UNIVERSITY OF CALIFORNIA
AT LOS ANGELES

AMERICAN SCIENCE SERIES

AN INTRODUCTION

GENERAL BIOLOGY

BY

WILLIAM T. SEDGWICK, PH.D.

Professor of Biology in the Massachusetts Institute of Technology, Boston
AND

EDMUND B. WILSON, PH.D.

Professor of Zoology in Columbia College, New York

SECOND EDITION, REVISED AND ENLARGED

47139

NEW YORK
HENRY HOLT AND COMPANY

^636 7 l8"

Copyright, 1886, 1895,

BY

HENRY HOLT & CO.

QH

r

PEEFACE TO THE FIKST EDITION.

j SEVERAL years ago it was our good fortune to follow, as grad-
« uate students, a course of lectures and practical study in General
j Biology under the direction of Professor Martin, at Johns Hop-
j kins University. So interesting and suggestive was the general
method employed in this course which, in its main outlines, had
been marked out by Huxley and Martin ten years before, that
we were persuaded that beginners in biology should always be in-
troduced to the subject in some similar way. The present work
thus owes its origin to the influence of the authors of the
"Elementary Biology," our deep indebtedness to whom we
gratefully acknowledge.

It is still an open question whether the beginner should pur-
sue the logical but difficult course of working upwards from the
simple to the complex, or adopt the easier and more practical
method of Mrorking downwards from familiar higher forms.
Every teacher of the subject knows how great are the practical
difficulties besetting the novice, who, provided for the first time
with a compound microscope, is confronted with Yeast, Proto-
coccus, or Amoaba ; and on the other hand, how hard it is to sift
out what is general and essential from -the heterogeneous details
of a mammal or a flowering plant. In the hope of lessening the
practical difficulties of the logical method we venture to submit a
course of preliminary study, which we have used for some time
with our own classes, and have found practical and effective.

It has not been our ambition to prepare an exhaustive trea-
tise. We have sought only to lead beginners in biology from
familiar facts to a better knowledge of how living things are
built and how they act, such as may rightly take a place in gen-

iii

iv PREFACE TO THE FIRST EDITION.

eral education or may afford a basis for further studies in General
Biology, Zoology, Botany, Physiology, or Medicine.

Believing that biology should follow the example of physics
and chemistry in discussing at the outset the fundamental prop-
erties of matter and energy, we have devoted the first three
chapters to an elementary account of living matter and vital en-
ergy. In the chapters which follow, these facts are applied by
a fairly exhaustive study of a representative animal and plant, of
considerable, though not extreme, complexity— a method which
we believe affords, in a given time, a better knowledge of vital
phenomena than can be acquired by more superficial study of a
larger number of forms. We are satisfied that the fern and the
earthworm are for this purpose the best available organisms, and
that their study can be made fruitful and interesting. The last
chapter comprises a brief account of the principles and outlines,
of classification as a guide in subsequent studies.

After this introductory study the student will be well pre-
pared to take up the one-celled organisms, and can pass rapidly
over the ground covered by such works as Huxley and Martin's
"Practical Biology," Brooks's "Handbook of Invertebrate
Zoology," Arthur, Barnes and Coulter's "Plant Dissection," or
the second part of this book, which is well in hand and will
probably be ready in the course of the following year.

The directions for practical study are intended as suggestions,
not substitutes, for individual effort. We have striven to make
the work useful as well in the class-room as in the laboratory ,
and to this end have introduced many illustrations. The gener-
osity of a friend has enabled us to enlist the skill of our friend
Mr. James H. Emerton, wrho has drawn most of the original
figures from nature, under our direction. We have also been
greatly aided in the preparation of the figures by Mr. William
Glaus of Boston.

SEPTEMBER, 1886.

PREFACE TO THE SECOND EDITION.

IT was originally our intention to publish this work in two
parts, the first, which appeared in 1886, being intended as an
introduction, while the second was to form the main body of the
work and to include the study of a series of type-forms. The
pressure of other work, however, delayed the completion of the
second part, and meanwhile several laboratory manuals appeared
which hi large measure obviated the need of it. Nevertheless
the use of the introductory volume by teachers of Biology,
and its sale, slowly but steadily increased. It soon appeared,
however, that in some cases the work was being employed not
merely as an introduction, as its authors intended, but as a
complete course in itself; though the wish was often expressed
that the number of types were somewhat larger. These facts,
and the many obvious defects in the original volume, induced
us to undertake the preparation of a second and extended edition.

With increased experience our ideas have undergone some
change. We are as firmly convinced as ever that General Biol-
ogy, as an introductory subject, is of the very first importance ;
but we are equally persuaded that it must not trespass too far
upon the special provinces of Zoology and Botany. The present
edition, therefore, differs from the original in these respects:
first, while the introduction has been extended so as to in-
clude representatives of the unicellular organisms (Amoeba,
Infusoria, Protococcus, Yeasts, Bacteria), the publication of a
second volume has been abandoned. It is hoped that the work
as thus extended may serve a double purpose, viz., either to
be used as an introduction to subsequent study in Zoology, Bota-
ny, or Physiology ; or as a complete elementary course for
general students to whom the minutiae of these more special sub-
jects are of less importance than the fundamental facts of vital
structure and function. We believe that a sound knowledge of

vi PREFACE TO THE SECOND EDITION.

these facts can be conveyed by the method of study here out-
lined ; but we must emphatically insist that neither this nor any
other method will give good results unless rightly used, and that
this work is not designed to be a complete text-book. Probably
few teachers will find it desirable to go over the whole of the
ground here laid out, and we hope that still fewer will be inclined
to confine their work strictly to it. Even in a brief course the
student may, after going over certain portions of this work, be
made acquainted with the leading types of plants and animals ;
and this may be rapidly accomplished if the introductory work, '
however limited, has been carefully done. In extended courses
we have sometimes found it desirable to postpone certain parts of
the introductory work, returning to them at a later period.

A second modification consists in placing the study of the
animal before that of the plant, which plan on the whole appears
desirable, especially for students who have not been well trained
in other branches of science. The main reason for this lies in the
greater ease with which the physiology of the animal can be ap-
proached ; for there is no doubt that beginners find the nutritive
problems of the plant abstruse and difficult to grasp until a cer-
tain familiarity with vital phenomena has been attained ; while
most of the physiological activities of the animal can be readily
illustrated by well-known operations of the human body.

The third change is the omission of the laboratory directions,
these having been found unsuitable. The needs of different
teachers differ so widely that it is impossible to draw up a scheme
that shall answer for all. In place of the laboratory directions for
students we have therefore given, in an appendix, a series of prac-
tical suggestions to teachers, leaving it to them to work out de-
tailed directions, if desired, by the help of the standard labora-
tory manuals. These suggestions are the result of a good deal of
experience on the part of many teachers besides ourselves, and
we hope they will be found useful in procuring and preparing
material (often a matter of considerable difficulty), and in decid-
ing just what the student may reasonably be expected to do.

For the rest, the original matter has been thoroughly revised,
numerous errors have been corrected, and many additions made,
particularly on the physiological side.
SEPTEMBER, 1895.

TABLE OF CONTENTS.

CHAPTER I.

INTRODUCTORY.

Living things and lifeless things. The contrast and the likeness between
living matter and lifeless matter. The journey of lifeless matter
through living things. Analogy between a fountain, a flame or a
whirlpool, and a living organism. Living matter is lifeless matter in
a peculiar state or condition. Its characteristic properties. Biology,
its scope and its subdivisions. The Biological sciences. The relations
of Biology to Zoology and Botany, Morphology and Physiology.
Definitions and inter-relations of the biological sciences. Psychol-
ogy, Sociology. Definition of General Biology ..... „,,?..., ........

CHAPTER II.

THE STRUCTURE OF LIVING THINGS.

Their occurrence and their size. Organisms composed of organs. Func-
tions. Organs composed of tissues. Differentiation. Tissues com-
posed of cells. Definitions. Unicellular organisms. Living organ-
isms contain lifeless matter. Lifeless matter occurs in living
tissues and cells. Examples. Lifeless matter increases relatively
with age. Summary statement of the structure of living things.
The organism as a whole — the Body — more important than any of its
parts o o

CHAPTER III.
PROTOPLASM AND THE CELL.

Protoplasm " the physical basis of life." Historical sketch. The com-
pound microscope and the discovery of cells in cork. The achromatic
objective. The cell-theory of Schleiden and Schwann. Virchow
and Max Schultze. Modern meaning of the term " cell." The dis-
covery of protoplasm and sarcode and of their essential similarity.

vii

TABLE OF CONTENTS,

PAQE

Purkinie. Von MohL Cohn. Schultze. Appearance and structure
^protoplasm. A typical cell. Itsparts. Cytoplasm and the nucleus.
The origin of cells. Segmentation of the egg, differentiation of the
tissues the genesis of the " body," and the physiological division of
labor Protoplasm at work. Muscular contractions. Amoeba on i
travels. "Rotation" in Nitella and Anackaris. " Circulation "of
the protoplasm in hair-cells of spiderwort. Ciliary motion. The
sources of protoplasmic energy. Metabolism and its phases. Vital
energy does not imply a "vital force." The chemical relations of
protoplasm: proteids, carbohydrates, and fats. Physical Relations:
temperature, moisture, electricity, etc. The protoplasm of plants and
of animals similar but not identical 20

CHAPTER IV.

THE BIOLOGY OF AN ANIMAL: THE COMMON EARTHWORM.

A representative animal. Earthworms taken ns a type. Their wide dis-
tribution. The common earthworm. Its name ; habitat ; habits ;
food; castings; influence on soils; burial of objects; senses. Its
differentiation: autero- posterior and dorso-ventral. Its symmetry:
bilateral and serial. Plan of the earthworm's body. Organs of the
body and the details of their arrangement in systems : alimentary ;
circulatory; excretory, respiratory; motor; nervous; sensitive; etc.. 41

CHAPTER V.

TEE BIOLOGY OF AN ANIMAL: THE COMMON EARTHWORM (Continued).

Definition of reproduction. The germ-cells. Sexual and asexual repro-
duction. Regeneration. The reproductive system of the earthworm.
Its copulation and egg-laying. The process of fertilization, and the
segmentation or cleavage of the egg. The making of the body. The
gastrula. The three germ-layers : ectoblast, entoblast, mesoblast.
Brief statement of the phenomena of cell-division, and of nuclear
division or karyokinesis. The making of the organs. The fate of
the germ-layers. The germ-plasm . „ 73

CHAPTER VI.

THE BIOLOGY OF AN ANIMAL: THE COMMON EARTHWORM (Continued).

The microscopic anatomy or histology of the earthworm. The funda-
mental animal tissues and their constituent cellular elements. Epi-
thelial, muscular, nervous, germinal, blood, and connective tissues,
and their distribution in the various organs. Microscopic structure
of the body-wall ; of the alimentary canal ; of the blood-vessels ; of
the dissepiments ; of the nervous system, ganglia ; etc. 90

TABLE OF CONTENTS

CHAPTER VII.
THE BIOLOGY OF AN ANIMAL: THE COMMON EARTHWORM vO

FAQS

General Physiology. The animal and its environment. Definitions.
Adaptation, structural and functional, of organism to environment.
Origin of adaptations. Effect of their persistence and accumulation.
Natural selection through the survival of the fittest. The need of an
income of food to supply matter and energy. Nature of the income.
The food and its journey through the body. Alimentation. Diges-
tion and absorption. Circulation. Metabolism. The outgo. Inter-
action of the animal and the environment. Summary 97

CHAPTER VIII.
THE BIOLOGY OF A PLANT: THE COMMON BRAKE OR FERN.

A representative plant. Ferns taken as a type. Their wide distribution.
The common brake. Its name, habitat, size, etc. General morphol-
ogy of its body. Its differentiation, autero-posterior and dorso-ventral.
Its bilateral symmetry. The underground stem. Origin and arrange-
ment of the leaves. Internal structure of the rhizome and the three
great tissue-systems. The elementary tissues of plants. Histology of
the rhizome. Roots and branches. Embryonic tissue and the apical
cell. How the rhizome grows. The frond or leaf of Pteru and its
structure. Chlorophyll-bodies. Stomata. Veins 105

CHAPTER IX.

THE BIOLOGY OF A PLANT: THE COMMON BRAKE (Continued).

The various methods of reproduction in Pteris. Sporophore^ and
oOphore. Alternation of generations. Sporangia. Spores. Ger-
mination of the spores. Protonema. Prothulliiim. The sexual
organs. Antheridia. Male germ-cells. Archegonia. Female germ-
cells. Fertilization. Segmentation. Differentiation of the tissues.
The making of the body 130

CHAPTER X.

THE BIOLOGY OF A PLANT: THE COMMON BRAKE (Continued).

Physiology. The fern and its environment. Its adaptation. A defini-
tion of life. The need of an income of matter and energy. Income
of Pteris. Its power of making foods, especially starch. The circu-
lation of foods through the plant-body. Metabolism. Outgo. Res-
piration. Interaction of the fern and the environment. Special

x TABLE OF CONTENTS.

PA6»

physiology of the tissue-systems and of reproduction The question
of old a?e A comparison of the fern with the earthworm and of
plan* in gentral with animals in general. The physiological im-
portance of the chlorophylless plants

CHAPTER XI.

THE UNICELLULAR ORGANISMS.

The multicellular body. Its origin in continued, but incomplete, cell-
division. The unicellular body. Its origin traced to compete cell-
division The multicellular body and the unicellular body as
individuals. Unicellular forms physiologically " organisms." Special
importance of their structural simplicity. " Organisms redu to
their lowest terms. " l

CHAPTER XII.
UNICELLULAR ANIMALS.

A. AM<EBA.

General Account. Habitat, Form. The " Proteus animalcule." Ap-
pearance. Pseudopodia. Locomotiou. Foods. The encysted state.
Structure of the unicellular body. Cytoplasm. Nucleus. Vacuoles.
Reproduction by fission. Physiology. The fundamental physiological
properties of protoplasm as displayed in Amoeba. The question of
old age. Related forms. The Rhizopoda or pseudopodial Protozoa.
Arcella. Difflugia. The "sun-animalcule." The Foramenifera.
The Radiolaria • 158

CHAPTER XIII.

UNICELLULAR ANIMALS (Continued).
B. INFUSORIA.

General account. Habitat. The "slipper-animalcule." The "bell-
animalcule." Paramcecium. Its form, structure, and habits. Cyto-
plasm; trichocysts; vacuoles; nuclei; mouth; oesophagus; anal spot.
The encysted state. Reproduction by again ogenesis; by conjugation;
amphimixis. Vorticella. Its form, structure, etc. Its reproduction
by fission, endogenous division, and conjugation. Microgamete and
macrogamete. Related forms. Euglena; Zoothamnion ; Carchesium;
Epistylis; etc. Physiology of the Infusoria. Herbivorous, carniv-
orous, and omnivorous infusoria. Analogy with higher forms. The
problem of chlorophyll in animals. Symbiosis. Vegetating animals.
The claim of unicellular animals to be regarded as unicellular "or-
ganisms"; organs in the cell; etc

TABLE OF CONTENTS. XI

CHAPTER XIV.

UNICELLULAR PLANTS.

A. PROTOCOCCUS.

PAGB

General account. Habitat. Morphology. Structure. Motile and non-
motile states. Reproduction by fission. Cell-aggregates. Physi-
ology. Income and outgo. The making of starch from inorganic
matters. The fundamental physiological properties of protoplasm as
displayed by plants Comparison of Protococcus with Amoeba, and
chlorophyll-bearing plants in general with animals in general. Other
unicellular chlorophyll-bearing plants: diatoms; desinids; Chroococ-
cus; Glceocapsa; etc 178

CHAPTER XV.

UNICELLULAR PLANTS (Continued).
B. YEAST.

General account. Wild yeast and domesticated yeast. Microscopical
examination of a yeast-cake. Morphology of the yeast cell. Cyto-
plasm and nucleus. Reproduction by budding and by spores. Physi-
ology. Yeast and the environment. Dried yeast. Income. Meta-
bolism. Outgo. The minimal nutrients of yeast compared with
those of Protococcus and Amoeba. Why yeast is regarded as a plant.
Top yeast Bottom yeast. Wild yeasts. Red yeast. Fermentation
and ferments. Unicellular plants not necessarily at the bottom of
the scale of life; etc 184

CHAPTER XVI.

UNICELLULAR PLANTS (Continued).
C. BACTERIA.

The smallest, most numerous, and most ubiquitous of known living
things. Their abundance in earth, air, milk, water, etc. Comparison
of their work in soils with that of earthworms. Parasitic and sapro-
phytic bacteria. Their botanical position. Sanitary and economic
importance. Morphology. Structure. Cytoplasm and nucleus.
Cilia. Their size. Swarming and the resting stages. Reproduction.
Endospores. Arthrospores. Physiology. Income. Metabolism.
Outgo. Ferments. Fermentation. Putrefaction. Disease. One
species capable of living upon inorganic matter. Related forms.
Why bacteria are regarded as plants. The relations of bacteria to
temperature, moisture, poisons, etc. Sterilization, Pasteurizing,
disinfection, filtration, etc 192

PAGE

XJi TABLE OF CONTENTS.

CHAPTER XVII

A HAY INFUSION.

General account. Results of microscopical examination. Turbidity.
Odor. Color. Constituents. The scene of important physical,
chemical, and biological phenomena. Previous history of the hay
and the water. Effect of bringing them together. Causes of tur-
bidity, color, odor, etc. Aerobic and anaerobic bacteria thrive.
Infusoria multiply and devour them. Carnivorous infusoria attack
the herbivorous. The struggle for existence. Hay a green plant
and the source of food. Quiet finally supervenes. How nutritive
equilibrium may be preserved or disturbed. The hay-infusion an
epitome of the living world .......................................

APPENDIX.
SUGGESTIONS FOR LABORATORY STUDIES AND DEMONSTRATIONS.

Books for the laboratory. Time required for General Biology ....... ---- 205

Special suggestions for laboratory work, etc., upon the subjects treated
in the several chapters as outlined above, viz.:

Chapter I. Introductory ..................................... 205

II. Structures of Living Organisms .................... 206

III. Protoplasm and the Cell ............................ 307

IV. -VIII. The Earthworm ............................. 210

IX.-XI. The Fern .......................... . .......... 213

XII. Amoeba ........................................... 216

XIII. Infusoria .......................... . .............. 217

XIV. Protococcus ....................................... 220

XV. Yeast ............. . .............................. 221

XVI. Bacteria ........................................... 223

XVII. A Hay Infusion ................................... 223

INSTRUMENTS AND UTENSILS ......................... „ .............. 220

REAGENTS AND TECHNICAL METHODS ................................. 221

INDEX .............................. ...... .................... .. 227

GENERAL BIOLOGY.

CHAPTER I.
INTRODUCTORY.

WE know from common experience that all material things
are either dead or alive, or, more accurately, that all matter is
either lifeless or living ; and so far as we know, life exists only
as a manifestation of living matter. Living matter and lifeless
matter are everywhere totally distinct, though often closely as-
sociated. The most careful studies have on the whole rendered
the distinction more clear and striking, and have demonstrated
that living matter never arises spontaneously from lifeless matter,
but only through the immediate influence of living matter already
existing. And so, whatever may have been the case at an earlier
period of the earth's history, we are justified in regarding the
present line between living and lifeless as one of the most
clearly defined and important of natural boundaries.

The Contrast between Living Matter and Lifeless Matter is made
the ground for a division of the natural sciences into two great
groups, viz. : the Biological Sciences and the Physical Sciences,
dealing respectively with living matter and lifeless matter. The
biological sciences (p. 7) are known collectively as Biology
(/?z'os, life; Ao^o?, a discourse), which is therefore often de-
fined as the science of life, or of living things, or of living mat-
ter. But living matter, so far as we know, is only ordinary
matter which has entered into a peculiar state or condition.

2 INTROD UCTOR T.

And hence biology is more precisely defined as the science which
treats of matter in the living state.

The Relationship between Living and Lifeless Matter. Al-
though living matter and lifeless matter present this remarkable
contrast to one another, they are most intimately related, as a
moment's reflection will show. The living substance of the human
body, or of any animal or plant, is only the transformed lifeless
matter of the food which has been taken into the body and has
there assumed, for a time, the living state. Lifeless matter in
the shape of food is continually streaming into all living things
on the one hand and passing out again as waste on the other.
In its journey through the organism some of this matter enters
into the living state and lingers for a time as part of the body-
substance. But sooner or later it dies, and is then for the most
part cast out of the body (though a part may be retained within
it, either as an accumulation of waste material, or to serve some
useful purpose). Matter may thus pass from the lifeless into the
living state and back again to the lifeless, over and over in never-
ending cycles. A living plant or animal is like a fountain or a
flame into which, and out of which, matter is constantly stream-
ing, while the fountain or the flame maintains its characteristic
form and individuality. It is " nothing but the constant form of
a similar turmoil of material molecules, which are constantly
flowing into the organism on the one side and streaming out on
the other. . . . It is a sort of focus to which certain material par-
ticles converge, in which they move for a time, and from which
they are afterward expelled in new combinations. The parallel
between a whirlpool in a stream and a living being, which has
often been drawn, is as just as it is striking. The whirlpool is
permanent, but the particles of water which constitute it are in-
cessantly changing. Those which enter it on the one side are
whirled around and temporarily constitute a part of its indi-
viduality ; and as they leave it on the other side, their places are
made good by newcomers. ' ' (Huxley. )

How then is living matter different from lifeless matter ?
The question cannot be fully answered by chemical analysis, for
the reason that this process necessarily kills living matter, and
the results therefore teach us little of the chemical conditions ex-
isting in the matter when alive. Analyses, nevertheless, bring

LIVING MATTER. 3

to light several highly important facts. It is likely that living
matter is a tolerably definite compound of a number of the
chemical elements, and it is probably too low an estimate to say
that at least six elements must unite in order that life may ex-
ist. Moreover, only a very few out of all the elements are able,
under any circumstances, to form this living partnership.

The most significant fact, however, is that there is no loss of
weight when living matter is killed. The total weight of the
lifeless products is exactly equal to the weight of the living sub-
stance analyzed, and if anything has escaped at death it is im-
ponderable, and, having no weight, is not material. It follows
that living matter contains no material substance peculiar to it-
self, and that every element found in living matter may be found
also, under other circumstances, in lifeless matter.

Considerations like these lead us to recognize a fundamental
fact, namely, that the terms living and lifeless designate two
different STATES or CONDITIONS of matter. We do not know, at
present, what causes this difference of condition. But so far as
the evidence shows, the living state is never assumed except
under the influence of antecedent living matter, which, so to
speak, infects lifeless matter and in some way causes it to as-
sume the living state.

Distinctive Properties of Living Matter. Those properties of
living matter which, taken together, distinguish it absolutely
from every form of lifeless matter, are :

1. Its chemical composition.

2. Its power of waste and repair, and of growth.

3. Its power of reproduction.

Living matter invariably contains substances known as pro-
teids, which are believed to constitute its essential material basis
(see p. 33). Proteids are complex compounds of Carbon, Oxy-
gen, Hydrogen, Nitrogen, Sulphur, and (in some cases at any
rate) Phosphorus.

It has been frequently pointed out that each of these six elements is
remarkable in some way : oxygen, for its vigorous combining powers ;
nitrogen, for its chemical inertia ; hydrogen, for its great molecular
mobility ; carbon, sulphur, and phosphorus, for their allotropic properties,
etc. All of these peculiarities may be shown to be of significance when
considered as attributes of living matter. (See Herbert Spencer, Principles
of Biology, vol. i.)

4 INTRODUCTORY.

It is not, however, the mere presence of proteids which is
characteristic of living matter. White-of-egg (albumen) contains
an abundance of a typical proteid and yet is absolutely lifeless.
Living matter does not simply contain proteids, but has the
power to manufacture them out of other substances ; and this is
a property of living matter exclusively.

The waste and repair of living matter are equally character-
istic. The living substance continually wastes away by a kind
of internal combustion, but continually repairs the waste. More-
over, the growth of living things is of a characteristic kind, dif-
fering absolutely from the so-called growth of lifeless things.
Crystals and other lifeless bodies grow, if at all, by accretion, or
the addition of new particles to the outside. Living matter
grows from within by intussusception, or the taking-in of new-
particles, and fitting them into the interstices between those
already present, throughout the whole mass. And, lastly, liv-
ing matter not only thus repairs its own waste, but also gives
rise by reproduction to new masses of living matter which,
becoming detached from the parent mass, enter forthwith upon
an independent existence.

We may perceive how extraordinary these properties are by
supposing a locomotive engine to possess like powers : to carry-
on a process of self- repair in order to compensate for wear ; to
grow and increase in size, detaching from itself at intervals
pieces of brass or iron endowed with the power of growing up
step by step into other locomotives capable of running them-
selves, and of reproducing new locomotives in their turn. Pre-
cisely these things are done by every living thing, and nothing
like them takes place in the lifeless world.

Huxley has given the best statement extant of the distinctive properties
of living matter, as follows :

" 1. Its chemical composition— containing, as it invariably does, one
or more forms of a complex compound of carbon, hydrogen, oxygen, and
nitrogen, the so-called protein (which has never yet been obtained except
as a product of living bodies), united with a large proportion of water,
and forming the chief constituent of a substance which, in its primary
unmodified state, is known as protoplasm.

l| 2. Its universal disintegration and waste by oxidation, and Us con-
comitant reintegration by the intussusception of new matter. A process
of waste resulting from the decomposition of the molecules of the proto-

LIVING MATTER. 5

plasm in virtue of which they break up into more highly oxidated products,
which cease to form any part of the living body, is a constant concomitant
of life. There is reason to believe that carbonic acid is always one of these
waste products, while the others contain the remainder of the carbon, the
nitrogen, the hydrogen, and the other elements which may enter into the
composition of the protoplasm.

" The new matter taken in to make good this constant loss is either a
ready-formed protoplasmic material, supplied by some other living being,
or it consists of the elements of protoplasm, united together in simpler
combinations, which constantly have to be built up into protoplasm by the
agency of the living matter itself. In either case, the addition of molecules
to those which already existed takes place, not at the surface of the living
mass, but by interposition between the existing molecules of the latter. If
the processes of disintegration and of reconstruction which characterize
life balance one another, the size of the mass of living matter remains sta-
tionary, while if the reconstructive process is the more rapid, the living
body grows. But the increase of size which constitutes growth is the
result of a process of molecular intussusception, and therefore differs alto-
gether from the process of growth by accretion, which may be observed in
crystals, and is effected purely by the external addition of new matter ; so
that, in the well-known aphorism of Linnaeus, the word ' grow ' as applied
to stones signifies a totally different process from what is called ' growth '
in plants and animals.

" 3. Its tendency to undergo cyclical changes. In the ordinary course
of nature, all living matter proceeds from pre-existing living matter, a
portion of the latter being detached and acquiring an independent exist-
ence. The new form takes on the characters of that from which it arose ;
exhibits the same power of propagating itself by means cf an offshoot ;
and, sooner or later, like its predecessor, ceases to live, and is resolved
into more highly oxidated compounds of its elements.

"Thus an individual living body is not only constantly changing its
substance, but its size and form are undergoing continual mollifications,
the end of which is the death and decay of that individual ; thecoontinua-
tion of the kind being secured by the detachment of portions which tend
to run through the same cycle of forms as the parent. No forms of matter
which are either not living or have not been derived from living matter
exhibit these three properties, nor any approach to the remarkable phe-
nomena defined under the second and third heads." (Encyclopaedia Bri-
tannica, 9th ed., art. " Biology," vol. iii. p. 679.)

For the purposes of biological study life must be regarded as
a property of a certain kind of compounded matter. But we
are forced to regard the properties of compounds as the result-
ants of the properties * their constituent elements, even though
we cannot well imagine how such a relation exists ; and so in the

Q INTRODUCTORY.

long-run we have to fall back upon the properties of carbon,
hydrogen, nitrogen, oxygen, etc., for the properties of living

matter.

Scope of Biology. The Biological Sciences. It follows from
the broad definition given to Biology that this science includes
the study of whatever pertains to living matter or to living
things. It considers the forms, structures, and functions of living
things in health and in disease ; their habits, actions, modes of
nutrition ; their surroundings and distribution in space and time,
their relations to the lifeless world and to one another, their
sensations, mental processes, and social relations, their origin and
their fate, and many other topics. It includes both zoology and
botany, and deals with the phenomena of animal and vegetal life
not only separately, but in their relations to one another. It
includes the medical sciences and vegetal pathology.

The field covered by biology as thus understood is so wide as
to necessitate a subdivision of the subject into a number of principal
branches which are usually assigned the rank of distinct sciences.
These are arranged in a tabular view on p. 7. The table shows
two different ways of regarding the main subject, according as
the table is read from left to right or vice versa. Under the more
usual arrangement biology is primarily divided into zoology and
botany, according as animals or plants, respectively, form the
subject of study. Such a division has the great advantage of
practical convenience since, as a matter of fact, most biologists
devote their attention mainly either to plants alone or to animals
alone. From a scientific point of view, however, a better sub-
division is into Morphology (yuop0//, form', Adyo?, a discourse)
and Physiology ((frvais, nature; Xoyos, a discourse). The
former is based upon the facts of form, structure, and arrange-
ment, and is essentially statical ; the latter upon those of action
or function, and is essentially dynamical. But morphology and
physiology are so intimately related that it is impossible to sepa-
rate either subject absolutely from the other.

Besides the sub-sciences given in the table a distinct branch
called Etiology is often recognized, having for its object the in-
vestigation of the causes of biological phenomena. But the sci-
entific study of every phenomenon has for its ultimate object the
discovery of its cause. ^Etiology is therefore inseparable from

THE BIOLOGICAL SCIENCES.

Anatomy. ") 1

The science of struc-

ture ; the term being

usually applied to the

coarser and more ob-

vious composition of
plants or animals.

Histology.

Microscopic anatomy.

The ultimate optical

analysis of structure

by the aid of the

m i c r o s c op e ; sepa-

rated from anatomy

only as a matter of

convenience.

Taxonomy or Classifi-

cation.

The classification of

living things. Based

chiefly on phenomena

' Morphology.

of structure.

Botany.

The science

Distribution.

The science

of form,
structure,

Considers the position
of living things in

of vegetal
living

etc.

Essentially
statical.

space and time, their
distribution over the
present face of the

matter or
• plants.

earth and their distri-

bution and succession

at former periods, as

displayed in fossil re-

mains.

•

Embryology.

Biology.

The
science of
all living
things ;
i.e., of

The science of develop-
ment from the germ.
Includes many mixed
problems pertaining
both to morphology
and physiology. At
present largely mor-
phological.

Biology.

The
science o£
all living
things ;

matter in
the living
state.

Physiology.
The special science of

i.e., of
matter in
the living
state.

the functions of the

Physiology.

individual in health
and in disease ; hence

The science

including Ikthvlogy.

Zoology.

of action or
function.
Essentially
dynamical.

Biychology.

The science of mental
phenomena.

The science
of animal
living
matter or

animals. -

Sociology.

The science of social

life, i.e., the life of

communities, wheth-

er of men or of lower

animals.

g INTRODUCTORY.

any of the several branches of biology and need not be assigned
an independent place.

Psychology and Sociology are not yet generally admitted to
constitute branches of biology, and it is customary and con-
venient to set them apart from it. The establishment of the
theory of evolution has clearly shown, however, that the study
of these sciences is inseparable from that of biology in the ordi-
nary sense. The instincts and other mental actions of the lower
animals are as truly subjects of psychological as of physiological
inquiry ; the complex social life of such animal communities as-
we find, for instance, among the bees and ants are no less truly
problems of Sociology.

It will be observed that in the scheme morphology and physi-
ology overlap; that is, there are certain biological sciences ia
which the study of structure and of action cannot be separated.
This is especially true of embryology, which considers the suc-
cessive stages of embryonic structure and also the modes of
action by which they are produced. And finally it must not be
forgotten that any particular arrangement of the biological sci-
ences must be in the main a matter of convenience only ; for it
is impossible to study any one order of phenomena in complete
isolation from all others.

The term General Biology does not designate a particular
member of the group of biological sciences, but is only a con-
venient phrase, which has come into use for the general introduc-
tory study of biology. It bears precisely the same relation to
biology that general chemistry bears to chemistry or general
physics bears to physics. It includes an examination of the gen-
eral properties of living matter as revealed in the structures and
actions of particular living things, and may serve as a basis for
subsequent study of more special branches of the science. It
deals with the broad characteristic phenomena and laws of life as
illustrated by the thorough comparative study of a series of
plants and animals taken as representative types; but in this
study the student should never lose sight of the fact that all the
varied phenomena which may come under his observation are in
the last analysis due to the properties of matter in the living
state, and that this matter and these properties are the real goal
of the study.

CHAPTER II.
THE STRUCTURE OF LIVING THINGS. ORGANISMS.

LIFELESS tilings occur in masses of the most various sizes
and forms, and may differ widely in structure and chemical com-
position. Living things, on the other hand, occur only in rela-
tively small masses, of which perhaps the largest are, among
plants, the great trees of California and, among animals, the
whales ; while the smallest are the micro-organisms or bacteria.
Moreover, the individual masses in which living things occur
possess a peculiar and characteristic structure and chemical com-
position which have caused them to be known as organisms, and
their substance as organic. All organisms are built up to a
remarkable extent in the same way and of the same materials,

FIG. 1. (After Sachs.)— Longitudinal section through the growing apex of a young
pine-shoot. The dotted portion represents the protoplasm, the narrow lines be^
ing the partition-walls composed of cellulose (C«H|oO6). (Highly magnified.)

and we may conveniently begin a study of living things with the
larger and more complex forms, which exhibit most clearly
those structural peculiarities to which we have referred.

Organisms composed of Organs. Functions. It is character-
istic of any living body — for example, a rabbit or a geranium —
that it is composed of unlike parts, having a structure which
enables them to perform various operations essential or accessory
to the life of the whole. The plant has stem, roots, branches,
leaves, stamens, pistil, seeds, etc. ; the animal has externally

9

10 THE STRUCTURE OF LIVING THINGS.

head trunk, limbs, eyes, ears, etc., and internally stomach, in-
testines, liver, lungs, heart, brain, and many other parts of

FIG. 2.— Cross-section through part of the young leaf of a fern (Pferte aquttina)f
showing thick-walled cells ; most of the walls are double. The granular sub-
stance is protoplasm. Most of the cells contain a large central cavity (vacuole)
filled with sap, the protoplasm having been reduced to a thin layer inside the
partitions. Nuclei are shown in some of the cells, and lifeless grains of starch
in others : ?i, nuclei ; 8, starch ; v, vacuole ; w, double partition-wall. ( X 500.)

the most diverse structure. These parts are known as organs,
and the living body, because it possesses them, is called an or-
ganism.

The word organism, as here used, applies best to the higher animals
and plants. It will be seen in the sequel that there are forms of life so.
simple that organs as here denned can scarcely be distinguished. Such
living things are nevertheless really organisms because they possess-
parts analogous in function to the well-defined organs of higher form.
(See p. 157.)

Since organisms are composed of unlike parts, they are said
to be heterogeneous in structure. They are also heterogeneous
in action, the different organs performing different operations-
called functions. For instance, it is the function of the stomach
to digest food, of the heart to pump the blood into the vessels,
of the kidneys to excrete waste matters from the blood, and
of the brain to direct the functions of other organs. A similar
diversity of functions exists in plants. The roots hold the

ORGANS AND TISSUES.

11

Fio. 3. (After Sachs.)-Cros8-section
through a group of dead, thick-
walled wood-cells from the stem of
maize. The cells contain only air or
water. (Highly magnified.)

plant fast and absorb various substances from the soil ; the stem
supports the leaves and flowers
and conducts the sap ; the leaves
absorb and elaborate portions of
the food; and the reproductive
organs of the flower serve to
form and bring to maturity seeds
destined to give rise to a new gen-
eration.

Heterogeneity of the kind
just indicated, accompanied by a
division of labor among the
parts, is one of the most char-
acteristic features of living things,
and is not known in any mass of
lifeless matter, however large and
complex.

Organs composed of Tissues. Differentiation. In the next
place, it is to be observed that the organs also, when fully
formed, are not homogeneous, but are in turn made up of
different parts. The human hand is an organ which consists
of many parts, differing widely in structure and function. On
the outside are the skin, the hairs, the nails ; inside are bones,
muscles, tendons, ligaments, blood-vessels, and nerves. The leaf
of a plant is an organ consisting of a woody framework (the
" veins ") which supports a green pulp, the whole being covered
on the outside by a delicate transparent skin. In like manner
every organ of the higher plants or animals may be resolved
into different parts, and these are known as tissues. The
tissues of fully formed organs are often very different from one
another, as in the cases just mentioned ; that is, they are well
differentiated; but frequently in adult organs, and always in those
which are sufficiently young, the tissues shade gradually into
one another, so that no definite line can be drawn between them.
In such cases they are said to be less differentiated. For ex-
ample, in the full-grown leaf of a plant the woody framework, the
green cells, and the skin exist as three plainly different tissues.
But in younger leaves these same tissues are less different, and
in very young leaves, still in the bud, there are no visible differ-

12 THE STRUCTURE OF LIVING THINGS,

ences and the whole organ is very nearly homogeneous In this
case the tissues are ^differentiated, though potentml y capable
of differentiation. In the same way, the tissues of the embry-

FIG. 4.— Cross-section through dead wood-like cells from the underground stem of a
fern (Pferi* aqu\\\na}. The walls are uncommonly thick and the protoplasm has
disappeared. The channels shown served in life to keep the cells in vital con-
nection, (x -t50.)

onic human hand are imperfectly differentiated, and at a very
early stage are undifferentiated.

Tissues composed of Cells. Finally, microscopical examina-
tion shows every tissue to be composed of minute parts known
as cells, which are nearly or quite similar to one another through-
out the whole tissue, and form the ultimate units into which the
tissues and organs, and hence the whole organism, become more
or less perfectly divided, somewhat as a nation is divided into
states and these into counties and townships.

CELLS. 13

It will be shown beyond that these ultimate units or cells
possess everywhere the same fundamental structure ; but they
differ immensely in form, size, and mode of action, not only in
different animals and plants, but even in different parts of the
same individual. As a rule, the cells of any given tissue are
closely similar one to another and are devoted to the same func-
tion, but differ from those of other tissues in form, size, arrange-
ment, and especially in function. Indeed, the differences be-
tween tissues are merely the outcome of the differences between
the cells composing them. The skin of the hand differs in ap-
pearance and uses from the muscle which it covers, because skin-
cells differ from muscle-cells in form, size, color, function, etc.
Hence a tissue may be denned as a group of similar cells hav-
ing a similar function.* As a rule, each organ consists of
several such groups of cells or tissues, but, as stated above, young
organs are nearly or quite homogeneous ; that is, all of the cells
are nearly or quite alike. It is only when the organ grows
older that the cells become different and arrange themselves in
different groups, — a process known as the differentiation of the
tissues. In the case of some organs — for instance the leaf of a
moss — the cells remain permanently nearly alike, somewhat as
in the embryonic condition, and the whole organ consists of a
single tissue.

What has been said thus far applies only to higher plants
and animals. But it is an interesting and suggestive fact that
there are also innumerable isolated cells, both vegetal and
animal, which are able to carry on an independent existence as
one-celled plants or animals. Physiologically these must cer-
tainly be regarded as individuals; but it is no less certain that
they are equivalent, morphologically, to the constituent cells of
ordinary many-celled organisms. It will appear hereafter that
the study of such unicellular organisms forms the logical ground-
work of all biological science. (See p. 157.)

Since organisms may be resolved successively into organs,
tissues and cells, it is evident that cells must contain living
matter. And a cell may be denned as a small mass of living
matter either living apart or forming one of the ultimate units

* Tissues frequently contain matters deposited between cells ; but these
have usually been directly derived from the cells, and vary as the cells vary.

14 THE STRUCTURE OF LIVING THINGS.

ofanorganism. The cell is an ^ orgamc individual of the first

uVtag td liLess Matter in the Living Organism. Since our
own bols and those of lower animals and of p ants are coin-
Ted of matter, it may be supposed, from what has been said
in the last chapter, that they are composed of living
matter This, however, is true only in part.
strictly true that every plant or animal contains living
matter but a little reflection will show that it contains
lifeless matter also. In the human body lifeless mat-
ter is found in the hairs, the ends of the nails, and
the outer layers of the skin, —structures which are
not simply devoid of feeling, as every one knows them
to be but are really lifeless in every sense, although
formino- part of a living body. Nor is lifeless mat-
ter confined to the exterior of the body. The mineral
matter of the bones is not alive; and this is true,
though less obviously, of many other parts, such as
the liquid basis or plasma of the blood, the fat (which
is never wholly absent), and various other forms of mat-
ter occurring in many parts of the body.

In lower animals examples of this truth occur on
every hand. The calcareous shells of animals like the
snail and the oyster ; the skeletons of
corals and sponges ; the hard outer crust
of insects, lobsters, and related animals ;
the scales of fish and reptiles; the
feathers, claws, and beaks of birds ; the
fur of animals — these are a few of the
countless instances of structures com-
posed wholly or in part of lifeless mat-
FIG.S. (AfterRanvier.)-Mus-ter ^^^ nevertheless enter into the

cle-cells. A, from the intes- 4 . ,

tine of a dog, in cross-sec- composition oi living animals.

won; B, single isolated ceil, Among plants like facts are even

from the intestine of a rab- & ^ r

bit, viewed from the side, more conspicuous. No one can doubt
that the outer bark of an oak is devoid

of life. The heart-wood of a tree is entirely dead, and even
in the so-called live wood, through which the sap flows, not only
is the solid part of the wood lifeless, but also the sap itself.

LIFELESS MATTER BETWEEN CELLS.

15

FlO. * (After SchSfer.)— Human cartilage (from head of metatarsal bone), c, cells ;
w, lifeless matrix. (X 600.)

FIG. 7. (Modified from Ranvier.)— Blood of frog, showing two forms of cells (cor-
puscles), one flattened and oval, one branched, floating in the lifeless plasma.
(X650.)

16 THE STRUCTURE OF LIVING THINGS.

Lifeless Matter in the Living Tissues. In the tissues the liv-
ing cells are seldom in contact one with another, but are more or
less completely separated by partitions of lifeless matter. This
may be seen in a section through some rapidly growing organ
like a young shoot (Fig. 1). The whole mass is formed of
nearly similar, closely crowded units or cells separated by very
narrow partitions. Each cell consists of a mass of granular,
viscid, living substance known as protoplasm, and a more solid,
rounded body, the nucleus.

In such a group of cells no tissues can be distinguished ; or,
rather, the whole mass consists of a single tissue (meristem),
which is almost entirely composed of living matter (protoplasm).
In older tissues the partitions often increase in thickness, as
shown in Fig. 2. In every case the partitions are composed of
lifeless matter which has leen manufactured and deposited by
tJie living protoplasm constituting the bodies of the cells. In
still older parts of the plant certain of the lifeless walls may
become extremely thick, the protoplasm entirely disappears, and
the whole tissue (wood) consists of
lifeless matter enclosing spaces filled
-"^J-^ -'Sy- witli a'r or water (Figs. 3 and 4).
V . ';$p : Among animals analogous cases

||. are common. The muscles of the
small intestine, for instance, (Fig.
5,) consist of bundles of elongated
V^':^: v cells (jilrefi) each of which is com-
posed of living matter surrounded
<i|H by a very thin covering (sheath) of
^^> lifeless matter. In cartilage or
J >v V ;T ^ -Silt gristle' wnicn covers the ends of
^ ;ft> . ;^ man-v 1)011CS (Fi£- °)' tlie oval cells
..->•' are very widely separated by the

^S; - •:/. deposition between them of large

FIG. 8. (Modified from Schenk )-Sec (*UantitieS °f 8°lid ^less "latter
tion of bone from the human femur forming what is known as the

SStflSJlSrss't*^:"** in Hood (Fig. 7) ti,e

•ru. Dmgram.tic. flattened or irregular cells (cor-

flllH , , pmdei) are separated by a lifeless

1 (plasma) m winch they float. In bone (Fig. s) the cell.

LIFELESS MATTER WITHIN CELLS.

17

have a branching, irregular form, and are separated by solid
calcareous matter which is unmistakably lifeless. These ex-
amples show that the lifeless matters of the body often occur in
the form of deposits between living cells by which they have
been produced. In all such cases the embryonic tissue consists,
at first of living cells in direct contact, or separated by only a
very small quantity of lifeless matter. In later stages the
cells may manufacture additional lifeless substance which
appears in the form of firm partition-walls between the cells,
or as a matrix, solid or liquid, in which the cells lie. When,
solid walls are present they are often perforated by narrow chan-
nels through which the protoplasmic cell-bodies remain in con-
nection. (See Figs. 4, 8, and 50.)

Lifeless Matter within Living Cells. Equally important with
the deposit of lifeless matter between cells is the formation of life-
less matter icithin cells, either (a) by the deposition of various sub-
stances in the protoplasm, or (fy by the direct transformation of
the whole mass of protoplasm. Examples of the first kind are

Fio. 9.— A group of cells from the stem of a geranium
(Pelargonium), showing lifeless substances (starch
and crystals) within the protoplasm. As in Fig. 2,
each cell contains a large central vacuole, filled
with sap ; c, groups of crystals of calcium oxalate ;
i.e., intercellular space ; n, nucleus; s, granules of
starch, (x 300.)

Fio. 10. (After Ranvier.) —
Group of "adipose cells"
from the tissue beneath the
skin ("subcutaneous con-
nective tissue") of an em-
bryo calf, showing drops of
fat in the protoplasm. /, fat-
drops (black) ; n, nuclei
(X550.)

mineral crystals (Fig. 9), grains of starch (Fig. 9), drops of
water, and many other substances found within the cells of
plants. Among animals drops of fat (Fig. 10) and calcareous

18 THE STRUCTURE OF LIVING THINGS.

or siliceous deposits are similarly produced. Indeed, there is
scarcely any limit to the number of lifeless substances which
may thus appear within the cells both of plants and animals.

The second case is of less importance, though of common
occurrence. A good example is found in the lining membrane
of the oesophagus of the dog (Fig. 11), which like the human
skin is almost entirely made up of closely crowded cells. Those

— P

FIG. ll.-Section through the inner coat of the gullet of a dog, showing : p, living
cells of the deeper layers; s, lifeless cells of the superficial layers; n, nucleus.

in the deepest part consist chiefly of living protoplasm very
similar to that of the young pine shoot (compare Fig. 1).
Above them the cells gradually become flattened until at the
surface they have the form of flat scales. As the cells become
flattened their substance changes. The protoplasm diminishes
in quantity and dies; so that near the surface the cells are
wholly dead, and finally fall off. In a similar manner are
formed the lifeless parts of nails, claws, beaks, feathers, and
many related structures. A hair is composed of cells essentially
like those of the skin. At the root of the hair they are alive,
but as they are pushed outwards by continued growth at the
root, they are transformed bodily into a dead, horny substance
forming the free portion of the hair. Feathers are only a com-
plicated kind of hair and are formed in the same way.

It is a significant fact that the quantity of lifeless matter in
the organism tends to increase with age. The very young plant
or animal probably possesses a maximum proportion of proto-
plasm, and as life progresses lifeless matter gradually accumulates
within or about it, — sometimes for support, as in tree-trunks and

THE STRUCTURE OF LIVING THINGS. 19

bony skeletons ; sometimes for protection as in oyster- and snail
shells ; sometimes apparently from sheer inability on the part of
the protoplasm to get rid of it. Thus we see that youth is lit-
erally the period of life and vigor, and age the period of com-
parative lifelessness.

Summary. The bodies of higher animals and plants are
subdivided into various parts (oi^gans) having different structure
and functions. These may be resolved into one or more tissues,
each of which consists of a mass of similar cells (or their deriva-
tives) having a similar function. The cells are small masses of
living matter, or protoplasm, which deposit more or less lifeless
matter either around (outside) them or within their substance.
In the former case the protoplasm may continue to live, or it
may die and be absorbed. In the latter case it may likewise live
on for a time, or may die, either disappearing altogether or leav-
ing behind a residue of lifeless matter.

The Organism as a Whole. Up to this point we have con-
sidered living organisms from an anatomical and analytical stand-
point, and have observed their natural subdivisions into organs,
tissues, and cells. We have now only to remark that these parts
are mutually interdependent, and that the organism as a whole
is greater than any of its parts. Precisely as a chronometer is
superior to an aggregate of wheels and springs, so a living organ-
ism is superior in the solidarity of its parts to a mere aggregate of
organs, tissues, and cells. We shall soon see that in the living
body these have had a common ancestry and still stand in the
closest relationship both in respect to structural continuity and
community of interest.

CHAPTER III

PROTOPLASM AND THE CELL.

IT has been shown in the last chapter that life is inherent in
a peculiar substance, protoplasm, occurring in definite masses or
cells. In other words, protoplasm is the physical basis of life,
and the cell is the ultimate visible structural unit. Protoplasm
and the cell deserve therefore the most careful consideration;
but because of the technical difficulties involved in their study
only such characteristics as are either obvious or indispensable to
the beginner will here be dwelt upon.

Historical Sketch. Organs and tissues are readily visible, but
in order to resolve tissues into cells something more than the
naked eye was necessary. The compound microscope came into
use about 1650, and in 1665 the English botanist Robert Hooke
announced that a familiar vegetal tissue, cork, is made up of
"little boxes or cells distinct from one another." Many other
observers described similar cells in sections of wood and other
vegetal tissues, and the word soon came into general use. It
was not until 1838, however, and as a consequence of a most
important improvement in the compound microscope, viz., the
invention of the achromatic objective, that cellular structure
came to be recognized as an invariable and fundamental charac-
teristic of living bodies. At this time the botanist Schleiden
brought forward proof that the higher plants do not simply con-
tain cells but are wholly made up of them or their products ; and
about a year later the zoologist Schwann demonstrated that the
same is true of animals. This great generalization, known as
the " cell-theory" of Schleiden and Schwann, laid the basis for
all subsequent biological study. The cell-theory was at first de-
veloped upon a purely morphological basis. Its application to
the phenomena of physiological action was for a time retarded

30

HISTORY OF "CELL" AND "PROTOPLASM." 21

by the misleading character of the term "cell." The word itself
shows that cells were at first regarded as cavities (like the cells
of a honeycomb or of a prison) surrounded by solid walls ; and
even Schleiden and Schwann had no accurate conception of their
true nature. Soon after the promulgation of the cell-theory,
however, it was shown that both the walls and the cavity might
be wanting, and that therefore the remaining portion, namely,
the protoplasm with its nucleus, must be the active and essential
part. The cell was accordingly defined by Virchow and Max
Schultze as " a mass of protoplasm surrounding a nucleus, ' ' and
in this sense the word is used to-day.* The word cell became
thereafter as inappropriate as it would be if applied to the honey
within the honeycomb or to the living prisoner in a prison-cell.
Nevertheless, by a curious conservatism, the term was and is re-
tained to designate these structures whether occurring in masses,
as segments of the plant or animal body, or leading independent
lives as unicellular organisms.

Protoplasm was observed long before its significance was
understood. The discovery of its essential identity in plants and
animals and, ultimately, the general recognition of the extreme
importance of the role which it everywhere plays, must be reck-
oned as one of the greatest scientific achievements of this cen-
tury. It was Dujardin who in 1635 first distinctly called atten-
tion to the importance of the "primary animal substance" or
"sarcode" which forms the bodies of the simplest animals.
Without clearly recognizing this substance as the seat of life, or
using the word protoplasm, he nevertheless described it as en-
dowed with the powers of spontaneous movement and con-
tractility. The word protoplasm (^pc5ros, first; n\acr)ji<x,
form) was apparently first used for animal substance by Purkinje
in 1839-40, and next by II. von Mohl, in 1840, to designate
the granular viscid substance occurring in plant-cells, although
both workers were ignorant of its full significance. In 1850
Colin definitely maintained not only that animal sarcode and
vegetal protoplasm were essentially of the same nature, but
also that this substance is the real seat of vitality and hence to
be regarded as the physical basis of life. To Max Schultze

* It is possible that in some of the lowest and simplest organisms even the
nucleus may be wanting as a distinctly differentiated body. See p. 193.

