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THE

BIOLOGICAL PROBLEM OF TO-DAY

HERTWIG

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THE BIOLOGICAL PROBLEM

OF TO-DAY

PREFORMATION OR EPIGENESIS?
THE BASIS OF A THEORY OF ORGANIC

DEVELOPMENT

0 RlC-iUfcC C

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BY

PROFESSOR DR. OSCAR HERTWIG

DIRECTOR OF THE SECOND ANATOMICAL INSTITUTE OF THE UNIVERSITY OF BERLIN

ut^oriseb translation

BY

P. CHALMERS MITCHELL, M.A.

WITH AN INTRODUCTION BY THE TRANSLATOR
AND A GLOSSARY OF THE TECHNICAL TERMS

'Cr

THE MACMILLAN CO.
NEW YORK

PREFACE

SHORTLY after the appearance of Dr. Oscar Hert-
wig's treatise ' Praformation oder Epigenese ?' I
published in Natural Science (1894) a detailed
abstract of it. But the momentous issues involved
in the problem of heredity, and the great interest
excited by Dr. Weismann's theories, make it de-
sirable that a full translation should appear. By
the kindness of Dr. Hertwig and his German pub-
lisher, this is now possible. I have prefixed an
introduction, written for those who are interested
in the general problem, but who have little acquaint-
ance with the technical matters on which the
argument turns. In the actual translation I have
tried no more than to give a faithful rendering of
the German. After no little perplexity, I have
rendered the German word Anlage as 'rudiment.'
It is true, a double meaning has been grafted upon
the English word, and it is widely employed to mean
an undeveloped structure, without discrimination
between incipient and vestigial character. I use it
in the etymological sense, as an incipient structure.
For the difficult words, Erbgleich and Erbungleich,
a succession of new terms have been suggested.
Here I use for the first term the word ' doubling,'

for the second ' differentiating.'

P. C. M.

TRANSLATORS INTRODUCTION

INQUIRY into the problems of heredity is beset with
many difficulties, of which not the least is the
temptation to argue about the possible, or the
probable, rather than to keep in the lines of obser-
vation. Setting out from a laborious and beautiful
series of investigations into the anatomy of the
Hydromedusse, Weismann came to think that the
organic material from which the sexual cells of
these animals arose was not the common protoplasm
of their tissues, but a peculiar plasm, distinct in its
nature and possibilities. In the course of several
years, Weismann not only continued his own in-
vestigations in the many directions that his con-
ception suggested, but made abundant use of that
new knowledge of the nature and properties of cells
which has been the feature of the microscopy of
the last decade. His theory of the germplasm
gradually grew, undergoing many alterations, so
that even in its present form he regards it as tenta-
tive. Neglecting the numerous modifications and
accessory hypotheses by which he has sought to
adapt the theory to the phantasmagorial complexity
of organic nature, the main outline of the theory is

viii TRANSLATORS INTRODUCTION

as follows : A living being takes its individual origin
only where there is separated from the stock of the
parent a little piece of the peculiar reproductive
plasm, the so-called germplasm. In sexless repro-
duction one parent is enough ; in sexual repro-
duction equal masses of germplasm from each
parent combine to form the new individual. The
germplasm resides in the nucleus of cells, and Weis-
mann identifies it with the nuclear material which
microscopists have named chromatin, on account
of the avidity with which it absorbs certain dyes.
Like ordinary protoplasm, of which the bulk of
cell-bodies is composed, the germplasm is a living
material, capable of growing in bulk without altera-
tion of structure, when it has access to appropriate
food. But it is a living material much more com-
plex than protoplasm. In the first place, the mass
of germplasm which is the starting-point of a new
individual consists of several, sometimes of many,
pieces termed ids, each of which contains all the
possibilities — generic, specific, individual — of a new
organism. Each id is a veritable microcosm,
possessed of a historic architecture that has been
slowly elaborated during the multitudinous series
of generations that stretch backwards in time from
every living individual. This microcosm, again,
consists of a number of minor vital units called
determinants, which cohere according to an orderly
plan. A determinant exists for every part of the
adult organism which is capable of being different
in different individuals. And, lastly, each deter-
minant consists of a number of ultimate particles

TRANSLATOR'S INTRODUCTION ix

called biophores, which eventually pass into the
protoplasm of the cells in which they come to lie
and direct the vital activities of these cells. A
most important part of the theory is what it
supposes to occur during the embryological develop-
ment of the individual. The mass of germplasm
derived from the germplasm of the parent lies in a
mass of ordinary protoplasm. Both the protoplasm
and the germplasm, by the assimilation of food,
gradually increase in bulk until the adult size of
the organism is reached. Along with the increase
of size there occurs a gradual specialisation, during
which the tissues, organs, and structure of the
creature are attained. The simplest conception of
this process is to regard the initial mass as a single
cell, the nucleus of which is composed of the
parental germplasm. The nucleus and the proto-
plasm increase in size, and then, first the nucleus
and next the protoplasm divide, so that there are
formed two cells, each with a nucleus. Each of
these again divides, and the process goes on con-
tinuously, the new-formed cells gradually being
marshalled into their places to form the adult
tissues and organs, and they gradually assume the
special characters of these tissues and organs.
Now, Weismann's theory supposes that the first
division of the germplasm is what is called in
this translation a doubling division (Erbgleiche
Theilung). The mass has grown in bulk, without
altering its character, so that each resulting mass
is precisely like the other. One of the two portions
subsequently increases in bulk, and may again

x TRANSLATOR'S INTRODUCTION

divide repeatedly, but always by doubling division.
It therefore remains unaltered germplasm, and
eventually is marshalled to the part of the adult
from which new organisms are to arise, becoming,
for instance, in the case of a woman, the nuclear
matter of the ovary. Thus, the germplasm is
handed on continuously from generation to genera-
tion, forming an unbroken chain, through each
individual, from grandparent to grandchild. This
is the immortality of the germ-cells, the part of the
theory which has laid so strong a hold on the
popular imagination. And with this also is con-
nected the equally celebrated denial of the inherit-
ance of acquired characters. For, at first, it seemed
a clear inference that, if the hereditary mass for the
daughters were separated off from the hereditary
mass that was to form the mother, at the very first,
before the body of the mother was formed, the
daughters were in all essentials the sisters of their
mother, and could take from her nothing of any
characters that might be impressed upon her body
in subsequent development. As this treatise
touches only indirectly upon the question of
acquired characters, it is necessary only to mention
that while his early sharp denial of the possibility
of inheritance of acquired characters has led to a
damaging criticism of supposed cases, Weismann,
in the riper development of his theory, has found a
possibility for the partial transference of influences
that affect the mother to the germplasm contained
within her.

It is with the fate of the other portion coming

TRANSLATOR'S INTRODUCTION xi

from the first division of the germplasm that we
are concerned here. It is set apart to form the
nuclear matter, and so to control the building up
of the actual individual. Weismann supposes that
the subsequent divisions it undergoes are what I
call in this translation differentiating divisions
(Erbungleiche Theilung). According to his theory,
in each of these divisions the microcosms of the
germplasm are not doubled, but are slowly dis-
integrated, the division differentiating among the
determinants, and marshalling one set into one
portion, the other set into the other portion. The
differentiating process occurs in an order deter-
mined by the historic architecture of the micro-
cosms, so that the proper determinants are liberated
at the proper time for the modelling of the tissues
and organs. Ultimately, when the whole body is
formed, the cells contain only their own kind of deter-
minants. It follows, of course, from this that the
cells of the tissues cannot give rise to structures
containing less disintegrated nuclear material than
their own nuclear material, and least of all to
reproductive cells, which must contain the undis-
integrated microcosms of the germplasm. As
special adaptations for the formation of buds and
for the reconstruction of lost parts, cells may be
provided with latent groups of determinants to
become active only on emergency. But with these
exceptions, the nuclear matter of the cells of the
body contains only what is called idioplasm, a
differentiated portion of the germplasm peculiar to
cells of their own order, and it can give rise only to

xii TRANSLATORS INTRODUCTION

idioplasm of the same or of a lower order. And here
we come round again to the original observations
from which Weismann set out. For he found that
among the Hydrornedusae, although the sexual
cells seemed to arise in very different topographical
positions, there had always been a migration to
these localities of a material which he would now
call the germplasm. And here also, that the point
may be made plain, there may be mentioned the
observations of surgeons and physicians, who insist
that the growths of disease always conform strictly,
in their cellular nature, to the tissues from which
they arose, and that in the healing of wounds like
only grows from cellular like.

Dr. Oscar Hertwig is a scientific naturalist of the
very first rank, and his name is peculiarly asso-
ciated with many of the most important advances
in our knowledge of cells and of embryology. To
him chiefly, for instance, is due the discovery of the
intimate nature of fertilisation — that it consists in
the union of the nuclear matter of a cell from the
male with the nuclear matter of a cell from the
female. With the exception of Francis Balfour, no
man has laboured more patiently, or achieved more
wonderful results, in the investigation of the origin
and marshalling of cells by which the egg changes
into the adult. From his own experience, and from
his study of the observations made by others, he
has been led to doubt the validity of apparently
fundamental parts of Weismann's conception. In
the first place, he thinks that there is no evidence
for the existence of differentiating as opposed to

TRANSLATOR'S INTRODUCTION xiii

doubling divisions, and that there is evidence that
divisions always are doubling divisions. He thinks,
in fact, that when a portion of germplasm divides,
the daughter- cells receive portions of germplasm
exactly alike and exactly like the original portion
in the parent-cell. The cells, indeed, become
different from each other as the organism grows,
some becoming muscle-cells, others nerve-cells,
others digestive-cells, and so forth. Weismann
thinks that the differences occur because, in the
disintegration of the germplasm - microcosms,
according to a prearranged plan, only the deter-
minants for nerve-cells are marshalled into nerve-
cells, only those for muscle-cells into muscle-cells,
and so forth. The development is an evolution, an
unfolding or unwrapping of little rudiments that
lie in the germplasm. Hertwig insists that every
cell receives the same kind of germplasm, but that,
according to the situations in which they come to
lie, different characters are impressed upon them.
The development is an epigenesis, or impressing on
identical material of different characters by dif-
ferent surrounding forces. His second line of
argument against Weismann leads to a similar
conclusion. A large number of the characters that
arise in an organism during its development are
due to the combination of many cells. They
cannot come into existence until the multiplication
of cells has made their existence possible, and he
thinks, therefore, that they cannot have rudiments
inside a single cell as their determining cause.
It is no part of my present purpose to insist,

xiv TRANSLATORS INTRODUCTION

even to the extent that in this treatise Hertwig

o

nimself insists, upon the points of agreement
between the two views. We are only at the
beginning of inquiry into the problems of heredity,
and the protagonists of the opposing views, like
all those who care more for knowledge than for
argument, are concerned more for truth than for
the establishment of a modus vivendi. Reconcilia-
tion is the parent of slothful thinking and of
glosses ; it is by sharp contrasting of the opposing
views that we are like to have new facts elicited,
and new lines of inquiry suggested.

As many are interested in the problems who

have little acquaintance with the technical facts

of embryology, a simple account of the early

stages in the development of an animal may be

useful for reference. I shall choose back-boned

animals, as, from the inclusion of man among them,

they are of more general interest. The process

begins with the fertilisation of the egg- cell by the

fusion with its nucleus of the nucleus or head of a

male-cell or spermatozoon. At their first origin

the nuclei of the sperm and of the egg may be

of very different appearance, while that of the

sperm is invariably smaller than that of the egg.

But before or during the process of fertilisation,

changes take place, the result of which is that

the fusing nuclei are exactly alike in morphological

character. The chromatin, or peculiar substance

of the nuclei, is transformed into a number of

bodies known as chromosomes, which are of the

same number, form, and size, in the two sexes.

TRANSLATORS INTRODUCTION xv

Form, size, and number are different in different
animals, but there is reason to believe that they
are normally the same in all the individuals of a
species. The fertilised nucleus, thus consisting of
chromosomes from male and female, then divides
by a complicated process known as karyokinesis,
in which each chromosome splits longitudinally, one
half passing to each daughter-nucleus. Through-
out the whole process of embryonic and post-
embryonic growth, the chromatin is gradually
increasing in bulk, and being distributed by
karyokinesis. The normal character of these
divisions is as follows : A daughter-nucleus, after
separation, passes through a resting phase, in
which the chromosomes, as detinite structures,
disappear, and in which growth of the nuclear
matter occurs. Then chromosomes of definite
size and form, and corresponding in number to
those present in the fertilised egg-cell, again
appear. These split longitudinally, and a half of
each passes to each daughter-nucleus. The simi-
larity of these processes among all living creatures,
vegetable and animal, and their extreme complica-
tion, suggests that karyokinesis is the chief factor
in distributing the hereditary mass to the growing
organism. Weismann and some others think that
there is evidence for a difference in the nature of
the process, which may in some cases correspond to
his distinction between doubling and differentiating
divisions, but it may be said at once that the record
of observations is yet too conflicting for any such
general interpretation.

xvi TRANSLATOR'S INTRODUCTION

Along with the increase in bulk and distribution
of the nuclear matter, there goes an increase in
bulk and segregation of the ordinary protoplasm.
The simplicity of the actual development of most
back-boned animals is disguised by provision for
the nutrition of the growing embryo. In a large
number of cases, as, for instance, in birds and
reptiles, the egg-cell, a microscopic structure at its
first formation, is bloated out into the large eggs
with which we are familiar, by the addition of
quantities of food-yolk. These eggs, although
morphologically single cells, do not divide as cells.
A small disc of protoplasm, surrounding the
nucleus, floats upon the surface of the yellow yolk,
and, when the nucleus divides, furrows appear in
this between the daughter-nuclei, but stretch very
little way into the inert food-yolk. The subse-
quent marshalling of the cells is disguised by their
association with a preponderating mass of inert
material. In a far-distant period in the history of
evolution, the eggs of mammals like man were
large, and contained, as in the lowest existing
mammals, a store of food-yolk. Now the food-
yolk is not formed, as the developing embryo
obtains its nourishment from the blood of the
mother. But the course of development is dis-
torted, partly as a legacy from the old large-yolked
condition, and still more to suit the new method of
nutrition. Some of the simpler animals even
among existing vertebrates still exhibit a marshal-
ling of cells common among invertebrates, and to
be traced under the complications of higher forms.

TRANSLATORS INTRODUCTION xvii

In these, now, as in the marine ancestors of all the
vertebrates, the fertilised egg is a tiny cell provided
with very little yolk, and set adrift in the sea-
water. The first division of the nucleus, and each
subsequent division of the daughter-nuclei, is at
once followed by division or segmentation of the
whole cell. The plane between the two cells thus
formed is called the first cleavage-plane, and is
regarded as vertical. The second cleavage-plane
is at right angles to the first, and is also vertical, so
that the little embryo consists of four cells, all on
the same horizontal plane. The third cleavage-
plane is horizontal, and divides the four cells into
an upper and lower tier of four cells. In the course
of a series of divisions the eight cells come to form
a hollow sphere — the blastosphere — enclosing a
cavity known as the cleavage or segmentation
cavity.

The first great modelling then occurs. At one
side the single layer of cells, of which the wall of
the blastosphere is composed, begins to bend
inwards, just as a dimple forms in a hollow india-
rubber ball if a pin-prick allow some of the con-
tained air to escape. Further cell-divisions occur,
and the invagination becomes deeper, until the in-
vaginating wall nearly touches the wall which has
retained its primitive position. The embryo has
thus become a hollow cup, the walls of which are
double. The cup elongates, and its mouth, origin-
ally wide open, becomes more and more narrow,
until it forms a small pore opening into an elongated
blind sack. The embryo in this stage is known as

xviii TRANSLATORS INTRODUCTION

a gastrula. The central cavity becomes the cavity
of the gut; the pore leading into it marks the
hind end of the future animal, in the case of verte-
brates, and is known as the blastopore. The layer
of cells lining the cavity of the sack is known as
the hypoblast, and gives rise chiefly to the cells
lining the alimentary canal of the future animal.
The outer layer of cells is known as the epiblast,
and forms the outer layer of the skin, and, along
the future dorsal line, gives rise to the nervous
system. The muscles and skeleton and the repro-
ductive cells arise from a set of cells known as the
mesoblast, that are formed chiefly from the hypo-
blast, and that push their way in between the
hypoblast and epiblast.

This general course of development may be
traced in all members of the vertebrate group, and,
with slight modifications, may be applied to a large
number of invertebrates. As the modelling of the
general contour of the whole body and of the
separate organs proceeds, the protoplasm of the
cells gradually assumes the characters of the sub-
stance of muscle-cells, liver-cells, nerve-cells, blood -
cells, and so forth. The problem of this book will
become clearer if it be considered with special re-
ference to what goes on in these early stages.
Hertwig says that all the cells of the epiblast,
hypoblast, mesoblast, and of the later derivatives
of these primary layers, receive identical portions
of germplasm by means of doubling nuclear
divisions. The different positions, relations to each
other and to the whole organism, and to the

TRANSLATOR'S INTRODUCTION xix

environment in the widest sense of the term, cause
different sides of the capacities of the cells to be
developed, but they retain in a latent form all
the capacities of the species. Weismann says that
the nuclear divisions are differentiating, and that
the microcosms of the germplasm, in accordance
with their inherited architecture, gradually liberate
different kinds of determinants into the different
cells, and that, therefore, the essential cause of the
specialisation of the organism was contained from
the beginning in the germplasm.

CONTENTS

PAGE

PREFACE - - V

TRANSLATOR'S INTRODUCTION - vii

INTRODUCTION 1

PART I.

WEISMANN'S THEORY OF THE GERMPLASM AND DOCTRINE

OF DETERMINANTS - 17

PART II.

THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT

OF ORGANISMS ..... 1Q1

31244

THE

BIOLOGICAL PEOBLEM OF TO-DAY

INTRODUCTION.

WHAT is development? Does it imply preforma-
tion or epigenesis ? This perplexing question of
biology has reappeared recently as a problem of the
day. Of late years there have been set forth con-
tradictory doctrines, each seeking to explain the
process by which the fertilised egg-cell, an ap-
parently simple beginning, gives rise to the adult
organism, which often is exceedingly complicated,
and which has the capacity of producing new
beginnings like that from which it itself arose.

The opposing views of to-day were in existence
centuries ago, and they are known in the history
of science as the theory of preformation or evolu-
tion, and the theory of epigenesis. That most
of the great biologists of the seventeenth and
eighteenth centuries were decided upholders of
evolution was the natural result of the con-
temporary knowledge of facts. For they knew
only the external signs of the process of develop-

1

2 THE BIOLOGICAL PROBLEM OF TO-DA Y

ment. All they saw was the embryo becoming
adult, the bud growing out into a blossom, as the
result of a process in which nutrition transformed
smaller to greater parts. And so they regarded
development as a simple process of growth result-
ing from nutrition. Their mental picture of the
germ or beginning of an organism was an exceed-

o o o o

ingly reduced image of the organism, an image
requiring for its development nothing but nutrition
and growth. That the material eye failed to
recognise the miniature they attributed to the
imperfection of our senses, and to the extreme
minuteness and resulting opacity of the object.

That it might satisfy our human craving for
final causes, the theory of preformation had to be
accompanied by a corresponding explanation of the
origin of the miniatures. Biologists had already
abandoned the error of such spontaneous generation
as the origin of flies from decaying meat, and, in
its place, had accepted the doctrine of the con-
tinuity of life, formulating it in the phrase, Omne
vivum e vivo (Each life from a life), and in the
similar phrase, Omne vivum ex ovo (Each life from
an egg). One creature issued from another, within
which it had lain as a germ, and the series was
continuous. Thus, the theory of preformation gave
rise to the conception that living things were a
series of cases or wrappings, germ folded within
germ. The origin of life was relegated to the
beginning, at the creation of the world : it became
the work of a supernatural Creator, who, when He
formed the first creatures, formed with them, and

INTRODUCTION 3

placed within them, the germs of all subsequent
creatures.

To reckon at their proper value the theory of
preformation, and, still more, the doctrine of en-
folded germs, the standard of appreciation must
not be the present range of our knowledge. They
must be viewed historically, in the light of the
knowledge of these days.

Nowadays it is not so much pure reason as a
wider empirical knowledge of nature, with its con-
sequent transformation of ideas, that makes the
doctrine of enfoldment difficult. Abstract thought
sets no limit to smallness or greatness ; for mathe-
matics deals with the infinitely small and with the
infinitely great. So long as actual observation had
not determined the limits of minuteness in the
cases in question, there were no logical difficulties
in the doctrine of enfolded germs. The biology of
earlier centuries had not our empirical standard.
What appeared then to be a simple organic
material we have resolved into millions of cells,
themselves consisting of different chemical materials.
The chemical materials have been analysed into
their elements, and chemistry and physics have
determined the dimensions of the ultimate molecules
of these. It is only because the minute constitu-
tion of matter is no longer a secret to us that the
theory of germ within germ now touches the
absurd.

It was very different in earlier days ; the acutest
biologists and philosophers were evolutionists, and
an epigenetic conception of the process of develop-

4 THE BIOLOGICAL PROBLEM OF TO-DA Y

merit could find no foothold alongside the apparent
logical consistency of the theory of preformation.

Wolff 's T/ieoria Generationis (1759) failed to
convince his contemporaries, because he could bring
against the closed system of the evolutionists only
isolated observations, and these doubtful of inter-
pretation ; and because, in his time, on account of
the rudimentary state of the methods of research
in biology, men attached more importance to ab-
stract reasoning than to observation. His effort
was the more praiseworthy in that it was observa-
tion bearing witness against abstract and dogmatic
conceptions. By means of actual observation he
tried to expose the fallacy in preformation, to show
that the organism was not fully formed in the germ,
but that all development proceeded by new forma-
tion, or epigenesis ; that the germ consisted of
unorganised organic material, which became formed
or organised only little by little in the course of its
development, and that Nature really was able to
produce an organism from an unorganised material
simply by her inherent forces.

It is interesting to display the essential contrast
between preformation and epigenesis in the poetical
words of Wolff' himself. ' You must remember,'
so run his words in the second argument against
the probability of preformation, ' that an evolution
would be a phenomenon formed in its real essence
by God at the Creation, but created in condition
invisible, and so as to remain invisible for long
before it would become visible. See, then, that a
phenomenon of enfolding is a miracle, differing

INTRODUCTION 5

from ordinary miracles only in these : first, it was
at the creation of the world that God produced it ;
second, it remained invisible for long before it
became visible. In truth, therefore, all organic
bodies would be miracles. Would not this change
for us the presence of Nature ? Would it not spoil
her of her beauty ? Hitherto we had a living
Nature, displaying endless changes by her own
forces. Now it would be a fabric displaying change
in seeming only, in truth and essence remaining
unchanged and as it was constructed, save that
it gradually becomes more and more used up.
Formerly it was a Nature destroying herself and
creating herself anew, only that endless changes
might become visible and new sides be brought to
light. Now it would be a lifeless mass shedding off
piece after piece until the stock should come to an
end.'

None the less, who seeks in Wolff's ' Theoria
Generationis ' an account of the means or forces by
which Nature builds up organic forms will seek in
vain. The vis essenticdis (inherent force) with
which Wolff endowed his plastic organic material, or
the nisus formativus (formative force), afterwards
suggested to science by Blumenbach- -what are
they but empty words by which men seek to
grasp in thought what has eluded them ? Wolff's
epigenesis was not a complete explanation— indeed,
from its fundamental conception it could not
possibly be such. For investigation of the natural
forces by which development proceeds can advance
only slowly and step by step, and for long will con-

6 THE BIOLOGICAL PROBLEM OF TO-DAY

stitute the foremost task of biology. The prose-
cution of biological investigation will continuously
endow the theory of epigenesis with a fuller and
fuller meaning, but will never transform it into a
solution final in the sense of the theory of pre-
formation.

It seems to me that the significance of Wolff's
doctrine lies in this : it rejected the purely formal
theory of preformation because actual observations
were against it. Thereby Wolft' freed research
from the straitened bonds of prejudice, and entered
the only possible path by which science can advance
-the path along which the biology of our century
has made so great advances.

Biologists of to-day approach the problem of
organic development equipped with incomparably
greater knowledge and with more delicate methods
of research. But in our thoughts to-day, as we
discuss the essential nature of the process of organic
development and the mutual causal relations be-
tween rudiments and their products, the same con-
tradictory views are present, altered only as our
methods of expression have altered.

In a striking fashion Roux1 has contrasted the
opposing ideas inherent in our modern conception
of development, but yet identical with those which
formerly found expression in the theories of pre-
formation and epigenesis.

c By the term " embryonic development," in its

1 "VVilhelm Roux in Zcitsclirift fiir Pioloylc, vol. xxi. (1SS5) :
Zilr Ori^ntinnui iicJicr rinlai- Proll^m,- <I<T Embryondleii Ert1 iclcJc-
luny.

INTRODUCTION 7

ordinary acceptation, we understand the appear-
ance of visible complexity. But when we speak of
the visibility of the resulting complexity, we use a
subjective term, the value of which is relative to
the human eye. Going further into the matter, we
must break up the conception into two parts, and
distinguish between the actual production of com-
plexity and the mere transformation of complexity
from a condition invisible to us into complexity
visible to our senses.

' The two kinds of development I have indicated
bear a relation to each other that recalls the old
opposing doctrines of preformation and epigenesis,
the alternatives of a time when it was a task —
perhaps the only possible task — to record the com-
pleted results of the stages in development as they
became complete — in fact, to record the externally
visible changes of shape. In this descriptive in-
vestigation of the development of external form,
epigenesis, the successive formation of new shapes,
gained a complete victory over evolution, the mere
becoming visible of pre-existing details of shape.

' The closer investigation of embryonic develop-
ment that is necessary in a search for causes
brings us once more against the old alternatives,
and compels us to a closer scrutiny of them.

' In this, if we still retain the old terms, epigenesis
would mean not merely the building up of com-
plicated form through the agency of a substratum,
apparently simple, but perhaps with an extra-
ordinarily complicated, minute structure, but, in the
strictest sense of the term, the new formation of

THE BIOLOGICAL PROBLEM OF TO-DA Y

complexity, an actual increase of complexity.
Evolution, on the other hand, would imply the
mere becoming visible of pre - existing latent
differentiation. Clearly, according to these general
definitions, occurrences which outwardly exhibit
epigenesis may be in reality partial or complete
evolution. In fact, the deepest consideration leads
us again to the original question : Is embryonic
development epigenesis or evolution ? Is it the
new formation of complexity, or is it the becoming
visible of complexity previously invisible to us ?'

Thus, in our own days, after the controversy has
been at rest for long, biologists are assembled in
opposing groups, one under the standard of epi-
genesis, another under that of preformation.

