius. The olfactory nerve is peculiar in the origin of its
neurites, which, like the neurites of primitive neurosensory cells, develop
as processes of sense cells located in the olfactory epithelium. Their
terminal arborizations effect synapses with neurons located in the

ANTERIOR

PERFORATED

SUBSTANCE

OPTIC CHIASMA

TUBER CINEREUM

MAMMILLARY BODIES

LATERAL GENICULATE
BODY

/»_--MASri CATOR NERVE
^ -V

CEREBELLUM

'i MEDULLA
f OBLONGATA

DECUSSATION OF PYRAMIDS

SPINAL CORD

Fig. 437.

-Human brain stem, showing nerve connexions,
after Allen Thomson.)

(Redrawn from Morris,

olfactory lobes of the brain. By means of a neuron chain, connexions
are established with the cells in the hippocampus. That the "terminal
nerve" of Pincus is an independent segmental nerve has not been demon-
strated. That it is a ganglionic division of the olfactory nerve is sup-
ported by the evidence of its structure, development and relations.

n. Opticus. The so-called optic nerve is not a true segmental nerve
comparable with other cranial nerves. Its mode of development proves
that it is a specialized fiber tract of the forebrain, since the neurosensory
cells which form it are originally a part of the brain wall, and their terminal
arborizations lie not in the skin but in the retina which develops from the

THE NERVOUS SYSTEM 527

brain wall. Like the trochlear, the optic nerve has a chiasma which lies
below the brain immediately anterior to the infundibulum. Half its
fibers enter the chiasma and cross to the opposite side of the brain, the
other half effect central connexions with the thalamus and optic lobe
of the side from which they enter. In lower vertebrates, the central
connexions of the optic are with the optic lobes of the mesen-
cephalon. But in mammals they are shifted mostly to the occipital
lobes of the hemispheres, which they reach indirectly by way of the lateral
geniculate body of the thalamus.

III. Oculomotorius. The motor nucleus of the oculomotor lies in the
somatic motor column at the base of the midbrain, median to the cerebral
peduncle. It contains somatic efferent and visceral efferent fibers. It is
an eye muscle nerve innervating four eye muscles; it also innervates the
ciliary muscle within the eye by way of the ciliary ganglion, a para-
sympathetic ganglion attached to the oculomotor and ophthalmic nerves.
Like the other eye muscle nerves the oculomotor nerve remains relatively
unchanged from fishes to man.

IV. Trochlearis. The trochlear, another eye muscle nerve, has its
nucleus in the base of the metencephalon, and its root of origin from the
dorsal constriction which separates the optic lobes from the meten-
cephalon. In cyclostomes its root is said to be connected with the lateral
wall of the medulla between those of the ophthalmicus and trigeminal
nerves, the roots of which are separate in cyclostomes. In elasmobranchs
its fibers form a chiasma over the isthmus behind the midbrain. In
mammals and man its chiasma lies posterior to the inferior colliculus.
Although in having a chiasma, the trochlearis is unique among motor
nerves, we must class it as a somatic motor nerve, since its fibers are
connected with cells in the somatic motor column of the brain posterior to
those of the oculomotor. Moreover, the muscle which it innervates
develops from a mesodermal somite (somite 2).

V. Trigeminalis. In most vertebrates the trigeminal arises from the
lateral wall of the oblongata by two roots, the larger sensory, the smaller
motor. Its sensory ganglion, the Gasserian or semilunar, sends an
ophthalmic branch to the skin of the forehead, a maxillary branch to the
upper lip and the teeth of the upper jaw, and a mixed mandibular branch
to the teeth of the lower jaw and to the tongue. In man and mammals all
three branches have somatic afferent fibers. (Fig. 438)

The motor root is that of the masticator nerve, which supplies special
visceral efferent fibers to the chewing muscles. In man, the nucleus of
the masticator nerve lies in the visceral motor column in the wall of the
oblongata, median to the pons. General visceral or proprioceptive fibers
enter the semilunar ganglion from the chewing muscles, and pass to the
mesencephalic nucleus in the roof of the midbrain. Such sensory fibers

528 COMPARATIVE ANATOMY

arising from cells within the brain are unique in amniotes, although such
relations are not uncommon in invertebrates and amphioxus. (Fig. 516)
In the anamnia the ophthalmic branch of the trigeminus divides into
a superficial branch, which innervates the skin of the snout but not the
lateral line sense organs of that region, and a deep or profundus branch

Fig. 438. — The twelve cranial nerves shown as if projected upon a median section
of the head. I. Olfactory lobe. II. Optic. III. Oculomotor, IV. Trochlearis. V.
Trigeminus. VI. Abducens. VII. Facialis. VIII. Acusticus. IX. Glossopharyngeus.
X. Vagus. XI. Accessorius. XII. Hypoglossus.

which passes through the orbit to the snout. Both contain general
somatic sensory fibers. As stated above, in cyclostomes the deep ophthal-
mic is an independent nerve with a separate ganglion and root and is
therefore regarded by many morphologists as a "segmental" nerve. In
most anamnia, however, the root of the profundus nerve joins that of the
rest of the trigeminus and persists in amniotes as a branch of that nerve.

THE NERVOUS SYSTEM 529

VI. Abducens. The roots of the abducens nerve in man emerge from
the ventral side of the medulla near the posterior boundary of the pons.
In lower vertebrates it is a meta-otic nerve. Its somatic efferent fibers
come from a nucleus in the somatic motor column near the midventral
line of the medulla, and innervate the external rectus eye muscle. The
eye muscle nerves (III, IV, VI) are an especially conservative group,
which, like the muscles they innervate, differ little from fishes to man.

VII. Facialis. The facial is a mixed nerve with two roots. Its major
root is purely motor, the fibers, which belong to the special visceral motor
group, innervating the superficial facial and scalp muscles, the posterior
belly of the digastric muscle, and the stylohyoid. Its motor nucleus lies
lateral and posterior to that of the abducens, the fibers of the facial looping
around those of the abducens to form the genu of the facial.

The minor root of the facial forms the glossopalatine or intermediate
nerve of the facial. Most of it enters the chorda tympani nerve, which
consists of special visceral sensory fibers from the anterior taste buds of the
tongue. Some general visceral efferent fibers innervate the submaxillary
and sublingual salivary glands. The sensory fibers of this nerve come
from cells in the geniculate ganglion. With the disappearance of lateral
line organs in land amphibians, the seventh nerve loses its special somatic
sensory (lateralis) components.

Vm. Acusticus. The auditory nerve is relatively short, with two
branches, cochlear and vestibular, which have their root or origin near
that of the facialis. The fibers which develop from the spiral and ves-
tibular ganglia belong to the special somatic sensory group. The cochlear
is the nerve of hearing; the vestibular, of equilibration. The dendrites
of both receive their nervous impulses from receptor hair-cells. Their
central connexions are with the cochlearis and vestibularis nuclei in the
oblongata. The centers of hearing, however, are located in the temporal
lobes. Ganglion cells of both nerves are primitive and unique among
amniote afferent neurones in being bipolar.

IX. Glossopharjmgeus. The glossopharyngeal is a mixed nerve which
contains special and general visceral fibers, both sensory and motor. Its
roots are posterior to those of the acusticus and in line with those of the
vagus. The sensory fibers come from the petrosal ganglion and are
distributed to the posterior part of the tongue and to the pharynx. The
central nucleus is that of the tractus solitarius which lies between the
cochlear and vestibular nuclei. The secondary cortical connexions and
the centers of taste are unknown. In some fishes the glossopharyngeus
receives special somatic afferent fibers from the supratemporal line of
sense organs.

The motor fibers of the glossopharyngeus come from the inferior
salivatory and ambiguus nuclei in the medulla. The general visceral

530 COMPARATIVE ANATOMY

fibers connect with the otic ganglion and, by postganglionic fibers, with
the parotid gland. The special visceral efferent fibers innervate the
stylopharyngeus muscle. In cyclostomes, fishes, and urodeles the
glossopharyngeus forks over the first gill slit to form post- and pre-
trematic branches. With the disappearance of gills in land amphibia,
the glossopharyngeal nerve does not lose its post- and pre-trematic
branches. The latter, which is the larger, persists as the pharyngeal
nerve.

X. Vagus. The vagus or pneumogastric is a mked nerve, which has
the widest and most diverse distribution of any of the cranial nerves.
It arises by an extended series of fine roots from the medulla posterior to
the roots of the glossopharyngeus.

The sensory components arise in the ganglion jugulare and ganglion
nodosum and are distributed to the larynx, trachea, lungs, esophagus,
stomach, intestine, and gall bladder. Their central connexions are with
the ala cinerea in the floor of the medulla. The vagus in cyclostomes,
fishes and urodeles receives special somatic afferent fibers from the lateral
line organs of the trunk and tail.

Some of the motor fibers come from the nucleus ambiguus and some
from the dorsal motor nucleus of the vagus near the nucleus ambiguus.
They innervate the muscles of the larynx, esophagus, stomach, small
intestine, and part of the large intestine. Inhibitory fibers pass to the
heart and secretory fibers to the gastric glands and pancreas.

The lateral line components of the vagus are lost in the land amphibia
and appear in none of the higher vertebrates.

XI. Accessorius. The spinal accessory, purely motor, combines
characteristics of cranial and spinal motor nerves. Some of its fibers come
from the vagus and ambiguus nuclei and join vagus fibers in the innerva-
tion of thoracic and abdominal viscera. Another group of fibers of
similar medullary origin innervate striated muscles of the pharynx and
larynx. A third set of somatic motor fibers innervate sternocleidomastoid
and trapezius muscles. The accessory nerve of amniotes is in part a
somatic motor spinal nerve which has been added to the ten cranial nerves
of anamnia as a result of the union of cervical vertebrae with the occipital
region of the skull, and in part visceral motor like the vagus with which
it is associated.

XII. Hypoglossus. The hypoglossal is a somatic motor nerve which
supplies the tongue muscles. Its roots of origin lie posterior to those of
the abducens between the pyramid and the olive. The nucleus is medial
and ventral to that of the vagus, and in series with the spinal division
of the accessory. Like the accessory, the hypoglossus is a spinal nerve
which has become cranial as the result of the extension of the occipital
region of the skull.

THE NERVOUS SYSTEM

531

OLFACTORY BULB-

OCCIPITAL LOBE-

SYMPATHETIC GANGLIA'

RAMI COMMUNICANTE

FROrvTTAL LDBE

SYMPATKETIC TRUNK

SPLANCHNIC NERVE-

-TEMPORAL LOBE

CEREBELLUM
1ST CERVICAL NERVE

^

J BRACHIAL PLEXUS

1ST THORACIC NERVE

SPINAL GANGLION

1ST LUMBAR NERVE

;lumbar plexus
sacral nerve

FILUM TERMINALE'' ' ' \COCCYGEAL NERVE

Fig. 439. — The brain and spinal cord of man in ventral aspect shown in relation to
nerve roots and the chief autonomic ganglia. (Redrawn from Morris, after Allen
Thomson.)

532 COMPARATIVE ANATOMY

Evolution of the Spinal Nerves. In man thirty-one pairs of nerves are
connected with the spinal cord. Of these, eight are cervical, twelve
thoracic, five lumbar, five sacral, and one coccygeal. Unlike cranial
nerves, each spinal nerve is connected with the central nervous system
by two roots, one dorsal, sensory and ganglionated; the other ventral,
motor and non-ganglionated. The fibers of the dorsal root are outgrowths
of the nerve cells which form the ganglion; the nerve cells of the ventral
root lie in the ventral horn of the spinal cord. The two roots unite near
the ganglion to form a mixed nerve which has its exit from the vertebral
canal by way of intervertebral foramina. As a spinal nerve passes
towards the periphery, it divides into dorsal and ventral branches, and a
visceral branch, the ramus communicans, which connects with a sympa-
thetic ganglion. (Fig. 439)

The similarity of vertebrate spinal nerves, in their metameric repeti-
tion, to the nerves of amphioxus has been one of the reasons for assuming
amphioxus to be the prototype of the vertebrates. Except the first two
pairs, which are in some respects peculiar, all the segmental nerves of
amphioxus are similar, each having a dorsal mixed root and a ventral
motor root, which do not unite. Another peculiarity of amphioxus is the
absence of a ganglion on the dorsal root. The cell bodies from which
sensory neurites arise lie either in the dorsal wall of the cord or scattered
along the course of the nerve. It is, however, asserted that some of the
cells of the dorsal nerves are neuroepithelial cells located in the skin.
The general somatic afferent fibers of the dorsal nerves pass directly
to the skin by way of the myocommata. The general visceral efferent
fibers leave the dorsal nerve by way of intestinal branches which supply
the muscles of the intestine.

The ventral nerves emerge from the cord by numerous fibrillar roots,
and connect directly with the myotomes opposite. The dorsal nerves are,
therefore, intermetameric, and the ventral metameric.

In myxine the dorsal and ventral nerves unite peripherally as they
do in all higher vertebrates; and only in petromyzon among cyclostomes
are all dorsal and ventral nerves separate. But in the head region of all
vertebrates, the somatic motor nerves, oculomotor, trochlear, abducens,
and hypoglossus do not unite with sensory nerves. In this regard, as in
the mixed character of such nerves as V, VII, IX and X, the cranial nerves
of higher vertebrates retain a primitive characteristic which is lost by
the spinal nerves.

Plexuses. Although the spinal nerves of all chordates show a
segmental one-to-one correspondence with myotomes and vertebrae, this
regular metamerism is modified in the region of the paired appendages by
the union of a number of ventral rami into an interlaced network or
plexus. Four such plexuses are recognized in vertebrates, cervical,

THE NERVOUS SYSTEM

533

brachial, lumbar, and sacral. Frequently the four unite in pairs to form
cervico-thoracic and lumbo-sacral plexuses. In cyclostomes and fishes
the group of postoccipital nerves forms a plexus which innervates the
hypobranchial muscles. In many forms this cervical plexus unites with
the brachial plexus. In amniotes the two plexuses become separate and
the plexus of hypobranchial nerves forms the hypoglossus (XII) nerve.

Fig. 440. — A, diagram of collector nerve; B, of a nerve plexus. (After Braus.)
C, brachial plexus of Salamandra maculata. (From Kingsley's " Comparative Anatomy
of Vertebrates," after Ftirbringer.)

In different species, and even in different individuals of the same
species, there is considerable variation in the number of nerves in a plexus
and in their interconnexions, so that exact homologies are impossible.
The largest number of nerves in the cervico-thoracic plexus, twenty-five,
occurs in skates. In higher vertebrates the number is usually reduced to
four or five, and in man, the cervical plexus includes only the first four
spinal nerves. The brachial plexus in man usually involves the lower
four cervical and the first thoracic nerves. The first four lumbar nerves
of man unite in the lumbar plexus; the fifth lumbar and the five sacral
nerves form the sacral plexus. But generalization is difficult because of
the large amount of variation.

Such plexuses are presumably adaptive, since by the interlacing of
fibers any muscle may be innervated by more than a single nerve and a
summation of stimulation effected. This does not imply, however, that

534

COMPARATIVE ANATOMY

an individual muscle fiber is innervated by more than one nerve. Evi-
dence of such plural innervation of a single muscle fiber is lacking. In
fishes other than teleosts, the lumbo-sacral plexus receives fibers from a
collector nerve, which unites anterior spinal nerves with the plexus.

The problem of the origin of a nerve plexus was difficult in the days
when a primary and indissoluble connexion between nerve and muscle was
assumed. It is simpler, now that neuro-muscular connexions are known
to be secondary. Even in those regions where no plexuses are formed,
each spinal nerve innervates at least three myotomes, its own and parts
of the two adjacent. This tendency to muscle piracy is carried to a

Sensory nerve fibers.
Muscle fibers.

Motor plate.

Medullated nerve fibers. ''

Nerve fiber bundle.

Fig. 441. — Motor nerve endings of intercostal muscle ilbcrs of a rabbit. X150.
(From Bremer's "Text Book of Histology.")

greater extreme in the nerve plexuses, in which fibers from one nerve may
innervate more than three adjacent myotomes.

The numerous variations in plexuses, therefore, demonstrate that the
peripheral distribution of a given neurite is not unalterably predetermined,
though it must be admitted that the factors which determine the direction
of growth of a given neurite are still uncertain. In the light of the evi-
dence which we now possess, it does not seem necessary to explain the
formation of a plexus by assuming a crowding of adjacent nerves in the
formation of an appendage. The crowding is secondary, not primary.

Since both brachial and lumbosacral plexuses have their origin and
meaning in relation to appendages, it is significant that both brachial
and lumbo-sacral plexuses occur in snakes.

In mammals the cord ends in a terminal thread, the filum terminale,
the length of which is proportional to that of the tail. The spinal cord
in the fetus stops growing sooner than the vertebral column. As a result,
the more posterior spinal nerves, which have their foramina of exit in

THE NERVOUS SYSTEM

535

the lumbar and sacral vertebrae, are drawn out into a bundle of nerves
parallel to the spinal cord and lying within the spinal canal. From its
resemblance to a horse's tail, this is known as the Cauda equina. Similar
relations are found in anura and in some teleosts.

THE AUTONOMIC NERVOUS SYSTEM

In the higher chordates a special system of nerves, the autonomic, is
distributed to the smooth muscles of the digestive and circulatory systems

I Clarke's column

S0MAT1C.<A^„-H
MOTOR
GANGLION,
CELLS

M_50MATIC
■^ MUSCLE

VISCERAL MOTOR
FIBERS

Fig. 442. — A diagram of neurons of the spinal cord and spinal nerves shown in their
relations to one another and to their end-organs. Somatic sensory fibers are shown by
continuous lines, somatic motor fibers by fine dots. Visceral sensory fibers are indi-
cated by short broken lines, visceral motor by long broken lines. (Redrawn after
Plate.)

and to many other organs. This autonomic system is most evident in the
chain of S3nnpathetic ganglia which lie along the dorsal aorta from the
neck to the sacrum. These, however, are only a portion of the autonomic
system, which is distinguished by functional rather than anatomical
characteristics, for the cerebrospinal system is so intimately connected
with autonomic nerve fibers that the two systems cannot be separated

536 COMPARATIVE ANATOMY

anatomically. The vagus nerve, for example, which seems to be a part
of the cerebro-spinal system, contains many autonomic fibers connected
with the nervous plexuses of the viscera. Moreover, each sympathetic
ganglion of the trunk has fiber connexions with a spinal nerve and with
the plexuses of the intestine. (Figs. 439, 442, 443)

Autonomic nerves are connected not only with the digestive and
circulatory systems but also with respiratory and urogenital systems,
endocrinal and other glands, and the skin, so that there are few parts of
the body which autonomic fibers do not reach. Except possibly the
autonomic fibers connected with the ciliary muscle of the eye, autonomic
nerves, although markedly influenced by the emotions, are not under the
control of the will.

Two kinds of autonomic nerves, sympathetic and parasjrmpathetic,
may be distinguished on the basis of their antagonistic action and their
different response to drugs.

A sharp distinction between the sympathetic and parasympathetic
fibers cannot be drawn on the basis of function. The sympathetic
fibers are usually excitatory; the parasympathetic are usually inhibitory.
Most organs of the body have this double innervation and the action of
the two kinds of nerves is antagonistic. But some parasympathetic
fibers, as for example those in the vagus nerve, are excitatory.

The two sets of nerves differ in their reaction to atropin and adrenalin.
Atropine is stimulating to parasympathetic nerves but not to sympa-
thetic. Adrenalin stimulates sympathetic nerves but not parasympa-
thetic. Possibly this latter reaction might be expected, since adrenalin is
secreted by cells which have their origin from the sympathetic anlagen.

To the sympathetic group belong the vasomotor and secretory fibers
which connect with the sympathetic ganglia of the neck and trunk, from
the first thoracic to the fourth lumbar. Parasympathetic fibers are
found in the oculomotor nerve connected with the ciliary ganglion, and
in the chorda tympani and vagus nerves, and in the second, third, and
fourth sacral nerves. In this system must also be grouped all the neurons
with which these nerves have synaptic connexions.

It is evident, therefore, that there are three distinct groups of auto-
nomic nerve fibers, a cranial group, a thoracico-lumbar group, and a sacral
or pelvic group. Of these, the cranial and sacral elements are para-
sympathetic, and hence are grouped together as the craniosacral division
of the autonomic system. The thoracico-lumbar division constitutes the
sympathetic portion of the autonomic system.

Autonomic nerves may also be classified, on the basis of their distribu-
tion, into somatic fibers which innervate the blood vessels of the body
wall and the smooth muscles of skin and sweat glands, and visceral
fibers which supply the glands and smooth muscles of the viscera.

THE NERVOUS SYSTEM

537

TESTIS/ UTERUSl I , \^^^^^^

UTERINE PLEX! \haemORHOID PLEX

nUM TERMINALE ^^^^^

SYMPATHETIC I pQgj

■^'BERSlPRE.'

I POST --

PARASYMPATHETIC i.

Fig. 443. — The autonomic nervous system in man. Autonomic ganglia are lettered,
autonomic nerves given arabic numerals, cranio-spinal nerves are indicated by Roman
numerals. Relations to brain and spinal cord are shown to the right by a series of
cross sections taken at various levels, i, Lacrimal nerve; 2, Oculomotor nerve;
3, Nasociliary nerve; 4, Gasserian ganglion; 5, ramus maxillaris; 6, posterior nasal
nerve; 7, vidian nerve; 8, superficial petrosal n.; 9, deep petrosal n.; 10, chorda tympani
n.; II, minor superficial petrosal n. ; I2, lingual nerve; 13, vagus nerve; 14, inhibitor
cordis nerve; 15, broncho-dilator nerves; 16, accelerator cordis n.; 17, vertebral nerve;
18, major splanchnic nerve; 19, minor splanchnic nerve; 20, hypogastric nerve; 21, pelvic
nerve; 22, nervus erigens. A, ciliary ganglion; B, spheno-palatine gang; C, otic gang-
lion; D, carotid ganglion; E, sublingual ganglion; F, superior cervical ganglion;
G, stellate ganglion; H, celiac ganglion; /, inferior mesenteric ganglion. (Redrawn
from Ariens Kappers, after L. R. Miiller.)

538

COMPARATIVE ANATOMY

The somatic fibers act upon the hairs to stimulate their erection and
cause "goose flesh." They also serve the important function of regulating
temperature by influencing the tonus of the capillaries in the skin and
thus, by changing the rate of blood flow, altering the amount of secretion
of the sweat-glands. (Fig. 136)

The course followed by sympathetic and parasympathetic fibers within
the central nervous system is almost unknown. Evidence is not lacking
that stimulation of the cerebral cortex may be followed by reactions of
the viscera.

visceral motor
somatic motor

visceral sensory

somatic sensory
sympathetic.

Pig. 444. — Diagram of the relations of the sympathetic system. The character of
the different fibers is shown by conventional lines, hv, blood-vessel; eg, chain ganglion;
d, dorsal ramus; dr, dorsal root; g, gland; gr, gray ramus; pc. Pacinian corpuscle; pg,
peripheral ganglion; pvg, prevertebral ganglion; sg, ganglion of dorsal root; st, sym-
pathetic trunk; v, ventral ramus; vi, visceral ramus; vr, ventral root; wr, white ramus.
(From Kingsley's "Comparative Anatomy of Vertebrates," based on Huber.)

Three kinds of autonomic fibers connect with sympathetic ganglia.
Preganglionic fibers are visceral efferent fibers which come from ganglion
cells located in the lateral column of the spinal cord and have their ter-
minations in sympathetic ganglia. Post-ganglionic fibers are also
visceral efferent and have their cell bodies within sympathetic ganglia
and their telodendria upon smooth muscles of the intestine and of the
blood vessels. The preganghonic fibers are medulla ted and form the
white rami communicantes which connect spinal nerves with sympathetic
ganglia. The post-ganglionic fibers are rarely medullated. They pass

THE NERVOUS SYSTEM 539

either to the viscera by way of sympathetic nerves or to the body wall
and skin by way of the spinal nerves with which they are connected
through the gray rami communicantes.

Visceral afferent fibers, the third type, with cell-bodies in the dorsal
ganglia, carry impulses directly from visceral muscles to the gray matter
of the cord. Visceral afferent fibers having cell-bodies in the sympathetic
ganglia have not been demonstrated, the cells of sympathetic ganglia
being exclusively motor.

Most, if not all, actions of the autonomic system are reflexes mediated
through the brain or cord. Some intestinal reactions, however, may
occur after all nerve connexions with the cord and brain have been
severed. It is, therefore, possible that some visceral reflexes pass through
the intestinal plexuses only.

Three kinds of autonomic ganglia may be distinguished, ganglia of the
sympathetic trunk, collateral ganglia such as the coeUac and mesenteric
located in the wall of stomach and intestine, and terminal ganglia hke
the ciliary and cardiac located in the organs which they innervate.

Evolution of the Autonomic System

The nervous system of coelenterates is a plexus of primitive ganglion
cells connected with neurosensory cells and smooth muscle fibers and
located between the two primary body layers. This persists as an
intestinal plexus in other invertebrates, from flatworms to moUuscs and
insects. That the intestinal plexus of vertebrates is homologous with
that of invertebrates has not been demonstrated beyond a reasonable
doubt, but may be assumed in the absence of evidence to the contrary.
The late ontogenetic appearance of the plexus in vertebrates does not,
however, harmonize with this assumption, since, in the light of the funda-
mental law of biogenesis, we should hardly expect the most ancient part
of the nervous system to be one of the last to appear in the embryo.
On the other hand, we may have here another example of retarded develop-
ment, of which there are numerous examples in ontogenesis. Moreover,
the relations of the myenteric and submucous plexuses of the walls of
the stomach and intestine resemble those of the invertebrate intestinal
plexuses, and both are equally autonomic in their functions. Finally,
in elasmobranchs, sensory ceUs in the wall of the aUmentary canal form a
part of the system as in invertebrates. Evidence of similar cells in
mammals is wanting.

In invertebrates and vertebrates alike the evolution of the autonomic
system keeps pace with that of the digestive and circulatory systems.
Sympathetic and parasympathetic systems are recognized in arthropods;
but no structures are homologous with the autonomic ganglia of
vertebrates.

540

COMPARATIVE ANATOMY

As a system, therefore, the sympathetic of vertebrates is a new addi-
tion which arises late both ontogenetically and phylogenetically. Kap-
pers, however, calls attention to the fact that in arthropods the intestinal
plexuses, Uke the parasympathetic system of
vertebrates, are limited to the cerebral and caudal
part of the intestine.

So far as sympathetic ganglia are concerned,
chorda tes appear to start with a clean slate, since
there is no evidence of sympathetic ganglia in any of
the protochordates. The autonomic system is,
however, represented in protochordates, as in
vertebrates, by the visceral nerves, motor and
sensory. In the myxinoids the intestinal plexuses
develop exclusively from the brain region, mostly
from the vagus nerve. Tretjakoff has found in
petromyzon autonomic fibers in all spinal nerves.

Some sympathetic and parasympathetic ganglia
are found in elasmobranchs where, as in the higher
vertebrates, the ciliary ganglion is parasympathetic
and connected with the oculomotor and ophthalmic
nerves. Several sympathetic ganglia occur in the
trunk in connexion with a limited number of spinal
nerves. The metameric arrangement seen in the
embryos is modified in the adult through fusion.
No longitudinal connectives are found in elasmo-
branchs. But Allis has described in the head region
of teleosts segmental autonomic ganglia chained
together by connectives. Nothing similar has been
found in other vertebrates.

The autonomic system in tetrapods is essentially
similar to that of man. A shift in the relations of
the autonomic fibers occurs in phylogenesis. In
anamnia, visceral motor fibers have their cell-
bodies in the lateral horn and have their exit from
the tube by way of the dorsal roots; those in the
thoracico-lumbar and sacral region of amniotes,
on the other hand, enter the ventral or somatic
motor roots. The meaning of this shift is not clear.
In the head region, the connexion with dorsal roots is maintained through-
out the vertebrate series.

Pig.

445. — Sym-

pathetic system of right
side of a frog. Somatic
nerves dotted, sym-
pathetic black, a,
atlas; ai, common in-
testinal artery; ao,
aorta; c, coccyx; cr,
crural nerve; j, jugal
ganglion; i, sciatic
nerve; r, radices aortae;
s, base of skull; sp,
splanchnic nerve; st,
sympathetic trunk; ih,
ilio-hypogastric nerve;
II— XI, second to elev-
enth trunk nerves.
(From Kingsley's
"Comparative Anat-
omy of Vertebrates,"
after Gaupp.)

DEVELOPMENT OF THE BRAIN

The central nervous system of vertebrates arises as a thickened placode
of dorsal ectoderm anterior to the blastopore. This placode is known as

THE NERVOUS SYSTEM 541

the neural plate. Next to the notochord, the nervous system is the first
organ to develop. By the elevation of its edges, the neural plate is
converted into a neural groove bordered by neural folds. The anterior
more widely expanded portion forms the brain, and the narrower posterior

Fig. 446. — The sympathetic system in a i6-mm. human embryo. The ganglionated
trunk is heavily shaded. The first and last cervical, thoracic, lumbar, sacral and coccy-
geal spinal ganglia are numbered, a, Aorta; ace, accessory nerve; car, carotid artery;
cil, ciliary ganglion; coe, coeliac artery; Ht, heart; nod, nodose ganglion; ot, otic ganglion;
pel, petrosal ganglion; s-m, submaxillary ganglion; s.mes. superior mesenteric artery;
sph-p, sphenopalatine ganglion; spl, splanchnic nerve; st, stomach. (From Bremer's
"Text Book of Histology," after Streeter.)

portion the spinal cord. The transition between the two is, however, in
most animals, gradual rather than abrupt.

As the neural folds rise, they bend towards the median plane and
finally unite to form a neural tube with an anterior enlarged brain and a
posterior constricted spinal cord. The closure of the neural tube begins
in the neck region and proceeds craniad and caudad. But even before
the neural folds in the cephalic region unite, a series of three expansions

542

COMPARATIVE ANATOMY

appear, corresponding with forebrain, midbrain, and hindbrain, the fore-
brain differing from the other two in being anterior to the notochord.
From these, by processes of local unequal growth, all the parts of the
definitive brain are differentiated. Experiments demonstrate that the
position of the three brain divisions is predetermined in the open neural
plate. Soon after their closure and expansion as midbrain and hindbrain
vesicles, the lateral walls become divided by a longitudinal sulcus into a
ventral basal plate and a dorsal alar plate. Less clearly seen is a narrow
floor plate in the mid ventral line, and a roof plate in the mid-dorsal line.

= -" . 1a"l>r pl>te

ibasal plate

ifloor plate

- ' MESEINCEPHALON

EPIPHYSIS

TELENCEPHALON

METENCEPHALON

MYELENCEPHALON

TERMINALIS.

PREOPTIC RECESS

Fig. 447. — Diagrams of the development of the brain. A, Early neural plate before
closure, with zones marked; B, Longitudinal section of early brain tube; C, Later stage
with parts diiTerentiated. The dorsal zone (alar plate) is finely stippled; the ventral
zone (basal plate) is coarsely stippled; the floor plate is cross-hatched. Cer., cere-
bellum; c.str., corpus striatum; inf., infundibulum; mam., mammillary recess; olf.,
olfactory lobe; pal., pallium; thai., thalamus. (After Kingsbury modified.)

Since this sulcus does not develop in the forebrain, and ends where the
notochord ends, it appears that the forebrain consists of alar and roof
plates only. By the time a human embryo is a month old, the primitive
forebrain vesicle has begun to divide into the anterior telencephalon
and posterior diencephalon. In a five weeks embryo, the hindbrain has
begun to divide into the anterior metencephalon and posterior myelen-
cephalon. The undivided midbrain is the mesencephalon. These five
brain regions occur in all vertebrates, and from them all the parts of the
adult brain are formed.

Brain Flexures. While the subdivision of the primary vesicles is
taking place in ontogenesis, the brain undergoes in amniote embryos three
successive flexures, the cephalic or primary, the pontine, and nuchal or
cervical. The cephalic flexure occurs in the midbrain region, the other
two in the region of the hindbrain. All three flexures are in a vertical
plane, but the bend of the pontine is the reverse of the other two. The

THE NERVOUS SYSTEM

543

Mesencephalon

Shombencephalon
Myelencephalon

Amnion (cut)

Mesodermal segment 14

Open neural groove

Prosencephalon

Stomodaum

Amnion {cut)

Yolk sac

Body stalk

Fig. 448. — Human embryo of 2.4 mm. showing the neural tube partially closed.
(From Ranson's "The Anatomy of the Nervous System," after KoUmann; courtesy of
W. B. Saunders Company.)

CEPHALIC FLEXURE
OlENCEPHALON
TELENCEPHALON
I.FOREBRAIN

OPTIC VESICLE ^_ jf,
OLFACTORY LOBE-^*^
II.MIDBRAIN INFUNDIBULUM

MAMMILLARYBODY
OTIC CAPSULE

III.HINDBRAIN

CEREBELLUM

MESENCEPHALON

EPIPHYSIS
DIENCEPHALON
TELENCEPHALON
METENCEPHAUON

YELENCEPHALON

OLFACTORY LOBE

PONTINE FLEXURE
CERVICAL FLEXURE

SPINAL CORD

A. B, C.

Fig. 449. — Three stages — A, B, and C — in the development of the human brain,
showing the brain vesicles and flexures. A is an early stage, dorsal aspect, B the brain
of a three-weeks embryo in lateral aspect, C that of an eight-week embryo in lateral
aspect. (Redrawn after His and Hardesty.)

544

COMPARATIVE ANATOMY

bending is presumably the result of the elongation of the brain in limited
space, since the brain elongates more rapidly than does the head itself.
The cephalic flexure is well marked in the embryos of anamnia, but the
pontine and nuchal flexures scarcely appear. They become increasingly
evident as we pass from lower to higher amniotes. (Fig. 449)

Brain Vesicles and Their Derivatives
(After Keibel and Mall)

Primary vesicles

Secondary
vesicles

Derivatives

Ventricles

Telencephalon

Olfactory lobes
Corpora striata
Cerebral cortex
Optic thalami

Lateral (ist & 2nd)

ventricles
Anterior part of 3rd

ventricle

Forebrain

Diencephalon

Epithalamus (pineal
gland, etc.)

Thalamus

Hypothalamus

Infundibulum

Tuber cinereum

Mammillary bodies

Posterior lobe of pi-
tuitary

Posterior part of
3rd ventricle

Midbrain

Mesencephalon

Corpora quadrigemina
Tegmentum
Crura cerebri (pe-
duncles)

Aqueduct

Hindbrain

Metencephalon

Cerebellum
Pons

Fourth ventricle

Myelencephalon

Medulla

Telencephalon. The division of the primary forebrain into telen-
cephalon and diencephalon is effected by a dorsal constriction. The
hemispheres develop from the telencephalon by paired lateral expansions
of the alar plates. The roof plate takes no part in this, but persists
between the hemispheres as the lamina terminalis of the adult brain.
The paired hemispheric enlargements become conspicuous in a human
embryo of six weeks and by the fifth month, have overgrown the cere-
bellum. In a six weeks embryo two chief divisions appear in each hemi-
sphere, a dorsal expanded pallium and a ventro-lateral thickening, the
corpus striatum. The palHum is indistinctly divided into a more ventral
and median archipaUium, and a more dorsal neopallium. (Fig. 447)

THE NERVOUS SYSTEM

545

The neopallium, by great enlargement, forms most of the hemispheres;
the archipaUium develops into the olfactory and hippocampal lobes.

The wall of the brain consists primarily of a simple columnar epithe-
lium, which in the region of the basal and alar plates rapidly becomes
many-layered. Soon three strata may be distinguished, an inner epithe-
lial or ependyma layer where the most rapid multiplication of cells occurs,
a middle relatively thick mantle layer of compact epithelial cells, and an
outer marginal layer of nerve fibers. As the hemispheres enlarge, cells
migrate from the mantle layer into the marginal zone where they form
the cortex, a layer which, lacking in lower vertebrates, appears first in
Reptiles. The cortical cells of the definitive hemispheres become arranged
in five layers of multipolar neurons.

Fig. 450. — Median section of brain of pig 15.5 mm. long, showing* flexures of the
brain. C, primary flexure; cs, corpus striatum; CP, chorioid plexus of fourth ventricle;
h, hypophysis; i, infundibulum; M, mid-brain; A^, nuchal flexure; nc, notochord and
vertebrae;' F, pontal flexure; RO, optic recess; T, 'twixt-brain. (From Kingsley's
"Comparative Anatomy of Vertebrates.")

During its early expansion, the surface of the hemisphere is smooth.
In the fourth month, fissures arise, and eventually cut the hemisphere into
lobes and gyri. The first to appear are those between the olfactory lobes
and the pallium. Later, the hippocampal, lateral, and other fissures
develop successively. These are followed during the last two months of
fetal life by shallower sulci. The chief factor in forming these grooves is
the more rapid increase of cortex, compared with that of the underlying
white fibrous matter.

The lumen of each hemisphere persists as the lateral ventricle of the

adult brain.

The corpora striata develop as local thickenings of the ventro-lateral
walls of the telencephalon. As the dorsal pallium expands into the
enlarged neocortex, the thickness of the corpus striatum steadily increases
by cell multiplication. Consequently, in a six weeks embryo, each striate
body projects into the lateral ventricle as a conspicuous swelUng. Pri-

546

COMPARATIVE ANATOMY

marily, the thickening of the striate body is the anterior continuation of
that of the thalamus. As a result of the local enlargement of each, the
two become separated by a deep groove until the end of the third month.
The growth and elongation of the hemisphere carries the corpus
striatum back over the thalamus, so that it rests on the thalamus like a
saddle on a horse; and the two merge into what is structurally and func-

notochord . mandibular arch

dorsal aorta \ / Seessell's pocket

pharynx ^
thyrc-gloGsal duct

Rathke's pocket

tuberculura
posterius
infundibulum

ir.esonephi os
soinite

amnion {

Pig. 451. — Diagram of median longitudinal section of four-day chick. Due to a
slight bend in the embryo the section is para-sagittal in the mid-dorsal region but for
the most part it passes through the embryo in the sagittal plane. (From Patten's
"Embryology of the Chick.")

tionally a unified organ which becomes a pathway for fibers to and from
the cerebral cortex. In the corpus striatum these fibers form the internal
capsule which divides the corpus striatum into a median caudate nucleus
and a lateral lenticular nucleus. At the end of the fourth month, the
relations resemble those of the adult brain.

Olfactory Lobes. The olfactory lobes grow out from the base of the
hemispheres. At first they are hollow, but as they become differentiated
into bulb and tract, the cavity is lost. Fibrous connexion with that part
of the pallium which becomes the hippocampus is effected by neurites
from cells in the bulb.

THE NERVOUS SYSTEM 547

Commissures. The morphological changes which the brain under-
goes are accompanied by the development of fiber tracts which connect
the various parts of the brain and effect its functional unity. Among
these, none are more important than the commissures which unite the
lateral halves of the brain. These are formed by bundles of neurites
which grow from neuroblast cells in each side of the brain to corresponding
nuclei in the other side. The anterior and hippocampal commissures
develop in the lamina terminalis. Following the elongation of the
cerebral hemispheres, the hippocampal commissure is carried caudad
and incorporated in the posterior part of the fornix. The fibers of the
anterior commissure connect the olfactory lobes with the hippocampus of
the opposite side; those of the hippocampal commissure mostly connect
the hippocampus with the hypothalamus of the opposite side. (Fig. 425)

In correlation with the growth of the cerebral cortex, the corpus
callosum develops just above and anterior to the hippocampal commissure.
When it first appears, the corpus callosum is semicircular in cross section;
but as development proceeds, it assumes, in a five months embryo, the
elongated sickle-shape characteristic of the adult. Between the fornix
and corpus callosum, the thin median walls of the hemispheres are stretched
to form the median septum pellucidum. Between the two layers of this
septum there frequently develops a cavity erroneously known as the
fifth ventricle. (Fig. 383)

Chorioid Plexus. From the roof of the forebrain, in the region where
telencephalon and diencephalon merge into one another, a chorioid plexus,
which involves numerous blood vessels as well as brain wall, grows down
into the cavities of the brain. This plexus penetrates the paired ven-
tricles of the hemispheres and the median unpaired third ventricle of the
diencephalon.

Diencephalon. The wall of the posterior half of the primary fore-
brain differentiates four zones, roof plate, epithalamus, thalamus, and
hypothalamus. The roof-plate forms a chorioid plexus, which is con-
tinuous with that of the telencephalon, and which pushes into the ven-
tricle. An outgrowth from the epithalamus becomes the pineal gland.
A sulcus divides the thickened lateral wall into thalamus and hypo-
thalamus. The thalamus becomes a massive thickening and unites with
the corpus striatum.

From the hypothalamus develop the tuber cinereum, mammillary
bodies, infundibulum, and the neural lobe of the pituitary gland. The
tuber cinereum and mammillary bodies, thickenings of the posterior wall
of the infundibulum, become connected with the hippocampus by means
of a fiber tract, and are therefore believed to have an olfactory function.
The infundibulum is formed as a funnel-shaped pit with apex towards the

548

COMPARATIVE ANATOMY

roof of the mouth. The pars nervosa of the pituitary is proliferated from
its ventral extremity during the third month.

Mesencephalon. The inesencephalon undergoes relatively little
change during ontogenesis. As a result of thickening the lateral walls,
the central cavity is reduced to a slender aqueduct which connects third

metacoele
( ventricle tV)
thin roof of myelcncephalon

myelocoele
/ ventricle IV )

posterior commissure
esodiencephalic fold

(ventricle III ;

foramen of Monro

(Sylvian aqueduct)

Fig. 452. — Diagrams to show the topography of the brain of a four-day chick.
A, plan of sagittal section. The arbitrary boundaries between the various brain
vesicles (according to Kupffer) are indicated by broken lines. B, dextral view of a
brain which has been dissected free. C, schematic frontal section plan of brain. The
flexures of the brain are supposed to have been straightened before the section was cut.
(From Patten's "Embryology of the Chick.")

and fourth ventricles. From the dorsal portion of the midbrain arise
the corpora quadrigemina, which contain optic and auditory reflex
centers. From the dorsal plate of the mesencephalon develop also the
red nuclei, which receive fibers from the cerebellum and send fibers by
the rubrospinal tract into the spinal cord.

The motor nucleus of the oculomotor nerve develops in the basal
plate. Posterior to it at the same level is the nucleus of the trochlear
nerve which belongs originally, in elasmobranchs, with the metencephalon.

THE NERVOUS SYSTEM 549

Along the ventral wall of the mesencephalon develop two parallel fiber
tracts, the cerebral peduncles, visible on the ventral side of the brain stem,
which contain most of the ascending and descending fibers connecting the
hemispheres with the lower parts of the body. Just dorsal to these,
the tegmentum is differentiated as a region of interwoven fibers inter-
spersed with masses of nerve cells.

Metencephalon. The chief modifications of the vesicle of the meten-
cephalon are the cerebellum which develops from the alar plates, and the
pons which is differentiated from the basal plates.

Four anlagen are involved in the cerebellum. From a pair of small
thickenings near the median line the vermis is developed, while a larger
pair of lateral masses form the cerebellar hemispheres. The formation
of the cerebellar cortex resembles that of the cerebral. The relatively
rapid growth of the cortex results in the formation of lobules and fissures
of the adult cerebellum. The dentate nucleus is a cellular mass which
develops within the white matter of the cerebellum. Three large bundles
of fibers grow from the cerebellum, the brachium pontis, brachium con-
junctivum, and the restiform body.

The cavity of the metencephalon forms the anterior part of the fourth
ventricle.

Myelencephalon. The fifth and last of the brain vesicles forms the
medulla oblongata. Its ventricle enlarges to form the posterior part of
the fourth ventricle, which is partly obliterated by a chorioid plexus
developed from the thin dorsal roof of the medulla. Early in develop-
ment, the alar and basal plates of the medulla show a series of five expan-
sions, the rhombomeres or neuromeres, which disappear in ontogenesis
and have been interpreted as evidence of a primitive metamerism corre-
sponding to that of the trunk and possibly, it is suggested, inherited from
the metameric ganglia of invertebrate ancestors. The motor fibers of the
trigeminal, abducens, facial, glossopharyngeal, vagus, accessory, and
hypoglossal nerves arise from nuclei located in the medulla. The growth
of nerve fibers in all directions within the wall of the medulla results in the
reticular formation which is strikingly shown in cross section. Among
the nuclei which develop in the medulla are the nucleus cuneatus and
nucleus gracilis, the sensory centers of the fibers which pass into the
medulla from the spinal cord by way of the fasciculus cuneatus and
fasciculus gracilis.

DEVELOPMENT OF THE SPINAL CORD

When the neural plate becomes the neural tube, its wall is a simple
columnar epithelium. In consequence of more rapid cell proliferation,
limited to a layer of germinal cells which lie near the lumen of the cord,

550

COMPARATIVE ANATOMY

the lateral walls become greatly thickened, while the mid-dorsal and
midventral portions remain as thin roof and floor plates.

This thickened lateral wall becomes divided by a median longitudinal
sulcus into a dorsal alar plate and a ventral basal plate.

syncytium

r'^-m^^

- germinal
cell

B

) germinal
' cells

■^.iis^ii^

n

mesenchyme

germinal
•/ cells

DO

ext. limiting

membrane

nuclei of
mantle layer

int. limiting
membrane

marginal
layer

mantle layer

ependymal
layer

Fig. 453. — Stages in the histogenesis of the spinal cord. A, From open neural plate
of rabbit embryo. B, From wall of recently closed neural tube, 5 mm. pig embryo.
C, From neural tube of 7 mm. pig. D, From neural tube of 10 mm. pig. (All drawings
X550.) (From Patten's "Embryology of the Pig," after Hardesty.)

Three layers are differentiated in the lateral walls of the cord, an
epend)nnal layer next to the lumen, a thick mantle layer of spindle-shaped
cells, and an outer marginal layer of fibers free from cells. The marginal
layer increases in thickness by the addition of fibers which grow lengthwise
of the cord. By the addition of myelin sheaths to these fibers, the
marginal zone is converted into the white matter of the cord.

As the lateral walls increase in thickness, the lumen of the cord is
narrowed down so that in a cross section it appears as a slit which, how-

THE NERVOUS SYSTEM

551

ever, widens out slightly at the level of the longitudinal sulcus. Finally,
the lateral walls fuse together, except in the ventral region, where a
portion of the lumen is left as the central canal of the cord. The plane
of fusion of the ependymal cells persists in the adult as the dorsal septum.
On the ventral side of the cord, however, a median fissure is formed as a
result of the increase in thickness of the lateral walls and the failure of the
floor-plate to grow.

The mantle layer becomes the gray matter of the cord. As a result
of unequal growth of this layer, dorsal, lateral and ventral gray columns

ependymal
layer

D. 100mm. E. Adult

Fig. 454. — Transverse sections through spinal cord of the pig at various ages. Note
especially the parts of the adult cord derived from the ependymal, mantle, and marginal
layers of the embryonic neural tube. (From Patten's "Embryology of the Pig.")

develop. By the time the embryo is three months old, the gray matter
has assumed in cross section its characteristic H-shape.

In the early stages of the spinal cord, two kinds of cells are differ-
entiated, germinal cells which become neuroblasts, and non-nervous
spongioblast cells. Following the outgrowth of the neurite from a neuro-
blast, a number of dendrites grow in the opposite direction. According
to the theory of neurobiotaxis of Kappers, the neurite is stimulofugal,
that is, grows away from the source of stim.ulus, and the dendrites are
stimulopetal, that is, they grow towards the stimulus. Nerve cells of two
kinds arise, motor, which are Hmited to the anterior and lateral gray
columns, and association cells, which may lie in the dorsal column.

Two types of supporting cells develop from the spongioblasts, ependy-
mal cells, with elongated processes which extend radially from the central
canal to the periphery of the cord, and neuroglia cells, which have shorter

552 COMPARATIVE ANATOMY

and more numerous processes, and which do not extend through the
entire thickness of the wall. Some of the spongioblasts form transient

neurolemma cells which enclose the
neurites while the myelin sheaths
are formed. Most of these dis-
appear in the adult cord. Some of
the ependymal cells persist as the
epithelial lining of the lumen of the
cord.

The marginal layer gradually
thickens by the addition of libers,
some of which grow craniad and
some caudad, and most of which
soon acquire a myelin sheath.

"fig. 4S5.-A diagram illustrating the ^ith the appearance of dorsal and
theory of neurobiotaxis of Ariens-Kappers. Ventral nerve roots in Connexion

The diagram represents a series of motor ^j^j^ ^^^ j^^j ^^^^ ^-^^ marginal
neuroblast cells lying m the wall of the _^ ' °

spinal cord as activated to form neurites zone of white fibers bccomes divided

by a longitudinal bundle of neurites As ^^^^ ^^^g^j lateral, and Ventral
the bundle of neurites passes a neuroblast, ... .

a stimulofugal process is formed. Later by f unicuH. Since the time of myelin-

a reverse or stimulopetal process the ^^^^^ q£ ^^ers differs in the several

neuroblast is drawn towards the activating

bundle and dendritic processes grow towards tracts, depending upon the time

the bundle. Successive stages in the proc- .^^^en they begin tO function, some,

ess are shown beginning at the top of the' .

diagram. (Redrawn after Ariens-Kappers.) like the pyramidal, become medul-

lated only after birth.

DEVELOPMENT OF PERIPHERAL NERVES

The problem of how nerves become connected with their end-organs
in skin, muscle, or gland has been a controversial issue down to the present.
There is, however, no doubt that all nervous tissues are of ectodermal
origin. Three theories of neurogenesis may be mentioned.

The Hensen Theory. According to the Hensen theory, which has
been revived in this century, the connexion between nerve and muscle is
primary. This hypothesis is based upon the assumption that an embryo
is not a mosaic of cells but a syncytium or mass of protoplasm containing
numerous nuclei but without cell boundaries. Hensen asserts that the
protoplasm of a vertebrate embryo is continuous and unbroken; that
when cells divide, the division is never quite complete, but connexion is
retained by means of protoplasmic threads or plasmodesms. Not all of
these plasmodesms, however, become nervous. Some degenerate and
disappear during ontogenesis. The plasmodesms which later become
nerves do so as a result of nervous activity and consequent enlargement.
Within them eventually appear neurofibrillae. The mesenchyma cells

THE NERVOUS SYSTEM

553

which form the neurolemma sheaths migrate into the nerve anlagen. The
medullary sheaths are secreted later by the nerve fibers. The Hensen
hypothesis has recently been supported by J. Graham Kerr in his text-book
of vertebrate embryology.

In support of the Hensen hypothesis, Kerr advances evidence derived
from the study of fish embryos. The first connexions between myotome
and neural tube which he is able to discover are fine plasmodesms which,
without evidence, Kerr assumes to be primary. Later, these connexions
increase in length and diameter, become fibrillar, and finally are invaded
by the mesenchyma cells which form the neurolemma. The elongation

PLASMODESMS

EURAL TUBE

NERVE ANLAGE

NOTOCHORD

SCLEROTOME
-MYPOCHORDA

1MEURAL CREST

NEURAL TUBE

MYOTOME-^jj^WI'
NEUROBLAST S WTI" vji^

NERVE ANLAGE^ ' "

NEURITES-^^\xy^x^^^^>;

CHORDA ■ ■

SCLEROTOME
HYPOCHORDA-

A.PLASMODESM THEORY B. CELL-CHAIN THEORY. C. PROCESS THEORY

Fig. 456. — Diagrams illustrating the three chief theories of neurogenesis, based
upon sections of elasmobranch embryos in the trunk region. According to the plasmo-
desm theory of Hensen (.4) motor nerves are bundles of primary plasmodesmatous
connexions between nerve and muscle which have become enlarged through functional
activity. According to Balfour (5) nerves arise from chains of cells. The process
theory of Kupffer (C) holds that neurites are processes of neuroblast cells. The evi-
dence, experimental and other, supports the Kupffer theory.

of nerve fibers, which has been cited by others as evidence in favor of the
outgrowth theory of nerve development, Kerr interprets as evidence of
stretching of the primary connexion. In favor of the theory of primary
connexion of nerve and muscle, Kerr calls attention to the fact that, from
the time of their first appearance in the embryo, neural tube and myotome
are in close proximity to one another and hence might easily be connected
by plasmodesms. As evidence in favor of the Hensen hypothesis, Kerr
mentions the results of an experiment of Spemann who transplanted the
limb bud of a frog to its cheek. The leg muscles became innervated by
nerves from the cheek. It might appear that such an experiment proves
that nerve and muscle may become secondarily interconnected, but Kerr
interprets the evidence as favoring the Hensen hypothesis !

Of the presence of plasmodesms in vertebrate embryos, there can be
not the slightest doubt. The fact that they are more conspicuous in
some embryos than in others, and that they are exaggerated by some

554

COMPARATIVE ANATOMY

methods of fixation by no means proves that they are artifacts. The
evidence, however, that they are primary in origin is not convincing.
Moreover, the supporters of the hypothesis have failed to show that
plasmodesms become nervous.

The Balfour H3rpothesis. The distinguished British embryologist,
Balfour, maintained that embryonic nerves are cellular and that nerves
are modified cell chains. He concluded that all nerve cells are ectodermal
in origin; that sensory nerves develop from chains of cells derived from
the neural crest, and motor nerves come from cells which emerge from

SPINAL CORD

SENSORY
■^r.;:ir— GANGLION
CELLS

SCLEROTOME

MESENCHYME

SOMATIC

MOTOR

NEUROBLASTS

r-T-T^ r- NOTOCHORD

Fig. 457. — A portion of a cross section of a chick embryo showing the effect of the
ordinary embryological method of preservation and staining (on the left) in contrast
with that of a special neurological method such as that of Cajal (on the right). It is
not surprising that the chain theory of the origin of nerve fibers long seemed supported
by the study of sections of embryos prepared by the earlier methods. (Redrawn after
B. Patten.)

the brain or spinal cord. The connexions of nerve and end organ are
therefore secondary.

The histogenesis of a nerve fiber, according to Balfour, involves first
the arrangement of spindle-shaped ectodermal cells in a chain extending
from the central nervous system to the end organ. Each spindle-shaped
cell becomes differentiated into an axial thread and a peripheral proto-
plasmic non-nervous sheath. The axial threads unite end to end to form
the neurite, while the enclosing nucleated protoplasm becomes the neuri-
lemma sheath which is thus formed of a series or chain of cells as in the
adult nerve fiber. In medullated nerve fibers, the fatty medullary sheath
arises later between the neurite and the neurilemma.

The Balfour hypothesis leaves unsolved the question how the chain
of cells which forms a nerve fiber is directed in its development towards

THE NERVOUS SYSTEM

555

the end-organ. But this difficulty confronts any hypothesis which
assumes a secondary connexion between nerve and end organ. The
Balfour hypothesis had respectable standing only as long as special
methods of staining embryonic nerves had not been invented, or were
unused. When methods of staining which were specific for embryonic
nerves were finally employed by Golgi, Cajal, Bielschowsky, Ranson,
and others, the foundations of the Balfour hypothesis were gradually
undermined. It was found that embryonic nerves are fibrillar from the

EPIDERMIS

MYOTOME- }V^

SOMATIC
MOnOR NERVE

SCLEROTOME

NEURAL TUBE

NEUROBLAST
CELLS

NOTOCHORD

Fig. 458. — A portion oi a bt-Liuai o. a 7 mm. elasmobranch embryo in the trunk
showing an early stage m the de\ elopmcnt of a somatic motor nerve, Bielschowsky
preparation. The nervous connexion is secondary and not formed by the enlargement
of a plasmodesm. The nervous character of the connexion is attested by its affinity
for special nerve stains. That the nerve anlage is formed by processes of neuroblast
cells in the spinal cord is further evidenced by the fact that processes of medullary
neuroblasts may be traced into the nerve anlage. Moreover, deeply staining neuro-
blasts are differentiated in the wall before the nerve anlage appears, as would be expected
if they form the nerve.

time of their first appearance, and that sheath cells are a secondary
addition. Finally, the experiments of Harrison were decisive against this
hypothesis and in favor of the process theory of Kupffer.

The Kupffer Theory. The German anatomist, Kupffer, was the
first to suggest that a nerve fiber becomes connected with its end-
organ by means of protoplasmic outflow from a neuroblast cell either in
the spinal cord or in the neural crest cells. Hence his theory became
known as the process theory of nerve development. Kupffer's theory
of neurogenesis may be better understood by taking as an example the
development of a somatic motor nerve fiber in a vertebrate embryo.
The neurite of such a fiber is formed by the protoplasmic outflow from a
neuroblast cell located in the ventro-lateral waU of the neural tube.
Neural tube and myotome are primarily unconnected. The neuroblast
cell before it forms a process is a spindle-shaped epithelial cell in the wall

556

COMPARATIVE ANATOMY

of the neural tube, distinguished by its affinity for specific nerve stains,
which reveal a deeply staining network within the cytoplasm of the
neuroblast. Subsequently, an amoeboid process extends from the cell
into the liquid-filled space between neural tube and myotome. This
space is narrow and connexion between neuroblast cell and myotome is
quickly effected. From the time of its first appearance, the nervous
character of this process is attested by its affinity for specific nerve stains.
The subsequent changes in this neurite involve its elongation along the
median surface of the myotome or its penetration into the mass of the
muscle fibers. Other neurites are added after the manner of the one

MYOTOME-

FiG. 459. — A camera drawing of a portion of a cross section of a 6 mm. elasmobranch
embryo showing a Rohon- Beard "giant" ganglion cell, stained by the vom Rath
method. In the specimen represented the body of the Rohon-Beard cell is located in
the dorsal wall of the spinal cord and its neuraxon process extends to the level of the
myotome. Periferally the neuraxon terminates in numerous amoeboid processes.
Sectioned material provides no better evidence than this of the correctness of the process
theory of Kupffer.

first formed, until the connexion between tube and myotome becomes a
bundle of naked neurites. Cells from the surrounding mesenchyma are
soon added to the nerve anlage. In Elasmobranchs, cells migrate from
the wall of the neural tube to form the neurolemma of the differentiated
nerve. Finally, medullary sheaths appear, apparently as a result of a
reaction between neurite and neurolemma.

One of the early objections raised against the process theory was that
it left unexplained the problem of how nerves find their way to their
end-organs. This difficulty, however, does not appear to be serious in
the case of most somatic nerves, since these are separated by a minimal
distance from their end-organ, the myotome. The difficulty is more
serious for such a nerve as the trochlearis, the fibers of which grow from
the floor of the metencephalon, emerge from the roof of the isthmus, and
grow a long distance through the mesenchyma surrounding the brain,
until they reach the anlage of the superior oblique muscle. We have
today no theory which satisfactorily explains this circuitous route; but
such are the facts even if we cannot explain them. A similar phenomenon

THE NERVOUS SYSTEM 557

is seen in the outgrowth of the neurites of the Rohon-Beard cells in the
spinal cord of fishes. These transient nerve fibers extend, devoid of
sheath cells, from cells in the roof of the spinal cord to the skin in the
extra-embryonic blastoderm.

