The Evolution Theory, Vol. 1 of 2

by August Weismann

From all these results we can at least infer so much, that chemical changes and influences may determine whether the ripe ovum shall go on to embryonic development or not, and that these influences, may be very diverse in nature in different cases. I shall return later to these important facts.

When we sum up the facts we have cited with reference to the reduction of the number of chromosomes, it appears that nature is, as it were, striving to keep the number constant for each species; that in germ-cells which are destined for amphimixis they are reduced to half the normal number, but that this halving of the number is suppressed where fertilization is always absent, or that the reduction to half is compensated for again in various ways, whether by subsequent fusion of the two daughter-nuclei, which have arisen from the process of reduction, or by an independent duplicating of the chromosomes in the segmentation nucleus.

We might perhaps be inclined to conclude from all this that the occurrence of development depended on the presence of the normal number of chromosomes; and I used to regard this as possible. But facts which have been more recently brought to light have excluded this view. Above all, we now know that every nuclear division depends on the presence of a dividing apparatus, a centrosphere, but that this organ degenerates in the ova of most animals and is completely lost after the second polar division has been effected. The mature ovum is therefore in itself incapable of entering on its embryonic development, no matter how many chromosomes its nucleus contains; it is only capable of further division when the fertilizing sperm-cell brings with it its dividing apparatus or centrosphere. In thread-like sperms this lies in the median portion ([Fig. 68]C), and after the tail-piece has been dissolved, which happens soon after the sperm enters the egg, the central corpuscle, at first very small, can be recognized in front of the sperm-nucleus, where it is soon transformed into an 'aster' and divides into two. Then both spheres move apart ([Fig. 75]D, p. 296) and form the nuclear spindle between them by the confluence of their rays.

From this the division of the ovum into the two first embryonic cells proceeds. The two pronuclei in the ovum, the male and the female, are thus exactly alike as to number of the chromosomes, and frequently at least as to size and appearance ([Fig. 75]C). But they differ in the possession or absence of a dividing apparatus, and in the great majority of cases it is the male nucleus that brings with it the central corpuscle which seems to be indispensable to embryonic development ([B], cspt). Hitherto, at least, only two exceptions to this are known. In the little segmented worm, Myzostoma, which is parasitic on sea-lilies or Crinoids, Wheeler observed that the ovum retained its central corpuscle even after the polar divisions, while the sperm-cell which penetrated into the egg had none. More recently Conklin made the interesting discovery that in the egg of a marine Gasteropod (Crepidula) both the egg-nucleus and the sperm-nucleus retain their centrosphere and together form the segmentation spindle, one lying at one pole and the other at the opposite.

All these observations confirm the view that the sperm and the egg-cell are alike in this respect also. Each of them can, in certain circumstances, bring with it the dividing apparatus indispensable to development, though it is usually the sperm-cell that does so.

I should indeed assume that the sperm-cell and the egg-cell were essentially alike, even although there were no exception to this rule, that is, although the centrosome of the ovum perished in all eggs which were fertilized. For this is obviously a secondary arrangement, an adaptation to fertilization, that the ovum should be incapable of development without fertilization, and it is made so by the disappearance of its centrosome. In all other cells, as far as is known, the central corpuscle persists after division, so that this remarkable cell-organ is transmitted from cell to cell just like the nucleus, and like it, never rises de novo. It is only in the egg-cell that it disappears, though even there often very late, for it may be present, as an aster, even after the sperm has penetrated into the ovum and disclosed its own central body, or even brought it the length of dividing into two (Fig. 80, A and B). But the ovum-centrosome disappears as soon as the second polar division is accomplished.

That this disappearance is really a secondary arrangement, which may be again departed from, is proved by the case of those eggs which are able to develop parthenogenetically, for in them the central body does not disappear, but persists in the ovum after the first polar division, as Brauer showed in Artemia. It then behaves exactly like the sphere of the sperm-nucleus in the fertilized ovum, that is, it duplicates itself and forms the segmentation spindle.

Fig. 80. Fertilization of the ovum of a Gasteropod (Physa), after Kostanecki and Wierzejski. A, the whole spermatozoon lies in the ovum. sp, its already divided centrosphere. Rk 1, the first polar body. Rsp 2, the second directive spindle. B, spk, the sperm-nucleus, the second directive spindle still has its centrosphere, which afterwards disappears. The first polar body (Rk 1) has divided into two. Highly magnified.

Thus the beginning of embryonic development in the ovum depends not on a definite number of chromosomes, but on the presence of an apparatus for division. Upon what the awakening of this to activity just at that time depends cannot as yet be exactly stated; we can only indicate that all parts of the cell have interrelations with each other, and that, therefore, the division mechanism is dependent on the condition of the rest of the cell-parts at the moment, and on the substances which they contain or produce. From what we know experimentally in regard to artificial parthenogenesis it is not difficult to imagine that some sort of chemical substances are necessary to stimulate the central corpuscle to activity. In any case, the whole nutrition of the central corpuscle depends on the cell in which it lies, as is shown by the fact that the sperm-nucleus, whose centrosome before the entrance of the sperm into the ovum was inactive and scarcely recognizable, grows rapidly after entrance and forms a large aster round itself—is, in short, in the highest degree active (Fig. 80). As the chromosomes certainly play an important part in the life of the cell, and materially help to determine its various phases, it cannot be disputed that they also may share in awakening the activity of the central corpuscle. But this influence is only indirect; it is not the mere number of chromosomes that decides whether the central corpuscle is to become active or remain inactive. This cannot be assumed, because we have in the maturation divisions a proof that division may take place with a double number of chromosomes as well as with the undoubled number; while in the divisions of the mother-egg-cells and the mother-sperm-cells we have proof that a doubled number of chromosomes does not in itself compel to division.