appears to be. If we may assume that in eggs which are capable of two
kinds of development the second polar body is not expelled until the
entrance of a spermatozoon has taken place, the explanation of the
possibility of parthenogenetic development when fertilization does not
occur would be forthcoming. Now we know, from the investigations of O.
Hertwig and Fol, that in the eggs of _Echinus_ the two polar bodies are
even formed in the ovary, and are therefore quite independent of
fertilization, but in this and other similar cases a parthenogenetic
development of the egg never takes place. There are, however,
observations upon other animals which point to the fact that the first
only and not the second polar body may be formed before the
spermatozoon penetrates into the egg. It can be easily understood why
it is that entirely conclusive observations are wanting, for hitherto
there has been no reason for any accurate distinction between the first
and the second polar body. But in many eggs it appears certain that the
second polar body is not expelled until the spermatozoon has
penetrated. O. Schultze, the latest observer of the egg of the frog, in
fact saw the first polar body alone extruded from the unfertilized egg:
a second nuclear spindle was indeed formed, but the second polar body
was not expelled until after fertilization had taken place. A very
obvious theory therefore suggests itself:—that while the formation of
the second polar body is purely a phenomenon of maturation in most
animal eggs, and is independent of fertilization,—in the eggs of a
number of other animals, on the other hand, and especially among
Arthropods, the formation of the second nuclear spindle is the result
of a stimulus due to the entrance of a spermatozoon. If this suggestion
be confirmed, we should be able to understand why parthenogenesis
occurs in certain classes of animals wherever the external conditions
of life render its appearance advantageous, and further, why in so many
species of insects a sporadic parthenogenesis is observed, viz. the
parthenogenetic development of single eggs (Lepidoptera). Slight
individual differences in the facility with which the second nuclear
spindle is formed independently of fertilization would in such cases
decide whether an egg is or is not capable of parthenogenetic
development. As soon, however, as the second nuclear spindle is formed,
parthenogenesis becomes impossible. The nuclear spindle which gives
rise to the second polar body, and that which initiates segmentation,
are two entirely different things, and although they contain the same
quantity, and the same kind of germ-plasm, a transformation of the one
into the other is scarcely conceivable. This conclusion will be
demonstrated in the following part of the essay.
II. The Significance of the Second Polar Body.
I have already discussed the physiological importance of the first
polar body, or rather of the first division undergone by the nucleus of
the egg, and I have explained it as the removal of ovogenetic nuclear
substance which has become superfluous and indeed injurious after the
maturation of the egg. I do not indeed know of any other meaning which
can be ascribed to this process, now that we know of the occurrence of
a first division of the nucleus in parthenogenetic as well as in sexual
eggs. A part of the nucleus must thus be removed from both kinds of
eggs, a part which was necessary to complete their growth, and which
then became superfluous and at the same time injurious. In this respect
the observations of Blochmann[254] upon the eggs of _Musca vomitoria_
seem to me to be very interesting. Here the two successive divisions of
the nuclear spindle arising from the egg-nucleus take place, but true
polar bodies are not expelled, and the two nuclei corresponding to them
(one of which divides once more) are placed on the surface of the egg,
surrounded by an area free from yolk granules; and they break up at a
later period. The essential point is obviously to eliminate from the
egg-cell the influence of nucleoplasm which has been separated from the
egg-nucleus as the first polar body; and this condition is satisfied
whether the elimination is brought about by a process of true
cell-division, as is the rule in the eggs of most animals, or by the
division and removal of part of the egg-nucleus alone. The occurrence
of the latter method of elimination certainly constitutes a still
further proof of the physiological importance of the process, and this,
taken together with the universal occurrence of polar bodies in all
eggs—parthenogenetic and sexual—forces us to conclude that the process
must possess a definite significance. No one of the various attempts
which have been made to explain the significance of polar bodies
generally is applicable to the _first_ polar body except that which I
have attempted.
But the case is different with the significance of the _second_ nuclear
division, or the _second_ polar body. Here it might perhaps be possible
to return to the view brought forward by Minot, Balfour, and van
Beneden, and to consider the removal of this part of the nucleus as the
expulsion of the male part of the previously hermaphrodite egg-cell.
