the possibility, that, under certain circumstances, the substance of
the egg-nucleus may be larger, so that it is capable of forming four
loops. Eggs which thus possess sufficient nucleoplasm, viz. germ-plasm,
for the formation of the requisite four loops of normal size, (namely,
of the size which would have been produced by fertilization), can and
must develope by the parthenogenetic method.
Of course the assumption that four loops must be formed has only been
made for the sake of illustration. We do not yet know whether there are
always exactly four loops in the segmentation nucleus[163]. I may add
that, although the details by which these considerations are
illustrated are based on arbitrary assumptions, the fundamental view
that the development of the egg depends, _ceteris paribus_, upon the
quantity of nuclear substance, is certainly right, and follows as a
necessary conclusion from the ascertained facts. It is not unlikely
that such a view may receive direct proof in the results of future
investigations. Such proof might for instance be forthcoming if we were
to ascertain, in the same species, the number of loops present in the
segmentation nucleus of fertilization, as compared with those present
in the segmentation nucleus of parthenogenesis.
The reproductive process in bees will perhaps be used as an argument
against my theory. In these insects, the same egg will develope into a
female or male individual, according as fertilization has or has not
taken place, respectively. Hence, one and the same egg is capable of
fertilization, and also of parthenogenetic development, if it does not
receive a spermatozoon. It is in the power of the queen-bee to produce
male or female individuals: by an act of will she decides whether the
egg she is laying is to be fertilized or unfertilized. She ‘knows
beforehand’[164] whether an egg will develope into a male or a female
animal, and deposits the latter kind in the cells of queens and
workers, the former in the cells of drones. It has been shown by the
discoveries of Leuckart and von Siebold that all the eggs are capable
of developing into male individuals, and that they are only transformed
into ‘female eggs’ by fertilization. This fact seems to be incompatible
with my theory as to the cause of parthenogenesis, for if the same egg,
possessing exactly the same contents, and above all the same
segmentation nucleus, may develope sexually or parthenogenetically, it
appears that the power of parthenogenetic development must depend on
some factor other than the quantity of germ-plasm.
Although this appears to be the case, I believe that my theory
encounters no real difficulty. I have no doubt whatever, that the same
egg may develope with or without fertilization. From a careful study of
the numerous excellent investigations upon this point which have been
conducted in a particularly striking manner by Bessels[165] (in
addition to the observers quoted above), I have come to the conclusion
that the fact is absolutely certain. It must be candidly admitted that
the _same_ egg will develope into a drone when not fertilized, or into
a worker or queen when fertilized. One of Bessels’ experiments is
sufficient to prove this assertion. He cut off the wings of a young
queen and thus rendered her incapable of taking ‘the nuptial flight.’
He then observed that all the eggs which she laid developed into male
individuals. This experiment was made in order to prove that drones are
produced by unfertilized eggs; but it also proves that the assertion
mentioned above is correct, for the eggs which ripen first and are
therefore first laid, would have been fertilized had the queen been
impregnated. The supposition that, at certain times, the queen produces
eggs requiring fertilization, while at other times her eggs develope
parthenogenetically, is quite excluded by this experiment; for it
follows from it, that the eggs must all be of precisely the same kind,
and that there is no difference between the eggs which require
fertilization and those which do not.
But does it therefore follow that the quantity of germ-plasm in the
segmentation nucleus is not the factor which determines the beginning
of embryonic development? I believe not. It can be very well imagined
that the nucleus of the egg, having expelled the ovogenetic
nucleoplasm, may be increased to the size requisite for the
segmentation nucleus in one of two ways: either by conjugation with a
sperm-nucleus, or by simply growing to double its size. There is
nothing improbable in this latter assumption, and one is even inclined
to inquire why such growth does not take place in all unfertilized
eggs. The true answer to this question must be that nature generally
pursues the sexual method of reproduction, and that the only way in
which the general occurrence of parthenogenesis could be prevented, was
by the production of eggs which remained sterile unless they were
fertilized. This was effected by a loss of the capability of growth on
the part of the egg-nucleus after it had expelled the ovogenetic
nucleoplasm.
The case of the bee proves in a very striking manner that the
difference between eggs which require fertilization, and those which do
not, is not produced until after the maturation of the egg, and the
removal of the ovogenetic nucleoplasm. The increase in the quantity of
the germ-plasm cannot have taken place at any earlier period, or else
the nucleus of the egg would always start embryonic development by
itself, and the egg would probably be incapable of fertilization. For
the relation between egg-nucleus and sperm-nucleus is obviously based
upon the fact that each of them is insufficient by itself, and requires
completion. If such completion had taken place at an early stage the
egg-nucleus would either cease to exercise any attractive force upon
the sperm-nucleus, or else conjugation would be effected, as in Fol’s
interesting experiments upon fertilization by many spermatozoa; and, as
in these experiments, malformation of the embryo would result. In
_Daphnidae_ I believe I have shown[166] that the summer-eggs are not
only developed parthenogenetically, but also that they are never
fertilized; and the explanation of this incapacity for fertilization
may perhaps be found in the fact that their segmentation nucleus is
already formed.
