OF THE VERTEBRATE OVARY. 6ll 



of finding transitional forms between them and ordinary cells of 

 the germinal epithelium, caused me at one time to seek (un- 

 successfully) for a different origin for them. Any such attempts 

 appear to me, however, out of the question in the case of 

 Mammals. 



TJie egg membranes. The homologies of the egg membranes 

 in the Vertebrata are still involved in some obscurity. In 

 Elasmobranchii there are undoubtedly two membranes present, 

 (i) An outer and first formed membrane the albuminous 

 membrane of Gegenbaur which, in opposition to previous ob- 

 servers, I have been led to regard as a vitelline membrane. (2) 

 An inner radiately striated membrane, formed as a differentiation 

 of the surface of the yolk at a later period. Both these mem- 

 branes usually atrophy before the ovum leaves the follicle. In 

 Reptilia 1 precisely the same arrangement is found as in Elasmo- 

 branchii, except that as a rule the zona radiata is relatively 

 more important. The vitelline membrane external to this (or as 

 it is usually named the chorion) is, as a rule, thin in Reptilia ; 

 but in Crocodilia is thick (Gegenbaur), and approaches the 

 condition found in Scyllium and other Squalidae. It appears, as 

 in Elasmobranchs, to be formed before the zona radiata. A 

 special internal differentiation of the zona radiata is apparently 

 found (Eimer) in many Reptilia. No satisfactory observations 

 appear to be recorded with reference to the behaviour of the two 

 reptilian membranes as the egg approaches maturity. In Birds 2 

 the same two membranes are again found. The first formed 

 and outer one is, according to Gegenbaur and E. van Beneden, 

 a vitelline membrane ; and from the analogy of Elasmobranchii 

 I feel inclined to accept their view. The inner one is the zona 

 radiata, which disappears comparatively early, leaving the ovum 

 enclosed only by the vitelline membrane, when it leaves the 

 follicle. All the large-yolked vertebrate ova appear then to 

 agree very well with Elasmobranchs in presenting during 

 some period of their development the two membranes above 

 mentioned. 



Osseous fish have almost always a zona radiata, which it 

 seems best to assume to be equivalent to that in Elasmobranchs. 



1 Gegenbaur, loc. cit.; Waldeyer,./^. cit.; Eimer, loc. cit.; and Ludwig, loc. cit. 

 - Gegenbaur, Waldeyer, E. van Beneden, Eimer. 