The decisive proof of the truth of the process theory, however, was
afforded by the beautiful experiments of Harrison, who removed a piece
of the spinal cord of a frog or salamander embryo before somatic motor
nerves had appeared. Such a fragment, when placed in a sterile nutrient
solution, produced elongated protoplasmic processes such as are formed
by the Rohon-Beard cells within the normal embryo. No more con-
vincing proof of the process theory could be asked and, as a result, this
theory is all but universally accepted by students of neurogenesis.

The origin of the neurolemma and the medullary sheaths, however,
presents another problem which also Harrison tried to solve by experi-
ment. Observations of sectioned material suggested that, in amphibian
embryos, the cells present in embryonic nerves are derived from the
neural crest. Harrison therefore removed the skin and neural crest
from the back of a salamander larva. The result was striking. Larvae
from which the neural crest had been thus removed not only lacked
sensory nerves, which develop from the crest, but their somatic motor
nerves were wholly devoid of cells. The conclusion seems reasonable that,
in amphibia at least, many, if not all, sheath cells are derived from the
neural crest and that, in this group, medullary cells do not migrate from
the cord into the somatic motor nerves. Later, Harrison was able to
observe in the tail of living tadpoles neurolemma cells migrating along
naked neurites and thus to confirm the conclusion that neurolemma cells
migrate to their definitive position.

Harrison left unsolved the problem of the origin of the medullary
sheath. Speidel, on the basis of observations upon the tails of living
tadpoles, has made it seem likely that, in this region, the myelin sheath is
formed as a result of a reaction between the neurolemma and the neurite.
He has found that the myelin appears soon after the neurolemma encloses
the neurite, and the naked neurite seems unable to secrete myelin
unaided.

Histogenesis of Motor Nerves. Somatic motor nerves arise as
protoplasmic processes in the manner just described. The neurites
become secondarily connected with myotomes by protoplasmic outflow
from neuroblast cells, located in the ventral gray column of the neural
tube, which are neurofibrillar from their first appearance. The researches
of Coghill and others lead to the conclusion that this protoplasmic outflow
is a response to stimulus from a bundle of longitudinal nerve fibers which
grow caudad along the neural tube. Following the outgrowth of the
neurite to the muscle, the dendrites grow in the opposite direction towards
the source of stimulus.

558

COMPARATIVE ANATOMY

Neurolemma cells are added to the nerve either by migration from
the neural tube or the dorsal ganglia or from the loose mesenchyma which
surrounds the nerve anlage. The migration of medullary cells from the
cord appears to be absent in amniote embryos. Some embryologists
assume that all neurolemma cells are derived from the dorsal ganglia.
The appearance of the medullary sheath is correlated with the enclosure
of the neurite by the neurolemma cells. The telodendria of each neurite
are associated with the end-plate on the surface of one or more muscle
fibers.

ectoderm

neural crest

neural plate

neural primordium

crest of dorsal

Fig. 460. — Drawing showing closure of the neural tube and formation of the neural
crest. From pig embryos of: — A, 8 somites; B, 10 somites; C, 1 1 somites; D, 13 somites.
X135. (From Patten's "Embryology of the Pig.")

Visceral motor fibers develop in a manner similar to that of somatic
motor fibers, except that the nerve cells are located in the lateral column
dorsal to the somatic motor column. Both the nerve cell in the neural
tube and the peripheral fiber from it are relatively small and the myelin
thin. In the trunk region, the telodendria acquire synaptic connexions
with the ganglion cells of the sympathetic ganglia.

Histogenesis of Sensory Nerves. All sensory neurites develop as
outgrowths of cells located in the dorsal ganglia which, in turn, are groups
of cells derived from the neural crest.

When the neural groove closes over and its edges fuse together during
the formation of the neural tube, some loose cells are left between skin
and neural tube. These ectodermal cells form the neural crest. Whether
by this process the neural crest cells are derived from the dorsal wall
of the neural tube or from the ectoderm of the skin above matters little,
since they are derived, like the neural tube itself, from the ectoderm.
The neural crest cells, soon after they first appear, multiply by division.

THE NERVOUS SYSTEM

559

and become concentrated into two parallel strands dorsal to the spinal
cord. The crest is continuous throughout the entire length of the central
nervous system, except for an interruption in the head region between
the fifth and seventh nerves. As development proceeds, each strand
of neural crest cells migrates en masse ventrad. In the trunk region
the cells become more and more concentrated and metamerically arranged
as a series of ganglionic masses median to the myotomes. For each
myotome, there develops both a somatic motor root and a dorsal sensory
ganglion. Within the sensory ganglia, two kinds of cells become differen-
tiated. Toward the center, most of the cells become nerve cells, from
each of which processes grow in two directions, one process into the dorsal

A B

Fig. 461. — A, diagram of early spinal cord; B, later, showing increase in size and
consequent ventral fissure, c. central canal; e, ectoderm; /, floor plate; g, anlage of
spinal ganglion; nc, neural crest; r, roof plate; 5, sulcus limitans; v, ventral fissure.
(From Kingsley's "Comparative Anatomy of Vertebrates.")

column of the tube and the other ventrally to join the somatic motor
nerves which have already formed. The resulting mixed nerve gives
off dorsal and ventral rami, to skin and muscles. The relations of the
fibers of the sensory roots have already been described. Most of the
peripheral cells of the ganglion become neurolemma cells. Possibly
all neurolemma cells of sensory nerves are so derived.

The development of the neural crest in the head region differs in
some details from that of the spinal nerves just described. Olfactory
nerve fibers, except those in the nervus terminalis, develop not from
the neural crest but from the olfactory epithelium; and, since the optic
nerve is strictly a fiber tract of the brain, its development also is not from
the neural crest. The oculomotor nerve is somatic motor, like the
trochlearis, abducens, and hypoglossus, and its development is like that
of the ventral root of a spinal nerve. The trigeminal is a mixed nerve
with visceral motor fibers and with sensory fibers developed from the
neural crest. There is evidence that some of the neural crest cells of all
the visceral arches enter into the formation of connective tissue of the
arches. The sensory elements of the facial and acoustic come from a
common ganglion. Indeed the acoustic nerve develops as a branch
of the seventh and only secondarily acquires an independent root. The

r5o COMPARATIVE ANATOMY

glossopharyngeal and vagus nerves are mixed, like the trigeminal, and
have a similar double origin. Their sensory elements come from the
neural crest and skin, while their motor fibers arise as processes (neurites)
of motor cells located in the lateral wall of the medulla. One peculiarity
of the V, VII, IX and X nerves is that their ganglia receive cellular
increments from the skin just above the visceral pouch with which the
nerve is associated. The patches of thickened ectoderm which proliferate
these cells are known as epibranchial placodes. They have been inter-
preted, perhaps erroneously, as rudiments of sense organs which have
degenerated.

C. NODOSUM\
C. PETROSUI
OTIC G

ACCeSSOPY NERVE

CERVICAL GANGLIA
/AORTA

Fig. 462. — The autonomic system of a 16 mm. human embryo. The sympathetic
trunk is shown in solid black. The intestine is stippled. (Redrawn from Bremer,
after Streeter.)

Development of Sympathetic Ganglia. Before the embryological
facts were known, it was assumed that the sympathetic ganglia of verte-
brates represent the chain of ventral ganglia of invertebrates. Their
origin in vertebrates proves that this homology is incredible. The sym-
pathetic ganglia of vertebrates are derived, like the neurolemma cells,
from the dorsal ganglia by the migration of neural crest cells ventrally
along the nerves toward the dorsal aorta. They first appear as clusters
of cells, each cluster connected with the nerve from which it arose, at
the level of the aorta. In the head the ciliary, sphenopalatine, otic,
and submaxillary ganglia are formed in this way. In the trunk the
superior and inferior cervical ganglia, and the series of vertebral and
prevertebral ganglia belonging to the sympathetic are derived from the
neural crest by the prolonged migration of nerve cells. In the sympa-
thetic ganglia the nerve cells "spin" the postganglionic fibers to the blood

THE NERVOUS SYSTEM

561

vessels and viscera. Connexions with the nerves from which the sympa-
thetic ganglia arise persist as the rami communicantes. The metameric
ganglia become secondarily connected by a sympathetic cord which
runs parallel with the dorsal aorta. The prevertebral plexuses, cardiac,
coeliac, and hypogastric, arise by the more extensive migration of cells
from the ganglia of the vagus nerve. The most extensive cellular migra-
tion leads to the formation of the myenteric and submucous plexuses

PLEXUS OF AUERBACH/^

.LONGITUDINAL
MUSCLES

-PERITONEUM

Fig. 463. — A stereogram of a portion of the small intestine, showing the arrangement
of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are
shown in black, sensory cells with white nuclei. (Redrawn after Kahn.)

(the plexuses of Auerbach and of Meissner). But, however remote from
their source such sympathetic cells may be, they retain fibrous connection
with the rest of the nervous system. (Fig. 463)

The whole sympathetic system is well established in a three months
human embryo.

MENINGES

The spinal cord of Amphioxus is surrounded by loose connective
tissue. In cyclostomes this tissue shows the beginnings of differentiation
into compact outer and inner layers with loose tissue between, the three
representing possibly the three meningeal layers of higher vertebrates.
In fishes the cranium and the vertebrae are lined by compact periosteum
or perichondrium, between which and the brain or cord the connective
tissue is loose, except where the connective tissue comes in contact with
the central nervous system. There it becomes the highly vascular
meninx primitiva. Such connective tissue membranes or meninges
surrounding the central nervous system (brain and cord) serve both for
protection and for nourishment.

562

COMPARATIVE ANATOMY

Two such meninges surrounding brain and cord occur in Amphibia,
a pia mater primitiva next to the brain and cord and, outside this, a
dura mater. The wide space between dura mater and periosteum is
bridged by connective tissue trabeculae.

In mammals, three meninges are differentiated. Innermost is the
pia mater, thin and highly vascular, from which connective tissue processes
grow into brain and cord carry in blood vessels, and support the nervous

CORPUS CALLOSUM

TELENCEPHALON

DURA MATER

THIRD VENTRI

OPTIC NERVE

SUBDURAL CAVITr
ARACHNOID;

LATERAL VENTRICLE

•CISTERNA SUPERIOR

rEMTORIUM
XREBELLI

SUft+ INF. COLLI CULI

CEREBELLUM

CHORIOID PLEXUS

CISTERNA

FOURTH VENTRICLE

B

PIA MATER

DURA MATER

ENTRAL CANAL

SUBARACHNOIDAL SPACE

^^ SUBDURAL CAVITY

FILUM TERMINALS

Fig. 464. — Diagram showing the relations of the meninges to the central nervous
system, as shown in median longitudinal section and in cross section. (Redrawn after
Rasmussen's "Principal Nervous Pathways," The Macmillan Co.)

tissue. Outside this is the arachnoid, which as its name suggests, is a
delicate web-like tissue. Only its outer layer is organized into a mem-
brane. Outermost of the three is the dura mater, thickest and toughest
of all, and more or less closely attached to the periosteum which lines
the cranium and the vertebral canal, so that this periosteum is sometimes
reckoned as a part of the dura mater.

Where the dura mater of mammals penetrates between the cerebral
hemispheres it forms the falx cerebri. A similar fold of the dura mater
grows between the hemispheres and the cerebellum to form a tentorium
cerebelli.

All three meninges develop from the loose mesenchyma which sur-
rounds the embryonic neural tube.

•±1 LIBRARY 2

\^V MASS. .

CHAPTER 14
THE SENSE ORGANS

The sense organs of animals consist of sensory cells or groups of cells
adapted to respond to stimuli and transmit an impulse to the nerves.
The essential property of sense cells is, therefore, irritability. Like
nerve cells they are able to transmit impulses caused by physical and
chemical changes, either in the environment or within the organism.

The receptor organ may be either the neurosensory cell itself, or a
secondary sensory cell which transmits a stimulus to the nerve. In
higher animals, moreover, sense organs, in addition to receptor cells,
have various mechanisms for protecting and supporting the sensory cells
or conveying stimuli to them so that great diversity of receptor cells
and of sensory nerve terminations has arisen during the course of evolution.

The result is a considerable number of special senses. These may be
classified as major or minor, according to their importance. Major
senses are touch, taste, smell, hearing, and sight. Minor, in addition
to the muscular and visceral senses, include heat, cold, pain, hunger,
thirst, fatigue, sex, and equilibrium. It is evident that this division
of the senses rests on their conscious accompaniments.

On the basis of the source of stimulus, senses are Exterior — sight,
hearing, taste, smell, pressure, heat, cold; or Interior — pain, hunger,
thirst, the muscular and visceral senses, equilibrium, lust.

Some physiologists recognize as many as thirty-six special senses,
separated for the most part by differences in sensation. Certain it is
that we have many more than the traditional five!

EVOLUTION OF SENSE CELLS

Sense organs and sensory nerves alike begin in responsiveness to
stimuli, such as is manifested by amoeba, which reacts to changes in
pressure, light, heat, chemical substances, and electricity.

The first sense cells differentiated are the neurosensory cells of coelen-
terates, which establish nervous connexion with underlying muscle
cells by means of protoplasmic processes, while the body of the cell
remains in the external epithelium. Each cell may have a stiff sensory
bristle or hair.

An advance towards conditions in higher animals is taken when the
body of the cell recedes from the surface, retaining connexion by means

563

564

COMPARATIVE ANATOMY

THE SENSE ORGANS

56s

of an elongated process. Usually the outer termination of these cells
also is beset with one or more hair-like processes. Neurosensory cells
of this sort have a wide distribution in the animal kingdom. Those of
the olfactory epithelium of vertebrates are at this stage.

A third stage is represented by sensory neurons which have lost
their connexion with the external surface, but retain "free" nerve termina-
tions in the epidermis. Such a cell is bipolar in form with both cutaneous
and central connexions. A neuron of this sort may also terminate
within the underlying corium, branch freely in the connective tissue,

A. NEUROSENSORY CELLS
OF ANNELID.

B. t^UROSENSORY CELLS
OF OLFACTORY MEM-
BRANE OF MAN.

C. PRIMARY SENSORY CELL D. SECONDARY SENSORY
WITH FREE NERVE CELLS CONVEY IMPULSES

TERMINATION. TO THE PRIMARY SENSORf CELL.

Fig. 466. — Stages in the hypothetical evolution of secondary sense cells. A and B,
diagrams of neurosensory cells in invertebrate (.4) and vertebrate (B). C, a sensory
cell (neuron) with free nerve terminations. D, secondary sense cells convey impulses
to the primary sensory cell. The series assumes that the definitive receptor cells are
secondary. The possibility that the neurosensory cells become the definitive sense cells
and that the sensory nerve is secondary must be admitted. (Redrawn after Kahn's
"Das Leben Des Menschen," W. Keller & Co.)

end between tactile cells, or become surrounded by a connective tissue
capsule.

A definitive evolutionary stage is attained when a secondary sensory
cell becomes the receptor element by means by which a stimulus is
transferred to the dendrites of a neuron. The receptor cells of the
taste-buds and the hair-cells of the cochlea represent a final stage of
this sort. The substitution of a secondary sensory cell for the primary
neurosensory one presents a problem which has never been satisfactorily
solved. That such an evolutionary change has occurred seems indisput-
able, but transitional stages are wholly conjectural.

Among the factors which have led to the formation of the sense
organs of higher animals, may be noted the tendency of the neurosensory

566 COMPARATIVE ANATOMY

cells, which were primarily scattered and separate, to become concentrated
in clusters to form the sense organs. Secondary sense cells show the
same tendency, and the cephalization of the nervous system is correlated
with the concentration of the sense organs in the head region. Attention
has already been called to the fact that the three primary divisions of
the brain are associated with three major sense organs, the nose, the eye,
and the ear.

CUTANEOUS SENSES

At least four senses, pressure or touch, pain, warmth, and cold,
are based upon nerve terminations in the skin. The experiments of
Goldschneider and others demonstrate that corresponding with these
four sensations are four different sorts of nerve terminations. These
may be either free or encapsuled. Usually, the free nerve terminations
are located in the basal layers of the epidermis, and are therefore more
superficial, while the encapsuled terminations lie in the corium below.

Evolution of Cutaneous Sense Organs

It may be assumed that the various cutaneous sense organs were
derived from neurosensory cells of the epithelium, which by the outgrowth
of a neurite become connected with the nerve net or cord. When the
body of the cell gradually migrated into the underlying connective tissue
and the epidermis became many-layered, all connexion with the surface
was lost, and the neurosensory cell was converted into a sensory neuron,
with free nerve terminations in the lower layers of the epidermis and
central connexions with the nerve cord. As a last step, connexion with
the epidermis was lost, and the peripheral termination of the sensory
cell was buried in the corium.

All these stages are represented in chordates. The skin of amphioxus,
for example, is beset with many neurosensory cells, both single and in
clusters, many of which possess a stiff terminal bristle which projects
above the general surface. Amphioxus also has sensory nerves with
free nerve terminations branching among the epithelial cells.

The encapsuled nerve terminations have apparently followed two
independent lines of evolution. On the one hand, free nerve terminations
in the corium have become encapsuled by concentric layers of connective
tissue cells, as represented in the corpuscles of Pacini, Krause, and Golgi-
Mazzoni. On the other hand, some nerve terminations are associated
with tactile cells, which primarily were located in the basal layer of the
epidermis, but which later migrated into the corium. In some instances,
a single lenticular tactile cell may rest upon a cup-shaped termination
of a sensory neurite, or the nerve may branch among a cluster of such
cells. As a final evolutionary stage, a cluster of tactile cells connected

THE SENSE ORGANS

567

with the dendrites of a sensory nerve may become encapsuled by con-
nective tissue to form a Meissner's corpuscle.

Free nerve terminations occur in the skin of all classes of chordates,
usually in the form of multiple arborizations or dendrites. These may
lie in the epidermis or in the corium, in either case, are located where
they may respond to changes in pressure. Such free nerve terminations
in the skin are found in all classes of vertebrates, and are believed to be

CLUB-SHAPED

TACTILE CORP.

(KRAUSE)

ENCAPSULED
NERVC- KNOT
(GOUGI-MAZZDNl)

ENCAPSULED
GROUP OF
TACTILE CELLS
(MEISSNER)

DIAGRAMS B.-D CUTANEOUS SENSE ORGANS IN CRANIATES WITH SECONDARY SENSE CELLS.

Fig. 467. — Varieties of cutaneous sense organs in chordates. A, B, C show sensory
terminations in the skin of acrania. B' to D^ show varieties of terminations in craniotes.
C to C* and D' to D* respectively represent stages in the hypothetical evolution of
encapsuled nerve terminations. (Redrawn after Plate.)

the sensory mechanism of painful sensations arising in the skin. The
sense of touch is believed to depend chiefly upon the tactile cells or corpus-
cles in the corium, of which various forms occur.

Meissner's corpuscles, present only in primates, are located in the
corium papillae of the palms and soles, and in the external genital organs.
Each corpuscle consists of a group of tactile cells surrounded by a rela-
tively thin envelope of connective tissue and connected with one or more
nerve fibers. The non-medullated nerve fibers twists spirally among the
tactile cells, each of which is in contact with a reticular nerve termination.

In birds and reptiles nerve endings are connected with tactile cells of
Merkel but without a connective tissue capsule. These tactile cells

568 COMPARATIVE ANATOMY

are sometimes solitary, sometimes clustered. On the other hand, the
Grandry's corpuscles of birds are encapsuled, and the nerve termination
lies between two tactile cells.

In the relatively small corpuscles of Krause and in the large one of
Pacini, both found in mammals, the nerve termination is club-shaped
and encapsuled. In a Pacini corpuscle, are as many as eight concentri-
cally arranged lamellae. A secondary or adjunct nerve fiber penetrates
the capsule and forms a varicose network within the inner lamella.
Pacini's corpuscles are located not only in the deeper layers of the skin,
but also in the mesenteries, tendons, and periosteum. Those present in
tendons give rise to sensations which serve to indicate the position of a
limb. Those in the deeper parts of the body probably give rise to painful
sensations associated with disease.

The Golgi-Mazzoni corpuscles are spherical or club-shaped tactile
corpuscles, in which the coiled nerve termination is enclosed by, but not
in contact with, the surrounding capsule. In them, both chief and
adjunct nerve fibers are present. These corpuscles occur in the corium,
the peritoneum, and in the conjunctiva of the eye. The genital cor-
puscles found in the corium of the glans penis are supposed to be the
sensory mechanism associated with sexual desire.

Development of Cutaneous Sense Organs

Free nerve terminations are formed by the outgrowth of sensory
nerve fibers from the sensory ganglia. Of these, some remain free, some
effect connexion with tactile cells, some become enveloped by connective
tissue capsules.

LATERAL LINE ORGANS

Lateral line organs are a specialized type of cutaneous sense organs
limited to fishes and water-dwelling amphibia. It is believed that they
respond to currents of water and to sudden changes in pressure. Although,
among cutaneous sense organs, the arrangement in rows is peculiar
to the lateral line organs, this linear arrangement is presumably second-
ary, since both invertebrates and vertebrates have, scattered over the
surface of the body, sensory papillae or neuromasts similar to those
of the lateral line organs.

Dorsal, lateral, and ventral rows of lateral line organs occur in verte-
brates on each side of the body. Usually all three are present only in
embryonic and larval stages. The lateral rows persist in the trunk
region of adult fishes and urodeles. Three lines, however, are present
in the head, supraorbital, infraorbital, and mandibular, innervated by
branches of the facial and vagus nerves, and exceptionally by a branch

THE SENSE ORGANS

569

of the glossopharyngeal nerve. A supratemporal line may connect the
systems of the two sides across the posterior part of the skull.

A sense organ of the lateral line consists of a cluster of pear-shaped
hair cells, each of which is connected with a branch of the lateral line nerve.
Usually the cluster of sense cells is encircled by a ring of columnar epi-
thelial cells. The sensory cells of neuromasts, unlike those of taste-buds,
do not extend to the base of the epithelium.

Fig. 468. — Head of pollack, showing lateral-line canals and nerves of the lateralis
system. Lateralis nerves black, canals and brain dotted, b, buccalis ramus of VII
nerve; dl. dorsal ramus of lateralis of X nerve; h, hyomandibularis nerve; hm, hyo-
mandibular line of organs; io, infraorbital line; I, lateral-line canal; n, nares; o, olfactory
lobe; op, operculum; os, ophthalmicus superficialis nerve; soc, commissure connecting
lines' of the two sides; so, supraorbital line of organs; st, supratemporal part of lateral
line; vl, ventral ramus of lateralis of X nerve; x, visceralis part of X nerve. (From
Kingsley's "Comparative Anatomy of Vertebrates," after Cole.)

Evolution of Lateral Line Organs

It is generally assumed that neuromasts have evolved from clusters of
neurosensory cells like those of lower invertebrates. Similar clusters
occur in the oral tentacles of amphioxus. Later in phylogenesis, however,
sensory cells of the secondary type become the sensory elements of the
neuromasts. How this change occurred is problematic. At present,
there is no evidence adequate to settle the question whether this change
involves the replacement of primary sensory cells by secondary ones,
or by the retention of the cell-bodies of the primary cells and the substitu-
tion of new nervous connexions for the primary ones. Since neuromasts
with secondary sense cells like those of chordates occur also in leeches
of the genus Glossosiphonia, it is evident that the substitution of secondary
for primary neurosensory cells has occurred independently in vertebrates
and invertebrates.

570

COMPARATIVE ANATOMY

The stages in the evolution of neuromasts are repeated in ontogenesis
and also represented in the lateral line organs of adult craniotes. Pri-
marily, the neuromasts are scattered over the surface of the body, but
appear first in the head region. Those which become components of
the lateral line organs sink into grooves, which extend from the ear region
both craniad and caudad. There is evidence to suggest that neuromasts
originally had a metameric arrangement, but this metamerism is usually
lost through the subdivision and multiplication of the primary clusters
of sensory cells.

In cyclostomes and tailed amphibia each neuromast sinks into a
separate pit. In the fishes lines of neuromasts sink below the surface.
In chimaera the grooves remain open throughout life. In other groups of

Fig. 469. — Stereogram of lateral-line organs of a fish, c, lateral-line canal; In,
lateralis nerve; p, pores connecting with the exterior; 5, scales in skin; so, sense organs
of lateral line. (From Kingsley's "Comparative Anatomy of Vertebrates.")

fishes, this being the definitive condition, the grooves are converted into
mucus-filled canals with occasional pores opening to the surface.

Lateral line organs, although limited to the ichthyopsida, are of
special interest to morphologists since the evidence leads some to believe
that the nose and ear are modifications of these. The reasons for this
conclusion are more convincing for the ear than for the nose, since the
hair cells of the nose resemble primary neurosensory cells of the inverte-
brate type, not the secondary sensory neuromast cells characteristic of
lateral line organs.

Development op Lateral Line Organs

In fish embryos, the first indication of lateral line organs is a local
thickening of the ectoderm in the ear region above the first visceral
pouch. By local thickening the placode becomes extended anteriorly
and posteriorly. Later stages resemble those of phylogenesis already
described. Differentiation of the canals proceeds both craniad and
caudad. Grooves are formed in the elongated placode, which later close
over, leaving apertures at regular intervals. The epithelial cells at the

THE SENSE ORGANS

571

bottom of the groove become differentiated into clusters of neuromast
cells surrounded by indifferent supporting cells. Mucus cells also develop,
and fill the canal with their secretions. Nervous connexions are second-
ary, and are formed by neurites from neural crest cells, usually of the
vagus and facial ganglia, but sometimes of the glossopharyngeal ganglion.

MUSCLE SPINDLES

Muscle spindles form a special set of sensory nerve terminations
upon muscle fibers, by means of which we are able to sense the degree of
contraction of a muscle and the position of parts of the body. Such
neuromuscular spindles are found in vertebrates, beginning with the
Amphibia. As their name suggests, they are spindle-shaped structures
located among the fibers of a muscle, usually near where the muscle
joins a tendon.

MUSCLE FIBERS

N

DENDRITES RINGS

SPIRALS SHEATH

-.-.--/V-U.— ^

.. .... • /• -.v. -1 ■' -.■=?.<

^■■" "*"

SHEATH

Fig. 470.^A portion of a muscle spindle from a gold chloride preparation of a cat
muscle. The sensory nerve terminations take the form of spirals, or rings, or branched
dendrites. A connective tissue sheath surrounds the spindle. (Redrawn from Jordan
and Ferguson, after Ruffini.)

Each spindle usually consists of several striped muscle fibers attached
to a tendon at one end and to intramuscular connective tissue at the other.
One or more nerve fibers connect with the spindle. Each nerve fiber
terminates within the spindle in the form of a spiral encircling a muscle
fiber.

Neurotendinous spindles resemble muscular, but are connected with
tendons. These also serve to indicate the amount of muscular work and
the resistance overcome.

OLFACTORY ORGANS

There are two chemical senses, smell and taste, but distinction between
the two is difficult to draw in lower animals in which differentiated organs
are wanting. For smell and taste alike chemical substances, in order to
affect sense receptors, must be dissolved in water. This response to dis-
solved chemicals is a fundamental property of organisms, as is shown by
such facts as that an amoeba will engulf a protein particle but not a piece

572

COMPARATIVE ANATOMY

of glass, and that injurious substances swept into the gullet of a Para-
mecium cause a reversal of ciliary action. A similar sensitivity is shown
by multicellular forms. A sea anemone responds differently to pieces of
meat and of blotting paper, yet it has no specialized olfactory or gustatory
organs.

Beginning with this fundamental property of organisms, evolutionary
changes in the chemical sense have followed two paths, one leading to
the differentiation of an olfactory epithelium and the other to the forma-
tion of taste-buds. Both types of sense organ occur in aquatic animals,

and they become still further differentiated in
land animals, one responding to chemicals
suspended in the air and the other to
substances dissolved in water. Of the two,
smell is far more delicate. In us the two
senses are much confused in experience, since
much that we eat is not only tasted but also
smelled. In general we do not taste flavors
such as those of onion or coffee, but smell
them.
(^/oLut)!'"showfng^oronasS ^c may^ infer that smells and tastes are
groove (o) leading from naris not distinguished by coelenterates, since they
S's*' Complmtil! A^atSiTof ^^^ , ^ot attracted to food at a distance.
Vertebrates," after Jordan- Actinians react to food placed in the tentacles,
vermann.) I^^^ ^^^ ^^ food placed near the mouth.

The chemical sense receptors are therefore probably neurosensory cells
located in the tentacles.

Some flatworms search for bait placed at a distance and are therefore
credited with an olfactory sense. In some species, the so-called olfactory
organs are located upon tentacles, while in others they take the form of
ciliated pits near the anterior end of the body. Both tentacles and pits
have hair cells which are connected by nerves with the brain. Some tape-
worms have at the anterior end of the body a transverse ciliated groove to
which an olfactory function is ascribed. In nemerteans, a tubular pit,
the cerebral organ, which is lined by ciliated epithelium and terminates
near the brain, has been assumed to have a like function.

Annelids are believed to have two sorts of chemical sense organs,
paired ciliated "olfactory organs" near the anterior end of the body, and
sense buds formed of clusters of cells, scattered in various parts of the
body, each cell terminating in a sensory bristle. These latter are of
special interest, since they resemble roughly the neuromasts of vertebrates.
Both cuttle fish and snails go in search of food, guided by their sense
of smell. In land snails the olfactory organs are located, not only on the
tentacles, but also over the general surface of the body. The receptors

THE SENSE ORGANS

573

are slender neurosensory cells with swollen terminations beneath the
cuticula, especially abundant at the tips of the tentacles. Some molluscs
have osphradia or "false gills," which are believed to have an olfactory
function.

The olfactory sense is so keen in land molluscs that snails were used
as gas detectors in the trenches of the world war, the animals withdrawing
into their shells in response to a
quantity of poison gas so minute as
to be imperceptible to man, yet
dangerous when its action is pro-
longed.

The olfactory organs of insects
and Crustacea have the form of
olfactory hairs or bristles on the
appendages, especially in the head
region.

That the olfactory organs of
vertebrates have evolved from any
of the many sense organs of inverte-
brates has not been demonstrated. Fig- 472.— Head of chick of s^i days,

-, . , •■Li • -i^ i_ showing development of oronasal canal.

It IS, however, possibly significant f/,chorioid fissure ;Z, thickening for lacrimal

that the olfactory receptors of chor- duct; «, nasal pit; on, oronasal groove.

1 ^ 11 1 • 1 (From Kingsley's " Comparative Anatomy

dates are neurosensory cells which, ^f Vertebrates," after Keibel.)

like those of invertebrates, spin

their own neurites, but persist as constituent elements of the olfactory

epithelium.

The olfactory epithelium of most vertebrates is of the simple columnar
type, in which neurosensory cells are uniformly distributed among sup-
porting non-nervous epithelial cells. Each receptor terminates on the
surface in a brush of fine hairs, and is prolonged from its basal end in a
neurite, which breaks up in numerous telodendria within the olfactory
bulb. As a special modification in some fishes and amphibia, the sensory
cells are aggregated into olfactory buds, separated from one another by
intermediate areas of indifferent non-sensory cells. In the higher verte-
brates, however, the sensory cells have their original uniform distribution
in the olfactory epithelium.

Olfactory organs seem to be lacking in the protochordates, yet these
respond to chemical stimulation, so that they must be assumed to have
chemo-receptors.

Amphioxus also lacks a specialized olfactory organ and does not hunt
for its food, but lies buried in the sand. Koelliker's pit, which was
formerly assumed to be an unpaired olfactory organ, is the remnant of the
larval neuropore, and is non-nervous. Hatschek's pit, in the roof of the

574

COMPARATIVE ANATOMY

mouth is not nervous, as was once assumed, but glandular. A chemical
sense, however, amphioxus indisputably has, since it responds to chemical
stimulation. Neurosensory cells occur in clusters on the oral cirri and
scattered over the surface of the body, and these may be assumed to be
the chemoreceptors.

In cyclostomes, as in all the higher vertebrates, are paired olfactory
pits innervated by olfactory nerves. The erroneous notion that cyclo-
stomes are monorhinal is based upon the fact that the olfactory pits open
into the median unpaired hypophysis, the external opening of which lies

on the dorsal side of the snout. This
evidence is, however, misleading, since
the monorhinal condition in the
embryo results from the fusion of
primarily paired anlagen, and the
olfactory pits develop as paired organs
connected with paired olfactory lobes
of the brain. The proximity of the
olfactory pits and their connexion
with the hypophysis may be explained
as the result of the great enlargement
of the upper lip of the sucking mouth.
The connexion of the olfactory pits
with the pharynx by way of the
hypophysis in myxinoids is not to be
compared with the paired posterior
nares or choanae of air-breathing
vertebrates. Paired narial passages
are not present in most fishes and
make their first appearance in dipnoi.
The olfactory epithelium of cyclo-
stomes is a many-layered ciliated
membrane, beset with neurosensory
cells like those of worms. By the
folding of this epithelium, as in fishes, the number of hair-cells is multiplied,
and the sensitiveness of the organ correspondingly increased. In petro-
myzon the bellows-like action of the pharyngeal muscles forces water in
and out of the hypophysial pit as if it were a pipette.

In elasmobranchs the olfactory organs are paired pre-oral pits, lined
with plicated olfactory epithelium, and as in other fishes generally, uncon-
nected with the mouth. In some genera such as Pristiurus, however,
nasobuccal grooves extend from the nasal pits to the corners of the mouth.
Morphologists see in these grooves the beginnings of the narial passages,
which in higher vertebrates, beginning with the dipnoi, connect the nasal

Fig. 473. — Head of human embryo
with pharyngeal floor removed. Cut
surfaces lined. Compare with Fig.
471; b, lung; cs, cervical sinus; e, eye;
h, hyoid arch; hd, hypophysial duct
(Rathke's pocket) ; /, lung, Ig, lacrimal
groove; md, mandible; n, naris; on,
oronasal groove; tr, trachea. (From
Kingsley's " Comparative Anatomy of
Vertebrates," after Hertwig.)

THE SENSE ORGANS

575

pits with the pharynx. As a special adaptation to aquatic life the nasal
pits of elasmobranchs become incompletely divided by a transverse fold
of skin, so that as the fish swims, water flows into the anterior and out of
the posterior opening.

The dipnoi take an important step forward in the evolution of the nose
by accjuiring a connexion between the olfactory pits and the mouth.
True choanae opening into the pharynx first appear in this group, and
thus the functions of smelling and breathing become associated. In the
embryo, the narial passages are formed by the closure of the edges of
paired nasobuccal grooves, such as are seen in adult elasmobranchs.

ECTOTURBINALS:

ENDOTURBINALS.
SEPTUM.

Fig. 474. — Diagrams of cross sections of the narial passage in A, Ruminant and B,
Man, showing the contrast in the number of turbinal bones. Since olfactory epithelium
covers these bones, the keenness of the sense of smell is proportional to the number and
size of turbinals. Compared with many mammals the sense of smell in man is
degenerate.

The connexion between olfactory organs and respiratory passages
which was invented by dipnoi, is also found in amphibia; but the lamellated
olfactory epithelium disappears as unsuited to life in the air. Narial
passages enlarge in amphibia, and become divided into a more dorsal
olfactory region and a more ventral respiratory passage. Olfactory hair-
cells are limited to the upper region. A supplementary olfactory organ,
the organ of Jacobson, arises in connexion with each narial passage and
thus with the mouth. It may serve to test the food taken into the mouth.
A lacrimal duct from each eye opens into the narial passage, and serves to
moisten the olfactory epithelium.

Some reptiles, lizards for example, have added to the narial passage a
more expanded and glandular vestibule, which is apparently a mechanism
for eliminating dust from the air taken into the lungs. Novel also in this
group are the paired turbinal bones or conchae, which project into the
nasal passages, and serve to increase and support the olfactory membrane.
A nasopharyngeal cavity distinct from the mouth cavity also first appears
in this group, as the result of the ingrowth and extension of the palatine

576

COMPARATIVE ANATOMY

bones. Thus a bony palate is formed, and the narial passages open by
secondary choanae into the pharynx. The vomer bone, therefore, no
longer lies in the roof of the mouth, but in the naso-pharyngeal passage.
By the formation of the palate, the nasopharyngeal cavity is both enlarged
and elongated. (Fig. 183)

In birds there are three pairs of conchae, and Jacobson's organ
disappears.

In mammals there is an enormous enlargement of the narial passages,
and a corresponding multiplication of conchae, the single pair of reptilian
conchae persisting as the maxillo-turbinals, and ethmo-turbinals, upper

CORPUS CALLOSUM

CYRUS DCNTATUS

MAMMILLARY BODY
TUBER CINEREUM

Fig. 475. — A diagram of the nervous relations of the olfactory system of fibers.
(Redrawn after Rasmussen's "Principal Nervous Pathways," The Macmillan Co.)

and lower, being added. Jacobson's organ nearly disappears in the higher
mammals. The contrast between the upper olfactory and the lower
respiratory region persists.

That the olfactory organ of man is degenerate, is evidenced by the
reduction of the conchae in size and number, and in the ontogenetic loss
of three pairs of turbinal bones present in the embryo. What is left of
Jacobson's organ in man enters into the formation of the incisive canal,
which connects the anterior part of the nasal passage with the mouth
cavity. (Fig. 474)

Development of Olfactory Organs

In elasmobranchs, each olfactory organ develops from a placode-like
thickening at the anterior termination of the series of lateral line organs;
Subsequently each placode, by invagination, is converted into a pit, which
lies in close apposition to the telencephalon vesicle. In man a groove
similar to the nasobuccal groove of elasmobranchs connects each olfactory

THE SENSE ORGANS

577

pit with the corner of the mouth. The narial passages, however, are not
formed by the fusion of the edges of this groove, as in some amphibia and
fishes, but by the backward extension of the olfactory pits, which acquire
a secondary connexion with the mouth. Hare-lip in man results from
the imperfect obliteration of the nasobuccal groove. The primary
openings of the narial passages into the mouth correspond with those of
amphibia and reptiles. The secondary and definitive choanae rise poste-
rior to the primary pair, when paired palatine processes unite in the middle
of the roof of the mouth, and thus separate the naso-pharyngeal cavity

NASO-OPTIC.
GROOVE

NASO-BUCCAL
GROOVE

MANDIBULAR
PROCESS

DOUBLE HARE-LIP

Fig. 476. — The development of the narial passages in A, Chick and B, Man. In
phylogenesis the narial passages are believed to have arisen through the approximation
and closure of the edges of the nasobuccal grooves. Such grooves appear in ontogenesis.
Failure of such grooves to close over is the best explanation of hare-lip and perforate
palate shown in B. In normal growth in the human embryo, however, the narial
passages are not formed by the closing over of grooves but by the backward growth of
an ectodermal cord which grows from the nasal pit to the mouth cavity. (A redrawn
after B. Patten, and B after Corning.)

from the mouth cavity. Palatine processes appear first in a two-months
human embryo, and the formation of the palate is completed at five
months.

The inferior or maxillary conchae arise early in ontogenesis. Five
more pairs are formed by outgrowth from the ethmoid. Of these, three
later disappear, leaving the maxillary and two ethmoid characteristic of
the adult.

TASTE ORGANS

The second of the chemical senses is taste which, as we have seen, has
a common origin with smell. All animals respond in one way or another
to substances dissolved in water. Specialized taste organs in the form

578

COMPARATIVE ANATOMY

of taste-buds occur in annelids, in which they are scattered over the
surface of the body, but are more numerous in the mouth region; and
experiment demonstrates that leeches can taste. In molluscs, neurosen-
sory cells connected with subcutaneous ganglia surround the mouth. A
few insects have gustatory hairs or bristles on the antennae, and some
have them around the mouth. With the exception of the taste-buds,
none of the invertebrate sense organs appears to have a genetic relation
with those of chordates.

The organs of taste of chordates are taste-buds, with a core of sensory
hair cells and an outer rampart of supporting cells. Unlike the cells of
the neuromasts, however, both sensory and supporting cells of taste-buds
are of equal length, and the sensory as well as the supporting cells rest

SENSORY
tPELLS

(SENSORY BRrSTLE

SENSORY
SENSORY BRISTLE ^^'■'^

.SENSORY BRISTLE

'rEPIOERMIS

NERVE FIBERS

A. ANNELID B. FISH C. MAN

Fig. 477.— Diagrams of taste-buds in A, Annelid, B, Fish, and C, Man, showing
their fundamental similarity. (Redrawn after Kahn's "Das Leben Des Menschen,"
W. Keller & Co.)

upon the basement membrane of the epidermis. Each hair cell of a
taste-bud is a secondary sense cell, which is supplied by the dendritic
terminations of sensory nerves. Taste-buds differ little in lower and
higher vertebrates, though tending to be less widely distributed over the
bodies of higher forms. (Fig. 477)

In amphioxus, clusters of hair-cells connected with sensory nerves
occur in the velum and the oral cirri. A chemical sense, whether smell
or taste would be difficult to say, may therefore be ascribed to this animal.
That similar sense organs are generally distributed over the surface of the
body has not been demonstrated. Consequently, if the arrangement of
taste-buds in amphioxus may be taken as primitive, the chordates start
their phylogenesis with a high degree of concentration in the distribution
of their organs of taste.

Pharyngeal taste buds occur in larval cyclostomes, while in the aciult
they are present also on the surface of the head. In elasmobranchs the
taste-buds are limited to the lining of the mouth and pharynx, chiefly
on the surface of papillae. In ganoids and teleosts they are found on the

THE SENSE ORGANS

579

surface of the head as well as on the pharynx. A few teleosts such as
amiurus have taste buds on the surface of the trunk, fins, and mouth.

In amphibia, with the assumption of a land life, taste buds become
limited to the tongue and the roof of the mouth. In man, they are found
on the tongue, especially on the sides of the vallate papillae, upon the soft
palate, and upon the posterior surface of the epiglottis.

A taste-bud is an ovoid cluster of columnar epithelial cells, each of
which extends from the basement membrane to the free surface of the

EPIGLOTTIS.

FOLIATE
PAPILLAE

Fig. 478. — The dorsal surface of the tongue. The sulcus terminalis divides the
body or apex of the tongue from the root. The two regions have a different embryonic
origin. (Redrawn after Sobotca.)

epithelium. The cells of the outer layer are arranged like the sections of a
melon, so that the ends of the cells are brought close together around a
small pore, which opens on the surface. Two kinds of cells are differen-
tiated, peripheral supporting cells, and sensory taste-cells, which form
the core. The sensory cells stain more intensely, and are distinguished
by the bristle-like process in which each cell terminates. (Fig. 99)

To stimulate the taste cells, substances must be dissolved, so that they
can penetrate the pore-like opening of the bud. Only four kinds of sub-
stances can be tasted by us, sweet, sour, bitter, and salt. Other flavors
are smelled, not tasted.

580 comparative anatomy

Development of Taste Organs in Man

The lingual papillae upon which the taste buds develop first appear in
a nine weeks embryo as minute elevations on the surface of the tongue.
The formation of circumvallate papillae involves not only similar eleva-
tions of the surface, but also the ingrowth of the epidermis around each
papilla. By the subsequent splitting of this ring of epidermis into an inner
and outer layer separated by a circular groove, the papilla becomes vallate.
The taste buds are differentiated on the sides of the papillae, which are
covered by a stratified epidermis of endodermal origin. Each taste bud
appears primarily as a local thickening of the epidermis, due to the elonga-
tion of the cells which are to become the sensory and supporting cells.
During the fifth to the seventh month of fetal life, taste buds are more
numerous than later, and occur very generally over the surface of the
mouth and pharynx. Later the number diminishes and each taste bud
requires an external pore opening to the surface.

VISUAL ORGANS

Sensitivity to light is a wide-spread, if not a universal, property of
living cells. An amoeba will not enter strong light; and sea anemones,
among other creatures, react to light, although they have no specialized
photoreceptors. All flagellates which are phototropic have a red pigment
spot or stigma like that of Euglena; and in general, in multicellular crea-
tures, only those cells respond to light which contain some such pigment
as visual purple, which is altered by light.

The so-called eye-spots of coelenterates are pigment cells which
respond to changes in intensity of light. Many medusae have clusters of
pigmented columnar epithelial cells on the margin of the umbrella, which
are interpreted as photic organs, since the animal ceases to be phototropic
when these are removed. The jelly-fish Nausithoe has a lens associated
with each of these pigment spots.

Many species of flatworms have paired pigment spots closely associated
with the brain, which are interpreted as eyes. In some species, beaker-
like clusters of pigmented cells surround the terminations of sensory nerves,
and may, therefore, be regarded as among the most primitive of true eyes.

While, however, photoreceptors occur in coelenterates and all phyla
above, true vision is limited to those forms in which the photoreceptors are
aggregated into eyes capable of forming an image on a retina. Among
invertebrates two types of eyes predominate, beaker eyes and vesicular
eyes. Beaker eyes get their name from their shape. They usually, have
a core or plug of cuticula. Vesicular eyes, however, have a liquid-filled
cavity around which the photoreceptor cells are arranged.

The eyes of annelids are varied. Some free-swimming forms have
beaker-eyes with spherical lenses and a layer of retinal cells connected by

THE SENSE ORGANS

581

nerve fibers with the brain. In some, the beaker-like eyes are connected
with the epidermis; in others the eye sinks below the skin. Nereis has
vesicular eyes. The differences between the various types of annelid eyes
are so great that it is impossible to believe that they are genetically related
to one another.

The eyes of molluscs are sometimes beaker-eyes, sometimes vesicular
eyes with a lens. On the edge of the mantle of pecten, are complex vesic-
ular eyes, which have many features in common with those of vertebrates.
Each eye is partly surrounded by a layer of opaque pigmented epithelium,
which in front of the eye becomes a translucent cornea. Beneath the

CARTILASE

HUSCUE

Fig. 479. — A diagram of median sections of the eyes of vertebrates and cuttlefish.
While the two types of eye resemble one another in many fundamental characters, the
retina of the vertebrate eye is inverted, while that of the cuttlefish is not. Bergson
has challenged the assumption of Darwinians that two eyes of such divergent origin
could by chance variations alone come to resemble one another so closely.

cornea and adherent to it, is a biconvex lens. Like the vertebrate eye,
that of Pecten has a liquid-filled chamber. But the retina of Pecten,
unlike that of vertebrates, has two layers of photoreceptors, each photo-
receptor connected with a nerve fiber. Back of the retina lies an inner
layer of pigment epithelium. There is no reason to doubt that such an eye
is able to form an image.

The eyes of cephalopods, especially those of sepia and loligo, bear a
striking resemblance to those of vertebrates. But the fact that the retina
is not inverted as in vertebrates, proves that the two cannot be genetically
related. Bergson has cited this instance of similar results, notwith-
standing diverse conditions, as proof of the existence of a vital factor or
"elan vital" which distinguishes the living from the lifeless. Since Berg-
son appears to be substituting one mystery for another, the elan vital
hypothesis has not made a very strong appeal to biologists. Nevertheless,
it is equally difficult to accept the assumption that eyes so alike could
have been independently produced merely by the selection of random
variations of so many elements as enter into the composition of cephalopod
and vertebrate eyes.

582 COMPARATIVE ANATOMY

Urochordates have pigment spots and an unpaired eye, neither of
which can be compared with those of vertebrates. Such forms as salpa
have a median vesiculated eye derived from the brain and usually pig-
mented. Only in their origin from the brain do such eyes resemble those
of vertebrates.

Among cephalochordates the so-called eye of amphioxus is a pigment
spot located at the anterior end of the nerve cord. There is obviously
very little resemblance between such a structure and the eye of a verte-
brate. The reasons for calling it an eye are that pigment is associated
with eyes, and that this special pigment spot is associated with the anterior
expansion of the nerve cord, which is generally homologized with the fore-
brain of vertebrates. On the basis of such slight resemblance, it is
scarcely possible to derive the paired vertebrate eyes from this unpaired
pigment spot.

In the floor of the nerve cord of amphioxus, throughout its length, are
photoreceptors partially enclosed by pigment capsules. Since removal
of the pigment spot from the so-called brain of amphioxus does not affect
its response to light, it is assumed that the true light-recipient organs of
this animal are these photoreceptors of the nerve-cord.

The study of invertebrates reveals that their eyes have all the histolog-
ical elements of the paired eyes of vertebrates, but never in the same
combinations. Some invertebrates have photoreceptors in the form of
rods and cones associated with pigment spots and with the brain. But
vertebrates alone have eyes with an inverted retina formed as an out-
growth of the brain wall, and surrounded by a mesenchymatous capsule.
Comparative anatomy throws little, if any, light upon the past history
and origin of paired eyes, since the eyes of the cyclostomes are in all essen-
tials like those of the highest vertebrates. Eyes, therefore, appear to
spring into existence full-formed, and we are compelled to draw phylo-
genetic conclusions from the facts of ontogenesis.

These facts appear to justify the conclusion of Ray Lankester (1880)
that paired eyes of vertebrates are paired pigmented depressions in the
anterior part of the neural plate. It has been asserted that the parietal
eye has a similar paired origin, from pits anterior and lateral to those which
form the paired eyes; and the conclusion has been drawn that, when the
neural plate was converted into a neural tube, the unpaired eye was
formed by the fusion of the lateral paired pits, which subsequently grew
out as a stalked vesicle. Thus the parietal eye looks upwards, while the
paired eyes, bulging laterally from the brain wall are receptors of light
from the sides and below. In most vertebrates, the median eye degen-
erates ; but the paired eyes enlarge and become the definitive organs of vision .

It is fairly easy to imagine the conditions which led to the lateral
outgrowth of the paired eyes. Among the factors involved was presum-

THE SENSE ORGANS

583

ably the enlargement of the head and the recession of the brain from the
surface. Less light, consequently, would reach the photoreceptors in the
brain wall. When the skin became pigmented, eyes in the brain wall
would become useless.

On the other hand, we are still painfully ignorant as to the factors
which would start the growth of the eyes towards the skin and convert
the skin into a lens. Possibly the invagination of the optic cup was
primarily determined by the enlargement and ingrowth of the lens. Yet

NEURAL
TUBE

PHOTORECEPTOR CELLS'

Fig. 480. — Diagrams illustrating Boveri's theory that the paired eyes of vertebrates
have evolved from lateral outgrowths of the brain-wall. The theory accords well with
embryological evidence.

experiment shows that lens formation in the embryo is stimulated by the
optic vesicle, in the absence of which no lens develops, and that if the
optic vesicle is removed from the brain and grafted under the skin of
the trunk, it will cause a lens to develop there.

As the eye became a two-layered cup enclosing a lens and surrounded
by a connective tissue capsule, the sclerotic, it acquired nerve connexion
with other parts of the brain. The eye muscles became attached to the
eyeball and were preserved, while other pre-otic muscles disappeared.
That the lens is a modified lateral line organ has been plausibly urged
both from its position of origin and from its mode of development.

The phylogenesis of the paired eyes of vertebrates may be briefly
summarized. First these were open beaker eyes, like those of such inverte-
brates as nautilus, with photoreceptors towards the source of light and
with pigment, but without lens. Later by invagination the eyes were

584

COMPARATIVE ANATOMY

carried into the anterior brain vesicle, so that the retina became inverted
and each eye grew in the form of a hollow ball towards the skin and the
source of light. The photoreceptors, however, come to lie on the side of
the retina farthest from the source of light, and hence are "inverted" in
position. Possibly a lateral line organ was converted into a lens, which by
enlarging and pushing into the optic vesicle, converted this into a two-
layered cup with an inner retinal, and an outer pigment layer. Finally,
the surrounding mesenchyma was converted into a sclerotic layer and the
eye-ball thus formed became connected with the eye muscles.

The paired eyes of all vertebrates are essentially similar. Except in
size, the eye of petromyzon is like that of mammals. Having once
invented a camera-like eye in cyclostomes, "nature" has pursued the
policy of letting well enough alone. A description of the human eye will,
therefore, serve for vertebrates generally.

CCONJUNCTIVA.

FOVEA CENTHALIS

Fig. 481. — A diagram of a median section of the eye. (Redrawn after Sobotta.)

The Eye of Man

Comparison of the eye with a camera holds for many details. Both are
mechanisms by which an image is focused upon a sensitive layer, the film
or the retina, the opaque sclerotic corresponding with the box of the
camera. Each has a lens to form the image, and an iris diaphragm to
regulate the amount of light. The eye, however, is filled with liquid,
the humors of the eye. That between the translucent cornea and the
lens is the aqueous humor ; the semisolid material between lens and retina
is the vitreous humor. A ray of light entering the eye passes first through
the cornea, then successively through aqueous humor, pupil, lens, vitreous

THE SENSE ORGANS

:)"D

humor, and retina. The curved cornea serves to converge rays of light,
and the lens increases the convergence.

The lens is a biconvex translucent and elastic body surrounded by an
elastic capsule. Fibers which extend from the periphery of the lens to the
ciliary body form a suspensory ligament which holds the lens in position
back of the iris. The ciliary body is a ring of vascular and muscular
tissue, wedge-shaped in cross section, which projects into the cavity of the
eye-ball, just back of the iris. The ciliary muscles, circular and meridional,
act on the suspensory ligament of the lens, and accommodate the eye for

RADIAL MUSCLE FIBERS

CILIARY BODY'

-CIRCULAR MUSCLE

LENS-
PUPIL-T-~-l

CORNEA— \^

SUSPENSORY LIGAMENTS

CIRC. MUSC. CONTRACTED

■>— CILIARY BODY

RADIAL MUSCLE FIBERS

Fig. 482. — A diagram illustrating the mechanism of accommodation of the eye by-
means of the change of shape of the lens. The lens is flattened through the tension of
the suspensory ligaments. When the circular muscle of the iris contracts tension is
relaxed and the lens assumes a more spherical shape as a result of its natural elasticity.
A shows the lens adapted for distant vision, B the accommodation of the lens for near
vision. (Redrawn after Kahn's "Das Leben Des Menschen," W. Keller & Co.)

near or far vision. When the meridional fibers contract, the suspensory
ligament tightens, and flattens the lens for distant vision. When the
circular muscle contracts, the ligament is loosened, and the lens by its
own elasticity becomes more nearly spherical. When, in later life, this
elasticity is lost, we put on convex glasses.

The iris is a muscular diaphragm, usually pigmented, which projects
from the ciliary body into the cavity of the eye, between lens and cornea,
and is perforated at its center by the pupil. It has two sets of muscles,
a radial and a circular, which are controlled by the sympathetic system.
In dim light, the radial muscles contract and the pupil enlarges. In
strong light, the circular muscles contract and reduce the opening.

586

COMPARATIVE ANATOMY

The retina lines the posterior chamber, but thins out where it covers
the ciliary body. In the region where light strikes the retina, at least
eight layers are distinguishable. Beginning with the innermost, these
are: a layer of nerve fibers, a ganglion cell layer, an inner reticular layer,
an inner nuclear layer, an outer reticular layer, an outer nuclear layer, a
layer of rods and cones, and a pigment epithelium. (Fig. 483)

By using special nerve stains, such as that of Golgi, the retina has been
found to consist of three layers of neurones chained together and con-
nected with the brain by the optic nerve. One set of neurones, the rods
and cones, is the true sensory layer. The rods are sensitive to light of
low intensity; the cones are affected by light waves of different lengths.

Fig. 483. — Mammalian retina; above the general appearance, below the diagram-
matic relations; the lens toward the left, c, cone; cc, cone cell; g, ganglion cells; ig,
inner granular layer; im, inner molecular layer; m, basal membrane; nf, nerve fibres;
og, outer granular layer; om, outer molecular layer; r, rod; re, rod cell. (From Kingsley's
"Comparative Anatomy of Vertebrates.")

and are regarded as the mechanism of color vision. There are in the
human retina four times as many rods as cones. Both rods and cones
have synaptic connexions with the neurones which form the inner nuclear
layer, and these in turn are connected with the dendrites of the ganglion
cell layer, the neurites of which form the optic nerve.

When the retina is flooded by intense light processes of the cells of the
pigment epithelium penetrate between the rods and cones. In the dark
these processes are withdrawn. When the eye is focussed upon an object,
the region of sharpest vision is a small spot near the center of the retina,
the yellow spot, or macula lutea. At its center, is a depression, the fovea
centralis, where the nuclear and reticular layers are absent and the retina
is thin. The greater sensitivity of this area is, therefore, due to the fact
that light strikes the rods and cones without passing through the other
layers.

A vascular chorioid layer surrounds retina and pigment epithelium.
Besides many pigment cells, this layer contains many blood vessels.

THE SENSE ORGANS

587

branches of the central retinal and of the anterior and posterior ciliary

arteries, and veins. The retina, however, is supplied by its central

PALLIUM

rECTUM OPTICUM

'CEREBELLUM

MAMMAL

Fig. 484. — A diagram illustrating the course of optic nerve fibers in A, a fish, B, a
reptile, and C, a mammal. The definitive centers of vision lie in the occipital lobes
of the cerebral hemispheres. The primitive centers in the optic lobes persist in mam-
mals as reflex centers located in the superior colliculi. (Redrawn after Monakow.)

vessels. Most of the blood leaves the eye ball by way of four to six
vorticose veins.

The ganglion cells of the retina give off neurites which converge towards
the "blind spot," where the optic nerve fibers leave the retina, and rods
and cones are absent. Of the fibers of the optic nerve, half pass to the

588 COMPARATIVE ANATOMY

thalamus of the opposite side of the brain and half to that of the same side.
The sensory centers of vision are located in the occipital lobes.

The outer layer of the eyeball is the sclerotic or tunica fibrosa, com-
posed of interwoven bundles of compact connective tissue fibers. This
layer is opaque, except the portion which forms the cornea. A many-
layered epithelium or conjunctiva covers that part of the cornea which is
exposed to the air, and also lines the eyelids.

Eyelids occur only in land forms, as an adaptive device for protecting
the eyeball. They are folds of skin above and below the eyes, movable
except in serpents. They are opened by levator and closed by the
orbicularis muscle. The upper lid is more movable than the lower. Along
the edges of both lids, a series of sebaceous Meibomian or tarsal glands
form a film of oil, which keeps the tears from flowing over the lower lids.

The lacrimal glands are compound tubular glands, located at the upper
border of the upper eyelid. Their secretions are poured into the conjunc-
tival sac, and are carried over the surface of the eyeball to the lacrymal
ducts, one in each eyelid, near the nose. The two lacrimal ducts of each
side unite to form the nasolacrimal duct, which opens into the nasal
passage below the inferior concha of the same side.

Development of the Vertebrate Eye

The vertebrate eye has a twofold origin. The retina, optic nerve and

pigment epithelium are derived from the brain, and therefore from the

^^.^gr232iiEi7>^ ectoderm. The lens and conjunctiva are

^^^^^^^^^^^^1 j and the connective tissue surrounding the
Mm^I^^^^^^^H I eye-ball, all come from the mesenchyma.

Fig. 485. — Stereogram of de- branchs and amphibia, the anlagen of the
;rc«rw:,rof ftt'lll.'LTi °P''^ resides appear as paired depressions
of lens; oc, optic cup; os, optic of the ncural plate. As the optic vesicles
•S^l ',!^"Jn",t"er'tFrom continue their lateral expansion, the con-
Kingsiey's " Comparative Anatomy nexion with the brain becomes constricted

of Vertebrates.") ^^ ^^ ^p^.^ ^^^^j^^ ^^^^^ ^^^^^ -^ converted

into a shallow trough to guide the fibers of the optic nerve as they grow
from the retina to the brain wall.