We may therefore conclude that, in bees, the nucleus of the egg, formed
during maturation, may either conjugate with the sperm-nucleus, or else
if no spermatozoon reaches the egg may, under the stimulus of internal
causes, grow to double its size, thus attaining the dimensions of the
segmentation nucleus. For our present purpose we may leave out of
consideration the fact that in the latter case the individual produced
is a male, and in the former case a female.
It is clear that such an increase in the germ-plasm must depend, to a
certain extent, upon the nutrition of the nucleus, and thus indirectly
upon the body of the egg-cell; but the increase must chiefly depend
upon internal nuclear conditions, viz. upon the capability of growth.
We must further assume that the latter condition plays the chief part
in the process, for everywhere in the organic world the limit of growth
depends upon the internal conditions of the growing body, and can only
be altered to a small extent by differences of nutrition. The phyletic
acquisition of the capability of parthenogenetic development must
therefore depend upon an alteration in the capability of growth
possessed by the nucleus of the egg.
This theory of parthenogenesis most nearly approaches Strasburger’s
views upon the subject, for he also explains the non-occurrence of
parthenogenetic development by the insufficient quantity of nucleoplasm
remaining in the egg after the expulsion of polar bodies. The former
theory differs however in that the occurrence of parthenogenesis is
supposed to be only due to an increase of this nucleoplasm to the
normal size of the segmentation nucleus. Strasburger assumes that
‘specially favourable conditions of nutrition counteract the deficiency
of nuclear idioplasm,’ while it seems to me that nutrition must be
considered as only of secondary importance. Thus in bees, as above
stated, the same egg may develope parthenogenetically or after
fertilization, the nucleus being subject to the same conditions of
nutrition in both cases. Strasburger[167] considers that
parthenogenesis may be interpreted by one of three possible
explanations. First, he suggests that especially favourable nutrition
may lead to the completion of the nuclear idioplasm. But if this
assumption be made, we must ask why a part of the idioplasm should be
previously expelled, when immediately afterwards the presence of an
equal amount becomes necessary. Such a view can only be explained by
the above-made assumption that the expelled nucleoplasm has a different
constitution from that possessed by the nucleoplasm which is afterwards
formed. It is true that we do not yet certainly know whether a polar
body is expelled in eggs in which parthenogenesis occurs, but we do
know that the egg of the bee passes through the same stages of
maturation whether it is to be fertilized or not. I can hardly accept
Strasburger’s second suggestion, ‘that under some favourable conditions
of nutrition half [or perhaps better, a quarter] of the idioplasm of
the egg-nucleus is sufficient to start the processes of development in
the cyto-idioplasm.’ Finally, his third suggestion, ‘that the
cyto-idioplasm, nourished by its surroundings and thus increased in
quantity, compels the nucleus of the egg to enter upon division,’
presupposes that the cell-body gives the impulse for nuclear division,
a supposition which up to the present time remains at least unproved.
