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Thread by thread the patient hand

Till they bind us, neck and wrist;

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Must untwine ere free we stand.

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—JOHN BOYLE O’REILLY

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hae ome we oe 8 Bw

BY THE SAME AUTHOR

Text-book of the Embryology of Man and Mammals

Translated from the Third German Edition by
Epwarp L. Mark, Ph.D., '
Hersey Professor of Anatomy in the Harvard University.

With 339 Figures and 2 Lithographic Plates. 21s. ($5.25)

Lonpon: SWAN SONNENSCHEIN & co
New, York: MACMILLAN & CO

i

1/
THE CELL

OUTLINES OF

GENERAL ANATOMY AND PHYSIOLOGY

BY

DR. OSCAR HERTWIG

Professor Extraordinarius of Anatomy and Comparative Anatomy, Director of the
Lf, Anatomical Institute of the University of Berlin

TRANSLATED BY M. CAMPBELL, AnD EDITED BY

HENRY JOHNSTONE CAMPBELL, M.D

Assistant Physician to the City of London Hospital for the Diseases of the Chest
and to the East London Hospital for Children
Sentor Demonstrator of Biology and Demonstrator of Physiology tn Guys Hospital

WITH 168 ILLUSTRATIONS

London
SWAN SONNENSCHEIN & CO

NEW YORK: MACMILLAN & CO
1895

BS

Burien & TANNER, é ,
Tue SELwoop PrintInG Works, it es
FROME, AND LONDON. : een

TO HIS FRIEND AND COLLEAGUE

Wo WALDEYVI-R

‘¢ Each living being must be considered a microcosm, a small universe, which
is formed from a collection of organisms, which reproduce themselves, which
are extremely small, and which are as numerous as the stars in heaven.”

Darwin.

A GLANCE at the numerous text-books on histology shows us that
many questions of great interest in scientific investigation are
scarcely mentioned in them, whilst many branches of knowledge
which are closely connected with histology are more or less
excluded. The student is taught the microscopic appearances
which are presented by the cell and the tissues, after these have
been prepared according to the different methods which are most
suitable to each, but he is taught very little of the vital properties
of the cell, or of the marvellous forces which may be said to
slumber in the small cell-organism, and which are revealed to us
by the phenomena of protoplasmic movements, of irritability, of
metabolism, and of reproduction. With regard to the different
subjects which he studies, if he wish to be in touch with the
progress of science, and to understand the nature and attributes
of the cell-organism, he must read the works of specialists.

It is not difficult to discover the reason for this; it is chiefly
due to thé division of what was previously one subject into two,
namely, into anatomy and physiology. This sub-division has
been extended to the cell, and, it seems to me, with rather un-
fortunate results; for the separation which, in spite of the many
disadvantages which are naturally attached to it, is in many
respects a necessity in the investigation of the human body as a
whole, is not practicable in the study of cells, and has in reality
only brought about the result, that the physiology of the cell has
been dogmatically treated as a part of descriptive anatomy, rather
than as a science, and that in consequence much that the diligence
of scientists has brought to light is barren of results. In this book
I have avoided the beaten track, and in order to emphasise this

vii

Vill AUTHOR'S PREFACE

fact, I have added to the principal title of the whole work, “ The
Cell and the Tissues,” the secondary title “Outlines of General
Anatomy and Physiology.” Further, I am able to say, as I said
of my Text-book of Embryology: Man and Mammals, that it has
been produced in close connection with my academical labours.
The contents of the first part, in which I have endeavoured to
sketch a comprehensive picture of the structure and life of the
cells, were the subject of two lectures which I delivered at the
University of Berlin four years ago, under the titles of ‘“‘ The Cell
and its Life,” and “The Theory of Generation and Heredity.”

Besides wishing to communicate to a larger circle of readers
the views which I had often expressed verbally, I had the further
desire of giving a comprehensive review of results obtained by
private research, some of which were recorded in various Journals,
whilst others appeared in the six papers on “ The Morphology and
Physiology of the Cell,” which I wrote in conjunction with my
brother.

Finally, a third reason which induced me to write this book
was, that it should supplement my Teat-book of Embryology: Man
and Mammals. In it I have endeavoured to state the laws which
underlie animal formation, according to which cells, formed from
the fertilised egg-cell by repeated division, split up, as a result
of unequal growth, the complicated layers and outgrowths into
germinal folds, and finally into individual organs.

In addition to the distribution of cell-masses and to the
arrangement of cells, that is to say, in addition to the morpho-
logical differentiation, a second series of processes, which may be
grouped together under the term histological differentiation, takes
place during development. By. means of histological differentia-
tion, the morphologically separated cell material is capable of
performing the different functions into which the vital processes
of the developed collective organism may be divided.

In my Text-book of Embryology, it was impossible to deal ex-
haustively with the second or more physiological side of the pro-
cess of development. The Anatomy and Physiology of the Cell,
forms a necessary complement to it, as I mentioned above. This
will be especially noticed by the student in the first part of
the book, which deals with the cell alone. For not only is there,
in the seventh chapter, a detailed description of the anatomy and
physiology of reproduction, which is ultimately a cell pheno-
menon, but at the end of the book, in the ninth chapter, there

AUTHOR’S PREFACE 1x

is a section entitled “ The Cell as the Elemental Germ of an
Organism,” in which both the older and more recent theories of
heredity are dealt with.

The second part of the complete work, which is to deal with the
tissues, will be of about the same length, and will form to a greater
extent a supplement to the Teat-book of Embryology. For in
addition to a description of the tissues, especial emphasis will be
laid upon their origin of histogenesis and upon the physiological
causes which underlie the formation ; the other side of the process
of development, that is to say, histological differentiation, will also
be discussed.

In the account, which I have endeavoured to make as intelligible
as possible, scientific views have primarily guided me. What I
have striven to do to the best of my ability is, to fix the scientific
stand-point occupied at present by the doctrines of cell and tissue
formation. Further, I have tried to delineate the historical course
of the development of the more important theories. With regard
to disputed points I have frequently compared various opinions.
If, as is natural, I have placed my own views in the foreground,
and, moreover, if I have occasionally differed from the views and
explanations of prominent and highly-esteemed scientists whose
opinions I value extremely, it is only due to them to say that I do
not on that account consider the conceptions preferred by me to be
unconditionally correct, still less do I wish to belittle the views
from which I differ. Antagonistic opinions are necessary to the
life and development of science; and, as I have observed in
studying the history of the subject, science progresses most
rapidly and successfully in proportion to the diversity of the
opinions held by different authorities. As is only human, almost
all observations and the conclusions deduced from them are one-
sided, and hence continually need correction. How necessary then
must this be in the subject of the present inquiry, that is to say,
in the cell, which is a marvellously complicated organism, a small
universe, into the construction of which we can only laboriously
penetrate by means of microscopical, chemico-physical and experi-
mental methods of inquiry.

Oscar Hertwic.

Berlin, October, 1892.

hy See oe
ED Sis wh
seca gh

if eae -
Ag fee ahs

EDITOR'S PREFACE

THE translation of Professor Hertwig’s book has been no easy
task. The extreme complexity of much of the matter treated, in
addition to the large number of subjects referred to, has often
rendered it extremely difficult to express the author’s meaning in
readable English. Of one thing there can be no doubt, and that is,
that the subject matter is of very great importance; moreover, it
cannot but prove most useful to the student who does not read
German fluently, to possess in English so comprehensive an
account of the Anatomy and Physiology of the Cell, as the one
contained in Professor Hertwig’s book.

In many cases it has been extremely difficult to find equivalents
for terms used in the German. Amongst these the word
“ Anlage ” may be specially mentioned. Various terms have been
used by different translators to express the meaning of this word,
but none of them seems to be applicable to all cases. Professor
Mark has introduced the word “‘fundament,” and Mr. Mitchell has
suggested the term “ blast,’’ but neither of these appears to express
the meaning of the German word sufficiently accurately to justify
the use of either of them exclusively. Hence, we thought it best
in some cases to employ the somewhat cumbrous expression,
“elemental germ,” although it is undoubtedly open to objection ;
however, it frequently seemed to us to convey the author’s idea
most correctly. On other occasions we have thought better to
make use of a paraphrase.

Several additions have been made to the Bibliography of papers

x

xii EDI'TOR’S PREFACE

that the English student might wish to consult. The frequent
quotations from English authors have in most cases been verified
by reference to the originals; but in some cases, despite careful

search, we have been unable to find the passages in question.

H. Jounstone CaMpBett.

54, Welbeck Street, London, W.

CON TENES

CHAPTER I.
Introduction
The History of the Cell Theory:
The History of the Protoplasmic Theory
Literature . ; : : 2

CHAPTER II.
THE CHEMICO-PHYSICAL AND MorpHoLoGicaL PROPERTIES OF THE CELL

I. The Chemico-physical and Morphological Properties of the Proto-
plasm : P
(a) Justification of the a of the Tern Pesvontaen
(b) General Characteristics of Protoplasm
(c) Chemical Composition of Protoplasm .
(ad) The more minute Structure of Protoplasm
(e) Uniformity of Protoplasm. Diversity of the Cell .
(f) Various examples of the Structure of the Cell-body
1. Cells consisting almost entirely of Protoplasm
2. Cells which contain several different substances in
their Protoplasm :
Il. The Chemico-physical and Moepnorogieal Properties of itis Nudlens
(a) The form, size and number of Nuclei é
(b) Nuclear Substance . : : : . : 6
(c) The Structure of the Nucleus: Examples of its various
Properties ,
III. Are there Elementary Orvantams existing Woiont Nuclei?
IV. ‘lhe Central or Pole Corpuscles of the Cell
V. Upon the Molecular Structure of Organised Bodies
Literature ;

CHAPTER III.
Tus Viran PRopERTIES OF THE CELL

The Phenomena of Movement
I. Protoplasmic Movements
(a) The Movements of naked Pearonlagn
(b) The Movements of Protoplasm inside the Cell. hckahrans

(c) Theories concerning Protoplasmic Movements
xiii

xiv CONTENTS

PAGE

II. Movements of Flagella and Cilia . : 3 ; : : Ae
(a) Cells with Flagella . 5 ; i : A - . Abe

(v) Cells with numerous Cilia. -. 83

III. The Contractile Vacuoles, or Vesicles, re Unicellular Gesaniatun Ae 19)
IV. Changes in the Cell vee passive movement . ~ ; 3 Oe

Literature s j ; 5 : , R B02 SD
CHAPTER IV.
Tue Virat Properties oF THE CELL ; ps ‘ ; x J Spee: jt
_ Phenomena of Stimulation . ; 5 S ; : B x «Ok
I. Thermal Stimuli . . 2 . : . : . 94
Il. Light Stimuli : . 3 : . ; v : a Br
‘III. Electrical Stimuli . pies A ; : : : . 106
Phenomena produced by Gavduditopian 5 s é A s - 108
IV. Mechanical Stimuli : A y ‘ is : R A + 219
VY. Chemical Stimuli . 4 a : >.
(a) Chemical Stimuli which affect the whole ag : 112

(b) Chemical Stimuli which come into contact with the Cell. body
at one spot only . : A ; . ° . + op

1. Gases’ . : ie : c : : : ee

2. Liquids : ‘ ; . A : ; Fees ti hf

Literature ; ; : : z : : ‘ ‘ : A - 123
CHAPTER V.

Tue VitTaL PRopERTIES OF THE CELL 7 . ‘ 4 : - - 126

‘Metabolism and Formative Activity : ; ; : Zz : . 126

I. Absorption and Excretion . : ; - 128

1. The Absorption and Excretion of Gasdoes Matotial - - 128

2. The Absorption and Excretion of Fluid Substances ; . 138

3. The Absorption of Solid Bodies . 4 ; - lad

II. The Assimilative and Formative Activity of the Cell : é 545

1. The Chemistry of Assimilation . 2 : x - 146

2. The Morphology of Metabolism . , 3 é , . 154

(a) Internal Plasmic Products . : ‘ : ‘ - 154

(b) External Plasmic Products . : : 3 ‘ - 166

Literature. : . 2 ‘< A ; , : j : . 174

CHAPTER VI.
THE ViraL PHENOMENA OF THE CELL é 3 ‘ % : ; mer Wig

Reproduction of the Cell by division . . ° . . . ewe if
I. History of Cell-formation A ‘ £ . : Pe ‘ Piya yh
- It. Nuclear Division . ; ; ° m ‘ i ‘ ‘ «179

CONTENTS

1. Nuclear Segmentation. Mitosis (Flemming) ; gaiiaey
(Schleicher)
(a) Cell division as it occurs in Silemenapa eicdata é
First Stage. Preparation of the Nucleus for Division.
Second Stage of Division
Third Stage of Division
Fourth Stage of Division
(b) Division of the Egg-cells of Asian is fiagatscenhata and
Toxopneustes lividus .
(c) Division of Plant Cells : ;
(d) Historical remarks and unsolved piohlenan concerning
Nuclear Segmentation . ; 3 P :
2. Direct Nuclear Division. Pemeneied. hraitoaie
3. Endogenous Nuclear Multiplication, or the Formation of
Multiple Nuclei .
III. Various methods of Cell Multiplication
1. General Laws .
2. Review of the Various Modes of Cell Division:
la. Equal Segmentation
1b. Unequal Segmentation
le. Cell-Budding .
2. Partial or Meroblastie Bepmentation
3. So-called Free Cell Formation
4. Division with Reduction : : :
IV. Influence of the Environment upon Cell Division: Dovenkenuoa :
Literature

CHAPTER VII.
Tae ViraL PROPERTIES OF THE CELL

The Phenomena and Methods of Fertilisation . ,
I. The Morphology of the Process of Fertilisation .
1. The Fertilisation of the Animal Egg
(a) Echinoderm Eggs
(b) Eggs of Ascaris egadccephank
2. The Fertilisation of Phanerogamia
3. The Fertilisation of Infusoria ,
4. The various forms of Sexual Cells ; euaiyalenies of artic?
pating Substances during the Act of Fertilisation ; Con-
ception of Male and Female Sexual -Cells
5. Primitive and Fundamental modes of Sexual Generation
and the first appearance of Sexual Differences
Il. The Physiology of the Process of Fertilisation
1. The Need of Reproduction of Cells
(a) Parthenogenesis .
(b) Apogamy
2. Sexual Affinity

PAGE

179

182

239

278

291

xvi CONTENTS

(a) Sexual Affinity in general
(b) More minute discussion of Sexual Affinity, and its
different gradations
a. Self-fertilisation :
8. Bastard Formation, or Hybridisation
vy. The Influence of Environment upon Sexual Affinity
6. Recapitulation and Attempted Explanations .
Literature ° ; ; : ‘ ; ; .

CHAPTER VIII.

Mertasponic CHANGES OCCURRING BETWEEN Protopriasm, NUCLEUS AND CELL
Propucts . ; . z

I. Observations on the Position of the Nucleus, as an indication of its
participation in Formative and Nutritive Processes .
Il. Experiments proving Reciprocal Action of Nucleus and Pistonleien
Literature

CHAPTER IX.

Tue CELL AS THE ELEMENTARY GERM OF AN ORGANISM. THEORIES OF
HEREDITY

I. History of the older Theories of Development ‘ F
II. More Recent Theories of Roproduction and Development
III, The Nucleus as the Transmitter of Hereditary Elemental Gevitig's
1. The Equivalence of the Male and Female Hereditary Masses
2. The equal Distribution of the Multiplying Hereditary Mass
3. The Prevention of the Summation of the Hereditary Mass .
4. Isotropy of Protoplasm
IV. Development of the Elemental Germs
Literature : : : ; x
Tadex .

- B57

PAGE

301

305
306
310
313
316
320

323

324
330
332

334

335
339
344
345
346
350
354

361
363

DA © Ei

—++—

CHAPTER I
INTRODUCTION

Boru plants and animals, although they differ so widely in their
external appearance, are fundamentally similar in their anatomical
structure; for both are built up of similar elementary units,
which, as a rule, are only to be seen with the microscope. These
units, in consequence of a hypothesis which was once believed in,
but is now discarded, are called cells; and the view that plants
and animals are built up in a similar manner of these extremely
minute particles is called the cell-theory. The cell-theory is
rightly considered to be one of the most important and funda-
mental theories of the whole science of modern biology. In the
study of the cell, the botanist, the zoologist, the physiologist, and
the pathologist go hand in hand, if they wish to search into the”
vital phenomena which take place during health and disease.
For it is in the cells, to which the anatomist reduces both plant
and animal organisms, that the vital functions are executed;
they, as Virchow has expressed it, are the vital elementary units.

Regarded from this point of view, all the vital processes of a
complex organism appear to be nothing but the highly-developed
result of the individual vital processes of its innumerable variously
functioning cells. The study of the processes of digestion, of the
changes in muscle and nerve cells, leads finally to the examination
of the functions of gland, muscle, ganglion, and brain. And just
as physiology has been found to be based upon the cell-theory, so
has the study of disease been transformed into a cellular pathology.

Hence, in many respects, the cell-theory is the centre around which
the biological research of the present time revolves.

Further, it forms the basis of the study of minute anatomy,
now more commonly called histology, which consists in the exami-
nation of the composition and minute structure of the organism.

: B

2 THE CELL

The conception or idea connected with the word “ cell,” used
scientifically, has been considerably altered during the last fifty
years. The history of the various changes in this conception, or
the history of the eell-theory, is of great interest, and nothing
could be more suitable than to give a short account of this history
in order to introduce the beginner to the series of conceptions
connected with the word “cell”; this, indeed, may prove useful
in other directions. For whilst, on the one hand, we see how
the conception of the cell, which is at present accepted, has
developed gradually out of older and less complete conceptions,
we realise, on the other hand, that we cannot regard it as final or
perfect ; but, on the contrary, we have every ground to hope that
better and more delicate methods of investigation, due partly to
improved optical instruments, may greatly add to our present
knowledge, and may perhaps enrich it with a quite new series of
conceptions.

The History of the Cell-Theory. The theory, that organ-
isms are composed of cells, was first suggested by the study of
plant-structure. At the end of the seventeenth century the
Italian, Marcellus Malpighi (I. 15), and the Englishman, Grew (I.
9), gained the first insight into the more delicate structure of _
plants; by means of low magnifying powers they discovered, in
the first place, small room-like spaces, provided with firm walls,
and filled with fluid, the cells ; and in the second, various kinds of
long tubes, which, in most parts of plants, are embedded in the
ground tissue, and which, from their appearance, are now called
spiral ducts or vessels.

Much greater importance, however, was attached to these facts
after the investigations, which were carried on in a more philo-
sophical spirit by Bahn towards the end of the eighteenth century,
were published.

Caspar Friedrich Wolff (I. 34,.13), Oken (I. 21), and others,
raised the question of the development of plants, and endeavoured
to show that the ducts and vessels originated in cells. Above all,
Treviranus (I. 32) rendered important service by proving in his
treatise, entitled Vom inwendigen Bau der Gewiichse, published in
1808, that vessels develop from cells; he discovered that young
cells arrange themselves in rows, and become transformed, by the
breaking down of their partition walls, into elongated tubes; this
discovery was confirmed and established as a scientific fact by the
subsequent researches of Mohl in 18380.

THE HISTORY OF THE CELL-THEORY 3

The study of the lowest plants has also proved of the greatest
importance in establishing the cell-theory. Small alge were
observed, which during their whole lifetime remain either single
cells, or consist of simple rows of cells, easily to be separated
from one another. Finally, the study of the metabolism of plants
led investigators to believe that, in the economy of the plant, it is
the cell which absorbs the nutrient substances, elaborates them,
and gives them up in an altered form (Turpin, Raspail).

Thus, at the beginning of our century, the cell was recognised
by many investigators as the morphological and physiological
elementary unit of the plant. This view is especially clearly
expressed in the following sentences, quoted from the Text-book of
Botany (1. 16),.published by Meyen in 1830: “ Plant-cells appear
either singly, so that each one forms a single individual, as in the
case of some alge and fungi, or they are united together in greater
or smaller masses, to constitute a more highly-organized plant.
Even in this case each cell forms an independent, isolated whole;
it nourishes itself, it builds itself up, and elaborates the raw
nutrient materials, which it takes up, into very different sub-
stances and structures.” In consequence, Meyen describes the
single cells as “ little plants inside larger ones.”

These views, however, only obtained general acceptance after the
year 1838, when M. Schleiden (I. 28), who is so frequently cited
as the founder of the cell-theory, published in Miiller’s Archives
his famous paper “ Beitrage zur Phytogenesis.” In this paper
Schleiden endeavoured to explain the mystery of cell-formation.
He thought he had found the key to the difficulty, in the discovery
of the English botanist, R. Brown (I. 5), who, in the year 1833,
whilst making investigations upon orchids, discovered nuclei.
Schleiden made further discoveries in this direction; he showed
that nuclei are present in many plants, and as they are invariably
found in young cells, the idea occurred to him, that the nucleus
must have a near connection with the mysterious beginning of the
cell, and in consequence must be of great importance in its life-
history.

The way in which Schleiden made use of this idea, which was
based upon erroneous observations, to build up a theory of phyto-
genesis, must now be regarded as a mistake (I. 27) ; on the other
hand, it must not be forgotten that his perception of the general
importance of the nucleus was correct up to a certain point, and
that this one idea has in itself exerted an influence far beyond the

4 THE CELL

narrow limits of the science of botany, for it is owing to this that
the cell-theory was first applied to animal tissues. For it is just
in animal cells that the nuclei stand out most distinctly from
amongst all the other cell-contents, thus showing most evidently
the similarity between the histological elements of plants and
animals. Thus this little treatise of Schleiden’s, in 1838, marks
an important historical turning-point, and since this time the
most important work, in the building up of the cell-theory, has
been done upon animal tissues.

Attempts to represent the animal body as consisting of a large
number of extremely minute elements had been made before
Schleiden’s time, as is shown by the hypotheses of Oken (I. 21),
Heusinger, Raspail, and many other writers. However, it was
impossible to develop these theories further, since they were
based upon so many incorrect observations and false deductions,
that the good in them was outweighed by their errors.

It was not until after some improvements had been made in
optical instruments, during the years from 1830-1840, that work
justifying the application of the cell-theory to animal tissues was
accomplished.

Purkinje (I. 22) and Valentin, Joh. Miiller (I. 20) and Henle
(I. 11), compared certain animal tissues with plant tissues, and
recognized that the tissue of the chorda dorsalis, of cartilage, of
epithelium and of glands, is composed of cells, and in so far is
similar in its construction to that of plants. Schwann (I. 31),
however, was the first to attempt to frame a really comprehensive
cell-theory, which should refer to all kinds of animal tissues,
This was suggested to him by Schleiden's “ Phytogenesis,” and
was carried out by him in an ingenious manner.

During the year 1838 Schwann, in the course of a conversation
with Schleiden, was informed of the new theory of cell-formation,
and of the importance which was attached to the nucleus in plant-
cells. It immediately struck him, as he himself relates, that
there are a great many points of resemblance between animal and
vegetable cells. He therefore, with most praiseworthy energy,
set on foot. a comprehensive series of experiments, the results
of which he published in 1839, under the title, Mikroscopische
untersuchungen tiber die Uebereinstimmung in der Structur und
dem Wachsthum der Thiere und Pflanzen. This book of Schwann’s
is of the greatest importance, and may be considered to mark an
epoch, for by its means the knowledge of the microscopical

THE HISTORY OF THE CELL-THEORY 5

anatomy of animals was, in spite of the greater difficulty of
observation, immediately placed upon the same plane as that of
plants.

Two circumstances contributed to the rapid and brilliant result
of Schwann’s observations. In the first place Schwann made the
greatest use of the presence of the nucleus in demonstrating the
animal cell, whilst emphasizing the statement that it is the most
characteristic and least variable of its constituents. As before
mentioned, this idea was suggested to him by Schleiden. The
second, no less important circumstance, is the accurate method
which Schwann employed in carrying out and recording his obser-
vations. As the botanists by studying undeveloped parts of
plants traced the development of the vessels, for instance, from
primitive cells, so he, by devoting especial attention to the history
of the development of the tissues, discovered that the embryo, at
its earliest stage, consists of a number of quite similar cells; he
then traced the metamorphoses or transformations, which the cells
undergo, until they develop into the fully-formed tissues of the
adult animal. He showed that whilst a portion of the cells retain
their original spherical shape, others become cylindrical in form,
whilst yet others develop into long threads or star-shaped bodies,
which send out numerous radiating processes from various parts
of their surface. He showed how in bones, cartilage, teeth, and
other tissues, cells become surrounded by firm walls of varying
thickness; and, finally, he explained the appearance of a number
of the most atypical tissues by the consideration that groups of
cells become, so to speak, fused together; this again is analogous
to the development of the vessels in plants.

Thus Schwann originated a theory which, although imperfect
in many respects, yet is applicable both to plants and animals, and
which, further, is easily understood, and in the main correct.
According to this theory, every part of the animal body is either
built up of elements, corresponding to the plant cells, massed
together, or is derived from such elements which have undergone
certain metamorphoses. This theory has formed a satisfactory
foundation upon which many further investigations have been
based.

However, as has been already mentioned, the conception which
Schleiden and Schwann formed of the plant and animal element was
incorrect in many respects. They both defined the cell as a small
vesicle, with a firm membrane enclosing fluid contents, that is to say,

6 THE CELL

as a small chamber, or cellula, in the true sense of the word. They
considered the membrane to be the most important and essential
part of the vesicle, for they thought that in consequence of its
chemico-physical properties it regulated the metabolism of the
cell. According to Schwann, the cell is an organic crystal, which is
formed by a kind of crystallisation process from an organic mother-
substance (cytoblastema).

The series of conceptions, which we now associate with the
word “cell,” are, thanks to the great progress made during the
last fifty years, essentially different from the above. Schleiden
and Schwann’s cell-theory has undergone a radical reform, having
been superseded by the Protoplasmic theory, which is especially
associated with the name of Max Schultze.

The History of the Protoplasmic theory is also of supreme interest.
Even Schleiden observed in the plant cell, in addition to the cell
sap, a delicate transparent substance containing small granules ;
this substance he called plant slime. In the year 1846 Mohl
(I. 18) called it Protoplasm, a name which has since become so
significant, and which before had been used by Purkinje (I. 24)
for the substance of which the youngest animal embryos are
formed. Further, he presented a new picture of the living
appearances of plant protoplasm ; he discovered that it completely
filled up the interior of young plant cells, and that in larger and
older cells it absorbed fluid, which collected into droplets or
vacuoles. Finally, Mohl established the fact that protoplasm, as
had been already stated by Schleiden about the plant slime, shows
strikingly peculiar movements ; these were first discovered in the
year 1772 by Bonaventura Corti, and later in 1807 by C. L.
Treviranus, and were described: as “ the circulatory movements of
the cell-sap.”

By degrees further discoveries were made, which added to the
importance attached to these protoplasmic contents of the cell.
In the lowest alge, as was observed by Cohn (I. 7) and others,
the protoplasm draws itself away from the cell membrane at the
time of reproduction, and forms a naked oval body, the swarm-
spore, which lies freely in the cell cavity; this swarm-spore soon
breaks down the membrane at one spot, after which it creeps out
through the opening, and swims about in the water by means of
its cilia, like an independent organism; but it has no cell mem-
brane.

Similar facts were discovered through the study of the animal

THE HISTORY OF THE PROTOPLASMIC THEORY 7

cell, which could not be reconciled with the old conception of the
cell. A few years after the enunciation of Schwann’s theory,
various investigators, Kolliker (I. 14), Bischoff (I. 4), observed
many animal cells, in which no distinct membrane could be dis- .
covered, and in consequence a lengthy dispute arose as to whether
these bodies were really without membranes, and hence not cells,
or whether they were true cells. Further, movements similar to
those seen in plant protoplasm were discovered in the granular
ground substance of certain animal cells, such as the lymph cor-
puscles (Siebold, Kolliker, Remak, Lieberkiihn, etc.). In con-
sequence Remak (I. 25, 26) applied the term protoplasm, which
Mohl had already made use of for plant cells, to the ground
substance of animal cells.

Important insight into the nature of protoplasm was afforded
by the study of the lowest organisms, Rhizopoda (Amcebe),
Myxomycetes, etc. Dujardin had called the slimy, granular,
contractile substance of which they are composed Sarcode. Sub-
sequently, Max Schultze (I. 29) and de Bary (I. 2) proved, after 7.
most careful investigation, that the protoplasm of plants and |
animals and the sarcode of the lowest organisms are identical.

In consequence of these discoveries, investigators, such as
Nageli, Alexander Braun, Leydig, Kolliker, Cohn, de Bary, etc.,
considered the cell membrane to be of. but minor importance in com-
parison to its contents ; however, the credit is due to Max Schultze,
above all others, of having made use of these later discoveries in
subjecting the cell theory of Schleiden and Schwann to a search-
ing critical examination, and of founding a protoplasmic theory.
He attacked the former articles of belief, which it was necessary
to renounce, in four excellent though short papers, the first of
which was published in the year 1860. He based his theory that
the cell-membrane is not an essential part of the elementary
organisms of plants and animals on the following three facts:
first, that a certain substance, the protoplasm of plants and
animals, and the sarcode of the simplest forms, which may be
recognised by its peculiar phenomena of movement, is found in
all organisms ; secondly, that although as a rule the protoplasm
of plants is surrounded by a special firm membrane, yet under
certain conditions it is able to become divested of this membrane,
and to swim about in water as in the case of naked swarm-spores ;
and finally, that animal cells and the lowest unicellular organisms
very frequently possess no cell-membrane, but appear as naked

Bs THE CELL

protoplasm and naked sarcode. It is true that he retains the
term “ cell,” which was introduced into anatomical language by
Schleiden and Schwann; but he defines it (I. 30) as: a small mass
of protoplasm endowed with the attributes of life.

Historical accuracy requires that it should be mentioned that
in this definition Max Schultze reverted to the older opinions held
by Purkinje (I. 22-24) and Arnold (I. 1), who endeavoured to
build up a theory of granules and masses of protoplasm, but with-
out much result, for the cell theory of Schwann was both more
carefully worked out, and more adapted to the state of knowledge
of the time.

The term, a small mass of protoplasm, was not intended by
Max Schultze and other investigators even then to mean so simple
a matter as appears at first. The physiologist, Briicke (I. 6),
especially came to the correct conclusion, gathered with justice
from the complexity of the functions of life, which are inherent in
protoplasm, that the protoplasm itself must be of a complex con-
struction, that is must possess “an extremely intricate structure,’
into which, as yet, no satisfactory insight has been gained owing
to the imperfections of our means of observation. Hence Briicke
very pertinently designated the “ultimate particle” of animals and
plants, that is the mass of protoplasm, an elementary organism.

Hence it is evident that the term “ cell” is incorrect. That it,
nevertheless, has been retained, may be partly ascribed to a kind of
loyalty to the vigorous combatants, who, as Briicke expresses it,
conquered the whole field of histology under the banner of the
cell-theory, and partly to the circumstance, that the discoveries
which brought about the new reform were only made by degrees,
and were only generally accepted at a time when, in consequence
of its having been used for several decades of years, the word cell
had taken firm root in the literature of the subject.

Since the time of Briicke and Max Schultze, our knowledge of
the true nature of the cell has increased considerably. Great
insight has been gainéd into the structure and the vital properties
of the protoplasm, and in especial, our knowledge of the nucleus,
and of the part it plays in cell-multiplication, and in sexual repro-
duction, has recently made great advances. The earlier definition,
“the cell is a little mass of protoplasm,’ must now be replaced. by
the following : “the cell is a little mass of protoplasm, which contains
in its intertor a specially formed portion, the nucleus.”

The history of these more recent discoveries will be entered

THE HISTORY OF THE PROTOPLASMIC THEORY 9

into later, being only incidentally mentioned here and there in
the following account of our present knowledge of the nature of
the elementary organism.

The enormous amount of knowledge which has been acquired
through a century of investigation will be best systematically
arranged in the following manner :—

In the first section the chemico-physical and morphological
properties of the cell will be described.

The second section will treat of the vital properties of the cell.
These are, (1) its contractility, (2) its irritability, (3) the phe-
nomena of metabolism, (4) its power of reproduction.

Further, in order to complete and amplify our account of the
nature of the cell, two sections more speculative in character will
be added, one treating of the relationship between the proto-
plasm, the nucleus, and the cell products, and the other of the
cell considered as the germ of an organism.

Literature lI.

1. Fr. Arnotp. Lehrbuch der Physiologie des Menschen. 2 Theil. Ziirich.
1842. Handbuch der Anatomie des Menschen. 1845.
2. DE Bary. Myxomyceten. Zeitschrift f. wissenschaftl. Zool. 1859.
3. Lionen S. Beate. On the Structure of the Simple Tissues of the Human
Body. 1861.
4. Biscuorr. Entwicklungs-geschichte des Kanincheneies. 1842.
5. R. Brown. Observations onthe Organs and Mode of Fecundation in Orchidee
and Asclepiadee. Transactions of the Linnean Soc., London. 1833.
6. Bricker. Die Elementarorganismen. Wiener Sitzungsber. Jahrg. 1861.
XLIV. 2. Abth.
CLELAND. On Cell Theories. Quar. Jour. Microsc. Sc. XIII., p. 255.
7. Coun. Nachtrige z. Naturgeschichte des Protococcus pluviatilis. Nova acta.
Vol. XXII., pp. 607-764.
8. Bonaventura Corti. Observazioni microsc. sulla Tremella e sulla circola-
zione del fluido in una pianta acquaiola. 1774.
Datuincer and DryspaLE. Researches on the Life History of the Monads.
Month. Mic. Journ. Vols. X.-XIII.
9. Grew. The Anatomy of Plants.
10. Harcxen. Die Radiolarien. 1862. . Die Moneren.
11. Henue. Symbole ad anatomiam villorum intestinalium. 1837.
12. Oscar Hertwic. Die Geschichte der Zellentheorie. Deutsche Rundschau.
13. Huoxuey. On the Cell Theory. Monthly Journal. 1853. ,
14. Koxuixer. Die Lehre von der thierischen Zelle. Schleiden u. Ndgeli
Wissenschaftl. Botanik. Heft 2, 1845.
Kéuiiker. Aanual of Human Histology, trans. Sydenham Society. 1853.

10.

15.
16.
17.
18.
19.
20.
21.
22.
23.
21.
25.

26.

27.

28.

29.
30.

31.

32.

33.

34.

THE CELL

MaxriecH1. <Anatome plantarum.

Meyen. Phytotomie. Berlin. 1830.

H. v. Monn. Ueber die Vermehrung der Pflanzenzellen durch Theilung.
Dissert. Tiibingen. 1835. Flora. 1837.

H. v. Moun. Ueber die Saftbewegung im Innern der Zellen. Botanische
Zeitung. 1846.

H. v. Moun. Grundziige der Anatomie und Physiologie der vegetabilischen
Zelle. Wagners Handwirterbuch der Physiologie. 1851.

J. Miuurr. Vergleichende Anatomie der Myxinoiden.

OxeNn. Lehrbuch der Naturphilosophie. 1809.

PurxinyzE. Bericht tiber die Versammlung deutscher Naturforscher und
Aertzte in Prag im September, 1837. Prag, 1838, pp. 174, 175.

Purkinje. Uebersicht der Arbeiten und Verinderungen der schlesischen
Gesellschaft fiir vaterliindische Cultur im Jahre, 1839. Breslau, 1840.

PorxinsE. Jahrbiicher fiir wissenschaftliche Kritik. 1840. Nr 5, pp.
33-38,

Remax. Ueber extracellulire Entstehung thierischer Zellen und iiber Ver-
mehrung derselben durch Theilung. Miillers Archiv. 1852.

Remax. On the Embryological Basis of the Cell Theory (trunslated).
OQ. FMS. AT. pearls

Sacus. Geschichte der Botanik. 1875.

Marruias ScHLEweENn. Beitriige zur Phytogenesis. Millers Archiv. 1838.
Principles of Scientific Botany, translated by Lankester. 1849.

Max Scuuuze. Das Protoplasma der Rhizopoden und der Pflanzenzelle.

Max Scuuuze. Ueber Muskelkirperchen und was man eine Zelle zu nennen
habe. Archiv fiir Anatomie und Physiologie. 1861.

Tu. Scuwann. Mikroscopische Untersuchungen tiber die Uebereinstimmung
in der Structur und dem Wachsthum der Thiere und Pflanzen. 1839.

Scuwann und ScHLeIDEN. Microscopical Researches, trans. Sydenham Soe.
1837.

C. L. Tkeviranus. Vom inwendigen Bau der Gewiichse, 1806.

R. Vircnow. Cellular Pathology as based upon Physiological and Patho-
logical Histology, trans. by Chance. 1860.

Casp. Friepr. Woirr. Theorie von der Generation. 1764.

CHAPTER II

THE CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES OF
THE CELL

The cell is an organism, and by no means a simple one, being built
up of many different parts. To ascertain with accuracy the true
nature of all these constituents, which, for the greater part, elude
our observation at present, will remain a problem for biological
research for a long time. Our position, with regard to the cell, is
similar to that of investigators towards the whole animal or vege-
table body a hundred years ago, before the discovery of the cell
theory. In order to penetrate more deeply into the secrets of the
cell, optical instruments, and, above all, methods of chemical
examination, must be brought to a much higher degree of perfec-
tion than they have attained at present. It seems best to me to
jay stress on these points to start with, in order that the student
may have them always before his mind’s eye in reading the follow-
ing account.

In each cell there is invariably to be seen one specially well-
defined portion, the nucleus, which throughout the whole of the
animal and vegetable kingdom is very uniform in appearance;
evidently the nucleus and the remaining portion of the cell have
different functions to perform in the elementary organism. Hence
the examination of the chemico-physical and morphological proper-
ties of the cell becomes naturally divided into two sections, the
examination of the protoplasm and of the nucleus.

To these, three short sections are added. The first deals with
the question, Are there cells which possess no nuclei? The
second treats of the pole or central corpuscles, which are at times
found as special cell-structures in addition to the nucleus; and in
the third a short account is given of Nageli’s theory of the mole-
cular structure of organic bodies.

I. The Chemico-physical and Morphological Properties
of the Protoplasm. Some animal and plant-cells appear to

differ so much from one another as to their form and contents,
1

12 ' PHE CELL

that, at first sight, they seem to have nothing in common, and
that hence it is impossible to compare them. For instance, if a
cell at the growing-point of a plant be taken and compared
with one filled with starch granules from the tuber of a potato,
or if the contents of an embryo cell from a germinal disc be com-
pared with those of a fat cell, or of one from the egg of an
Amphibian filled with yolk granules, the inexperienced observer
sees nothing but contrasts. Nevertheless, all these exceedingly
different cells are seen on closer examination to be similar in
one respect, z.e. in the possession of a very important, peculiar mix-
ture of substances, which is sometimes present in large quantities,
and sometimes only in traces, but which is never wholly absent
in any elementary organism. In this mixture of substances the
wonderful vital phenomena, which are dealt with later, may very
frequently be observed (contractility, irritability, etc.); and, more-
over, since in young cells, in lower organisms, and in the cells of
growing-points and germinal areas, it is in the cell-substance alone
(the nucleus of course being excepted) that these properties have
been observed, this substance has been recognised as the chief
supporter of the vital functions. It is the protoplasm or “forming
matter” of the English histologist, Beale (I. 3).

a. Justification of the Use of the Term Protoplasm.
In order to know what protoplasm is, it is advisable to examine it
in those cells in which it is present in large quantities, and in
which it is as free as possible from admixture with other bodies ;
and amongst such the most suitable are those organisms from the
study of which the founders of the protoplasmic theory formed
their conception of the nature of this substance. Such organisms
are, young plant-cells, Amcebe, and the lymph corpuscles of
vertebrates. After the student has learnt to recognise the cha-
racteristic properties of protoplasm in such bodies, he will be able
to discover it in others, in which it is only present in small

_ , quantities and is more or less concealed by other substances.

It has been proposed (II. 10) to give up altogether the use of
the term protoplasm, since it has been associated with such
mistaken views; for the word has now come to be used in so
indefinite and vague a manner, that it may be questioned whether
it is not at present more misleading than useful.

However, this proposition cannot be considered to be advisable
or even justifiable in the present condition of affairs, for, although
it must be admitted that the word is frequently used incorrectly ;

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 13

and that further, it is impossible in a short phrase to give an
adequate definition of its meaning; and finally, that frequently it
is difficult to determine what part of the cell really consists of
protoplasm, and what does not; yet, in spite of all this, the
necessity of the conception remains. Similar objections could be
raised against a number of other words which we use for certain
definite compounds present in organic bodies. For instance, to
designate a certain portion of the nucleus we use the term nuclein
or chromatin, which is considered fairly adequate by many people.
And yet the microscopist is bound to admit that it is impossible to
state exactly which part of a resting nucleus consists of linin, and
which of nuclein, or to determine in any special case whether too
much or too little has been stained.

Now the term protoplasm is quite as necessary in speaking
about the constituent parts of a cell. Only it must be stipulated
that the word protoplasm must not be understood to designate a
substance of definite chemical composition.

The word protoplasm is a morphological term (the same is true in
a greater or less degree of the word nuclein, and of many others) ;
it is an expression for a complex substance, which exhibits a
variety of physical, chemical, and biological properties. Such ex-
pressions are absolutely necessary in the present state of our
knowledge. Anyone whois acquainted with the history of the cell
knows what a number of observations and how much logical
thought were necessary before this conception was arrived at, and
further is quite aware that with the creation of this expression the
whole theory of cells and tissues gained in depth and significance.
How much wordy warfare was necessary before it was established
that the cell contents, and not the cell membrane, constitute the
essential portion of the cell, and further that amongst these cell
contents a peculiar substance is invariably present, which takes
part in the vital processes in quite a different way from the cell
sap, the starch granules, and the fat globules.

Thus we see that the use of the word protoplasm is not only
justifiable from an historical point of view, but also from a
scientific one, and we will now proceed to endeavour to explain
what is meant by the term.

b. General Characteristics of Protoplasm. The proto-
plasm of unicellular organisms, and of plant and animal cells (Figs.
1 and 2), appears as a viscid substance, which is almost always
colourless, which will not mix with water, and which, in con-

14 THE CELL

sequence of a certain resemblance to slimy substances, was

called by Schleiden the slime of the cell,

Its refractive power is

greater than that of water, so that the most delicate threads of
protoplasm, although colourless, may be distinguished in this
medium. Minute granules, the microsomes, which look only like

Fie. 1.—Parenchyma cells, from the cortical layer of the
root of Fritillaria imperialis ; longitudinal sections (x 550);
after Sachs([I. 33), Fig.75: A very young cells, as yet without
cell-sap, from close to the apex of the root; B cells of the same
description, about 2mm. above the apex of the root—the cell-
sap (s) forms in the protoplasm (p) separate drops, between
which are the partition walls of the protoplasm ; C cells of the
same description, about 7-8 mm. above the apex; the two lower
cells on the right-hand side are seen in a front view, the large
cell on the left side is seen in optical section, the upper right-
hand cell is opened by the section; the nucleus (#y) has a
peculiar appearance, being distended with water which it has
absorbed ; k nucleus; kk nucleolus ; h membrane,

dots, are always
present in greater
or less numbers
in all protoplasm,
and may be seen
with a low power
of the microscope
to be embedded
in a homogeneous
ground sub-
stance. Accord-
ing to whether
there are few or
many of these
microsomes in
the protoplasm, it
is more trans-
parent (hyaline)
or darker and
more granular in
appearance.

The distribu-
tion of these
granules in the
body of the cell
is rarely regular.
Generally a more
or less thin outer
zone remains free
from granules.
Now as this layer
appears to be
somewhat firmer
in consistence
than the more
watery granula

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 1§

mass, it has been thought advisable to distinguish two kinds of
protoplasm, the ectoplasm or hyaloplusm, and the endoplasm or
granularplasm (Fig. 2, ek, en).

Many investigators, such as Pfeffer, de Vries, etc., are inclined
to consider that this peripheral layer is a specially differentiated organ
of the cell and is endowed with special functions. The following
experiment which I have made seems to bear out this view,

Some ripe eggs of Rana temporaria, which had entered the
oviduct and were surrounded with a gelatinous coating, were care-
fully pierced with the exceedingly fine point of a glass needle.
The puncture thus made was not visible externally after the
operation, nor was any yolk seen to exude through the holes.
However, some time after fertilisation of the eggs had taken place, a
fair quantity of yolk began to make its way out of all the punctured
eggs, and to form a more or less large ridge (extraovat, Roux)
between the membrane of the egg and the yolk. This welling out
of the yolk substance was induced by the act of fertilisation, for the
entrance of the spermatozoon stimulates the surface layer to con-
tract energetically, as may be easily demonstrated under suitable
conditions. Hence the puncture must have caused a wound in the
peripheral layer, which had not time to heal before fertilisation
took place, and through which the yolk was only pressed out after
the contraction caused by the fertilisation had taken place. Now
since between the piercing of the eggs and their fertilisation a
fairly long interval, which however I did not accurately measure,
had elapsed, this experiment seems to show that the peripheral
layer possesses a structure differing somewhat from that of the rest
of the cell contents, and also that it has properties Peounae to
~ itself,

c. Chemical Composition of Protoplasm. Our know-
ledge of the chemical nature of protoplasm is most unsatisfactory. It
has sometimes been described as an albuminous body, or as “ living
albumen.” Such expressions may give rise to utterly incorrect
conceptions of the nature of protoplasm. On this account I will
recapitulate what I said in section a: Protoplasm is not a/
chemical, but a morphological conception; it is not a single,
chemical substance, however complex in composition, but is com-
posed of a large number of different chemical substances, which
we have to picture to ourselves as most minute particles united
together to form a wonderfully complex structure.

Chemical substances exhibit similar properties under different

16 THE CELL

circumstances (as, for instance, hemoglobin, whether present as a
constituent of the blood corpuscles, or dissolved in water, or in the
form of crystals). Protoplasm, on the other hand, cannot be
placed under different conditions without ceasing to be protoplasm,
for its essential properties, in which its life manifests itself,
. depend upon a fixed organisation. For as the principal attributes
of a marble statue consist in the form which the sculptor’s hand
has given to the marble, and as a statue ceases to be a statue if
broken up into small pieces of marble (Nageli II. 28), so a body
of protoplasm is no longer protoplasm after the organisation,
which constitutes its life, has been destroyed; we only examine
the considerably altered ruins of
the protoplasm when we treat
the dead cells with chemical re-
agents.

It is possible that after a time
our knowledge of chemistry may
have advanced sufficiently to en-
able us to produce albuminous
bodies artificially by synthesis.
On the other hand, the attempt
to make a protoplasmic body
would be like Wagner's en-
deavour to crystallise out a
homunculus in a flask. For, as

plasmic bodies are only reproduced
from existing protoplasm, and in
Fria. 2.—Ameba Proteus (after Leidy ; no other basic: fe hence the present

from Rich. Hertwig): n nucleus; cv con- organisation of protoplasm is the
tractile vacuole; » food vacuoles; en
endoplasm ; ek ectoplasm.

cess of development.

It is very difficult to determine the chemical nature of the sub-
stances which are peculiar to living protoplasm. For setting
aside the fact that the bodies are so unstable that the least inter-
ference with them essentially alters their constitution, the
difficulty in analysing them is considerably increased by the
presence in each cell of various waste products of metabolism,

which it is not easy to separate from the rest of the cell contents.

Amongst these complex substances the proteids, as the true sus-
tainers of the vital processes, are of especial importance ; these

far as we know at present, proto- |

result of an exceedingly long pro-

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 17

proteids are the most complex of all known organic substances,
but up till now very little has been determined as to their chemical
structure. This complex structure depends, in the first place, upon
the very remarkable chemical properties of carbon (Haeckel II. 15).
In proteids carbon occurs combined with four other elements,
hydrogen, oxygen, nitrogen, and sulphur, in proportions which,
it has been endeavoured to express by the following formula: C7?
H1°6N18S0*2 (composition of a molecule of egg-albumen).

Amongst the various kinds of proteid bodies (albumins, globu-
lins, fibrins, plastins, nucleins, etc.) plastin alone seems to be pecu-
liar to protoplasm (Reinke II. 32; Schwarz II. 37; Zacharias
IT. 44) ; plastin is insoluble in water, in 10 per cent. salt solution,
and in 10 per cent. solution of sulphate of magnesia; it is pre-
cipitated by weak acetic acid, whilst concentrated acetic acid
causes it to swell up; it is precipitated in concentrated salt
solution; it resists both pepsin and trypsin digestion. It is hardly,
or not at all, stained by basic aniline dyes, but is stained by
acid ones (eosin and acid fuchsine).

In addition, globulins and albumins are present in smaller
quantities; these are also found in solution in the cell-sap of
plants.

Protoplasm is very rich in water, which, as Sachs (II. 33) states,
is built up into the structure of its molecule, in the same sense as,
for example, the water of crystallisation is a necessary constituent
of many crystals, which lose their characteristic form if the water
of crystallisation is withdrawn. Reinke (II. 32) found 71°6 per
cent. of water and 284 per cent. of solid substances in fresh
sporangia of the Mthalium septicum (66 per cent. of this water
could be squeezed out).

Further, a number of various salts are present in protoplasm ;
these remain as ash when the protoplasm is burnt ; in the case of
the Althalium septicum the ash contains the following elements:
chlorine, sulphur, phosphorus, potassium, sodium, magnesium,
calcium, and iron.

Living protoplasm is distinctly alkaline in reaction; red litmus
paper is turned blue by it, as is also a red colouring matter, which.
is obtained from a species of cabbage, and which has been used by
Schwarz. This is also the case with plants, although the cell-sap,
as a rule, has an acid reaction. According to the investigations
of Schwarz (II. 37) on plants, this alkaline reaction is due to the

presence of an alkali, which is united with the proteid bodies in
Cc

18 THE CELL

living protoplasm. Reinke (II. 32) states that the Athaliwm
septicum gives off ammonia after it has been dried.

Moreover, the most different metabolic products are always to
be demonstrated in protoplasm; these are produced either by
progressive or retrogressive metamorphosis. There is a great
similarity shown between the substances occurring in plant and in
animal cells. For example, the following substances are found in
both,—pepsin, diastase, myosin, sarcin, glycogen, sugar, inosit,
dextrin, cholesterin and lecithin, fat, lactic acid, formic acid, acetic
acid, butyric acid, etc.

_ As an example of the quantitative composition of a cell includ-
ing its nucleus, Kossel (II. 35) quotes in his text-book, the
analysis of pus-corpuscles which was made by Hoppe-Seyler.
According to this statement, 100 parts by weight of organic
substance contain :

Various albuminous substances . ; : ; . 13°762
Nuclein 8 z . : ; k ‘ P . 34:257
Insoluble substances . f ; , : . 20°566
Lecithin and fat . A ; : fe 3 2 - 14:383
Cholesterin . . : . ° ‘ : : - 7400 |
Cerebrin . 3 : : : : A 3 . ip ASe
Extractives A . : A é Z ; - 4°433

Phosphorus, sodium, iron, magnesium, calcium, phosphoric
acid and chlorine were found in the ash.

As regards the physical properties of protoplasm, streaming
protoplasmic threads are sometimes noticed in which double re-
fraction is seen, the movements being for the most part in a
direction such that their optical axes coincide (Engelmann).

d. The more minute Structure of Protoplasm. Proto-
plasm was defined above as a combination of substances, the most
minute particles of which we must picture to ourselves as united
together to form a complex structure. Investigators have en-
deavoured to discover more about this marvellous structure, partly
by speculation, and partly by microscopical observation.

As to the first, Nigeli has made some important suggestions,
a more detailed account of which is given later in the section
entitled ‘‘ The Molecular Structure of Organised Bodies.”

As to the second, numerous investigators, amongst whom From-
mann, Flemming, Biitschli and Altmann are conspicuous, have
recently been working at the subject. Living protoplasm, as well
as that which has been killed by special reagents, has been

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 19

examined; in the latter, its most minute structure has been
rendered visible, by means of various staining reagents; thus we
have already a considerable amount of literature on the subject
of the structure of protoplasm.

Starting with the assumption that protoplasm consists of a
mixture of a small quantity of solid substances with a large
quantity of fluid, to which circumstance it owes its peculiar
viscid property as a whole, the question might be raised as to
whether it be possible, by using the strongest lenses, to distinguish
optically the solid particles from the fluid which contains them,
and to recognise their arrangement into special structures. A
priort, it does not seem to be necessary to distinguish them
from one another, since the solid particles are so very small, and
since they differ so little from the fluid in their refractive power.
Thus, according to Nageli’s micellar theory, which will be de-
scribed in detail later on, they are supposed to be arranged as
a framework, which, however, in consequence of the minute size of
the hypothetical micelle, escapes our observation. In a word, it is}
possible that protoplasm may have a very complicated structure, _
although it appears to us to be a homogeneous body. Hence the |
expression homogeneous protoplasm does not necessarily imply that
protoplasm does not possess a definite structure or organisation. 1

Recent observations, for which powerful oil immersion lenses
have been successfully used, point more and more to the conclusion
that protoplasm possesses a structure which may be optically
demonstrated ; however, individual microscopists differ so essen-
tially in their views upon the nature of this structure, that it is
impossible to come to any definite decision upon the subject.

At the present time, at least four conflicting theories hold the
field; these may be described as the framework theory, the foam
or honeycomb theory, the filament theory, and the granula theory.

The framework theory has been advocated by Frommann (II. 14),
Heitzmann (II. 17), Klein (II. 21), Leydig (II. 26), Schmitz
(II. 36), and by others. According to this theory, protoplasm
consists of a very fine network of fibrille or threads, in the inter-
stices of which the fluid is held. Thus, roughly speaking, it is.
like a sponge, or, shortly expressed, its structure is spongiose.
The microsomes, which are seen in the endoplasm (granular
plasma), are nothing but the points where the fibrille intersect.

A glance over the literature on this subject shows the reader
that very different appearances are sometimes described under the

20 THE CELL

title, ‘The spongiose structure of protoplasm.” Sometimes the
description refers to coarser frameworks, which, being due to the
deposition in the protoplasm of various kinds of substances, should
not be considered as pertaining to protoplasm, nor should they be
included in its description. This holds true, for example, of the
description of the goblet cells of List (II. 48) (see p. 36, fig. 17).
This subject is more fully discussed later on.

Sometimes net-like structures are described and depicted, which,
as they are evidently caused by coagulation (due to some pre-
cipitation process), must be considered as artificial products.
For instance, artificial framework structures may be easily pro-
duced, if a solution of albumen or gelatine be caused to coagulate
by the addition of chromic acid, picric acid, or alcohol. Thus
Heitzmann (II. 17) demonstrates, in a somewhat diagrammatic
manner, the presence of networks in the most various cells of the
animal body, which does not correspond to actual fact. Biitschli
also remarks in his abstract of the literature on the subject
(II. 7b, p. 113): “Above all, it is frequently very difficult to
determine whether the net-like appearances described by earlier
observers are really delicate protoplasmic structures, or whether
they are caused by coarser vacuolisation. Since the same appear-
ance is produced in either case, it is only possible to form a fairly
correct opinion by considering their relative sizes.” Biitschli
found that in all cases the spaces in the meshes of the protoplasm
measured barely 1 p.

Thus, although no doubt many statements may be legitimately
questioned, yet it is undeniable that many investigators (From-
mann, Schmitz, Leydig) have really based their descriptions upon
the more delicate structures of the cell.

In the explanation of these so-called net-work appearances,
Bitschli takes up a position which is-different from that of the
other observers who have been mentioned, and which has caused
him to advance a foam or honeycomb theory of protoplasm (II. 7a,
7b).

He succeeded in producing a very delicate emulsion by mixing
inspissated olive oil with K,CO,, common salt, or cane-sugar.

This emulsion consists of a groundwork of oil, containing an
exceedingly large number of spaces, which are completely closed in
and filled with watery liquid ; ifthe emulsion is too fine to be seen
except under the microscope, the diameter of the spaces is
generally less than ‘001mm. In appearance they are very like

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 2)

the cells of a honeycomb, being in the form of very varying poly-
hedra ; they are separated from one another by the most delicate
lamelle of oil, which refract the light somewhat more strongly
than the watery liquid does. As a result of physical laws, only
three lamelle can touch at one
edge. Hence it appears in optical
section, that only three lines meet in
any one point. If before the for-
mation of the emulsion fine par- Fie. 3.—Optical section of the edge of
a drop of an emulsion made with olive
ticles of lamp-black are distributed 6) ana salt; the alveolar layer (alv.) is
throughout the oil, these collect at very distinct, and relatively deep. (x
: nie : : 1250: after Biitechli, Pl. ILI., Fig. 4.)
the point of intersection. Finally,
the superficial layer is composed of a delicate froth, the frame- —
work of which is arranged in a peculiar fashion, the partition
walls of oil, which touch the surface, being perpendicular to it,
and thus appearing parallel to one another in optical section.
Biitschli describes this as the alveolar layer (Fig. 3 alv.).
Biitschli considers that the protoplasm of all plant and animal
cells (Figs. 4, 5) possesses a structure which is similar to this.

Fie. 4. Fie. 5.
Fig. 4.—Two living strands of plasma from a hair-cellof a Mullow. (x about 3,000:
after Biitschli, Pl. II., Fig. 14.)
Fie. 5.—Web-like extension, very distinct in structure, from the pseudopodic net of a
Miliola from life. (x about 3,000: after Biitschli, Pl. II., Fig. 5.)

22 THE CELL

His opinion is based upon his experiments on living objects, which
he treated with various reagents. In his opinion there is a frame-
work of plasma corresponding to the lamelle of oil, which, in the
artificial emulsion, separate the droplets of fluid from one another.
Similarly here also granules (microsomes) are collected together at
the points of intersection. Further the protoplasmic body is fre-
quently differentiated externally to form an alveolar layer. The
appearance, described by other observers as a thread or net-like
structure with. spaces which communicate and contain fluid,
Biitschli considers to be due to the presence of a froth or honey-
comb structure, in which the cavities are closed in on all sides; he
himself, however, remarks that, in consequence of the minuteness
of the structures in question, it is impossible to decide finally,
simply by the appearance under the microscope, whether a net-like
or honeycomb structure really exists (II. 7b, p. 140), since “ in
either case the appearance under the microscope is the same.”’

Now it seems hardly justifiable, that this similarity to an
artificially prepared froth, although it has caused Biitschli
finally to make up his mind, should be allowed to settle the
question.

Two objections to this theory of Biitschli’s must be mentioned.
The first is that it does not apply to nuclear substance, which
without doubt is similar in its organisation to protoplasm. For
during the process of nuclear division threadlike arrangements in
the form of spindle-threads and nuclein-threads are so distinctly to
be seen, that their existence certainly cannot be questioned by
any one.

The second objection is more theoretical in nature. The oil
lamellae are composed of a fluid which does not mix with water.
Now if the comparison between the structure of this emulsion and
that of protoplasm is to depend upon something more than a mere
superficial similarity, the plasma lamelle, corresponding to the
oil lamella, must be composed of a solution of albumen or of liquid
albumen. Now this cannot be the case, for a solution of albumen
is capable of mixing with water, and hence would of necessity mix
with the contents of the spaces ; hence the albuminous froth would
have to be prepared with air. In order to get over this difficulty,
Biitschli assumes that the chemical basis of the framework sub-
stance is a fluid, composed of molecules of albumen combined with
those of a fatty acid (Il. 7b, p. 199); this supposition, and
especially the theory that the framework substance is a fluid, is

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 23

not likely to meet with much support. For on many accounts it
seems to be true that the structural elements of protoplasm,
whether they form the threads of a net, or the lamelle of a honey-
comb, or granules, or what not, must be solid in their nature.
Protoplasm does not consist of two non-miscible fluids, such as
water and oil, but of a combination of solid organic particles with
a large quantity of water. Hence quite different physical condi-
tions are necessarily present. (Compare section on molecular
structure, p. 58.)

The third of the above-mentioned views, or the filament theory, is
connected with the name of Flemming (II. 10).

Whilst examining a large number of living cells (cartilage, liver,
connective tissue, and ganglion cells, etc.), Flemming observed in
the protoplasm (Fig. 6) the presence of extremely delicate threads
which have somewhat greater refractive power than the inter-
vening ground substance. These threads vary in length, being
longer in some cells than in others; sometimes larger numbers are
present than at others. It seemed im-
possible to determine with certainty
whether they are separated from one
another all along their length, or
whether they join together to form a
net; if they do form a net, then its
meshes must be very uneven in size.
Hence Flemming considers that two —
different substances occur in proto- _ ¥%6- 6.—living cartilage cell of

- a Salamander larva, much mag-
plasm, a thread substance and an inter- nified, with clearly marked fila-
stitial substance, or a filamentous and an ‘™entous substance: after Flem-
; é ming (from Hatschek, Fig. 2).
interfilamentous substance (mitome and
paramitome) ; upon the chemical nature of these substances and
upon their general condition Flemming does not enlarge. How
much importance should be attached to this structure, about
which at present nothing further can be stated, it remains for the
future to reveal. ©

In this section, ‘‘On the Structure of Protoplasm,” the ray-like arrangement of
the protoplasm which is observed at certain stages of the division of the
nucleus, or the striated appearance which is exhibited by the protoplasm of
secretory cells, might be more fully described. Since, however, such structures
only occur under special conditions, it has been considered more advisable to
defer their consideration to a later period.

Fourthly, and finally, come the attempts of Altmann (II. 1) to

=

24 " THE CELL

demonstrate a still more minute structure of protoplasm (granula
theory). By means of a special method of treatment, this in-
vestigator has succeeded in rendering minute particles visible in
the body of the cell; these he calls granula. He preserves the
organ in a mixture of 5 percent. solution of potassium bichromate
with 2 per cent. solution of perosmic acid; he then prepares thin
sections of the organ and stains them with acid fuchsine,
finally treating them with alcoholic solution of picric acid, by
means of which the differentiation is rendered more distinct. The
result of these staining reactions is to render visible a large num-
ber of very minute dark-red granules. Sometimes they are seen
to be isolated, sometimes more densely packed; sometimes they
are near together, sometimes further apart ; or they may be united
in rows to form threads.

In consequence of these observations, Altmann has propounded a
very important and far-reaching hypothesis. He considers these
granules to be still more minute elementary organisms, of which
the cell itself is composed; he calls them bioblasts, attributes to
them the structure of organised crystals, and looks upon them
as equivalent to the micro-organisms which, as individuals,
arrange themselves in masses to form a zooglea, or in rows to form
threads. ‘‘As in a zooglea the single individuals are connected
together by means of a gelatinous substance secreted by them-
selves, and at the same time are separated from one another by it,
so in the cell the same might occur with the granula; in this
case also we must not consider that there is merely water and salt
solution surrounding the granula, but similarly that a more
gelatinous substance (intergranula substance) is present; this is
sometimes liyuid, and sometimes fairly viscid in consistency. The
great mobility, peculiar to most protoplasm, renders the former
probable. If this intergranula substance becomes collected with-
out granula at any point in the cell, a true hyaloplasm may be
formed, which, being free from living elements, does not really
deserve the name of protoplasm.”

Thus Altmann defines protoplasm as “a colony of bioblasts, the
individual elements of which are grouped together either in a
zooglea condition or in the form of threads, and which are con-
nected by an indifferent substance.” ‘Hence the bioblast is the
much-sought-after, morphological unit of all organic substances,
with which all biological investigation must finally deal.” How-
ever, the bioblast is not able to live alone, but dies with the cell

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 25

in which, according to Altmann, it multiplies by fission (omne
granulum e granulo).

Many objections may be raised to this hypothesis of Altmann’s,
in so far as it refers to the interpretation of recorded observations.
Firstly, the most minute micro-organisms of a zooglea are connected
by means of a great number of forms, which are intermediate as to
size, with the larger fission and yeast fungi; and since these are
not to be distinguished from cells in their construction, they also
must, according to Altmann, be colonies of bioblasts. Further,
Butschli has shown that the larger micro-organisms are most
probably divided into nucleus and protoplasm, and hence are
similar in structure to other cells. The flagella, also, which have
been demonstrated in many micro-organisms, must be considered to
be cell organs. Secondly, we have not been sufficiently enlightened
upon the nature and function of the granula in the cell, excepting
that for some reason or other we are to conclude that they are
its true vital elements. According to Altmann’s hypothesis, the
relative importance which has been attached to cell-substances is
completely reversed. The substance which he calls intergranula
substance, and which in its physiological importance he considers
to correspond to the gelatinous substance of the zooglea, is to all
intents and purposes the protoplasm of the generally accepted
cell theory, that is to say, the substance which is considered to form
the most important generator of the vital processes ; on the other
hand, the granula belong to the category of protoplasmic contents,
and as such have had a much less important réle ascribed to them.
Thus Altmann designates the melanin granules of a pigment cell
as the bioblasts, and the connecting protoplasm as the inter-
granula substance. Similarly he completely reverses the physio-
logical importance of the substances in the nucleus, as will be
shown later on, in that he considers that his granula are con-
tained in the nuclear sap, whilst his intergranula substance corre-
sponds to the nuclear network, containing the chromatin.

Under the term granula, Altmann has, according to our opinion,
classed together substances of very different morphological im-
portance, some of which should be considered as products of the
protoplasm. However, he has rendered important service by faci-
litating the investigation of protoplasm by means of new methods,
although his bioblastic theory, which is based upon these experi-
ments, is not likely to attract many supporters. (See the conclu-
sion of the ninth chapter.)

J

~~

26 THE CELL

e. Uniformity of Protoplasm. Diversity of the Cell.
A great uniformity of appearance is manifested by protoplasm
in allorganisms. With our present means of investigation we are
unable to discover any fundamental difference between the proto-
plasm present in animal cells and that in plant cells, or unicellular
| organisms. This uniformity is of necessity only apparent, being due
to the inadequacy of our methods of investigation. For since
the vital processes occur in each organism in a manner peculiar
to itself, and since the protoplasm, if the nucleus be excepted, is
the chief site of the individual vital processes, these differences
must be due to differences in the fundamental substance, that is to
say, in the protoplasm. We must therefore accept, as a theory,
that the protoplasm of different organisms: varies in its material,
composition and structure. Apparently, however, these important
differences are due to variations in molecular arrangement.

In spite of the uniform appearance of the protoplasm, the in-
dividual cell, of which after all the protoplasm forms only a more
or less important part, when taken as a whole, may vary very
much in appearance; this is due partly to variations in external
form, but chiefly to the fact, that sometimes one, and sometimes
another substance is stored up in the protoplasm, in such a manner
as to be distinguishable from it. Sometimes this occurs to. so
great a degree that the whole cell appears to be composed almost
entirely of substances which under other circumstances are not
present in protoplasm at all. If we imagine that these substances
have been eliminated, a number of larger and smaller gaps would
be naturally produced in the cell, between which the protoplasmic
groundwork of the cell would be seen as partition walls and frame-
works, which are sometimes extremely delicate. This arrangement
of the protoplasm, as has been already mentioned (p. 19), must
not be confused with the network structure, which, according to
the opinion of many investigators, is inherent to protoplasm itself,
and which was more fully described in the chapter on the structure
of protoplasm. _

The names deutoplasm (van Beneden) and paraplasm (Kupffer,
II. 24) have been proposed for these adventitious substances.
Since, however, the idea of an albuminous substance is always con-
nected with the word plasm—and these substances may consist of
fat, carbohydrates, sap, and of many other bodies—the use of the
above terms does not seem desirable, and it is better either to class
them generally as intraplasmic products and adventitious cell contents

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 27

or, according to their significance, as reserve material and secretions,
or indeed to specify them, as yolk granules, fat globules, starch
granules, pigment granules, etc.

The difference between the protoplasm and these substances,
which may be classed together as cell contents, is the same as that
between the materials of which the organs of our body are com-
posed and those substances which in the first place are taken up
as food by our bodies, and which later on are circulated in a
liquid form as a nutrient fluid through all the organs; the for-
mer, which are less dependent upon the condition of nourishment

of the body for the time being, and hence are less subject to
variations, are called in physiological language tissue substances,

the latter circulating substances. The same distinction may be

applied to the substances which compose the cell. Protoplasm |

is the tissue material, whilst the adventitious bodies are circulating
substances. 5

f. Various examples of the structure of the cell body.
In connection with the chemico-physical and morphological pro-
perties of the cell, a few especially pertinent examples may be
of use in order to explain the general statements. For this pur-
pose we will compare various lower unicellular organisms, both
plant and animal, choosing first, cases in which the body consists
almost entirely of protoplasm, and secondly, those in which the
cells also contain considerable quantities of various adventitious
substances, and hence are very much altered in appearance.

Unicellular organisms, which live in water or on damp earth,
such as Amcebe, Mycetozoa, and Reticularia, form very useful
subjects for examination in studying the-cell; in addition, lymph
corpuscles, the white blood corpuscles of vertebrates, and young
plant cells are most suitable objects for investigation.

l. Cells consisting almost entirely of Protoplasm. An Ameba
(Fig. 7) is a small mass of protoplasm, from the surface of
which, as arule, a few short irregular processes (pseudopodia)
or foot-like organs are extended. The body is quite naked, that
is to say, it is not separated from the surrounding medium by
any special thin coating or membrane; the only differentiation
being that the superficial layer of the protoplasm (ectoplasm), ek,
is free from granules, and hence is transparent, like glass; this
ectoplasm is most marked in the pseudopodia; below the ectoplasm
hes the darker and more liquid endoplasm (en), in which the
vesicular nucleus (7) is embedded.

aa

28 THE CELL

Very similar in appearance to the Ameeba, but much smaller in
size, are the white blood corpuscles and the lymph corpuscles of the
vertebrates (Fig. 8). If they are examined just after they have
been taken from the body of the living animal, they are seen to
be more or less globular masses of protoplasm, each one consisting
of a scarcely visible hyaline layer, enclosing a granular internal
portion in which the nucleus is situated. However, whilst the
Specimen is fresh, this nucleus can hardly be distinguished, and
‘sometimes even is quite invisible. After a time, the little body
begins to push out from its surface, processes similar to the pseudo-
‘podia of the Ameeba.

Fie. 7. : Fie. 8.
Fie. 7.—Ameba proteus (after Leidy: from R. Hertwig, Fig. 16): n nucleus; cv con-
tractile vacuole; n food vacuoles ; en endoplasm; ek ectoplasm,
Fie. 8.—A leucocyte of the Frog, containing a Bacteriwm which is undergoing the
process of digestion; the Bacteriwm has been stained with vesuvine. The two figures re-
present two successive changes of shape in the same cell. (After Metschnikoff, Fig. 54.)

Myxomycetes and Reticularia, which also consist of naked proto-
plasm, are very different in appearance. The Myxomycete, which
is best known to us, is the Wthalium septicum, which forms the
so-called flowers of tan and grows over large portions of the surface
of tan-pits, during its vegetative condition, like a thin coherent
skin of protoplasm (plasmodium).

Chondrioderma is another slime fungus which is nearly allied to
the above. A small piece of its edge is represented in Fig. 9.

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 29

Towards its edge the plasmodium becomes broken up into a
number of threads of protoplasm, which are sometimes exceedingly
thin, and sometimes somewhat thicker, and which unite together
to form a fine network. In the thicker threads it is possible to
distinguish both a thin layer of homogeneous ectoplasm, and
also the endoplasm which it encloses; these cannot, however, be
made out in the thinner ones. Throughout the whole mass of
protoplasm, which is sometimes very extensive, a large number
of minute nuclei are seen to be distributed.

Amongst the Reticularia, of which many different kinds occur
in fresh and salt water, Gromia oviformis (Fig. 10) is especially
well known, in consequence of the experiments which have been
made upon it by Max Schultze (I. 29).
Part of the granular protoplasm, which
contains a few small nuclei, lies within
the oval shell, in which there is a wide
Opening at one pole, whilst the re-
mainder protrudes through this open-

g, covering the surface of the shell
with a thin layer. If the organism
has not been disturbed, very delicate
threads of protoplasm (pseudopodia)
stretch out from this layer into the
water in every direction; sometimes

ing

these pseudopodia are exceedingly

long; many become forked, others Fie. 9.—Chondrioderma difforme
5 ; ; _ (from Strasburger): f part of a

break up into numerous minute ‘ fairly old plasmodium; a dry

spore; b the same, swollen up in

threads, whilst yet others send off side
branches, which unite with neighbour-
ing pseudopodia.

water ; c spore, the contents of
which are exuding ; d zoospore;
e amoeboid forms, produced by

the transformation of zoospores
which are commencing to unite
together to form a plasmodium.
(In d and e the nuclei and con-
tractile vacuoles may be seen.)

Dujardin gave the name of sarcode to the
peculiar substance of which the bodies of the
lower organisms, described above, are com-
posed, because, like the muscle-substance of
the higher animals, it is capable of exhibiting movements. Influenced by
Schleiden and Schwann’s cell theory, investigators attempted to prove that
sarcode was composed of a number of minute cells, so that the sarcode
organisms might be included in the cell hyp thesis. However, the solution
to the difficulty was found to be in quite another direction. Investigators like
Cohn (I. 7) and Unger were the first to compare sarcode with the protoplasmic
contents of a plant-cell, in consequence of the similarity of the vital phenomena.
Finally, Max Schultze (I. 29), de Bary (I. 2), and Haeckel (I. 10) established

30 THE CELL

{ beyond a doubt the identity of sarcode with the protoplasm of plant and animal
cells; and this discovery was most helpful to Max Schultze in working out his

Fic. 10.—Gromia oviformis. (After M. Schultze.)

cell theory, and in estab-
lishing his theory of pro-
toplasm (p. 6).

In Ameeba, lymph
cells, Mycetozoa, and
Reticularia, we have
learnt to recognise
naked cells; those of
plants on the contrary
are almost invariably
enclosed by a well-
defined layer, which is
sometimes very thick
and firm; this is also
very frequently the
case with animal cells
(membrane, intercel-
lular substance), and
thus in such cases a
little chamber, or cell,
in the true sense of
the word is formed.
Young cells from the
neighbourhood of the
growing point of a
plant, and cartilage
cells from a Salaman-
der larva, are very
good examples of this.

The celis at the
growing point of a
plant (Fig. 12 A),
where they multiply
very rapidly, are very
small, and are very
similar to animal
cells. They are only
separated from one
another by very thin

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 31

cellulose walls. The small cell spaces are completely filled up
with the cell-substance, which, with the exception of the nucleus
and chlorophyll, consists solely of finely granular protoplasm.

Flemming recommends cartilage cells from young Salamander
larvee as affording the best and most reliable material for the
study of the structure of living proto-
plasm (Fig. 11). The cell-substance,
which during life, as in the young
plant-cells, completely fills the spaces
in the cartilaginous ground-substance,
is traversed by wavy threads of fairly
high refractive power; these are less
than 1 pin diameter, and are generally
most numerous, and at the same time Fre. 11.—Living cartilage cell

if 2 of a Salamander larva, much
most wavy, in the neighbourhood of magnified, with distinctly marked
the nucleus; sometimes the periphery threads. (After Flemming: from
of the cell is nearly, if not entirely, aguas eka
free from threads, but sometimes they are present in great num-
bers here also.

2. Cells which contain several different substances in their
protoplasm. In plants, and in unicellular organisms, the pro-
toplasm frequently contains drops of fluid, in which salt, sugar,
and albuminates are dissolved (circulating substances). The
further we go (Fig. 12 A) from the growing-point of a plant, where
the minute elementary particles of pure protoplasm as described
above are grouped, the larger do the individual cells (c) appear, until
they are frequently seen to be more than a hundred times as large
as they were originally, whilst, in addition, their cellulose wall has
become considerably thicker. However, this growth depends only
to a very small extent upon any marked increase of the proto-
plasmic substance. The cavity of such a large plant cell is
never seen to be completely filled with granular protoplasmic
substance. The increase in the size of the cell is due much
more to the way in which the small amount of protoplasmic
substance, which was originally present at the growing point,
takes up fluid, which in the form of cell-sap separates out into
small spaces in the interior, called vacuoles. By this means a
frothy appearance is produced (Fig. 12 B, s).

More or less thick protoplasmic strands stretch out from the
mass of protoplasm in which the nucleus is embedded. These
strands serve to separate the individual sap vacuoles from one

32 THE CELL

another, and in addition they unite together on the surface to form
a continuous layer (primordial utricle), which adheres closely to
the inner surface of the enlarged and thickened cellulose

membrane.

Two different conditions which are found in the fully grown

Fie. 12.—Parenchyma cells from the cortical layer of the
root of Fritillaria imperialis (longitudinal sections, x 550: after
Sachs IT. 33, Fig. 75): A very young cells, as yet without
cell-sap, from close to the apex of the root; B cells of the same
description, about 2 mm. above the apex of the root; the cell-
sap (0) forms in the protoplasm (p) separate drops between
which are partition walls of protoplasm; C cells of the same
description, about 7-8 mm. above the apex; the two lower
cells on the right hand side are seen in a front view; the
large cell on the left hand side is seen in optical section ; the
upper right hand cell is opened by the section ; the nucleus (ay)
has a peculiar appearance, in consequence of its being dis-
tended, owing to the absorption of water; k nucleus; kk nu-
cleolus ; h membrane,

plant cell are the
result of this
arrangement.
Through the fur-
ther increase of
the cell-sap, the
vacuoles are en-
larged, and the
partition wall at-
tenuated. Finally
the latter par-
tially breaks
down, so that the
separate spaces
are connected by
openings, and
thus form one
continuous vacu-
ole. Consequent-
ly part of the
protoplasmic sub-
stance becomes
transformed into
a fairly thin layer
lying close to the
cellulose mem-
brane, and the
rest into more or
less numerous
strands and
threads travers-
ing the large con-
tinuous vacuole
which is filled
with finid (Fig.
12, right side, and

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES oo

Fig. 13). Finally, in other cases, even these strands of protoplasm
in the interior of the cell may disappear. Then the protoplasmic
substance is represented solely by a thin skin, which lines the
interior of the little chamber, to use an expression of Sachs
(IT. 33), as the paper covers the walls of a room, and which con-
tains one single large sap vacuole (Fig. 12 C, left lower cell, and
Fig. 59). In very large cells this coating is sometimes so thin that,
except for the nucleus, the presence of protoplasm can hardly be
demonstrated at all in the cell, even when a high power of the
microscope is used, so that special methods of investigation are
necessary in order to render it visible.

Fie. 13.—A cell from a hair ona Fie. 14.—CEdogonium, during process of form-
Staminal filament of Tvadescantia ing zoospores (after Sachs; from R. Hertwig’s
virginica (x 240: after Strasburger, Zoologie, Fig. 110): A a portion of the thread
Practical Botany, Fig. 15). of the alga, with the cell contents just escap-

ing ; C zoospore, which has reached the exterior ;
D stationary spore undergoing germination.

It was by the study of such cells, that the earlier investigators,
such as Treviranus, Schleiden, and Schwann, arrived at their
conception of the cell. Hence it is not surprising that they con-
sidered that the cell membrane and the nucleus constituted the
essential portions of the cell, and quite overlooked the importance’
of the protoplasm. That this latter is the true living body in the
plant-cell too, and that it is able to exist independently of the

D

34 : THE CELL

membrane, has been proved beyond a doubt by the following
observation, which has played such an important part in the
history of the cell theory (I. 7). In many algex (dogonium,
Fig. 14), at the time of reproduction, the protoplasmic substance
becomes detached from the cellulose cell-wall, and, whilst parting
with some of its fluid contents, contracts up into a smaller volume,
so that it no longer quite fills up the cavity; it thus forms a
naked swarmspore, which is either globular or oval in shape (A).
_ After a time this swarmspore breaks down the original cell-wall,
and, escaping through the opening it has made, reaches the
exterior. It then develops cilia (0) upon its surface, by means of
which it moves about pretty quickly in the water, until after a
time it comes to rest (D), when it differentiates a delicate new
membrane upon its surface. Thus Nature herself has afforded us
the best evidence that the protoplasmic body is the true living
elementary organism.

A similarly great formation of vacuoles and separation of sap, as
is found in plant-cells, is also seen in the naked protoplasm of the
lower unicellular organisms, especially in certain Reticularia and
Radiolarians ; thus the Actinospheriwm, which is depicted in Fig.
15, presents quite a frothy appearance, resembling the fine froth
which is produced when albumen or soap-suds are beaten up. An .
immense number of larger and smaller vacuoles, filled with fluid,
are distributed throughout the whole body. These are only
separated from one another by delicate partition walls of proto-
plasm, which are sometimes too thin to be measured. The
protoplasm consists of a homogeneous poh substance, in which
granules are embedded.

The result of this formation of vacuoles is that the protoplasmic
substance becomes broken up, so that surfaces of it become exposed
to the nutrient solutions in the vacuoles, in consequence of which
diffusion can take place between them. Evidently the whole
arrangement adds considerably to the facility with which
materials are taken up and given out. This internal increase of
surface may be compared with the external increase of surface,
which is shown in the formation of many-branched pseudopodia
(Fig. 10), and indeed it answers the same purpose.

In animal-cells, on the contrary, the formation of vacuoles and
the secretion of sap only take place extremely rarely, for instance,
in notochordal cells; on the other hand, adventitious substances,
such as glycogen, mucin, fat globules, albuminous substances, etc.,

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 35

are more frequently found; these either distend the cell or render
it somewhat solid. When there has been a considerable develop-
ment of such substances, the protoplasm may again assume a
frothy appearance, as in <Actinospherium (Fig. 15), or it may
become transformed into a network structure, as in a T'radescantia
cell (Fig. 15), the only difference being that the interstices are
filled with substances denser than sap.

Fie. 15.—Actinospherium Eichhorni (after R. Hertwig, Zoologie, Fig. 117): M medullary
substance, with nuclei (n) ; R peripheral substance, with contractile vacuoles (cv); Na
nutrient substances,

The most perfect examples are often seen in animal egg-cells
The exceedingly large size, which is attained by many of these, is
not so much caused by an increase of protoplasm, as by the storing
up of reserve materials, which vary very much as to their chemical
composition, being sometimes formed and sometimes unformed
substances, and which are intended for future use in the economy
of the cell. Very often the egg-cell appears to be almost entirely
composed of such substances. The protoplasm only fills up the
small spaces between them, like the mortar between the stones of

36 THE’ CELL

a piece of masonry (Fig. 16) ; if a section be made of an egg, the
protoplasm is seen to be present in the form of a delicate net-
work, in the larger and smaller meshes of which these reserve
substances are deposited. The only place where it is collected
together into a thick, cohesive layer is on the surface of the °B8,
and in the neighbourhood of the nucleus.

Another gcod example of a protoplasmic framework structure,
caused by the deposition of various substances, is afforded us by the
mucous cells of vertebrates (Fig. 17) and invertebrates. The
section varies according as to whether it is taken from the
epithelial surface, or from the base of the goblet. In the former
case it is wider, and is seen to consist chiefly of homogeneous
shining secretion, the mucilaginous substance, which is evacuated

Fie. 16.—An egg of Ascaris megalocephala, Fie. 17. — Goblet-cell from the

which has just been fertilised (after Van Bene- bladder epithelium of Squatina vul-
den; from O. Hertwig, Fig. 22): sk spermato- - garis, hardened in Miiller’s fluid,
zoon, with its nucleus which has just entered ; (After List, Plate I., Fig. 9.)

f glistening fatty material of spermatozoon;

kb female pronucleus.

from time to time by the cell, through a small opening at its free

end, and transformed into mucin. The protoplasm traverses the

mass of secretion in the form of fine threads, which join together

to make a wide meshed network, only forming a compact body at
‘the lower extremity of the cell, in which also the nucleus is

situated.

II. The Chemico-physical and Morphological Properties

of the Nucleus. The nucleus is quite as important as the
» protoplasm in the economy of the cell. It was first discovered,
} in 1833, by Robert Brown (I. 5), in plant-cells ; soon afterwards
( Schleiden (I. 28) and Schwann (I. 31) made it the foundation

stone of their theory of cell formation ; after that the study of

the nucleus remained for some time. in the background, as the

{TS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 37

interesting vital phenomena of the protoplasm became more fully
known. During the last thirty years, however, one discovery after
another has been made about the nucleus, the rasult of which is
that this neglected body has been shown to be of as much import-
ance to the elementary organism as the protoplasmic substance.

It is of interest that the history of the nucleus is analogous
in some respects to that of the cell. The nucleus was also con-
sidered at first to consist of a vesicle; indeed, it was even held to be
a smaller cell inside the larger one. But just as it came to be
recognised that the protoplasm is the vital substance of the cell,
so by degrees it came to be seen that the form of the nucleus is of
minor importance, and that its vitality depends far more upon the
presence in it of certain substances, the arrangement of which may
vary very considerably according as to whether the nucleus is in
an active or a passive condition.

Richard Hertwig (II. 18) was the first to enunciate this
clearly in a short paper entitled, “‘ Beitrage zu einer einheitlichen
Auffassung der verschiedenen Kernformen,” in the following
words: “‘ It is necessary to state at the commencement of my
observations, as the most important point to be considered in
classifying the various nuclear forms, that they all possess
a certain uniformity in composition. Whether the nuclei of
animals, plants, or Protista be under examination, it is invariably
seen that they are composed of a larger or smaller quantity of a
material which, like the earlier writers, I shall call nuclear/
substance (nuclein). We must commence with the properties of |
this substance in the same way as he who wishes to describe the
important characteristics of the cell must begin with the cell
substance, 7.e. protoplasm.”

Hence the nucleus is now defined, not, according to Schleiden
and Schwann’s idea, as a vesicle in the cell, but as a portion of a!
special substance which is distinct from the protoplasm, and to a\
certain extent separate from it, and which may vary considerably, as
to form, both in the resting and in the actively dividing condition.

We will now consider the form, the size, and the number of
nuclei in a cell, and then the substances contained in the nucleus,
and their various modes of arrangement (the structure of the
nucleus).

a. The form, size and number of Nuclei. As arule the
nucleus in plant and animal-cells appears as a round or oval body
(Figs. 1, 2, 6, 16), situated in the middle of the cell. Since it is

38 THE CELL

frequently richer in water than protoplasm is, it may be dis-
tinguished from the latter even in the living cell, appearing as a
bright spot with indistinct outlines, or as a vesicle or vacuole.
But this is not always the case. In many objects, such as lymph
corpuscles, corneal cells, and the epithelial cells of gills of Sala-
mander larve, no nuclei can be distinguished during life, although
they immediately become visible when coagulation, induced either
by the death of the cell, or by the addition of distilled water or
weak acids, occurs.

In many kinds of cells; and in the lower organisms, the nucleus
may assume very various shapes. Sometimes it is in the shape of
a horse-shoe (many Infusoria), sometimes of a more or less twisted

Fie. 18.—(After Paul Mayer, from Korschelt, Fig. 12.) A A piece of the seventh appen-
dage of a young Phronima, 5 mm. in length (x 90). B A piece of the sixth appendage of a
half-grown Phronimella (x 90). CA group of cells froma gland in the sixth appendage of
a Phronimella; the nucleus is only shown in two cells (x90).

strand (Vorticella), and sometimes it is very much branched,
stretching into the protoplasm in every direction (Fig. 18 B, C).
This latter form chiefly appears in the large gland-cells of many
insects (in the Malpighian tubes, in the spinning and salivary
glands, etc.), and similarly in the gland-cells of the crustacean
Phronima.

The size to which the nucleus attains is generally proportional
to the size of the mass of protoplasm surrounding it; the larger
this is, the larger is the nucleus. Thus, in the great ganglionic

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 39

celis of the spinal cord, extremely large vesicular nuclei are seen.
Similarly, enormously large nuclei occur in immature egg-cells,

which themselves are of a great size.

Sometimes the nuclei of

immature eggs of Fishes, Amphibians, and Reptiles are perceptible
to the naked eye as small spots; under these circumstances they
can be easily extracted with needles and isolated. Yet there are
exceptions to this rule; for even these same eggs which, when

immature, have such immense nuclei,
when they are mature and fertilised
contain such minute nuclei, that they
can only be demonstrated with the
greatest difficulty.

The lowest organisms, when of a con-
siderable size, frequently possess one
single large nucleus. It is sometimes
enormously large in the central capsules
of many Radiolarians.

As regards the number present, as a
general rule there is only one nucleus in
each cell in plants and animals. To this
rule, however, there are some exceptions ;
there are frequently two nuclei in liver
cells, whilst a hundred or more have
been observed in the giant cells of bone
marrow. Osteoclasts and the cells of
many tumours, the cells of several Fungi,
and of many of the lower plants, such as
Cladophora (Fig. 19) and Siphonee (Bo-
trydium, Vaucheria, Caulerpa, etc.), are
remarkable for this plurality of nuclei,
as has been described by Schmitz.

Similarly, a large number of the
lowest organisms, such as Myxomycetes,
many Mono- and Poly-thalamia, Radio-
larians, and Infusoria (Opalina ranarum),
possess many nuclei in each cell. Fre-
quently in these cases the nuclei are so
minute, and are distributed in such
numbers throughout the protoplasm,

Fie. 19.—Cladophora glomer-
ata. A cell from athread ina
chromic acid carmine prepara-
tion (after Strasburger, Pract.
Botany, Fig. 75): n nucleus;
ch chromatophores ; p amyloid
bodies (pyrenoids); a starch
grenules (x 540).

that they have only been demonstrated quite recently by means of
the most improved methods of staining (Myxomycetes).

40 THE CELL

b. Nuclear Substance. As regards its composition, the
nucleus is a fairly fixed body. Two chemically distinct proteid
substances, which can be distinguished from one another with the
microscope, are always present; very often there are more. The
two constant ones are nuclein or chromatin, and paranuclein, or
pyrenin ; in addition, linin, nuclear sap, and amphipyrenin are
generally to be found.

Of these, NUCLEIN, or chromatin, is the most characteristic pro-
teid of the nucleus, and it generally preponderates as regards
quantity. When fresh it resembles non-granular protoplasm
(hyaloplasm), but it can be easily distinguished from this substance
by its behaviour towards certain staining solutions. After it has
been caused to coagulate by means of reagents, it takes up the
colouring matter from suitably prepared staining solutions (solu-
tions of carmine, hematoxylin, aniline dyes), as has been discovered
by Gerlach. This occurs to a more considerable extent during the
stages preceding division, and during division itself, than when the
nucleus is in a resting condition. Whether this is due to chemical
or to physical causes has not yet been worked out. The art of
staining is now so fully understood that it is quite easy to make
the nuclein of the nucleus stand out clearly from the rest of the
nucleus and the protoplasm, which are either quite colourless or
are only slightly stained. In this manner even small particles of
nuclein, only about as large as Bacteria, may be rendered visible
in comparatively speaking large masses of protoplasm, as, for
example, the minute heads of spermatozoa, or the chromosomes of
the direction spindles in the centres of large egg-cells.

