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DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 



just before the ovum leaves the ovary, this membrane, previously so delicately thin, 

 becomes perfectly distinct and very thick ; and that the chorion, imbibing fluid into 

 its interior, becomes somewhat distended, so that a minute space is visible between 

 the membrana vitelli and the chorion. This thickening of the proper membrane of 

 the yelk, and the distention of the chorion, subsequently proceed much farther, as is 

 proved by the state of ova found in the Fallopian tube. I find also, that the mem- 

 brana vitelli is still visible, and has considerable thickness in minute ova met with in 

 the uterus. This subject will be entered into more fully in a future paper. 



The true Chorion, a Structure superadded within the Ovary in the Class Mammalia. 



52. The figures above referred to, in comparing the ovisac of Mammalia with that 

 of Birds, Amphibia, and Fishes, present also up to a certain period a perfect analogy 

 between the rudiments of the ovum itself in these four classes. We have seen in all, 

 the germinal vesicle and its contents, as well as the yelk, and proper membrane of 

 the yelk. Here, however, the analogy is terminated by the formation, within the 

 ovary, in Mammalia, of a membrane to which there is no corresponding structure 

 within the ot^ori/ in other Vertebrata. This membrane appears to be the true chorion. 

 In the ovary of Birds, Amphibia, and Fishes, it is, I believe, allowed that there is no 

 membrane formed external to the membrana vitelli-f-. The body therefore expelled 

 from the ovary in these animals is not an ovum, but a yelk-ball. The subject will be 

 illustrated by the following Table, showing the parts present (in a mature state) in 

 the ovary of Mammalia on the one hand, and of Birds, &c. on the other : — 



t Professor Rathke, however, finds that in certain Fishes, not provided with an oviduct, the " schaalenhaut" 

 is a production of the ovary. (Burdach's Physiologic, 1837, Band II. § 339.) 



X In other Amphibia and most of the osseous Fishes, the peritoneum does not enter into the formation of 

 the calyx (47. Note). 



§ To be described in Part II. of this Memoir (64. 80. 72.). 



