significance: it could only be looked upon as the last remnant of a
process which formerly possessed some meaning, but which is now devoid
of any physiological importance. We are indeed compelled to admit that
a process does occur in connexion with the true polar bodies of animal
eggs, which we cannot explain on physiological grounds; I mean the
division of the polar bodies after they have been expelled from the
egg. In many animals the two polar bodies divide again after their
expulsion, so as to form four bodies, which distinctly possess the
structure of cells, as Trinchese observed in the case of gastropods.
But, in the first place, this second division does not always take
place, and, secondly, it is very improbable that a process which occurs
during the first stage of ontogeny, or more properly speaking, before
the commencement of ontogeny, and which is, therefore, a remnant of
some excessively ancient phyletic stage, would have been retained up to
the present day unless it possessed some very important physiological
significance. We may safely maintain that it would have disappeared
long ago if it had been without any physiological importance. Relying
on our knowledge of the slow and gradual, although certain,
disappearance, in the course of phylogeny, of organs which have lost
their functions, and of processes which have become meaningless, we are
compelled to regard the process of the formation of polar bodies as of
high physiological importance. But this view does not exclude the
possibility that the process possessed a morphological meaning also,
and I believe that we are quite justified in attempting (as
Bütschli[154] has recently done) to discover what this morphological
meaning may have been.
Should it be finally proved that the expulsion of polar bodies is
nothing more than the removal of histogenetic nucleoplasm from the
germ-cell, the opinion (which is so intimately connected with the
theory of the continuity of the germ-plasm) that a re-transformation of
specialised idioplasm into germ-plasm cannot occur, would be still
further confirmed; for we do not find that any part of an organism is
thrown away simply because it is useless: organs that have lost their
functions are re-absorbed, and their material is thus employed to
assist in building up the organism.
III. On the Nature of Parthenogenesis.
It is well known that the formation of polar bodies has been repeatedly
connected with the sexuality of germ-cells, and that it has been
employed to explain the phenomena of parthenogenesis. I may now,
perhaps, be allowed to develope the views as to the nature of
parthenogenesis at which I have arrived under the influence of my
explanation of polar bodies.
The theory of parthenogenesis adopted by Minot and Balfour is
distinguished by its simplicity and clearness, among all other
interpretations which had been hitherto offered. Indeed, their
explanation follows naturally and almost as a matter of course, if the
assumption made by these observers be correct, that the polar body is
the male part of the hermaphrodite egg-cell. An egg which has lost its
male part cannot develope into an embryo until it has received a new
male part in fertilization. On the other hand, an egg which does not
expel its male part may develope without fertilization, and thus we are
led to the obvious conclusion that parthenogenesis is based upon the
non-expulsion of polar bodies. Balfour distinctly states ‘that the
function of forming polar cells has been acquired by the ovum for the
express purpose of preventing parthenogenesis[155].’
It is obvious that I cannot share this opinion, for I regard the
expulsion of polar bodies as merely the removal of the ovogenetic
nucleoplasm, on which depended the development of the specific
histological structure of the egg-cell. I must assume that the
phenomena of maturation in the parthenogenetic egg and in the sexual
egg are precisely identical, and that in both, the ovogenetic
nucleoplasm must in some way be removed before embryonic development
can begin.
Unfortunately the actual proof of this assumption is not so complete as
might be desired. In the first place, we are as yet uncertain whether
polar bodies are or are not expelled by parthenogenetic eggs[156]; for
in no single instance has such expulsion been established beyond doubt.
It is true that this deficiency does not afford any support to the
explanation of Minot and Balfour, for in all cases in which polar
bodies have not been found in parthenogenetic eggs, these structures
are also absent from the eggs which require fertilization in the same
species. But although the expulsion of polar bodies in parthenogenesis
has not yet been proved to occur, we must assume it to be nearly
certain that the phenomena of maturation, whether connected or
unconnected with the expulsion of polar bodies, are the same in the
eggs which develope parthenogenetically and in those which are capable
of fertilization, in one and the same species. This conclusion depends,
above all, upon the phenomena of reproduction in bees, in which, as a
matter of fact, the same egg may be fertilized or may develope
parthenogenetically, as I shall have occasion to describe in greater
detail at a later period.
Hence when we see that the eggs of many animals are capable of
developing without fertilization, while in other animals such
development is impossible, the difference between the two kinds of eggs
must rest upon something more than the mode of transformation of the
nucleus of the germ-cell into the first segmentation nucleus. There
are, indeed, facts which distinctly point to the conclusion that the
difference is based upon quantitative and not qualitative relations. A
large number of insects are exceptionally reproduced by the
parthenogenetic method, e. g. in Lepidoptera. Such development does not
take place in all the eggs laid by an unfertilized female, but only in
part, and generally a small fraction of the whole, while the rest die.
