OVAHY AND OVARIAN OVA IN CKRTAIN MARINE FISHES. 133 



that the degeneration of the vitelline nucleus, that body being 

 the centrosome of tlie egg, accounts for the absence of the 

 centrosome in tlie fertilised egg; and for the fact, asserted by 

 Boveri, that the centrosomes of the segmenting egg are both 

 derived from the spermatozoon. 



These views are open to several serious objections. There 

 is first the uncertainty with respect to the origin of the vitelline 

 nucleus. Balbiani argues that since the vitelline nucleus arises 

 from the nucleus proper, the centrosome in an ordinary cell 

 has a similar origin, a fact which he thinks explains the im- 

 portant part taken by the centrosome in the division of the 

 cell. But the latest and most definite researches indicate the 

 constant presence of the centrosome in the cytoplasm, and if 

 this is correct then it ought to be possible to trace back the 

 vitelline nucleus into continuity with the centrosome of the 

 germ- cell. An equal difficulty presents itself in the interpre- 

 tation of the later history of the egg. There is ample evidence 

 that the vitelline nucleus disappears in the yolk, and takes no 

 further part in fertilisation or segmentation. Yet the polar 

 bodies are formed bv mitotic division before fertilisation. If 

 the vitelline nucleus is the effete centrosome of the ovum, — and 

 if centrosomes are such a constant and important feature in 

 mitotic division, — how is it that the nucleus of the ovum is able 

 to divide twice in the formation of the polar globules ? Boveri 

 (3) concluded that the centrosome of the ovum disappeared 

 before fertilisation, and pointed out that in some eggs the direc- 

 tive spindle was destitute of polar radiations at its extremities. 

 But numerous observers have seen radiations and centrosomes 

 at the extremities of the directive spindle, and Boveri suggests 

 that the female centrosome, when present, is no longer capable, 

 after the formation of the polar globules, of performing its 

 functions, i. e. of taking part in cell division ; it disappears, 

 and the male centrosome takes its place. This suggestion has 

 been demonstrated to be actually correct by A. D. Mead (16) 

 in a paper published in 1895. This observer states that he 

 followed the degeneration and disappearance of the egg 

 centrosome, — that is to say, the inner centrosome of the 



