Essays Upon Heredity and Kindred Biological Problems

But before the full bearing of the phenomena could be considered, it was necessary to ascertain how far they were of general occurrence. There were two ways in which this might be achieved, and in which it was possible to prove that parthenogenetic eggs expel only one polar body, while sexual eggs expel two. We might attempt to observe the phenomena of maturation in both kinds of eggs in a species which reproduces itself by the parthenogenetic as well as the sexual method. This would be the simplest way in which the question could be decided, if it were possible to make such observations on a sufficient number of species. But the other method was also open, a method which would have been the only one, if we did not know of any animals with two kinds of reproduction. We might attempt to investigate the phenomena of maturation in a large number of parthenogenetic eggs, if possible from different groups of animals, and we might compare the results with the facts which are already certain concerning the expulsion of polar bodies from the sexual eggs of so many species.

I have followed both methods, and by means of the second I have arrived already, indeed some time ago, at the certain conclusion that the above-mentioned difference is really general and without exception. The first polar body only is formed in all the parthenogenetic eggs which I investigated, with the valuable assistance of my pupil, Mr. Ischikawa of Tokio. On the other hand, an extensive examination of the literature of the subject convinced me that there is not a single undoubted instance of the expulsion of only one polar body from eggs which require fertilization, and that there are very numerous cases known from almost all groups of the animal kingdom in which it is perfectly certain that two polar bodies are formed, one after the other. A number of the older observations cannot be relied upon, for the presence of two polar bodies is mentioned without any explanation as to whether they are expelled from the egg one after the other, or whether they have merely resulted from the division of a single body after its expulsion. In parthenogenetic eggs two polar bodies are also formed in most cases, but they arise from the subsequent division of the single body which separates from the egg. But such subsequent division is only of secondary importance as far as the egg itself is concerned, and is also unimportant in the interpretation of the process. The essential nature of the process is to be found in the fact that the nucleus of the egg-cell only divides once when parthenogenesis occurs, but twice when fertilization is necessary, and it is of no importance whether the expelled part of the nucleus of the cell-body atrophies at once, or after it has undergone division. We have, therefore, to distinguish between primary and secondary polar bodies. If this distinction is recognized, and if we leave out of consideration all doubtful cases mentioned in literature, such a large number of well-established observations remain, that the existence of two primary polar bodies in sexual eggs, and neither a smaller nor a larger number, may be considered as proved.

Hence follows a conclusion which I believe to be very significant,—the difference between parthenogenetic and sexual eggs lies in the fact that in the former only one primary polar body is expelled, while two are expelled from the latter. When, in July, 1886, I published a short note248 on part of the observations made upon parthenogenetic eggs, I confined myself to facts, and did not mention this conclusion. I took this course simply because I did not wish to bring it forward until I had made sufficient observations in the first of the two ways described above. I had hoped to be able to offer all the proofs that can be obtained before undertaking to publish the far-reaching consequences which would result from the above-mentioned conclusion. Unfortunately the material with which I had hoped to quickly settle the matter, proved less favourable than I had expected. Many hundred sections through freshly laid winter-eggs of Bythotrephes longimanus were made in vain; they did not yield the wished for evidence, and although continued investigation of other material has led to better results, the proofs are not yet entirely complete.

I should not therefore even now have brought forward the above-mentioned conclusion, if another observer had not alluded to this idea, referring to my observations and also to a new discovery of his own. In a recent number of the ‘Biologische Centralblatt,’ Blochmann249 gives an account of his continued observations upon the formation of polar bodies. It is well known that this careful observer had previously shown that polar bodies do occur in the eggs of insects, although they had not been found before. Blochmann proved that they are found in the representatives of three different orders, so that we may indeed ‘confidently hope to find corresponding phenomena in other insects.’ This discovery is most important, and it was naturally very welcome to me, as I had for a long time ascribed a high physiological importance to the process of the formation of polar bodies, and it would not be in accordance with such a view if the process was entirely wanting from whole classes of animals. To fill up this gap in our knowledge, and to give the required support to my theoretical views, I had proposed to one of my pupils, Dr. Stuhlmann250, that he should work out the maturation of the eggs of insects; and it is a curious ill-luck that he, like many other observers, did not succeed in observing the expected expulsion of polar bodies, in spite of the great trouble he had taken. It may be that the species selected for investigation were unfavourable: at all events, we cannot now doubt that a division of the egg-nucleus is quite universal among insects, for Blochmann, in his latest contribution to the subject, proves that the Aphidae also form polar bodies. He examined the winter-eggs of Aphis aceris, and ascertained that they form two polar bodies, one after the other. Even in the viviparous Aphidae, thin sections revealed the presence of a polar body, though Blochmann could not trace all the stages of its development. It appears that the polar body is here preserved for an exceptional period, and its presence can still be proved when the blastoderm has been formed, and sometimes when development is even further advanced. Skilled observers of recent times, such as Will and Witlaczil, have not been able to find a polar body in the parthenogenetic eggs of the Aphidae, and Blochmann’s proof of its existence seems to me to be of especial value, because the eggs of Aphidae are in many respects so unusually reduced; for instance, the primary yolk is absent and the egg-membrane is completely deficient, so that we might have expected that if polar bodies are ever absent, they would be wanting in these animals—that is, if they were of no importance, or at any rate of only secondary importance.

Hence the presence of polar bodies in Aphidae is a fresh confirmation of their great physiological importance. As bearing upon the main question dealt with in this essay, Blochmann’s observations have an especial interest, because only one polar body was found in the parthenogenetic eggs of Aphis, while the sexual eggs normally produce two. The author rightly states that this result is in striking accordance with my results obtained from the summer-eggs of different Daphnidae, and he adds the remark,—‘It would be of great interest to know whether these facts are due to the operation of some general law.’ To this remark I can now reply that there is indeed such a law: not only in the parthenogenetic eggs of Daphnidae, but also, as I have since found, in those of the Ostracoda and Rotifera251, only one primary polar body is formed, while two are formed in all eggs destined for fertilization.

Before proceeding to the conclusions which follow from this fact, I will at once remove a difficulty which is apparently presented by the eggs which may develope with or without fertilization. I refer to the well-known case of the eggs of bees. It might be objected to my theory that the same egg cannot be prepared for development in more than one out of the two possible ways; it might be argued that the egg either possesses the power of entering upon two successive nuclear divisions during maturation, and in this case requires fertilization; or the egg may be of such a nature that it can only enter upon one such division and can therefore form only one polar body, and in that case it is capable of parthenogenetic development. Now there is no doubt, as I pointed out in my paper on the nature of parthenogenesis252, that in the bee the very same egg may develope parthenogenetically, which under other circumstances would have been fertilized. Bessel’s253 experiments, in which young queens were rendered incapable of flight, and were thus prevented from fertilization, have shown that all the eggs laid by such females develope into drones (males) which are well known to result from parthenogenetic development. On the other hand, bee-keepers have long known that young queens which are fertilized in a normal manner continue for a long time to lay eggs which develope into females, that is to say, which have been fertilized. Hence the same eggs, viz. those which are lowest in the oviducts and are therefore laid first, develope parthenogenetically in the mutilated female, but are fertilized in the normal female. The question therefore arises as to the way in which the eggs become capable of adapting themselves to the expulsion of two polar bodies when they are to be fertilized, and of one only when fertilization does not take place.

But perhaps the solution of this problem is not so difficult as it appears to be. If we may assume that in eggs which are capable of two kinds of development the second polar body is not expelled until the entrance of a spermatozoon has taken place, the explanation of the possibility of parthenogenetic development when fertilization does not occur would be forthcoming. Now we know, from the investigations of O. Hertwig and Fol, that in the eggs of Echinus the two polar bodies are even formed in the ovary, and are therefore quite independent of fertilization, but in this and other similar cases a parthenogenetic development of the egg never takes place. There are, however, observations upon other animals which point to the fact that the first only and not the second polar body may be formed before the spermatozoon penetrates into the egg. It can be easily understood why it is that entirely conclusive observations are wanting, for hitherto there has been no reason for any accurate distinction between the first and the second polar body. But in many eggs it appears certain that the second polar body is not expelled until the spermatozoon has penetrated. O. Schultze, the latest observer of the egg of the frog, in fact saw the first polar body alone extruded from the unfertilized egg: a second nuclear spindle was indeed formed, but the second polar body was not expelled until after fertilization had taken place. A very obvious theory therefore suggests itself:—that while the formation of the second polar body is purely a phenomenon of maturation in most animal eggs, and is independent of fertilization,—in the eggs of a number of other animals, on the other hand, and especially among Arthropods, the formation of the second nuclear spindle is the result of a stimulus due to the entrance of a spermatozoon. If this suggestion be confirmed, we should be able to understand why parthenogenesis occurs in certain classes of animals wherever the external conditions of life render its appearance advantageous, and further, why in so many species of insects a sporadic parthenogenesis is observed, viz. the parthenogenetic development of single eggs (Lepidoptera). Slight individual differences in the facility with which the second nuclear spindle is formed independently of fertilization would in such cases decide whether an egg is or is not capable of parthenogenetic development. As soon, however, as the second nuclear spindle is formed, parthenogenesis becomes impossible. The nuclear spindle which gives rise to the second polar body, and that which initiates segmentation, are two entirely different things, and although they contain the same quantity, and the same kind of germ-plasm, a transformation of the one into the other is scarcely conceivable. This conclusion will be demonstrated in the following part of the essay.