22 PROTOPLASM AND THE CELL.

(1860) is generally assigned the credit of having finally placed
this conclusion upon a secure basis; and by him the meaning of
the word Protoplasm was so extended as to include all living
matter, whether animal or vegetal. In this sense the word is
now universally employed.

Appearance and Structure. Protoplasm and cells differ
greatly in appearance in different plants and animals, as well as
in different parts and different stages of development of the
same individual. The appearance of protoplasm and the consti-
tution of the cell are as a rule

m ~----^/<x<vT^ most easily made out in very

young structures, such as the
eggs of some animals or in
the cells of young vegetal
shoots. The egg of the star-
fish, for example, (Fig. 12), is
a single isolated cell of nearly
typical form and structure.
It is a minute, nearly spheri-

Fio. 12.-Slightly diagrammatic figure of ca] ^Ody (_L. inch diameter)
the egg or ovum of a star-fish, showing the . , . , ,

structure of a typical cell, m, membrane; m wlllCJl tlirCC parts may be
n, nucleus ; p, protoplasm (cytoplasm). distinguished, VIZ.: (1) the

cell-body, which forms the bulk of the cell ; (2) the nucleus, a
rounded vesicular body suspended in the cell-body ; (3) the mem-
brane or cell-wall, which immediately surrounds the cell-body.
Of these three, the nucleus and cell- body are mainly composed
of protoplasm, while the membrane is a lifeless deposit upon the
exterior. The protoplasm of the cell-body is generally called
cell-plasm, or cytoplasm, that of the nucleus nudeoplasm; that
is, the living matter of the cell is differentiated into two different
but closely related forms of protoplasm, cytoplasm and nucleo-
plasm.

The Cytoplasm appears as a clear semifluid or viscid sub-
stance, containing numerous minute granules and of a watery
appearance, though it shows no tendency to mix with water.
Under very high powers of the microscope, especially after treat-
ment with suitable reagents, the clear substance is found to have
a definite structure, the precise nature of which is in dispute.
By some observers it is described as a fibrous meshwork or retic-

THE MINUTE ANATOMY OF THE CELL.

23

ulum, like a sponge ; by others as more nearly like an emulsion
or foam, consisting of a more solid framework enclosing innu-
merable minute separate spherical cavities tilled with liquid ; by
others still as composed of unbranched threads running in all
directions through a more liquid basis ; but its real nature is still
unknown.

It is evident that the visible structure of protoplasm gives no
hint of its marvellous powers as the seat of vital action, and we
are therefore compelled to infer that it is endowed with a chemi-
cal and molecular constitution extremely complex, and probably
far exceeding in complexity that of any lifeless substance.

The Nucleus is a rounded body suspended in the cell-sub-
stance ; it is distinguishable from the latter by its higher refrac-
tive power, and by the intense color it assumes when treated
with staining fluids. It is surrounded by a -very thin membrane,
and consists internally of a clear substance (achromatiri), through
which extends an irregular network of fibres (chromai/m). It
is especially these fibres which are stained by dyes. In the

FIG. 13. (After Sachs.)— Young growing cells from the extreme tip of a stonewort
(Chara). m, membrane; «, nuclei; p, protoplasm; i\ vacuole filled with sap.
(X550.)

meshes of the network is suspended in many cases a second
rounded body known as the nudeolus, which stains even more
deeply than the network itself.

The Membrane or Wall of the cell forms a rather thick sac,

24 PROTOPLASM AND THE CELL.

composed of a soft, lifeless material closely surrounding the cell
substance.*

As a second example we choose the growing point of a com-
mon water-plant (Chard), Fig. 13. This structure is composed
of cells which are more or less angular in outline as a result of
mutual pressure, but show otherwise an unmistakable similarity
to the egg-cell just described. They difl'er mainly in the fact
that the protoplasm of the larger cells contains rounded cavities,
known as vacuoles, filled with sap (v} ; also in the chemical com-
position cf the cell-walls (here consisting of "cellulose," a sub-
stance of rare occurrence among animals).

Origin of Cells and Genesis of the Body. The body of every
higher plant or animal arises from a single germ-cell (" egg,'*
" spore," etc.) more or less nearly similar to that of the star-
fish, described above, and originally forming a part of the parent
body. The germ-cell, therefore, in spite of endless variations in
detail, shows us the model after which all others are built ; for
it gives rise to all the cells of the body by a continued process-
of segmentation as follows :

The first step (Fig. 14) consists in the division of the egg
into two similar halves, which differ from the original cell only
in lacking membranes, both being surrounded by the membrane
of the original cell. Each of the halves divides into two, mak-
ing four in all ; these again into two, making eight, and so on
throughout the earlier part of the development. By this process-
(known as the cleavage or segmentation of the egg) the germ-
cell gives rise successively to 2, 4, 8, 1C>, 32, 64, etc., de-
scendants, forming a primitive body composed of a mass of
nearly similar cells, out of which, by still further division and
growth, the fully-formed body of the future animal is to be
built up. These cells are only slightly modified, but differ in
most animals from the typical germ-cell in having at first no sur-
rounding membranes. The membrane of the original germ-
cell meanwhile disappears.

* The word cell Las been used in Cbap. I and elsewhere to denote the
living matter within the membrane, the latter being considered a product of
the cell rather than an integral part of it. It is more usual to include the
membrane in a definition of the cell, and as a matter of convenience it is «>
included here.

DEVELOPMENT AND DIFFERENTIATION OF CELLS. 25

The embryonic body or embryo of every higher plant and ani-
mal is derived from the genii-cell by a process essentially like that
just described, though both the form of the cells and the order of
division are usually more or less irregular. In animals the cells

Fio. 14.— Cleavage or segmentation of an ovum, showing successive division of the
germ-cell (a) into two (b), four (c), and eight (<?)• Later stages are shown ate
and /. The first four figures are diagrammatic ; e and / are after Hatschek's fig-
ures of the development of a very simple vertebrate (A

thus formed are usually naked at first, though they often ac-
quire a membrane in later stages. Among plants, on the con-
.trary, the cells usually possess membranes from the first, prob-
ably because their need for a firm outer support is greater than
the need for free movement demanded by animals.*

Modification of the Embryonic Cells. Differentiation. The
close similarity of the embryonic cells does not long persist. As
development proceeds, the cells continually increasing in number
by division become modified in different ways, or differentiated,
to fit them for the many different kinds of work which they have
to do. Those which are to become muscle-cells gradually assume
an entirely different form and structure from those which are to
become skin-cells; and the future nerve- or gland-cells take
on still other forms and structures. The embryonic cells are
gradually converted into the elements of the different tissues —
this process being the differentiation of the tissues which has

* For a more precise account of cell-division see p. 83.

26 PROTOPLASM AND THE CELL.

already been mentioned on p. 11— and are in this way enabled
to effect a physiological division of labor.

The variations in form and structure which thus appear are
endlessly diversified. Cells may assume almost any conceivable
form, and there are even cells (e.g., Amahs, or the colorless
corpuscles of the blood) which continually change their form
from moment to moment. The variations in structure may in-
volve any or all of the three characteristic parts of the typical
cell, being at the same time accompanied by variations of form.
It is easy to understand, therefore, how cells may vary endlessly
in appearance, while conforming more or less closely to the same
general type.

Meanwhile the protoplasm itself undergoes extensive altera-
tion. Even in young cells, or in the germ-cell itself, it may
,~~ - j contain an admixture of other substances,

and these may entirely change their
character or (as is especially common in
plant-cells) may become more abun-
dant as the cell grows older, taking the
shape of fluid, solid, or even gaseous de-
posits. Common examples of such de-
posits are drops of water, oil, and resin
~n granules of pigment, starch, and solid
no. 15. (After Ranvier.)- Protekl mattfre> ^"1 crystals of mineral

Part of a single fibre of vol- Substances like Calcium OXalate, pllOS-

rSr£X£±! P''ate a"(1 "r'-onate, an.l silica. Bub-
n, nucleus. (xToo.) hies of gas sometimes appear in the pro-

toplasm, but this is exceptional. The living substance itself
often changes in appearance as the cells become differentiated.
The protoplasm of voluntary muscles (Fig. 15) is firm, clear,
non-granular, highly refractive, and arranged in alternating
bands or stripes of darker and lighter substance. In some cases
(e.g., the outer portions of the skin, or of a hair, as explained
in Chap. II) the modifications of the cell-substance becomes so-
groat that both its physical and chemical constitution are entirely
altered, and it is no longer protoplasm, but some form of lifeless
matter.

Protoplasm in Action. We may now briefly consider proto-
plasm from the dynamical or physiological point of view. We

fwarmlac fif rii rrm nnt ttiirr»l< flml snli<

PROTOPLASMIC MOVEMENTS.

27

know that living things are the seat of active changes, which
taken together constitute their life. In the last analysis these
changes are undoubtedly chemical actions taking place in the
protoplasm, which may or may not produce visible results.
There is no doubt that extensive and probably very complex
molecular actions go on in the protoplasm of young growing
cells, though it may appear absolutely quiescent to the eye, even
under a powerful microscope. In other cases, the chemical
action produces perceptible changes in the protoplasm, — for in-
stance, some form of motion, — just as the invisible chemical
action in an electrical battery may be made to produce visible
effects (light, locomotion, etc.) through the agency of an electrical
machine.

A familiar instance of protoplasmic movement is the contrac-
tion of a muscle. This process is most likely a change of molec-
ular arrangement, causing the muscle, while keeping its exact
bulk, to change its form, the two ends being brought nearer
together (Fig. 16). The visible change
of form is here supposed to be due to an
invisible change of molecular arrange-
ment, and this in turn to be coincident
witli chemical action taking place in the
living substance.

A striking and beautiful example
of movement in protoplasm occurs in
the simple organism known as Amoeba
(Fig. 84, p. 150). The entire body of
this animal consists of a mass of naked
protoplasm enclosing a nucleus, or
t sometimes two ; in other words, it is a

Fio/i6.-change of form in a &™&e »aked cell. The protoplasm of
contracting muscle. A, mus- an active Amceba- is in a state of cease-

cle in the ordinary or extend- ,.

ed state; B, the same muscle less movement, contracting, expanding,

when contracted. (Diagram.) flowmg? an(J changing the form of the

animal to such an extent that it is known as the "Proteus.^
animalcule. The whole movement is a kind of flux. A portion
of the protoplasm flows out from the mass, making one or more
prolongations (pseudopods) into which the remainder of the
protoplasm finally passes, so that the whole body advances in the

2g PROTOPLASM AND THE CELL.

direction of the flow. If particles of food be met with, the
protoplasm flows around them, and when they have been digested
within the body, the protoplasm flows onward, leaving the refuse
behind. Hour after hour and day after day this flowing may
go on, and there is perhaps no
more fascinating and suggestive
spectacle known to the biologist.
A similar change of form is ex-
hibited by the colorless corpuscles
of amphibian and other blood, in
which it may be observed, though
far less satisfactorily, if Amaebm
cannot be obtained. Among plants,
protoplasmic movements of perhaps
equal beauty may be observed.
One of the simplest is known as the
rotation of protoplasm, which may

FIG. 17.— A cell of a stonewort (Ifitd-
la) showing the rotation of proto-
plasm ; the arrows show the direc-
tion of the flow, m, membrane of
the cell; ri, nucleus, opposite to
which is a second ; p, protoplasm ; i\
large central vacuole filled with sap.

n

FIG. Yin.— Two cells and a part of a
third from the tip of a "leaf" of a
stonewort, showing rotation of the
protoplasm in the direction of the
arrows.

be studied to advantage in rather young cells of stoneworts (Chara
or Nitella). These cells have the form of short or elongated
cylinders which are often pointed at one end (Fig. 17). The

PROTOPLASMIC MOVEMENTS. 39

protoplasm is surrounded by a delicate membrane which thus
forms a sac enclosing the protoplasm. In very young cells the
protoplasm entirely tills the sac ; but as the cell grows older a
drop of liquid appears near the centre of the mass and increases
in size until the protoplasm is reduced to a thin layer (jsrimor-
dial utricle), lining the inner surface of the membrane (compare
Fig. 2). In favorable cases the entire mass of protoplasm is
eeen to be flowing steadily around the inside of the sac, as in-
dicated by the arrows in Fig. 17. It moves upwards on one
gide, downwards on the opposite side, and in opposite directions
across the ends, forming an unbroken circuit. The flow is ren-
dered more conspicuous by various granules and other lifeless
masses floating in the protoplasm and by the large oval nucleus
or nuclei, all of which are swept onward by the current in its
ceaseless round. A similar rotation of protoplasm occurs in many
other vegetal cells, one of the best examples being the leaf-cells
of Anacharis.

A second and somewhat more intricate kind of movement in
vegetal protoplasm is known as circulation. This differs from
rotation chiefly in the fact that the protoplasm travels not only in
a peripheral stream but also in strands which nin across through
the central space (vacuole) and thus form a loose network. Cir-

Fio. 18.— Flower-cluster (a) and single stamen (ft) of a cultivated spiderwort (Trades'
cantia). h, hairs upon the stamen, a, slightly reduced ; b, slightly enlarged

dilation is well seen in cells composing the hairs of various plants,
such as the common nettle (Urtica), the spiderwort (Trades-

30

PROTOPLASM AND THE CELL.

), the hollyhock (Althaea), and certain species of gourds
Mta). It may be conveniently studied in the hairs upon
the stamens of the cultivated spiderwort (Tradeacantia). The
flower of this plant is shown in Fig. 18, a, and one of the
stamens with its hairs at I. Each hair consists of a single row

.— Enlarged cells of the hairs from the stamens of the spiderwort. A, five
cells, somewhat enlarged, protoplasm not shown ; B and G, cells much more en-
larged, showing the circulation of protoplasm as indicated by the arrows; n,
nucleus.

of elongated cells covered by delicate membranes and connected
by their ends. As in Nitella, the protoplasm does not fill the
cavity of the sac, but forms a thin lining (primordial utricle)

CILIARY ACTION.

31

on its inner face (Fig. 19). From this layer delicate threads of
protoplasm reach into and pass through the central cavity, where
they often branch and are connected together so as to form a
very loose network. The nucleus (w) is embedded either in the
peripheral layer or at some point in the network, and the threads
of the latter always converge more or less regularly to it. In
active cells currents continually flow to and fro throughout the
whole mass of protoplasm. In the threads of the network gran-
ules are borne rapidly along, gliding now in one direction, now
in another ; and although the flow is usually in one direction in
any particular thread, no system can be discovered in the com-
plicated movements of the whole. In the larger threads the
curious spectacle often appears of two rapid currents flowing in
opposite directions on opposite sides of the same thread. The
currents in the thread may be seen to join currents of the pe-
ripheral layer which flow here and there, but without sthe regu-
larity observed in the protoplasm of Nitella. The protoplasmic
network also, as a whole, undergoes a slow but steady change of
form, its delicate strands slowly
swaying hither and thither, while
the nucleus travels slowly from
point to point.

Finally, we may consider an
example of a form of protoplas-
mic movement known as ciliary
action, which plays an important
role in our own lives and those
of lower animals and of some
plants. The interior of the tra-
chea, or windpipe, is lined by
cells having the form shown in
Fig. 20. At the free surface of
the cell (turned towards the cavi-
ty of the trachea) the protoplasm
is produced into delicate vibra-
tory filaments having a sickle-
shape when bent ; these are known as cilia (cilium, an eyelash).
They are so small and lash so vigorously as to be nearly or quite
invisible until the movements are in some way made sluggish.

FIG. 20. (After Klein.) -Three isolated
ciliated cells from the interior of the
windpipe of the cat. c, the cilia at the
free end; n, the nucleus; p, the proto-
plasm. (Highly magnified.)

32 PROTOPLASM AND THE CELL.

The movement is then seen to be more rapid and vigorous in one
direction than in the other, all the cilia working together like
the oars of a row-boat acting in concerted motion. By this
action a definite current is produced in the surrounding medium
(in this case the mucus of the trachea) flowing in the direction
of the more vigorous movement. In the trachea this movement
is upwards towards the mouth, and mucus, dust, etc. , are thus
removed from the lungs and windpipe. In many lower animals
and plants, especially in the embryonic state, cilia are used as
organs of locomotion, serving as oars to drive the organism
through the water. The male reproductive germs of plants and
animals are also propelled in a similar fashion.

In all these forms of vital action the protoplasm is visibly at
work. In most cases, however, no movements of the protoplasm
in cells can be detected. But it is certain from indirect evidence
that protoplasm is no less active in those modes of physiological
action that give no visible outward sign, as for example in an
active nerve-cell or a secreting cell. This activity being molec-
ular arid chemical is beyond the reach of the microscope, but it
is none the less real ; and the play of these invisible molecular
actions is doubtless far more tumultuous and complicated than the
visible movements of the protoplasmic mass displayed in Nitella,
or in a nettle-hair. It is of the utmost importance that the stu-
dent should attain to a full and vivid sense of the reality and
energy of this invisible activity even in protoplasm which (as is
ordinarily the case) under the closest scrutiny appears to be abso-
lutely quiescent.

The Sources of Protoplasmic Energy. Whence comes the
power required for protoplasmic action, and how is it expended?
The answer to this question can be given at this point only in
very general terms. It is certain that protoplasm works by
means of chemical actions taking place in its own substance;
and it is further certain that these actions are, broadly speaking,
processes of oxidation or combustion; for in the long run all
forms of protoplasmic action involve the taking up of oxygen
and the liberation of carbon dioxide. Energy is therefore set
free in living, active protoplasm somewhat as it is in the com-
bustion of fuel under the boiler of a steam-engine, and in this
process the protoplasm, like the coal, is gradually used up, disin-

CHEMICAL RELATIONS OF PROTOPLASM. 33

tegrates, and wastes away, giving off as waste matter the various
chemical products of the combustion, and liberating energy as
heat and mechanical work. The loss of substance is, however,
continually made good (much as the coal is replenished) by the
absorption of new substance in the form of food, which may
consist of actual protoplasm, derived from other living beings,
or of substances convertible into it. These substances are in
some unexplained way converted into protoplasm and thus
built into the living fabric.

To this dual process of waste (li fcatafiolism") and repair
("anabolism") is applied the term metabolism, which must be
considered as the most characteristic and fundamental property
of living matter. It is evident from the foregoing that meta-
bolism involves on the one hand a destructive action (katabol-
ism) through which protoplasm disintegrates and energv is set
free, and on the other hand a constructive action (anabolisni)
whereby new protoplasm is built up from the income of food and
fresh energy is stored. It is a most remarkable fact that as far
as known the constructive action resulting in the formation of
new protoplasm never takes place except through the immediate
agency of protoplasm already existing. In other words, there is
no evidence that k " spontaneous generation" or the production
of living from lifeless matter without the influence of antecedent
life ever takes place.' Xor is there any evidence that any energy
can be ' ' generated, ' ' but rather that the vital energy of living
things is only the transformed energy of their food, and that
"vital force" having an origin elsewhere than in such energy
does not exist.

Chemical Relations. We know nothing of the precise chemi-
cal composition of living protoplasm, because, as has been said
(p. 2), living protoplasm cannot be subjected to chemical analy-
sis without destroying its life. But the results of chemical ex-
aminations leave no doubt that the molecules of protoplasm are
highly complex and are probably separated 'from one another by
layers of water.

A. PROTEIDS. It has already been stated (p. 3) that the
characteristic products of the analysis of protoplasm are the
group of closely-related substances known &sproteids. But pro-
teids form only a small part of the total weight of any plant or

34

PROTOPLASM AND THE CELL.

animal, being always associated with quantities of other sub-
stances. Even the white of an egg, which is usually taken for
a typical proteid, contains only twelve per cent of actual proteid
matter, the remainder consisting chiefly of water. The follow-
ing table shows the percentage of proteids and other matters in
a few familiar organisms and their products :

PROXIMATE PERCENTAGE COMPOSITION OF SOME COMMON

SUBSTANCES.*
Arranged according to richness in Proteids.

1

2
3
4
5

6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2«
27

Water.

Pro-
.teids.

Carbo-
hy-
drates.

Fats.

Other
Sub-
stances.

Apples
Indian corn, aerial portion fresh.
Oysters, shells included
Turnips..
Melons

84.8
84 3
15.4
91.2
95 2
75 8
10.0

27^3
81.0
75.0
81.5
87 4

0.4
0.9
10
1.0
1.1
1.5
1.9
2.0
2.1
2.3
3.0
3 2
3.4
5 2
5.4
6.0
7.3
9.9
11.1
12.5
14 3
14.9
20.1
20.7
23.2
27.1
38.3

14.3
13.7
0.6
69
25
21.1
0.1
21.3
1.8
15 3
19.9
13.8
4 8
0.0

22i5
0.5

6^5

'<J! 5

57.4
2 4
9.0

0.0
0.5
0.2
0.2
06
0.4
0.1
0.2
0.1
0.5
0.8
0 6
3.7
0.3
0.5
1.6
0.9
1.1
10.8
293
1.1
24.8
5.4
8.1
2.1
i54
6.8

05
0.6
82 8
0.7
0.6
1.2

1:!

69.2
0.9
0.8
0.8
0.7
67.3
60 9
0.5
56.2
48.7
12.0
16.9
42.4
6.9
15
11.2
3.6

11

Sweet potatoes
Crayfish, whole

Clams, round, shells included...
Oats, aerial part fresh
Wrass, " " "

Peas, " " "

Cow's milk.

Flounder, whole. . .

27.2
33.0
70.0
34.1
40.3
656
41.3
42.2
534
09 5

'•-Ml

13.7
31.2
41.3

Poplar and elm leaves, fresh ...
Crab, whole
Brook trout, whole
Hen's eggs, shells included
Mutton 'rchops"
Chicken, whole .

Beef, heart.
Beef, liver

Beefsteak, round, lean
Beans
Cheese..
Cheese from skimmed milk .

All proteids have nearly the same chemical composition and
similar physical properties, however different may be the forms
of protoplasm in which they occur. The analysis of protoplasm,
or rather of the proteids which are its basis, teaches us really
nothing of its vital properties, but serves only to show the
chemical composition of the material basis by which these aro
manifested.

Proteids are so called from their resemblance to protein
(7rpo3ro?, first], a hypothetical substance first described and

ohnson'

., 1883.

PROTEIDS.

35

named by Mulder. According to Hoppe-Seyler they have ap-
proximately the following percentage composition :

c.

H.

N.

O.

S.

?oom

51.5

54 5

f:S

15.2
17 0

20.9
23 5

03

A small quantity of phosphorus is also very frequently present.
Associated with these elements are always small quantities of
various mineral substances which remain as the ash when proto-
plasm is burned ; but the nature of their relations to the other
elements is uncertain. The ash varies both in quantity and
chemical composition in different animals and plants. In the
white-of-egg the chief constituents of the ash are potassium chlo-
ride (KC1) and sodium chloride (XaCl), the former being much
in excess. The remainder consists of phosphates, sulphates, and
carbonates of sodium and potassium, with minute quantities of
calcium, magnesium, and iron, and a trace of silicon. Many
other mineral substances occur in association with other kinds of
proteids, but always in very small proportion. These salts are in
some way essential to the activity of protoplasm, as we know by
familiar experience. Man, like other animals and the plants,
requires certain mineral substances (e.g. common salt), but we
have no knowledge of the part these play in protoplasm.

It is important to note the close chemical similarity of animal
and vegetal proteids, because this is one reason for regarding
vegetal and animal protoplasm as essentially similar in other re-
spects. The following table, from Johnson after Gorup-Besanez
and Bitthausen, shows the percentage composition of various pro-
teids, and proves that the difference between vegetal and animal
proteids is chemically no greater than that between different
kinds of vegetal or different kinds of animal proteids :

PERCENTAGE COMPOSITION OF PROTEIDS.

C.

H.

N.

O.

S.

Animal albumen .

53.5

7.0

15.5

22.4

1.6

Vegetal "

53 4

7.1

15.6

23.0

0.9

Animal casein

53.6

7.1

15.7

22.6

1.0

Vegetal " ...

50 5

6.8

18.0

24.2

0.5

Animal (flesh) fibrin
Vegetal (wheat) "

54.1
54.3

7.3

7.2

16.0
16.9

21.5
20.6

1.1
1.0

Animal (blood) "

52.6

7.0

17.4

21.8

1.2

36 PROTOPLASM AND THE CELL.

There is a corresponding likeness in the general properties and reaction*
of proteids. They are colloidal or non-diffusible, i.e., they will not pass
through the membrane of a dialyser, or only with great difficulty ; they
are rarely crystalline ; they rotate the plane of polarized light to the left.
Though not all soluble in water, they may be dissolved by the aid of heat
in strong acetic acid and in caustic alkalies, but are insoluble in cold ab-
solute alcohol and in ether. They may be precipitated from solution by
strong mineral acids, etc. Many proteids are precipitated by heat (a pro-
cess which is called coagulation} ; and it is worthy of note that tempera-
tures which produce coagulation of proteids (40°— 75° C.) produce also the
death of most organisms. "Amongst the organic proximate principles
which enter into the composition of the tissues and organs of living beings,
those belonging to the class of proteid or albuminous bodies occupy quite
a peculiar place and require an exceptional treatment, for they alone are
never absent from the active living cells which we recognize as the pri-
mordial structures of animal and vegetable organisms. In the plant, whilst
we recognize the wide distribution of such constituents as cellulose and
chlorophyl, and acknowledge their remarkable physiological importance,
we at the same time are forced to admit that they occupy altogether a
different position from that of the proteids of the protoplasm out of which
they were evolved. We may have a plant without chlorophyl, and a vege-
table cell without a cellulose wall, but our very conception of a living,
functionally active, cell, whether vegetable or animal, is necessarily asso-
ciated with the integrity of its protoplasm, of which the invariable organic
constituents are proteids.

" In the animal, the proteids claim even more strikingly our attention
than in the vegetable, in that they form a very much larger proportion of
the whole organism, and of each of its tissues and organs. We may indeed
say that the material substratum of the animal organism is proteid, and
that it is through the agency of structures essentially proteid in nature
that the chemical and mechanical processes of the body are effected. It is
true that the proteids are not the only organic constituents of the tissues
and organs, and that there are others, present in minute quantities, which
probably are almost as widely distributed, such as for instance phosphorus-
containing fatty bodies, and glycogen, yet avowedly we can (at the most)
only say probably, and cannot, in reference to these, affirm that which we
may confidently affirm of the proteids — that they are indispensable constit-
uents of every living, active, animal tissue, and indissolubly connected
with every manifestation of animal activity." (Gamgee, Physiological
Chemistry, Chap. I.)

The molecular instability of proteids is proved by the ease
with which they may be decomposed into simpler compounds ;
their complex constitution by the numerous compounds, them-
selves often highly complex, which may thus be derived or
split off from them.

CARBOHYDRATES AND FATS. 37

Amongst the other matters found in protoplasm or closely
associated with it those of most frequent occurrence and greatest
physiological importance are two groups of less complex sub-
stances, viz. , carbohydrates and fats. These contain carbon, hy-
drogen, and oxygen, but no nitrogen ; they do not appear to be
closely related to proteids in chemical constitution, but they
occur to some extent almost everywhere in living organisms, and
in many instances are known to be of great importance, espe-
cially in nutrition. They are rich in potential energy and mo-
bile in molecular arrangement ; hence it is not strange that they
figure largely in food, and are often laid by as reserve food-
materials in the organism.

- -B. CARBOHYDRATES. These substances are so called because,
besides carbon, they contain hydrogen and oxygen united in the
same proportions as in water. They include starch, various
kinds of sugar, cellulose, and glycogen. Starch (C,H10O6) is of
very frequent occurrence in plant-cells, where it appears in the
form of granules embedded in the protoplasm (Fig. 9). Cel-
lulose, having the same chemical formula as starch, but quite
different in physical properties, almost invariably forms the basis
of the cell-membrane in plants.

C. FATS. These are of especial importance as reserves of
food-materials (e.g., in adipose tissue and in seeds). They con-
tain much less oxygen than the carbohydrates; are therefore
more oxidizable, and richer in potential energy.* They com-
monly occur in the form of drops suspended in the protoplasm
(Fig. 17), and are especially common in animal cells, though by
no means confined to them.

Physical Relations. The appearance, consistency, etc., of
protoplasm have already been described ; but it still remains to
speak of certain of its other physical properties, and especially
of the manner in which its activity is conditioned by various
physical agents.

Relations of Vital Action to Temperature. It is a general
law that within certain limits heat accelerates, and cold dimin-
ishes, the activity of protoplasm. We know that cold tends to

* According to careful researches, one pound of butter contains 5654 foot-
ions, and a pound of sugar 2755 foot-tons, of energy. A pound of proteid is
nearly equivalent in this respect to a pound of carbohydrate.

47133

38 PROTOPLASM AND THE CELL.

benumb our own bodies (provided they become really chilled), and
in lower animals the heart beats more slowly, the movements be-
come sluggish or cease, breathing becomes slow and heavy,— in
a word, all of the vital actions become depressed,— whenever
the ordinary temperature is sufficiently lowered. If we chill
the rotating protoplasm of Chara or Nitella, the vibrating cilia
of ciliated cells, or an actively flowing Amoeba , the movements
become slower, and finally cease altogether.

On the other hand, moderate warmth favors protoplasmic
action. Benumbed fingers become once more nimble before the
warmth of the fire. In a hot room the frog's heart beats more
rapidly, cilia lash more energetically, the Amoeba flows more
rapidly, and the protoplasm of Chara courses more swiftly. In
the winter months the protoplasm of plants and of many animals
is in a state of comparative inactivity. Most plants lose their
leaves and stop growing ; many animals bury themselves in the
mud or in burrows, and pass the winter in a deep sleep (hiberna-
tion), during which the vital fires burn low and seem well-nigh
extinguished. The warmth of spring re-establishes the activity
of the protoplasm, and in consequence animals awake from their
sleep and plants put forth their leaves.

But this law is true only within certain limits. Extreme
heat and cold are alike inimical to life, and as the temperature
approaches these extremes all forms of vital action gradually or
suddenly cease. The limits are so variable that it is not at
present possible to formulate any exact law which shall include
all known cases. For instance, many organisms are killed at
the freezing-point of water (0° C.); but certain forms of life
have withstood a temperature of — 87° C. (— 123° F.), and re-
cent experiments show that frogs and rabbits may be chilled to
an unexpected degree without fatal results.

The upper limit is also inconstant, though less so than the lower.
Most organisms are destroyed at the temperature of boiling
water (100°C.), but the spores of bacteria have been exposed to
a much higher temperature without destruction (120°-125° C.).
As a rule, protoplasm is killed by a temperature varying from
40° to 50° C. , the immediate cause of deatli being apparently
due to a sudden coagulation (p. 36) of certain substances in the
protoplasm. Thus, if a brainless frog be gradually heated,

PROTOPLASM AND PHYSICAL AGENTS. 39

death ensues at about 40° C., and the body becomes stiff and
rigid (rigor caloris) from the coagulation of the muscle-sub-
stance. The lower forms of animal life agree well with plants
in their " fatal temperatures," which in many cases lie between
40° and 50° C.

Lastly, it appears to be true that there is a certain most
favorable or optimum temperature for the protoplasm of each
species of plant and animal, this optimum differing considerably
in different species. Probably the highest limit occurs among
the birds, where the uniform temperature of the body may be
as high as 40° C. The lowest occurs among the marine plants
and animals of the Arctic seas, or of great depths, where the
temperature seldom rises more than a degree or two above the
freezing-point. Between these limits there appears to be great
variation, but 35° C. may perhaps be taken as the average op-
timum.

Moisture. Protoplasm always contains a large amount of water, of
which indeed the lifeless portion of living things chiefly consists. (Se
table on p. 34.) All plants and animals are believed to be killed by com-
plete drying, though some of the simpler forms resist partial drying for a
long time, becoming quiescent and reviving again when moistened, some-
times even after the lapse of years. Hence water appears to be an essen-
tial constituent of protoplasm, although, as in the case of mineral matters,
we do not know the nature of its connection with the other elements or
compounds present.

Electricity. It has been shown that many forms of vital action are ac-
companied by electrical disturbances in the protoplasm. It is therefore
not surprising that the application of electricity to living protoplasm should
have a marked effect on its actions. If the stimulus be very slight, proto-
plasmic movements are favored. Colorless blood-corpuscles creep more
actively, and ciliary action increases in vigor. Stronger shocks cause a
spasmodic contraction of the protoplasm (tetanus), from which it may or
may not be able to recover, according to the strength of the shock.

Poisons. Towards certain agents protoplasm is indifferent or seemingly
80, but towards others it behaves in a very remarkable manner. The mat-
ters known as poisons modify or destroy its activity, as is well known from
the familiar effects of arsenic, opium, etc. Disease may also interfere with
its normal activity ; but the consideration of these phases of the subject
belongs to the more exclusively medical sciences, such as toxicology and
pathology.

Other Physical Agents. The more highly specialized forms of proto-
plasm are affected by a great variety of physical agents, such as light,

40 PROTOPLASM AND THE CELL.

sound, pressure, etc., and upon this susceptibility depend many of the
higher manifestations of life. For instance, waves of light or of sound,
acting upon special protoplasmic structures in the eye and ear, call forth
actions which ultimately result in the sensations of sight and hearing.
Similar considerations apply to the senses of smell, taste, and touch ; but
the discussion of all these special modes of protoplasmic action must be
deferred. Enough has been said to show that living organisms (that is,
the protoplasm which is their essential part) are able to respond to many
influences proceeding from the world in which they live. Upon this prop-
erty depend the intimate relations between the organism and its environ-
ment, and the power of adaptability to the environment which is one of the
most marvellous and characteristic properties of living things.

Non-diffusibility. Living protoplasm, like most of the various proteid
matters which it yields (p. 36), is indiffusible. It will be seen eventually
that osmotic processes play a leading role in the lives of plants and animals,
since they are in large part the means by which nutriment is conveyed to
the living substance. In view of this fact, the non-diffusibility of proto-
plasm as well as of ordinary proteids is a fact of much significance.

Vegetal and Animal Protoplasm. The protoplasm of plants is es-
sentially identical with that of animals in chemical and physical relations,
and manifests the same fundamental vital properties. But it would mani-
festly be absurd to suppose this identity absolute, for if it were so, plants
and animals would also be identical ; and furthermore, the protoplasm
of every species of plant and animal must differ more or less from the
protoplasm of every other species. What is meant is that the differences
between the many kinds of protoplasm are far less important than the
fundamental resemblances which underlie them.

CHAPTER IY.
THE BIOLOGY OF AN ANIMAL.

The Common Earthworm.

(Lumbricus terrestris, Linnaeus.)

WE now advance to a more precise examination of the living
body considered as an individual. It is a familiar fact that
living things fall into two great groups, known as plants and
animals. We shall therefore examine a representative of each
of these grand divisions of the living world, and inquire how
they resemble each other and how they differ. Any higher
animal would serve as a type, but the common earthworm is a
peculiarly favorable object of study, because of the simplicity of
its structure, the clearness of its relation to other animals stand-
ing above and below it in the scale of organization, and the ease
with which it may be procured and dissected. Earthworms, of
which there are many kinds, are found in all parts of the world,
extending even to isolated oceanic islands. In the United States
there are several species, of which the most common are L.
communis (Allolobophora mucosa, Eisen), L. terrestris, and Z.
fixtidus (Allolopobhora fcetida, Eisen). The first two of these
are found in the soil of gardens, etc., L. terrestris being the
larger and stouter species and readily distinguishable by the
flattened shape of the posterior region. L. ftxtidus, a smaller
red species, transversely striped, and having a characteristic
odor, occurs in and about compost-heaps.

Mode of Life, etc. Earthworms live in the earth, burrow-
ing through the soil at a depth varying from a few inches to
several feet. Here they pass the daytime, crawling out at
night or after a shower. The burrows proceed at first straight
downwards, and then wind about irregularly, sometimes reach-

41

42 THE BIOLOGY OF AN ANIMAL.

ing a depth of six or eight feet. The earthworm is a nocturnal
animal, and during the day lies quiet in its burrow near the sur-
face, extended at full length, head uppermost. At night it
becomes very active, and, thrusting the fore end of the body
far out, explores the vicinity in all directions, though still clinging
fast, as a rule, to the mouth of the burrow by the hinder end.
In tliis way the worm is able to forage, seizing leaves, pebbles,
and other small objects, and dragging them into the burrow.
Some of these are devoured ; the remainder (including the peb-
bles, etc.) are used to line the upper part of the burrow, and to
plug up its opening when the worm retires for the day. Be-
sides bits of leaves and animal matter, earthworms swallow large
quantities of earth, which is passed slowly through the alimentary
canal, so that any nutritious substances contained in it may be
digested and absorbed. This earth is generally swallowed at a
considerable distance below the surface of the ground, and is
finally voided at the surface near the opening of the burrow.
In this way arise the small piles of earth (" castings " or faeces}
which every one has seen, especially in the morning, wherever
earthworms abound. Very large quantities of earth are thus,
brought to the surface by earthworms — in some cases, accord-
big to Darwin's estimates, more than eighteen tons per acre in
a single year. In fact, most soils are continually being worked
over by worms; and Darwin has shown that these humble
creatures, in the course of centuries, have helped to bury huge
rocks and the ruins of ancient buildings.*

The earthworm has no ears, eyes, or any other well-marked
organs of special sense. Nevertheless — and this is a point of
great physiological interest — the fore end of the body is sensi-
tive to light ; for if a strong light be suddenly flashed upon this
part of the worm as it lies stretched forth, it will often " dash
like a rabbit into its burrow." The animal has a keen sense of
touch, as may be proved by tickling it ; and its sense of taste
must be well developed, since the worm is somewhat fastidious
in its choice of food. Earthworms appear to be quite deaf, but
possess a distinct, though feeble, sense of smell.

* Darwin, Vegetable Mould and Earthworms. Appleton, N. Y., 1882. See
also White's Natural History ofSelborne, Index, references to " Earthworms/*

ANTERO-POSTERIOR DIFFERENIIA TION.

43

GENERAL MORPHOLOGY.

Attention will first be directed to certain features of the
BODY seemingly of little importance, but really full of meaning
when compared with like features in other
animals higher or lower in the scale of
organization.

Antero- posterior Differentiation. The
body (Fig. 21) has an elongated cylindrical
form, tapering to a blunt point at one end,
obtusely rounded and flattened at the other.
As a rule, the pointed end moves for-
wards in locomotion, and the mouth opens
near it. For these and other reasons
the pointed end might be called the head-
end, and the other the tail-end. But the
worm has really neither head nor tail, and
hence the two ends may better be distin-
guished as tliefore end and the hinder end,
or still better as anterior and posterior.
And in scientific language the fact that the
worm has anterior and posterior ends
which differ from each other is stated by
saying that it shows antero-posterior differ-
entiation. This simple fact acquires great
importance in the light of comparative
biology; for it may be shown that the
antero-posterior differentiation of the earth-
worm, insignificant as it seems, is only the
begining of a series of important modifica-
tions extending upwards through more and
more complex stages to culminate in man
himself.

FIG. 21. — Enlarged view of the anterior and posterior
parts of the body of an earthworm as seen from the
ventral aspect, an, anus ; c, clitellum ; (/.p., glandular
prominences on the 36th somite ; in, mouth ; o.d, exter-
nal openings of the oviducts ; p.*., prostomium ; s, setae ;
S.r., openings of the seminal receptacles ; s.d., external
openings of the sperm-ducts. The form of the body
varies greatly in life according to the state of expan-
sion. The specimen here shown is from an alcoholic
preparation. (Slightly enlarged.)

44 THE BIOLOGY OF AN .ANIMAL.

Dorso-ventral Differentiation. In living or well-preserved spe-
cimens, the body is not perfectly cylindrical, but is somewhat
flattened, particularly near the posterior end, and has a slightly
prismatic four-sided form. One of the flattened sides, slightly
darker in color than the other, is habitually turned upwards, and
is therefore called the back, the opposite or lower side, commonly
turned downwards, being the belly. For the sake of accuracy,
however, biologists are wont to speak of the dorsal aspect (back)
and ventral aspect (belly) of the body ; and the fact that an animal
has a back and belly differing from each other in structure or
function, or both, as in the earthworm, is expressed by saying
that the body exhibits dorse-ventral differentiation. This, like
antero-posterior differentiation, is very feebly expressed in the
external features, though clearly marked in the arrangement of
the internal parts of the earthworm. In higher animals it
becomes one of the most conspicuous features of the body.

Bilateral Symmetry. When the body is placed in the natural
position, with the ventral aspect downwards, a vertical plane
passing longitudinally through the middle will divide it into
exactly similar right and left halves. This similarity is called
two-sided likeness, or bilateral symmetry. Though not very
obvious externally, this symmetry characterizes the arrangement
of all the internal parts; and it may be gradually traced up-
wards in higher animals, until it becomes as striking and perfect
as in the human body.

Thus a very superficial examination reveals in the earth-
worm two fundamental laws of organization, viz., differentia-
tion or the law of difference, and symmetry or the law of like-
ness. And these laws are of interest for the reason among
many others that earthworms, like other organisms, have as a
race had a history, have come to be by a gradual process (cf.
p. 99). And biology must strive to answer the questions how and
why certain parts have become symmetrical and others differ-
entiated. Without entering into a full discussion of the ques-
tion at this point, it may be said that the main cause of sym-
metry or differentiation has probably been likeness or unlikeness
of function, or of relation to the environment. Earthworms
show antero-posterior and dorso-ventral differentiation, because
the anterior and posterior extremities, or the dorsal and ventral

METAMERISM. 45

aspects, have been differently used and exposed to different con-
ditions of environment. And on the other hand the organism is
bilaterally symmetrical, because the two sides have been similarly
used and have been exposed to like conditions of environment.

Metamerism. Another general feature of the earthworm is
of great importance in view of the conditions existing in other
animals, including the higher forms. The body is marked off
by transverse grooves into a series of similar parts like the joints
of a bamboo fishing-rod, or like the joints of lingers (Fig. 21).
These parts are called metameres, or more often somites, and
the body is consequently said to have a metameric structure, or
to exhibit metamerism. From the outside, the somites appear to
be produced simply by regular folds in the skin, like the
wrinkles between the joints of our fingers. But as the wrinkles
of the fingers are only the external expression of a more funda-
mental jointed structure within, so the external folds separating
the somites, represent an internal division into successive parts,
which affects all the organs of the body, and is a result of some
of the most important phenomena of development.

The explanation of metamerism or " serial symmetry" is one of the
most difficult problems of morphology. But it will be seen farther on that
metamerism, so clearly and simply expressed in the earthworm, can be
traced upward in ever-increasing complexity to the highest forms of life,
and suggests some of the most interesting and fundamental problems with
which biology — and especially morphology — has to deal. Indeed, the
comparative study of the anatomy of most higher animals consists very
largely in tracing out the manifold transformations of their complicated
somites, which under many disguises can be recognized as fundamentally
like the simpler somites of the earthworm.

Modifications of the Somites. The somites differ considerably
in different parts of the body. The extreme anterior end is
formed by a smoothly-rounded knob called the prostomium,
which is shown by its mode of development not to be a true
somite. It forms a kind of overhanging upper lip to the mouth,
which lies just behind it on the ventral aspect. Behind the
mouth is the first somite, in the form of a ring,* interrupted
above by a backward prolongation of the prostomium.

* In numbering the somites the prostomium must never be reckoned, the
first somite being behind the mouth.

46 THE BIOLOGY OF AN ANIMAL.

The somites from the 1st to the 27th are rather broad,
and gradually increase in size. A variable number cf the
somites lying between the 7th and 19th are often swollen on
the ventral side, forming the so-called capsulogenous glands.
Between the 28th and 35th (the number and position vary-
ing slightly in different specimens) the somites are swollen
above and on the sides, and the folds between them are
scarcely defined except on the ventral aspect. Taken together,
they form a broad, conspicuous girdle called the clitelluin
(Fig. 21, c), whose function is to secrete the capsule in which
the eggs are laid, and also a nutritive milk-like fluid for the use
of the developing embryos. (The clitellum is not present in
immature specimens.) Behind the clitelluin the somites are
narrower, somewhat four-sided in cross-section, and ilattened
from above downwards. This flattening sometimes becomes
very conspicuous towards the posterior end. Towards the very
last they decrease in size rather abruptly, and they end in the
anal somite, which is perforated by a vertical slit, the anus
(Fig. 21, an). All the somites are perforated by small openings
leading into the interior of the body, and forming the outlets of
numerous organs ; the position of these openings will be de-
scribed in treating of the organs. Each somite, excepting the
anterior two or three and the last,
gives insertion to four groups of
short and minute bristles or setce^
which are arranged in four longi-
tudinal rows along the body. T\vo
s -- j ^ - of these rows run along the ventral

FIG. 22.— Diagram to illustrate the aspect, two are more Upon the

.

the seta and its muscles when from the interior of the body,

J^tSSS SEEL"1 where th*y are supplied with small

muscles by which they can be

turned somewhat either forwards or backwards, and can also be
protruded or withdrawn (Fig. 22). The setse are of great use
in locomotion. When pointed backwards they support the worm
as it crawls forwards ; when they are turned forwards the worm
can creep backwards. They are of interest, therefore, as repre-
senting an extremely simple and primitive limb-like organ.

GENERAL PLAN OF THE BODY.

47

Plan of the Body. The body of the earthworm (Fig. 23),
like that of all higher animals, consists of two tubes, one (al\
within the other and separated from it by a considerable space
or cavity (cce). The inner tube is the alimentary canal, open-
ing in front by the mouth and behind by the anus ; the outer
tube is the body-wall, and its cavity is the body-cavity or ccelom.

FlG. 23. — A, diagram of the earthworm as seen in a longitudinal section of the body,
showing the two tubes, the ccelom, and the dissepiments. B, diagram of cross-
section : nl, alimentary tube ; OH, anus ; c<», ccelom ; m, mouth. C, diagram
showing the arrangement of some of the principal organs : m, mouth ; a», anus ;
a?, alimentary canal; d#, dissepiments; d.r., dorsal blood-vessel; r, ventral or
sub-intestinal vessel ; c.r., circular vesselb; n, nephridia or excretary organs; e.g.,
cerebral ganglia ; t\0., ventral chain of ganglia ; o.d., oviduct; o.rf., ovary. The
arrows indicate the course of the circulation of the blood.

The ccelom is not, however, a free continuous space extending
from end to end, but is divided transversely by a series of thin
muscular partitions, the dissepiments, into a series of nearly
closed chambers traversed by the alimentary canal. Each com-
partment corresponds to one somite, the dissepiments being
opposite the external furrows mentioned on p. 45. All the
organs of the body are originally developed from the walls of
these chambers, and some of them (e.g., the organs of excretion)
project into the cavities of the chambers, that is into the co3lom.

48 THE BIOLOGY OF AN ANIMAL.

In the median dorsal line of each somite (excepting the first
two or three) is a minute pore (the dorsal pore) which perfo-
rates the body-wall and thus places the coelom in connection
with the exterior.* Other pores that pass through the body-
wall into the cavities of various organs will be described fur-
ther on.

Organs of the Animal Body. Systems of Organs. The body of
the earthworm consists essentially of protoplasm, and in order that
so large a mass of living matter may continue to exist and carry
on the ordinary life of an earthworm it must be able to obtain
a sufficient supply of food; to digest and absorb it, and dis-
tribute it to all parts of the body ; to build up new protoplasm
and remove waste. It must be sensitive to external and internal
influences ; capable of motion and locomotion. Above all, each
part must act with reference to, and in harmony with, every
other part, so that the organism may not be merely an aggregate
of organs, but one body acting as a unit or a whole.

These functions are fulfilled by the ORGANS,- respectively, OF

ALIMENTATION, DIGESTION, ABSORPTION, OIRCUI-ATION, EXCRETION,

SENSATION, MOTION, and COORDINATION. All of these minister to
the welfare of the individual. The REPRODUCTIVE function, on
the other hand, and its corresponding organs, serve to perpet-
uate the species, thus ministering rather to the race than to the
individual.

Sets of organs devoted to the same function constitute sys-
tems / as the alimentary system, the circulatory system, etc.
Those which are more immediately concerned with the income
and outgo of matter — namely, the alimentary, digestive, absorp-
tive, circulatory, and excretory systems — are sometimes called the
vegetative systems or systems of nutrition / while those which
have to do more immediately with the relation of the body to
its environment, rather than the individual itself, are called sys-
tems of relation. Examples of the latter are the systems of
organs of support, motion (including locomotion), sensation, and
coordination ; and even the reproductive system, as relating chiefly
to other individuals, finds a place here.

* If living worms be irritated they will often extrude a rnilky fluid from
these pores, but the use of the latter is not well understood.

ALIMENTARY SYSTEM. 49

A. SYSTEMS OF NUTRITIVE ORGANS: THEIK SPECIAL MOR-
PHOLOGY AND PHYSIOLOGY. (For 13 see p. 62.)

Alimentary System (Organs of Alimentation). Earth-worms
feed mainly upon leaves or decaying vegetable matter, but
will also eagerly devour meat, fat, and other animal sub-
stances. They also swallow large quantities of earth from
which they extract not only any organic materials that it may
contain, but probably also moisture and a small amount of vari-
ous salts. The most essential and characteristic part of their
food is derived from vegetal or animal matter in the form of
various organic compounds, of which the most important are
proteids (protoplasm, albumen, etc.), carbohydrates (starch,
cellulose), and/ate. These materials are used by the animal in
the manufacture of new protoplasm to take the place of that
which has been used up. It is, however, impossible for the ani-
mal to build these materials directly into the substance of its
own body. They must first undergo certain preparatory chemi-
cal changes known collectively as digestion • and only after the
completion of this process can all the food be absorbed into the
circulation. For this purpose the food is taken not into the
body proper, but into a kind of tubular chemical laboratory
called the alimentary canal through which it slowly passes,.
being subjected meanwhile to the action of certain chemical sub-
stances, or reagents, known as digestive ferments. These sub-
stances, which are dissolved in a watery liquid to form the diges-
tive fluid, are secreted by the walls of the alimentary tube.
Through their action the solid portions are liquefied and the food
is rendered capable of absorption into the proper body.

The alimentary canal is divisible into several differently con-
structed portions playing different parts in the process of alimen-
tation. Going backwards from the mouth these are as follows :

1. The pharynx (Fig. 24, ph\ an elongated barrel-shaped
pouch extending to about the 6th somite. Its walls are thick
and muscular, and from their crelomic surface numerous small
muscles radiate on every side to the body- wall. When these
muscles contract, the cavity of the pharynx is expanded ; and if
the mouth has been previously applied to any solid object, such
as a leaf or pebble, the pharynx acts upon it like a suction-pump.

THE BIOLOGY OF AN ANIMAL.

FIG. 24.— Dorsal view of the anterior part of the body of I/umbrfctw, as it appears
when laid open along the dorsal aspect, ao, aortic arch ; c, crop ; c.g, cerebral
ganglia; c.gl, calciferous glands; d, dissepiment; d.r, dorsal vessel; 0, gizzard ;
(B, 03sophagus ; ph, pharynx ; ps, prostomium ; s.i, stomach-intestine, showing
the lateral pouches; s.r, seminal receptacles; ».•».*, s.u.1, 8.r.», the three pairs of
lateral seminal vesicles.

