158 J. T. CUNNINGHAM. 



which spawning has never occurred. Here also they are aborted 

 dead ova, which are undergoing disintegration and absorption. 

 They are scattered singly in the ovarian tissue, and their de- 

 velopment is arrested at an early stage, before the formation 

 of yolk has made any progress, if even it has commenced. 

 In those in which death has only recently occurred the germinal 

 vesicle is seen to be shrunken, and to contain a single large 

 spherical nucleolus. 



The vitelline nucleus is first seen as a stained corpuscle in 

 contact with the germinal vesicle. I have not been able to 

 follow the mitosis of the germ-cells, or to trace back the vitel- 

 line nucleus, but consider it most probable that the latter is 

 identical with the centrosome which remains in the ovum after 

 the last division of the germ-cell. 



In ova of plaice and flounder the vitelline nucleus separates 

 from the germinal vesicle, moves towards the surface of the 

 ovum, and is afterwards found at the inner border of the yolk- 

 layer. It becomes surrounded with yolk, and ceases to be 

 visible. 



In Syngnathus acus there are often two or even more 

 vitelline nuclei in a single ovum. These are probably pro- 

 duced by the division of a single body. 



If the vitelline nucleus is the centrosome, its disappearance 

 forms an interruption to the persistence of the centrosome as 

 an extra-nuclear body, since the directive spindle is provided 

 with new centrosomes. 



The germinal vesicle in the Teleostean ova examined con- 

 sists at first of a single large nucleolus, a nuclear network, 

 and a surrounding membrane. I could not resolve the net- 

 work into a continuous filament, or into separate chromo- 

 somes. 



At the next stage the vesicle is larger, and there are several 

 nucleoli in contact with the nuclear membrane. 



In still larger ova the nucleoli are still at the periphery, but 

 there is a central region in the vesicle distinguished by the 

 presence of separate feathery fibrils, the centrosomes of 

 Rvickert. 