Weismann1 leads the van for preformation ; for
the last ten years he has occupied himself with the
theoretical discussion of the questions set forth
above ; and now, in a recent treatise, The Germ-
plasm, he has combined his views, already many
times modified, in a coherent theory. Now he ex-
plains candidly that he has been driven to the
view that epigenetic development does not exist.
* In the first chapter of my book,' he remarks, ' will
be found an actual proof of the reality of evolution,
a proof so simple and obvious that I can scarcely
understand to-day how it could have escaped my
notice so long' (Germplasm, p. 14). Elsewhere he
writes : c I believe that I have established that

1 See Weismann's Collected Essays, Clarendon Press (2nd edit. ),
vol. i. , 1891, vol. ii., 1892 ; and Weismann's Germplasm, Walter
Scott's Contemporary Science Series-, 1893. The references in this
translation are to the latter volume.

9

ontogeny can be explained only by evolution, and
not by epigenesis.'

A mental process, which consciously or uncon-
sciously plays a great part with evolutionists, and
helps to determine their conclusions, is characteristic
of the direction of their inquiries. They set out
from the fact that the characters of the parents,
often to the smallest detail, are transmitted to
children bv means of the germ or rudiment ; they
conclude that the active causes of all the complexity
that arises must be contained in the apparently
homogeneous germ, embryological differentiation
being a spontaneous process. It follows that the
apparent homogeneity is, in reality, latent com-
plexity which becomes patent during the progress
of ontogeny. Latent complexity implies a material
substratum, consisting of actual particles for which
manv different names have been found. As our

•j

senses can give us no experimental knowledge of
these particles, which are so small as to be invisible,
modern evolutionists attempt to picture them, in
imagination, by reflecting all the visible characters
of the perfected organism upon the undivided egg-
cell, so peopling that globule of yolk with a system
of minute particles corresponding in quality and in
spacial arrangement with the larger parts of the
adult.

Weismann has practised this art in the true
spirit of a virtuoso, and has elaborated it into a novel
mode of biological investigation. Take an example ; — •
( It would be impossible,' he says in The Germplasm
(p. 138), 'for any small portion of the human skin

10 THE P. 10 LOGICAL PROBLEM OF TO-DAY

to undergo a hereditary and independent change
from the germ onwards, unless a small vital element
corresponding to this particular part of the skin
existed in the germ substance, a variation in this
element causing a corresponding variation in the
part concerned. Were this not the case, birth-
marks would not exist.'

Thus, in a slightly altered fashion, we come again
to the position of the evolutionists of last century,
for whom the germ was an extremely small
miniature of the adult creature. The new evolu-
tion, as Weismann in especial has established it,
seems to me to differ from the old doctrine only
in two important points ; and these must be placed
to the credit of the greater scientific knowledge of
our centuiy. The first point concerns the relative
positions of the parts in the patent and latent con-
ditions. The older evolutionists assumed that
these were identical, that the germ was a true
miniature. It is true that Weismann regards his
almost countless germinal particles as being held
together in an architectural structure of almost in-
conceivable complexity. For him the germ is an
exceedingly complicated living being, a microcosm
in the truest sense, in which every independently
variable part that ever appears throughout the
whole life is represented by a living particle, and
in which each of the living particles is endowed
with a definite, inherited position, a constitution, and
the power of rapid multiplication. It is upon the
qualities of these ultimate particles that he makes
depend the qualities of the corresponding parts of

INTRODUCTION 11

the adult, the parts that are cells as well as the parts
built of many cells. As, however, during visible
development the parts of the embryo undergo
many changes of position and metamorphoses,
Weismann is compelled to make the assumption
that the germ, as a micro-organism, is not simply
a miniature of the adult, but that its minute
particles have an arrangement totally different from
that of the corresponding parts in the adult

organism.

The second point is the origin of each new
generation. To explain the continuity of develop-
ment, the old evolutionists held that the genera-
tions lay enfolded one within another. Weismann
avoids this difficulty by endowing his germs with
divisibility, but he gives us no proof that division
could possibly take place in the case of structures
composed of innumerable particles built up into a
definite and most complicated architectural system.

Although the new evolution differs from the old

o

in the points mentioned above, the two theories
obviously agree very closely in the nature of their
arguments and conclusions. When, to satisfy our
craving for causality, biologists transform the visible
complexity of the adult organism into a latent com-
plexity of the germ, and try to express this by
imaginary tokens, by minute and complicated
particles cohering into a system, they are making
a phantasmal image which, indeed, apparently may
satisfy the craving for causality (to satisfy which
it was invented), but which eludes the control of
concrete thought, by dealing with a complexity

12 THE PJOLOKICAL PROBLEM OF TO-DAY

that is latent, and perhaps only imaginary. Thus,
craftily, they prepare for our craving after causality
a slumbrous pillow, in the manner of the philoso-
phers who would refer the creation of the world to
a supernatural principle.

But their pillow of sleep is dangerous for
biological research ; he who builds such castles in
the air easily mistakes his imaginary bricks, invented
to explain the complexity, for real stones. He
entangles himself in the cobwebs of his own

o

thoughts, which seem to him so logical, that finally
he trusts the labour of his mind more than Nature
herself.

' Experiment,' says Weismann in The Germ-
plasm, ' is not the only way to reach general views,
nor is it always the safest means of discrimination,
although at first it seems conclusive. . . -1 It
seems to me that in this case we can draw more
prudent conclusions from the general facts of in-
heritance than from the results of experiments that
are neither quite clear nor undubious, although in
themselves they are most valuable, and deserve
the most careful consideration. If one remembers
what was said in my section on the architecture of
the germplasm as the basis of the theory of deter-
minants, it will be agreed with me that ontogeny
must find its explanation in evolution, and not in
epigenesis.'2

I take up a more epigenetic position, and years
ago I attacked evolutionary doctrines in many of

1 The Germplasm, p. 137. 2 Ibid., p. 138.

INTRODUCTION 13

their modifications.1 Thus, in the Studien zur
Blatter Theorie, published by Richard Hertwig and
myself, I combated the supposed law that the
germinal layers histologically were primitive organs.
Next, in a pamphlet entitled The Problem of Fer-
tilisation: a Theory of Heredity, I attempted to
disprove the principle of His that there were
organ-building foci in the germ. In my treatise
Oil Ovogenesis and Spermatogenesis in the
Nematodes, I declared against the suppositions
involved in Weismann's doctrine of the germplasm,
and sharply distinguished the theory, simul-
taneously propounded by Strasburger and myself,

1 The ideas expressed in this book may be found, in an elemen-
tary condition, in various publications of my own, and written in
conjunction with my brother, Richard Hertwig : Oscar and Richard
Hertwig, Die Actinicn ; Jena, 1879 (pp. 203-217). Oscar HertAvig,
Das Problem dcr Befruchtung und dcr Isotropic dcs Eies. cine
Tlicoricdcr Vcrcrbung ; Jena, 1884. Oscar Hertwig, Vcrgleich dcr
Ei- und Samcnbildungbei Ncmafoden, Arch. f. Mikrosk. Anatomic,
vol. xxxvi., 1890, pp. 77-128. Oscar Hertwig, Una and und Spincc
bifida, Arch. f. Mikrosk. Anatomic, vol. xxxix., 1892, pp. 476-492.
Oscar Hertwig, Aeltcrc und ncuere Entiricklungsthcoricn •; Berlin,

1892. Oscar Hertwig, The Cell: Sonuenschein ; London, 1895.
Oscar Hertwig, Ueber den Wertli dcr ersten Furchungszcllen fur die
Oryartbitdtuig des Embryo, Arch. f. Mikrosk. Anatomic, vol. xlii.,

1893. The chief other writers to whom I refer are : Herbert
Spencer, Principles of Biology. Darwin, Pangcnesis, a Provisional
Hypothesis (in Variation of Plants and Animals under Domestica-
tion}. Haeckel, Die Perigcnesis der Plastidulc. Weismann, loc. ciL,
p. 8. Naegeli, Median isch - physiologische Theorie dcr Abstam-
nmngslehre ; Miinchen, 1884. Strasburger, Neue Untersuchungcn
ueber den Bcfruchtungsvorgang bei den Phanerogamenals GruncUagc

fill- cine Theorie der Ze-ugung, 1884. H. de Yries, Intracellulare
Pangenesis. W. His, Unserc Kdrpcrform and dux pliysiologische
Problem Hirer Entstehung, 1874. W. Roux, loc. cit., p. 6.
Driesch, loc. cit., p. 48.

14 THE BIOLOGICAL PROBLEM OF TO-DAY

that the nucleus is the bearer of the hereditary
material, from the evolutionistic interpretation
given it by Weismann.

A paper on ' The Blastopore and Spina Bifida/
and an occasional lecture on ' Old and New Theories
of Development/ gave me the opportunity of deal-
ing with Roux's mosaic theory, although that not
only shows learning, but apparently is the out-
come of experiment. I advocated in its place the
theory that ' the embryological development of an
organism is no mosaic work. The parts of an
organism develop in relation to each other, the
development of a part depending upon the develop-
ment of the whole.' The labours of Roux, as well
as the valuable researches of Driesch, induced
me to carry out a series of experiments with the
object of getting a surer basis for my epigenetic
conception of development. The results of these
were published recently under the title, On the
Value of the First Cleavage-cells in the Formation
of the Organs of Embryos.

In the latter treatise I confined myself advisedly
to the exposition and interpretation of the results
of my investigations, having in view a subsequent
discussion of the more theoretical bearings of my
results. It is this that sees the light in the present
book.

As for many years I have occupied myself with
the problem of development, pursuing observation
and framing theory, there is due to nryself and to
others an exposition of the position I have assumed
in many of my treatises, but in a more connected

INTRODUCTION 15

and elaborated fashion than has been possible
hitherto. This course is the more imperative, as
in his recent magnum opus on the germplasm
Weismann has propounded a theory of evolution
wrought with the greatest care and acuteness, and
totally irreconcilable with my conclusions. The
chief differences between my views and those of
Weismann have now become clearer and more
tangible than ever. It is true that in my text-
book, On the Structure and Function of Cells,1
published in the autumn of 1892, I gave a short
account of my theory of heredity in chapter ix.,
' The Cell as the Material Beginning of the
Oro-anisni.' But in that I could not deal with

o

Weismann's work, which appeared simultaneously,
and, moreover, in a text-book it was impossible to
do more than sketch my views.

My present task is twofold ; it has both a positive
and a negative side. First, I have to examine the
arguments recently alleged in favour of the theory
of preforrnation, testing them to reveal their in-
herent weaknesses, and to controvert their fallacies.
As Weismann unquestionably is the chief of those
who have advocated preformation, and has made a
closed system of it again, it is necessary for me to
take special notice of his conception as it is set
forth in The Germplasm. Although I am no friend
of polemic, the case demands it. For the decision
of a question so momentous as the relative scopes
of evolution and epigenesis in embryology must

1 An English translation, The Cell, was published by Swan
Souneuschein and Co. in 1895.

16 THE BIOLOGICAL PROBLEM OF TO-DAY

have an important bearing on the future of biology,
upon its aim and the method of research.

But criticism of Weismann's hypothesis is not to
be an end in itself; I am more anxious to show the
lines upon which, as I think, the real meaning of
the process of organic development will come to be
learned. In a second section, therefore, I shall
explain my own views in greater detail, and, as I
hope, place them on a firmer foundation than
formerly was possible.

PART I.

WEISMANN'S THEORY OF THE GERMPLASM
AND DOCTRINE OF DETERMINANTS.

As may be seen in his essays, On Life and Death,
On the Duration of Life, etc., Weismann believes
himself to have established a fundamental dis-
tinction between unicellular and multicellular
organisms. Unicellular organisms (he would have
it) do not undergo natural death, but, since they
are able to reproduce themselves continuously
by a process of simple division, are immortal.
Multicellular organisms, on the other hand, must
perish after a definite duration of life, and so are
mortal. He makes an exception of the sexual
cells, which, like unicellular organisms, are able to
multiply indefinitely, and so are immortal. Thus
Weismann came to make a distinction between the
mortal (somatic) cells and the immortal (germ) cells
of multicellular organisms. The latter he regarded
as arising directly from the egg-cell, and never
from somatic cells.

Nussbaum has given utterance to similar views,
holding that the dividing egg at a very early period
cleaves into the cells from which the individual

2

18 THE BIOLOGICAL PROBLEM OF TO-DA Y

grows and the cells for the maintenance of the

o

species. He has enunciated the proposition that,
when the sexual cells have been separated from the
cells of the young embryo, the material of the germ
has been divided into shares for the individual and
shares for the species ; that the sexual cells take
no part in the formation of the body, and that
body-cells never give rise to ova or spermatozoa.

Weismann differs from Nussbaum in one im-
portant point. He lays no stress on the direct
origin of the sexual cells, as cells, from the egg at
the beginning of its development. He found, for
instance, that, in the case of hydroids, the sexual
cells did not arise in such a fashion. He considers,
therefore, that the chain of events is as follows :
The whole of the protoplasm of an egg- cell is not
required to build up the new being, aud the super-
fluous part remains unaltered to form the sexual
cells of the new generation. Unlike Nussbaum,
then, he asserts a continuity, not for the sexual
cells, but for the germinal protoplasm which he
believes to pass along definite cell- tracks until it
forms the sexual cells. From this germinal proto-
plasm, which makes the germ-cells, he distinguishes
the somatic protoplasm which makes the mortal,
somatic cells.

The germplasm theory entered a new phase in
the year 1885, after the independent appearance in
1884 of essays by Strasburger and by me, in which
we gave reason for thinking that the cell nucleus
was, as I expressed it, the bearer of the characters
which were transmitted by parents to their off-

WEISMANN'S THEORY OF THE QERMPLASM 19

spring ; that, in fact, the nucleus was the material
basis of heredity.

Weisinann laid hold of this idea, but transmuted
it to fit in with his original theory of the germ-
plasm. Shortly put, his view is as follows : The
whole of the nuclear material is not hereditary
material, but only a definite part is such, and this
part, throughout the development of the individual,
remains unaltered in composition, and finally
becomes the starting-point for the generations to
come. The remaining and greater part of the
nuclear material does not remain in an unaltered
condition. The layers of cells, first formed in the
embryo, grow unlike each other, and give rise to
different organs and tissues ; Weismann draws the
inference that the nuclear substance as well alters
during the process of development, transforming
itself in a regular, orderly fashion, until, finally,
each different kind of cell in the whole body
contains a specific nuclearplasm. This segrega-
tion and transformation begins with the process of
cleavage itself, and thus ( the two daughter- cells
that arise from the first cleavage of the egg-cell
become different, so that the one contains all the
hereditary characters for the ectoderm, the other
for the endoderm. In further course the ectodermal
nuclear-plasm divides into that containing the
primary germs of the nervous system, and that
containing the similar constituents for the outer
skin. By further cellular and nuclear divisions the
inherited germs for the nervous system separate
into those for the sense organs, those for the central

20 THE BIOLOGICAL PROBLEM OF TO-DA Y

nervous system, and so forth, until there are sepa-
rated the germs for all the separate organs, and for
the production of the minutest histological differ-
entiation.'

Weismann calls the diverging nuclearplasms into
which the primitive germplasm is gradually trans-
formed histogenous, because they determine the
specific characters of the tissues. He assumes that
the primitive, original germplasm has a most com-
plicated molecular structure, while the histogenous
nuclearplasms for tissue-cells, like muscle-cells,
nerve-cells, sense-cells, gland-cells, and so forth,
have relatively simpler structures. As, during the
growth of the embryo, the germplasm becomes
transformed into the histogenous plasms, its mole-
cular structure becomes simpler in proportion to
the fewer different possibilities of development each
separated portion of it comes to contain.

Following out this chain of ideas, Weismann
attributes only to those cells which contain un-
altered germplasm the power of giving rise to
complete new individuals, while cells with histo-
genous nuclearplasm, whether these be embryonal
cells or cells of the ectoderm or of the endoderm,
he regards as having lost this capacity, because
nuclearplasm of a simpler molecular structure
cannot retransform itself into that with the more
complicated structure. The further conclusion is
necessary that a part of the nuclearplasm of the
original nucleus of the fertilised egg- cell must
remain unaltered throughout the various nuclear
divisions, although it may be mingled with the

WEISMANN' S THEORY OF THE GERMPLASM 21

nuclearplasms of certain series of cells. For these
reasons, ova and spermatozoa can arise only when
the germplasm which has been handed on from
the original nucleus to certain cells is able to over-
come the histogenous plasm of these cells. In this
respect Weismann has amended his original propo-
sition that the germ-cells were immortal, like
unicellular organisms. In a strict and literal inter-
pretation such a proposition would be incorrect, for
the germ-cells are immortal only so far as they
contain the germplasm, the immortal part of the

organism.

In its further elaboration Weismann's conception
was influenced considerably by publications of
Naegeli, De Tries, and Wiesner. These dealt with
the composition of the hereditary material, and
they contained new hypotheses concerning the
primary structure of the cell-body. Weismann
avowedly accepted the suggestion of De Yries, who
had rehabilitated and modernized Darwin's doctrine
of pangenesis, according to which gemmules, small
particles endowed with the power of division, were
the material bearers of hereditary characters.

From these different sources Weismann has now
worked out, in minutest detail, a theory to which
he considers his former writings but as the preface ;
none the less, he has taken from his own writings
the most essential and characteristic sequences of
idea, in a fashion but slightly modilied. Let me
give the most important parts of his conception.

The substance which is the bearer of the here-
ditary character of a species (the idioplasm of

22 THE BIOLOGICAL PROBLEM OF TO-DAY

Naegeli) lies not in the general protoplasm of the
ovum and spermatozoon, but in their nuclear
matter (hypothesis of Hertwig and Strasburger).
Weismann calls this the germ plasm, so altering
the previous connotation of the word. The germ-
plasm of every species has an extremely compli-
cated, stable architecture, an architecture that has
been elaborated gradually in the course of past
time. In this he distinguishes simple and complex
component parts, the biophores, determinants, ids,
and idants.

The biophores are his smallest material units,
and to them are due the fundamental qualities of
life — assimilation, metabolism, and reproduction by
division. Thus, they correspond to Herbert Spencer's
physiological units, Darwin's gemmules, De Yries'
pangenes, and Hertwig's idioblasts. They are the
bearers of the various characters of cells, and there
are present in the germplasm a very large multitude
of different kinds of them, corresponding to the
number of cells with different characters.

The determinants are units of the rank next
higher ; they have qualities of their own, but are
composed of groups of several kinds of biophores.
They, too, have the power of division which is
associated with, and comes about by, multiplication
of the coherent company of biophores which lies
within them.

The histological character of every cell in a
multicellular organism is determined by a single
determinant (cell-determinants). Weismann has
framed his conception of determinants so as to

WEISMANN'S THEORY OF THE GERMPLASM 23

avoid the supposition that every single cell is
represented in the germplasm by its own biophores.
There are small parts in the body in which the
cells are all alike, and for these parts a single
determinant suffices, afterwards multiplying by
division. On the other hand, each cell or cell-
group in the body, that is independently variable,
must have its special determinant in the germ-
plasm. And so the germplasm of a species must
possess as many determinants, or guiding par-
ticles, as there are in the organism cells or cell-
groups that are independently variable in the
germ or in later stages (hereditary pieces or deter-
minates).

As every cell or group of cells which corresponds
to determinants has a definite position in the body,
Weismann infers that the determinants are defi-
nitely placed in the germplasm, and form an ordered,
complicated community. He has given the name
•id to these communities, which are higher units
with definite constitution and with complicated
architecture. These ids are bodies containing all
the determinants necessary to build up the indi-
vidual of a species, and correspond to what Weis-
mann previously called ancestral plasms. Every
id must be able to grow and multiply, for it is by
their multiplication that the germplasm for new
individuals is formed.

A single id would suffice for the conduct of a
single life - history ; Weismann, however, in the
pursuit of a chain of thought connected with the
relation of sexual reproduction to heredity, and

24 THE BIOLOGICAL PROBLEM OF TO-DA Y

which I shall not discuss here, regards the germ-
plasm as being still more complicated, and consist-
ing of many, sometimes more than a hundred,
ancestral plasms or ids, which have been derived
from near or distant ancestors, the peculiarities of
whose structures they retain, and may at some time
actually produce (explanation of atavism).

But how does this fabric, endowed with an archi-
tecture so complicated, actually produce the de-
velopment of the adult from the egg ? The natural
mechanism for this purpose is cell division and
nuclear division.

According to Weismann's supposition — a sup-
position which forms, as we shall see, a chief
corner stone of his system — there are two kinds of
nuclear division, the difference between which has
not been observed, but is a corollary from the
difference between their results. The one kind is
denoted as integral, or doubling division ; the other
as differential, or differentiating division. The first
method has only an incidental importance in Weis-
mann's hypothesis : it consists of the doubling by
growth of the rudiments, and of a perfectly fair
division of them between the half-chromosomes;
it occurs in tissues-cells, where parent-cells divide
into daughter-cells exactly similar to each other
and to their parents.

On the other hand, in differentiating division the
rudiments become irregularly grouped during their
growth ; consequently, on division of the ids, which
are composed of determinants, totally different com-
binations of the determinants are included in the

WEISMANN'S THEORY OF THE GERMPLASM 25

daughter-ids. This method of division of the
germplasm plays the chief part in the trans-
formation of the egg into the adult. It has to
take place so that the numberless determinants,
or guiding particles, of the germplasm may be dis-
entangled and brought forward at the time and

o o

place necessary for them to guide the formations of
the determinates, or independently variable parts
of the adult body.

To take an example : Weismann's hypothesis
requires that when the egg first divides into two,
the germplasm should divide into two halves, each
containing only one half of the total assemblage of
determinants. In each subsequent cell - division
this process of segregation is continued, so that the
ids, as the phases of embryonic growth occur, con-
tain more and more few different kinds of deter-
minants. Supposing the germplasm to be composed
of a million determinants at one stage, in the next
it would contain only half a million, and in the
next, again, only a quarter-million. In this manner
the architecture of the ids becomes simpler and
simpler, reaching the simplest conceivable con-
dition in the active cells of the adult body. In
these the germplasm consists only of the kind of
determinants peculiar to the cells in which they
lie ; and these determinants are broken up into
biophores, or bearers of cell qualities.

' The disintegration of the germplasm,' says
Weismann,1 ' is a wonderfully complicated process ;
it is a true " development," in which the idic stages

1 TllC CrCT///X(/x//'; P1K &S, 69.

26 THE BIOLOGICAL PROBLEM OF TO-DA Y

necessarily follow one another in a regular order,
and thus the thousands and hundreds of thousands
of hereditary parts are gradually formed, each in
its right place, and each provided with the proper
determinants. The construction of the whole body,
as well as its differentiation into parts, its segmenta-
tion, and the formation of its organs, and even the
size of these organs — determined by the number of
cells composing them — depend upon this com-
plicated disintegration of the determinants in the
id of germplasm. The transmission of characters
of the most general kind — that is to say, those
which determine the structure of an animal as well
as those characterising the class, order, family, and
genus to which it belongs — are due exclusively to
this process.'

This mechanism of differentiating division fails
to explain the phenomena of reproduction and of
regeneration. For these Weisrnann has the follow-
ing ancillary suppositions :

The first is the already-described hypothesis of
continuity of the germplasm. As the disintegration
of the germplasm into determinants, occurring in the
development of an egg into an organism, is a process
which cannot be retraced, and, as the future repro-
ductive cells of the organism must contain undisinte-
grated, perfect germplasm, it follows that the germ-
plasm in the germ-cells of the child must have come
directly from the original germplasm of the parent.
During the development, as Weismann assumes, only
a few of the ids, each of which contains all the
necessary germs, break up by differentiating division

WEISMANN'S THEORY OF THE GERMPLASM 27

into the determinants which control the course of
the ontogeny, and decide the final characters of
the cells. Another set of ids remains undisin-
tegrated, with their determinants fast bound
together, and, in the cell divisions, is not broken
up into dissimilar groups. The first set of ids is
the active, disintegrating germplasm ; the second set
is a passive, latent germplasm, which may be de-
scribed as accessory germplasm (NebenJceimplasma).
The active ids are his explanation of the embryonic
events, which they direct ; the accessory germ-
plasm is reserved to form the germ-cells, and, in
fast-bound condition, is handed on through a short
or long series of cell-divisions alongside the active
germplasm. Handed on in this passive state, it
finally reaches a group of cells which may be many
or few generations distant from the original egg-
cell, and impresses upon them the character of
sexual cells. This transfer of germplasm from the
egg to the sexual cell occurs in orderly fashion,
along prescribed series of cells which Weismann
has called the germ-tracks. Only these cells, which
contain part of the perfect, undisintegrated germ-
plasm, serve for the preservation of the species and
are immortal ; the other cells, since, from the dis-
integration due to differentiating division, they
contain only fragments of the perfect plasm (groups
of determinants or single determinants) , are mortal,
somatic cells.

The formation of buds is explained in much the
same way as the origin of germ -cells. There is
handed along from the egg, through prescribed

28 THE BIOLOGICAL PROBLEM OF TO-DAY

series of cells, a quantity of accessory, or bud,
idioplasm.

The phenomena of alternation of generations
require the supposition that in those animals and
plants in which it occurs ' two kinds of germplasm
exist, both of which always are present in the egg
or in the bud, but of which one only is active at
any time and rules the ontogeny, while the other
remains inactive.' The alternating activity of these
two produces the alternation of generations. So
also dimorphism, which is exhibited most frequently
as differences between the sexes, is explained by
the assumption that ' double determinants ' are
present in the germplasm for all the cells, cell-
groups, or entire organisms which have different
characters in the male and female. One set of
these double determinants remains latent, the other
becomes active.

Finally, to explain the phenomena of regenera-
tion, it is assumed that in the complicated cases
where large parts of the body, like the head, the
tail, or a bone, can be replaced after accidental
loss, the cells with this power of regeneration con-
tain, in addition to the determinants proper to
them, supplementary determinants, which contain
the germs needed for regeneration of the lost parts.
These were handed on, during the ontogeny,
through definite series of cells, in a passive con-
dition, to become active when the conditions for
their growth are supplied by the loss of the parts
they can replace.

WEISMANN'S THEORY OF THE GERMPLASM 29

CRITICISM OF THE GERMPLASM THEORY.1

At first sight, much of Weisrnann's fabric of
hypotheses gives the impression of being a closed
system, thought out as a whole, and it has been
treated as such in most of the notices and criticisms
which I have seen. As a matter of fact, Weismann
has spared no pains in the elaboration of his
system, and has attempted to bring under his
theory the many different phenomena of heredity
and development, as well as alternation of genera-
tions, regeneration, atavism, and so forth. But, on
the other hand, he has been careless in testing the
stability and security of the foundations upon
which he has built. It is on solid foundations that
lie deep in the earth, and that avoid all reproach
of being scamped or superficial work, that the
durability of a structure depends. In this criticism
the details of the superstructure will be disregarded,
but the foundation will be tested thoroughly.

