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
distinction 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 grows and the cells for the maintenance of the 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 important 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, and
the superfluous 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 protoplasm, 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
offspring; that, in fact, the nucleus was the material basis of heredity.
Weismann laid hold of this idea, but transmuted it to fit in with his
original theory of the germplasm. 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 segregation 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 nuclearplasm 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 nervous system, and so forth, until
there are separated the germs for all the separate organs, and for the
production of the minutest histological differentiation.'
Weismann calls the diverging nuclearplasms into which the primitive
germplasm is gradually transformed _histogenous_, because they determine
the specific characters of the tissues. He assumes that the primitive,
original germplasm has a most complicated 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 molecular 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 unaltered germplasm the power of giving rise to complete new
individuals, while cells with histogenous 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 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
overcome the histogenous plasm of these cells. In this respect Weismann has
amended his original proposition that the germ-cells were immortal, like
unicellular organisms. In a strict and literal interpretation 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 Vries, 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 Vries, 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
modified. Let me give the most important parts of his conception.
The substance which is the bearer of the hereditary character of a species
(the idioplasm of 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 germplasm, so altering the previous
connotation of the word. The germplasm of every species has an extremely
complicated, 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 Vries' 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 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 germplasm. And so the
germplasm of a species must possess as many determinants, or guiding
particles, as there are in the organism cells or cell-groups that are
independently variable in the germ or in later stages (hereditary pieces or
determinates).
As every cell or group of cells which corresponds to determinants has a
definite position in the body, Weismann infers that the determinants are
definitely 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
individual of a species, and correspond to what Weismann 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 which I shall not discuss
here, regards the germplasm as being still more complicated, and consisting
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 architecture so complicated,
actually produce the development of the adult from the egg? The natural
mechanism for this purpose is cell division and nuclear division.
According to Weismann's supposition--a supposition 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 Weismann'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 combinations of
the determinants are included in the daughter-ids. This method of division
of the germplasm plays the chief part in the transformation of the egg into
the adult. It has to take place so that the numberless determinants, or
guiding particles, of the germplasm may be disentangled and brought forward
at the time and 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, contain more and more few
different kinds of determinants. 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 condition 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,[7] 'is a wonderfully
complicated process; it is a true "development," in which the idic stages
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 segmentation, and the formation of its organs, and even the size of
these organs--determined by the number of cells composing them--depend upon
this complicated 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 Weismann has the following
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 reproductive cells of the organism
must contain undisintegrated, perfect germplasm, it follows that the
germplasm 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 into the determinants which
control the course of the ontogeny, and decide the final characters of the
cells. Another set of ids remains undisintegrated, 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
described as accessory germplasm (_Nebenkeimplasma_). The active ids are
his explanation of the embryonic events, which they direct; the accessory
germplasm 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 germplasm, serve for the preservation of the species and
are immortal; the other cells, since, from the disintegration 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 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 regeneration, 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 contain, 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 condition, to become active
when the conditions for their growth are supplied by the loss of the parts
they can replace.
CRITICISM OF THE GERMPLASM THEORY.[8]
At first sight, much of Weismann'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
generations, 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 Vries and others, and I consider that the course taken by
Naegeli has made his position untenable.
Naegeli would make his theory of the idioplasm 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),
micellae, 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.'
Although, 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
literature 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 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 conceptions.
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 pangenesis 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 particles that may be divided
into two groups, organised and unorganised. The latter are free, or in
solution; they are such as albuminates, fats, carbo-hydrates, water,
salts, and they serve as material 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 absorption of different material, may transform themselves
into the plasomes, just as these last, by a similar process, may produce
the plasma-products. In my view, the nucleus is the bearer of the idioplasm
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.[9]
A mass of protoplasm with several nuclei (like the myxomycetes,
coeloblasts, etc.) has the morphological 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 associated with complicated
changes of the nuclear contents, appears essentially to consist of the
following: The elementary units of the cell (centrosomes, chromatin bodies
in the case of nuclear division), being endowed with special energy
resulting from the processes of growth, divide, and the elementary products
of division separate into two groups, which move from the middle line;
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 important bases of Weismann's germplasm theory.
For convenience of exposition these may be divided into two groups:
Objections to the hypothesis of differentiating division; objections to
Weismann's doctrine of determinants.
I. OBJECTIONS TO THE HYPOTHESIS OF DIFFERENTIATING DIVISION.
A corner-stone of Weismann's theory is his assumption of nuclear divisions
which are differentiating. 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 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 idioplasm for regular and spontaneous change)
'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 dissimilar as regards their hereditary value, although
they are all young and embryonic, and are not infrequently quite similar in
appearance. It therefore 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 considering 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 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 differentiation of the body is due to all the determinants
except _one_ particular one remaining dormant in a regular order, and that,
apart from special adaptations, 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
alternatives.
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 characters of their protoplasm, the possession of
numerous latent rudiments which become active when opportunity presents
itself.
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 consequence 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,[10]
'cannot _become_ different in the cells of the fully formed organism; the
differences in the chromatin controlling the cells must begin with the
development of the egg-cell and must increase as development 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 himself that[11] '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 involves 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, unchanged 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 unequal 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 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 structure is a complex, but a perfectly
definite one, which, originating in the _id_ of germplasm, is transferred
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 development.