The second polar body is only expelled when the egg is to be
fertilized, and at first sight it appears to be quite obvious that such
a preparation of the egg for fertilization must depend upon its
reduction to the female state. I believe however that this is not the
case, and am of opinion that the process has an entirely different and
much deeper meaning.
How can we gain any conception of this supposed hermaphroditism of the
egg-cell, and its subsequent attainment of the female state? What are
the essential characteristics of the male and female states? We know of
female and male individuals, among both animals and plants: their
differences consist essentially in the fact that they produce different
kinds of reproductive cells; in part they are of a secondary nature,
being adaptations of the organism to the functions of reproduction;
they are intended to attract the other sex, or to ensure the meeting of
the two kinds of reproductive cells, or to enable the fertilized egg to
develope and sometimes to guide the development of the offspring until
it has reached a certain period of growth. But all these differences,
however great they may sometimes be, do not alter the essential nature
of the organism. The blood corpuscles of man and woman are the same,
and so are the cells of their nerves and muscles; and even the sexual
cells, so different in size, appearance, and generally also in motile
power, must contain the same fundamental substance, the same idioplasm.
Otherwise the female germ-cell could not transmit the male characters
of the ancestors of the female quite as readily as the female
characters, nor could the male germ-cell transmit the female quite as
readily as the male characters of the ancestors of the male. It is
therefore clear that the nuclear substance itself is not sexually
differentiated.
I have already previously pointed out that the above-mentioned facts of
heredity contain the disproof of Minot’s theory, inasmuch as the
egg-cell transmits male as well as female characters. Strasburger[255]
has also raised a similar objection. I consider this objection to be
quite conclusive, for there does not seem to be any way in which the
difficulty can be met by the supporters of the theory. The difficulty
could indeed be evaded until we came to know that the essential part of
the polar body is nuclear substance, and that the latter must be
regarded as idioplasm,—as the substance which is the bearer of
heredity. It might have been maintained that the male part, removed
from the egg, consists only in a condition, perhaps comparable to
positive or negative electricity; and that this condition is present in
the substance of the polar body, so that the removal of the latter
would merely signify a removal of the unknown condition. I do not mean
to imply that any of those who have adopted Minot’s theory have had any
such vague ideas concerning this process, but even if any one were
ready to adopt it, he would be unable to make any use of the idea. He
would not be able to support the theory in this way, for we now know
that nuclear substance is removed with the polar body, and this fact
requires an explanation which cannot be afforded by the theory, if we
are right in believing that the expelled nuclear substance is not
merely the indifferent bearer of the unknown principle of the male
condition, but hereditary substance. I therefore believe that Minot’s,
Balfour’s, and van Beneden’s hypothesis, although an ingenious attempt
which was quite justified at the time when it originated, must be
finally abandoned.
My opinion of the significance of the second polar body is shortly
this,—a reduction of the germ-plasm is brought about by its formation,
a reduction not only in quantity, but above all in the complexity of
its constitution. By means of the second nuclear division the excessive
accumulation of different kinds of hereditary tendencies or germ-plasms
is prevented, which without it would be necessarily produced by
fertilization. With the nucleus of the second polar body as many
different kinds of idioplasm are removed from the egg as will be
afterwards introduced by the sperm-nucleus; thus the second division of
the egg-nucleus serves to keep constant the number of different kinds
of idioplasm, of which the germ-plasm is composed during the course of
generations.
In order to make this intelligible a short explanation is necessary.
From the splendid series of investigations on the process of
fertilization, commenced by Auerbach and Bütschli, and continued by
Hertwig, Fol, Strasburger, van Beneden, and many others, and from the
theoretical considerations brought forward by Pflüger, Nägeli, and
myself, at least one certain result follows, viz. that there is an
hereditary substance, a material bearer of hereditary tendencies, and
that this substance is contained in the nucleus of the germ-cell, and
in that part of it which forms the nuclear thread, which at certain
periods appears in the form of loops or rods. We may further maintain
that fertilization consists in the fact that an equal number of loops
from either parent are placed side by side, and that the segmentation
nucleus is composed in this way. It is of no importance, as far as this
question is concerned, whether the loops of the two parents coalesce
sooner or later, or whether they remain separate. The only essential
conclusion demanded by our hypothesis is that there should be complete
or approximate equality between the quantities of hereditary substance
derived from either parent. If then the germ-cells of the offspring
contain the united germ-plasms of both parents, it follows that such
cells can only contain half as much paternal germ-plasm as was
contained in the germ-cells of the father, and half as much maternal
germ-plasm as was contained in the germ-cells of the mother. This
principle is affirmed in a well-known calculation made by breeders of
animals, who only differ from us in their use of the term ‘blood’
instead of the term germ-plasm. Breeders say that half of the ‘blood’
of the offspring has been derived from the father and the other half
from the mother. The grandchild similarly derives a quarter of its
‘blood’ from each of the four grandparents, and so on.
Let us imagine, for the sake of argument, that sexual reproduction had
not been introduced into the animal kingdom, and that asexual
reproduction had hitherto existed alone. In such a case, the germ-plasm
of the first generation of a species which enters upon sexual
reproduction must still be entirely homogeneous; the hereditary
substance must, in each individual, consist of many minute units, each
of which is exactly like the other, and each of which contains within
itself the tendency to transmit, under certain circumstances, the whole
of the characters of the parent to a new organism—the offspring. In
each of the offspring of such a first generation, the germ-plasms of
two parents will be united, and every germ-cell contained in the
individuals of this second sexually produced generation will now
contain two kinds of germ-plasm—one kind from the father, and the other
from the mother. But if the total quantity of germ-plasm present in
each cell is to be kept within the pre-determined limits, each of the
two ancestral germ-plasms, as I may now call them, must be represented
by only half as many units as were contained in the parent germ-cells.
In the third sexually produced generation, two new ancestral
germ-plasms would be added by fertilization to the two already present,
and the germ-cells of this generation would therefore contain four
different ancestral germ-plasms, each of which would constitute a
quarter of the total quantity. In each succeeding generation the number
of the ancestral germ-plasms is doubled, while their quantities are
reduced by one half. Thus in the fifth sexually produced generation,
each of the sixteen ancestral germ-plasms will only constitute 1/16 of
the total quantity; in the sixth, each of the thirty-two ancestral
germ-plasms, only 1/32, and so on. The germ-plasm of the tenth
generation would be composed of 1024 different ancestral germ-plasms,
and that of the n^{th} of 2^n. By the tenth generation each single
ancestral germ-plasm would only form 1/1024 of the total quantity of
germ-plasm contained in a single germ-cell. We know nothing whatever of
the length of time over which this process of division of the ancestral
germ-plasms may have endured, but even if it had continued to the
utmost possible limit—so far indeed that each ancestral germ-plasm was
only represented by a single unit—a time would at last come when any
further division into halves would cease to be possible; for the very
conception of a unit implies that it cannot be divided without the loss
of its essential nature, which in this case constitutes it as the
hereditary substance.
In the diagram represented in Fig. I. I have tried to render these
conclusions intelligible. In generation I. each paternal and maternal
germ-plasm is still entirely homogeneous, and does not contain any
combination of different hereditary qualities, but the germ-plasm of
the offspring is made up of equal parts of two kinds of germ-plasm. In
the second generation this latter germ-plasm unites with another
derived from other parents, which is similarly composed of two
ancestral germ-plasms, and the resulting third generation now contains
four different ancestral germ-plasms in its germ-cells, and so on. The
diagram only indicates the fusion of ancestral germ-plasms as far as
the offspring of the fourth generation, the germ-cells of which contain
sixteen different ancestral germ-plasms. If we imagine the germ-plasm
units to be so large that there is only room for sixteen of them in the
nuclear thread, the limits of division would-be reached in the fifth
generation, and any further division into halves of the ancestral
germ-plasms would be impossible.
