Part 38

If we are forced to the conclusion in regard to multicellular organisms that amphimixis does not supply the power of development to the ovum, but that, on the contrary, the power of development is withdrawn from the ovum, so that amphimixis can, so to speak, be forced, must we not assume something similar for unicellular organisms also? May not amphimixis be made compulsory in their case also, in that the Infusorians in preparation for conjugation go through changes which make their unlimited persistence possible only on condition that they conjugate? In my opinion the division of labour in the nucleus, which is differentiated into a macronucleus and a micronucleus, and the transitory nature of the former, may be regarded as an adaptation in this direction. In any case, it is striking that an organ which otherwise persists without limit among unicellular organisms, the nucleus, is here subject to natural death after the manner of the body of multicellular organisms, that it breaks up and must be reformed from the micronucleus which in this case is alone endowed with potential immortality. I am inclined to regard this as an arrangement for compelling conjugation, since it is only after conjugation that the micronucleus forms a new macronucleus, although the latter is indispensable to life, as we see from experiments in dividing Infusorians artificially.

Suppose we had to create the world of life, and it was said to us that amphimixis must--wherever possible--be secured periodically to all unicellular and multicellular organisms, what better could we do than arrange devices which should exclude individuals which, by chance or constitution, could not attain to amphimixis from the possibility of further life? But would amphimixis then be the cause of persistence or a principle of rejuvenescence?

I do not see that there can be any ground for such an assumption other than the tenacious and probably usually unconscious adherence to the inherited and deep-rooted idea of the dynamic significance of 'fertilization,' no longer, perhaps in its original form, which regarded the sperm as the vital spark which awakened new life in the dead ovum, but in the modified form of the 'rejuvenating' power of amphimixis.

Quite recently an attempt has been made to modify the idea of the 'rejuvenating' effect of amphimixis so that it should mean only an advantage, not an actual condition of persistence. Hartog, in particular, admits so much, that the occurrence of purely asexual and purely parthenogenetic reproduction excludes the possibility of our regarding the process of amphimixis as a condition of the maintenance of life. But then we must also cease to regard the 'ageing' and dying off of Infusorians which have been prevented from conjugating as an outcome of the primary constitution of the living substance, and should entirely abandon the misleading expression 'rejuvenescence.'

If we fix our attention on the numberless kinds of cells in higher organisms and on multicellular organisms as intact unities, we see that they all die off, that they are subject to a natural death, that is, a cessation of vital movement from internal causes, yet no one is likely to refer their transitoriness to the fact that they do not enter into amphimixis. We find it quite 'intelligible' that the cells of our body should be used up sooner or later as a result of their own function, though we are very far from being able to demonstrate the necessity for this, and so really to 'understand' it.

It is only from the standpoint of utility that we can understand the occurrence of natural death; we see that the germ-cells _must_ be potentially immortal like the unicellular organisms, but that the cells which make up the tissues of the body _may_ be transient, and indeed _must_ be so in the interests of their differentiation--often great and in one direction--which determines the services they render to the body. They required to become so differentiated that they could not continue to live on without limit, and they did become so differentiated because only thus could an ever-increasing functional capacity of the whole organism be rendered possible; but they die not because 'rejuvenescence through amphimixis is denied them, but because their physical constitution is what it is.' And we must explain the death of the whole many-celled individual in a similar way. When we were trying in a previous study to establish the unlimited continuance, the potential immortality, of unicellular organisms, we noted that an eternal continuance of the life of the body of multicellular organisms could certainly not be a necessity, since the continuance of these forms of life is secured by their germ-cells. A continuance of the body cannot even be regarded as useful from any point of view. And what is not useful for a form of life _does not arise as a lasting adaptation_, which is of course not to say that an immortality of multicellular organisms, such as they are now, would even have been possible. If these organisms were to attain to such a high degree of functional capacity and of structural complexity as they now exhibit, they obviously could not also have been adapted at the same time to an eternal persistence of life.

This is in perfect harmony with our whole conception of the impelling forces in the development of the organic world; the ever-increasing functional capacity of the structure arose from the advantage which this afforded in the struggle for existence, in comparison with which the apparent advantage of the endless life of the individual was of no account whatever.