The following fact, which is emphasised by Fol (II. 13), may at
some future period-prove to be of far-reaching importance: “ that
the staining of the nucleus with neutral staining solutions always
produces the same shade of colour as the dye in question assumes
when a small quantity of a substance of basic reaction is added to
it. For example, red alum carmine becomes lilac when the solu-
tion is rendered slightly alkaline, B6hmer’s violet haematoxylin
becomes blue, red ribesia (black currant juice) bluish-green, whilst
the red dye made from red cabbage turns green. Now, it has been
observed that nuclei of tissue-cells, stained with neutral solutions
of these substances, exhibit a corresponding colouration; that is to
say, they become lilac in alum carmine, blue in hematoxylin, light
blue in ribesia, green in the colouring matter of red cabbage.
That part of the nucleus which can be stained (the nuclein) behaves,

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 41

as a rule, towards the staining substance united to it, like a weakly
alkaline body” (Fol).

Further, nuclein exhibits characteristic chemical reactions,
which must not be forgotten in preparing nuclear structures for
preservation (Schwarz II. 37, Zacharias II. 43, 45). It swells up
in distilled water, in very dilute alkaline solutions, and in 2 or
more per cent. solution of common salt, of sulphate of magnesia,
or of monopotassium phosphate and of lime-water. If solutions
of from 10 per cent. to 20 per cent. of the above-named salts are
used, the nuclein, whilst swelling gradually, becomes quite dis-
solved. Similarly, it dissolves completely in a mixture of ferro-
cyanide of potassium and acetic acid, or in concentrated hydrochloric
acid, or if it is subjected to pancreatic digestion. It becomes pre-
cipitated in a fairly unaltered form if treated with acetic acid
from 1 to 50 per cent. in strength, when it can be very clearly
distinguished from the protoplasm by its greater refractiye power,
and by a glistening appearance which is peculiar to it.

Fig. 20.—A resting nucleus of a spermato-genetic cell of Ascaris megalocephala
bivalens. B Nucleus of a sperm-mother-cell from the commencement of the growth-zone
of Ascaris megalocephala bivalens. C Resting nucleus of a sperm-mother-cell of the growth
zone of Ascaris megalocephala bivalens. D Bludder-like nucleus of a sperm-mother-cell of
Ascaris megalocephala bivalens, from the commencement of the dividing zone, shortly
before division,

In the nuclear vesicle (Fig. 20), the nuclein sometimes appears as
isolated granules (A), or as delicate network (B, C), or as threads (D).

Miescher (II. 49) has attempted to obtain pure nuclein from
pus corpuscles and from spermatozoa, in the heads of which it is
present. An important ingredient in its composition is phosphoric
acid, of which at least 3 per cent. is always present. Several
facts seem to indicate that the nuclein of. the nucleus “consists of
a combination of an albuminous body with a complex organic com-
pound containing phosphoric acid (Kossel II. 35). This latter has
been called nucleic acid, and Miescher has calculated its formula
to be CogHygNgP30op.

“Tf subjected for a long time to the action of weak acids or
alkalies, or even if kept in a damp condition, nuclein becomes de-

42 THE CELL

composed, albumen and nitrogenous bases being formed, whilst in
addition phosphoric acid separates out. The two latter decom-
position products are also formed from nucleic acid. The bases
are: adenin, hypoxanthin, guanin, and xanthin.”

PARANUCLEIN, or pyrenin, is a proteid substance, which is always
present in the nucleus; however, the part it plays in the vital
functions of the latter has not yet been worked out, much less being
known about it than about nuclein. It occurs in the nucleus in
the form of small granules, which are described as true nucleoli
or nuclear corpuscles (Fig. 20).

These paranuclein bodies resist the action of all the media
(distilled water, very dilute alkaline solutions, solutions of salt,
sulphate of magnesia, potassium phosphate, lime-water) in which
nuclein substances swell up. Whilst the latter disappear from
view in the nuclear cavity, which has become homogeneous in
appearance, the former often stand out with greater clearness.
They are invariably more easily seen after death than during life.

This explains the fact that these nuclear corpuscles were well
known long ago to the older histologists, Schleiden and Schwann,
who always examined their tissues in water.
Osmic acid is a very useful reagent for rendering these corpuscles
visible, for it very much increases their refractive power, whilst
rendering the nuclein structures paler.

Paranuclein and nuclein behave quite differently towards acetic
acid (1 to 50 per cent.). Whilst the latter coagulates, and in-
creases in refractive power, the nuclear corpuscles swell up more
or less, and may become quite transparent; however, they do not
become dissolved, for if the acetic acid is washed away, they
shrink up, and become visible again.

In addition, it must be pointed out that paranuclein, in
contradistinction to nuclein, is insoluble in 20 per cent. solution
of common salt, in a saturated solution of sulphate of magnesia,
in 1 per cent. and 5 per cent. solutions of potassium phosphate,
of ferrocyanide of potassium plus acetic acid, and of copper
sulphate ; finally, it is very resistent to the action of the pan-
creatic juice.

Further distinct differences are shown in their behaviour to-
wards staining solutions. As Zacharias has observed, and as I can
corroborate as a general rule from my own experience, nuclein
bodies become especially clearly and intensely coloured in acid
staining solutions (aceto-carmine, methyl green, and acetic acid),

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 43

whiist paranuclein bodies remain almost unaffected ; on the other
hand, the latter become better stained in ammoniacal staining solu-
tions, such as ammonia, carmine, etc. Many substances, such as
eosin, acid fuchsine, etc., have a greater affinity for paranuclein.
Hence it is possible, by using two staining solutions at the same
time, to stain the nuclein bodies a different colour from the para-
nuclein ones, thus bringing about a so-called contrast staining
(fuchsine and solid green, haematoxylin and eosin, Biondi’s stain) ;
however, since the nature of staining processes is as yet very im-
perfectly understood by us, it is not possible at present to lay down
general rules concerning the staining properties of these two nu-
clear substances.

I consider that nuclein and paranuclein are the essential constituents
of the nucleus, and that its physiological action depends in the first
instance upon their presence. They seem to me to be correlated
in some way or other. Flemming (II. 10) has suggested, that the
nucleoli may consist of nuclein in a special condition of develop-
ment and density, thus representing a preliminary chemical phase
of it. The material that we have at present for examination is not
sufficient to enable us to decide these questions.

The three other substances which may be distinguished in the
nucleus, linin, nuclear sap, and amphipyrenin, appear to me to be of
much less importance ; it is possible also that they are not always
present.

The name LININ has been applied by Schwarz (II. 37) to the
material of which the threads, which frequently form a network
or framework in the nuclear cavity, consist ; these threads are not
affected by the ordinary staining reagents used for the nucleus,
and can by this means, as well as by their different chemical re-
actions, be easily distinguished from the nuclein, which is deposited
upon them in the form of small particles and granules (Fig. 20
A, C). In many respects it resembles the plastin of proto-
plasm, and indeed Zacharias has called it by that name.

Nvucrear SAP may be present in larger or smaller quantities ;
it fills up the interstices left in the structures composed of nuclein,
linin, and paranuclein. It may be compared to the cell-sap which is
contained in the vacuoles of the protoplasm, and no doubt functions
in a similar manner, by nourishing the nuclear substances, just as
the cell-sap nourishes the protoplasm. By the action of several
reagents, such as absolute alcohol, chromic acid, etc., finely granu-
lar precipitates are caused to make their appearance in the nuclear

44 THE CELL

sap; these, being artificial products, must not be confused with the
normal structures. Hence cell-sap must contain various substances
in solution, amongst which albuminates are probably present;
Zacharias has grouped these together under the common name of
paralinin, a term which may well be dispensed with.

The name AMPHIPYRENIN has been applied by Zacharias to the
substance of the membrane which separates the nuclear space
from the protoplasm, just as this latter is separated from the ex-
terior by the cell membrane. In many cases it is as difficult to
demonstrate the presence of this nuclear membrane, as to decide
the vexed question whether a large number of cells are enclosed
by membranes or no. It is most easily seen in the large germinal
vesicles of many eggs, such as those of Amphibians, where it is at
the same time somewhat dense in consistency. It is on this
account that it is so easy to extract the nucleus quite intact from
immature eggs with a needle. The nuclear membrane can be
ruptured, as a result of which its contents flow out, and may
be spread out in the liquid in which the examination is taking
place. But it seems to me to be equally certain that, in other
cases, a true nuclear membrane is absent, so that the nuclear sub-
stance and protoplasm come into direct contact. Thus Flemming
(II. 10), in the blood cells of Amphibians, and I myself, in the
sperm-mother-cells of Nematodes at a certain stage of their develop-
ment (Fig. 20 B), have failed to discover a nuclear membrane.

Aurmann has endeavoured, by means of a special staining process with
cyanin, to demonstrate a granula structure in the nucleus as well asin the
protoplasm. By means of this process he has succeeded in intensely staining
the sap which fills up the interstices in the nuclear network, and in thus
showing up granula, whilst the nuclear network remains uncoloured, and is
designated intergranula substance. In this manner Altmann has obtained a,
so to speak, negative impression of the nuclear structure, as it becomes re-
vealed by staining the nuclear network with the usual nuclear staining
reagents. Since he considers that the granula form the most important part
of the nucleus, his opinion of the relative importance of the nuclear sub-
stances differs from the one which is generally accepted, and according to
which the nuclear sap is of less importance than the nuclein and paranu-
clein.

c. The Structure of the Nucleus. Examples of its
various Properties. Theabove-mentioned substances, of which
nuclein and paranuclein at any rate are never absent, occur in
very different forms in the nuclei of various plant and animal
cells ; this is especially true of nuclein, which may be present as

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 45

fine granules, as large masses, as fibrils, as a framework, or in the

form of a honeycomb structure.

Further, one such structure

may develop into another during the various vital phases of the

cell’s life-history.

Hence in formulating a definition of the nucleus, its varying
form must be quite disregarded; the difficulty consists in defining
the active substances contained in it, similarly as, in defining the

cell, the difficulty les in describing protoplasm.
The nucleus consists of a mass of substances,
which are peculiar to it, and which, to a cer-
tain extent, differ from protoplasm, and may
be distinguished from it. On this account, in
all definitions of the nucleus, more importance
should be attached to the properties of its

structural Sg aac than is usually the

case.

The following selection of typical examples
will serve to show what a multiplicity of
forms may be assumed by the internal struc-
ture of the resting nucleus.

It is beyond dispute that the Sie struc-
ture —disregarding the molecular conditions
discussed later—is seen in the nuclei of mature
sperm-cells. When the sperm-cells, as is the
rule, assume a thread-like form, being the one
most suitable for boring their way into the
egg-cells, the nuclei constitute the anterior
ends or heads of the threads. In the Sala-
mandra maculata the head is like a sword,
terminating in a sharp point (Fig. 21 k); it
consists of dense nuclein which, even when
most highly magnified, is still homogeneous in
appearance. A short cylindrical body, the so-
called middle portion (m), which also appears
homogeneous, is joined on to the head; this
portion reacts like paranuclein. Hence, ap-
parently, it must be considered to form part
of the nuclear portion of the sperm-thread ;
this, however, can only be finally proved when
its further development has been observed.

‘p

Fie. 21. — Spermato-
zoon of Salamandra
maculata: k head; m
middle portion; ef ter-
minal portion; sp apex;
wu undulating mem-
brane.

Further, in sperm elements, where the form of the cell has been

46 THE CELL

retained, the nucleus appears as a compact globular mass of
nuclein; this is the case in the sperm elements of Ascaris
megalocephala (Fig. 22), which, when immature, are shaped like
fairly large, round cells, and when mature

assume the form of a thimble.
Pon8 Having examined this simple condition
k of the nucleus, as it occurs in sperm-cells,
b and where it is composed almost entirely
Fira. 22.—Sperm-cell of of active nuclear substances, being nearly
Ascaris megalocephdla (after tree from the admixture of. other sub-
Van Beneden; from O.
Hertwig’s Embryology, Fig. stances, we may now proceed to examine
ks re a ben other nuclear forms. In these we see that
itself totheegg; fshining the chief cause for the variety in form, which
Pateieros veering 0. has been observed in plant and animal cells, is
the fact, that the active nuciear substances evince a great inclination
to take up liquid, with the substances dissolved in tt, and to store it up,
generally to such an extent, that the whole nucleus acquires the ap-

pearance of a bladder enclosed in protoplasm.

Thus in the nucleus, a process takes place similar to that which
occurs in protoplasm, where the cell-sap becomes collected in
vacuoles or large sap-cavities. This circumstance bears the same
significance in either case. These vacuoles are concerned in the
metabolism both of the cell and of the nucleus, for they contain
in solution nutrient materials, which can be easily taken up by
the active substances, in consequence of the great superficial de-
velopment of the vacuoles.

This process of sap absorption may be directly observed when,
after fertilisation has taken place, the nucleus of the sperma-
tozoon, in performance of its function, enters the egg-cell. In
many cases it begins to swell up gradually, until it becomes ten
to twenty times as large as it was originally ; this is not due to
any increase of its active substances, which remain absolutely
unaltered in quantity, but entirely to the absorption of fluid
substances which were held in solution in the yolk. In such a
nucleus, which has become transformed into a vesicular body, the
nuclein is spread out in fine threads to form a net; in addition,
one or two globules of paranuclein (nucleoli) are now to be seen.
A similar process occurs each time a nucleus divides, when the
daughter nuclei are being reconstructed.

According as to whether the nucleus has absorbed a greater or
less quantity of nuclear sap, its solid constituents, which on account

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 47

of their chemical properties have been distinguished above as linin
and nuclein, arrange themselves in the form of a more or less fine
framework structure. Figs. 23-26 show us examples of the various
modifications which may occur.

Fig. 23 represents the nucleus of a cilio-flagellate organism. It
consists, like the chief nucleus of the Infusoria, of a small-meshed
framework of nuclein. Biitschli (II. 5) considered that it is in
the form of a small delicate honeycomb; in his opinion the nucleus
is composed of extended faviform chambers, with three or more
sides, separated from one another by very delicate partition walls
-of nuclein, and enclosing the nuclear sap, which is only slightly
affected by staining reagents. Similarly their upper surfaces are
separated from the protoplasm by means of a delicate layer of
nuclein, there being no distinct true nuclear membrane. The points

Fie. 23. — Nuclens of Ceratium tripos, Fie. 24,—Nucleus of a connective
in which the faviform structure is very tissue cell from the peritoneum of a
plainly shown (after Biitschli, Pl. 26, Fig. Salamander larva, with central cor-
14): A ventral view; B lateral view. Both puscles lying nearit. (After Flemming,
illustrations represent optical sections only. Fig. 4.)

where the partition walls meet are thickened like columns. The
appearance varies according to the point of view from which the
nucleus is seen, in consequence of the extended form of the faviform
chambers, which lie parallel to one another; a glance at Fig. 23 A,
B, explains this. One or two nucleoli are to be seen in the cavity.

Fig. 24 represents the nuclear framework of a connective tissue cell
of a Salamander larva. It consists of a fairly close network com-
posed of extremely delicate threads. A few denser swellings
occur here and there, usually where several threads cross ; these
swellings retain the stain with especial tenacity. They consist of
collections of nuclein, and may look very like true nucleoli, which

48 THE CELL

consist of paranuclein, and on this account Flemming has called
them net-knots, in order to distinguish them from nucleoli.

The framework of the nuclei of the various animal tissue cells
may be fine or coarse. In the latter case it consists of only a
few strands, so that “it hardly deserves the name of a net or
framework.” As a rule, the nuclei of young, embryonic and
growing tissues possess, as Flemming has observed, networks’
coarser than those of similar tissues in the adult.

For the most part the nuclear framework is composed of two different
- substances, linin and nuclein; of these the latter alone is capable of
absorbing and retaining the ordinary staining reagents. The two
substances are generally so arranged that the nuclein, in the
form of coarser and finer granules, is evenly distributed upon and
throughout the colourless linin. When the meshes of the frame-
work are very fine (Fig. 24) it may be very
difficult, or indeed impossible, to distinguish
the two substances from one another. In a
coarser network, such as is represented in
Fig. 25, it is much easier to do so; here a
resting nucleus from the protoplasmic lining of
the wall of the embryo-sac of Fritillaria im-
perialis is portrayed. \ According to Stras-
burger’s description, the delicate framework
threads as a rule do not become stained; hence

Fic. 25.— Fritilaria they must consist of linin. Coloured nuclein
imperialis. A resting granules of varying size are seen to be de-
nucleus (after Stras- - aan
putger, Wig. WE A posited upon them. In addition a number of

variously sized nucleoli are to be seen.

If any one should wish to convince himself of the fact that a
special framework of linin is present in the nucleus, he cannot do
better than examine the nuclei of the sperm-mother-cells, of the
round worm of the horse (Fig. 26). During the early stages of
division, all the nuclein is gathered into eight bent hook-shaped
rods, which collect together into two bundles; they are, as it were,
suspended in the nuclear cavity, for colourless threads of linin con-
nect them both to the nuclear membrane and to one another. It
is impossible for these threads to be coagula in the nuclear sap,
produced by the use of reagents, since they are invariably regu-
larly arranged. Similarly their chemical reaction and their be-
haviour during the process of division show that they are composed
of a substance which differs somewhat from nuclein and para-
nuclein.

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 49

Moreover, the nuclein is not always spread out upon a frame-
work. For example, the large vesicular nuclei of Chironomus larve
(Fig. 27) enclose, as Balbiani (II. 2) has discovered, a single thick
nuclear thread ; this is variously twisted, and in stained prepara-
tion is seen to be composed of regular alternately stained and
unstained layers. This has also been observed by Strasburger in
some plants. The two ends of the thread terminate in nucleoli.

Further, in other cases the greater part of the nuclein is collected
into a large round body, which looks like a nucleolus, but which is
really very different from the above-described true nucleoli, which
contain paranuclein (p. 42). In order to avoid confusion it is best
to call such bodies nuclein corpuscles. As an example of this class
the nucleus of Spirogyra may be mentioned; the nuclei of many of
the lower organisms are very similar to it in structure. It consists

Fig. 26. Fie. 27.

Fig. 26.—Nncleus, about to divide, of a cell from Ascaris megalocephala bivalens, with the
eight nuclear segments arranged in two bundles, and with two pole corpuscles. (Hertwig
LTS; Labs 11. Bice 18.)

Fig. 27.—Structure of the nucleus of a cell from the salivary gland of Chironomus. (After
Balbiani, Zoolog. Anzeiger, 1881, Fig. 2.)

of a vesicle which is separated from the protoplasm by a delicate
membrane, and which contains a fine nuclear framework. Since
this is incapable of retaining the dyes of staining solutions, it is
evident that it consists chiefly of linin, upon which only a few
nuclein granules are deposited. One large nuclein body is present
in the framework ; occasionally, however, it is divided into two
smaller ones. That this body really consists of nuclein is proved
partly by its behaviour towards staining solutions, but chiefly by
the fact that during nuclear division its substance breaks up into
granules, thus forming the nuclear segments.

Similar nuclein bodies, which in literature generally go under
the name of nucleoli, play a very important part in the structure of

the germinal vesicles of animal egg-cells. These germinal vesicles
Ez

me THE CELL

‘differ considerably in their structure from the nuclei met with in
ordinary tissues, as may be seen from Figs. 28, 29, 30.

Fig. 28 represents the immature egg of a sea urchin ; if it is ex-
amined when alive, an exceedingly coarse network of rather
thick isolated threads can be distinguished. These, as is shown
by their micro-chemical properties, consist chiefly of linin. The
stained material is nearly all collected into a single large round
body, the “ germinal spot,” which lies in a net-knot of the frame-
work, where the greatest number of linin threads intersect.

In the enormously large germinal vesicles, for which the large
eggs of Fishes, Amphibians, and Reptiles, which are so rich in yolk,
are remarkable, the number of germinal spots increases consider-

Fig. 28. Fig. 29.

Fie. 28.—Immature egg from the ovary of an Echinoderm. In the large germinal vesicle

there is a network of threads, the nuclear net, in which the germinal spot can be seen.
(O. Hertwig, Embryology, Fig. 1.)

Fig. 29.—Germinal ve-icle of a small immature egg from the Frog. Ina dense nuclear

net (kn) a very large number of germinal spots, mostly peripheral (kf), are to be seen,

(O. Hertwig, Embryology, Fig. 2.)

ably during the growth of the cell, until finally they may number
some hundreds; whether this multiplication takes place by division
or in some other fashion is not yet known. The position of the
germinal spots varies at different times; generally, however, they
are situated on the surface of the vesicle, being distributed at even
distances over the membrane, as is shown in Fig. 29, where the
nucleus of a rather small immature egg of a frog is depicted.

The shape of the germinal spots also varies; they may be round—
this is especially the case when they are isolated—or oval; some-
times they are somewhat extended, at others they are constricted
in the middle; occasionally they are irregular in outline, and when

It'S CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 51

they are very numerous, they show considerable differences in their
size. Very frequently a few small vacuoles filled with fluid are to
be seen. The examination of living egg-cells shows that these
vacuoles are not artificially produced. Additional vacuoles may
be formed after the death of the egg, whilst those already present
may increase in size, as has been pointed out by Flemming (II. 10,
ps Lol), ;

These germinal spots differ in their chemical properties from true
nucleoli, which consist of paranuclein and do not become stained
with the usual nuclear staining reagents. On the other hand, it
has not yet been discovered whether their substance is quite iden-
tical with the nuclein of the framework. Up to the present
this point has not yet been satisfactorily worked out, in spite
of the numerous experiments which have been made upon the
nucleus. One thing alone can be accepted as certain—that the
more or less rounded bodies present in various plant and animal
nuclei, which in scientific literature are classed together, for the
most part incorrectly, under the name of nucleoli, show material
differences amongst themselves. This has been proved beyond a
doubt by the investigations made by Flemming (II. 10), Carnoy
(II. 8), myself (II. 19a), Zacharias (II. 45), and others. Hither
such very different bodies should not be called by the same name,
or if, merely on account of their similarity in form, the common
name of nucleolus or nuclear body is retained for all round nuclear
contents, at any rate in each case an accurate description of the
chemical nature of the nucleolus in question should be given.
Above all, as has been already remarked, in all examinations of the
nucleus, more attention should be paid to the chemical properties
of its individual constituents than to their form and arrangement,
which are always of comparatively little importance. For the
function of a framework in the nucleus composed of linin threads
differs considerably from that of one consisting of nuclein, or of a
combination of the two substances, and similarly the function of the
nucleolus varies according to the material of which it is composed.

I will conclude this discussion of nucleoli with the remark that
germinal spots exist which are most evidently built up of two\
different substances. This circumstance was first observed by
Leydig in a lamellibranchiate Mollusc, and his statement has since
been verified by Flemming (II. 10) from observations on the same
animal, and by myself (II. 19) from those on other objects. I here
quote the description as it is given by Flemming.

52 THE CELL

In Cyclas cornea and in the Naiadez a principal nucleolus, in
addition to a few smaller secondary nucleoli, is present in the
germinal vesicle. ‘The former consists of two differently consti-
tuted portions ; these may be seen in Fig. 30 as a smaller, strongly
stained more refractive part, and a larger, paler, less chromatic one,
which swells up more in acids, In Anodon these two portions
are closely coherent; in Unio they very frequently only just touch
each other, or, indeed, may lie apart. The smaller secondary
nucleoli, which lie in the meshes of the framework, show the same
power of refracting light, of swelling up, and of becoming stained,
as the larger portion of the principal nucleolus. If water is added,
this larger portion disappears,
as well as the small nucleoli,
amongst the strands of the
framework; the small, strongly
chromatic portion of the prin-
cipal nucleolus alone remains ;
this becomes more sharply de-
fined, shrinking up somewhat,
and developing a clearly marked
outline. The addition of strong
acetic acid (5 per cent. or more)
causes the larger paler portion
of the principal nucleolus to
swell up rapidly and to dis-
appear, whilst the smaller shin-
ing portion, though also swell-

Fie. 30.—(Afier Flemming, Fig. E’, p. 104.)
a Nucleus of anegg from the ovary of Unio;

it has just emerged from the cell into the
ovarian fluid. Nucleolus with two :pro-
tuberances. A small portion of the nuclear
framework is visible; a a similar nucleus
after 5 per cent. acetic acid has been added.
The framework strands stand out more
clearly; the larger paler portion of the
principal nucleolus, as well as the minor
nucleoli, have similarly become swollen up
and faded; the smaller portion of the
principal nucleolus is also swollen up, but
to a less degree. b Nucleolus of an egg
of Tichogonia polymorpha ; the principal
glistening portion rests like a cap upon the
larger one. 6 Diagrammatic representation
of an optical section of above.

many animals.

ing up somewhat, remains visi-
ble.” ‘When nuclear staining
reagents are used, both portions
of the nucleolus, and also the
secondary nucleoli, become
coloured to a considerable ex-
tent; the most strongly refrac-
tive part of the former, however,
is especially intensely stained.”
“Such a differentiation of the
principal nucleolus into two
parts occurs in the egg-cells of

In Dreissena polymorpha the strongly refractive

chromatic portion covers the paler one like a hollow cap,”

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 53

I have observed (II. 19) that the germinal spot is composed of
two substances in Helix, Tellina, and Asteracanthion, as well as in
Anodon. Asteracanthion (Fig. 31) is of special interest, as the
separation into two substances (pn,u) only becomes distinctly
visible when the germinal vesicle commences to break up and to
form the polar spindle out of its contents.

Finally, in the description of the structure of the resting nucleus,
attention must be drawn to one other important point. According to
the uge or stage of development of a cell, the resting nucleus may present
very considerable variations in all its separate parts: as to the appearance
of its framework, and as tothe number, size, and peculiarities of its wucleolt.
Thus, as Flemming (II. 10) remarks, ‘‘In young eggs from the
ovaries of Lamellibranchs, this twofold composition of the large
nucleolus is not to be seen; it only develops in the mature egg.”
Aboveall, the germinal vesicles of the eggs undergo during their de-
velopment important metamor-
phoses, which at present have
been but little investigated,
whilst their significance is still
less understood. The same is
true of the nuclei of sperm
mother cells. I have
deavoured to follow accurately
these changes of form in such
cells obtained from the testis of

en-

Fie. 31.-—Section from an egg of Astevrias
glacialis showing the degeneration of the
germinal vesicle. This begins to shrink up,

Ascaris megalocephala (II. 19 b),
which are very suitable for the
purpose.

As is shown in Fig. 32, form A

whilst a mass of radiated protoplasm («)
forces its way into the interior, breaking

down the membrane. The germinal spot
(kf) is still visible, but is divided into two
substances, nuclein (nn) and paranuclein

gradually becomes transformed (yn). (0. Hertwig, Embryology, Fig. 12.)

into form B, and this during the process of development of the
spermatozoon into form C; the youngest sperm mother cells (B)
have naked nuclei containing dense nuclear frameworks, and
superficially-placed nucleoli; this form develops in older cells (C)
into a vesicular nucleus with a distinctly marked membrane. In
the vesicle a few linin threads are extended through the nuclear
sap, the nuclein heaped up into one or two irregular masses,
amongst which the more or less globular nucleolus is situated. In
cells which are not yet mature, the nuclein is collected chiefly at
one spot of the nuclear membrane in the form of a thick layer,
whilst granules of varying size lie upon the surface of the linin

54 : THE CELL

threads, a few of which are extended throughout the nuclear
space. A considerable time before division occurs, the nuclein

Fie. 32.—A Resting nucleus of a primitive sperm cell of Ascaris megalovephalu bivalens ;
B nucleus of a sperm mother cell from the commencement of the growth zone; C resting
nucleus of a sperm mother cell from the growth zone; D vesicular nucleus of a sperm
mcther cell from the commencement of the dividing zone just before division.
becomes arranged in definite threads (D). A nucleolus is always
present in the meshes of the framework.

Ill. Are there Elementary Organisms avisittig without
Nuclei? The important question, as to whether the nucleus
is an indispensable portion of every cell, follows naturally on
the description of the chemical: and morphological properties of
the nucleus. Are there elementary organisms without nuclei ?
Formerly investigators were not at a loss to answer this question.
For since, in consequence of the inadequacy of former methods of
examination, no nuclei had been discovered in many of the lower
organisms, the existence of two different kinds of elementary cells
was assumed: more simple ones, consisting only of a mass of
protoplasm, and more complex ones, which had developed in
their interior a special organ, the nucleus. The former were
called cytodes by Haeckel (I. 10; II. 15), to the simplest, solitary
forms of which he gave the name of Monera; the latter he called
cellule, or cytes. But since then the aspect of the question has
become considerably changed. Thanks to the improvements in
optical instruments, and in staining methods, the existence of
organisms without nuclei is now much questioned.

In many of the lower plants, such as Alge and Fungi, and
in Protozoa, Vampyrella, Polythalamia, and Myxomycetes, all
quoted formerly as examples of non-nucleated cells, nuclei may
now be demonstrated without much trouble. Further, since
the nucleus has been discovered in the mature ovum (Hertwig
TI. 19a), we may safely say that, in the whole animal kingdom,
there is not a single instance where the existence of a cell with-
out a nucleus has been proved. I shall probably be confronted
with the red corpuscles of Mammals. It is true that they contain
no nuclei, but then neither do they contain any true proto-

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 55

blood discs of Mammals are not true cells, but only the products
of the metamorphosis, or of the development of former cells, may
be defended for many reasons. /

The only remaining instance of cells in which, on account of

plasm, and hence the theory, more fully described later, that the |

their extreme minuteness, no differentiation into protoplasm and
nuclear substance can be demonstrated, is furnished by Bacteria
‘and other allied forms. However, even here Biitschli (II. 6) has
endeavoured to prove the existence of a nuclear-like body. Thus
in Oscillaria and in others (Fig. 33 A, B), he has pointed out
bodies which are not digested by gastric juice, and which contain
a few granules, which stain intensely (probably nuclein granules) ;
these make up the greater part of
the cell substance, the protoplasm
being present only as a delicate
envelope. Biitschli’s views are for

the most part shared by Zacharias A
(II. 47). wr
Even if it is objected that the Leni
above statement is at present un- tig
proven, it caunot be denied that yt
the supposition that Bacteria con- Brece

sist entirely, or principally, of nu-
clear substance, seems at any rate
as probable, if not more so, as the Fig. 33.—A Oscillaria : Optical section
one that they are minute masses of a cell from a thread, killed with
alcohol and stained with hematoxylin
of pure protoplasm. The extra- (after Biitschli, Fig. 12a). B Bacterium
ordinary affinity of these organisms limeola (Cohn), in optical section, killed
: Pest : 2 with alcohol and stained with hema-
for staining reagents is very much toxylin (after Bitschli, Fig. 3a).
in favour of the first view.

IV. The Central or Pole Corpuscles of the Cell. Long
ago an exceedingly minute object, which, on account of its
function, is of the greatest importance, was observed in addition
to the nucleus in the protoplasm of some cells; this is the central
or pole corpuscle (centrosome). This was first noticed during cell
division (which is described later on in Chapter IV.), and here it
plays a most important part, as it forms a central point for the
peculiar radiated appearances, and above all functions as the centre
of the cell, around which the various cell contents are, to a certain

extent, arranged.
As to size, it is only just visible, and is frequently much smaller

he. THE CELL

than the most minute micro-organism. As to its composition, it
appears to consist of the same substance as the so-called neck or
middle portion of the seminal thread, to which, further, during
the process of fertilisation, genetic functions have-been ascribed
(vide Chap. VII, 1). When the ordinary methods for staining
the nucleus are employed it does not absorb any of the dye;
if, however, special reagents, especially acid aniline dyes, such as
acid fuchsine, safranin, and orange, are used, it becomes vividly
coloured. This is the only way to distinguish the central cor-
puscle from the other granules in the cell (microsomes) unless it
is enclosed by a special radiation sphere or envelope. If we dis-
regard the processes of cell division and of fertilisation, which are
treated of in later sections, the central corpuscles have been, up
till now, most frequently observed in lymph cells (Flemming II.
11, 12 b, and Heidenhain IJ. 16), in the pigment cells of the
Pike (Solger II. 38), and in the flattened epithelial, endothelial,
and connective tissue cells of Salamander larve (Flemming II.
12 b).

As arule there is only one central corpuscle present in each
lymph cell (Fig. 34); this can be seen without having been
stained, since the protoplasm in its im-
mediate neighbourhood assumes a distinctly
ray-like appearance forming the radiation,
or attraction sphere, which later on will
occupy so much of our attention. The cen-
tral corpuscle is sometimes situated in an
indentation of the nucleus, or, if the latter,
has broken down into several pieces, a con-
dition which is frequently seen in lymph
cells, it lies between them and some portion
or other of the protoplasmic body.

In pigment cells (Fig. 35), Solger (II. 38)
was able to make out the radiation sphere
as a bright spot between the pigment gran-

Fie. 34. — Leucocyte

from the peritoneum of a
Salamander larva. For
the sake of clearness in
the figure, the central cor-
puscle, surrounded by its
radiation sphere, has been
distinguished by a bright
ring, which is not really
present in nature. (After
Flemming, Fig. 5.)

ules, and in consequence he concluded that _
the central corpuscle was present.
In the epithelium of the lung, and in the
endothelium and connective tissue cells of
the peritoneum of Salamander larve (Fig.
36 A, B), Flemming found, almost without
exception, that instead of a single central

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES o7'

corpuscle, two were present, lying
close together, either in the im-
mediate neighbourhood of the
resting nucleus, or in an indenta-
tion of it, directly in contact
with the nuclear membrane. As
a rule no radiation sphere was to
be seen in these cases; some-
times the two central corpuscles,
instead of touching each other
closely, were somewhat separated
from one another, and under Poe

: Fig. 35.—Pigment cell of the Pike, with
these circumstances the first com- iy nuciei, and one sa Soranaclot nie

mencement of a spindle formation ronnded by a radiation sphere. (After
Solger, Fig. 2.)

ys
|
\

between them was visible.

Fig. 36.—A Nucleus of an endothelial cell from the peritonenm of a Salamander larva,
avith the pole corpuscle lying near (after Flemming, Fig. 2). B Nucleus of a con-
nective tissue cell from the peritoneum of a Salamander larva, with the pole corpuscle lying
near (after Flemming, Fig. 4).

Van Beneden (II. 52) first advanced the theory that the central
corpuscle, like the nucleus, is a constant organ of each cell, and that it
must be present in the cell in some portion of the protoplasm near
the nucleus. The property possessed by the central corpuscle of
being able to multiply itself by spontaneous division (vide Chap.
VI.) seems to be in support cf the first part of this view, as is also
the réle it plays in the process of fertilisation (vide Chap. VII. 1);
but the second portion of this theory, although it is very generally
accepted, that the central corpuscle belongs to the protoplasm,
appears to me, on the contrary, less certainly true.

58 THE CELL

.

T have for some time held the opinion, which, for reasons that
I will state later (vide Chap. VI.), I still hold to be worthy of con-
sideration, that the central corpuscles are generally constituent
parts of the resting nucleus, since after division has taken place
they enter its interior, and whilst it is preparing for division come
out again into the protoplasm. Only in rare cases do the central
corpuscle or corpuscles remain in the protoplasm itself, whilst the
nucleus is resting, and then to a certain extent they represent a
subordinate nucleus in addition to the principal one. This theory
would explain the fact that, even with the more recent methods and
most improved optical instruments, the central corpuscles as a rule
cannot be demonstrated near the resting nucleus in the protoplasm of
the cell.

V. Upon the Molecular Structure of Organised Bodies.
In order to explain the chemico-physical properties of organised
bodies, Nigeli (V. 17,18; II. 27, 28) has advanced a micellar
theory, which, although undoubtedly to a great extent hypothetical,
is very useful in rendering many complicated conditions more
easy of comprehension, and above all more easily pictured to the
imagination. A short abstract of this micellar theory, which de-
serves attention, if only on account of the strictly logical manner
in which it has been worked out, will not be out of place
here. |

One of the most remarkable properties of an organised body is
its capacity of swelling up, that is to say, of absorbing into its
interior a large, though not unlimited, quantity of water, with the
substances dissolved init. This may take place to such an extent
that in an organised body only a small percentage of solid sub-
stances may be present.

The body increases in size in proportion to the amount of
water absorbed, shrinking up again when the water is expelled.
Hence the liquid is not stored up in a _pre-existent. cavity,
which before was filled with air, as in a porous body, but
becomes evenly distributed amongst the organised particles,
which, as the body swells up, must become farther and farther
pushed apart, being separated from one another by larger and
larger envelopes of water. In spite of the absorption of so much
water, none of the organised substance becomes dissolved. In this
respect the phenomenon differs from that which takes place with a
crystal of salt or sugar, which on the one hand does not possess
the power of swelling up, and on the other becomes dissolved

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 59

in the water, its molecules separating from one another, and dis-
tributing themselves evenly throughout the water.

Its power of swelling up and its non-solubility in water are
the most important properties of an organised body, without
which it is inconceivable that the vital processes could proceed.

Many organised bodies may be dissolved if treated according
to special methods, as for example starch and gelatine-producing
substances, which become dissolved when they are boiled in
water. But even these starch and gelatine solutions differ very
much in their chemical properties from solutions of salt or sugar.
The latter diffuse easily through membranes, whilst the former
either do not do so at all, or only to a very small extent, whilst
their solutions are slimy or viscous. Graham distinguishes
between the two groups of substances, which exhibit such different
properties in solution, by calling them erystalloids and colloids.

Now Nageli has attempted to explain all these phenomena as
being due to differences in the molecular structure of the various
bodies. As atoms combine together to form molecules, thus pro-
ducing so great a variety of chemical substances, so he considers
that the molecules unite together in groups to form still more
complex units, the micelle, and that in this manner the complex
properties of organised bodies arise. In comparison with that of
the molecule, the size of the micella is considerable, although too small
to be seen with the microscope; it may be built up, not only of
hundreds, but even of many thousands of molecules.

Nageli ascribes a crystalline structure to these micelle, in con-
sequence of their power of double refraction, which further is ex-
hibited by many organised bodies, such as cellulose, starch, mus-
cular substance, and even protoplasm itself in polarised light. In
addition, great differences may be present in their outward appear-
ance as well as in their size.

The micelle have an affinity for water as well as for each
other; hence their power of swelling up. In a dry organised
body the micelle lie close together, being only separated by
delicate envelopes of water; as more water becomes absorbed,
these envelopes increase considerably in size, since at first the
micelle have a stronger affinity for water than for each other.
Thus they become pushed apart from each other by the penetrating
water as with a wedge; ‘‘ however, organised bodies cannot become
really dissolved, for the molecular attraction of the micelle for
the water diminishes with distance at a proportionally greater

60 THE CELL

rate than that of the micelle for each other, and hence when the
envelopes have reached a certain size a condition of equilibrium,
the limit of the power of the body to swell up, is reached.”

When, however, by means of special methods of treatment, the
attraction of the micelle for each other is quite overcome, a
micellar solution is obtained. This solution is cloudy and opal-
escent, which is an indication that the light is unevenly refracted.
Niageli compares this with the slimy opalescent masses produc d
when Schizomycetes. are crowded together in large numbers.

Niageli explains the differences, which Graham has described
as existing between crystalloids and colloids, by the statement
that in the former isolated molecules are distributed amongst the
particles of water, whilst in the latter crystalline groups of mole-
cules or isolated micelle are so distributed. Hence numbers of
the one group form molecular solutions, and those of the other
micellar solutions (such as egg-albumen, glue, gum, etc). The
micelle themselves have considerable power of preventing the
substance from breaking down into molecules. Such a breaking
down is generally accompanied by chemical transformation.
Thus starch, after it has been converted into sugar, is capable of
forming a molecular solution, as is also the case with proteids and
gelatine-yielding substances after they have been converted into
peptones.

In organised bodies the micelle unite together to form regu-
larly arranged colonies, in which the individual micelle may
consist of similar or different chemical substances, and may vary
as to size and form; further, they may unite in smaller or larger
groups of micelle within the colony itself. he micelle within
these micellar colonies appear asa rule to hang together in chains,
which further unite together to form a frame or network structure
with more or less wide meshes. In the gaps or micellar interstices the
water is enclosed. ‘“ Only in this manner is it possible to have a
firm structure, composed of a large quantity of water and a small
quantity of solid matter, such as is seen in a jelly.”

The water, which-is contained in organised bodies, may be
found in three conditions, distinguished by Nageli under the names
water of coustitution or of crystullisation, water of adhesion, and
cupillary water. By the first are understood the molecules of
water, which, as in a crystal, are united firmly to the molecules
of the substance in a fixed proportion, thus entering into the
structure of the micella, _

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 61

The water of adhesion consists of molecules of water, which are
held closely to the surface of the micella by molecular attraction.
‘The concentric layers of water, which compose the spherical
envelope surrounding the micella, vary considerably as to their
density and their immobility ; they are naturally most dense and
firmly attached when they are in direct contact with the surface
of the micella” (Pfeffer).

The capillary water finally is outside the sphere of attraction
of the individual micelle and fills up the gaps in the micellar net-
work.

“These three kinds of water show considerable variation as
to the degree of motility shown by their molecules. The mole-
cules of capillary water are as free in their movements, as those
of free water; in the water of adhesion the progressive move-
ments of the molecules are more or less diminished, whilst the
molecules of the water of constitution are fixed and non-motile.”
Hence only the waters of capillarity and of adhesion can pass
through a membrane by osmosis.

Just as water particles may be firmly held upon the surface of
the micelle by molecular attraction, other substances (calcium
and silicon salts, colouring matter, nitrogenous compounds, etc.),
having been taken up in solution into the organised body, may
be deposited upon them. The growth of organised matter by
intussusception is explained by Nageli, by the supposition that
particles of material in solution make their way into the organised
body, such as, for example, molecules of sugar into a cellulose
membrane, where they may either become deposited upon the
micelle which are already present, thus adding to their size, or
to a certain extent they may crystallise out to form new micelle
situated between the ones already present. As an example of
this, the phenomenon of sugar'molecules becoming converted into
cellulose molecules may be quoted.

This micellar hypothesis of Nageli is frequently referred to in
later chapters, as it often is of great use in forming a mental
picture of the complex arrangement of matter in the elementary
organism.

Literature II.
1. Anrmann. Dis Elementarorganismen u. ihre Beziehungen zu den Zellen.
Leipzig. 1890.
2. Jun. Arnonp. Ueber feinere Structur der Zellen unter normalen und
pathologischen Bedingungen. Virchows Archiv. Bd. 77, 1879, p. 181.

62

7B.

UB Fe
14a.

14s.

15.
16.

Ay

18.

THE CELL

Baupianrt. Sur la structure du noyau des cellules salivaires chez les larves
de Chironomus. Zoologischer Anzeiger, 1881, p. 637.

van Brenepen et Neyr. Nouvelles recherches sur la fécundation et la
division mitosique chez Vascaride mégalocéphale. Leipzig. 1887.

Birscuut. inige Bemerkungen iiber gewisse Organisationsverhdltnisse
der sogenannten Cilioflagellaten und der Noctiluca. Morph. Jahrbuch.
Bd. X. 1885.

Birscuut. Ueber den Bau der Bakterien und verwandter Organismen.
Leipzig. 1890.

. Birscnur. Ueber die Structur des Protoplasmas, Verhandlungen des

Naturhist.-Med.-Vereins zu Heidelberg. N.F., Bd. IV., Heft 3. 1889.
Heft 4. 1890. (See Quar. Jour. Mic. Soc., 1890.)

Birscuxur. Untersuchungen iiber mikroskopische Schiume u. das Proto-
plasma. 1892.

Carnoy. Several papers in La Cellule. Recueil de Cytologie et @histo-
logie générale.

La cytodiérése chez les Arthropodes, T. I.

La vésicule germinative et les ylob. polaires chez divers nématodes.

See also Conférence donnée @ la société belge de microscopie, T. III.

See also A. B. Lex. On Carnoy’s cell researches. Quar. Jour. Mic. Soc.

Vol. XXVI., pp. 481-497.

_Encetmann. Ueber den fasrigen Bau d. contractilen Sustanzen,

Piliigers Archiv. Bd. 26. Physiology of Protoplasmic Movement,
trans. Quar. Jour. Mic. Soc. Vol. XXIV., p. 370.
Fremmine. Zellsubstanz, Kern und Zelltheilung. Leipzig. 1882.
Fiemmine. Ueber Theilung u. Kernformen bei Leukocyten und iiber deren
Attractionssphiren. Archiv. f. mikroskop. Anat. Bd. 37, p. 249.
Fuiemmine. Neue Beitriige zur Kenntniss der Zelle. Il. Theil. Archiv. f.
mikroskop. Anat. Bd. 37, p. 685.

. Fuemmine. Attractionssphiren und Centralkirper in Gewebszell n und

Wanderzellen. Anatomischer Anzeiger. Bd. VI.

See also Joan E. 8S. Moore. On the Relationships and Réle of the
Archoplasm during mitosis in the Larval Salamander. Quar. Jour. Mic.
Soc. Vol. XXXIV., p. 181.

Fou. Lehrbuch der vergleichen mikroskop. Anatomie. Leipzig. 1884.

Frommann. Zur Lehre von der Structur der Zellen. Jenaische Zeit-
schrift f. Med. und Naturw. Bd.9. 1875.

Frommann. Zelle. Realencyklopdidie der gesammten Heilkunde. 2 Aufl.
1890.

Harckent. Générale Morphologie.

Marrin Herpennarn. Ueber Kern und Protoplasma. Festschrift fiir
Kélliker. 1892.

See also W. D. Hattrsurton. Gulstonian Lectures on the Chemical
Physiology of the Animal Cell. Brit. Med. Jour. Vol. I. 1893.

C. Herrzmann. -Untersuch. iiber Protoplasma. Wiener Sitzungsver. math.
naturw. Classe. Bd. LXVII. 1873.

Ricuarp Hertwiec. Beitriige zu ein r einheitlichen Auffassung der ver-
schiedenen Kernformen, Morphol. Jahrbuch. Bd. 2. 1876.

ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 63

194. Oscar Hertwic. Beitriige zur Kenntniss der Bildung, Befruchtung und
Theilung des Thierischen Eies. Morphol. Jahrbuch. Bd. I., l., 1V.

198. Oscar Hertrwic. Vergleich der Ei- u. Samenbildung bet Nematoden.
Archiv. f. mikroskop. Anatomie. Bd. 36. 1890.

20. HormetistER. Die Lehre von der Pflanzenzelle. Leipzig. 18€7.

21. KE. Kuen. Observations on the Structure of Cells and Nuclei. Quar. Jour,
Mic. Soc. Vol. XVIII., 1878, p. 315.

22. Kouiiker. Handbuch der Gewebelehre. 1889.

23. Kosser. Zur Chemie des Zellkerns. Zeitschrift fiir physivlog. Chemie
von Hoppe Seyler. 1882. Bd. 7.

Untersuchungen tiber die Nucleine und ihre Spaltungsprodukte. Strassburg.
1881.
Kanruack and Harpy. Proceedings of the Royal Society. Vol. LIT.

24. C. Kuprrer. Ueber Differenzirung der Protoplasma an den Zellen thier-
ischer Gewebe. Schriften des naturwissenschaftl. Vereins fiir Schleswig-
Holstein. Bd. I., p. 229. Heft 3. 1875.

25. Leypie. Untersuchungen zur Anatomie u. Histologie der Thiere. Bonn.
1883.

26. Leypie. Zelle und Gewebe. Bonn. 1885.

27. NAGELI u. SCHWENDENER. The Microscope. Theory and Practice, trans.
London.

28. C. Nkcrext. Mechanisch-physiologische Theorie der Abstammungslehre.
Miinchen und Leipzig. 1884.

29. Prirzner. Beitriige zur Lehre vom Bau des Zellkerns u. seinen The l-
ungserscheinungen. Archiv. f. mikrosk. Anatomie. Bd.22. 1883.

J. Priesttey. Recent Researches on the Nuclei of Animal and Vegetalle
Cells. Quar. Jour. Mic. Soc. Vol. XVI., pp. 131-152.

30. v. Rarg. Ueber eine eigenartige polycentrische Anordnung des Chromatins
Zoolog. Anzeiger. 1890.

31. Ravusper. Neue Grundlegungen zur Kenntniss der Zelle. Morph. Jahrb.
VuI. 1882. : ,

32. Reinke u. H. Ropewaup. Studien iiber das Protoplasma. Untersuch-
ungen aus dem botanischen Institut der Universitat. Géottingen. Heft 2.
1881.

83. Sacus. Textbook of Botany, Morphological and Physiological, trans. by
S. H. Vines. 1832.

34. Scuirer and E. Ray Langester. Discussion on the Present Aspect of
the Cell Question. Nature. Vol. XXXVI. 1887.

See also ScHAFER in Quain’s Anatony, Vol. I., pt. 2. 1891.

35. ScHIEFERDECKER u. KosseL. Gewebelehre mit besondere Beriicksichtigung
des menschl. Kérpers.

36. Scumirz. Untersuchungen iiber die Structur des Protoplasmas und der
Zelikerne der Pflanzenzellen. Sitz..Ber. der Niedenh. Gesellsch. f.
Natur u. Heilk. Bonn. 1880.

37. Frank Scuwarz. Die morphologische und chemische Zusammensetzung des
Protoplasmas. Beitriige zur Biologie der Pflanzen. Bd. IV. Breslau. 1887.

38. Soncer. Zur Kenntniss der Pigmentzellen. Anatomischen Anzeiyger. Jahrg.
Vie 82:

48,

49,
50.

THE CELL

Srraspurcer. Zellbildung und Zelltheilung. 2 Aufl. Jena. 1876.
SrraspurGER. Studien iiber das Protoplasma. Jenaische Zeitschrift. 1876.
Bd. X. ;

SrrasBpurGER. . Practical Botany, trans. by Hillhouse.’ London.

Wiesner. LElementarstructur und Wachsthum der lebenden Substanz.

Zacwanias. Ueber den Zellkern, Botanische Zeitung, 1882, p. 639.

Zacnarias. Ueber Hiweiss, Nuclein und Plastin, Botanische Zeitung.
1883. ‘

Zacuarias, Ueber den Nucleolus. Botanische Zeitung. 1885.

Zacuarias. Beitriige zur Kenntniss des Zellkerns u. der Sexualzellen.
Botan. Zeitung, 1887. Bd. 45.

Zacwarias. Ueber die Zellen der Cyanophyceen. Botan. Zeitung. 1890.

See also HaturpurTon loc. cit.

List. Untersuch. iiber das Cloakenepithel der Plagiostomen. Sitzungsber,
der kaiserl. Acad. der Wissensch. zu Wien, Bd. XCII. III., Abth.
1885.

Mriescuer. Verhandl. der naturforschenden Cesellschaft in Basel; 1874,

ANEB\CH. Organologische Studien. Heft I. 1874.

CHAPTER III.
THE VITAL PROPERTIES OF THE CELL.

I. The Phenomena of Movement. All the mysteries of
life, which are exhibited by plants and animals, are present, as it
were in a rudimentary form, in the simple cell. Each individual
cell, like the whole complex organism, has an independent life of
its own. If we wish to study more deeply the true nature of
protoplasm, we must above all things investigate its most important
properties, its so-called vital properties. However, life, even the
life of the simplest elementary organism, is a most complex
phenomenon, which it is most difficult to define ; it manifests
itself, to use a wide generalisation, in this, that the cell in conse-
quence of its own organisation, and under the influence of its
environment, experiences continual changes and develops powers,
by means of which its organic substance is being continually
broken down and built up again. During the former process,
energy is set free. The whole vital process, as Claude Bernard
(IV. 1a) expresses it, depends upon the continual co-relation of
this organic destruction and restoration.

It is most convenient to classify these most complex phenomena
under four heads. Thus each living organism exhibits four
different fundamental functions or properties, by means of which
its life is made manifest : it can alter its form, and exhibit move-
ments ; it reacts to certain external stimuli in various ways, that
is to say, it is irritable; it has the power of nourishing itself, it
can by absorbing and transforming food material, and by giving
up waste products, form substances, which it utilises for growth,
for building up tissues, and for special vital functions ; finally, it
can reproduce itself.

Hence we will discuss the vital properties of the cell in four
chapters, which we will take in the following order :

1. Phenomena of movement.

2. Phenomena of irritability.

3. Metabolism and formative activity.

4, Reproduction.
65 F

66 THE CELL

In addition there will be a special chapter on the process of
fertilisation.

The cell may exhibit several kinds of movement, as is seen if an
extensive comparative study is made. We will here distinguish
between: (1) true protoplasmic movements ; (2) ciliary or
flagellar movements ; (3) the movements of the pulsating
vacuole ; (4) the passive movements and changes of shape
exhibited by cells. A

In addition to these four, there are a few special phenomena of
motion, of which it will be best to treat in later chapters, for
example, the formation of the receptive protuberance which appears
in the egg-cell in consequence of fertilisation ; the radiation figures
which are seen in the neighbourhood of the spermatozoon after it
has penetrated into the ovum, and those which occur during the
process of cell division, when the cell body splits up into two or
more parts.

Protoplasmic Movements. Although it is probable that
movements take place in all protoplasm, yet in most cases, with
our present means of observation, they cannot be perceived on
account of their great slowness ; hence in only a few objects in
the plant and animal kingdoms can this phenomenon be studied
-and demonstrated. The movement manifests itself partly in
changes in the external form of the cell, and partly in the arrange-
ment of the structure enclosed in the protoplasm, the nucleus, the
granules, and the vacuoles.

These movements differ somewhat according as to whether they
are manifested in naked protoplasm, or in that which is enclosed
by a firm membrane.

a. The Movements of naked Protoplasm. Small uni-
cellular organisms, white blood corpuscles, lymph corpuscles,
connective tissue cells, etc., exhibit movements which,; in con-
sequence of their similarity to those seen in the Amoeba, are
termed amoeboid.

If a lymph corpuscle of a Frog (Fig. 37) is observed under suit-
able \circumstances, it is seen to undergo continual changes of
form. Small processes of protoplasm, the -foot-like processes, or
pseudopodia, are protruded from its surface ; at first as arule they
consist of hyaloplasm alone, but after a time granular protoplasm
streams intothem. By this means the pseudopodia are increased
in size; they become broader, and may in their turn extend new,
more minute processes from their surface. Or the protoplasm may

THE VITAL PROPERTIES OF THE CELL 67

flow back again, thus causing them to
decrease in size, until finally they are com-
pletely withdrawn, whilst new processes
are being protruded from another portion
of the body. By means of these alternate
protrusions and retractions of their pseudo-
podia, the small bodies of protoplasm are
enabled to move from place to place, crawl-
ing over the objects to whose surfaces they
cling at a rate which can only be measured
under the microscope. Amebe are able to
traverse a distance of $ mm. in a minute.

In this manner the white blood cor- |
puscles during inflammation are able to
pass through the walls of the capillaries
and of the smaller vessels, and the lymph
corpuscles make their way as wandering Ww;
cells into the connective tissue spaces, such Maw e onere ie. oF
as the interlamellar spaces of the cornea, the Frog containing « Bac-
where the resistance to be overcome is not "tm which is undergoing

: the process of digestion.
great, or they force their way between ‘he Bacterium has been
epithelial stained with vesuvine. The
two figures represent two
cells, and successive changes of shape

so reach _ in the same cell. (After ©
Metschnikoff, Fig. 54.)

the sur-
face of an epithelial membrane.

. This extension and retraction
of pseudopodia is most marked in
a small Ameba (Fig. 38), which
was described as far back as 1755
by Roesel von Rosenhof, who on
account of its energetic changes
of form called it the small Pro-
teus.

Somewhat different movements
take place in Myxomycetes, and
in Thalamophora, Heliozoa, and
Radiolaria.

Fia. 38.—Anteba proteus (after Leidy ; The plasmodia of some species
from R. Hertwig, Fig.16): nucleus; of Myxomycetes, such as the

cv contractile vacuole; N food vacuoles; : ; °
en endoplasm; ek ectoplasm. _ Aithalium septicum, often spread

635 THE CELL

themselves out over the object upon which they rest, in large
masses about the size of a fist. In order to make a suitable pre-
paration for observation of such a plasmodium, it is best to hold
a moistened slide near to its edge in an oblique position, and to
cause a stream of water by means of a special contrivance to flow
slowly down the slide. The plasmodia of the Avthalium possess
the property of moving in a direction opposite to that of the
stream of water (rheotropism) ; hence they protrude innumerable
pseudopodia, and by this means crawl up on to the moistened
slide, where they spread themselves out, and, by uniting neigh-
bouring pseudopodia together by means of transverse branches,
they form a delicate transparent net-
work (Fig. 39). When this network is
examined with a high power, it can be
seen to exhibit two kinds of move-
ments.

At first .the granular protoplasm
which is present in the threads and
strands, where it is surrounded by a
thin peripheral layer of hyaline proto-
plasm, is seen to have a quick, flowing
movement, which is chiefly observable
because of the movement of the small
granules, and which resembles the cir-
culation of the blood in the vessels of
a living animal. There is no distinct

Fie. 39. — Chondrioderma dif-

forme (after Strasburger). Part
of a fairly old plasmodium. a Dry

spore; b the same, swollen up in
water; c spore, the contents of
Rim are exuding; d zoospore;

moeboid forms produced by
vue transformation of zoospores,
which are commencing to unite
together to form a plasmodium,
(In d and e the nuclei and con-
tractile vacuoles may be distin-
guished.)

boundary line between the motile endo-
plasm and the non-motile ectoplasm,
for the granules at the edge of the
stream move much more slowly than
those in the centre; indeed, sometimes
they may keep quite still for a time,
to be later on again caught up by the

stream and carried along with it. In
the thinner threads there is always only one stream flowing longi-
tudinally, but in the thicker branches there are often two flowing
along side by side in opposite directions. ‘In the flat membrane-
like extensions’ which are developed here and there in the net-
work, “there are generally a large number of branched streams
flowing either in the same or in different directions; not infre-
quently we find streams flowing along side by side in opposite

THE VITAL PROPERTIES OF THE CELL 69

directions.” Further, the rate of movement may vary in different
places, or it may gradually alter; it may be so great that under
a powerful lens the granules appear to travel so fast that the eye
can scarcely follow them; on the other hand, it may be so small
that the granules scarcely appear to change their place.

The second kind of movement consists of a change of form in
the individual threads and in the network as a whole. As in
the Amcbu, processes are protruded and withdrawn from various
places, a mass of homogeneous protoplasm being first protruded,
into which the granular protoplasm flows later on. Occasionally,
when the streaming movements are very powerful, it appears as
though the granular endoplasm is pressed forcibly into the newly
formed processes. By this means the plasmodium can, like the
Ameba, crawl slowly along over a surface in a given direction;
new processes are continually being protruded from the one edge,
towards which the endoplasm chiefly streams, whilst others are
withdrawn from the opposite one.

Gromia oviformis (Fig. 40) is a classical object amongst the
Reticularia, for the study of protoplasmic movements (see p. 29).
If the little organism has not been disturbed in any way, a large
number of long fine threads may be distinguished stretching out
from the protoplasm, which has made its way out of the capsule,
and spreading themselves out radially in every direction into the
water; here and there lateral branches are given off, and oc-
casionally all the threads are united together into a network by
such branches. Even the most delicate of these threads exhibit
movements. As Max Schultze (I. 29) aptly describes it, “ a glid-
ing, a flowing of the granules which are imbedded in the threar
substance,” may be seen with a high power; ‘ they move alot’,
the thread, more or less quickly, either towards its periphery or
in the other direction; frequently streams flowing in both direc-
tions may be seen at the same time even in the finest threads.
When granules are moving in opposite directions, they either
simply pass by each other, or else move round one another for a
time, until after a short pause they either both go on in their
original directions, or one takes the other along with it. All the
granules in a thread do not move along at the same rate; hence
sometimes one may overtake another, either passing it or being
stopped by it.” Many evidently pass along the outermost surface
of the thread, beyond which they can be plainly seen to pro-
ject. Frequently other larger masses of substance, such as spindle-

70 THE CELL

shaped swellings or lateral accumulations in a thread, may be
seen to move in a similar manner. Even foreign bodies which

Fic. 40.—Gromia oviformis. (After M. Schultze.)

adhere to the thread

substance, and have

been taken in by it,
are seen to join in
this movement, the
rate of which may
attain to ‘02. mm. per
second. Where
several threads over-
lap each other gran-
ules may be seen pass-
ing from one into the
other. At such places
broad flat surfaces
may be produced by
the heaping up of the
thread substance.

A special kind of
protoplasmic move-
ment is described by
Engelmann (III. 5, 7)
under the name of
gliding movement
(Glitschbewegung).
It has been observed
chiefly in Diatoms and
Oscillaria. In the
former the proto-
plasm is surrounded
by a siliceous shell, in
the latter by a cellu-
lose membrane. How-
ever, outside this
covering there is an
exceedingly delicate -
layer of hyaloplasm,

quite free from gran-

ules, which cannot be
seen in the living ob-

THE VITAL PROPERTIES OF THE CELL eB

ject, but which may sometimes be demonstrated by means of
reagents. Hence, since this layer moves in a certain direction
over the siliceous shell, or cellulose membrane, the small organisms
can “move in a gliding or creeping fashion over a solid surface”
(Engelmann).

b. The movements of Protoplasm inside the Cell Mem-
brane. This kind of movement is chiefly seen in the vegetable
kingdom, and as a rule is best observed in the cells of herbaceous
plants rather than in those of shrubs and trees. According to
de Vries (III. 25), these movements are never totally absent in
any plant-cell, but frequently they are so slow as to escape direct
observation. They are best seen in vascular tissues, and in those
where materials have been stored up, and further at such times
when considerable quantities of plastic substances are being
transported in order to supply the material necessary for the
continuation of growth, for local accumulations, and for special
needs (de Vries). Hence this movement of the protoplasm ap-
pears to be directly of importance during the conveyance of
materials from one part of the plant to another. More rarely
it may be seen in the lower organisms, and in the animal king-
dom, as in Noctiluca in the vesicular cells in the centre of the
tentacles of Cawlenterata, etc.

Two kinds of movements may be distinguished in plants,
Rotation and Circulation.

These movements of rotation were first observed in 1774 by
Bonaventura Corti (I. 8); after that they were lost sight of for a
time, but were re-discovered by Treviranus. The most suitable
objects for studying them are afforded us by the Characee ; root-
hairs of the Hydrocharis morsus rane, and of Trianea bogotensis,
leaves of Vallisneria spiralis, etc., are also very convenient for
observations. In the large cells of the Characez, the protoplasm,
as has already been described on p. 33, is spread out as a thick
cohesive layer upon the inner surface of the cellulose membrane,
surrounding the large quantity of cell-sap like a closed sac. In
this lining two distinct layers of protoplasm can always be dis-
tinguished: an outer one, touching the cellulose membrane, and
an inner one, in contact with the cell-sap. The former is always
motionless; in Hydrocharis it is very thin, in Characez it is
somewhat thicker, and it also contains a greater number of
chlorophyll grains, which remain motionless. This immotile
layer gradually passes over into the inner motile one, which in

72 THE CELL

Chara contains no chlorophyll corpuscles, but only nuclei and
granules. The protoplasm of the inner layer, which, compared
to that of the outer layer, appears to be richer in water, exhibits
rotatory streaming movements, which take place in the following
manner. The current passes up along the longitudinal wall of an
elongated cell, then, turning round past a transverse wall, flows
down the opposite longitudinal side, until, curving round again
at the second transverse wall, it reaches the starting point,
when the cycle recommences. Between the upward and downward
streams there is a more or less broad neutral strip where the protoplasm
is at rest, and where as a rule it is reduced to a very thin layer.
In Nitella there are no chlorophyll corpuscles along this neutral
strip in the outer layer.

A connecting link between the rotatory movement and true
circulation is afforded us by the so-called “ fountain-like rotation”
(Klebs III. 14). This, which as a rule but rarely occurs, is
found in young endosperm cells of Ceratophyllum, in young wood
vessels of the leaf-stem of Ricinus, etc., etc. Here the protoplasm,
in addition to spreading itself out in a thick layer over the inner
surface of the cellulose wall, stretches itself in the form of a
thick central strand along the longitudinal sap-cavity of the cell.
Under these circumstances a single stream flows along this central
strand, spreading itself out in all directions like a fountain upon
the transverse wall, upon which it impinges; then streaming
down the sides of the cell, it collects again at the opposite trans-
verse wall, where it re-enters the main axial stream.

The motion which is described as circulation is observed in
those plant and animal cells in which the protoplasm spreads
itself out, both as a thin layer’ beneath the membrane, and also in
the form of more or less delicate threads, which traverse the
sap-cavity and are united together to form a net-like structure.

The objects which have been most examined are the staminal
hairs of the various kinds of T'radescantia, and young hairs of the
stinging nettle, and of pumpkin shoots.

The phenomenon of circulation resembles that observed in the
protoplasmic nets of Myxomycetes, and of the delicate pseudo-
podia of the Rhizopoda. Circulation consists of two kinds of
movements. In the first place attention must be drawn to the
streaming movements of the granules. In the thinnest threads
they move more or less quickly over the surface of the walls in one
direction, whilst in broader bands several separate streams may

THE VITAL PROPERTIES OF THE CELL 73

circulate quite close together, sometimes in the same, sometimes in
opposite directions. The
nucleus, as well as_ the
chlorophyll and _ starch
grains, which lie embedded
in the protoplasm, are car-
ried slowly along by the
current. Similarly in this
case the most external hy-
aline layer of protoplasm,
which is in contact with
the cellulose membrane, is,
comparatively speaking, at
rest. In the second place,
the whole body of proto-

:
gs
a Ye
z

& Xe:
ie
VE
. <
ik
‘ A
Hat 2
i 5.
( .

LE
‘

plasm itself slowly moves
along, in consequence of
which it changes its form.
Broad bands become nar-
rowed, and may after a
time disappear, delicate
threads increase in size, and

new processes are formed,
just as new pseudopodia
are protruded to the ex-
terior by Myxomycetes and
Rhizopoda. Large masses

Fie. 41.—A B, cells of a staminal hair of Tra-
of protoplasm become dsecantia virginica, A Undisturbed streaming
heaped up here and there movements of protoplasm. B Protoplasm which
peas Pare Gai th has run together into ball-like masses in con-
npor Oo ByCr sins sd sequence of irritation: a cell-wall, b transverse
cell-wall, whilst at other wall of two cells; ¢d protoplasm which has
laces the coating becomes massed itself together into small balls. (After
F ues pC ORNS ee Kiihne; from Verworn, Fig. 13.)
thinner.

c. Theories concerning Protoplasmic Movements.
Attempts have lately been made by various investigators, Quincke
(LIL. 17), Biitschli (II. 78), Berthold (III. 2), and others, to com-
pare these protoplasmic movements with those exhibited by a
mixture of inorganic substances, and thus to explain them.

Quincke has carefully investigated the movements which occur
at the areas of contact of various fluids. He placed in a glass
containing water a drop of a mixture of almond oil and chloroform,

Se eae THE CELL

the specific gravity of which is slightly greater than that of
water, and then, by means of a fine capillary tube, he caused a
drop of 2 per cent. solution of soda to approach the globule
of oil. This latter then exhibited changes in shape, which are
‘similar to those observed with the microscope in certain Amebe.
The explanation of this is that the soda solution gradually
spreads itself out over the surface of the oil, forming a soap.
Quincke is of opinion that the protoplasmic movements are
analogous to these. In the plasmolysis of plant cells, the proto-
plasm frequently breaks up into two or more balls, which
spread themselves out, and then either re-unite, or remain
separated from one another by an even surface, just as two
soap bubbles of equal size which are placed in contact may
touch each other, without uniting. In consequence of these
appearances he is of opinion that, considering the physical pro-
perties of delicate solid or fluid lamelle, the protoplasm must
be surrounded by a very delicate fluid membrane, just as in
the soap bubble the air is enclosed in a thin skin layer of
soap solution. “The substance of the membrane surrounding
the protoplasm,” as Quincke proceeds to state, “‘must be a
fluid which forms drops in water. Since of all the substances
known in nature oil is the only one which possesses this pro-
perty, the membrane must consist of an oil, that is to say of a fluid
fat. The thickness of this layer may be most minute, less than
‘0001 mm., and hence it is not perceptible even with the micro-
scope.” Through the action of ‘the albumen upon this oil, a
substance is produced upon the areas of contact, which is soluble
in water, and spreads itself out just like the soap produced
by the combination of soda and oil. Hence itis called albuminous
soap.
Thus Quincke considers the cause of the protoplasmic move-
ments to be a periodic spreading out of albuminous soap upon the
inner surface of the envelope of oil surrounding the protoplasm.
This soap, in being continually re-formed on the area of contact
as fast as it is dissolved and diffused throughout the surrounding
fluid, remains constant in quantity; thus, since the presence of
oxygen is necessary in this chemical process, the fact is explained,
that, in its absence, the protoplasmic movements are arrested, and
similarly their cessation at extreme temperatures may be ascribed
to chemico-physical conditions.
Bitschli, being stimulated by these investigations of Quincke,

THE VITAL PROPERTIES OF THE CELL 70

has undertaken some interesting experiments based on the assump-
tion of his foam or emulsion theory of protoplasm, and these, as it
appears to him, throw lght upon the cause of the protoplasmic
movements. He prepared frothy structures of oil in various ways.
The most delicate and instructive masses were obtained by mix-
ing a few drops of olive oil, which had been kept for some time
in a warm chamber, with some finely powdered K,COs, until a
viscous mass was produced; a small drop of this mixture was
then introduced into water. The emulsion which is produced in
this manner is milky white in appearance, and consists of minute
vacuoles, filled with the solution of soap, which is formed at the
same time: it may be cleared by adding to ita few drops of dilute
glycerine. By this means active streaming. movements are pro-
duced, which, in a successful preparation, last for at least six days,
and which are certainly surprisingly like the protoplasmic move-
ments of an Ameba. ‘‘From one place on the edge the current
flows through the axis of the drop; it then streams away from the
edge down both sides, in order to unite again, gradually to form
the axial current again. Here and there a blunt process is pro-
truded and withdrawn, and so on; indeed, individual drops may
exhibit fairly active locomotive powers for a time.” Biitschli, in
accordance with Quincke’s experiments, explains these phenomena
of movement in the following manner: “On some place on the
surface some of the delicate chambers of the froth structure burst,
and thus the soap solution at this region is able to reach the sur-
face of the drop, which is composed of a very thin lamella of oil.
The necessary consequence of this is a diminution of surface-
tension at this spot, and hence a slight bulging and out-streaming
occur. Both of these induce a flow of foam-substance from the
interior to this spot. A few more meshes may be broken down
by this current, and so on, the result being that a streaming, once
induced, is persistent unless considerable obstacles present them-
selves.” Biitschli is quite convinced that the streaming move-
ments seen in these saponified fat drops are identical in all
essentials with amceboid protoplasmic movements.

These experiments of Quincke and -Biitschli are of the greatest
interest, for they prove that very complex phenomena of move-
ment may be induced by means of comparatively simple methods.
On the other hand, various objections may be raised against their
deduction, that in protoplasmic movements similar processes
occur. Even the hypothesis, that the protoplasmic substance is

76 THE CELL

enveloped by a delicate lamella of oily substance, is exceedingly
doubtful. For if we only take into account the single fact that
protoplasm is composed of a great number of chemical substances,
which, during the metabolic processes upon which its life depends,
are continually undergoing chemico-physical changes, we cannot
but think that conditions much more complex in their nature
must be necessary for its movements, than those required for a
moving drop of saponified oil, and, indeed, the complexity of these
conditions must be proportionate to the immense difference in the
complexity of the chemical composition and organisation of the
two substances in question [cf. statements: already made’on this
subject on p. 22; and Die- Bewegung der lebendigen Substanz by
Verworn (III. 24)]. Further, all the protoplasmic movements
—the streaming movements, the radial arrangement round attrac- _
tion centres, the movements of cilia and flagella, and muscular
contraction — together constitute a single group of correlated
phenomena which demand a common explanation. This, however,
is not afforded us by the experiments of either Quincke or
Bitschli. The movements, induced by them in a mixture of sub-
stances, bear the same relation to the movements of living bodies,
as the structure of the artificial cell produced by Traube does to
the structure of the living cell.

Fig. 42, which is taken from a paper by Verworn (III. 24),
shows what very different. appearances, closely resembling the
various kinds of pseudopodic extensions, may be produced by the
simple spreading out of a drop of oil upon a watery solution; a—d
is a drop of salad oil which has spread itself out upon soda solu-
tions of different. degrees of concentration; in a it has assumed
the form of Ameba, guttula, in b and ¢ of Ameba proteus, and in d
of a plasmodium of a Myxomycete. Figs. e and f, which repre-
sent drops of almond oil, resemble the formation of pseudopodia
in Heliozoa and Radiolaria, whilst g is taken from Lehmann’s
Molecular Physics, and represents a drop of creasote in water, in
which it has assumed a'form resembling a typical Actinospheerium
(Verworn IIT. 24, p. 47).

Other attempts to explain the protoplasmic movements (Engel-
mann III. 6, Hofmeister II. 20, Sachs) lead us into the domain
of theories upon the molecular structure of organised bodies, since
the cause of the movements is supposed to lie in the changes of »
form of the most minute particles. A discussion of Verworn’s
latest attempt (III. 24) would lead us too far in another direction.

THE VITAL PROPERTIES OF THE CELL Le

Once for all, it must be admitted that none of the hypotheses
which have, up till now, been propounded, are able to furnish us
with a satisfactory conception of the causes and mechanical con-
ditions of the plasmic movements, and that, therefore, we must
confine ourselves to a simple description of observed conditions.
This, however, is not to be wondered at, when we consider what

Fie. 42.—Different appearances assumed by drops of oil, which have spread themselves
out. (After Verworn, Fig. 11.)
a number of different opinions are held with regard to the ultimate
structure of protoplasm itself (see pp. 18-26), and this must of
course affect the explanations tendered of its movements.

II. Movements of Flagella and Cilia. Unicellular organ-
isms, by means of their flagella and cilia, are able to move from

78 THE CELL

place to place much more rapidly than can be effected by means
of pseudopodia. Flagella and cilia are delicate hair-like processes,
which extend in greater or less numbers from the surface of the
cell. They are composed of a homogeneous, non-granular sub-
stance, and in this respect resemble short, thin pseudopodia,
when these consist of hyaloplasm alone. However, they differ
from pseudopodia in two respects : firstly, they move in a different
and more energetic fashion, and secondly, they are not transitory,
but permanent organs, being neither protruded nor withdrawn.
Fundamentally, however, the movements of flagella and pseudo-
podia are identical in kind, as is shown by the observations made
by de Bary (1. 2) on swarmspores of Myxomycetes, and by
Haeckel, Engelmann, R. Hertwig (III. 128), and others on
Rhizopoda.

Many of the lower organisms reproduce themselves by means
of small spores, which resemble Amwbe in their appearance and
in their mode of movement (Fig. 43). After a time such spores
usually protrude two thread-like pseudopodia (Fig. 43 4), which
exhibit slow oscillatory movements, and develop into flagella,
whilst the remainder of the
body withdraws all its other
pseudopodia, and so becomes
spherical in shape. As _ the
movements become stronger,
the spore travels more and

Fre. 43.—Microgromia socialis. An amo-
boid cell (a) which has been produced by
division, and has wandered from the
colony; and which, having withdrawn all
its pseudopodia, with the exception of two,
which have developed into flagella, be-
comes transformed into a swarmspore (b).
(From Hertwig, Pl. 6, d and e.)

more rapidly, by means of its:
two flagella, through the water
(Fig. 43 6); thus a “swarm-
spore” has developed out of the
little amceboid cell.

It may be safely deduced from
these discoveries that flagella

are developed from delicate protoplasmic processes, which become
especially contractile, and in consequence differ somewhat im their
properties from the remaining protoplasm. Hence they may be
considered as constituting a special plasmic product or cell-organ,
composed of contractile substance.

Flagella and cilia always arise directly from the body of the
cell. If the cell is enveloped by a membrane, they protrude
through pores in it. At their bases they are always somewhat
thickened, frequently starting from the surface of the protoplasm

THE VITAL PROPERTIES OF THE CELL 79

as small button-like protuberances, whilst at their free ends they
gradually become reduced to fine points.

Ciliary organs may occur in large or small numbers. In the
latter case, when only from one to four are present, and when
they are generally longer and more powerful, they are called
flagella; in the former case, they cover the whole surface of the
cell in large numbers, thousands being frequently present, they
are then smaller and more delicate, and are called cilia.

a. Cells with Flagella. Flagella occur either at the anterior
or posterior end of the body, producing a correspondingly different
movement in the body. In the first case the flagella travel
forwards, dragging the body along after them; in the second
they propel it from behind. The former mode of locomotion has

GC

Fie. 44.—A Euglena viridis (after Stein): n nucleus; c contractile vacuole; o pigment-
spot. B Hexamitus inflatus (after Stein). C Chilomonas paramecium (after Biitschli):
oe cytostome; v contractile vacuole; n nucleus. (From Hertwig, Figs. 130-132.)
been chiefly observed in Flagellata and kindred organisms
(Fig. 44 A, B,C), in many kinds of Bacteria (Fig. 33 6), in
antherozoids (Mosses, Ferns, Equisitacee), and in swarmspores,
under which name the reproduction bodies of many Alge and
Fungi are included; the latter method of locomotion occurs in the
spermatozoa of most animals (Fig. 45).

The ciliary organs of unicellular organisms have a double

80 THE CELL

A B function to perform. Firstly, they have to
vo keep the cell body afloat by means of their
es activity, since its specific gravity is some-

what greater than that of the surrounding
medium. This is proved by the fact that
dead swarmspores and spermatozoa sink to
the bottom of the vessel. Secondly, they
have to propel the body in a certain direc-
tion by means of their movements.

Rake Matare human Niigeli (III. 16) has made most careful
spermatozoon from two observations upon the mechanism of the move-
points. of view. It is s ;
composed of k head; m ‘ents of the motile cells of plants. Accord-
middle portion; and s ing to this investigator, the oscillations of
ac the flagella impart a two-fold movement of
the body—a forward, and at the same time a rotatory movement.
Hence the resultant motion resembles that of a ball shot out of a
rifle. Such motions may be divided, into three types :—

‘‘Many motile cells travel forwards in a straight or somewhat
eurved line, the anterior and posterior ends of their axes remaining
exactly in the same direction ; these swim steadily forward, with-
out deviating from a fixed path. With others it may be distinctly
seen that they describe a straight, or somewhat bent spiral, in
which one revolution round the axis always corresponds to one
turn of the spiral (a given side of the cell always facing out-
wards), whilst the axis of the cell is parallel to that of the
spiral. Finally, there are other cells whose anterior ends describe
spirals, whilst their posterior ends proceed in a straight line, or
in a spiral of smaller diameter. The nature of the second and
third of these movements can only be distinguished if they are
very slow. If they are rather quicker, only a kind of wavering
can be made out, which, especially in the third, is of a peculiar
character.”

The direction in which the motile cells rotate about their
longitudinal axis generally remains constant for each kind, species,
or family; many rotate from south to west (Ulothrix), others
from south to east (antherozoids of Ferns), others are somewhat
uncertain in their rotations, turning now from south to east, and
now from south to west (Gonium). If motile cells strike against
any object, they cease for a time their forward movements, but
continue to rotate about their longitudinal axes; then, ‘as a rale,
they commence to retreat, their posterior ends being in advance,

THE VITAL PROPERTIES OF THE CELL 81

and to rotate themselves in an opposite direction.” This backward
movement never lasts for long, and is always slower than the
forward one; however, the cell soon returns to its normal mode
of progression, which usually takes place in a somewhat oblique
direction.

In consequence of his investigations, Nigeli is of opinion that
if zoospores and spermatozoa be quite regular in form, if their
substance be evenly distributed throughout their mass, and
further, if the medium be quite homogeneous, they must travel
in a perfectly straight line, and hence that all deviations from this
straight line, both as regards rotation round the axis and forward
progression, must be ascribed either to the circumstance that they
are not symmetrical in form, and that their centres of gravity
are not in the centres of their bodies; or to the fact that the
frictional opposition which they encounter is not equal in every
direction.

By means of flagella a far greater speed is attainable than by
means of pseudopodia. According to Nageli, zoospores usually
proceed at the rate of one foot per hour; the quickest, however,
take only a quarter of an hour to traverse the same space; whilst
a man, at ordinary speed, traverses a distance of rather more than
half his length in a second, a swarmspore in the same time
covers a distance of nearly three times its own diameter. How-
ever, although the rate at which they move appears, when they
are seen under the microscope, to be very great, we must take
into account the fact that the distance is also magnified, and that
in consequence they appear to move much more rapidly than they
doin reality. As a matter of fact, their movements are exceed-
ingly slow. ‘ Without magnification, even if the organisms could
be plainly seen, no movement could be perceived on account of its
slowness.” ; ee

Spermatozoa (Fig. 45) may be distinguished from the zoospores
of plants by their possessing one single thread-like flagellum,
situated at the posterior end of the body. The spermatozoon,
being propelled by it, advances by means of snake-like move-
ments, resembling those of many fishes. In some cases the
structure is more complicated, a delicate contractile or undulating
membrane, which may be compared to the edge of a fish’s fin,
being present. This is especially well developed on the posterior
end of the large spermatozoa of the Salamander and the Triton
(Fig. 46).

G

82 THE CELL

If this undulating membrane be examined with a very high
power of the microscope, waves are seen to travel continually over
its surface, passing from the front to the back. ‘ These,” as
Hensen explains, “are caused by each successive transverse
portion passing one after the other from one
extreme position (Fig. 47) to the other. For
instance, if at the initial period a portion of
the edge, which is seen from above, occupies
position J to I! (Fig. 47), it is seen at the
end of the first quarter of the period to have
assumed position II to IJ, or, which amounts
to the same thing, position IJ! to II®. At
the end of the second quarter the portion
IT* to II? is in the position III to III' or,
which is the same, JJJ! to IJ1?. At the end
of the third quarter IJI' to III? has passed
into the position IV to IV}, whilst_at the end
of the whole period it has again taken up
position I to J!. The movements follow after
each other with a certain degree of force and
speed; it remains now to be seen how a for-
ward motion results from them. Any one
point on the surface of the undulating border
(Fig. 47) moves, as is indicated by the arrow,
from 6 to y with the force x=ay. This force
can be resolved into its two components af
and By. The force af is exerted in the direc-
tion of the border, compressing it, and ap-
parently producing no further effect. Force
By may. be again split up into yéd and ye.
ye exerts a direct backward pressure on the
water, and hence, in consequence of the re-
sistance of the water, propels the body in a

Fig. 46.— Spermato-
zon of Salamandra forward direction. Force yd would cause the

maculata: k head ; m . :
middle portion; ef tail; body to rotate on its own axis; but opposed

sp anteriorend; u un- to it is the opposite force, which is developed
ee ORS at all the places where the arrows point in an
opposite direction (as for instance over D). Further the same
force ye is present-in Fig. D as in Fig. C, only the shaded por-
tions of Fig. A develop the forces which are opposed to ye. It is
seen, however, that the size of the surfaces in question, and hence

THE VITAL PROPERTIES OF THE CELL 83

of their force components, is invariably of minor importance ’

(Hensen III. 11).

Fig. 47.—Explanation of the mechanism of the movements of spermatozoa (after
Hensen, Fig. 22). A The four phases of position assumed by the border of the flagellum
when an undulation passes overit. I to I’, the first; II to If} to II?, the second; III to
IT} to [1I?, the third; IV to IV}, the fourth stage of the bending of the border in a longi-
fidinal undulation. B Section of the thread-like tail and membrane, in its two positions
of greater elongation. C and D resolution of forces. E Movement of an ordinary sper-
matozoon ; a bc various phases of this movement.

b. Cells with numerous Cilia. The
Infusoria are chiefly to be distinguished
from other unicellular organisms by the
large number of cilia they possess, on which
account they are called Ciliata (Fig. 48).
Cilia are much smaller than flagella, be-
ing, as a rule, about ‘1 to ‘3 p thick, and
about 15 » long. They may number
many thousands. For example, it has
been calculated, that the Paramecium
aurelia possesses approximately 2,500.
As for the Balantidium elongatum, which
is parasitic in the Frog, and which is very
thickly ciliated, Bitschli (III. 3) is of
opinion that it has nearly ten thousand
cilia; these are generally arranged in
several longitudinal rows, which either
encircle the body in spirals, or are con-

fined to a certain portion of its sur- Fie. 48,.— Stylonychia my-
face tilus (after Stein; from Claus’
; ie ws ; Zoology) seen from the ventral

In addition to the cilia, many Infusoria surface. Wz Adoral zone of
possess special large organs of locomotion, “i#s © contractile vacuole;

eat i N nucleus; N?} nucleolus; A
cirri, and undulating membranes. The anys.

84 THE CELL

former may be distinguished from cilia by their greater thickness
and length, and by the fact that they are somewhat wide at the
base, whilst they taper off to a fine point (Fig. 48). Further, like
other special contractile tissues (muscular fibres), they exhibit a
fibrillar differentiation, so that they may be split up into many
delicate fibrils (Biitschli). These cirri occur with especial
frequency in hypotrichous Infusoria, being situated chiefly around
the mouth. The undulating membranes also terminate at the
mouth cavity. They are locomotive organs which have been
developed superficially; they may frequently be seen to be dis-
tinctly marked with delicate strie extending from the base to the
free edge, and hence they, like the.cirri, must possess a fibrillar
structure.

Infusoria have various methods of locomotion. As a rule the
body, when it moves freely through the water, revolves about its
longitudinal axis. It has the power of changing the direction in
which it travels; the rate at which the cilia move may-suddenly
be altered, being either slowed or quickened; the body may even
keep still for a short time, without any apparent external cause.
Hence various kinds of movements take place, suggesting the
idea of volition. In addition, it is remarkable that the cilia, often
thousands in number, of one and the same individual, always act
together in a strictly co-ordinate fashion. ‘They do not only
always oscillate at the same rate, and with the same amplitude of
beat (rhythm), but they always strike the water in the same direc-
tion, and in the same order” (Verworn). This co-ordination is
carried out to such an extent, that two individuals which have
been produced by the division of a parent cell always exhibit
uniform and synchronous movements as long as they are united
by a bridge of protoplasm. Hence it follows, that although the
cilia possess the power of spontaneous contraction, yet their work-
ing together is regulated by stimuli from the protoplasmic body
itself.

The ectoplasm seems to play an especially important part in
the transference of these stimuli, as is shown by an experiment
made by Verworn (IV. 40). He made a slight incision with a
lancet in Spirostomum ambiguum (Fig. 49) and in Stentor
ceruleus in the ectoplasm supporting the rows of cilia. “ Under
these circumstances it could be plainly seen that the ciliary
waves did not cross the area of the incision, but were confined to
the one side, and could not be seen on the other.” Occasionally

THE VITAL PROPERTIES OF THE CELL 85

also he observed that the mean position through which the cilia

oscillated was different for a time in one half
of the rows of cilia from that seen on the
other side.

Ill. The Contractile Vacuoles, or
Vesicles, of Unicellular Organisms.
Contractile vacuoles occur very frequently in
Amobe, Reticularia, Flagellata (Figs. 7, 43,
44), and Ciliata (Fig. 50 cv). In the last,
where they have been most accurately ex-
amined, there is generally only one single
vacuole in the whole body; occasionally two
are present (Fig. 50), or rarely a few more ;
they are always situated just below the sur-
face of the body, under the ectoplasm. They
may be easily distinguished from the other
fluid vacuoles, of which
large numbers may be
distributed throughout
the body, by the fact,
that at regular intervals
they discharge their con-
tents to the exterior, and
then gradually fill up
again. Hence they tem-
porarily disappear (Fig.
50 cv) to reappear again
in a short time (cv'),

— SSS Te .

AsrLs sas ase La swssdrraa sis 153

PLeeee ate

Fie. 49,—Spirostomum
ambiguum. The con-
tinuity of the surface
which bears the peri-
stomatic cilia has been
interrupted by an in-
cision. (After Verworn
(VI. 40), Fig. 25.)

Fie. 50.— Paramecium
caudatum semi-diagram-
matic (R. Hertwig, Zoo-
logie, Fig. 139): K nu-
cleus ; nk secondary nu-
cleus; 0 mouth aperture
(cytostome); na! food
vacuole in process of for-
mution; va food vacuole ;
cv contractile vacuole,
contracted; cv! the same
contractile vacuole, dis-
tended ;_ t trichocysts,
t! the same with their
threads ejected.

The evacuation takes place through one or
more special pores, which can be observed
on the surface of the infusorian immediately
over the vacuole. “ Hach pore appears as a
rule as a minute circle, the border of which
is dark, but which is bright in the centre ;
this brightness of the centre is due to the re-
fractive power of the pellicular and alveolar
layer. Sometimes each pore is connected to
the vacuole by means of a fine excretory
tube. In addition, it is not uncommon to
find special conducting canals (1, 2, or more)
regularly arranged in its neighbourhood. In

£6 THE CELL

Paramecium aurelia and Paramecium caudatum (Fig. 50), there
is a system of conducting canals, which have been known for
a long time, and have been worked at more than any others;
from each of the two dorsal vacuoles about eight to ten fairly
straight tubes radiate ; their course may be traced almost all over
the whole body. However, the two systems remain independent
throughout their whole extent.” They are thickest in the neigh-
bourhaod of the vacuoles, becoming gradually finer distally.

The Paramecium affords us an excellent subject for a closer
study of the working of this peculiar apparatus. When both the
contractile vacuoles have attained their greatest size, their whole
contents are suddenly and energetically ejected to the exterior
through their efferent canals and pores, so that for a time the
vacuole cavities quite disappear. This condition, as with the
heart, is termed the systole, whilst the period during which the
vacuoles become again filled with fluid, and hence distended and
visible, is called the diastole. m

They become filled in the following manner: Even before the
systole has commenced, the above-described conducting canals
have collected fluid from the endoplasm of the body of the infuso-
rian; this fluid probably is charged with carbonic acid and other
decomposition products. According to Schwalbe (III. 21) the
process occurs in consequence of ‘“ the condition of pressure of the
fluid in the animal’s body, this pressure being due to the ever-in-
creasing amount of water which is continually being taken in by
the mouth.” The conducting canals can be easily seen, at this
time being full of water. They become swollen in the neighbour-
hood of the contractile vacuole, which is now fully distended, so
that they look like a circle of rosette-shaped vacuoles surrounding
it ; these have been called formative vacuoles by Biitschli. In
consequence of their being in this condition, the contractile vacuole
cannot, during its systole, discharge its contents back through
them, but only forwards to the exterior. As soon as the diastole
again occurs, the distended formative vacuoles empty themselves
into the contractile vacuole, which in consequence becomes visible
again; it then gradually distends itself until it reaches its maxi-
mum size. Hence at the commencement of the diastole the emp-
tied formative vacuoles disappear for a time; however, they con-
tinue to collect fluid from the parenchyma of the body until the
commencement of the next systole.

