5/2 THE STRUCTURE AND DEVELOPMENT 



changes which take place in the follicular epithelium. The zona 

 is the first to disappear, and the vitelline membrane next be- 

 comes gradually thinner. Finally, when the egg is nearly ripe, 

 the follicular epithelium is separated from the yolk by an im- 

 measurably thin membrane the remnant of the vitelline 

 membrane only visible in the most favourable sections (PI. 25, 

 fig. 23 v /.). When the egg becomes detached from the ovary 

 even this membrane is no longer to be seen. 



Both the vitelline membrane and the zona radiata are found 

 in Raja, but in a much less developed condition than in Scyllium. 

 The vitelline membrane is for a long time the only membrane 

 present, but is never very thick (PI. 25, fig. 31). The zona is not 

 formed till a relatively much later period than in Scyllium, and 

 is always delicate and difficult to see (PI. 25, fig. 32). Both 

 membranes atrophy before the egg is quite ripe ; and an ap- 

 parently fluid layer between the follicular epithelium and the 

 vitellus, which coagulates in hardened specimens, is probably the 

 last remnant of the vitelline membrane. It is, however, much 

 thicker than the corresponding remnant in Scyllium. 



Though I find the same membranes in Scyllium as Alexander 

 Schultz did in other Squalidae, my results do not agree with his 

 as to Raja. Torpedo I have not investigated. 



It appears to me probable that the ova in all Elasmobranch 

 Fishes have at some period of their development the two mem- 

 branes described at length for Scyllium. Of these the inner one, 

 or zona radiata, will probably be admitted on all hands to be a 

 product of the peripheral protoplasm of the egg. 



The outer one corresponds with the membrane usually 

 regarded in other Vertebrates as a chorion or product of the 

 follicular epithelium, but, by tracing it back to its first origin, I 

 have been led to reject this view of its nature. 



The follicular epithelium. The follicular epithelium in the 

 eggs of Raja and Acanthias has been described by Gegenbaur 1 . 

 He finds it flat in young eggs, but in the larger eggs of Acanthias 

 more columnar, and with the cells wedged in so as to form a 

 double layer. These observations are confirmed by Ludwig 2 . 



Alexander Schultz 3 states that in Torpedo, the eggs are at 

 first enclosed in a simple epithelium, but that in follicles of 



1 Loc. n't. * Loc. at. 3 Loc. cit. 