The ascertained facts appear to me to indicate rather that the
cell-body serves only as a medium for the nutrition of the nucleus, and
Fol’s recently mentioned observations, which have been especially
quoted by Strasburger in support of his theories, seem to me to rather
confirm my conclusions. If supernumerary sperm-nuclei penetrate into
the egg, they may, under the nutritive influence of the cell-body,
become centres of attraction, and may take the first step towards
nuclear and cell-division by forming amphiasters. Such nuclei cannot
control the whole cell-body and force it to divide, but each one of
them, having grown to a certain size at the expense of the cell-body,
makes its influence felt over a certain area. Strasburger is quite
right in considering this process as a ‘partial parthenogenesis.’ Such
partial parthenogenesis presumably occurs in all egg-nuclei, but the
latter cannot attain to complete parthenogenesis when, as in Fol’s
supernumerary sperm-nuclei, their powers of assimilation are
insufficient to enable them to reach the requisite size. As before
stated, the cell-body does not force the nucleus to divide, but _vice
versa_. It would, moreover, be quite erroneous to suppose that
parthenogenetic eggs must contain a larger amount of nutritive material
in order to facilitate the growth of the nucleus. The parthenogenetic
eggs of certain _Daphnidae_ (_Bythotrephes_, _Polyphemus_) are very
much smaller than the winter-eggs, which require fertilization, in the
same species. It is also an error for Strasburger to conclude that ‘it
has been established with certainty that favourable conditions of
nutrition cause parthenogenetic development in _Daphnidae_, while
unfavourable conditions cause the formation of eggs requiring
fertilization.’ It is true that Carl Düsing[168], in his notable work
upon the origin of sex, has attempted, in a most ingenious manner, to
prove, from my observations and experiments on the reproduction of
_Daphnidae_, ‘that winter or summer-eggs are formed according to the
nutritive condition of the ovary.’ I do not, however, believe that he
has succeeded in this attempt, and at all events it is quite clear that
the validity of such conclusions is not fully established. I have
observed that the maturing eggs break up in the ovaries and are
absorbed in those _Daphnidae_ (_Sida_) which are starved because
sufficient food cannot be provided in captivity. Hence such animals
live, as it were, at the expense of their descendants; but it would be
quite erroneous to conclude with Düsing, from the similarity which such
disappearing egg-follicles bear to the groups of germ-cells which
normally break up in the formation of winter-eggs, that with a less
degree of starvation winter-eggs would have been formed. Düsing further
quotes my incidental remark that the formation of resting-eggs in
_Daphnia_ has been especially frequent in aquaria ‘which had been for
some time neglected, and in which it was found that a great increase in
the number of individuals had taken place.’ He is entirely wrong in
concluding that there was any want of food in these neglected aquaria;
and if I had foreseen that such conclusions would have been drawn, I
might have easily guarded against them by adding that in these very
aquaria an undisturbed growth of different algae was flourishing, so
that there could have been no deficiency, but, on the contrary, a great
abundance of nutritive material. I may add that since that time I have
conducted some experiments directly bearing upon this question, by
bringing virgin females as near to the verge of starvation as possible,
but in no case did they enter upon sexual reproduction[169].
An author must have been to some extent misled by preconceived ideas
when he is unable to see that the manner in which the two kinds of eggs
are respectively formed, directly excludes the possibility of the
origin of sexual eggs from the effects of deficient or poor nutrition.
The resting eggs, which require fertilization, are always larger, and
require for their formation far more nutritive material, than the
parthenogenetic summer-eggs. In _Moina_, for instance, forty large
food-cells are necessary for the formation of a resting egg, while a
summer-egg only requires three. And Düsing is aware of these facts, and
quotes them. How can the formation of resting eggs depend upon the
effects of poor nutrition when food is most abundant at the very time
of their formation? In all those species which inhabit lakes, sexual
reproduction occurs towards the autumn, and in such cases the resting
eggs are true winter-eggs, destined to preserve the species during the
winter. But at no time of the year is the food of the _Daphnidae_ so
abundant as in September and October, and frequently even until late in
November (in South Germany). At this period of the year, the water is
filled with flakes of animal and vegetable matter in a state of partial
decomposition, thus affording abundant food for many species. It also
swarms with a large number of species of Crustacea, Radiolaria, and
Infusoria; and thus such Daphnids as the _Polyphemidae_ are also well
provided for. Hence there is no deficiency in the supply of food. Any
one who has used a fine net in our fresh waters at this time of the
year must have been at first astonished at the enormous abundance of
the lower forms of animal life; and he must have been much more
astonished if he has been able to compare such results with the scanty
population of the same localities in spring. But it is during the
spring and summer that these very _Daphnidae_ reproduce themselves
parthenogenetically. I am far from believing that my experiments on
_Daphnidae_ are exhaustive and final, and I have stated this in my
published writings on the subject; but it seems to me that I have
established the fact that direct influences, whether of food or of
temperature, acting upon single individuals, do not determine the kind
of eggs which are to be produced; but that such a decisive influence is
to be found in the indirect conditions of life, and especially in the
average frequency of the recurrence of adverse circumstances which kill
whole colonies at once, such as the winter cold, or the drying-up of
small ponds in summer. It is unnecessary for me to controvert Düsing in
detail, as I have already taken this course in the case of Herbert
Spencer[170], who had also formed the hypothesis that diminished
nutrition causes sexual reproduction.