But among the latter there are some which enter upon embryonic
development without being able to complete it, and the stage at which
development may cease also varies. It is also known that the eggs of
higher animals may pass through the first stages of segmentation
without having been fertilized. This was shown to be the case in the
egg of the frog by Leuckart[157], in that of the fowl by
Oellacher[158], and even in the egg of mammals by Hensen[159].
Hence in such cases it is not the impulse to development, but the power
to complete it, which is absent. We know that force is always bound up
with matter, and it seems to me that such instances are best explained
by the supposition that too small an amount of that form of matter is
present, which, by its controlling agency, effects the building-up of
the embryo by the transformation of mere nutritive material. This
substance is the germ-plasm of the segmentation nucleus, and I have
assumed above that it is altered in the course of ontogeny by changes
which arise from within, so that when sufficient nourishment is
afforded by the cell-body, each succeeding stage necessarily results
from the preceding one. I believe that changes arise in the
constitution of the nucleoplasm at each cell-division which takes place
during the building-up of the embryo, changes which either correspond
or differ in the two halves of each nucleus. If, for the present, we
neglect the minute amount of unchanged germ-plasm which is reserved for
the formation of the germ-cells, it is clear that a great many
different stages in the development of somatic nucleoplasm are thus
formed, which may be denominated as stages 1, 2, 3, 4, &c., up to _n_.
In each of these stages the cells differ more as development proceeds,
and as the number by which the stage is denominated becomes higher.
Thus, for instance, the two first segmentation spheres would represent
the first stage of somatic nucleoplasm, a stage which may be considered
as but slightly different in its molecular structure from the
nucleoplasm of the segmentation nucleus; the four first segmentation
spheres would represent the second stage; the succeeding eight spheres
the third, and so on. It is clear that at each successive stage the
molecular structure of the nucleoplasm must be further removed from
that of the germ-plasm, and that, at the same time, the cells of each
successive stage must also diverge more widely among themselves in the
molecular structure of their nucleoplasm. Early in development each
cell must possess its own peculiar nucleoplasm, for the further course
of development is peculiar to each cell. It is only in the later stages
that equivalent or nearly equivalent cells are formed in large numbers,
cells in which we must also suppose the existence of equivalent
nucleoplasm.
If we may assume that a certain amount of germ-plasm must be contained
in the segmentation nucleus in order to complete the whole process of
the ontogenetic differentiation of this substance; if we may further
assume that the quantity of germ-plasm in the segmentation nucleus
varies in different cases; then we should be able to understand why one
egg can only develope after fertilization, while another can begin its
development without fertilization, but cannot finish it, and why a
third is even able to complete its development. We should also
understand why one egg only passes through the first stages of
segmentation and is then arrested, while another reaches a few more
stages in advance, and a third developes so far that the embryo is
nearly completely formed. These differences would depend upon the
extent to which the germ-plasm, originally present in the egg, was
sufficient for the development of the latter; development will be
arrested as soon as the nucleoplasm is no longer capable of producing
the succeeding stage, and is thus unable to enter upon the following
nuclear division.
From a general point of view such a theory would explain many
difficulties, and it would render possible an explanation of the
phyletic origin of parthenogenesis, and an adequate understanding of
the strange and often apparently abrupt and arbitrary manner of its
occurrence. In my works on _Daphnidae_ I have already laid especial
stress upon the proposition that parthenogenesis in insects and
Crustacea certainly cannot be an ancestral condition which has been
transmitted by heredity, but that it has been derived from a sexual
condition. In what other way can we explain the fact that
parthenogenesis is present in certain species or genera, but absent in
others closely allied to them; or the fact that males are entirely
wanting in species of which the females possess a complete apparatus
for fertilization? I will not repeat all the arguments with which I
attempted to support this conclusion[160]. Such a conclusion may be
almost certainly accepted for the _Daphnidae_, because parthenogenesis
does not occur in their still living ancestors, the Phyllopods, and
especially the _Estheridae_. In _Daphnidae_ the cause and object of the
phyletic development of parthenogenesis may be traced more clearly than
in any other group of animals. In _Daphnidae_ we can accept the
conclusion with greater certainty than in all other groups, except
perhaps the _Aphidae_, that parthenogenesis is extremely advantageous
to species in certain conditions of life; and that it has only been
adopted when, and as far as, it has been beneficial; and further, that
at least in this group parthenogenesis became possible, and was
adopted, in each species as soon as it became useful. Such a result can
be easily understood if it is only the presence of more or less
germ-plasm which decides whether an egg is, or is not, capable of
development without fertilization.
