SEGMENTATION. 243 



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

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

 branchs. 



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 1 . 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 recognized 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 Gerbe 2 . 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 3 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 4 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 segmentation, 

 but my observations upon the early stages of this process render 

 it clear that no division of the germinal disc exists subsequently 



1 Rochen u. Haie. It is here mentioned that Coste observed the segmentation in 

 these fishes. 



2 "Recherches sur la segmentation des products adventifs de 1'ceuf des Plagios- 

 tomes et particulierement des Raies." Robin, Journal de r Anatomic et de la Phy- 

 siologic, p. 609, 1872. 



3 "Die Eier von Raja quadrimaculata innerhalb der Eileiter." Sitz. der k. Akad. 

 IVlen. Vol. LXXIII. 1873. 



4 Loc. cit. My denial of the existence of this membrane naturally applies only to 

 the egg after impregnation, and to the genera Scyllium and Pristiurus. 



