OVART AND OVARIAN OVA IN CERTAIN MARINE FISHES. 135 



appearance — must be a new modification of some part of nucleus 

 or cytoplasm. 



If centrosomcs can be formed anew thus on the occasion 

 of one mitosis, there is apparently no reason why they should 

 not be newly formed at each mitosis. Further, we know that 

 the centrosome of the ripe egg — that is to say, the inner centro- 

 some of the second directive spindle — degenerates. It may be 

 urged that this is a special case, that this centrosome dege- 

 nerates as a preliminary to fertilisation, because the centre- 

 somes of the first segmentation spindle are supplied by the 

 sperm. But since the centrosomcs of the directive spindle are 

 formed anew, the preceding centrosome must have disappeared. 

 Therefore there is nothing to contradict the suggestion that 

 this preceding centrosome is the vitelline nucleus, which dege- 

 nerates. And if the centrosome degenerates after one mitosis, 

 there is no reason why it should not degenerate after every 

 mitosis. On these grounds the theory might be suggested 

 that the centrosome is not persistent, but is a structure formed 

 by the changes which precede mitosis, and that after division 

 it disappears, more or less gradually in the cytoplasm. This 

 theory does not, however, appear to be applicable to all mitoses, 

 for in the segmentation of the fertilised ovum, according to 

 the latest observations, the centrosome and archoplasm persist 

 after each division, remain during the reconstitution of the 

 resting nucleus, and then divide to form the new spindle of 

 division and the centrosomcs of its two extremities. It is 

 definitely and particularly stated by A. D. Mead that this 

 occurs in the segmentation of the ovum of Chsetopterus. 



We must conclude, then, that although the extra-nuclear 

 persistence or continuity of the centrosome has been observed 

 in the segmentation of the ovum, and may occur in the mitosis 

 of all somatic cells, it certainly does not occur in the matura- 

 tion of the ovum. The centrosomcs of the directive spindles 

 are not formed from a pre-existing extra-nuclear body. In 

 this case the centrosome of the ovum must have passed into 

 the interior of the germinal vesicle, or must have degenerated 

 in the cytoplasm. There is no evidence of the inclusion of 



