S2 SEGMENTATION. 



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 segmentation, which were caused by the non-penetration of 

 my hardening 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 compe- 

 tent 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 \ 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^. He merely states 

 that he has observed the segmentation, and confirms Professor 

 Schenk's statements about the amoeboid movements of the 

 germinal disc. 



1 Loc. cit. 



^ Die Embryonal Aniage der Selachier. Vorlanjlge Mittheilung, Centralhlattf. 

 Med. Wiss. No. 33, 1875. 



