author recognizes the truth of the fundamental law of development, and
explains the stages of ontogeny as an abbreviated recapitulation of
phyletic stages, he nevertheless explains the latter by a different
principle from that which he employs to explain the former. According
to Nägeli, the stages of phylogeny are based upon true qualitative
differences in the idioplasm: the germ-plasm of a worm is qualitatively
different from that of _Amphioxus_, a frog, or a mammal. But if such
phyletic stages occur crowded together in the ontogeny of a single
species, they are said to be based upon different ‘conditions of
tension and movement’ of one and the same idioplasm! It seems to me to
be necessary to conclude that if the idioplasm, in the course of
phyletic development, undergoes any alteration in specific
constitution, such alterations must also take place in ontogeny; so far
at least as the phyletic stages are repeated. Either the whole phyletic
development is based upon different ‘conditions of tension and
movement,’ or if this—as I believe—is impossible, the stages of
ontogeny must be based upon qualitative alterations in the idioplasm.
Involuntarily the question arises—how is it that such an acute thinker
fails to perceive this contradiction? But the answer is not far to
seek, and Nägeli himself indicates it when he adds these words to the
sentence quoted above: ‘It follows therefore that if a cell is detached
as a germ-cell in any stage of ontogenetic development, and from any
part of the organism, such a cell will contain all the hereditary
tendencies of the parent individual.’ In other words, if we are
restricted to different ‘conditions of tension and movement’ as an
explanation, it seems to follow as a matter of course that the
idioplasm can re-assume its original condition, and therefore that the
idioplasm of any cell in the body can again become the idioplasm of the
germ-cell; for this to take place it is only necessary that the greater
tension should become the less, or _vice versa_. But if we admit a real
change in constitution, then the backward development of the idioplasm
of the cells of the body into germ-cells appears to be very far from a
matter of course, and he who assumes it must bring forward weighty
reasons. Nägeli does not produce such reasons, but considers the
metamorphosis of the idioplasm in ontogeny as mere differences in the
‘conditions of tension and movement.’ This phrase covers the weak part
of his theory; and I look upon it as a valuable proof that Nägeli has
also felt that the phenomena of heredity can only find their
explanation in the hypothesis of the continuity of the germ-plasm; for
his phrase is only capable of obscuring the question as to how the
idioplasm of the cells of the body can be re-transformed into the
idioplasm of germ-cells.
I am of the opinion that the idioplasm cannot be re-transformed, and
I have defended this opinion for some years past[118], although I
have hitherto laid especial stress on the positive aspect of the
question, viz. on the continuity of the germ-plasm. I have attempted
to prove that the germ-cells of an organism derive their essential
nature from the fact that the germ-plasm of each generation is
carried over into that which succeeds it; and I have tried to show
that during the development of an egg into an animal, a part of the
germ-substance—although only a minute part—passes over unchanged into
the organism which is undergoing development, and that this part
represents the basis from which future germ-cells arise. In this way
it is to a certain extent possible to conceive how it is that the
complex molecular structure of the germ-plasm can be retained
unchanged, even in its most minute details, through a long series of
generations.
But how would this be possible if the germ-plasm were formed anew in
each individual by the transformation of somatic idioplasm? And yet if
we reject the ‘continuity of the germ-plasm’ we are compelled to adopt
this latter hypothesis concerning its origin. It is the hypothesis
adopted by Strasburger, and we have therefore to consider how the
subject presents itself from his point of view.
I entirely agree with Strasburger when he says, ‘The specific qualities
of organisms are based upon nuclei’; and I further agree with him in
many of his ideas as to the relation between the nucleus and cell-body:
‘Molecular stimuli proceed from the nucleus into the surrounding
cytoplasm; stimuli which, on the one hand, control the phenomena of
assimilation in the cell, and, on the other hand, give to the growth of
the cytoplasm, which depends upon nutrition, a certain character
peculiar to the species.’ ‘The nutritive cytoplasm assimilates, while
the nucleus controls the assimilation, and hence the substances
assimilated possess a certain constitution and nourish in a certain
manner the cyto-idioplasm and the nuclear idioplasm. In this way the
cytoplasm takes part in the phenomena of construction, upon which the
specific form of the organism depends. This constructive activity of
the cyto-idioplasm depends upon the regulative influence of the
nuclei.’ The nuclei therefore ‘determine the specific direction in
which an organism developes.’
