Dr. M. Barry's Researches in Embryology, 459 



not hitherto described. The ovum of all vertebrated animals is con- 

 tained in a vesicle (the " chorion " of some authors, found in Birds, 

 Amphibia, and Fishes), which is essentially the same in structure 

 wherever found, and which he thinks it desirable universally to 

 denominate an ovisac. This vesicle is the "couche interne" of 

 the Graafian vesicle, as described by Professor Baer. ITie Graafian 

 vesicle of Mammalia is nothing more than an ovisac that has 

 acquired a covering or tunic, susceptible of becoming highly 

 vascular, which covering is the " couche externe " of the Graafian 

 vesicle as described by Baer. The ovisac of Birds, Amphibia, and 

 Fishes ("chorion" of some authors), acquires in like manner a 

 covering or tunic, susceptible of becoming highly vascular ; and by 

 the union of the ovisac with this covering, there is constituted a 

 structure analogous to the Graafian vesicle of Mammalia. The quan- 

 tity of yelk in the former being large, 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 external tunic, the 

 ovarian stroma, and the peritoneal investment, — are together called 

 the calyx. From this it will be obvious that the Graafian vesicle 

 is not a structure peculiar to Mammalia, as it has been sup- 

 posed. 



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 examined 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 avithor proposes to denominate jyarcfszVic? 

 ovisacs. The ovisac is often found in a cavity proper to itself, with 

 the walls of which it has no organic union. The granules forming 

 the envelope of the germinal vesicle above referred to, and sub- 

 sequently 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 a formation of others, 

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

 when completely formed 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 diflfusion of granules throughout the fluid of the latter. 

 The ovisac about the same time begins to acquire a covering or tunic. 