I will not here follow out this idea. I have merely touched on it in order to make clear that the death of individuals in all multicellular organisms gives us no ground for thinking of the unlimited life of the germ-cells as dependent on a special artifice of nature, such as amphimixis is often supposed to be. Let us always remember that there is parthenogenesis, and that there are unicellular germs (spores) which are never fertilized, and that the reproduction of many species of animals and plants occurs in this way without the intervention of amphimixis at all.

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Attempts have recently been made to prove that parthenogenesis is a kind of self-fertilization, and these have been based on the observations of Blochmann and Brauer, which showed that in the bee and in the salt-water Crustacean, _Artemia salina_, the reducing second maturation division of the ovum-nucleus is not suppressed, but is regularly accomplished, and that the two daughter-nuclei which result from this division unite with each other subsequently. I have already noted that these statements do not hold true, at least with regard to the bee. In this case the second maturing division takes place without any subsequent fusion of the two daughter-nuclei. According to the observations of Dr. Petrunkewitsch, which I have already mentioned, and for the exactness of which I can vouch, the second maturation-spindle is unusually long, so that the two daughter-nuclei are pushed very far apart (Fig. 79, _Rsp 2_), and only the inner of the two nuclei (_K 4_) becomes a segmentation nucleus, while the outer undergoes a remarkable fate; it unites with the inner nucleus which results from the division of the _first maturation cell_ (_K 2_), and from this union the primitive _genital cells of the animal appear to arise_--an observation the eventual theoretical importance of which can only be estimated later.

Meantime all we can gain from it is a certain mistrust of the interpretation of the processes of maturation in _Artemia_ which have hitherto been given; at least we are tempted to suppose that the copulation of two nuclei which Brauer observed in _Artemia_ may not have led to the formation of the segmentation nucleus there either, but may have had some other significance.

But, even if we leave this point entirely out of account, there remain all the cases of regular parthenogenesis in which this mode of reproduction occurs alone and not in alternation with the sexual mode. In these only one maturing division is undergone, and only one polar body is formed, and thus there can lie no possibility of supposing a self-fertilization of the ovum.

It is possible that we may yet discover species among unicellular organisms which multiply without limit in the absence of any amphimixis. R. Hertwig has recently observed phenomena in Infusorians which he is inclined to refer to the suppression of an earlier habit of conjugation, and so to a kind of parthenogenesis. But even if it should be shown that amphimixis plays a part regularly and without exception in the life of _all_ unicellular organisms, the facts in regard to multicellular organisms are not affected; and, finally, the process of amphimixis is one which we have not the slightest ground for assuming to be either an awakener or a maintainer of life, and so I return to the most essential part of the whole problem, the meaning of the chromatin structures, the combination of which is the undoubted result of amphimixis. Do they really represent, as we assumed earlier, _the hereditary substance_, and what do we mean by this term?

As far as I know the literature and the development of biological theories, the botanist Nägeli was the first to deduce, from the considerable difference in size between the egg-cell and the sperm-cell, the conclusion that the material basis on which the hereditary tendencies depend must be a _minimal_ quantity of substance. The difference is especially great in animals, even in those species whose eggs may be called small, for instance, those of sea-urchins or of mammals; even in these the mass of spermatozoon is scarcely a thousandth part, often scarcely a hundred-thousandth part of the mass of the ovum. And yet the inheritance from the father and from the mother is equally great. Now as we know that vital powers have always a material basis, a minute quantity, such as is contained, for instance, in the spermatozoon of Man, must have implicitly in it all the hereditary tendencies of the father; and the conclusion is inevitable that in the ovum there can only be an equally minimal quantity of substance which is the bearer of the hereditary powers, for if there were a larger quantity of hereditary substance in the ovum its power of transmission would also be greater[16].

[16] The improbable assumption that the hereditary substance of the father may be in quality altogether different from that of the mother, and so may have the same power of transmission, and yet take up much less room, I leave out of the question altogether.

If we inquire as to the part of the spermatozoon which bears this hereditary substance, we may exclude both the tail-thread and the middle piece (Fig. 68), the former because it obviously fulfils quite a specialized physiological function and is histologically adapted to this function, the latter because, from observation on the spermatozoon which has made its way into the ovum, we know that it contains the centrosome, the dividing apparatus of the nucleus. Thus there only remains the 'head' of the spermatozoon, which includes the nucleus, as the possible vehicle of the heritable substance. Therefore we are led to seek for the hereditary substance in the nucleus. But the hereditary substance cannot be a perishable substance which may at need be dissolved, in the literal sense of the word, and be formed anew; therefore we cannot look for it in the nuclear membrane, and just as little in the 'nuclear sap' which fills the meshes of the nuclear network, since the material on which heredity depends must necessarily be solid. Nägeli has clearly shown that we must assume a stable, that is, a solid molecular architecture. There thus remains only the nuclear reticulum with its chromatin granules, and when we remember what we have learnt of the behaviour of this chromatin substance during division and amphimixis we can entertain no doubt that the sought-for bearer of the inheritance is contained in the substance of the chromosomes.

