244 DEVELOPMENT OF ELASMOBRANCH FISHES. 



to the commencement of segmentation, and that the cavity 

 discovered by Schenk can have no connection whatever with 

 the segmentation cavity. I am indeed inclined to look upon 

 this cavity as an artificial product I have myself met with 

 somewhat similar appearances, after the completion of segmen- 

 tation, which were caused by the non-penetration of my harden- 

 ing reagent beyond a certain point. 



Without attempting absolutely to explain the appearances 

 described by Professor Schenk, I think that his observations 

 ought .to be repeated, either by himself or some other competent 

 observer. 



Several further facts are recorded by Professor Schenk in 

 his interesting paper. He states that immediately after im- 

 pregnation, the germinal disc presents towards the yolk a 

 strongly convex surface, and that at a later period, but still be- 

 fore the commencement of segmentation, this becomes flattened 

 out. He has further detected amoeboid movements in the disc 

 at the same period. As to the changes of the germinal disc 

 during segmentation, his paper contains no facts of importance. 



Next in point of time to the paper of Schenk, is my own 

 preliminary account of the development of the Elasmobranch 

 Fishes 1 . In this a large number of the facts here described in 

 full are briefly alluded to. 



The last author who has investigated the segmentation in 

 Elasmobranchs, is Dr Alexander Schultz 2 . He merely states 

 that he has observed the segmentation, and confirms Professor 

 Schenk's statements about the amoeboid movements of the 

 germinal disc. 



EXPLANATION OF PLATE 6. 



Fig. i. Section through the germinal disc of a ripe ovarian ovum of the Skate. 

 gv. germinal vesicle. 



Fig. 2. Surface-view of a germinal disc with two furrows. 



Fig 5 - 3> 4) 5- Surface-views of three germinal discs in different stages of segmen- 

 tation. 



1 Loc. cit. 



2 "Die Embryonal Anlage der Selachier. Vorlaufige Mittheilung," Centralblalt f. 

 Med. Wiss. No. 33, 1875. 



