BELL'S SCIENCE SERIES

Edited by
PERCY GROOM, M. A. (Cantab, et Oxon.), F.L.S.

Lecturer on Botany in the Royal Indian Engineering College,
Cooper's Hill

AND

G. M. MINCHIN, M.A., F.R.S.
Professor of Applied Mathematics in the same College

THIS Series has been designed to supply the wants of
Students in the upper forms of Schools, and Candidates
for the various examinations, not distinctly elementary,
at which scientific subjects are now sejt.

ELEMENTARY BOTANY. By PERCY GROOM, M.A., F.L.S.,
sometime Examiner in Botany to the University of Oxford.
Second Edition. With 275 Illustrations. Crown 8vo, 35. 6d.

AN INTRODUCTION TO THE STUDY OF THE
COMPARATIVE ANATOMY OF ANIMALS. By G. C.
BOURNE, M.A., Fellow and Tutor of New College, Oxford.
With numerous Illustrations. Vol. I. 45. 6d.

THE STUDENT'S DYNAMICS. By Professor G. M.
MINCHIN, M.A., F.R.S. [Shortly.

ELEMENTARY GENERAL SCIENCE. By D. E. JONES,
D.Sc., Science Inspector, formerly Professor of Physics in the
University College of Wales, Aberystwith, and D. S. M'NAIR,
Ph.D., B.Sc. U" the Press.

PHYSIOGRAPHY. By H. N. DICKSON, F.R.S.E., F.R.Met.
Soc., F.R.G.S. t/» the Press.

CHEMISTRY. By JAMES WALKER, D.Sc., Professor of Chem-
istry in University College, Dundee.

ELECTRICITY AND MAGNETISM. By OLIVER J. LODGE,
D.Sc., F.R.S., LL.D., M.I.E.E., Professor of Physics in
University College, Liverpool.

LONDON: GEORGE BELL & SONS

COMPARATIVE ANATOMY
OF ANIMALS

GEORGE BELL & SONS

LONDON : YORK STREET, COVENT GARDEN
NEW YORK : 66 FIFTH AVENUE, AND
BOMBAY : .... 53 ESPLANADE ROAD
CAMBRIDGE I DEIGHTON BELL AND CO.

INTRODUCTION TO THE
STUDY OF THE

COMPARATIVE ANATOMY
OF ANIMALS

GILBERT C. BOURNE, M.A., F.L.S.

Fellow and Tutor of New College, Oxford
University Lecturer in Comparative Anatomy

VOL. I.

ANIMAL ORGANISATION. THE
PROTOZOA AND CCELENTEKATA

8PECJMEN.COPY

LONDON

GEORGE BELL & SONS
1900

PREFACE

IN the following Introduction to the study of the Comparative
Anatomy of Animals, I have necessarily been guided by the
requirements of the elementary examinations at the leading
universities of Great Britain.

Having found by experience that beginners find great
difficulty in apprehending the full meaning of the cell-theory
at the commencement of their studies, I have departed from
the course usually pursued in lectures and practical instruction,
and instead of beginning with the study of cells, I have taken
the common frog as a type of animal organisation. The general
anatomy of the frog is first described in some detail; the
microscopic structure of its organs and tissues is next explained,
and then the cell-theory and the phenomena of the cell-division
are dealt with. In this way I have attempted to lead the
student gradually from familiar to new and unfamiliar concep-
tions.

I have attempted, as far as possible, to verify by personal
observation the statements of fact contained in this book, but
want of time and opportunity has prevented me from repeating
the long and laborious researches of many investigators on
such subjects as the reproduction of the Protozoa. Where
personal observation has been wanting I have tried to give
an adequate account of the best and most recent researches
on the subject. In certain cases, however, I have preferred
the results of older to those of more recent observers. For
example, I have adhered to Maupas' account of the phenomena
of conjugation in the Ciliata, because in my judgment the
results of the latest researches on this matter require inde-
pendent confirmation before they can be held to overthrow
the results of such careful and consistent work as that of
Maupas. Since this book is in part a record of my own
observations and not wholly a compilation, I have in several
cases departed from the accepted accounts of certain
phenomena. Thus the description of the truncus arteriosus

x PREFACE

of the frog is based upon models made from my series of
sections, and the account of the histology of Hydra is largely
new, and based upon my as yet unpublished researches. I
have not thought it desirable to burden the elementary student
with a list of references to literature, and I must ask original
authors to pardon me for making use of their facts and
arguments without acknowledgment. But in every case in
which figures have been taken from published works full
acknowledgment is made. The illustrations have been drawn
specially for this book ; many by Mr P. J. Bayzand, the skilful
artist of the Linacre Department at Oxford. Others are by
my friend and pupil Mr E. H. Schuster of New College, Oxford,
and the rest have been drawn by myself. Where not other-
wise stated the figures are from my own sketches and
preparations.

There is a considerable difference of opinion as to the
limits of elementary teaching in Comparative Anatomy. For
my own part, I consider that the more elementary the teaching
is in this subject the fuller it should be, and I have not
hesitated to enter fully into details where a detailed description
seemed necessary, and have discussed certain questions of
theoretical importance at considerable length. Students
seldom begin the study of Comparative Anatomy at an early
age, and they should never begin it until they have mastered
the elements of Physics and Chemistry. I have therefore
addressed myself, not to children, but to persons whose
education is well advanced, and whilst I have tried to write
simply and intelligibly, I have not attempted to evade the
difficulties of technical language. All technical terms, where
used for the first time, are printed in thick type, and are
sufficiently explained in the context.

Whilst the faults of this book are entirely my own, I must
attribute any merit it may possess to the influence of the
three successive occupants of the Linacre chair of Comparative
Anatomy at Oxford under whom I have had the honour to
serve, the late Professor H. N. Moseley, Professor E. Ray
Lankester, and Professor W. F. R. Weldon. Nor must I
forget the many lessons I have learned from my whilom
colleagues at Oxford, Professor W. Blaxland Benham and
Professor E. A. Minchin.

PREFACE xi

The present volume deals with animal organisation as re-
presented by the Frog, with the Protozoa, and the Coelenterata.
The second volume will deal with the Ccelomate Metazoa.
I had hoped to bring out the two volumes simultaneously, but
have been called out on military duty since the outbreak of
the war in S. Africa, so the completion of the second volume
has been delayed.

GILBERT C. BOURNE.

THE BARRACKS, TIPPERARY,
March 1900.

CONTENTS

PAGE

INTRODUCTION . . . . . . . 1-15

CHAPTER I.
The Common Frog — External Features ; Muscles; Skeleton . 16-37

CHAPTER II.

The Frog (continued) — Internal Anatomy ; Crelom ; Lymph ;
Gut ; Lungs ; Spleen ; Kidneys ; Reproductive Organs ;
Heart ; Arteries ; Veins ; Capillaries ; Circulation of Blood ;
Nervous System ; Spinal Cord ; Brain ; Cranial and Spinal
Nerves ; Sympathetic System ; Organs of Special Sense . 38-72

CHAPTER III.

Histology ; Blood ; Protoplasm and Cells ; Structure of Cell ;
Epithelia ; Nerve Cells and Fibres ; Muscular Tissue ; Con-
nective Tissue ; Cartilage ; Bone ; Adipose Tissue ; the Cell
Theory ; Mitosis ; Maturation of Ovum ; Spermatogenesis ;
Fertilisation ; Segmentation of Ovum ; Early stages of
Development . . . . . .73-125

CHAPTER IV.

The Rhizopoda, Amreba proteus; Structure of Amoeba; Structure
of Protoplasm ; Reproduction of Amoeba ; Groups of Rhizo-
poda ; Formation of Chalk ..... 126-137

CHAPTER V.

The Sun- Animalcule, Actinosphaerium Eichornii ; Structure of

Actinosphaerium; Reproduction; Mitosis; Encystment . 138-146

CHAPTER VI.

The Mycetozoa, Badhamia utricularis ; Plasmodium ; Sclerotium ;
Spore Formation ; Flagellulae ; Amcebulse ; Affinities to
Plants . . 147-153

CHAPTER VII.

Monocystis agilis andjMonocystis magna ; Structure ; Conjuga-
tion ; Sporoblasts, Pseudonavicellae and Sporozoites ;
Parasitism . . . . .154-159

xiii

xiv CONTENTS

CHAPTER VIII.

The Flagellata, Euglena viridis ; Structure ; Nutrition ; Repro-
duction ; Mitosis ; Encystment. Bodo saltans ; Structure,
Conjugation, and Reproduction. Polytoma uvella ; Structure ;
Nutrition ; Reproduction ; Exhaustion following on Con-
tinuous Reproduction by Binary Division . . . 160-177

CHAPTER IX.

The Volvocinse. Pandorina morum, Eudorina elegans, and Vol-
vox globator. Macrogametes and Microgametes ; Somatic
and Generative Cells ; Parthenogonidia ; Relationship of
the Flagellata . . . . . .178-186

CHAPTER X.

The Ciliata, Paramecium aurelia and caudatum. Structure,
Reproduction, and Conjugation of Paramecium. Alter-
nation of Generations ; Immortality of Protozoa . . 187-204

CHAPTER XL

The Ciliata (continued) — Vorticella monilata. Structure, Repro-
duction, and Conjugation of Vorticella. Sex foreshadowed
in Vorticella . . . . .205-214

CHAPTER XII.

Protozoa and Metazoa. Structure and Development of Zootham-
nium. Somatic and Germ Cells. Multicellular Organisms.
Evolution of Metazoa . . . . .215-220

CHAPTER XIII.

The Ccelenterata, Hydra fusca and Hydra viridis. Structure of
Hydra. Ectoderm, Mesogkea, and Endoderm. Nematocysts.
Reproduction and Development of Hydra . . . 221-240

CHAPTER XIV.

Obelia geniculata. Structure of the Colony. The Medusa, its
Structure and Development. Comparison of Medusa '
and Polype. Sexual Development of Obelia. Theory of
Individuality ....... 241-254

CHAPTER XV.

Classification. Phylum, Class, Order, Family, Genus and
Species. System of Linnaeus. Genealogy of Animals.
Progress in Classification ..... 255-258

LIST OF ILLUSTRATIONS

PAGE

Fig. I. Skull and Vertebral Column of Frog ... 25

,, 2. Shoulder Girdle of Frog . .... 29

,, 3. Fore Limb of Frog ..... 31

,, 4. Hind Limb of Frog ..... 34

» 5- Typical Fore and Hind Limbs of Pentadactyle Vertebrate 36

,, 6. Transverse Section of Body of Frog • • • 39

,, 7. Heart and Truncus Arteriosus of Frog ... 47

,, 8. Arteries of Frog ...... 50

,, 9. Venous System of Frog ..... 53

,, 10. Spinal and Sympathetic Nerves of Frog . . . 59

,, II. Brain and Cranial Nerves of Frog ... 64

,, 12. Right Membranous Labyrinth of Frog's Ear . . 70

,, 13. Red and White Blood Corpuscles of Frog and Man . 74

,, 14. Stratified, Pavement, Ciliated, and Columnar Epithelia. 84

,, 15. Sensory Epithelia ..... 89

,, 16. Nerve Ganglion Cells and Nerve-Fibre ... 91

,, 17. Unstriped Muscle-Fibre ..... 95

,, 1 8. Embryonic Striped Muscle- Fibre and Cardiac Muscle-
Fibre ....... 98

,, 19. Areolar Connective Tissue . . . .100

,, 20. Hyaline Cartilage ..... 102

,, 21. Structure of Bone ..... 105

,, 22. Adipose Tissue . . . . . .108

,, 23. Diagrams of the Essential Phenomena of Mitosis . 113

,, 24. Spermatogenesis in Ascaris megalocephala . . 119

,, 25. Maturation of the Ovum in Ascaris megalocephala . 121

,, 26. Segmentation of the Frog's Ovum . . . 123

,, 27. Sections through segmenting Ovum of Frog . . 124

„ 28. Amceba proteus, and Nuclear Division in Amoeba crystal-

ligera . . . . . .128

,, 29. Actinosphoerium Eichornii . . . 139

xvi LIST OF ILLUSTRATIONS

Fig. 30. Cysts and Mitosis in Actinosphrerium
,, 31. Plasmodium and Spore Cases of Badhamia
,, 32. Free Individuals and Cyst of Monocystis agilis .
, , 33. Conjugation of Monocystis agilis
,, 34. Structure and Reproduction of Euglena viridis .
,, 35. Mitosis in Euglena .....

„ 36. Bodo saltans, Structure, Conjugation, and Spore Forma-
tion .......

,, 37. Polytoma uvella, Structure and Reproduction .

,, 38. Pandorina morum .....

,, 39. Eudorina elegans .....

,, 40. Volvox globator ......

,, 41. Paramecium caudatum .....

, , 42. Stages in the Conjugation of Paramecium

,, 43. Diagrams of the Course of Conjugation in Paramecium

caudatum and Colpidium colpoda . . . zoo

,, 44. Vorticella monilata . . . . 207

,, 45. Stages in the Conjugation of Vorticella . . .211

,, 46. Diagram of the Course of Conjugation in Vorticella

monilata . . . . . .213

,, 47. Hydra fusca and Hydra viridis .... 222

,, 48. Longitudinal Sections of Body and Tentacle of Hydra.

Nematocysts ...... 225

,, 49. Isolated Cells from Hydra fusca .... 229

,, 50. Development of Hydra ..... 239

,, 51. Colony, Medusa and Enlarged Bases of Tentacles of

Obelia geniculata ..... 242

,, 52. Longitudinal Section through a hydranth of Obelia

geniculata ...... 244

,, 53. Development of the Medusa in Podocoryne carnea . 247

COMPARATIVE ANATOMY

INTRODUCTION

IT is necessary, even in the study of Natural Science, to have
something of the nature of a creed, an abiding belief in some
fixed principle, which may regulate and give coherence to the
mass of information and ideas which we accumulate in the
course of our studies. Without such a belief to guide us
we embark on our journey of investigation without rudder,
compass, or pilot.

In those sciences which occupy themselves with the explana-
tion of the existing order of things on the earth, the funda-
mental beliefs are, firstly, that the course of Nature has been
uniform — that is to say, that in past times the same forces have
operated, possibly with varying intensities, but always in the
same manner as those which are in operation to-day. Secondly,
that that which is, is the outcome of that which has been, and
is the forerunner of that which will be. The history of the
world and its inhabitants presents itself to our imaginations as
an unbroken series of successive states, each differing somewhat
from its predecessor and successor, but as truly derived from
its predecessor as the child is derived from its parent, and
as truly the antecedent of its successor as the parent is the
antecedent of the child. This belief is what we express by the
word Evolution.

The Science of Comparative Anatomy is founded on these
principles. It believes that living things, ever since their first
appearance on the earth, have been essentially the same as
they are now, and that the concourse of living animals which
now peoples the globe is the result of a long-continued process
of evolution, reaching far back into geological time. The as-
semblage of animals at any given period of the earth's history,
with all its diversities of form, habit, and structure, was directly
descended from the somewhat different assemblage of the pre-
ceding period, and, in its turn, gives rise to the assemblage
of the period next following, just as one generation of men

2 COMPARATIVE ANATOMY

succeeds another, every generation differing somewhat, both
collectively and in the individual characteristics of its com-
ponent members. In fact, the successive generations of
mankind are but a part of the successive generations of living
beings ; and, whilst we use them as an illustration, we are only
specifying a part of that universal evolution which it is our
business to trace more closely.

Whilst these principles of uniformity and evolution must be
our guides through the very intricate paths of the study of
animal organisation, it must not be supposed that they are
obvious and elementary truths, which can be arrived at by
a priori reasoning. On the contrary, they have been attained
to only as the result of an immense amount of study and reflec-
tion on the details of animal organisation, and they cannot be
apprehended without a very considerable amount of study and
reflection on the part of every individual. It is the object of
the present course to justify our belief in these principles by
the study of just so much of the structure and development of
certain animals as will suffice to bring conviction to our minds.

Superficially, the animal world seems to be in a condition
of comparative stability. The least observant person already
recognises in early childhood a considerable number of kinds
of animals. The individuals of each kind are approximately
like to one another, they come and go, are born and die, but
they leave behind them progeny which continues to exhibit
the same characteristics of form and structure, and we learn
from books that these same kinds of animals with which we
are familiar, have existed with these same characteristics ever
since man began to describe the natural objects by which he
is surrounded.

Daily experience and written record, therefore, combine to
impress upon our minds the notion of the fixity of the form
and structure of the different kinds of animals. The fixity is
not absolute, for we recognise individual differences between
animals of the same kind, yet these differences are so small
and the likenesses so great that we have no hesitation in
describing animals having such and such characters as being
of one kind, or, as zoologists say, of one species.

We observe then that the earth is tenanted by countless
species of animals, that each species remains constant and has
remained constant as far back as human records go, yet we

INTRODUCTION 3

are asked to believe that this constancy, so evident to our
experience, is a delusion, and that change is the rule in the
animal world. If we come to think of it, change is the rule,
even among individuals. The individual man passes from
infancy to childhood, from childhood to youth and manhood,
and finally to senility and death. All these stages in his
career mean change, and we recognise the same changes as
occurring in animals. Further reflection assures us that even
during the stages when the body seems to have arrived at a
condition of stability of some duration, there is daily change
going on. A man gains or loses in weight according to the
amount of work he does and the amount and form of nourish-
ment that he takes. It is a familiar fact that if we would
do an increased amount of work we must take a larger amount
of food, that if we do less work we must take less food, or pay
the penalty of becoming stout. The same thing applies to
the animals which we domesticate and compel to work for us,
and though we cannot follow the steps so closely in the case
of wild animals, we have no doubt that they too are subject
to the same conditions.

In short, animal life is the outcome of a never-ceasing series
of chemical changes. An animal is constantly taking in
substances as food, converting these substances into the living
material of its own body, and then, when it exhibits energy
of any kind, this living material undergoes chemical change,
is broken down into substances of comparatively simple
chemical constitution, which, being of no further use in the
body, are forthwith expelled as excreta.

This condition of constant exchange of material is character-
istic of all living things, whether animal or vegetable, and marks
them off sharply from dead or inanimate objects. A living
organism is constantly manifesting energy, whether in the form
of motion or of heat. The source of this energy is chemical
change in the constituents of its body. These constituents,
of complex and unstable chemical constitution, enter into
combination with the oxygen of the air, are broken down into
substances of less complex and more stable constitution, which
are ejected from the body, and if the loss thus occasioned
were not made good the body would waste away. The loss
is made good by food, which is converted into new living
tissue to be broken down afresh ; and so the process goes on.

4 COMPARATIVE ANATOMY

During infancy and youth the gain by food is greater than the
loss, and so the animal grows ; during maturity the income
and expenditure are practically balanced. In old age the
expenditure is in excess of the income, so that decay sets in
and the organism finally perishes.

It perishes, but not entirely, for it has the capacity, which
is another great characteristic of living things, of reproducing
its like. On reaching maturity a part of it, generally an
infinitesimal part, is separated : this part lives, absorbs
nourishment and grows into a new organism which resembles
in every essential particular the parent form from which it was
separated. It is in virtue of the resemblance of the offspring
to the parent, commonly called heredity, that species retain
that fixity of form and structure to which we have alluded.

These facts are sufficiently familiar to all, but the con-
sequences of them are very seldom understood. The fact
that food is necessary, both to animals and plants, makes
them dependent on those external physical conditions which
are generally summed up in the term environment. Plants
are as much, and even more dependent on their environment
than animals, but their relations to it are quite different.

A plant is able to obtain its food in a manner which is quite
impossible to animals.* The substance of which all living
things, both plants and animals, are composed is known as
protoplasm. We shall study it more closely hereafter, and
need only state here that it is an albuminous body of great
chemical perplexity, composed of the elements Carbon,
Hydrogen, Oxygen, Nitrogen, Sulphur, Phosphorus. It is
obvious that an organism, in order to repair the material of its
body must be supplied with all these elements. Plants
are able, in virtue of the property possessed by their green
colouring matter, chlorophyll, to obtain their carbon from
the minute quantities of carbon dioxide present in atmospheric
air. So part of their food is gaseous. The other constituents
they obtain in solution in water, by means of their roots, so that
the remainder of their food is absorbed in a liquid form. If
a plant is supplied with a weak solution of certain mineral
substances, and is kept in the air and in sunlight (for
chlorophyll is only active in sunlight), it is able to construct
protoplasm out of these simple mineral and gaseous com-
* With the few exceptions to be mentioned later.

INTRODUCTION 5

pounds. But no animal can do this. An animal supplied
with the substances suitable for plant food would starve and
perish from inanition as quickly as if they were withheld from
it. It must have food already elaborated, in the first instance
through the agency of plants. It requires, in addition to
water, proteid or albuminous substance whether derived from
vegetable or animal sources, starches and sugars, and fats.
These are solid substances which the animal takes into itself,
or, as we say ingests, and elaborates within its body into the
more complex substance of living tissue.

It is by reason of this fundamental difference in nutrition
that plants and animals differ so widely from one another in
structure. The plant, dependent on gases and liquids for its
food, seeks to expose as much surface as possible to these
media. It spreads its leaves in the air, and thus presents a
great surface for the assimilation of carbon from the carbon
dioxide of the air. Its roots penetrating into the earth seek
out the water which, precolating through the soil, has
dissolved and holds in solution the mineral matters necessary
to its sustenance. Fixed thus in the earth the plant is
independent of locomotion, of sense organs, of means of
capturing and swallowing its food, and, though it is true that
some of the lowest plants are locomotory, the proposition just
stated holds good for the vast majority of the members of the
vegetable kingdom.

Animals, on the other hand, are under the necessity of
moving in search of their food. They require the means of
recognising it, of seizing and swallowing it, and since they do
take it into themselves or swallow it, they require some sort of
cavity or receptacle in which it may be contained and further
elaborated. Clearly, then, the relations of an animal to its
environment are far more complex than those of a plant ; and
they are the more complicated because animals stand in every
kind of relation to one another, and are endowed, as the case
may be, with special powers of offence or defence, with means
of protection, concealment, attraction, and even of deception.

Looking at an animal from a general point of view, and
studying its necessities to some extent in the light of our own,
we may enumerate the several activities indispensable to its
existence.

Each of these activities is effected, in higher animals like

6 COMPARATIVE ANATOMY

ourselves, through the means of a special living mechanism,
and each such mechanism is called, in technical language, an
organ. An organ is not, as a rule, a simple homogeneous
substance of like material throughout, but is a composite
structure, formed of special kinds of living material, and each
kind of living material entering into the composition of an
organ is called a tissue.

Since an animal is locomotory, or at least endowed with the
power of movement, we may expect to find a special set of
motile organs or tissues, and we find them in muscular tissue.
The substance commonly known as flesh is muscular tissue.
It exhibits in an eminent degree the phenomenon of con-
tractility— by which is meant the power of decreasing in one
dimension whilst it increases in another. There must be no
confusion between the vital muscular contraction and the
physical phenomenon known by the same name. Put the
poker in the fire, and it will expand as it becomes heated ;
remove and it will contract as it cools. In expansion and in
contraction its bulk increases or diminishes in every dimension,
and that to so slight a degree that special means are required
to measure the change. But nobody ever heard of a poker
suddenly, as the result of a shock or blow maybe, diminishing
in length to a very appreciable amount, and at the same time
swelling up in the middle, so that its bulk remained the same,
but its shape altered. This, however, is what a muscle does,
and everybody can observe this property in the biceps muscle
of his own arm.

Muscles contract with considerable force, and, by pulling or
pushing on or against some fixed object, effect movements of
the animal's body. In many animals they are attached to a
system of levers, which form a part of what is known as the
skeleton of the animal. In ourselves, and in animals allied to
us, these levers are internal, and clothed by muscle ; but in
many other animals such as crabs and lobsters, spiders and
insects, the hard moveable parts are outside and the muscles
inside. The skeleton, when present, is a very important part
of the animal economy, and besides affording a point (Fappui
for the muscles, it gives support to the whole body and pro-
tects some of the delicate organs from injury. Skeleton and
muscles taken together form far the greater part of the bulk
of the higher animals.

INTRODUCTION 7

An animal ingests bulky food in a crude state ; before this
food can be made serviceable, and can be converted into
actual living matter, or stored up in some form in which it
may be serviceable hereafter (e.g. in the form of fat), it has
to undergo a process of chemical elaboration. The great
majority of animals have a special aperture for taking in food,
the mouth. This is very commonly furnished with special
mechanisms for seizing and comminuting the food — viz. with
jaws and teeth. The mouth leads into a more or less
capacious tube, the gut, in which the process of elaboration
goes on. The gut is a sort of laboratory, and is provided with
the necessary stock of chemical solvents poured into it from,
special receptacles, the glands, with which it is abundantly
provided. The substances poured in by the glands are known
as secretions : they act chemically upon the food, rendering it
soluble and capable of passing through the relatively thin walls
of which the gut is composed. Those parts of the food which
are insoluble are expelled, usually through a special aperture,
the vent or anus. The food, altered and rendered soluble,
passes through the walls of the gut. It has to be distributed
to all parts of the body — many being situate at some distance
from the gut. To this end we find a mechanism for distribu-
ting the nutriment to all parts, commonly in the form of a
system of tubes, containing a fluid in which certain solids are
suspended. There are two such fluids in ourselves, the blood
and the lymph ; each with its proper system of channels and
vessels. By the blood and the lymph the nutriment is con-
veyed to every tissue, and in the tissues themselves it under-
goes those further changes which may be described as trans-
formation into living substance of unstable composition and
high potential energy. The blood is kept in movement by
the contractions of a hollow muscular sac, the heart. The
blood, however, is much more than a carrier of nutriment.
The complex living substance liberates energy in undergoing
further chemical change, on breaking down again into simpler
substances, and this change is at the bottom a process of
oxidation. A supply of oxygen is therefore necessary in every
part of the body, and this is provided for by a special respira-
tory mechanism. There are various kinds of respiratory
mechanisms, but they are all alike in this respect, that they
afford a means of interchange between the gases dissolved in

8 COMPARATIVE ANATOMY

the blood and those contained in the air (or, in the case of
aquatic animals, air dissolved in water). In ourselves the
respiratory mechanism consists of a pair of sacs, the lungs,
with thin walls in which exceedingly fine blood-vessels ramify.
By means of special muscular arrangements the lungs are
alternately expanded and compressed like bellows ; air rushes
in and out, and in the cavities of the lungs it is separated by
such very fine membranes from the blood that exchange of
gases readily takes place. The blood absorbs oxygen from
the air and carries it to the tissues ; the oxygen combines with
tissue materials : these, being oxidised, break down into simpler
substances and liberate energy. The simpler substances must
be got rid of, or they would accumulate and clog the machine.
The blood is again the agent for carrying away waste matter.
The principal waste matters are carbonic acid, water, and nitro-
genous bodies — e.g. urea. Carbonic acid, a gas, is got rid of
by the lungs, water by the lungs, the skin and the kidneys, the
nitrogenous bodies by the kidneys. Thus we find that there
are special mechanisms for getting rid of waste matters, or
excreta, as they are called, and we recognise the skin and
kidneys as excretory ; the lungs are in part excretory, but
they also serve for introducing oxygen into the system.

Thus there is a continual exchange of material going on in
the animal body. Fresh material is coming in, waste material
is going out, and the blood is the medium of exchange. It
carries the fuel to the places where it is wanted, and removes
the ashes ; it also carries the torch, in the shape of oxygen,
which kindles the flame and burns up the material. The whole
series of exchanges is expressed by the word metabolism.
We can distinguish two phases of metabolism, the building-
up processes from food to living matter, called anabolism ;
the breaking - down processes, with formation of carbonic
acid, water, and urea, called katabolism. The several organs
which we have spoken of are not always in action, nor do they
act at haphazard. On the contrary, they perform their functions
when and as they are required, and that in a definite order
and sequence. They work harmoniously, not each organ for
its own ends, but all combining for the good of the whole
individual of which they form the parts. This co-ordination
of functions is effected by the nervous system, which may be
likened to a great telegraph system with a central office from

INTRODUCTION 9

which wires pass to every part. The wires are the nerves, the
head office is the central nervous system ; in ourselves the
brain and spinal marrow. There are wires for bringing
messages in (afferent nerves) and wires for taking messages
out (efferent nerves), and messages are continually travelling
along them. News comes to the central office of some change
or occurrence without or in some part of the body, and
forthwith messages are flashed out to the appropriate parts,
ordering muscles to contract, glands to secrete, etc. But how
are the messages sent? All living substance is to a certain
extent irritable — that is to say, it responds to a shock or
stimulus of any kind, and this response is not the passive
result of pull or push, but an active manifestation of energy,
often disproportionate to the amount of the disturbing in-
fluence. Nervous tissue is the irritable tissue par excellence,
and stimuli are conveyed to it by means of the organs of
special sense — viz. touch, sight, hearing, smell, and taste.
Touch organs are distributed all over the skin, in the form of
minute bulbs, each connected with a fine nerve fibril. The
eye is the organ of sight, and it consists essentially of end
organs of extreme minuteness, which are capable of being
affected by luminiferous undulations of aether. The bulk
of the eye in ourselves is made up of arrangements for
focussing the light on the actual end organs. The ear
similarly has end organs, which are affected only by sound
waves, and in addition, is an apparatus for communicating the
vibrations of the air to the end organs. Smell and taste are
localised patches of end organs of similar kind. In addition,
we may speak of a muscular sense, and a temperature sense.

Through these channels comes all the information that we
have of the outside world, and, in addition, a great deal of
information comes to our brains about the working operations
of our own bodies, but this information is not recorded in our
consciousness and we are not aware of its being sent. The
brain, however, does the needful, and sends out the messages
appropriate to keeping up the working of the machine.

The various organs must not only work together, but must
also be held together, and they are bound up and connected
by a special form of tissue appropriately called connective
tissue. Moreover, every free surface of the body and its
organs is covered by a protecting membrane known as an

io COMPARATIVE ANATOMY

epithelium, or if it is an internal surface an endothelium, of
which more hereafter. Finally, there is the reproductive
system. The animal casts off a minute and formless part of
its substance, and this part under appropriate conditions
grows, assumes the shape and structure of the whole form
which it was cast off, and becomes a new animal, like to its
parent. Reproduction is a complex process which will be
better understood at a later stage.

An animal, then, of a degree of organisation comparable to
our own, has muscular, skeletal, digestive, glandular, blood-
vascular, respiratory, excretory, nervous, sensory, and repro-
ductive organs, each performing its special function, and all
working together to the ends of the individual. The detailed
study of the functions of the animal body, and the exposition
of the mechanisms and chemical actions by which those
functions are performed belongs to the science of animal
physiology. We are only concerned with the materials of
which the organs are constructed, what are their form and
composition, how they are combined, and how the organs are
fashioned and put together in different kinds of animals. In
short, the comparative anatomist is concerned with the archi-
tecture of the building, and gives but a passing heed to the
uses to which the parts of the building are put.

Comparative anatomy, then, is the science which treats of the
architecture of animals, and we shall see that just as there are
several styles of architecture, Classical, Byzantine, Gothic, etc.,
each with its subdivisions, as Gothic into Pointed, Perpen-
dicular, Decorated, etc., so there are several styles of animal
architecture, and in each style there is infinite variety, though
the general plan, the " motive," is the same.

But even as all buildings resemble one another in the most
general way because, whatever their style, they are put to
similar uses, so do all animals resemble one another in a very
general way because, being animals, they have similar functions
to perform, and those functions are performed by analogous
organs.

Not by any means, however, by organs which have any
degree of exact similitude. Let Us compare two or three
familiar kinds of animals with one another ; a dog, a fish, and
a lobster. The fish (say, a cod or pike) and the lobster are
both aquatic animals, and it might be supposed that they

INTRODUCTION n

would more nearly resemble one another than do the fish and
the dog. But the slightest analysis of their structure will show
that they do not.

The fish has a distinct head provided with jaws, and in the
jaws are teeth. On its snout are the openings of the organs of
smell, usually called olfactory organs, and they have the form
of a pair of pits with external apertures. Behind them is a
pair of eyes, placed in sockets, moveable by muscles fixed on
a definite hollow eyeball. Behind the eyes we find, embedded
in the skull, a pair of organs of hearing — the auditory organs.
All these structures are present in the dog also, and though
the details differ largely, the general arrangement is the same.
The muscles which move the eyeballs are almost identical.
In both the dog and the fish the head is largely formed of a
bony and gristly case or box, which contains a mass of nervous
structure — the brain. The fish has no distinct neck, the head
passing without any distinct intervening region into the trunk
or body. The dog has a neck. Both the fish and the
dog have a distinct backbone, or, as anatomists call it, a
vertebral column, composed of numerous short bony joints
or pieces, arranged in a row down the middle of the back,
ending behind in a tail and jointed in front to the brain-case
or skull. And we find that in both dog and fish there is a
little arch placed on the top of each joint of the backbone,
and in the canal formed by all these arches there lies a long
nervous cord, the spinal marrow or spinal cord, which is
directly continuous in front with the brain. The dog has two
pairs of limbs used for walking and running, the fish has two
pairs of fins used as aids in swimming. The fins are not at
all like the dog's legs, but they occupy the same relative posi-
tions, and are comparable to some extent. Other noticeable
features show considerable differences between dog and fish.
The dog has a coat of hair covering its body — the fish a coat
of scales. The dog breathes air by lungs, the fish breathes
water by gills, which are really a series of apertures passing
from the throat to the exterior, the walls which separate the
apertures being covered with folds of tissue richly supplied
with blood-vessels. A legion of other characters, some of
similarity, some of difference, between dog and fish might be
enumerated, but enough has been said to show that, different
as dog and fish are, they do resemble one another in funda-

12 COMPARATIVE ANATOMY

mental points of structure; they belong in fact to the same
style of architecture.

Now, consider the lobster. To begin with, it has a hard
outside armour (often but erroneously called its shell), but
neither true hairs nor scales. Break open the armour, and
you find no bones inside ; no skull, no vertebral column.
The foremost end of the lobster may be called its head,
but how different to the head of the dog or fish ! In the
first place it is fused with, and can scarcely be distinguished
from, the breast or thoracic region. You look in vain for a
nose, an olfactory organ. There are two pairs of long jointed
feelers borne in front of the head region, and these have
numerous bristle-like structures, some of which are considered
to serve as olfactory organs, but how different are they from
the similarly named organs in dog and fish ! The " ear " or
auditory organ of the lobster does not lie behind the eyes, but
is placed on the basal joint of the first pair of feelers, and is
not at all like our ear. The eyes of the lobster have no eye-
ball, are placed on a pair of moveable stalks, and each is com-
posed of a number of minute facets, very strikingly different
from the arrangement of the dog's or fish's eye. The jaws of
the lobster do not work up and down, but from side to side ;
there are several pairs of them, and a little study convinces us
that they are nothing more than so many modified pairs of
limbs. The lobster walks and swims. It walks by four pairs
of jointed legs, in front of which a quite similar pair of limbs
is modified to form the great nippers or claws. The posterior
region of its body is composed of hard rings, which move on
one another. On the under surface of these rings are as many
pairs of little limbs used in swimming, the last pair being
large, expanded, and forming with the last joint of the body, a
posterior expansion used in vigorous swimming movements.
Lastly, we find that there is no skull with its enclosed brain in
the lobster, and no vertebral column and spinal cord, There
is a little nervous mass in front of the mouth, but posteriorly
the nerve cord is on the lower instead of on the upper side of
the body. The lobster does breathe water by structures called
gills, but these are quite unlike the gills of the fish, and are
simply bottle-brush-like appendages borne on the body-wall
or on some of the legs, and covered over by a fold of the
thoracic region of the body-wall. In so far as the jaws, the gills,

INTRODUCTION 13

and the limbs of the lobster perform the same kinds of func-
tions as in the fish, they may be said to resemble one another.
But the resemblance is only one of use and function, not of
form, and organs which perform similar functions whilst differ-
ing in plan in different animals are said to be analogous.

On the other hand, the eyes, skull, backbone, and numerous
other parts of the dog and fish clearly resemble one another
in plan of structure. Such organs are said to be homologous.

It is most important that the student should learn to distin-
guish between organs which are merely analagous and those
which are homologous. Comparative anatomy is largely con-
cerned in establishing homologies between organs which often
differ considerably from one another in superficial aspect and in
function. In these days we consider organs to be homologous
when we can produce evidence to show that they are modifica-
tions of some pre-existing organ belonging to a remote ancestor
from which all the animals possessing the organs in question
have descended. The nature of this evidence, and the reason-
ing which enables us to assert that the identity in the
architectural plan of certain animals is due to their having
inherited their leading features from a common ancestor,
whilst varying the details in an almost infinite degree, will
become apparent in the course of this work.

We must now pass to the more detailed consideration of
a single animal type, and learn from it what is the structure
of the organs about which we have been talking, and what
relative positions they hold to one another. As we are more
familiar with the general structure of our own bodies than that
of other animals we should do best if we took the human
subject as a type ; but the study of human anatomy is in
many ways inconvenient, and we shall take as our type the
common frog, an animal easily obtained, of a size convenient
for dissection and sufficiently like the human subject to
enable us to learn all the lessons necessary for the elementary
comprehension of our subject.

Let it be understood here that the student will make no
progress in the study of comparative anatomy unless books and
lectures are supplemented by thorough painstaking practical
work. No words that ever were spoken or written, no draw-
ings or diagrams, however artistic or simple, can ever bring
home to the mind the structure of the animal body without

i4 COMPARATIVE ANATOMY

dissection and microscopical study. The student will find
full instruction for the detailed examination of the common
forms of animal life dealt with in this book in the excellent
practical manuals, * published by the late Professor A. Milnes
Marshall. As this work is intended to supplement, not to
supersede, practical work carried on by aid of these books,
attention will be given rather to the lessons which may be
learned and the conclusions which may be drawn from
anatomical and embryological facts than to the detailed ex-
position of the facts themselves. The frog, however, being
taken as a type of animal structure, will be described in
detail.

We must bear in mind, before we embark on our practical
studies, that the object of our practical and literary studies in
Zoology is to furnish the mind with clear ideas on animal
structure and organisation, and to this end our ideas must be
arranged in an orderly and methodical fashion. We must
accustom ourselves, from the outset, to think of the organs
and tissues of the animal body in a definite order, so that
they may readily be called up to our minds without haziness
or omission. The order in which we shall deal with the
anatomical facts exposed by dissection will not necessarily be
the same as that in which they are discovered in the course of
practical work. In any animal of complex structure the
various organs are so interwoven with one another, and in
such different manners in different types of organisation, that
it is generally necessary to take into account several systems
of organs in the course of any piece of dissection. Moreover,
it will frequently be necessary to cut away and destroy one
organ in order that another may be exposed. Hence the
lessons learned from dissection are apt to confuse us at first
unless we deal with them in a methodical manner, keeping
our attention fixed upon one set of organs at a time, and
eventually constructing a plan of that system from the results
obtained by our various dissections. But, whilst we deal with
organs and systems of organs in a methodical manner, we

* THE FROG : an Introduction to Anatomy, Histology and Embryology.
By A. Milnes Marshall. Sixth Edition. Edited by G. Herbert Fowler.
London : D. Nutt. 1897.

A JUNIOR COURSE OF PRACTICAL ZOOLOGY. By A. Milnes Marshall
and C. Herbert Hurst. Fifth Edition. London : Smith, Elder & Co.

INTRODUCTION 15

must remember that we do so for the sake of memory and
of precision ; we must by no means shut our eyes to the fact
that the relations of one system to another, as we discover them
in the course of dissection, are of at least as much importance
as the plans of those systems considered apart from one another
and the rest of the body. Our object, then, is twofold : to get
an accurate idea of each system of organs considered by itself,
and to form a conception of the manner in which those
systems are united together to make up the sum-total of the
organisation of the animal in question.

It is perhaps the most convenient plan to consider the
different systems of organs in the order in which they make
their appearance in the course of development, or, to speak
more accurately, according to the germinal layer from which
they are developed, for by so doing we shall bring together the
results of anatomical and embryological study, and make the
one support and illustrate the other. But, as the study of the
adult anatomy of a vertebrate type must precede that of its
development, we shall begin by describing the organs of the
frog in the order in which they are most conveniently studied
in the course of dissection.

CHAPTER I

THE COMMON FROG-RANA TEMPORARIA AND
RANA ESCULENTA

THE frog commonly found in pools and ditches in England
is the brown or grass-frog, Rana temporaria. A larger
form, more abundant on the Continent than in England, is
known as the edible frog, Rana esculenta. A third species,
intermediate between the two, has been distinguished under
the name Rana oxyrhinus. The differences between these
three common species of European frogs are unimportant, and
may be neglected at this stage of our studies.

In a frog belonging to any of these species the following
external characters may be noted without dissection. The
body is divisible into two regions, the head and trunk.
There is no neck, the head passing without any distinct break
into the trunk. There is no tail. P'urther, we may recognise
sides and surfaces which correspond to those of our own
bodies and are similarly named. There is an upper surface,
corresponding with our back, which we shall call the dorsal
surface ; an under side or ventral surface ; an anterior end
which indicates the direction in which the animal moves, and
a posterior end opposite to it. The attachments of the two
pairs of limbs mark the right and left sides of the body.

The whole surface of the body is covered by a smooth
moist skin of a greenish yellow hue, mottled on the dorsal
surface with dark, almost black patches. The ventral surface
is generally yellow.

Frogs have the power of varying their colour within certain
limits, so that the same specimen may present very different
appearances under different circumstances. The changes of
colour enable the frog to approximate its colour to that of
surrounding objects, and thus to conceal itself in some measure
from its enemies. The mechanism by which the changes are
effected lies in the structure of the pigment bodies of the skin,

16

THE FROG 17

which are under the control of the central nervous
system.

The frog differs much from man and all " beasts," from
birds, lizards, snakes, tortoises, and fishes, in the fact that its
skin is devoid of hair, feathers, or scales, and there are no
claws or nails on the toes and ringers. This, however, whilst
true of English frogs, does not hold good for all frogs, nor for
the class Amphibia to which the frog belongs. The bull-frog,
Ceratophrys, for example, has flat bony plates in the skin of
the dorsal surface. There are only two median openings on
the surface of the body — viz. the mouth, a large slit-like
opening situated at the anterior end of the head, and the anus
or vent (more strictly called the cloacal aperture) which lies at
the hinder end of the trunk, between the hinder legs, and
nearer to the dorsal than the ventral surface. There is but one
set of paired apertures — viz. the external nares or nostrils,
a pair of small holes placed upon the snout at the anterior end
of the head. A bristle passed through one of the nostrils will
be found to pass into the cavity of the mouth by an internal
nostril. On either side of the head is an eye, which has an
eyeball, a coloured portion, or iris, and a pupil, as in ourselves.
Each eye is protected by two eyelids, of which the upper
is thick, pigmented like the rest of the skin, and nearly
immovable; the lower is thin, semi-transparent, and freely
movable.

Just behind the eye, on either side of the head, is a patch of
dark colour, in the middle of which a circular membranous
area, bounded by a firm, somewhat raised, ring, can be seen.
This is the tympanic membrane, or drum of the ear. The
frog has no external valve of the ear as have man and beasts,
nor has it a passage leading down from the exterior to the
ear-drum ; the latter is on the surface.

The frog's limbs have a general correspondence with our
own, as may be seen at a glance. The front limb or arm is
divisible into an upper-arm, technically called the brachium, a
fore-arm or ante-brachium, and a hand or manus. The wrist, or
carpus, is hardly distinguishable externally, but we shall learn
more of it when we study the skeleton.

The hind limb comprises a thigh or femur, a leg or cms, a
much elongated ankle or tarsus, and a foot or pes.

On the hand there are four fingers or digits corresponding

1 8 COMPARATIVE ANATOMY

to the four fingers of our own hand. A rudiment of a thumb
is present, but it is very small and hidden under the skin.
Anatomists count the fingers from thumb to little finger, and
sometimes distinguish them by the following names : i. Pollex,
2. Index, 3. Medius, 4. Annularis, 5. Minimus. In the
male frog, the index develops a rough, cushion-like swelling in
the breeding season. The fingers of the frog are not webbed.
Though the foot has only five apparent toes a close inspection
reveals the presence of six. The first, counting from the
inner side, is extremely small, and almost hidden beneath the
skin. It is a moot point whether it should be regarded as the
representative of the great toe or hallux of man, or whether it
should be regarded as an accessory toe, not represented in man.
It is usual to consider the toe next to it, the shortest of the
remaining five, as the great toe. Of the five apparent toes the
fourth is the longest : they are united by a membranous ex-
pansion, and are said to be webbed. The frog's mouth is
furnished with distinct jaws ; the upper jaw is a forward con-
tinuation of the head, and is not independently movable ; the
lower jaw is hinged on to the posterior region of the head, and
is freely movable in a vertical direction. Jaws of this kind,
opening vertically, are only found in vertebrates — that is, in
animals which have a back-bone, and in them only in the
division (which comprises nearly the whole group) known
as the Gnathostomata. The mouth is furnished with fine
pointed teeth arranged in a single row on the edges of the
upper jaw, and there are also two small patches of teeth on the
fore-part of the roof of the mouth, called vomerine teeth. There
are no teeth on the lower jaw. The toad, so like the frog in
most respects, has no marginal teeth on the upper jaw. On
opening the jaws the wide buccal cavity is seen, which narrows
posteriorly to form the gullet or oesophagus. Notice (i) the
posterior nares, to the outside of and in front of the patches
of vomerine teeth; (2) the Eustachian tubes, a pair of
relatively large apertures, one on each side of the hinder part
of the buccal cavity : each leads into a cavity, the tympanic
cavity, which is closed externally by the tympanic membrane ;
(3) the glottis, a slit-like aperture on the floor of the hinder
part of the buccal cavity : it leads through a short larynx into
the lungs; (4) the prominences on the roof of the buccal
cavity, formed by the projection of the lower sides of the eye-

THE FROG 19

balls ; (5) the tongue, thin and fleshy, attached to the front
part of the floor of the mouth with its free end, which is
" forked," projecting backwards towards the throat.

Whilst there is a general correspondence between the mouth
and buccal cavity of the frog and our own, there are consider-
able differences, of which the most important are, the absence
in the frog of mobile lips ; the marginal teeth of the upper
jaw correspond in position to ours, but they are more
numerous, are not implanted in sockets, are all alike simple
and pointed, instead of being of various shapes as ours are.
The vomerine teeth of the frog have no counterpart in man.
The posterior nares, situated right forward in the upper jaw of
the frog, are carried backwards in man by the formation of the
"roof of the mouth" or hard palate, followed by a fleshy
curtain or soft palate, so that they open far back at the
entrance to the throat. The hard palate in man also excludes
the orbits altogether from the region of the mouth. The
tongue of man, fixed at the back of the mouth, with its free end
projecting forward, is obviously different from that of the frog.

Already, after an examination of the external features of the
frog, we are able to make a comparison of its structure with
our own, and to recognise a general correspondence of plan,
with numerous differences in detail. We cannot escape from
the conclusion that the frog is built on the same principle as
ourselves ; and yet not a single organ or feature that we have
examined is exactly or even nearly the same as it is in man.
The science of comparative anatomy takes account, both of
the resemblances and the differences, and embodies the results
of its comparison in a system of classification. The frog, because
of the resemblances which we have already noted, and many
others of greater importance relating to the internal organs, is
placed in the phylum vertebrata along with fishes, reptiles,
birds, and beasts. But the differences compel us to place it
far apart from man in a class comprising many other animals
such as newts and salamanders : in this class, Amphibia,
the component members resemble one another in all their
more important characters, and differ only in those which are
relatively unimportant. But comparative anatomy does more
than this. It seeks to give an explanation of the resemblances
and of the differences, and it finds the explanation in the
doctrine of the common descent of all the animals composing a

20 COMPARATIVE ANATOMY

phylum from an ancestor which embodied all the fundamental
peculiarities of structure which distinguish the phylum from
all other phyla. But the justification of this doctrine can only
be understood after a considerable study of the internal as well
as the external anatomy of different kinds of animals.

Turning now to the internal anatomy of the frog, we will
begin by a general consideration of the principal organs and
systems of organs, without at first going into very exact details
concerning them.

The body, we have seen, is covered by a smooth moist skin
or integument, which lies rather loosely so that it can easily
be pinched up in folds without including the underlying parts.
Cutting through the skin with knife or scissors we find that it
is attached by sheets of a white glistening substance to the
flesh beneath. This white substance is called connective
tissue, and it does not tie down the skin evenly in all places,
but only along certain lines, so that large spaces are left
between skin and flesh which are filled with a colourless fluid
called lymph. The most important of these spaces, known as
lymph-sacs, are (i) a dorsal lymph-sac, extending over nearly
the whole of the dorsal surface of the head and trunk, and
separated on either side by a septum from (2) the lateral lymph
sacs, which occupy the flanks of the animal; (3) the ventral
lymph-sac, a large triangular sac extending from the breast
region over the abdomen on the ventral surface ; (4) the
pectoral lymph-sac, lying over the region of the breast ; and (5)
the submaxillary lymph-sac, occupying the ventral surface of the
head and throat. There are other lymph-sacs on the fore and
hind limbs. These large sacs are connected with other lymph-
spaces lying between the muscles and organs of the body, and
the fluid which they contain is eventually carried into the
blood stream.

After the skin has been stripped off, the body is seen to be
covered with muscle, commonly called the " flesh'" of the
animal. The use and mode of contraction of the muscles has
already been explained (p. 6). We need only notice here
that the muscles which are exposed on the removal of the skin
belong to the system known as voluntary muscles; by their
means all the voluntary movements of the body are effected.
Over the back, flanks, and belly the muscles have, for the most
part, the form of flat sheets of fleshy tissue, composed of

THE FROG 21

numerous fibres which either run parallel with one another or
diverge outwards in a fan-shaped manner from the point of
attachment to the line of insertion (e.g. the muscles of the
breast). In the limbs the muscles are more rounded or
spindle-shaped, their component fibres being gathered into
bundles which terminate at each end in a tendon. The
general character of a muscle may most conveniently be
studied in the calf-muscle or gastrocnemius of the frog. This
muscle lies on the inner surface (that surface which in man
would be hindermost) of the leg. It is stout and spindle-
shaped, tapering towards either extremity, but most markedly
towards the lower end, where it passes into a stout glistening
white tendon, the tendo Achillis. It is attached at the upper
end by a broad tendon to the bone of the thigh, and to the
upper end of the bone of the leg ; the tendon at the lower end
passes round the place where the heel would be, if such a
structure were present in the frog, and is continuous with a
broad sheet of connective tissue spread over the sole of the
foot. Such a sheet of connective tissue serving for the attach-
ment of muscle or tendon is called an aponeurosis, and this
particular one is known as the aponeurosis plantaris. The
gastrocnemius muscle by its contraction pulls upon the back
of the ankle and the sole of the foot, and tends to bring the
foot and ankle in a line with the leg. The bones to which the
upper end of the muscle is attached are not moved by its con-
traction, and this fixed end of the muscle is called its origin.
The opposite end, which is moved when it contracts, is called
its insertion. Most of the voluntary muscles are attached, at
one of their ends at least, to bones. The bones of an animal
constitute its skeleton, and afford a system of levers actuated
by the muscles, and they also form a solid framework which
supports and protects some of the organs.

We have already been able to recognise the fact that the
skeleton of the frog is internal, in this respect resembling our
own, and differing from the skeleton of such animals as lobsters
and insects, which have their skeleton on the outside.

In the frog's skeleton we may recognise two main parts, an
axial and an appendicular skeleton. There is also a branchial
skeleton, well developed in the tadpole, but reduced and of
minor importance in the adult frog. The axial skeleton, so
named because it lies along the main longitudinal axis of the

22 COMPARATIVE ANATOMY

body, consists of the skull and the vertebral column. We
will first consider the vertebral column. It is made up of ten
pieces, nine being peculiarly shaped rings of bone called
vertebrae, the tenth being a long rod-shaped piece called the
urostyle ; it is the representative of a number of fused
vertebrae.

A typical vertebra (the fourth or fifth in the frog will serve
as a type) consists of a short cylinder of bone called the body
or centrum, on the top of which is placed a flat bony arch
in such wise that the centrum below and the arch above
enclose a space through which in life the spinal marrow passes ;
hence it is called the neural arch. The bodies of the vertebrae
are jointed together, and the arches are also connected by slid-
ing joints called zygapophyses (Greek Cwyov, a yoke ; aTro^uo-i?,
a process), and so the nine arches enclose a canal, the neural
canal, or canal of the spinal cord. Examining the vertebra
more closely it will be seen that its posterior end is a rounded
knob covered with smooth white cartilage, whilst its anterior
end presents a concavity lined with cartilage. The bodies of
the vertebrae are, in fact, connected by ball and socket joints,
and the ball is on the hinder, the socket on the anterior, face
of each vertebra, excepting the first, eighth, and ninth. The
centrum of the eighth vertebra is concave at both ends ; that of
the ninth is convex anteriorly, and posteriorly it has two small
rounded processes for articulation with the urostyle. The first
vertebra will be more particularly described further on. A
vertebral centrum which is concave in front and convex behind
is called procoelous ; one which is, like the eighth vertebra,
concave both anteriorly and posteriorly, is called amphicoelous ;
and one which is convex in front and concave behind (not
represented in the frog) is called opisthoccelous (TT/X>, in
front of; oVicr^e, behind ; ap/u, on both sides ; KotAo?, hollow).
If the centrum of a vertebra is sawn across the middle it will he
found to contain a central cavity filled with a peculiar tissue.
This is the remnant of the notochord or chorda dorsalis, the
primitive skeletal rod which formed the backbone of the
embryo, and has been replaced by bone. A notochord is the
first part of the skeleton to be formed in the embryos of all
Vertebrates. In some it is persistent throughout life ; in others
it is surrounded by, or replaced by, cartilage or bone, but traces,
larger or smaller, of it are generally to be found. A word of

THE FROG 23

warning is here necessary. Since the vertebral column is
sometimes called the spinal column, or simply the spine, and
since the column of nervous tissue which is enclosed by the
arches of the vertebral column is generally called the .spinal
cord, the beginner is apt to confuse the notochord with the
spinal cord. It must be clearly understood that the nervous
structure, the spinal cord, lies above and not inside the
centrum, and that it is the notochord, the embryonic structure,
which occupies the cavity when present in the vertebral
centrum.

The arch of the vertebra (fourth or fifth) consists of a flat dorsal
expansion, united on either side to the centrum by a narrow
pedicle. When the vertebrae are jointed together in the natural
position, spaces are left between the pedicles of successive
vertebral arches through which, in life, the spinal nerves
emerge from the neural canal. From each side of every arch
a stoutish bony piece, the transverse process, projects
outwards.

Each arch is yoked to its fellows in front and behind by a
pair of articular processes, already referred to as zygapophyses.
These processes project forwards and backwards from the outer
angles of the arch. Their articular surfaces are flat, and it is
an invariable rule in the Vertebrata that the articular faces of
the anterior zygapophyses look upwards or inwards, those of
the posterior zygapophyses downwards or outwards. Thus
one can tell the anterior end of a single vertebra by mere
inspection of the zygapophyses. In addition to the processes
already detailed, each arch has a ridge running along its
median dorsal line. This is called the neural spine, and it is
much smaller in the frog than in many other Vertebrata.
The transverse processes do not all stand at right angles to the
axis of the vertebral column, but point in various directions.
There is no transverse process in the first vertebra ; those of
the second are directed downwards and forwards ; those of the
third very slightly backwards; those of the -fourth much more
backwards ; those of the fifth, sixth, seventh, and eighth stand
nearly straight out ; and those of the ninth are very large and
stout and point very markedly backwards. Each transverse
process has a little tip of cartilage called an epiphysis.

The first or atlas vertebra differs markedly from the rest.
It has a very thin centrum, much compressed from above

24 COMPARATIVE ANATOMY

downwards, and a wide ring-like arch. Posteriorly the
centrum bears an articular head, but anteriorly, instead of a
single concavity, there are two oval, concave, articular surfaces,
separated from one another by a median projection. These
concavities receive the occipital condyles of the skull. There
are no transverse processes. The urostyle is nearly as long as
the rest of the vertebral column. It is a rod-like bone with a
ridge on the dorsal surface, which is high and thick in front,
and becomes lower and thinner behind, eventually disappear-
ing altogether. The front end of the urostyle is thick and
broad, and has two concavities for articulation with the two
posterior prominences of the ninth vertebra. The anterior
portion of the ridge of the urostyle is hollow, and in life con-
tains a prolongation of the spinal cord known as the filum
terminale. The anterior end of the urostyle presents an
opening on either side through which nerves pass outwards
from the spinal cord ; and just in front of each opening a small
process may generally be found which represents the transverse
process of an ordinary vertebra. The lateral openings repre-
sent the spaces which are left between the pedicles of the
arches of the successive vertebrae of the column, and similarly
serve for the transmission of the spinal nerves. The vertebrae
are bound together by ligaments, and the whole column is
capable of a certain amount of flexion, in virtue of the articula-
tions of the centre and arches of its component vertebra. The
column, especially its neural and transverse processes, serves
for the attachment of some of the most important muscles of
the trunk.

The skull is articulated to the first vertebra by two pro-
minences, known as the occipital condyles, which fit into its
two concavities.

The skulls of all the craniate vertebrates (by which we
mean those vertebrate animals which have a distinct head)
present certain features in common, and though that of the
frog is peculiar in- some respects, it is fairly illustrative of the
fundamental features of all skulls.

A skull comprises the following parts : (i.) A brain case or
cranium proper; a cartilaginous or bony, or, as in the frog,
partly cartilaginous, partly bony, box, which contains the brain.
At the back of the brain-case is a large aperture, the foramen
magnum, by means of which the spinal cord passes into the brain.

Fig. I.

A, The skull and vertebral column of the Frog, viewed from the dorsal surface.
B, The same from the ventral surface. C, Lateral view of the urostyle ; a bristle
is passed through the foramen for the tenth spinal nerve. D, The branchial
skeleton of the Frog. (All the figures after Ecker.) O, orbital fossa; pmx,
premaxilla ; mx, maxilla; q-j, quadrato-jugal ; na, nasal; pf, parieto frontal ;
ex, exoccipital ; fni, foramen magnum ; pro, pro-otic ; sy, squamosal ; sp. et,
sphenethmoid ; par, parasphenoid ; pal, palatine ; vo, vomer ; ptg, pterygoid ;
av, atlas vertebra ; c, centrum ; ar, neural arch ; zyg, zygapophysis ; trv, trans-
verse process; ur, urostyle; H, body of the hyoid; Ha, anterior cornu ; Hp,
posterior cornu of the hyoid.

26 COMPARATIVE ANATOMY

(2.) The sense capsules, of which there are three pairs. The
olfactory capsules, which contain the organs of smell, are
situated anteriorly; the auditory or otic capsules, containing
the organs of hearing, are situated one on each side of the
posterior region of the cranium. Both the auditory and
olfactory capsules are firmly united to, and continuous
with, the cranium. The optic capsules, comprising the
organs of sight, are never united to the cranium, but pro-
foundly modify that part of the skull in which they lie.
(3.) The jaws, present in nearly all craniate vertebrates,
comprise an upper and a lower jaw. The latter is always
articulated to the skull, and falls away from it in the dried
skeleton (except in the badger and sea-otter) ; the upper jaw-
in the majority of vertebrata is firmly fastened to the cranium,
and appears to form part of it.

The frog's head is large, flat, triangular in shape, with the
blunt apex forming the snout. But the cranium is relatively
small, the large size of the head being due to the wide sweep
of the jaw-bones and the large size of the cavities for the eyes.

In the tadpole the brain case is entirely composed of
cartilage, but in the adult frog certain regions of the
cartilaginous case become ossified — turned into bone — but
the greater part remains cartilaginous throughout life.

On either side of the great hole — the foramen magnum — at
the back of the cranium, is an ossification known as the
ex-occipital. The ex-occipitals are separated from one another
by a cartilaginous piece above and below in the middle line,
but elsewhere form the border of the foramen magnum, and
they bear two cartilaginous articular heads, the occipital
condyles, for articulation with the atlas vertebra.

The roof, the sides, and the floor of the middle part of the
cranium are cartilaginous, but anteriorly it is completed by a
single bone, shaped somewhat like a dice box, one cavity of
which, the posterior, lodges the fore part of the brain ; the other,
divided into two by a median vertical partition, affords a passage
for the nerves of smell, and lodges part of the olfactory organs.
This bone, the sphenethmoid, is peculiar to Batrachia.

