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. [Illustration: 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.]