Chapter XIV: Preface (4)

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._

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Footnotes for Essay IV.

Footnote 94:

Häckel, ‘Ueber die Wellenzeugung der Lebenstheilchen etc.,’ Berlin,
1876.

Footnote 95:

Darwin, ‘The Variation of Animals and Plants under Domestication,’
vol. ii. 1875, chap. xxvii. pp. 344-399.

Footnote 96:

His, ‘Unsre Körperform etc.,’ Leipzig, 1875.

Footnote 97:

Brooks, ‘The Law of Heredity,’ Baltimore, 1883.

Footnote 98:

Galton’s experiments on transfusion in Rabbits have in the mean time
really proved that Darwin’s gemmules do not exist. Roth indeed states
that Darwin has never maintained that his gemmules make use of the
circulation as a medium, but while on the one hand it cannot be shown
why they should fail to take the favourable opportunities afforded by
such a medium, inasmuch as they are said to be constantly circulating
through the body; so on the other hand we cannot understand how the
gemmules could contrive to avoid the circulation. Darwin has acted
very wisely in avoiding any explanation of the exact course in which
his gemmules circulate. He offered his hypothesis as a formal and not
as a real explanation.

Professor Meldola points out to me that Darwin did not admit that
Galton’s experiments disproved pangenesis (‘Nature,’ April 27, 1871,
p. 502), and Galton also admitted this in the next number of ‘Nature’
(May 4, 1871, p. 5).—A. W. 1889.

Footnote 99:

Weismann, ‘Ueber die Vererbung.’ Jena, 1883; translated in the
present volume as the second essay ‘On Heredity.’

Footnote 100:

E. Roth, ‘Die Thatsachen der Vererbung.’ 2. Aufl., Berlin, 1885, p.
14.

Footnote 101:

Jäger, ‘Lehrbuch der allgemeinen Zoologie,’ Bd. II. Leipzig, 1878.

Footnote 102:

M. Nussbaum, ‘Die Differenzirung des Geschlechts im Thierreich,’
Arch. f. Mikrosk. Anat., Bd. XVIII. 1880.

Footnote 103:

I have since learnt that Professor Rauber of Dorpat also expressed
similar views in 1880; and Professor Herdman of Liverpool informs me
that Mr. Francis Galton had brought forward in 1876 a theory of
heredity of which the fundamental idea in some ways approached that
of the continuity of the germ-plasm (‘Journal of the Anthropological
Institute,’ vol. v; London, 1876).—A. W., 1888.

[A less complete theory was brought forward by Galton at an earlier
date, in 1872 (see Proc. Roy. Soc. No. 136, p. 394). In this paper he
proposed the idea that heredity chiefly depends upon the development
of the offspring from elements directly derived from the fertilized
ovum which had produced the parent. Galton speaks of the fact that
‘each individual may properly be conceived as consisting of two
parts, one of which is latent and only known to us by its effects on
his posterity, while the other is patent, and constitutes the person
manifest to our senses. The adjacent and, in a broad sense, separate
lines of growth in which the patent and latent elements are situated,
diverge from a common group and converge to a common contribution,
because they were both evolved out of elements contained in a
structureless ovum, and they, jointly, contribute the elements which
form the structureless ova of their offspring.’ The following diagram
shows clearly ‘that the span of each of the links in the general
chain of heredity extends from one structureless stage to another,
and not from person to person:—

Structureless elements {...Adult Father... } structureless elements
in Father {...Latent in Father...} in Offspring.’