The ex-occipitals and sphenethmoid are the only ossifications
of the cranium proper, but there is a pair of ossifications in
the ear-capsules which are continuous with, and scarcely to be
differentiated from, the cranial box. The ear-capsules are

THE FROG 27

placed one on either side of the posterior region of the
cranium, where they form prominent projections. The
anterior and lower face of each projection presents an ossifica-
tion known as the pro-otic bone.

Anteriorly, the cartilage of the cranium is continued directly
into the cartilage of the olfactory chambers or nose ; so we see
that the central portion of the frog's skull is a cartilaginous
brain-box, with which the cartilaginous olfactory capsules are
continuous in front, and the auditory capsules at the side
and behind, and the cartilage is replaced by bone in the
ex-occipital, the sphenethmoid, and the pro-otic regions. But
the brain-box and sense organs are further protected by bones
above and below, which are not ossifications in cartilage, but
are formed in membrane as sub-dermal ossifications in the
integument covering the head. These bones over- or under-
lie the cartilaginous cranium, and can easily be stripped off
it, whereas the ossifications in cartilage cannot be stripped off.
Above, running from the occipital or posterior region of the
skull as far forwards as the sphenethmoid, is a pair of bones
called the fronto-parietals. In front of these a pair of
triangular nasals roof over the cartilage of the chambers of the
nose. Below, the whole floor of the cranium is protected by
a large dagger-shaped bone, the cross-pieces of the handle
reaching beneath the auditory capsules. This is the
parasphenoid. The floor of the olfactory region is occupied
by a pair of small irregularly-shaped bones, the vomers, which
bear each a patch of teeth.

The broad triangular skull is completed by the jaws
and the suspensory apparatus by which the jaws are
attached to the cranium. The gape of the upper jaw
is bordered on either side, by a pre-maxilla in front and
a slender maxilla behind, both bearing teeth. These two
bones are ossifications in the sub-dermal membrane and
therefore are membrane bones. The maxilla is attached to
the cranium before and behind by bony and cartilaginous
struts. From the ethmoid region of the skull, just in front
of the sphenethmoid bone, a bar of cartilage runs out on
either side. This bar is covered below by a slender bone,
the palatine. It is usual to reckon the palatine among the
"cartilage bones" of the skull because in most animals it is
preformed in cartilage. But, as a matter of fact, in the frog it

28 COMPARATIVE ANATOMY

is a membrane bone, which largely or wholly replaces the
ossification in cartilage. The same is the case with a large
three-headed bone, the pterygoid, which reaches from the point
where the palatine unites with the maxilla to the auditory
process of the skull, and gives off outwards and backwards
a process towards the articular surface for the lower jaw.
This posterior limit of the pterygoid lies beneath a rod of
cartilage which extends from the pro-otic region outwards and
backwards, and is known as the quadrate cartilage. Exter-
nally it presents a concave articular surface, the glenoid cavity
for the lower jaw; it is covered below by the process of the
pterygoid as described, and above by a T-shaped bone, the
squamosal. Finally, the posterior end of the maxilla is
connected with the outer end of the quadrate cartilage by a
small splint-shaped bone, the quadrato-jugal. It should be
noticed that the palatine and pterygoid, with the cartilage
which overlies them, form the front and exterior borders of a
cavity which is completed internally and posteriorly by the
cranium and auditory process. In this cavity the eye is
lodged, and it is known as the orbital cavity, or, more shortly,
the orbit. It is open below, so that the eyeballs are only
separated from the mouth-cavity by the integuments lining the
latter and by a thin sheet of muscle. The lower jaw or
mandible consists of two bars of cartilage, one on each side,
frequently known as Meckel's cartilages, united in the middle
line in front by ligament. Each cartilage ends in front in a
little ossification, the mento-Meckelian (cartilage-bone), and is
ensheathed by two membrane bones, the angulo-splenial,
inside and below, and the dentary covering the upper and
outer sides of the anterior half of Meckel's cartilage. The last
named widens out behind and forms the articular head which
fits into the glenoid cavity of the suspensorium.

The Branchial skeleton is very much reduced in the adult
frog, and comprises what is known as the hyoid apparatus.
This lies on the ventral surface of the throat and consists of a
broad thin cartilaginous plate, the body of the hyoid, from
which processes are given off anteriorly and posteriorly. The
anterior processes or cornua are cartilaginous and run first
forward and then upwards and backwards as two -slender
curved cartilaginous rods which are united with the cartilage
of the auditory region. The posterior cornua are bony rods

THE FROG

29

which diverge from one another and enclose between them the
larynx. The frog has no ribs, unless the little cartilaginous

Fig. 2.

A, The shoulder girdle of the Frog; the scapula and suprascapula are turned
outwards. ep, episternum ; as, omosternum ; ep, c, epicoracoids ; tnes,
mesosternum ; xi, xiphisternum ; s, sc, suprascapula ; sc, scapula ; gl,
glenoid cavity ; cor, corncoid ; cl, clavicle. B, The pelvic girdle of the
Frog, viewed from the side. //, ilium ; Isch, Ischium ; Pu, cartilaginous
pubis ; Ac, acetabulum. C, The pelvic girdle of the Frog, viewed from
below. (All the figures after Ecker.)

epiphyses at the ends of the transverse processes of the
vertebrae are to be considered as rudiments of such. But

30 COMPARATIVE ANATOMY

there is a distinct sternum or breast-bone with which the
lower ends of the shoulder-girdle articulate.

In the mid-ventral line of the breast there lies anteriorly a
flat circular plate of cartilage called the episternum. Posterior
to this is a bony rod, the omosternum. This is succeeded by
a pair of cartilaginous strips, the epicoracoids, which belong to
the shoulder girdle ; and these again are succeeded by the
mesosternum, a rod of cartilage ensheathed in bone; and
hindmost of all the xiphisternum, a flat plate of cartilage,
somewhat heart-shaped, with a bifid apex.

The shoulder-girdle proper is to be thought of as two half-
rings of bone and cartilage which encircle the fore -part of the
body, being united below in the sternum but separate above.
Each half-ring fe divided by an articular cavity, the glenoid,
into an upper or scapular and a lower or coracoid moiety.
The scapular moiety comprises an upper expanded portion
formed of calcified cartilage, the- suprascapula, and a lower
bony portion, the scapula. In the coracoid moiety we dis-
tinguish a coracoid, a stout bony rod, expanded at its two ends
and joined to its fellow of the opposite side by the interven-
tion of the epicoracoid cartilages already mentioned. In front
of the coracoid a rod of cartilage known as the pre-coracoid
connects the scapula with the anterior ends of the epicoracoids,
and this rod is ensheathed by a splint-like bone generally
described as the clavicle or collar-bone.

In the fore-limb there is one bone, the humerus, in the
upper arm. It has a shaft and two articular surfaces, one at
each end. That of the upper or proximal extremity of the
bone is called the head, and fits into the glenoid cavity of the
shoulder-girdle. The distal extremity has a rounded knob,
internal to which is a small flat articulating surface, the
trochlea, placed upon a little eminence called the internal
condyle. There is a similar external condyle on the outer
side of the articular process.

The fore-arm has two bones, the radius and the ulna, but
they are firmly and immovably united together throughout
their length so as to form one bone, the radio-ulna. The
upper end of the double bone has a large concavity for articu-
lation with the humerus, beyond which a hook-shaped process,
the olecranon, projects from the ulnar component of the bone.
At its distal end the radio-ulnar bears two articular surfaces

THE FROG 31

belonging to the ulnar and radial components respectively.
In ourselves the ulna and radius are separate ; the former
takes the main articulation with the humerus, the latter the
main articulation with the wrist. The radius can be crossed
over the ulna, carrying the wrist and hand with it, and thus we
are able to turn our hands palm upwards when the bones are

Fig. 3-

A, Humerus of a female Frog, seen from below, h, head ;
sh, shaft; ar, distal articular knot; /, trochlea. B,
Radio-ulna of the right side, o, olecranon ; r,
radius ; u, ulna. C, Forearm and hand of the right
side, seen from above, ru, radio-ulna; / — V the five
digits; r, radiale; itn, intermedium; u, ulnare ; a,
first distal carpal bone ; b, second distal ; c, third
distal. (All the figures after Ecker.)

parallel (supination), or palm downwards when the bones are
crossed (pronation). In the frog the radius and ulna, though
fused together, are fixed half-way towards pronation.

The wrist of the frog comprises six little carpal bones
arranged in two rows. In the proximal row there are three
bones : one corresponding to the radius, which we call the
radiale ; one corresponding to the ulna, the ulnare, and one
between the two, the intermedium. In the distal row we find,

32 COMPARATIVE ANATOMY

beginning on the radial or thumb side, two small bones cor-
responding with the rudimentary thumb and the second digit,
which we call distals i and 2 ; and there is a much larger cres-
centic bone which represent distals 3, 4, and 5 fused together.
The hand consists of a set of five proximal bones joined on to
the wrist ; these are the metacarpals, and the first is very small.
To the metacarpals succeed the phalanges, forming the fingers.
The two longest fingers, IV and V, have each three phalanges ;
the third and second have only two. The thumb is only
represented by the metacarpal.

The hind-limb, like the front, is swung on the body by
means of a girdle, the pelvic girdle. There is this important
difference between the pectoral and pelvic girdles, that where-
as the former is not directly attached to the vertebral column,
and is only kept in its place by muscles, the latter is arti-
culated to the transverse processes of one or more vertebrae,
known as the sacral vertebrae. In the frog there is one sacral
vertebra, the ninth, and to its broad and long transverse
processes a pair of long and rather slender bones, the ilia,
are articulated. Each ilium curves downwards and inwards
so as to approach its fellow of the opposite side, and, ex-
panding into a broad plate posteriorly, it is firmly bound to
the similar expansion of its fellow by ligament. Each ilium
has a crest on its upper edge, and the external surface of its
posterior expansion is occupied by the half of a deep articular
concavity, the acetabulum, into which the head of the thigh-
bone fits. The acetabulum is completed by an irregularly-
shaped bone, the ischium, above and behind, and a wedge
shaped piece of cartilage, the pubis, in front and below.
Both ischium and pubis are placed back to back with their
fellows of the opposite side ; and are firmly united to them by
ligament. The whole pelvis of the frog has thus somewhat
the shape of the "merry-thought" of a bird (but the latter has
nothing to do with the pelvis) and is abnormal in . structure.
Each half of a typical pelvic girdle consists of an ilium above
united to one or more sacral vertebrae. This constitutes the
dorsal moiety of the girdle, and is analogous with the scapula
in the pectoral girdle. The ventral moiety consists of an
ischium, which inclines backwards, and a pubis, which inclines
forward. The ischia and pubes of opposite sides commonly
meet, and are joined together in the mid-ventral line ; the points

THE FROG 33

of union are known as the ischial and pubic symphyses. The
dorsal moiety is separated from the ventral by the acetabulum,
just as in the pectoral girdle the scapular half is divided from
the coracoid half by the glenoid, and all three bones, ilium,
ischium, and pubis enter into the composition of the aceta-
bulum. The peculiarity of the frog's pelvis consists in the
great relative length of the ilia, the reduction in size of the
ischia and pubes, and the flattening and approximation of
the members in the region of the acetabulum.

The thigh-bone or femur is a long cylindrical bone with
a slight S-shaped curvature. The shaft is ossified; the upper
extremity has a rounded cartilaginous articular head placed
directly on the shaft; and the extremity also has a rounded
cartilaginous surface for articulation with the leg-bone.

The leg of those vertebrates which have limbs with five
fingers or toes has typically two bones. Both are present
in the frog, but so firmly and intimately are they fused
together that they seem to form a single bone, the os cruris.
But, if it is cut across the middle, two cavities are seen, showing
that the bone is double ; and its double nature is further ex-
pressed by longitudinal grooves on the surface. We shall
therefore call it the tibio-flbula. Its upper surface has a
grooved articular surface for the femur, and its lower ex-
tremity a transversely elongated surface for articulation with
the ankle.

The ankle of the frog is peculiar. Its proximal portion
consists of two rather long bones, separated from one another
in the middle ; but their extremities approach one another
'and are bound together proximally and distally by cartilaginous
articular epiphyses. These two bones represent the proximal
row of the ankle or tarsal bones, and correspond to the
heel-bone (calcaneum) and ankle-bone (astragalus) in man.
The further row of tarsals is very much reduced, consisting of
two tiny pieces of calcified cartilage. One, a flat piece, lies
between the common epiphysis of the astragalus and cal-
caneum and the metatarsal bones of the foot, and is generally
considered to correspond to the cuboid of human anatomy.
The other piece is a mere nodule on the inner or astragalar
side, and is compared with the navicular bone of human
anatomy. The foot has six toes. The first is minute with
a very small proximal piece articulating with the navicular,

34

COMPARATIVE ANATOMY

and probably representative of the metatarsal. This is suc-
ceeded by a flat crescentic piece representing a phalanx. The
first toe is generally called a supplemental toe, and, as we June
already seen, the innermost of the longer toes is usually
reckoned as the hallux or great toe. There are five fairlv
long metatarsals in the foot, and to these succeed the

Fig. 4.

A, Femur of the Frog, p, proximal; d, distal articulating surfaces; s,
shaft. B, Tibio-fibula, seen from below. /, proximal ; d, distal
articulating surfaces ; t, tibial half of the bone separated by a groove
fromyj the fibular half. C, The right ankle and foot of the Frog, seen
from below. This figure is drawn to a smaller scale than A and B.
a, astragalus ; c, calcaneum ; / — l^, the five principal digits ; X, the
minute accessory digit. (All the figures after Ecker.)

phalanges of the toes. The first and second toes have two
phalanges, the third three, the fourth four, and the. fifth three.

It is obvious that the fore and hind limbs are built on
essentially the same plan. The humerus of the arm has its
analogue in the femur of the leg. The radius and ulna are
represented respectively by the tibia and fibula in the leg.
The wrist corresponds to the ankle, and the bones of hand
and foot are very similar. Paired structures, which are
repeated in this way, are said to be serially homologous, but

THE FROG 35

we shall find better examples of serial homology among
the Invertebrates.

The limbs of the frog have undergone considerable modi-
fication in connection with its swimming and leaping habits ;
the limbs of man are more typical, but the best examples of
typical limbs are to be found among the long-tailed amphibia
and the reptiles.

When we say that a limb or any other organ is typical, we
mean that it conforms to a pattern which we recognise as
being the fundamental standard to which all the limbs belong-
ing to members of the group may be referred. This funda-
mental pattern does not always exist in Nature, though it does
in the case of limbs. It may be, in the case of other organs,
an abstraction, a design, of which we form the idea after the
comparison of a great number of limbs, all resembling one
another in some particulars, but differing from one another
in this character and in that. In the case of limbs, the
differences are chiefly in the suppression or fusion of parts :
it is rare that there is an addition of parts as in the case of the
supplemental toe of the frog's foot.

The examination of a large number of cases leads to the
recognition of the following plan of structure for the limbs of
amphibia, reptiles, birds, and beasts; those vertebrata which
we consequently class together as Pentadactyla.

Suppose the animal to be placed on the table, belly down-
wards, and its head pointing away from the observer, a
straight line drawn from the mouth to the tip of the tail
divides it into equal and symmetrical right and left halves.
This line is called the principal axis of the body, and as the
halves into which it divides the animal are alike, the animal
is called bilaterally symmetrical. The skull and vertebral
column coincide with the principal axis, hence we have
referred to them as the axial skeleton. Let two lines be
drawn at right angles to the principal axis from the points
where the limbs are joined on to the body ; these will be the
two secondary axes. Let us suppose the limbs to be
straightened out along the secondary axes, as in the diagram,
palms downwards. Then we find that the humerus in the
arm, and the femur in the leg, correspond with the secondary
axes. In the fore-arm the radius lies in front of the secondary
axis, and is called pre-axial ; the ulna lies behind it, and is called

36 COMPARATIVE ANATOMY

post-axial. Similarly, in the leg, the tibia is pre-axial, the
fibula post-axial. In the wrist we find a proximal row of three
bones — radiale lying beyond the radius, ulnare lying beyond
the ulna, and a median bone, the intermedium, lying between
them. The distal row has five bones, and we begin counting
from the pre-axial or radial side. The thumb, therefore, is

Fig- 5-

Diagram of the typical ore and hind limbs of a pentadactyle vertebrate. The
arrow represents the principal axis of the body, the point directed anteriorly.
The limbs are represented as stretched straight out at right angles to the
principal axis, palms downwards. Sc, scapula ; //, humerus ; Ra, radius
(pre-axial with regard to the secondary axis) ; 67, ulna (post-axial) ; r,
radiale ; «, ulnare ; i, intermedium ; c, centrale ; i, 2, 3, 4, 5, the distal
row of five carpals ; /, //, ///, IV, V, the five digits ; //, ilium ; /v,
Femur ; Ti, Tibia (pre-axial) ; fv, Fibula (post-axial) ; i, tibiale ;_/, fibulare.

on the pre-axial side. Between the proximal and distal row
of carpals a central bone is wedged in. This condition is
realised in the hand of the water-tortoise. In the ankle we
similarly find a tibiale (pre-axial), a fibulare (post-axial), and
between them an intermedium. There is a centrale and
a distal row of five bones. This condition is realised in the

THE FROG 37

foot of the spotted salamander. But in most animals some of
the carpal and tarsal bones are fused together, as in the frog.
In man we find in the carpus that the radiale, intermedium,
and ulnare are present as separate bones, and they are called
scaphoid, lunar, and cuneiform, respectively. The centrale
is present in the foetus, but becomes fused with the third bone
of the distal row to form the os magnum. The first and
second bones of the distal row, called the trapezium and
trapezoid, are separate ; the fourth and fifth are fused together,
and are called the unciform. In the foot the tibiale and
intermedium are fused to form the astragalus, the fibulare
forms the calcaneum. The centrale remains separate, and is
called the navicular; the first, second, and third distals are
called the ento-cuneiform, meso-cuneiform, and ecto-cuneiform,
respectively ; the fourth and fifth are fused together to form
the cuboid. After this explanation the limb bones of the frog
should present no difficulty.

CHAPTER II
THE FROG (continued)

WE have treated the skeleton of the frog in detail, but for the
present we will pass the remainder of its anatomy in somewhat
rapid survey, leaving details for further study at a later time.

On making an incision through the muscular sheet which
covers the abdomen, we open up a spacious cavity, in which
the gut and other organs lie. Extending the cut farther
forwards, and cutting through and removing the breast-bone, we
find that the cavity is prolonged forward into the region of the
heart, and that the heart lies in a part of it which is cut off
from the abdominal part by a partition formed by the insertion
of some of the abdominal muscles on the oesophagus. This
cavity is the pleuro-peritoneal cavity, or coelom. It must be
noticed that it does not contain blood, and that it is lined by
a smooth, glistening, pigmented membrane — the peritoneum.
Tracing the peritoneum round to the dorsal side, we find that
under the vertebral column, in the mid-dorsal line, it is folded
ventralwards on each side, and the two folds are so closely
applied that they form an apparently single sheet of trans-
parent tissue, the mesentery, by which the gut is suspended in
the ccelom. The other organs which lie in the coelom — ovaries,
liver, etc. — are similarly suspended from the dorsal wall by a
reflection of the peritoneal membrane, which passes right
round them. It should be clearly understood that the gut
and other organs which appear to lie in the ccelom are in
reality external to it, being separated from it by the' mesenterial
fold. This will be clear if we liken the abdomen of the frog
to a stout bag with an inner lining representing the peritoneum.
The gut, etc., do not lie in the cavity, enclosed by the lining,
but are placed between the lining and the outer wall of the
bag. The gut lies in a deep fold of the lining which projects
into the cavity, and the upper or suspensory limbs of the fold
are closely pressed together, and form the mesentery. These

38

ANATOMY OF THE FROG 39

relations are exhibited in the diagram (fig. 6), in which
the peritoneum and the mesenterial folds are represented
by a broken line. The coelom, then, is a cavity which
contains a fluid, the peritoneal fluid, but not blood; the
viscera project into this cavity, but are in all cases separated
from it by folds of the peritoneum. The peritoneum, over

Fig. 6.

Diagrammatic transverse section of the body of a female Frog,
taken at the posterior end of the eighth vertebra, to show
the relation of the peritoneal membrane, represented by a
broken line, to the viscera. K, vertebral centrum; JVC,
spinal cord; n n, nerves; 6"^, the great dorsal lymph
spaces; Ao, aorta; / KC, interior vena cava ; K K, kidneys;
Ov Ov, ovaries; OdOd, oviducts; G, gut ; /, the niesen-
tery, a double fold of peritoneal membrane suspending the
gut ; 2, the mesovarium, a similar peritoneal fold suspend-
ing the ovary; 3, the mesometrium, a similar peritoneal
fold suspending the oviducts ; Ds, dorsal subcutaneous
lymph space ; L S, lateral subcutaneous lymph space ; l^s,
ventral subcutaneous lymph space. The skin is represented
by a thick black line.

the greater part of the surface of the abdomen, is closely
adherent to its muscular walls, except for the mesenterial and
other folds referred to. On either side of the kidney, on the
dorsal side, it is separate from the muscular walls of the
abdomen, leaving a space, which is filled with lymph. The
peritoneum in the region of this space is found, on careful
microscopical examination, to be perforated by numerous tiny
apertures called stomata, putting the coelom into communica-

40 COMPARATIVE ANATOMY

tion with the lymph-space. This last communicates with the
general system of lymph spaces in the body, which, in their
turn, communicate, by means of two pairs of small pulsatile
vesicles called lymph hearts, with the blood system. One pair
of lymph hearts lie just behind the transverse processes of the
third vertebra, the other pair alongside of the urostyle.

Thus we see that the ccelom, though it does not contain
blood, is, in a roundabout way, in communication with the
blood-vessels.

The most obvious organ, when the body-cavity is opened, is
the gut, with its great glandular appendages, the liver and
pancreas. The buccal cavity passes into a wide tube, the
gullet or oesophagus, which passes straight backwards under-
neath the vertebral column in the anterior part of the body,
narrowing somewhat as it goes. It then widens out again
rather suddenly to form a pear-shaped sac with rather stout
muscular walls — the stomach. At the end of the stomach a
distinct constriction — the pylorus — marks the commencement
of the small intestine, the first portion of which, known as the
duodenum, is bent sharply forwards so as to lie parallel to the
stomach, to which it is attached by a membranous fold — the
gastro-duodenal omentum. In this fold lies a small whitish-
yellow elongated mass — the pancreas — which will be described
farther on.

The small intestine bends sharply back again towards the
posterior end of the animal, is thrown into several loops — its
total length when straightened out being from four to five
inches, — and then it passes suddenly into a much wider tube
about an inch and a quarter long, the large intestine. The
large intestine passes without any obvious line of demarcation
into a short terminal portion, the cloaca, which opens to the
exterior by the anus. A large thin-walled sac, the urinary
bladder, opens into the ventral side of the cloaca, which receives
other ducts connected with the kidneys and reproductive
organs, to be described farther on.

In order to examine the oesophagus, stomach, and intestines,
it has been necessary to turn aside a large lobulated reddish-
brown organ which occupies the ventral region of the anterior
half of the abdominal cavity. This is the Liver. It consists of
three main lobes, a right, a left, and a median, united across
the middle line by a comparatively narrow bridge of liver

ANATOMY OF THE FROG 41

tissue. The left lobe is much the largest, and is divided into
two secondary lobes by a deep fissure extending forwards from
its hinder margin. If the lobes of the liver be turned forwards,
a round or oval vesicle, the gall-bladder, may be seen in the
deep fissure between the right and left lobes. It has thin
walls, and is generally full of a green fluid, the bile, which
imparts its colour to the whole bladder. The gall-bladder
receives bile from the liver by means of three ducts, which pass
from the lobes of the liver, unite together, and enter the gall-
bladder at its upper end as a single vessel, the cystic duct.
The bile is carried from the liver and gall-bladder to the
duodenum by a single duct, the bile duct, or ductus choledochus,
which is formed by the union of three vessels arising from the
cystic duct, and is joined, at some distance from its origin, by
several vessels from the middle lobe of the liver.

The bile duct, in its passage towards the duodenum, traverses
the whitish lobular gland which we have already noticed lying
between the stomach and duodenum. This is the pancreas, a
gland which secretes a solvent fluid, the pancreatic juice. The
ducts of the pancreas join the bile duct about the middle of
its course, and the common biliary and pancreatic duct opens
into the duodenum on its dorsal surface about half-an-inch
beyond the pylorus.

The liver and pancreas are the two glands which discharge
their secretions into the alimentary canal by ducts. Both of
them were formed, in the course of development, as outgrowths
of the embryonic gut. The stomach and intestine are also
glandular, their lining membrane being thrown into longitudinal
folds for increase of surface ; and the walls of these folds are
studded with innumerable minute orifices, the openings of
microscopically small gastric and intestinal glands, which dis-
charge their secretions into the cavity of the gut. These
glands are of very simple construction, being nothing more
than finger-shaped depressions lined by the mucous membrane
lining the gut. We shall study their characters more closely
hereafter.

In studying the buccal cavity of the frog we noticed, on the
floor of its hinder end, a slit-like aperture, the glottis. This
opens into a short, wide tube whose walls are strengthened by
a rather complex set of cartilages. This tube is the larynx.
The cartilages are five in number : a single curiously shaped

42 COMPARATIVE ANATOMY

hoop, with various processes, called the cricoid; a pair of
crescentic cartilages, placed in front of it, called the arytenoids ;
and a smaller pair, connected with these, called the pre-
arytenoids. These cartilages are moved by special muscles,
and they bear a pair of flat bands of connective tissue — the true
vocal cords — lying parallel to one another, with a slit-like space
between — the rima glottidis. The cartilages and muscles are
so disposed that the edges of the vocal cords forming the sides
of the rima glottidis may be approximated or separated, whilst
the cords themselves are tightened or slackened. The cords
are thrown into vibration by the passage of air over their
tightened and approximated edges, and thus a sound — the well-
known croak of the frog — is produced. The male frog croaks
much more loudly than the female, and its laryngeal cartilages
are larger, thicker, and stronger. The croak of the male is
further intensified by a pair of resonators, in the form of vocal
sacs, which open one in each corner of the floor of the hinder
part of the mouth. When the animal croaks the sacs are
inflated, and bulge out under the angles of the mouth.

The lungs take their origin from the posterior part of the
laryngeal tube, their openings being kept extended by a pair
of curved cartilaginous processes of the cricoid. Each lung
starts as a contracted tube from the larynx, and then swells out
into a pear-shaped bag which lies in the body cavity dorsal to
the liver. The lungs have highly elastic walls, which, by their
contraction, expel the air in a dead frog and leave the lungs
shrunken and collapsed. They are easily inflated by means of
a blow-pipe inserted in the glottis. The internal structure of
the lungs is somewhat complicated. The cavity of the sac,
especially its upper part, is broken up into chambers by a
number of folds or partitions passing into the interior. These
chambers are further subdivided by secondary folds starting
from their walls, and the smaller chambers so formed may
again be subdivided in a similar manner into minute chambers
termed alveoli. The whole of the upper part of the lung thus
assumes a sort of honeycomb structure, but in its lower
pointed end the chambers are less developed, and at the
extreme apex are absent altogether. The honeycomb arrange-
ment provides a large surface for exposure to the air contained
in the lungs ; and, as the partition walls are exceedingly thin, the
blood which circulates through them is brought so close to the

ANATOMY OF THE FROG 43

air that an exchange of the gaseous constituents of blood and
air is readily effected, the blood taking up oxygen and giving
in return carbonic acid gas. The lungs, as appears from
this description, are offsets of the alimentary tract, and they
are formed, in the course of the growth of the tadpole into
the frog, as an outgrowth from the ventral wall of the gut in
the throat region, the outgrowth soon becoming bi-lobed to
form the pair of lung sacs.

The other abdominal viscera of the frog are the spleen,
the kidneys with the adrenal bodies, and the reproductive
organs.

The spleen is a small, rounded-oval, dark-red body lying in
the mesentery near the commencement of the large intestine.
It belongs to the lymphatic system, and has no duct.

The kidneys are two elongated, flattened, dark-red bodies
lying one on each side of the vertebral column towards the
hinder end of the abdominal cavity. They lie in the large
lymph space formed behind the peritoneal lining of this part
of the crelom, and, as they are covered over by the peritoneum,
they are, like the gut and other organs which we have studied,
outside the ccelom. Each kidney has a straight border turned
towards the vertebral column, and a convex outer margin.
The kidney consists, essentially, of a number of convoluted
tubules, the uriniferous tubules, bound together by fibrous
connective tissue, and abundantly supplied with blood-vessels.
The tubules open into collecting ducts, which unite together to
form a single duct, the ureter, leading to the cloaca. The
ureters are short, straight tubes which arise from the outer
edges of the kidneys, and run backwards, opening into the
cloaca on its dorsal side, opposite the opening of the bladder.
In the male frog each ureter presents a spindle-shaped dilata-
tion on its outer side, the vesicula seminalis.

The adrenal bodies are found on the ventral sides of the
kidneys, near their outer borders. They are small yellow
patches, of somewhat doubtful function and significance.

The reproductive organs consist of the testes and their
ducts in the male, the ovaries and their ducts in the female.
The testes are a pair of ovoid pale-yellow bodies attached to
the dorsal wall of the body cavity by a fold of the peritoneum,
the mesorchium. They lie on the ventral sides of the kidneys,
to which they are attached by a variable number of very fine

44 COMPARATIVE ANATOMY

ducts, called the vasa efferentia. The sperm, which is
formed in the testes, passes through the vasa efferentia into
the collecting tubes of the kidney, and thence by the ureter
to the vesicula seminalis and to the exterior. Thus in the
male frog there is a common duct for both generative and
urinary products, a fact of which we shall see the significance
hereafter. The ovaries vary very much in size according to
the season of the year. In the breeding season in early
spring they are very large, and fill the body cavity to
distention ; in summer and autumn they are much
smaller. They correspond in position with the testes — i.e.
they are attached to the dorsal wall of the body cavity by
a peritoneal fold, the mesovarium, and lie on the ventral sides
of the kidneys. Each ovary is a large irregularly-lobed sac,
which, when viewed externally, seems to be filled by a mass of
spherical black and white bodies about the size of No. 5
shot. These are the ova or eggs. There are no ducts
continuous with the ovaries ; the ova, when ripe, escape by
rupture of the ovarian walls, and fall into the body cavity,
whence they are carried to the exterior by a pair of much
coiled ducts, with funnel-like mouths opening into the body
cavity. These are the oviducts. They lie external to the
kidneys and ovaries, their mouths opening on either side into
the anterior part of the body cavity not far from the middle
line. Behind the mouth each oviduct has the form of a
rather slender white tube, which gradually expands, maintains
an even size throughout a much coiled course towards the
posterior end of the body, and then widens out suddenly to
form a pear-shaped dilation- — the so-called uterus. The two
uteri open close together into the cloaca just in front of the
openings of the ureters. The walls of the coiled part of the
oviducts are abundantly furnished with glands, which swell
up during the breeding season and cause the oviducts to
increase greatly in size. The peritoneal fold by which the
ovaries are suspended is known as the mesometrium. (See
fig. 6.)

The cloaca, into which the urinary and generative ducts
open, is a short tube continuous with the rectum. From
what precedes, it follows that it has, besides the opening of the
bladder on the ventral side, one pair of orifices on the dorsal
side in the male — viz. those of the common uro-genital duct ;

ANATOMY OF THE FROG 45

and two pairs in the female — viz. those of the ureters and
oviducts.

The fat-bodies should be noticed; they are finger-shaped
fatty masses of a bright yellow colour lying in front of the
testes or ovaries.

All parts of the frog's body are supplied. with a nutrient
fluid, the blood, by means of a system of blood-vessels which
take their origin from a hollow muscular organ, the heart. In
order that the relations of the blood-vessels to one another and
to the heart may be clearly understood, it should be borne in
mind that the blood is the great medium of exchange in the
body. It brings nutrient material, collected from the walls of
the alimentary canal to the organs for their repair, and receives
in exchange effete waste material which it carries away to
organs — the kidneys and lungs — whose function it is to separate
out the waste material and expel it from the body. A large
part of this waste matter is carbonic acid gas, which is exhaled
through the lungs ; and the blood, in its passage through the
lungs, receives in return oxygen which it carries to all parts of
the body.

Bearing these facts in mind, we may study the mechanism
by means of which the circulation of the blood is effected.

The heart is a hollow muscular organ, of roughly triangular
shape, the apex pointing backwards, lying in the mid-ventral
line, immediately above the ventral ends of the shoulder-girdle
and below the oesophagus. It is enclosed in a very thin
membranous sac, the pericardium, on the ventral walls of
which some muscular slips from the great internal oblique
muscle of the abdominal wall are inserted. The pericardial
cavity is a portion of the ccelom, which has been cut off from
the remainder by the growth of a partition, in the substance of
which run the great veins leading to the heart. Its relations
are rather complicated, and will be better understood at a later
stage ; but it may here be noticed that the pericardial cavity is
a ventral portion of the ccelom, cut off by the above-mentioned
partition from a dorsal portion continuous behind with the
abdominal ccelomic space, in which the lungs lie. The heart
seems to lie in the pericardial cavity, but in reality it is outside
of it, just as the gut and other abdominal viscera are seemingly
inside, but really outside, of the abdominal portion of the
ccelom. The pericardium has an extremely thin lining

46 COMPARATIVE ANATOMY

membrane, which is reflected over the heart, and so excludes
it from the cavity which it lines.

The heart itself consists of (i) two auricles, forming the
wider anterior portion : they are readily distinguished by
their thinner walls and darker colour, due to the blood being
seen through their walls ; (2) a single ventricle, situated
posteriorly, with thick muscular walls, which are of a paler
colour than those of the auricles ; (3) the sinus venosus, a
thin -walled sac, lying on the dorsal or hinder region of the
heart, and formed by the union of three great veins ; (4) the
truncus arteriosus, a stout cylindrical vessel which arises from
the base of the ventricle at its right side on the ventral surface.
Examining the structure of the heart more closely we find that
the sinus venosus is a thin-walled sac of triangular shape, the
apex of the triangle pointing backwards. At its apex it
receives a large blood-vessel, the inferior vena cava, and at
each of its remaining angles, right and left, it receives a large
vessel, the right and left superior venae cavae. The sinus
venosus communicates by a transverse opening, guarded by
two valves, with the right auricle. The valves at this opening
are so disposed that they admit blood from the sinus venosus
into the auricle, but prevent its passage in a reverse direction.

The two auricles together form a large thin-walled sac, of
rather irregular but generally hemispherical shape, which
opens below by an elongated slit into the ventricle. The sac
is divided into two unequal parts by the auricular septum
passing from the anterior wall to the auriculo-ventricular
aperture, where it ends in a free concave border. Of the two
auricles the left is decidedly the smaller, and in exceptional
cases it is much smaller than the right. It receives at its
upper dorsal surface, close to the auricular septum, a vein — the
pulmonary vein — which brings blood back from the lungs.

The ventricle is much thicker than the auricles, through the
presence of abundant muscular tissue in its walls. Tts cavity
is not smooth, but rather spongy in appearance, through the
presence of numerous muscular ridges which project into it.
The cavity does not extend far into the apex of the ventricle,
is rather narrow dorso-ventrally, but elongated from right to
left. The auriculo-ventricular aperture, by which the ventricle
communicates with the auricles, is rather a wide opening,
divided into two by the free edge of the auricular septum, and

ANATOMY OF THE FROG

47

guarded, on its ventricular side, by a pair of valves which have
the form of flaps of membrane springing from the walls of the
heart. One of these flaps is on the dorsal, the other on the
ventral side of the aperture, and each is connected with the
muscular ridges of the ventricular wall by about a dozen very
fine fibrous cords the cordse tendineae. The length of these
cords is such that they allow the membranous flaps to rise up

Fig. 7-

A, The frog's heajt dissected from the ventral surface. B, an enlarged view of
the truncus arteripsus. R A , right auricle ; L A , left auricle ; y, ventricle ;
T, truncus arteriosus, divided into Py, pylangium, and Sy, synangium ;
& ' y, opening of the sinus venosus into the right auricle; Pv, opening of the
pulmonary vein into the left auricle ; Aw, auriculo-ventricular valve ; slv,
semi-lunar valves guarding the passage from the ventricle to the pylangium ;
C, carotid arch ; S, systemic arch ; P, pulmo-cutaneous arch ; spv, spiral
valve in the pylangium ; po, opening of the right pulmo-cutaneous arch ; co,
opening of the carotid arches.

and meet one another in the middle line so as to close the
aperture, but they prevent their rising farther into the cavity
of the auricles.

The left-hand anterior corner of the ventricle is prolonged,
on its ventral side, into a stout vessel, which passes forwards
and to the left across the auricles, and divides at the upper
limits of the latter into two vessels, right and left, each of

48 COMPARATIVE ANATOMY

which almost immediately divides again into three. The
large single vessel is called the truncus arteriosus. Though
it appears single up to its bifurcation, the distal part of the
truncus really consists of vessels closely, united together, hence
the truncus is conveniently subdivided into a proximal single
portion, the pylangium, and a distal multiple portion, the
synangium. The opening of the pylangium into the ventricle
is large, and ' is guarded by a pair of membranous ralves,
shaped like watch-pockets with their mouths turned towards
the cavity of the pylangium. The free edges of each of these
valves— known as the semi-lunar valves— are tied to the inner
walls of the pylangium by cordse tendineae, similar to those
of the auriculo-ventricular valve. Further, the cavity of the
pylangium is incompletely divided into two by a membranous
fold which commences on the ventral side close above the
opening into the ventricle, runs forward with a spiral course,
becoming deeper as it goes, and it ends on the dorsal and
right side of the pylangium, where it is fused to one of the
three semi-lunar valves, similar to those of the ventricular
aperture, which guard the opening from the pylangium into
the synangium. The ventral border of this fold hangs free
in the cavity of the pylangium, and extends across two-thirds
of its diameter when fully extended.

The synangium is the wide but very short part of the truncus
which lies in front of the three valves separating it from the
pylangium. The middle valve is the smallest, and just above it
a horizontal partition springing from the spiral valve separates
a dorsal from a ventral chamber. The dorsal chamber, which
is much the smaller, twists round to the right and passing for-
wards branches right and left to form the carotid arches. The
ventral chamber is divided by a vertical partition into two wide
right and left passages, and each of these is again subdivided
by a partition into a systemic aortic arch above and a pulmo-
cutaneous arch below (fig. 7, £). We have already noticed
that the truncus arteriosus bifurcates at its anterior end, and that
each bifurcation very shortly divides into three vessels. The
bifurcations themselves are triple in constitution, as may be
seen by cutting them across, when each is observed to be made
up of three vessels — an inner and upper, the carotid ; a median,
the systemic ; and a lower and outer, the pulmo-cutaneous.
It is by means of these vessels that blood is conveyed from

ANATOMY OF THE FROG 49

the heart to all parts of the body. Vessels which carry blood
away from the heart are known as arteries. They have firm,
elastic walls, which do not collapse when empty, and retain
their circular section when cut across ; their firmness is due
to the relatively large amount of elastic tissue contained in
their walls. Veins are vessels which carry blood back to the
heart ; they have softer and less elastic walls than arteries, and
collapse when empty or when cut across. The arteries and
veins are united in the tissues by exceedingly fine vessels,
called capillaries, which can only be studied by the aid of the
microscope. The arterial system of the frog starts from the
truncus arteriosus, and the three main branches on either side
into which it divides.

Of these three arches the carotids are the most anterior.
Each runs round the oesophagus towards the dorsal surface,
and, shortly after its origin, expands into a small almost spherical
dilatation — the carotid gland. Just before it expands into the
carotid gland the arch gives off a small branch, which runs
inwards and forwards over the throat, nearly parallel to its
fellow of the opposite side, giving off branches to the hyoid
apparatus and the tongue. This is the lingual artery. The
carotid artery arises from the outer border of the carotid
gland, runs round the oesophagus towards its dorsal surface, and
there turns forward to run beneath the base of the skull, where
it divides into (i) the external carotid artery, whose branches
supply the pharynx, palate, and orbit, and (2) the internal
carotid, which passes through a foramen in the base of the
skull and supplies the brain. Just before it turns forward to
run under the base of the skull, the carotid gives off a small
branch backwards which joins the systemic or second arch,
and is known as the ductus Botalli. This duct, which is
open in the very young frog, usually becomes occluded in the
adult and no longer admits the passage of blood. The second
or systemic arch runs round the oesophagus on either side
towards the dorsal surface, and then turns inwards and back-
wards to join its fellow in the middle line just below the
vertebral column, at about the level of the sixth vertebra. The
right arch is directly continuous with the dorsal aorta,
which runs straight back beneath the vertebral column and
the urostyle; the left arch opens into the dorsal aorta by
a small opening, and is continued as the coeliaco-mesenteric

50 COMPARATIVE ANATOMY

artery, passing to the stomach and intestines. The systemic
arches give off on either side before they unite to form the
dorsal aorta (i) a laryngeal artery, a small vessel arising
from the proximal part of the arch, and passing forwards to
supply the larynx; (2) an ffisophageal artery, arising from the
dorsal side of the upper part of the arch, and turning down-
wards to supply the oesophagus ; (3) an occipito-vertebral
artery, arising from the dorsal side of the arch opposite the
transverse process of the second vertebra, and dividing after
a short course into two branches (a) the occipital artery,
which courses forward, supplying the sides of the head and the
jaws, (ft) the vertebral artery, running backwards parallel to the
vertebral column, and dorsal to its transverse processes : it gives

The Frog dissected from the right side to show the distribution of the chief
arteries ; semi-diagrammatic. H, heart ; tr, truncus arteriosus ; ca! ', carotid
arch ; /, lingual artery ; ca, carotid artery ; sy, systemic arch ; a?, oesophageal
artery ; oc.v, occipito-vertebral artery ; sc, subclavian artery, cut short ; ao,
dorsal aorta ; ccel, coeliaco-mesenleric artery, supplying the viscera ; its
three main branches are indicated but all its subdivisions cannot be shown ;
ra, uro-genital arteries ; //, iliac arteries ; PC, pulmo-cutaneous arch, divid-
ing into /, the pulmonary artery supplying the lung, and cu, the cutaneous
artery, cut short ; La, larynx ; L, lung ; S, stomach ; /, intestine ; R,
rectum ; Bl, bladder; M, muscular abdominal wall turned back ; A", kidney.

off branches to the muscles of the body and to the spinal cord ;
(4) a subclavian artery, arising from the arch close behind
the occipito-vertebral, and supplying the shoulder and fore-
limb.

ANATOMY OF THE FROG 51

The coeliaco-mesenteric artery, which arises from the left
arch just at its point of union with its fellow, soon divides
into two branches, the coeliac and the mesenteric. The
cceliac artery is further divided into the gastric artery supply-
ing the stomach, and the hepatic, supplying the liver. The
mesenteric artery divides into an anterior and a posterior
branch and into the splenic artery, which goes to the spleen. In
its further course backwards the dorsal aorta gives off from
four to six small vessels from its ventral side, to supply the
kidneys and generative organs, and some small vessels from its
dorsal side, which supply the muscles of the back and sides.
These are the uro-genital and lumbar arteries. Opposite the
middle of the urostyle the dorsal aorta bifurcates to form the
iliac arteries which are continued onwards as the sciatic
arteries into the hind limbs.

The third or pulmo-cutaneous arches divide, after a short
course on either side of the oesophagus, into the pulmonary
artery, which supplies the lungs, and the cutaneous artery,
a large vessel running up to pass close behind the ear, and
then turning sharply backwards to be distributed over the
skin of the trunk.

The principal arteries which have been detailed above
divide and subdivide in the various regions of the body to
which they are distributed ; the walls of their branches
become thinner and less elastic, till finally they end in net-
works of extremely fine vessels with very thin walls, called
capillaries. The capillaries may easily be studied, by the aid
of the microscope, in the web of the frog's foot when the toes
are stretched apart. They present the form of a network of
fine tubes running through • the tissues of the web. The
meshwork is irregular, but, speaking generally, the capillaries
form numerous loops of various sizes, and the capillary tubes
themselves unite freely with one another. Their walls are so
thin that they are hardly apparent, but the blood stream
coursing through their cavities is readily recognisable. The
capillary tubes vary in diameter from ^-^V^-th to y/oiyth of an
inch.

The capillaries are continuous, on the one hand, with the
ultimate subdivisions of the arteries. On the other hand, they
unite to form larger, but still very small vessels, the ultimate
subdivisions of the veins, also called venules. Both arterioles

52 COMPARATIVE ANATOMY

and venules are distinguished from capillaries by their walls,
which are thickened by the presence of muscular fibres and
connective tissue. The elastic layer, which is a conspicuous
element in the walls of larger arteries, dies out in the smallest
arterioles.

The venules unite together and are gathered into larger and
larger vessels, the veins. Veins are vessels which convey
blood back to the heart. Their walls have much the same
structure as those of arteries, but there is much less elastic
tissue and much more connective tissue in them ; hence they
are less resilient, but at the same time tougher than arteries.
We may consider the veins under three heads: (i) those
bringing blood back to the heart from the head, throat, and
fore-limbs; (2) those bringing back blood from the viscera, the
trunk, and hind limbs ; and (3) those bringing back blood from
the lungs.

Blood is brought back from the tongue and from the lower
jaw by two veins, known as the lingual and mandibular veins
(/ and m fig. 9). These unite on the ventral wall of the
throat to form a single vessel, the external jugular vein, which,
after a short course, is joined by an internal jugular vein
bringing blood back from the inside of the skull. It leaves
the skull by an aperture at the hinder border of the orbit.
Just before it unites with the external jugular the internal
jugular receives a small subscapular vein bringing back blood
from the shoulder and back of the fore-limb. The short
section lying between the union of the subscapular with the
internal jugular and the external jugular is generally dis-
tinguished as the innominate vein. The single vessel formed
by the union of the external jugular and innominate is almost
immediately joined by a third large vein, the subclavian. The
subclavian vein, which, as its name implies, lies under the
clavicle and precoracoid, is itself formed by the union of two
veins — the brachial which brings blood from the fore-limb, and
the musculo-cutaneous, a very large vein which takes its origin
in the nose, runs straight backwards as far as the middle of the
trunk, receiving branches from the skin and muscles of the
side of the body, and then turns sharply forwards to pass up
the ventro-lateral wall of the body and join the brachial. The
three veins, external jugular, innominate, and subclavian, unite
to form the superior vena cava of their side of the body, a

ANATOMY OF THE FROG

53

Fig. 9.

Diagram of the venous system of the Frog. The apex of the
heart is turned forwards to show the sinus venosus. »z,
mandibular vein ; /, lingual vein ; //, internal jugular vein ;
S.sc, subscapular vein ; Br, brachial vein ; C. musculo-
cutaneous vein ; VCS, vena cava superior; VC /, inferior
vena cava ; H, hepatic veins ; f, femoral vein ; Sc, sciatic
vein; R A, renal portal vein; ilc, ram us communicans
iliacus; A A V^ anterior abdominal vein, formed by the
union of two pelvic veins ; P, portal vein. The pulmonary
circulation is not represented in the figure.

54 COMPARATIVE ANATOMY

wide vessel which runs into an anterior angle of the sinus
venosus. The systems of the right and left venae cavse
anteriores are similar, and the above description serves for
either side of the body.

There are, then, two anterior venae cavse opening into the
two anterior angles of the triangular sinus venosus. But there
is only one posterior vena cava opening into the posterior
angle of the sinus venosus and bringing back blood from the
abdominal viscera, the trunk, and hind limbs. The system of
the posterior vena cava is complicated. It arises between the
kidneys, where it is formed by the union of four or five renal
veins from each side, and by the ovarian or testicular veins in
the female or male. It passes straight forwards in the median
line, is partly enveloped by the substance of the liver, and, just
before it enters the sinus venosus, it receives the right and
left hepatic veins from the liver. The blood from the hind
limbs, and from the other abdominal viscera, reaches the vena
cava only after an indirect course through either the liver or
kidneys.

The blood from the hind limb is returned to the pelvic
region by two veins — the femoral, which twists round from back
to front of the thigh to reach the ischial region of the pelvis,
and the sciatic, which comes up from the back of the thigh to
pass under the iliac bone of its side. The sciatic vein is
almost directly continuous with a vein known as the renal
portal vein, which passes forward to the outer border of the
kidney, and there breaks up into small vessels which open into
the capillaries of the kidney. The femoral vein divides at the
root of the thigh into two branches. The dorsal branch,
known as the iliac vein, runs forward in the dorsal side of the
posterior part of the abdominal cavity, and unites with the
sciatic vein to form the renal-portal. The femoral and sciatic
veins are further connected by a small vessel, the ramus com-
municans iliacus, which runs transversely from one to the
other at the base of the thigh. The second or ventral branch
of the femoral vein is known as the pelvic vein ; it passes
inwards towards the mid-ventral line of the abdominal wall,
and unites, just in front of the pubic cartilages, with its fellow
of the opposite side to form the anterior abdominal vein.
This is a large, median, unpaired vessel, which receives at its
point of origin the vesical vein from the bladder, and then

ANATOMY OF THE FROG 55

courses forwards in the middle line of the ventral wall of the
abdomen till it reaches the xiphoid cartilage of the sternum.
It then turns inwards and divides into three branches, two
lateral and one median. The two lateral branches pass to the
right and left lobes of the liver respectively, and break up in
its substance into capillaries ; the median branch unites with
the hepatic portal vein.

The last-named vessel brings blood from the stomach and
intestines. It is formed by the union of the intestinal, gastric
and splenic veins, runs forward in the mesentery, and passes
into the left lobe of the liver, where it breaks up into
capillaries. Just before it enters the liver the hepatic portal
vein is joined by the descending branch of the anterior
abdominal. The blood from the liver is collected by the right
and left hepatic veins, which open into the inferior vena cava
just before it passes into the sinus venosus. We have seen
that the lungs are supplied with blood by the two pulmonary
arteries, each of which courses down to the outer surface of
the lung sac to which it belongs from base to apex, giving off
a number of branches as it goes. The blood, after passing
through the lung capillaries, is gathered by venules, which unite
to form the venous vessel lying along the inner side of the
base of each lung. These right and left pulmonary veins run
above the corresponding venae cavae, and unite to form a
single trunk, the common pulmonary vein, which opens into
the left auricle. We are now in a position to review the
course of the circulation of the blood in the frog. The blood
from the head and fore limbs, as well as that brought back by
the musculo-cutaneous veins, passes straight into the sinus
venosus by way of the superior venae cavae. But the blood
coming from the hind limbs and from the posterior part of
the trunk and the abdominal viscera takes a much more
complicated course.

The blood brought from the hind limbs by the femoral
vein may take one of two paths. It may either go by way
of the iliac veins to join the blood brought by the sciatic
veins, and pass on by the renal portal veins to the kidneys ;
or it may go by way of the pelvic and anterior abdominal
veins to the liver. In either case it has, whether in the
substance of the kidney or in the substance of the liver, to
pass a second time through a system of capillaries, and to

56 COMPARATIVE ANATOMY

mingle with the blood brought to those organs by the renal
and hepatic arteries respectively. Similarly, the blood
returned from the stomach and intestines has to pass a
second time through capillaries in the liver. Eventually,
the blood from the capillaries of the kidneys and liver finds
its way by the inferior vena cava into the sinus venosus.
When veins do not pass direct to the heart from the organs
and tissues in which they originate, but go to some other
organ and sub-divide in it to join its capillary system, they are
called portal veins, and the whole system of double capillary
circulation is called a portal system. Thus in the frog we
have a renal-portal and a hepatic-portal system, and renal-
portal and hepatic-portal •vein's. These portal systems are
very characteristic of the great group of Vertebrate animals,
but a renal-portal system is only found in its lower members.
The higher Vertebrata have only a hepatic-portal system.

The walls of the sinus venosus are feebly muscular and
contractile. The beat of the frog's heart starts in the sinus
venosus, which, by its contraction, forces blood through the
sinu-auricular aperture into the right auricle, whence its return
is prevented by the valves by which that aperture is guarded.
At the same time the left auricle is filled with blood returned
from the lungs by the pulmonary veins. The blood brought
into the sinus by the venae cavae, and passed thence into the
right auricle, has passed through the capillaries of the body,
where it has been robbed of a considerable portion of its
oxygen, and has received a quantity of carbonic acid. In
consequence of its poverty in oxygen it has a dark purplish
hue, and is known as venous blood. But that coming from
the lungs has parted with its excess of carbonic acid, and has
taken up oxygen from the air contained in the lungs ; it is of
a bright scarlet colour, and is known as arterial blood.

The muscular contraction which started in the sinus
venosus spreads over the auricles, which contract simul-
taneously, and drive the blood forward through the wide
auriculo-ventricular aperture into the single ventricle. The
return of blood from ventricle to auricles is prevented by the
membranous valve described above. As the right auricle
contains venous and the left auricle arterial blood, the
transversely-elongated cavity of the ventricle is filled with both
kinds, the venous blood being on the right side nearest the

ANATOMY OF THE FROG 57

opening of the truncus arteriosus, the arterial blood farthest
away from it on the left. In the middle of the ventricular
cavity the blood is, of course, mixed. The contraction of the
ventricle follows immediately upon that of the auricles. The
auriculo-ventricular valves close, and the blood is forced into
the truncus arteriosus, the venous blood on the right side
passing first. The wave of contraction passes forwards over
the truncus from the base of the ventricle to the roots of the
aortic arches, and any reflux of blood into the ventricle is
prevented by the closing of the semi-lunar valves which guard
the two ends of the pylangium.

On entering the pylangium the blood is necessarily directed
by the spiral valve. It therefore passes from the dorsal side
round to the right and thence to the ventral side of the
truncus, and the venous blood passing over the free border
of the upper end of the spiral valve enters the synangium.
Now, the arteries are already distended with blood under
considerable pressure, which was only prevented from re-
turning into the truncus by the semi-lunar valves at the
distal end of the pylangium. This pressure is overcome
by the blood forced forward at greater pressure by the con-
tractions of the ventricle and pylangium, and the semi-
lunar valves are forced open. In the synangium the blood
newly arrived follows the path of least resistance, which is
found in the wide aperture of the pulmonary artery. The
first gush of nearly entirely venous blood therefore passes
to the lungs, but as it fills the vessels of the lungs the
resistance in them becomes as great as or greater than that in
the synangium and other aortic arches. The next succeeding
portion of the blood, which is now mixed arterial and venous
from the middle of the ventricular cavity, finds the least
resistance in the wide openings of the right and left systemic
arches, which are accordingly filled with mixed blood until
their resistance becomes as great as, and greater than, that of
the carotid arches, the resistance in which has been consider-
able, both because of their small diameter and because the
flow of blood through the carotid arteries is impeded by the
carotid glands. Thus the last portion of the blood, consisting
of nearly pure arterial blood from the left end of the ventricular
cavity, passes into the carotids, and is distributed to the head.
It seems probable, from the relation of the carotid opening

58 COMPARATIVE ANATOMY

to the distal row of semi-lunar valves, that so long as the
pressure in the carotid arches is greater than that in the
systemic and pulmonary arches, the edges of the semi-lunar
valves are kept pressed together so as to bar the passage
to the carotids.

We have seen how the arterial blood in the carotids is
distributed to the head, there to pass into the capillaries of
the tissues, to part with its oxygen, take up carbonic acid,
and eventually to return by the superior vena cava to the sinus
venosus. Similarly, the mixed blood in the systemic arches is
distributed to the trunk, viscera, and hind-limbs by the dorsal
aorta and its branches, and is collected and returned by the
veins in the manner already described. The venous blood
which enters the pulmonary artery goes to the lungs, takes up
oxygen, parts with its excess of carbonic acid, and is returned
to the left auricle by the pulmonary veins.

The frog, therefore, has a greater and a lesser (pulmonary)
circulation, the blood in the former case making its circuit
through the tissues of the body, in the latter through the
shorter circuit of the lungs. The two circuits, however, are
not closed off from one another, as they are in the higher
vertebrates, but have a common starting-place in the single
ventricle of the heart.

There remains but one set of organs and tissues for us to
consider, the nervous system and the organs of special sense
associated with it.

The nervous system consists of the cerebro-spinal axis and
its nervous branches, and the sympathetic ganglion chain.
The cerebro-spinal axis lies, as we have already seen, in the
cranium, and in the canal formed by the neural arches of the
vertebral column. That part which lies in the cranium is the
brain; and that part which lies in the neural canal of the
vertebral column is the spinal cord.

The spinal cord is a cord of nervous tissue extending from
the foramen magnum into the urostyle. It is not circular in
section, but somewhat flattened dorso-ventrally, and is traversed
both dorsally and ventrally by a median longitudinal narrow
cleft or fissure — the dorsal and ventral fissures, — but neither
of these extend to its most posterior part. Nor is the cord
of the same diameter throughout, but it is swollen in the
regions of the second and third, and again in the regions of

A dissection of the Frog from the ventral surface to show the origins of
the spinal nerves and their connection with the sympathetic system.
The nerves are drawn black ; the dorsal aorta and its branches are
left white. /, the first spinal or hypoglossal nerve ; 2, the second
spinal or brachial nerve joined by 3, the third spinal nerve ; 4, 5, 6,
successive spinal nerves ; 7, S, q, three large spinal nerves uniting to
form Sc, the great sciatic nerve, which also receives a branch from
jo, the tenth spinal nerve ; Sy, the main sympathetic trunk ; Si —
SS, successive sympathetic ganglia united to the spinal nerves by
communicating branches of varying length ; A V, annulus of
Vieussens (the loop has been drawn too large) ; Ca, calcareous
bodies near the exits of the spinal nerves ; Cr, the crural nerve.

60 COMPARATIVE ANATOMY

the fifth and sixth vertebrae to form the brachial and sacral
enlargements. Posteriorly to the seventh vertebra the cord
thins out rapidly, and is continued into the urostyle as a
slender thread, the filum terminate.

Ten pairs of nerves are given off from the spinal cord to the
trunk and limbs. The first of these, which marks the com-
mencement of the spinal cord (for there is no other demarca-
tion between it and the hindmost part of the brain), passes out
by the inter-vertebral foramen between the first and second
vertebrae. It courses round the side of the throat, coming
near to the surface just behind the angle of the lower jaw, and
there it turns forwards and runs to the root of the tongue.
This nerve is known as the hypoglossal. The second spinal
nerve is large; it leaves the neural canal in the foramen
between the second and third vertebrae, and passes straight
outwards, supplying branches to the shoulder-girdle and arm ;
hence it is called the brachial nerve. It is joined more or
less completely, at a little distance from its exit from the verte-
bral column, by the third spinal nerve. The two nerves supply
a number of small branches beyond the point where they unite;
and these branches, inosculating with one another, form a net-
work known as the bracMal plexus. The fourth, fifth, and
sixth spinal nerves run obliquely backwards from the inter-
vertebral foramina proper to them, and supply the muscles of
the trunk. The seventh, eighth, ninth, and tenth spinal nerves
(the last-named passing out of the foramen of the urostyle)
run obliquely backwards to the region of the pubis, and there
the seventh and eighth unite to form a single trunk, which is
joined a little farther back by the ninth, and, a little farther
back still, by a branch of the tenth. The large nerve so
formed is the sciatic, and the network formed by the union
of the nerves which are combined in it is called the sciatic
plexus.* As the spinal cord narrows rather suddenly in the
region of the sixth nerve, the roots of the seventh, eighth,
ninth, and tenth start from the spinal cord considerably in
front of the inter-vertebral foramina through which they succes-
sively pass to the exterior, and they form, together with the
filum terminale, a bunch of parallel cords within the posterior
part of the neural canal, which is called the cauda equina.

* The details of the sciatic plexus are subject to some variation, but the
arrangement described here may be considered normal.

6i

The crural nerve arises from the seventh spinal at or just
before its union with the eighth, and runs to the ventral and
outer border of the thigh, supplying the muscles and skin of
that region.

