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Heinemann's Scientific Handbooks
THE BIOLOGICAL PROBLEM OF TO-DAY
HERTWIG
Heinemann's Scientific Handbooks
THE BIOLOGICAL PROBLEM OF TO-DAY
_PREFORMATION OR EPIGENESIS? THE BASIS OF A THEORY OF ORGANIC DEVELOPMENT_
BY
PROFESSOR DR. OSCAR HERTWIG DIRECTOR OF THE SECOND ANATOMICAL INSTITUTE OF
THE UNIVERSITY OF BERLIN
Authorized Translation
BY
P. CHALMERS MITCHELL, M.A.
_WITH AN INTRODUCTION BY THE TRANSLATOR AND A GLOSSARY OF THE TECHNICAL
TERMS_
LONDON
WILLIAM HEINEMANN
1896
[_All rights reserved_]
PREFACE
Shortly after the appearance of Dr. Oscar Hertwig's treatise 'Praeformation
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 desirable that a full translation should appear. By the kindness of Dr.
Hertwig and his German publisher, this is now possible. I have prefixed an
introduction, written for those who are interested in the general problem,
but who have little acquaintance 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.
TRANSLATOR'S 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 observation. Setting out from
a laborious and beautiful series of investigations into the anatomy of the
Hydromedusae, 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 investigations in the many directions that his conception
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 tentative.
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 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 reproduction one parent is enough; in sexual
reproduction equal masses of germplasm from each parent combine to form the
new individual. The germplasm resides in the nucleus of cells, and Weismann
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
alteration of structure, when it has access to appropriate food. But it is
a living material much more complex 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
_determinant_ consists of a number of ultimate particles 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 development 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 protoplasm 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 continuously, 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 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
generation, 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 connected the equally celebrated denial
of the inheritance 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 on 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 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 disintegrated,
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 determined by the historic
architecture of the microcosms, 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 determinants. 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 undisintegrated 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 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 Hydromedusae,
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 associated 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 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 determinants 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 different 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, even to the extent that in
this treatise Hertwig himself 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_. Reconciliation 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. 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. Throughout 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 definite 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 similarity of these processes among all living
creatures, vegetable and animal, and their extreme complication, 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.
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 subsequent 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 distorted, 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
marshalling of cells common among invertebrates, and to be traced under the
complications of higher forms. 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 contained air to escape. Further cell-divisions
occur, and the invagination becomes deeper, until the invaginating 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, originally 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 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 vertebrates, 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 reproductive cells arise from a set of cells known as the mesoblast,
that are formed chiefly from the hypoblast, 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 substance 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 reference 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 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