204 Dr Barry's Researdtes in Embryology. 



vesicle of Mammalia. The quantity of yelk in the former beingf larg>e, 

 that portion of the ovary which contains the structure here referred to 

 (as analogous to the Graafian -vesicle of Mammals) becomes pendent; 

 and now the united coverings of the yelk-ball — viz. the ovisac,, its exter- 

 nal tunic, the ovarian stroma, and the peritoneal investment — are to- 

 gether called the calyx. From this it will be obvious that the Graafian 

 vesicle is not a structure peculiar to Mammalia, as it has been supposed. 



The ovisac has at first an elliptical or ellipsoidal form, becomes more 

 spherical, and in Mammalia is often met with somewhat tapered at one 

 end. The structure of the ovisac in some of the Mammalia may be ex- 

 amined when it does not exceed in length the 50th or even the 100th 

 part of a Paris line, that is, in the latter case the 1125th of an English 

 inch. Myriads of ovisacs with their contents are formed that never 

 reach maturity. Some of the ovisacs which do not reach maturity 

 are situated in the parietes of Graafian vesicles in Mammalia, or of 

 the corresponding structures in other Vertebrata; being sometimes 

 formed in this situation, and sometimes included within the covering 

 which the larger ovisac acquires. The minute ovisacs so situated 

 the author proposes to denominate parasitic. The ovisac is often 

 found in a cavity proper to itself, with the walls of which it has no or- 

 ganic union. The granules forming the envelope of the germinal vesi- 

 cle above referred to^ and subsequently found in the fluid of the ovisac, 

 are very peculiar in their appearance, contain a nucleus, and sometimes 

 also a pellucid fluid, and are intimately connected with the evolution of 

 the ovum. These granules are present in largest quantity in the ovisac 

 of Mammalia ; yet granules essentially the same exist in an early stage 

 in the ovisac of Birds, and are sometimes met with in that of Fishes. 



A continual disappearance of ova, and the formation of others, arc 

 observable even at a very early age. The ovum of Mammalia when 

 completely/ormed is at first situated in the centre of the ovisac. It is at 

 this period supported in the centre of the ovisac by an equable diffusion 

 of granules throughout the fluid of the latter. The ovisac about tlic 

 same time begins to acquire a covering or tunic, 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 jiecu- 

 liar granules of the Graafian vesicle arrange themselves to form three 

 structures, viz. the memhrana granulosa of authors, and two structures 

 not hitherto described, one of which the author proposes to name tlie 

 tunica granulosa, and the other, which is rather an assemblage of struc- 

 tures than a single structure, the retinaculu. The tunica granulosa is 

 a spherical covering proper to the ovum, and its presence explains why 

 the outer line in the double contour of the thick chorion remained so 

 long unobserved. At a certain period this tunic, in some animals at 

 least, is seen to have tail-like appendages, consisting of granules similar 

 to its own. The retinacula consist of a central mass containing the 



