460 Royal Society. 



by which addition, as already stated, there is constituted a Graafian 

 vesicle ; and of the latter, the ovisac is now the inner membrane. 

 After this period, then, it is proper to speak, not of an ovisac, but of 

 a Graafian vesicle. The peculiar granules of the Graafian vesicle 

 arrange themselves to form three structures, viz. the membrana gra- 

 nulosa of authors, and two structures not hitherto described, one of 

 which the author proposes to name the tunica granulosa, and the other, 

 which is rather an assemblage of structures than a single structure, 

 the retinacula. The tunica granulosa is a s])herical covering proper to 

 the ovum, and its presence explains why the outer line in the double 

 contour of the thick chorion has remained so long unobserved. At a 

 certain period this tunic, in some animals at least, is seen to have tail- 

 like aj^ijendages, consisting of granules similar to its own. The re- 

 tinacula consist of a central mass containing the ovum in its tunica 

 granulosa, and of cords or bands extending from this central mass to 

 the membrana granulosa. These structures at a certain period 

 became invested by a membrane. The offices of the retinacula ap- 

 pear to be, — first, to suspend the ovum in the fluid of the Graafian 

 vesicle, — next, to convey it to a certain part of the periphery 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 con- 

 veyed, 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 Baer is made up of the parts called by Dr. Barry the 

 tunica granulosa and the central portion of the retinacula ; and the 

 band-like portions, collectively, of what Dr. Barry calls the retina- 

 cula, mainly contribute to produce the appearance 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 

 Mammalia, viz. : 



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



peculiar granules. 



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



envelope 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. 



