436 F. M. BALFOUR. 



minous mpinbraiie of Gegenbaur— wliicli, in opposition to pre- 

 vious observ( r!5, I have been led to regard as a vitelline mem- 

 brane. (2) An inner radiately striated membrane, formed as a 

 (iitierentiation of the surface of the volk at a later }>eriod. Both 

 the.se membranes usually atrophy before the ovum leaves the 

 follicle. In Rf-ptilia ^ precisely the same arrangement is found 

 as in Elasmobranchii, 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 

 lleptilia ; but in Crocodilia is thick (Gegenbanr), and approaches 

 the condition found in Scyllium and other Squalida. It ap- 

 pears, as in Elasmobranchs, to be formed before the zona 

 radiata. A special internal differentiation of the zona radiata 

 is apparently found (Eimer) in many lieptiha. No satisfac- 

 tory observations appear to be recorded with reference to the beha- 

 viour of the two reptilian membranes as the egg approaches matu- 

 rity. In Birds * 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 Elasmo- 

 branchii 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 Elasmo- 

 branchs. Interiial to this is a thin membrane, the equivalent, 

 according to Eimer, of the membrane found by the same author 

 within the zona in lleptilia. A membrane equivalent to the thick 

 vitelline membrane of Elasmobranchii would seem to be absent in 

 most instances, though a delicate membrane, external to the zona, 

 has not infrecjuently been described ; Eimer more especially asserts 

 tiiat such a membrane exists in the perch within the peculiar mucous 

 covering of the egg of that fish. 



In Petromyzon, a zoi\a radiata appears to be present,'"* which is 

 divided in the adult into two layers, both of them perforated. 

 The inner of the two perhaps corresponds with the membrane 

 internal to the zona radiata in other ty|)es. In Amphibia the 

 single late formed and radiately striated (Waldeyer) membrane 

 would appear to be a zona radiata. If the suggestion on page 

 431 turns out to be correct the ova of Mammalia possess 



' Gegenbaur, !oc. cit. ; Waldeyer, loc. cit. ; Einicr, loc. cit. ; and Lud- 

 wig, loc. cit. 



^ Gegenbaur, Waldeyer, E. van Benoden, Eimer. 

 ' ' Carllicila, ' Zeil. 1'. wi&s.. Zool.,' lid. \\\. 