When several vacuoles are present they winepally: empty them-

THE VITAL PROPERTIES OF THE CELL 87

selves in turn, with the result that the water is discharged as
regularly as possible. The frequency with which these evacuations
take place varies considerably in different species. According to the
observation of Schwalbe (III. 21) the following law may be stated :
that the smaller the vacuoles are, the more frequently are they
emptied. For instance, in Chilodon cucullulus they contract about
13 to 14 times in two minutes, in Paramecium aurelia, only 10 or
11 times in the same period, whilst in Vorticella microstoma, only
once or twice. In Stentor and Spirostomum the contractions occur
less frequently still. Of all the above-mentioned animals, the two
last have the largest contractile vacuoles, next comes Vorticella,
then Paramecium aurelia, and lastly Chilodon cucullulus, whose
vacuoles are only half as large in diameter as those of Paramzec-
dum, where the diameter is about ‘0127 mm.; in Vorticella it is
°0236 mm (Schwalbe).

The interval which elapses between the two evacuations is very
regular at the same temperature; it is, however, considerably
affected if the temperature is raised or lowered (Rossbach III. 19,
Maupas). For instance, with Euplotes charon, the interval between
the contractions is 61 seconds; at 30° Celsius, it has diminished
to 23 seconds (Rossbach); that is tosay, the frequency has become
nearly trebled.

The amount of water which in this-manner passes through the
animal is extremely great. According to the computations of
Maupas, Purameciwm aurelia, for example, evacuates, in 46
minutes at 27° Celsius, a volume of water equal to its own
volume.

From the above-mentioned observations, it may be concluded
that contractile vacuoles are not merely simple variable drops of water
in the plasma, but that they are permanent morphological differentia-
tions in the body of the Protozoon ; that 7s to say, true cell organs,
which appear to perform an important function in the carrying en of
breathing and excretion. The energy with which the vacuole dis-
charges its contents, so that it completely disappears, indicates that
its walls, which consist of hyaline substance resembling the flagel-
lum substance, must be contractile to an exceptional degree, and
by means of this property are to be distinguished from the endo-
plasm of the infusorian body. It must, however, be admitted that
no special membrane, clearly defined from the remainder of the
body mass, can be seen microscopically, just as with smooth muscle
fibres the contractile substance and the protoplasm are not sharply

88 . THE CELL

defined from one another, and further as flagella pass over imper-
ceptibly at their base into the protoplasm of the cell.

Therefore I agree with Schwalbe (II[.21) and with Engelmann,
that the vacuoles possess contractile walls although they are not
clearly defined from the rest of the protoplasm. In addition, it is
well known that delicate membranes are often imperceptible with
the microscope although they are undoubtedly present. In many
plant cells it is impossible to see the so-called primordial utricle
as long as it adheres closely to the cellulose membrane ; its exist-
ence, however, cannot be doubted, as its presence can be proved by
plasmolysing it.

In this opinion, however, I find myself in opposition to Biitschli
(III. 3). He considers that the contractile vesicle is simply a drop
of water in the plasma. ‘‘ Hach vacuole after evacuation ceases
to exist as such. The one that takes its place is a new formation,
a newly created drop, which in its turn only exists until it has
discharged itself.”” In his opinion they are due to the flowing to-
gether of several formative vacuoles, which separate out as small
drops in the plasma, where they increase in size until, by break-
ing down the partition walls, they coalesce. However, the exist-
ence of the conducting and afferent canals, described by Biitscbli
himself, the fact that the number of vacuoles present remains
constant, and the circumstance that during the diastole the vacuole
is seen to occupy the same position as during the systole, and
moreover, that the frequency of contraction bears a fixed relation
to changes of temperature, all appear to me to support the former
view, and to be opposed to Biitschli’s theory. The fact that at the
end of the systole the vacuole, having evacuated its contents, is
for a moment invisible, does not seem to weigh much against the
theory of its constancy, especially if one considers that even large
lymph spaces and capillary blood vessels in vertebrates elude per-
ception in an uninjected condition.

IV. Changes in the Celi during passive movement. In
order to complete the subject of the movements of protoplasm, it
is necessary to consider finally the changes of form which, to a
certain extent, the cell may experience ‘in consequence of passive
movements. Under these circumstances, the cell is in the same
condition as a muscle which, being excited by an external stimulus,
bécomes extended and then contracted again.

In this manner the cells of an animal body may become con-
siderably altered in form, in adapting themselves to all the

THE VITAL PROPERTIES OF THE CELL 89

changes of shape which an individual organ experiences as a
consequence of muscular action or of distension through a col-
lection of fluid or nutriment. Thread-like epithelial cells have to
become cylindrical, and cylindrical ones to become flat, when the
surface increases in size through the distension of an organ,
whilst, on the other hand, the reverse takes place when the
whole organ, including its surface, decreases in size.

How powerful and sudden may be the changes of form which
the protoplasm of a cell, in consequence of passive movement,
may experience without damage to its delicate structure, can be
best seen in Coelenterata, in which extended portions of the
body, like palpocils, may sud- d RB
denly shorten by about a tenth
or more of their length, in con-
sequence of sudden energetic
muscular contraction (I1I. 12
a). The form which an epi-
thelial cell assumes varies very
considerably, according as to
whether it has been taken from
a portion of a body which is
moderately or strongly con-
tracted, as may be seen by Fig. 51.—Muscular epithelial cell from the

: : endodermal surface of the tentacle of an
comparing Fig. 51 A, B. The Actinia (Sagartia parasitica) (after O. and R.
former was takenfrom theten- Hertwig, Pl. VI., Fig. 1); from Hatschek,
tacle of an Actinia, which was ct i). A extended Sa es of tentacle ;

strongly contracted condition of same.
only moderately contracted,
since by means of chemical reagents it had been rendered non-
sensitive before it was killed ; the second was derived from the ten-
tacle of another individual which had contracted strongly in death.

Literature III.
1. pE Bary. Die Mycetozoen, Zeitschrift f. wissenschaftl. Zoologie. Bd. 10.
1860.
2. G. Bertruoup. Studien iiber Protuplasmamechanik. Leipzig. 1886.
3. Btrscuur. Protozven. Hirst Volume of Bronn’s ‘ Classen und Ordnungen
des Thierreichs.” 1889.
4. Avex. Ecker. Zur Lehre vom Bau u. Leben der contractilen Substanz der
niedersten Thiere. Zeitschrift f. wissenschaftl. Zoologie. Bd. I, 1849.
ENGELMANN. Protoplasm and Ciliary Movement, trans. by Bourne from
Hermann’s “ Handbuch der Physiologie.” Bd. I. Quar. Jour. Mic.
Soc. 1880.

or

90

13.

Iie

THE CELL

ENGELMANN. Contractilitdét und Doppelbrechung. Archiv. f. die gesammte
Physiologie. Bd. XI.

See also E. A. Scuirer. On the Structure of Ameboid Protoplasm, etc.,
with a Suggestion as ta the Mechanism of Ciliary Action. Proc. Roy.
‘Soc. 1891.

J.CxuarK. Protoplasmic Movements and their Relation to Oxygen Pressure.
Proc. Roy. Soc. 1809.

Encretmann. Ueber die Bewegungen der Oscillarien wnd Diatomen.
Pfltigers Archiv. Bd. XIX.

Encetmann. Ueber die Flummerbewegung. Jenaische Zeitschrift f. Medi-
cin und Naturwissenschaft. Bd. 1V. 1868.

Frommann. Beobachtungen iiber Structur und Bewegungserscheinungen —
des Protoplasmas der Pflanzenzelle. Jena, 1880.

Frommann. Ueber neuere Erkliirungsversuche d. Protoplasmastrémungen
u. tiber Schaumstructuren Biitschli’s. Anatom. Anzeiger. 1890.

HeEnsEN. Physiologie der Zeugung. Handbuch der Physiologie. Bd.
iV. 1881.

. O. and R. Hertwie. Die Actinien. Jena. 1879.
. Kicnarp Hertwie. Ueber Mikrogromia socialis, eine Colonie bi'dende

Monothalamie des siissen Wassers. Archiv. f. mikruskop. Anat. Bd. X.
1874.

JURGENSEN. Ueber diein den Zellen der Vallisneria spiralis stattfinden-
den Bewegungserscheinungen. Studien des Physiol. Instituts zu Bres-
lau. 1861. Heft 1.

Kurss. Form und Wesen der Pflanzlichen Protoplasmabewegung.
Biologisches Centralblatt. Bd. I.

Kotrimann. Ueber thierisches Protoplasma. Biol. Centralblatt. Bd. IT.

C. Nigent. Die Bewegung im Pflanzenreiche. Beitriige zur wissen-
schaftiichen Botanik. Heft 11. 1860.

Nicenr. Rechts und links. Ortsbewegungen dir Pflanzenzellen und
ihre Theile.

G. QuinckE. Ueber periodische Ausbreitung an Fliissigkeitsoberflichen
u. dadurch hervorgerufene Bewegungserscheinungen. Sitzungsber. der
Akademie der Wissenschaften zu Berlin. 1888.

Purxkinye u. Vatentin. De phaenomeno generali et fundamentali: motus
vibratorti continui. 1835.

Rosspaca. Die rhythmischen Bewegungserscheinungen der einfachsten
Organismen und ihr Verhalten gegen physikalische Agentien u Arznei-
mittel. Arbeiten a. dem. zool. zoot. Institut zu Wiirzhurg. 1874.

Sacus. Eaperimentalphysiologie der Pflanzen. Leipzig. 1865.

ScuwaLBe. Ueber die contractilen Behditer der Infusorien. Archiv.
fur mikroskopische Anatomie. Bd. Il.

VELTEN. Hinwirkung strémender Elektricitiét auf die Bewegung des
Protoplasmas, etc. Sitzuwngsber. d. Wiener Akademie. 1876. Bd. 73.

Verworn. Studien zur Physiologie der blimmerbewegung. Pfliigers
Archiv. Bd. 48. 1890.

VERWoRN. Lie Bewegung der lebendigen Substanz. Jena. 18:2.

DE VriEs. Ueber die Bedeutung der Circulation und der Rotation des

Protoplasmas fiir den Stofftransport in der Pflanze. Botanische “Zeitung. 1885.

CHAPTER IV.
THE VITAL PROPERTIES OF THE CELL (continued).

Phenomena of Stimulation./ The most remarkable pro-
perty of protoplasm is its power of reacting to stimuli:—its
Irritability.

By this is understood, as Sachs (IV. 32a) expresses it, ‘“ the
power possessed by living organisms alone of reacting to the
most various external stimuli in one way or another.” It is
chiefly through this irritability that living objects can be distin-
guished from non-living ones, and in consequence the earlier
natural philosophers considered that it was the exprestion of a
special vital force which was only to be seen in organised nature.

Modern science has discarded the theory of vitalism (vitalismus) ;
instead of explaining irritability by means of a special vital
force, it is considered to be a very complicated chemico-physical
phenomenon, differing only in degree from other chemico-physical
phenomena of inanimate nature. That is to say, the external
stimuli come into contact with a substance very complex in
structure, an organism, which is an exceedingly complicated
material system, and in consequence they give rise to a series of
very complex phenomena.

However, care must be taken in accepting this mechanical
conception not to fall into the very common mistake of trying to
explain vital processes as being due directly to mechanical causes,
in consequence of their analogy to many phenomena seen in

1 Claude Bernard (IV. 1a), in his lectures on vital phenomena, arrives at the
same conclusion, his opinion being based on a nuwber of considerations :
“ Arrivés an terme de nos études, nous voyons qu’elles nous imposent une
conclusion trés-générale, fruit de lexpérience, c’est, a savoir, quentre les
deux écoles qui font des phénonémes vitaux quelque chose d’absolument
distinct des phénouémes physico-chimiques ou quelque chose de tout a fait
identique a eux il ya place pour une troisiéme doctrine, celle du vitalisme
physique, qui tient compte de ce qu'il y a de spécial dans les manifestations de
la vie et de ce qu’il y ade conforme a ]’action des forces générales: lélément
ultime du phénoméne est phy ee l'arrangement est vital!”

Bee es ab Covrdolitimng
ote into

92 THE CELL

inanimate objects. It must never be forgotten that there is no
substance in inanimate nature which remotely approaches the
living cell for complexity of structure, and that hence the reactions
of such a substance are of necessity correspondingly complex in
character.

The field of the phenomena of irritability is exceedingly wide,
since it embraces all the correlations which take place between the
organism and the outer world. The stimuli which act upon us

_ from without are innumerable. For the sake of clearness, we will
consider them under five heads: (1) thermal stimuli, (2) light
stimuli, (3) electrical stimuli, (4) mechanical stimuli, (5) the
almost infinite variety of chemical stimuli.

The manner in which an organism responds to one of these
stimuli is called its reaction. This may vary very considerably
with different individuais even when they are exposed to the same
stimulus. It depends entirely upon the structure of the organism,
or upon its finer properties, although these may not be perceptible
tous. Different organisms, to use a simile of Sachs (1V. 32a), may
in this respect be compared with variously constructed machines,
which, when set in motion by the same external force, heat, pro-
duce different useful effects according to their internal structures.

J Similarly, the same stimulus may produce quite different effects in

| different organisms, according to their specific structure.

We shall see later on that many ‘protoplasmic bodies are to
a certain extent attracted, whilst others are repelled, by light ;
a similar difference will be seen when the action of chemical
reagents, etc., on protoplasm is studied. The terms positive und
negative heliotropism, positive and negative chemotropism, galvanotro-
pism, and geotropism are used to describe these varying effects.

Another phenomenon, in some respects the exact opposite of the
ones described above, must also be explained by the varying
specific strncture of the stimulated substance; the term spec/fic
energy has been used to describe this phenomenon. | Whilst, as
described above, we see that protoplasmic bodies, differing in
structure, react in various ways to the same stimulus, we find, on
the other hand, that similareffects are produced upon the same
protoplasmic body by very different stimuli, such as_ light,
electricity, or mechanical injury.

A muscle cell responds to all kinds of stimuli by contracting, a
gland cell by secreting; an optic nerve can only experience the
sensation of light, whether stimulated by light waves, electricity,

THE VITAL PROPERTIES OF THE CELL 93

or pressure. Similarly, as Sachs has pointed out, plant cells also
are furnished with their specific energies. Tendrils and roots
bend themselves in a manner peculiar to themselves, whether
stimulated by light, gravitation, pressure, or electricity. The effect
of a stimulus bears the specific stamp, so to speak, of the special
structure of the stimulated substance, or, in other words, irritability is
a fundamental property of living protoplasm, but it manifests itself in
specific actions according to the specific structure of the protoplasm
under the influence of the external world.

The same idea is expressed by Claude Bernard (IV. 1a) in the
following words: ‘ La sensibilité, considerée comme propriété du
systéme nerveux, n’a rien d’essentiel ou de specifiquement distinct;
c'est lirritabilité spéciale au nerf, comme la propriété de contrac-
tion est l'irritabilité spéciale au muscle, comme la propriété de
sécrétion est lirritabilité spéciale a l’élément glandulaire. Ainsi,
ces propriétés sur lesquelles on fondait la distinction des plantes
et animaux ne touchent pas 4 leur vie méme, mais seulement aux
mécanismes par lesquels cette vie s’exerce. Au fond tous ces mé-
canismes sont soumis & une condition générale et commune,
Virritabilité.”

In speaking generally of irritability, another peculiar pheno-
menon deserves especial attention, namely the transmission or con-
duction of stimuli. If a small portion of the surface of a
protoplasmic body is stimulated, the effect produced is not limited
to this point alone, but extends to far outlying ones. Hence the
changes produced by the stimulus at the point of contact must be
more or less quickly shared by the rest of the body. Stimuli, as
a rule, are more quickly transmitted in animal than in vegetable
bodies; in human nerves, for example, the rate is 34 metres per
second ; it is always slower in plant protoplasm.

We imagine that the substance which is capable of receiving
stimuli forms a system of exceedingly elastic particles in a condi-
tion of unstable equilibrium. In such a system it is sufficient for
one of the particles to receive a slight shock, in order toset all the
others in motion, since each transmits its movement to another.
This theory explains the phenomenon, that exceedingly great effects
are often produced by very slight stimuli, just as a small spark,
by setting on fire a single grain of powder, may cause a powder
magazine to explode.

Finally, another peculiarity of organic matter is its capacity of
returning more or less completely to its original condition, after a

94 THE CELL

period, varying in length, of rest or recuperation has elapsed since
the cause of irritation was removed. I say advisedly more or less
completely, for very often the organic substance is permanently
altered in its structure and reacting powers by the application, for
a considerable period, of a stimulus, or by the repeated action of
the same stimulus. The phenomena thus produced are spoken of
as the after-effects of stimulation.

As a rule, we are not in a position to determine whether or no a
body can be stimulated, that is to say, whether it reacts to changes
in its environment, since most of the effects due to stimulation are
imperceptible to us. Sometimes the protoplasm responds by exhibit-
ing movements, or by striking changes of form; but, as has been
just remarked, such phenomena constitute only a small and limited
portion of the results produced, although naturally they are the
most important to the investigator, since they are apparent to his
perception. In consequence, in the following pages, we will chiefly
consider the way in which protoplasm responds, by means of move-
ments, to the stimuli, which have been grouped into the above five
classes. I have therefore decided to commence my considerations
of the vital properties of the elementary organism with contrac-
tility.

I. Thermal Stimuli. One of the essential conditions for the
vital activity of protoplasm is the temperature of its environment.
This temperature can only vary between certain fixed limits; if it
oversteps either of these, the protoplasm invariably dies immedi-
ately. These limits, it is true, are not the same for all protoplas-
mic bodies; some are able to withstand extremes of temperature
better than others.

The maximum temperature for plants and animals is generally
about 40° C. Exposure for a few minutes to such a temperature
suffices to cause the protoplasm to swell up and become coagulated,
and thereby its irritable structure and its life are destroyed. If an
Ameba is placed in water at 40°, it dies immediately ; it draws in
its pseudopodia and “converts itself into a globular vesicle, whose
sharply defined double contour encloses a large, turbid mass which,
by transmitted light, looks brownish in colour” (Kiihne IV. 15).
The same temperature causes “death from heat” in Mthalium
septicum, coagulation being induced. In Actinophrys, however,
instantaneous death occurs at a temperature of 45°, whilst the
cells of J'radescantia and Vallisneria are only killed by a tem-
perature of 47-48° C. (Max Schultze I. 29).

THE VITAL PROPERTIES OF THE CELL 95

The protoplasm of organisms which live in hot springs is able
to sustain much higher temperatures. Cohn found specimens of
Leptothriz and Oscillaria in the Karlsbad springs at 53° C., whilst
Ehrenberg observed Algz in the warm springs of Ischia.

But even in these cases we have not arrived at the extreme limit
of heat which can be sustained for a time by living substance. For
endogenous spores of Bacilli, which are furnished with unusually
resistent envelopes, remain capable of germination after they have
been heated for a short time in a liquid at a temperature of 100°.
Many even can endure 105-130° (de Bary IV. 56, p. 4). It is only
after a substance has been exposed to the action of dry heat of 140°
for a period of three hours that we can assume with certainty that
all life has been completely destroyed in it.

It is even more difficult to determine the lower limit at which
‘death from cold” occurs. Asa rule, temperatures below 0° are
less injurious to protoplasm than high ones. It is true that if the
egys of Hchinodermata, which are about to divide, are placed in a
freezing mixture at a temperature of from 2° to 3° C., the pro-
cess of division is temporarily arrested (IV. 12); but division
recommences and proceeds in a normal fashion when the eggs are
slowly warmed, even if they have been kept in the freezing mix-
ture for a quarter of an hour. Indeed, the greater number of
the eggs are found to be uninjured even if they have been kept
at this temperature for two hours. Plant-cells may be frozen
until ice crystals develop in the sap, and yet, after they have been
thawed, they exhibit the streaming movements of protoplasm
CEV...15);

Sudden exposure to temperatures below zero produces striking
changes of form.in the protoplasm of plants; however, it reverts
to its normal condition on being thawed. When Kiihne (IV. 15) ex-
amined in water cells of Tradescantia, which had been kept for
a little more than five minutes in a freezing mixture at 14° C., he
found, in the place of the ordinary protoplasmic net, a large number
of isolated, round drops and globules. These, after a few seconds,
began to show active movements, and in a few minutes commenced
to join themselves one to another, and thus to gradually become
reconstructed into a network, in which active streaming movements
soon commenced to take place.

Me

Kiihne describes in the following words another experiment :—

“If a preparation of Vradescantia cells is kept for at least one
hour in a space which is maintained by means of ice at a tempera-

96 THE CELL

ture of 0°, the protoplasm is found to exhibit an inclination to
break up into separate drops. Even where the network still per-
sists, it is composed of extremely fine threads, which are studded
here and there with large globules and drops ; several other glo-
bules float about freely in the cell fluid, in which, without moving
much from place to place, they revolve about their own axes with
active, jerking movements. After a few minutes, the free globules
are seen to unite themselves to the delicate threads, or to amalga-
mate themselves with some of the globules hanging on to the
threads, until the appearance of the streaming protoplasmic net-
‘work is quite restored.”

In plants, as a rule, their power of resistance to cold is inversely
in proportion to the amount of water they contain; seeds which
have dried in the air, and winter-buds, the cells of which consist
almost entirely of pure protoplasm, can withstand intense cold,
‘whilst young leaves, with their sap-containing cells, are killed
/even by frosty nights. However, the power of resistance to cold
| varies according to the specific organisation of different plants, or
rather of their cells, as is proved by daily experience (Sachs IV.
32p).

Micro-organisms are able to resist exceedingly low tempera-
tures. Frisch has discovered that the spores, and indeed the
vegetative cells of the Anthrax bacillus do not lose their capacity of
development by being cooled down in a liquid to a temperature of
— 110° C., from which they were extracted after it had been thawed.

Before reaching the above-mentioned extreme temperatures, at
which death by heat or cold is produced, phenomena known as heat
rigor or heat tetanus, and cold rigor, occur; when the protoplasm
is in either of these conditions, all the attributes which show it to
be alive, especially those of movement, are arrested so long as
the temperature in question is maintained ; but when this is either
raised or lowered, as the case may be, after a period of rest, they
again manifest themselves.

Cold rigor generally occurs at a temperature of about 0° C.,
whilst heat rigor sets in at a temperature only a few degrees lower
than that at which immediate death results; in both cases the
protoplasmic movements become gradually slower and slower, until
at last they quite cease. Amcbe, Reticularia, and white blood
corpuscles draw in their pseudopodia and become converted into
globular masses. Most plant cells assume the appearance described
above by Kiihne. If the temperature is either slowly raised or

THE VITAL PROPERTIES OF THE CELL 97

lowered, as the case may be, the vital appearances gradually become
normal, It is true that if the condition of rigor produced by cold
is maintained for a considerable time, death may ensue, although
cold is better withstood, and for a longer time, than heat. When
the protoplasm dies it becomes coagulated and turbid, whilst com-
mencing to swell up and to decompose. At the temperatures lying
between these extremes, the vital processes are performed in a
manner which varies in intensity with the degree of temperature.
This is especially true of the movements which take place at dif-
ferent speeds, increasing in rate up to a certain point, as the tem-
perature rises, until they reach a certain fixed maximum speed.
This occurs at the so-called optimum temperature, which is always
several degrees below that at which heat rigor is produced. As
the temperature passes this limit, the protoplasmic movements are
seen to slacken, until at last rigor sets in.

White blood corpuscles have been much used in studying the
effects produced by heat; for this purpose Max Schultze’s warm
stage, or Sachs’ warm cells, are most suitable. Ina fresh drop of
blood the corpuscles are seen to be motionless and globular in

form. If the drop is warmed—the necessary precautions being of

course observed—the corpuscles gradually commence to extend
pseudopodia, and to move about. As the temperature approaches
the optimum for the time being, these changes of shape become
more rapid. In Myxomycetes, Rhizopoda, and plant cells, the
effect produced by an access of heat is exhibited by an increase of
rapidity of the streaming movements of the granules. Thus,
according to the measurements of Max Schultze (I. 29), the
granules in the hair-cells of Urticu and Tradescantia travel at
ordinary temperatures at a rate of ‘004-005 mm. per second,
whilst if the temperature is raised to 35° C., their speed is in-
creased to ‘00J mm. per second. In Vallisneria the rate of
circulation may be increased to ‘(015 mm., and in a species of
Chara even to ‘04 mm. per second. The difference between the
slow and accelerated movements may be so great that whilst with

the former the length of a foot is traversed in fifty hours, with.

the latter the same distance may be covered in half an hour.
Niigeli (III. 16) has expressed the acceleration produced by an
accession of heat in the granular streaming movements in the cells

of Nitella by the following figures: in order to traverse a distance |
of ‘1 mm. the granules require 60 seconds at 1° C.; 24 seconds at |

5° C.; 8 seconds at 10° C.; 5 seconds at 15° C.; 3°6 seconds at
H

98 THE CELL

20° C.; 2:4 seconds at 26° C.; 1 5 seconds at 31° C.; and *65 seconds
at 37° C. From these figures it is apparent that “each consecu-
tive degree of temperature produces a corresponding slight
acceleration’ (Nigeli, Velten).

Finally, it is necessary to mention the remarkable behaviour of
protoplasm towards sudden great fluctuations of temperature, and
also towards partial or uneven heating.

Fluctuations of temperature may be either positive or negative,
that is to say, they may be caused by a raising or a lowering of
temperature. The consequence of a violent thermal stimulation
is a temporary cessation of allmovements. However, after a time, ©
the motion recommences at a rate corresponding to the tempera-
ture (Dutrochet, Hofmeister, de Vries). The accuracy of these
observations has been questioned by Velten (IV. 38). According
to his experiments, fluctuations of temperature between the neces-
sary limits produce neither a cessation nor a slackening of the
protoplasmic movements, which, on the contrary, immediately
proceed at a rate corresponding to the temperature which has
been attained.

Stahl (1V. 35), in his experiments upon the plasmodia of

| Myzxomycetes, has made some very interesting discoveries concern-
ing the effects produced by partial heating. If a portion of such
« plasmodium, which has spread its network out over an even
surface, be cooled, the protoplasm is seen to travel gradually from

| the cooler to the warmer part, so that the one portion of the net-

work is seen to shrink up, whilst the other becomes swollen. The
experiment may be conducted in the following manner: Two
beakers, one filled with water at 7°, and the other with water at
30°, are placed quite close to one another ; a wetted strip of paper
over which a plasmodium has spread itself is then placed over
their contingent edges, so that one of its ends dips into each
beaker; the temperature of the water in the beakers is not allowed
to vary. After a time the plasmodium, by stretching out and
drawing in its protoplasmic thread, succeeds in creeping over to
the medium which is best adapted to it.

No doubt most free-living protoplasmic bodies move somewhat
in this fashion, for as a rule their movements are regulated by
expediency, that is to say, they take place in order that the life of
the organism may be maintained. For instance, flowers of tan
sink down during the autumn toa depth of several feet into the
warmer layers of the tan, in order to pass the winter there.

THE VITAL PROPERTIES OF THE CELL 99

Then during the spring, as the temperature rises, they move in an }
opposite direction, ascending to the warmer superficial layers.

II. Light Stimuli. In many cases light, like heat, acts as a
stimulus to animal and plant protoplasm. It induces character-
istic changes of form in individual cells, and causes movements in
fixed directions in free-living unicellular organisms. Botanists
have obtained especially interesting results in this department.

The plasmodia of Athalium septicum only spread themselves
out on the surface of the tan in the dark; in the presence of light
they sink down below the surface. If a small pencil of light is
allowed to fall upon a plasmodium which has spread its network
upon a glass slide, the protoplasm is immediately seen to stream
away from the illuminated portion, and to collect in the parts
which are in shadow (Barenezki, Stahl IV. 35).

Pelomyzxa palustris, an organism resembling the Ameba, is
actively motile in shadow, extending and protruding broad
pseudopodia. If a fairly powerful ray of light impinges upon it,.
it suddenly draws in all its pseudopodia, and transforms itself into
a globular body. Only after it has rested quietly in the shade
for a time does it gradually recommence its amoeboid movements.
“Tf, on the other hand, daylight is admitted gradually during a
period of rather Jess than a quarter of an hour, no effects of stimu-
lation are to be perceived; this is also the case when, after a
prolonged illumination, the light is suddenly withdrawn” (Engel-
mann IV. 6b).

The star-shaped pigment cells of many invertebrates and verte-
brates, which have been described under the name of chromat»-
phores (IV. 3, 29, 30, 33), react very actively to light; they are
the cause of the changes of colour so often seen in many Fishes,
Amphibians, Reptiles, and Cephalopods. For example, the skin of
a Frog assumes a lighter shade of colour when under the influence
of light. This is due to the fact that the light causes the black
pigment cells, which extend their numerous processes through the (
thick skin, to contract up into small black points. In addition, as ,
they become less prominent, the green and yellow pigment cells, °
which do not contract, become more easily seen.

Further, the pigment cells of the retina become considerably [
altered in form under the influence of light, both in vertebrates |
(Boll) and in invertebrates, for instance in the eyes of Cephalopoda
(Rawitz IV. 31).

It is a well-known fact that many unicellular organisms which

100 THE CELL

propel themselves by means of cilia or flagella, such as Flagel-
lata, Ciliata, the swarm-spores of Alge, etc., prefer to collect
either on that side of the cultivation dish which is nearest the
window, or on the one which is away from it.

This may be easily proved by means of a simple experiment
described by Nigeli (III. 16). A piece of glass tubing three feet
in length is filled with water containing green swarm-spores of
Alge (tetraspores), and.is placed perpendicularly. Then, if the
upper part of the tube is covered with black paper, and light is
allowed to fall upon the lower portion, it is seen after a few hours
that all the spores have collected in this lower portion, leaving
the upper part colourless. If now the upper portion is uncovered,
and the paper is transferred to the lower part, all the swarm-
spores ascend the tube, and collect on the surface of the water.

Euglena viridis is exceedingly sensitive to light (Fig. 44 A,
1V. 8). If a drop of water containing Huglene is placed upon a
slide, and only a small portion of it is illuminated, all the
individuals collect in this area, which, to quote an expression
of Engelmann’s, acts like a trap. This organism is especially
interesting, because the perception of light is restricted to a
definite portion of the body. Each Euglena consists of two
portions, a large posterior one containing chlorophyll, and a
colourless anterior, flagella-bearing one, in which there is a red
pigment spot. Now, it is only when this anterior portion comes
into contact with light, or is placed in shadow, that the organism
is seen to react by altering the direction of its movements
(Engelmann). Hence, in this case, a certain part of the body
functions to a certain extent as an eye.

Stahl (IV. 34) and Strasburger (IV. 37) have investigated most
fully the action of light upon swarm-spores. The former sums up
his results in the following words :—“ Light effects an alteration

is the direction of the movements of swarm-spores by causing

_ them to make their longitudinal axes coincide approximately with
the light. The colourless flagellated end may be directed either
towards or away from the source of light. Either position may
become exchanged for the other under otherwise unaltered
external conditions, and, indeed, this occurs at very different
degrees of light intensity. The intensity has the greatest influence

yorer relative positions. When the light is very intense, the
anterior end is directed away from the source; when it is less
strong, the swarm-spores move towards the light.”

THE VITAL PROPERTIES OF THE CELL 101

This sensitiveness towards light varies considerably both in
different species and in individual members of the same species;
indeed, even in the same individual, considerable differences may
be seen under different external conditions. This varying power
of reaction in swarm-spores has been called phototonus or light-
tone by Strasburger.

Swarm-spores of the Botrydiwm and Ulothrizx, which react some-
what differently under the influence of light, are very suitable
for experiments on this subject.

If some swarm-spores of Botrydiwm are placed in a drop of
water upon a coverglass, and are kept in shadow, they spread
themselves out evenly in the water. If a light is allowed to fall
on them, they are seen to immediately direct their anterior ends
towards the source of light, and to hurry in fairly parallel paths
towards it. After a short time, at most from one and a half
to two minutes, almost all of them have collected at the illuminated
side of the drop, which, for the sake of brevity, Strasburger has
named the positive edge, to distinguish it from the opposite or
negative edge. Here they are seen to intermingle and to conjugate
in large numbers. If the slide is now turned round through an
angle of 180°, all the spores which are still capable of movement
immediately forsake the edge of the drop, which is now turned
away from the light, and hasten back towards the light. If
the microscope is fitted with a rotating stage, it is possible by
turning the latter to make the swarm-spores continually keep |
changing their course. They always travel in a straight line |
towards the light.

Ulothriz zoospores behave in a somewhat different manner.
“‘ These also travel quickly, and in approximately straight paths
towards the positive edge of the drop; however, as a rule, they do
not all move in this manner; on the contrary, it is generally the
case that a larger or smaller number of individuals in each prepara-
tion are seen to move rapidly in the opposite direction, that is
to say, towards the negative edge. A most peculiar spectacle is
thus produced, for the spores, since they go in opposite directions,
appear to travel at double speed as they pass each other. If the
preparation is turned through an angle of 180°, the spores which
had collected on the side which was positive are seen to hasten
to the other edge, whilst the others, which were collected on the
side which was negative, travel in the opposite direction, and
having arrived at their destination, they commence to move about

102 THE CELL

amongst themselves, keeping more or less close to the edge of
the drop, according to the condition of the preparation. Continu-
ally, individual spores are seen to suddenly forsake the side, either
positive or negative, at which they were stationed, and to hurry
through the drop to the opposite one. Such an exchange is
continually taking place between the two sides. Indeed, it
frequently occurs that certain individuals, which have just left
one side and arrived at the other, hasten back to the one from
which they originally came. Others become arrested in the middle
of their course, and then return to their starting-point, in order
eventually to oscillate backwards and forwards for a considerable
time like a pendulum.”

The following experiment, described by Strasburger, shows
how sensitively and quickly the zoospores react to light:—“ If a
piece of paper is placed between the microscope and the source of
light, just as the zoospores are on their way from one edge of the
drop to the other, they immediately turn to one side, many
rotating in a circle; this, however, only lasts for a moment,
after which they continue to move in the same direction as before
(interruption movements).” Strasburger (IV. 37) has named those

/ zoospores which hasten towards the source of light light-seekiny
(photophylic), and those which travel from at light-avoiding (photo-
\ phobic).

As has been already remarked, the way in which the spores
collect at one or other side of the drop, thus indicating their
special kind of phototonus, depends upon external circumstances,
such as the intensity of the light, the temperature, the aération of
the water, and their condition of development.

It is possible to entice spores, which under intense illumination
have collected on the negative side, to come over to the other side.
The intensity of the light must be gradually diminished in pro-
portion to their phototonus by introducing one, two, three or more

f sereen of ground glass between the preparation and the source
of light. The same result may be more easily obtained by moving
the microscope slowly away from the window, and thus rendering
the illumination less intense.

. The temperature of the environment often has a considerable
influence upon the degree of sensitiveness to light which is evinced
by many zoospores. When the temperature is raised they become,
so to speak, attuned to a greater degree of sensitiveness ; whilst,
at the same time, their movements are rendered more active: the

THE VITAL PROPERTIES OF THE CELL 103

reverse is the case when the temperature is lowered. In the first
case they also become more photophylic (light-seeking), and in
the latter-more photophobic (light-avoiding).

“In addition, zoospores alter as regards their phototonus during
the course of their development, for they appear to be able to
withstand greater intensity when they are young than when they
are old.”

As is shown by the experiments of Cohn, Strasburger, and
Others, not all the rays of the spectrum are able to exert an
influence upon the direction of the movements of the spores, i
being only those which are strongly refracted (blue, indigo and violet)
that produce stimulation.

If a vessel containing a deep-coloured solution of ammoniated
copper oxide, which only transmits blue or violet rays, be placed
between the source of light and the preparation, the spores are
seen to react just as if they came in contact with ordinary white
light; on the other hand, they do not react at all to light which
has passed through bichromate of potassium solution, through the
yellow vapour of a sodium flame, or through ruby-red glass.

Another very important and complex manifestation of the

effects due to light is seen in the movements of the chlorophyll |

corpuscles in plant cells. The light acts as a stimulus to proto-
plasm, which contains chlorophyll, causing the latter to collect by
means of slow movements in suitable places within the cellulose
membrane.

The most suitable object for the study of these phenomena is the
Alga, Mesocarpus, upon which Stahl (IV. 34) has made some most
convincing observations.

In the cylindrical cells, which are united together to form long
threads, a narrow band of chlorophyll is extended longitudinally
along the middle of the vacuole, which is thus divided into two
equal parts; the ends of this band pass over into the protoplasmic
lining of the wall. Now this chlorophyll band changes its position
according to the direction of the impinging light. If it is exposed
directly from above or below to weak daylight, it turns its surface
towards the observer. If, however, on the contrary, it is arranged
so that only such rays as are parallel to the stage of the micro-
scope are allowed to reach the preparation from one side, the
green plates are seen to turn about through an angle of 90°, so
that they take up an exactly vertical position, assuming now an
appearance of dark green longitudinal stripes, stretching them-

104 THE CELL

selves through the otherwise transparent cell. The band is able
to assume every possible intermediate position in its endeavour to
place its surface at right angles to the impinging light. Ona
warm summer's day this change-of position is effected in a very
few minutes, being brought about by the active movements which
the protoplasm makes inside the cell membrane.

The effect produced varies in this case also, as with the Z00-
spores, according to the intensity of the light. Whilst diffuse
daylight has the effect described above, direct sunlight brings
about a quite opposite result, for in this case the chlorophyll
bands turn one of their edges to the sun. Hence we can educe
the following: “Light exerts an influence upon the position
of the chlorophyll bands of Mesocarpus. If the light is fairly
weak, the bands turn themselves at right angles to the path of the
rays; if, however, it is intense, they place themselves in the same
direction as the rays.” Stahl calls the first arrangement surface
position, and the second, profile position.

If illuminated intensely for a considerable period, the whole
band contracts to form a dark green vermiform body; it is,
however, under favourable conditions capable of resuming its
original form.

The purpose of all these various movements of the protoplasm —
\ under the influence of light is, on the one hand, to bring the
_ chlorophyll bands into a favourable position for the exercise of
their functions ; and, on the other, to protect them from the in-
jurious action of a too powerful illumination.

Further, the plant-cells which contain chlorophyll granules, and
which are connected to form tissues, are also subjected to the
influence of light, as is so plainly seen in Mesocarpus. Only in
this case the phenomena are somewhat more complex (Fig. 52).

Sachs was the first to notice that the colour of leaves is lighter
when they are exposed to direct sunlight, than when they are in
shadow, or when the light is less intense. In consequence of this
digcovery, Sachs was able to produce light pictures upon leaves,
by partially covering them with strips of paper, and exposing
them to intense light (IV. 32a); after a certain time the strips of
paper were removed, and it was then seen that the portions which
they covered appeared as dark-green stripes upon a light-green
background. .

This phenomenon may be explained by the law which was laid
down in the case of Mesocarpus; this has been proved by the

THE VITAL PROPERTIES OF THE CELL 105

investigation of Stahl (IV. 34), which he conducted on the lines
laid down by Famintzin, Frank, and Borodin. When the illumina-
tion is faint, or when the leaves are in shadow, the protoplasm
moves so that the chlorophyll granules are arranged upon those
external surfaces of the cells which are turned towards the light
(Fig. 52A), having completely forsaken the side-walls. On the
other hand, the protoplasm, under the influence of direct sunlight,
streams away towards the side-walls, until the external surface is
quite free from chlorophyll granules, that is to say, in the first

Fig. 52.—Transverse section through the leaf of Lemna trisulca (after Stahl): A surface
position (position assumed in diffused sunlight); B arrangement of chlorophyll granules
under the influence of intense light ; C position assumed by chlorophyll granules in the
dark.

case, the whole chlorophyll-bearing substance, as in Mesocarpus,
assumes a surface position towards the impinging light, and in the
second, a profile position; hence the varying colour of the leaves.

106 THE CELL |

In addition, the chlorophyll granules themselves, when under the
influence of intense light, alter their shape, becoming smaller and more
globular.

All these occurrences serve to accomplish the same end:

'“Chlorophyll granules protect themselves by turning on their
axes (Mesocarpus), by migration, or by altering their shapes from
intense illumination.” “If the illumination is weak, the largest
surfaces are turned towards the light, in order that as much of it
may be received as possible. The behaviour is exactly the oppo-

_ site when the light is strong, a smaller surface being then exposed

| to the light.”

III. Electrical Stimuli. As has been shown by the experi-

ments of Max Schultze (I.
29), of Kiihne (IV. 15), of
Engelmann, and of Ver-
worn (IV. 39), electrical
currents, both constant and
induced, act as stimuli upon
protoplasm, when they flow
directly through it.

If some staminal hairs of
Tradescantia (Fig. 63) are
placed between non-polar-
isable electrodes which are
close together, and are then
stimulated by means of
weak induction shocks, the
granular streaming moye-
ments can be seen to have
been influenced in that por-
tion of the protoplasmic net
through which the current

flowed. Irregular masses
and globules develop upon
the protoplasmic threads ;
these separate off at the

Fig. 53.—A, B cell of a staminal hair of Tra- thinnest places, and become
descantia virginica, A Normal condition of proto- ; j
plasm before it has been disturbed. B'The proto- absorbed abe neighbouring
plasm, in consequence of stimulation, has massed’. threads. After a short
itself into balls; a cell-wall; b transverse wall of ;
two cells; c,d balls of protoplasm, (After period of sie, Bho: 001s
Kiibne; from Verworn, Fig. 13.) ments recommence, the

THE VITAL PROPERTIES OF THE CELL 107

masses and globules being gradually taken up by the neighbouring
streams of protoplasm, carried along by them, and finally split up.
If strong shocks are repeatedly administered, so that the whole
cell is affected, a return to the normal condition is impossible, for
the protoplasmic body, by becoming partially coagulated, has been
transformed into turbid flakes and masses.

In Amebe and white blood corpuscles the streaming motions of
the granules and the crawling movements of the whole cell are
both arrested for a time by slight induction shocks; after a while
they are resumed and proceed ina normal fashion. If stronger
induction shocks are administered, the result is that the pseudo-
podia are quickly withdrawn, and the body contracts up into a
ball; finally, very strong shocks cause the bursting and consequent
destruction of the contracted spherical body.

If the induction current is applied for a considerable time to one of
the lower unicellular organisms, it can be gradually destroyed bit by
bit, and thus diminished in size. In Actinuspherium the process 1s
as follows: the pseudopodia, which are parallel to the current,
soon exhibit varicosities; they are gradually completely with-
drawn, whilst the protoplasm becomes massed together to form
little balls and spindles (Fig. 54); then at this place the surface
of the body becomes gradually destroyed by a process resembling
to a certain extent a kind of melting down, during which the
vacuoles, which are con-
tained in the proteplasm,
burst. On the other hand,
those pseudopodia which are
at right angles to the cur-
rent are unaffected. When
the stimulus is removed, the
body, which has thus been
reduced to about a half or a
third of its original size,
gradually recovers, and re-
produces the parts which

have been destroyed. Fie. 54.—Actinospherium Eichhornii, action

The action of the constant of an interrupted current. Progressive de-
th Acti struction of protoplasm is equal at both poles.
coeEeuy ee . eh (After Verworn, Tab. 1, Fig. 5.)

spherium (Fig. 55), Actino- |
phrys, Pelomyxa, and Myxomycetes, is similar to this. When the
circuit is closed, an excitation occurs at the positive pole or anode

108 THE CELL

(in Fig. 55,+) which is manifested by the retraction of the pseudo-
podia, and, if the stimulus lasts
long, by the destruction of the
protoplasm at the place where
the current enters. When
communication is broken, the
destructive process at the anode
immediately ceases, whilst, on
the other hand, a transitory

~ contraction occurs at the sur-
face which is turned towards
the cathode.

Perhaps even more interest-
ing and important than these
processes are the phenomena

See produced by Galvanotrop-

Bis: 8—Actinnaphavion Biboris Ye igm, which have been observed

short time after the closing of the current, by Verworn in a number of
granular destruction of the protoplasm
commences at the anode (+). At the
cathode the pseudopodia have become 40).
normal again. (After Verworn, Tab. 1,
Fig. 2.)

unicellular organisms (IV. 39,

Many organisms, in conse-
quence of the influence of the
constant current, are caused to move in certain fixed directions,
just as they move when stimulated by a ray of light (heliotropism).
“Tf a drop, containing as many Paramecia aurelia as possible, is
placed upon a slide between two non-polarisable electrodes, and the
constant galvanic circuit is closed, it is seen that the Paramecia
immediately leave the anode in a mass, and hurry in a dense
swarm to the cathode, where they collect in great numbers.
After a few seconds the rest of the drop becomes completely
free from Protista, whilst at the cathode there is a dense seething
crowd of them. Here they remain as long as the current persists.
When connection is broken, the whole swarm immediately forsakes
the cathode to swim back in the direction of the anode. How-
ever, they do not all collect at the anode, part of them re-
maining scattered about in the drop; at first they do not come
near to the cathode, but after a time they gradually approach it,
until finally all the Protista are again evenly distributed through-
out the drop.”

If pointed electrodes are employed, the Paramecia swarm
inwards to form a galvanic figure around the cathode (Fig. 56 A).

THE VITAL PROPERTIES OF THE CELL 109

An appearance similar to that produced when iron filings are
attracted by a magnetis seen. ‘Under the circumstances,” as

Fie. 56.—On completing the circuit of the constant current all the Paramecia in a drop
of water swim within the curve of the electric current towards the negative pole (A), until
after a time they collect on the other side of the pole (B). (After Verworn IV. 40, Fig. 20.)

Verworn remarks, “it may be observed that after all the
Paramecia have wandered over to the negative pole, the largest
collection is formed behind, that is to say—reckoning from the
positive pole—on the other side of the negative pole, and that
only a few remain on this side of the pole (Fig. 56 B). When the
connection is broken the Protista swim back again, in the manner
described above, towards the positive pole, keeping at first, just as
before, well within the curve of the electric current, until gradually
the movement, and with it the division into groups, becomes ir-
regular again.”

In the same manner, a number of other Ciliata (such as
Stentor, Colpoda, Halteria, Coleps, Urocentum) and Flagellata
(such as T'rachelomonas, Peridinium) are galvanotropic.

Amebe react in a similar manner. At the first moment after
the circuit of the constant current has been completed a cessa-
tion of the streaming movements of the granules occurs; very
soon, however, the hyaline pseudopodia are suddenly protruded
from the end which is turned towards the cathode, and, whilst
the remainder of the body substance flows in the same direction,
and keeps continually stretching out new psendopodia, the Ameba
creeps towards the cathode. When the current is reversed it is
seen that the granular streaming movements are also immediately
reversed, and the Ameba commences to creep in the opposite
direction.

The movement towards the cathode may be called negutire
galvanotropism. As there exist both negative and _ positive
heliotropism and thermotropism, so we occasionally find isolated

110 THE CELL

instances of positive galvanotropism. It has been observed by
Verworn in Opalina ranarum, and in a few Bacteria and Flagellata
such as Oryptomonas and Chilomonas. When the circuit is com-
pleted the above-named species travel towards the anode instead
of towards the cathode, and collect there. If Ciliata and Flagel-
lata are present side by side in one drop, they are seen under the
influence of the constant current to hasten in opposite directions,
so that finally two distinct groups are to be seen, the Flagellata
being at the anode, and the Ciliata at the cathode. If the current
is now reversed they advance like two hostile armies upon one
another, until they assemble again at the opposite poles. Each
time the current. was made it produced in a few seconds a dis-
tinct sorting out of the crowd of Infusoria, which were otherwise
in inextricable confusion.

IV. Mechanical Stimuli. Pressure, violent shaking, crushing,
all these act as stimuli to protoplasm. Weak mechanical stimula-
tions only produce an effect upon the point of contact; strong stimuli
affect a larger area and produce a more rapid and more powerful
effect than weak ones. If a cell of a Tradescantia or Chara or the
plasmodium of an A’thalium be violently shaken, or pressed upon
at one place, the granular movement is temporarily arrested,
whilst swellings and knots may even appear on the protoplasmic
threads, such as are produced by the electrical current. Hence it
frequently occurs, that in preparing the slide for observation all
the protoplasmic movements may be brought to a standstill,
simply by putting on the coverglass. They gradually return after
a period of rest.

Amebe and white blood corpuscles withdraw their pseudopodia
and assume a globular form when they are violently shaken.
Reticularia, which have extended their long processes, often with-
draw them with so much energy that the ends which were
attached to the slide are torn off (Verworn). A localised stimulus
can be produced at a given point with a fine needle. If the
stimulus is weak the effect is confined to this point, a varicosity
being formed and a shortening of the pseudopodium being pro-
duced. Strong and repeated stimuli cause neighbouring
pseudopodia, which were not directly touched, to contract (Fig.
57 B),

If an Infusorian or other small animal comes in contact with
an outstretched pseudopodium, it is firmly grasped by it, and
becomes surrounded by the protoplasm. As the pseudopodium

THE VITAL PRUPERTIES OF THE CELL ERY

SSI RSH B
SLIT XR |
ft iis
Vea f | \
/ /
TA \ l
y | :

NN

/
a

ae
ne

}
| Vy

Fic. 57.—Orbitolites. A portion of the surface with its psendopodia: A undisturbed; B
the whole has been stimulated by repeated shaking. (After Verworn III. 24, Fig. 7.) This
is of importance to Rhizopoda in absorbing food.

gradually shortens itself, a motion in which the neighbouring
threads eventually participate, the Infusorian is gradually drawn
into the centre of the protoplasmic mass, where it undergoes
’ digestion.

V. Chemical Stimuli. <A living cell is able to a certain
extent to adapt itself to chemical changes in its environment. For
this, however, one thing is most important, namely that the
changes should be made gradually, not suddenly.

MHthalium plasmodia flourish in a 2 per cent. solution of grape-
sugar, if the latter is added in gradually increasing quantities to
the water (IV. 35). If they were to be transferred straight from
pure water into this chemically different environment, the sudden
change would result in their death; this would also occur if they
were to be suddenly placed back into pure water from the 2
per cent. sugar solution. It is evident that the protoplasm needs
time to adapt itself to its altered condition, probably by increasing
or diminishing the amount of water it contains.

Marine Amcebe and Reticularia remain alive after the water
which contains them, in consequence of being in an open vessel,
has evaporated so much that it contains 10 per cent. of salt.
Fresh water Amcoebe can gradually accustom themselves to a 4
per cent. solution of common salt, whereas, if they are suddenly
immersed in a 1 per cent. solution, they contract into balls, and in
time become broken up into glistening droplets. During the pro-
cess of adaptation to a new chemical environment, the individual

112 THE CELL

cells may undergo greater or less changes in their structure and
vital properties. When such changes are apparent to us, we
speak of the effects of chemical stimulation. These phenomena, which
are so exceedingly numerous, may vary considerably, according as to
whether the whole, or only part, of the cell-body is affected by the
stimulus.

a. First group of experiments. Chemical stimuli which
affect the whole of the body. In order to throw light upon
this first group of phenomena, the behaviour of protoplasm towards
certain gases, which are grouped under the common name of
anesthetics, must be investigated.

The protoplasmic movements of a plant cell soon become
arrested, if, instead of being put into water, it is placed in a drop
of olive oil, by which means the air is excluded (IV. 15). After
the oil has been removed, the movements are seen to gradually
recommence. ;

The streaming movements may ina similar manner be slackened
and finally completely stopped, if the air is replaced by carbon di-
oxide or hydrogen. For these experiments special slides with gas,
chambers have been constructed through which a current of
carbon dioxide or hydrogen may be conducted. If the plant
cellis kept from 45 minutes to an hour ina current of carbon
dioxide, the movements are as a rule completely stopped; when
hydrogen is used, a longer time must be allowed (III. 5). This
protoplasmic paralysis may, if it has not been allowed to last too
long, be removed by the addition of oxygen. “ Apparently living.
protoplasm unites chemically with the oxygen of its environment.
The definite oxygenated compound thus produced, of which under
ordinary conditions a considerable amount must be assumed to
exist in every protoplasmic body, is continually broken down
during the movements, whilst carbon dioxide is probably given
off” (Engelmann III. 5). Hence the removal of oxygen has a
paralysing effect upon the irritability, and indeed upon all the.
vital activities of the protoplasm.

Such anesthetics, as chloroform, morphia, chloral-hydrate, etc.,
have a marked influence upon the vital activities of the cell.
These substances do not affect the nervous system alone, as is
frequently believed, but all the protoplasm of the body. The
difference is only a matter of degree ; the irritability of the
nerve-cells is more quickly lowered and finally destroyed than
that of the protoplasm of other cells. Further, when narcotics

THE VITAL PROPERTIES OF THE CELL Pi3

are employed medicinaily, the attempt is made to act upon the
nervous system alone, for if all the elementary cells were affected,
a cessation of the vital processes would result, and death might
ensue. However, the following examples will prove clearly that
the irritability of animal and vegetable protoplasm may be
temporarily destroyed without permanent harm.

The sensitive plant, or Mimosa pudica, is very easily affected by
mechanical stimulation. When a leaflet is shaken a little, it
immediately closes itself up, and forsaking its upright position,
droops downwards. In addition, it forms an example of the rapid
manner in which a stimulus is conducted in plants, in which, since
no nerves are present, it must be simply transmitted by each pro-
toplasmic cell quickly conveying the impulse to its neighbour.
In consequence of this, if the stimulus is sufficiently strong, not
only do the leaves which were directly touched close up, but also
those on the same branch, and eventually even the whole plant,
are affected. In consequence of the stimulation, certain mechanical
arrangements, not suitable for present discussion, come into play.

In order to study the effect of anesthetics, a sensitive plant, in
a condition of normal irritability, should be placed under a bell-
jar, and when the leaves are fully extended, a sponge soaked with
chloroform or ether should be inserted (Claude Bernard IV. 1).
After about half an hour it is seen that the chloroform or ether
vapour has caused the protoplasm to lose all its irritability.
When the bell-jar is removed, the leaves, which are spread out
as usual, may be touched, or even severely crushed or cut, without
any reaction being produced; the result is the same as that pro-
duced on one of the higher animals provided with nerves. And
yet, if proper precautions have been taken, it is found that the
protoplasm has not been killed, for after the sensitive plant has
been for a short time in the fresh air, the narcosis gradually
disappears ; at first, individual leaves gradually close up when
they are roughly handled, until finally complete irritability is
restored.

Ova and spermatozoa may be subjected to the action of narcotics
inasimilarmanner. When Richard Hertwig and myself (IV. 12a)
placed the actively motile spermatozoa of a sea-urchin in a ‘5 per
cent. solution of chloral-hydrate in sea water, we found that after
five minutes, their motions were completely arrested; however,
these soon recommenced, after the cbloral solution had been diluted

with pure sea water. Further, those spermatozoa which had been
I

114 _ ‘THE CELL

temporarily paralysed in this manner united with ova when they
were brought to them, almost as quickly as fresh spermatozoa.
When they were kept for half an hour in the chloral solution,
a more marked paralysis was produced, which persisted for a
long time after the noxious agent had been removed. It was
not until some few minutes had elapsed that certain individual
isolated spermatozoa commenced to exhibit snake-like movements,
which gradually became more active. Even when they were
brought into the neighbourhood of ova, it was observed, that after
ten minutes none of these were fertilised, although several
spermatozoa had attached themselves to their surfaces, and had
bored their way in. But even in this case fructification and the
subsequent normal division of the eggs took place finally.

Similarly, egg-cells become affected, as regards their irritability,
by a°2 to ‘5 per cent. solution of chloral hydrate or of some similar
drug ; this may be recognised by the abnormal manner-in which,
after the seminal fluid has been added, the process of fertilisation
takes place. For whilst under ordinary circumstances only one
single spermatozoon penetrates into the ovum, with the result that
a firm yolk membrane is immediately formed, which prevents the
entrance of other spermatozoa, in chloralised eggs multiple fertilisa-
tion takes place. It has been proved that, according to the inten-
sity of the action of the chloral, that is to say, the stronger the
solution, and the longer it is allowed to act, the greater is the
number of spermatozoa which make their way into the ovum
before the formation of the yolk and membrane. Evidently the
effect of this chemical reagent is to lower the power of reaction
of the egg plasma, so that the stimalus which is produced by the
entrance of one spermatozoon is now no longer sufficient, but the
ovum must be stimulated by the entrance of two, three, or even
more spermatozoa, before it is sufficiently excited to form a mem-
brane.

Finally, another example will show that the chemical processes
of “the cell may also be hindered by anesthetics. As is well known,
the yeast fungi (Saccharomyces cerevisie) produce alcoholic fer-
mentation in a solution of sugar, and during this process bubbles
of carbon dioxide rise through the fluid. When Claude Bernard
(IV. 1) added chloroform or ether to the solution of sugar,
before adding the yeast, no fermentation took place, although
in other respects the circumstances were favourable. But when
the yeast, after having been filtered out from the chloroform

THE VITAL PROPERTIES OF THE CELL PS

solution, and rinsed with clean water, was placed in pure sugar
solution, he found that fermentation soon occurred; hence the
yeast had recovered its power of converting sugar into alcohol
and carbon dioxide, this power having, by the action of the
chloroform and ether, been temporarily suspended.

In a similar manner the functions which the chlorophyll per-
forms in plants, and the dependent process of giving off oxygen in
the sunlight, may be arrested by means of chloroform (Claude
Bernard).

b. Second Group of Experiments. Chemical Stimuli
which come into contact with the cell-body at one spot
only. Very interesting and varying phenomena are produced
when chemical substances, instead of coming into contact with the
body all round, only impinge upon it, at a definite fixed point.
Such stimuli may produce changes in form, and movements in a
definite direction, which phenomena have been classed under the
name of Chemotropism (Chemotazis).

Chemotropic movements may be directed towards the stimulating
source, or, on the contrary, away from it. In the first case the chemi-
cal substance is said to attract, and in the second to repel, the
protoplasmic body. This depends partly upon the chemical
nature of the substance, partly upon the individual properties of
the special kind of plasma, and, finally, upon the degree of conden-
sation of the chemical substance. A substance, which when
dilute may attract, may repel when the solution is strong. Here,
as with strong and weak light, special differences are present.
Just as heliotropism may be positive or negative, so may chemotro-
pism be positive or negative.

We will first examine the action of gases,.and next that of
solutions ; at the same time we will become acquainted with a very
ingenious method of investigation, for which we must especially
thank the botanist Pfeffer (LV. 26).

1. Gases. Oxygen has great attractive powers for freely
moving cells, as has been shown by the experiments of Stahl,
Engelmann, and Verworn.

Stahl has made experiments upon the plasmodia of Avthaliwm
septicum (IV. 35). He half filled a glass cylinder with thoroughly
boiled water, which, in order to exclude the air, he covered with
a very thin layer of oil. He then took a strip of filter paper, over
which a plasmodium had extended itself, and placed it along the
side of the cylinder in snch a manner that one half of it was

116 THE CELL

immersed in the water. The strands of protoplasm, which were
placed in the non-oxygenated water, were seen to grow gradually
thinner, until after a time all the protoplasm had crept up above
the layer of oil, which, except in excluding the air, had no
deleterious effect upon it, to the upper portion of the cylinder,
where it could come into contact with the oxygen of the air.
Another method of performing the same experiment is to place a
plasmodium in a cylinder which is quite full of thoroughly boiled
water; to close the opening with a perforated cork, and then to
place the cylinder upside down in a plate of fresh water. Very
soon the plasmodium is seen to have wandered through the small
hole in the cork into the medium which contains oxygen.

Engelmann (IV. 7) has made some very interesting experiments
upon the directing influence exerted by oxygen upon the move-
ments of bacteria. Heshows that many species of bacteria may be
used as a very delicate test for minute quantities of oxygen. If into a
fluid which contains certain bacteria, a small alga or diatom is
introduced it is seen after a short time to be surrounded with a
dense envelope of bacteria, which
have been attracted by the oxy-
gen set free by the action of its
chlorophyll.

Verworn (IV. 40) saw a dia-
tom quite enclosed by a wall of
motionless Spirochete whilst
the rest of the preparation was
quite free from them (Fig. 58).
Suddenly the diatom moved a
short distance away, getting out
of the crowd of Bacteria. The
Spirochetx, so suddenly left in
the lurch by the producer of
oxygen, remained quiet for a
second, but soon commenced to
move about quickly, and to
swim after the diatom in dense
masses. After a minute or two
they had nearly all reassembled

Fig. 58.—A large diatom (Pinnularia) round about it, after which they
surrounded by a large number of Spiro- remained motionless as before.

chete plicatilis, (After Verworn IV. 40, in f
Fig. 14.) This attractive power pos-

THE VITAL PROPERTIES OF THE CELL 17

sessed by oxygen explains the fact that in microscopic prepara-
tions almost all Bacteria, Flagellata, and Ciliata are found
collected together round the edges, or round any air bubbles
which may be present in the water.

Verworn describes a most instructive experiment (IV. 40). A
large number of Paramecia are placed in a test-tube, which is
filled with water, poor in oxygen. The test-tube is then reversed
and placed under mercury. Very soon the movements of the
cilia commence to slacken, in consequence of the lack of oxygen.
If now a bubble of pure oxygen is introduced through the mer-
cury into the test-tube, it will be seen after a few seconds to be
surrounded by a thick white envelope of Paramecia, “ which,
driven by their thirst for oxygen, throw themselves energetically
upon the bubble of this gas.”

2. Liquids. Stahl and Pfeffer have made systematic experi-
ments upon the stimulating action of fluid substances.

Stahl (IV. 35) has again made great use of flowers of tan.
Upon this organism even pure water has a stimulating effect,
a phenomenon described by Stahl as positive and negative hydro-
tropism. If a plasmodium is evenly spread out over a strip of
damp filter paper, it is seen, as soon as the paper commences to
dry, that the plasmodium makes its way to the dampest parts. If,
whilst the drying process is going on, a slide covered with gelatine
is held perpendicularly at about two mm. distance above the
paper, a few branches are seen to extend themselves upwards to-
wards the gelatine, attracted by the water vapour it gives off,
until finally they reach it and spread themselves out upon it
possibly, during the course of a few hours, the whole plasmodium
may transfer itself to the damper surface. When Myxomycetes
are about to fructify, negative instead of positive hydrotropism
takes place. Under these conditions the plasmodia seek the
driest portions of the environment, and withdraw themselves
from any damp gelatine or moistened filter paper which may be
brought into their neighbourhood.

These phenomena of hydrotropism are easily explained by the
fact that protoplasm contains a certain quantity of imbibition
water, which may fluctuate up to a certain extent, and may even
increase or decrease during the development of the cell-body.
The more saturated the protoplasm is with water, the more active
as arule are its movements. During the vegetative period the
plasmodium of the d/thalium tends to increase its supply of water,

118 THE CELL

and hence it moves towards the source of water; when the re-
productive period commences, it shuns moisture, because, at the
time when spores are being formed, it diminishes its water supply.

Many chemical substances attract, whilst others repel plasmodia.
If a net of Athalium, which has spread itself out upon a moist
substratum, is brought into contact with a ball of filter paper,
which is saturated with an infusion of tan, individual strands of
plasma immediately commence to creep towards the nutrient
medium. After a few hours all the spaces in the paper ball are
filled up with the slime fungus.

In order to study negative chemotropism, a crystal of common
salt or of saltpetre, or a drop of glycerine, may be brought to the
edge of the piece of damp filter paper upon which the slime fun-
gus has spread itself out. It can then be seen how, as the con-
centrated solution of salt or of glycerine gradually creeps along
the filter paper, the protoplasm shrinks away from the source of
stimulation in ever-widening circles.

Hence the naked plasmodia, which are so easily destroyed,
possess the marvellous property, on the one hand, of avoiding
harmful substances, and, on the other, of searching all through
the medium in which they are, for substances which are of value
to them for purposes of nutrition, and of absorbing them. “For
instance, if one of the numerous branches of a plasmodium, by
chance comes across a place which is rich in nutriment, an influx
of plasma immediately occurs to this favourable spot.”

Pfeffer has very accurately examined the chemotropism of small,
freely motile cells, such as spermatozoa, Bacteria, Flagellata, and
Ciliata, in some pioneering investigations that he has made, and
by this means has discovered a very simple and ingenious method
of investigation.

He takes some fine glass capillary tubes from 4 to 12 mm.
long; one end of each tube is closed, whilst at the other there is
an opening varying in inside diameter from ‘03 to 15 mm., ac-
cording to the size of the organism to be examined. He fills
these tubes for about a half or a third of their length with the
stimulating substance, there being a space filled with air at the.
closed end.

In order to explain their use, we may quote the following: ex-
periment. Pfeffer has discovered that malic acid has a strong
affinity for the antherozoids of Ferns, and that probably it is on
this account that it is secreted normally by the archegonia. A

THE VITAL PROPERTIES OF THE CELL 119

capillary tube is filled with ‘01 per cent. of malic acid, and after its
surface has been most scrupulously cleansed, is reversed and care-
fully placed in a drop of water containing a large number of Fern
antherozoids. With a magnifying power of 100 to 200 diameters,
it can be seen that some antherozoids immediately begin to make
their way towards the opening of the tube, from which the malic
acid commences to diffuse itself throughout the water. They
soon force their way right into the tube itself, until after five or
ten minutes several hundreds of them have collected there. After
a short time there are only a few left outside of the tube.

If experiments are made with solutions of malic acid of varying
strengths, a law similar to that of the effect produced by various
degrees of heat upon protoplasmic streaming movements may be
deduced. Beyond a certain minimum concentration (about ‘OOL per
cent.) which may be considered to coustitute the stimulative starting
point, every increase in concentration produces a corresponding in-
creased effect, until a certain fixed point 1s reached, when the optimum
or maximum result is produced; if the concentration is increased
above this point the attraction of the malic acid for the anthero-
zoids decreases, until finally the positive chemotropism is con-
verted into negative chemotropism.

Hence a very strong solution produces an exactly opposite effect
to that produced by a weak one, the antherozoids being repelled
instead of attracted. How small a quantity of malic acid is
necessary to prcduce a result may be seen from the fact that in
a capillary tube which contains a ‘001 per cent. solution only
0000000284 milligramme, or zso05o00 Of a milligramme, of
malic acid is present.

As has been already stated, if the chemical stimulus is to pro-
duce movements in a certain direction, it must only be strongly
applied at one point, or at any rate from one side. This is the
case in the above experiment, for as the malic acid becomes dif-
fused through the opening in the surrounding water, the anthero-
zoids, passing through the opening and making their way up the
tube, come into contact with solutions gradually increasing in
strength. The diffusion causes an unequal distribution of the
stimulus about the bodies of the antherozoids: “thus varying
with its varying degrees of concentration, the malic acid exerts a
stimulus which causes a movement in a fixed direction.”

The antherozoids, as might be expected, are distributed evenly
throughout a homogeneous solution, yet even under these condi-

120 THE CELL

tions a specific stimulative effect is exerted upon them. This,
however, can only be perceived indirectly, and can only be ex-
plained by the supposition that the attitude, so to speak, of the
antherozoids towards malic acid has experienced some modifica-
tion. Pfeffer is able in this case to demonstrate a relation simi-
lar to that expressed by the Weber-Fechner law for the mental
perceptions of man: “‘ Whilst the stimulus increases in geometrical
progression, the perception or reaction increases in arithmetical
progression.”

This ratio, which in many respects is very important, can be
observed in the behaviour of antherozoids towards malic acid.

To the fluid, containing the fern antherozoids, some malic acid
is added in such a quantity that when the two are well mixed to-
gether a solution of ‘0005 per cent. is produced. If now a capil-
lary tube containing a solution of ‘001 per cent. is inserted, attrac-
tive influence, as was the case when the antherozoids were in pure
water, can be perceived. The tube must now contain a ‘015 per
cent. solution in order to produce an effect, and if the water, in
which the antherozoids are, contains ‘05 per cent. of malic acid,
the solution in the tube must be 15 per cent. in strength.
Or more generally expressed, the solution in the tube must be thirty
tumes as strong as that from which the antherozoids are to be attracted.
The sensitiveness to stimuli, or the stimulation tone of the antherozoids,
is affected, if they are present in a liquid which contains a certain
proportional amount of the substance which is to act as the stimulus.
Thus it is possible in an artificial way to render them non-
sensitive towards weak solutions of malic acid, which under
ordinary circumstances constitute excellent stimuli, whilst on the
other hand they may be made susceptible to attraction from
strong concentrations of malic acid, which would repel antherozoids
accustomed to living in pure water.

Individual cell bodies behave very variously towards chemical
substances, just as they do towards light. Malic acid, which
exerts such a powerful attraction upon fern antherozoids, does
not affect those of Feather-moss at all. For these, however, a1 per
cent. solution of cane sugar acts as a stimulus, whilst on the other
hand neither of these substances has any effect on Liverwort or
Characece.

A 1 per cent. solution of meat extract or of Asparagin exerts
a strong attraction upon Bacterium termo, Spirillum undula, and
many other unicellular organisms. Even after a short period,

THE VITAL PROPERTIES OF THE CELL 121

varying from two to five minutes, a distinct plug of bacteria is
seen to have collected at the mouth of a capillary tube, which has
been placed in a drop of water containing these micro-organisms.

On account of the different ways in which various cell bodies
react towards different chemical stimuli, the method, which
Pteffer has perfected and used with various reagents, may be
employed, not only to attract one individual organism sensitive to
one special reagent, but also to separate different species which are
mixed together, as has also been done by means of galvanotropism
or heliotropism. Glass tubes provided with suitable attractive
material, and inserted in fluids, may be used as traps for Bacteria
or Infusoria.

Farther, it follows from the above-mentioned experiments, that
organisms which are specially sensitive towards a given chemical
substance may be used as reagents to indicate the presence of
this stimulating substance. Thus, according to Engelmann (IV.
7), certain Schizomycetes form an excellent test for oxygen, of
which such a minute portion as one trillionth of a milligramme
is sufficient to attract them.

Not every substance which attracts an organism is useful to it
as food, or is even innocuous to it; many, such as sodium salicylate,
saltpetre, strychnine, or morphia, even cause the immediate death
of the organisms which they have enticed. However, as a rule
the substances which are hurtful to protoplasm generally repel
it; this is the case with most acid and alkaline solutions. Even
2 per cent. solutions of citric acid and sodium carbonate exert a
distinctly repellent influence.

Hence, within the above-mentioned limitations, the general rule
may be stated that organisms are, through positive chemotropism,
enabled to seek suitable nutriment, whilst in consequence of
negative chemotropism they avoid hurtful substances.

These phenomena of chemotropism are of the greatest import-
ance in understanding many processes in the bodies of man and
of other vertebrates. Here also there are cells which react to
chemical stimuli by changes of shape, and movements in special
directions. These cells are the white blood cor Beciee and lymph
cells (leucocytes or wandering cells).

The chemical irritability of leucocytes has been established as
a fact by the experiments of Leber (IV. 17a, b) ; Massart and
Bordet (IV. 20, 21); Steinhaus (IV. 36); Gabritschevsky (IV.
10); and Buchner (IV. 2). If, in accordance with Pfeffer’s

122 THE CELL

method, fine capillary tubes, filled with small quantities of some
“irritating substance,” are introduced into the anterior chamber
of the eye or the lymph sac of a frog, they become filled in a short
time with leucocytes, whilst tubes filled with distilled water exert
no attractive power upon the leucocytes. When introduced into
the subcutaneous connective tissue the tubes cause the out-
wandering of the leucocytes from the neighbouring capillary
vessels (diapedesis), and under certain conditions produce sup-
puration.

Amongst substances which will set up inflammation, many
micro-organisms and their metabolic products are in the first
rank. Thus, Leber found during his experiments that. an extract
of Staphylococcus pyogenes proved very effectual as an inflamma-
tory agent. Hence the study of chemotropism is of the greatest
importance in the investigation of the diseases produced by the
presence of pathogenetic micro-organisms. Accurate knowledge of
the former will no doubt explain many apparently contradictory
phenomena, which are met with in the study of infectious
diseases.

It may be taken for granted at the outset, that if leucocytes
can be stimulated by means of chemical substances produced by
micro-organisms, such stimulation can only occur in accordance
with laws similar to those which have been established generally
with regard to cells. Positive and negative chemotropism—ex-
citation, and the variations which may occur in it owing to the
even distribution of the existing agent—the effects of stimulation
—all these must be taken into account.

Hence the behaviour of the leucocytes towards the stimulating
‘substance assumes the form of a complicated process, which may
vary very considerably according to the special conditions. For
the metabolic products excreted by micro-organisms may, accord-
ing to their nature and state of concentration, exert an attractive
or repellent influence. In addition, the effect produced may vary
according as to whether these products are restricted to the region
where they are produced, and from which they attack the leuco-
cytes, or whether they are in addition evenly distributed through-
out the blood. For in the latter case the presence of the bacterial
products in the blood will modify the way in which the leucocytes
react towards those which are collected in considerable quantities
‘near the diseased spot; and as was the case with the antherozoids
and malic acid (pp. 118-120), the result will depend upon the rela-

THE VITAL PROPERTIES OF THE CELL 123

tive proportions of the stimulating substance which is present in
each region.

The numerous possibilities may be grouped under two heads.

First group.—The metabolic products are evenly distributed or
approximately so throughout the blood and the diseased tissues.
Since under these conditions there can be no special point of
stimulation, it stands to reason that the leucocytes cannot wander
away from the diseased spot.

Second group.—The collections of products are unequal in con-
centration, and further, the difference in their concentration is
sufficient to give rise to an effective stimulation. Two alter-
natives may occur. Hither the higher degree of concentration is
present at the seat of the disease, or in the blood-vessels. In the
first case only will the leucocytes collect around the affected
tissue.

The consideration of these relative conditions appears to me to
explain many interesting phenomena, which have been observed
by certain French investigators, Roger, Charrin, Bouchard (IV.
lb), etc., during their various experiments with the catabolic
products of the Bacillus pyocyaneus, of the Anthrax bactilus, etc. ;
and by Koch in his observations upon the action of Twberculin.
I have endeavoured to explain such phenomena in a short popular
paper: ‘Ueber die physiologische Grundlage der Tuberculin
wirkung, eine Theorie der Wirkungsweise bacilliirer Stoffwechsel-
producte” (IV. 13), to which I refer the reader for information
with regard to physiological experiments and the explanation of
the special phenomena of disease.

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botan. Institut zu Tiibingen. Bad. I.

W. Prerrer. Ueber chemotactische Bewegunyen von Bakterien, Flagellaten
und Volvocineen. Untersuch. aus d. botan. Institut zu Tiibingen.
Basle:

Grorce PoucHer. D'un ewil véritable chez les Protozoaires. C. R. soc.
Biol. No. 36.

GrorcE Poucuet. Du réle des nerfs dans les changements de coloration des
poissons. Journ. de V'anat. et de la phys. 1872.

GeorcE Poucuer. Note sur l’influence de Vablation des yeux sur la colora-
tion de certaines espéces animales. Journ. de Vanat. et dela phys. T. X.
1874.

F. A. Poucuer. Surla mutabilité dela coloration des reinettes et sur la
structure de leur peau. Compt. rend. T. 26.

F. E. Bepparp. Animal Colouration. London.

Povu.Ton.

Rawirz. Zur Physiologie der Cephalopodenretina. Archiv f. Anat. u.
Physiologie. 1891.

A. Rurrer. On the Phagocytes of the Alimentary Canal. Quar. Journ.
Mic. Soc, 1891. :

A. RurFrer. Immunity against Microbes. Quar. Journ. Mic. Soc. 1891.

. Sacas. Lectures on the Physiology of Plants, trans. by Marshall Ward.

Oxford. 1887.

- Sacas. Handbuch des Experimentalphysiologie der Pflanzen. 1865.

Sacus. Text-book of Botany, trans. by Bennet and Dyer. 1875.

SEerpuirz. Bevtriige zur Descendenztheorie. Leipzig. 1876.

Srauu. Ueber den Einfluss von Richtung u. Stirke der Beleuchtung auf
einige Beweygungeserscheinungen im Pflanzenreich. Botan. Zeitung. 1880.

Sraut. Zur Biologie der Myromyceten. Botan. Zeitung. 1884.

Sremngaus. Die Aetiologie der acuten Eiterungen. Leipzig. 1889.

Srrasporcer. Wirkung des Lichts und der Wairme auf die Schwdrmsporen.
Jena. 1878.

S. Vines. Lectures on the Physiology of Plants. Cambridge. 1886.

VELTEN. LEinwirkung der Temperatur auf die Protoplasmabewegungen.
Flora, 1876.

Verworn. Die polare Erregung der Protisten durch den galvanischen
Strom. Pfliigers Archiv. Bd. XLV.u. XLVI.

Verworn. Psycho-physiologische Protisten-Studien. Jena. 1889.

CHAPTER V.
THE VITAL PROPERTIES OF THE CELL (continued).

Metabolism and Formative Activity.

Geverat Cuaracreristics. Each living cell exhibits the pheno-
mena of metabolism; it absorbs nutrient material, which it
elaborates, retaining certain portions of it within its body, whilst
it rejects others; it resembles a small chemical laboratory, for
the most varying chemical processes are almost continually taking
place in it, by means of which substances of complex molecular
structure are on the one hand being formed, and on the other are
being broken down again. The more intense is the vitality of the
cell, the more considerable are these processes of destruction and
reconstruction, the latter keeping pace with the former. In the
chemistry of the cell these two principal phenomena must be
clearly kept apart, namely the phenomena of progressive and
retrogressive metabolism, or, as Claude Bernard (IV. la) ex-
presses it, “les phénoménes de destruction et de création
organique, de décomposition et de composition.”

During its destruction the living substance, as a result of its
own decomposition, passes through a series of intermediate stages
of more simple chemical combinations, the precise nature of
which is at present unknown, Carbon dioxide and water are the
simplest final products of this decomposition. Tension (potential
energy) is converted into active vital force (kinetic energy).
Intra-molecular heat becomes free, and represents the living force,
which is the essential condition for the production of work in the
cell body. The fact that the slightest shock often suffices to call
forth great changes and to cause work to be done shows that vital
substances are exceedingly unstable in composition: as Pfliiger
(V. 25, 26) remarks: “Are not the forces which act in a ray of
light truly inconceivably small ? and yet they produce most marked
effects upon the retina and the brain. How infinitesimal are the

forces which serve to excite the nerves; how extremely minute
126

THE VITAL PROPERTIES OF THE CELL 127

the amount of certain poisons which suffices to kill a large living
animal.”

In the reconstruction of living substance, or in progressive
metabolism, new material is taken up from outside, to replace that
which has been used up; these substances become incorporated
and transformed into new chemical combinations. During the
execution of this work, more or less heat is rendered latent, and
is converted into potential energy; this latent heat is derived
partly from the intramolecular heat, which is released by the
process of decomposition, partly, and in the case of plants chiefly,
from the vivifying heat of the sun’s rays, by means of which
a large amount of kinetic energy is conveyed to the organic
world, and is converted in the protoplasmic body into potential
energy. The substances taken up from outside, and the heat rays
from the sun, supply in the last instance the material and energy
required for the carrying on of the vital processes of alternate
decomposition and reconstruction.

According to Pfliiger’s definition,—‘‘ The vital force is the intramolecular
heat. The highly unstable molecules of albumen, which are built up in the cell
substance, and which become decomposed through a splitting up of the
molecules—carbon dioxide, water, and nitrogenous bodies being chiefly formed
—becoming continually regenerated and rearranged.”

In spite of the great variety of metabolic processes which occur
in a single individual, there is a series of fundamental processes,
which are common to all organic bodies, and which take place
in the lowest unicellular organisms, as well as in the bodies of
plants and animals. Thus the unity of the entire organic kingdom
is exhibited in these fundamental processes of metabolism, just
as in the phenomena of movement and of reaction to stimuli.

Up to this point they may be included in the general anatomy
and physiology of the cell. This uniformity is especially note-
worthy in the following three points :—

1. Each cell, whether plant or animal, respires, that is to say,
it is essential to it, to take up oxygen from its environment, by
means of which it oxidises the carbo-hydrates and albuminous
substances of its own body, and produces as end products carbon
dioxide and water.

2. In both organic kingdoms to a large extent, corresponding
substances make their appearance during metabolism, such as
pepsin, diastase, myosin, xanthin, sarcin, sugar, inosit, dextrin,
glycogen, lactic acid, formic acid, acetic acid, and butyric acid.

128 THE CELL

3. In both kingdoms a great many. identical, or at any rate
very similar, processes occur, by means of which complex chemical
combinations are produced. These, however, differ essentially
from the synthetical methods employed by chemists for the pro-
duction of different organic compounds. In the chemistry of
the cell, whether plant or animal, ferments play an important
part (diastase, pepsin, trypsin, etc.). By the term ferment is
understood an organic substance, produced by the living cell,
of which an exceedingly minute quantity is sufficient to bring
about a considerable chemical effect, and which, without being
itself, to any appreciable extent, consumed, is able to produce
characteristic chemical changes both in carbo-hydrates and
albuminous bodies.

“Le chimisme du laboratoire est exécuté a l’aide d’agents et
d’appareils que le chimistre a créés, et le chimisme de l’étre
vivant est exécuté A l’aide d’agents et d’appareils que l’organisme
a eréés”” (Claude Bernard IV. la).

In the following pages we will consider the individual phenomena
of metabolism, chiefly from a morphological point of view, with-
out entering more fully into the chemical processes, which for the
most part are very complicated, and as yet to a great extent
obscure. During the course of metabolism three stages may be
recognised: the absorption of new material, the consequent trans-
formation effected in the interior of the protoplasm, and the
excretion of waste products. We will first consider together
the first and third of these stages, and later on the second by
itself.

I. Absorption and Excretion. All cells absorb gases, and
also substances in a fluid or dissolved, and hence diffusible, con-
dition ; finally many cells can make use of solid substances as food.
These three series of phenomena must be considered apart.

1. The Absorption and Excretion of Gaseous Material. Proto-
plasm can absorb the most various kinds of substances in a
gaseous condition (oxygen, nitrogen, hydrogen, carbon dioxide,
carbon monoxide, nitrous oxide, ammonia, chloroform, ether, and
a large number of similar substances).

Amongst these substances, oxygen and carbon dioxide are the
only ones of general importance in metabolism, and of these
oxygen is the more important.

Without the absorption of oxygen, that is to say without
respiration, life cannot continue. With very few exceptions

THE VITAL PROPERTIES OF THE CELL 129

(anaérobic Bacteria, etc.) the respiration of oxygen is a funda-
mental characteristic of the whole of organic nature, being abso-
lutely necessary for the continuance of the metabolic processes
upon which life depends, and through which by the oxidising of
complex molecular compounds the vital forces must be produced.
Asarule the lack of oxygen very quickly arrests the functions
of the cell (its irritability, powers of movement, etc.): and finally
death of necessity ensues.

Some of the fermentation organisms, the fission and pullulating fungi, appear
to form an exception to this fundamental process of respiration. For they are
able to grow and multiply in a suitable nutrient fluid when completely shut off
from oxygen. In this case, however, the oxygen necessary for the oxidation
processes in the protoplasm is obtained through the decomposition of the fer-
menting substance. Similarly intestinal parasites are able to exist in an
environment comparatively free from oxygen by splitting up of compounds of
which a superfluity is supplied to them (Bunge V. 2).

What is the part played by the oxygen after it has been taken
up by the cell?

It was formerly believed that the oxygen directly oxidised the
living material, so that, as it was figuratively expressed, a pro-
cess of combustion was called forth, as the result of which heat
was given off. However, there seems to be little doubt but that
the forces which result in the combination of the oxygen origi-
nate in the vital substance itself. In this mixture of special
albuminous bodies, and their derivatives, which goes under the
name of protoplasm, and in which, moreover, fats and carbo-
hydrates are stored up, important molecular re-arrangements | and
re-groupings of atoms, often the result of very minute exciting
causes, take place; amongst these, decomposition and dissocia-
tion occur. ‘Under these circumstances many decom position
products continually develop an affinity for free oxygen (oxidative
decomposition), and it is in this way that oxygen takes part in the
process of metabolism” (Pfliiger V. 25, 26). Hence in conse-
quence of respiration, and at the cost of the organic substance,
combinations rich in oxygen are produced ; and finally, through
the repeated dissociation and oxidation of these substances, carbon
dioxide and water, the most important final products of the de-
structive processes of living substance during respiration are
produced.

This is true for every animal and every plant cell.

If plant cells (staminal hairs of Tradescantia, cells of Characee),
K

130 | THE CELL

in which active streaming protoplasmic movements are taking
place, are immersed in a drop of pure olive oil, the movements, in
consequence of the exclusion of the oxygen, soon commence to
slacken, and finally quite cease. The same occurs when plant-
cells are introduced into an atmosphere consisting exclusively of
carbon dioxide or of hydrogen, or of a mixture of the two. At
first the functions of the protoplasm are only arrested, and if the
_ olive oil, carbon dioxide, or hydrogen, be soon removed, the irrita-
bility and movements return gradually after a period of rest. If
however the cells are deprived of oxygen for a considerable time,
their functions become paralysed, until finally death, accompanied
by the turbidity, coagulation, and decomposition of the protoplasm,
ensues.

In a similar manner each animal cell respires. If a hen’s egg,
which has been incubated, and which, being in an early stage of
development, consists simply of small cells, is placed in an atmos-
phere of carbon dioxide, or if its porous shell is so saturated
with oil that no interchange of gases can take place between the
embryo and the outer air, the egg dies in a few hours.

The oxygen which is absorbed by man through the lungs serves
to satisfy the need of oxygen evinced by all the cells contained in
the various tissues of our bodies. This last process is designated
in animal physiology internal or tissue respiration, in contradis-
tinction to the taking in of oxygen or lung respiration.

In the whole organic kingdom, respiration is united with the excre-
tion of carbon dioxide and with the production of heat. The follow-
ing is a simple chemical law: “A certain amount of heat is
evolved during respiration, just as it is produced in every other
case when carbon and hydrogen are oxidised into carbon dioxide
and water” (Sachs IV. 32a). Plant cells expire carbon dioxide
and evolve heat, just like animal cells.

The formation of heat is most easily demonstrated in portions
of plants which are growing rapidly; such as in germinating
seeds. It can be especially well detected in the flowers of Aroidex.
These become heated to as much as 15° C. above the temperature
of their surroundings.

The living cell itself is able, by means of its respiration, to
regulate the amount of oxygen which it consumes. This depends
simply upon the degree of its functional activity, to which the
decomposition of organic substance is proportionate. An unfer-
tilised egg-cell and-a resting plant seed breathe in very minute

THE VITAL PROPERTIES OF THE CELL 131

quantities of oxygen; however, after the egg-cell has been fer-
tilised, and division is proceeding rapidly, or when the plant seed
germinates, the amount of oxygen which is absorbed increases.
This absorption of oxygen is one of the functions of active living
protoplasm (Sachs). Thus the following is easily explained, that
the absorption of oxygen by the living cell “is, within certain
wide limits, quite independent of the gasecus tension of the
oxygen” (Pfliger).

One important phenomenon must be described before closing
this chapter on respiration. Even when oxygen is absent the cells
are able to excrete carbon dioxide and evolve heat for a longer or
shorter time. If germinating plants are introduced into a
Torricellian vacuum, they continue to exhale a normal quantity of
carbon dioxide for about an hour, after which the quantity gradu-
ally decreases.

According to Pfliiger’s experiments, Frogs can live for ey
hours in a bell-jar which is free from oxygen and filled with
nitrogen, during which time they exhale a considerable quantity
of carbon dioxide.

Both these experiments prove, that for a time, without direct
access to oxygen, but simply through the decomposition of organic
substances, carbon and oxygen atoms may unite together in the
cell to form carbon dioxide.

This process is termed intramolecular respiration. As long as
this persists, the cell lives, and remains irritable and capable of
performing its functions, although with continually decreasing
energy, by using up a portion of the oxygen contained in combina-
tion in its substance. However, when oxygen is withheld for a
considerable time, death invariably ensues.

Upon these phenomena of intramolecular respiration the pro-
position already mentioned rests: “that the first impulse to the
chemical processes of respiration is not given by the oxygen
which enters from without, but that first and primarily a decom-
position of albumen molecules resulting in the formation of carbon
dioxide takes place inside the protoplasm, and that hence the
incoming oxygen effects a restitutio in integrum.”

In fermentation processes, during which the ferments grow, multiply, and
evolve carbon dioxide, without having access to oxygen, we see an instance
which resembles intramolecular respiration ; to this Pfeffer (V. 22) has called
especial attention.

132 THE CELL

Whilst the absorption of oxygen and the giving up of carbon
dioxide indicate the beginning and end of a series of complicated
processes which belong chiefly to retrogressive or destructive
metabolism (catabolism), the absorption and elaboration of carbon
dioxide in the cell afford us an insight into the opposite pro-
cess, progressive metabolism (anabolism), or the reproduction of
organic substance. This process, in contradistinction to respira-
tion, is termed assimilation.

Respiration of oxygen and assimilation of carbcn dioxide are in
every respect opposite processes. The former is a fundamental
phenomenon common to nearly the whole organic kingdom, the
latter is confined to the vegetable kingdom alone, and even here
occurs only in such cells as contain chlorophyll or xanthophyll in their
protoplusm. The respiration of oxygen conduces to oxidation
decomposition provesses, whilst on the contrary the assimilation
of carbon dioxide causes the reduction of the latter, and the
synthetic formation of complex molecular organic substances.
These are carbo-hydrates, especially starches, which are found
deposited in the form of small granules in the green portions of
plants (chlorophyll corpuscles and chlorophyll bands).

The individual stages of the synthetic processes which take
place in the plant-cell during the assimilation of carbon dioxide
are as yet unknown. Only so much may be said: carbon dioxide
and water form the initial material for the synthesis ; further, as
a result of the reduction of the carbon dioxide and water, oxygen is
evolved, and is given off largely in the form of a gas. This trans-
formation can only take place in protoplasm when chlorophyll is
present ; but it is possible that other chemical substances are also
concerned in the process. Finally, carbon dioxide assimilation
can only occur under the influence of light. Heat is necessary
in order to liberate the oxygen from the molecules of carbon
dioxide and water. In this point also carbon dioxide assimilation
and oxygen respiration are opposed: in the latter heat is evolved
through oxidation, which is a process of combustion, and vital
force is set free; in the former heat is used up in reducing the
carbon dioxide, and as potential heat is rendered latent in the
assimilation products. The heat required for this process is af-
forded by the sun’s rays. |

If an aquatic plant is introduced into water containing carbon
dioxide, and is placed in the sunlight, innumerable small bubbles
of gas are soon seen to rise; if these are collected in a bell-jar,

THE VITAL PROPERTIES OF THE CELL 133

they can be shown by chemical analysis to consist chiefly of oxy-
gen. The amount of oxygen exhaled is in proportion to the carbon
dioxide which is simultaneously absorbed out of the water, and the
carbon of which is elaborated into carbo-hydrates. It has already
been mentioned in a previous chapter (p. 103), that the living
protoplasm, which is sensitive to light, endeavours to bring the
chlorophyll corpuscles into favourable positions for receiving the
direct powerful rays of light.

