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.
This explanation of the necessity for fertilization does not exclude 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.
Similarly, the germ-cells of female Daphnidae, which at first do not exhibit the smallest differences, must really differ in that their nuclei must contain different kinds of nucleoplasm, which are present in different proportions. Germ-cells which are to produce a finely granular, brick-red, winter yolk (Moina rectirostris) must possess an ovogenetic nucleoplasm of a somewhat different molecular structure from those germ-cells which have only to form a few large blue fat-globules, as in the summer-eggs of the same species. It is further probable that different proportions obtain between germ-plasm and ovogenetic nucleoplasm, in these two kinds of germ-cells; and it would be a very simple explanation of the otherwise obscure part played by the food-cells, if we were to suppose that they do not contain any germ-plasm at all, and on this account do not enter upon embryonic development, but are arrested after growing to a certain size. Such an explanation, however, would not by itself show why they subsequently undergo gradual solution in the surrounding fluids. But since we know that egg-cells also begin to undergo solution as soon as the parent Daphnid is poorly nourished, we can hardly help also referring the solution of the food-cells to insufficient nourishment, occurring as soon as the egg-cell, after the attainment of a certain size, exercises a superior power of assimilation. But hitherto we could not in any way understand why the third out of a group of germ-cells should always gain this superior power and become an egg-cell. If it could be shown that its position is more highly favoured in respect of nutrition, we could understand why it outstrips the other three in development, and thus prevents them from further growth. But nothing of the kind can be shown to occur with any degree of probability, as I have previously mentioned in my works on the subject. At that time, having no better explanation, I adopted the view in question, although only as a provisional interpretation. It was not possible for me to seek in the substance of those four apparently identical cells for the cause of their different development; but now I am justified in offering the supposition that during the division of a primitive germ-cell into two, and afterwards into four germ-cells, an unequal division of the nucleoplasms takes place, in that one of the four cells receives germ-plasm as well as ovogenetic nucleoplasm, while the other three receive the latter alone. Similarly, the fact that the second cell of the group may occasionally become an egg is also intelligible, although this fact remained quite inexplicable by my former interpretation. The fact that true egg-cells, or even the whole ovary with all its germ-cells, may break up and become absorbed when the animal has been starved for a certain period of time, seems to me to be no objection to our present view, any more than the fact that an Infusorian may die from starvation would be an objection to the supposition of the immortality of unicellular organisms. The growth of an organism is not only arrested by its constitution, but also by absolute want of food; but it would be very foolish to explain the differences in size of the various species of animals as results of the different conditions of nutrition to which they were subject. Just as a sparrow, however highly nourished, could never attain the size or form of an eagle, so a germ-cell destined to become a summer-egg could never attain the size, form, or colour of a winter-egg. It is by internal constitutional causes that the course of development is determined in both these cases; and in the latter, the cause can hardly be anything more than the different constitution of the nucleoplasms.
All these considerations depend upon the supposition that the egg-nucleus contains two kinds of idioplasm, viz. germ-plasm and ovogenetic nucleoplasm. I have not hitherto brought forward any direct evidence in favour of this assumption, but I believe that such proofs can be obtained.
It is well known that there are certain eggs in which the polar bodies are not expelled until after the entrance of spermatozoa. Brooks[[172]] has already made use of this fact as evidence against Minot’s and Balfour’s theory; for he quite rightly concludes that if the polar bodies really possess the significance of male cells, we cannot understand why such eggs are unable to develope without fertilization, when they still possess the male half of the nucleus necessary for development. But such eggs (e.g. that of the oyster) do not develope, but always die if they remain unfertilized.
This argument can only be met by a new hypothesis, the construction of which I must leave to the defenders of the above-mentioned theory. But the observation in question seems to me to furnish at the same time a proof of the co-existence of two different nucleoplasms in the egg-nucleus. If the nucleoplasm of the polar bodies was also germ-plasm, we could not understand why such eggs are unable to develope parthenogenetically, for at least as much germ-plasm is contained in the unfertilized egg as would have been present after fertilization.