The opinion—derived from the recent study of the phenomena of
fertilization—that the nucleus impresses its specific character upon
the cell, has received conclusive and important confirmation in the
experiments upon the regeneration of Infusoria, conducted
simultaneously by M. Nussbaum[119] at Bonn, and by A. Gruber[120] at
Freiburg. Nussbaum’s statement that an artificially separated portion
of a _Paramaecium_, which does not contain any nuclear substance,
immediately dies, must not be accepted as of general application, for
Gruber has kept similar fragments of other Infusoria alive for several
days. Moreover, Gruber had previously shown that individual Protozoa
occur, which live in a normal manner, and are yet without a nucleus,
although this structure is present in other individuals of the same
species. But the meaning of the nucleus is made clear by the fact,
published by Gruber, that such artificially separated fragments of
Infusoria are incapable of regeneration, while on the other hand those
fragments which contain nuclei always regenerate. It is therefore only
under the influence of the nucleus that the cell substance re-developes
into the full type of the species. In adopting the view that the
nucleus is the factor which determines the specific nature of the cell,
we stand on a firm foundation upon which we can build with security.
If therefore the first segmentation nucleus contains, in its molecular
structure, the whole of the inherited tendencies of development, it
must follow that during segmentation and subsequent cell-division, the
nucleoplasm will enter upon definite and varied changes which must
cause the differences appearing in the cells which are produced; for
identical cell-bodies depend, _ceteris paribus_, upon identical
nucleoplasm, and conversely different cells depend upon differences in
the nucleoplasm. The fact that the embryo grows more strongly in one
direction than in another, that its cell-layers are of different nature
and are ultimately differentiated into various organs and
tissues,—forces us to accept the conclusion that the nuclear substance
has also been changed in nature, and that such changes take place
during ontogenetic development in a regular and definite manner. This
view is also held by Strasburger, and it must be the opinion of all who
seek to derive the development of inherited tendencies from the
molecular structure of the germ-plasm, instead of from preformed
gemmules.
We are thus led to the important question as to the forces by which the
determining substance or nucleoplasm is changed, and as to the manner
in which it changes during the course of ontogeny, and on the answer to
this question our further conclusions must depend. The simplest
hypothesis would be to suppose that, at each division of the nucleus,
its specific substance divides into two halves of unequal quality, so
that the cell-bodies would also be transformed; for we have seen that
the character of a cell is determined by that of its nucleus. Thus in
any Metazoon the first two segmentation spheres would be transformed in
such a manner that one only contained the hereditary tendencies of the
endoderm and the other those of the ectoderm, and therefore, at a later
stage, the cells of the endoderm would arise from the one and those of
the ectoderm from the other; and this is actually known to occur. In
the course of further division the nucleoplasm of the first ectoderm
cell would again divide unequally, e.g. into the nucleoplasm containing
the hereditary tendencies of the nervous system, and into that
containing the tendencies of the external skin. But even then, the end
of the unequal division of nuclei would not have been nearly reached;
for, in the formation of the nervous system, the nuclear substance
which contains the hereditary tendencies of the sense-organs, would, in
the course of further cell-division, be separated from that which
contains the tendencies of the central organs, and the same process
would continue in the formation of all single organs, and in the final
development of the most minute histological elements. This process
would take place in a definitely ordered course, exactly as it has
taken place throughout a very long series of ancestors; and the
determining and directing factor is simply and solely the nuclear
substance, the nucleoplasm, which possesses such a molecular structure
in the germ-cell that all such succeeding stages of its molecular
structure in future nuclei must necessarily arise from it, as soon as
the requisite external conditions are present. This is almost the same
conception of ontogenetic development as that which has been held by
embryologists who have not accepted the doctrine of evolution: for we
have only to transfer the primary cause of development, from an unknown
source within the organism, into the nuclear substance, in order to
make the views identical.