The great care with which the chromosomes are halved by means of the complicated division apparatus led us earlier to regard them as a substance of complex and manifold qualities and of great physiological importance; their constant number in any one species, and the reduction of that number to half by means of the reducing divisions, justify us in concluding that they are permanent structures, physiological and morphological units, which undergo no more than an apparent irregular dispersion during the resting state of the nucleus. Finally, the fact that these supposed vehicles of inheritance occur in equal numbers in each of the conjugating germ-cells, and that this number is _always_, both in animals and in plants, half of the normal number occurring in somatic cells, is decisive. The logical necessity that the hereditary substance of both parents should be transmitted to the offspring in equal quantity could not be more precisely met than it is by the fact that half the normal number of chromosomes occurs in each of the sex-nuclei in the ovum. Personally, I have long been certain, on these grounds, that the chromosomes of the nucleus are the hereditary substance, and I expressed my conviction on this point almost simultaneously with Strasburger and O. Hertwig[17].

[17] More precisely, my conclusions were published several months later than those of the investigators named (1885). I think, however, that no one who is familiar with my writings for the years immediately preceding, which are collected in _Aufsätzen über Vererbung und verwandte biologische Fragen_ (Jena, 1892), will dispute that the idea was reached by me independently. I attach importance to this because all my later work is based upon this idea.

But there is also a physiological proof of the meaning of the nuclear substance; and this we owe, again, to the simultaneous and independent researches of two investigators, M. Nussbaum and A. Gruber, the latter working in the Zoological Institute here (in Freiburg), and at my request. They made experiments on regeneration in unicellular organisms, and found that Infusorians which were artificially divided into two, three, or four pieces were able to build up a whole animal out of each piece, provided that it contained a portion of the nucleus (macronucleus). The large blue trumpet-animalcule, _Stentor cœruleus_, is well suited for such experiments, not only on account of its size, but because it possesses a very long rosary-like nucleus, which can be easily cut two or three times. When a piece is cut off which does not contain a portion of the nucleus, it may indeed live for some days and swim about and contract, but it is incapable of reconstructing the lost parts, and thus of forming a whole animal, and it perishes. It is in the nucleus, therefore, that we have to look for the substance which stamps the material of the cell-body with a particular form and organization, namely, the form and organization of its ancestors. But that is exactly the conception of a hereditary substance or idioplasm (Nägeli). Some modern biologists deny that there is any hereditary substance _per se_, and believe that the whole of the germ-cell, cell-body and nucleus together effects transmission. But though it must be admitted that the nucleus without the cell-body cannot express inheritance any more than the cell-body without the nucleus, this is dependent on the fact that the nucleus cannot live without the cell-body; if it be removed from the cell and put, say, into water, it bursts and is dissolved. But the cell-body without the nucleus lives on, though of course only for a few hours or days, and its metabolism ceases only when it is brought to a standstill by the failure to replace by nutrition the used-up material. Thus the argument used by those who deny the existence of a hereditary substance would be paralleled if we denied that Man possesses a thinking substance, and maintained that he thinks with his whole body, and even that the brain cannot think by itself without the body.

I am convinced that it is just as mistaken to maintain that every part of an organism must contain the hereditary tendencies in the same degree, or that in unicellular organisms the cell-body is as important in inheritance as the nucleus (Conklin). If one feels any doubt on this point, one has only to call to mind Nägeli's inference, from the minuteness of the spermatozoon, that the hereditary substance must be minimal in quantity. But even theoretically there is not the smallest ground for the assumption that the cell-body as well as the nucleus contains the hereditary qualities, since we find in general that functions are distributed among definite substances and parts of the whole organism, and it is just on this division of labour that the whole differentiation of the body depends. And why should this principle not have been employed just here where the most important of all functions is concerned? Why should all living substance be hereditary substance? Although Nägeli thought of his 'idioplasm' otherwise than we now think of hereditary substance, although he wrongly imagined it in the form of strands running a parallel course through the cell-substance and forming a connected reticulum throughout the whole body, he recognized at least so much quite correctly, that there are two great categories of living substance--hereditary substance or idioplasm, and 'nutritive substance' or trophoplasm, and that the former is much smaller in mass than the latter. We now add to this, that the idioplasm must be sought for in the cell-nucleus, and indeed in the chromatin granules of the nuclear network and of the chromosomes.