Cells and cell-properties are essential parts of
Weismann's theory ; while Naegeli has attempted
to make his theory of the idioplasm independent of
the whole conception of cells. In this matter I
agree with Weismann, as, indeed, with De Yries
and others, and I consider that the course taken by
Naegeli has made his position untenable.

Naegeli would make his theory of the idioplasm

1 The following treatises contain criticisms of Weismann's
theories: W. Haacke, Gestaltung und Vererbung ; Leipzig, 1893;
Herbert Spencer, articles in Contemporary Review (1893-94) ;
Romanes, An Examination of Wcismannism ; Longmans, 1893.

30 THE BIOLOGICAL PROBLEM OF TO-DAY

quite independent of the theory of cells, because,
while cells are important units in morphological
structure, independently of this they cannot be
regarded as important units. ' By a unit,' he
insists, ' we must understand, in a physical sense,
a system of material particles. In the organic
world there are very many kinds of higher and
lower units; vegetable and animal individuals,
organs, tissues, groups of cells (in the vegetable
kingdom, for instance, vessels and sieve- tubes),
cells, parts of cells (plant cell-membranes, plasma,
granules, and crystalloids, starch - grains, fat-
globules, and so forth), micelke, molecules, atoms.
In morphology and physiology, sometimes one kind
of unit, sometimes another, comes characteristically
and notably into evidence. That being so, there is
no reason why a special kind of unit should be
exalted in a general theory.'

Athough, with Naegeli, we must recognise and
keep in view the presence of a large number of
higher and lower units in the organic world, a fact
upon which I shall lay considerable emphasis later,
we must none the less recognise that, among all
elementary units, cells are most the conspicuous,
morphologically and physiologically, in the whole
organic realm. In actual research this is avowed
very practically, as a glance at the biological litera-
ture of the last thirty years will show. Especially
in the study of heredity, the cell is a unit that
cannot be neglected, for it has been established that
spores, ova, and spermatozoa, the units by which
species are preserved in reproduction, both in the

'S THEORY OF THE GERHPLASM 31

animal and in the vegetable kingdom, have the
morphological value of cells.

In this point I am in opposition to Naegeli,
although otherwise I agree with much in his con-
ceptions.

A theory of heredity must be reconciled with the
cell theory. In investigating Darwin's pangenesis,
Galton's doctrine of the stirp, Naegeli's idioplasm,
Weismann's germplasm, the intracellular pange-
nesis of De Vries, His' doctrinal of germinal foci
for the formation of organs, or Roux's mosaic
theory, I believe that one must face the question :
How far do these doctrines agree with what we
know about the structure and function of the cell ?
Moreover, in deciding between the alternatives —
preformation and epigenesis — I believe that it will
profit us to start our critical investigation with the
cell itself. With this object, I shall now sum up in
a few sentences as much of our present knowledge
of the life of cells as, I believe, must be reckoned
with in any theory of propagation.

The cell, which consists of protoplasm and a
nucleus, is an elementary organism, that, by itself,
or in combination with other cells, forms the basis
of all animal and vegetable organisation. In minute
structure it is so extraordinarily complicated that
its essential constitution (its micellar or molecular
structure) eludes our observation. It is a medley,
composed of numerous, chemically distinct par-
ticles that may be divided into two groups,
organised and unorganised. The latter are free,
or in solution ; they are such as albuminates, fats,

32,1 THE BIOLOGICAL PROBLEM OF TO-DAY

:" iA

ifcarbo - hydrates, water, salts, and they serve as

jnaterjal for the nutrition and growth of the cell.
The former make up the living cell body (in the
narrow sense). They are able to multiply by growth
and division, and they are therefore the elementary
parts, units of life of lower rank, of which the cell,
a unit of higher rank, is composed. They are the
gemmules of Darwin, the physiological units of
Spencer, the bioblasts of Altmann, the pangenes of
De Vries, the plasomes of Wiesner, the idioblasts of
Hertwig, and the biophores of Weismann.

The cells of every organic species possess a proper,
specific organisation, more or less complicated, and,
in correspondence with this, they are composed of
more or less numerous and varied organised
particles.

The nucleus is a special organ of cells, which is
always present. It displays a collection of numerous,
peculiar, elementary living units, the idioblasts.
These show chemical, morphological, and functional
differences from the plasomes, the living units of
the protoplasm ; but perhaps the idioblasts, by ab-
sorption of different material, may transform them-
selves into the plasomes, just as these last, by a
similar process, may produce the plasma-products.
i Jn my view, the nucleus is the bearer of the idio-
y plasm or hereditary material, that is to say, of a
substance that is more stable than ^protoplasm,
and, because it is less subject to influences of the
outer world, it stamps its specific character upon
the organism.1

1 Notwithstanding the objections raised by Bergh, Verworn,
and Haacke, I abide by the supposition that the nucleus of repro-

WEISMANN'S THEORY OF THE GERMPLASM 33

A mass of protoplasm with several nuclei (like
the myxomycetes, cceloblasts, etc.) has the morpho-
logical value of a number of cells (synergides),
corresponding to the number of the nuclei.

The means by which the continuity of life is
maintained is the capacity of the cell to manifold
itself by division, so forming two or more separate
pieces. The process, which in most cases is asso-
ciated with complicated changes of the nuclear
contents, appears essentially to consist of the
following : The elementary units of the cell (centro-
somes, chromatin bodies in the case of nuclear
division), being endowed with special energy re-
sulting from the processes of growth, divide, and
the elementary products of division separate into
two groups, which move from the middle line ;

ductive cells contains the hereditary mass or germinal material.
My reasons may be found in my text-book on TJie Cell (English
edit., p. 274). Briefly they are: 1. The equivalence of the male
and female hereditary masses. 2. The equal distribution of the
growing nuclear mass of the primary egg-cell among the daughter-
cells that, arising from it, build up the organism. 3. The preserva-
tion of a constancy of bulk of the hereditary mass when fertilization
occurs. 4. The isotropism of protoplasm. Following Pfliiger, I
mean by isotropism that the protoplasm of the egg does not contain
local areas for the formation of different organs ; but that, according
to the conditions, any part of the protoplasm may be employed in
the formation of any organ. Isotropism is merely the negation of
His' doctrine of the presence of local areas for definite organs, and
without losing its meaning, is compatible with the fact that many
eggs have their poles different, and that others have a bilateral
symmetry which determines the plane of the first division. 5. The
fact that the first stages of many embryonic developments consist
in the multiplication of the nuclear material and its distribution
in the yolk, following which the yolk-mass cleaves into cells.

3

34 THE BIOLOGICAL PROBLEM OF TO-DAY

upon this there follows a division of the general
body of the cell, i.e., of the protoplasm and its
contents.

From the point of view of cells, I believe myself
compelled to raise several objections to most im-
portant bases of Weismann's germplasm theory.
For convenience of exposition these maybe divided
into two groups : Objections to the hypothesis of
differentiating division; objections to Weismann's
doctrine of determinants.

I. OBJECTIONS TO THE HYPOTHESIS OF DIFFER-
ENTIATING DIVISION.

A corner-stone of Weismann's theory is his
assumption of nuclear divisions which are differ-
entiating. Proof of this fundamental assumption
may be sought in vain in Weismann's writings.
Instead of that, a series of abstract arguments are
brought forward in favour of it. Thus on p. 31
(of the English translation) Weismann treats the
chromatin in the nucleus of the fertilised egg as
the substance which accomplishes inheritance,
and he denotes all the nuclei of the organism
arising from the nucleus of the egg by divisions as
the chromatin - tree, and then goes on to ask
whether or no the pieces of hereditary material
that make up the chromatin-tree of an organism
are like each other or different. ' It can easily be
shown/ the answer runs, ' that the latter must be
the case.' For ' the chromatin is in a condition to
impress the specific character on the cell in the

WEISM ANN'S THEORY OF THE GERMPLASM 35

nucleus of which it is contained. As the thousands
of cells which constitute an organism possess very
different properties, the chromatin which controls
them cannot be uniform ; it must be different in
each kind of cell.'

Moreover, on p. 45 (of the English edition), ' The
fact itself ' (the capacity on the part of the idio-
plasm for regular and spontaneous change) f is
beyond doubt. When once it is established that
the morphoplasm of each cell is controlled, and its
character decided, by the idioplasm of the nucleus,
the regular changes occurring in the egg-cell, and
the products of its division in each embryogeny,
must then be referred to the corresponding changes
of the idioplasm.'

Finally, on p. 205 (of the English edition), ' The
cells of the segmenting ovum are completely dis-
similar as regards their hereditary value, although
they are all young and embryonic, and are not in-
frequently quite similar in appearance. It there-
fore seems to me to follow from this, as a logical
necessity, that the hereditary substance of the egg-
cell, which contains all the hereditary tendencies of
the species, does not transmit them in toto to the
segmentation cells, but separates them into various
combinations, and transmits them in groups to the
cells. I have taken account of these facts in con-
sidering the regular distribution of the determinants
of the germplasm, and the conversion of the latter
into the idioplasm of the cells in the different
stages of ontogeny.'

In the different propositions I have quoted, we

36 THE BIOLOGICAL PROBLEM OF TO-DA Y

have to deal with what is merely a fallacy in
rhetorical disguise. For, from the premiss that
the chromatin has the power of impressing specific
character upon the protoplasm of the cell, it by no
means follows that two cells, distinguishable by the
nature of their plasma-products, must therefore
contain different kinds of protoplasm. There are
other possibilities to be reckoned with. Weismann
himself knows that there is no logical necessity for
the conclusion, for he himself suggests another
possibility in the following : ' If we wished to
assume that the whole of the determinants of the
germplasm are supplied to all the cells of the
entogeny, we should have to suppose that differ-
entiation of the body is due to all the determinants
except one particular one remaining dormant in a
regular order, and that, apart from special adapta-
tions, only one determinant reaches the cell, viz.,
that which has to control it. If, however, we do
make the assumption,' etc. (p. 63, English edition).

Here, then, Weismann himself points out that
what in other places he has attempted to represent
as a necessary conclusion is but one of two alter-
natives.

Not only does he grant the possibility of the
alternative, but uses it himself in explanation of the
phenomena of reproduction and development. He
attributes to certain series of cells, in addition to
the active rudiments controlling the normal cha-
racters of their protoplasm, the possession of
numerous latent rudiments which become active
when opportunity presents itself.

WEISMANN'S THEORY OF THE GERMPLASM 37

This non sequitur in his argument Weismann
excuses with the remark that the presence of latent
rudiments in special cases ' depends, as I believe,
upon special adaptations, and is not primitive, at
any rate not in higher animals and plants. Why
should Nature, who always manages with economy,
indulge in the luxury of always providing all the
cells of the body with the whole of the determinants
of the germplasm, if a single kind of them is
sufficient ? Such an arrangement will presumably
have occurred only in cases where it serves definite
purposes ' (p. 63, English edition). Here, again, is a
rhetorical flourish instead of a proof.

But the dilemma which we are examining is not
yet at an end. Supposing for the moment that we
accept the assumption that different character in
cells implies different character in their nuclear
matter, we have at once a new and important
decision to make. Does the nuclear matter in the
different cells, that has arisen by division from the
nuclear matter of the egg-cell, become unlike by
the process of division itself ? or is it only after the
division that it becomes different, and in con-
sequence of the action of outer forces upon the
nuclei ?

Weismann decides boldly — but again without
bringing forward proof — in favour of the former
interpretation. ' For the chromatin,' he remarks,1
' cannot become different in the cells of the fully
formed organism ; the differences in the chromatin
controlling the cells must begin with the develop-

1 English edition, p. 32.

38 THE BIOLOGICAL PROBLEM OF TO-DAY

ment of the egg- cell and must increase as develop-
ment proceeds ; for otherwise the different products
of the division of the egg-cell could not give rise to
entirely different hereditary tendencies. This is,
however, the case/ Weismann represents to him-
self that1 * the changes .of the idioplasm depend on
purely internal causes, which lie in the physical
nature of the idioplasm. In obedience to these, a
division of the nucleus accompanies each qualitative
change in the idioplasm, in which process the
different qualities are distributed between the two
resulting halves of the chromatin rods.'

I shall proceed to show that this conception in-
volves material difficulties and contradictions. It
will be found that characters totally contradictory
are ascribed to Weismann's idioplasm. On the one
hand, it is credited with being a stable substance,
possessing a coherent, complicated architecture ; in
the form of ancestral plasms it is supposed to be
handed on, from one individual to another, un-
changed through many generations ; on the other
hand there is ascribed to it a labile architecture,
that allows a free and perpetual casting loose of
rudiments, of such a kind that at each division
there is caused a complete rearrangement and un-
equal division of these rudiments. In the one case,
the inner forces produce a reciprocal, coherent bond
between the numerous rudiments ; in the other
case, permit change of their position and relations
to one another, and this not only once but in
orderly, definite fashion, different in each of many

1 English edition, p. 34.

WEISMANN'S THEORY OF THE GERMPLASM 39

successive divisions, so that the id comes to
possess a completely altered architecture. ' Each
id in every stage ' (p. 77 of the English edition),
' has its definitely inherited architecture ; its struc-
ture is a complex, but a perfectly definite one,
which, .originating in the id of germplasm, is trans-
ferred by regular changes to the subsequent idic
stages. The structure exhibited in all these stages
exists potentially in the architecture of the id of
germplasm : to this architecture is due, not only
the regular distribution of the determinants — that
is to say the entire construction of the body from
its primary form.'

Unfortunately, Weismann's hypothesis tells us
nothing at all about these internal causes, that
depend upon the physical nature of the idioplasm ;
that is to say, nothing at all about the causes
which, working in a fashion so contradictory and
astonishing, really produce the whole develop-
ment.

In such a state of affairs it is better to turn to
Nature herself, and to see whether or no the
occurrence of differentiating division of the nucleus
in the organic world is at all supported by the
actual observations and investigations of those who
study cells.

We shall examine (1) Unicellular organisms ;
(2) Lower multicellular organisms ; (3) The phe-
nomena of generation and regeneration ; (4) altera-
tion of structural growth due to external inter-
ferences (heterornorphosis) ; (5) A nurnber of
physiological indications that cells and tissues, in

40 THE BIOLOGICAL PROBLEM OF TO-DA Y

addition to their patent characters, contain latent
characters which have reached them by doubling
division, and which are representative of the
species.

i

FIRST GROUP OF FACTS. — UNICELLULAR ORGANISMS.

Doubling division alone exists, or could exist,
among unicellular organisms. The maintenance of
the species depends upon this. Our belief that a
species produces only its own species, that like
begets only like, a belief that finds continual con-
firmation all through the study of systematic and
embryological natural history, would disappear, were
it possible that in the division of unicellular organ-
isms the hereditary mass should be split into two
unequal components and be bestowed unequally
upon the daughter-cells. All research shows that
unicellular fungi, algae, infusoria, and so forth, in
dividing, transmit specific characters so strongly and
in detail so minute that their descendants, a million
generations off, resemble them in every respect.
No one has doubted the fact, and Weismann him-
self recognises that division, among unicellular
organisms, is always doubling. The process of
division, as such, appears never to be the means by
which new species are called into existence among
unicellular organisms. This is a fundamental pro-
position of cell-life, not to be doubted, and to be
taken into account in the presentation of theories
of heredity.

From the proposition that like begets only like

WEISMANN'S THEORY OF THE GERMPLASM 41

the corollary by no means follows that mother- and
daughter-cells must appear identical from the
beginning. For the identity under consideration
belongs only to the substance that is the bearer of
specific characters, to the hereditary mass ; besides
that, a unicellular organism contains other sub-
stances, substances that change from time to time
during its life. Many unicellular organisms pass
through a regular series of developmental stages ;
the stages themselves being inherited, and follow-
ing each other as infallibly as in the case of
embryonic stages of higher animals.

The following will serve as examples of this.
Podophrya gemmipara, an Acinetan, in the adult
condition is attached by a long stalk, while the
free end, at which is the mouth, is provided with
suctorial tentacles. It reproduces by giving rise to
many little buds, ciliated on the upper surface like
free-swimming, hypotrichous infusoria. These, in
appearance, are quite unlike the parent organism,
and, after a vagrant existence in the water for some
time, they attach themselves to a surface and pro-
duce a stalk, tentacles with suctorial pseudopodia,
and so for the first time attain the maternal form.

Some Gregarines are large, jointed cells, divided
into two pieces, a protomerite and a deutomerite ;
they are clad with a cuticle, under which lies a layer
of muscular fibrils. After conjugation they encyst,
the nucleus divides, and they break up into
numerous peculiarly-shaped boat-like structures,
(pseudonavicellffi), which afterwards are set free as
small, sickle-shaped embryos. These exceedingly

42 THE BIOLOGICAL PROBLEM OF TO-DAY

small germ-cells afterwards develop into the very
different, adult gregarine- cells.

If the characters of a species be associated with
a hereditary mass, an actual substance that is
handed on from the parent-cell to the offspring, it is
clear that the infusoria-like vagrant young of the
Acinetan, and the sickle-shaped embryos of the
Gregarine possess it, although for some time they
are quite unlike the parent organism. For at last
they become an Acinetan or a Gregarine, exactly like
the parent- cell from which they arose as embryos.

These circumstances, among unicellular organ-
isms, are a weighty indication of the error of con-
cluding, with Weisniann, in the case of multicellular
forms, that because cells are unlike in outward
appearance, the hereditary mass, or, as I call it,
the nuclear matter, within them is also unlike.
Such an assumption would involve us in the
greatest contradictions. For the supposition that
the nucleus is the hereditary mass transmitting
the characters of the species necessitates the con-
clusion, in the case of unicellular forms, that the
hereditary mass remains in possession of all the
rudiments of the cell while it passes through the
various phases of its cycle of development. Other-
wise, these phases would have to be acquired anew
in each case. We must, therefore, represent the
possibilities of exchange between the nucleus, in its
capacity of bearer of the hereditary mass, and the
protoplasm as being such that all the rudiments
are not simultaneously in activity, but that some of
them can remain latent for a time.

THEORY OF THE GE&MPLASM 43

SECOND GROUP OF FACTS. — THE LOWER MULTI-
CELLULAR ORGANISMS.

Although in the development of unicellular
organisms the way by which like begets like is plain
and intelligible enough, at least in the cases dealt
with, it is different with multicellular organisms,
which have reached a higher grade of development.
Among them we have to do with a continuous
process of development, in which the highly-
differentiated, multicellular organism arises from
an egg, and in turn gives rise to an egg, and so on in
unending sequence. But the succeeding stages of
the sequence are so exceedingly dissimilar in appear-
ance that the question how one step of the series
turns into the next, and, above all, the question how
the similarity of organisms, separated by the egg-
stage, can be transmitted through the egg-stage,
form the deepest riddle offered to biological investi-
gation. Here, in a completeness so wonderful that
our intelligence can hardly apprehend it, are pre-
sented to us the qualities of the organic material of
which cells are made. Here lies that dark secret
into which the various theories of generation try
to direct a beam of light, and seek to find out the
direction in which explanation may be found.

An intermediate stage which may serve towards
the explanation of these circumstances is presented
by the lower multicellular organisms, such as
threadlike algse, fungi, and other simple creatures.
In them cells arise by division from the egg or
from the spore, and become united into an in-

44 THE BIOLOGICAL PROBLEM OF TO-DAY

dividual of a higher rank ; these cells resemble one
another so completely in appearance and in qualities
that there can be as little doubt as in the case of
unicellular organisms that they arose by doubling
division.

It is certain, then, that there exist multicellular
bodies, often consisting of many thousand cells, in
which each part retains the qualities of the egg
from which it arose by doubling division, and
which, as that method implies, possess the rudi-
ments of the whole of which each is a part.

In this category there naturally fall the multi-
nucleated masses of protoplasm, sometimes highly
organised, in which every nucleus, surrounded by
a shell of protoplasm, is capable of reproducing the
whole. I am thinking of the slime-fungi (Myxo-
mycetes), with their peculiar formation of repro-
ductive bodies ; of the ' acellular plants,' which in
some cases closely resemble multicellular species in
their formation of leaf and root, and in their mode
of growth, as, for instance, Caulerpa, the multi-
nucleated Foraminifera and Radiolarians. For, ac-
cording to our definition of the cell, a multinucleated
organism potentially is a multicellular organism.

In this matter Weismann has assumed a position
which leads to peculiar consequences. In his
opinion, somatic cells and germ-cells were sharply
distinct at their first appearance in evolution, and
have remained so ever since. Transitional forms
between them are nowhere to be found. It would
be inconsistent with his theory of the germplasm
had somatic cells contained germplasm as their

WEISM ANN'S THEORY OF THE GERHPLASM 45

idioplasm, even when the soma first came into
existence. The phyletic origin of the somatic cells
depended directly upon an unequal separation of
the determinants contained in the germplasm. It
would totally contradict his presentation if the
somatic cells, even at their first origin in phylogeny,
contained, in addition to their patent special
qualities, the qualities common to the whole species
in a latent condition.

Weismann's conception, therefore, implies that
many of the lower multicellular organisms, having
no somatic-cells, have no body. Take the closely-
allied creatures Pandorina morum and Volvox
globator, which Weismann himself brings forward
as instances for his view ; the latter has a body,
the former has no body, as all its cells are able to
serve for reproduction !

It is enough to have pointed out how contra-
dictory are the interpretations in this matter.
Enlarging upon them may be postponed at present,
for we are concerned here not with the interpreta-
tion of individual cases, but with the principles
involved in the question, and, therefore, we must
pass on to show further reason for considering the
existence of differentiating division highly im-
probable in the whole organic world.

THIRD GROUP OF FACTS. - - THE PHENOMENA OF
REPRODUCTION AND REGENERATION IN PLANTS
AND ANIMALS.

The numerous phenomena of reproduction and
regeneration appear to support the principle of

46 THE BIOLOGICAL PROBLEM OF TO-DAY

doubling division — that is, of division in which the
germinal substance is handed on to every part of
the organism. Our review may be short, as the
phenomena are matters of common knowledge.

In nearly all plants there exist, widely spread
through the body, cells and cell-groups, which may
be induced, by inner or outer influences, to give
rise to a bud ; the bud grows out into a shoot,
ultimately producing flowers and genital products.
Such happens both in parts of the plant above the
ground and below it ; in the latter case shoots
arise from roots, and reproduce the species in the
ordinary sexual fashion by bearing sexual products.

Thus, in the case of Funaria hygrometrica, a
little moss, one may chop up the plant into tiny
fragments, scatter these on damp earth, and see
numerous moss-plants reproduced from the little
groups of cells. By cutting little pieces from a
willow, an experimenter may cause the production
from slips of thousands of willow-trees, each with
all the characters of the species, so that there must
have been contained in each of the little pieces of
tissue hereditary masses with the characters of the
whole plant. Separate pieces of the leaves of many
plants, as of the begonia, produce buds from which
the whole plant may grow out.

An aptitude for reproduction like that in plants
exists in many ccelenterates, worms, and tunicates.
The polyps of hydroids and of bryozoa, the stolons
of an ascidian (Glavellina lepadiformis), may give
rise to buds in many places, and these grow up
into the perfect hydroid, bryozoon, or ascidian.

WEISMANN'S THEORY OF THE GERMPLASM 47

There must, then, be contained in the cells of the
bud the germinal rudiments of the whole animal ;
this conclusion is more necessary as the individuals,
produced from the buds, in due course bear sexual
products.

Although in many higher animals and plants
one sees that cells with the capacity for repro-
duction are limited to special areas, still, the
capacity for regeneration often is very great. In a
wonderful fashion animals will reproduce lost parts,
sometimes of most complicated structure ; just as
a crystal, from which a corner has been chipped,
will perfect itself again when brought into a solu-
tion of its own salt. A Hydra, from which the
oral disc and tentacles have been cut off, a Nais
deprived of its head or of its tail, a snail of which
a tentacle with its terminal eye has been amputated,
will reproduce the lost parts, sometimes in a very
short time. The cells lying at the wounded spot
begin to bud, producing a layer or lump, the cells
of which resemble embryonic cells. From this
embryonic mass of cells the lost organs and tissues
arise — in Hydra, the oral disc with its tentacles ;
in Nais, the anterior end with its sense-organs and
special groups of muscles ; in the snail, the tentacle
with its compound eye built up of elements so
different as retinal-rods, pigment-cells, nerve-cells,
lens, and so forth.

Even among vertebrates, in which the capacity
for regeneration is the least, as in the restoration of
the wounded parts small defects occur, lizards
can reproduce a lost tail, tritons an amputated

48 THE BIOLOGICAL PROBLEM OF TO-DAY

limb. From a bud of embryonic tissue there are
elaborated in the one case whole vertebrae, with their
muscles and tendons, and part of the spinal cord
with its ganglia and nerves, in the other case, the
numerous, differently-shaped, skeletal pieces of the
hand or foot, with their appropriate muscles and
nerves. The regeneration, moreover, is in strict
conformity with the characters of the species con-
cerned. Thus, from the facts of regeneration also,
we must infer that cells in the vicinity of these
casual wounds possess not only the special qualities
which they possess as definite parts of a definite
whole, but also the characters of the whole, and
thus have the power of becoming buds, from which
a complicated part of the body may be reproduced
with the appropriate characters of the species.

FOURTH GROUP OF FACTS. — THE PHENOMENA OF

HETEROMORPHOSIS.1

Of all the facts brought forward here, the
phenomena of heteromorphosis perhaps bear most
strongly in favour of my conception, and offer

1 In tliis section upon heteromorphosis I rely upon the follow-
ing treatises, which have appeared recently. Loeb, Untersuchungen
zur pliysiologischen Morphologic der Thiere. Organ-bildung und
Wachsthum. Heft, 1 and 2 (1891-1892). H. de Vries, Intra-
ccttulare Pangenesis (1889). H. Driesch, Entiviclclungsmechanische
Studien, i. -vi. ; Zeitschrift f. ivissenschaft, Zool., vol. liii. -Iv. The
same, Zur Theorie der thierischen FormUldung. Biol. Central-
blatt, vol. xiii., 1893. Chabry, Contribution a I' embryologie
normale et Urcdologiqiie des Ascidies simples. Jour, de VAnat. et
de Physiol. (1887). Wilson, Amphioxus and the Mosaic Theory.
Journal of Morpli. (1893). See also Anatomischer Anzeigcr
(1892).

WEISMANN'S THEORY OF THE GERMPLASM 49

difficulties most irreconcilable with Weismami's
theory.

Loeb uses the word ' heteromorphosis ' to denote
the ability possessed by organisms, under the
stimulus of external forces, to produce organs on
parts of the organism where such do not occur
normally, or the power to replace lost parts by
parts unsimilar to them in form and function-
Kegeneration is the reproduction of parts like those
lost ; heteromorphosis is the reproduction of parts
unlike those lost.