The process of assimilation proceeds in such an energetic man-
ner under the influence of sunlight that, in comparison to it, the
respiration of oxygen and the exhalation of carbon dioxide, which
are absolutely essential for the maintenance of the vital processes,
are placed quite in the background, so much so, indeed, that in
former times they were quite overlooked. But in plants which
are placed in the dark, the expiration of oxygen and, to an equal
degree, the absorption of carbon dioxide are immediately arrested,
whilst respiration continues in precisely the same manner as when
the plants were in the light. The gas now given off is seen to be
carbon dioxide, the quantity of which, however, is much less than
that of the oxygen in the preceding experiment.

Claude Bernard (IV. la) has drawn attention to a very interest-
ing difference existing between the respiration of oxygen and the
assimilation of carbon dioxide in plants. He narcotised water-
plants by means of chloroform or ether, and then found that they
no longer gave off oxygen in direct sunlight. Thus the function
of the chlorophyll, the capacity of forming starch by synthesis
from carbon dioxide and water, is absolutely suspended during
narcosis, just as the irritability and power of motion are arrested
in the protoplasm. This capacity returns when the plants are
transferred into pure water. But it is still more remarkable that
respiration, including the exhalation of carbon dioxide, is uninter-
rupted during narcosis. ‘This difference may be probably traced
back to the fact thatrespiration, and the decomposition in connection
with it, stand in a much closer relationship to the whole vital eco-
nomy, and hence can only be quite extinguished with the life of
the cell itself. But long before this occurs, the functions of the
cell are paralysed during narcosis, and with them the chlorophyll
function.

2. The Absorption and Excretion of Fluid Substances. Most of
the substances concerned in metabolism are taken up by the organ-
ism in a fluid condition. Unicellular and aquatic plants extract

134 THE CELL

them from the fluid by which they are surrounded, whilst terres-
trial plants take them up with their roots from the soil, which is
saturated with moisture. The cells of the higher animals nourish
themselves by absorbing substances held in solution in fluid media,
which must first, by means of complicated processes, be introduced
by them into their bodies. These fluid media are the chyme of the
intestinal canal, blood, chyle, and lymph. They play the same
part in the economy of the animal cell as the water and moisture
of the earth do in that of the lower organisms and of plants.

In opposition to the antiquated physiological view that the prin-
cipal metabolic processes take place in the fluids of the body, too
much stress cannot be laid upon the following proposition,—that
the cells are the site of the absorption, excretion, and transforma-
tion of material; the fluids only function in conveying the nutrient
material in a fluid condition to the cells, and in carrying away the
waste products. :

Between the cell and its surrounding medium, there exist the
most complicated physical and chemical conditions of interchange.
Their investigation is a most difficult undertaking, and can only
be entered into here to a very limited extent.

Each cell adapts itself most closely in its organisation to the
surrounding medium, any considerable variation in the concentra-
tion or composition of which causes its death. However, in many
cases, great alterations may be permanently endured, provided
that the consecutive stages are allowed to merge slowly and gradu-
ally into one another, so that the cell has time to adapt itself to its
new conditions.

As has’ been already mentioned in the chapter on chemical
stimuli (p.111), fresh-water Amebx are.able to accustom them-
selves to living in salt water, whilst marine animals can adapt
themselves to the presence of a greater or less percentage of salt
in the water surrounding them. Apparently they adapt them-
selves by adjusting the fluid they contain to the surrounding
medium. It is on this account that when the changes are made
suddenly, death immediately ensues, the protoplasm either swelling
up, or shrinking and coagulating.

Since in Vertebrates the cells which are bathed in the tissue-
fluids exist under such extremely complex conditions, it is difficult
to keep small portions of tissue alive, even for a short time, when
once they have been separated from the rest of the body; for even
the tissue-fluids become quickly altered as soon as they are sepa-

THE VITAL PROPERTIES OF THE CELL 135

rated from the living body. Hence, in examining a tissue outside
of the body, blood serum, aqueous humour, amniotic fluid, iodised
serum, or artificially prepared mixtures resembling these fluids, only
function, to a certain extent, as indifferent, supplementary fluids.
As a matter of course, they cannot at all supply the natural condi-
tions for the cell.

In endeavouring to understand the relationship which exists be-
tween the cell and the fluid which bathes it, care must be taken at
the outset to avoid the idea that the former is simply saturated by
the latter. Such a conception is wholly fallacious; on the con-
trary, each cell is an independent unity which selects certain sub-
stances from the mixture of fluids surrounding it, and absorbs a
varying quantity of them, whilst others it quite rejects. In all
these respects different cells behave very differently : in a word,
the cells, to a certain extent, make a selection from the substances
offered them.

Such selective powers, often very different in character, may be
easily demonstrated by the following :—

Amongst the lowest unicellular organisms there are some which
possess silicious skeletons, whilst others construct theirs out of
carbonate of lime. Hence they exhibit quite opposite powers of
selection towards these two substances, both of which occur in
small quantities in solution in water, and by this means very im-
portant effects have been produced in the formatiun of chalk, and
of the geological strata, consisting of silicious shells. Similarly,
different plants, which thrive side by side under similar conditions
and in the same water, take up from it very different salts, and
these in very varying quantities. The relative proportions which
occur may be easily computed by drying and burning the plants,
and then reckoning out the proportion which the ash bears to the
whole of the dried substance, and further the proportion the
separate constituents of the ash bear to the pure ash.

The ashes of several kinds of Fucus which were collected on the
west coast of Scotland were examined, and the results obtained
were tabulated by Pfeffer (V. 23) in his Plant Physiology.

136 THE CELL

Fucus Fucus Fucus Laminaria

vesiculosus. nodosus. serratus. digitata.

Pure ash . per cent. 13°89 14°51 13°89 18°64
20. ; $3 15 23 10°07 4°51 22°40
Na,O je 24°54 26°59 31:37 24:09
CaO ” 9°78 12-80 16°36 11°86
MgO < 7°16 10°93 11-66 7°44
Fe,03 ‘4 33 *29 *B4 62
P.Us ¥9 1:36 1-52 4-40 2°56
SO, AS 28°16 26°69 21-06 13°26
Si0, Ns 1:35 1:20 *43 1:56
Cl ” 15°24 12°24 11°39 17°23
I F 4 31 46 1:13 3°08-

'

Marine plants show most clearly, in what very unequal propor-
tions, they absorb from the multitude of salts offered them in sea-
water, the ones which are necessary to them. For instance, they
only store up very small quantities of common salt, of which about
3 per cent. is present in the water, whilst,-on the contrary, they
take up relatively large amounts of potassium, magnesium, and
calcium salts, of which there are only traces. And in a similar
manner, the analysis of the ashes of different land-plants which
have flourished side by side in the same earth yields very different
results.

Investigation of the metabolism occurring in the animal
body leads to the same conclusion. Only certain cells have
the tendency to take possession of the lime-salts, which. are
present in almost inappreciable amounts in the fluids of the
body, and to deposit them in the osseous tissues; other groups
of cells, such as those in the kidneys, take up the substances
from the blood, and excrete them in the form of urine; others
store up fat, etc., etc.

The factors concerned in this absorption and non-absorption of
matter are at present quite beyond our comprehension. It is
curious that the need which is evinced by the economy of a cell
for a certain substance does not always imply that this will be
taken up. Cells may absorb materials which are either directly
hurtful or completely useless to them. In this respect the very
different ways in which living plant'cells take up aniline dyes are.
very instructive (Pfeffer V. 22b).

Although solutions of methylene blue, methyl violet, cyanin,
Bismark brown, fuchsine and safranin, are absorbed, those of
nigrosin, aniline blue, methyl blue, eosin, and congo-red, are not.

THE VITAL PROPERTIES OF THE CELL 137

As to whether a given substance will be absorbed or not can, ac-
cording to Pfeffer, who has carefully studied the subject, only be
decided empirically.

The substances excreted by cells also vary. Just as with
absorption, excretion depends upon the special individual properties
of the living cell body. The red or blue-coloured petals of
phanerogamic flowers do not allow the concentrated solution of
colouring matter which they contain to become diffused into the
surrounding water as long as they are alive. However, as soonas
the cells die, the colouring matter commences to pass through
the cell-wall.

In order to really understand all these complicated phenomena,
it would be necessary to possess an exhaustive knowledge of the
chemistry and physics of the cell. For the property, which I have
designated above as the power of selection, must in the last instance
be traced back to the chemical affinities of the very numerous
substances which, being formed during the process of metabolism,
are present for a time in the cell. The same thing, doubtless,
occurs here as with the absorption of oxygen and carbon dioxide,
which can only take place when, through metabolic processes, sub-
stances with chemical affinities for them are set free. It is on
this account that no carbon dioxide is taken up by plants in the
dark, although it is immediately absorbed, if, under the influence
of direct sunlight, the chemical process for which it is necessary is
started.

The same thing occurs when living cells absorb aniline dyes.
Azolla, Spirogyra, the root-hairs of Lemna, etc., gradually draw
up into themselves so much colouring matter out of a very weak
solution of methylene blue, that they acquire a deep blue coloura-
tion, such as is seen in a 1 per cent. solution. The methylene blue
does not stain the protoplasm itself, but simply passes through it,
thus forming in the cell sap a solution of ever-increasing strength.
Hence the death of the cell, which would inevitably occur if the
poisonous methylene blue were to be collected in such quantities
in the protoplasm itself, does not ensue. This storing up in the
cell sap is caused by the presence in it of substances which, with
the aniline dye, form compounds, which osmose with difficulty.
Pfeffer considers that the tannin which is so frequently found in
plant cells is a substance of this nature. This tannin, with the
aniline colour, forms compounds which are sometimes insoluble,
and hence are precipitated in the cell sap (methylene blue, methyl

| 138 THE CELL

violet), and sometimes are more or less soluble (fuchsine, methyl
orange, tropeolin).

Further, animals afford us good examples of this storing up in
living cells. Fertilised eggs of Echinovdea acquire a more or less
intense blue colouration, if they are placed for a short time in
a very dilute solution of methylene blue (Hertwig IV. 12b). A
small accumulation of colouring matter does not arrest the process
of segmentation, which still continues, although somewhat slowly,
in a normal fashion, and in some cases may go on even until the
gastrula is formed. Here the colouring matter is chiefly deposited
in the endoderm cells, which points to the conclusion that it is by
the agency of the yolk material that the accumulation takes
place. Living Frog and Triton larve become of an intense blue
colour if they are left for from five to eight days ina weak solution
of methylene blue. In this case the colouring matter combines
with the granules in the cells (Oscar Schultze V. 44). After
remaining for days in pure water they commence to become
colourless again. If indigo-carmine is injected directly into the
blood of a mammal, it is soon taken up both by the liver-cells and
by the epithelium of the convoluted tubules of the kidney, and
then is excreted either into the biliary ducts, or into the kidney
tubules (Heidenhain V. 42). If methylene blue is injected into
the blood, it combines with the substance of the nerve fibres,
imparting to them a dark blue colouration (Ehrlich V. 41).
Alizarin is stored up in the ground substance of the bones.

Next to the chemical affinities, which exist between the par-
ticles of matter within the cell and those outside of it, the study
of the physical processes of osmosis is of the greatest importance
for the comprehension of the absorption and rejection of matter.
We must here observe whether the membrane, when present, is
more or less permeable. As a rule it is much more permeable to
dissolved substances than is the protoplasmic substance itself.
This latter is separated from the exterior by a peripheral layer
(cf. p. 15), which, according to Pfeffer, plays a most important
part in the process of osmosis. If some substance in solution is to
be taken up into the protoplasm, it must first be imbibed by the |
peripheral layer; that is to say, its molecules must become
deposited between the plasmic particles, and from there be trans-
ferred to the interior. Further, a substance in solution can, even if
it be not actually absorbed, produce an osmotic action by exerting
an attraction upon the water contained in the cell, and by thus

THE VITAL PROPERTIES OF THE CELL 139

inducing a flow of water towards the exterior. ‘ Essentially
osmosis consists in this, that two fluids simultaneously pass
through a membrane in opposite directions; with regard to an
endosmotic equivalent (a term expressing the proportionate inter-
change, upon which there is frequently too much stress laid), this
cannot be spoken of in such cases where only water is diosmosed
through a membrane” (Pfeffer V. 25).

On account of their fragility and small size, experiments upon
osmosis can only be made in animal cells with great difficulty.
Hence the osmotic processes have been investigated chiefly by
botanists in plant cells, which are much more suitable, and our

Fig. 59.—1. A young, at most half-grown, cell from the cortical parenchyma of the flower
peduncle of Cephalaria leucantha. 2. The same cell immersed in a 4 per cent. solution. 3.
The same cellina6per cent. solution. 4. The same cell in a 10 per cent. solution (Nos. 1
and 4 are taken from nature, Nos. 2 and 3 are diagrammatic ; all in optical longitudinal
section). h Peripherallayer; p protoplasmic coating of wall; k nucleus; ¢ chlorophyll
granules; s cell sap; e salt solution which has penetrated into the interior. After de
Vries (V. 36).

knowledge has been especially advanced by the following experi-
ments.

If plant cells containing a large sap space are placed in a 5 to
20 per cent. solution of a suitable salt, or of sugar or glucose
(Fig. 59), they are seen to diminish somewhat in size from having
given up water from the interior to the exterior; in consequence,
as this process of water abstraction proceeds, the protoplasmic
coating becomes separated from the cellulose membrane, which, on
account of its greater firmness, is unable to shrink any more

(de Vries V. 36).

140 THE CELL

Thus the salt or sugar solution must make its way through the
cellulose membrane, after which it continues to abstract more
water from the protoplasm, which shrinks more and more accord-
ing to the concentration of the solution, so as to occupy a smaller
and smaller space. The sap which it encloses becomes corre-
spondingly more concentrated. In spite of these changes, which
are grouped together under the same plasmolysis, the protoplasm
may remain alive for weeks, and exhibit its usual streaming
movements; it may even surround itself with a new peripheral
layer, although it remains in its contracted condition.

Two conclusions may be deduced from the process of plas-
molysis: (1) that the cellulose membrane is pervious to the salt
solutions which were used ; (2) “that the amount of dissolved salt
which diosmoses through the peripheral layer is not worth
mentioning, for if a considerable quantity penetrated into the
protoplasm, or into the cell sap, an increase in the quantity of the
substances setting up osmosis would be produced within the proto-
plasmic membrane, and thus an increase in the volume of the
pretoplasmic body would result ” (Pfeffer).

If the cells which have become flaccid through plasmolysis are
carefully removed and placed in pure water, the reverse process
occurs. The sugar solution which was enclosed within the cellu-
lose membrane becomes diffused into the water. In consequence,
the peripheral protoplasm layer becomes distended, because its
cell sap is now richer in osmotolytic substances than its environ-
ment, and so water is caused to flow in the opposite direction.
This distension gradually increases, as the water becomes ab-
sorbed, until the peripheral layer of protoplasm comes into close
contact with the cellulose membrane, and until finally the cell has
dilated to its original size. :

Other experiments have shown that the sap contained in the
plant cell is under a considerable pressure, often of several atmos-
pheres. This produces the natural turgescence of certain por-
tions of plants. The cause is,—that powerfully osmotolytic sub-
stances are present in the cell sap, such as saltpetre, vegetable
acids, and their potassium salts, which have a strong affinity for
water (Pfeffer V. 23; de Vries V. 36).

Therefore under these conditions the protoplasmic coating con-
taining the cell sap may be compared to a very elastic thin-walled
bladder, which is filled with a concentrated salt solution. If such
a bladder is put into pure water, the solution attracts the water,

THE VITAL PROPERTIES OF THE CELL 141

and so produces a current, the result being that the bladder swells
up in consequence of the increased pressure of its contents, and its
wall grows thinner and thinner. The distension of the bladder
only ceases when the external and internal liquids are in osmotic
equilibrium. Thus the protoplasmic coating of many plant-cells
would be very much distended in consequence of the internal
pressure (turgor) were it not that a limit is set to its distension
by the less elastic cellulose membrane.

Equilibrium between the cell-sap and the surrounding fluid
might be established, if the osmotic substances were to become
diffused into the water, so as to remove the cause of the internal
pressure. However, this is prevented by the properties of the
living plasmic membrane. As the plasmic membrane, if the ex-
pression may be allowed, decides whether a body may be admitted
into the interior of the cell or no, similarly it has the important
power of retaining in the cell-sap dissolved substances which
otherwise would be washed out by the water bathing the cell; of
this property mention has already been made, and an instance
cited (Pfeffer V. 23).

That, in fact, the cell-sap exists under a pressure greater than
that of its environment, for instance, that the pressure in aquatic
plants is greater than that of the surrounding water, may be
easily proved by some simple experiments, as has been shown by
Nageli (V. 16). If a cell of Spirogyra be opened by an incision,
so that part of its contents flows out, the transverse walls of the
two neighbouring cells bulge out towards the cavity of the injured
one. Hence the pressure in the uninjured cells must be greater
than that in the injured one, the tension of which has sunk down
to the level of that of the surrounding water.

3. Absorption of Solid Bodies. Cells, which either are not
surrounded by a special membrane, or possess apertures in
their membranes, are able to take solid bodies up into their
protoplasm, and to digest them. Thus Rhizopoda capture other
small unicellular organisms with which their widely outstretched
pseudopodia come into contact (Figs. 10, 60). The pseudopodia
which have seized the foreign body contract, and so gradually
draw it into the mass of the protoplasm; here the nutrient sub-
stances are extracted, whilst the indigestible remains, such as
skeletal structures, are after a time ejected to the exterior. Even
solid substances, which possess but small nutritive value, are taken
up. If carmine or cinnabar granules are introduced into the water,

142 THE CELL

the Rhizopoda eagerly seize upon them, so that after a short time
their whole bodies are quite filled with them.

Infusoria (Fig. 50) eat Flagellata, unicellular Algee and Bacteria,
conveying them into their endoplasm through an opening in their
cuticle which functions as a mouth. Here a vacuole filled with
fluid forms itself round each foreign body, which undergoes
digestion.

as a
Soe cael

Fie. 60.—Actinospherium Eichhorni (after R. Hertwig, Zool., Fig. 117): M medullary
substance with nuclei (n); RB cortical substance with contractile vacuoles (cv); Na
nutrient material.

In a similar manner to that shown by unicellular organisms,
many tissue cells of Metazoa devour solid substances offered to
them, and digest them.

Intracellular digestion, as it has been termed by Metchnikoff
(V.12), occurs very frequently in Invertebrates; it may be best
demonstrated by means of feeding experiments with easily recog-
nisable substances, such as granules of colouring matter, globules
of milk, spores of fungi, etc. In some Coelenterata the ectoderm
as well as the endoderm takes up foreign bodies. The tentacular
ends of Actinia may load themselves with carmine granules, which

THE VITAL PROPERTIES OF THE CELL 143

may also be found distributed throughout the whole endoderm of
Actinia larve after suitable feeding.

But white blood corpuscles, lymph cells and the migratory cells
of the mesoblast, in both Vertebrates and Invertebrates, afford us
the best material for observation, in consequence of their power
of absorbing and digesting solid bodies. This important fact was
first observed by Haeckel (V. 4a), who injected a molluse (Tethys)
with indigo, and found after a short time that indigo granules
were present inside the blood corpuscles.

Metchnikoff (V. 12) has further investigated the phenomenon
most thoroughly. He found that if powdered carmine were injected
under the skin of another species of mollusc (the transparent
Phyllirhoé), the smaller granules were eaten up by some of the
migratory cells, while the larger ones attracted a number of other
migratory cells around them, which surrounded them like an
envelope, and fused themselves together to form a plasmodium
or multinucleated giant cell.

That the same thing occurs in Vertebrates may be easily proved
by injecting some carmine into the dorsal lymph sac of a Frog,
and, after a short time has elapsed, removing some drops of lymph,
and examining them with the microscope. Further, the eating
process can be directly followed under the microscope if powdered
carmine or a little milk be added to some fresh drops of lymph or
of blood which have been carefully drawn off, certain precautions
having been observed. If the blood has been taken from man or
some other mammal, the preparation must be carefully heated on
Max Schultze’s warm stage until it has attained a temperature
of 30-35° Celsius (V. 43). The white blood corpuscles now
commence to show amceboid movements; they seize with their
pseudopodia the carmine granules, or milk globules with which
they come in contact, and draw them into their bodies. On this
account Metchnikoff designates them as phagocytes, and the whole
process as phagocytosis.

This capacity of the amceboid elements of the animal to take
up solid substances is of great physiological importance ; for herein
the organism possesses a means of ridding itself of foreign and
noxious organic particles which are present in its tissues. There
are three different conditions of the body, partly normal and
partly pathological, when the phagocytes exercise this function.

Firstly, during the process of development in many Inverte-
brates and also in Vertebrates, certain larval organs lose their

144 THE CELL

importance, and undergo fatty degeneration. Thus, during the
metamorphosis of Hchinoderm larve and of Nemertines, certain
portions disappear; and, similarly, the young Frog during its
development loses its conspicuous tail, which acted as a rudder.
In all these cases the cells of these degenerating organs undergo
a fatty metamorphosis, die and disintegrate. In the meantime a
large number of migratory cells or phagocytes have collected in
their neighbourhood, and these commence to devour and digest
the degenerated tissue, as can be plainly seen during life in trans-
parent marine animals, ’

Secondly, just as during the normal processes of development,
the phagocytes occupy themselves in reabsorbing particles, the
death or disintegration of which has been brought about either by
normal or pathological conditions. Red blood corpuscles become
destroyed after they have circulated in the blood for a certain
time. In splenic blood their remains have been seen in the bodies
of white corpuscles, which here again fulfill their function of
getting rid of dead material. When in consequence of a wound
an effusion of blood occurs in the tissue, and thousands of blood
corpuscles and elementary particles are destroyed, the migratory
cells again set to work, and produce reabsorption and healing.

Thirdly, and lastly, the phagocytes during infectious diseases
constitute a body-guard to the organism, in opposing the spread
of the micro-organisms in the blood and tissues.

Metchnikoff has rendered great service in drawing attention to
this circumstance (V. 13-15, 1V. 22). He succeeded in showing
that the Cocci of erysipelas, the Spirilla of relapsing fever, and the
Bacilli of anthrax were eaten up by the wandering cells, and thus
rendered harmless (Fig. 61). The micro-organisms, of which as
many as from ten to twenty may be present in one cell, after a
certain time show distinct signs of degeneration. If the micro-
organisms are present in the blood, they are destroyed, especially
in the spleen, liver, and red bone marrow. If they succeed in
settling down in some place in the tissue, the body endeavours to
get rid of the intruders by collecting as the result of inflammatory
processes a large number of migratory cells to the spot.

As Metchnikoff expresses it, between micro-organisms and
phagocytes an active war is raging. This is settled in favour of
one or other party, resulting, as the case may be, in the recovery
or death of the affected animal.

The power possessed by migratory cells of destroying certain

THE VITAL PROPERTIES OF THE CELL 145

species of micro-organisms appears to vary
considerably in different animals, and to
depend largely upon the most varying
conditions ; for instance, chemical stimuli
play an especially important part, as has
been already mentioned on p. 121 (negative
and positive chemotropism; Hertwig IV.
13). Apparently it is upon this that the
greater or less immunity of organisms
from many infectious diseases depends.
This discovery opens a wide vista in the
field of the comprehension and treatment
of infectious diseases.

Il. The Assimilative and Forma-
tive Activity of the Cell. The gases,
the fluids, and the solid substances, which
are introduced into the protoplasm as food,

and through respiration, compose the very
Fig. 61.—A leucocyte of

varying raw materials which are elaborated
in the chemical workshop of the cell, and
which are converted into an exceedingly
large number of substances.
these the most important for both plants

and animals are: carbo-hydrates, fats, pro-

Amongst

a Frog, enclosing a Bac-
terium, which is undergo-
ing digestion. The Bac-
terium is stained with
vesuvine. The two figures
represent two stages of one
and the same cell. (After
Metchnikoff, Fig. 54.)

teids, and their numerous compounds.

Similarly the ways in which they are utilised in the vital pro-
cesses of the cell vary very considerably. They serve partly to
replace the substances, which, during the vital process, become
decomposed in the cell, such as the substance which is oxidised
during respiration, and which thus furnishes the vital energy
necessary for the activity of the cell. They are also utilised for
that growth and increase of the protoplasm which is absolutely
indispensable for the function of reproduction. Further, some
of the substances formed in the chemical laboratory are stored up
for future use in the cell-body in some form or other, thus consti-
tuting reserve material. Finally they may be set aside to fulfil
some function inside or outside the cell.

Thus arise the different materials which, especially in the
animal kingdom, are very numerous, and upon which the dif-
ferentiation of tissues depends: glandular secretions, which are
passed to the exterior, membranes, and intercellular substances of

L

146 THE CELL

very varying chemical composition, and muscle and nerve fibres,
which, in consequence of their peculiar organisation, are endowed
in a special manner with contractility and the power of conduct-
ing stimuli. In the last case the chemical activity of the cell
assumes a character which Max Schultze has designated as its
formative activity. The protoplasm makes use of the raw ma-
terial which is brought to it, and prepares from it often very
wonderfully constructed substances, which answer special pur-
poses. In this activity the cell appears, to a certain extent, like a
builder, or, as Haeckel (V. 4b) has it, like a modeller or sculptor.

This formative activity of the cell, or, as it is better expressed,
the power of the protoplasmic body to create different structures,
is of extreme importance; for it is solely due to this power that
there is so great a diversity of elementary particles, in consequence
of which the animal body is able to attain to so high a degree of
perfection. The division of labour, which is so successful amongst
cells, is based solely upon this foundation, and by its means the
capacity for work of the cell community is rendered much greater.

Hence this subject of the assimilation of material must be
examined from two points of view; the first is a chemical one, in
so far as it treats of the formation of innumerable substances by
means of the protoplasm, whilst the second is more morphological,
in so far as the various substances present in the protoplasm may
be seen to differ from it, to occupy a definite position, to have a
fixed form and structure, and to obey special laws of development.

One of the most important tasks for the biological chemist of
the future is to render accessible to morphological investigation
the various substances distributed throughout the cell body by
means of differential staining mixtures.

1. Chemistry of Assimilation. The chemical processes of the
cell, which are at present shrouded in mystery, can only be
treated here in so far as they are connected with fundamental
problems, such as the synthesis of carbo-hydrates, fats, and pro-
teids out of more simple elementary substances.

The chemical processes in the animal kingdom appear to differ
considerably from those occurring in the vegetable kingdom.
Only that protoplasm present in plant cells, which contains chloro-
phyll, is able to make high molecular ternary compounds out’ of
carbon dioxide and water; the protoplasm which does not contain
chlorophyll, and which is present in animals and certain colourless
portions of plants, is only able to undertake further synthesis

THE VITAL PROPERTIES OF THE CELL 147

with this original material, and thus to produce quaternary com-
pounds.

It is as yet impossible to say what chemical processes occur in
the green protoplasm, when, under the influence of the sun’s vital
energy, carbon dioxide and water are taken up, and oxygen is
given off. The first product of assimilation, which can be
definitely made out, is starch, or perhaps, as a preliminary stage,
sugar. Itis almost inconceivable that either of these could be
formed by a direct synthesis of carbon and water; apparently a
number of intermediate substances are formed during the course
of a complicated process. ‘“‘ Indeed, it is not impossible,”’ as Sachs
(IV. 32a) remarks, “ that certain closely-connected constituents of
the green plasma themselves participate in the process; that, for
example, the molecules of the green protoplasm become split up,
and that certain atoms are given up and others substituted for
them. The theory has a certain degree of probability from the
observation that in many, though not all cases, the mass of
chlorophyll substance gradually decreases, and finally quite dis-
appears, whilst the starch granules which it contains become
larger and larger.”

The carbo-hydrates (starch) which, by means of the chloro-
phyll function, have accumulated in the body of the plant, form
the material which is converted in the protoplasm into the
vegetable oils. The ternary non-nitrogenous, organic compounds
supply further the basis for the synthesis of quaternary albamin-
ous substances, and thus assist in the completion and increase of
the protoplasm. However, for these processes, nitrates and sul-
phates are necessary, and these are obtained by the plants from
the earth by means of their roots.

That proteid substances can be formed by the living cell out of
such material has been experimentally proved by Pasteur. He
cultivated low Schizomycetes, such as Mycoderma acett, Yeast, etc.,
in artificially prepared nutrient solutions. Thus he showed that
Mycoderma aceti can multiply actively in the dark, if only a few
cells are placed in a nutrient solution, composed of a salt of
ammonia, phosphoric acid, potash, magnesia, water, and alcohol or
acetic acid of suitable strength. Hence the fungi cells, if they
have multiplied to a considerable extent, must have formed proteid
materials by means of the decomposition of these substances, in
addition to cellulose and fats.

Thus plants, which by means of their chlorophyll produce carbo-

148 THE CELL

hydrates, and convert these again into fats and albuminous sub-
stances, supply to the animal organism the ternary and quater-
nary substarices which are necessary for its nutriment, and which
it is unable to elaborate, as the plants do, from such simple sub-
stances. In this manner the vegetable and animal kingdoms con-
stitute a life cycle, in which they assume opposite positions and
complement cach other. This antithesis may be formulated as
follows :—

In the green plant cell the organic substance is formed syn
thetically from carbon dioxide and water, whilst the vital force
which is obtained from the sunlight becomes potential; on the
other hand, the animal cell uses as nutriment the ternary and
quaternary compounds formed in the vegetable kingdom, for the
most part oxidising them. By this means it reconverts the
potential energy stored up in the complex compounds into vital
energy whilst performing work and evolving heat. The plant,
whilst its chlorophyll is exercising its function, absorbs carbon di-
oxide, and gives off oxygen; the animal breathes in oxygen, and
breathes out carbon dioxide. In the chemical processes of the
plant reduction and synthesis predominate, whilst in those of the
animal oxidation, combustion and analysis are most important.

However, from this one example of antithesis occurring in the
economy of nature between the animal and vegetable kingdoms,
it must not be concluded that plant and animal cells are quite
opposed in all their ordinary vital phenomena; for this is not
true. Close investigation shows that there is universal unity in
the fundamental processes of the whole organic world. The
above-mentioned difference is only due to the fact that the plant
cell has developed a special faculty which is lacking in animal
cells, namely, the power of decomposing carbon dioxide by means
of its chlorophyll. With the exception of this one function, exer-
cised by chlorophyll, many of the metabolic processes which
are essential for the maintenance of life are performed in the
protoplasm in a perfectly similar manner in both plant and
animal cells.

In both the protoplasm must breathe, take up oxygen, evolve
heat, and give up carbon dioxide if the vital processes are to be
carried on. In both plants and animals the decomposition and
reconstruction of protoplasm follow one another, and complicated
processes of correlated chemical analysis and synthesis occur.

This similarity can be more easily understood when it is re-

THE VITAL PROPERTIES OF THE CELL 149

membered that a large proportion of plant cells, namely all those
which do not contain chlorophyll, are in a position similar to that
occupied by animal cells; these also, since they cannot assimi-
late directly, must obtain from the green cells, the material neces-
sary for the maintenance of their life, for their growth, and for
their reproduction. Thus the same antithesis, which is present
in the economy of nature between plants and animals, also exists
in the plant itself between its colourless and its chlorophyll-con-
taining cells.

Claude Bernard has shortly and in a striking way expressed the
relationship in the following words :

“Tf, in the language of a mechanician, the vital phenomena,
namely the construction and destruction of organic substance,
may be compared to the rise and fall of a weight, then we may
say that the rise and fall are accomplished in all cells both plant
and animal, but with this difference, that the animal element
finds its weight already raised up to a certain level (niveau), and
that hence it has to be raised less than it subsequently falls. The
reverse occurs in the green plant cells. In a word, ‘Des deux
versants, celui de la descente est prépondérant chez l’animal;
celui de la montée, chez le végétal’” (Claude Bernard, IV. la,
vol. ii. p. 514). .

Now, having placed the subject of the chlorophyll function in
its true position, we will proceed to examine the important
uniformity which exists in the chemistry of metabolism between
plant and animal cells.

We must first lay stress upon the fact that. a large number of
the materials made use of in progressive and retrogressive meta-
morphosis are common to both plants and animals.

Further, the means by which certain important processes in
plant and animal cells are carried out appear to be similar.
Carbo-hydrates, fats and albuminous substances are not adapted
in every condition for direct use in the laboratory of the cell and
for conversion into other chemical compounds. It is necessary
to prepare them by transforming them into a soluble and easily
diffusible form. This occurs, for instance, when starch and glyco-
gen are converted into grape sugar, dextrose and levulose; when
fat is split up into glycerine and fatty acids, or when proteids are
peptonised.

Sachs (IV. 32a) describes the above-mentioned modifications of
carbo-hydrates, fats and proteids as their active condition, in dis-

150 THE CELL

tinction to their passive condition, when they either remain
accumulated in the cell as fixed reserve materials—starch, oil, fat,
albumen crystals—or are taken up as nourishment by animals.
It is only when they are in the active condition that the plastic
materials in both plant and animal bodies can accomplish their
migrations, by means of which they reach the places where they
are either to be temporarily stored up or immediately used.

For instance, the starch, which is accumulated in seeds or in
portions of plants which are underground, such as tubers, was
not assimilated at these spots. It originated in the assimilating
green cells, from which it was transported, often through long
distances, by means of intermediate cells to the tubers or seeds.
Now, since starch grains cannot pass through the cell-membrane,
this migration can only occur when the substances are in a soluble
form (sugar) ; when they reach the place where they are to be
stored up, they are re-converted into the insoluble form (starch).
If now the germ develops, either in the tuber or in the seed, the
passive reserve materials assume the active form and make their
way to the place where they are needed, namely, to the cells of
the developing germ. Similarly the carbo-hydrates, fats and pro-
teids which enter the body in the form of food, must be rendered
soluble, so that they may be able to reach the place where they
will be used, and the fats which are stored up in fatty tissues
must be altered before they can be used in any part of the
body.

In plant and animal cells this important transformation of
carbo-hydrates, fats and proteids from a passive into an active
condition is efficiently accomplished by means of very peculiar
chemical substances called ferments.. These are ‘allied to the
albumens, and indeed are derived from them; they are present in
very minute quantities in the cell, but nevertheless produce
powerful chemical effects, and induce chemical processes without
being essentially altered themselves. This process of fermenta-
tion is very characteristic of the chemistry of the cell. There
are special ferments for carbo-hydrates, others for proteids, and
others for fats,

Whenever starch is rendered soluble in plants, the process is
effected by means of a ferment, diastase, which can easily be ob-
’ tained from germinating seeds. Its efficacy is so great, that one
part by weight of diastase is sufficient to convert in a short time
2,000 parts of starch into sugar. Another ferment, invertin,

THE VITAL PROPERTIES OF THE CELL Tok

which acts upon carbo-hydrates, is present in some fission fungi
and moulds; it splits cane sugar up into dextrose and levulose.

The salivary ferment in the animal, ptyalin, which converts
starch into dextrin and maltose, corresponds to the diastase in the
plant. Similarly the non-diffusible glycogen, which in conse-
quence of its properties has been called animal starch, must, if it
is to be utilised further, be converted by means of a sugar-form-
ing ferment, wherever it occurs, into sugar (liver, muscles).

Albuminous bodies are peptonised before they can be absorbed.
In the animal body this takes place chiefly by means of a ferment,
pepsine, which is secreted by the cells of the gastric glands. A
small quantity of pepsine is able either in the stomach or in a
test-tube to dissolve a considerable amount of coagulated albu-
men in the presence of free hydrochloric acid, thus converting it
into such a form that it is able to diffuse through membranes.

Peptonising ferments have been also demonstrated in plant
cells. For example, one has been extracted in the form of a
digestive juice from those organs of carnivorous plants which are
adapted for the capture of insects, such as the glandular hairs of
the leaves of the Drosera; in this manner the small dead animals
are partially dissolved and absorbed by the plant cells. A fer-
ment resembling pepsine has also been demonstrated in germi-
nating plants, where it serves to peptonise the proteid bodies
which are stored up as reserve material in the seed. The pepto-
nising ferment from the milky juice of the Carica papaya and of
other species of Carica is well known on account of its energetic
action. Finally, a similar ferment has been discovered in the
body of the Myxomycetes by Krukenberg.

In the animal body fats are split up into glycerine and fatty
acids. This result is effected mainly by the pancreatic juice.
Claude Bernard endeavoured to trace this back to a fat decom-
posing ferment secreted by the pancreas. Further, it is supposed
that during the germination of fat-containing plant seeds the oils
are split up into glycerine and fatty acids by means of ferments
(Schiitzenberger).

Thus even from these few data it may be seen that, although
at present so little is known about the subject, there appears to
exist a far-reaching uniformity throughout the whole organic
kingdom as regards the elaboration of material in the cell.

One of the points which is least understood concerning the
metabolism of the cell is the part played by the protoplasm.

Wee THE CELL

This is especially true of all the processes which are described
above as belonging to the formative activity of the cell. What
relationship does the protoplasm bear to its organised products,
such as the cell membrane, the intercellular substance, etc.?

Two quite opposite views have been suggested upon this sub-
ject. According to the one, the organised substances are formed
by the transformation of the protoplasm itself, that is to say,
through the chemical rearrangement or splitting up of the proto-
plasmic molecules ; according to the other, on the contrary, they
are supposed to be formed of plastic materials, carbo-hydrates,
fats, peptonised proteids, etc., which are taken up during meta-
bolism by the protoplasm, conveyed to the place where they are
required, and there brought into a suitable condition for secre-
tion.

This difference may be best explained by an example, such as
the formation of the cellulose membrane of the plant cell.

According to a hypothesis which has been strongly supported by
Strasburger (V. 31-33) amongst others, the microsome containing
protoplasm becomes directly transformed into cellulose lamelle ;
that is to say, cellulose, as a firm organised substance, is formed
directly out of the protoplasm.

Another theory is, that some non-nitrogenous plastic substance,
such as glucose, dextrin, or some other soluble carbo-hydrate, forms
the materials from which the cell membrane is constructed.
These materials are conveyed by the protoplasm to the place
where they are required, and are here converted into an insoluble
modification, cellulose. Since this cellulose acquires a fixed struc-
ture from the beginning, the protoplasm must, in a manner at
present unknown to us, assist in its construction; this process is
described by the expression ‘“ formative activity.”

According to the first hypothesis, the cellulose membrane may
be described shortly as a metabolic product of the protoplasm,
and, according to the second, as a separation product of it.

The question of the formation of chitinous skin, of the ground
substance of cartilage and bone, of calcareous and gelatinous sub-
stances, may also be regarded from the same two points of view ;
in fact, all conceptions of the metabolism of the cell present the
same difficulty.

Claude Bernard (IV. la) described this relationship in the
following words: “From a physiological standpoint it may be
conceived that in the organism only one synthesis occurs, that of

THE VITAL PROPERTIES OF THE CELL | es!

protoplasm, which grows and develops itself at the expense of
the substances which it absorbs. Then, from the splitting up
of this most complex of all organised bodies, all the complicated
ternary and quaternary compounds must arise, the formation of
these being ordinarily ascribed to a direct synthesis. Hence
Sachs was obliged to allow that it was possible, although he con-
sidered it improbable, that in the assimilation of starch decompo-
sition and restitution occur in the molecules of the green proto-
plasm.”

These remarks show how difficult the whole subject is in so
far as it concerns the chemical processes in question.

If it is allowable to draw conclusions from analogous cases, I
must certainly decide in favour of the second hypothesis, accord-
ing to which the protoplasm participates more indirectly than in
the first in the formation of the greater number of intercellular
substances. For in the cases where organisms construct a sili-
cious or calcareous membrane the nature of the substance itself
distinctly shows that it could not proceed directly as a firm
organised substance out of protoplasm. This. latter in such a
case, in consequence of its chemical composition, can only play
the part of an intermediary, by selecting the substances from its
environment, absorbing them, accumulating them at the places
where they are required, and depositing them in a distinct form
as firm compounds, which are invariably joined to an organic
substratum.

Such a conception appears to me to be nearer the truth in the
case of the formation of the cellulose membrane also, if the facility
with which various carbo-hydrates become transformed into one
another is taken into account, as well as the complicated process,
which would be necessary if protoplasm were to be converted into
cellulose. And even those intercellular substances which are
chemically more nearly related to protoplasm, such as chondrin,
gluten, etc., may be governed by the same laws of construction.
For, apart from the organised proteid substances, protoplasm and
nuclear substance, there are always present in each cell a large
number of unorganised proteids; these serve as formative
material, and occur in a condition of solution in the cell sap of
plant cells, in the nuclear sap, and in the blood and lymph of
animals. Instead of the protoplasm itself being directly seized
upon and used uv in the formation of nitrogenous intercellular
substances, it is possible that the unorganised proteid materials

154 THE CELL

may be utilised by the formative activity of the cell, in the same
way as has been suggested above, that other substances are used
for the formation of the cellulose membrane.

In what way the protoplasm executes its above-mentioned
function of adoption is quite beyond our comprehension at this
present time, when the majority of the bio-chemical processes
escape our observation. This function of the protoplasm, however,
may consist in this, that certain particles of its substance may
unite, through molecular addition, with particles of other sub-
stances present in the nutrient solutions, and thus become trans-
formed into an organic product. Thus soluble silicious compounds
may unite with molecules of organic substance to form a silicious
skeleton ; thus particles of cellulose may be formed through the
influence of particles of protoplasmic substance from soluble
carbo-hydrates, forming with them a compound (prebably per-
manent, but possibly only temporary), and becoming organised to
form a cell-membrane. This conception is quite in accordance
with the fact that in many objects freshly-formed layers of
cellulose are found to pass imperceptibly into the neighbouring
protoplasm.

2. The Morphology of Metabolism. The formative activity of
the Cell. The substances which are formed during the meta-
bolism of the cell may be included under the head of morpho-
logy, in so far as they can be optically distinguished from the
protoplasm. They may be differentiated out in a formed or
unformed condition, either in the interior of the protoplasm, or
upon its surface; according to their position they are distin-
guished as internal or external plasmic products. However, as is
80 often the case in biological classifications, a sharp line of dis-
tinction cannot be drawn between the two groups.

a. Internal Plasmic Products. Substances dissolved in water
may separate out as larger or smaller drops in the protoplasm,
and thus cause cavities or vacuoles. These play a most important
part, especially in the morphology of plants. As has already been
described in detail on p. 31, a plant cell (Fig. 62) is able by
secreting sap to increase its size in a short time more than a
hundred-fold. It is by means of the simultaneous action of a
large number of such cells that in spring-time certain organs of
plants are able to grow to such a considerable size. The solid
substance contained by a plant very rich in water may be as little
as 5 per cent., or even only 2 per cent.

THE VITAL PROPERTIES OF THE CELL

The cell sap,
however, is not
pure water, but
a very complex,
nutrient solution
containing veget-
able acids and
their. salts, - nit-
rates and phos-
phates, sugar, and
small quantities
of dissolved pro-
teids, etc. Thus
between the pro-
toplasm and the
sap material is
interchanged to
a considerable ex-
tent, substances
for use being ex-
tracted from the
one, which in
receives
other substances

return

in exchange.
Since the sap re-
presents a con-
centrated — solu-
tion of osmotic
substances, it ex-
erts a powerful
attraction
water,

upon
and also
an internal pres-
sure, which is of-
ten considerable,
upon the envelope

Fie. 62.—Parenchyma cells from the cortical jlayer of the
root of Fritillaria imperialis (longitudinal sections, x 650:after
Sachs II. 33, Fig. 75): A very young cells, as yet without
cell-sap, from close to the apex of the root; B cells of the same
description, about 2 mm. above the apex of the root; the cell-
sap (0) forms in the protoplasm (p) separate drops between
which are partition walls of protoplasm ; C cells of the same
description, about 7-8 mm. above the apex ; the two lower
cells on the right hand side are seen in a front view; the
large cell on the left hand side is seen in optical section ; the
upper right hand cell is opened by the section ; the nucleus (xy)
has a peculiar appearance, in consequence of its being dis-
tended, owing to the absorption of water; k nucleus; kk nu-
cleolus; h membrane.

surrounding it, thus producing a tense condition, which was
described on p. 141 as turgor.

Many botanists, especially de Vries (V. 35) and Went, consider
the vacuoles to be special cell organs, which are not of accidental

156 THE CELL

formation in the cell-body, but which can only be produced by
division. Even in the youngest plant-cells, according to their
opinion, minute vacuoles are present, which multiply continually
by fission, and which are distributed amongst the daughter cells
when cell division occurs. Here all the vacuoles of the whole
plant would originate from those of the meristem. This theory
however is disputed by other investigators. Just as the proto-
plasm is bounded externally by a peripheral layer, the vacuoles,
in de Vries’ opinion, possess a special wall (the tonoplast), which
regulates the secretion and accumulation of the dissolved sub-
stances present in the cell sap.

a SS
ed ee

< \ Aa

Fra. 63.—Actinosprerium Richhorni (after R. Hertwig, Zoologie, Fig. 117): M medullary

substance ‘with nuclei (n); R peripheral sabstance with contractile vacuoles (cv); Na
nutrient material.

: The formation of vacuoles also occurs to a considerable extent
in the lower organisms. In Actinospherium, for example, the
protoplasmic body has quite a foamy appearance, in consequence
of the large number of great and small vacuoles present in it.
A few vacuoles, the number of which is constant, acquire a
specially contractile peripheral layer; they are then described as

THE VITAL PROPERTIES OF THE CELL 157

contractile vacuoles or reservoirs (p. 85). This occurs with
especial frequency in Ciliata.

Finally, it occasionally, although rarely, happens that the sap
collects into special vacuoles; this may occur in various kinds of
animal cells, and especially in structures which have a supporting
function in the body. In the tentacles of many Coelenterates, in
certain appendages of Annelids, and also in the chorda dorsalis of
Vertebrates, there are comparatively large vesicular cells, which
are separated from the exterior by a thick membrane, and which
contain hardly anything but cell sap, only a very minute quantity
of protoplasm being present. This is spread out in a very thin
layer over the membrane, extending threads here and there across
the sap space; the nucleus is generally embedded in a somewhat
denser collection of protoplasm, either in the peripheral layer, or
in the network. Here also, as in plants, the firm cell-wall is
tensely distended in consequence of the osmotic action of the
substances in the sap. Although no experimental investigations
have yet been made concerning the turgescence of the organs in
question, yet it can only be explained in this manner: that the
notochord functions in the body of a Vertebrate as a supporting
organ. The very numerous small turgescent notochord cells
being built up into one organ, and also shut off from the exterior
by means of a firm elastic sheath, their individual tensions are
summed up, and through the internal pressure of the sheath the
structure is kept rigid.

The absorption and secretion of sap occur in nuclear substance,
just as in protoplasm. ‘The sap serves the same purpose in both
cases, namely to offer a large surface to the active substances,
and to put them into direct communication with the nutrient
fluid.

Although the formation of sap vacuoles occurs but rarely in
animal cells, various substances, such as fat, glycogen, mucin,
albuminates, etc., frequently separate out from the protoplasm.

The fat is seen to occur at first as small drops in the proto-
plasmic body, resembling the drops of cell sap in young plant
cells. Just like such vacuoles, the droplets increase in size, and
run together, producing, finally, one single large drop, which fills
the whole internal space of the cell, and which is surrounded by
a delicate cell-membrane, and by a thin layer of protoplasm,
which contains the nucleus.

Glycogen collects in separate particles in the liver cells; these

158 THE CELL

drops, when a solution of iodine in iodide of potassium is added
to them, acquire a mahogany-brown coloration, by means of which
they can be easily seen.

Mucigenous substances often fill up the interior
of the cells, by which they are secreted (Fig.
64) in such quantities that the cells swell up
into vesicles, or assume the form of goblets.
The greater part of the protoplasm is collected
at the base of the cell, where the nucleus also is
situated, whilst the remainder surrounds the
mucigenous substance with a thin envelope, and ~
extends into it a few threads which unite together

to form anet. The mucigenous substances can
Ciipeaenaghee’ be clearly distinguished from protoplasm when

der epithelium of the cell is stained with one of several aniline
Squatina vulgaris, .

hardened in Mal “YeS:

ler’s fluid. (After The internal plasmic products very frequently

ree acquire greater solidity in egg-cells, which are
loaded in the most various ways with reserve materials. These
are grouped according to their form as yolk-globules (Fig. 65),
yolk granules, and yolk lamelle, and from a chemical point of
view chiefly consist of a mixture of albuminates and fats. The
more numerous, small, and closely packed these yolk-elements
are, the more the plasmic body assumes a foamy or net-like ap-
pearance.

Fre. 65.- Yolk elements out of a Hen’s egg © Balfour): A @ yolk spheres ;
B white yolk spheres.

Many plasmic products are crystalline in character, such as the
guanin crystals, to which the glistening silvery appearance in the
skin and peritoneum of fishes is due, or as the pigment granules in
the pigment cells.

Plasmic products, similar to those in animal cells, occur also in
plant cells; however, in this case they are generally present in a
few special organs, which are utilised either for the storing up of
reserve material, or, as with seeds, for purposes of reproduction.

THE VITAL PROPERTIES OF THE CELL 159

Under such circumstances the cells are filled with drops of oil
(oily seeds), with granules of various albuminous substances
(vitellin, gluten, aleuron), with crystalloids of proteinaceous sub-
stance, or with starch granules, about which more will be said
later.

The above-mentioned internal plasmic products being only tem-
porarily accumulated during metabolism before being utilised,
vary considerably in composition, but there are others which
attain a higher degree of organisation, and which participate
permanently in the functions of the cell. To such belong the
internal skeletal structures of the protoplasm, the various sub-
stances in plant cells, described under the common name of
trophoplasts, the cnidoblasts of Coelenterata, and, finally, the
sheaths of the muscle and nerve fibres, etc.

Internal — skeletons
are found in the bodies
of a large number of
Protozoa, but especi-
ally in great variety
and beauty in Radio-
larians. They consist
sometimes of regularly
arranged spicules,
sometimes of a fine,
open trellis-work, and
sometimes of a com-
bination of the two
kinds of structures (Fig.
66). In some families

Fre. 66.—Haliomma erinaceus (from R. Hertwig, Zool.,
of Radiolarians they Fig. 82): a external, i internal trellis work ; ck central

capsule ; wk soft extra capsular body; n internal vesicle

are composed of an or- (nucleus).

ganic substance which
is soluble in acids and alkalies, but in most cases, on the contrary,
they consist of silicious material which is united to an organic
substratum, just as, in the bones of Vertebrates, the phosphates
are united with the ossein. In each species the skeleton has a
constant and characteristic structure, and follows certain fixed
laws during the process of its development (Richard Hertwig,
5, 40).

Under the name trophoplasts, the highly organised differen-
tiated products of vegetable protoplasm are included ; these occur

160 THE CELL

as constantly as the nucleus, and possess great functional independ-
ence. They are of great importance in the nutrition of plants,
for the whole process of assimilation and the formation of starch
takes place in them (Meyer V. 9-11).

Trophoplasts are small bodies, which are generally either
globular or oval in shape; they are composed of a substance very
similar to and yet distinct from protoplasm. They are easily de-
stroyed, whilst the preparation is being made, by either water or
reagents, and are most successfully fixed by means of tincture of
iodine, or concentrated picric acid. They acquire a steely blue
coloration in nigrosin, and thus stand out clearly from the proto-
plasmic body. They often occur in great numbers.in the cell, and
may actively change their form. According to the investigations
of Schmitz (V. 29), Schimper (V. 27, 28), and Meyer (V. 9-11),
trophoplasts are not direct new formations in the protoplasm, but
on the contrary reproduce themselves, like nuclei, from time to
time by division. According to this conception, all the tropho-
plasts in the generations of cells which spring from the original
vegetable egg cell are derived from those trophoplasts which
were originally present.

Various kinds of trophoplasts may occur, fulfilling various
functions ; these are distinguished as starch-forming corpuscles, as
chlorophyll corpuscles, and as pigment-granules (amylo- or leuco-
plasts, chloroplasts, chromoplasts).

Most starch-forming corpuscles
(amyloplasts) (Fig. 67) occur
in the non-assimilating cells of
young plant organs, and in all
underground portions, as also in
stems and _ petioles. In_ the
pseudo-tubers of Phajus grandi-
folius, which are especially suitable
for investigation, they form, when
viewed on the flat, ellipsoidal
finely granular discs, whilst when
viewed from the side they look
like small rodlets; these when -

Hie. 67.—Phajus gronitjstine, anylo. treated with picro-nigrosin stain a
Ramis ‘tuber(after Strasburger, steely blue colour, and so stand
> Mig. 30): 4,C, out clearly from the surrounding

D, and Eare seen from the side, B from
above, FE is coloured green, (x 540.) protoplasm. On one of the flat

THE VITAL PROPERTIES OF THE CELL 161

sides of the disc, a starch granule is situated. When this is small,
it is completely covered with a thin coating of the substance of
the amyloplast; when it is somewhat larger, only the side turned
to the amyloplast is so coated. Further, a concentric stratification
may occur; under these conditions the hilum, which is surrounded
by the concentric layers, is situated near the surface, which is
turned away from the amyloplast. Hence the layers on this sur-
face are very thin, becoming gradually thicker and thicker as they
approach the starch-forming corpuscle, which is only natural,
since they grow out of it, and are formed by it. Frequently a
rod-shaped crystal of albumen may be seen embedded in the
substance of the amyloplast, on the surface which is turned away
from the starch granule.

Now since starch, as has been already mentioned, can only be
produced synthetically in the green portions of plants, these white
amyloplasts cannot be regarded as its true places of origin. It
is much more likely to be true that they have obtained the starch,
in a soluble form, probably as sugar (Sachs), from those places
where assimilation occurs, so that their only function is to re-
convert this soluble substance into a solid, organised body.

The chlorophyll granules (Fig. 68) must be
closely connected with the starch-forming
-corpuscles, since the latter may be converted
directly into them—this occurs when chloro-
phyll under the influence of sunlight develops
in them. In such a case the amyloplasts turn
green, increase in size, and part with their
starch granules, which become dissolved. In an Gas A
addition, chlorophyll granules are formed of Funariahygrometrica,
from the colourless trophoplasts, which are ee ee
developed at the growing points in the form givision. (x 510: after
of undifferentiated corpuscles; finally they See Pract. Bot.,
multiply by division in the following manner reg
(Fig. 68): to start with, their substance increases in size, and
they elongate themselves; they next become biscuit-shaped, and
finally divide into two equal portions.

The chlorophyll granules consist of two substances: a ground
substance, which reacts like albumen, and a green colouring matter
(chlorophyll), which saturates the stroma. This may be extracted
by means of alcohol, when it is seen to be distinctly fluorescent, ap-

pearing green with transmitted, and bluish red with reflected, light.
M

162 THE CELL

Several small starch granules are generally enclosed in the
chlorophyll corpuscles, being formed in them through assimilation.
They are most easily seen, if, when the chlorophyll has been ex-
tracted by means of alcohol, tincture of iodine is added to the
preparation.

As has been proved by Stahl’s investigations, the chlorophyll
granules, quite apart from the changes of position brought about
by the streaming movements of the protoplasm (vide p. 104), are
able to change their shape under the stimulating influence of the
sun’s rays, toa surprising extent. Whilst in diffused daylight they
assume the shape of polygonal discs with their broad sides directed
towards the source of light, in direct sunlight they contract up
into little round balls or ellipsoidal bodies. By this means they
effect a change which is necessary for the performance of the chloro-
phyll function, by “ offering to direct sunlight a small.surface, and
to diffused daylight a larger one, for the absorption of the rays of
light. In this, they offer us an insight into the high degree of
the differentiation that they have attained which we could never
have arrived at simply by the study of their chemical activity”
(de Vries V. 46). As regards their mode of multiplication by
division, their active motility, their functions in the processes of
assimilation, etc., they appear, like nuclei, to be very highly
specialised plasmic products.

Finally another variety of trophoplasts, the colour-granules,
must be mentioned: the red and orange red coloration of many
flowers is caused by their presence. They consist of a proto-
plasmic substratum which may assume very various forms, oc-
curring sometimes in the shape of a spindle and sometimes of a
sickle, a triangle or a trapezium. In this substratum crystals of
colouring matter are deposited. In this case also colourless tro-
phoplasts may, in suitable objects, be seen to develop gradually
into colour granules. Further Weiss has observed spontaneous
movements and changes of form in these granules also.

We will conclude this review of the various kinds of tropho-
plasts by describing in more detail the structure of the starch
grains, which have acquired considerable theoretical importance
in consequence of Nageli’s (V. 17, 20) researches, and the con-
clusions which have been deduced from them.

The starch grains (Fig. 69) in a plant cell may vary consider-
ably as to size. Sometimes they are so small that even with the
strongest powers of the microscope they only appear as minute

THE VITAL PROPERTIES OF THE CELL 163

points, whilst at others they may be as large as 2 mm. in circum-
ference. Their reaction towards iodine solution is characteristic ;
they become either dark or
light blue according to the
strength of the solution.
In warm water they swell
up considerably, and if fur-
ther heated turn into a
paste.

Their shape also varies,
being sometimes oval, some-
times round, and sometimes
irregular. When strongly
magnified they are seen to
be distinctly stratified, and
in an optical section bright
broad bands are seen to

alternate with more narrow Fie. 69.— Starch grains from a Potato tuber
dark ones. Nageli explains (after Strasburger, Pract. Bot., Fig. 3): A simple
this appearance by the sup- ste At eg ae i and D com
position that the starch
grain is composed of lamelle of starch substance, which are alter-
nately rich and poor in water. Strasburger (V. 31), on the other
hand, is of opinion, that “the darker lines represent the specially
marked adhesion surfaces of consecutive lamellz, which,’ he con-
siders, ‘“‘are more or less identical with each other in composition.”

The lamelle (Fig. 69) are arranged round a hilum, which is
either situated in the centre of the whole grain (B, C) or, as is
more frequently the case, is eccentric in position (A). Further it
is not rare to find starch grains, which consist of two (B, C) or
three (D) systems of lamellae, united together; these are termed
compound grains, in contradistinction to others which contain one
single hilam. When the hilum is in the centre, the strata of starch
surrounding it are fairly uniform in thickness. On the other
hand when its position is eccentric, only the inner layers surround
it completely, whilst the peripheral layers are of greatest thick-
ness on that side which is turned away from the hilum, and grow
thinner and thinner as they approach it, becoming finally so
narrow, that they either fuse with neighbouring lamellx, or end
freely.

In each starch grain the amount of water contained is greatest

164 THE CELL

re

at the centre, and diminishes as the surface is approached. The
hilum is richest in water, whilst the superficial layer, bordering
on the protoplasm, is most dénse in composition. To this cause
we can trace the fissures which occur in the hilum of the starch
grain as it dries, and which extend outward from it towards the
periphery (Nageli V. 17).

As has been already mentioned, the starch grains of plants do
not, as a rule, arise directly in the protoplasm, but in certain
special differentiation products of it, the starch-forming corpuscles
(amyloplasts, and chlorophyll bodies). According to the investiga-
tions of Schimpfer (V. 27), the special variety of stratification
which occurs in the grain depends upon whether it is situated in
the interior or upon the surface of one of these corpuscles. In the
first case, the starch lamelle arrange themselves evenly around
the hilum since they receive equal accretions on every side from
the starch-forming corpuscle. In the second case, that portion of
the grain, which adjoins the free surface of the amyloplast, is
under less favourable conditions for growth, for the surface of the
grain, which is directed towards the centre of the starch-forming
corpuscle, acquires the most substance, and in consequence the
layers are thicker at this point, and grow gradually thinner as
they approach the opposite side. '

Hence the hilum, about which the layers are arranged, becomes
pushed further and further beyond the surface of the amyloplast,
assuming a more and more eccentric position in the stratification.

That the starch grains grow by the deposition of new layers
upon the surface, that is by apposition, may be deduced from a
statement of Schimpfer’s. He observed, that around the corroded
centres of starch grains whose surfaces had been dissolved away
new layers had been deposited.

Strasburger is of opinion that starch grains may be occasionally
produced in the protoplasm itself, without the intervention of
special starch-forming corpuscles. He found them in the cells of
the medullary rays of Conifer, during their early stages of
development, as minute granules, embedded in the strands of the
plasmic network. As they grew larger they were to be plainly ;
seen situated in the plasmic cavities. These cavities have highly
refracting walls, upon which microsomes are situated.

One of the most remarkable of the internal plasmic products
is the nematocyst (Fig. 70), which functions in Cwlenxterala as
& weapon of attack, in the cnidoblasts, which are distributed

THE VITAL PROPERTIES OF THE CELL 165

throughout the ectoderm. It consists of an oval capsule (a and
b), which is formed of a glistening substance, and which has an
opening in that end which is directed towards the external sur-
face. The internal surface is lined with a delicate lamella which,
at the edge of the opening, merges with the sheath of the cap-
sule ; the structure of this sheath is frequently very complicated
(cf. Fig. 70 a, b). In the figure, this sheath consists of a very
delicate filament and of a broad, conical,
proximal portion, which is situated in the
interior of the capsule, and is provided with
shorter and longer barbs. The filament
stretches from the end of the conical por-
tion, and is wound spirally round and
round it several times; the free, internal
cavity is filled with an irritating secretion ;
the protoplasm, which borders on the ne-
matocyst, is differentiated to form a con-
tractile envelope, which also has an open-
ing to the exterior (Schneider V. 45).

Near the opening of the capsule a rigid, —_ Fre. 70.—Thread cells of
glistening, hair-like process, the cnidocil, Se es eee
stretches out from the free surface of the 4 cell with cnidocil, and
cell. If this is touched by any foreign the thread coiled il ee
body, it communicates the stimulus to the sal alga

nated from the capsule,

protoplasm. In consequence, the cnido- and armed at its base with
barbs ; ¢ prehensile cell of
a Ctenophore.

blast, enclosing the nematocyst, contracts
suddenly and forcibly, thereby compressing
it, and forcing out the thread which is in the interior, so that it is
turned inside out, like the finger of a glove (Fig. 70 0). At first
the conical proximal portion is protruded with the barbs extended
outwards, next comes the delicate, rolled-up thread. The irritat-
ing secretion is apparently poured out through an opening in the
capsule.
Some light is thrown upon the formation of this extraordinary
apparatus by the history of its development. First of all, an
oval secretion cavity is formed in the cnidoblast; this cavity 1s
separated from the protoplasm by a delicate membrane, then a
delicate protoplasmic process grows into the secretion cavity from
the free end of the cell; it gradually assumes the position and
form of the internal thread apparatus, separating upon its surface
the delicate enclosing membrane. Finally, the shining, tough, ex-

166 THE CELL

ternal wall of the capsule, with its opening, becomes differentiated,
and around it the contractile sheath develops.

b. External Plasmic Products. The external plasmic pro-
ducts may be divided into three groups,— cell membranes, cuticular
formations, and intercellular substances.

- Cell membranes are structures which separate out, and envelop
the whole surface of the cell-body. In the vegetable kingdom
they are very important, and easily seen, whilst in the animal
kingdom they are frequently absent, or are so slightly developed
that they can hardly be made out even with the strongest powers
of the microscope.

In plants, the cell membrane is composed of cellulose, a carbo-
hydrate very nearly allied to starch. The presence of this sub-
stance may generally be easily demonstrated by a very character-
istic reaction. If a section of a plant tissue, or a single plant cell,
is saturated first with a dilute solution of iodine in potassic iodide,
and then (after the excess of the iodine solution has been removed)
the preparation is immersed in sulphuric acid (2 parts acid to
1 part water), the cell membranes assume a lighter or darker blue
coloration. Another reaction for cellulose is seen when chlorzinc-
iodine solution is used (Schulze’s solution).

The membranes of plant cells often become thick and firm, and
then they show, in section, a distinctly marked striation, being
composed, like starch grains, of alternate bands of high and low

A

SN

~

Fig. 71, Fia. 72.
Fre. 71] -—Transverse section through the thallus of Caulerpa prolifera at the place where
a branch is inserted. (After Strasburger, Pl. I., Fig. 1.)
Fia@. 72.—A Portion of a fairly old pith cell, with six layers from Clematis vitalba (after

Strasburger, Pl. L., Fig. 13); B a similar cell after it has been swollen up by sulphuric
acid. (After Strasburger, Pl. I., Fig. 14.)

THE VITAL PROPERTIES OF THE CELL 167

refractive power (Figs. 71, 72 A and B). However, when the
surface is examined, a still more delicate structure can frequently
be seen. The cell membrane is faintly striated, looking as though
it were composed of a large number of parallel layers; these are
crossed by others running in an opposite direction. They runeither
longitudinally and transversely—that is to say, like rings round
the cell—or are arranged diagonally to the longitudinal axis of the
cell. Nageli and Strasburger hold different opinions concerning
the relation of this delicate striation towards the separate cellulose
lamellee.

Niigeli (V. 19) considers that both systems of striation are
present in each lamella; further that, as in starch grains, the
lamelle, as well as the intersecting bands, consist of substances
alternately rich and poor in water, and hence are alternately dark
and light in appearance. In consequence, a lamella is, as it were,
divided into squares or rhomboids, like a parquetted floor. ‘‘ These
may assume one of three appearances; they may consist of sub-
stances of greater, of less, or of medium density, according as to
whether they occur at the point of intersection of two denser, of
two less dense bands, or of one dense and one less dense band.”
Hence Niageli is of opinion that the whole cell membrane “ is
divided in three directions into lamelle, which consist of sub-
stances alternately rich and poor in water, and which intersect in
a manner similar to that seen in the intersecting lamine of a
crystal. The lamine in one direction compose the layers, those
in the others the two striated systems. These latter may intersect
at almost any angle; they both meet the lamelle of the layers,
apparently, in most cases at right angles.”

On the other hand, in opposition to Nigeli, Strasburger (V.
31-33) and other botanists, whose statements are not to be dis-
puted, consider that intersecting strie never belong to the same
lamella ; they think it much more likely that if one lamella is
striated in a longitudinal direction, the next one is striated trans-
versely, and so on alternately. Strasburger does not believe that
the difference, either in the lamelle or the striw, is due to the
varying amount of water which they contain. The lamelle and
the striez in them are separated from one another by their
surfaces of contact, which, in consequence of being seen at
different angles (cross section and surface view), appear as
darker lines. Thus the arrangement is similar, in the main, to
that seen in the cornea, which consists of laminew formed of

168 THE CELL

bundles of white fibres which cross one another at right angles in
alternate laminee.

Not infrequently cellulose membranes show delicate sculptur-
ings, especially upon the inner surface. Thus thickenings may
originate in the interior ; these may run into each other to form
a spiral, or may be arranged in large:numbers transversely to
the long axis of the cell, or finally, may be united together in an
irregular fashion to form a network. On the other hand, the
thickenings may be absent at various places, where neighbouring
cells touch, and thus pits or perforations are produced (Vig. 72 A),
by means of which neighbouring cells can interchange nutrient
substances with greater ease.

Moreover, as regards its composition, the cell-wall can alter its
character in various ways soon after its original formation; this
may be produced by the deposition of various substances upon it,
or by its transformation into wood or cork.

Lime salts or siliceous substances are not infrequently deposited
in the cellulose, thus producing greater solidity and hardness of
the walls. When portions of such plants are burnt, the cellulose °
is destroyed and a more or less perfect skeleton of lime or silica
remains in the place of the framework of the cell. Lime is
deposited in Corallinew, in Characee, and in Oucurbitacee; and
silica in Diatomacee, Equisitacew, Grasses, ete.

Similarly the cell-wall obtains very great strength through the
formation of wood. Here the cellulose becomes mingled with
another substance, woody substance (lignin and vanillin), this
may be dissolved away by: means of potassic hydrate, or with a
mixture of nitric acid and chlorate of potash, after which a frame-
work, which gives the reaction, of cellulose remains.

In the formation of cork the cellulose becomes united in larger
or smaller quantities with corky substance or suberin. In this case,
also, the physical properties of the cell-wall are altered, it being
no longer permeable to water. Thus cork cells are formed on the
surface of many parts of plants in order to prevent evaporation.

Whilst it is evident, that in the deposition of lime and silica, the
particles of these substances must be conveyed by the protoplasm
to the place where they are required, and where they are de-
posited between the particles of cellulose, whereupon molecular
combinations are again called into play, two explanations may
be given concerning the formation of wood and cork. Hither the
wood and cork substances are constructed in a soluble form, by.

THE VITAL PROPERTIES OF THE CELL 169

means of the protoplasm, and, like the lime and silica particles,
are deposited as an insoluble modification in the cellulose mem-
brane, or both substances originate on the spot, through a chemical
transformation of the cellulose. This is another problem which
must be decided by means of physiological chemistry rather than
through morphological investigations (vide p. 153).

The question as to how the cell membrane grows is a very im-
portant problem, and has led to much discussion; it is very diffi-
cult to come to any decision on the subject. Two methods of
growth may be distinguished, a superficial and an interstitial
method. The delicate cellulose coating, which at first is scarcely
measureable, may by degrees attain a very considerable thickness,
growing by the addition of numerous lamine, the number of which
varies with the thickness. It is most probable that layer after
layer is deposited by the protoplasm of the outer layer which was
at first differentiated off. This method of growth is termed
‘growth by apposition,’ in contradistinction to “ growth by in-
tussusception,” which, according to Nageli, is the way in which
the cell-wall grows, that is to say, by deposition of particles in
the interstices between the particles already present.

The apposition theory is supported by the following three ob-
servations: (1) Before the ridge-like thickenings are formed upon
the inner surface of a cell-wall, the protoplasm is seen to collect
together at those places, where thickening of the wall is about to
occur, in masses, which exhibit active streaming movements. (2)
When, in consequence of plasmolysis, the protoplasmic body has
receded from the cell-wall, a new cellulose membrane is seen to
appear on its naked surface (Klebs IV. 14). If the plasmolysing
agent be removed, and the cell-body be made to increase in size by
the absorption of water, so that its new cellulose membrane comes
into close contact with the original cell-wall, they unite with one
another. (3) When a plant cell divides, it may often be plainly
seen that each danghter cell surrounds itself with a new wall of
its own, so that the two newly-formed walls of the daughter-cells
are enclosed by the old wall of the mother-cell.

It is more difficult to explain the growth in superficial area of
the cell-wall. This may be effected by two different processes,
working either singly or in unison. The membrane may become
stretched, like an elastic ball which is inflated with air; or it
may grow by intussusception, that is to say, by the deposition of
new cellulose particles between the old ones.

170 THE CELL

That such a stretching of the cellulose membrane does actually
occur is proved by several phenomena. The turgescence already
mentioned causes distension.. When a cell is: plasmolysed it at
first contracts somewhat as a whole, in consequence of the loss of
water, before the outer layer of the protoplasm becomes separated
from the cell-wall. This indicates that it was subjected to in-
ternal pressure. It may be observed in many Alge, that the celln-
lose lamellz, which are first formed, are eventually ruptured by
the stretching, and discarded (Rivularia, Gleocapsa, Schizochlamys
gelatinosa, ete ). Each distension and contraction must be con-
nected with a change of position of the most minute particles,
which become located either on the surface or in the deeper layers.

Thus the way in which a membrane increases in size when
stretched offers many points of resemblance to growth by intus-
susception. The difference consists in this, that in the first case
particles of cellulose already present are deposited in the surface,
whilst in the second case particles in process of formation are so
deposited.

However, I do not wish to totally disregard growth through
intussusception, as Strasburger formerly did (V. 31). On the
contrary, I consider it to form, in addition to apposition, a second
important factor in the formation of the cell-wall, although it is
certainly not the only factor, as is dogmatically stated in Nigeli’s
theory.

Many phenomena in cell-growth may be most easily explained
by means of intussusception, as has been done by Nageli, whilst
the apposition theory presents numerous difficulties.

It does not often occur that the cell-wall becomes ruptured by
stretching, and yet the increase in size which occurs in nearly all
cells from their initial formation until their full growth, is quite
out of proportion to the elasticity of the cell-wall, which, as it is
composed of cellulose, cannot be assumed to be very great. Many
plant cells grow until they are a hundred or even two hundred
times as long as they were originally (Chara).

The fact that many cells are very irregular in form would be
very difficult to explain if the cell membrane were considered to
increase superficially solely by stretching, like an indiarubber
bladder. For example, Caulerpa, Acetabularia, etc., are apparently
differentiated, like multicellular plants, into root-like, stem-like,
and leaf-like structures, although each plant consists of only a
single cell-cavity. The growth of each of these parts proceeds

THE VITAL PROPERTIES OF THE CELL ua |

according to a law of its own. Many plant cells grow only at
one point: either at the apex or near the base, or they develop
lateral outgrowths and branches. Others undergo during growth
complicated changes of direction, as in the internodes of the
Characee.

Finally, Nageli states, as a point in favour of the theory of
growth by intussusception, that many membranes increase con-
siderably both superficially and in thickness after they have
become separated from the protoplasmic body, in consequence of
the formation of special membranes around the daughter-cells ;
‘* Glaocapsa and Glewocystis appear first as simple cells with a thick
gelatinous cell-wall. The cell divides into two, whereupon each
develops for itself a similar enclosing cell-wall, and in this manner
the enveloping process proceeds.” The outermost gelatinous cell-
wall must in consequence become larger and larger. According
to Nageli’s computation, their volume during successive develop-
mental stages may increase from 830 cubic micromillimetres to
2,442, to 5,615, and finally to 10,209 cubic micromillimetres.

In another species the gelatinous cell-wall was seen to increase
from 10 to 60 micromillimetres, that is to say, it became six times
as thick. “In Apiocystis the pear-shaped colonies, which consist
of cells embedded in a very soft gelatinous matrix, are surrounded
by a thicker membrane. In this case, moreover, the membrane
increases with age, not only in circumference but also in thick-
ness; for whilst in smaller colonies it is barely 3 micromilli-
metres thick, in larger ones it is 45 micromillimetres thick; in
the former it is 27,000 square micromillimetres in area, and in the
latter 1,500,000 square micromillimetres. Thus the thickness of
the sheath increases at a ratio of 1 to 15, the superficial area of
1 to 56, and the cubic contents of 1 to 833. That apposition
should take place upon the inner surface of this sheath is out of
the question, for its smooth internal surface never comes into con-
tact with the small spherical cells, or only does so in a few isolated
spots.”

In all these cases I am obliged to agree with Nigeli, who con-
siders that. we have to make too many improbable assumptions, if
we attempt to explain the superficial growth of the cell membrane
solely by the deposition of new layers, whereas the above-men-
tioned “ phenomena (variations in furm and direction, uneven growth
of various parts, torsions) may be explained in the simplest and
easiest fushion by intussusception. Everything depends upon this,

172 THE CELL

that the new particles become deposited in definite positions, in definite
quantities, and in definite directions, between those already present.”

Moreover, the process of intussusception is not to be disre-
garded in those cases where calcium and silicon salts are deposited
in the cell-wall, for this mostly occurs at a later period, the salts
being frequently only found in the superficial layers. It could
only be proved that it is impossible for particles of cellulose to be
deposited in a similar manner, if it could be shown that cellulose
is actually only produced by the direct metamorphosis of layers
of protoplasm. However, up till now this is anything but proved;
and, moreover, it seems that the study of plant anatomy, by means
of microscopic observation alone, is insufficient to establish this
theory, and that in addition a very much improved and advanced
knowledge of micro-chemistry must be reached, as in the case
mentioned on pp. 153, 154. Consideration of the statements made
there shows especially, that under certain conditions in the for-
mation of cellulose there is not the marked difference that is
frequently considered to exist between growth by apposition and
growth by intussusception.

Cuticular structures are the skin-like formations with which a
cell covers its external surface—not all over, however, but only
on one side. In the animal kingdom, those cells which are situated
on the surface of the body, or which cover the internal surface of
the alimentary canal, are frequently provided with a cuticle, which
protects the underlying protoplasm from the hurtful influences -
of the surrounding media. The cuticle usually consists of thin
lamelle, intersected by fine parallel pores, into which delicate
processes stretch from the underlying protoplasm. As cuticular
formations of a peculiar kind, which exhibit at the same time a
very marked structure, the outer portions of the rods and cones
in the retina may be cited.

Cuticular membrane-like formations, consisting of cells united

Fic, 73.—Epithelium with cuticle of a Saw-fly (Cimbex coronatus) (from R. Restores ‘
Fig. 24f): ¢ cuticle; e epithelium.

THE VITAL PROPERTIES OF THE CELL Pic

together, form by their coalescence extensive structures (Fig. 73),
which, especially in Worms and Arthropods, serve as a protection
to the whole surface of the body. This skin consists chiefly of
chitin, a substance which is only soluble in boiling sulphuric acid.
In its minute structure it very closely resembles cellulose mem-
branes, especially in its stratification, which indicates that growth
has taken place by the deposition of new lamelle upon the inner
surface of those already formed.

Occasionally the old chitinous sheaths are ruptured and dis-
carded after they have developed beneath them a younger, more
delicate skin to take their place; this process is termed sloughing.
Calcium salts may be deposited, by means of intussusception, in the
chitinous skin in order to strengthen it.

Finally, intercellular substances are formed, when numerous
cells secrete from their entire surfaces solid substances, which,
however, do not remain isolated as in cell membranes, but which
coalesce to form a coherent mass, it being
impossible to recognise from which cells
the various portions of it originated (Fig.
74). Thus, in tissues with intercellular
substance, the individual cells cannot be
separated from one another, as they can be
in plant tissue. In the continuous ground-
substance, which may consist of very differ-
ent chemical substances (mucin, chondrin,
glutin, ossein, elastin, tunicin, chitin, etc.),
and which further may be either homo-
geneous or fibrous, small spaces are present,
which contain the protoplasmic bodies.

: tee Fie. 74.—Cartilage (after
Now, since the area of intercellular sub- Berenesne oe mupertoial

stance in the neighbourhood of the cell taser; b intermediate layer
space is controlled to a considerable extent ee oe eeauees.
by the protoplasmic bodies it contains, it :

has been called by Virchow (I. 33) a cell territory. Such a cell
territory, however, is of necessity not marked off from neighbour-
ing ones.

Amongst the cell products, which may be classed as external or
internal according to their position, the muscle and nerve fibres
must be mentioned. Being composed of protein substance, they
come next after protoplasm in the consideration of the substances
of which tissues are composed; they must be classed with the

174° THE CELL

above-mentioned structures, since they are quite distinct from
protoplasm, and may be described as peculiar formations which
perform a definite function in the life of the cell. Their more
delicate structure will be discussed in another volume dealing with
the tissues.

Literature V.

1. Baumann. Ueber den von O. Liw und Th. Bokorny erbrachten Nachweis
ron der chemischen Ursache des Lebens. Pfliigers Archiv. Bd. XXIX.
1882.

2. Buner. Physiological and Pathological Chemistry, trans. by Wooldridge.

8. Encenmann. Neue Methode zur Untersuchung der Sauerstoffausscheidung
pflanzlicher und thierischer Organismen. Botan. Zeitung. 1881.

4, Haxcxen. Die Radiolarien. 1862.

Harckxen. Générale Morphologie. ‘

5. Hess. Untersuchungen zur Phagocytenlehre. Virchows Archiv. Bd. 109.

6. Laneuaxs. Beobachtungen iiber Resorption der Extravasate und Pigment-
bildung in denselben. Virchows Archiv. Bd. 49. . 1870.

7. Léwuv. Boxorny. Die chemische Ursache des Lebens. Miinchen. 1881.

8. Marcuanp. Ueber die Bildungsweise der Riesenzellen um Fremdkorper.
Virchows Archiv. Bd. 93. 1883.

9. ArtHuR Meyer. Ueber die Structur der Stirkekirner. Botan. Zeitung.
1881.

10. ArtHur Meyer. Ueber Krystalloide der Trophoplasten und iiber die
Chromoplasten der Angiospermen. Botan. Zeitung. 1883.

11. Arruur Meyer. Das Chlorophyllkorn in chemischer, morphologischer und
biologischer Beziehung. Leipzig. 1883.

12. Merrcuntxorr. Untersuchung iiber die intracellulare Verdauung bei
wirbellosen Thieren. Arbeiten der zoologischen Institute in Wien. Bd. V.
Heft 2.

13. Mercunixorr. Ueber die Beziehung der Phagocyten zer Milzbrand-bacillen.
Archiv. fiir patholog. Anatomie u. Physiologie. Bd. 96 u.97. 1884.

14. Mercnnrxorr. Ueber den Kampf der zellen gegen Erysipelkokken. Ein
Beitriig zur Phagocytenlehre. Archiv. fiir patholog. Anatomie uw.
Physiologie. Bd. 107.

15. Mercunixorr. Ueber den Phagocytenkampf bei Riickfalltyphus. Vir-
chows Archiv. Bd. 109.

Mercunikorr. Lectures on Inflammation, trans. by Starling. 1893.

16. Naceut. (1) Primordialschlauch. (2) Diosmose der Pflanzenzelle. Pflanzen-
physiologische Untersuchungen. 1855.

17. Nicent. Die Stirkekorner. Pflanzenphysiologische Untersuchungen.
Heft 2. 1858.

18. Nicer. Theorie der Gihrung. 1879.

19. Nicen1, Ueber der inneren Baw der vegetabilischen Zellenmembran.
Sitzungsber. der bairischen Akademie. Bd. I.u.II. 1864.

20.

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26.

27.

28.

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35.

36.

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38.

39.

THE VITAL PROPERTIES OF THE CELL E75

Nicent. Das Wachsthum der Stirkekérner durch Intussusception. Botan.
Zeitung. 1881.

Nicent. Ernihrung der niederen Pilze durch Kohlenstoff- u. Stickstoff-
verbindungen. Untersuch. tiber niedere Pilze aus dem pflanzenphysiolog.
Institut in Miinchen. 1882.

Pathological Society’s Transactions. Discussion on Phagocytosis and
Immunity. Vol. XLUT, 1892.

. W. Prerrer. Ueber intramoleculare Athmung. Untersuchungen aus dem

botan. Institut zu Tiibingen. Bd. I.

. W. Prerrer. Ueber Aufnahme von Anilinfarben in lebende Zellen.

Untersuchungen aus dem botan. Institut zu Tiibingen. Bad. II.

W. Prerrer. Pflanzenphysiologie. 1881.

W. Prerrer. (1) Ueber Aufnahme und Ausgabe ungelister Kirper. (2) Zur
Kenntniss der Plasmahaut und der Vacurlen nebst Bemerkungen iiber den
Aggregatzustand des Protoplasmas und iiber osmotische Vorgéinge.
Abhandl. der Mathemat. physik. Classe d. kgl. stichs. Gesellsch. d.
Wissenschaft. Bd. XVI. 1890.

Priicer. Ueber die Physiolog. Verbrennug in den lebendigen Organismen.
Archiv. f. Physiologie. Bd. X. 1875.

Pricer. Ueber Wiirme und Oxydation der lebendigen Materie. Pfliigers
Archiv. Bd. XVIII. 1878.

W. Scarmrer. Untersuchungen iiber das Wachsthum der Stiirkekérner.
Botan. Zeitung. 1883.

W. Scuimper. Ueber die Entwickelung der Chlorophyllkirner und Farb-
korner. Botan. Zeitung. 1883.

Fr. Scuuirz. Die Chromatophoren der Algen. Vergleichende Untersuch.
tiber Bau und Entwickelung der Chlorophyllkérper und der analogen.
Farbstoffkirper der Algen. Bonn. 1882.

ScutrzenBERGER. Die Gédhrungserscheinungen. 1876.

SrrasBurRGER. Ueber den Bau und das Wachsthum der Zellhiute. Jena.
1882.

SrraspurcEer. Ueber das Wachsthum vegetabilischer Zellhdute. Histolo-
gische Beitrige. Heft 2. 1889.

SrraspureerR. Practical Botany, trans. by Hillhouse.

A. Weiss. Ueber spontane Bewegungen und Formiinderungen von Farb-
stoffkorpern Sitzungsber. d. kgl. Akademie d. Wissensch. zer Wien. Bd.
XC. 1884.

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Pringsh. Jahrb. f. wissensch. Botanik. Bd.16. 1885.

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treckung. 1877.

Went. Die Vermehrung der normalen Vacuolen durch Theilung. Jahrb.
f. wissensch. Botanik. Bd.19. 1888.

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Athmung der Pflanzen. Arbeiten des botanischen. Instituts zu Wiirz-
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Wiesner. Die Elementarstructur u. das Wachsthum der lebenden Substanz.
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46.

THE CELL

Ricuarp Hertwic. Die Radiolarien.

Esruicu. Ueber die Methylenblaureaction der lebenden Nervensubstanz.
Biologisches Centralblatt. Bd. VI. 1887.
R. Herennain. Physiologie der Absonderungsvorginge. Handbuch der

Physiologie. Bd. V.

Max Scuuuzx. Lin reizbarer Objecttisch u. seine Verwendung bei Unter-
suchungen des Blutes. Archiv. f. mikrosk. Anatomie. Bd. I.

Oscar Scuutze. Die vitale Methylenblaureaction der Zeilgranula. Anat.
Anzeiger, 1887, p. 684.

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sichtigung des Nervensystems der Hydropolypen. Archiv. f. mikrosk.
Anatomie. Bd. XXXV.

Hueco ve Vries. Intracellulare Pangenesis. Jena. 1889.

CHAPTER VI.
THE VITAL PHENOMENA OF THE CELL.

I. Reproduction of the Cell by Division.—One aittri-
bute of the cell, which is of the greatest importance, since the
maintenance of life depends upon it, is its power of producing new
forms similar to itself, and by this means maintaining its species.
It is becoming daily more and more clearly evident, as the result
of innumerable observations, that new elementary organisms can
only arise through the division of the mother-cell into two or more
daughter-cells (Omnis cellula e cellula). This fundamental law,
which is of paramount importance in the study of biology, has
only been established after much laborious work along the most
diverse lines, and after many blunders.

l. History of Cell Formation. Schleiden and Schwann (I.
28, 31), in developing their theories, asked themselves the natural
question, “ How do cells originate?” Their answer, based upon
observations both faulty and insufficient, was incorrect. They
held that the cells, which they were fond of comparing to crystals,
formed themselves, like crystals, in a mother-liquor. Schleiden
named the fluid inside the plant cell Cytoblastem. He considered
it to be a germinal substance, a kind of mother-liquor. In this
the young cells were supposed to originate a solid granule, the
nucleolus of the nucleus developing first, around which a layer of
substance was precipitated; this, they considered, became trans-
formed into the nuclear membrane, whilst fluid penetrated between
it and the granule. The nucleus thus formed constituted the cen-
tral point in the formation of the cell, in consequence of which it
was termed the Cytoblast. The process of cell development was then
supposed to be similar to the one described above when the nucleus
was formed round the nucleolus. The cytoblast surrounded itself
with a membrane which was composed of substances precipitated
from the cell-sap. This membrane was at first closely in contact
with the nucleus, but later on was pushed away by the in-pressing

fluid.
177 N

178 THE CELL

Schwann (I. 31), whilst adopting Schleiden’s theory, fell into a
second, and still greater error. He considered that the young
cells developed, not only within the mother-cell (as propounded
by Schleiden), but also outside of it, in an organic substance, which
is frequently present in animal tissues as intercellular substance,
and which he called also Cytoblastem. Thus Schwann taught
that cells were formed spontaneously both inside and outside of
the mother-cell, which would be a genuine case of spontaneous
generation from formless germ substance.

These were indeed grave fundamental errors, from which, how-
ever, the botanists were the first to extricate themselves. In the
year 1846 a general law was formulated in consequence of the
observations of Mohl (VI. 47), Unger, and above all, Nageli
(VI. 48). This law states, that new plant cells only spring from
those already present, and further that this occurs in such a
manner, that the mother-cell becomes broken up by dividing into
two or more daughter-cells. This was first observed by Mohl.

It was much more difficult to disprove the theory, that the cells
of animal tissues arise from cytoblasts, and this was especially the
case in the domain of pathological anatomy, for it was thought
that the formation of tumours and pus could be traced back to
cytoblasts. At last, after many mistakes, and thanks to the
labours of many investigators, amongst whom Kolliker (VI.
45, 46), Reichert (VI. 58, 59), and Remak (VI. 60, 61) must be
mentioned, more light was thrown upon the subject of the genesis
of cells in the animal kingdom also, until finally the cytoblastic
theory was absolutely disproved by Virchow, who originated the
formula, ‘ Omnis cellula e cellula.”” No spontaneous generation of
cells occurs either in plants or animals. The many millions of
cells of which, for instance, the body of a vertebrate animal is
composed, have been produced by the repeated division of one
cell, the ovum, in which the life of every animal commences.

The older histologists were unable to discover what part the
nucleus played in cell-division. For many decades two opposing
theories were held, of which now one and now the other obtained
temporarily the greater number of supporters. According to the
one theory, which was held by most botanists (Reichert VI. 58 ;
Auerbach VI. 2a, etc.), the nucleus at each division was sup-
posed to break up and become diffused throughout the protoplasm,
in order to be formed anew in each daughter-cell. According to
the other (C. E. v. Baer; Joh. Miiller; Remak VI. 60; Leydig;

THE VITAL PHENOMENA OF THE CELI, 179

Gegenbaur; Haeckel V. 4b; van Beneden, etc.), the nucleus
was supposed to take an active part in the process of cell-division,
and, at the commencement of it, to become elongated and con-
stricted at a point, corresponding with the plane of division which
is seen later, and to divide into halves, which separate from one
another and move apart. The cell body itself was supposed to
become constricted, and to divide into two parts, in each of which
one of the two daughter-nuclei formed the attraction centre.