The sciatic nerve is the largest in the frog's body, and is of
great importance in certain physiological experiments. It
passes into the. leg close to the posterior end of the urostyle,
and runs down between the muscles of the thigh on the inner
or hinder side, following, in its course, the biceps muscle of
the thigh. In the upper part of its course it gives off several
branches to the thigh muscles, and a little way above the knee
it divides into two branches, the peroneal and the tibial nerves.
The former runs down the crus on the outer side of the tibio-
fibula, and is eventually distributed to the foot. The tibial
nerve runs down the crus on the inner border of the gastroc-
nemius muscle, giving off branches to it, and eventually supplies
the sole of the foot.

The tenth spinal nerve, which passes through the small
coccygeal foramen in the urostyle, besides contributing a
branch to the sciatic, sends twigs to the bladder, the cloaca,
and the adjacent parts.

Each of the spinal nerves arises from the spinal cord by two
roots, a dorsal or posterior, and a ventral or anterior root. The
dorsal root is also called sensory or afferent, because it is com-
posed of fibres, which only transmit impulses from without
inwards — from the tissues to the spinal cord and brain. The
ventral roots are. also known as motor or efferent, because their
fibres only transmit impulses in the reverse direction, from the
cerebro-spinal axis to the tissues. The two roots unite to
form one just as they pass out through the inter-vertebral
foramina, and the dorsal afferent root has an enlargement or
ganglion on it just before it joins the ventral root. The spinal
nerves are connected with the sympathetic nervous system,
but before describing this it will be well to consider the brain
and cranial nerves.

The brain is directly continuous with the spinal cord, and
a small canal, which traverses the centre of the latter, expands
into the brain to form a series of chambers known as the
ventricles of the brain.

The brain may be considered as consisting of three parts —
the hind-brain, the mid-brain, and the fore-brain.

62 COMPARATIVE ANATOMY

The hind-brain of the frog is a somewhat enlarged but
direct forward continuation of the spinal cord. It has two
divisions, the medulla oblongata (also called the bulb, or
myelencephalon) and the cerebellum (metencephalon). The
spinal cord, as it passes forwards into the bulb, widens out,
its floor becomes thicker, its roof very much thinner, and its
central canal widens out to form a triangular cavity, the
fourth ventricle, whose exceedingly thin roof is covered over
by a very vascular membrane. The cerebellum, relatively
large in many vertebrates, is represented in the frog by a
narrow band of nervous tissue lying transversely over the most
anterior part of the bulb. The mid-brain (mesencephalon) lies
immediately in front of the hind-brain, and in a line with it.
Its roof is formed by a pair of ovoid swellings, called the
optic lobes or corpora bigemina ; its floor, which is thick, con-
sists chiefly of nerve fibres running forward from the bulb to
the fore-brain. Each optic lobe is hollow, and its cavity com-
municates with a narrow passage called the Sylvian aqueduct,
leading from the cavity of the fourth ventricle to the cavity of
the fore-brain in front. This passage is sometimes called the
"iter." (Iter a tertio ad quartum ventriculum.)

The fore-brain consists of two parts, the thalamencephalon
behind, and the cerebral hemispheres or prosencephalon in
front. The side walls of the thalamencephalon are thickened
to form the optic thalami, but its roof and floor are thin, and
the former is covered over by a very vascular membrane,
just as is the roof of the fourth ventricle. By reason of the
thickening of its lateral walls, and the thinning out of its floor
and roof, the cavity of the thalamencephalon, known as the
third ventricle, is deep dorso-ventrally but narrow from side
to side. Its floor is produced into a conical depression, the
infundibulum, and its roof is produced into a hollow finger-
like projection, on the top of which is borne a rounded
vascular body usually known as the pineal gland. It appears,
however, that what is usually called the pineal gland is nothing
more than a thickened portion of the choroid plexus or
vascular membrane which covers this part of the head. The
true pineal gland is a small vesicle lying outside the skull
beneath the skin. In the embryo this vesicle is connected by
a pedicle with the roof of the thalamencephalon, but in the
adult frog the connection is lost, and there is no foramen

ANATOMY OF THE FROG 63

between the parietal bones whereby the pineal gland can make
communication with the brain. All that remains inside the
skull, therefore, is the hollow pedicle. The pineal body is
a structure of much interest which occurs in the brains of all
Craniata. The older anatomists fancifully described it as
being the seat of the soul, but it really is the relic of a once
well-developed and functional sense-organ, having the character
of an eye, with lens, retina and nerve, the last being re-
presented by the pedicle. Such an eye, in a more or less
degenerate condition, actually occurs in several living reptiles,
and in them there is a distinct foramen between the parietal
bones through which the nerve or pedicle passes to the eye.
This parietal foramen is obsolete in living Batrachia, but was
universally present in certain extinct Amphibia — the Stego-
cephalia, for example, — and it is inferred that in them the
pineal eye was well developed and functional. It is worthy
of remark that the pineal organ, though it has the structure
of an eye, does not resemble the paired eyes of Vertebrata,
but rather resembles those of certain Invertebrata.

In front the third ventricle is bounded by a wall of nervous
tissue called the lamina terminalis. Right and left of this a
passage leads into the cavities of the foremost division of the
fore-brain, the cerebral hemispheres, which together constitute
the prosencephalon. The hemispheres are ovoid bodies of
considerable size relatively to the other parts of the brain.
Their smaller ends are directed forwards, and produced in
front into two corresponding rounded swellings, the olfactory
lobes, or rhinencephala. The hemispheres are united together
before and behind by the fusion of their inner walls, but are
completely separated from one another in their middle portions
by a deep vertical cleft extending from the dorsal to the
ventral surface. The rhinencephala are fused together by
their inner walls, but both in them and in the fused portions
of the cerebral hemispheres shallow median furrows on the
dorsal and ventral surfaces mark their double origin.

The cavities of the cerebral hemispheres are known as the
lateral ventricles. They are separate from one another, but
each opens by a short narrow passage into the third ventricle
just behind the lamina terminalis. The Y-shaped opening
thus produced is known as the foramen of Munro. The
lateral ventricles are ovoid cavities with rather thin walls in the

64 COMPARATIVE ANATOMY

embryo, but, as growth proceeds, the anterior and inner part

Fig. ii.

The Brain, Cranial Nerves, and anterior part of the Spinal Cord
of the Frog dissected from the dorsal surface. Cb, cerebral
hemispheres ; Thai, Thalamencephalon ; in the middle of
it is seen the hole by which the stalk of the pineal body
communicated with the cavity of the third ventricle ; O L,
corpora bigemina (mid-brain) ; Cer, cerebellum ; B, the
bulb or medulla oblongata ; 4V, the fourth ventricle ; /,
The olfactory or first pair of cranial nerves ; //, the optic,
second pair of cranial nerves ; l-'a, the ophthalmic branch of
the trigeminal or fifth pair of cranial nerves ; Vi>, the
maxillary branch, and Vc, the mandibular branch of the
trigeminal. The two last branches spring from a large
swelling on the main stem of the nerve ; this is the
Gasserian ganglion ; VII, the main branch of the facial
or seventh pair of cranial nerves crossing over Col, the
columella auris, and just beyond this sending back a branch
to connect with the ninth nerve ; rp, the ramus palatinus
of the seventh nerve, passing below the eyeball and
distributed to the skin on the snout and anterior part
of the palate ; VIII, the auditory or eighth pair of cranial
nerves ; IX, the glossopharyngeal or ninth pair of cranial
nerves ; X, the pneumogastric, vagus, or tenth pair of
cranial nerves. The vagus has a large ganglion on its
root which is joined by the anterior extension of the
sympathetic system coming from the first sympathetic
ganglion lying under Hy, the hypoglossal or first pair of
spinal nerves : Br, the brachial second pair of spinal
nerves. The third, fourth, and sixth pair of cranial nerves
passing to the muscles of the eyeball are not shown in the
figure.

of the wall of each becomes much thickened, and bulges into

ANATOMY OF THE FROG 65

the cavity, reducing it to a crescentic slit in its lower portion.
The thickened prominences are known as the corpora striata.

Ten pairs of cranial nerves are given off from the brain.

The first pair springs from the anterior end of the
rhinencephala, and passes straight to the olfactory chambers ;
these are the olfactory nerves. The second or optic pair
arises from the sides of the brain beneath the optic lobes.
Each nerve starts as a broad band of fibres which runs forward
and downward to meet its fellow of the opposite side on the
under surface of the thalamencephalon just in front of the
infundibulum. Here most if not all of the fibres of the optic
nerves cross over to the opposite side, the point of their
decussation being called the optic chiasma. From the chiasma
each nerve runs outwards through a foramen in the cranial
wall, and passes into the eyeball.

The third, fourth, and sixth pairs of cranial nerves are dis-
tributed to the muscles of the eye.

The eyeball is moved by six muscles passing from its equator
to the walls of the orbit. Four of these, attached close
together to the inner posterior angle of the orbit, are known
as the recti muscles, and are attached respectively to the
upper (rectus superior), lower (rectus inferior), posterior
(rectus posterior), and anterior (rectus anterior) sides of the
eyeball. In addition to these a muscle arising from the
anterior inner angle of the orbit passes obliquely backwards,
and is inserted on the lower surface of the eyeball (inferior
oblique), and another (the superior oblique), arising close to
the origin of the inferior oblique, passes obliquely upwards and
backwards and is inserted on the upper surface of the eyeball.
These muscles occur in all craniate vertebrates ; the frog has in
addition' a musculus retractor bulbi, which partly surrounds
the optic nerve, and lies within the cone formed by the four
recti muscles. The third nerve, called the motor oculi,
supplies the recti superior, inferior, and anterior, and the
obliquus inferior. The fourth nerve supplies the superior
oblique, and is known as the pathetic or trochlear nerve. The
sixth, or abducens nerve, passes to the posterior rectus and gives
off a branch to the retractor bulbi. The third nerve rises from
the floor of the mid-brain near the median line, the sixth pair
from the ventral surface of the medulla, and also close to the
median line ; but the fourth pair differs from all the other cranial

66 COMPARATIVE ANATOMY

nerves in arising from the dorsal side of the brain between the
medulla and optic lobes.

The fifth pair of nerves, called the trigeminal, is the largest
of the cranial nerves of the frog. The nerve of each side
arises from the side of the anterior end of the medulla
oblongata, runs outwards and forwards to the cranial wall, and
passes through a foramen which is partly bounded by a notch
in the inner side of the pro-otic bone. Just before it reaches
the foramen the trigeminal swells out to form the large
Gasserian ganglion. After passing through the foramen the
nerve runs along the anterior face of the auditory capsule and
divides at once into two large branches, the ophthalmic branch
and the maxillo-mandibular branch. The ophthalmic branch
runs forward close to the cranial wall between it and the
eyeball. At the front end of the orbit it divides into two
nerves which pass through apertures in the walls of the olfactory
capsule and supply the nose and the skin of the front of the
head. The maxillo-mandibular nerve, after a short course in
front of the auditory capsule, divides into two branches — an
upper, the maxillary nerve, which runs forwards and outwards
between the eyeball and the lower and outer wall of the orbit
to the margin of the upper jaw, and the lower mandibular
nerve, which turns backwards, outwards, and downwards
to the squamosal bone, and passes across to the lower
jaw, where it runs forward on the outer side to the
symphysis.

The seventh, or facial nerve, arises from the medulla, close
behind the fifth, and runs forward close to the Gasserian
ganglion, with which it unites. It leaves the cranium in close
company with the mandibular branch of the trigeminal, and
divides at once into (i) a palatine branch, which runs forward
on the floor of the inner side of the orbit, and eventually
makes connections with the maxillary branch of the fifth, and
supplies the roof of the mouth ; (2) a hyomandibular branch,
which runs outwards and backwards across the wall of the
auditory capsule to reach the columella auris ; crossing over
the inner end of this, it turns down to the eustachian tube and
angle of the mouth. The seventh is joined close to the
columella by a branch from the ninth.

The eighth, or auditory nerve, arises from the medulla,
immediately behind the seventh, goes straight through a

ANATOMY OF THE FROG 67

foramen in the auditory capsule to the ear, and supplies the
organ of hearing.

The ninth, or glossopharyngeal nerve, arises, in common
with the tenth nerve, from a number of roots on the side of the
medulla, behind the auditory nerve. It passes, in company
with the tenth nerve, through a foramen at the back of the
cranium just in front of the occipital condyle, and divides into
two branches, an anterior, which runs forward to join the facial
nerve close to the columella, and a posterior, which curves
downwards and inwards to the floor of the pharynx, along
which it runs, supplying the petrohyoid muscles and the
mucous membrane of the tongue and pharynx.

The tenth, pneumogastric or vagus nerve, is a very
important nerve with complicated relations. After leaving the
skull in company with the ninth nerve, it enlarges to form a
ganglion, the ganglion nervi vagi. The nerve runs backwards
and ventralwards along the side wall of the pharynx, and
finally divides into several branches, of which the most
important are : (i) the nervous recurrens or laryngeus, which
arrives at the posterior cornu of hyoid, and turns forward to
pass beneath it and the pulmo-cutaneous artery, whence it runs
forward close to the middle line, to end in the larynx; .(2)
the ramus cardiacus, which runs along the dorsal surface of
the pulmonary artery and the superior vena cava towards the
sinus venosus, where it joins its fellow of the opposite side, and
the two pass on to the auricular septum of the heart; (3) the
rami gastrici, which run through the partial diaphragm formed by
the insertion of the internal oblique muscle upon the walls of the
03sophagus, and are distributed to the walls of the stomach ; (4)
the rami pulmonales, which also perforate the partial diaphragm,
and then follow the course of the pulmonary artery to the lung.

The sympathetic nervous system (see fig. 10) is intimately
connected with both spinal and cranial nerves. It is a chain of
paired nervous ganglia, joined together by longitudinal cords,
lying on either side of the vertebral column, and each ganglion
makes connection with a spinal nerve by a short branch, the
ramus communicans. In the anterior part of the body the
sympathetic chains lie right and left of and parallel to the
vertebral column ; at about the level of the sixth vertebra they
approach one another in the middle line, and become closely
connected with the dorsal aorta, alongside of which they run.

68 COMPARATIVE ANATOMY

The first sympathetic ganglion lies on the hypoglossal nerve,
close to its exit from the first inter-vertebral foramen. Its
ramus communicans is represented by a few fine and very
short fibres connecting the ganglion with the nerve. From
the ganglion two or three nervous strands pass backwards, one
passing beneath and one or two above the brachial artery, so
as to encircle it, as it were, with a ring. This ring is known
as the annulus of Vieussens. The second ganglion lies on
the brachial nerve ; it is the largest of the whole series, and,
like the first, is connected with the spinal nerve by a few fine
fibres which do not form a distinct ramus communicans.
The third ganglion is usually fused with the second, but has
a short and distinct ramus communicans with the third spinal
nerve. After this point the sympathetic chain follows the
course of the systemic aortic arch, and when this joins its
fellow to form the dorsal aorta the sympathetic chain follows
the course of the latter. Being thus farther removed from the
vertebral column, the rami communicantes become longer.
There are as many sympathetic ganglia as there are spinal
nerves, and each makes communication by its ramus com-
municans with the spinal nerve proper to it. Thus the fourth,
fifth, sixth, and successive spinal nerves up to the tenth,
supply each a ramus communicans to a sympathetic ganglion,
the tenth nerve being peculiar in making several connections —
as many as ten or twelve in some cases— with the sympathetic
chain, but the number is not constant. The seventh, eighth,
and ninth are also said to have two or three communications
a-piece with the sympathetic chain.

The sympathetic chains of each side are connected by
numerous fine twigs, which surround the dorsal aorta and
form a plexus around it, and from this plexus very fine nerves
pass to the adjoining organs. The sympathetic system is
characterised by the fact that its branches divide and sub-
divide, and the sub-divisions interlace and anastomose with
one another to form networks or plexus which include
numerous ganglia. The two most important of these are : (i)
the cardiac plexus, formed by nerves arising from the first
ganglion of the chain. It lies on the auricles and surrounds
the great blood-vessels at their openings into the heart ; (2)
the solar plexus, formed by branches from the third, fourth,
and fifth ganglia, and lying on the dorsal side of the stomach.

ANATOMY OF THE FROG 69

Anteriorly the sympathetic chain makes communications with
the cranial nerves by a branch which passes on either side
from the first ganglion to the ganglion nervi vagi, where some
of its fibres enter the vagus, the remainder pursuing their
forward course to enter the Gasserian ganglion of the fifth
nerve.

We may here consider two collections of nervous ganglia
in the heart which are connected with the vagi and the
sympathetic system. The two vagi pass to the sinus venosus
and enter a nervous ganglion situated in its wall and known
as Eemak's ganglion. Thence the vagi pass into the auricular
septum, one in its dorsal and the other in its ventral portion,
and run backwards to the region of the auriculo-ventricular
groove where they enter another collection of nerve-cells
known as Bidder's ganglia. From these ganglia fibres are
distributed to the rest of the heart. These two minute collec-
tions of nervous matter are of importance to physiologists.

The eyeball of the frog is attached to the skull by muscles
which have already been described. The eyeball is not
spherical, but is flattened on its outer side, more convex on its
inner side. It consists of the following parts : (i) a firm outer
wall, the sclerotic, formed of cartilage and dense white con-
nective tissue ; (2) the cornea, a transparent area on the ex-
posed part of the eyeball, through which light is admitted into
its interior : it is continuous with the sclerotic ; (3) a coloured
curtain, the iris, lying behind the cornea, and surrounding a
central aperture, the pupil. The iris is provided with circular
and radical muscle fibres, by means of which the size of the
pupil can be diminished or enlarged. In the interior of the
eye a firm transparent spheroidal body lies behind the iris,
attached to its outer margin. This is the lens, and it serves to
focus light upon the back of the eye. Between the corner and
the lens is a small space, the anterior chamber of the eye, filled
with a watery fluid, the aqueous humour. Between the lens
and the back of the eye is a much larger space, the posterior
chamber of the eye, filled with a gelatinoid material, the
vitreous humour. The sclerotic is lined by a black pig-
mented layer called the choroid, which is continued in front
into the iris. Inside of this, lining the chamber of the eye, is a
delicate transparent membrane, the retina, on which the
ramifications of the optic nerve are spread. It is the sensitive

70 COMPARATIVE ANATOMY

portion of the eye, and the cornea, iris, lens, etc., are structures
adapted to focus the light upon it.

The frog's ear is embedded in the periotic capsule, which, as
we have seen, is largely cartilaginous, with an anterior ossifica-
tion, the pro-otic. The essential organs of hearing are deeply
embedded in the cartilage, and only communicate with the
surface by an accessory apparatus consisting of the tympanic
cavity, the eustachian tube, and columella.

The tympanic cavity, bounded externally by the tympanic
membrane, is seen, after removal of the latter, as a shallow,

The right membranous labyrinth of the frog's ear
seen from the inner side. U, utriculus, from which
spring ace, the anterior semicircular canal with
its ampulla aa ; pec, the posterior circular canal
with its ampulla amp \ ext.cc, the external
horizontal semicircular canal, with its ampulla
amp, represented as showing through the trans-
parent walls of the utriculus ; .9, the sacculus; de,
the ductus endolymphaticus ; lag, lagena ; pbc,
pars basilaris cochleae. (Adapted from Retzius.)

funnel-shaped cavity, lined by a pigmented mucous membrane,
and communicating by the wide, short passage of the eusta-
chian tube with the pharynx. The inner and deeper part
of the cavity presents a small aperture leading into the inner
ear. This aperture, called the fenestra ovalis, is blocked
by the enlarged cartilaginous end of a partly bony, partly
cartilaginous rod, the columella auris, which traverses the
tympanic cavity and is fixed by its outer end in the tympanic
membrane. The tympanic cavity and eustachian tube
together constitute the middle ear, and are derived from the
most anterior of the gill-clefts of the tadpole. The inner
ear consists of a membranous sac lying in a corresponding

ANATOMY OF THE FROG 71

cavity of the cartilage of the auditory capsule. Between the
cartilage and the sac is a fluid called perilymph. The sac
also has fluid contents called endolymph. A constriction
partly divides the sac into two portions: (i) an upper and
larger division, called the utriculus ; (2) a lower and smaller divi-
sion, with three small dilatations on its posterior face, called
the sacculus. 1'he last-named also gives off from its inner
and upper border a tubular offset ending in a thin-walled
dilatation. This is called the ductus endolymphaticus.

The utriculus has more complex relations. Three semi-
circular canals, an anterior, a posterior, and an external, open
into it. Of these the anterior semi-circular canal lies in the
median or sagittal plane of the head ; at its anterior end, just
where it joins the utriculus, it is dilated to form an ampulla,
and its posterior end joins the posterior semi-circular canal
which opens, in common with it, into the utriculus. The
posterior semi-circular canal lies in the transverse plane open-
ing by its upper end into the utriculus, in common with the
anterior canal, and at its lower end it dilates into an ampulla be-
fore opening into the utriculus. The external canal lies in the
horizontal plane, and has an ampulla at its anterior end. Thus
the three canals lie at right angles to one another in the three
dimensions of space; they open into the utriculus at both
ends, and each has an ampulla at one end. The auditory
nerve, passing through an aperture in the inner wall of the
auditory capsule, divides into branches, which are distributed to
the utricle, the saccule, and the ampullae. In the interior of
the membranous labyrinth are the elements sensitive to sound-
waves, in the form of modifications of the lining epithelium
bearing stiff sensory hairs. There are also peculiar calcareous
concretions, called otoliths, especially abundant in the ductus
endolymphaticus.

The olfactory organs of the frog consist essentially of a pair
of sacs separated from one another by a median cartilaginous
septum, and opening to the exterior by the anterior nares, into
the buccal cavity by the posterior nares. The cavity of each
sac is divided up and complicated by the projection into it of
cartilaginous offsets of the walls of the nasal chambers, and
three main subdivisions or sinuses have been recognised in
each sac — a dorsal, a ventral, and a lateral sinus. These
sinuses are lined by an epithelial membrane which in the

72 COMPARATIVE ANATOMY

olfactory region is modified to serve as the percipient
element.

We have thus far treated of the anatomy of the frog as it
may be learned from simple dissection ; and we have not, in
dealing with the various tissues and organs, considered the
ultimate elements of which these are composed. But we have
already gathered important information respecting the plan of
structure of the frog's body. We have seen that it has a
special cavity, the coelom, in which the abdominal viscera
appear to be suspended, so we recognise the frog as belonging
to the group Coelomata. From its possessing a vertebral
column with a central nervous tube placed dorsally to it we
recognise it as being a Vertebrated animal ; from its having a
distinct head we recognise it as being a member of the
Craniate branch of the vertebrata. As it has distinct jaws it
must be placed among the Craniata G-nathostomata ; and, as
it has five-fingered arms and legs, and not fins, it must be a
member of the sub-grade Pentadactyla of that group. Finally,
the smooth glandular skin, the absence of nails or claws, the
fact that it breathes when a tadpole by gills, when adult by
lungs, show it to be a member of the class Amphibia.

CHAPTER III
THE HISTOLOGY OF THE FROG

WE must now consider shortly the ultimate elements of which
the complex mechanisms of the frog are built up, and we shall
begin best by taking the simple case of the blood.

The blood differs slightly in colour according as it is taken
from an artery or a vein. In the former case it is bright
scarlet in colour, in the latter dark and almost purple in hue,
but in both cases the microscopical characters are the same.
The blood is an opaque viscid fluid, slightly alkaline in re-
action, and consists of a fluid matrix, the plasma, in which
solid particles of minute size, the corpuscles, are contained.

The plasma is a transparent fluid, of a pale yellow colour,
containing albumens and some salts in solution. The cor-
puscles are of two kinds, red and white, and the white
corpuscles may further be divided into three kinds. The
red corpuscles, or haematids, are far more numerous than the
white. Each is a flattened oval disc, measuring some 0x323
mm. in its widest and some 0-014 mm- m its shortest
diameter ; it is pale red or yellow in colour, and presents a
central bulging due to the presence of a specialised central
portion, the nucleus. If a film of blood is treated with certain
dyes, such as carmine, haematoxylin (a preparation of log-
wood), magenta, etc., the flattened part or body of the
corpuscle is but little affected by the stain, but the nucleus
absorbs it readily, and may be seen to contain a network of a
substance which stains more deeply than the rest. This intra-
nuclear network, because of its strong affinity for certain
colouring matters, is called chromatin. The body of the
corpuscle may also be observed to be made up of a network
of a denser substance containing a more fluid substance in its
meshes. The red colour of the body of the corpuscle is due
to the presence of an iron-holding compound named haemo-
globin, which can be separated out by shaking up blood with

73

74 COMPARATIVE ANATOMY

ether. When separated, the haemoglobin crystallises out in
prisms. The hsematids do not spontaneously alter their shape,
but they are very elastic, so that they can easily be squeezed
through passages smaller than themselves, and regain their
shape after the passage, as may be seen in the capillaries of
the frog's foot.

The white corpuscles or leucocytes are much fewer in
number than the red, though their relative number varies con-

"*•«$'

Fig. 13-

a, Red blood corpuscles (haematids) of the frog, stained with safranin and
much magnified, to show the nucleus and nuclear network. _ bi, an
amoeboid coarsely granular leucocyte from the frog's blood, showing trifid
nucleus ; 62, 1>J, l>4, other forms of leucocyte from the frog's blood, c,
discoid non-nucleated hjematids from human blood, much magnified ;
ci, C2, cj, different forms of leucocytes from human blood.

siderably in different individuals, and in the same individual
at different times. A leucocyte, as it appears in freshly-drawn
frog's blood, when viewed under the microscope, is a minute
colourless body of smaller size than the red corpuscles, measur-
ing some 0-017 mm- m diameter. It consists, like the hrematid,
of a protoplasmic body, containing a nucleus. The nucleus is
not easily seen in a living corpuscle, but it becomes apparent
on treatment with acetic acid, and it stains deeply with carmine
and other colouring matters, whilst the protoplasmic body is

75

scarcely affected. Some of the leucocytes contain only a
single nucleus, but, as a rule, they have two, three, or even four
contained in a single protoplasmic envelope. The body of
the leucocyte has no definite outer limiting membrane, and no
constant shape. It usually contains granules of different
kinds, and a few clear spaces called vacuoles, filled with fluid.
Simple as its structure is, the leucocyte is living, endowed
with the power of spontaneous movement, and capable of ex-
hibiting by itself some of the fundamental properties of living
organisms. In a sense a leucocyte may be said to be an
independent organism, but its independence is not complete.
It is incapable of separate free existence apart from the blood
which forms its normal environment, and therefore we cannot
isolate leucocytes, and study them apart from the blood, and
deal with them as independent living beings. But within the
blood as it courses through the tissues, or in blood freshly
drawn from the body, the leucocyte behaves as a separate
organism complete in itself. That it is contractile, and capable
of change of shape, is easily seen on simple inspection of a
sample of freshly-drawn blood. The leucocytes do not remain
still, nor do they retain a constant form as do the hsematids,
but are continually in movement, putting forth irregular blunt
processes of their protoplasm in various directions, and again
withdrawing them. These blunt movable processes are called
pseudopodia, and the movement due to them, from its resem-
blance to that of a minute organism, the Amceba, which we
shall study presently, is called amoeboid. That the leucocytes
are "automatic" — that is to say, endowed with the power of
spontaneous movement — is shown by their constant changes of
form ; changes which, so far as can be determined, are called
forth by no definite external stimulus. But the corpuscle is
also irritable; for if the blood be very gently warmed the
activity of the pseudopodial movement is increased ; if the
temperature be much lowered it will be decreased or stopped
altogether, to recover when the temperature is raised again.
Certain drugs also, such as quinine, retard the activity of the
leucocytes. The leucocytes are assimilative, for they surround
and take into their substance foreign substances; many of
them are seen to be filled with fat granules, and it has been
shown that these are ingested, swallowed, as it were, during the
passage of the blood through the intestine. And leucocytes

76 . COMPARATIVE ANATOMY

ingest many other substances such as bacteria, this pheno-
menon being known as phagocytosis. The fat granules
may be subsequently extruded from the corpuscle, or they
may be used up and converted into its substance, and
therefore the leucocyte is assimilative and metabolic; for,
although we cannot measure the expenditure of energy nor
trace the formation and rejection of waste material in so small
a body, we are certain that the exhibition of energy in the
form of pseudopodial movement must involve a chemical
change, a decomposition of the chemical substances of which
the corpuscle is composed, and the production of waste matters
as a result of the decomposition. And the leucocytes are
reproductive, for they may be observed to divide into two,
each portion containing a part of the nucleus, and the daughter
leucocytes thus formed carry on the existence of the parent
form which gave origin to them, and lead independent exist-
ences as individual leucocytes. In short, the leucocyte mani-
fests all the activities characteristic of a living being, and
withal it is nothing more than a speck of that peculiar life-
substance which we call protoplasm, showing scarcely any
structure, beyond that it is divisible into nucleus and cell-body
— for the leucocyte is a fairly typical example of what is called a
cell ; and, before we consider the different kinds of leucocytes
which occur in the blood and lymph of a frog, we may pause
for a moment to consider what a cell is.

" A cell is a corpuscle of protoplasm which contains within
it a specialised component particle, the nucleus." Such is the
definition of a cell given by one of the most notable living
authorities on the subject, and, for our present purposes, the
definition may be accepted. But it must be remembered that
it is well-nigh impossible to frame satisfactory definitions
concerning living things. " A cell is a corpuscle of proto-
plasm containing a nucleus." But we have just seen that a
leucocyte, a single corpuscle of protoplasm, may contain two or
more nuclei. Is it still a cell ? or, does the increase of the
number of nuclei remove it from our definition ? In the case
of the leucocyte, certainly not. The double, treble, or
quadruple nucleus is clearly brought about by division of a
single nucleus, and the parts are smaller than the whole from
which they were formed. And it appears that when a multi-
nuclear leucocyte reproduces itself by division each nuclear

HISTOLOGY OF THE FROG 77

fragment does not divide prior to the division of the cell-
body, so that the new daughter cells may contain as many
nuclei as there were in the parent cell, but that a half of the
nuclei pass over to one cell, the other half to the other cell
formed by division. Moreover, the occurrence of truly multi-
nucleate leucocytes is much rarer than is commonly supposed ;
for, even where two or three nuclei seem to be present, close
inspection reveals the fact that they are united together by
strands of nuclear substance. In fact, the nucleus of the
leucocytes is rather irregular and lobate than divided into
separate portions, and it is the irregular lobes folded over
one another which give the appearance of several nuclei. Such
irregular nuclei are in other respects similar to normal spherical
or sub-spherical nuclei, contain a network of the deep-staining
substance, chromatin, and in division behave as do ordinary
nuclei. (See p. 76.)

But, as we shall see farther on, there are many unequivocal
instances of numerous independent nuclei being enclosed in
a single mass of protoplasm. Some authors regard these as
single cells, because the protoplasmic mass, the cell-body, is
single. Others hold an opposite opinion, and it is not an
easy matter to decide which of the two opinions is to be
preferred. But, as a matter of convenience, it is well to call
a single mass of protoplasm containing more than one nucleus
by a special name — viz. coenocyte.

We have said that a cell consists of a mass of protoplasm
containing a nucleus. But this is not a satisfactory definition ;
for protoplasm is a vague general term which at the present
day has no very exact meaning attached to it. Protoplasm,
the material basis of life, was first discovered by Dujardin, who
called it sarcode. Dujardin stated precisely what he meant
by sarcode. "It is that," he said, "which other observers
have called a living jelly — that gelatinous, diaphanous structure,
insoluble in water, which contracts into globular masses,
attaches itself to one's dissecting needles, and can be drawn out
in strings like mucus." Further than this, Dujardin, recognised
that sarcode must be organised, must possess a structure. Not
that he was able, with the imperfect microscopes of the day, to ob-
serve and describe a structure, but he felt bound to postu-
late one, because of the vital activity of the substance. " Sar-
code is without visible organs, and without the appearance

78 COMPARATIVE ANATOMY

of cellulosity, but it is nevertheless organised, since it emits
diverse prolongations carrying granules with them, extending
and retracting themselves alternately — in a word, it has life."

The word " protoplasm " is due to a botanist, Hugo von
Mohl, who, in 1846, described the constitution of the so-called
primordial utricle in plant cells — a semi-fluid, gelatinoid,
living substance, similar to the sarcode already described by
Dujardin. But, curiously enough, the identity between the
sarcode of animals and the protoplasm of plants, does not
seem to have struck the physiologists of the time. One
distinguished anatomist, Robert Remak, did indeed perceive
the identity between them, and applied the term protoplasm to
the substance composing animal cells, but his example was
not generally followed, and it was not until 1861, that the
modern conception of protoplasm was expounded by Sigis-
mund Max Schultze, to whom also we owe the definition of
a cell as given above — "a corpuscle of protoplasm within which
lies a nucleus" (Ein Kliimpchen Protoplasma, in dessen
Innerem ein Kern liegt).

Max Schultze showed, by means of many beautiful and
convincing experiments, the identity in the behaviour of the
protoplasm of plants and animals ; that both consist of a
hyaline, contractile, jelly-like ground substance containing
minute granules ; that in both animals and plants the
granules may be seen to be in constant movement, streaming
in definite courses through the ground substance ; that the
effects of heat, electric stimulus, and chemical action are the
same in plant as in animal protoplasm.

But still the term protoplasm has remained vague, and at
the present day there are several rival theories as to its con-
stitution. The name itself has become too general, and
though it remains in use, it is on the tacit understanding
that it means the substance of the cell-body of such primitive
cells as we call undifferentiated. The cell-body of the
leucocyte is a good instance of what is meant ; but nowadays
it is usual to refer to the material of the cell-body as cytoplasm,
and to that of the nucleus as nucleoplasm, it being understood
that all cytoplasms are not the same, and that both it and
nucleoplasm are not simple and homogeneous, but complicated
mixtures.

We may regard the terms " protoplasm " and " cytoplasm "

HISTOLOGY OF THE FROG 79

as being nearly synonymous, the former having a slightly
wider and more general meaning than the latter.

The ultimate structure of protoplasm, as we have said, is
a matter hotly debated at the present time, and it would be
out of place to enter here into a discussion of the different
views of different observers. All are agreed on one thing, that
"protoplasm" is not a chemical compound of definite
formula, but a mixture of several different kinds of albumin-
ous bodies, each of great molecular complexity. Speaking
generally, it may be said that protoplasm is a mesh-work, or
rather a sponge-work, of a denser albuminous substance, in
the spaces of which a more fluid substance is contained. The
granules so generally present are of various kinds, and may be
fatty, albuminous, or starchy; some of them are of complex
constitution, and have been described as behaving like a
miniature cell within the cell. The nucleus is described as
having a surrounding membrane composed of a substance
called amphipyrenin, the chemical distinctness of which is,
however, very doubtful. In the nucleus there is a network
of fibrils, composed of a substance called linin, which does
not stain with the usual dyes ; and there is a network of
coarser fibrils, composed of a deeply-staining substance
called chic-matin or nuclein. We know something of the
chemical constitution of nuclein ; it is a compound of a
complex organic acid — nucleic acid, — rich in phosphorus, with
albumen. Linin is probably an allied body, less rich in
phosphorus, and the amphipyrenin is probably identical with
linin. Other substances have been described in the nucleus,
but they need not detain us. The cell, then, is a very
complicated body, and it has an organisation, a complex
constitution, such as Dujardin postulated for his sarcode.
It consists essentially of a cell-body or cytoplasm and a
nucleus, and both cytoplasm and nucleus are mixtures of
several very complex chemical compounds whose formulae
have not yet been worked out.

It is clear that both kinds of corpuscles contained in the
frog's blood, the hsematids and the leucocytes, are cells. Both
have a cell-body and a nucleus, yet they differ in a marked
degree from one another. The leucocyte is comparatively
formless, and its characters are such as to afford an excellent
illustration of what we mean when we speak of an animal cell.

8o COMPARATIVE ANATOMY

It is, in fact, an embodiment of our idea of a cell, and is there-
fore what we call typical or generalised. The hagmatid, on the
other hand, has characters all its own. Its structure departs
from the general conception, because it is altered in connection
with its function as a carrier of haemoglobin, and it is what we
call specialised or differentiated. Its cell-body is not simple
protoplasm, but is in large part changed into something that
is not protoplasm but has been formed by or out of protoplasm.
This circumstance shows us that a cell has activities, that its
constituent protoplasm has the power of changing itself into
some special substance, or of manufacturing some special
substance, and depositing it within the limits of the cell. A
cell, therefore, has formative powers, in virtue of that mysterious
attribute of its constituent protoplasm which we call life. Life
is the sum-total of the activities of the cell.

The truth of this statement is borne in upon us when we
examine further the constitution of the other tissues of the
frog's body. All of them, skin, bone, cartilage, nervous tissue,
muscular tissue, gland tissue, are formed either of cells, or of
the union of the products of cells. But there are few of the
constituent cells of the frog's body which are as simple and
generalised as the leucocytes of its blood.

We may first of all study the composition of the membranes
which cover the surface or line the cavities of the body.
These are called epithelia ; and, since all epithelia are com-
posed of cells, the latter are known as epithelial cells. The
most simple kind of epithelium is perhaps that variety known
as columnar. Columnar epithelium is found in the intestine
of the frog, lining its cavity, and forming the coverings of the
minute processes, called villi, which project into the cavity.
Each component cell of a columnar epithelium is a prism
of greater or less height, the body of the prism being
cytoplasm, in the interior of which lies a nucleus. These
prismatic cells usually rest on a very fine basement membrane,
which is not cellular, but produced by the cells. The prisms
form a single layer, are pressed close together, and owe their
prismatic shape to mutual pressure. They are not so closely
packed, however, but that there are minute intercellular spaces
between adjoining cells, and these spaces, which communi-
cate with the lymph system, are generally traversed by
exceedingly fine protoplasmic fibrils passing from cell to cell,

HISTOLOGY OF THE FROG 81

and so establishing protoplasmic continuity throughout the
whole tissue. But, in spite of this protoplasmic continuity,
we recognise that the tissue is not a single tract of protoplasm
containing many nuclei, but is really composed of individual
cells, just as we should recognise that a house is really com-
posed of a number of rooms, though they all communicate
by means of doors. Columnar cells may be tall and thin,
or short and broad, or mid-way between these two extremes.
Their nuclei are usually placed mid-way between their free
ends and their bases.

Such cells as those just described are simple columnar
epithelial cells ; but there are many varieties of columnar
epithelium. One of the most common of these is ciliated
columnar epithelium, to be obtained from the frog's palate, or,
better, from the windpipe of higher vertebrates, or from the
gills of such a mollusc as the fresh-water mussel or the oyster.

If a minute scraping be taken with the point of a knife
from the roof of the buccal cavity of a freshly-killed frog,
and the whitish material thus obtained be mounted on a
glass slip in normal salt solution and examined under the
microscope, it will be seen to consist of a number of trans-
lucent rounded cells, in most of which a large nucleus can
be seen. There are several kinds of these cells, but some of
them are conspicuous because of a continual shimmering
movement on one of their surfaces ; if such a cell be isolated
from the remainder of the mass it will be seen to gyrate and
spin round and round in the salt solution. After some time
the movement becomes slower, and then it can be seen that
the cell bears on one surface a tuft of very fine, hair-like
transparent processes of highly contractile protoplasm, called
cilia. When the movements have slowed down, it may be
seen that the cilia work in unison, with a sort of lashing
movement, bending very sharply in one direction, and then
more slowly relaxing till they regain their straightness, when
they again bend sharply as before. Tracts of these cells cover
the frog's palate, and their cilia, by their united action, drive
mucus and small particles of foreign matter along in a given
direction. If the ciliated cells are killed by a drop of very
dilute osmic acid, and afterwards stained with carmine, they are
seen to be shaped like those represented in fig. 14, D. Each cell
is shaped like a truncated club ; its broader end is turned out-

82 COMPARATIVE ANATOMY

wards, and its edge is formed by a border of highly refracting
contractile substance differing in appearance from the cytoplasm
of which the rest of the cell -body is composed. From this
border the cilia project into the cavity of the pharynx. The
cell tapers gradually inwards, its inner end being fine and
pointed or drawn out into two or more irregular processes
which rest upon a basement membrane. The nucleus is
relatively large, placed in about the middle of the length of
the cell, which is rather bulged out by it. The ciliated cells
found on the gills of oysters and mussels are larger and have
longer cilia than those of the frog's palate, and afford a
very convenient means of studying ciliary action. The great,
flat, plate-like gills of these animals are covered with a ciliated
epithelium, whose cilia, constantly in motion, drive currents
of water through the gills in a definite direction. The cells
are long and columnar, tapering internally, and with large
nuclei. The refracting border on the free surfaces of the cells
may be seen to be made up of a number of minute refractive
knobs placed close together. Each knob bears a cilium on
its outer side, and internally it is prolonged into a fine varicose
fibril which runs down into the cell-body past the nucleus,
eventually uniting with similar fibrils from the other knobs
to form a single thread which runs into the pointed inner
extremity of the cell (fig. 14, D 3).

Several phenomena may be noticed in a fragment of the gill
of an oyster or mussel examined under the microscope. The
ciliary motion appears to sweep over the surface in waves, just
as a field of corn is thrown into waves by the wind passing
over it, and the waves always pass in the same direction. The
ciliary motion continues for some considerable time in the
excised piece of gill, and even in small fragments or single
cells which have been broken away with needles. This shows,
that the activity of the cilia is not due to impulses reaching
the cells by nerves from the nervous centres, but that it is
dependent on the activity of the cells themselves, which
remain living for some time after their removal from the body.
If the temperature is lowered the activity of the cilia is
diminished, and eventually is stopped altogether at some
degrees below freezing point, but it is revived on warming.
Up to a certain point warmth increases the activity of the
cilia, but above a certain point it retards, and soon stops their

HISTOLOGY OF THE FROG 83

motion altogether. The cilia pass into a stiffened condition,
known as heat rigor, from which they do not recover. In
warm-blooded animals the optimum temperature for ciliary
motion is about 45 deg. C. In cold-blooded animals the tempera-
ture of the body appears to be the optimum, but as this varies
according to the temperature of the surrounding medium, we
find the cilia more active in summer heat than in winter cold.
Oxygen, at least in its free state, is not essential to the action
of cilia, as may be shown by the fact that it will go on for
some time in water, which has been deprived of all its dis-
solved oxygen by boiling. This experiment, however, does not
prove that the source of energy is anything else than the oxida-
tion of the substance of the ciliated cells, but rather that the cells
can store up oxygen in a combined form for use when required.
Carbonic acid gas has a definite effect in arresting ciliary motion,
as can be seen if a stream of the gas is passed over a prepara-
tion of active ciliated cells. Their motion is at first accelerated,
but soons slows down, and may be arrested altogether. It
soon is restored, however, on the readmission of fresh air.
Chloroform vapour acts in the same way as carbonic acid gas.
Ciliated cells are found in various parts of the frog's body :
in the pharynx and olfactory passages ; on the edges of the
partitions separating the alveoli of the lungs ; in the central
canal of the spinal cord, and the ventricles of the brain ; in the
uriniferous tubules of the kidney, and in the ureters ; in the
oviducts, and, in the tadpole, on the epithelium covering the
transitory external gills.

Both columnar and ciliated epithelia belong to the variety
of epithelium known as simple, because their component cells
are arranged in a single layer. Another variety of simple
epithelium is that forming the thin membranes lining internal
cavities, such as the ccelom, the heart and blood-vessels, the
lymph-spaces, etc. This variety is often referred to under the
name endothelium, but may more conveniently be described
as pavement epithelium. -Pavement epithelium may be very
advantageously studied in the mesentery of the frog. Both
sides of the mesentery are covered with a very thin membrane
continuous with that lining the pleuro-peritoneal cavity. This
membrane is composed of very thin, flat, scale-like cells, which
may be polygonal, and fit together by their edges, like the tiles
in a pavement, but more usually have serrated or sinuous

84 COMPARATIVE ANATOMY

edges which dovetail into one another, like the wooden puzzles
of children (fig. 14, B\ Each scale-like cell has a distinct
nucleus which bulges out its central portion. In order to see
the outlines of pavement-epithelial cells it is necessary to treat
the membrane with nitrate of silver and expose it to the light.
The intercellular substance between the cells has the property

A

Fig. 14.

A, stratified epithelium from the oesophagus of the rabbit, seen in section. In the lower
part of the figure the connective tissue and muscular layers are shown. B, pavement
epithelium from the mesentery of the Frog, silver nitrate preparation ; /•.'/, E>,
goblet cells from the frog's mouth ; Dl, Dz, isolated ciliated epithelium cells from
the frog's mouth; Dj, an isolated ciliated cell from the gill of the mussel : (',
columnar epithelium from the intestine of the frog. (From a drawing by Mr K. H.
Schuster.)

of being able to reduce the nitrate in the sunlight, and is
accordingly filled with particles of metallic silver, which mark
out the outlines of the cells in black.

Stratified epithelium is distinguished from simple by the
fact that its component cells are arranged in several layers or
strata, it is commonly said like a heap of tiles ; but the simile

HISTOLOGY OF THE FROG 85

is not exact, for the tiles at the bottom of the heap would have
to differ from those at the top to make the comparison good.
Stratified epithelium is found in the epidermis or outer skin,
the cornea, and other parts of the frog. In the epidermis there
are several layers of epithelial cells ; the lowest are more or
less columnar in form, arranged in a definite row, and have
abundant cytoplasm in the cell-body : this layer is commonly
known as the stratum malpighi. The cells of the middle
layer become progressively more flattened and polygonal in
outline, and the outermost layer consists of quite flat trans-
parent scales, in which the nucleus is still present, but the
cell-body has been converted into a horny substance. The
skin of man is formed of a somewhat similar stratified
epithelium, with a more abundant horny outer layer ; this
extends into the mouth, and it is easy to scrape some of the
superficial layer from the roof of one's mouth, and observe the
horny scales of which it is composed. The epithelium lining
the urinary bladder of the frog (and of mammals) presents a
variety intermediate between columnar and stratified epithelium.
There are three layers of cells, but they do not over-lie one
another, nor are those of the outer layer horny and flattened ;
they are dovetailed together by their wedge-shaped and
serrated upper and lower ends, those of the outermost
layer, lining the cavity of the bladder, being almost columnar
in form. This is known as transitional epithelium.

Both in the epidermis and in the alimentary canal of the
frog there are numerous glands of simple structure ; and the
liver and pancreas are large glands of more complicated
structure. The essential part of these glands is the epithelium
lining their cavities, its cells being modified to form what
is known as glandular epithelium. There are also many
uni-cellular glands amongst the columnar and ciliated cells in
the buccal cavity, and amongst the stratified cells of the
epidermis. Excellent examples of these single-gland cells
are to be found amongst the ciliated cells scraped from
the palate. Each has the form of a modified columnar
cell with a broad free extremity and a narrow fine-pointed
internal extremity. The goblet or chalice-cell, as it is
called, elaborates a substance by the activity of its protoplasm,
obtaining the necessary material from the blood, or rather
from the lymph which surrounds it. This substance (which in

86 COMPARATIVE ANATOMY

the case of the chalice-cells of the palate is mucus or slime) is
called the secretum, and is stored up in granules in the distal
end of the cell between its nucleus and its free border. When
secretion takes place, the free wall of the cell bursts, and the
secretum is poured out; the cavity which it occupied in the
distal end of the cell remains as a hemispherical cup, which,
situated as it is on the attenuated inner extremity, gives the
whole cell the appearance of a goblet (fig. 14, E). After
secretion, the protoplasm of the cell again enters into activity,
and a fresh secretum is produced, to be discharged in due
course. The simplest kind of multi-cellular gland is found in
the stomach and intestines of the frog and other vertebrate
animals. The mucous membrane lining these organs, when
examined superficially with a microscope, is seen to be pitted
with innumerable minute orifices placed close together. These
are the mouths of simple tubular glands, each of which has
the form of a shorter or longer tube formed as a finger-like
recess in the connective tissue layer or corium underlying
the epithelium of the alimentary tract. These tubes, which
are longer in the stomach and shorter in the intestine (in this
last situation they are known as the crypts of Lieberkiihn),
are lined within by an epithelium, which is a continuation
of the columnar epithelium lining the cavity of the
alimentary tract. At the mouths of the glands the cells are
high and columnar, having the same characters as those
lining the cavity of the gut. In the next section of the gland,
known as the neck, the cells become progressively shorter and
more cubical, and in the deeper part of the gland they become
larger and polyhedral, and change their character, in that they
have contents in the shape of numerous granules, most
abundant in the ends turned towards the lumen of the tube.
The opposite ends of the cells are formed of ordinary
cytoplasm, and the nucleus lies in the borderland between
the granular and the protoplasmic part. The granules are the
secretum of the cell, and are formed by the activity of its
protoplasm. They are, however, not considered to be the
actual secretum, but rather a precursor of the secretum
peculiar to the cell ; and, as the cells of the gastric glands
secrete a ferment or enzyme, the granules which are the
precursor of that ferment, are called zymogen granules.
When secretion takes place the granules are discharged from

• HISTOLOGY OF THE FROG 87

the cells into the lumen of the gland, become converted into
ferment, and this is poured into the cavity of the gut. Amongst
the gland cells of the frog's stomach mucus-secreting chalice
or goblet cells are found. In the pyloric end of the frog's
stomach the cubical epithelium is continued nearly down to
the blind end of the gland, its terminal portion only being
occupied by a few large gland-cells filling up its cavity.

In the mammalian stomach the simple tubular glands are
slightly complicated by the branching of their extremities, so
that two or more tubes open into a single short neck or duct,
and in the cardiac end of the stomach the glands have two
kinds of cells in the walls of their deeper portions, chief cells
in the form of normal gland-cells, and parietal cells, which are
ovoid, filled with darkish granules, and lie between and behind
the chief cells, between them and their basement membrane.
The chief-cells secrete the digestive fluid pepsin, the parietal
cells secrete the acid (hydrochloric) found in the gastric juice,
and hence are also known as oxyntic cells.

Such glands as the pancreas, or the salivary glands of
mammals, are of the kind known as racemose. They may
easily be derived from the branched tubular condition of the
gastric glands, by supposing the branches of the latter to divide
and subdivide, the proximal portions of the duct and branches
being lined by simple non-glandular cubical epithelium whilst
the distal ends are dilated to form the so-called acini lined by
glandular cells. The whole structure is called racemose from
its resemblance to a bunch of grapes, the stalk and its divisions
being the ducts, the grapes the secreting portions or acini.

In the case of the liver the bile-ducts are lined with a
columnar epithelium which changes as the ducts break up
into finer and finer ramifications, the columnar cells passing
insensibly into the hepatic cells, whilst the cavities of the
ductules become no more than irregular spaces running
between the liver cells, and are known as bile-capillaries.
The hepatic cells form the bulk of the liver ; they are large,
polygonal in shape, and possessed each of a single nucleus.
These liver-cells, especially if taken from an animal killed not
long after a meal, may be seen to contain numerous granules
composed of the carbohydrate, called glycogen. Glycogen is
a substance belonging to the group of Amyloses or starches
[formula (C6Hl0O5)n]. It gives a port-wine red colour with

88 COMPARATIVE ANATOMY

iodine, and the granules may easily be stained with this
reagent in fresh liver-cells.

In certain parts of the body individual epithelial cells are
modified in connection with sensory surfaces. Such modified
epithelia are known as sensory or neuro-epithelia, and
the cells composing them may conveniently be described as
end-cells. They are found in the frog in the forms of taste-
cells on the tongue and palate, olfactory cells in the olfactory
region of the nasal cavities, rods and cones in the retina of
the eye, hair-cells in the labyrinth of the ear, especially in
patches on the ampullae of the semi-circular canals, and touch-
cells on the touch-spots of Merkel, which are best seen on the
swelling on the thumb of the male during the breeding season.
There are also end-cells in the sense organs of the lateral
line, highly developed in the tadpole, but degenerate in the
adult frog.

We may take the olfactory cell as a good example of a neuro-
epithelial cell (fig. 15, D). In the olfactory region of the
nasal chamber the epithelium is mostly composed of columnar
epithelial cells, produced internally into long branched pro-
cesses. Amongst these are placed cells, each of which possess
an ovoid body enclosing a large nucleus. From the body
a peripheral or distal and a central or proximal process is
given off. The former reaches the surface of the surrounding
epithelium, and bears at its end a few (5 to 8) stiff hair-like
processes, which may be modified or fused cilia, but, unlike
cilia, are immovable. The central process of the cell consists
of a very fine varicose fibre, which passes into a sub-epithelial
plexus composed of similar fibres from other olfactory cells
and the inner branched ends of the epithelial cells. Each
fibril of an olfactory cell is continued into a fibril of the
olfactory nerve.

The rods and cones of the retina are peculiar structures,
each being a product of rather than a metamorphosed
epithelial cell. A rod is a cylindrical rod-shaped body
about o-o5 mm. in length, consisting of two parts, a longer,
highly refractive, outer limb, and a shorter, more homogeneous,
less refractive, inner limb, separated by a small plano-convex
structure known as the lenticular body (fig. 15, C). At the
base of each rod is an oval nucleus surrounded by a very thin
sheath of granular protoplasmic matter which is continued inter-

HISTOLOGY OF THE FROG 89

nally as a fine fibril ; this probably becomes continuous with,
or comes in contact with, a terminal fibril of the optic nerve.

Hair-cells from the crista acustica of an ampulla of the ear
are shown in fig. 15, B. Each is a pyriform cell, the narrower
end turned towards the cavity of the ampulla, and bearing
on its truncated free surface a long stiff hair-like process which

Fig- 15-

Various forms of sensory epithelium. A, two forked cells from the tongue
of the frog ; B, B', two forms of auditory hair cells from the crista
acustica of the anterior ampulla of the frog's ear ; C, three rods and a
cone from the retina of the frog's eye ; D, two olfactory cells, with
their epithelial supporting cells from the frog's nose. (All the figures
after Haslam in Ecker's " Anatomy of the Frog." Engl. Ed.)

rests by a broad base on the cell. The opposite broader end
of the cell usually appears rounded, but sometimes a fine
fibril may be seen springing from it, and this probably
becomes continuous with a terminal fibril of the auditory
nerve though the connection has not been actually traced.
The characters of taste-cells may be learned by an inspection
of fig. 15, A.

9o COMPARATIVE ANATOMY

It should be noticed that epithelia play a very important
part in the economy of the animal, and that the cells of the
outer skin, as well as those lining the gut, are modified in
diverse ways for the performance of diverse functions. The
endothelia lining the coelom, the cavities of the heart and
blood-vessels, the lymph-spaces, etc., must be considered as
belonging to a different category from the other two epithelia,
since they are formed from the middle of the three embryonic
layers, and are differentiated in one respect only — viz. in respect
of their function as components of a limiting membrane.

In the nervous system of the frog two principal elements
are to be distinguished, nerve ganglion cells and nerve-fibres.
The former are found in the cortical substance of the brain
and in the so-called grey matter surrounding the central canal
of the spinal cord and forming the floor of the fourth ventricle
in the medulla oblongata. They are also found in the various
ganglia which have been described ; such as the ganglia of
the dorsal roots of the spinal nerves and the sympathetic
ganglia. Nerve ganglion cells, however, may be most easily
studied in the spinal cord of a larger animal, such as the
ox. In that part -of the grey matter known as the anterior
horn of the spinal cord a few large cells may be seen in
transverse section or may be separated by suitable methods
of maceration. Each ganglion cell has a body composed of
a finely granular cytoplasm, which sometimes appears to be
indistinctly striated or reticular in structure. In the centre
of the cell-body is a nucleus, which usually has the form
of a relatively large, clear, rounded vesicle with a distinct
nucleolus. The intra-nuclear network of chromatin is not
usually well defined. The most characteristic feature in a
nerve-cell is the prolongation of the cell-body into long
branching processes, which show a distinct but fine longi-
tudinal striation, the striae of one process being continued over
the peripheral parts of the cell-body to unite with the similar
strise of an adjacent process. A nerve-cell from the anterior
horn of the spinal cord of the ox or other mammal has
numerous processes (see fig. 16, A], which break up into
branches at a little distance from the cell-body, and the bran-
ches sub-divide again till they end in very fine nerve-fibrils,
which inosculate with fibrils from adjoining cells. One process,
however, is not branched, and may be traced into direct con-

HISTOLOGY OF THE FROG 91

tinuity with the axial fibre of an efferent nerve of the ventral
root. The nerve-cells of the anterior horn are large, and are
called multi-polar because of their numerous processes, but
nerve-cells from other parts may be much smaller, and may
vary both in the number and the mode of attachment of their
processes. Thus there are bi-polar nerve-cells with two pro-
cesses, or uni-polar^ cells with one process only ; and in the
superficial layer or cortex of the cerebellum of mammals there

Fig. i 6.

A , multipolar nerve ganglion cells from the spinal cord of an ox ; B,
portion of a medullated nerve fibre from the sciatic nerve of the
frog, highly magnified; ax.f, axial filament; N R, Node of
Ranvier ; n, neurilemma ; Sch, medullary sheath or white
sheath of Schwann ; Nu, nucleus. The preparation was treated
with osmic acid, which blackens the medullary sheath, and the
breaks in its continuity as shown in the figure are probably due
to the action of the reagent.

are remarkable ganglion cells, called the corpuscles of Purkinje,
having a pyriform body, produced at the narrow end into
one or two stout processes, which immediately branch and
break up into a number of fine fibres, running with more or
less straight courses towards the surface of the cortex. The
broad end of the cell is usually produced into a single fine
fibril. A curious form of nerve-cell is found in the sym-
pathetic ganglia of the frog and of mammals. The cell-body

92 COMPARATIVE ANATOMY

is pear-shaped and enclosed in a distinct sheath. From its
narrower end two processes are given off, of which one, the
larger, starts from the deeper part of the body of the cell
and pursues a straight course, being, in fact, the direct con-
tinuation of a nerve -fibre. The second smaller fibre starts
from a plexus of fine fibrillse on the surface of the cell-body,
and is wound spirally round the larger fibre, eventually quit-
ting it and taking an opposite course.

Nerve-fibres are of two kinds, white or medullated nerve-
fibres, and grey or non-medullated nerve-fibres. The latter
are chiefly found in the sympathetic system, but also occur,
though not abundantly, in the cerebro-spinal nerves. They
are transparent fibres with a faint longitudinal striation, and
exhibit nuclei at frequent intervals on their courses. It is
doubtful whether .the nuclei lie outside the fibre, enclosed in
a delicate investing sheath, or whether they are embedded in
the superficial layer of the fibre itself.

White medullated nerve-fibres form the bulk of the white
matter of the brain and spinal cord, and of the cerebro-spinal
nerves. They are called "white" because, shortly after death,
a peculiar fatty substance forming their sheaths sets hard and
white, but during life it is liquid, and the fibres have then a
pale transparent aspect. A single white fibre may be roughly
compared to an insulated telegraph wire. The transmitting
part, the actual wire, is represented by a pale strand of tissue,
having the characters of a non-medullated fibre, and known
as the axial fibre or axis-cylinder. It is nearly certain that
every axis-cylinder is a direct prolongation of a process of a
nerve ganglion cell, and it is to be remarked that the axis-
cylinder of a nerve fibre runs an uninterrupted course from
the nerve-cell from which it originates to its peripheral ter-
mination. The insulating gutta-percha sheath of a telegraph
wire is represented in the nerve fibre by a sheath of a medul-
lary substance, or myelin, composed chiefly of the peculiar
phosphorised fat, lecithin, and the calico or canvas outer
wrapping of the telegraph wire is represented in the nerve-
fibre by a delicate external homogeneous covering, called the
primitive sheath of Schwann, or the neurilemma.

