DEVELOPMENT OF ELASMOBRANCH FISHES. 31 



from the lower parts of the gerrainal disc which in Osseous Fish 

 is more distinctly marked off from the food-yolk than in 

 Elasmobranchs. 



I conclude my description of the segmentation by a short 

 account of what other investigators have written about its 

 features in these fishes. One of the earliest descriptions of 

 this process was given by Leydig\ To his description of the 

 germinal disc, I have already done full justice. 



In the first stage of segmentation which he observed 20 — 30 

 segments were already visible on the surface. In each of these 

 he recognised a nucleus but no nucleolus. 



He rightly states that the segments have no membrane, and 

 describes the yolk-spherules which fill them. 



The next investigator is Gerbel I have unfortunately been 

 unable to refer to this elaborate paper, but I gather from an 

 abstract that M. Gerbe has given a careful description of the 

 external features of segmentation. 



Schenk^ has also made important investigations on the sub- 

 ject. He considers that the ovum is invested with a very 

 delicate membrane. This membrane I have failed to find a 

 trace of, and agree with Leydig* in denying its existence. 

 Schenk further found that after impregnation, but before seg- 

 mentation, the germinal disc divided itself into two layers, 

 an upper and a lower. Between the two a cavity made its 

 appearance which Schenk looks upon as the segmentation 

 cavity. Segmentation commences in the upper of the two 

 layers, but Schenk does not give a precise account of the 

 fate of the lower. I have had no opportunity of investigating 

 the impregnated ovum before the commencement of segmen- 

 tation, but my observations upon the early stages of this 

 process render it clear that no division of the germinal disc 

 exists subsequently to the commencement of segmentation, and 

 that the cavity discovered by Schenk can have no connection 



1 Rochen u. Haie. It is here mentioned that Coste observed the segmenta- 

 tion in these fishes. 



2 Eecherches sur la segmentation des products adventifs de Vceuf des Plagxos- 

 tomes et particuUerement des Bates. Eohin, Journal de VAnatomie et de la 

 Physiologie, p. 609, ]872. 



'^ Die Eier von Raja quadrimaculata innerhalb der Eileiter. Sitz. der k. 

 ATiad. Wien. Vol. lxxiii. 1873, 



^ Loc. cit. My denial of the existence of this membrane naturally applies 

 only to the egg after impregnation, and to the genera Scylhum and Pristinrus. 