Again Galton states—‘Out of the structureless ovum the embryonic
elements are taken ... and these are developed (_a_) into the visible
adult individual; on the other hand ..., after the embryonic elements
have been segregated, the large residue is developed (_b_) into the
latent elements contained in the adult individual.’ The above quoted
sentences and diagram indicate that Galton does not derive the whole
of the hereditary tendencies from the latent elements, but that he
believes some effect is also produced by the patent elements. When
however he contrasts the relative power of these two influences, he
attaches comparatively little importance to the patent elements. Thus
if any character be fixed upon, Galton states that it ‘may be
conceived (1) as purely personal, without the concurrence of any
latent equivalents, (2) as personal but conjoined with latent
equivalents, and (3) as existent wholly in a latent form.’ He argues
that the hereditary power in the first case is exceedingly feeble,
because ‘the effects of the use and disuse of limbs, and those of
habit, are transmitted to posterity in only a very slight degree.’ He
also argues that many instances of the supposed transmission of
personal characters are really due to latent equivalents. ‘The
personal manifestation is, on the average, though it need not be so
in every case, a certain proof of the existence of latent elements.’
Having argued that the strength of the latter in heredity is further
supported by the facts of reversion, Galton considers it is safe to
conclude ‘that the contribution from the patent elements is very much
less than from the latent ones.’ In the later development of his
theory, Galton adheres to the conception of ‘gemmules’ and accepts
Darwin’s views, although ‘with considerable modification.’ Together
with pangenesis itself, Galton’s theory must be looked upon as
_preformational_, and so far it is in opposition to Weismann’s theory
which is _epigenetic_. See Appendix IV. to the next Essay (V.), pp.
316-319.—E. B. P.]

Footnote 104:

Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’
München u. Leipzig, 1884.

Footnote 105:

O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung und
Theilung des thierischen Eies.’ Leipzig, 1876.

Footnote 106:

Fol, ‘Recherches sur la fécondation, etc.’ Genève, 1879.

Footnote 107:

Kölliker formerly stated, and has again repeated in his most recent
publication, that the spermatozoa (‘Samenfäden’) are mere nuclei. At
the same time he recognizes the existence of sperm-cells in certain
species. But proofs of the former assertion ought to be much stronger
in order to be sufficient to support so improbable a hypothesis as
that the elements of fertilization may possess a varying
morphological value. Compare Zeitschr. f. wiss. Zool., Bd. XLII.

Footnote 108:

F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 69.

Footnote 109:

Arch. f. mikr. Anat., Bd. 23. p. 182, 1884.

Footnote 110:

Born, ‘Biologische Untersuchungen,’ I, Arch. Mikr. Anat., Bd. XXIV.

Footnote 111:

Roux, ‘Beiträge zum Entwicklungsmechanismus des Embryo,’ 1884.

Footnote 112:

O. Hertwig, ‘Welchen Einfluss übt die Schwerkraft,’ etc. Jena, 1884.

Footnote 113:

[Our present knowledge of the development of vegetable ova (including
the position of the parts of the embryo) is also in favour of the
view that it is not influenced by external causes, such as
gravitation and light. It takes place in a manner characteristic of
the genus or species, and essentially depends on other causes which
are fixed by heredity, see Heinricher ‘Beeinflusst das Licht die
Organanlage am Farnembryo?’ in Mittheilungen aus dem Botanischen
Institute zu Graz, II. Jena, 1888.—S. S.]

Footnote 114:

E. van Beneden, ‘Recherches sur la maturation de l’œuf,’ etc., 1883.

Footnote 115:

M. Nussbaum, ‘Ueber die Veränderung der Geschlechtsprodukte bis zur
Eifurchung,’ Arch. Mikr. Anat., 1884.

Footnote 116:

Eduard Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang
bei den Phanerogamen als Grundlage für eine Theorie der Zeugung.’
Jena, 1884.

[It is now generally admitted that, in the Vascular Cryptogams, as
also in Mosses and Liverworts, the bodies of the spermatozoids are
formed by the nuclei of the cells from which they arise. Only the
cilia which they possess, and which obviously merely serve as
locomotive organs, are said to arise from the surrounding cytoplasm.
It is therefore in these plants also the nucleus of the male cell
which effects the fertilization of the ovum. See Göbel, ‘Outlines of
Classification and Special Morphology,’ translated by H. E. F.
Garnsey, edited by I. B. Balfour, Oxford, 1887, p. 203, and Douglas
H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden,’ in
Berichte d. deutschen bot. Gesellschaft, vol. v (1887), p. 120.—S. S.]

Footnote 117:

O. Hertwig, ‘Das Problem der Befruchtung und der Isotropie des
Eies.’ Jena, 1885.

Footnote 118:

This opinion was first expressed in my lecture, ‘Ueber die Dauer des
Lebens,’ Jena, 1882, translated as the first essay in the present
volume.

