THE PROCESS OF FERTILIZATION 309 



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, Mt/zostoma, which is 

 parasitic on sea-lilies or Crinoids, Wheeler observed that the ovum 

 retained its central corpuscle even after the polar divisions, Avhile 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 (Crepidida) 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. Eacli 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 oliviously a secondary arrange- 

 ment, an adaptation to fertilization, that the ovum should be incapable 

 of development without fertilization, and it is made so by the dis- 

 appearance of its centrosome. In all other cells, as far as is known, 

 the central corpuscle persists ai;ter 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 dis- 

 appears, though even tliere often very late, for it may be present, as 

 an aster, even aft^r tlie 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-esntrosome 

 disappears as soon as the second polar division is accomplished. 



That tliis 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 Artemki. 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. 



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 