The neurilemma forms a continuous covering to the nerve-
fibre, but the medullary substance is broken up into segments
by constrictions, placed at regular intervals, and known as the

HISTOLOGY OF THE FROG 93

nodes of Ranvier ; and, in this respect, and in others, the simile
of the telegraph wire will help us no further. Each node of
Ranvier is formed by a constricting band, in the form of an
annular ring extending inwards from the neurilemma to the
axial-fibre. The nature of this constricting band is not very
well understood, but its staining properties suggest that it
belongs to the class of intercellular substances, and if this view
be correct each internode would appear to be formed as a
single cell. This view is strengthened by the fact that in
each internode there is a single nucleus, surrounded by a small
quantity of granular matter, and lying underneath the neuri-
lemma, between it and the medullary sheath. But the axial-
fibre is continued without interruption across each node of
Ranvier, and it has been shown, both in development and in
the regeneration of a nerve-trunk which has been divided,
that the axial-fibre, which we must regard as the essential part
of a medullated nerve-fibre, is formed as an outgrowth of a
nerve-cell, and is therefore to be regarded as a very much
elongated process of a nerve-cell, and not as a product of a
number of cells represented by the nuclei of the internodes.
This much, at any rate, is certain : that every nerve-fibre, con-
tinuous as it is with a process of a nerve-cell, is dependent on
that cell for its nutrition, and dies if it is divided off from the
cell. It is furthermore certain that, in the course of develop-
ment, nerve-fibres first appear as pale fibres, destitute both of
medullary sheath and neurilemma. The next thing to make
its appearance is the neurilemma, which is nucleated, and is
known at this stage as the nucleated sheath of Schwann, and
lastly, the medullary sheath makes its appearance compara-
tively late in embryonic life. It is supposed by some authori-
ties that the medullary sheath is formed by the activity of the
cells composing the nucleated sheath of Schwann, and that, in
the adult nerve-fibre, the medullary sheath and neurilemma of
each internode represent the results of the activity of a single
cell derived from the nucleated sheath of Schwann, whilst the
axial-fibre which they surround is something different — namely,
a process of the nerve-cell proper to the fibre. Other authors,
however, hold that the medullary sheath is formed from the
axial fibre, and the neurilemma alone from the nucleated
sheath of Schwann ; and the phenomena of regenerating nerve-
fibres certainly lend support to their view. The subject is com-

94 COMPARATIVE ANATOMY

plicated, and cannot be profitably discussed in this place ; but
it is important to remember, firstly, that the axis fibre is an
outgrowth from, and is nourished by, a nerve cell ; secondly,
that the nodes of Ranvier mark off the limits of cells which
have become secondarily related to the axis fibre, and are
represented by the nuclei found in each internode. It may
remain an open question whether the cells in question form
both medullary sheath and neurilemma or the latter only.

A further peculiarity may be mentioned, that in most pre-
parations the medullary sheath is seen to be broken up into
segments, with conical or funnel-shaped ends which fit into
one another. There may be many such in a single internode,
and it is a moot point whether they exist in the living nerve or
whether they are the result of post-mortem changes, induced
perhaps by the reagents used in the preservation of the nerves.
Towards their terminations, both afferent and efferent nerves
lose their medullary sheath, and the axis cylinder breaks up
into minute ramifying fibrils which exhibit characteristic
varicosities. These terminal ramifications are distributed in
various ways ; as, for instance, over the surface of a muscle in
the case of efferent nerves, or a fibril may be continuous, as
we have seen, with the internal process of a sense-cell, in the
case of afferent nerves.

The nervous tissues are the eminently irritable, the muscular
tissues the eminently contractile. Muscular tissue is of two
kinds, plain or non-striated, and striated or striped muscle.
The latter presents two important varieties ; the one found in
the heart is known as cardiac muscular tissue, the other forms
the skeletal muscles. As plain muscle-fibre is usually removed
from the control of the will it is also called involuntary, and
the striped muscle, being under the control of the will, is called
voluntary ; but these names are not satisfactory, since cardiac
muscle, though striated, is involuntary.

Plain muscular tissue (fig. 17) is found in the 'walls of the
alimentary tract, in the walls of the arteries and veins, in the
bladder, and in other viscera. It may occur in bundles or in
layers, and is formed of distinct cellular elements, which may
be separated from one another by suitable methods. A plain
muscle-fibre is of elongate fusiform shape, thickest in the
middle, and pointed, more rarely forked, at the ends. The
bulk of the fibre is composed of a highly contractile, doubly-

HISTOLOGY OF THE FROG

95

refracting substance, which is not cytoplasm, but the product
of the cytoplasm of the cell from which the fibre was formed.
The cellular character of each fibre is shown by the nucleus,
which is generally elongated and oval, and exhibits an intra-
nuclear network of chromatin. At each end of the nucleus are

Fig. 17.

Group of unstriped muscle fibres from the bladder. «, the nuclei ; /, the
granular remains of the cell protoplasm ; f, the longitudinally striated
contractile portion.

a few granules, the remains of the cytoplasm from which the con-
tractile substance was formed. The plain fibres exhibit a faint
longitudinal striation, and each is contained in a very delicate
homogeneous sheath, which can only be seen when the fibre is
twisted or torn across. The individual fibres fit closely side by
side, and are gathered into larger or smaller bundles, which are
attached to the connective tissue layers of the visceral organs.

Striped muscular fibre (fig. 18, A) from a skeletal muscle
differs greatly in appearance from unstriped. The individual
fibres are generally gathered together into bundles or fasciculi,
and the bundles are united together to form the whole muscle.
The last-named is invested by a sheath of connective tissue,
called the perimysium, within which the bundles extend from
end to end of the muscle, being only partially separated from
one another by inward prolongations of the perimysium, the
so-called endomysium. The bundles may be large or small,
according to the muscle from which they are taken, and each
consists of a number of fibres varying from *i to -oi mm. in
diameter and 250 to 350 mm. in length in the human subject.

96 COMPARATIVE ANATOMY

Each muscle-fibre is invested by a transparent, homogeneous,
elastic membrane, called the sarcolemma, which is particularly
thick and strong in fishes and Amphibia. The individual
fibres are not as long as the bundle which they compose, but
end with tapering extremities, which cohere with neighbouring
fibres, or at the ends of a muscle may be affixed to a tendon.
Muscle fibres rarely branch, but an exception is found in the
muscles of the frog's tongue, and also in the tongues and facial
muscles of mammals.

The sarcolemma may be regarded as a tube whose contents
are the muscular substance. This substance is of soft, semi-
fluid consistency during life, but after death it becomes
coagulated and firm. The most characteristic thing in the
muscle-substance is its cross-striation. When viewed under
the microscope each fibre exhibits a number of parallel cross
stripes alternately light and dark. The fibre further exhibits
a fine longtitudinal striation, and certain methods enable us to
break it up into a number of fine longitudinal strands, which
run from end to end of the fibre, and are called sarcostyles.
They are prismatic in section, and are separated from one
another by a more fluid substance known as sarcoplasm, so
that a transverse section of a fibre gives the appearance of a
number of polygonal areas separated by lines representing the
intervening sarcoplasm. Each isolated sarcostyle exhibits the
same cross-striations as the whole muscle-fibre, and this
appearance is due to the fact that the sarcostyle itself is made
up of a number of segments, called sarcomeres, separated from
one another by fine membranes, called the membranes of
Krause, which in optical section appear as fine lines running
across the middle of every bright band. An individual
sarcomere consists of a median darker portion, which is called
the sarcous element, and hyaline extremities abutting on the
membranes of Krause. When the muscle-fibre is fully
extended, the sarcous element divides into two portions, and
in so doing leave a clear cross stria between their separated
portions which is known as the line of Hensen. When the
muscle-fibre is contracted the lines of Hensen become
obliterated by the apposition of the halves of the sarcous
elements, and the whole sarcomere being shortened, the
sarcous elements encroach upon the hyaline areas abutting
on the membranes of Krause. At the same time each

HISTOLOGY OF THE FROG 97

sarcomere becomes bulged out laterally, so that the whole
sarcostyle has a moniliform appearance. It is held that the
sarcous elements are composed of a denser protoplasmic
substance, spongioplasm, and that the clear areas are occupied
by a more fluid substance, the hyaloplasm, and that in the
process of contraction the more fluid hyaloplasm is forced into
or flows into the pores which permeate the spongioplasm of
the sarcous elements. This, at least, is one explanation of the
structure of striated muscular fibres, but it is held by other
authors that a muscle-fibre is composed of a very regular intra-
cellular network of modified highly contractile protoplasm,
holding in its meshes a more fluid hyaloplasm, and that the
optical effect of cross-striation is due to the regular arrange-
ment of the network. It would be beyond the scope of this
work to enter into a discussion of the relative merits of the
rival theories, and it need only be remarked here that the
theory of an intra-cellular network accords better with our
present knowledge of the structure of protoplasm, than does
the elaborate theory of sarcomeres, sarcous elements, etc.,
divided up by membranes. Both theories resemble one
another in assuming the existence of a denser, actively-con-
tractile spongioplasm, and a passive, more fluid hyaloplasm.

Cardiac muscular tissue (fig. 18, B and C) is, in some
respects, intermediate between plain and striated muscular
fibre. Its cellular components are not fused together, but
remain distinct, so that the muscle appears to be made up of
a number of oblong nucleated cells, joined end to end to form
fibres, and communicating by short processes with the cells of
adjacent fibres. Each cell exhibits cross striations not unlike
those of striated voluntary fibres, but less distinct.

Ordinary striped muscle-fibres are formed from somewhat
elongated embryonic cells, which are at first simple and
uni-nucleate. At the time of embryonic life when muscle is
formed each cell elongates and its nucleus multiplies, so that
a long multi-nucleate protoplasmic fibre is formed devoid of
striae. The last-named first appear as longitudinal fibrils
running along one side of the cell, and the whole has now the
appearance of the very primitive epithelio-muscular cells found
in some of the lower multi-cellular animals. At about this
time the sarcolemma is formed as a delicate membrane
bounding the fibre. Presently cross as well as longitudinal

98

COMPARATIVE ANATOMY

striae make their appearance and invade the whole periphery
of the fibre; the nuclei imbedded in a core of granular
protoplasm occupying the centre. Eventually the striation
and fibrillation extends to the centre, and the nuclei move

A, mbryonic striped muscle fibre from the tail of a tadpole,
bowing the nuclei nn, and the protoplasm /, of the
oenocyte from which the fibres are developed. The fibres
xhibit alternate dark and light bands, and in the centre of
ach dark band is a light line, the line of Hensen ; />',
ardiac muscle-fibre showing the short branched nucleated
ells; C, a single cell from cardiac muscle- fibre, more
highly magnified, showing the cross striation and the
nucleus n. (A original; B and C from Schafer.)

outwards to take up a superficial position under the
sarcolemma. Thus it appears that, although the striated
muscle-fibre is often spoken of as a cell-fusion it is in reality
not a product of many cells but of a coenocyte.

HISTOLOGY OF THE FROG 99

The various organs and tissues which we have enumerated
are supported by and attached to the skeleton, and are bound
together and pervaded by connective tissue. The skeleton is
composed of bone and cartilage, which are classified as varieties
of connective tissue ; but we must regard the white membranous
or fibrous material, which forms the sheaths of muscles and
nerve-trunks, ties down the skin to the underlying tissues, and
enters into the composition of tendons and aponeuroses, as
constituting connective tissue proper, bone and cartilage
standing somewhat apart from these.

Connective tissue may be studied to more advantage in
mammals than in the frog. On cutting through the skin of a
rabbit and stripping it from the body it is seen to be connected
to the subjacent parts by a soft, white, tenacious, fleecy
material known as areolar tissue (fig. 19). If a portion of
this be carefully spread out as a film on a slide, and examined
under the microscope, it is seen to be composed of a soft homo-
geneous ground substance or matrix in which are imbedded
cells and bundles of extremely fine transparent white fibres,
which traverse the matrix in all directions. These fibres do
not branch, nor do those of one bundle cross over to and
unite with those of another bundle, but keep a direction
parallel with the remaining fibres of the bundle. They swell
up in acetic acid, and are converted into gelatin on boiling.
Each bundle usually pursues a wavy course, its component
fibres maintaining their parallelism throughout the undulations.
In addition, larger fibres may be distinguished which run
nearly straight, or are thrown into large bold curves unlike
the minute undulations of the white fibres. They differ also
from the latter in the fact that they branch and anastomose
with one another to form an open network, and also, when
broken across, their ends tend to curl up. These are the
yellow elastic fibres; they are not affected by acetic acid,
and do not yield gelatin on boiling, being composed of a
different chemical substance called elastin.

The ground substance of areolar connective tissue stains,
like intercellular substances in general, with silver nitrate, and
it then exhibits a number of irregular cavities pervading a
homogeneous basis. By staining connective tissue with other
dyes it may be shown that these spaces are occupied by
branching cells, the so-called connective tissue corpuscles.

TOO COMPARATIVE ANATOMY

There are three kinds of these cells, the flattened or lamellar,
the granular, and the vacuolated or plasma-cells. The first-
named have a large oval nucleus and a finely granular
protoplasm ; their cell-bodies are produced into numerous
branching processes, and the processes of adjacent cells may
often be observed to unite with one another. They lie either
embedded in the matrix, where they occupy the spaces above

Fig. 19-

Subcutaneous Areolar connective tissue from the rabbit, highly magnified,
showing numerous wavy bundles of parallel white fibre_s, crossing
one another in all directions ; el, branched fibres of elastic tissue, some
broken ends of which are curled up on the left of the figure ; M, branched
connective tissue corpuscles; g, granular corpuscles ;f, a fibrous cor-
puscle.

referred to, or they may frequently lie on the surfaces of the
bundles of white fibres, or between bundles which cross one
another.

The granular cells are generally subspherical in shape, with
an oval nucleus and a relatively small cell-body, in which are
numerous coarse granules of a proteid nature. ' The granules
stain deeply with certain acid dyes, such as eosin, and hence
these cells are sometimes called eosinophilous. The plasma
cells are distinguished by the fact that their cytoplasm is filled
with minute vacuoles, so that it has a bubbly or frothy
appearance. That the corpuscles are concerned in the
nutrition of the connective tissue there can be no doubt,
but the exact part played by each kind is not fully under-

HISTOLOGY OF THE FROG 101

stood. They are often called fixed corpuscles, in opposition to
intrusive leucocytes which are frequently found in the areolar
tissue. But it seems probable, from the analogy of lower
animals, that some, at least, of the connective tissue cells may
have a migratory character.

Tendons and ligaments are a modified form of connective
tissue, known as white fibrous tissue. In them the bundles of
white fibres run in parallel courses instead of crossing one
another irregularly, and are greatly preponderant over the
elastic fibres. The corpuscles of fibrous tissue are all of the
flat or lamellar variety, and tend to be disposed in rows or
chains following the parallel arrangement of the bundles.

Some ligaments, such as the ligamentum nuchae of the
ox and other mammals, are composed almost entirely of
yellow elastic tissue, and they are very elastic and extensible.
Ordinary white tendons, on the other hand, are scarcely
extensible.

It is clear that the bulk of a connective tissue consists of an
intercellular substance which has been formed by the activity
of the cells contained in it. In the course of development,
connective tissue appears as an accumulation of amoeboid
undifferentiated cells, which presently send out ramifying
processes which become united with one another to form a
network. The meshes of the network are occupied by an
albuminous fluid, which afterwards becomes changed to form
the ground substance. The ground substance is at first homo-
geneous, but presently fibres are formed in it ; how they are
formed is not clear. Some think that they are the result of
the actual conversion of cells into bundles of fibres ; others
think that the substance of the fibres is simply deposited in
the matrix, the cells contributing the material without being
themselves converted and used up in the process. The latter
view is more probably correct, though there is little positive
evidence on the subject.

The substance popularly known as gristle, but scientifically
named cartilage, has many features in common with connec-
tive tissue. Like the latter, it consists of a homogeneous
matrix in which cells are imbedded. If a thin slice of the
white or pearly and translucent cartilage from the articular
head of a bone or from the hyoid, sternum, or supra-scapula of
a frog be examined under the microscope, it is seen to consist

102 COMPARATIVE ANATOMY

of nucleated cells imbedded in a matrix of firm gelatinous
consistency, which can easily be cut with a knife, but is at the
same time highly elastic ; it may be bent, twisted, or com-
pressed, but readily recovers its size and shape when freed
from the stress to which it was subjected. Like the inter-
cellular substance of connective tissue, the matrix of cartilage
is readily stained by nitrate of silver.

There are several kinds of cartilage, hyaline, calcified, and
fibrous cartilage ; the last-named may be yellow or white,

Fig. 2O.

Part of a section through the xiphisternum of a frog to show the structure of
hyaline cartilage. Several cartilage cells, many of them in groups of
two, are imbedded in a hyaline matrix.

according as the fibres in it are elastic or simply white fibres.
We will concern ourselves chiefly with hyaline cartilage, so-
called because of its homogeneous translucent matrix. The
cells imbedded in it are rarely disposed singly and at equal
distances from one another, but usually in groups or lines of
two, four, or more, and it is often noticeable that in a group
of two the cells are separated by a very small amount of inter-
vening matrix, and that their adjacent sides are flattened as
if by mutual pressure. Each cartilage cell is a subspherical
or lenticular corpuscle of protoplasm, containing a rounded
nucleus with a well-defined chromatin network, and a cell-
body, in which fine granules and interlacing fibrils may often

HISTOLOGY OF THE FROG 103

be distinguished. The arrangement of the cells in groups is
the expression of the fact that they multiply by division, and
every group is made up of the daughter products of a single
cell which originally occupied a central position within the
group. The whole arrangement is best understood after a
consideration of the development of cartilage. Primordial
cartilage consists of an accumulation of polygonal cells
(derived in the first instance from undifferentiated embryonic
cells), each surrounded by a capsule (the product of the
activity of the cell-protoplasm) of a transparent gelatinous-
looking substance. The capsules of contiguous cells cohere
together, and presently become fused, thus forming a scanty
matrix. The cells meanwhile enlarge and undergo division,
and at each division the daughter cells, lying in the single
cavity of the parent cell, form each one a capsule for itself.
The secondary capsules after a time cohere to and become
blended with the matrix, and then each of the two daughter
cells again divide, and each of the four cells of the second
generation again forms a capsule for itself, which in its turn is
blended with the matrix ; and so on, as long as growth and
active multiplication of the cells proceeds. But after a time
the cells cease to multiply rapidly, and then each becomes
surrounded by a number of concentric capsules which are
successively blended with the matrix, and increase its mass.
Thus it is evident that the matrix is a product of cartilage
cells, and is probably secreted from their surfaces.

In many forms of cartilage the cells are, as described,
rounded and isolated from one another in the matrix. It is
difficult to understand how they are nourished, for the matrix
is not vascular, nor can lymph channels be detected in it. In
other forms, however, and especially in the cartilage of sharks
and rays, the cells are not rounded but branched, and their
prolongations ramify through minute canals in the matrix,
and form a network of protoplasmic strands uniting the
cells.

A great part of the supra-scapula of the frog consists of
calcified cartilage. Here we find that granules of carbonate
of lime have been deposited in the matrix in greater or less
number. The lime can be removed by acids, when the
structure of hyaline cartilage is made manifest, though it is
somewhat altered by the presence of the cavities occupied by

io4 COMPARATIVE ANATOMY

the calcareous granules. Calcified cartilage is found in
abundance in the skeletons of sharks and rays, and it must not
be mistaken for bone, which is not present in these fishes ; nor
must any confusion be made between calcified cartilage and
" cartilage bone," such as has been described as occurring in
the skull of the frog. A cartilage bone is one which has taken
the place of pre-existing cartilage, but has not been formed out
of it. The replacement of cartilage by bone will be described
after a consideration of the structure of bone itself.

Bone consists of an inorganic or earthy and an organic or
animal part, intimately blended together. If a bone is steeped
in dilute acid the earthy salts are dissolved out, and a model
of the bone is left in a flexible, tough substance which is
converted into gelatin on boiling. This last properly is
sufficient to distinguish the animal part of bone from
cartilage, for the latter, after prolonged boiling, yields a
substance of different chemical character called chondrin.
The earthy parts of bone may be separated by burning out
the animal part in a fire. They consist chiefly of phosphate
of lime, with a small proportion of carbonate and a trace of
fluoride of lime. Bone may be compact or spongy ; in the
latter case it is called cancellous. Cancellous bone, however,
is made up of fine bars and partitions which have the same
structure as compact bone.

If a thin slice of bone is viewed under the microscope it no
longer appears dense and homogeneous, but is seen to be
perforated by a number of holes, and a longitudinal section
shows that these are the sections of a number of short
longitudinal canals called Haversian canals. The calcareous
substance is arranged round each Haversian canal in a series
of concentric lamellae. The concentric systems of adjacent
Haversian canals nearly or quite touch one another, but are
generally separated by vertical layers, which run parallel to
the long axis of the bone and extend to its surface, where
they form a peripheral layer. The arrangement of the
lamellae is made conspicuous by the presence of numerous
little dark cavities, which are disposed conformably to the
lamellae in which they occur. These dark spaces are called
the lacunae. Each is connected centrally with the Haversian
canal, peripherally with other lacunae by numerous fine
branching passages called the canaliculi. The Haversian

HISTOLOGY OF THE FROG 105

canals are occupied during life by blood-vessels — an arteriole
and a venule to each — and a small amount of connective
tissue containing branched cells. Some of the larger canals
also contain marrow. Each of the lacunae contains a small,
nucleated, branched cell or bone-corpuscle, which com-

Fig. 21.

Portion of a transverse section through the femur of a pig to show the
structure of bone. The section shows two concentric Haversian
systems, each with its central Haversian canal, surrounded by
numerous lacuna; with their canaliculi. At the top of the figure
the external or periosteal layer is shown, in which the lacunae are
elongated in a direction parallel with the surface of the bone.

municates by means of protoplasmic processes running
through the canaliculi with the processes of adjacent
corpuscles, and eventually with the branched connective
tissue cells of the Haversian canal. The outermost canaliculi
of a Haversian system do not, as a rule, communicate with
those of an adjoining system, but bend round to join the
peripheral lacunae of their own system. The elaborate system
of blood-vessels and nucleated cells with communicating
protoplasmic processes shows that the nutrition of bone is
dependent on the presence of living protoplasm, and the whole
structure of bone may be compared to that of connective
tissue, the gelatin plus lime salts forming the matrix, the bone-

106 COMPARATIVE ANATOMY

corpuscles being homologous with ordinary connective tissue-
cells. In fact, true bone is formed in connective tissue,
whether it be what we call " cartilage bone," or whether it be
"membrane bone." It must be borne in mind that when we
speak of a " cartilage bone," such as the ex-occipital or pro-
otic of a frog's skull, we do not mean that the cartilages of the
cranium or of the otic capsule have been transformed into
bone. We only mean that the bone has been preceded by
cartilage, which latter has been absorbed and replaced by bone
in the manner to be described. There must be no confusion
between calcified cartilage and true bone. Cartilage is a non-
vascular tissue ; bone, with its system of blood-vessels running
in the Haversian canals, is highly vascular. Bone, as has been
said, may be developed directly from membrane, without being
preceded by cartilage. To take, as an example, the parietal
bone of a mammal, such as a sheep. Between the dura mater,
as the connective tissue layer surrounding the brain is called,
and the outer integument, is a layer of connective tissue, com-
posed of a matrix with fibres and numerous granular corpuscles.
It is in this membrane that each parietal bone appears in the
form of a number of spicules of bone radiating outwards from
a " centre of ossification," and joined together at intervals by
short, irregularly-placed tangential bars. The outer end of
each radial spicule is prolonged for some distance beyond, the
actual calcareous spicule in the form of a bundle of parallel or
slightly divergent transparent fibres, the so-called osteogenic
fibres. They resemble white connective tissue fibres, except
that they are straighter, stiffer, and are less distinctly fibrillated.
The osteogenic fibres are covered by a layer of granular cells
called osteoblasts, applied to the surfaces of the fibres and
occupying their interstices when they diverge from one
another. The osteoblasts appear to form the osteogenic
fibres, and also to supply the calcareous deposit which is laid
down in the form of minute granules in the matrix between
the fibres. Eventually, by their coalescence, the granules form
the substance of the bone, the latter first appearing as a
network of spicules and splinters, but as ossification proceeds
the spaces between are gradually filled up by the continued
deposition of calcareous granules. In many cases, however,
the interstices are occupied by blood-vessels, and these are
surrounded by the advancing bone, and eventually form the

HISTOLOGY OF THE FROG 107

canals of the Haversian systems. Some of the osteoblasts
become imbedded in the advancing bone, and remain as the
bone-corpuscles of the future lacunae.

The so-called ossification in cartilage can be very well
studied in a long bone such as the tibia or femur. In the
first place, a small model of the future bone is laid down in
cartilage, and this is surrounded by a vascular membrane
which will become the future covering or periosteum of the
bone. When ossification is about to commence the cartilage-
cells in the middle of the model become much larger and
flatter, and are arranged in a number of rows or columns
parallel to the long axis of the bone. The matrix between
them becomes calcified by deposition of calcareous granules.
In the meantime true bone is being formed outside the
cartilage, between it and the periosteum. The last-named
membrane is vascular, and contains numerous granular cells,
chiefly on its inner surface nearest to the cartilage. From
these osteoblasts, osteogenic fibres are formed, and true bone
containing blood-vessels and bone-corpuscles is formed just
as in the case of membrane bone. At this stage the develop-
ing bone consists of a core of calcified cartilage enclosed in
its middle portion by a sheath of membrane bone. Presently
the vascular and osteoblastic tissues break through the sheath
of .newly-formed bone and burrow their way into the cartilage,
which is for the most part absorbed by the agency of certain
giant-cells containing several nuclei, and known as osteoclasts.
A network of bony spicules is developed from the intrusive
osteoblasts in the cavities formed by the absorption of the
cartilage, and this network carries blood-vessels in its meshes
just as is the case in membrane bone. Gradually the whole of
the cartilage of the shaft is absorbed and replaced by a sponge-
work of bone, which for a long time remains spongy or
"cancellous," but eventually is solidified in the peripheral
parts of the shaft by the deposition of concentric layers round
the blood-vessels, whilst the central portion is absorbed, and
a central cavity, the medullary space, is left in the middle of
the bone.

Before leaving the subject of connective tissue, mention
may be made of fat-cells, which, although their function is
essentially metabolic, are nothing more in their origin than
connective tissue corpuscles in which droplets of fatty matter

io8

COMPARATIVE ANATOMY

have been formed by the activity of the cell protoplasm.
Fatty or adipose tissue appears under the microscope as a
number of minute oval vesicles, generally gathered into a
cluster called a lobule, which receives an afferent artery and
an efferent vein. Each vesicle is filled with an oily matter
which can be dissolved out by the action of ether, and it can
then be seen that the vesicle consists of a very delicate
structureless membrane, on one side of which is a flattened
nucleus, surrounded by a few granules of .protoplasm. In some

fc

Fig. 22.

Part of a fat lobule from the mesentery of a rabbit, highly magnified, to
show the structure of adipose tissue. (From a drawing .by Mr
E. H. Schuster.)

of the vesicles a wide-meshed reticulum of delicate proto-
plasmic strands may be seen, in the cavities of which the
oil drops were lodged. A fat-cell is a good example of the
endoplastic formation of material by a living cell. '

We have now studied the microscopical characters of the
tissues sufficiently to enable us to comprehend the fact that
the whole organism is composed of a number of minute living
units called cells, which, indeed, are seldom to be found in a
primitive and undifferentiated condition in the adult animal,
but are variously modified, changed, and adapted to particular
purposes, and consequently exhibit every variety of form and

ACTIVITIES OF CELLS 109

every sort of relation to one another. It is true that there are
some tissues, such as cartilage and bone, the bulk of which is
not formed of cellular elements but of a non-oellular, more or
less homogeneous matrix. It is evident, however, that this
matrix has been formed by the activity of the cells which
remain enclosed within it ; and these tissues do not contradict
the general statement that the body of the frog, as indeed of
all animals, is composed of cells. It is of the utmost impor-
tance that the part played by cells in the formation of animal
and vegetable tissues should be clearly apprehended by the
student. It has been seen that most of the cells are
specialised, or, as it is generally said, are differentiated. This
means that the cell-protoplasm has been active in various
ways, and has given rise to substances which are either stored
up within the body of the cell (as, for example, the fat-cell),
when they are said to be endoplastic products, or are formed
on the outside of the cell as a sort' of envelope (as in the case
of cartilage- or bone- cells), in which case they are called ecto-
plastic products. And in all cases we see that differentiation
means the exaltation of one of the fundamental properties
or functions of living matter at the expense of the others.
Thus nerve-cells are essentially irritable, this fundamental
property of protoplasm being exalted, whilst others, such as
contractility, metabolism, etc., are depressed. Gland-cells are
essentially metabolic and secretory, muscle-cells are essenti-
ally contractile ; and so forth. In every case, also, the exaltation
of a special function or attribute of the protoplasm is accom-
panied by a change of form, a morphological differentiation.
The nerve-cell differs from the muscle-cell, and both differ
from the gland - cell in their obvious structural qualities.
Further, we can see that amongst the countless cells of which
such an organism as the frog is composed, large groups of
cells have become specialised for the performance of special
functions, some undertaking one special kind of work, some
another kind ; yet they are not independent units acting for
themselves without regard to the other kinds with which they
are associated, but, contrariwise, they act and interact harmoni-
ously, as it seems purposefully, for the benefit of the organism
as a whole. A multi-cellular animal has often been described
as a cell-colony, in which there are cell-castes, the members
of which undertake special kinds of labour ; and so it has

no COMPARATIVE ANATOMY

become common to speak of a physiological division of labour
amongst the cell-castes, with a corresponding morphological
differentiation of form. The illustration is a useful one, but
it ceases to be valuable if pushed too far. The cells compos-
ing the frog's body are not independent units in the sense that
the individual men composing a colony are individual units.
Their life not only contributes to the life of the whole, but
is indissolubly connected with it ; and in this connection it
should be borne in mind that, however much the other func-
tions of an individual cell may be depressed in order that
its special function may be exalted, all cells retain and must
retain one vital attribute, that of assimilation. All cells, whilst
they retain their vitality, must be nourished, and they are
nourished by the blood through the intermediary of the
lymph. Each cell has its own kind of metabolism; but the
metabolism of each, whilst it contributes to the whole, is
merged into the metabolism of the whole, and ceases when
it is removed from it, except in one case, to be considered
directly.

All these considerations and facts are collected together in
what is known as the Cell- Theory, which was first propounded,
for the vegetable kingdom, by Schleiden, a botanist, in 1838.
Schleiden's views were adopted and applied to the animal
kingdom by Theodore Schwann in 1838 and 1839.

The continuous researches of botanists and zoologists have
added much to the cell-theory since the days of Schwann, and
at the same time have rendered his position less tenable.
Schwann made a fundamental error regarding the genesis of
cells. He supposed that they were formed, in a manner
analogous to crystallisation, out of a nutrient matrix, which
he called the cytoblastema. He supposed that the cell was a
hollow vesicle containing another vesicle, the nucleus, and that
this again surrounded a smaller vesicle, the nucleolus. The
nucleolus, according to his views, was first deposited in the
structureless or minutely granulous cytoblastema, and it formed
a centre of attraction round which other molecules were
deposited to form the nucleus. This, again, continued to attract
fresh molecules, which formed the cell-body.

The error of this view was pointed out in 1851 by Robert
Remak, who showed that all the cells of which an animal body
is composed are formed by the continuous and repeated sub-

DIVISION OF CELLS in

division of an originally single and simple cell, the ovum or egg-
cell, and that there is no evidence of cells being formed
autogenously, like crystals out of their mother liquor, as
Schwann had supposed. The continued labours of many
investigators have confirmed Remak's conclusions, and it is
now an established fact, universally assented to, that every cell
arises from the division of a pre-existing cell, a generalisation
neatly summed up in Virchow's aphorism, Omnis cellula e
cellula.

It was observed at a comparatively early date that the
nucleus took an important if not a predominant share in the
reproduction of cells by division, but the significance of this
fact was for a long time obscure, and it seemed of less im-
portance because it was long believed that there were many
simple uni-cellular organisms destitute of a nucleus. Improved
methods of observation, however, have demonstrated the exis-
tence of a nucleus, or at least of the essential constituent of
a nucleus, chromatin, in many animal and vegetable cells in
which its existence was formerly denied, and there now remains
but a very small number of cases in which the presence of
chromatin has not been satisfactorily demonstrated.

The part played by the nucleus assumed a new significance
after the discovery by Anton Schneider in 1873 of remarkable
form-changes undergone by the nuclei of tissue-cells at the
time of reproduction by division. Schneider's observations
were followed up by a host of observers, amongst whom the
names of Fleming, Fol, Ed. van Beneden, and Hertwig must
always occupy an honourable position ; and it has been demon-
strated that the phenomena first observed by Schneider are of
normal occurrence, not only in dividing tissue-cells, but also
in the germ-cells, the developing ova and spermatozoa of
multi- cellular animals, and in a large number of simple uni-
cellular animals, or Protozoa.

The phenomena are generally known as karyo-kinesis, but
the word mitosis is also used to denote them, and it is, on the
whole, the more convenient term, though it must yield priority
to the other.

As has been already said, division of the cell-body is always
accompanied by and generally preceded by division of the
nucleus. In certain, but comparatively rare, cases the nucleus
divides quite simply. It becomes elongated, and then dumb-

ii2 COMPARATIVE ANATOMY

bell shaped. The strand connecting its swollen ends becomes
thinner, and finally breaks, so that two new nuclei are formed.
The cell-body becomes constricted in its middle and eventu-
ally is separated into two halves, each containing a portion of
the divided nucleus. Such a moue of nuclear division is
known as simple or amitotic. It occurs in some Protozoa, in
some leucocytes, and characteristically in certain cells forming
morbid growths. But in the great majority of cases the
process is of a far more complex kind, and it differs somewhat
in tissue-cells and in germ-cells. We will take the former case
first, as being the simpler.

A resting nucleus has the characters depicted in figure 23, A.
Its nuclear membrane is evident ; and within the membrane
the deeply-staining chromatin is distributed in the form of a
reticulum or network, which often exhibits thickenings or
nodes at the points where the chromatin threads are, as it
were, knotted together to form the network. These thicken-
ings are not to be confounded with the nucleolus, which does
not seem to take an active share in the process of mitosis.

Outside of the nuclear membrane, lying close to it in the
cytoplasm, is a very minute but very important body, known
as the centrosome. It is often surrounded by a little
specialised mass of cytoplasm, known as the centrosphere.
The division of the cell appears to be heralded by changes
in the centrosome and the cytoplasm immediately surrounding
it. The centrosome divides into two parts, which travel round
the periphery of the nucleus to take up provisions at opposite
poles. As it divides and travel's round, each moiety of the
centrosome becomes surrounded by a number of radiating
fibrillae stretching into the adjoining protoplasm, and so form
ing a characteristic star-shaped figure to which the name of
the astral figure has been given, the fibrillae being known as
the astral rays. There is some doubt as to whether the astral
rays are formed entirely from the centrosphere or from the
ordinary cytoplasm.

Meanwhile important changes have been going on in the
chromatic reticulum of the nucleus. The network breaks up
and resolves itself into a convoluted thread composed entirely
of chromatin, which now stains even more intensely than
before. The convoluted thread at this stage is known as the
skein or spireme. In a short time the spireme breaks up into

MITOSIS

113

a number of segments of equal length, which may be straight
and rod-shaped, or curved into a horseshoe shape, or may
be simply spherical or ovoid masses. In some cases the

Fig- 23.

Diagrams representing the essential phenomena of mitosis. A , a cell with resting
nucleus containing a chromatic reticulum and a single nucleolus. The
centrosome is double and surrounded by the centrosphere. B, the centro-
somes are separating and each is surrounded by astral rays ; the chromatin
forms a convoluted thread or spireme. C, The spireme is broken up into a
number of V-shaped chromosomes, the polar spindle is formed between the
now widely separate centrosomes. /?, The chromosomes attached to the
spindle-fibres are arranged at the equator of the spindle. E, division of the
chromosomes, which are viewed end on. f, divergence of the chromosomes.
G, chromosomes collecting at the poles of the spindle, the space between
them occupied by interzonal fibres ; commencement of division of the cell-
body. H, /, complete division of the cell, and reconstitution of the nuclei.
In / the centrosomes are dividing preparatory to a new mitosis. Note
A-D-propha.se; .£=metaphase ; F, (7 = anaphase. H, /=telophase.

curved rods may be joined together by their ends to form
rings. But their shape is relatively unimportant, the essential
thing is that the chromatin is resolved into a definite number

1 14 COMPARATIVE ANATOMY

of equal masses, called chromosomes, and it is a striking and
important fact that every species of plant or animal has a
fixed number of chromosomes, which number recurs in every
division of every tissue-cell in the body. The number may be
very large— it is said to be one hundred and sixty-eight in a
certain crustacean known as Artemia salina, — or it may be very
small, as in the thread-worm parasitic in the intestines of the
horse (Ascaris megalocephala\ but in all animals which are
reproduced sexually it is an even number.

At the time when the chromosomes are formed the nuclear
membrane has generally been absorbed, and has disappeared,
leaving the chromosomes apparently free in the cytoplasm,
and at the same time an important and remarkable structure
makes its appearance in the position formerly occupied by the
now broken-up nucleus. This is the spindle, composed of a
number of fibrillar threads converging at each end of the
spindle towards the centrosomata, and diverging from one
another in the centre, or, as we may call it, the equator of the ,
spindle. There is some doubt as to whether the spindle is
formed out of the substance of the cytoplasm or out of the
achromatic substance of the nucleus. In a large number of
cases, at all events, the spindle fibres appear to be formed in
two groups which have different origins: an internal group,
forming the axis of the spindle, is formed from the linin threads
of the nucleus, and an external group, forming the periphery
of the spindle, is derived from the cytoplasm, and probably
from that modified part of it which has been described as the
attraction sphere or centrosphere. It should be noticed that
in some cases — e.g. the tissue-cells of the salamander — the
spindle at its first origin lies wholly outside the nucleus, and
is formed entirely under the influence of the centrosomata.

The .spindle and the two asters surrounding the centro-
somata do not easily stain with the ordinary dyes, and hence
are often called the achromatic figure, or amphiaster, but the
centrosomata themselves stain readily and intensely with
certain dyes.

However the spindle may be formed, its eventual relation to
the chromosomata is the same. The latter become attached
to the spindle fibres in such a manner as to form an equatorial
ring round the spindle. The whole of the stages which have
been described constitute what is known as the prophase of

MITOSIS 115

mitosis, and may be regarded as preparatory to the next and
most essential step, the metaphase. This consists in the divi-
sion of the chromosomata into equal halves. (Fig. 23, E.) If
they are rod-shaped or horse-shoe shaped the chromosomata
split into two lengthwise, if round or ovoid they simply divide
into two, and each half so-formed travels in opposite directions
along the spindle-threads towards the poles of the spindle. The
metaphase now passes into the final stages of the process
known as the anaphase. The divergent groups of chromo-
somata become closely crowded into a mass at each pole of
the spindle in the centre of the astral rays, being connected,
across the space previously occupied by the spindle, by a
bundle of fibres, known as the connecting or interzonal fibrts ;
these are supposed to represent the central fibres of the
spindle. In plant cells, and in some animal cells (e.g. in
cartilage) the interzonal fibres are thickened in the equatorial
region of the spindle to form the so-called equatorial plate.
The last stages of mitosis are known as the telophase. The
groups of chromosomata at each pole of the spindle are
reconstituted into a new daughter-nucleus, usually going
through the processes of the prophase in a reversed direction.
Thus the chromosomata become united to form a spireme,
and the spireme breaks up into a chromatic reticulum, the
nuclear membrane re-appearing in the spireme stage. Whilst
this is going on, the cell-body is divided into two in a plane
passing through the equator of the spindle. In plant cells, and
some animal cells in which an equatorial plate is formed, the
division is effected by the formation of a septum across the
cell-body in the plane of the equatorial plate. But in most
animal cells division of the cell-body is effected by a simple
constriction which gradually deepens and divides the cell into
two. The asters generally disappear, or are reduced to a
spherical mass surrounding the centrosome, constituting a
centrosphere. The results of this truly remarkable process
are obvious. The nucleus of each daughter-cell received
exactly half the chromatin of the nucleus of the mother-cell.
Nor is this all ; the halves into which the chromatin of the
mother-nucleus is divided, are not only equal halves, but,
because of the longitudinal splitting of the chromosomata, are
like halves. When division of the cell is complete a resting
stage follows, during which the nucleus of each daughter-cell

n6 COMPARATIVE ANATOMY

grows to the size of the original mother-nucleus, the chromatin
sharing in the growth of the whole nucleus ; and when the
normal size is attained the prophase is again entered into, and
a fresh mitosis with a new division of the cell takes place.
The behaviour of the centrosome demands some attention.
This minute body seems to lead the way in cell-division. The
two halves into which it divides form the centres of the asters
and the poles of the spindle ; they determine, by the positions
which they take up, the direction of the spindle, and conse-
quently the plane in which the cell-division will take place.
It is evident that this plane must always be at right angles to
the long axis of the spindle. The centrosomata sometimes
divide very precociously during the telophase or even during
the anaphase of mitosis, and the two products then remain in
each daughter-cell throughout the resting stage. On the other
hand, there are cases in which the division of the centrosome
is retarded until after the formation and even after the segmen-
tation of the spireme. We cannot, therefore, regard the
centrosome as a " dynamic centre," in the sense of its giving
an impulse to cell-division; it seems rather to exercise a
directing influence, inasmuch as it determines the direction of
the long axis of the achromatic figure.

The details of mitosis differ in different animals and plants,
but the end result in tissue-cells is always the same, the
chromatin of the mother-nucleus is divided into two equal and
like halves which are distributed to the daughter-nuclei. The
only real exceptions occur in abnormal and pathological cases,
such as cancer-cells, which need not detain us here. But,
whilst the equal division of the chromatin is the rule for tissue-
cells, the case is different for germ-cells.

In a tadpole of the frog of about 10 mm. in length, the
primordia of the generative organs are first found as a pair of
ridge-like thickenings of the peritoneal epithelium lying along
the dorsal side of the coelom close to the mesentery. Each
genital ridge is simply a thickening or modification of the
peritoneal epithelium, whose cells, elsewhere flattened, here
become cubical or columnar. As age advances, the genital
ridges become thickened, partly by the division of the cubical
germ-cells, which come to form a layer several cells thick,
partly by the ingrowth of connective tissue amongst the
developing cells. At this period there is no distinction of

MITOSIS IN GERM CELLS 117

sex, but further changes may lead to the formation of ova in
the female or spermatozoa in a male frog. Whilst the frog is
still in the tadpole stage, certain of the epithelial cells of the
genital ridge become larger and more rounded than their
neighbours, and form definite germ-cells or primitive ova.
Around these the adjacent epithelial cells are arranged so as
to form a number of capsules or follicles which form slight
projections from the surface of the genital ridge. The
germinal epithelium gives rise to new primitive ova, or they
may be formed by division of those already existing. The
so-called primitive ova may give rise either to ova or sper-
matozoa, and the sexual differentiation only appears at about
the time of the metamorphosis. In the female frog the
process of differentiation is comparatively simple. The
primitive germ cells, which by continued multiplication by
division have become very numerous, enter into a period of
rest and growth, during which they grow much larger and
become filled with a number of granules of nutritive reserve
material known as the deutoplasm or food-yolk. The per-
manent ova are enclosed in follicles like the primitive ova,
and it is by the activity of the follicular cells that the food-
yolk is elaborated from the blood and lymph and passed on
to the ovum. The nucleus grows much larger, and its nuclear
membrane and chromatic reticulum become more distinct.
When the ovum has attained a size of about -5 mm., a thin
structureless envelope, the vitelline membrane, is formed around
it, and rather later black pigment is deposited at first over the
whole surface, but later it is restricted to one hemisphere. By
continued deposition of yolk within their cell-bodies the ova
grow till they reach the size of small shot, and then they
project in bunches from the surface. They then go through
a process known as maturation, during which each ovum
divides twice into two very unequal portions, the nucleus being
divided at each division, but in a manner different to that
which obtains in tissue - cells. The two smaller portions
formed by division of the ovum are known as polar bodies.
After maturation the ova are discharged into the body-cavity
by the bursting of their follicles ; they are passed along partly
by muscular contractions of the abdominal walls, partly by
ciliary action, to the mouths of the oviducts, and are passed
down the oviducts to the uteri. During their passage down

it8 COMPARATIVE ANATOMY

the oviducts the ova obtain gelatinous coats secreted by the
glands with which the oviducal walls are abundantly supplied.
The ova accumulate in great numbers in the highly distensible
uteri, and eventually are expelled through the cloaca into the
water, in which the gelatinous investment swells up to form
the well-known jelly of frog's spawn.

At the time of their passage to the exterior the ova are
fertilised by the simultaneous discharge of the semen of the
male frog. Without fertilisation they are incapable of further
development ; but if fertilisation takes place each ovum goes
through a series of divisions which result in the formation of a
multi-cellular aggregate, leading up to the tadpole and cul-
minating in the adult frog.

The spermatozoa of the male are formed from the follicles
containing primitive ova. These become converted into the
seminiferous tubules of the adult testis. Briefly told, the
history of the primitive ova in the male is this. They become
spermatogonia, which divide and sub-divide, till, after a time,
they come to rest and increase in size. They are now known
as spermatocytes. Each spermatocyte divides into two, and
each of these again divides into two, making four spermatids
derived from each spermatocyte. Each spermatid is directly
changed into a spermatozoon.

The changes accompanying the maturation of the ovum
are rather complicated in the frog, and the formation of the
spermatozoa is not very well understood, so it will be better
to describe the simplest possible case of spermatogenesis and
maturation of the ovum as it occurs in the round worm of the
horse, Ascaris megalocephala. It has already been stated
that this animal, or at least that variety of it known as
univalens, has only two chromosomata. Another variety
known as bivalens has four chromosomata in its nuclei, and
this will be chosen for the description of the processes of
maturation of the germ-cells. Beginning with the sper-
matozoon— it must be explained that the spermatozoa of the
thread-worms (Nematoidea), of which Ascaris is an example,
are not filamentous, like those of most other animals, but are
simply cellular, and therefore are very suitable objects for study.
The primitive germ-cells have, like the rest of the tissue-cells of
the body, four chromosomata each. They multiply by division
again and again, the nucleus each time undergoing normal

SPERMATOGENESIS 119

mitosis so that the spermatocytes eventually formed have
each the typical number of four chromosomata. But in the
spermatocyte preparing for division the chromatin, instead of
dividing into four chromosomes, splits up into eight rod-shaped
bodies, united by linin threads into two groups of four, or
tetrads. On the formation of the spindle the tetrad groups

Fig. 24.

Spennatogenesis in Ascaris megaloccphala var. Invalens. .4,3. sper-
matogonium with four rod-shaped chromosomes. />', a spermatocyte
preparing for division. Instead of four chromsomata there, are two
groups of four, or tetrads. C, first division of the spermatocyte ;
two members of each tetrad are passing to opposite poles of the
spindle. I), first division complete ; two secondary spermatocytes
are formed, each containing two dyads. £, second division of the
spermatocytes, showing the halving of the dyads, f, the division
completed, resulting in four spermatids, each containing two chro-
mosomes. (After Boveri.)

are arranged at the equator, and two members of each group
of four pass to the opposite poles of the spindle. At the close
of the division there are two daughter-spermatocytes, each
containing two groups of two, or dyads. These two cells
immediately divide again ; the dyad groups are arranged at
the equators of the spindles as the tetrad groups were in the
preceding division, and one member of each of the two dyad

120 COMPARATIVE ANATOMY

groups passes to opposite poles of the spindle. At the close
of division the two daughter-spermatocytes have given rise to
four spermatozoa, each of which contains two chromosomata —
i.e. half the characteristic number. Such a mitosis as this,
resulting in the halving of the number of chromosomata, is
known as a reducing division.

In the ovum the process is essentially the same. The
primitive ova divide and sub-divide for some time with
normal mitoses. At length a stage is reached when each
cell, now called an oocyte, ceases to divide, and enters on
a period of growth, during which it becomes enormously
larger through the accumulation of food-yolk in its cytoplasm.
Then two tetrad groups are formed preparatory to a division.
The division takes place in the same manner as in the case
of the spermatocyte, so far as the behaviour of the tetrad
groups is concerned, but only a very minute portion of
cytoplasm containing two dyad groups is segmented off from
the ovum. This minute cell is called the first polar body.
It must be noticed that both it and the ovum contain two
dyad groups, just as the two daughter-spermatocytes contained
each two dyad groups. Both ovum and polar body im-
mediately prepare for a second division, and in each the
two dyad groups are halved in the process of mitosis. The
end result is the formation of (i) an ovum, containing two
chromosomata; (2) three polar bodies, one formed by the
second division of the ovum, the other two by the division
of the first polar body, each containing two chromosomata.
The polar bodies are not destined for further development.
They may persist for some time under the vitelline membrane,
but they eventually disintegrate and disappear. The ovum,
however, is ready for fertilisation.

Fertilisation consists essentially in the union of the nucleus
of the spermatozoon (best called the sperm-nucleus) with that
of the ovum (best called the egg-nucleus). Each of these,
as the result of the reducing divisions, has half the number
of chromosomata characteristic of the species. By their
union the full number is restored, and the ovum is capable
of further division and development.

The essential facts of fertilisation in Ascaris megalocephala
are as follows : —

Whilst the ovum is dividing to form the second polar body,

OOGENESIS

121

the spermatozoon, surrounded by a small cell- body consisting
of cytoplasm with centrosphere and centrosome, enters the
ovum. It moves towards the egg-nucleus, and its two
chromosomata become resolved into a tangled skein of
chromatin. The chromosomata of the egg-nucleus, after the

Fig. 25.

Maturation of the ovum in Ascaris ineglocephala var. bivalens. A, the ovum,
with the spermatozoon, Spz, entering it. The nucleus contains two
tetrads. B, the nucleus has approached the surface and the tetrads are
dividing. C, the nucleus has divided to form pb' ', the first polar body ;
both egg-nucleus and polar body contain two dyads. D, the dyads
in the egg-nucleus rotating preparatory to a second division. E, division
of the dyads in the egg-nucleus and in the first polar body pb' . F, for-
mation of the second polar body and division of the first polar body. There
are three small cells (the polar bodies), and one large cell (the ovum),
each containing two chromosomes. The spermatozoon has entered the
ovum, and its nucleus, covered by a granular cap of archoplasm, is shown at
pn (J . (Adapted from Boveri.)

formation of the second polar body, follow suit, and the
centrosome and attraction sphere of the egg-nucleus seem
to disintegrate and disappear altogether. As the sperm-
nucleus approaches the egg-nucleus, the centrosome of the

122 COMPARATIVE ANATOMY

former divides, and forms, with its attraction sphere, also
divided, an achromatic figure of the normal type. The skein
in each of the germ-nuclei now resolves itself into two
chromosomata having the form of bent rods, and the four
chromosomata become attached to the equator of the spindje
of the achromatic figure. Each is split into two longitudinally,
and a half of each passes to opposite poles of the spindle.
Division of the body of the ovum then takes place in the
normal manner, and two embryo-cells or blastomeres are
formed, each of which contains four chromosomata, half
derived from the male, half from the female nucleus. The
processes of maturation of the ovum, development of the
spermatozoa, and fertilisation are essentially the same in the
frog as those described for Ascaris, though differing in details
which appear to be unimportant. The number of chromo-
somata in the cells of the frog is twenty-four. These are
reduced in the reducing divisions of ova and spermatozoa to
twelve, and the number is restored on union of the sperm
and egg-nuclei to twenty-four. The spermatozoon of the
frog, however, is very different to that of Ascaris, consist-
ing of (i) a head, (2) a minute segment immediately behind
the head, which appears to consist of archoplasm with a
contained centrosome, (3) a whip-lash-like tail or flagellum,
by whose movements the spermatozoon -is propelled through
liquid. The head consists entirely of nuclear matter, except
for a very thin layer of cytoplasm, which probably forms an
investment to it.

The ovum of the frog, as has been said, is abundantly
provided with food-yolk, stored up chiefly in one hemisphere
of the cell. The first segmentation following upon fertilisa-
tion divides the ovum into two equal halves, or blastomeres.
A pause follows, and the ovum divides again, the plane of
the second division being at right angles to the first. Thus
four blastomeres are formed, each consisting of a smaller
upper pigmented protoplasmic part, and a lower colourless part
filled with deutoplasm. The third segmentation is described
as equatorial, but it does not pass through the equator of
the spherical ovum, but is placed nearer the pigmented pole.
It nearly completely cuts off four pigmented upper blastomeres
from four lower heavily-yolked blastomeres. Hitherto the
planes of division have passed right through the ovum,

DEVELOPMENT OF THE FROG 123

giving rise to an eight - celled stage, and the next divi-

J£

''

Fig. 26.

Segmentation of the frog's ovum and formation of the blastopore (after Morgan).
A, eight-cell stage resulting from two meridional and one equatorial
divisions. B, beginning of sixteen-cell stage. C, Thirty-two-cell stage.
D, Forty-eight-cell stage, showing the smaller cells at the upper pole and
larger yoke-laden cells at the lower pole. E, F, two sides of the same ovum
in later stages of segmentation. G, a still later stage of segmentation. //,
the smaller cells are growing over the larger cells but are involuted along a
crescentic line /'/, the dorsal lips of the blastopore. /, external view of the
blastula, showing the circular blastopore.

slon is described as meridional. Each upper pigmented
blastomere is divided into two longitudinally, and the division

I24

COMPARATIVE ANATOMY

lines are continued downwards into the yolk blastomeres.
But these, being so largely composed of inert deutoplasm,
divide slowly, and their division is completed some time
after it has been effected in the upper blastomeres. The
next divisions may be described as equatorial, and affect
all the sixteen blastomeres, dividing each of them into two,
as is shown in fig. 26, C. The embryo now consists of thirty-

Fig. 27.

A, vertical section through a segmenting ovum at about the stage
represented in fig. 26 D. B, C, and D, similar sections through
later stages. Bl, segmentation cavity or blastocoele, />/, bjastopore.
(After Morgan.)

two cells, and the succeeding divisions become irregular, and
are no longer synchronous. The upper pigmented cells divide
much faster than the lower yolk-cells, and the final result of
the first phase of development (usually called the segmentation
phase) is a vesicle containing a rather flattened and eccentric
cavity, whose roof is composed of two layers of small pig

DEVELOPMENT OF THE FROG 125

mented cells, and the floor of much larger irregularly-poly-
gonal yolk-cells which occupy the whole of the lower
hemisphere of the embryo. The small cells pass, without any
sharp line of demarcation, into the yolk-cells. Such a hollow
vesicle is known as a blastula, and its cavity is known as the
segmentation cavity or blastoccele. Further changes, brought
about by repeated cell-division, result in the formation of
a three-layered embryo, and all the organs and tissues of the
tadpole, and, finally, of the adult, are derived from these three
layers.

The actual formation of the three layers is somewhat ob-
scured in the frog because of the large preponderance of
inert food-yolk, and it will be better to defer a consideration
of the details of further development till a later stage.

CHAPTER IV
THE RHIZOPODA— AMCEBA PROTEUS

OUR studies have hitherto been directed to the elucidation of
the anatomy and development of a complex animal provided
with tissues and organs which subserve different purposes in
its economy, and are modified structurally in accordance with
the functions which they have to perform. ,We have seen that
all its tissues (which are comparable to our own) are formed
either by, or out of, certain ultimate form-elements called cells,
and that the diversity and complexity of the animal's structure
is to be referred to the diversity of structure obtaining among
its component cells. Further, we have seen that this remark-
ably complex animal, composed of countless cells, is brought
into being from the simplest possible beginning— namely, a
germ-cell, which, after undergoing the processes which have
been described as maturation and fertilisation, possesses the
power of giving rise to the entire organism. The course of
the development of the germ-cell has only been lightly touched
upon ; but we have seen that it consists essentially in the
multiplication of the fertilised germ-cell or oosperm by division,
the products of division cohering and forming a hollow embryo
composed of many cells, which are ultimately differentiated to
form the tissues of the adult.

We have learned in the course of these studies that all the
phenomena of the frog's life (exclusive of its psychical pheno-
mena, which baffle our powers of analysis) are to be referred
to the activities of its cells. We now can turn to the study
of a large group of organisms of the simplest possible con-
stitution, the Protozoa, whose vital phenomena are manifested
within the limits of a single cell.

In commencing the study of the Protozoa, we cannot do
better than begin, as is customary, with the study of a common
pond animalcule, known to science by the name of Amoeba
proteus. This minute organism, pregnant with meaning to

126

THE RHIZOPODA 127

the biologist, is found in fresh-water ponds and in damp
places. There are many kinds of Amoebae, but that kind
known as proteus is selected because of its relatively large
size and unequivocally simple structure. Amoeba proteus is
about '25 mm. (y^th of an inch) in diameter. It is a shape-
less mass of protoplasm, which, under moderate magnifying
powers of the microscope, appears to be divisible into an outer
transparent colourless layer, generally referred to as the ecto-
plasm, and an inner, more fluid mass, the endoplasm, contain-
ing numerous granules of a dark colour. Closer inspection
reveals the presence of a small, subspherical, denser body
within the endoplasm ; this is the nucleus, and, if the animal
is killed and stained, it is seen, like the nuclei of tissue-cells,
to take up the staining fluid with more avidity than the
cytoplasm. The nucleus, in fact, contains chromatin, which,
in this particular species, is scattered throughout the substance
of the nucleus in the form of minute spherules. In addition
to the nucleus, clear spaces may be seen in the endoplasm
filled with a watery fluid. They are not altogether permanent
structures, but are formed or absorbed so slowly that they
may be credited with persistent characters, and hence are
called permanent vacuoles. Besides these, there is a structure
called the contractile vacuole. If a living Amceba is carefully
watched under the microscope, a clear round space, larger
than the permanent vacuoles, is seen to arise in the cytoplasm.
It grows in size, just as a soap-bubble grows when blown from
the bowl of a pipe, and its optical characters leave no doubt
that it is filled with a watery fluid. After reaching a certain
size the vacuole ceases to expand, and, after a period of rest,
it is suddenly obliterated by the closing in of its walls from
all sides. The obliteration of the vacuole is clearly brought
about by the active contraction of the cytoplasm forming its
walls, and in the process its fluid contents are expelled, though
no obvious pores or canals can be seen through which they
effect their escape.

These are the only definite organs of the Amceba ; but its
most characteristic feature is its constantly-changing shape.
The creature, under normal circumstances, is never at rest,
but is constantly protruding processes of its cytoplasm, now
in one direction, now in another. These blunt, shapeless
processes are called pseudopodia, and the characteristic move-

128

COMPARATIVE ANATOMY

ment produced by their extension and retraction is called
amoeboid movement.

Careful observation of pseudopodial movement shows us
that the formation of a pseudopod is initiated by a slight
bulging of the hyaline edge of the cytoplasm; the bulge
increases in size, and for a while the internal granular mass
is not affected. Then, rather suddenly, the granular mass
seems to flow into the hyaline process ; and, as it flows with an

*i

Fig. 28.

,

Amcel>a crystalligera, highly magnified, showing the central so-called nucleolus
surrounded by an envelope of chroraatin. F, dividing nucleus of the same species,
highly magnified. (A after Leidy ; B-F after Schaudinn.)

active streaming movement of its granules, the hyaline process
spreads and extends, the internal granular stream keeping pace
with its extension, until a large lobe-like process is formed, as
large it may be as the whole of the remaining body of the

AMCEBA 129

Amoeba. Continued formation of pseudopodia in one direc-
tion results in locomotion in a definite direction, but the move-
ment is seldom definite in Amoeba proteus. After any given
pseudopod has attained a certain size it ceases to extend, the
streaming of the granules in its interior becomes slower, and
eventually the flow is reversed. Another pseudopod has
been forming in some different direction, and the granular
" endoplasm " streams off into it, the older pseudopod
diminishing in size and finally becoming obsolete.

These phenomena have led zoologists to describe a definite
division of the body of the Amceba into two parts, an outer
hyaline and firmer ectoplasm, and a more fluid endoplasm
containing granules and vacuoles, and also various foreign
substances which have been taken in as food. But recent
researches make it doubtful whether there is any permanent
differentiation of the cytoplasm into ectoplasm and endoplasm.