THE SENSE ORGANS

589

Where the optic vesicle, as it expands, comes into contact with the
ectoderm, a local thickening of the ectoderm forms as the anlage of the
lens. This placode thickens, invaginates, and sinks below the surface to
form a small hollow vesicle, which eventually loses connexion with the
skin. The lens vesicle is finally converted into a solid lens by the thicken-
ing of its medial wall, the epithelial cells of which become elongated into
fibers and arranged in layers like the coats of an onion.

Fig. 486. — Sections of successive stages in the development of the lens of the eye
from the first thickening of the ectoderm (ec) to the complete separation of the lens, /.
(From Kingsley's "Comparative Anatomy of Vertebrates.")

While the lens is undergoing these changes, it becomes enclosed by
the optic cup, formed by invagination of the optic vesicle. Possibly the
ingrowth and enlargement of the lens is a factor in this invagination. The
optic vesicle is thus converted into a two-layered optic cup, attached to
the brain by the optic stalk. The cup is, however, incomplete, for a fissure

CONJUNCTIVA-
CORNEA-
AQUEOUS HUMOR-
IRIS

WITREOUS BODV

Fig. 487. — Diagram illustrating the growth of optic nerve fibers from the retina
along the optic stalk into the brain. Some of the fibers cross below the brain to form
the optic chiasma, while the remaining pass into the thalamus of the same side.

divides it along its lower side, and extends as a groove along the lower side
of the optic stalk.

Of the two layers of the optic cup, the outer persists as the single-
layered pigment epithelium, and the inner thickens to form the three
layers of neurons of the definitive retina. Neurites from the inner
ganglion cell layer grow along the optic stalk and gradually fill its groove.
Half of these fibers cross to the opposite side of the brain to form the
chiasma of the optic nerve. The cells of the optic stalk are converted
into neuroglia cells.

59© COMPARATIVE ANATOMY

The remaining parts of the adult eye are derived from mesenchyma.
The cellular elements of the aqueous humor, and possibly also some of
those of the vitreous body are mesenchyme cells which enter the optic
cup by way of the optic fissure. But the fact that fibers connect the lens
and retina has led some investigators to infer that the vitreous body is
derived from the retina and therefore is ectodermal. On the outside of the
eye, the chorioid and fibrous tunics are added from the surrounding mesen-
chyma. The eye muscles of man have a like origin, although in lower
vertebrates they arise from the walls of the head "cavities." Folds of
skin form the eyelids, which unite temporarily but in man separate again
before birth.

Unpaired Eyes

Two kinds of median eyes are recognized, the parietal and the pineal.
Each, when present, arises from the roof of the diencephalon, and lies
beneath an unpigmented "apical spot" on the upper surface of the head.

That parietal and pineal eyes are light-recipient organs is indicated by
the presence of a retina with photoreceptors, pigment, ganglion cells,
nervous connexions with the brain, and sometimes a lens. Experimental
evidence is somewhat conflicting. But lizards show a muscular response
to photic stimulation of the parietal organ, and teleosts react to stimulation
of the pineal organ.

The history of the median eyes of vertebrates is one of reduction and of
functional change. Among the possible factors in the degeneration of these
organs may be the increased importance of the lateral eyes and the great
enlargement of the cerebral hemisphere, which in mammals overlie the
diencephalon.

Cyclostomes have both parietal and pineal organs, in the form of
epithelial vesicles immediately below an unpigmented "apical spot"
between the lateral eyes. The pineal vesicle, which lies above the parietal,
is considerably larger, and its upper epithelium is thin and translucent,
while its lower layer is thickened, and consists of columnar epithelial
cells, some of which contain pigment. For this reason, the lower layer is
regarded as a retina. This inference is supported by the fact that the
photoreceptors of the retina are supplied with nerve fibers from adjacent
ganglion cells. Neurites of these ganglion cells unite to form a pineal
nerve, the fibers of which may be traced by way of the posterior commis-
sure to the mesencephalon.

The lower or parietal vesicle resembles the pineal vesicle in essentials,
but rests upon a segregated portion of the left habenular ganglion. The
nerve fibers of the parietal nerve connect with the left habenular ganglion.
Whether parietal and pineal organs are serially homologous, one anterior
and the other posterior, or whether they are members of a pair which

THE SENSE ORGANS

591

have in time shifted their position to the present antero-posterior arrange-
ment, remains a disputed question. That they were formerly paired

PARAPHYSIS SUPERIOR

A CYCLOSTOME HABENULAR
/\. «_T(_L.<JilUME. COMMISSURE.

PINEAL TRACT

B. ANURAN

PARIETAL EYE

PINEAL ORGAN

CORIUM^EPII^ERMIS

M I DBRA I N
iPOSTERIOR COMMISSURE

DORSAL SAC— &^

SUPERIOR HABENULAR/

COMMISSURE

POSTER I C
COMMISSURE

D. MAMMAL

MIDBRAIN

C REPTILE

Fig. 488. — Parietal and pineal organs of vertebrates as seen in median longitudinal
section. A, Cyclostome; B, Anuran; C, Reptile; D, Mammal. Both parietal and pineal
organs have served as median eyes. In amphibia it is the pineal organ, in reptiles the
parietal organ. In man and mammals the pineal organ becomes a gland of doubtful
function. (Redrawn from Oppel, after Studnicka.)

organs is suggested by the fact that in geotria, a cyclostome, each has a
lateral position in relation to the other ; and the parietal organ has nervous

Fig. 489. — Diagrammatic longitudinal section of brain, ac, anterior commissure
in lamina terminalis; aq, aqueduct; c, cerebral hemisphere; cb, cerebellum; cp, chorioid
plexus; cs, corpus striatum; cv, cerebral ventricle; h, hypophysis; he, habenular com-
missure; i, inf undibulum ; ip, inferior chorioid plexus; m, mesencephalon; ml, myelen-
cephalon; p, pinealis; pa, paraphysis; pc, posterior commissure; pe, parietal eye; v,
anterior medullary velum; vl, velum trans versum with aberrant commissure; III and
IV, third and fourth ventricles. (From Kingsley's "Comparative Anatomy of
Vertebrates.")

connexion with the left habenular ganglion, while the pineal organ is con-
nected with the right habenular ganglion; and also by the fact that the
anlagen of the two in the embryos of fishes, amphibia, and reptiles lie in

592

COMPARATIVE ANATOMY

the same cross section. On the whole, these facts tend to suggest that
cyclostomes have a right pineal eye and a left parietal eye. In myxine,
however, the pineal organ is wanting.

The median eyes of fishes are degenerate. The parietal organ is
lacking, and the pineal does not perforate the skull. A tendency for the
organ to become glandular is shown by the folding of the wall of the vesicle.
In the elasmobranch embryos, the anlage of the parietal organ appears
but later degenerates.

Among the amphibia, the anura have a translucent "apical" spot like
that of cyclostomes. Beneath this spot, lies the vesicle of the pineal eye,
connected by a nerve with the brain. The basal layer of the vesicle forms

Fig. 490. — Parietal eye of Anguis fragilis. ct, connective-tissue cells around nerve;
gc, ganglion cells; I, lens; n, nerve fibers; pn, parietal nerve; pc, pigment cells; r, retinal
cells; vb, vitreous body. (From Kingsley's "Comparative Anatomy of Vertebrates"
after Nowikofl.)

a thickened retina. A parietal organ is wanting. In the fossil stego-
cephala, the roof of the cranium is perforate between the two parietal
bones and from this a functional pineal eye in these animals is inferred.

In reptiles the tendency of the epiphysis to become glandular is evident.
But in crocodiles, the pineal organ wholly disappears. The parietal organ
of this group assumes even more markedly than in the lower vertebrates
the characteristics of the vesicular eye of invertebrates. In sphenodon
and many lizards, the upper layer of cells of the vesicle forms a lens-like
thickening, which focusses light upon the retina. The skull is perforate.

Among the reptiles the parietal eye of anguis fragilis is most highly
differentiated. The terminal vesicle in this animal Hes immediately
below a translucent apical spot, and is connected with the left habenular
ganglion by a ganglionated parietal nerve. The vesicle is lined by
columnar epithelial cells, which become locally thickened on the upper

THE SENSE ORGANS 593

side to form a biconvex lens, which concentrates light upon the retinal
layer below. Three kinds of cells are distinguishable in this retinal layer,
sensory flagellated cells, pigment cells alternating with the sensory cells,
and ganglion cells connected with the brain by the parietal nerve. Except
in the region of the lens, the entire vesicle is surrounded by a single layer
of pigment epithelium. The lumen of the vesicle is filled with translucent
processes of the surrounding epithelial cells, and thus is analogous with the
vitreous body of the lateral eyes.

In mammals a parietal organ is wanting. The pineal organ is glan-
dular. Its function is problematic, but is commonly assumed to be

Fig. 491. — Section through brain of a Squalus embryo, 15 mm. long, showing the
early appearance of two epiphysial structures, the parietal organ {p) which is already
degenerating, and the pinealis (e) ; h, hypophysis, growing in from the oral epithelium;
i, inf undibulum ; n, notochord; v, velum transversum. (From Kingsley's " Comparative
Anatomy of Vertebrates.")

endocrinal. The rich vascular supply of the organ suggests this, but
experiments to demonstrate the secretion of hormones have failed, and
the removal of the gland does not cause death or affect health.

DEVELOPMENT OF THE PINEAL ORGAN IN MAN

The pineal organ arises in a four-months embryo as a hollow tubular
outgrowth from the roof of the diencephalon. By the formation of
secondary lateral outgrowths, the anlage is converted into a compound
vesiculated structure richly supplied with blood vessels. The vesicular
structure later disappears, and the adult organ consists of solid masses of
polygonal epithelial cells of which various sorts may be distinguished by
their different staining properties. No ganglionic or nervous elements
can be recognized. Mulberry-like concretions sometimes occur.

594

COMPARATIVE ANATOMY

STATIC AND AUDITORY ORGANS

A wide-spread and remarkable trait of animals is the ability to respond
to gravitation, and thus to orient their bodies in space. The long axis
of the body is usually kept horizontal, but occasionally, as in man, vertical.

With few exceptions, animals have
dorsal and ventral sides. But the
ventral side does not lie nearer the
earth because it is heavier, for when
a fish dies in the water it turns
ventral side up. Orientation in
relation to gravity is a reflex act,
which involves nervous and mus-
cular mechanisms and special sense
organs. Such organs are known
as static organs.

Some microscopic floating
plankton organisms are indifferent
to gravity. When these maintain
a constant position the stable
balance is due to the greater weight
of one side. Where special static
organs are lacking, orientation may
be maintained through eyes or
^. ^TTT^ u tactile organs or both. Some

Fig. 492. — Diagram of the membranous o . .

labyrinth of a vertebrate, the sensory areas crustacea, for example, Will SWim

dotted, ac, anterior semicircular canal; j^g ^^^^ when the body is

ap, ampullae; ca, cristae acusticae m the t^ a i 1, u

ampullae; de, ductus endolymphaticus; he, illuminated from below. Although

horizontal (external) canal; /. lagena; -^ organs are generally absent

ml, mn, ms, mu, maculae (of lagena, neglecta, o _ » i j

sacculi and utriculi) ; pc, posterior semi- in insectS, mOSt inseCtS, placed On
circular canal; s, sacculus; se, saccus
endolymphaticus; sue, sacculo-utricular

canal; u, utriculus

"Comparative Anatomy of Vertebrates

the back, will right themselves,
(From Kingsley's probably through the action of
^ nerves connected with tactile hairs.
The normal position of many animals such as worms brings the ventral
side of the body in contact with the bottom. When this contact is lost,
reactions tending to restore it take place. The sensory basis of this
orientation is the sense of touch. In the orientation of the mammalian
body, tactile organs, muscular spindles, and the semicircular canals of the
ear are all involved.

Among invertebrates most static organs have a hollow sac or statocyst
which contains one or more statoliths, which are granules of calcium
carbonate or sulphate mixed with organic matter. Frequently, loose
crystals or otoconia occur in statocysts, and serve by their motion to

THE SENSE ORGANS

595

stimulate the hair cells which are the sensory elements of the organ.
Grains of sand occasionally replace the otoliths.

The diversity of static organs in invertebrates is, however, so great
that we must conclude that they have been independently acquired in
the different groups. The fact that similar statocysts occur in such
diverse forms as echinoderms, annelids, and molluscs points in the same
direction.

Auditory organs have not been demonstrated in aquatic invertebrates.
Indeed, it has not been demonstrated that any fishes can hear. Fishes
respond to blows upon the surface of the water, but this may involve the

Fig. 493. — Diagram of mammalian ear. a, ampullae of semicircular canals; an,
acoustic nerve; en, cochlear nerve; em, external auditory meatus; eu. Eustachian tube;
ft, fenestra tympani; i, incus; m, malleus; p, perilymph space (black); pd, perilymph
duct; ph, pharynx; s, stapes; sc, sacculus; sg, spiral ganglion; sm, st, su, scalae media,
tympani, et vestibuli; t, tympanic cavity; tm, tympanic membrane; u, utriculus; vn,
vestibular nerve. (From Kingsley's "Comparative Anatomy of Vertebrates.")

lateral line organs or the sense of touch, not the ear. Auditory organs
may be unnecessary among animals which are themselves unable to
produce sounds.

Among invertebrates, true auditory structures are represented by the
chordotonal and tympanic organs of insects. But neither these nor any
other organs of invertebrates have a. genetic relation to the ears of
vertebrates.

Static organs occur in the free-swimming urochordates. In the larvae
of ascidia and phallusia, a static organ with ciliated sensory epithelium,
statolith, and nervous connexions projects into the brain cavity. Nothing
similar is found in vertebrates. Amphioxus lacks a static organ, and is

596 COMPARATIVE ANATOMY

quite unable to maintain its balance in swimming. Plate's suggestion
that the ciliated infundibular organ of amphioxus is a static organ has little
in its favor.

All vertebrates have a static organ, which is a novelty in this group
and not a structure inherited from invertebrate forbears. In fishes and
aquatic urodeles, the membranous labyrinth of the ear appears to be
exclusively a static organ. But in land forms, beginning with the
amphibia, the ear has the double function of equilibration and hearing.

Of vertebrates, the cyclostome myxine has the simplest static organ,
which in shape is not unlike an inflated inner tube of an automobile tire
with some inequalities of expansion. From its nerve supply, it is thought
to correspond to the utriculus and the two vertical semicircular canals
of the higher vertebrates. Homology with two semicircular canals of
vertebrates rests on the presence of two ampullae, each of which contains
an elongated cluster of hair cells, the crista, innervated by branches of the
auditory nerve. Each of the semicircular canals of higher vertebrates
has, however, only a single crista and ampulla, with three sensory maculae
representing the single macula of myxine. A macula is a cluster of sensory
cells with short hairs located either in the utriculus or sacculus. A rudi-
mentary endolymphatic duct which extends dorsally towards the skin is
present.

The statocyst of petromyzon is slightly more complex than that of
myxine. A ventral sacculus is partly separated from the utriculus by a
circular constriction. There are three maculae instead of one. The
statolith is represented by a mass of calcareous particles encased in a
mucous matrix, and lying in contact with the hairs of the sensory cells of
the maculae. None of the cyclostomes has a horizontal semicircular canal.

In elasmobranchs, the cavity of the statocyst retains its primary
connexion with the outside, through the persistent invagination canal,
which is sometimes erroneously homologized with the endolymphatic
duct of higher vertebrates. The statocyst is filled with sea water, instead
of endolymph secreted by its own cells; and, in some species of sharks and
rays, grains of sand replace calcareous statoliths. All three semicircular
canals are present, as in higher vertebrates. The sacculus becomes
further separated from the utriculus, and connexion between the two is
effected by a canalis reuniens. The lagena is formed as an outgrowth of
the sacculus, but it has not been demonstrated that it has an auditory
function. The entire membranous labyrinth is enclosed in a cartilaginous
capsule, which fuses with the cranium.

In teleosts, the invagination canal degenerates, and is replaced by an
endolymphatic duct, which grows out from the sacculus. The cavity of
the bony auditory capsule opens into that of the cranium, and the peri-
lymph surrounding the statocyst is identical with the cerebrospinal fluid.

THE SENSE ORGANS

597

The cavity of the otic capsule in amphibia becomes closed and inde-
pendent of that of the cranium. Near the lagena, another outgrowth of
the sacculus, the basilar papilla, is formed. From this and the lagena,
arises the cochlea of the higher vertebrates. To its static function, the
ear of the amphibia now adds an auditory one. (Fig. 494)

The loss of gills in land amphibia is associated with important changes
in the visceral arches. The hyomandibular cartilage ceases to be a sus-
pensory apparatus of the jaw, and slips into the spiracular passage as the
stapes or columella. Amphibia are, therefore, the first animals to add to

Fig. 494. — The left membranous labyrinth of vertebrates as seen in lateral view.
A, Myxine; B, Petromyzon; C, Teleost; D, Prog; E, Crocodile; F, Bird; G, Mammal.
The series represents fairly closely an evolutionary series. (Redrawn after Hesse.)

the inner membranous ear structures corresponding to the middle ear of
mammals. (Fig. 501)

In reptiles the lagena and basilar papilla unite in an elongated cochlea,
which in crocodiles becomes spirally wound. By the attachment of the
cochlear duct to the bony labyrinth along two sides, the perilymphatic
cavity is divided into two portions, the scala vestibuli and the scala
tympani. Further advance towards the mammalian ear is seen in the
appearance of a membrane-covered window, the fenestra vestibuli, to
which the stapes is attached. As a result of these advances, the hearing
of reptiles is noticeably keener than that of creatures lower in the scale.

In mammals, the length of the cochlea varies from a half-turn in
echidna to three and a half turns in the deer. The keenness and range
of the sense of hearing differ correspondingly in the two animals. In
correlation with the elongation of the cochlea, the length of Corti's organ

598

COMPARATIVE ANATOMY

is increased and, with it, the range of audible sounds. A second mem-
brane-covered window, the fenestra cochleae, is added to the inner ear.
The malleus and incus are added to the stapes, to form a chain of bones so
arranged that the amplitude of the vibrations of the ear drum is reduced
and their intensity increased as they pass from the drum to the fenestra

vestibuli. The efficiency of the apparatus is
increased by the addition of two muscles,
the stapedial, the smallest of skeletal muscles,
and the tensor tympani which tightens the
drum. (Fig. 501)

The conclusion of morphologists that the
vertebrate membranous ear is a modified
lateral line organ, or a group of such organs,
seems justified by the fact that the mem-
branous ear develops, like a lateral line organ,
from a thickened placode of ectoderm on the
side of the head, and that its later ontogenetic
changes resemble those of a lateral line organ.
In both cases, the skin sinks below the surface,
and patches of sensory cells are differentiated.
Moreover, the eighth nerve develops as a
branch of the facial, a nerve which supplies
lateral line organs. In the elasmobranchs, the
external apertures of the invagination canal of
the statocyst lies near the openings of the
occipital row of lateral line organs. A similar
separation of lateral line organs also occurs
in the case of the ampullae of Lorenzini
and the vesicles of Savi in elasmobranchs.
Both from their development and nerve
relations these organs are obviously differen-
tiated lateral line organs. The ear, it is be-
lieved, has had a similar history.

The Human Ear

The ear consists of three parts, external, middle, and internal. The
last is the true sensory organ, which has the double function of equilibra-
tion and hearing.

The External Ear. The external ear has two parts, an auricle or pinna,
supported by cartilages, and an auditory meatus, which extends to the ear
drum. Sound waves are collected by the pinna, and conveyed by the
meatus to the ear-drum which lies about an inch below the surface of the
head. The deeper portion of the auricle, which forms a vestibule to

Fig. 495. — Labyrinth of
human embryo, 30 mm. long.
a, ampulla; ac, anterior canal;
c, cochlea; cr, crus; de, endo-
lymph duct; nc, cochlear
nerve; s, sacculus; se, endo-
lymph sac; u, utriculus; v.
vestibular nerve and its gan-
glion. (From Kingsley's
"Comparative Anatomy of
Vertebrates," after Streeter.)

THE SENSE ORGANS

599

the meatus, is the concha. The opening of the meatus is guarded by two
processes, a ventral tragus next the cheek and a dorsal antitragus opposite.
The incurved outer rim of the auricle is the helix. The antihelix is a
smaller ridge which bounds the concha dorsally. The walls of the meatus
are supported laterally by fibro-cartilage and medially by bone and lined
by skin. The meatus is beset with hairs, and contains many tubular
glands, which secrete the ear-wax.

The Middle Ear. A tympanic membrane or ear-drum separates the
external meatus from the tympanic cavity or middle ear, within which lie
the ear bones. Leading from the tympanic cavity to the pharynx, is the
auditory or Eustachian tube, which serves to equalize the atmospheric

-COCHLEAR NERVE

"~"'~^'' i#j55*^-COCHLEAR DUCT

EUSTACHIAN TUBE

Fig. 496. — The human ear. "Sound waves" impinge upon the drum and are
carried by the three ear bones to the fenestra vestibuli and thus to the scala vestibuli
and the cochlear duct. (Redrawn from W. H. Howell after Czermak.)

pressure on the two sides of the tympanic membrane, so that this may
vibrate freely. Temporary deafness occurs when the auditory tube is
closed because of inflammation caused by a cold. Functionally, however,
the most important elements of the middle ear are the three ear bones.

The Internal Ear. The membranous sac or true organ of equilibration
and hearing is so complex that it is frequently called the membranous
labyrinth. The otic bone in which it is embedded takes on its shape and
is equally complex, and hence is known as the bony labyrinth. Except in
the region where the membranous labyrinth is attached to the bone, it is
surrounded by a cavity iilled with the perilymph. The sac itself is filled
with endolymph. (Fig. 495)

The Organ of Equilibration. The function of equilibration in man is
served, as in lower vertebrates, by the utriculus and the three semicircular
canals connected with it. All three are hollow membranous canals lined

6oo

COMPARATIVE ANATOMY

by columnar epithelium and loosely attached by connective tissue to the
periosteum of the otic bone. Connected at both ends with the utriculus,
each canal swells at one end into an ampulla. On one side of each ampulla,
an elongated cluster of hair-cells forms a crista, which is innervated by a
branch of the vestibular nerve.

The elongated hairs of the sensory cells of the crista extend into the
endolymph, the movements of which are communicated to the hairs, and
thus indirectly to the nerves connected with the hair-cells. Similar
patches of hair cells, the maculae, with shorter hairs, occur on the sides
of the utriculus and sacculus and, like the cristae, have a static function.

COCHLEAR DUCT

OUTER
/\HA1R CELLS

^gP^-NERVE FIBERS
GANGLION CELL

|/DE ITERS
CELLS

SCALA TYMPANI

Fig. 497. — A stereogram of a portion of the organ of Corti (spiral organ) of man.
The precise function of the tectorial membrane is uncertain. It may serve as a damp-
ener or it may intensify the action of the "sound waves" upon the sensory hair cells.
(Redrawn after Kahn's "Das Leben Des Menschen," W. Keller & Co.)

Proof of the static function of utriculus, sacculus, and semicircular
canals has been obtained by e.xtirpation experiments upon lower animals.
An animal without these parts of the ear is unable to orient its body in
space. The centers of equilibration in the brain are closely associated
with those of the vagus nerve; so that the sea sickness which follows con-
tinuous and violent disturbance of the static organ results from its relation
to the nerve which innervates the stomach. The action of the endolymph
upon the hairs of the cristae and maculae is intensified by the presence
of small calcareous crystals or otoconia in the endolymph.

The Organ of Hearing. The organ of hearing is the cochlear duct;
otherwise known as the scala media, which is attached to the outer side
of the spiral bony cochlea. The cochlear duct is connected with the
sacculus by means of a narrow tubular ductus reuniens. The sacculus is

THE SENSE ORGANS

60 1

also connected with a slender endolymphatic duct, which terminates in a
swollen saccus endolymphaticus near the dura. The utriculus connects
with the sacculus through the utriculosaccular duct by means of the
endolymphatic duct. (Fig. 494, G)

Throughout the two and a half turns of its extent, the membranous
duct of the cochlea is triangular in cross section, and is attached at its

SCALA VESTIBUU

SOUND WAVES IN
PER I LYMPH

PILLAR CELLS

SCALA TYMPAN

COCHLEAR DUCT

COCHLEAR NERVE \^^^^^ ^^ ^^^^,

Fig. 498. — A stereogram of a portion of the cochlear duct of man. The figure
suggests how "sound waves" are conveyed to the hair cells of the organ of Corti.
(Redrawn after Kahn's "Das Leben Des Menschen," W. Keller & Co.)

apex and base to the surrounding bone. The side lying towards the
greater curvature of the cochlea consists of a much thickened periosteum,
the spiral ligament. The upper side, the vestibular membrane, consists
of a thin sheet of connective tissue, covered on both sides by flattened
epithelium. As in reptiles, the apex of the triangle is fastened to the thin
bony lamina which projects into the perilymphatic cavity, and partly
divides this into a dorsal cavity, the scala vestibuli, and a ventral cavity,
the scala t5mipani, both of which are filled with perilymph and connected
with one another at the apex of the cochlea. (Fig. 498)

The third and lower side of the cochlear duct is the essential auditory
organ, the spiral organ of Corti, which rests upon a connective tissue

6o2 COMPARATIVE ANATOMY

basilar membrane. In the region of the spiral organ, the epithelial lining
of the cochlear duct becomes a much thickened columnar epithelium, in
which are sensory hair-cells and supporting cells. Two kinds of support-
ing cells occur, pillar cells arranged like the rafters of a house, and Deiter's
cells, which support the sensory hair cells. Between them, is a liquid-
filled cavity, the tunnel. (Fig. 497)

The hair cells do not extend through the entire thickness of the epithe-
lium, but are suspended with their rounded bases hanging between the
supporting cells. Four to six rows of hair cells extend through the entire
length of the cochlea. An inner row is separated by the pillar cells from
the outer rows.

These hair-cells are estimated to be sixteen thousand in number.
Each has on its free surface about forty stiff hairs which project into the
endolymph. The base of each hair cell is supplied with the dendritic
terminations of a nerve fiber from the cochlear nerve.

Suspended in the endolymph above, and possibly in contact with the
hair, is the fibrous membrana tectoria, the function of which is prob-
lematic. It has been suggested that its vibrations are communicated to
the hairs. Others believe that like a soft pedal it acts as a dampener to
reduce vibration.

Between the liquid-filled cavity of the internal ear and the air-filled
tympanic cavity, are the two membrane-covered openings, the fenestra
vestibuli or fenestra ovalis, to which the stapes bone is attached, and the
fenestra cochleae or rotimda. By means of the stapes, vibrations are
transmitted through the fenestra vestibuli to the perilymph and to the
top of the scala vestibuli. These vibrations, passing down the scala
tympani cause similar vibrations of the fenestra cochleae. It is still
a disputed question, whether the vibrations are communicated to the
hair-cells through the endolymph and the tectorial membrane, or through
the vibrations of the basilar membrane of the spiral organ.

The auditory centers of the brain are located in the temporal lobes
of the cerebral hemispheres.

Development of the Human Ear

The membranous sac of the ear begins in the two-somite embryo as a
thickened patch of ectoderm lateral to the hindbrain. By the time the
embryo has acquired eleven somites, this placode has begun to bend in
towards the brain wall to form a pit. In a five-weeks embryo, the otic
pit is converted into a spherical liquid-filled otic vesicle, which has lost
its original connexion with the ectoderm. This vesicle elongates dorso-
ventrally, and an endolymphatic duct grows from the dorsal side, not as a
remnant of the primitive invagination canal, but as a new formation.

THE SENSE ORGANS

603

By the elongation and spiral twisting of the ventral part of the vesicle, the
cochlear duct is formed.

The dorsal portion of the vesicle, following the outgrowth of the
endolymphatic duct, becomes the utriculus. The two vertical canals
develop from a single dorsal hollow outpocketing of the utriculus; but
the lateral hollow outgrowth from which the horizontal semicircular duct
develops arises later, as might be expected from its phylogenetic history.
Each semicircular duct is formed by a partial fusion of the lateral walls
of its hollow anlage, by which the cavity is obliterated except in its
periphery, where it persists as the cavity of the definitive duct connected

ABC D

Fig. 499. — Lateral or external surfaces of models of the membranous portion of the
left internal ear from human embryos. Different enlargements. A, from an embryo
of 6.9 mm.; B, 10.2 mm.; C, 13.5; and D, 22 mm.; am., ampulla; c.v., cecum vestib-
ulare of d.c, cochlear duct; d.e., endolymphatic duct; d.s.L, d.s.p., and d.s.s., lateral,
posterior, and superior semicircular ducts; sac, sacculus; ut., utriculus. (After His, Jr.)
(From Bremer's "Text Book of Histology.")

with the utriculus at each end. At one of these ends the duct swells
out into an ampulla.

That portion of the membranous sac which is intermediate between
the utriculus and the cochlea is converted into the sacculus. Its separa-
tion from the utriculus is initiated by a horizontal constriction, which
finally reduces the connexion to the slender utriculosaccular duct. By a
similar constriction, the connexion between the cochlear duct and the
sacculus becomes a ductus reuniens.

From the beginning, the otic vesicle is directly attached to that part
of the neural crest which forms the ganglion of the facial nerve. This
connexion is retained by the auditory nerve, which arises as a branch
of the facial, although the roots of the two subsequently become separated.
Throughout the changes which convert the otic vesicle into the membra-
nous labyrinth, the connexions of the auditory nerve are retained. Two
main branches are differentiated, the vestibular, connected with utriculus

6o4

COMPARATIVE ANATOMY

and semicircular canals, and the cochlear, which innervates the hair-cells
of Corti's organ.

Most of the membranous labyrinth retains throughout life its primitive
epithelial character. In local patches, however, it is converted into
sensory columnar epithelium, the cells of which are connected with
branches of the auditory nerve. Each ampulla develops a ridge-like
crista composed of hair cells. The utriculus and sacculus develop larger
cushion-like maculae. One side of the cochlear duct is modified as the
spiral organ of Corti.

The development of the tectorial membrane, like its function, has
long been a controversial question. Primarily, this membrane is closely

Fig. 500. — Diagram of developing human labyrinth from 6 to 30 mm. long, am,
ampulla; c, cochlear region and cochlea; au, ampullo-utricular region; d, endolymph
duct; e, endolymph region; sc, semicircular canal; 5^, endolymph sac; 5, sacculus; u,
utriculus; us, utriculo-saccular canal; v, vestibule. (From Kingsley's "Comparative
Anatomy of Vertebrates," after Streeter.)

attached to the columnar epithelium from which the organ of Corti arises.
The intimacy of the connexion is gradually reduced during ontogenesis,
so that eventually the membrane loses its connexion with the cells of
the spiral organ and floats above them in the endolymphatic fluid.

The differentiation of the elements of the spiral organ is not completed
at the time of birth, so that an infant is stone deaf until it is some weeks
old.

The mesenchyma surrounding the membranous labyrinth becomes
differentiated into an inner connective tissue membrane which lies in
contact with the epithelium of the sac, and an outer cartilaginous capsule
which encloses the membranous ear. Later, the portion of the cartilage
near the membranous sac disappears and is replaced by liquid perilymph
around the sac. As the cochlear duct develops, it becomes triangular in
cross section.

THE SENSE ORGANS

605

In the higher vertebrates the cartilaginous otic capsule is subsequently
converted into bone by the process of endochondral bone formation.
But in the axis of the spiral cochlea the connective tissue is converted
directly into bone, after the manner of membrane bone formation. The
contrast between the modes of formation of the inner and outer portions
of the bony part of the cochlea seems to have no phylogenetic significance.

HYOMANDIBULAR

-ARTICULAR--^^

DENTAR

MANDIBULAR
A. ELASMOBRANCH

HYOMANDIBULAR CCOLUMELLA)

QUADRATE

ARTICULAR w_^
DENTARX^
MANDIBULAR

2ND
MANDIBULAR

B. TELEOST.

STYLOID PROCESS

(HYOID^ r

STAPES CHYOMANDIBULARJ

INCUS CQUADRATEJ—

MALLEUS CARTICULAR)

MECKEL'S

CABTILAGE CMANDIBULAR^^

HYOID

DENTARY-
STYLOHYOID LIGAMENT CHYOID) -

HYOID BONE

C. AMPHIBIAN AND REPTILE. D. MAMMAL.

Fig. 501. — Diagrams of the first and second visceral arches in A, Elasmobranch,
B, Teleost, C, Amphibian and Reptile, and D, Mammal, illustrating the transformation
of the hinge of the jaw of lower vertebrates into the malleus and incus of the mammal.
The third earbone, the stapes, comes from the hyomandibular. (Redrawn after Gegen-
baur and Stempell.)

The middle ear and the Eustachian tube which leads from it to the
pharynx are both derived directly from the spiracular pouch, a diverticu-
lum of the pharynx. Consequently, both, like the pharynx from which
they evaginate, are lined by endoderm. To meet this endodermic
diverticulum, the ectoderm invaginates to form the external auditory
meatus. The two-layered membrane between the two invaginations,

6o6 COMPARATIVE ANATOMY

Strengthened secondarily by the ingrowth of mesenchyme between the
two layers, forms the ear-drum.

The three ear bones are primarily embedded in the connective tissue
dorsal to the tympanic cavity in which the spiracular pouch terminates.
This connective tissue subsequently disappears, and the tympanic cavity
is correspondingly enlarged and surrounds the earbones. The bones
thus become enclosed by the endodermic epithelium, which persists
throughout life. The origin of the three ear bones from elements of
the visceral skeleton is clearly evidenced by the facts of ontogenesis.

The external auricle develops from materials supplied by the mandibu-
lar and hyoid arches. Three small hillocks arise on each of these arches,
and each hillock acquires a cartilaginous support. By the enlargement
and fusion of these six hillocks and their cartilaginous supports, the
external ear develops. The three nodules of the mandibular arch form
tragus and helix; those of the hyoid arch become antitragus and antihelix.
The claim that the cartilages of the ear are derived from those of the
hyoid arch has not been confirmed.

CHAPTER 15
THE HEAD PROBLEM

The striking contrast between head and trunk has not prevented
morphologists from comparing the two regions. To those who assumed
the origin of vertebrates from annelid ancestors, the fundamental similar-
ity seemed self-evident. Curiously enough, the first attempt to demon-
strate that the head contains segments like those of the trunk was made
in pre-Darwinian days, even by those who denied evolution. Now,
however, for more than a century, it has been assumed that the present
differences between head and trunk are coenogenetic, and that the two
regions were differentiated from one another in the immediate ancestors of
vertebrates.

The "head problem" is a threefold one. First, is the head, like the
trunk, metameric? Second, if metameric, are segments of head and
trunk fundamentally similar? Third, how many segments are contained
in the head? The complexity of this inquiry and diversity of answers
have combined to make it one of the persistent problems of vertebrate
morphology.

The problem is not identical with that of the ancestry of vertebrates,
although its elucidation may throw light upon the latter. Even if it were
demonstrated that metamerism characterizes both head and trunk of
vertebrates, this would by no means prove that vertebrates are derived
from metameric ancestors. For metamerism, like any other animal
characteristic, may be a convergent trait acquired de novo within the
chordate group.

Historically, the head problem has passed through three phases,
which may be called the transcendental, the anatomical, and the
embryological.

Transcendental Phase. In its first phase, the head problem was
limited to the skeleton, and the rest of the head region was ignored. The
distinguished German poet Goethe was the first to conceive that the skull
consists of a number of enlarged and united vertebrae. In 1790, Goethe
wrote to his friend Karoline Herder that the study of a sheep's skull found
in the Jewish cemetery near Venice had given him a new notion in regard
to the structure of the head. Goethe was familiar with Greek authors
and, through them, with the evolution theory. In fact, he had already
suggested that a flower is a shortened and modified branch, a theory now
universally accepted by botanists. Goethe, however, did not publish

607

6o8

COMPARATIVE ANATOMY

his vertebral theory of the skull until 1823. In the meanwhile a German
naturalist Oken published, in 1807, a similar conception of the skull as a
composite of enlarged vertebrae. Quite unrestrained by sound morpho-
logical principles, Oken compared the upper jaw with the arms, the lower
jaw with the legs, and the fingers and toes with the teeth.

BASISPHENOIO

ORBITOSPHENOID
PRESPHENOID
TRONTAL
VOMER
PREFRONTAL.
NASAL

SUPRAOCOPITAL

EXOCCIPITAL
BASIOCCIPITAL

A. MAMMAL

ALISPHENOip
ORBITOSPHE NOIP

PRESPHENOI

PREFRONTAL.
VOMER

SUPRAOCQPITAL
OCCIPITAL

B.MAN

Fig. 502. — Owen's figures illustrating the Goethe-Oken vertebral theory of the skull.
Owen believed that he could find in the mammalian skull four enlarged vertebrae, the
components of which he identified with the elements of a trunk vertebra. Not knowing
the embryology of the skull, he did not realize that vertebrae lack the membranous
elements which are so conspicuous in the skull. (Redrawn from Wilder.)

In England, the distinguished anatomist Owen adopted Oken's con-
ception and attempted to demonstrate in the bones of the skull all the
t^-pical elements of a trunk vertebra. Owen's familiar figure of an
"archetypal vertebrate," with four vertebrae in the skull and each cranial
vertebra with typical vertebral elements, is shown in Fig. 503. During

THE HEAD PROBLEM

609

the first half of the nineteenth century, the vertebral theory formed an
essential part of the orthodox creed of biologists. The brilliant morpholo-
gist Johannes Mueller defended it in Germany, while Carl Vogt opposed
it. Rathke and Reichert supported it on embryological grounds. In
France, Cuvier was coolly indifferent. In America, Louis Agassiz opposed
it.

It was Thomas Huxley who, in 1858, a year before the publication of
the Origin of Species, dealt the vertebral theory of the skull its death
blow. In his famous Croonian lecture, Huxley pointed out that the fish
skull develops primarily as a cartilaginous box, and not as a series of
vertebral elements. He thus turned against the vertebral theory embryo-
logical evidence which Rathke and Reichert had interpreted in its favor.
No vertebra-like structures are present in the cartilaginous skull of
elasmobranch fishes, in which, according to hypothesis, vertebrae ought

NEURAPOPHYSES.

.NEURAL

'SPINES.

mM6&M&&MtMMmMm

?IBS. "^STERNEBRAE.

Fig. 503. — A diagram of Owen's archetypal vertebrates based upon the skeleton,
modified vertebrae are assumed to form the skull.

Four

to be more evident than in higher forms. Although the vertebral theory
of the skull never recovered from Huxley's attack, it was Huxley who
converted the head problem from a problem of the morphology of the skull
only, to one of the structure of the entire head.

Anatomical Phase. In 1869, Huxley, abandoning the vertebral theory,
demonstrated that evidence of head metamerism is found in the series
of cranial nerves and the gill-slits with which they are associated. The
facial nerve forks over the spiracular cleft, the glossopharyngeus over the
first branchial, and the vagus over the more posterior clefts. Huxley
assumed that the gill-sHt proper to the trigeminal nerve is the mouth
which, in his opinion, might have been formed by the coalescence of a
pair of gill-slits. He also suggested that the ophthalmicus profundus nerve
represents a pre-oral segment, and that the embryonic orbito-nasal groove
to which it is related is the remnant of a gill slit. Huxley recognized
altogether nine head segments, four in front of the ear and five behind.

Two years later, Gegenbaur confirmed the conclusions of Huxley and
added the cartilaginous visceral arches as criteria of metamerism. To
supply skeletal elements for pre-oral segments, he used the labial cartilages
of the elasmobranchs. Gegenbaur, however, erroneously compared
visceral arch cartilages with trunk ribs and consequently regarded head

6io

COMPARATIVE ANATOMY

and trunk segments as serially homologous. It has since been demon-
strated that visceral arches differ from ribs, both in development and in
relations to other parts, and hence are not homologous. Gegenbaur
called attention to the fact that the presence of membrane bones in the
skull makes it impossible to compare the skull with a vertebra. Like
Huxley, Gegenbaur counted nine segments in the head. These he
regarded as serially homologous with those of the trunk.

Head Segments According to Gegenbaur

Primary visceral
skeleton

I St arch.
2nd arch

3rd arch.
4th arch.
5th arch.
6th arch.
7th arch.
8th arch.
9th arch.

Modified visceral
skeleton

ist upper labial cartilage
2nd upper and ist lower

labial cartilages
Mandibular arch
Hyoid arch
ist gill arch
2nd gill arch
3rd gill arch
4th giU arch
5 th gill arch

Nerve

^ Trigemini

Ramus 21

Ramus 3 ;
Facial

Glossopharyngeal
Ramus, i vagi
Ramus, 2 vagi
Ramus, 3 vagi
Ramus, 4 vagi

Embryological Phase. With the rise of the science of embryology
during the latter part of the nineteenth century, the head problem entered

SCLEROTOME-
HYPOCHORDA

V^ERMATOME
Ist-MTOTOME

SCLEROTOME
HYPOCHORDA

MESENTERY

NEURAL CREST

NEURAL TUBE

EPIMERE

'SOMATIC MOTOR NERVE
"^zES-V— -""^ NOTOCHORD-'"

(.;;^^]T__— DORSAL AORTA

MESOMERE

ECTODERM

— ENDODERM

ENTERON

HYPOMERE

COELOM

SUBINTESTINAL
BLOOD VESSEL

A. HEAD B. TRUNK

Fig. 504. — Diagrams of cross sections in A, head and B, trunk regions of an elasmo-
branch embryo showing the fundamental similarity of the two regions. The discovery
that the coelom of elasmobranch embryos extends throughout head and trunk and that
in this respect the two regions are alike was made by the English embryologist, Francis
Balfour.

its third and last phase. The assumption which underlies the interpreta-
tion of embryological evidence, as applied to the head problem, is von
Baer's law, the fundamental law of biogenesis, that the development of the
individual recapitulates briefly the past history of the race. The truth

THE HEAD PROBLEM

6ii

of this assumption has been persistently challenged, and experience has
demonstrated that it should not be pressed too far. In a very general
way, however, the law has proved helpful, especially in relation to the
head problem.

In 1878 Francis Balfour made the important discovery that in elasmo-
branch embryos the coelom extends continuously throughout head and
trunk. He thus demonstrated that in this fundamental respect head
and trunk are similar to a degree not previously realized. Balfour also
stressed the fact that, by the formation of gill-pouches, the cephalic
coelom is divided into a series of mesodermal "cavities," which Balfour

CEREBELLUM

HEAD CAVITIES
Y

MOUTH
HYPOPHYSIS
OPTIC RECESS

^THYROID

Fig. 505. — A parasagittal section of a squalus embryo showing the three "head
cavities" from which the six eye muscles develop. The somatic motor nerve relations
are also shown. (After Neumayer modified.)

erroneously compared with trunk somites. The falsity of this comparison
was subsequently recognized when it was demonstrated that the trunk
somites develop from the epimere, while the mesoderm of the visceral
arches comes from the hypomere.

In 1 881, Balfour's pupil, Marshall, emphasized the distinction between
the segmentation of the somites and that of the mesoderm of the visceral
arches, and asserted the independence of the two. Only segmentation
of the dorsal somites of head and trunk are comparable. The mesodermal
cavities within the visceral arches are peculiar to the head. This contrast
between "mesomerism" and "branchiomerism" has been recognized by
later morphologists. Whether or not these two kinds of segmentation
originally corresponded has remained a controverted issue to this day.
Marshall also showed that the first or premandibular "cavity" gives
rise to the four eye muscles innervated by the oculomotor nerve, while
the external rectus muscle comes from the third somite.

6l2

COMPABATIVE ANATOMY
ISr SOMITE SOMrTE 10

ANTERIOR
CAVITy
ENDOSTYLE
CLUB-SHAPED GLAND

A. AMPHIOXUS EN/1BRY0

1ST PERM. GILL-SLIT

FACIALIS GANGLION PTIC CAPSULE
1ST SOMITE

LENS

SOMITE 10

THYROID
ST PERM.GILL-SUT

B. PETROMYZON

FACIALIS GANGLION
1ST SOMITE

VI

OTIC CAPSULE

SOMITE 10

ANTERIOR CAVITY

HEART

C.SQUALUS

Fig. so6. — Diagrams of the mesodermal segmentation in the head region of chordate
embryos. The somites are stippled and nerve ganglia shown in solid black, visceral
pouches cross-hatched. A, Larval Amphioxus; B, Petromyzon (cyclostome) ; C,
Squalus (elasmobranch) . The neuromeres are numbered with Roman numerals.

THE HEAD PROBLEM
Head Segments According to Marshall

613

Segment

Nerve

Visceral
cleft

Visceral
arch

I Preoral

I. Olfactory

III. Oculomotor

IV. Trochlearis

V. Trigeminal
VII. Facial

VI. Abducens

IX. Glossopharyngeal

X. Vagus, ist branch
X. Vagus, 2nd branch
X. Vagus, 3rd branch
X. Vagus, 4th branch
X. Vagus, 5th branch
X. Vagus, 6th branch

Olfactory

Lacrimal
Buccal

Spiracular
ist branchial
2nd branchial
3rd branchial
4th branchial
5th branchial
6th branchial
7th branchial

2 Preoral

■2 Oral

Maxillary
Mandibular

Hyoid

ist branchial

6 Postoral

2nd branchial

3rd branchial

8 Postoral

4th branchial

5 th branchial

10 Postoral

6th branchial

1 1 Postoral

The year following the publication of Marshall's paper, van Wijhe
discovered in elasmobranch embryos an unbroken series of somites
beginning with the premandibular and extending throughout head and
trunk. He was thus able to demonstrate an "acraniote stage" of the
vertebrate embryo, which, so far as the mesodermal segmentation is
concerned, resembles the acraniote amphioxus. While all the myotomes
of amphioxus persist throughout life, in elasmobranchs some of those in
the region of the ear degenerate. As a result, the three anterior somites,
which form eye muscles, become separated from the metaotic. In
cyclostomes, however, according to Koltzoff ('02), all the embryonic
myotomes form muscles, which persist in the adult lamprey as in amphi-
oxus. There is, therefore, in the lamprey no hiatus between prootic and
metaotic muscles. (Figs. 507, 508)

Head Segments According to van Wijhe

Somite

Muscles from the somite

Rect. sup., inf., int., and inf.
oblique
Superior oblique

Rectus externus

None

None

Very rudimentary

Muscles from skull to shoul-
der girdle, with anterior
part of sternohyoid

Ventral nerve root

Dorsal nerve root

Oculomotor

Trochlearis

Abducens/

None )

None

Not recognizable/

Hypoglossus )

Ophthalmicus profundus

Trigeminus less ophth.
prof.

Acusticofacialis

Glossopharyngeal

Vagus

6i4

COMPARATIVE ANATOMY

Van Wijhe confirmed the conclusions of Marshall in regard to the fate
of the first and third somites, and also asserted that the superior oblique
muscle is derived from the second somite. In the metaotic region the
first permanent myotome develops from the seventh somite, and van
Wijhe concludes that altogether nine segments enter into the formation
of the head, a conclusion very similar to that reached by Huxley and
Gegenbaur on anatomical grounds. In a later paper, van Wijhe ('22)
adds another metaotic segment, making a total of ten in the head of
elasmobranchs.

Since van Wijhe's first paper, many embryologists have investigated
the mesodermal segmentation of embryos in all classes of vertebrates.

PROFUNDUS Yn
SOMITE

FOREBRAIN

MANDIBULAR IS V.
SOMITE 3.

ACUSTICO-FACIALIS.

OTIC PIT

SOMITE 4.

GLOSSO-
PHARYNGEUS.

VAGUS.

Fig. 507. — A diagram of a Squalus embryo, showing the mesodermal somites of van
Wijhe. The nerve anlagen are stippled. (Redrawn after Braus.)

While their observations and conclusions have not been wholly consistent,
it may be said that van Wijhe's general results have been confirmed.
Somites like those of elasmobranchs have been found in cyclostomes and
amphibia. Their presence indicates that the ancestors of vertebrates,
like the Hving amphioxus, were segmented throughout the length of the
body, that the metamerism was primarily muscular, and that head seg-
ments were like those of the trunk.

The conclusion that the mesodermal segments in the head region
are comparable with those of the trunk has by no means been accepted by
morphologists without opposition. Among the objections raised are
that the head segments are due to the mechanical influence of surrounding
parts such as the gill-pouches; that the head somites are irregular in size;
that the continuity of trunk and head segments does not prove their
serial homology; that the head segments do not develop in succession

THE HEAD PROBLEM

615

with those of the trunk, as would be expected if they were members of a
continuous series; that the prootic segments do not become differentiated
into myotome and sclerotome as do trunk somites; that the muscles
in the trunk region are differentiated from the median wall of the somite,
while the musculature of the pre-otic segments arises in large part from
the lateral wall; that the mesenchyma of trunk somites comes from a
constricted region of the somite, while in the head segments it comes
from the entire median wall; that relations of the nerves to the head
segments differ from those of spinal nerves to the trunk somites. In
addition to these general objections, special objections have been raised
against the inclusion of the firSt and second pro-otic segments as true
somites. Most of these assertions have proved to be erroneous.

EAR PLACODE'

POSTERIOR

HEAD FOLD/

Fig. 508. — A parasagittal section of an eight-day Petromyzon embryo viewed from
the left side showing the mesodermal somites. The first somite is shown in an adjacent
section, not in the figure. According to Koltzoff all of the head somites of Petromyzon
form permanent muscles. The first three form the eye muscles, while the fourth is the
first myotome of the lateral trunk muscles. (Redrawn after Koltzoflf.)

That the segments discovered by van Wijhe in elasmobranch embryos
are true somites homologous with those of the trunk, now seems sufficiently
estabhshed by the fact that their presence in diverse groups of vertebrates
has been repeatedly confirmed; that their segmentation is independent
of the visceral segmentation represented in the visceral arches; that they
form a series continuous with that of the trunk; that they are dorsal in
relation to the dorsal aorta and notochord; that they develop consecu-
tively, beginning in the neck region; that they become differentiated like
trunk somites into myotome and sclerotome; that their muscles develop
from the median wall; and that they correspond numerically with the
neuromeric segments. Consequently, the conclusion that they form
the most convincing evidence of the fundamental similarity of the seg-
mentation in head and trunk seems warranted.

Van Wijhe noticed in the embryo of Galeus, an elasmobranch, a pair
of mesodermal segments or "cavities" which he interpreted as an anterior

6i6

COMPARATIVE ANATOMY

prolongation of the premandibular cavity. Later, Julia Piatt found these
segments in squalus embryos, and observed that they develop anterior
to the premandibular cavities and independent of them. Finding
rudimentary muscle fibers in the walls of these cavities, she concludes
that they are serially homologous with mesodermal somites, and named
them anterior cavities (somites). If her interpretation of the anterior
cavities as somites is correct, this adds another segment to the series
discovered by van Wijhe.

The existence of the anterior somites has been repeatedly confirmed
by subsequent investigators, but opinion is divided as to their significance.
Van Wijhe, Dohrn, and Goodrich doubt their segmental value and assume

SOMITE 3

SOMITE 2
1ST GILL-POUCH

SOMITE 4

HYPOMERE
SOMITE I

a CROSS SECTION X.
C. CROSS SECTION Y.

SOMITE 2

A. PARASAGITTAL SECTION.

1ST GILL-POUCH-

CYCLOSTOME EMBRYO.

Fig. 509. — Diagrams of sections of an eight-day Petromyzon (cyclostome) embryo,
showing the segmentation of the mesoderm in the head region. The "anterior" somite
of Miss Piatt, which is present in elasmobranch embryos, is wanting in cyclostomes.

that they are derivatives of the premandibular somites, Goodrich urges
that in any scheme of cephalic metamerism the anterior cavities should be
ignored, since "they soon disappear and form no permanent structure,
are scarcely or not at all developed in other elasmobranchs, have not
been found at all in other gnathostomes, and are absent in petromyzon."
None of these objections, however, appears to invalidate Miss Piatt's
interpretation. In making them, Goodrich seems to have overstrained
his point. If, for example, every embryonic feature should be ignored if
it "soon disappears and forms no permanent structure," we should have to
ignore also van Wijhe's fifth and sixth somites which, like the anterior
somite, break up into mesenchyma. If we are to ignore the anterior
somites because they are " scarcely or not at all developed in other elasmo-
branchs," to be consistent, we should have to ignore also the tubular

THE HEAD PROBLEM

617

connexion between the premandibular cavity and the hypophysis dis-
covered by Goodrich ('17) in Torpedo. The fact is that in the develop-
ment of the anterior cavities, as in many other respects, Squalus and
Galeus present more primitive characteristics than do most elasmobranch
embryos. While petromyzon, as Goodrich says, does not have anterior

ANT. ENDODERMIC DIVERTICULUM

ANT ENCXDDERM.
DIVERTICULUM

A.

.BRAIN

ENTERON'

a: b.' c'

Fig. 510. — Diagrams of frontal and cross sections of Amphioxus (cephalochordate),
Amia (ganoid), and Squalus (elasmobranch) embryos, showing the exact homology of
the anterior endodermic diverticula of Amphioxus, the adhesive organs of Amia, and
the anterior somites of Squalus. A, A', A^, A^ sections of amphioxus; B, B' sections of
Amia; C, C sections of Squalus.

cavities, there are mesenchyma cells anterior to the premandibular somite
which may represent the anterior cavities.

The assertion that the anterior cavities "have not been found at all
in other gnathostomes" may be questioned, since Reighard ('02) has
shown that the adhesive organs of amia, a ganoid fish, resemble the
anterior cavities both in development and in their relations to the preoral
gut and to the premandibular somite, and must therefore be considered

6i8

COMPARATIVE ANATOMY

to be homologous with them. There is evidence also that they are
possibly homologous with the pit-organs of vipers.

The facts of development do not bear out the assertion that the
anterior cavities are segregated portions of the premandibular cavities.
It would be equally misleading to state that the premandibular cavity
is a segregated portion of the mandibular cavity. The three structures

ANLAGE

ENDODERMIC

DIVERTICULUM

SOMITES'
A. AMPHIOXUS

/(SOMITES

C. SQUALUS ^YOUK

Fig. 511. — Parasagittal sections showing the similarity of the relations of the
anterior endodermic diverticula of Amphioxus (A) to those of the adhesive organ of
Amia (B) and of the anterior somites of Squalus (C). Each of these "cavities" arises
immediately in front of the first permanent myotome (somite i).

are, in fact, serially homologous — at least in part — and have equal seg-
mental value. This conclusion is supported by the resemblance of the
anterior cavities to the anterior endodermic diverticula of amphioxus.
Like the latter they are the anteriormost cavities of the body and develop
immediately anterior to the first permanent myotomes. The resemblance
of the adhesive organs of amia to the anterior endodermic diverticula
of amphioxus is even more striking. Both arise as paired endodermic
diverticula of the anterior end of the preoral intestine. Both open by
pores to the exterior, the left cavity in amphioxus forming the preoral pit.

THE HEAD PROBLEM

619

In the light of the evidence we are compelled to add another segment
to those enumerated by van Wijhe, so as to make eleven segments in the
elasmobranch head, instead of the ten recognized by Goodrich.

F ^ Yi w

in

I ! ■- ACUSflCO-FACIALlS GANGLION .
3Ssr7SB»cJs8«4Sfe*Ci:^ ' f-SOM ITE .

Fic;. 512. — A parasagittal section of a 33-hour chick embryo showing the brain neuro-
meres. The neuromeres are indicated by Roman numerals.

Among the criteria used to determine head segmentation, some
morphologists have leaned heavily upon the so-called neuromeres or
segments of the central nervous system. Neuromeres are evanescent

'NEURAL CREST CELLS/
NEUROMERES

Fig. 513. — A reconstruction of the head region of a 5 mm. Squalus embryo showing
neuromeres and van Wijhe's somites as viewed from the left side. Neural crest cells
are indicated by heavy black dots. Roman numerals number the neuromeres.

embryonic structures, first described by Bischoff ('42) in the medulla
of mammal embryos. The assumption that neuromeres are reliable
criteria of segmentation has been based partly upon the belief that
vertebrates are the descendents of annelids which have a segmented

620

COMPARATIVE ANATOMY

nervous system. In annelids and arthropods each metamere typically
contains a well-marked ganglion which serves as a nerve center for the
segment. In some arthropods in which the metameres have been tele-
scoped together, the original segmentation is still represented in a series
of metamerically arranged gangUa. Here the neuromeric segmentation
persists when other signs of metamerism are all lost. The fact of the
transiency of vertebrate neuromeres has not counted against their value
as criteria of segmentation, but in their favor, as vanishing remnants of the
metamerism of invertebrate ancestors.

A.AMPHIOXUS

C. SQUALUS.

B.PETROMYZON D. CALLUS

Fig. 514. — Diagrams of the neuromeres in chordate embryos. Wanting in Amphioxus
(A), they appear with increasing distinctness in Petromyzon (B), Squalus (C), and
Chick (D). This evidence does not accord with the assumption that they are pre-
chordate structures which appear transiently in vertebrate embryos.

Compared, however, with the results based upon mesomeres (somites),
the study of neuromeres has proved disappointing. Students of neuro-
meric segmentation agree as to the existence of hindbrain neuromeres
or rhombomeres, but as to little else. The discrepancies in their results,
combined with the increasing distrust of the theory of the annehd ancestry
of vertebrates, have served to weaken confidence in conclusions based
upon the study of neuromeres. Moreover, most morphologists are
convinced that metamerism began in the muscular, not in the nervous
system. Nervous metamerism is, in their judgment, a secondary adapta-
tion to muscular segmentation.

Locy ('94) and his pupil Hill ('00) claim to have found a uniformly
beaded segmentation throughout the length of the brain and spinal cord.
Their highly diagrammatic figures have found their way into recent
textbooks, in spite of the fact that the majority of investigators have
failed to confirm their results. As a matter of fact, the serial homology

THE HEAD PROBLEM

621

of neural divisions anterior and posterior to the neuromeres of the medulla
has not been demonstrated.

A number of facts appear to invalidate the assumption that neuro-
meres are reliable criteria of the primitive segmentation of the head.
Neuromeres are more conspicuous in the embryos of higher than in those
of lower vertebrates, and it was evidence similar to this that led to the
abandonment of the vertebral theory of the skull. Neuromeres in cyclo-
stomes appear to be limited to the medulla and even there are identified

"ST yn

' "Orr^NOTOCHORD.

i— NOSE
MOUTH PLACODE—^ KeopoQc'^
Fig. 515. — A median section of a young Petromyzon embryo, showing the neuro-
meres. The neuromeric segmentation of Petromyzon appears to correspond with the
primary neuromeric segmentation of elasmobranchs and birds. It is, however, less
marked in Petromyzon than in these higher forms. (Redrawn after KoltzofE.)

with difl&culty; but they become increasingly prominent as we pass in
the vertebrate series from fishes to mammals. Such evidence does
not accord mth the assumption that neuromeres have phylogenetic
significance.

As stated above, little resemblance is seen between rhombomeres
(neuromeres of the medulla oblongata) and the neural segments anterior
and posterior to them. Since rhombomeres appear chiefly in the lateral
plates of the medulla, the only structures comparable anteriorly are the
forebrain and midbrain vesicles, not secondary subdivisions of these.
The so-called neuromeres in the trunk region are transient effects produced
by the pressure of the mesodermal somites in early stages of their develop-
ment, and affect only the ventro-lateral portions of the spinal cord. Such
evidence leads to the conclusion that a rhombomere is a new structure,
possibly developed in relation to the visceral arches and clefts.

Furthermore, the cranial nerves do not have simple one-to-one rela-
tions with neuromeres, as might be expected if they are metameric

622

COMPARATIVE ANATOMY

Structures. If, for example, each rhombomere were metameric, it should
be connected with a visceral arch by such a cranial nerve as V, VII, IX or
X. At least the motor fibers of each nerve should have a center in a
single neuromere. It is found, however, that the mandibular branch of
the fifth nerve has its motor centers in two rhombomeres, the second and
third; that the centers of the seventh nerve lie in four rhombomeres,
the fourth to seventh inclusive, and that those of the ninth have their
nucleus in the sixth and seventh rhombomeres. Of the somatic motor
nerves, the relations of the abducens (VI) are even less clearly metameric.

METEN,

13 MM. SQUALUS EMBRYO -PARASAGITTAL SECTION.

Fig. 516. — A diagram of the motor nerve relations of the hind-brain neuromeres,
based upon parasagittal sections of a 13-mm. squalus embryo, treated by the Ranson-
Cajal silver method. Similar nerve relations have been shown by Graper to occur in
bird and mammal embryos. The evidence does not strengthen the assumption of a
primary metameric correspondence between neuromere, nerves, myotomes and visceral
arches.

Although the abducens innervates a single myotome, that of the third
head cavity, the motor nucleus stretches through three rhombomeres,
the fifth, sixth, and seventh. Since these relations occur both in elasmo-
branchs and in mammals, it may be a^ssumed that they occur in all other
vertebrates. While these confused nerve relations by no means prove
that the neuromeres are without metameric value, they are certainly not
the relations which would be expected in metameric structures. If
the relations of cranial nerves to muscles and visceral arches were originally
metameric, they have obviously been profoundly modified in the course of
phylogenesis.

Doubt as to the value of neuromeres as criteria of metamerism also
arises out of the failure of students of neuromerism to agree on the number
of neuromeres in the head. This divergence of opinion is partly based
upon disagreement in regard to what structures may be counted as

THE HEAD PROBLEM

623

neuromeres. While some hold that the segmentation visible in the open
neural plate is the true neural segmentation, others assume that the
secondary subdivisions of the brain vesicles are to be counted.

Opinions differ also as to the criteria of neuromeres. Some observers
find the neural segments inconstant and asymmetrical. It is, therefore,
not surprising that the majority of those who have studied neuromeric
segmentation are skeptical of their metameric value, those of the hindbrain
possibly excepted. If any brain divisions anterior to those of the hind-
brain may be counted as neuromeres, they are the primary forebrain and

Fig. 517. — Diagram of the segments (neuromeres, myotomes, etc.) of the head
A, anterior myotome; a, abducens nerve; b, branchial clefts; /, facial nerve; g, glosso-
pharyngeal nerve; h, hypoglossal nerve; I, lens, surrounded by layers of eye; n, nasal
pit, near it the terminalis nerve; o, oculomotor nerve; op^, ophthalmicus profundus part
of fifth nerve; os^, ophthalmicus super facialis part of fifth nerve; ot, otocyst; s, spiracular
cleft; t, trigeminal nerve; ta, truncus arteriosus; tr, trochlearis nerve; I-VIII, neuromeres;
1-6, myotomes. (From Kingsley's "Comparative Anatomy of Vertebrates," after
Neal.)

midbrain vesicles, which owe their primary appearance to their functional
association with the olfactory organs and the paired eyes. It is possibly
significant that these two vesicles, like the neuromeres of the hindbrain
show a numerical correspondence with mesodermal somites, including
Miss Piatt's anterior somites. If this numerical correspondence seen
in elasmobranch embryos is not wholly accidental, the neuromeres, hke
the mesodermal somites, may have a metameric value. (Fig. 517)

In 1885 Beard and Froriep independently discovered that the ganglia-
of the cranial nerves receive cellular accessions from epidermal placodes
above the gill-pouches. To these placodes Beard gave the name epi-
branchial sense-organs. Since they may be assumed to have a primary
numerical correspondence with gills, they were used by Beard as criteria
of the primitive segmentation of the head. (Fig. 518)

624

COMPARATIVE ANATOMY
Head Segments According to Beard

Dorsal
nerve root

Branchial
cleft

Nature of

sense-organ

of cleft

Ganglion

Head
cavity

Ventral
nerve
root

I

Olfactory

None

Olfactory
organ

Olfactory

None

None

II

Radix longa
of ciliary

None, or
hypo-

Branchial

Ciliary

First

Oculomotor

III

ganglion
Trigeminus

physis
Mouth

Branchial

Gasserian

Second

Trochlear

IV >

V S

Facial ■]

Absent
Branchial

Branchial
Branchial

■Facial

Third

?

Abducens
None

VI

Auditory

None

Auditory
organ

Auditory

None

None

VII

Glosso-
pharyngeal

I St

branchial

Branchial

Glosso-
pharyngeal

?

None

VIII

Vagus I

2nd
branchial

Branchial

Vagus I

None

None

IX)

3d, 4th, &

(Branchial

) Vagus II,

Vagus II, III,
and IV

5th bran-
chial

< Branchial
( Branchial

\ III, and

None

None

In cyclostome embryos, Kupffer ('95) found thirteen such epibranchial
sense organs, five anterior to the facial ganghon and seven posterior.
Six of these are associated with the visceral branches of the vagus nerve.
Kupffer 's results, however, have never been confirmed, and relatively

N. FACIALIS
N. TRIGEMINUS
?

N. GLOSSOPHARYNGtUS
EAR/ ,N. VAGUS /LATERAL LINE NERVE

NOTOCHORD

NOSE /

EPIBRANCHIAL

GANGLIA (x-an)

HYPOPHYSIS

Fig. 518.— A diagram of the head of a Petromyzon (ammocoetes) embryo seen from
the left side showing the series of epibranchial placodes in solid black. Beard and
Kupffer have suggested that such placodes or "epibranchial sense organs" might be
used as criteria of the primitive segmentation of the head. (Redrawn after Kupffer.)

few schemes of cephalic segmentation make use of epibranchial ganglia
as criteria of metamerism.

In the attempt to compare a head segment with a trunk segment, a
number of mistakes have been made. Gegenbaur's homology of a
skeletal visceral arch with a rib is invalidated by the fact that the two

THE HEAD PROBLEM 625

resemble one another neither in development nor in relations. The same
objection may be made against the attempt to compare one of the aortic
arches with one of the intercostal arteries. Balfour's failure to distinguish
between the mesoderm of the visceral arches and that of the somites has
misled many, and this error has been perpetuated by H. E. Ziegler ('15).
Van Wijhe's head somites develop from the epimere, while the mesoderm
of the visceral arches comes from the hypomere. The segmentation of
the somites has been shown to be independent of that of the visceral
arches. There is nothing in the trunk region comparable with a visceral
arch. Branchiomerism is confined to the head. If branchiomeric
segmentation were the only evidence of head metamerism, we should have
to conclude that the head is a region sui generis. Whether or not there
was, or is, a correspondence between mesomerism and branchiomerism
remains today an open question. Their topographic relations in amphi-
oxus and cyclostome embryos justify the assumption of a primitive
correspondence, but the tendency of visceral arches and pouches to be
reduced and modified, makes them less satisfactory criteria of the primi-
tive segmentation than the somites.

A number of objections have been made against the use of cranial
nerves as criteria of head segmentation and against the attempt to com-
pare them with spinal nerves. The dissimilarities of the two have been
emphasized. It has been asserted that cranial nerves, such as V, VII,
IX and X, are mixed motor and sensory, while the dorsal spinal nerves
are purely sensory; that the cranial nerves mentioned receive cellular
contributions from epibranchial ganglia, while spinal nerves do not;
that the ganglia of cranial nerves lie lateral to the somites, while those of
spinal nerves are medial; that spinal nerves have a metameric distribution,
while cranial nerves have an extensive polymeric distribution.

To meet these objections, it has been pointed out that the dorsal
nerves of amphioxus are mixed in function, and that in this respect
cranial nerves retain an ancestral trait which has been lost by the dorsal
roots of spinal nerves. As a matter of fact visceral motor fibers occur
in the dorsal roots of spinal nerves of fishes and amphibians. They
disappear from these roots in amniotes. The difference in the rela-
tions of dorsal nerves to the somites and to epibranchial ganglia may be
explained as a result of the reduction in size of cranial as compared with
trunk somites, and the limitation of gills, with which epibranchial ganglia
are associated, to the head region. The wide peripheral distribution of
cranial nerves is regarded as a coenogenetic adaptation. There is some
evidence of metamerism in cranial nerves, however, in the relations of the
visceral branches of cranial nerves to the series of gill-clefts.

The relations of the somatic motor cranial nerves, the oculomotor,
trochlearis, and abducens, resemble those of spinal somatic motor nerves.

626

COMPARATIVE ANATOMY

The hypoglossus is evidently a spinal nerve which has become cranial in
amniotes. It is a reasonable conclusion that originally cranial and spinal
nerves were similar and serially homologous. Amphioxus presents the
living evidence that a serially homologous segmentation extended through-
out the entire length of the chordate ancestor from which vertebrates
evolved. The differences between head and trunk, therefore, are coeno-
genetic, and are due largely to the concentration of the major senses
in the head and the high degree of differentiation of cranial organs.

All who have studied head metamerism agree that the head is seg-
mented. Some, however, have concluded that the prootic region is a
region sui generis. Yet the great majority compare prootic and metaotic
segments with those of the trunk. Some disagreement appears in esti-
mates of the total number of segments. This is not surprising, since the
number of occipital segments varies in different classes. That vertebrae
have been added to the skull is shown by the fact that the accessory and
hypoglossal nerves are cranial in amniotes but spinal in anamnia.

SUMMARY OF THE EVIDENCE

The resemblances and differences between head and trunk metameres
may be summarized thus:

I. Resemblances

Head
Coelom
Myotome
Sclerotome

Somatic efferent nerve
Visceral efferent nerve
Somatic afferent nerve
Visceral afferent nerve
Sympathetic
Neuromere
Vertebrae (occipital)

Aortic arch
Visceral cleft
Epibranchial sense organ
Visceral skeleton
None

II. Differences

Trunk
Coelom
Myotome
Sclerotome

Somatic efferent nerve
Visceral efferent nerve
Somatic afferent nerve
Visceral afferent nerve
Sympathetic
Neuromere
Vertebrae

None
None
None
None
Kidney tubule

It is, therefore, evident that head and trunk segments in vertebrates
are fundamentally similar. To prove this comparability it is not necessary
to demonstrate that there are no differences in regions which are so
highly specialized, since the observed differences might arise through
adaptive modification of parts originally alike. The most convincing
evidence of head metamerism is that manifested in the mesodermal
somites of anamniote embryos.

THE HEAD PROBLEM

627

There appear to be four or five prootic metameres. The number of
metaotic segments varies in different vertebrates, depending on the
number of occipital vertebrae added. In elasmobranchs five or six have
been counted. From this evidence of metamerism in both head and
trunk region of vertebrates, most morphologists conclude that verte-
brates have evolved from metameric ancestors. Such a metameric

TRIG.

Ill / iv

GLOSSOPHARYNGEAL _

PROF

V,

N.TERM. V GILL SLITS 6-8

AORTIC ARCHES

Fig. 519. — A diagram of the primitive segmentation of the vertebrate head, based
chiefly upon elasmobranch embryos. The majority of morphologists are skeptical of
the assumption that there ever was a gill-slit between the mouth and the spiracle or
first gill-slit.. Possibly amphioxus embryos present such evidence. Neuromeres
numbered by Roman numerals, somites stippled, gill-pouches cross-hatched. The
cranial nerves have two kinds of ganglion cells — dorsal and epibranchial as shown in the
diagram.

ancestor is best represented today by amphioxus. But, like vertebrates,
amphioxus is a chordate. Were the ancestors of chordates metameric
animals? Does the metamerism of amphioxus warrant the inference that
chordates have evolved from metameric ancestors like the annelids? Or
is the metamerism of chordates and annelids a convergent character?
The discussion of the head problem thus leads to that of the ancestry of
vertebrates.

CHAPTER 1 6

THE ANCESTRY OF THE VERTEBRATES

The histories of the various vertebrate organs, as set forth in this
text, may seem to involve certain inconsistencies. Nematodes, for
example, appear in the account of the digestive system, but are not
mentioned in describing the phylogenesis of the nervous system. Simi-
larly, in discussing the excretory system of vertebrates, that of annelids is
significant, while for the phylogenesis of the nervous system that of
anneHds may have no meaning. The gonadic sacs of nemerteans maybe
genetically related to the coelomic segments of vertebrates while it is dif-
ficult, if not impossible, to compare their muscles with those of chordates.

Clearly then, for the genesis of one system of vertebrate organs, the
condition in one group of invertebrates may be most enlightening, while
for another system some quite different group may reveal the ancestral
relations. This implies the paradoxical notion that a particular inverte-
brate phylum may be "ancestral" to the vertebrates for one set of organs
but not for another. It is as if each set of organs were an independent
variable in the evolution of the various phyla. For this reason, morpholo-
gists have ceased searching for a single common ancestor of all chordates.

None of the several species of prehistoric men — Java, Sussex, Peking,
Neanderthal — appears to be directly in the line of ancestry of Homo
sapiens. None is "the missing link." None the less they serve to
bridge the gulf which separates man from his mammalian relatives. One
fossil type has retained certain characteristics possessed by the common
ancestor of both itself and Homo sapiens, which Homo sapiens has lost.
Another fossil type has retained a different feature. All throw some
Ught on the hypothetical course of evolution of modern man. All are
near the ancestral line. None is completely on it. Their relations must,
therefore, be expressed graphically by a branching diagram.

One may, then, in considering both human origins and the larger
problem of vertebrate ancestry, draw quite wrong conclusions, unless
one keeps always in mind what is meant by "ancestor" or "ancestral
stage," noting always that a group may become greatly specialized along
one or more lines, while it retains in other organs primitive and ancestral
traits. When, therefore, one speaks, for example, of an annelid stage of
vertebrate ancestry, this should not be understood to mean that any
vertebrate ancestor ever was, in all respects, a typical annelid, or that
any annelid stands in the direct line of vertebrate descent. All that is

628

THE ANCESTRY OF THE VERTEBRATES

629

meant is that annelids retain certain traits which more or less closely
match those which the ancestors of vertebrates are thought to have

1 1

4

;| |i

-r:

;

4 m

■ r :

n m

r;:

H SH

- z.

• E 3l

: -T ■

H s

:r:

:ii m

:--

Ik J

r

COELENTERATE

FLATWORM

BLASTULAE
Fig. 520. — A diagram showing hypothetical stages in the ancestry of sponges,
coelenterates, flatworms and chordates. Such a series is evidently based chiefly upon
embryological evidence. If the diagram represents correctly the early phases of
phylogenesis, chordates have split off from other phyla earlier than morphologists
generally have assumed and not even flatworms can be said to be ancestral to chordates.
(Redrawn after H. E. Ziegler.)

possessed. Since, then, a group treated as ancestral may resemble an
ancestor in one respect, but depart widely from the ancestral condition
in others, the conventional family tree may be misleading, unless it is

630 COMPARATIVE ANATOMY

clearly understood that the so-called ancestors are near, not on, the
family line.

In such a phylogenetic series or family tree for vertebrates, morpholo-
gists agree that protozoa and coelenterates are near the root. In addition
nearly all are convinced that in many respects flatworms lie near the
main line of ancestry. Great diversity of opinion, however, prevails in
regard to the groups which may be thought to link flatworms to verte-
brates. There is great uncertainty, even, as to the group which may be
considered as immediately ancestral to vertebrates.

Among the immediate precursors of vertebrates have been included
arthropods, annelids, urochordates, hemichordates, cephalochordates,
and even the trochophore larva of annelids. The evidence which has
been advanced by the proponents of these various theories may be briefly
summarized.

ANNELID THEORY

Since vertebrates are strikingly metameric, it is not surprising that
early speculations generally assumed that vertebrates have descended
from metameric invertebrate ancestors. Saint Hilaire (1822) sought to
derive vertebrates from insects. The necessity of reversing the upper and
lower sides of the arthropod in order to bring the nervous system on the
dorsal side as in vertebrates has not deterred morphologists from making
such comparisons.

As phylogenetic speculation became more restrained, however, the
origin of vertebrates from such highly specialized forms as insects began
to seem less probable. Biologists, therefore, sought for more generalized
invertebrates as hypothetical ancestors. To some, annelids seemed to
meet the necessary requirements. Delsman, a recent supporter of the
annelid hypothesis of vertebrate ancestry writes, (1922) "Hardly two of
the great subdivisions or phyla of the animal kingdom show such a close
agreement, even in the details of their structure, as annelids and verte-
brates, and in no other case can the structure of one be derived so com-
pletely from that of the other."

In 1875, Semper and Dohrn attempted independently to demonstrate
the origin of chordates from annelids.

Both annelids and chordates are coelomate and metameric; both
elongate by means of terminal growth; circulatory and urogenital systems
are fundamentally similar; resemblances of nervous and muscular systems
may be found; and the differences between annelids and chordates may
be interpreted as coenogenetic.

The reversal of upper and lower sides necessary to convert an annelid
into a vertebrate is not regarded by supporters of the theory as a serious
difficulty. They point to the fact that some animals swim either side

THE Ancestry of the vertebrates 631

up indifferently, and that dorsal and ventral sides may be reversed in
relation to the earth in different genera of the same phylum. Among
Crustacea for example many phyllopods swim with the neural side upper-
most. This is the normal position of most adult cirripeds. Amphioxus
in swimming maintains no fixed orientation of its body in relation to
gravitation.

The main supports of the annelid hypothesis are found in metamerism,
terminal growth, and the circulatory and urogenital systems. Both
annelids and vertebrates have dorsal and ventral longitudinal blood
vessels, interconnected around the pharynx by aortic arches. If the
dorsal and ventral sides of the annelid are reversed, the flow of blood is
similar, and the contractile dorsal vessel of the worm becomes the heart
of the vertebrate. The heart of amphioxus and that of vertebrate
embryos is, as would be expected from the annelid theory, a median
longitudinal tube. In both phyla, similar visceral and somatic vessels
occur. The cardinal veins of chordates, however, are wanting in annelids.

Semper, in supporting the annelid theory, laid great stress upon the
similarity of segmental tubules in annelids and chordates. The pro-
tonephridia of amphioxus are identical with those of annelids, and the
coelomoducts of annelids resemble the renal tubules of vertebrates
both in development and in metameric relations. Some annelids have
both nephridia and coelomoducts in the same metamere. Apparently,
nephridia have persisted in cephalochordates and coelomoducts in verte-
brates. The failure of earlier morphologists to recognize the difference
between nephridia and coelomoducts should, therefore, not prejudice
the case against the annelid theory.

The presence of a longitudinal collecting duct, into which the nephridia
open in such annelids as allolobophora, has been cited as evidence in
support of the annelid theory. Superficially this collecting duct might
seem to resemble the primitive duct of vertebrates. But the resemblance
is misleading, since the collecting duct of annelids is ectodermal in origin,
while the primitive duct of vertebrates is mesodermal. The two ducts,
therefore, cannot be homologous. The evidence, instead of supporting
the annelid theory, actually weakens it, since it shows that structures not
genetically related may closely resemble one another. In other words,
this evidence appears to favor the alternative theory that the resemblances
between annelids and chordates illustrate the principle of convergence.
If the ectoderm of annelids and the mesoderm of vertebrates can produce
organs so similar in form and function, we are impressed more by the
plasticity of organisms than by their genetic relationship.

On the other hand, the gonads of annelids and chordates appear to be
comparable. But there are no facts which invalidate the assumption
that the gonads of annelids and chordates have evolved independently

632 COMPARATIVE ANATOMY

from the gonadic sacs of fiatworms. The metamerism which is so charac-
teristic of the gonads of amphioxus is lost in vertebrates.

The attempt has been made to compare the mesodermal bands of
annelids and chordates. These in annelids arise from a pair of teloblastic
cells located in the posterior end of the body. In amphioxus, which is
usually taken as the prototype of chordate development, the anterior
portion of the mesoderm arises as an outpocketing of the primary endo-
derm. A similarity of origin is, therefore, not easily demonstrated.
It is true, however, that the mesoderm of annelids and of amphioxus
undergoes a similar total segmentation into a series of metameric divisions.
The coelomic cavity of amphioxus is primarily connected with the enteron
and is therefore an enterocoele, while that of annelids is not. It must be
admitted that the case for the annelid theory is not greatly strengthened
by the study of the mesoderm.

One of the most striking contrasts between annelids and chordates is
afforded by the notochord. If chordates are the descendants of annelids,
it is necessary to assume either that the notochord is a novelty or that an
homologous structure is present in annelids. Some morphologists prefer
the first alternative. Others assume that the notochord has developed
from the " Faserstrang" of annelids, which is a bundle of connective
tissue fibers extending along the ventral nerve cord. Both nerve cord
and faserstrang are enclosed in a common connective tissue sheath.
Histologically, however, there is little resemblance between faserstrang
and notochord, and there is no evidence of similarity in development.

The annelid theory throws no Hght upon the origin of either the
cartilaginous or the bony skeleton of vertebrates.

The absence of gill-slits in annelids and their presence in chordates
has led to considerable speculation on the part of supporters of the
annelid hypothesis. The most plausible guess as to their origin has been
that gill apertures have arisen from paired diverticula of the pharynx,
which reached the skin and became perforated. It has also been sug-
gested by Dohrn that nephridia have been converted into gill-slits by
the attachment of their inner ends to the pharynx, and subsequent
shortening. The elongated tubular ectodermal passages of the gills of
myxinoids have been mentioned as evidence in support of this assumption.
Since, however, the gills of cyclostomes are wholly endodermal, and since
in all vertebrates the development of gills is initiated by endodermal
pouches, the suggestion that ectodermal nephridia have been converted
into gill-slits seems quite incredible. It is true, however, that the ecto-
derm is not wholly passive in the development of gills, and that in fishes
the ectoderm participates in the formation of internal and external gills,
but this evidence by no means justifies the supposition that nephridia
have been converted into gills.

THE ANCESTRY OF THE VERTEBRATES 633

Some supporters of the annelid hypothesis compare the external gills
of fishes and amphibia with the filamentous gills attached to the parapodia
of annelids. To convert these into internal gills, it is assumed that they
wander into the openings of the nephridia. Most suggestions of this
sort are couched in Lamarckian terms which make little appeal to the
morphologists of this generation.

Dohrn ('75) and Marshall ('79) regard the olfactory pits of vertebrates
as the anteriormost pair of pre-oral gill-pouches, and hence unlike any
structures in annehds. Balfour ('81) and Gegenbaur ('87), however,
opposed the assumption that olfactory pits are gill-pouches, on the ground
that olfactory pits are ectodermal, while gill-pouches are endodermal.
Advocates of the annelid hx-pothesis are inclined to compare the olfactory
pits of vertebrates with a pair of ciliated pits located in the dorsal half
of the prostomium of annelids. With a reversal of surfaces, the pits
would be brought into their definitive position on the ventral side of the
snout of fishes.

Eisig ('87) believed that the lateral line organs of vertebrates a'C
represented in annelids by certain sense-buds found in capitellid worms.
These sense-organs are metamerically arranged in annelids as are the
lateral line organs of teleost embryos. Their structure is also similar.
Their innerv^ation, however, is different, for lateral line organs are inner-
vated by cranial nerves which extend through many metameres, the
segmental sense organs of annelids by segmental nerves. To explain this
difference, it is assumed that a collector nerve, which originally connected
the segmental nerves of the trunk, has become the lateralis branch
of the vagus nerve. The epibranchial ganglia of vertebrates are supposed
to be comparable with the lateral or parapodial ganglia of annelids,
which have been preserved only in the head region of vertebrates.

The intestinal muscles of annelids are arranged like those of chordates
in circular and longitudinal layers. The resemblances of the somatic
muscles is not so striking. Except possibly in the gill region, the circular
muscles of annelids are lacking in chordates. Annelids have longitudinal
metameric muscles on both dorsal and ventral sides of the body. In the
trunk region of chordates, muscles are primarily lacking in the ventral
half of the body; but are secondarily supplied by migration from the
dorsal side. Acceptance of the annelid theory, therefore, makes it
necessary to assume that the dorsal muscles of annelids have disappeared
in the course of phylogenesis.

Some morphologists regard the transient neuromeres of vertebrate
embryos as evidence in favor of the annelid theory, and interpret neuro-
meres as remnants of the segmental ganglia of the annelid nerve-cord.
It has, however, not been demonstrated that neuromeres are metameric
structures. Serial repetition of structures does not demonstrate that

634 COMPARATIVE ANATOMY

they are metameric in the strict sense of the word metameric. The
reliability of neuromeres as criteria of metamerism rests upon the assump-
tion that vertebrates come from metameric ancestors like the annelids.
Those who hold this view explain the absence of neuromeres in amphioxus
as the result of degeneration. There is, however, a suspicion that we
have here another instance of reasoning in a circle. Until we have a more
general agreement among students of neuromerism in regard to the
existence of neuromeres in the trunk region, their presence in the hind-
brain region does little to strengthen the belief in the annelid ancestry of
vertebrates.

It has generally been assumed by advocates of the annelid hypothesis
that annelids are derived from non-metameric ancestors like the flatworms
and actinians. The protonephridia of annelids resemble those of flat-
worms. In some ways the trochophore larva of annelids resembles a
flatworm. The mesenchyme of the larva is comparable with that of
the flatworm, and the coelomic cavities of the adult may be compared
with the gonadic sacs of nemerteans. The flatworms in turn may be
derived from the actinian by the partial union of the borders of its slit-
like mouth. By the persistence of the anterior and posterior portions
of the actinian mouth after the intermediate region had united, the
mouth and anus of the ancestor of annelids, it is assumed, were formed.
In this way, the nerve-ring which surrounded the mouth of the actinian
would be brought into the relations characteristic of annelids.

Objections to the AnneUd Theory. One of the fundamental difficulties
which has turned some biologists away from the annelid theory is the
difference in the fate of the blastopore in annelids and chordates. Indeed,
had this difference been known when the annelid hypothesis was first
promulgated, it is doubtful whether the theory would have seemed so
plausible. It was Goette who, in 1895, first found the difference in the
fate of the blastopore an insuperable objection to the annelid theory.
Comparison of annelid and chordate is difficult, if not impossible, Goette
says, since in annelids the blastopore becomes, at least in part, the mouth,
while in chordates it becomes the anus, or lies near the anus. It was this
difference which led Grobben to classify animals into two great groups,
Protostomians and Deuterostomians. If this evidence has the weight
which it seems to have, the separation of annelids and chordates occurred
long before either of their adult types of organization had appeared,
possibly even before such an organism as a trochophore larva emerged
from a coelenterate ancestor.

To meet this objection, supporters of the annelid hypothesis minimize
the importance of the fate of the blastopore and assert that even in
different genera of the same phylum the fate of the blastopore may differ,
so that its fate cannot have a phylogenetic significance.

THE ANCESTRY OF THE VERTEBRATES 635

Another objection raised against the annelid hypothesis has been
the impossibility of comparing the mouth of the annelid with that of
the vertebrate. The acceptance of the annelid hypothesis makes it
necessary to assume that the transformation of the annelid into the
vertebrate has involved the loss of the annelid mouth and the formation
of a new mouth ventral to the nervous system.

Ley dig, in 1864, was the first to suggest that the esophagus formerly
passed through the vertebrate brain in the region of the fourth ventricle,
between the crura cerebri. The late appearance of the mouth in the
vertebrate embryo has been interpreted as supporting this assumption.
Dohrn (1875) pointed out the fact that the embryo of a vertebrate is
almost completely formed, that all other organ systems are developed,
and that circulation has begun before the mouth opens, while in inverte-
brates the mouth is one of the first organs formed. To some morpholo-
gists, however, there appears to be insurmountable difficulty in assuming
that the gut of vertebrates once pierced the brain, either in the region
of the infundibulum or between the crura cerebelli. Of such a relation,
ontogenesis affords not the slightest evidence.

To meet this difficulty. Beard 1888 assumed that when the annelid
became a vertebrate the supra-esophageal ganglion disappeared, so that
the nerve cord came wholly from the ventral cord of annelids. The
assumption that the esophagus of vertebrates once pierced the nerve
cord was thus rendered unnecessary.

Speaking of the annelid hypothesis, Minot (1897) says, "As regards
the head, however, even theories have hitherto failed us, and no hypothesis
as to the evolution of the vertebrate head from the annelidan type has
been brought forward, which could not be shown to encounter insuperable
objections, or at least which appeared insuperable to those who were
opposed to the annelid theory."

As his contribution of the solution of this problem, Minot suggested
that, in the evolution of a vertebrate, the paired lobes of the supra-
esophageal ganglion of the annelid failed to unite in the median plane,
and were converted into the lateral eyes of vertebrates. "We are thus
led to the supposition that eyes and optic nerves represent in vertebrates
the supraesophageal ganglia and the esophageal commissures of the
articulates." Thus, according to Minot, the brain of vertebrates arose
from the subesophageal and adjacent ventral ganglia of the annelid.
Its enlargement would cause the brain to expand forward and carry
the mouth to a new position on the hemal or heart side, and at the same
time prevent the eyes and preoral ganglia from meeting in the median
line and force the visual organs into lateral positions. "We thus reach a
natural and simple explanation of the difference between the annelid and
vertebrate head." Minot stresses the similarity of the ectodermal invagi-

636

COMPARATIVE ANATOMY

nation which forms the mouth in annelids to the ectodermal hypophysis in
cyclostomes.

Minot's hypothesis involves three assumptions:

1. The preservation of the ectodermal stomodeum of annelids as
the naso-hypophysial invagination of vertebrates.

2. The formation of the vertebrate brain from several of the post-
oral ganglia of the ventral chain.

3. The conversion of the visual apparatus and supra-esophageal
ganglia, which are prevented from fusing in the median line by the
enlargement of the vertebrate brain, into the vertebrate retina, the
esophageal commissures persisting as optic nerves. In Minot's opinion,
this hypothesis obviates those difficulties of the Dohrn-Semper annelid
theory which at present are the most serious. He explains the absence
of eyes in amphioxus as the result of degeneration. Concerning the
failure of embryology to confirm his speculations Minot is silent.

MOUTH STOMODAEUM CARDIOPORECBLASTOPORE;
J

PREORALLOBE

A. Annelid

ANUS
NEURENTERIC
DORSAL NEURAL TUBE CANAL

B.ACRANIATE.
ARCHEJ4CEPHAL0N

NEURAL TUBE

NEURENTERIC CANALCBLASTOPORE)

NEUROPORE

C. Craniate
Fig. 521. — Diagrams of A, Annelid, B, Acraniote, C, Craniote. According to Dels-
man the ectodermal stomodoeum of annelids is converted in vertebrates into the neural
tube. Amphioxus is a transitional form between annelids and vertebrates. The
blastopore of annelids becomes the neurenteric canal of chordate embryos. (Redrawn
after Delsman slightly modified in lettering.)

DELSMAN'S THEORY

A modified annelid hypothesis has been advanced by Delsman (19 13),
not, however, as an attempt to render the views of Dohrn and Semper
acceptable, but as an entirely new theory which grew out of embryological
researches on invertebrates. The gist of Delsman's theory is that the
neural tube of vertebrates was originally a part of the alimentary canal
of invertebrates and that it corresponds to the ectodermal stomodeum
of invertebrates such as the annelids.

Kowalewsky (1877) was the first to suggest that the neural tube may
originally have functioned as a part of the digestive system. Sedgwick

THE ANCESTRY OF THE VERTEBRATES 637

(1884) accepting this hypothesis assumed that the neural tube originated
in animals as a dorsal groove which secondarily became converted into a
canal opening into the archenteron. Van Wijhe (1884) reached inde-
pendently the same conclusion, but assumed that the neural tube may
have functioned as an ingress for food and as an outlet for wastes. T. H.
Morgan (1890) has supported this view.

Delsman finds many points of agreement between the medullary tube
of chordate embryos and the stomodeum of Protostomians. In the
development of both groups, the border of the blastopore, originally
wide and large, contracts to a very narrow opening, the definitive blasto-
pore. In both annelids and chordates, there is formed, in connexion with
the blastopore, a tube of ectodermal origin, which at one end opens to the
exterior, and at the other end leads to the archenteron. In annelids this
ectodermal tube is the stomodeum, the outer opening being the mouth,
the inner the "cardiac pore" which is the opening of the stomodeum into
the annelid stomach. In chordates the ectodermal tube is the neural
tube with its neuropore and neurenteric canal. The fundamental simi-
larity of the relations in the two groups is obscured by the fact that the
"cardiopore" of the annelid lies at the anterior end of the enteron, while
the neurenteric canal of chordates lies at the posterior end.

As additional proof of the comparability of chordates and annelids,
Delsman calls attention to the fact that the stomodeum of annelids,
like the neural tube of a young amphioxus, is ciliated. Morgan ('90) has
found that the neural tube of frogs is ciliated. Delsman sees significance
in the fact that "while all other developmental processes of segmented
animals proceed from in front backwards, the formation of the neural
tube in lower chordates has its starting point at the blastopore and
proceeds in a forward direction."

If we assume that in chordates the stomodeum of invertebrates has
become elongated so as to carry the "cardiac pore" backwards to the
hinder-end of the body, the agreement between stomodeum and the
embryonic neural tube becomes almost complete. Delsman asserts
that in ontogenesis of vertebrates the backward shifting of the blastopore
is quite evident. Thus the conditions arise which necessitate the forma-
tion of a new mouth. That the definitive vertebrate mouth is a new
mouth is almost universally agreed.

"The primary mouth, being that of annelids," says Delsman, "is
represented by the neuropore of amphioxus, and this itself again is
phylogenetically secondary in respect to the "Urmund," the mouth of the
hydroid polyps, which in annelids we find again in the cardiac pore, and
in chordates in the neurenteric canal, both representing the former
blastopore. Thus in the ontogeny of vertebrates we see the three suc-
cessive mouths appear in the same succession as they appeared in phylo-

638 COMPARATIVE ANATOMY

geny: the blastopore ('Urmund'), the neuropore (the Annelidan mouth),
and finally the definitive vertebrate mouth."

In deriving the neural tube of vertebrates from a part of the alimentary
canal of pre-chordates, Delsman's theory may appear to resemble that of
Gaskell, which derives the ependymal lining of the neural tube from the
intestine of arthropods. The similarity, however, is superficial. Gas-
kell's theory assumes that the entire alimentary canal of the arthropod
becomes the lining of the neural tube, so that an endodermal structure
is converted into an ectodermal one. Delsman's hypothesis holds that
only a part, and that an ectodermal part, of the invertebrate intestine
becomes the chordate neural tube.

The conditions under which an ectodermal stomodeum might be
converted into a neural tube have been stated by H. E. Ziegler (1908).
If we start with a gastrula, he says, we must assume that it was primarily
nourished through the blastopore. The ciliated dorsal or neural plate
swept the food towards the blastopore, and consequently might have
acquired the function of a sensory epithelium. As the sensory plate
was converted into a neural tube, the stream of water entered the neuro-
pore and was carried through the neurenteric canal to the enteron. Then
came the formation of an anus, and later that of the definitive mouth and
gill slits. The neuropore and neurenteric canal became superfluous and
disappeared. Thus the neural tube, which had acquired a sensory func-
tion, became exclusively nervous and served as the central nerve cord.

In support of this speculation of Ziegler, Delsman calls attention to
the fact that in some gastropods the buccal ganglia of the nervous system
are derived as a proliferation of the stomodeum.

Others also have been impressed by the evidence that the neural
tube was formerly a part of the digestive system. Huntsman (1913)
writes, "It may be noticed that the condition in the tunicate strongly
supports the view that the neural tube was originally not nervous but a
part of the digestive system."

In agreement with most supporters of the annelid hypothesis, Delsman
compares the ventral side of the annelid with the dorsal side of the chor-
date. But, if the stomodeum of the annelid forms the nerve cord of
chordates, then what becomes of the chain of nerve ganglia of the annelid
when transformed into a chordate? According to Delsman, the annelid
ganglia are represented in the dorsal ganglia of vertebrates. These are
formed, as would be expected, between the stomodeum and the skin in
annelids, and in chordates from the neural crest. The fact that the
ganglia of chordates are purely sensory, while those of annelids contain
motor cells, is an objection to this homology which Delsman has attempted
to meet. He points out that dorsal nerves in chordates are primarily
mixed in function, and that the motor cells of the sympathetic ganglia are

THE ANCESTRY OF THE VERTEBRATES 639

mostly, if not exclusively, derived from the dorsal ganglia. Hence the
possibility should not be denied that the ganglia of annelids and chordates
are homologous.

The fact remains, however, that the chordate basis for a rellex action
is the spinal cord and not the spinal ganglia. Consequently, the accept-
ance of the Delsman hypothesis compels us to assume that the reflex
system of invertebrates has been lost, and a new reflex system invented
when annelids became chordates. The seriousness of this difficulty
seems not to have been fully appreciated by Delsman.

With Hatschek (1878), Delsman suggests that both the dorsal longi-
tudinal musculature and the circular muscles of annelids have disappeared
in the metamorphosis of the annelid into the chordate. But he thinks
that the transverse muscle of amphioxus may correspond to the circular
muscle of annelids, although their position is not similar.

The double muscular innervation, sensory and motor, demonstrated
by Boeke (191 1) in vertebrates is also found in annelids (Fraipont 1884).

The absence of an ectodermal foregut in vertebrates, Delsman takes
to accord with expectation, if his theory is a correct one.

Delsman derives the lateral eyes of vertebrates from the pit-eyes
of the preoral lobe of annelids. This conclusion, he thinks, is supported
by the embryological evidence that vertebrate eyes appear as pigmented
depressions of the neural plate in the region of the forebrain previous
to its closure. He assumes with Balfour (1881) that the forebrain of
vertebrates originates from the preoral lobe of annelids and amphioxus.
The preoral lobe or episphere of the trochophore larva contains paired
pigment spots which Delsman thinks can be traced into the paired eyes
of vertebrates. A step in the vertebrate direction is taken by those
annelids in which the pit-eyes become vesicular eyes. The absence of
eyes in amphioxus is interpreted, in agreement with Ray Lankester (1880)
and many others, as a result of degeneration. Delsman holds that verte-
brate eyes became secondarily included within the brain and, as the
animal became more opaque, grew out towards the skin. If, then,
the prechordal part of the vertebrate embryo is the homolog of the
preoral lobe of amphioxus, the prechordal forebrain and the eyes, would,
according to Delsman, be derived from the same source which in anneUds
gives rise to the cerebral ganglia and the eyes. The otic vesicles of some
annelids are paired organs lying just behind the mouth, in a position which
corresponds to that of the otic vesicles of vertebrate embryos.

Hatschek (1909) defended the assumption that the infundibulum
of craniotes represents the anterior boundary of the acraniote brain.
But Hatschek thought that the prechordal portion of the vertebrate
brain is an outgrowth from the epichordal region. This is not Delsman's
view. Delsman assumes a forward extension and incorporation of ecto-

640 COMPARATIVE ANATOMY

derm in front of the notochord. Consequently, Delsman stresses the
contrast between a prechordal archencephalon and an epichordal deuter-
encephalon. In early stages of vertebrate ontogenesis, the contrast
between the two regions is very evident. The line of demarcation between
the two is the plica ventralis which, according to Gegenbaur ('72), indi-
cates the limit between the segmented "vertebral" and the unsegmented
"prevertebral" part of the skull. Thus it follows, thinks Delsman, that
the brain vesicle of Amphioxus is to be compared with the deuteren-
cephalon of the vertebrate brain.

The similarity of larval amphioxus and annelid is seen in the relations
of the mouth of the annelid and the neuropore of larval amphioxus.
Both openings lie just posterior to the preoral lobe and just in front of
the first mesodermal segment.

Delsman regards the anterior endodermal diverticula of amphioxus
as the anteriormost pair of gill pouches, and assumes that the mesoderm
of the preoral lobe, like that in annelids, is formed by the forward exten-
sion of the first myotome. Thus the mesoderm of the preoral lobe is,
he concludes, secondary.

The notochord of amphioxus ends primarily at the level of the neuro-
pore, while in vertebrates the forebrain lies anterior to the notochord.
Hence it follows, according to Delsman, that the forebrain of vertebrates
has been formed from the ectoderm of the preoral lobe of amphioxus.

Delsman appears to think that comparison of annelid with vertebrate,
and more particularly the homology of the annelid mouth with the
neuropore of amphioxus, compels him to deny metamerism in the preoral
region and to assume that the mandibular segment with the prostomium
form not two segments but a single segment. Thus the anterior endo-
dermic diverticula of amphioxus become gill-pouches and not mesodermal
somites. Consequently, the series of three divisions which form the
"brain" of annelids have not the value of segmental ganglia and do not
represent as many segments. The trigeminus to him becomes a single
nerve, the first somatic, which belongs with the mandibular segment.
The definitive vertebrate mouth is assumed to be a fused pair of gill-slits.

Objections to the Delsman Hypothesis. Since Delsman assumes that
amphioxus is intermediate between annelids and vertebrates, he is
confronted with the difficulty of explaining the absence of well-marked
spinal ganglia in amphioxus, in which they should be more evident than
in vertebrates. He attempts to meet this difficulty by assuming that
either the ganglia of amphioxus have fused with the neural tube, or have
been lost by degeneration, as the cerebral ganglia have been lost.

Another difficulty arises out of the development of the neural tube.
From the standpoint of the Delsman hypothesis, we might expect the
neural tube to arise as a tubular invagination of the ectoderm, afterwards

THE ANCESTRY OF THE VERTEBRATES

641

lengthening from in front backwards, and thus pushing its inner end and
the former blastopore under the neural body-wall to the caudal end of the
embryo. This is not the case, as Delsman himself points out. But,

CAR0IOPOB£

rroMOOEuMi

^e^ PR£0«AL CIUATED

PROTONEPHRIOIUM-

MESODERM

Fig. 522. — A diagram of a trochophore larva. According to Delsman's theory of
the origin of vertebrates, the cardiopore (blastopore) of the trochophore larva is homolo-
gous with the neurenteric canal of vertebrates. In other words the relatively short
stomodeum of the annelid is stretched to become the neural tube of vertebrates.
(Redrawn after Hatschek.)

he claims, processes which occur in early development serve to explain
why the neural tube does not elongate in this fashion. The backward
growth of the blastopore in vertebrate embryos involves the extension

CHORDATE TROCHOPHORE ANNELID

NEUROPORE(

BLASTOPORE,
NEURAL TUBE\ ENTERON

FOREGUT
MOUTH

VnuS ! ''BLASTOPORE

EYE

^^-ANUS

MOLLUSC

Fig. 523. — Diagram illustrating the theory of Delsman that by change in the loca-
tion of the growth center a trochophore (A) may be converted into either a chordate
(B), or a mollusc (C), or an annelid (L>).

of the neural plate caudad. According to Delsman this is not a process
of concrescence, as is sometimes assumed, but a true process of backward
growth. This precocious backward growth takes the place of the back-
ward elongation of the neural tube. The result is the same as if there
had been an elongation of the neural tube— the blastopore is carried
to the posterior end of the body, where it persists for a while as the

642 COMPARATIVE ANATOMY

neurenteric canal of the embryo. The facts of ontogenesis, therefore,
according to Delsman, support his theory that the neural tube is the
homolog of the stomodeum of annelids. (Fig. 55)

In one of his preliminary papers, Delsman (1913) makes a suggestion
of vertebrate ancestry which will appeal to some who remain unconvinced
by the theory just outlined. This suggestion is that the characteristic
differences which separate phyla, such as the chordates, annelids, and
molluscs, may have arisen by changes in the region of growth by which a
larval form like the trochophore is converted into an adult. Morphology
has tended to lean heavily upon evidence from comparative anatomy
for phylogenetic conclusions, especially in the absence of decisive paleonto-
logical evidence. But the growing conviction that evolutionary progress
has been dependent primarily upon changes in the chromatin of germ
cells has made speculations which assume the evolution of one phylum
from another by the transformation of highly differentiated adults seem
less probable than they did to earlier morphologists. The suggestion
that three great phyla may have arisen from a simple larval form like a
trochophore, therefore, sounds attractive. Delsman suggests that, just
as the elongated annelid develops from the trochophore by the elongation
of the region posterior to the blastopore separating mouth and anus as
far as possible from one another at opposite ends of the body, precisely
so molluscs and chordates may have originated from a trochophore-like
ancestor by change of the growth center. Most molluscs grow, not
antero-posteriorly, but dorsally. In them, the mouth and anus remain
relatively near one another, while an elongated dorsal sac is formed, in
which the intestine and liver are bent into a U-shaped tract. Again,
by the elongation of the region anterior to the blastopore so as to separate
this from the anterior end of the body, a chordate form like the larval
amphioxus is produced. Such assumptions as these avoid the pitfalls
which are met in attempts to convert a complex adult of one phylum into
that of a different phylum. Furthermore, they seem to accord with
recent discoveries in genetics and embryology. If, then, this suggestion
of Delsman is actually in accordance with the facts of ontogenesis, it
follows that the divergence of the chordate phylum from the other inverte-
brate phyla, occurred before the annelid phylum made its appearance.
But Delsman himself appears to have abandoned his attempt to derive
chordates and other phyla from a trochophore larva and to favor the
opinion that the adult annehd is in the direct line of chordate ancestry.
(Figs. 521, 523)

THE ARTHROPOD THEORY

The old theory of Geoffrey St. Hilaire (1822) that an arthropod might
be converted into a vertebrate by reversing dorsal and ventral sides and

THE ANCESTRY OF THE VERTEBRATES 643

acquiring a new mouth has, in more recent years, been espoused by two
distinguished biologists, Gaskell (1908) and W. Patten (191 2), who have
followed the comparison of arthropods and vertebrates into greater
detail than has been done by other students of vertebrate phylogenesis.
Both lean heavily upon paleontological evidence. Both assume that
evolutionary change occurs through the transformation of one dominant
group into a higher dominant group. Both authors assume that the
most highly differentiated types of invertebrates such as the arachnids
are the progenitors of the vertebrates, contrary to prevalent opinion
which assumes that evolutionary change has affected chiefly generalized
types.

It is significant that the two reach fundamentally different conclusions
as to the most primitive types of vertebrates. These, according to
Gaskell, are the cyclostomes, while Patten regards the ostracoderms as
the immediate descendents of the sea scorpions. Both succeed in finding
an amazing degree of resemblance between limulus and a fish, and present
a great mass of evidence to prove this similarity. This resemblance,
however, seems to be more for the details of anatomical structure than
for fundamental characteristics.

Both authors agree that hemichordates, urochordates, and cephalo-
chordates are degenerate, not ancestral types. Both assert that the
comparative anatomy of the central nervous system affords the strongest
structural evidence of the similarity of arthropods, and more especially
arachnids, with vertebrates.

Gaskell's Hypothesis

According to Gaskell the neural tube of chordates is derived directly
from the alimentary canal of arthropod ancestors. The ventricles of
the vertebrate brain, Gaskell asserts, have evolved from the cavity
of the cephalic portion of the arachnid stomach. Their ependymal lining
corresponds to the mucous lining of the arthropod stomach. Differences
in their germ-layer origin do not disturb Gaskell, who is a physiologist!

Gaskell finds the brain of both scorpions and vertebrates to consist
of three chief divisions, forebrain, midbrain, and hindbrain. The con-
strictions which separate these represent ontogenetically two nerve
commissures, which cross the cephalic stomach in the prevertebrate stage,
as the result of the mid-dorsal position of the pineal eyes and of the
insertion of the superior oblique muscle. The anterior of the two is
the commissure between the two supra-esophageal ganglia. The anterior
end of the cephalic stomach of the arthropod becomes the lamina termi-
nalis of the vertebrate brain. The evolution of the vertebrate brain
from that of the arthropod is the result of the growth and extension of
nervous material, which was originally ventral to the stomach, around

644

COMPARATIVE ANATOMY

and over the epithelial wall of the cephalic stomach. (See Fig. 524.)
The presence of neuromeres in the brains of embryonic arthropods and
vertebrates is taken as another point of similarity between the two phyla.
According to Gaskell, the great enlargement of the brain in arthropods
made necessary a new mouth. Just before vertebrates appeared, the
arthropods were in a "terrible dilemma." They were compelled to
choose between the capacity for taking in food without intelligence
to capture it, or intelligence sufRcient to capture food and no power to
consume it. The enlargement of the brain, through which the esophagus

DORSAL

CORPUS STRIATUM'

VENTRICLES
OF BRAIN.
> INFRA-INFUNDIBULAR BRAIN.
^■'iNFUNDIBULUM
'CRURA CEREBRI

^^SPINAL
CORD.
SEGMENTAL NERVES

VENTRAL .

OGRSAL

SUPRA-ESOPHAGEAL
GANGLION

•ESOPHAGUS-
\CIRCUM-ES0PHAGEAL
, COMMISSURE.

VENTRAL.

Fig. 524. — Gaskell's diagrams (redrawn) illustrating how the alimentary canal
and the nerve cord of an arthropod (B) may be — in his opinion has been — converted
into the central nervous system of a vertebrate (A) without assuming a reversal of
dorsal and ventral surfaces. The arthropod stomach becomes the ventricles of the
vertebrate brain. The process obviously leaves the animal with the necessity of invent-
ing a new alimentary canal. How this is effected is illustrated in Fig. 527.

passed, meant the strangulation of the esophagus, while the atrophy
of the brain meant degeneration. Only a new intestine would solve
the problem. But since the skin cells of arthropods and cyclostomes
have been shown to have digestive powers, there is to Gaskell no more
difficulty in getting a new digestive system from the skin than there is
for an animal to gain a new respiratory or circulatory system. All of
this sounds teleological, but is probably not intended to imply conscious
purpose on the part of the animal.

As the result of the changes which occurred, the anterior ganglia
of the arthropod formed the cyclostome brain and their cephalic stomach
became the ventricles and ependymal lining of the brain, while the
intestine and nerve cord became the tubular spinal cord.

Gaskell believes that the assumption that cyclostomes are degenerate
forms is not substantiated by the evidence. The transformation of the

THE ANCESTRY OF THE VERTEBRATES
A,

64:

>-M

ANDROCTONUS

E. F AMMOCOETES G.

Fig. 525. — Diagrams illustrating the hypothetical evolution of the vertebrate brair
from the nerve cord of a primitive arthropod such as Branchippus (fairy shrimp).
Supraesophageal ganglia are shown in solid black, prosomatic ganglia stippled, meso-
somatic cross-hatched. A, olfactory nerve; B, optic nerve; C, nerve to median eye;
P, prosomatic nerves; M, mesosomatic nerves. (Redrawn after Gaskell.)

646 COMPARATIVE ANATOMY

ammocoetes larva into petromyzon proves that the animal is not degener-
ate. The resemblance of limulus and ammocoetes is attested by many
anatomical details. Both, for example, have two lateral and two median
eyes.

In agreement with many morphologists, Gaskell traces arthropods
back to an original coelenterate stock in which the central nerve cord
consists of a ring of nervous material which surrounds the mouth or the
umbrella. By the union of the two halves of the coelenterate umbrella
along a median line, the bilaterally symmetrical worm is formed. The
phylogenetic series then leads through annelids to arthropods. But, as a
consequence, the arthropod esophagus pierces the central nerve cord,
and divides it into a supraesophageal and a subesophageal ganglion.
Certain details of resemblance between annelids and vertebrates, such as
coelom, metamerism, coelomoducts, aortic arches, etc., have led to the
attempt to derive vertebrates directly from annelids. Since, however,
the central nerve cord of annelids is ventral while that of vertebrates is
dorsal, the annelid hypothesis of vertebrate ancestry necessitates the
reversal of dorsal and ventral sides of the body, and a new ventral mouth
to replace the old one which by reversal has been carried to the dorsal
side. In Gaskell's opinion "the difl&culties in the way of accepting such
reversal of surfaces have proved insuperable." By imagining the enclo-
sure of the arthropod stomach and intestine by the nerve cord and the
migration of the two towards the dorsal side, Gaskell seeks to avoid
the necessity for such reversal of surfaces. The necessity for a new
mouth, however, cannot be escaped, and his solution of that problem
will be stated later.

It is, however, obvious that changes such as are postulated by Gaskell
might quite as readily take place in annelids as in arthropods. Gaskell's
reason for deriving vertebrates from arthropods is the higher development
of the arthropod brain.

Gaskell writes: "If there is one organ more than another which
increases in complexity as evolution proceeds, which is the most essential
organ for upward progress, surely it is the central nervous system, espe-
cially that portion of it called the brain. This consideration points
directly to the origin of vertebrates from the most highly organized
invertebrate group — the Arthropods — for among all the groups of animals
living on the earth in the present day they alone possess a central nervous
system closely comparable with that of vertebrates. Not only has an
upward progress taken place in animals as a whole, but also in the tissues
themselves a similar evolution is apparent, and the evidence shows that
the vertebrate tissues resemble more closely those of the arthropod than
of any other invertebrate group.

THE ANCESTRY OF THE VERTEBRATES

647

"The evidence of geology points to the same conclusion. Evolution
takes place from one dominant stock to another dominant stock — man
from mammals, mammals from reptiles, reptiles from amphibia, amphibia

OBLIQUE MUSCLE

CEPHALIC STOMACH

A.

■ ESOPHAGUS

PHARYNX
PINEAL EVE

OLD MOUTH

OLFACTORY NERVE

OLFACTORY PIT

COXAL GLAND

ALIMENTARY CANAL

B.

BRAIN VENTRICLE
TROCHLEAR NERVE
INFUNDIBULUM
PINEAL EYE

SACCUS
VASCULOSUS

OLFACTORY NERVE

HYPOPHYSIAL TUBE

FACIAL NERVE

NEURAL TUBE

PITUITARY BODY VAGUS NERVE ' NOTOCHORD

BRAIN VENTRICLE

NEURAL TUBE

*-• FACIAL NERVE

TROCHLEAR NERVE

PINEAL EYE
OLFACTORY NERVE

NASAL TUBE

UPPER LIP
MOUTH

PITUITARY BODY VAGUS NERVE NOTOCHORD

Fig. 526. — Diagram depicting the hypothetical evolution of a vertebrate from an

arachnid (Eurypterus). A, Eurypterus as seen in median longitudinal section, B,

Ammocoetes (larval cyclostome), C, Adult Cyclostome (Petromyzon). (Redrawn

after Gaskell.)

from fishes. We are therefore forced to conclude that fishes came from
pre-Devonian paleostracan arthropods, the ancestors of Crustacea and
arachnids. These in turn came from trilobites. The earliest fishes

648 COMPARATIVE ANATOMY

Strongly resemble the paleostracan forms. The limulus or king-crab is a
living representative of the paleostracan ancestors of fishes. From such a
form vertebrates may have evolved without the necessity of reversal of
dorsal and ventral sides. This is possible if we assume that the infundibu-
lar tube of the vertebrate brain represents the esophagus of the arachnid,
that the vertebrate forebrain is the supraesophageal ganglion, the crura
cerebri the esophageal commissures, the remainder of the brain the
subesophageal ganglia. The ventricles of the brain correspond to the
cephahc stomach of the arthropod. In the arthropod, the cephalic
stomach leads directly into the straight narrow intestine; in the vertebrate
the fourth ventricle leads into the straight narrow canal of the spinal cord.

"In the arthropod the intestine terminates in the anus; in the verte-
brate embryo the canal of the spinal cord terminates in the anus by way
of the neurenteric canal. Keep the animal unreversed, and immediately
the whole mystery of the tubular nature of the central nervous system
is revealed, for it is seen that the nervous matter, which corresponds bit
by bit with that of the arthropod, has surrounded to a greater or less
extent and amalgamated with the tube of the arthropod alimentary canal,
and thus formed the so-called central nervous system of the vertebrate.

"The manner in which the nervous material has invaded the walls
of the tube is clearly shown both by the study of the comparative anatomy
of the central nervous system in the vertebrate, and also by its develop-
ment in the embryo.

"This theory implies that the vertebrate alimentary canal is a new
formation necessitated by the urgency of the case, and, indeed, there
was cause for urgency, for the general plan of the evolution of the inverte-
brate from the coelenterate involved the piercing of the anterior portion
of the central nervous system by the esophagus, while, at the same time,
upward progress meant their .development; brain development meant
concentration of nervous matter at the anterior end of the animal, with
the result that in the highest scorpion and spider-like animals, the brain
mass has so grown round and compressed the food-tube that nothing but
fluid pabulum can pass through into the stomach; the whole group have
become blood-suckers. These kinds of animals, the sea scorpions, were
the dominant race when vertebrates first appeared; here in the natural
competition among members of the dominant race the difficulty must
have become acute. Further upward evolution demanded a larger and
larger brain with the ensuing consequence of a greater and greater dif-
ficulty of food-supply. Nature's mistake was rectified and further
evolution secured, not by degeneration in the brain region, for that
means degradation and not upward progress, but by the formation of a
new food-channel, in consequence of which the brain was free to develop
to its fullest extent. Thus the great and mighty kingdom of the verte-

THE ANCESTRY OF THE VERTEBRATES

649

brates was evolved with its culminating organism — man — whose massive
brain with all its possibilities could never have been evolved if he had
still been compelled to pass the whole of his food through the narrow
esophageal tube, still existent in him as the infundibular tube."

^NOTOCHORDAL GROOVE .

NOTOCHORD.

AUMENTARY CANAL,

HEART

FAT BODY.

NERVOUS SYSTEM

MESONEPHROS

NOTOCHORD

-MYOTOME

APPENDAGE

ATRIAL CHAMBER

METACOELE

PLEURON .

c

Fig. 527. — A diagram illustrating how a trilobite-like animal (.4) might evolve into
a vertebrate (D). B and C are hypothetical stages transitional between A and D.
All figures represent cross sections. Since Gaskell's theory of the origin of vertebrates
assumes that the arthropod intestine has become the lining of the brain and spinal cord
of vertebrates, a new alimentary canal and a notochord have to be invented. These,
according to Gaskell, arise by infolding of the skin along the mid-ventral line, first the
notochord and later the alimentary canal. Germ layers mean nothing to Gaskell.
Pleural appendages unite to form the body-wall. (Redrawn after Gaskell.)

As further evidence of the genetic relationships of arthropods and
vertebrates, Gaskell points out that eurypterids and limulus, like ostra-
coderms, have both median and lateral eyes. Furthermore, arachnids,
like vertebrates, have an inverted retina, while other invertebrates have

650 COMPARATIVE ANATOMY

a simple upright retina. The resemblance holds, it is asserted, for the
finer histological details of the retina.

Gaskell is much impressed by the evidence of the similarity of the
skeletal elements of limulus and ammocoetes. "The evidence of the
origin of the cartilaginous skeleton of the vertebrate points directly to
the origin of the vertebrate from the Palaeostraca, and is of so strong a
character that, taken alone, it may almost be considered as proof of such
origin." Branchial cartilaginous bars are found in both. The branchial
skeleton of ammocoetes begins where that of the arachnids, represented
by limulus, leaves off.

The branchiae of ammocoetes are homologized with the branchial
appendages of limulus, which are "sunk-in" during ontogenesis. Since,
as a matter of fact, the branchial appendages of limulus are covered with
ectoderm, while the epithelial covering of the gills of cyclostomes is
endodermal, morphologists find it difficult to accept this homology.
Germ layers, however, mean little to Gaskell.

The cranial nerves of vertebrates are identified with those which
innervate the branchial appendages.

Gaskell has more difficulty in finding in vertebrates a homologue of the
limulus heart. Since the heart of limulus is dorsal, a new heart has to
be formed when arachnids become vertebrates. Gaskell assumes, there-
fore, that a pair of ventral venous sinuses in limulus unite to form a median
ventral heart in vertebrates. This change is recapitulated in the onto-
genesis of the vertebrate heart. The remnants of the limulus heart are
found by Gaskell in the so-called fat-column, which lies dorsal to the
spinal cord in ammocoetes but is lacking in higher vertebrates.

Gaskell appreciates the difhculty involved in attempting to derive
the heart and circulatory system of a vertebrate from that of an arthropod.
He calls attention to the fact that the invertebrate heart is systemic and
drives the arterial blood directly to the different organs of the body.
But the vertebrate heart is branchial and drives the blood to a ventral
aorta from which it is carried to the gills. The distributing systemic
vessel of vertebrates is the dorsal aorta, not the heart, which belongs
essentially to the ventral venous system. The invertebrate heart is,
therefore, not homologous with the vertebrate heart and the vertebrate
heart is new.

Gaskell derives the vertebrate thyroid gland from the uterus of the
paleostracan ancestor. This homology is made without regard to the
fact that the thyroid is endodermal in origin while the crustacean uterus
is ectodermal. The endocrinal influence of the thyroid upon the gonads
is cited as evidence in favor of this homology.

The hypophysis of petromyzon, with its associated olfactory pits,
is compared by Gaskell with the olfactory tube of scorpions. Coelomic

THE ANCESTRY OF THE VERTEBRATES 65 1

cavities resembling those of vertebrate embryos are found in limulus.
Limulus has seven prootic segments, and a similar number should be found
in vertebrates — all supplied by the trigeminal nerve. Of these seven
segments, the first is represented in vertebrates by Miss Piatt's anterior
somite, segments 2 to 5 by the pre-mandibular cavity, 6 and 7 by the
mandibular cavity. The chiasma of the trochlearis nerve is explained
by the fact that, in scorpions, the homologue of the superior oblique
muscle, alone of all the head muscles, crosses the mid-dorsal line to be
attached to the other side, carrying its nerve with it.

The tentacles of ammocoetes are assumed to be remnants of the
prosomatic appendages of the paleostracan ancestor. The mucocartilage
head-shield of ammocoetes is considered to be the exact counterpart of the
head-shield of cephalaspis.

Gaskell interprets the flabellum of limulus and the pectens of scorpions
as homologues of the auditory organ of vertebrates. The various endo-
crinal organs of vertebrates — pituitary, thymus, etc. — are assumed to
come from the coxal glands of Protostraca, or primitive arthropods, which
were the common ancestors of arachnids and Crustacea.

Gaskell's speculations necessitate the invention of a notochord and a
new alimentary canal to replace the old one which formed the neural
tube. He imagines that among the Protostraca were forms somewhat
resembling trilobites but with annelid affinities, which, like Apus, possessed
a deep ventral groove extending from one end of the body to the other,
and also pleural fingers, as in many trilobites. This groove on the ventral
side of the body was converted into a tube, and so gave rise to the noto-
chord, while the appendages were still free and the pleurae had not met
to form a new ventral surface. (See Fig. 527.)

Passing from this protostracan to a paleostracan stage, oral and
respiratory chambers were formed, not communicating with each other.
A ventral groove, however, connected the respiratory chamber and cloaca.
Finally, with the conversion of this groove into a tube to form an alimen-
tary canal, the opening of the oral into the respiratory chamber, and the
formation of an atrium by the ventralward growth of the pleural folds,
the vertebrate was completed. The pharynx was formed by the fusion
and concrescence of the crustacean legs, the spaces between the legs
becoming gill slits. Because of the digestive power of the epidermis,
Gaskell sees no difficulty in converting skin into a digestive canal.

The suggestion that the notochord may have originated as an accessory
digestive tube accords with the evidence that it is formed in some verte-
brates in connexion with the alimentary canal.

Gaskell asserts that since the germ-layer theory is discredited by
embryologists themselves, it may not properly be raised against his theory,
which ignores it. Gaskell attacks the germ-layer theory as fallacious.

652 COMPARATIVE ANATOMY

It asserts that the aUmentary canal is homologous in metazoa because it is
formed of endoderm. But there is no definition of endoderm except that
it is always the layer which forms the definitive alimentary canal. He
therefore concludes that the guts of vertebrates and Crustacea are not
homologous.

There appear to be two fatal objections to the Gaskell arachnid hypo-
thesis of vertebrate ancestry. First, the theory ignores the evidence of
the homology of the germ layers, by deriving the neural tube from the
alimentary canal. Second, the theory leaves a heart between the pharynx
and the nervous system, so that in order to meet this difficulty it is
necessary to assume that the crustacean heart disappears and that a new
one takes its place. There is, however, no embryological evidence to
support this assumption.

It may be further urged against the theory that it pictures the
ependyma of the vertebrate neural tube as receiving cellular increments
from adjacent tissue to form the nervous tissue of the brain and cord.
In ontogenesis, however, the mantle and marginal layers of the neural
tube are not formed by additions from without, but by the proliferation of
cells of the ependymal layer itself.

Of Gaskell's theory. Patten, who has an arachnid theory of his own,
says: "Gaskell at least makes a valiant fight to save the pieces of the
invertebrate nervous system, even if he does annihilate the rest of the
animal in the attempt."

Patten's Arachnid Hypothesis

According to Patten (191 2), the structural plan of an arachnid such
as limulus and a primitive vertebrate such as an ostracoderm is "all the
same," but the arachnid head and body correspond to the vertebrate head
only. Nearly all of the vertebrate body consists of a new generation of
metameres, not represented in arachnids. (Figs. 10, 17)

Patten, although agreeing with Gaskell that the vertebrate mouth is
new, differs in his explanation of the closure of the old mouth. In Patten's
view, the closure of the old mouth is due to the "conditions created by
apical growth, by cephalization, and by the increase in the volume of the
yolk sphere." The new mouth is derived from an ancient arthropod organ
known as the "dorsal organ," which in limulus becomes the "cephalic
navel." What its original significance may be is not apparent.

Patten sees no difficulty in converting the nerve cord of arachnids into
the tubular cord of vertebrates. In the brains of both he finds the same
five divisions. The resemblance holds good even for their subdivisions
and their histological details. He lays much stress upon the numerical
correspondence between the neuromeric divisions of the brain described
by Locy and Hill and those which he finds in limulus. There are, he

THE ANCESTRY OF THE VERTEBRATES

653

says, thirteen neuromeres in the arachnid brain, but these do not corre-
spond with the number of metameres in the head. Arachnid and verte-

Ms. a

Br.C

st.co. ..."

Fig. 528. — Diagrams "showing the five characteristic body regions of arthropods,
and their progressive concentration to form the head of a vertebrate." A and B,
Insects; C, Arachnid; D, Vertebrate. The principal points illustrated are: The enlarge-
ment and concentration of the anterior cephalic neuromeres; the closure of the old
mouth and the formation of a new one; the transfer of locomotor organs from the
mesocephalon to the postbranchial metameres. (From Patten's "Evolution of Verte-
brates and Their Kin.")

brate brains are also in essential agreement in the distribution of the main
fiber tracts. The fact that the arachnid has nothing comparable with
the vertebrate trunk makes it seem to Patten impossible to compare the

654 COMPARATIVE ANATOMY

cranial and spinal nerves, since they have diverse origins. In the head,
however, he recognizes in arachnids the twelve cranial nerves of verte-
brates from the olfactory to the hypoglossal.

The peripheral nervous system of arachnids attains a condition similar
to that in vertebrates. The neural (ventral) nerves of arachnids like
the dorsal nerves of vertebrates, are ganglionated, and the hemal (dorsal)
nerves are non-ganglionated, like the ventral nerves of vertebrates.

In order to compare arachnid and vertebrate hearts, Patten assumes
that the arachnid heart represents the ventricular portion of vertebrates,
and that the posterior portion of the pericardial cavity of arachnids
corresponds with the atrium and sinus venosus of vertebrates.

But even Patten, who is so impressed by the resemblances of scorpions
and vertebrates, is compelled to admit that comparison of arachnid with
vertebrate circulation is difficult. He is, however, curiously reticent in
regard to the fact that the blood in limulus flows to the heart from the gills,
and not the reverse as in vertebrates. In limulus five veins bring blood
from the gills to the pericardial cavity. It is difficult to understand why
Patten should label the last of these, and only the last, the ductus Cuvieri.
According to Patten, the auditory organ of vertebrates is represented
in the limulus embryo by a large discoidal placode which lies opposite
the fourth pair of legs. There is, however, no evidence that this is
auditory, or even sensory, and it disappears early in development.

Both parietal and lateral eyes of vertebrates are homologous with
those of arachnids. During the evolution of vertebrates from arachnids.
Patten assumes, there was a considerable period during which the lateral
eyes were adjusting themselves to their new position inside the brain
chamber, and when they were in functional abeyance. When the lateral
eyes again became functional, the parietal eye began to decrease in size
and efifectiveness.

"The characteristic shape of the arthropod eye and the arrangement
of its retinal cells is retained in an exaggerated form in the vertebrate
retina, and affords us the only satisfactory explanation of its inversion,
its contour and mode of growth, its choroid fissure, its arrangement of
rod and cone cells, and its centrally located optic nerve.

"The agreement between the olfactory organ of limulus and that of
vertebrates may be traced in respect to so many different characters that
the existence of a genetic relationship between the marine arachnids and
the vertebrates is placed beyond a reasonable doubt. Indeed, there is a
greater difference in respect to this organ, between limulus and other
invertebrates than there is between limulus and vertebrates."

The olfactory apparatus arose in the higher arachnids through the
secondary modifications of pre-existing organs which had some other
function or meaning.

THE ANCESTRY OF THE VERTEBRATES

stc.

p.e. p Of g- St c-

655

Thp^ ?i'i„/t?;T°Vi?''^™u '"ustratmg the principal stages in the evolution of segmented animals.
rln=r^„ ^f fvfi li substitution of one organ for another. The most striking illustration is the
,>, ?l»^.» ■ ?! "IT^^"- f-n'i stomodeum and the formation of a new opening into the mesenteron
k thl ^t^^Zi- *''«, dorsal organ ; the substitution of lungs for gills. The most striking innovation
^PtTJilJl^^- % * D "If "^ ^^P^''^^"? gill sacs and enteric diverticula; the rise and decline of
metamerism. (From Patten's "Evolution of Vertebrates and Their Kin.")

656 COMPARATIVE ANATOMY

The segmental taste organs and slime buds of the arachnids are the
forerunners of the special cutaneous organs of vertebrates, the taste organs
of the arachnids corresponding with the taste-buds of the vertebrates,
and the slime buds probably in part with the neuromasts of lateral line
organs.

Nothing resembling the dermal skeleton of limulus occur in any other
invertebrate. But this skeleton resembles that of the ostracoderm,
Pteraspis, more than does the skeleton of any other known anim.al,
vertebrate or invertebrate. Since ostracoderms are vertebrates, the
dose affinity of limulus to vertebrates is thus attested. The ostracoderm
skeleton is a highly specialized one, produced by an exaggeration of the
type of skeleton seen in limulus. Patten, however, admits the possibility
that the exoskeleton of ostracoderms may be mesodermal and not, like
that of Hmulus, ectodermal. But in his opinion, very slight changes
would be necessary to convert the skeleton of limulus into that of verte-
brates. He is inclined to assume that the mesenchyme which forms
dermal bone in vertebrates is of ectodermal origin. The. fact that the
ectodermal part of the skeleton of limulus is chitinous while that of verte-
brates is enamel does not disturb Patten, since in his opinion they "have
essentially the same structure and mode of growth."

Patten also finds a cartilaginous endocranium characteristic of the
arachnid-vertebrate stock. "The fully developed arachnid endocranium
is in every essential respect a duplicate of the primordial cranium of a
primitive vertebrate embryo." They agree, he says, in position, general
form, mesodermal origin and histological structure, absence of metamer-
ism, and in their planes of growth.

The resemblance also holds for the branchial skeleton. In limulus,
as in vertebrates, the "branchial bars" are postrcranial, and may be
united by longitudinal bands. The gill-bars of both consist of muco-
cartilage and not, like the cranium, of fibro-cartilage. The pre-auditory
region is without gills and gill bars.

The neural arches of limulus represent the initial stages in the forma-
tion of a vertebral column. They agree in general form and in the direc-
tion of their processes, with the neural arches of vertebrates. The
homologue in arachnids of the notochord of vertebrates is the bothroidal
cord or lemmatochord. Lemma tochord and notochord have according to
Patten a similar embryonic origin and similar relations to the nerve cord
in the adult.

Objections to the Arachnid Hypothesis of Patten. It is customary
to object to the arachnid hypothesis on the ground that arachnids, such as
Hmulus, are highly specialized animals, while evolution proceeds by the
transformation of generalized forms. The very fact that limulus has
changed little, if at all, in miUions of years might seem to weaken the

THE ANCESTRY OF THE VERTEBRATES 657

case based upon the assumption that vertebrates are descendants of a
limulus-like arachnid ancestor.

Patten's hypothesis is based upon the assumption that ostracoderms
are primitive vertebrates. But morphologists are generally of the opinion
that ostracoderms are highly specialized descendants of elasmobranchs,
more closely related to teleosts than to cyclostomes.

In order to reconcile his homologies of arthropod and hemichordate
embryos, Patten is obliged to ignore the fact that the primary blastopore
in hemichordates becomes anus and not mouth. He assumes that the
water pore of hemichordates is the primitive mouth or "neurostoma,""
and that the blastopore is a " telopore." Likewise, in spite of the evidence
of the origin of the notochord from the primary endoderm of amphioxus,
Patten asserts that the notochord is ectodermal in origin, apparently
because the lemmatochord of limulus is ectodermal.

Patten's theory compels him to locate the primitive mouth in the floor
of the neural plate, at the place where later the infundibulum develops.
That this is a late and secondary development seems not to matter to him.
Neither does the fact that embryology presents not the slightest evidence
that the foregut was ever open to the exterior at this point. Also, in
order to substantiate his hypothesis, Patten is compelled to assume that
echinoderms, urochordates, balanoglossus, and cephalodiscus are degen-
erate offshoots of a common arthropod-vertebrate stock. The evidence
of this degeneration is, to say the least, not convincing.

Patten's argument for the close blood relationship of arachnids and
vertebrates is based largely, if not exclusively, upon details of anatomical
structure rather than upon fundamental resemblances. The striking
contrast in basic features of the two groups does not accord with expecta-
tion, if arachnids and vertebrates are closely related phyla. This diffi-
culty has left biologists unimpressed by the mass of evidence of detailed
resemblance gathered by Patten.

Consequently, the evidence of similarity in some, or even many,
details of structure fails to convince zoologists of their genetic affinities.
Furthermore, the argument that one dominant group of vertebrates has
succeeded another dominant vertebrate group does not justify the con-
clusion that fishes have come from a dominant group of invertebrates.
When two able biologists such as Gaskell and Patten, on the basis of
identical material, reach diametrically opposite conclusions, it is not
surprising that a skeptical attitude toward phylogenetic speculation
characterizes this generation of zoologists.

The Nemertean Hypothesis

In 1883, Hubrecht advanced the hypothesis that nemerteans are
the long-sought ancestors of vertebrates. Among advantages of this

658

COMPARATIVE ANATOMY

theory is that it derives vertebrate from invertebrate without reversing
dorsal and ventral sides. Hubrecht assumes that the paired nerve
cords of the Nemertean unite in the mid-dorsal line to form the nerve cord

of vertebrates. The paired cere-
bral lobes of the nemertean become
the brain of the vertebrate.
Hubrecht compares the intestinal
diverticula of the flatworm with
the coelomic outpocketings of the
amphioxus embryo. The more
anterior diverticula are possibly the

Fig. 530. — Diagrams illustrating Hu- r 1 -n 1 r

brecht's theory of the origin of the noto- homologues of the gill pouCheS ot

chord from the proboscis sheath of rhordates

nemerteans. A, diagram of a nemertean, ,■ ■, i • r j-rc

B. of a chordate. The proboscis of the One of the chief difficulties in

nemertean according to Hubrecht becomes making Comparison between

the vertebrate hypophysis. °

vertebrates and invertebrates has
been the absence in invertebrates of any structure similar to the
notochord. By comparing the proboscis sheath of nemerteans with
the notochord, Hubrecht thinks that he has removed this obstacle

ECTODERM

CIRCULAR
MUSCLES

DORS NERVE-CORD
LONG. MUSCLES

PROBOSCIS SHEATH
V- PROBOSCIS

r GONAD

LATERAL
NERVE-CORD

LATERAL
BLOOD-VESSEL

PARENCHYMA

NEMERTEAN-CROSS SECTION
Fig. 531. — A cross section of a Nemertean. While the nemertean lacks a coelom,
the gonadic sacs have been compared with the coelomic sacs of coelomate animals.
(Redrawn after Lang.)

and has thus demonstrated that nemerteans are the connecting link.
The nemertean proboscis, which is of ectodermal origin, is homo-
logized with the vertebrate hypophysis. Hubrecht points out that the

THE ANCESTRY OF THE VERTEBRATES

659

hypophysial ingrowth is directed towards the anterior termination of the
notochord. If it were to grow into the notochord, its relations would be
similar to those of the proboscis to its sheath.

Hubrecht, however, fails to show that the proboscis sheath is endo-
dermal. It is in fact mesodermal, while the notochord is endodermal.
The two structures are, therefore, not comparable. This objection
remains, even if the notochord sometimes arises as a hollow outgrowth
and therefore may have been, like the proboscis sheath, originally hollow.
Two hollow structures may be compared, but only if derived from the
same germ layer. Further difficulty appears in the fact that in the
lowest vertebrates, the myxinoids, the hypophysis connects posteriorly
with the pharynx, not with the notochord.

NERVE CORD

Fig. 532. — Balanoglossus, the typical genus of the Hemichordates, seen in left lateral
aspect. The possession of both dorsal and ventral nerve-cords links hemichordates
on the one hand with invertebrates and on the other with vertebrates. (Redrawn after
Stempell.)

Nemerteans certainly do resemble vertebrates in having paired seg-
mentally arranged nerves connected with the paired nerve-cord, and
with a dorsal nerve-cord, when this is also present. There is, however,
not a regular alternation of sensory and motor nerves in nemerteans as in
amphioxus and vertebrates.

The case which Hubrecht is able to make in favor of the nemertean
ancestry of vertebrates is obviously not very convincing. This, however,
should not blind us to the fact that, in some ways, such as the presence of
gonadic sacs, the flatworms exhibit traits which may be considered
ancestral to those of vertebrates.

BALANOGLOSSUS THEORY

Bateson (1886) was the first to suggest that bilaterally segmented
animals have evolved in two directions. In one group, the alimentary
canal pierced the nervous system, leading to annelids and arthropods;

66o

COMPARATIVE ANATOMY

in the other, the nervous system was dorsal to the alimentary canal,
leading to chordates. Balanoglossus, the representative genus of hemi-
chordates, belongs in the latter group and exhibits many traits which are
transitional between those of non-chordates and vertebrates. The found-

APICAL PLATE

COLLAR CAVITIES

GILL SLITS^

CAVITY
DIVERTICULA)

COLLAR>v^,l

IRCULAR BAND
OF CILIA

''TRUNK CAVITIES

Fig. 533. — Balanoglossus embryos. .4. A horizontal section of a young embryo,
showing the origin of mesodermal pouches. MacBride and others have noted the
similarity of this section to that of a young amphioxus embryo as evidence of the close
affinity of the.se two forms. B. A young Balanoglossus larva with five pairs of gill slits
viewed from the left side. The gill slits of Balanoglossus bear a striking resemblance
to those of Amphioxus. On the other hand the young larva of Balanoglossus is strik-
ingly like the larva of echinoderms. (Redrawn after Bateson.)

ations of the balanoglossus theory of vertebrate ancestry are found by
Bateson in the presence of a rudimentary notochord, gill slits resembling
those of amphioxus, a dorsal nerve cord which is tubular in the anterior

GILL-SUT

•x.s.

B. HARRIMANIA - STEREOGRAM OF COLLAR REGION.

A. ESOPHAGEAL REGION -

Fig. 534. — Harrimania, a Hemichordate. .4. The dorsal portion of a cross section
of Harrimania in the region of the esophagus. The resemblance of this cross section
to one of amphioxus is striking and serves to demonstrate the close genetic affinities
of these two chordates. In Harrimania the notochord is present not only in the preoral
lobe as in other hemichordates but also in the collar and anterior pharyngeal regions.
B. A stereogram of Harrimania in the collar and anterior pharyngeal region, showing the
presence of the notochord in these regions. Such evidence tends to remove the doubt
that a true notochord exists in Hemichordates. (Redrawn after Ritter.)

portion, a preoral lobe with epithelial cavities resembling those of the
pre-oral lobe of amphioxus, segmentally arranged gonads like those of
amphioxus, three pairs of enterocoelic cavities, and the resemblance of
the tornaria larva of balanoglossus to the early embryo of amphioxus.
All of these features show that the affinities of balanoglossus are with

THE ANCESTRY OF THE VERTEBRATES

66l

chordates, but, unfortunately, do not connect it with any phylum of
invertebrates.

That the so-called notochord of balanoglossus is a true chorda dorsalis
like that of other chordates has been doubted. But Ritter (1900), on
the basis of his studies on Harrimania, a close relative of balanoglossus,
says that "there can be no serious doubt that we have in this organ the
immediate genetic forerunner of the vertebrate notochord." Its peculiar-
ities, in Ritter's opinion, do not warrant giving it another name, as has
been suggested. In Harrimania, two regions of the notochord are dis-
tinguishable, an anterior nuchal and a posterior esophageal. The poste-

NOTOCHORD^

-DORSAL ARTERY

^^ PHARYNX^^^

METAPLEURAL FOLD,

, NERVE CORD.

/^ 1/ GONAD--' I I ^

w \c

•^prBIRRANPHIAL CAVITY. ^^^

PERIBRANCHIAL CAVITY.

GILL APERTURE'.

I VENTRAL ARTERY.

GILL APERTURE

VENTRAL ARTERY^'

DIGESTIVE GROOVE.

A. AMPHIOXUS. B.PTYCHODERA.

Fig. 535. — Diagrams illustrating divergent methods by which the peribranchial
cavity is formed. In Amphioxus {A) the pleural folds are separated from the pharynx
by paired folds which extend dorsally from the ventral side. In the hemichordate
Ptychodera {B), on the other hand, the paired folds begin to form at the dorsal side of
the worm and extend ventrally. The peribranchial cavity in urochordates arises in a
similar manner. As frequently happens in animals, a similar end-result is attained by
divergent means. (Redrawn after Gaskell.)

rior is embryonic and transient in other genera of hemichordates, but
persists in Harrimania, which must therefore be regarded as a more
primitive type. In the possession of a lumen, the notochord of balano-
glossus is peculiar; but in some chorda te embryos as in Harrimania the
notochord arise as a grooved outpocketing of the median dorsal wall of
the endoderm. (Fig. 534)

While the connexion of balanoglossus with lower vertebrates is obscure,
the afhnities of its tornaria larva with the larvae of echinoderms and
with such primitive types as Cephalodiscus and Rhabdopleura are sug-
gestive of relationship with lower invertebrates.

APPENDICULARIA THEORY

Following Kowalewsky's (1868) researches upon the embryology of
urochordates and amphioxus, it was supposed that the long-sought
ancestor of vertebrates had been found, and the tunicate larva was pro-

662

COMPARATIVE ANATOMY

claimed to be the missing link. The pendulum of opinion, however, later
swung away from this view, when Dohrn and Ray Lankester made it
seem probable that tunicates are degenerate vertebrates. But . iks
(1893), after a thorough investigation of the anatomy and develop
of pelagic tunicates, stated that he was unable to find any t
degeneration in Appendicularia, a pelagic larval type of Urv

PROBOSCIS PORE'

NERVE STRAND

.CONAO

NOTOCHORO' MOUTH' I ^"*''^''*

IPHARYCEAL POOCH

CEPHALODISCUS.
Fig. 536. — A diagram of Cephalodiscus viewed from the left side as if in median
optical section. The presence of a notochord in the pre-oral lobe is one of the reasons
for placing this animal among hemichordates. While not regarded as a form "ancestral "
to vertebrates, cephalodiscus interests morphologists as a primitive chordate. (Redrawn
after W. Patten.)

As a result, Brooks totally rejected the dogma that vertebrates are
modified annelids and the assumption that tunicates are degenerate
vertebrates. He regards the free-swimming appendicularia as the type
from which chordates have evolved, and thus revives the tunicate theory
of vertebrate ancestry. The many points of resemblance of appendicularia

STATIC ORGAN V

>ENDODERM STRAND

PHARYNX-
GILL SLITS'"

OCELLUS) (HE^T

Fig. 537. — Diagram of a larval urochordate. The similarity of the larval uro-
chordate to the embryo of a cephalochordate (amphioxus) suggests that a form like
this lies near the main line of vertebrate ancestry. (Redrawn after von Beneden and
Julin modified.)

to amphioxus, and the similarity of their development, give a strong
foundation to this hypothesis. Appendicularia, like the larvae of other
tunicates, is obviously a chordate with fundamental chordate traits —
notochord, hollow dorsal nerve cord, expanded "brain" vesicle, gill-
slits, endostyle, and ventral heart. Indeed, the resemblances between
tunicates and amphioxus led Minot to suggest that the urochordates and
cephalochordates should be grouped together as Atriozoa.

THE ANCESTRY OF THE VERTEBRATES

663

The relations of the thyroid gland in cyclostomes, and its derivation
from a hypostyle-like structure in the embryo, strongly suggest that
vertebrates came from an animal like the cephalochordates and uro-
chordates, in which food was drawn by ciliary action into an enlarged and
elongated pharynx and collected by the mucus-secreting endostyle.

OPENING OF PERIBRANCH.CAV.
! /ANUS

A. LARVAL UROCHORDATE

/DORSAL NERVE

GILL SLITS'

ANUS
POSrn OPENING OF
PERIBRANCHIAL CAVITY

B. CEPHALOCHORDATE

,DORS. KtRVE CORD

_^„--,-/^ / \ \. ,WF~ ^INTESTINE ANUSi

THYROID' l-fciLL SUTS ^HEART OLIVER

C. VERTEBRATE CCYCLOSTOME)
Fig. 538. — Diagrams of A, Larval urochordate, B, Cephalochordate (Amphioxus)
and C, Vertebrate (Petromyzon), illustrating the fundamental characteristics of
chordates redrawn after Hesse-Doflein. If the so-called liver of the cephalochordate
be homologized with the intestine of urochordates the similarity of the two groups
appears to be greater. The endodermal strand in urochordates which is comparable
with the intestine of cephalochordates degenerates and disappears during metamorphosis
when the tail is lost. The branchial basket of both urochordates and cephalochordates
becomes surrounded by a peribranchial cavity. The evidence appears to justify the
assumption of a common genetic origin of the two groups.

Appendicularia meets all the requirements of such a hypothetical ancestor
of chordates.

Brooks suggests that gill slits originated in tunicates, not as respiratory
organs, but to allow water to escape from the pharynx, so that food which
passes to the stomach and intestine can be more concentrated. Dohrn's
hypothesis that gill slits are modified segmental nephridia lacks proof.
Dohrn himself admits that he is unable to "assign any reason (sic) why
segmental organs should unite with the gut." The suggestion obviously
is based on the assumption that vertebrates are derived from annelid

664

COMPARATIVE ANATOMY

ancestors and that urochordates are degenerate vertebrates. Brooks
denies the truth of both of these assumptions.

The objection most frequently raised against the appendicularia
theory is that tunicates are chordates, and that this theory does not
carry us beyond the Hmit of the phylum. The purpose of a theory of
vertebrate ancestry, however, is to reveal the non-chordate predecessors of
chordates. But there appear to be no invertebrates which can be con-

LATERAL TRUNK MUSCLES

SPINAL CORD
NOTOCHORD

DORSAL AORTA-

PRECARDINALV

EPIBRANCHIALGROOVE ||-^:S?£

GILL LAMELLAE

CARTILAGE BAR
GILL-RODS

TRANSVERSE MUSCLE
VENTRAL AORTA
PERIBRANCHIAL CAVITY

HYPOBRANCHIAL MUSCLE
METAPLEURAL FOLD

A. AMPHIOXUS B. AMMOCOETES.

Fig. 539. — Cross sections of A, Amphioxus and B, Ammocoetes (larval Petromyzon)
through the pharyngeal region showing their fundamental resemblance. Their simi-
larities even in details of histological structure seem too great to be the result of con-
vergence of genetically unrelated forms. Just as the search for the most primitive
forms of vertebrates leads from elasmobranchs to cyclostomes, so in turn Amphioxus
shows characters similar to, but simpler than, those of vertebrates. It is therefore not
surprising that most morphologists place amphioxus near the main line of vertebrate
ancestry.

sidered the immediate predecessors of chordates — no forms transitional
between chordates and the much simpler coelenterates.

CONCLUSION

A survey of these various attempts to solve the problem of the ancestry
of vertebrates seems to suggest that the most promising clue to the solu-
tion is to be found in the lower chordates. Cyclostomes, more especially
their larval stages, lead us in the direction of Amphioxus, which "if it
hadn't existed, would have had to be invented." The similarity of
amphioxus embryos to larval tunicates strongly suggests their common
origin. Since this seems the most reasonable interpretation of the facts,
we may conclude that metamerism has been attained de novo by chor-

THE ANCESTRY OF THE VERTEBRATES

665

dates, and not inherited from metameric invertebrates. Hesitation to
accept this conclusion seems to be due chiefly to the fact that no pre-

HOMO

SINANTHROPUS

AUSTRALOPITHECUS-

PROPLIOPITHECUS«

INSECT IVOR

MONOTREMES-
STEGOCEPHALA
DIPNOI

MOLLUSCS
ARTHROPODS

ANNELIDS

MOLLUSCOIDS

ROTIFERS

THREADWORMS

CYCLOSTOMES
.UROCHORDS

FLATWORMS

DRYOPITHECUS
LEMUROIDS

'MARSUPIALS

•THEROMORPHS

CROSSOPTERYGIANS

ELASMOBRANCHS

CEPHALOCHORDS
HEMICHORDS

ECHINODERMS

OELENTERATES

SPONGESv

PROTOZOA

Fig. 540. — A diagram of the phylogenesis of man, based on the assumption that the
Protochordates resemble vertebrate ancestors more closely than do the annelids.
Annelids and other Proterostomians have, it is assumed, diverged from the main line
of vertebrate ancestry very early in animal phylogenesis. Before man emerged in the
late Tertiary transitional forms between man and the lower Primates made their
appearance. Among these the best known are Propliopithecus, Dryopithecus and
Australopithecus, which are true "connecting links."

chordates can be found which resemble larval chordates. Thus the
chordate clue seems to lead us into a blind alley out of which the most
promising exit is the way back.

666 COMPARATIVE ANATOMY

The absence of bony skeletal structures in the simpler chordates
makes it appear unlikely that paleontological evidence can be used in
the solution of the problem. Students in the past have possibly depended
too much upon comparative anatomy. The study of genetics has not
tended to support the orthodox assumption of the transformation into
chordates of adult invertebrates of the types now extant. Future stu-
dents of phylogenesis will turn to embryological evidence more than has
been done in the past. Delsman's suggestion of a common origin of
annelids, molluscs, and chordates by alteration in the growth centers of a
trochophore-like ancestor sounds plausible.

For the present, morphologists must resort to the eclectic method
used in this text, and select the hypothetical stages in the evolution of the
various organ systems in whatever invertebrate types they may be found.
However, in view of the evidence of the convergent and independent
appearance of similar structures in different phyla, phylogenetic "series"
are necessarily speculative and tentative.

Among the invertebrate groups with traits resembling those of the
hypothetical ancestors of vertebrates this text has mentioned Protozoa,
Coelenterates, Platyhelminths (including Nemerteans), Nematodes, and
Annelids. More closely related to vertebrates than any of these are
Hemichordates, Urochordates, and the Cephalochordates, which may
therefore be regarded as having some of the ancestral traits. None of
these, however, lies in the main line of vertebrate descent, as is indicated
by the phylogenetic tree of the animal kingdom shown in Fig. 540. The
main line of vertebrate phylogenesis is today better represented in larval
stages like the trochophore than in any adult forms. On the whole,
the evidence supports the conclusion that the separation of animal phyla
occurred farther down in the phylogenetic series than has hitherto been
assumed. See Fig. 520.

GLOSSARY

abdomen. The portion of the body between thorax and pelvis.

abducens. The sixth cranial nerve.

abduction. The withdrawal of a part from the median plane.

aberrant. Wandering from the usual.

abnormality. Deviation from the normal.

abomasum. The fourth stomach of a ruminant.

aboral. Opposite to the mouth.

abortion. Expulsion of a fetus before it can live.

acetabulum. The socket in the coxal bone in which the head of the femur articulates.

acidophilic. Easily stained with acid dyes.

acidosis. Reduction of alkali reserves in the body.

acinus. A grape-like terminal subdivision of a gland or lung.

acoelomate. Without a body cavity (coelom) .

acrania. Chordate animals without a brain-case.

acrodont. With teeth fastened to edge of jaw and not lodged in sockets.

acromegaly. A disease which involves an enlargement of bones due to over-functioning

of pituitary gland.
acromion. The lateral extension of the spine of the scapula.
adduction. The act of drawing a part towards the median plane.
adenoid tissue. Lymphocyte-forming tissue.

adrenal (suprarenal) gland. An endocrine organ located near the kidney.
adrenin. The suprarenal medullary hormone.
afferent. Centripetal, conveying towards the center.

after -birth. Extra-embryonic membranes discharged from uterus after the child is born.
air bladder (sac). Respiratory or hydrostatic organ in fishes.
ala cinerea. The vagal eminence which projects into the fourth ventricle.
alar plate. The dorso-lateral portion of the embryonic neural tube.
alisphenoid. That part of the embryonic cartilage cranium which forms most of the

great wing of the sphenoid.
allantois. A hollow outgrowth of the embryonic hind-gut.
alveolus, i. A tooth-socket. 2. A terminal acinus of a gland. 3. A respiratory sac

of the lung.
ameloblast (adamantoblast). A cell which secretes enamel.
ammocoetes. The larval stage of Petromyzon.
amnion. The liquid-filled sac which encloses the embryos of reptiles, birds and

mammals.
amniota. The animals the embryos of which are enclosed in an amnion.
amphicoelous. Biconcave like the centrum of a fish vertebra.
ampulla. A flask-like dilatation.

amylopsin. A starch-splitting enzyme secreted by the pancreas.
analogy. Resemblance based on similarity of function.
anamnia. Vertebrates the embryos of which lack an amnion.
anastomosis. The communication of two vessels or connexion of two nerves,
angioblast. One of the cells from which blood and blood vessels develop.

667

668 COMPARATIVE ANATOMY

angular (e). A membrane bone of the lower jaw.

anisotropic. Doubly refracting or polarizing.

ankylosis. Consolidation or fusion of the bones of a joint or suture.

anlage. The embryonic fundament of an organ.

anterior. Toward the head. In human anatomy toward the ventral side.

anthropoids. The man-like apes.

antibodies. Chemical substances formed by a body in reaction to foreign substances

introduced,
antihelix. The inner curved edge of the external ear.
antithrombin. A substance in the blood which prevents clotting.
antitoxin. A substance in the blood serum antagonistic to a poisonous substance or

toxin.
antitragus. An ear prominence opposite the tragus.
antrum. A bone cavity.
anus. The egestive opening of the intestine.
aorta. The chief artery which leaves the heart.
aponeurosis. A connective tissue membrane or fascia which surrounds or attaches a

muscle.
appendices epiploicae. Fatty pouches attached to the colon.
appendicularia. A free-swimming pelagic genus of urochordates.
aqueous hiunor. The refractive liquid between cornea and lens of the eye.
arachnoid. A web-like membrane between the dura mater and pia mater of brain and

spinal cord.
arbor vitae. The tree-Uke arrangement of hber tracts seen in section of the cerebellum.
arch. .\ bent or curved structure.
archenteron. The primitive digestive cavity.
archencephalon. The primitive forebrain of chordates.
archetype. An ideal original form.

archicortex ('archipallium). The olfactory cerebral cortex including the hippocampus,
archinephros. The primitive kidney or mesonephros (Wolffian body).
archipterygium. The original appendicular skeleton of vertebrates.
areolar tissue. .\ fibrous tissue containing minute interspaces,
arrectores pilorum. Cutaneous muscles attached to hairs,
articulare. The bony articular element of the lower jaw of lower vertebrates,
arytenoid. A pitcher-like cartilage of the larynx or voice-box.
assimilation. Constructive metabolism,
astragalus. The ankle bone which articulates with the tibia,
ateliotic dwarf. A normal dwarf or midget,
atlas. The first cervical vertebra.
atrium, i. An auricle of the heart. 2. The chamber surrounding the gills of lower

chordates.
atrioventricular bundle. A muscle which connects auricle and ventricle,
atrophy. The wasting of a part or organ.

atropine. A poisonous alkaloid which stimulates the sympathetic,
auditory bulla. A capsule-like portion of the tympanic bone,
auditory meatus. The external meatus extends from the drum to the outside,
auditory tube. A passage from the middle ear to the pharynx.
Auerbach's plexus. A sympathetic plexus in the wall of the intestine,
auricle, external. The pinna of the ear.

auricularis magnus nerve. A sensory nerve distributed to face, ear, and neck.
Australopithecus. A South .\frican fossil anthropoid.
autonomic system. The sympathetic and parasympathetic nervous systems.

GLOSSARY 669

axilla. The arm-pit.

axis (epistropheus). The second cervical vertebra.

axolotyl. The larval, and sometimes permanent, stage of the salamander Ambystoma.

Bartholin, glands of. The vulvo-vaginal glands.

basalia. The proximal elements of the cartilaginous skeleton of the extremity.

basihyal. The ventral element of the hyoid skeletal arch.

basilar membrane. The basal membrane of the organ of Corti.

basilar plexus. A venous plexus in the dura mater which lines the occipital bone.

basioccipital. The basal portion of the occipital ring.

basophilic. With affinity for basic dyes.

Bell's law. The dorsal roots of spinal nerves are sensory, the ventral motor.

biceps brachii. The arm muscle which flexes the forearm.

bicuspid. A tooth (premolar) with two cusps. The left atrioventricular valve of the

heart.
bilateral. A tj^pe of symmetry such that one plane, and only one, will divide a body

into equal halves.
biogenesis. Life comes from life, not from the lifeless.
biogenesis, fundamental law of. Ontogenesis repeats phylogenesis.
blastocoele. The cavity of the blastula.

blastoderm. The membrane from which the embryo develops.
blastopore. The external orifice of the gastrula.
blastula. The one-layered stage of ontogenesis.

body-stalk. The mesodermal bridge which connects the embryo with the chorion.
Bowman's capsule. The globular dilatation of an uriniferous tubule enclosing a

glomerulus.
brachium conjunctivum. The superior peduncle of the cerebellum.
brachium pontis. The middle peduncle of the cerebellum.
branchial bars. The gill or visceral skeletal arches.
branchiomerism. The metamerism represented in the visceral arches and pharyngeal

pouches.
broad ligament. The peritoneal fold which supports uterus and ovary.
bronchiolus. One of the branches of a bronchus.
bronchus. One of the two branches of the trachea.
Brunner's glands. Submucous glands of the duodenum.
bulbus. The enlarged origin of the aorta.
bulbo-urethral glands. Cowper's glands of the urethra.
bulbus urethrae. An elongated swelling of the urethra.
bursa. A sac-like cavity.
buttock. The prominence formed by the gluteus muscle.

calcaneum. The heel bone.

calcareous. Composed of lime (calcium) salts.

callosity. A local thickening of the horny layer of the skin.

calyx (pi. calyces). One of the recesses of the renal pelvis which encloses the pyramids.

canaliculus. One of the fine canals which connects bone lacunae.

canalis reuniens. The duct which connects the cochlear duct with the sacculus.

cancellous bone. Spongy bone.

canine tooth. The single cuspid tooth between lateral incisor and first premolar.

capillaries. The minute vessels which connect arteries and veins.

capitate. The os magnum of the carpus.

capitulum costae. The head of a rib.

cardinal veins. The paired veins which drain head and trunk in lower vertebrates.

670 COMPARATIVE ANATOMY

camassial teeth. The last upper premolar and the first lower molar tooth of carnivora.

carnivorous. Flesh-eating animals.

carpus. The wrist.

cartilage. An elastic connective tissue with cells embedded in a homogeneous matrix.

castrate. To remove the testes.

catalyzer. A substance which by its presence changes the velocity of a chemical

reaction.
caudad. Toward the tail.

cecum. That diverticulum of the colon into which the vermiform appendix opens,
cenogenetic. Of recent, as contrasted with ancestral, origin.
centrum. The body of a vertebra.
cephalic. Pertaining to the head.

cerebellum. The little brain, a coordinating center above the fourth brain ventricle,
cerebrum. The chief division of the brain.
cheiropterygium. The skeleton of the fingered appendage.
chemoreceptor. A sensory cell which responds to chemical stimulation.
chiasma. A decussation or x-shaped crossing of nerve fibers within the central nervous

system.
chief cells. The pepsinogen secreting cells of the gastric glands.

chitin. The hornynitrogenous substance which forms mostof the skeletonsof arthropods.
choana. A funnel-shaped opening.

cholesterol. A fat-like substance which forms the usual type of gallstones.
chondrin. A gelatin-like protein found in cartilage.
chorda dorsalis. The notochord, the primary chordate axial skeleton.
chorda tympani. A mixed branch of the facial nerve.
chordae tendineae. The tendons of the heart-valves.
chorio-allantois. The fused membranes of the allantois and chorion.
chorioid (choroid). A chorion-like vascular membrane. In the eye a layer between

retina and sclera.
chorioid fissure. A defect in the optic cup and a groove along the optic stalk.
chorion. The outer protective extra-embryonic membrane of amniotes.
chromaffin cells. Cells of sympathetic origin having an afiinity for chrome salts,
chromatophore. A pigment cell.

chyme. The partly digested material which passes from the stomach to the duodenum,
ciliary body. A local thickening of the choroid layer of the eye to which the lens is

attached.
cingulum. An association fiber tract which encircles the corpus callosum near the

median plane.
cistema chyli. A receptacle or enlargement at the lower extremity of the thoracic

duct.
Clarke's column. A tract of nerve cells in the dorsal column of gray matter of the

cord,
cleft palate. Congenital fissure in the roof of the mouth.
cleithrum. A membrane bone of the shoulder girdle of fishes and amphibia,
clitoris. An erectile organ of the female homologous with the penis.
cloaca. The cavity into which digestive and urogenital organs open,
cochlea. The spiral tube of the inner sensory ear.

celiac (solar) plexus. A large sympathetic plexus in the epigastric region.
coelom. The chordate body-cavity.

coelomoducts. Paired segmental reproductive ducts of annelids,
collagen. The chief organic constituent of bone and connective tissue.
collateral. A side branch of an axon.

GLOSSARY 671

colliculus. One of the divisions of the corpora quadrigemina.

colloid. Glue-like.

colon. The large intestine to the rectum.

columella auris. The ear bone of amphibia and sauropsida.

commissure. A bundle of nerve fibers which connect right and left halves of central
nervous system.

Concrescence Theory, i. The theory that separate primordia unite in the median
plane to form the right and left halves of the embryo. 2. The theory that com-
pound teeth are formed by the fusion of simple conical teeth.

condyle. A rounded articular surface of a bone.

congenital. E.xisting at or before birth.

conjunctiva. The delicate membrane which covers the eyeball and lines the eyelid.

conus arteriosus, i. The conical portion of the right ventricle which joins the pulmo-
nary artery. 2. The valvular region of the ventral aorta.

copulation. Sexual congress.

coracoid. The posterior of the two ventral elements of the pectoral girdle.

coriimi. The deeper mesodermal layer of the skin.

cornea. The translucent anterior portion of the sclera of the eyeball.

comified. Converted into horny tissue.

corona radiata. The projection fibers which radiate from the internal capsule to the
cerebral cortex.

coronary. Encircling like a crown.

coronoid bone. A membrane bone of the lower jaw.

corpora bigemina (quadrigemina). The midbrain centers of optic reflexes.

corpora cavernosa penis. The paired masses of erectile tissue of the penis.

corpus albicans. The fibrous tissue formed after the discharge of the ovum.

corpus callosum. A group of commissural fibers which connect the two cerebral
hemispheres.

corpus cavemosmn urethrae. The spongy tissue surrounding the urethra.

corpus luteum. The yellowish tissue formed in the Graafian follicle when an ovum is
discharged and fertilized.

corpus striatum. The basal ganglion of the cerebral hemisphere.

cortin. The hormone secreted by the adrenal cortex.

coxal bone. The hip bone.

cranium. The brain case.

cretin. A type of idiotic dwarf supposedly caused by deficient thyroxin secretion.

cribriform plate. A sieve-like portion of the ethmoid bone.

cricoid. One of the laryngeal cartilages which resembles a seal ring.

crista. A crest or ridge of sensory hair-cells.

crura cerebri. The brain peduncles formed by descending fiber tracts from the
hemispheres.

crypt. A pit or follicle.

cryptorchism. The condition of undescended testicles.

cuboid. The most lateral distal tarsal bone of the foot.

cuneiform bones. Three wedge-shaped bones of the distal tarsals.

cusp. A conical projection of a tooth.

cutaneous. Pertaining to the skin.

cuticle. The epidermis or outer layer of the skin.

cystic duct. The duct from the gall bladder to the common bile duct.

deciduous placenta. The primitive placenta in which chorion and uterine mucosa are
loosely associated.

672 COMPARATIVE ANATOMY

decussation. A decussation of nerve fibers occurs when thej'^ cross the median plane to
connect unlike centers on the two sides.

Deiter's cells. The cells in Corti's organ which support the outer hair cells.

delamination. The splitting of a cell layer into two or more layers of cells.

demersal eggs. Eggs (of fishes) which sink to the bottom of the water.

demilunes. Crescent-shaped cells of mucus-secreting acini.

dendrite. One of the branched processes of a neuron which carries impulses towards
the cell body.

dentary. The membrane bone of the lower jaw to which teeth are attached.

dentine. The bone-like substance which forms most of the material of a tooth.

dentition. The kind, number, and arrangement of the teeth.

depressor. A muscle which lowers an organ. A nerve which inhibits action.

dermatome. That portion of the epimere which forms corium.

deuterencephalon. That part of the embryonic brain which includes mid- and hind-
brain.

deuterostomia. Animals in which the blastopore forms the anus or lies near the anus.

deutoplasm. The passive nutritive portion of the germ-cell.

development. The process by which an egg changes into an adult.

diabetes. A disease which is marked by excessive excretion of urine.

diaphragm. The muscular partition which in mammals separates thorax and abdomen.

diapophysis. A process of the neural arch which articulates with the tubercle of a
rib.

diastema. A space between the teeth, especially between the canine and the lateral
teeth.

diencephalon. The second of the five successive divisions of the brain.

differentiation theory. The theory that compound teeth have evolved by differentia-
tion from simple conical teeth.

digestion. The process by which foods are made soluble.

dioecious. Having sexes separated in two individuals.

diphyodont. Having two sets of teeth.

diploe. The cancellous bone between the two layers of compact bone of the cranium.

diploid. The double number of chromosomes characteristic of body cells.

diplospondyly. The condition of double centra in vertebrae.

distal. Opposed to proximal.

dorsal. Pertaining to the back or dorsum.

duodenum. The anterior portion of the small intestine.

dura mater. The tough outermost membrane which surrounds the central nervous
system.

ectoderm. The outermost germ layer.

ectosarc. The outer layer of the Prptozoon cell.

effector. The end-organ, muscle or gland, which responds to nervous stimulation.

efferent. Away from a center or organ.

egg. The animal ovum.

ejaculatory duct. The duct which unites the seminal vesicle and ductus deferens with

the urethra.
"elan vital." Bergson's name for the "vital factor."
endobranchiate. Forms with endodermal gills.
endocardiimi. The epithelial membrane which lines the heart.
endochondral bone. Bone which develops within cartilage.
endocranivmi. The dura mater of the brain.
endocrine gland. A gland which secretes internally into the blood.

GLOSSARY 673

endoderm. The innermost germ layer.

endolymph. The liquid contained in the membranous sac of the inner ear.

endometrium. The mucous lining of the uterus.

endoneurium. The connective tissue which subdivides a nerve into funiculi.

endoskeleton. The internal skeleton as distinguished from the dermal skeleton.

endostyle. A ciliated groove in the floor of the pharynx of lower chordates.

endothelium. The thin membrane which lines blood vessels and lymphatics.

end-plate. The expanded termination of a motor nerve on a muscle fiber.

ensiform. Shaped like a sword.

enterocoele. A mesodermal cavity formed as an outpocketing of the endoderm.

enteron. The cavity of the alimentary canal.

enterokinase. An enzyme secreted by the small intestine which converts trj^asinogen

into trypsin.
enzjmie. A catalytic organic compound which facilitates chemical changes such as

the splitting of foods into simpler substances in the alimentary canal.
eosinophile. A leucocyte with an aftmity for eosin.
epaxial. Dorsal to the vertebrate axis.

ependyma. The lining membrane of the central nervous system.
epibranchial organs (ganglia or placodes) . Ectodermal masses above the gill-pouches of

vertebrate embryos.
epicardium. The outer covering of the heart.
epidermis. The outer ectodermal layer of the skin.

epididjmiis. The mass of convoluted tubules and duct attached to the testis.
epiglottis. The lidlike structure which covers the entrance to the larynx.
epimere. The dorsal portion of the mesoderm.

epinephrine (adrenin). The hormone secreted by the medulla of the suprarenal,
epineurium. The connective tissue covering of a nerve.
epipharynx. The nasal portion of the pharynx.

epiphysis. The terminal portion of a long bone. The pineal organ.
epithalamus. The dorsal portion of the diencephalon.
epithelium. The tissue which covers a surface,
epitrichium. The outer layer of the fetal epidermis,
epoophoron. A rudiment of the mesonephros near the ovary.

ereptose (erepsin). A peptone-splitting enzyme secreted by the intestinal mucosa.
erythrocytopoietic tissue. Tissue forming red blood corpuscles.
esophagus. The portion of the alimentary canal between pharynx and stomach.
ethmoturbinals. The superior and middle turbinated bones.
ethmoid. The sieve-like bone at the front of the skull.
eunuch. A castrated individual.

eustachian tube. The passage which connects the tympanic cavity with the pharynx,
excretion. The elimination of liquid wastes.

exoskeleton. That part of the skeleton which is derived from the skin.
exteroceptive. Receptive to external stimuli.
extirpation. The complete removal of a part.

extravasation. The escape of blood from a vessel into surrounding tissues.
extremity. A limb.
extrinsic. Originating outside an organ.

falciform ligament. A fold of peritoneum between liver and diaphragm.
Fallopian tube. The uterine tube or anterior portion of Muellerian duct.
falx cerebri. That portion of the dura mater between the hemispheres.
fasciculate layer. The middle layer of the suprarenal cortex.

674 COMPARATIVE ANATOMY

fasciculus. One of the divisions of a funiculus of the spinal cord

femur. The thigh bone.

fenestra cochleae. The round window of the internal ear.

fenestra vestibuli. The oval window to which the stapes bone is attached.

fertilization. The union of sperm and egg nuclei within the ovum.

fetus. The child in the womb after the end of the third month.

fibril. A cytoplasmic thread.

fibrin. The clot-forming substance of the blood.

fibrocyte. A flat elongated connective tissue cell.

fibula. The outer shin bone.

filum terminale. The thread-like termination of the spinal cord.

fimbriae tubae. The fringe-like end of the uterine tube.

fission. Asexual division into equal parts.

fissure. A deep fold of the cerebral cortex which involves the entire thickness of

the brain wall.
fistula. A deep ulcer-like opening, usually into a hollow organ.
fiabellum. A fan-like set of radiating fibers in the corpus striatum.
flagellate. Having whip-like processes. A protozoon with flagellae.
flagellum. A whip-like protoplasmic process of a cell.
flame-cell. An excretory cell having one or more flagellae.
flexion. The condition of being bent.
flexure. A bend or fold.

flocculus. A small lobe on the lower side of the cerebellum.
floor plate. The flooi- of the neural tube,
foliate papillae. Leaf-like folds on the sides of the tongue.
follicle. .\ small secretory sac or gland. The integumentary sac enclosing the base

of a hair or feather.
fontanelle. One of the unossified regions in the skull of the infant.
foramen. A small opening, usually in a bone.

fornix. A band of commissural fibers ventral to the corpus callosum.
fossa. A pit or depression.
fovea centralis. A pit in the macula lutea of the retina where the layer of nerve fibers

is lacking.
freemartin. The sterile female twin of a male calf.
frenulum linguae. A median fold between tongue and mandible.
friction-ridge pattern. A concentric arrangement of fine ridges on the hands and

feet.
frontal. The bone of the forehead. A sectional plane which divides a bilateral body

into dorsal and ventral divisions.
function. The normal activity of an organ.
fundus. The part of a hollow organ opposite its opening.
fungiform. Mushroom-shaped.
funiculus. One of the three chief divisions of white matter on the sides of the cord.

gall bladder. The pear-shaped sac which stores the bile secreted by the liver.

gamete. A sexual cell — ovum or spermatozoon.

ganglion. A functional group of nerve cells outside of the central nervous system.

Gartner's duct. The rudiment of the Wolftian duct in the female mammal.

Gasserian ganglion. The semilunar ganglion of the trigeminal nerve.

"gastraea" theory. The theory that the gastrula stage represents a coelenterate stage

in vertebrate phylogenesis.
gastric glands. The glands of the stomach wall.

GLOSSARY 675

gastrula. The two-layered stage of ontogenesis.

gelatin. The translucent protein which forms jelly and glue.

genetic. Pertaining to origin or birth.

geniculate body. A tubercle-like body in the lower part of the optic thalamus.

geniculate ganglion. The ganglion of the facial nerve.

genital. Pertaining to the organs of reproduction.

gestation. Pregnancy. The condition of being with child.

gigantism. Excessive growth sometimes due to disease of the anterior lobe of the

pituitary.
gingiva. The gum. The tissue which covers the jaw and surrounds the necks of the

teeth.
gizzard. The muscular grinding stomach of birds and reptiles.
gladiolus. The middle chief portion of the sternum.
glans penis. The swollen terminal portion of the penis.
glenoid. The concave depression in the scapula in which the head of the humerus

articulates.
globus pallidas. The pale interior of the lenticular nucleus of the corpus striatum.
glomerulus. The knot of capillaries of a renal corpuscle,
glottis. The pharyngeal opening of the larynx.

glycogen. Animal starch, a carbohydrate stored in the liver and other tissues.
gnathostomes. Fishes with biting jaws in contrast with cyclostomes.
goblet-cells. Mucus-secreting cells of the intestine.
goiter. Enlarged thyroid gland.

Golgi-Mazzoni corpuscles. Special tactile corpuscles of the finger-tips.
gonad. A gamete-producing gland.

gonotome. That part of the mesoderm which forms the gonad or germ-gland.
goose flesh. The formation of skin papillae due to the action of cutaneous muscles on

hairs.
Graafian follicle. An ovarian vesicular sac containing an ovum and secreting an

hormone (estrin).
Grandry's corpuscles. Special tactile corpuscles of tongue and mouth. The same as

Merkel's corpuscles.
granulocyte. A leucocyte which contains coarse granules.
gray matter. That portion, mostly cellular, of the central nervous system which lacks

the white myelin.
groin. The part of the abdominal wall adjacent to the thigh.
gubemaculum testis. The fetal cord which attaches the testis to the scrotal sac.
gullet. The esophagus.
gustatory. Pertaining to the sense of taste.

gynandromorph. An organism having both male and female characteristics.
gyrus. A fold or convolution of the brain cortex.

hagfish. The cyclostome myxine.

habenular commissure. The "superior" commissure which connects the habenular

ganglia in the roof of the diencephalon.
habenular ganglia. Olfactory centers anterior to the pineal body.
hamate. The distal carpal bone adjacent to the fifth metacarpal.
haploid. The reduced number of chromosomes found in mature germ-cells.
harelip. A congenital cleft in the upper lip, rarely double.
Harrimania. A genus of hemichordates.
harvest men. Long-legged spiders or "daddy-long-legs."
Hassall's corpuscles. Clusters of concentrically arranged epithelial cells of the thymus.

676 COMPARATIVE ANATOMY

Hatschek's pit. A preoral pore in amphioxus which opens into the left anterior cavity.

haustra. Sacculations of the wall of the colon.

Haversian canal. A bone canal which contains a blood vessel and nerves.

"heat." Estrus or se.x ardor in animals.

helix. The margin of the pinna of the ear.

hemal. Pertaining to the blood.

hemibranch. The anterior or posterior half of the respiratory portion of a visceral arch.

hemoblast. A primitive blood cell.

hemoglobin. The coloring matter of a red blood cell.

hemopoietic. Blood-forming.

Henle's loop. A loop of a uriniferous tubule.

Hensen theory. The theory that nerve fibers are enlarged plasmodesms.

hepatic. Pertaining to the liver.

hermaphrodite. An individual with both male and female characteristics.

hernia. The protrusion of an organ through an abnormal opening.

heterodont. Having specialized kinds of teeth.

heterogamy. Reproduction which involves the union of unlike gametes.

heterolateral. Relating to opposite sides.

Highmore's antrum. A cavity or sinus of the maxillary bone.

hilus (hilum). The pit of an organ where blood vessels and nerves enter.

hindbrain. The posterior of the three primary embryonic brain vesicles.

hippocampus. A gyrus in the floor of the lateral ventricle which constitutes the greater

portion of the olfactory centers.
histogenesis. The ontogenetic differentiation of a tissue.
holobranch. An entire fish gill.
homodont. Having teeth all alike.
homolateral. On the same side.

homologous. Having the same structure, development, and relations.
hormone. The secretion of an endocrine gland which affects the activity of one or more

other organs.
hyaline. Translucent.
hydatid of Morgagni. The rudiment of the Miillerian duct attached to the oviduct or

testis.
hydrostatic organ. An organ of aquatic organisms which serves to adjust internal to

external pressure.
hymen. The membrane which more or less completely closes the external opening of the

vagina.
hyoid. The second visceral arch of vertebrates. The hyoid bone of mammals develops

from skeletal elements of the second and third visceral arches.
hyomandibula. The dorsal element of the hyoid skeletal arch.
hypaxial. Ventral to the chief axis.
hypertrophy. The abnormal enlargement of an organ.
hypobranchial muscles. The muscles ventral to the gills.
hypochorda. A transient cord of cells ventral to the notochord.
hypocone. A fourth tubercle of a compound tooth.
hypodermic. Administered beneath the skin.
hypomere. The ventral portion of the mesoderm.
hypoparathyroidism. Insufficient secretion of the parathyroid glands.
hypophysis. Usually identified with the pituitary gland but strictly forming only the

anterior lobe of the latter.
hypothalamus. The ventral portion of the diencephalon.
hypo thenar. The ridge on the ulnar side of the palm.