II. The Significance of the Second Polar Body

I have already discussed the physiological importance of the first polar body, or rather of the first division undergone by the nucleus of the egg, and I have explained it as the removal of ovogenetic nuclear substance which has become superfluous and indeed injurious after the maturation of the egg. I do not indeed know of any other meaning which can be ascribed to this process, now that we know of the occurrence of a first division of the nucleus in parthenogenetic as well as in sexual eggs. A part of the nucleus must thus be removed from both kinds of eggs, a part which was necessary to complete their growth, and which then became superfluous and at the same time injurious. In this respect the observations of Blochmann254 upon the eggs of Musca vomitoria seem to me to be very interesting. Here the two successive divisions of the nuclear spindle arising from the egg-nucleus take place, but true polar bodies are not expelled, and the two nuclei corresponding to them (one of which divides once more) are placed on the surface of the egg, surrounded by an area free from yolk granules; and they break up at a later period. The essential point is obviously to eliminate from the egg-cell the influence of nucleoplasm which has been separated from the egg-nucleus as the first polar body; and this condition is satisfied whether the elimination is brought about by a process of true cell-division, as is the rule in the eggs of most animals, or by the division and removal of part of the egg-nucleus alone. The occurrence of the latter method of elimination certainly constitutes a still further proof of the physiological importance of the process, and this, taken together with the universal occurrence of polar bodies in all eggs—parthenogenetic and sexual—forces us to conclude that the process must possess a definite significance. No one of the various attempts which have been made to explain the significance of polar bodies generally is applicable to the first polar body except that which I have attempted.

But the case is different with the significance of the second nuclear division, or the second polar body. Here it might perhaps be possible to return to the view brought forward by Minot, Balfour, and van Beneden, and to consider the removal of this part of the nucleus as the expulsion of the male part of the previously hermaphrodite egg-cell. The second polar body is only expelled when the egg is to be fertilized, and at first sight it appears to be quite obvious that such a preparation of the egg for fertilization must depend upon its reduction to the female state. I believe however that this is not the case, and am of opinion that the process has an entirely different and much deeper meaning.

How can we gain any conception of this supposed hermaphroditism of the egg-cell, and its subsequent attainment of the female state? What are the essential characteristics of the male and female states? We know of female and male individuals, among both animals and plants: their differences consist essentially in the fact that they produce different kinds of reproductive cells; in part they are of a secondary nature, being adaptations of the organism to the functions of reproduction; they are intended to attract the other sex, or to ensure the meeting of the two kinds of reproductive cells, or to enable the fertilized egg to develope and sometimes to guide the development of the offspring until it has reached a certain period of growth. But all these differences, however great they may sometimes be, do not alter the essential nature of the organism. The blood corpuscles of man and woman are the same, and so are the cells of their nerves and muscles; and even the sexual cells, so different in size, appearance, and generally also in motile power, must contain the same fundamental substance, the same idioplasm. Otherwise the female germ-cell could not transmit the male characters of the ancestors of the female quite as readily as the female characters, nor could the male germ-cell transmit the female quite as readily as the male characters of the ancestors of the male. It is therefore clear that the nuclear substance itself is not sexually differentiated.

I have already previously pointed out that the above-mentioned facts of heredity contain the disproof of Minot’s theory, inasmuch as the egg-cell transmits male as well as female characters. Strasburger255 has also raised a similar objection. I consider this objection to be quite conclusive, for there does not seem to be any way in which the difficulty can be met by the supporters of the theory. The difficulty could indeed be evaded until we came to know that the essential part of the polar body is nuclear substance, and that the latter must be regarded as idioplasm,—as the substance which is the bearer of heredity. It might have been maintained that the male part, removed from the egg, consists only in a condition, perhaps comparable to positive or negative electricity; and that this condition is present in the substance of the polar body, so that the removal of the latter would merely signify a removal of the unknown condition. I do not mean to imply that any of those who have adopted Minot’s theory have had any such vague ideas concerning this process, but even if any one were ready to adopt it, he would be unable to make any use of the idea. He would not be able to support the theory in this way, for we now know that nuclear substance is removed with the polar body, and this fact requires an explanation which cannot be afforded by the theory, if we are right in believing that the expelled nuclear substance is not merely the indifferent bearer of the unknown principle of the male condition, but hereditary substance. I therefore believe that Minot’s, Balfour’s, and van Beneden’s hypothesis, although an ingenious attempt which was quite justified at the time when it originated, must be finally abandoned.