In a large number, if not in all Amoebae, it may be shown
that the cytoplasm, the hyaline border as well as the granular
central part, has the structure of an exceedingly fine sponge-
work, or rather, let us say, a foam. Imagine a foam or froth
composed of bubbles of extremely minute size. Imagine that
the skins of these bubbles are composed of a somewhat denser
tenacious material and that their cavities are occupied, not by
air, but by a watery fluid. The bubbles in the centre of the
froth would be polygonal through mutual pressure ; and if the
whole thing could be hardened and cut into slices the sections
of the walls of the bubbles would look like a network having
somewhat irregular polygonal meshes. Such a foam may be
made by mixing up finely-pounded common salt with olive oil
which has been kept for a long time so as to be slightly rancid
and of a thick consistency like treacle. A small droplet of
this mixture, placed in water and examined under the micro-
scope, exhibits a structure which is marvellously like that of
the cytoplasm of an Amoeba. It is also divisible into an outer
more hyaline border and an inner granular mass, and it has
been shown that the outer hyaline border is due to the special
form assumed by the alveoli or bubbles in consequence of the
surface tension produced by the contact of two immiscible
liquids of different densities. A similar arrangement of the
alveoli is found in the so-called ectoplasm of the Amoeba, and
it is permissible to assume that the similar effects observed

1 3o COMPARATIVE ANATOMY

in the two cases are produced by like causes. But the
resemblance of the oil-drop to the Amoeba does not end here.
Under favourable conditions the former may be seen to move,
active circulation of its contained granules takes place,
exactly simulating the characteristic "streaming movements"
of protoplasm, and it may even emit pseudopodial processes
and withdraw them, just as an Amceba does. All these move-
ments are to be explained, in the case of the oil and salt
mixture, on purely physical grounds, and it has been suggested
that the movement of protoplasm is due to and may be
explained by the same physical causes as those which bring
about similar phenomena in the artificial mixture. The
question cannot be discussed here, and the reader who
wishes to learn more about the remarkable analogies between a
simple chemical mixture and living protoplasm should consult
Professor Biitschli's work.* It must be borne in mind, how-
ever, that with the pseudopodial movement and a few other
phenomena of secondary importance the resemblance between
the mixture and the Amoeba, the non-living and the living
matter, ends. The droplet of oil and salt does not ingest food,
does not assimilate, does not grow and reproduce its kind.
The most essential vital phenomena are wanting, and it can-
not be said, in any sense, to live. The value of Professor
Biitschli's experiments consists in their having opened the
way for an understanding of the real structure of protoplasm,
and it is evident that the analogy of the oil and salt mixture
throws great deal of doubt on the propriety of dividing the body
of the Amoeba into endoplasm and ectoplasm.

That the outer layer is nothing more than a temporary
rearrangement of the superficial alveoli is further suggested
by the manner in which an Amceba feeds. Like all indisput-
able animals it takes into its interior, or, as we say, ingests
solid food, consisting of vegetable matter, Oscillaria, Diatoms,
and even other Protozoa. These substances may generally
be seen in the central part of the cytoplasm, and careful
examination shows that the separate food-particles are en-
closed in a space filled with fluid, called a food-vacuole. The
vacuole, however, is formed subsequent to the ingestion of

* "Investigations on Microscopic Foams and on Protoplasm," by
O. Biitschli. Eng. Trans, by E. A. Minchin. London : A. & C. Black,
1894.

AMCEBA 131

the food, and is not merely a droplet of water taken in along
with it, but a secretum of the protoplasm. There is no
special orifice nor even a soft spot in the protoplasm
where solid food is taken in. The pseudopodia come into
contact with some food -substance; they embrace it, flow
round it, meet above and below it, and finally enclose it.
Blochmann has described how Amoeba proteus catches the
agile infusorian Cydidium glaucoma in this simple manner.
The Cyclidium seems to be fatally attracted to the Amoeba,
swims towards it, and lies between its pseudopods. In this
position it is enclosed above and at the sides by the flowing
movement of the protoplasm ; it seeks awhile to escape from
the death-chamber, but it is finally enclosed; and, when enclosed,
a vacuole is secreted round it. It is evident that the walls of
the death -chamber formed of fused pseudopodial processes,
were originally part of the exernal layer or ectoplasm, and that
in enclosing the Cyclidium or other prey they have become
internal, and are eventually merged into the inner substance, or
endoplasm, of the Amoeba. .Thus ectoplasm can become
endoplasm and vice versa. It would not, however, be correct
to say that all Amcebse consist of naked protoplasm without
any external limiting coat. Amoeba bi-nucleata, so named
because it has two nuclei, is a form with a distinct though very
thin pellicle outside the external alveolar layer, and this
pellicle may be made evident by certain dyes. Pelomyxa
palustris, a very large and rather peculiar Amceba, is described
as being covered all over with minute non-vibratile hair-like
appendages ; and Amoeba villosa has a tuft of similar append-
ages at one end of its body, pseudopodia of simple form being
formed only at the opposite end.

It has recently been demonstrated that the contents of the
food-vacuoles, at the time of their formation, are acid, and
eventually they exercise a solvent action upon proteid sub-
stances, but not, it would seem, upon starches and fats. Thus
an Amoeba may be said to be strictly carnivorous. It is most
probable that the fluid contents of a food-vacuole are a special
secretion of the protoplasm, containing a ferment or enzyme,
which has an action analogous to that of the gastric secretion
of higher animals. The proteid morsel enclosed in a food-
vacuole is gradually dissolved, and disappears, only a small
granular residue remaining. This residue is eventually re-

132 COMPARATIVE ANATOMY

jected, together with other non-nutritious substances which
have been taken in, as it were, accidentally. There is no special
aperture for the ejection, as there is none for the ingestion, of
solid matter. One may say that the Amoeba flows away from
its indigestible contents, leaving them behind.

Since Amoebae live in muddy and sandy bottoms amidst all
sorts of inorganic non-nutritious grains and fragments, it is
clear that they must exercise some sort of selection in ingest-
ing foreign material ; for if they did not, they would take in
every particle of convenient size that they meet, and they
do not. Moreover, an Amoeba living in water which contains
numerous diatoms of relatively large size may be seen to
engulf a number of these apparently inconveniently large
objects almost to the exclusion of other matters, thus making
a distinction between things good for food and things not
good. But its powers of discrimination are not great, for it
will sometimes ingest grains of quartz sand, and such non-
nutritious substances as powdered carmine, or powdered
litmus. None the less we must credit the animal, lowly as it
is, with a certain power of selection which constitutes a part
of its sensibility or irritability.

In common with all living things, the Amoeba exhibits that
response to external stimuli which we call irritability. Thus
its movements are retarded by cold, and up to a certain point
are made more active by heat. But if the temperature of the
water in which it is contained is raised above this point, its
movements become less active and cease altogether between
30° and 35° C., but begin again when the temperature is
lowered. At about 40° C. the protoplasm is coagulated, and
the Amoeba is killed. The organism is also responsive to
electrical stimuli. If an Amoeba is placed in a drop of water
between the electrodes of an induction battery, and a weak
shock is passed, it may be seen to withdraw its pseudopodia
and contract itself into a ball, gradually recovering its shape
and resuming its pseudopodial movement after the effects of
the shock have passed away. There is also evidence that
Amoebae are sensitive to light. Pelomyxa palustris has been
observed to avoid the light, creeping under the mud at the
bottom of the vessel in which it was contained during the day,
and crawling out on the sides of the vessel during the night.
In these diurnal migrations the animalcule traversed a dis-

AMCEBA 133

tance of 20 cm. during the twenty-four hours. On the
introduction of water-weeds into the vessel the Pelomyxse
were seen to crawl up them during the night, and on being
suddenly exposed to bright sunlight they contracted them-
selves into balls and dropped off the weed to the bottom of
the vessel, where they hid themselves under the mud. But
after the water-weed had been in the vessel for some time the
Pelomyxse no longer left the mud but stayed buried in it both
by day and night. It would seem that, before the introduction
of the water-plant, the water became deficient in oxygen, and
that the deficiency was greatest in the mud at the bottom of
the vessel. During the day the animalcule's aversion to light
proved stronger than its intolerance of an insufficient supply
of oxygen, so it remained buried in the mud, but during the
night left its hiding place in search of oxygen. When the
whole contents of the vessel became appropriately oxygenated
the migration ceased.

Thus Amoeba, simple as its constitution is, exhibits the
characteristic vital phenomena of irritability, automatism,
assimilation, and excretion. We cannot doubt that it is also
metabolic. The pseudopodial movements are manifestations
of energy ; they imply waste of material, and we can infer
from the fact that the animalcule takes in and digests food
that the waste of its substance is made good by the elaborated
products of digestion. In so small an organism the actual
processes of waste and repair cannot be followed out in detail,
none the less we can assert that they are performed. So, too,
we can assert that the Amoeba is respiratory, that the energy
exhibited in pseudopodial movement is the result of the
oxidation of the tissues, and that one of the waste products
is carbon dioxide. There is no definite respiratory organ,
unless, indeed, the contractile vacuole functions as such, and
the amount of carbonic acid gas given off is too small to be
collected and measured. But the same observation which
showed that Pelomyxa is sensitive to light shows also that it
requires oxygen.

Lastly, Amoeba is reproductive. It propagates its kind
in the simplest and most primitive manner by binary fission.
Seeing how common an object of study Amoeba is, it is re-
markable that the process of binary fission is but rarely
observed, and that even now we are not sure whether mitotic

134 COMPARATIVE ANATOMY

or amitotic division of the nucleus during binary fission is the
rule. It is, at all events, certain that amitotic nuclear division
does occur. In Amoeba crystalligera the nucleus consists of
a central mass or kernel, which stains but feebly with ordinary
dyes. This has been described as a nucleolus, but the name
is hardly applicable, for it is nothing like the nucleolus of
other cells — e.g. ovicells. Around this kernel is an envelope
of material which stains deeply, and is undoubtedly chromatin ;
and outside of this again is a cortical layer which scarcely
stains at all. The characteristic alveolar structure of proto-
plasm is distinguishable in all three elements of the nucleus.
(Fig. 28, E.) When division is about to take place, the nucleus,
previously spherical, becomes oval, and is drawn out into an
elongated dumb-bell shape (fig. 28, C and F), all three layers
sharing in the elongation without obvious change. The connect-
ing thread between the two swollen ends of the nucleus becomes
attenuated, and eventually snaps, upon which the two ends
round themselves off and become new nuclei. At the same
time, a deep constriction appears in the cytoplasm, passing
towards the place where the nuclear thread divided. This
constriction deepens, and eventually divides the body of the
Amoeba into two halves, each containing one - half of the
original nucleus. In this case there is no trace of centrosomes,
spindle, chromosomes, or astral rays, and the division is
undoubtedly amitotic. But in Amoeba bi-nucleata a primitive
kind of mitosis has been observed. In this species both
nuclei act together, and undergo the same changes during
division, in such a manner that each daughter form has two
nuclei, one derived from each of the two nuclei present in the
parent form. The nuclei have very firm nuclear membranes,
inside which is a nuclear sap containing numerous chromatin
masses. In the prophase of mitosis, the chromatin masses
break up, and their minute fragments are scattered through the
substance of the nucleus. The nucleus itself becomes some-
what flattened, and at each of its two flatter poles a small
mass of hyaline structureless protoplasm, apparently the
representative of the archoplasm, makes its appearance as a
flat cap. Meanwhile the chromatin granules collect in the
equatorial plane of the nucleus and form a plate. A true
spindle has not been observed, but in the later stages of
division fine longitudinal threads were seen stretching between

AMCEBA 135

the chromatin particles, now separated into two groups, of
which one travels towards each pole of the nucleus. After
the separation of the chromatin groups, the two nuclei divide
to form four, and division of the cell-body follows. Indica-
tions of mitosis have been described in Amoeba proteus and
A. verrucosa. In view of this conflicting evidence, it cannot
be decided whether amitotic division is of normal occurrence
or not in Amoebae. Some authors maintain that when once
an amitotic division has taken place the two resulting nuclei
are incapable of further division, and on this view mitotic
division would be the rule, amitotic the exception in Amoebae.
But there is no evidence of the truth of the statement, and
the most that can be asserted is that both mitotic and
amitotic division have been observed. In some Rhizopods,
which differ only from Amcebae in the fact that they form
a test 'or shell for themselves, mitotic division appears to be
the rule. (Euglypha, Arcella.}

Occasionally an Amoeba is seen to withdraw its pseudopods,
to become spherical, and surround itself with a firm coat,
and so pass into a resting-stage, or as is generally said, to
encyst itself. After a time the cyst wall is absorbed, and
pseudopodial movement resumed, so that there is no necessary
connection between encystment and reproduction. It has
been stated, however, that encystment is sometimes accom-
panied by the breaking up of the nucleus into small fragments,
which become surrounded with cytoplasm, and are set free
as minute reproductive bodies called spores. It is further
said of one species, Paramceba eilhardi, that the spores emerge
from the cyst as minute actively swimming bodies, each pro-
vided with a nucleus and a pair of long vibratile flagella or
cilia. Eventually the spores are said to lose their flagella
to become amoeboid, and grow into new Amcebae. But the
observations on which these statements are founded are not
entirely satisfactory, and it must remain doubtful for the
present whether true Amcebae ever reproduce themselves by
spore formation. Nor is there any evidence to show that
conjugation — i.e. the fusion of two individuals, or at any rate
the fusion of nuclear matter derived from two separate
individuals — is either a necessary or even an occasional pre-
lude to reproduction in Amcebae.

Amoebae and their allies, distinguished from other Protozoa

136 COMPARATIVE ANATOMY

by the possession of pseudopodia, are classed together as
Rhizopoda. Amoeba is one of the most simple members of
the group, and may be taken as a type of it, but it must not
be supposed that all Rhizopoda have so simple a structure.
Some of the nearest allies of Amoeba, having, like it, blunt
lobose pseudopodia, have the power of forming shells of
characteristic form for the protection of their bodies. Thus
Difflugia, a common fresh-water Rhizopod, protects itself by a
case formed of particles of sand glued together by a secretion
of the protoplasm. Arcella, another common fresh-water
form, has a watch-glass-shaped shell formed of a chitinoid
substance. The concavity of the watch-glass is covered in
by a plate with a central hole through which the pseudopodia
are protruded. Not far removed from these, and differing
from them chiefly in the fact that their pseudopodia are long
and thread-like, frequently branched at their extremities, and
anastomosing so as to form a protoplasmic network outside
the cell-body, are Euglypha, a fresh-water Rhizopod, with an
ovoid shell formed of hexagonal siliceous plates, and Micro-
gromia with a siliceous shell which is not made up of plates,
but is continuous. Closely allied to these, again, are the
Foraminifera, a very large group of rhizopodous Protozoa,
with calcareous shells, many of which are of great beauty and
complexity. The Foraminifera are all marine, and are ex-
tremely abundant. Some of them are pelagic — that is to say,
they float at or near the surface in the open ocean far away
from land; and the bottoms of those oceans whose depth
does not exceed 2000 fathoms are generally covered with
deposits of a grey mud, which is chiefly composed of the
calcareous shells of the countless Foraminifera which have
lived and died in the waters above. At depths greater than
2000 fathoms, the proportion of Foraminifera in deep-sea
deposits becomes less and less, and at great depths the
deposits are largely composed of the siliceous' skeletons of
another class of marine Rhizopods, the Eadiolaria. In the
greater depths the fragile calcareous shells of the Foraminifera
have been dissolved by the action of the carbonic acid dis-
solved in the sea-water; it seems that the solvent action is
greater the greater the pressure. But the flinty skeletons of
the Radiolaria are not dissolved, and hence they take the
place of the Foraminifera at great depths. There are, of

THE RHIZOPODA 137

course, Radiolarian skeletons amongst the calcareous deposits
in moderate depths, but they are scarcely distinguishable
among the greatly preponderant mass of calcareous shells.
The chalk, which in some parts of England is over a
thousand feet in thickness, is almost entirely composed of
the shells of Foraminifera, and in the Barbadoes there are
deposits of considerable thickness formed almost entirely of
the flinty skeletons of Radiolaria.

It is beyond the purpose of this book to enter into details
concerning all the different groups of the Protozoa, however
important they may be from a geological point of view. But
the reader should remember that animalcules whose structure,
except for the presence of a calcareous or flinty skeleton, is
scarcely more complex than that of an Amoeba, have played
and are still playing a very important part in the formation
of the earth's crust.

CHAPTER V

THE SUN ANIMALCULE

ACTINOSPHuEEIUM EICHOENII

IN many fresh-water pools, particularly in the warm months,
a number of glistening white specks, about as large as the
head of a small pin, may be seen floating in the water.
Examined under the microscope each of these specks is found
to be animalcule having the structure represented in fig. 29. It
has a spherical protoplasmic body from which radiate out on
all sides a number of stiff ray-like processes, the pseudopodia
whence the animal derives its name — Actinosphaerium (OLKTIVOS,
a ray ; o-</>cup?7, a sphere). Examined more closely, the spherical
body is seen to be composed of naked protoplasm, so richly
beset with clear spaces or vacuoles that the animal may not
inaptly be compared to a speck of foam. It must be borne
in mind, however, that the vacuoles of Actinosphaerium differ
from foam bubbles in containing not air but fluid. When
viewed by reflected light Actinosphaerium appears brilliantly
white because of the reflection of light from all parts of its
body, and the details of its organisation cannot well be
distinguished, but when examined by transmitted light, especi-
ally if it be subjected to gentle pressure, the details of its
structure can easily be made out. The body is then seen
to consist of a peripheral or cortical lighter and more tran-
sparent layer, sometimes called the ectosarc, and a central
darker medullary portion, sometimes called the endosarc.
The difference is really due to the size of the vacuoles, which
are large and bounded by very thin envelopes of granular
protoplasm in the ectosarc, but smaller, with much thicker
protoplasmic walls, in the endosarc. The protoplasm of the
endosarc further shows a difference in that it is more granular
and therefore less transparent than that of the ectosarc.

138

THE SUN ANIMALCULE 139

In any collection of Actinosphaeria there is a considerable
difference in size between individuals. They vary from o-4
to '04 mm. in diameter. The smaller are the younger forms,
and in them the large vacuoles of the ectosarc form a single
peripheral layer, separated by thin radial walls of protoplasm.

Fig. 29.

Actinosphaerium Eichornii, viewed by reflected light, showing
the radiating pseudopodia, the lighter ectosarc, and the
darker central endosarc containing numerous nuclei. Two
contractile vacuoles are seen on the edge of the ectoplasm.

In the larger and older forms the vacuoles of the ectosarc
are generally two or three layers deep.

Even in the living animal, but more easily in one which
has been killed with osmic acid and stained with picrocarmine,
one can distinguish a number of minute spherical bodies,
deeply stained by picrocarmine, each of which contains a more
darkly stained central spot. The spherical bodies are the
nuclei, the central spots their nucleoli. The nuclei are always

140 COMPARATIVE ANATOMY

numerous, but more numerous in the larger and older
than in the smaller and younger animals. In the former
as many as 150 may be present, and some authors describe
an even larger number. The nuclei always lie in the more
peripheral part of the endosarc, never in its centre nor in
the ectosarc. This constant position of the nuclei is indicative
of a differentiation between the protoplasm of the ectosarc
and endosarc ; a differentiation which is also expressed in the
more granular appearance of the latter. Further, we find
that the processes of digestion are accomplished wholly in
the endosarc. Actinosphasria, like all true animals, ingest solid
food, and in any specimen one may observe foreign bodies,
infusoria, diatoms, algae, etc., lying in the endosarc and
surrounded each by a. food-vacuole. The formation of food-
vacuoles has not been studied with the same care in Actino-
sphserium as in Anroeba, but their form and optical characters
differ from those of the ordinary permanent vacuoles with
which the body is beset, and there can be little doubt that, as
in Amoeba, each food-vacuole is secreted round the solid
particle contained in it. That the contents of the food-
vacuoles exercise a solvent action upon the diatoms, infusoria,
and other bodies taken in as food is shown by the various stages
of disintegration exhibited by the latter. The darker more
granular constitution of the protoplasm of the endosarc is
doubtless due in large part to its being loaded with the
products of digestion. The endosarc, then, is the seat of
the digestive processes, and it may be remarked that the
fact that these processes are here carried on in the same
region as that in which the nuclei are found, confirms the
conclusion supported by many other facts, that the nucleus
plays an important part in the function of assimilation in the
cell.

In order that we may understand the manner in which
Actinosphaerium captures and ingests its food, which, it must
be remarked, consists of active living organisms, we must first
consider the structure and action of its pseudopodia. They
are not, like those of Amoeba, of constantly changing form,
nor are they blunt and lobose, but are semi-permanent
structures radiating outwards in every direction, like fine
needles. Careful examination with a high power of the
microscope shows that every pseudopod is provided with a

THE SUN ANIMALCULE 141

very slender axial filament or rod which runs down its centre,
passes through the ectosarc, and ends, at some considerable
distance from the centre of the body, in the endosarc. These
needle-like supports of the pseudopodia form the skeleton of
the animal. They end peripherally in extremely fine points ;
proximally they are somewhat thicker, and their internal ends
are wedge-shaped. They give the stiff ray-like appearance to
the pseudopods ; but they are not stiff, but elastic and easily
bent, being formed of an organic substance which has been
called elastin. This substance may be absorbed by the
protoplasm and the whole pseudopod withdrawn, and if the
pseudopod is again protruded its axial rod is reformed. The
radiating rods are clothed with a layer of protoplasm, thick-
est at the base of the pseudopod, and tapering to a very
fine thread at its end. The protoplasm does not invest the
skeletal rod as an even coat, but here and there it may show
swellings and prominences which come and goj and it may
easily be seen, on close examination, that the protoplasm of the
pseudopods exhibits the streaming movements which have
already been described as characteristic of the protoplasm of
many cells. Granules may be seen flowing up one side of a
pseudopod and down the other; and a similar streaming of
granules can be seen in the protoplasmic walls of the vacuole,
both of ectosarc and endosarc. If some small and active
infusoria are introduced into the drop of water in which an
Actinosphserium is imprisoned under observation, its mode
of catching and ingesting its prey may be seen. When the
infusorian swims among the pseudopods and touches them
they bend suddenly inwards so as to enclose it in a trap ; the
infusorian struggles for a while, but appears soon to be
paralysed, though it is not clear why it is. As its struggles
become less violent, knots of protoplasm appear on the distal
extremities of the pseudopodia enclosing it, and these travel
centripetally down the axial filaments, drawing the infusorian
with them till it reaches the ectosarc into which it is passed,
and so through the ectosarc into the endosarc where it is
surrounded by a food-vacuole. Food may be ingested at any
part of the body, and it is evident that the stiff radiating
pseudopodia form a trap-like apparatus, enabling the inert
looking Actinosphaerium to capture organisms much more
agile than itself. The active bending movements of the

142 COMPARATIVE ANATOMY

pseudopods are manifestations of the contractility of the
protoplasm, and contractility is further exhibited by the
ectosarc ; this, when the animal is subjected to a weak electric
shock, a sudden jar, or other stimulus, may be seen to
contract, diminishing the size of the vacuoles, which, as
the effects of the stimulus pass away, recover their normal
dimensions.

An Amoeba, as we have seen, possesses a single contractile
vacuole which is credited with excretory functions. Actino-
sphserium has two or more contractile vacuoles, situated
superficially on the surface of the ectosarc. They expand
slowly, swelling till they project like bubbles from the surface
of the body, and then their outer walls collapse inwards,
expelling their fluid contents, though no definite channels for
their escape can be observed. At times two or more
Actinosphseria may be seen to come into contact, and their
pseudopodia fuse together. The process of fusion goes on
until their bodies come into contact and also fuse, so that
a double individual or a colony of several partially-fused
individuals is formed. This phenomenon has been called
plastogamy. Its significance is doubtful; no fusion of the
nuclei has been observed to accompany the fusion of the
cytoplasm of the cell-bodies, nor does the phenomenon appear
to be in any way connected with reproduction, so it cannot be
regarded as a process of conjugation, such as will be more
particularly described in other Protozoa.

Normally Actinosphaerium reproduces itself by binary
division, and a celebrated observer of the life-histories of
Protozoa states that if a number of these animalcules are kept
without food for a time, and are then provided with a super-
abundance of food in the shape of the common ciliated
infusorian Stentor, they multiply rapidly by division. In
binary division the globular body is simply divided into two
halves by a constriction, each half containing a number of
nuclei. But both before and after division the number of
nuclei is constantly increasing by mitotic division. The
mitosis, however, differs from that which has been described
as typical for the cells of multi-cellular animals, and since it
may be regarded as a type of the modified form of mitosis
characteristic of Protozoa, it will be worth while to consider it
in some detail.

THE SUN ANIMALCULE 143

The resting nuclei present some variety of form, but they
may in general be described as spherical bodies from '012 to
•018 mm. in diameter consisting of a nuclear membrane, a
network of achromatic (i.e. not easily stainable) substance,
containing a more fluid substance in its meshes, and a single,
relatively large deeply-staining body, often called the nucleolus.
This so-called nucleolus is described as consisting of two
different substances, namely chromatin and a material which
stains differently to chromatin, forms the true nucleoli of tissue-
cells, and is called plastin. The central body, or "nucleolus,"
of Actinosphoerium varies very much in shape, being sometimes
compact and oval or crescentic, sometimes stellate, variously
branched, or broken up into minute chromatin particles which
are scattered through the nucleus. When a nucleus is about
to divide, conical accumulations of homogeneous cytoplasm
make their appearance at two opposite poles, forming the
so-called plasmatic cones. The nucleus, previously spherical,
becomes flattened between the plasmatic cones, so as to
be shaped like a bi-convex lens (fig. 30, D). Meanwhile the
chromatin of the central body has become distributed along
the threads of the achromatic network in the form of branch-
ing strings of minute granules. At the same time peculiar
structures known as the polar plates make their appearance at
the two poles of the nucleus underlying the bases of the
plasmatic cones. These polar plates are thin curved plates of
a homogeneous, highly-refracting, nuclear substance, and it
should be noticed that they arise inside the nuclear membrane,
and cannot therefore be homologised with the archoplasm nor
yet with the centrosomes of tissue-cells. Similar pole-plates
have been observed in the dividing nuclei of Amoeba bi-
nudeata, in Euglypha, Paramecium and several other Protozoa.
The meshes of the achromatic network now become elongated
and stretched between the two pole-plates to form the achro-
matic spindle (fig. 30, £). When the spindle is established
the chromatin granules, previously distributed along the meshes
of the achromatic network, become aggregated at the equator
of the spindle in the form of numerous little lumps or rods of
irregular shape, each of which may be regarded as a chromo-
some. Each chromosome divides into halves which diverge
from one another to the opposite ends of the spindle, there
to form by fusion with their fellows two continuous plates

1 44

COMPARATIVE ANATOMY

Fig. 30-

A, Encysted Actinosphasrium, the primary cysts dividing to form secondary cysts.
B, a pair of secondary cysts, more highly magnified, showing the cost of
siliceous spicules ; the nuclei are in the resting stage previous to division to
form the polar bodies. C, normal mitosis of a non-encysted Actinosphaerium
showing the plasmatic cones. D, E, F, successive stages of normal mitosis.
G, resting nucleus of a primary cyst, the centrosome dividing preparatory to
mitosis. H, /, successive stages of mitosis in a secondary cyst during the
formation of the polar bodies. (After R. Hertwig.)

THE SUN ANIMALCULE 145

of chromatin underlying the pole-plates (fig. 30, F). The
achromatic spindle, and with it the whole nucleus, have mean-
while become elongated, and finally a constriction divides the
nucleus into two equal halves, each of which contains a pole-
plate, a half of the spindle, and a chromatin plate. From
each half a daughter-nucleus is reconstituted. This description
only applies to the nuclear division of free living Actinosphaeria;
in the processes about to be described three other varieties of
mitosis have been observed, the details of which need not
occupy our attention except in so far as they are of importance
for the understanding of the phenomena under consideration.

At the onset of winter Actinosphserium ceases to multiply
by binary fission, and undergoes a process known as en-
cystment with spore formation. The axial filaments of its
pseudopodia are absorbed ; the pseudopodia themselves are
withdrawn; the vacuoles in the cytoplasm disappear; and a
gelatinous, sticky envelope of relatively considerable thickness
is secreted as a protective coat to the protoplasm. A number
of round or oval platelets, or granules with thickened edges,
make their appearance in the cytoplasm, rendering it turbid
and opaque. These granules seem to serve as reserve material
for the spores which are shortly afterwards formed, and they
may therefore be called yolk-granules. At the same time
a number of minute siliceous spicules are formed in the
cytoplasm, and the majority of the nuclei disintegrate and are
absorbed, only about five per cent, of the original number
remaining. These remaining nuclei increase in size, and
presently the cytoplasm is divided into as many corpuscles as
there are nuclei, each corpuscle surrounding itself with a
special gelatinous capsule into which the siliceous spicules
previously scattered through the cytoplasm are collected.
These corpuscles are known as the primary cysts. The
nucleus of each divides mitotically, and division of the nuclei
is followed by the division of each primary cyst into two
secondary cysts. The mitosis of the nuclei of the primary
cysts differs from that already described in the fact that
centrosomes are formed from the chromatin of the nuclei.
The secondary cysts remain side by side in the pairs in which
they were formed, and the nucleus of each divides twice to
form two polar bodies. The process may be shortly described
as follows : The nucleus divides mitotically, and one of its

146 COMPARATIVE ANATOMY

halves is expelled from the cyst as the first polar body. The
other half increases in size and again divides mitotically to
form the second polar body. Both polar bodies consist only
of nuclear material, and speedily disintegrate and disappear.
After the extrusion of the polar bodies the members of each
pair of secondary cysts fuse together again, nucleus with
nucleus and cytoplasm with cytoplasm, to form as many
spores as there were primary cysts. The spores, however,
differ from the primary cysts in being denser and more opaque,
in having firm spore coats, formed by the union of the loosely
scattered siliceous spicules of the previous stages, and in having
a tough resistant membrane within the siliceous coat. The
spores remain in this condition for weeks together, and then on
the onset of warmer weather the spore coats, and also the
gelatinous cyst wall, are ruptured, and from each spore a young
Actinosphaerium emerges. These young forms are vacuolated
and protrude pseudopodia. Each has several nuclei, formed
by mitotic division of the single spore nucleus either imme-
diately before or during emergence, but it would appear that
before growing any further it divides into as many pieces as
there are nuclei, so that the end result of this long and
complicated process is a brood of young uni-nuclear Actino-
sphaeria, which proceed forthwith to feed and grow, the single
nucleus dividing to form many as growth proceeds.

CHAPTER VI

THE MYCETOZOA (SLIME-FUNGI)
BADHAMIA UTRICULAEIS

IN damp woods the trunks of old and decaying trees,
especially elm -trees, are often seen to be infested with a
greyish yellow parasitic fungus Stereum hirsutum projecting
like a shelf from the surface of the bark. Similar fungi may
be found on fallen trees, damp, decaying timber, old garden
seats or arbours, and other such places. A search under the
bark or on the surface of such timber will generally reveal the
presence of a bright yellow slimy substance spreading over the
surface of the wood or beneath the bark. This yellow gelatinous
and shapeless mass is the plasmodium of Badhamia, and the
Stereum is its food. If a mass of the bright yellow slime be
carefully removed, whilst still attached to the wood or to a
piece of Stereum, and kept in a dry place, it loses its slimy
character, dries up, and forms a thin yellow incrustation which
can be chipped off in flakes resembling so much yellow sealing
wax. This dry condition of the Mycetozoon is known as the
sclerotium. The protoplasm is not dead, but has simply
passed into a resting or encysted condition, enabling it to
withstand prolonged drought. Pieces of the sclerotium of
Badhamia which have been kept for as long as three years
may be revived by being brought into a moist chamber.
Examination shows that the sclerotium is formed of a number
of cysts or chambers, separated from one another by firm
partition walls, each cyst containing some ten or twenty nuclei.
If a piece of sclerotium be soaked in water for about twelve
hours and then placed on a moist surface (wet blotting paper
placed under a bell jar serves very well for the purpose) it will
gradually revive and pass again into the plasmodium stage.
The opaque horny walls of the cysts become soft and semi-

148 COMPARATIVE ANATOMY

transparent, the contours of the fragment become rounded and
irregularly lobed and puckered, and presently it reverts to the
slime-like condition, and begins to move, but so slowly that the
motion is imperceptible to the eye. In fact, it awakes once
more into life. A plasmodium may be of considerable size,
covering as much as six square inches, or it may be small, of
the size of a sixpence or threepenny bit, but, whether small or
large, it shows the same structure. It spreads out over the
surface on which it crawls in the form of a network of
protoplasm, or rather of a thin sheet of protoplasm traversed
by numerous thicker veins of various dimensions which
anastomose with one another, and give the whole a reticulate
appearance. In some places the veins are not united by the
thin sheet of protoplasm, so that the meshes are empty and a
true reticulum is formed. The edges of the plasmodium are
usually thickened and puckered in an irregular manner, and
occasionally processes of protoplasm, pseudopodia, may be
seen to be protruded from them.

If a small plasmodium is examined under the microscope,
for which purpose it can easily be persuaded to crawl over a
glass slip, it is seen to consist of protoplasm, the outermost
layer of which is clear and transparent, and may be called the
ectosarc. The inner mass is opaque and granular, the veins
being especially rich in granules. Amongst the granules may
be seen particles of inorganic matter, carbonate of lime, and
also a large number of round nuclei, each of which has a
nucleolus and an achromatic network to which minute
chromatin grains are attached. The plasmodium of Badhamia
—or any other Mycetozoon — affords an admirable object for
studying the characteristic streaming movements of proto-
plasm. When watched under the microscope the granules of
the endosarc are seen to be in a constant state of movement,
streaming first in one direction, then in another. The move-
ment is most active in the veins, and is perfectly rhythmical ;
the granules stream steadily in one direction for one or two
minutes, then slow down and come to rest, and immediately
the flow is reversed and they stream back in the direction
whence they came.

The plasmodium has been described as crawling slowly over
the surface on which it lives. This creeping movement appears
to be associated with the internal flow of protoplasm, for the

BADHAMIA

149

Fig. 31-

a portion of a plasmodium of Badhaniia utricularis, showing the frilled
edge and the reticulated protoplasmic mass. B, a small portion of the
plasmodium highly magnified, showing the vacuolated protoplasm and the
numerous nuclei. C, spore fruits of Badhamia Magnet ; sp, spore cases ;
cap, capillitium. (C, after Lister.)

150 COMPARATIVE ANATOMY

streaming movements last longest in the direction in which
locomotion is taking place. It is easy to observe that this
slimy sheet of protoplasm, the plasmodium, is irritable, selective
and assimilative. If the surface on which it crawls becomes
too dry it will move off to a wetter portion of it. If exposed
to a bright light it will move off to any shaded corner which
is available ; unless, indeed, it is about to form spores, when it
will move out of the shade and seek the light; and under
the same circumstances it will leave a damp for a drier situa-
tion. Still more characteristic are its movements in search of
food. If a plasmodium is creeping along one edge of some
moist surface — a sheet of wet blotting paper or a piece of wood —
and a piece of its food fungus, Stereum, be placed at the opposite
edge, the whole plasmodium will move off towards it, flow over
it, and engulph it. Small pieces of the fungus or individual
hyphse are soon seen to be ingested, and each ingested piece
is enclosed in a food-vacuole which is probably secreted round
it, just as in Amoeba. The protoplasmic contents of the
hyphse are digested and assimilated, the indigestible remains
being expelled much in the same way as in Amoeba. These
movements in search of moisture, shade, or food are so slow as
to be imperceptible ; but if a plasmodium, with a piece of its
food-fungus placed a few inches away from it, be left for a
while, it will be found, after a few hours, to have advanced up
to its prey, and to be surrounding it. A plasmodium may be
killed in osmic acid, or any other of the ordinary killing and fix-
ing reagents, and stained for examination of the nuclei. These
are very numerous, and they continue to multiply, as long as
the plasmodium is active and well fed, by mitotic division. In
mitosis a spindle of achromatic fibres is formed, the chromatin
is gathered at the equator of the spindle in the form of a
number of chromosomes, which divide, and their products pass
to opposite poles of the spindle in the usual manner. There
is some evidence that in an actively-moving arid streaming
plasmodium the nuclei also multiply amitotically, but actual
division of the nucleus without mitosis has not been seen.
The reproduction of Badhamia is so similar to that of some
of the lowest forms of plants, that it and its allies are described
in botanical works as plants. When reproduction is about to
take place, the plasmodium emerges from under the bark or
crevice of wood in which it may be hidden, and seeks the

BADHAMIA 151

light. The protoplasm is concentrated and accumulated at
certain points where it may be seen to pulsate in a rhythmic
manner. The waves of contraction advance and recede, but
gradually the advancing movement predominates, and little
prominences are formed, the basal part of each contracting to
form a stalk consisting of tube of a tough hyaline substance
through which the protoplasm continues to flow till all the
contents of the neighbouring parts of the plasmodium become
aggregated on its summit in the form of a spherical mass.
The outer layer of this mass hardens and thickens to form a
wall surrounding the more fluid contents, and part of the
calcareous granules which were scattered through the plas-
modium are incorporated into the substance of the wall. The
fluid contents become differentiated into two structures. First
a part of the protoplasm gives rise to a branching system of
flattened threads, spreading like a network from the base of
the chamber to its wall. The threads are expanded where
they unite together, and contain a number of lime granules
evenly distributed through their substance. The whole system
of branching threads is called the capillitium, and it is not
until it is established that the residual protoplasm and the
nuclei contained in it are formed into reproductive bodies or
spores. Previous to spore-formation all the nuclei in each
chamber divide simultaneously, and the protoplasm breaks up
into a number of lobed masses each containing from one to
ten nuclei. Eventually these masses are divided into as many
corpuscles as there are nuclei. Each corpuscle containing a
single nucleus is a spore ; it acquires a cell-wall composed of
a substance resembling the cutin of cuticularised vegetable
cells, and a period of rest ensues. Each globular chamber
borne on its stalk is called a sporangium. The sporangia of
Badhamia utricularis are ovoid or globular, of a grey or
iridescent violet colour, clustered and borne on membranous
straw-coloured branching stalks. The capillitium is a network
of flat bands, with broad thin expansions at the angles,
and lime granules evenly distributed through the strands.
The spores themselves are from 9 to 12 /x in diameter, of a
bright brown colour.

After some time the external wall of the sporangium breaks
down, and the spores are exposed hanging on the threads of
the capillitium. They are gradually scattered and disseminated,

152 COMPARATIVE ANATOMY

and if kept dry retain their vitality for an almost indefinite
time. But if they are wetted the spore coats are ruptured, and
their contents emerge as a number of pellucid globules, which
lie quiescent for a few minutes and then begin to put forth
pseudopodia and exhibit amreboid movements. A few minutes
more and each animalcule becomes pear-shaped, develops a
contractile vacuole, a single flagellum at its narrow end, and
swims off wit^i a dancing movement, the flagellum at the
narrow end being in advance, whilst pseudopodial processes
are given off from the broad posterior end. These active
bodies are known as flagellulae, and they feed actively on
living bacteria caught by the pseudopodia at the hinder end.
The flagellulas, if well fed, multiply rapidly by binary division,
but after a time they become sluggish in movement,- withdraw
their flagella, and creep about by their pseudopodia. In this
condition they are known as amcebulse. When two amcebulae
come into contact they coalesce, the protoplasm of the two
running together, whilst the nuclei remain separate. Other
amcebulae are attracted to the spot, often in great numbers, and
after a time the cytoplasm of all becomes confluent, and a new
plasmodium is formed. It should be noticed that the union
of the amcebulae to form a plasmodium is not a case of
conjugation; there is no union of nuclei but only of cyto-
plasm, whereas conjugation consists essentially in the union
of nuclear matter from two different cells. Such a mass as
the plasmodium of Badhamia, formed by the secondary fusion
of a number of cells originally separate, is often called a
syncytium, and it differs in its mode of origin, if not in its
ultimate constitution, from such a body as Actinosphaerium,
whose many nuclei are the result of continued division of the
originally single nucleus without corresponding division of the
cytoplasm. But in Badhamia also, as soon as the plasmodium
is formed the nuclei begin to multiply rapidly, so that the
distinction between syncytium and ccenocytium breaks down.
Badhamia is a member of a large class of organisms which
live on decaying organic substances. It alone, in the plas-
modial condition, feeds upon living fungi, though the flagellulae
of a large number of species have been observed to feed on
living bacteria. Now, the ingestion of solid organic substances
and their digestion within the body are essentially animal
characteristics, and seem to justify the claim of zoologists to

BADHAMIA 153

the possession of Badhamia. But many of its allies, such as
the common Fuligo septica, or " flowers of tan," found in old
tan-pits, feed on decaying vegetable matter only, and in this
respect rather resemble the Fungi in their mode of nutrition.
The whole group of the Mycetozoa is further held to resemble
the lower plants, because of the manner in which the sporangia
are formed, and because the spores themselves have coats of
cutin and sometimes of cellulose, two characteristically vegetable
products, though cellulose is not unknown in the animal
kingdom. But a discussion as to whether they are animal or
vegetable would be fruitless. Organic nature does not lend
itself to sharp distinctions and the Mycetozoa, of which our
Badhamia is taken as an example, afford one of the best in-
stances of the convergence of the two great types of organic
structure, plants and animals, so widely different in their
higher forms.

CHAPTER VII
MONOCYSTIS AGILIS AND MONOCYSTIS MAGNA

WHILST most of the Protozoa are free-living, and obtain solid
food by various devices, the Gregarines or Sporozoa, to
which order Monocystis magna and Monocystis agilis belong,
are endoparasites, spending their existence within the body
of another animal known as the host, and deriving their
nutriment from its tissues or secretions. Both Monocystis
magna and agilis are found in the sperm-sacs of the
earthworm, and may nearly always be found if the contents
of the sperm-sacs are teased up, with the addition of some
salt solution, on a glass slide. Monocystis agilis is the
smaller but more common species, and the following descrip-
tion will apply chiefly to it : —

The free mature individual is a single elongate ovoid cell,
consisting of a granular, opaque central or medullary protoplasm,
containing a single spherical nucleus and an external hyaline
denser layer, usually called the cortex. This cortex, which
must not be confounded with the so-called ectoplasm of
Amceba or Badhamia, is a specially differentiated contractile
external layer of the protoplasm, and its consistence, admit-
ting of no pseudopodial movement, confers a more or less
definite shape on the body ; a shape, however, which alters
somewhat as a result of the contractility of the cortex. This
contractility is manifested as a constriction appearing at one
end of the body, and travelling slowly to the other, giving
rise to frequent changes of form. These sluggish move-
ments are termed euglenoid, because they are highly char-
acteristic of the green animalcule Euglena, which will be
described in the next chapter. The Gregarines are probably
degenerate in consequence of their parasitic habit, and
accordingly we find that Monocystis has no pseudopodia,
no flagella or cilia, and no contractile vacuole. After moving

MONOCYSTIS

155

about for a short time in the worm's sperm-sacs, the free
and mature Monocystis conjugates and encysts. Two
individuals come together, and are attached to one another
by their broader ends. In this condition each individual

Fig. 32.

, a free specimen of Monocystis agilis\ nu, nucleus. 6", sperm-
mother cells adhering to the parasite. ii, a free monocystis
covered with ripe spermatozoa, and thus appearing as if covered
with a coat of long cilia. C, a ripe cyst of Monocystis agilis
containing numerous pseudonavicellae/ra. (A and £, original ;
C, after Lankester.)

is termed a zygote (Greek £uy>), a yoke, because the two
are yoked or joined together). The zygotes contract into
spherical form, and each contributes to the formation of a
common transparent protective case or cyst-wall, composed
of chitin-like substance. At first the contours of the cyst

156 COMPARATIVE ANATOMY

wall betray its double origin, but it soon becomes spherical,
and forms a single chamber, containing the two closely-apposed
zygotes. These are separated by a sharp line of division
at the surface of contact, and, whilst the cyst-wall is being
formed, each forms within its substance a number of minute
highly-refracting granules, which give a blue colour when
treated with sulphuric acid and iodine, and are therefore
probably composed of a starchy substance akin to cellulose.
It is supposed that these granules serve as a reserve of food
material during the changes about to be described. When
the cyst-wall is established, the nucleus of each zygote travels
to the side farthest from the partition wall, and there
elongates to form a mitotic spindle. The chromatin is
gathered at the equator of the spindles as a number of very
minute chromosomes, mitotic division takes place, and one
half of the divided nucleus is extruded from the cell as a
polar body. The other half, reduced to nearly half the size
of the original nucleus, travels back towards the partition
separating the two zygotes, and lies close against it. The
middle part of the partition is absorbed, and the nuclei of
the two zygotes meet in the hole thus formed, and fuse
together. The next stage has not been seen, but it seems
certain that the combination nucleus, formed by the union
of the nuclei of the two zygotes, divides, and each product
of division passes back to the centre of the body of a
zygote. Here it divides repeatedly by mitosis, and thus
a number of nuclei are produced, peripherally placed,
and each surrounded by an aggregation of denser proto-
plasm. Eventually the nuclei, with the denser protoplasm
around them, become separated off as corpuscles, or sporo-
blasts, and the remainder of the protoplasm of each
zygote disintegrates and disappears. Probably it goes to
form reserve material. Each of the sporoblasts now sur-
rounds itself with a lemon-shaped coat,* and becomes a
spore of the kind commonly known as a pseudonavicella
(fig. 32), from its supposed resemblance to a boat or canoe.
The nucleus of each spore now undergoes three successive
nuclear divisions, giving rise to eight minute curved bodies
arranged round a central core of residual protoplasm, much

* The chemical composition of this coat is not known. It is not
siliceous as was once supposed, nor does it appear to be chitinous.

MONOCYSTIS

157

as the divisions of an orange are arranged round the central
fibrous axis. These minute bodies, called the sporozoites,
or sometimes the falciform (i.e. sickle-shaped) young, are

Fig. 33-

A , Conjugation of two zygotes of Monocystis agilis showing, the
nuclei and the partition wall between the zygotes. B,
formation of the polar body, visible in one of the zygotes
only. C, approximation of the nuclei of the two zygotes
after formation of the polar bodies. D, two zygotes
showing division of the nuclei derived from the combination
nucleus. £, Two zygotes surrounded by a capsule, f, a
pseudonavicella containing eight sporozoites. (After
Wolters.)

liberated, and enter the sperm-mother cells of the same, or
another, earthworm ; but their actual history at this stage
has not been traced. It is probable that, after rupture of the
cyst-wall, the sporozoites escape by bursting out of the

158 COMPARATIVE ANATOMY

pseudonavicella ; that some of them make their way into the
sperm-mother cells of their host ; whilst others are passed
to the exterior by the seminal ducts, and remain alive for a
while in damp earth. If they are then swallowed by another
earthworm, they pass through the walls of its gut into the
sperm-sacs, and find their way into the sperm-mother cells
of their new host.

The spermatozoa of the earthworm are formed by repeated
divisions of a sperm-mother cell ; but, instead of the whole of
the protoplasm going to form spermatozoa, a central cylinder
of residual protoplasm, the sporophore, is left, to which
the heads of the spermatozoa are attached. It is in this
sporophore that the sporozoite lives, and, absorbing its con-
tents, grows up into a mature Monocystis. The development
of the spermatozoa proceeds, in spite of the presence of the
parasite, and, eventually, the latter appears to be clothed with
a coat of long cilia, which are, in fact, the tails of the sper-
matozoa, now withering up for want of nutriment. Finally
the Monocystis, after absorbing all the available nourishment,
quits its cell-host, and is set free in the sperm-sac in a state
ready for conjugation.

The chief interest of the Gregarine Monocystis lies in its
adaptation to a parasitic mode of existence. Relying entirely
for its nutriment upon the diffusible substances in the cell
(sporophore), in which it spends the greater part of its exist-
ence, its organisation is of the simplest possible character.
The absence of an aperture for ingestion of food, of pseudo-
podia, cilia, and contractile vacuole, point to a degradation of
organisation in connection with the parasitic habit. Still more
striking is the provision for perpetuation of the species by
means of an inordinately large number of young forms. A
large number of spores are formed in the first instance from
a pair of conjugating individuals, and, as each spore gives
rise to eight young forms, or sporozoites, the number of the
latter must be enormous. Though it is a matter of chance
whether any one of them will find its way to a new host, or to
a new sperm-mother cell in the same host, the fecundity of the
zygote ensures that some few at least shall find their way to a
new cell-host, and survive. The sporozoites themselves repre-
sent the active phase of existence. Each is an actively mobile
animalcule, with a broader blunt extremity containing the

MONOCYSTIS 159

nucleus, the other extremity being produced into a highly-
contractile tapering process, the lashing movements of which
propel the organism. Once it finds its way into a sperm-
mother cell, the sporozoite enters upon an indolent phase,
grows in size, stores up nutriment, and only emerges when
ready for sporulation, the final preparation for this process
consisting in the extension of a part of its nuclear substance,
as a so-called polar body. The act of conjugation consists in
the commingling of the nuclear material of two perfectly similar
zygotes. One cannot distinguish an active from a passive
form, a male from a female ; and, after the commingling has
taken place, each zygote recovers a half of the combination
nucleus, and gives rise to spores. We shall see that far more
complicated processes take place in other Protozoa.

CHAPTER VIII

THE FLAGELLATA— EUGLENA VIRIDIS, BODO
SALTANS, AND POLYTOMA UVELLA

IT was known to the earliest microscopists that water contain-
ing organic matter in solution soon becomes tenanted with
swarms of microscopic animalcules, and that of these different
kinds succeed one another in a more or less regular order.
The first stage is that of putrescence, a phenomenon due to
the development of a vast number of Bacteria, which after a
time become quiescent, form spores, and seem to disappear.
They are succeeded by swarms of other animalcules of larger
size, usually characterised by the possession of one or more
relatively long, transparent, and very fine processes of proto-
plasm, which by their lashing movements propel the organisms
through the water. These protoplasmic "processes have been
aptly likened to whip-lashes, and hence the organisms possess-
ing them, have been called the Mastigophora, or whip-bearers,
whilst a subdivision of the group is known by the name of the
Flagellata. One of the commonest as also one of the largest
of these forms is an animalcule of a deep green colour, which
is frequently found in such abundance in puddles of rain-water
impregnated with decaying vegetable matter that the water
appears of a uniform dark green tint, easily distinguished from
the green colour imparted to ponds by the presence of fila-
mentous algae, since the latter are visible to the naked eye
when a glass vessel containing them is held up to the light.
Our animalcule, indistinguishable by the naked eye, is known
as Euglena. There are many species of Euglena, differing in
the shape and proportions of their bodies, but agreeing in all
essential particulars with the species called Euglena viridis,
which we will take as a type. An individual of this species
has the form depicted in fig. 34. Its body consists of a single
elongated cell, tapering to a point at one extremity. The

160

A , Euglena. viridis ; m, the so-called mouth ; «, the nucleus ; e, the
stigma ; r, reservoir ; c.v., contractile vacuoles ; chr, chromatophors ;
am, pyrenoids with sheaths of pararriylum. B, another specimen,
showing change of shape and diagonal striation of the cuticle.
C and D, outlines to show various stages of contraction. E, a free
swimming specimen undergoing longitudinal division. F and G,
division of an encysted form. (A—D, original ; E — G, after Stein.)

1 62 COMPARATIVE ANATOMY

opposite extremity is blunt and obliquely truncated, and, as it
is always directed forward in swimming, it may be called the
anterior end. From it projects a single thread-like transparent
process of protoplasm in which no definite structure can be
made out even with the highest powers of the microscope.
This process is the flagellum, and although it presents no
obvious structure, it is highly contractile, and when the animal
is in active movement executes a series of rapid bending move-
ments, which can best be compared to the motion of the
finger in beckoning. By these movements of the flagellum
the organism is propelled through the water in the direction of
the flagellum, and the latter organ has sometimes been called
a tractellum, because it draws the organism after it. A little
observation shows that the flagellum does not spring from the
surface of the anterior extremity, but emerges from a conical
depression situated somewhat excentrically on the blunted
snout-like end. The flagellum originates from one of the walls
of this depression and clearly is a process of the ectoplasm,
since it cannot be traced deeper than the most external firmer
layer of the protoplasmic body. Though Euglena viridis has
the characteristic shape here described, it is eminently con-
tractile, and therefore capable of considerable change of shape.
An individual, if watched for a short time, may be seen to go
through the various phases of contraction shown in fig. 34.
Now it is swelled up at the hinder, now at the anterior ex-
tremity, and now it is swelled up in the middle so as to
resemble a top. These contractions are combined with
sinuous worm-like movements, the whole being so character-
istic that the name "euglenoid" is applied to them, a word
which we have already used in describing the movements of
Monocystis. Just as in the last-named genus we saw that the
contractile power resided in a specialised external layer, the
ectoplasm, so we find that the cell body of Euglena is com-
posed of an external layer of firmer and denser ectoplasm,
whilst the central more fluid portion of the cell-body is known
as the endoplasm. In addition to the ectoplasm, Euglena is
clothed by a delicate external cuticle, which is continued at
the anterior end into the conical depression from which the
flagellum springs. This cuticle exhibits a number of fine
oblique parallel striations, which are thought by some authors
to be the seat of the contractile movements — to be specially

EUGLENA 163

differentiated fibres of contractile protoplasm. This, however,
can hardly be the case, since the cuticle can easily be isolated
by crushing the organism, and then the striae can be seen to
be thickenings of the cuticle and not differentiations of the
ectoplasm. The cuticle is obviously very elastic, or the
euglenoid movements would be impossible.

The nucleus of Euglena is a spherical vesicular body, con-
taining a darker spot called, but probably incorrectly, the
nucleolus. The nucleus maintains a constant position about
one-third of the whole length of the animal from its hinder
end. Its structure will be described in detail when we come
to consider the reproductive process.

One of the most striking characteristics of Euglena is its
green colour. Viewed with a low power of the microscope,
the cell-body appears uniformly green, with the exception of
the blunt anterior extremity and the flagellum, which are
colourless. But examination with a higher power shows that
the green tint is due to the presence of a number of circular
or oval discs, known as the chromatophors. In many species
of Euglena the chromatophors lie in an even layer imme-
diately under the external surface, and, since they do not
change their position, it is probable that they are embedded
in the ectoplasm. In Euglena viridis^ however, the chromato-
phors radiate outwards from a heap of granules, situated in the
centre of the body. The green colour of the chromatophors
is due to the presence of chlorophyll, each chromatophor con-
sisting of a matrix of protoplasm of firm consistency, through
which the chlorophyll is evenly diffused. Just as green plants
are able, through the agency of their chlorophyll corpuscles,
to decompose the carbonic acid in the air, setting free oxygen
and combining the carbon with water to form starch, so
Euglena is able through the agency of its chromatophors to
decompose carbonic acid, but the product is not starch but an
allied body of the same chemical composition — viz. C6 HIO
O5. This substance, called paramylum, differs from starch
in several respects, particularly in giving no colour with iodine,
whereas starch gives a deep blue colour.

When Euglense are kept in a vessel in bright sunlight,
bubbles are abundantly formed in the water in which they are
contained, and these bubbles, if collected, can be shown to
consist of oxygen. At the same time the heap of granules

164 COMPARATIVE ANATOMY

from which the chromatophors radiate can be seen to have
increased in size. On the other hand, if Euglense are kept in
the dark the granules diminish. These granules consist of
paramylum, and they are arranged round a central corpuscle
of rather dense proteid material, which is considered to be
equivalent to the proteid structures found in many Flagellata,
and known as pyrenoids. True pyrenoids are found in only
a few of the green Euglenoids, and when present they are
always closely associated with the chromatophors. For every
chromatophor there is a single pyrenoid, projecting from its
surface, and consisting of a central proteid mass surrounded
by a sheath of paramylum. In other Flagellates the sheath
consists of true starch. It should be noted with respect to
both chromatophors and pyrenoids, that these minute com-
ponents of cell structure behave in certain respects like cells
themselves. They never arise independently, but always as a
result of the division of pre-existing bodies of the same nature ;
pyrenoid arising from pyrenoid, and chromatophor from
chromatophor. Hence there are authors who would carry
Virchow's aphorism still farther (see p. in), and would say not
only "omnis cellula e cellula," but also "omne granulum e
granule " ; a generalisation true enough for the bodies we
have just been considering, but not yet applicable to all the
kinds of granules found in cells.

From what precedes it is obvious that in its nutrition
Euglena differs from animals in general, and resembles the
vast majority of plants. The question is whether its plant-
like, or, as it is called, holophytic, mode of nutrition is supple-
mented by the true animal mode of ingesting solid food, called
holozoic nutrition ? It has been asserted that the action of the
flagellum causes a vortex in the small conical depression from
which it springs, and that minute particles of solid matter,
swept in by the vortex, are ingested by the Euglena and serve
as food. Hence the opening has been styled a mouth, the
depression a gullet. But, as a matter of fact, there is but one
well authenticated instance of the ingestion of solid food by a
green Flagellate, and that not by a Euglenoid, but by a species
known as Chrysomonas flavicans. On the other hand, it would
seem that in Euglenae the holophytic nutrition is supplemented
by another mode known as saprophytic. It has been observed
that Euglena will live and apparently flourish in complete

EUGLENA 165

darkness for as long as thirty-nine days, and, as chlorophyll is
only active in sunlight, this raises the presumption that the
organism is able to obtain food in some other way. Experi-
ments have shown that if they are kept in carefully filtered
water Euglense will not flourish, even in the sunlight, but that
they will recover on the addition of small quantities of
albuminous matter to the water. Hence it is concluded that
they absorb organic substances in solution by the whole
surface of the body, and it is supposed that during the daylight
they form paramylum, and during the night they absorb
organic food in solution. It must be confessed, however,
that there is no very satisfactory evidence on the subject of
the nutrition of Euglena, except the well-established fact that
it decomposes carbonic acid and forms paramylum.

A noticeable feature in Euglena is the presence of a speck
of bright red colour immediately behind the anterior end of
the body and close to the inner end of the conical depression
called the gullet. This spot is the stigma. It has a some-
what quadrangular outline, lies close against a spherical clear
cavity which will be described in connection with the con-
tractile vacuoles, and consists of a meshwork of plasma, con-
taining a number of minute red granules composed of a
substance called hsematochrome. This red pigment is common
in many flagellates, and is particularly abundant in a red
species known as Euglena sanguinea. It becomes blue black
with iodine, is soluble in ether or alcohol, but is not affected
by ammonia or acetic acid. According to a recent author the
stigma is covered anteriorly by one or more strongly refracting
granules of paramylum, and the whole structure serves as a
rudimentary organ of vision, the paramylum granules forming
the lenses, the stigma the percipient elements. It is at the
best, doubtful whether the stigma is specially sensitive to light,
but it is significant that the haematochrome disappears in
individuals kept in the dark, and that it has the same reactions
as the pigments found in the eyes of many higher animals.

The clear vesicular space against which the stigma appears
to be flattened has often been confused with the contractile
vacuole. It is not, however, contractile, but serves as a
chamber or reservoir into which several minute contractile
vacuoles discharge their contents. These minute contractile
vacuoles have much the same structure as in Amoeba ; there

1 66 COMPARATIVE ANATOMY

may be one only, or several, as in Euglena viridis, and when
there are several they contract in succession. The reservoir
into which they discharge is in connection with the hinder
end of the so-called gullet, and slowly empties its contents into
it. Euglena viridis, if kept sufficiently long under observa-
tion, may be seen to multiply by binary division, the plane of
division always passing through the long axis of the body.
Transverse division has never been observed in any species
of Euglena. The division of the body begins at the anterior,
and passes gradually to the posterior end, so that in the middle
of the process one sees two organisms joined together, and it
would be difficult to say, if one did not follow it to the end,
whether there were two individuals conjugating or one in-
dividual dividing. Division of the cell-body is accompanied
by division of the nucleus, which goes through a peculiar form
of mitosis. It has already been said that the nucleus lies
nearer the posterior end of the body, that it is of oval shape,
and contains in its centre a body generally described as a
nucleolus ; but, as its behaviour differs from that of an ordinary
nucleolus, it will be best to call it by a special name — viz. the
nucleocentrosome. The nucleocentrosome itself stains in-
tensely with certain aniline dyes, but not with those which
ordinarily stain chromatin, and thus it can easily be differenti-
ated from the true chromosomes, which are rod-like structures
arranged radially around it, and are so small and so numer-
ous that they cannot be counted. A nuclear membrane
is present. Preparatory to division the nucleocentrosome
elongates and becomes dumb-bell-shaped, and the chromosomes
are at first disposed with regard to it like the barbs of a feather
to its axis. Eventually, as the nucleocentrosome continues to
elongate, the chromosomes come to lie parallel with it, and
when it assumes the shape shown in fig. 35, C, they are collected
in a ring round the middle of the fine thread which connects
its now swollen ends. At this stage the chromosomes split
longitudinally, and eventually their halves separate and travel
to the opposite ends of the nucleocentrosome. The thread
connecting the two ends of the latter then breaks, the whole
nucleus divides, and the chromosomes take up a resting position
around the two new nucleocentrosomes. The whole process
of mitosis goes on inside the nuclear membrane, as is usual in
Protozoa, and the nucleocentrosome appears to take the

EUGLENA 167

place, or, at any rate, to fulfil the functions, of the centrosomes
in normal mitoses. Previous to division, the flagellum of

C.

