Page:Proceedings of the Royal Society of London Vol 4.djvu/97

Rh 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 denom.ma.te parasitic 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 diffusion of granules throughout the fluid of the latter. The ovisac about the 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 peculiar granules of the Graafian iesicle arrange themselves to form three structures, viz. the membrana gra- nulosa of authors, and two structures not hitherto described, one of which the author proposes to name the tunica granulosa, and the other, which is rather an assemblage of structures than a single structure, the retinacula. 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 has 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 re- tinacula consist of a central mass containing the ovum in its tunica granulosa, and of cords or bands extending from this central mass to the membrana granulosa. These structures at a certain period became invested by a membrane. The ofiices of the retinacula ap- pear to be, — first, to suspend the ovum in the fluid of the Graafian vesicle, — next, to convey it to a certain part of the periphery of this vesicle, — and subsequently to retain it in the latter situation, and also to promote its expulsion from the ovary. The particular part of the periphery of the Graafian vesicle to which the ovum is con- veyed, is uniformly that directed towards the surface of the ovary. The mass of granules escaping with the ovum on the bursting of a Graafian vesicle under the compressor, is composed chiefly of the tunica granulosa and the ruptured retinacula. The " cumulus " of Professor Baer is made up of the parts called by Dr. Barry the tunica granulosa and the central portion of the retinacula ; and the band-like portions, collectively, of what Dr. Barry calls the retina- cula, mainly contribute to produce the appearance denominated the " flat disc " by Professor Baer.