DEVELOPMENT OF ELASMOBRANCH FISHES. 9 



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

 know, has not been done. After the disappearance of the 

 germinal vesicle, and before the first act of division, a fresh 

 nucleus frequently appears [ — vide — Auerbach (Ascaris nigro- 

 venosa), Fol (Geryonia), Kupffer (Ascidia canina), Strasburger 

 (Phallusia mamillata), Fleming (Anodon), Gotte (Bombinator 

 igneus)], which is generally stated to vanish before the appear- 

 ance of the first furroAV ; but in some cases (Kupffer and Gotte, 

 and as studied with especial care Strasburger) it is stated to 

 divide. Upon the second nucleus, or upon its relation to the 

 germinal vesicle, I have no observations ; but it appears to me 

 of great importance to determine whether this fresh nucleus 

 arises absolutely de novo, or is formed out of the matter of the 

 germinal vesicle. 



The germinal vesicle is situated in a bed of finely divided 

 yolk-particles. These graduate insensibly into the coarser yolk- 

 spherules around them, though the band of passage between the 

 coarse and the finer yolk-particles is rather narrow. The mass 

 of fine yolk-granules may be called the germinal disc. It is 

 not to be looked upon as diverging in any essential particular 

 from the remainder of the yolk, for the difference between the two 

 is one of degree only. It contains in fact a larger bulk of active 

 protoplasm, as compared with yolk-granules, than does the 

 remainder of the ovum. The existence of this agreement in 

 kind has been already strongly insisted on in my preliminary 

 paper ; and Schultz [loc. cit) has arrived at an entirely similar 

 conclusion, from his own independent observations. 



One interesting feature about the germinal disc at this 

 period is its size. 



My observations upon it have been made with the eggs of 

 the Skate (Haja) alone ; but I think that it is not probable that 

 its size in the Skate is greater than in Scy Ilium or Pristiurus. 

 If its size is the same in all these genera, then the germinal 

 disc of the unimpregnated ovum is very much greater than that 

 portion of the ovum which undergoes segmentation, and which 

 is usually spoken of as the germinal disc in impregnated ova. 



I have no further observation on the ripe ovarian ovum; 

 and my next observations concern an ovum in which two furrows 

 have already appeared. 