C

Fig. 35-

Mitosis in Euglena. A, elongation of the nucleocentrosome, radial arrangement of the
chromosomes. B, further elongation of nucleocentrosome ; pennate arrangement
of chromosomes. C, chromosomes arranged in a ring round the dumb-bell-shaped
nucleocentrosome. D and E, the chromosomes split and their halves travel in
opposite directions, f, final stage of mitosis. G, reconstitution of the daughter
nuclei. (After Keuten.)

Euglena is doubled ; not, it would appear, as the result of the
splitting of the original flagellum, but by growth of a new one.

1 68 COMPARATIVE ANATOMY

It is not known whether there is any limit to the power of
multiplication by longitudinal binary division in Euglenae, but
probably there is, for at times the individuals come to rest,
absorb or cast off their flagella, and each becomes rounded
and surrounds itself with a gelatinous cyst wall of some
thickness. Often great numbers of Euglenge may be observed
in this encysted condition, and, the cysts of adjacent individuals
fusing together, they form a scum on the surface of the water,
or at the bottom around stones and other objects. After a
period of quiescence the encysted organism divides longitudin-
ally, the nucleus passing through the mitotic changes already
described. Usually there is only one division, and its products,
after developing flagella, escape from the cyst and swim off as
young Euglenae. But it not infrequently happens that after
the first division each of the daughter products secretes a new
cyst for itself within the parent cyst, and rests awhile. Each
may then divide, so that four cells are formed within the parent
cyst, and these four may develop each one a cyst, and, after a
further period of rest, divide again, forming eight. Usually
the process does not go further, but the parent cyst and the
daughter cysts burst, and the eight young Euglenae are
liberated. Occasionally, however, the parent cell has been
observed to divide repeatedly without the formation of special
daughter cysts, as many as thirty-two young Euglenae being
formed and eventually set free. A recent observer has
described the formation of numerous flagellated young by the
repeated division of an encysted individual, an observation
which confirms the description given above of division into
thirty-two.

Encystment, though commonly, is not necessarily followed
by division. Euglena viridis not infrequently throws off its
flagellum, surrounds itself with a gelatinous cyst wall, and,
after a period of rest, develops a new flagellum, and emerges to
resume an active existence.

There is no well-authenticated case of conjugation in
Euglenoids.

Our next type of the Flagellata, Bodo saltans (sometimes
misnamed Heteromita rostrata), or the springing Monad, is one
of the organisms commonly found in infusions of animal
matter which have been allowed to stand for some months.

BODO 169

It seems to have a special predilection for long-standing infu-
sions of cod's head. It is a minute organism, only measuring
•008 mm. in its longest diameter, and has the shape repre-
sented in fig. 36, A. The body is ovoid, somewhat compressed,
and drawn at one extremity into a point. This pointed
extremity is always directed forward in swimming. Immedi-
ately beneath it, rather to one side of the middle line, is a pit-
like depression continued along the flattened side of the body
into an oblique furrow. Instead of having a single flagellum
like Euglena, Bodo has two, both arising from the bottom of
the depression at the anterior end of the body. One of them,
always the shorter, is directed forwards, and is the active agent
in the free swimming movements of the animal. The second
flagellum, some two or three times longer than its fellow, is
known as the trailing flagellum, because during active
swimming movements it lies partly in the oblique furrow
which traverses the flattened side of the body and partly trails
out behind like a rope. The flattened side is usually called
the ventral, the opposite side the dorsal, but, as the former is
always directed upwards towards the observer during swimming
movements, the terms are not very well chosen. When
swimming, Bodo saltans does not progress in any definite
direction, but simply zigzags about in an apparently aimless
manner.

The body consists of colourless protoplasm containing a
few granules and foreign substances which have been ingested
as food. Towards the middle of the body there is a vesicular
nucleus, and at the anterior end, close beneath the insertions
of the flagella, there is a contractile vacuole. A minute open-
ing, having the appearance of a vacuole, lying between the
bases of the flagella, is the mouth. It appears that this opening
is sometimes closed by confluence of its walls, so that it must
be regarded as a temporary structure. From time to time
Bodo saltans ceases to swim about, anchors itself by its trailing
flagellum to some particle floating in the water, and then
executes a series of movements from which it has gained the
name of the springing Monad. The trailing flagellum is
spirally coiled, and then suddenly straightened out, and as
quickly coiled into a spiral again, so that the animalcule bobs
up and down in the water in a peculiar and very charac-
teristic manner. During these movements the anterior

170

COMPARATIVE ANATOMY

Fig. 36.

Bodo saltans. A , an anchored, and B, a free swimming form showing nucleus,
contractile vacuole, and two flagella. C, transverse division of a free
swimming form. D, a free-swimming form coming into contact with an
anchored form. £, conjugation of the individuals in D, with fusion of
the nuclei. F, triangular zygote resulting from conjugation. G, zygote
bursting and emitting spores from three corners. H, young forms
developed from the spores. (Modified from Dallinger and Drysdale.)

BODO 171

flagellum is inactive, and often is wound round the body
in a loose coil.

There are several species of Bodo, of which Bodo saltans
alone exhibits the peculiarity of attaching itself by its ventral
flagellum and executing springing movements. The signific-
ance of this habit is doubtful, but it is supposed, that it is
during its anchored state, that it ingests solid food. Living as
it does in organic infusions impregnated with proteids and
other nutrient substances, it is more than probable that the
nutrition of Bodo saltans is largely a process of simple diffusion
through the whole surface of the body — that is, a saprophytic
mode of nutrition. But whether this be the case or not, it is
certain that the animalcule ingests solid food, for bacteria and
similar foreign particles can be seen in its protoplasm in various
stages of disintegration. As has been said, it is probable that
this solid food is ingested during the anchored state, though
the process has never been actually seen because of the cease-
less rapid movements of the creature. But many of the other
species have been seen to feed in true animal fashion. One
of the most interesting cases is Bodo angustatus, an organism
found in the cells of decaying potatoes. Normally it has two
flagella and the structure of a typical flagellate. But it
frequently passes into an amoeboid condition and emits a
number of long and very fine pseudopodia : it is uncertain
whether the flagella are always withdrawn in this state or not.
In the amoeboid form Bodo angustatus attacks the starch
granules of the potato. Gradually spreading its body over a
granule as large as or larger than itself, it completely envelops
it, and is often reduced to so thin a film of protoplasm as to
be hardly visible. Sometimes a flagellate form may be seen
to engulph a starch granule in the same way, without previously
passing into an amoeboid condition. In Bodo caudatus there
is a distinct mouth in the same position as the temporary
mouth of Bodo saltans, and it is produced into a short passage
or gullet leading into the internal protoplasm. By means of
this mouth Bodo caudatus can not only swallow bacteria and
such like minute nutritious particles, but can also attack other
and larger Protozoa, fastening on to them and sucking out their
substance. It is established, then, that the Bodonime are not
only holozoic but also predaceous and carnivorous, and in this
respect they offer a strong contrast to the holophytic Euglena.

172 COMPARATIVE ANATOMY

The reproduction of Bodo saltans is effected in the manner
usual among Protozoa, by binary fission. The plane of
division is longitudinal, and the process begins at the anterior
end and passes gradually to the posterior. The details of
nuclear division during fission have not been studied. It has
been said that the flagella share in the division, each being
split longitudinally into two, but this is doubtful. It is more
probable that the flagella are doubled immediately before or
during division. Tranverse fission, with division' of the trail-
ing flagellum only, has also been described. In addition to
binary division in the free swimming condition, a process of
encystment with subsequent spore formation has been de-
scribed, preceded by conjugation. It is said that a free swim-
ming form comes into contact with an anchored form and fuses
completely with it, the nuclei fusing as well as the cell-bodies.
The result of this act of conjugation is a zygote, a roughly
triangular body with a pair of flagella at each of two of the
angles. The zygote swims about awhile by its two pairs of
flagella, and then comes to rest, its nucleus having meanwhile
disappeared, and its protoplasm having become clear and
hyaline. After a period of quiescence the zygote bursts at its
three corners, and its contents escape as a mass of particles
so minute that they can hardly be seen with the best powers
of the microscope. These particles are known as spores, and
the observers who describe this method of reproduction allege
that they were able to follow the growth of these spores, and
to see them grow into small oval bodies which acquired first
a ventral flagellum, then a pointed extremity with an anterior
flagellum projecting from it, and finally by continued growth
in size arrived at the condition of an adult Bodo. This is the
only positive account of a process of conjugation followed by
spore formation in Bodo saltans. It has been seen only once,
by a pair of investigators working in concert, and, though their
observations are now many years old, they have never been
verified. Many authorities, indeed, are inclined to doubt their
truth altogether. It must be remembered, however, that the
difficulty of following the life-history of so minute and active an
organism as Bodo saltans is very great, and that one piece of
positive evidence on such a subject is of more importance than
any amount of negative evidence. A similar process of conju-
gation has been described in Bodo caudatus, but it is denied by

POLYTOMA 173

recent and very trustworthy observers. On the other hand, the
same species has been seen to form a spherical cyst with a thin
envelope, inside which the protoplasm became divided into
several young forms, which emerge on rupture of the cyst. In
Bodo angustatus several individuals have been seen to come
together and fuse to form a plasmodium just as the amcebulse
of Badhamia fuse to form the plasmodium of that genus. The
plasmodium then forms a cyst wall and enters into a resting
stage. Any undigested food particles are extruded, and the
contents of the cyst divide into numerous small spores which
are set free in the shape of the parent form. A similar mode of
conjugation has been described in Bodo globosus; but it is
questionable whether the fusion of several individuals has any
importance, for in both species free swimming individuals may
encyst and form spores without any previous conjugation of two
individuals. Seeing that conjugation followed by encystment
and spore formation undoubtedly occurs in closely allied species,
there is good ground for accepting the account given for Bodo
saltans as correct in the main ; and the process as described in
this species is of importance, for not only do we find conjuga-
tion, but the conjugating individuals or gametes are in so far
dissimilar that one is anchored, the other free swimming ; and
we have here the first suggestion of a differentiation of sex, the
stationary form being perhaps comparable to a female the free
swimming form to a male gamete.

Polytoma uvella is a flagellate found often in association
with Bodo in long standing infusions of organic matter. It is
slightly larger than Bodo, and of different shape, being almost
perfectly oval, with a pair of flagella of medium length projecting
from one of its ends. Both flagella are used equally in
swimming, and the organism darts with great rapidity through
the water, pursuing a more or less definite course. Whereas
the protoplasm of Bodo is naked, that of Polytoma is invested
by a delicate but distinct membrane or envelope. Usually the
protoplasmic body completely fills the cavity of the envelope, so
that the latter is difficult to see ; but if the organism is badly
nourished the protoplasm shrinks, and the envelope becomes
obvious. This fact shows that the envelope is not a differentia-
tion of the most external layer of protoplasm but a true
protective covering secreted by but distinct from the protoplasm.

174

COMPARATIVE ANATOMY

The two flagella project through pores in the envelope. The
chemical constitution of the envelope is not known, but it does
not give the reactions of cellulose. There is no mouth, but
two contractile vacuoles may be seen close to the bases of the
flagella. A nucleus is placed near the hind end of the body,
and generally a small stigma or nodule of haematochrome may
be distinguished in the anterior half of the body. Though

Fig- 37-

Polytoma uvella.. A, an individual showing the envelope and con-
tained protoplasmic body, central nucleus, stigma, two flagella, two
contractile vacuoles, and posterior starch granules. B — F, succes-
sive binary divisions within the envelope giving rise to four young
Polytomae.

Polytoma has no chromatophors, is colourless and devoid of
chlorophyll, it has the power, usually possessed only by
organisms which contain chlorophyll, of storing up amylum in
its body. Close observation shows that the posterior half of
every well-nourished individual is filled with small granules,
which, when treated with iodine, give the characteristic reactions
of starch. If a Polytoma is starved by being transferred to a
liquid devoid of nutrient organic matter, the starch granules

POLYTOMA 175

gradually diminish in size, and finally they disappear and the
animal dies. The exact mode of formation of these starch
granules is not understood. They are not ingested after the
manner of the starch grains swallowed by Bodo angustatus, for
Polytoma has no mouth, and has never been seen to ingest solid
particles, nor are any foreign matters excepting these starch
granules to be seen in its protoplasm. Its nutrition must be
described as saprophytic, for it lives immersed in a nutrient
fluid, and imbibes it by the whole surface of its body. It
evidently does not get its carbon from carbon dioxide, for, as
we have just seen, it becomes enfeebled and dies if it is taken
from the nutrient solutions in which it lives, and therefore it
must form starch in some other manner than its chlorophyll-
containing relatives. Now, in the higher animals a form of
amylum, known as gfycogen or animal starch, is found in some
abundance in the liver and in muscle. Numerous experiments
have shown that glycogen is formed, in the dog for instance,
when the animal is kept on an exclusively proteid diet, and it is
therefore proved that starch may be formed by the activity of
living tissue from proteid material. It cannot be doubted that
the protoplasm of Polytoma has this same power of converting
the albuminous substances which it absorbs from organic
solutions into starch. Attention should be paid to the existence
of starch in Polytoma uvella, for it is sometimes incorrectly said
that organisms devoid of chlorophyll never contain starch.
Polytoma reproduces itself by a process of continued division
within the cell envelope, either in the free swimming or in a
resting condition. The act of division may nearly always be
seen in individuals which are kept under continuous observation.
In a free swimming form the nucleus divides mitotically, and a
transverse constriction of the protoplasmic body follows, not
involving the envelope. The two flagella of the parent form
remain attached to the anterior product of division and the
organism continues to swim about actively, consisting now of
two cells lying inside a common membranous envelope, the
more anterior of the two cells alone bearing flagella. The next
division is at right angles to the first, and therefore parallel to
the long axis of the original parent cell. Four cells are thus
produced, still enclosed within the envelope of the original
parent form, and still the two flagella are active and attached to
one only of the four daughter cells. Another division may

176 COMPARATIVE ANATOMY

take place, producing eight daughter-cells, but more usually
when four cells are formed the maternal flagella are withdrawn,
each of the four cells acquire a new pair of flagella, the envelope
in which they are contained is dissolved, and they are set free
as four young Polytomse, each of which acquires a new envelope
on liberation. This process is repeated again and again for a
period of from four to six days, by which time the reproductive
activities seem to be exhausted and to require a stimulus for
their renewal. This stimulus is afforded by the act of conjuga-
tion. The Polytomse come together in pairs, each individual
of a pair constituting a gamete. The gametes fuse, nucleus
with nucleus and cell-body with cell-body, and form a spherical
zygote, which surrounds itself with a thick envelope and passes
into a resting condition. After a time the zygote divides into
two, each of the two again divides, and the four cells thus pro-
duced divide once again, so that eight are formed within the cyst
wall. Each of the eight cells develops a pair of flagella, the
thick cyst wall is dissolved, and the eight young Polytomae
emerge to resume an active existence. They divide, usually
into four, in the free-swimming condition, and the life cycle is
repeated. Occasionally a free swimming Polytoma will throw
off its flagella, secrete a thick envelope, and pass into the resting
stage without conjugation. In this case no division of the
contents of the cyst has been observed.

As has just been remarked, the reproductive power seems
to become exhausted after a period of repeated multiplication
by division. We shall see in the next chapter that this is a
very common phenomenon amongst the Protozoa, yet it cannot
be called a universal one, since we have been unable to record
any exhaustion following upon repeated binary fission, either
in Amoeba or in Euglena. In neither of these forms does it
appear to be necessary that at regularly recurring periods
individuals should conjugate to form zygotes which, after a
period of rest, enter upon a new lease of reproductive life.
Yet the phenomenon of conjugation is so common both in
the lower plants and animals that we are tempted to believe
that it is a very necessary condition of existence and that it
has simply been missed, that we have failed to observe it, in
the forms in which it is recorded. We have seen that it is
of normal occurrence in Monocystis, that the nuclei of the
cysts of Actinosphaerium conjugate in a very peculiar way in

POLYTOMA 177

couples, that there is a fusion of amcebuke to form a plas-
modium in Badhamia (though as the nuclei do not unite in
this case it cannot be called a true act of conjugation), and
that true conjugation has been recorded in Bodo. In all
these forms the conjugating individuals or gametes are either
perfectly similar, or, as in the case in Bodo saltans, differ
from one another only in a temporary change of habit.
But the case is not always thus : in some of the near allies
of Polytoma the gametes are very dissimilar, and we get a
forecast of the differentiation of sex which is so characteristic
both of the higher animals and plants.

Polytoma uvella is colourless and devoid of chlorophyll,
but the majority of its nearest allies are green, having one or
more chromatophors containing chlorophyll. Their nutrition
is holyphytic, and they are so plant-like in structure and
in function that they are always described as plants in
botanical works. Yet the zoologist, recognising their affinities
to Flagellata, whose behaviour is clearly animal, cannot afford
to leave them out of consideration, and hence they are
classed in zoological works under the name Phytomastigoda,
or plant-like Flagellates. Some of them are of so much interest
and importance that they must engage our attention for a
while.

M

CHAPTER IX
THE VOLVOCIN^E

THE three genera, Pandorina, Eudorina, and Volvox, are of

special interest because they form a progressive series of
increasing complexity both in their organisation and in their
mode of reproduction. All three are found in stagnant water.
Whereas in the Protozoa which we have hitherto studied
the products of cell division separate sooner or later from one
another and lead a free and independent existence as separate
cells, or, as is the case in Badhamia, the cells after a short
period of separate existence fuse together and form a plas-
modium in which the cell outlines are no longer distinguishable,
we find in the Volvocinse that a certain number of the cells
remain adherent, and form a cell-colony.

One of the simpler forms of these colonies is illustrated
by Pandorina morum. The unit of organisation is an
ovoid cell, much resembling Polytoma in form, since it is
furnished with a pair of flagella of equal length, a mem-
branous envelope, a contractile vacuole, a stigma composed
of haematochrome, and a vesicular nucleus. It differs,
however, from Polytoma in possessing a large chromato-
phor coloured green with chlorophyll. These units, in-
stead of living separately, are aggregated together to form a
simple colony composed of sixteen, more rarely of thirty-two,
similar individual cells. The colony is spherical or oval (fig.
38); the cells composing it are wedge-shaped, their broader
ends external, their pointed ends meeting together in the centre
of the colony. Each cell has its own membranous envelope,
but in addition there is a colonial envelope of like structure
and composition in which the individual components are
imbedded. The colony swims about by the united action
of the flagella of its members, and its nutrition is holophytic.

Multiplication of the colony is effected in two ways. The

178

PANDORINA

179

first and simpler method is by continued binary division
of all the units composing the colony. Each cell divides into

Fig. 38.

Pandorina. inoruin. A , a colony of sixteen flagellated
cells enclosed in a common colonial envelope. B,
a colony in which the cells have given rise by
repeated binary division to daughter colonies, still
enclo,sed_ in the common colonial envelope ; cv,
contractile vacuole ; st, stigma ; nu, nucleus ; en,
colonial envelope. (After Stein.)

two, the two into four, the four into eight, and the eight into
sixteen. The colonial envelope is gelatinised and finally

i8o COMPARATIVE ANATOMY •

dissolved, as also are the envelopes of the individual mother
cells of the daughter colonies. Thus each daughter group of
sixteen is set free, develops new cell envelopes and a new
colonial envelope, and forms a free-swimming colony.

After repeated multiplication by this means a generation of
Pandorina colonies arises which enters upon a process of
conjugation before reproduction. Preparatory to this process
the sixteen individuals of a colony divide each into eight cells,
the mother colony ceases to swim, falls to the bottom, loses its
flagella, and the colonial as well as the individual membranous
envelopes gelatinise and are dissolved. The dissolution of the
envelopes proceeds slowly, so that it is some time before the
groups of eight cells, formed by division of the sixteen cells
composing the original colony, are set free. But eventually
each member of a group develops flagella and an envelope
and escapes as a single free swimming gamete. Two gametes
meet by their anterior ends and conjugate, fusing completely
with one another to form a zygote, which at first bears the four
flagella of the two united gametes. Presently the flagella are cast
off, the zygote surrounds itself with a thick envelope of red colour
and enters into a resting stage. Before it develops further it
is necessary that the zygote should be dried, which normally
takes place when the pool of rain-water in which it lives
becomes dried up in the summer. When the pool fills again
after rain the zygote develops. Its envelope thins out on one
side and forms a projection from the surface which breaks
through and allows the zygote to escape in the form of a naked
cell, which immediately develops flagella and an envelope,
divides into sixteen adherent cells surrounded by a colonial
envelope, and thus a new Pandorina colony is produced.

It is not certain whether there is any differentiation between
the gametes of Pandorina. It appears that there are larger
and smaller colonies, producing larger and smaller gametes,
and also forms intermediate in size between these. It is stated
that the larger gametes never conjugate with one another, but
that the smaller and middle-sized gametes conjugate with the
larger and also with one another. If this is the case, there is
at the most the beginning of a differentiation into macrogametes
and microgametes, or, as we may say, into female and male
forms, in Pandorina.

EUDORINA

i8i

from

Eudorina elegans is not very different in structure
Pandorina. Its colonies, measuring from 'i to '15 mm. in
diameter, are spherical or oval, composed of thirty-two, rarely
of sixteen cells, which differ from those of Pandorina in being
rounded, imbedded at some distance from one another at
regular intervals in the colonial envelope, and in the fact that
their inner ends do not reach to the centre of the colony
(fig. 39). The colonies multiply by repeated division of their

Fig. 39-

Eudorina elegans. The colony consists of thirty-two
flagellate cells, situated at some distance from one
another, and enclosed in a common colonial envelope.
cv, contractile vacuoles ; ttu, nucleus ; am, amylum
bodies ; en, colonial envelope. (After Stein.)

component individual cells, so that daughter colonies are
formed much in the same way as in Pandorina. The process
of cell-division is somewhat different, but the details need not
be considered here. During the formation of the daughter
colonies the envelope of the mother colony becomes very thin,
and eventually it bursts and liberates the daughter colonies
already provided with their flagella and colonial envelopes.
After multiplying for several generations in this manner a
generation of Eudorina colonies appears, the members of which

1 82 COMPARATIVE ANATOMY

do not differ in structure from ordinary colonies, but show a
marked differentiation in the course of their further history.
Certain of these colonies, which we may speak of as female
colonies, consist of thirty-two cells which we may call macro-
gametes, differing from the normal only in their somewhat
larger size and oval shape. The remaining colonies undergo
a process of cell-division leading to the formation of male cells
or microgametes. Each individual cell of a male colony
divides to form a flat plate composed of sixteen or thirty-two
yellow cells which become rounded, and secrete an external
gelatinous wall round the plate. Gradually these cells become
elongated and spindle-shaped, they develop each a pair of
flagella at one end, and may now be called microgametes.
All the flagella of the cells composing a plate are turned in the
same direction, and eventually the composite plate moves about
by the action of its flagella, bursts out of the parent envelope,
and swims freely in the water. Meanwhile the female colonies
have come to rest (though their flagella remain attached and
execute slow waving movements), and their colonial envelopes
swell up and become gelatinised. When a free swimming plate
of microgametes meets a female colony in this condition it
attaches itself to it, the plate is resolved into its component
microgametes, and these bore their way into the gelatinised cyst-
wall of the female colony. Each microgamete works its way
towards a macrogamete and conjugates with it, the two fusing
to form a zygote which immediately becomes enclosed in a
thick cyst-wall. The further development of the zygote has
not been followed, but it probably does not differ much from
that of the zygotes of Pandorina.

In both Pandorina and Eudorina all the cells composing the
colonies take their share in the reproductive processes, but
in the latter there is a differentiation into male and female
colonies, into microgametes and macrogametes, which, at the
most, is only obscurely indicated in the former species. But
in Volvox we find that a considerable advance has been
made, for the cells composing a colony are differentiated into
those which are nutritive but not reproductive, and those
which are reproductive but not nutritive, or, as it is said, into
somatic and generative cells. Volvox globator is tolerably
common in ponds and ditches. It has the form of a hollow
sphere of considerable size as compared to Pandorina and

VOLVOX 183

Eudorina, measuring from '2 to "j mm. in diameter. The
wall of the sphere consists of a thick membranous envelope
in which numerous cells, each provided with its own cell-
envelope, are imbedded. There may be as many as 12,000

Fig. 40.

Volvox globator. A, A sexually ripe colony, showing microgametes
O ' and macrogametes, O. , in various stages of development.
B, A portion of the edge of the colony highly magnified, showing
three flagellate cells united by protoplasmic threads, and a single
reproductive cell, rp. st, stigma ; cv, contractile vacuole. (After
Kolliker.)

of these cells. They form a single layer, and when viewed
from the surface present the structure shown in fig. 40, A.
.The cell-bodies do not fill their envelopes, but are separated
from them by a relatively considerable space, and every cell

1 84 COMPARATIVE ANATOMY

envelope, pressed up against the envelopes of six adjacent
cells, has a hexagonal outline. The bodies of adjacent cells
are in direct organic continuity by means of fine processes
of protoplasm, six of which radiate from every cell, perforate
the centres of the six side-walls of the envelope, and are
continued into the similar processes radiating from the six
adjacent cells. Every one of these cells has a nucleus, a con-
tractile vacuole, a chromatophor, an amylum body, and a pair
of flagella, which last project from the surface of the colony
and serve, in conjunction with the similar flagella of the other
cells, as the locomotor organs of the colony. These ordinary
or somatic cells multiply by division and thereby increase
the number of units composing the colony, but they are
unable to reproduce the colony. This function is limited to
a small number of cells, distinguished from the somatic cells
by their larger size and the absence of flagella. In Volvox
globator eight of such cells are capable, by repeated binary
division, of giving rise to new colonies, and, since this mode
of reproduction is not preceded by conjugation, the cells in
question are known as parthenogonidia. Without entering
into the details of division, it may be stated shortly that the
cell-body of each parthenogonidium undergoes repeated division
within its special envelope, giving rise to a spherical colony
which at first projects into the central cavity of the mother
colony. Eventually the envelope of the parthenogonidium
gives way, and the young Volvox, which has by this time
developed flagella, falls into the cavity of the mother colony.
Here it remains for a while, but finally escapes by rupture
of the maternal wall. This method of asexual reproduction
is continued for several generations, after which individual
colonies make their appearance which differ from the normal
asexual colonies in that they possess some fifty large non-
flagellate cells which in their structure and size resemble
parthenogonidia. Their fate, however, is different. Some of
them simply increase in size, acquire a deep green colour,
and project into the central cavity of the colony. These
are the macrogametes or ova, and they are incapable of further
development unless they are fertilised.

The microgametes or spermatozoa are formed from large
non-flagellate cells which at first are exactly similar to those
which give rise to the female cells. These cells divide

AFFINITIES OF FLAGELLATA 185

repeatedly, in much the same manner as in Eudorina, to
form a flat plate or bundle of as many as 128 flagellate
spindle-shaped microgametes. In Volvox globator the bundle
now breaks up (the case is somewhat different in the closely
related Volvox minor) and the microgametes pass into the
central cavity of the colony. They swim by means of their
flagella towards a macrogamete, and bore their way into its thick
envelope. A single microgamete having passed through the
envelope, fuses with the macrogamete, and the result is a fertilised
ovum or zygote. The zygote surrounds itself with a double
cyst-wall, not composed of cellulose, the outer wall being red
and provided with spines and projections. The zygote itself
forms a large number of starch granules in its protoplasm. A
long period of winter rest follows, and in the spring the zygote
develops by a repeated process of cell-division into a new
Volvox colony.

The Flagellata are perhaps the most instructive group of
the Protozoa. On the one hand they are connected with the
lowliest forms of life, on the other hand they indicate, through
the Volvocinae, the transition from the uni-cellular to the
multi-cellular condition, from Protozoa to Metazoa. It is
specially important to note how many members of the group
are able to assume an amoeboid stage and to return again to the
flagellate condition. We have seen this to be the case in Bodo
angustatus, but there are many other instances, the best known
being a form called Mastigamoeba, which passes regularly from
the flagellate to the amoeboid, and from the amoeboid back to
the flagellate condition again. When we consider further that
the young of Badhamia and also the young of many Heliozoa
(though not of Actinosphaerium) are flagellate, we recognise that
there is no sharp dividing line between the Rhizopoda and
the Flagellata; the one form is capable of passing into the
other, and there can be little doubt that they stand in the
closest relationship to one another. Again, we see in the
Flagellata organisms which are exclusively holophytic and
undistinguishable from plants, organisms which are holozoic,
and therefore unquestionably animals, and organisms which are
saprophytic, and therefore resemble Fungi. We stand, as it
were, in neutral territory, belonging to neither the animal nor
to the vegetable kingdom, but inhabited by the natives of
both. It is tempting to suppose that the Flagellates represent

186 COMPARATIVE ANATOMY

the group from which both animals and plants have sprung,
but this we can hardly do. Were such a supposition true we
should expect the group to be still giving rise to new forms of
animals and plants ; for if it was capable of evolving higher
forms in the past, why not in the present and the future ? We
have no evidence that the Flagellates are giving, or have
recently given birth to new forms of higher organisation, and
we must regard the living representatives of the group as
possibly representing the early forms of life from which animals
and plants sprung, but themselves so far specialised and
adapted to particular conditions of life as to be now incapable
of evolution outside the limits of their own special organisa-
tion. The Volvocinae certainly suggest to us the successive
steps through which the multi-cellular condition was evolved
from the uni-cellular, and the importance of such a series
cannot be over-estimated. But again, if we would be scientific
we must be cautious, and we must not say that these are the
steps through which Protozoa developed into Metazoa or
Protophyta into multi-cellular plants. There is, in fact, no
evidence that Volvox leads us any farther in the scale of
organisation, and, indeed, there is some reason to suppose that
in the Volvocinae we have a line of evolution which has reached
its acme in Volvox itself. The Flagellata suggest much, but
they prove little, except that there are no distinct boundaries
in living nature. A group which merges on the one hand
into the Rhizopoda, on the other hand passes by insensible
gradations into the multi-cellular condition, a group which
belongs neither to the animal nor to the vegetable kingdom,
but equally to both, brings to our minds the conviction of the
unity of nature. And this idea, of the unity of organic life, is
one of the most important in enabling us to apprehend the
full meaning of the doctrine of Evolution.

CHAPTER X
THE CILIATA— PARAMECIUM AND VORTICELLA

THOUGH the Protozoa which we have studied present many
differences of form and habit, none of them show any great
complexity of structure. The Volvocinae, indeed, seem to form
an exception to this statement, but even in them the unit
of organisation, the flagellate cell, is simple enough. We have
now to consider a group of which the individual members,
whilst retaining the characters of a single cell, attain to a very
considerable degree of complication, and may be spoken of as
being highly differentiated. But this elaboration of structure,
be it clearly understood, is the outcome of specialisation of the
protoplasm composing the cell-unit, and as such is different
from that other mode of elaboration which, as we have seen in
the case of the frog, is the outcome of the aggregation of many
cells differing in kind and interwoven so as to produce tissues
and organs of great complexity. In other words, the cell-body
of the ciliate Infusorian is a microcosm ; parts of it are
differentiated for the performance of the several vital functions,
whereas, in the higher multi-cellular animals, each component
cell of a tissue is specialised for the performance of only one of
those functions. Thus we may say that in the Ciliata the cell
attains its greatest complexity of visible structure.

The species known as Paramecium caudatum and Parame-
cium aurelia are most conveniently selected for the study of
ciliate structure and reproduction. Both are equally common and
may nearly always be found in stagnant pools containing dead
leaves and other decaying organic matter. Usually they are
not very abundant in pools, but large quantities of them may
easily be reared by collecting confervae and water-weeds in
summer weather, placing them in a jar of rain-water covered
over by a glass plate and leaving them to rot. For the first two
187

1 88 COMPARATIVE ANATOMY

or three days the water will swarm with Bacteria, these will give
way to Flagellates, and then the Paramecia contained in the
water will feed greedily on the Flagellates and multiply with
great rapidity.

Paramecium caudatum and P. aurelia are species so similar to
one another that they have often been called by the same name.
They are relatively large, attaining a length of '3 mm., so that
they are visible to the naked eye as whitish specks in the water
when the glass containing them is held up to the light. We
will confine our attention chiefly to P. caudatum, but the
account given here will apply, as far as structure is concerned,
almost equally well to P. aurelia. The latter species differs from
the former principally in having two micronuclei instead of one,
and also its posterior extremity is rounded and blunt, whilst
that of P. caudatum is pointed. A third species Paramecium
bursaria is almost as common as the other two, but it is much
less convenient for study, as its protoplasm is rendered opaque
by the presence of numerous round green corpuscles, which
are minute algae living in the body of the infusorian and sharing
its nourishment. The algae, whilst they find protection in its
body, are also of direct benefit to their host, for they manufac-
ture starch through the agency of their chlorophyll, and from
time to time their starch-laden bodies are swept into the
endoplasm of the infusorian, are swept round in the food
current, and digested. This intimate union of an animal and
plant is paralleled in several other groups of the animal
kingdom, and the phenomenon is technically called Symbiosis.

The first thing to observe in Paramecium caudatum is its
shape. Unlike the Protozoa which we have hitherto studied
it preserves a perfectly definite and constant form as long as
it is alive, never entering into an amoeboid condition, nor
exhibiting euglenoid or pseudopodial movements. It is,
however, flexible and elastic, and may be observed to bend
its body in passing round an obstacle, and to squeeze itself
through an aperture smaller than its own diameter.

The general shape of the animal is that of a spindle
pointed at one extremity and rounded off at the other. (Fig.
41, A.) It always moves with the blunt end foremost, so this
may at once be distinguished as the anterior, the opposite
pointed end as the posterior extremity. Further, it has a

PARAMECIUM 189

definite ventral surface indicated by a groove which begins
as a shallow furrow at the anterior extremity and on the left,
and curves over with a slight spiral twist towards the right,
becoming narrower and deeper as it passes backwards. After
traversing rather more than the anterior half of the body,
the groove ends in the middle line in a relatively large and
deep funnel-shaped depression leading into the soft protoplasm
of which the centre of the body is composed. This depression
is the mouth, or, as some authors prefer to call it, the
cytostome, and the groove leading into it, the peristome. The
anterior half of the body is slightly twisted in connection with
the peristomial groove, somewhat as a leaden rifle-bullet is
twisted after it is fired through a grooved rifle-barrel; and
in consequence of this twist the animal spins round and
round on its long axis as it swims through the water, a
movement which makes it hard for a beginner to realise
its exact shape.* If it is killed Paramecium swells out and
loses its characteristic outlines. A living specimen, if its
movements are not artificially restricted, moves about with
great rapidity across the field of the microscope. It progresses
in a tolerably straight course with a uniform velocity, very
different to the jerky motions of a Flagellate. After going
for some distance in one direction it stops, turns, seems to
hesitate for a moment, and then darts off in a new direction.
These movements are entirely due to ciliary action. In a
specimen which is held fast by pressure, or, better, by being
placed in a thick solution of gum, the cilia can readily be
seen all round the margins of the animal. They are all of
equal size, except, perhaps, those at the posterior extremity,
which are slightly longer than elsewhere, and completely
clothe the external surface of the body. They extend also
into the mouth and line the narrow tube through which the
mouth opens into the endoplasm. Examination with a very
high power of the microscope shows that the cilia are
arranged in a definite order in rows. In the posterior half

* Paramecium may easily be kept still by the use of a freshly-made
solution of gum-arabic. A few specimens should be placed in the smallest
possible drop of water on a glass slide. A droplet of methylene blue or
neutral red may be added to the water, and then a drop of fairly thick
gum. The coverslip is now put on, and the animals are kept nearly still
by the viscous mixture, but are not injured, and will live for some time
and gradually be stained by the dye.

1 90

COMPARATIVE ANATOMY

of the body the rows are nearly longitudinal, but in the anterior
half their course is modified in connection with the spiral
twist of the body, and they curve inwards towards the peris-
tomial groove on the ventral surface. Paramecium and its

Paramecium caudatum. A, a view of the entire animal; m, mouth; an,
anus ; cv, contractile vacuole in diastole ; cv' ', contractile vacuole in
systole ; »ta, macronucleus ; mi, micronucleus. B, the hinder portion of
the same animal more highly magnified to show the mouth leading by a
funnel-shaped gullet into the endoplasm ; a food vacuole is forming at the
extremity of the gullet ; tun, undulating membrane ; an, anus ; tr,
trichocysts. The arrows indicate the direction of the flow of the granules
in the endoplasm. C, a diagrammatic view of a Paramecium from the
ventral surface, showing the mouth and peristomial groove and the striations
of the cuticle. (A and B, original ; C, after Biitschli.)

near allies are placed in a group called Holotricha, characterised
by the uniform covering of fine cilia.

The cell-protoplasm of Paramecium is differentiated into an
external elastic and delicate membrane or cuticle, within this
is a cortical layer (ectoplasm) of relatively dense consistency,
and the centre of the body is occupied by a softer endoplasm.
The mouth, as has already been mentioned, leads by
way of a short ciliated tube into the endoplasm. There
is a large nucleus situated nearly in the middle of the

PARAMECIUM 191

length of the animal, and rather to one side so as to lie in
the cortical layer or ectoplasm. It is called the Macronucleus,
to distinguish it from a smaller ovoid body lying alongside of
it and called the Micronucleus. There are two contractile
vacuoles, one placed at about the end of the first quarter, the
other at the end of the third quarter of the length of the body,
in the ectoplasm on the dorsal side. We may now consider
these structures in detail.

Examination with a very high power of the microscope shows
that the cuticle is mapped out into a number of very minute
hexagonal areas, from the centre of each of which a single cilium
projects. These areas are slight elevations of the cuticle
separated by fine grooves from one another, and they are so
disposed that, under a lower magnification, the grooves appear
as two sets of lines crossing each other obliquely and sweeping
round towards the peristome and mouth in the manner indicated
in fig. 41, C. (The lines are drawn much too large, for clear-
ness' sake.) Since each elevated area bears a single cilium, it
is obvious that the arrangement of the cilia corresponds exactly
with that of the apparent striation, and the figure therefore gives
a clear idea of the manner in which the cilia are distributed over
the body. It will be observed that in the region of the mouth
and peristome they are arranged so as to create a current in
the direction of the mouth.

The cuticle itself is not a fine homogeneous membrane or
pellicle like the similarly-named structure in the Flagellata, but
appears rather to be of the nature of a specialised external
layer of the protoplasm in which, on the alveolar theory of
protoplasmic structure, the minute alveoli are arranged side by
side, so that their outer walls form a definite surface layer.

The cortical layer or ectoplasm underlying the cuticle con-
sists of a dense semi-fluid protoplasm with a fine alveolar
structure. It is relatively of considerable thickness, and contains
the more important structures of the cell-body. The two con-
tractile vacuoles lie in the deeper part of the cortical layer and
have the form and position shown in fig. 41, A. Each contracts
with great regularity at intervals of about 10-20 seconds, the
contractions of the two not being synchronous but rather
alternate. Just before contraction (systole) the vacuole appears
as a large clear space in the ectoplasm ; in systole its walls
come together and its fluid contents are expelled to the exterior,

iQ2 COMPARATIVE ANATOMY

but no visible aperture for their passage can be detected. The
ensuing pause, during which the vacuole is refilled with fluid,
is known as the diastole. Immediately after the systole a
number, varying from 6-10, of fine canals make their appear-
ance in the ectoplasm, radiating from the spot where the vacuole
disappeared by closure as the spokes radiate from the hub of a
wheel. These canals may reach for some considerable distance
into the surrounding cell-body, but those of one vacuole do not
communicate with those of the other. The canals become
swollen with fluid at their inner ends and slowly void their
contents into the vacuole, which reappears, gradually filling with
the fluid poured into it by the different canals till it reaches
its largest dimensions, and then it suddenly contracts again.
Towards the close of the diastole the different canals, having
emptied their contents into the vacuole, become altogether or
nearly indistinguishable.

Immediately beneath the cuticle is a layer of spindle-shaped
rods lying in the most external layer of the ectoplasm. These
rods are known as the Trichocysts. They form a tolerably
uniform layer immediately below the whole surface of the cell-
body, and are disposed with their long axes at right angles to
the surface. Each trichocyst is about 4 //. (/x = TWO" millimetre)
in length, is somewhat darker than the surrounding protoplasm,
and betrays no sign of structure, appearing as a simple spindle-
shaped homogeneous body, even under the highest powers of
the microscope. But under certain circumstances they undergo
a remarkable change. If a trace of acetic or osmic acid is added
to the water in which a Paramecium is contained the trichocysts
are suddenly projected to about eight times their former length
in the form of long thread-like filaments. In the genus
Paramecium the trichocysts are promptly shot out on stimulation
by an irritating fluid, but those of some allied Infusoria
can only be made to extend with great difficulty. As a rule the
trichocysts are only discharged at or immediately before the
death of the animal, and a specimen killed for preservation is
nearly always seen lying in the midst of a cloud of fine
filaments formed by the exploded trichocysts. In Paramecium
the exploded trichocysts are generally cast right out of
the body on explosion, but sometimes they remain
sticking in it by one extremity, and the animal then looks as if
it were clothed with a coat of very long cilia. There is no

PARAMECIUM 193

relation, however, between cilia and trichocysts, and after the
the latter have been exploded, the much shorter and finer cilia
can be seen between the inner ends of the trichocyst threads.
Nothing is known of the mechanism by which the trichocysts
are extended, and very little is known about their function.
They are believed to be organs of offence and defence whereby
a Paramecium seizes and paralyses its prey or defends itself
against enemies. But it must be confessed that hours of patient
watching fail to demonstrate that a living and actively feeding
Paramecium uses its supposed weapons for capturing prey,, nor
does it turn them against the many living organisms which may
run up against it. The trichocysts, however, are so like the
thread-cells or nematocysts which are the undoubtedly offensive
and defensive weapons of ccelenterates and of some flat-worms
and mollusca, that it is difficult to avoid the conclusion that
they have a similar function.

Before entering into details about the macronucleus and
micronucleus it will be well to consider the other organs. The
endoplasm, occupying the centre of the body, forms by far the
larger part of its bulk. There is really no sharp distinction
between endoplasm and ectoplasm or cortical layer. Both
have the alveolar structure characteristic of undifferentiated
protoplasm, and the two pass gradually into one another, the
passage being sometimes more gradual, sometimes more distinct.
As a rule the endoplasm may be distinguished by the rotatory
streaming movement or cyclosis of the granules and food
particles which it contains. These may be seen to move
steadily round a course which begins just above and behind the
so-called gullet near the posterior end; passes forward along the
dorsal side, and turns downward at the anterior extremity to
return again towards the mouth on the ventral surface. This
course is indicated by the arrows in fig. 41, B.

Whilst the ectoplasm only contains structural constituents
of the cell-body,* such as the trichocysts and the macro- and
micronuclei, the substance of the endoplasm is loaded with a
number of granules and particles, the products of assimilation

* An exception, however, must be made in the case of Paramecium
bursaria, in which the green symbiotic algae, though they are not, strictly
speaking, structural constituents of the cell-body, are situated in the
ectoplasm, and, when they pass into the endoplasm, are digested and serve
as food.

194 COMPARATIVE ANATOMY

and metabolism. The most obvious foreign substances in the
endoplasm are the food-vacuoles and little balls of foreign
substance taken in as food. The food-vacuoles, which, of
course, must not be confounded with the contractile vacuoles,
are clear spherical spaces looking like bubbles in the protoplasm,
and filled sometimes with fluid only, sometimes with solid food
particles surrounded by fluid. The structure and behaviour
of the food-vacuoles of Paramecium are the same as those of
Amoeba (p. 131). Besides the food-pellets there are numerous
minute crystalline particles which are probably excretory
products. It has also been shown that the endoplasm of
Paramecium contains diffused animal starch or glycogen,
giving the characteristic port-wine red colour on treatment
with iodine. Since the contents of the food-vacuoles and the
food-pellets are in various stages of disintegration, and since
glycogen is shown, in higher animals, to be a reserve material
formed by the activity of protoplasm from the substances taken
in as food, we can have no doubt that a portion at any rate of
the food of Paramecium is converted into glycogen in the
endoplasm, is further built up as required into the substance
of the body, and that the waste materials resulting from the
breaking down of the protoplasm are excreted partly by the
contractile vacuoles, partly in the form of the crystalline
particles found in the endoplasm. The endoplasm is clearly
the seat of the digestive and to a great extent of the anabolic
activities of the animal. Surrounded as it by the firm
ectoplasm and the external cuticle, solid food can only be
introduced into it by means of the mouth. Paramecium feeds
chiefly on minute infusoria and flagellata which are swept into
the mouth by the cilia lining the peristomial groove and the
entrance to the mouth itself, and it has already been shown
that the cilia in these regions are so disposed as to direct the
currents towards the mouth. A drawing of the mouth as seen
under a magnification of 1000 diameters is given in fig. 41, B.
It commences as a wide funnel-like cavity at the posterior end
of the peristomial groove. The cuticle is reflected so as to line
the cavity, and the cilia are also continued into it. The mouth
narrows somewhat rapidly to form a narrow ciliated tube which
is directed at first upward, then turns downward with a slight
spiral twist and ends in the endoplasm of the posterior third
of the body. The entrance to the narrow tube is guarded by

PARAMECIUM 195

the so-called undulating membrane. This structure (um. in
fig. 41) has the form of a fine transparent membrane, starting
like a frill near the anterior border of the mouth and passing
along its dorsal wall into the commencement of the narrow
tube or gullet. The free edge of the frill hangs down into the
cavity of the mouth, and its other edge is obliquely inserted in
the dorsal wall of the mouth cavity. The membrane vibrates
rapidly with an undulating movement, and is evidently formed
from a row of fused cilia, for its free edge is often frayed in the
manner shown in the drawing. The inner end of the gullet is
furnished with sparse and rather stout cilia, which move with
a relatively slow undulating movement, recalling the ciliary
action of the so-called flame-cells in the excretory organs of
certain Metozoa. The ingestion of food by Paramecium may
easily be studied by adding finely-powdered carmine or sepia
to the water in which it is contained, or, very frequently, if the
specimen under observation is imprisoned in freshly-made
gum-arabic, it will swallow the gum. Particles may be seen
to be driven into the mouth and down the gullet by the action
of the cilia, their course being apparently guided by the un-
dulating membrane. At the extremity of the gullet a vacuole
is gradually formed in the endoplasm in which the solid par-
ticles are accumulated : the vacuole grows larger and larger
till it reaches the size represented in fig. 41, B. Then there
is a sort of gulp, a contraction of the surrounding protoplasm,
the vacuole is separated from the end of the gullet and passes
with its solid contents into the endoplasm, where it is at once
carried away by the cyclotic current, and a new vacuole forth-
with begins to form in its place.

Just as a special aperture or mouth is required to admit of
the entrance of solid food into the endoplasm, so a special
exit is required for the passage of insoluble remnants of food
and solid excretory substances to the exterior. This is
furnished by the so-called temporary anus, a spot situated a
little way behind the mouth opening on the ventral surface.
The anus is not a permanent aperture, or at least it is not
permanently open, and it can only be detected at the moment
when excreta are being discharged through it. Then it has
the appearance of a minute circular orifice placed upon a
•small papillary projection of the body. But as soon as the
solid matter is voided the aperture and papilla vanish, and no

196 COMPARATIVE ANATOMY

trace of them can be seen. Whether the anal orifice is really
temporary, and its lips fuse together after an evacuation, or
whether it is permanent, and its lips are only pressed together,
is a matter of conjecture.

The macronucleus and micronucleus lie just above the
mouth in the ectoplasm. The macronucleus is a relatively
large structure, measuring from 30-35 /x in its longer, and
15-20 ft in its shorter diameter. It has a definite nuclear
membrane which, in the resting condition, appears to be filled
with minutely granular contents staining deeply with the
ordinary dyes. The micronucleus, in the resting condition, is
a small ovoid body, measuring some 10 //, by 7 /A, placed
alongside of and close to the macronucleus. When at rest it
stains less intensely than the macronucleus, but its staining
capacity varies in the different phases which it goes through in
the course of division.

There is good reason to suppose that the macronucleus is
chiefly concerned with the nutritive, the micronucleus chiefly
with the reproductive processes of the animal, and the evidence
for this statement will become apparent during the description
of the processes of reproduction and conjugation.

Reproduction in Paramecium is a simple process of trans-
verse binary fission. Both macronucleus and micronucleus
elongate prior to the division of the cell-body, and undergo
the modified form of mitosis shown in fig. 42. They first
become fusiform, and at either pole of the spindle polar plates
are formed connected by fibrils running the whole length of
the spindle. The spindle elongates, becomes dumb-bell
shaped, and finally the two swollen ends are connected by a
fine thread, which presently snaps in the middle, and the
nuclear division is complete. At the same time a constriction
divides the cell-body into two equal halves, which separate and
form two new Paramecia. A swarm of Paramecia, well
supplied with food, will continue to multiply by binary fission
for many generations. Careful observation has shown that the
divisions take place two or three times every twenty-four hours,
and that they will continue at this rate for some four or five
days. But not longer. Towards the close of this period the
offspring of a single Paramecium, some four thousand in
number, become smaller and begin to show signs of decay.
Eventually, if they are unable to conjugate, they undergo

CONJUGATION IN PARAMECIUM 197

degeneration and die. Similarly, if the food supply runs
short, the members of a swarm of Paramecia become weak
and diminished in size ; but under normal circumstances the
onset of degeneration is arrested by conjugation. Prior to
conjugation the individuals move hither and thither in a rapid
and excited manner as if in search of one another. After a

F

Fig. 42.

Stages in the conjugation of Paratneciutn caudatum. A, Stage A, the micro-
nucleus in each gamete, preparing for division. B, Stage B, the daughter
nuclei in each gamete dividing. C, Four micronuclei in each gamete. D,
Three of the four micronuclei are disintegrating ; the surviving nucleus in
each gamete has divided to form (J the male, and ^ the female pro-
nucleus. £, The male pronuclei crossing over, f, Conjugation effected ;
separation of the gametes and division of the combination nucleus. (After
Maupas.)

while they come together in couples and conjugate. Two
individuals become attached together mouth to mouth, and
are closely united by fusion or, at any rate, the intimate union
of their ventral cuticular surfaces. The two conjugating
individuals or gametes are exactly like one another, though of
small size as compared with normal free-swimming forms. The

198 COMPARATIVE ANATOMY

usual length of Paramecium caudatum is 300-325 /x, the
gametes rarely exceed 210 /x in length. As a rule the process
of conjugation begins during the late hours of the night and
the early hours of the morning, and it lasts till late in the
following afternoon. The first consequence of conjugation is
that the micronucleus of each gamete goes through what may
be called the first stage of its evolution. The result of which
is that its diameter is doubled. Before conjugation it measured
some 10 ^1x7 \L. After a period of 4-5 hours it measures
20-24 A1 x 12-14 V" During this time it has undergone
considerable changes of shape, becoming first elongate, then
spindle-shaped, and then doubled up to form a crescent, whilst
its chromatin granules are aggregated in rows to form longitu-
dinal fibrils. But eventually it resumes its original ovoid
condition. This period of growth is distinguished as stage A,
and is represented in the annexed diagram (fig. 43) by the
space enclosed between the two lowest horizontal lines. The
two next stages, B and C, are occupied by the division of the
micronucleus. This structure, increased as it is to twice its
former diameter and eight times its original volume, passes
through the mitosis represented in fig. 42 and divides into two,
and each product of division immediately undergoes mitosis
and again divides, so that there are now four micronuclei. The
macronucleus meanwhile has undergone no change. The four
micronuclei in each gamete are of equal size and to all appear-
ances equivalent, and all of these begin to prepare for a new
division by elongating to form fibrous spindles ; but only one,
and it is always the micronucleus which happens to be nearest
to the mouth, passes through the further stages of mitosis and
divides. The three others are arrested at the spindle stage, and
then gradually degenerate and are absorbed, leaving not a trace
behind. The surviving micronucleus in each gamete completes
its division so that at the close of stage D each gamete has two
micronuclei and one as yet unaltered macronucleus. Though
these micronuclei are quite similar to one another and, except
for their size, to the micronuclei which preceded them, they take
up definite positions which would seem to determine their
future fate. In each gamete one of the micronuclei is placed
close against the mouth, and may be called, in anticipation, the
male, whilst the other is the female pronucleus. Both male
and female pronuclei elongate and form fibrillated spindles,

CONJUGATION IN PARAMECIUM 199

and then a transference of micronuclear material is effected,
the male pronucleus of one gamete passing over and fusing with
the female pronucleus of the other, and vice versa. The
all-important act of conjugation, the transference of nuclear
material has now been effected, and the gametes, which
have diminished in size during conjugation, shortly afterwards
separate from one another, begin to feed again and recover
their original dimensions. Each possesses the original
unaltered macronucleus and a rather large micronucleus, now
called the Combination nucleus, formed by the union of the
male pronucleus of the other gamete with its own female
pronucleus. Immediately after separation the original macro-
nucleus undergoes changes which lead to its final disappearance.
It becomes deformed by the appearance of numerous folds
on its surface, and at a later stage splits up into a number
of irregular shreds which elongate to form long interlacing
ribbons. Eventually these ribbons divide into a number of
spherical corpuscles, which are either cast out of the body or
incorporated with the new macronuclei subsequently formed,
according as the progeny of the ex-gamete are well fed or
starved. Meanwhile the combination nucleus in each ex-gamete
has undergone three successive mitotic divisions (stages F, G1
G2 in the diagram) and there are eight products in the com-
bination nucleus arranged in two groups of four, one group at
the anterior and another at the posterior end of the body. The
four nuclei of the anterior group increase in size and eventually
become the macronuclei of the progeny of the ex-gamete. Of
the posterior group three members atrophy and disappear, one
alone surviving as a micronucleus. Some twenty-four to thirty
hours after separation the ex-gamete divides in such a manner
that two of the new macronuclei pass into one product of
division and two into the other, whilst the micronucleus
(which at this stage is not very much smaller than the macro-
nuclei) divides mitotically, one of its products entering each of
the two daughter Paramecia. A second division quickly follows
accompanied, as before, by mitotic division of the micronucleus,
whilst the macronuclei are again passively distributed among
the products of division. The end result is that each ex-gamete
has produced four normal Paramecia containing each a macro-
nucleus and a single micronucleus, both derived from the
' combination nucleus. These Paramecia feed and multiply by

200 COMPARATIVE ANATOMY

transverse division at the normal rate of two or three divisions
in the twenty-four hours.

The course of conjugation is in all essentials the same in the
other members of the genus Paramecium as in P. caudatum,
but the details are slightly different. In P. aurelia the process
is complicated by the presence of two micronuclei. In P.
bursaria the final stages are somewhat simplified, and resemble

To.ramecxu.Tn caudatum

Fig. 43-

co

lpoda

Diagrams of the course of conjugation in the two infusoria Paratneciutn caudatum and
Colpidium colpoda. The transverse lines represent the different stages into which
the process may be divided. The black circles represent persistent nuclei, the
empty circles stand for those which disintegrate and disappear. The division of the
gametes are represented by the ovals enclosing the nuclei at the top of the figure.
(After Maupas.)

those of Colpidium colpoda, a ciliate infusorian whose conjuga-
tion is represented in the diagram (fig. 43) for comparison. It
will be observed that the behaviour of the micronucleus up to
stage G1 is identical with that of P. caudatum, but that four
daughter nuclei are formed from each combination nucleus
instead of eight, and at the first division two of these become
the macronuclei and two the micronuclei of the daughter forms.
In P. bursaria the original macronucleus does not break up

SIGNIFICANCE OF CONJUGATION 201

as it does in P. caudatum, and it is doubtful whether it is
eventually resorbed or whether it fuses with one of the new
macronuclei.

The phenomena of conjugation in Paramecium caudatum
are worthy of most careful study, for they have been worked
out more carefully in this species than in any other Protozoan,
thanks chiefly to the brilliant researches of Maupas. Com-
paring the course of events with those observed in Bodo,
Polytoma and other Protozoa, it will be observed that in
Paramecium there is no distinction of sex as far as the two
gametes are concerned. Both are exactly alike and equal in
size. But there is an indication of differentiation in the
micronuclear products, so that we have not hesitated to call
the active product the male and the passive one the female
pronucleus. Again, it is remarkable that the conjugation is
only temporary, the two gametes separating and dividing, each
on its own account, after the mutual transfer of micronuclear
material has been effected. There is no fusion of cytoplasm,
and no zygote is formed. Perhaps the most instructive com-
parison is that between the conjugations of Paramecium and
Monocystis. The abortive products of the first two micro-
nuclear of divisions in Paramecium are comparable with the
so-called polar bodies of Monocystis, and in the latter form
the conjugating individuals preserve their individuality to some
extent, though not as completely as in Paramecium. In Mono-
cystis the gametes encyst before any exchange of nuclear
material takes place, and are therefore called zygotes, in contra-
distinction to the gametes of Paramecium which do not encyst.
Perhaps the most striking result of Maupas' researches is
the evidence, more surely founded than in the case of Polytoma
or Bodo, that in Paramecium and a large number of other
ciliate infusoria there is a definite limit to the reproductive
faculty, and when this limit is reached death ensues if conjuga-
tion does not intervene to restore the lost fertility. It would
appear that in these lowly organised creatures there is, as in
the higher animals, a period of growth, of bloom, and of decay ;
that the organism wastes and wears out in the exercise of its
functions, and requires rejuvenation from time to time in order
that its activities may not be brought altogether to a stop.
This rejuvenation is effected, in a manner unknown to us, by
the act of conjugation, and Paramecium shows, better than

202 COMPARATIVE ANATOMY

any other example, that the essential thing in conjugation is
the mingling of nuclear material derived from two different
individuals. It is possible that more than this is necessary ;
that the nuclear material must be derived from two different
stocks. Maupas has shown that, in one infusorian, individuals
derived from the repeated division of a single parent will not
conjugate with one another but only with the progeny of
another parent. Nothing is certainly known on this point
in Paramecium, but if it should prove to be a general rule
it makes the correspondence between the conjugation of the
Protozoa and the process of fertilisation in the Metazoa very
striking. For we know that even in those members of the
latter group which are hermaphrodite there are numerous
devices for avoiding self-fertilisation, and fertilisation, as has
been seen in the case of the frog, is essentially the same thing
as conjugation, the mingling of nuclear material of two different
cells.

In a Protozoon like Paramecium or Polytoma there is a
distinct life cycle, which comprises the alternation of a sexual
or conjugate generation with a series of asexual generations
reproducing their like by simple binary fission. The products
of fission of a zygote separate from one another, lead free
individual existences, and form a stock or family which has a
limited term of existence, for its members degenerate and die
if they do not conjugate at the proper time. In a Metazoon
the product of fertilisation is the fertilised ovum or oosperm,
comparable to the zygote of a Protozoon. The oosperm re-
produces itself, just as Protozoa reproduce themselves, by
binary division, but its progeny, instead of separating, remain
adherent, forming an integral whole or individual. Countless
numbers of cells are formed by repeated binary division, and
the whole, of which they form a part, grows into what we call
the adult organism. This we may compare with the sum of
the numerous individuals forming a stock or family of Protozoa.
Even as the Protozoon family has a limit of existence, degenera-
ting and eventually dying if that limit is overpassed, so the
Metazoon body, composed of the innumerable progeny of the
oosperm, has its well-marked phases of growth, maturity, decay,
and death. In both cases the extinction of the race is provided
against by the act of conjugation or fertilisation, by means of
which the forces of life — to use a vague expression — are as it

IMMORTALITY OF PROTOZOA 203

were rejuvenated and started on a new cycle of activity by the
commingling of the nuclear material of two simple cells.
Further than this we see a close parallel in the manner in
which the nuclear material is halved, before fertilisation, by the
formation of the polar bodies, or by the division of the sperma-
tocytes in the Metazoan ovum and spermatozoon and the
similar formation of the polar bodies in Monocystis and Actino-
sphaerium, or the division of the micronucleus with the abortion
of all but one of its products in Paramecium. The alternation
of generations, so obvious in many Protozoa, would seem to
be masked in the Metazoa by the coherence and integration of
the cell progeny of the fertilised ovum.