If we now examine the foundations of this hypothesis we shall find that
we may at once accept one of its assumptions, viz. that fluctuations
occur in the quantity of germ-plasm in the segmentation nucleus; for
there can never be absolute equality in any single part of different
individuals. As soon therefore as these fluctuations become so great
that parthenogenesis is produced, it may become, by the operation of
natural selection, the chief mode of reproduction of the species or of
certain generations of the species. In order to place this theory upon
a firm basis, we have simply to decide whether the quantity of
germ-plasm contained in the segmentation nucleus is the factor which
determines development; although for the present it will be sufficient
if we can render this view to some extent probable, and show that it is
not in contradiction with established facts.
At first sight this hypothesis seems to encounter serious difficulties.
It will be objected that neither the beginning nor the end of embryonic
development can possibly depend upon the quantity of nucleoplasm in the
segmentation nucleus, since the amount may be continually increased by
growth; for it is well known that during embryonic development the
nuclear substance increases with astonishing rapidity. By an
approximate calculation I found[161] that, in the egg of a _Cynips_,
the quantity of nuclear substance present at the time when the
blastoderm was about to be formed, and when there were twenty-six
nuclei, was even then seven times as great as the quantity which had
been contained in the segmentation nucleus. How then can we imagine
that embryonic development would ever be arrested from want of nuclear
substance, and if such deficiency really acted as an arresting force,
how then could development begin at all? We might suppose that when
germ-plasm is present in sufficient quantity to start segmentation, it
must also be sufficient to complete the development; for it grows
continuously, and must presumably always possess a power equal to that
which it possessed at the beginning, and which was just sufficient to
start the process of segmentation. If at each ontogenetic stage, the
quantity of nucleoplasm is just sufficient to produce the following
stage, we might well imagine that the whole ontogeny would necessarily
be completed.
The flaw in this argument lies in the erroneous assumption that the
growth of nuclear substance is, when the quality of the nucleus and the
conditions of nutrition are equal, unlimited and uncontrolled. The
intensity of growth must depend upon the quantity of nuclear substance
with which growth and the phenomena of segmentation commenced. There
must be an optimum quantity of nucleoplasm with which the growth of the
nucleus proceeds most favourably and rapidly, and this optimum will be
represented in the normal size of the segmentation nucleus. Such a size
is just sufficient to produce, in a certain time and under certain
external conditions, the nuclear substance necessary for the
construction of the embryo, and to start the long series of
cell-divisions. When the segmentation nucleus is smaller, but large
enough to enter upon segmentation, the nuclei of the two first
embryonic cells will fall rather more below the normal size, because
the growth of the segmentation nucleus during and after division will
be less rapid on account of its unusually small size. The succeeding
generations of nuclei will depart more and more from the normal size in
each respective stage, because they do not pass into a resting-stage
during embryonic development, but divide again immediately after their
formation. Hence nuclear growth would become less vigorous as the
nuclei fell more and more below the optimum size, and at last a moment
would arrive when they would be unable to divide, or would be at least
unable to control the cell-body in such a manner as to lead to its
division.
The first event of importance for embryonic development is the
maturation of the egg, i. e. the transformation of the nucleus of the
germ-cell into a nuclear spindle and the removal of the ovogenetic
nucleoplasm by the separation of polar bodies, or by some analogous
process. There must be some cause for this separation, and I have
already tried to show that it may lie in the quantitative relations
which obtain between the two kinds of nucleoplasm contained in the
nucleus of the egg. I have suggested that the germ-plasm, at first
small in quantity, undergoes a gradual increase, so that it can finally
oppose the ovogenetic nucleoplasm. I will not further elaborate this
suggestion, for the ascertained facts are insufficient for the purpose.
But the appearances witnessed in nuclear division indicate that there
are opposing forces, and that such a contest is the motive cause of
division; and Roux[162] may be right in referring the opposition to
electrical forces. However this may be, it is perfectly certain that
the development of this opposition is based upon internal conditions
arising during growth in the nucleus itself. The quantity of nuclear
thread cannot by itself determine whether the nucleus can or cannot
enter upon division; if so, it would be impossible for two divisions to
follow each other in rapid succession, as is actually the case in the
separation of the two polar bodies, and also in their subsequent
division. In addition to the effects of quantity, the internal
conditions of the nucleus must also play an important part in these
phenomena. Quantity alone does not necessarily produce nuclear
division, or the nucleus of the egg would divide long before maturation
is complete, for it contains much more nucleoplasm than the female
pronucleus, which remains in the egg after the expulsion of the polar
bodies, and which is in most cases incapable of further division. But
the fact that segmentation begins immediately after the conjugation of
male and female pronuclei, also shows that quantity is an essential
requisite. The effect of fertilization has been represented as
analogous to that of the spark which kindles the gunpowder. In the
latter case an explosion ensues, in the former segmentation begins.