Now however minute the units may be, there is not the least doubt that
the limits of possible division have been long since reached by all
existing species, for we may safely assume that no one of them has
acquired the sexual method of reproduction within a small number of
recent generations. All existing species must therefore now contain as
many different kinds of ancestral germ-plasms as they are capable of
containing; and the question arises,—How can sexual reproduction now
proceed without a doubling of the quantity of germ-plasm in each
germ-cell, with every new generation?
There is only one possible answer to such a question:—sexual
reproduction can proceed by a reduction in the _number_ of ancestral
germ-plasms, a reduction which is repeated in every generation.
[Illustration: Fig. I.]
This _must_ be so: the only question is, how and when does the supposed
reduction take place.
Inasmuch as the germ-plasm is seated, according to our theory, in the
nucleus, the necessary reduction can only be produced by nuclear
division; and quite apart from any observation which has been already
made, we may safely assert that there _must_ be a form of nuclear
division in which the ancestral germ-plasms contained in the nucleus
are distributed to the daughter-nuclei in such a way that each of them
receives only half the number contained in the original nucleus. After
Roux’s[256] elaborate review of the whole subject, we need no longer
doubt that the complex method of nuclear division, hitherto known as
karyokinesis, must be considered not merely as a means for the division
of the total quantity of nuclear substance, but also for producing a
division of the quantity and quality of each of its single elements. In
by far the greater number of instances the object of this division is
obviously to effect an equal distribution of nuclear substance in the
two daughter-nuclei, so that each of the different qualities contained
in the mother-nucleus is transferred to the two daughter-nuclei. This
interpretation of ordinary karyokinesis is less uncertain than perhaps
at first sight it may appear to be. We cannot, it is true, directly see
the ancestral germ-plasms, nor do we even know the parts of the nucleus
which are to be looked upon as constituting ancestral germ-plasm; but
if Flemming’s original discovery of the longitudinal division of the
loops lying in the equatorial plane of the nuclear spindle is to have
any meaning at all, its object must be to divide and distribute the
different kinds of the minutest elements of the nuclear thread as
equally as possible. It has been ascertained that the two halves
produced by the longitudinal splitting of each loop never pass into the
same daughter-nucleus, but always in opposite directions. The essential
point cannot therefore be the division of the nucleus into absolutely
equal quantities, but it must be the distribution of the different
qualities of the nuclear thread, without exception, in both
daughter-nuclei. But these different qualities are what I have called
the ancestral germ-plasms, i.e. the germ-plasms of the different
ancestors, which must be contained in vast numbers, but in very minute
quantities, in the nuclear thread. The supposition of a vast number is
not only required by the phenomena of heredity but also results from
the comparatively great length of the nuclear thread: furthermore it
implies that each of them is present in very small quantity. The vast
number together with the minute quantity of the ancestral germ-plasms
permit us to conclude that they are, upon the whole, arranged in a
linear manner in the thin thread-like loops: in fact the longitudinal
splitting of these loops appears to me to be almost a proof of the
existence of such an arrangement, for without this supposition the
process would cease to have any meaning.
This is the only kind of karyokinesis which has been observed until
recently; but if the supposed nuclear division leading to a reduction
in the number of ancestral germ-plasms has any real existence, there
must be yet another kind of karyokinesis, in which the primary
equatorial loops are not split longitudinally, but are separated
without division into two groups, each of which forms one of the two
daughter-nuclei. In such a case the required reduction in the number of
ancestral germ-plasms would take place, for each daughter-nucleus would
receive only half the number which was contained in the mother-nucleus.
Now there is more evidence for the existence of this second kind of
karyokinesis than the fact that it is demanded by my theory; for I
believe that it has been already observed, although it has not been
interpreted in this sense.
It is very probable that this is true of van Beneden’s[257] observation
on the egg of _Ascaris megalocephala_: he found that the nuclear
division which led to the formation of the polar body differs from the
ordinary course of karyokinesis, in that the plane of division is at
right angles to that usually assumed. Carnoy[258] has confirmed this
observation in its main features, and he has made the further
observation that out of the eight nuclear loops which are found at the
equator of the spindle, four are removed with the first polar body, and
that half of the remaining four are removed with the second polar body.
