Page:Proceedings of the Royal Society of London Vol 4.djvu/96

Rh to all vertebrated animals, but hitherto regarded as the inner mem- brane of the " foUiculus Graafianus " in Mammalia, and by some authors denominated the " chorion " in other Vertebrata. He also describes the real nature of the " foUiculus Graafianus," and its re- lation to the calyx of the Bird ; the germinal vesicle and its con- tents, as being the most primitive portion of the ovum ; the order of formation of the several other parts of the ovarian ovum ; and the true chorion of Mammalia as being a structure superadded within the ovary.

In the second part the author describes a granulous tunic of the ovum of Mammalia not hitherto observed ; the manner of origin of the " membrana granulosa " of authors ; the different situations of the ovum in the Graafian vesicle at certain periods ante coitum, not hitherto observed ; and certain structures by means of w^hich the ovum is made to occupy these several situations.

The follovi^ing are the principal facts made known by Dr. Barry in this memoir ; but other facts are also mentioned, which he in- tends to make the subject of a future communication. In Mammalia and in Birds the germinal vesicle and its contents are those parts of the ovum which are first formed. The germinal vesicle at an early period is surrounded by peculiar granules, forming an envelope 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. The 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 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 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