Thus the ovum and spermatozoon might be regarded as the
sexual generation of cells, the tissue-cells derived from them
as the numerous asexual generations, and the cycle is com-
pleted on the reappearance of the ovum and spermatozoon.
Such comparisons open up a wide field of inquiry, the interest
in which is further increased when one studies the parallel
series of events in plants; and the gradations exhibited by
Pandorina, Eudorina, and Volvox seem to supply a real basis
to speculation. But at present the whole subject is too obscure
to be discussed with profit. It must not be forgotten that no
process of conjugation has yet been observed in Amoeba and
Euglena ; that the fusion of the amcebulse to form the plas-
modium of Badhamia is not an act of conjugation, for there is
no fusion of nuclei ; and that, in Actinosphgerium, the gametes
are the identical pairs of so-called secondary cysts formed by the
binary division of the primary cysts, modified only to the extent
of their having ejected part of their nuclear matter in the form
of polar bodies. Amongst so many seeming contradictions it is
impossible to lay the foundations of a solid theory.

Much has been said of late years of the potential immortality
of Protozoa. But the facts described for Polytoma and Para-
mecium, as well as those about to be described for Vorticella,
show that these creatures, like the higher forms of life, are
subject to the inexorable laws of decay and death. If it should
be established beyond doubt that Amoeba (or any other
Protozoon) is capable of multiplying itself indefinitely by binary
division, without loss of functional activity calling for repair by
means of conjugation, then indeed it could be said that these
organisms are immortal in that decay and death would not

204 COMPARATIVE ANATOMY

come to them of necessity, but as an accident, through de-
privation of food or violence. But we know too little of the
life-history of Amoeba to allow of our making any positive
assertion about it, and the same may be said of most other
Protozoa. We must be content at present with the positive
evidence afforded by the life-histories of Paramecium and
allied forms, which demonstrate that the organism would wear
out and perish were it not refreshed, rejuvenated from time to
time, by the act of conjugation. How it is that conjugation
rejuvenates the enfeebled organisation we cannot say.

But it must be remembered, whilst we admit decay and
death as the natural accompaniments of existence, that the
life-stuff, protoplasm in its widest sense, is immortal. All the
evidence at our disposal forces us to believe that living matter
can only be born from living matter. Organisms are not bred,
as was once believed, from slime and mud, but only from other
organisms, and all that now lives on the earth is descended
from that original life-stuff whose origin will ever be a mystery
to us.

CHAPTER XI
THE CILIATA (continued)— VORTICELLA

THE genus which we are now going to study is very common
and rich in species. Specimens may be collected during the
warmer months in almost any fresh-water pool or ditch, where
they are generally found adhering to the stalks of water-weeds.
Or one may nearly certainly obtain a cultivation of Vorticella
by making an infusion of hay and dead leaves in soft water
and leaving it to stand for a time. After the first stages of
putrescence have passed a thick brown scum will form on the
surface, and numerous specimens of Vorticella will be found
adhering to the under side of this scum.

There, are many species of Vorticella, differing from one
another in so small a degree that it is often very difficult to
identify them. The following description refers chiefly to
Vorticella monilata, a species which frequently makes its ap-
pearance in vegetable infusions, and is easily recognisable
because of the warty excrescences with which its body is
studded.

In shape Vorticella may be compared to an inverted bell,
with a very long handle. Imagine the rim of the bell to be
thickened and beset with a circle of cilia, and its mouth to be
almost closed by a discoidal plug, placed to one side so as to
leave between it and the thickened rim a slit leading into the
interior. Further, imagine that the handle is hollow and
traversed by a contractile cord thrown into a loose spiral so
that on contraction of the cord it is shortened into a close
corkscrew-like spiral, and one has a very fair idea of the
general appearance of the organism.

In the genus Vorticella the individuals may be closely

crowded together or may stand at some little distance apart on

the weed or other substance to which the stems are attached,

but they are always separate and independent of one another.

205

206 COMPARATIVE ANATOMY

In the closely allied genera Zoothamnium and Carchesium the
stems are branched, and each branch bears a bell-shaped
animalcule at its extremity, so that there is a colony com-
posed of numerous individuals organically connected with one
another by means of the branching stems. The bell-shaped
expansion will be called the body, the handle the stalk.
The contractile cord traversing the stalk is the contractile
filament ; the discoidal plug nearly filling up the orifice of the
bell-shaped body is the disc ; the groove formed between the
thickened rim and the disc is the peristomial groove ; and the
orifice leading into the interior of the body is the vestibule.

Having settled on these terms, we may proceed to examine
the structure of the animal in detail. The first thing to be
observed is the distribution of the cilia. They are not
scattered over the whole body, as in Paramecium, but they are
reduced to two circles or whorls, one placed on the thickened
marginal rim, the other on the edge of the disc. In addition,
there are cilia in the narrowed tube or gullet into which the
vestibule leads, and there is an undulating membrane of large
size forming a sort of triangular flap, one side of which is
attached along the peristome and the entrance to the vestibule.
(Fig. 44, A, un.) The size and arrangement of the cilia can
easily be understood by reference to the accompanying figure.
Because of the restriction of the cilia to the vestibular region,
Vorticella and its allies are placed in an order Peritricha.

The bell-shaped body consists, as in Paramecium, of cuticle,
cortical layer, or ectoplasm, and a soft internal endoplasm
occupying its cavity. The cuticle, like that of Paramecium,
must be regarded as the surface layer of the protoplasm, in
which the alveoli, because of their superficial position, assume
a particular arrangement. The cuticle is variously striated or
otherwise ornamented in different species. In Vorticella
monilata, as has already been said, it is covered with closely-
set warty prominences.

The ectoplasm is the layer immediately beneath and con-
tinuous with the cuticle. It exhibits the usual alveolar
structure of protoplasm, and differs from the ectoplasm of
Paramecium in being devoid of trichocysts. But in Vorticella
one can recognise structures of which there was no trace in
Paramecium — namely, a layer of very fine contractile fibres
lying immediately beneath the cuticle. These fibres are not

VORTICELLA

207

a product of the ectoplasm, but a differentiation of the walls of
the alveoli of the cuticular layer. They begin as a number of
very fine and rather widely separated fibrillse close underneath
the peristomial ring, and run longitudinally down the body
towards the stalk. At about one-third of the length of the
body from the hinder end the cuticular layer is thickened so

Fig. 44.

A, a specimen of l-'orticella monilata viewed as a transparent object ;
un, undulating membrane ; g, gullet ; inacr, macronucleus ; micr,
micronucleus ; r, reservoir; c.v., contractile vacuole ; _/, food
vacuoles; msc, mucular fibrillae at the base; c.f., contractile
filament. B, another specimen of the same species showing the
warty prominences characteristic of the species ; and mic g, a
microgamete which has just attached itself. C, a specimen partly
retracted to show how the peristome is infolded over the disc. Z>,
a specimen dividing ; the macronucleus and micronucleus drawn
out into fibrillated spindles. £, a microgamete of V. nebulifera.
(A to D original ; E after Claparide).

as to form a ring encircling this part of the body, and the
fibrillse appear to be attached to this ring and then to bend
rather suddenly inwards, traversing the thickened ectoplasm in
the hinder part of the body. Converging together, the fibrils
unite where the stalk is joined to the body and are continued
into it as the contractile filament. Usually the converging

208 COMPARATIVE ANATOMY

fibrils at the hinder extremity of the body are easily recog-
nisable, whilst those at the anterior end are difficult to see.

Anyone who watches a living Vorticella will easily see that
the creature not only springs to and fro by the contractions of
its stalk, but that it also has the power of expanding and
unfolding its peristomial disc. When it is fully expanded the
peristomial rim is everted, somewhat like the mouth of a bell,
the disc is protruded, and the beautiful whorls of cilia situated
on these organs are fully displayed. In some species the disc
is protruded far above the margin of the peristome ; in others,
as in V. monilata, it hardly rises to the level of the peristomial
rim.

In contraction the disc is withdrawn and the peristomial
ring is contracted and folded inwards over it. The whorls of
cilia are turned inwards and covered over by the infolded
margins of the peristomial rim, and the whole body assumes the
shape of a pear. It has been said that the contraction of the
peristome is effected by means of a circular layer of contractile
fibres or sphincter lying in the peristomial ring. Such fibres
may be present— one can hardly imagine how the contraction
of the peristome could take place without them — but many of
the best observers have failed to distinguish them, and the
writer has been equally unsuccessful.

The contractile filament, formed, as has been shown, by the
union of the basal contractile fibres of the disc, traverses the
stalk as a homogeneous doubly refracting thread, which in the
expanded condition forms a very loose spiral, in the contracted
state is tightly coiled up. The filament is ensheathed in a
transparent elastic tube which is not a continuation of the so-
called cuticle of the body, but an ectoplastic product secreted
by the posterior end of the animal, and probably of a chitinous
nature. The space between this tube and the contractile
filament is occupied by a transparent homogeneous mass of
gelatinous consistency, and it is probably due to the presence
of this substance that on contraction of the filament the whole
stalk is coiled up in a spiral, and the body drawn down towards
the point of attachment. The stalk being closed below, the
contractile filament does not project beyond it.

Returning to the structure of the body, we find that the
vestibule is continued into a narrowed ciliated gullet, which,
in V. monilata^ turns through a half-spiral and ends in the

VORTICELLA 209

endoplasm near the middle of the body. The endoplasm has
much the same character as that of Paramecium, con-
taining food-vacuoles, pellets, and granules, which move round
and round in a regular cyclosis.

Close alongside of the gullet is a large clear space, which is
stationary and generally distinguished from the food-vacuoles
by its larger size. This cavity is filled with fluid, and com-
municates by a very narrow passage with the vestibule ; it is a
reservoir like that already described in Euglena. Close beside
it is a contractile vacuole, whose contents at every contraction
are poured into the reservoir, whence they are slowly emptied
into the vestibule, and so to the exterior.

Like Paramecium, Vorticella is in need of a special aperture
for the expulsion of undigested matter and solid excreta. This
aperture is situated in the vestibule close to the opening of the
reservoir, and is generally known as the anus. It is often a
permanent opening with a short and narrow tube leading to
the endoplasm, but these relations vary very much in different
species.

A macronucleus and a micronucleus are present. The former
is very large and elongated. In some species it has the form
of a horse-shoe, in others it is like a twisted riband. Its shape
and position in V. monilata can be better understood from an
inspection of fig. 44, A, than from any description. The micro-
nucleus is small, and is placed close alongside of the macro-
nucleus in the anterior end of the body. The micronucleus
stains with great difficulty, whereas the macronucleus seizes on
the ordinary dyes with great avidity and obscures all the
structures lying against it. Hence the micronucleus is difficult
to see, but it can usually be demonstrated in living specimens,
and better still in specimens cleared in glycerine after being
killed in a 10 % solution of corrosive sublimate to which a few
drops of acetic acid have been added.

Vorticella reproduces itself by binary longitudinal fission.
The peristome is contracted, and the body becomes depressed
and transversely elongated. The macronucleus is straightened
and comes to lie transversely across the middle of the body,
forming a fibrillated spindle like that of a Paramecium preparing
for division. The micronucleus elongates to form a spindle
placed alongside of the macronuclear spindle and both become
dumb-bell shaped and divide. (Fig. 44, Z>.) A constriction

210 COMPARATIVE ANATOMY

beginning at the vestibular end divides the body into two halves,
both of which are at first attached to the same stalk. But one
moiety develops an additional ring of cilia at its posterior end,
this cilated ring being always coincident in position with the
thickening of the cuticle described in connection with the con-
tractile fibrils. It then becomes detached from the stalk, swims
away by means of its ciliated band, and after a time attaches itself
to some object by its posterior end, forms a new stalk for itself,
loses its posterior circlet of cilia, expands its peristomial field,
and so develops into a new Vorticella. The other product of
division remains attached to the stalk, and thus one may
speak of it as the parent form, whilst the detached free-
swimming individual may be called the child. No such
distinction was possible in the Protozoa described in the
preceding pages.

Though our information on the subject is somewhat meagre
there is no doubt that Vorticella is as little able to propagate
itself indefinitely by binary division as Paramecium. Speci-
mens cultivated in an organic infusion will flourish and multiply
by division for three or four days, but will then dwindle and
disappear if conjugation does not take place.

The conjugation of Vorticellids presents many special features
which must be described in detail. It is easy enough to obtain
specimens in the act of conjugating, but it is very difficult to
follow out all the steps, partly because of the attachment of
the creatures to foreign substances, partly because the macro-
nucleus breaks up at an early stage into a number of minute
fragments and thus obscures the micronuclei. The various
steps have been followed more completely in Vorticella moni-
lata than in any other species, and again we owe the most
complete and accurate account to Maupas.

The gametes of Vorticella are of two kinds, macrogametes
and microgametes. The former are stalked individuals differ-
ing neither in size nor in any other obvious feature from normal
specimens. The microgametes, as their name implies, are
much smaller, and they differ further from normal individuals
in the possession of a posterior circlet of cilia. They have no
stalk, but swim freely through the water by means of this ac-
cessory ciliated band.

In V. monilata the microgametes are formed by the binary
division of an ordinary stalked form. Sometimes the products

CONJUGATION IN VORTICELLA 211

of division may again divide, giving rise to four microgametes,
but a single binary division is the rule.*

The microgametes when set free from the parent stalk swim
through the water. Conjugation begins towards the hour of

Fig. 45-

Stages in the conjugation of Vorticella monilata ; mac, macrogamete ; taic, micro-
gamete. M, fragmented macronucleus of the macrogamete. M1, fragmented
macronucleus of the microgamete. IV, water accumulated below the peristome
of the macrogamete ; m, micronucleus of the macrogamete ; m', micronucleus of the
microgamete ; en, combination nucleus. For further description see text. (After
Maupas.)

daybreak and lasts from thirteen to fourteen hours. A micro-
gamete coming into contact with a fixed macrogamete attaches
itself by its posterior end to the hinder region of the body of
the latter, and immediately becomes intimately fused with it.

* It is so commonly stated that Vorticella gives rise by three successive
binary divisions to eight microgametes that it is well to insist on the fact
that Maupas has found that the microgametes are formed by equal binary
division in V. monilata, V. putriua, and V. nebulifera ; by unequal binary
division in V. microstoma ; and only by repeated binary division in the case
of Carchestum polypinum.

212

So long as it is swimming about freely the microgamete has a
single micronucleus of fusiform shape, measuring some 4-5 [J.
in its longer and 3 ^ in its shorter diameter. Scarcely is it
fixed, however, to a macrogamete, and before actual fusion has
taken place, than the micronucleus undergoes mitosis and
divides into two. The micronucleus of the macrogamete mean-
while remains inert. In the next stage the micronuclei are un-
altered, but the microgamete loses its posterior circlet of cilia
and becomes intimately fused with the macrogamete. When
this is accomplished the micronuclei in both gametes increase
greatly in size, become spindle-shaped, and divide by mitosis.
A second division follows, and as there were two micronuclei
in the microgamete and one in the macrogamete the result is
that there are eight micronuclear corpuscles in the former and four
in the latter. Up to this time the micronucleus of the macro-
gamete and its products have been situated in the anterior
region of the body. The peristome being closed, water now
accumulates below it and forces the four micronuclear products
towards the posterior end, so that they are closely contiguous
to the micronuclear corpuscles of the microgamete. Seven of
the last-named then disintegrate and disappear, the one re-
maining moving close up against the partition which still exists
between the bodies of the macrogamete and microgamete.
Similarly three of the four micronuclear corpuscles of the
macrogamete disappear, and the one remaining takes up a
position opposite to the surviving corpuscle in the micro-
gamete. These corpuscles increase notably in size, the dividing
wall separating them disappears ; they come into contact, and,
on doing so, both divide mitotically. The result of the division
is that one product of each micronucleus is pushed back into
the body of the microgamete, there to disintegrate and be ab-
sorbed without leaving a trace. The two remaining products,
constituting the male and female pronuclei, are pushed into the
body of the macrogamete, where they fuse together to form a
combination nucleus.

As soon as the fusion is complete the water collected beneath
the peristome of macrogamete is ejected, the protoplasm moves
forward to take its place, carrying with it not only the com-
bination nucleus but also the protoplasmic contents of the
microgamete. The latter is now reduced to a mere membranous
appendage, which shrivels up and eventually falls off and is lost.

CONJUGATION OF VORTICELLA 213

The combination nucleus soon elongates to form a fibrous
spindle, and divides by mitosis. Two more divisions follow, so
that the macrogamete contains eight similar nuclear corpuscles.
One of these does not grow any further, but assumes the char-
acters of a micronucleus. The other seven become spherical,
and grow rapidly till each consists of a central mass of granular
matter separated by a clear space from a well-defined nuclear

Fig. 46.

Diagram of the course of conjugation in Vorticella monilata, constructed on the
same principle as fig. 43, which should be compared. (After Maupas.)

membrane. The fertilised macrogamete, if well fed, is now
ready to divide. The large corpuscles, which may now be
called macronuclei, do not divide, but are distributed to the
offspring, four passing into one and three into the other of the
two individuals ' resulting from the first division. The micro-
nucleus, on the contrary, divides, one of its products passing
into each of the daughter Vorticellae. The rest of the process
has not been determined with absolute certainty, but it is

214 COMPARATIVE ANATOMY

probable that it follows the course indicated in the diagram,
fig. 46. The young Vorticellid with four macronuclei under-
goes two more divisions, each accompanied by a division of
the micronucleus, whilst macronuclei are simply distributed
equally among the progeny. The young Vorticellid with three
macronuclei behaves similarly, but as there are only three
macronuclei to be distributed, only three young forms are pro-
duced in the manner shown in the diagram.

The fate of the original macronuclei of the two gametes has
already been alluded to. At a very early stage they are broken
up by abstriction into a number of fragments, those of the
microgamete being always smaller than those of the macro-
gamete. The fragments continue to break up into smaller
and smaller pieces which are finally absorbed and disappear
altogether.

One cannot fail to observe that the conjugation of Vorticella
is a step further advanced towards a differentiation of sex than
in Paramecium, or, indeed, in any other of the unequivocally
animal Protozoa previously described in these pages. Volvox
exhibits a still more marked sexual differentiation, but, whilst
it is allied to animal forms, it has all the characters of a plant.
Whereas in Paramecium the gametes are similar, the conjuga-
tion temporary, and both of the ex-gametes reproduce their
kind, in Vorticella the gametes are dissimilar, the conjugation
permanent and complete, and the resulting zygote is the sole
reproductive individual. We cannot hesitate to ascribe to the
active microgamete the role of the male, to the passive macro-
gamete the role of the female.

CHAPTER XII
PROTOZOA AND METAZOA

WHILST hunting for Vorticella in sea or fresh water one fre-
quently comes across little branching colonies composed of
numerous Vorticella-like individuals united together to form a
sort of tree or bush. These belong to the genera Epistylis,
Carchesium, and Zoothamnium. The genus Epistylis may
readily be recognised, because the individuals forming the
colony have no contractile filament and therefore cannot
retract themselves by coiling their stalks. There is no organic
connection between the individuals ; they are merely associated
together and grouped in such a way as to form a plant-like
growth. In the genus Carchesium the individuals are similarly
associated to form a bush-like colony, but each has a well-
developed contractile filament, and can retract itself inde-
pendently of its fellows. A strong stimulus may cause all the
members of the colony to retract themselves at once, but
generally some are retracted whilst the others remain fully
extended and expanded. In the genus Zoothamnium there
is a main stem with branches which again sub-divide so
that the colony has something the shape of an espalier pear-
tree. The contractile filament runs through the main stem,
and, sub-dividing, is continued into its branches, the ultimate
sub-divisions terminating in the Vorticella-like individuals.
Thus all the members of the colony are organically united by
means of the branched contractile filament, and a stimulus
affecting any one member is immediately communicated to all
the others, causing the whole colony to contract into a nearly
globular form. Further than this, in certain species of
Zoothamnium, the individuals of a colony are not all alike.
The greater number of them, forming the termination of
the twigs and branches, do not differ materially in structure
from Vorticella, and measure about 0-08 mm. in diameter.
215

216 COMPARATIVE ANATOMY

Other individuals of much larger size, measuring as much as
o'i2 mm. in diameter, are situated in the axils of the branches
and are distinguished as the macrogonidia. They are globular
in shape, provided with a nucleus, contractile vacuole, and a
posterior circlet of cilia, but their peristomial field is permanently
closed.

The arborescent form of a colony of Zoothamnium is due to
the fact that longitudinal binary division of the ordinary in-
dividuals involves not only the body but also the distal half
of the contractile filament, and consequently repeated binary
divisions give rise to a dichotomously-branched colony. But,
since both products of a division remain fixed on the branched
stalk instead of one of them swimming away, there would be
no provision for the foundation of new colonies unless some
means were especially adapted to that end. These means are
supplied by the macrogonidia, whose sole function it is to
reproduce new colonies. It is not quite certain whether a
macrogonidium must undergo fertilisation before it reproduce
a new colony or not. At any rate it may undergo fertilisation,
that is to say, it may conjugate with a microgamete. The
microgametes of Zoothamnium, so far as is known, are produced
by multiple division of an ordinary individual, and they have
much the same appearance as the microgametes of a Vorticella.
They swim away and conjugate with a macrogonidium, never
with an ordinary individual. When conjugation is effected
the fertilised macrogonidium is liberated from the colony,
swims about for a while, and then attaches itself and divides
rapidly and repeatedly, giving rise to a new colony. It also
seems to be possible that a macrogonidium may be able
to detach itself and give rise to a new colony without
fertilisation.

An interesting comparison may be drawn between Zootham-
nium and Volvox. In both these forms, though they belong
to different classes of the Protozoa, the cell individuals formed
by successive binary divisions remain in organic connection
with one another and give rise to a compound organism, a
stock or colony. In Volvox the cell units are so intimately
connected that their individualities are to a great extent merged
into the higher individuality of the stock of which they form a
part, whilst in Zoothamnium the union is of a looser kind, and
the individualities of the cells seem to us to preponderate over

SOMATIC AND GERM CELLS 217

the individuality of the colony. But the difference is unim-
portant, the main feature in each case being the tendency
for the products of cell-division to adhere together and form a
compound organism. In consequence of this adherence the
reproductive faculty of the ordinary cells is limited to the
increase of the bulk of the colony, and in order that the species
may be perpetuated special individuals are produced to which
the function of reproducing new colonies is delegated. Thus
the union of cells into a stock or colony involves, it would seem
as a necessary consequence, a differentiation into two distinct
kinds of individuals. The one kind, comprising what we have
styled the ordinary individuals, are mainly nutritive in function
and may henceforth be called the vegetative, or, better, the
somatic cells. The other kind are almost exclusively repro-
ductive, and may henceforth be called the germ cells. It
should be noticed in passing that in Volvox and possibly also
in Zoothamnium, the germ cells are themselves divided into
separate kinds, asexual and sexual. The asexual germ cells are
the parthenogonidia of Volvox and the unfertilised macro-
gonidia (which, if they really exist, might be called by the same
name) of Zoothamnium. The sexual cells are the male and
female gonidia of Volvox, the microgametes and macrogonidia
of Zoothamnium.

Now it has been shown that uni-cellular organisms — in most
cases at any rate — are incapable of propagating themselves in-
definitely by binary division. Sooner or later they must con-
jugate or they will wear out and perish. What is true of
uni-cellular organisms is true of the individual cells composing
a colony or a multi-cellular organism. After a more or less
prolonged exhibition of their vital activities they will degenerate
and perish if they do not receive a fresh impulse from conjuga-
tion. But the somatic cells are incapable of conjugation;
that function is limited to the male and female germ cells.
Hence the union of cells to form a composite colony or in-
dividual, and their differentiation into somatic and germ cells
is fraught with the inevitable consequence of decay and death
for the somatic cells.

It follows that the continued propagation of a race of multi-
cellular organisms, or cell colonies, in which there is a differen-
tiation into somatic and germ cells, is dependent upon the germ
cells, for they alone can conjugate and so rejuvenate the en-

218 COMPARATIVE ANATOMY

feebled powers of the organism. They are the survivors, whilst
the somatic cells perish.

By following up this course of reasoning we can understand
why it is that in the higher animals, the Metazoa, there is so
close a connection that it amounts to identity between the
processes of conjugation and reproduction.

It has been shown that in all essential respects conjugation
is the same thing as fertilisation : the essential act is the ming-
ling of nuclear material of two separate individual cells.

It has further been shown that a recurrence of conjugation
(or fertilisation, which is the same thing) at no long intervals
of time is essential to the maintenance of the vital activities.

If then in a cell-composite, or, as we call it, a multi-cellular
individual or Metazoon, the capacity for conjugation is restricted
to certain cells only — viz. to the germ cells — it becomes evident
that in proportion as organisation advances from the uni-cellular
to the multi-cellular condition, and from homogeneity of cell-
structure to heterogeneity of cell-structure, so will the primitive
distinction between reproduction and conjugation become less
and less, until finally only those cells are reproductive which
are capable of conjugating — namely, the ova (macrogametes)
and spermatozoa (microgametes).

We learn, then, that the progress from uni-cellular to multi-
cellular structure involves a division of labour amongst a com-
munity of cells, a differentiation in the first instance into
somatic and germ cells. The somatic cells, incapable of
conjugation, run through a certain course of vital activity and
then wear out and perish. But before they perish they give
rise to other cells, germ cells, which are capable of conjugation.
When two of these have united, the product, a fertilised ovum,
enters upon a new career of activity, divides, gives rise to a
new composite of somatic cells, and the cycle is again repeated.

The steps through which this condition came to be estab-
lished in the course of evolution are indicated by' the series
Gonium, Pandorina, Eudorina, and Volvox, and much more
obscurely by the series Vorticella, Carchesium, Zoothamnium.

It is not permissible to suppose that either of these series
represents the actual ancestral form of the Metazoa (see p. 186),
but they enable us, with good reason, to trace the possible
course of evolution from one great division of the animal
kingdom to the other.

PROTOZOA AND METAZOA 219

It is very probable that Protozoa developed into Metazoa,
not once, but many times over, and that the classes of Metazoa
which we recognise to-day may have sprung from different
uni-cellular groups. In this connection it is interesting to note
that, although the Flagellata and Ciliata are distinct classes of
the Protozoa, and have developed along characteristic lines,
they both exhibit similar tendencies in evolution. In both we
find the successive steps of simple adhesion of equivalent cells,
followed by organic connection with a differentiation into
vegetative and reproductive, somatic and germ cells, the series
culminating in the one case in Volvox, in the other in
Zoothamnium. Volvox, however, must be ranked as a plant ;
and it should be noticed that the life cycle of plants is not so
simple as that of animals. Plants retain the capacity of re-
producing themselves asexually, as Volvox does by means of
its parthenogonidia, and an alternation of asexual and sexual
generations has been established, the one succeeding the other
with great regularity. It is by no means certain that the
macrogonidia of Zoothamnium can develop without fertilisation,
or, as it is said, parthenogenetically, but if they can the fact is
exceptional in the animal kingdom. Some few animals, it is
true, are capable of reproducing themselves parthenogenetically
by means of reproductive cells which do not require fertilisa-
tion, and in them there is a more or less regular alternation of
generations. But in the great majority there is no such alter-
nation but an unbroken succession of sexual generations.

It must now be evident that although we attempt to draw
a sharp distinction between the Protozoa and the Metazoa,
defining the one group as uni-cellular the other as multi-cellular,
the existence of the two series which we have been considering
breaks down the barrier implied by our definitions. If a
Metazoon is a multi-cellular organism reproducing itself asexu-
ally by ova and spermatozoa, it would be hard to say why
Volvox (leaving its vegetable affinities out of the question)
should not be classed among the Metozoa. It is, as a matter
of fact, placed among the Protozoa because it is directly
connected through Eudorina and Pandorina with the uni-
cellular Flagellates, and is not similarly connected by transi-
tional forms with any higher animal or plant. So it is simply
classed amongst its nearest kinsmen. And it is obvious
that, if the theory of evolution is true, and we could recover

22o COMPARATIVE ANATOMY

all the animal forms which have become extinct, it would be
impossible to fix the point where the Protozoa ceased and the
Metozoa began. The one group must have passed imperceptibly
into the other, and it is only because the intermediate forms
have become extinct that we are led to draw a hard and fast
line between uni-cellular and multi-cellular organisms. And
even now the Flagellata, and in a lesser degree the Ciliata, are
there to remind us that our definitions are, after all, arbitrary
and artificial.

But, after all, our conception of a Metazoon is not quite
simply that of a multi-cellular organism, propagating itself by
sexual reproduction, as will appear from the study of the
animals described in the following chapters.

CHAPTER XIII

THE CCELENTEEATA— HYDEA FUSCA AND
HYDRA VIEIDIS

THE little fresh-water polyps, known by the generic name of
Hydra, so common in our pools and streams, are by common
consent chosen as the type of the simplest form of Metazoan
structure. To depart from well-established custom and choose
another type would occasion inconvenience, so custom must
be adhered to in spite of the fact that Hydra is far from being
as simple and primitive in structure as was at one time supposed.
One of the simpler forms of sponges, abundant enough in
certain localities on our coasts, would serve better as an
illustration of a primitive type of multi-cellular organisation.

Two species of Hydra are abundant in streams and pools in
Great Britain. The one, of a yellow or light brown colour, is
known as Hydra fusca. The other is called Hydra viridis
because of its bright green hue, due to the presence of in-
numerable green chlorophyll-containing corpuscles in its
tissues. A third nearly colourless species has been recognised
and named Hydra grisea, but it differs so little and in such
unimportant characters from Hydra fusca, that it need not
engage our attention.

Hydra fusca being selected for descriptive purposes, the
general shape of the animal may be described as that of a
cylindrical sac forming the body, attached by one end to
water-weed or other object. At the opposite distal end the sac
opens to the exterior by a small orifice, the mouth, situated on
the top of a conical elevation termed the hypostome. A
circlet of simple, tapering, arm-like processes springs from the
junction of the hypostome with the body wall. These processes
are the tentacles. They vary in number in different specimens,
but there are seldom less than six and never more than nine or
ten.

222

COMPARATIVE ANATOMY

A living Hydra adheres pretty firmly by its proximal end —
often called the foot, or, better, the basal disc — to the object to
which it is attached. It is not permanently fixed, however, but

Fig- 47-

On the left hand a specimen of Hydra fusca, fully extended ;
tt, testes ; mi, ovum with horny case. On the right hand a
specimen of Hydra viridis, half extended ; 6, a bud.

can detach itself and move slowly from place to place, either
creeping along by means of its tentacles, or by a series of
movements resembling those of the looping caterpillar. In the
latter case, the body is extended as far as possible and bent

HYDRA 223

downwards till the mouth touches the weed or other object to
which the animal is attached. The mouth being fixed to the
surface the basal disc is detached and drawn up, and again fixed
close to the point where the mouth is attached. The mouth
then lets go, the body is again extended, a fresh hold is taken
by the mouth ; and so on.

A living Hydra may easily be examined in a watch glass full
of water under a simple lens or a low power of the microscope.
After being detached it soon fixes itself again to the sides of
the glass, and, if undisturbed, extends itself to a considerable
length, spreading out its tentacles like a net.

Both body and tentacles are exceedingly contractile and
extensible. The body of the large species Qi Hydra fusca when
fully extended, may attain a length of 7 or 8 mm. or more,
whilst its diameter is reduced in proportion as its length is
increased. The tentacles, when fully extended, project some
6 or 7 mm. beyond the body, and are reduced to the
thickness of the finest thread. When the animal is irritated or
alarmed, the body is contracted into a mere lump some 2
mm. long, and its diameter increases in proportion, whilst the
tentacles are reduced to circlets of blunt finger-like projections
surrounding the hypostome. All sorts of intermediate con-
ditions between the extremes of contraction and extension
may be observed. When the body is fairly extended it can
be seen that the trunk is not of uniform thickness but rather
club-shaped. The proximal moiety is slender and nearly
colourless, corresponding to the handle of a club, whilst
the distal moiety is thicker and more deeply coloured. The
external difference in form corresponds with an internal differ-
entiation of cell-structure, as will be described presently.

Hydra is carnivorous, and feeds on the " water-fleas " — i.e.
small Crustacea, chiefly Daphnids and Copepods, which abound
in fresh-water pools. If some of these organisms are placed in
the watch-glass along with a Hydra, the manner in which the
animal captures and swallows its prey may easily be observed
A Daphnid swimming within reach of the circle of extended
tentacles is seized and held fast by them. After a short
struggle it appears to be paralysed, and is drawn down-
wards towards the mouth by the contraction of the tentacles.
The hypostome being expanded and the mouth widely
opened, the Daphnid is slowly swallowed and passed down

224 COMPARATIVE ANATOMY

into the digestive cavity. The distal moiety of the body
is often swelled up by the presence of one or more water-
fleas, which have been swallowed and are undergoing digestion,
but they are seldom to be seen in the proximal moiety.
Crustaceans have hard chitinous exoskeletons which cannot
be digested. These and other indigestible matter are afterwards
expelled by the mouth.

The general structure of Hydra can be studied by subjecting
the animal to slight pressure under a cover-glass and examining
it under a moderate power of the microscope ; but in order to
study details one must have recourse to very thin longitudinal
and transverse sections and to the method of maceration. By
the first method of examination it can be seen that the body
is a simple sac, having a single spacious internal cavity, the
gastrovascular cavity, whose walls are formed of two layers of
cells separated by a fine membrane. The external layer of
cells, forming the outside skin of the body and tentacles, is
colourless, not very thick, and studded, especially on the distal
moiety of the body and on the tentacles, with numerous small
ovoid capsules, the nematocysts. This outer layer is known
as the ectoderm. The inner layer, or endoderm, is notably
thicker, and is full of granules and small brown corpuscles
which render it opaque. The colour both of H. fusca and H.
viridis is due to the brown or green corpuscles contained in
the endoderm cells, and the distinction between the colourless
ectoderm and the endoderm stuffed full of green corpuscles is
particularly well marked in the latter species.

Between ectoderm and endoderm is a very thin layer of
colourless gelatinoid material, distinguishable as a transparent
line in optical section. This is the jelly, or mesoglaea, very
scantily developed in Hydra, but attaining great thickness and
forming the greater part of the bulk of the body in many
Ccelenterates. It is not formed of cells as the ectoderm and
endoderm are, but is formed as a secretion of (probably) both
of these layers.

The general structure of the tentacles is the same as that of
the body. They are simply hollow processes of the latter,
each containing a prolongation of the gastrovascular cavity,
bounded by the two cellular layers ectoderm and endoderm,
with the mesogloea between. When retracted a tentacle is
thick and bluntly finger-shaped, its surface thrown into a

HYDRA

225

number of coarse transverse wrinkles. When fully extended
it is attenuated and filiform, the wrinkles disappear, and the

Fig. 48.

A , a longitudinal section of Hydra viridis, semi-diagrammatic ; in, mouth ;
bd, basal disc ; en, endoderm ; ec, ectoderm ; gv, gastrovascular cavity.
The mesogloea is represented by a black line. B, portion of a longitudinal
section through an extended tentacle, highly magnified, to show, the
batteries of nematocysts ; mgt mesogloea ; n, nucleus of the ectoderm
cell containing a battery ; en, endoderm ; en, cnidocil. C, a nematocyst
of the large oval kind enclosed in a cnidoblast ; the thread partly everted.
C1, a large oval nematocyst with the thread coiled up, enclosed in a
cnidoblast ; en, cnidocil. C*, C3, stages in the development of a nematocyst.
D, an elongate oval nematocyst, showing the short relatively thick thread
everted. E, a small oval nematocyst with everted coiled up thread ; the
cnidoblast is produced below into a long contractile process.

surface is covered by a number of warty projections beset with

226 COMPARATIVE ANATOMY

minute stiff hair-like projections. Further examination shows
that each warty prominence is a lump of protoplasm, a single
ectoderm cell, in fact, in which a number of highly refringent
oval and cylindrical capsules are imbedded. These capsules
are the nematocysts, organs of offence and defence possessed
by all members of the phylum Crelenterata to which Hydra
belongs. The stiff hair-like processes projecting from the
surface are called cnidocils, and it is easy to determine that
each nematocyst has a single cnidocil associated with it.
If a drop of acetic acid or magenta is run under the coverslip
some of the nematocysts will be shot out and will assume the
forms shown in fig. 48, C, D, and E. There are three kinds of
nematocysts, the largest having the form of a pear-shaped sac,
the narrow end of which is produced into a short tapering
tube which narrows rapidly to form a long thread or filament.
The base of the filament is armed with three stiff projecting
spines and a number of smaller spines. The two other kinds
of nematocysts have shorter and thicker threads destitute of
spines, and the threads are usually curled up into a close cork-
screw-like spiral. Before the application of the stimulus the
thread in each nematocyst was introverted and coiled up inside
the cavity of the sac. To understand the meaning of " intro-
version " the reader should imagine an india-rubber sac pro-
longed at one end into a closed tube. If a thread were passed
up the inside of the tube and fastened to its inside at the tip,
by pulling on the thread the whole tube could be drawn back
into the cavity of the sac. If, then, the sac being closed and
filled with fluid, pressure was exerted on its walls, the intro-
verted tube would, in consequence of the pressure transmitted
to it, be everted and thrown out sharply to its original length.
There is no internal thread in a nematocyst but in other re-
spects its structure resembles that of the imaginary india-rubber
ball and tube. The contents of a nematocyst sac, however,
do not appear to contain a fluid but a semi-solid gelatinoid
material which stains deeply with hsematoxylin and many other
dyes. This substance is hygroscopic, and it is due to the
absorption of water by it that the pressure inside the sac is
increased and the thread ejected. Hydra is able to sting and
paralyse its prey by means of its nematocysts, and it is probable
from the paralysing effects observed in the case of the water-flea
that these structures contain a poisonous secretion in their sacs.

STRUCTURAL PLAN OF HYDRA 227

It is clear, from what has preceded, that the structural plan,
the architecture of Hydra, is of the simplest kind. It is an
elongated sac closed at one end and open at the other. The
walls of the sac are formed of two distinct cell layers cemented
together, as it were, by an intermediate structureless substance,
the mesoglcea. An imaginary line drawn through the mouth
to the centre of the surface of attachment marks out the
longitudinal axis of the body. The tentacles are disposed
radially with regard to this axis. There is a single internal
cavity communicating with the exterior only by the mouth, and
this cavity is at once the receptacle of food, the organ of diges-
tion, and the means whereby the products of digestion are
distributed. This structural plan, exhibited in its simplest
form in Hydra, is diagnostic of the Ccelenterata, but is variously
modified and disguised in the different classes into which that
phylum is divided. Particular attention should be given to
the fact that there are only two primary cell layers, ectoderm
and endoderm, in the ccelenterate body, and that these corre-
spond with the outermost and innermost germ-layers — the
epiblast and hypoblast of the frog's embryo. The third middle
layer, or mesoblast, is entirely absent in Ccelenterates (for the
mesoglcea is not a cellular layer), and hence these animals are
sometimes distinguished as diploblastic or two-layered animals.
The name Ccelenterata calls attention to the important char-
acteristic of a single internal cavity which, from its fulfilling
the functions of digestive and circulatory systems is called the
gastrovascular cavity.

Careful microscopical examination of the ectoderm and
endoderm shows that these two layers are differentiated so as
to perform various functions which in the higher three-layered
animals are performed by the mesoblast. Both ectoderm and
endoderm, being layers of cells spread over external or internal
surfaces, come under the definition of epithelia (p. 80).
The ectoderm, covering the outside of the body, is essentially
the protective and sensory layer; the endoderm, lining the
internal cavity, is essentially the digestive and secretory layer.
The two layers become continuous with one another at the
lips of the mouth.

Partly because of their delicacy, partly because of the
intimate union of their component cells, the tissues of Hydra
are very difficult to study, Two methods are available. The

228 COMPARATIVE ANATOMY

animal may be killed and hardened by plunging it suddenly
in a mixture of acetic acid and corrosive sublimate, or, prefer-
ably, in the mixture of chromic, osmic, and acetic acids, known
as Flemming's fluid. It is then imbedded in paraffin, cut into
very thin sections with a microtome, and stained. A combination
of borax-carmine, and picronigrosin following corrosive sub-
limate, or safranin and light green following Flemming's fluid,
gives the best results. But in either case the action of the
reagents and the paraffin has such a destructive effect on the
tissues that many of the finer details are lost.

The second method is that of isolation. The animal is
placed from five to ten minutes in a very weak mixture of osmic
and acetic acids,* and, after being washed for a minute in
distilled water, is placed for at least 24 hours in a mixture
of equal parts of dilute glycerine and Ranvier's picrocarmine.
It is then carefully removed and placed on a glass slide in a
fresh drop of dilute glycerine. Slight shaking suffices to
separate the cells from one another, or, if they still adhere, the
layers may be stripped off with a fine hair mounted in a brush-
holder. The coverslip should be put on cautiously and
supported by a hair. If the preparation is successful, the
individual cells will come apart and can be very satisfactorily
studied as they are stained by the picrocarmine.

The ectoderm differs in structure in the tentacles, basal disc,
and body wall. That of the body wall will be described first.
Each cell is somewhat the shape of an inverted cone, the
broad end external and the narrow internal end produced into
a tapering process which rests upon, or rather is continued into,
one or more contractile fibres placed at right angles to the long
axis of the cell. (Fig. 49, A.} The contractile fibre is as much as
•38 mm. long, and lies close against the surface of the mesoglcec
parallel to the long axis of the animal's body. It is invested
throughout its length by a fine coat of protoplasm, continuous
with the protoplasm of the body of the cell. The protoplasmic
investment is raised into a number of little prominences, and
so looks jagged and indented on its lower surface. The body
of the cell is composed of coarsely alveolar protoplasm, the

* Schneider recommends the following mixture : A '02 % solution of
osmic acid, I part. A 5 % solution of acetic acid, 4 parts. This mixture
gives good results, but macerates rapidly. The tissues should not be left
in it longer than four minutes.

HISTOLOGY OF HYDRA

229

alveolar walls forming a reticulum of denser substance, which,
in the lower moiety of the cell, is drawn out into a network of
fibrils which may be traced into connection with the basal
contractile fibre. A large nucleus, with a distinct nucleolus
(sometimes there are two nucleoli) and a network of very fine
chromatin particles, occupies the middle of the cell. The ex-
ternal moieties of adjacent cells are fused together by intimate
union of their protoplasmic walls, hence it is very difficult to

ii

Fig. 49.

Isolated cells from Hydra fusca. A, a group of ectoderm cells from the body
wall showing the muscular fibres. B, an ectoderm cell from the basal disc
showing the secretory granules. C, two interstitial cells from the ectoderm.
D, an interstitial cell dividing, very highly magnified. £, an endoderm
cell, not very highly vacuolated with muscular process, nucleus, and
plastids. F and G, two forms of gland cell from the endoderm of the body
wall.

isolate a single cell, and if one is isolated its edges look torn
and jagged, as is shown in the outermost cells in fig. 49, A.

The cells are limited externally by a cuticle which, under the
highest powers of the microscope, is seen to contain a number
of minute refringent particles. When the animal is fully
extended the cuticle forms a continuous even layer over the
surface. When it is contracted, and its walls are thrown into

230 COMPARATIVE ANATOMY

furrows and wrinkles, the cuticle dips down into the valleys
between the wrinkles. To sum up, the ectoderm of Hydra is
mainly composed of columnar epithelial cells, differing from an
ordinary epithelium chiefly in the fact that each cell has formed
at its base one or more long contractile muscular fibres. Each
cell, then, partaking of the characters of an epithelial and an
unstriped muscular cell, is called an epithelio-muscular cell.
It should be borne in mind that the muscle fibres are relatively
very long, and that therefore they extend upwards and down-
wards among the similar muscle-fibres of adjacent cells,
forming a regular longitudinal layer of muscle-fibres closely
applied to the mesoglcea.

The other elements of the ectoderm of the body wall are the
sub-epithelial or interstitial cells and the nematocyst-forming
cells or cnidoblasts. As the epithelio-muscular cells taper
towards their bases spaces are left between them which in the
distal moiety of the body wall are occupied by numerous round
or cubical cells, each having a nucleus with a nucleolus. Their
nuclei are considerably smaller than those of the epithelio-
muscular cells. These rounded cells may be called indifferent,
because they are as yet undifferentiated and are destined to
give rise to other elements, cnidoblasts and sexual cells.
Frequently they may be seen in active division, and their
products may be observed to develop into cnidoblasts or into
ova or spermatozoa. The majority go to form cnidoblasts.
A nematocyst, whether of the large oval, cylindrical or small
oval kind, is always the product of a single cell, and is formed
inside that cell. In the upper part of the body wall one can
see nematocysts in all stages of growth. First a clear space
appears in an interstitial cell; this space enlarges, it acquires
a definite wall, and its contents stain deeply. Presently it
elongates, and one end is produced to form the thread, which
at its first appearance is everted and coiled round the outside
of the sac. After a time the thread is introverted — it is not
quite clear how — and the nematocyst assumes its final form.
When nearly ripe a nematocyst, still contained in its mother-
cell or cnidoblast, migrates into the inside of an epithelio-
muscular cell and approaches the surface. The external end
of the cnidoblast is produced to form a cnidocil which per-
forates the cuticle, and the gun is, so to speak, loaded and
cocked, ready to explode on the pressure of the trigger — the

HISTOLOGY OF HYDRA 231

cnidocil. It is a most remarkable thing that, whilst nematocysts
occur in all parts of the ectoderm, with the exception of the
basal disc, and especially in the tentacles, the developing
stages of nematocysts are almost exclusively confined to the
distal moiety of the body wall, where the indifferent interstitial
cells are most abundant. The exploded nematocysts of the
tentacles must be renewed, and since they are not formed in
situ, they can only be replaced by the new capsules formed in
the distal region of the body wall. How this replacement is
effected is not known. It can only be by migration of the
cnidoblasts, but the fact of migration has not been satisfactorily
established, though as it has been shown that the upper moieties
of the epithelio-muscular cells are confluent and form a practic-
ally continuous sheet of protoplasm, there appears to be no
obstacle to migration.

In the late spring and summer months, when Hydra repro-
duces itself sexually, the indifferent interstitial cells at certain
defined spots multiply rapidly by division and form masses of
germ cells bulging out the ectoderm. Their further develop-
ment will be described later on.

Observation of a living Hydra has shown that it is highly
irritable. Irritation of the tentacles will generally cause the
whole animal to contract. This would seem to imply the
existence of a mechanism whereby impulses generated at one
part of the body may be conducted to all other parts and cause
the muscle-fibres of the epithelio-muscular cells to contract in
unison. In short, it would seem to imply the existence of a
nervous system. It is impossible to find a trace of a nervous
system in sections. Nor has the writer been able, after numerous
attempts with the aid of the most approved methods of isola-
tion, to demonstrate the existence of structures which could
unhesitatingly be called nerve-fibres or nerve ganglion-cells.
It is extremely doubtful whether any differentiated nervous
elements occur in Hydra, but a nervous system has been
described in the form of a sub-epithelial network of branching
cells whose processes form a layer of varying thickness lying
close on the muscle-fibres in all parts of the body. Some of
the branches are said to run upwards between the epithelio-
muscular cells and end in or upon them ; others to make a
similar connection with the cnidoblasts. These nerve cells are
said to be found between the lower ends of the endoderm as

232

well as the ectoderm cells, and to be so abundant on the hypo-
stome as to form an ill-defined nerve ring near the bases of
the tentacles.

The ectoderm of the tentacles consists almost exclusively
of epithelio-muscular cells : they are large, rather flat, and
provided with several longitudinally -disposed muscle-fibres.
When the tentacle is contracted each cell is transversely
elongated and bulged outwards, giving rise to the transverse
wrinkles already noted. When the tentacle is extended the
central part of each cell forms a projecting lump, whilst its
periphery is thinned out to a thin protoplasmic layer. The
central lump contains a large nucleus with a distinct nucleolus,
and a variable number, often as many as twelve, of nemato-
cysts imbedded in the cell substance. Usually there is one
— sometimes two or three — large oval nematocysts occupying
the centre of the swelling, and around these are several of
the two other kinds of nematocysts — viz. cylindrical and small
oval. The arrangement can best be understood by an examina-
tion of fig. 48, B. It will be noticed that each nematocyst is
contained in a cnidoblast which is itself imbedded in the
epithelio-muscular cell. Each cnidoblast is furnished with a
cnidocil projecting through the cuticle, the cnidocils of the
cylindrical and small oval nematocysts being longer than those
of the large oval form. The cell-bodies of the smaller cnido-
blasts are produced internally into a fibril which, on reaching
the mesoglcea, bends sharply and runs backwards amongst the
muscle-fibres. The large cnidoblasts have shorter and less
conspicuous fibres. There can be little doubt that these fibres
are contractile, and belong to the same category as the muscular
processes of the epithelio-muscular cells.

The ectoderm of the basal disc consists almost entirely of
columnar epithelio-muscular cells which are filled with small
refringent granules and have no external cuticle. A few
interstitial cells may be found among their bases, but no
nematocysts. A single cell from the basal disc is shown in
fig. 49, B. Its outer half is longitudinally fibrillated and along
each fibril is arranged a row of bright refringent corpuscles
which are the secretum of the cell. Thus the basal cells are
glandular as well as epithelial and muscular. Their function
is to secrete a sticky substance by means of which the animal
can adhere to a foreign body. Occasionally the basal cells

HISTOLOGY OF HYDRA 233

send out pseudopodial projections from their free surfaces, and
possibly the animal is able to effect slight changes of position
by means of these pseudopodia.

The endoderm lines the whole of the gastrovascular cavity,
including the cavities of the tentacles, and consists primarily
of two kinds of cells, epithelio-muscular digestive cells and
secretory cells devoid of muscular processes.

The character of the endoderm differs in different regions of
the body. In the tentacles it consists wholly of much-vacuolated
epithelial cells which are prismatic and columnar when the
tentacles are retracted, flattened and elongate-oval in section
when the tentacles are extended. Each cell contains a single
large nucleus with a conspicuous nucleolus, and, in addition, a
number of rounded corpuscles and granules. The round
corpuscles in H. fiisca are yellow in colour ; they stain tolerably
deeply with certain dyes but do not show any trace of internal
structure. They are specialised protoplasmic structures,
capable of multiplying by division, and are connected in some
way unknown to us with the metabolism of the cell. In
Hydra viridis the corpuscles are very numerous and bright
green in colour. Each consists of a central protoplasmic body
containing one or more clear spaces with a minute central
corpuscle, which resembles a nucleolus not only in appearance
but also in the fact that it stains very brightly with certain dyes.
The central body is covered by an envelope of a somewhat
different protoplasmic substance containing chlorophyll. The
envelope is sometimes continuous, sometimes in the form of
two or three cap-like plates. The whole corpuscle recalls the
chromatophors found in so many Protozoa, and may be called
by the same name. Experiments have shown that Hydra
viridis is able to decompose carbonic acid in sunlight, setting
free bubbles of oxygen gas. It is not known in what form
the carbon is assimilated, for if an animal which has been
exposed for several hours to bright sunlight is killed, placed
in spirit to dissolve out the cholorophyll, and treated with
iodine, no trace of starch can be discovered either in the
chromatophors or in the endoderm cells in which they are
contained.

A single nutritive endoderm cell from the body of H. fusca
is shown on fig. 49, E. It is an elongate claviform epithelio-
muscular cell containing a large nucleus with a nucleolus and

234 COMPARATIVE ANATOMY

a number of granules and corpuscles of different kinds. Some
of these are yellow plastids, others are spherical balls made up of
a number of highly-refringent angular brown particles, prob-
ably products of metabolism. After a meal such a cell will
be loaded with bright yellow glistening globules, evidently of
a fatty nature, for they blacken with osmic acid. The endoderm
cells lining the proximal moiety of the gastrovascular cavity,
and, in a starving Hydra, all the endoderm cells, are vacuolated
to such an extent that the protoplasm is reduced to a mere
envelope forming the outside of the cell and a loose network
around the nucleus. The internal free end of the cell,
turned towards the gastrovascular cavity, has no cuticle, and
the naked protoplasm often sends pseudopodial processes into
the cavity. Normally each cell bears a pair of fairly long
flagella, whose lashing movements may be observed in a
transparent living specimen subjected to pressure. It seems
probable that the flagella can be withdrawn and pseudopodia
protruded in their place, but nothing is certainly known on
this head. At its outer end, abutting on the mesogloea, the
nutritive cell widens out suddenly and gives rise to a muscular
fibre resembling in all respects that of an ectodermic cell,
except that it is shorter, rather finer, and disposed transversely
to the long axis of the body. Hence whilst the ectodermic
fibres form a longitudinal, the endodermic fibres form a circular
muscular coat. The endoderm cells of the tentacles do not
appear to have muscular fibres.

A nutritive cell of H. viridis would be exactly the same as
the one described except that the yellow plastids would be
replaced by much more numerous chromatophors.

The endoderm cells are closely packed together, leaving
room for only a very few interstitial cells between their basal
ends. Being wider at their free ends than at their bases, they
are packed in bunches which look fan-shaped in section, as is
shown in the diagram fig. 48, A, When the animal is con-
tracted the endoderm cells are much elongated and the bunches
separated by deep valleys, project far into the gastrovascular
cavity. When the animal is extended the cells are correspond-
ingly stretched out, the valleys become shallow, and the whole
endoderm has the appearance of a rather irregular columnar
epithelium.

The gland-cells are absent from the endoderm of the

HISTOLOGY OF HYDRA 235

tentacles and basal disc, present, but in scanty number, in
the proximal region of the body, abundant in the distal region,
and very abundant in the hypostome. Two forms of gland-cell
are shown in fig. 49, F and G. Both have a wine-glass-shaped
body, the broader end turned towards the gastrovascular cavity,
the opposite end tapering to form a fine filament which runs
between the nutritive endoderm cells and is inserted by a
slight basal swelling on the mesoglcea. Near the thinner end
of the wine-glass-shaped body is a nucleus similar to but
smaller than the nuclei of the digestive cells. In the one kind
of gland-cell the body consists of dark finely-alveolar protoplasm
containing numerous minute granules and occasionally a few
larger globules. In the other kind, the free end of the cell — i.e.
the end nearest the gastrovascular cavity — consists of large
vacuoles separated by thin walls of transparent protoplasm, and
each vacuole is filled with a globule of highly-refringent
substance, the secretum of the cell. It is probable that the
finely-granular cells are simply transitional stages of the
coarsely-granular kind, which have discharged their secretion
and are forming a fresh supply.

The tissues of a water-flea swallowed by Hydra are rapidly
dissolved by the action of the secretion poured out by the
gland-cells, and if a Hydra is killed whilst this process of
digestion is going on, the nutritive cells are seen to be loaded
with the products of digestion in the form .of globules of a
proteid and fatty nature. There is also evidence that the
nutritive cells have the power of seizing solid particles by their
pseudopodia and ingesting them after the manner of an Amoeba.
If finely-divided carmine is injected into the gastrovascular
cavity, the endoderm is soon afterwards found to be full of
minute granules of carmine, generally aggregated in the form
of little balls and enclosed in a vacuole. Very frequently,
also, nematocysts are found embedded in the endoderm cells.
Usually they are somewhat shrunken and altered in appearance
as if acted upon by the digestive secretions of the cells, and
three or four are often rolled together in a ball and contained
in a large vacuole. Most probably these nematocysts have
been swallowed with the food and ingested by the endoderm
cells, to be afterwards ejected with other indigestible matters.
There is no evidence that nematocysts are formed in the
endoderm.

236 COMPARATIVE ANATOMY

Hydra multiplies itself in two ways — asexually, by means of
buds, and sexually, by the development of fertilised ova. In
budding the animal gives rise to a new individual very much
as a plant gives rise to a new shoot. At a spot about half-way
between the mouth and basal disc the ectoderm becomes
thickened by a proliferation of its interstitial cells, and then
all three layers, ectoderm, mesoglcea, and endoderm, are bulged
outwards to form a hollow projection from the surface. The
hollow projection enlarges to form a conical outgrowth, at the
distal end of which a ring of small knobs, the rudiments of the
future tentacles, make their appearance. The tentacles en-
large, a mouth is formed in their midst, and the outgrowth has
assumed the shape of a small Hydra whose tissues and gastro-
vascular cavity are continued into those of the parent form.
Several such buds may be formed at one time on a single
Hydra, and some of them may give rise by a similar process
to secondary buds, so that a little composite stock or colony
of Hydrae is produced, all having their gastrovascular cavities
in communication with that of the parent. Sooner or later,
however, the buds drop off and lead independent existences
as free individual Hydrae. It should be noticed that the method
of reproduction by budding, whilst undoubtedly asexual, differs
from the asexual method of reproduction observed in Volvox
and described as characteristic of the majority of plants. In
the latter case a single cell is produced which, without previous
fertilisation, segments and develops into a new organism. In
budding, the tissues of the parent pass over into the like tissues
of the offspring, ectoderm into ectoderm and endoderm into
endoderm.

The summer months appear to be the season of sexual re-
production for Hydra viridis, October and November for H.
fusca, but specimens bearing reproductive organs are never
very abundant. As a rule, both male and female organs are
produced by the same individual ; hence Hydra is said to be
hermaphrodite or monoecious. But several authors have de-
scribed broods of Hydra which produced either exclusively
male or exclusively female organs, so Hydra may be said to
be occasionally dioecious.*

* pdvos, single ; 8fe, twice ; OLKOS, a house.

DEVELOPMENT OF HYDRA 237

The gonads, whether male or female, may easily be distin-
guished from rudimentary buds by the fact that the latter
always contain a central fold of coloured endoderm, the former
never. The ovaries, of which there is generally only one, but
occasionally several, are developed at the upper end of the
posterior third of the body. The testes, generally two or three
and frequently more in number, are borne on the upper third
of the body wall, rarely extending downwards to the middle
and posterior thirds.

An ovary originates as an ectodermic swelling caused by
the rapid multiplication of indifferent interstitial cells. In this
manner a mass of primitive ova or oogonia is formed, but of
these only one undergoes further development and becomes an
oocyte. (See p. 120.) The remainder are arrested in growth
and go to form food material for the oocyte, which increases
greatly in size, takes up a central position amongst its fellows,
and begins to emit pseudopodial processes. As it grows it
seizes upon and ingests the arrested oogonia as an Amoeba
ingests its prey, storing up their digested products in the form
of a number of dark spherical corpuscles, which have been
called pseudocells, but which we had better call yolk corpuscles
or deutoplasts. Whilst these processes are going on — they
last for several days — the ectoderm cells in the neighbourhood
of the developing ovary swell up and form a covering for the
oocyte and the oogonia or reserve cells on which it feeds. By
the time that all the reserve cells are eaten up, the ovum has
increased enormously in size and the ectoderm cells covering
it are stretched till they form only a thin envelope around it.
The ovum withdraws its pseudopodia, assumes a hemispherical
shape, and by two successive unequal divisions gives rise to
two polar bodies. Whilst these are being formed a structure-
less gelatinoid substance is secreted between the ovum and its
covering cells. After the formation of the polar bodies the
ovum becomes quite spherical except for a flattened area where
it rests upon the mesoglcea. The covering cells become very
thin, break at the distal pole of the ovum, and shrink back-
wards towards its base, leaving its protoplasm naked except
for an envelope of the gelatinoid material secreted at the
previous stage. (Fig. 50, A.) The ovum is now ready for
fertilisation, which is effected in the normal manner, by the
entrance of a single spermatozoon.

238 COMPARATIVE ANATOMY

A testis originates in the same way as an ovary — viz. as a
proliferation of indifferent interstitial cells of the ectoderm
giving rise to a little mass of germ cells — spermatogonia. The
mass is enclosed by several epithelio-muscular covering cells,
whose expanded outer ends form the wall of the testis, their
elongated bodies forming columns which traverse the mass of
spermatogonia and imperfectly divide it up into compartments.
The spermatogonia develop into spermatocytes, and each
spermatocyte divides to form four spermatids, which develop
directly into spermatozoa. (See p. 118.) A spermatozoon of
Hydra has a conical head, consisting almost entirely of nuclear
matter, a small protoplasmic middle-piece, and a fairly long
vibratile flagellum.