The first of these two divisions would have to be looked upon as a
reduction, if it is certain that each of the eight nuclear loops
consists of different ancestral germ-plasms; but this assumption is
impossible, although on the other hand it cannot be _directly_
disproved: for we are not able to see the ancestral germ-plasms. But it
must nevertheless be maintained that the removal of the first four
loops does not imply a reduction in the number of ancestral germ-plasms
in the nucleus; because, as I have already argued, two successive
divisions of the number of ancestral germ-plasms into halves is
inconceivable; and because the first polar body is also present in
parthenogenetic eggs in which such division into halves cannot take
place. But the karyokinetic process can readily be looked upon as a
removal of ovogenetic nucleoplasm, for we know from the observations of
Flemming and Carnoy, that, under certain circumstances, subsequent
divisions may occur, involving an increase in the number of nuclear
loops to double their number. These subsequent divisions of course take
place in the daughter-nuclei. This fact proves, as I think, that there
are nuclei in which the same ancestral germ-plasm occurs in two
different loops: but such loops, identical as regards the composition
of their ancestral germ-plasms, may very well contain different
ontogenetic stages of this substance. This will be the case in the
instance alluded to, if four loops of the first nuclear spindle are to
be looked upon as ovogenetic nucleoplasm, and the four others as
germ-plasm. It is therefore unnecessary to regard the first division of
the egg-nucleus as a ‘reducing division’: it may be looked upon as an
‘equal division’[259] entirely analogous to the kind of division which,
in my opinion, directs the development of the embryo. This conclusion
would receive direct proof if it were possible to show that the eight
loops of the first division have arisen by the longitudinal splitting
of four _primary_ loops: for a longitudinal splitting of the nuclear
thread would be the means by which the different ontogenetic stages of
the germ-plasm could be separated from one another, without leading to
any reduction in the number of ancestral germ-plasms in the
daughter-nuclei. Thus I have previously attempted to prove that the
ontogenetic development of the egg must be connected with a progressive
transformation of the nucleoplasm during successive nuclear divisions,
and this transformation will very frequently (but not always) occur in
such a way that the different qualities of the nucleoplasm are
separated from one another by the nuclear division. The nucleoplasm of
the daughter-nuclei will be identical if the two daughter-cells are to
potentially contain corresponding parts of the embryo; as for instance
the first two segmentation spheres of the egg of the frog, which
according to Roux[260] correspond to the right and left halves of the
future animal. But the nucleoplasm must be unequal if the products of
division are to develope into different parts of the embryo. In both
cases, however, karyokinesis is connected with a longitudinal splitting
of the nuclear threads, and we may conclude from this fact (which is
also confirmed by the phenomena of heredity) that all such nuclei,
whether they have entered upon the same or different ontogenetic
transformations of their nucleoplasm, are identical as regards the
ancestral germ-plasm which they contain. During the whole process of
segmentation and the entire development of the embryo, the total number
of ancestral germ-plasms which were at first contained in the
germ-plasm of the fertilized egg-cell must still be contained in each
of the succeeding cells.
Thus no objection can be raised against the view that the four loops of
the first polar body contain the ovogenetic nucleoplasm, that is to
say, an idioplasm which contains the total number of ancestral
germ-plasms, but at an advanced and highly specialized ontogenetic
stage.
The formation of the second polar body may be rightly considered as a
‘reducing division,’ as a division leading to the expulsion of half the
number of the different ancestral germ-plasms, in the form of two
nuclear loops, for no reason can be alleged in support of the
assumption that the four loops of the second nuclear spindle are made
up of identical pairs. Furthermore the facts of heredity require the
assumption that the greatest possible number of ancestral germ-plasms
is accumulated in the germ-plasm of each germ-cell, and thus that the
small number of loops not only means an increase in quantity but a
multiplication in the number of different ancestral germ-plasms present
in each of them. If this conclusion be correct, there can be no doubt
that the second division of the egg-nucleus means a reduction in the
above-mentioned sense.