One of my observations seems, indeed, to support such a view, but only
when it is considered as an isolated example. I refer to the behaviour
of the genus _Moina_. Females of this genus which possess sexual eggs
in their ovaries, and which would have continued to produce such eggs
if males had been present, enter in the absence of the latter upon the
formation of parthenogenetic summer-eggs, that is, if the sexual eggs
have not all been extruded, but have been re-absorbed in the ovary. At
first sight, indeed, such a result appears to indicate that the
increase in nutrition, produced by the breaking-up of the large
winter-egg in the ovary, determines the formation of parthenogenetic
eggs. This apparent conclusion seems to be further confirmed by the
following fact. The transition from sexual to parthenogenetic
reproduction only occurs in one species of _Moina_ (_M. rectirostris_),
but in this species it occurs always and without exception, while in
the other species which I have investigated (_M. paradoxa_),
winter-eggs, when once formed, are always laid, and such females can
never produce summer-eggs. But in spite of this fact, Düsing is
mistaken when he explains the continuous formation of sexual eggs in
the latter species as due to the absence of any great increase in the
amount of nutrition, such as would have followed if the egg had broken
up in the ovary. In many other _Daphnidae_ which have come under my
notice, the females frequently enter again upon the formation of
parthenogenetic summer-eggs, after having laid fertilized resting eggs,
upon one or more occasions. This is the case, for instance, in all the
species of _Daphnia_ with which I am acquainted, and such a fact at
once proves that the abnormal increase in nutrition produced by the
absorption of winter-eggs cannot be the cause of the succeeding
parthenogenesis. It also supports the proof that a high or low
nutritive condition of the whole animal can have nothing to do with the
kind of eggs which are produced, for in the above-quoted instance, the
nutrition has remained the same throughout, or at all events has not
been increased. It is erroneous to always look for the explanation of
the mode of egg-formation in the direct action of external causes. Of
course there must be direct causes which determine that one germ shall
become a winter-egg, and another a summer-egg; but such causes do not
lie outside the animal, and have nothing to do with the nutritive
condition of the ovary: they are to be found in those conditions which
we are not at present able to analyze further, and which we must, in
the meantime, call the specific constitution of the species. In the
young males of _Daphnidae_ the testes have precisely the same
appearance as the ovaries of the young females[171], but the former
will, nevertheless, produce sperm-cells and not ova. In such cases the
sex of the young individual can always be identified by the form of the
first antenna and of the first thoracic appendage, both of which are
always clawed in the male. But who can point to the direct causes which
determine that the sexual cells shall become sperm-cells in this case,
and not egg-cells? Does the determining cause depend on the conditions
of nutrition? Or, again, in the females, can the state of nutrition
determine that the third out of a group of four germ-cells shall become
an egg-cell, and that the others shall break up to serve as its food?
It is, I think, clear that these are obvious instances of the general
conclusion that the direct causes determining the direction of
development in each case are not to be looked for in external
conditions, but in the constitution of the organs concerned.
We arrive at a like conclusion when we consider the quality of the eggs
which are produced. The constitution of one species of _Moina_ contains
the cause which determines that each individual shall produce
winter-eggs only, or summer-eggs only; while in another species the
transition from the formation of sexual eggs to the formation of
summer-eggs can take place, but only when the winter-egg remains
unfertilized. The latter case appears to me to be notably a special
adaptation, in this and other species, to the deficiency of males,
which is apt to occur. At all events, it is obvious that it is an
advantage that an unfertilized sexual egg shall not be lost to the
organism. The re-absorption of the winter-egg is an arrangement which,
without being the cause, is favourable to the production of summer-eggs.
This subject is by no means a simple one, as is proved by the behaviour
of the small group of _Daphnidae_. Thus in some species, the
winter-eggs are produced by purely sexual females, which never enter
upon parthenogenesis; in others, the sexual females may take the latter
course, but only when males are absent; in others, again, they
regularly enter upon parthenogenesis. In my work on _Daphnidae_, I have
attempted to show that their behaviour in this respect is associated
with the various external conditions under which the different species
live; and also that the ultimate occurrence of the sexual period, and
finally the whole cyclical alternation of sexual and parthenogenetic
reproduction, depend upon adaptation to certain external conditions of
life.
With the aid of my hypothesis that the egg-nucleus is composed of
ovogenetic nucleoplasm and germ-plasm, I can now attempt to give an
approximate explanation of the nature and origin of the direct causes
which determine the production, at one time of parthenogenetic
summer-eggs, and at another time of winter-eggs, requiring
fertilization. But in such an explanation I should also wish to include
a consideration of the causes which determine the formation of the
nutritive cells of the egg and of the sperm-cells to which I have
alluded above.
I believe that the direct cause which determines why the apparently
identical cells of the young testis and ovary in the _Daphnidae_
develope in such different directions, is to be found in the fact, that
their nuclei possess different histogenetic nucleoplasms, while, if we
neglect individual differences, the germ-plasm remains precisely the
same. In the sperm-cells the histogenetic nucleoplasm is spermogenetic,
in the egg-cells it is ovogenetic. This must be conceded if our
fundamental view is correct, that the specific nature of the cell-body
is determined by the nature of its nucleus.