After fertilisation, the ovum divides, still remaining attached
by a short stalk or foot to the parent. The segmentation is
total and regular. Two successive meridional divisions divide
the ovum into blastomeres, and the next equatorial division
produces an eight-celled stage. At this period, a space — the
segmentation cavity or blastoccele — appears between the inner
ends of the blastomeres. The final result of segmentation is a
hollow sphere or blastula, having a wall composed of a single
layer of cells surrounding a central cavity, the blastocoele.
(Fig. 50, £.}

It is a striking fact that in two animals so far apart in the
animal scale as the frog and Hydra the result of segmentation
is the same — viz. the formation of a blastula.

The next step is the formation of a two-layered embryo,
which is effected in Hydra by a process known as multipolar
immigration. During the growth of the blastula the planes of
division of the blastomeres were radial, but now several of the
cells undergo tangential divisions and the innermost of their
products pass into the blastoccele. Other cells, again, slip
bodily from their positions in the blastula wall and pass into
the blastocoele, and eventually the latter cavity is completely
filled up by cells which have been derived by these two
methods from the blastula wall. Immediately after this the
outermost layer of cells — i.e. those of the blastula wall —
divide rapidly by radial divisions and form a definite layer of
columnar epithelium sharply marked off from the inner mass of
cells. (Fig. 50, D, ec.) This outer layer may now be called the
ectoderm, the inner mass the endoderm.

DEVELOPMENT OF HYDRA

239

Fig. 50.

Development of Hydra. A, the ovum filled with deutoplasts ; ec,
ectoderm of the parent Hydra; mg, mesogloea ; en, endoderm ;
B, blastula stage, in which some cells of the blastula wall are
passing in towards the interior ; blc, segmentation cavity or
blastocoele. C, the blastoccele nearly filled up by immigrant cells.
D, an embryo further developed with sh, an external spiny, and ski,
an internal protective case ; ec, ectoderm ; i, inner solid mass of
cells. E, the embryo still enclosed in its double protective case,
flattening itself out. F, the embryo creeping out of its protective
cases ; lettering as in A and D. G, the empty case after escape of
the embryo. (D, original ; the rest after Brauer.)

24o COMPARATIVE ANATOMY

All this while the embryo remains attached to the parent,
and has no external protection save the thin gelatinoid coat
described above. The ectoderm cells now secrete a thick
external chitinous envelope covered all over with thorny pro-
cesses, and one result of their activity is that the deutoplasts
which they contained are absorbed and disappear. The
endoderm cells are still full of deutoplasts, and so the two
layers are still more sharply marked off from one another.
After the thick chitinous shell is formed, a second thin, homo-
geneous elastic envelope is formed within it, and about this
time the embryo is detached from the parent and falls to the
bottom. It is now a solid aggregate of cells, divided into an
outer layer with finely granular protoplasm, and an inner mass
stuffed with reserve material in the form of deutoplasts. The
whole is enclosed . by two protective envelopes. In this con-
dition the embryo passes into a resting stage and undergoes
no change for several weeks. The first sign of further
development is the appearance of a number of small rounded
cells between the inner ends of the ectoderm cells. These are
the interstitial cells. There is as yet no trace of a mesoglcea.
A further resting period follows the development of the
interstitial cells, and then the embryo, which has hitherto
been subspherical, takes the form of an egg, the smaller end
turned upwards, the broader resting on the bottom. The shell
cracks and its upper half comes away in pieces, allowing the
embryo to push the smaller end of its body out into the water.

The embryo is now more transparent than heretofore ; the
ectoderm and endoderm are well defined, and the mesoglcea
is established as a boundary between them. A clear space,
the beginning of the gastrovascular cavity, makes its appear-
ance in the endoderm of the emergent upper half of the body,
and this space gradually extends, probably as a result of the
disintegration of the central cells of the endodermic mass, to the
lower end. As the embryo protrudes itself further from the
shell its upper end expands and the tentacles are formed as a
ring of small hollow outgrowths. The mouth is formed as a split
in the centre of the ring of tentacles, placing the gastrovascular
cavity in communication with the exterior. The young Hydra
is now complete. It creeps out of the half of the shell in
which it has up till now been seated, attaches itself to some
other object and enters upon its adult career.

CHAPTER XIV
OBELIA GENICULATA

EVERYBODY who has been to the seaside in the summer
months is familiar with the large jelly-fishes which often float
in millions past the shores. But everybody is not so
familiar with the much smaller transparent jelly-fishes,
measuring some 3 mm. in diameter, which really are more
abundant. They can easily be caught in a fine muslin net
towed slowly behind a boat, and they are beautiful little
objects when turned into a glass jar or aquarium. So unlike
is one of these little jelly-fishes, or Medusae, to a Hydra that
nobody would suspect that it is at once the child and the
parent of a Hydra-like form, yet such is the case as the
present chapter will show.

The organism known as Obelia geniculata is a little stock
or colony formed by the repeated budding of a Hydra-like
person produced from the ovum. We have seen in the
last chapter how Hydra gives rise to lateral buds, and that
sometimes these may produce secondary buds before they
drop off from the parent stem. Now if a Hydra were to
grow to a considerable length and produce a single bud
which did not drop off but lengthened out and in turn pro-
duced another bud, and if this secondary bud produced
a tertiary and the tertiary a quaternary, and so on, a simple
colony of hydriform persons would result, all of whose
members would be united together and have their
gastrovascular cavities in connection. This condition is
realised in Obelia. There are several species in the genus
differing from one another chiefly in their mode of budding,
and consequently in the eventual form of the plant-like
colonies which result from budding. The species known as
Obelia geniculata is at once the simplest and most common
of the genus. It is to be found in great abundance just
Q 241

below low-water mark, generally growing on the broad fronds
of the oar-weed (Laminaria). A portion of a colony is shown

Fig- 5i-

A , part of a colony of Ol'dia geniculata magnified ; ///, hydrotheca contain-
ing a hydranth ; gt, a gonotheca enclosing a blastostyle with medusa buds ;
p, perisarc ; /, terminal growing point. B, a sexually mature female
medusa, seen from below; m, mouth; re, radial canal; cc, .circular or
ring canal ; go, gonads. C, diagrammatic longitudinal section through a
medusa ; m, mouth ; ml', manubrium ; gv, gastrovascular cavity ; re,
radial canal ; cc, ring canal ; el, endoderm lamella ; oc, ocellus ; at, otocyst.
The section is supposed to pass through a radial canal on the left side and
an adradial tentacle on the right. Kndoderm black ; mesogloca shaded ;
ectoderm represented by a broken line. D, the bases of two tentacles
magnified, showing oc, ocelli ; ot, an otocyst on an adradial tentacle ; cc,
ring canal.

in fig. 51, A. The colony consists of a root-like branching
stem, the hydrorhiza, which adheres to the oar-weed and

ANATOMY OF OBELIA 243

sends off at frequent intervals little branches some 10-
20 mm. in height. Each branch consists of a somewhat
zig-zag axis, the hydrocaulus, which at every bend gives off a
very short ringed branch ending in a little terminal polype
which, from its resemblance to a flower, is called a hydra-flower
or Hydranth. The hydrocaulus and its branches consist of a
delicate tube with a triple wall of ectoderm, mesoglcea, and
endoderm protected externally by a relatively stout but
transparent chitinous investment, the perisarc. The soft
tissues do not fill the cavity of the perisarc, but are bound
to it at intervals by processes of the ectoderm cells. At every
joint the perisarc is ringed, as shown in the figure, and the
little branchlets which bear the hydranths are made up of
three or four such rings. At the base of each hydranth the
perisarc is expanded to form a pretty vase-like cup, known as
the hydra-case or hydrotheca, into which the hydranth can be
withdrawn.

An individual hydranth does not differ very much in
structure from a Hydra, but its cell-elements are smaller and
in some particulars simpler. The tentacles are numerous,
twenty-eight to thirty in number, arranged in a single circle,
and they differ from those of Hydra in being solid instead
of hollow, the axis of each being occupied by a single row
of large discoid endoderm cells with stout walls of cartilaginoid
consistency. The interior of each of these cells is greatly
vacuolated, the protoplasm being reduced to a little column
surrounding the nucleus placed in the centre of the cell, and
a few radiating strands. The ectoderm is thin, but loaded
with nematocysts at the extremity of each tentacle.

The hypostome is enormous, forming a sort of ante-chamber
to the gastrovascular cavity, and the endoderm lining it is
richly supplied with gland-cells. Further details may be
learned from an inspection of fig. 52. The proximal part
of the hydranth is flat and rests upon a sort of shelf near
the bottom of the hydrotheca. The centre of this shelf is per-
forated by the neck of the hydranth or hydrocope, very narrow
at first, but soon swelling up to pass into the hydrocaulus.
A glance at the figures shows that the cavities of all the
hydranths communicate with the cavity of the hydrocaulus,
and so with one another.

The hydranths are solely nutritive persons. Their function

244

COMPARATIVE ANATOMY

is to catch, swallow, and digest prey, and, since they are all
in communication, there is a community of nutrition among

:V

Fig. 52-

Longitudinal section through a hydranth of Obelia
geniculata ', m, mouth ; Ay, placed in the cavity of
the hypostome ; gv, gastro vascular cavity ; ec,
ectoderm ; en, endoderm ; mg, mesogloea ; nc, nemato-
cysts on the tentacles ; en' , large vacuolated endoderm
of the solid tentacle; hi, hydrotheca.

the members of the colony. But a hydranth has nothing to
do with reproduction, except that it gives rise to a bud
in prolongation of the colony. (For it must be understood

ANATOMY OF MEDUSA 245

that although the apparent stem and flowers are called by
different names they are not organically different structures.
The hydranth itself only represents the upper end of a polype
the rest of whose body consists of the section of the hydrocaulus
between it. and the hydranth next following.) When the
colony has attained to a certain size special reproductive
persons are formed as buds in. the axils of the branches — i.e.
in the angles between the hydranths and the hydrocaulus.
These buds give rise to tubular outgrowths called blastostyles,
each blastostyle being nothing more than a degenerate
hydrifofm person without mouth and tentacles. It is enclosed
in a special investment of the perisarc called a gonotheca;
that of Obelia geniculata is shaped like a Greek urn. The
gonotheca and blastostyle together are sometimes called a
gonangium. A large number of buds is formed on the
sides of the blastostyle, and these, developing in a peculiar
manner, give rise to Medusae, which are set free and escape
from the terminal aperture of the gonangium into the water,
there to swim about as independent persons.

The Medusa-person of Obelia is shaped like an umbrella
with a very short, thick handle. It is quite transparent, and
its structure can easily be made out without dissection. The
upper convex surface of the umbrella is called the ex-umbrella,
the lower concave surface the sub-umbrella, the handle the
manubrium. The margin of the umbrella is fringed with-
tentacles whose number varies according to the age of the
medusa. At the time of liberation from sixteen to twenty-four
are present in the medusa of O. geniculata, and as growth
proceeds the number is increased by the development of new
tentacles between those already existing.

At the extremity of the manubrium is a cross-shaped opening,
the mouth, leading into a large gastrovascular cavity occupying
the whole extent of the manubrium. This cavity is covered
in above by the central part of the dome of the umbrella, but
it gives off peripherally four radial gastrovascular canals which
run at right angles to one another through the substance of
the umbrella and communicate with a ring-canal running all
round its margin. The four radial canals coincide in direction
with the four arms of the cruciform mouth and define the
principal secondary or radial axes of the medusa. As is the
case in all the Ccelenterata, the primary or chief axis of the

246 COMPARATIVE ANATOMY

body is defined by an imaginary line passing through the
mouth and the centre of the dome of the umbrella. The
secondary axes are disposed radially with regard to the chief
axis, and we may recognise : (i.) The perradial axes defined
by the four radial canals. At the extremity of each canal is
a tentacle, the base of which is swollen and covered by a
thickened patch of ectoderm containing pigment and nervous
cells. These patches are supposed to be sensitive to light, and
are called ocelli. (2.) The interradial axes are defined by
four imaginary straight lines bisecting the angles formed by
the perradii. At the end of every such imaginary line is
found an interradial tentacle having an ocellus at its base.
(3.) The adradial axes are defined by eight imaginary lines
bisecting the angles between the interadii and perradii. At
the end of each adradius is a tentacle having not only an
ocellus but also a little hollow vesicle containing calcareous
concretions at its base. The walls of this vesicle are formed
by modified ectoderm cells having fine hairs projecting from
their inner ends into the cavity of the vesicle. As these
vesicles are analogous to structures found in the auditory
organs of higher animals, the function of hearing has been
ascribed to them, and they are therefore called otocysts. It
is, however, much more probable that they are organs of balance
and not of hearing. The perradial, interradial, and adradial
tentacles are always developed at the time the medusa is
liberated from its blastostyle, and even before that time, and
always during subsequent growth, new cycles of tentacles are
formed, one in each interspace between tentacles already
existing. These newly-added tentacles have ocelli but not
otocysts at their bases. In Obelia, then, every tentacle has
an ocellus at its base, but only eight — namely, the adradials —
have otocysts in addition to ocelli. The presence of otocysts
on the margin of the umbrella is characteristic of the group of
Hydromedusae known as Leptomedusce. In the closely-allied
Anthomedusa ocelli are always present, otocysts never.

Fig. 51, C, is a diagrammatic representation of a median
longitudinal section through the medusa of Obelia. It will be
seen that the bulk of the umbrella is made up by the greatly
thickened jelly or mesoglcea. The ex-umbrella, sub-umbrella,
and manubrium are clothed externally by ectoderm, which
passes into the endoderm at the lips of the mouth. The

ANATOMY OF MEDUSA

247

endoderm lining the cavity of the manubrium is thick, and
composed of columnar nutritive, and numerous gland-cells,
but it is reduced to a layer of simple cubical cells in the
radial and ring canals. The diagram shows a feature which
is of importance in enabling a comparison to be established

en.

re -

Fig- 53-

Development of the medusa in Podoeoryne carnea, a hydroid allied to
Obelia ; ec, ectoderm ; 'en, endoderm ; bn, bell nucleus ; re, radial
canal ; cc, ring canal ; be, sub-umbrellar cavity ; tti, manubrium ; el,
endoderm lamella ; t, tentacles ; v, velum, a circular curtain running
round the mouth of the bell, not developed in the medusa of Obelia ;
ov, ova developing on the wall of the manubrium. For further
description of the figures see text.

between the structure of a medusa and that of an ordinary
Hydroid polype. The section is represented as being slightly
oblique, so that on the left side of the picture it passes through
a radial canal, but on the right side it passes through the solid
substance of the umbrella. It can be seen that the mesoglcea
is traversed by a thin sheet of solid endoderm continuous

248 COMPARATIVE ANATOMY

centrally with the endoderm of the manubrium and peripherally
with the endoderm of the ring-canal. This sheet, known
as the endo derm-lamella, extends right round the medusa,
and the radial canals and ring-canal are nothing more than
channels hollowed out in it. These relations will perhaps be
better understood after a consideration of the development of
a medusa as a bud on the blastostyle as represented by the
diagrams, fig. 53, A — E. In A the bud is shown as a simple
hollow outgrowth of the blastostyle wall. In B the ectoderm
cells at the extremity of the bud have multiplied and separated
into ec, an external epithelial layer continuous with the rest of
the ectoderm, and bn, a solid mass of cells pressed close
against the sac-like outgrowth of the endoderm. This solid
mass of cells is known as the bell-nucleus. In C the bell-
nucleus has increased in size, and a cavity has appeared in its
centre. As a consequence of its increase the centre of the
endodermic outgrowth has been further pushed in, whilst its
periphery embraces the bell-nucleus, so that the latter structure
lies like a ball in the mouth of a double-walled cup of endoderm.
In the next figure D the cavity of the bell-nucleus has increased
greatly in size, and the ectoderm cells surrounding it have
arranged themselves as an epithelial layer against the mesogloea
covering the walls of the endodermic cup. The last-named
has correspondingly increased in size, and forms a deep cup
with double walls. The space between the double-walls is a
part of the gastrovascular cavity, and is still in free communica-
tion with the cavity of the blastostyle. A further structure has
made its appearance at the bottom of the cup in the shape of
a simple finger-like outgrowth of endoderm pushing before it
the mesogloea and the ectoderm of the bell - nucleus. The
essential parts of the medusa are now established. The cavity
of the bell-nucleus is the sub-umbrellar cavity. The walls of
the endodermic cup, appearing as two horns in longitudinal
section, are the endoderm lamella, still double, and enclosing a
cavity reaching all round the walls of the bell or umbrella.
The outgrowth at the bottom of the cup is the primordium of
the manubrium. In the next figure E the medusa is nearly
complete. The tissues at the extremity of the bud have
thinned out and finally broken through, converting the cavity
of the bell-nucleus into the sub-umbrellar cavity. The manu-
brium has increased in size, and projects freely into the sub-

COMPARISON OF MEDUSA AND POLYPE 249

umbrellar cavity; a mouth will be formed at its extremity.
The double walls of the endoderm lamella have fused to-
gether over the greater part of the umbrella, but remain
separate at its margin and also in four radially disposed lines
reaching from the margin to the manubrium. Thus the ring-
canal and radial canals are the relics of a space which has else-
where been obliterated by the fusion of the endodermic walls
containing it. The first four tentacles are formed as out-
growths of the margin of the umbrella at the ends of the four
radial canals ; the four interradial tentacles are next formed
and then the eight adradials. The medusa is now complete,
but still attached by a short pedicle to the blastostyle. The
cavity in this pedicle is obliterated, the pedicle breaks across
and the medusa is set free, escapes through the mouth of the
gonotheca, and propels itself through the water by the rhythmical
contractions of the umbrella.

We are now in a position to make a comparison between
the structure of a medusa and that of an ordinary hydranth.
Imagine a hydranth, like that shown in fig. 52, to have its
body compressed in the longitudinal axis and the margins to
which the tentacles are attached to be pulled out so as to
form a shallow cup. Then it is clear that the body will
correspond to the umbrella of a medusa, the hypostome
to the manubrium, and the tentacles of the flattened
hydranth to the marginal tentacles of a medusa. Further-
more, it is clear that the gastrovascular cavity of the com-
pressed hydranth corresponds to the gastrovascular cavity of
a developing medusa, and that identity will be established
by the fusion of the two layers of endoderm, excepting in the
regions of the ring-canal and radial-canals, and by a great
increase in the thickness of the mesoglcea. The hydranth
and the medusa, much as they seem to differ from one
another, are therefore built upon the same plan, and their
several organs are homologous. But the free-swimming
medusa differs from the sessile Hydranth in its greater
histological differentiation. In the latter there are epithelio-
muscular ectoderm cells like those of Hydra. In the medusa
such cells occur on the sub-umbrella, but at the margins of
the umbrella and on the tentacles a more complete division
of labour is often established. Part of the ectoderm cells
take up a deeper situation and give rise to a layer of muscle-

250 COMPARATIVE ANATOMY

cells, the body of the cell being almost entirely used up in
forming a long fusiform contractile fibre resembling the
unstriped muscle- fibre described in the frog, but differing
from it in exhibiting a faint transverse striation. The rest
of the ectoderm cells form an epithelial layer lying over the
muscular layer. Further, the nervous system is better de-
veloped in the medusa than in the hydranth, consisting in
the former of a ring of branched ganglion cells running round
the margin of the umbrella near the bases of the tentacles
and a number of scattered ganglion cells lying close upon
the muscular layer of the sub-umbrella surface, their branched
processes inosculating with one another and with processes
of the ganglion cells of the marginal ring.

The free medusa is the sexual member of the hydroid
colony. At the time of its liberation it shows no trace of
reproductive organs, but after a while a finger-like down-
growth involving both ectoderm and endoderm, is developed
on each of the four radial canals. The reproductive organs
are formed on the walls of these down-growths. In Obelia
geniculata the generative cells, oogonia or spermatogonia,
originate in the ectoderm of the manubrium, migrate into
the endoderm, pass along the radial canals to the down-
growths, and there go through the early stages of their matura-
tion whilst still in the endoderm, but in the later stages they
lie between the ectoderji -and mesoglcea. The medusae are
dioecious, ova being developed in one individual and sperma-
tozoa in another. When the ova and spermatozoa have
arrived at maturity they are dehisced into the water by
rupture of the walls of the gonads, and, as medusae float
together in crowds near the surface of the sea, some of the
spermatozoa are sure to come in contact with the ova to
which, indeed, they are attracted as it were by a strong
chemical affinity (chemiotaxis). The ovum, after fertilisation,
divides and forms a hollow blastula like that of Hydra. The
further steps of development have not been followed out in
the case of Obelia geniculata, but are well-known in an allied
species, Clytia flavidula. In this species the blastoccele is
filled up, as in Hydra, by immigration of cells from the
blastula wall. But whereas in Hydra the immigrating cells
are derived from all parts of the blastula wall, it is stated
that in Clytia flavidula they are derived from one pole only,

DEVELOPMENT OF OBELIA 251

so that we have a process which very nearly resembles gastru-
lation — i.e. the folding of one half of a hollow blastula into
the other half so as to produce a two-layered sac or gastrula
opening to the exterior by a single aperture called the
blastopore. But in Clytia there is no blastopore. The result
of immigration is a solid embryo, having an external layer of
columnar cells forming an ectoderm and an internal solid
mass representing an endoderm. This embryo elongates
and becomes pear-shaped and the ectoderm cells acquire a
covering of cilia. In this stage it is known as a planula, a
larval form very characteristic of the majority of Ccelenterata
but not found in Hydra. The planula swims about for some
time by means of its cilia, and during its free existence a split
appears in the central mass of endoderm cells, forming the
first rudiment of the gastrovascular cavity. After a time it
settles down by its broader end on some convenient sub-
marine object, throws off its coat of cilia and begins to grow
into a hydriform person, the founder of a new colony. The
surface of attachment spreads out to form a disc clinging to
the weed or stone to which it is fastened, and the edges of
the disc soon become divided up into lobes which grow out
and form the branches of the root-like hydrorhiza. The
embryo elongates, its distal portion swells into a club shape,
it loses its coat of cilia and the gastrovascular cavity enlarges
whilst the endoderm cells take on the characters of the adult
endodermic epithelium. A mouth is soon formed at the
distal extremity, a circlet of tentacles grows out round the
mouth, and a chitinous tube, the perisarc, is secreted by
the ectoderm. The first hydra person is now complete. It
soon gives off a bud, and by continued repetition of the
budding process a new colony is formed ready to go through
the same life cycle as that just described.

A life-history as complicated as that of Obelia presents
many problems for consideration. In the foregoing descrip-
tion, by calling the different members of the colony by the
name of " persons " it has been tacitly assumed that they are
really individuals, which have only become secondarily united
to form a composite organism. And indeed one kind of
person, the medusa, displays a high degree of individuality
Inasmuch as it leads a free and separate existence. But why
are the other members of the stock or colony to be regarded

252 COMPARATIVE ANATOMY

as individuals united together rather than as analogues of the
members — the leaves and flowers — of a plant? Clearly be-
cause of the conditions which obtain in Hydra. When we
see a Hydra bearing one or more buds we do not regard the
aggregate as an individual body having members, because we
know that, if we watch it long enough, we shall see the buds
drop off and lead independent existences. Therefore we
ascribe individuality to the buds, even whilst they are
attached to the parent form. Nor, if a hydra -bud were
accidentally to remain in permanent connection with its
parent, should we hesitate to call it an individual and to
regard the united Hydra as a compound organism, a sort of
Siamese twin made up of two individuals joined together.
But if this reasoning holds good for Hydra it clearly holds
good for all hydroid colonies in which buds are produced in
the same manner as in Hydra. Therefore the members of a
hydroid stock, hydranths, blastostyles, and medusae (as well as
those other kinds of members which are found in allied
genera, such as the defensive persons in Hydractinia), are
regarded as individuals, and, since they differ in kind, the
colony formed by their union is called Polymorphic.

Now, reproduction or generation means simply this, that
one individual gives rise to another individual. Budding,
such as occurs in the Hydromedusas, is therefore an act of
reproduction ; and, as it is an asexual form of reproduction,
there is an alternation of generations, an indefinite number of
asexual generations alternating with a sexual generation,
represented in Obelia by the medusa.

This line of argument is universally adopted by zoologists.
It seems unexceptionable, but it has this result, that what
they mean by alternation of generations is something quite
different to what botanists mean by the same term. In
plants the asexual generation or sporophyte produces repro-
ductive cells or spores, each of which is capable, without
fertilisation, of giving rise to a new organism. This new
organism is not the same as the parent sporophyte, but a
different form called the gametophyte, producing reproductive
cells of two kinds, ova and spermatozoa, which are not by
themselves able to give rise to a new organism. But if the
two kinds conjugate, the product of their fusion, the oosperm,
is capable of reproduction, and gives rise not to the parent

ALTERNATION OF GENERATIONS 253

gametophyte but to the sporophyte. Thus there is a regular
alternation of sporophyte and gametophyte generations such
as is not found in any animal. At the same time either the
sporophyte or gametophyte generation (more rarely both)
in a plant is capable of vegetative multiplication of its
members, multiplication being effected by a process of bud-
ding analogous to that of the Hydroid stocks. And in many
plants members produced by vegetative multiplication may be
detached from the parent and grow into independent organ-
isms just as the buds of a Hydra are detached and grow into
independent Hydrae. Now, the process of budding in Hydra
and Obelia is clearly a process of vegetative multiplication, and
it is quite open to us to consider a composite stock resulting
from this process as an individual body of which the so-called
persons, the hydranths, blastostyles, and medusa, are the
members, just as the parts of a plant, the leaves and floral
organs, are regarded as the members of the individual plant
body. On such a view the medusae set free from a Hydroid
stock would be regarded as the analogues of the gemmae and
bulbils set free from so many plants, and we should apply the
term "individual" not to the various members of the stock
but to the whole aggregate of members, whether united or
detached, which have been produced by the continued growth
of the fertilised egg. In this case it would not be possible to
speak of an alternation of generations in Obelia and its allies ;
for it would be absurd to speak of a part of an individual as a
generation : and if we limit the term individual to the totality
produced from the egg, and consider, as we must, that "indi-
vidual" and "generation" are convertible terms, then there is
only one generation in Obelia, and that a sexual one.

Whilst zoologists still hold to the opinion that a hydroid
colony is a composite made up of many individuals joined
together, and therefore is polymorphic and exhibits alternation
of generations, it is by no means clear that the opposite view
of the individuality of the so-called colony is not preferable.
It is certainly more consistent with the conclusions arrived at
by a discussion of the theory of individuality. It is not
possible here to enter into the very complicated arguments
arising out of the question, What is an individual ? But the
.only answer which does not quickly land us in contradictions
and absurdities is the one given by the late Professor Huxley —

254 COMPARATIVE ANATOMY

viz. that "the individual animal is the sum of the phenomena
presented by a single life : in other words, it is all those animal
forms taken together which proceed from a single egg." Since
all the hydranths, blastostyles, and medusae which make up or
are derived from a colony of Obelia proceed from a single egg,
they comprise one individual; there is only one generation,
and consequently there can be no alternation of generations in
this or any similar Hydromedusan organism.

CHAPTER XV
ON CLASSIFICATION

HYDRA and Obelia both belong to the same class, Hydro-
medusas, of the phylum Coelenterata. A coelenterate animal is
radially symmetrical with a mouth opening into a single gastro-
vascular cavity serving alike for digestion and circulation ; its
body wall is formed by the two primary cell layers, ectoderm
and endoderm, and between these is a layer, more or less
thick, of a jelly-like substance, the mesoglcea. In the Hydro-
medusae the mouth opens directly into the gastrovascular
cavity, and the latter is not complicated by the presence of
partitions, ridges, or filaments in its interior. The other classes
of the Coelenterata are the Acalephse, the Anthozoa, and the
Ctenophora. The large jelly-fishes which abound in our seas
in the warm months belong to the Acalephse. In this class
the medusa is the dominant phase in the life-history of the
organism, and the gastrovascular cavity is furnished with gastral
ridges or gastral filaments. The Anthozoa comprise the sea-
anemones and corals. In them the mouth does not open directly
into the gastrovascular cavity, but into a short tube lined by
ectoderm which hangs down in and opens below into it, The
gastrovascular cavity is further complicated by the presence
of a number of radial partitions called mesenteries passing
from its walls very nearly to its centre. The Ctenophora are
transparent marine animals which swim through the water by
the united action of a number of comb-like plates of cilia.
In structure and. development they differ a good deal from
other Coelenterata, and must be regarded as an aberrant offshoot
of the phylum. Amongst other peculiarities they do not
possess nematocysts, but are provided instead with peculiar
adhesive structures known as lasso-cells.

In the accepted system of classification a phylum is divided
into classes ; a class into orders ; an order into sub-orders, and

255

256 COMPARATIVE ANATOMY

these into families ; a family into genera ; and a genus into
species. This system of classification and the nomenclature
universally used was invented by the great Swedish naturalist,
Carl Linnaeus, in the eighteenth century. Before his time there
was no system for naming animals, and no recognised subordina-
tion of groups into which animals could be placed according
to their affinities. Zoology and Botany alike were in confusion,
and no progress was possible. The classes, orders, and
families established by Linnaeus were arbitrary and artificial,
and have been replaced by new groupings as naturalists came
to recognise more and more clearly the natural affinities
between different animals. But the system of nomenclature
introduced by Linnaeus still is and probably always will be in
use. The system consists in giving every animal a Christian
name and a surname. The surname is the name of the genus
to which the animal belongs, and, in accordance with the
practice of the Latin language, it is put first. The Christian
name is the name of the species, and it is generally a trivial
name alluding to some small peculiarity of the species, or it
may commemorate the name of a naturalist or may be alto-
gether fanciful. Thus Obelia is the name of the genus,
geniculata is the name of the species, given to it because it is
bent at every joint like a knee (Latin gemt, a knee). A family
is a group of closely-allied genera, and is usually named after
one of the characteristic genera included in it. Thus the
genus Obelia belongs to the family Campanularidae, called
after the genus Campanularia closely allied to Obelia. An
order is a group of closely -related families, and a class is a
group of closely-related orders. This methodical arrangement
of groups subordinate to groups has obvious advantages : it
assists the memory, enables us to take a general survey of the
animal kingdom, and it tells us at a glance what animals are
more closely and what more distantly related to one another.

But the beginner should not forget that a modern system
of classification, in which animals are arranged like the
divisions, brigades, battalions, and companies of an army,
aims at much more than a mere methodical grouping for
the sake of precision and convenience. A good classification
attempts to arrange animals as far as is possible according
to their blood relationships ; in other words, to group them
like the branches of a genealogical tree. To use a homely

CLASSIFICATION 257

illustration, a genus may be compared to a number of human
families related to one another in the first degree of cousin-
ship. A zoological family may be compared to groups of first
cousins related to one another in more distant degrees of
cousinship. An order, again, to groups of distant cousins
related by remoter ties of tribal kinship. A class may be
compared to a number of tribes forming a nation, and a
phylum to a number of nations forming a great race, like the
Aryan or Mongolian race. The complete genealogy of a
tribe would show the descent of all the families composing
it from a common ancestor, the founder of that tribe, and
it would show also the various degrees of kinship not only
between the surviving families of the tribe but between all
the families existing at any given time in the past. Similarly,
the complete genealogy of a zoological order would show the
descent of all the species contained in it from a common
ancestor, and would show the degrees of kinship of all
existing species and of all pre-existing species at any given
period of time. But it need hardly be said that such a
complete genealogy does not exist and never will exist.
There is no continuous record. Zoologists can only infer
the degrees of affinity of species by their greater or less
structural resemblance to one another, aided by the very
imperfect and fragmentary evidence of descent furnished by
extinct forms whose remains have been preserved in a fossil
state. Any system of classification, therefore, depends, in
the first place, on the extent of our knowledge of the structural
differences which obtain among species, and in the second
place, upon the judgment shown by the framer of the system
in grouping together or separating species according to their
greater or less resemblances and differences. Consequently
there is no such thing as a final scheme of classification of
any group. Systems must change with the increase of our
knowledge and according to the varying judgments of the
authors who frame them. It also follows that there is more
certainty about the affinities of species and genera than of
the higher divisions, for it is easier to recognise near than
remote relationships. In fact, the limits assigned to classes,
orders, and families are somewhat arbitrary, and these divisions
in different phyla of the animal kingdom do not imply equal
degrees of affinity. Thus the classes in one phylum may

258 COMPARATIVE ANATOMY

stand in much closer relationship to one another than classes
in another phylum. If there were a complete genealogical
record it would be possible to make the various grades of
classification correspond to degrees of relationship; but, as
the record cannot be complete, such a correspondence is
impossible, and we can only deal according to the best of
our judgment with the evidence at our disposal. A classifica-
tion, then, can only be regarded as an approximate representa-
tion of animal affinities ; but, such as it is, it should represent
the fullest extent of our knowledge. Classifications must
change as knowledge advances.

I NDEX

[The names of species, genera, families, orders, etc., are printed
in Italics.]

Abducens nerve, 65
Acalephce, 255
Acetabulum, 32, 33
Achromatic figure, 144
Acini, 87

Actinosphcerium, 138
Activities, of animals, 5

— of cells, 80
Adipose tissue, 108
Adradial axis, 246
Adrenal bodies, 43
Afferent nerves, 9, 61
Alternation of Generations, Hydra,

219
Obelia, 252

— Protozoa, 202

Alveolar structure of Protoplasm,

191
Alveoli, of lungs, 42

— microscopic, 129

Amitotic division of nucleus,

Amoeba, 134
Amceba binudeata, 131, 134, 143

— crystalligera, 1 34

— verrucosa, 135

— villosa, 131

Amoeboid condition, Bodo, 171

— movement, 75, 128
Amosbulse, Badhamia, 151
Amphiaster, 114
Amphibia, 17, 19, 72
Amphiccelous vertebrae, 22
Amphipyrenin, 79
Ampulla of ear, 71
Anabolism, 8
Analogous organs,: 13
Anaphase, 115
Angulosplenial bone, 28
Animals, characters of, 5

259

Annulus of Vieussens, 68
Antebrachium, 17
Anterior abdominal vein, 54
Anthomedusce, 246
Anthozoa, 255
Anus, 7

— of Frog, 40

— of Paramecium, 195

— of Vorticella, 209
Aponeurosis plantaris, 21
Appendicular skeleton, 21
Aqueous humour, of eye, 69
Arcella, 135, 136
Architecture of animals, 10
Areolar tissue, 99 •
Artemia salina, 114
Arterial blood, 56
Arteries, 49

Arytenoid cartilages, 42

Ascaris megalocephala, 114, 118, 120

Asexual reproduction, 184

— in Hydra, 236
Astragalus, 33, 37
Astral figure, 112

— rays, 112
Atlas vertebra, 23
Auditory capsules, 26

nerve, 66

Auricle, of heart, 46
Auricular septum, 46
Auriculo-ventricular aperture, 46

— valves, 57
Axial skeleton, 21

Axis cylinder, of nerve, 92
Axis, primary and secondary of
body, 35

Bacteria, 160
Badhamia, 147

z6o

INDEX

Basal disc, Hydra, 221, 232
Bell-nucleus, 248
Beneden, E. van, 1 1 1
Bidder's ganglia, 69
Bilateral symmetry, 35
Bile capillaries, 87

duct, 41

Binary division, Actinosphserium,
142

Amoeba, 133

Bodo, 172

Euglena, 166

Pandorina, 179

Paramecium, 196

— Vorticella, 209

— Zoothamnium, 216
Bipolar nerve cells, 91
Blastocoele, 125, 238
Blastomeres, 122
Blastostyle, 245
Blastula, 125, 238
Blochmann, 131
Blood, 7, 45, 73

Bodo angustattts, 171
caudatus, 171

— globosus, 173

— saltans, 1 68
Bone, 104

Bone-corpuscles, 105
Brachial nerve, 60
plexus, 60

— vein, 52
Brachium, 17
Brain, 9

— of Frog, 58, 6 1

— case, 24

Branchial skeleton, 21, 28
Buccal cavity, 18
Budding, in Hydra, 236

— in Obelia, 241
Bulb (see Medulla), 62
Biitschli, 130

Calcaneum, 33, 37
Calcined cartilage, 102, 103
Campanulartdce, 256
Canaliculi, 104
Cancellous bone, 104
Capillaries, 49, 51
Capillitium, 151
Carbonic acid gas, 8, 45

Carchesium, 206, 215
Cardiac muscular fibre, 94, 97
Cardiac plexus, 68
Carotid arch, 48

— arteries, 48, 49, 57, 58

gland, 49, 57

Carpal bones, 31

Carpus, 17

Cartilage, 101

Cartilage bone, 26, 104, 106

Cauda equina, 60

Cell, definition of, 76, 80

Cell -colony, 109

Cell-division, in

Cell-theory, no

Cellulose, 153, 156

Centrale, 36

Central nervous system, 9

Centrosome, 116

Centrosphere, 112

Ceratophrys, 17

Cerebellum, 62

Cerebral-hemispheres, 62, 63

Cerebro-spinal axis, 58

Chalice cell, 85

Chalk, formation of, 137

Chemical change, 3

Chemiotaxis, 250

Chief cells, gastric glands, 87

Chlorophyll, 4

in Euglena, 163

— corpuscles in Hydra, 233
Chondrin, 104

Chorda dorsalis, 22
Choroid coat of eye, 69
Chromatin, 73, 79, in, 1 12
Chromatophors, in Euglena, 163

— in Hydra, 233
Chromosomes, 114
Chrysomonasjlavicans, 164
Cilia, 81, 189

Ciliary action, 82
Ciliata, 187
Ciliated Epithelium, 81
Circulation of blood, 58
Class, 19, 255
Classification, 255
Clavicle, 30
Cloaca, 40, 44
Clytiaflavidula, 250
Cnidoblast, 226, 230, 232

INDEX

261

Cnidocil, 226, 230
Coeliac artery, 5 1
Cceliaco-mesenteric artery, 49
Coelom, 38, 40, 45, 72
Ccelontata, 72
Ccenocyte, 77
Colony, Eudorina, 181

— Hydra, 236

— Obelia, 241

— Pandorina, 178

— Volvox, 184

— Zoothamnium, 216
Colour changes, in Frog, 16
Colpidium colpoda, 200
Columella auris, 7°
Columnar epithelium, 80
CombinationnucleusinParameciurn,

199

in Vorticella, 212

Comparative Anatomy, definition

of, 10

Compound organism, 217
Condyles, of humerus, 30
Conjugation, in Amoeba, 135

— in Bodo, 172

— in Eudorina, 182

— in Monocystis, 155, 159

— in Pandorina, 180

— in Paramecium, 197

— in Polytoma, 176
in Vorticella, 210

Connective tissue, 9, 20, 99
Connective tissue corpuscles, 100
Contractile fibres, in Hydra, 228

— in Vorticella, 207
Contractile filament in Vorticella,

206, 208
Contractile vacuole in Amreba, 127

— in Actinosphaerium, 142
in Bodo, 169

— in Euglena, 165

— in Paramecium, 191

— Polytoma, 174

— Vorticella, 209
Contractility, 6
Convergence, 153
Coracoid, 30
Cordre tendineoe, 47
Corium, 86
Cornea, 69
Cornua of hyoid, 28

Corpora bigemina, 62
Corpora striata, 65
Corpuscles of blood, 73, 74
Corpuscles of Purkinje, 91
Cortex, of Monocystis, 154
Cranial nerves, 65
Craniata, 24, 72
Cranium, 24,
Cricoid cartilage, 42
Crural nerve, 6 1
Crus, 17

Crypts of Lieberkuhn, 86
Ctenophora, 255
Cuboid bone, 33, 37
Cuneiform bone, 37
Cutaneous artery, 51
Cuticle, in Euglena, 162
in Hydra, 229

— in Paramecium, 190

— in Vorticella, 206
Cutin, 153

Cyclidium glaucoma, 131
Cyclosis, 193

Cyst, of Polytoma, 176
Cystic duct, 41
Cytoblastema, no
Cytoplasm, 78
Cytostome, 189

Death, 202, 217
Degeneration, 197, 202
Dentary bone, 28
Deutoplasm, 117
Development of Frog, 122

of Hydra, 237

— of Obelia, 250
Diastole, 192
Differentiation, 109
Difflugia, 136
Digits, of Frog, 34
Dioecious, 236
Diploblastic animals, 227
Disc, of Vorticella, 206, 208
Dog, 10

Dorsal aorta, 49

Dorsal fissure of spinal chord, 58
Dorsal root of spinal nerve, 61
Dorsal surface, of animals, 16
Ductus botalli, 49
Ductus choledochus, 41
Ductus endolymphaticus, 71

262 INDEX

Dujardin, 77
Duodenum, 40
Dura mater, 106
Dyads, 119

Ear of Frog, 70
Ectocuneiform bone, 37
Ectoderm, 224, 227, 228, 238
Ectoplasm of Amoeba, 127, 129

— of Euglena, 162

— of Paramecium, 190

— of Vorticella, 206
Ectoplastic products, 109
Ectosarc of Actinosphserium, 138

— of Badhamia, 148
Efferent nerves, 9, 61
Egg-nucleus, 120
Elastic fibres, 99
Elastin, 99, 141

Electric stimuli, effect of on Amoeba,

132

Embryo of Hydra, 240
Embryonic cells, 97, 103
Encystment in Actinosphrerium, 145

— in Amoeba, 135

— in Euglena, 168

— in Monocystis, 155
End cells, 88

Endoderm, 224, 227, 233, 238
Endoderm lamella, 248
Energy, manifestation of, 3
Endolymph, 71
Endomysium, 95
Endoplasm of Amoeba, 127, 129

— of Euglena, 162

— of Paramecium, 190, 193

— of Vorticella, 206, 209
Endoplastic products, 109
Endosarc of Actinosphserium, 138

— of Badhamia, 148
Endothelium, 83
Entocuneiform bone, 37
Envelope of Polytoma, 173
Environment, 4
Enzyme, 86

Eosinophilous granules, 100
Epicoracoid, 30
Episternum, 30
Epistylis, 215

Epithelio-muscular cell, 230
Epithelium, 10, 80

Equatorial cleavage, 124
Ethmoid, 27
Eudorina elegans, 181
Euglena viridis, 1 54

— sanguinea, 165
Euglenoid movement, 154, 162
Euglypha, 135, 136, 143
Eustachian tube, 70
Evolution, i, 219
Exchange of material, 3, 8
Excreta, 8

Exhaustion, 176
Exoccipital bone, 26
Expenditure of living body, 4
External carotid artery, 49
Exumbrella, 245
Eye of Frog, 17, 69

Facial nerve, 66

Falciform young, 157

Family, 256

Fat bodies, 45

Fat cells, 107

Female pronucleus, 198, 212

Femoral veins, 54

Femur, 17, 33

Fenestra ovalis, 70

Ferment, 86

Fertilisation, 118, 120, 216, 218,

237

Fibrous cartilage, 102
Fibulare, 36
Filum terminate, 24, 60
Fish, 10

Flagellata, 1 60, 185
Flagelluke, 151
Flagellum, 162, 169, 234
Fleming, in
Foams, microscopic, 129
Fol, in

Follicle, ovarian, 117
Food vacuole, of Ac'tinospho;ruim,

140

— of Amoeba, 130, 131

— of Paramecium, 194
Food yolk, 117

Foot of Frog, 33
Foramen magnum, 24, 26
Foramen of Munro, 63
Foraminifera, 136
Formative powers of cells, 80

INDEX

263

Fourth ventricle, 62
Pronto-parietal bone, 27
Fuligo sept tea, 153

Gall bladder, 41
Gametes, 176, 180, 197
Gametophyte, 252
Ganglion nervi vagi, 67
Gasserian ganglion, 66
Gastric artery, 51
Gastric glands, 41
Gastric vein, 55
Gastrocnemius muscle, 21
Gastro-duodenal omentum, 40
Gastrovascular cavity, 224
Gelatin, 99, 104
Genealogical tree, 256
Generative cells, 182
Genital ridge, 116
Genus, 256
Germ cells, 116, 217
Glands, 7, 85
Gland cells of Hydra, 234
Glandular epithelium, 86
Glenoid cavity, of shoulder, 30

— of skull, 28
Glossopharyngeal nerve, 67
Glottis, 18, 41
Glycogen, 18, 175, 194
Gnathosfomata, 18, 72
Gonads of Hydra, 237
Gonangium, 245
Goblet cell, 85
Gonotheca, 245
Gregarines, 154

Hsematids, 73
Hsematochrome, 165, 174
Haemoglobin, 73) 74
Hair cells, 88, 89
Hallux, 1 8

Haversian canals, 104
Heart, 7, 45, 46, 47
Hepatic artery, 51
Hepatic portal vein, 55
Hepatic vein, 54, 55
Heredity, 4
Hertwig, in
Heteromita (see Bodo}
Holophytic nutrition, 164
Holotricha, 190

Holozoic nutrition, 164, 171
Homologous organs, 13
Humerus, 30
Huxley, T. H., 253
Hyaline cartilage, 102
Hyaloplasm, 97
Hydra fusca, 221

grisea, 221

— virtdis, 221
Hydranth, 243
Hydrocaulus, 243
Hydromedusa, 255
Hydrorhiza, 242
Hydrotheca, 243
Hyoid, 28
Hyomandibular, branch of 7th

nerve, 66

Hypoglossal nerve, 60
Hypostome, 221, 243

Ilia, 32

Iliac artery, 51

Iliac vein, 54

Immigration, 238

Immortality of Protozoa, 203

Income of living body, 4

Individuality, theory of, 251, 252

Inferior oblique muscle, 65

Infundibulum, 62

Ingestion, 5, 130, 141, 150, 171, 195

Innominate vein, 52

Insertion of muscles, 21

Intermedium, 31, 36

Internal carotid artery, 49

Interradial axis, 246

Interzonal fibres, 115

Intestinal glands, 41

Intercellular substance, 101

Intracellular network, 97

Introversion, 226

Iris, 17, 69

Irritability, 9

Ischium, 32

Isolation, method of, 228

Iter (see Sylvian aqueduct)

Jaws, 7, 26

Jelly (see Mesoglcea)

Jugular veins, 52

Karyokinesis, 1 1 1

264 INDEX

Katabolism, 8
Kidneys, 43

Lacunae, bone, 104
Lamina terminalis, 63
Large intestine, 40
Laryngeal artery, 50
Larynx, 1 8, 41

Lateral ventricles of brain, 63
Lens, 69

Lenticular body, 88
Leptomedusce, 246
Leucocytes, 74, 75
Ligaments, 101
Ligamentum nuchre, 101
Line of Hensen, 96
Lingual artery, 49

— vein, 52
Linin, 79

Lin nreus, 256
Liver, 40
Liver cells, 87
Lobster, 10, 12
Lower jaw, 28
Lumbar artery, 5 1
Lunar bone, 37
Lungs, 8, 42
Lymph, 7, 20
Lymph hearts, 40

— Sacs, 20

Macrogametes of Eudorina, 182

— of Pandorina, 180

— of Volvox, 184

— of Vorticella, 210, 213
Macrogonidia, 216
Macronucleus of Paramecium, 191,

196

— of Vorticella, 209
Male pronucleus, 198, 212
Mandible, 28
Mandibular nerve, 66
Mandibular vein, 52
Manubrium, 245, 248
Mastigamaba, 185
Mastigophora, 160
Matrix of cartilage, 99
Maturation of ovum, 117, 118
Maupas, 201, 210
Maxilla, 27

Maxillary nerve, 66

Max Schultze, 78
Meckel's cartilage, 28
Medulla oblongata, 62
Medullary substance, nerve, 92
Medullated nerve fibre, 92
Medusa, 241, 245, 248, 249
Membrane bones, 27, 106
Membranes of Krause, 96
Mento-Meckelian bone, 28
Meridional cleavage, 123
Mesenteric artery, 51
Mesentery, 38
Mesocuneiform bone, 37
Mesoglcea, 224
Mesometrium, 44
Mesorchium, 43
Mesosternum, 30
Mesovarium, 44
Metabolism, 8, 133
Metacarpals, 32
Metaphase, 115
Metatarsals, 34
Metazoa, 185, 218
Metencephalon, 62
Microgametes of Eudorina, 182

— of Pandorina, 180

— of Volvox, 184
of Vorticella, 210

— of Zoothamnium, 216
Micronucleus of Paramecium, 191,

196

— of Vorticella, 206, 212
Microgromia, 136
Mitosis, in

— in Actinosphrerium, 143, 145

— in Amoeba, 134
in Badhamia, 150

— in Euglena, 166

— in Monocystis, 156

— in Paramecium, 196

— in Vorticella, 212
Mohl, H. von, 78
Monocystis agilis, 1 54

— magnet, 154
Monoecious, 236
Motor oculi nerve, 65
Motor root of nerve, 61
Mouth, 7, 17, 169, 189, 194, 221
Multicellular animals, 186
Multicellular glands, 86
Multipolar ganglion cells, 91

INDEX

265

Muscles, 20

Muscle fibres, Hydra, 230

— Medusa, 250
Muscular tissue, 6, 94
Musculo-cutaneous vein, 52
Musculus retractor bulhi, 65
Mycetozoa, 147, 153
Myelencephalon, 62
Myelin, 92

Nares, 17

Nasal bones, 27

Navicular bone, 33, 37

Nematocysts, 193, 224, 226, 230

Nerve, 9

Nerve fibres, 90

Nerve ganglion cells, 90

Nervous system, 8, 58, 231

Nervus recurrens, 67

Neural arch, 22

Neural canal, 22

Neural spine, 23

Neurilemma, 92

Neuroepithelium, 88

Node of Ranvier, 93

Non-medullated nerve fibre, 94

Non-striated muscle fibre, 94

Nostrils (see Nares)

Notochord, 22

Nucleated sheath of Schwann, 93

Nucleic acid, 79

Nuclein, 79

Nucleocentrosome, 166

Nucleolus, 112

Nucleoplasm, 78

Nucleus, 73, 74, 78, 79, in, 112

Nucleusof Actinosphrerium, 139, 143

— ofBadhamia, 148, 150

— of Bodo, 174,

— of Euglena, 163
Nutrition of plants and animals, 5

— of Euglena, 164

— of nerve fibre, 93
Nutritive cells of Hydra, 233

Obelia geniatlafa, 241
Occipital condyles, 24, 26
Occipital artery, 50
Occipito-vertebral artery, 50
Ocelli, 246
Oesophageal artery, 50

Oesophagus, 18, 40

Olecranon, 30

Olfactory capsules, 26

Olfactory cells, 88

Olfactory lobes, 63

Olfactory nerves, 65

Olfactory organ, 71

Omosternum, 30

Oocyte, 1 20

Oosperm, 126

Opisthocoelous vertebra', 22

Ophthalmic branch of 5th nerve, 66

Optic capsules, 26

Optic chiasma, 65

Optic lobes, 62

Optic nerves, 65

Optic thalami, 62

Orbital cavity, 28

Order, 255

Organ, 6

Organic continuity of cells, 184

Organisation of protoplasm, 77, 79

Origin of muscle, 21

Os cruris, 33

Os magnum, 37

Ossification, in cartilage, 107

— in membrane, 106
Osteoblasts, 106
Osteoclasts, 107
Osteogenic fibres, 107
Otocysts, 246
Otoliths, 71
Ovarian veins, 54
Ovaries, 43, 44, 237
Oviducts, 44

Ovum, 44, 117, 120, 218
Oxygen, 8, 45, 163
Oxyntic cells, 87

Palate, 19
Palatine bone, 27
Palatine nerve, 66
Pancreas, 40, 87
Pancreatic duct, 41
Pandorina niorum, 177
Paramecium aurelia, 187

— btirsaria, 1 88

— eaudatutn, 187
Paravicfba eilhardi, 135
Paramylum, 163
Parasitism, 158

266

INDEX

Parasphenoid bone, 27

Parietal cells, 87

Parthenogonidia, 184

Pathetic nerve, 65

Pavement epithelium, 83

Pelomyxa palustris, 131, 132

Pelvic girdle, 32

Pelvic vein, 54

Pelvis, 32

Pentadactyla, 35, 72

Pericardial cavity, 45

Pericardium, 45

Perilymph, 71

Perimysium, 95

Periosteum, 107

Peristome, 189

Peristomial groove, 206

Peritoneal fluid, 39

Peritoneum, 38

Peritricha, 206

Permanent vacuoles, 127

Peroneal nerve, 61

Perradial axis, 246

Pes, 17

Phagocytosis, 76

Phalanges, 32, 34

Phylum, 19, 255

Physiological division of labour,

no

Physiology, 10
Phytomastigoda, 177
Pineal eye, 63
Pineal gland, 62
Plain muscle fibre, 94,
Plants, characters of, 4
Planula, 251
Plasma, 73
Plasma cells, 100
Plasmatic cones, 143
Plasmodium, 147, 148, 173
Plastids, 234
Plastin, 143
Plastogamy, 142
Pleuroperitoneal cavity, 38
Pneumogastric nerve, 67
Polar bodies, 117, 120, 146
Polar plates, 143
Polymorphism, 252
Polytoma uvella, 173
Portal veins, 56
Post-axial structures, 36

Pre-arytenoid cartilages, 42
Preaxial structures, 35
Premaxilla, 27
Primary cysts of Actinosphcerium,

145

Primitive ova, 117
Primitive sheath of Schwann (see

Neurilemma)
Primordial cartilage, 103
Procoelous vertebras, 22
Pronation, 31
Prootic bone, 27
Prophase, 114
Prosencephalon, 62, 63
Protoplasm, 4

Protoplasm, structure of, 76, 79
Protoplasmic continuity, 81
Protozoa, 126
Pseudonavicella, 156
Pseudopodia, 75, 127, 140
Pterygoid bone, 28
Pubis, 32

Pulmo-cutaneous artery, 48
Pulmonary artery, 5 1
Pulmonary vein, 46, 55
Pupil, of eye, 17, 69
Pylangium, 48, 57
Pylorus, 40
Pyrenoids, 164

Quadrate cartilage, 28
Quadrato-jugal bone, 28

Racemose glands, 87

Radiale, 31, 36

Radial canals, 245, 248

Radiolaria, 136

Radio-ulna, 30

Radius, 30

Rami gastrici of roth nerve, 67

Rami pulmonales of loth nerve, 67

Ramus communicans iliacus, 54

Ramus communicans of sympathetic

nerves, 67

Ramus cardiacus of loth nerve, 67
Rana esculent a, 16
oxyr&tttus, 16

— ieniporaria, 16
Recti muscles of eyeball, 65
Reducing division, 120
Regeneration of nerve fibre, 93

INDEX

267

Rejuvenation, 201
Remak, R., 78, no
Remak's ganglion, 69
Renal portal veins, 54
Renal veins, 54
Reproduction, 4

— of Actinosphaerium, 142

— of Amoeba, 133

— of Badhamia, 1 50

— of Euglena, 166

— of Hydra, 236

— of Paramecium, 196

— of Polytoma, 175

— of Vorticella, 209
Reproductive organs, Obelia, 250
Reproductive system, 10
Reservoir, 165, 209
Respiratory mechanism, 7
Resting stage, 168, 176, 180
Retina, 69
Rhinencephala, 63
Rhizopoda, 136
Ribs, 29

Rima glottidis, 42
Ring canal, 245, 248
Rods and cones, 88

Sacral vertebra, 32

Sacculus, 71

Saprophytic nutrition, 164, 171, 175

Sarcode, 77

Sarcolemma. 96, 97

Sarcomeres, 96

Sarcoplasm, 96

Sarcostyles, 96

Sarcous elements, 96

Scaphoid bone, 37

Scapula, 30

Schleiden, no

Schneider, A., in

Schwann, Th., no

Sciatic artery, 51

Sciatic nerve, 60

Sciatic plexus, 60

Sciatic vein, 54

Sclerotic, 69

Sclerotium, 147

Secondary cysts, 145

Secretions, 7

Sections, method of, 228

Segmentation cavity, 125

Segmentation of ovum, 122

Semicircular canals, 71

Semilunar valves, 48, 57, 58

Sense capsules, 26

Sensory root of nerve, 61

Serial homology, 35

Sex, 173, 177, 201, 214

Sexual development of Hydra, 236

Shoulder girdle, 30

Sinus venosus, 46, 56

Skein, 112

Skeleton, 6

Skull, 22, 24

Solar plexus, 68

Somatic cells, 182, 217

Special senses, 9

Species, 2, 256

Spermatids, 118

Sperm nucleus, 120

Sperm sacs, 158

Spermatocytes, 118

Spermatogonia, 118

Spermatozoa, 117, 118, 122, 218

Sphenethmoid bone, 26

Spinal cord, 58

Spinal nerves, 60, 61

Spindle, mitotic, 114

Spiral valve, 48

Spireme, 112

Spleen, 43

Splenic artery, 51

— vein, 55
Spongioplasm, 97
Sporangium, 151
Spores, 135, 145, "S1. 17*
Sporoblasts, 156
Sporophore, 158
Sporophyte, 252
Sporozoa, 154
Sporozoites, 157
Springing Monad (see Bodo)
Squamosal, 28
Stalk of Vorticella, 206
Starch, in Infusoria, 163, 175
Stegocephalia, 63
Stereum hirsutum, 147
Sternum, 30
Stigma, 165
Stock (see Colony)
Stomach, 40
Stomata, 39

268

INDEX

Stratified epithelium, 84

Stratum Malpighi, 85

Streaming movements, 130, 141,

148, 193

Striped muscle, 94, 96
Subclavian artery, 50

vein, 52

Subscapular vein, 52

Subumbrella, 245, 248

Superior oblique muscle, 65

Supination, 31

Suprascapula, 30

Suspensory apparatus of jaw, 27

Sylvian aqueduct, 62

Symbiosis, 188

Sympathetic ganglia, 68

Sympathetic nerve system, 58, 67

Symphysis, 33

Synangium, 48, 5?

Syncytium, 152

Systemic aortic arch, 48, 57

Systole, 191

Tadpole, 28

Tarsus, 17

Taste cells, 88, 89

Teeth, 7, 18

Telophase, 115

Tendo Achillis, 21

Tendons, 21, 101

Tentacles of Hydra, 221, 224

— of Obelia, 230, 243
Testes, 43, 237
Testicular veins, 54
Tetrads, 119
Thalamencephalon, 62
Third ventricle, 62
Tibiale, 36
Tibial nerve, 61
Tibio-fibula, 33
Tissue, 6
Toad, 18
Toes, 1 8
Tongue, 19
Touch cells, 88
Touch spots of Merkel, 88
Tractellum, 162

Transverse process of vertebra, 23
Trapezoid bone, 37
Trapezium bone, 37
Trigeminal nerve, 66

Trochlea, 30
Trochlear nerve, 65
Transitional epithelium, 85
Trichocysts, 192
Truncus arteriosus, 46, 48, 57
Tympanic cavity, 18, 70
Tympanic membrane, 1 7
Typical organs, 35

Ulna, 30
Ulnare, 31, 36
Unciform bone, 37
Undulating membrane, 195, 206
Unicellular animals, 186
Unicellular structure, 218
Unipolar nerve cells, 91
Unity of Nature, 186
Urea, 8
Ureter, 43
Urinary bladder, 40
Uriniferous tubules, 43
Urogenital artery, 51
Urostyle, 22, 24
Uterus, 44
Utriculus, 71

Vacuolated cells, 234

Vagus nerve, 67

Vasa efferentia, 44

Vegetative cells, 217

Vegetative Multiplication, 253

Veins, 49, 52

Vena cava inferior, 46, 54

— superior, 46, 52
Venous blood, 56
Vent, 17

Ventral fissure of spinal cord, 58
Ventral root of nerve, 61
Ventral surface, 16
Ventricles of brain, 61
Ventricle of heart, 46
Venules, 51
Vertebra, 22
Vertebral artery, 50
Vertebral column, n, 22
Vertebrata, 18, 72
Vesical vein, 54
Vesicula seminalis, 43
Vestibule of Vorticella, 206
Virchow, ill
Vision, in Euglena, 165

INDEX 269

Vitelline membrane, 117 White fibrous tissue, 101

Vitreous humour of eye, 69

Vocal chords, 42 Xiphisternum, 30

Vocal sacs, 42,

Vol-vocince, 178

Volvox globator, 182 Yolk, 117, 122

Volvox minor, 185 ^ olk granules, 145

Vomerine teeth, 18

Vomer, 27 Zoothamnium, 206, 215

Vorticella monilata, 205 Zygapophyses, 22, 23

Zygote, 155, 172, 176, 185

White fibres in cartilage, 99 Zymogen granules, 86

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