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 phenomena of generation and regeneration; (4) alteration
of structural growth due to external interferences (heteromorphosis); (5) A
number of physiological indications that cells and tissues, in addition to
their patent characters, contain latent characters which have reached them
by doubling division, and which are representative of the species.
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 confirmation all through the study of
systematic and embryological natural history, would disappear, were it
possible that in the division of unicellular organisms 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 himself 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 proposition 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 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 substances,
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 following 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 produce 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, (pseudonavicellae), which afterwards are set free as
small, sickle-shaped embryos. These exceedingly 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 organisms, are a weighty indication
of the error of concluding, with Weismann, 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 conclusion, 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. Otherwise, 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.
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 appearance 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 investigation.
Here, in a completeness so wonderful that our intelligence can hardly
apprehend it, are presented 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 algae, fungi, and other simple creatures. In them cells arise by
division from the egg or from the spore, and become united into an
individual 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 rudiments of the whole of which each is a part.
In this category there naturally fall the multinucleated 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 (_Myxomycetes_), with their peculiar formation
of reproductive 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
multinucleated _Foraminifera_ and _Radiolarians_. For, according 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 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 contradictory are the interpretations
in this matter. Enlarging upon them may be postponed at present, for we are
concerned here not with the interpretation 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 improbable 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 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
coelenterates, worms, and tunicates. The polyps of hydroids and of
bryozoa, the stolons of an ascidian (_Clavellina lepadiformis_), may give
rise to buds in many places, and these grow up into the perfect hydroid,
bryozoon, or ascidian. 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 reproduction 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 solution 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 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 concerned. 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.[12]
Of all the facts brought forward here, the phenomena of heteromorphosis
perhaps bear most strongly in favour of my conception, and offer
difficulties most irreconcilable with Weismann'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.
Regeneration is the reproduction of parts like those lost; heteromorphosis
is the reproduction of parts unlike those lost.
Heteromorphoses are well known in plant physiology. 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 order, by artificially shading the upper
surface, and strongly illuminating the lower surface.
Among the most interesting heteromorphoses 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 stimulation 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
systematically.
Blumenbach has already brought forward the existence of galls as an
argument against preformation, holding them to be structures produced
epigenetically, and, therefore, unrepresented by rudiments in the germ. I,
also, consider them witnesses against Weismann's germplasm. 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 pathological conditions,
retains the power of producing 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 experimentally upon many lower animals,
among which were _Tubularia_, _Cerianthus_, and _Cione intestinalis_.
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 horizontally 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 prevented coalescent growth. These buds
gave rise to inner and outer tentacles, and an oral disc was produced.
Thus, artificially, an animal with two mouth-openings or two heads was
produced; and, similarly, animals with a row of three or more heads may be
produced.
[Illustration: FIG. 1.--CERIANTHUS MEMBRANACEUS, in which a second oral
aperture, surrounded by tentacles, has appeared as the result of an
artificial slit. (_After Loeb._)]
[Illustration: FIG. 2.--CIONE INTESTINALIS, in which eye-specks resembling
those surrounding the mouth have appeared in the neighbourhood of an
artificial opening (_a_).]
The third animal in which heteromorphosis was produced artificially was
_Cione intestinalis_, a solitary ascidian, an animal more highly organized.
In _Cione_ (Fig. 2) the edges of the mouth-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 exhalent tube; after a time
eye-spots appeared round the edges of the cut; then the margin of the
artificial 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 simultaneously.'
In the three cases, the cut surfaces, from which in _Tubularia_, a head, in
_Cerianthus_, tentacles, and in _Cione_, eye-spots, took their origin, were
made in different parts of the bodies and in different directions. 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
abnormal 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 development 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 fertilised 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 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 developmental 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 specification 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 interest. The frog's egg has the poles different, and so has
a definite orientation. Weismann and Roux 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, it follows that the _id_ of germplasm itself possesses a bilateral
structure, and that it also divides so as to give rise to the determinants
of the right 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.'
Roux[13] has based his mosaic theory upon experiments 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 broken up into
unlike halves, by which the development of the corresponding cells is
directed diversely, _i.e._, is determined in a specific fashion.
[Illustration: 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 developing eggs. B. Eggs developing under compression by
horizontal plates. C. Eggs developing under compression by vertical
plates.]
The error in these representations of Weismann and of Roux has been shown
by varied experiments 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 dorsoventral
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 distribution 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.[14]
The diagrams show the positions of the segmentation 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 distinguished 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
------------
/ \
3 4
------ ------
/ \ / \
7 8 9 10
--- --- --- ---
/ \ / \ / \ / \
15 16 17 18 19 20 21 22
2
------------
/ \
5 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 A1 and B1 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 agreement 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 B2
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.
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 multiplication 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
protoplasmic 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 protoplasm 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 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 subsequent
development of these separated segmentation spheres.
[Illustration: FIG. 4.--NORMAL AND FRACTIONAL GASTRULAE AMPHIOXUS.
(_After Wilson._)
A Gastrula from a whole egg; B, C and D, gastrulae 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.]