attempt to ascertain the meaning of the formation of polar bodies. I
based my views upon the idea, which was just then gaining ground, that
Nägeli’s idioplasm was to be sought for in the nucleus, and that the
nucleoplasm must therefore contain the substance which determines the
form and functions of the cell. Hence it followed that the
germ-plasm—the substance which determines the course of embryonic
development—must be identified with the nucleoplasm of the egg-cell.
The conception of germ-plasm was brought forward by me before the
appearance of Nägeli’s work[242] which is so rich in fertile ideas; and
germ-plasm does not exactly coincide with Nägeli’s idioplasm[243].
Germ-plasm is only a certain kind of idioplasm—viz. that contained in
the germ-cell—and it is the most important of all idioplasms, because
all the other kinds are merely the results of the various ontogenetic
stages into which it developes. I attempted to show that the molecular
structure in these ontogenetic stages into which the germ-plasm
developes would become more and more unlike that of the original
structure of this substance, until it finally attains a highly
specialized character at the end of embryonic development,
corresponding to the production of specialized histological elements.
It did not seem to me to be conceivable that the specialized idioplasm
contained in the nuclei of the tissue cells could re-transform itself
into the initial stage of the whole developmental series—that it could
give up its specialized character and re-assume the generalized
character of germ-substance. I will not repeat the reasons which
induced me to adopt this opinion; they still seem to me to be
conclusive. But let the above-mentioned theory be once accepted, and
there follows from it another interesting conclusion concerning the
germ-cell, or at least concerning those germ-cells which, like most
animal eggs, possess a specific histological character. For obviously,
such a character presupposes the existence of an idioplasm with a
considerable degree of histological specialization, which must be
contained in the nucleus of the egg-cell. We know, on the other hand,
that when its growth is complete, after the formation of yolk and
membranes, the egg contains germ-plasm, for it is capable of developing
into an embryo. We have therefore, as it were, two natures in a single
cell, which become manifest one after the other, and which, according
to our fundamental conception, can only be explained by the presence of
two different idioplasms, which control the egg-cell one after the
other, and determine its processes of development. At first a
nucleoplasm leading to histological specialization directs the
development of the egg and stamps upon it a specific histological
character; and then germ-plasm takes its place, and compels the egg to
undergo development into an embryo. If then the histogenetic or
ovogenetic nucleoplasm of the egg-cell can be derived from the
germ-plasm, but cannot be re-transformed into it (for the specialized
can be derived from the generalized, but not the generalized from the
specialized), we are driven to the conclusion that the germ-plasm,
which is already present in the youngest egg-cell, first of all
originates a specific histogenetic or ovogenetic nucleoplasm which
controls the egg-cell up to the point at which it becomes mature; that
its place is then taken by the rest of the unchanged nucleoplasm
(germ-plasm), which has in the meantime increased by growth; and that
the former is removed from the egg in the form of polar bodies—a
removal which has been rendered possible by the occurrence of nuclear
division. Hence the formation of polar bodies signified, in my opinion,
the removal of the ovogenetic part of the nucleus from the mature
egg-cell. Such removal was absolutely necessary, if it is impossible
that the ovogenetic nucleoplasm can be re-transformed into germ-plasm.
Hence the former substance cannot be made use of after the maturation
of the egg, and it must even be opposed to the commencement of
embryonic development, for it is impossible that the egg can be
controlled by two forces of different kinds in the same manner as it
would have been by one of them alone. I therefore concluded that the
influence of the ovogenetic idioplasm must be removed before embryonic
development can take place. In this way it seemed to me that not only
the ordinary cases of ovogenetic and embryonic development became more
easily intelligible, but also the rarer cases in which one and the same
species produces two kinds of eggs—‘summer and winter eggs.’ Such eggs
not only differ in size but also in the structure of yolk and
membranes, although identical animals are developed from each of them.
This result presupposes that the nucleus in both eggs contains
identical germ-plasm, while the formation of different yolks and
membranes requires the supposition that their nucleoplasm is different,
inasmuch as the two eggs differ greatly in histological character.
The fact that equal quantities are separated during nuclear division,
led me to conclude further that the expulsion of ovogenetic nucleoplasm
can only take place when the germ-plasm in the nucleus of the egg-cell
has increased by growth up to a point at which it can successfully
oppose the ovogenetic nuclear substance. But we do not know the
proportion which must obtain between the relative quantities of two
different nuclear substances in order that nuclear division may be
induced; and thus, by this hypothesis at least, we could not conclude
with certainty as to the necessity for a single or a double division of
the egg. It did not seem to be altogether inconceivable that the
ovogenetic nucleoplasm might be larger in amount than the germ-plasm,
and that it could only be completely removed by means of two successive
nuclear divisions. I admit that this supposition caused me some
uneasiness; but since nothing was known which could have enabled us to
penetrate more deeply into the problem, I was satisfied, for the time
being, in having found any explanation of the physiological value of
polar bodies; leaving the future to decide not only whether such
explanation were valid, but also whether it were exhaustive. The
explanation seems to have found but little favour with some of our
highest authorities. Hensen[244] does not consider that my reasons for
the distinction between germ-plasm and histogenetic nucleoplasm are
conclusive, and it may be conceded that this objection was perhaps, at
that time, well founded. O. Hertwig does not mention my hypothesis at
all in his work on embryology[245], although he states in the preface:
‘Among current problems I have chiefly taken into consideration the
views which seem to me to be most completely justified, but I have not
left unmentioned the views which I cannot accept.’ Minot’s hypothesis
is discussed by Hertwig, but Bütschli’s[246] is preferred by him,
although these two hypotheses are not strictly opposed to each other;
for the former is a purely physiological, the latter a purely
morphological explanation. I desire to lay especial stress upon the
fact that my hypothesis is simply a logical consequence from the
conclusion that the nuclear substance determines the nature of a cell.
How this takes place is quite another question, which need not be
discussed here. If it is only certain that the nature of a cell is thus
determined, it follows that a cell with a certain degree of
histological specialization must contain a nucleoplasm corresponding to
the specialization. But the mature egg also contains germ-plasm, and
there are only two possibilities by which these facts can be explained:
either the ovogenetic nucleoplasm is capable of re-transformation into
germ-plasm, or it is incapable of such re-transformation. Now, quite
apart from the arguments which might be advanced in favour of one of
these two possibilities, the fact that a body is undoubtedly expelled
from the mature egg seems to me of importance, while it is of even
greater importance that this body contains nucleoplasm from the
germ-cell.
It may be thought that the process, as supposed by me, is without
analogy, but such a conclusion is wrong, for during every embryonic
development there are numerous cell-divisions in which unequal
nucleoplasms are separated from one another, and in all these cases we
cannot imagine any way in which the process can take place, except by
supposing that the two kinds of nucleoplasm were previously united in
the mother-cell, although their differentiation probably took place
only a short time before cell-division. Perhaps the new facts which
will be mentioned presently, and the views derived from them, will make
my hypothesis upon the histogenetic nucleoplasm of the germ-cells
appear in a more favourable light to the authorities above-named.
My hypothesis has at all events the one merit that it has led me to
fruitful investigations.
If the formation of polar bodies really means the removal of ovogenetic
nucleoplasm from the mature egg, they must also be found in
parthenogenetic eggs; inasmuch as the latter possess a specific
histological structure equal to that found in eggs requiring
fertilization. If, therefore, it were possible to observe the formation
of polar bodies in eggs which develope parthenogenetically, such an
observation would not form a proof of the validity of my
interpretation; but it would be a fact which harmonized with it, and
negatived a suggestion which, if confirmed, would have been fatal to
the hypothesis. Minot, Balfour, and van Beneden, from the point of view
afforded by their theories, were compelled to suppose that polar bodies
are wanting in parthenogenetic eggs; and the facts which were known at
that time favoured such an opinion, for in spite of many attempts, no
one had ever succeeded in proving the formation of these bodies by
parthenogenetic eggs.
During the summer of 1885 I first succeeded in ascertaining that a
single polar body is expelled from the parthenogenetic summer-egg of
one of the _Daphnidae_,—_Polyphemus oculus_[247]. Thus my
interpretation of the process in question received support, while it
seemed to me that Minot’s interpretation of polar bodies had been
refuted; for if these bodies are formed in the parthenogenetic eggs of
a single species, just as in eggs which require fertilization, it
follows that the expulsion of polar bodies cannot signify the removal
of the male element from the egg.
The desire to throw light upon the significance of polar bodies has
been the only cause of my investigation. At the same time I hoped by
this means to gain further knowledge as to the nature of
parthenogenesis.
In the third part of the essay on ‘The Continuity of the Germ-plasm’
(see p. 225) I attempted to make clear the nature of parthenogenesis,
and I arrived at the conclusion that the difference between an egg
which is capable of developing without fertilization, and another which
requires fertilization, must lie in the quantity of nucleoplasm present
in the egg. I supposed that the nucleus of the mature parthenogenetic
egg contained nearly twice as much germ-plasm as that contained in the
sexual egg, just before the occurrence of fertilization; or, more
correctly, I believed that the quantity of nucleoplasm which remains in
the egg, after the expulsion of the polar bodies, is the same in both
eggs, but that the parthenogenetic egg possesses the power of doubling
this quantity by growth, and thus produces from within itself the same
quantity of germ-plasm as that contained in the sexual egg after the
addition of the sperm-nucleus in fertilization.
This was only an hypothesis, and the considerations which had led to it
depended, as far as they went into details, upon assumptions; but the
fundamental view that the _quantity_ of the nucleus decides whether
embryonic development takes place with or without fertilization seemed
to me, even at that time, to be correct, and to be a conclusion
required by the facts of the case. Indeed, I thought it not unlikely
that its validity might be proved by direct means: I pointed out that a
comparison of the quantities of the nuclei in parthenogenetic and
sexual eggs, if possible in the same species, would enable us to decide
the question (_l. c._, p. 234).
I had thus set myself the task of making this comparison. The result of
this investigation was to show that, as already mentioned, polar bodies
are formed in parthenogenetic eggs. But even the first species
successfully investigated revealed a further fact, which, if proved to
be wide-spread and characteristic of all parthenogenetic eggs, was
certain to be of extreme importance:—the maturation of the
parthenogenetic egg is accompanied by the expulsion of _one_ polar
body, or, as we might express it in another way, the substance of the
female pronucleus is only _once_ divided, and not _twice_, as in the
sexual eggs of so many other animals. If this difference between
parthenogenetic and sexual eggs was shown to be general, then the
foundations of my hypothesis would indeed have been proved to be sound.
The quantity of nuclear substance decides whether the egg is capable of
undergoing embryonic development. This quantity is twice as large in
the parthenogenetic as in the sexual egg. I had, however, been mistaken
in a matter of detail; for the difference in the quantities of nuclear
substance is not produced by the expulsion of two polar bodies, and the
reduction of the nuclear substance to a quarter of its original amount,
in both eggs, while the parthenogenetic egg then doubles its nuclear
substance by growth; but the difference is produced because the
reduction of nuclear substance originally present is less in one case
than it is in the other. In the parthenogenetic egg the nuclear
substance is only reduced to one-half by a single division; in the
sexual egg it is reduced to a quarter by two successive divisions. It
is an obvious conclusion from this fact, if proved to be wide-spread,
that the significance of the first polar body must be different from
that of the second. Only one polar body can signify the removal of
ovogenetic nucleoplasm from the mature egg, and the second is obviously
a reduction of the germ-plasm itself to half of its original amount.
This very point seemed to me to be of great importance, because, as I
had foreseen long ago, and as will be shown later on, the theory of
heredity forces us to suppose that every fertilization must be preceded
by a reduction of the ancestral idioplasms present in the nucleus of
the parent germ-cell, to one-half of their former number.
But before the full bearing of the phenomena could be considered, it
was necessary to ascertain how far they were of general occurrence.
There were two ways in which this might be achieved, and in which it
was possible to prove that parthenogenetic eggs expel only one polar
body, while sexual eggs expel two. We might attempt to observe the
phenomena of maturation in both kinds of eggs in a species which
reproduces itself by the parthenogenetic as well as the sexual method.
This would be the simplest way in which the question could be decided,
if it were possible to make such observations on a sufficient number of
species. But the other method was also open, a method which would have
been the only one, if we did not know of any animals with two kinds of
reproduction. We might attempt to investigate the phenomena of
maturation in a large number of parthenogenetic eggs, if possible from
different groups of animals, and we might compare the results with the
facts which are already certain concerning the expulsion of polar
bodies from the sexual eggs of so many species.
I have followed both methods, and by means of the second I have arrived
already, indeed some time ago, at the certain conclusion that the
above-mentioned difference is really general and without exception. The
first polar body only is formed in all the parthenogenetic eggs which I
investigated, with the valuable assistance of my pupil, Mr. Ischikawa
of Tokio. On the other hand, an extensive examination of the literature
of the subject convinced me that there is not a single undoubted
instance of the expulsion of only one polar body from eggs which
require fertilization, and that there are very numerous cases known
from almost all groups of the animal kingdom in which it is perfectly
certain that two polar bodies are formed, one after the other. A number
of the older observations cannot be relied upon, for the presence of
two polar bodies is mentioned without any explanation as to whether
they are expelled from the egg one after the other, or whether they
have merely resulted from the division of a single body after its
expulsion. In parthenogenetic eggs two polar bodies are also formed in
most cases, but they arise from the subsequent division of the single
body which separates from the egg. But such subsequent division is only
of secondary importance as far as the egg itself is concerned, and is
also unimportant in the interpretation of the process. The essential
nature of the process is to be found in the fact that the nucleus of
the egg-cell only divides once when parthenogenesis occurs, but twice
when fertilization is necessary, and it is of no importance whether the
expelled part of the nucleus of the cell-body atrophies at once, or
after it has undergone division. We have, therefore, to distinguish
between primary and secondary polar bodies. If this distinction is
recognized, and if we leave out of consideration all doubtful cases
mentioned in literature, such a large number of well-established
observations remain, that the existence of two primary polar bodies in
sexual eggs, and neither a smaller nor a larger number, may be
considered as proved.
Hence follows a conclusion which I believe to be very significant,—the
difference between parthenogenetic and sexual eggs lies in the fact
that in the former only one primary polar body is expelled, while two
are expelled from the latter. When, in July, 1886, I published a short
note[248] on part of the observations made upon parthenogenetic eggs, I
confined myself to facts, and did not mention this conclusion. I took
this course simply because I did not wish to bring it forward until I
had made sufficient observations in the first of the two ways described
above. I had hoped to be able to offer all the proofs that can be
obtained before undertaking to publish the far-reaching consequences
which would result from the above-mentioned conclusion. Unfortunately
the material with which I had hoped to quickly settle the matter,
proved less favourable than I had expected. Many hundred sections
through freshly laid winter-eggs of _Bythotrephes longimanus_ were made
in vain; they did not yield the wished for evidence, and although
continued investigation of other material has led to better results,
the proofs are not yet entirely complete.
I should not therefore even now have brought forward the
above-mentioned conclusion, if another observer had not alluded to this
idea, referring to my observations and also to a new discovery of his
own. In a recent number of the ‘Biologische Centralblatt,’
Blochmann[249] gives an account of his continued observations upon the
formation of polar bodies. It is well known that this careful observer
had previously shown that polar bodies do occur in the eggs of insects,
although they had not been found before. Blochmann proved that they are
found in the representatives of three different orders, so that we may
indeed ‘confidently hope to find corresponding phenomena in other
insects.’ This discovery is most important, and it was naturally very
welcome to me, as I had for a long time ascribed a high physiological
importance to the process of the formation of polar bodies, and it
would not be in accordance with such a view if the process was entirely
wanting from whole classes of animals. To fill up this gap in our
knowledge, and to give the required support to my theoretical views, I
had proposed to one of my pupils, Dr. Stuhlmann[250], that he should
work out the maturation of the eggs of insects; and it is a curious
ill-luck that he, like many other observers, did not succeed in
observing the expected expulsion of polar bodies, in spite of the great
trouble he had taken. It may be that the species selected for
investigation were unfavourable: at all events, we cannot now doubt
that a division of the egg-nucleus is quite universal among insects,
for Blochmann, in his latest contribution to the subject, proves that
the _Aphidae_ also form polar bodies. He examined the winter-eggs of
_Aphis aceris_, and ascertained that they form two polar bodies, one
after the other. Even in the viviparous _Aphidae_, thin sections
revealed the presence of a polar body, though Blochmann could not trace
all the stages of its development. It appears that the polar body is
here preserved for an exceptional period, and its presence can still be
proved when the blastoderm has been formed, and sometimes when
development is even further advanced. Skilled observers of recent
times, such as Will and Witlaczil, have not been able to find a polar
body in the parthenogenetic eggs of the _Aphidae_, and Blochmann’s
proof of its existence seems to me to be of especial value, because the
eggs of _Aphidae_ are in many respects so unusually reduced; for
instance, the primary yolk is absent and the egg-membrane is completely
deficient, so that we might have expected that if polar bodies are ever
absent, they would be wanting in these animals—that is, if they were of
no importance, or at any rate of only secondary importance.
Hence the presence of polar bodies in _Aphidae_ is a fresh confirmation
of their great physiological importance. As bearing upon the main
question dealt with in this essay, Blochmann’s observations have an
especial interest, because only one polar body was found in the
parthenogenetic eggs of _Aphis_, while the sexual eggs normally produce
two. The author rightly states that this result is in striking
accordance with my results obtained from the summer-eggs of different
_Daphnidae_, and he adds the remark,—‘It would be of great interest to
know whether these facts are due to the operation of some general law.’
To this remark I can now reply that there is indeed such a law: not
only in the parthenogenetic eggs of _Daphnidae_, but also, as I have
since found, in those of the Ostracoda and Rotifera[251], only one
primary polar body is formed, while two are formed in all eggs destined
for fertilization.
Before proceeding to the conclusions which follow from this fact, I
will at once remove a difficulty which is apparently presented by the
eggs which may develope with or without fertilization. I refer to the
well-known case of the eggs of bees. It might be objected to my theory
that the same egg cannot be prepared for development in more than one
out of the two possible ways; it might be argued that the egg either
possesses the power of entering upon two successive nuclear divisions
during maturation, and in this case requires fertilization; or the egg
may be of such a nature that it can only enter upon one such division
and can therefore form only one polar body, and in that case it is
capable of parthenogenetic development. Now there is no doubt, as I
pointed out in my paper on the nature of parthenogenesis[252], that in
the bee the very same egg may develope parthenogenetically, which under
other circumstances would have been fertilized. Bessel’s[253]
experiments, in which young queens were rendered incapable of flight,
and were thus prevented from fertilization, have shown that all the
eggs laid by such females develope into drones (males) which are well
known to result from parthenogenetic development. On the other hand,
bee-keepers have long known that young queens which are fertilized in a
normal manner continue for a long time to lay eggs which develope into
females, that is to say, which have been fertilized. Hence the same
eggs, viz. those which are lowest in the oviducts and are therefore
laid first, develope parthenogenetically in the mutilated female, but
are fertilized in the normal female. The question therefore arises as
to the way in which the eggs become capable of adapting themselves to
the expulsion of two polar bodies when they are to be fertilized, and
of one only when fertilization does not take place.