It appears at first sight that the knowledge which has been gained by
studying the indirect division of nuclei is opposed to such a view, for
we know that each mother-loop of the so-called nuclear plate divides
longitudinally into two exactly equal halves, which can be stained and
thus rendered visible.
In this way each resulting daughter-nucleus receives an equal supply of
halves, and it therefore appears that the two nuclei must be completely
identical. This at least is Strasburger’s conclusion, and he regards
such identity as a fundamental fact, which cannot be shaken, and with
which all attempts at further explanation must be brought into accord.
How then can the gradual transformation of the nuclear substance be
brought about? For such a transformation must necessarily take place if
the nuclear substance is really the determining factor in development.
Strasburger attempts to support his hypothesis by assuming that the
inequality of the daughter-nuclei arises from unequal nutrition; and he
therefore considers that the inequality is brought about after the
division of the nucleus and of the cell. Strasburger has shown, in a
manner which is above all criticism, that the nucleus derives its
nutrition from the cell-body, but then the cell-bodies of the two _ex
hypothesi_ identical daughter-nuclei must be different from the first,
if they are to influence their nuclei in different ways. But if the
nucleus determines the nature of the cell, it follows that two
identical daughter-nuclei which have arisen by division within one
mother-cell cannot come to possess unequal cell-bodies. As a matter of
fact, however, the cell-bodies of two daughter-cells often differ in
size, in appearance, and in their subsequent history, and these facts
are sufficient to prove that in such cases the division of the nucleus
must have been unequal. It appears to me to be a necessary conclusion
that, in such an instance, the mother-nucleus must have been capable of
splitting into nuclear substances of differing quality. I think that,
in his argument, Strasburger has over-estimated the support afforded by
exact observations upon indirect nuclear division. Certainly the fact,
discovered by Flemming, and more exactly studied by Balbiani and
Pfitzner, that, in nuclear division, the loops split longitudinally, is
of great and even of fundamental importance. Furthermore, the
observations, conducted last year by van Beneden, on the process of
fertilization in _Ascaris_, have given to Flemming’s discovery a
clearer and more definite meaning than could have been at first
ascribed to it. The discovery proves, in the first place, that the
nucleus always divides into two parts of equal quantity, and further
that in every nuclear division, each daughter-nucleus receives the same
amount of nuclear substance from the father as from the mother; but, as
it seems to me, it is very far from proving that the quality of the
parent nucleoplasms must always be equal in the daughter-nuclei. It is
true that the fact seems to prove this; and if we remember the
description of the most favourable instance which has been hitherto
discovered, viz. the process of fertilization in the egg of _Ascaris_,
as represented by van Beneden, the two longitudinal halves of each loop
certainly impress the reader as being absolutely identical (compare,
for instance, loc. cit. Plate XIX, figs. 1, 4, 5). But we must not
forget that we do not see the molecular structure of the nucleoplasm,
but something which we can only look upon (when we remember how complex
this molecular structure must be) as a very rough expression of its
quantity. Our most powerful and best lenses just enable us to make out
the form of separate stainable granules present in a loop which is
about to divide: they appear as spheres and immediately after division
as hemispheres. But according to Strasburger, these granules, the
so-called microsomata, only serve for the nutrition of the nuclear
substance proper, which lies between them unstainable, and therefore
not distinctly visible. But even if these granules represent the true
idioplasm, their division into two exactly equal parts would give us no
proof of equality or inequality in their constitution: it would only
give us an idea of their quantitative relations. We can only obtain
proofs as to the quality of the molecular structure of the two halves
by their effect on the bodies of the daughter-cells, and we know that
these latter are frequently different in size and quality.