But incontrovertible proof of the fact that the nuclear substance _alone_ is the hereditary substance was furnished when it was found possible to introduce into a non-nucleated piece of a mature ovum of one species the nucleus of another related species, and when it was seen that the larva that developed from the ovum so treated belonged to the _second_ species. Boveri made this experiment with the ovum and spermatozoon of two species of sea-urchin, and believed that he had succeeded in getting from non-nucleated pieces of the ovum of the first species, fertilized with the sperm of the second, larvæ of this second species; but, unfortunately, later control-experiments made by several investigators, especially by Seeliger, have shown that this result cannot be regarded as quite certain and indubitable.

I must emphasize again that I am far from regarding the cell-protoplasm of the ovum as an indifferent substance. It is certainly not only important but indispensable for the development of the embryo, and it has assuredly its own specific character, as in every other kind of cell. It represents, so to speak, the matrix and nutritive environment in which alone the hereditary substance can unfold its wonderful powers; it has developed historically, like every other kind of cell, but it contains nothing more than the inherited qualities of this one kind of cell-protoplasm, not those of the other cells of the body.

But although the essence of fertilization lies, as we have seen, in the union of the hereditary substance of two individuals, and not in a 'quickening' of the ovum, we may quite well speak of a quickening by fertilization in another sense, if we mean the impulse to embryonic development, for this is really supplied by the entrance of the sperm-nucleus with its centrosphere into the ovum. But even this impulse can, under certain circumstances, be given in another way, and certainly the awakening of it is not the _end_ of fertilization, but only the condition without which the end, the union of two kinds of nuclear substance, could not be attained. There is no indication whatever that this 'quickening' of the ovum would be necessary for any other reason except that _the ovum was previously made incapable of development_. There would be no 'fertilization' were not the mingling of hereditary substances of fundamental importance for the organic world.

Moreover, an ovum, or a fragment of an ovum, may also develop of itself, having only _one_ of the sex-nuclei, and the union of the hereditary substance of two cells is therefore not indispensable for the mere production of a new individual.

What has been observed in regard to fragments of ova is particularly interesting in this connexion. Ernst Ziegler first succeeded in halving a newly fertilized sea-urchin ovum, so that one half contained the female and the other the male pronucleus. The latter alone contained a centrosphere, and developed a blastula larva. Delage carried these experiments further, and cut an unfertilized but mature sea-urchin ovum into pieces, and then 'fertilized' the non-nucleated pieces with spermatozoa. These pieces developed and yielded young larvæ of the relevant species; so it is clearly seen that even a piece of mature ovum-protoplasm may undergo embryonic development, provided that a nucleus furnished with a dividing apparatus penetrates into it. Unfortunately it is technically impossible to cut such a non-nucleated and then fertilized fragment of ovum so that one half shall contain the male nucleus the other its centrosphere. Even without this _experimentum crucis_ we may say that the half with the male nucleus would not multiply by division, and that the other probably would, though it would not go through the regular course of segmentation processes, because the hereditary substance absolutely necessary for these was wanting.

But these and similar experiments prove something more, namely, that the nuclei of the sperm-cell and egg-cell do not, as was formerly believed, stand in a primary and essential contrast to each other, which may be described as male and female, but that both are alike in their deeper essence, and may replace each other. They only differ from each other as far as the cells to which they belong differ, in this, namely, that they are mutually attractive; they find each other and unite, and then go on to develop, which each was previously unable to do by itself. Widely as the sperm-cell and egg-cell differ in size, constitution, and behaviour, in regard to essential character they are alike; they bear the relation--as I expressed it twenty years ago--of 1:1; that is, _they both contain an equal quantity of essentially similar hereditary substance_, and the quality of this substance is only individually variable. We should, therefore, speak not of a 'male' and 'female,' but of a 'paternal' and a 'maternal' nucleus.