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.