My opinion of the significance of the second polar body is shortly this,—a reduction of the germ-plasm is brought about by its formation, a reduction not only in quantity, but above all in the complexity of its constitution. By means of the second nuclear division the excessive accumulation of different kinds of hereditary tendencies or germ-plasms is prevented, which without it would be necessarily produced by fertilization. With the nucleus of the second polar body as many different kinds of idioplasm are removed from the egg as will be afterwards introduced by the sperm-nucleus; thus the second division of the egg-nucleus serves to keep constant the number of different kinds of idioplasm, of which the germ-plasm is composed during the course of generations.

In order to make this intelligible a short explanation is necessary.

From the splendid series of investigations on the process of fertilization, commenced by Auerbach and Bütschli, and continued by Hertwig, Fol, Strasburger, van Beneden, and many others, and from the theoretical considerations brought forward by Pflüger, Nägeli, and myself, at least one certain result follows, viz. that there is an hereditary substance, a material bearer of hereditary tendencies, and that this substance is contained in the nucleus of the germ-cell, and in that part of it which forms the nuclear thread, which at certain periods appears in the form of loops or rods. We may further maintain that fertilization consists in the fact that an equal number of loops from either parent are placed side by side, and that the segmentation nucleus is composed in this way. It is of no importance, as far as this question is concerned, whether the loops of the two parents coalesce sooner or later, or whether they remain separate. The only essential conclusion demanded by our hypothesis is that there should be complete or approximate equality between the quantities of hereditary substance derived from either parent. If then the germ-cells of the offspring contain the united germ-plasms of both parents, it follows that such cells can only contain half as much paternal germ-plasm as was contained in the germ-cells of the father, and half as much maternal germ-plasm as was contained in the germ-cells of the mother. This principle is affirmed in a well-known calculation made by breeders of animals, who only differ from us in their use of the term ‘blood’ instead of the term germ-plasm. Breeders say that half of the ‘blood’ of the offspring has been derived from the father and the other half from the mother. The grandchild similarly derives a quarter of its ‘blood’ from each of the four grandparents, and so on.

Let us imagine, for the sake of argument, that sexual reproduction had not been introduced into the animal kingdom, and that asexual reproduction had hitherto existed alone. In such a case, the germ-plasm of the first generation of a species which enters upon sexual reproduction must still be entirely homogeneous; the hereditary substance must, in each individual, consist of many minute units, each of which is exactly like the other, and each of which contains within itself the tendency to transmit, under certain circumstances, the whole of the characters of the parent to a new organism—the offspring. In each of the offspring of such a first generation, the germ-plasms of two parents will be united, and every germ-cell contained in the individuals of this second sexually produced generation will now contain two kinds of germ-plasm—one kind from the father, and the other from the mother. But if the total quantity of germ-plasm present in each cell is to be kept within the pre-determined limits, each of the two ancestral germ-plasms, as I may now call them, must be represented by only half as many units as were contained in the parent germ-cells.

In the third sexually produced generation, two new ancestral germ-plasms would be added by fertilization to the two already present, and the germ-cells of this generation would therefore contain four different ancestral germ-plasms, each of which would constitute a quarter of the total quantity. In each succeeding generation the number of the ancestral germ-plasms is doubled, while their quantities are reduced by one half. Thus in the fifth sexually produced generation, each of the sixteen ancestral germ-plasms will only constitute 1/16 of the total quantity; in the sixth, each of the thirty-two ancestral germ-plasms, only 1/32, and so on. The germ-plasm of the tenth generation would be composed of 1024 different ancestral germ-plasms, and that of the nth of 2n. By the tenth generation each single ancestral germ-plasm would only form 1/1024 of the total quantity of germ-plasm contained in a single germ-cell. We know nothing whatever of the length of time over which this process of division of the ancestral germ-plasms may have endured, but even if it had continued to the utmost possible limit—so far indeed that each ancestral germ-plasm was only represented by a single unit—a time would at last come when any further division into halves would cease to be possible; for the very conception of a unit implies that it cannot be divided without the loss of its essential nature, which in this case constitutes it as the hereditary substance.

In the diagram represented in Fig. I. I have tried to render these conclusions intelligible. In generation I. each paternal and maternal germ-plasm is still entirely homogeneous, and does not contain any combination of different hereditary qualities, but the germ-plasm of the offspring is made up of equal parts of two kinds of germ-plasm. In the second generation this latter germ-plasm unites with another derived from other parents, which is similarly composed of two ancestral germ-plasms, and the resulting third generation now contains four different ancestral germ-plasms in its germ-cells, and so on. The diagram only indicates the fusion of ancestral germ-plasms as far as the offspring of the fourth generation, the germ-cells of which contain sixteen different ancestral germ-plasms. If we imagine the germ-plasm units to be so large that there is only room for sixteen of them in the nuclear thread, the limits of division would-be reached in the fifth generation, and any further division into halves of the ancestral germ-plasms would be impossible.