ORGANS OF ALIMENTATION. 51

In this way the animal lays hold of the various objects, nutri-
tious and otherwise, which it devours or draws into its burrow.

Embedded in the muscular walls of the pharynx are a
number of small ' ' salivary ' ' glands of whose function nothing
is definitely known, though they doubtless pour a digestive fluid
into the pharyngeal cavity.

2. The (Esophagus («?), a slender, thin-walled tube extending
from the 6th to the 15th somite. Through this the food is
swallowed, being driven slowly along by wavelike (peristaltic)
contractions (p. 55). In the region of the llth and 12th
somites are three pairs of small pouches opening at the sides of
the O3sophagus. These are the calciferous glands (c.gl.). They
contain solid masses of calcium carbonate, and Darwin conjec-
tures that their use is partly to aid digestion by neutralizing the
acids generated during the digestion of leaves, and perhaps
partly to serve as an outlet for the excess of lime in the body,
especially when worms live in calcareous soil.

3. The crop (c), about the 16th somite; a thin-walled, sac-
like dilatation of the alimentary canal, which serves as a reser-
voir to receive the swallowed food.

4. The gizzard (g), about the 17th somite; a cylindrical,
firm and muscular portion, lined by a horny membrane. In this
the food is rolled about, squeezed and ground to prepare it for
digestion in the following portion, viz. :

5. The stomach-intestine (s.i.\ which corresponds physio-
logically to both the stomach and intestine of higher animals.
This is a straight thin- walled tube, extending from the gizzard
to the anus, without convolutions, not differentiated into stomach
and intestine, and devoid of distinct glandular appendages such
as the liver or pancreas existing in the higher animals. The
digestive fluid is secreted by the walls of the alimentary canal
itself, the surface of which is much increased by the presence of
lateral pouches or diverticula, one on either side in each somite.
In front these are large and conspicuous, but behind they gradu-
ally diminish in size until scarcely perceptible.

The inner surface of the stomach-intestine is further increased by a
deep inward fold, called the typhlosole, running longitudinally along the
dorsal median line. The typhlosole is not visible on the exterior, but is
seen by opening the stomach-intestine from the side or below, or upon

52 THE BIOLOGY OF AN ANIMAL.

making a cross-section. It is richly supplied with Hood-vessels that pass
down into its cavity from the dorsal vessel (Fig. 39), and its main func-
tion is probably to increase the surface for the absorption of food (cf. the
" spiral valve " in the intestine of sharks.)

The outer surface of the stomach-intestine is covered with pigmented,
yellowish-brown "chloragogue-cells." These were formerly supposed to be
concerned with the secretion of the digestive fluid, and hence are often
called " hepatic cells." This, however, is probably an erroneous interpreta-
tion, and they are now believed to be concerned with the process of excre-
tion (p. 61).

Digestion. Digestion begins even before the food is taken
into the alimentary canal ; before being swallowed, the leaves,
etc., are moistened by digestive fluid poured out from the
mouths of the worms. The main action, however, doubtless goes
on in the anterior part of the stomach-intestine and diminishes
as the food passes backward. It has been proved by experiment
that the digestive fluid acts on at least two of the three principal
varieties of organic food-stuffs, viz. , on proteids and on starch
(carbohydrate), and in so far resembles the pancreatic fluid of
higher animals, which it further resembles in having an alkaline
reaction. Analogy leads us to believe that the digestive fluid
has some action also on fats ; but this has not been proved.

Krukenberg and Fredericq have shown that the digestive fluid of the
earthworm contains at least three ferments ; and according to the former
author these occur only in the stomach-intestine. They are as follows :

1. Peptic ferment, which has the property in an acid medium of con-
verting proteids into soluble and diffusible peptones; this is therefore
analogous to the pepsin of the gastric juice in higher forms.

2. Tryptic ferment, having a similar action on proteids, but only in an
alkaline medium— hence analogous to the trypsin of pancreatic juice.

3. Diastatic ferment, which converts starch into glucose (grape-sugar)
in an alkaline medium — hence analogous to the ptyalin of saliva and the
amylolytic ferment of pancreatic juice.

Absorption. The ferments of the digestive fluid convert the
solid proteids into soluble and diffusible peptones, the starchy
matters into sugar (glucose). These products dissolve in the
liquids present and are then gradually absorbed by the walls of
the intestine as the food passes along the alimentary canal. The
precise mechanism of absorption is not yet thoroughly understood,
but it is probable that much of the nutriment passes by diffusion
(osmosis) into the walls of the stomach-intestine and thence into

ORGANS OF CIRCULATION.

the blood for distribution to all parts of the body. The refuse
remaining in the alimentary canal (and which has never been a
part of the body proper) is finally voided through the anus as
castings or faeces. This process of " defaecation " must not be
confounded with that of excretion, which will be described later.

Circulatory System. The food, having been absorbed, is
distributed throughout the body by two devices.

1. Cwlomic Circulation. The cavity of the coslom is filled
with a colorless fluid (" coslomic fluid ' ') which must be regarded as a
kind of lymph or blood. By the contractions of the body-wall, as
the worm crawls about, the ccelomic fluid is driven back and forth
through all parts of the coelom,
through irregular openings in the
dissepiments. As the digested
food is absorbed from the stomach-
intestine a considerable part of it is
believed to pass into the coelomic
fluid, and is thus conveyed directly
to the organs which this fluid
bathes. The coelomic fluid is com-
posed of two constituents, viz., a
colorless fluid called the plasma,
and colorless isolated cells or cor-
puscles which float in the plasma,
and are remarkable for the fact
that they undergo constant though
slow changes of form. In fact they
closely resemble certain kinds of
Amaebce, and we should certainly
consider them to be such if we
found them occurring free in stag-
nant water. We know, however,
that they live only in the plasma, and have a common origin
with the other cells of the body ; hence we must regard them
not as individual animals, but as constituent cells of the earth-
worm. The coalomic fluid is in fact a kind of tissue consisting
of isolated colorless cells floating in a fluid intercellular substance.
These free floating cells are probably the scavengers (phagocyte*}
of the body, devouring and destroying waste matters. Some

FIG. 25.— Phagocytes, from the coe-
lomic fluid of the earthworm. A,
agglomeration of phagocytes,
surrounding a foreign body; B,
single phagocyte, with vacuoles.
(After Metschnikoff.)

54 THE BIOLOGY OF AN ANIMAL.

suppose that they also attack invading parasites such as bacteria.

2. Vascular Circulation. Besides the coelomic circulation
there is another and more complicated circulatory apparatus con-
sisting of branching tubes, the Hood-vessels, which form a com-
plicated system ramifying throughout the body. Through these
tubes is driven a red fluid analogous to the red blood of higher
animals, and like it consisting of plasma and corpuscles, the latter
being flattened and somewhat spindle-shaped. The red color is
due to a substance, haemoglobin, dissolved in the plasma and not (as
in higher forms) contained in the corpuscles, which are colorless.

The earthworm is not provided with a special pumping-
organ or heart for the propulsion of the blood, sucli as we find
in higher animals. In place of this certain of the larger blood-
vessels (viz., the "dorsal vessel" and the "aortic arches")
have muscular contractile walls, which propel the blood in a con-
stant direction by wave-like contractions that run along the
vessel from one end to the other (" peristaltic " contractions, cf.
p. 51) at regular intervals and thus give rise to a "pulse."
The contractile vessels give off other non-contractile trunks
which divide and subdivide into tubes of extremely small calibre
and having very thin walls. The ultimate branches, known as
capillaries, permeate nearly all the organs and tissues, in which
they form a close network. The stream of blood after passing
through the capillaries is gathered into successively larger vessels
which after a longer or shorter course finally empty into the
original contractile trunks and complete the circuit. Thus the
vascular system is a closed system of tubes, and there is reason to
believe that the blood follows a perfectly definite course, though
this is not yet precisely determined.*

We may now consider the arrangement of the principal
trunks. The largest of them, which is also the most important
of the contractile vessels, is :

^ The dorsal vessel (Fig. 24, d.v.\ a long muscular tube
lying upon the upper side of the alimentary canal. In the liv-
ing worm it may be distinctly seen through the semi-transparent

* It should be noted that in the absence of a heart it is difficult to distin
guish between "arteries" and "veins." We may more conveniently distin-
guish " afferent vessels," carrying blood towards the capillaries, and ""efferent
vessels," carrying blood away from them.

BLOOD • VESSELS. 55

skin as a dark-red band, which is tolerably straight when the
worm is extended, but is made zigzag by contraction of the body.
If it be closely observed, a sort of wavelike contraction is often
seen running from behind forwards. This may be very clearly
observed in a worm stupefied by chloroform, especially if it has
been laid open along the dorsal side. The dorsal vessel then
appears as a deep-red, somewhat twisted, tube running along the
upper side of the alimentary canal. Wavelike contractions
continually start from its hinder end and run rapidly forwards,
one after another, to the anterior end, where the dorsal vessel
finally breaks up on the pharynx into a large number of branches
(Fig. 24).

The result of these orderly progressive contractions is that
the fluid within the tube is pushed forwards — very much as the
fluid in a rubber tube is forced along when the tube is stripped
through the fingers. It is still better illustrated by the action
of the fingers in the operation of milking. This action of the
vessels is a typical example of peristaltic contraction.

b. Sub -intestinal vessel. This is a straight vessel which
runs along the middle line on the lower side of the alimentary
canal, parallel to the one just described, It returns to the
hinder part of the body the fluid which has been carried
forwards by the dorsal vessel. On the pharynx it breaks up
into many branches, which receive the fluid from corresponding
branches of the dorsal vessel. •

0. Circular or commissural vessels, metamerically repeated
trunks which run from the dorsal vessel downwards around the
alimentary canal and ultimately connect with the ventral vessel.
They are of several kinds, of which the most important are as
follows :

1. The aortic arches or circumoesophageal vessels, often
known as ' ' hearts, ' ' since like the dorsal vessel they are con-
tractile and with the latter furnish the entire propulsive force
for the circulation. These are five pairs of large vessels en-
circling the oasophagus in somites 7 to 11 inclusive. Theso
vessels pass directly from the dorsal to the ventral vessel, giving
off no branches. During life they perform powerful peristaltic
contractions, receiving blood from the dorsal vessel and pumping
it into the sub-intestinal or ventral.

5(j THE BIOLOGY OF AN ANIMAL.

2. Dorso-intestinal vessels, passing from the dorsal vessel
into the wall of the gut in the region of the stomach-intestine.
Of these vessels there are two or three pairs in each somite.
They are thickly covered (like the dorsal vessel in this region)
with pigmented " chloragogue- cells, " so that their red color is
usually not apparent. Unlike the aortic arches these vessels
break up on the wall of the intestine into capillaries which are
continuous with branches from the ventral vessel.

3. Dorso-tegumentary vessels, passing from the dorsal vessel
along the dissepiment into the body-wall on each side. These
are small vessels that pass directly around the body to connect
with a longitudinal trunk (" sub-neural ") lying below the ven-
tral nerve-cord (see below), and giving off branches to the body-
wall, dissepiments, and nephridia.

Course of the Blood. The precise course of the blood in
Zumbricus is still in dispute, though its more general features are
known. It is certain that the bulk of the blood passes forward in
the dorsal vessel, downward around the gut through the aortic arches
into the ventral vessel, and thence backwards towards the pos-
terior region. Its path thence into the dorsal vessel is doubtful.
The most probable view is that the blood proceeds from the ven-
tral vessel through ventro-intestinal vessels to the capillaries of
the intestine and thence to the dorsal vessel through the dorso-
intestinal vessels. It is possible, however, that the return path
is through the dorso-tegumentary vessels and that the dorso
intestinal carry blood from the dorsal vessel to the intestine.

In the foregoing account only the more obvious features of the blood-
vessels have been mentioned, and many important details have been passed
over. The circular vessels of the stomach-intestine can be followed for
only a short distance out from the dorsal vessel, where they seem to break
up into a large number of small parallel vessels lying close together and
running around to the lower side. The efferent vessels do not directly join
the sub-intestinal, but empty into a sinus or vessel which runs parallel to
tne latter, closely imbedded in the wall of the stomach-intestine. The sub-
intestinal vessel proper is quite separate from the stomach-intestine, and
communicates by short branches (usually two in each somite) with the
vessel lying above it. This may be clearly seen in the region of the gizzard.
On this there is a variable number of small lateral vessels, which break up
partly into a branching network, and are partly resolved into extremely
fine parallel vessels surrounding the organ. On the crop are three or four
pairs of lateral branches from the dorsal vessel which branch out into a

BLOOD-VESSELS. 67

fine network, but do not break up into parallel vessels as on the gizzard.
In the two somites (13th and 14th) in front of the crop there are usually
two pairs of vessels running around the oesophagus. In the llth and 12th
somites a small branch is given off to each calciferous gland. The most
anterior pair of circular vessels are in the 6th somite, and are very small.
In front of this the dorsal vessel breaks up into the pharyngeal network.
In front of the llth somite there are three sub-intestinal vessels. The two
additional vessels lie, one on either side of the primary one and break up
into branches at the sides of the pharynx. The aortic arches empty into
the middle vessel, and at the point of junction there is a communication
with the lateral vessel of the corresponding side.

Besides the dorsal and sub-intestinal vessels there are three other minor
longitudinal trunks (Fig. 26). Two of these are very small, and lie on

Fia. 26.— Dorsal view of part of the ventral nerve-cord, showing the arrangement of
the vessels of the ventral region, ds, dissepiment ; si, sub-intestinal or ventral
blood-vessel ; sh.n., sub-neural ; .«p.n., supra-neural. The sub-intestinal receives on
either side the ventro-laterals (r.l) from the nephridia, of which it forms the ef-
ferent vessel (e.f). The sub-neural is joined on each side by a continuation of the
dorso-tegurnentary (d.t.); a/, afferent branch to the nephridium (cf. Fig. 27).

either side above the nerve-cord (p. 66), sending fine branches out from
each ganglion along the lateral nerves. These are the supra-neural trunks
(.9.W.). The third longitudinal vessel (sub-neural) lies below the nerve-cord.
(See Fig. 26.) It receives on each side the termination of the dorso-tegu-
mentary vessel (d.t., Fig. 26) which in its course is connected with the
capillary networks of the body-wall and the dissepiment, and gives off a
large branch to the nephridium (cf. Fig. 27).

gg TEE BIOLOGY OF AN ANIMAL.

Besides the lateral vessels from the sub-neural and supra-neural a pair
of « ventro-lateral" (v.L, Figs. 26 and 27) are given off in each somite from
the sub-intestinal to the nephridium, probably receiving from it the blood
originally entered through a branch of the dorso-tegurnentary.

>CQS

FIG. 27.— Nephridia of LMmhricu*. A showing the regions of the tube, B the vascular
supply. /, II, III, the three principal loops.

A. /.funnel; »i.f, the "narrow tube"; m.f, middle tube; u\f, wide tube: m.p, mus-
cular tube or end-vesicle ; cte, dissepiment. The narrow tube extends from a to Q
and is ciliated between a and b, at c, and from d to c. The middle (ciliated) tub0
extends from g to /i , the Vide tube from h to /c, where it opens into th« muscular
part ; «r, external opening.

B, Letters as before ; d.t, dorso-tegumentary vessel, bringing blood ft om the dorsal
vessel, receiving at s a branch from the body- wall, sending an afferent branch to
the nephridium., and finally joining the sub-neural («.»); r.f, ventro-lateral vessel
carrying the blood from the nephridium to the sub-intestinal or ventral vessel
(s.i) ; r.w, ventral nerve-cord. (After Benham ; the direction of the blood-cur-
rents according to Bourne.)

Excretory System. It is the office of the excretory 'system to
remove from the body proper the waste matters ultimately re-

ORGANS OF EXCRETION. NEPHRIDIA.

Of

convoluted

suiting from the breaking down of living tissue. This does not
mean the passing away of the refuse of digestion through the
anus (defsecation, p. 53), for such matters have never been
absorbed and therefore have never really been within the body
proper. Excretion means the removal from the body of matter
which has really formed a part of its substance, but has been
used up and is no longer alive. In higher animals this function
is performed chiefly by the kidneys, the lungs, and the skin, the
waste matters passing off in the urine, the breath, and the sweat.
In the earthworm it is principally performed by small organs
called iwphridia, of which here are two in each somite, except-
ing the first three or four (Fig. 29).

Each nephridium (Fig. 27) consists
tube, attached to the hinder face of a
dissepiment, and lying in the coelom at
the side of the alimentary canal. At
one end the tube passes through the
body -wall and opens to the exterior by a
minute pore situated between the outer
and inner rows of setae (p. 46). The
other end of the tube passes through the
dissepiment very near to the point
where this is penetrated by the nerve-
cord (p. 66), and opens by a broad,

funnel-like expansion into the cavity of Fm 8g>_A nephpldlal funnel
the next somite in front (/", Fig. 27).
The margins of the funnel and the inner
surface of the upper part of the tube are
densely covered with powerful cilia (Fig. 28), whose action tends
to produce a current setting from the coelom into the funnel and
through the nephridium to the exterior.

The coils of the nephridium are disposed in three principal loops (I, II,
III in Fig. 27). The tube itself comprises five very distinct regions, as
follows :

1. The funnel or nephrostome ; much flattened from above downwards,
with the opening reduced to a horizontal chink. It is composed of beau-
tiful ciliated cells set like fan-rays around its edge. It leads into

2. The " narroiv tube " (n.t.), a very delicate thin-walled contorted tube
extending from the nephrostome through the first loop and a part of the
second. In certain parts of its course (a to 6, at c, and from d to e) this

much enlarged, showing the
cilia, the beginning of the
ciliated canal (c), and the
outer sheath (»).

60 THE BIOLOO Y OF AN ANIMAL.

tube contains cilia which are arranged in two longitudinal bands on the
inner surface. At g it passes into the

3. " Middle tube" (m.t.) (g toft), extending straight through the second
loop, of greater diameter, ciliated throughout, and with piginented walls.
At h it opens into the

4. " Wide tube" (w.t.). This is of still greater calibre, with granular
glandular walls and without cilia. It extends through the second loop
(from h to i, II) into and through the first from i to j, and finally into the
third, opening at k into the

5. Muscular part or duct (m.p.) which forms the third loop and opens to
the exterior at ex. This, the widest part of the entire nephridium, has
muscular walls and forms a kind of sac or reservoir like a bladder, in
which the excreted matter may accumulate and from which it may be
passed out to the exterior.

The various parts of the nephridium are held together by connective
tissue (p. 90), and are covered with a rich network of blood-vessels, the
arrangement of which is shown in Fig. 27, B. The smaller vessels usually
show numerous pouchlike dilatations which must serve to retard the flow
of blood somewhat. The vessels supplying the nephridium are connected
(Fig. 27, B) on the one hand with the sub-intestinal vessel through the
ventro-lateral trunks (v.l.) ; on the other hand with the sub-neural (s.n.) and
dorsal vessels, through the dorso-tegumentary (d.t.). The course of the
blood is somewhat doubtful. According to the view here adopted (cf. p. 56)
the blood proceeds from the dorso-tegumentary trunk to the nephridia and
thence through the ventro lateral to the sub-intestinal, as shown by the
arrows in the figure. Benham (from whom the figures are copied) adopts
the reverse view. The development of the nephridium shows that its
ciliated and glandular portions arise from a solid cord of disk-shaped cells
which afterwards becomes tubular by the hollowing out of its axial portion.
The tube is therefore comparable to a drain-pipe in which each cylinder
represents a cell. Its cavity is not intercellular (between the cells, like the
alimentary cavity), but intracellnlar (witliin the cells, like a vacuole).

The mode of action of the nephridia is as yet only partially
understood, though there is no doubt regarding their general char-
acter. It is certain that their principal office is to remove from
the body waste nitrogenous matters resulting from the decompo-
sition of proteids ; and there is reason to believe that these waste
matters are passed out either as urea ( [KH8]SCO) or as a nearly
related substance, together with a certain quantity of water and
inorganic salts.

Excretion in Lwnbricus appears, however, to involve two quite distinct
actions on the part of the nephridia. In the first place the glandular walls
of the tube, which are richly supplied with blood-vessels, elaborate certain
liquid waste substances from the blood and pass them into the cavity of

BREATHING. 61

the tube. In the second place the ciliated funnels are believed to take up
solid waste particles floating in the coelomic fluid and to pass them on into
the tube, whence they are ultimately voided to the exterior together with
the liquid products described above. It is nearly certain that these parti-
cles are derived from the breaking up of " lymphoid " cells, some of which
may have been phagocytes (p. 53), floating in the coelomic fluid, and that
most if not all of these cells arise from " chloragogue cells " set free from
the surface of the blood-vessels and of the intestine.

Respiration. Kespiration, or breathing, is a twofold operation,
consisting of the taking in of free oxygen and the giving off of
carbon dioxide by gaseous diffusion through the surface of the
body. Strictly speaking, this free oxygen must be regarded as
food, while carbon dioxide is to be regarded as one of the excre-
tions. Hence respiration is tributary both to alimentation and to
excretion ; but since many animals possess special mechanisms to
carry on respiration, it is convenient and customary to treat of
it as a distinct process.

Kespiration is essentially an exchange of gases between the
blood and the air, carried on through a delicate membrane lying
between them. The earthworm represents the simplest condi-
tions possible, since the exchange takes place all over the body,
precisely as in a plant. Its moist and delicate walls are every-
where traversed by a fine network of blood-vessels lying just
beneath the surface. The oxygen of the air, either in the
atmosphere or dissolved in water, readily diffuses into the blood
at all points, and carbon dioxide makes its exit in the reverse
direction. Freed of carbon dioxide and enriched with oxygen,
the blood is then carried away by the circulation to the inner
parts, where it gives up its oxygen to the tissues and becomes
once more laden with carbon dioxide.

In higher animals it has been proved that the red coloring
matter (haemoglobin) is the especial vehicle for the absorption
and carriage of the oxygen of the blood, entering into a loose
chemical union with it and readily setting it free again under the
appropriate conditions. This is doubtless true in the earthworm
also.

It is interesting to study the various devices by which this function is
performed in different animals. In the earthworm the whole outer surface
is respiratory, and no special respiratory organs exist. In other animals
such organs arise simply by the differentiation of certain regions of the

62 THE BIOLOG Y OF AN ANIMAL.

general surface, which then carry on the gaseous exchange for the whole
organism. In many aquatic animals such regions bear filaments or flat
plates or feathery processes known as gills or branchial, which are bathed
by the water containing dissolved air, though in many such animals
respiration takes place to some extent over the general surface as well. In
insects the respiratory surface is confined to narrow tubes (trachea:) which
grow into the body from the surface and branch through every part, but
must nevertheless be regarded as an infolded part of the outer surface.
In man and other air-breathing vertebrates the respiratory surface is
mainly confined to the lungy, which are simply localized infoldings of the
outer surface specially adapted to effect a rapid exchange of gases between
the blood and the air.

It is easy to see why special regions of the outer surface have in higher
animals been set aside for respiration. It is essential to rapid diffusion
that the respiratory surface should be covered with a thin, moist membrane,
and it is no less essential that many animals should be provided with a
firm outer covering as a protection against mechanical injury or desicca-
tion. Hence the outer surface becomes more less distinctly differentiated
into two parts, viz., a protecting part, the general integument ; and a
respiratory part, which is usually preserved from injury by being folded
into the interior as in the case of lungs or tracheae, or by being covered
with folds of skin as in the gills of fishes, lobsters, etc. This covering or
turning in of the respiratory surfaces brings with it the need of mechanical
arrangements for pumping air or water into the respiratory chamber ; and
thus arise many complicated accessory respiratory mechanisms.

/ B. ORGANS OF KELATION. (For A see p. 49.)

Motor System. The movements of the body have a twofold
purpose. In the first place they enable the animal to alter its
relation to the environment, to move about (locomotion), to seize
and swallow food, and to perform various adaptive actions in
response to changes in the environment. In the second place,
the movements may alter the relation of the various parts of the
body one to another (visceral, movements and the like), such as
the movements which propel the blood, drive the food along the
alimentary canal and roll it about (p. 49), those which expel
waste matters from the nephridia, discharge the reproductive
products, etc.

Most of these movements are performed by structures known
as muscles, which consist of elongated cells (fibres) endowed in a
high degree with the power of contractility — i.e., of shortening,
or drawing together (cf. p. 27). Ordinary "muscles" are in

MUSCLES. 68

the form of long bands or sheets of parallel fibres, such as those
that form the body-wall, that move the setse, and dilate the
pharynx. Other muscular structures, however, do not form dis-
tinct ' ' muscles, ' ' but consist of muscular h' bres more or less
irregularly arranged and often intermingled with other kinds of
tissue. Of this character are the muscular walls of the contrac-
tile vessels, and of the muscular portions of the nephridia and
dissepiments. It is clear from the above that the muscular sys-
tem is not isolated, but is intimately involved in many organs.

The muscles of the body-wall are arranged in two concentric layers
below the skin. In the outer layer the muscles run around the body, and
are therefore called circular muscles. Those of the inner layers have a
longitudinal course, — i.e., parallel with the long axis of the body, — and
are arranged in a number of different bands. The most important of these
are :

1. The dorsal bands (Fig. 39), one on either side above, in contact at
the median dorsal line, and extending down on either side as far as the
outer row of setae.

2. The ventral bands, on either side the middle ventral line and occupy-
ing the space between the two inner (lower) rows of setae.

3. The lateral bands, occupying the space on either side between the
two rows of setae.

All these vary greatly in different regions of the body, and in some parts
become more or less broken up into subsidiary bands. There is also a
narrow band traversing the space between the two setae of each group.

The seta, which may be reckoned as part of the motor system, are pro-
duced by glandular cells covering their inner ends, and they grow con-
stantly from this point, somewhat as hairs grow from the root. After
being fully formed, and after a certain amount of use, the setae are cast
off and replaced by new ones which have meanwhile been forming. In.
each group we find, therefore, setae of different sizes. At their inner ends
they are covered by a common investment of glandular cells which appears
as a slight rounded prominence when viewed from within. These prom-
inences are called the setigerous glands. When a worm is laid open from
above, the glands are seen in four parallel rows, two of which lie on either
side of the nerve-cord (see Fig. 29).

Each group of setae is provided with special retractor or protractor
muscles, and a narrow muscular band passes from the upper to the lower
group on each side internal to the body-wall.

Cilia. A second set of motor organs are cilia (their mode of action has
been referred to on p. 31), which are of the utmost importance in the
life of the earthworm. They cover the inner surface of the stomach-intes-
tine (where they doubtless assist in the movements of the food) play the
important part in excretion already described, collect and help to discharge

64 THE BIOLOGY OF AN ANIMAL.

the reproductive elements (p. 74), and, assist in the fertilization of the egg
(p. 74). Their action, like that of the muscle-fibres, is doubtless due to the
property of contractility, the protoplasm alternately contracting on opposite
sides of the ciliuin and thus causing its whiplike action.

White Blood-corpuscles. Amoeboid Cells. Lymph-cells. Phagocytes.
Besides muscle-cells and ciliated cells there is a third variety which display
contractility and movement, These are the ccelomic corpuscles referred to
above (p. 53). Until recently their function was wholly unknown, but it
is now generally believed that they are the scavengers of the body, devour-
ing the dead tissues or foreign bodies which invade the organism. Whether
they also attack and devour living parasites such as Qregarina and Bacteria
is not yet fully determined. They move their parts much as Amoebae do,
engulfing particles about them by a kind of flux.

Nervous System. Organs of Coordination.

Introduction. The general office of the nervous system of
organs is to regulate and coordinate the actions of all the other
parts in such wise that these actions shall form an harmonious
and orderly whole. Through nervous organs the worm receives
from the environment impressions which pass inwards through
the nerves as sensory or afferent impulses, to the nervous centres ;
and through other nervous organs impulses (efferent or motor)
pass outwards from the centres to the various parts so as to
arouse, modify, or suspend their activities. Thus the animal is
enabled to call forth movements resulting in the two kinds of
adjustments referred to on p. 62, viz., (a) adjustments of the
body as a whole to changes in the environment (e.g., the with-
drawal of the earthworm into its burrow at the approach of day) ;
and (b) adjustments between the parts of the body itself, so that
a change in one part may call forth answering changes in other
parts (e.g., the increased supply of blood to the alimentary canal
during digestion, or vigorous movements of the fore end of the
body when the hind end is irritated).

These functions are always performed by one or more nerve-
tells, which give off long slender branches known as nerve-fires
usually gathered together in bundles, the nerves, extending into
all parts of the body. In all higher animals the main bulk of
the nerve-cells are aggregated in definite bodies known as
ganglia, out of which, into which, or through which, the nerves
proceed ; and as a matter of convenience it is customary to desig-
nate the most important of these ganglia collectively as the cen-

NERVES AND GANGLIA. 65

tral nervous system. The remaining portion, which consists
mainly of nerve-fibres, though it may also contain many nerve-
cells and small sporadic ganglia, is known as the peripheral
nervous system.

General Anatomy of the Nervous System. In the earth-
worm the central system consists of a long series of double ganglia,
metamerically repeated, and connected by nerve-cords known as
commissures. The most anterior pair of ganglia, known as the
8upra-cesophageal or cerebral ganglia, lie on the dorsal aspect of
the pharynx, a short distance behind the anterior extremity
(Figs. 24, 29). From each of them a slender cord, the circum-
cesophageal commissure, passes down at the side of the pharynx
to end in the sub-cesophageal or first ventral ganglion on the
lower side, forming with its fellow a complete ring or pharyn-
geal collar around the alimentary canal. From the sub-o3sopha-
geal ganglion a long double ventral nerve-cord proceeds backwards
in the middle ventral line. The ventral cord consists of a series
of double ganglia, one to each somite, connected by commissures
and giving off lateral nerves.'3*

Internally the cerebral ganglia and the ventral cord (com-
missures as well as ganglia) consist of both nerve-cells and nerve-
fibres as described on p. 04.

Peripheral Nervous System. To and from the central sys-
tem just described run the nerves which constitute the peripheral
system. These are as follows :

1. A pair of nerves running out on either side of each ven-
tral ganglion and lost to view among the muscles of the body-
wall.

2. A single nerve proceeding from the ventral commissures
on each side immediately behind the dissepiment to which it is
mainly distributed.

3. A pair of nerves from the sub-oesophageal ganglion.

4. A nerve from each half of the pharyngeal collar just
beyond its divergence from its fellow. (Origin incorrectly
shown.)

5. Two large cerebral nerves, which run forwards from the

*So closely are the two halves of the ventral cord united that its double
nature can scarcely be made out without sections.

THE BIOLOGY OF AN ANIMAL.

Fia. 29.— Anterior portion of the earthworm laid open from above, with the alimen-
tary and circulatory systems dissected away. c.c., circum-cesophageal com-
missure ; e.g., cerebral ganglia ; (fe, dissepiment : /, funnel of nephridium ; np
nephridium; o, ovary; od, oviduct; pft, pharynx; ps, prostomium ; r.s., seminal
receptacle; «.d., sperm-duct; «.f., sperm-funnel; 8.V.I., lateral seminal vesicle;
t, testis; v.g., and v.n.c., ventral nerve-cord.

NER VE-IMP ULSES. 67

cerebral ganglia, break up into many branches, and are dis-
tributed to the anterior part of the body.

Besides the main ganglia of the central system, there are many smaller
ganglia in various parts of the body. Of these the most important are the
pharyngeal ganglia — 3 to 5 in number — which lie on the wall of the
pharynx on each side just within the pharyngeal collar. They are con-
nected with the latter by fine branches, and send minute nerves out upon
the walls of the pharynx. This series of ganglia is often inappropriately
called the sympathetic system.

Physiology of the Nervous System. Nerve - impulses.
What is the origin and nature of a nerve-impulse? Under nor-
mal conditions the impulse is set up as the result of some dis-
turbance, technically called a stimulus, acting upon the end of
the fibre. A touch or pressure upon the skin, for example, acts
as a stimulus to the nerve-fibres ending near the point touched —
that is, it causes nerve-impulses to travel inwards along the fibres
towards the central system. The nerves may be stimulated by
a great variety of agents : — by mechanical disturbance, as in the
case just cited, by heat, electricity, chemical action, and in
special cases by waves of light or of sound, and upon this prop-
erty of the nerves depends the power of the worm to receive as
afferent impulses impressions from the outer world. But, besides
this, nerve-fibres may also be stimulated by physiological changes
taking place within the nerve-cells, which may thus send out
efferent impulses to the various organs and so control their ac-
tion.

Regarding the precise nature of the nerve-impulse we are ignorant, but
it is probably a chemical or molecular change in the protoplasm, travelling
rather rapidly along the fibre, like a wave.* We know that the nature
of the impulse is not in any way dependent upon the character of the stimu-
lus. The stimulus can only throw the nerve into action ; and this action
is always the same whatever be the stimulus — as the action of a clock
remains the same whether it be driven by a weight or by a spring.

Co-ordination. The activities of the various organs are co-
ordinated by a chain of events which in its simplest form is known
as a reflex action, and which lies at the bottom of most of
the more complicated forms of nervous action. Its nature is

* In the frog the nervous impulses travel at the rate of about 28 metres per
second ; in man it is considerably more rapid.

68 THE BIOLOGY OF AN ANIMAL.

illustrated by the diagram (Fig. 30). Co-ordination be-
tween S and Jf (two organs) is not effected by a direct nervous

connection, but indirectly
through a nerve-centre, 67,
which is a nerve-cell or group
of nerve-cells situated in one
of the ganglia, with which both
S and M are separately con-
nected by nerve -fibres. If S
be thrown into action, an affer-
ent impulse travels to Ct ex-
cites the nerve-centre, and

FIG. 30.— Diagram of simple reflex action, causes an efferent impulse to

S, skin to which stimulus is applied; a/, t j t t jr j j j j tj
the afferent nerve-fibre ; C, nerve-centre ;

c/, efferent nerve-fibre; Jf, muscle in by thrown into action also, OF

is modified in respect to actions

already going on. Thus the actions of S and M are co-ordi-
nated through the agency of C\ the whole chain of events
constituting a reflex action.

For example, let S be the skin and M a certain group of
muscles. If the skin be irritated, afferent impulses travel in-
wards to nerve-centres in the ganglia (6*), which thereupon send
forth efferent impulses to the appropriate muscles. Muscular
contractions result, and the worm draws back from the unwel-
come irritation.

This chain of events involves three distinct actions on the
part of the nervous system which must be carefully distinguished,
viz. : (a) the afferent impulse; (I) action of the centre; (c)
the efferent impulse. It must not be supposed that the afferent
Impulse passes unchanged out of the centre as the efferent impulse,
i.e., is simply "reflected," like a ball thrown against a wall, as
the word ' ' reflex ' ' seems to imply. The afferent impulse as such
ends with the nerve-centre, which it throws into activity. The
efferent impulse is a new action set up by the agency of the
centre.

There is reason to believe that many if not all nerve-centres
are connected with a number of different afferent and efferent
paths, and also with other centres, as shown in the diagram
Fig. 31. Efferent impulses may therefore be sent out from

SENSES OF THE EARTH-WORM. 69

the centre in various directions, and the precise path chosen
depends on some unknown-
action taking place in the
centre. The action of the
centre moreover may be
modified by efferent impulses
arriving from other centres,
and thus we can dimly per-
ceive how reflexes may be-
contr oiled and guided, and
how even the most compli-
cated forms of nervous ac-
tivity may be Compounded FlG' K--™**"M representing three nerve.
* * centres and connections. Arrows represent

OUt of elements similar to the possible direction of nerve-impulses.
is .• a/, one afferent path ; ef, one efferent path.

There is reason to believe that in the earthworm each ven-
tral ganglion presides over the somite to which it belongs, and
is probably in the main a collection of reflex centres from whose
action the element of consciousness is absent. But there is also
some reason to believe that the cerebral ganglia occupy a higher
position, since they probably receive the nerves of sight, taste,
and smell, besides those of touch, while the ventral ganglia re-
ceive only those of touch. Experiment has shown further that
the cerebral ganglia exercise to a certain limited extent a con-
trolling action over those of the ventral chain by means of im-
pulses sent backwards through the commissures, though this
action is far less conspicuous here than in higher metameric ani-
mals such as the insects.*

The Sensitive System. (Organs of Sense.) The sensitive
system is distinguished from the nervous system as a matter of
convenience of description, since most of the higher animals
possess definite " sense-organs" which receive stimuli and throw
into action the sensory nerves proceeding from them. Although
the earthworm possesses the ' ' senses ' ' of touch, taste, sight,
and smell, it has no special organs for these senses apart from
the general integument covering the surface of the body, and

* For a fuller discussion the student is referred to special works on Phy
ology.

70 THE BIOLOGY OF AN ANIMAL.

hence caii hardly be said to possess any proper sensory system.
We do not know, moreover, whether the so-called "sensations"
of the earthworm are really states of consciousness as in ourselves,
for we do not even know whether earthworms possess any form
of consciousness. When, therefore, we speak of the earthworm
as possessing the "sense" of touch or of sight we mean simply
that some of the nerves terminating in the skin may be stimu-
lated by mechanical means or by rays of light, without necessa-
rily implying that the worm actually feels or sees as we feel and
see.

It has recently been shown that the skin contains many cells each of
which gives off a single nerve-fibre that may be traced directly into the
ventral nerve-cord. These "sensory cells " may be regarded as "end-
organs " through which the stimuli are conveyed to the fibres. It has also
been shown that these cells are aggregated in minute groups thickly scat-
tered over the surface of the body. Each of these groups may be regarded
as a simple form of sense-organ.

The sense of touch extends over the whole surface of the
body. That of taste is probably located in the cavity of the
mouth and pharnyx ; the location of the sense of smell is un-
known. Darwin's experiments have shown that the earth-
worm's feeble sense of sight is confined to the anterior end of
the body. It is probable that the nerves of sight, taste, and
smell enter the cerebral ganglia -alone, while those of touch run
to other ganglia as well.

Systems of (Organs of) Support, Connection, Protection, etc.
The structure and mode of life of many animals are such as to
require some solid support to the soft parts of the body. Such
supporting structures are, for instance, the bones of vertebrata,
the hard outer shell of the lobster or beetle, and the coral
which forms the skeleton of a polyp. The earthworm has,
however, nothing of the sort, and it is obvious that a hard sup-
porting-organ would be not only useless, but even detrimental.
The power of creeping and burrowing through the earth depends
upon great flexibility and extensibility of the body; and with
this the presence of a skeleton might be incompatible.

The connecting system consists simply of various tissues by
which the different organs are bound firmly together. These
can only be seen upon microscopical examination. The most
important of them is known as connective tissue.

DEFENCES OF THE EARTHWORM. 71

As to protective structures, the earthworm is probably one of
the most defenceless of animals. Nevertheless there are certain
structures which are clearly for this purpose. The cuticle which
covers the surface is a thin but tough membrane which protects
the delicate skin from direct contact with hard objects. It
passes into the mouth and lines the alimentary canal as far down
as the beginning of the stomach-intestine. In the gizzard,
where food is ground up, the cuticle is prodigiously thick and
tough, and must form a very effective protection for the soft
tissues beneath it. The main defence of the animal lies, how-
ever, not in any special armor, but in those instincts which lead
it to lie hidden in the earth during the day and to venture forth
only in the comparative safety of darkness.

CHAPTEK Y.

THE BIOLOGY OF AN ANIMAL (Continued).

The Earthworm.
KEPRODUCTION. EMBRYOLOGY.

Reproduction. The life of every organic species runs in
regularly recurring cycles, for every individual life has its limit.
In youth the constructive processes preponderate over the de-
structive and the organism grows. The normal adult attains a
state of apparent physiological balance in which the processes of
waste and repair are approximately equal. Sooner or later,
however, this balance is disturbed. Even though the organism
escapes every injury or special disease the constructive process
falls behind the destructive, old age ensues, and the individual
dies from sheer inability to live. Why the vital machine should
thus wear out is a mystery, but that it has a definite cause and
meaning is indicated by the familiar fact that the span of natural
life varies with the species ; man lives longer than the dog, the
elephant longer than man.

It is a wonderful fact that living things have the power to
detach from themselves portions or fragments of their own
bodies endowed with fresh powers of growth and development
and capable of running through the same cycle as the parent.
There is therefore an unbroken material (protoplasmic) continuity
from one generation to another, that forms the physical basis of
inheritance, and upon which the integrity of the species depends.
As far as known, living things never arise save through this
process; in other words every mass of existing protoplasm is
the last link in an unbroken chain that extends backward in the
past to the first origin of life.

The detached portions of the parent that are to give rise to
offspring are sometimes masses of cells, as in the separation of
branches or buds among plants, but more commonly they are single

72

REPRODUCTION. 73

cells, known as germ- cells, like the eggs of animals and the
spores of ferns and mosses. Only the germ-cells (which may
conveniently be distinguished from those forming the rest of the
body, or the somatic cells), escape death, and that only under
certain conditions.

All forms of reproduction fall under one or the other of two
heads, viz., Agamogenesis (asexual reproduction) or Gamogenesis
(sexual reproduction). In the former case the detached portion
(which may be either a single cell or a group of cells) has the
power to develop into a new individual without the influence of
other living matter. In the latter, the detached portion, in this
case always a single cell (ovum, oosphere, etc.), is acted upon
by a second portion of living matter, likewise a single cell, which
in most cases has been detached from the body of another in-
dividual. The germ is called the female germ-cell; the cell act-
ing upon it the male germ-cell / and in the sexual process the
two fuse together (fertilization, impregnation] to form a single
new cell endowed with the power of developing into a new in-
dividual. In some organisms (e.g., the yeast-plant and bacteria)
only agamogenesis has been observed ; in others (e.g. , vertebrates)
only gamogenesis ; in others still both processes take place as in
many higher plants.

The earthworm is not known to multiply by any natural
process of agamogenesis. It possesses in a high degree, however,
the closely related power of regeneration / for if a worm be cut
transversely into two pieces, the anterior piece will usually make
good or regenerate the missing portion, while the posterior piece
may regenerate the anterior region. Thus the worm can to a
certain limited extent be artificially propagated, like a plant, by
cuttings, a process closely related to true agamogenesis.* Its
usual and normal mode of reproduction is by gamogenesis, that
is, by the formation of male germ -cells (spermatozoa) and female
germ-cells (ova). In higher animals the two kinds of germ-
cells are produced by different individuals of opposite sex. The
earthworm on the contrary is hermaphrodite or bisexual; every

* Many worms nearly related to Lumbricus — e.g., the genus Dero, and other
Naads — spontaneously divide themselves into two parts each of which becomes
& perfect -animal. This process is true agamogenesis, though obviously closely
related to regeneration.

74

THE BIOLOGY OF AN ANIMAL.

individual is loth male and female, producing both eggs and
spermatozoa. The ova arise in special organs, the ovaries, the
spermatozoa in spermaries or testes.

The ripe ovum (Fig. 33, JB) is a relatively large spherical
cell, agreeing closely with the egg of the star-fish (Fig. 12), but
having a thinner and more delicate membrane. It is still cus-
tomary to apply to ova the old terminology, calling the cell-
substance vitellus, the membrane vitelline membrane, the nucleus
germinal vesicle, and the nucleolus germinal spot.

The ripe spermatozoon (Fig. 33, C) is an extremely minute
elongated cell or filament thickening towards one end to form
the head (n), which contains the nucleus of the cell enveloped by a,
thin layer of protoplasm. This is followed by a short " middle
piece ' ' (in) to which is attached a long vibratory fiagellum or tail
(t). The tail is virtually a long cilium (p. 31), which by vigorous
lashing drives the whole cell along head-foremost, very much as
a tadpole is driven by its tail.

Since the ovaries and spermaries give rise to the germ-cells,
they are called the essential organs of
reproduction. Besides these, Lumbricus,
like most animals, has accessory organs of
reproduction which act as reservoirs or
carriers of the germs, assist in securing
cross-fertilization, and minister to the
wants of the young worms.

Essential Reproductive Organs. The
ovaries are two in number and lie one on
either side in the 13th somite attached to
the hinder face of the anterior dissepiment
(ov, Fig. 29). They are about 2mra in
length, distinctly pear-shaped, and at-
fl ai tached by the broader end (Fig. 32). The

.-'•:^j narrow' extremity contains a single row of
FIG 33— Th ova and is called the egg-string (es). In

enlarged, b, the basal part; this the ova are ripe or nearly so; behind

teSg jmn^tuTovaT^ ^ ^^ °ff intO th°Se mOT6 and m°re

egg-string; or, ripe ovum immature, till these are lost in a mass of

ready to fall off. -, ,./,. . , ,,/...

nearly unamerentiated cells (jprimitive
ova), constituting the great bulk of the ovary. Each of these,

REPRODUCTIVE ORGANS. 75

however, is surrounded with still smaller cells constituting its
nutrient envelope or follicle. As the ova mature the follicles
still persist, and they may be detected even in the eggstring.
When fully ripe the ovum bursts the follicle and is shed from
the end of the egg-string into the body-cavity. It is ultimately
taken into the oviduct and carried to the exterior.

The development of the ovary shows it to be morphologically
a thickening of the peritoneal epithelium. The eggs therefore
are originally epithelial cells.

The spermaries or testes (t,t, Fig. 29) are four in number and
in outward appearance are somewhat similar to the ovaries.
They are small flattened bodies with somewhat irregular or lobed
borders, lying one on either side the nerve-cord in a position
corresponding with that of the ovaries, but in somites 10 and 11.
Like the ovary the testis is a solid mass of cells, which are shed
into the body-cavity and are finally carried to the exterior.
The sperm-cells leave the testis, however, at a very early period
and undergo the later stages of maturation within the cavities of
the seminal vesicles described below.

Accessory Reproductive Organs. The most important of the
accessory organs are the genital ducts, by which the germ-cells
are passed out to tlje exterior. Both the female ducts (oviducts}
and the male (sperm-ducts) are tubular organs opening at one
end to the outside, through the body-wall, and at the other end
into the coelom by means of a ciliated funnel somewhat similar
to a nephridial funnel, but much larger. By means of these
ciliated funnels the germ-cells after their discharge from the
ovary or testis are taken up and passed to the exterior.

The oviducts (od, Fig. 29, Fig. 23) are two short trumpet-
shaped tubes lying immediately posterior to the ovaries and pass-
ing through the dissepiment between the 13th and 14th somites.
The inner end opens freely into the cavity of the 13th somite,
by means of a wide and much-folded ciliated funnel, from the
centre of which a slender tube passes backward through the
dissepiment, turns rather sharply towards the outer side and,
passing through the body-wall, opens to the outside on the 14th
somite (see p. 43). Immediately behind the dissepiment the
oviduct gives off at its dorsal and outer side a small pouch,
richly supplied with blood-vessels. In this, the receptaculum

76 THE BIOLOGY OF AN ANIMAL.

ovorum, the ova taken up by the funnel are temporarily stored
before passing out to the exterior.

It is probable that the eggs never float freely in the coelom,
but drop out of the ovary at maturity directly into the mouth of
the funnel. They pass thence into the receptaculum, where they
may remain for a considerable period.

The sperm-ducts (vasa deferentia) (sd, Fig. 29) are very
long slender tubes, open like the oviducts at both ends. The
outer opening is a conspicuous slit surrounded by fleshy lips
(Fig. 21), on the ventral side of the 15th somite. From this
point the duct runs straight forwards to the 12th somite, where
it branches like a Y, the two branches passing forwards to ter-
minate, one in the llth somite, the other in the 10th. • Near its
end each branch is twisted into a peculiar knot and finally ter-
minates in an immense ciliated funnel (the so-called "ciliated
rosette"), the borders of which are folded in so complicated a
manner that they form a labyrinthine body, the true nature of
which can only be made out in microscopic sections.

The two pairs of sperm-funnels (Fig. 29) lie in the 10th
and llth somites, immediately posterior to the respective testes,
i.e., they have essentially the same relation to the testes as that
of the oviduct-funnels to the ovaries.

The testes and sperm-funnels can be readily made out only in young
specimens. In mature worms they are completely enveloped by the semi-
nal vesicles described below.

Seminal vesicles. These, the most conspicuous part of the
reproductive apparatus, are voluminous pouches in which the^
sperm-cells undergo their later development, after leaving the
testis. They are large white bodies lying in somites 9 to 12 and
usually overlapping the oesophagus in that region. In all cases
there are three pairs of lateral seminal vesicles, viz. , an anterior
pair in somite 9, a middle pair in somite 11, and a posterior pair
in somite 12. In immature specimens these six are entirely
separate, and allow the testes to be easily seen. In mature
worms (as shown in Fig. 29) the posterior pair of lateral
vesicles grow together in the middle line, thus forming a pos-
terior median vesicle lying below the alimentary canal in the
llth somite. In like manner an anterior median vesicle is
formed in the 10th somite by the union of the two anterior pairs

EGG-LA TING. 77

of lateral vesicles. The two median vesicles thus formed envelop
the testes and sperm- funnels of their respective somites and hide
them from view.

The sperm-cells leave the testis at a very early period and float freely
in the cavities of the seminal vesicles, where many stages of their develop-
ment may easily be observed. They are developed in balls known as
gpermatospheres, each of which consists of a central solid mass of proto-
plasm surrounded by a single layer of sperm-cells. When mature the
spermatozoa separate from the central mass and are drawn into the fun-
nels of the sperm-ducts. The manner in which this action is controlled is
not understood.

The seminal receptacles are accessory organs of reproduction
in the shape of small rounded sacs or pouches, open to the out-
side only, at about the level of the upper row of setae. They
lie between the 9th and 10th, and 10th and llth somites (s.r,
Figs. 24 and 29), where their openings may be sought for (Fig.
21). Their function is explained under the head of copulation.

Accessory glands. Besides all the structures so far described
there are many glands which play a part in the reproductive
functions. The setigerous glands from about the 7th to about
the 19th somite (sometimes fewer, sometimes none at all) are
often greatly enlarged, and form the glandular prominences men-
tioned at p. 46. They seem to be used as organs of adhesion
during copulation. The clitellum is filled with gland-cells which
probably serve in part to secrete a nourishing fluid for the young
worms, and in part to provide a tough protecting membrane to
cover them.

Copulation. Egg-laying. Inasmuch as each individual earth-
worm produces both ova and spermatozoa, it might be supposed
that copulation, or the sexual union of two -different individuals,
would not be necessary. This, however, is not the case. The
ova of one individual are invariably fertilized by the spermatozoa
of another individual after a process of copulation and exchange
of spermatozoa, as follows : During the night-time, and usually
in the spring, the worms leave their burrows and pair, placing
themselves so that their heads point in opposite directions and
holding firmly together by the enlarged setigerous glands and the
thickened lower lateral margins of the clitellum. During this
act the seminal receptacles of each worm are filled with sperma-
tozoa from the sperm-ducts of the other, after which the worms

78 THE BIOLOGY OF AN ANIMAL.

separate. [The spermatozoa thus received are. simply stored up
and do not perform' their function until the time of egg-laying.]
When the worm is ready to lay its eggs the glands of the
clitellum become very active, pouring out a thick glairy fluid
which soon hardens into a tough membrane and forms a girdle
around the body. Besides this a large quantity of a thick jelly-
like nutrient fluid is poured out and retained in the space be-
tween the girdle and the body of the worm. The girdle is
thereupon gradually worked forward toward the head of the
worm by contractions of the body. As it passes the 14th somite
a number of ova are received from the oviducts, and between
the 9th and llth somites a quantity of spermatozoa are added
from the seminal receptacles where they have been stored since
the time of copulation, when they were obtained from another
worm. The girdle is next stripped forwards over the anterior

end and is finally thrown
* completely off. As it
passes off its open ends
immediately contract
tightly together, and the
girdle becomes a closed
capsule (Fig. 33) contain-
ing both ova and sperma-
A ° tozoa floating in a nutri-

Fio. 33.— A, egg-capsule enlarged 5 diameters . a .., „,]

(a few eggs, or, enlarged to the same scale are tlVC lUlld Or milK.

shown near by on the right) ; B, an ovum very membraiie SOOn assumes a

much enlarged ; C, a spematozoon, enormously _

magnified ; n, head ; m, middle piece ; t, tail. light yellowish Or LrOWn

color, becomes hard and tough, and serves to protect the de»
veloping embryos. The capsules may be found in May or June
in earth under logs or stones, or especially in heaps of manure.
Within the capsules the fertilization and development of the ova
take place.

Fertilization and Embryological Development. The sperma-
tozoa swim actively about in the nutrient fluid of the capsule,
approach an ovum, and attach themselves to its surface by their
heads. Several of the spermatozoa then enter the vitellus (cf .
p. 80), but it has been proved that only one of these is con-
cerned in fertilization, the others dying and becoming absorbed
by the ovum.

FERTILIZATION OF THE EGG. 79

It is probable that the tail plays no part in the actual fertili-
zation, but is merely a locomotor apparatus for the head (nucleus)
and middle-piece.

Within the ovum the head of the spermatozoon persists as
the sperm-nucleus (or male pro-nucleus), while the protoplasm in
its neighborhood assumes a peculiar and characteristic radiate
arrangement like a star, probably through the influence of the
middle-piece.

After the entrance of the spermatozoon the egg segments off

FIG. 34. — Fertilization of the ovum. A, entrance of the spermatozoon (in the sea-
urchin, after Fol). .B, the sea-urchin egg after entrance of the spermatozoon;
' within and to the left is the egg-nucleus ; above is the sperm-nucleus, with a cen-
trosome near it (modified from Hertwig). C, diagram of the ovum after extrusion
of the polar cells (p.c.), and union of the two pro-nuclei to form the segmenta-
tion-nucleus. The smaller and darker portion of the latter is derived from the
sperm-nucleus. Two asters or archoplasm-spheres are shown near the nucleus.
These arise by the division of a single aster derived from the middle-piece of the
spermatozoon. D, two-celled stage of the earthworm, after the first fission of
the ovum. (After Vejdovsky.)

at one side two small cells, one after the other, known as the
polar cells or polar bodies. These take no part in the formation
of the embryo, and their formation probably serves, in some way
-not yet wholly clear, to prepare the egg for the last act of
fertilization. After the formation of the polar cells the egg-
nucleus (now often called \h& female pro-nucleus) and the sperm-
nucleus approach one another and finally become intimately

gO THE BIOLOGY OF AN ANIMAL.

associated to form the segmentation- or cleavage-nucleus / by this
act fertilization is completed.

The process of fertilization appears to be essentially the same among
all higher animals, and in a broader sense to be identical with the sexual
process among all higher and many lower plants (compare the fern. p. 139),
but its precise nature is still in dispute. It is certain that one essential
part of it is the union of two nuclei derived from the two respective parents.
This has led to the view, now held by many investigators, that inheritance
has its seat in the nucleus, and that chromatiu (p. 23), is its physical
basis. Later researches have shown that another element known as the
archoplasm- or attraction-sphere is concerned in fertilization, and this is
apparently always derived from the middle-piece. It is not yet certain
whether the archoplasm is to be regarded as a nuclear or a cytoplasrnic
structure, and it is equally doubtful whether it plays an essential or merely
a subsidiary role in fertilization and inheritance (cf. p. 84).

Cleavage of the Fertilized Ovum. Soon after fertilization the
ovum begins the remarkable process of segmentation which
has already been briefly sketched on p. 25. The segmen-
tation-nucleus divides into two parts, and this is followed by
a division of the vitellus, each half of the original nucleus becom-
ing the nucleus of one of the halves of the vitellus ; that is, the
original cell divides into two smaller but similar cells (see Fig.
34). These divide in turn into four, and these into eight, and
so on, but yet remain closely connected in one mass. In the
case of the earthworm, the cells do not multiply in regular
geometrical progression, but show many irregularities ; and more-
over they become unequal in size at an early period.

The blastula (pp. 25, 85,) shows scarcely any differentiation
of parts, though the cells of one hemisphere are somewhat smaller
than the others. From this time forwards the whole course of
development is a process of differentiation, both of the cells and of
the organs into which they soon arrange themselves. One of
the first steps in this process is a flattening of the embryo at the
lower pole — i.e., the half consisting of larger cells (Fig. 35, D).
The large cells are then folded into the segmentation-cavity so
as to form a pouch opening to the exterior ; at the same time
the embryo becomes somewhat elongated (Fig. 35, E, F\

This process is known as gastndation, and at its completion
the embryo is called the gastrula. The infolded pouch (called
the archenteron} is the future alimentary canal ; -its opening (now
known as the Uastopore) will become the mouth ; and the layer

THE GERM-LAYERS.

81

of small cells over the outside will form the skin or outer layer
of the body-wall.

The embryo very soon begins to swallow, through the blasto-
pore, the milklike fluid in which it floats, and to digest it with-
in the cavity of the archenteron.

It is obvious that the embryo already shows a distinct differ-

Fio. 35.— Diagrams of the early stages of development in the earthworm. A, accu-
rate drawing of the blastula, surrounded by the vitelline membrane (after Vej-
dovsky) ; B, blastula in optical section showing the large segmentation.-cavity
(8.C.), and the parent-cell of the mesoblast (m.); C, later blastula, showing forma-
tion of mesoblast-cells ; D, flattening of the blastula preparatory to imagination ;
.E, the gastrula in side view ; as the infolding takes place the two mesoblast-
bands are left at the sides of the body, in the position shown by the dotted lines;
F, section of E along the line s-s, showing the mesoblast-bands and pole-cells.

entiation of parts which perform unlike functions. In fact we
may regard the gastrula as composed of two tissues still nearly
similar in structure though unlike in function. One of these-
consists of the layer of cells which forms the outer covering;
this tissue is known as the ectoblast (ec, Fig. 35). The second
tissue is the layer of cells forming the wall of the archenteron ;
it is called the entoblast (en). The ectoblast and entoblast to-
gether are known as the primary germ-layers.

Meanwhile changes are taking place which result in the for-
mation of a third germ-layer lying in the segmentation-cavity
between the ectoblast and entoblast and therefore called the
mesoblast (m, .Figs. 35, 36). In some animals the mesoblast
does not arise until after the completion of gastrulation. In

g2 TUB BIOLOGY OF AN ANIMAL.

Lwnlricus, however, it goes on during gastralation and begins
even before gastrulation. Even in the blastula stage two large
cells may be distinguished which afterwards give rise to the
mesoblast and are hence called the primary mesoUastie cells.
They soon bud forth smaller cells into the segmentation-cavity,
and as the blastula flattens they themselves sink below the sur-
face At this period, therefore, the mesoblast forms two bands
of cells (mesoblast-bands) each terminating beliind in the large
mother-cell or pole-cell. Throughout the later stages the pole-
cells continue to bud forth smaller cells which are added to the
hinder ends of the mesoblast-bands (Figs. 35, 36).

ec

n

FIG. 36.— Diagrams of later embryonic stages. A, late stage in longitudinal section,
showing the appearance of the cavities of the somites ; B, the same in cross-sec-
tion ; E, diagram of a young worm in longitudinal section after the formation of
the stomodeeum, proctodeeum, and anus; C, the same in cross-section, showing
the beginning of the nervous system ; D, cross-section of later stage with the
nervous system completely established, al, alimentary canal ; ar, archenteron :
on, anus; cce, coelom; ec, ectoblast; en, entoblast; m1, primary mesoblastic cells;
•m", mesoblast; m/i, mouth; n, nervous system; *, cavity of somite; s.m, somatic
layer of the mesoblast, which with the ectoblast forms the somatopleure ; «p!.m,
splanchnic layer of the mesoblast, which with the entoblast forms the splanch-
nopleure.

After each division the pole-cells increase in size, so that up
to a late stage in development they may be distinguished from

CELL-DIVISION. KARYOKINESIS.

83

the cells to which they give rise. T^he two masses of mesoblastic
cells gradually increase in size andjmally fill the segmentation-
cavity. ^

The internal phenomena of cell-division are of great complexity and
can here be given only in outline. The ordinary type of cell-division, as
shown in the segmentation of the ovum and in the multiplication of most
tissue-cells, involves a complicated series of changes in th'e nucleus known
as karyokinesis or mitosis. These changes, which appear to be of essen-
tially the same character in nearly all kinds of cells, and both in plants and
in animals, are illustrated by the following diagrams :

C D

FIG. 37. — Diagrams of indirect cell-division or karyoKinesis.

A. Cell just prior to division, showing nucleus (n) with its chromatic reticulum and
the attraction-sphere and centrosome (c).

B. First phase ; the attraction-sphere has divided into two, which have moved
180° apart ; the reticulum has been resolved into five chromosomes (hlack), each
of which has split lengthwise.

C. Second phase; fully developed karyokinetic figure (amphiaster), with spindle
and asters; the chromosome-halves are moving apart.

D. Final phase ; the cell-body is dividing, the spindle disappearing, the daughter-
nuclei about to be formed.

In its resting state the nucleus contains a network or reticulum of
chromatin (Fig. 37, A). As the cell prepares for division a small body (c)

84 THE BIOLOGY OF AN ANIMAL.

makes its appearance near the nucleus, known as the attraction- sphere or
archoplasm-mass, and in its interior there is often a smaller body, the
centrosome. The first step in cell-division is the fission of the archoplasm-
mass into two, each containing a centrosome (derived by fission of the
original centrosome); after this the two masses move apart to opposite
poles of the nucleus (Fig. 37, B). The reticulum now becomes, in most
cases, resolved into a thread coiled into a skein (not shown in the figure),
which finally breaks up into a number of bodies known as chromosomes.
Their form (granular, rodlike, loop-shaped) and number (two, eight, twelve,
sixteen, etc., or often much higher numbers) appear to be constant for
each species of plant and animal. The second principal step is the longi-
tudinal splitting of each chromosome into halves (Fig. 37, B) and the
disappearance of the nuclear membrane.

In the third place starlike rays (aster) appear in the protoplasm around
the archoplasm-masses, a spindle-shaped structure appears between them
(Fig. 37, C), and the double chromosomes arrange themselves around the
equator of the spindle. The structure thus formed is known as theamphi-
aster or Jtopffekinetic figure.

Fourthly, the two halves of each chromosome move apart towards the
respective poles of the spindle and the entire cell-body then divides in a
plane passing through the equator of the spindle. Each group of daughter-
chromosomes now gives rise to a reticulum, which becomes surrounded with
a membrane and forms the nucleus of the daughter-cell. The spindle dis-
appears, and in some cases the archoplasm-mass, with its star- rays (aster),
seems to disappear also. In other cases, however, the archoplasm-mass and
centrosome persist and may be found in the resting cell (e.g., in leucocytes
and connective-tissue cells), lying near the nucleus in the cytoplasm.

It appears from the foregoing description that each daughter-cell re-
ceives exactly half the substance of the mother-nucleus (chromatic), mother-
archoplasm, and mother-centrosome. In many cases the cytoplasm also
divides equally, in other cases unequally.

It has been proved in a considerable number of cases that in the fer-
tilization of the ovum each germ-cell contributes the same number of chro-
mosomes, and the wonderful fact has been established with high probability
that the paternal and maternal chromatic substances are equally distributed
to the two cells found at the first segmentation of the ovum. It is further
probable that this equal distribution continues in all the later divisions ;
and if this is true, every cell in the whole adult body contains material
directly derived from both parents, and hence may inherit from both.

Gastrulation. Germ-layers. Differentiation. Origin of the
Body. Almost from the first the cells arrange themselves so as
to surround a central cavity known as the segmentation-cavity.
This cavity increases in size in later stages, so that the embryo
finally appears as a hollow sphere surrounded by a wall consist-

DEVELOPMENT OF THE ORGANS. 85

ing of a single layer of cells. This stage is known as the llastula
(or Uastosphere) (A, B, Fig. 35).

The formation of the GERM-LAYERS is one of the most im-
portant and significant processes in the whole course of develop-
ment. Germ-layers like those of Lumbricus, and called by
the same names, are found in the embryos of all higher ani-
mals ; and it will hereafter appear that this fact has a profound
meaning.

Development of the Organs. (Organogeny.) The embryo gradu-
ally increases in size and at the same time elongates. As it
lengthens, the blastopore (in this case the moutJi) remains at one
end, which is therefore to be regarded as anterior, and the
elongation is backwards. The cells of all three germ-layers
continually increase in number by division, new matter and
energy being supplied from the food, which is swallowed by the
embryo in such quantities as to swell up the body like a bladder.
The archenteron enlarges until it comes into contact with the
ectoblast and the segmentation-cavity is obliterated.

The two primary mesoblastic cells are carried backwards,
and always remain at the extreme posterior end (m, Fig. 36).
The mesoblast is in the form of two bands lying on either side
of the archenteron, and extending forwards from the primary
mesoblastic cells.

This is clearly seen in a cross-section of the embryo, as in
Fig. 36, J?, C. The mesoblastic bands are at first solid, but
after a time a series of paired cavities appears in them, con-
tinually increasing in number by the formation of new cavities
near the hinder end of the bands as they increase in length. A
cross-section passing through one pair of these cavities is shown
at B, Fig. 35. As the bands lengthen they also extend up-
wards and downwards (C", Fig. 35), until finally they meet above
and below the archenteron. The cavities at the same time
continue to increase in size, and finally meet above and below
the archenteron, which thus becomes surrounded by the body-
cavity or co3lom (Z)). The cavities are separated by the double
partition-walls of mesoblast. These partitions are the dissepi-
ments, and the cavities themselves constitute the co3lom. The
outer mesoblastic wall of each cavity is known as the somatic
layer (s.m.); it unites with the ectoblast to constitute the body-

THE BIOLOGY OF AN ANIMAL.

wall (somatopleure). The inner wall, or splanchnic layer
(stpl.m), unites with the entoblast to constitute the wall of the
alimentary canal (splanchnopleure). An ingrowth of ectoblast
(stomodceum) takes place into the blastopore to form the pharynx,
and a similar ingrowth at the opposite extremity (proctodceum)
unites with the blind end of the arckenteron to form the anus
and terminal part of the intestine.

As to its origin, therefore, the alimentary canal consists of
three portions, viz. : (1) the arckenteron, consisting of tke

d.v j
ch s hV

n

n.

s.i.v

FIG. 38. — Diagram of a cross-section of Lumhriciis, showing the relation of the
various organs, etc., to the germ-layers. Ectoblastic structures shaded with fine
parallel lines, entoblastic with coarser parallel lines, mesoblastic with cross-lines;
o/.c, alimentary canals; c/i, chloragogue layer; c<r, ccelom; c.m, circular muscles
of body-wall; c.ma, circular muscles-of alimentary wall; ep, lining epithelium of
alimentary canal; il.v, dorsal vessel; fij/i hypodermis or skin; l.m, longitudinal
muscles of body-wall ; l.m.a, longitudinal muscles of alimentary wall ; ?i, central
part of nerve-cord ; np, nephridium ; JM, sheath of nerve-cord ; p.c, peritoneal
epithelium ; r, reproductive organs ; g.i.v, sub-intestinal vessel.

original entoblast; (2) the stomodaeum or pharyngeal region,
lined by ectoblast; and (3) the proctodaeum or hindmost part,
also lined by ectoblast. These three parts are called the fore-
gut (stomodseum), mid-gut or mensenteron (archenteron), and
hind-gut (proctodaeum), and it is a remarkable fact that these
same parts can be distinguished in all higher animals, not ex-
cepting man.

The body now becomes jointed by the appearance of trans-
verse folds opposite the dissepiments, and the metamerism of the
body becomes evident on the exterior. The young worm has
thus reached a stage (E^ Fig. 36) where its resemblance to the

FATE OF THE GERM-LAYERS. 87

adult is obvious. It has an elongated, jointed body, traversed
by the alimentary canal, which opens in front by the mouth and
behind by the anus. The metamerism is expressed externally
by the jointed appearance, internally by the presence of paired
cavities (coalom) separated by dissepiments. Both the body-wall
and the alimentary wall consist of two layers : the former of
ectoblast without and somatic mesoblast within; the latter of
splanchnic mesoblast without (i.e., towards the body-cavity),
and either entoblast or ectoblast within, according as we con-
sider the mid-gut on the one hand, or the fore- and hind-gut on
the other. This is shown in Fig. 38, which represents a cross-
section of the embryo through the mid-gut. If this be clearly
l>orne in mind the development of all the other organs is easy to
understand, since they are formed as thickenings, outgrowths,
•etc., of the parts already existing. For instance, the blood-
vessels make their appearance everywhere throughout the meso-
tlast, and the reproductive organs are at first mere thickenings
on the somatic layer of the mesoblast, afterwards separating
more or less from it so as to lie in the cavity of the coelom.
The nervous system is produced by thickenings and ingrowths
from the ectoblast. The origin of the different parts is shown
in the following scheme : —

THE GERM-LAYERS AND THEIR DERIVATIVES.

Ectoblast.

Outer skin (Hypodermis and Cuticle).
Nerves and Ganglia.
Lining membrane of pharynx (fore-gut).
Lining membrane of anus and hinder part of intestine (hind -gut).

Mesoblast.

Muscles.
Blood-vessels.
Reproductive organs.
Outer layers of alimentary canal.

Entoblast.

Lining membrane of greater part of the alimentary canal

(mid-gut).

The above statements * as to the origin of the various organs
acquire great interest in view of the fact that they are essen-

* The nephridia have been omitted since their precise origin is in dispute.
It is certain that the outer portion of the tube (muscular part) is an ingrowth
from the ectoblast. The latest researches seem to show that the entire ne-
phridium has the same origin, though some authors describe the inner portion
as arising from mesoblast.

88 THE BIOLOGY OF AN ANIMAL.

tially true of all animals above the earthworm, as well as of
many below it — of all, in a word, in which the three germ-
layers are developed, i.e., all those above the Ccdenterata, or
polyps, jelly-fishes, hydroids, sponges, etc. In man, as in the
earthworm and all intermediate forms, the ectoblast gives rise
to the outer skin (epidermis), the brain and nerves, fore- and
hind-gut ; the entoblast gives rise to the lining membrane of the
stomach, intestines, and other parts pertaining to the mid-gut;
while the somatic and splanchnic layers of the mesoblast give
rise to the muscles, kidneys, reproductive organs, heart, blood-
vessels, etc. It is now generally held that the germ-layers
throughout the animal kingdom (with the partial exception of
the Codenterata already mentioned) are essentially identical in
origin and fate. This view is known as the Germ-layer Theory.
It is one of the most significant and important generalizations-
which the study of Embryology has brought to light, since it
recognizes a structural identity of the most fundamental kind
among all the higher animals.

Sooner or later the young earthworm bursts through the
walls of the capsule and makes its entry into the world. When
first hatched it is about an inch long and has no clitellum.

It is a curious fact that in certain species of Lumbricus the young
worms are almost always hatched as twins, two individuals being derived
from a single egg by a process which is described by Kleinenberg in the
Quarterly Journal of Microscopical Science, Vol. XIX., 1879. It often
happens that the twins are permanently united by a band of tissue, as in
the case of the well-known Siamese twins.

We have now traced roughly the evolution of a complex
many-celled animal from a simple one-celled germ. It is im-
portant to notice at this point a few general principles which are
true of higher animals in general.

1. The embryological history is a true process of develop-
ment,— not a mere growth or unfolding of a pre-existing rudi-
ment as the leaf is unfolded from the bud. Neither the ovum
nor any of the earlier stages of development bears the slightest
resemblance to an earthworm. The embryo undergoes a trans-
formation of structure as well as an increase of size.

2. It is a progress from a one-celled to a many-celled con-
dition.

SUMMARY OF DEVELOPMENT. 89

3. It is a progress from relative simplicity to relative com-
plexity. The ovum is certainly vastly more complex than it
appears to the eye, but no one can doubt that the full-grown
worm is more complex still.

4. It is a progress from a slightly differentiated to a highly
differentiated condition. The life of the ovum is that of a
single cell. The blastula is composed of a number of nearly
similar cells, which in the gastrula become differentiated into
two distinct tissues. In later stages the cells become differenti-
ated into many different tissues, which in turn build up different
organs performing unlike functions.

5. Lastly, the development forms a cycle, beginning with
the germ-cell, and after many complicated changes resulting in
the production of new germ-cells, which repeat the process and
give rise to a new generation. All other cells in the body must
sooner or later die. The germ-cells alone persist as the starting-
point to which the cycle of life continually returns (cf. p. 73).
Their protoplasm, the " germ-plasm," is the bond of continuity
that links together the successive generations.

CHAPTEK VI.

THE BIOLOGY OF AN ANIMAL (Continued).

The Earthworm.
MICROSCOPIC STRUCTURE OR HISTOLOGY.

WE have followed the development of the one-celled germ
through a stage, the llatfula, in which it consists of a mass of
nearly similar cells out of which the various tissues of the adult
eventually arise. The first step in this direction is the differen-
tiation of the germ-layers or three primitive tissues (p. 84).
As the embryo develops, the cells of these three tissues become
differentiated in structure to fit them for different duties in the
physiological division of labor. And when this process of dif-
ferentiation is accomplished and the adult state is reached we
find six well-marked varieties of tissue, as follows : —

PRINCIPAL TISSUES OF Lumbricus.

I. Epithelial. Layer of cells covering free surfaces.

(a) Pavement Epithelium. Cells thin and flat, arranged like the

stones of a pavement.
(6) Columnar Epithelium. Cells elongated, standing side by side,

palisade-like,
(c) Ciliated Epithelium. Columnar or cuboid, and bearing cilia.

II. Muscular. Cells contractile and elongated to form fibres. Often
arranged in parallel masses or bundles.

III. Nervous. Cells pear-shaped or irregular, with large nuclei ; hav-
ing processes prolonged into slender cords or fibres, bundles of which con-
stitute the nerves.

IV. Germinal. Including the germ-cells. At first in the form of epi-
thelial cells covering the coelomic surface, but afterwards differentiated
into ova and spermatozoa.

V. Blood. Isolated cells or corpuscles floating in a fluid intercellular
substance, the plasma.

VI. Connective Tissue. Cells of different shapes, often branched but
sometimes rounded, separated from one another by more or less lifeless
(intercellular) substance in the form of threads or homogeneous material.

90

ARRANGEMENT OF TISSUES.

91

These six kinds of tissue constitute the main bulk of the
earthworm, as of higher animals generally ; but there are in ad-
dition other tissues which will be treated of hereafter.

Arrangement of the Tissues. The simplest and most direct
mode of discovering the arrangement of the tissues is by the mi-
croscopical study of thin transverse or longitudinal sections. A

,c

FIG. 39.— Transverse section of the body behind the clitellum. a.c, cavity of the ali-
mentary canal ; c, cuticle ; car, coelom ; c.m, circular muscles ; c.r, circular vessel ;
cf.r, dorsal vessel; /»[/, hypodermis; Lm, longitudinal muscles; n.c, ventral nerve-
chain; p.f, peritoneal epithelium; s, seta; «.(/, setigerous gland; s.i.r, sub-intes-
tinal vessel ; s.m, muscle connecting the two groups of setse on the same side ; ty,
typhlosole.

transverse section taken through the region of the stomach-
intestine is represented in Fig. 39. Its composition is as
follows : —
A. BODY-WALL.

This consists of five layers, viz. (beginning with the out-
side),—

1. Cuticle (c). A very thin transparent membrane, not
composed of cells and perforated by fine pores. It is a product
or secretion of the —

92 THE BIOLOGY OF AN ANIMAL.

2. Hypodermis (hy) (epidermis or skin). A layer of colum-
nar epithelium, composed of several kinds of elongated cells, set
vertically to the surface of the body. Some of these, known as
gland-cells, have the power of producing within their substance
a glairy fluid (mucus), which exudes to the exterior through the
pores in the cuticle. Others (sensory cells) give oif from their
inner ends nerve-fibres which may be traced inwards to the
ganglia (Fig. 43).

The Clitellum is produced by an enormous thickening of the hypoder
mis, caused especially by a great development of the gland-cells. Three
forms of these may be distinguished, which probably produce different
secretions. The tissue is permeated by numerous minute blood-vessels
•which ramify between the cells.

3. Circular Muscles (c.m\ A layer of parallel muscle-
fibres running around the body. On the upper side they are
intermingled with connective-tissue cells containing a granular
brownish substance (pigment) which gives to the dorsal aspect
its darker tint.

4. Longitudinal Muscles (l.m). A layer of muscle-fibres
running lengthwise of the body. They are arranged in compli-
cated bundles, which in cross-sections have a feathery appear-
ance. In longitudinal sections they appear as a simple layer, and
resemble the circular fibres as seen in the cross- section.

The circular muscles are arranged in somewhat similar bun-
dles, as may be seen in longitudinal sections.

5. Ccelomic or Peritoneal Epithelium (p.e.). A very thin
layer of flattened cells next the co?lomic cavity.

The hypodermis, and therefore also the cuticle to which it
gives rise, is derived from the ectoblast. The other layers (3,
4, 5) arise from the somatic layer of the mesoblast.
B. ALIMENTARY CANAL.

The wall of this tube appears in cross-section as a ring sur-
rounded by the coelom. The typhlosole (ty) is seen to be a deep
infolding of its upper portion. In the middle region the wall is
composed of five layers as follows, starting from the alimentary
cavity (Fig. 40) : —

1. Lining Epithelium (ep}. A layer of closely packed, nar-
row ciliated columnar cells with oval nuclei.

2. Vascular Layer (v.l). Numerous minute blood-vessels.

HISTOLOGY OF THE ALIMENTARY CANAL.

93

3. Circular Muscles (c.ni). A thin layer of muscle-fibres
running around the gut.

4. Longitudinal Muscles (l.m). A thin layer of muscle-
fibres running along the gut.

5. Chlw-agogue Layer (cK). Composed of large polyhedral
or rounded cells containing yellowish-green granules. The cells
fill the hollow of the typhlosole, and cover the surface of the
dorsal and lateral blood-vessels. This layer represents the
splanchnic part of the peritoneal epithelium.

The same general arrangement exists in all parts of the alimentary
canal, but is sometimes greatly modified. For instance, the gizzard and
pharynx are lined by a tough, thick cuticle, and the muscular layers are
enormously developed. In a part of the gizzard the chloragogue-layer is
nearly or quite absent and the typhlosole disappears. A fuller description
of these modifications will be found in Brooks's Handbook of Invertebrate
Zoology, and a complete account in Claparede, Zeitschrift fur wissen-
schaftlicJie Zoologie, Vol. XIX., 1869.

The lining epithelium is derived from the entoblast. The
remaining layers arise by differentiation of the splanchnic layer
of inesoblast.

FIG. 40. — Highly magnified cross-section through the wall of the alimentary canal,
eft, chloragogue layer ; c.m, circular muscles ; e.p, lining epithelium ; Z.wi, longi-
tudinal muscles ; v.l, vascular layer.

Blood-vessels appear in the section as rounded or irregular
cavities bounded by thin walls. They consist of a delicate lining
epithelium covered by a thin layer of muscle-fibres. In the
walls of the stomach-intestine the vessels are often completely
invested by chloragogue-cells, which radiate from them with

94

THE BIOLOGY OF AN ANIMAL.

great regularity (Fig. 39). The finer branches have no muscu-
lar layer, consisting of the epithelium alone.

Dissepiments. These often appear in cross or longitudinal
sections. They consist chiefly of muscle-fibres irregularly dis-
posed, intermingled with connective-tissue cells and fibres, and
covered on both sides with the peritoneal epithelium.

Nervous System. A cross-section of a ganglion (Fig. 41)
shows it to be composed of two distinct parts, viz. , (1) the gan-

FIG. 41. — Highly magnified cross-section of a ventral ganglion, g.f, giant-flbres; I.n,
lateral nerve; «.c, nerve-cells; s, muscular sheath of the ganglion; s.v, sub-neu-
ral vessel ; s.n.r, supra-neural vessel.

glion proper on the inside, and (2) a sheath which envelops it.
The sheath (.s, Fig. 41) consists of two layers, viz. : —

1. Peritoneal Epithelium. On the outside.

2. Muscular Layer, or sheath, a thick layer of irregularly
arranged muscle-fibres intermingled with connective tissue. Im-
bedded in it are the sub-neural blood-vessel on the lower side
and the supra-neural blood-vessels on each side above. In the
middle line are three rounded spaces (g, f, Fig. 41), which are
the cross-sections of three hollow fibres running along the entire
length of the ventral nerve-chain. They are called "giant-
fibres, ' ' and possibly serve to support the soft parts of the nerve-
cord.

The Ganglion proper is distinctly bilobed, and consists of
two portions, viz. : —

1. Nerve-cells (n.c). Numerous pear-shaped nerve-cells near
the surface, with their narrow ends turned towards the centre,
into which each sends a single branch or nerve-fibre. They are
confined chiefly to the ventral and lateral parts of the ganglion.

HISTOLOGY OF THE NERVOUS SYSTEM.

95

2. Fibrous Portion. This occupies the central part. It
consists of a close and complicated network of nerve-fibres inter-
mingled with connective tissue. Some of these fibres communi-
cate with branches of the nerve-cells, as stated above ; others
run out into the lateral nerves, while still others run along the
commissures to connect with fibres from other ganglia.

Fio. 42.— Two of the ventral ganglia (I, II) of Lumbricus with the lateral nerves,
showing some of the motor nerve-cells and fibres (black), a sends fibres for-
wards and backwards within the nerve-cord ; fr, a fibre into one of the double-
nerves on its own side ; c and d, fibres that cross to the nerves of the opposite side.
(After Retzius.)

According to the latest researches (of Lenhossek and Retzius) most if
not all of the nerve-cells of the ventral cord are motor in function. Near
the centre of each ganglion (Fig. 42, e) in a single large multipolar cell of
doubtful nature. All the other cells are either bipolar or unipolar, in the-
latter case sending out a single branch which soon divides into two. In
every case one of the branches breaks up into fine sub-divisions within the
cord. The other branch in most cases passes out of the cord through one
of the lateral nerves to the muscles or other peripheral organs, either

96

THE BIOLOGY OF AN ANIMAL.

crossing within the cord to the opposite side of the body or making exit
on its own side. Some of the cells, however, are purely " commissural,"
le., neither branch leaves the cord.

The sensory fibres entering from the periphery terminate freely (not in
nerve-cells), breaking up into numerous fine branches on the same side of
the cord. (Fig. 43.)

The nerves leaving the central system are mixed, i.e., they contain both
sensory and motor fibres.

71. C

FIG. 43.— Transverse section of ventral part of the body, showing the nervous con-
nections. 7i.c, ventral ganglion, giving off a lateral nerve at l.n. ; p.f., peritoneal
epithelium ; I.m., longitudinal muscles; 7ij/, hypodermis ; «, seta. A single motor
nerve-cell (black) is shown sending a fibre into the nerve towards the left. In
the nerve to the right are sensory fibres proceeding inward from the sensory cells
(black) of the hypodermis, and terminating in branching extremities. (After
Lenhossek.)

Sections through the ventral commissures are similar to those through
the ganglia, but the central portion (i.e., that within the sheath) is smaller,
is divided into two distinct parts, and the nerve-cells are less abundant.

Sections through the nerves show them to consist only of parallel fibres
surrounded by a sheath which gradually fades away as the nerves grow
smaller, and finally disappears, the muscular layer first disappearing, and
then the epithelial covering.

"With tliis brief sketch of the histological structure of the
earthworm we conclude our morphological study of the animal.
Those who desire fuller information on the histology will find a
geneial treatment of it in the work of Claparede, already cited
at p. 93. Many later works have been published on the de-
tailed histology.

CHAPTEE YII.

THE BIOLOGY OF AN ANIMAL (Continued.)
Physiology of the Earthworm.

IN the preceding pages brief descriptions of many special
physiological phenomena have been given in connection with the
detailed descriptions of the primary functions and systems. It
now remains to consider the more general problems of the life of
the animal, and especially its relations to the environment, and
the transformations of matter and energy which it effects.

The Earthworm and its Environment. The earthworm is an
organized mass of living matter occupying a definite position in
space and time, and existing amid certain definite and character-
istic physical surroundings which constitute its ' 6 environment. ' y

As ordinarily understood the term environment applies only
to the immediate surroundings of the animal — to the earth
through which it burrows, the air and moisture that bathe its
surface, and the like. Strictly speaking, however, the environ-
ment includes everything that may in any manner act upon the
organism — that is, the whole universe outside the worm. For
the animal is directly and profoundly affected by rays of light
and heat that travel to it from the sun ; it is extremely sensitive
to the alternations of day and night, and the seasons of the year ;
it is acted on by gravity; and to all these, as well as to more
immediate influences, the animal makes definite responses.

We have seen that the body of the earthworm is a compli-
cated piece of mechanism constructed to perform certain definite
actions. But every one of these actions is in one way or an-
other dependent upon the environment and directly or indirectly
relates to it. At every moment of its existence the organism is
acted on by its environment ; at every moment it reacts upon
the environment, maintaining with it a constantly shifting state
of equilibrium which finally gives way only when the life of the
animal draws to a close.

Adaptation of the Organism to its Environment. In its rela-
tions to the environment the earthworm embodies a fundamental

97

98 THE BIOLOGY OF AN ANIMAL.

biological law, viz., that the living organism must be adapted to
•its environment, or, in other words, that a certain harmony
between organism and environment is essential to the continu-
ance of life, and any influence which tends to disturb or destroy
this harmony tends to disturb or destroy life. The adaptation
may be either passive (structural) or active (functional). Struc-
tural adaptation is well illustrated, for instance, by the general
shape of the body, so well adapted for burrowing through the
earth. Again, the delicate integument gives to the body the
flexibility demanded by the peculiar mode of locomotion; it
affords at the same time a highly favorable respiratory surface —
a matter of no small importance to the worm in its badly-venti-
lated burrow ; and yet this delicate integument does not lead to
desiccation, because the animal lives always in contact with moist
«arth. The alimentary canal, long and complicated, is most
perfectly fitted for working over and extracting nutriment from
the earthy diet. The reproductive organs are a remarkable in-
stance of complex structural adaptation in an animal which on
the whole is of comparatively simple structure.

Functional adaptation is perhaps best shown in the instinctive
actions or "habits" of the worm. Its nocturnal mode of life
{functional adaptation to light) and its "timidity" protect it
from heat, desiccation, from birds and other enemies. In win-
ter or in seasons of drought it burrows deep into the earth.

A striking instance of adaptation is shown in the care which
is taken to insure the welfare of the embryo worms. Minute,
delicate, and helpless as they are, they develop in safety inside
the tough, leathery capsule (p. 78), floating in a milklike
liquid which is at once their cradle and their food.

Origin of Adaptations. The development of the earthworm
shows that its whole complex bodily mechanism takes origin in a
single cell (p. 74), and that all the remarkable adaptations ex-
pressed in its structure and action are brought about by a gradual
process in the life-history of each individual worm. There is
reason to believe that this is typical of the ancestral history (de-
scent) of the species as a whole, and that adaptation has been
gradually acquired in the past, We know that environments
change, and that to a certain extent organisms change corre-
spondingly through functional adaptation, provided the change of

NUTRITION OF THE ANIMAL. ." 99

environment be not too sudden or extreme. In other words,
the organism possesses a certain plasticity which enables it to
adapt itself to gradually -changing conditions of the environment.
Now there is good reason to believe that as environment
has gradually undergone changes in the past, organisms have
gradually undergone corresponding changes of structure. Those
which have become in any way so modified as to be most per-
fectly adapted to the changed environment have tended to sur-
vive and leave similarly-adapted descendants. Those which
have been less perfectly adapted have tended to die out through
lack of fitness for the environment ; and by this process — called
by Darwin ' ' Natural Selection ' ' and by Spencer the ' ' Survival
of the Fittest*" — the remarkable adaptations everywhere met
with are believed to have been gradually worked out.

It should be observed that Natural Selection does not really explain the
origin of adaptations, but only their persistence and accumulation. The
theory of evolution is not at present such as to enable us to say with cer-
tainty what causes the first origin of adaptive variations.

Nutrition. The earthworm does work. It works in travel-
ling about and in forcing its way through the soil ; in seizing,
swallowing, digesting, arid absorbing food; in pumping the
blood ; in maintaining the action of cilia ; in receiving and send-
ing out nerve-impulses; in growing; in reproducing itself — in
short, in carrying on any and every form of vital action. To
live is to work. Now work involves the expenditure of energy,
and the animal body, like any other machine, while life con-
tinues, requires a continual supply of energy. It is clear from
what has been said on p. 32 that the immediate source of the
energy expended in vital action is the working protoplasm itself,
which undergoes a destructive chemical change (katabolism or
destructive metabolism) having the nature of an oxidation. From
this it follows on the one ban 1 that the waste products of this
action must be ultimately passed out of the body as excretions,
and on the other hand that the loss must ultimately be made
good by fresh supplies entering the animal in the form of food.
It is further evident that the income must equal the outgo if the
animal is merely to hold its own, and must exceed it if the ani-
mal is to grow.

100

THE BIOLOGY OF AN ANIMAL.

Thus it comes about that there is a more or less steady flow
of matter and of energy through the living organism, which is
itself a centre of activity, like a whirlpool (p. 2). The chemical
phenomena accompanying the flow of matter and energy through
the organism are those of nutrition in the widest sense. This
term is more often restricted especially to the phenomena accom-
panying the income, while those pertaining to the outgo are
regarded as belonging to excretion. The intermediate processes,
directly connected with the life of protoplasm are put together
under the head of metabolism; they include both the construc-
tive processes by which protoplasm is built up (anabolism) and
the destructive processes by which it is broken down (katabolism}
in the liberation of energy.

Income. It is difficult to determine the exact income of
Lumbricus, but it may be set down approximately as follows : —

INCOME OF LUMBRICUS.

MATTER.

WHENCE DERIVED.

1. Proteids.

From vegetal or animal matters taken in through the mouth.

2. Fats.

From vegetal or animal matters taken in through the mouth.

3. Carbohydrate*.

From vegetal or animal matters taken in through the mouth.

4. Water.

Taken in through the mouth, or perhaps to" some extent ab-
sorbed through the body-walls.

5. Free oxygen.

Absorbed directly from the atmosphere or ground-air by dif-
fusion through the body-walls. Sometimes from water in
which it is dissolved.

6. Sotta.

Various inorganic salts taken along with other food-stuffs.

ENERGY.

Potential.

In the food.

The food-stuffs are converted by the animal into the sub-
stance of its own body (protoplasm and all its derivatives), and
they must therefore be the ultimate source of energy. It fol-
lows that the animal takes in energy only in the potential form
(i.e., in the chemical potential between the oxidizable proteids,
carbohydrates and fate, and free oxygen). It is true that the

DIGESTION AND ABSORPTION. 101

animal may under certain circumstances absorb kinetic energy in
the form of heat, but this is available only as a condition, not as
a cause of protoplasmic action. In this inability to use kinetic
energy the earthworm is typical of animals as a whole.

Of the organic portion of the food proteids are a sine qua
non, and in this respect again the worm is a type of animal life
in general. Either the fats or the carbohydrates may be omitted
(though the annual probably thrives best upon a mixed diet in
which both are present), but without proteids no animal, as far
as is known, can long exist.

General History of the Food. Digestion and Absorption.
Lumbricus takes daily into its alimentary canal a certain amount
of necessary food-stuffs, but these are not really inside the body
so long as they remain in the alimentary canal ; for this is shown
by its development to be only a part of the outer surface folded
in to afford a safe receptacle within which the food may be
worked over. Before the food can be actually taken into the
body, or absorbed, it must undergo certain chemical changes col-
lectively called digestion (cf. p. 49). A very important part
of this process consists in rendering non-diffusible substances dif-
fusible, in order that they may pass through the walls of the
alimentary canal into the blood. Proteids, for example, have
been shown to be non-diffusible (Chap. III). In digestion they
are changed by the fluids of the alimentary canal into peptones
— substances much like proteids, but readily diffusible. In
like manner the non-diffusible starch is changed into diffusible
sugar and becomes capable of absorption. It is highly probable
that all carbohydrates are thus turned into sugar. The fats are
probably converted in part into soluble and diffusible soaps which
are readily absorbed, but are mainly emulsified and directly passed
into the cells of the alimentary tract in a finely divided state.
Nothing, however, is known of this save by analogy with higher
animals. In all cases digestion takes place outside the body, and
is only preliminary to the real entrance of food into the physio-
logical, or true, interior.

Metabolism. After .absorption into the body proper the
incoming matters are distributed by the circulation to the ulti-
mate living units or cells, and are finally taken up by them and
built into their substance. There is reason to believe that each

102 THE BIOLOGY OF AN ANIMAL.

cell takes from the common carrier, the blood, only such ma-
terials as it needs, leading a somewhat independent life as to its
own nutrition. It co-operates with other cells under the direc-
tion of the nervous system (co-ordinating mechanism), but to a
great degree is independent in its choice of food — just as a sol-
dier in a well-fed army obeys orders for the common good, but
yet takes only what he chooses from the daily ration supplied to
all.

What takes place within the cell upon the entrance of the
food is almost wholly unknown, but somehow the food- matters,
rich in potential energy, are built up into the living substance
probably by a series of constructive processes culminating in pro-
toplasm. Alongside these constructive processes (anabolism) a
continual destructive action goes on (katabolism) ; for living mat-
ter is decomposed and energy set free in every vital action, and
vitality or life is a continuous process. It must not be supposed,
however, that either the synthetic or the destructive process is a
single act. Both probably involve long and complicated chemi-
cal transformations but the precise nature of these changes is at
present almost whqlly unknown. It is certain that the destruc-
tive action is in a general way a process of oxidation effected by
aid of the free oxygen taken in in respiration. We may be
sure, however, that it is not a case of simple combustion (i.e., the
protoplasm is not " burnt"). It is more probably analogous to
an explosive action, the oxygen first entering into a loose asso-
ciation with complex organic substances in the protoplasm, and
then suddenly combining with them under the appropriate stim-
ulus to form simpler and more highly-oxidized products. Of
the precise nature of the process we are quite ignorant.

Outgo. Just as the income of the animal represents only the
first term in a series of constructive processes, so the outgo is
the last term of a series of destructive actions of which we really
know very little save through their results. The outgo is shown
in the accompanying table.

Both energy and matter leave the cells, and finally leave the
body — the former as heat, work done, or energy still potential
(in urea and other organic matters); the latter as excretions,
which diffuse freely outwards through the skin and nepliridial
surfaces.

THE ANIMAL AND ITS ENVIRONMENT.
OUTGO OF LUMBRICUS.

103

MATTER.

MANNER OF EXIT.

Carbon dioxide (CO,).

Mainly by diffusion through the skin.

Water (HaO).

Through the skin, through the nephridia, and in the faeces.

[7refl[(NH,),CO],and
its allies.

Through the nephridia.

Salts.

Dissolved in the water.

Proteids and other
organic matters.

In the substance of the germ-cells, the egg-capsules, and
the contained nutrient fluids.

ENERGY.

Potential.

A small amount still remaining in urea, in the germ-cells,
etc.

Kinetic.

Work performed. Heat.

Of the daily outgo the water, carbon dioxide, and salts are
•devoid of energy, but the urea contains a small amount which is
a sheer loss to the animal. Were the earthworm a perfect ma-
•chine it could use this residue of energy by decomposing the urea
into simpler compounds [viz., ammonia (NH3), carbon dioxide
{CO,), and water (H,O)] ; but it lacks this power, though there
are certain organisms (Bacteria) which are able to utilize the last
traces of energy in urea (p. 107). To the daily outgo must be
added the occasional loss both of matter and of energy suffered
in giving rise to ova and spermatozoa, and in providing a certain
amount of food and protection for the next generation.

Interaction of the Animal and the Environment. The action
•of the environment upon the animal has already been sufficiently
.stated (p. 97). It remains to point out the changes worked by
the animal on the environment. These changes are of two
kinds, mechanical (or physical) and chemical. The most impor-
tant of the former is the continual transformation of the soil
which the worms effect, as Darwin showed, by bringing the
•deeper layers to the surface, where they are exposed to the at-
mosphere, and also by dragging superficial objects into the bur-
rows. The chemical changes are still more significant. The

104 THE BIOLOGY OF AN ANIMAL.

general effect of the metabolism of tlie animal is the destruction
by oxidation of organic matter ; that is, matter originally taken
from the environment in the form of complex proteids, fats, and
carbohydrates is returned to it in the form of simpler and more
highly oxidized substances, of which the most important are car-
bon dioxide and water (both inorganic substances). This action
furthermore is accompanied by a dissipation of energy — that is,
a conversion of potential into kinetic energy.

On the whole, therefore, the action of the animal upon the
environment is that of an oxidizing agent, a reducer of complex
compounds to simpler ones, and a dissipator of energy. And
herein it is typical of animals in general.

CHAPTER YIIL
THE BIOLOGY OF A PLANT.

The Common Brake or Pern.

(Pteris aquilina, Linnaeus.)

FOE the study of a representative vegetal organism some
plant should be chosen which may be readily procured and is
neither very high nor very low in the scale of organization.
Such a plant is a common fern.

Ferns grow generally in damp and shady places, though
they are by no means confined to such localities. Some of the
more hardy species prefer dry rocks or even bold cliffs, in the
crevices of which they find support ; others live in open fields
or forests, and still others on sandy hillsides. In the northern
United States there are altogether some fifty species of wild
ferns, but those which are common in any particular locality are
seldom more than a score in number. Throughout the whole
world some four thousand species of ferns are known, but by
far the greater number are found only in tropical regions, where
the climate is best suited to their wants. At an earlier period
of the earth's history ferns attained a great size, and formed a
conspicuous and important feature of the vegetation. At
present, however, they are for the most part only a few feet in
height. Nearly all are perennial ; that is, they may live for an
indefinite number of years. Most of them have creeping or
subterranean stems ; but some of the tropical species have erect,
aerial stems, sometimes rising to a height of fifty feet or more
and forming a trunk which is cylindrical, of equal diameter
throughout, and bears leaves only at the summit, like a palm
(tree-ferns).

Of all the ferns perhaps the commonest and most widely
distributed is the " brake " or " eagle-fern," which is known to
botanists as Pteris aquilina^ Linnaeus, or Pteridium aquilinum,

105

106 THE BIOLOGY OF A PLANT.

Kuhn. This plant is not only common, but of comparatively
simple structure ; it is of a convenient size, and has been much
studied. It may therefore be taken both as a representative
fern and as a representative of all higher vegetal organisms.

Habitat, Name, etc. The brake occurs widely distributed in I
the United States, under a great variety of conditions; e.g., in
loose pine groves, especially in sandy regions ; in open wood-
lands amongst the other undergrowth ; on hillside pastures and
in thickets — indeed almost everywhere, except in very wet or
very dry places. It appears to be equally common elsewhere ;
for, according to Sir "W. J. Hooker, Pteris aquilina grows
" all round the world, both within the tropics and in the nortli
and south temperate zones. ... In Lapland it just passes,
within the Arctic circle, ascending in Scotland to 2000 feet,
in the Cameroon Mountains to 7000 feet, in Abyssinia to 8000
or 9000 feet, in the Himalayas to about 8000 feet." (Synopsis-
Filicum.}

"Pteris (nTepis, the common Greek name for fern), signify-
ing wing or feather, well accords with the appearance of Pteri»
aquilina, the most common and most generally distributed of
European ferns. It is possible that this fern may rank as the
most universally distributed of all vegetable productions, extend-
ing its dominion from west to east over continents and islands in
a zone reaching from Northern Europe and Siberia to New
Zealand, where it is represented by, and perhaps identical with,
the well-known Pteris esculenta. The rhizome of our plant
like that of the latter is edible, and though not employed in
Great Britain as food, powdered and mixed with a small quan-
tity of barley-meal it is made into a kind of gruel called gofo,
in use among the poorer inhabitants of the Canary Islands. "-
(Sowerby.)

The specific name aquilina (aquila, eagle) and a popular
name, "eagle-fern," in Germany, etc., have come from a,
fanciful likeness of the dark tissue seen in a transverse section
of the leaf-stalk to the figure of an outspread eagle. The same
figure has, however, been compared to an oak-tree, and has al><>
given rise to the name of " devil' s-foot fern," from its alleged
resemblance to "the impression of the deil's foot," etc., etc.

The popular designation of this plant as ' ' the brake ' ' testi-

THE PLANT BODY. 107

fies to its great abundance ; for a brake is a dense thicket or
undergrowth — as for example a cane "brake."

"When fully grown (Fig. 44) the common brake has a leafy
top supported by a polished, dark-colored, erect stem, which in
New England rises to a height of from one to four feet above
the ground. In this climate, however, it appears to be some-
what undersized, for it grows to a height of fourteen feet in
the Andes,* and in Australia attains to twice the height of a
man, forming a dense undergrowth beneath tree-ferns 40-100
feet high.f In Great Britain it is from six inches to nine feet
high (Sowerby), or even larger in exceptional cases. " In dry
gravel it is usually present, but of small size; while in thick
shady woods having a moist and rich soil it attains an enormous
size, and may often be seen climbing up, as it were, among the
lower branches and underwood, resting its delicate pinnules
on the little twigs, and hanging gracefully over them."
(Newman.)

GENERAL MORPHOLOGY OF THE BODY.

The body of the fern, like that of the earthworm, consists
of cells, grouped to form tissues and organs. Their arrange-
ment, however, differs widely from that in the animal, for the
plant-body is a nearly solid mass, and there are no extended
internal cavities enclosing internal organs. The organs of the
plant are for the most part external, and arise by local modifica-
tions of the general mass. Like many higher plants the body
of the fern consists of an axis or stem-bearing branches, from
which arise leaves. The fern differs form ordinary trees, how-
ever, in the fact that the stem, with its branches, lies horizontal
beneath the surface of the ground. Only the leaves (fronds)
rise into the air. (Fig. 44.) It is convenient to describe the
body of the brake, accordingly, as consisting of two very dif-
ferent parts — one green and leaflike, which rises above the
ground ; the other black and rootlike, lying buried in the soil.
These will henceforth be spoken of as the aerial and the under-
ground parts.

The underground part lies at a depth of an inch to a foot

* Hooker, I, c.

f Krone, Botan. Jahresbericht, 1876 (4), 346.

Im.

' — •* 1 \ «•> x j~

FIG. 44.— The Brake (Pteris aquiUna\ showing part of the underground stem (r.h)
and two leaves, one (!>), of the present year, in full development; the other
(Is), of the past year, dead and withered, a.b, apical bud at the extremity of a
branch which bears the stumps of leaves of preceding years and numerous
roots; l>, mature active leaf ; 1", dead leaf of preceding year ; Z.m, lamina of leaf ;
p, pinna ; r./i, portion of main rhizome ; -.r, younger pinna, which is shown en-
larged at B. This pinna is nearly similar to the pinnules of older pinnae. (X J.»

AERIAL AND UNDERGROUND PARTS. 109

below the surface, and brandies widely in various directions.
It may often be followed for a long distance, and in such cases
reveals a surprisingly complicated system of underground
branches. At first sight, the underground portion of the fern
appears to be the root, but a closer examination shows it to be
really the stem or axis of the plant, which differs from ordinary
stems chiefly in the fact that it lies horizontally under the
ground instead of rising vertically above it. The aerial portion,
which is often taken for stem and leaf, is really leaf only. The
true roots are the fine fibres which spring in great abundance
from the underground stem. Underground stems more or less
like that of Pteris are not uncommon — occurring, for instance,
in the potato, the Solomon' s-seal, the onion, etc. In Pteris,
and in certain other cases, the underground stem is technically
called the rootstock or rhizome, and in this plant it constitutes
the larger and more persistent part of the organism. In the
specimen shown in Fig. 45 the rhizome was about eight feet
long and bore two leaves. It was dug out of sandy soil on the
edge of a woodland, and lay from one to six inches below the
surface. It was crossed and recrossed in all directions, both
above and below, by the rhizomes of its neighbors, the whole
constituting a coarse network of underground stems loosely fill-
ing the upper layer of the soil.

