Thus amphimixis is seen to be really a condition of development. But we now know that the ovum can emancipate itself from this condition, by only going through a part of the processes of maturation which are related to the subsequent amphimixis, and by thus retaining its own centrosome. Nothing is more instructive in this connexion than the cases we have already briefly discussed of facultative or occasional parthenogenesis. We have seen that in some insects, for instance in the silk-moths, there are sometimes, among thousands of unfertilized eggs, a few that develop little caterpillars. If we examine a large number of such unfertilized eggs we not infrequently find among them several which, although they have not gone through the whole course of development, have at least gone through the earlier stages, and others which may have advanced somewhat further and then come to a standstill; in short, we can see that several of these eggs were capable of parthenogenetic development, although in varying degrees.
The cause of this parthenogenetic capacity has not as yet been definitely determined by observation, but we shall hardly go wrong if we seek it in the fact that the centrosphere of the ovum does not always perish immediately and completely during maturation, and may persist, rarely in its integrity, but sometimes in a weakened state. Future observations will probably reveal some differences in the size or aster-forming power of the centrospheres of such eggs; in any case it is of the greatest interest that stimuli of various kinds—mechanical or chemical—can strengthen the disappearing centrosphere of the ovum, although as yet we are far from being able to say how this comes about.
The experiments already mentioned of Tichomiroff, Loeb, and Winkler give us at least an indication how we must picture to ourselves the origin of parthenogenesis, namely, through the fact that the breaking up of the apparatus for division, introduced for the sake of compelling amphimixis, is prevented. Minute changes in the chemistry of the ovum, similar to those caused artificially in the ova of the sea-urchin by the introduction of an infinitesimal quantity of chloride of magnesium (Loeb), in the ovum of the silk-moth by friction or by sulphuric acid (Tichomiroff), or in the sea-urchin ovum by an extract of the sperm of the same animal (H. Winkler), will effect this modification, and normal parthenogenesis is induced.
For the ovum, therefore, amphimixis is certainly not a life-renewing or rejuvenating factor; it only appears as such because the process has in the course of nature been made compulsory by making the two uniting cells each incapable of developing by itself. As we have seen, this is true also of the sperm-cell, for although it contains a centrosphere, and would be capable of division as far as that is concerned, yet in almost all animals and plants it consists of such a minimal quantity of living matter that it is unable to build up a new multicellular organism by itself. Only in one alga (Ectocarpus siliculosus) has it been observed that not only the female germ-cell can develop parthenogenetically under certain circumstances, but that the male-cell may also do so. In this case, however, the difference in size between the two is not great, and it is noteworthy that the male plant, in correspondence with the smaller size of the zoosperm, tends to be a somewhat poorly developed organism.
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.