The only objection which can be raised against this conclusion depends upon the supposition that the nucleoplasm of the sperm-cell is qualitatively different from that of the egg-cell. I have already dealt with this view, but I should wish to refer to it again rather more in detail. Some years ago I expressed the opinion[[173]] that the physiological values of the sperm-cell and of the egg-cell must be identical; that they stand in the ratio of 1 : 1. But Valaoritis[[174]] has brought forward the objection that if we consider the function of a cell as the measure of its physiological value, it is only necessary to point to the respective functions of ovum and spermatozoon in order to show that their physiological values must be different. ‘The egg-cell alone, by passing more or less completely through the phyletic stages of the female parent, developes into a similar organism; and although the presence of the spermatozoon is in most cases required in order to render possible such a result, the cases of parthenogenesis prove nevertheless that the egg can do without this stimulus.’ This objection appeared to be fully justified as long as fertilization was looked upon as the ‘vitalization of the germ,’ and so long as the sperm-cell was considered as merely ‘the spark that kindles the gunpowder,’ and further so long as the germ-substance was believed to be contained in the cell-body. But now we can hardly give to the body of the egg-cell a higher significance than that of the common nutritive soil of the two nuclei which conjugate in fertilization. But these two nuclei ‘are not different in nature,’ as Strasburger says, and as I fully believe. They cannot differ in kind, for they both consist of germ-plasm belonging to the same species of animal or plant; and there cannot be any deeper contrast between them such as would correspond to the differences between mature individuals. They cannot, from their essential nature, exercise any special attraction upon each other, and when we see that sperm-cell and egg-cell do nevertheless attract each other, as has been shown in both plants and animals, such a property must have been secondarily acquired, and has no other significance than to favour the union of sexual cells—an arrangement which may be compared to the vibrating flagellum of the spermatozoon or the micropyle of the egg, but which is not fundamental, and is not based upon the molecular structure of the germ-plasm. In lower plants, Pfeffer has proved that certain chemical stimuli emanate from the egg and attract the spermatozoid; and according to Strasburger, the synergidae in the upper part of the embryo-sac of Phanerogams secrete a substance which is capable of directing the growth of the pollen-tube towards the egg-cell. In animals it is only known as yet that spermatozoa and ova do attract each other, so that the former find the latter and bore their way through its membranes. It has also been shown that the substance of the egg-body moves towards the penetrating spermatozoon (‘cones d’exsudation’ in Asteridae: Fol); and that it sometimes enters upon convulsive movements (Petromyzon). Here therefore a mutual stimulation and attraction must exist; and perhaps we must also assume that there is an attraction between the two conjugating nuclei, for we cannot readily understand how the cytoplasm alone could direct the one to the other, as Strasburger supposes. According to Strasburger’s hypothesis, we must suppose that part of the specific cytoplasm of the sperm-cell continues to surround the nucleus after it has penetrated into the body of the egg. But however this may be, the assumed attraction between the conjugating nuclei certainly cannot depend upon the molecular structure of their germ-plasm, which is the same in both, but it must be due to some accessory circumstance. If it were possible to introduce the female pronucleus of an egg into another egg of the same species, immediately after the transformation of the nucleus of the latter into the female pronucleus, it is very probable that the two nuclei would conjugate just as if a fertilizing sperm-nucleus had penetrated. If this were so, the direct proof that egg-nucleus and sperm-nucleus are identical would be furnished. Unfortunately the practical difficulties are so great that it is hardly possible that the experiment can ever be made; but such want of experimental proof is partially compensated for by the fact, ascertained by Berthold, that in certain Algae (Ectocarpus and Scytosiphon) there is not only a female, but also a male parthenogenesis; for he shows that in these species the male germ-cells may sometimes develope into plants, which however are very weakly[[175]]. Furthermore the process of conjugation may be considered as a proof that this view as to the secondary importance of sexual differentiation is the true one. At the present time there can hardly be any hesitation in accepting the view that conjugation is the sexual reproduction of unicellular organisms. In these the two conjugating cells are almost always identical in appearance, and there is no evidence in favour of the assumption that they are not also identical in molecular structure, at least so far as one individual of the same species may be identical with another. But there are also forms in which the conjugating cells are distinctly differentiated into male and female, and these are connected with the former by a gradual transition: thus in Pandorina, a genus of Volvocineae, we are unable to make out any differences between the conjugating cells, while large egg-cells and minute sperm-cells exist in the closely allied Volvox. If we must suppose that the conjugation of two entirely identical Infusoria has the same physiological effect as the union of two sexual cells in higher animals and plants, we cannot escape the conclusion that the process is essentially the same throughout: and that therefore the differences, which are perhaps already indicated in Pandorina and are very distinct in Volvox and in all higher organisms, have nothing to do with the nature of the process, but are of quite secondary importance. If we further take into account the extremely different constitution of the two kinds of sexual cells in size, appearance, membranes, motile power, and finally in number, no doubt remains that these differences are only adaptations which secure the meeting of the two kinds of conjugating cells: that in each species they are adaptations to the peculiar conditions under which fertilization takes place.
NOTE.
It is of considerable importance for the proper appreciation of the views advanced in the present essay, to ascertain whether a polar body is or is not expelled from eggs which develope parthenogenetically. I wish therefore to briefly state that I have recently succeeded in proving the formation of a polar body of distinctly cellular structure in the summer-eggs of Daphnidae. I propose to publish a more detailed account in a future paper.
A. W.
June 22, 1885.