Even now, many authorities are inclined to refer the polar repulsion
manifested in the nuclear division which immediately follows
fertilization, to the antagonism between male and female elements. But,
according to the important discoveries of Flemming and van Beneden, the
polar repulsion in each nuclear division is not based on the antagonism
between male and female loops, but depends upon the antagonism and
mutual repulsion between the two halves of the same loop. The loops of
the father and those of the mother remain together and divide together
throughout the whole ontogeny.
What can be the explanation of the fact that nuclear division follows
immediately after fertilization, but that without fertilization it does
not occur in most cases? There is only one possible explanation, viz.
the fact that the quantity of the nucleus has been suddenly doubled, as
the result of conjugation. The difference between the male and female
pronuclei cannot serve as an explanation, even though the nature of
this difference is entirely unknown, because polar repulsion is not
developed between the male and female halves of the nucleus, but within
each male and each female half. We are thus forced to conclude that
increase in the quantity of the nucleus affords an impulse for
division, the disposition towards it being already present. It seems to
me that this view does not encounter any theoretical difficulties, and
that it is an entirely feasible hypothesis to suppose that, besides the
internal conditions of the nucleus, its quantitative relation to the
cell-body must be taken into especial account. It is imaginable, or
perhaps even probable, that the nucleus enters upon division as soon as
its idioplasm has attained a certain strength, quite apart from the
supposition that certain internal conditions are necessary for this
end. As above stated, such conditions may be present, but division may
not occur because the right quantitative relation between nucleus and
cell-body, or between the different kinds of nuclear idioplasm, has not
been established. I imagine that such a quantitative deficiency exists
in an egg, which, after the expulsion of the ovogenetic nucleoplasm in
the polar bodies, requires fertilization in order to begin
segmentation. The fact that the polar bodies were expelled proves that
the quantity of the nucleus was sufficient to cause division, while
afterwards it was no longer sufficient to produce such a result.
This suggestion will be made still clearer by an example. In _Ascaris
megalocephala_ the nuclear substance of the female pronucleus forms two
loops, and the male pronucleus does the same; hence the segmentation
nucleus contains four loops, and this is also the case with the first
segmentation spheres. If we suppose that in embryonic development, the
first nuclear division requires such an amount of nuclear substance as
is necessary for the formation of four loops,—it follows that an egg,
which can only form two or three loops from its nuclear reticulum,
would not be able to develope parthenogenetically, and that not even
the first division would take place. If we further suppose that, while
four loops are sufficient to start nuclear division, these loops must
be of a certain size and quantity in order to complete the whole
ontogeny (in a certain species), it follows that eggs possessing a
reticulum which contains barely enough nuclear substance to divide into
four segments, would be able to produce the first division and perhaps
also the second and third, or some later division, but that at a
certain point during ontogeny, the nuclear substance would become
insufficient, and development would be arrested. This will occur in
eggs which enter upon development without fertilization, but are
arrested before its completion. One might compare this retardation
leading to the final arrest of development, to a railway train which is
intended to meet a number of other trains at various junctions, and
which can only travel slowly because of some defect in the engine. It
will be a little behind time at the first junction, but it may just
catch the train, and it may also catch the second or even the third;
but it will be later at each successive junction, and will finally
arrive too late for a certain train; and after that it will miss all
the trains at the remaining junctions. The nuclear substance grows
continuously during development, but the rate at which it increases
depends upon the nutritive conditions together with its initial
quantity. The nutritive changes during the development of an egg depend
upon the quantity of the cell-body which was present at the outset, and
which cannot be increased. If the quantity of the nuclear substance is
rather too small at the beginning, it will become more and more
insufficient in succeeding stages, as its growth becomes less vigorous,
and differs more from the standard it would have reached if the
original quantity had been normal. Consequently it will gradually fall
more and more short of the normal quantity, like the train which
arrives later and later at each successive junction, because its
engine, although with the full pressure of steam, is unable to attain
the normal speed.
It will be objected that four loops cannot be necessary for nuclear
division in _Ascaris_, since such division takes place in the formation
of the polar bodies, resulting in the appearance of the female
pronucleus with only two loops. But this fact only shows that the
quantity of nuclear substance necessary for the formation of four loops
is not necessary for all nuclear divisions; it does not disprove the
assumption that such a quantity is required for the division of the
segmentation nucleus. In addition to these considerations we must not
leave the substance of the cell-body altogether out of account, for,
although it is not the bearer of the tendencies of heredity, it must be
necessary for every change undergone by the nucleus, and it surely also
possesses the power of influencing changes to a large extent. There
must be some reason for the fact that in all animal eggs with which we
are acquainted, the nucleus moves to the surface of the egg at the time
of maturation, and there passes through its well-known transformation.
It is obvious that it is there subjected to different influences from
those which would have acted upon it in the centre of the cell-body,
and it is clear that such an unequal cell-division as takes place in
the separation of the polar bodies could not occur if the nucleus
remained in the centre of the egg.