GLOSSARY 077

hypothesis. An attempted explanation.

ichthyopsida. The sub-phylum which includes the fish-like forms — Fishes and

Amphibia.
ichthyopterygium. The skeleton of the extremity of fishes.
ileum. The posterior portion of the small intestine.
ilium. The dorsal element of the pelvic girdle.
imbricate. A tile- or shingle-like arrangement of alternating parts,
immunity. The condition of resistance to infection.
impulse, nervous. The change transmitted along an active nerve fiber.
incisive canal. The passage in the maxillary bone from the nasal fossa to the mouth

cavity.
incisor. One of the four front teeth of either jaw.
incus. The anvil-like middle earbone.
infundibulimi. The funnel-like portion of the hypothalamus which connects with the

pituitary gland.
inguinal canal. The passage in the groin which contains the spermatic cord.
inscriptiones tendineae. Fibrous bands which partially divide the belly of a muscle.
insertion. The attachment of a muscle to the bone which it moves.
insula. A triangular portion of the cerebral hemisphere covered by the temporal lobe.
insulin. The hormone secreted by the islands of Langerhans in the pancreas,
integration. The coordination of functions.
intercalated. Placed between or interposed.
intercostal. Between the ribs.
intergrade, sex. An individual with characteristics intermediate between those of the

sexes.
interoceptors. Sense organs in the lining of the alimentary canal,
interrenal body. The tissue which in higher vertebrates forms the adrenal cortex,
interstitial tissue. The endocrinal tissue of the testis.
intestinal portal. The transient embryonic anterior opening of the hindgut and the

posterior opening of the foregut which appear during the formation of the floor of

the intestine.
intima. The innermost lining of an artery.
intrinsic. Situated within a part or organ.

invaginate. The enclosure of a part by another portion of the same thing,
iris. The circular pigmented membrane between the cornea and lens of the eye.
ischium. The posterior of the two ventral elements of the pelvic girdle.
islands of Langerhans. Endocrinal organs embedded in the substance of the pancreas

which secrete the hormone insulin.
isogamy. The union of similar gametes.

Jacobson's organ. An accessory olfactory organ which opens into the nasal cavity,
jejunum. The middle of the three divisions of the small intestine.
jugular ganglion. The ''root" ganglion of the vagus nerve.

keratin. An insoluble protein which forms the base of horny structures.
kidney. The chief excretory organ of amniotes.

Koelliker's pit. The rudiment of the embryonic neuropore of Amphioxus.
Krause's corpuscles. Rounded sensory corpuscles located in mucous membranes.

labial cartilages. Rudimentary cartilages of elasmobranchs.
labitun. A lip or lip-like structure.

678 COMPARATIVE ANATOMY

lab5rrinth. The internal ear of vertebrates.

lacrimal. Pertaining to the tears.

lacteal. Pertaining to milk. An intestinal lymphatic.

lacuna. A pit or hollow. In bone the cavity filled by a bone cell.

lagena. An outgrowth of the sacculus of the ear.

lamella. A thin leaf or plate as of bone.

lamina terminalis. The thin membrane which forms the anterior wall of the third
brain ventricle.

lanugo. The fine hairy covering of the fetus of man.

larva. An immature but active stage in the development of an organism.

larynx. The cartilaginous organ which encloses the vocal cords.

lateral line. A series of sense organs which extends along the sides of the body of
fishes and some amphibia.

lemmatochord or bothriochord. An arthropod structure which W. Patten has com-
pared with the notochord.

leucocyte. A "white" blood corpuscle.

Lieberkuehn's glands or crypts. Tubular mucous glands of the intestine.

linea alba. The tendon in the median line of the abdomen.

lingual. Pertaining to the tongue.

lipase (steapsin). A fat-splitting enzyme secreted by the pancreas and some other
digestive organs.

lipoids. Fat-like cell constituents soluble in alcohol and ether.

lobule. A normal small division of a lobe.

Lorenzini's ampullae. Tubular sensory organs of Elasmobranchs.

luciferin. A substance which combined with luciferase in luminous animals produces
light.

lumen. The cavity of a hollow organ.

lunula. The whitish crescent at the root of a nail.

luteal hormone. The hormone progestin secreted by a corpus luteum.

lymph. The coagulable liquid of the lymphatic vessels.

maculae acusticae. The sensory patches of the sacculus and utriculus.

macula lutea. The point of clearest vision at the center of the retina.

malar bone. The zj'gomatic or cheek bone.

malleolus. The hammer-headed process of a bone (tibia and fibula).

malleus. The ear ossicle which is attached to the drum.

maltase. The enzyme which splits maltose into dextrose.

mamma. The breast or mammary gland.

mammillary bodies. Paired rounded bodies posterior to the tuber cinereum.

mandible. The horseshoe-shaped lower jaw.

mandibular arch. The anteriormost visceral arch.

mantle. The body wall of urochordates. The shell-secreting organ of molluscs.

manubriimi. The anterior division of the sternum.

marsupial pouch. The abdominal pouch of marsupials in which the young are carried

after birth.
mastoid. A process of the temporal bone.

maturation. The process by which homologous chromosomes are segregated.
maxilla. The upper jaw bone,
maxillo-turbinals. The inferior turbinated bones,
meatus. A passage or opening.

Meckel's cartilage. The lower jaw of cartilaginous fishes.
mediastinum. The thick partition which divides the two pleural cavities.

GLOSSARY 679

medulla. The marrow or core of an organ.

medulla oblongata. The posterior brain division which contains the fourth
ventricle.

Meibomian glands. Sebaceous glands of the eye lids.

Meissner's corpuscles. Tactile corpuscles with thick capsule and brush of nerve
terminations.

melanophore. A pigment cell containing melanin.

membrane bones. Bones which are not preformed in cartilage.

meninges. The three membranes which enclose the brain and spinal cord.

menopause. The time in life when menstruation normally ends.

menstruation. The monthly flow of women.

men turn. The chin.

Merkel's corpuscles. Tactile corpuscles of the tongue and mouth.

mesencephalon. The third or mid-brain vesicle.

mesenchyme. The embryonic connective tissue.

mesentery. The peritoneal membrane which attaches the intestine to the body wall.

mesethmoid. The median cribriform portion of the ethmoid bone.

mesocardium. The membrane which connects the embryonic heart with the body wall.

mesoderm. The middle germ layer.

mesogaster. The peritoneal membrane which attaches the stomach to the body wall.

mesoglea. The gelatinous middle tissue of sponges and coelenterates.

mesomere fnephrotome). That portion of the trunk mesoderm which connects each
somite (epimere) with the ventral mesoderm or h>T3omere.

mesomerism. The segmentation of the epimeric or dorsal mesoderm into somites.

mesonephros. The middle kidney or Wolffian body, the functional kidney of anamnia.

mesopterygium. The middle basal cartilage of the elasmobranch pectoral fin.

mesorchium. The peritoneal membrane which attaches the testis to the dorsal body
wall.

mesorectum. The mesentery of the rectum.

mesothelium. The cellular layer which encloses the coelom.

mesovarium. The peritoneal membrane which connects the ovary to the body wall.

Mesozoic. The middle life era of geologic time.

metabolism. The chemical cycle of matter in living organisms.

metacarpus. The group of five bones between the wrist and the fingers.

metacone. The postero-external cusp of the upper molar teeth of mammals.

metaconid. The postero-internal cusp of the lower molar teeth of mammals.

metamere. One of the serial divisions of the body of a segmented animal.

metamorphosis. A striking change of form during development, as seen in the trans-
formation of the tadpole into a frog.

metanephros. The definitive kidney of the amniotes.

metaotic. Posterior to the ear vesicle.

metapleural folds. Paired folds on the ventral side of the body of Amphio.xus.

metapterygium. The posterior basal cartilage of the pectoral fin of elasmobranchs and
some ganoids.

metastemum. The posterior or xiphoid process of the breast bone.

metatarsus. The five bones between the ankle and the toes.

metazoa. Many-celled animals.

metencephalon. The fourth brain division which forms cerebellum and pons.

midbrain. The third or mesencephalon division of the brain.

mitral valve. The left atrioventricular valve of the heart.

moa. An extinct running bird of New Zealand.

molar teeth. The three grinding teeth of each half jaw.

68o COMPARATIVE ANATOMY

monocytes. Large mononuclear leucocytes with kidney-shaped nuclei.
monoecious. Having male and female flowers separated on the same plant,
monophyletic theory. The theory that all forms of blood cells arise from a common

primitive type of cell or hemoblast.
monophyodont. Having a single permanent set of teeth.
monorhine. Having a single narial aperture.

monosaccharid. A sugar the molecules of which have only six carbon atoms,
mons pubis. The pubic eminence or mons veneris.

morphology. The science which treats of form and structure of organisms.
morula. The mulberry stage of segmentation of the egg.
motor (efferent) neuron. A nerve cell which conveys impulses away from a nervous

center,
mucosa. The lining of the intestine.
Muellerian duct. The oviduct or uterine tube,
multitubercular. Having many cusps as the molar teeth do.
muscularis mucosae. The layer of smooth muscle fibers in the mucosa,
myelencephalon. The fifth or posterior division of the brain — the medulla oblongata.
myelin. The fat-like substance which forms the Schwann's sheath of nerve fibers,
myelocytes. Cells of red bone marrow usually with granular cytoplasm,
myenteric plexus. The network of sympathetic fibers on the muscles of intestine.
myocarditim. The muscular layer of the heart,
myocoele. The coelom of the somite.

myocomma. The connective tissue between two myotomes.
myofibril. One of the fine fibrils within a muscle fiber.
myotome. The muscular portion of a somite.
myxine. The hagfish, a genus of cyclostomes.

nail wall. The skin which covers the base of the nail.

nares. The nostrils.

navicular. The scaphoid bone of the carpus and tarsus.

Neanderthal. A gorilla-like tj'pe of fossil man.

necrobiotic. Dying as a result of functional activity.

neocortex. The major portion of the cerebral hemispheres.

neostoma. The definitive mouth of vertebrates.

neostriatimi. An olfactory center, near the corpus striatum, which becomes in mammals

the nucleus amygdalae.
nephridiimi. The ectodermal excretory tubule of annelids and amphioxus.
nephrostome. The opening of a nephridium into the body cavity.
nephrotome. The intermediate portion of the mesodermal somite which gives rise to

a renal tubule.
nerve. A bundle of nerve fibers which connect the central nervous system with some

part of the body.
nerve fiber. The axon process of a neuron plus sheaths when such occur.
nerve nucleus (nidulus). A group of nerve cells within the central nervous system.
nerve tract. A bundle of nerve fibers of similar origin and function within the central

nervous system.
nervus terminalis. A sensory nerve associated with the olfactory.
neiiraxon. The nerve fiber or axon process of a nerve cell.
neurenteric canal. The blastoporic opening which in chordate embryos connects the

neural tube with the enteron at the posterior end of the body,
neurolemma. The cellular sheath of an axon.
neurite. The axon process of a neuron which carries impulses away from the cell body.

GLOSSARY 68l

neurobiotaxis. The migration of the cell bodies of neurons towards the source of

stimulation.
neuroblast. The embryonic neural cell which forms an axon process.
neuro-epithelial cell. A sensory receptor cell the body of which is located in an

epithelium.
neurofibrillae. The fine fibrils within a neuron.
neurogenesis. The differentiation of nervous tissue in ontogenesis.
neuroglia. The ectodermal supporting tissue of the central nervous system.
neurohumor. A nerve secretion which stimulates an effector cell.
neuromast. A cluster of sensory cells in the skin such as is represented in a lateral line

organ.
neuromere. An embryonic segmental division of the central nervous system.
neuromuscular spindle. Specialized sense organs located in muscles.
neuron. The functional unit of the nervous system.
neuropore. The anterior opening of the embryonic neural tube.

neurosensory cell. A sensory or receptor cell the cell body of which lies in an epithelium.
neurostoma. The hjpothetical mouth (Delsman) of vertebrate ancestors, 'which is

represented in the neuropore of chordate embryos.
neurotendinous spindle. A special type of sensory nerve termination connected with

a tendon.
neutrophile. A blood cell with an affinity for neutral stains.
Nissl bodies. Large protein granules with an affinity for basic dyes found in nerve

cells.
nodosum ganglion. The ganglion of the vagus below the jugular ganglion.
non-deciduate placenta. The type of mammalian placenta which does not involve the

uterine mucosa at birth.
non-meduUated. Devoid of myelin sheath.

Nordic. The blond teutonic type of man of northern Europe and Africa,
normoblast. A nucleated stage in the histogenesis of a red blood corpuscle.
notochord. The axial rod between the chordate nervous system and the dorsal aorta,
nuchal. Pertaining to the nape of the neck.

obturator foramen. The opening between the pubis and ischium.

occipital lobe. The posterior lobe of the cerebral hemispheres.

occlusion. The state of being closed.

odontoblast. One of the dentine-secreting cells.

odontoid process. The tooth-like process of the axis (epistropheus) vertebra.

oestrus (estrus). The period of "heat" or receptivity in the female.

olecranon process. The process of the ulna at the elbow.

oliva. A prominence of the medulla oblongata lateral to the pyramid.

omasum (psalterium). The third division of the ruminant stomach.

omentum. A sac formed by the doubling of the mesentery.

ontogenesis. The development of the individual from the egg.

oocyte. The immature egg.

operculum. The gill-cover of fishes.

opisthonephros. The posterior metanephros-like portion of the mesonephros of

anamnia.
optic vesicle. The hollow lateral outpocketing of the forebrain which forms the retinal

and pigment layers of the eye.
oral hood. The funnel-like membrane which in Amphioxus bears the tentacles.
orbit. The bony socket which surrounds the eye.
OS uteri. The orifice of the uterus.

682 COMPARATIVE ANATOMY

osculum. A minute aperture.

osmosis. The passage of dissolved substances of different concentration through a

semipermeable membrane which separates them.
ossification. The formation of bone.
osteoblast. A bone-secreting cell.

ostixun tubae. The opening of the oviduct or uterine tube into the body cavity.
otic. Pertaining to the ear.
otoconia (or otoliths). Crystals of calcium carbonate contained in the endolymph of

the membranous ear.
ovariotomize. To remove an ovary or ovarian tumor,
oviduct. The egg duct or uterine tube.
oviparous. Egg-laying.

ovulation. The discharge of an egg from the ovary.
ox3rtocin. A pituitary hormone which affects the uterine muscle.

pabulum. Food.

Pacini's corpuscles. Large tactile corpuscles with single dendrite and many-layered

capsule.
palate. The roof of the mouth — hard and soft.

palatine. The membrane bone which forms the posterior part of the hard palate,
paleocortex. The primitive cortex, or olfactory portion of the hemispheres.
paleontology. The science which treats of fossils.
paleostoma. The hypothetical primitive mouth of vertebrates — possibly represented

in the hypophysial duct.
paleostriatvmi. The primitive corpus striatum (basal ganglion) as distinguished from

the Epi- and Neostriatum.
paleozoic. The ancient life era from the Cambrian to the Permian period,
pallivmi. The cerebral cortex or layer of gray matter which covers the cerebral

hemispheres.
pancreas. A combined digestive and endocrinal gland between stomach and duodenum.
panniculus camosus. A layer of integumentary muscles represented in man by the

platysma of the neck.
papilla. A small nipple-shaped elevation.

parachordal. A cartilage which lies along the anterior end of the notochord.
paracone. The anterior external cusp of an upper molar tooth.
paraconid. The anterior cusp of a lower molar tooth.
paradidjTnis. A rudiment of the mesonephros near the testis.
paraphysis. A hollow outgrowth of the anterior dorsal roof of the diencephalon.
parapodium. One of the paired appendages of annelids.
parapophysis. A transverse process from the centrum of a vertebra.
parasite. An organism which depends upon another for its living without paying

board.
parasphenoid. A membrane bone of the roof of the mouth of fishes and amphibia,
parasympathetic. The craniosacral portion of the autonomic nervous system.
parathyroid glands. Ductless glands, usually four, which lie near the thyroids.
parietal organ (eye) . The anterior epiphysial outgrowth from the diencephalon.
paroophoron. A rudiment of the mesonephros near the mammalian ovary.
parotid. The serous salivarj^ gland below the ear.
patella. The knee-pan.

pecten. A comb-like structure in the vitreous body of the eyes of reptiles and birds,
pfectoral. Pertaining to the chest.
pedal. Pertaining to the foot.

GLOSSARY 683

pedicle. The bony connexion between the lamina and centrum of a vertebra.

peduncle. A fiber tract which connects the cerebellum with the brain stem.

Peking man. A fossil species of man intermediate between the Java man and the

Neanderthal.
pelvis. The basin-shaped ring of bone which connects back and leg bones.
penis. The male intromittent organ.
pepsinogen. A zymogen which when combined with hydrochloric acid forms

pepsin.
peptone. A soluble derived protein formed by the hydrolysis of protein,
perennibranch. With persistent gills.

pericardium. The membranous sac which encloses the heart.
perichondrium. The connective tissue membrane which surrounds cartilage,
periderm. The transient external layer of the mammalian embryonic epidermis,
perilymph. The liquid in the space between the membranous and the skeletal labyrinth

of the internal ear.
perimysium. The delicate connective tissue membrane which surrounds a bundle of

muscle fibers.
perineum. The floor of the pelvic outlet.
perineurium. The connective tissue covering a nerve cord,
periosteum. The connective tissue membrane which surrounds a bone,
peristalsis. The wave of contraction which passes along the intestine,
peritoneum. The serous membrane which lines the body cavity and covers the viscera,
petrosal. The petrous portion of the temporal bone.
Pfiueger's egg tubes. Cords of peritoneal cells which are said to grow into the stroma

of the ovary.
phagocyte. A cell which ingests bacteria,
phalanx. One of the finger or toe bones,
phallus. The penis.

pharynx. That part of the alimentary canal which connects mouth and esophagus,
photophore. A luminous organ,
photoreceptor. A sensory cell sensitive to light.

phylogenesis. Racial history. The evolution of higher from lower animals.
pia mater. The innermost and most vascular of the three coverings of the central

nervous system.
pilaster cells. Columnar supporting cells peculiar to fish gills.
pillar cells. Special columnar cells of Corti's organ located between the inner and

outer rows of hair cells.
pineal body. The posterior epiphysis of the diencephalon.
pinna. The projecting portion of the external ear.

pisiform. A pea-shaped proximal carpal bone on the ulnar side of the wrist.
pit organs. A pair of sensory pits anterior to the eyes of vipers.
pituitary gland. An endocrine organ attached to the infundibulum.
pituitrin. Extract of posterior lobe of pituitary gland, used to stimulate contraction of

smooth muscle (uterus, etc.).
placenta. The mammalian organ of attachment and nutrition of the embryo,
placode. A local disc-like thickening formed as an anlage of an organ,
placoid scale. The typical elasmobranch scale with enamel and dentine layers.
plankton. Floating organisms which may be collected with a tow-net.
plantigrade. Flat-footed.

plasma. The liquid portion of the blood, the serum and fibrinogen.
plasmodesm. A fine protoplasmic thread which connects two cells.
plastron. The ventral bony shield of the turtle.

684 COMPARATIVE ANATOMY

platysma. The integumentary muscle of the neck.

pleura. The serous membrane which lines the chest and covers the lungs.
plexus. A network of nerves or blood vessels.
plica. A fold or pleat.

polymerization. The chemical synthesis of two or more molecules to form a new com-
pound without the production of a secondary compound.
poljrmorphonuclear leucocyte. A white blood corpuscle which has a nucleus with

irregular constrictions.
polyp. The sessile or attached stage of a coelenterate.

polyphyletic theory. The theory that the various blood cells have had a multiple origin.
polyphyodont. Having more than two sets of teeth.
pons. A bridge of fibers below the cerebellum which connects cerebrum, cerebellum

and medulla oblongata.
postanal gut. That part of the embryonic digestive tract posterior to the anus,
postfrontal. A roofing bone of the skull posterior to the frontal.
posttrematic branch. That division of a cranial nerve which forks behind a gill-slit.
precoracoid. The anterior of the two ventral elements of the pectoral girdle,
pregnancy. The condition of being with child. Gestation.
premandibular cavity. The second somite which forms the superior oblique eye muscle

and — it is asserted — a part of the external rectus.
premolar. A bicuspid tooth.

preoral gut. That part of the embryonic intestine which is anterior to the mouth,
prepuce. The skin fold which covers the glans penis or the clitoris.
pretrematic branch. The nerve branch which forks in front of a gill-slit.
primitive duct. The pronephric duct.

primitive streak. The elongated and closed blastopore of amniote embr^'os.
process theory. The theory that nerve fibers (axons) develop as processes of neuroblast

cells.
proctodeum. That portion of the hindgut which is lined by ectoderm,
progestin. A luteal hormone which affects the endometrium of the uterus,
progynon. A proprietary name for the female sex hormone.
pronation. The act of turning the palm of the hand downwards.
pronephroi. The primitive vertebrate kidneys.

proprioceptors. The mechanisms for receiving stimuli from within the body,
propterygium. The anterior basal element of the fish extremity.
prosencephalon. The anterior division of the embryonic brain which forms the cerebral

hemispheres.
prosimian. Pertaining to primitive apes,
prostate gland. A muscular gland which surrounds the urethra where it leaves the

bladder.
prostomium. The preoral lobe of Annelids.
proterostomia. Those animals in which the blastopore becomes the mouth or lies near

the mouth.
proteose. A soluble protein formed by hydrolytic cleavage of a protein.
protochordate. The primitive chordates which do not acquire a vertebral column.
protocone. The inner cusp of an upper molar.
protoconid. The outer cusp of a lower molar.
protonephridia. Primitive excretory tubules without nephrostomes and with soleno-

cytes.
prototype. The original form or t>'pe from which others evolve.
proximal. Towards the body.
pseudocoelom. A false body cavity not lined by peritoneum.

GLOSSARY 685

pterygoid. A portion of the mandibular arch which doubtfully becomes the pterygoid

process of the mammalian sphenoid.
ptyalin. The starch-splitting enzyme found in the saliva.
puberty. The age at which reproductive organs start to function.
pubis. The anterior of the two ventral arms of the pelvic girdl.e.
Purkinje cells. Large much-branched neurons of the cerebellar cortex,
putamen. The outer darker portion of the lenticular nucleus of the corpus striatum.
pylorus. The aperture at the posterior end of the stomach.
pyramids, renal. The conical masses present in the medulla of the kidney.
pyramids of the medulla. Paired eminences on the ventral side of the medulla.
pyramidal cells. Neurons with pyramid-shaped cell-bodies in the cerebral cortex.

Rathke's pouch. The hypophysis of amniote embryos.

ray-finned fishes. Fishes with bony and horny rays — includes most fishes.

recapitulation theory. The theory that individual development repeats briefly the

evolutionary history of the race.
receptaculum (cisterna) chyli. A chamber for the storage of lymph at the lower end

of the thoracic duct.
receptor, A sensory cell.

rectum. The lower six to eight inches of the large intestine.
red nucleus. A nervous center in the tegmentum of the midbrain. Its cells contain

a red pigment.
reduction. Meiosis. The process by which the haploid number of chromosomes is

attained.
reflex action. An action which involves a reflex arc, i.e., a sensory and a motor neuron

connected within a central nervous system.
renal corpuscle. The ex-panded termination of an excretory tubule containing a

glomerulus.
respiration. The regulated burning of carbon compounds within living cells.
restiform bodies (inferior peduncles). A fiber tract which connects sensory spinal

nerves with the cerebellum.
rate testis, A network of fine ductules between the seminiferous tubules and the

ductuli efferentes.
retina. The sensory innermost layer of the eye.
Rhodesian man, A prehistoric t>^e of man having affinities with the Neanderthal

man.
"rhomboid." A kite-shaped area where four hair currents meet.
rhombomere. A hindbrain neuromere.

roof plate. The median dorsal wall of the embryonic neural tube.
rods and cones. The receptor cells of the retina.
rubrospinal tract. A longitudinal fiber tract which connects the red nucleus with the

somatic motor cells of the spinal cord.
rumen. The anteriormost stomach of a ruminant.

sacctilus. The membranous sac connected with the cochlear duct of the ear.

sacrvun. The five fused vertebrae to which the ilium is attached.

sagittal plane. A median longitudinal vertical section.

Santorini's duct. The accessory duct of the pancreas.

sarcolemma. The delicate elastic membrane which surrounds a muscle fiber.

sarcoplasm. The interfibrillar substance of a striped muscle fiber.

Savi's vesicles. Cranial sensory vesicles of Torpedo without external openings.

scala media. The cochlear duct.

686 COMPARATIVE ANATOMY

scala tympani. The ventral or descending staircase of the cochlea.

scala vestibuli. The dorsal or ascending staircase of the cochlea.

scapula. The shoulder blade. The dorsal arm of the pectoral girdle.

Schwann's sheath. The neurilemma of a nerve fiber.

sclerotic (sclera). The tough white outer covering of the eye-ball.

sclerotome. The part of the somite which forms the vertebral column.

scrotum. The integumentary sac in which the testes of mammals are lodged.

scute. A scale-like skeletal structure.

sebaceous gland. A gland which produces a greasy secretion.

secretin. An intestinal hormone which activates the pancreas.

secretion. A product of cell metabolism which is thrown out as waste or is used by

the organism in its normal activity.
sella turcica. The pituitary fossa in the sphenoid bone,
semen. The external secretion produced by the male in coitus.
semicircular canal. One of the ducts of the membranous ear used in equilibration,
semilunar valves. Crescentic valves at the root of the aorta and pulmonary artery

which prevent blood from returning to the ventricles.
seminal vesicle, A secretory gland and reservoir for spermatozoa connected with the

ductus deferens.
seminiferous tubule. One of the secretory tubules of the testis.

septula testis. The connective tissue partitions which divide the testes into compart-
ments.
septiun pellucidum. A double membrane which lies in the median plane of the brain,

between the corpus callosum and the fornix.
septum transversimi. A transverse partition which separates pericardial and peritoneal

cavities of vertebrates.
serous gland, A gland which secretes a thin watery secretion (serum) .
Sertoli cells. Columnar cells of the seminiferous tubules, which serve as a base of

attachment for spermatozoa.
serum. The clear liquid portion of the blood devoid of cells and of fibrinogen.
sessile. Fixed or attached.
sex. The differentiation of two kinds of reproductive cells and of the individuals which

produce them,
shell membrane. A tough double membrane attached to the shell of the hen's egg.
sigmoid flexure. The bent lower part of the colon to the rectum.
Sinanthropus. The Peking species of fossil man.
sino-auricular node. A group of fibers at the root of the precava which in contraction

serves as "pace-maker of the heart" beat.
sinus. A cavity or hollow space.
sinusoid. Relatively large anastomosing blood spaces lined by endothelium without

adventitia.
solenocyte. An elongated flagellated excretory cell associated with the protonephridia

of annelids.
somatic muscle. Derived from the somite or epimere in contrast with the visceral

muscle which is derived from the h>T)omere.
somatopleure. The combined layers of ectoderm and parietal mesoderm,
somite. A segment of the dorsal or epimeric mesoderm.

spermatogonia. Immature germ-cells which by enlargement form the primary sperma-
tocytes.
sphenopalatine ganglion, Meckel's ganglion of the autonomic and facial nerves.
splanchnopleure. The double embryonic layer formed by the visceral layer of mesoderm

and the endoderm.

GLOSSARY 687

spongioblast cells. The cells of the neural tube which form ependyma and glia cells.

squamosal. The bone which forms the thin vertical element of the temporal.

stapes. The innermost earbone which is attached to the foramen vestibuli.

static organ. An organ of equilibration, a statocyst.

status thymicolymphaticus. A edematous condition associated with enlarged th3'mus.

steapsin (lipase). A fat-splitting enzyme secreted by the pancreas.

Steno's (Stensen's) duct. The duct of the parotid gland.

stemebra. One of the segments of the sternum.

stimulofugal. Reacting by motion or growth away from a stimulus.

stomodeum. The ectodermal invagination which leads to the formation of the mouth.

stratum comeum. The outer horny layer of the epidermis.

stratum germinativum. The lowest layer of the epidermis from which the other layers

are derived.
stroma. The connective tissue matrix of an organ,
styloid. Long and pointed.

sublingual gland. A salivary gland below the tongue.

submaxillary gland. A mixed salivary gland below the angle of the lower jaw.
submucosa. The connective tissue layer below the mucous lining of the intestine.
sulcus. A groove or fissure especially of the brain.
superciliary. Pertaining to the region of the eyebrows.

superior colliculi. The anterior larger pair of swellings of the corpora quadrigemina.
supination. Turning the palm upwards.
Sussex man. A prehistoric type of man the remains of which were found at Piltdown,

Sussex, England.
suture. The line of junction of two cranial bones.
sympathin. A neurohumor secreted by sympathetic nerves.
symphysis. The line of junction of primarily separate bones.
sjmapse. The histological connexion between two neurons.
synarthrosis. The fusion of two bones with resultant elimination of a joint.
syncytium. A multinucleate mass of protoplasm.
systole. The contraction of the heart.

talus (astragalus). The ankle bone which articulates with the tibia.

tarsal (Meibomian) glands. Sebaceous glands of the eyelid.

tarsus. The ankle or instep with its seven bones.

taste-bud. A cluster of chemoreceptors which form a taste-organ.

Taimgs man-ape. A fossil ape skull discovered in South Africa.

taurodont. Having a small pulp-cavity and thus resembling a human tooth.

tectorial membrane. The colloidal membrane which covers Corti's organ of the
cochlear duct.

tectum. The roof of the midbrain.

tegmentum. The gray matter which covers each of the brain peduncles.

telencephalon. The anteriormost of the five brain vesicles which forms the cerebral
hemispheres.

teloblast cells. The posterior pair of cells which in annelids forms the mesodermal
bands.

telodendria. The motor end-plates or terminations of a neurite.

tendon. The fibrous attachment which connects a muscle with a bone or other struc-
ture.

teniae coli. Three longitudinal muscle bands of the colon.

tentoriimi cerebelli. The dura mater partition between cerebrum and cerebellum.

terminal nerve. A ganglionated portion of the olfactory nerve.

688 coMPARArrvE anatomy

tetany. Painful muscular spasms associated with calcium deficiency.

thalamus. Gray matter located in the lateral walls of the diencephalon.

thecodont. That type of dentition in which the teeth are lodged in sockets.

theelin. The female sex hormone.

thoracic duct. The main lymph channel which conveys lymph from the lower half of

the body to the left jugular vein.
thrombin (fibrinogen). A hj'pothetical enzyme in clotted blood which converts

fibrinogen into fibrin.
thymus. A ductless gland, largest in infancy, which lies below the sternum in the

mediastinum.
thyroglossal duct. A rudiment of the connexion of the thyroid gland with the pharynx,
thyroid gland. An endocrine organ located on the sides of the trachea.
thyroxine. The iodine-containing hormone secreted by the thyroid gland.
tibia. The shin bone which carries the weight of the body to the ankle.
tigroid substance. The deeply-staining granular substance which forms the Nissl

bodies.
tonus. The normal condition of tension of a muscle.
Tomaria. The free-swimming larva of Balanoglossus (Hemichordate).
trabeculae. The embryonic prechordal cartilages which enter into the formation of

the basicranium.
trachea. The cartilaginous tube which connects the larynx with bronchial tubes.
tragus. The cartilaginous projection before the opening of the auditory meatus.
transverse septum. The connective tissue partition which separates the pericardial

cavit}' from the abdominal cavity.
triconodont. A primitive t>^e of dentition in which three cusps in line occur,
tricuspid valve. The right atrio-ventricular valve.
trigone. A triangular area as of the bladder.
trilobite. A fossil crustacean extinct since the Carboniferous,
triquetrmn. A proximal carpal bone between the lunate and the pisiform.
trochanter. One of the two processes below the head of the femur.
trochlea. A pulley-like structure.

trochophore. The swimming larval stage of some annelids.
trophoblast. The ectodermal portion of the fetal villi of mammalian embryos.
tr5T)sin. The protein-splitting enzyme secreted by the pancreas.
tuber cinereiun. A conical process of the subthalamus between the mammillary bodies

and the infundibulum.
tuberculum impar. The anlage of the apex of the tongue.
tuberosity. A tuber-like process of a bone,
tunica. A covering tissue or coat.

ultimobranchial (postbranchial ) bodies. Glands of unknown function derived from

the last pair of pharyngeal pouches.
vunbilicus. The scar which marks the abdominal connexion of the umbilical cord.
imcinate. Hooked.

imcus. The backward-bent anterior portion of the hjppocampal gyrus,
ungulate. Hoofed.
urachus. The canal which leads to the allantois and which becomes the median

umbilical ligament.
ureter. The duct which connects the metanephros (kidney) with the bladder,
urethra. The outlet of the bladder. In the male the urinogenital passage.
uterus. The "womb" in which the fetus develops.
utriculus. That part of the static organ with which the semicircular canals connect.

GLOSSARY

689

uvula. The median posterior pendulous process of the soft palate.

vagina. The tubular passage from the uterus to the outside.

valvulae conniventes. Transverse folds of the lining of the small intestine.

varicose. Tortuous and swollen.

vas deferens. The efferent duct of the testis.

vasa eflferentia. The ductules which connect the testis with the ductus (vas) deferens.

vasopressin. An hormone secreted by the posterior lobe of the pituitary, which

stimulates contraction of smooth muscle.
veliger. The trochophore larva of molluscs.

ventricle. A small cavity such as is found in the brain and heart.
vermis. 'l"he median lobe of the cerebellum,
vemix caseosa. The waxy covering of the fetal skin.
vesicle. A liquid-tilled sac or cavity.

vitamin. A food substance of unknown composition necessary for growth and health,
vitelline. Pertaining to vitellus or yolk.

vitreous himior. The translucent semisolid substance between retina and lens.
viviparous. Giving birth to living young.
volvox. A genus of flagellate algae.

Wharton's duct. The duct of the submaxillary.
Wirsung's duct. The duct of the pancreas.

Wolffian ducts. The ducts of the mesonephros. In the male the ductus deferentes.
Wolffian folds or ridges. Paired longitudinal ridges of the embryo from which the
paired appendages are differentiated.

xiphistemum. The xiphoid cartilage of the sternum.

zygapophysis. An articular process of a vertebra,
zygomatic bone. The malar bone,
zymogen. An enzyme-forming substance.

INDEX

All numbers refer to pages — those in bold face type to pages with figures.

A

Abdominal, 29

aorta, 392

cavity, 353

pores, 44, 404, 410, 411

skin, 53, 54

vein, 351, 352-356
Abducens nerve, 482, 483, 513, 523, 529,

622, 625
Abductor digiti minimi, 283, 284

hallucis, 284

pollicis brevis, 283
longus, 282
Abomasum, 312
Abortion, 451
Absorption, 65, 314

Acanthocephala (hook-headed worms), 9
Accessory duct, 326

nerve, 482, 530, 626
Acetabulum, 250

acetabular notch, 250
Achilles, tendon of, 276, 283
Acid, hydrochloric, 310
Acidity, 313
Acidosis, 447
Acinous, 140, 326
Acinus, 302, 327
Acoelomate, 30
Acoustic nerve, 482, 523, 529
Acrania, 20
Acraniote stage, 613
Acrodont, 186
Acromegaly, 463
Acromion, 247, 248
Actinia (n), 634
Adapidae, 33
Adaptation, 58
Adaptive changes, 80

evolution, xiv

mutation, xi
Addison's disease, 452

Adductor brevis m., 283

longus m.,. 283

magnus m., 283

pollicis m., 283
Adenoid tissue, 386
Adhesions, 322
Adhesive discs, 47

organs, 617, 618

papillae, 19
Adipose tissue, 158
Adrenal (suprarenal), 451-455
Adrenal artery, 377
Adrenal glands, 453, 454

evolution of, 454
Adrenin (epinephrine), 453, 454, 536
Adult, 450
^pyornis, 40, 43
Aerated (blood), 388, 389
Afferent arteries, 351

nerve, 520,
"After-birth," 121
Agassiz, Louis, 62, 609
Air bladder (sac), 89, 328, 329, 344, 346

breathers, 25

tubes (tracheae), 14, 89, 223, 226, 338
Ala cinerea, 530
Alar plate, 542
Albino, 179

Albumen, 38, 40, 41, 46, 55
Alcohol, 451
Algae, I
Alimentary canal, 7, 8, 9, 11, 12, 30, 244,

648, 651, 659
Alisphenoid, 222
Allantoic (umbilical) artery, 53, 377, 393

placenta, 27

sac, 53, 54

stalk, 317

vein, 371, 371, 376, 379, 382, 384
Allantois, 52-56, 113, 114, 115, 118, 119,

317
Alligator(s), Si> 52
AUis, 540
AUolobophora, 631

6gi

692

INDEX

Alpine, 33

Alternation of generations, 35

Alveolar ducts, 338, 339

glands, 140, 230, 232, 303
limbus, 232
sacs, 338, 339

Alveolus, 193, 197

Amber eye, 178

Ambystoma tigrinum, 51, 154

Ameloblast, ig6

Amia, 617

Amino-acid, 326

Amiurus, 579

Ammocoetes, 23, 646, 647, 651

Ammonites, 14

Amnion(ic), 25, 31, 52-56, 113, 114, 115,
119

Amniota(es), 31, 86, 87, 89, 95, 96, loi

Amoeba, 3, 4, 467

Amphibia(n), 2, 25, 25, 31, 34, 36, 39, 40,
45, 49, SO, 51, SS, 56, 57, 60,
62, 66, 69, 70, 74, 75, 75, 76,
86, 87, 88, loi, 186, 246, 411

Amphicoelous (biconcave), vertebrae, 204

Amphioxus, 18, 20, 21, 22, 31, 59, 60,
61, 62, 63, 64, 66, 69, 70, 71,
74. 75, 76, 81, 81, 83, 84, 84,
85, 86, 87, 88, 333, 351, 410,
661, 663, 664

Amphitherium, 188

Ampullae of ductus deferens, 428
of ear, 596, 600, 603, 604
of Lorenzini, 598

Amygdaloid nucleus, 504

Amylopsin, 326

Anal canal, 318
muscle, 8
opening, 8, 10, 88

Anamnia, 31, 95, 96, loi

Anastomosis, 382

Anatomy, vii, 35, 126
comparative, vii, viii

Ancestor, i, 3, 5, 9, 20, 25

Ancestral tree, 7, 628

Ancestry, xiv, 9

of vertebrates, 628-666

Anconeus m., 281

Angioblast, 366, 367

Anguillidae, 48

Anguis fragilis, 592

Angular(e), 221, 222, 231

Animal, ix, i, 4, 30

evolution, 13

hemisphere, 63, 70

kingdom, 1,2, 3, 5

pole, 41, 60, 61, 62, 66
Anisotropic, 144
Ankle, 252
Ankylose, 257
Anlage(n), 236, 325

Annelid ancestr>' of vertebrate theorj',
350, 607, 628, 630-634
objections to, 634
Annelida(s), 2, 4, 8, 10, 11, 12, 14, 15,

30, 409, 628, 629, 630, 634
Ansulate commissure, 511
Anteater, 28
Anterior cavity, 263, 265, 266, 616, 640

crescent, 76

somite, 267, 651
Anthropoid(s)(ea), 29, 30, ^2, 33
Antibodies, 348
Antihelix, 599, 606
Antithrombin, 348
Antitoxins, 348
Antitragus, 599, 606
Antler, 171
Antrum, 227, 228

of Highmore, 232
Anura, 25, 45, 50, 56, 331
Anus, 4, 6, 7, 8, 9, II, 12, 20, 21, 30, 65, 82,
84, 88, 90, 91, 300, 301, 315, 318
Aorta, 352, 374, 390

abdominal, 392

development of, 373, 374

dorsal, 10, 26, 330, 351, 352, 353,
354, 356, 376, 377, 390

thoracic, 392

ventral, 352, 376
Aortic arch(es), 26, 31, 350, 351, 352, 360,
374. 376, 377, 390, 625
development of, 375
evolution of, 359
Ape, 29, 30, 38
Apex, apices, 303
Apical spot, 590, 592
Appendage(s), 14, 30

integumentary, 169
Appendices epiploicae, 315
Appendicular muscles, 288

skeleton, 236-258
development of, 253
in man, 247

INDEX

693

Appendicularia, 334, 662

theory, 661-664
Appendix epididymidis, 445

fimbriae, 445

ovarii, 445

testis, 442

vermiformis, 315, 316
Apus, 651
Aquatic, 28, 328
Aqueduct, 490, 511, 548
Aqueous humor, 584, 589
Arachnid hypothesis, 643-657

objection to, 656-657
Arachnids, 2, 24, 30, 643, 649, 651, 653,

653

Arachnoidea, 15

Arbor vitae, 512

Arboreal, 29, 246

Archencephalon, 640

Arch(es) aortic, 26, 31, 350, 351, 352,
360, 374, 376, 377, 390, 625
branchial, 230, 329, 332
hemal, 204, 206, 244
hyoid, 230, 232, 246, 330, 606
mandibular, 229, 606
neural, 23, 204, 206, 244, 656
visceral, 609, 610, 621, 624
skeletal, 228—236

Archenteric cavity, 87

Archenteron (enteron), 64, 65, 68, 72,

73, 74, 75
Archetypal, ix

vertebrate, 217, 608, 609
Archetype, 608, 609
Archicortex (primordium hippocampi),

490, 493, 498, 502, 544
Archinephros, 405
Archipallium (archicortex), 544
Archipterygium, 243
Archistriatum, 491, 503, 504
Arcuate artery, 418

vein, 418
Area opaca, 76

pellucida, 76
Areolar tissue, 152
Arey, viii
Arm, upper, 247
Armadillo, 27
Armenoid, 33
Armor, 201
Arrectores pilorum, 1 76

Arteries, adrenal, 377
afferent, 351
aorta abdominal, 392
arch of, 390
ascending, 390
descending, 390
dorsal, 10, 26, 330, 351, 353, 354,

376, 377, 390

thoracic, 392

ventral, 352, 376
arch, volar, 378, 392
arcuate, renal, 418
auricular, posterior, 390
axillary, 392
axis, thyroid, 390
basilar, 377, 390
brachial, 377, 392
bronchial, 340, 390
carotid, common, 352, 354, 356, 390
external, 352, 376
internal, 352, 376
celiac (axis), 352, 355, 377, 392
centraUs retinae, 587
cerebral, 375
cervical, superficial, 390
ciUary, 587
circumflex, 391, 392
coUc, 393
coronary, 389
costal (intercostal), 377
costo-cervical, 390
digital, 377, 379, 392
dorsalis pedis, 395
ductus arteriosus. 355, 376, 384
epigastric, 393
facial, 391

femoral, 378, 379, 393
gastric, 392
gastroduodenal, 391
gluteal, 393

hemorrhoidal middle, 393
hepatic, 323, 392
humeral, anterior circumflex, 392

posterior circumflex, 392
hypogastric (internal iliac), 391, 393
iliac, 353, 354, 356

common, 393

external, 377, 393

internal, 393
iliocolic, 392

694

INDEX

Arteries, iliolumbar, 393

innominate (brachiocephalic), 390
intercostals, 392, 625
interlobar, 418
interlobular, 418
interosseous, 377
intersegmental, 377
intestinal, 392
ischiadic (peroneal), 378
lingual, 390
lumbar, 377, 392
mammary, internal, 390
maxillary, external (facial), 390

internal, 390
median, 377
mesenteric, 352, 354, 356

inferior (posterior), 377, 393

superior (anterior), 322, 355, 356,
377, 382, 392
obturator, 393
occipital, 390

omphalomesenteric, 348, 349, 371
ovarian, 377, 393
pancreatico-duodenal inferior, 392

superior, 392
peroneal, 379

pharyngeal, ascending, 390
phrenic, 377

inferior, 392
plantar, lateral, 39s

medial, 395
popliteal (saphenous), 379, 393
profunda (deep femoral), 391, 393
pudendal, internal, 393
pulmonary, 354, 356, 376, 384, 389
radial, 378, 392
renal, 354, 356, 377, 393, 418
sacral, 377, 393

lateral, 393
saphenous (popliteal), 379
spermatic, internal, 377, 393
splenic (lienal), 391, 392
sterno-cleido-mastoid, 390
subclavian, 352, 354, 356, 376, 377,

390, 392
subscapular, 392
suprarenal, 451
systemic, 355
tarsal, 391

temporal, superficial, 390
testicular, 393

Arteries, thoracic, lateral, 392

superior, 392
thoraco-acromial, 392
thyroid axis (thyreo-cervical), 390
thyroid, inferior, 455, 458

superior, 390, 455
thyroidea ima, 455
tibialis, anterior, 379, 395

posterior, 379, 395
truncus arteriosus, 351, 353, 354,

356, 371, 373

ulnar, 377, 392

umbilical (allantoic), 53, 377, 393

uterine, 393

vertebral, 377, 390

vesical, inferior, 393

vitelline, 376, 377

volar, 378, 392
Artery(ies), 53, 347, 378, 389, 391

evolution of, 361
Arteriole, 418
Arthrodires, 25
Arthropod(a), 4, 14, 15, 30, 630, 647, 651

theory of vertebrate ancestry, 642
Articulare, 231, 235, 246
Articulata, 14

Artificial parthenogenesis, 44
Artiodactyla, 28, 32
Arytenoid, 233, 236
Ascending fibers, 513, 517
Ascidia, 18, 18
Ass, 28

Assheton, 77, 79
Assimilation, 292
Association cell, 472
Association fiber tracts, 495, 499

neurons, 516, 517, 520
Astragalus (talus), 252
Ateliotic dwarf, 463
Atlantic salmon, 55
Atlas, 26, 205, 207
Atmospheric pressure, 342
Atrial cavity, 19

siphon, 19
Atriopore, 20, 21
Atrioventricular bundle, 388

valve, 353, 354, 358, 371, 387, 388
Atriozoa, 662
Atrium(a), 335, 338, 353, 354, 358, 37i,

387, 388, 389, 651
Atropine, 536

INDEX

695

Auditory, 89, 602

external meatus, 605

nerve, 482, 483, 603

organs, 594, 595, 654
Auerbach's plexus, 486
Auricle, 384, 387

external, 606
Auricular artery, 390

vein, 395
Auricularis magnus, 274
Australia, 27
Australian, ^7,
Australopithecus, 32
Autonomic fibers, 149, 537

ganglia, 53 1, 537

nerves, 536, 537

system, 85, 495, 522, 531, 535, 537
evolution, 539
Aves, 26, 31, 57
Axial, 84, 95

skeleton, 203, 232
Axillary, 386

artery, 392

vein, 396
Axis, 205, 207
Axolotyl, 51

Axon (neuraxon), 470, 471, 471
Aye-Aye, 33
Azygos vein, 356, 357, 361, 382, 396

B

Baboon, 29, 33

Backbone (vertebral column), 246

Bacteria, 348

Balanoglossus, 17, 30, 659, 660, 661

theory, 659, 660, 661
Balfour, Francis, 553, 554, 610, 611,
625, 633, 639

hypothesis, 554
Barb(s), 173
Barbule(s), 173
Barnacle, 14

Bartholin, glands of, 426
Basal plate, 170
Basalia, 241

Basement membrane, 133
Basihyal, 305
Basilar artery, 377, 390

membrane, 602

papilla, 597

Basilar plexus, 462

Basilic vein, 396

Basioccipital, 228

Basophiles, 162, 364

Bat, 29

Bateson, 17, 659

Bayliss, 447

Bdellostoma, 23. 31, 229, 299, 336, 359, 458

Beak, 13

Beaker eyes, 580

Bear, 28

Beard, 453

Beard, 297, 623, 624, 635

"Bearded lady," 453

Beaver, 28

Beetle, 16

Behavior, xiv

Bell's law, 485

Belly, 259

Bergson, 581

Betula, 2

Biceps, 249

brachii, 281

femoris, 283
Bichir, 345

Biconcave (amphicoelous), 204
Bicuspid, 193

valve, 193, 373, 389
Bielschowsky, 555
Bilateral, 9

symmetry, 7, 10, 30
Bile, 322

capillaries, 324, 325

duct, 90, 99, 323, 324, 326, 327
Bimana, 30, 32
Binomial system, 2
Biochemistry, xiii

Biogenesis, fundamental law of, 610
Biophysics, xiii
Bipolar, 147
Birches, 2
Bird(s), 2, 25, 26, 34, 39, 40, 45, 51,

52, 56, 57, 58, 62, 69, 70, 76
Birthmarks, 168, 177
Bischoff, 619
Biting jaw, 31, 244
Bivalve, 9, 13
Bladder, 324, 407, 415, 419, 435, 436, 439

air, 344
Blastocoele, 60, 62, 62, 63, 64, 66, 68
Blastoderm, 40, 62, 63, 69, 70, 76, 78, 80
Blastodermic vesicle, 77, 78

696

INDEX

Blastoporal region, 86, 87
rim, 75, 76, 77, 87
streak, 87
Blastopore, 4, 10, 17, 63, 64, 65, 68, 70,
71, 74, 75, 76, 78, 79, 81, 82,
87, 88, 634, 637, 638, 641
Blastula, 58, 60, 62, 63, 65, 66, 69, 71
Blind spot, 587

Blood, 10, 12, 52, 53, 158, 159, 160,
347, 351, 366, 367, 376
aerated, 389
arterial, 26
capillary, 325, 329
corpuscles, 348, 368
functions of, 348
islands, 365, 366
plasma, 159, 348
plates, 162
pressure, 464
serum, 348
sucker, 10
system, 8

vascular system, 350, 353, 3^6
venous, 26, 351, 384, 389
vessel, 7, 10, II, 52, 54, 83, 98, 99,
326, 348, 366, 631
"Blue" baby, 384
Boas, 18
Body, 8

cavity (coelom), 7, 8, 9, 44, 63, 83, 410
muscle, 94
of penis, 429
stalk, 543
temperature, 26
of uterus, 424

wall, 5, 8, 10, II, 30, 31, 52, 84,
Boeke, 639
Bolk, 188

Bone(s), 95, 154, 156, 156, 219, 225, 244
development, 158
marrow, 157
membrane, 170, 219
primary, 106
secondary, 106
Bony armor, 23
fishes, 34, 45
labyrinth, 599
skeleton, 31
Bothroidal cord, 656
Boveri's theory, 583

Bowman's capsule, 98, 405, 417, 433, 438
Brachial artery, 377, 392

Brachial v., 396

Brachialis m., 281

Brachiopods ("lamp-shells), 9

Brachioradialis m., 283

Brachium conjunctivum, 512, 513, 549

pontis, 512, 513, 549
Brain, 7, 10, 12, 18, 19, 21, 27, 82, 84,
85, 246, 475, 478, 481, 486,
487, 487, 492, 493, 495, 503.
514, 636, 643
case, 22, 31, 216-225, 244, 531
development, 540-549
flexures, 542, 543, 545
region, 86
size, 225, 505
stem, 526

vesicles, 85, 544, 623
Branchiae, 650
Branchial arch, 230, 329, 332
bars, 329, 334
cleft, 609
placenta, 57
system, 329-337
Branchiata, 14

Branchiomeric muscles, 269, 291
Branchiomerism, 291, 336, 625
Braus, viii, 309, 310, 322. 614
Breast(s), 450
Breathing, 328, 331
Brehm, 245
Bremer, viii, 37
Bristle, sensory, 10
Broad ligament, 421, 424
Broman, 327, 343
Bronchial artery, 340, 390
tubes, 390
vein, 340
Bronchioli, 338, 339
Bronchus(i), 89, 338, 340, 34i
Brood-pouch, 47, 47, 57
Brooks, 662
Broom, 244
Brunner's glands, 313
Bryozoa, 9

Buccal glands, 301, 3°3
Buccalis nerve, 483, 484
Buccinator muscle {see Muscle)
Bufo marinus, 49
Bulbils, 351

Bulbo-cavernosus m., 284, 425
Bulbo-urethral gland, 426, 428

INDEX

697

Bulbus, cordis, 358, 373
urethrae, 427, 429

Bundle, 8

Burrow, 53

Bursa copulatrix, 7
omental, 321, 322

Butterflies, 16

Caecilians (gymnophiona) , 45, 49
Cajal, 555
Calcaneus, 252
Calcareous, 3, 5, 12, 38
Calcified cartilage, 156

structures, 135
Calcium, 458

carbonate, 158, 193

phosphate, 158, 193
Callorhynchus, 47
Calyces, 419, 434

major, 419, 434

minor, 419, 434
Calyx, renal, 419
Cambrian, 34
Camel, 28
CanaUculus(i), 156
Canalis centralis, 86
Canalis reuniens, 596
Cancellous bone, 156, 205
Canine, 28, 186, 192
Caninus muscle (^ee Muscles)
Cannon, 454
Capillary, 323, 347, 350, 354

network, 330, 351, 382
Capitate, 249
Capitellid worms, 633
Capitulum, 249
Capsule, 6

Bowman's, 98, 405, 417, 433, 438

internal, 504

otic, 483, 605

sensory, 228
Caput (head) epididymidis, 428
Carapace, 171
Carbon dioxide, 328, 339
Cardiac glands, 310

muscle, 145, 387

orifice, 309

plexus, 561

pore, 637

stomach, 309

Cardinal v., 352, 353, 631

common, 358, 376, 379, 382

post-, 352, 353, 356, 376, 382

pre-, 352, 353, 356, 376, 382

sub-, 382

supra-, 382
Cardiopore, 4, 637, 641
Carinatae, 26
Carnassial tooth, 190
Carnivora, 28, 32, 190
Carnivorous, xiv
Carotid a., 351, 352,, 354, 356, 376, 390

plexus, 537
Carpus, 249
Carrel, xii

Cartilage, 95, 103, 103, 104, 154, 154,
202, 229, 244

bone, 202

ray, 237, 238
Cartilaginous cranium, 228, 609

skeleton, 23, 24, 31, 239, 609
Casein, 311
Castrate, 448
Cat, 3, 28
Cat-fish, 46, 47
Catalyzers, 292
Catarrhina, 30, 32, 33
Cattle, 28
Caucasian, 192
Cauda epididymidis, 428

equina, 468
Caudal artery, 351, 352, 353, 356

fin, 20

glands, 8

muscles, 272

vein, 351, 353, 356
Caudate lobe, 323

nucleus, 504, 546
Causation, xiii
Cause, x, xiv
Cavity anterior, 263, 265, 266, 616, 640

atrial, 19
Cebidae, 33

Cecum(a), 28, 315, 316, 319
Celiac artery (axis), 352, 355, 377, 392

ganglion, 539

plexus, 561
Cells afferent, 467

association, 372, 551

Deiter's, 602

effector, 467

698

INDEX

Cells efferent, 467

enucleated, 368

ependymal, 515, 551

flame, 399

ganglion, 467, 469, 472, 472, 482, 516

germ, 449

germinal, 551

giant, 556

hair, 600, 602

intercalary, 472

interstitial, 449

of Joseph, 478

layer, 498

mesenchymal, 617

motor, 467, 516, 551

nerve, 467

neurochord, 515

neuroglia, 551

neuromuscular, 469

neurosensory, 467, 469, 470, 563,
565, 566

proliferation, 63, 66

Purkinje, 480

receptor, 467

Rohon-Beard, 516, 556

sense, 467, 469

supporting, 602

sustentacular (Sertoli), 37

taste, 579

transmittor, 467
Cellulose, 18, 31
Cement of teeth, 194, 195, 197
Cement gland, 10
Cenozoic, 34
Centers, equilibratory, 513

reflex, 501
Centipede, 15
Central cells, 326

nervous organ, 145

system, 65, 350, 470, 495, 509, 619,
646
Centralis retinae artery, 587
Centrifugal flow, 87
Centrum(a), 204, 206, 244
Cephalaspis, 24, 651
Cephalic, 85

coelom, 611

metamerism, 616

navel, 652

stomach, 644, 647, 648

vein, 396

Cephalochorda(ta), 2, 4, 20, 31, 334,

630, 643, 663
Cephalodiscus, 17, S33j 661, 662
Cephalopoda, 13, 14, 581
Cercopiths, 33
Cerebellar cortex, 512, 513
Cerebellum, 480, 487, 490, 493, 495,

511, 512, 514, 531, 549
Cerebral cortex, 499, 508, 520
eye, 22

ganglion, 7, 12
hemispheres, 491, 493, 494, 495 >

500, 602
organ, 572
peduncles, 549
Cerebrospinal nerves, 522
Cerfontaine, 59, 64, 72
Cervical, 516
artery, 390
enlargement, 490
myotomes, 377
nerves, 537
plexus, 489
vein, 395
Cervix of the uterus, 424
Cetacea(eans), 28, 32
Cestracion, 203, 230
Chaetognatha (arrow worms), 9
Chalaza, 40
Chalk, 3

Chase, A. F., 267
Cheek teeth, 189
Cheiropterygium, 241, 243, 246
Chelonia(n), 26, 171, 412, 414
Chemical compounds, xiii
regulation, 447
specificity, 163
Chemo receptors, 572
Chiasma, optic, 482, 510, 589

trochlearis, 651
Chick, 62, 92, 93, lOl
Chief cells, 311
Chimpanzee, 29
Chin, 232
China, 178
Chinaman's cap, 13
Chiroptera, 29, 32
Chitin, 3, 39
Chitinous, 14
Chiton(s), 12
Chlamydoselachus, 230
Chloragogue, 11