Each of these theories, so diametrically opposed, contains a
grain of truth, although neither describes the real process, which
remained hidden from the earlier histologists, chiefly on account
of the methods of investigation used by them. It is only during
the last two decades, that our knowledge of the life of the cell has
been materially advanced by the discoveries made by Schneider
(VI. 66), Fol (VI. 18, 19), Auerbach (VI. 2a), Biitschli (VI. 81),
Strasburger (VI. 71, 73), O. and R. Hertwig (VI. 30-38), Flem-
ming (VI. 13-17), van Beneden (VI. 4a, 4b), Rabi (VI. 53), and
Boveri (VI. 6, 7). These discoveries have revealed to us the
extremely interesting formations and metamorphoses, which are
seen in the nucleus during cell-division. These investigations, to
which I shall have occasion to refermfrequently in this section,
have all pointed to the same conclusion, that the nucleus is a
permanent and most important organ of the cell, and that it
evidently plays a distinct réle in the cell life during division.
Just as the cell is never spontaneously generated, but is produced
directly by the division of another cell, so the nucleus is never
freshly created, but is derived from the constituent particles of
another nucleus. The formula, “ omnis cellula e cellula,’ might be
extended by adding “ omnis nucleus e nucleo” (Flemming VI. 12).

After this historical introduction, we will consider more in detail,
first, the changes which take place in the nucleus during division,
and next, the various methods of cell multiplication.

II. Nuclear Division.—The nucleus plays an important and
most interesting part in each process of cell-division. Three
methods of nuclear reproduction have been observed: indirect, or
nuclear segmentation, direct (Flemming), or nuclear fission, and
endogenous nuclear formation.

1. Nuclear Segmentation. Mitosis (Flemming). Karyokinesis
(Schleicher). The phenomena which occur during this process
are very complicated ; nevertheless they conform to certain laws
which are wonderfully constant in both plants and animals.

180 THE CELL

The main feature of the process consists in this, that the various
chemical substances (vide p. 40), which are present in the resting
nucleus, undergo a definite change of position, and the nuclear
membrane being dissolved, enter into closer union with the proto-
plasmic substance. During this process the constant arrangement
of the nuclein becomes especially apparent; and, indeed, the
changes, which occur in this substance, have been most carefully
and successfully observed, whereas we are still very much in the
dark concerning what takes place in the remaining nuclear sub-
stances.

The whole mass of nuclein in the nucleus becomes transformed
during division into fine thread-like segments, the number of
which remains constant for each species of animal. These seg-
ments are generally curved, and vary in form and size according
to the individual species of plant or animal; they may appear as

loops, hooks, or rodlets, or if they are. very small,.as granules.
Waldeyer (VI. 76) proposed the common name of chromosomes for
all these various forms of nuclein segments. As a rule I shall
employ the more convenient name of nuclear segments, which
applies equally to them all, whilst, at the same time, the expres-
sion indicates the most important part of the process of indirect
division, which consists chiefly in this, that the nuclein breaks up
into segments. Similarly the term nuclear segmentation appears
to me to be preferable to the longer and less significant expression
of indirect nuclear division, or the terms mitosis and karyokinesis,
which are incomprehensible to the uninitiated.

During the course of division each nuclear segment divides
longitudinally into two daughter segments, which for a time lie
parallel to one another, and are closely connected. Next, these
daughter segments separate into two groups, dividing themselves
equally between the two danghter-cells, where they form the
foundation of the vesicular daughter nuclei. :

The following phenomena ave also characteristic of the ‘process
of nuclear segmentation: (1) the appearance of the two so-called
pole corpuscles (centrosomes), which function as central points,
around which all the cell constituents arrange themselves; (2) the
formation of the so-called nuclear spindle; and (3) the develop-
ment of the protoplasmic radiation figures around the centrosomes.

As regards the two centrosomes, they make their appearance
’ in the vesicular nucleus at an early stage, before the membrane
/ has been dissolved, being situated in that portion of the proto-

THE VITAL PHENOMENA OF THE CELL 181

plasm which is directly in contact with the membrane. At this
period they are close to one another, and are in the form of two
extremely small spherules. They are composed of a substance
which is only stained with difficulty, and which is, perhaps, de-
rived from the substance of the nucleolus. These spherules are
the pole or central corpuscles (corpuscules, poles, centrosomes),
which have been already described. Gradually they separate
from one another, describing a semicircle round the upper sur-
face of the nucleus, until they take up their position at opposite
ends of the nuclear diameter.

The nuclear spindle develops itself between the centrosomes.
It consists of a large number of very delicate fibrils, which are
parallel to one another, and which are probably derived from the
linin framework of the resting nucleus. These fibrils diverge
somewhat at their centres, and converge at their ends towards the
centrosomes, in consequence of which the bundle assumes more
or less the shape of a spindle. At first, when the centrosomes
are just commencing to separate, the spindle is so small, that it
can only be made out with difficulty, as a band connecting them
together. However, as the centrosomes separate from one
another, the spindle increases in size, and becomes more clearly
defined.

The protoplasm also commences to arrange itself around the
poles of this nuclear figure as though attracted by them. Thus
an appearance, similar to that seen at the ends of a magnet,
which has been dipped in iron filings, is produced. The proto-
plasm forms itself into a large number of delicate fibrils, which
group themselves radially around the centrosome as a middle
point or centre of attraction. At first they are short and confined
to the immediate neighbourhood of the attraction centre. How-
ever, during the course of the process of division they increase in
length, until finally they extend throughout the whole length of
the cell. This arrangement of the protoplasm around the pole is
variously described as the plasmic radiation, radiated figure, star,
sun, etc., in consequence of its resemblance to the rays of light,
attraction spheres, etc.

These are briefly the various elements out of which the nuclear
division figures are built up. The centrosomes, the spindle,
and the two plasmic radiations have been grouped together by
Flemming under the name of the achromatin portion of the dividing
nuclear figure, in contradistinction to the various appearances

182 THE CELL

which are produced by the re-arrangement of the nuclein, and
which constitute the chromatin portion of the figure.

All the individual constituent portions of the division-figure as
a whole vary according to fixed laws, by grouping their elements
in various ways during the course of the process of division.

For the sake. of convenience it is well to distinguish four
different phases, which succeed each other in regular sequence.

During the first stage the resting nucleus undergoes changes
preparatory to division, resulting in the formation of the nuclear
segments and the nuclear centrosomes, whilst at the same time
the spindle commences to develop. During the second stage the
nuclear segments, after the nuclear membrane has become dis-
solved, arrange themselves into a regular figure, midway between
the two poles, at the equator of the spindle. During the third the
daughter-segments, into which during one of the former stages the
mother-segments have divided by longitudinal fission, separate
_ into two groups, which travel in opposite directions from the
equator until they reach the neighbourhood of the centrosomes.
During the fourth stage reconstruction takes place, vesicular
resting daughter nuclei being formed out of the two groups of
daughter-segments, whilst the cell body divides into two daughter-
cells. In the next few sections a more minute description will be
given of the process of cell division as it occurs in some individual
cases, and finally a special section will be devoted to the discussion
in detail of certain disputed points.

The most convenient, and at the same time the commonest, sub-
jects for examination in the animal kingdom are the tissue cells of
young larve of Salamandra maculata, of Triton, the spermatozoa
of mature animals, the segmentation spheres of small transparent
eggs, especially of Nematodes (Ascaris megalocephala), and of
Echinoderms (Tocopneustes lividus). Amongst plants the proto-
plasm of the endosperm of the embryo sac, especially of Fritil-
laria imperialis,and the developing pollen cells of Liliacesx, are
especially to be recommended.

a. Cell. division, as it occurs in the Salamandra
maculata, as an example of the division of the sperm-
mother-cell.

First Stage. Preparation of the Nucleus for Division.

In the Salamandra maculuta certain preliminary changes occur

in the resting nucleus some time before division actually com-

THE VITAL PHENOMENA OF THE CELL 183

mences. The nuclein granules, which are distributed all over the
linin framework (Fig. 75 A), collect together at certain places and
arrange themselves into delicate spiral threads, which are covered

Fie. 75.—A Resting nucleus of a sperm-mother-cell of Salameandra maculata (after
Flemming, Pl. 23, Fig. 1; from Hatschek), B Nucleus of a sperm-mother-cell of Sala-
mandra maculata. Coil stage. The nuclear threads are already commencing to split
longitudinally (diagrammatic, after Flemming, Pl. 26, Fig. 1; from Hatschek).

with small indentations and swellings. From these, innumerable
most delicate fibrils branch off at right angles; these fibrils, which
consist of strands of the linin framework, only become visible as
the nuclein withdraws itself from their surface. Later on the
nuclein threads become still more clearly defined, and, as the in-
dentations and swellings disappear, develop a perfectly smooth sur-
face (Fig. 75 B). Now since they
surround the nuclear space on
every side, they produce an ap-
pearance described by Flemming
as the coil figure (spirem, skein).
The coil is much more dense in
the epithelial cells of Salamandra
than in sperm cells, whilst at the
same time the threads are much
finer and longer (Fig. 76).

It is as yet undecided, whether
at the outset the coil consists of a
single long thread or of several
such threads. I agree with Rabl %
(VI. 53) that the latter is more Fra. 76.—Nucleus of an epithelial cell
probable. atthe commencement of division; from

A striking difference is now seen Setamontey are, ine cfm
in the way the various nuclear still present. (After Flemming.)

184 THE CELL

constituents absorb staining solutions, compared to that observed
in former stages. The more distinctly and sharply defined the
threads grow, the more strongly stained do they become, and the
more energetically do they retain the colouring matter, whereas
the network of the resting nucleus exhibits these properties to a
much less degree. This may be especially well demonstrated if
Graham’s method of staining be employed, for whilst the resting
nuclei are completely decolourised, those that are preparing to
divide, or are actually undergoing the process, are so strongly
stained that they cannot fail to attract the attention of the
observer.

During the first stage of coil formation the nucleoli are still
present ; however, they gradually diminish in size, until after a
short time no trace of them can be seen. Up till now it has not
been determined with certainty what is formed from them.

Whilst the coil is developing, careful observation reveals a small
spot on the surface of the nucleus. This becomes more and more
distinctly defined as the process progresses: it has been designated
by Rabl the polar area (Fig. 77). The opposite surface of the
nucleus is the anti-polar area. The
nuclein threads become gradually
more and more distinct, and ar-
range themselves so as to point

ST SIA
ZAR NWS

Fie. 77.—Diagrammatic representa-
tion of a nucleus with a polar area, in
which the two centrosomes and the
spindle are developing. (After Flem-
ming, Pl. 39, Fig. 37.)

towards these two areas.

Starting from the anti-polar
region they collect in the neigh-
bourhood of the polar area. ‘ Here
they bend round upon themselves
in a loop-like fashion, and then
return, by means of several small,
irregular indented loops, to the
neighbourhood of their starting
point.” Later on the threads be-

come shorter and correspondingly thicker ; they are less twisted,
and cling less closely together, so that the whole skein looks much
looser. In the meantime their arrangement in loops gradually
grows more and more distinct. In favourable cases it has been
ascertained that there are twenty-four such loops or nuclear
segments ; this number is constant for the tissue cells and sperm-
mother-cells of Salamandra and Triton.
Meanwhile the two centrosomes and the spindle— most im-

THE VITAL PHENOMENA OF THE CELL 185

portant portions of the nuclear figure—have developed in the
polar area. However, on account of the difficulty in staining
them, and their minute size and extreme delicacy, these appearances
are not easily made out at this stage; further, they may be more
or less concealed by granules, which collect in the protoplasm in
their neighbourhood. According to Flemming and Hermann, two
centrosomes may be made out in successful preparations. These
are situated very close together, and have probably been formed
by the division of an originally single centrosome. Between them
the connecting fibrils, which later on develop into the spindle, can
be seen.

Second Stage of Division.

The second stage may be said to date from the time when the
nuclear membrane grows indistinct and dissolves. The nuclear
sap then distributes itself evenly throughout the cell body, whilst
the nuclear segments come to lie freely in the middle of the pro-
toplasm (Fig. 78). The two
centrosomes, which are now
further apart from one an-
other, are situated near
them. The spindle increases
proportionately in size and
distinctness, and is seen to
consist of a number of most
delicate fibrils, stretching
continuously from one cen-
trosome to the other, as is

2 b]
clearly shown in Hermann’s Frc. 78.—Nucleus of a sperm-mother-cell of

preparation represented in Salamandra maculata preparatory to division.
The spindle is situated between the two centro-

Fig. 78. The centrosomes of gomes. (After Hermann (VI. 20), Pl. 31, Fig. 7.)

the nuclear figure commence
at this stage to exercise an influence upon the surrounding proto-
plasm. Around each centrosome as centre, innumerable proto-
plasmic fibrils group themselves radially, stretching out principally
towards that region where the nuclear segments are situated, and
appearing to adhere to their surface. From now on, the spindle
commences to increase rapidly in size until it has attained the
dimensions seen in Fig. 79.

Meanwhile the chromatin figure becomes markedly altered (Fig.
79). The nuclear segments have grown considerably shorter and

186 THE CELL

thicker, and are grouped-around the spindle in the form of a com-
plete ring, the arrangement being that described by Flemming as
the mother-star. The loop-like shape
of the segments is now most clearly
defined. They are invariably so ar-
ranged that the angle of the loop is
directed towards the axis of the
spindle, whilst its arms point towards
the surface of the cell. All of the
twenty-four loops lie pretty accu-
rately in the same plane, which, since
it bisects the spindle at right angles,
iis Wak Reeth eatin ating. is called the equatorial plane; it is

sentation of the segmentation ofthe identical with the plane of division
nucleus (after Flemming). Stage
in which the nuclear segments are
airanged in the equator of the from either of the poles the chromatin
spindle,

which develops later. When seen

figure has ‘““the shape of a star whose
rays are formed of the arms of the V-shaped loops, and whose
centre is traversed by the bundle of achromatin fibrils which
compose the nuclear spindle.” This point of view is the most
convenient one for counting the nuclear segments, and for de-
termining their number to be twenty-four.

Another most important process occurs during the second stage.
If the nuclear segment of a well-preserved preparation be. ex-
amined with a high power of the microscope, it will be seen that
each mother segment is cleft longitudinally, and is thus split up
into two parallel daughter segments, which lie close together.
Now since no sign of this longitudinal division could be seen in
the original nuclear network, it follows that it must have occurred
after karyokinesis had commenced. Generally the longitudinal
cleft may be first seen when the nuclear threads have arranged
themselves in the form of a coil (Fig. 75 B), but it is always
completed during the second stage (mother-star), when it is most
clearly defined. This was first observed by Flemming (VI. 12,
13), in Salamandra; and his statements have been corroborated
by v. Beneden (VI. 4a), Heuser (VI. 39), Guignard (VI. 23),
Rabl (VI. 53), and many others, who made observations upon the
same and other objects. This longitudinal splitting appears to
occur invariably in indirect nuclear division, and is of the greatest
importance for the comprehension of the process, as will be
shown later on, when the subject is discussed theoretically.

THE VITAL PHENOMENA OF THE CELL 187

Third Stage of Division.

The third stage is characterised by the division of the single
group of mother-segments in the equatorial plane into two groups
of daughter-segments, which retreat in opposite directions from
one another, until they are situated in the neighbourhood of the
two poles of the nuclear figure (Fig. 80 A, B, C). The two

Fra. 80.—Diagrammatic representation of nuclear segmentation (after Flemming). The
daughter-segments are retreating in two groups towards the poles. (From Hatschek.)

daughter-stars are formed, as Flemming expresses it, from the
mother-star. The details of the process, which can only be ob-
served with difficulty, are as follows :—

The daughter-segments, which have been produced by the
splitting of a mother-seyment, separate from one another at the
angle of the loop, which is directed towards the spindle, and com-
mence to retreat towards the poles, whilst for a time the ends of
the arms of the loop remain undivided. Finally these also split
up. From out of the 24 original loops two groups, each contain-
ing 24 daughter-loops, have developed; these move towards the
centrosomes, until they come quite close to them, when they
stop, for they never actually reach the poles themselves. Be-
tween these two groups fine “connecting fibrils” stretch ; these
are probably derived from the spindle fibrils.

Each loop, or daughter-segment, has “its angle directed towards
the pole, whilst its free ends are turned either obliquely, or per-
pendicularly, to the equatorial plane.” As might be expected, to
start with, they are much thinner than the mother-segments ;
however, they soon begin to shorten and to become proportion-
ately thicker. When the daughter-star is first formed, the
segments lie somewhat far apart, but they soon begin to draw

188 THE CELL

more closely together, so that it becomes very difficult to count
them and to trace their further development ; in fact, it can only
be accomplished in exceptional cases.

Fourth Stage of Division.

During this stage each group of daughter-segments becomes
gradually re-transformed into a vesicular resting nucleus (Fig. 81).
The threads draw still more closely to-
gether, become more bent and thicker ;
their surfaces grow rough and jagged, and
small processes become developed exter-
nally upon them, whilst a delicate nuclear
membrane develops around the whole
group. The radiated appearance around
the centrosomes gradually grows less and
less distinct, until it soon quite disappears.
Finally, also, the centrosomes and the
spindle fibrils can no longer be distin-
guished. It has not yet been decided what
they develop into. In fact, their origin and

Fie. 81.— Diagrammatic : :
meivegeninsion OF amclbak their disappearance are equally shrouded

segmentation (after Flem- in mystery. Near to the place where the

ming). The resting nucleus ; i
geen Saag ay centrosome was situated a depression may

self up out of the danghter. be seen in the newly forming daughter
cet (From Hat- nucleus. Rabl considers it to be the above-
described polar area of the nucleus which
1s seen preparatory to division, and is of opinion that the centro-
some has ensconced itself within it, being enclosed in the proto-
plasm of the cell-body. The nucleus gradually swells up more
and more through the absorption of nuclear sap, and becomes
globular in form, whilst the framework of the resting nucleus,
with its irregularly distributed nuclein granules of various sizes,
18 reconstructed. Further, one or more nucleoli haye made their
appearance in the framework during the process of reconstruction
but as yet no one has succeeded in discovering their origin.

When, at the commencement of the fourth stage, the two
daughter-stars are separated as far as possible from ee another
and have taken the preliminary steps towards becoming eee
formed into the resting daughter nuclei, the cell-body itself begins
to, divide. The radiations at the centrosomes have sou ae
tained their greatest size. At this period a small furrow becomes

THE VITAL PHENOMENA OF THE CELL 189

visible on the surface of the cell-body, corresponding to a plane,
which passes perpendicularly through the centre of the nuclear
axis, uniting the two centrosomes; this has already been
referred to as the plane of division. “The furrow commences on
one side, and gradually extends itself round the equator; how-
ever, it remains somewhat deeper on the side where it commenced
than on the opposite one” (Flemming). This ring-like constric-
tion gradually cuts more and more deeply into the cell body, until
finally it divides it completely into two nearly equal parts, each
of which contains a daughter nucleus, undergoing the process of
reconstruction. As soon as division is complete, the polar radia-
tions commence to fade away.

The above-mentioned connecting fibrils between the daughter
nuclei may be distinguished, in many objects, until division is
completed. They are then severed in their centres by the cutting
through of the cell-body. Sometimes a number of spherical
swellings, which become intensely stained, may be seen at this
time to develop at the centres of the spindle fibrils; these Flem-
ming (VI. 13”) has named separation bodies, and he considers
that they probably represent the equatorial plates of plants,
which are much better developed.

b. Division of the egg-cells of Ascaris megalocephala and
Toxopneustes lividus. ‘The nuclei of the eggs of Ascaris are re-
markable for the size and distinctness of their centrosomes, and
for the small number of their nuclear segments, of which in one
species only four, and in another only two, are present. Another
very important phenomenon, the multi-
plication of the centrosomes by division,
may be especially clearly seen in this
object. It is best to commence our in-
vestigations at that point when the egg
has just developed the furrow, and when
the four nuclear loops on either side of
the plane of division have transformed
themselves into a vesicular nucleus of
irregular outline (Fig. 82). The side of
the nucleus, which is directed towards
the pole, has several ragged processes,

Fie. 82. — Egg of Ascaris
megalocephala undergoing the
the nuclein being spread out upon its process of double division.
loose network. The centrosome may stil] Nuclei are resting; the cen-

2 - trosomes as yet undivided.
be distinguished in the neighbourhood of (after Boveri, Pl. IV., Fig. 74.)

190 THE CELL

what was formerly the pole of the division figure; it is enclosed
in granular protoplasm, which contracts with the yolk substance
of the egg, and has been named by van Beneden the attraction
sphere, and by Boveri the archoplasm. me
Before the nucleus has quite returned to the resting condition,
and even sometimes before the first division is completed, it com-
mences to make preparations to divide a second time; these start
with changes in the centrosome (Fig. 84), which extends itself

Fig. 33. Fie. 84.

Fie. 83.—Dividing egg of Ascaris megalocephala. The nuclei are preparing to divide;
the centrosomes are divided. (After Boveri, Pl. IV., Figs. 75, 76.)

Fig. 84—Two daughter-niuclei with lobulated processes commencing to reconstruct
themselves. The centrosomes are multiplying by self-division. (After van Beneden «nd
Neyt, Pl. VL, Fig. 13.)
longitudinally parallel to the first division plane, becomes biscuit-
shaped, and divides itself by a constriction into two daughter
centrosomes, which for a time are enclosed by one common granu-
lar sphere; these phenomena were discovered by van Beneden.
(VI. 4b) and Boveri (VI. 6, 1888). Next, the two centrosomes
separate somewhat from one another (Fig. 83), in consequence of
which their common radiation sphere becomes converted into two
spheres.

This division of the centrosomes gives the signal, as it were,
for the occurrence of the following changes in the nucleus,
although the latter is not yet completely at rest (Fig. 83). The
nuclein withdraws itself out of the framework, and collects in
four long loops, the surfaces of which are at first uneven, but
later on become smooth. The four loops are turned in the same
direction as the daughter-segments after the first division, so that
Boveri (IV. 6) agrees with the opinion expressed by Rabl (VI. 53),
that they are derived directly from the substance of the seg-
ments, and that even when the nucleus is resting they have an

THE VITAL PHENOMENA OF THE CELL 191

independent individuality. The angles of the loop are turned
towards the original pole (the polar area in the Salamandra),
whilst the ends of the loop, which are knob-like and swollen, are
directed towards the region of the anti-pole.

The second stage of division now commences. The centro-
somes, with their spheres, separate and travel for some distance,
until their common axis lies either somewhat obliquely or parallel
to the first division plane. The nuclear membrane dissolves.
The four segments arrange themselves in the equator between the
two centrosomes in the manner described above, whilst a dis-
tinct radiation develops around the centrosomes in the proto-
plasm, so that the appearance, seen from the pole, resembles that
depicted in Fig. 85 A. The four segments then split longitudinally

I.
c “Ir

Fie. 85.—A Four mother-segments seen from the pole of the nuclear figure (after van
Beneden and Neyt, Pl. VI., Fig. 16). B Longitudinal splitting of the four mother-seg-
ments into eight daughter-segments (after van Beneden and Neyt, Pl. VI., Fig. 17).
—that is to say, the third stage commences (Fig. 85 B). The
daughter segments thus formed separate from one another, and
travel towards opposite poles. E. van Beneden (VI. 4b) and Boveri
(VI. 6) consider that the spindle fibrils play an active part in this
process. In their opinion, the spindle in Ascaris is composed of
two independent portions, each of which consists of a large
number of protoplasmic fibrils. These
converge towards the ceatrosome and
attach their ends to it, whilst the op-
posite ends diverge, approach the nuclear
loops, and fasten themselves at various
points to the daughter-segments, which
are turned towards them. These threads
by gradually contracting, and thus_be-

coming shortened. cause, in van Beneden’s Fig. 86.—The construction
of the spindle out of two half-
spindles, the fibrils of which
the four daughter-segments, which are have attached themselves to
thus gradually drawn towards the cen- ‘he daughter-segments. (After

van Beneden and Nept, Pl.
trosomes. VL., Fig. 8.)

and Boveri’s opinion, the separation of

192 THE CELL

During the fourth stage the cell-body divides, and the daughter-
nucleus becomes built up again. This, according to van Beneden,
takes place in the following manner (Fig. 87) : the four chromatin

C

Fie. 87.—A A group of four daughter-segments seen from the pole, the swellings at the
ends, forming the loops, are especially well marked (after van Beneden and Neyt, Pl. VL,
Fig. 19). B Reconstruction of the nucleus from the four daughter-segments, diagrammatic
(from van Beneden and Neyt, Pl. VI., Fig. 20). C Resting condition of the nucleus, seen
from the pole (from van Beneden and Neyt, Pl. VI., Fig. 21).
loops (A) absorb fluid, which becomes nuclear sap, out of the
protoplasm; they become saturated with it, as.a sponge with
water, and thus swell up into thick vesicular bodies (B). The
nuclein divides up into granules, which are connected together by
delicate threads, which are situated chiefly upon the surfaces of
these vesicles. The inner surfaces of these latter come close
together and fuse. Thus a vesicular nucleus, irregular in shape,
and saturated with nuclear sap, is formed; it is separated from the
protoplasm by a membrane, and contains a delicate framework,
upon which the chromatin substance is distributed.

The eggs of Ascaris afford us special advantages for the study
of centrosomes and nuclear segments, but-the small eggs of
Lichinoderms (Hertwig VI. 30a;
Fol VI. 19a) are also of great use,
particularly for observing radia-
tion phenomena in the protoplasm
of the living cell. More will be
said about this later on.

In the egg-cell of a living
Lichinoderm, a few minutes after
fertilisation (Fig. 88), the small
globular cleavage-nucleus is seen
to be situated in the centre of the
yolk; it looks like a clear vesicle,

Fie. 88.—Egg of a Sea-urchin just
after fertilisation has been completed a
(from O. Hertwig, Embryology, Fig. 20). ° and 1s surrounded by rays of proto-
ase ses hag sperm nucleus are plasm, : like a sun with rays of
use orm the cleavage nucleus (fk) ; : Pee . ,
which occupies the centre of a proto- light. This radiation 18 80 dis-

plasmic radiation, tinct in this object during life,

THE VITAL PHENOMENA OF THE CELL 193

as the large number of small granules, which are situated in
the yolk, are arranged in rows, passively following the arrange-
ment of the protoplasm. After a short time this radiated appear-
ance, which is the result of the processes which occur during
fertilisation, begins to fade, and to become metamorphosed into
two radiated systems, which are found at opposite points of the
nucleus. These are small at first, but become momentarily larger
and more distinct, until finally they extend all over the whole
yolk-sphere, dividing it up into two radiated masses, each arranged
around its own attractive centre (Fig. 89).

A small homogeneous spot can be
distinguished in the middle of each
radiation from the very beginning;
this spot adheres closely to the nuclear
surface, and is free from granules. It
contains the centrosome, which, how-
ever, cannot be distinguished at all in
the living object.

As the radiations become more dis-
tinct and more spread out, the collec- —
tions of homogeneous non-granular Fie. 89.—Egg of a Sea-urchin
protoplasm im the neighbourhood of Prepeting to divide; taken from

F the living object (from O. Hert-
the centrosomes become larger, whilst wig, Embryology, Fig. 27). The
at the same time they gradually re- nucleus is invisible, the dumb-
treat farther and farther apart, carry- Gil tien aca arcane ae
ing the poles with them. At this period the nucleus loses its
vesicular properties, and assumes the spindle structure which has
been described in other objects, but which, on account of its
minuteness, cannot be distinguished here during life. In conse-
quence, the very characteristic dumb-bell appearance, depicted in
Fig. 89, develops in the granular yolk. The two collections of
homogeneous protoplasm, enclosing the poles of the division figure,
form the heads of the dumb-bell; the non-granular connecting
portion indicates the place where, during the preceding stages,
the now invisible nucleus was situated. This has been replaced
by the spindle, the ends of which extend right up to the centro-
somes. The granular yolk mass is arranged in two radial
systems around this homogeneous dumb-bell figure. ‘These sys-
tems have been named amphiaster, or double star, by Fol.

The egg, which at the outset was perfectly round, now com-
mences to extend itself longitudinally in the direction of the axis

Oo

194 THE CELL

of the dumb-bell, and quickly enters the last stage of division
(Fig.90 A). A ring-like furrow corresponding to a plane, which

Fie. 90.—Egg of a Sea-urchin when division is just taking place (from O. Hertwig, Embryo-
logy, Fig. 29). A A circular furrow cuts into the yolk and divides it ina plane which is
perpendicular to the centre of the nuclear axis and to the long axis of the dumb-bell. B
Egg of a Sea-urchin after division has taken place. In each of the division products a
vesicular danghter nucleus has been formed. The radial arrangement of the protoplasm
is commencing to become indistinct. Both figures are drawn from the living object.
might be carried through the dumb-bell at right angles to its
longitudinal axis, develops upon the surface of the egg. This
rapidly penetrates more and more deeply into the egg-substance,
quickly dividing it into two equal portions, each of which contains
half of the spindle with a group of daughter segments, that is to
say half of the dumb-bell, and a radial system of protoplasm.

When the division in two is nearly completed, the two portions
of the egg are in contact at a small portion only of their surfaces,
at the middle of the handle of the dumb-bell. When, however,
cleavage is quite finished, the whole of their division surfaces come
closely into contact with one another, so that they flatten each
other into nearly hemispherical bodies (Fig. 90 B).

Meanwhile the nucleus has become visible in the living object.
Somewhere near the place where the head and the handle of the
dumb-bell merge, that is to say, at some little distance from the
centrosome, a few small vacuoles make their appearance, being
caused by the saturation of the daughter nuclear segments with
nuclear sap. After a short time these fuse together to form a
globular vesicle, the daughter nucleus (Fig. 90 B). The radiated
arrangement of the protoplasm grows gradually less distinct, and

makes way, if the cell prepares to divide a second time, for a new
donble radiation.

THE VITAL PHENOMENA OF THE CELL 195

For examination with reagents, and especially for studying
chromatin figures, the eggs of Hchinoderms are not so suitable as
those of Ascaris. The loop-like nuclear segments are especially
small and numerous in them, so that even with the strongest
powers they only look like small granules. Fig. 91 represents a
spindle, which has been treated with reagents and staining solu-
tions; it corresponds somewhat to Fig. 89, where the living egg
is depicted, and may therefore be considered to complete it.

The process of segmentation may take a fairly long time in very
large eggs, such as Frogs’ eggs, where a considerable amount of

=,

ae ae

—-s oe, “3 oh
AL EAN

Sn pr ll aN
AS fifth -
Ca ies.
afi

Fig. 91. Fie. 92.

Fig. 91.—Nuclear figure of an egg of Strongylocentrotus, one hour and twenty minutes
after fertilisation. Reagents have been used.

Fig. 92.—A portion of the upper hemisphere of an egy of Rana temporaria a quarter
of an hour after the appearance of the first furrow, when the coronal radiation is most
sharply and plainly defined. (After Max Schultze, Pl. I., Fig. 2.)

yolk has to be divided. Consequently a second process of division
may commence before the first is completed. In Frogs’ eggs an
interesting appearance may be observed, which has been described
under the name of the coronal furrow (VI. 68) (Fig. 92). This
first furrow commences to appear on a small area of the black
pigmented hemisphere of the egg, which is directed upwards; as
it penetrates into the substance, it increases in length, and, during
the course of half an hour, extends itself round the whole peri-
phery of the globe, appearing last upon the bright surface, which
is turned downwards. At this place it penetrates less deeply into
the yolk. When it first appears, it is not smooth in appearance,

196 THE CELL

but is seen—most distinctly at that period when it has extended
itself around one third of the circumference of the egg—to be pro-
vided with a large number of small grooves, which open into it on
both sides for the most part at right angles (60-100 on either side,
Fig. 92). Thus a very pretty picture is produced, like a long deep
valley in the mountains, with a large number of shorter, narrower
valleys opening into it on either side. As the process of division
progresses, and the main furrow deepens, the side furrows diminish
in number, and finally quite disappear.

The appearance of this peculiar and clearly marked coronal fur-
row is a phenomenon which is connected with the contraction of
the protoplasm during cleavage.

c. Division of Plant Cells. The protoplasmic coating of
the wall of, the embryo-sac of Fritillaria impertalis affords an in-
structive illustration of the great uniformity of the process of
nuclear division as it occurs in plants and animals. This, as well
as the embryo-sacs of other Liliacee, is particularly suitable for the
study of nuclear figures, for the layer of protoplasm is extremely
thin, and, if examined at the right time, is seen to contain a large
number of nuclei at various stages of division (Strasburger VI.
71-73; Guignard VI. 23).

The large resting nucleus contains a linin framework with small
meshes (Fig. 93°A), upon the surface of which a large number of
small nuclein granules are pretty evenly distributed. In the
majority of cases nucleoli are present. These vary in size, and
lie between the meshes of the framework, to which they are
attached. Strasburger is of opinion that, when the nucleus is
preparing to divide, the whole framework becomes transformed
into a few fairly thick threads, which are much twisted; he de-
scribes in them a diagonal striation (c) similar to that observed by
Balbiani (II. 3) in the nuclei of Chironomus larve (Fig. 27). He
accounts for this. striation by the statement, that each thread is
composed of numerous discs of nuclein arranged one after the
other, and separated by their partition walls of linin.

In the course of time, as the process advances, the nuclear mem-_
brane dissolves, and the nucleoli break up into smaller granules and
disappear, whilst the nuclein threads grow shorter and thicker, and
produce twenty-four nuclear segments ; a typical spindle composed
of a large number of most delicate fibrils develops, in the centre of
which the nuclear segments arrange themselves in a circle (Fig.
93 D). Guignard has lately demonstrated the presence of two

THE VITAL PHENOMENA OF THE CELL 197

centrosomes with their radiation spheres situated at either end of
the spindle.

Fig. 93.—Fritillavia imperialis. A resting nucleus and other nuclei at various stages of
division, taken from the free protoplasmic lining of the wall of the erabryo-sac depicted in
Fig. 128 (after Strasburger, Practical Botany, Fig. 191). A A resting nucleus; B a coil of
thick threads, as yet unsegmented; C a portion of a nuclear thread, more highly magni-
fied ; Da nuclear spindle, with segments longitudinally split ; E the separation and change
of position of the daughter-segments. A, B, D, Ex 800; Cx1100.

When the process of division has reached its highest point, the
nuclear segments split longitudinally. The daughter segments
then travel towards the two poles, twenty-four on each side (F),
and thus form the foundation for the daughter nuclei, which
develop in a manner similar to that described as occurring in
Salamandra maculata. As soon as the daughter nuclei become
vesicular, several nucleoli appear in them.

Up to this point the resemblance shown by the process to that
seen in animal nuclear division has been complete; however, now,
at the end of the whole process, a peculiar and interesting devia-

198 THE CELL

tion is shown in the formation of the so-called cell plate. In order
to study this phenomenon, it is better to watch the process of
division as it occurs in pollen mother-cells, and in various other
objects, rather than to study the embryo-sac of Fritillaria, which
up till now has formed the basis of our description; for in the latter
nuclear division is not immediately followed by cell division.

The following description refers to pollen mother-cells of
Fritillaria persica (Fig. 94). After the daughter-segments have

Fig 94.—Three stages in the division of the pollen mother-cells of Fritillaria persica
(after Strasburger, Fig. 114, Eng. Edition) : f separation of the daughter segments ; g for-
mation of daughter coils and of the cell-plate; h position of the nuclear segments in the
daughter nuclei and in the developed partition wall. (x 800.)

separated into two groups, delicate connecting fibrils are seen to
be stretched between them; these, according to Strasburger
(VI. 73), are derived from the central portions of the spindle fibrils
(Fig. 94). After a time, in the middle of the connecting fibrils,
small swellings, which look like glistening granules, are formed
(Fig. 94g). They are most regularly arranged, so that they are
seen in optical section to lie close to one another in a row. Thus
collectively they form a disc, composed of granules, and situated in
the division plane between the two daughter-nuclei; this disc has
been called the cell plate by Strasburger. Flemming (VI. 18")
considers, that these are represented in a rudimentary form in
animal cells in the above-mentioned (p. 189) central granules
which are found in a few objects. The cell plate is of the qeskhnes
importance in plants, in connection with the formation of the
cellulose partition wall, which is the final stage in the whole
process of division (Fig. 94h). ‘The cell plate,” as described by
Strasburger, “ ultimately extends over the whole diameter of the
cell, its elements fusing together ‘to form a partition wall, which
divides the mother-cell into two daughter-cells.” A thin layer of
cellulose may soon be distinguished. Meanwhile the connecting

THE VITAL PHENOMENA OF THE CELL 199

fibrils disappear, first around the daughter-nuclei, and then also
in the neighbourhood of the cellulose partition wall.

The minute, definite particles, which collect as granules in the
middle of the connecting fibrils, and form a cell plate, may be
designated as cell-wall formers, in accordance with the above-
mentioned conception, which will be entered into at more detail
later on.

d. Historical remarks and unsolved problems concern-
ing nuclear segmentation.—In the commencement of the year
1870, in consequence of the labours of Biitschli (WII. 6), Stras-
burger (VI. 71), Hertwig (VI. 30a), and Fol (VI. 19a), the
changes experienced by the nucleus during division were
described on the whole correctly, although somewhat vaguely.
The fibrillous nuclear spindle, the collection of shining granules,
which is stained with carmine, in its centre (Strasburger’s nuclear
plate), the subsequent division of the granules into two groups, or
two daughter nuclear plates, and the development of the vesicular
daughter nuclei from these latter, had all been discovered by then.
Further, the radiation figures—stars, or amphiaster (Fol)—at the
ends of the spindle were known, and Fol and myself had already
described the presence of more strongly glistening granules, the
centrosomes, in them; diagrams had been made of them, and
their functioning as attraction centres had been pointed out.
Further it had been satisfactorily established that during cell-
division the nucleus did not become dissolved (karyolysis,
Auerbach, VI. 2a), but became metamorphosed. Further,
through my investigations on mature eggs, especially on those of
Asteracanihion and Nephelis, and in consequence of the discovery
of the internal phenomena which occur during fertilisation, I
showed, at the same time, that the nucleus is not a new develop-
_ ment in the egg, but that it is derived from definite portions of the
germinal vesicle, which united themselves with the male pro-
nucleus, derived from the head of the spermatozoon (the altered
nucleus of the sperm cell), to form the division nucleus. As a
result, the important proposition was formulated that all nuclei
may be traced back in an unbroken line of descent from the
nucleus of the egg-cell, just as all cells of the animal organism
are derived from a fertilised egg-cell (Omnis nucleus e nucleo.
Flemming VI.).

The theory of nuclear and cell division, which was founded in.
consequence of the above-mentioned investigations, has been

200 THE CELL

proved subsequently to be right in the main, whilst at the same
time it has formed a good foundation for many further discoveries,
and has suggested a number of problems, which have not yet been
definitely solved. These problems may be expressed in a single
sentence: it was necessary, and to a certain extent is still
necessary, to follow more closely in every detail the movements
which, during nuclear division, and during the formation of the
characteristic figures, take place in the individual micro-chemical
particles of substance, which can be distinguished in the nucleus
and in the division figures ; that is to say, to trace the rearrange-
ments which occur in the nuclein granules, the linin framework,
the spindle fibrils, the centrosomes, and the nucleoli, etc. The
discovery of suitable objects for examination, such as the nuclei of
tissue cells of Salamander larve (Flemming), and the eggs -of
Ascaris megalocephala (van Beneden), as well as the use of the
newer oil immersion and apochromatic lenses, and the improve-
ment in the manipulation of reagents and staining solutions, have
rendered progress in this direction possible.

The greatest advance has at present been made in the investiga-
tion of the figures produced by the changes of place of the nuclein,
thanks in the main to the excellent experiments of Flemming
(VI. 12-17), and the supplementary investigations of van Beneden
(VI. 4), Rabl (VI. 53), Boveri (VI. 6), Strasburger (VI. 71-73),
and Guignard (VI. 23).

Flemming, who has made his observations chiefly upon tissue
cells of Salumander larve, distinguishes clearly between the
achromatin and chromatin portions of the nuclear figure, that is to
say, the unstainable spindle fibrils and plasmic radiations, and the
stainable nuclear loops, or segments, which rest upon their sur-
faces. He was the first to make the important discovery that
these latter split longitudinally. The explanation of these in-
teresting phenomena was afforded by the discoveries of Henser,
Guignard, van Beneden, and Rabl, who all observed independent] y;
on different objects, that the halves of the divided segments
(chromosomes) separate, and move towards the nuclear poles,
forming the foundation for the daughter-nuclei.

The changes of position of those substances, which are connected
with the development of the spindle and the centrosomes, and
with the disappearance of the nucleoli, have been much less ac-
curately investigated.

As concerns the spindle, very various Opinions are held, both as

THE VITAL PHENOMENA OF THE CELL 201

to its construction and origin. Whilst the first observers considered
that the spindle consisted of most delicate fibrils, which stretched
continuously from pole to pole, van Beneden (VI. 46) and Boveri
(VI. 6) are of opinion that these fibrils are broken at the equator,
and that, in consequence, the spindle is
composed of two separate and distinct
half-spindles (Fig. 95). They contend that
the half-spindles are attached directly
with the ends of their fibrils to the nu-
clear segments, and in consequence are
of mechanical use in nuclear division, in
that they shorten or contract like muscle Wie ge <= Conateaction oe
fibres after the segments have divided the spindle out of two half-
into daughter-segments, and thus draw Sue eae ERG

the daughter-segments, which are at- segments. (From van Bene-
den and Neyt, Pl. VI., Fig. ®.)

tached to them, in opposite directions.

On the other hand, Flemming (VI. 14) for the tissue cells of
Salamandra, and Strasburger (VI. 72) for plants, still adhere to
their old theory, that spindle fibrils, stretching uninterruptedly
from pole to pole, do exist. The observations made by Hermann,
which have been already mentioned, are especially convincing
concerning the undivided condition of the spindle; they call
to mind my description and representation of the formation of
the spindle in the germinal vesicle of Asteracanthion (VI. 30a,
Pl. VIII, Figs. 3, 4). In both cases a very small, undivided
spindle may be observed between the poles, which are situated
near to one another (Fig. 96), at that period when the nuclear
segments are a good way
off, and so cannot hide it
at all; it is seen to grow
gradually, as its fibrils in-
crease in length, until it
reaches its full size.

The explanation of this
discrepancy, as has_ been
suggested by Hermann, is
that the structure described
by van Beneden and Boveri
as the half-spindle is some- Fig. 96.—Nucleus of a sperm-mother-cell of
thing quite different from Salamandra maculata preparing to divide. Posi-

; 2 tion of the spindle between the two centrosomes.
the spindle of the earlier (after Hermann, Pl. XXXI., Fig. 7.)

202 / THE CELL

observers. The half-spindles, described by van Beneden and
Boveri, consist of a portion of the protoplasmic radiation figure
proceeding from the poles, namely, all those fibrils which are
situated in the equator around the nuclear segments. The true
spindle lies in the centre of these protoplasmic fibrils and nuclear
segments. Hermann, to distinguish it from van Beneden’s
spindle, has given it the name of central spindle. The prefix
“central,” however, appears to me to be quite superfluous ; for
one thing, it is better to decide to limit the name of spindle once
for all to this portion of the nuclear figure, and to give, if
necessary, some other name to the protoplasmic polar rays, which
are connected with the nuclear segments, and which are described
by van Beneden and Boveri as half-spindles ; indeed, the name
spindle is not suitable to them.

Another moot point is the derivation of the spindle fibrils.
Many investigators are inclined to trace them back to that
protoplasm, which forced its way in between the nuclein threads
when the nuclear membrane was dissolved (Strasburger VI. 72;
Hermann VI. 29, etc.). I have already advocated, and am still
inclined to hold the view, that, with the exception of the polar
radiations, which belong to the protoplasmic body of the cell, the
various structural portions of the nuclear figure are derived from
the various substances in the resting nucleus. I consider that the
substance of the spindle and of the connecting fibrils is derived
from the linin framework. This view is supported also by Flem-
ming, and to some extent by the micro-chemical investigations of
Zacharias. However, the most important facts in its favour
appear to me to be the following :—

In many unicellular organisms the nuclei, during certain stages
of division, remain separated from the protoplasm by a delicate
membrane; this occurs in Huglypha (Schewiakoff VI. 65b), and in
the nuclear divisions of Ciliata and Actinospheria (Rich. Hertwig,
VI. 82, 83). Under these conditions there can be no doubt but
wren: Les fod eae mene aa from the achromatin portion
tk ae ee ses are occasionally met with in
lirhoth, va Pol e rae . Insome molluses (Pterotrachea, Phyl-

é), as Fo (VI. 19a) and I myself (VI. 30a) have observed, the
polar spindle, as long as the nuclear membrane remains, is situated
- the interior of the germinal vesicle (Fig. 97 A, B), which, in
siete if bce size.. The assumption that, under these cir-

, plasm has made its way into the nuclear space

THE VITAL PHENOMENA OF THE CELL 203

from the exterior, appears to me, at the least, forced. Further, in
my opinion, it can no longer be doubted that the connecting

A

Fie. 97.—A A germinal vesicle, in which a spindle is developing, taken from a newly
laid egg of Phyllirhoé. Acetic acid preparation (Hertwig, Pl. XI., Fig. 2). B Germinal
vesicle from a freshly laid egg of Phyllirhoé, in which the spindle is seen in optical section.
Acetic acid preparation (Hertwig, Pl. XI., Fig. 6).

threads, which, in the dividing sperm-mother-cells of Ascaris, ex-
tend between the separating nuclear segments, are derived from
the linin framework. I was not able to observe a typical spindle
development in this object.

Another point under discussion is the origin of the centrosomes.
These were first described and depicted at the commencement
of the year 1870, but they were only brought into prominence as
a distinct component part of the nuclear division figure by van
Beneden (VI. 4a), when he succeeded in differentiating them
clearly from their environment by means of a staining solution of
aniline dyes dissolved in 33 per cent. glycerine solution. Soon
afterwards both van Beneden and Boveri made simultaneously and
independently of each other (VI. 4b, 6) the important discovery,
that centrosomes multiply by self-division ; later on I was able to
verify this statement for the sperm cells of Ascaris (VI. 34). Van
Beneden came to the following conclusion as a result of his
observations: that the centrosomes, like nuclei, are permanent
organs of the cell, and must therefore always occur in the proto-
plasm as independent forms. This view was supported to a
certain extent by the discoveries of Flemming (VI. 17), Solger
(VI. 70), and Heidenhain (II. 16), who stated that in many
kinds of cells, such as lymph corpuscles and pigment cells, a
centrosome with a radiation sphere may be demonstrated in the
protoplasm, even when the nucleus, which is frequently situated
some little distance off, is completely at rest. (See p. 56, Figs.
34-36.)

204 THE CELL

Our knowledge of the centrosomes was as early as 1834
much advanced by the study of the processes of fertilisation. I
expressed the opinion (VI. 85) that during fertilisation a cen-
trosome was introduced into the egg with the spermatozoon, and
that to all appearance it was really the so-called middle portion, or
neck, which functions as the attraction centre in the protoplasmic
radiation preceding the sperm nucleus. I compared this to “the
small quantity of substance present at the end of the nuclear
spindle (the polar substance and the centrosome), which, although
only stained with difficulty, can yet be distinguished from the
protoplasm,” and hence I came to the conclusion that if the com-
parison is correct, the radiations of the protoplasm, which occur
during fertilisation and cell-division, have a common cause in the
presence of one and the same substance.

Richard Hertwig (VI. 84) repeatedly pointed out that the polar
substance, the middle portion of the spermatozoon, and the sub-
stance of the true nucleoli are similar in composition. Boveri
(VI. 7) was of opinion that the spermatozoon carried a pole
corpuscle or centrosome with it into the egg. The question was
definitely decided by Fol (VII. 14) and Guignard (VI. 23b), whose
important discoveries will be described later on. According to
them the nucleus of the egg, as well as that of the spermatozoon,
has a centrosome of its own. Whilst the nuclei coalesce, each
centrosome splits up into two parts; half of the one then unites
with one half of the other, and thus the two new centrosomes,
which are situated at the ends of the division spindle, are formed.

In spite of this discovery, one problem still remains unsolved.
Are the centrosomes to be regarded as permanent cell organs
of the protoplasm, and if so, are they contained in it during rest,
only coming into correlation with the nucleus during division; or
are they to be regarded as special elementary portions of the
nucleus, such as the nuclear segments, spindle threads, nucleoli,
etc.? In the latter case they must be enclosed during rest in the
nucleus itself, and only come into relation with the protoplasm
during division.

The material for observation, which we have at present, does not
suffice for the solution of this question. It is extremely difficult
to follow the movements of the centrosomic substance during and
after nuclear division as closely as we can observe those of the
nuclear substance, for the centrosomes are so excessively small ;
and further, it is not always possible to be sure of rendering them

THE VITAL PHENOMENA OF THE CELL 205

visible under all circumstances by means of certain definite stain-
ing solutions. During division they are chiefly recognised by
means of their radiation figures, but these are not seen during rest.

Several data seem to point to the conclusion that the centro-
somes originate in the nucleus; firstly, with a few exceptions,
nothing corresponding to a centrosome can be found in the proto-
plasm during rest; secondly, at the commencement of division,
the centrosome is seen to be in immediate contact with the surface
of the nuclear membrane (Fig. 98), and only later on to move
further away from the nucleus into the
protoplasm; thirdly, subsequent to this
appearance of the centrosome, the nuclear
membrane frequently collapses, just as if
nuclear sap had exuded through a small

aperture; and fourthly, in many objects

the appearance of the centrosome is simul-
taneous with the disintegration of the nu-
cleoli.

I have frequently occupied myself with
this question of the origin of the centro-
somes, and have expended in vain a great

Fie. 98.— Nucleus of a
sperm-mother-cell of Ascaris
megaloceyhala bivalens. The
nuclein substance is ar-
ranged in threads which
are separated from one
another in two groups.
Appearance of the centro-

somes. Breaking up of the

deal of energy upon it. Latterly, during ancleoiua, (Pl IIL, Fig. 7.)

my experiments upon the construction of

the eggs and spermatozoa of Nematodes, I have again gone into
the subject, but have been unable to arrive at any definite con-
clusions. However, although at the present time the majority of
investigators consider that they belong to the protoplasm, yet a
certain amount of importance must be attached to the opposite
view, namely, that they have a nuclear origin.

Finally, another point, which is as yet unexplained, is the fate
of the nucleoli, which disappear at the commencement of nuclear
division,and reappear in the daughter nuclei. What interchanges
of substanees can have occurred in this process? There are
exceptional difficulties in the way of the solution of this question,
since in many cases the nucleoli are composed of two chemically
different substances (vide p. 51).

It appears probable to me that if we disregard the above-
mentioned connection with the centrosomes, the nucleoli, during
the preparation for division, become split up into small portions,
and become distributed upon the nuclear segments.

In sperm-mother-cells of Ascuris, which have been hardened

206 THE CELL

with Flemming’s weak solution, the nuclein loses its power of
becoming stained, whilst the nucleoli become stained dark red in

Fie. 99.—A Nucleoli, with granules, which are dissolving (PI. III., Fig. 4). B Nucleus
of a sperm-mother-cell of Ascaris megalocephala bivalens from the end of the growth zone.
Preserved in Flemming’s weak solution of chromo-osmic acid. Stained with acid fuchsine
(Pl. IIL, Fig. 5). OC Nucleus of a sperm-mother-cell of Ascaris meg locephala bival
from the middle of the division zone. Preserved in Flemming’s weak solution of chromo-
osmic acid. Stained with acid fuchsine (PI. III., Fig. 9).

acid fuchsine (Fig. 99 A,B). By this means I was able to observe
that during the preparatory stages the nucleolus breaks up into
several pieces, that small portions of these dissolve off, and that
similar particles, stained a deep red, are deposited upon the nuclear
threads. Later on, when the nuclear segments are fully formed,
and the nucleolus has quite disappeared (Fig. 99 ©), the centro-
somes become visible upon the surface of the nucleus, and more-
over, each nuclear segment is seen to enclose a dark red granule,
which reacts towards staining solutions like the substance of the
nucleolus.

Several interesting reactions with staining solutions seem to
point to the fact that the nucleolar substance is taken up into the
nuclear segments, although probably in an extremely finely divided
state. As Wendt has discovered by his experiments on plants,
the nuclein framework of the nucleus from the embryo sac of any
one of several species of the Liliacee is stained blue green when
treated with fuchsine iodine-green, whilst the nucleoli are coloured
red. On the other hand, during the division stages, when the
nucleoli are dissolved, the nuclear segments are stained violet.
Further, later on, after the nucleoli have reappeared in the
daughter nuclei, the nuclear threads are again stained bluish green.
Wendt explains this varying reaction towards staining solutions
by assuming that during division the nuclear segments absorb the
nucleolar substance, and give it up again after division, so that
the nucleoli may be found in the daughter nuc'ei.

THE VITAL PHENOMENA OF THE CELL 207

Flemming (VI. 13, 1891) and Hermann, by means of double
staining with safranin-hematoxylin, safranin-mauvine, safranin-
gentian, etc, have obtained a similar alteration of staining re-
actions in auimal cells, varying according to the condition of the
nucleoli. “It appears to me important,” says Flemming on this
occasion, “that in those stages when nucleoli are still present, or
have only just disappeared, or have just reappeared, the chromatin
figure inclines towards a blue coloration, whereas in those cases
where the nucleoli are quite disintegrated the figures are distinctly
safranophil, just like the nucleoli.”

2. Direct Nuclear Division. (Direct nuclear multiplication,
fragmentation, amitosis, amitotic division.)

As a contrast to the complicated processes connected with seg-
mentation, nuclear division may take place apparently in a very
simple manner. This is called fragmentation, or direct nuclear
division, and is seen in a few kinds of cells. Under these cir-
cumstances spindle threads, nuclear segments, and protoplasmic
radiations are not seen. The division of the nucleus appears
rather to proceed in a manner resembling that described by the
earlier histologists. It can be most easily observed in the lymph
corpuscles, both when alive, and when fixed by means of reagents.

There are various ways in which good preparations may be
made: a drop of lymph may be drawn up from the dorsal lymph
sac of a. Frog into a fine capillary tube, and then placed upon a
slide and covered with a cover-glass, the edges of which should
be smeared with paraffin, in order to protect the preparation from
evaporation. Or asmall glass chamber may be prepared accord-
ing to Ziegler’s method, by fastening together by their four corners,
or by two of their sides, two extra thin cover-glasses, so that there
is a capillary space between them. The glass chamber is then
placed for one or more days in the dorsal lymph sac of a Frog,
during which time a large number of lymph cells make their way
between the two cover-glasses, where they undergo changes. The
third method, recommended by Arnold, is to place a thin pervious
dise of elder pith in the lymph sac. After a few hours numbers
of leucocytes have attached themselves to its surface, and are thus
available for observation. Later on, thin layers of fibrin, pro-
duced by coagulation, are deposited upon the disc of elder pith;
these may be removed, and, with the cell elements which are
attached to them, may be easily examined.

Ranvier (VI. 54) observed all the phenomena of division take

208 THE CELL |

place in a lymph cell during the course of three hours, the pre-
paration being kept at a temperature varying from 16° to 18°.
Arnold (VI. 1) and others have verified his statements, and have
amplified them in various ways. The vesicular nucleus can change
its form actively, and can cover itself with excrescences and pro- ©
tuberances. Under such circumstances constrictions frequently
oceur, after which the nuclei break up into two, three, or more
pieces (Fig. 100 A, B). The nuclear fragments move apart
from one another, not infrequently remaining joined together for
a considerable time by delicate connecting threads. Cell division
often closely follows nuclear division, as is seen in Figs. 100 A, B.

Fra. 100.—A A migratory cell from a disc of elder pith which has lain for ten days in
the lymph sac of a Frog. When first observed the nucleus was somewhat constricted in
its middle, whilst its ends were bilobed. After five minutes the nuclear division was com-
pleted (after Arnold, Pl, XII., Fig. 1). B Migratory cell during division. Fig. A de-
veloped into Fig. B during the course of thirty minutes (after Arnold, Pl. XII., Fig. 3).

The protoplasmic body also becomes constricted between the
nuclear fragments, which move apart, but are still joined by a
fine thread. The two nuclear fragments move in opposite direc-
tions by means of a large number of ameeboid processes. In
consequence, the connecting bridge between them is sometimes
drawn out to a long fine thread, after the daughter-nuclei have
separated from one another.

“No law can be laid down as to the time when the various
stages of division follow one another during fragmentation; very
frequently nuclei and cells linger in one or other stage’ (Arnold).

THE VITAL PHENOMENA OF THE CELL 209

It is in consequence of this delay in completing the process of
cell division after the nucleus has divided that cells containing
several nuclei are found. Sometimes, during inflammatory pro-
cesses, such cells become so large that they are called giant cells
(Fig. 101); the small nuclei vary considerably both as to form
and arrangement. Sometimes they are
globular vesicles, sometimes oval, sausage-
shaped, or lobulated bodies ; they may occur
singly and evenly distributed throughout
the protoplasm, or they may be arranged
in chains and circles; finally, isolated small
nuclei are occasionally found arranged one
after another in rows. As time goes on,
small cells may become detached from the
giant cells, as has been observed by Arnold.
This may occur in one of two ways. Fic. 101.—A large multi-
‘Sometimes the giant cell protrudes knob- 2ucleated cell, with nucle-
: ae ; = ated cells becoming con-
like processes containing nuclei, which, gtrictea off peripherally.
after having been withdrawn and again (After Arnold, Pl. XIV.,
protruded several times, sooner or later a
become separated ; sometimes they become detached without any
or only very slight movement on the part of the cell.”

Cell division, accompanied by the phenomenon of direct nuclear
division, has been observed in epithelial cells, as well as in lymph
corpuscles ; this occurs with especial frequency in Arthropods.
They have been described by Johnson (VI. 41) and Blochmann
(VI. 86) in the embryonic cells of the Scorpion; by Platner
(VI. 52) in the cells of the Malpighian tubes, and by other in-
vestigators in other objects.

A peculiar method of nuclear constriction has been described
by Goppert (VI. 22), Flemming (VI. 16), von Kostanecki (VI. 46),
and others. The most suitable object for observing it appears to
be the lymphoid tissue on the surface of the liver of Amphibians.
According to Géppert, the nucleus of a lymph cell develops a
funnel-shaped invagination, which grows deeper and deeper until
it reaches the opposite surface of the nuclear membrane, where
it opens to the exterior by a minute aperture (Fig. 102 A, B).
Thus a ring-shaped nucleus, perforated by a narrow canal, is formed.
This ring becomes first constricted, and then cut asunder at a
certain point, whilst at the same time it transforms itself into a
semicircle, which becomes divided by superficial constrictions

p

210 THE CELL

into several portions (Fig. 102 C). As the disintegration pro-
gresses, it may be broken up into a larger number of smaller

A B

Fra. 102,—A Side view of a perforated nucleus from the lymphatic peripheral layer of
the liver of Triton alpestris. The nucleus is flattened in the direction of the perforation
(after Géppert, Pl. XX., Fig. 4). B Perforated nucleus with distinct radial arrangement
of the nuclein framework (after Géppert, Pl. XX., Fig. 4). C Ring-shaped nucleus ot
a lymph cell divided into several portions by constrictions (after Goppert, Pl, XX., Fig. 10).

nuclei, which are sometimes connected for a long time by delicate
connecting bridges. Similar “ perforated nuclei” have been ob-
served in other objects by Flemming (VI. 16); for instance, in the
epithelium of the Frog’s urinary bladder. However, in this case,
division of the cell body does not appear to occur.

Direct nuclear division occurs also occasionally in the vegetable
kingdom. Certain objects, like the long internodal cells of the
Characee, or older cells of more highly organised plants, are most
suitable for observing it; thus Strasburger (II. 41) observed in
the older internodes of Tradescantia more or less irregular nuclei
which are divided into portions of varying size and shape. “If
the indentation is one-sided, the cell nuclei appear kidney-shaped ;
but if they are indented all round, they look biscuit-shaped, or
irregularly lobulated. In many cases the fragments have quite
separated from one another, either still remaining in contact, or
lying at a greater or less distance from one another. These
nuclear fragments may number as many as eight to ten in one
cell.” In Characee the nuclei may temporarily assume the
appearance of a string of pearls in consequence of several con-
strictions having occurred. This appearance passes away when
the fragmentation is completed.

However, even if constrictions of the nucleus are observed, it
cannot be immediately taken for granted that direct division is
commencing, unless this method of multiplication has been already
observed in all its stages in the object in question. Thus in ova
and in sperm-mother-cells, mulberry-shaped or irregularly

i

THE VITAL PHENOMENA OF THE CELL 211.

lobulated nuclei are frequently seen, and yet fragmentation does
not appear to occur in these cases, so that the lobulation must not

A

Fig. 103.—Tradescantia virginica,

Cell nuclei of older internodes undergoing direct

division (after Strasburger, Fig. 193): A from life; B after treatment with acetic-acid-

methyl] green.

be considered to be the commencement of direct division. It is
apparently connected with metabolic processes in the nucleus (cf.

what is said upon the subject in Chapter VIII.).

Nuclear multiplication
by direct division occurs
also amongst Protista ;
it is seen with especial
frequency in the group
of Acinetz, of which the
Podophrya —gemmipara
(Fig. 104), described on
p- 229, is an instructive
example.

5. Endogenous Nuclear
Multiplication, or the
Formation of Multiple
Nuclei.

A third, very different

Fig. 104,— Cell-budding. Podophrya gemmipara
with buds (R. Hertwig, Zoology, Fig. 21): a buds
which are becoming detached and developing into
zoospores b: N nucleus.

912 THE CELL

method of nuclear multiplication, to which I should like to attach
the above name, has been observed by Richard Hertwig (VI. 36y
amongst a group of Radiolarians, the Thalassicollide ; these ob-
servations have been corroborated by Car] Brandt (VI. 8), who
has followed them up in greater detail.

The Thalassicollide, which are the largest in size of all the
Radiolarians, the diameter of their central capsule being nearly as
long as that of the Frog’s egg, possess during the greater part
of their lives one single highly differentiated giant nucleus, the
so-called internal vesicle; this is about } mm. in diameter, and
possesses a thick porous nuclear membrane. It is very similar to
the multinucleated germinal vesicle of a Fish or of an Amphibian.
A large number of variously shaped nuclein bodies, generally
compressed together into a heap in the centre, are present in its
interior (Fig. 105). Amongst these, a
bright central corpuscle (centrosome),
surrounded by a radiation sphere, may
very frequently be seen. This was
observed and depicted by R. Hertwig,
and has recently been more closely in-
vestigated by Brandt. The latter ob-
server was able to follow how, at the
time of reproduction, the centrosome,
which appears to me to correspond
with the body of that name in plant
and animal cells, betakes itself to the
surface of the internal vesicle, drawing
the radiation sphere after it. Here,
after passing through the nuclear
membrane, it enters into the surround-
ing protoplasm of the central capsule ;
however, as yet nothing has been re-

Wig, 30h Wg an gateek te ported as to its further fate.
section through a great vesicular About this time a large number of
meray Aas esti ie small nuclei make their appearance
with funicular internal bodies Outside of the internal vesicle, being
bargepen omc which radiate. situated in the protoplasm of the cen-

mon point, (R. Hert- 3 ons
wig, Pl. V., Fig. 7.) tral capsule, which originally was quite
free from nuclei; these function as
centres around which nucleated zoospores develop, whose number
finally may amount to some hundreds of thousands. Meanwhile,

THE VITAL PHENOMENA OF THE CELL 213

the internal vesicle begins to shrink up and loses its nuclei,
which pass into the protoplasm outside. Finally it is quite dis-
solved. Brandt has observed that this nuclear multiplication
varies according to whether isospores or anisospores are formed.
From the whole process R. Hertwig and Brandt draw the
following conclusion, which is certainly correct: that the nuclei
which function in the formation of zoospores, and which occur in
the central capsule, at first but sparsely, but which gradually
increase in number, are derived from the substance of the internal
vesicle (nuclear corpuscles). ‘This explanation,’ remarks R.
Hertwig, “leads me to adopt a theory of nuclear multiplication
which differs fundamentally from the generally accepted one, and
which is not supported by any observations which up till now have
been made in animal or vegetable histology. For if we try to
explain this process histologically, we must conclude not only that
nuclei can multiply by division or budding, but that they may
be produced by the nuclear substance of a nucleus multiplying
itself by division, the portions thus produced making their way
into the protoplasm to which they belong, and there developing
into independent nuclei. Hence such a cell containing many
nucleoli may be regarded as potentially multinuclear, just asa
multinucleated cell may be regarded as potentially multicellular ;
and thus the gradual transition between individual cells, and the
groups of cells which are derived from them by division, is by
these intermediate stages rendered easier than it would otherwise

be 9

The extraordinary phenomena of nuclear multiplication, observed by Fol
(VI. 20), Sabatier, Davidoff (VI. 87), and others, in rather young immature eggs
of Ascidians, and which have been shown to be connected with the develop-
ment of follicle cells, may be mentioned here. Compare also the similar
processes observed by Schafer (VI. 65a) in young mammals.

III. Various Methods of Cell Multiplication.
1. General Laws.

In addition to the process mentioned in the last section under
the names of nuclear segmentation, direct nuclear division, and
endogenous nuclear formation, cell multiplication may assume
very various appearances according to the way in which the
protoplasmic body behaves during division. Before classifying
the various kinds of cell multiplication, it is necessary to mention

OT4 ; THE CELL

certain general relationships which exist between the nucleus ,
and the protoplasm, and to which I have drawn attention in my
paper upon the influence exerted by gravitation upon cell
division (VI. 31).

In the resting cell the nucleus may occupy various positions ;
it may also change its place, as, for instance, in plant cells, where
it may be carried along by the protoplasmic stream. However,
under certain conditions, of .which only those connected with cell
division will be entered into here, whilst others will be mentioned

‘ Jater on in Chapter VIII., the nucleus occupies a definite constant
position in relation to the protoplasmic body.

Certain interactions take place between the protoplasm and the
nucleus during division, similar to those which (to use a familiar
illustration) exist between iron filings and a magnet suspended
loosely over them. The magnetic influence polarises the iron
filings, causing them to group themselves radially about the poles.
On the other hand, the whole mass of the polarised particles of
iron has a directing influence upon the position of the magnet.
These metastatic reactions between protoplasm and nucleus re-
ceive their evident expression in the appearance of the pole centres
and the radiation figures, which have been already described.
The result of the reaction is that the nucleus always endeavours |
to occupy the centre of the reaction sphere. |

No objects are more suitable for demonstrating this than animal
ova, which may vary considerably as regards size, shape, and in-
ternal organisation.

In most small ova, in which protoplasm and yolk substance are
more or less evenly distributed, the nucleus, before fertilisation
(Fig. 106 4), does not occupy any definite position. On the other
hand, when, after fertilisation, it commences to be active and to
divide (Fig. 106 B), it places itself exactly in the geometrical
median point, that is to say, if the egg is spherical in the centre,
or if it is oval (Fig. 110) in the point of intersection of the two
longitudinal axes. The nucleus surrounded by a radiation sphere
may be seen to travel through the protoplasm to this point.

Variations from the normal are seen when the protoplasm and
yolk granules, of which the latter, as a rule, have the greater
specific gravity, are unevenly distributed in the egg cavity. Very
frequently the eggs undergo a polar differentiation, which is partly
Produced directly by gravity, the various substances being sepa-
rated out according to the weights, and partly by other processes

THE VITAL PHENOMENA OF THE CELL 215

such as are brought about by the fertilisation and the maturation
of the ova.

Fig. 106.—A Mature Egg of an Echinoderm, containing in its yolk a very small nucleus
(ek) (O. Hertwig, Embryol., Fig. 14). B Egg of a Sea-urchin, immediately after the close
of fertilisation. Female pro-nucleus and male pro-nucleus have united to form the
cleavage nucleus (fk), which occupies the centre of a protoplasmic radiation.

Polar differentiation consists in this, that the lighter protoplasm
collects at one pole, and the heavier yolk substance at the other.
They may be more or less sharply separated from one another.
For instance, sections through the eggs of Amphibians do not show
any striking separation, the only thing being, that in the one half
the yolk plates are smaller, and are separated from each other by

kb k,sch

Fig. 108.
Fie. 107.

Fie. 107.—Diagram of an Egg with the nutritive yolk in a polar position (O. Hertwig,
Embryol., Fig. 3). The formative yolk constitutes at the animal pole(A, P) a germ disc
(k, sch), in which the germinal vesicle (kb) is enclosed. The nutritive yolk (nd) fills the
rest of the egg up to the vegetative pole (V, P).

Fic. 1¢8.—Egg-cell (yolk) of the Hen, taken from the ovary (O. Hertwig, Embryol.,
Fig. 6 A): k, sch germinal disc; kb germinal vesicle; g, d yellow yolk; w,d white yolk;
d, h vitelline membrane.

216 THE CELL

a larger amount of protoplasm than in the other half, where they
are larger and more closely packed together.

In other cases a small portion of protoplasm, more or less free
from yolk, has separated itself from the yolk-containing portion of
the egg, and, as in birds and reptiles (Fig. 108 k, sch), has assumed
the form of a disc.

The two poles in an egg are distinguished from one another by
the names animal and vegetative ; at the former most of the proto-
plasm collects, and at the latter most of the yolk substance ; hence
the former has a smaller specific gravity than the latter. In
consequence, eggs in which polar differentiation has occurred must
always endeavour to attain a certain position of equilibrium.
Thus, whilst in small cells, in which the substance is equally
divided, the centre of gravity coincides with the centre of the
sphere, the result being that the eggs can readily take up different
positions, in eggs, on the other hand, in which polar differentia-
tion has taken place, the centre of gravity has become eccentric,
having approached the vegetative pole to a greater or less degree.
Hence the egg so arranges itself in space that the animal pole is
directed upwards, and the vegetative downwards. A line joining
the two poles, the egg-axis, must, if the egg is allowed to move
freely, assume a perpendicular position.

Frogs’ eggs and Hens’ eggs furnish us with useful examples of
this. In the Frog’s egg (Fig. 115) the unequal portions can be
clearly distinguished externally, since the animal part is pigmented
and of a deep black colour, whereas the vegetative is whitish
yellow in appearance. If such an egg is placed in water after
fertilisation has occurred, in a few seconds it takes up a position
of equilibrium, the dark side being always turned upwards, and
the specifically heavier light side downwards.

Similarly, in whatever way a Hen’s egg (Fig. 108) may be
turned about, the germinal disc (k, sch) will be seen to occupy the
highest point in the yolk sphere, for the latter rotates in its
albuminous sheath with every movement, keeping its vegetative
pole always directed downwards.

Polar differentiation occurs both in oval and spherical eggs.
The egg of the worm Fabricia (Fig. 109) may serve as an example.
Here, at the one end more protoplasm is seen, at the other more
yolk substance.

In eggs with polar differentiation it is useless to look for the
cleavage nucleus in the place where it is seen in eggs poor in yolk.

THE VITAL PHENOMENA OF THE CELL DEF

However, this is only an apparent exception to the law already
mentioned, for reflection shows that the nucleus, in seeking to
occupy the centre of its sphere of action,
only affords an example which confirms the
law. Interactions take place between the
- nucleus and the protoplasm, not between
it and the yolk-substance, for the latter
during all the processes of division behaves
like an inert mass. Thus the unequal dis-
tribution of the protoplasm must, in con-
sequence of the above law, affect the position
of the nucleus, forcing it to make its way

: Fie. 109.—Egg from Fab-
to those places where the protoplasm is ‘icia (after Haeckel): A

chiefly collected, that is to say, away from mal portion; V vegeta-

tiv rtion.
the centre of gravity. The nearer the ive portion

latter approaches the vegetative pole, the nearer the cleavage
nucleus approaches the animal pole.

Actual examination shows the truth of this statement. In the
Frog’s egg (Fig. 115), the cleavage nucleus is somewhat above
the equatorial plane of the sphere in the animal half, whilst in
eggs, where the protoplasm is more sharply differentiated as a
germinal disc from the yolk (Fig. 108), the cleavage nuclens has
risen quite close to the animal pole, and has taken up a position
inside the germinal disc itself (Reptiles, Birds, Fishes, etc.).
Similarly in the egg of Fabrivia (Fig. 109), the cleavage nucleus
has been pushed towards that portion of the oval body which is
rich in protoplasm.

Further, the reaction between protoplasm and nucleus, affect-
ing the position of the latter, becomes more marked from the
moment when the poles develop. Thus the second general law
may be stated here, that the two poles of the division figure
come to lie in the direction of the greatest mass of protoplasm,
somewhat in the same way as the poles of a magnet are in-
fluenced as to their position by the iron filings in their neigh-
bourhood.

According to the second law, in a spherical egg, for instance, in
which protoplasm and yolk are evenly distributed, the axis of the
centrally laid nuclear spindle may coincide with the direction of
any radius whatever; whereas, on the contrary, in an oval proto-
plasmic body it can only coincide with the longest diameter. In
a circular protoplasmic disc the spindle axis is parallel to the

218 THE CELL

surface in any of the diameters, but in an oval disc it is parallel
only to the longest diameter.

The phenomena observed during cell division, and especially
during the formation of the furrows, are almost without exception
in accordance with these laws. ‘T'wo facts, however, are especially
confirmatory of the truth of the second law; one was discovered
by Auerbach, through his experiments on the eggs of Ascaris
nigrovenosa and Strongylus auricularis. (VI. 2), and the other by
Pfliger.

The eggs of both the Nematodes investigated by Auerbach are
oval in shape (Fig. 110), so that two poles can be distinguished in

D

Fig. 110.—Eggs of Ascaris nigrovenosa, in four different stages of fertilisation. (After
Auerbach, Pl. IV., Figs. 8-11.)

them, and these two poles play different roles during fertilisation.
At the one at which the germinal substance of the egg is situated,
the pole cells are formed, and the female pro-nucleus develops,
whilst at the other pole, which faces the mouth of the uterus, the
spermatozoon enters, and fructification occurs; further, the male
pro-nucleus makes its appearance here (vde Chap. VII.).

Whilst gradually increasing in size, both pro-nuclei approach
each other, travelling in a straight line, which coincides with the
axis of the egg; finally, after having grown into two vesicles of
considerable size, they meet in the centre of the axis; they then
come into such close contact that their contingent surfaces become
flattened (Fig. 110 A).

As a rule, during the conjugation of the sexual nuclei, the axis
of the spindle, which develops out of them, and at the ends of
which the centrosomes are situated, lies somewhere in the

THE VITAL PHENOMENA OF THE CELL 219

plane of the contingent surfaces, that is to say, in the so-called
conjugation plane. If this were to occur here, the spindle axis,
contrary to the above-mentioned law, would cut the longitudinal
axis at right angles, the centrosomes would be placed in the
neighbourhood of the least amount of protoplasm, and finally, the
first division plane would have to divide the egg longitudinally.

A proceeding so contrary to law does not occur here, for the
protoplasm and nucleus, whilst reacting on each other, subse-
quently regulate their finally assumed positions, which are in
accordance with the conditions present. The original position of
the conjugating pair of nuclei, which is brought about by the pro-
cess of fertilisation, and which is quite unsuitable for the purposes
of division, becomes changed, whilst the two poles become more
clearly defined. The nuclear pair commence to turn themselves
through a right angle (Fig. 110 B), until the conjugation plane co-
incides with the longitudinal axis of the egg (Fig. 110 C).

‘Sometimes they rotate in the same direction as the hands of a
watch, sometimes in the opposite direction” (Auerbach).

In consequence of this interesting phenomenon of rotation, the
two poles of the division figure come to be in the neighbourhood
of the largest accumulation of protoplasm, in accordance with the
law, whilst the smallest amount is situated near the division plane,
which develops later (Fig. 110 D).

A second instance of the truth of this law is afforded by the
experiments of Pfliiger (VI. 49, 50) upon Frogs’ eggs. He care-
fully compressed a freshly-fertilised egg between two vertical
parallel glass plates, thus giving to it pretty nearly the form of
“a much-flattened ellipsoid, of which the longest axis is horizontal,
the one of medium length vertical, and the shortest again horizon-
tal and perpendicular to the longest.” In nearly every case the
first division plane was vertical to the surface of the compressed
plate, and at the same time perpendicular. Hence the nuclear
spindle must again in this case, in accordance with the above-
mentioned law, have placed itself in the direction of the longest
diameter of the ellipsoid.

From this law, that the position of the nuclear axis in division
is determined by the differentiation and form of the surrounding
protoplasmic body, so that the poles place themselves in the
direction of the greatest collection of protoplasm, we can deduce
a third law, which Sachs (VI. 64) arrived at from a study of plant
anatomy, and has described as the law of rectangular intersection

220 THE CELL

of the dividing surfaces in bipartition. For, having once learnt
the causes which determine the position of the spindle axes, we
ean know beforehand how the division plates must lie, in order to
intersect the spindle axes at right angles.

As a general rule, unless the mother-cell is exceptionally long
in any one direction, it happens that in each division that axis
of the daughter-cell, which lies in the same direction as the
chief axis of the mother-cell did, has become the shortest. Hence
the axis of the second division spindle would never in such a case ~
place itself in the direction of the preceding division spindle, but
rather at right angles to it, according to the form of the proto-
plasmic body. In consequence, the second division plane must
intersect the first at right angles.

Generally, the consecutive division surfaces of a mother-cell
(which becomes split up into 2, 4, 8, and more daughter-cells by
successive bipartitions) lie in the three directions of space, and so
are more or less perpendicular to each other.

This is often very plainly to be seen in plant tissues, because
here firm cell-walls, corresponding to the division planes of the
cells, rapidly develop, and thus, so to speak, fix the places to a
certain degree permanently. But in animal cells, which in the
absence of a firm membrane frequently change their form during
the processes of division, this is not the case; in addition the
position of the cells to one another may change. “Fractures and
displacements” of the original portions into which the mother-
cell splits up occur, examples of which are afforded us by the
study of the furrowing of any egg cell. This is entered into more
fully on p. 224.

In botany, these three directions of space are designated as
tangential or periclinal, transverse or anticlinal, and radial (Figs.
111, 112). Periclinal or tangential walls are parallel to the
surface of the stem. Anticlinal or transverse walls intersect the
periclinal walls, and at the same time the axis of growth of the
stem at right angles. Finally radial walls, whilst being also at
right angles to the periclinal ones, lie in the same plane as the
axis of growth of the stem.

In order to render this clear by an example, we will select a
somewhat difficult object, namely, the growing-point of a shoot.
Sachs demonstrates the truth of his law with reference to this
object in the following sentences which are taken from his lectures
on plant physiology (II. 33) :—

THE VITAL PHENOMENA OF THE CELL 221

“‘ Suitably prepared longitudinal and transverse sections of the
growing-points of roots and shoots show characteristic cell-wall
networks and cell arrangements, which agree with the type, even
in the most various plants. This depends essentially upon the
fact that the embryonic substance of the growing-point, as it
increases in volume on every side and at all parts, becomes divided
up into compartments or chambers by cell-walls, which intersect
one another at right angles. The longitudinal section of a growing-
point always shows a system of periclinal walls, intersected by
anticlinal walls, which in their turn represent the right-angled
trajectories of the former. If only the growing-points of flat
structures be considered, then there will be only two systems of
cell-walls present ; if, however, the growing-point is hemispherical
or conical, or of some other similar shape, that is to say not flat, but
forming a solid mass, a third system of cell-walls must be taken
into account; namely, the longitudinal walls, which stretch out in
a radial direction from the longitudinal axis of the growing-point.”

“Tt will facilitate a clear comprehension of the subject, if
before proceeding farther we examine a diagram, which has been
constructed arbitrarily, although according to fixed laws, and

XxX
A A

E Ee
P P Pp PP Ue 4 -;

Fig. 111.—Diagram of the cell arrangement at a growing-point. (After Sachs, Fig. 28!.)

for this purpose it will be well to consider as a starting-point a
median longitudinal section through the growing-point (Fig. 111).
Confining our attention, therefore, to our figure, of which the out-
line L E represents the longitudinal section through a conical
growing-point—which resembles fairly closely those met with in
nature—it will be seen that it has the form of a parabola and

999 THE CELL

that the space occupied by the embryonic substance is partitioned
out, so that anticlinal and periclinal walls intersect at right angles.
This being granted, the network of cells in Fig. 111 may be con-
structed according to a well-known geometrical law. Let a 2 re-
present the axis, and y y the direction of the parameter, then all
the periclines, denoted by P p, form a group of confocal parabolas.
Similarly, all the anticlines, 4A a, form another group of confocal
parabolas, whose focus and axis coincide with those of the pre-
ceding group, but which run in the opposite direction. Two such
systems cut one another everywhere at right angles.

“Let us now observe whether a median longitudinal section made
through a dome-shaped, and approximately parabolic growing-
point, does not present an arrangement of cells which corresponds
in all essentials with our geometric diagram. We see at once, if we
examine such a section, made from the growing-point of a Larch
for example (Fig. 112), that the internal structure is identical, if

1

oy* Se
CHINTZ ND
on

ee. Ce
SP ise
57) f

Fie. 112.—Longitudinal section through the growing-point of a winter bud of Abies

pectinata (x about 200) (after Sachs, Fig. 285): S a ex of i i
aye cee » Fig ) pex of growing point; b b youngest

we disregard the two protuberances, b b, which interfere somewhat
with the symmetry of the figure. These are young leaf-rudiments,
budding off from the growing-point. We recognise at once the
two systems of anticlines and periclines, which it can scarcely be
doubted cut each other at right angles, as in the diagram; that is
to say, the anticlines are the right-angled trajectories of the peri-
clines. As in the diagram, further, only a few periclines under
the apex S run round the common focus of all the parabolas; the
others, which come from below, only reach the aviphbourteas of

THE VITAL PHENOMENA OF THE CELL O25

the focus; that is to say, the corresponding cell divisions only
occur after the periclines below the centre of curvature have
become sufficiently far apart from one another for it to be neces-
sary for new periclines to intercalate themselves between them;
and the same is true of the anticlines. It is easy to see in the
diagram (Fig. 111), that the curvatures of the construction lines
are especially sharp around the common focus of all the anticlines
and periclines.”

“ Hundreds of median longitudinal sections, through the growing-
points of roots and shoots, have been made by various observers,
before the fundamental principle was at all understood, and all
of these correspond with the construction which I have given, and
thus prove its accuracy.”

Finally, in order to understand certain variations from normal
cell division, a fourth law must be mentioned, which has been
formulated by Balfour (VI. 3) in the following words: “ The
rapidity with which a cell divides is proportional to the concentra-
tion of the protoplasm it contains. Cells rich in protoplasm
divide more quickly than those which are poor in protoplasm and
rich in yolk.” This Jaw is explained by the fact that, in the
process of division, it is the protoplasm alone which is active, the
yolk substance stored up in it being passive, and, so to speak,
carried along by the active protoplasm. The greater the amount
of yolk present, the more work is there for the protoplasm in
division ; indeed, in many cases there may be more to do than the
protoplasm can accomplish. This occurs frequently in eggs, in
which polar differentiation has occurred, the greater part of the
protoplasm being concentrated at the animal pole. Then division
is confined to this portion of the cell, the vegetative part being no
longer broken up into cells. Thus an incomplete or partial division
has resulted instead of a complete one. Both extremes are united
in nature by intermediate forms.

2. Review of the Various Modes of Cell Division.
The following classification, upon which I have based my detailed
accounts, may be made of the various methods of cell division.

I. CompeLere or Honopnastic SEGMENTATION.

a, EQuat.
b. UNEQUAL.
c. CELL-Boppina.

224 THE CELL

II. Parrran or Merosiastic SEGMENTATION.
IIL. So-cattep Free CEeLL-ForMATION.
IV. Division witH ReEpDvUcTION.

The most instructive examples of the various methods of cell
division are afforded, for the most part, by animal egg-cells ; for
here the divisions follow so quickly one upon another, that the
normal conditions may be clearly observed.

Ta. EQuaL SEGMENTATION.

In equal division the egg, if, as is generally the case, it is
spherical, is first split up into two hemispheres. According to:
the law explained above, in the division which follows, the nuclear
spindle must place itself parallel to the base of the hemisphere, so
that the latter is divided into two quadrants. Further, the spindle
axis must coincide with the longitudinal axis of each quadrant, so
that in each case a division into two octants is produced. In con-
sequence, during the second and third stages of the cleavage
process, the relative positions occupied by the second and third
division planes towards one another, and towards the first division
plane, are strictly according to law; that is to say, the second
cleavage plane cuts the first at right angles, and halves it, whilst
the third is perpendicular to the two first, and passes through the
centre of the axis in which they intersect. If now the ends of
this axis are considered as the poles of the egg, the two first
division planes may be regarded as meridional, and the third as
equatorial.

In many cases, after the second cleavage, the four portions may
be seen to separate somewhat from one another, the result of
which is that the furrows produced by
the second division no longer intersect
in one point, but meet the first formed
meridional furrow at a little distance
from the pole (Fig. 113). Thus a
transverse line, the cleavage line, which
varies in length, is produced. I have
found this especially well marked (VI.
30b) in the eggs of Sagitta (Fig. 113).

A short time after the termination
Fig. 113.—A four segmented of the second division of the egg of
SS Re ee ge 3 trom ne Sagitta, the four cells so arrange them-

. ; Hertwig,
Pi. V., Fig. 6.) selves (Fig. 113) that only two of them

THE VITAL PHENOMENA OF THE CELL 225

touch each other. At the animal pole they meet in a short trans-
verse furrow, the animal cleavage line. The pointed ends of the
two remaining cells, which do not come in contact with the pole,
meet this line at its extremities. A similar arrangement is seen
at the vegetative pole: here the two cells, which did not touch
the animal pole, meet along a vegetative cleavage line, which is
always in such a position that if both lines were projected upon
a common plane they would intersect at right angles. Here
the four cells, which are obtained by quartering the original
cell, are not of the shape of ordinary quarters of a sphere.
Each has a blunt and a pointed end, the latter being directed
towards the pole of the egg. Each pair of
cells formed from a hemisphere are so ar-
ranged that similar ends point in opposite
directions.

A corresponding arrangement of the first
four cleavage cells has been described by von
Rabl in the eggs of Planorbis, and by von
Rauber (VI. 56) in Frogs’ eggs. The latter
has entered into more details than the former.

Similarly in oval eggs, in which, according
to our law, the first division plane is transverse
to the longitudinal axis, distinct separations
of the cells from each other occur during the
second cleavage, which is vertical to the first. Fie. 114.—An egg of

. Ascaris nigrovenosa with
In consequence, well-marked cleavage lines four segments. (After

appear, as is seen in Fig. 114 in the egg of paar Pl. IV., Fig

Ascaris nigrovenosa.

Ib. Unequat SEGMENTATION.

Unequal division comes naturally after equal. It is most
generally caused by the unequal distribution of the protoplasm
and yolk substance in the cell. The Frog’s egg, in which polar
differentiation has occurred, will serve as an example of this.
There, as has already been stated, the nucleus is situated in the
upper or animal half of the sphere (p. 217). Now when division
is about to occur, the axis can no longer lie in any one of the
radii of the egg, for, in consequence of the unequal division of the
protoplasm in the egg space, it is influenced by that part of
the egg, which is pigmented and rich in protoplasm ; this portion
rests like a skull-cap upon the more transparent deutoplasm-con-

Q

226 THE CELL

taining portion, and, on account of its smaller specific gravity,
floats upwards, and is spread out horizontally (Fig. 115A). The

Fre. 115.—Diagram of the division of the Frog’s egg (O. Hertwig, Embryology, Fig. 31):
A first division stage; B third division stage; the four portions of the second stage
of division are beginning to be divided by an equatorial furrow into eight portions; P
pigmented surface of the egg at the animal pole; pr that part of the egg which is richer
in protoplasm ; d that part of the egg which is richer in deutoplasm,; sp nuclear spindle.

nuclear spindle, however, lies horizontally, in a horizontal dise of
protoplasm; hence the division plane must develop vertically.
At first a small furrow appears at the animal pole, since this
latter is especially influenced by the nuclear spindle which has
approached it, and further because it contains more protoplasm,
in which the movements occurring during division commence.
The furrow slowly deepens, cutting downwards towards the:
vegetative pole. |
The two hemispheres produced by this first, division consist of
an upper portion, rich in protoplasm, and of a lower portion, poor
in protoplasm, By this means, in the first place the position of
the nucleus, and in the second place its axis, are absolutely de-
termined before it commences to divide a second time. The
nucleus is to be looked for, according to the above-mentioned law,
in that quadrant which is richest in protoplasm. The axis of the
spindle must here lie parallel to the longitudinal axis of the
quadrant, that is to say, it must lie horizontally. Hence the
second division plane, like the first, is perpendicular, cutting
the latter at right angles. ‘
At the end of the second cleavage the amphibian égg consists
of four quadrants which are separated from one another by verti-
cal division planes, and which possess two unequal poles, the
upper one being lighter and richer in protoplasm, and the lower
one heavier and richer in yolk substance. In an egg where equal
cleavage occurs, we saw that at the stage of the third division

THE VITAL PHENOMENA OF THE CELL 237

the axes of the nuclear spindles arrange themselves so as to be
parallel to the longitudinal axis of the quadrants. The same
thing occurs here in a somewhat modified form (Fig.115 B). On
account of the greater amount of protoplasm present in the upper
half of each quadrant, the spindle is unable to lie in the centre, as
in an egg in which equal cleavage occurs, but must approach
nearer to the animal pole of the egg. Further, it is exactly per-
pendicular, for, on account of the unequal weight of their halves,
the quadrants of the amphibian egg are firmly fixed in the egg
space. In consequence, the third division plane must now be
horizontal (Fig. 116 A), and further, it must be placed above the

A B

Sa
SAP
22)
rp

Fie. 116.—-Stages in the cleavage of Petromyzon. (From Hatschek, Fig. 72; A, B
after Shipley; C, D, after M. Schultze.)

equator of the sphere of the egg, being situated more or less
towards the animal pole. The portions thus produced are very
dissimilar both in size and constitution, and this is why this form
of cleavage has been called unequal. The four upper portions
are smaller, and poorer in yolk; the four lower much larger,
and richer in yolk. They are called animal and vegetative cells
according to whether they are directed towards the animal or
vegetative pole.

As development proceeds (Fig.116 B,C,D),
the difference between the animal and the
vegetative cells grows greater and greater,
for the more protoplasm a cell contains, the
more quickly and frequently does it divide,
as has been already mentioned above.

Unequal cleavage can also occur in oval
eggs. For instance, the egg of Fabricia
(Fig. 117), as has been already mentioned
(Fig. 109), in consequence of the collection peers eee pane i
of yolk around one pole, divides into one cells. (After Haeckel.)

228 THE CELL

smaller cell, richer in protoplasm, and a larger one, richer in
yolk; in these segmentation proceeds at different rates.

Ic. Cetu-BupvinG.

When one of the portions produced by division is so much
smaller than the other, that it appears as though it were only
a small appendage to the original cell, scarcely causing any
diminution of its substance, the process is called ‘“cell-budding, or
gemmative segmentation,” the smaller portion being called the
bud, and the larger the mother-cell. . Two kinds of cell-budding
are distinguished, according to whether one or more buds are
formed.