Heteromorphoses are well known in plant physi-
ology. When one cuts a slip from a willow, one
may make the cut at the bottom of the slip and
the cut at the top in any part of the willow- twig,
yet still the lower end of the slip always produces
rootlets, which are organs not normal to that part
of the twig, while shoots will rise from the upper
end. Moreover, either end of the slip may be
made the root portion, and it is clear, therefore,
that in every small area there are cell-groups
present able to bear roots or shoots according to
the determining conditions ; and therefore that, in
addition to the characters active at any time, there
are present the germinal rudiments for shoots and
roots, and, indeed, for the whole organism, since
the shoots ultimately may bear genital products.

When the prothallus of a fern has developed
normally, it is a flattened leaf-like structure which
bears rootlets and male and female genital organs
on the lower surface, i.e., on that turned from
the light. But the experimenter may reverse this

4

50 THE BIOLOGICAL PROBLEM OF TO-DAY

order, by artificially shading the upper surface, and
strongly illuminating the lower surface.

Among the most interesting heteroraorphoses are
the galls, produced upon young plants when certain
insects lay eggs on them, or when plant-lice irritate
their tissues. From these abnormal stimuli there
result active masses of cells which grow into organs
of definite form and of complex structure. The
galls, moreover, differ widely, in correspondence
with the specific stimulus which was their initial
cause, and with the specific substance, the stimula-
tion of which resulted in the formation of a gall.
By the action of different insects upon the same
plant different galls are produced, and the galls of
different plants may be distinguished systemati-
cally.

Blumenbach has already brought forward the
existence of galls as an argument against preforma-
tion, holding them to be structures produced epi-
genetically, and, therefore, unrepresented by rudi-
ments in the germ. I, also, consider them witnesses
against Weismann's gerrnplasm. They teach us
that the cells of the plant- body may serve purposes
quite different from those arranged for in the course
of development ; that cells modify their form in
correspondence with novel conditions, and that
they are forced into forming special structures, not
by special determinants in the germ, but by external
stimulants.

Galls exhibit yet another instructive kind of
heteromorphosis.

Even the tissue of a leaf, turned into a gall by

WEISMANN'S THEORY OF THE GERMPLASM 51

pathological conditions, retains the power of pro-
ducing roots. Beyerinck has shown that galls of
Salix purpurea, planted in moist earth, bear
rootlets identical with those of the normal plant.
As the roots of all woody plants are able to bear
adventitious buds, De Vries thinks it probable that
one could rear a whole willow-tree from a gall.
That would imply that all the inheritable characters
of the willow were contained even in the gall.

Loeb has produced heteromorphoses experi-
mentally upon many lower animals, among which
were Tubularla, Cerianthus, and Clone intesti-
nalis.

In Tubularia mesembryanthemum, a hydroid
polyp, there are stalk, root, and polyp-head. If
one cut off the head, a new head will be formed in
a few days, this being a case of regeneration. On the
other hand, a heteromorphosis may be produced by
modifying the experiment as follows : Both root
and head must be cut from the stem ; if the lopped
piece of the stem be stuck in the sand of the
aquarium by the end that bore the head, then the
original aboral pole in a few days produces a head ;
if the lopped piece of stem be supported hori-
zontally in the water, then each end of it produces
a head.

In a Cerianthus membranaceus (Fig. 1), the
body was opened by a cut some distance below the
mouth, whereupon buds appeared on the lower
edge of the slit, where the experimenter had pre-
vented coalescent growth. These buds gave rise
to inner and outer tentacles, and an oral disc was

52

THE BIOLOGICAL PROBLEM OF TO-DA Y

produced. Thus, artificially, an animal with two
mouth-openings or two heads was produced; and,

FIG. 1. — CERIANTHUS MEMBEANACEUS, in which a second oral
aperture, surrounded by tentacles, has appeared as the result of
an artificial slit. (After Locb.)

FIG. 2. — CIOXE INTESTINAL:*, in which eye-specks resembling
those surrounding the mouth have appeared in the neighbour-
hood of an artificial opening («)•

/

similarly, animals with a row of three or more
heads may be produced.

WEISMANN'S THEORY OF THE GERMPLASM 53

The third animal in which heteromorphosis was
produced artificially was Clone intestinalis, a
solitary ascidian, an animal more highly organized.
In done (Fig. 2) the edges of the month-opening
and of the cloaca are provided with numerous,
simple eye-spots. Loeb, in a series of experiments,
made incisions either into the inhalent or the ex-
halent tube ; after a time eye-spots appeared round
the edges of the cut ; then the margin of the arti-
ficial oral opening grew out into a tube, even longer
than the normal oral tube. 'If several incisions
be made simultaneously at different places on the
same animal, then several new tubes arise simul-
taneously.'

In the three cases, the cut surfaces, from which
in Tubularia, a head, in Cerianthus, tentacles, and
in done, eyespots, took their origin, were made in
different parts of the bodies and in different direc-
tions. Thus, again, we have an indication that
there are present in most regions of the body cell-
groups, which may give rise to complex organs
in unnatural positions, and yet bearing the specific
stamp.

These examples might easily be multiplied, and
they serve to show that heteromorphosis in plants
and animals implies the presence of numerous
latent characters in cells and tissues, in addition to
the characters proper to their normal position in
the organism. These latent characters, under the
impulse of stimulation from without, manifest
themselves in abnormal formation of organs in
abnormal situations. Save that they are in ab-

54 THE BIOLOGICAL PROBLEM OF TO-DAY

normal situation, the induced organs conform to
the specific type in all respects, and indicate that
all the cells of an organism contain, as the result
of doubling division, the characters of germinal
rudiments of the whole organism. On the other
hand, heteromorphoses bear heavily against the
doctrine of determinants. For it is impossible that,
in the architecture of the germplasm, there can be
provision, in the form of special determinants, for
events so foreign to the natural course of develop-
ment as these arbitrary, outer stimulants.

Heteromorphosis may be extended to include
more than Loeb intended by reckoning under it
artificially-produced modification of the early stages
in the cleavage of the egg. I have in mind those
experiments by Driesch, Wilson, and myself, in
which the first cells of the embryonic history were
induced to form parts of the embryo, to which in
the normal course they would not have given rise.
In these cases heteromorphosis begins from the
first cleavage of the egg.

In an ingenious way Driesch compressed fer-
tilised echinoderm eggs between glass plates, and
so secured that the first sixteen cells were separated,
not by alternate vertical and horizontal planes, as
in the normal development, but only by vertical
planes. In the resulting one-layered plate of cells
the nuclei had relative positions quite different
from the normal. As, notwithstanding this, the
distorted eggs developed into normal plutei larvae,
Driesch inferred that the cell material composing
the earliest cells of echinoids is equivalent in all

WEISMANN' 8 THEORY OF THE GERMPLASM 55

the cells, and that the cells may be pushed over one
another like a heap of balls without disturbing in
the slightest their capacity to develop. Such a
permutation could be without injury to the develop-
mental product only if one nucleus had the same
qualities as another ; that is to say, only if all the
nuclei had arisen from the nucleus of the fertilized
egg by doubling division.

Driesch is right to regard these experiments as
incompatible with Weismann's theory. ' Only
consider,' he remarks, ' how great a number of
" supplemental hypotheses," how many " accessory
determinants," would be required to make speci-
fication of the early stages of a development in
which any nucleus may take the place of any other
nucleus in the whole embryo/

I myself have carried out similar experiments
upon frogs' eggs — experiments with a double in-
terest. The frog's egg has the poles different, and
so has a definite orientation. Weismann and Eoux
themselves have used these objects to support their
view that, at the first cleavage, nuclei with different
qualities are formed.

On p. 64 of the English edition Weismann
remarks : ' The fact that the right and left halves
of the body can vary independently in bilaterally
symmetrical animals points to the conclusion that
all the determinants are present in pairs in the
germplasm. As, moreover, in many of these
animals — e.g., in the frog — the division of the
ovum into the two first embryonic cells indicates a
separation of the body into right and left halves,

56 THE BIOLOGICAL PROBLEM OF TO-DA Y

it follows that the id of germplasm itself pos-
sesses a bilateral structure, and that it also divides
so as to ofive rise to the determinants of the risrht

o o

and left halves of the body. This illustration may
be taken as a further proof of our view of the
constant architecture of the germplasm.'

Roux1 has based his mosaic theory upon ex-
periments upon frogs' eggs. According to the
theory, the first two segmentation spheres contain
not only all the formative material for the right
and left halves of the embryo respectively, but also
the differentiating and elaborating forces for these,
so that on the destruction of one cell, the other
can give rise only to one lateral half of the embryo
(hemiembryo lateralis). Roux, therefore, considers
that by the first cleavage the nuclear material is

1 Roux tried to give experimental evidence in favour of his
mosaic theory in a treatise On the Artificial Productions of Half-
Embryos "by the Destruction of one of the first two Cleavage-Cells,
and on the Reconstruction of the Lost Parts. Vircliows Archiv.,
vol. cxiv., 1888. Roux defends his mosaic theory against Driesch
and myself in (1) Ueber das entwicklungsmechanische Vermogen
jeder der beiden ersten Furchungszellen des Eies. Verhandl. der
Anat. Grcsdlsch. der 6tol Vcrsamml. in Wien, 1892. (2) Ueber
Mosaikarbeit und neuere Entwicklungshypothesen. Anatomische
Hefte von Merkel und Bonnet (1893). Also in Biol. CcntralUatt
(1893) ; in the Anatom. Anzeiger (1893), and in the treatise Die
Methoden zur Erzeugung halbcr Froschcmlriionen und sum Nachweis
der Beziehung der erstcnFurchujirjschenen des Froscheies zur Mcd'nt ,><•-
bene des Embryo. Anatom. Anzeiger. (1894) ; Nos. 8 and 9.

If, as would appear from the last treatise, Roux would avoid
being reckoned with evolutionists, he must abandon his mosaic
theory, and this he has not done. I think in the present essay,
on theoretical and experimental grounds I have vshown the un-
tenability of Roux's mosaic theory.

WEISMANN'S THEORY OF THE GERMPLA8M 57

broken up into unlike halves, by which the
development of the corresponding cells is directed

FIG. 3. — DIAGRAMS OF THE EGGS OF FROGS, which show how
alteration of the cleavage process changes the mode in which the
nuclear material is distributed. The nuclei indicated by the
same numbers have the same descent in all the diagrams. All
the eggs are viewed from the animal pole. A. Normally develop-
ing eggs. B. Eggs developing under compression by horizontal
plates. C. Eggs developing under compression by vertical plates.

diversely, i.e., is determined in a specific fashion.
The error in these representations of Weismann

58 THE BIOLOGICAL PROBLEM OF TO-DA Y

and of Roux has been shown by varied ex-
periments of my own. The eggs of frogs on the
point of cleaving were flattened to a disc between
vertically or horizontally placed glass-plates. In
the first case they were flattened in the dorso-
ventral direction, i.e., the axis passing through the
animal and vegetative pole was shortened ; in the
second case an axis at right angles to this was
shortened. In both cases the course of cleavage,
and the resulting distribution of the nuclei in the
yolk, was artificially modified.

The diagrams A, B, C (Fig. 3) will make the
results plain to the reader. A, represents the dis-
tribution of the nuclei after normal cleavage ;
B, the same, when the egg was pressed between
horizontally- arranged parallel glass-plates ; C, the
same, where the flattening was produced by
vertically-placed parallel glass-plates.1

The diagrams show the positions of the seg-
mentation spheres and of the contained nuclei as
seen from the animal pole. In stages where two
layers of cells as a result of division lay one above
the other, the cells of the lower layer are distin-
guished in the figure by shading. In the three
diagrams the nuclei are numbered so that the
reader may know how far they are removed from
the nuclei of the first two segmentation spheres.
The numbers are further exhibited in the following
two genealogical trees :

1 The terms vertical and horizontal refer to the vertical axis of
the egg, which passes through the animal and vegetative poles. -
Translator s note.

WEISMANN'S THEORY OF THE GERMPLASM 59

7 8 9 10
15 16 17 18 19 20 21 22

6

11 12 13 14

23 24 25 26 27 28 29 30

In the three diagrams the nuclei with the same
numbers have the same rank in descent, and
therefore, according to the theory of Roux and
Weismann, have the same qualities, while the
nuclei with unlike numbers differ in qualities.

Let us now notice how the nuclei in the three
processes of division, of which two are abnormal,
are placed in the mass of the egg.

After the first division, the nuclei are alike in
all three cases ; after the second difference appears.
In Al and Bl nuclei 3 and 5 lie to the left ; 4 and
6 to the right of the second cleavage-plane, which,
according to Roux's hypothesis, corresponds to the
median-plane of the future embryo ; while in C
they are forced into two layers, one above the
other, nuclei 4 and 6 being dorsal, 3 and 5 ventral.

In the third cycle of division there is no agree-
ment between the three cases.

In the diagrams A2 and B2 the nuclei still lie
similarly to the right and left of the middle line ;
but in A2 they are arranged in two layers, in B3
in a single layer. The nuclei 8, 10, 12, and 14,
which compose the upper layer in A2, form the
middle of the disc in B2 ; and 7 and 9, 11 and 13,
the ventral nuclei of A2, occupy the ends of the
single-layered disc of B2, being closely pressed
against each other.

60 THE BIOLOGICAL PROBLEM OF TO-DAY

In the diagram C2 there is actually no median-
plane after the third cycle of division. The nuclei
9, 10, 14, 13, which in A and B form the right side
of the mass, here form a dorsal layer with nuclei
7, 8, 12, 11, forming a ventral layer. In the fourth
cycle of division the nuclear matter is still more
variously distributed through the mass, as may be
seen from comparison of diagrams A3, B3, C3.

Although, under normal conditions, the multi-
plication and division of the nuclear material occurs
in an almost invariable and definite fashion, the
mere altering of the spherical form to a cylinder or
to a disc produces a method of division completely
different, so far as the nuclei are related to each
other in a genealogical tree. In the one and the
other method of division the nuclei are brought
into relation with different regions of the proto-
plasmic mass, and are united with these regions to
form cellular individuals.

I had quite enough reason for what I said in my
essay : ' If the doctrine of Roux and Weismann be
true, and the successive divisions by which nuclei
arise really place different qualities in the nuclei —
qualities according to which the masses of proto-
plasm surrounding them become different and
definite parts of the embryo — what a pretty set of
malformations must result from eggs in which the
nuclear matter has been shuffled about so wantonly!
As such malformations do not occur, it is plain
that the doctrine is untenable.'

We reach the same conclusion from consideration
of the interesting experiments made by Driesch

WEISMANN'S THEORY OF THE GERMPLASM 61

and Wilson upon the early stages of segmentation
of the egg. In the cases of an echinoid and of
amphioxus (Fig. 4) they succeeded in shaking apart
the first two and the first four cells that arose in
division of the egg ; and they traced the subse-
quent development of these separated segmentation
spheres.

From one of the first two segmentation spheres
of an echinoid egg, Driesch was able to rear suc-
cessive embryonic stages (Gastrula and Pluteus),
which were normal in shape, but one-half the usual
size. Wilson's results, obtained by shaking apart

4. — NOKMAL AND FRACTIONAL GASTPOJL.E AMPHIOXUS.

(After Wilson.}

A Gastrula from a whole egg ; B, c and D, gastruLe from single
cells artificially separated, (B) from the two-celled stage, (c) from
the four-celled, and (D) from the eight-celled stages of normal
development.

the segmentation spheres, were even more interest-
ing, as they were performed upon amphioxus, a more
highly-organized animal. He reared gastrulss and
older embryos with notochord and nerve - tube,
which were perfect and normal, except in size.
They were one-half, one-quarter, or one-eighth of
the usual size, according as they were reared from

62 THE BIOLOGICAL PROBLEM OF TO-DAY

cells isolated from the two, four, or eight-celled
stage of the segmenting egg.

Results which Chabry and I gained by destroy-
ing, by puncture, one of the first two segmentation
spheres, assist the present argument. Although
one-half of the mass had been destroyed, Chabry
obtained, in the case of an ascidian, and I obtained,
in the common frog, embryos with notochord and
nerve-plate. These developed directly and normally,
although, in the case of the frog, there was a slight
defect at the ventral posterior part of the body,
where the arrested protoplasmic mass came to lie.

All these experiments show that the first two
(and in some cases the first four) results of division
can assume a quite different bearing as regards
their function in the mechanical building of the
embryo, according to whether they remain bound
with each other into a whole or are separated and
develop by themselves. In the former case, each
forms only one-half (in some cases only a fourth) of
the whole. In the latter case, each by itself pro-
duces the whole. The half and the whole, then, of
the first cleavage-cells are identical in real nature,
and, according to the circumstances, can develop,
now in this way, now in that.

Even if Weismann were to admit the correctness
of these experiments, perhaps he would not con-
sider that they contradicted his theory of the germ-
plasm and the segregation of the hereditary mass,
but would make a supplemental hypothesis, which,
from the spirit of his theory, could be none other
than this : each of the first cleavage - cells, in

WEISMANN'S THEORY OF THE GERMPLASM 63

addition to its specific part of the hereditary mass,
the part that controls its normal course of develop-
ment, possesses an accessory idioplasm, an undivided
fragment of the germplasm, left behind to be ready
for unforeseen emergencies ; this part takes com-
mand when, in consequence of violence, a separated
part develops into the whole.

But such an assumption does not go far enough,
if it be confined to the first cleavage-cells. By
compression of the frog's egg, I have shown that
the pole passing through the blastopore, which
coincides with the chief axis of the future embryo,
may assume different relations to the first segmenta-
tion-plane, sometimes coinciding with that, some-
times making a right or an acute angle with it. It
is clear that in each of these cases the embryonal-
cells take a different share in the formation of the
regions of the body, and that they must be fore-
endowed with the capacity of playing different
parts.

The developmental history of double monsters
enforces the same doctrine ; such are common
among the embryos of fish, and rather less common
among chicks. From causes of which we are
ignorant two, instead of one, gastrula stages may
arise at separate regions of the germinal layer of
the egg. According to the position of these two
invaginations, which may be regarded as crystal-
lisation-points for the formation of the future
embryo, the cells of the germinal disc will be drawn
into the process of development, and, falling into
groups, will build up organs. In relation to this

64 THE BIOLOGICAL PROBLEM OF TO-DA Y

double gastrulation, there may arise, for instance,
four instead of two primitive ears, eyes, and nasal
organs ; and these arise from cell - groups, the
choice of which is determined by their relation to
the position of the gastrula-invagination.

From various other experiments, conducted so
as to distort the normal course of development, I
have obtained parallel results.

Taking frogs' eggs immediately after fertilisation,
I compressed them strongly between parallel,
horizontally placed glass plates. I then inverted
them, so that the vegetative pole came to lie
uppermost. In spite of their unnatural relation to
gravity, they developed further, and became ab-
normal, quite unsymmetrical embryos.

In another experiment, taking a triton's eggs
after they had divided into two spheres, I sur-
rounded them with a silk thread in the plane of
the first cleavage, and tightened the thread until
the embryo assumed the form of a sand-glass. The
deformity of the resulting larvse was very different,
and perhaps depended on the tightness of the con-
striction. Some became greatly elongated, and
had developed so that the thread surrounded the
dorsal nerve- cord. In other cases the dorsally-
placed organs arose only from one-half of the sand-
glass-shaped embryo, while the other half gave rise
to the ventral part of the body. In this case the
dorsal organs (nerve -tube and notochord) were
doubled over like a snare, the head and tail ends,
the mouth and the region of the anus, being bent
in at the position of the constricting thread.

WEISMANN'S THEORY OF THE GERMPLASM 65

*

The important point is that in both the experi-
ments, in the case of the frog and of the triton, the
cell-material, separated at the first cleavage, was
turned to a use quite different to its use in the
formation of a normal embryo.

We may conclude with a very convincing proof.
In the above-mentioned abnormal development of
the frog's egg it happened that one edge of the
blastopore, on account of its weight, was very much
bent outwards. In consequence of this the cleft of
the blastopore lay between the normal blastopore-
lip and the everted border of the other lip. When
the notochord and the nerve-plate appeared, as a
result of this abnormal condition, they grew from a
cell-material that was quite different to that which
gives them origin in normal cases.1

In these cases Weismann cannot apply his
accessory conception, the existence of supple-
mentary idioplasm, only to the nuclei arising from
the first division ; he must extend it to the thou-
sands of embryonic cells that arise by division up to
the time for the appearance of the nerve-tube and
notochord. The behaviour of these cells under
fortuitously changed conditions shows them all to
be endowed with the capacity of development in
different directions.

1 Further details concerning these experiments may be found in
HERTWIG, Ueber den Werth der ersten Furclmngszellen fur die
Organbildung des Embryo. Experimentelle Studien am Frosch-
und Tritonei. Archiv. fur Mikrosk, Anatomic, vol. xlii., 1893,
p. 710 ; Plate xli. ; Figs. 1, 2, 27.

66 THE BIOLOGICAL PROBLEM OF TO-DA Y

FIFTH GROUP OF FACTS. — PHENOMENA OF
VEGETATIVE AFFINITY.1

Many considerations, taken from the region of
general physiology, support the view that all the
cells of an individual, of any species, are alike, and
are to be distinguished from one another only by
the special development of one character.

1 For the facts in this section I rely in particular upon the
writings of Vochting, Bert, Oilier, Trembley, Landois, Ponfick, and
others :

H. VOCHTING : Ueber Transplantation auf PflanzenJcorper. Un-
tersuchungen zur Physioloyie und Pathologic ; Tubingen, 1892.

VON GARTNER : Versuche und Beobachtungen ueber die Bastardcr-
zeugung im Pflanzenreich, 1849.

LEOPOLD OLLIER : Recherches experimentales sur la production
artifieielle des os au moyen de la transplantation du 2^ioste, etc.
Journal de la physiologic de I'homme et des animaux, torn, ii.,
1859, pp. 1, 169, 468.

LEOPOLD OLLIER : Recherches experimentales sur les greffes
osseuses. The same, torn, iii., p. 88, 1860.

PAUL BERT : Recherches experimentales pour servir a Vhistoire
de la vitalitc propre des tissus animaux. Annalcs des Sciences
naturclles, Ser. V., Zoologie, torn, v., 1886.

VON RECKLINGHAUSEN : Die Wiedererzeugung (Regeneration}
und die Ueberpflanzung (Transplantation}. Handbuch d. All-gem.
Pathologic des Kreislaufs aus Deutsche Chirurgie, 1883.

TREMBLEY : Memoires pour servir d Vhistoirc d'un genre de
Polypes d'eau douce, 1744.

LANDOIS: Die Transfusion des Blutes ; Leipzig, 1875.

ADOLF SCHMITT : Ueber Osteoplastik in klinischer und experi-
menteller Bcziehung. Arbeiten aus der chirurgischenklinik der
Konigl. Universitdt, Berlin.

PONFICK : Experimcntcllc Beitrage zur Lehre von der Transfusion.
Virchow's Archiv., vol. Ixii.

BERESOWSKY : Ueber die histologischen Vorgdnge bei der Trans-
plantation von Hautstilcken auf TJiiere einer andercn Species.
Ziegler's Beitrage zur pathologischen Anatomie und zur allgemeincn
Pathologic; Jena, 1893.

WEISMANN'S THEORY OF THE GERMPLASM 67

Formerly, indeed, many biologists, relying upon
the optical appearances presented in microscopical
investigation, have been inclined to the view that
the visible qualities of a tissue, as revealed by the
microscope, were the only, or the chief, distinctive
characters. For instance, by microscopical investi-
gation one cannot distinguish the tendons, nerves,
bones, and cartilages of a dog from the corre-
sponding tissues in a horse. So far as their special
use in the organism goes, one might interchange
the corresponding parts in these two mammals. A
tendon from the dog, if large enough, might be
attached to the muscle of a horse, and would
transmit the pull of the muscle on the bone just as
well, and would completely satisfy the mechanical
duties of the horse's tendon. The same might
happen in the case of a bone, of a cartilage, or of a
nerve-fibre.

As a matter of fact, the idea that parts of the
tissues of different animals may serve to replace
one another has been employed repeatedly in
science, especially in the science of medicine. But
I believe that our ideas are not yet clear upon the
matter. The erroneous impression to which I have
alluded has arisen because we do not bear in mind
that each tissue, each part of an organ, each cell,
possesses, in addition to its obvious characters, very
many characters that are invisible to us. Such
characters are inherent in the tissue-cells because
these are parts of a definite organism. In conse-
quence of their specific tissue characters, which are
visible to us, we assign cells their place in histo-

68 THE BIOLOGICAL PROBLEM OF TO-DA Y

logical classification ; in contrast, we may denote
the other characters as constitutional, or species,
characters.

No doubt tissue cells are in the same case as
genital cells. So far as microscopical characters go,
egg cells and spermatozoa are wonderfully alike in
all the mammalia ; in many cases we could not dis-
tinguish between those of different animals. But,
because they bear the specific characters, we cannot
doubt but that they are as distinct as are the
species, although invisibly to us.

The products of the sexual cells show us clearty
enough that out of each kind of egg only its own
species of organism can be developed. Certainly it
is not so plain that, besides their visible micro-
scopical characters, the tissues and organic parts
are in possession of more general characters,
identical in all the differently -specialised tissues of
a single organism ; but we may infer the existence
of such latent characters, at least partly, from the
results obtained, in the case of plants, by grafting,
in the case of animals, by transplantation and
transfusion.

In the case of plants one may graft a twig cut
from one tree upon the stem or lower part of
another tree of the same kind, and so bring about a
firm and lasting union between the two. In a short
time the corresponding tissues of the parts brought
into connection quietly unite. Thus from two
different individuals a single living organism may
be produced artificially.

WEISMANN'S THEORY OF THE GERMPLASM 69

One would expect, therefore, that a twig and
stem, chosen from two closely allied species, such
as, for instance, the pear and the apple, would
unite when the suitable tissues were put together.
But this does not happen. Successful grafting
depends far less on the conjunction of obviously
appropriate parts than upon characters unrecognis-
able by us, such as deep-seated kinship between
the parts, and the specific characters of their cells ;
while in the case of individuals of the same species
two pieces will unite even if they are not brought
together in appropriate conjunction, or when they
belong to different parts of the organism, as, for
instance, to the root and the leaf; yet in the
absence of deep-seated kinship union will not take
place.

Generally this kinship, which has been called
vegetative affinity, depends, like sexual affinity,
upon the degree of systematic relationship. It
appears that the same condition of things occurs
as when, in ordinary fertilisation, sexual cells from
different varieties, or species, are united. In both
cases it happens, on the average, that union is the
more to be expected the more closely the plants
concerned are akiu, in a natural system of classifica-
tion.

But in grafting, as in cross-fertilisation, unex-
pected exceptions to this rule occur. Relying upon
these, Naegeli thought that the external distinguish-
ing tokens do not always indicate correctly the
intrinsic constitutional differences. Frequently

70 THE BIOLOGICAL PROBLEM OF TO-DA Y

union will not take place between plants most
near akin in classification, most alike in external
characters ; while it will occur between plants
most different in outward aspect and belonging to
different genera or even families. In other words,
external characters give no certain index to the
degree of vegetative affinity or of sexual affinity
between two kinds of plants.

As an example of this, Vochting, in his treatise
upon transplantation of plant-tissues, takes the
tribes of pear-trees. Grafting between these and
apple - trees takes place only with difficulty,
although the apple is a close kinsman and belongs
to the same genus. On the other hand, most of
them graft easily upon the quince, although that
belongs to a different genus. In this case, also,
there is no sexual affinity between the pollen and
the ova. Hybrids are not formed between the pear
and the apple.

It seems probable to me, although as yet I can-
not get complete proof of it, that sexual and vege-
tative affinity, that is to say, the relationship
between the egg- cell and the pollen of two species,
and the relation between twig and stem, depend
upon the same intrinsic qualities of that elementary
organism the cell.

Vochting distinguishes as harmonic or dis-
harmonic the modes of union between twig and
stem, according to whether or no they reach the
formation of functional unity. Among cases of
disharmony there are several interesting grada-
tions. Generally speaking, in the case of plants

WEISMANN'S THEORY OF THE GERMPLASM 71

not adapted to each other, no attempt at union
occurs, and the grafted twig speedily perishes ;
sometimes even the stem dies, as if it had been
poisoned by the graft. In other cases the dis-
harmony is not shown so strongly. The twig and
the stem begin to unite, but. sooner or later, dis-
turbances occur, and complete destruction results.
According to Vochting, in the case of some Cruci-
ferce the disturbances are as follows : the twig
begins to form roots at its lower end, and these
QTOW into the stem of the host. Through them the

O o

twig uses as food the juices and salts of the stem,
refusing to unite with the stem so as to form a
single individual. As Vochting says, this forma-
tion of roots simply is an attempt on the part of
the twig to complete its own individuality. Instead
of growing into corporate union with the stem, the
twig attempts to become a parasite upon it. A
further consequence often is, that the stem, too,
begins to respond to the unadaptive stranger's in-
fluence. Thus, when Vochting grafted a Rhipsalis
paradoxa on an Opuntia laJbouretiana, he found
that round the roots of the graft the tissues of the
host threw out a protective sheath of cork, or
turned in places to a gelatinous mass.

In some cases experimenters have overcome dis-
harmony between two species, A and B, by making
use of a third species, C, with a vegetative affinity
for both A and B. Thus, an intermediary between
the two disharmonic forms is made, and by such an
arrangement a single functional individual is pro-
duced from pieces of three different species. Thus,

72 THE BIOLOGICAL PROBLEM OF TO-DA Y

upon A, as stock, a shoot of C is grafted, while
upon this shoot of C, as stock, a shoot of B in
turn is grafted.

In the matter of these different grades of dis-
harmony, a comparison may be made between
sexual and vegetative affinities. In many cases
the spermatozoa of one species will not impregnate
the eggs of another species. In other cases, the
alien spermatozoon may penetrate the egg and
unite with its nucleus, making, however, an un-
satisfactory combination in various degrees of
infertility. Sometimes the fertilised egg divides
only a few times and then dies ; sometimes develop-
ment proceeds to the stage of the blastula, the gas-
trula, or even further ; but it then comes to an
end, through intrinsic causes beyond our ken, and,
finally, complete destruction follows.

Our acquaintance with what happens in trans-
plantation of animal tissues is smaller than in the
sphere of botany.

Long ago, Trembley attempted to cause, by graft-
ing, the union of two pieces of hydroid polyps into
a single individual. He divided, across their
middles, two specimens of Hydra fusca, and then,
in a watch-glass, applied the upper end of one to
the lower end of the other. In one case he was
rewarded by the occurrence of complete union ; for,
after a few days, on feeding the upper end with a
worm, it was passed on into the lower end. Later
on buds arose, both above and below the point of
union. Trembley, however, was unable to graft on
each other parts of different species, parts of the

WEISMANN*S THEORY OF THE GERMPLASM 73

green hydra, Hydra viridis, upon the common
hydra.

Transplantations of single tissues or organs have
been made more often, and by several investigators.
I shall mention only the older results of Oilier and
M. Bert, and those made in 1893 by A. Schmitt and
Beresowsky.

Oilier exposed the bone of an animal, and, care-
fully removing a part of the periosteum, planted it
in the connective tissue under the skin in another
part of the body. The consequences differed
according as the transplanted tissue was imbedded
in another animal of the same species, or of another
species. In the first case the piece of periosteum
grew, obtaining a supply of blood from vessels
which grew out into it from the surrounding con-
nective tissue in which it was embedded. In a
short time lamellae of bone were formed by the
layer of osteoblasts, so that a small plate of bone
was formed under the skin. This, however, proved
always but a temporary structure, for, being formed
in an inappropriate spot, and, therefore, being
functionless, it was soon reabsorbed. In the second
case, however, in which the piece of periosteum
was removed from the bone of a dog and planted
in a cat, rabbit, goat, camel, or fowl (or vice versa),
formation of bone did not occur ; either the piece

7 0.

of periosteum was absorbed, or set up suppuration
around it, or became enclosed in a cyst.

Paul Bert's experiments were the following. He
removed pieces two or three centimetres long from
the tails of white rats a few days old, skinned each

74 THE BIOLOGICAL PROBLEM OF TO-DA Y

piece, and planted it in the connective tissue under
the skin of the same animal. In a few days circu-
lation of blood was established in the pieces of the
tails, by union with vessels from the connective
tissue in which they were embedded. Muscles and
nerves degenerated, but the other tissues, bones,
cartilages, and connective tissue, grew vigorously,
so that, in animals killed and examined a month
after the operation, the pieces of tail, implanted
when they were two or three centimetres long, had
grown five to nine centimetres long.

The result was totally different when the trans-
plantation was made from one species to another.
When the tip of the tail of a Mus decumanus or
a Mus rattus was transplanted to a squirrel, guinea-
pig, rabbit, cat, dog (or vice versa), either extensive
suppuration took place, and the piece was extruded,
while sometimes the subject of the experiment
died ; or, after a less turbulent course, the alien
piece was absorbed. The continuance of life and
growth in the piece only took place when the
two animals concerned were allied very closety.
Thus success followed transplantation from Mus
rattus to Mus decumanus (or vice versa), but not
when it was from Mus sylvaticus to Mus rattus.

The recent experiments of A. Schmitt and Bere-
sowsky lead to the same conclusion. The former
succeeded in making pieces of living bone ' take '
only when the transplantation was from one indi-
vidual to another of the same species, or to another
part of the same individual. Beresowsky trans-
planted pieces of frog's skin to the dog and the

WEISMANN'S THEORY OF THE GEPMPLASM 75

guinea-pig, and pieces of dog's skin to the guinea-
pig, and always found that they died, or were thrust
out as foreign bodies.

Precisely the same results follow transfusion of
blood between animals of different species. There
is complete agreement among investigators. When
the blood is made to flow directly from the vessels
of one animal to the vessels of an animal of a
different species, as from the dog to rabbit, or from
dog to sheep (or vice versa) ; or when it has been
first freed from fibrin and then injected, the result
is always the same. ' We have always found,' says
Ponfick, summing up the results of the investiga-
tion, ' not only that blood of another species acts in
strong doses as a poison, and in weaker or smaller
doses is harmful, but that (and this seems to me my
most important result) in every case the blood-
corpuscles are destroyed almost completely, pro-
bably quite completely.' In a very few minutes,
in the case of disharmonic kinds of blood, the red
corpuscles degenerate, and the haemoglobin, be-
coming dissolved in the blood-plasma, soon appears
in the urine. In the case of transfusion of similar
blood between individuals of the same or of very
closely related species, the haemoglobin does not
appear in the urine except after very large doses ;
and Ponfick infers that the red blood-corpuscles,
either all of them or most of them, remain un-
changed in the new animal.

Landois has carried out transfusion between the
remotest species, between different families of
mammals, and between mammals, birds, and

76 THE BIOLOGICAL PROBLEM OF TO-DA Y

amphibia ; from these he drew ' the inference,
important for classification of animals, that those
animals anatomically most nearly allied have their
blood most closely alike.' In fact, ' the destruction
of the foreign blood happens the more slowly the
more nearly the animals are allied.' ' Thus, in
doubtful cases, experiments on transfusion might
settle degrees of relationship. Between individuals
of the same species transfusion is a complete
success ; when the species are closely allied, the
transfused blood disappears only very gradually,
and large quantities may be transfused without
harm. The further apart the animals may be, in
a system of classification, the more violently the
destruction of the foreign blood takes place, and
the smaller is the quantity that can be endured in
the vessels. Thus, in the extent to which blood
transfusion may occur, I see a step towards the
foundation of a Darwinian theory applied to cells.'

As yet, transplantations and transfusions between
animals of different species have been considered
with a view to their importance in surgery and in
medicine, rather than from their purely physio-
logical side. From the results given above, in
which I believe, although there might be drawn
from literature contradictory results — in which,
however, I cannot feel confident — I am prepared
to extend a conclusion to the animal kingdom that
is better supported in botany : the conclusion that
the cells and tissues possess, in addition to their
definite microscopical characters, more general, in-
trinsic, specific characters, and that one may speak

WEISM 'ANN'S THEORY OF THE GERMPLASM 77

of the vegetative affinities between tissues exactly
as one speaks of the sexual affinities between repro-
ductive cells.

SUMMARY OF THE CONCLUSIONS IN THE FIRST

SECTION.

Summing up what has been said in the pre-
ceding pages, we find a large series of facts sup-
porting our contention that cells multiply only by
doubling division. First comes the fundamental
circumstance that single-celled organisms exhibit
only doubling division, as by that alone the per-
manence of species, which experience shows us to
exist, is possible.

Secondly, some facts of reproduction were con-
sidered. The formation of germinal tissues, and,
in the case of lower plants and animals, the occur-
rence of budding in almost any part of the bod}7,
are easily intelligible if every cell, like the egg- cell,
has been formed by doubling division, and so
contains the rudiments of all parts of the organism;
and if thus, on the call of special conditions, every
cell may become a germ-cell again.

Thirdly, great stress is to be laid on those ex-
periments in which the process of development was
interfered with at different stages, as these showed
that the separate cells which arose by division were
not predestined unalterably for a particular role,
according to a predetermined plan (facts of re-
generation and heteromorphosis).

Fourthly, the results of grafting, transplantation,
and transfusion indicate that the cells and tissues

78 THE BIOLOGICAL PROBLEM OF TO-DAY

of an organism possess, in addition to their patent
microscopical characters, latent characters, which
show themselves to be peculiar to the species.

How does Weismann attempt to reconcile his
hypothesis of differentiating division with these
facts ? By the provision of different complementary
hypotheses, which, as we have seen, amount to
this, that he allows the set of rudiments which he
had turned out by differentiating division of the
cell to creep in again by a back-door. He accom-
plishes this by his idea that the germplasm may un-
dergo, simultaneously, doubling and differentiating
division. In these cases cell-division has a double
aspect. According to Weismann, this is possible,
because the egg contains many, sometimes as many
as a hundred, ids, each of which is a combination
representing the species. Weismann believes that
in an egg, while it is preparing for its first division,
the ids are arranged in two groups — an active army
and a reserve army. By differentiating division
the active army is broken up into the divisions,
brigades, and regiments of determinants appropriate
to the separate groups of cells, and so the course
of the development is conducted according to a
preconceived plan. On the other hand, the passive,
reserve army multiplies by doubling division, and
is sent along with definite parts of the active army
as baggage in a fixed or inactive condition, so that
it has no influence upon the normal course of
development nor upon the characters of the cells
(fixed germplasm, inactive, accessory idioplasm,
bud-idioplasm).

WEISMANN 'S THEORY OF THE GEEMPLASM 79

In spite of this purely arbitrary, complementary
hypothesis, the facts seem to me to show that
Weismann assumed an untenable position when he
attributed a reserve army of ' stable plasma ' only
to the sets of cells in which it was necessary to
suppose its existence. The experiments of Driesch,
Wilson, and myself show that a complete embryo
may spring from a half or quarter of the egg, and
that the set of nuclei first to arise may be shifted
about in the egg like a heap of billiard-balls. In
the face of such facts there seems nothing left for
the theory of Weismann but to endow every cell
with accessory germplasro. to prepare it for un-
foreseen events. This, however, would sterilize the
other part of the theory, the doctrine of deter-
minants, and the mechanism of development de-
pendent on a rigid architecture of the germplasm.
Consider the confusion that would arise when the
deploying of the active army was disarranged by
external influences, now in one fashion, now in
another, if the reserve army, with its store of latent
rudiments, had to come to the help of the broken
pieces. What would compel the rudiments dis-
posed to activity according to the prearranged
plan to become latent where they were no longer
wanted ? And what would stir into activity in the
necessary places the originally quiescent rudiments
of the reserve army? In fact, if the roles of activity
and quiescence are even once to be exchanged by
the rudiments in the cell, what object is there in
drawing a distinction so sharp between the two
armies — the active army which carries out the

80 THE BIOLOGICAL PROBLEM OF TO-DA Y

process of development according to a plan pre-
arranged in its minutest details, and a passive
reserve army ordered into quiescence and carried
as baggage ?

But here we come upon the scarlet thread that
continuously has traversed the theory of germplasm
in all its changes. Weismann attaches the greatest
importance to the distinction. The twofold nature
of the process of development is a cardinal point
in his theory, linked to his doctrine of immortality
for unicellular organisms and germ- cells and
mortality for somatic cells.

Between somatic cells and reproductive cells
Weismann places a gulf that cannot be bridged.
Only the reproductive cells contain real germ-
plasm, and only these contain the conditions for
maintaining the species, as they alone serve for
the starting of new generations of development.
The somatic cells, on the other hand, are endowed
only with fragments of germplasm, and hence they
are incapable of preserving the species, and are
doomed to death. The reproductive cells, like
unicellular organisms, are regarded as immortal,
the somatic cells as mortal, According to Weis-
mann, cells cannot pass from the one category to
the other.

As I see Nature, this contrast has been arti-
ficially reasoned into her. From several reasons, I
do not think that it exists. In the first place,
I consider that the facts I have given show the
hypothesis of a differentiating division of cells and
germplasm to be not proven and arbitrary. Next,

WEISM ANN'S THEORY OF THE GERMPLASM 81

the reproductive-cells must be considered as much
a part of the organism as any other tissue. Some-
times they form the greater part of the body, as in
many parasites, and, like the other tissues, they
are subject to death, unless the conditions necessary
to their further development have occurred in
time. But under such conditions other cell-com-
plexes may have death averted from them, as, for
instance, when a slip cut from a willow-tree is
planted. Thirdly, the reproductive cells are
derived from the egg- cell just in the same way as
other tissue cells are derived from it. Like tissue
cells in multicellular organisms, they arise by the
specialisation of material separated from the egg-
cell, and, like every other organ, attain the position
assigned them in the plan of development in the
course of the general metamorphosis of position
that all the cells pass through. Often the sexual
cells, like those of other tissues, appear at a
distance of several cell-generations from the egg.
The intervening generations are specially numerous
in those animals and plants in which several sexless
generations come between the sexual generations
(e.g., many plants, coelenterates, worms, tunicates).
I cannot agree to the existence (in Weismann's
sense) of special germ-tracks. Naturally, I do not
deny that the sexual cells arise from the egg after
definite sequences of cell - divisions ; but this
happens in the case of all specialised cells, such as
muscle, liver, kidney, and bone cells. The con-
ception of special germ-tracks has no more sig-
nificance than there would be in the conception of

6

82 THE BIOLOGICAL PROBLEM OF TO-DA Y

muscle, liver, kidney, and bone tracks. Though
Weismann associates with germ- tracks the idea
that germplasm travels along them, proof of this
has yet to be brought forward.

Finally, a word about the meaning of ' immortal.'
In a scientific work the word must be used in a
philosophical sense. In calling a being immortal
one implies both individuality and indivisibility.
This, at least, was the view of the old philosophers,
who have defined the idea of immortality. Thus
says Leibnitz in his Theodice: ' I hold that the
souls which one day become the souls of men
existed already in the seed, that they have existed
always in organised form in the ancestors, back to
Adam — that is to say, to the beginning of things.'

In his doctrine of immortality, Weismann has
not concerned himself with the two implications —
individuality and indivisibility. He calls a uni-
cellular organism immortal, simply because its life
is preserved in the organisms arising from it by
division. The immortality of the unicellular forms
depends upon their divisibility, upon a property
which, according to the philosophical use of the
word, is incompatible with immortality. According
to Weismann, one immortal organism gives rise to
several immortal organisms, but, as these are sub-
ject to destruction by external agents, the separate
individuals are mortal. The unicellular organism
is not immortal in itself, but only in as much as it
may give rise to other organisms. In this way
Weismann comes in conflict with the idea of
individuality, and is compelled to transform his

WEISMAtfN'S THEORY OF THE GERMPLASM 83

conception. For he says ' that among unicellular
organisms there are not individuals separated from
each other in the sense of time, but that each living
being is separated into parts so far as space is con-
sidered, but is continuous with its predecessors and
successors, and is, in reality, a single individual
from the point of view of time.' Consequently
Weismann must take the same view of the germ-
cells, which, according to his theory, are immortal
in the same way as unicellular organisms, and, in the
same sense, he must make a single individual of all
the germ cells arising from a single germ cell, and,
with them, of ail the organisms developed out of
them. Adam is immortal quite as much as uni-
cellular organisms, for he survives in his successors.

In brief, \Yeismann assigns immortality not to
the unicellular individual, but to the sum of all the
individuals arising from it, all the individuals of
the same species, living contemporaneously and
successively— in fact, to the conception of a species,

In my view, what Weismann has tried to express
by the word ' immortality ' is no more than the
continuity of the process of development. So he
himself says in the course of a defence in which,
however, he did not intend to give up the stand-
point he had taken ; he wishes to imply, by the
immortality of unicellular organisms, only ' the
deathless transformation of organic material,' or ' a

O '

transformation of organic material that always
comes back to its original form again.'

Thus, Weismann himself really has implied that
his distinction between immortal unicellular organ-

o

84 THE BIOLOGICAL PROBLEM OF TO-DA Y

isms, immortal germplasm, and mortal somatic
cells, is a misconception. For the continuity of
the process of development, or the mode of trans-
formation of organic material, depends upon the
continual formation and eventual destruction of
newly-formed material, but in no way implies the
continuous existence of the organised material in a
state of organisation. From this point of view, the
immortality of unicellular organisms and of the
germplasm breaks down, and, above all, the artificial
distinction between somatic cells and reproductive
cells. For, in the latter, the organic process of
development, with its transformation of organic
material, also occurs.

Here I may give the conclusion of this division
of my argument. Cells multiply only by doubling
division. Between somatic cells and reproductive
cells there is no strong contrast, no gulf that cannot
be bridged. The continuity of the process of
development depends upon the power of the cells
to grow and to divide, and has already been set
forth in the sayings — Omniscellulaecellida, omnis
nucleus e nucleo. Whatever novelty the doctrine
of the continuity of the germplasm brings into this
saying depends upon error, and is in contradiction
to known natural facts.

II. ARGUMENTS AGAIN ST THE DOCTRINE OF
DETERMINANTS.

Weismann has united his doctrine of determinants
with his assumption of a differentiating division.
He conceives that every little group of cells in the

WEISMANN'S THEORY OF THE GERMPLASM 85

adult body possessed of definite character and of
definite position in the body — in fact, every group
of cells that is independently variable — is represented
in the egg and in the spermatozoon by a number of
little particles - - the biophores - - and that these,
joined in a system, form the determinants. The
innumerable determinants, he thinks, are so
arranged in the germplasm, and are endowed with
such powers, that, during the process of develop-
ment, they reach, at the right time, the right place
for their expansion into cells. For instance, in the
case of a mammal with parti-coloured fur, as many
architecturally arranged determinants would be
present as there were different spots and stripes in
the fur, due to colour and length of the hairs.

This chain of ideas, made sharp and definite by
Weismann, has recurred again and aofain in theo-

7 O O

retical biological literature in a vague way. In my
view, it rests upon a false use of the conception of
causality, and upon a false implication given to the
relation between the rudiment and the product of
the rudiment, each mistake involving the other.

o

Because, if its development be not interfered with,
a definite egg necessarily gives rise to a definite
kind of animal, a complete identity between the
rudiment and the product, between cause and con-
sequence, has been assumed more or less con-
sciously. The conception of the sequence has been
as if an organism caused its own development in a
closed system of forces, in a kind of organic per-
petual motion. It has been overlooked that, in
the course of the development, many other con-

86 THE BIOLOGICAL PROBLEM OF TO-DAY

ditions must be fulfilled, as without them the
product never would come from the rudiment.

That the same adults may come from the eggs
depends upon the egg-cells, in the ordinary course
of events, being in similar conditions of anabolism
and katabolism, being affected by gravity, light,
temperature, and so forth, in the same way. Thus,
when we are attempting to grasp the fundamental
nature of the course of organic development, we
must not omit the part played by these factors.

We may dwell for a moment upon this weighty
point, as its significance is commonly misunder-
stood.

The course of each organic development depends,
in the first place, upon the absorption and meta-
morphosis of matter. Inorganic matter perpetually
is being turned into organic material to serve for
the growth and development of the rudiments.
Thus, what in one stage of the development is mere
inorganic material, and an external condition of the
development of the rudiment, in the next stage is
become a part of the rudiment. The food-yolk of
an egg, for instance, like the oxygen of the atmo-
sphere, appears, in its relation to the material of the
rudiments, to be something supplied from outside,
an external condition of the development ; yet it is
continually passing into the rudiments and altering
them, even though the alteration may be purely
quantitative. From this follows the very simple
inference that during the course of an organic
development external matter is always being
changed into internal matter, or that the rudiments

WEISMANN'S THEORY OF THE GERMPLASM 87

are continually growing and changing at the ex-
pense of the surroundings.

Now, let one reflect that the egg and the adult
are two terminal states of organised material, and
that they are separated from each other by an
almost inconceivably long series of connecting,
intermediate states ; consider that each stage of
the development is the rudiment and the producer
of the succeeding stage, of the stage that follows,
as the consequence of it ; consider that what was
external in each antecedent stage has entered the
rudiment and become part of it in the succeeding
stage. Then it will be understood that it is a
logical error to assume that all the characters
present in the last link of the chain of develop-
ment have their determining causes in the first
link of the chain. The mistake lies in this : in
the failure to distinguish between the causes con-
tained in the egg at the beginning of the develop-
ment, and the causes entering it during the course
of development from the accession of external
material in the various stages. As there can be
no absolute identity between rudiment and pro-
duct, it is erroneous to transmute the visible com-
plexity of the final stage of the development into
an invisible complexity of the first stage, as the old
evolutionists did, and as the new evolutionists are
attempting to do.

But there is another error in the doctrine of
determinants. This is in intimate union with the
error just discussed, and, to put it shortly, consists
in attributing to a cell — and the egg and sperma-

88 THE BIOLOGICAL PROBLEM OF TO-DA Y

tozoon are cells — the possession of characters not
peculiar to cells, but resulting from the co-operation
of many cells.

The characters of an adult active organism, like
a plant or an animal, are exceedingly numerous,
most varied in their nature, and essentially differ-
ent. Some characters depend upon the healthy
co-operation of nearly all the parts of the body, or
of a group of organs ; others are peculiar to an
organ, and may be referred to its shape, structure,
position, function, and so forth. Others, again,
depend upon individual cells, or even upon sepa-
rate parts of cells. Is it really possible that all
these characters, so many and so heterogeneous,
have special, material bearers in the germ, and
that these bearers are either simple biophores or
determinants — that is to say, groups of biophores ?

I can conceive a cell as endowed only with the
material bearers of such characters as really belong
to a cell itself. Thus, a reproductive cell might have
material particles as the rudiments for producing
horn, chitin, chondrin, ossein, pigment, or chloro-
phyll, or for nerve-fibrils, muscle-fibrils ; but not
for producing a hair, or a separate ganglion of the
spinal cord or the biceps muscle. The rudiments
for hairs, nerve-ganglia, muscles, and so forth, must
be groups of cells, for only groups of cells, and not
specially arranged groups of particles within a cell,
are able to grow into hairs, spinal ganglia, or muscles.

In a short statement, made in 1892, I said : ' The
mistake into which speculations upon the nature of
organic development has led so many investigators

WEISMANN'S THEORY OF THE GERMPLASM 89

is this : they reflect the characters of the adult
upon the undivided egg, and so people that sphere
of yolk with a system of tiny particles, corresponding
to the parts of the adult, qualitatively and in spacial
relations. But in this method of thinking, it is left
out of count that the egg is an organism which
multiplies by division into numerous organisms
like itself, and that, in each stage of the develop-
ment, it is only by the mutual action of all these
numerous elementary organisms that the develop-
ment of the whole organism slowly proceeds.'

Weismann himself, in a discussion of the pan-
genes of De Vries, has partly shown that one
cannot assume the existence in the cell of material
particles that are the bearers of qualities foreign to
the nature of a cell and transcending it. In refer-
ence to the attempt to explain zebra-striping by
pangenes, he says (Germplasm, English edition,
p. 16) : ' There can be no "zebra-pangenes," because
the striping of a zebra is not a cell character. There
may perhaps be black and white pangenes, whose
presence causes the black or white colour of a cell ;
but the striping of a zebra does not depend on the
development of these colours within a cell, but is
due to the regular alternation of thousands of

o

black and white cells arranged in stripes.' Again
(p. 17), he says: ' The serrated margin of a leaf,
for instance, cannot depend on the presence of
" serration-pangenes," but is due to the peculiar
arrangement of the cells. The same argument
would apply to almost all the obvious " characters '
of the species, genus, family, and so on. For

90 THE BIOLOGICAL PROBLEM OF TO-DAY

instance, the size, structure, veining, and shape of
leaves, the characteristic and often absolutely

•/

constant patches of colour on the petals of flowers,
such as orchids, may be referred to similar causes.
These qualities can only arise by the regular co-
operation of many cells.'

Notwithstanding so correct a declaration, Weis-
mann himself, in his doctrine of determinants, has
fallen into the error he himself has exposed. To
represent characters of the adult due to groups of
cells and organisms, he imagines in the egg-cell,
not simple particles like pangenes, but architec-
turally arranged groups of particles, determinants.

No real change has been made. Conditions are
reflected upon the cell that in their real nature
surpass its possibilities. With right and reason one
may adduce, against his own determinants, what
Weismann has said about pangenes, for exactly the
same reasons : ' There cannot be zebra-determinants
or serration-determinants, because zebra- striping,
like the serrated edge of a leaf, is no cell character.'

The error in Weismann's doctrine of determinants
may be made clearer by an analogy.

The human state may be conceived as a high
and compound organism that, by the union of
many individuals, and by their division into classes
with different functions, has developed into a form
always becoming more complicated. To carry out
our comparison better, let us assume that all the
individuals united in the human state arose from a
single pair. The single pair would be the rudiment
of the whole state, and would bear the same signifi-

WEISMANN'S THEORY OF THE ftERMPLASM 91

cance in the development of the state, as the ferti-
lised egg bears to the development of the adult.
The characters of the state, its different organisa-
tions for protection, for tilling the soil, for trade,
for government, and for education, must be explained
causally from the characters of the first pair, which
we take as the human rudiment, and from the outer
conditions under which that pair and the generations
that arose from it had to live.

As the state develops, urban and district com-
munities, unions for husbandry and manufactures,
colleges of physicians, parliaments, ministries,
armies, and so forth, appear. All this visible com-
plexity depends upon individuals associated for
definite purposes and specialised in different direc-
tions. It would certainly not occur to anyone to
explain the growth of this complexity in the de-
veloping state by the assumption that this secondary
complexity was preformed as definite material par-
ticles present in the first pair, although the first
pair is the rudiment of the whole. Much comment
is unnecessary ; everyone must feel that this
attempt to explain the causal relations is on the
wrong track, that it is perverse to try to explain
the complex characters of the human state by a
system of architecturally arranged particles stored
within the first pair. The organisations arising
from the co-operation of many men are something
new, and cannot be regarded as present in the
organizations of one man. No doubt they depend,
in the last resort, upon human nature, but by no
means in this crude, mechanical fashion,

92 THE BIOLOGICAL PROBLEM OF TO-DA Y

But what applies to the causal relations between
the state-organism and men applies also, ceteris
paribus, to the explanation of the causal relations
between the rudiments in the egg and the organism
to which the egg gives rise. For these an explana-
tion cannot be expected on the lines of Weismann's
doctrine of determinants, as that implies a funda-
mentally erroneous assumption. It refers organi-
zations that depend upon cell-communities to
organizations of material particles within a cell.

'To understand inheritance,' says Naegeli, with
truth, c we require not an independent, special
symbol for every difference resulting from time,
space, and quality, but a substance that, by the
linking of the limited number of elements in it,
can exhibit every possible combination of differ-
ences, and that by permutation can pass into
another combination of differences.'

This standpoint is clearer when interpreted in
terms of cells. The hereditary masses contained
in the egg and spermatozoon can be composed only
of such particles as are the bearers of cell-characters.
Every compound organism can inherit characters
only in the form of cell-characters. The innu-
merable, and endlessly variable, characters of plants
and animals are of composite nature. They find
their expression in differences of shape, structure,
and function in the organs and tissues, and in the
special methods in which these are interrelated.
They depend upon the co-operation of many cells,
and, for this reason, cannot be carried into the
hereditary mass of any cell by material bearers.

WEISMANN'S THEORY OF TTtE GERMPLASM 93

They are secondary formations, that can arise only
after the multiplication of cells, and from the
varied combination of cell-characters that accom-
panies the multiplication of cells.

In the foregoing pages I have attempted to prove
the untenability of the doctrine of determinants
from general considerations. I shall now attempt
the same by analysis of a concrete case. The frog's
egg may serve for this. It is a familiar object,
frequently studied. Consider its mode of division,
and the formation of the blastula, gastrula, and
germinal layers.

In cleavage the nucleus plays the chief part, and
thus has been accepted as the bearer of the
hereditary mass. But no single, special determinant
gives the impulse for cleavage ; rather, the co-
operation of all the particles that are essential to
the nature of the nucleus. The chromosomes,
which we may regard as independently growing
and dividing units, must have doubled by assimila-
tion of food material from the yolk ; perhaps, also,
the centrosorne may have doubled in the same
way before the nucleus is in a condition to divide.
This condition itself appears the necessary result
of many different processes of nutrition and growth,
as the result of complicated chemical processes that
run their course within the separate, elementary,
vital units of the nucleus.

The multiplication of the nucleus into two, four,
and eight daughter-nuclei, and so forth, gives the
impulse for the breaking up of the yolk into a
corresponding number of cells. In that process

94 THE BIOLOGICAL PROBLEM OF TO-DA Y

the direction of the cleavage-planes, the relative
positions and the different sizes of the cells exhibit,
under normal conditions, the most marked regu-
larity. But it may be shown directly that this
regularity is not the result of special determinants
lying within the nucleus. For ail these phenomena,
which are characteristic in the cleavage of the
frog's egg, as well as in the cleavage of ail other
eggs, are determined directly by the qualities of
the yolk surrounding the nucleus.

In several publications I have shown clearly that
the external form, of an egg and the arrangement
of its contents, according to the different specific
gravities of the component particles, determine the
position of the nucleus and of the successive planes
of division. Similarly, the different sizes of the
ceils first formed and the unequal rate of division
shown at the two poles of the egg depend upon
the constitution of the yolk, upon the cleavage of
the yolk into a portion richer in protoplasm and a
portion poorer in protoplasm, and upon the differ-
ences in the bulk of protoplasm that in this way
reaches each of the first-formed cells.

In many cases it has been shown that there is a
constant relation between the first three cleavage -
planes of the egg and the long axis of the animal
that arises from the egg. Weismann and Roux
make this a proof that, in nuclear division, the
nuclei that arise have different qualities ; that the
protoplasmic masses lying to the right and left of
the median plane are set apart to build up the
right and left halves of the embryo ; that, similarly,

WEISMANN'S THEORY OF THE GERMPLASM 95

the lirst transverse and horizontal cleavage-planes
divide the protoplasm of the egg into pieces pre-
determined for the formation of the anterior and
posterior, dorsal and ventral, parts of the embryo.

But I think I have shown beyond possibility of
doubt that these events are due not to the exist-
ence of special, mysteriously working groups of
determinants within the nucleus, but merely to the
specific shape of the whole egg and to the segregation
of the yolk. It is self-evident that, as the body of the
embryo builds itself up from the actual material of
the egg, the way in which the material of the egg
is disposed must be of great influence upon the
formation of the shape of the embryo. And so, in
a recently published work, I stated that the grow-
ing embryo, especially in its early stages, must
conform in many ways to the shape of the fer-
tilised egg.

Thus, to bear out what I have been saying by
actual examples, the distribution of the actual
particles of the fertilised egg must correspond to
the disposition of the bulk of material in the
blastosphere ; for, in the breaking up into cells,
the spacial arrangement of the substances of
different weights undergoes no change. Thus,
amphibia, the eggs of which have the poles
different in character, produce blastospheres the
poles of which are unlike ; while eggs, like those
of the fowl, where the yolk does not divide, give
rise to blastospheres with unsegmented yolk. In
such cases the more or less complete segregation ot
the yolk and gravity, which causes a separation ot

96 THE BIOLOGICAL PROBLEM OF TO-DAY

the contents of the egg according to the weights of
the particles, are agencies determining the par-
ticular kind of development. It is no case of
special groups of determinants within the nucleus.

Thus, an oval and an elongate egg produce
respectively an oval and an elongate blastosphere.
The blastosphere determines the orientation of the
gastrula, and so forth. In fact, the original dis-
tribution of mass in the material of the egg is
carried directly on to the following stages of develop-
ment (oval eggs of triton, insects, etc.).

So, finally, in many eggs, where, in addition to a
polar differentiation, there is also a bilateral sym-
metry in the distribution of substances of different
specific gravities and of different physiological
value, the resulting blastospheres, from the reasons
given above, assume a bilaterally symmetrical form.
Although, then, in eggs with polar differentiation,
which have either one axis longer or are bilaterally
symmetrical, under normal conditions the planes of
the first two segmentations may correspond to the
principal axes of the future embryo, the cause for
this agreement lies in the structure of the egg, and
is not to be looked for, as Roux and Weismann
suppose, in differentiating processes of cleavage,
undergone by the nuclei in their first divisions.
It is in this way that there are to be explained the
investigations made by Van Beneden and Jiilin
upon the eggs of ascidians, by Wilson upon the egg
of Nereis, by Roux upon the egg of Rana esculenta,
and by me on the egg of Triton.

As it fails with the process of cleavage, so Weis-

WEISM ANN'S THEORY OF THE GERMPLASM 97

mann's doctrine of determinants fails when we
analyse the formation of the blastosphere, the
gastrula, and the germinal layers.

The formation of the blastosphere seems to me
to be due to the co-operation of the following
processes :

(1) In the division of the egg- cell cavities arise
between the four, eight, and sixteen pieces, and
thus the whole contents of the egg become arranged
more loosely. (2) The more the cells multiply by
division and become smaller in circumference, the
more closely they apply their lateral surfaces to
each other, especially at the outer surface of the
whole, so assuming the arrangement of cell-epithelia.
(3) By the secretion of iiuid, a constantly growing
central cavity is formed pari passu with the
approximation of the superlicial cells, and this
probably also brings with it an increase of the
internal pressure, and a wider curvature of the wall
of the sphere.

Now, is there any part of these processes that has
to do with the breaking of the nuclear contents
into groups of determinants with different qualities ?
By no means. The egg divides into many pieces,
because such division is a general property of cells,
and it is not associated with separate, special
material bearers. The appearance of spaces between
the cells, resulting from division, is due to forces
some of which reside within the single cells, some
of which come from without. In especial, the
assumption of a spherical shape — an assumption
occurring also to a greater or less degree when the

7

98 THE BIOLOGICAL PROBLEM OF TO-DA Y

results of division leave each other — is caused by
the yolk actively arranging itself round the two
nuclei as centres of attraction. The attempt to
become spherical is opposed by other forces, in
accordance with which the ceils resulting from
division press against each other. These forces
that press the cells together seem to increase ^as
the size of the cells diminishes, so that the cells
approximate their lateral faces continually more
closely. The secretion of fluid into the interior of
the sphere and the resulting increase of the outer
surface results from the characters of the whole
wall, and cannot be explained by single, specially
determined cells.

Finally, to take the case of the special kinds of
blastospheres (e.g., of amphioxus, amphibia, reptiles,
birds, and so forth), it has been already shown
that these are produced by the shape of the egg,
by the bulk of the yolk, and by the segregation of
the yolk-particles under the influence of gravity;
that, in fact, the shapes are determined by the general
gross conditions of the structure of the egg.

Plainly, the blastosphere cannot be pre-existing
as a structure of particles in the fertilised nucleus ;
there cannot be blastosphere determinants. The
conditions for the origin of the blastosphere come
into existence only by the process of segmentation,
and it is only by its capacity to divide that the
egg contains the conditions for blastosphere forma-
tion. Here we have epigenesis — the appearance of
a new formation, not the becoming visible of pre-
existing complexity.

WEISMANN'S THEORY OF THE GERMPLASM 99

The conditions of gastrulation and of the forma-
tion of the germinal layers are similar. The
invagination of the blastosphere comes about by
the co-operation of all the cells of its wall, by
local differences in the rates of growth in that wall,
from dissimilarities in its curvature, from many
causes which have not yet been sufficiently sought
out and investigated. As cell division itself depends
not upon special particles, but upon changes in the
entire nuclear contents, it follows that the growth
of the blastosphere- wall, which is merely the sum
of the growth of all the cells in it, cannot be
determined by special groups of determinants.

As an attempt to explain gastrulation, the origin
of the germinal layers and many other events of
development, the doctrine of determinants has re-
versed cause and effect. Certain cells do not
become invaginated into the segmentation cavity
because they possess groups of determinants that
impel them to the assumption of inner layer
characters. The reverse is the truth. Local condi-
tions of growth cause the invagination of a set of
the cells of the blastosphere-wall. This invaginated
layer of cells, brought into a new position with regard
to its environment, becomes the endoderm and re-
ceives the stimulus to assume the character appro-
priate to the new environment. It is unlogical to
speak of endoderm in the fashion of many text-
books and treatises on embryology, while the so-
called endoderm cells still form part of the outer
surface of the blastosphere, or even while they are
still in process of formation by cleavage. For

100 THE BIOLOGICAL PROBLEM OF TO-DAY

' inner germinal layer ' implies a condition of
position winch is created by the invagination.

In fact, it is impossible, in thinking of the
gastrula as in thinking of the blastosphere, to
conceive that in the egg, which is a simple cell,
there can be preformed by material particles in the
nucleus a condition which implies the existence
of two layers of cells.

Thus analysis of a special case leads to the same
conclusion as is reached by the general reasoning
of the earlier part of this section.

PART IT.

THOUGHTS TOWARDS A THEORY OF THE
DEVELOPMENT OF ORGANISMS.1

Now that criticism of the germplasm theory has
given us a bias in the right direction, it is necessary
to map out more clearly the path along which

1 The second section contains references to the following treatises :

C. V. NAEGELI : Mechanisch-physiologische Theorie der Abstam-
mungslehre (1884).

HERTWIG, OSCAR : Lehrbuch der Entwicklungsgeschichte des
AFensclien und der Wirbelthiere ; 4th edit.

SACHS : Lectures on Plant Physiology ; English edition, Clarendon
Press.

VOECHTIXG : Ueber die Theilbarkeit im Pflanzenreich und die
JVit'kung innerer und dusserer Krdfte auf Organbildung an Pflan-
zentheilen. Pflugers Archiv., vol. xv. , 1877.

Ibid. : Ueber Organbildung im Pflanzenreich, 1, 2 ; Bonn, 1878,
1884.

GOEBEL : Beitrdge zur Morphologic und Physiologic des Blattes.
Bot. Zeit., 1880.

PFLUGER : Die tcleologische Mechanik der lebendigen Natur ;
Bonn, 1877.

MAITPAS : Sur le determinisme de la sexitalite chez THydatina
senta. Comptes rendus des seances de VAcadlmie des Sciences ;
Paris, 1891.

WEI.SMAXN : DieAllniachtder Naturzilchtung. Eine Erwiderung
an Herbert Spencer ; Jena, 1893.

102 THE BIOLOGICAL PROBLEM OF TO-DAY

solution of the problem may be sought. In general
terms, our problem is the necessary origin from an
egg, always of the same organism, with its manifold
characters, and the explanation must avoid the
attribution to the egg of characters foreign to its
nature as a cell. This is the more necessary as
Weismann objects to the supposition that cell-
division is doubling, holding that the supposition
allows neither an explanation, nor even the beginning
of an explanation, of the differences that arise among
cells while the differentiation of the body occurs.
1 Any explanation must in the first place account for
this differentiation/ says Weismann (Germplasm,
p. 224) ; ' that is to say, the diversity which always
exists amongst these cells and groups of cells
arising from the ovum must be referred to some
definite principle. In fact, no one could even look
at it as giving a partial solution of the problem, if
differentiation is supposed to be due to that part
alone of the germplasm becoming active which is
required for the production of the cell or organ
under consideration. But the higher we ascend
in the organic world, the more limited does the
power of producing the whole from separate cells
become, and the more do the numerous and varied

HERBERT SPENCER : A Rejoinder to Professor Weismann. Con-
temp orary Review, 1893.

Ibid. : Die Unzulangliclikeit der l NatiirZichen Zuchtwahl.' BioL
Centralblatt, vol. xiv. , No. 6.

EMERY: Die JSntstehung und Ausbildung dcs Arbeiterstandes I>CL
den Ameisen. BioL Centralb., vol. xiv., No. 2, 1894.

HAACKE ; Gestaltung und Vererbung (1894).

THEORY OF THE DEVELOPMENT OF ORGANISMS 103

differentiations of the soma claim our attention and
require an explanation in the first instance. The
presence of idioplasm in all parts containing the
primary constituents does not help us in this
respect.'

With this I cannot agree. Naturally, Naegeli,
De Vries, Driesch and I assume that, of the many
rudiments present in every cell, only some come to
activity in each special case, and that the selection
of those that become active is due to causes arising
in the course of development. Our conception of
the nature of these causes, and of their place of
origin, is diametrically opposed to Weismann's.

Weismann would make the causes of this orderly
development of the rudiments reside in the germ-
plasm itself; for he considers that to be not only
the material but the motive force of the course of
development. According to him, every cell must
have become what it is, because it was provided
only with the definite rudiments assigned it before-
hand, according to the plan of the development of
the germplasm.

On the other hand, we regard the development of
the rudiments as depending upon motive forces or
causes that are external to the germplasm of the
ovum, but that none the less arise in orderly
sequence throughout the course of the development.
The causes we recognise are first, the continual

O

changes in mutual relations that the cells undergo
as they increase in number by division, and second,
the influence of surrounding things upon the
organism.

104 THE BIOLOGICAL PROBLEM OF TO-DAY

One may group together as centrifugal coMses
of the process of development the characters of the
fertilised cells and the interrelations between the
products of their divisions, and distinguish them
from the centripetal causes, or motive forces that
are provided by the action of surrounding things.
None the less, it must be borne in mind that there
is no sharp distinction between centrifugal and
centripetal forces. On page 86 I showed how
what is external in one stage of the process
becomes internal in the succeeding stage. The
external constantly is becoming internal, and the
sum of the internal factors increases only at the
expense of external factors.

From the physiological point of view I regard
the divergent differentiation of cells as a reaction of
the organic material to unlike impelling forces —
that is, to factors shown by experimental physiology
to be actually present and to rule the building up
of the organism. ' It were superfluous to detail/
as Naegeli says, * how continually other forces
external to the idioplasm, but belonging to the
individual, influence the idioplasm; every cell,
indeed, as it grows and divides, takes up a definite
place in the growing whole, and finds itself in a
peculiar combination of conditions of organisation.'
' Not only influences within the individual affect
the idioplasm, as that may be altered by external
influences, and so may be forced to grow in a new
direction.1 * The influence of surroundings in
determining which of the rudiments contained in

o

the idioplasm shall achieve development is shown

THEORY OF THE DEVELOPMENT OF ORGANISMS 105

in the following example: it depends on their
nutrition whether certain trees shall bear foliage
or flowers ; while in an unpropitious climate many
plants refuse to bear flowers at all, but content
themselves with vegetative reproduction.'

This principle indicates the path along which
explanation of the differentiation of cells is to be
sought. Although in no single case is it yet
possible to refer a known action to its appropriate
cause — in other words, to show a definite stimulus
producing a definite reaction upon the rudiment —
this failure is not to be attributed to error in the
principle. It is the natural result of the enormous
difficulties besetting an attempt to understand
the highly involved events of development. We
can only ask whether or no our general prin-
ciple is harmonious with the facts displayed in
nature.

In the following pages I shall try to develop
this view, taking, as formerly, a few instances. I
shall now proceed further with suggestions I made
in my treatise on Old and New Theories of Develop-
ment. I start from the conception that the ovum
is an organism that multiplies by division into
numerous organisms like itself. I shall explain the
gradual, progressive organisation of the whole
organism as due to the influences upon each other
of these numerous elementary organisms in each
stage of the development. I cannot regard th^
development of any creature as a mosaic work. I
hold that all the parts develop in connection with
each other, the development of each part always

106 THE BIOLOGICAL PROBLEM OF TO-DA Y

being dependent upon the development of the
whole.

The power of the egg to multiply by division is a
chief and most important factor in the production
of complexity during the course of development.
It is only because the nuclear material, by a series
of intricate, chemical changes, assimilates reserve
material from the egg and oxygen from the atmo-
sphere that it can give rise to continually increas-
ing complexity within itself. The increase in bulk
results in a cleavage into two, four, eight, and
sixteen pieces, and so forth. The cleavage produces
a constantly changing distribution in space of the
nuclear material. The two, four, eight, and sixteen
nuclei that arise by division diverge from each
other and take up new positions inside the egg, in
definite relations to each other. At first the
particles of the egg were arranged around the
fertilised nucleus, which was a single centre offeree ;
they become grouped around as many centres of
forces as there are nuclei, and so become segregated
into as many cells. Clearly enough, the egg, in its
single-celled condition, changes its quality in a
marked degree when it becomes multicellular, even
although the change has occurred by doubling
division.

This, so clear in the early stages of development,
continues to occur throughout the later stages of
growth. The continued cell-multiplication causes
not only changes of bulk, but also from time to
time changes in quality ; for each shape is bound
up with definite conditions. When the conditions

THEORY OF THE DEVELOPMENT OF ORGANISMS 107

alter, the organic material, by its power of reaction,
changes its shape in a corresponding fashion.

As the nature of architectural plans depends
upon the properties of the wood, stone, or iron, as
they must correspond with the material to be em-
ployed (i.e., the span of a roof, the construction of
a bridge depend upon the material in shape and
weight), so the nature of the organic material
determines to a large extent the shapes assumed in
the course of growth.

Shape in many respects appears to be a function
of growth in an organic material.

A few examples will make clear this important
relation. A limit is set to increase in the size of a
blastosphere by the nature of the material of its
walls. Its wall is a membrane, composed of one
or more layers of cells ; that this may preserve its
curvature, a definite pressure from within must be
maintained, proportioned to the cohesive force of
the cells ; at the same time the wall of the sphere
must be able to withstand the strain and pressure
put upon it by external forces. All these, and
many other factors less easy to conceive, must be
delicately adjusted to one another. If in any
direction a definite limit be exceeded, then either
the structure will be destroyed by disintegration
of the component parts, or a new shape will be
assumed. The latter is the event in the case of a
living substance capable of reaction. The blasto-
sphere, growing beyond its limits, folds into a cup-
shaped organism. Did we know all the influences
affecting the wall of the blastosphere, then we

108 THE BIOLOGICAL PROBLEM OF TO-DAY

would understand the causes by which growth
beyond a definite limit must result in invagination.
From the occurrence of the gastrula in all the
divisions of the animal kingdom, we may conclude
that it is a temporary phase, inevitable in the
growth of animals.

There may be noticed here a second connection
between shape and organic growth, exceedingly
simple in its nature, but of fundamental importance
in its consequences. It may be stated in this saying :
Growth always must be such as to produce the
greatest possible extension of surface. The reason
of this is simple, depending on the different natures
of inorganic material and living organic material.

A crystal in its mother liquor grows by attract-
ing new particles and depositing them upon its
outer surface, according to the kind of crystallisa-
tion peculiar to the material of which it is com-
posed. These particles, once crystallised, retain
their position even when new layers are deposited
on their outer surfaces, and remain unchanged,
perhaps, like rock crystals, for thousands of years,
until changed outer forces loosen the bonds that
bind them.

Organised material cannot grow in this fashion ;
it takes up material from without, not, like the
crystal, arranging it on the outer surface, but in-
gesting it. Protoplasm cannot become fixed in any
condition without being destroyed ; it exhibits
perpetual interchanges with the outer world ; un-
ceasing intake and output is a necessary accompani-
ment of its life. ' The growth of idioplasm,' as

THEORY OF THE DEVELOPMENT OF ORGANISMS 109

Naegeli strikingly says, ' implies a constancy oi
perpetual change.'

Thus, growing protoplasm can assume only such
shapes as allow it to remain in constant touch with
the outer world. A cubical or spherical mass of
cells could not grow by the formation of new layers
of cells on the outside, for these layers would
deprive the centrally placed masses of cells of their
conditions of existence. Similarly, an extended
membrane of cells or an epithelial layer cannot add
indefinitely to its thickness, else would the ceils
furthest removed from the outside be injured in
their relations to surrounding things. To satisfy
its essential conditions, protoplasm can grow only
with a proportionate extension of its external
surfaces. This is secured by the cells becoming
arranged in threads and membranes, and its result
is that the threads by branching, and the mem-
branes by folding, produce structures whose com-
plexity increases with growth.

This conception that the shape of growing
organisms is in many respects the necessary con-
sequence of the specific characters with which
protoplasm is endowed, explains the great contrast
between animals and plants in their general
organisation. The contrast is the result of the
difference between animal and plant metabolism,
and between the ways in which animals and plants
obtain their food. Plant cells elaborate protoplasm
from the carbonic acid of the air, water, and easily
diffusible solutions of salts, obtained from the sea or
from the soil. For the chemical work of combining

110 THE BIOLOGICAL PROBLEM OF TO-DAY

these, they require the active energy of sunlight.
We can now see the chief requirements to which the
constitution and arrangement of the cells in a
multicellular plant must be adapted. Plant cells
may become clothed in a thick membrane, as that
would prove no hindrance to the passage of gases
and easily diffusible salts ; but they must be
arranged so as to present the greatest possible
surface to the surrounding media (i.e., to the soil
and the water, the air and the sunlight) whence is
drawn their supply of matter and force. The cells
must turn a broad face to the outside ; this they
do by becoming arranged in branching rows, or in
leaf-shaped flattened organs. That they may suck
up water and salts from the soil, the ceils are
arranged as a highly branched system of roots,
covered with delicate hairs, and penetrating the soil
in every direction. To inhale the carbonic acid
from the air, and to be subjected to the influence
of sunlight, the aerial part of the plant stretches
out its branches towards the light, and becomes
folded into the flat leaves, the structure of which
reveals a suitability for assimilation. Thus the
whole architecture of a plant is superficial and
visible; internal differentiation into organs and
tissues either is wanting, or, compared with animals,
is very scanty. It is only in the higher plants that
the internal fibro-vascuiar tissues appear; these
serve a double purpose : they act as channels along
which the sap passes, so bringing together the
different materials absorbed by roots and leaves ; and
they have the mechanical function of strengthening

THEORY OF THE DEVELOPMENT OF ORGANISMS 111

the stem and branches. The different mode of
nutrition of animals results in a totally different
structural plan. Animal cells absorb material that
is already organised, and that they may do so their
cells are either quite naked, so affording an easy
passage for solid particles, or they are clothed only
by a thin membrane, through which solutions of
slightly diffusible, organic colloids may pass.
Therefore, unlike plants, multiceliular animals
display a compact structure with internal organs
adapted to the different conditions which result
from the method of nutrition peculiar to animals.
A unicellular animal takes organic particles bodily
into its protoplasm, and forming around them
temporary cavities known as food vacuoles, treats
them chemically. The multiceliular animal has
become shaped so as to enclose a space within its
body into which solid organic food -particles are
carried and digested, thereatter, in a state of solution,
to be shared by the single cells lining the cavity. In
this way the animal body does not require so close
a relation with the medium surrounding it ; its food,
the first requirement of an organism, is distributed
to it from inside outwards. In its further complica-
tion the animal organisation proceeds along the same
lines. The system of internal hollows becomes
more complicated by the specialisation of secreting
surfaces, and by the formation of an alimentary
canal, and of a body cavity separate from the
alimentary canal.

