220 DEVELOPMENT OF ELASMOBRANCH FISHES. 



only the membrane and not the contents were extruded from 

 the egg, it would be possible to frame a consistent account of 

 the behaviour of the germinal vesicle throughout the animal 

 kingdom, which may be stated in the following way. 



The germinal vesicle usually before, but sometimes imme- 

 diately after impregnation undergoes atrophy and its contents 

 become indistinguishable from the remainder of the egg. In 

 those cases in which its membrane is very thick and resistent, 

 e.g. Osseous and Elasmobranch Fishes, Birds, etc., this may be 

 incapable of complete resorption, and be extruded bodily from 

 the egg. In the case of most ova, it is completely absorbed, 

 though at a subsequent period it may be extruded from the egg 

 as the Richtungskorper. In all cases the contents of the 

 germinal vesicle remain in the ovum. 



In some cases the germinal vesicle is stated to persist and to 

 undergo division during the process of segmentation ; but the 

 observations on this point stand in need of confirmation. 



My investigations shew that the germinal vesicle atrophies in 

 the Skate before impregnation, and in this respect accord with 

 very many recent observations. Of these the following may be 

 mentioned. 



(i) Oellacher (Bird, Osseous Fish). (2) Gotte (Bombinator 

 igneus). (3) Kupffer (Ascidia canina). (4) Strasburgef 

 (Phallusia mamillata). (5) Kleinenberg (Hydra). (6) Metsch- 

 nikoff (Geryonia, Polyzenia leucostyla, Epibulia aurantiaca, and 

 other Hydrozoa). 



This list is sufficient to shew that the disappearance of the 

 germinal vesicle before impregnation is very common, and I am 

 unacquainted with any observations tending to shew that its 

 disappearance is due to impregnation. 



In some cases, e.g. Asterocanthion 1 , the germinal vesicle 

 vanishes after the spermatozoa have begun to surround the egg; 

 but I do not know that its disappearance in these cases has 

 been shewn to be due to impregnation. To do so it would be 

 necessary to prove that in ripe eggs let loose from the ovary, but 

 not fertilized, the germinal vesicle did not undergo the same 

 changes as in the case of fertilized eggs ; and this, as far as I 



1 Agassiz, Embryology of the Star- Fish. 