In the animal kingdom this process of cell-budding occurs when
the egg is mature, causing the development of the directive cor-
puscles, or polar bodies (polar cells). By this term we understand
two or three small spherules, which are composed of protoplasm
and nuclear substance, and hence are of the same value as small
cells; they are frequently situated at the animal pole of the egg,
within the vitelline membrane. The course of the process of cell-
budding is as follows :—

Whilst the germinal vesicle is becoming broken up, a typical

Fig. 118.—Formation of the polar cells in Asterias glacialis (O. Hertwig, Embryol., Fig.
13), In Fig. I. the polar spindle (sp) has advanced to the surface of the egg. In Fig. dt,
a small protuberance (rk!) has been formed, which receives halfof the spindle. In Fig. III.
the protuberance is constricted off, forming a polar cell (rk!). Out of the remaining half of
the original spindle, a second complete spindle (sp) has developed. In Fig. IV. a second
protuberance has bulged out below the first polar cell, which in Fig. V. becomes constricted

pola cell (i k?) In Fi . V I out of the r
. g. .
off to form the second 01 r e emainder of the spindle the

THE VITAL PHENOMENA OF THE CELL 229

nuclear spindle, with a polar radiation at each end, develops out of
its contents. This changes its position in the yolk (Fig. 118 L.),
raising itself gradually towards the animal pole, until its end
touches the surface. It then arranges itself with its longitudinal
axis in the direction of a radius of the egg. Cell-budding soon
commences at the place where one of the poles of the nuclear
figure touches the surface; the yolk arches itself up to form a
small knob, into which half of the spindle protrudes itself (Fig.
LES hy,

The protuberance then becomes constricted at its base, and,
with half of the spindle, separates itself from the yolk, forming a
very small cell (Fig. 118 IIJ.). Then the whole process repeats
itself (Fig. 118 IV.-VI.), the half of the spindle which has re-
mained in the egg, without previously passing through a resting
vesicular or nuclear condition, developing first into a complete
spindle. This process, as far as it refers to the nuclear spindle,
will be entered into at more detail on p. 237.

Cell-budding occurs frequently amongst certain species of uni-
cellular organisms. I will select from amongst these a second
example, which has been examined by Richard Hertwig (VI.
35), the Podophrya gemmipara, a marine Acineta, which attaches
itself by means of a stalk at its posterior end to other objects.
From eight to twelve cell-buds not infrequently develop at its
free end, which is provided with prehensile tentacles and suction
tubes; these cell-buds are grouped in a ring around the centre of
the free surface. In this case; the nucleus divides in a peculiar
fashion. As long as the
Podophrya is young, and
has not yet commenced
to bud, the nucleus has,
as in so many Ciliata,
the form of a long horse-
shoe-shaped twisted band
(Fig. 119 6). Later on,
a large number of pro-
cesses grow out in a ver-
tical direction, towards
the free surface of the

body ; their ends soon Fig. 119. — Cell-budding, Podophrya gemmipara
; i ig, Zoology, Fig. 21); a buds,

swell eat with buds (O. Hertwig,

pike on BEC knobs, which become detached and form zoospores b; N

nucleus.

N

whilst the portion of the

230 THE CELL

band connecting them with the main part of the nucleus generally
becomes as fine as a hair. Small protuberances develop on the
free surface whenever the knob-like nuclear ends touch it. Thus,
as these ends grow, each is contained by a special protuberance
or cell-bud of its own. The whole cell-bud then increases some-
what in size, and becomes constricted at its base from the
mother-cell; the part of the nucleus, which it contains, takes the
form of a horse-shoe, separating itself from the delicate connecting
thread which united it to the mother-nucleus. The cell-buds are
now mature, and after detaching themselves from the mother
organism, move about for a time in the sea-water as zoospores.

II. Parrsat orn Merosiastic SEGMENTATION.

If we disregard the case of certain Protozoa (Noctiluca), partial
segmentation occurs only in egg-cells. It may conveniently be con-
sidered after unequal division. It is found in all cases where the
amount of yolk present is extremely great, and where the proto-
plasm is clearly separated from it, being collected together in a disc
at the animal pole (Fig. 108). The nucleus, which is situated in the
centre of this disc, must assume a horizontal position when it de-
velops into a spindle. Hence the first division plane is in a ver-
tical direction, and appears first at the animal pole in the centre of
the dise (Figs. 120 A,121 A), as in an egg, in which unequal cleay-

Fab 120.—Surface view of the first cleavage sta ge of a Hen’s egg (after Coste): a edge
of germinal disc ; b vertical furrow; ¢ small central portion ; d large peripheral portion.

age occurs (Fig. 92). Whilst, however, it gradually deepens and
sinks in until it has cut its way through to the vegetative pole, the
germinal dise is divided into two equal segments, which rest like
two buds, with their broad bases upon the undivided yolk-mass,

THE VITAL PHENOMENA OF THE CELL 231

and are thus connected with one another. Soon afterwards a
second vertical furrow makes its appearance, crossing the first at
right angles, and terminating in a similar manner at the germinal
disc, which is now split up into four segments (Figs. 120 B, 121 B).
In this case also a cleavage line is formed.

Fig. 121.—Discoidal cleavage of the egg of a Cephalopod (after Watase; from Hertwig,
Fig. 99).

Each of the four segments is again halved by a radial furrow.
The segments so produced correspond to sectors, whose pointed
ends meet in the centre of the germinal disc, and whose broad
ends are turned towards the periphery. The pointed ends are
separated from the rest of the segment by a diagonal furrow, or
by one which is parallel to the equator of the egg-sphere; and
in consequence smaller central segments cut off from the yolk
in every direction, and larger peripheral portions still connected
with the yolk, may be distinguished (Fig. 120 C). From now on,
furrows which are radial, and ones which are parallel to the equa-
tor, alternately make their appearance, so that the germinal disc
becomes more and more split up, the segments being so arranged
that the smaller ones are in the centre of the disc, and the larger
ones on its circumference (Fig. 121 C). Many of the segments
which are still attached to the yolk become constricted, so that the
nuclear spindle is slanting or vertical, the consequence of which is,
that when division occurs one of the daughter nuclei is situated
in the yolk-mass. In this manner the yolk-nuclei are produced by
partial cleavage; an especially large number of them are em-
bedded in the superficial layers of yolk on the periphery of the
segmented germinal disc. Compare the interesting observations
of Riickert (VII. 36), and Oppel (VII. 34), from which it appears
that in Selachians and Reptiles yolk-nuclei develop in consequence
of over-impregnation.

232 THE CELL

III. So-cattep Free Ceit-ForMATION.

The peculiarity of this form of multiplication consists in this,
that the nucleus in a cell subdivides several times consecutively,
whilst the protoplasmic body remains undivided for a considerable
time without showing the least inclination towards even a partial
cleavage. After bipartition has been repeated several times, the
number of nuclei in a single protoplasmic body may amount to
several hundreds. These arrange themselves at regular distances
from one another. Finally a period arrives when the many-nucle-
ated mother-cell bécomes either suddenly or gradually split up
into as many daughter-cells as there are nuclei in it.

Free cell-formation occurs chiefly, in both plants and animals,
during the development of the sexual products. In order to
demonstrate it, I will select three examples: the superficial
segmentation of the centrolecithal eggs of Arthropoda, the for-
mation of the endosperm in the embryo-sac within.the ovule of
Phanerogamia, and the formation of spores in the sporangia of
Saproleqnia.

The yolk mass is generally collected in the centre of the egg
in Arthropoda, being’ surrounded by a thin peripheral layer of
protoplasm. Hence the eggs are called centrolecithal, i.e. eggs
with yolk in the centre, in distinction to telolecithal eggs, in which
the yolk is situated at the poles (Balfour VI. 3). The cleavage
nucleus, surrounded by a protoplasmic envelope, is generally in
the centre of the nutritive yolk ; here it divides into two daughter-—
nuclei, whilst the division of the egg itself does not immediately

; Fie. 122.—Buperficial cleavage of the egg of an insect (Pieris crataegi) (after Bobretsky ;
rom R. Hertwig, Fig, 100): A division of the cleavage nucleus; B the nuclei raise

rene age and commence to form a germinal layer (blastoderm); C formation of blas-

THE VITAL PHENOMENA

follow. These daughter-nuclei
(Fig. 122 A) then divide into four,
these four into eight, the eight
into sixteen, and so on, whilst the
egg as a whole remains unseg-
mented. Later on the nuclei sepa-
vate from one another, and for the
most part move gradually to the
surface (Fig. 122 B), penetrating
into the protoplasmic envelope,
where they arrange themselves at
equal distances from one another.
Not until this has occurred does
the egg commence to segment, the
peripheral layer splitting up into
as many cells as there are nuclei in
it, whilst the central yolk remains
intact, or is only split up at a
much later period. This latter
occurs when in the eggs of insects,
as in telolecithal eggs, the yolk
contains yolk nuclei, or merocytes
(Fig. 122 C).

The wall of the embryo-sac in
Phanerogamia is coated with a
protoplasmic lining, which at a
certain stage of development con-
tains several hundred regularly
arranged nuclei; these were for-
merly considered to develop like
erystals in a mother-liquor; but
we know now, that they are pro-
duced by the repeated bipartition
of a mother nucleus, as in the eggs
of Arthropoda (Fig. 123). The divi-
sions occur almost simultaneously
in any one region of the embryo-
sac. If the preparation is suc-
cessful, nuclei in numerous stages
of division may be observed at one
time in a small space (Fig. 123).

OF THE

CELL

233

A strip showing all the phases of nuclear division.

(After Strasburger, Botan. Prakticum, Fig. 190.)

Protoplasmic lining from the embryo-sac,

Fig, 123.—Frilillaria imperialis.

234 THE CELL

After a sufficient number of nuclei have developed, a further
stage supervenes, when cells are formed (Fig. 124). Between the
nuclei, which are arranged at regular distances from one another,
the protoplasm differen-
tiates itself into radial
fibrille. Further it de-
velops connecting
threads in all directions,
which thicken at their
centres, and form cell-
plates. Jn the cell-plates
the cellulose walls make’
their appearance in the

manner already de-
scribed. These swell up

a
=e
eS

SS

“ easily, and owing to their
EZ formation, a portion of

the protoplasmic lining
becomes encapsuled
around each nucleus to
form the protoplasm of
the cell. Sometimes

Fig. 124.—Resedo odorata. Protoplasmic lining of E A
the embryo-sac at the commencement of free cell- two nuclei are enclosed

formation, (x 240; after Strasburger, Botan. Prakti- in one cell; these sub-
cum, Fig, 192.) : °
sequently are either

separated from one another by a partition wall, or, as in Corydalis
cava, fuse together to form a single cell.

The sporangium of Saprolegnia is, to commence with, a long
cell filled with protoplasm. Later on the nuclei in it increase
very much in number through bipartitions, which for the most
part occur simultaneously. After a time they distribute themselves
evenly throughout the cell-space. The protoplasm in the neigh-
bourhood of each nucleus then differentiates itself into a small
mass, which surrounds itself with a firm glistening envelope;
by this means the cell contents split up simultaneously into as
many spores as there are small nuclei present in the cell. Later

on these are passed to the exterior by the bursting of the mother-
cell, the sporangium.

The formation of swarm-spores in Radiolaria, which has been
already mentioned, affords us another peculiar instance of so-
called free cell-formation.,

THE VITAL PHENOMENA OF THE CELL 235.

IV. Drtvision with Repwction.

During the final development of ova and spermatozoa, certain
peculiar processes of division occur, which have for their function
the preparation of the sexual cells. The essential characteristic of
this is, that in the double division that occurs the second follows
the first so quickly, that the nucleus has no time to enter the rest-
ing condition. The result is, that the groups of nuclear segments
produced by the first division are immediately split up into
two daughter-groups without previously undergoing longitudinal
cleavage. Hence, at the end of the second division, the mature
egg- and sperm-cells only contain half the number of nuclear
segments, and half as much nuclein substance, as are present in
the nuclei produced by ordinary cell division in the same animal
(Hertwig VI. 34). To this phenomenon the name of ‘division
with reduction” has been given (Weismann VI. 77). Division
with reduction is most easily followed in the sperm- and egg-cells
of Ascaris megalocephala.

In the testis tube a certain number of cells are differentiated off
to form the sperm-mother-cells. In the large vesicular nucleus
(Fig. 125 I.), eight long nuclear threads develop out of the

10

ae ©

ar 0

OS A90
Cc

o

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OG,
O os
De

i) 0)
/ O
o 50°0 MMe:
Oe ray a,
e) is . eS
35 ee
or 0 OS
25 Pn SO.
. °
292300

Fig. 125.—Four nuclei of sperm-mother-cells of Ascaris megalocephala bivalens at various
stages of preparation for division.
chromatin substance. (Ascaris megalocephala bivalens has been
selected for description.) These are arranged in two bundles, and
are connected with the nuclear membrane by linin threads, which
stretch out in every direction. Whilst the nucleolus splits up
into separate spherules, two centrosomes, surrounded by a small
radiation sphere (Fig. 125 II.), make their appearance near to
one another in the protoplasm, close to the outer surface of the
nuclear membrane (Fig. 125 IJ.). The segments then become

236 THE CELL

shorter and thicker (Fig. 125 IJ., III). The centrosomes separ-
ate from one another, until finally they are situated at oppo-
site sides of, and at some distance from, the vesicular nucleus.
By this time, the rest of the nucleolus has disappeared; the
nuclear membrane becomes dissolved, and the two bundles, each
containing four nuclear segments, arrange themselves in the
equator between the centrosomes ; then each bundle splits up
into two daughter-bundles containing two nuclear segments,
which separate and move towards the poles (Figs. 125 IV.,
126 I.). The sperm-mother-cell now becomes constricted into

yf IT, TE.

Fie. 126.—Diagram showing the development of sperm-cells from a sperm-mother-cell
of Ascaris megalocephala bivalens. I. Division of the sperm-mother-cell into two sperm-
daughter-cells. II. The two sperm-daughter-cells (A, B) immediately prepare to divide
a second time. III. The sperm-daughter-cell A divides into two grand-daughter-cells.
B and C are grand-daughter-cells, which have been produced by the division of the
daughter-cell B of Fiy. IT.

two daughter-cells of equal size (Fig. 126 IJ.). Whilst this
process of constriction is taking place, the changes commence
which lead up to the second division (Fig. 126 I.), the cen-
trosome of each daughter-cell splits up into two parts which
travel, each surrounded by its own radiation sphere, in opposite
directions, which are parallel to the first division plane (Fig. 126
A, B). The nuclear segments produced by the first division
immediately afford the material for the second division, without
passing through the vesicular resting condition. They move
until they are situated between the newly-developed poles of the
second division figure (Fig. 126 IL, B), and then divide into two
groups, each of which contains two nuclear segments; these groups
then separate, and move towards the poles, after which the second
constriction commences (Fig. 126 III., A). Whilst after the first
division each daughter-cell contains four of the eight nuclear
segments, which have developed beforehand in the resting nucleus
each grand-daughter-cell contains only two. For, in dansauinee a

THE VITAL PHENOMENA OF THE CELL 237

of the second division following so closely on the first that the
resting condition was missed, an augmentation of nuclear sub-
stance, and an increase in the number of the nuclear segments,
through longitudinal cleavage, have been unable to take place.
In consequence, the number of segments has been diminished or
reduced to half the normal number.

In exactly the same way division with reduction occurs in the
egg of Ascaris megalocephala during the process of ripening.

‘The sperm-mother-cell corresponds to the unripe egg, or egg-
mother-cell. Here also eight nuclear segments, arranged in two
bundles, develop in the germinal vesicle (Fig. 127 I.). After the
nuclear membrane has been dissolved, they arrange themselves in
the equator of the first direction spindle, which rises up to the
surface of the yolk (Fig. 127 J/.), and in the manner already

rie. ;
ay,
sey

Vag a
oO Hgos
2970 5
.

st,

AP

Fie. 127.—Diagram of the development of polar-cells and the fertilisation of the egg of
Ascaris megalocephala bivalens.

described (p. 228) forms the first polar-cell. This process corre-
sponds to the division of the sperm-mother-cell into two daughter-
cells. As before (Fig. 126 I.), each of the two unequally large
products of division, viz. the egg-daughter-cell and the polar-cell

238 THE CELL

which was produced by budding, receive from the two bundles of
four segments two daughter-bundles each containing two seg-
ments.

Here also the second division follows the first so closely, that
the resting stage is omitted. Out of the material of that half of
the spindle which remained behind in the egg-daughter-cell, a
second complete spindle develops directly, containing only four
segments, arranged in pairs. A second budding produces both
the second polar-cell (Fig. 127 IV.), and the grand-daughter egg-
cell, or the mature egg, each division product containing only two
nuclear segments.

If we disregard the fact that the division products, when the
egg is ripe, are very unequal in size (budding), the processes which
take place resemble so exactly those already described as occurring
during sperm formation, that through them some light is thrown
upon the raison d’étre of the polar-cells. Whilst-on the one
hand four spermatozoa (Fig. 126 I/I., A, B, C) develop out of a
sperm-mother-cell (Fig. 126 J.), on the other only one egg capable
of being fertilised (Fig. 127 V.) and three abortive eggs arise out of
an egg-mother-cell. These latter still remain in a rudimentary
form, although they play a part in the physiologically important
division with reduction.

It has been noticed in many other objects besides Nematodes,
_ that the mature sexual products only possess half as many nuclear

segments as the tissue cells of the organism in question; this was
observed by Boveri (VI. 6) in the mature egg-cells of the most
various classes of the animal kingdom, by Flemming (VI. 13 IZ),
Platner (VI. 52), Henking (VI. 27), Ishikawa (VI. 40), Hicker
(VI. 24), vom Rath (VI. 55), in mature spermatozoa of Salamandra,
Gryllotalpa, Pyrrhocoris, Cyclops, etc., and by Guignard (VI. 23 b),
in the nuclei of the polar-cells, which are formed during fertilisa-
tion, and in the nucleus of the mature egg-cell of Phanerogamia.

Maupas (VII. 30) and Richard Hertwig (VII. 21) observed
that a reduction of nuclear substance occurs also in Infusoria
before fertilisation; however, further details on this subject are
given later, on p. 269 (Chapter VII.).

In all the above-mentioned cases, the reduction of nuclear sub-
Stance occurs before the egg-cell is fertilised by the spermatozoon.
It appears, however, that the reduction of nuclear substance may
occur after fertilisation has taken place, as a priort appears quite
possible, as a result of the first division. . At any rate that is the

* THE VITAL PHENOMENA OF THE CELL 239

way in which I explain the interesting observations of Klebahn
(VI. 43) upon two species of low Alge, Closteriwm and Cosmariwm.
A more detailed account is given in the chapter on the process of
fertilisation, p. 279.

IV. Influence of the Environment upon Cell-Division.
The complex play of forces, exhibited to the spectator at each cell-
division, can, just like the phenomena of protoplasmic movements,
which have been already described, be influenced to a considerable
extent by external agencies. Only here, for obvious reasons, the
conditions are more complicated than with the protoplasmic move-
ments, because bodies differing in structure, such as protoplasm,
nuclear segments, spindle threads, centrosomes, etc., are concerned,
and these can be altered in very various ways. As yet very little
experimental work has been done upon the subject. If the ques-
tion is raised as to how the processes of nuclear division are
affected at any individual stage by thermal, mechanical, electrical
or chemical stimuli, the answer is but unsatisfactory. The most
complete experiments that have been made at present have been
upon Echinoderm eggs, whose reactions during division to thermal
and chemical stimuli have been carefully observed.

It is generally accepted that tne rate of cell-division is affected
by the temperature, but what are the exact maximum and
minimum temperatures, and what changes in the nuclear figure are
produced by temperatures exceeding the maximum, have not yet
been accurately determined.

I (VI. 32, 33) have conducted a series of experiments upon the
inflnence of temperature from 1° to 4° Celsius below zero.

If dividing Echinoderm eggs are cooled down for about 15 to 20
minutes from 1° to 4° Celsius below zero, after a few minutes
the whole achromatin portion of the nuclear figure becomes dis-
integrated and destroyed, whilst the chromatin portion forming the
nuclear segments experiences only small or unimportant changes.
The most instructive figures are seen when the nuclear segments
are arranged in the equator (Fig. 128 A), or when they have
already migrated to the two poles, as can be seen from Fig. 128
B; the protoplasmic radiations and the spindle threads have abso-
lutely disappeared, whilst the radiation spheres in the neighbour-
hood of the centrosomes are marked by bright portions in the
yolk. The nuclear segments alone are unaltered in appearance
and position.

240 THE CELL

As long as the eggs are under the influence of the cold, the
nuclear figures remain in this condition; however, the rigidity
RB gradually disappears when

the eggs are placed in a
drop of water upon an ob-
ject glass, and gradually
warmed up to the tempera-
ture of the room. After 5
or 10 minutes the two polar
radiations develop again at
the same places as_ before,
at first being only faintly
seen, but finally being as
S distinct as ever; the spindle
HAAS threads reappear between

Fig, 128.— A Nuclear figure of an egg of the two poles, ‘and division
Strongylocentrotus, one hour and twenty minutes . l
man-
after fertilisation. B Nuclear figure of an egg proceeds in the usua
of Strongylocentrotus; this was killed afterhav. ner. In such cases the cold
ing been kept for two hours and fifteen minutes has acted only as a check,
in a freezing mixture, with a temperature of ea é
_ 9°, in which it was placedoneandahalf hows the process of division sim-

after the occurrence of fertilisation. ply going on from the point.
at which it was arrested by the cold.

A greater effect is produced if the eggs are subjected for about
2 to 3 hours to a temperature of from 2° to 3° Celsius below zero.
The whole nuclear figure is then fundamentally altered, and hence,
when the cold rigor is over, it is obliged to reconstruct itself en-
tirely, on which account a longer period of recuperation is neces-
sary. The nuclear segments either become fused together to form
an irregularly-lobulated body, or they develop into a small vesi-
cular nucleus, such as is formed during the reconstruction process
after division. Then changes begin anew, which result in the for-
mation of polar radiations, and frequently‘of more or less abnormal
nuclear division figures. In fact the division of the egg-body is
not only considerably delayed, but even pathologically altered.

Similarly certain chemical substances exert a marked effect
upon the process of division (‘05 solution of sulphate of quinine
and 5 per cent. chloral hydrate). If eggs which have developed
spindles, and which exhibit the equatorial arrangement of the nu-
clear segments, are subjected for about 5 to 10 minutes to the
action of the above-mentioned substances, the pole radiations soon
commence to disappear completely. However, after a short period of

THE VITAL PHENOMENA OF THE CELL Q41

rest, they reappear, and division proceeds as usual. If, however, the
substances are allowed to act upon the eggs for from 10 to 20
minutes, a still greater disturbance is produced, resulting in many
cases in a very peculiar and, in its way, typical course of the division
process. Not only are the pole radiations completely destroyed, but
the nuclear segments become gradually transformed into the vesi-
cular resting condition of the nucleus (Fig.129 A). This constitutes
the starting point of a new but considerably modified process of
division (O. and R. Hertwig VI. 38).

Fie. 129.—Nuclei of eggs of Strongylocentrotus which, one and a half hours after the act
of fertilisation has occurred, have been placed in ‘025 per cent. solution of quinine sulphate,
where they remained for twenty minutes. A Nuclear figure of an egg, which was killed one
hour after it was removed from the quinine solution; B nuclear figure of an egg, killed
somewhat later; C nuclear figure of an egg, killed two hours after it was removed from
the quinine sulphate solution.

Instead of two radiations, four develop immediately upon the
surface of the nuclear vesicle (Fig. 129 B, in which one radiation
is obscured). If treated with quinine, these soon become sharply
defined ; when, however, chloral is used, they remain permanently
faint, and confined to the immediate neighbourhood of the nucleus.
The nuclear membrane next becomes dissolved ; five spindles de-
velop between the four poles, and upon these the nuclear seg-
ments distribute themselves equatorially, thus producing a cha-
racteristic figure (Fig. 129 C). The nuclear segments then move
towards the four poles, and form the basis for four vesicular
nuclei, which separate from one another and travel towards the
surface of the yolk. The egg then begins, by means of two cross
furrows, to become constricted into four corresponding segments,

However, as a rule, this division into four portions is not com-
pleted until after the four nuclei have begun to make preparations
for dividing again by forming spindles with two pole radiations

R

249 THE CELL

At the same time, the furrows already mentioned deepen, so that
each spindle comes to lie in a protuberance or bud. Now the
splitting up becomes either pretty well completed, or the four
spindles, before the furrows have penetrated far into the yolk,
commence to divide, the nuclear segments travelling towards the
poles. The result of this is that the four first protuberances.
begin to become constricted a second time and to separate from
one another (cell-budding, bud formation).

The most striking of the phenomena described above is the
sudden appearance of the four pole radiations, for which, accord-
ing to our present knowledge, an equal number of centrosomes
must have served as bases. An explanation of this is afforded us
by the processes connected with the fertilisation of the Echinoderm
egg, which are discussed on p. 259.

Modifications of the form of nuclear transformation, shown in
Fig. 129 C, occur not infrequently; these are due to one of the
radiations being somewhat separated from the three others (Fig.
130). In this case the three that are situated close to one another

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16. 130. Fie. 131.

aoe aa 131.—Nuclear figures with four poles from Strongylocentrotus eggs, which, ore
da ha hours after the act of fertilisation, have been placed for twenty minutes in -(5.
per cent, solution of quinine, and which have been killed two hours after their removal

from the quinine solution,

are united by the three spindles to form a triaster. In the centre

of the equilateral triangle thus formed, the three nuclear planes

intersect, producing another regular figure. The fourth radiation,

which is situated at one side, is connected by a single spindle with

the radiation nearest to it.

as bese may be regarded as an intermediate stage between
igs. 129 and 130. Here the radiation x, which lies somewhat

THE VITAL PHENOMENA OF THE CELL 243

apart, is connected by means of two spindles to the remaining
portion of the figure, which forms a triaster. Of these two
spindles one is only faintly and imperfectly developed, and is
further remarkable for the small number of its nuclear segments.
Apparently it would never have made its appearance if radiation
x had been at a somewhat greater distance from radiation y.

Nuclear figures with three, four or more poles (triaster, tetraster,
polyaster, multipolar mitoses), have been frequently observed by
pathological anatomists in tissues altered by disease (Arnold,
Hansemann, Schottlander, Cornil, Denys, etc., VI. 1, 10, 11, 25,
67); they occur with especial frequency in malignant tumours,
such as carcinoma, and resemble to a remarkable extent those
produced artificially in egg-cells, such as are represented in Figs.
129 to 131. Apparently the cause for the abnormal appear-
ances may be traced to chemical stimuli. Thus Schottlinder
(VI. 67) was able to excite pathological nuclear division in the
endothelium of Descemet’s membrane by cauterising the trans-
parent cornea of the Frog’s eye with chloride of zinc solution of a
certain strength, and thus inducing inflammation. It is remarkable
how much the number of nuclear segments may vary in individual
spindles. For instance, Schottlinder found as many as twelve
segments in some spindles, and in others only six or even three;
the same was observed in Echinoderm eggs.

Further, multipolar nuclear figures may apparently be due to
other causes, about which at present extremely little is known to
us. For instance, a very common cause is the presence of several
nuclei in one cell. Such a condition can be easily produced
artificially by injuring egg-cells in some suitable way, and by
subsequently fertilising them (Fol VI. 196; Hertwig VI. 30a,
32, 33, 38). Under these circumstances instead of one single
spermatozoon entering in the usual manner, two, three, or more
make their way into the yolk. The consequence of this kind of
over-fertilisation (polyspermia) is the formation of several sperm
nuclei, corresponding in number to the spermatozoa which entered.
Some of these approach the egg nucleus, and since each of them
has brought a centrosome with it into the egg, a corresponding
number of pole radiations develop around the egg nucleus. And
thus, according to the number of spermatozoa, the egg nucleus
becomes transformed into a nuclear division figure with three, four,
or more radiations.

Further, those sperm nuclei which are not in contact with the

244 THE CELL

egg nucleus, but which remain isolated in the yolk, very frequently
give rise to peculiar, multipolar nuclear figures. They next
become transformed into small sperm spindles. Neighbouring
spindles then frequently approach each other, so that two pole
radiations, and consequently the centrosomes which they contain,
are fused together to form one. In this manner the most various
collections of spindles may be produced according to the amount
of coalescence which occurs, especially when over-fertilisation has
taken place to a high degree. Further the multi-radiated figure,
proceeding from the over-fertilised egg nucleus, may become yet
still more complicated in structure by the formation of male
nuclear spindles.

The interesting discoveries of Denys on the giant cells of bone
marrow, and of Kostanecki (VI. 46) on those in the embryonic
livers of mammals, may be explained in asimilar manner. Several
centrosomes, proportionate in number to the nuclei, are present in
the cell. Hence when the whole cell contents commence to divide,
several centrosomic radiations have to develop, and amongst
these the nuclear segments, which under certain circumstances
may number several hundreds, arrange themselves in peculiarly
branched nuclear plates, such as have been depicted by Kostanecki
in Fig. 132. When subsequently the mother-segments split up
into daughter-segments, these move off in groups towards the

Fig. 132.

Fig. 132.—Multicentrosomic nuclear division figure, with several groups of mother-
segments, from a giant cell from the liver of a mammalian embryo. (After Kostanecki.)

Fie. 133.—Multicentrosomic nuclear division -figure of a giant cell from the liver of a
mammalian embryo; the daughter-segments form several groups, which bave travelled
towards the numerous centrosomes. (After Kostanecki.) :
poles of the complicated nuclear division figure, where they form
a large number of small spheres (Fig. 133). Later on, a nucleus
develops out of each sphere; finally the giant cell splits up into
as many portions as there were nuclei—that is to Say, spheres
consisting of daughter-segments present in the cell.

The observations of Henneguy (VI. 28) on Trout eggs belong to

THE VITAL PHENOMENA OF THE CELL Q45

the same category. It is well known, that a large number of
nuclei (merocytes) are scattered throughout the yolk layer; this
is situated below the germinating cells in eggs, which are partially
segmented by furrows. Occasionally some of them collect to-
gether to form small spindle aggregations, whilst at the same time
they are making preparations
for division. Hence it is
very instructive to see, that
in the following case, de-
scribed by Henneguy (Fig.
134), the centrosomes act as
attraction centres. Two me-
rocytes, which are in the act
of dividing, lie close together
in the common mass of yolk,
so that the longitudinal axis
of spindle B would, if pro- Fie. 134.— Two nuclear spindles from the

7 G ° yolk of the germinal disc of a Trout’s egg: the
gee cae spindle A in its centrosome is exerting a disturbing iufluence
equator ; we see also that uponthe arrangement and distribution of the

the one centrosome b is very daughter-segments of the second spindle.
: (After Henneguy.)
near to spindle A. In con-
sequence, the arrangement of the daughter-segments of spindle A
has been disturbed to a considerable extent. Instead of their being
arranged in two groups near the centrosomes, a, a, as would occur
normally, a number of those which are within the attraction sphere
of the centrosome } of the neighbouring foreign spindle have been
drawn towards it. In a word: the centrosome of the one spindle
has evidently exerted a disturbing influence upon the arrangement
and distribution of the daughter-segments of the other spindle.
Henneguy has observed triasters, such
as the one depicted in Fig. 135, and also
tetrasters, in the germinal cells of the
same object; these gradually separated
themselves from the layer of merocytes.
At the close of this fourth section we
may mention the degeneration processes,
which sometimes occur in cell nuclei,
apparently as the result of injurious
influences. specially in the sexual Fre. 135.—Cell with a tri-
St alee centrosomic nuclear figure :
orgaus, individual germ cells, or groups Aorta Prout emibryroe, (teas
of them, appear to degenerate before Henneguy.)

246 _ THE CELL

they have reached matnrity, as has been observed by Flemming
and Hermann in Salamandra maculata, and by myself in Ascaris
megalocephala. The framework of the nuclei disintegrates, and
the nuclein collects together into a compact mass, which is re-
markable for its strong affinity for the most various stains. The
protoplasm diminishes in quantity, in proportion to that present
in similar normal germ cells. Such a stunted cell with a com-

B

Fie. 136.—A Sperm cell with a degenerated nucleus from the testis of a Salamandra
maculata (from Flemming, Pl. 25, Fig. 51a). B Residuary body (corps résiduel) from the
testis of Ascaris megolocephala. Nuclear degeneration.

pletely disorganised nucleus is depicted in Fig. 136. A is a
germinal cell from the testis of Salamandru; B, a germinal cell
of Ascaris, such as is found both in the testis and ovary, and
which is known by the name of corps résiduel, or residuary body.
Wasielewski, by injecting turpentine into the testes of mammals,
has succeeded in inducing experimentally a similarly degenerated
condition in the nuclei of germ cells.

Concerning the physiological importance of the nuclear division processes,
compare Chapter IX., section 3, especially that portion dealing with the equal
distribution of the multiplying inherited mass amongst the cells proceeding
from the fertilised egg.

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48.

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67.

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69.

THE VITAL PHENOMENA OF THE CELL 249

Nicewt. Zellkern, Zellbildung und Zellenwachsthum bei den Pflanzen.
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1883.

Pritcer. Ueber die Einwirkung der Schwerkraft u. anderer Bedingunen
auf die Richtung der Zelltheilung. 3, Abh. Archiv f. d. gesammte
Physiologie. Bd. XXXIV. 1884.

PuatNeR. Die Karyokinese bei den Lepidopteren als Grundlage fiir eine
Theorie der Zelltheilung. Internationale Monatsschrift. Bd.ILI. 1885.

PuatNer. Beitrége zur Kenntniss der Zelle u. ihrer Theilungserscheinungen.
Archiv f. mikroskop. Anatomie. Bd. XXXIII. 1889.

Rais. Ueber Zelltheilung. Morpholog. Jahrb. Bd. X. 1885, und Anat.
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Ranvier. Technisches Lehrbuch der Histologie. Leipzig. 1888.

O. vom Ratu. Zur Kenntniss der Spermatogenese von Gryllotalpa vulg.
mit besonderer Beriicksichtigung der Frage nach der Reductionstheilung.
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Vermehrung derselben durch Theilung. Miiller’s Archiv. 1852.

Remax. Untersuchungen iiber die Entwicklung der Wirbelthiere. 1855.

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82.

83.

THE CELL

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98.

THE VITAL PHENOMENA OF THE CELL 251

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(translati-ns). Oxford, Clarendon Press. 1889.

CHAPTER VII.
THE VITAL PROPERTIES OF THE CELL.

V. Phenomena and Methods of Fertilisation. Cell
reproduction by means of cell division, such as is described in
Chapter VI., does not, at least for the majority of organisms, ap-
pear to be able to continue for an indefinite period ; the process of
multiplication, after a shorter or longer period, comes to a stand-
still, unless it is stimulated afresh by the excitatery processes,
which are grouped together under the name of fertilisation. Only
the very lowest organisms, such as fission fungi, appear to be able .
to multiply indefinitely by repeated divisions; for the greater
part of the animal and vegetable kingdoms the general law may
be laid down, that after a period of increase of mass through cell
division a time arrives when two cells of different origin must
fuse together, producing by their coalescence an elementary
organism which affords the starting-point for a new series of
multiplications by division. =

Hence the multiplication of the elementary organism, and with
it life itself, resolves itself into a cyclic process. After generations
of cells have been produced by division, the life-cycle returns to
the same starting-point, when two cells: uahust unite in the act of
fertilisation, and thus constitute therhselves the foundation of a
new series of generations. Such cycles are termed generation
cycles. They occur in the whole organic kingdom in the most
various forms.

In unicellular organisms, for instance, the generation cycle
consists of a large number of independent individuals, which
sometimes amount to thousands. The fertilised elementary
organism multiplies by repeated divisions, producing descendants,
which do not require fertilisation, until a period arrives when a
new generative act occurs between the generations which have been
produced asexually. These phenomena have been most carefully

observed in Infusoria. Thus Maupas (VII. 30, p. 407) has proved
252

THE VITAL PROPERTIES OF THE CELL. 2535

by a number of experiments upon Leucophrys patula, a species of
Infusorian, that only after 300 generations have been produced
from a fertilised individual does the generation cycle close, the
descendants now showing for the first time the inclination and
capacity for sexual conjugation. In Onychodromus grandis this

Fig. 137.—Development of Pandovina morum (after Pringsheim; from Sachs, Fig. 411):
I a swarming colony; II the same, split up into sixteen daughter-colonies; III a
sexual family, through the gelatinous envelope of which the individual cells are passing
out; IV, V conjugation of the swarm-spores; VI a newly-formed zygote; VII a full-
grown zygote; VIII transformation of the contents of a zygote into a large swarm-
cell; IX the same, after having been set free; X the young colony derived from the
swarm-cell.

condition occurs after the 140th generation, and in Stylonichia
pustulata, after the 130th generation.
In multicellular organisms the cells, which are produced by the

254 THE CELL

division of a fertilised egg, remain associated together, forming a
colony of cells or an organic individual of a higher order. Re-
garded from the common point of view, from which we here treat
the sexual question, they may be compared to the collection of
cell individuals, multiplying asexually by division, which are
derived from a fertilised mother Infusorian. The generation
cycle closes here, when in the multicellular organism sexual cells,
which have become mature, unite after the processes of fertilisa-
tion have occurred, and thus form the starting-point for new
generations of dividing cells. The generation cycle may, in this
case, present a very different picture, being sometimes very
complicated in character. The simplest form is seen in many of
the lower multicellular Algw, such as Hudorinn, or Pundorina. A
cell colony (Fig. 137) is produced by the repeated division of the
fertilised cell. After having lived for a definite period, all the
cells become sexual cells. In order that conj ugation may occur,
the whole colony produced by cell division splits up into in-
dividuals, which serve as starting-points for new generation cycles.

The capacity, which each cell thus exhibits of reproducing the
whole multicellular organism, is not seen when the organism is
somewhat more highly developed. The cell substance, which has
been derived from a fertilised egg, and which has multiplied by
division to an immeasurable extent, then separates itself into two
masses, one of which consists of cells which serve to build up the
tissues and organs of the plant or animals, and the other of those
destined to function in reproduction. In consequence the or-
ganism generally remains unaffected in itself when it reaches
sexual maturity; it continues to detach the sexual elements from
itself, so that they may start new generation cycles, until in con-
sequence of the deterioration of the cells of its own body, or from
any other cause, it succumbs to death (Nussbaum VII. 33;
Weismann VII. 48),

In its purest form, a fixed and definite cycle is only to be met
with in the higher animals, in which multiplication of individuals
is only possible through sexual reproduction. In many species of
the animal and vegetable kingdoms sexual and asexual multipli-
cation take place simultaneously. In addition to the cells which
require fertilisation, there are others which do not need it, and
which, having detached themselves from the organism in the
ay of spores or pseud-ova, or as large groups of cells (buds,
' 8, etc.), give rise to new organisms solely by repeated

THE VITAL PROPERTIES OF THE CELL PA

divisions, without sexual intercourse (vegetative reproduction).
Or, to speak generally, between two acts of fertilisation a large
number of events, which are the result of cell division, are intro-
duced; these, however, need not belong to a single highly
developed physiological individual, but may be shared by
numerous individuals. This may occur in one of two ways.

In the one case the organism proceeding from the fertilised
egg is unable itself to form sexual cells; it is only able to mul-
tiply by means of buds, spores, or parthenogenetic ova. These,
or their asexually produced descendants, then become sexually
mature, and develop the capacity of producing ova and sperma-
tozoa. Such a cycle of events is called a regular alternation of
generations (Hydroid polyps, Trematodes, Cestodes, partheno-
genesis of Aphides, Daphnids, etc. Higher Cryptogams).

In the second case the organism derived from the fertilised
egg multiplies both sexually and asexually. The consequence of
this is, that even in the same species of plant or animal the
generation cycle must vary considerably. Between the comple-
tion of the first and the commencement of the second act of
fertilisation, either, on the one hand, only cell descendants arise
which belong to the single individual from which the fertilised
egg was derived, or one or more generations, the number in some
cases being very large, intervene, until finally the eggs of an
individual, produced by budding, become fertilised. In conse-
quence, fertilisation here assumes the character of a facultative
process, which is not absolutely necessary for the continuation of
the species, at any rate, so long as it has not been proved that
there are definite limits to vegetative multiplication. At present
this cannot be demonstrated in numerous plants, which appear to
be able to multiply indefinitely by means of runners, tubers, etc.

When we consider such cases, we must admit that the vital
processes may continue indefinitely simply by repeated division
of the cells themselves, without the intervention of the act of fer-
tilisation ; still, on the other hand, we are bound to conclude, on
account of the wide distribution throughout the whole organic
kingdom of the phenomenon of fertilisation, that this institution
is of essential importance amongst the vital processes, and that it
is fundamentally connected with the life of the cell. Fertilisa-
tion is in fact a cellular problem.

Our present subject is most closely connected with the study
of the cell, especially as concerns its irritability and divisibility.

956 THE CELL

Hence this chapter may be divided into two sections: the Mor-
phology and the Physiology of the process of fertilisation.

I. The Morphology of the Process of Fertilisation.
Up till now the process of fertilisation has been thoroughly worked
out to the most minute details in three objects: in the animal egg,
in the embryo-sac of Phanerogams, and in Infusoria. Although
‘ these three objects belong to different kingdoms of the organic
world, they show a marked similarity in all the processes
peculiar to fertilisation. It is, therefore, most suitable to com-
mence this section by investigating these three objects. We will
then occupy ourselves with the more general points of view pro-
vided by a study of comparative morphology, discussing :—

l. The different forms of sexual cells, the relative importance

4 Ay Stet
. 3. eer
Cote = se ese

Fig. 138.—A, B, C small sections from the eggs of Asterias glacialis (after Fol). The
spermatozoa have already penetrated into the gelatinous sheath covering the ova. In
A a protuberance is commencing to raise itself to meet the most advanced spermato-
zoon. In B the protuberance and spermatozoon have met.. In C the spermatozoon has
entered the ovum. By this time a yolk membrane with a funnel-shaped opening has

developed,
of the cell-substances, which are concerned in the generative act,
and the idea of ‘ male and female sexual cells.”

2. The original and fundamental forms of sexual generation,
and the derivation of sexual differences in the animal and vege-
table kingdom.

1. Fertilisation of the Animal Egg. Echinoderm ova
(Hertwig VI. 30; Fol. VI. 19, VII. 14) are classical subjects for the
study of the processes of fertilisation, as also are the eggs of
Ascaris megalocephala (van Beneden VI. 4.a,4b; Boveri VI. 6, ete.).
They complement each other, for some phases of the process
are more easily to be demonstrated in the one, whilst others are
more plainly to be seen in the other. .

THE VITAL PROPERTIES OF THE CELL 257

a. Echinoderm Eggs. In most Echinoderms, the minute trans-
parent ova are laid in sea-water, in a completely mature con-
- dition, having already budded off the pole cells (p. 229), and
developed a small egg nucleus. They are surrounded by .a soft
gelatinous sheath, which can be easily penetrated by the sper-
matozoa (Fig. 138 A).

The spermatozoa are exceptionally small, and consist, as is the
case in most animals, of (1) a head resembling a conical bullet; (2)
a small spherule, the middle portion or neck; and (3) a delicate,
contractile, thread-like tail. The head contains nuclein, the
middle portion paranuclein, whilst the tail consists of modified
protoplasm, and may be compared to a flagellum.

If ovaand spermatozoa are brought together in sea-water, several
of the latter immediately attach themselves to the gelatinous
envelope of each ovum. Of these, however, only one normally
fertilises each egg, namely, that one which, by means of the
undulating movements of its tail, was the first to approach its
surface (Fig. 138 4-C). At the spot where the apex of the head
impinged, the hyaline protoplasm constituting the peripheral
layer of the ovum raises itself up to form a small protuberance,
the receptive protuberance. Here the head, impelled by the
undulating movements of the tail, bores its way into the ovum,
which at this moment, excited by the stimulus, excretes a deli-
cate membrane, the vitelline membrane, upon its surface (Fig.
138 C), and, apparently by means of the contraction of its contents,
presses some fluid out of the yolk. In consequence, a gradually
increasing intervening space, which commences at the receptive
protuberance, develops between the yolk and the yolk. mem-
brane. By this means the entrance of another spermatozoon is
prevented. ;

Processes occurring in the interior of the yolk follow the external
union of the two cells; these may be grouped together under the
common name of internal fertilisation.

The tail ceases to move, and soon disappears from view; the
head, however, slowly pushes its way into the yolk (Fig. 139 A) ;
meanwhile, it absorbs fluid (Fig. 139 B), and swells up to form a
small vesicle, which may be called the sperm-nucleus, or male
pro-nucleus, since its essential constituent is the nuclein of the
head of the spermatozoon ; hence it becomes intensely stained by
carmine, etc. Fol has lately shown that immediately in front of
it, on the side which is directed to the centre of the egg (Fig.

s

258 THE CELL

139 A, B), there is a much smaller spherule, around which the
yolk commences to arrange itself in radial strie (Fig. 140 A),
forming a radiated figure (a star); this star grows gradually more
distinct, and at the same time extends itself farther away from
the spherule. Since it seems to be derived from the neck of the

iy se |

cs a aoe
fo Bete YS

Fig. 139.—A and B represent portions of a section of a fertilised egy of Asteracanthion.
A centrosome (sperm-centrum) has moved out a little in advance of the sperm-nucleus,
(After Fol.)
spermatozoon, Fol has called it the sperm-centrum (male centro-
some). A corresponding spherule can be seen close to the egg-
nucleus, on that side which is turned away from the sperm-
nucleus ; Fol has called this the ovo-centrum (female centrosome).

B

Fia.140.—A Fevrtilised egg of a Sea-urchin (O. Hertwig, Embryology, Fig. 18). The
head of the spermatozoon, which has penetrated into the egg, has been converted into a
Sperm-nucleus (sk) surrounded by a protoplasmic radiation, and has approached the egg-
nucleus (ek). B Fertilised egg of a Sea-urchin (0. Hertwig, Embryology, Fig. 19). The
Sperm-nucleus (sk) and the egg-nucleus (ek) have approached each other, and are both
surrounded by a protoplasmic radiation.

An interesting phenomenon now commences to attract attention
(Fig. 140 A, B). The egg- and sperm-nuclei (male and female
pro-nuclei) mutually attract each other, as it were simultaneously,
and travel through the yolk towards each other with increasing
velocity; the sperm-nucleus (sk) with its radiation containing
the centrosome always moving in front of it, travels more quickly
than the egg-nucleus (ek) with its ovo-centrum. Soon they.

THE VITAL PROPERTIES OF THE CELL 259

meet in the centre of the egg, to become surrounded by an
aureole of non-granular protoplasm, outside of which there is a
radiation sphere, common to them both (sun-like figure and
aureole of Fol).

During the course of the next twenty minutes the egg-nucleus
and the sperm-nucleus fuse together to form a single germinal or
cleavage nucleus (Fig. 141 I-IV); at first they lie close to one
another, flattening their contingent surfaces (Fig. 141 IJ), until
finally the lines of demarcation disappear, so that they unite to
form a common nuclear vesicle. In this the substance derived
from the spermatozoon may be distinguished for a considerable
time as a distinct granular mass of nuclein, which eagerly absorbs
staining solutions.

The fusion of the centrosomes (Fig. 141 I) follows closely on
the union of the nuclei. They lie, surrounded by the homo-
geneous protoplasmic area, on opposite sides of the cleavage
nucleus (Fig. 141 IT); they then spread themselves out tangen-
tially upon its surface, assuming the shape of a dumb-bell, and
finally divide into halves, which move off in opposite directions
(Fig. 141 III), and travel over one quarter of the circumference
of the cleavage nucleus. By means of these circular movements
(Fol’s quadrille), half of each male centrosome approaches a cor-
responding half of a female centrosome; the plane in which they
meet finally intersects at right angles the one in which they were
first represented as lying (Fig. 141 IV). Here they fuse together
to form the centrosomes of the first division figure. This con-
cludes the process of fertilisation, since all further changes are
connected with the division of the nucleus. ;

b. Eggs of Ascaris megalocephala. Further knowledge of the
process of fertilisation may be gained from the study of the eggs
of Ascaris megalocephala. Here the spermatozoon penetrates into
the egg before the development of the pole-cells (cf. Fig. 127, and
the text on p. 237), arriving finally at the centre (Fig. 142 1) ;
‘meanwhile the germinal vesicle, after changing itself, in the
manner already described, into the pole spindle, mounts up to the
surface of the yolk, and gives rise to several pole cells. Two
vesicular nuclei develop, one derived from the nuclear substance
of the spermatozoon, which has entered, and the cther from one
half of the second polar spindle (Fig. 142 J). Egg-nucleus and
sperm-nucleus (Fig. 142 IZ) then approach each other; in this
case, however, the male nucleus is in the centre, whilst the female

{HE CELL

260

261

THE VITAL PROPERTIES OF THE CELL

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262 THE CELL |

one makes its way in from the surface, whereas just the reverse
occurs in Echinoderm eggs; further, both nuclei are approxi-
mately of the same size, and lie close together, although for a
time they do not coalesce, but pass through a period of rest.
Indeed, even after they have begun to prepare for the formation
of the first division spindle, they do not commence to fuse. In
consequence of this, and of the further circumstance, that in
Ascaris megalocephala during nuclear division there develop only a
few nuclear segments, which are of considerable size, and hence
are easy to count, van Beneden (VI. 4a, 4b) was able to supple-

Fig. 142.—(I-III). Three diagrams depicting the course of the processes of fertilisation
in Ascaris megalocephala bivalens,

ment our knowledge of the process of fertilisation by the following
fundamental discovery :—

During the preparation for the first division spindle, the nuclein
in the egg- and sperm-nuclei, whilst these are still separated from
one another, becomes transformed into a delicate thread which
spreads itself out in many coils in the nuclear space. Kach
thread then divides into two twisted loops of equal size, the
nuclear segments (Fig. 142 II). On either side of the pair of
nuclei a centrosome makes its appearance; however, up till now,
no one has been so fortunate as to observe whence these are
derived. The line of demarcation between the two nuclei and the
surrounding yolk now disappears.

Between the two centrosomes (Fig. 142 III), which are sur-
rounded by a radiation sphere, spindle fibrils develop ; these are
at first faint, but later on are distinctly visible; they arrange
themselves about the four nuclear segments, which have been set
free by the breaking up of the nuclear vesicles, so that they rest
externally upon the middle of the spindle.

THE VITAL PROPERTIES OF THE CELL 263

Thus in the egg of the round worm of the horse the union of
the two sexual nuclei, which is the final stage of fertilisation,
only occurs during the formation of the first division spindle, in
which process they take an important part. The important
principle enunciated by van Beneden is as follows: Half of the
nuclear segments of the first division are derived from the egg-
nucleus, and half from the sperm-nucleus, hence they may be
distinguished as male and female. Now since in this case, as
before in nuclear division, the four segments split longitudinally,
and then separate, and move towards the two centrosomes, two
groups of four daughter-loops are formed, of which two are of
male and two of female origin. Each group then transforms
itself into the resting nucleus of the daughter-cell. Thus it is
indisputably proved, that each daughter-nucleus in each half of
the egg produced by the first division process contains two equal
quantities of nuclein, one of which is derived from the egg-nucleus,
and the other from the sperm-nucleus.

2. The Fertilisation of Phanero-
gamia. The discoveries which have been
made concerning the processes of fertilisa-
tion in Phanerogamia correspond most
completely with those which have been ob-
served in the animal kingdom. Stras-
burger (VII. 38) and Guignard (VII. 15)
stand in the first rank of investigators.
The most suitable objects for examination
are the Liliaceze, especially Liliwm martugon
One of the cells,
into which the pollen grain divides in

and Fritillaria imperialis.

Phanerogams, corresponds to the sperma-
tozoon, whilst the vegetable egg-cell, which
in the ovule is enclosed in the ovary of the
gynoecium, forms the most important por-
tion of the embryo-sac, and corresponds to
the animal egg.

When the pollen grain has reached the
stigma of the style, its contents commence
to emerge through a weakened portion of
the membrane, and to develop into a long
tube (Fig. 143), which penetrates into the
style until it reaches an embryo-sac. Here

Fie. 143.—Section through
the embryo-sac of Lilium
martagon (after Guignard

XV., Fig. 75). At the endof
the pollen-tube, whose weak-
ened wall is allowing its
contents to escape, the
sperm-nucleus may be seen
with its two centrosomes.
The egg-nucleus is also pro-
vided with two centrosomes,
On the right, at the end of
the pollen tube, a synergida
may be distinguished which
has commenced to disinte-
grate.

264

THE CELL

it presses between the two synergide right into the egg-cell.

The pollen grain and the pollen tube contain two nuclei, the
vegetative one, which takes no part in fertilisation, and the sperm-
nucleus. This latter comes to lie at the end of the pollen tube,
after this has made its way to the egg-cell; thence it passes
through the weakened cellulose wall into the protoplasm of the
egg, whilst two centrosomes advance in front of it; these latter
were discovered by the French investigator, Guignard (Fig. 143).
It soon meets the egg-nucleus, which is somewhat larger, and on
whose surface also a pair of centrosomes may be distinguished.

Fie. 144.—Egg from Lilium martagon (after Guignard XVI., Figs. 80 and 81): Aa
short time after the union of the egg- and sperm-nuclei; Ba Jater stage. The fusing of
ithe centrosomes is nearly completed.

The two nuclei (Fig: 144) then coalesce, as do also the four

Fie. 145. — Egg - cell
from the embryo-sac of
Lilium martagon, with its
nucleus undergoing divi-
sion. The nuclear plate
consists of twenty -four
nuclear segments, (After
Guignard XVI. Fig. 83,)

centrosomes ; these latter unite so as to form
two new pairs, of which each is composed
of one element of male and one of female
origin. The new pairs are situated on op-
posite sides of the cleavage nucleus, and
there develop into the two centrosomes of
the first nuclear spindle (Fig. 145),

In the same way as in animal sexual cells,
the nuclein and the number of nuclear seg-
ments derived from it are decreased during
the formation of the pollen-cell and of the
egg-cell to one half of the quantity present
in a normal nucleus. For instance, whilst
in Lilium martagon the normal nucleus de-
velops during its division 24 nuclear seg-
ments which split up into 48 daughter-

THE VITAL PROPERTIES OF THE CELL 265.

segments, in the nuclei of egg- and sperm-cells there are but 12.
It is only when the two nuclei unite that they form a complete
nucleus, from which arises the first division spindle with its 24
mother-segments, 12 being of male and 12 of female origin.

As concerns the centrosomes, a slight difference is shown by
Kchinoderms and Phanerogams. In the former, the centrosome
at the beginning is single in both egg- and sperm-nuclei, and |
only becomes doubled through division; in the latter, on the other
hand, two centrosomes are seen at a very early period both in the
pollen-tube and in the egg-cell.

If we compare the results mentioned on the preceding pages
(256-264), we may lay down the following fundamental laws re-
ferring to the process of fertilisation as it occurs in animals and
‘phanerogamous plants :—

During fertilisation morphological ‘processes, plainly to be
demenstrated, occur. The most important and essential of these
is the coalescence of the two nuclei which are derived from differ-
ent sexual cells, that is to say, the coalescence of the egg- and the
sperm-nuclei.

During the act of fertilisation two important processes of coa-
lescence occur :—

1. Equivalent quantities of male and female stainable nuclear
substance (nuclein) unite together.

2. Each of the halves obtained by the division of a male centro-
some unites with a corresponding half of a female centrosome,
by means of which the two centrosomes of the first nuclear
division figure are obtained.

In the male and female alike, the stainable nuclear substance
has been reduced to one half of the normal quantity, both as re-
gards mass and the number of nuclear segments which it contains.
Hence it is only after they have fused together that the full
amount of substance and the complete number of segments con-
tained by a normal nucleus are again present.

3. The Fertilisation of Infusoria. Certain Infusoria afford us
especially important objects for the investigation of the subject
of fertilisation. The sexual processes occurring in them were
discovered by Balbiani and Biitschli (VII. 6), who were pioneers
in this work, and they have lately been rendered much clearer by
the classical labours of Maupas (VII. 30) and of Richard Hertwig
CV ER ZL),

Infusoria, as it is well known, differ from other lower organisms

266 THE CELL

in one very interesting peculiarity, namely, that their nuclear
apparatus has split up into two kinds of nuclei, which differ
physiologically, i.e. into the chief nucleus (macro-nucleus) (Fig.
146 k), and into one or more sub-nuclei or sexual nuclei (n, i)
(micro-nuclei). If plenty of nourishment be present, the Infu-
soria, which may be cultivated for observation in a small drop of
water, multiply by means of the usual transverse division (Fig.

Fia. 147.

Fria. 146.—Pai ium dat (semi-diagrammatic) (R. Hertwig, Zool., Fig. 139):
k nucleus; nk paranucleus; 0 mouth aperture (cytostom) ; na’ food vacuole during process
of formation; na food vacuole; cv contractile vacuole in contracted condition; cv’ contrac-
tile vacuole in extended condition ; t trichocysts; t’ the same extended. .

Fig. 147.—Paramecium aurelia, undergoing process of division. Fig. 2 shows how at
an earlier stage the cytostom of the lower animal is formed by means of constriction from
the upper one (R. Hertwig, Zool., Fig. 140): k, nk, 0, nucleus, paranucleus, and mouth
seal of upper portion; k’, nk’, o’, nucleus, paranucleus, and mouth aperture of lower
portion.

147), when the macro- and micro-nuclei extend themselves simul-
taneously ina longitudinal direction and divide.

This asexual multiplication is so energetic under favourable
conditions that a single individual may, during the period of six
days, divide thirteen times, and thus produce about 7,000 or 8,000
descendants.

However, it has been shown, especially by the culture experi-
ments of Maupas and Richard Hertwig, that an Infusorian is un-
able to maintain the species for any length of time, and to continue
to multiply by simple division, even if nourishment be supplied to
it. The individuals undergo changes with regard to the nuclear
apparatus ; they may even completely lose it, when they no longer

THE VITAL PROPERTIES OF THE CELL 267

divide, but die, as a result of the changes induced by age, or,
as Maupas has expressed it, of senile degeneration. In order to
maintain the species, it seems to be absolutely necessary that
after definite periods two individuals should unite together in a
sexual act. In cultures such acts occur simultaneously through-
out the colony, so that a conjugation epidemic may be said to occur
occasionally.

During an epidemic, which lasts for several days, the observer
sees hardly any isolated Infusoria in the culture glass, for they
are nearly all joined together in pairs. Maupas states that con-
jugation occurs in Lewcophrys patula in the 300th generation, in
Onychodromus in the 140th, and in Stylonichia in the 120th genera-
tion. By a diminution of the amount of nourishment, the onset of
an epidemic may be hastened; by an increase it may be postponed,
or even permanently prevented, in which case the individuals
perish from senile degeneration.

If, after these preliminary remarks, we examine more closely
the process of fertilisation, we find that, during a period of several
days, the following peculiar and interesting changes take place
in the couples of Infusoria. We will take as the basis of our
description the Paramecium caudatum, for, since it possesses but
one nucleus and one single paranucleus, it presents simpler condi-
tions than those seen in most other species (Fig. 148).

When the inclination for conjugation arises, “two paramecia
come close together; at first only their anterior ends toug', but
later on their whole ventral surfaces are in contact, their mouth
openings being opposite to one another” (Fig. 148 I, 0). An ir-
regular thickening develops over a small area in the neighbourhood
of these latter, if conjugation lasts for any considerable period.
Meanwhile the nuclear apparatus, including both the chief nucleus
and the paranucleus, has undergone fundamental changes.

The chief nucleus becomes somewhat enlarged, its surface being
at first covered with protuberances and depressions (Fig. 148 [/-—
IV, k); these protuberances extend themselves into longer pro-
cesses, which later on become separated off, and then gradually
split up into still smaller pieces (V, VI, k). Thus the whole chief
nucleus becomes broken up into several small segments, which
distribute themselves all over the body of the Infusorian (V/Z),
and finally become dissolved and absorbed. In a word, the main
nucleus, having played its part, becomes completely disintegrated,
during and after conjugation. )

268 THE CELL

I.

Fie. 148.—Conjugation of Paramecium (R. Hertwig, Zool., Fig. 141): nk paranuclei; k
nuclei of conjugating animals. I The paranucleus transforms itself into a spindle; in left-
hand animal the sickle-stage, in right-hand animal the spindle-stage, are represented. II
Second division of paranucleus into chief spindle (marked 1 in left, and 5 in right) and
subsidiary spindles (2,3, 4in left, and 6, 7,8inright). III Subsidiary spindles show de-
generation (2, 3, 4 in left, 6, 7, 8in right), the chief spindles divide into male and female.
spindles (1 into 1 mand 1 w in left, and 5 into 5 mand 5 winright). IV Transmigration of
male spindles nearly completed (fertilisation), One end remains in the mother animal,
whilst the other has united itself with the female spindle of the other animal (1 m with
5 w,and 5m with lw). The main chief nucleus has become converted into segments.
V The primary division spindle resulting from the union of the male and female spindles
divides into secondary division spindles t/ and t”’. VI, VII After the termination of
conjugation. ‘Lhe secondary division spindles separate from one another, and come to lie
amongst the rudiments of the new paranucleus (nk’), and of the new chief nucleus (pt,
placente). The degenerated original nucleus commences to disintegrate. Since the Para-
meciwm caudatum has been selected to demonstrate the initial stages, and P. aurelia
the final stages, I-III represent the former, and IV-VII the latter. The difference be-
tween the two consists in this, that P. caudatum has only one paranucleus, whilst Ee

aurelia has two, and also that in the latter, nuclear disintegration commences even in the
first stage (stage 1). . :

THE VITAL PROPERTIES OF THE CELL 269

During the retrogressive metamorphosis of the chief nucleus,
the small paranucleus undergoes most important changes, which
always recur in the same manner, and which may be compared to
the phenomena of maturation and fertilisation seen in animal eggs.
It enlarges itself by taking up fluid from the protoplasm, its con-
tents assume a filiform appearance, until finally it transforms
itself into a little spindle (Fig. 148 I, nk). This spindle divides
into two parts, which soon develop into two new spindles; these
in their turn become constricted and divide into two, so that
finally four spindles, which have developed out of the paranucleus,
are present in the neighbourhood of the main nuclens, which is
undergoing transformation (Fig. 148 IJ, 1-4, 5-8).

During the further course of development, three of these four
paranuclear spindles disintegrate (JJ, 2, 3, 4,6, 7,8). They
become transformed into globules, which finally cannot be dis-
tinguished from the segments of the chief nucleus, whose fate
they share. They strikingly recall the formation of the pole cells
during the maturing of animal eggs, and in consequence have been
compared to them by many investigators.

The fourth or chief spindle alone persists (IJ, 1 and 5); it takes
part in the process of fertilisation, and serves as the foundation
for the new formation of the whole nuclear apparatus in the body
of the Infusorian. Which of these four spindles, derived from
the original paranucleus, eventually becomes the chief spindle,
depends, according to Maupas, solely and entirely upon its position.
They are all four precisely alike as regards structure. The one
which happens to be nearest to the above-mentioned zone of ir-
regular thickening becomes the chief spindle (JJ, 1 and 5). Here
it places itself at right angles to the surface of the body, extends
itself longitudinally, and again divides into two (III, lw, 1m;
ow, om).

Each of the halves contains apparently only about half as many
spindle fibrils, and half as many chromatic elements as one of the
earlier spindles. According to the observations made by Richard
Hertwig, during the division of the chief spindle the number of
spindle fibrils has been reduced to one half, a process similar to
that occurring in the nuclei of animal and plant sexual cells.
Hence these very characteristic nuclei play the same part as those
of ova and spermatozoa, and may be distinguished as male and
female, or as migratory and stationary nuclei.

Further, which of the two nuclei is to be migratory end which

270 THE CELL

stationary cannot be foretold from their structure, for it depends
solely and entirely upon their position and their consequent role
during the process of fertilisation. Thus the portions which are
situated nearest to the zone of thickening become the migratory
nuclei (III, 1m, 5m); the two conjugating bodies exchange
these migratory nuclei; these pass each other across the proto-
plasmic bridge, which has been formed for this purpose. During
this exchange, the male migratory nuclei possess the structure
of spindles (IV, 5m, 1m). After the exchange has been com-
pleted, each male nucleus coalesces with a stationary or female
nucleus, which is also in the form of a spindle (IV, 1w, 5w), so
that now each animal possesses only one spindle—the division
spindle (vt)—if we disregard the segments of the chief nucleus,
and the paranucleus, which are gradually undergoing disinte-
gration.

The similarity to the process of fertilisation, as it occurs in
Phanerogamia and animals, is striking. In Paramecia, the
stationary and migratory nuclei unite to form a division spindle,
just as in plants and animals the egg- and sperm-nuclei unite to
form the germinal nucleus. The division spindle serves to replace
the old nuclear apparatus, which is becoming dissolved. It in-
creases considerably in size (Fig. 148 V, t). The chromatin ele-
ments inside it arrange themselves into a plate; they then divide
and move apart towards opposite ends of the spindle, almost up to
the poles, thus forming the daughter-plates (V, right ¢’ ¢’). The
two halves remain united for a considerable time by a connecting
strand. They then develop in a roundabout fashion into chief
nucleus and paranucleus; in Paramecium aurelia (Fig. 148 VI)
for example, the daughter-spindles (¢', ¢’), which have been
formed outiof the primary division spindles, re-divide, and so pro-
duce four spindles (VI), two of which develop into paranuclei
(nk’, nk’), whilst the other two coalesce to form the chief nucleus
(pt). Thus, in Infusoria, “ fertilisation brings about a complete
re-organisation of the nuclear apparatus, and at the same time of
the Infusorian” (Richard Hertwig).

Sooner or later, after the exchange of migratory nuclei, the two
individuals separate from one another (Fig. 148 VJ, VII).
A longer period is necessary for the reabsorption of the useless
portions of the nucleus, and for their replacement by new for-
mations. The individuals, which have thus become rejuvenated,
have regained the capacity of multiplying enormously by means

THE VITAL PROPERTIES OF THE CELL Py a

of division, until again the necessity for a new “ conjugation
epidemic ”’ arises.

The conjugation period at the same time causes a somewhat
lengthy cessation of multiplication in the life of the Infusorian,
as Maupas, for instance, has plainly shown in the case of Onychro-
dromus grandis, where, if the temperature is kept at from 17° to
18°, an interval of six and a half days occurs between the com-
mencement of conjugation and the first subsequent division.
During this period, if conjugation is not taking place, a single
individual, when provided with sufficient nourishment, divides
thirteen times; that is to say, it produces from 7,000 to 8,000
descendants.

In most Infusoria, as in the cases described here, both con-
jugating individuals behave in the same way, each functioning
towards the other as male and female, that is to say, both impart-
ing and receiving. Fixed forms of Infusoria, however, such as
Vorticelle, etc., behave in an interesting and somewhat different
fashion.

The LEpistylis umbellaria
(Fig. 149) may serve as an
example. When a conjuga-
tion period is approaching,
several individuals of the
colony of Vorticelle divide
rapidly and repeatedly, thus
producing a generation of
individuals (r) very inferior
in size to the mother organ-
ism. Other individuals of
the colony remain undivided
and of normal size. The
former are called microga-

Fig. 149.—Epistylis umbellaria (ufter Graeff;
from R. Hertwig, Fig. 142) : portion of a colony
metes, and the latter macro- in the act of conjugation; + microzoids pro-

duced by division ; k microgametes in conjuga-
tion with macrogametes.

gametes; they differ from
one another sexually.

Each microgamete detaches itself from its stalk, swims round
in the water, and after a short time attaches itself to a macro-
gamete in order to conjugate with it (Fig. 149k). Changes
occur in the nuclear apparatus similar to those described in detail
above in the Paramecium, and migratory nuclei are exchanged here
also. However, the macrogamete alone continues to develop, the

272 THE CELL

migratory and stationary nuclei of the primary division spindle
coalescing, whilst the corresponding structures in the microgamete
are, as it were, paralysed, and, instead of fusing and developing
further, degenerate and become dissolved, like the fragments of
the chief nucleus and the subsidiary spindles.

In this manner the microgamete loses its independence and
individuality, and becomes gradually absorbed into the macro-
gamete, increasing the size of the latter.

Thus, in consequence of the stationary mode of life of Vorticella,
a peculiar sexual dimorphism has developed, resulting in the ab-
sorption of the smaller of the conjugating individuals, after it has
functioned to a certain extent as a male element in fertilising the
macrogamete. However, the resemblance to ova and spermatozoa
is not. complete, although both in Vorticella and Paramecium
fertilisation commences with the interchange of nuclear material,
and only results later on in the formation of a single effective
individual.

4. The various forms of sexual cells ; equivalence of participating
substances during the act of fertilisation; conception of male and
female sexual cells, Having shown in various instances, that the
course of the process of fertilisation, and especially the behaviour
of the nucleus during the process, is essentially uniform in animals,
plants, and Protozoa, we will now proceed to state more clearly a
difference which can be perceived in the cells participating in the
act of fertilisation in most organisms, and to point out the im-
portance of this difference. It consists in the unequal size and
form of male and female germinal cells. The larger, stationary,
and hence receptive cell, is called the female; the male cell, on
the contrary, is much smaller, often extremely minute ; it is either
motile, approaching the egg-cell actively by amceboid movements
or by means of flagella, or so small that it is conveyed passively
through the air or water to the egg-cell.

What is the importance of this difference? Is it an essential
product of the process of fertilisation, or is it brought about by
causes of a subsidiary and secondary nature, due to incidental and
secondary causes? It is of the greatest importance, in order to
decide this question, to determine in what substance and in what
portion of the two sexual cells this variation manifests itself.

Each cell consists of protoplasm and nuclear substance. Of
these the amount of protoplasm present in the sexual cells may.
vary considerably, as may be immediately recognised by their apy

THE VITAL PROPERTIES OF THE CELL 273

pearance; the spermatozoon often-contains less than +5555 of
the protoplasm present in the ovum. Thus, according to Thuret’s
computation, the ovum of Fucus is as large as from 30,000 to
60,000 antherozoids. In animal sexual cells, the difference is
usually still greater, especially when the egg-cells are copiously
laden with reserve materials, such as fat-globules, yolk-granules,
etc. Indeed, in typically developed spermatozoa the presence of
protoplasm at all may be doubted; for the tail, which is attached
to the middle portion, consists of contractile substance, which, like
muscle fibres, is a differentiation product of the protoplasm of the
sperm-cell. In immature spermatozoa, protoplasm is present in the
form of drops of various sizes, which, having served their purpose
during development, eventually disappear.

Nuclear substance behaves in quite a different way. However
much the ovum and spermatozoon may vary as to size, they still
invariably contain equal quantities of active nuclear substance.
The trath of the above statement cannot be proved by a simple
comparison of the two sexual cells, but if the course of the process
of fertilisation and of the development of the mature ovum and
sperm-cell be watched, it will be seen that they both contain an
equal quantity of nuclein, and that during the process of matura-
tion they develop an equal number of nuclear segments. For
example, the sperm-nucleus of Ascaris megalocephala bivalens con-
sists, like the egg-nucleus, of two nuclear segments of the mother
cell; each during fertilisation contributes similar elements, which
are utilised in the formation of the germinal nucleus (Fig. 142
JI). In the same way each nucleus contributes the same amount
of polar substance, the male and female centrosome both of which,
in the manner described on p. 262, take part in the process of
fertilisation (Fig. 141).

In opposition to these conclusions, it might be stated, that the
nuclear portions of both egg and sperm-cells before their union
are usually very different in appearance, and vary more or less in
size. This, however, is easily explained by the fact, that the passive
fluid substances may be mixed in greater or less quantities with
the active nuclear substance. The minute head of the sperma-
tozoon consists of fairly compact, and hence strongly stainable,
nuclein. In the egg-nucleus, which is much larger, the same
amount of nuclein is saturated with a quantity of nuclear sap,
throughout which it is distributed in the form of minute granules
and threads, the result being that the egg-nucleus as a whole is

.

274 THE CELL

less dense and does not become so strongly stained as the head of
the spermatozoon. 7

This difference in size and consistency soon disappears during
the course of the process of internal fertilisation; for the sperm-
nucleus, which was at first small, whilst on its way to the egg-
nucleus, soon swells up to the same size as the latter by absorb-
ing fluid out of the yolk (Fig. 142 IT), as is seen in the eggs of
most Worms, Molluscs, and Vertebrates. It is true that in iso-
lated cases, as in the eggs of the Sea-urchin (Fig. 141), the nuclei
are of different sizes, when they unite; under these circumstances
the sperm-nucleus has taken up a smaller quantity of sap than
usual, and is consequently somewhat denser in consistency ; so
that, in spite of the difference in size, we may still assume that
an equal amount of solid active constituents is present in both.

It may be demonstrated in suitable objects, that the relative
size of egg and sperm-nuclei depends chiefly upon the time at
which the egg-cell was fertilised, whether before, during, or after
the formation of the polar cells. For instance, if spermatozoa be
brought into contact with an egg of Asteracanthion whilst the
polar cells are developing, the sperm-nucleus must remain for a
considerable time in the yolk before fusion commences, and in
consequence it swells up during this period by absorbing nuclear
sap, until it is of the same size as the egg-nucleus, which develops
after the second polar cell has separated off. On the other hand,
if fertilisation occurs after the egg-cell is provided with both the
polar cells and the egg-nucleus, the sperm-nucleus remains for
only a few minutes as an independent body in the yolk, com-
mencing almost immediately after its entrance to fuse with the
egg-nucleus. Under these circumstances it keeps small in size, for
it is not able to saturate itself in the same way with nuclear sap.

Thus we may consider the following important law as proved,
z.e. that the two sexual cells, in spite of the fact that frequently
they vary considerably in appearance and contain such unequal
quantities of protoplasm, contribute equal amounts of nuclear
substance (nuclein, in a definite number of nuclear segments,
paranuclein, in the ovocentrum and spermcentrum) during the
process of fertilisation, and in so far are equivalent.

From this law I deduce the following: the nuclear substances
which are derived in equal quantities from two different indi-
viduals are invariably the only active substances, upon whose
union the act of fertilisation depends ; they are the true fertilisa-

THE VITAL PROPERTIES OF THE CELL 275

tion substances. All other substances (protoplasm, yolk, nuclear
sap, etc.) are not concerned in fertilisation as such.

This proposition is supported by two important facts :—

Firstly, the complicated processes of preparation and matura-
tion which the sexual cells must undergo. As follows from the
statements given on pp. 235-239, the chief result of these processes
is not that the nuclear substances are increased through fertilisa-
tion, but that they remain constant in amount for the species of
plant or animal in question.

Secondly, the phenomena of fertilisation seen in Infusoria. Here,
as Maupas and Richard Hertwig both assert, similar individuals
remain in contact for a sufficient period in order to exchange
halves of equal nuclei. When this exchange of migratory nuclei
has been effected, the process of fertilisation is completed, and
the two animals separate. Hence it is evident, that the ultimate
result of the complicated processes consists in this, that after the’
fusion of the migratory and stationary nuclei the nucleus in each
fertilised individual is composed of nuclear substance derived
from two different sources.

If the important substance of fertilisation is contained in the
nucleus, the question arises whether the nuclear substance of the
spermatozoon differs from that of the egg-cell. This question has
been answered in very different ways. Formerly it was generally
considered, as Sachs expressed it, that the male element intro-
duced into the ovum a substance which it did not contain before.
One view especially has obtained many adherents; it may be de-
scribed as the doctrine of the hermaphroditism of nuclei and the
theory of restitution.

Many investigators consider that the cells possess hermaphro-
dite nuclei, that is to say, nuclei with both male and female
properties. For instance, according to van Beneden’s hypothesis,
which has been the most clearly worked out, immature egg and
sperm-cells are hermaphrodite; they only gain their sexual
character after the egg-cell has lost its male, and the sperm-cell
its female constituents of their normal hermaphrodite nuclear
apparatus. The male nuclear constituents are expelled-from the
egg in the nuclear segments of the polar cells. The reverse pro-
cess occurs in a similar manner with sperm-cells. Thus the egg
and sperm-nuclei, being halved, become pronuclei, and possess
opposite sexual characteristics.

Regarded from this point of view, fertilisation consists essenti-

276 THE CELL

ally in the replacement of the male elements, which have been
expelled from the egg, by an equal number of similar elements,
which are introduced by the spermatozoon.

More careful investigation shows that these theories are not
tenable. For the empirical foundation, upon which they were
based, is destroyed by the fact which was proved on p. 237, namely
that the polar cells are morphologically nothing but egg-cells,
which have become rudimentary. This follows from a comparison
of the development of egg and sperm-cells in Nematodes. Hence
the nuclear segments, expelled from the egg in the polar cells,
cannot be the discharged male constituents of the germinal vesicle,
as is stated in.the restitution theory.

Apart from this, we are unable, with the methods of investiga-
tion at our command, to discover the least difference between the
nuclear substances of the male and female cells. Nuclein and
centrosomic substance are identical, both as regards quantity and
composition. There is no specific male or female fertilising
material. The nuclear substances, which come into contact with
one another during the process of fertilisation, differ only in this,
that they are derived from two different individuals.

Now, if, in consequence of this, it can no longer be allowed that
the egg and sperm-nuclei are sexually opposed in the way under-
stood by the supporters of the restitution theory, what meaning
must be attached to the terms male and female sexual cells or
male and female nuclei ?

These terms do not really touch the essential part of fertilisa-
tion, and do not express an opposition based upon fundamental
processes of reproduction; they refer rather to secondary differ-
ences of minor importance which have developed between the
conjugating individuals, between the sexual cells and their nuclei,
and which must be classed as secondary characteristics. Hence
we will state at once that the formation of two separate sexes is
not the cause of sexual generation, as might be concluded from a
superficial investigation, but that the reverse is really true. All
sexual differences, if we trace them back to their sources, have
arisen because the union of two individuals of one species, which
originally were similar, and hence sexless, is advantageous to the
maintenance of the vital processes; without exception, these
differences only serve one purpose, namely to facilitate the com-
bees of two cells. On this account solely have the cells de-

ped the differences which are termed male and female.

THE VITAL PROPERTIES OF THE CELL PAE |

The theory built up by Weismann, Strasburger, Maupas,
Richard Hertwig, and myself may be worked out more in detail in
the following manner. During fertilisation two circumstances
must be considered, which work together and yet are opposed to
one another. In the first place, it is necessary for the nuclear
substances of the two cells to become mixed ; hence the cells must
be able to find one another and to unite. Secondly, fertilisation
alfords the starting point for a new process of development and a
new cycle of cell divisions; hence it is equally important that
there should be present, quite from the beginning, a sufficient
quantity of developmental substance, in order to avoid wasting
time in procuring it by means of the ordinary processes of nutri-
tion.

In order to satisfy the first of these conditions, the cells must be
motile, and hence active; in order to satisfy the second, they must
collect these substances, and hence increase in size, and this of
necessity interferes with their motility. Hence one of these
causes tends to render the cells motile and active, and the other to
make them non-motile and passive. Nature has solved the dith-
culty by dividing these properties—which cannot of necessity be
united in one body, since they are opposed to one another—between
the two cells which are to join in the act of fertilisation, according to
the principle of division of labour. She has made one cell active
and fertilising, that is to say male, and the other passive and
fertilisable, or female. The female cell or egg is told off to supply
the sabstances which are necessary for the nourishment and
increase of the cell protoplasm during the rapid course of the pro-
cesses of development. Hence, whilst developing in the ovary, it
has stored up yolk material, and in consequence has become large
and non-motile. Upon the male cell, on the other hand, the second
task has devolved, namely of effecting a union with the resting
egg-cell. Hence it has transformed itself into a contractile sperma-
tuzoon, in order to be able to move freely, and, to as large an
extent as possible, has got rid of all substances, such as yolk
material or even protoplasm itself, which would tend to interfere
with this main purpose. In addition it has assumed a shape which
is most suitable for penetrating through the membrane which
protects the egg, and for boring its way through the yolk.

We may transfer the terms male and female from the cell ele-
ments, which are thus differentiated sexually, to the nuclei
which they contain, even when these are equal both as regards

278 THE CELL

mass and composition. Only we must understand by the ex-
pression male or female nucleus nothing more than a nucleus
derived from a male or female cell. In the same way, in In-
fusoria, the migratory nucleus may be termed male and the
stationary nucleus female, in the sense of the above definition,
since the former seeks the latter.

This difference, which has developed in sexual cells for the
purpose of division of labour, and to fit them for their special
work, is repeated in the whole organic kingdom, whenever the indi-
viduals in which the male and female sexual cells develop differ
from one another in sexual characteristics. In all the arrange-
ments referring to sex, one and the same object is aimed at:
measures are taken on the one hand to facilitate the meeting of
the sexnal cells, and on the other to arrange for the nourishing
and protection of the egg. The one organisation we call male,
and the other female. All these relationships are secondary, and
have nothing to do with the process of fertilisation itself, which
is a true cell phenomenon.

Fertilisation is an union of two cells, and, above all, a fusing
of two equivalent similar nuclear substances, which are derived
from two cells, but it is not a combination of sexual opposites, for
the differences depend solely upon structures of subsidiary import-
ance.

The truth of the above law may be still more clearly demon-
strated, if we compare the generative processes throughout the
whole organic kingdom, and thereby endeavour to determine how
the differences have gradually developed between the cells which
unite for the purpose of fertilisation. Amongst unicellular organ-
isms and plants, we find innumerable instructive examples of the
elementary and primitive forms of sexual generation and of the
origin of sexual differences in the plant and animal kingdoms.

d. Primitive and fundamental modes of sexual generation and
the first appearance of sexual differences. The study of the
lowest organisms, such as Noctilucee, Diatomacee, Gregarinx, Con-
jugatee, and other low Algx, shows that in many of them the con-
jugation of two individuals occurs in regular cycles, and this we
must regard as a process of fertilisation.

In Noctiluca conjugation commences by two individuals,
which are of the same size, and do not differ from one another in
any respect, placing themselves side. by side, with their mouth
apertures opposite one another, and beginning to fuse, whilst their

THE VITAL PROPERTIES OF THE CELL 279

cell membranes become dissolved. A connecting bridge, which
continually grows broader, develops; after which the proto-
plasmic masses stream together from all sides, until the two in-
dividuals become transformed into a single large vesicle. The
two nuclei, each accompanied by a centrosome, travel towards
each other, and place themselves in contact, but, according to
Ishikawa, do not fuse (VII. 25). After a time, the conjugating
pair of Noctiluce again divide into two cells, a partition membrane
having developed between them. At the commencement of this
division, the pair of nuclei, which have united together, become
extended ; they then become constricted in the middle, and divide
into two, after which they separate again, the result being that
each Noctiluca contains half of each nucleus. Thus the result
of conjugation is the production again of two individuals, each
of which possesses a nucleus of twofold origin. Fertilisation is
followed sooner or later by active multiplication by means of
budding and spore formation.

The Conjugate (VII. 11) are of especial importance in the
study of primitive’: modes of fertilisation. This order is sub-
divided into three families : the Desmidiacew, the Mesocarpex, and
the Zygnemacex.

Klebahn (VII. 27) has discovered the minute details of the
process of fertilisation in two species of Desmidiacexe: the Clos-
terium and Cosmarium.

Two Closterium cells, which are shaped somewhat like bent
sickles, lie lengthwise against each other, being kept in contact
by a gelatinous secretion; each then develops a protuberance near
its centre. The two protuberances come closely into contact and
fuse, whilst the wall separating them dissolves, to form a conju-
gation canal common to both. Here all the protoplasm from both
the conjugating Closterium cells gradually collects, and, detaching
itself from the old cell membrane, fuses to
form a single globular body, which finally
becomes surrounded by a membrane of its
own.

This zygospore or zygote, which has been
produced by the fusion of two similar indi-
viduals, now passes through a resting stage,
which lasts for several months (Fig. 150). Fie. 150.— Zygote of
It contains two nuclei, which were derived lsterivm, just __ before

: . germination. (After Kle-
from the two cells, and which remain apart pann, Pl. XIIL., Fig. 3.)

280 THE CELL

during the whole of the resting period. It is not until the
spring, when a new vegetative period recommences, that the
nuclei come close together, and fuse to form a germinal nucleus.
At this period the zygote, which is surrounded by a delicate
membrane, makes its way through the old cellulose wall, whilst
its germinal nucleus transforms itself into a large spindle, of
somewhat unusual appearance (Fig. 151 I). This divides into
two half-spindles (Fig. 151 II), which, however, do not enter into
the resting condition, but immediately prepare to divide again

: co rapa ce: germinal stages of Closterium. (After Klebahn, Pl. XIII., Figs. 6b, 8,
, > 3.

(Fig. 151 IIT). Thus the germinal nucleus divides into four
nuclei, by means of two divisions, the second of which succeeds the
first without a pause (Fig. 151 IV).

Meanwhile the protoplasm of the zygote has divided into two
hemispheres (Fig. 151 IV), each of which contains two nuclei,
which have been produced by the division of one spindle. The
two nuclei soon develop differences in appearance, the one (ac-
cording to Klebahn, the large nucleus) becoming large and vesi-
cular, whilst the other (the small nucleus) remains small, and
finally quite disappears. The small nucleus becomes much more
intensely Stained than the large one. It seems to me that the
former disintegrates and dissolves, just like the fragments of the
chief nucleus and the subsidiary spindles in Infusoria. Before

THE VITAL PROPERTIES OF THE CELL 281

the process of dissolving is quite completed, the
two halves of the zygote gradually assume the
shape of a Closteriwm cell (Fig. 152).

What is the significance of this second
division, which occurs immediately after the
first, without any intermediate resting stage ?
It appears to me that by its means the same
result is obtained, although in a different
manner, as is produced by the division, with ic. 152. — Two
reduction, which occurs during the maturing l»steria, which have

. developed from a
of egg and sperm-cells. In both cases by ygospore, vefore
means of the double division the nuclear sub- they have escaped
stance is reduced to one half of that contained anne eae
by a normal nucleus, and thus an increase of
nuclear substance is avoided when, in consequence of fertilisation,
two nuclei coalesce. Similarly in Desmidiacewe a reduction of
nuclear substance occurs after fertilisation, and thus the double
amount of nuclear substance, produced by the conjugation of
two complete, fully developed nuclei, is reduced to a normal
quantity. The germinal nucleus, instead of dividing into two
daughter-nuclei, splits up in consequence of the two divisions,
which follow immediately upon one another, into four grand-
daughter-nuclei. The protoplasmic body, however, is halved,
each portion containing only one functional nucleus; the other

two, being useless, disappear.

This supposition might be proved to be correct, if the nuclear
segments were accurately counted at the various stages. One
circumstance, which may be mentioned in its support, has fre-
quently been observed by Klebahn, namely that in Cosmarium
the four granddaughter-nuclei, which are derived from the ger-
minal nucleus, are distributed unequally between the halves of
the zygote, the one half containing one single active nucleus, and
the other containing three, two of which degenerate. It does not
matter whether the two degenerating nuclei fall to the share
of one or both cells during division, since they behave like yolk
contents. ya te

In Desmidiacee we have observed conjugation as it occurs in
isolated living cells; the Zygnemacee teach us its method ‘of pro-
cedure in a colony of cells, where several individuals have joined
together in rows to form long threads.

When, in the thick felt-like masses with which the Alge cover

282 THE CELL

the top of the water, two threads lie in contact with one another
for any considerable portion of their length, conjugation occurs
between neighbouring cells. As a rule all the cells prepare for
reproduction at the same time by sending out lateral processes to-
wards each other. These fuse at the point of contact, whilst the
separating wall dissolves, and thus transverse canals are formed,
which connect the conjugating threads at regular distances, and

ae f

alla
‘ee%)

Re

fi

Fie. 153.—Spirogyra longata (after Sachs, Fig. 410). To the left, several cells of two fila-
ments, which are about to conjugate: they show the spiral chlorophyll bands, in which
crown-like arrangements of starch grains are lying, as well as small drops of oil. The
nucleus of each cell is surrounded by protoplasm, from which threads stretch to the cell-
wall, b, preparatory to conjugation. ‘To the right, A, cells engaged in conjugation: the
protoplasm of the one cell is just passing over into the other at a; inbthe two proto-
plasmic masses have already united, In B, the young zygotes are surrounded by a wall.

resemble the rungs of a ladder (Fig. 153).
bodies of the cells then contract away from t
and after a time fuse together.

Differences which in themselves are trifling, but which on that

The protoplasmic
heir cellulose wall,

THE VITAL PROPERTIES OF THE CELL 283

very account are interesting, are seen in various species of
Zygnemacee; they are worth noticing, for they teach us the way
in which sexual differences may at first develop.

For instance, in Monjeotia, as in the Desmidiacex, the two proto-
plasmic bodies enter the conjugation canal and there fuse together
to form a zygote, which becomes globular, expresses fluid, and
surrounds itself with a membrane. In this case both cells
behave exactly alike; neither can be termed male or female.

In other species, such as Spirogyra (Fig. 153), one cell remains
passively in its membrane, and is sought out by the other, which
in consequence may be called the male. It wanders into the
conjugation canal, and, passing through it, reaches the female cell,
as though attracted by it; they then fuse to form a zygote (Fig.
153 A, a). When the zygote is treated with reagents and staining
solutions, it can be further established, that soon after the union
of the cells their nuclei approach each other, and unite to form the
germinal nucleus. Since in a thread all the cells act either as
males or females, one of the two conjugating threads generally
has all its cells empty, whilst the other contains a zygote in each
cavity (Fig. 153 B). The zygote surrounds itself with a separate
cell-wall, after which it generally rests until the next spring,
when it commences to germinate, and finally, by means of trans-
verse divisions, develops into a long Spirogyra thread.