Footnote 119:

M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fur
Natur- und Heilkunde.’ Dec. 15, 1884.

Footnote 120:

A. Gruber, ‘Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5.

Footnote 121:

According to the observations of Nussbaum and van Beneden, the egg of
_Ascaris_ departs from the ordinary type, but I think that the latter
observer goes too far when he concludes from the form of the nuclear
spindle (of which the two halves are inclined to each other at an
angle) that we have before us a process entirely different from that
of ordinary nuclear division.

Footnote 122:

Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti
Acad. Lyncei (3) vii. 1879, Roma.

Footnote 123:

M. Nussbaum, ‘Archiv für Mikroskopische Anatomie,’ Bd. XVIII und
XXIII.

Footnote 124:

Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882.

Footnote 125:

Kölliker, ‘Die Bedeutung der Zellkerne,’ etc.; Zeitschr. f. wiss.
Zool. Bd. XLII.

Footnote 126:

‘Compt. rend.’ Tom. LIV. p. 150.

Footnote 127:

‘Entwicklung der Dipteren.’ Leipzig, 1864.

Footnote 128:

‘Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866).

Footnote 129:

‘Compt. rend.’ Nov. 13, 1882.

Footnote 130:

Grobben, ‘Arbeiten d. Wien. Zool. Instituts,’ Bd. II. p. 203.

Footnote 131:

Bütschli, ‘Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409.

Footnote 132:

‘Science,’ vol. iv. No. 90, 1884.

Footnote 133:

Among unicellular organisms, encysted individuals are often called
germs. They sometimes differ from the adult organism in their smaller
size and simpler structure (_Gregarinidae_), but they represent the
same morphological stage of individuality.

Footnote 134:

Compare Bütschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’
Bd. I. p. 777.

Footnote 135:

Gustav Jäger, ‘Lehrbuch der Allgemeinen Zoologie,’ Leipzig, 1878; II.
Abtheilung. Probably on account of the extravagant and superficial
speculations of the author, the valuable ideas contained in his book
have been generally overlooked. It is only lately that I have become
aware of Jäger’s above-mentioned hypothesis. M. Nussbaum seems to
have also arrived at the same conclusion quite independently of
Jäger. The latter has not attempted to work out his hypothesis with
any degree of completeness. The above-mentioned observations are
followed immediately by quite valueless considerations, as, for
instance, that the ontogenetic and phyletic groups are in concentric
ratio! The author might as well speak of a quadrangular or triangular
ratio!

Footnote 136:

[Facts of the same kind are also known in the Vascular Cryptogams,
Muscineae, Characeae, Florideae, etc.—S. S.]

Footnote 137:

Weismann, ‘Die Entstehung der Sexualzellen bei den Hydromedusen.’
Jena, 1883.

Footnote 138:

[I adopt this term, suggested by E. Ray Lankester and G. C. Bourne,
as the name of the supporting lamina of Coelenterata. See ‘Quart.
Journ. Microsc. Sci.’ Jan. 1887, p. 28.—E. B. P.]

Footnote 139:

Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei
Obelia.’ Freiburg, Inaugural Dissertation: see also ‘Zeitschrift für
wissenschaftliche Zoologie.’ Bd. XLI. 1884.

Footnote 140:

English translation, by H. Marshall Ward. Oxford, 1887, Clarendon
Press.

Footnote 141:

[Such gland-cells are known in both animals and plants. See W.
Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting
cells of _Blechnum occidentale_ L., and _Osmunda regalis_ L., ‘Annals
of Botany,’ vol. i. p. 49.—S. S.]

Footnote 142:

Thus in 1877 Bütschli thought that ‘the chief significance of the
formation of polar bodies lies in the removal of part of the nucleus
of the egg, whether this removal is effected by simple expulsion or
by the budding of the egg-cell.’ ‘Entwicklungsgeschichtliche
Beiträge;’ Zeitschrift für wissenschaftliche Zoologie, Bd. XXIX. p.
237, footnote.

Footnote 143: