Dr Barry's Researches in Embryology. 205 



ovum in its tunica granulosa, and of cords or Lands extending from 

 this central mass to the membrana granulosa. Tliese structures at a 

 certain period become invested by a membrane. The offices of the 

 retinacula appear to be, — first, to suspend the ovum in the fluid of the 

 Graafian vesicle, — next, to convey it to a certain part of the peripher}- 

 of this vesicle, — and subsequently to retain it in the latter situation, 

 and also to promote its expulsion from the ovary. The particular part 

 of the periphery of the Graafian vesicle to which the ovum is conveyed, 

 is uniformly that directed towards the surface of the ovary. The mass of 

 granules escaping with the ovum on the bursting of a Graafian vesicle 

 under the compressor, is composed chiefly of the tunica granulosa and 

 the ruptured retinacula. The " cumulus " of Professor Bacr is made up 

 of the parts called by Dr Barry the tunica granulosa and the central por- 

 tion of the retinacula ; and the band-like portions, collectively, of what 

 Dr Barry calls the retinacula, mainly contribute to produce the ap- 

 pearance denominated the " flat disc" by Professor Baer. 



In Mammalia a thick and highly transparent membrane, — the true 

 chorion, — is formed external to the proper membrane of the yelk, while 

 the latter is in the ovary. The inner part of the substance of the chorion 

 in its early stages is in a fluid state, so that the yelk-ball moves freely in 

 it ; but it subsequently acquires more consistence. There is not any 

 structure corresponding to the chorion in the ovary of other vertebrated 

 animals. 



The following appears to be the order of formation, as to time, of the 

 more permanent parts of the ovum and the Graafian vesicle in Mamma- 

 lia, viz. : — 



1. The germinal vesicle, with its contents, and its envelope of pecu- 



liar granules. 



2. The proper membrane of the ovisac, which forms around this en- 



velope of granules. 



3. The yelk which forms around the germinal vesicle. 



4. The proper membrane of the yelk, which makes its appearance 



while the yelk is still in an incipient state. 



5. The chorion. 

 The covering or tunic of the ovisac; and about the same time, 



the peculiar granules of the ovisac arrange themselves to 

 form 



< 



/'The tunica granulosa. 



The retinacula, and 

 v.The membrana granulosa. 

 Such of these structures as are present in the ovary of other Vertebra- 

 ta, appear to originate in the same order as to time. 