INDEX

699

Chlorophyll, i, 4
Choanae, 342, 574, 575

secondary, 576, 577
Cholesterol, 324
Chondrin, 202

Chondrocranium, 216, 217, 217, 228
Chorda dorsalis, 16

tympani, 484
Chordae tendineae, 373, 388
Chordate(a), 3, 10, 16, 22, 30, 57, 59,

71, 79, 244, 627, 630
Chordotonal organs, 595
Chorio-allantoic membrane, 120
Chorioid fissure, 87, 588

layer, 586

plexus, 480, 481, 490, 507, 510, 513,

547
Chorion (serosa), 52, 53, 54, 55, 113, 114,

IIS, 119
Chorionic villi, 379

Chromaffin cells, 452, 453, 453, 454, 486
Chromatin, 43, 59

constitution, 449
Chromatophore, 153, 177
Chromosome complex, 449
Chrysophrys aurata, 36
Chyme, 326
Chymosin, 311
Ciliary, 334

artery, 587

body, 585

ganglia, 489

muscles, 585
Ciliated, 9, 11

groove, 21
Cilium(a), 3, 9, 10, 19, 30, 84, 138, 329,

334, 400
Cingulum, 188
Ciona, 18

Circular muscles, 11

Circulation, 7, 8, 10, 12, 53, 347, 348,
349, 383, 654

embryonic, 348, 349, 376

intercellular, 348

intracellular, 348
Circulatory organs, 106

system, 9, 14, 17, 21, 350, 631

vessels, 106
Circumesophageal, 10

ring, 8, 9, 12, 477
Circumvallate papillae, 579, 579, 580
Cirripeds, 631

Cirrus, cirri, 21

Cisterna chyli, 363, 386, 398

Cladoselache, 239, 241

Clam, 9, 13

Clam worm (nereis), 10

Clarke's column, 517, 520

Class(es), i, 3

Classification of animals, 30

Clavicle, 28, 238, 239, 240, 247, 248

Claw(s), 29, 30, 171

Cleavage, 58, 59, 60, 60, 61, 61, 62, 63, 77

"Cleft palate," 577

Cleithrum, 244

Clendening, 391, 416

Climbing foot, 30

Clitellum, 11

Clitoris, 415, 425, 426, 445

Cloaca, 5, 26, 45, 90, 91, 96, 317, 319,

407, 412, 413, 414
Cloacal aperture, 88

glands, 45

membrane, 317, 318
Cobb, 8

Coccygeus muscle, 284
Coccyx, 205, 209, 251
Cochlea, 597, 600, 601, 603, 605
Cochlear duct, 600, 601, 601, 604

nerve, 523, 529, 602, 604
Cod, 36, 46
Coelenterata, 2, 4, 5, 6, 7, 30, 65, 66, 66,

407, 630
Coelom(ic), 9, 10, 11, 12, 14, 21, 30,

83, 84, 92, 93, 94, 96, 98
Coelomate, 9, 10, 11, 30
Coelomic pouch(es), 7, 16, 408

sacs, 12, 83

spaces, 83, 87, 94, 96, 99
Coelomoducts, 11, 400, 401, 401, 408,

409, 631
Coghill, 557
Coitus, 77
Cold, 563
Colic artery, 393
Collagen(ous), 152
Collar, 17

bone, 238

region, 660
Collateral(s), 147

Collecting tubules, 434, 437> 438, 631
CoUiculus, inferior, 511

superior, 511
Colloids, 292

700

INDEX

Colon, 314, 315, 318, 322, 416

ascending, 315

descending, 315

sigmoid, 315

transverse, 315
Colonial, 5, 9

protozoa, 4
Colonies, 3, 5
Color, 178, 179

Columella (stapes), 230, 246, 597
Columnar epithelium, 131, 604
Columns, 516

lateral, 516

posterior, 516

ventral, 516
Commissures, 12, 480, 508, 547

ansulate, 511

anterior, 480, 490, 547
pallial, 490, 495, 510

dorsal, 510
pallial, 510

gray, 516

habenular, 480, 490, 506, 508

posterior, 480, 490, 510

pallial (hippocampal) , 490, 510, 547
Components, nerve, 535

general cutaneous, 484

lateralis, 484

somatic motor, 522
sensory, 523

visceral motor, 484, 524
sensory, 484, 524, 539
Concha(e), 232, 575, 599
Concrescence theory, 188
Conduction, 467, 517
Conductor, 145
Condyle, 26, 251, 252
Condyloid, 232
Conjunctiva, 588
Conjunctival sac, 588
Connecting link, 225

Connective tissue, 102, 151, 237, 325, 332
Connectives, 12, 475
Conscious, xiv
Consciousness, xii
Contractile, 10, 350, 631

vacuole, 3, 399
vesicle, 10
Contractility, 261
Contraction, 84, 329, 388
Conus, 353, 354, 371
arteriosus, 358

Convergence, 57, 359
Cony, 28, 189
Cook, 332
Cope, 187
Copulation, 45
Copulatory apparatus, 46

organ, 414
Coraco-brachialis m., 281
Coracoid, 239, 240, 247
Coral(s), 5, 6, 39
Cord nerve, 475

spinal, 514, 531
Corium (dermis), 95, 164, 165, 166, 167,
168

papillae, 167
Cornea, 584

Corniculate, 233, 236, 340
Corning, viii, 376, 378
Corona radiata, 42
Coronary artery, 389

sinus, 382, 389

vein, 358, 389, 396
Coronoid bone, 231

process, 232, 249
Corpora bigemina, 511

cavernosa penis, 415, 429

quadrigemina, 493, 511, 548
Corpus albicans, 420, 422, 450

callosum (ant. pall, comm.), 495,
510, 547

cavernosum urethrae, 27, 414, 415,
429

hemorrhagicum, 420

luteum, 420, 422, 449, 450

penis, 429

restiforme, 549

striatum, 487, 495, 501, 50S) 544, 545
Corpuscle, 21, 322

of Golgi-Mazzoni, 566, 568

of Grandry, 568

of Hassall, 461

of Krause, 566, 568

of Merkel, 567

of Meissner, 567

of Pacini, 566, 568

renal, 417

tactile, 567

thymic, 461, 461
Corrugator m., 274
Cortex, 494, 495, 498, 545

nodular, 386

renal, 418

INDEX

701

Corti, organ of, 597, 600, 601, 601, 604
Cortical enlargement, 495

rays, 416
Corticospinal tract, 518, 520
Cortin, 452
Costal artery, 377

cartilage, 210, 212

groove, 212

processes (ribs), 204, 212
Costo-cervical artery, 390
Cotyledonary placenta, 120
Cotylosaurian, 173
"Cowlick "-(vortex), 174
Cowper's gland, 426, 428
Coxal bone, 241, 250, 251

glands, 651
Crab(s), 14

horseshoe or king, 15
Cranial nerves, 481, 482, 484, 488, 489,
490, 5i3> 514, 522, 609, 621, 623,
625
Cranio ta, 22
Cranium, 228, 499

bones of, 225, 226, 227

development of, 227

evolution of, 216, 217, 218, 219, 220,
221, 222, 223, 224, 225
Crayfish, 14

"Creative synthesis," xiii
Cremaster muscle (externus, internus),

426
Creodonts, 28
Crescent, 302
Cretaceous, 34
Cretin, 456
Crew, 449

Cribriform plate, 227
Cricoid, 233, 236, 340
Cricothyroid m., 341
Cristal, 137, 596, 600, 604
Crocodilia, 26, 34, 52, 412
Cro-Magnon man, 32, 224, 225
Croonian lecture, 609
Crossopterygians, 24, 25, 31, 244, 328
Crown, 193
Crura cerebri, 635

cerebeUi, 512

penis, 429
Crustacea, 2, 14, 30, 631, 651
Crypt, 313

Cr>T)tobranchus (hellbender), 39, 40

Cryptorchism, 443

Ctenophore, 7

Cuboid(al), 252

Cuneiform, 233, 236, 252, 340

Curvatures cervical, 206, 207, 207

greater, 309, 312

lesser, 309, 312

lumbar, 206, 207, 207

sacral, 206, 207, 207

thoracic, 206, 207, 207
Cusps, 193
Cutaneous, 336

glands, 179

sense organs, 566, 567, 568
development of, 568
Cuticle, II

Cuticula, 8, 9, 21, 135
Cutis (corium), 167
Cuttlefish, 13, 14
Cuvier, 14, 609

duct of, 352, 356, 357, 376, 382, 654
Cyclostome(ata), 2, 20, 23, 31, 44, 88,

332, 353. 409, 643, 647
Cynognathus, 223, 231
Cystic duct, 323

vein, 397
Cytology, 127
Cytoplasm, 36, 38, 40, 42, 59

D

Darwin, vii, ix, x, xiv, 219

Darwinian principles, 179

Dasyurus, 27

Dean, 23, 24

Death, 3

Deciduous placenta, 121

teeth, 198
Decussation, 511
Deer, 28

Degeneration, 8, 20
Deiter's cell(s), 602
Delamination, 70, 75> 80
Delsman, 297, 630, 636-642

hypothesis, 636-640
objections to, 640-643
Deltoideus m., 247, 248
Demersal eggs, 46
Demilunes, 302

702

INDEX

Dendrite(s), 147, 469, 47°, 499. 5i6, 551,

552
Dental arcade, 192

canaliculi, 193, 197

formula, 192

papilla, 196

ridge (lamina), 195, 199

sac, 195, 198
Dentalium, 12
Dentary, 26, 231, 246
Dentate nucleus, 513, 549
Dentine (ivory), 170, 184, 193, 197, 199
Dentition, 26, 27, 28

acrodont, 186

diphyodont, 187

heterodont, 186

homodont, 186

monophyodont, 187

pleurodont, 186

polyphyodont, 187, 188

thecodont, 186
Depressor, 332
Dermal armor, 244

bones, xiv, 105, 105, 219, 236

sac, 51

scales, 219

skeleton, 244
Dermatome, 95, 102, 168, 209
Dermis(corium), 95
Descending fibers, 515, 517
Descensus cordis, 373

of gonads, 443
Deuterencephalon, 478, 640
Deuterostomia, 2, 4, 10, 17, 300, 634
Deutoplasm, 38

Development, 12, 44, 46, 58, 59, 77
Developmental, 52

mechanism, 80

methods, 80

potentialities, 44

processes, 70
Devilfish, 13, 14
Devonian, 25, 34, 244
DeVries, xi, xiv, 219
Diabetes, 447
Diaphragm, 26, 35, 107, 108, 272, 280,

280, 323, 342
Diapophysis, 204
Diastema, 192
Dibranchiates, 14
Didelphia, 27, 27

Diencephalon, 479, 505, 507, 510, 542, 544

547
Differentiation, 4, 35, 36, 55, 83, 94

antero-posterior, 7, 83

dorso-ventral, 7

theory, 187
Difflugia, 3
Diffuse placenta, 120
Diffusion, 51, 328
Digastric m., 275
Digestion, 6, 65, 89, 100, 292
Digestive cavity, 65, 82, 87

epithelium, 89

organ, 90, 326

outlet, 27

system, 292-327, 644

tube, 52, S3, 66, 71, 84, 89, 91, 99

wastes, II, 90
Digit, 29, 250
Digital artery, 377, 379, 392

vein, 397
Dimetrodon, 223, 231
Dinosauria, 26, 34
Dinotherium, 191
Dioecious, 36
Diphyodont, 187
Diploe, 395
Diploid, 43
Diplospondyly, 204
Dipnoi, 25, 31, 243, 329, 354
Discoidal cleavage, 62

placenta, 120
Diseases, 4

Diverticulum(a), 7, 325, 333, 605, 618
Divisive (churning) movements, 3^4 .
Doglish, 35, 38, 58
Dogs, 28
Dohrn, 297, 318, 616, 630, 632, 633, 635,

662
Dohrn-Semper theory, 636
Dolphin, 28
Dorsal, 8

aorta, 10, 26, 330, 351, 353, 354.
376, 377. 390

lip, 68

organ, 652
Dorsalis pedis artery, 395
Dorsum, 303
Dragonfly, 16
Dromatherium, 187, 188
Dryopithecus, 33

INDEX

703

Duckbill, 27, 53
Duct(s), 99

accessory (Santorini's), 326

cystic, s^S

ejaculatory, 428

of Gartner, 445

hepatic, 323

Muellerian, loi, 102, 411, 413

nasolacrimal, 588

pancreatic (Wirsung's), 326

thoracic, 363, 386, 398

thyroglossal, 457

Wharton's, 301

Wirsung's, 326, 327

Wolffian, 97, 98, loi, 102, 404, 411
Ductless gland, 303, 446
Ductuli aberrantes, 427, 441

deferentes, 441
Ductus aberrans, 407

arteriosus, 355, 376, 384

choledochus, 323

Cuvieri (common cardinal), 352,
356, 357, 376, 382, 654

deferens, 407, 412, 413, 427, 428, 434,

441, 443

ejaculatorius, 428

epididymidis, 426, 427, 428, 441

reuniens, 600, 603

venosus, 382, 384
Dugong, 28
Duodenal glands, 313
Duodenum, 313, 323, 324, 326
Dura, 601

mater, 562
Duval, 62, 76
Dwarf, 456

E

Ear, 86, 523, 595, 598, 599, 602

external, 598, 606

internal, 227, 598, 599, 602, 603

middle, 227, 228, 234, 598, 599, 605
Ear-bone, 26, 232, 246, 599, 599

-drum, 234, 598, 599, 599, 606

-wax, 599
Earthworm, 10, il
East Indian manis, 1 7 1
Echidna, 27, 41, 46, 53, 90
Echinodermata, 2, 4, 6, 10, 16, 17, 30
Echiuroids, 365
Ectobranchiate, 332

Ectoderm, 56, 64, 65, 66, 68, 71, 72, 73,
4, 75, 81, 82, 84, 85, 86, 87, 89,

91, 95
Ectoplasm (ectosarc), 3, 164
Edentates, 27
Eel, 23
Effect, xiii
effector(s), 145

neuron, 520
Efferent ducts, 426

ductules, 433
Egg(s), 5, 8, 26, 38, 40, 44, 49, 50, 51, 52,

54, 55, 56, 57, 59, 61, 80
Egg cases, 39, 39, 45, 47
Egg-laying, 26, 27
Eisig, 633

Ejaculatory duct, 428
"Elan vital," 581
Elasmobranch, 22, 24, 25, 31, 34, 48, loi,

235, 237, 244, 353
Elastic cartilage, 156

fibers, 152
Elasticity, 153, 585
Elastin, 152
Elephants, 28, 43

Embryo, 7, 25, 26, 51, 55, 64, 84, 87
Embryological, xiv
Embryology, vii, 35
Embryonic membranes, 113, 121

shield, 78, 79, 79

tissue, 52

waste, 53
Emergent conception of evolution, xi
Emulsification of fats, 322
Enamel, 28, 135, 170, 184, 185, 194, 197,
198, 199

cells, 198

organ, 195, 196, 199

prisms, 197

pulp, 198
Endobranchiate, 332
Endocardial, cushions, 373
Endocardium, 107, 387
Endochondral bone, 103, 205, 605
Endocranium, 656
Endocrinal function, 449

organs, 446
Endocrine (ductless) glands, 90, 303, 329,

446
Endoderm, 5, 6, 8, 64, 65, 66, 68, 70, 72,
74, 75, 76, 80, 81, 86, 87, 88, 90,
91, 652

704

INDEX

Endodermal lining, 8

Endodermic diverticula, 332, 333, 618,

618, 640
Endolymph, 599, 600, 604
Endolymphatic duct, 596, 601, 602
Endometrium, 424, 425
Endoneurium, 150, 150, 471
End-organ, 552
Endoskeleton, 22, 201
Endostyle, .18, 19, 19, 21, 335, 457, 458,

663
Endothelium, 63, 106, 130, 366, 367, 369
End-plate, 523
Energy, xii, 46
Ensiform, 214
Enteric aperture, 87, 88

cavity, 87, 88, 94

endoderm, 82, 84, 89, 90, 100

tube, 84, 87, 88
Enterocoele, 10, 632
Enterocoelic cavity, 660
Enterokinase, 326
Enteron, 4, 6, 7, 8, 10, 11, 30, 82, 84, 87,

89, 90. 93
Enteropneusta, 16
Environment(al), x, 46, 64, 65
Enzyme, 292, 348
Eoanthropus Dawsoni, 32, 33
Eocene, 33
Eosinophiles, 364
Epaxial muscles, 268
Ependyma, 644
Ependymal cells, 515

layer, 481, 545, 550
Epiboly, 68
Epibranchial ganglia, 624, 625, 633

placodes, 560

sense-organ(s), 623, 624
Epicardium, 107, 369, 387
Epiderniis(al), 8, n, 22, 71, 95, 129, 134,

164, 165, 167, 176
Epididymis, 96, loi, 407, 412, 426, 428

caput (head), 428

ductus, 426, 427, 428, 441
Epiglottic cartilage, 340
Epiglottis, 233, 305, 341
Epimere, 94, 95
Epimeric muscle, 261
Epimyocardium(al) {see Epicardium)
Epinephrine (adrenin), 453
Epineurium, 150, 150, 471
Epiotic, 228

Epipharyngeal groove, 21

Epiphysis(es), 103, 480, 488, 490, 506, 507,

592
Epistriatum, 487, 489, 501, 502, 503
Epithalamus, 505, 506, 547
Epithelial bed, 176

bodies, 337

tissues, 89, 129
Epithelioid tissues, 130
Epithelio-muscular tissues, 141
Epithelium, 63, 129, 324

columnar, 131

cuboidal, 131

germinal, 441

glandular, 136

olfactory, 136

sensory, 136, 137

simple, 131

squamous, 131

stratified, 132, 133
Epitrichial layer, 168
Epoophoron, 407, 413, 422, 442
Equilibration, 596
Equilibratory centers, 513
Equilibrium, 563
Erect(ion), 415
Erectile fold, 414

tissue, 412
Erector nerves, 429
Ereptose (erepsin), 326
Erinaceus, 188
Eruption of teeth, 198
Erythrocytes, 159, 160, 160, 348, 367
Erythrocytopoietic organ, 365
Eskimo, 178
Esophagus, 19, 307
Estrin (theeUn), 450
Estrus (oestrus), 450
Ethmoid(al), 225, 227, 228

conchae, 577
Ethmo-turbinals, 576, 577
Euglena, 580
Eunuch, 448
European, 178
Eurypterid, 15, 647, 649
Eustachian tube, 227, 307, 599, 605
Eusthenopteron, 244, 246
Eutheria(n), 119

Evolution, vii, ix, x, xiv, i, 4, 7, 57> 58,
187, 607, 628

of cutaneous appendages, 177

glands, 179

INDEX

705

Evolution, integument, 164
mouth, 295
teeth, 183
Evolutionary, x, 55
change, 80
conservatism, 80
Excretion, 9, 60, 98
Excretory, 9
duct, 408

organ, 10, 12, 331, 399
tubule, 438
vessel, 8
Exoccipital, 228
Exophthalmic goitre, 456
Exoskeleton(al), 14, 22, 23, 201, 656
Extensor muscles, 282, 283, 284
carpi radialis brevis, 282
longus, 282
ulnaris, 282
digiti quinti proprius, 282
digitorum brevis, 282, 284
communis, 282
longus, 282, 283
hallucis longus, 283
indicis proprius, 282
pollicis brevis, 282
longus, 282
Externa, 389
External fertilization, 45
genitals, 414, 415
nares, 342
Exteroceptor, 138

Extra-embr>'onic membranes, 116, 117, 118
Extremity, 241
Extrinsic muscles, 269
Eye(s), 6, 7, 12, 14, 20, 86, 87, 480, 580-
592, 635, 654
beaker, 580
cephalopod, 581
lid, 588

muscles, 264, 266, 284, 488, 611
paired, 13, 86, 480, 654
parietal, 590, 654
pineal, 590
pit, 639
unpaired, 590
vesicular, 580

Facial angle, 225, 246
ganglion, 624

Facial nerve, 483, 484, 489, 523, 609

vein, 395
Falciform ligament, 323, 325
Fallopian tube, 413
Falx cerebri, 562
Family (ies), 7
Family tree, 629, 630
Fasciculate layer, 453
Fasciculus, 515, 517

cuneatus, 517, 520, 549

gracilis, 517, 520, 549

spinocerebellaris dorsalis, 517
"Faserstrang," 632
Fat, 158, 326
Fatty acid, 326

secretion, 180
Fatigue, 563
Feather, 26, 172, 173, 176

down, 172, 173
Feeler, 10
Felis, 3
Felix, 92
Female, 5, 52, 448

sex glands, 449
Femoral artery, 378, 379, 393

vein, 397
Femur, 89, 239, 240, 243, 250, 251, 25:
Fenestra cochleae (rotunda), 598, 602

ovalis (vestibuli), 234, 597, 602
Fertilization, 43, 45, 59, 62, 64

external, 45

internal, 5, 45
Fetal, 25, 54

membranes, 113, 119, 121
Fetus, 382
Fiber(s), 8, 519

ascending, 517

descending, 517

postganglionic, 538

pregangUonic, 538

somatic, 521, 523

visceral, 521

tract, 509, S13, 515
Fibril, 141, 148
Fibrin, 159
Fibrinogen, 159, 348
FibrocartUage, 156
Fibrocytes, 152

Fibrous connective tissue, 102, 514
Fibula, 243, 250, 252
Filamentous, 11, 57
Filiform papillae, 303

7o6

INDEX

Filum terminale, 516, 534

Fimbriae, 423

Fin, 21, 24, 241, 246

paired fins, 236

Ta.y, 21
Fin-fold theory-, 236, 237, 238
Final common path, 520
F"inger, 29, 608

nail, 30

pad, 169

prints, 168
Fingered appendages, 246
Fish(es), 2, 24, 25, 31, 45, 51, 55, 57, 89
Fish-fin skeleton, 243

skull, 609
P'ission, 3, 30
Fissures, 91, 516, 551

cerebral, 500
Fistula(e), 337
Flagellum(a), 3, 5, 138, 400
Flame-cells, 9, 399, 400
Flatfish, 46
Flatworm(s), 2, 4, 7, 8, 349, 409, 630, 632,

634
Flexion, of heart, 370, 371
Flexor brevis digiti minimi, 283, 284
carpi radialis, 282

ulnaris, 282
digitorum brevis, 284
longus, 283
profundus, 282
sublimis, 282
hallucis longus, 283, 284
pollicis brevis, 283
longus, 282
Flexures, 542, 543
cephalic, 542
nuchal, 542
pontine, 542
Flocculi cerebellar, 513
Floor plate, 542
Fluctuating variation, xi
Flying fox, 29
Foliate papillae, 303
Follicle, 7, 37

Graafian, 420, 449
FolKcular cells, 42
hormones, 450
layer, 421, 421
liquid, 449
Fontanelle, 228

Food, I, 9, 46, 51, 53, 54, 55

vacuole, 3
Foot (feet), 11, 12, 28, 30, 246, 250
P"oot-pads, 168
Foramen cecum, 303, 457

costotransverse, 390

epiploicum, 321

interventricular, (f. of Munro), 501

intervertebral, 207, 522

lacerum, 227

magnum, 226

obturator, 240, 241

ovale, 372, 384

Panissae, 356

of Winslow, 321
Foraminifera, 3
Forearm, 247
Forebrain, 542

vesicle, 21, 85, 479
Foregut, 16, 325
Foreskin, 445
Form, 35
Fornix, 547
Fossil(s), 5, 23, 25, 25, 26, 231

men, 224

skulls, 222, 224
Fovea centralis, 586
Fraipont, 639
Freckles, 176

Free extremities, evolution of, 241
"Free" nerve terminations, 139, 566
"Freemartin," 449
Frenulum, 445

Friction-ridge pattern, 168, 169
Frog, 25, 38, 39, 40, 49, SI. 60, 62, 67,

68, 88, 90
Frontal, 218, 225, 226, 228, 232

lobe, 495, 500, 501
Froriep, 623

Fundamental law of biogenesis, xiv
Fundus uteri, 423
Fungiform papillae, 303
Funiculus(i), 515, 516, 517

Galen, 328

Galeus, 615

Gall bladder, 323, 324, 326

stones, 324
Gametes, 407

INDEX

707

Ganglion(a), 522, 620, 623

autonomic, 531, 537

basal (corpus striatum), 501

cervical, 531, 537

ciliary, 489, 537, 540

collateral, 539

dorsal, 638

epibranchial, 624, 625, 633

Gasserian (semilunar), 527, 537

geniculate, 529

habenular, 480, 506

jugular, 484, 540

motor, 515

nerve, 638

nodosum, 530

parapodial, 633

petrosal, 529

sensory, 515, 516

sphenopalatine, 537

spinal, 85, 473, 531, 640

subesophageal, 10, 476, 635, 636, 643,
646, 648

supraesophageal, 10, 476, 635, 636,
643, 646, 648

sympathetic, 25, 489, 531, 535, 536,
537, 538

terminal, 539
Ganoid, 24, 25, 31, 34, 89, 238, 244
Garman, 230
Garpike, 31

Gartner, duct of, 413, 442
Gas detectors, 573
Gaskell, 15, 638, 643, 657
Gaskell's hypothesis, 643-652
Gasserian, ganglion, 483, 484
"Gastraea" theory, 65
Gastric artery, 392

glands, 10, 310

nerve, 484

juice, 310

pits, 310

vein, 396
Gastrocnemius m., 283
Gastropoda, 13
Gastrotheca, 50, 57
Gastrula, 7, 63, 64, 65, 66, 66, 68, 70, 71,

77, 127
Gastrulation, 64, 66, 67, 68, 69, 70, 75, 76,

87, no
Gauguin, ix

Gegenbaur, 230, 234, 237, 605, 609, 610,
614, 624, 633, 640

Gelatin, 152
Gelatinous, 60

substance, 39
Gemelli m., 283
Genetic, vii, i, 57, 85
Geniculate ganglion, 484

nuclei, 507
Genioglossus m., 275
Geniohyoid m., 275
Genital(s), 444, 445

artery, 352

external, 425, 425, 444, 445

folds (ridges), 440, 444

internal, 445

pore, 7, 44

ridges (folds), 100, loi, 412, 440, 444

tubercle (prominence), 444

vein, 352
Genus (genera), i, 3
Geologic time, 34
Geological, ix, x
Gephyreans, 365
Germ-cells, xi, 11, 100, 409, 449

disc, 41, 42, 62, 62

layer theory, 651

layers, 71, 80, 185

ring, 64, 66, 67, 68, 75
Germarium, 7
Germinal bodies, 36

cells, 551

center, 314, 315

epithelium, 441

protoplasm, 80
Gestation, 54
Giant fibers, 477, 481

ganglion (nerve) cells, 17, 477, 479,
513, 5i5> 556, 556
Gigantism, 463

Gill(s), II, 12, 14, 24, 25, 31, 51, 57, 89,
230, 246, 328, 329, 336, 632, 660

apertures, 24, 229

arch, 229, 230, 237, 244, 246

basket, 229

clefts, 82, 83, 84, 329

external, 329, 331

filaments, 330

internal, 89, 329

lamellae, 331, 332

pharyngeal, 329

pouch, 7, 335, 611, 614

ray, 330

region, 17, 21

7o8

INDEX

Gill(s), slit, i8, 19, 21, 23, 329, 331, 332,

333, 335, 609, 663
Gingiva, 195
Giraffe, 28
Girdle, 238

pectoral, 238

pelvic, 238
Gladiolus (mesosternum), 214
Gland (s), 139

acinous, 140, 303

adrenal (suprarenal), 451-455

alveolar, 140, 230, 232, 303

anal, 180

Bartholin's, 426

Brunner's, 313

buccal, 303

bulbo-urethral, 426, 428

cardiac, 310

caudal, 8

cement, 10

compound, 140, 303

Cowper's glands, 428

cytogenic, 139

digestive, 303

ductless, 303, 446

endocrine, 90, 303, 329. 446-466

excretory, 139

fundus (gastric), 310

gastric (fundus), 310

greater vestibular (Bartholin's), 426

intermaxillar}', 302

intestinal, 314, 318

labial, 301

lacrimal, 180

Lieberkiihn's, 313, 315

lingual, 303, 305

lymph, 363

mammary, 180, 181

Meibomian, 180

molar, 301

mucous, 165, 302

multicellular, 139

oral, 303

palatine, 303

parathyroid, 458

para-urethral, 445

parotid, 301, 303

pineal, 547

pituitary, 462

preputial, 180

prostate, 428

pyloric, 310

Gland (s), rectal, 319
salivary, 301
sebaceous, 176, 180
secretory, 139
serous, 302
sex, 407-412, 420, 421, 426, 427, 448-

451, 464

simple, 303

sublingual, 302

submaxillary, 301

subneural, 18

suprarenal (adrenal), 451-455

sweat, 166, 180

tarsal, 180

thymus, 460

thyroid, 455

tubular, 140, 303

tubulo-acinous, 302

unicellular, 139

vaginal, 8, 180
Glans, 412, 414, 444

cUtoridis, 415, 444

penis, 415, 427, 429, 444
Glass-fish (Leptocephalus), 48
Glenoid fossa, 247
Globus pallidus, 504
Glomerular layer of adrenal, 452, 453

glomerulus(i), 98, 404, 417, 438, 523
inner, 405
Glossopalatine arch, 306
Glossopharyngeus nerve, 483, 484, 523,

609
Glottis, 305, 340
Gluteal muscle, 393

vein, 397
Gluteus, 251
Glycerine, 326
Glycogen, 447
Glyptemys insculpta, 62
Gnathostomes, 305, 332
Goat, 28
Goblet-cells, 313
Goethe, 217, 607
Goette, 343, 344, 634
Goiter, 456
Goldschneider, 566
Golgi, 517

method, 324, 555
Golgi-Mazzoni, corpuscles of, 566, 568
Gonad(s), 8, 9, 10, 11, 12, 12, 19, 21, 35,
36, 90, 99, 100, 411, 412, 443,
448, 631, 660

INDEX

709

Gonad(s), descensus of, 443, 443

primary, 412
Gonadic ducts, 9

ridge, 439

sacs, 17, 21, 408, 409, 632, 634, 658,

659
Gonotome, 409
Goodrich, viii, 243, 616, 617
Gopher, 28
Goppert, 377
Gorilla, 33, 191, 245
"Graafian follicle, 420, 421, 449
Gracilis m., 283
Grandry's corpuscles, 568
Grantia, 5
Granule(ar), 59
Granulocyte, 365
Grasshopper, 16
Gravitation, 594
Gray commissure, 516

matter, 514, 515, 516

rami, 539
Gregory, W. K., 219
Grobben, 4, 634
Growth, S3

zone, 64
Gubernaculum testis, 427, 443
Guinea pig, 28
Gullet (esophagus), 307, 308
Gums, 195
Guppy (Lebistes), 45
Gymnophiona, 25, 45, 50
Gynandromorph, 412
Gyrus(i), 499, 500

angular, 501

anterior central, 501

cinguli, 501

dentatus, 491

fusiform, 501

hippocampal, 501

inferior, 500, 501
parietal, 501

insula, 501

lateral, 501

lingual, 501

long, 501

middle, 500, 501

orbital, 500

posterior central, 501

rectus, 500

short, 501

submarginal, 501

Gyrus(i), superior, 500, 501
parietal, 501
uncus, 501

H

Habenular commissure, 480, 490, 506, 508

ganglia, 480, 506, 507, 508, 590
Haddock, 46
Haeckel, Ernst, 65, 124
Hagfish, 23, 31, 36, 336
Hair(s), 26, 28, 173, 174, 174, 176

arrangement, 175

bulb, 174

cells, 600, 602

column, 176

contour, 174

cortex, 174

development, 175, 176

direction, 174

follicle, 174

histogenesis, 175

root, 174

shaft, 175

sheath, 176
Hake skull of, 221
Haldane, J. S., 339
Hamate, 249

"Hamstring" muscles, 283
Hand, 30, 246, 247
Hapalidae, 33
Haploid, 43
Hare lip, 344, 577
Harrimania, 660, 661
Harrison, 555, 557
Harvest men, 15
Harvey, 347

Hassall's corpuscles, 461, 461
Hatching, 53, 56

Hatschek, 60, 81, 83, 84, 334, 639
Hatschek's pit, 574
Haustra, 315
Haversian canal, 157

system, 157
Head, 7, 31, 36, 107

cavities, 6ll

kidney, 430

problem, 607-627

segments, 607-610, 613, 614, 623, 624,
626
Hearing, 479, 563, 596

organ of, 596

yio

INDEX

Heart, 9, 12, 18, 23, 24, 25, 26, 52, 85,
106, 107, 246, 347, 384, 387, 388,
631, 650, 654
development, 368-370
evolution, 358
muscle, 144
"Heat," 450, 563
Hedgehog, 27, 78
Heidelberg man, 32, 233
Heider, 19
Helix, 599, 606
Hemal arch, 204, 206, 244
Hemapophysis, 206
Hemiazygos v., 356, 357, 361, 382, 396
Hemibranch, 330
Hemichordata, 2, 4, 9, 16, 21, 30, 31, 333,

630, 643, 660
Hemisphere animal, 61
cerebral, 495
vegetal, 61
Hemoblast(ic), 366

cell, 367
Hemoglobin, 160, 348, 352
Hemopoietic organ, 367
tissue, 363
evolution, 363
Hemorrhoidal artery, 393

vein, 397
Henderson, L. J., x, xiii
Henle's loop, 418
Hensen theory, 552
Hepatic, 90

artery, 323, 392

duct, 323

portal system, 378, 379

vein, 351, 352, 354, 356, 382, 396
vein, 352, 354, 356, 382, 396
Heptanchus, 24, 229, 230, 336
Herder, Karoline, 607
Hermaphrodite (ditic), 12, 18, 23, 31, 35,

36, 412
Hernia, 52
Herrick, viii
Herring, 36, 46
Heterodont deutition, 186
Heterogamy, 407
Hexanchus, 336
Highmore's antrum, 232
Hill, 620, 652
Hilum (hilus), 416
Hindbrain, 85, 542
Hind-foot, 29

Hind-gut, 317
Hipbone, 241, 250

girdle, 240
Hippocampal lobe, 493
Hippocampus, 491, 493» 495, 498, S02,

508, 546
Hippopotamus, 28
His, W., 130
Histogenesis of hair, 175

of nerves, 552, 558

of ovary, 440

of spinal cord, 550
Histogenesis of testis, 440
Histological specificity, 162
Histology, 126, 127
Holobranch, 330
Holocephali, 47
Hominidae, 3
Homo Heidelbergensis, 32, 628

Neanderthalensis, 32, 628

Rhodesiensis, 32, 628

Sapiens, 3, 32, 628
Homodont dentition, 186
Homology, 91, 620
Hoof(s), 28, 171, 172, 172
Hooton, 33, 233
Hormone, 54, 177, 446, 450

follicular, 450

luteal, 450

pituitary, 450
Horn(s), 39, 171, 232
Horny, 5, 135
Horny-scaled, 25
Horny scales, 170
Horse, 28
Horse-tails, 34
Hoskins, 464
Howell, A. Brazier, 238
Howell, W. H., 506, 507
Hubrecht, 657, 657, 658
Humeral artery, anterior circumflex, 392

posterior circumflex, 392
Humerus, 89, 243, 244, 249
Humming-bird, 40, 43
Humor, 584

aqueous, 584

vitreous, 584
Hunger, 563

Huntington and McClure, 240, 357, 382
Huntsman, 638

Huxley, Thomas, ix, 217, 609, 614
Hyaline cartilage, 155, 156

INDEX

711

Hydatid of Morgagni, 422, 427

stalked, 445
Hydra, 5, 65, 66
Hydrocarbons, xiii
Hydrochloric acid, 311, 447
Hydrostatic organ, 507
Hylidae, 49
Hylobates, 33
Hymen, 425, 442
Hyoglossus m., 275
Hyoid, 228, 229, 230, 232, 236

arch, 230, 232, 246, 330, 606

nerve, 484

septum, 336
Hyomandibular cartilage, 229, 230, 246

nerve, 483, 484
Hypaxial m., 287

lateral, 287

prevertebral, 287
Hypertrophy, 306, 336
Hypobranchial m., 264, 288, 485, 489
Hypochorda, 457
Hypocone, 188
Hypoconid, 188
Hypogastric artery, 391, 393

plexus, 561

vein, 397
Hypoglossal m., 288
Hypoglossus nerve, 485, 489, 493, 514, 523,

626
Hypomere, 94, 96, 99, 290
Hypomeric mesoderm, 332
Hypoparathyroidism, 458
Hypophysial duct, 465, 466

recess, 466
Hypophysis, 20, 227, 465, 466, 507, 617,

650, 658
H>'postyle, 663

Hypothalamus, 505, 507, 508, 547
Hypothenar, 169
Hypothesis, x
Hyrax, 28, 32, 189

Ichthyopsida, 25, 31

Ichthyopterygium, 241, 243, 246

Ideals, xii

Ideas, xii

Ihle, viii, 343

Ileocolic valve, 212

Ileum, 313

Iliac artery, 353, 354, 356, 377, 393

fossa, 315

vein, 353, 354, 356, 357, 382, 386, 396
Iliocostalis m., 277
Iliolumbar artery, 393

vein, 397
Iliopsoas m., 283
Ilium, 238, 239, 240, 250
Imbricate(d), 170
Implantation, 423, 449
Impregnation, 45
Impulse, nervous, 467
Incisive canal, 232
Incisor, 28, 186, 192
Incisure, 207
Incubate, 53

Incus, 227, 232, 233, 235, 246, 598
Indian, 178
Indifferent cells, 440
Indigenous, 27
Inelastic fibers, 152
Inertia, 80

Infraspinatus m., 247
Infraspinous fossa, 247
Infundibular organ, 478
Infundibulum, 507, 508, 547

cerebri, 465, 480

tubae, 422
Infusoria, 4
Inguinal canal, 426, 428, 443, 443

hernia, 443
Inheritance, x, 44, 70
Inhibitor nerves, 536
Innominate artery, 390

vein, 358, 382, 395
Inorganic, i

Inscriptiones tendineae, 277
Insect(s)(a), i, 2, 15, 30, 34, 630
Insectivora, 27, 31, 33, 34, 168, 169
Insertion, 259
Insula, 501
Insulation, 471
Insulin, 326, 327, 447
Integration, 446

chemical, 446

nervous, 446
Integrative function, 467
Integument, 164
Integumental muscles, 272
Integumentary, 55

system, 164
Interauricular septa, 384

712

INDEX

Interbranchial septa, 330
Intercalated discs, 145

ducts, 301, 327
Intercentra, 205
Intercondylar fossa, 251
Intercostal arteries, 392, 625

muscles, external, 280, 287
internal, 280

vein, 396
Interference color, 178
Intergrade, sex, 412
Interlobar arteries, 418

veins, 418
Interlobular artery, 418

ducts, 323

veins, 323, 418
Intermandibularis m., 269, 270
Intermaxillary gland, 302
Intermediate cell mass, 430, 433

cords, 440

lobe, 465, 465
Intermyotomic, 237
Internal fertilization, 45, 46
Interoceptors, 145
Interosseous artery, 377

muscle, 283, 284
Interparietal, 228
Interpeduncular nucleus (ganglion), 480,

511
Interpleural space (mediastinum), 337
Interradial, 10
Interrenal body, 454
Intersegmental artery, 377
Inter-spinales m., 277
Interstitial cells, 37, 440, 448

tissue, 426, 427
Intersubcardinal anastomosis, 357
Intertransversarii m., 277
Intertrochanteric crest, 251

line, 251
Interventricular foramen, 501

septum, 372
Intervertebral discs, 209
Intestinal artery, 392

diverticula, 333

glands, 12, 313

plexus, 489

portal, 317, 325
Intestine, 7, 8, 9, n, 21, 90, 312, 316, 318,
322, 326

large, 314

small, 312

Intima, 389

Intracellular circulation, 348

organs, 126
Intralobular veins, 325
Intrauterine, 54, 57
Intromittent organ, 412, 414
Invagination, 63, 64, 66, 67, 68, 70, 90

canal, 596
Invertebrate, 10
Involuntary muscles, 142
Iodine, 456
Iris, 585

Irritability, 467, 563
Ischiadic artery, 378
Ischio-cavernosus m., 429

-pubis, 238
Ischium, 239, 240, 250
Islands of Langerhans (pancreatic), 327,

447
Isogamy, 407
Isotropic (bands), 144
Isthmus faucium, 306

uteri, 424
Italians, 179
Ivory, 193

Jacobson, organ of, 575, 576

Jaeckel, 239

Jamaican tree frog, 49

Jammes, 388

Java man, 32, 225, 495, 628

Jaw, 23, 26, 31, 231, 232, 608

cartilage, 230
Jejunum, 313
Jellyfish, 5, 6, 7
Jennings, xiv
Johnson, Fred, 29
Joints, development of, 257
Jordan & Ferguson, 502
Joseph, cells of, 478
Jumbo, 191
Jugular ganglion, 484, 540

vein, 352, 353, 382, 386, 395
Jung, II
Jurassic, 34, 187

Kahn, 251, 505, 601

Kangaroo, 27, 54, 55

INDEX

713

Kappers, 475, 540, 551

Keel(ed), 26, 29

Keibel and Mall, 291

Keith, 353

Kellicott, viii

Keratin, 135

Kerr, J. Graham, 553

Kidney, 53, 85, 90, 94, 98, 102, 246, 403,

415, 416, 416, 447, 451
"head" (pronephros), 430
tubules, 96, 98
Kingsley, viii, 87, 91, 92, 94, 95, 96, 97, 98,

loi, 229, 230
Kirkaldy, 21
Koelliker's pit, 573
Koltzoff, 613, 615, 621
Korscheldt (and Heider), 19
Kowalewsky, 297, 636, 661
Krause, corpuscles of, 566, 568
Kupffer, 297, 553, 555, 624

Labia majora, 415, 425, 445

minora, 415, 425, 445
Labial cartilage, 230, 609

gland, 301

swelling, 444
Labyrinth, 604

bony, 599

membranous, 596, 599, 604
Labyrinthodonts, 186
Lacerta muralis, 96
Lacertilia, 26
Lacrimal bone, 225, 227

duct, 575

gland, 588
Lacteal, 314

Lacuna(ae), 78, 105, 154, 157
Lacunar blood system, 10, 17, 349, 350

spaces, 7, 12, 14, 19, 350
Lagena, 596
Lamarck, xiv, 219
Lamarckian, x
Lamella(e'), 156, 251, 330, 332

branchial, 330
Lamellated olfactory epithelium, 574
Lamellibranch, 13
Lamina, 195

labial, 195

lingual, 195

terminalis, 490, 544

Lamprey, 23, 229

"Lamp-shells," 9

Lancelet, 20

Langerhans, islands of, 327

Lankester, Ray, 582, 639, 662

Lantern of Aristotle, 183

Lanugo, 176

Larva(ae), xiv, 5, 10, 11, 18, 19, 40, 50,

55, 56, 57
Larvacea, 20
Larval, period, 23, 48, 50, 51, 55, 84

stage, 51
Laryngeal cartilage, 228
Laryngo-tracheal groove, 342
Larynx, 230, 233, 340, 341
Latebra, 40
Lateral, 9

line centers,

nerves, 484, 485

organs, 513, 523, 568-571, 633

nerves, 8

trunk muscles, 262, 286

umbilical ligament, 384, 419
Lateralis components, 484

fibers, 523

nerve, 483, 485, 490
Latissimus dorsi muscle, 281
Lavoisier, 328
Leech, 10
Leg, 10, 246, 250
Lemmatochord, 656
Lemur, 28, 29

Lemuroid(s) (ea) , 29, 32, 33, 34
Lens, 86, 584, 585, 589, 589
Lenticular, 546
Lepidosiren, ix

Lepidosteus (garpike), 31, 204
Leucocyte(s), 159, 162, 348, 367, 368

mononuclear, 364
Levator, 332

ani m., 284

palpebrae superioris m., 274

scapulae m., 275
Leydig, 635

cells, 448
Lieberkuehn, glands of, 313, 315
Life, X, xii, xiii

history, 8, 35
Ligament(s), broad, 421, 424

falciform, 323, 325

lateral umbilical, 419

middle umbilical, 439

714

INDEX

Ligament(s), round, 424

suspensory, 421, 585, 585
Ligamentum arteriosum, 384, 389
ovarii, 421, 424
teres, 384
testis, 445
venosum, 384
Limb(less), 25, 29
lower, 250

development of, 256
upper, 247

development of, 255
Lime, 164
Limestone, 3

Limulus, 15, 643, 646, 648, 649
Linea alba, 277
aspera, 251
Lingual artery, 390
gland, 301, 305
papilla, 303, 304
tonsil, 306
vein, 395
Lingualis m., 275
Linnaeus (Linne), i, 3
Lion (Leo), 3

Lipase (steapsin), 311, 322, 326
Lipoid, 451
Lips, 301
Liver, 9, 17, 21, 25, 90, 99, ^22-326, 382

fluke, 7
"Living fossil," 26
Lizard, 52, 57, 96
Llama, 28
Lobe caudate, 323
frontal, 500, 501
occipital, 500

olfactory, 480, 489, 495, 508, 546
parietal, 500, 501
preoral, 639, 640
temporal, 500, 501
Lobe-fins, 25
Lobster, 14
Lobule, 323, 325
Locomotion, 12, 244
Locomotor organs, 246
Locy, 620, 652
Loligo, 581

Longissimus m., 276, 277
Longitudinal, 8, 11

fibers, 516
Longus capitis m., 277
cervicis m., 277

Lophophore, 30
Lorenzini, ampullae of, 598
Lorises, 33
Luciferin, 181
Lumbar, 516

artery, 377, 392

enlargement, 490

plexus, 533

vein, 396

vertebra, 377
Lumbo-costalis m., 271
Lumbrical m., 283, 284
Lumbricus, 11

Lumen, 82, 98, 308, 325, 384, 489, 514
Luminous organs, 180
Lunate, 249, 250
Lung, 12, 25, 31, 89, 246, 328, 329, 337:

345, 354, 387

development of, 342

origin of, 344

phylogenesis of, 345

sacs, 384
Lung-fishes, 25, 34, 354
Lunula, 171
Lust, 563

Luteal hormone, 450
Lymph, 162, 347, 397, 398

capillaries, 397, 398

glands, 363, 363, 386

heart, 363

node, 384, 386, 398

nodule, 398

sac, jugular, 386

sinus, 398
Lymphatic(s), 314, 347. 35°, 362, 384, 385,

397
system, 362, 384, 397

evolution of, 362
trunks, 353, 386
vessel, 385, 386
Lymphocytes, 153, 162, 364, 365, 398
Lymphoid (adenoid) tissue, 363
Lynx, 190

M

MacBride, viii, 660
Mackerel, 36, 46
Macropus major, 54
Macula(ae), 137, 596, 600, 604
Macula lutea, 586
Madagascar, 40

INDEX

715

Major senses, 563
Malar bone, 227
Malaria, 302
Male, 448, 449

characteristics, 449
Malleolus, 248

Malleus, 227, 232, 233, 235, 246, 598
Malpighi, 347, 350
Malpighian corpuscle, 98, 98
Maltase, 301
Mammal(ia), 2, 3, 26, 27, 27, 31, 34. 4i>

53> 54, 56, 57, 58, 79, 80, 493
Mammary, 26, 55

artery, 390

fetus, 54

glands, 29, 53, 54, 55, 181, i8i

pocket, 182

vein, 396
Mammillary bodies, 508, 547
Man, I, 29, 169
Manatee, 28

Mandible, 225, 227, 232, 234
Mandibular, 229, 231

arch, 229, 606

cavity, 618

fossa, 232
Mandibularis(V) nerve, 483
Manis, 171

Mantle, 11, 13, 18, 18, 30, 498
Mantle-cavity, 11, 12, 13, 30

layer, 481, 545, 55°
Manubrium (presternum), 214, 248
Marginal layer, 481, 545, 550
Marmoset, 29, 30
Marrow, 157

red, 157, 367

yellow, 157
Marshall, 61, 88, 611, 613, 614, 633
Marsupial(s), 26, 27, 31, 34, 51, 54, 55, 57

pouch, 27, 50, 51
"Marsupial" tree-frog, 50, 57
Marsupium, 50, 53, 54, 57
Masseter m., 275
Mastication, 232, 301
Mastodon, 28
Mastoid, 227, 228
Materialism, xiv
Maternal, food, 53
Matrix, 151, 1 55
Matter, xii
Maturation, 43
Maturity, 450

Mauer, 33
Maurer, 331
Maxilla, 227, 232, 234
Maxillaris nerve, 483
Maxillary artery, 390

bone, 225, 230

conchae, 577

sinus, 232, 386
Maxillo-turbinals, 576
McClure, CFW, 357
McJMurrich, 378
Meatus, 598, 605

Mechanical conception of evolution, xi
"Mechanical mythology," xiv

support, 150
Mechanism, 43

Meckels' cartilage, 229, 234, 235
Media, 389
Median artery, 377
Mediastinum (interpleural space), 337
Mediterranean, 33
Medulla, 513, 619, 621

oblongata, 480, 513, 517, 549

renal, 418
Medullary cords, 386, 441

plate, 73

sheath, 149, 471, 471, 489, 515, 517,

557

tube, 637
MeduUated nerves, 149
Meibomian (tarsal) glands, 588
Meissner, corpuscles of, 567
Melanin, 153
Melanophores, 153
Membrana tectoria, 600, 602, 604
Membrane bones, 105, 170, 202, 218, 220,

221, 231, 610
Membranous labyrinth, 596, 604

sac, 602
Meninge(s), 561
Meninx primitiva, 561
Menopause, 450

Menstruation, 425, 424, 450, 451
Mental foramen, 232
Mentum, 232

Merkel, tactile cells of, 567
Merostomata (xiphosura), 15
Mesencephalon, 479, 510, 542, 544, 548,

548
Mesenchyma(atous), 327, 332, 385, 617
Mesenchyme, 102, 151, 325

7t6

INDEX

Mesenteric a., 352, 354, 356

inferior (posterior), 377, 393
superior (anterior), 322, 355, 356,

377, 382, 392
veins, 352, 354, 356, 382, 396
Mesentery, 93, 99, 100, loi, 312, 319, 320,

321,321, 322, 323
Mesethmoid, 227
Mesocardium(a), 370
Mesocolon, 321, 322

Mesoderm, 5, 8, 71, 72, 75, 75, 76, 78, 83,
84, 86, 91, 93, 94
splanchnic, 369
Mesodermal bands, 632
cavity, 611, 615
folds, 74, 369
pouch, 72, 73, 74, 75, 81, 81, 82, 83,

84
segmentation, 611, 612, 614
somites, 74, 82, 91, 92, 96, 336, 614,
616
Mesogaster, 321
Mesogloea, 5, 7
Mesomere, 94, 95, 96, 98, 620
Mesomerism, 336, 622, 625
Mesonephric (Wolffian) duct, 98, loi, 411,
413, 432, 432, 442, 442
tubules, 98, loi, 404, 432, 432, 433,

433, 434
Mesonephroi, 354

Mesonephros, 96, 97, 98, 100, 404, 405,
411, 412, 431, 432, 432, 439, 442
development of, 432-434
Mesopterygium, 241
Mesorchium, loi, 440
Meso rectum, 321
Mesothelium, 130
Mesovarium, loi, 420, 421, 440
Mesozoic, 34
Metabolism, 53, 58, 63, 399

basal, 456
Metabolic rate, 58, 59
Metacarpal v., 396
Metacarpus, 250
Metacone, 187
Metaconid, 188

Metameres, 8, 14, 620, 626, 652, 653
Metameric, 10, 30, 31, 607

muscles, 262

structures, 634
Metamerism, 20, 21, 22, 336, 405, 607, 609,
614, 620, 622, 627, 631, 632, 655

Metamorphosis, 10, 11, 12, 18, 19, 20, 23,

48, 50, 51, 55, 56, 412
Metanephridia, 400, 401
Metanephros(i), 96, 98, 404, 406, 434,

442
Metaotic, 614, 627

muscles, 266
Metapleura(l), 21

folds, 20, 236
Metapterygium, 241
Metasternum (xiphoid process), 214
Metatarsus, 250, 252
Metatheria (marsupials), 27
Metazoa, 4, 30, 65, 66
Metencephalon, 479, 511, 513, 542, 544,

549
Midbrain, 85, 542
"Midget," 463
Milk, 53, 54
field, 182
line, 181

-secreting organs, 181
Mind, ix, xii, i
Minor senses, 563
Minot, 297, 635, 636
Mitral (bicuspid) valve, 373, 389
Moa, 40, 43
Modifications, x, xi
Molar (s), 186, 192
gland, 301
teeth, 186, 192
Mole (nevus), 176, 177
Moles, 27

Mollusca, 2, 4, II, 12, 14, 30
Molluscoida, 2, 4, 9, 30

Mongolian, 32

Mongoloid, 33

Monhystera, 8

Monkey, ix, x, 29, 30, 169

Monocyte, 365

Monodelphia, 27

Monoecious, 36

Mononuclear leucocyte, 364

Monophyletic theory, 367

Monophyodont dentition, 187

Monopyramidal kidney, 416

Monorhine, 31

Monosaccharid, 326

Monotreme, 25, 31

Mons pubis, 425

Morgagni, hydatid of, 422, 427

Morgan, 60, 61, 637

INDEX

717

Morphological, vii
Morphologist, xv, 8
Morphology, xiv, xv
Morris, viii, 504
Morula, 77
Mosquitoes, 302
Moth(s), 16
Mother, 26

of pearl, 13
Motor cell (neuron), 148

fibers, 622

nerve, 516

plate, 105, 105

reaction, 65
Mouse (mice), 38

egg, 37, 42
Mouth, 4, 6, 7, 9, 10, II, 12, 21, 30, 55, 65,
82, 84, 87, 232, 294, 335, 634,
63s, 652

cavity, 25, 88, 294
Mucosa, II

Mucous glands, 303, 310
Mucus, 313
Mudfish, ix

Mueller, Johannes, 609
Mueller's (Muellerian) ducts, loi, 102,
411, 413, 441, 442

fibers, 513, 515
Multangular, greater, 249

lesser, 249
Multicellular, 4, 5, 9, 30
Multifidus m., 277
Multipyramidal kidney, 416
Multitubercular, 187, 188
Mumps, 301
Muscle, 6, 8, 11, 65, 106, 237

branchiomeric, 269, 291

bud, 237, 288, 289

bundles, 7

cell, 141, 142

fibers, 19, 142

insertion, 259

layer, 63

origin, 359

smooth, 142

spindle, 57i, 57i. 594

striated, 142, 143

tissue, 83, 84
Muscles, abdominal, 277

abductor, digiti minimi, 283, 284
hallucis, 284

Muscles, abductor, pollicis brevis, 283
longus, 282
adductor brevis, 283

hallucis, 284

longus, 283

magnus, 283

pollicis, 283
anal, 8

anconeus, 281
appendicular, 288
arrectores pilorum, 176
auricularis, 274
biceps, 249

brachii, 281

femoris, 283
brachialis, 281
brachioradialis, 283
branchiomeric, 269, 291
buccinator, 274
bulbo-cavernosus, 284, 425
caninus, 274
cardiac, 145, 387
caudal, 226, 272
circular, 11
coccygeus, 284
coraco-brachialis, 281
corrugator, 274
cremaster, 426, 443

externus, 443

internus, 443
cricothyroid, 341
deltoideus, 281
development of, 284-291
diaphragm, 280, 280
digastricus, 275
ear, 274
epaxial, 287
evolution of, 261, 633
extensor carpi radialis brevis, 282
longus, 282
ulnaris, 282

digitorum brevis, 282, 284
communis, 282
longus, 282, 283

digiti quinti proprius, 282

indicis proprius, 282

hallucis longus, 283

pollicis brevis, 282
longus, 282
extrinsic, 269

7i8

INDEX

Muscles, flexor brevis digiti minimi, 283,
284
carpi radialis, 282

ulnaris, 282
digitorum brevis, 284
longus, 283
profundus, 282
sublimis, 282
hallucis longus, 283, 284
pollicis brevis, 283
longus, 282
frontalis, 274
gastrocnemius, 283
gemelli, 283
geniohyoid, 275
genioglossus, 275
gluteus maximus, 283
medius, 283
minimus, 283
gracilis, 283
"hamstring," 283
hyoglossus, 275
hypaxial, 287
lateral, 287
prevertebral, 287
hypobranchial, 264, 288, 305, 489
hypoglossal, 288
ilio-costalis, 277

psoas, 283
infraspinatus, 281
integumental, 272
intercostal, external, 280, 287

internal, 280
intermandibularis, 269, 270
interosseus, 283, 284

ventral interossei, 284
inter-spinales, 277
intertransversarii, 277
intrinsic, 269

ischio-cavernosus, 284, 429
laryngeal, 291, 343
lateralis, 633
lateral trunk, 286
latissimus dorsi, 281
levator ani, 284

palpebrae superioris, 274
scapulae, 275
lingualis, 275
longissimus, 276, 277
longitudinal, 11
longus capitis, 277
cervicis, 277

Muscles, lumbo-costalis, 271
lumbricales, 283, 284
masseter, 275
mimetic, 273
multifidus, 277
mylohyoid, 275
nasalis, 274

non-striated (smooth), 142
obliquus capitis, 276
externus, 277
inferior, 264
internus, 277
superior, 614
obturator externus, 283

internus, 283
occipitalis, 274
omohyoid, 277
opponens digiti minimi, 283, 284

pollicis, 283
orbicularis oculi, 274

oris, 274
palmaris brevis, 283

longus, 282
panniculus carnosus, 272
papillary, 373, 389
pectineus, 283
pectoralis major, 281

minor, 281
pelvic, 284
peroneus brevis, 283
longus, 283
tertius, 283
pharyngeal, 275
piriformis, 283
plantaris, 283
platysma, 275
popliteus, 283
prevertebral, 277
pronator quadratus, 283

teres, 282
psoas major, 277

minor, 277
pterygoid, 275
pyramidalis, 278
quadratus femoris, 283
lumborum, 277
plantae, 284
quadriceps femoris, 251, 283
radio-dorsal, 282
rectus abdominis, 277, 287
capitis, 276
externus (posterior), 286, 611

INDEX

719

Muscles, rectus femoris, 283

inferior, 264

internus, 264

superior, 264
rhomboideus, major, 281

minor, 281
risorius, 274
rotators, 276
sacro-coccygeal, 277

spinalis, 277
sartorius, 283
scaleni, anterior, 277

middle, 277

posterior, 277
semimembranosus, 283
semispinalis, 276
semitendinosus, 283
serratus anterior, 281

posterior, 280
skeletal, 261, 523
smooth, 142, 290
soleus, 283
somatic, 284
sphincter ani externus, 284, 316

colli, 301

urethrae, 284, 419
spinalis, 277
splenius, 276
stapedius, 598
sterno-cleido-mastoid, 227
sternohyoid, 248, 277
sternomastoid, 248
sternothyroid, 277
striated (striped), 142
styloglossus, 275
stylohyoid, 232, 236, 275
stylopharyngeus, 307
subclavius, 281
subscapularis, 247, 281
supinator, 282
supraspinatus, 247, 281
temporalis, 226, 295
tensor fasciae latae, 283

tympani, 598

veli palatini, 291
teres major, 281

minor, 281
thoracic, 280, 281, 281
thyro-arytenoid, 341
thyro-hyoid, 277
tibialis anterior, 283
posterior, 283

Muscles, tongue, 288

transverso-spinalis, 277, 287

transversus, 277
perinei, 284
thoracis, 280

trapezius, 247, 248, 275, 281

triangularis, 274

triceps, 249, 281

ulno-volar m., 282

vastus intermedius, 283
laterahs, 283
medialis, 283

ventral, 8

visceral, 261, 290

vocalis, 341

zygomaticus, 274
Muscular, sense, 571

system, 259-291

tissue, 141-145
Muscularis mucosae, 307, 310
Mutability, ix
Mutation, xi

Myelencephalon, 479, 513, 542, 544, 549
Myelin, 522

sheath, 149, 471, 514, 557
Myelocytes, 369
Myenteric plexus, 539, 561
Mylohyoid m., 275
Myocardium, 107, 369, 387, 388
Myocoele, 83, 93, 94
Myocomma(ta), 22, 83, 262
Myofibril, 141
Myomere, 21
Myotomes, 21, 82, 83, 84, 85, 94, 95, 209,

