134 J. T. CUNNINGHAM. 



second directive spindle. These facts, assuming them to 

 be firmly established, have a very important bearing on 

 Balbiani's theory of the significance of the vitelline nucleus. 

 Since, at any rate in the eggs of many animals, the directive 

 spindles are provided with centrosomes, of which the one 

 belonging to the female pronucleus disappears before the for- 

 mation of the first segmentation spindle, it is clear that the 

 disappearance or degeneration of the vitelline nucleus can have 

 nothing to do with the absence of the female centrosome in 

 fertilisation. We must consider the question of the identity 

 of the vitelline nucleus with a centrosome on other grounds. 



If the centrosome were a permanent organ of the cell, and 

 the vitelline corpuscle were identical with the centrosome of 

 the ovum, the vitelline corpuscle having disintegrated, or, at 

 any rate, having been removed from the neighbourhood of the 

 nucleus, fixed in a distant part of the cytoplasm, and usually 

 surrounded by yolk, the directive spindle could not possess 

 centrosomes. For it is certain that the vitelline nucleus does 

 not return to the vicinity of the germinal vesicle and again 

 take part in its changes. If follows, therefore, either that the 

 centrosome is not a permanent part of the cell, or that the 

 vitelline nucleus is not to be identified as the centrosome. Now 

 if the vitelline nucleus is not the centrosome it is certain that 

 there is no other body visible in the ovum outside the germi- 

 nal vesicle which can be identified with the centrosome. The 

 suggestion that the centrosome exists within the germinal 

 vesicle is inconsistent with the theory of the persistence of the 

 centrosome outside the nucleus. The history of the ovum, then, 

 between its first definite constitution and the formation of the 

 polar bodies, actually disproves the theory of the unbroken 

 continuity of the centrosome as an extra-nuclear body. It is 

 an observed fact that the directive spindle, whether first or 

 second, possesses centrosomes, and yet no body can be dis- 

 covered in the egg at an earlier stage which can be identified 

 as a centrosome, except the vitelline nucleus, which is known 

 to degenerate and disappear. The centrosomes of the directive 

 spindle, then, must be formed as such at the tiine of theif 