In plants, it is the external surface that is in-
creased as much as possible* In animals, in obedi-

112 "THE BIOLOGICAL PROBLEM OF TO-DAY

m

ence to their different requirements, increase takes
place in the internal surface. The specialisation of
plants displays itself in organs externally visible — in
leaves, twigs, flowers, and tendrils. The specialisa-
tion of animals is concealed within the body, for the
internal surface is the starting-point for the forma-
tion of the organs and tissues.

Comparative embryology shows that, however
varied the forms and functions of the numerous
animal organs may be, the method of their develop-
ment is remarkably similar. There are required
only the slightest variations of a few simple general
laws. For these I may refer readers to a series of
special investigations (Studies on the tierm-layer
Theory, Oscar and Kichard Hertwig), and to the
fourth chapter of my Embryology, ' General Dis-
cussion of the Principles of Development.'

In these works and in the foregoing pages I have
tried to show that the multiplication of the egg-
cell by division is itself a source of increasing com-
plexity and an active principle in the determination
of form, since the products of the division unite to
form a higher unity. But in another way the
multiplication of cells leads to differentiation among
the cells arising from the egg. Although each of
these resembles the parent egg, from which they
arose by doubling division, yet they differ from it
in one point : they are no longer a whole, but have
become the subordinate parts of a higher unity,
that is, of a higher organism. A cell that is no
longer a whole, but the part of a whole, has entered
upon reciprocal relations with other cells, and in

THEORY OF THE DEVELOPMENT OF ORGANISMS 113

the functions of its life is limited by these others and
by the whole. The further this is carried the more
the cell falls short of its independence as an
elementary organism, and appears only as a part
with its functions subordinate and in dependence
upon the whole.1

Although from the point of view of morphology
it has become more and more imperative to regard
the cell as the unit of the higher organism, still,
from the physiological point of view the higher
organisms must be regarded as masses of material
acting as wholes, and composed of several grades
of structural parts, subordinate in function to the
whole, and displaying only a limited division of
capacities. And so the cell theory, according to
which the cell was exalted unduly as the unit of life,

1 The assumption of doubling division does not involve the
assumption that the germinal mass is unalterable. Although I do
not regard the process of division as a mechanism for breaking up
the idioplasm into dissimilar groups of determinants, I regard the
idioplasm — and here I agree with Naegeli — as only relatively stable.
In course of time external and internal forces may slowly alter it.
On the one hand, the idioplasm of the reproductive cells in the
course of generations may slowly alter, while, on the other hand,
the idioplasm of cell groups in an organism may acquire a local
character in correspondence with their different topographical and
functional positions in the whole creature, and in relation to their
place in the organic division of labour, just as in human com-
munities individuals become altered by the lifelong exercise of some
calling.

Nor does the doctrine of doubling divisions conflict with those
conclusions of pathology according to which, in the process of
regeneration, cells and tissues give rise only to cells and tissues of
their own order. For further details see my treatise, Ei und
Samen-Bildung lei Nematoden, pp. 97-99. These slight sugges-
tions are only to prevent misconceptions.

8

114 THE BIOLOGICAL PROBLEM OF TO-DAY

the centre of life, the elementary organism, must
take limitation and correction from these wider
views. This has already been insisted upon by
many physiologists of insight — for instance, by
Naegeli (see p. 30), by Sachs, and by Yochting.

1 Cell formation,' declares Sachs (Physiology of
Plants, p. 73), ' is a phenomenon very general, it is
true, in organic life, but still only of secondary
significance ; at all events, it is merely one of the
numerous expressions of the formative forces which
reside in all matter, in the highest degree, however,
in organic substance.' ' Essentially, every plant,
however highly organized, is a continuous mass of
protoplasm, surrounded externally by a cell wall
and penetrated internally by numerous transverse
and longitudinal partitions.'

My conception receives strong support from the
way in which Vochting set forth the relations of
the cell to the whole :

1 Is the circumstance that a cell, separated from
the organism, is able to survive and build up the
whole again a proof of the independent life of the
cells while in the organism ? I believe it to be
only a proof that the life of the organism is always
dependent upon the cell, that the life is inherent
in the cell, and that the life of a compound
organism is merely the resultant of the vital
phenomena of its single cells ; but by no means
that the cell when isolated displays the same
functions as while it is a part of the organism.
The cell while in the organism and the cell
separated from the organism and self-sufficing, are

THEORY OF THE DEVELOPMENT OF ORGANISMS 115

quite different. We must regard the functions of
a cell that is part of an organism, disregarding
external influences, as determined by the whole
organism, and only by the cell itself, in so far as
that forms a greater or less part of the whole
organism. When not part of an organism, the cell
is independent, and entirely determines its own
function. Nowhere is it easier than in this case to
confuse possibilities with facts, and nowhere is the
confusion more fatal. From a morphological point
of view, one may confidently regard the cell as an
individual ; but it must be borne in mind that an
abstraction has been made. Physiologically con-
sidered, the cell is an individual only when it is
isolated from a complex and is independent ; of
this no abstraction can be made/

According to the conception I have been explain-
ing, cells merge their independent individuality in
that of the whole, and so the force that directs
their ultimate development, and that leads to their
appropriate elaboration, cannot be within them,
cannot reside in special groups of determinants,
in the sense of Weismann. It is given by the
relations in which the cells come to stand to the
whole organism and to the various parts of the
organism, and, on the other hand, to surrounding-
things. Naturally, such relations differ with the
place or position occupied by cells in the whole
organism, and in this way there come to be in-
numerable conditions making for diverging direc-
tions of development, for division of labour, and
for dissimilar, histological differentiation. The

116 THE BIOLOGICAL PROBLEM OF TO-DAY

part played by a cell, as Vochting puts it, will
depend upon the position it comes to assume in the
whole living unit. To use an expression of Driesch's,
dissimilar differentiation of cells is a ' function of
position.' Such a conception my brother and I, ia
our Studies on the Germ-layer Theory, sought to
establish clearly by many examples from the
histology of the coelenterates and of higher animals ;
such a conception for long has been clearly ex-
pressed in physiological botany.

The simpler nature of plants in structure and
function makes it easy to conduct experimental
observations upon this point.

I have already described how either side of the
prothallus of a fern may be made to produce male
or female organs, according as it is kept in the
light or in the dark. Similarly, taking a willow
slip, roots may be made to appear at one end by
moisture and darkness, while they will not appear
on the end kept in the light.

The experiments of botanists and of fruit-growers
show that young buds and the rudiments of roots
are indifferent structures, the further growth of
which depends entirely upon the conditions in
which they are placed. ' One and the same bud
may grow to a long or short vegetative shoot, to a
floral shoot, to a thorn, or may remain undeveloped.
The same root rudiment may grow to a main tap-
root or may form a secondary lateral root. The
conditions that determine the mode in which these
structures will develop are quite within the power
of the experimenter. We have shown already

THEORY OF THE DEVELOPMENT OF ORGANISMS 117

and could show further, that he is able to determine
the mode of growth by cutting, bending, tying in a
horizontal position, and so forth/ For such reasons,
Yochting describes plants as masses of tissue,
practically plastic, and which may be moulded at
the discretion of the investigator. ' For instance,
in the case of Prunus spinosa, a branch may be
produced in place of a thorn by cutting a growing
shoot at the proper height, in spring. The buds
below the point where the cut was made turn to
shoots like the rest of the plant and complete the
interrupted growth, while on an uncut stem they
would have grown to thorns. Thus, the rudiment
of a thorn has been changed to that of a shoot '
(Yochting).

Although it is more difficult to carry out experi-
ments upon animals, some good instances are
known. If a piece cut from the stem of Antennu-
laria (a hydroid polyp) be placed vertically, in a
short time new branches and new ' roots ' spring
from it. In this case, again, the position of the
new growths is determined by the relation in which
the stem is placed to gravity. * The tentacles arise
only at the end turned towards the zenith ; the
" roots " from the parts directed towards the ground '
(Loeb).

A similar example may be taken from among
vertebrates. The notochord arises from a set of
cells which are in close relation with the fused tips
of the blastopore. By exposing developing frog's
eggs to abnormal conditions, I was able, in some
cases, to produce a hypertrophy of; one of the lips

118 THE BIOLOGICAL PROBLEM OF TO-DA Y

of the blastopore. When fusion of the lips took
place the normal lip united with the rim of the
protruding hypertrophied lip. As a result of this
the notochord and the nerve plate came to arise, not
from the usual set of cells, but from those cells that,
by the abnormal condition, had come to lie in the
place for the notochord. The protruding cells,
which normally would have developed into noto-
chord and nerve plate, grew into a simple fold of the
external skin.

Moreover, it is well known in pathology that
mucous membranes may lose their proper char-
acter and assume the qualities and aspect of the
external skin, when, as in cases of prolapse, fistula,
etc., they have been exposed for some time to the
air.

The relations of different parts to each other and
to the whole are known as correlations. Correla-
tion exists in all the stages of the development
of an organism, sometimes in one way, sometimes
in another. One must note very carefully that
Weismann's doctrine of determinants, according to
which all that happens in development follows a
prearranged plan, is entirely in opposition to this
correlative character of the changes that occur
during development.

Here I shall give a few quotations from botanical
and zoological writers :

1 If the stem of a plant be cut so that it retains
its roots, but is deprived of leaves and shoots, then
the adventitious buds will produce new leaves
and shoots. If, however, the stem be cut so as to

THEORY OF THE DEVELOPMENT OF ORGANISMS 119

deprive it of roots, then the same cells that in the
other case produced leaves and shoots will now
produce roots. Precisely the same occurs with a
piece of the root. In fact, it appears as if the idio-
plasm knew what parts of the plant were wanting,
and what it must do to restore the integrity and
vital capacity of the individual.' 'The idioplasm
in the remaining part of a plant must be affected
when an important part has been removed, because
the idioplasm of the lost part is no longer capable
of having influence.' * It is clear enough that
necessity acts as a stimulus, and that each definite
need calls into existence the appropriate reaction.'

These are Naegeli's views, and they have been
elaborated by Pfliiger in his important treatise on
The Teleological Mechanism of Living Nature
(1877).

Vochting writes in similar fashion :
' In a tree that is growing under normal condi-
tions, without being subjected to injury, all the
organs appear in definite relation to each other:
so many leaves correspond to a definite number
of twigs and branches. These spring from a stem
of proportionate thickness, and the stem passes
into a definitely proportioned tap-root, from which
arise a due array of lateral roots. In normal
conditions all these organs are in equilibrium. An
apple-tree, growing on the line where tilled garden
ground meets a lawn, grows more vigorously on
the side towards the garden. If one of the roots of
an apple-tree with three main roots and three
branches be amputated, then the correspond-

120 THE BIOLOGICAL PROBLEM OF TO-DA Y

ing branch will lag behind in growth, although it
may not absolutely perish.' * The equilibrium
varies according to the specific nature of the tree.
It is shown in one way in the oak, in another in the
beech, and is different in the varieties of a species.'

Finally, consider this statement from Goebel's
Treatise on the Morphology and Physiology of the
Leaf: 'The fact that lateral buds do not develop
while the axial bud is still growing vigorously
depends upon the relation between the two. That
I denote as correlation of growth.'

The dependence of parts upon each other, and
upon the whole, is specially clear and instructive in
cases where different plant individuals are united by
budding or grafting. To limit the growth of a
tree, and to induce it to become dwarfed, it is
necessary only to graft it upon a nearly allied but
dwarf variety. When a pear-tree is grafted upon
the quince, which is characterized by its dwarf- like
growth, the vegetative growth of the pear is re-
duced exceedingly. It produces shorter and
weaker shoots ; all the dwarf varieties of the pear
employed as wall fruits, or growing into the little
pyramids spoken of in the trade as ' cordon '-trees
and potting-trees, could not have been produced
unless the gardener had had the quince as a
natural dwarf stock (Vb'chting). With the dwarf-
ing is associated a freer and earlier production of
fruit. Other kinds of fruit-trees, apples, apricots,
and so forth, show the same course.

* The capacity to withstand external influences
and the duration of life may be altered in the same

THEORY OF THE DEVELOPMENT OF ORGANISMS 121

way. The pistachio (Pistazia vera), cultivated in
Frankfort, which is destroyed by a temperature
lower than 7*5 degrees of frost, will survive 12'5
degrees if it has been grafted upon P. terebinthus.
Moreover, when it is grown from a seedling, it may
reach the age of 150 years; but when it has been
grafted upon P. terebinthus its length of life is in-
creased to 200 years ; while, grafted on P. lentiscus,
it reaches only about 40 years ' (Yochting).

Vochting's experiments upon beetroot are still
more characteristic. ' The stem of a beet plant
that bore young buds gave rise to vegetative
shoots when it was united with a young, still grow-
ing root, but to a blossoming stem when it had
been grafted, in spring, upon an old root.'

Similarly, animal growth is correlative in all its
stages. When a muscle becomes unusually large it
sets up corresponding correlations of growth in
many other parts of the body. The bloodvessels
and nerves supplying it become larger, and the
increase in the nerves leads to corresponding in-
crease in the nerve centres. The tendons of
origin and of insertion, and the parts of the skeleton
to which these are attached, must react to the in-
creased size of the muscle by growing larger ; in
fact for all the parts of the animal body the con-
clusions which Naegeli and other physiologists
drew from plants are applicable. All the different
elements of the body are in definite and intimate
touch with each other.

This is shown most beautifully and clearly in the
extraordinarily interesting phenomena called di-

' THE BIOLOGICAL PROBLEM OF TO-DA Y

orphism and polymorphism. These seem to me
V to show how very different final results may grow
from identical rudiments, if these, in early stages
/* of development, be subjected to different external
« /• influences.

Finally, I have a little to say about the sexual
dimorphism that occurs so generally in the animal
kingdom.

Nearly all kinds of animals appear as male or
'as females. These differ from each other not only
in that they produce eggs or spermatozoa, but
frequently in a number of more or less striking
characters affecting different parts of the body, and
known as secondary sexual characters. In fact,
the difference between the sexes may be so great
that a systematic naturalist, unacquainted with the
mode of development of the creatures, might place
them in different species, genera, or even families,
on account of the striking differences in external
characters.

As an instance, take Bonellia, a gephyrean, the
strange case of which has been remarked upon by
Hensen and by Weismann. The male is about a
hundred times smaller than the female, in the
respiratory chamber of which it lives as a kind of
parasite, and appears, so far as outward shape goes,
more like a turbellarian than a gephyrean. None
the less, male and female are alike not only while
they are in the egg, but as larvse, and it is only
towards the period of sexual maturity that the
great difference between them begins to appear.
So also is it with the dwarf males of the cirripedes.

THEORY OF THE DEVELOPMENT OF ORGANISMS 123

Males and females, whether they be more or less
unlike, arise from the same germinal material.
The germinal material itself is sexless ; that is to say,
there is not a male and a female germinal material.
The phenomena of inheritance in the sexual genera-
tion of hybrids show this clearly. Characters appro-
priate both to males and to females are transmitted
either by eggs or by spermatozoa. In partheno-
genetic animals both male and female individuals
appear at definite times from eggs produced without
sexual commerce. Whether the male or the female
forms be produced depends, not upon any difference
in the germinal material, but on the external influ-
ences, just as external influences determine whether
the bud on a twig shall give rise to a vegetative or
to a flowering shoot, to a thorn or to a stem.
The influence of food, of temperature, or probably
of other agencies, determines in which direction
the germinal material shall grow.

The experiments of a distinguished French
investigator, M. Maupas, on the determination of
sex in Hydatina senta, a rotifer, have given
striking results.

In Hydatina, under normal conditions the eggs
of certain individuals give rise always to males, of
others always to females. By raising or lowering
the temperature at the time when the eggs are being
formed in the germaria of the young females, the
experimenter is able to determine whether these
eggs shall give rise to males or to females. After
that early time the character of the egg cannot be
altered by food, light, or temperature.

124 THE BIOLOGICAL PROBLEM OF TO- DA Y

In one experiment, in which five females not yet
fully grown were kept in a room at the tempera-
ture of 26 to 28 degrees centigrade, Maupas found
that, of 104 eggs only 3 per cent, gave rise to
females, while in the case of other five young
females of the same brood, but kept in a cold
chamber at a temperature of 14 to 15 degrees
centigrade, 95 per cent, of females were produced.
In another experiment, young animals were kept
for a few days in the cold, and then, until death, in
a higher temperature. Of the eggs produced while
in the cold, 75 per cent, produced females, of those
deposited in the warmth, 81 per cent, became males.

With these results may be compared what
happens with many plants. Melons and cucumbers,
which produce on the same stem both male and
female flowers, bear only male flowers in high
temperatures, only female flowers when subjected
to cold and damp.

In the case of many insects in which partheno-
genesis occurs, the determination of sex depends
upon fertilisation. Thus, among bees, unfertilised
eggs give rise to drones, fertilised eggs to females.

Sexual dimorphism in still another way reveals
the intimate interactions existing between all the
parts of an organism in every stage of develop-
ment. It is well known, for instance, that among
animals the early removal or destruction of the
sexual organs hinders the development of the
secondary sexual characters, or even may occasion
the appearance of the characters of the other sex.
Old hens become cock-feathered ; human eunuchs

THEORY OF THE DEVELOPMENT OF ORGANISMS 125

have the high-pitched voice and the peculiarities
of the larynx found in women.

As much as sexual dimorphism, the phenomena
of polymorphism show the enormous influence
exerted by external forces upon correlated varia-
tion of the parts during development, and in this
way upon the final structure.

In the question of polymorphism it is worth
while to discuss at some length the extreme poly-
morphism exhibited in the case of some of the
colonial animals — first, because the matter has
recently occasioned an important controversy be-
tween Herbert Spencer and Weismann ; and,
secondly, because the discussion will serve to
make still more clear the difference between my
views and those of Weismann upon the nature of
the process of development.

Among the colonial insects there arise, in
addition to males and females, sexless individuals
known as neuters. These in certain cases are very
different from both males and females in structure
and in social instincts.

Among bees there are the queens, sexually
mature females ; the workers, females whose sexual
organs are rudimentary, and parts of whose bodies
— the stings, the wings, the hind legs, with their
pollen-collecting apparatus — are peculiarly formed;
and, lastly, the males, or drones.

In many of the ant and termite colonies still
greater differences exist between the different sets
of individuals. In addition to males and females,
there are sexless workers, and these in many species

126 THE BIOLOGICAL PROBLEM OF TO-DAY

are of two kinds, known as workers and soldiers.
The divergences of structure among the three or
four forms are shown, frequently by considerable
differences in size, by the presence and absence of
wings, by differences in the sense-organs, the brain,
and the structure of the head. In the common
ant — Solenopsis fugax, for instance, as Weismann
quotes from Forel — the males have more than
four hundred facets on their eyes, the females
about two hundred, and the workers from six to
nine. Many soldiers possess enormously large and
heavy heads, with massive jaws, and naturally,
with the appropriate muscles much enlarged.

But as workers and soldiers, on account of the
rudimentary state of their sexual organs, cannot
reproduce themselves, all the three or four kinds
of ants in the colony must be developed from eggs
deposited by the females. In this Weismann finds
the most convincing proof of the omnipotence of
natural selection, and, I venture to add, for the
omnipotence of his doctrine of determinants.

He says (Contemporary Review, vol. Ixiv.,
p. 313) : ' It fortunately happens that there are
animal forms which do not reproduce themselves,
but are always propagated anew by parents which
are unlike them. These animals, which thus
cannot transmit anything, have nevertheless varied
in the past, have suffered the loss of parts that
were useless, and have increased and altered
others ; and the metamorphoses have at times
been very important, demanding the variation of
many parts of the body, inasmuch as many parts

THEORY OF THE DEVELOPMENT OF ORGANISMS 127

must adjust themselves so as to be in harmony
with them.' 'None of these changes' (p. 318)
' can rest on the transmission of functional varia-
tions, as the workers do not at all, or only ex-
ceptionally, reproduce. They can thus only have
arisen by a selection of the parent ants, dependent
on the fact that those parents which produced the
best workers had always the best prospect of the
persistence of their colony. No other explanation
is conceivable, and it is just because no other ex-
planation is conceivable that it is necessary for us
to accept the principle of natural selection.'

According to Weismann's conception, ' every
part of the body of the ant' (Ioc. cit., p. 326) 'that
is differently formed in the males, females, and
workers is represented in the germplasm by three
(sometimes four) corresponding determinants ; but
on the development of an egg never more than
one of these attains to value — i.e., gives rise to the
part of the body that is represented — and the
others remain inactive.' This structure of the
germplasm Weismann attributes to the operation
of selection. ' For in the ant state ' (Ioc. cit., p. 326)
' the barren individuals or organs are metamor-
phosed only by the selection of the germplasm,
from which the whole state proceeds. In respect
of selection, the whole state behaves as a single
animal. The state is selected, not the single indi-
viduals, and the various forms behave exactly like
the parts of one individual in the course of ordinary
selection.'

Naturally, from the views on the germplasm

128 THE BIOLOGICAL PROBLEM OF TO-DAY

theory and on the doctrine of determinants that I
have expressed in this book, I cannot accept the
explanation Weismann thus gives of the facts. It
is true that Weisniann holds his own explanation
to be the only conceivable explanation. ' For there
are only two possible a priori explanations of
adaptations for the naturalist, namely, the trans-
mission of functional variations and natural
selection ' (loc. cit, p. 386) ; ' but as the first of these
can be excluded' (on account of the infertility
of workers and soldiers), 'only the second remains.'

But are the alternatives really only as Weis-
mann suggests ? Is there no choice left for the
naturalist ?

When I was reading his All-sufficiency of
Natural Selection, kindly sent me by the author,
it came into my mind that I could not accept his
dilemma. For the different individuals in the
insect states may be explained in a third way — in
a way overlooked by Weismann. This third ex-
planation is nothing more than the subject of all
this treatise of mine. It is that, in obedience to
different external influences, the same rudiments
may give rise to different adult structures.

I am glad that the same answer has been made
to Weismann's All-sufficiency of Natural Selection
by two biologists, Herbert Spencer and Emery,
simultaneously with mine. Emery, a specialist
upon the structure of ants, and Herbert Spencer,
relying upon the investigations of several English-
men, have sought to prove that the differences
between the individuals in the colonies of ants,

THEORY OF THE DEVELOPMENT OF ORGANISMS 129

bees, and termites, have been slowly called into
existence by the operation of external influences
affecting the egg in its situation and food during
development.

It has been shown fully by experiment and by
observation that the fertilised eggs of the queen
bee may become either workers or queens. This
depends merely on the cell in the hive in which
the egg is placed, and on what food the embryo is
reared. In the specially large cells, known as
queens' chambers, and with specially nutritious
diet, they become queens. With poor food, and
in smaller cells, they become workers. Even if
worker larvae be supplied in time with a richer
diet, they may be turned into queens.

Similarly, the differences that exist among
termites and ants, as Emery shows, may be de-
scribed as polymorphism due to food. The Italian
zoologist, Grassi, has shown that termites have it
in their power to alter the relative numbers of
workers and soldiers, and to produce as many of
the latter as may be required, and they are able to
accelerate the sexual maturity of other individuals
by supplying nourishment suitable for stimulating
the maturation of the genital organs.

Emery explains this polymorphism by attri-
buting it to the general laws of growth in the
insect organism under the influence of different
external stimuli. He thinks that ' the production
of workers depends upon a special capacity of the
germplasm to respond to the abundance or scanti-
ness of certain nutritive materials by a greater

9

130 THE BIOLOGICAL PROBLEM OF TO-DAY

growth of certain parts of the body, and a lesser
growth of other parts. Workers' food stimulates
growth in the jaws and brain, retards growth in
the wings and sexual cells. Queens' food has the
opposite action.' There is a correlation between
retardation of the sexual glands and acceleration
of the development of the head, just as in verte-
brates there is a correlation between the sexual
glands and the secondary sexual characters. ' The
characters by which the workers differ from the
queens, therefore, are not innate, but are produced
secondarily/

Quite independently, but simultaneously, Herbert
Spencer has suggested the same explanation as
Emery. Moreover, he has used the conditions that
exist among the state-forming insects as a strong
argument against Weismann's doctrine of deter-
minants. The observations of many careful persons,
such as Charles Darwin, Emery, and others, show
that in many species of ants the extreme types of
individuals are connected by many intermediate
forms. (Apud Emery, this is the case in many
Myrmicidce, in most Oamponotidce, and in Azteca.)
These forms are transitional, not only in general
size, but in the degree to which the genital organs
have been arrested, and in the peculiarities of
the jaws.

Spencer explains these transitional forms, and I
agree with him, by supposing that the stoppage in
food supply has taken place at different times after
development has begun. ('It must happen that
the stoppage of feeding will be indefinite/) Thus,

THEORY OF THE DEVELOPMENT OF ORGANISMS 131

the existence of transitional forms presents no
difficulty on the theory of the agency of food. But
how can the doctrine of determinants be applied
to it ? ' If he is consistent ' (says Spencer, Con-
temporary Revieiv, Ixiv., p. 901), ' he must say that
each of these intermediate forms of workers must
have its special set of " determinants," causing its
special set of modifications of organs ; for he cannot
assume that while perfect females and the extreme
types of workers have their different sets of deter-
minants, the intermediate types of workers have
not. Hence we are introduced to the strange con-
clusion that, besides the markedly distinguished
sets of determinants, there must be, to produce
these intermediate forms, many other sets slightly
distinguished from one another — a score or more

o

kinds of germplasm, in addition to the four chief
kinds. Next comes an introduction to the still
stranger conclusion, that these numerous kinds of
germplasm producing these numerous intermediate
forms are not simply needless, but injurious —
produce forms not well fitted for either of the
functions discharged by the extreme forms, the
implication being that natural selection has origin-
ated these disadvantageous forms. If, to escape
from this necessity for suicide, Professor Weismann
accepts the inference that the differences among
these numerous intermediate forms are caused by
arrested feeding of the larvae at different stages,
then he is bound to admit that the differences
between the extreme forms, and between these and
perfect females, are similarly caused. But if he

*» iw THE BIOLOGICAL PROBLEM OF TO-DA Y

V &

does this, what becomes of his hypothesis that the
''[several castes are constitutionally distinct, and
</ result from the operation of natural selection ?'
JjjtfS' My course of thought leaves me with little to
add to this criticism by Spencer. In this case, as
in many others that I have pointed out, Weismann
makes his usual mistake. He incorporates in the
rudiment what really are stimuli coming from
external conditions during the process of develop-
\ jt-ment; he makes a grave confusion between the

LI i rudiment and the conditions of its development.
r iy • . f , .

r . In my view, in these cases ot polymorphism in

'"the colonies of insects Nature exhibits a series of
most important experiments, and their plain
meaning is that the same germinal material, when
subjected to different external influences, may
roduce very different final products. .When from

?,

neutral germinal material of an insect egg

w „**„,.», is produced a male or female creature, or a
tr i/ worker or soldier (as this or that influence acts),

Jr JM .

