328 DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 



The Tunica granulosa and Retinacula of Mammals compared with the Memhrana 



chalazifera and Chalazoe of Birds. 



92. After what has been stated on the subject of the tunica granulosa and retina- 

 cula, it may by some be supposed that these structures are analogous to the mem- 

 brana chalazifera and chalazse of the Bird. They differ however in several respects. 

 Thus the tunica granulosa and retinacula invest the chorion, whWe^ the membrana cha- 

 lazifera invests the proper membrane of the yelk. The former exist ante coitum, the 

 latter are formed post coitum. The retinacula and chalazse may agree in the func- 

 tion of both having reference to the position of the ovum, but the object of the posi- 

 tions they respectively influence is essentially different, the one determining the 

 situations of the ovum in the ovary, and assisting in its expulsion from that organ ; 

 the other (as it is supposed) performing a function which has reference to the parent 

 heat in incubation. 



Recapitulation. 



93. In Mammalia and in Birds the germinal vesicle and its contents are those 

 parts of the ovum which are first formed. This is probably the case in ova through- 

 out the animal kingdom (12 — 22.). 



94. The germinal vesicle at an early period is surrounded, in Mammalia and in 

 Birds, by oil-like globules and peculiar granules, forming an envelope not hitherto 

 described (14.). 



95. The ovum of all vertebrated animals is contained in a vesicle, (the "chorion" 

 of some authors, as found in Birds, Amphibia, and Fishes,) which is essentially the 

 same in structure wherever found, and which it is desirable universally to denomi- 

 nate an ovisac (2. 47.)- This vesicle is the "couche interne'''' of the Graafian vesicle 

 of Mammalia as described by Baer (24. 25.). 



96. The Graafian vesicle of the Mammalia is an ovisac that has acquired a proper 

 covering or tunic, susceptible of becoming highly vascular, which covering is the 

 "couche externe"" of the Graafian vesicle as described by Baer (25.). 



97. The ovisac of Birds, Amphibia, and Fishes ("chorion" of some authors) acquires 

 in like manner a proper 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 (47.)- The quantity of yelk in the 

 former being large, that portion of the ovary which contains the structure here re- 

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

 now the united coverings of the yelk- ball, viz. the ovisac, its proper tunic, the ova- 

 rian substance, and in some instances the peritoneal investment, are together called 

 the calyx (32. 45. 47.)- From this it will be obvious that the Graafian vesicle is not, 

 as it has been supposed to be, a structure peculiar to Mammalia; nor is it correct to 

 consider the Graafian vesicle of Mammalia analogous to the whole calyx of other 

 animals (48.). 