262, 336, 613, 615, 618
Myotomic buds, 288, 289
Myriapod, 15, 16, 30
Myxine, 23, 31, 36, 336
Myxinoids, 296, 540

N

Nail(s), 29, 171

Nail wall, 172

Nares external, 342

Narial passages, 25, 227, 344, 577

Narwhal, 190

Nasal aperture, 23

bone, 227

passages, 328, 342, 343

pits, 575

scute, 218

720

INDEX

Nasal septum, 30, 225, 227

Nasalis m., 274

Nasobuccal grooves, 343. 574. 577? 577

Nasohypophysial invagination, 636

Nasolacrimal duct, 227, 588

groove, 574, 577

passage, 576
Nasopharyngeal cavity, 575
Natural selection, ix, x
Nautilus, 14
Navicular, 249, 250, 252
Neanderthal, 32, 33, 224, 225, 628
Neck, 107, 193
Necrobiotic, 180

Necturus (mud puppy), 25, 39, 51
Negrito, 33

Negro(id), 32, 33, 179, 192
Nemathelminths (thread worms), 4, 8, 9,

Nematode (round worm), 8, 9, 628
Nemerteans, 7, 8, 349, 628

hypothesis, 657-659
Neocortex (neopallium), 498, 510, 544
Neopallium (neocortex), 491, 493, 498, 544
Neostoma, 297, 298
Neostriatum, 491, 503, 504
Nephridiopores, 11, 400
Nephridial, 10
Nephridium(a), 9, 10, 11, 12, 21, 351, 400,

409, 631, 632
Nephrogenic cord, 433, 434

tissue (cord), 406, 435, 435, 437, 438
Nephrostome, 11, 21, 96, 98, 400, 401,

403, 404, 431
Nephrotome, 405, 430
Nerve(s), 21, 99, 147, 148, i49> ^-5°, 47i.
634, 654

cell, 146

chain, 9

components, 147, 150, 522-524

cord, 7, 8, II, 18, 475, 476, 476, 491,
652

cranial, 482

development, 552-560

fiber, 85, 146, 413, 516

histogenesis, 552, 558

net, 470, 471

nucleus, 527, 529, 530

ring, 8, 10

root, 514

terminations, 565, 566, 567

tract, 517

Nerves, abducens, 482, 513, 523, 529, 622,625
accelerator cordis, 537
accessory, 482, 530, 626
acoustic (auditory), 482, 523, 529
afferent, 520

auditory (acoustic), 482, 483, 603
autonomic, 536, 537
bronchodilator, 537
buccaUs, 483, 484
cerebrospinal, 522, 537
cervical, 537

chorda tympani, 529, 537 '
coccygeal, 537
cochlear, 523, 529, 602, 604
collector, 534, 633
cranial, 482, 488, 489, 490> 522, 524,

609, 621, 623, 625, 654
dorsal, 8, 478
erector, 429, 537
erigens, 537

facialis, 482, 489, 523, 529, 609
gastric, 484
glossopharyngeal, 482, 484, 485, 523,

529, 609
hyoid, 484
hyomandibular, 484

hypogastric, 537

hypoglossal, 482, 485, 489, 493, 514,
523, 53°, 626

inhibitor cordis, 537

lacrimal, 537

lateral, 8, 480, 482, 483, 484, 485, 49°

lingual, 537

lumbar, 537

masticator, 527

maxillaris, 483, 484, 537

motor, 516

nasal, posterior, 537

nasociliary, 537

oculomotor, 482, 489, 523, 527, 537,
611, 625

olfactory, 482, 526

ophthalmic, 482, 484, 489, 537, 609

optic, 482, 510, 526, 636

palatine, 484

parasympathetic, 536

parietal, 592

pelvic, 537

petrosal deep, 537
superficial, 537

pharyngeal, 530

phrenic, 342

INDEX

721

Nerves, pineal, 591, 592

pneumogastric (vagus), 530

profundus, 482, 489

sacral, 537

sensory, 516, 563

somatic motor, 489, 522
sensory, 523

spinal, 85, 518, 532, 537, 625, 654
accessory, 482, 493, 514, 523

splanchnic, 531, 537

sympathetic, 486, 537

terminal, 478, 482

thoracic, 537

trigeminal, 482, 489, 523, 527, 609

trochlearis, 482, 510, 523, 527, 625

vagus, 482, 484, 485, 523, 530, 537,
609, 624

vasoconstrictor, 390

vasodilator, 390

ventral, 7, 478

vertebral, 537

vestibular, 529, 600, 603

vidian, 537

visceral motor, 485, 524
sensory, 524
Nervous center, 7

elements, 467

system, 5, 6, 8, 9, 10, 12, 17, 20, 21,
30, 65, 467-562, 630
autonomic, 535-54°, 537
sympathetic, 486, 535-540, 537

tissue, 145
Nervus terminalis, 482
Nest, 47, 49, 52
Nest-building fishes, 47
Nettle cell, 6, 30
Neural arch, 23, 204, 206, 244, 656

crest, 85, 87, 558, 603

fold, 88, 541

gland, 20

groove, 541

plate, 72, 73, 75, 81, 82, 85, 541, 623

tube, 72, 73, 84, 85, 86, 86, 87, 88, 91,
541, 558, 637, 640, 642, 651
Neurapophysis, 206

Neuraxon (axon or neurite), 146, 147, 470
Neurenteric canal, 4, 82, 84, 88, 637, 638
Neurite (neuraxon), 467, 470, 470, 521,

552
Neurobio taxis, 551, 552
Neuroblasts, 551, 552
Neurochord cell(s), 515

Neurofibrils (fibrillae), 146, 148, 469
Neurogenesis, 552
Neuroglia, 149
Neurohumors, 447
Neurolemma, 471, 523, 556, 557

sheath, 149, 489, 522
Neuromasts, 568, 569, 656
Neuromeres, 619, 619, 621, 622, 623, 633,

634, 644, 653, 653
Neuromeric, 615, 620
Neuromerism, 634
Neuromuscular spindle, 571
Neuron, 147, 469, 472, 472, 494, 499, 515,
520, 522, 565

afferent (sensory), 469, 472, 520, 565

association, 516, 517, 520

efferent (motor), 469, 470, 472
Neuropore, 82, 84, 478, 640
Neurosensory cells, 467, 469, 472
"Neurostoma," 657
Neurotendinous spindle, 571
Neutrophiles, 162, 364
Nevus, 176, 177
Newman, 27
Newts, 25, 336
New World monkeys, 32, igi
Nipple, 54, 180

supernumerary, 181, 182
Nissl bodies, 148, 522
Nitrogenous compounds, 399

wastes, 399
Node of Ranvier, 149, 523

sino-auricular, 357
Nodosum ganglion, 530
Nodule, 314

Non-deciduate placenta, 121
Non-epithelial tissues, 140
Non-medullated nerve fibers, 149
Non-metameric, 11, 30
Non-placentals, 26, 27
Nordic, 33, 179
Normoblast, 367
Notharctidae, 33

Notochord, 16, 17, 19, 20, 21, 23, 23, 30,
31, 72, 74, 75, 81, 82, 84, 86, 87,
91, 153, 204, 209, 244, 515,632,
651, 656, 657, 658, 660, 66r
Nuchal flexure, 542, 543
Nucleus (Nerve) amygdaloid, 504

caudate, 504, 546

cuneatus, 549

dentate, 513, 549

INDEX

722

Nucleus dorsalis, 517

geniculate, 507

gracilis, 549

lenticular, 546

olivary, 513

pulposus, 209

ruber (red), 511, 520, 548
Nutriment, 51, 55
Nutrition, 41, 46, 54, 65
Nutritive, 65

O

Obesity, 464
Oblique muscle, 614
external, 277
inferior, 277
internal, 277
superior, 614
vein (of Marshall), 382
"Obstetric Toad" (Alytes obstetricans), 49
Obturator artery, 393
externus m., 283
foramen, 250
internus m., 283
vein, 397
Occipital, 225, 226, 228, 626
artery, 390
condyle, 26, 226, 246
lobe, 500

vertebrae, 493, 627
Occipitalis muscle, 274
Occluded, 308, 318
Octopus, 14

Oculomotor n., 482, 489, 511, 523,611,625
Odontoblast, 195, 196, 197, 199
Odontoid process, 207
Oestrus (estrus), 450
Oken, 217, 608
Old World monkeys, 30, 191
Olecranon fossa, 249

process, 249
Olfactory bulb, 488

lobe, 480, 489, 495, 5o8, 546
nerve, 482

organ, 86, 503, 508, 571-577, 623, 654
pit, 22, 25, 574
reflexes, 480
tract, 486
Oligocene, 33
Oliva, 513
Olivary nucleus, 513

Omasum (psalterium), 312
Omentum(a), 319, 321, 321

greater, 312
Omohyoid m., 277
Omphalomesenteric (vitelline) vein, 371,

371

Ontogenesis, vil, xiv, 10
Ontogeny, 65, 66
Onychophora, 30
Oocyte, 412

Operculum, 13, 31, 196, 330, 331
Ophidia, 26

Ophthalmicus profundus, 483, 484, 609
superficialis V, 484, 489
VII, 483
Opisthonephros, 406
Opisthotic, 228
Opossum, 27, 27
Opponens digiti minimi m., 283, 284

pollicis m., 283
Opposable thumb, 29, 30
Optic chiasma, 482, 510, 589
cup, 589
fissure, 589
lobes, 480, 493, 510
nerve, 86, 482, 510, 511
stalk, 588, 589
thalami, 507
vesicle, 589
Oral, 30

glands, 302
hood, 21

membrane, 87, 89
siphon, 19
Orang-utan, 192
Orbicularis oculi m., 274

oris m., 274
Orbit, 227, 232
Orbito-nasal groove, 609
Ordovician, 34
Organ, i, 6

of Corti, 597, 600, 601, 601, 604
of equilibration, 6, 599
of hearing, 594, 595, 654
of Jacobson, 575, 576
photic, 580
Organic, 194

evolution, x, xiii
Organized, 5
Organized whole, xiii
Organism, ix, i, 4, 64
Organization, xiii, 5, 16, 163

INDEX

723

Organogenesis, 80, 81, 85, 109
"Origin of Species," 609
Origin, ix, x

muscle, 259
Ornithodelphia, 26
Ornithorhynchus, 27, 27, 41, 46, 53, 90,

492
Os entoglossum (basihyal), 305
Os uteri, 425
Osborn, 187
Osculum, 5
Osmotic pressure, 44
Osphradia (or "false gills"), 573
Ossification, 103

Osteoblast, 103, 104, 158, 195, 205
Osteoclast, 104, 158, 205
Ostium tubae, 421, 432, 441
Ostracoderm(ata)(i), 15, 23, 24, 31, 34,

643, 652, 656, 657
Ostriches, 55
Otic bone, 599

capsule, 483, 605
pit, 602

vesicle, 602, 603
Otoconia, 600
Otolith, 595
Ovarian artery, 377
cavity, 45
ligament, 421, 424
vein, 396
Ovariotomize(d), 450
Ovary, 7, 8, 10, 18, 35, 36, 37, 54, loi,

412, 420, 421, 441, 449
Oviducal gland, 45
Oviduct, 7, 44, 45, 51, 53, loi. ^°^> 4ii,

412
Oviparity, 45, 55, 56, 57
Oviparous, 39, 45, 47, 51, 53, 55, 57
Ovoid, 64

Ovulation, 450, 451
Ovum(a), 36, 38, 41, 42, 45, 412, 420, 421,

421, 423
Owen, 217, 608, 609
Oxidation, 46, 328

Oxygen, 46, 48, 51, 52, 53, 54, 160, 328
Oxytocin, 464
Oysters, 13

Pacini, corpuscles of, 566, 568
Pads, digital, 169

Pads, hypothenar, 169
interdigital-, 169

thenar, 169
Pain, 563

Paired appendages, ir, 23, 23, 31, 85
evolution of, 236

eyes, 86, 623

fins, 236, 237, 244, 353
Palate, 227, 344, 577

soft, 306
Palatine bone, 235, 227, 232

glands, 301

nerve, 483, 484

tonsil, 306
Palato-pterygo-quadrate, 229
Paleocortex, 4S9, 498
Paleontological, xiv
Paleontology, vii
Paleopallium, 502
Paleostoma, 296
Paleostracan, 651

Paleostriatum, 489, 501, 502, 503, 504
Paleozoic, 34

Pallium, 487, 489, 491, 498, 544
Palm, 168, 249
Palmaris brevis m., 283

longus m., 282
Pancreas, 90, 99, 322, 326, 327, 416, 447
Pancreatic duct, 326

islands, 447, 448
Pancreatico-duodenal artery, 392
Panniculus carnosus, 272
Papilla(e), 10, 19, 303

basilar, 597

circumvallate, 579, 579, 580

filiform, 303, 304, 579

fungiform, 303, 304, 579

lingual, 580

taste, 184, 579

vallate, 303, 304
Papillary ducts, 435

muscles, 373, 389
Paracasein, 311
Parachordal cartilage, 228
Paracone, 187
Paraconid, 188

Paradidymis, 407, 412, 427, 441
Paramecium, 3
Paraphysis, 505
Parapithecus, 33
Parapodium(a), 11
Parapophysis, 204

724

INDEX

Parasitic, 4, 8, 9, 14
Parasphenoid, 220
Parasympathetic, 539

nerves, 536

plexus, 472
Parathyroid, 90, 337, 458

endocrine, 459

extract, 458, 459
Parental protection, 46, 56, 58

tissue, 51
Parietal, 82, loi, 218, 225, 226, 228

cells, 310, 311

eye, 507, 590, 591, 592, 592, 654

lobe, 500, 501

nerve, 592

organ, 491, 507, 590, 591, 592

outgrowth, 505, 591

pleura, 340
Parker, G. H., xiii, 229
Paroophoron, 407, 413, 422, 442
Parotid, 301, 326, 327
Parrot, 38

Pars intermedia, 462, 463, 464, 465, 465,
466

nervosa, 462, 465, 466, 548

tuberalis, 462, 464, 465, 466
Patagonian, 178
Patella, 13, 251, 252, 257
Patten, B., viii, 93, 379, 380, 381, 383, 509,

518
Patten, W., 15, 24, 643, 652-657
Patten's hypothesis, 652-657
Patterns, 168
Pearl (s), 13
Peccaries, 219
Pectineus m., 283
Pectoral, 246

fin, 236, 241

girdle, 238, 241, 244, 247
Pectoralis major m., 281

minor m., 281

muscle, 248
Pedal ganglion, 1 2
Pedicle, 207
Peduncle cerebral, 513, 549

inferior (restiform bodies), 512

middle, 512

superior, 512
ventral, 503
Peking man, 32, 495, 628
Pelagic (eggs), 46
Pelecypoda, 13

Pelvic, 29, 246

fin, 236, 238, 414

girdle, 238, 239, 240, 246

muscles, 284

vein, 354
Pelvis, 251, 434

renal, 416, 419
Penis, 7, 4I3j 4i5, 427. 428, 429

crura, 429

glans, 415, 427, 429, 444
Pennsylvanian, 34
Pepsin, 311

Pepsinogen granules, 310, 311
Peptone, 311, 326
Perennibranch, 230
Perforation, 87

Peribranchial cavity, 18, 19, 20, 335, 661
Pericardial cavity, 12, 19, 107, 353, 387
Pericardium, 107, 343
Pericellular, 523
Perichondral, 103

bone, 103, 104
Perichondrium, 103, 104, 155
Periderm, 168

Perilymph, 596, 599, 601, 602, 604
Perilymphatic cavity, 601
Perimysium, 145
Perineum, 318, 437
Perineurium, 150, 150
Periosteum, 104, 157, 562, 601
Peripatus, 15, 300, 301
Peripheral, 87

nervous system, 65, 495, 522

sinus, 386
Periphery, 62
Perissodactyls(a), 28, 32
Peristalsis, 309, 322
Peritoneum(al), 11, 21, 63, 84, 94, 99,

100, 102, 325
Peritoneal layer, loi, 416
"Permanent larvae," 51
Permian, 34, 187
Peroneal artery, 379
Peroneus brevis m., 283

longus m., 283

tertius muscle, 283
Petromyzon, 23, 23, 31, 299, 336, 458,

494, 663
Petromyzon marinus, 229
Petrosal, 227, 228, 236

ganglion, 529
Pflueger's egg tubes, 441

INDEX

725

Phagocytic, 162
Phalanx, (ges), 250, 252
Phallus, 412, 414, 415, 444
Pharyngeal artery, 390

cleft, 89

derivatives, 337, 459

gills, 328, 329, 333

gill slits, 16

pouches, 89, 90, 234, 329, 337
Pharynx, 7, 8, 10, 11, 12, 19, 20, 21, 89,

306, 326, 329, 331
Phoronida, 9
Phosphorescent, 7
Phosphorus metabolisrri, 459
Photophore, 180
Pho'toreceptor, 580, 581, 582
Phototropic, 580
Phrenic artery, 377, 392

nerve, 342

v., 396
Phyllopods, 631
Phylogenesis, vii, 14, 26
Phylogenetic, xv, 621

series, 666
Phylogenetic tree, 2, 5
Phylogeny, 66
Phylum(a), i, 3, 7, 8, 9, 30
Physics, XV
Physiological, vii
Physostomous fishes, 344
Pia mater, 562

primitiva, 562
Pig(s), 189
Pigeon "milk," 55
Pigment, 176, 177, 178

cell, 151, 580

epithelium, 581, 586, 589

layer (of retina), 589

spot, 22, 580, 582
Pigmentation, 176, 452
Pilaster cells, 330
PiUar cells, 602
Pineal body, 463
Pineal eye, 590, 591, 591, 592

gland, (epiphysis), 490, 495, 507, 590,

591, 593

nerve, 590, 591

organ, 590, 591, 591, 592, 593
Pinna, 232, 598
Pipa americana, 50
Pipe-fish, 47, 57
Piriformis m., 283

Pisces, 24, 31
Pisiform, 249
Piston, 30s
Pit(s), 87

eyes, 639

gastric, 310

organs, 618
Pithecanthropus, 32, 33
Pituitary gland, 227, 462, 465, 465, 481,

507, 547
Placenta, 27, 49, 54, 57, 80, 120, 121, 376
Placental(s), 26, 27, 31, 34, 55, 56
Placental mammals, 77, 79

sharks, 48

structures, 46
Placode, 540, 560, 623
Placoid scale, 169, 170, 177, 183, 184,

185, 332
Planarian, 7
Planets, xii
Plantar artery, 395
Plantar v., 397
Plantaris m., 283
Plantigrade, 28
Plasma, 348, 367, 397
Plasmodesm(s), 133, 467, 552
Plastron, 171
Plate, viii, 494, 496, 497, 519, 596

alar, 542

basal, 542

floor, 542, 549

roof, 542, 549
Piatt, Miss Julia, 267, 616, 623, 651
Platyhelminths, 4, 7, 30
Platypus (duck bill), 41
Platyrrhina, 30, 32, 33
Platysma, 274
Pleistocene, 33, 34
Pleura, 340
Pleural cavity, 337

fingers, 651

folds, 651
Pleurodont, 186
Pleuroperitoneal cavity, 342
Plexus, 470, 473, 532, 533

of Auerbach, 486, 561

brachial, 468, 531, 533

cardiac, 561

cervical, 468, 486, 489, 532, 533

cervico-thoracic, 489, 533

celiac, 561

chorioid, 490, 547

726

INDEX

Plexus, hypogastric, 561

intestinal, 489, 539, 540

lumbar, 531

lumbo-sacral, 468, 489, 534

Meissner's, 561

myenteric, 539, 561

prevertebral, 561

sacral, 533, 534

submucous, 539, 561

thoracic, 4S9
Plica(e) circularis (semilunaris), 314, 315

ventralis, 640
Pliocene, 33
Pliotrema, 336
Plume, 172
Pluralist, xiv
Poison fangs, 186

gas, 573

glands, 303
Polar axis, 64

body, 59, 64
Polarity, 60, 469
Polarization, 42, 62
Polarized, 61

cells, 469

light, 144
"Polliwog," 50
Polymastism, 181
Polymorphonuclear leucocyte, 162
Polynuclear gland cells, 515
Polyp(s), 5, 6
Polyphyletic theory, 367
Polyphyodont dentition, 187
Polypterus, 24, 89, 345
Polysaccharid (glycogen), 322
Polyzoa, 9

Pons (of Varolius), 480, 495, 511, 513, 549
Popliteal artery, 379, 393

vein, 397
Popliteus m., 283
Porcupine, 28
Pore (porous), 11, 30, 53
Porifera, 4, 30
Porpoise, 28
Portal v., 352, 382

hepatic, 352

renal, 352, 354, 356
Portuguese man-of-war, 5
Postanal gut, 298, 300, 318
Postbranchial bodies, 460, 459
Postcardinal veins, 352, 353, 356, 376, 382

Postcava, 25, 323, 352, 354, 356, 361, 382,

387, 396
Postfrontal bone, 218
Postganglionic fibers, 538
Post-natal, 55, 56
Posttrematic branch, 485
Postzygapophysis, 207
Potassium iodide, 456
Potentialities, 44
Pouch(es), 83

marsupial, 27, 50, 51
Precardinal vein, 352, 363, 356, 37C, 382
Precava, 352, 353, 356, 387, 395
Prechordal cartilage, 228
Precoracoid, 239, 240
Preganglionic fibers, 524, 538
Pregnancy, 449, 451
Premandibular cavity, 611, 616, 617, 618

somite, 617
Premaxillary bone, 219, 230
Premolar(s), 30, 189, 192
Pre-natal, 56
Pre-oral, 30, 230, 609, 618

gut, 617

lobe, 17, 639, 640, 660
Prepuce, 429, 445
Preputial sac, 415
Pressure, 487, 507
Pretrematic branch, 485
Prezj'gapophysis, 207
Primary bone, 106
Primate, 3, 28, 29, 30, 32, 34, 191
Primitive duct, 430, 431, 432

ganglion cells, 469, 472

groove, 76

streak, 76, 76, 78, 78, 79, 79, 86, 87
Primordial, 228

blood cells, 365

germ cells, 3(1, 100, 409, 440

skull, 216
Primordium hippocampi, 490
Proboscidian (s), 28, 32, 190, 191
Proboscis, 17, 658

cavitj', 660

pore, 17

sheath, 658, 658
Pro-cartilage, 229
Process(es), accessory, 208

articular, 208

condyloid, 223

coronoid, 248

costal, 212

INDEX

727

Process(es), diapophysis, 204, 206
mammillary, 20S
odontoid, 207
parapophysis, 206
spinous, 206, 207
styloid, 248
transverse, 206, 207
xiphoid (ensiform), 214, 215
zygapophysis, 204, 206
Process theory (Kupffer's), 555
Prochymosin, 311
Proctodeal ectoderm, 88

perforation, 90
Proctodeum, 88, 90, 318
Profundus nerve, 482, 489
Progestin, 450, 451
Progynon, 451
Prolan a and b, 450
Proliferation, 86, 87
Pronation, 282
Pronator quadratus, 283

teres m., 282
Pronephric chamber, 405
duct, 96, 411, 431, 432
tubule, 92, 96, 431
Pronephros, 96, 97, 98, 355, 403, 404, 442

development of, 430-432
Pronucleus(ei), 59
Prootic, 228, 615, 626

metameres, 626, 627
Propliopithecus, ^^y 665
Proprioceptors, 145
Propterygium, 241
Prosencephalon, 478
Prostate gland, 428
Prostatic concretions, 428, 429

utriculus, 428
Prostomium, 633
Protection, 46
Protective, 6

membranes, 39
shell, 52
Protein, 326
Proteose, 326

Proterostomia, 10, 634, 637
Proterostomians, 2, 4, 300
Protochordate, 31, 409
Protocone, 187
Protoconid, 188

Protonephridia, 9, 20, 400, 402, 631, 634
Protoplasm, xiii, 58, 60, 61, 65, 70
Protoplasmic activity, 58
bridges, 133

Protopterus, 489

Protostraca, 651

Prototheria, 26

Prototypes, 400

Protozoa, 2, 3, 4, 9, 30, 630

Pseudocoelom, 8, 9, 10, 14, 30, 350

Pseudopodium(a), 3, 467

Psittacus, 38

Psoas, major, 277

minor, 277

muscle, 251
Pteraspis, 656
Pterichthys, 24
Pterygoid, 226, 228, 229
Ptyalin, 301
Ptychodera, 661

Puberty, 341, 449, 451, 461, 462
Pubic symphysis, 250
Pubis, 240, 250
Pudendal artery, 393

vein, 397
Pulmonary, 329

arter>s 354, 356, 374, 374, 376, 384,

389

circulation, 389

system, 337, 344

vein, 354, 355, 356, 387
Pulmonate molluscs, 344
Pulp, 193, 194, 196, 197

cavity, 199

chamber, 194
Pulse, 392
Pupil, 584, 585
Purkinje cells, 511, 513
Putamen, 504
Pyloric glands, 310

stomach, 309, 311
Pylorus, 309, 323
Pyramid (s), 513
Pyramidal cells, 146, 491, 518
Pyramidalis m., 278
Pyramids of the kidney, 416

of the medulla, 504, 513
Python, 239, 241

Q

Quadrate, 229, 231, 232, 235, 246, 323

lobe, 323
Quadratus femoris m., 283

lumborum m., 277

plantae m., 284
Quadriceps femoris m., 251, 283

728

INDEX

R

Rabbit, 28, 77, 78
Race(s), 178
Raccoon, 28
Radasch, 324
Radial, 10

artery, 378, 392
canal, 5

cartilage, 237, 238, 241, 243
notch, 249
symmetry, 6, 10, 30
vein, 396
Radio-dorsal m., 282
Radius, 243, 249
Radula, 183
Ramus-rami, 232

buccalis, 483, 484
communicans, 489, 538, 561
ophthalmicus profundus, 483, 484, 609
superficialis V, 484, 489
VII, 483
Rana, 38, 494
Ranson, viii, 521, 555

-Cajal method, 622
Ranvier, nodes of, i49) 523
Raphe, 426, 444
Rasmussen, 510
Rat, 28
Rathke, 609

Rathke's pouch, 462, 464, 465
Ratitae, 26
Rattlesnake, 171
Ray(s), 31
Ray-finned, 25
Reality, xiv

Recapitulation theory, 124
Receptaculum chyli, 363, 398
Receptor, 86, 138, 145, 467
neuron, 148
organ, 563
Rectal cavity, 413

gland, 319
Rectum, 90, 91, 315, 316, 318
Rectus abdominis m., 277, 287
capitis m., 276
femoris m., 283
externus m., 286, 611
inferior, 264
internal, 264
superior, 264

Red corpuscles, 159
man, 178

nucleus, 511, 520, 548
Reflex action, 474

arc, 469, 472, 472, 477, 477, 520
centers, 515

nervous system, 467, 639
Reichert, 609
Reighard, 617
Renal, 90

artery, 354, 356, 377, 4^8
calyces, 434
columns, 416

corpuscle, 98, 98, 417, 433
ducts, 419
fascia, 416
pelvis, 419, 437
portal v., 353, 354, 356
pyramids, 416
sinus, 416

tubule, 96, 404, 417, 631
vein, 352, 354, 356, 382, 418
Rennin (chymosin), 311
Reproduction, 35, 53, 55, 56, 57
Reproductive, 51, 56, 411
ducts, 441, 442

organs, 4, 5, 9, 12, 19, 421, 426, 439
system, 407
Reptile(ia), 2, 25, 26, 27, 31, 34, 39, 45,
51, 52, 53, 56, 57, 62, 69, 70, 76,
80, 186, 246
Reptilian, 53, 79
Resident, ix
Respiration, 11, 12, 13, 46, 52, 54, 60, 65,

89, 232, 246, 328
Respiratory center, 447
gases, S3
membranes, 89
organs, 10
system, 328-346
Restiform bodies (inferior peduncles),

512, 513, 517, 549
Rete ovarii, 441

testis, 426, 440
Reticular layer, 452

tissue, 152
Reticulum, 312
Retina, 14, 86, 137, 580, 586

inverted, 581, 582
Retzius, 38

Rhabdocoele turbellarian, 7
Rhabdopleura, 17, 333, 402, 661

INDEX

729

Rhinoceros, 28
Rhodesian, 32, 33, 225
"Rhomboid," 174
Rhomboideus m., major, 281

minor, 281
Rhombomere(s), 549, 620, 621
Rhynchocephalia, 26
Ribs, 210, 211, 211, 342, 610

evolution of, 212

floating, 211

true, 212
Rickets, 179
Ring cartilage, 236
Risorius m., 274
Ritter, 660, 661

Rod and cone cells, 137, 582, 586, 586
Rodents, 28, 31, 190
Rohon-Beard cells, 516, 556
Romanes, 241
Romer, 222, 223, 224, 247
Roof plate, 542
Root, 28, 193, 303
Rotator m., 276
Rotifera, 2, 4, 9, 10, 30
Ruber (red) nucleus, 511, 520
Rubrospinal tract, 520
Rumen, 312
Ruminauts, 312
Running birds, 26

Saber-toothed tiger, 190

Sac, 25

Sacculation, 315

Sacculus, 596, 597, 600, 601, 603, 604

Saccus endolymphaticus, 601

vasculosus, 487, 490, 507
Sacral artery, 377, 393

canal, 209

vein, 397

vertebrae, 204, 239, 241, 246
Sacro-coccygeal m., 277

spinalis m., 277
Sacrum, 206, 209, 246, 251
Sagitta, 9

Saint Hilaire, 630, 642
Salamander, 25, 49, 51, 57
Salivary glands, 7, 12, 301
Salmon, 44, 46, 51
Samoyads of Siberia, 178
Sand-burrowing, 20

Santorini's duct, 326, 327
Saphenous artery, 379

vein, 397
Sarcolemma, 144, 523
Sarcoplasm, 143

Sartorius m. — "tailor muscle," 283
Sauripterus, 244

Sauropsida, 25, 31, 58, 69, 70, 77
Savi, vesicles of, 598
Scala media, 600

tympani, 597, 601, 602

vestibuli, 597, 601, 602
Scale(s), 23, 25, 26, 169, 170, 170, 171, 177

placoid, 169, 170, 177, 183, 184, 185
Scaleni, anterior muscle, 277

middle, 277

posterior, 277
Scallop (pecten), 13
Scaly skin, 24, 31
Scapula, 238, 239, 240, 247, 248
Schwann's sheath, 146, 149
Sclerotic (sclera), 588
Sclerotome, 95, 102, 209, 615
Scomberomorus, 24
Scorpions, 15, 651
Scrotal sac, 412, 443, 449

swellings, 444
Scrotum, 28, 415, 426, 443, 443, 444
Scutes, 218
Scymnosuchus, 231
Sea-anemone, 5, 6

-cucumber, 10

-horse (hippocampus), 47, 57

-lily, 10

-scorpions, 643

-squirt, 18

-urchin, 10

water, 10, 52, 348
Seal, 28

Sebaceous glands, 176, 180
Secondary sense cell, 469
Secretin, 447
Secretion, 100, 165
Secretory cells, 90
Sedgwick, 300, 636
Sedimentary, 9
Seeliger, 19
Segment(s), 11, 616

head, 607, 609, 610, 614, 623, 624

trunk, 607, 610
Segmental tubules, 631

730

INDEX

Segmentation, 76, 91, 92, 92, 94, 95, 611,

612, 613, 616, 619, 620
Selection, xi
Self-fertilization, 36
Sella turcica, 227, 462
Semen, 412
Semicircular canals, 513, 594, 596, 600

duct, 603
Semilunar notch, 249

valves, 389
Semimembranosus m., 283
"Seminal" groove, 412, 414

ridge, 412

vesicles, 7, 427, 428
Seminiferous tubules, loi, 426, 440, 448,

449
Semipermeable membranes, 164, 328

Semispinalis m., 276
Semitendinosus m., 283
Semnopiths, 33
Semon, loi
Semper, 630, 631
Sense(s), 563

buds, 572

cells, 563, 578

cutaneous, 566

exterior, 563

interior, 563

major, 563

minor, 563

organs, 6, 12, 22, 85, 563, 564, 565

567
Sensory cells, 563, 565, 579

ganglion, 515, 516

cells, 472, 482, 515, 516

nerve, 516
Seps, 57
Septula, 426

Septum(a), 9, 30, 83, 371, 384, 49°, 49i>
502, 516, 551

pellucidum, 547

transversum, 107, 325, 343, 353
Serosa, 309, 340, 343
Serous, 388

gland, 303

membrane, 387
Serpent, 240
Serranus, 36
Serratus anterior m., 281

posterior m., 280
Sertoli (sustentacular) cells, 37, 427
Serum, 159, 34S

Sessile, 5, 9
Setae (bristles), 10, 11
SewertzofT, 18, 235, 332
Sex(es), 5, 9, 10, 11, 35, 464, 563
characters, 448
differences, 449
glands, 448
traits, 449, 450
Sexual maturity, 51

reproduction, 407
Shaft, 173
Shagreen, 183
Shank, 250

Shark, 25, 31, 38, 45, 57, 89, 241
Sheath cells, 557

of Schwann, 146, 149
Sheep, 28

Shell, 9, 12, 38, 40, 41, 51, 52, 53, 55
gland, 45

membrane, 38, 40, 41
Sherlock Holmes, xv
Shipley, 28
Shoulder, 247

Shoulder blade {see Scapula)
Shrew, 27
Shrimp, 14
Siamang, 33
Sight, 479, 563

Sigmoid(al) flexure, 205, 246, 358
SUica (siliceous), 3, 5
Silurian, 34
Sinanthropus, 32, 33
Sino-auricular node, 357
Sinus(es), 226

pocularis, 445

urogenital, 318, 411, 413, 437
venosus, 351, 352, 353, 357, 358, 371,
376, 379
Sinusoid, 323
Siphon, 13, 19
Sirenians, 28, 32
Sivapithecus, 33
Skate, 25, 31, 38, 45, 57, 236
Skeletal evolution, summary of, 244
Skeletal muscles, 261, 523

system, 201
Skeleton, 3, 5, 24, 103, 201-258, 650, 656
appendicular, 202
axial, 84, 202
bony, 31
human, 211, 213

INDEX

731

Skin, 10. II, 63, 66, 84, 164, 165

color, 176

elasmobranch, 169

human, 166
Skull, 25, 216, 217, 218, 219, 220, 221, 222,
223, 224, 228, 244, 246, 607, 610
Sloth, 27
Slug, 13

Smell, 470, 498, 563, 572
Smooth muscle fiber, 142, 290
Snail, 12, 13
Snake, 52

Sobotta, 226, 248, 250, 323, 503
Socket, 2 7,2, 244
Soft palate, 306
Sole, 168

Solenocyte, 21, 400, 402
Soleus m., 283
Somatic afferent nerves, 523

efferent nerves, 522

(parietal] layer, 82, 83, 84, 99

motor cells, 516
fibers, 489

muscles, 284

sensory fibers, 523
Somatopleure, 84, 92
Somite, 11, 83, 613, 614, 615

anterior, 6r6

mesodermal, 82, 91, 92, 93, 96

trunk, 611, 615
Sound-conduction, 232, 235, 246, 329
South American frog (Rhinoderma), 49

monkeys, 30
Sowbug, 14
Spalacotherium, 188
Spaulding, xiii
Special creation, 236
Specialization, 65

Species, vii, ix, i, 5, 7, 8, 9, 10, 11, 55
Specific gravity, 46
Specificity, 162
Spectroscope, xii
Speech, 232

center, 495
Speidel, 557
Sperm, 411, 414

duct, II, 44, loi
Spermatheca, 7
Spermatic artery, 377, 393

cord, 428, 443

vein, 396
Spermatid, 37

Spermatocyte(s), 37

Spermatogonia, 37, 440

Spermatophore, 45

Spermatozoon, (oa), 8, 35, 36, 37, 38, 42,

44, 45, 59, 426
Sphenodon, 26
Sphenoid, 225, 226, 227, 228
Sphenopalatine, 227

ganglion, 537
Sphincter ani m., 284, 316

ani externus m., 284, 316

colli m., 301

urethrae, 284
Spinal accessory nerve, 493, 514, 523

cord, 19, 20, 82, 481, 489, 512, 514,
515, 516, 520, 531

development of, 549, 550, 550

gangha, 85, 473

nerve, 85, 518, 532, 537, 625, 654
Spinalis m., 277
Spindle, neuromuscular, 571

neurotendinous, 571
Spine, 246, 247 ■
Spinocerebellar tract, 518
Spinothalamic tract, 511, 518
Spinous process, 244
Spiny, 10, 30

Spiny ant-eater (Echidna), 27, 41, 53
Spiracle, 89, 331, 335, 336
Spiracular cleft, 609
Spiracular pouch, 336, 605
Spiral ligament, 601

organ of Corti, 597, 600, 601, 601,
604

valve, 324
Splanchnic arteries, 351

(visceral) mesoderm, 83, 84, 99, 369
Splanchnopleure, 84, 92
Spleen, 365
Splenic artery, 391, 392

veins, 382, 396
Splenius muscle, 276
Sponge(s), 2, 4, 5, 6
Spongioblast cells, 551
Squalus acanthias, 22, 38
Squamosal, 26, 218, 231, 232, 246
Squamous, 227, 228
Squid, 13, 14
Squirrel, 28
Stalk, 9
Stapedius m., 598

732

INDEX

Stapes, 227, 230, 232, 233, 236, 597, 598

602
Starch, 326
Starfish, 10
Starling, 447
Static center, 487

organs, 20, 594, 600
Statocyst, 594, 596
Statolith(s) (otoconia), 594, 59S
Status thymicolymphaticus, 461
Steapsin (lipase), 322, 326
Stegocephala, 25, 25, 31, 187, 219
Stempell, viii, 8, 10, 234, 605
Stendell, 465

Stenon's (Stensen's) duct, 301
Stensio, 23
Stentor, 3
Sterility, 443
Sternebrae, 214

Sterno-cleido-mastoid artery, 390
Sterno-cleido-mastoid m., 227
Sternohyoid m., 248, 277
Sternomastoid m., 248
Sternothyroid m., 277
Sternum, 26, 29, 214, 215, 215, 246
development of, 214, 214
evolution of, 214, 215
Stickleback, 47
Stigma (ta), 19, 334
Stimulofugal, 552
Stimulopetal, 552
Stimulus(i), 563
Stolons, 18

Stomach, 9, 10, 12, 18, 19, 28, 308, 324
development of, 311
evolution of, 312
Stomodeal ectoderm, 88
Stomodeum, 87, 295, 636, 637, 638, 642
Stratum corneum, 134, 135, 165, 166, 167,
170
germinativum, 134, 165, 166, 167
granulosum, 134, 166, 167
lucidum, 134, 166, 167, 171
Straus, viii, 416

Striate body, 498, 501, 503, 504, 507
Stroma, ovarii, 421
Structure, 35
Struggle for existence, xi
Sturgeon, 31
Styloglossus m., 275
Stylohyoid m., 232, 236, 275
Styloid, 228, 232, 249

Stylopharyngeal m., 307
Subcardinal anastomosis, 357

vein, 357
Subclavian artery, 353, 356, 376, 377, 39°.

392
vein, 353, 356, 382, 396
Subclavius m., 281
Subcutaneous connective tissue, 152
fat, 103

network (plexus), 470
Subesophageal ganglion, 10, 476, 635,

636, 643, 646, 648
Subgerminal cavity, 62, 63
Subintestinal, 11

vein, 3SI
Sublingual gland, 302
Submaxillary, 301
Submucosa, 307, 309
Submucous plexus, 539, 561
Subneural gland, 18
Subscapular(is) artery, 392

muscle, 247, 281
Sub-supracardinal anastomosis, 382
Sub-umbrellar nerve ring, 476
Subunguis, 172
Subungulates(a), 28, 32
Sugar, 326, 447
Sulcus(i), 500, 517
coronarius, 387
longitudinal, 542
terminalis, 303, 304
Superciliary crests (ridges), 225
Superior colliculi, 511
Supernumerary nipple, 181, iS-"
Supination, 282
Supinator m., 282
Supporting cell, 602

rod, 244
Supracardinal veins, 357
Supra-esophageal ganglion, 10, 635, 636,

646, 648
Supraoccipital, 228
Supraorbital line, 489
Suprapericardial bodies, 337
Suprapostcardial bodies, 459, 460
Suprarenal (adrenal), 451, 452, 453
artery, 45 1

cortex, 452, 452, 453, 454
development of, 454
glands, 451-455
evolution of, 454
medulla, 452, 452, 453, 454

INDEX

733

Suprarenal, physiology of, 452

vein, 451
Suprascapula, 239, 240
Supraspinatus m., 247, 281
Supraspinous fossa, 247
Survival, 58
Suspensorium, 246
Suspensory ligament, 421, 585, 585
Sussex man, 32, 628
Sustentacular cells, 37
Sweat glands, 166, 180
"Sweetbreads," throat, 460
Swine, 28
Symmetry, bilateral, 7, 10, 30

radial, 6, 10, 30
Sympathetic, 505, 560, 561

ganglia, 25, 454, 489^ 49i> 5i5> 539

nerves, 479, 53^

plexus, 561

system, 22, 486, 495, S35-540, 538,
540, 541, 560, 561
development of, 560, 561
Sympathin, 454
Synapse, 474

Synaptic connexions, 516, 520
Synapticulae, 334
Synarthrosis, 257
Syncytium, 143
Syphilis, 4
System(s), xii, i
Systemic artery, 355

circulation, 390

veins, 380
Systole of heart, 388

Tactile cells of Merkel, 567

hairs, 594

organs, 594
Tadpole, 49, 50, 51
Tail(less), 25, 31, 36, 107, 108, 246
Tailed forms, 25
Talgai, 33
Talon, 188

Talus (astragalus), 252
Tapeworm, 7
Tapir, 28
Tarsal glands, 588
Tarsier(s), 29
Tarsioids, 33
Tarsius, 33

Tarsus, 250, 252
Taste, 563, 572, 577

buds, 137, 185, 304, 332, 578, 656
cells, 579
organs, 577, 656
papilla, 184, 579
Taungs man-ape, 32
Taurodont, 193
Taxonomists, i, 7, 10
Teat, 26, 181

abdominal teats, 29
false teats, 182
true teats, 182
Tectorial membrane, 600, 602, 604
Tectum, 481

Teeth, xiv, 26, 28, 183, 185-200, 230, 608
calcareous, 183
carnassial, 190
compound, 187
development of, 195-200
evolution of, 187
germ, 188
horny, 183
mammalian, 189
milk, 186, 189, 196, 199
papilla, 199

permanent, 186, 196, 199
taurodont, 193
Tegmentum, 480, 511, 513
Telencephalon, 479, 480, 501, 505, 507,

542, 544
Teleology, xiv
Teleost(ei), 24, 25, 31, 36, 44, 45, 5<'' 9<=>y

186, 230
Teloblastic cells, 632

Telodendria, 146, 469, 518, 520, s^i, 552
"Telopore," 657
Temperature, 61, 348, 399

regulation, 179
Temporal artery, 390
bone, 225, 227, 228
lobe, 500, 501, 602
Temporalis m., 226, 295
Tendon, 151, 153

of Achilles, 276, 283
Teniae, 315

Tensor fasciae latae, 283
tympani muscle, 598
veli palatini m., 291
Tentacle, 6, 7, 13, 19, 21, 65, 651
Tentorium cerebelli, 562
Teredo (ship worm), 13

734

INDEX

Teres, 251

ligament, 384
major m., 281
minor m., 281
Terminal arborization, 147
growth, 630
lamina, 508
Terminal nerve, 478, 482
Tertiary, 34, 231

Testisfes), 7, 18, 28, 35, 36, 96, loi, 411,
411, 412, 426, 441, 441, 448, 449
cords, 440
histogenesis of, 440
Tetany, muscular, 458
Tetrabelodon, 191
Tetrabranchiata, 14
Tetrapods, 31, 240, 244, 246
Thalamus(i), 480, 503, 504, 505, 506, 508,

518, 546, 547
Thecodont dentition, 186
Theelin (estrin), 450, 451
Theory, ix, x
Theriodonts, 187
Theromorpha, 26, 31, 187, 246
Thigh, 250
Thirst, 563
Thoracic aorta, 392
artery, 392

duct, 363, 363, 386, 398
plexus, 489
vertebra, 392
Thoraco-acromial artery, 392
Thread worms, 2, 8
Thrombin, 348
Thumb, 29, 30, 250
Thymic corpuscles, 461
Thymus, 90, 337, 460-462

cervical, 462
Thyreo-cervical art. (thyroid axis), 390
Thyro-arytenoid m., 341
Thyroglossal duct, 457
Thyro-hyoid m., 277
Thyroid, accessory, 455
artery, 390, 455, 455
cartilage, 233, 236, 340
gland, 19, 90, 337, 455. 650, 663
anatomy, 455
development, 456, 457
evolution, 457
histology, 455, 456
physiology, 456
vein, 395, 455

Thyroidea ima, 455
Thyroxine, 456
Tibia, 243, 250, 251
Tibialis artery, 379, 395
anterior, 379, 395
Tibialis posterior, 379, 395
Tibialis m., anterior, 283
posterior, 283
vein, 397
Tiger, 3

Tigroid substance, 469
Time, 35
Tissue, 126

cells, 59
Toad, 25, 49, so
Toe, 28, 608

great, 29, 30
nail, 30
Toed appendages, 241
Tongue, 246, 303, 305, 579
bar, 333

muscles, 230, 305
Tonsil lingual, 306

palatine, 306, 337
Tornaria, 17, 660
Torus(i), 511
Touch, 563, 594
Trabecula(e), 228, 373
Trachea, 89, 223, 236, 338, 340, 341
Tracheae, 15
Tracheata, 14, 15
Tract, 517

lateral corticospinal (crossed pyra-
midal), 518
rubrospinal, 520
spinothalamic, 511, 518
ventral corticospinal (direct py-
ramidal), 520
spinocerebellar, 518
Tragus, 599, 606
Transmitter cell, 467
Transverse foramen, 207
process, 206
septum, 107
Transverso-spinalis m., 277, 287
Transversus, 277
perinei m., 284
thoracis m., 280
Trapezius muscle, 247, 248, 275, 281
Tree shrews, 33
Tretjakoff, 540
Triangularis m., 274

INDEX

735

Triassic, 34, 187

Triceps m., 249, 281

Triclad turbellarian, 7

Triconodont, 188

Tricuspid valve, 373, 389

Trigeminal, 483, 484, 489, 523, 527, 609

Trigone, 419

Trilobites, 15, 34, 647, 651

Tripartite, 238, 246

Triquetrum(al), 249, 250

Triradii, 169

Triticeous, 340

Tritubercular, 188

Trochanter, 251

Trochlea, 249

Trochlearis chiasma, 267, 483, 510, 651

nerve, 483, 510, 523, 625
Trochophore, 9, 10, 11, 12, 30

larva, 630, 634, 639, 641
Trophoblast (trophoderm), 77, 78
Trout, 46
Truncus, 354

arteriosus, 351, 353, 354, 356, 371, 373
Trunk, 25, 31, 607

metameres, 626

region, 94

segments, 607, 610, 626

somites, 611, 615
Trypsin, 326
Trypsinogen, 326
Tube(s) medullary, 637

neural, 72, 73, 84, 85, 86, 86, 87, 88,
9i> 541, 558, 651

uterine, 422
Tuber cinereum, 508, 547
Tubercle, 188
Tuberculum, 212

impar, 304, 305
Tubular glands, 303
Tubules, II, 37, 98

collecting, 417, 418, 438

convoluted, 438

renal, 417, 418

uriniferous, 438
Tubuli recti, 426
Tubulo-acinous glands, 302
Tunica adventitia, 307, 309

albuginea, 426, 440

fibrosa, 588

mucosa, 307

muscularis, 307, 309

propria, 307

Tunica serosa, 309

submucosa, 307, 309
Tunicata (urochordates), 18, 18, 35, 661,

662
Tunicin, 18
Tupaia, 27

Turbinals (conchae), 227, 575
Turtle, 62, 221
Tusk.(s), 190
Tympanic cavity, 231, 232, 599, 602, 606

membrane, 599

organs, 595
Tympanum, 234
Typhlosole, 11

U

Ulna, 243, 249

Ulnar artery, 377, 392

vein, 396
Ulno-volar m., 282
Ultimobranchial bodies, 337, 460, 459, 460

evolution, 460
Ultraviolet light, 177
Umbilical (allantoic) artery, 53, 377, 393

stalk, 118

(allantoic) vein, 371, 376, 379, 382,

384
Umbrella, 6
Uncus, 501
Unguis, 172
Ungulate, 189
Unicellular, 30

algae, i
Unipolar, 147
Univalve, 13
Universe, ix, xi
Unpaired eyes, 590-593
Unsegmented, 9
Urachus, 419

Ureter, 98, loi, 413, 415, 419, 436
Urethra, 413, 415, 419, 444

cavernous, 419, 429

membranous, 419, 429

prostatic, 419, 429
Urethral folds, 444

groove, 444

sphincter m., 284, 419
Urinary, 411

canal, 415

duct, 407

system, 399, 415, 43°

736

INDEX

Urinary tubule, 438

Urinogenita! egion, loi

"Urmund," 1.37

Urochordata, 2, 4, 18, 18, 19, 21, 21, 31,

35, 630, 643, 661, 663
Urodeles, 25, 25, 45, 50, 51, 88, 94, 331,

336, 354
Urogenital, 27, 90, 411

cavity, 413

duct, 90, loi

folds, 433

papilla, 411

ridge (fold), 440

sinus, 318, ATI, 413, 437

system, 39c,, ^06, 414, 415, 631
development of, 430-445
Uterine artery, 393

tissue, 54

tubes (Fallopian tubes), 413 422, 442

walls, 54, 80
Uterovaginal vein, 397
Uterus, 29, 45, 413, 413, 423, 424, 424,
442, 449

bicornis, 414

bipartitus, 414

duplex, 414

masculinus, 411, 428, 442

simplex, 414, 423
Utriculo-saccular duct, 601, 603
Utriculus, 596, 597, 600, 601, 603, 604
Uvula, 306

Vagina, 7, 413, 425, 442
Vaginal gland, 8

sac, 443
Vagus lobe, 513

nerve, 324, 483, 484, 485, 523, 609,
624
Vallate papillae, 303
Values, xii
Valves, 9, 331

atrioventricular, 358, 372

bicuspid (mitral), 373

semilunar, 358

spiral, 324

tricuspid, 373
Valvula, 511

Valvulae conniventes, 313
Van Cleave, 7

Van Wijhe, 334, 335, 335, 613, 613, 616,

62s, 637
Variation, xi, 407

Vas deferens (ductus deferens), 7, loi
Vasa efferentia, 411
Vasa vasorum, 389
Vascular, 51, 52, 53, 54, 57, 89
placenta, 26
system, 9, 53, 347, 349-398

development of, 366
trunk, 350
Vasoconstrictor nerves, 390
Vasodilator nerves, 390
Vasopressin, 464
Vastus intermedins m., 283
lateralis m., 283
medialis m., 283
Vegetal hemisphere, 63, 65, 66, 68, 69

pole, 41, 59, 60, 61, 62, 66
Vegetative, 4
Veins, 53, 347, 385, 395

abdominal, 351, 352-356
lateral, 352, 354,356
ventral, 355, 356
allantoic (umbilical), 371, 371
arcuate, 418
auricular, 395
anterior, 395
posterior, 395
axillary, 396

azygos, 357, 361, 382, 396
basilic, 396
brachial, 396
bronchial, 340
cardinal, 352, 631

common, 352, 353, 356, 376, 379,

382
pre-, 352, 353, 356, 376, 382
post-, 352, 353, 356, 376, 382
sub-, 382
supra-, 382
cephalic, 396
cervical deep, 395
circumflex of humerus, 394

of femur, 394
coronary of heart, 358, 389

of stomach, 396
cystic, 397
digital, 397

ductus Cuvieri, 352, 356, 357, 376, 382
ductus venosus, 382, 384
epigastric, 394

INDEX

737

Veins, evolution of, 361
facial, anterior, 395

common, 395

posterior, 395
femoral, 397
gastric (coronary), 396
genital, 352
gluteal, 397

inferior, 397

superior, 397
hemiazygos, 357, 361, 382, 396

accessory, 396
hemorrhoidal, 397
hepatic, 352, 354, 356, 382, 396

portal, 352, 354, 356, 396
hypogastric (internal iliac), 397
iliac, 353, 356, 357, 382, 396, 397

external, 397

internal (hypogastric), 397
iholumbar, 397
innominate (brachiocephalic), 358,

382, 395
intercostal, 396
interlobar, 418
interlobular, 323, 418
intersubcardinal anastomosis, 357
intervertebral, 396
intestinal, 396
jugular, 382

external, 395

internal, 353, 386, 395
lingual, 395
lumbar, 396

ascending, 396
mammary, internal, 396
mesenteric, 352, 354, 356

inferior, 396

superior, 382, 396
metacarpal, 396
oblique (of Marshall), 382
obturator, 397
ovarian, 396
pelvic, 354
peroneal, 394
phrenic, inferior, 396
plantar, 397
popliteal, 397
portal, 352, 382, 396

hepatic, 352

renal, 352, 354, 356
postcava, 25, 323, 352, 354, 355, 356,
361, 382, 387, 396

Veins, precava, 352, 353, 356, 387, 395

pudendal, 397

pulmonary, 354, 355, 356, 387

radial, 396

renal, 352, 354, 356, 382, 396, 418

renal portal (advehent), 352, 356
efferent, 354

retinal, 587

sacral lateral, 397
middle, 397

saphenous, great, 397
small, 397

sinus venosus, 351, 352, 353, 357,
358, 371, 376, 379

spermatic, 396

splenic (lienal), 382, 396

subcardinal, 357

subclavian, 352, 353, 356, 382, 396

subintestinal, 351

supracardinal, 357

suprarenal, 451

systemic, 380

thymic, 395

thyroid, inferior, 395, 455
middle, 455
superior, 395, 455

thyroidea ima, 395, 455

tibial, anterior, 397
posterior, 397

ulnar, 396

umbilical (allantoic), 371, 376, 379,
382, 384

uterovaginal, 397

vertebral, 396

venae comitantes, 396

vesical, 397

vitelline (omphalomesenteric), 367,
376, 379, 382

vorticose, 587
Veliger larva, 12
Velum, 229, 336

transversum, 505
Venous blood, 26, 351, 384, 389
Ventral aorta, 352, 376

motor root, 85

nerve, 8

plate, 172

spinocerebellar tract, 518
Ventricles, of brain, 480, 493, 498, 502, 503

of heart, 353, 358, 371, 384, 387, 388,
389

first, 480

738

INDEX

Ventricles, fourth, 480
lateral, 480, 507
second, 480

third, 480, 501, 506, 507
Ventricular septum, 388
Venus fly-trap, i
Vermiform appendix, 315, 316
Vermis, 512, 549
Vernix caseosa, 168

Vertebra(e), 23, 31, 154, 204, 206, 210,
217, 607, 610
amphicoelous, 206
amphiplatyan, 206
atlas, 26, 207

axis (epistropheus), 205, 207
caudal, 204
cervical, 204, 207
coccygeal, 207
cranial, 608
lumbar, 204, 207, 208
opisthocoelous, 206
procoelous, 206
sacral, 204, 207
thoracic, 207, 208
trunk, 204, 608
Vertebrae, development of, 209, 210
Vertebral artery, 377, 390
canal, 207

column, 22, 84, 203, 207, 244, 246
lamina, 207

theory of the skull, 608, 608, 609, 621
Vertebrata, ix, 4, 16, 24, 25, 31, 55, 56, 57,

84, 85, 86, 95, 629, 630
Vertebrate ancestry, 15, 20, 628-666
Vesicle(s), 86, 544
artery, 393

forebrain, 85, 621, 623
hindbrain, 85, 621, 623
midbrain, 85
of Savi, 598
vein, 397
Vesico-urethral portion, 437
Vesicular eye, 580
Vessel, 8, {see Blood vessel)
Vestibular, greater v. glands (glands of
Bartholin), 426
membrane, 601

nerve, 529, 600, 603
Vestibule, 294, 426, 445
Vestige(s), ix
Villus(i), 54, 313
Viper, 618

Viscera, 99, 251

Visceral afferent (sensory) hbers, 524

arch, 229, 232, 234, 23s, 609, 610,

621, 624
cartilage, 230
cleft, 89, 621

efferent (motor) fibers, 524, 625
ganglion, 12
mesoderm, 83, 84, 99
muscle, II, 261, 290
sense, 563

skeleton, 216, 228, 232-235, 244, 246
Vision, 498
Visual organs, 580-592

purple, 580
Vitalists, xii

"Vitally secretory," 180
Vitellarium, 7, 10
Vitelline (omphalomesenteric), 53
artery, 376, 377
duct, 318
membrane, 38, 41
vein, 325, 367, 371, 376, 379> 382
Vitreous humor (body), 581, 584
Viviparity, 45, 46, 47, 48, 5°, 52, 55, 56-

57, 79
Viviparous, 39, 44, 45, 46, 52, 53, 55j 57,
411
fish, 48
Vocal cord, 246, 341, 343
pouch, 49, 50, 51, 55
Vocalis m., 341
Vogt, Carl, II, 609
Voice-box, 232, 233, 246
Volar arch, 378, 392
Volvox, 4

Vomer ("plough share"), 225, 227
Von Baer, 125, 191, 610
"Vortices" ("cowlicks"), 174
Vorticose v., 587
Vulva, 8

W

Waldeyer, loi

Walking foot, 30

Walrus, 189

Ward & Whipple, 8

Warm-blooded, 52

Wastes, 9, 13, 53, 9^

Water fleas, 14

Water-vascular system, 10, 30

INDEX

739

Watson, 231, 231
Wax glands, 180
Weasels, 28
Web, 29

Whale(s), 28, 32, 43, 55, 190
Whalebone, 29, 55
Wharton's duct, 301
White corpuscles, 159

man, 178

matter, 516

rami, 539
Wiedersheim, 237, 413
Wierstratz, 352
Wilder, viii, 240, 608
Windpipe, 341
Wing, 26, 29
Wirsung's duct, 326, 327
Wirsung's duct(s), 99, 326, 327
Wolffian ducts, 97, 98, loi, 102, 404, 411,

432, 432

folds, 236
Woodger, xii
Woodruff, 352
Worm, 9, 30

capitellid worms, 633
Wrist, 249

Xenarthra, 27, 31
Xiphisternum, 214

Xiphoid (ensiform) process, 214
Xiphosura (merostomata), 15

YeUow pigment, 178

spot (macula lutea), 586
Yolk, 38, 40, 51-57, 59. 60, 61, 62, 70,
79, 80, 85, 88, 108, no

-cells, 67, 68

mass, 54, 62, 63, 80

plug, 68

-sac, 22, 53, 54, 112, 382, 411
placenta, 27, 411
Young & Robinson, 376

Zebra, 28

Ziegler, H. E., 625, 629, 638
Zona pellucida, 41, 42
Zonary placenta, 1 20
Zone ependymal, 550

mantle, 550

marginal, 550
Zygapophysis, 204
Zygomatic bone, 225, 227, 232
Zygomaticus m., 274
Zymogen granules, 310, 311, 326
Zymogenic cells, 313