The above-mentioned distinction between male and female
Spirogyra threads by no means invariably occurs. For instance,
it may happen that a thread bends back on itself, so that one end
comes into the neighbourhood of the other. Under such condi-
tions, cells situated at the opposite ends of the same thread con-
jugate together, so that those which under other circumstances
would have functioned as male cells now play the part of female
cells. |

In the above-mentioned families of Noctiluce and Conjugatx
and in others, such as Diatomacex, Gregarine, etc., the large pro-
toplasmic bodies are enclosed in membranes ; these pair, after
having passed through periods of vegetative multiplication by
simple division. A second series of primitive modes of sexual
reproduction is afforded us by lower plant organisms, such as
some of the Algse. For purposes of reproduction they develop
special cells, the swarm-spores, which are distinguished from the
vegetative cells by their small size, by the absence of a cell
membrane, and by the presence of two flagella or numerous cilia,

284 THE CELL

by means of which they move about independently in the water.
They are of especial interest, for they show us how, by means of
gradual differentiation and division of labour in opposite directions,
they have developed more highly differentiated forms, namely,
typical eggs and typical antherozoids. 4

Swarm-spores are small, motile, naked cells, generally pear-
shaped (Figs. 154, 155, 157, 158). The pointed end is anterior
and goes in front, whilst the spore moves through the water; it
consists of hyaline protoplasm, and frequently contains a red or
brown pigment spot (the eye-spot) ; the remainder of the body is
hyaline, or coloured green, red, or ‘brown with
colouring matter, according to the species ; it con-
tains one or two contractile vacuoles (Fig. 154).
The swarm-spore moves along by means of flagella,
which spring from the hyaline anterior portion ;

there are generally two flagella (Fig. 154), but
Beers cs sometimes there is only one; occasionally there are
Microgromia so- four or more (Fig. 14).
Peet The swarm-spores are derived at certain times

. &.)

from the contents of a mother-cell, either by means
of repeated bipartitions, or by the splitting up of the mother-cell
into several portions (pp. 232-234). When division into two
occurs, the number of swarm-spores is small, being 2, 4, 8, or
16; when, however, many cells are produced, the number is very
great, for in that case the mother-cell is of considerable size, and
may produce as many as from 7,000 to 20,000 daughter-cells.
When the wall of the mother-cell ruptures at one place, the broad
end of the swarm-spore escapes first to the exterior.

There are two kinds of swarm-spores, which are developed at
different times. The one kind multiply asexually, giving rise to
young A/gx, whilst the others require fertilisation. The mother-
cell, from which the former are derived, is termed by botanists the
sporangiwm, that giving rise to the latter gametangium.

We will only consider sexual Spores or gametes here. In many
of the lower Alge conjugating swarm-spores (Fig. 155 a, h, c, d)
cannot be distinguished from one another in any respect, either as
regards their sizes, mode of movement, or behaviour (Ulothriz,
Bryopsis, Botrydium, Acetabularia, etc.). On the other hand, in
other species sexual differences develop, which enable us to dis-
tinguish between male and female gametes. In the first case we
speak of isogamous, and in the second oogamous fertilisation.

THE VITAL PROPERTIES.OF THE CELL - 285

_ We may take either Botrydium or Ulothriz (Fig. 155) as an
example of isogamous fertilisation. If minute swarm-spores from
different sources are placed in a drop of water and examined with
a high power of the microscope, some of them are seen to approach
each other immediately, their
hyaline anterior ends (b) com-
ing into contact; and after a
short time they commence to
fuse together. At first they
touch each other laterally (c),
after which they grow to-
gether, the fusion commencing
at their anterior ends and gra-
dually extending backwards.
The couple (d) hurry about
for some time in the water
with an intermittent and stag-
gering movement. After a
short time the fusion is so far
advanced that the two gametes
form a single thick oval body,
which, however, betrays its
derivation from two _ indi-
viduals by containing two pig-

Fie. 155.—Botrydium granulatum (after
Strasburger, Fig. 139): A free plant of
ment spots, and four flagella medium size (x28); B swarm-spore, fixed

; with iodine solution (x540); C isogametes.
(e,f). The zygote now gradu- aa single individual; b two isogametes

ally slackens its movements, which have just come into contact; c, d,
until finally it comes to rest ; and e the same lying side by side; if: zygote,
x 2 produced by the complete fusion of the
it then loses its four flagella, gametes (x 5410).

which are either drawn in or

thrown off, becomes globular in shape, and surrounds itself with
a cell-wall.

Frequently the resting stage begins only a few minutes after
the commencement of pairing ; in other cases, however, the zygote
may swim round in the water with its four flagella for three
hours, in a naked condition, without a membrane, until finally it
draws in its flagella, and sinks to the ground.

The gradual appearance of sexual differentiation can be
followed still better in the very numerous species of lower Alge,
in which the fertilisation of gametes occurs.

As in Spirogyra (Fig. 153), one of the two individuals, which

286 THE CELL

in other respects. are absolutely similar, may be called female,
since it remains at rest, and must be sought for by the other for
the purposes of conjugation. Thus a relationship, similar to that
seen in Phxosporex and Cutleriacex, is produced.

In some species of Pheosporex, the male and female swarm-
spores cannot be distinguished from one another when they are
evacuated from the mother-cell; they are of the same size, and
are each provided with a pigment spot and two flagella; they
do not pair whilst they are swimming about. However, a
difference between the gametes soon becomes apparent. Some
come to rest earlier than others; each of these attaches itself by
the point of one of its flagella to some solid object, to which it
draws up its protoplasmic body by shortening and contracting
the connecting flagellum; it then retracts its second flagellum.
These resting swarm-cells may be termed female; their capacity
for becoming fertilised is only retained for a few minutes; they
appear to exert, as Berthold expresses it, ‘‘a powerful attraction ”’
upon the male gametes, which are swimming about in the water,
so that in a few seconds one egg may be surrounded by hundreds
of swarm-spores, one of which fuses with it (VII. 51).

Sexual differentiation is still more marked in Cutleriacex. Here
the sexual swarm-cells become: different in size before they are
separated from the parent, the female one developing singly, and
the male in groups of eight. In this genus the difference in size
of the sexual cells is fairly striking. Both kinds of gametes
swim about in the water for a time; fertilisation, however, can
only occur after the female swarm-spore has come to rest, has
drawn in its flagella, and has become spherical. Upon the egg,
which is now capable of becoming fertilised, a hyaline spot
appears, which was produced by the drawing in of the anterior
beak-like end. This is the so-called reception spot. It is the
only point at which one of the small male swarm-spores, which
soon come to rest around the female cell, can fertilise it. When
fertilisation is complete, the zygote surrounds itself with a
cellulose cell-wall.

In Fucacex, Characese, and other Algz the difference is still
more marked than in Outleriacew. Here the female cells, which
attain a considerable size, do not even pass through the swarm-
spore stage. They are either expelled to the exterior in a mature
condition as globular immotile egg cells (Fucacex, Fig. 156 @),
or they are fertilised at the place where they. originated, that is,

THE VITAL PROPERTIES OF THE CELL 287

in the oogonium. The male cells, on the contrary (Fig. 156 F),
are even smaller and more motile than those already described,
and have assumed the characteristic properties of antherozoids ;

G

Fig. 156.—Spermatozoids of Fucus (x 540). Egzs, with adhering spermatozoiis. (After
Strasburger, Fig. 87 G and F.)
they are composed almost entirely of nuclear substance, and are
provided with two flagella, which function as organs of locomo-
tion.

The view that eggs and spermatozoids of the higher Algx are
derived genetically from swarm-cells, which differentiate them-
selves sexually in opposite directions, and gradually assume a
specific male and female form, is still more strongly supported by
the phenomena observed in the little family of Volvocinee than
by comparing various species of Alge.

This family is especially interesting and important in the
consideration of the problem in question, since some of the various
species, which in their whole appearance are extremely similar
(Pandorina morum, Eudorina elegans, Volvox glubator), exhibit
marked differences in their sexual cells, whilst others show no
difference at all, and in yet others an intermediate stage can be
observed. The whole relationship is so clearly demonstrated
that it is worth while to consider it more in detail.

Pandorina morum, which is especially well known—for as early
as 1869 Pringsheim (VII. 35) discovered the pairing of its
swarm-spores—forms small colonies of about sixteen cells, which
are enclosed in a common gelatinous sheath (Fig. 157 IJ). Each
cell bears two flagella on its anterior end; these stretch out
beyond the surface of the gelatinous sheath, and are used for
locomotion.

During sexual reproduction each of the sixteen cells splits up
generally into eight portions, which aftar a time are set free, and

983 THE CELL -

Fic. 157.—Development of Pandorina morum (after Pringsheim ; from Sachs, Fig, 411):
I aswarming family; II a similar family, divided into sixteen daughter-families; IJI a
sexual family, the individual cells of which are escaping the gelatinous investment; IV,
V conjugation of pairs of swarmers; VI a zygote, which has just been completed; VII
a fully grown zygote; VIII transformation of the contents of a zygote into a large swarm-
cell; IX the same after being set free ; X a young family developed from the latter.

swim about independently (Fig. 157 IJI, 1V). These swarm-
cells, which are oval, and (with the exception of the anterior,
somewhat pointed, hyaline end) are green in colour, possess a red
pigment spot and two flagella; they are somewhat unequal in
size. However, in this respect a marked sexual differentiation
is not apparent in Pandorina. For when swarm-cells from two
different colonies approach each other, it is seen amongst the
crowd that sometimes two small ones, sometimes two large ones,
and sometimes one large and one smail unite together (Fig. 157

IV, V).

THE VITAL PROPERTIES OF THE CELL 289

When two swarm-spores meet, they first touch each other with
their points (IV), and then fuse together to form a biscuit-shaped
body, which gradually draws itself up into a ball (VJ, VII, X).
This surrounds itself, a few minutes after fertilisation, with a
cellulose cell-wall, and then, as a zygote, enters into a resting
condition, during which its original green colour becomes brick-
red.

A sexual difference is seen in Hudorina elegans, a species which
is very similar in other respects to Pandorina, being also a
gelatinous sphere containing from sixteen to thirty-two cells
(Fig. 158). At the time of fertilisation the colonies become
differentiated into male and female.

Fre. 158.—Eudorina elegans, female colony (Cenobium), around which antherozoids, Sp,
are swarming (after Goebel ; from Sachs, Fig. 412): M,—M, bundle of antherozoids.

In the female colonies the individual cells transform themselves
without further division into globular eggs; in the male colonies,

on the contrary, each cell splits up by means of repeated divisions
U

290 THE CELL

into a bundle of from sixteen to thirty-two spermatozoids (Fig. 158
M’). They are “extended bodies, bearing anteriorly two cilia, the
original green colour of which has been transformed into yellow.”
The individual bundles separate from the mother-colony, and swim
about in the water. “If they meet a female colony, the cilia on
both sides become entangled; by this means the male colony is
fixed; it however soon falls to pieces, after which the individual
spermatozoids, which become considerably longer, bore their
way into the gelatinous vesicle of the female colony. They then
make their way to the egg-cells, to which, after they have crept
round them, they attach themselves, often in great numbers. We
may assume that, as has been observed in many other cases, one
of these spermatozoids makes its way into each egg-cell ”’ (Sachs).

AG:

i

Sree ore

St one >
Bos

ax

AN oie as
RCA Gran Yh
i ‘ - Ree

{ 4% X ae

Fig. 159.—Volvow globator, sexual, hermaphrodite colony, somewhat diagrammatic repre-
sentation constructed from figures by Cienkovsky and Biitschli (after Lang, Fig. 21): s
male gamete (spermatozoid); O female gametes (eggs).

Finally, in Volvox globator (Fig. 159) the differentiation is
greater than ever, for amongst the very numerous cells which
constitute the globular colony some remain vegetative, whilst
others become transformed into sexual cells. Further the eggs
(O) are still larger than in Eudorina, and are fertilised by very
small male elements (s), which swim about with two flagella.

If we take all these numerous facts into account, we may surely
consider the following law as established, i.e. that egg and sperm-
cells are derived from reproductive cells, which, to start with, are
similar and not to be distinguished from one another, but which
become differentiated by developing in opposite directions.

Il. The Physiology of the Process of Fertilisation.
Having discussed the morphological phenomena which have been

THE VITAL PROPERTIES OF THE CELL 291

observed in the organic kingdom during the process of fertilisa-
tion, we must now turn our attention to a still wider and more
difficult subject--the examination of the properties which the
cells must possess in order to unite themselves in the reproduc-
tive act, and thus to constitute a starting point for a new cycle of
development.

In the first instance it is evident, that not all the cells of a
multicellular organism are capable of fertilising or of becoming
fertilised, and that even the sexual cells are only suitable for
reproductive purposes for, in many cases, quite a limited time.
Hence definite characteristics must be developed in the cells;
these we will provisionally group under the common name of
“need for reproduction.”

This need for reproduction alone is in itself far from sufficient
to ensure the occurrence of fertilisation. This is proved by the
fact that, if mature eggs and spermatozoa from different organ-
isms are brought together, they do not pair. Hence a second
factor is necessary: the cells which are to unite sexually must
suit one another in their organisation, and in consequence must
have the inclination to combine with one another. This we will
designate as sexual affinity.

Thus the physiology of the process of fertilisation may be
separated into two parts :—

1. Investigation of the need for reproduction.

2. Examination of the sexual affinity of the cells.

In a third section various hypotheses, which have been started
by various investigators, concerning the nature and aim of fertili-
sation, will be investigated.

1. The “Need for Reproduction” of Cells. By the expression
‘‘need for reproduction,’ we understand a condition of the cell,
when it has lost the capacity of carrying on the vital processes by
itself, although it regains the power to a still greater degree after it
has fused with a second cell in the act of fertilisation. At present
we entirely lack a deeper insight into the nature of this condition ;
for it is one of the inherent properties of living matter, and as
such is outside of the domain of our perceptive powers, since these
properties can only be recognised by their results. Similarly the
physiological side of the subject is completely unknown, since it
as yet has not been subjected to systematic investigation. Hence
we can only here mention certain observations, which must be
extended and widened in future by means of physiological investi-

292 THE CELL

gation. We expect by this means to increase our knowledge by
the study of the lowest organisms chiefly, since in them the indi-
vidual cells possess an absolute, or at any rate a large, degree of
independence, and are not, as in the higher organisms, related
to and dependent upon the other cells of the body. Hence in
them the fundamental vital phenomena are more clearly to be
recognised.

The facts which we know at present may be summed up under
the following heads :—

(1) The need for fertilisation occurs periodically during the life
of the cell; (2) it invariably lasts only a short time; (3) it de-
pends to a certain extent upon external conditions; and hence (4)
in many cases it may be suspended and transformed into partheno-
genesis and apogamy.

That the need for fertilisation is a phenomenon occurring periodi-
cally in the life of the cell may be demonstrated experimentally
through the study of Ciliata. Maupas (VII. 30) has carried out
a large number of very instructive experiments upon this subject.

During the life of one of the Ciliata, two periods can be dis-
tinguished—an-asexual one and one of sexual maturity or need of
fertilisation. The first commences after two animals have ferti-
lised one another and moved apart; multiplication then occurs by
the rapid and repeated division of the cells. During this period,
individuals from different cultures may be brought together, and
the most favourable conditions for conjugation be provided, and
yet pairing never occurs. However, after a considerable time,
they again experience a need for fertilisation. If at this time
individuals from two cultures are brought together under suitable
ee pairing occurs to a considerable extent for a few

ays.

Thus Maupas has established the fact, that in Leucophrys patula
ouly individuals of the 300th to 450th generation after the act
of fertilisation has taken place can reproduce themselves sexually.
In Onychodromus this sexual period occurs between the 140th and
230th generations, and in Stylonichia pustulata between the 130th
and 180th.

The second law runs : This condition of “need for fertilisation ”
is invariably of short duration. If cells capable of fertilisation are
not fertilised at the right time, they soon perish. This may be

demonstrated with Ciliata, swarm-spores of Algw, and animal egg-
cells. .

THE VITAL PROPERTIES OF THE CELL 293

If single individuals of Onychodromus, of a generation between the
140th and the 250th,.or specimens of Stylonichia pustulata of a
generation between the 130th and the 180th, do not have the op-
portunity of pairing, they become old sexually, or over-mature.
It is true that they continue to multiply by means of division, and
indeed are able to pair, but no result is produced. For, in spite of
their pairing, they degenerate and succumb to a gradual decay of
their organisations, as Maupas expresses it, “in consequence of
senile degeneration.”” The commencement of this stage may be
recognised by characteristic changes in the nuclear apparatus.

Swarm-spores or gametes of Alge often die off, after swimming
about in the water for a few hours, without having succeeded in
pairing with suitable individuals. The receptive capacity of the
large female gamete of the species Cutleria, after it has come to
rest, and has become capable of functioning as an egg, only lasts
for a comparatively short time. Falkenberg (VII. 10) has per-
formed a large number of experiments which show “that, whilst
on the third day after they have come to rest almost all the eggs
are capable of becoming fertilised, on the fourth day only half
are in that condition. Further, after this period all the eggs lose
their receptive capacity, and although spermatozoids are placed in
their neighbourhood, commence to die off, exhibiting the same
changes as those eggs which were completely shut off from the
fertilising cells.”

Finally, mature animal egg-cells, even when under normal con-
ditions in the ovary or in the oviducts, live only for a short time;
they soon become over-mature (Hertwig VI. 32). ‘Their normal
functions become weakened, as is seen by the fact that, although
they can still undergo fertilisation for a time, yet this occurs in
an abnormal fashion; several spermatozoa make their way into
the egg, the result being an abnormal process of development.
Without doubt, this phenomenon is analogous to the senile de-
generation of Ciliata which have been prevented from pairing at
a suitable period.

The third law, that the commencement of the need for fertilisa-
tion may be hastened or postponed by external circumstances,
may be clearly proved in some cases.

Thus, if nourishment be continually and abundantly supplied to
cultures of Ciliata, pairing can be prevented (Maupas VII. 30).
They continue to divide until the whole culture dies off in con-
sequence of senile degeneration. On the other hand, cultures of

294, THE CELL

Infusoria, which are approaching sexual maturity, may be induced
to pair by withholding nourishment. “ Une riche alimentation,”

as Maupas observes, “endort l’appétit conjugant ; le jetine, au
contraire, l’éveille et l’excite.”

Similarly Klebs (VII. 28) has observed in Hydrodictyon, that
changes in the environment influence the development of sexual
cells, by either inducing or hindering the process.

Klebs has induced the formation of gametes in “ nets,” which
were growing naturally, by cultivating them in a 7 to 10 per cent.
solution of cane sugar. After from five to ten days, the net fell”
completely to pieces, gametes having developed in nearly all the
cells. Further, the inclination for the formation of gametes was
increased in the cells by cultivating fresh nets in shallow glass
dishes, which contained a relatively small quantity of water, and
which were placed in a sunshiny window. According to Klebs,
the influence of chamber culture is “to arrest growth, but not to
interfere with the production of organic compounds by means of
assimilation ; at the same time, however, a certain poorness in
nutrient salts is produced.”

On the other hand, sexual reproduction may be suppressed, as
in Ciliata. For this purpose it is only necessary to place a net,
the cells of which have just commenced to form gametes, in a 5
to 1 per cent. nutrient solution composed of 1 part sulphate of
magnesia, 1 part phosphate of potassium, 1 part sulphate of potas-
sium, and 4 parts sulphate of calcium. After a short time, asexual
swarm-spores develop, especially if the net is put back into fresh
water.

Kidam has observed that a small fungus, Basidiobolus ranarum,
when cultivated from conidia in a nutrient medium, develops a
firm mycelium, which produces simultaneously both asexual re-
productive cells (conidia) and sexual cells. In an exhausted
nutrient medium, on the contrary, the conidia produce only a
loose mycelium, which immediately and exclusively gives rise to
sexual cells, which unite together to form zygospores.

Abundant nourishment in plants is conducive to vegetative in-
crease, as the experience of gardeners teaches us, but hinders the
formation of seed, whereas the development both of bloom and
seed is increased by restricting vegetative growth (cutting off
roots and shoots), and thus diminishing the absorption of nourish-
ment.

The same phenomenon has also been observed in animals, which

THE VITAL PROPERTIES OF THE CELL 295

multiply parthenogenetically. When nutriment is withheld from
the Phylloxera vastatriz, the winged sexual forms, as Keller (VII.
26) has shown experimentally, soon make their appearance, and
fertilised eggs are laid.

In many cases, especially amongst the lower organisms, the
need for fertilisation is only relative.

When the female gamete of the Alga Ectocarpus (VII. 51) comes
to rest, for a few minutes it becomes receptive. “If the egg is
not fertilised at this time, it draws in its flagella completely, be-
comes spherical, and excretes a cellulose membrane. After from
twenty-four to forty-eight hours parthenogenetic germination first
begins to make its appearance.” Even the male gametes are
capable of spontaneous development, although in a less degree
than the female. After they have swum round for several hours,
they finally, as Berthold states, come to rest, “ but only a portion
of them develop slowly into very weak and tender embryonic
plants, whilst the remainder become immediately, or after the
course of one or two days, disintegrated.”

A very peculiar facultative relation is seen in Bees, whose eggs,
whether fertilised or not, develop into adults. But the unfertilised
eggs produce drones, and the fertilised, female Bees (working
and queen-Bees). Sometimes, as is stated by Leuckart, herma-
phrodites are derived from eggs which were fertilised too late for
the development in the male direction to be entirely set aside.
The possibility of accelerating, or, on the contrary, of delaying the
need of fertilisation in sexual cells by interference from with-
out, throws light upon the phenomena of parthenogenesis and
apogamy, which we are now about to discuss in detail.

a. Parthenogenesis. In most cases sexual cells, both in the
animal and vegetable kingdoms, perish quickly, unless they are
fertilised at the right time. Although they consist of a substance
which is eminently capable of development, yet if this one con-
dition fails they cannot develop.

Till a short time ago the majority of scientists were so con-
vinced of the impossibility of the spontaneous development of
the egg-cell, that they received the theory of parthenogenesis
with incredulity, because they perceived in it an offence against a
law of nature. And, indeed, it may be accepted as a law of
nature for mammals, and for the majority of other organisms,
that their male and female sexual cells are absolutely incapable
of development by themselves. Any single species of mammal

296 THE CELL

would unquestionably die out, if its male and female individuals
did not unite in the act of generation. Nevertheless, it cannot be
stated as a general law of nature, that ova are always incapable
of development unless they are fertilised.

Both in the vegetable and the animal kingdoms, numerous in-
stances occur of cells being formed in special sexual organs, which
were, as far‘as we can judge by their design, originally destined
to develop by means of fertilisation as eggs; but which have sub-
sequently lost their need for fertilisation, and in consequence
behave exactly like vegetative reproductive cells, that is to say,
like spores.

Only female specimens of Chara crinita, one of the higher

' Algee, are to be found in Northern Europe. In spite of this, ova,

—

which develop without fertilisation into normal fruits, are formed
in the oogonia.

Still more instructive are the cases of parthenogenesis which
occur in the animal kingdom. They have been observed chiefly
in smallanimals belonging to the Arthropoda, in Rotatoria, Aphides,
\Daphnidee, Lepidoptera, etc. At one time females produce in
their ovaries only ova which develop without fertilisation, and at
another the same individuals form those which require fertilisa-
tion. Ova, with such different physiological attributes, generally
\ differ in appearance. The parthenogenetic ones are exceptionally
small and poor in yolk, and in consequence develop in a shorter
time and in greater numbers; whilst, on the other hand, those
which require fertilisation are much larger and contain much
more yolk, and consequently require a longer time for their de-
velopment. Since the former are only produced in summer and
the latter at the commencement of the cold season, they have
been distinguished as summer and winter eggs. The latter are also
called retarded eggs (Dauereier), because they have to pass
through a somewhat lengthy period of rest after fertilisation,
whilst the summer eggs (Subitaneier) immediately enter upon
the process of development.

The development of the parthenogenetic summer eggs, and of
the winter eggs, which require fertilisation, may be affected by
external conditions. In Aphides, abundant nourishment favours
the formation of summer eggs, whilst a diminished supply of
nourishment causes the production of ova requiring fertilisation. —
Daphnidee are also evidently affected by the environment, although
the individual factors can be less easily established experimentally.

THE VITAL PROPERTIES OF THE CELL 297

This may be concluded from the fact, that, in certain species, the
generation-cycle assumes a different appearance, according to the
conditions of life under which the animals are living.

The inhabitants of small shallow pools, which readily dry up,
produce only one, or at most a few generations of females, which
multiply asexually; after this ova requiring fertilisation are
produced, so that in the course of a year several generation-cycles
{consisting of unimpregnated females and sexual animals) suc-
ceed each other. The inhabitants of lakes and seas, on the-other
hand, produce a long series of unimpregnated females before de-
positing ova, which require fertilisation; this occurs towards the
end of the warm season.’ A generation-cycle, therefore, in this
case occupies a whole year (polycyclical and monocyclical species
of Weismann).

Weismann (VII. 39), who investigated the whole subject most
thoroughly, remarks: ‘“ That asexual and bi-sexual generations
alternate with one another in various ways in Daphnidx, and
that the mode of their alternation stands in a remarkable relation
to their environment. According to whether the causes of de-
struction (cold, desiccation, etc.) visit a colony several times
during the year, or once, or not at all, we find Daphnoids which
exhibit several cycles within a year, others which have only one
cycle, and finally there are species which do not exhibit any
generation-cycle at all; hence we can distinguish between poly-
cyclical, monocyclical and acyclical forms.”

In many species, which are exposed to frequently changing con-
ditions, we notice, that some of the ova, which are formed in the
ovary, develop into summer eggs, whilst others have a tendency
to become winter eggs. According to Weismann, “a war, as it
were, goes on to a certain extent in the body of a female between
the tendencies to form these two kinds of eggs.”

In Daphnia pulex, the germ of a winter egg may often be re-
cognised amongst several summer eggs in the ovary; this grows
for a few days, even beginning to accumulate the finely granular,
characteristic yolk; but then it is arrested in its development,
becomes gradually dissolved, and finally completely disappears.
If winter eggs have been developed, but owing to the absence of
the males, have not become fertilised, they disintegrate after a
time, and summer eggs are again formed.

How can it be explained, then, that, amongst eggs which have
been developed one after another in the same ovary, some

298 THE CELL

should require fertilisation and others not ? Weismann (VII. 40),
Blochmann (VII. 44), Platner (VII. 47), and others, have made
the interesting discovery, that parthenogenetic ova, and those
requiring fertilisation, exhibit an important and fairly essential
difference in the matter of the formation of the polar cells (vide
p- 236); whilst in the case of the latter two polar cells are divided
off in the usual manner, in that of the former the development
of the second polar cell, and consequently also the reduction of
the nuclear substance, which is otherwise connected with this
process, do not occur. Hence the egg-nucleus of the summer egg,
of a Daphnia, for instance, possesses without fertilisation the
whole nuclein mass of a normal nucleus.

However, this interesting behaviour by no means explains the
nature of parthenogenesis.. For the summer egg has the ten-
dency to develop without fertilisation, before it begins to form
the polar cells, as is seen from the small amount of yolk it con-
tains, the different nature of its membranes, etc. Hence the
ovum does not become parthenogenetic because it does not form
the polar cell; but, on the contrary, it does not form the polar
cell because it is already destined for parthenogenetic develop-
ment; it does not develop it because, under these conditions, the
reduction of the nuclear mass, which presupposes subsequent
fertilisation, is unnecessary.

Many peculiar phenomena connected with parthenogenesis
have been observed, the closer study of which will probably con-
tribute much to the explanation of this question. Such a
phenomenon, the importance of which cannot at present be esti-
mated, is the fact, that the preparatory process for fertilisation
can be retraced, even after the polar cell has been formed.

In many animals, the ova, if they are not fertilised, commence to
develop parthenogenetically, at the normal time. Attempts are
made by the ova of many worms, of certain Arthropods and
Echinoderms, and even of some Vertebrates (birds) to begin to
segment in the absence of male elements, and eventually to form
(germinal dises; but at that point they come to a standstill in
, their development and die off. Abnormal external circumstances
seem to favour the occurrence of such parthenogenetic phe-

‘nomena in individual instances, as, for example, in Asteracanthion.
_ The following remarkable occurrence has been observed by Boveri

in Nematodes and Pterotrachea, and by myself in Asteracanthion,
during the formation of the polar cells.

THE VITAL PROPERTIES OF THE CELL 299

After the separation of the first polar cell, that half of the
spindle, which was left behind in the ovum, develops into a com-
plete spindle again, just as if the second polar cell were going
to be divided off. However, this does not occur; for the second
spindle only divides into two nuclei, which remain in the ovum
itself. After some time they fuse together in this place, and
drifting towards the middle of the yolk, again produce a single
nucleus as it were by self-fertilisation; by means of this nucleus
the parthenogenetic processes, which quickly follow, are introduced.
Thus, in this case, the second division, the purpose of which is to
reduce the nuclear mass and to prepare for subsequent fertilisation,
is abortive. That by this means no sufficient compensation is made
for the absence of fertilisation is evident from the subsequent
course of the parthenogenetic process of fertilisation, 7.e. from the
more or less premature death of the ovum.

From the circumstance, that in parthenogenetic development
the formation of the second polar cell does not occur or is abortive,
we might conclude, that development invariably becomes im-
possible after the nuclear mass has been reduced to one half of
its normal amount, unless a fresh stimulus is given to the organism
by means of fertilisation. However, at present, this conclusion,
which perhaps contains some truth, cannot be said to be generally
applicable. For Platner (VII. 47), Blochmann (VII. 46), and
Henking (VII. 17) have observed, that the ova of certain
Arthropods (Liparis dispar, Bees) develop in a parthenogenetic
manner into normal animals, although, like ova which require fer-
tilisation, they have produced two polar cells. In these cases a
more careful investigation of the circumstances with reference to
the number of the nuclear segments is certainly desirable.

Hence, at any rate, it must be admitted, that it is possible for
ova, which contain reduced nuclei as a result of the formation of
the two polar cells, to develop further in a parthenogenetic
manner; for nuclei, which contain a reduced amount of nuclein,
have in no way lost their capacity for division, as may be easily
supposed. An experiment, conducted by Richard Hertwig and
myself (VI. 38, 32), upon the ova of the sea-urchin, proves this in
a striking manner.

By shaking the ova of sea-urchins violently, they can be split up
into small portions, which do not contain nuclei; these then be-
come globular, and exhibit signs of life for a fairly long time;
further they may be fertilised by spermatozoa. By this means

300 THE CELL

we can definitely prove that the sperm-nucleus, or, as is more
frequently the case, the sperm-nuclei, which have penetrated into
one of the fragments of the ovum, become metamorphosed into
small typical nuclear spindles with a radiation at each pole. The
sperm-nucleus now splits up into daughter-nuclei, which for their
part again multiply by indirect division, so that the fragment of
the ovum breaks up into a number of small, embryonal cells.
Boveri (VIII. 2) has pursued this observation further, and has
discovered the important fact, that out of a rather large non-
nucleated fragment of an ovum, which has been fertilised by a
single spermatozoon, a normal, although proportionately small,
larva can be develcped. ,

b. Apogamy. The phenomena, which de Bary (VII. 2) has
included under the name Of apogamy, have a close relationship
to parthenogenesis, and may be conveniently treated now.

Apogamy has been observed in certain Ferns ; it is well known
that in the course of their development there is an alternation of
generations. Minute plants, the prothallia, are derived from the
vegetative reproductive cells, or spores; the function of these
prothallia is to develop male and female sexual organs, the latter
of which produce egg-cells. These, when fertilised, produce an
asexual Fern-plant, which develops spores in a vegetative manner.

In Pieris cretica and Asplenium filix-femina cristatum and fal-
catum, the law of alternation of generations, which is generally so
constant in Ferns, is broken through. The prothallia of these
three species either produce no sexual organs at all, or only such
as are no longer functional, i.e. have become rudimentary; on the
other hand, a new Fern arises from the prothallium by means of
vegetative budding.

Since these three species of Ferns have been affected by culti-.
vation, it is possible that the development of cells requiring
fertilisation has been suppressed by excessive nourishment,
whilst the vegetative mode of reproduction has been favoured. ©

2. Sexual Affinity. By sexual affinity we understand the re-
ciprocal influences which are exercised by cells of related species
requiring fertilisation upon each other. This takes place in such
a manner, that, when the cells are brought within a definite dis-
tance of one another, they exert a mutual attraction upon each
other, and combine, fusing into one, like two chemical bodies,
between which unsatisfied chemical affinities existed. If both

THE VITAL PROPERTIES OF THE CELL 301

sexual cells are able to move, they precipitate themselves upon
each other; if however one cell, as ovum, has become fixed, the
reciprocal attraction is evinced by the movements of the sperma-
tozoon. But sexual affinity continues to operate even after the
two cells have fused, being seen in the attraction which the egg
and sperm-nuclei, with their centrosomes, exercise upon each other,
the result of which is, that they come into contact and coalesce as
described above. ;

Thus two points remain to be proved in this section: firstly,
that reciprocal influences between cells requiring fertilisation
really do exist; these we will designate by the name of sexual
affinity ; and secondly, that this affinity is only evinced between
cells of a definite kind ; and this suggests the question as to what
are the special attributes which these cells requiring fertilisation
must possess.

a. Sexual Affinity in General. That sexual cells at a
certain distance from one another exert upon one another a
definite influence may be concluded from numerous observations,
made by reliable observers. I will confine myself to a few especi-
ally instructive examples, which have been described by Falken-
berg, de Bary, Engelmann, Juranyi and Fol.

Falkenberg (VII. 10) investigated the process of fertilisation in
a low species of Alga, Cutleria. To the receptive ova of Cutleria
adspersa which have come to rest, he added actively motile
spermatozoids of the nearly allied species Cutleria multyida; these
two species can only be distinguished from one another by small
external differences. ‘In this case the spermatozoids, as seen
under the microscope, wandered aimlessly about, and finally died,
without having fertilised the ova of the allied species of Alga.
It is true, that individual spermatozoids, which by chance came
into contact with the quiescent ova, remained attached to them
for a few moments, but they soon detached themselves again.
However, a very different result was obtained as soon as a single
fertilisable ovum of the same species was introduced into the
vessel containing the spermatozoids. After a few moments, all
the spermatozoids from all sides had gathered around this ovum,
even when the latter was several centimetres distant from the
place at which they were chiefly collected.’ In doing this they
even overcame the attractive force exerted by the rays of lght
falling upon them, and moved in a direction opposed to the one
which they would otherwise have chosen.

=

302 THE CELL

Falkenberg concludes from his observations, that the attraction
between the ova and spermatozoids of Cutleria makes itself felt at
a relatively great distance, that this attractive force must have its
seat in the cells themselves, and further that the attraction is only
exerted between sexual cells of the same species.

De Bary (VII. 2 b), investigating the sexual reproduction of
Peronosporex, observed that, in the interlacing hyphe, the oogonia
at first lie alongside of each other. Somewhat later, the anther-
idia develop, but this invariably occurs in the immediate neigh-
bourhood of the egg-cell only; they are frequently derived from
hyphe, which have no connection -with the one from which the
oogonium is formed. De Bary concludes from this, that the
oogonium must exert an influence over a limited area, and that
this influence induces the hyphe to form an antheridium. A
peculiarly striking instance of this influence exerted at. a distance
is seen in the circumstance, that the branch on which the an-
theridium is developed is diverted from its natural direction of
growth; for, in order to approach the oogonium, it bends over with
its end towards it, and then lies close to it. De Bary estimates
that the distance at which the oogonium is able to exert this
attraction is almost as great as its own diameter, and remarks
further, that “the above-described divergence of the lateral
branches can be ascribed to no other cause than the special
attributes of the oogonium.”

Not less interesting, and worthy of notice, are the statements
which Engelmann (VII. 9) has made about the conjugation of
Vorticella microstoma. In‘ this case small male zoospores are
formed by budding (p. 228) ; these, just like spermatozoa, fertilise
the large female individuals (p. 271). Engelmann succeeded four
times in tracing the bud after its separation from the mother-cell,
until it had united with another individual.

Engelmann describes his observations as follows: “ At first the
bud, always rotating on its longitudinal axis, wandered with fairly
constant rapidity (cir. ‘6~1 mm. per sec.), and, as a rule, in a fairly
straight line through the drop. This lasted for from five to ten
minutes, or even longer, without anything especial happening ;
then the scene was suddenly changed. Coming by chance into the
neighbourhood of an attached Vorticella, the bud changed its
direction, occasionally even with a jerk, and dancing, like a butter-
fly which plays round a flower, approached the fixed form; it then,
as if it were feeling it, glided round about it, meanwhile always

THE VITAL PROPERTIES OF THE CELL 303

rotating on its own longitudinal axis. After this had been going
on for several minutes, and had been repeated with several fixed
individuals one after the other, the bud at last attached itself to
one of them, generally at the aboral end, near the stalk. Aftera
few minutes the fusion might be definitely observed to be taking
place.”

In connection with the above-mentioned description, Engelmann
remarks: “At another time I observed a still more striking
physiological or even psychophysiological exhibition. A free bud
crossed the path of a large Vorticella, which was travelling with
great rapidity through the drop, and which had abandoned its
stalk in the usual manner. At the moment of meeting, although
there was absolutely no contact, the bud suddenly changed its
course, and followed the Vorticella with the greatest rapidity ; then
a regular chase ensued, which lasted for about five seconds.
During this time the bud kept ata distance of about 7; mm.
behind the Vorticella; however, it did not succeed in overtaking
it, but lost it in consequence of its making a sudden side move-
ment. Hereupon the bud continued its path at its original slower
pace.”

This phenomenon of influence exerted at a distance has also
been observed by Fol (VI. 19 a) in animal cells, such as the ova of
the Star-fish. Each ovum is surrounded by a thin gelatinous
envelope. When fresh spermatozoa of the same species approach
the surface of the envelope, the one which is most in advance
exercises a distinctly perceptible influence upon the yolk (Fig. 160).

Fre. 160.—A, B, C Sections of ova of Asterias glacialis, after Fol. The spermatozoa have
already penetrated the gelatinous envelopes of the ovum. In A, a prominence is just
beginning to rise up to meet the most advanced spermatozoon. In B, the promin-
ence and spermatozoon have'met. In C, the spermatozoon has penetrated the egg which
has formed a yolk-membrane with a crater-like aperture.

304 THE CELL

Its hyaline superficial layer raises itself up as a small protuber-
ance, thus projecting a receptive prominence (cone d’attraction)
towards the spermatozoon. Sometimes this protuberance is soft,
and drawn out in the form of a needle or tongue, and sometimes it
is broad and short. After contact with the spermatozoon has
taken place, it is withdrawn.

Fol considers that it is impossible to doubt the accuracy of this
observation, and remarks further: “Since we cannot deny the
fact that the spermatozoon exercises an influence upon the yolk,
from which it is separated by a relatively great distance, we must
accept the theory that. influence at a distance (action a distance)
is a possibility.”

_ I will contine myself to the above-mentioned observations, the
number of which could be easily greatly multiplied, and will only
quote the following words of the botanist Sachs (II. 33) :-—

“Influence at a distance, or the mutual attraction of sexual cells
for one another, is one of the most startling facts connected with
the processes of fertilisation. I have chosen this term for the facts
about to be more minutely described, as it is not too long, and, at
any rate, realistic. We must not, however, take the words, in-
fluence at a distance and mutual attraction, exactly in the same
sense as in physics.

“In the numerous descriptions which various observers have
given of the behaviour of antherozoids in the neighbourhood of
the oosphere, and of wandering gametes and antherozoids in the
neighbourhood of oogonia, we meet with the, most definite asser-
tions, that the sexual cells: always exert a certain influence upon
one another, which makes itself felt over a certain distance, and
which always tends to induce the union of the two. This occur-
rence is the more remarkable, in that this mutual attraction
immediately disappears after fertilisation has taken place.”

The question naturally arises as to what are the forces to which
the phenomena can be attributed. Pfeffer has expressed the
view, based upon the above-mentioned experiments (p. 117), that
in the objects examined by him the antherozoids are attracted to
the egg-cells by chemical substances, which the latter secrete. Too
great an importance, however, must not be attached to these
opinions, as would be the case if we considered that the conjuga-
tion of two sexual cells was explained by them. In my opinion,
the chemical substances, which are secreted by the egg-cells, only
exert a secondary influence upon fertilisation; they play a part

THE VITAL PROPERTIES OF THE CELL 305

similar to that performed by the mucoid and gelatinous envelopes
of many ova which retain the antherozoids.

On the other hand, they in no wise explain conjugation itself,
i.e. the processes peculiar to fertilisation. This may be proved in
avery simple manner. According to the researches of Pfeffer,
malic acid is secreted in the archegonia of the most different Ferns.
Nevertheless, only the antherozoids of the same species will fuse
with the oosphere, those of a different species being as a rule un-
able to fertilise them. Thus we see, that there are relations exist-
ing between the sexual products which cannot be explained by
the action of irritating chemical secretions. The same is true of the
conjugation of gametes, of the formation of the receptive promin-
ence in animal ova, and of the mutual attraction of egg- and
sperm-nuclei.

Nageli (IX. 20) suggests that electrical forces may be the cause _

of sexual attraction, and this seems to me to be an explanation
of far-reaching importance. But, until this conjecture has been
definitely proved, it is better to attribute the sexual phenomena in

general to the reciprocal action of two somewhat differently |

organised protoplasmic bodies, and to call this reciprocal action
sexual affinity. We must be content with such a general ex-
pression, since we cannot accurately analyse the forees which come
into activity. Presumably it is not a question here of a simple
phenomenon, but of a very complicated one.

This may be rendered still clearer by an investigation of the
second point, namely, what is the nature of the cells requiring
fertilisation, and between which there is sexual affinity ?

b. More minute discussion of sexual affinity, and its
different gradations. The possibility of the occurrence of
fertilisation, and the results produced by it, are to a great extent
determined by the degree of relationship which exists between the
sexual cells. But since a near relationship implies a greater or
less similarity in their organisation, these differences in their
organisation must be the determining factor.

The degree of relationship between the two cells may vary con-
siderably. It is nearest when both the cells to be fertilised are
descended directly from the same mother cell; it is more distant
where many cell-generations have developed asexually from the
mother-cell, the final products at last producing sexual cells.
Here, too, cases of nearer or more distant relationship are possible.

If we take as an example one of the higher flowering plants, we
x

306 THE CELL

see that the male and female sexual cells may be derived from the
same sexual apparatus, i.e. from one blossom, or they may spring
from different blossoms of the same shoot, or, finally, from different
shoots; in this way, three different degrees of relationship are
obtained. In hermaphrodite animals they may belong to the
same individuals, or to different individuals of the same species. '

The degree of relationship is still more distant when the sex-
ual products are derived from two different individuals of the
same species. In such cases also, many degrees of relationship
are possible, according to whether the producing individuals are
descendants of common: parents, or are more distantly related.
Finally, we may have the union of sexual products derived from
parents which differ so much in their organisation, that they have
been classified as varieties of a species, or as belonging to different
species, or even to different genera. .

The innumerable possibilities, which the above-mentioned
series affords, are generally treated under three heads: (1) self-
fertilisation and in-breeding, (2) normal fertilisation, and (3)
hybridisation. There are, however, great differences of opinion
concerning the classification of individual cases under one or
other of the three heads. Further, there is no rule by means
of which we can estimate the various degrees of relationship of
the sexual cells, and which is equally applicable to all members of
the organic kingdom, -

A review of the facts connected with the subject teaches us,
that when the relationship of the reproductive cells—I use the
expression, relationship, in its widest sense—is either too near
or too distant, sexual affinity is either lessened or entirely done
away with; therefore we may state, as a general rule, that a
moderate degree of relationship, which is more or less distant
according to the species, is the one most likely to render fertili-
sation possible.

Further, we may also notice here, that sexual affinity is
affected by the environment. We will first disenss the ques-
tion of self-fertilisation, then that of hybridisation, and finally
we will investigate the influence exerted by the environment
upon these two.

a. Self-fertilisation. Self-fertilisation occurs under the
most various conditions. In many cases there is no sexual
affinity between cells needing fertilisation, which are nearly re-
lated to one another, being derived more or less directly from

THE VITAL PROPERTIES OF THE CELL 307

a common mother-cell or from the same highly differentiated
multicellular mother-organism. Lower Algsxe, Infusoria, Phanero-
gamia and all hermaphrodite animals supply us with a large
number of examples of this.

In Acetabularia, sexual reproduction takes place in such a
manner, that swarm-spores are derived in very great numbers
from the contents of resting-spores. According to Strasburger
and de Bary, conjugation only takes place between two swarm-
spores if they are descended from two different resting-spores,
whilst those that are derived from the same parent avoid each
other.

Strasburger (VII. 38) says: ‘‘ About mid-day I saw two
neighbouring spores, which were absolutely indistinguishable
from one another, rupture under my eyes, and the swarm-spores
of both hurry straight to that edge of the drop which was
nearest the window. Soon an extraordinary sight presented
itself. I observed that the swarm-spores, which were derived
from the same resting-spore, kept at equal distances from one
another and evidently avoided each other, whilst at the same
time conjugation groups,—if I may use the expression,—that
is to say, heaped-up collections of conjugating-spores, were
formed, into which the individual swarm-spores, as it were,
precipitated themselves. From these conjugation centres, pairs
of united swarm-spores were continually hurrying away.”

In his investigations upon Infusoria, Maupas (VII. 30), by
means of several hundred experiments on four different species
(Leucophrys, Onychodromus, Stylonichia, Loxophyllum), has estab-
lished the fact, that even when fertilisation is necessary con-
jugation only takes place when individuals of different generation
cycles are brought together.

Maupas remarks: “In many pure cultures of nearly related
individuals, the fast, to which I subjected them, resulted either in
their becoming encysted, or in their dying of hunger.

“Tt was not until after senile degeneration had already begun to
make inroads in the culture that I noticed that the conjugation
of nearly related individuals occurred in the experimental cultiva-
tions. However, all such conjugations ended with the death of
the Infusoria, which had paired, but which were unable to develop
further, or to reorganise themselves after they had fused. Such
pairings are, therefore, pathological phenomena due to senile
degeneration.”

308 THE CELL

Hence Maupas is of opinion that cross fertilisation between
individuals of different origin is necessary for Infusoria also.

The ineffectuality of self-fertilisation has also been proved in
certain cases amongst Phanerogamia. Hildebrandt (VII. 24, p.
66) says of Corydalis cava: “If a flower of this plant, in which
the opened anthers lie close to the stigma, be protected from
fertilisation by insects, no fruit is ever formed in it; that this
is not due to the circumstances of the pollen not coming in con-
tact with the susceptible part of the stigma may be seen from the
fact that even those flowers, whose stigmata were powdered with
the pollen of the surrounding anthers, were non-fertile.”

“ A perfect fruit can only develop when the pollen of the flowers
~ of one plant is placed on the stigma of another; it is true that.
fruit is formed when the flowers of the same vine are crossed ;.
but the resulting plants produce a much smaller number of
seeds than is normal, and further they do not always come
to perfect maturity.”

A similar absence of result after self-fertilisation has been
observed in a few other plants, i.e. certain species of Orchids,
Malvacex, Reseda, Lobelia and Verbascum.

Unfortunately, no very thorough investigation concerning the
behaviour of hermaphrodite animals has been made; the diffi-
culties of such research would be very great. No doubt cases
would be also found here in which no fertilisation occurs between
the eggs and spermatozoa of the same individual when they are
artificially brought into contact; with snails, for instance, this
must be the case. ;

However, in opposition to the above-mentioned examples,
we find others, which prove both that complete sexual affinity
does exist, and also that normal development by self-fertilisation
does take place between sexual cells, which are very nearly re-
lated to one another.

Thus in the case of certain Conjugate (Rhynchonema) sister-
cells unite with one another, or, as in Spirogyra, cells which
belong to the same filament conjugate together (vide p. 283).

Further, in many Phanerogams not only can the egg-cells be
fertilised with the pollen of the same flower, but the resulting
plants are strong and healthy; and, moreover, this in-breeding
co ps ibe through many generations with equally happy

Between the two extremes—the absence of any sexual affinity

THE VITAL PROPERTIES OF THE CELL 309

and the presence of strong mutual attraction in nearly related
sexual cells—there are many gradations.

Amongst the numerous egg-cells which are contained in an
ovary, only a few develop and become ripe seeds, where self-
fertilisation with the pollen of the same flower is induced
artificially. From this we may conclude that the individual
egg-cells possess somewhat different sexual affinities; that is
to say, that whilst some may be fertilised with the pollen
of their own flower, others cannot; thus they exhibit differences
similar to those which we shall come across in hybridisation.

Finally, it may be possible for egg-cells to be fertilised, to
begin to develop, and then to die off prematurely. In support
of this view, the phenomenon may be quoted, that many flowers,
which have been induced artificially to fertilise themselves, fade
more quickly than those which have been fertilised in a natural
manner. Indeed, the flowers of certain Orchids become black
and necrotic when treated in this fashion. This is probably
due to the premature death and disintegration of the embryos
which were about to be developed (Darwin VII. 8).

The seeds, which develop as a result of self-fertilisation, fre-
quently produce only weakly plants, showing some defect or
other in their constitution; further, the pollen grains are often
imperfectly developed.

From these three facts, namely, that in many organisms nearly
related sexual cells do not combine; that in others, even if fertili-
sation does take place, the embryo is arrested in its development,
and soon dies; and that finally, even if development proceeds
uninterruptedly, the evolved organisms are weakly ; we are able
to draw the general conclusion, that self-fertilisation on the
whole acts disadvantageously. It is true, that in individual
cases this disadvantage cannot be perceived, yet these excep-
tions do not disprove the accuracy of this statement any more
than the occurrence of parthenogenesis can be taken as an
argument against the theory, that great advantage is to be
derived from fertilisation.

That there must be something detrimental in self-fertilisation
may be inferred from a cursory glance over the organic kingdom.
As Darwin (VII. 8) says, nature evidently abhors frequent self-
fertilisation, for we see constantly on every side, that most com-
plicated arrangements have been made in order to prevent its
occurrence.

310 THE CELL

These arrangements are: (1) the distribution of the sexual
organs over two different individuals, so that one produces only
female sexual cells, and the other only male; (2) the reciprocal
fertilisation of hermaphrodite individuals; (3) the different times:

\ at which the maturation of the ova and spermatozoa occurs, as
in Pyrosoma, many molluscs, etc.; and (4) the peculiarities in the
organisation of hermaphrodite flowers of phanerogams (both dicho-
gamy and heterostylism), and the part played by insects, which,
in carrying the pollen from one flower to the other, induce cross.
fertilisation, as has been observed and described by Koelreuter,.
Sprengel, Darwin (VII. 8), Hildebrandt (VII. 24), H. Miller
(VII. 49), and others. These arrangements for the prevention of
self-fertilisation are so many-sided and striking, especially im
flowering plants, that Sprengel was able, in his book, to speak of

| “the discovered secret of nature, the fertilisation of flowers by
| insects,” and to say: “ Nature does not seem to have wished that
a single hermaphrodite plant should be fertilised with its own

‘ pollen.”

8B. Bastard Formation, or Hybridisation. The opposite
of self-fertilisation and in-breeding is hybridisation. By this is
meant the union of several products of individuals, which are so
different in their organisation, that they are classified into different
varieties, species, or genera.

As a rule, the principle, that the sexual products of individuals,
which are very different from one another, do not unite with one
another, is correct. Everybody considers it impossible for the
ovum of a mammal to be fertilised by the spermatozoon of a
fish, or for that of a cherry-tree by the pollen of a conifer. But
as the individuals become more closely related, whether they
belong to different families or species, or even only to different
varieties of the same species, the more difficult does it become to
prophesy « priori as to the result of cross-fertilisation. This can
only be discovered. by means of experiment, which has shown
that the various species in the animal and vegetable kingdoms do
not always behave in a similar manner towards hybridisation, in
that individuals which resemble one another in their form, down to
the minutest details, often cannot be crossed, whilst between others
which are much more dissimilar bastard fertilisation vs possible.

Briefly, sexual affinity does not always march pari passu with

the external similarity which can be perceived between the
individuals in question.

«

THE VITAL PROPERTIES OF THE CELL 311

Although the only difference between Anagallis arvensis and A.
cerulea is in the colour of their blossoms, they cannot be induced
to fertilise each other. No hybrids have been obtained from
apple and pear-trees, or from Primula officinalis and P. elatior;
whilst, on the other hand, hybrids have been successfully obtained
between species which belong to different orders, such as Lychnis
and Silene, Rhododendron and Azalea, ete.

Sachs says: “ The absence of correspondence between sexual
affinity and systematic relationship is shown in a more striking
manner, in that occasionally varieties of the same species are
either quite unable to fertilise each other, or can only do so toa
partial extent ; thus Silene inflata var. alpina cannot conjugate with
var. angustifolia, nor var. latifolia with var. litoralis, and so on.”

In both the animal and the vegetable kingdoms we find certain
orders the species of which can be easily crossed, whilst there are
others whose species offer the most obstinate resistance to all at-
tempts. In the vegetable kingdom, Liliacee, Rosacee, Salicacee ;
and in the animal kingdom, Trout, Carp, Finches, etc., readily
produce hybrids. Many dogs, too, which differ considerably in
bodily structure, such as the dachshund and the pointer, the
retriever and the St. Bernard, produce mongrels.

Further we see how unaccountable are the factors which are
dealt with in hybridisation when we consider the following
phenomenon: very frequently the ova of species A may be fer-
tilised with the spermatozoa of species B; whilst, on the other
hand, the ova of B cannot be fertilised with the spermatozoa of A.
Thus sexual affinity between the sexual cells of two species is
present in the one case and absent in the other. It seems to me
that the determining factor should be sought for in the organisation
of the ovum, as may be concluded from the experiments cited
below.

A few examples of one-sided crossing may be quoted. The ova’
of Fucus vesiculosus may be fertilised with the antherozoids of |
Fucus serratus, but the reverse cannot occur. Mirabilis Jalapa
produces seed when fertilised with the pollen of Mirabilis longi-
flora, whilst the latter remains unfruitful, if the attempt be made ,
to fertilise it with pollen from the former.

Similar cases often occur in the animal kingdom, and amongst
these the most interesting are met with in those species in which
fertilisation can be induced artificially by mixing the sexual pro-
ducts.

312 THE CELL

My brother and I (VII. 20) attempted to cross different species
of Echinoderms, and found that when the ova of Hehinus micro-
tuberculatus were mixed with the spermatozoa of Strongylocentrotus
lividus, fertilisation took place in every case after a few minutes,
the egg-membrane raising itself up from the yolk. After an hour
and a half all the ova were regularly divided into two. On the
following day glistening germ vesicles had appeared ; on the third,
gastrule ; and on the fourth, the calcareous skeleton had deve-
loped. Cross-fertilisation in the opposite direction yielded varying
results. When the spermatozoa of Hchinus micro-tuberculatus were
mixed in a watch-glass with the ova of Strongylocentrotus, the
greater number of the ova remained unchanged, the egg-mem-
brane raising itself from the yolk in only a few cases. After two
hours only a few isolated ova were divided into two. Amongst
these the egg-membrane lay fairly close to the yolk im some, and
in others was raised alittle. The next day a few glistening germ
vesicles were apparent in the watch-glass, but the majority of the
ova were quite unchanged.

Pfliiger (VIT. 50) observed a similar relationship between Rana
fusca and Rana esculenta. Ova of the former species, when sus-
pended in a watery extract of the testis of Rana esculenta, always
remained unfertilised. When, however, the ova of Rana esculenta
were mixed with the spermatozoa from the testis of Rana fusca,
the greater number of the former developed in a regular manner,
only a few dividing abnormally; however, after the blastula-
stage was reached, they all, without exception, died.

In many respects the results of hybridisation, seen later in
the development of the product of crossing, resemble those of
self-fertilisation. For instance, when fertilisation does take place,
the embryos in many cases die prematurely, or are of a weakly
constitution. ;

The embryos, which develop when certain Echinoderms are
crossed, do not live beyond the gastrula-stage. In the same way,
Pfliiger saw the ova of Rana fusca, which had been fertilised with
the spermatozoa of Rana esculenta, die as germ vesicles. The re-
productive organs of animal hybrids generally atrophy before the
age of sexual maturity is reached, and hence the animals are sterile.

A still larger number of examples is to be found in the vege-
table world. It is true, that seeds may develop, as a result of
hybridisation, but they are defective in their development, and
sometimes even incapable of germination. If, however, germina-

THE VITAL PROPERTIES OF THE CELL 313

tion does take place, the seedlings may be either weakly or
vigorous. Hybrids of widely different species are often very
delicate, especially in youth, so that it is difficult to rear them.
On the other hand, hybrids of*nearly related species are usually
uncommonly luxuriant and vigorous; they are distinguished by
their size, rapidity of growth, early blooming, long life, wealth of
blossoms, strong powers of multiplying, the unusual size of
individual organs, and similar properties.

Hybrids of different species develop a smaller quantity of
normal pollen grains in their anthers than plants of pure descent;
frequently they produce neither pollen nor ovules. In hybrids
of nearly related species, this weakening of the sexual reproduc-
tive powers is not usually to be observed.

Asa general rule, the closer the relationship of the parents,
and the greater their sexual affinity, the better does their hybrid
product thrive. In individual cases it may get on even better
than that of a normally fertilised ovum. For example, when egg-
cells of Nicotiana rustica are crossed with pollen of N. Californica,
a plant is produced which, as regards height, stands to its parents
in the ratio of 228: 100 (Hensen VII. 18).

y. The Influence of the Environment upon Sexual
Affinity. We have seen in our experiments upon self-fertilisa-
tion and hybridisation, that the sexual affinity of the egg and
sperm-cells is a factor which cannot be reckoned upon with cer-
tainty, and with which a series of the most different resulting
phenomena is connected; such as fertilisation or non-fertilisation,
development which has been prematurely hindered and weakened,
or which has been rendered more vigorous, etc. We shall find,
however, that the phenomenon of sexual affinity is still more
complicated by the fact that in many cases it may be influenced
by external circumstances.

Most peculiar facts concerning hybridisation have been dis-
covered by means of experimental researches upon certain
Echinoderms (VII. 20). The unfertilised ova are naked, but
in spite of this, fertilisation does not usually take place when
spermatozoa, which are of nearly related species, and are exactly
similar in appearance, are placed in their neighbourhood, although
these latter settle upon the surface of the ova, and make boring
movements. In this case the non-fertilisation can only be ex-
plained by imagining, that the ovum, if I may use the expression,
refuses to admit the unsuitable spermatozoon.

314 THE CELL

This, however, does not invariably occur. In cross-fertilisations,
which were made between Strongylocentrotus lividus and Spher-
echinus granularis, it was found, that out of the hundreds of ova,
which were experimented upon at various times, a varying num-
ber of eggs was produced, which had been fertilised by the strange
spermatozoa, whilst the large majority of ova were unaffected.
Thus we see, that the ova of the same animal differ from one
another, just as swarm-spores of the same species may react differ-
ently to light, some seeking the positive edge of the drop, others
the negative, and others, again, oscillating between the two (vide
p. 101).. As swarm-spores exhibit different light reactions, the ova
of the same animal present different sex reactions, and what.
is still more extraordinary, these sex reactions can be largely
influenced and altered by external circumstances.

The experiment is a very simple one. The mature ova of
Echinoderms, after their evacuation from the ovaries, can be pre-
served in sea water in an unfertilised condition for 2448 hours
without losing their capacity for development. But, during this
time, changes take place in them, which manifest themselves in
their behaviour towards foreign spermatozoa.

Two different methods were adopted in the experiments, one of
which may be described as the method of successive after-fer-
tilisations. It consisted in this, that the experimenters crossed
the same egg-mass several times with foreign spermatozoa. In
doing this the following important result was obtained: all the
ova, which were crossed immediately after their evacuation from
the distended and full ovary, with extremely few exceptions,
refused the foreign. spermatozoa; but after 10, 20, or 30 hours,
that is to say, after the second, third, or fourth crossing, an
increasingly large proportion of the ova behaved differently,
becoming cross-fertilised, and subsequently developing normally.
The same result was always produced, whether the ova of
Strongylocentrotus lividus were covered with the spermatozoa of
Spherechinus granularis, or of Echinus micro-tuberculatus, or
whether the ova of Spherechinus granularis were crossed with
the spermatozoa of Strongylocentrotus lividus.

The success or failure of hybridisation cannot in these cases be
attributed to a difference in the spermatozoa, since they were each
time taken afresh from a distended and full testis, and may,
therefore, be considered to be a relatively constant factor in the
experiments. In this case, without doubt, it was the egg-cell

THE VITAL PROPERTIES OF THE CELL 315

alone that altered its behaviour towards the foreign sperma-
tozoa.

Hence, if changes take place, or can be induced artificially to
take place, in the egg-cell, by means of which hybridisation is
rendered practicable, we must conclude, from a theoretical point
of view, that it is also possible to induce so complete a hybridisa-
tion between the sexual products of two species, which have a
certain degree of sexual affinity for one another, that scarcely any
ova should remain unfertilised. Thus, according to the conditions
under which the sexual products are brought together, a maximum
or a minimum of hybridisation may be obtained.

In order to establish these relations, it is best, in making the
experiments, to divide the egg-material of a female into several
portions, which are fertilised at different times. The smallest per-
centage of hybrids is always obtained when the foreign spermatozoa
are added to the ova immediately after these latter have been
evacuated from the ovaries. The later fertilisation takes place,
whether after 5, 10,20 or 30 hours, the greater is the percentage
of the hybridised ova, until the maximum of hybridisation is
reached. This is called the stage at which the addition of foreign
spermatozoa produces normally the greatest possible number of
eggs. This period is of short duration, since imperceptible
changes in the ova are uninterruptedly taking place. After that,
the percentage of the ova which, in consequence of the bastard
fertilisation, develop normally, begins to decrease ; and this is due
to the fact, that a steadily increasing number of ova are caused to
segment in an abnormal fashion and to become malformed, in
consequence of several spermatozoa having penetrated into each of
them.

The results obtained by fertilising eggs at different times may
be represented by a curved line, the summit of which corresponds
to the maximum of hybridisation. The results obtained by cross-
ing the ova of Spherechinus granularis with the spermatozoa of
Strongylocentrotus serve as an illustration. When fertilisation
takes place a quarter of an hour after the eggs have been evacu-
ated from the ovary (minimum hybridisation), only a very few
individual ova are developed. After two and a quarter hours 10
per cent. can be fertilised, after six and a quarter hours about
60 per cent., whilst after ten and a quarter hours almost all the
ova, with the exception of about 5 per cent., are affected; in the
latter case they generally develop normally (maximum hybridisa-

316 THE CELL

tion). If the ova are fertilised after twenty-five hours, some
develop normally, and a not inconsiderable number irregularly,
in consequence of multiple fertilisation, whilst a small number re-
main unaffected.

The results obtained with Echinoderm ova seem to me to offer an
explanation of the fact, that domesticated animal and vegetable
species are generally more easily crossed than nearly related species
in the state of nature. The entire constitution seems to be altered
and rendered less stable by domestication. The changes are most
evident in the sexual products, since the generative apparatus is
sympathetically affected by any variations which take place in the
body.

In self-fertilisation, as in hybridisation, sexual affinity is in-
fluenced by the environment. Darwin (VII. 8) has pointed out,
that Eschscholtzia californica cannot be induced to fertilise itself in
Brazil, whilst it can in England; moreover, if seeds from England
are taken back to Brazil, they quickly become useless for self-
fertilisation. Further, various individuals behave in different
manners. Just as in Echinoderms, in which some of the ova of an
ovary may be fertilised with foreign spermatozoa, and others not,
so we find experimentally, that some individuals of Reseda odorata
can fertilise themselves whilst others cannot. In a similar manner
we must attribute to individual differences of the egg-cells of an’
ovule the circumstance that in many plants far fewer seeds are
produced by self-fertilisation and hybridisation than by normal
fertilisation. A certain number of egg-cells either are not receptive
to the foreign pollen, or if they do become fertilised, die prema-
turely.

Recapitulation and attempted Explanations. If we now
review the facts described in the last chapter, there can be no
doubt but that the necessity of fertilisation of the sexual cells and
sexual affinity, which is closely connected with it, are extremely
complicated, vital phenomena. The factors which are influential
here are beyond our knowledge. Many circumstances seem to
' point to the fact, that the conditions, under which the egg-cells

are able to develop either parthenogenetically or in connection
with a sperm-cell, must be sought for in small differences of
molecular organisation. Similarly, we can only explain the facts,
that sometimes self-fertilisation and cross-fertilisation are possible,
and at others not, that the egg-cells of the same individual often
behave differently during self-fertilisation and cross-fertilisation,

THE VITAL PROPERTIES OF THE CELL 317

that the need for fertilisation and parthenogenesis, or the success
of self-fertilisation and cross-fertilisation, may often be influenced
by external circumstances, and that the well-being of the pro-
ducts of generation is dependent upon the mode of fertilisation,
by the presence of these same differences of molecular organisa-
tion.

What now must be the molecular organisation of the sexual cells
which renders them suitable for the purposes of fertilisation ?
Some help towards solving this problem may be obtained by com-
paring the phenomena of self-fertilisation and bastard fertilisation
with normal fertilisation.

As is evident from numerous observations, the result of fertilisa-
tion is essentially determined by the degree of relationship which
the male and female sexual cells bear to one another. The
process of fertilisation is prejudiced by a relationship which is
either too near or too distant ; or, as we may express it, by a tot
great similarity, or a too great difference. Hither the sexual cells
do not unite at all, since they exhibit no sexual affinity towards
each other, or the mixed product of both, i.e. the embryo pro-
duced by fertilisation, is unable to develop ina normal manner. In
the latter case the embryo may either die during the first stages of
development, or it may live as a weakly product; or further, this
weakly product, owing to the destruction of its capacity for re-
production, may be useless for the preservation of the species.
In all cases the product of reproduction thrives best when the
generative individuals, and consequently their sexual cells, differ
only slightly in their constitution and organisation.

Darwin (VII. 8) rendered science a great service when, by means
of his extensive experiments and investigations, he laid the
foundations of this knowledge, and first clearly formulated these
theories. I will quote three of his sentences: “ The crossing of
forms only slightly differentiated favours the vigour and fertility
of their offspring . . . and slight changes in the conditions |
of life add to the vigour and fertility of all organic beings, |
whilst greater changes are often injurious.” The act of crossing |
in itself has no beneficial effect, but “the advantages of cross-
fertilisation depend on the sexual elements of the parents having
become in some degree differentiated by the exposure of their
progenitors to different conditions, or from their having inter-
crossed with individuals thus exposed, or lastly from what we
call in our ignorance ‘spontaneous variation.’” The need of

318 THE CELL

fertilisation consists in “mixing slightly different physiological
units of slightly different individuals.” *

Herbert Spencer (IX. 26) availed himself of these experiments
of Darwin’s, in order to build up a molecular theory of the nature
of fertilisation, which deserves notice as a preliminary attempt.

Spencer, to a certain extent, states as an axiom, that the need
of fertilisation of the sexual cell “recurs only when the organic
units (micelle) are approximating to equilibrium—only when
their mutual restraints prevent them from readily changing their
arrangements in obedience to incident forces.” *

If this hypothesis, which appears to me to be at present but a
possibility, could be proved, we could certainly accept without
further consideration Spencer’s explanation: ‘“ Gamogenesis
(sexual. reproduction) has for its main end, the initiation of a new
development by the overthrow of that approximate equilibrium
arrived at amongst the molecules of the parent organism.”? For
“by uniting a group of units from the one organism with a group
of slightly different units from the other the tendency towards
equilibrium will be diminished, and the mixed units will be ren-
dered more modifiable in their arrangements by the forces acting
on them; they will be so far freed as to become again capable of
that redistribution which constitutes evolution.” ¢ -

In this sense, fertilisation may be considered to be a process of
rejuvenation, to employ the expression used by Biitschli (VII. 6),
Maupas (VII. 30), and others.

Spencer’s statement at present lacks an exact and scientific
foundation, but it seems to deserve notice as a preliminary at-
tempt to solve this extremely difficult question.

An important conclusion may be deduced from the above-
mentioned principle, that the process of fertilisation consists in
the “mixing of slightly different physiological units of slightly
different individuals.” If sexual reproduction is a mingling of the
properties of two cells, it must result in the.development of inter-
mediate forms.

Thus reproduction, so to speak, strikes a balance between

' The first of these quotations is taken from Darwin’s Origin of Species,
p. 432, and the second and third from Darwin’s Cross- and Self-fertilisation of
Plants, pp. 462, 463.

* Principles of Biology, by Herbert Spencer, vol. i. p. 275.

3 Thid., p. 284.

* Ibid., p. 277.

THE VITAL PROPERTIES OF THE CELL 319

differences by producing a new individual, which occupies a mean
position between its parents. By this means numberless new
varieties are developed, which only differ slightly from one
another. Hence Weismann (1X. 34) is of opinion that fertilisa-
tion is an arrangement by means of which an enormous number
of varying individual combinations arise; these supply the
material for the operation of natural selection, the result being
that new varieties are produced.

Whilst agreeing with the first part of this. principle, I cannot
support the second. The individual differences which are called
into being by fertilisation, and which furnish the basis for
natural selection, are as a rule only of an insignificant nature, and
are always liable to become suppressed, weakened, or forced into
another direction, by some subsequent union. A new variety can
only be formed, if numerous members of a species vary in a
definite direction, so that a summation or strengthening of their
peculiarities is arrived at, whilst other individuals of the same
species, which preserve their original characters, or vary in another
direction, must be prevented from uniting sexually with them.
Such a process presupposes the presence of an environment which
always acts in a constant manner, and the existence of a certain
intervening space between the two sets of individuals belonging
to the species, which is destined to divide into two new species.

Sexual reproduction, therefore, seems to me to influence the
formation of a species in a manner opposed to that suggested by
Weismann. By creating intermediate forms, it continually re-
conciles the differences which are produced by external circum-
stances in the individuals of a species; thus it tends to make the
species homogeneous and to enable it to retain its own peculiar
features. Here, too, sexual affinity, that mysterious property of/
organic substance, by preventing a combination, or at any rate a
successful one, between substances which are either too similar or
too dissimilar, acts as an important factor. For, if the sexual
products, on account of their different organisation and their
slight sexual affinity, cannot mingle successfully, the species and
orders in question are kept apart.

Darwin and Spencer express the same opinion. According to
the former, “intercrossing plays a very important part in nature, in
keeping the individuals of the same species or of the variety true
and uniform in character.’ And Spencer remarks : “ In a species
there is, through gamogenesis, a perpetual neutralization of those

320 THE CELL

contrary deviations from the mean state, which are caused in its
different parts by different sets of incidental forces; and it is
similarly by the rhythmical production and compensation of
these contrary deviations that the species continues to live.” }

Literature VII.

1. Averpacn. Ueber einen sexuellen Gegensatz in der Chromatophilie der
Keimsubstanz, etc. Sitzungsber. d. kgl. Preuss. Akad. d. Wissensch.
Nr. 35:

2a. A. pe Bary. Ueber apogame Farne u, die Erscheinungen der Apogamie im
Allgemeinen. Botanische Zeitung. Bd. XXXVI. 1878.

2n. A. pe Bary. Beitriige zur Morphologie u. Physiologie der Pilze. Abhandl.
d, Senkenberg. naturf. Gesellschaft. 1881.

3. vAN BENEDEN, Siehe Capitel VI.

4, Bium. Ueber Reifung u. Befruchtung des Eies von Petromyzon. Arch. f.
mikrosk. Anatomie.’ Bd. XXXII.

5. Boum. Die Befruchtung des Forelleneies. Sitzungsber. d. Gesellsch. f.
Morph. u. Physiol. zu Miinchen. 1891. :

6. Birscuut. Ueber die ersten Entwicklungsvorgiinge der Hizelle, Zellthei-
lung u. Conjugation der Infusorien. Abhandl. der Senkenberg. naturf.
Gesellsch. Bd, X. 1876.

7. Cauperta. Befruchtungsvorgang beim Ei von Petromyzon Planeri. Zeit-
schrift fiir wissenschaftl. Zoologie. Bd. XXX.

8. Darwin. Results of Cross and Self-Fertilisation in the Vegetable Kingdom.
London. 1879.

9. Enearumann. Ueber Entwicklung und Fortpflanzung von Infusorien. Mor-
pholog. Jahrbuch. Bd. I.

10. P. Fanxenserc. Die Befruchtung und der Generationswechsel von .Cut-
leria. Mittheilungen aus der zoologischen Station zu Neapel. 1879.

11. P. Faukenperc. Die Algen im weitesten Sinn. Schenk’s Handb. der
Botanik. Bd. II. 1882.

12. Focxr. Die Pflanzen-Mischlinge. Botanische Zeitung. 1881.

13. H. Fou. Archives des sciences physiques et naturelles. Généve, 15. Oct.,
1883.

14. H.Fou. Le quadrille des centres, wn épisode n dans Uhistoire de la
fécondation. Archives des scienc. phys. et nat. Géneve. Troisieme pér.
Tom. XXV. 1891.

15. L. Guienarp. Nouvelles études sur la fécondation: Comparaison des
phénoménes morpholog. observés chez les plantes et chez les animaux.
Annales des sciences natur. Tom. XIV. Botanique. 1891.

16. M. Hartoc. Some Problems of Reproduction: a Comparative Study of
Gametogeny and Protoplasmic Senescence and Rejuvenescence. Quar.
Journ. Mic. Soc. 1891.

* Principles of Biology, by Herbert Spencer, vol. i. p. 286.

28.

29.

30.

37.

38.

THE VITAL PROPERTIES OF THE CELL 5 |

HENKING. Untersuchungen iiber die ersten Entwicklungsvorginge in den
Eiern der Insekten. Zeitschr. f. wissenschaftl. Zoologie. Bd. 49, 51, 54.
HENsEN. Die Physiologieder Zeugung. Handb. der Physiologie. Bd. VI.

Oscar Hertwic. See Cap. VI., Nr. 30a, 32, 33, 34.

Oscar Hertwie u. Ricnoarp Hertwic. Experimentelle Untersuchungen
tiber die Bedingungen der Bastardbefruchtung. Jena. 1885.

Ricwarp Hertwic. Ueber die Conjugation der Infusorien. Abhandl. der
bayer. Akad. der Wissensch. Cl. 1I. Bd. XVII. 1889.

R. Hertwie. Ueber die Gleichwerthigkeit d. Geschlechtskerne bei den
Seeigeln. Sitzungsber. d. Gesellsch. f. Morphol. u. Physiol. in Miinchen.
Bd.IV. 1888.

R. Hertwic. Ueber Kernstructur u. ihre Bedeutung f. Zelltheilung uw.
Befruchtung. Ebenda.

HILpEeBrandD. Die Geschlechter-Vertheilung bei den Pflanzen, etc. Leipzig.
1867.

Isaixawa. Vorldufige Mittheilungen iiber die Conjugationserscheinungen
bei den Noctiluken. Zoolog. Anzeiger. Nr. 353. 1891.

Keuuer. Die Wirkung des Nahrungsentzuges auf Phylloxera vastatriz.
Zoolog. Anzeiger. Bd. X. p. 583. 1887.

KureBaHn. Studien iiber Zygoten: Die Keimung von Closterium und
Cosmarium. Pringsheim’s Jahrbiicher f. wissenschaftl. Botanik. Bd.
XXII.

Kuirss. Zur Physiologie der Fortpflanzung. Biolog. Centralblatt. Bd.
IX. 1889.

E. L. Marx. Maturation, Fecundation, and Segmentation of Limax cam-
pestris. Bullet. of the Museum of Comp. Zool. at Harvard College. Vol.
VI. 1881.

E. Mavpas. Le rajeunissement karyogamique chez les ciliés. Arch. de
Zool. expér. et génér. 2e série. Vol. VII.

C. Niceni. Die Bastardbildung im Pflanzenreiche. Sitzungsber. der kgl.
bayer. Akad. d. Wissensch. zu Miinchen. 1865. Bd. II. p. 395.

C. Nicer. Die Theorie der Bastardbildung. Sitzungsber. der kgl. bayer.
Akad. der Wissensch. zu Miinchen. 1866. Bd. I.

Nusspaum. Zur Differenzirung des Geschlechts im Thierreich. Arch. f.
mikroskop. Anatomie. Bd. XVIII.

OprgL. Die Befruchtung des Reptilieneies. Arch. f. mikroskop. Anat.
Bd. XXXIX. 1892.

. PrincsHeim. Ueber die Befruchtung der Algen. Monatsber. d. Berliner

Akad. 1855.

. PRINGSHEIM. Jeber Paarung von Schwéirmsporen, die morphologische

Grundform der Zeugung im Pflanzenreich. Ebenda. 1869.

Rickert. Ueber physiologische Polyspermie bei meroblastischen Wirbel-
thiereiern. Anat. Anzeiger. Jahrg. VII. Nr. 11. 1892.

Setenkxa. Befruchtung der Eier von Voxopneustes variegatus. Lvipzig.
1878.

SrrasBurGER. Neue Untersuchungen iiber den Befruchtungsvorgang bet
den Phanerogamen als Grundlage fiir eine Theorie der Zeugung. Jena.
1884.

» 4

41.

42.

43.

44,

45.

46,

47.

48.

49,

50.

51.

52.
53.

54,
55.
56.

THE CELL

Weismann. Beitrige zur Naturgeschichte der Daphnoiden. Zeitschr. f.
wissenschaftl, Zorlogie. Bd. XXXIIT.

Weismann. On the Number of Polar Bodies and their Significance in
Heredity, trans. by Schinland ; Essays upon Heredity, trans. by Poulton,
Schinland, and Shipley. Oxford. 1889.

Wersmann u. Isurxawa. Ueber die Bildung der Richtungskérper bei thie-
rischen Eiern. Berichte der naturforsch. Gesellsch. zu Freiburg. Bd.
III. 1887.

WEIsMANN and Isuikawa. Weitere Untersuchungen zum Zahlengesetz der
Richtungskirper. Zoolog. Jahrbiicher. Bd. IIT., Abth. f. Morph.

Orro Zacwartas. Neue Untersuchungen iiber die Copulation der Gesch-
lechtsproducte und den Befruchtungsvorgang bei Ascaris megalocephala.
Archiv f. mikroskop. Anat. Bd. XXX. 1887.

Buiocumann. Ueber die Richtungskirper bei Insecteneiern. Mor phol.
Jahrb. Bd. XII.

Brocamann. Ueber die Reifung der Eier bei Ameisen u.Wespen. Festschr.
zur Feier des 30 Ojahr. Bestehens der Univers. Heidelberg. 1886. Med.
Theil. i

Btocumann. Ueber die Zahl der Richtungskirper bei befruchteten und un-
befruchteten Bieneneiern. Morphol. Jahrb. Bd. XV.

Pratner. Ueber die Bildung der Richtungskérperchen. Biolog. Central-
blatt. Bd. VIII. 1888-89.

WeIsMANN. On Heredity, trans. by Shipley ; The Continuity of the Germ-
Plasm as the Foundation of a Theory of pase trans. by Schénland ;
Essays on Heredity. Oxford., 1889.

Herm. Miuurr. Die evuchtuss der Blumen durch Insecten. Leipzig.
1873.

Priicer. Die Bastardzeugung bei den Batracheiern. Archiv f. die ges.
Physiologie. Bd. XXIX.

BrrruowD. Die geschlechtliche Fortpflanzung der eigentl. Phaeosporeen.
Mittheil. aus der zool, Station zu Neapel. Bd. II. 1881.

Darwin. The Origin of Species. London. 1869.

Darwin. Variation of Animals and Plants under Domestication. London.
1875.

Herserr Spencer. Principles of Biology. London. 1864.

Ray Lanxester. Art. Protozoa, Encyclopedia Britannica. London. 1891.

Herperr Spencer. First Principles. London. 1870.

CHAPTER VIII.

METABOLIC CHANGES BETWEEN PROTOPLASM, NUCLEUS, AND
CELL PRODUCTS.

Aut the morphologically different parts of a living organism
necessarily stand to one another in a definite relation, as regards
metabolic changes. In most cases it is extremely difficult to
understand these relations, on account of the complexity of the
vital processes. However, some knowledge has already been
gained upon the subject, by means of observation and experiment,
and the fact that protoplasm takes part in all formative processes,
such as the formation of the cell-wall, of intercellular substance,
etc., is indicated by various circumstances, which can scarcely be
explained in any other manner.

In plants the main portion of the protoplasm is always massed
together at those parts, where growth is chiefly taking place: e.g.
at the ends of growing root-hairs, in the growing hyphe, with
Fungi, etc., and at the growing points of multicellular and uni-
cellular plants, such as Caulerpa. Further, the protoplasm, in /
individual cells, always accumulates in the regions of greatest |
activity.

Sometime before the cellulose membrane of a plant-cell forms
thickenings or sculpturings, the protoplasm undergoes prepara-
tory changes, by collecting in the places where the most rapid
growth is taking place. Further, whilst these thickenings are
being formed, continuous streams of granular protoplasm are seen
to pass along them.

If a small portion of Vaucheria is cut off, the protoplasm im-
mediately tries to repair the injury. ‘Granular plasma can be
seen to collect in dense masses about the wound, and to close up
to form a layer, which is sharply defined externally. A cell-
membrane immediately commences to develop upon this layer.”
(Klebs.)

If the protoplasm of a plant-cell has by means of plasmolysis

been separated from its membrane, without damage having been
323

324 i THE CELL

done to its vital functions, it soon develops upon its surface a new
cellulose layer, which becomes stained red when congo-red is
added to the water. ;

As long as cells are young and growing vigorously, they contain
a large quantity of protoplasm, whilst older cells, especially those
in which formative activity has been arrested, only contain a small
quantity of it. For instance, the protoplasmic layer, on the inner
surface of the cellulose membrane of large and fully developed
plant cells, may be so extremely thin that its presence, as a dis-
tinct stratum, can only be demonstrated by means of plasmolysis.
Similarly, only minute traces of protoplasm are present in the
notochordal cells of animals, ete.

The relations that the nucleus bears to the remaining com-
ponent parts of the cell are at present attracting great attention.
It has already been shown (p..214) that very remarkable meta-
bolic interactions take place between the nucleus and the proto-
plasm during the processes of division. But it is evident, that
the nucleus plays an important physiological part at other times,
as well, in the life of the cell; all the formative and nutritive
processes seem to be dependent upon it, and to bear a close re-
lationship to it. The true nature of this relationship, however,
cannot at present be more exactly defined, as may be deduced
from the observations of Haberlandt and Korschelt, which will be
described later, as well as from the experiments of Gruber, Nuss-
baum, Balbiani, Klebs and Hofer.

1. Observations on the position of the nucleus, as an
indication of its participation in formative and nutritive
processes. According to the extensive and important observa-
tions of Haberlandt (VIII. 4) the nucleus of young and developing
plant-cells is “ situated in that portion of the cell where growth is
most active, or lasts longest. This is true both for the growth of
the cell as a whole and for the increase in volume and superficial
area of the cell-membrane in especial. If the cell is growing in
more than one place, the nucleus takes up a central position, so
that it is about equidistant from the regions of most active growth
(Fig. 161, ID). Occasionally the nuclei are connected with the
places of most active growth by means of protoplasmic strands,
which are as short as possible. The nucleus only rarely retains
its original position in fully developed cells. As a rule it has left
the place which it occupied in the growing cell, and generally has

METABOLIC CHANGES Byes

no definite position. In other cases, however, its position is
fixed.”

I will cite a few especially instructive examples from the
numerous observations, on which Haberlandt has based his laws.

The epidermal cells of many plants often exhibit thickenings
on the surface of their walls; this may occur either on those
pointing outwards or on those pointing inwards. The nucleus
here lies near to the one in which the thickening occurs, being
always close to the middle of the latter. The examples given in
Fig. 161 show this very distinctly: No. [., a row of cells from the
epidermis of a foliage-leaf of Cypripedium insigne; No. III. an
epidermal cell of the fruit-scale of Carex
panicea, and No. IV. a young epidermal cell
of a foliage-leaf of Aloé verrucosa.

A second series of investigations have
been made upon the development of plant-
hairs, growing both above and_ below
ground.

EL

Fie. 161. Fre. 162.

Fre. 161.—I Epidermal cells of a foliage leaf of Cypiipedium insiqne (after Haberlandt,.
Pl. I., Fig. 1). II Epidermal cells of Luzula maxima (after Haberlandt, Pl. I., Fig. 3). III
Epidermal cells of the fruit-scale of Carex panicea (after Haberlandt, Pl. I., Fig.14). IV
Young epidermal cells of a foliage leaf of Aloé verrucosa (after Haberlandt, Pl. I., Fig. 7).

Fig. 162.—A Root-hair of Cannabis sativa (after Haberlandt, Pl. II., Fig. 26). B Forma-
tion of root-hairs of Pisum sativum (after Haberlandt, Pl. IL., Fig. 22).

326 THE CELL

The tender root-hairs of plants exhibit a characteristic struc-
ture at their growing points. Hence the nucleus, as long as
growth continues (Fig. 162 A), is situated at the free end, whilst
when the hairs are old and fully developed, it is higher up.
When a root-hair is developing out of an epidermal cell, a protu-
berance is always formed upon that part of the external wall,
which is situated over the cell-nucleus (Fig. 162 B). In many
plants (Brassica oleracea) the root-hair cell may form branches,
into one of which the single nucleus enters. This one becomes at
once the richest in protoplasm and also the longest, whilst the
other branches leave off growing.

The hairs that grow above ground, differ from the root-hairs,
in that they exhibit a basipetal, or intercalary growth, as Haber-
landt has established by measurements. In consequence of this,
the nucleus is not situated at the apex, but near t0 the place,
where the secondary, basal growing-point is situated, and where
longitudinal growth persists longest.

Stellate hairs (Fig. 163) are peculiar, unicellular structures,
which split up at their peripheral end into several radially diver-
gent branches. Under these circumstances the nucleus, as long as
the formative processes continue, is situated in the middle of the
radiation, but after growth is finished it returns to its former
position near to the base.

Contirmatory evidence of this participation of the nucleus in the
formative processes is furnished us by the examination of Fungi
and Alge. In the multi-nucleated
hyphe of Saprolegnia lateral
branches develop; these are always
found immediately over a nucleus,
which is situated close to the cell-
wall. In Vaucheria and other multi-
nucleated Alge, as in the higher
plants, special growing points are
present, at which growth chiefly oc-
curs ; at each of these, immediately
underneath the cellulose membrane,
there is an accumulation of small
nuclei, after which comes a layer

F 14. 163.—Young stellate hair of Au- gi poe eraies Satie! ashe ee
trictia delloidea (after Haberlandt, Pi.it,, 28 Portions of the cell the positions
Fig, 28), of these bodies are reversed.

METABOLIC CHANGES 327

Phenomena, which are still more remarkable, and which indi-
cate the part played by the nuclei in the formation of the cell-
wall, are to be observed during the healing of wounds in
Vaucheria. Numerous small nuclei appear in the protoplasm,
which collects round about the wound, thus approaching the
upper surface, whilst the grains of chlorophyll are forced back
in exactly the opposite direction. By this means the nuclei and
chlorophyll grains exchange places. This observation immedi-
ately refutes the objection, which might otherwise easily be
raised, namely, that the nucleus or nuclei are present in those
places to which the protoplasm flows in greater quantities, be-
cause they are carried along by the protoplasmic stream. For, if
this were the case, we should expect to find the chlorophyll grains
also in the same places, since these are much smaller than the
nuclei, and may even be induced to change their positions by
variations in illumination, which have no effect upon the nuclei.

“Thus we see,” as Haberlandt remarks, ‘‘that the nuclei and
chlorophyll grains exhibit quite independent changes of position,
which, if we assume that they are passive, cannot in any way be
influenced by the movements of the granular plasma as a whole.
These phenomena—that the streaming protoplasm to a certain
extent selects the bodies, which it carries along with it, in the one
case taking the larger cell-nucleus, and leaving the smaller chro-
matophores and neglecting the cell nuclei, which are as small or
even much smaller—can only be explained by supposing, that
their réle is to effect definite accumulations, which depend upon
the functions of the nuclei and the chromatophores.”

Korschelt (VIII. 8) has demonstrated, that relations, similar to
those described by Huaberlandt, as existing between the position
and the function of the nuclei in plant cells, are also present in
animal cells.

Ova increase considerably in size, by absorbing large quantities
of reserve materials. In these, the germinal vesicle is frequently
found in that place, where the absorption of material must of
necessity take place. Thus, for instance, in one species of Coelen-
terates, the ova are derived from the endoderm and are nourished
by the gastrovascular system by means of endodermal cells. In
conformity with the above-stated law the germinal vesicles of
young ova are situated superficially near to the surface of that
wall, which is turned towards the gastric cavity (Fig. 164). In
many Actinie (Hertwig, VIII. 5b) the ova, for a considerable

328 : THE CELL

period, protrude a stalk-like (peduncular) process right up to the
surface of the intestinal epithelium (Fig. 165). This process has
a regular fibrillary (rodded) structure, as is always seen, when an
active exchange of material takes place in definite directions ;
it may, therefore, be considered to be a special nutrient apparatus
of the ovum. In this case, too, the germinal vesicle is always
situated in immediate contact with the base of the nutrient ap-
paratus. ;

Va Retr cerry eae eae sink
Fie. 164. Fie. 165.

Fig. 164.—Immature ovum of Actinia parasitica. (x 145: after Korschelt, p. 47, Fig. 8.)

Fig. 165.—Transverse section through the peripheral end and through the stalk of egg-
cells of Sagartia parasitica (after O. and R. Hertwig); from Korschelt, Fig. 10. The
striated stalk of the egg-cell has penetrated into the epithelium at the top of the figure.

A similar condition is found in the tubular ovaries of Insects,
which are divided into germ compartments and yolk compart-
ments. In this case the germinal vesicle is either again placed
close to the yolk compartment, or, which is more interesting, it
extends towards this compartment numerous pseudopodic pro-
cessess, by which means it considerably increases its superficial
area in that region, where the absorption of material is taking
place. Here, too, the yolk in the neighbourhood of the germinal
vesicle ‘begins to separate off numerous dark granules, which have
been derived from the nutritive cells. .

In most animals the ova are nourished by means of the follicular
cells. Thus Korschelt has found that, as.long as the formation of
the yolk and chorion is proceeding, the nuclei of the follicular
cells in Insects are situated in immediate contact with that surface

METABOLIC CITANGES 329

which is directed towards the ovum, whilst after the chorion has
been completed, they retreat into the middle of the cell.

Still more striking is the behaviour of the nuciei in the so-called
double cells, which occur in the eggs of water-bugs (Ranatra and
Nepa, Fig. 167 A, B). These develop radiating chitinous pro-
cesses on the chorion. The protoplasmic bodies of the two cells,
between which a radiation figure develops, coalesce. During this
process both of the very large nuclei extend numerous fine pro-
cesses towards that side, which is turned towards the radiated
figure.

Fie. 166. Fig. 167.

Fie. 166.—Egg-follicle of Dytiscus marginalis with neighbouring yolk compartment, in
which a large number of granules are being separated off. The germinal vesicle of the
ovum is extending processes towards the accumulations of granules. (After Korschelt,
Pld. Fig520:)

Fig. 167.—A Transverse section of a secreting double cell from the egg-follicle of Nepa
cinerea L. The formation of the radiation figure is still taking place (x 270: after Kors-
chelt, Pl. V., Fig.120). B Longitudinal section of a double cell from the egg-follicle of Nepa.
Commencement of the development of the radiation figure (x 195: after Korschelt, Pl. V.,
Fig. 121).