The aerial portion (the frond or leaf) is likewise divisible
into a number of parts, comprising in the first place the leaf-
stalk or stipe, and the leaf proper or lamina. The latter is subdi-
vided like a feather (pinnately) into a number of lobes (pinnae,
Fig. 44), which vary in form according to the state of de-
velopment of the leaf. In large leaves the two lower pinnae are
often larger than the others, so that the leaf appears to consist
of three principal divisions, and is said to be " ternate ' ' or trip-
ly divided (Fig. 44, A). Each pinna is in turn pinnately sub-
divided into pinnules (pinnulce) or leaflets (Fig. 44, £), each of
which is traversed down the middle by a thickened ridge or
rod, the midrib. The leaflets sometimes have smooth outlines,
but are usually lobed along the edges, as in Fig. 44, B. In
this case their form is said to be pinnatifid. Each lobe is like-
wise furnished with a midrib. The stipe enlarges somewhat
just below the surface of the ground, then grows smaller and

THE BIOLOGY OF A PLANT.

joins the rhizome. The enlarge-
ment is of considerable interest,
for it occurs at precisely the
point of greatest strain when the
leaf is bent by the wind or other-
wise, and must serve to strength-
en the stipe.

It will appear from the fol-
lowing description that the plant
body exhibits in some measure
certain general forms of sym-
metry and differentiation which
in a broad sense may be regarded
as analogous to those occurring in
the animal. The rhizome grows
only at one end, and in its struc-
ture suggests the antero-posterior
differentiation of the animal. It
also shows a slight differentiation
between the upper and lower
surfaces, which appears both in
the external form and in the ar-
rangement of the internal lines.
It is furthermore distinctly bilat-
eral, a vertical 'plane dividing it
into closely similar halves. These
features are, however, far less
prominent in the fern than in
the earthworm, and in plants
they never attain a high degree
of development, while in the
higher animals they are among
the most conspicuous and im-
portant features of the body.
Fro. 45.— An entire Of more general importance in
* leaves0"! the fern is the repetition of
and a comparison of similar parts (branches, roots,

the figure wiWi Fig. ,

44 will show some of leaves) along the axis, which

the differences be-
tween leares of dii- suggests, perhaps, a certain an-

AXIS AND APPENDAGES. Ill

alogy to animal metamerism, though not usually recognized
or designated by the same term. All of these conditions of
differentiation and symmetry are more easily made out by an
examination of the aerial portion.

The plant as a whole, may be regarded as consisting of
an axis (the rhizome and its branches) which bears a number
of appendages in the form of roots and leaves. The axis forms-
the central body or trunk of the plant, and in it most of its mat-
ter and energy are stored ; the appendages are organs for taking
in food, for excretion, for respiration, for reproduction, etc.

The Underground Stem, or Rhizome, and its Branches. The
rhizome is a hard black, elongated, and brandling stem, gener-
ally flattened somewhat in the vertical direction as it lies in the
earth, and expanded slightly on either side to form well-marked
lateral folds — the lateral ridges. Its thickness is seldom more
than half an inch, and usually considerably less. In transverse
section it has the outline shown in Fig. 48, and the marginal
part only is black. The branches repeat in all respects the form
and structure of the main axis. Both the main axis and the
branches end either in conical, pointed, and fleshy structures*
about two inches long, or in blunt, yellowish knobs, plainly de-
pressed in the centre. At these ends the rhizome grows ; hence
they are called the growing points or apical buds (Figs. 44, 47).

Besides the apical buds the rhizome bears nearly always one
or more dead, decaying tips. These arise in the following man-
ner : After attaining a certain length both the rhizome and its-
branches gradually die away behind. Death of the hinder part
follows at about the same rate with which growth advances at
the apical buds ; so that the total length may not change mate-
rially from year to year. It is obvious that this process must
result in the gradual and successive detachment of the branches-
from the main axis. Each branch, now become an independ-
ent rhizome, repeats the process; and in this manner a single
original rhizome may give rise to large numbers of distinct
plants, all of which have been at some time in material connec-
tion with an ancestral stock. This process is evidently a kind of
reproduction, (though it is not the most important or most obvi-
ous means for the propagation of the plant), and in this way a
large area may be occupied by distinct, though related, plants

112

THE BIOLOGY OF A PLAN1.

whose branching rhizomes cross and recross, making the subter-
ranean network already described, p. 109.

Origin of Leaves upon the Rhizome and its Branches. The
young plant of Pteris puts up a number of leaves (7-12) yearly,
but the adult generally develops one only, which grows very
slowly, requiring two years before it unfolds. Towards the end
of the first year it is recognizable only as a minute knob at the
bottom of a depression near the growing point. At the begin-
ning of the second year it is perhaps an inch high, the stalk

D.

ep. s.p.fb

FIG. 46. (After Sachs.)— Developing leaf, etc., of Ptfris. A, end of a branch show-
ing the apical bud and the rudiment of a leaf ; 7?, a rudimentary leaf ; C, a
similar leaf in longitudinal section, showing the infolded lamina (I), the attach-
ment to the rhizome, and the prolongation of the tissues of the latter into the
leaf; D, lamina of a very young leaf ; K, horizontal section through a growing
point which has just forked to form two apical buds. a.h. apical bud ; ep, epi-
dermis and underlying sclerotic parenchyma ; f.tt, flbro-vascular bundles ; I,
lamina ; r, root ; s.p, sclerotic prosenchyma ; x, an adventitious bud at the base
of the leaf.

only having appeared. At the end of the second year the lamina
is developed, and hangs down as shown in Fig. 46, C. Early in
the spring of the third year it breaks through the ground, and
grows rapidly to the fully-matured state.

LEAVES AND RHIZOME.

113

The leaves usually arise near the apical buds of the main
axis or of the branches. Behind each mature leaf remnants of
the leaves of preceding years are often to be found, alternating
on the sides of the rhizome in regular succession, and showing
various stages of decay. The first of these (which is on the
opposite side of the rhizome from the living leaf) was alive the
previous year ; the next (on the same side with the living leaf) is
the leaf of the year before that ; and so on. Fig. 47 shows an
example of this sort. The leaf of the present year, f, is fully

li

FlG. 47. (After Sachs.)— Branch of a rhizome of Pteris, showing the apical bud (rt.7>),
the stumps of a number of successive leaves (V, Is, Is, etc.), and a part of the main
rhizome (rh>. r, root.

developed ; arid the relics of the leaves of the preceding years
are indicated at Z3, V, etc. ; I1 is the rudiment of next year's leaf.

Internal Structure of the Rhizome. The rhizome is a nearly
solid mass, consisting of many different kinds of cells, united
into different tissues, and having a very complicated arrange-
ment. Its study is somewhat difficult. Nevertheless the ar-
rangement of the cells is definite and constant, and merits careful
attention, since it has many features which are characteristic of
the cellular structure of the stems of higher plants. We shall
first examine its more obvious anatomy as displayed in transverse
and longitudinal sections, afterwards making a careful micro-
scopical study of the cells and tissues.

Seen with a hand-lens or the naked eye, a transverse section
of the rhizome (Fig. 48) presents a white or yellowish back-

114

THE BIOLOGY OF A PLANT.

ground bounded by a black margin (the epidermis) and marked
by various colored or pale spots and bands ; the latter are differ-
ent tissues, or systems of tissue. These different stnictures are
arranged in three groups or systems of tissue, which are found

fp

s.p

FIG. 48.-Cross-section of the rhizome of Pterts. Lr, lateral ridges; f.p, fundamental
parenchyma; s.p, sclerotic parenchyma; s.pro, sclerotic prosenchyma: /.ft, x$
nbro-vascular bundles.

among all higher plants in essentially the same form, though
differing widely in the minor details of their arrangement.
These are : —

I. The Fundamental System of Tissues.
II. The Epidermal System.
III. The Fibro- vascular System.

The Fundamental system consists in Pteris of three tissues :
(a} fundamental parenchyma (Fig. k$,f.p), the soft whitish
mass forming the principal substance of the rhizome ;

(ft) sclerotic parenchyma (s.p), the brown hard tissue lying
just below the epidermis, from which it is scarcely distinguish-
able;

(c) sclerotic prosenchyma (s.pro), black or reddish dots and
bands of extremely hard tissue, most of which is contained in two
conspicuous bands lying one on either side of a plane joining
the lateral ridges.

THE GREAT TISSUE-SYSTEMS. 115

The sclerotic parenchyma and the sclerotic prosenchyma both
arise through a transformation (hardening, etc.) of portions of
originally-soft fundamental parenchyma. In most plants above
'the ferns the fundamental system contains neither of these tissues.

The Fibro-vascular system is composed of longitudinal
threads or strands of tissue known as ihejibro-vascular bundles,
and these in one form or another are characteristic of all higher
plants. They appear here and there in the section (Fig. 48, f.b)
as indistinct, pale or silvery areas of a roundish, oval, or elon-
gated shape. Closely examined they show an open texture, en-
closing spaces which are sections of empty tubes, or vessels and
fibres, from which the bundles take their name.

The Epidermal system consists of a single tissue, the epider-
mis, which covers the outside of the rhizome.

By a simple dissection of the stem with a knife the sclerotic
prosenchyma and the fibro- vascular bundles may be seen to be
long strands or bands, coursing through the softer fundamental
tissues.

It should be clearly understood that these three systems are,
in general, not single tissues, but groups of tissues which are
constantly associated together for the performance of certain
functions. *

MICROSCOPIC ANATOMY (HISTOLOGY) OF THE EHIZOME.

General Account. Microscopic study of thin sections of the
rhizome shows the various tissues to be composed of innumerable
closely-crowded cells, which differ very widely in structure and
in function. In studying these cells the student should not lose
sight of the fact that they are objects having three dimensions,
of which only two are seen in sections. And hence a single sec-
tion may give an imperfect or entirely false impression of the
real form of the cells, — just as the face of a wall of masonry may
give only an imperfect idea of the blocks of which it is built.

* This classification of the tissues is only a matter of convenience, and Las
little scientific value. By many botanists it has been rejected altogether ; but
no apology for its use need be made by those who, like the authors, have
found it useful, so long as it is defended by Sachs (who first introduced it) and
its value for beginners is conceded by De Bary.

116

THE BIOLOGY OF A PLANT.

For this reason many of the cells can only be understood by <a
comparison of transverse and longitudinal sections, and these
should be studied together until their relations are thoroughly
mastered.

The following table gives brief definitions of the leading
vegetal tissues and is good not only for Pteris but for all
plants : —

PRINCIPAL ADULT VEGETAL TISSUES.

TISSUES.

CHARACTERISTICS.

1. Epidermis.

Cells in a single layer covering the outer surface.

2. Parenchyma.

Masses of cells, rounded, prismatic or polyhedral, usually incom-
pletely joined at the angles, thus leaving intercellular spaces.
Not much longer than broad. Thin-walled.

3. Prosenchyma.

Cells elongated, typically massed, without intercellular spaces.

4. Sieve-tubes.

Cells elongated, thin-walled, panelled with perforated areas,
containing proteids.

5. Tracheids.

Cells thick-walled, elongated, pointed, hard ; walls pitted ; filled
with air.

6. Tracheae or

vessels.

Cells very slender, elongated, opening into one another at their
ends, often spirally thickened, and filled with air.

These BIX tissues are not only found in the rhizome, but ex-
tend throughout the roots and the fronds as well. Moreover,
all the tissues not only of the fern but of all higher plants are
varieties of them.

Special Account. It must not be forgotten that the differences
between tissues are only the outcome of the differences between
their component cells (p. 13). So that the study of the histology
of the rhizome, even if preceded (as it may well be) by a dissec-
tion, and a naked-eye examination of some of the tissues, event-
ually resolves itself into the careful microscopic study of the
several kinds of cells composing those tissues.

The mature parts of the rhizome contain at least nine very
different kinds of cells, the characteristics and grouping of
which are shown in the following table. In the apical buds,
however, this arrangement disappears, and all the cells appear
closely similar.

HISTOLOGY OF TUB RHIZOME. 117

MINUTE ANATOMY OF THE RHIZOME OF PTERIS AQUILINA.

SYSTEM.

TISSUES.

CHARACTERISTICS.

I. Epidermal

1. Epidermis.

Cells polygonal in cross-section, empty. Walls
hard, thickened, especially towards the outside.

II. Funda-
mental.

2. Fundamental
parenchyma.

»

Cells rounded or polygonal in cross-section, color-
less. Thin-walled, containing protoplasm, nu-
cleus and starch. Intercellular spaces present.
(Fig. 52, /.p.)

3. Sclerotic par-
enchyma.

Cells polygonal or semi-fusiform in section, nearly
empty. No intercellular spaces. Walls hard
and brown, thickened. (Fig. 49.)

4. Sclerotic pros-
enchyma (or
. sclerenchyma)

Cells fusiform, empty. Walls thick, red. (Fig. 50.)

III. Fibro-
vascular.

5. Wood - paren -
chyma.

Like the fundamental parenchyma, but with more
elongated cells. (Figs. 52, 53.)

6. Phloem-paren-
chyma.

Precisely like 5, differing only in position.

1. Phlnem-prosen-
chyma, or
bast-fibres.

Cells fusiform, rich in protoplasm, colorless. Walls
thick, soft. (Figs. 52, 53.)

8. Sieve-tubes.

Having the ordinary characters (see preceding
table). (Figs. 52-51.)

9. Trachrids {lad-
der-cells).

Pits jtransversely elongated (scalariform). (Figs.

10. Trachea or re*-
seUs (»piral).

Very slender, with one or two internal spiral thick-
enings. (Fig. 52.)

Besides the above-mentioned tissues, the rhizome contains
certain other secondary varieties which will be described further
on.

Epidermal System. Epidermis. It is the function of the
epidermis (aided in this case by the underlying sclerotic paren-
chyma) to protect the inner tissues from contact with the soil
and to guard against desiccation of the rhizome during droughts.
The cells (Fig. 49) are dead and empty, with enormously thick,
hard walls perforated by numerous branching canals. The outer
wall is especially thick.

Fundamental System. The tissues of this system form the
main body of the plant, and in the fern have two widely differ-

118

THE BIOLOQT OF A PLANT.

FIG. 49.— Section showing the epidermis (ej>) and the underlying sclerotic paren-
chyma (s.p) of the rhizome of Pteris cufuilina. Canals, sometimes branching, are
everywhere seen. These served to keep the once-living cells in material con-
nection.

Fio. 50.-Cross-section of sclerotic prosenchyma of the rhizome of Pteris aquttina.
The enormously thickened walls consist of three layers, are perforated by canals,
and are UffnAfled or turned into wood

HISTOLOGY OF THE RHIZOME.

119

ent functions. The fundamental parenchyma is a kind of store-
house in which matter and energy are stored — mainly in the
form of starch, C6H10O5 — and in which active chemical changes
take place. The cells are thin- walled and soft, and are rather
loosely joined together, leaving numerous intercellular spaces
(Figs. 52, 53). They contain protoplasm and a nucleus, and
very numerous rounded grains of starch. This starch is stored
up by the plant during the summer as a reserve supply of food
— just as hibernating animals store up fat in their bodies for use
during the winter. Accordingly, starch increases in quantity
during the summer and decreases in the spring when the plant
resumes its growth, before the leaves are unfolded. The paren-
chyma probably has also the function of conducting various sub-
stances (especially dissolved sugar) through the plant by diffusion
from cell to cell.

The sclerotic parenchyma and sclerotic prosencJiyma (Figs. 49, 50) are
dead, and hence play a passive part in the adult vegetal economy. The
former co-operates with the epidermis ; the
latter probably serves in part to support the
soft tissues, and to some extent affords a
channel for the conveyance of the sap. The
sap, however, does not flow through the
cavities, but passes slowly along the sub-
stance of the porous walls. The cells of
both these sclerotic tissues have very thick,
hard, brown walls, perforated here and
there by narrow canals. The cells of the
parenchyma are prismatic or polyhedral ;
those of the prosenchyma elongated, and
pointed at their ends. In both, the proto-
plasm and nuclei disappear when the cells
are fully formed. Towards the apical buds
both fade into ordinary fundamental paren-
chyma.

Fibro-vascular System. The fibro-

vascular bundles (p. 115) are long FIG. si. (After Sachs.)— view of
strands or bands of tissue which ap- the rhizome, which is supposed

to be transparent so as to show

pear in CrOSS-Section as isolated Spots the network of the upper fibro-
(Fig. 48). The bundles are not vascular bundles. Z, a leaf.

really isolated, however, but join one another here and there,
forming an open network (Fig. 51), which can only be seen in a

120

TEE BIOLOGY OF A PLANT.

lateral view of the rhizome. From this network bundles are
given off which extend on the one hand into the roots and on
the other into the leaves, branching in the latter to form the
complicated system of veins to be described hereafter (p. 129).

Each bundle consists of a number of different tissues which,
broadly speaking, have the function of conducting sap from one
part of the plant to another.

fP

. 53.— Highly magnified cross-section of a flbro-vascular bundle surrounded by
the fundamental parenchyma, /.p. f, scalariform tracheids ; h.*, bundle-sheath ;
p.s, phloem-sheath ; h.f, bast-fibres ; s.t , sieve-tubes ; p.p, phloSm-parenchyma ;
w.p, wood (xylem) parenchyma; jt.r, spiral vessel.

These tissues have the following definite arrangement. Beginning with
the outside of a bundle, we find (Figs. 52, 53) —

1. Bundle- sheath ; a single layer of elongated cells enveloping the
bundle, probably derived from and belonging to the fundamental system.

2. Phloem-sheath ; a single layer of larger parenchymatous cells con-
taining starch in large quantities.

3. Bast-fibres; soft, thick-walled, elongated, pointed cells containing
protoplasm and large nuclei.

4. Sieve-tubes; larger, soft, thin-walled, elongated cells containing
protoplasm and having the walls marked by areas perforated by numerous
fine pores (panelled). They join at the ends by oblique panelled partitions-
(shown in Figs. 52 and 53).

HISTOLOGY OF THE RHIZOME.

121

5. Phloem-parenchyma; ordinary parenchymatous cells filled with
starch, scattered here and there among the bast-fibres and sieve-tubes.

6. Tracheids (scalariform) or "ladder-cells" ; occupying most of the
central part of the bundle. Their structure calls for some remark. They
are empty or air-filled fusiform tubes, whose hard, thick walls are in the
young tissue sculptured with great regularity into a series of transverse
hollows or pits, which finally become actual holes. The walls of the
tracheid are therefore continuous at the angles, but along their plane sur-

fP.

FIG. 53.— Longitudinal section of a nbro-vascular bundle, surrounded by the fun-
damental parenchyma. &./, bast-fibres; b.s, bundle-sheath; f.p, fundamental
parenchma ; p.p, phlegm-parenchyma ; p.s, phloem-sheath ; s.t, sieve-tubes ; t,
scalariform tracheids or ladder-cells ; w.p, wood-parenchyma.

faces become converted into a series of parallel bars, making a grating of
singular beauty. The slits between the bars are not rectangular passages
through the wall, but are rather like elongated, flattened funnels, opening
outwards. The sides of the funnels are called the borders of the pits; and
pits of this sort are called bordered scalariform pits (cf. Fig. 53).

7. Trachece or vessels (spiral) ; scattered here and there among the
tracheids, and hardly distinguishable from them in cross-section. They
are continuous elongated tubes filled with air, and strengthened by a beau-
tiful close spiral ridge (sometirnes double) which runs round the inner face
of the wall (Fig. 52).

The tracheids and vessels are of great physiological importance, being
probably the main channels for the flow of sap. Sap is water holding
various substances in solution. The water enters by the roots, flows prin-
cipally through the walls of the vessels and tracheids, and not through
their cavities, which are filled with air, and is thus conducted through the
rhizome and upwards into the leaves.

8. Wood-parenchyma; cells like those of the phloem-parenchyma (5)
scattered between the vessels and tracheids.

122

THE BIOLOGY OF A PLANT.

Branches of the Rhizome These repeat in all respects the
structure of the main stem. They are equivalent members of
the underground part, and differ in no wise, excepting in their
origin, from the main stem itself.

Roots. The roots may easily be recognized by their small
size and tapering form, and their lack of the lateral ridges of the

iv.

FIG. 54. (After De Bary.)— Sieve-tubes from the rhizome of Pterte aquilina, show-
ing: A, the end of a member of a sieve-tube ; B, part of a thin longitudinal sec-
tion. The section has approximately halved two sieve-tubes, S1 and S* , which are-
so drawn that the uninjured side lies behind. The broad posterior surface of S*
is seen covered with sieve-plates connecting with another sieve-tube. S1, on the
contrary, abuts by a smooth non-plated surface upon parenchymatous cell*
which are seen through it. «', sections of walls bearing sieve-pits ; j, section of
a non-plated wall abutting upon parenchyma.

stem and branches. They arise endogenously from the main
stem or its branches, i.e., by an outgrowth of the internal tissues,
and not (as in the case of the false roots or rhizoids of the pro-
thallium, shortly to be described) by elongation of superficial
cells of the epidermis. True roots, of which those of Pteris are
good examples, arise always as well from the fundamental and
fibro-vascular regions, and include all the systems found in the
stem itself. Hence cross-sections of Pteris roots differ but
slightly from those of the stem or the branches, and the root in
general is clearly a member of the plant body. As in all true
roots, the free end is covered by a special boring tip called the

STRUCTURE OF TEE APICAL BUDS.

123

root-cap, but this is apt to be lost in removing the specimen
from the earth.

The Embryonic Tissue or Meristem of the Rhizome. The
mature rhizome remains at the tip nearly undifferentiated into
tissues. At this point the epidermis may be distinguished, but
it remains very delicate, and the underlying cells continue to
grow and multiply, producing continued elongation of the mass.
In this way the apical bud is formed. Lateral buds are given
off right and left to constitute the embryos of leaves, branches,
or roots, which, always retaining their soft and delicate tips, are
capable of further growth.

Behind these "growing points" the epidermis and other
tissues grow more and more slowly, and soon reach their maxi-
mum size, whereupon rapid growth ceases. The power of
growth is henceforward mainly confined to the apical buds, and
the growing tissue of which they are composed is known as em-
bryonic fissue ormeristem.

The Apical Cell of the Rhizome. Close examination reveals
the fact that each apical bud contains a remarkable cell which is
especially concerned in the function of growth, viz., the apical
cell, which lies in a hollow at the apex of the bud. In the
apical buds of the rhizome or branches this cell has somewhat the

a.c,

FIG. 55A. (After Hofmeister.)— Apical cell
of the rhizome in a vertical longitudinal
section, a.c, apical cell ; It, hair ; m, meri-
stem.

FIG. 55 B. (After Hofmeister.)—
Apical cell of the rhizome in hori-
zontal longitudinal section, a.c,
apical cell.

form of a wedge with its base turned forwards and its thin edge
backwards, the latter placed at right angles to a plane passing
through the lateral ridges. It continually increases in size, but
as it grows repeatedly divides so as to cut off cells laterally

124

THE BIOLOGY OF A PLANT.

alternately on its right and left sides. These cells in turn con-
tinue to grow and divide, and thus give rise to two similar masses
of meristem, which together constitute the apical bud. From
the meristem by gradual, though rapid, changes the various tis-
sues of the adult rhizome are differentiated ; and longitudinal
sections passing through the lateral ridges show the mature
tissues fading out in a region of indifferent meristem about the
apical cell (Fig. 5 OB).

The apical cell lies at the bottom of a funnel-shaped depression at the
tip of the stem. It is shaped approximately like a thin, two-edged wedge
with an arched or curved base turned forwards towards the centre of the
funnel-shaped depression. The thin edge of the wedge is directed back-
wards, and its sides, which are also curved, meet in a 'vertical plane above
and below. A longitudinal section taken through the plane of the lateral

a-c

FTG. 56. (After Sachs.)— A vertical transverse section through the apical ceH, a.c,
showing a boundary of hairs and a second apical cell, I, belonging to a leaf.

ridges therefore shows the apical cell in a triangular form as in Fig. 55B.
A section taken at right angles to this — i.e., vertical and longitudinal —
shows the cell to be approximately rectangular and quadrilateral (Fig.
55A), while a transverse vertical section shows it in the form of a bi-convex
lens (Fig. 56).

The funnel-shaped depression is compressed vertically, and its walls are
thickly covered with erect branching hairs, which are closely fastened

Fio. CT.-Cross-section of an entire fertile leaflet, m.r. midrib; v, veins; ep, epi-
dermis ; m«, mesophyll ; sp, sporangia ; CM, indusium.

together by a hardened mucilage secreted by the apical bud. These hairs
entirely close the mouth of the funnel and shut off the delicate young

HISTOLOGY OF THE LEAF.

125

portions at its base from the outer world. Protected by these hairs, the
end of the stem forces its way through the toughest clay without injury to
the delicate bud buried in its apex. (Hofmeister.)

FIG. 58.— Cross-section, still more enlarged, passing throngh the midrib of a leaflet.
In the centre the circular nbro-vascular bundle, supported, especially above and
below, by thickened prosenchyma ( p>. On either side the parenchymatous, mes-
ophyll cells (shaded) and the intercellular spaces (i.*) opening by stomata (st);
epidermis <ep).

THE AERIAL PART OF THE BRAKE. THE FROND OR LEAF.

The external form of the leaf has been described on p. 109,
and it now remains to consider its internal structure. The
lamina is to be regarded as a flattened and altered portion of the
stipe, made thin and delicate in order to present a large surface
to the light and the air. The stipe, in turn, is a prolongation
of the rhizome, so that the whole plant body is a continuous
mass, throughout which extend the three systems of tissue vir-
tually unchanged. The transverse and longitudinal sections of
the stipe show only minor points of difference from correspond-
ing sections of the rhizome. In the leaf, however, all three

126

THE BIOLOGY OF A PLANT.

systems undergo great changes. The epidermis becomes very
thin, delicate, and transparent ; the fibro-vascular bundles break
up into an extremely fine and complex network forming the

.

FIG. 59.-Cross-section of part of a leaflet showing the microscopic structure. cpt
epidermis; st, stomata; f.s, intercellular spaces between the mesophyll-cells,
which are filled with (shaded) chlorophyll-bodies lying in the protoplasm.

veins; the sclerotic tissues become transparent and are found
only along the veins. The cells of the fundamental parenchyma
alter their form, lose their starch, and become filled with bright-
green, rounded bodies, called the chromatoplwrcs or cliloroplnjll-
bodies, which are composed of a protoplasmic basis colored by a
pigment known as chlorophyll. The green fundamental paren-
chyma of the leaf is sometimes called the mesophyll.

A cross-section of a leaflet (p. 100) is shown in Fig. 57.
The finer structure of the leaflet is shown in Figs. 58 and 59.
On the outside is the epidermis (V/>) ; within, the mesophyll and
midrib — the latter composed of thickened epidermal and sclerotic
fundamental tissue, and a large fibro-vascular bundle.

The mesophyll, or leaf-parenchyma, consists of irregular cells

HISTOLOGY OF THE LEAF,

127

which are loosely arranged on the lower side, leaving very large
intercellular spaces, but are closely packed, and leave few or no
intercellular spaces, on the upper (sunny) side. The cells have
very thin walls, contain protoplasm and a large central space

FIG. 60.— Epidermis from the under side of a leaflet, showing wavy cells ; elongated
(prnscnchymatoHfi) cells over the veins ; and stomata with their guard-cells, st,
stomata and guard-cells ; r, veins covered by thick and prosenchymatous epi-
dermal cells. Intermediate stages between wavy and straight cells are also
shown. (Surface view.)

(vacuole) filled with sap, and numerous chlorophyll-bodies im-
bedded in the protoplasm. These are especially numerous in.

128

THE BIOLOO T OF A PLANT.

the upper part of the leaf, as might be expected from their
functions in connection with the action of light (see page 147).

The epidermis, or skin of the leaf, consists of translucent,
greatly flattened cells having peculiar wavy outlines and rela-
tively thick walls (Figs. 58-61). Upon

A- ^\ y-^"rn'c' tne veins they become elongated, and
their walls are considerably thickened,
especially upon the midrib (Fig. 58,
They generally contain large, distinct
nuclei, and often considerable proto-

i.c. n a! ? plasm. The wavy epidermal cells,

particularly in young plants, contain

«^v \ ^^rCt^ some chlorophyll and starch, though

in this respect the feni is somewhat
exceptional.

In the rhizome the epidermis forms

gf '**~\) { a continuous layer over the whole sur-

FIG. ei. (After Sachs.)-Epi- face. In the leaf, however, this is not

dermal cells of Pteris flaM- ft c t|ie epj(]ermi8 O11 the lower

Ma, showing the development

of stomata. A, very young side being perforated by holes leading

mother-cell; s.r, sudsidiary Or stomata (singular, stoma) (Fig. 61).

*• These holes do not pass into the cells,

but are gaps or breaks between certain cells of the epidermis,
and open directly into the intercellular spaces, of which they are,
in fact, the ends. That portion of the intercellular labyrinth
which directly underlies the stoma is sometimes called the respira-
tory cavity. Each stoma is bounded, as in most plants, by two
curving guard-cells, which are generally nucleated, and, unlike
epidermal cells generally, contain abundant chlorophyll-bodies
and starch.

The guard-cells are capable of changing their form accord-
ing to the amount of light, the hygroscopic state of the atmos-
phere, and other circumstances, and thus open or close the hole
or stoma between them. This action is of great importance in,
the physiology of the plant (transpiration, p. 147).

In Pteris cretica and P. flabellata the stomata develop as follows : A
young epidermnl cell is divided by a curved partition into two cells, one of
which (Fig. 61) is called the initial cell of the stoma (e.c). This is again

VENATION. 129

divided by a curved partition into the mother-cell of the stoma (Fig. 61,
m.c) and a subsidiary cell (Fig. 61, s.c).

The mother-cell is then bisected into the two guard-cells, and the stoma
appears as a chink between them (Fig. 61, B).

The veins are the fibres or threads which constitute the
framework of the leaf. Each consists, essentially, of a small
fibro- vascular bundle branching from that of the midrib (Figs.
57, 58, 62). Above and below them the inesophyll and epi-
dermal cells are generally thickened and proseuchymatous, in this
way contributing alike to the form and the function of the
" vein."

FIG. 62. (After Luerssen.)— Venation of a leaflet of Ptetis aquilina

Their arrangement (veining or venation) is definite, and depends on the
mode of branching of the fibre-vascular strand which constitutes the prin-
cipal part of the midrib. Secondary strands (nerves) proceed from this at
an acute angle, then turn somewhat abruptly towards the edge of the
leaflet (or lobe), making an arch which is convex towards the distal ex-
tremity of the midrib (Fig. 62).

From this point, after branching once or twice, the delicate veins run
parallel to each other to the edge of the leaflet, where they join one another
or anastomose. This form of venation is known as Nervatio Neuropteri-
dis, and is more easily seen in the leaf of Osmunda regalis (cf. Luerssen,
RabenhorsVs Kryptogamen-Flora (1884), III., s. 12).

CHAPTER IX.

THE BIOLOGY OF A PLANT (Continued).

Keproduction and Development of the Brake or Pern.

Reproduction. Unlike the earthworm, the fern reproduces
both by gamogenesis (sexually) and agamogenesis (asexually).
Pteris possesses two modes of asexual reproduction, viz., the
detachment of entire branches from the rhizome and the con-
sequent establishment of independent plants, as already men-
tioned (p. Ill), and the formation of " adventitious buds " from
the bases of the leaf-stalks (Fig. 40). But besides these the
fern has a quite different method of reproduction, in which a
process of agamogenesis regularly alternates with gamogenesis
(alternation of generations). The following brief outline of
this important process may help to guide the student through
the subsequent detailed descriptions.

Upon some of the leaves are formed organs called sporangia
(Figs. 57, 63, 64), which produce numerous reproductive cells
called spores. The spores become detached from the parent and
develop into independent plants, the prothdttia (Fig. 70), which
differ entirely in appearance from the fern and ultimately pro-
duce male and female germ-cells. The female cell of the pro-
thallium, if fertilized by a male cell, develops into an ordinary
"fern," which again produces spores asexually. The forma-
tion and development of the spores is evidently a process of
agamogenesis, and the fern proper is therefore neither male nor
female — i.e., it is sexless or asexual. The formation and de-
velopment of the germ-cells, on the contrary, is a process of
gamogenesis; and the prothallium is a distinct sexual plant,
being both male and female (hermaphrodite or bisexual). In
general terms this is expressed by calling the ordinary fern the
spore-bearer, or sporopkore, and the prothallium the egg-
bearer, or oophore. The life-history of the fern, broadly

130

ALTERNATION OF GENERATIONS. 131

speaking, consists therefore in an alternation of the sporophore
(asexual generation) with the oophore (sexual generation) ; that
is, .it consists of an alternation of
generations. An essentially similar
alternation of sporophore with oophore
occurs in all higher plants, though in
most cases it is so disguised as to es-
cape ordinary observation.

The Sporangia and Spores. The
sporangia of Pteris (Figs. 63, 64) a.-
arise upon a longitudinal thickening
of tissue situated on the under side of
the leaflets near their edges, and in-
cluding a marginal anastomosis of the
veins. This swelling is known as
the receptacle. Hairs are not uncom- Fl«- 63. (After suminski.)— Spo-

, i j -i /. ,1 | ,. rangium of Pferfo serrulata. p,

mon Upon the Under Side of the leaf, pedicel; c, capsule; a, annulus;

and some are found upon or near the 8' 8P°re-
receptacle. On the latter arise structures, at first superficially
similar to hairs, which become enlarged at the tip, and finally
develop into the sporangia. Meanwhile the edge of the leaflet
is bent down and under so as to make a longitudinal band of
thin tissue composed of epidermis known as the outer veil or
indusium (Fig. 64, o.i). A similar thin sheet of epidermis
grows down from the under side of the leaf, and passing out-
wards to meet the former, constitutes the inner veil or true
indusium (Fig. 64, B, i.i).

In the Y-shaped space thus formed the sporangia are de-
veloped.

A superficial (epidermal) cell enlarges and becomes divided into a
proximal (basal) cell and a distal (apical) cell (Fig. 65, a). The former de-
velops into the future pedicel or stalk of the sporangium ; the latter gives
rise to the head or capsule within which the spores are formed (cf. Fig. 68).
The pedicel arises from the original pedicel-cell by continued growth and
subdivision until it consists of three rows of cells somewhat elongated.
The rounded capsule-cell is next transformed by four successive oblique
divisions into four plano-convex "parietal cells" and a tetrahedral central
cell, the archesporium, enclosed by the others. The capsule-cell is thus
divided by three planes inclined at about 120° (Fig. 65, 6, c). A fourth
(Fig. 65, d, e) passes nearly parallel to the top of the capsule and cuts off

132

THE BIOLOGY OF A PLANT.

from it the central cell or archesporium. In the parietal cells further
divisions follow, perpendicular to the surface, while the archesporium gives
rise to four intermediate or tapetal cells, parallel to the original parietal
group (Fig. 65, g). The sporangium now consists of a central tetrahedral
archesporium bounded by four tapetal cells, which in turn are enclosed by
the parietal cells, at this time rapidly multiplying by divisions perpen-
dicular to the exterior. Owing to the peculiar position of the planes of

A.

B.

O.I.

FIG. &4. (From Luerssen, after Burck.)— Indusia and receptacle of Pteris aquilina;
B (diagrammatic), seen from below ; A, in the section of the edge of a leaflet, o.t,
outer (false) indusium; i.t, inner (true) indusium; r, receptacle; 8, young
sporangia.

division the whole capsule is now somewhat flattened, and it becomes still
more so by the formation along the edge of a peculiar structure called the
ring or annulus, whose function is the rupturing of the capsule and the
liberation of the spores. The annulus is formed by a number of parallel
transverse partitions (Fig. 65, /, 7t, «,./), which subdivide the peripheral
cells of one edge of the capsule until a certain number of cells have been
formed. These then project upon the capsule (Fig. 65, j) and form an in
complete ring (Fig. 65, k).

Meanwhile the tapetal cells sometimes subdivide so as to form a double
row (Fig. 65, 7i), and soon afterwards are absorbed, space being thus left

DEVELOPMENT OF SPORANGIA.

133

FIG. 65. (After Luerssen.)— Development of the sporangia of Atpidium FiHx mas,
which is closely similar to that of Fteris. fl, the young sporangium standing
upon the epidermis-cell from which it has just been divided ; x, the proximal
cell cut off from the sporangium to form the pedicel and support the capsule ;
0, 1, the first partition in the capsule ; h, 1 and 2, the first and second partitions;
c, 1, 2, 4, the first, second, and fourth partitions ; d and e are cross-sections of the
capsule showing the oblique position of the partitions, and especially that of the
third ; /, a later stage ; g, the origin of the tapetal cells and the formation of the
archesporium ; 7i, division of the tapetal cells and the formation of the spore
mother-cells ; Z, four spores as they originate in the spore mother-cells ; ?, J, k,
the annulus and ripe sporangium, in surface view ; p, peripheral cells ; «r,
archesporium ; t, tapetal cells ; on, annulus.

134

THE BIOLOGY OF A PLANT.

for the growth and enlargement of the archesporium. The latter now
divides — first into 2, then into 4, 8, and finally 16 cells, the mother-cells
of the spores. These remain for a time closely united, but eventually
separate and again subdivide, each into 4 daughter-cells (Fig. 65, I). The
64 cells thus formed are the asexual spores. In their mature state they
have a tetrahedral form and certain external markings, indicated in Figs.
63, 66. Each spore acquires a double membrane, viz., an inner, endo-
sporium, delicate and white, and an outer, exosporium, yellowish brown,
hard, and sculptured over the surface with very close and fine, but
irregular, warty excrescences.

Germination of the Spores. Development of the Prothallium.
In the brake the spores ripen in July or August and are set
free by rupture of the sporangium under
the strain exerted by the elastic annulus, as
indicated in Fig. 63. Germination of the
spores normally occurs only after a considera-
ble period (perhaps not before the following
spring) ; it begins by a rupture of the exospo-

FIG. 66. (After FIG. 67. (After Suminski.V-Germinat- Fio. 68. (After Sumin-
Suminski.) - ing spores of Pteri* xerndata. A, in an ski.)— Very young pro-
early stage ; B, after the appearance thallium
of one transverse partition ; s, spore ;
p, protonema ; r, rhizoid.

Single spore of
Pteris serrula-
to.

of Pferte,

showing the spore («),
two rhlzoids (r), and
the enlarging extrem-
ity.

rium which is probably immediately due to an imbibition of
water. The spore bursts irregularly along the borders of the
pyramidal surfaces, and from the opening thus formed the endo-
sporium protrudes as a papilla filled with protoplasm in which
numerous chlorophyll-bodies soon appear.

This papilla is known as the protonema, or first portion of
the prothallium (Fig. 67). It develops very quickly into a stout
cylindrical protrusion divided into cells joined end to end.
Close to the spore one or more rhizoids are put down from the

DEVELOPMENT OF THE PROTHALLIUM.

135

growing protonema to serve as anchors and roots. At the oppo-

site or distal end longitudinal partitions soon appear (Fig. 68),

which speedily convert this portion into a broad flat plate at

first only one cell thick, but eventually several cells thick along

the median line. This thickening is the so-called ' ' cushion ' '

(see Fig. 70). The whole prothallium is now somewhat spatulate

(Fig. 69), but by further growth anteriorly, by an apical cell or

otherwise, the wider end becomes

still more flattened and heart-

shaped or even kidney-shaped.

Numerous rhizoids (so-called be-

cause they are not morphologi-

cally true roots) are put down,

and the whole structure assumes

approximately the appearance in-

dicated in Fig. 70. The spore-

membranes and protonema soon

fall away, and the prothallium

enters upon an independent exist-

ence, being rooted by its rhizoids

and having an abundance of

•chlorophyll. In the broad thin

plate of tissue no subdivision into

stem and leaf exists, and the

plant body closely resembles the

"thallus" of one of the lowest

plants. Since it is the precursor

of the ordinary "fern," it is

Called the "prothalluS " Or '"'"pro-
thallium"

The cushion forms a prominence on the lower side ; upon
its posterior part most of the rhizoids are borne.

Sexual Organs of the Prothallium. The prothallia of ferns
are as a rule bisexual or hermaphrodite ; that is, each individual
possesses both male and female organs. But the latter appear
somewhat later than the former, and poorly nourished prothallia
often bear only male organs, though they will frequently develop
female organs also if placed in better circumstances.

The Antheridia^ or male organs, are hemispherical promi-

young antheridia, and numerous
chlorophyll-bodies.

136

THE BIOLOO Y OF A PLANT.

nences occurring upon the posterior part and the under side of
the prothallium, often among the rhizoids. When fully formed
(Figs. 70, 71) an antheridium consists of a mass of rounded cells.
(spermatozoid mother-cells) enveloped by a membrane one cell
in thickness.

FIG. 70. (After Suminski, slightly modified.) -Adult prothallium of Ptcrti serrulata
seen from below, showing the rhizoids (r) at the posterior end, the depression at
the anterior end ; the cushion near the latter bearing (in this case) four arche-
3nia. Among the rhizoids are the (spherical) antheridia. The chlorophyll-
bodies only are shown in the cells of the broad plate of tissue constituting the
prothallium. Just above the anterior depression is seen a prothallium of the-

•m

FIG. 71. (After Strasburger.)-Mature an-
theridium of Pterix Kcrrulata. p, periphe-
ral cells; m, mother-cells of the sper-
matozoids.

FIG. 72.— Diagram to illustrate the ori-
gin of an antheridium. A, very-
young stage: B, older; a, original
epidermal cell enlarged ; h, mother-
cell of the entire antheridium.

MALE GERM-CELLS.

137

The mode of origin of the mother-cells differs considerably in different
ferns, but in all cases is essentially as follows : An ordinary cell on the
. lower side of the prothallium swells and forms a hemispherical or dome-
shaped projection, which is soon separated by a partition from the original
cell (Fig. 72). Further divisions then follow in the dome-shaped cell such,
that a central cell is left, surrounded by a
layer of peripheral cells (Fig. 73). By re-
peated divisions the central cell splits up
into the spermatozoid mother-cells (Fig. 71).
Within each mother-cell the proto-
plasm arranges itself in a peculiar
spiral body, the spermatozoid, which
is the male germ-cell.

When the mature antheridium is
moistened, the peripheral cells swell
and thus press out the mother-cells
and spermatozoids (Fig. 74). The
latter escape from the mother-cells and swim about very actively
in the water. They appear as naked single cells, of a peculiar
corkscrew shape, and bear upon the liner spirals numerous ex-
tremely active cilia (p. 31), by
which they are driven swiftly
through the water.

The Archegonia, or female

FIG. 73. (After Hofmeister.)—
Later stage in the development
of an antheridium of Pteris ser-
rulate, p, peripheral cell; c,
central cell from which the
spermatozoid mother - cells
arise.

Fro. 74. (After Luerssen.)— Bursting of
the antheridium and escape of the
spermatozoids. an, antheridium ; m.c,
spermatozoid mother-cells; sp, sper-
Tnatozoids.

FIG. 75. (After Strasburger.)— Mature
archegonium, showing the oosphere
(o), the neck (n), and mucus (m) is,
suing from the mouth of the canaL

138

THE BIOLOGY OF A PLANT.

organs (Figs. 70, 75), described for the first time by Suminski
in 186-i, likewise arise from single superficial cells of the pro-
thallium. They are situated almost exclusively upon the cushion
near its anterior or apical extremity, and hence at the bottom of
the anterior depression (Fig. 70). Since they appear later than
the antheridia, they are not likely to be fertilized by spermato-
zoids descended from the same spore. This phenomenon of
maturation of one set of sexual organs of a bisexual individual
before the ripening of the other set is a common feature among
plants, and is known as dic/togamy. There is reason to believe
that important advantages are gained by thus securing cross-fer-
tilization and preventing self-fertilization or ' ' breeding in and
in."

In the development of the archegonium the original cell enlarges, be-
comes somewhat dome-shaped, and divides by transverse partitions into
three cells : a proximal, im-
bedded in the tissue of the
prothallium, a middle, and a
distal dome-shaped cell (Fig.
76). The fate of the proximal
cell is unimportant. The dis-
tal cell gives rise by division
to a chimney-like structure,
the neck (Figs. 75, 77), which

B.

a.

6.

A.

FIG. 76.-Diagram to illustrate
the origin ot an archegonium.
A, an early stage; B, a later
stage; A, a, the original epi-
dermal cell enlarged ; B, o, the FIG. 77. (After Strasburger.)— Developing arche-

basal cell; b, the central or
canal cell; c, the neck-cell.

gonia of Pteris xerrulata. A, young stage ; B,
older ; n, neck ; c, canal ; o, oOsphere.

encloses a row of cells (canal-cells) derived from the original middle cell
(Figs. 75, 77). These afterwards become transformed into a mucilaginous-
substance filling a canal leading through the neck from the outside to the
oosphere (Fig. 77), which also arises from the original " middle" cell at its

FERTILIZATION AND DEVELOPMENT.

139

proximal end. The oosphere is the all-important female germ-cell to which
the " neck-" and " canal-cells " are merely accessory.

Fertilization or Impregnation. Fertilization, or the sexual
act, is performed as follows : Sper-
matozoids in vast numbers are at-
tracted to the mouths of the arclie-
gonia and there become entangled
in the mucilage (Fig. 78). In
favorable cases one or more work
their way down the mucilaginous
canal, and at length one penetrates
and fuses with the oosphere.

FIG. 78. (After Strasburger.)—
Mouth of an archegonium of Pte-
ris semtlata, crowded with sper«
matozoids striving to effect an en-
trance.

It is known that one spermatozoid is
enough to fertilize the oosphere, and
probably one only penetrates it ; but sev-
eral are often seen in the mucilaginous
canal. It has been shown that the muci-
lage contains a small amount (about 0.3?)
of malic acid, which probably acts both as an attraction to the spermato-
zoids and as a stimulus to their movements. Pfeffer has proved that
capillary tubes containing a trace of a malate in solution are as attractive
to the spermatozoids as is the mucilage in the central canal, and phe-
nomena of this kind (chemiotaxis) have recently been shown to be common
and highly important.

The entrance of the spermatozoid into the ovum and its
fusion with if- mark an important epoch in the life-history of the
fern. The oosphere is from this instant a new and very differ-
ent thing, viz., an embryo, and is known as the oospore. It is
now the first stage of the asexual generation, though it is still
maintained for some time at the expense of the sexual generation
or oophore (p. 130).

Growth of the Embryo. The oospore, or one-celled embryonic
sporophore (p. 130), now rapidly becomes multicellular by di-
viding first into hemispheres, then into quadrants, etc. (Fig. 80 ;
compare Fig. 14). The first plane of division is approximately
a prolongation of the long axis of the archegonium (Fig. 80).
The second is nearly at right angles to it, so that the quadrants
may be described as anterior and posterior to the first plane.
The fate of the quadrant-cells is of special importance. The

140 THE BIOLOGY OF A PLANT.

lower anterior quadrant as it undergoes further division grows
out into t\\Q first root; the upper anterior quadrant in like man-
ner gives rise to the rhizome and the first leaf. The mass of
cells derived from the two posterior quadrants remains connected
with the prothallium as an organ for the absorption of nutri-
ment from the latter, and is inappropriately called iliefoot.

FIG. 79. Fro. 80.

FIG. 79. (After Hofmeister.)— Development of the embryo. A, section showing the
closed neck (»i) and the planes of quadrant division of the oospore or embryo (em).
The fore end of the prothallium is to the right. JJ and f, stages of the embryo
later than A, showing the beginnings of apical growth ; /, foot ; /, leaf ; r, root;
rh, rhizome.

FIG. 80. (From Luerssen, after Kienitz-Gerloff.)— Development of the embryo of
Pterte serrulata. The figures are optical sections taken vertically in the antero-
posterior axis of the prothallium, passing through the long axis of the neck of
the archegonium ; except C and D, which are taken at right angles to the others.
A, «, and p are the anterior and posterior segments of the oospore after this has
divided into hemispheres. The former (a) forms the stem, the latter (p) the root.
F shows in a late stage the division of the quadrants, r going to form the root, 8
the stem or rhizome, I the leaf, and / the foot : r, I, and 8 soon take on apical
growth as indicated in H and I. •

In Pteris serrulata the development is slightly different. The lower
anterior cell becomes the first leaf ; the upper anterior becomes the first
portion of the rhizome, the lower posterior becomes the primary root, and
the upper posterior remains as the "/oo£."

The several parts now enter upon rapid growtli accompanied
by continued cell-multiplication, until a stage is reached repre-

GROWTH AND DIFFERENTIATION.

141

sented in C, Fig. 79. A stage somewhat later than this, with
its attachment to the prothallium, is shown in Fig. 81. After
this the leaf grows upwards into the air, the root downwards
into the earth, and the young fern begins to shift for itself.
Eventually it reaches a condition shown in Figs. 82 and 83.
The prothallium remains connected
with the young fern for some time,
and may readily be found in this
condition attached to ilower-pots in
hot-houses, etc. But sooner or
later it falls off, and the young fern
enters upon an entirely independent
existence. The appearance of the
plant and the shape of the leaf do FlQ
not always at first resemble those
of the adult fern; growth is also
more rapid at first, several leaves
(7—12) being developed successively in the first year (p. 112).
Differentiation of the Tissues. In the earliest stages the tissue
is nearly or quite homogeneous, i.e., meristemic. But very
early in development, as the leaf turns upwards and the root

(After Hofmeister.)— Young
embryo of Pterte a(fuiUna, showing
its attachment to the prothallium
by the foot ; I, leaf ; /, foot ; r, firs,*
root.

or.

rh.

FIG. 82. (After Sachs.)— Older embryo of maidenhair-fern (Adiantum) attached to
the prothallium. Seen in section. Z, leaf; r, first root; rh, beginning of the
rhizome ; p, prothallium ; rz, rhizoids ; or, archegonia.

downwards, changes take place, which lead directly to a differ-
entiation into the three great systems of tissue — epidermal, fibro-
vascular, and fundamental. The epidermal and fundamental
systems take on almost at once the peculiarities which have al-

142

THE BIOLOGY OF A PLANT.

ready been noted in the adult, p. 117. The fibro-vascular system
of tissues is differentiated a little later. Different as the tissues
of the three systems are, it is plain from their mode of origin
that all are fundamentally of the same nature because of their
descent from the same ancestral cell; hence every cell in the
plant partakes more or less completely of the nature of every
other cell. The resemblances are primary and fundamental, the
differences secondary and derived.
And what is true of the fern in this
respect is equally true of all other
many-celled organisms.

Course of the Fibro-vascular Bundles.
Certain features of -the disposition and
course of the fibro-vascular bundles in the
embryo and in the adult may conveniently
be studied at this point. From the point
of junction of the bundles of the first leaf
and first root (Figs. 79, 81, 82) is developed
one central bundle traversing the young
rhizome and sending branches into the new
leaves and roots until 7-9 leaves have been
formed. After this time the rhizome
forks, and the course of the fibro-vascular
bundles in each fork is henceforwards com-
FIG. 83. (After Sach8.)-Young d A lateral depression appears in

maidenhair-fern (Ailwmtum) at- *

tachert totheProthaiiium,p. I, the central bundle of each stem, rapidly

leaf: i, 2, the first and second increases in depth, and soon divides the

bundle into two, one upper and one lower,

which are best recognized in old specimens (Fig. 48). When the forked
shoots have reached a length of about three inches, these bundles send out
at a small angle towards the periphery thinner, forked branches which
soon unite again to form a network near the epidermis. The uppermost
of these branches, which passes in the median line above the axile bundles,
is usually somewhat more fully developed, and almost as broad as the lat-
ter. This structure is generally retained in the mature rhizome (Fig.
48, x). The number of peripheral bundles maybe as great as twelve in the
cross-section. They anastomose in the vicinity of the place of insertion of
each frond, and thus form a hollow, cylindrical network, having elongated
meshes ; but no connecting branches between them and the two axile
bundles are found anywhere in the rhizome. The latter follow an en-
tirely isolated course within the creeping stem;* branches from them

* See, however, De Bary, Comp. Anat. Phanerogams and Ferns, p. 295.
Oxford, 1884.

EXCEPTIONAL MODES OF DEVELOPMENT. 143

enter the leaves, and it is only inside the leaf-stalk that these ramifications
are met by branches from the peripheral network. The bundles of the
roots arise only from the peripheral bundles, but those of leaves, as already
said, receive branches from both axillary and peripheral bundles. Two
thick brown plates (sclerotic prosenchymd) lie between the inner and
outer systems of bundles, and are only separated from one another at the
sides by a narrow band of parenchyma. They are often joined on one side
or even on both, in the latter case forming a tube which separates the
two systems of bundles. (Hofmeister.)

Apogamy. Apospory. In rare cases, e.g., in Pteris cretica, the ordi-
nary alternation of generations in the life-cycle of ferns is abbreviated by
the omission of the sexual process, and the immediate vegetative outgrowth
of the sporophore from the prothallium (apogamy). In other cases there
is an omission of the spore stage, and immediate vegetative development
of the oophore from the frond (apospory). (cf. Farlow, Quart. Journ.

Mic. Science, 1874 ; De Bary, Botan. Zeitung, 1878; Druery, etc., Journ.

Royal Mic. Soc., 1885, pp. 99 and 491.)

CHAPTER X.

THE BIOLOGY OF A PLANT (Continued).
The Physiology of the Fern.

THE brake, like the earthworm, is a limited portion of organ-
ized matter occupying a definite position in space and time. It
is bounded on all sides by material particles, some of which may
be living, but most of which are lifeless. The aerial portion is
immersed in and pressed upon by an invisible fluid, the atmos-
phere, while the underground portion is sunk in a denser
medium, the earth, which likewise acts upon it. At the same
time the fern reacts upon the air and the earth, maintaining
during its life an equilibrium which is disturbed and finally gives
way as the life of the plant draws to a close.

The Fern and its Environment. Those portions of space,
earth, and air which are nearest to the brake constitute its imme-
diate environment. But in a wider and truer sense the environ-
ment includes the whole universe outside the plant. To perceive
the truth of this it is only necessary to observe how profoundly
and directly the plant is affected by rays of light which travel to
it from the sun over a distance of many millions of miles, or
how extremely sensitive it is to the alternations of day and night
or of •summer and winter. The plant is fitted to make certain
exchanges with its environment, drawing from it certain forms
of matter and energy, and returning to it matter and energy in
other forms. Its whole life is an unconscious struggle to wrest
from the environment the means of subsistence ; death and decay
mark its final and unconditional surrender.

Adaptation of the Organism to its Environment. We can dis-
tinguish in Pteris as clearly as in Lumbricm the adaptation of
the organism to its environment. The aerial part of Pteris
must be fitted to make exchanges with, and maintain its life in,
the atmosphere, while the underground part must be similarly
" adapted " to the soil in which it lives.

144

ADAPTATION TO THE ENVIRONMENT. 145

The aerial part displays admirable adaptation in its stalk, which
rises to a point of vantage for procuring air and light, and in its
broadly spreading top, which is covered by a skin, tough and
impervious, to prevent undue evaporation and consequent desic-
cation, yet translucent, to allow the sun's rays to reach the
starch-making tissue within. The rhizome also, with its pointed
terminal buds, its elongated roots, armed with boring tips, and
its thick, fleshy parenchyma for the storage of food, is admirably
adapted to its own special surroundings. In order to realize
this, we have only to imagine the fern to be inverted, the aerial
portion being planted in the earth, and the underground portion
lifted into the air and exposed to the winds and sunshine. Under
these circumstances the want of adaptation of the parts to their
respective environments would speedily become apparent.

Yet different as these parts now are, they have originally
sprung from the same cell. More recently they were barely dis-
tinguishable in a mass of tissue, part of which turned upwards,
into the air, while another part turned downwards into the earth.
But as development went on, the aerial and underground parts
were progressively differentiated, thus becoming more and more
perfectly adapted to the peculiar conditions by which each is
surrounded.

Thus it appears that the harmony between every part of the
plant and its environment is brought about, as in the animal, by a
gradual process in the history of each individual. We can here
clearly see also the functional adaptation of the plant to chang-
ing external conditions. The environment of Pteris changes
periodically with the regular alternation of summer and winter,
and the plant also undergoes a corresponding periodic change of
structure in order to maintain its adaptation to the environment.
During the summer the aerial part is fully developed, and, as a
result of its activity, starch is accumulated in the rhizome. At
the approach of winter the aerial part dies, and the plant is re-
duced to the underground part safely buried in the soil. During
the winter and spring the starcli is gradually consumed, and the
aerial part is put forth again as the aerial environment becomes
once more favorable to it. The plant, therefore, like the animal,
possesses a certain plasticity which enables it to adapt itself to
gradually changing conditions of the environment.

146 THE BIOLOGY OF A PLANT.

A little consideration will show that every function or action of living
things may be regarded as contributing to the same great end, viz., har-
mony with the environment ; and from this point of view life itself has
been defined as "the continuous adjustment of internal relations to ex-
ternal relations." *

Nutrition. The fern does work. In pushing its stem
through the soil, in lifting its leaves into the air, in moving
food-matters from point to point, in building new tissue, in the
process of reproduction, and in all other forms of vital action,
the plant expends energy. Here, as in the animal, the imme-
diate source of energy is the living protoplasm, which, as it
lives, breaks down into simpler compounds. Hence the need of
an income to supply the power of doing work.

The Income. The income of the fern, like that of the earth-
worm, is of two kinds, viz. , matter and energy, but unlike that
of the worm it is not chiefly an income of foods, but only of tfie
raw materials of food. Matter enters the plant in the liquid or
gaseous form by diffusion, both from the soil through the roots
(liquids), and from the atmosphere through the leaves (gases).
We have here the direct absorption into the body proper of food-
stuffs precisely as the earthworm takes in water and oxygen.
Energy enters the plant, to a small extent, as the potential energy
of food-stuffs, but comes in principally as the kinetic energy of
sunlight absorbed in the leaves. The table on p. 147 shows the
precise nature and the more important sources of the income.

Of the substances, the solids (salts, etc.) must be dissolved
in water before they can be taken in. Water and dissolved salts
continually pass by diffusion from the soil into the roots, where
together they constitute the sap. The sap travels throughout
the whole plant, the main though not the only cause of move-
ment being the constant transpiration (evaporation) of watery
vapor from the leaves, especially through the stomata. The
gaseous matters (carbon dioxide, oxygen, nitrogen) enter the
plant mainly by diffusion from the atmosphere, are dissolved by
the sap in the leaves and elsewhere, and thus may pass to every
portion of the plant.

The Manufacture of Foods— especially Starch. Pteris owes
its power of absorbing the energy of sunlight to the chlorophyll-

* Spencer, Principles of Biology, vol. i. p. 80. N. Y., Appleton, 1881.

INCOME OF THE PLANT.

147

todies or chromatophores ; for plants which, like fungi, etc. , are
devoid of chlorophyll are unable thus to acquire energy. Enter-
ing the chlorophyll-bodies, the kinetic energy of sunlight is ap-
plied to the decomposition of carbon dioxide (CO2) and water
(H.,0). After passing through manifold but imperfectly known
processes, the elements of these substances finally reappear as
starch (C6H10O6) often in the form of granules imbedded in the
chlorophyll-bodies, and free oxygen, most of which is returned

INCOME OF PTERIS.

MATTER.

WHENCE DERIVED.

Carbon.

Mainly from the atmosphere as carbon dioxide (COa), but per-
haps partly from dissolved organic matters (food).

Hydrogen.

Mainly from the soil as water (HaO), but perhaps partly
organic foods.

from

Oxygen.

Mainly from the soil as water (HaO) and from the air as
oxygen.

free

Nitrogen.

Mainly from the soil * as nitrates or ammonium compounds, or
organic foods.

Sulphur.

Mainly from the soil as sulphates.

Other elements.

Mainly from the soil as various salts.

ENERGY.

Kinetic.

Mainly from the sunlight through the leaves.

Potential.

Perhaps to a limited extent in food materials via the roots.

to the atmosphere. Thus the leaf of Pteris in the light is con-
tinually absorbing carbon dioxide and giving forth free oxygen.

Carbon dioxide and water contain no potential energy, since
the affinities of their constituent elements are completely sat-
isfied. Starch, however, contains potential energy, since the
molecule is relatively unstable, i.e., capable of decomposition
into simpler, stabler molecules in which stronger affinities are

* It has been generally believed that plants are unable to make use of free
atmospheric nitrogen, but recent investigations have disproved this view for
certain species.

148 THE BIOLOGY OF A PLANT.

satisfied. And this is due to the fact that in the manufacture
of starch in the chlorophyll-bodies the kinetic energy of sunlight
a was expended in lifting the atoms into position of vantage,
thus endowing them with energy of position. In this way some
of the radiant and kinetic energy of the sun comes to be xl«r< <J
up as potential energy in the starch. In short, Pteris, like all
green plants, is able by co-operation with sunlight to use simple
raw materials (carbon dioxide, water, oxygen, etc.) poor in en-
ergy or devoid of it, and out of them to manufacture food, i.e.,
complex compounds rich in available potential energy. We
shall see hereafter that this power is possessed by green plants
alone ; all other organisms being dependent for energy upon the
potential energy of ready-made food. This must in the first
instance be provided for them by green plants ; and hence with-
out chlorophyll-bearing plants animals (and colorless plants as
well) apparently could not long exist.

The plant absorbs also a small amount of kinetic energy, in-
dependently of the sunlight, in the form of heat; this, however,
is probably not a source of vital energy, but only contributes to
the maintenance of the body temperature.

Circulation of Foods. It is chiefly in the green (chlorophyll-
bearing) parts of the plants, and in the presence of sunlight, that
food-manufacture goes on. Somehow, then, the water absorbed
by the roots must be transported to the leaves, and the starch
made in the leaves must be conveyed to the subterranean tissues.
Exactly how these transfers of material are effected is uncertain,
but there is reason to believe that they take place mainly by the
slow processes of diffusion. It is certain that no distinct organs
of circulation or distribution, such as the blood-vessels of the
earthworm, exist in the fern.

Metabolism. Starch, as has just been seen, is first formed in
the chlorophyll-bodies. But the formation of starch, all-impor-
tant as it is, is after all only the manufacture of food as a pre-
liminary to the real processes of nutrition. These processes must
take place everywhere in ordinary protoplasm; for it is here
that oxidations occur and the need for a renewal of matter and
energy consequently arises (cf. pp. 32 and 33). Sooner or later
the starch grains are changed into a kind of sugar (glucose,
C.HjjO,), which, unlike starch, dissolves in the sap, and may

OUTGO OF THE PLANT. 149

thus be easily transported to all parts of the plant. Wherever
there is need for new protoplasm, whether to repair previous
waste or to supply materials for growth, after absorption into
the cells the elements of the starch (or glucose) are, by the liv-
ing protoplasm, in some unknown way combined with nitrogen
and sulphur (probably also with salts, water, etc.), to form proteid
matter. The particles of this newly-formed compound are incor-
porated into the protoplasm (by " intus-susception, " p. -i) and, in
some way at present shrouded in mystery, are endowed with the
properties of life. We do not know how long they may remain
in the living state, but sooner or later they are oxidized, and, as a
result of the oxidation, that energy is set free which enables the
fern to do work and prolong its existence. The oxidized prod-
ucts are afterwards eliminated (excreted) from the cells.

If a larger quantity of starch is formed in the chlorophyll
bodies than is immediately needed by the protoplasm for pur-
poses of repair or growth, it may be re-converted into starch
after journeying as glucose through the plant, and be laid down
as "reserve starch " in the parenchyma of the rhizome, or else-
where. Apparently, when this reserve supply is finally needed
at any point in the plant, it is again changed to glucose and trans-
ported thither. It is probable that new leaves and new tissues
generally, are always formed in part from this reserve starch,
and not solely from newly-formed starch.

In dealing with the metabolism of the fern we may safely
assume, as we have done already for the earthworm, a constructive
phase (anabolimi) and a destructive phase (katabolism) ; but
these terms represent merely probable events, not known facts.

The Outgo. The outgo, like the income, is of two kinds,
matter and energy, but it cannot be so readily tabulated.

The plant suffers annually a great loss both of matter and of
potential energy in the production of spores and in the autumnal
dying-down of the fronds. But matter also leaves the plant
daily as carbon dioxide (in small quantities), water, and oxygen,
both by diffusion through the epidermis and by transpiration
through the stomata. Strictly speaking, the term outgo should
be restricted to the output of matter which has at some time
actually formed a part of the living protoplasm ; hence it does
not apply to the oxygen, which is simply given off in the maim-

150

THE BIOLOGY OF A PLANT.

facture of starch, or to the bulk of the water of evaporation,
which passes straight through the plant without undergoing any
chemical change. Energy likewise leaves the plant continuously
both as heat and in the doing of mechanical work, both of which
are involved in every vital act.

Respiration. It has been remarked that in the light (i.e.,
when manufacturing starch) Pteris takes in carbon dioxide and
gives oft' free oxygen. But if the plant be deprived of light, as
at night, the reverse is true, and the plant takes in a small
amount of oxygen and gives off a corresponding amount of car-
bon dioxide. This latter process is the true breathing or respi-

PTERIS AQUILINA.
(Balance-Sheet of Nutrition.)

INCOME.

Matter.
Foods,

Inorganic salts.
Carbon dioxide.
Water,
Free oxygen.

Energy.

Sunlight absorbed by chlorophyll,
Potential energy in foods.

OUTGO.

Matter.

Carbon dioxide,
Water,

Excreted substances,
Reproductive germs,
Leaves, etc.,
Free oxygen — from decomposition

of carbon dioxide in light.
Energy.

Work performed.
Heat.
Potential energy in cast-off matters,

reproductive germs, etc.

Balance in favor of the living Pterte :
Matter.

Tissues, protoplasm, starch, cellulose, chlorophyll, etc.
Energy.

Potential energy in organic matters.

ration of the plant, and it must not be confounded with that
taking in of carbon dioxide and giving off of oxygen which is an
incident in the manufacture of starch. Respiration goes on in
the light also, probably with greater energy than in darkness,
but it is then largely obscured by the other and more conspicu-
ous process. We have seen that energy is set free in living mat-
ter by a decomposition of its own substance, which is really a
process of oxidation or combustion, where free oxygen plays
;an important part (p. 32, Chap. III.); hence the absorption of
free oxygen in respiration. Among the products of the combus-
tion, water and carbon dioxide are the most important ; and this

ACTION UPON THE ENVIRONMENT. 151

is the origin of the carbon dioxide given off. It will appear
beyond that precisely the same action takes place in the respi-
ration of animals, and that all living things breathe or respire in
essentially the same way.

It was for a long time believed that a leading difference between plants
and animals lay in the fact that the former give off oxygen and absorb
<jarbon dioxide, while the latter give off carbon dioxide and absorb oxygen.
But it is now known that both give off carton dioxide and both require
oxygen, and that only the chlorophyll-bearing parts of green plants are en-
dowed with the special function of decomposing carbon dioxide and water
and manufacturing starch — as a result of which they do (but in the light
only) give off oxygen as a kind of incidental- or by-product.

INTERACTION OF THE FERN AND ITS ENVIRONMENT.

The actions of the environment upon the fern have already
been sufficiently dwelt upon (p. 144). It still remains, however,
to consider the actions of the fern upon the environment.
These are partly physical, but mainly chemical. By pushing
its fronds into the air and slowly thrusting its rhizome, roots, and
branches through the soil, the atmosphere and the earth are alike
•displaced. But it is by its chemical activity that it most pro-
foundly affects its environment. Absorbing from the latter
water, salts, carbon dioxide, and other simple substances, as well
as sunlight, it produces with them a remarkable metamorphosis.
It manufactures from them as raw materials organic matter in
the shape of starch, fats, and even proteids. These it gives
back to the environment in some measure during life, and sur-
renders wholly after sudden death. But the most striking fact
is that the fern is on the whole constructive and capable of pro-
ducing and accumulating compounds rich in energy. In this
respect it is unlike the earthworm (p. 104) and is typical of green
plants in general. Thus, while animals are destroyers of ener-
gized compounds, green plants are producers of them. Ani-
mals, therefore, in the long run are absolutely dependent on
plants ; and animals and colorless plants alike upon green plants.
But it must never be forgotten that most plants are enabled to
manufacture organic from inorganic matter by virtue of the
chlorophyll which they contain. "Without this they are power-
less in this respect. (See, however, p. 107).

152 THE BIOLOGY OF A PLANT.

Physiology of the Tissue- Systems. The epidermal tissues
serve as the sole medium of exchange between the inner parts of
the plant and the environment ; they are also protective, and in
certain regions are useful for support. The function of repro-
duction also falls upon these tissues, as is shown by the develop-
ment of the sporangia, antheridia, and archegonia.

The fibro-vascular tissues serve in part as a supporting
skeleton, for which function their richness in prosenehyma
and their firm continuity admirably adapt them. An equally
important function, however, is their conductivity, since they
serve for the transportation of the water for evaporation by the
leaf (transpiration}, and for the movement (through the sieve-
tubes) of the undissolved and indiffusible proteids. The/'"/"/"-
mental tissues are devoted either to sharing the special duties
of the other systems, as in the case of the sclerotic parenchyma
abutting upon the epidermal tissue in the rhizome (p. 119), and
the sclerotic prosenchyma which appears to behave like the libm-
vascular tissues; or to nutritive and metabolic functions, as in
the mesophyll (p. 126) and the parenchyma of the rhizome.

The Physiology of Reproduction. It is not known whether the
brake ever dies of old age. Barring accidents, growth at the
apical buds seems to be unlimited, keeping pace with death of
the hinder parts of the rhizome (p. 111). But whether the indi-
vidual dies or not, ample provision against the death of the race
is made in the act of reproduction. Although reproduction ap-
pears to be useless to the individual, and even entails upon it
serious annual losses of matter and energy, yet to this function
every part of the plant directly or indirectly contributes. The
reproductive germs are carefully prepared ; are provided with a
stock of food sufficient for the earliest stages of development ;
and are endowed with the peculiar powers and limitations of
Pteris aquilina, which influence their life-history at every step
and are by them transmitted in turn to their descendants. They
are living portions of the parent detached for reproductive pur-
poses; they contain a share of protoplasm directly descended
from the original protoplasm of the spore from which the parent
came ; and thus they serve to effect that ' ' continuity of the
germ-plasm" to which we have already referred in dealing
with the earthworm. In short, reproduction is the supreme

PLANT AND ANIMAL COMPARED. 153

function of the plant. If we may paraphrase the words of
Michael Foster, the oosphere is the goal of individual existence,
and life is a cycle, beginning with the oosphere and continually
coming round to it again.

Comparison of the Fern and the Earthworm. To the super-
ficial observer the fern and earthworm seem to have little or
nothing in common, except that both are what we call alive. But
whoever has studied the preceding pages must have perceived
beneath manifold differences of detail a fundamental likeness
between the plant and animal, not only in the substantial iden-
tity of the living matter in the two but also in the construction
of their bodies and in the processes by which they come into
existence. Each arises from a single cell which is the result of
the union of two differently-constituted cells, male and female.
In both the primary cell multiplies and forms a mass of cells, at
iirst nearly similar but afterwards differentiated in various di-
rections to enable them to perform different functions, i.e., to
effect a physiological division of labor. In both, the tissues thus
provided are associated more or less closely into distinct organs
and systems, among which the various operations of the body
are distributed. And in botli the ultimate goal of individual
existence is the production of germ-cells which form the start-
ing-point of new and similar cycles.

This fundamental likeness extends also to most of the actions
(physiology) of the two organisms. Both possess the power of
adapting themselves to the environments in which they live.
Both take in various forms of matter and energy from the en-
vironment, build them up into their own living substance, and
finally break down this substance more or less completely into
isimpler compounds by processes of internal combustion, setting
free by this action the energy which maintains their vital ac-
tivity. And, sooner or later, both give back to the environment
the matter and energy which they have taken from it. In other
words, both effect an exchange of matter and of energy with
the environment.

Nevertheless the plant and the animal differ. They differ
widely in form, and the plant is fixed and relatively rigid, while
the animal is flexible and mobile. The body of the plant is
relatively solid ; that of the animal contains numerous cavities.

154 THE BIOLOGY OF A PLANT.

The plant absorbs matter directly through the external surface ;
the animal partly through the external and partly through an
internal (alimentary) surface. The plant is able to absorb simple
chemical compounds from the air and earth, and kinetic energy
from sunlight ; the animal absorbs, for the most part, complex
chemical compounds and makes no nutritive use of the sun's
kinetic energy. By the aid of this energy the plant manufac-
tures starch from simple compounds, carbon dioxide, and water ;
the animal lacks this power. The plant can build up proteids
from the nitrogenous and other compounds of its food ; the animal
absolutely requires proteids in its food. And by manufacturing
proteids within its living substance, the plant is relieved of the
necessity of carrying on a process of digestion in order to render
them diffusible for entrance into the body.

Still, great as these differences appear to be at first sight,
all of them, with a single exception, fade away upon closer ex-
amination. This exception is the power of making foods.
Plants and animals differ in form because their mode of life
differs ; but a wider study of biology reveals the existence of in-
numerable animals (corals, sponges, hydroids, etc.) which have
a close superficial resemblance to plants, and of many plants
which resemble animals, not only in form, but also in possessing
the power of active locomotion. The stomach of the worm, as
shown by its development, is really a part of the general outer
surface which is folded into the body ; and the animal, like the
plant, therefore, really absorbs its income over its whole surface
— oxygen through the general outer surface, other food-matters
through the infolded alimentary surface.

In like manner it is easy to show that not one of the differ-
ences between the plant and animal is fundamentally impor-
tant save t\\Q- power of making foods. The worm must have
complex ready-made food including proteid matter. So must
the fern ; but the fern is able to manufacture this complex food
out of very simple compounds. In terms of energy, the worm
requires ready-made food rich in potential energy; the fern,
aided by the sun's energy, can manufacture food from matters,
devoid of energy.

Hence it appears, broadly speaking, that the fern by the aid
of solar energy is constructive, and stores up energy ; the earth-

FOOD OF PLANTS AND ANIMALS. 155

worm is destructive, and dissipates energy. And this difference
becomes of immense importance in view of the fact that the
fern is typical in this respect of all green plants, as the earth-
worm is typical of all animals.

It will hereafter appear that even this difference, great as it
is, is partly bridged over by colorless plants like yeast, moulds,
bacteria, etc., which have no chlorophyll, are therefore unable
to use the energy of light, and hence must have energized food.
But these organisms do not, like animals, require proteid food,
being able to extract all needful energy from the simpler fats,
carbohydrates, and even from certain salts. When we consider
that the distinctive peculiarities of animals can thus be reduced
to the sole characteristic of dependence on proteid food, we can-
not doubt that the differences between plants and animals are of
immeasurably less importance than their fundamental likeness.

It has been the object of the foregoing chapters to give the
student a general conception of organisms, whether vegetal or
animal ; of their structure, growth, and mode of action ; of their
position in the world of matter and energy, and of their relations
to lifeless things. With this preliminary knowledge as a basis,
the student is prepared to take up the progressive study of other
organisms, selected as convenient types or examples. It is con-
venient to begin with low and simple forms of life and work
gradually upwards; and it is especially desirable to do so be
cause there is reason to believe that this course corresponds
broadly with the path of actual evolution.

CHAPTER XI,
THE UNICELLULAR ORGANISMS.

IT lias been shown in the foregoing pages that the complex
body of an adult fern or earthworm, or of any of the higher
forms of life, originates from a single cell of microscopic size.
This cell — the fertilized ovum or oosphcre — gives rise by divi-
sion to new cells which in their turn divide, generation after
generation, until a full-grown Ijody is formed, composed of
myriads of cells. But the process of cell-division does not in
this case go as far as complete cell-separation, and the cells do
not acquire a complete individuality. They do, it is true, ac-
quire a certain independence of structure and function ; and
their individual characteristics may even depart widely from
those of neighboring cells (differentiation). Nevertheless they
remain closely united by either material or physiological bonds to
form one body. The body is not, however, to be regarded as
merely an assemblage of independent individual cells. The body
is the individual / its more or less perfect division into cells is
only a basis for the physiological division of labor; of which
cell-differentiation is the outward expression.

All this is true, however, only in the higher types. At the
bottom of the scale of life there is a vast multitude of forms in
which the body consists, not of many cells but of only one, and is
therefore comparable in structure not to the adult fern or earth-
worm, but to the germ-cells from which these arise. Such forms
are known as unicellular organisms, in contradistinction to the
multwellular. Like other cells the unicellular organisms multi-
ply by division, but division is followed sooner or later by com-
plete separation ; the daughter-cells become entirely distinct and
independent individuals, and do not remain permanently asso-
ciated. In them a true multicellular body, therefore, is never
formed ; the cell is the individual, and the lody is unicellular.

156

THE UNICELLULAR BODY. 157

Nevertheless the one-celled organism performs all of the
characteristic operations of life. A single mass of protoplasm, a
single cell, unites in itself the performance of all the various
elementary functions which in the multicellular forms are distrib-
uted among many cells, differentiated into divers tissues and
organs. The unicellular forms are therefore in a physiological
sense as truly ' ' organisms ' ' as the multicellular forms ; and in
many cases the unicellular body shows a very considerable degree
of differentiation among its parts. But the unicellular forms
are organisms reduced to their lowest terms ; they present us with
the problems of life in their most rudimentary form. Hence
they may afford a kind of key to the more elaborate organization
of the higher types.

We shall find among unicellular forms representatives both
of animals and of plants, and to a detailed examination of some
of these we may now proceed.

CHAPTEK XII.
TJNICELLULAR ANIMALS (Protozoa).

A. Amoeba.

(The Proteus Animalcule.)

General Account. Amoeba is a minute organism occasionally
found in stagnant water, in the sediment at the bottom of ponds
and ditches, on the surface of water-plants, .in damp earth, in
organic infusions of various kinds — almost anywhere, in short, in
the presence of moisture, organic matter, and other favorable
conditions. There are many species of Amoeba, some living in
salt water, others in fresh. One of the largest and commonest
fresh- water forms is Amoeba Proteus, wliich forms the subject
of this account.*

Amoeba occurs in an active or motile state, and a quiescent or
encysted state. When active the body consists (Fig. 84) of a
minute naked mass of protoplasm which in the case of large
specimens is barely visible to the naked eye — i.e., half a milli-
metre (^ inch) or less in length. This mass creeps, or rather
flows, actively about by the continual protrusion of lobes or proc-
esses of its own substance, known as pseudopodia. These may
be put forth from any part of the surface and again merged into
the general mass; the body therefore continually changes its
shape, and hence the name ' ' Proteus. ' '

When the body is well extended the protoplasm is seen to
consist of a clear peripheral substance, the ectoplasm, and a cen-
tral substance, the entoplasm, filled with coarse granules which
give the body a highly characteristic granular appearance some-
times described as a "gray color." Within the ectoplasm the
more fluid entoplasm freely flows, as if confined in a tube or

* Other common forms are the smaller A. radiosa and A. verrucosa. The
large A. (Pelomyxa) villosa and A (Dinamceba) mirabttis are not infrequent.
See Leidy, Fresh-water Rhizopods of North America.

158

THE PROTEUS ANIMALCULE.

159

SfMiiflPP

^•^^^^v^^^^

Fio. 84.— ^.nuBba Proteus, from life X 300. The arrows indicate the direction of the
protoplasmic currents ; n, nucleus ; e.r, contractile vacuole ; /.r, food-vacuole ;
w.v, water-vacuole. A. shows the texture of the protoplasm. B is an outline of
the same individual four minutes later ; the upward currents at the right of Fig.
A have stopped, reversed, and the main flow is now towards the left.

160 UNICELLULAR ANIMALS.

sac but the two substances are not separated by any definite
boundary-line, and pass imperceptibly into one another. The
external boundary of the body is formed by the outermost limit
of the ectoplasm. There is no membrane, and the body is
quite naked. Nevertheless the protoplasmic mass shows no
tendency to mix with the surrounding water, and perfectly main-
tains its integrity ; it is an individual.

The formation of a pseudopod begins by the bulging out of
the ectoplasm to form a rounded prominence at some point on
the surface. Into its interior a sudden gush of entoplasm then
takes place and a steady outward stream ensues, the entoplasm
pushing the ectoplasm before it, and the substance of the body
flowing into the pseudopod. The whole substance of the body
may thus now onward into the pseudopod, which meanwhile forms
new pseudopods, and so the entire animal advances in the direction
of the flow ; or, the pseudopod after attaining a certain size may
be withdrawn into the body by reverse (centripetal) currents, the
main body having meanwhile flowed onward in another direction.

As a rule, the new pseudopodia are put forth near one end
of the body (hence called "anterior "), and the general direction
of advance is therefore fairly constant, not vague and indefinite,
as is often stated. The direction of flow fluctuates, however,
about a certain mean, being continually diverted this way or
that by the formation of new pseudopodia. Those which do not
form directly in the line of march either merge little by little
with the advancing ones, or are withdrawn by reversed currents
into the body. In the latter case they often leave shrivelled
wart-like remnants, and a group of similar warts is usually
found near the ' ' posterior ' ' end of the body (Fig. 84, p).
Definite changes in the general direction of advance are effected
by the diversion of the main current into lateral pseudopodia.

Amoeba feeds upon minute plants and animals or other or-
ganic particles. There is no mouth, and food-matters are bodily
ingulfed (at no definite point) by the protoplasm which closes
up beyond them.* The indigestible remains are passed out in

* This mode of cellular alimentation is of frequent occurrence in some cells
of multicellular, as well as in unicellular, animals. Cells exhibiting it are
known as phagocytes (eating-cells), and the process is referred to as pliagocytosia.
It is obviously only a prelude to intra-cellular digestion.

ENCYSTED STATE OF AMCEBA.

161

an equally primitive fashion, usually at some point near the
"posterior " end. Besides solid food-stuffs Amoeba takes in a
certain quantity of water (along with minute quantities of inor-
ganic salts dissolved in it), and it also breathes, by taking in
(mainly by diffusion) the free oxygen dissolved in the water and
giving off carbon dioxide.

Such is Amoeba in its active phase. The quiescent or en-
cysted state is entered upon under conditions not thoroughly
understood, but probably of an unfavorable nature, such as the

D C

FIG. 85.— A, Amoeba dividing by fission, nucleus not seen (after Leidy). C, Arnreba
after a full meal consisting of a large diatom (ilt). (After Leidy). Letters as in
Fig. 84. D, Encysted Amoeba, containing food-matters (after Howes).

lack of food, drying up of ponds, and the like. The pseudo-
podia are withdrawn, movement ceases, the body becomes
spherical and surrounds itself with a tough membrane (cell-wall)
(Fig. 85, D). The animal takes no food and all of its activities
are nearly suspended. It is like an animal asleep or hibernating,
and in this state it may long remain. Protected by its mem-
brane it is able to resist desiccation, and upon the evaporation of
the surrounding water it may, as a particle of ' ' dust, ' ' be trans-
ported by the winds, even to a great distance. When again
placed under favorable conditions the protoplasm bursts its
envelope, crawls forth from it, and reassumes its active phase.

162 UNICELLULAR ANIMALS.

Structure. Lying in the entoplasm, usually near the pos-
terior extremity, is a nucleus (w, Fig. 84), having the form of
a bi-concave disk and largely made up of coarse granules of
chromatin (cf. p. 23). Amoeba is therefore at once a single
cell and a unicellular organism, morphologically equivalent to a
single tissue-cell of a higher animal or to the germ-cell from
which every multicellular form arises. The body of Amoeba is
a one-celled body.

The protoplasm (cytoplasm) consists of a clear basis, and (in
the case of the entoplasm) of innumerable granules extremely
diverse in form and size, and frequently differing in character
in different individuals. Often they are in the form of rhom-
boidal crystalline bodies; in other cases they are rounded or
irregular. Their precise chemical composition is uncertain, but
they are probably complex organic compounds, a product of
metabolism and serving as reserve food-matter.*

Vacuoles. The protoplasm often contains rounded vacuoles
of which the three following kinds may be distinguished :

(a) Water-vacuoles (w.v. Figs. 84, 85), filled with water,
lying in the entoplasm and carried along in its currents.

(b) Food-vacuoles (f.v\ also lying in the entoplasm, con-
taining the solid food-matters that have been ingulfed. Within
them digestion takes place. When this process is completed
they approach the exterior — usually at some point near the
posterior end — the outer wall breaks through, and the innutri-
tions remnants are cast out, the ectoplasm closing up the breach
immediately afterwards. Thus Amceba has no mouth, ali-
mentary canal, or anus, but the general mass of protoplasm
plays the role of all three.

(c) Contractile vacuole (c.v). Usually single, sometimes
double, lying near the posterior end, and filled with liquid.
This is sharply distinguished from the other vacuoles by its
rhythmical pulsation, expanding (diastole') and contracting (sys-
tole) at regular intervals. During the diastole the vacuole slowly
fills with liquid which drains into it from the surrounding proto-
plasm. At the systole, which is very sudden, this liquid is forci-
bly expelled to the exterior through an opening that breaks

* In some species of Amceba the entoplasm may also contain innumerable
grains of sand taken in from the exterior, but this is not the casein A. Proteus.

PHYSIOLOGY OF AMCEBA. 163

through the ectoplasm, and immediately afterwards disappears.
The contractile vacuole is almost certainly to be regarded as a
simple kind of excretory apparatus, the water which collects in
it containing in solution various products of destructive metabo-
lism which are thus passed out of the body.*

Reproduction. However abundant the food-supply Amoeba
never grows beyond a certain maximum limit. After this limit
has been attained the animal sooner or later divides by "fission"
into two smaller Amoebae (Fig. 85, A). Thus the existence of
an individual Amoeba is normally terminated, not by death, but
by resolution into two new individuals. This process is the
simplest possible form of agamogenesis, and Amoeba is not known
to multiply in any other way.f The fission of Amoeba is a
process essentially of the same nature as the division of ordinary
tissue-cells, a division of the nucleus preceding that of the
cytoplasm. Whether the division of the nucleus is of the indi-
rect type (i.e.: passes through the phenomena of karyokinesis) is
not known by direct observation, but there is some reason to be-
lieve that it is so. In any case the successive fissions of Amoeba
are directly comparable with the successive cleavages of the egg
of a metazoon (p. 25). The progeny of the Amoeba, however,
separate and form independent individuals, while those of the egg-
cell remain intimately associated to form a single multi-cellular
individual. Morphologically, therefore, a metazoon is comparable
not with a single Amoeba, but with a multitude of Amwbce.

Physiology. The possible simplicity of animal structure is
well shown in Amoeba, which is morphologically an animal re-
duced to its lowest terms. Its physiological operations are cor-
respondingly primitive and rudimentary ; and by an analysis of
them we may discover what is essential and fundamental in the
physiology of animals in general. A survey of the various activ-
ities of Amoeba shows that these may all be reduced to a f ew funda-
mental physiological properties of the protoplasm,^; as follows:

* It may be recalled that the cavity of the nepliridiuin in the earthworm is
intra-cellular, like a vacuole (p. 60).

f It has been asserted that Amoeba conjugates and also that it multiplies by
endogenous division ; but the evidence on both these points is inconclusive.

J It is hardly necessary to remark that in common with all English-speak-
ing biologists we are indebted to Foster for the first comprehensive elaboration
of the " fundamental physiological properties " as exhibited by Amaha.

164 UNICELLULAR ANIMALS.

(1) Contractility, by means of which motion is effected.
This appears most clearly when the animal is stimulated by a
sudden jar, or by an electric shock, which causes the body to
contract into a ball. This property, precisely like the contraction
of a muscle (p. 27), is the result of a molecular rearrangement,
accompanied by chemical changes, which causes a change of
form in the mass without altering its bulk. The action of the
contractile vacuole is due to the contractility of the surrounding
protoplasm ; and in like manner the currents which cause the
protrusion and withdrawal of pseudopods, and so the locomotion
of the animal as a whole, are produced by localized contractions
of the peripheral layer of protoplasm which drive onwards the
more fluid central parts.

(2) Irritability (including Co-ordination), or the power to
be affected by, and to respond to, changes or " stimuli" acting
upon or within the protoplasm. The change of shape following
the application of an electric shock is actually effected by con-
tractility, but the power to be affected by the shock and to arouse
contractility, is irritability. To this property the animal owes its
power of performing adaptive actions in response to changes in
the environment, and also its power to co-ordinate the various
actions of its own body. To illustrate : It is a remarkable fact
that Amoeba is able to discriminate between nutritious and innu-
tritions matters, ingulfing the former, but rejecting the latter.
Physiologically this discrimination is a difference of response to
different stimuli — hence a phenomenon of irritability. Again,
the various actions (movements, etc.) of Amoeba, despite their
apparently vague character, are co-ordinated to form a definite
whole ; and co-ordination may be regarded as a phenomenon of
irritability, changes in one part serving as stimuli to other parts
and being brought into orderly relation with them. The property
of irritability lies at the base of all nervous activity in higher
forms (cf. p. 67) and is concerned in many other actions.

(3) Metabolism, the most fundamental of all vital actions,
since it lies at the root of all, is the power of waste and repair —
the destructive chemical changes in protoplasm (Jfatabolism)
whereby energy is set free, and the constructive actions (anabo-
lism) through which new protoplasm is built and potential
energy is stored (cf. p. 33). There is every reason to believe

PHYSIOLOGICAL PROPERTIES OF AMCEBA 165

that the metabolic phenomena of Aviceba are, broadly speaking,
similar to those of higher animals. The katabolic changes are in
the long run processes of oxidation, and although their products
have not yet been definitely ascertained in Amoeba, there can be
no doubt that they consist mainly of carbon dioxide, water, and
some form of nitrogenous matter (urea or a related substance).
Most of these waste matters are believed to be passed out (se-
cretion, excretion) by means of the contractile vacuole, but prob-
ably carbon dioxide leaves the body by diffusion through the
general surface (respiration in part).

The materials for the constructive process (anabolism) are
derived from organic food-matters — bodies or fragments of plants
and animals taken as food in the process of alimentation, and
absorption from the water and the inorganic salts dissolved in
it, and from the free oxygen that enters by diffusion through
the general surface (respiration in part). Proteid matter is an
indispensable constituent of the food, and Amceba is therefore
an animal.

Alimentation, absorption, secretion, digestion, and circula-
tion, all of which are only the prelude to metabolism, but which
in the higher animals are assigned to different organs, tissues,
and cells, are here performed by one and the same cell. The
capture of solid food here requires its entrance into the cell ;
and the fact that proteids cannot be absorbed by diffusion neces-
sitates intracellular digestion which in turn necessitates cellular
defecation. It will be observed that while there is no localized
or permanent mouth or anus, the whole surface of the cell is
potentially mouth or anus. In short, the protoplasm here ex-
hibits not the physiological division of labor, but its absence.

(4) Growth and Reproduction. Logically there is in the
case of Amoeba no good ground for a distinction between these
processes and metabolism ; for reproduction is directly or indi-
rectly an effect of growth, and growth is simply an excess of
anabolism over katabolism. Practically, however, the distinc-
tion is necessary ; for the tendency of living things to run in
cycles of growth and reproduction is one of their most obvious
and characteristic features.

Here, as in all protoplasmic structures, growth takes place
throughout the mass, by intussusception (p. 4), not by the ad-

166 UNICELLULAR ANIMALS.

ditions of superficial layers, as in the case with growth by accre-
tion (inorganic bodies, e.g., crystals). Under favorable condi-
tion of nutrition this process exceeds the destructive process so
that the body increases in size up to a limit, at which fission
takes place. What determines this limit is unknown, but the
cause is perhaps in some way connected with the geometrical
principle that the volume of the cell increases as the cube of its
diameter, whereas the surface, by which it absorbs nutriment,
and otherwise comes into relation with the outside world, in-
creases only as the square of the diameter. No great increase
in size, therefore, is possible without destroying the normal equi-
librium of the cell and hence the periodic reduction of size by
division. This principle is, however, too general to be of much
value. Different species of Amaiba differ in size-limit, and the
immediate cause lies in some subtle relation between organism
and environment that cannot at present be made out. It is not
known whether or not the Amoeba ever dies of old age.

These "fundamental physiological properties" of proto-
plasm lie at the basis of all physiology, and will be found ap-
plicable to all forms of life whether vegetal or animal.

Related Forms. Amoeba is a representative of a very extensive class of
Protozoa known as I-ihizo-poda, all characterized by the power to form
pseudopodia, and agreeing with Amtfba in many other respects. One of
the commonest fresh-water forms is the genus Arcella (Fig. 86, C), which
even in the active phase is surrounded by a brown horny membrane
("shell") perforated by a large rounded opening through which pseudo-
podia are protruded. Difflugia (Fig. 86, J5), also a common fresh -water
form, builds about itself a beautiful vase-shaped or retort-shaped shell
composed of sand -grains, or even, in some cases, of diatom-shells. In
Actinophrys, or the "sun-animalcule" (Fig. 86, A), the pseudopodia are
stiff needle-shaped processes radiating in every direction.

Among the marine forms two groups (orders) are of especial interest
and importance ; viz., the Foraminifera, which secrete a calcareous shell
perforated by numerous pores, and the Radiolaria, which have a siliceous
shell. Many of these forms float at the surface of the water, and their
cast-off shells have in former times accumulated at the bottom in such
•enormous quantities as to form beds of chalk in the case of Foraminifera,
while the remains of Radiolaria have made important contributions to the
formation of siliceous rocks.

FRESH-WATER RHIZOPOD8.

167

FIG. 86.— Group of common fresh-water Rhizopods (after Leidy) . A, Actinophrys
sol, the "sun-animalcule," filled with vacuoles and containing three food-bodies
(zoospores of an alga) ; a fourth is just being ingulfed. The nucleus is not seen.

B, Difltugia urceolata, with shell built of sand-grains and pseudopodia far ex-
tended.

C, Arcclla mitrata, a transparent individual showing the protoplasmic body sus-
pended within the shell ; several vacuoles are shown, but no nucleus.

(Highly magnified.)

CHAPTER XIII.
UNICELLULAR ANIMALS (PROTOZOA) (Continued).

B Infusoria.

(Paramaecium, Vorticella, etc.)

INFUSORIA are minute unicellular animals found like Amoeba
in stagnant water or in organic infusions (see p. 201) (hence
"Infusoria"). In the leading features of their organization
they are closely similar to Amoeba and its allies, from which
they differ, however, in having a much higher degree of differ-
entiation, in moving by means of cilia instead of pseudopodia>
and in showing the first indication of gamogenesis (amphimixis).
Paramcecium (the slipper-animalcule) is an actively free-
swimming form often found in multitudes in hay-infusion or
water containing the decomposing remains of Nitella and other
water-plants. Vorticella (the " bell -animalcule ") is commonly
attached by a slender stalk to duck-weed (Lemna) and other
water-plants, or to other submerged objects; at other times it
breaks loose from the stalk and swims for a while actively about.
The two forms are constructed upon essentially the same plan,
but Vorticella shows in some respects a much higher degree of
differentiation.

Paramoecium. — The slipper-shaped body (Fig. 87) is covered
with cilia by means of which the animal rapidly swims about.
Morphologically the bod^y is a single cell, having the same gen-
eral composition as in Anweba, but possessing in addition a deli-
cate surrounding membrane ("cuticle") or cell- wall. The
differentiation of the protoplasm into ectoplasm and entoplasm
is very sharply marked, and the former contains numerous
peculiar rod-like bodies (trichocysts) from which long threads may
be thrown out. Their function is probably that of offence and
protection. As in Amoeba the protoplasm contains water-vacu-
oles (w.v) and food-vacuoles (f.v) (both of which are carried

THE SLIPPER-ANIMALCULE.

169

FIG. 87.— Paramcetium caudatum. A, from the left side, showing the anal spot; B,
from the ventral side, showing the vestibule en face; arrows inside the body in-
dicate the direction of protoplasmic currents, those outside the direction of
water-currents caused by the cilia.

an, anal spot; c.i\ contractile vacuoles; /r, food-vacuoles ; uu\ water vacuoles; m.
mouth; moo, macronucleus ; mic, micronucleus ; ce, oesophagus ; v, vestibule. Tne
anterior end is directed upwards.

170 UNICELLULAR ANIMALS.

about by currents in the entoplasm), and two very large contrac-
tile vacuoles (c.v) occupying a constant position, one near either
end of the body. The nucleus (as in Infusoria generally) is
differentiated into two distinct parts, viz. , a large oval macro-
nucleus (mac.) and a much smaller spherical micronucleus (mic.)
(double in some species) lying close beside it.

Unlike Amceba, Paramcecium possesses a distinct mouth (m)
and (esophagus (a?) which open to the exterior through an oblique
funnel-shaped depression known as the vestibule (v) situated at
one side of the body. Minute floating food-particles are drawn
by the cilia into the mouth and accumulate in a ciliary vortex at
the bottom of the oesophagus. From time to time a bolus or
food -mass is thence passed bodily into the substance of the en-
toplasm, forming a food-vacuole within which digestion takes
place. The indigestible remnants are finally passed out not
through a permanent opening or anus, but by breaking through
the protoplasm at a definite point, hence known as the anal
spot, which is situated near the hinder end (Fig. 87). The
contractile vacuoles of Paramcecium are especially favorable for
study, showing at the moment of contraction, or just before it,
a pronounced star-shape, with long canals running out into the
protoplasm. Through these liquid is supposed to flow into the
vacuole.

Like Amceba, Paramo3cium occurs both in an active and in
an encysted state. In the former state it multiplies by trans-
verse fission, division of both macronucleus and micronucleus
preceding or accompanying that of the protoplasmic body (Fig.
88, A). Under favorable conditions division may take place once
in twenty-four hours, or even oftener. This process, which is a
typical case of agamogenesis, may.be repeated again and again
throughout a long period. But it appears from the celebrated
researches of Maupas that even under the most favorable con-
ditions of food and temperature the process has a limit (in the
case of Stylonichia, a form related to Paramcecium, this limit
is reached after about 300 successive fissions). As this limit is
approached the animals become dwarfed, show various signs of
degeneracy, and finally become incapable of taking food. The
race grows old and dies.

In nature, however, this limit is probably seldom if ever

CONJUGATION OF PAUAMCECIUM. 171

readied, and the degenerative tendency seems to be checked by
a process known as conjugation. In this process two individuals
place themselves side by side, partially fuse together, and remain
thus united for several hours (Figs. 88, B, C}. During this
Union an exchange of nuclear material is effected, after which
the animals separate, both macronucleus and micronucleus now

Fio. 88.— A. Fission of Paramcecium. (From a preparation by G. N. Calkins), mac,

macronucleus ; mfc, micronucleus ; rn, mouth.

S. First stage of conjugation. The animals are applied by their ventral sur-
faces; the only change thus far is the enlargement of the micronuclei.
C. Conjugation at the moment of exchange of the micronuclei (less magnified).
The macronuclei are degenerating. Each individual contains two micronuclei
(now spindle-shaped), one of which remains in the body, while the other crosses
over to fuse with the fixed micronucleus of the other individual (After Maupas.)

consisting of mixed material derived equally from both individ-
uals. Separation of the two animals is quickly followed by
fission in each.

In each individual the macronucleus breaks up and disappears. The
micronucleus of each divides twice, and of the four bodies thus produced
three disappear. The fourth divides again into two, one of which remains
in the body, while the other crosses over and fuses with one of the micro
nuclei of the other individual, after which the animals separate. This
process being reciprocal, each individual now contains a micronucleus con-

172

UNICELLULAR ANIMALS.

taining an equal amount of material from each individual. This micro-
nucleus now divides twice and gives rise to four bodies, two of which be-
come macronuclei and two micronuclei. Fission next occurs, and is there-
after continued in the usual manner.

This is a process clearly analogous to the union of the gi-rm-
cells of higher animals. It cannot, however, be called gamo-
genesis or even reproduction ; it is only comparable with one of
the elements of gamogenesis. In the metazoon a fusion of two

.11

FIG. 89.— Group of Vorticettce, in various attitudes, attached to the surface of a
water-plant.

cells (fertilization) is followed by a long series of cell-divisions
(cleavage of the ovum), the resulting cells being associated to
form one new individual. In the Infusoria temporary fusion
(conjugation) is likewise followed by a series of cell-divisions,
but the cells become entirely separate, eacli being an individual.
Vorticella agrees with Paramaecium in general structure, but
differs in many interesting details, most of which are the expres-

THE BELL-ANIMALCULE.

173

mac

Fio. 90.— A single head of Vortlcella, highly magnified, ex, contractile axis of the
stalk; c, cuticle; c.u, contractile vacuole; d, disk; et, ectoplasm; en, entoplasm;
ep, epistome; f.v, food-vac uole ; m, mouth; mac, macronucleus ; mic, micronu-
cleus ; ce, oesophagus ; p, peristome ; r, vestibule ; w.i\ water- vacuoles ; a;, point at
which epistome and peristome meet at one end of the vestibule.

174 UNICELLULAR ANIMALS.

sion of higher differentiation. The body is pear-shaped or coni-
cal, attached at its apex by a long slender stalk. The latter
consists of a slender contractile axial filament, by means of
which the stalk may be thrown into a spiral and the body drawn
down, and an elastic sheath (continuous with the general cuticle)
by which the stalk is straightened (Fig. 90). The cilia are con-
fined to a thickened rirn, the peristome (p), surrounding the
base of the cone, which may be termed the disk. At one side
the disk is raised, forming a projecting angle covered with cilia,
and known as the epistorne (ep). At the same side the peristome
dips downwards, leaving a space between it and the epistmnr.
This space is the vestibule (-y), and into it the mouth opens. In
it likewise is situated an anal spot like that of Paramcecium.
The cilia produce a powerful vortex centering in the mouth, by
means of which., food is secured. The macronucleus (i/atr) is
long, slender, and horseshoe-shaped ; the small spherical micro-
nucleus (mic) lies near its middle portion. There is usually
but one contractile vacuole.

Vorticella multiplies by fission, division of the protoplasm
being accompanied by that of the macronucleus and micronu-
cleus (Fig. 91). The plane of fission is vertical (thus dividing
the peristome into halves), but extends only through the main
body, leaving the stalk undivided. At the close of the process,
therefore, the stalk bears two heads. One of these remains
attached to the original stalk, while the other folds in its peri-
stome, acquires a second belt of cilia around its middle (Fig. 91),
breaks loose from the stem, and swims actively about as the so-
called " motile form." Ultimately it attaches itself by the base,
loses its second belt of cilia, develops a stalk, and assumes the
ordinary form. By this process dispersal of the species is en-
sured. Under unfavorable conditions similar motile forms are
often produced without previous fission, the head simply acquir-
ing a second belt of cilia, dropping off, and swimming away to
seek more favorable surroundings. Vorticella may become en-
cysted, losing its peristome and mouth, becoming rounded in
form, acquiring a thick membrane, and having no stalk. In
this state it is said sometimes to multiply by endogenous division,
breaking up into a considerable number of minute rounded
bodies (spores) each of which contains a fragment of the

CONJUGATION OF VORTICELLA. 175

; nucleus. These are finally liberated by the bursting of the
membrane, acquire a ciliated belt, and after swimming for a
-time become attached, lose the ciliated belt, and develop^ stalk
and peristome.

Vorticella goes through a process of conjugation which has
some interesting peculiarities. (1) Conjugation always takes
place between a large attached individual (the macrogamete) and
a much smaller free-swimming individual (the microgamete}

FIG. 91.— Fission and conjugation of Vorttetlla. A. Early stage of fission, showing-
division of micronucleus (mic) and macronucleus (mac) ; p, peristome. (After
Blitschli. )

.B, C, D. Successive stages of fission ; in B and C the nuclei have completely di-
vided and fission of the cell-body is in progress; r.r, contractile vacuoles. In
D fission is complete; the right-hand individual has acquired a belt of loco-
motor cilia at x, and is ready to swim away.

E. Conjugation of a fixed macrogamete (ma) with a free-swimming microgamete
(ml) ; p, peristome, ep, epistome. (After Green5.)

(Fig. 91 , E]. The microgamete is formed either by the unequal
fission of an ordinary individual, the smaller moiety being set
free, or by two or more rapidly succeeding fissions of an ordinary
individual. (2) Conjugation is permanent and complete, the
body of the microgamete being wholly absorbed into that of the

176 UNICELLULAR ANIMALS.

macrogamete. Within the body of the latter, after complicated
changes, the nuclei fuse together, and this is followed by fission.
The analogy of conjugation to the fertilization of the egg is here
complete. The conjugating cells show a sexual differentiation,
one being like the ovum, large and fixed, the other like the
spermatozoon, small and motile,

As in Paramcecium the raacronuclei entirely disappear, fusion takes
place between derivatives of the micronuclei, and from the resulting body
both macronuclei and micronuclei are derived.

Euglena and Other Simpler Infusoria. Besides forms like
Paramo3cium and VortioeUa which bear numerous cilia, there
are many Infusoria which possess only one large lash mflagellum.
Of these Euylena, which is sometimes found in stagnant water,
sewage-polluted pools, etc. , is one of the most interesting, inas-
much as it contains chlorophyll, possesses an ' ' eye-spot ' ' of red
pigment, and under certain conditions exhibits amcebiform
movements.

Compound or "Colonial" Forms. In a number of forms,
closely related to VorticeUa, the individuals (" zooids") formed
by fission do not immediately separate, but remain for a time
united to form a "colony" which may contain hundreds of
zooids. Zoothamnion, a common species, thus forms a beautiful
tree-like organism, consisting of a single central stalk with nu-
merous branching offshoots from its summit, each twig terminat-
ing in a zooid. The entire system of branches is traversed by a
continuous contractile axis. Carchesium is similar, but the axis
is interrupted at the beginning of each branch. In Epistylis
the entire axis is non-contractile.

Such colonial forms are of high interest as indicating the
manner in which true multicellular forms may have arisen.
From the latter, however, they differ not only in the fact that
the association of the cells is not permanent, but in the absence
of any division of labor among the units.

Physiology. Most Infusoria are true animals, agreeing with
Amoeba in the essential features of their nutrition, and having
the power to digest not only proteids, but also carbohydrates and
fats. Paramcecium and Vorticella are herbivorous forms,
feeding upon minute plants, and especially upon the bacteria-

CHLOROPHYLL- CONTAINING INFUSORIA. 177

Other forms are omnivorous (e.g., Stentor, Rursarid), feeding
both on vegetable and on animal food. Others still are car-
nivorous and lead a predatory life, often attacking herbivorous
forms much larger than themselves, precisely as is the case with
carnivores among the mammalia. Thus the unicellular world
reproduces in miniature the essential biological relations of
higher types.

It is a remarkable fact that some species of Infusoria (e.g.,
Paramwcium bursar ia, Vorticella viridis) contain numerous
chlorophyll -bodies embedded in the entoplasm. Much discus-
sion has arisen as to whether these bodies are to be regarded as
an integral part of the animal, i.e., differentiated out of its own
protoplasm, or as minute plants living " symbiotically " (i:e. as
mess-mates) within the animal. In the former case (which is
the most probable) the animal would to a certain extent be
nourished after the fashion of a green plant (cf. p. 148).

It will now be clear to any one who has carefully considered
the phenomena described in the foregoing pages that the uni-
cellular animals are "organisms" by right, and not merely by
courtesy. In some of the Infusoria, for example, differentia-
tion within the single cell may go so far as to give rise to primi-
tive sense-organs (as in the case of the eye-spot of Euglend) ; a
rudimentary oesophagus and definite mouth (as in Paramcecium
and Vorticella) ; organs of locomotion (cilia, flagella] ; organs
of excretion (contractile vacuoles) etc. , etc.

CHAPTER XIV.
UNICELLULAR PLANTS.

A. Protococcus.

(Protococcus, Pleurococcus, Chlorococcus, Hcematococcus, etc.)

UNICELLULAR plants, like unicellular animals, are very com-
mon, although as individuals mostly invisible on account of their
microscopic size. In the mass, however, they are often visible
either as suspended or floating matter, causing "turbidity" in
liquids (yeast, bacteria, diatoms, desmids, etc.) or discolorations
on tree-trunks, earth, stones, roofs, and flower-pots. (Pro-
tococcus, Glwocapsa, etc.).

Under the term Protococcus (rrpoTOS, first, KOKKO;, bt-rrt/)
we may for our present purposes include a number of the simplest
spherical forms, generally green in color and of uncertain affin-
ities in classification, but very similar in structure, living for the
most part in quiet waters or on moist earth, stones, tree-trunks,
or old roofs, or in water-butts, roof-gutters, and the like.
Sometimes the color which they exhibit is yellowish-greea
sometimes bluish-green, and sometimes, though less often, reddish,
according to the species.

One of the commonest and most conspicuous is a species
often seen on the shady side of old tree-trunks where, when
abundant, it forms a greenish dust-like coating or discoloration,
scarcely visible when dry but becoming a rich bright green dur-
ing prolonged rains or after warm showers. If pieces of bark
covered with this form of Protococcus are moistened, the green-
ish coating may be observed at any time. It is granular in tex-
ture and after moistening is easily loosened by a camel' s-hair
brush.

Morphology. Microscopical examination shows that the
tides detached consist of rounded yellowish-green cells occurrii
either singly or in groups of two, three, four, or even more.

178

PROTOCOCCUS. 179

Each single cell is a complete individual, capable of carrying on
an independent life. It fairly represents the green plant (such
as Pteris) reduced to its lowest terms. (Fig. 92.)

Like Amoeba and the Infusoria Protococcus, at least in some
species, occurs both in a motile or active state in which it moves
about, and a quiescent or non-motile state analogous to the en-
cysted state of the unicellular, animals. In the latter the motile
or active state is the usual or dominant condition and the en-
cysted state is rarely assumed. In Protococcus, on the other
hand, the motile state is rare, and the ordinary activities of the
plant are carried on in the non-motile state.

Structure. In structure Protococcus is a nearly typical cell
(p. 22). It consists essentially of an approximately spherical
mass of protoplasm enclosed within a thin woody layer of cellu-
lose (cell-wall or cell-membrane), and contains a single nucleus.
It also includes one or more chlorophyll-bodies (ckromatophores)
(p. 126) by virtue of which it is able to manufacture its own
foods, very much after the fashion of the green cells of Pteris.

In those forms which possess a motile stage the latter con-
sists of a spherical, egg-shaped or pear-shaped cell having chro-
matophores and a membrane through which two flagella protrude.
In the oval forms these are placed near the narrowed end of the
cell, and in all cases they are locomotor organs and propel the
cell swiftly through the water. (Fig. 92).

Reproduction. The ordinary method of reproduction in the
unicellular plants, as in the unicellular animals, is by cell-division.
In Protococcus the sphere becomes divided by a partition into
two cells which eventually separate completely one from the
other. Very often, however, the separation being incomplete
or postponed until after each daughter-cell has in turn become
divided, groups or aggregates of cells arise which suggest the
first steps in the formation of tissue in the development of higher
forms. In the end, however, separation is total and complete,
and each cell is therefore not a unit in a body, but is itself a
body and an individual (see p. 156). (Fig. 92.)

The daughter-cells thus produced are the young, or offspring,
which have the power to grow and ultimately to divide in their
turn. Under favorable circumstances generation may thus fol-
low generation in quick succession. Each young cell is actually

180

UNICELLULAR PLANTS.

FIG. 92.— Protococcus (Pleurococcus) from the bark of an elm tree, in active vegeta-
tion and showing aggregation into masses of cells. A, Pleurococcun in the dried
condition. B, An&wtccw (?), showing endogenous division into two cells and (C)
into four. D, E, F, motile forms of Protocorciw (after Cohn).

NUTRITION OF PROTOCOCCUS. 181

one half of the parent cell and contains a moiety of whatever
that contained. Here, therefore, as in Amoeba, the problems
of heredity, uncomplicated by the occurrence of sex, are reduced
to their lowest terms.

In some kinds of Protococcus the quiescent cells, under
special circumstances, which are not well understood, give rise
to the motile forms (zoospores) referred to above. Cilia, or
rather flagella, are formed, and the protoplasmic mass with its
included chromatophores swims actively about in the water.
After a time these motile cells may come to rest, lose their fla-
gella and divide into two or more daughter-cells, each of which
in its turn may become a motile cell and repeat the process, or,
under other conditions, develop into the ordinary quiescent cell.

In some species of Protococcus in which there is a motile
stage another form of reproduction, a kind of rudimentary
gamogenesis, has been observed. In this process two of the
motile cells (gametes) meet, fuse (conjugation], lose their flagella,
become encysted (see p. 161), and ultimately give rise to the
ordinary cells of Protococcus, both non-motile and motile.
This process, however, has not yet been observed in the species
under consideration.

Physiology. Our actual knowledge of the physiology of
Protococcus is very small. But the study of comparative plant
physiology gives every reason to believe that the essential phys-
iological operations of this simple plant are fundamentally of
the same character as in the higher green plants, such as Pteris.

Nutrition. The income of Protococcus, when growing in
its natural habitat on tree-branches, moist bricks, and the like,
is difficult to determine. But as it is able to live also in ordi-
nary rain-water, we are able to set down its probable income
under those conditions with some degree of accuracy. There
is do doubt that it absorbs water and carbon dioxide by dif-
fusion through the cellulose wall, and that these substances
are used in the manufacture of starch, which, if stored up,
makes its appearance in the form of small granules within the
chromatophores. This process takes place only in the light and
through the agency of the chlorophyll, and is attended by a
setting free of oxygen precisely as in Pteris. Nitrogen is prob-
ably derived from nitrates or ammoniacal compounds, minute

182

UNICELLULAR PLANTS.

quantities of which are dissolved in the water, and other neces-
sary salts (sulphates, chlorides, phosphates, etc.) as well as free
oxygen are procured from the same source. These substances
may be derived from dust blown or washed by the rain into the
water, or from the walls of the vessel. To the process of starch-
making, attended by the absorption of CO, and H,O and the
liberation of O, the term ' ' assimilation ' ' is generally given.
Like other plants, moreover, Protococmis probably breathes
by absorbing free oxygen and setting free CO, (respiration).

The income and outgo of Protococcus may then be displayed
by the following diagram :

II20

FreeO

lncojne

Outgo

It should be understood that this only represents the broad
outlines of the process and under the simplest conditions. It is
quite possible that under other conditions Protococcus may use
more complex foods. The facts remain, however, (1) that
Protococcus is dependent on the energy of light; (2) that its
action is on the whole constructive, resulting in the formation of
complex compounds (carbohydrates, proteids) out of simpler
ones. In these respects it shows a complete contrast to Am&ba,
which is on the whole destructive, breaking down complex com-
pounds into simpler ones, and is independent of light, since it
derives energy from the potential energy of its food. The
relations between Protococcus and Amoeba are therefore an
epitome of the relations between Pteris and Lumbricus, and
between green plants and animals generally.

The Fundamental Physiological Properties of Plants. In con-
sidering the physiology of Amoeba we found it possible to re-

PROTOCOCCUS AND AMCEBA COMPARED. 183

duce its vital activities to a few fundamental physiological proper-
ties, namely, contractility, irritability, metabolism, growth and
reproduction, common to all animals. A little reflection will,
show that the same properties are manifested also by Proto-
coccus. Contraction and irritability are difficult to witness in
the quiescent stage of Protococcus, but obvious enough in the
rarer motile forms. Metabolism, growth and reproduction, on
the other hand, are evident accompaniments of normal life, even
in the quiescent condition. And precisely as Protococcus differs
from Amoeba in respect to contractility and irritability, of which
it possesses relatively little, so plants in general differ in these
respects from animals in general. Animals are eminently con-
tractile and irritable, while plants are but feebly specialized in
these directions. On the other hand, as we have already seen
in comparing Pteris with Lumbricus (p. 154), and as we see
once more in comparing Protococcus with Amoeba, in respect to
metabolism, the green plant is pre-eminently constructive, while
the animal is preeminently destructive, of organic matter.

In their modes of nutrition, as stated above, Amoeba
and Protococcus represent two physiological extremes. We
pass now to the study of Yeasts and Bacteria, which are plants
destitute of chlorophyll and in a certain sense may be regarded
as occupying a middle ground between these extremes.

Other Forms. There are innumerable species of unicellular green
plants. A vast group of peculiar brownish forms covered with transparent
glass-like cells composed of siliceous material is known as the Diato-
maceoe or diatoms. In these the chlorophyll is masked by a brown pig-
ment, but is nevertheless present. Another group is that known as the
Desmidice or desmids. These often have the individual cells peculiarly
constricted in the middle so that at first sight the two halves appear to be
two separate cells. More closely resembling Protococcus in many respects
are some members of the Cyanophycece or "blue-green algae," among
which Chroococcus and Glceocapsa differ from Protococcus chiefly, in the
former case, in having a blue-green instead of a yellow-green pigment,
and, in the latter, not only in this respect, but also in the fact that the
single cells are widely separated by transparent mucilage.

CHAPTER XV.
UNICELLULAR PLANTS (Continued).

B. Yeast.
(Saccharomyces.)

UNDER the general name of yeast are included some of the
simplest forms of vegetal life. Some yeasts are "wild," liv-
ing upon fermenting fruits or in fruit juices, and commonly

FIG. 93.— Yeast-cells. Brewer's (top) yeast actively vegetating. The large internal
vacuolcs and the small fat-drops are shown, as are also buds, in various stages of
development, and the cell-wall. Nuclei not visible. (Highly magnified.)

occurring in the air; others are "domesticated," or cultivated,
such as those regularly employed in brew-ing and in baking.

If a bit of "yeast-cake " (either "compressed "or " dried"
yeast) is mixed with water, a milky fluid is obtained which
closely resembles the so-called baker's or brewer's yeast.

184

STRUCTURE OF YEAST. 185

Microscopical examination proves that the milky appearance
of liquid yeasts is due chiefly to the presence of myriads of
minute egg-shaped suspended bodies, and that pressed yeast is
almost wholly a mass of similar forms. These are the cells of
yeast ; which is therefore essentially a mass of unicellular organ-
isms. For reasons which will soon appear yeast is universally

FiO. 94.— Yeast-cells. Brewer*s (bottom) yeast showing structure— protoplasm, cell-
walls, vacuoles, fat-drops. (Nuclei not shown.)

regarded as a plant, and the single cell is often spoken of as the
yeast -plant.

Morphology. The particular yeasts which we shall consider are
the common cultivated forms of com-
merce. The cells of an ordinary cake
of pressed yeast are spherical, sphe-
roidal, or egg-shaped in form, and con-
sist of a mass of protoplasm enclosed
within a well-defined cell-wall. By
appropriate treatment the latter may
be shown to consist of cellulose ; and
it is distinctly thicker in old or resting F™spores) Four spores inacell

Cells than in young Ones Or those Vlg- of brewer's yeast (Saccliaromyces

orously growing. Within the granular cerwfete)-
protoplasm (cytoplasni) are usually a number of vacuoles (con-
taining sap) and minute shining dots (probably fat-droplets), but

-Spor

Yeast (As-

186

UNICELLULAR PLANTS.

no chlorophyll is present and no starch. Until recently the yeast-
cell was supposed to be destitute of a nucleus, but it isnow known
that each cell probably possesses a large and characteristic nucleus.
This, however, can be demonstrated only by special reagents and,
is rarely or never seen in the living cell (Fig. 96).

Reproduction. The ordinary mode of reproduction of yeast
is by a modification of cell-division called 'budding. Under

FIG. 96.-The Nuclei of Yeast-cells and the Process of Budding. (Drawn by J. EL
Emerton from specimens prepared by S. C. Keith, Jr.) The upper left-hand figure
shows the nucleus in a specimen treated with Delafl eld's haematoxylin. The
other figures in the upper row and those in the lower (from left to right) show
cells in successive stages of budding, together with the appearance, position, and
movements of the nucleus. It will be observed that the bud is formed before the
nucleus divides. (Iron-haematoxylin method.)

favorable circumstances in actively growing yeast a local bulging
of the wall takes place, usually near, but not precisely at, one
pole of the cell. Protoplasm presses into this dilatation or
" bud " and extends it still further. At this time we have still
but one cell, although it now consists of two unequal parts and.
the separation of a daughter-cell is clearly foreshadowed. Event-
ually the connection between the two parts is severed and the
daughter-cell or " bud " is detached from the original or parent-
cell ; but detachment may or may not occur until after the bud

FORMATION OF SPORES IN YEAST. 187

has begun to produce daughter-cells in its turn, and more than
one bud may be borne by either or both parent- or daughter-
cells. In very rapid growth the connection may persist between
the cells even during the formation of several generations of
buds ; but this is unusual, and in cases where a number of cells
remain apparently united together forming tree-like forms there
is often no real connection, the cells separating readily on agita-
tion.

Endospores (Ascospores). Some yeasts in addition to the
method of reproduction by budding exhibit another mode known

Fio. 97.— Spores of Yeast (Ascospores). Three- and two-celled stage of spore for-
mation in S. cerevisice.

as endogenous division or ascospore formation. Under certain
circumstances not yet entirely understood there are formed
within the yeast-cell two, three, or four rounded shining spores.
These become surrounded by thick walls and thus give rise
eventually to a group of daughter-cells within the original cellu-
lose sac. To the latter the term ascus (sac) has been applied,
and to its contained daughter-cells the term ascospores.

It is not yet allowed by all botanists that this terminology, which im-
plies a relationship of yeasts to the Ascomycetous fungi, is sound ; but it
is commonly used.

Each ascospore is capable under favorable circumstances of
sprouting and starting a new series of generations of ordinary
yeast-cells. It should be particularly observed that the endo-
spores of yeast are reproductive bodies, and that the process of
their formation is one of multiplication — not merely one of de-
fence or protection, as is the case with the so-called "spores "
of bacteria described beyond (p. 194).

188 UNICELLULAR PLANTS.

Physiology. Like all other organisms the yeast-plant occu-
pies a definite position in space and time; it possesses an en-
vironment with which it must be in harmony if it is to live,
from which it derives an income, and to which it contributes an
outgo of matter and energy ; it manufactures its own substance
from foods (anabolism), and like all living things it wastes by
oxidation of its substance (katabolism). It is not obviously con-
tractile or irritable, but it is highly metabolic and reproductive.

Yeast and its Environment. Yeast is an aquatic form, and,
as might be supposed, cultivated yeast thrives best in its usual
habitat, the juices of fruits, such as apples or grapes, and the
watery extracts of sprouted seeds, such as barley, corn, and
rye (wort, mash, etc.). It lives, however, more or less success-
fully in many other places (such as the dough of bread), and can
even endure much dryness, as is shown by the commercial
" dried - yeast. " It appears to prefer a temperature from
20° to 30° C. ; it is usually killed by boiling, but if dried, it can
endure high temperatures. Its action is inhibited by very low
temperatures, but like most living things it endures low temper-
atures better than high. It is killed by many poisons (anti-
septics).

Income. Owing to its industrial importance yeast has been
perhaps more thoroughly studied in respect to its nutrition than
any other unicellular organism. And yet it is impossible to
give accurate statistics of its normal income and outgo. It is
believed that the ordinary income of a yeast-cell living in wort
(the watery extract of sprouted barley -grains) consists of #, dis-
solved oxygen / i, nitrogenous bodies allied to proteids, but diffusi-
ble and ablo to pass through the cellulose wall ; £, carbohydrates,
especially sugary matters ; and d, salts of various kinds.

It was supposed for a long time by Pasteur and others that
yeast could dispense with free (dissolved) oxygen in its dietary.
It now appears that this faculty is temporary only, and that if
yeast is to thrive it must, like all other living things, be sup-
plied, at least occasionally, with free oxygen.

Metabolism. Out of the income of foods just described yeast
is able to build up its own peculiar protoplasm (anabolism}, and,
doubtless, to lay down the droplets of fat which often appear in
it. There is good reason to believe that its substance also breaks

NUTRITION OF TEAST. 189

down, with the production of carbon dioxide, water, and nitro-
genous waste (katdbolism\ and the concomitant liberation of
energy. The work to be done by the yeast-cell is plainly
limited. The manufacture of new and of surplus protoplasm
and the protrusion of buds require work, partly chemical,
partly mechanical; but most of the liberated energy probably
appears as heat. In point of fact, great activity of yeast is
accompanied by a rise of temperature, as may beproved by
placing a thermometer in " rising" dough or fermenting fruit-
juice.

Outgo. Barring the outgo of energy already mentioned, and
the probable excretion of carbon dioxide and nitrogenous waste,
but little can be said concerning the outgo of a yeast-cell. The
ordinary excretions are so masked by the presence of foreign
matters in the liquids which yeast inhabits that little is known of
the real course of events. To the consideration of conditions
which entail these difficulties we may now pass, merely pausing
to caution the student against the supposition that the evolution
of carbon dioxide in fermentations represents to any great ex-
tent the normal respiration of the yeast cells.

Mineral Nutrients of Yeast. It has been shown (pp. 148, 181)
that Pteris and Protococcus, inasmuch as they possess chlorophyll
can live upon simple inorganic matters such as CO2, HaO, and
nitrates, out of which they are able to manufacture for them-
selves energized foods such as starch. Yeast is unable to do
this, as might be supposed from the fact that it is destitute of
chlorophyll. And yet yeast does not require proteid ready-
made as all true animals do, for experiments have shown that it
can live and grow in a liquid containing only mineral matters
plus some such compound of nitrogen as ammonium tartrate
(C4H4(NH4),O.). Upon a much less complex organic compound
of nitrogen such as a nitrate it cannot thrive, thus showing its
inferiority in constructive power to Protococcus and all green
plants, on the one hand, and its superiority to Amoeba and all
animals, on the other.

Pasteur's fluid, composed of water and salts, among which is ammonium
tartrate (above), will suffice to support yeast. It will support a much more
vigorous growth if sugar be added to it. But if ammonium nitrate is sub-
stituted for ammonium tartrate yeast will refuse to grow in the fluid.

190 UNICELLULAR PLANTS.

Yeast is a Plant. The superior constructive faculty of yeast,
just described, separates it fundamentally from all animals in
respect to its physiology, and allies it closely to all plants. Its
inferiority to the chlorophyll-bearing plants or parts of plants, on
the other hand, in no wise separates it fundamentally from
plants ; for it must not be forgotten that the power, even of
plant-cells to utilize mineral matters as raw materials and from
them to manufacture foods like starch, ordinarily resides exclu-
sively in the chlorophyll bodies, and is operative only in the
presence of light. It follows, therefore, that most of the cells,
even of the so-called green plants, and a considerable portion of
the contents of the so-called green cells, must be destitute of
this synthetic power. Considerations of this kind show how
exceedingly localized and special the starch -making function is,
even in the "green" plants; and yeast probably compares very
favorably in its synthetic powers with many of the colorless cells
of such plants, or even with the colorless protoplasmic portions
of chromatophore-bearing cells.

But yeast is vegetal rather than animal, morphologically as
well as physiologically. Its structure more nearly resembles
that of some undoubted plants (fungi) than any animal. Its
wall is composed of a variety of cellulose, called fungus-cellulose ;
and cellulose, though occasionally occurring in animal structures,
is, broadly speaking, a vegetal compound. Finally, in its
methods of reproduction by budding, and by spores, yeast is
allied rather to plants than animals.

Top Yeast. Bottom Yeast. In the process of brewing two well-
marked varieties of yeast occur, known as "top" and " bottom" yeast.
The former is used in the making of English ale, stout, and porter ; the
latter in the making of German or " lager " beer. The top yeast is culti-
vated at the ordinary summer temperature of a room, without special at-
tention to temperature ; the latter in rooms artificially cooled so that even
in summer, icicles often hang from the walls. The two yeasts also show
obvious differences in form, size, and structure ; and how much they must
differ in their function is plain from the very different products to which
they give rise.

Wild Yeasts. Besides the commercial or cultivated yeasts there are
also wild yeasts, and to them are due in the main the fermentations of
apple-juice, of grape-juice, and other fruit juices. A drop of sweet cider
shows under the microscope a good example of one of these species ; and
Pasteur long ago proved that the outer skins of ripe grapes and other fruits

VARIETIES OF JEAST. 191

are apt to harbor yeast-cells in the dust which lodges upon them. More
recently it has been shown that wild yeasts often live under apple-trees
upon the surface of the earth. In a dry time the wind easily lifts the dust
containing them and conveys them- over great distances (cf. Amoeba,
Infusoria, etc.). The domesticated yeasts of to-day are probably the de-
scendants of similar wild yeasts.

Red Yeast. One of the finest of the wild yeasts is the so-called "red
yeast," which is furthermore very easy to study. Red yeast, and many
others not red, grow luxuriantly upon a jelly, made by thickening beer-
wort with common gelatine. In this way "pure" cultures— that is, cul-
tures free from other species of yeasts, or bacteria, and consisting of one
kind only — can be easily made and studied. The microscope shows that
the cells of red yeast, which form red dots upon such jelly, are not them-
selves colored, but the pigment appears to lie between the cells, as in the
case of the " miracle germ " (Bacillus prodigiosus).

Fermentation. To the processes where yeast is employed to
produce chemical changes in various domestic, agricultural, and
industrial operations the term fermentation, or more often
alcoholic fermentation, is applied. In the "raising" of bread
or cake, in brewing, cider-making, etc., yeast acting upon
sugar produces from it an abundance of alcohol and carbon
dioxide. Both products are sought for in brewing, and carbon
dioxide is especially desired in bread-making.

But alcoholic fermentation is only one example of a large
class, and yeast is only one of many ferments. We may, there-
fore, postpone further consideration of fermentation to the next
chapter.

Related Forms. It has been shown by the researches of Hansen that
ordinary commercial yeast is seldom one single species, as was formerly
supposed, but rather a mixture of several species. It is therefore no
longer safe to speak of commercial yeast as SaccJiaromyces cerecisice, unless
careful examination by the modern methods has shown it to be such ; and
to determine what species exist in any particular specimen is often a labori-
ous and difficult matter.

Inasmuch as the natural position of yeast in the vegetal kingdom is
not established beyond all doubt, it is impossible to state precisely what
are its near relatives. There are numerous unicellular colorless plants, but
they are not necessarily closely related to yeast ; and the student must not
conclude for plants any more than for animals that because an organism
unicellular it is necessarily at the very bottom of the scale of life.

CHAPTER XVI.

UNICELLULAR PLANTS (Contimted).

C. Bacteria.

(Schizomycetet.)

THE smallest, and the most numerous, of all living things are
the bacteria. Bacteria occur almost everywhere : they are lifted
into the atmosphere as dust particles, in it they float and with its
currents they are driven about; water — both fresh and salt —
often contains large numbers of them ; and the upper layers of
the soil teem with them. But they are most abundant in liquids
containing dissolved organic matters, especially such as have stood
for a time — for example, stale milk and sewage, these fluids
often containing millions of individual bacteria in a single cubic
centimetre.

In respect to their abundance in the surface layers of the
earth (one gram of fertile soil often containing a million or more),
and the work which they do there in producing the oxidation of
organic matters and changes in the composition of the soil, bac-
teria may well be compared with earthworms (cf . p. 42). They
are also of much general interest because some are what are
known as " disease-genns. " Most bacteria, however, are not
parasitic, but saprophytic, i.e., live upon dead organic matters,
and therefore are not merely harmless, but positively useful in
rendering back to the inorganic world useless organic matters.
Some species such as the vinegar bacteria are commercially
important.

In systematic botany bacteria constitute a well-defined group,
the Schizomycetes (fission-fungi}, their near allies being the
Cyanophycew or "blue-green algae."

Morphology. Under the microscope bacteria appear as
minute rods (Bacilli) (Fig. 98), balls (Cocci) (Fig. 100), or spii
(Spirilla) (Fig. 104), sometimes at rest, but often, at least in
the case of the rods and spirals, in active motion. Little or no

192

SHAPES OF BACTERIA.

193

structure can be made out in them by the beginner, to whom
they usually appear at first sight like pale, translucent or watery
bits of protoplasm. Investigation has shown, however, that they
possess a cell-wall (probably composed of cellulose) and a non-
homogeneous protoplasm. Unlike Protococeus, but like yeast-
cells, the cells of bacteria contain no chlorophyll. Nuclear mat-

FlG. 98. — Bacillus Megaterium.
Rods (unstained) in various
aggregations as commonly seen
with a high powor after their
cultivation in bouillon and
while rapidly growing and mul-
tiplying by transverse divi-
sion.

FIG. 99. — Bacilli from
Hay I ifusion (unstain-
ed). The filaments at
the left in a condition
of active vegetation.
The middle filament
forming spores. The
filament to the right
contains five spores
enclosed in otherwise
empty cells, the walls
of which bulge, proba-
bly from the absorp-
tion of water.

ter is present, either scattered about, or, if the views of Biitschli
be accepted, composing most of the protoplasmic body itself.
Many bacteria bear appendages in the shape of flagella or
cilia; but these can only be demonstrated in special cases, and
by special methods. They are believed to be locomotor organs,
and in some cases have been seen in active motion (Fig. 103).

194 UNICELLULAR PLANTS.

The minuteness of bacteria is extraordinary. Many bacilli are
not more than .005 mm. (yuW inch) in length or more than .001
mm. Gnriinr incn) m breadth. Some are very much smaller.

Most bacteria are at some tune free forms ; but like other
unicellular organisms many of them have the power to pass
from a free-swimming (swarming) into a quiescent (resting)
condition. In the latter some undergo a peculiar change, in
which the cell-wall becomes mucilaginous, and by the aggrega-
tion of numerous individuals or by repeated division lumps of
jelly-like consistency (zooyloea) arise. If the jelly mass takes
the shape of a sheet or membranous skin (as happens in the
mother-of-vinegar), it is sometimes described as Mycoderma
(fungus-skin) (Fig. 102).

Reproduction. The bacteria increase in numbers solely by
transverse division. Growth takes place and is followed by trans-
verse division of the original cell, usually into halves. Each half
then likewise grows and divides in its turn. In this way multi-
plication may go on in geometrical progression, and with almost
incredible rapidity. It has been stated that such repeated divi-
sions may follow only an hour apart, and on this basis it is easy
to compute the enormous numbers to which a single cell may
give rise in a single day.

If separation after division is complete, strictly unicellular
forms arise. If actual separation is postponed, long rods, chains,

or plates (in the case of cocci)
may appear. Different names
are given to the resulting forms.
Streptococcus is a moniliform
or necklace-like arrangement;
Staphylococcus, single cocci ;
DiploccoccuSy cocci in pairs;
Leptothrix, a filament of
bacilli ; Sarcina, a plate of
cocci resembling a card of bis-
cuit, or two or more cards

FIG. lOO.-Micrococci FIG. lOl.-Short
(unstained) from hay Bacilli (un- Superposed ; etc. , etc.

lnfU8ion' stained) from Spores. Some bacteria pro-

hay infusion. r

duce so-called spores (• »</<>-
spores) in the following way: The contents of the cell

00

SPORES OF BACTERIA.

195

FIG. 102.-The Mother-of-
Vinegar. The edge of a
film of zoogloea of mother-
of-vinsgar as it appears
under a high power. The
bacteria are seen imbedded
in the jelly /which they
have secreted.

withdraw from the wall and condense into a (usually oval)
mass at one end of the cell, leaving the rest of it empty
It is at this time that the cell- wall
is best seen. The condensed mass
now becomes dark and opaque, appa-
rently from the deposit upon itself of a
greatly thickened and peculiar wall ; it
refuses to absorb stains which the origi-
nal cell would have taken, and becomes
exceedingly resistant to extremes of
heat, cold, and dryness (Fig. 105). To
these spores the Germans give the
excellent term Dauersporen, i.e.,
enduring
spores,
often called

resting spores. When brought under
favorable conditions, these sprout
and, the ordinary bacterium cell
having been produced, growth and
fission proceed as before. Obviously
these spores are very different in
function from those of Pteris (p.
130), since they are protective
merely, and not reproductive. They
correspond, doubtless, to that phase
of animal life which is known as the
' ' encysted ' ' state. Another mode
of spore-formation in bacteria is that
known as the production of arthro-
spores, in which a long slender cell
may become constricted and detach
daughter-cells from one or both ends.
This is obviously a special case of
PIG. 108,-Ciiiated Bacteria. The unequal cell-division, but if it exists

bacillus of typhoid fever, showing t ]} , j j j j b doubted) it

cilia. (From a specimen prepared ™ ' .

by s. c. Keith, jr. Drawn by J. H. clearly approaches agamogenesis
Emerton') in such forms as Pteris.

Physiology. Income, Metabolism, and Outgo. The bacteria

196 UNICELLULAR PLANTS.

show a surprising diversity in the precise conditions of their
nutrition, and it is therefore difficult to make for them a
satisfactory general statement. As a group, however, and dis-
regarding for the moment certain important exceptions, they are
to be regarded as colorless plants living for the most part upon
complex organic compounds from which they derive their in-
come of matter and energy and which they decompose into
simpler compounds poorer in poten-
tial energy. In so doing they
bring about certain chemical
changes in the substances upon
which they act which are of the
highest theoretical interest, and
sometimes of great practical im-
portance. Perhaps the most pecul-
iar feature of the physiology of
bacteria is the fact that while they
are themselves individually invisi-
. ble, they collectively produce very

wl'iot-spwuum'unduu!' Spiral conspicuous and important changes
bacteria deeply stained. Drawn m their environment. For exam-

from the first photographic repre- . .

sentation of bacteria ever pub- pie, vinegar bactena act upon

viz that of Robert Koch, a^ho] (m cider, etc.) and by a

in Cohn a Beitrayc, 1876.) *

process of oxidation slowly convert
it into acetic acid and water, thus : —

Here it is not the bacteria that are most conspicuous, but the
effect which they produce. It is clear that the alcohol can be
only one factor in the nutriment of the organism, because it
contains no nitrogen, and the above reaction cannot represent
more than a phase in the nutrition of the bacterium. That this
is indeed the ca«e is proved by the fact that if the conditions be
somewhat changed the same bacteria may go further and convert
the acetic acid itself into carbonic acid and water : —

4O, + O4 = 2CO, + 2H,O.
Chemical changes of this kind in which the effect upon the en-

FERMENTS AND FERMENTATION. 197

vironment is more conspicuous than, and out of all proportion to
the change in the agent are in some cases known wo, fermen-
tations, and the agent effecting the change is described as a
ferment. Some ferments are organized or living, and some are

B C G

Fio. 105.— Bacillus megaterium (X 600). Spore formation and germination. A,
a pair of rods forming spores, about 2 o'clock P.M. B, the same about an hour
later. C, one hour later still. The spores in C were mature by evening ; the one
apparently begun in the third upper cell of A and B disappeared ; the cells in 0
which did not contain spores were dead by 9 P.M. D, a five-celled rod with three
ripe spores, placed in a nutrient solution, after drying for several days, at 12.30,
P.M. E, the same specimen about 1.30 P.M. F, the same about 4 P.M. G, a pair of
ordinary rods in active vegetation and motion. (After De Bary.)

unorganised or lifeless. Of the former the vinegar bacterium
and yeast are good examples. Of the latter the digestive fer-
ments, like pepsin, ptyalin, and trypsin, and certain vegetal
ferments, like diastase of malt are familiar instances.

As a rule the bacteria seem to prefer neutral or slightly
alkaline nitrogenous foods. They therefore decompose more
readily meats, milk, and substances (such as beef-tea) made of
animal matters ; less readily acid fruits, timber, etc. If in the
course of their activity they decompose meats, or fish, eggs, etc.,
with the production of evil-smelling gases or putrid odors, the
process is known as putrefaction. Rarely, bacteria invade the
animal (or plant) body and act upon the organic matters which
they find there. In such cases disease may result, and the
bacteria concerned are then known as disease germs.

But while bacteria appear to prefer highly organized nitrog-
enous (proteid) food, they are by no means dependent upon it.
Experiments have shown that many . species can thrive upon
Pasteur's fluid, a liquid containing only ammonium tartrate and
certain purely inorganic substances ; and one bacterium, at least
(the "nitrous"), according to Winogradsky, can thrive upon
ammonium carbonate. If this proves to be true for other spe-
cies, it will show that bacteria can not only obtain their nitrogen
from the inorganic world, but their carbon also. Enough has

198 UNICELLULAR PLANTS:

been said already to prove that the bacteria are plants, for only
plants can live upon inorganic food. But if the experiments
just referred to are correct, bacteria are not only plants, but, in
spite of their lack of chlorophyll, some at least appear t«» l>e
able, like green plants to manvfacture their own food out of
the raw materials of the inorganic world. The importance of
this fact in studies of the genealogy of organisms is very great,
for we are no longer obliged to suppose all chlorophylless plants
to be degenerate forms. They may have been the primitive
forms of life.

As was the case with yeast and Protococcus, it is extremely
difficult to make any precise statement concerning the income or
outgo of bacteria. It is believed, however, that the income
always includes salts and water, and the outgo CO,,H,O and
some nitrogenous compound or, possibly, free (dissolved) nitro-
gen. In more favorable cases the income appears to include
proteids, fats, and carbohydrates or their equivalents. Sugar is
freely used under some circumstances; and fats (when saponified)
and proteids peptonized, or otherwise altered, might readily be
absorbed. It is probable that soluble ferments are excretr«l by
the bacteria, which dissolve, and make absorbable, solid matters,
such as meat or white of egg; and if this is true, bacteria exhibit
a kind of external digestion. However this may be, it is certain
that bacteria can live and multiply upon an amount of food ma-
terials so small as almost or quite to elude chemical analysis : and
it is fair to say that they are among the most delicate of all
reagents.

It must not be inferred from what has been said above that bacteria are
always oxidizing agents. Broadly speaking and in the long run they are
such, and in this respect they resemble animals. Like the latter they are
unable (because of want of chlorophyll) to utilize solar energy, and there-
fore must obtain their energy by oxidizing their food. Yet under certain
circumstances bacteria act as. reducing agents, as, for example, when they
reduce nitrates to ammonia. This action only takes place, however, in
the presence of organic matter, and appears to be merely an incidental
effect, the oxygen of the nitrate being needed for the oxidation of carbon.
What at first sight appears to be an exception, therefore, proves in the end
to be a part of a general law that bacteria, like animals, are oxidizing
agents, are dependent for their energy upon the potential energy of their
foods, and are unable to utilize solar energy (p. 104).

METHODS OF STERILIZING. 199

It has recently been shown that many bacteria under circumstances
otherwise favorable are killed by exposure to sunlight.

* Related Forms. According to our present ideas of classification the

bacteria form a somewhat isolated group, their nearest relatives being the

slime-moulds (Myxomycetes) and especially the Myxobacteria of Thaxter, on

| the one hand, and the Cyanophycece the "blue-green" or "fission" algje

i on the other. Neither of these, however, need be considered here.

Why Bacteria are Considered to be Plants. The bacteria were

| formerly regarded as infusorial animalcules (because they abound

! in infusions, and many have the power of active movement).

j They are still regarded by some as animals. Most biologists,

j however, regard them as plants, because they can live without

proteid food (which no animal, so far as known, can do), and

because in their method of reproduction and in their growth -

forms they more nearly resemble the Cyanophycece than they do

any animal. There is also reason to think that their cell- wall is

composed of cellulose.

Bacteria and their Environment. The relations of organisms to tem-
1 perature and moisture have been more thoroughly studied for the bacteria
j than for any other unicellular organisms on account of their bearing upon
i modern theories of infectious disease. In general, temperatures above
) 70° C. are fatal to ordinary bacteria. In general, as is shown by common
i experience with the "keeping" of foods in cold storage, bacteria are be-
I numbed but not killed by moderate cold. But in special cases, particu-
larly when they are dried slowly, bacteria may withstand even prolonged
I boiling or freezing or the action of poisons, so that the removal or destruc-
I tion of the last traces of bacterial life is often very difficult.

Sterilization and Pasteurizing. The removal of all traces of living
matter from any substance, and in particular the destruction of all bac-
terial life, is known as sterilization. To free organic substances from the
larger forms of life is a comparatively easy matter; but bacteria are so
1 minute and so ubiquitous that scarcely anything is normally free from
them, and they are so hardy that it is exceedingly difficult to destroy them
without at the same time destroying the substances which it is desired to
sterilize. They are not normally present in the living tissues of plants or
animals which are sealed against their entrance by skins or epithelia ; but
after these are broken or cut open (as in wounds) bacteria speedily invade
the tissues. Ordinary earth, as has been said above, teems with bacteria,
which are easily dried and disseminated in dust driven by the wind. What-
ever is in contact, therefore, with the air or exposed to dust or dirt is never
free from bacteria, and meat or milk which in the living animal are nor-
mally sterile, if exposed to the air soon become contaminated with bacteria.
Sterilization (such as is required to preserve canned goods, for example)

200 UNICELLULAR PLANTS.

may be effected by heat and continued, after cooling, by exclusion of
germ-laden air. Disinfection, which is the destruction of bacterial life by
powerful poisons, is another form of sterilization. Still another is filtra-
tion through media impervious to germs, such as occurs in the well-
known clay, or porcelain, water-filters. In the last case the pores of the-
filter are large enough to allow the water very slowly to pass, but too small
for the bacteria.

In some cases, especially those in which disease-producing (pathogenic}
germs may be present and yet it is impossible to use poisons and undesira-
ble to use a high temperature, Pasteurization is resorted to. This con-
sists in heating to a temperature (usually 75° C.) high enough to destroy
the particular pathogenic germs supposed to be present, but not high
enough to alter the digestibility or other valuable properties of the liquid
in question.

For the medical, economic, and sanitary aspects of problems relating
to the bacteria, reference must be had to the numerous treatises upon
Bacteriology, perhaps the youngest, and certainly one of the most impor-
tant, of the biological sciences.

CHAPTER XYII.
A HAY INFUSION.

IF a wisp of hay is put into a beaker of water and the mix-
ture allowed to stand in a warm place there is soon formed what
is known as a hay infusion. Microscopical examination of a
drop of the liquid at the end of the first hour or two reveals
little or nothing, and if the beaker be held up to the light the
liquid appears clear and bright. But after some hours a marked
change is found to have taken place. The liquid, originally
clear, has become cloudy, and a drop of it examined microscop-
ically will be found to be swarming with bacteria. A day or
two later, the cloudiness meanwhile increasing, the microscope
generally reveals not only swarms of bacteria, but also numerous
infusoria. At the same time the color of the liquid has deep-
ened, it begins to appear turbid, a scum forms on the surface,
and the odor of hay, which was present at the outset, is replaced
by the less agreeable odors of putrefaction. The simple ex-
periment of bringing together hay and water has, in fact, set in
motion a complicated series of physical, chemical, and biological
phenomena.

The Composition of a Hay Infusion. A hay infusion consists
of two principal constituents, hay and water. But neither of
these is chemically pure. Hay is only dried grass which for
weeks, and even months, was exposed in the field to wind and
dust. Covered with the latter — often the pulverized mud of
roads and roadside pools — hay is richly laden with dried bacteria
and other micro-organisms; while water, such as is ordinarily
drawn from a tap, frequently contains not only an abundance of
free oxygen and various salts in solution, but also numerous bac-
teria, infusoria, algae, diatoms, and other micro-organisms in
suspension. In the making of a hay-infusion, therefore, numer-
ous factors co-operate, and a series of complicated reactions
follow one another in rapid succession. At the start both

201

202 A HA T INFUSION.

hay and water are in i state of comparative rest or equilib-
rium, but upon bringing them together action and reaction,
begin. First, the dust on the hay is wetted and soaked,
and any micro-organisms in it or adhering to the hay are set free,
and float in the water ; next, the water finds its way into the
stems and leaves of the hay, causing them to swell and resume
their original form. At the same time various soluble constitu-
ents of the daad grass, such as salts, sugars, and some nitrog-
enous substances, diffuse outward into the water, while from
such cells as have been crushed or broken open during drying
or handling, solid proteid or starchy substances may pass out and
mingle with the water. These simple physical reactions obvir
ously involve a disturbance of the chemical equilibrium of the
water. Originally able to support only a limited amount of life
(such as exists in drinking-waters), it is now a soil enriched
by what it has gained from the hay. The bacteria, extremely
sensitive to variations in their environment, and especially to
their food-supply, immediately proceed to multiply enormously,
so that a biological reaction follows closely on the heels of the
chemical change. But as a result of their metabolic activity the
bacteria set up extensive chemical changes, which in their turn
involve physical disturbances. For example, the dissolved oxy-.
gen with which the liquid was saturated soon disappears, so that
more oxygen must, therefore, diffuse into the liquid from the
atmosphere. Carbonic acid is generated in excess, and some
may pass outwards to the air. Also, as a result of the vital
activity of the micro-organisms the temperature of the infusion
may rise a fraction of a degree above that of the surround-
ing atmosphere.

We are concerned, however, chiefly with the biological
results. In consequence of the exhaustion of the oxygen supply
in the lower parts of the liquid, many of the bacteria which
require abundant oxygen for their growth (aerobes) find
their way to the surface, where some pass into a kind o£
resting stage (zooglcea) and form a scum or skin (mycoderm) on
the surface of the liquid. Others, for which free oxygen is not
necessary or to which it is even prejudicial (anaerobes), live and
thrive in the deeper parts of the beaker. But, meantime, an-

PHYSIOLOGICAL CYCLE IN THE INFUSION. 203

other phenomenon has occurred. The infusoria, originally few
in number, finding the conditions favorable, have multiplied
enormously, and after a day or two may be seen darting in and
out among the bacteria, especially near the surface, and feeding
upon them. Among the infusoria, however, are some which
feed upon their fellows, so that we soon have the herbivorous
infusoria pursued by carnivorous forms, the whole scene illus-
trating in one field of the microscope that struggle for existence
which is one of the fundamental facts of biology.

Obviously, this chain of life is no stronger than its weakest
part. The hay is the source of the food-supply for all these
forms, and this supply must eventually become exhausted.
When this happens, the bacteria cease to multiply, the herbivo-
rous infusoria which depend upon them perish or pass into a rest-
ing stage, the carnivorous infusoria likewise starve, and all the
biological phenomena must either come to an end or change
their character.

Up to this point the action is purely destructive. But sooner
or later microscopic green plants may appear on the scene, —
Protococcus, it may be, or its allies, — and a constructive action
begin, the waste products of the animals and of the bacteria be-
ing rebuilt by the green plants into complex organic matter. By
this time, also, the dissolved organic matter will have been
largely extracted from the liquid, the bacteria for the most
part devoured by the infusoria, and the latter may more or less
completely have given way to larger forms — to rhizopods, roti-
fers, small worms, and the like. The putrefying infusion has
run its course, and the ordinary balance of nature has been
restored.

Thenceforward an approximate equilibrium is maintained.
The green plants build complex organic matter and store up
the energy of light. The animals feed upon the plants, or
upon one another, break down the complex matter, and dissi-
pate energy. The ever-present bacteria break down all the
refuse, extract soluble organic matter from the water, decom-
pose the dead bodies of the animals or plants, and in the end,
it may be, themselves fall victims to devouring infusoria. The
physiological cycle is complete.

204 A HAT INFUSION.

A hay infusion thus affords in miniature a picture of the liv-
ing world. The green plants are constructive, and in the sun-
light build up matters rich in potential energy. These as foods
support colorless plants (such as bacteria) or animals. On these,
again, herbivorous and carnivorous animals feed ; and so, in the
world at large, as in the hay infusion, omnivorous as well as
carnivorous animals, in the long run, feed upon herbivorous
animals, and the latter upon plants — either colorless or green —
which thus stand as the bulwark between animals and starvation.

APPENDIX.

SUGGESTIONS FOE LABOKATORY STUDIES AND
DEMONSTRATIONS.

The " Laboratory Directions in General Biology," published
and copyrighted by Prof. E. A. Andrews of Johns Hopkins
University, will be found extremely useful and practical. Also
the following : Huxley and Martin's "Practical Biology " (Howes
and Scott), and the accompanying ' ' Atlas of Biology, ' ' by Howes ;
Marshall and Hurst's "Practical Zoology," Colton's "Practical
Zoology," Bumpus's "Invertebrate Zoology," Dodge's "Ele-
mentary Practical Biology," Brooks' s "Handbook of Inverte-
brate Zoology." According to our experience, the periods for
the course should be so arranged as to afford laboratory work
and recitations or quizzes in about the proportions of three to
two (for example, three periods of laboratory work and demon-
stration to two of quiz), for a half-year.

CHAPTER I. (INTRODUCTORY.)

It is convenient to give at the outset one or more practical
lessons on the microscope, affording the student an opportunity to
learn its different parts, use its adjustments, test the magnifying
power of the various combinations, etc. A good object for a
first examination is a human hair, which serves as a convenient
standard of size for comparison with other things. Other good
objects are starches, the scales from a butterfly's wing (sketch
under different powers), a drop of milk or blood, and powdered
carmine or gamboge rubbed up in water (to show the Brownian
movement). The student should compare the same object as
seen under the simple and the compound microscope (to show

205

206 APPENDIX.

reversal of the image in the latter), and should during the course
learn the use of the camera lucida (Abbe's camera, of Zeiss, the
best). The stage-micrometer may also be examined at this time
or later, and the student taught to prepare a scale (see Andrews)
by drawing the lines, with camera, on a card under different
powers (A + 2, D + 2, D -f- 4, of Zeiss), and labelling each
with the names of lenses and actual size of the spaces, as stated
on the micrometer.

Pencil-drawing should begin as soon as the first specimen is
in focus, and sketches should be made, from the very first exercise
onward, of everything really studied. It is absolutely indis-
pensable to keep a laboratory note-look, which ought at any time
to give tangible evidence that the laboratory study is bearing
fruit ; and in the very first laboratory exercise a beginning should
be made in this direction.

The preliminary microscopy of one or two laboratory peri-
ods, corresponding to the time spent in conferences upon the first
chapter of the text-book, leads naturally up to the easy micro-
scopical studies required in connection with the second chapter.

CHAPTER II. (STRUCTURE OF LIVING ORGANISMS.)

The laboratory work may be made very brief and simple,
and the facts shown largely by illustration. The principal
organs of a plant and of a live or dissected animal may be shown
and some of the more obvious tissues pointed out. A frog under
a bell-glass, and a flowering plant (geranium) in blossom, placed
side by side on the demonstration-table will serve to suggest
materials for the lists of organs and the comparisons called for.

The skin of a Calla leaf is easily stripped off and demon-
strated to the naked eye as one form of tissue. It may then be
cut up and distributed for microscopic study and for proof that
it is composed of cells. (During this process air is apt to replace
water lost by evaporation, and must be displaced by alcohol,
which in turn must be removed by water.)