This point is so important that I must illustrate it by a few more
examples. The so-called polar bodies (to be treated more in detail
below) which are expelled during maturation from the eggs of so many
animals, are true cells, as was first proved by Bütschli in Nematodes:
their formation is due to a process of undoubted cell-division usually
accompanied by a typical form of indirect nuclear division[121]. If any
one is still in doubt upon this point, after the observations of Fol
and Hertwig, he might easily be convinced of its truth by a glance at
the figures (unfortunately too little known) which Trinchese[122] has
published, illustrating this process in the eggs of certain gastropods.
The eggs of _Amphorina coerulea_ are in every way suitable for
observation, being entirely translucent, and having large distinct
nuclei which differ from the green cytoplasm in colour. In these eggs
two polar bodies are formed one after the other: and each of them
immediately re-divides: hence it follows that four polar bodies are
placed at the pole of the egg. But how is it that these four cells
perish, while the nucleus, remaining in the yolk and conjugating with
the sperm-nucleus, makes use of the whole body of the egg and developes
into the embryo? Obviously because the nature of the polar body is
different from that of the egg-cell. But since the nature of the cell
is determined by the quality of the nucleus, this quality must differ
from the very moment of nuclear division. This is proved by the fact
that the supernumerary spermatozoa which sometimes enter the egg do not
conjugate with the polar bodies. According to Strasburger’s theory, the
objection might be urged that the different quality of the nuclei is
here caused by the very different quantity of cytoplasm by which they
are surrounded and nourished; but on the one hand the smallness of the
cell-bodies which surround most polar globules must have some
explanation, and this can only be found in the nature of the nucleus;
and on the other hand the quantity of the cell-body which surrounds the
polar globules of _Amphorina_ is, as a matter of fact, somewhat larger
than the sphere of green cytoplasm which surrounds the nucleus of the
egg! The difference between the polar bodies and the egg-cell can thus
only be explained on the supposition that, in the division of the
nuclear spindle, two qualitatively different daughter-nuclei are
produced.
There does not seem to be any objection to the view that the
microsomata of the nuclear loops—assuming that these bodies represent
the idioplasm—are capable of dividing into halves, equal in form and
appearance, but unequal in quality. We know that this very process
takes place in many egg-cells; thus in the egg of the earth-worm the
first two segmentation spheres are equal in size and appearance, and
yet the one forms the endoderm and the other the ectoderm of the embryo.
I therefore believe that we must accept the hypothesis that, in
indirect nuclear division, the formation of unequal halves may take
place quite as readily as the formation of equal halves, and that the
equality or inequality of the subsequently produced daughter-cells must
depend upon that of the nuclei. Thus during ontogeny a gradual
transformation of the nuclear substance takes place, necessarily
imposed upon it, according to certain laws, by its own nature, and such
transformation is accompanied by a gradual change in the character of
the cell-bodies.
It is true that we cannot gain any detailed knowledge of the nature of
these changes in the nuclear substance, but we can very well arrive at
certain general conclusions about them. If we may suppose, with Nägeli,
that the molecular structure of the germ-idioplasm, or according to our
terminology the germ-plasm, becomes more complicated according to the
greater complexity of the organism developed from it, then the
following conclusions will also be accepted,—that the molecular
structure of the nuclear substance is simpler as the differences
between the structures arising from it become less; that therefore the
nuclear substance of the segmentation-cell of the earth-worm, which
potentially contains the whole of the ectoderm, possesses a more
complicated molecular structure than that of a single epidermic cell or
nerve-cell. These conclusions will be admitted when it is remembered
that every detail in the whole organism must be represented in the
germ-plasm by its own special and peculiar arrangement of the groups of
molecules (the micellae of Nägeli), and that the germ-plasm not only
contains the whole of the quantitative and qualitative characters of
the species, but also all individual variations as far as these are
hereditary: for example the small depression in the centre of the chin
noticed in some families. The physical causes of all apparently
unimportant hereditary habits or structures, of hereditary talents, and
other mental peculiarities, must all be contained in the minute
quantity of germ-plasm which is possessed by the nucleus of a
germ-cell;—not indeed as the preformed germs of structure (the gemmules
of pangenesis), but as variations in its molecular constitution; if
this be impossible, such characters could not be inherited. Nägeli has
shown in his work, which is so rich in suggestive ideas, that even in
so minute a space as the thousandth of a cubic millimetre, such an
enormous number (400,000,000) of micellae may be present, that the most
diverse and complicated arrangements become possible. It therefore
follows that the molecular structure of the germ-plasm in the
germ-cells of an individual must be distinguished from that of another
individual by certain differences, although these may be but small; and
it also follows that the germ-plasm of any species must differ from
that of all other species.