XjyXf /.the process is no other, and presents no greater

fir( Difficulties, than when an experimenter, taking the

J/voung bud of a plant, according to the conditions

*• yA\io which he subjects it, can turn it into a vegeta-

W ^j/fcive or into a reproductive shoot, a thorn or a root ;

^ J* tio different to what occurs when the investigator,

L/cutting into a Cerianthus, produces a second or

})(* third mouth, surrounded by tentacles, or in the

J/p case of Clone surrounded by eye-spots.

tf'f '$ \ ^ ^as t>een shown, I think, in these pages that
n 4f^' Vmuch of what Weismann would explain by deter- ijf
^ niinants within the egg must have ° ^^^^ ^nfcirio / /

THEORY OF THE DEVELOPMENT OF ORGANISMS 133

the egg. The chief factors in the process of
development we have found to be : (1) The multi-
plication of cells by division (growth as a moulding
factor) ; (2) the relations of cells to their external
environment (position in its widest sense as a
factor); (3) the interrelations of the parts of a
whole (cells, tissues, and organs) to one another
and to the whole (correlative development). There
remains to be considered the extent to which the
germinal material in the egg determines the course
of development of the organism. Here, before all
things, it must be insisted that the individual
nature of the cell determines the specific fashion in
which the cell will react to the varying stimuli
coming from varying conditions. The same agency
produces very different results upon different
organisms. These differences must be attributed
to the differences in the nature (different intimate
structure) of the active material.

Sachs speaks strikingly on this point (Physiology
of Plants, p. 602) : ' If the same external cause
induces exactly opposite effects in the organs, the
explanation of this must simply be sought in the
different structure of the organs. If one organ,
when illuminated from one side, becomes curved
so as to be concave on the side turned towards the
centre of light, while another becomes convex on
that side, the cause can only lie in the internal
structure of the organ. But it is just on such
differences of structure that the great variety of
reactions which the most different plant organs
exhibit towards the same external influences

134 THE BIOLOGICAL PROBLEM OF TO-DAY

depends; and, fundamentally, all that we term
biology — the mode of life of organisms — depends
upon the fact that different organisms react dif-
ferently towards the same external influences, and
these reactions differ not only qualitatively, but
also quantitatively, the finest gradations existing in
both cases.'

For instance, in a plant-embryo roots are pro-
duced at the lower end under the influence of the
soil and of gravity. But it is upon the specific
nature of the protoplasm of different kinds of
plants that the special shape of the whole root
system depends : whether, for instance, the root
system ramifies superficially or strikes deep into
the soil; whether the rootlets grow quickly or
slowly ; in what fashion they fork, and whether or
no they form special structures like bulbs.

Thus, even from my point of view, explanation
of the process of development requires the as-
sumption of the existence of different kinds of
germinal material in different kinds of organisms.
These germinal substances must be possessed of an
extraordinarily complex organisation, and must be
able to react in specific fashion — that is to say, in
a fashion different in each species — to all the
slightest internal and external stimuli encountered
from time to time as the organisation becomes
formed by cell division.

In this sense I agree with what Naegeli says :

' The egg-cells contain all actual specific char-
acters as truly as the adult organisms ; when they
exist in the condition of eggs, organisms are as

THEORY OF THE DEVELOPMENT OF ORGANISMS 13»

distinct from each other as in the adult condition.
The species is present as truly in the fowl's egg as
in the fowl, and the egg of a fowl differs as much
from the egg of a frog as the fowl differs from the
frog. Men, rodents, ruminants, invertebrates display
more or less important and outwardly visible dif-
ferences in constitution ; so also the sexual cells to
which they give rise, since they represent the
rudiments of the future adults, must be different
from each other in the constitution of the rudi-
ments, although we are not yet able to prove these
differences by observation.'

In this assumption of a specific and highly-
organized germinal substance with which a develop-
ment begins, I agree with evolutionists ; but in its
details my conception is quite different from their
conception. For I can ascribe to the germinal
substance only such characters as are appropriate
to the true nature of a cell, but I cannot ascribe to
it the numerous characters that can come into
existence only by the interrelations of many cells
and the action of the environment.

Haacke, in his recently-published book (Gestal-
tung und Vererbung), has expressed a doubt that
my conception of development is, after all, a pre-
formational theory. 'For preformation,' he says,
' it is not necessary to imagine that the egg contains
a miniature of the adult. If only, like Hertwig,
one assumes to be present in the germinal material
a prearrangement of qualitatively different idio-
blasts, one has steered into the harbour of pre-
formation with all sails set.'

136 TEE BIOLOGICAL PROBLEM OF TO-DAY

In reply, I plead that, like Naegeli, De Vries,
Driesch, and others, I have tried to blend all that
is good in both theories. My theory may be called
evolutionary, because it assumes the existence of a
specific and highly-organised initial plasm as the
basis of the process of development. It may be
called epigenetic, because the rudiments grow and
become elaborated, from stage to stage, only in the
presence of numerous external conditions and
stimuli, beginning with the metabolic processes
preceding the first cleavage of the egg-cell, until
the final product of the development is as different
from the first rudiment as adult animals and plants
differ from their constituent cells.

To explain more clearly my conception of the
nature of the process of development, especially in
the relations that I conceive to exist between the
rudiment and the adult, I shall conclude by revert-
ing to my comparison between a human community
and an organism.

As a man arises from an egg-cell by cell
multiplication and cell differentiation, so the
human community, a composite organism of a
still higher nature, has arisen from separate human
beings as its starting-point.

Culture and civilization are the wonderfully com-
plicated results of the co-operation of many in-
dividuals united in society. By the manifolding
of their relations and their combinations, men in
society have brought about a higher complexity
than man, left by himself, ever would have been
able to develop from his own individual properties —

THEORY OF THE DEVELOPMENT OF ORGANISMS 137

a complexity that has arisen by the interaction of
the same characters of many men in co-operation.

Similarly the activity of the egg in growth
and cell-formation is an inexhaustible source of
new complexity ; for the self-multiplying systems
of units, always binding themselves into higher
complexes, continually enter into new interrelations,
and afford the opportunity for new combinations of
forces — in fact, of new characters.

Both cases — the course of the development of
the egg- cell into a man, and of men into a state —
depend upon epigenesis, not upon evolution.

The comparison may be carried into details.

The more complex and higher organisation of
human society occurs in this fashion : of the
numerous single individuals, all of which are
endowed with the various incipient human char-
acters, some individuals elaborate some incipient
characters, others other characters, and these come
to play correspondingly different parts. The special
differentiation undergone by any individual depends
upon the special place he comes to occupy in the
whole of which he is a part, not upon really
different organisation residing in him from his
birth. Beside those characters which have de-
veloped specially in his case, there lie dormant the
rudiments of all the characters possessed by men,
and, under different conditions, these might have
come to development.

Differentiation in multicellular organisms takes
a similar course. Every cell, by doubling division
of the egg, receives all the rudiments of its kind ;

138 THE BIOLOGICAL PROBLEM OF TO-DAY

of these rudiments, some in one set of cells, others
in another, come to develop, according to the part
of the whole in which the cells come to lie during
the progress of the development, and according to
the relations to the whole they come to assume.
Thus, here they assume the characters of the external
skin; there, they become gland-cells of the intestine ;
here, muscle-fibres; there, sense-cells or nerve-cells ;
in one place they serve the whole organism, in the
form of blood- corpuscles, as agents for nutrition
and respiration ; there, becoming connective tissue
or bone, they form skeletal elements of the body.

Thus, during the course of development, they are
forces external to the cells that bid them assume the
individual characters appropriate to their individual
relations to the whole ; the determining forces are
not within the cells, as the doctrine of determinants
supposes. The cells develop those characters that
are suggested by their relation to the external
world and their places in the whole organism.

But I must insist here that the subordination of
the cells to the whole organism, in both multicellular
animals and in plants, is much more complicated
than that of the units to the human state. In the
latter case, the individuals are separate from one
another; they are independent organisms and are
bound together only in social relations. None the
less, consider how in a civilized state the apparently
sovereign individual is conditioned in all his circum-
stances; how each change in the general state
exercises an influence on the individual's disposi-
tion freedom of will, and method of life (dwelling,

THEORY OF THE DEVELOPMENT OF ORGANISMS 139

food, institutions, health) ; then reflect how much
greater in the animal and the plant is the domina-
tion of the whole, and the subordination of the units,
as in them cell is directly joined to cell — indeed,
in most cases united materially by threads of proto-
plasm. In such cases the self-sufficiency of the
cell as an elementary, living organism is so far pre-
vented, that it becomes a subordinate part, with its
function in dependence on the whole.

One other point our comparison will make
clearer : I refer to the relation of the specific nature
of the rudiment to the specific nature of the pro-
duct of the rudiment.

The different organisations and qualities of the
communities formed by different animals may be
explained by the special characters of the animals
forming them. Those of the bee colonies depend
on the nature of bees ; of ant colonies on the
nature of ants; of the societies of men on the
nature of men ; indeed, in the latter case we see how
they differ as they are formed by Italians, Germans,
Slavs, Turks, Chinese, or Negroes. Similarly, the
specific organisation of the cell determines the kind
of animal which may be built up by it.

In my theory two assumptions of totally con-
trasting nature are made : I assume a germplasm
of high and specific organisation, and I assume that
this is transformed into the adult product by epige-
netic agencies. To a certain extent, therefore, I
reconcile the opposition between evolution and epi-
genesis, these opponents so prominent last century.

But my theory does not pretend to explain all

140 THE BIOLOGICAL PROBLEM OF TO-DA Y

the many problems involved in the course of organic
development. In this respect it differs from
Weismann's doctrine of determinants, as that
is a closed system, finding within itself a formal
explanation of all development. So far it seems to
me an abandonment of explanation rather than an
explanation ; for it explains by signs and tokens
that elude verification and experiment, and that
cannot encounter concrete investigation. His ex-
planation is no more than a description, in other
words, of the visible events of development. To be
more than this, it would be necessary to explain
how in each case the biophores and determinants
and ancestral plasms are constituted, and how they
are arranged in the architecture of the germplasm
so as to produce the development of the egg- cell in
this or that fashion. It must, at the least, offer
such possibilities as the structural formulae of
chemists offer. But in the present stage of our
knowledge Weismann's method is unpromising ;
it merely transfers to an invisible region the
solution of a problem that we are trying to solve,
at least partially, by investigation of visible char-
acters ; and in the invisible region it is impossible
to apply the methods of science. So, by its very
nature, it is barren to investigation, as there is no
means by which investigation may put it to the
proof. In this respect it is like its predecessor,
the theory of preformation of the eighteenth
century.

INDEX AND GLOSSARY

ACINETA, a group of protozoa,

development of, 41.
Acquired characters, question

of their inheritance, x.
Arnphioxus, a marine animal,

representative of the primi-

tive vertebrate stock, experi-

ments on eggs of, 61.
Anabolism, the formation of

more complex chemical

bodies by the agency of pro-

toplasm, 86.
Animal cells, characteristic

mode of growth, 111.
Antennularia, Loeb's experi-

ment, 117.

Ants, polymorphism in, 125.
Ascidians, tunicate animals, 46.
Atavism, the occurrence in an

organism of a character ab-

normal in it, but normal in

an ancestor, 24.

B

Bees, polymorphism in, 125.
Beetroot, grafting experiments,

Begonia, reproduction from
leaves, 46.

BERT, experiments on rats, 73.

BERESOWSKY, skin-grafting, 75.

BEYERINCK, upon galls, 51.

Biophores. Each determinant,
according to Weismann, is
composed of a number of
ultimate living pieces, the

biophores, which are the
active agents that direct the
functions of a mature cell,
ix, 22.

Blastosphere, an early stage in
embryonic development ; the
embryo consists of a hollow
sphere, the walls of which
consist of a single layer of
cells, and the cavity of which
is called the segmentation
cavity, xvii ; explanation of
formation, 97, 98.

Blood, transfusion of, 75.

BLUMENBACH, nisus forma-
tivus, 5 ; upon galls, 50.

Bone-grafting, 73, 74.

Bonelfia, sexual dimorphism in,
122.

Bryozoa, a group of minute
animals which form encrus-
tations on seaweeds and
stones, 46.

Buds, origin of, 28 ; reproduc-
tion and regeneration by, 46.

C

Cell, description of, 31 ; charac-
ters possible in, 88 ; differen-
tiation of, in development,
112 ; as units in morphology
and physiology, 113 ; Sachs
on, 114; Vochtiiig on, 114,
116.

Cell theory, relation of, to
heredity, 31.

Centrosome, an organ of cells

142

INDEX AND GLOSSARY

most obvious during nuclear
division, 93.

Cerianthus, experimental het-
erornorphoses, 51.

CHABRY, destruction of segmen-
tation sphere, 62.

Chromatin, a material found
in the nucleus of cells, so
called because it absorbs
stains with avidity : germ-
plasm and, viii, xiv ; relation
of, to specific character of
cells, 36, 37.

Chromosomes, definite, visible
bodies, as which the chroma-
tin of a dividing nucleus
appears, xiv, 93.

Crystal, growth of, compared
with organic growth, 108.

Cione, experimental hetero-
morphoses, 52.

Clavelliiia, reproduction from
buds, 46.

Cleavage - planes, the planes
separating the daughter-
nuclei, or daughter-cells, in
the early division of a fer-
tilised egg-cell, xvii ; relation
between appearance of, and
structure of eggs, 95.

Coelenterata, a major division
of multicellular animals, in-
cluding such creatures as
sea-anemones, corals, and
jelly-fish, 46.

Continuity of the germplasm,
26.

Continuity of life, the doctrine
opposed to spontaneous gene-
ration, 2.

Correlations, 118, 121.

D

DARWIN, pangenesis, 21.
Determinants. Each id of

germplasm is supposed by
Weismann to be composed
of minor pieces, arranged in
a complicated fashion that
is the result of the past
history of the species. For
every part of the body, large
or small, that may be dif-
ferent in different individuals
or species, there is, at least,
one determinant in the id.
The determinants are so
grouped in the id that they
are liberated and become
active when the time comes
for the development of that
part of the body they con-
trol, viii, 22 ; arguments
against, 82; relation to cells,
87.

Determinates, the smallest
parts of an organism which
vary independently, and
which are supposed by Weis-
mann to be represented in
the germplasm by special
pieces, 23, 25.

Differentiating division, such a
division of the nucleus as
would result in daughter-
nuclei unlike each other, and
unlike the parent nucleus.
The qualities of the parent
nucleus are supposed to have
been distributed between the
daughter-nuclei, xi ; absence
of visible evidence for, xv,
25 ; objections to occurrence
of, 34, 78.

Dimorphism, the appearance
of the same species in two
different forms, sexual di-
morphism, 122, 124.

Disharmonic union in graft-
ing, 70.

INDEX AND GLOSSARY

143

Double monsters, as examples
of heteromorphosis, 63.

Doubling division. When an
amoeba reproduces by simple
division, the daughter-amoe-
bae are identical, and each
is identical with the parent
except in size ; from one
amoeba two have been
formed. A doubling division
of the nucleus is such as
would result in the forma-
tion of two nuclei alike in
every respect, ix ; visible
evidence for, xv, 24; in
unicellular organisms, 40 ;
occurrence of, with differen-
tiating division, 78.

DRIESCH, experiments on eggs,
54 ; separation of segmenta-
tion spheres, 60.

E.

Echinoderms, a group of marine
animals, of which the star-
fish is the most familiar type,
eggs of, 54.

Echinoidea, a group of echino-
derms, 61.

Ectoderm, the tissue in an
adult derived from the epi-
blast (which see), 19.

Egg, relation between structure
and division of, 94 ; specific
character of, 135.

EMERY, on polymorphism in
ants, 128.

Endoderm, the tissue in an
adult, derived from the
hypoblast (which see), 19.

Enfoldment. See Evolution.

Epiblast. In the development
of all multicellular animals,
the young embryo soon be-
comes divided into two sets

of cells, the epiblast and
hypoblast ; where a gastrula
is formed, the outer layer of
cells is the epiblast, the inner
layer the hypoblast, xviii.

Epigenesis, the doctrine that
the formation of a new indi-
vidual is not the mere out-
growing of particles hidden
in the egg-cell, but the result
of moulding external forces,
xiii ; Roux's definition of, 7 ;
Weismann's denial of, 9 ;
epigenetic explanation of
stages in development, 98 ;
summary of Hertwig's ac-
ceptance of, 136.

Evolution. Originally the
term was applied, not to the
origin of existing forms of
life from common ances-
tors, but to the doctrine that
every living creature con-
tained within it the whole
series of its future descend-
ants, and that the growth of
a living creature was evolv-
ing of one of these enfolded
miniatures, xiii, 1, 2, 3 ;
Eoux's contrast of, with epi-
genesis, 6 ; the new evolu-
tion, 10 ; Hertwig's partial
agreement with, 135, 136.

Experiment, Weismann's cau-
tion against, 10.

F.

Fertilisation, the union of the
nuclear matter of a male cell
with the nuclear matter of a
female cell, xii, xiv.

Foraminifera, a group of pro-
tozoa provided with shells,
44.

FOREL, on eyes of ants, 126.

144

INDEX AND GLOSSARY

Frogs' eggs, Hertwig's experi-
ments upon ; development of,
under compression, 57-60.

Funaria, reproduction from
chopped pieces, 46.

G.

Galls, 50.

Gastrula, an early embryonic
stage, most simply formed
from the blastosphere by the
invagination of one side of
the wall, and consisting of
a hollow sac, the walls of
which are formed by two
layers of cells, xviii, 60 ; for-
mation of, 99.

Gernmules. See Pangenesis.

Germ, the youngest embryonic
stage of an individual or
organ, 10.

Germplasm, the substance sup-
posed to be the material
bearer of inherited qualities :
Weismann's conception of,
viii, 20; identification of,
with nuclear matter, 21 ;
account of Weissmann's
theory, 21-28.

Germ-tracks, the hypothetical
paths along which germ-
plasm passes in an un-
altered condition during
development, 27 ; objections
to, 81.

GOBBEL, on plasticity of plants,
120.

Grafting, 68, 70 ; of Hydra, 72 ;
bone -grafting, 73, 74 ; skin-
grafting, 74, 120, 121.

GRASSI, polymorphism due to
food, 129.

Gregarines, a group of parasitic
protozoa, development of,
41.

H.

HAACKE, declaration that Hert-
wig is evolutionary, 135.

Haemoglobin, the red colour-
ing matter of blood, 75.

Harmonic union in grafting,
70.

Heteromorphosis, explanation
of, 49 ; cases of, 51, 52 ;
embryonic cases, 54.

His, presence of foci in the
germ, 13.

Histogenous, producing micro-
scopical characters, 20.

Histology, study of the micro-
scopical characters of cells
and tissues, differentiation,
115.

Hydatina, determination of
sex, 5 ; temperature, 123.

Hydra, regeneration in, 47 ;
grafting of, 72.

HydroniedusEe, a group of in-
vertebrate animals, the
typical members of which
are branched colonies of
polyps : Weismann's in-
vestigations on, viii, xii.

Hypoblast. See Epiblast, xvi.

Hypotrichous infusoria, a group
of protozoa, 41.

Ids, hypothetical individual
pieces, a number of which
are supposed by Weismann to
be present in the germplasrn
of every sexual cefi, and each
of which is supposed to con-
tain the inherited material
necessary for a complete new
organism. It has been sug-
gested that tiny beads seen
within the chromosomes of

INDEX AND GLOSSARY

145

a sexual cell are the ids, viii,
23, 33.

Idioblasts, Hertwig's name for
hypothetical ultimate units
of living matter, 22, 32 ; the
ultimate units of living
matter, according to De
Vries, 22.

Idioplasm, as opposed to germ-
plasm, which is the nuclear
material of gerin-cells ; idio-
plasm is the nuclear material
of tissue-cells, xi, 38.

Immortality, definition of, 82 ;
of germ-cells, ix ; of uni-
cellular organisms, 17 ; of
germ-cells, 80.

Individuality of cells, 115.

Invagination, the infolding of
a layer of cells, as, for
instance, in the transforma-
tion of a blastosphere into a
gastrula, xvii.

Isotropism, explained in foot-
note, 33.

K

Karyokinesis, a complicated
process of nuclear division,
xiv.

Katabolism, the formation of
less complex chemical bodies
by the agency of protoplasm,
86.

Labile, unstable, constantly
changing, 38.

LANDOIS, experiments on trans-
fusion of blood, 75.

LEIBNITZ, on immortality, 82.

LOEB, on heteromorphoses, 49 ;
on plasticity of animals, 117.

M

MAUPAS, experiments on sex
of rotifers, 123.

Melons, determination of sex
by temperature, 124.

Mesoblast, in the development
of the cceloniata, or three-
layered multicellular ani-
mals ; a third set of cells,
themesoblast, arises between
the epiblast and hypoblast,
xviii.

Monsters, relation of, to divi-
sion of egg-cell, 63.

Mosaic theory of Eoux, 56.

Morphoplasrn, the general
protoplasm of a cell, 35.

Multicellular organisms, those
in which the body is com-
posed of many cells, spe-
cialized in different direc-
tions ; cell- division in, 43.

Mus, experiments on grafting
among mice and rats, 74.

Myxomycetes, sometimes
called ' slime fungi,' a group
of low organisms, consisting
of creeping masses of proto-
plasm with many nuclei,
33.

N

NAEGELI, biological units, 30 ;
cross-fertilization and graft-
ing compared, 69 ; heredity,
92 ; environment in develop-
ment, 104 ; on plasticity of
plants, 119 ; on specific
characters of eggs, 134.

Nais, regeneration in, 47.

Notochord, formation of, from
unusual cells, 117.

Nucleus, a specialized portion
of the protoplasm of cells,
different in chemical and

10

146

INDEX AND GLOSSARY

physical properties (see
Chromatin, Chromosomes)
as the bearer of heredity, 19

NUSSBAUM, views on origin o
germ- cells, 17.

Nutrition, influence of, on de-
velopment, 2.

O

OLLIER, bone-grafting, 73.

Ontogeny, the development of
an individual from the egg
upwards, 9.

Osteoblasts, cells which are the
active agents in bone-forma-
tion, 73.

Ovogenesis, the formation of
egg-cells in the ovary, 13.

Pangenesis, Darwin's provi-
sional hypothesis, that the
sexual cells were composed
of minute particles (gem-
mules), given off by all the
cells of the body, 21.

Periosteum, a cellular sheath
of bones, 73.

Physiological units, Herbert
Spencer's name for hypo-
thetical ultimate units of
living matter, 22.

Pistachio, influence of tem-
perature on, 121.

Plant-cells, mode of growth,
110.

Plasomes, Hertwig's name for
theoretical units of proto-
plasm, 32.

Plasticity of plant tissues, 117,
119, 120.

Pluteus, a free - swimming
larval stage in the develop-
ment of echinoderms, 54.

Podophrya, reproduction of, 41.

Polymorphism, the appearance
of the same species in several
different forms in ants and
social insects, 125.

PONFICK, on transfusion of
blood, 75.

Preformation, identical with
the original meaning of evo-
lution, which see.

Prothallus, the leaf - shaped
green organism that grows
from the spore of a fern and
produces sexual organs, 49.

Pseudopodia, extensions of pro-
toplasm beyond the general
contour of the cell, 41.

R

Radiolaria, a group of pro-
tozoa, 44.

Regeneration in plants and
animals, 45, 47.

Ehipsalis grafted on Opuntia,
71.

Roux, contrast between epi-
genesis and evolution, 6 ;
mosaic theory of, 56.

Rudiment, used here as a
translation for the word
anlage, which means the
first plotting-out or begin-
ning of a living structure.
Darwin showed that rudi-
mentary organs in adult
creatures were for the most
part vestiges of organs that
had lost their use. In this
treatise ' rudiment ' is ap-
plied to an organ or struc-
ture in its incipient condi-
tion, whether that incipient
state be visible in a young
embryo, or a hypothetical
structure in the germplasm,
6 ; latent rudiments, 37.

INDEX AND GLOSSARY

147

S

SACHS, on cells, 114 ; on re-
action and protoplasm, 133.

Salix purpurea, reproduction
from galls, 51.

SCHMITT, bone-grafting, 74.

Segmentation, the early divi-
sion of a developing egg,
xvii.

Segmentation spheres, the cells
resulting from the early di-
visions of a developing egg

separation of, by Wilson and
Driesch, 60.
Segmentation cavity. See

Blastosphere.
Sex, determination of, by

temperature, 123, 124.
Sexual cells (spermatozoa in
male, ova or egg-cells in
female), the nucleated pieces
of protoplasm which are the
starting-point of the new
generation in sexual repro-
duction, origin of, 18.
Soma, the body of a plant or
animal as contrasted with
the reproductive cells con-
tained within it, 45.
Somatic cells, the cells of the

soma ; mortality of, 17.
SPENCER, HERBERT, contro-
versy withWeismann on poly-
morphism in insects, 125.
Spermatogenesis,the formation
of spermatozoa in the testis,
13.

Spontaneous generation, 2.
Stolon, a strand of tissue con-
necting the individuals of
colonial animals, 46.
STRASBURGER, the value of the
nucleus in heredity, 13, 18.

T

Termites, polymorphism in,
125.

Transfusion of blood, 75.

Transplantation of bone, 73,
74.

TREMBLEY, grafting of Hydra,
72.

Triton, an amphibian, experi-
ments on the egg by con-
striction, 64.

Tubularia, experimental hetero-
morphoses, 51.

Tuiiicata, a group of marine
animals clad with a leathery
tunic, 14.

U

Unicellular organisms, animals
(protozoa) and plants (proto-
phyta) with the simplest
structure, each being a single
ceU : immortality of, 17 ;
division doubling in, 40.

Unit, definition of a biological,
30.

Vegetative affinity, 66 et seq.

Vertebrates, regeneration of
lost parts, 47.

VOECHTIXG, experiments on
grafting, 70 ; harmonic and
disharmoiiic union, 70 ; on
cells, 114, 116 ; on plasticity
of plants, 117, 119 ; on
grafting, 120.

W

WEISMANN and preforrnation,
8-10 ; caution against experi-
ment, 12 ; sources of his
theory, 20, 21; Hertwig'
description of his theory, 22

148

INDEX AND GLOSSARY

absence of proof for differ-
entiating division, 34 ; sym-
metry of egg and adult, 55 ;
immortality of germ-cells,
17, 80, 82 ; germ-tracks, 83 ;
doubling division, 102 ; con-
troversy with Spencer, 125.

Willow, reproduction from
slips, 46.

WILSON, separation of seg-

mentation spheres of am-

phioxus egg, 60.
WOLFF, Theoria Generationis ,

4.
Wounds, healing of, in relation

to idioplasm, xii.

Yolk, nutritive material stored
in an egg- cell, xvi.

THE END.

BILLING AND SONS, PRINTERS, GUILDFORD.

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