From these and similar observations, Haberlandt and Korschelt
draw the following conclusions, respecting the function of the
cell-nucleus :— R

1. “The fact that the nucleus is generally found in a definite
position in the immature and developing cells, indicates that its
function is connected chiefly with the developmental processes of
the cell.” (Haberlandt.)

2. “From its position it may be concluded that the nucleus
plays a definite part during the growth of the cell, especially
during the thickening and increase in superficial growth of the

330 THE CELL

cell-wall. This does not prevent it from eventually fulfilling
other functions in the fully developed cell.” (Haberlandt.)

3. The nucleus takes part both in the excretion and absorp-
tion of material. This is shown by its position, and also by the
fact that the nucleus increases its superficial area by extending
numerous processes towards the place where excretion and absorp-
tion are occurring.

II. Experiments proving the reciprocal action of the
nucleus and protoplasm. The experimental researches of
Gruber, Nussbaum, Hofer, Verworn; Balbiani, and Klebs have
led to the same results. ‘Their method was to divide by some
means or other, a unicellular organism or a single cell into two
portions, one nucleated and the other non-nucleated, and then to
follow and compare their future behaviour.

By means of plasmolysis in 16 per cent. sugar solution, Klebs
was enabled (IV. 14; VIII. 7) to divide the cells of Spirogyra
threads into one nucleated part and several non-nucleated portions.
Although these latter sometimes live for six weeks before they
disintegrate, the vital processes occurring in them differ con-
siderably from those taking place in the nucleated ones, the latter

continuing to grow and to surround themselves with a new cell-wall, -

which stains easily with congo red, and can thus be rendered
visible. The former on the other hand remain globular in form,
do not increase in size, and develop no cell-wall. That the
_ latter process is considerably influenced by the presence of the
nucleus, is clearly shown by the fact that, when the fragments
obtained by means of plasmolysis, are connected by a thin
bridge of protoplasm, the non-nucleated part is able to form
cellulose.

However, certain metabolic processes take place in protoplasm
without the presence of the nucleus; for instance, the non-
nucleated parts are still able to assimilate, to dissolve, and to form
starch, provided that they contain a portion of the chlorophyll-
band. If they are kept for a considerable time in the dark, they
become free from starch, because they have used up the stock of
stored-up granules; when they are brought back again into the
light, the chlorophyll bands recharge themselves with - newly-
assimilated starch ; indeed, in this case the accumulation of starch
1S even greater than in the nucleated part, probably because its

e . . . . e P
consumption, whilst all the other vital functions are in abeyance,
is reduced to a minimum.

he

METABOLIC CHANGES 331

Non-nucleated portions of Funaria hygrometrica behave some-
what differently, in that they are able to dissolve starch, but
cannot develop it, even if they remain alive for six weeks.

When a Vaucheria thread is divided into various sized masses
of protoplasm, scme of which contain nuclei, we find that the
vital activity of these, as well as the separation of a new cellulose
membrane, depends upon the presence in each, of at least one
cell-nucleus. (Haberlandt, VIII. 4.)

Results, which are no less important than those obtained with
plants, are observed when Ameabe, Reticularia and Ciliata are cut
up. Nussbaum (VIII. 9), Gruber (VIII. 3), Hofer (VIII. 6), and
Verworn (VIII. 10) all agree that only nucleated parts are able to
replace organs which they had lost, and thus to reconstruct them-
selves into normal individuals, that grow and multiply. Non-
nucleated portions, even when they are larger than the nucleated
ones, are unable either to replace the lost organs or to grow, but
for some time, often for more than fourteen days, appear to lead a
kind of psendo-existence ; eventually, however, they disintegrate.
Thus the formative activity of protoplasm appears to be primarily
influenced by the nucleus. This is less certainly established in
the case of the other functions of the cell, viz. power of move-
ment, irritability and processes of digestion. As regards these the
opinions of different observers vary.

Hofer observed that a non-nucleated portion of an Ameba,
after the first stage of irritability occasioned by the operation had
passed off, exhibited for from fifteen to twenty minutes, move-
ments which were nearly normal. He ascribes this to an after-
effect of the nucleus, which, he considers, exerts a regulating
influence upon the movements of the protoplasm. For whilst,
further, the nucleated part extends pseudopodia like a normal
individual, and propels itself forwards, the non-nucleated part
contracts up into a round body, and only occasionally, after pauses
of many hours’ duration, makes abnormal, jerky movements ; it
does not attach itself to the bottom of the glass, as crawling
Amebe do, and in consequence vibrates upon the slightest move-
ment of the water.

Verworn discovered that the protoplasm in Diflugia was still
more independent of the nucleus. Hven small non-nucleated por-
tions extended long finger-like pseudopodia in a manner character-
istic of an uninjured Rhizopod, and continued their movements
even for five hours. Further, they were unimpaired as regards

352 : THE CELL

irritability, reacting to mechanical, galvanic, and chemical stimuli
by contracting their bodies.

According to Verworn, Culiata, too, which have developed
special locomotive organs, such as cilia, flagella, cirrhi, etc.,
assume, when cut up, a complete autonomy and independence of
the nucleus.

In Lacrymaria, each part, when deprived of its nucleus, ex-
hibits, after its separation from the body, the same movements
as it was performing before. Small portions of Stylonichia,
which are furnished with a number of ventral cilia, continue to
make with them the movements peculiar to their species. Even
the minutest portion of protoplasm, which is furnished with only
one bristle-like cilium, continues to make with it characteristic
movements. If it was directed backwards, it is suddenly from
time to time jerked forwards, by which movement the portion
receives a short jerk backwards ;. thereupon the cilium returns
again to a state of rest, and so on.

The contractile vacuoles of the Protista are, like cilia and
cirrhi, remarkable for complete autonomy. Even in non-nucleated
portions they can be observed to contract rhythmically for days
together (Verworn).

Finally, an important difference is noticeable between non-
nucleated and nucleated portions, as regards digestion. Whilst
small Infusoria, Rotifera, etc., are normally digested by nucleated
portions, in non-nucleated parts digestion is considerably dimin-
ished, both as regards time and intensity. It may, therefore,
be concluded that protoplasm can only produce digestive secre-
tions with the assistance of the nucleus (Hofer, Verworn).

It is not surprising that diversities of opinion, as mentioned in
Chapter VII., should exist upon this subject, when the difficulty
of the problems to be solved be taken into account.

Literature VIII.

1. Bauprant. Recherches expérimentales sur la mérotomie des Infusoires ciliés.
Prém. part. Recueil. Zool. Suisse. 1889.

2. Boveri. Hin geschlechtlich erzeugter Organismus ohne miitterliche Eigen-
schaften. Gesellsch. f. Morphol. u. Pysiol. zu Miinchen. 1889.

3. Grouper. Ueber die Einflusslosigkeit des Kerns auf die Bewegung, die —
pbhcead u. das Wachsthum einzelliger Thiere. Biolog. Centralblatt.

Grouper. Ueber kiinstliche Theilung bei Infusorien, Biolog. Centralbdl.
Bd. IV.u. V.

5a.

5B.

METABOLIC CHANGES 333

HaBerntannt. Ueber die Beziehungen zwischen Function und Lage des Zell-
kerns bei den Pflanzen. Jena. 1887.

Oscar u. Ricaarp Hertwic. Ueber den Befruchtungs- u. Theilungsvorgang
des thierischen Eies unter dem Einfluss Giusserer Agentien. Jena. 1887.

Oscar u. Richarp Hertwic. Die Actinien, anatomisch und histologisch mit
besonderer Beriicksichtigung des Nervenmuskelsystems untersucht. Jena.
1879.

Horer. Experimentelle Untersuchungen iiber den Einfluss des Kerns auf
das Protoplasma. Jenaische Zeitschrift f. Naturwissenschaft. Bd. ay:

Kuess. Ueber den Einfluss des Kerns in der Zelle. Biolog. Centralbl. Bd.
VII. 1887.

KORSCHELT. Beitriige zur Morphologie u. Physio'ogie des Zellkerns. Zovl.
Jahrbiicher. Abth. f. Anatomie. Bd. IV. 1889.

Nusspaum. Ueber die Theilbarkeit der lehendigen Materie. Archiv. f.
mikroskop. Anatomie. Bd. XXVI. 1886.

Verworn. Die physiologische Bedeutung des Zellkerns. Archiv. f. d. ges.
Physiologie. Bd. LI. 1891.

Vines. Students’ Text-book of Botany. London. 1895.

Cuakk, J. Protoplasmic Movements and their relation to Oxygen Pressure.
Proceedings of the Royal Society, XLVI. 1889.

WoopgEaD AnD Woop. The Physiology of the Cell considered in relation to
its Pathology. Edinburgh Medical Journal. 1890.

CHAPTER IX.

THE CBLY AS THE ELEMENTAL GERM OF AN ORGANISM
(THEORIES OF HEREDITY).

f We are forced to the conclusion, that the cell is a highly or-
\ ganised body, composed of numerous, minute, different parts, and
that hence it is in itself to a certain extent a small elementary
~ organism, when we consider, that it is capable of executing move-
ments, and of reacting in a constant manner to the most various
external stimuli, which may be chemical, mechanical, or caused by
heat or light; and further that it can execute complicated chemical
processes and can produce numerous substances of definite com-
( position.

This idea is still more impressed upon us, when we take into
account the fact, that egg- and sperm-cells form by their union
the elemental germ which develops into an organism, the latter
reproducing on the whole the attributes of the parents, even often
to the most insignificant characteristics. Hence we must conclude,
that the egg- and sperm-cells possess all the constituent proper-
ties which are necessary for the production of the final result of
the developmental process. It is true that these properties elude
our perception, but that they are anything but simple, is evident
from the complex composition which is attained by the final
product of development in the highest organisms. » The sexual
cells must therefore, of necessity, possess a large number of attri-
butes and characteristics, which are concealed from us, but whose
presence renders the formation of the final product possible. These
hidden or. latent properties, which only gradually become evident
during the process of development, are called fundamental con-
stituent attributes. These attributes, taken collectively, to a
certain extent foredhadow, or potentially determine the matured
organism.

At a certain stage of their development, when they are simple
cells, all organisms are extremely alike. The ova of man, of ro-
dents, of ruminants, and even of many invertebrate animals, do not

differ from one another in any essential points; they resemble one |
334

THE CELL AS THE ELEMENTAL GERM OF AN ORGANISM 335

another more closely than do the egg- and sperm-cells of the same

animal.

However, these similarities and differences in form appear to be
of less importance when we go more deeply into the subject. For,
as men, rodents, ruminants, and invertebrate animals present to
us more or less important external differences, the sexual cells
originating from them must differ in a corresponding manner as
regards their fundamental attributes, in so far as they represent
the embryonic stage of the subsequent complete organism. The
only thing is that, at present, the essential differences lie beyond
our perception. On the other hand the egg- and sperm-cells of
the same organism, although they differ so much in external
appearance, must resemble one onother in their essential properties,
since they must contain aaa all the characteristics of the
fully-developed animal.

Nageli pertinently remarks (Ix. 26): ‘The egg cells must
contain all the essential characteristics of the mature organism,
and hence they must differ as much from one another, when they
are in this early stage, as when they are more fully developed.
The Hen’s egg must possess the characteristics of its species as
completely as the Hen, and hence must differ as much from the
Frog's egg as the Hen does from the Frog.”

What is true of the egg is equally true of individual cells and
collections of cells, which, being detached from the mother or-
ganism, either as spores or buds, are able to reproduce the parent.
They, too, must possess all the essential properties of the whole,
in an embryonic condition, although they are imperceptible to us.

( What idea can we form to ourselves of these invisible properties

< of the cells, which predetermine the complex organism? What is

/ the connection between the developed and undeveloped stage ?

' These problems are amongst the most difficult which the theory
of life presents. Scientists and philosophers have occupied them-
selves with these questions for centuries, and have formulated
their conclusions in hypotheses, which have frequently influenced
enquiry. We will mention shortly those theories which are most
important historically, since they are both of general interest,
and will serve as a suitable introduction to the consideration of
the views, which are suggested by modern research.

c

I. History of the older Theories of Development. Two
important scientific theories which are directly opposed to one

336 THE CELL

another, were advanced up to the beginning of this century; viz.,
the theory of Preformation or Evolution and the theory of Epiyenesis.

The theory of Preformation was embraced by such well-known
authorities of the 17th and 18th centuries, as Swammerdam, Mal-
pighi, Leeuwenhoek, Haller, Bonnet (1X. 3), and Spallanzani (cf.
His IX. 14). They held the opinion, that the germ, as regards
structure, absolutely resembles the mature organism, and that
hence it must, from the very first, possess similar organs, which,
although extremely minute, must be in the same positions and
similarly related to one another. Since, however, it was impossible
by means of the microscopes at their command, actually to observe
and demonstrate these organs, which they assumed to be present
in the egg at the beginning of its development, they took refuge
in the theory, that certain parts, such as the nervous system,
glands, bones, etc., were present not only in a minute, but also in
a transparent condition.

In order to render the process more comprehensible, the de-
velopment of the butterfly from the chrysalis, and the flower from
the bud, were quoted as examples. Just as a small bud of green,
tightly closed sepals, contains all the parts of the flower, such as
stamens and coloured petals, and as these parts grow in secret, and
then suddenly, when the sepals unfold, become revealed, so the
“ Preformists ” considered, that the minute parts, which are sup-
posed to be present in a transparent condition, grow, gradually
reveal themselves, and become perceptible to our eyes.

Hence the old name of the “ theory of Evolution or Unfolding,”
in the place of which the more pertinent, intelligible, designa-
tion of the “ theory of Preformation” has been adopted. For the
peculiarity of this doctrine, is that nothing is supposed to be
newly formed at any period of development, each part being
present or preformed from the beginning, and that, therefore, the
true nature of development or growth is denied. ‘There is no
new development,” says Haller, in his Elements of Physiology ;
“no part in the animal body is formed before the other; all are
created at the same time.”

The theory of Epigenesis is directly opposed to the theory of
Preformation. Its chief supporter was Caspar Friedrich Wolff
(IX. 36), who lived in the middle of the 18th century. In his
important paper, entitled “ Vheoria Generationis,” published in the
year 1759 (Germ. ed. 1764), he enunciated the following axiom,
which was in opposition to the generally accepted dogma of pre-

AS THE ELEMENTAL GERM OF AN ORGANISM 337

formation, namely, “that what cannot be perceived by the senses,
is not present in a preformed state in the germ; that the germ at
the outset is nothing but unorganised matter, excreted from the
sexual organs of the parents, which in consequence of fertilisation,
gradually becomes organised during the process of development.”
He states further that the organs differentiate themselves one after
another out of this unorganised germinal substance, and he tried
to actually demonstrate this process in individual cases. Thus he
showed how various plant organs gradually differentiate them-
selves out of the germinal substance, and in so doing undergo
alterations in their shape, and he pointed out that the intes-
tinal canal of a chick develops out of a leaf-shaped embryonic
structure.

By thus basing his arguments upon accurate observation, in-
stead of upon preconceived notions, Wolff laid the foundation-
stone of the important hypothesis, which, based upon the theory
of development, has been gradually built up during the course of
this century.

If\we carefully compare these two theories, we see that neither
can be accepted in its entirety. Both have their weak points.

The theory of Preformation is open to attack from the stand-
point of the evolutionists, since, in the higher organisms, each
individual is produced by the co-operation of two members of
separated sexes. When, later on, Leeuwenhoek discovered the
existence of spermatozoa as well as ova, an animated discussion
arose as to whether the egg or the spermatozoon constituted the
preformed germ.

The hostile schools of the Ovists and Animalculists existed for
a century. The Ovists, such as, for instance, Spallanzani, stated
that the unfertilised ovum of a Frog was a diminutive Frog,
being of opinion that the spermatozoon only acted as a stimulating
agent, exciting vital activity and growth. The Animalculists, on
the other hand, by means of the magnifying glasses at their dis-
posal, discovered the presence of heads, arms, and legs in the
spermatozoon. They therefore considered that the egg was only
a suitable nutrient medium, which was necessary for the develop-
ment of the spermatozoon.

Further, the theory of Preformation, more logically worked ont,
leads to very serious difficulties. One such obstacle, which even
Haller and Spallanzani did not think could be overcome, was the
consideration that the germs of all the subsequent animals would

Z

338 THE CELL

have to be stored up or contained in one germ. This principle
would necessarily follow from the fact, that sexual animals
develop in. unbroken sequence from one another. Therefore, the
natural outcome of the Preformation theory, is the pill-box theory,
or, as Blumenbach (IX. 2) expresses it, the theory of the ‘ im-
prisoned germs.” The eagerness of its supporters actually carried
them so far, that they reckoned out how many human germs were
boxed up in the ovary of mother Eve, and put down the number
as, at the very least, 200,000 millions (Elemente der Physiologve, by
Haller).

On the other hand, the theory of Epigenesis in its older form,
when worked out more fully, also presents difficulties. For the
question suggests itself how nature, with the forces that we know
of at her command, can produce in a few days or weeks, out of
unorganised matter, an animal organism resembling. its progeni-
tors. On this point no theory, which regards the organism as. a
completely new creation, can supply us with an acceptable and
satisfactory solution.

Blumenbach (XI. 2), therefore, took refuge in the conception of
a peculiar “nisus formativus,” or formative instinct, which was
supposed to cause the unformed or unorganised male and female
fluids to assume a “ formation,” ¢.e. a definite form, and later on to
replace any parts that had been lost. But if we accept the exist-
ence of an especial formative instinct, we have obtained nothing
more than an empty expression, in the place of an unknown thing.

The cell theory, which has been gradually worked out during
the latter half of this century, has furnished us with new funda-
mental facts, upon which to base more accurate theories of genera-
tion and heredity. These facts are, first, that ova and spermatozoa
are simple cells, which free themselves from the parent organism
for the purposes of reproduction, and that the developed organisms
are only organised combinations of a very large number of such
cells, which are able to function in various ways, and which are
produced by the repeated division of the fertilised egg-cell. A
second, and still more advanced principle, is, that the cell in it-
self is an extremely complex body, that is to say, that it is an
elementary organism.’ Thirdly, we have gained a fuller know-
ledge of the process of fertilisation, of nuclear structure and
nuclear division (longitudinal division and arrangement of the
nuclear segments), whilst the discovery of the fusion of the egg
and sperm nuclei, of the equivalence of the male and female

AS THE ELEMENTAL GERM OF AN ORGANISM 339

nuclear masses, and of their distribution amongst the daughter-
cells, has given us a greater insight into the complicated pro-
cesses of egg and sperm maturation, and the reduction of the
nuclear substance thus produced.

II. More recent Theories of Reproduction and De-
velopment. The new theories of generation have been worked
out chiefly by Darwin (IX. 6), Spencer (IX. 26), Nageli (IX. 20),
Strasburger (IX. 27, 28), Weismann (IX. 31-34), de Vries (IX.
30), and myself (1X. 10-13). The sharp antagonism which ex-
isted between the theories of Preformation and Epigenesis has
been diminished in these theories, in that in certain respects they
resemble both; so that they could be designated from one point of
view, as the continuation of preformatory, and from another, as a
further extension of epigenetical views. The new theories, al-
though they hardly deserve more than the name of hypotheses,
differ from the old, in that they are based upon a large. collection
of well-substantiated facts, which are to a certain extent funda-
mental.

It would take too long to mention the different views of the
above-mentioned scientists, who, though they agree in many
essential points, differ considerably as to details. I will, therefore,
limit myself to a short description of what seems to me to be the
essential part of the modern theories of generation and develop-
ment.

All the numerous attributes of the developed organism are
present in an embryonic condition in the sexual products since
they are passed on from the parent to the offspring. They may be
considered to constitute an hereditary mass (idioplasm, Nageli).
Each act of generation or development, therefore, does not result
in a new formation, or epigenesis, but produces a transformation
or metamorphosis of an elemental germ, or of a substance which
was provided with potential forces, converting it into a developed
organism; this, again, in its turn produces elemental germs,
similar to those from which it was derived.

If the matured organism be considered to be a macrocosm, the
hereditary mass on the other hand represents a microcosm, com-
posed of numerous regularly arranged particles of material of
different kinds, which, each being provided with its own peculiar
forces, are the bearers of the hereditary properties. Just as the
plant or animal can be divided into milliards of elementary parts,

340 THE CELL

“ viz. cells, so each cell is composed of numerous, small, hypothetical

- elementary particles.

Darwin, Spencer, Nigeli, and de Vries have called these hypo-
thetical units by different names, although they mean the same
thing by them. Darwin (IX. 6) in his provisional hypothesis of
Pangenesis, calls them little germs or gemmule; Spencer (IX.
26), in his Principles of Biology, speaks of physiological units ;
Nigeli (IX. 20), of particles of idioplasm or groups of micelle ;
and de Vries, in his essay upon Darwin’s Pangenesis, calls them
Pangene.

What then are these small elementary portions of the cell,
which I will in future call idioblasts, in accordance with Nageli’s
views, who, in my opinion, has most ably criticised the subject in
question ?

It must be borne in mind, in answering this question, that no
precise definition of an idioblast can at present be given, like that
given by chemists and physicists of the terms atoms and mole-
cules. We are still on unknown ground, like the scientists of the
eighteenth century, who tried to prove that animal bodies were
constructed out of elementary units. Naturally, the danger of
going astray increases, the more we try to work this hypothesis
out in detail. I will, therefore, confine myself as far as possible
to the most general considerations.

The hypothetical idioblasts are the smallest particles of
material into which the hereditary mass or idioplasm can be
divided, and of which great numbers and various kinds are
present in this idioplasm,

They are, according to their different composition, the bearers
of different properties, and produce, by direct action, or by various
methods of co-operation, the countless morphological and physio-
logical phenomena, which we perceive in the organic world. Me-
taphorically they can be compared to the letters of the alphabet,

_ which, though small in number, when combined form words,
which, in their turn, combine to form sentences; or to sounds,
which produce endless harmonies by their periodic sequence and
simultaneous combinations.

De Vries remarks that “just as physicists and chemists have
| es ee to resort to atoms and molecules, the biologist has been
ced to presuppose the existence of certain units, in order to ex-

plain by means of them the various vital phenomena.”

In Nageli’s opinion, “ the characteristics, organs, structures, and

AS THE ELEMENTAL GERM OF AN ORGANISM 341

functions, all of which are only perceptible to us collectively, are
resolved into their true elements in the idioplasm.” Such
elements, according to de Vries, are the particles which are able
to form chlorophyll, the colouring matter of flowers, tannic acid
or essential oils, and we may add muscular tissue, nerve tissue,
etc.

Similar ideas are expressed in a somewhat different form, and regarded from
other points of view, by Sachs (IX. 25) in his essay ‘‘ Stoff und Form der
Pflanzenorgane.” Here he says, ‘‘ we are forced to assume the presence of as
many specific formative materials as there are definite forms of organs to be
distinguished in a plant.” We must therefore imagine that “very small
quantities of certain substances are able so to influence those masses of
materials, with which they are mixed, that they induce them to set into
ditferent organic forms.”

Although at present we cannot with any degree of certainty
detine the specific nature of a single idioblast, we are able to draw
fairly definite conclusions regarding some of their common
properties.

It is, of course, first necessary to consider, that the hypothetical
idioblasts must possess the power of multiplying by means of
division, like the higher elementary units, the cells. For the egg
imparts to each of the two cells into which it divides, and these
again to the daughter-cells, which are derived from them, certain
particles, which are the bearers of specific properties. Hence a
multiplication of these particles must take place during the differ-
ent processes of development ; they must further be able to go on
dividing, and in consequence must possess also the power of growth,
without which continuous divisibility is inconceivable. Darwin,
Niageli, and de Vries, therefore, logically assume that their gem-
mule, particles of idioplasm, and pangene, are both able to grow
and to divide.

This assumption enables us to draw another conclusion about the
nature of the idioblasts, viz. that by their very nature they can-
not be identical with the atoms and molecules of the chemist and
physicist ; for the former are indivisible, and the latter, although
divisible, split up into portions, which no longer possess the
properties of the whole. A definite molecule of albumen cannot
grow without changing its nature, for when it takes up new groups
of atoms, it enters into new combinations, by which means its
properties are altered. Neither can it break itself up into two

342 THE CELL

similar molecules of albumen, since the portions obtained by
dividing a molecule, consist of groups of atoms of unequal value.
On this account idioblasts are not identical with the plastidules,
the existence of which is assumed by Elsberg and Heckel (1X. 8 b).
For, according to Heckel, the latter possess all the physical pro-
perties, which physicists ascribe to molecules, or to collections of
atoms, in addition to especial attributes, which belong exclusively .
to themselves, viz. “the vital properties which distinguish the
living from the dead, and the organic from the inorganic.”

Our units, therefore, the gemmule of Darwin, the pangene of
de Vries, and the physiological units of Spencer, must be complex
units, or, at any rate, groups of molecules. In this fundamental
view, all the above-mentioned scientists agree. Thus, according
to Spencer, there is nothing left but to assume, that chemical
units combine together to form units of an infinitely more
complex nature than their own, complex though this be, and
that in every organism the physiological units, produced by |
such combinations of highly complex molecules, possess various
characters.”

If Nigeli’s hypothesis of the molecular structure of organised
bodies be accepted, it is easy to imagine that the nature of the
idioblasts is as follows: “They can as little be single micelle
(crystalline molecule-groups), as molecules; for even-if, as a
mixture of different modifications of albuminates, they possess
different properties, they would still lack the capacity of multi-
plying and forming new similar micelle. Insoluble and stable
groups of albuminous micelle alone afford all the necessary
conditions for the construction of the gemmule; they alone, in
consequence of their varying composition, can acquire all the
necessary properties, growing indefinitely by storing up micelle,
or multiplying by means of disintegration. Hence, the pangene
or gemmulg must consist of small masses of idioplasm.”

Now comes the question: What is the size and number of
the idioblasts contained in a complete germ ? :

As regards size, the idioblasts must certainly be exceedingly small,
since all the hereditary elemental germs of a highly-developed
organism must be present in the minute spermatozoon. Niageli
has attempted to make an approximate calculation on this impor-
tant point. He starts with the assumption, that the hypothetical
albumen formula of chemists, with seventy-two atoms of carbon
(Cz2HogMgSO.2), does not represent a molecule of albumen, but a

AS THE ELEMENTAL GERM OF AN ORGANISM 343

micella of crystalline construction composed of several molecules.
Its absolute weight is the trillionth part of 3°53 mg. The specific
weight of dry albumen is 1:°344. Hence, 1 cubic micro-millimetre
contains about 400 million micelle. Nageli, basing his calculations
on some further hypotheses, considers that the volume of such a
micella is 0000000021 cub. mic. mil. Further, upon the supposi-
tion that micelle are prismatic, and are only separated from one
another by two layers of molecules of water, 25,000 micelle would
occupy a superficial area of ‘1 sq. mic. mil. Hence, in a body of
the size of a spermatozoon there would be room for a considerable
_ number of micelle, united together in groups. Thus, no difficul-
. ties present themselves on this point.

Logically thought out ideas are especially valuable, when they
harmonize with perceptible facts. The following observations are
in support of the above-mentioned hypothesis, ¢.e. that idioblasts |
multiply by growth and sub-division ; the capacity of self-division |
does not only apply to the individual cell as an elementary organ-
ism, but also to the above-mentioned masses of special material,
which are enclosed in the cell. Chlorophyll, starch, and pig-
ment formers multiply by direct division; the centrosomes, which
are only just perceptible with the microscope, also divide, when
nuclear segmentation occurs; the nuclear segments split up longi-
tudinally into daughter-segments, and this is attributed by many
to the presence in the mother-thread of qualitatively different
units (mother-granules), which are arranged in a row one behind
the other; each of these is supposed to divide directly into two,
after which the daughter-granules thus obtained, distribute them-
selves evenly amongst the daughter-segments.

Even if the idioblasts, which we have supposed to be of a much
smaller size, do not themselves take part in these divisions, we may
assume that groups of idioblasts are so concerned; the importance
of these observations, as concerns our theory, consists in this, that
they teach us how small masses of material grow in the cell by
themselves, and are able to multiply by division.

Finally, another aspect of this theory may be mentioned here.
If the elemental germs, taken in the aggregate, give rise to a
definite organism, the individual constituents must evolve in
regular sequence, during the process of development. As sentences,
with logical meanings, are formed of words, and these of letters ; /
and similarly, as harmonies, and whole musical compositions,
consist of individual notes, suitably arranged, so we must also

344 THE CELL

assume that the idioblasts are arranged in a constant regular
manner. This portion of the theory is the most difficult to under-
stand.

In the above, certain logical principles for the formation of a
physiological molecular theory of generation and heredity have
been deduced, in accordance with Nageli’s views. We must leave
the proof of the correctness of these assumptions to future ob-
servers and experimenters, who will thereby establish the relation
between the theory, and the facts which are perceptible to our
senses. The physiological idea of the creation of the organic
world from elementary units, and of the essential agreement in .
the structure of plants and animals, have been of real service in
building up the cell and protoplasm theories; in a similar manner
we must hope to obtain a corresponding position for the theory
of heredity. Several attempts have already been made in this
direction, connected with the observations made upon the fertili-
sation in animals, plants, and Infusoria. =

Ill. The Nucleus as the transmitter of Hereditary
Elemental Germs. The hypothesis that the nuclei are the
transmitters of the hereditary properties, was suggested to both
Strasburger and myself by the study of the process of fertilisation
and of the theoretical considerations connected with it; thus we
have assigned to the nuclear substance a function, which is
different from that of protoplasm. A short time before, Nigeli
had been compelled, solely on logical grounds, to assume, that two
different kinds of protoplasm were present in the sexual cells, the
one sort which occurs in exactly equal proportions in the egg and
sperm cell, conveying the hereditary properties, and the other,
which is stored up in great quantities in the ovum, functioning
chiefly as a nutritive medium. He calls the first idioplasm, and
the second somatoplasm, and assumes that the former is more
solid in consistency, the micelle being regularly arranged, whilst
the latter contains more water, and hence its micelle are less
closely united. He imagines that the idioplasm is extended like
a fine network throughout the whole cell body.

If it be admitted, that the assumption of a separate idioplasm is
logically justifiable, it cannot be denied that the nuclear substance
probably constitutes the hereditary mass. 7

Farther, by means of this theory, a practical interpretation has
been given to Niigeli’s deduction, which was based simply upon

AS THE ELEMENTAL GERM OF AN ORGANISM 345

reasoning, and which in consequence could neither be verified by
observation nor developed further.

In order to establish the hypothesis, that the nucleus is the
transmitter of the hereditary elemental germs, four points have to
be considered :—

1. The equivalence of the male and female hereditary masses.

2. The equal distribution of the multiplying hereditary mass
upon the cells, which are derived from the fertilised ovum.

3. The prevention of the summation of the hereditary masses.

4. The isotropism of protoplasm.

1. The Equivalence of the Male and Female Hereditary Masses.
It is evidently true, and hence must be accepted, as an axiom, that
the egg and sperm cells are two similar units, each of which, being
provided with all the hereditary properties of its kind, transmits
an equal quantity of hereditary material to the offspring. The
offspring is in general a mixed product of both its parents ; it
receives from both father and mother an equal number of idio-
blasts, or active particles, which are the bearers of hereditary
attributes.

However, it is only in the lowest organisms that the sexual
cells resemble each other in size and composition; in the higher
organisms, they present in both respects the greatest differences,
so that in extreme cases an animal spermatozoon may be even
smaller than the hundred-millionth part of an egg. It is, however,
inconceivable, that the carriers of the elemental germs, which,
a priori, must be assumed to be equal both as to number and
attributes, can present such differences in their volume. On the
contrary, the fact that two cells, which are quite different as
regards mass, can possess equal hereditary potentialities, can be
easily explained by the assumption, that they may contain at the
same time substances of very different hereditary value, i.e. for
idioblastic and non-idioblastic substances.

We must, therefore, endeavour to find this idioplasm in the egg
and spermatozoon, and to isolate it from the other substances.

First of all, there is no doubt that the reserve materials—fat
globules, yolk platelets, etc., must be included in the category of
germ substances, which are useless as regards heredity. But
even if we discard these, the egg and sperm cells still remain
unequal, as regards the quantity of their other constituents.
For the protoplasm which is present in a large egg-cell, even
after all the contents of the yolk have been abstracted, is much

346 THE CELL

greater in volume than the total substance of a spermatozoon ;

hence protoplasm cannot be the idioplasm. Only one substance

fulfils all the necessary conditions, namely, the nuclear substance.

The study of the phenomena of fertilisation in the animal and
_vegetable world proves this irrefutably.

As was described in chapter seven, the essence of the process of

| fertilisation consists in this, that the sperm and egg nuclei, i.e. one

_ nucleus derived from the spermatozoon, and one derived from the

sient

egg-cell, each accompanied by its centrosome, place themselves in

_ contact, and, fusing together, form a germ-nucleus, from which

subsequently, one after another, all the nuclei of the developed
organism are obtained by repeated divisions. In Ciliata, two
individuals only lay themselves alongside of each other for a short
time, so as to exchange migratory nuclei, each of which subse-
quently fuses with the stationary nucleus of the other organism.

As far as the most careful observation shows, the egg and
sperm nuclei contribute exactly equal quantities of material to-
wards the formation of the germ-nucleus, that is to say, equal
quantities of nuclein, and of polar substance, which I include
amongst the nuclear substances.

Fol (VII. 14) has proved the equivalence of the polar substance,
which is contributed by the two conjugating individuals, whilst
the observations of van Beneden (VI. 4b) upon the process of
fertilisation, as seen in Ascaris megalocephala, demonstrate irre-
futably the equivalence of the nuclein so obtained.

We, therefore, draw the following important conclusion from
the facts observed during the process of fertilisation: since in
fertilisation the. nuclear substances (nuclein and polar substance)

_ are the only materials which are equivalent in quantity, and which

unite to form a new fundamental structure, the germinal nucleus,

\ they alone must constitute the hereditary mass which is transmitted

from parent to child. We cannot at present decide what is the
exact relation borne by the nuclein and the polar substance to the
idioplasm.

2. The equal Distribution of the multiplying Hereditary Mass,
amongst the Cells, proceeding from the fertilised Egg. We are
obliged to assume that the multiplying hereditary mass is evenly
distributed amongst the descendants of the egg-cell, when we
consider the various phenomena of reproduction and regeneration ;
for instance, the circumstance that each new organism produces
numerous egg or sperm cells, which contain the same hereditary.

AS THE ELEMENTAL GERM OF AN ORGANISM 347

mass as the sexual cells, from which the organism was derived,
renders this assumption absolutely necessary.

Secondly, we are forced to this conclusion, when we consider the
fact, that in many plants and lower animals, even an extremely
small group of cells is able to reproduce the complete organism.
When a Funaria hygrometrica, is chopped up into very small pieces,
and placed upon damp soil, a complete plant grows out of each
minute fragment. Similarly, if the fresh water Hydra is cut up)
into small portions, each develops into a complete Hydra, possess-
ing all the properties of its species. Buds may be formed from)
the most different parts of a tree by the growth of the vegetative
cells; these buds develop into shoots, which, if separated from
the parent, and planted in the earth, can take root and grow into
complete trees. In Coelenterata, in many worms and T'unicates,
the asexual mode of multiplication is similar to the vegetative
mode, since at each part of the body a bud can be formed, which is
able to develop into a new individual. In Bougainvillea ramosa,
for instance (Fig. 168), new animals are developed, not only as
side branches of the
hydroid stock, but
also as stolons, which
extend themselves
like roots upon any
surface, and serve to
attach the colony.

Thirdly, many
processes of re-

‘generation, or re-
'placement of lost
parts, prove that in
addition to the pro-
perties, which are
evidently exercised,
- there must be others
which are latent, but
which are capable of
development under
abnormal conditions.

For instance, if a

willow twig is eut Fie. 168.— Bougainvillea ramosa (from Lang): h hy-
dranths, which develop into medusa buds mk; m free
off and placed 11 medusa Margelis ramosa.

348 THE CELL

water, it develops root-forming cells at its lower extremity; thus
the cells are here executing functions, very different from their
original ones, which proves that they possessed this capacity
potentially. Further, on the other hand, shoots can develop from
severed roots, and even subsequently can produce male and female
sexual products. In this case, therefore, sexual cells proceed
directly from the component parts of a root-cell, and hence serve
‘for the reproduction of the whole. Certain hydroid polyps,
according to von Loeb (IX. 17), display similar powers.
Most botanists agree with the theory, recently advanced by de
\ Vries (IX. 30), in opposition to Weismann, which states that all,
or at any rate by far the greater number, of the cells of a vegetable
body contain all the hereditary attributes of their species in a
latent condition. The same is true of the lower animal organisms,
although we are unable to prove it for the higher ones. However,
on this account, it is not necessary to conclude that the cells of
the higher,and lower organisms differ so much from one another,
that the latter possess all the attributes in a latent condition,
and therefore the whole hereditary mass, whilst the former only
contain a part of it. For it is quite as likely that the incapacity
of most of the cells of the higher animals to develop latent
properties, is due to their external conditions, which have produced
a great differentation of the cell-body, in which the hereditary
mass is enveloped, or to other similar conditions.
Johannes Miiller (IX. 18), has raised the question: ‘* How does
it happen, that certain of the cells of the organised body, although
‘they resemble both other cells and the original germ.cell, can
produce nothing but their like, i.e. cells which are capable of
, developing into the complete organism? Thus epidermal cells
}can only, by absorbing material, develop new epidermal cells,
‘and cartilage cells only other cartilage cells, but never embryos
or buds.” To which he has made answer: “This may be due to
the fact, that these cells, even if they possess the power of
forming the whole, have, by means of a peculiar metamorphosis
of their substance, become so specialised, that they have entirely
lost their germinal properties, as regards the whole organism, and
when they become separated from the whole, are unable to lead
an independent existence.”
Whatever opinion is held as regards the conditions present in
the higher animal, it is quite sufficient for our purpose to acknow-
ledge, that in the plants and lower animals, all the cells which are

~

AS THE ELEMENTAL GERM OF AN ORGANISM 349

derived from the ovum, contain equal quantities of the hereditary
mass. Hence this must grow and multiply in the cell before
division takes place. All idioblasts must divide and must be
transmitted to the daughter-cells, in equal proportions both as
regards quality and quantity.

Nageli (IX. 20, p. 531) has enunciated the same view: “ Idio-
plasm, by continuously and proportionately increasing, splits itself
up during cell-division—by means of which the organism grows
into as many parts as there are individual cells.” Therefore,
“each cell of the organism is capable, as far as the idioplasm is
concerned, of becoming the germ of a new individual. Whether
this potentiality ever becomes a reality, depends upon the nature
of the nutrient plasm (somatoplasm).”

If we look upon the vital processes of the cells from this second
point of view, there can be no doubt that the nuclear substance is
the only one amongst all the constituents of the cell, which is able
to fulfil all the conditions in every respect.

The nucleus is strikingly uniform in all plant and animal
elementary tissues. If we disregard a few exceptious, which
require a separate explanation, the nuclei of all the elementary
tissues of the same organism resemble each other closely, as
regards shape and size, whilst the protoplasm differs in quantity
toa marked degree. In an endothelium cell, or in a portion of
muscle or tendon, the nucleus has almost the same characters and
contains the same substances as an epidermal, liver, or cartilage
cell, whilst, in the former case, the protoplasm is barely distin-
guishable, and, in the latter, is present in large quantities.

The striking aud complicated phenomena of the process of
nuclear division, are both more important and more comprehensi-
ble, when regarded in the light of our theory. The arrangement
of the substance into fibrillea, which consist of small microsomes,
arranged alongside of each other, the formation of loops and
spindles, the longitudinal halving of the fibrils, and the mode of
their distribution amongst the daughter-nuclei, can only serve one

——l—

purpose, namely, to halve the nuclear substance and to apportion

it equally amongst the daughter-cells.

Roux, from another stand-point, has already pertinently de-
nominated ‘ the nuclear division-figures as mechanisms, by means
of which it is possible to divide the nucleus, not only accord-
ing to its own volume, but according to the volume and nature
of its special constituents. The essential part of the process

—

350 THE CELL

of nuclear division is the division of the mother-granule; all the
other processes only serve to convey one of the daughter-granules,
which have been derived by division from the same mother-
granule, into the centre of each daughter-cell.” If we replace the
term “mother-granule” by the expression “idioblast,” we have
established a connection between the process of nuclear segmenta-
tion and the theory of heredity.

This conception of the nuclear substance as an hereditary mass
is important, since it offers some explanation of the facts that the
nuclear substance takes less part in the coarser processes of
metabolism, than the protoplasm does, and. that, for its better
protection, it is enclosed in a vesicle provided with a special
membrane. :

3. The Prevention of the Summation of the Hereditary Mass. I
consider the third point, viz. the prevention of the summation of
the hereditary mass, during sexual reproduction, to be a most
important point in the argument. In consequence of the nature
of the process of nuclear division, each cell receives the same
quantity of nuclear substance as the fertilised egg-cell, A. Now
when two of its descendants unite, as sexual cells, the product of
generation, B, ought to contain twice as much nuclear substance
as the cell A originally did. Then when members of the third
generation conjugate, the product C ought to contain twice as
much nuclear substance as B, or four times as much as A, and
thus with each new act of fertilisation the nuclear mass would
increase by geometrical progression. Sucha summation, however,
must be prevented by nature in some way or other.

This would also be true of the idioplasm, if the full quantity of
it were transmitted to each cell, and if it were doubled each time
by the act of fertilisation. By this means, its nature, per se,
would not be changed. For instead of twice, each individual
elemental germ would be represented four, eight, or even more
times. Thus, although the quantity would be increased, the
quality would always remain the same. But it is self-evident
that the mass cannot thus increase to an unlimited extent. Nageli,
and especially Weismann, have laid stress upon this difficulty, and
have tried to solve it.

Nigeli remarks: “ If during each act of reproduction by means
of fertilisation, the volume of the idioplasm of whatever constitu-
tion it may be, were to become doubled, after a few generations
the idioplasmic bodies would have increased so much, that there

AS THE ELEMENTAL GERM OF AN ORGANISM 351

would not be room for them ina spermatozoid. It is, therefore,
unavoidable, that in bisexual reproduction, the union of the
parental idioplasmic bodies must take place without causing a
corresponding and permanent increase of their substance.”

Nageli has attempted to overcome this difficulty by assuming,
that idioplasm consists of strands, which are fused together in
such a peculiar way, that the transverse section of the product of
fusion remains the same as that of the simple thread, whilst the
length of the whole is increased (1X. 20, p. 224).

Weismann (IX. 32-34) has investigated this snbject most care-
fully, and has attempted to demonstrate, that a summation of the
hereditary mass is prevented by means of a process of reduction,
it being halved before each act of fertilisation. He considers that
theoretically it is so absolutely necessary for reduction to take
place in each generation, “that the processes by which it is brought
about must be discoverable, even if they are not to be deduced
from the facts already mentioned.”

Weismann has been led to these conclusions by considering the
nature of idioplasm; however, his views do not agree with the
ones I have mentioned above. He groups them under the common
name of ‘“‘ancestral plasma theory,” to the essential points of which
I will refer later.

The enquiry into the processes of fertilisation and of nuclear
division proves logically, on the one hand, that the two hereditary
masses must fuse, and must subsequently be re-distributed amongst
the cells, and on the other that a summation of the nuclear sub-
stance of the hereditary mass must be avoided. The unanimity of
opinion as regards the assumption, that the nuclear substance is
the hereditary mass sought for, may certainly be taken as evidence
in its favour, especially if, during the fusion of the nuclei, pro-
cesses can be demonstrated, which correspond in every respect
to the necessary conditions.

A priori, there are only two possible means of preventing the sum __
of the equal quantities from being greater than either of the added
parts. Hither the quantities, which are to be added together, must
be halved beforehand, or their sum must be halved subsequently.
Both methods appear to have been adopted during the process of
fertilisation.

The one course occurs in phanerogamous plants and in animals.
When the male and female sexual products are mature, the nuclear
mass of both the egg and sperm mother cell, as was described at

352 THE CELL

length on p. 235, under the title of division with reduction, is so
distributed amongst the four grand-daughter cells, that each of
them only contains half the nuclear mass of an ordinary cell, and
hence only half the normal number of nuclear segments.

The second course occurs during the process of fertilisation in
Closterium. Here, according to the observations of Klebahn (VII.
27), the germinal nucleus, formed by the fusion of two nuclei,
divides consecutively twice without entering into a state of rest,
just as when pole-cells are formed. Of the four vesicular nuclei,
two disintegrate, so that each half of the original mother-cell
contains only one nucleus, which possesses only a fourth part of
the germ-nucleus, instead of one half, as in normal division (see
the description and figures on pp. 280, 281).

If, according to our assumption, the nuclear mass is identical
with the hereditary mass, we must conclude, arguing from the
process of division with reduction, that the hereditary mass may be

/ divided up to a certain point, without losing its power of reproducing
the whole out of itself. The question then arises, as to how far this
‘conception is admissible.

Weismann and I both lay emphasis upon the necessity of a
reduction of mass, but we have arrived at different conclusions as
regards particulars. . .

In his ancestral germ-plasm theory, Weismann starts with the
, supposition, that in the hereditary mass the paternal and maternal
portions having kept themselves apart, form units, which he calls
‘ancestral germ plasms. He assumes that these are very compli-
cated in structure, being composed of extremely numerous biological
units. At each new act of fertilisation still more numerous ancestral
germ-plasms come together.’ Supposing that we revert to the
beginning of the whole process of fertilisation, then in the tenth
generation 1024 different ancestral plasms must have taken part
in the formation of the hereditary mass. But since the total mass
of the latter does not double itself with each act of fertilisation,
Weismann makes the ancestral plasms divisible in the first stages
of the process, and supposes that they are transmitted to the
following generation, reduced each time by one half; “at last,
- however,” he continues, “the limit of this constant diminution of
the ancestral plasms must be reached, and this must occur when
the mass of substance, which is necessary in order that all

elemental germs of the individual may be contained therein, has
reached its minimum.”

AS THE ELEMENTAL GERM OF AN ORGANISM Bs

After this period, which, by the way, would be reached in a few
years in the case of low, quickly-multiplying organisms, formation
of the hereditary mass would be obliged to take place with each
fresh act of fertilisation, in consequence of the impossibility of
diminishing the ancestral plasms any further, unless some other
arrangement be made. Weismann considers, that this new arrange-
ment consists in this, that, when the sexual products are mature,
half of the ancestral plasms are ejected from the hereditary mass
in the pole-cells, before fertilisation occurs. In place of the
division of the individual ancestral plasms, therefore, the division
of the total number of plasms takes place after they have become
no longer divisible as units.

Thus, according to Weismann’s assumption, the hereditary mass |
is an extremely complicated piece of mosaic, composed of innumer-
able units, the ancestral plasms, which, by their very nature are‘
indivisible and incapable of mixing with other units, and each of
which in its turn is composed of numerous elemental germs, which
are necessary for the production of a complete individual.

Thus, every hereditary mass, in consequence of its composition,
would have to produce countless individuals, if each ancestral
plasm were to be active. The essential nature of the process of
fertilisation lends itself to a combination and elimination of an-
cestral plasms. Further, if the ancestral plasm theory were
true, elemental germs of equal value would accumulate in the
hereditary mass. In fact the generative individuals belonging
to the same species are essentially similar in their properties, if
we disregard small individual differences of coloration. All the
ancestral plasms must, therefore, contain essentially the same
elemental germs. These various germs are represented in the
hereditary mass as many times as there are ancestral plasms,
the majority being similar to one another, and only presenting
differences of shade. But all these similar, or slightly different,
elemental germs would stand in no direct relation to each other,
since they must remain integral component parts of the ancestral
plasms, for which we have assumed indivisibility.

The question of heredity, instead of being simplified by Weis-
mann’s theory of ancestral plasms, is rendered more complicated
by it, especially by the assumption that the paternal and maternal
hereditary masses are incapable of mixing with one another.

I cannot see that this theory of Weismann’s is of any great
use, since it leads to so many difficulties, which appear to be

AA

oa

354 THE CELL

_ entirely superfluous. Neither Nageli nor de Vries consider that

the ancestral plasms have this construction ; they assume rather
that the units contained in the two hereditary masses are capable
of mixing with one another. Neither can I imagine that, during
the process of hereditary transmission, the idioblasts of paternal
and maternal origin continue as parts of two separated elemental
germs, it seems more likely that they unite together in some
way or other to form a compound elemental germ.

How then, on this supposition, is the summation of the here-
ditary mass, occasioned by the act of sexual generation, to be
avoided? I do not think that there is the slightest difficulty
if we assume the divisibility of the hereditary mass as a whole.
Even Weismann has assumed that this is possible at the beginning
of sexual generation, otherwise, a summation of the ancestral
plasms, could not have taken place without causing an increase of
the hereditary mass.

But the hereditary mass can only be divided, without its pro-
perties being altered, if several individual units of each different
kind are present init. Since the progeny are produced from two
almost equal combinations of elemental germs, derived from
the parents, there must be at least two individuals of every kind
of idioblast in the embryo. Nothing prevents us, however, from
conceiving that, instead of two individuals of each kind, there may
be four, eight, or speaking generally, a number of equivalent
idioblasts in the hereditary mass. Then it is self-evident, that a
reduction of mass, without the essential nature of the idioplasm
itself being altered, is possible in the same manner, as has been
observed during the maturation of the sexual products, and there-
fore any further complicated hypotheses are superfluous.

In order to explain the so-called reversion to an, ancestral type,
we need not assume the existence of ancestral plasms, for, as
will be seen later, the elemental germs may themselves remain
latent.

4. Isotropy of Protoplasm. Various investigators have at-
tempted to ascribe to the whole egg a very complex organisa-
tion, namely, that it is composed of very minute particles, the
arrangement of which corresponds to that of the organs of
the mature animal. The clearest conception of this subject
is that formulated by His in his “ Princip der organbildenden
Keimbezirke.” According to this author, “on the’ one hand,
every point in the embryonal area of the germinal disc must cor-

AS THE ELEMENTAL GERM OF AN ORGANISM 355

respond to an organ which develops later, or to part of such an
organ, and on the other hand, every organ developed from the
germinal area must have its preformed germ in a definite region
of thisarea. The material for the germ is already present in the
flat germinal disc, but it is not morphologically distinct, and hence
is not to be recognised as such at this stage. By tracing the
mature organs back to their elemental form, we shall be able to
discover the situation of each during the period of incomplete
morphological separation, and indeed, if we wish to be consistent,
we must apply this method to the fertilised and even to the un-
fertilised ovum also.”

It is hardly necessary to emphasise how sharply opposed this
principle of the formation of organs in the germinal area is to the
above-mentioned theory of heredity. One of the first points to —
be noticed is, that the influence of the paternal elemental germs,
upon the formation of the embryo, is entirely left out of account.
For this reason alone, the theory is evidently untenable. But, in
addition, various experimental facts, which, as Pfliiger has pointed
out, indicate that the egg is isotropous, entirely disprove it.

By the term isotropy of the egg, Pfliiger (VII. 50), wishes to
imply, that the contents of the egg are not arranged in such a
manner as that the individual organs can be traced back to this
or that portion of it. He draws his conclusions from experiments
made upon Frog’s eggs. The Frog’s egg is composed of two
hemispherical portions, one of which, the animal half, is pig-
mented black, whilst the other, or vegetative portion, is clear or
colourless, and is, at the same time, specifically heavier. In conse-
quence of this difference in specific gravity, the eggs, immediately
after fertilisation, assume a definite position in the water, the
pigmented portion always being directed upwards, so that the
egg-axis, which connects the animal with the vegetative pole, is
vertical. It is possible, however, to experimentally force the
eggs which have just been fertilised to take up an abnormal
position, that is to say, to prevent them from rotating in the yolk-
membrane by applying friction to it. The experimenter, for
instance, can force the egg to assume such a position that the
egg-axis shall lie horizontally, instead of vertically. Now when
the process of division begins, the first division plane, in spite of
the changed position of the egg, is in a vertical direction, for its
position depends on that of the nuclear spindle, as shown on p. 219.
As Born (IX. 37), has minutely described, however, although the

356 THE CELL

nucleus and the specifically lighter portion of the egg have been
forced to change their position, the first division plane takes anew
a vertical direction. This plane cuts the horizontal egg-axis at
various angles. For instance, Pfliiger often saw that it separated
the egg into a black and a white hemisphere. Under such cir-
cumstances, therefore, the hemispheres evidently do not contain
the same particles of material, as when they are under normal
conditions. Nevertheless, a normal embryo is developed out of
the egg. Even after the formation of the notochord and spinal
cord, one half of the body can be seen to be darker than the
other. Thus, according to the position of the original cleavage
plane, the individual organs must be composed of different parts
of the egg contents. The experiments made by Richard Hertwig
and myself (VI. 38), by Boveri (IX. 4), by Driesch (IX. 7), and
by Chabry (IX. 5), all furnish additional proof of the isotropy of
the egg.

Richard Hertwig and I found, that the ova of Echinoderms can
be divided by violent shaking into small portions; these become
spherical in form, and may be fertilised by spermatozoa. Boveri
indeed has succeeded in raising a few dwarf larval forms from
such small fertilised portions. Driesch, by shaking normally
developed and dividing Echinoderm ova, was able to separate
from one another the two first cleavage segments; these he then
isolated, and was thus able to establish the fact that a normally
shaped though somewhat small blastula, followed by a gastrula,
and even in some cases by a pluteus, developed from each half.

Chabry has obtained a corresponding result. He destroyed, by
pricking it, one of the two, or, when it had divided into four, one
of the four cells of the ovum of an Ascidian. In many cases he
succeeded in raising from such mutilated ova, absolutely normal
larvee, which only occasionally, were without subordinate organs,
such as otoliths or attachment papille. From all these experi-
ments the fundamental proposition is proved, that the cell-nucleus,

_ which may be enclosed in any part of the yolk, is able to produce

a complete organism. This isotropy of the egg negatives the
hypothesis that there is a germinal region from which organs are
developed. Moreover, at the same time, it supplies an additional
proof that the idioplasm is not to be found in the protoplasm, but
in the nucleus; and further, it allows us to draw some conclu-
sions as to the construction of protoplasm and nuclear substance.
Protoplasm must consist of loosely-connected particles of mi-

AS THE ELEMENTAL GERM OF AN ORGANISM 307

celle, which are more similar to one another than those of the nu-
cleus. For, firstly, fragments of a cell, which contain the nucleus,
are capable of normal development (vide experiments, p. 330).
Secondly, the first division plane can be induced, by means of ex-
ternal influences, to divide the contents of the egg in the most
various directions, without causing any deviation from the normal,
in the product of development. Thirdly, considerable changes of
position of the egg substance may be induced, by means of gravity,
in F'rog’s ova which have been forced into an abnormal position,
without causing any difference in their subsequent development.
Fourthly, we are able to infer, that the micelle are loosely con-
nected together from the streaming movements of protoplasm, in
which, of necessity, the groups of micelle are obliged to push past
one another in the most different directions, and apparently with-
out any method. On the other hand the complicated phenomena
of the whole process of nuclear segmentation indicate a more
stable arrangement of the nuclear substance,

Nageli has assumed that there is a similar difference between
his hypothetical trophoplasm and idioplasm. He states (pp. 27,
41): “If the arrangement of the micelle determines the specific
properties of the idioplasm, the latter must be composed of a fairly
solid substance, in order that the micelles may not be displaced in
consequence of active forces in the living organism, and in order
to secure to the new micelle, which become deposited during multi-
plication, a definite arrangement, On the other hand, ordinary
plasma consists of a mixture of two kinds, fluid and solid, the two
modifications easily merging into one another, whilst the micelle,
or groups of micelle of the insoluble form, are more easily able to
push past one another, as must be assumed to be the case when
the streaming movements occur.” Nigeli, therefore, makes the
assumption, which however cannot be proved off-hand, that the
idioplasm is spread out like a connected net throughout the whole
organism.

IV. Development of the Elemental Germs. Having
assumed that there is a special germ substance or idioplasm in the
cell, we must next enquire how the individual idioblasts become
active, and thus determine the specific properties or the character
of the cell as a result of their development.

It has been suggested, that during the process of development
of the ovum, the idioplasm is qualitatively divided unequally by

358 THE CELL

means of the process of nuclear division, so that different parts of
the cells acquire the different properties, which are subsequently
developed in them. According to this view, the essential nature
of development would consist in gradually separating all the
elemental germs, taken collectively, which the idioplasm or the
- fertilised egg contains, into constituent parts, and of distributing
them differently, both as regards time and place. Only those cells,
which function in the reproduction of the organism, are supposed
to be exceptions to this rule, and to receive again the whole collec-
tion of the elemental germs during the processes of development.
Hence a twofold mode of distributing the idioplasm is assumed to
occur, one by the growth and halving of similar germs, and one by
the resolution into different component parts of dissimilar ones.

It is difficult to imagine how such a process can actually take
place in any concrete case. Further, this assumption does not
agree with the above-mentioned facts of reproduction and regener-
ation; for instance, in plants and in the lower animals, almost any
collection of cells is able to reproduce the whole; and again, cells
may alter their functions, as seen in the phenomena of regeneration.

Therefore, the views which I have frequently upheld (IX. 10-13),
and which agree with those held by Nageli and de Vries, etc.,
seem to be more probably true, that as a rule each cell of an
organism receives all the different kinds of elemental germs from
| the egg-cell, and that its especial nature is solely determined by its
_ conditions, only certain individual elemental germs or idioblasts
becoming active, whilst the others remain latent.

But in what manner can individual idioblasts become active,
and thus determine the nature of the cell? Two hypotheses have
been suggested in answer to this question, a dynamic one by
Nageli (IX. 20), and a material one by de Vries (IX. 30). In
order to explain the specific activity of idioplasm, Nageli assumes
that “occasionally a definite colony of micelle, or a combination ;
of such colonies, become active,” that is, “are thrown into definite
conditions of tension or motion,” and he considers that “ this local
irritation, by means of dynamic influence, and the transmission of
peculiar conditions of oscillation acting at a microscopical distance,
governs the chemical and plastic processes.” ‘It produces fluid
trophoplasm in enormous quantities, and by its help effects the
formation of non-albuminous constructive material, of gelatinous,
elastic, chitinous, cellulose-like substances, etc., and it gives to this
material the desired plastic form, Which micella group of the.

AS THE ELEMENTAL GERM OF AN ORGANISM 359

idioplasm becomes active during development depends upon its
shape, upon the stimulation. it has previously received, and finally,
upon the position in the individual organism in which the idio-
plasm is placed.”

In place of this dynamic hypothesis, de Vries (1X. 30) assumes
that the character of the cell is affected ina more material fashion.
He is of opinion that, whilst the majority of the idioblasts or “pan-
gene” (de Vries) remain inactive, others become active, and grow
and multiply. Some of these then migrate from the nucleus into
the protoplasm, in order to continue here their growth and multipli-
cation in a manner corresponding to their functions. This out-
wandering from the nucleus can, however, only take place in such
a fashion as to allow of all the various kinds of idioblasts remain-
ing represented in the nuclear substance.

This hypothesis of de Vries appears at present to bea simpler
explanation and to be more in accordance with the many pheno-
mena that have been observed. Thus, for instance, as described
above, there are separate starch-forming corpuscles, chroma-
tophores, and chlorophyll grains, which function in a specific
manner and multiply independently of the rest of the cell, and
are transferred at each cell-division from one cell to another. De
Vries calls this “ transmission outside the cell-nuclei.”’ According
to his hypothesis, some of the transmitted idioblasts are those
which have become active, have reproduced themselves in the
protoplasm, and have united together to form larger units, whilst
in addition there are similar idioblasts present in the nucleus (in
the germinal substance). The same would be true of the centro-
somes, if it were not that the balance of proof is already in favour
of their belonging to the nucleus.

By means of the hypothesis of “intracellular pangenesis,” the
intrinsic difference, which was apparently revealed by the theory
of heredity, between nuclear substance and protoplasm, is more or
less modified, without the fundamental character of the theory
being interfered with ; further, it has been shown how a cell can
contain the whole of the attributes of the complex organism, in
a latent condition, whilst at the same time it can discharge its
own special functions.

The transmission and development of characteristic potentialities
are, as de Vries rightly remarks, very different. The transmission
is the function of the nucleus, and the development, that of the
protoplasm. In the nucleus all the various kinds of idioblasts of

360 . THE CELL

the individual in question are represented ; therefore, the nucleus
is the organ of heredity ; the remaining protoplasm of the cell
contains practically only those idioblasts which have become
active in it and which can multiply rapidly in an adequate man-
ner. We have, therefore, to distinguish between two modes of
multiplication of the idioblasts; the one referring to all of them,
which results in nuclear division and in their equal distribution
amongst the two daughter cells ; and the other, which toa certain
extent, is a multiplication connected with function ; and this latter
only affects those idioblasts which have become active ; moreover,
it is connected with the material changes which occur in them
and it takes place chiefly in the protoplasm, outside the nucleus.

This conception is another indication that the protoplasm is
composed of small elementary units of substance, as has been
assumed latterly by several investigators, who have started various
theories ; as for instance Altmann (II. 1), in his theory of bio-
. blasts, and Wiesner (IX. 35), in his recent work “ Die Hlementar-
structur und das Wachsthum der lebenden Substanz.” The proto-
plasm, like the nucleus, consists of a large number of small
particles of material, which differ as to their chemical composition,
and which have the power of assimilating material, of growing
and of multiplying by division. (Omne granulum e granulo, as
Altmann expresses it.) Material for growth is supplied by the
fluid, which bathes the nucleus and protoplasm, and in which
‘plastic materials of the most different kinds (albumen, fats, carbo-
hydrates, salts) are dissolved.

In order to distinguish the idioblasts of the nucleus from those
of the protoplasm, we will call the latter “ plasomes,” a name
which has been used by Wiesner.

As the plasomes (or as it were the active idioblasts) are, accord-
ing to the theory of “ intracellular pangenesis,” supposed to be
derived from the idioblasts of the nucleus, so they may also form
the starting-point of the organic products of the plasma, since
according to their specific characters, they join to themselves
various substances; for instance, certain kinds of plasomes, by
combining with carbo-hydrates, might produce the cellulose mem-
brane, or by combining with starch the starch granules; hence
they might be designated, the cell-membrane formers or starch
formers.

Thus the most different occurrences in cell life may be regarded,
from a common point of view, as vital processes taking place in

AS THE ELEMENTAL GERM OF AN ORGANISM. 361

the most minute organised, dissimilar particles of matter, which
multiply indefinitely and which are found in the nucleus, in proto-
plasm, and in the organised plasmic products, according to the
different phases of their vital activity.

Wiesner has formulated his conception, which is in accordance (
with the above, in the following sentences: “ The assumption, that ,
protoplasm contains organised separate particles, which are cap- _
able of division, and that it, in fact, entirely consists of such living, \
dividing particles, is forced upon us as the result of recent en-
quiry.” By means of the division of these particles ‘ growth is
brought about,” and “all the vital processes occurring in the |
organism depend on them.” ‘ They must, therefore, be considered
to be the true elementary organs of life.”

Literature IX.

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2. Buumenpacw. Ueber den Bildungstrieb und das Zeugungsgeschift. 1781.

3. Bonner. Considérations sur les corps organisés. Amsterdam, 1762.

4. Boveri. Hin geschlechtlich erzeugter Organismus ohne miitterliche Eigen-
schaften. Gesellschaft f. Morphol. u. Physiol. zu Miinchen. 1889.

5. CuHasry. Contribution a l'embryologie normale et tératologique des Ascidies
simples. Journal de Vanat. et de la phys. 1887.

6. Darwin. Animals and Plants under Domestication. Vol. IT.

7. Driescu. Entwicklungsmechanische Studien. Der Werth der beiden ersten
Furchungszellen in der Echinodermenentwicklung. Experimentelle Erze-
ugung von Theil- und Doppelbildungen. Zeitschr. f. wissenschajtl.
Zoologie. Bd. LIT. Leipzig, 1891.

8. Haxrcxen. Generelle Morphologie. Der Perigenesis der Plastidule.

9. V. Hensen. Die Grundlagen der Vererbung nach dem gegenwirtigen
Wissenskreis. Landwirthschaftl. Jalrbiicher. Bd. XIV. 1885.

10. Oscar Herrwie. Das Problem der Befruchtung und der Isotropie des
Kies, eine Theorie der Vererbung. Jena, 1884.

11. O. Herrwic. Vergleich der Ei- und Samenbildung bei Nematoden. Eine
Grundlage fiir celluliire Streitfrayen. Archiv f. wissenschaftl. Anatomie.
Bd. XXXVI. 1890.

12. O. Herrwie. Urmund und Spina bifida. Archiv f. mikrosk. Anatomie.
Bd. XXXIX. 1892.

13. O. Hertwie. deltere und neuere Entwicklung theorieen. 1892.

14. W. His. Die Theorieen der geschlechtlichen Zeugung. Archiv f. Anthro-
polugie. Bd. IV.u. V. 1871, 1872.

15. W. His. Unsere Kérperform u. das physiologische Problem ihrer Entste-
hung. Briefe an einen befreundeten Naturforscher. 1874.

16. Kérutker. Bedeutung der Zelikerne fiir die Viryange der Vererbung.
Zeitschrift f. wissenschaftl. Zoologie. Bd. XLII.

362

AW.

18.
19.

20.

21.

22.
23.

36.
37.

THE CELL

Kéuurer. Das Karyoplasma und die Vererbung. Ewe Kritik der
Weismann’schen Theorie von der Continuitiét des Keimplasmas. Zeitschr.
f. wissenschaftl. Zoologie. Bd. XLIV. 1866.

Logs. Untersuchungen zur physiologischen Morphologie der Thiere.
Organbildung u. Wachsthum. 1892.

Jowannes Miuer. Handbuch der Physiologie des Menschen.

Josepx Miiuer. Ueber Gamophagie. Ein Versuch zum weiteren Ausbau
der Theorie der Befruchtung u. Vererbung. Stuggart, 1892.

Nicent. Mechanisch-physioloyische Theorie der Abstammungslehre.
Miinchen. 1884.

Nusspaum. Zur Differenzirung des Geschlechts im Thierreich. Archiv f.
mikrosk. Anatomie. Bd. XVIII.

Nussbaum. Ueber die Veriinderungen der Geschlechtsprodukte bis zur
Lifurchung, ein Beitrag zur Lehre von der Vereroung. Archiv f. mikrosk.
Anatomie. Bd. XXIII.

Prutcer. Loc. citat. Cap. VIT.

Rovx. Beitriige zur Entwicklungsmechanik des Embryo im Froschet.
Zeitschrift f. Biologie. Bd. 21. 1885.

Roux. Ueber die kiinstliche Hervorbringung halber Embryonen durch die
Zerstorung einer der beiden ersten Furchungskugeln. Virchow’s Archiv.
Bd. CXIV. 1888.

Sacus. Ueber Stoff und Form von Pflanzenorganen. Arbeiten des botan.
Instituts, Wiirzburg. Bd. II u. IIL.

Spencer. Principles of Biology. 1864.

SrraspurGER. Neue Untersuchungen iiber den BiteuiKagusinaes bet
den Phanerogamen als Grundlage fiir eine Theorie der Zeugung. Jena,
1884. :

SrraspurGER. Ueber Kern- und Zelltheilung im Pflanzenreich, nebst einem
Anhang tiber Befruchtung. Jena, 1888.

Vécutine. Ueber Organbildung im Pflanzenreich. Bonn, 1878.

Hueco pe Vries. Intracellulare Pangenesis. Jena, 1889.

Weismann. Ueber Vererbung. 1883.

Weismann. The Continuity of the Germ-plasm as the Foundation of a
Theory of Heredity, 1885. Translated by Schinland. 1889.

Weismann. The Significance of Sexual Reproduction in the Theory of
Natural Selection, 1886. Translated by Schinland. 1889,

Weismann. On the Number of Polar Bodies and their Significance in
Heredity, 1887. Translated by Schinland. 1889.

Weismann. Amphimixis, or the Essential Meaning of Conjugation and
Reproduction, 1891. Translated by Poulton and Shipley, etc. 1892.
Weisner. Die LElementarstructur und das Wachsthum der lebenden

Substanz. 1892.

Caspar Friepr. Wourr. Theorie von der Generation. 1764.

Born. Ueber den Einfluss der Schwere auf das Froschei. Arch. f,
mikrosk, Anatomie. Bd. 24. :

PON ADV Bee

Abortive eggs, 238.

Acetabularia, 307.

Achromatin, 181.

Actinospherium, 35.

Adventitious substances in the cell, 27.

Aithalium septicum, 17, 99, 111, 115,
17

(.
Affinity, sexual, 300.
sexual influence of environment, 313.
Albumen, building up of, 150.
circulation of, 31.
crystals of, 150, 159.
molecule, 17.
Se yas Of 151.
Alga, 3, 6, 34
Alternation of generations, 255.
Alveolar layer, 21.
Auwitosis, 207.
Ameeba, structure of, 27.
movements of, 67.
stimulation of, 107, 111.
Amphiaster, 193.
Amphipyrenin, 44.
Amyloplasts, 160, 164.
Anesthetics, 112.
action of, upon Mimosa, ova and
spermatozoa, 118.
Analysis of pus corpuscles, 18.
of ash of Fucus, 1386.
Ancestral plasma theory, 351.
Aniline dyes, absorption of by living
cell, 136.
Animalculists, 337.
Antheridia, deviation of, 302.
Anticlinal division walls, 220.
Antipolar area, 184,
“Aphides, 296.
Apogamy, 295, 300.
Apposition, 164, 169.
Archoplasm, 190.
Aroidea, formation of heat in germinat-
ing seeds of, 130.
aeiskt! Pap ald eee corps résiduel,

division with reduction in sper-
matozoa of, 235.

division with reduction in ova of,
23

. fertilisation of, 259.
nuclear division of, 189.

Ascidians, multiplication of nuclei in

immature eggs of, 213.
Asexual condition in Ciliata, 292.
Ash, analysis of, in Fucus, 136.
Asparagin, attractive effect of, upon

Bacteria, 120.
Asplenium, apogamy of, 300.
Assimilation, 132.

Attraction centre, 245.
sphere, 181, 190.
Aureole (Fol), 259.

Bacteria, anzerobie, 129.
as tests for oxygen, 116.
traps, 121.
Basidiobolus ranarum, influence of nutri-
ment upon formation of sexual
cells, 294.
Bastard formation, 310.
Bees, 295.
Bibliography, 9, 61, 89, 123; 174
320, 332, 361.
Bioblasts of Altmann, 24,
Botrydium, 101, 288.

, 246,

Cane sugar as a stimulant to anthero-
zoids, 120.
Carbo-hydrates, 147.
Carbon dioxide, absorption of, 132.
Carica papya, 151.
Carnivorous plants, 151.
Cartilage cell, 31.
Cell-budding, 223.
contents, 26, 27, 31, 35.
definition of (Briicke), 8 is
definition of (Schleiden& Schwann),5.
definition of fgets
division, equal, 224
division, influence of the environ-
ment upon, 239.
division, partial, 230.
division, unequal, 225.
membrane, 5
nutritional Bienes OF S2Ts
permanent substances of, 27.
plate, 189, 198, 234,
sap, 6, 31, 154.
territories, 173.
theory, history of, 2
Cellular pathology, 1.
Cellulose, formation of, 152.
reaction of, 166.
Cell-wall, 166.
corky change of, 168.
deposition upon, 168.

growth of, 169.
woody change of, 168.
Central corpuscles (see Centrosomes).
Central spindle, 202.
Centrolecithal ees: 232.
Centrosomes, 55, 1
division ‘of, 189, "199, 259.
in Echinoderm fertilised ova, 258.
in lymph corpuscles, 56
in over fertilised eggs, UL.

364

Centrosomes in ovum of Ascaris, 262.
in Phanerogams, 264.
in pigment cells, 56.
in Radiolaria, 212.
female, 258, 265.
male, 258, 265.
multiple division of, 242, 244.
origin of, 203.
quadrille of; 259.
Characez, nuclei of, 210.
parthenogenesis in, 296.
rotation in, 71.
Chemical stimuli, 111.
Chemistry of assimilation, 146.
’ Chemotaxis, 115.
Chémotropism, 92.
in Aithalium, 115.
in antherozoids, 119.
in Bacteria and Infusoria, 116.
in leucocytes, 121.
Chief nucleus in Infusoria, 267, 269.
Chief spindle in Infusoria, 269.
Chloral, effect upon nuclear division, 240.
effect upon ovaand spermatozoa, 118.
Chloroform, 113.
Chlorophyll, 161.
corpuscles, 161.
effects of chloroform upon, 1138, 133.
function of, 132, 146.
movements of corpuscles under in-
fluence of light, 103.
Chorda dorsalis, 157.
Chromatic nuclear figures, 182.
Chromatin, 18, 181.
Chromatophores, 99.
Chromatoplasts, 160.
Chromosomes, 180, 200.
Cilia, 77.
formation of, 77, 83.
movements of, 77.
Ciliata, fertilisation of, 265.
galvanotropism in, 108.

_ heed for fertilisation of, 292.
Circulation in protoplasm, VAS 73:
Cleavage line in segmentation, 225.
Cleavage nucleus, 259.

Closterium, 279, 352.
Cold rigor, 96.
Colloids, 59.
Colour granules in plants, 162. j
Colouring matter, absorption of by
living cell, 136.
Conjugation, 278.
epidemics in Infusoria, 267.
Constant current, effect upon protoplasm,
of 107.
Cork formation in cell-wall, 168.
Coronal furrow in Frog’s egg, 196.
Corps résiduel in Ascaris, 246,
Corydalis cava, 308.
Cross-fertilisation in Acetabularia, 307.
in Amphibia, 312.
in Ciliata, 308.
in Echinoderms, 312.
in plants, 310.
need for, 318.

THE CELL

Crystalloids, 59.

Cuticle, formation of, 172...

Cutleriacee, fertilisation of, 286, 293.
sexual affinity in, 301.

Cytoblast, 177.

Cytoblastem, 6, 177.

Daphnoids, parthenogenesis in, 296.
Degeneration of animal egg-cells, 293.

of Infnsoria 267, 292, 307.

of nuclei, 245.

of swarm-spores of Algee, 245.
Desmidiacew, 279.
Deutoplasm, 26.
Development, theories of, 339.
Diapedesis, 122.
Diastase, 150.
Directive corpuscles, 228 (see Pole-cells).
Division of centrosomes, 189, 199, 259.

of chlorophyll granules, 161.

of egg-cell, 223-232.

of idioblasts, 341.

of nuclei, direct, 207.

of nuclei, indirect, 179.

of plasomes, 360.

of trophoplasts, 160.

with reduction, 235.

with reduction in Cosmarinm, 279.
Division plane, position of, in division of

egg-cell, 219. Be
Division plane, change of position
through external influences, 355.

Drosera, 151. ay
Dumb-bell figure in egg-cell division, 19.

Echinoderms, division of egg-cells of, 192.
Ectocarpus, 295.
Ketoplasm, 15.
Egg-cell, division of nncleus in, 199.
segmentation of, 223-232.
Electrical stimuli, 106.
Elementary organisms, 7, 24.
particles, 1, 8, 24, 340, 361.
units, 1, 3, 24, 340, 361.
Elemental germs, 334, ;
of an organism, 334, 339, 344.
development of, 357.
Embryo-sac of Phanerogams, 233, 263.
Endoplasm, 15.
Energy, kinetic, 126.
potential. 126.
Epigenesis, 336.
Kpistylis, fertilisation of, 271.
Equivalence of male and female heredi-
tary masses, 345.
Equivalence of nuclear substances in
fertilisation, 272.
Eudorina, 254.
fertilisation of, 289.
Kuglena viridis, reaction of light to, 100.

Fat, 151, 157.

Fertilisation, 252.
isogamous, 284.
methods of, 252.
need fur, 291.

INDEX

Fertilisation of Algx, 284.

of Ascaris megalocephala, 259.

of Botrydium, 285.

of Ciliata, 265.

of Cutleriacez, 286.

of Desmidiacee, 279.

of Echinoderm eggs, 256.

of Fucacer, 286.

of Infusoria, 265.

of Monjeotia, 283.

of Noctiluce, 278.

of non-nucleated portions of proto-
plasm, 299.

of Phanerogams (Lilium martagon),
283.

of Phzeosporee, 286.
of Spirogyra, 283.
of Vorticella, 281.
of Volvocines, 290.
of Zygnemacew, 281.
oogamous, 284
phenomena of, 289.
Filament theory (Flemming), 23.
Filamentous substance, 23.
Ferments, 128, 150
action of, 151.
Flagella, 77.
Foam theory of protoplasm (Biitschli),
20

Foam, structure of, 21.

Formative instinct (Blumenbach), 338.

Formative activity of the cell, 145.

Framework theory of protoplasm, 19.

Fritillaria imperialis, nuclear division in
the embryo-sac of, 196.

Fritillaria persica, nuclear division in
pollen grain of, 198

Fucacee, fertilisation of, 286.

Fucus, analysis of the ash of, 136.

Galvanotropism, 92, 108.
Gametangium, 284.
Gametes, 284, 293.
Gas chamber, 112.
Gemmule (Darwin), 340.
Generation cycle, 252, 297.
theories of, 339.
Geotropism, 92.
Germinal nucleus, 259.
Germinal spot, 50.
of Asteracanthion, 53.
of Molluses, 51, 52.
Germinal vesicle, 49.
Giant cells of bone marrow, 244.
Gliding movements of protoplasm, 70.
Goblet cells, 36.
Granula, 24, 25, 44.
theory (Altmann), 24.
Granular plasm, 15, 68.
Granule and mass theory (Arnold and
Purkinje), 8
Granules, streaming movements of, 68.
Gravity, effect of upon egg-cell division,
214.

Gromia oviformis, 29.
movements of, 69.

365

Growing point, arrangement of cells in,
221.

heaping up of protoplasm at, 323.
Guanin crystals, 158.

Heat production, a vital process, 130.
Heat rigor, 94
Heliotropism, 92.
Hereditary mass, 339.
combination of, 353.
distribution of, in the cell, 346.
division of, 352.
equivalence of male and female, 345.
prevention of the summation of, 350.
Heredity, theories of, 334.
Hermapbhroditism of the nucleus, 275,
History of the cell-theory, 2
History of the protoplasmic theory, 6.
Houeycomb theory of protoplasm (Biit-
schli), 20
Hyaloplasm, 15.
Hybrids, 313.
Hybridisation, 310.
Hydrocharis, 71.
Hydrodictyon, 294.
Hydrotropism, 117.

Idioblasts, 340.
arrangement of, 344.
division of, 341.
size and number of, 342.
Idioplasm, 339, 342, 357
Infusoria, fertilisation of, 265.
galvanotropism of, 108.
need for fertilisation of, 292.
Intercellular substance, 173.
Interfilamentous substance, 23.
Intergranula substance, 24.
Internal vesicle of Thalassicola, 212.
Intracellular digestion, 142.
pangenesis, 359.
Intramolecular heat, 127.
respiration, 131.
Intraplasmic products, 27.
Intussusception, 169.
Invertin, 150.
Irritability of the cell, 91.
of protoplasm, 91.
Isogamous fertilisation, 285.
Isotropy of protoplasm, 354.

Karyokinesis, 179.
Karyolisis, 199.

Latent properties, 334.
Leucocytes, absorption and digestion of
foreign bodies by, 1
chemotropism of, 121.
Leucopbrys patula, 253, 292.
Leucoplasts, 160.
Life-cycle in animals and plants, 148.
Light, action of, upon Aithalium, Pelo-
myxa, chromatophores, and pig-
ment cells of retina, 99.
action of upon Euglena and swarm-
spores, 100.

366

Light pictures produced upon leaves, 104.
stimulation,
tone (phototonus), 101, 102.

Lilium martagon, 26

Linin, 43.

Lymph corpuscles, centrosomes of, 203.
division of, 209.
movements of, 66.
perforated nuclei of, 209.
structure of, 28.

Macrocosm, 339.
Macro-gametes, 271.
Macro-nuclei of Ciliata, 266.
Malic acid as an attracting agent for
Fern antherozoids, 119.
Mechanical stimuli, 110.
Membrane of the cell, 5.
Meroblastic segmentation, 230.
Merocytes, 238, 245.
Mesocarpus, action of light upon, 104.
Metabolic products | of protoplasm, 18.
of micro-organisms, 122.
of the cell, 128.
Metabolism of the cell, 126-154.
progressive, 126.
retrogressive, 126.
Metastasis in plants, 150.
Micelle, 58, 340, 3438.
Micella theory, 19, 58.
solution, 60.
Microcosm, eure
Microgametes, 2
Miero-nuclei of Giiata, 266.
Micro-organisms, destruction of, by
phagocytes, 144.
metabolic products of, 122.
nuclei of, 55.
Microsomes, 14, 19, 22.
Middle portion or neck of spermatozoon,

Migratory nuclei of Infusoria, 269.
Mimosa pudica, 1
Mitome, 23.
Mitosis, 179.
Molecular structure, 58.
Monjeotia, 283.
Movements, changes in the cell during
passive movements, 88.
occurring in oil drops, 73.
of contractile pei 86.
of flagelia and cilia, 77
of protoplasm, 73-89.
of protoplasm during heat stimula-
tion, 94,
of protoplasm due to light stimula-
tion, 99. _
Mucous cells, 36.
Multiple fertilisation in chloralised egg-
cells, 114.
Multipolar giant cells, 244,
mitoses, 243
Muscle fibres, 173.
Mycoderma aceti, 147.
Myxomycetes, movements i in, 67.
structure of, 28.

THE CELL

Narcosis (of protoplasm, Mimosa, egg-
cells, and spermatozoa), 112-115.
Neck or middle portion of spermatozoon,

Nematodes, “nucleus of the fertilised
egg-cell of, 218.

Nematocysts, 164.

Nerve fibres, 173.

Net-like ciao of protoplasm, 238.
of nucleus, 47.

Nisus formativus (Blumenbach), 638.

Noctiluea, 278.

Non-nucleated cells, 54.

Nuclear framework, 47.
membrane,
sap, 43.
spindle, 181.

Nuclein, 40, 41.
bodies, 49.
in division, 180.
reaction of, 40).

Nucleoli, 42, 49, 52.
fate of, 205.

Nucleus, connecting fibrils in, 187, 198.
definition of, 37. ~
degeneration of, 245.
determination of ag oe of in the

cell, 214, 216, 2
discovery of, 3, 36.
division of, direct, 207.
division of, indirect, 179.
division of, influence of the environ-
ment upon, 239.
division of, in fertilised egg-cells,
263, 264, 973.
division of,
figures, 243
division of, pathclogical, 244.
division of, with reduction, 235.
fixed position of, in et cells, 325.
form, size, number of, 37.
germinal, 259.
history of, 37.
importance of, in segmentation, 349.
influence of, upon cell processes, 324,
330.

multipolar nuclear

in segmentation, 179.

longitudinal splitting of segments,
ot 186, 191

migratory, of Infusoria, 269.

multiplication of, 211.

of animal cells, 327.

of Bacteria, Oscillaria, ete., 54.

of Chironomons larva, 49.

of Ciliata, 4

of egg-cells, 50. :

of egg-cell of Dytiscus, 329.

of Fritillaria, 48.

of Salamander, 47.

of secreting cells of Nepa, 329.

of spermatozoa, 45.

of Psst mother cells of Ascaris,

of Spirogyra, 49.
segments during fertilisation, 263, .
264, 273

INDEX

Nucleus segments, number of, in division
with reduction, 235.
spindle, 180.
spindle, derivation of, 200, 202.
spindle, formation of, 185.
staining of, 40.
structure of, 44-54.
transmitter of hereditary elemental
germs, 344.
Nutrient plasm, 349.
solutions, 147, 294.
substances of the cell, 27.

(Edogonium, 34.

Onychodromis grandis, 253, 271, 293.

Oogamous fertilisation, 284.

Oogonium, 287, 302.

Osmosis, 138.

Over fertilisation, 243.

Over mature egg-cells, 293.

Ovists, 337.

Ovocentrum, 258, 274.

Oxygen, action upon Athalium, 115,
128.

action upon Bacteria and Ciliata,
116, 117.
action upon cells, 112.

Pandorina, 254.
fertilisation of, 287.
Pangenee (de Vries), 340, 359.
Pangenesis, 340.
intracellular, 359.
Paramecia, need for fertilisation of, 267. |
need for oxygen of, 117.
Paramitome, 23.
Paranuclein, 42, 257.
Paranucleus of Ciliata, 267, 269.
Paranuclear spindle, 269.
Paraplasm, 26.
Parthenogenesis, 255, 295.
Pelomyxa, 99.
Pepsin, 151.
Perforated nuclei, 210.
Periclinal division walls, 220.
Peripheral layer of protoplasm of the
cell, 15.
of Frog’s eggs, 15.
réle of in osmosis, 140.
Permanent material of the cell, 27.
Peronosporex, sexual affinity in, 302.
Pheeosporee, fertilisation of, 286.
Phagocytes, 143.
Phagocytosis, 143.
Photophobie spores, 102.
Photophylic spores, 102.
Phototonus, 101, 102.
Phylloxera, 295.
Physiological units (Spencer), 340.
Phytogenesis, 3.
Pigment granules, 158.
Pill-box theory of development, 338.
Plane of division, position of, in egg-cell,
219.

Plant anatomy, 2.

Plasmic products, 27, 154.

367

Plasmolysis, 140.
Plasmodium, 68.

light-stimulation of, 99.
Plasomes, 360.
Plastidule, 342.
Plastin, 17.

reaction of, 17.
Podophrya gemmipara, 229.
Polar area, 1

differentiation, 214.
Pole cells, 228, 237, 269.

of parthenogenetic ova, 298, 299.
Pollen grains, 263.
Pollen tube, 264.
Polyaster, 243.
Polyspermia, 114, 243.
Preformation theory, 336.
Primordial utricle, 32.
Pronuclei, 275.
Proteid substances, 17.
Protoplasm, adventitious substances in,

34

alkalinity of, 17
chemico-pk ysical and morphological
properties of, 11.
death from cold of, 95.
double refraction of, 18.
first use of the word, 6.
formation of, 16.
history of protoplasmic theory, 6.
of Amoeba, 28.
of lymph corpuscles, 28.
of Myxomycetes, 28
of Reticularia, 28.
structure of, 18.
Protoplasmic movements, 68, 73-89.
due to heat stimulation, 94.
due to light stimulation, 99.
metabolic products of, 18.
of Amecebee, 67.
of flagella and cilia, 77.
of Gromia oviformis, 69.
of lymph corpuscles, 66.
of Myxomycetes, 67.
of plant cells, 71.
simulated by drops of oil, 73-77.
theories concerning, 73
Protoplasmic threads, 238, 31.
Pseudopodia, 27, 28, 29, 66, 110.
Pteris cretica, 300.
Ptyalin, 151.
Pyrenin, reaction of, etc., 42.

Quadrille of the Centrosomes, 259.
Radiation figures in Echinoderm eggs,

92.
figures in protoplasm, 55, 181.
Balog 212.
Receptive protuberance, 257, 304.
spot in Algwe, 256.
Reduction of nuclear segments, 262, 264,
265.
of nuclear segments in Ciliata,
70

iu.
Regeneration, 346.

“368

Reproduction of the cell, 177.
theories of, j
Reserve materials, 26, 35, 150.
Respiration of the cell, 128.
intramolecular, 131.
Restitution theory, 276.
Retarded eggs, 296.
Reticularia, 28.
movements in, 69.
Rheotropism of Rr pajeues, 68.
Rotation in protoplasm, 71
Rotatoria, 296.

Saccharomyces cerevisiz, effect of ebloro-
form upon, 114.
Salamandra maculata, nuclear division
of, 1¢3.
Sarcode, 7, 29.
Segmentation ‘of the egg, 223-232.
equal, 224
meroblastic or partial, 230.
~ unequal, 225.
Selective powers ‘of the cell, 135.
Self-fertilisation, 299.
Separation bodies (Flemming), 189.
Sexual affinity, 300
characters, 276.
dimorphism in Vorticella, 272.
ear fundamental modes of,

taararty 3 in Ciliata, 292, 293.
nuclei, 266.
swarm-spores, 284.
Skeleton of the cell, 159.-
Somatoplasm, 349.
Specific energy, 92.
Sperm centrum, 258, 274.
nucleus, 199, 243, 257.
nucleus in non-nucleated fragments
of egg-cells, 300.
spindle, 244.
Spermatozoon, of Ascaris, 46.
movements of, 82.
narcosis of, 114, 147, 160.
of Echinoderms, 257.
structure of, 45.
Spindle aggregations, 245.
fibrils, 181, 202.
Spirogyra, 283.
Sporangium, 234, 284.
Staphylococcus, 122.
Starch formation, 132.
formation in plant cells, 160, 163.
granules, 162
Stationary nuclei of Infusoria, 269.
Stimulation, phenomena of, 91, 938.
after-effects of, 94.
Stimuli, chemical, 111.
electrical, 106.
kinds of, 92. °
light, 99.

“THE CELL.

Stimuli, mechanical, 110.
protoplasmic, 91
thermal, 94.
Streaming movements of oil drops, 73-

77

Stylonichia, 258, 292.

Suberin, 168.

Sommer eggs, 296.

Swarm- -spores, action of light upon,
100.

formation of, 234.
passing out from cell membrane of,

sexual a and asexual, 284.

Telolecithal eggs, 232.
‘Temperature, effect of, upon cell, 94.
maximum, mimimum, 94,
Tension (potential energy), 126.
Tetraster, 243.
Thermal stimuli, 94, 239.
T'radescantia, 72, 94, 106.
Transverse division plane, 220.
Trianea bogotensis, 71.
Triaster, 243.
Trophoplasm, 357. -
'lrophoplasts, 159.
division of, 160.
Tuberculin, mode of action of, 1238.

' Turgor (turgescence), 141, 155.°

Ulothrix, 101.

Vacuoles, 31, 34, 154,
contractile, 85.
Vallisneria, 71, 194.
Vaucheria, repair of, after injury, 328.
Vegetative reproduction, 255.
Vessels in plants, 2
Vital elementary units, 1.
force, 91.
properties of the cell, 65, 126.
processes, 128.
Vitalism, theory of, 91.
Vitelline membrane, 257.
Volvocines, 287.
Volvox globator, 290.
Vorticellze, 271, 302.

Winter eggs, 296.
Woody change of cell-wall, 1€8.

Xanthophyll, 132.

Yolk, 158.

granules, 158.
nuclei, 233.

Zooglea, 24.
Zygnemaceer, fertilisation of, 281.
Zygote, 279, 281, 283

Butler & Tanner, The Selwood Printing Works, Frome, and London.

FEB 5 1984
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