For a first microscopical examination of tissue there is no
better object than the leaf of a moss (a species having thin broad
leaves should be chosen) or a fern prothallium. Other good
objects are thin sections of a potato-tuber from just below the

LABORATORY STUDIES AND DEMONSTRATIONS. 207

surface (stained with dilute iodine to show nuclei and starch-
grains), and frog's or newt's blood, mixed with normal salt solu-
tion, and examined either fresh or slightly stained with dilute
iodine.

Thin sections of pith (elder, etc.), from which the air ha&
been displaced by alcohol, give good pictures of tissue composed
of empty cells. Fresh or alcoholic muscle from the frog's leg,
gently teased out, shows muscular tissue to be composed of elon-
gated cells (fibres). Finally, the student may prove that he
himself is composed of cells by gently scraping the inside of his.
lip or cheek with a scalpel, mounting the scrapings on a slide,
and after adding a drop of Delafield's haematoxylin, covering,
and examining in the usual way.

To show the lifeless matter in living tissue it suffices to ex-
amine frog's blood or human blood; sections of potatoes, es-
pecially if lightly stained with iodine ; sections of geranium stems.
(Pelargonium), which usually show crystals in some of the more
peripheral cells ; cartilage, stained with iodine, in which the life-
less matrix remains uncolored ; or prepared sections of bone, in
which the spaces once tilled by the living cells are now black and
opaque, being filled with dust in the grinding, or with air.

CHAPTER III. (PROTOPLASM AND THE CELL.)

Naked-eye Examination of Protoplasm. A drop of proto-
plasm is readily obtained from one of the long (internodal) cells.
of Nitella, after removing the superfluous water and snipping off
one end of the cell with scissors. The cell collapses and the
drop forms at the lower (cut) end. It may be transferred to a
(dry) slide and tested for its viscidity by touching it with a
needle. Microscopically it is instructive chiefly by its lack of
marked structure.

The Parts of the Cell. The structure of the cell is beauti-
fully shown in properly stained and mounted preparations of un-
fertilized star-fish or sea-urchin eggs, or of apical buds of Nitella.
If these are not available potato-cells or cartilage cells do very
well ; or sections of epithelium, glands, etc. , may be shown.

The class may also mount and draw frog's or newt's blood-
.cells, prepared and double-stained as follows. The blood is spread

208 APPENDIX.

out evenly on a slide and dried cautiously over a flame. Stain
with hsematoxylin for three minutes ; wasli thoroughly with water,
add strong aqueous solution of eosin, allow to stand one minute ;
wash this time very rapidly, remove the excess of water quickly
with filter-paper pressed down over the whole slide ; dry rapidly,
and examine with low power. If successful mount in balsam ; if
the specimen is not pink enough add more eosin and wash still
more rapidly than before. In good specimens the cells keep
their form perfectly, the cytoplasm is bright pink, and the nucleo-
plasm is light purple.

Epidermis from young leaves of hot-house lilies (" African "
lily, "Chinese" lily, and especially lily-of- the- valley) yields
cells showing finely the cell-wall, nucleus, and (in favorable
cases) cytoplasm. If stained with acetic acid and methyl-green
the nuclei are highly colored ; with Delafield's hsematoxylin the
cytoplasm is more easily seen.

Cell-divisions or Cleavage are easily observed in segmenting
ova or in fresh specimens of Protococeus (Pleurococcus) de-
tached from moistened pieces of bark which bear these algae.
(See p. 178).

Stages in the cleavage of the ovum may be seen in the seg-
menting eggs of fresh- water snails (Physa, Planorbis) which
are easily procured at almost any time by keeping the animals in
aquaria. The old egg-masses should be removed so as to ensure
the eggs being fresh. Or a supply of preserved segmenting eggs
(star-fish, sea-urchin) may be kept for demonstrating the early

Protoplasm in Motion. The best introduction to protoplasm
in motion is afforded by a superficial examination of Amoeba
(for procuring Amoeba see above, Chapter XII). If Amoeba, is
not available young living tips of Nitella or Chara may be used.
Anacharis and Tradescantia are useful, and often very beautiful,
but less easy to manage, as a rule. In mounting Nitella or
Chara care must be taken not to crush the cells, and as far as
possible pale fresh specimens rather than darker and older ones
should be chosen. If Anacharis is to be studied the youngest
leaves should be selected from the budding ends, and not, as is
sometimes recommended, leaves which are becoming yellow.
The movement in the cells of Anacharis leaves often begins

LABORATORY STUDIES AND DEMONSTRATIONS. 209

only after the leaf has been mounted for a half -hour or more •
but when once established affords one of the most beautiful and
striking examples of protoplasmic motion. If Tradescantia is to
be used, care must be taken to have, if possible, flowers just open
or opening. The morning is therefore preferable for work on
this plant. High powers are necessary.

In all these forms the movements may often be stimulated by
placing a lamp near the microscope or by cautiously warming
the slide over the lamp-chimney. Ciliary action is easily shown
in bits of the gills taken from fresh clams, mussels, or oysters, or
in cells scraped from the inside of the frog's O3sophagus. A
striking demonstration is easily given by slitting open a frog's
(or turtle's) O3sophagus lengthwise, pinning out flat, moistening
with normal salt solution, and placing tiny bits of moistened cork
on the surface. The progressive movement of the cork-bits i&
then very obvious. Muscular contractility is easily shown by
removing the skin from a frog's leg, dissecting out the sciatic
nerve, cutting its upper end, and then stimulating the lower end,
if possible, by contact with a pair of electrodes, otherwise by
pinching it with forceps. If the necessary apparatus is available
the regular muscle-nerve preparation may be shown (see Foster
and Langley's "Practical Physiology").

Food-stuffs Contain Energy. This may be shown (in dem-
onstrations) by sprinking finely powdered and thoroughly
dried starch, sugar, or flour upon a fire, or upon a platinum dish
or piece of foil heated to redness over a small flame. Oils and
dried and powdered albumen (proteid) may be similarly made to
burn with almost explosive violence if applied in a state of fine
division in presence of air.

The Chemical Basis, (a) ProUids ; Coagulation ; Rigor Mor-
tis ; Rigor Caloris. White- of -egg may be shown (in demonstra-
tion) and made to coagulate in a test-tube hung down into a
beaker of water under which is put a llame. A thermometer in
the test-tube may be read off from time to time as the experi-
ment advances, until finally coagulation begins, when the temper-
ature is noted. The death-stiffening (rigor mortis) comes on
very quickly in frogs killed with chloroform. Heat-stiffening
(rigor caloris) is well shown by immersing one leg of a decapi-
tated frog in a beaker of water at 40° C. The other leg re-

210 APPENDIX.

mains normal and affords a valuable means of comparison. It
is not worth while to make many chemical tests of proteids at
this point.

(b) Carbohydrates. A useful demonstration may be made
of various starches, sugars, and glycogen. The iodine-test may
be applied if desired. If time allows, the microscopical appear-
ance of potato-starch, corn-starch, Bermuda arrowroot, etc.,
may be dwelt upon in the laboratory-work. Cellulose is well
shown in filter-paper or absorbent cotton.

(c) Fats. A demonstration of animal fats and vegetable oils
may be made if tune allows. They may be examined microscop-
ically in a drop of milk, in an artificial emulsion made by shak-
ing up sweet oil in dilute white-of-egg, or in fresh fatty tissue
(from subcutaneous tissue of mouse, or fat-bodies of frog). It is
hardly worth while to examine these substances chemically, but
a few simple tests may be applied if desired.

Dialysis. A demonstration of dialysis is easily made by in-
verting a broken test-tube, tying the membrane over the flaring
end, filling the tube to a marked point with strong salt or glu-
cose solution, and immersing it in a beaker of distilled water.
After an hour or so the fluid will be found to have risen in the
test-tube against gravity.

Temperature and Protoplasm. The profound influence of
temperature on protoplasm is well shown by the frog's heart.
Decapitate a frog and destroy the spinal cord. Expose the
heart and count the beats at the room temperature. Then pour
upon the heart iced normal salt solution. Again count the beats.
Next pour upon it normal salt solution heated to 35° C. The
nmnber of beats will follow the fall and rise of temperature.

CHAPTERS IV TO VIII. (THE EARTHWORM.)

Large earthworms must he used or satisfactory results can-
not be expected. Pains should therefore be taken to procure
the large L. terrestris (not the common Allolobophora mucosd),
which is readily recognizable by the flattened posterior end.
This species is not everywhere common ; hence a supply should
be procured and kept in a cool place in barrels half full of earth,
on the surface of which is placed a quantity of moss. They will

LABORATORY STUDIES AND DEMONSTRATIONS. 211

thus live for months. Z. terrestris may be obtained in great
numbers between April and November, by searching for them
at night with a lantern in localities where numerous castings
show them to abound (a rather heavy but ricli soil will be found
most productive). They will then be found extended from their
burrows, lying on the surface of the ground, and may be seized
with the fingers. Considerable dexterity is needed, and it is
necessary to tread very softly or the worms take alarm and in-
stantly withdraw into their burrows.

For dissection fresh specimens are far preferable for most
purposes, though properly preserved ones answer the purpose.
Fresh specimens should be nearly killed by being placed for a
short time (about five minutes) in 70$ alcohol, and then stretched
out to their utmost extent in 50$ alcohol in a dissecting-pan,
the two ends being fastened by pins. They should then be at
once cut open along the middle dorsal line with scissors, the
flaps pinned out, and the dissection continued under the 50$
alcohol. (They must be completely covered with the liquid.)
By this method the minutest details of structure may be ob-
served, and many of the dissections should be done under a
watchmaker's lens.

For preservation (every detail of which should be attended
to) a number of living worms are placed in a broad vessel filled
to a depth of about an inch with water. A little alcohol is then
cautiously dropped on the surface of the water at intervals until
the worms are stupefied and become perfectly motionless and re-
laxed (this may require an hour or two). They are then trans-
ferred to a large shallow vessel containing just enough 50$
alcohol to cover them, and are carefully straightened out and
arranged side by side. After an hour the weak alcohol is re-
placed by stronger (70$), which should be changed once or twice
at intervals of a few hours; they are finally placed in 90$
alcohol, which should be liberally used. The trouble demanded
by this method will be fully repaid by the results. The worme
should be quite straight, fully extended, and plump, and they
may be used either for dissection or for microscopic study.

For the purposes of section-cutting worms should be carefully
washed and placed in a moist vessel containing plenty of wet
filter-paper torn into shreds. The worms will devour the paper,

212 APPENDIX.

which should be changed several times, until the paper is voided
perfectly clean. The worms are then preserved in the ordinary
way, and when properly hardened are cut into short pieces,
stained with borax-carmine, imbedded in paraffin, and cut into
sections with the microtome.

The living worms should first be observed — their shape,
movements, behavior to stimuli, pulsation of the dorsal vessel
(time the pulse and vary the rate by temperature changes).
Well-preserved specimens should then be carefully studied for
the external characters (draw through the fingers to feel the setae).
(Sketch.) Observe openings. The nephridial openings cannot be
seen, but if preserved worms be soaked some hours in water and
the cuticle peeled off they may be clearly seen in this. A
general dissection of a fresh specimen should now be made,
and the positions of the larger organs studied. (Make partial
sketch, to be filled out afterwards, as in Fig. 24.) The alimentary
canal and circulatory organs should now be carefully studied.
Even the smallest of the blood-vessels may easily be worked out
under the lens by using fresh specimens (killed in 70# alcohol
and afterwards dissected under water) and carefully turning aside
the alimentary canal.

The alimentary canal should afterwards be cut through be-
hind the gizzard and gradually dissected away in front, exposing
the nerve-cord and the reproductive organs (wash away dirt with
a pipette). No great difficulty should be found in making out any
of the parts, excepting the testes. These are difficult to find in
mature worms, but may be found with ease in those which have
no median seminal vesicles (usually the case with specimens hav-
ing no clitellum).

The contents of the seminal receptacles and vesicles from a
fresh worm should be examined with the microscope. Remove
an ovary (with forceps and small curved scissors), mount in water,
and study. (Stained in alum-carmine and mounted in balsam
the ovary is a beautiful object.) The student should also re-
move a fresh nephridial funnel and part of a nephridium, and
study with the microscope. (This may have to be shown by the
demonstrator, but should never be omitted, as the ciliary action
is one of the most striking things to see.) A careful dissection
of the anterior part of the nervous system should also be made.

LABORATORY STUDIES AND DEMONSTRATIONS. 213

If time presses, the detailed study of microscopical sections
mav be omitted, but a series of prepared sections should be kept
on hand and a demonstration given.

The embryological development is too difficult to study, but
very instructive demonstrations may be given by those who have
had some experience. In the neighborhood of Philadelphia egg-
capsules may be found in great numbers in old manure-heaps,
in May and June. One end of the capsule should be sliced oft'
with a very sharp scalpel and the contents drawn out, under
water, with a large-mouthed pipette. The mass may then be
mounted in water under a supported cover-glass and studied
with the microscope. The embryos may be preserved in
Perenyi's fluid, and either studied whole in the preserving fluid
or hardened in alcohol and cut into series of sections.

CHAPTERS IX TO XI. (THE COMMON BRAKE.)

Except when the ground is frozen Pteris may be dug up and
brought into the laboratory in a fresh state. Fronds may be
cut and dried in midsummer and considerably freshened (by a
moment's immersion in warm water) when needed to be used (in
the opening exercise) to illustrate the aerial portion of the plant.
Itliixomes may be obtained at convenience and kept in weak
alcohol (50$).

The Morphology of the Body. To illustrate this, one whole
a>«/ ' n tire plant should, if possible, be at hand for examination.
The aerial and the underground portions may then be sketched
in their normal relations. Branches, roots, and old leaf -stalks
should be pointed out, identified, and sketched.

The Anatomy of the Rhizome should first be made out with
the naked eye. The lateral ridges will be detected by the class,
which should be asked to draw the cross- section as seen with
the naked eye. For this preliminary work each student should
have a piece of rhizome two or three inches in length. (Care
should afterwards be taken that the drawing has been correctly
placed dorsoventrally.) A rough dissection with jack-knife or
large scalpel may next follow, with inferences as to the characters
of the several tissues found (as fibrous, pulpy, woody, etc.).

TJte Microscopic Anatomy of the Rhizome is interesting, and,

214 APPENDIX.

for the most part, easy, but demands much time. If time al-
lows, cross-sections of roots may be made and mounted in balsam.
They are readily cut in pith. Sections of the rhizome may be
made freehand with a razor or, better, with a microtome : but
the old stems are exceedingly hard and liable to injure the
knives.

The frond or Leaf may be obtained in fruit in July and
August and preserved in alcohol. From it sections of leaflets
may easily be got by imbedding in pith. Epidermis is obtained
with some difficulty (by beginners) after scraping. Fresh fern-
leaves from hothouses answer the purpose as well, are easier to
get, and more attractive. Keally good sections of fern-leaves are
not easy for beginners to make. They should be kept on hand.

Sporangia may be obtained in abundance from alcoholic.
specimens of Pteris, or upon hothouse ferns, even in midwinter.
Some of the many species of Pteris found in hot-houses answer
every purpose. The thin edge of a scalpel slipped under the un-
ripe indusium removes the latter, and generally also long ranks of
sporangia in all stages of development. In some sporangia spores
may be found. Sporangia and spores are always readily got,
but care must be taken to select fruit-dots which are not too old
or too young.

Sprouting the Spores. To obtain good specimens of sprout-
ing spores and TpYo\\\n\\\& free from dirt, we can recommend the
following procedure : Fill several small flower-pots, which have
been thoroughly cleaned inside and out, with clean line sand.
Sterilize the whole by baking in an oven or a hot-air sterili/er.
Set the pots into large (porcelain) dishes capable of holding water,
and keep the bottom of these dishes covered to the depth of one
inch with water; cover the pots completely with bell-glasses.
After twenty-four hours, or after the sand and the pots have In-
come thoroughly wet, inside and outside, dust thickly the sand
and the outsides of the pots with spores (obtained from fern-
houses by shaking fertile fronds over white paper). Care must
be taken to get spores, and not merely empty sporangia. A f'ter a
week or longer (sometimes several weeks) a bit of the surf 'ace-
layer of sand is removed to a drop of water on a slide and exam-
ined for sprouting spores. These will often be found in various
stages of development. After a month or two prothallia will ap-

LABORATORY STUDIES AND DEMONSTRATIONS. 215

pear on the outside of the pots ; and as these are clean, they may
be removed and examined (bottom side upwards) free of all
dirt.

Failing these, prothallia may almost always be found in fern-
houses on the tops or sides of the pots, and especially on the
moist earth under the benches. Care should be -taken not to
confound prothallia with the lighter green and relatively coarse
liverwort (Lunularia) often found in hothouses.

The Sexual Organs of Prothallia. With good clean speci-
mens these are easily found with a rather low power. Higher
powers are needed to make out details. If the archegoiiia and
and antheridia are young they are green ; if old, brown. On
young prothallia antheridia only are often found, and on very
old ones archegonia only.

Fertilization. This is not easy to observe, but the attempt
may be made by examining successively a number of very fresh
and vigorous prothallia in different stages. They must be
mounted carefully (not flooded with water), and spermatozoids
are generally more easily found swimming about after the speci-
men has been mounted a little while.

Embryology. Except in its general features, this is too dif-
ficult for the beginner. He may, however, observe the later
stages by studying old prothallia with the young fern just ap-
pearing, and young ferns with the old prothallia still adherent.

Chlorophyll and Starch. Vigorous prothallia afford excellent
examples of cells bearing chlorophyll-bodies in which starch is
easily detected. Some of the marginal cells should be examined
with the highest power, attention being given to the chloro-
phyll-bodies and their arrangement. In favorable cases one may
observe the opaque rod-like or oval grains inside the latter,
and prove by reagents that they are starch grains.

The student should also examine, at this point, the large
chromatophores of Nitella, which may be obtained by pressing
out a drop of the contents from an internodal cell, adding dilute
iodine solution, and examining with a high power. In favor-
able cases as many as a dozen starch grains, stained blue, may be
found inside a single elliptical chlorophyll-body.

216 APPENDIX.

CHAPTER XII. (AMCEBA.)

Amoeba is one of the most capricious of animals, appearing
and disappearing with inexplicable suddenness, and as a rule it
cannot be found at the time when needed, unless special prepara-
tions have been made in advance. It is never safe to trust to
chance for a supply of material. It is equally unsafe to trust to
the methods usually prescribed. Amoebae may, however, often,
be procured in abundance and with tolerable certainty as follows :
A month or six weeks beforehand collect considerable quantities-
of water-plants (especially Nitella or Chard} from various pools-
or slow ditches, with an abundance of sediment from the bottom.
It is important to select clear, quiet pools containing an abun-
dance of organic matter (such as desmids, diatoms, etc., in the
sediment) — not temporary rain-pools or such as are choked with
inorganic mud (dirt washed in by rain). The material thus pro-
cured should be distributed in numerous (10 to 20) open shallow
dishes (earthenware milk-pans) and allowed to stand about the
laboratory in various places — some exposed to the sun, others in
the shade. The contents of many, perhaps all, of the veeaeli
will undergo putrefactive changes and swarm with life — first with
bacteria, later with infusoria — and will then gradually become
clear again as in a hay-infusion. The sediment should now be
examined at intervals, and Am&bce are almost certain to appear,
sooner or later, in one or more of the vessels. Usually the small
A. radiosa appears first, but these should only be used if it i&
found impossible to procured. Proteus, which is far larger, clearer,
and more interesting. Experience will show that particular
pools always yield a crop of Amoebae, while others do not.
When once a productive source is found all trouble is ended.

If possible a sediment should be selected that swarms with
Amcebaz. It is very discouraging for students to pass most of
their time looking for the animals instead of at them. I.<tr<j>
cover-glasses should be used, and the material taken witli a
pipette from the very surface of the sediment (not from its
deeper layers). When first mounted the animals are usually con-
tracted, and only become fully extended after a time. Outline
sketches should be made at stated intervals, the structure <>t tin-
protoplasm carefully studied, the pulse of the contractile

LABORATORY STUDIES AND DEMONSTRATIONS. 217

vacuole timed (vary by varying temperature), and the effect of
tapping the cover-glass noted. It is practically useless to look
for fission, for encysted forms, or for the external opening of the
contractile vacuole; and the ingulfing of food or passing out
of waste matters is rarely seen. The formation of pseudopodia
should be carefully studied. After examining the living animals
they should be killed and stained with dilute iodine.

Arcella is almost always, and Difflugia sometimes, found
with Amoeba. These forms may be examined for comparison.

It is desirable also to compare white blood-corpuscles, which
may be obtained either by pricking the finger or, better, from a
frog or newt. A drop of blood, received upon a slightly warmed
elide, should be covered and sealed with oil around the edge of
the cover-glass. The white corpuscles are at first rounded, but
soon begin to show change of form. (No contractile vacuole, no
•differentiation into ectoplasm and entoplasin, often no nucleus
visible.)

CHAPTER XII. (INFUSORIA.)

JParamcecia are almost certain to appear in the earlier stages
of the Amoeba cultures, and in similar decomposing liquids or
infusions, and to ensure having them a large number of vessels
and jars containing an excess of vegetable matter should be pre-
pared a month or more beforehand. Their successful study is
very easy if they are procured in very large numbers (the water
should be milky with them), otherwise it is practically impossible.
Three slides of them should be prepared and set aside for a short
time (under cover, preferably, in a moist chamber) to allow the
animals to become quiet. One slide should contain simply a
drop of the infusorial water ; a second the same, with the addi-
tion of a little powdered carmine ; to the third add a drop or two
of an aqueous solution of chloral hydrate (made by dropping a
crystal or two into a watch-glass of water). The first slide
should be studied first ; and it will usually be found that after a
time the animals crowd about the edges of the cover, often lying
nearly or quite still. If this is not the case, the specimens para-
lyzed by chloral may be studied. The carmine specimens will
show beautiful food-vacuoles filled with carmine ; and by careful
study the formation of the vacuoles may be observed.

218 APPENDIX.

The general structure should be carefully studied, the con-
tractile vacuoles particularly examined (they are seen best in dying
specimens or in those paralyzed by chloral), and dividing or con-
jugating individuals looked for (they are often abundant). The
only really difficult point is the nucleus, which cannot be well
seen in the living animal. It may be clearly seen by mounting
a drop, to which a little dilute iodine or 2# acetic acid has been
added. The former shows the cilia well, the latter the tridio-
cysts. Osmic acid and corrosive sublimate also give good preser-
vation. The internal changes during fission and conjugation
must be studied in prepared specimens mounted in balsam. Such
preparations are often of great beauty and interest.

Vorticella must be sought for on duck- weed or other plants,
or on floating sticks, and the like. Zoijthamnion, Carclux />///,*,
etc. , are liable to appear at any time in the aquaria. All these
forms are easily studied. Conjugation is very rarely seen, but
fission and motile forms are common. The macronucleus is
especially well shown in dead or dying specimens.

CHAPTER XIV. (PKOTOCOCCUS.)

Protococcus (Pleurococcus) is found in abundance on the
northerly side of old trees in many parts of the United States.
In case it cannot be obtained in any region it may be procured,
during 1895 and 1896, from Prof. Sedgwick, Institute of
Technology, Boston, Mass., by mail. The laboratory-work with
it is too easy to require comment. See, however, Arthur,
Barnes & Coulter's "Plant Dissection" (Henry Holt & Co.v
New York).

CHAPTER XV. (YEAST.)

Bakers', brewers', compressed, and dried yeast may be had
in the markets. Brewers' yeast is to be preferred, as com-
pressed yeast-cakes contain starch, bacteria, and other extraneous
matters. All of the kinds may be cultivated to good advantage
in wort (to be obtained at breweries) or in Pasteur's fluids. (See
Huxley and Martin, chapter on Yeast.) Wild yeasts may be

LABORATORY STUDIES AND DEMONSTRATIONS. 219

found by examining sweet cider microscopically. For the fol-
lowing methods of demonstrating nuclei in yeast and obtaining
ascospores we are indebted to Mr. S. C. Keith Jr.

To Demonstrate Nuclei in Yeast. Any good actively-growing
yeast will answer, but a large (brewers') yeast is preferable. Mix
a little of the yeast with an equal amount of tap- water in a test-
tube and shake thoroughly. Add an equal volume of Hermann's
fluid and shake again. As soon as the yeast has settled pour off
the supernatant liquid and wash the yeast by decantation. Trans-
fer some of the cells to a slide, fix by drying, stain by Heiden-
hain's iron-hamiatoxylin method (see Centmlblatt far Bacteri-
ologie, xiv. (1893), pp. 358-360), wash, dehydrate with alcohol,
follow with cedar-oil, and mount in balsam. In successful speci-
mens the effect is very satisfactory. (See Fig. 96.)

A Simpler Method. To demonstrate nticlei in yeast more
quickly and very easily the following method may be used : Boil
(in a test-tube) for a moment an infusion of very vigorous yeast
in water, place a drop of the boiled infusion on a slide, add a
drop of very dilute "Dahlia" solution, cover, and after one or
two minutes examine with a high power. The nuclei in most of
the cells will be easily discoverable.

To Obtain Ascospores in Yeast. It has been usually recom-
mended to employ for this purpose blocks of plaster-of-Paris.
We have found the following method more trustworthy :

The yeast to be used should be the " top" yeast used in ale-
breweries. It should also be actively growing and fresh. If
fresh yeast cannot be obtained, some may be revived by cultiva-
tion for 24 hours at 25° C. in wort, and a little of the thick sedi-
mentary portion may then be placed in a very thin layer on dry
filter-paper which has previously been sterilized by baking. The
filter-paper is then placed on a layer of cotton about £ inch in
thickness lying on a plate or saucer, the cotton having previously
been thoroughly wetted with cold sterilized tap-water. The
whole is covered by a bell-glass and set in a rather warm place
(25° C.). In the course of two or three days spores will be found
in many of the cells. The lower the temperature the longer is
the time required for spore formation. If "bottom" yeast is
used instead of "top" yeast a much longer time is required, and
the results are far more uncertain.

220 APPENDIX,

CHAPTER XYI. (BACTERIA.)

For the study of Bacteria it is very desirable to have a largo
species, and for this purpose there is none better than Bacillus
megaterium, which may be obtained from almost any bacteriologi-
cal laboratory and grown in the bouillon used by bacteriologists.
During 1895 and 1896 it may be obtained from Boston (see
above). This form is very large, and produces spores readily.
(See l)e Bary, " Lectures on Bacteria ;" Sternberg, "Bacteriol-
ogy;" Abbott, "Principles of Bacteriology;" etc.) The pro-
longed study of bacteria is not suited to beginners. Vinegar
bacteria may be seen in the mother- of -vinegar by pressing a bit
of it out under a cover slip and examining with a high power.
The jelly of mother-of- vinegar is a good example of zov<jl«-<i.
The white scum which appears on aquaria and infusions i.- of
the same general character (zooglcea).

CHAPTER XVII. (A HAY INFUSION.)

To make a successful hay infusion care should be taken to
use water containing numerous and various organisms, and there-
fore distilled water, spring- waters, and well-waters, are in general
to be avoided. Tap- water should also be avoided if it is derived
from springs or wells. The best water for the purpose is that
drawn from ponds, rivers, lakes, or other surface sources.
Clean ditch or pool water is excellent. The choice of hay is less
important, but it is well to avoid old hay and hay that is \ »-ry
woody. The infusion should be warmed, but not heated or
boiled. It may be kept in a beaker in diffuse daylight, e.g., in
a north window, the beaker being loosely covered.

INSTRUMENTS AND UTENSILS.*

The student should have access to the following articles :
A compound microscope with two eyepieces and low and
high power objectives (i.e., about 1 in. and £ in., or objectives

* Most of the apparatus and reagents here mentioned may be obtained from
any first-class dealer in physical and microscopical apparatus, e.g., from The

LABORATORY STUDIES AND DEMONSTRATIONS. 221

A and D of Zeiss, or * and | inch of Bausch and Lomb- still
higher powers are desirable).

A simple dissecting microscope; a desirable form is an ordi-
nary watchmaker's lens provided with a support. An ordinary
pocket-lens; glass slides (3 X 1 in.), cover-glasses, watch-crystals
small gummed labels, needles with adjustable handles, camel' s-
hair brushes, blotting and filter paper, a good razor, pipettes
(medicine-droppers), glass rods and tubes, glass or porcelain
dishes for staining, etc., a set of small dissecting instruments
{small scalpel, forceps, and straight-pointed scissors), a section-
lifter, pieces of pith for section-cutting, thread, a shallow tin pan
lined with wax, long insect pins for pinning out dissected speci-
mens, drawing materials, and a note-book for sketches and other
records.

Each table should be furnished with a set of small reagent-
bottles, a Bunsen burner, wash-bottle, test-tubes, beakers, and a
bell-glass for protection from dust. Thermometers, a balance,
microtome, drying oven, and a paraffin water-bath should also be
accessible.

REAGENTS AND TECHNICAL METHODS.*

Alcohol. — Since biological laboratories belonging to incorpo-
rated institutions obtain alcohol duty free, it should be liberally
supplied and freely used. Alc'ohol of 100°, i.e., "absolute"
alcohol, may be purchased in 1-pound bottles. "Squibb's"
absolute alcohol may be obtained of any druggist, f but ordinary
alcohol of 90—95% answers nearly every purpose. "Cologne
spirits," i.e., alcohol of about 94$, may be obtained from the
distillers at 60c., or thereabouts, per gallon. It may then be

Bausch & Loinb Optical Co., Rochester, N. Y.; the Franklin Educational
Co., Hamilton Place, Boston; or Queen & Co., Chestnut Street, Philadelphia.
Chemical and other apparatus may be obtained from Eimer & Amend, 205-211
Third Avenue, N. Y.

* Every laboratory should be supplied with some of the standard books upon
this subject, e.g., Strasburger's Botanische Practicum, Jena; Whitman's
Methods of Research in Microscopical Anatomy and Embryology, Boston: Lee,
The Microtomist's Vade Mecum, last edition; Zimmerman's Botanical Micro-
technique (Humphrey), Holt, N. Y.

t See also Whitman, 1. c., p. 14.

222 APPENDIX.

diluted to 8(% 70$, 50$, etc. , as needed. For this purpose an
alcoholimeter is very convenient.

Acetic Acid. — One or two parts glacial acetic acid to 100 parts
water.

Acetic Acid and Methyl-green. — This is valuable for staining
nuclei in vegetal tissues. Dissolve methyl-green in one or two
per cent acetic acid until a rich deep color is obtained.

Borax-carmine. — Add to a 4$ aqueous solution of borax 2-3$
carmine, and heat until the carmine dissolves. Add an equal
volume of 70$ alcohol, and filter after 24 hours. After staining
(6-12 hours, or more for large objects, a few minutes for sec-
tions) place the object in acidulated alcohol (100 c.c. 35$ alcohol,
3-4 drops hydrochloric acid) and leave until the color turns from
dull to bright red (10—30 m.). Afterwards remove to 70$
alcohol.

Canada Balsam, Mounting in. — This invaluable substance may
be obtained in the crude condition, dried by prolonged heating,
and then dissolved in chloroform, benzole, or turpentine, for
use. The benzole solution is perhaps the best, and may be ob-
tained from most of the dealers. The principles of mounting in
balsam are very simple. It does not mix with water or alcohol,
but mixes freely with clove-oil, chloroform, benzole, etc. Ob-
jects are therefore generally treated, first with very strong alco-
hol, 95-100$, in order to remove the water ; then with clove-oil,
chloroform, or turpentine to remove the alcohol, and afterwards
mounted in a drop of balsam. This should usually be placed on
the cover-glass, which is thereupon inverted over the object.
The balsam gradually sets and the preparations are permanently
preserved.

Carmine. — Carmine may be obtained as a powder, which
when rubbed up thoroughly with water in a mortar passes into a
state of very fine subdivision. This property makes it available
for experiments with cilia, etc.

It is more often used in solution, as a staining agent. (See
Borax- carmine.)

Cellulose- test. — Saturate the object in iodine solution, wash in
water, and place it in strong sulphuric acid prepared by carefully
pouring 2 volumes of the concentrated acid into 1 volume of
water.

LABORATORY STUDIES AND DEMONSTRATIONS. 223

Collodion and Clove-oil.-Used for fixing sections to the slide
in order to prevent the displacement of delicate or isolated parts
in balsam-mounting. Mix one part of ether- collodion and three
parts of oil of cloves. In mounting, varnish a slide with the
mixture by means of a camel's-hair brush, lay on the sections
arid place the slide for a few minutes on the water-bath (i.e.,
until the clove-oil evaporates). Transfer the slide to a wide-
mouthed bottle of turpentine (to dissolve the paraffin), remove it
and drain off the turpentine, place a drop of Canada balsam on
the middle of a cover-glass, and invert it over the object.

Dahlia. — Dissolve in water.

^ Eosin.— Dissolve in water until a bright-red solution is ob-
tained. It should be diluted when used.

Glycerine, dilute. — Two parts glycerine, one part distilled
water.

Htematoxylin (Delafield's).— Add 4 c.c. of saturated alcoholic
solution of hsematoxylin to 150 c.c. of strong aqueous solution of
ammonia-alum; let the mixture stand a week or more in the
light, filter, and add 25 c.c. of glycerine and 25 c.c. of methyl
alcohol. The fluid improves greatly after standing some weeks
or months.

Haematoxylin (Kleinenberg's).— To a saturated solution of cal-
cium chloride in 70$ alcohol add an excess of pure alum ; filter
after 24 hours and add 8 volumes of 70$ alcohol, filtering again
if necessary. Add a saturated alcoholic solution of hsematoxylin
until the liquid becomes purple-blue. The longer the liquid
stands before using, the better. It should be diluted for use
with the alum-calcium-chloride solution in 70$ alcohol.

Hermann's Fluid. — See Lee's Vade Mecum.

Iodine Solution. — Dissolve potassium iodide in a small quantity
of water, add metallic iodine until the mixture assumes a dark-
brown color, and then dilute to a dark-sherry color. The solu-
tion should be kept from the light.

Magenta (Aniline Red). — Dissolve in water.

Methyl Green. — Used in aqueous or alcoholic solution or
with acetic acid.

Normal Fluid (Normal Salt Solution). — Dissolve 7.50 grams of
sodium chloride in 1 litre of distilled water.

Paraffin.— " Hard " and "soft" paraffins, i.e., those of high

224 APPENDIX.

and low melting-points, should be mixed in such proportions that
the melting-point lies between 50° and 55° C.

Perenyi's Fluid. — Ten-per-cent nitric acid 4 parts, 90# alco-
hol 3 parts, £# aqueous solution of chromic acid 3 parts. Not
to be used until the mixture assumes a violet hue. Leave objects
in the fluid 30 minutes to an hour, then 24 hours in 70# alcohol,
and finally place in 90 per cent alcohol.

Schultze's Macerating Fluid. — Dissolve a gram of potassium
chlorate in 50 c.c. of nitric acid. The tissue should be boiled
in the mixture and afterwards thoroughly washed in water.

Schulze's Solution. — Dissolve zinc in pure hydrochloric acid,
evaporate in the presence of metallic zinc, on a water-bath, to a
syrupy consistency, add as much iodide of potassium as will dis-
solve, and then saturate with iodine. (When heated with this
fluid cellulose turns blue.

Section-cutting. — Many objects can be cut by hand with a
razor (which must be very sharp). The object should be held in
the left hand while the razor is pointed away from the body, and
allowed to rest on the tips of the fingers with its edge turned
towards the left. It is then drawn gently towards the body so
as gradually to shave off the section. Small objects may be held
between two pieces of watchmaker's pith previously soaked in
water. In either case the razor should be kept wet.

Many objects, however, require more careful treatment by
one of the following methods :

A. Paraffin Method. — After hardening and staining, the
object is soaked in strong alcohol (95$ or more) until the water
is thoroughly extracted (2-12 hours, changing the alcohol at
least once), then in chloroform until the alcohol is extracted
(2-12) hours), and then in melted paraffin (not warmer than 55°
C.) on a water-bath for 15 to 30 minutes (too high a tempera-
ture or too long a bath causes excessive shrinkage). Some of the
paraffin is then poured into a small paper-box, or into adjustable
metal frames. The object is transferred to it and after the mass
has begun to set it is placed in cold water until quite hard. It
is then cemented (by paraffin) to a square piece of cork and
placed in the section-cutter or microtome.

The sections may be cut singly with the oblique knife or by

LABORATORY STUDIES AND DEMONSTRATIONS. 225

the ribbon-method,* the knife being kept dry in either case. In
mounting they should be fixed by the collodion-method. (See
Collodion and Clove-oil.)

B. Celloidin Method. — This is especially applicable to deli-
cate vegetal tissues. After dehydrating the object thoroughly in
alcohol, soak it 24 hours in a mixture of equal parts of alcohol
and ether. Make a thick solution of celloidin in the same mix-
ture and soak the object for some hours in it. It may then be
imbedded as follows : Dip the smaller end of a tapering cork
in the celloidin solution, allow it to dry for a moment (blowing
on it if necessary), and then build upon it a mass of celloidin,
allowing it to dry a moment after each addition. Transfer the
object to the cork and cover it thoroughly with the celloidin.
Then float the cork in 82-85$ (0.842 sp. gr.) alcohol until the
mass has a firm consistency (24 h.). It may then be cut in the
microtome with the oblique knife, which must be kept dripping
with 82-85$ alcohol. Keep the sections in 82-85$ alcohol until
ready to mount them, then soak them for a minute in strong
alcohol, transfer to a slide, pour on chloroform until the alcohol
is removed, drain off the liquid, quickly add a drop of balsam,
and cover. (See also Whitman, 1. c., p. 113.)

* See Whitman, 1. c. p. 71.

IKDEX.

Absorption, 48, 52, 101, 165.

Accretion, 166.

Acbromatin, 23.

Actinophrys, 166.

Adaptation, 97, 98, 144.

Adventitious buds, 130.

Probes, 202.

^Etiology, 6.

Agamogenesis, 73, 130, 163.

Albuminous bodies. 36.

Alimentation, 48, 105.

Alimentary canal, 82, 92.

Alimentary system, 49.

Allolobophora, 41.

Alternation of generations, 130.

Ainceba, 27, 158, 216.

Amoeboid cells, 64.

Ainphiaster, 84.

Amphimixis, 168.

Anabolism, 33, 100, 149, 164.

Anachtiris, 29.

Anaerobes, 202.

Anatomy, 7.

Animalcule, 158, 199.

Annulus, 132.
Anus, 46, 82, 165.
Antheridia, 135.
Aortic arches, 54, 55.
Apical buds, 111, 116, 123.
Apical cell, 123.
Apogamy, 143.
Apospory, 143.
Arcella, 166.
Archegonia, 137.
Archenteron, 80, 82, 85.
Archesporium, 131.
Archoplasm, 79, 80.
Arthrospore, 195.
Ascospore, 187.
Asexual reproduction, 73.
Assimilation, 182.
Aster, 79, 84.
Attraction sphere, 83, 84.
ATWATER, W. O., 34.

Bacilli, 192.
Bacteria, 64, 178, 192.
Bast-fibres, 120.

Biology, 1, 6, 7, 8.
Bisexual, 73, 130.
Blastopore, 80, 85.
Blastosphere, 85.
Blastula, 80, 90.
Blood, 15, 16, 90, 102.
Blood-vessels, 54.
Blue-green algae, 183, 192,
Body, 19, 24, 84, 107, 156.
Body-cavity, 47.
Bone, 16.
Botan.v, 6, 7.
Branches, 111, 122, 130.
Branchiae, 62.
Budding, 186.
Bursaria, 176.

Calciferous glands, 51.
CALKINS, G. N., 171.
Capillaries, 54.
Capsules of eggs, 78.
Capsulogenous glands, 46.
Carbohydrates, 37, 101.
Carchesium, 176.
Carnivora, 177, 203.
Cartilage, 15, 16.
Castings, 42, 53.
Cell, 12, 20.
Cell-division, 24, 83.
Cell-theory, 20.
Cellulose, 37.
Cell-wall, 22, 23.
Centrosome, 79, 83, 84.
Cerebral ganglia, 65. 69.
Chalk, 166,
Chara, 24.
Chemiotaxis, 139.
Chlorococcus, 178.
Chloragogue-cells, 52, 61, 93.
Chlorophyll, 126, 151, 215.
Chlorophyll -bodies, 179, 215.
Chroococcus, 183.
Chromatin, 23, 83.
Chromatophores, 147, 179.
Chromosomes, 83, 84.
Cilia, 31, 63, 74, 137, 192.
Circulation, 48, 53, 101, 165.
CLAPAREDE, 96.

227

228

INDEX.

Classification, 7.
Clitellum, 46, 77, 78, 88, 92.
Coagulation, 36, 39.
Cocci, 192.
Coelenterata, 88.
Ccelom, 47, 82.
Coalomic fluid, 53.
COHN, 21.
Cold storage, 199.
Colloidal, 36.
Colony, 176.
Commissures, 65.
Conjugation, 171, 181.
Connective tissue, 70, 90.
Consciousness, 69, 70.
Contractility, 62, 164.
Coordination, 48, 64, 67, 164.
Copulation, 77.
Cross-fertilization, 74.
Crystals, 17.
Cushion, 135.
Cuticle, 71, 91.
Cyanophycea?, 183, 192, 199.
Cyclical change, 5, 72, 89.
Cytoplasm, 22, 84.

DARWIN, 42, 51, 70, 99, 103.

Death, 152.

DE BARY, 115, 143.

Defalcation, 53, 165.

Desinids, 178, 183.

Dialysis, 36, 210.

Diastatic ferment, 52.

Diatoms, 178, 183.

Dichogamy. 138.

Differentiation, 11, 84, 141.

Differentiation, antero-posterior, 43,

110.

Differentiation, dorso-ventral, 43, 110.
Differentiation of the tissues, 25.
Difflitgia, 166.

Digestion, 48, 49, 52, 101, 165.
Diplococcut, 194.
Disease-germs, 192, 197.
Disinfection, 200.
Dissepiments, 47, 94.
Distribution, 7.

Division of latwr, 11, 26, 156, 165.
Dorsal pore, 48.
Dorsal vessel, 54.

DUJARDIN, 21.

Earthworm, 41.
Ectoblast, 81.
Ectoplasm, 158.
Egg, 24.
Egg laying, 77.
Egg-nucleus, 79.
Egg-string, 74.
Embryo, 25.
Embryology, 7, 72, 78.

Endospore, 187, 194.

Endosporium, 134.

Energy, 32, 99, 146, 151.

Entoblast, 81.

Entoplasin, 158.

Environment, 97, 103, 144, 151.

Epidermal system, 114.

Epidermis, Il4, 116.

Epistylis, 176.

Epithelium, 90.

Euglenn, 176.

Excretion, 48, 53, 59, 100, 165w

Exosporium. 134.

Eye-spot, 176.

Fa-ces, 53.

FARLOW, 143.

Fats, 17, 37. 101.

Feathers, 18.

Ferns, 105.

Ferment, 52.

Fermentation, 191. 197.

Fertilization, 73, 78, 139.

Fibro- vascular system, 114.

Fibro-vascular bundles, 143.

Filtration, 200.

Fission, 163.

Flagellum, 176, 192.

FOL, 79.

Foods, 146.

Foraminifera, 166.

Fore-gut, 86.

FOSTER, MICHAEL, 153, 163\

FREDERIC^, 52.

Frond, 125.

Functions, 9.

Fundamental system, 114.

Fungi, 147.

Gamete, 181.

Gamogenesis, 73, 130, 168.
Ganglion, 64, 94.
Gastrula, 80.
Gastrulation, 84.
Germ- cells, 24. 73, 90, 130.
Germination, 134.
Germ-layers, 81, 84,85.
Germ-layer theory, 88.
Germ-plasm, 89, 152.
Germinal spot, 74.
Germinal vesicle, 74.
Giant-fibres, 94.
Gills, 62.
Girdle, 78.
Gizzard, 51, 71.
Glceocapsa, 178, 183.
Glucose, 52.
Glycogen, 37.
Gregnrina, 64. •
Growth, 165.
Guard-cells, 128.

INDEX.

Hamatococcus, 178.
Haemoglobin, 54.
Hair, 18.

Hay infusion, 201.
Herbivora, 176, 203.
Heredity, 84.
Hermaphrodite, 73, 130.
HERTWIG, 79.
Hibernation, 38.
Hind-gut, 86.
Histology, 7.
HOOKE, ROBERT, 20.
HOOKER, SIR W. J., 106
HOPPE-SEYLER, 35.
HUXLEY, 2, 4.
Hypodermis, 92.

Impregnation, 73, 139.
Individual, 13, 156, 164.
Indusiuin, 131.
Infusions, 168.
Infusoria, 168, 217.
Inheritance, 80, 84.
Intussusception, 4, 165.
Irritability, 164.

JOHNSON, 35.

Katabolism, 33, 99, 149, 164.
Karyokinesis, 83.
KEITH, S. C., Jr., 186, 195.
KRUKENBERO, 52.

Lateral ridges, 111, 114.
Leaf, 11, 125.
LENHOSSEK, 95.
Leptothrix, 194.
LINNAEUS, 105.
Lumbricui, 41.
Lungs, 62.
Lymph, 58.
Lymph-cells, 64.

Macrogamete, 175.

Macro nucleus, 170, 171.

Malic acid, 139.

MAUPAS, 170.

Meristem, 123.

Mesoblast, 81.

Mesophyll, 126.

Metabolism, 33, 100, 101, 148, 164.

Metamerism, 45.

METCHNIKOPF, 53.

Microgamete, 175.

Micronucleus, 170, 171.

Micro-organisms, 201.

Middle-piece, 74, 79, 80.

Mid-gut, 86.

Mitosis, 83.

MOHL, H. VON, 21.

Morphology, 6, 7.

Mother-of -vinegar, 194 195
Mother-cells, 134 137 '
Motion, 48.
Motor system, 62.
Mouth, 49, 80, 85, 165.
Muscles, 14, 26, 27, 62, 90.
MULDER, 35.
Mycoderma, 194, 202.
Myxobacteria, 199.
Myxomycetes, 199.

Natural selection, 99.

Nephridia, 58, 59.

Nerves, 64, 90.

Nerve-cells, 94.

Nerve-centre, 68.

Nerve-impulses, 67.

Nervous system, 64, 82, 94, 102.

Nitetta, 28.

Nitrogen, 147.

Nucleolus, 23.

Nucleoplasm, 22.

Nucleus, 16, 23, 186.

Nutrition, 99, 146.

(Esophagus, 18.
Old age, 72, 152, 166.
OSphore, 130.
Oosphere, 73, 138.
OOspore, 139.
Organisms, 9.
Organogeny, 85.
Organs, 9.
Ovaries, 74.
Oviduct, 75.
Ovum, 73, 74, 89.

Parammcium, 168.
Parasites, 192.
Parenchyma, 116.
PASTEUR, 188.
Pasteurization, 200.
Pasteur's fluid, 189, 197.
Pathogenic, 200.
Pathology, 6, 7.
Peptic ferment, 52.
Peptone, 52. 101.
Peristaltic actions, 51, 54, 55.
PFEFFER, 139.
Phagocytes, 53, 61, 64, 158.
Pharyngeal ganglia, 67.
Pharynx, 49.

Physiological properties of proto-
plasm, 163, 182, 183.
Physiology, 6, 7, 166.
Physiology of the nervous system, 67.
Polar cells, 79.
Pole-cells, 82.
Poisons, 39.
Plasma, 53.
Pleurococcus, 178.

230

INDEX.

Primordial utricle, 29.
Proctodseum, 82, 86.
Pronucleus, 79.
Prosenchyma, 116.
Prostoinium, 45.
Protection, 71.
Proteids, 3, 33, 52.
Proteus animalcule, 27, 158.
Prothallium, 130, 135, 214.
Protococcut, 178.
Proton ema, 134.
Protoplasm, 16, 20, 207, 208.
Protozoa, 158.
Pseudopodia, 27, 158'
Psychology, 7, 8.
Pulse, 54.

Putrefaction, 197, 201.
PURKINJE, 21.

Radiolaria, 166.

Receptacle, 131.

Receptaculuin ovorum, 75.

Reflex action, 67.

Regeneration, 73.

Reproduction, 48, 72, 111, 180, 152, 165.

Respiration, 61, 150, 165.

RETZIUS, 95.

Rhizoids, 134.

Rhizome, 111, 140.

Rhizopoda, 166.

Rigor caloris, 39.

Rigor mortis, 209.

Roots, 122.

Saccharomyces, 184.
SACHS, 115.
Salivary glands, 51.
Sap, 14.

Saprophytes, 192.
Sarcina, '194.
Schizoniycetes, 192.
Sen LEIDEN. 20.
SCHULTZE, MAX, 21.

SCHWANN, 20.

Sciences, biological, 1, 6.

Sciences, physical, 1.

Segmentation, 24, 80.

Segmentation cavity, 84, 85.

Seminal receptacle, 77.

Seminal vesicle, 76.

Sensation, 48.

Sense organs, 42, 69.

Senses, 42, 69.

Sensitive system, 69.

Setae, 46, 63.

Setigerous glands, 63, 77.

Sexual reproduction, 73.

Sieve-tubes, 116.

Sight. 42, 69, 70

Skin, 128.

Slipper animalcule, 168.

Smell, 42, 69.
Sociology, 7, 8.
Somatic cells, 73.
Somatic layer, 85.
Somatopleure, 82, 86.
Somites, 45.

SPENCEK, HERBERT, 3, 99, 146.
Sperrnaries, 74, 75.
Sperinatosphere, 77.
Spermatozoid, 137.
Spermatozoon, 73, 74
Sperm-duct, 76.
Sperm-nucleus, 79.
Spiderwort, 29.
Spirilla. 192.
Splanchnopleure, 82, 86.
Spontaneous generation, 83.
Sporangia, 130.
Spores, 24, 130, 194,
Sporophore, 130.
Staphylococcus, 194.
Starch, 17, 37, 146.
Stentor, 176.
Sterilization, 199.
Stimulus. 67.
Stipe. 125.

Stomach-intestine, 51.
Stomata, 126, 128.
Stomodaeum, 82, 86.
Streptococcus, 194.
Struggle for existence, 203.
Mylonichia, 170.
Sugar, 87.

Sun-animalcule, 166.
Survival of the fittest, 99.
Symbiosis, 177.
Symmetry, bilateral, 44, 110.
Symmetry, serial, 45.
Sympathetic system, 67.

Taste, 42, 69, 70.
Taxonomy, 7.
Temperature, 38, 199, 210.
Testes, 74, 75.
Tissues, 11, 13.
Touch. 42, 69, 70.
Toxicology, 39.
Trachea;, 116.
Tracheids, 116.
Tradescnntia, 29.
Transpiration, 146.
Trichocysts, 168.
Tryptic ferment, 52.
Twins, 88.
Typhlosole, 51, 91.

Unicellular animals, 158.
Unicellular organisms, 156, 177.
Unicellular plants, 178.

Vacuoles, 24, 162, 170.

INDEX.

231

Vascular system, 54.
Vas deferens, 76.
Veins, 126.
VEJDOVSKY, 79, 81
Venation, 129.
Vessels, 116.
Vinegar, 196.

VlRCHOW, 21.

Vital energy, 33.
Vital force, 83.
Vitellus, 74, 78.
Vvrticella, 168, 173,

WHITE, 43.

White blood-cells, 64.
Whirlpool, 2.

WlNOGRADSKY, 197.

Yeast, 178.
Yeast, bottom, 190.
Yeast, red, 191.
Yeast, top, 190.
Yeast, wild, 190.

Zooglcea, 194, 195.
ZoOids, 176.
ZoOlogy, 6, 7.
Zoospores, 181.
Zoothamnion, 176.

8636 T *

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