These considerations lead us to conclude that the molecular structure
of the germ-plasm in all higher animals must be excessively complex,
and, at the same time, that this complexity must gradually diminish
during ontogeny as the structures still to be formed from any cell, and
therefore represented in the molecular constitution of its nucleoplasm,
become less in number. I do not mean to imply that the nucleoplasm
contains preformed structures which are gradually reduced in number as
they are given off in various directions during the building-up of
organs: I mean that the complexity of the molecular structure decreases
as the potentiality for further development also decreases, such
potentiality being represented in the molecular structure of the
nucleus. The nucleoplasm, which in the grouping of its particles
contains potentially a hundred different modifications of this
substance, must possess far more numerous kinds and far more complex
arrangements of such particles than the nucleoplasm which only contains
a single modification, capable of determining the character of a single
kind of cell. The development of the nucleoplasm during ontogeny may be
to some extent compared to an army composed of corps, which are made up
of divisions, and these of brigades, and so on. The whole army may be
taken to represent the nucleoplasm of the germ-cell: the earliest
cell-division (as into the first cells of the ectoderm and endoderm)
may be represented by the separation of the two corps, similarly formed
but with different duties: and the following cell-divisions by the
successive detachment of divisions, brigades, regiments, battalions,
companies, etc.; and as the groups become simpler so does their sphere
of action become limited. It must be admitted that this metaphor is
imperfect in two respects, first, because the quantity of the
nucleoplasm is not diminished, but only its complexity, and secondly,
because the strength of an army chiefly depends upon its numbers, not
on the complexity of its constitution. And we must also guard against
the supposition that unequal nuclear division simply means a separation
of part of the molecular structure, like the detachment of a regiment
from a brigade. On the contrary, the molecular constitution of the
mother-nucleus is certainly changed during division in such a way that
one or both halves receive a new structure which did not exist before
their formation.
My opinion as to the behaviour of the idioplasm during ontogeny, not
only differs from that of Nägeli, in that the latter maintains that the
idioplasm only undergoes changes in its ‘conditions of tension and
movement,’ but also because he imagines this substance to be composed
of the preformed germs of structures (‘Anlagen’). Nägeli’s views are
obviously bound up with his theory of a continuous network of idioplasm
throughout the whole body; perhaps he would have adopted other
conclusions had he been aware of the fact that the idioplasm must only
be sought for in the nuclei. Nägeli’s views as to ontogeny can be best
seen in the following passages: ‘As soon as ontogenetic development
begins, the groups of micellae in the idioplasm which effect the first
stage of development, enter upon active growth: such activity causes a
passive growth of the other groups, and an increase in the whole
idioplasm, perhaps to many times its former bulk. But the intensities
of growth in the two series of groups are unequal, and consequently an
increasing tension is produced which sooner or later, according to the
number, arrangement, and energy of the active groups, necessarily
renders the continuation of the process impossible. In consequence of
such disturbance to the equilibrium, active growth now takes place in
the next group, leading to fresh irritation, and this group then reacts
more strongly than all the others upon the tension which first
stimulated its activity. These changes are repeated until all the
groups are gone through, and the ontogenetic development finally
reaches the stage at which propagation takes place, and thus the
original stage of the germ is reached.’